Status and outlook of solid electrolyte membrane reactors for energy, chemical, and environmental applications

Abstract

Solid electrolyte membrane reactors (SEMRs) can be operated at high temperatures with distinct reaction kinetics, or at lower temperatures (300–500 °C) for industrially relevant energy applications (such as solid oxide fuel/electrolysis cells, direct carbon fuel cells, and metal–air batteries), chemical (such as alkane dehydrogenation, C–C coupling, and NH₃ synthesis), environmental (De-NO_x, CO₂ utilization, and separation), as well as their combined (one-step coupled CO₂/H₂O co-electrolysis and methanation reaction, power and chemical cogeneration) applications. SEMRs can efficiently integrate electrical, chemical, and thermal energy sectors, thereby circumventing thermodynamic constraints and production separation issues. They offer a promising way to achieve carbon neutrality and improve chemical manufacturing processes. This review thoroughly examines SEMRs utilizing various ionic conductors, namely O²⁻, H⁺, and hybrid types, with operations in different reactor/cell architectures (such as panel, tubular, single chamber, and porous electrolytes). The reactors operate in various modes including pumping, extraction, reversible, or electrical promoting modes, providing multiple functionalities. The discussion extends to examining critical materials for solid-state cells and catalysts essential for specific technologically important reactions, focusing on electrochemical performance, conversion efficiency, and selectivity. The review also serves as a first attempt to address the potential of process-intensified SEMRs through the integration of photo/solar, thermoelectric, and plasma energy and explores the unique phenomenon of electrochemical promotion of catalysis (EPOC) in membrane reactors. The ultimate goal is to offer insight into ongoing critical scientific and technical challenges like durability and operational cost hindering the widespread industrial implementation of SEMRs while exploring the opportunities in this rapidly growing research domain. Although still in an early stage with limited demonstrations and applications, advances in materials, catalysis science, solid-state ionics, and reactor design, as well as process intensification and/or system integration will fill the gaps in current high temperature operation of SEMRs and industrially relevant applications like sustainable clean chemical production, efficient energy conversion/storage, as well as environmental enhancement.

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Liangdong Fan

Dr Liangdong Fan is a Research Professor in the Department of New Energy Science and Technology, College of Chemistry and Environmental Engineering, Shenzhen University, China. He received two PhD degrees, one in Energy Technology from the Royal Institute of Technology (KTH, Sweden) in 2014 and another in Chemical Technology from Tianjin University (China) in 2012. He was a postdoctoral Research Fellow at Nanyang Technological University (2014.06–2015.11). At the end of 2015, Dr Fan joined Shenzhen University as a Lecturer/Research Associate Professor (Principal Investigator) and was promoted to Research Professor in Sept. 2023. His current research interests are developing inorganic solid materials for high temperature solid oxide cells for small molecule conversion into sustainable fuel, chemicals, and energy. He has published more than 100 works including in Nature Catalysis, Energy & Environmental Science, Chemical Science and Electrochemical Energy Reviews with more than 6600 times overall citations and an H index of 47.

Te-Wei Chiu

Prof. Te-Wei Chiu is presently working as a distinguished professor and department head at the Department of Material and Mineral Resources Engineering, National Taipei University of Technology, Taiwan. He received a BS in chemistry in 1995 from National Cheng-Kong University and an MS in chemistry in 1997 from National Chong-Chen University, Taiwan. He obtained a PhD degree in Materials Science Engineering from the Tokyo Institute of Technology, Japan in 2003. Later, he worked as a post-doctoral researcher at Tsukuba Research Centre for Interdisciplinary Materials Science, Tsukuba University (2003–2005). He worked at the National Institute of Advanced Industrial and Science Technology (AIST), Japan from 2005 to 2008. His areas of research interest include the application of nanomaterials and functional oxides for energy technology such as the synthesis of nanoparticles (0D), and fiber (1D), thin film (2D), porous (3D), and tubular architectures for environmental and energy device applications. He has published more than 170 research articles in international journals.

Peter D. Lund

Peter D. Lund is Professor in Advanced Energy Systems at Aalto University with past visiting positions in Germany and China. His main interest is in sustainable energy, including different nanomaterial systems. He is active in senior roles with EU energy initiatives: he chaired the Advisory Group Energy of European Commission and Energy Steering Panel of European Academies Science Advisory Council, and co-chaired the European Academies' Science Advice for Policy on the Energy Transition. He is past vice-chair of the Finnish Climate Panel, member of the Swedish Engineering Academy in Finland and the Finnish Academy of Science and Letters. He is Honorary Professor at Southeast University (Nanjing) and received the prestigious China Friendship Award in 2021. He is Editor-in-Chief of Oxford Open Energy and co-EIC of Wiley Interdisciplinary Reviews on Energy & Environment.

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1. Introduction

Environmental issues such as carbon emissions from fossil fuel-based energy production necessitate efficient energy conversion and use of clean energy sources. Intelligent process innovations will be important to achieve these goals. Electrification through ‘green’ electricity from renewable sources (e.g., wind and solar) linked to electrochemical processes is becoming economically attractive with a major potential to lower key greenhouse gas CO₂ and CH₄ emissions and even convert them to valuable chemicals and fuels.^1–3 Likewise, direct conversion of carbon-based fuel into electricity employing the electrochemical reaction path can lead to reduced CO₂ emissions.⁴ In parallel, chemical synthesis can also more smartly supply reaction energy through electrochemistry using ‘green electrons’.^5–7 In some cases, this has been prompted by new regulations that encourage electricity and electrode/catalyst use. For example, the emerging power-to-X conversion relies mostly on renewable power, where the X stands for H₂, CO, NH₃, CH₄, CH₃OH, C₂H₄, etc., or, liquid fuels such as methanol and ethanol from abundant small molecules, such as H₂O, CO₂, N₂, and CH₄, and also depends on the choice of reaction and reactor operation as well as their thermodynamics.^8–12 Farr and Vayenas first realized this concept in 1980 by producing NO from NH₃ feedstock in a solid oxide fuel cell (SOFC) using a noble-metal catalyst.¹³ Electrification as such has already been applied in chemical industries, e.g., in chloralkali industries, and similar success stories could be envisioned assuming that cheap renewable power would be abundantly available in the future.^5,6,14

To realize both efficient energy conversion and storage, reliable electrochemical technology is needed, in which a tailored combination and coupling of consumption and production of electrical, chemical, and thermal energy is preferred.¹⁵ Moreover, these technologies should be capable of directly coupling electrical and chemical processes to make their industrial use attractive. Furthermore, other benefits of the electrochemical process are as follows.^5,6

(i) Avoiding the reverse reaction balance limitation since the reactants are fed separately, not competing for the same catalytic sites, and have no thermodynamic limitation, which leads to efficient usage of reactants since they can be recirculated.

(ii) Electrochemical cells often result in natural product separation due to the use of a membrane that keeps reactants and products generated at the cathode and anode separately. For example, in water electrolysis, the reaction products H₂ and O₂ already appear at the opposite electrode sides of the membrane, and one of the products may be mixed with steam but it can be easily purified/recovered through condensation.

(iii) Electrochemical methods enable bond-formation steps using electricity, which offers an alternative/controllable and safe driving force namely voltage that can enable operation at lower temperature and pressure out of reach by the conventional catalytic process such as the thermal catalysis process.

(iv) Through intentional specific potential adjustment, the selectivity and conversion rate of the reaction are well controllable when combined with a proper choice of electrode catalyst.

Among the electrification technologies, fuel cells, and their reverse process, electrolysis cells, possess a unique capability of reversible operation, i.e., directly converting the chemical energy in fuels such as hydrogen and hydrocarbons to electricity, or in the electrolysis mode storing the energy from renewable electricity in chemicals, characterizing by high efficiency and large-scale capability since no intermediate process such as combustion/mechanical conversion process is involved. Among the different types of fuel cells, the solid oxide cell (SOC) operates at elevated temperatures as an all-solid-state cell showing the highest priority for such a purpose, also because of fuel flexibility.^16,17 In the all-solid-state form, there is no need for liquid cell materials to avoid corrosion and easy separation on the opposite electrode. Elevated temperature operation also brings multiple advantages vs. low temperature reaction:^9,18 (i) significantly improved reaction kinetics by improving exchange current density (j₀) at a high temperature which is essential for a high reaction conversion rate, leading to considerably improved technological electrical/energy efficiency; the total energy efficiency can be further improved to use highly valuable heat energy under fuel cell conditions and joule heat in the electrolysis mode; (ii) much reduced reliance on rare precious metal catalysts/active components; (iii) fuel flexibility, avoiding highly pure hydrogen needed in polymer exchange membrane fuel cells under close to room temperature operational conditions; a flexible bridge from current fossil fuel to future hydrogen economy can therefore be expected; (iv) reduced electrical energy consumption under solid oxide electrolysis operational conditions; moreover, by utilizing waste heat from different sources, the electrolysis process can be tuned into a chemical-electricity co-generation mode, significantly enhancing the energy utilization efficiency and realizing alternative purposes; (v) achieving homogeneous distribution of the reactant in the form of ions such as O²⁻ and H⁺ from the electrolyte lattice instead of O₂ or H₂ molecules direct contacting to the catalytic reaction under ideal conditions (such as temperature), debarring undesired side reactions such as over-oxidation of CH₄ activation products, improving the reaction selectivity and yields, holding economic and environmental significance; (vi) the same device can be reversibly operated in the fuel cell or electrolysis cell mode, which enables quick deployment of current joint fossil fuel and renewable power sources.

2. Fundamentals of SEMRs

Solid electrolyte membrane reactor (SEMR) development has advanced considerably in recent years and continues to progress rapidly due to the abovementioned characteristics.¹⁹ A general diagram of a SEMR is shown in Fig. 1. The reactor has two chambers that are separated by a gas-tight membrane in most cases, but sometimes a porous membrane is also applied. The membrane is made of a solid oxide material that is capable of transporting O²⁻ or H⁺ at elevated temperatures (typically 873–1273 K but extended to 573–1273 K with the evolution of cell materials and fabrication technology). The SOC can be operated in either SOFC or SOEC mode, depending on whether a current/potential is applied or not or it is operated using various ionic conductors like alkali ions and CO₃²⁻etc.^20,21 In fuel cell mode, fuel conversion takes place simultaneously with the power generation, whereas in the electrolysis mode, additional energy like electricity is supplied to the system to sustain nonspontaneous reactions. In other words, the same unit can make fuel or value-added chemicals through electrolysis during periods of excess power production like abundant solar energy storage in the form of electricity during the daytime, and then convert the fuel back to electricity when the grid needs to support the large electricity requirement. One of the unique characteristics of SEMR is its possibility to operate under transient load conditions over a broad range of feed gas compositions while maintaining faradaic efficiency of nearly 100%, which renders this particularly suitable in future scenarios of intermittent availability of renewables.²² The transfer of O²⁻/H⁺ through the solid membrane is essential for the function of SEMRs, generating a flux of ions to balance the electrons which are transferred through the external circuit, and either oxidize or reduce the reactant to produce fuels or value-added chemicals.

Fig. 1 The working principle of a SEMR for versatile energy, chemical, and environmental applications.

The increasing availability and affordability of renewable energy, mostly from solar and wind sources, has a large impact on the industrial process and transport sectors, which makes electrification and de-carbonization of the chemical industry reasonable and affordable.⁵ In this context, advanced technologies for energy storage, transportation, and direct conversion of electricity into chemical products are required. SOCs or SEMRs, enabling interconversion between chemical energy and electrical energy, can be therefore seamlessly integrated to overcome the challenges associated with the intermittent nature of renewable energy, combined with downstream chemical processes to produce various liquid fuels, rendering it a highly capable all-rounder.^23,24

The SOC is more than a simple power source or electrolysis device. The high operation temperature together with the closed electrode chamber with the catalyst turns the SOC into an ideal chemical reactor or SEMR. Various reaction parameters, such as gas atmosphere, conversion, and selectivity can easily be tuned by adjusting the operational mode, temperature, potential bias, and so on. This provides an attractive advantage compared to thermal and other conversion reactors. Moreover, such a unique characteristic may encourage the possibility of chemical–electrical coupling in SEMRs and open alternative avenues toward electrocatalytic synthesis of chemicals at higher temperatures.²⁵ SEMRs have been employed in many important chemical processes,^20,21,26,27 as follows:

(1) Highly efficient generation of green hydrogen through water electrolysis using renewable power sources or electricity through fuel oxidation

(2) Direct splitting of CO₂ to produce CO fuel and O₂ for de-carbonization and neutrality purposes space exploration

(3) Combining H₂O electrolysis or H₂O/CO₂ co-electrolysis to yield advanced alkanes

(4) Steam or dry-reforming or partial oxidation of hydrocarbons to form compositionally well-controllable syngas

(5) Oxidative coupling of methane (OCM) to form alkanes, olefins and even arenes

(6) Ammonia NH₃/oxynitride synthesis and de-NO_x technology

(7) Oxidative and non-oxidative dehydrogenation of alkanes, olefins and even arenes

(8) Gas separation for a highly purified gas product

(9) Power and chemical co-generation

(10) High temperature metal–air batteries

(11) Direct carbon fuel cells

(12) Many other integrated processes such as combined electrolysis with F–T synthesis in one step or one device…

Even though the water electrolysis cell using a SEMR was reported already in 1968,²⁸ limited interest was shown due to the insufficient performance. However, since the pioneering work in 1979,²⁹ there has been huge research progress on SEMRs for versatile chemical processes. Many excellent studies, review papers, and monographs that report, discuss, and analyze the applications and understand SEMR processes were reported. A broad range of aspects concerning fundamental operating principles and the applications of these solid-state electrolytes and these types of reactors in heterogeneous catalysis and chemical cogeneration (chemical products and energy) have well been documented.^1,30–33 However, most of the previous work focus on particular aspects of the fields like CO₂, H₂O, and H₂O/CO₂ co-electrolysis^34–36 methane coupling to produce C₂ hydrocarbon^{1,26,31,34,37–39} or two wide coverage,^27,40–42 and needed significant literature update^31,43,44 since considerable progress has been made in the last decades with the positive evolution of the key electrolyte material,^20,45,46 and catalyst/electrode components,⁴⁷ the cell geometries from typical construction from SOC with dense electrolyte and porous anode and cathode (alternative functional layer on/in electrode even porous electrolyte based),^48–50 never mentioned the progress in the system combined operation and coupling/integration.^51–53 Therefore, a timely and comprehensive review of the solid electrolyte membrane is needed.

Membrane reactors operating at low temperatures (300 °C) will not be discussed in this article. The focus will be on membrane reactors operating at a temperature between 300 and 800 °C because the majority of industrially important catalytic processes occur in this temperature range with favorable reaction thermodynamics and kinetics. The present work refers to SEMR-related scientific fields such as electrochemistry, materials science and technology, chemical reaction engineering, solid-state physics, and heterogeneous catalysis as well as calculation/quantum chemistry. The aim of the present review is not only to provide a survey of relevant studies but also to present the characteristic features, methods, and techniques used in this class of reactors. Hence a classification of SEMRs based on different ionic conductors (O²⁻, H⁺, mixed O²⁻ and H⁺, carbonate ionic composite conductor) and working modes (fuel cell or electrolysis, pumping or extracting mode) is presented, with case study demonstrations (Fig. 2). Discussion is moved from the typical reactions based on different ionic conductors to SEMRs with featured multi-functionality and/or specific geometrics. A particular section on the use of SEMR for gas separation is also highlighted using the specific properties of the solid electrolyte materials. The system and device integration or coupling or process intensification with SEMRs is also included. In addition, a typical phenomenon or an additional functionality regarding SEMRs, i.e., electrochemical promotion of catalysis (EPOC) effect is also included to highlight the fundamentals of accelerated reaction rate under the electrical/thermal coupling conditions. Finally, the conclusion and future outlook of this interesting yet important field are made and recommendation of those reactors for industrial application purposes is suggested.

Fig. 2 A schematic illustration of outlining the focus of this work.

3. Advances and challenges of SEMRs with various ionic conductors

Similar to the SOC, the types and compositions of the ionically conductive electrolytes in the SEMRs play key roles in reactor temperature, electrode/catalyst materials, detailed reactor classification, as well as reactor thermodynamics and kinetics. To ensure high faradaic efficiency, the ionic transport number of electrolytes should be close to unity and remain stable under operational conditions such as a harsh reducing atmosphere and high applied voltage, under which a number of electrolyte materials, such as the partially electron-conductive doped ceria electrolyte, cannot be employed even in a low temperature range. Currently, the typical oxygen ionic conductor is still the state-of-the-art yttrium-stabilized zirconia (YSZ). However, the high activation energy of ionic conduction requires an operating temperature above 700 °C. High temperature results in high reaction kinetics and favorable thermodynamics in the electrolysis mode. However, it also results in a deep reaction to produce untargeted chemicals, and the quick degradation of reactor performance due to cell component instability, incompatibility, and mechanical property issues. To reduce the reactor temperature, LaGaO₃ electrolytes typically doped with strontium and magnesium (denoted as LSGM), proton conductive BaCeO₃, and BaZrO₃ perovskite oxide and their solid solution state are developed. Particularly, proton conduction not only reduces the reactor temperature but also widely expands the reactor diversity, with the non-oxidative dehydrogenation of alkanes to olefins as a typical example.⁵⁴ In addition, the mixed conduction of the Ba(Zr,Ce)O₃ material is also used for functional purposes,⁵⁵ like coke removal⁴⁷ and double-chamber hybrid electrolysis.⁵⁶ In this section, the discussion will be on SEMRs with different ionic conductors and operational modes, like pumping and extractor modes.⁵⁷ A typical example will be given to show the advances.

3.1 SEMRs with oxygen ionic conductors

3.1.1 Oxygen extraction/electrolysis reactors. In the extractor mode, the oxygen atom in the gaseous oxygen-containing reactants such as H₂O and CO₂ will be extracted leaving behind valuable H₂, CO, their mixed compounds like syngas, as well as the upgraded electrochemical products like CH₄, CH₃OH, and NH₃. From the aspect of cell voltage, a potential or additional electrical energy is required and applied to the reactor to decompose the aforementioned chemicals, which leads to the storage of electrical energy in chemical molecules. Among these, green H₂ electrolysis products from water using renewable electricity are now considered as a key strategy to reduce carbon emissions, while CO from CO₂ is also considered important for carbon-neutral cycling or feedstock for chemical industry. A mixture of H₂/CO is a typical composition for syngas, which is also an important feedstock for fuel upgradation for the production of specialty and commodity chemicals, like alkanes, alkenes, and their oxygenated derivatives. There are already commercial plants that produce H₂ in the electrolysis, although most of them are based on low temperature processes with alkaline solutions or PEM systems.⁵⁸ Meanwhile, SEMR plants with SOECs have attracted increased attention because of their high efficiency in converting electricity into fuels and chemicals using renewable electrical energy and quick development of materials, cell structure, cell fabrication technology evolution as well as increased capability to operate SOC devices reversibly with a good round-trip efficiency.^59,60 Such reactors can use high operating temperatures to maximize electrical energy efficiency. In the SOEC, the oxygen ions are transported from the cathode across the electrolyte membrane to the anode, where they can be again used for other purposes, like hydrocarbon fuel auto-thermal reforming or partial oxidation. However, we would prefer to describe this kind of process as oxygen-pumping mode. At the cathode side, where the electrolysis reaction occurs, O²⁻ is electrochemically extracted from gaseous oxygen-containing molecules under an externally applied potential.⁶¹
3.1.1.1 Water electrolysis for green hydrogen production. In terms of excellent electrical efficiency and decoupling direct reaction process by electrolyte, and direct adaptation of SOC devices with improved reliability^62–64 and efficiency,⁸ green hydrogen generation based on SOECs is already entering the market. There are already a lot of excellent experimental studies and reviews to cover information on SOECs including steam electrolysis thermodynamics, SOEC electrolyte/electrode material/catalyst development, electrode structure design, cell fabrication technology, reaction kinetics/mechanism, as well as cell durability for long-term operation.^{8,38,42,62,65–71} The longest steam electrolysis operational time based on SOEC is 23 000 h (reaching the targeted period) using Ni-GDC/6Sc1CeSZ/GDC/LSCF cells with a voltage degradation rate of 0.57%/1000 h based on a 45 cm² cell study at 0.9 A cm⁻².⁶² The current research interests focus on the performance and durability improvement for commercial applications including the anode Ni-based cermet electrode's redox and poisoning issue and the anode/electrolyte interface delamination issue⁶¹ due to the high oxygen concentration and aggregation in this area. Strategies such as the application of mixed ionically and electronically conductive electrode materials,⁶¹ especially for materials with high oxygen capacity,^72,73 and modification of cell electrode microstructure, like using transitional functional layers⁷⁴ or straight pores⁷⁵ and the smart strategy of reversible operation between fuel cells and electrolysis mode⁷¹ have been proposed to alleviate these issues.
3.1.1.2 CO₂ utilization. Similar to steam electrolysis, extensive studies on the efficient reduction and conversion of CO₂ using SOECs were reported in the literature.⁷⁶ These include, e.g., the pioneering work on CO₂ electrolysis with SOECs for use on Mars to produce O₂ for life-support.⁷⁷ As a notorious greenhouse gas, its utilization and conversion to valuable chemicals are considered the most crucial approach to realizing carbon neutrality.²⁶ In this context, the industrial generation of carbon-based molecules—such as drugs, commodities, and polymers, in addition to chemical fuels—from CO₂ is subject to intense research using different conversion technologies including SEMRs, electrochemical reduction in aqueous solution, and photocatalytic reduction (Table 1), among which electrochemical reduction using SEMRs shows the most promising characteristics like low energy consumption, quick reaction, high Faraday efficiency, and energy efficiency, even though its product is only CO because of the thermodynamic limitation.^78,79 CO₂ reduction in the cathode of an O-SOEC generally follows three steps:^80–84 (1) adsorption of CO₂ on the surface of the electrocatalyst with lattice oxygen ions and formation of carbonate species or with an oxygen vacancy to form bicarbonate intermediates, (2) decomposition of the intermediates by electrons in sequence, which is normally considered as the overall rate-determining step, and (3) desorption of the CO molecule. However, the exact reduction process is highly dependent on the applied catalyst, for example, for a ceria-based cathode electrocatalyst, Ce^3+/4+ participates in all the elementary steps of CO₂ reduction.⁸⁵ Generally, CO is the thermodynamically favored product for CO₂ reduction at elevated temperatures. Recently, there has also been interest in adopting PCECs for CO₂ reduction, in which CO₂ acts as the precursor to react with H⁺ or H₂ at the electrode/electrolyte interface yielded by steam electrolysis at the cathode and transported through the electrolyte layer. Under such conditions, CO₂ could be electrochemically or thermally activated at the cathode to produce CO or upgrade to interesting low-carbon hydrocarbon fuels.⁸⁶ It should be mentioned that different from H₂O splitting, CO₂ reduction should overcome a larger energy barrier (750 kJ mol⁻¹) because of the large C=O binding energy. Moreover, CO₂ is a linear molecule; combined with its larger molecular volume, its diffusion, adsorption, and activation attract particular attention and require higher energy input and higher reduction temperature. Therefore, direct CO₂ electro-reduction in SOECs normally works above 700 °C. Under such harsh conditions, only stabilized zirconia and doped LaGaO₃ can be used, while ceria is normally used as a functional layer to overcome chemical compatibility, and electrode active species to increase the triple-phase boundary to improve reaction kinetics.

Table 1 Comparison of different electrochemical/photocatalytic CO₂ utilization/conversion technologies

	SEMRs	In aqueous electrolytes	Photocatalytic reduction
Energy consumption	Low	High	Low, sustainable
Conversion rates	Quick	Voltage dependent	Slow rate
		Slow rate
Product selectivity	Sole product CO	Rich products like CO/CH4/CH3OH/C2H5OH…
Others	High faradaic efficiency	Poor energy efficiency	Poor energy efficiency
	Good stability	Low yield	Insufficient stability
	High energy efficiency	Insufficient stability

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Considerable attention has been paid to the development of active cathode electrocatalysts to replace Ni, an active but fragile catalyst.^87–89 Alternative cathodes are related to perovskite oxides, a series of compositionally and structurally flexible complex oxides, because of the phase durability and high resistance toward the thermodynamically favored carbon deposition issue. However, their bulk CO₂ reduction activity is far way inferior to the Ni-based cathode. Therefore, current efforts focus on modifying the perovskite oxide through doping, constructing a hybrid catalyst, and optimizing the microstructure to adjust the electronic structure and increase the electrode adsorption capability, the active site, and reduce the gas diffusion barrier. For example, Xi et al.⁹⁰ reported that replacing Mo with Mg in Sr₂FeMoO_6−δ not only improved the electrode redox stability but also enhanced the electrolysis efficiency by 100%. The improved cell performance was ascribed to the reduced formation energy of oxygen vacancies induced by reduced d–p band coupling which is believed to be caused by the shortage of d electrons of the Mg element. In other words, the electronic structure of the active transition metal is modified by the doping approach which favors the CO₂ reduction rate-determining step, the activation, and the dissociation step. The same group also demonstrated that a proper increase of the Fe and Mo ratio in the same Sr₂FeMoO_6−δ double perovskite led to exceptional CO₂ reduction activity, comparable to the state-of-the-art Ni-based electrode. They confirmed that the Fe-ion oxidation state increases with a slight increase of Fe element that favors the metal–oxygen hybridization and shifts its bulk O p band energy toward the Fermi level. Such electronic structure variation reduces the O-vacancy formation and migration energy, resulting in more oxygen vacancy defects and favoring O ion transport, promoting the catalytic reaction kinetics.⁹¹ Different from the above cation replacement method, anion doping is also demonstrated to favor CO₂ adsorption and activation. For example, the replacement of O with F ions could reduce the CO₂ reduction electrode area polarization resistance from 1.130 to 0.656 Ω cm² at 800 °C.⁹² Such improved electrochemical performance is induced by the increased surface reaction rate constant and bulk diffusion coefficient by factors of 2–3 as reflected by the approximately doubly increased CO₂ adsorption capability, plus 35–37% oxygen vacancy concentration. A more interesting work reported adopted a single-site anchored perovskite electrocatalyst for CO₂ reduction in SOECs for the first time.⁹³ Through compositional control of the content of Pd raw materials, Pd single-site perovskite oxide materials instead of homogeneously doping are formed by a simple mechanical mixing and pyrolysis. The single site enables structurally Pd coordinative unsaturation, which results in plenty of oxygen vacancies. The latter facilitates CO₂ dissociative adsorption, electron transfer, and mass transport, thus leading to improved CO₂ electrolysis performance.

Doping generally leads to elevating the elementary reaction rate of the CO₂ reduction reaction. However, with the single-phase perovskite cathode, the final performance is limited by the bulk properties of perovskite oxide. The low electrical conductivity under the cathode atmosphere and the limited surface area and catalytic activity normally lead to insufficient activity. A common method is to prepare hybrid catalysts for CO₂ reduction reactions. For example, a composite material containing (La_0.75Sr_0.25)_0.97Cr_0.5Mn_0.5O₃ (LSCM) and Ce_0.6Mn_0.3Fe_0.1O₂ (CMF) was prepared using a simple mechanical mixing method and applied as a cathode on LSGM for CO₂ reduction.⁹⁴ The targeted SOECs show excellent CO₂ reduction performance of 2.64 A cm⁻² at 1123 K and applied voltages of 1.5 V in a 50% CO₂/50% CO atmosphere, which is the ever-best single-cell performance reported in the literature. The much-improved CO₂ reduction activity is ascribed to the enhanced CO₂ adsorption ability, surface oxidant catalytic activity, and electrocatalytic activity induced by Fe and Mn-doped ceria. It should be noted that singly doping Mn or Fe into ceria has limited solid solution capability. On the other hand, co-doping can unexpectedly increase the solid solution content, up to 40% in this case, which significantly changes the material's chemical and physical properties, leading to exceptional performance in this work. Besides the mechanical mixing to form the hybrid catalyst, recent self-assembly including exsolution methods also showed promising performance in SOECs for CO₂ reduction.^95–101 For example, recent work reported adopting the self-assembly method to prepare Pr(Ca)Fe(Ni)O_3−δ perovskite and Ca₂Fe₂O₅ brownmillerite dual-phase composite in a nominal perovskite phase of Pr_0.2Ca_0.8Fe_0.8Ni_0.2O_3−δ for CO₂ reduction.¹⁰² The Ca₂Fe₂O₅ in the double-phase material with a large highly ordered oxygen vacancy concentration is used to adsorb CO₂ while the perovskite oxide functions as the catalyst for CO₂ dissociation. The composite cathode with a synergistic effect has led to improved CO₂ reduction performance. Recently, a redox exsolution strategy has been widely used for CO₂ reduction in SOECs. The exsolution led to a well-manipulated nanoscale metal/oxide interface with enriched oxygen vacancy for CO₂ high temperature adsorption and conversion. Xie et al.^103,104 reported that the decomposition of oxide precursors Ni/Nb_1.33(Ti_0.8M_0.2)_0.67O₄ (M = Mn, Cr) yielded redox MnO_x with large oxygen vacancy concentration and Ni/Cu metal or alloy on ceria/CrO_x which are active for CO₂ reduction. The strongly anchored functional phase and oxygen vacancy at the interfaces not only improve the electrolysis performance but also lead to better durability under the redox cycle and long-term operation. Another exsolution example is the preparation of active metal or alloy nanoparticles on phase structural redox stable perovskite oxides. The metal/oxide interface formed by exsolving CoFe or RuFe alloy on the parent perovskite oxide can significantly improve the CO₂ electrolysis performance through enhanced adsorption and activation under SOEC conditions.^97,98,105 They also observed in situ the atomic scale exsolution of metal nanoparticles in the order of Co ions, Fe ions, and the final CoFe alloy nanoparticles using STEM. Moreover, the CO₂ reduction reaction performance can be retrieved during 12 redox cycles. More recently, considering the sluggish diffusion process of dopant cations and limited driving force for metal/alloy nanoparticle exsolution, they proposed to promote the exsolution of RuFe alloy nanoparticles via repeated redox operation which enriches the Ru ions underneath the perovskite oxide surface. With the repeated redox operation, more RuFe nanoparticles aggregated and resulted in 74.6% current density improvement as well as 1000 h durability at 1.2 V and 800 °C. Both the suggested exsolution method and in situ exsolution process observation are important steps for understanding and application of this promising technology. A similar study demonstrated that the exsolved metal/alloy nanoparticles on an oxide parent showed higher performance and electrolysis durability than the infiltrated one with a similar composition and operational conditions.¹⁰⁶ When used as the cathode for SOECs, NiFe nanoparticles exsolved (La_0.75Sr_0.25)_0.95Cr_0.5Fe_0.35Ni_0.15O_3−δ (LSCrF as), NiFe infiltrated LSCrF and the one without any nanoparticles showed the corresponding current densities of 1.15, 0.8 and 0.59 A cm⁻² at 800 °C and 1.5 V, and the former showed a degradation rate of 0.012% h⁻¹ during a tested period of 260 h while the intermediate one degraded in a rate of 0.15% h⁻¹ in 26 h due to the severe agglomeration of the infiltrated nanoalloy, suggesting the nanoparticle exsolved LSCrF as promising cathode material for efficient CO₂ utilization.

Apart from the development of active and reliable cathode materials for CO₂ reduction, electrode microstructure optimization in SEMRs is also an important approach to improving cell performance and durability by increasing gas diffusion and exposing more active sites for CO₂ reaction.^75,107,108 For example, microchannel structural cathode-supported SOECs were constructed for CO₂ reduction.¹⁰⁷ The microchannel not only allows quick CO₂ gas diffusion, reducing the concentration polarization but also facilitates high-content catalyst loading and distribution. The SOEC based on deposited Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) cathode showed an electrode polarization resistance of 0.25 Ω cm² and a current density of 1.1 A cm⁻² at 1.5 V in the dry CO₂ without safety gas as well as promising stability; these performances reached the highest level at that time.

Much progress has been made in SOECs for CO₂ reduction. However, the research and applications still lag behind steam electrolysis. Many perovskite oxide cathodes have already demonstrated comparable performance to the Ni-based cermet electrode. Moreover, the current CO₂ reduction in SOECs is still limited to high-temperature operation (≥700 °C), to maintain sufficient current output, and an intrinsic large electrical input, which make the conditions severe for system stability for practical applications. A reduction of the operational temperature to 300–600 °C will benefit the system stability, involve low investment, as well as extend the product range, and not just limit to CO at the elevated temperature.³⁶

3.1.1.3 CO₂/H₂O co-electrolysis.
3.1.1.3.1 Compositionally tailor-made syngas by co-electrolysis. Syngas is an important feedstock for various important industrial applications. Since SOECs can reduce both steam and CO₂, the co-electrolysis of H₂O and CO₂ to obtain syngas has received increased attention in recent years.^9,70,109 Generally, under the SOEC conditions, both the water splitting and CO₂ reduction reaction take place, and the water-splitting rate is considerably quicker than that of CO₂ reduction due to the low activation energy barrier and consequent higher kinetics,^110,111 which leads to the advantage of low thermo-neutral voltage, i.e., lower electrical consumption. For example, Torrell et al.¹¹⁰ reported that with H₂ safety gas, the electrolysis current densities at 1.7 V with pure steam and with H₂O : CO₂ ratios of 1 : 1 were 750 and 620 mA cm⁻². This suggested that the main reaction in the mixed gas is steam electrolysis. However, the presence of the reverse water shift reaction (RWSR): CO₂ + H₂ = =CO + H₂O (ΔH₂₉₈ = 41 kJ mol⁻¹) at the elevated temperature makes the final gas composition complex. In other words, both the electrochemical and thermal-catalysis processes contribute to the final products. Workable strategies to improve the individual CO₂ electrolysis rate with the H₂O/CO₂ mixed inlet gas are highly desirable. Either the application of a higher electrical field¹¹¹ or improving the operating temperature¹¹² was suggested to boost CO₂ electrolysis performance by considering the high energy barrier of CO₂ activation. Differently, recent work reported that a BaCO₃ nanoparticle infiltrated Ni-YSZ fuel electrode improved the electrolysis performances of SOECs with H₂O, CO₂, and H₂O–CO₂ inlet gas, respectively, because of the enhanced surface charge transfer process, while it degrades the SOFC performance.¹¹³ The highest improvement is achieved in CO₂ reduction, which largely contributes to the improved chemical adsorption of CO₂ with BaCO₃ infiltration.

Syngas with a controllable (diverse ratio range) or ideal composition that favors downstream applications like the Fischer–Tropsch (F–T) process to synthetic fuel production at an H₂/CO ratio of 2 : 1 is preferred. To reduce the RWSR process, co-electrolysis without safety gases like H₂ and CO is preferred. This can be achieved by using non-metal-based fuel electrocatalysts that do not require safety gases,^107,110 even the state-of-the-art Ni-cermet electrodes could be electrochemically reduced and activated.¹¹⁴ For example, syngas with an H₂/CO ratio close to 2 can be obtained from an efficient 20% CO₂–80% H₂O electrolysis without a safety gas on an Sr₂Fe_1.5Mo_0.5O₆ nanostructure infiltrated YSZ scaffold with a finger channel and a modified phase-inversion tape-casting method.¹⁰⁷ Such a nanostructure electrode also can directly use CO₂ as the fuel, yielding a current density of 1.1 A cm⁻² at 1.5 V at 800 °C, comparable to a Ni-based cermet electrode. Another work proposed adjusting appropriate inlet gas fraction, temperature, and electrolysis current to obtain a controllable H₂/CO ratio of approximately 2 in a symmetric SOEC using the same electrode materials as the above case for H₂O/CO₂ co-electrolysis.¹¹⁵ With an appropriate H₂/CO ratio, the upgradation to synthetic fuel will be facilitated in the commercialized F–T plants.

3.1.1.3.2 In situ methanation using O-SOECs with a higher round-trip efficiency. Another important reason to develop co-electrolysis technology is its capability to produce higher hydrocarbon fuels like CH₄ accompanied by the electrolysis process in the one-step pathway, instead of first producing syngas that is then processed downstream into chemicals. Compared with syngas, one step pathway to hydrocarbon fuel can store renewable power sources in higher energy density and use the existing infrastructure such as natural gas pipelines and end-use appliances.¹¹⁶ Generally, the methanation process was taken placed through the following reactions: CO + 3H₂ = =CH₄ + H₂O (ΔH₂₉₈ = −206 kJ mol⁻¹) and CO₂ + 4H₂ = =CH₄ + 2H₂O (ΔH₂₉₈ = −165 kJ mol⁻¹) with the obtained syngas from H₂O/CO₂ co-electrolysis. The second reaction is also known as the Sabatier reaction. Therefore, the methanation reaction does not require the reduction of CO₂, which can reduce significant energy. Instead, according to the literature, compared with the co-electrolysis in SOECs to obtain syngas and then integrating with the F–T process to produce synthetic fuel, the one-step process has multiple benefits:^117,118 (1) the energy utilization efficiency can be significantly improved in the one step process since it balances the heat in situ between the exothermic methane production reaction and the endothermic H₂O/CO₂ electrolysis reaction in a single device. Taking H₂O electrolysis to obtain H₂ and then combining with CO₂ to produce CH₄ as an example, the thermodynamic maximum round-trip efficiency of H₂O electrolysis is around 76% at 800 °C, which is already much lower than that required for direct CH₄ synthesis, both under the thermal neutral conditions; (2) reduced industrial facilities, free-from the F–T process and the corresponding instruments and investment; (3) improvement in system reliability. Moreover, this process could be economically competitive with the biomass-to-liquid process if the provided electricity is from nuclear power and surplus renewable power.^119,120

The methanation process through the co-electrolysis technology has been widely investigated using the O-SOEC, in which CO₂ and H₂O are co-fed into the cathode chamber, and the O element is extracted to form O²⁻ which is then transported through the electrolyte layer to the anode layer, where it is released electrons and moved to the cathode through an external circuit, forming O₂ gas. In the cathode chamber, CO and H₂ are produced which can react with the remaining H₂O and CO through the above equation to produce methane under an electric field or thermal–catalysis conditions. It was initially proposed by NASA in the manned space colonization on Mars to convert the Martian CO₂ atmosphere and H₂O into methane fuel and build a astronaut life-support system. CO₂ hydrogenation is first carried out using the YSZ solid electrolyte and Rh electrode in the configuration of a single-chamber reactor.^103,104,122 It is found that both CO and CH₄ were produced at temperatures of 346–477 °C and their yield rates have an inverse relationship with each other with the application potential, i.e., the rates of CH₄ were enhanced with positive potentials while the rate of CO formation with negative potentials,¹²² suggesting different electrochemical promotion effects. Later work developed a monolithic SOC (Rh/YSZ/Pt or Cu/TiO₂/YSZ/Au) reactor to investigate the hydrogenation of CO₂ at atmospheric pressure.¹²³ For the former cell, it is found that the selectivity of CH₄ can reach 12%, while in the case of Cu/TiO₂/YSZ/Au cells, the selectivity of CH₄ could reach ∼100% under open-circuit polarization conditions at temperatures of 220–380 °C. Those pioneering research studies highlighted the important roles of cell structure design and electrode catalyst selection.

It should be noted that the H₂O or CO₂ electrolysis reaction preferably takes place at the elevated temperature with high reaction kinetics, while the exothermic methanation reaction is more favored at the reduced temperature of recommended value of 300–500 °C. The mismatch in temperature makes the heat balance of those two reactions quite difficult. In 2010, Bierschenk et al.¹¹⁸ experimentally observed that methane-containing fuels are produced during electrolysis operation, which decreased the thermal-neutral voltage (SOEC applied voltage) and thereby allowed improved round-trip storage efficiency. They observed that the CH₄ yields are more favorable at reduced temperature (∼600 °C), enabled by the LSGM electrolyte to replace the conventional YSZ electrolyte, and/or increased pressure (∼10 atm) based on their thermodynamic calculations and preliminary experimental analysis. The obtained CH₄ yield can be improved from trace to 14.3% with the reduction of SOEC operational temperature from 750 °C to 600 °C based on the same material system, and it can be further improved to 26.7% by properly applying the operational pressure. This opened a new avenue for methanation R & D. Almost at the same time, Xie et al.¹²⁴ reported the use of a novel Fe–La_0.2Sr_0.8TiO₃ fuel electrode for direct synthesis of methane by combining the co-electrolysis of CO₂–H₂O in an oxygen-ion conducting SOEC and in situ Fischer–Tropsch-type synthesis in a synchronous process. A CH₄ rate of 0.0047 mL min⁻¹ with a faradaic yield of 2.8% at 650 °C with a feedstock of 2 mL per min H₂O and 1 mL per min CO₂ were obtained. The methanation yield is limited by the heterogeneous catalysis process which is against the thermodynamic behavior. Following these pioneering studies, Chen et al.¹²¹ smartly designed long enough tubular cells with two distinct temperature zones: one for H₂O–CO₂ co-electrolysis at elevated temperature with a high conversion rate and one for the F–T synthesis process at the reduced temperature (down to 250 °C) for the one-step methanation reaction through co-electrolysis (Fig. 3c). The yield of CH₄ was substantially improved to 11.84% and an overall CO₂ conversion ratio of 64.1% at atmospheric pressure. Based on a similar cell design, an increased CH₄ yield of 23.1% was demonstrated by optimizing the H : C ratio in the inlet gas by adding H₂ into the cathode chamber with H₂O–CO₂ mixed gas.¹²⁵ However, the split-zone approach engenders thermo-mechanical stress to the super-long cell and delicate electro-ceramic cell components and brings reliability concerns. A combination of efficient co-electrolysis and the F–T process at the reduced temperature is preferred.

Fig. 3 The proposed cell structure design for coupling the low temperature F–T process and high temperature co-electrolysis process.¹²⁴ Reproduced with permission.

Further work tried to improve CH₄ yield through reducing temperature and the addition of H₂ gas.¹²⁶ It was observed that without the H₂ gas, the CH₄ yield increased negligibly from OCV to the applied voltage of 1.5 V, while with the 20% H₂ fed into the cathode, a clear increase of the CH₄ yield by 3–4% is observed at 550 °C. With the application of the higher ionic conductive ScSZ electrolyte layer and Ni-YSZ cathode catalyst, a maximum CH₄ yield of 9.94% was achieved. Based on the two-dimensional multiscale electro-thermal tubular SOEC model calculation with parameter optimization on the materials, flow modes, and operating conditions, the CH₄ production ratio can be improved from 13% to 50% at 550 °C and 1.3 V for the same inlet gas composition, even though the temperature is considered still too high to obtain acceptable CO₂ to CH₄ yield. Furthermore, the applied model calculation results revealed that the H₂O/CO₂ co-electrolysis and methanation reactions can be simultaneously enhanced by feeding a cold gas in counter-flow mode and under pressurized conditions. At 29 bar, the tubular LSGM-based SOEC could realize a conversion ratio of 98.7% and an electricity-to-gas efficiency of 94.5% in the thermal neutral mode.¹²⁷ Such a theoretical calculation suggested that developing methane-integrated SOECs operating at 300–600 °C can efficiently convert CO₂/H₂O into more valuable chemicals, which will be a new research direction in the future. However, the concept of in situ methanations in SOECs is still at a nascent stage and requires significant advancements in SOEC materials, F–T catalysts, and cell/reactor geometric design, along with optimization of cell operating conditions (temperature, pressure, and applied potential) to maximize CH₄ yield below 550 °C. Another interesting but important branch for the one-step methanation process is realized by the PCECs due to the superior proton conductivity at low temperatures that are crucial to realizing high reaction kinetics and CH₄ yields, which will be discussed later.

3.1.1.3.3 Fuel-assisted electrolysis (FAE). Under electrolysis conditions, though the major voltage loss is mainly ascribed to the electrode polarization resistance of the fuel reduction reaction for both steam and CO₂ electrolysis, the polarization loss for the oxygen evolution reaction (OER) in the anode chamber should not be ignored. In the initial study, the OER was used to prepare oxygen sources for life support in space. It can be employed in further applications considering the high value-added pure and fresh O₂ and provides the opportunity for process intensification. An important application is reforming CH₄ or other less purified/valuable solid fuels like carbon^128,129 and coal;¹³⁰ gaseous fuels such as CO,¹³¹ natural gas,^132,133 and biofuel & biomass¹³⁴ were also used to obtain syngas feedstock or achieve chemical upgradation, besides the hydrocarbon fuel. This comprises an important and hot research topic, fuel-assisted electrolysis (FAE). A distinguishing characteristic of FAE is its strong coupling of the endothermic electrolysis reaction with the exothermic fuel reforming or upgradation reaction, and the uphill potential barrier for the OER is also avoided, which has consequently led to significantly reduced electrical power input (both cell voltage and electricity are reduced) while increasing the production of valuable gases. Moreover, with the increase in temperature to a certain point, such an FAE device can be turned into a combined chemical and power co-generation device.¹³⁵ According to the one-dimensional elementary reaction kinetic model analysis,¹³¹ the CO-assisted steam electrolysis cell could save 80% of electricity and produce better steam electrolysis performance. It was also found that the assisted electrolysis cell based on different fuels showed different anode polarization behaviors. Another study even proposed that such a cell can be operated at room temperature and can realize a higher hydrogen generation rate, suggesting promising application in wide cell temperature windows for chemical and power generation with low carbon emission advantage.¹³⁶ Although the overall reaction of the FAE process is the same as that for fuel steam/CO₂ reforming, it is more advantageous to perform such a process by an electrical-to-chemical energy conversion route based on the solid electrolyte membrane reactor, which is promising to serve as a load lever for energy storage between renewable electricity and fuel. In addition, FAE can be implemented as distributed syngas generator under atmospheric pressure, which is different from traditional reforming reactor that typically operates as a large chemical plant in pressurized operation.

3.1.1.3.3.1. Solid carbon/coal-assisted electrolysis reaction . In 1979, carbon-assisted electrolysis of water was first reported at room temperature in a H₂SO₄ electrolyte solution,²⁹ showing that the carbon-assisted process only required a thermodynamic energy of about 9.5 kcal and a reversible potential of only 0.21 V to take place. The coal provided the additional electron required for H₂O splitting and consequently CO generation. Moreover, the authors hinted at a more rapid and steadier oxidation rate at the temperature range of 200–600 °C. A more recent study demonstrated that a steam–carbon electrochemical cell with YSZ as an electrolyte and Pt as an electrode can produce carbon-free hydrogen and positive OCV at the temperature of 800–900 °C under a carbon/(H₂O–H₂) atmosphere.¹³⁷ The higher the H₂O concentration in the cathode chamber, the larger the cell potential. At a negative voltage of 0.6 V, the electrolysis current density could reach 17 mA cm⁻². Lei et al.¹³⁸ also reported a carbon-assisted CO₂–H₂O co-electrolysis cell using LSGM-supported SOECs. The thermodynamic calculation and experimental results (Fig. 4) demonstrated that the co-electrolysis potential can be reduced by about 1 V by adding carbon to the anode, and the electricity input can be reduced to less than 10% of that without carbon. For example, a potential of 1.5 V is required to reach an electrolysis current density of 1.2 A cm⁻² without carbon fuel, while it is reduced to 0.8 V with carbon (Fig. 4c). It was also found that CO was mainly formed through a so-called “CO shuttle” mechanism: simultaneous production of CO on the anode and syngas CO/H₂ on the cathode side through the well-known Boudouard reaction (C + CO₂ = 2CO) at elevated temperatures (Fig. 4d). Moreover, as seen in Fig. 4b, the theoretical electrical demand (ΔG) decreases with increase in temperature. When the temperature is higher than 900 K, ΔG becomes a negative value, that is, carbon reacts with H₂O to produce CO₂ and H₂, spontaneously. Such an integration of carbon gasification and SOECs provides a promising method for the efficient storage of renewable electricity using coal/biomass and CO₂/H₂O, and producing clean fuel.

Fig. 4 Thermodynamic calculation of the required energy for CO₂–H₂O co-electrolysis without (a) and with (b) carbon fuel in the anode, (c) comparison of the I–V curves of the electrolysis cell, and (d) temperature-dependence of anode outlet gas composition.¹³⁸ Reproduced with permission.

Another interesting study attempted to mitigate the large polarization resistance of H₂O electro-splitting using the carbon-assisted method.¹³⁹ Different from the above approach, they first decomposed CH₄ in the anode compartment to H₂ and carbon under OCV conditions; the deposited carbon penetrated the porous Pt/YSZ electrode and had intimate contact with the electrode catalyst, which is different from that of the conventional design where carbon fuel is on the top of the fuel electrode. After the decomposition period, steam is introduced to the other electrode with the Pt catalyst, which is electrolyzed to H₂ and O²⁻ under the much lower polarization conditions, the O²⁻ is then moved to the carbon electrode allowing the removal of the previously deposited carbon and regenerating the Pt/YSZ electrode. Such a coupling catalysis and electrocatalysis process enabled efficient H₂ production (CH₄ pyrolysis and H₂O electrolysis) and cell electrode regeneration with a much lower energy input compared with the general CH₄ steam reforming process. 18 cycles of durable operation of the applied cell materials and operational mode demonstrated the feasibility of such a novel process and device. Moreover, ethane and ethylene were observed during the regeneration step because of the electrocatalytic methane oxidative coupling reaction on the Pt/YSZ porous catalyst film.

3.1.1.3.3.2. Gaseous chemical-assisted electrolysis . Besides the solid fuel, gaseous chemicals, like CH₄, natural gas, and biogas with a higher diffusion capability and reactivity are also used in fuel-assisted SOECs. In 2000, the first kind of patent on a natural gas-assisted electrolyzer for hydrogen and syngas co-generation was filed.¹⁴⁰ An oxygen ionic conductor is used for the electrolyte, water is introduced into the cathode chamber where it is reduced to H₂, and the oxygen ions are then transported through the electrolyte and reach the anode chamber where they react with the supplied natural gas mainly CH₄ to form CO and H₂; with further injection of H₂O, the gas–steam shift reaction takes place and converts CO and H₂O to CO₂ and H₂. Moreover, the oxygen transportation rate can be accelerated by 1–2 orders of magnitude in the presence of a gaseous fuel at the anode.¹⁴¹ The natural gas-assisted process reduced the electricity input by 65% and concurrently improved the H₂ yield. However, such a technology has not received sufficient attention, probably due to the immature SOFC technology and the low hydrogen price of the mature technologies. In 2008, Jiang et al.¹⁴² reported adopting a perovskite oxide hollow-fiber membrane for the simultaneous production of hydrogen and synthesis gas by combining water splitting with partial oxidation of methane. Pure H₂ was obtained in the inner tube of the hollow fiber by H₂O decomposition and the oxygen ion is transported to the extern of the fiber to react with CH₄ to synthesize syngas due to the oxygen partial pressure difference. In fact, such a reactor is an evolution of the steam-reforming process setup. However, it showed a much higher H₂ yield due to the avoidance of the equilibrium reaction and presented higher system controllability since the reactants are separated in the two chambers. Following this work and the increased attention to the energy crisis, extensive efforts have been made in the last decade with a focus on the development of novel electrode materials, like redox stable Sr₂Fe_1.5Mo_0.5O_6−δ perovskite, with high activity and durability and alternative fuels to intensify the oxygen transport process to reduce the electricity demand.^{132,143–145} For example, a hybrid catalyst Ru-LSM/YSZ, in which Ru was used for the oxygen evolution reaction and LSM/YSZ catalytically oxidized the diluted CH₄ to form CO/H₂, was developed as a bifunctional anode electrode in a CH₄-assisted CO₂ electrolyzer.¹⁴³ Such an anode catalyst enables a higher CH₄ single pass conversion rate of 75% at only an applied electrolysis current density of 200 mA. Moreover, the products from each electrode chamber can be collected separately and/or accurately mixed to prepare compositionally adjustable syngas, allowing great flexibility for various final products. To further improve the electrode activity and overcome the potential carbon deposition issue under the above conditions, a metal/oxide hybrid was developed through the in situ exsolution process as a symmetrical electrode catalyst for electrochemical reforming of CH₄/CO₂ in an SOEC (Fig. 5a–d).¹⁴⁶ A NiCu alloy was precipitated on the surface of (La_0.75Sr_0.25)_0.9(Cr_0.5Mn_0.5)_0.9(Ni_xCu_1−x)_0.1O_3−δ which greatly favored the chemical adsorption of CO₂ (Fig. 5b) and oxidizing reforming activity of CH₄, and the exsolved catalyst with a nominal composition of Ni_0.5Cu_0.5 gave the optimized performance. The exsolved NiCu alloy, the in situ formed active interface (Fig. 5c and d), and the application of CH₄ in the anode compartment in FAE reduce the required voltage and the electrode polarization resistance; a maximum electrolysis current density of 2.5 A cm⁻² could be achieved at the applied voltage of 1.5 V with the introduction of CH₄ gas into the anode chamber, which is double that of the electrolysis cell without the assistance of CH₄ fuel (Fig. 5e). Moreover, the H₂/CO molar ratio in gas products and the CO₂/CH₄ conversion rate are highly dependent on the electrode composition, the applied electrolysis current density as well as the composition of inlet gas. An optimized ratio of 2 can be obtained by adjusting the electrode material composition and the cell operational parameters, accompanying CO₂/CH₄ conversion rate above 90% (Fig. 5f). The constructed electrochemical reforming cells also showed stable performance in a 300 h period and were kept inert for 10 redox cycling tests due to the strong interaction of the exsolved metal with the perovskite oxide parent.

Fig. 5 (a) Schematic illustration of the CH₄-assisted CO₂ electrolysis based on the SOECs with the symmetric electrode, (b) comparison of the CO₂ chemical adsorption at 800 °C using in situ FTIR spectroscopy on Ni and Cu co-doped LSCM materials, (c and d) SEM and TEM images of the NiCu alloy precipitation on the LSCM substrate, (e) comparison of the I–V curves and (f) H₂/CO flowing rate of the outlet gas in FAE with various electrode materials.¹⁴⁶ Reproduced with permission (open access).

The in situ exsolution idea was also employed and optimized for both electrodes in CH₄-assisted CO₂ electrolyzer cells¹³⁵ considering that both anode and cathode chambers are in a reducing atmosphere. Rh nanoparticles with a particle size of 2.1 nm and coverage density of 11 000 μm⁻² were precipitated on the surface by treating under 5% H₂/N₂ and 600 °C for 10 h, which led to a higher CH₄ conversion rate by 3.7–13.4% and gave a molar ratio of 1.8–2.0 for H₂/CO. Moreover, such a reactor can be operated in the power and chemical co-generation mode. For instance, when the applied voltage is −0.15 V at 850 °C, the syngas yield is 5.6 mL min⁻¹ cm⁻², and enables a power density of 30 mW cm⁻²; all are remarkably better than that without the Rh nanoparticle precipitation.

Another interesting work adopted Ni-based quasi-symmetrical electrodes for biofuel-assisted water electrolysis.¹³⁴ The application of biofuel not only increases energy conversion efficiency but also stores abundant biomass energy into valuable fuels. Such an FAE enabled a thermal-neutral operation at the electrolysis potential of 0.36 V, which is above 1 V lower than that of the steam electrolysis alone. To facilitate the biofuel, typically the ethanol, reforming (CH₃CH₂OH + 3H₂O → 2CO₂ + 6H₂), a channeled anode support was prepared by the mesh-assisted phase-inversion process and 5 wt% Ru/GDC fibrous catalysts were loaded into the well-aligned channeled pores. A recorded high electrolysis current density of 3.0 A cm⁻² was obtained at 1.3 V, which were well preserved in 300 h of testing, and no carbon deposition is observed under the electrochemical oxidation conditions, especially with a high O²⁻/ethanol ratio at the largely applied electrolysis current density.

3.1.1.3.3.3. Electrolysis in tandem with fuel upgradation . The extracted oxygen from CO₂/H₂O splitting is mainly used for the fuel partial oxidation reforming in the above cases, i.e., the oxygen from H₂O/CO₂ electrolysis is added to the fuels or converted to the oxygenated chemicals (electrolysis in tandem with fuel upgradation). There is also another typical reactor in which the fuels are converted to upgraded chemicals in tandem with the H₂O/CO₂ electrolysis, such as from oxygenated alkanes to form alkenes and water. A typical example is the oxidative coupling of methane (OCM) and oxy-dehydrogenation of an alkane to an alkene with CO₂/H₂O electrolysis for producing C₂ compounds (ethane and ethene) at an elevated temperature. In this type of reactor, the cathode activated gaseous H₂O/CO₂ into O²⁻ species, which permeated through the solid electrolyte; selective oxidation reactions then take place in the anode with permeated oxygen species. Integrating the OCM and CO₂/H₂O reduction processes in one electrolysis apparatus with full application of valuable O₂ and heat management can provide economic and environmental benefits.¹ Besides that, the transportation of oxygen species could help in C–H bond cleavage in CH₄ and suppress the carbon deposition issues that are not achievable by other methods. Therefore, the effective utilization of methane as a chemical resource has been strongly desired and extensive studies have been performed after the pioneering work by Keller and Bhasin.¹⁴⁷ However, it should be noted that a deep oxidation reaction seems to be a more favorable route both from the thermodynamic and kinetic aspects. A key to achieving high C₂ yields was to control the type and state of oxygen species^1,148–151 for the oxygenation process. Different approaches like a sequential feed of oxygen and methane prefer lattice oxygen involved processes using metal oxide instead of gaseous oxygen and using SEMRs. It has been demonstrated that the electrochemically permeated oxygen showed higher activity and selectivity for the production of C₂ compounds over other forms.^152–157 For example, the highest C₂ yields of 16.7% (11.5% C₂H₄ + 5.2% C₂H₆) and 82.2% C₂ selectivity, as well as exceptionally high durability in 100 h operation for O₂ electrolysis in conjunction with the OCM process using an O-SEMR based on the intentionally constructed Fe-oxide interface in the anode by the exsolution approach, were reported.¹⁵⁸ However, when in conjunction with the CO₂ electrolysis, a C₂ yield of only 5.08% was obtained at 850 °C, which is far from the standard required for commercialization. Recent studies reported adopting a CeO₂ single crystal or Ni-doped single crystal as the anode for CH₄ oxidation coupling or ethane dehydrogenation with CO₂ electrolysis at the cathode.^151,157 The application of a single crystal effectively improved the hydrocarbon selectivity without deep oxidation by selectively using the low active lattice oxygen instead of highly active adsorption oxygen.¹⁵⁷ The exsolution of Ni in the anode significantly improves the catalytic activity and C₂ selectivity.¹⁵¹ With 2.25% Ni doping into the single crystal CeO₂, the electrolysis current density reaches 1.2 A cm⁻² at 1.6 V. Moreover, a C₂ selectivity of up to 99.5% was reported with a CH₄ single pass conversion of 7%. To further improve the C₂ yield, the focus should also be given to the conversion of alkanes. For example, an active La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–Sm_0.2Ce_0.8O_2−δ was developed as the anode catalyst for ethane conversion. The conversion of ethane can be well adjusted by the applied current/voltage. However, the pristine anode showed a little bit of deep oxidation to yield a lot of CO₂. By intentionally adding a thin layer of Al₂O₃, ethylene selectivity can be improved over 90% (Fig. 6a–d). The addition of Al₂O₃ not only effectively reduced the amount of adsorbed oxygen species, leading to improved ethylene selectivity and stability, but also altered the electronic structure of the Fe active center by forming an Al–O–Fe interface, which increased the density of states around the Fermi level and downshift of the empty band, leading to the enhanced ethane adsorption and conversion. The suggested Al₂O₃ modified anode shows high durability under 200 h testing under a current of 20 mA and ethane flow rate of 4 mL min⁻¹ (Fig. 6e).

Fig. 6 (a) Schematic illustration of the coupling of electrochemical oxidative dehydrogenation of ethane and CO₂ electrolysis into an SOEC and TEM image of the LSCF–SDC anode infiltrated with γ-Al₂O₃, and electrochemical performance of SOEC under different operational conditions: (b) with different anodes at a current of 20 mA and ethane flow rate is 4 mL min⁻¹, (c) at different currents with the ethane flow rate of 4 mL min^—1, (d) at different ethane flow rates and a current of 60 mA on the LSCF–SDC + Al₂O₃ anode and (e) stability test at a current of 20 mA and ethane flow rate of 4 mL min⁻¹.¹⁵⁶ Reproduced with permission.

Different from the above two-chamber configurations, Caravaca et al.¹⁵² employed a single-chamber SMER of Pt/YSZ/Ag for water electrolysis in conjunction with an OCM reaction. The influence of the applied current (± values), reaction temperature, and feeding composition on the H₂ and C₂ production were systematically evaluated. They demonstrated that the possibility of controlling the rate of O²⁻ supply enables the controllability and optimization of the production rate of the desired compounds (H₂ and C₂). A maximum C₂ yield of 8% could be achieved under negative current and an inlet CH₄ concentration of 0.6–0.8% at 820 °C. Recently, Kyun Kim and their colleagues further proposed a novel design for the integration of CO₂ electrolysis and the OCM process.¹⁵⁴ A successive but two-stage zone reactor was designed (Fig. 7a). The first zone of the reactor is a typical SOEC using LSGM as an electrolyte, a SrTi_0.3Fe_0.55Co_0.15O_3−δ (STFC) anode electrode, and an STFC-GDC cathode. In this stage, CO₂ is split into CO and O₂ at 850 °C. The latter is mixed with CH₄ to reach the second zone of the reactor, where CH₄ conversion and C₂ selectivity are favored with the applied Mn–Na₂WO₄/SiO₂ catalyst at 825 °C. The assembled SOEC showed an electrolysis current density of 550 mA cm⁻² at the applied voltage of 1.5 V, which enables the production of O₂ at the rate of 1.9 mL min⁻¹ cm⁻² (Fig. 7b). A C₂ yield of 25.4% and a stable short-term (100 h) performance were achieved through the designed reactor capable of independently controlling the operating conditions of two reactions in one apparatus. A combination of efficient SOEC and active catalysts would be a promising method to further improve the C₂ yield of such an integrated process.

Fig. 7 (a) Schematic diagram of integrating the CO₂ electrolysis with the OCM process in a successive two-stage reactor design, (b) SOEC performance and (c) durability comparison.¹⁵⁴ Reproduced with permission.

3.1.1.4 Other reactions. Besides the aforementioned reactions based on the O-SEMR, the unique separated chambers also present many opportunities for other advanced synthesis.^159–162 Recent work reported synthesizing ammonia through the electrochemical reduction of nitric oxide and water using the oxygen-conducting (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ) SEMRs.¹⁶⁰ Generally, ammonia synthesis is achieved through the Haber process at elevated temperatures and chamber pressure. Recently, the low temperature electrochemical process using N₂ and H₂O precursors was also demonstrated. However, the competitive hydrogen evolution reaction and the strong triple bonds lead to low faradaic efficiency and low yields. In addition, the sluggish reaction kinetics at room temperature hinder its wide application.¹⁶¹ Inspired by those studies, Kwon et al.¹⁶⁰ innovatively proposed to use the SOEC with a perovskite oxide cathode catalyst SrTi_0.3Fe_0.55Co_0.15O_3−δ-GDC and BSCF-GDC anode to synthesize ammonia using NO and H₂O as the reactants at 650 °C. Benefiting from the improved kinetics at the elevated temperature, a recorded electrochemical NH₃ production rate of 1885 μmol cm⁻² h⁻¹ and a faradaic efficiency of 34.8% were achieved at 1.5 V (Fig. 8), both are significantly larger than that of the low temperature electrochemical approach. This work potentially provides a new approach to electrolysis-based NH₃ production from environmentally harmful NO gas. One potential barrier is that the enriching NO with high concentration should be performed since most NO sources from exhaust gas are diluted gas.

Fig. 8 Schematic illustration of the electrochemical synthesis of NH₃ from NO and H₂O using oxygen-conducting SEMRs and the comparison of NH₃ yield with other studies operated at low to higher temperatures.¹⁶⁰ Reproduced with permission.

There is also intensive interest in developing SEMRs for the removal of nitric oxide (NO_x, including NO and NO₂, such a process is called De-NO_x),¹⁶³ one of the typical contaminants in exhaust gas emissions, especially for coal-based power plants and a lean-burn gasoline engine for automotives. Different operational modes use SEMRs for De-NO_x, including (1) operating under open circuit conditions, the reducing gas (fuel) is only applied for construction of the electrical field which favors the NO_x decomposition to N₂ and O₂, while no fuel is consumed in this process.¹⁶⁴ It is interesting to see that complete emission control for zero pollution of the automotive engines was achieved¹⁶⁵ because of the electrochemical promotion of the catalysis (EPOC) effect or non-faradaic electrochemical modification of catalytic activity (NEMCA, will be detailed in Section 6); (2) NO_x is reduced in the cathode side to form N₂ and O²⁻, then the O²⁻ will be transported to the anode and oxidized to O₂ (ref. 166) and (3) similar to case 2, the obtained O²⁻ in NO reduction in the cathode will involve the anodic fuel oxidizing reaction of chemical fuels like H₂, CO, ethane and propane that results in power generation.¹⁶⁷ Since there is already excellent review work on the typical De-NO_x process,^166,168 attention will be given to the NEMCA effect later.

3.1.2 Oxygen pumping mode-fuel cell reactors. Besides electrolysis, SEMRs have been long pursued for power generation for a broad spectrum of applications, to meet the high energy conversion efficiency from portable electronics to stationary power plants. Under typical fuel cell conditions, the oxygen is pumped from the molecular oxygen/CO₂/NO to react and form oxygenated chemicals, like oxygenated hydrocarbons to yield H₂O, CO₂, and an important product, electrical power. From the aspect of reactor voltage, a potential difference is yielded due to the different chemical potentials of the applied fuel and oxidant, building on the specific ionic conductor. Among versatile SEMRs, SOFCs have received particular attention and have made significant progress in recent years because of the highest energy conversion efficiency and fuel flexibility enabled by the high temperature operation. Through the quick evolution of new cell materials/components and cell fabrication technologies, e.g., Bloom Energy Ltd could provide a commercial unit as a ‘black box’ with 200 kW power capacity using natural gas as fuel. Considerable progress has been made in cell component material development,^169–174 super-electrochemical performance generation,^173,175 and flexible fuel adaptation, especially for commercially available natural gas and gasoline.^169,171 Moreover, with the further development of SOFC technologies and understanding of the fuel cell reaction mechanism, the reduction of the operational temperature of SOFCs has become the major tendency in the communities to improve cell stability and reduce investment.^{171,175–177} Many excellent reviews have systematically analyzed the progress and well-addressed the challenges of the current SOFC system, the readers are suggested to read and refer to the cited references in those excellent studies and reviews.^21,177–180 Different from the previous review work, attention here will not be put on the discussion on the fuel cell efficiency with H₂ fuel and stability. Instead, we will focus on SOFCs that could combine power and chemical co-generation with a particular hydrocarbon application, which has received increased attention in recent decades because of the progress of the evolution of the electrification process with a large requirement for chemicals and fuels from the industry (Fig. 9). As mentioned in the aforementioned section, the integrated thermo-electrocatalysis in SEMRs enables significant reactant conversion and targeted fuel production. Pioneering studies on hydrocarbon-fed SOFCs focused on electrocatalysis and chemical production, rather than electric power generation.¹⁷⁹

Fig. 9 Publication numbers vs. time in Web of Science database by searching for “Cogeneration” and “solid oxide fuel cells” both in “Topic”, up to Jun. 01, 2024.

3.1.2.1 Advanced fuel electrodes and novel design for fuel oxidation. A distinct advantage over other SEMR technologies, more exactly SOFCs in this section, is their fuel flexibility because of the high temperature operation. The technology is more attractive in the current fossil fuel era, where sufficient conversion of hydrocarbon fuel with reduced CO₂ emission is required. SOFCs can directly use hydrocarbon fuel without transitional reforming, with high efficiency, and the CO₂ emission can be significantly reduced compared to other technologies. However, for the typical SOFCs, the Ni-based cermet anode cannot work properly due to the super-catalytic activity toward hydrocarbon cracking, leading to the carbon deposition and deactivation of the anode. An alternative anode catalyst is therefore urgently needed.
3.1.2.1.1 Modified Ni-cermet electrode. One of the first approaches is to modify the Ni-based anode catalysts.^181,182 For example, the mixture of Ni metal with other metals can reduce the crack capability while not reducing the total oxidation activity.¹⁸³ The addition of the transition metal to form an alloy can reduce the degree of crystallization of formed carbon, which could be easily removed through reacting with the products from hydrocarbon oxidation reactions, like CO₂ and H₂O. However, there is always a compromise between activity and anti-coke capability. It is found that catalysts with good water maintenance/adsorption capability can effectively promote the removal of carbon. For example, in 2009, Yang et al.¹⁶⁹ reported that the Ni catalyst anode composited with BaZr_0.1Ce_0.7Y_0.2−xYb_xO_3−δ (BZCYYb), a kind of claimed mixed O²⁻ and H⁺ conductor, worked effectively with wet C₃H₈ under OCV conditions and with dry C₃H₈ under ‘discharging’ conditions, but not with dry C₃H₈ under OCV conditions. Such unique performance response and experimental phenomena suggested that BZCYYb with the water adsorption capability can effectively remove the thermodynamically unstable carbon through the equation: H₂O + C → CO/CO₂ + H₂ reaction. Moreover, the same functionality also helps improve the sulfur tolerance to 20 ppm. The same group also intentionally loaded BaO islands on the Ni surface through an evaporation deposition method, which formed numerous nanostructure BaO/Ni interfaces to readily adsorb water and facilitate carbon removal reaction with the adsorbed water.¹⁸⁴ They experimentally demonstrated that the designed BaO/Ni interface could work stably in dry C₃H₈ and CO as well as gasified carbon with a current density of 500 mA cm⁻² at 750 °C for 100 h, while the Ni-YSZ anode cannot work properly within 1 h. The density functional theory calculations showed that the dissociated OH from the adsorbed H₂O on BaO reacts with thermodynamically formed carbon on Ni near the BaO/Ni interface to produce CO and H species, which are then oxidized at the triple-phase boundaries to form CO₂ and H₂O, achieving the purpose of carbon removal. It is interesting to see that the newly formed BaO/Ni gave higher peak power density over the pristine one when using humidified CO, suggesting another role of the modified anode, promoting the reforming of CO to yield easily oxidized fuels, like H₂. To accelerate the application of SOFCs and use the current public infrastructure, not hydrogen, Zhan et al.¹⁸⁵ reported attaching an additional catalyst layer of Ru–CeO₂ on the Ni-YSZ anode to internally reform octane, a typical high-purity compound similar to gasoline. The resulting cell gave a stable power density of 0.3–0.6 W cm⁻² at 770 °C for 50 h. The application of Ru–CeO₂ internally reforms 5% iso-octane/9% air/86% CO₂ fuel mixtures resulting in a large content of CO/H₂/CH₄, which is closer to direct oxidation under fuel cell operating conditions and avoids the carbon deposition issue. With the worldwide research tendency to reduce the operating temperature of SOFCs to intermediate to low temperature, the direct application of hydrocarbons meets even challenges, especially for CH₄ activation. In 2018, Chen et al.¹⁸⁰ adopted a similar idea by coating the Ce_0.90Ni_0.05Ru_0.05O₂ (CNR) catalyst on Ni-BZCYYb, which enabled the synergistic thermal catalytic and electrocatalytic reforming of CH₄ to H₂ and CO (Fig. 10a and b). Integrating the highly active nanofiber-like PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ cathode, the assembled SOFCs showed 0.37 W cm⁻² peak power density at 500 °C, which is higher than that reported in the literature work under similar conditions, and was comparable to the cell performance fueled by H₂. Moreover, the designed cell showed stable cell performance with CH₄ fuel for more than 380 h. Both the presence of Ni and Ru elements favor the improved CH₄ conversion and H₂/CO selectivity (Fig. 10c–e). The application of an additional layer with improved thermal catalytic and electrocatalytic activity enables low temperature high-performance with hydrocarbon fuel, which may accelerate the commercialization of the SOFC technology. The only issue of such cell design is the challenge of current collection if integrated into the fuel cell stack.

Fig. 10 (a) Schematics of a Ce_0.90Ni_0.05Ru_0.05O₂ catalytic layer modified classic BZCYYb SOFC, (b) cross-sectional SEM image, (c) the synergistic effect of Ni and Ru element for CH₄ and CO₂ activation using DFT calculations, (d) fuel cell performance, and (e) durability.¹⁸⁰ Reproduced with permission.

Another effective strategy to overcome the carbon deposition issue is to use hydrocarbon + air fuel in a single chamber. For example, Hibino et al.¹⁸⁶ reported the operation of a typical SOFC cell unit (Ni-YSZ anode/SDC electrolyte/Sm_0.5Sr_0.5CoO₃) with a mixture of C₂H₆ or C₃H₈ and air. It was found that the peak power density reached 403 and 101 mW cm⁻² at 500 and 350 °C without the carbon deposition issue. Such an encouraging performance is enabled by the selectively catalytic activity of Ni and Sm_0.5Sr_0.5CoO₃ for fuel oxidation and oxygen reduction, the presence of air in the fuel, and the resulting H₂O/CO₂ from the electrochemical products help remove the formed carbon. The authors also compared the influence of electrolyte type (YSZ, SDC, and LSGM) on the electrochemical performance. Among these, SDC showed the highest activity toward hydrocarbon oxidation since ceria also acts as a catalyst for electrochemical reactions and YSZ showed the lowest electrochemical performance due to the lowest ionic conductivity.

3.1.2.1.2 Alternative electrode catalyst. One of the solutions to avoid the carbon deposition issues in SOFCs with methane as the fuel is to replace Ni with other metals that favor the oxidation of hydrocarbon without carbon formation. In 1999, Perry Murray first reported adopting a layer of (Y₂O₃)_0.15(CeO₂)_0.85 (YDC) between Ni-YSZ (current collection layer) and YSZ electrolyte and selecting a suitable temperature in the range of 500–700 °C;¹⁸⁷ the carbon deposition issue induced both by the direct CH₄ pyrolysis and CO disproportionation reaction is well-treated, and the reduced YDC gave high catalytic activity toward CH₄ direct oxidation, which is comparable to the performance of hydrogen oxidation but with a distinct reaction mechanism based on the EIS responses. Such pioneering work demonstrated the feasibility of CH₄ direct oxidation with superior performance while being free from coke deposition issues. Following this work, Park et al.¹⁷⁸ developed a composite anode of Cu and ceria for the direct electrochemical oxidation of CH₄, C₂H₆, 1-butene, n-butene, and toluene without reforming and they did not result in any carbon formation in the anode. The much-reduced hydrocarbon pyrolysis capability of Cu combined with active ceria yielded the direct oxidation of the abovementioned hydrocarbon into CO₂ and H₂O as final products. No carbon formation when the cells was operated at 0.12 W cm⁻² in dry butane at 700 °C for 48 h. Besides transition metals with reduced activity that are composited with ceria catalyst, a big group of active catalysts, perovskite oxides, that can replace Ni-cermet are developed. Perovskite oxides are famous for their compositional & structural flexibilities and their unique mixed ionic and electronic conductivity, which introduce multiple benefits for direct/non-direct electrochemical oxidation of hydrocarbons. They also showed interesting anti-sulfur poisoning capability, which is more attractive with real hydrocarbon fuels, like municipal natural gas. One of the distinct advantages of perovskite oxides is their redox stability in an anode atmosphere, where the introduced fuel results in a super low oxygen partial pressure, i.e., the reducing atmosphere, while the oxidation reaction products like H₂O and CO₂ significantly improve oxygen partial pressure at the gas/electrode or active site surface, which leads to the redox cycling of the active site. For a typical Ni-based cermet anode, the big volume change cycle between metallic Ni and NiO causes large strain stress, as well as redox-induced aggregation, reducing the electronic conduction and active site and consequently the anode activity and cell performance degradation. The application of redox-stable perovskite oxide well addresses the above challenges. In 2003, Tao et al.¹⁸⁸ first reported that La_0.75Sr_0.25Cr_0.5Mn_0.5O₃, a chromate perovskite oxide, showed a comparable H₂ oxidation activity (ASR of 0.2 cm² at 900 °C) and stable structure in anode and cathode atmospheres. Such anode catalysts also demonstrated a maximum power density of about 0.2 W cm⁻² at 0.5 V at 900 °C for wet (3% H₂O) CH₄ oxidation with only a trace of carbon detected. Exceptional work by Huang et al.¹⁸⁹ identified Sr₂Mg_1−xMn_xMoO_6−δ (SMMO), a double perovskite oxide material, as an active and long-term stable anode catalyst for methane fuel. Such a double perovskite oxide shows interesting oxygen ion conduction, much improved electronic conductivity (10 S cm⁻¹) in H₂, and well-balanced activity and stability with the highly coordinated Mo in the lattice compared with other reported perovskite oxides. Thus, SOFCs based on this anode gave a peak power density of 438 mW cm⁻² at 800 °C in fry CH₄ fuel, which is reduced to 338 mW cm⁻² with the addition of 3% steam, suggesting the direct oxidation capability of SMMO for CH₄ direct oxidation. Moreover, such a double perovskite oxide showed high resistance toward sulfur poisoning and no performance degradation with 5 ppm H₂S in fuel. Another typical oxide is the layered perovskite which generally showed higher promise for hydrocarbon oxidation due to the intrinsic charge and structure capability. Work by Sengodan et al.¹⁹⁰ fully used those capabilities and first reported the application in direct hydrocarbon SOFCs. They obtained the double-phase perovskite oxide PrBaMn₂O_5+δ (PBM) by in situ annealing of the oxide precursor of Pr_0.5Ba_0.5MnO₃ in a reducing atmosphere. At 800 °C, layered PBM shows high electrical conductivity of 8.16 S cm⁻¹ in 5% H₂ and demonstrates peak power densities of 1.3 W cm⁻² at 850 °C using propane fuels; the latter was one of the highest performances at that time. Such an anode also showed encouraging sulfur resistance up to 50 ppm, and the PBM with the Co–Fe catalyst could run stably with C₃H₈ fuel for 500 h under a current density of 0.2 A cm⁻² at 700 °C. In another study, Yang et al.¹⁹¹ used a similar method to prepare a layered perovskite oxide but with a newly formed Co–Fe nano alloy during phase conversion. The resulting Co–Fe/perovskite oxide gave 0.6 W cm⁻² in CH₄ and 0.94 W cm⁻² in C₃H₈ at 850 °C and showed an anti-sulfur capability up to 100 ppm without a clear poisoning effect. The constructed cells could work stably at 800 °C and 0.6 A cm⁻² in H₂ with 50 ppm H₂S for more than 500 h and 0.2 A cm⁻² in CH₄ and 0.4 A cm⁻² in C₃H₈ for a total of 150 h as well as 20 times H₂–N₂–air cycles. The results demonstrated showed that the combination of active transition metal/alloy with phase stable perovskite oxides will be the future approach for high efficiency and direct utilization of hydrocarbon fuel^54,192–194 and other functional purposes, and bridging the gap between the current fossil fuel era and future hydrogen economic society.
3.1.2.1.3 Novel cell structure design. The applications of other than Ni based anode catalysts do bring improved anode anti-coking capability, but generally suffers from low fuel cell performance and requires a dedicated cell assembly procedure. As one may see the application of both perovskite oxide and Cu cermet as the anode supporter to the fabrication of a single cell is difficult or impossible because of the coarsening and evaporation issue during high temperature cosintering with electrolyte. A novel cell configuration design is therefore required. Moreover, the application of a porous while active catalytic layer on the Ni-based cermet anode is a good option. However, both the suspicions of carbon deposition under low current conditions and the difficulty of the current collection remain. Different from previous work, Barnett's group reported adopting a conducting oxide material as the cell anode support and built-in barrier layer to construct an anode supported cell with a Ni-cermet active layer for efficient natural gas (85% CH₄, 10% C₂H₆, and 5% C₃H₈) operation.¹⁹⁵ The cell gave a peak power density of over 560 mW cm⁻² at 800 °C and ran stably at the current density of 0.2, 0.4, and 0.8 A cm⁻² for 9 h, while the conventional cell failed in 6 h. With such a conducting oxide supporter, the single cell also showed high cell operational stability during 7 times redox cycling (first in H₂ for 45 min, then in air for 30 min). Sullivan et al.¹⁹⁶ also confirmed such results in tubular SOFCs with simulated biogas over 12 days of continuous operation. They explained that the increase in the local concentration of electrochemically produced H₂O and CO₂ throughout the anode resulted in higher local H₂O-to-C ratios which helped remove the formatted deleterious carbon.
3.1.2.2 Hydrocarbons to H₂/CO and power. As shown in Fig. 9, the application of SEMRs attracted increased attention during those years. More specifically, as the effective area and the resident time of the reactant in the anode chamber are quite short, the overall conversion of the chemical energy into electricity is impossible. In the practical operation, the fuel utilization efficiency is kept at 70–90% to ensure good electrical efficiency. By the way, electrochemical fuel oxidation is generally a highly exothermic reaction, the produced heat can be well managed to reform the remaining fuel with the produced H₂O and CO₂, or the remaining fuel can be catalytically partially oxidized in the anode active site, or downstream process to form syngas, with an oxygen ionic conductor, such a power and chemical cogeneration is frequently reported in the single chamber SOFCs (SC-SOFCs). The oxygen or air mixed with the hydrocarbon was co-fed into the SC-SOFCs. The anode and cathode layers selectively catalyze fuel oxidation reaction and oxygen reduction reaction for power generation, and the remaining fuel and oxygen, as well as the resultant H₂O/CO₂, react further to generate CO and H₂. For example, in 2014, Shao et al.¹⁹⁷ fabricated and tested the performance and syngas production of a flow-through (first outside of cell then inner of cells) tubular SC-SOFC with a mixed CH₄/O₂ inlet gas. SC-SOFC presented an open circuit voltage of 1.02–1.08 V at the temperature of 650–800 °C, a peak power density of 300 mW cm⁻², and a total power output of 1.5 W was achieved at 750 °C. Single-cell can be operated stably at 700 °C for 10 h at a constant current density of 250 mA cm⁻². With an additional GdNi/Al₂O₃ catalyst on the two sides of the inner tube, the system can realize a CH₄ conversion rate of 90.6%, CO selectivity of 95.4%, and H₂/CO ratio of 2.04 at a furnace temperature of 800 °C. The same group⁵⁰ reported the integration of SC-SOFC with a downstream thermal catalytic partial oxidation system for power and syngas cogeneration without greenhouse gas emission (Fig. 11a). A cell configuration of Ni/YSZ/YSZ/SDC/BSCF-SDC was assembled, in which the BSCF cathode showed high activity toward the ORR while poor activity for hydrogen oxidation. The peak power density of SC-SOFCs can reach an impressive value of 1500 mW cm⁻² at 700 °C using a CH₄–O₂ gas mixture. With the integration of the downstream catalytic layer, a methane conversion >95%, CO and H₂ selectivity higher than 98%, and a polarization current density independent H₂ : CO ratio = 2 were obtained, without any waste gas emission. Though the efficiency of SC-SOFCs is slightly lower than that of double chamber SOFCs, the ideal syngas production efficiency offsets 100%. Both the utilization rate and the yield of H₂/CO with a ratio close to 2 are very promising for future deployment.

Fig. 11 Different strategies for CH₄-fed SEMRs for power and syngas cogeneration: (a) single chamber-SEMR integration with a downstream catalytic partial reformer⁵⁰ and (b) thermal and catalytic coupling within the SEMR anode.¹⁹⁸ Reproduced with permission from ref. 50 and ¹⁹⁸.

The application of SC-SOFCs for CH₄ cogeneration resulted in interesting performance and fuel utilization efficiency. However, careful gas (fuel/oxidant mixture) injection should be rationalized to avoid the possible explosion of the CH₄/O₂ mixture. In contrast, double chamber SOFCs (general one) have distinct advantages; the fuel and oxidant are separated in two chambers; the oxygen can be electrochemically pumped from the cathode to the anode and severed as the oxidant for CH₄ reforming in the form of partial oxidation, both the reaction rate and safety can be well controlled. One difficulty in using traditional SOFCs is their limited CH₄ gas diffusion capability over H₂ (ref. 199) in the well-sintered anode layer. A well-designed anode structure without a CH₄ gas diffusion limitation should be developed. In 2020, Dong et al.^198,200,201 at Jinan University developed microchannel SOFCs with a loaded catalyst for CH₄ fuel conversion to electricity and syngas co-generation through catalytic partial oxidation (Fig. 11b). The microchannel anode was prepared by a mesh-templating phase-inversion method with one end open on the anode surface and the other end terminated by a porous layer, the latter is well-tailored for electrolyte coating and co-sintering. To improve the anode catalytic activity, a CeO₂ nanocrystal,²⁰⁰ or a dedicated nano fibrous Ni/CeO₂–Al₂O₃ catalyst^198,201 prepared by the electrospinning process is impregnated into the microchannel by the capillary force. The adoption of micro-channeled pores significantly improved the SOFC power output (increased to 2.5 times) with the feeding of 30% CH₄/Ar at 600 °C due to significantly improved gas diffusion, i.e., reduction of the concentration polarization. It is demonstrated that a minimum CH₄ concentration of 0.35 atm and 0.80 atm should be used in microchannel SOFCs and no-channel SOFCs, respectively. Moreover, they found that the introduction of the catalytic layer into the microchannel remarkably improved the syngas yields by almost by 6 times, and CH₄ conversion from less than 40% to above 75% and increase with the increase of CO₂ concentration, as well as H₂/CO ratio. The final one reached 2 but decreased with increase of CO₂ concentration in the anode gas mixture. The micro-channeled SOFC with the catalyst also showed impressive operational stability at a fixed current density of 1.05 A cm^—2. The voltage, ohmic and electrode polarization resistance, H₂ yield and CH₄ conversion rate were stable during 120 h testing.

3.1.2.3 Oxidative dehydrogenation of alkanes. In Section 3.1.1.3, we saw that ethylene is one of the most important building blocks in the chemical industry; the application of solid oxide electrolysis with ethane dehydrogenation brings multiple benefits like overcoming the thermodynamic limitation and the possibility of over-oxidation compared with the conventional steam reforming, high temperature pyrolysis, and direct oxidative hydrogenation methods, respectively. However, additional electrical energy is still required even though it can be adjusted to realize the power and chemical cogeneration at the much-improved temperature. Alternatively oxidative dehydrogenation of alkanes to light olefins is an attractive, low-energy, alternative route which could reduce the carbon footprint of its production.^202,203 The direct application of oxygen ions which are pumped from the cathode side with the source from oxygen molecules could still realize such functionality according to the following equations (taking C₂H₆ fuel as an example):

Cathode: O₂ + 4e⁻ = 2O²⁻

Anode: C₂H₆ + O²⁻ = C₂H₄ + H₂O + 2e⁻

Overall reaction: O₂ + 2C₂H₆ = 2C₂H₄ + 2H₂O

Both selectivity and yield will be enhanced by this “oxygen buffer” approach since it can avoids deep oxidation. Oxygen enhances the forward reaction by reducing H₂ content, therefore shifting the reaction equilibrium, reducing the side reaction of coking and cracking, and transforming the reaction from an endothermic process into an exothermic one. In 2002, Akin and Lin²⁰⁴ reported an ethene reaction yield of 56% with a selectivity of 80% at 875 °C by using a dense Bi_1.5Y_0.3SmO₃ tubular membrane reactor without catalyst over 11% yield at a steady state in a fixed bed reactor. Bi-based oxide with hetero-elemental doping is a good oxygen ionic conductor with partial electronic conduction under real reaction conditions, which allows the transport of O²⁻ by converting O₂ outside of the tubular SEMR reactor and then reacting with the inner C₂H₆ to form C₂H₄. The effect of the operational temperature and C₂H₆ partial pressure on the C₂H₄ selectivity and yield are carefully investigated. The SEMR operation is much more beneficial because of high selectivity and yield.²⁰⁵ Some other type of membrane reactors based on perovskite oxides with a mixed ionic and electronic conductor is also used and showed interesting C₂H₄ yield due to the catalytic function of the applied materials.^206,207 However, an additional gas separation process is required and most perovskite oxides cannot be stably operated under real conditions, and the total energy efficiency could be further improved. For example, the chemical energy during a partial oxidation reaction which is converted to heat can be managed to produce highly valuable electricity in SEMRs while wasted in other reactors. Therefore, Dogu et al.²⁰⁸ developed such SEMRs and exploited a La and Cl co-doped SrTiO₃ anode catalyst for improving the oxidative dehydrogenation of ethane to ethene at 550 and 600 °C. The reactor La_0.2Sr_0.8TiO_3±δCl_σ/YSZ/LSM-YSZ showed a one-time ethane conversion of 4–14% and an ethene yield of 4–10% at the current density of 0–3.5 mA cm⁻², which are 4–10 times higher than that with SrTiO_3±δ and 2–5 times better than that of La_0.2Sr_0.8TiO_3±δ. The improved catalytic activity was ascribed to the improved oxygen mobility, an abundance of Lewis acid sites, as well as more Brønsted acid sites as demonstrated by the multiple characterization techniques including DRIFTS, XPS, TPO-CO₂, and laser Raman spectroscopy. However, high ethene selectivity is ensured in this work; low ethane conversion due to the limited SOFC efficiency with quite low current density is observed, and optimization of the SOFC efficiency should be managed. Moreover, a high-C compound conversion that easily induces coking at the SOFC operating temperature other than C₁ and C₂ compounds should be developed. A “best of both worlds” scenario by simultaneously obtaining excellent power output and a high yield of high-C chemicals could be realized. This was addressed recently by Yan et al.²⁵ In their work, thin film O-SEMRs with a configuration of Co₇W₆@WOx core–shell catalyst loaded (La,Sr)TiO₃-YSZ|YSZ|YSZ-La_0.6Sr_0.4FeO₃ (LSF) are prepared. The cell was fed with a H₂ and n-butane mixture. Under the optimized cell operating parameters and catalyst design, the reactor reached a peak power density of 212 mW cm⁻² at 650 °C, and a C4 alkene (butenes and 1,3-butadiene) yield higher than 50% under a butane conversion higher than 80% was achieved; coke free, are limited CO/CO₂ are produced in this SEMRs. The reactor performance can be further improved by employing higher ionic conductive and thinner electrolytes. Similar work by Tan et al.²⁰³ reported the adoption of NiFe alloy nanoparticles (NPs) exsolved Pr_0.8Sr_1.2Ni_0.2Fe_1.3Mo_0.5O_6−δ as the electrode for oxidative dehydrogenation of propane. With the gradually exsolved NiFe alloy, the SEMRs reached a propane conversion of 71.4% and light olefin (C₂H₄ + C₃H₆) yield of 70.1% under a current density of 0.3 A cm⁻² (750 °C) with a propane flowing rate of 40 mL min⁻¹. The cell showed high resistance toward carbon deposition under constant current mode during 100 h of testing.

Another interesting study by Zhang et al.²⁰⁹ reported operating propane fuel in SOFC mode for power and chemical cogeneration. They developed an active CoFe alloy-perovskite oxide catalyst for proton oxidation with LSGM electrolyte. An excellent power density of 0.92 W cm⁻² has been achieved with C₃H₈ fuel (3 vol% H₂O) and the SOFC worked stably under OCV and constant current density of 0.4 A cm⁻² at 850 °C. Both the ¹H nuclear magnetic resonance (NMR) spectrum and infrared (IR) spectrum demonstrated that polycyclic aromatic hydrocarbons were produced in the exhaust stream of the anode, which successfully demonstrated the co-generation system and promising application of SEMRs.

3.1.2.4 Oxidative coupling of CH₄ to C₂ compounds. The oxidation coupling of methane (CH₄ + O₂ → C₂H₄ + 2H₂O, ΔG = −69 kcal mol⁻¹) was first proposed by Keller and Bhasin in 1982 using a thermal catalysis method with a metal oxide supported on a-Al₂O₃ as a catalyst, in which a CH₄ conversion of 4% was achieved.¹⁴⁷ Later, different groups tried to develop active catalysts to improve the CH₄ conversion and selectivity of C₂ compounds.^210,211 However, as mentioned in the previous sections, significant levels of oxidation parallel products are formed regardless of the kind of oxidant, like CO₂ and O₂, since the total oxidation of CH₄ is thermodynamically favored at high temperatures.²¹² Compared with thermal catalysis, the application of SEMRs enables the supply of reactants at the desired rate to improve selectivity, shift the equilibrium, and remove the undesired products during the reaction by controlling the electrode potential and external electron flow.²¹³ The controllability of oxygen partial pressure in SEMRs enables higher selectivity of C₂ or higher hydrocarbons since the latter follows the half-order dependence on O₂ partial pressure while deep oxidation shows first-order dependence. To improve the C₂ selectivity, SEMRs can be operated at a low partial pressure of oxygen.²¹⁴ In particular, for SEMRs, the oxidation of CH₄ not only produces electrical power but also yields highly valuable chemicals of C₂ compounds,¹ which make methane coupling reactions more attractive. In 2005, Kiatkittipong et al.¹⁵³ found that the SEMRs showed improved C₂ selectivity over heterogeneous catalysis. However, the yield of CH₄ to C₂ compounds is low because of the poor fuel cell efficiency which should be largely ascribed to the low activity of cell components even though the electrode selectivity could reach 96.5%.²¹⁵ With the quick evolution of novel fuel cell materials and technology, the CH₄ conversion has been remarkably improved. In ref. 216, a SOFC tubular design using a Scandia stabilized zirconia (ScSZ) electrolyte, LSM cathode multilayer cell structure with the Na–W–Mn–Ce/SiO₂ catalyst integrated was applied. The author demonstrated a 60.7% CH₄ conversion and a high C₂ selectivity (41.6%), which is close to a C₂ yield threshold of 50%.²¹⁷ Such results demonstrate considerable advantages over conventional heterogeneous catalysis. The results are much higher than that of Appamana's work based on a similar material system.²¹⁸ Both the application of tubular cells with extended CH₄ residence time and high ionic conductive ScSZ electrolyte over classic YSZ electrolyte enables such a much-improved energy conversion and C₂ compound yield. It is also interesting to see that a high ethylene-to-ethane ratio of 5.8 was obtained, which highlighted the best performance among SEMRs for the OCM reaction. However, further improvement of the C₂ yield looks difficult as the desired products are far more reactive with oxidants than with methane, i.e., the C₂ products are easily oxidized to CO₂/CO. Also because one-pass CH₄ conversion is hard to improve with a limited residence time of the CH₄ in the pores of the anode, an integrated gas recycle electrocatalytic reactor and separator system was proposed by Jiang et al.²¹⁷ in 1994. The C₂ products of SEMRs with YSZ electrolyte and Ag-based catalyst were extensively sieved (up to 100%) using an appropriate molecular sieve trap in the cycling loop, then the unreacted CH₄ and undesired ethane were recycled to the SEMRs for further conversion. Such a recycling operation with efficient removal of C₂ products resulted in an ethene selectivity of up to 88% and CH₄ conversion of 97%, i.e., the yield of ethene reached 85% in the final sample.

Besides the experimental operating condition optimization such as temperature and/or pressure, researchers also performed techno-economic analysis on the OCM process in membrane reactors.^219,220 Based on the developed 1D membrane reactor model, Cruellas et al.²²⁰ found that the increased yield from the optimized operation led to a positive impact on the economics and performance of the downstream separation, which resulted in the cost of ethylene production being 595–625 € per ton C₂H₄, 25–30% lower than the benchmark naphtha steam cracking process. Moreover, the CO₂ emissions of the OCM studied processes are also 90% lower than with the reference, exhibiting better environmental benefits. Those techno-economic analysis results confirm the competition of the OCM process based on membrane reactors over conventional technology, encouraging the efforts to achieve a larger prototype. The authors also mentioned that the price of the membrane is crucial for the OCM scale-up process. In addition, the operational durability as well as the system durability are also important factors in determining the large-scale application potential. Therefore, an effective integration of experimentation, modeling, and techno-economic analysis is of great importance for the future implementation of SEMRs.

3.1.2.5 Other SEMRs for co-generations. Interest is also given to methanol synthesis from CH₄ using the SEMRs since methanol is an important industrial chemical, energy carrier, and intermediate product. It should be noted that methane activation should be performed at elevated temperatures, while methanol may be subjected to decomposition at this temperature. The key to realizing methane-to-methanol conversion is to develop an active and selective transition metal oxide catalyst. Torabi et al.²²¹ from Fuel Cell Energy Inc. developed an intermediate temperature anode-supported SOFCs based on sprayed GDC-Li₂CO₃, LiNiO₂, and doped SrTiO₃ as the electrolyte, cathode, and anode, respectively. The composite electrolyte showed 100 times higher ionic conductivity over doped ceria alone. To screen active catalysts, they employed the tubular fixed reactor. The initial results demonstrated that the main product using SrTiO₃ is methanol with selectivity over 90% over the temperature range of 300–600 °C. When the CH₄ flow rate is in the range of 70–100 mL min⁻¹, the complete conversion of CH₄ is achieved within 1 minute. The application of lattice O²⁻ instead of molecular O₂ is believed to induce a superior performance. Another interesting application of SEMRs for fuel and electricity co-generation from a C1 precursor is reported by Neophytides et al.²²² They developed an Ag symmetric cell based on YSZ electrolyte at atmospheric pressure and temperature of 547–697 °C. Both the cell current density, the methanol partial pressure with He on the CH₃OH conversion, and CO, CO₂, and HCOH were investigated. The cells gave an open circuit over 1.0 V. It was demonstrated that the selectivity to HCOH is around 90% at methanol conversions ≥30%. These results confirmed that Ag is a selective catalyst for the targeted reaction and SEMRs can serve as a good platform for high-efficiency conversion C1 chemistry.

Similar to the previous discussion, the application of SEMRs is also highly interesting for higher hydrocarbon conversion even though the selectivity to the targeted chemical is much lower than that of low carbon-containing hydrocarbons due to the mismatched operating temperature for reaction and missing active catalyst for the selective reaction.²²³ In 2009, Ji et al.²²⁴ reported adopting MoV_0.3Te_0.17Nb_0.12O as an active and conductive anode catalyst for propane selectivity to acrylic acid at the temperature of around 400 °C. When combined with the low temperature high ionic conductive Bi₄Cu_0.2V_1.8O_11−δ electrolyte for SEMRs, the cell gave an open circuit voltage of 0.7 V and peak power density of 10 mW cm⁻² at 420 °C using the C₃H₈ : 12H₂O : 15He mixed gas. Moreover the C₃H₈ conversion was around 3.0–4.7% with an acrylic acid selectivity of 67–72.9%. Such results demonstrated the feasibility of SEMRs for application in high hydrocarbon conversion and combination with electricity conversion. One more interesting work reported by Zhang et al.²⁰⁹ shows that ever macromolecular compounds such as polycyclic aromatic hydrocarbons (PAHs) can be synthesized by accompanying high fuel-to-electricity conversion efficiency. A Co–Fe nano alloy exsolved Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O₃ perovskite material developed by the authors' group was used as a catalyst and LSGM as the electrolyte, operating at 750–850 °C using 3% H₂O humidified C₃H₈ as fuel. The cell presented a peak power density of 0.33–0.92 W cm⁻² and worked stably under both conditions of OCV and current density of 0.4 A cm⁻² for a total of more than 80 h. The analysis of liquid by-products in the exhaust stream by combined gas chromatography-mass spectrometry, IR, and NMR showed that it is a mixed PAH.

Previous attention is mainly given to low carbon-based fuels or chemicals for efficient energy conversion and storage. SEMRs are also widely adopted for other related chemicals like NH₃, HCN, NO, H₂S, and SO₂ synthesis or abandonment or used for environmental treatment, which has a significant role in economic and environmental impacts, which we think will promote the commercialization of such a promising technology. Since the pioneering work by Vayenas and Farr et al. on the use of SEMRs (Pt/YSZ/Pt) to directly convert NH₃ to NO with yields ≥60% and simultaneously generate electric energy at 900–1200 K, this field has received increased attention with much-improved progress.²²⁵ Another example of the use of SEMR is the synthesis of HCN from the methanol/methane ammonia steam mixtures (2CH₄ + 2NH₃ + 3O₂ → 2HCN + 6H₂O).^226–228 The SEMR adopted an active Ni–CGO–iron antimony oxide catalyst and was operated in the temperature range of 500–650 °C. Under the optimized cell operational conditions, a maximum yield of 40% (to methanol input) and selectivity for conversion of methanol to HCN of 47.5% was achieved with CO₂, N₂, H₂, and CH₄ as the side products. It was also found that the HCN was obtained by ammonolysis of methanol instead of electrochemical ammonia oxidation; the H₂ produced by the side reactions was oxidized to provide the electricity, and the electrochemical supply of oxygen to the fuel side had a positive effect on the cell selectivity to HCN. In those studies, the electricity output was relatively low. However, their promoted role in targeted sample synthesis is quite huge, as claimed by EPOC or NEMCA, which will be discussed in Section 6. The H₂S, SO₂, and/or NO_x are also used as oxidants for simultaneous greenhouse gas treatment and clean energy generation.^229–232 In 2016, Li Kang developed multi-channel tubular SOFCs and ran them with N₂O instead of air as the oxidant.²²⁹ An increase of 50% in power density was observed over that with air oxidant, such an integration suggested techno-economic feasibility to eliminate the cost penalty for N₂O abatement and yield highly valuable electrical energy.

3.1.2.6 Direct carbon fuel cells. One of the advantages of SOFCs is their fuel flexibility. SOFCs fueled with solid state carbon instead of gaseous and liquid fuel received particular attention because of the widely available carbon-based fuel and the >100% theoretical thermodynamic efficiency as well as the high energy density.^233,234 The major challenge in using carbon-based fuel is to improve the fuel–catalyst contact sites or continuous fueling like gaseous or liquid fuel. Another challenge is the reduced real efficiency because of the emission of thermodynamically favored CO in the anode direct oxidation reaction as well as the Boudouard reaction (C + CO₂ → 2CO), even though the performances of most DCFCs rely on CO oxidation, instead of the direct carbon oxidation. Nevertheless, this may suggest the chemicals (CO) and electric energy co-generation from this point of view (Fig. 12a). Prof. Liu's group from the South University of Technology performed a systematical investigation of such a co-generation system from active electrode material development,²³⁵ carbon fuel precursor selection, electrochemical and thermal catalytic mechanism, small stack construction, demonstration,^233,236 and modeling.²³⁷ Tubular DCFC single cells that unite with one close end or 2-cell stack were constructed by the dip coating technique (Fig. 12b). Then they were assembled into DCFC devices using LSM-YSZ as the cathode and Ag-GDC as the anode. Ag is an active catalyst for CO oxidation. Activated carbon with a 5% Fe catalyst to catalyze the Boudouard reaction is used as fuel. Such a DCFC 2-cell stack gave an open circuit voltage of 2.11 V and a peak power density of 319 mW cm⁻² at 800 °C, which is close to that with hydrogen as fuel (Fig. 12c), suggesting the high activity of the applied catalyst and suitable cell structure. When the cell is operated at a fixed current of 2 A, the 2-cell stack can be operated continuously over 1.1 h. During the oxidation period, the peak CO yield can reach 25 mL min⁻¹ though it gradually reduced with time. In contrast, CO₂ concentration increased with the applied time (Fig. 12d). The authors also carefully calculated the electrical energy conversion and overall conversion efficiency for three cells in single-cell configurations that were operated at different applied currents. Generally, the former is proportional to its operating voltage while inversely proportional to its operating current. When the current is 1 A, the electrical conversion efficiency could reach 40% and the overall efficiency could be over 70% (Fig. 12e).²³⁵

Fig. 12 Schematic illustration of electricity–gas cogeneration in DCFC: (a) single cell and (b) the corresponding exhausted gas composition and power response to the operating time of 2-cell stack operated at 2 A, at 800 °C and (c) electrical efficiency and overall conversion efficiency of the cells operated with different current at 800 °C.²³⁵ Reproduced with permission from ref. 235.

Considering that the electrochemical performance of DCFCs operated at elevated temperatures like 800 °C is contributed by the CO oxidation nature and currently most DCFCs are constructed on small size cells or tubular cells with special sealing, the same group invented a compact and seal-less direct carbon SOFC stack for potential application.²³⁶ The SOFC stack is composed of a 12-array cell on one side of a large-scale single YSZ electrolyte plate in series, leaving the anode exposed but without direct contact with the carbon fuel. Each opposed cathode and anode constitute a cell unit, thus there are 12 unit cells on the single electrolyte. When the cells are operated using activated carbon or wood powder (or sawdust) made from a Bauhinia tree branch as carbon-rich fuel with Fe loading, an OCV value of 10.8 V, peak power of 10.2 W and a discharge energy of 29 W h were achieved on the activated carbon fuel, and they were increased to 11.6 V and 13.1 W with the Bauhinia tree branch wood powder fuel. A similar cell design but in the form of a tubular structure was also used by the same group to make a rechargeable carbon air battery,²³³ which makes DCFC more adaptive in practical applications. Furthermore, numerical modeling was also performed to understand the physical/chemical processes of the CO and electricity co-generation in DCFC.²³⁷ The 2D model confirmed the feasibility of a non-contact design of carbon/anode, and a large distance between the carbon fuel and the porous anode still could give a good electrochemical performance. The DCFC gave a reduced performance at the reduced temperature mainly ascribed to the low Boudouard reaction kinetics. With careful control of the operational parameters, like the temperature and applied cell current, the molar fraction of CO can be well controlled. It is also found that the higher the cell current, the lower the CO generation. Thus, a commonly used anode configuration resulted in improved DCFC performance but decreased CO fraction in the exhausted gas. Thus, such a DCFC showed promising application in CO-electricity application.

The DCFC performance is contributed by CO oxidation; how to effectively utilize the CO from the oxidation reaction and Boudouard reaction is therefore investigated since most CO is released during the testing. In this regard, Prof. Shao from the Nanjing University of Technology developed a combined solid oxide YSZ electrolyte type based DCFC with CO₂ permeating membrane to improve electrochemical performance and carbon utilization efficiency. The CO₂ permeating membrane is made up of the doped ceria–carbonate composite, which keeps CO₂ at a suitable concentration but confines all the unreacted CO in the anode chamber, which favors the CO oxidation kinetics and utilization efficiency and subsequently improves DCFC performance. Such a DCFC presented an OCV of 1.056 V and a peak power density of 279.3 mW cm⁻² at 850 °C. Moreover, a 2-cell stack was also built and delivered continuous operation for 200 min at a constant current density of 300 mA. However, it should be noted that the carbon utilization efficiency is still only 14.36%, which is mainly ascribed to the de-activation of catalysts.²³⁸ The same group then tried to fix the active Boudouard reaction catalyst Fe_mO_n (active component)–K₂O (promoter) first on Al₂O₃ and pre-sintered at high temperatures. The modified DCFC showed similar performance to the previous one but with a much improved continuous operational time of 314 min at 750 °C and a high carbon utilization efficiency of up to 98.7%, which is ever the highest one in the literature.²³⁹

People also tried to improve the DCFC performance by increasing the contact area of the carbon fuel with the anode catalyst by creating a nanoporous electrode^{238,240–244} or using a redox mediator,^245,246 especially at the reduced temperature where the effect of the Boudouard reaction is not so significant. The poor point-like solid–solid contact not only restricts the electrode kinetics but also does not favor electronic conduction. In this context, hollow nanofibers of Ce_0.6Mn_0.3Fe_0.1O₂ by the electrospinning method,²⁴⁴ Cu-modified Ni foam,²⁴⁰ and honeycombed porous, carbon fuel size-matching architecture anode²⁴³ with a three-dimensional open structure were developed. These 3D anodes not only increased the contact area with carbon and CO but also facilitated mass and electron transport, which significantly improved the DCFC electrochemical performance. A peak power density of 765 mW cm⁻² was obtained based on the carbon fuel size-matching architecture anode at 800 °C, which is one to two times higher than the previous cases. Besides the development of an active electrode with optimized cathode microstructure, in 2011, the Gorte group from the University of Pennsylvania reported adopting a molten Sb₂O₃/Sb redox mediator for DCFCs.²⁴⁵ In such a novel design, fuel cell performance is realized through the anode oxidation of metallic Sb at the electrolyte interface, producing Sb₂O₃, that is then reduced by the fuel in a separate step in the anode. In other words, the chemical energy of carbon is first stored in molten Sb, then further oxidized by the O²⁻ from the cathode. Therefore, the overall cell open circuit voltage is ascribed to the Sb oxidation, not to carbon oxidation anymore. It is worth noting that the Nernst potential of the Sb–Sb₂O₃ mixture is only 0.75 V, which is a little bit lower than that of carbon oxidation. However, the electrode resistance associated with Sb fuel is impressively low, approximately 0.06 Ω cm² because of the molten state of fuels, which has close contact with the anode catalyst. Therefore, the peak power density reaches 350 mW cm⁻² with an electrolyte-supported cell made from ScSZ on different carbon fuels, like sugar char, rice starch, carbon black, and graphite, and the redox moderated DCFC showed improved operating stability and service life compared with previous cases. Another interesting method reported by Jiang et al.²⁴⁶ who proposed and demonstrated a hybrid molten carbonate/solid oxide direct carbon fuel cell, in which molten carbonate is mixed with carbon and refined in the anode chamber, the liquefied “fuel in molten” effectively reduces the electrode polarization resistance of the anode. With an optimized carbonate–carbon composition, the hybrid DCFC showed a maximum power density of 390 mW cm⁻² using an LSM cathode, which was further improved to 878 mW cm⁻² using an LSC cathode under flowing air, comparable to extant fuel cell technologies. The exceptional DCFC electrochemical performance makes a solid step toward the wide application of DCFC.

Exceptional cases of the direct oxidation of carbon in DCFCs should be highlighted. Inspired by the nanoporous/size matching anode catalyst and microstructure development, as well as the “liquid fuel” concept, Ding et al.^241,242 developed a highly efficient, 3D solid-state architectured anode which was templated by fabric textile coupons and combined with ceria–carbonate, a low temperature superior ionic conductive electrolyte to directly oxidize carbon fuel below 600 °C. DCFCs showed maximum power densities of 143, 196, and 325 mW cm⁻² at 500, 550, and 600 °C, respectively. The cell also operated steadily with a rated power density of ≈0.13 W cm⁻² with a superior carbon utilization efficiency of over 85.5%. Moreover, the peak power density was continuously improved to 392 mW cm⁻² at 600 °C based on the same cathode catalyst but the optimized microstructure of 3D ceramic textiles. The DCFCs with direct oxidation of carbon to CO₂ while without going through intermediate fuel CO can remarkably improve energy conversion efficiency, which should become a future research interest in the academic and industry field.

3.1.2.7 Solid oxide metal–air batteries. Similar to the fuel cell, metal–air batteries received increased attention in recent years due to their high energy storage density and capability to perform fast and deep charge/discharge cycles.²⁴⁷ This has positioned the metal–air battery as a front-runner in the commercial development of large-scale energy storage devices. However, the current metal–air battery system is mainly investigated at room temperature or in the liquid electrolyte solution. The large electrode polarization resistance induced by the oxygen reduction remains the major challenge and it requires high loading of the precious metal catalyst. In the previous section, we showed that the SOCs operated at elevated temperatures showed much-reduced electrode and total polarization loss. So, metal–air batteries operated at an elevated temperature should hold a higher potential for energy conversion and storage. In recent years, metal–air batteries, like Fe–air,^247–251 Mo–air,²⁵² Si–air,²⁵³ W–air,²⁵⁴ Mg–air,²⁵⁵ Li–air,²⁵⁶ and Sb–air batteries²⁵⁷ have been reported, in which Fe–air batteries received particular attention because of the abundant and non-toxic nature of Fe fuel. In 2011, a redox flow battery concept based on the solid oxide membrane was first demonstrated.²⁴⁷ Such a battery comprises two key decoupled components: a reversible solid oxide cell (RSOC) and a metal/metal oxide redox couple. In a typical operation, Fe is first loaded on the surface of the anode, when water/steam is introduced and it reacts with Fe to form/release H₂, and Fe is oxidized to FeO_x. The resulting H₂ is then electrochemically oxidized to generate electricity in SOFC mode, i.e., the discharging process in a battery. When all Fe is oxidized, the discharging process of battery is stopped; then the discharging process is initiated. H₂O is recycled and electrochemically split into H₂, which is fed into the anode chamber to reduce FeO_x, regenerating metallic Fe to close the redox cycle. When considering the whole process, the overall reaction is the reverse reaction 2M + xO₂ ⇔ 2MO_x (Fig. 13). Since the flow of reaction gas resembles the flow of electrodes in a conventional liquid Redox Flow Battery (RFB), this is termed a “Solid Oxide RFB (SORFB)”.²⁴⁷ Inoishi et al.²⁴⁸ also demonstrated such an RFB with H₂O/H₂ as a redox mediator. Such a novel RFB can produce an energy capacity of 348 W h per kg-Fe and a round-trip efficiency of 91.5% over twenty reliable charge/discharge cycles, demonstrating the feasibility and promising further deployment. One critical issue with SORFBs, similar to that of DCFCs, is the limited O²⁻ transportation from the electrode to FeO_x. If directly oxidized, the reaction product FeO_x is a semiconductor with low electronic conductivity, which will hinder further reaction of the remaining Fe. So similar to the redox media-assisted DCFC, gas-mediated SORFBs received increased attention.^{249–251,258,259} Another consideration of a solid oxide Fe–air battery is improving reversibility and durability. Huang proposed multiple strategies in this aspect.²⁴⁹ For example, iron carbide replaced the metallic Fe. The reaction product while using iron carbide is CO and CO₂, which will create a unique porous structure to improve the reaction kinetics; the excess carbon in the anode can act as additional active fuel to engage in the redox reaction for additional storage capacity. With H₂O/H₂ mediation, the battery was continuously cycled at a current density of 10 mA cm⁻² for 10 consecutive cycles (each for 10 min), producing a constant discharge specific energy of 1258 W h per kg of Fe, which is 93% of the theoretical specific energy. The calculated round-trip efficiency is 83.3%. The cell also showed good stability for more than 100 cycles at 550 °C and j = 10 mA cm⁻². To further address the slow kinetics in iron/iron oxide redox couples, the Pd catalyst²⁵⁹ and a combined proton-conducting BaZr_0.4Ce_0.4Y_0.1Yb_0.1O₃ (BZC4YYb) and reduction-promoting catalyst Ir²⁵¹ were developed to improve the H⁺ diffusion and FeO_x-reduction kinetics. When the battery operated under 550 °C, 50% Fe-utilization, and 0.2C, it exhibited a discharge-specific energy density of 601.9 W h per kg-Fe with a round-trip efficiency of 82.9% for 250 h of a cycle duration of 2.5 h; at a reduced temperature of 500 °C, the same battery exhibits 520 W h per kg-Fe discharge energy density with an RTE of 61.8% for 500 h. Such exceptional durability and reversibility are much higher than those of a 10 h duration energy storage target that can only be marginally achieved using the most dominated energy storage technologies, like pumped-hydro storage, redox flow batteries, lithium-ion batteries, and sodium–sulfur batteries.²⁵¹ Further work from the same group recently demonstrated the feasibility of such a novel rechargeable battery for applications requiring both large current density and fast charge/discharge cycle capability through improving the FeO_x-to-Fe reduction kinetics and reducing the RSOC high electrode potential.²⁵⁸ With the adoption of the classic three-electrode configuration, the polarization resistance contribution of the oxygen evolution reaction and other sources are well deconvoluted and determined. Combining all the improvements made in battery materials including the FeO_x–Fe reduction kinetics, the group can realize 12.5 cycling with a discharging specific energy density of 625 W h per kg-Fe and a round trip efficiency of 87%, such a big step may move this RFB a step closer to practical applications. Further scaling of the solid oxide Fe–air battery may realize longer stability, like daily, weekly, monthly and even seasonal cycles.

Fig. 13 (a) Cell structure, working principle and cycling performance with different anode fuel materials at U_Fe = 50% and 0.2C (10 mA cm⁻²) at 550 °C (b) and 500 °C (c) of a solid oxide iron–air battery.²⁵¹ Reproduced with permission (open access).

3.1.3 Reversible conversion. As mentioned in the above section, SEMRs can be reversibly operated under both the fuel cell and electrolytic cell models, which are also called reversible SOCs (RSOCs, Fig. 14a). The application of the RSOCs addresses the time dependence on energy/fuel demand, realizing electrical load balancing in addition to fuel production, as well as reversible power-to-gas systems. Moreover, the intentional reversible operation of SOCs can eliminate the degradation issue, especially for the electrolytic operation, where the anode delamination issue induced by the oxygen partial pressure gradient across the anode/electrolyte interface causes insufficient long-term stability.²⁶⁰ The first demonstration of this gifted idea was performed by Graves C.⁷¹ It was found that electrolytic performance gradually decreased when the SOEC was operated at a galvanostatic mode (−1 A cm⁻²) at 800 °C. In particular, the cell voltage increased from 1.33 V to 1.73 V during 420 h, and the ohmic resistance and electrode polarization resistance both increased by a factor of 2.8 times, whereas in the reversible mode, the cell voltage during the −1.0/+0.5 A cm⁻² cycle with a period of 1 h and 5 h, respectively, remained stable at 1.33 V during the same period (Fig. 14b and c). An accelerated reversible operation with only a fraction of the time electrolysis operation during 120 cycles (1100 h of test time) also confirmed negligible degradation. The post-characterization of the tested ROSCs suggested the elimination of the microstructural degradation occurring near the oxygen electrode/electrolyte interface. The findings of this work not only well address the balance of the fluctuation of electrical load and the intermittent nature of renewable power, but also suggest a promising method to improve the device lifespan through controlling operational conditions to reverse or repair deteriorating materials and interfaces. Furthermore, such a reversible SOC operation could find unique application in the current period with increased attention to conserving and utilizing CO₂ at the present carbon emission peak and carbon neutralization period and realizing the important role of hydrogen economics for future green and sustainable development.^20,118,261

Fig. 14 (a) Schematic illustration of an RSOC, comparison of (b) voltage and (c) impedance spectra responses of an SOC during a constant-current electrolysis test, and a reversible cycling test.⁷¹ Reproduced with permission.

3.2 SEMRs with proton conductors

SEMRs with oxygen ions are characterized by high temperature operation due to the large ionic radius, atom weight, and two negative electrons. The high temperature operation leads to quick kinetics. However, it also brings the challenges of strict material requirements and subsequent high operational costs. Another additional drawback is the low degree of coupling with other thermal, chemical, and electrochemical processes. For example, the co-electrolysis cell with H₂ and CO co-generation with optimized volume ratio could be further in situ utilized to produce hydrocarbon fuel in one step through the F–T process. However, the thermodynamically favored temperature range for F–T process is 300–500 °C, much lower than that of the efficient electrolytic temperature. Direct integration of the exothermal F–T process and endothermic electrolysis is favored and the integration will also mean reduced reactor and space occupation.

In 1981, Iwahara first observed proton conduction in oxide materials and demonstrated their application in solid oxide cells.^30,262 In 2002, the principle of an H₂ pump to purify the H₂ gas using proton conductive SrCe_0.95Yb_0.05O_3−δ at 800 °C was demonstrated by Matsumoto.²⁶³ Those studies open a new research field.²⁶⁴ However, in the following years, the research progress on H-SEMRs (Fig. 15) was much lower than that on its counterpart, the oxygen ionic conductor, due to the lack of stable electrolytes and efficient cathode materials,⁶⁶ even though it has higher electrochemical conversion efficiency since water is produced in the cathode in the power generation mode or water is mixed with the oxidant in the electrolysis mode, while the fuel, hydrogen, will not be diluted and it can be directly recycled if properly pressurized and electrochemically compacted.^15,265 However, the research activity of H-SEMRs is comparable because of the significant room for process integration, especially for the combined heat and power cogeneration through the hydrogenation or dehydrogenation reaction by adding or stripping hydrogen from organic compounds (CH₄ to C₂H_4–6; C–C to C=C; C=C to C–C),⁴⁷ and electrochemical ammonia synthesis,²⁶⁶ CO₂ conversion²⁶⁷ and electrochemical hydrogen pressurization¹⁵ because of the matched operating temperature and reaction kinetics. Remarkable progress in last two decades has been made in the advanced proton conductor development,^169,268 active electrode materials,^269,270 in particular, the triple conductive materials,^{20,21,171,269,271} quick evolution of the cell fabrication technology,¹⁷¹ and cell interface optimization,^173,272 which made the H-SEMRs more reliable and more efficient;^66,273,274 these related research areas have become hot subjects/topics in the energy, environmental and chemical industrial field.

Fig. 15 The different working principles of H-SEMRs: (a) fuel cell, (b) electrolyzer cell, (c) CO₂/H₂O co-electrolysis, and (d) co-electrolysis reactor based on co-ionic conductor.^66,265 Reproduced with permission.

It should be noted that efficient proton conductors and active electrode materials are pre-conditions of efficient SEMRs. Much progress has been made in those aspects.^275–277 Prof. Liu Meilin's group from Georgia Institute of Technology contributed enormously to the development of active oxide proton conductors. In 2006, they first reported BaZr_0.1Ce_0.7Y_0.2O_3−δ as an active proton conductor for low temperature SOFCs which showed the highest ionic conductivity among known electrolyte materials.²⁷⁷ The same group later revealed the doping of Yb in the above composite and realized hybrid ion (H⁺/O²⁻) conducting materials in a single-phase oxide. The unique hybrid ionic conduction property grants the SOFC high anti-coking and sulfur poisoning capability.¹⁶⁹ Another exceptional case of active electrolyte material exploration is revealed by Zhou and his colleagues, they found that a typical electronically conductive material SmNiO₃ was changed with active proton conductor materials through H⁺ filling-controlled Mott transition.²⁶⁸ However, the compromise of the electrolyte materials' conductivity and stability and the lack of active electrode material reduce their potential. Until recently, O'Hayre developed a combined strategy of developing a reactive active sintering technique and infiltrated active electrode material BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ for superior performance and durable PCFCs, reaching 455 and 142 mW cm⁻² at 500 °C in the case of hydrogen fuel and CH₄, respectively, as well as more than 1400 h performance durability.¹⁷¹ Such a breakthrough work renewed the interest and activity of H-SEMR fields. With further extensive work on scaling up cell fabrication²⁷³ and addressing the interface properties, fuel flexibility,²¹ poisoning-induced durability issues^173,272,276 as well as reversible operation,²⁷⁸ H-SEMR or electrifying chemistry with protonic cells²³ made significant progress in superior energy conversion efficiency^20,21 and targeted reactions or chemical generation yields.^47,279 Since high-performance PCFCs are not the major concern of this work, readers are suggested to refer to the related reviews on this subject, we will focus more on the chemical generation or system integration in SEMRs using proton conductors.

3.2.1 Dehydrogenation. Electrochemical dehydrogenation using proton conductors offers a sustainable path for the utilization of fuels and their conversion to useful and important feedstocks compared with the common thermal catalyst and oxidative dehydrogenation process because of the improved reaction controllability and product selectivity. Generally, dehydrogenation can be divided into two major types. In the first type the proton from the hydrocarbon dehydrogenation reaction in the anode reacts with the oxidant. Thus, both chemicals and power are produced in one reactor; it is called the co-generation process. In the other type the proton is released as an H₂ molecule, where highly concentrated/compacted hydrogen can be obtained. Depending on the exact reaction, heat or electricity is required to initiate the chemical reaction.
3.2.1.1 Chemical and/or power cogeneration.
3.2.1.1.1 Alkane to alkene. Light olefins such as propylene and ethylene have drawn worldwide attention since they are the most important raw materials in the petrochemical industry. For example, they are the major intermediate chemicals for polymers, their global demand growth is estimated to be 4–5% per annum. While the typical manufacture is based on the steam cracking of hydrocarbons from fossil fuels on a large scale, both the low yield and large CO₂/NO_x gas emissions as well as the energy-intensive process are the major barriers. The oxidative dehydrogenation of alkanes has been researched intensively.²⁸⁰ However, it is not easy to control the degree of deep oxidation of the precursors to CO₂ and thereby achieve high ethylene selectivity. The application of H-SEMRs allows easy control of the reaction rate and overcomes the deep oxidation issue as well as the chemical equilibrium limitation, which has become one of the hot research topics although it is still in the early scaling stages (Table 2). Since 2007, a research group led by Prof. JingLi Luo performed extensive work on the dehydrogenation process from ethane and propane to propylene and ethylene using H-SEMRs by developing active anode catalysts, studying the carbon deposition phenomenon and dehydrogenation mechanism, to improve the process efficiency and operational stability with the final target of putting them into application. In their first run C₃H₈–O₂ H-SEMRs with BaCe_0.85Y_0.15O₃ and Pt symmetric electrode at the temperature range of 550–650 °C²⁸¹ achieved a selectivity of C₂H₄ and C₃H₆ higher than 60% and 20%, respectively at a conversion rate of up to 40%. Moreover, the migrated H⁺ can react with cathodic O₂, generating a peak power density of 35 mW cm⁻² at 650 °C. Such pioneering work demonstrated the feasibility of H-SEMRs for light olefin production from an alkane. In parallel, they investigated the possible carbon deposition issue during propane dehydrogenation with the newly developed electrode catalyst, Cr₂O₃.²⁸² They found that the carbon deposition under fuel cell ‘discharging’ conditions was much less than that under OCV conditions, the former can be further reduced with the addition of the K promotor. Careful adjustment of the ionic conductivity of the electrolyte through optimization of sintering parameters, precursor particle size, and application of active catalysts, like nano size Cr₂O₃,²⁸³ MoC_x,²⁸⁴ Cu–Cr₂O₃,²⁸⁵ Cr₃C₂-WC,²⁸⁶ and Co₂CrO₄ (ref. 287) or optimizing the anode/cell microstructure/configuration^283,288 was beneficial for the efficiency and fuel conversion.^289,290 A typical example is that this group developed a metal/alloy-perovskite oxide composite anodic catalyst for H-SEMR feeding with ethane.^194,291 The CoFe alloy nanoparticles were well socketed on the surface to perovskite oxide parents after being annealed in a reducing atmosphere. The SEMR based on such a novel anode catalyst gave a peak power density of 348.84 mW cm⁻² at 750 °C with ethane fuel. Moreover, high ethylene yields of up to 41.5% and a selectivity of 91% as well as no CO₂ emission were achieved. Durable testing over 100 h with a current density of 0.65 A cm⁻² was also demonstrated. A critical challenge to such high temperature H-SEMRs is their low efficiency, as you may see from the above case that the yields are mostly lower than 50%. To address the above challenge, Yan presented an ingenious combined approach of nano engineering of TPB by loading Co₇W₆@WOx core–shell nanoparticles on an anode matrix and co-feeding of hydrogen with the chemical feedstocks.²⁵ They reported that the yield of butenes and 1,3-butadiene from the n-butane dehydrogenation reaction at 650 °C reached 52% with a selectivity of 93.7%. Besides the simple dehydrogenation, a deep alkane crack reaction was also observed using Ni-exsolved perovskite oxide as a catalyst.²⁹² Conversion selectivity to propylene and ethylene increased with the applied current densities, reaching 36% and 32%, respectively, at 300 mA cm⁻² and 700 °C using the H-SEMRs. In their work, the peak power density of the reactor could reach 450 mW cm⁻² at 750 °C, suggesting a promising approach for electricity and light olefin cogeneration. In a quite recent study, Lei coupled the nitrous oxide decomposition into the above alkane to alkene process, in which NO_x was used to replace oxygen as the oxidant.^291,293 Both ethane conversion (from 44.9% to 45.2%) and fuel cell performance (from 200 mW cm⁻² to 208 mW cm⁻² at 750 °C) were improved with NO_x utilization efficiency of 19.0%. Thus, such a multi-win technology is promising for fuel upgradation and greenhouse gas reduction.

Table 2 Literature data on the performance of alkane reaction for power and chemical cogeneration

Anode	Electrolyte	Fuel	Flowing rate (mL min^−1)	Conversion	Selectivity	Yield	Power density (mW cm^−2)	Ref.	Duration
Pt	BaCeY0.15O3	C3H8	20–80	5–30% (550–650 °C)	90–95% for CH4 + C2H4 + C3H6	—	17–35 (600–650 °C)	281	—
Cr2O3 + Ag	BaCe0.8Y0.15Nd0.05O3−δ	C2H6	150	—	—	8–31% for C2H4	51–118	283	50 h (650–750 °C)
MoCx	BaCe0.7Zr0.1Y0.2O3−δ		100	7.7–42.6%	97.5–87.2%	—	65–215	284	100 h
Cu–Cr2O3			100	10–40%	90–100%		81–170	285	10 h
Cr3C2 and WC			75	10–40%	91–96%	—	−185	286	80 h
Co–Cr2O3	BaCe0.8Y0.15Nd0.05O3−δ		100	—	91.6%	31.2%	173	287	No CO2
La0.6Sr0.4Fe0.8Nb0.1Cu0.1O3−δ	BaCe0.7Zr0.1Y0.2O3−δ		30	43.3% (750 °C)	94%	—	90	288	12 h
Co/Fe-(Pr0.4Sr0.6)3 (Fe0.85Mo0.15)2O7			100	15–50% (650–750 °C)	90–100%	13.2–41.5% (650–750 °C)	349 (750 °C)	291	750 °C for 100 h
FeNi3-(PrBa)0.95(Fe0.8Ni0.2)1.8Mo0.2O6−δ	BaZr0.1Ce0.7Y0.2O3		30	50.9%	92.1%	46.9%	318 (750 °C)	295	50 h
Au	BaCe0.7Zr0.1Y0.2−xNdxO3−δ		100	3–23% (600–700 °C)	100–90%	23% (700 °C)	123 (700 °C)	296	—
Pr0.6Sr0.4Fe0.8Nb0.1Cu0.1O3−δ	BaZr0.1Ce0.7Y0.1Yb0.1O3−δ		30	45.2	92.8	—	209 (750 °C)	293	N2O as oxidant/24 h
Co–SrMo0.8Co0.1Fe0.1O3−δ	BaCe0.7Zr0.1Y0.2−xNdxO3−δ		100	12.5–41.3 at 650–750 °C	>91%	11.9–37.8%	258 (750 °C)	194	50 h
Aligned carbon nanotube forests	BaZr0.4Ce0.4Y0.1Yb0.1O3−δ		20	37.6 (650 °C)	>90	—	111 (650 °C)	297	90 h
PtGa/ZSM-5-(Pr0.3Sr0.7)0.9Ni0.1Ti0.9O3		C3H8	50	15%	>90	—	172 (550 °C)	298	20 h
(Pr0.3Sr0.7)0.9Ni0.1Ti0.9O3 catalyst layer	BaZr0.3Ce0.5Y0.2O3−δ	C3H8 + H2O	10	20–70%	68% for C3H6 + C2H4 (700 °C)	—	94–450 (650–750 °C)	292	50 h
Co7W6@WOx-La0.4Sr0.6TiO3	YSZ	n-C4H8	—	80% (650 °C)	65% for C4 alkenes	50%	212	25	7 h

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3.2.1.1.2 C–C coupling/upgradation. Direct conversion/upgradation of light olefins like methane to C₂ aromatics through the electrochemical method using the “green electrons” from renewable sources received increased interest because of the largest production volume and energy consumption based on the conventional thermal catalysis process.^5,148 One more additional benefit is the electrochemical approach can well avoid the severe thermodynamic constraint. The application of proton conductors compared with the mixed ionic and electronic conductors or pure oxygen ionic conductors in SEMRs also holds advantages through a non-oxidative dehydrogenation process, and the product selectivity is much higher than in other processes.

In light olefin conversion, methane is the most investigated molecule. One of the key issues is the high chemical stability of CH₄ due to the absence of a dipole moment and the small polarizability (2.84 × 10⁻⁴⁰ C² m² J⁻¹), high temperature is preferred.¹ While the direct conversion of CH₄ to C_xH_y in the absence of oxidants is thermodynamically unfavorable in a large range of temperatures.¹² In factor, CH₄ can be non-oxidatively activated to the couple as C₂–C₆ hydrocarbon based on the following equations, the exact products are dependent on the applied temperature, the operational mode,¹⁴⁹ and the last but most important one, the catalyst.²⁹⁴

2CH₄ → 2H₂ + C₂H₄, ΔH⁰_25°C = +202 kJ

6CH₄ → 9H₂ + C₆H₆

As one may see from the above equations, these processes are not spontaneous, so SEMRs are normally operated in the electrolytic or pumping mode at elevated temperature to efficiently force the H⁺ to move to the low concentration side, while too high temperature may result in carbon deposition through the CH₄ pyrolysis reaction. Therefore, maintaining high H⁺ conductivity and high stability of the CH₄ coupling reaction is the key to realizing high efficiency. Similar to the previous discussion, the main proton conductor in this section is still the perovskite oxide. In 1992, Hibino was the first to adopt SrCe_0.92Yb_0.05O_3−x perovskite oxide H-SEMRs with Ag catalyst for CH₄ activation and conversion to C₂ hydrocarbon.²⁹⁹ As expected, under the OCV conditions, the conversion rate of CH₄ was limited, which was significantly improved when the galvanostatic current was used; the formation rate of C₂H₆ and C₂H₄ increased with the increase of the applied current, and the evolved hydrogen in the cathode was in agreement with the theoretical value, suggesting that the proton transfer number of the applied electrolyte is close to unity. Moreover, no CO₂/CO was observed in the products. The same group later showed that a similar material system reached C₂-compound formation rates of 0.23 to 0.52 μmol min⁻¹ cm⁻² when the cathode Ar gas was replaced by air since H₂ was oxidized to water which speeded up the transportation of proton.³⁰⁰ The results demonstrate the feasibility of the application of H-SEMRs for CH₄ coupling. A similar MAE system was also utilized by adding a composite of Au–5Ce₅Na₂WO₄/SiO₂ to promote the conversion of CH₄ and selectivity of C₂ hydrocarbon.³⁰¹ Moreover, a CH₄–steam–O₂ mixture was introduced to prevent the problem of carbon formation. The reaction products were C₂H₄, C₂H₆, H₂, CO, and CO₂. Such an anode fuel composite also allowed the SEMRs to be operated under the fuel cell conditions, which showed higher resistance toward carbon deposition induced by pyrolysis. It was found that 95% of hydrogen was transported and a 20% increase of C₂ production was achieved.

In 2016, Morejudo and colleagues⁴⁷ reported one of the most significant developments using mainly proton conducting solid-state electrolytes for the direct conversion of CH₄ to aromatics with continuous H⁺ removal while being free from carbon deposition issues (Fig. 16a). The SEMRs were assembled with a 25 μm-thick BaZr_0.7Ce_0.2Y_0.1O_3−d (BZCY) supported on a Ni-BZCY cathode, Cu anode plus a critical shape-selective Mo/zeolite catalyst (Fig. 16b). When CH₄ was introduced into the tubular anode chamber, it was catalytically converted to aromatics with a constant yield of 10% at 710 °C, a current density of 40 mA cm⁻², and a space rate of 150 mL g⁻¹ h⁻¹ at 1 bar and a methane conversion rate of ∼11% (Fig. 16c and d). The yield can be further improved to over 20% with the increase of applied current density, or the H₂ extraction rate, while it reduced from 12% to 1.5% under the fixed bed reaction conditions after 45 h operation, against 9% under SEMR condition (Fig. 16e). In other words, the degradation rate of CH₄ to aromatics in SEMRs was one order of magnitude lower than that observed in the conventional fixed bed reactor; improved resistance to carbon deposition was ascribed. Such an exceptional anti-degradation capability was ascribed to the mixed H⁺ and O²⁻ (0.3% of H₂ conductivity in this case) conductivity of BZCY at the elevated temperature.¹⁶⁹ Minor CO formation in the anode chamber could be a good indicator of such O²⁻ conduction property, since the scavenging of reactive carbon from the catalyst surface occurred via the steam reforming reaction C + H₂O → CO + H₂, in which H₂O is formed from O²⁻ with H₂ in the anode dehydrogenation process.

Fig. 16 Co-ionic conductive SEMRs for CH₄ to aromatics conversion: (a) tubular cell configuration and material system, (b) SEM image of MEA based on BZCY electrolyte, (c) percentage of H₂ extracted and O₂ injected versus current density at 700 °C, (d) aromatic yields vs. time and (e) CH₄ conversion.⁴⁷ Reproduced with permission.

One may see that the one-pass conversion of CH₄ was ∼11% in the above case, which resulted in a relatively low aromatics yield. Recently, Sakbodine et al.³⁰² designed a new tubular H-SEMR for CH₄ conversion to C₂ and aromatic products. The SEMRs are composed of a mixed ionic–electronic SrCe_0.7Zr_0.2Eu_0.1O_3−δ thin film (≈20 μm) supported on the outer surface of a one-end capped porous SrCe_0.8Zr_0.2O_3−δ tube. And the internal tube was filled with a coke-resistant Fe©SiO₂ catalyst.³⁰³ In this case, the residence time of CH₄ in the reactor is extended so that the conversion rate and C₂ and aromatics product yields are much improved. Under the tested conditions, a stable ≈30% C₂₊ single-pass yield, with up to 30% CH₄ conversion and 99% selectivity to C₂ (C₂H₄ and C₂H₂) and aromatic (C₆H₆ and C₁₀H₈) products were obtained. Furthermore, the selectivity of C₂ and aromatics could be intentionally manipulated by adding H₂ or changing the H₂ extraction rate during the cell operation. The same group later demonstrated that replacing N₂ at the outside of the reactor with O₂ can realize the energy balance between the endothermic dehydrogenation reaction and exothermic H₂ combustion on opposite sides of the membrane and overcome the common carbon deposition issue for CH₄ pyrolysis reaction at the elevated temperature.³⁰⁴

3.2.1.1.3 Hydrocarbon to syngas. The direct application of hydrocarbon fuel on PCFCs is limited due to the quick carbon deposition issue even though currently most proton conductors possess mixed proton and oxide ionic conduction and hydration capability.¹⁶⁹ To effectively utilize hydrocarbon fuel, sufficient reforming chemicals like CO₂, H₂O, and/or O₂ are required to overcome the carbon deposition issue.²¹ In this case, the reaction of in-site reforming hydrocarbon fuel takes place in the anode chamber to produce H₂ and CO mainly, in which H will be oxidized to form H⁺ and transported through the electrolyte layer and reduce O₂ in the cathode in the presence of electrons from the external circuit, yielding electricity. During the reforming process, H₂O and CO₂ will also be produced due to the presence of limited O²⁻ conduction in the electrolyte, which will react with hydrocarbon to produce CO and H₂. In other words, the hydrocarbon/reforming agent co-fed PCFCs may serve as a potential platform for producing syngas. Moreover, with the proper control of the feed gas competition and cell operational parameters, an ideal CO : H₂ molar ratio of 1 : 2 could be achieved. One of the critical issues to effectively realizing hydrocarbon fuel reforming and hydrogen oxidation reaction while free from carbon deposition issues is the lack of an active anode catalyst. The state-of-the-art anode catalyst Ni generally suffers from carbon poisoning and low redox stability. Precious metals like Rh have high activity but resources are limited resource. Other impurities like H₂S may degrade the anode activity with time. To address the above critical issues, Hua et al. proposed two methods of anode engineering to improve PCFC performance and fuel compatibility.^305,306 In 2016, they first reported intentionally adding an anodic functional layer, consisting of an Ni_0.8Co_0.2–La_0.2Ce_0.8O_1.9 (NiCo–LDC) composite on a Ni cermet anode surface.³⁰⁵ Such multiple-twinned bimetallic nanoparticles loaded on LDC proton conductors have superior catalytic activity toward in situ dry reforming and also CO₂ resistance, which helps improve the H₂ concentration in the anode active layer and electrolyte stability. It was found that the CO₂ conversion reached 91.5% at 700 °C and the SOFC operated stably for more than 100 h under galvanostatic conditions (1 A cm⁻²) in a CH₄–CO₂ freestream. Moreover, the selectivity and conversion of CO₂ and CH₄ as well as oxidation of hydrogen resulted in no CO₂ but CO-concentrated syngas in the anode, so this process can be considered an important process for CO₂ conversion and utilization. Considering the difficulties of the current collection and the complexity of additional layer fabrication induced by the anode catalytic layer, the same group further developed an even simpler method to address the above-mentioned technique's difficulties. The cell is still based on the Ni-based cermet anode but is successively infiltrated with a redox stable and catalytically active PBM (9 wt%, the fuel is named FC-PBM) and Ni₄Co bimetal catalyst (1.5%) on the anode surface (denoted as FC-NiCo/PBM).³⁰⁶ It is interesting to see that all infiltrations were selectively deposited on the surface of the BZCYYb grain because of its highly hydrophilic nature (Fig. 17a). The infiltrated nanocatalyst showed exceptional catalytic activity toward CH₄–CO₂ dry reforming to produce H₂ and CO and prevented the corrosion of BZCYY by the H₂O/CO₂ (Fig. 17b). The fuel cell performance of the CH₄–CO₂ co-fed PCFCs was improved from 0.4 W cm⁻² to 0.8 W cm⁻² and then over 1.1 W cm⁻² after the successive infiltration of PBM and Ni₄Co catalysts even in the presence of 50 ppm H₂S (Fig. 17c). Moreover, the cell could be operated stably for 36 h at a constant current density of 1 A cm⁻². Nearly 100% CH₄ conversion was achieved, and no CO₂ but CO-concentrated syngas was continuously obtained (Fig. 17d). These results will benefit the commercialization of PCFCs and SEMRs. Another interesting study by Dr Chuancheng Duan recently further explored the potential of PCFCs by fully using the exclusive proton conducting properties below 650 °C to realize exceptional power generation (0.30–0.94 W cm⁻² at 55–650 °C), chemical (syngas) production and CO₂/CH₄ gas mitigation.³⁰⁷ The authors developed a novel anode catalyst Sm_0.2Ce_0.7Ni_0.1Ru_0.05O_2−δ which in situ forms exsolved Ni and Ru on SDC. Ni in the hybrid catalyst is responsible for CH₄ activation, while Ru favors CO₂ activation and coke resistance by changing the reaction pathway, enabling superior fuel cell performance, quick CO₂/CH₄ reaction, and system durability. The wise design can be fully exploited for application of the catalyst and SOFC reactor in chemical manufacturing and energy sector as well as environmental protection. The same group³⁰⁸ reported that steam-fueled SOECs can selectively convert CO₂ to CO without hydrocarbon fuels like methane production at a wide range of temperatures of 400–600 °C and applied potentials. A specific electrode catalyst, in situ exsolved Ni–Fe/Sr₂Fe_1.4Mo_0.5O_6−δ nanoparticle electrocatalyst, was developed to replace the typical Ni-based cermet anode which effectively inhibited the formation of formic species, and subsequently selective production of CO. Therefore, this work suggests the importance of the rational design of electrode catalysts for selectively manufacturing chemicals. Recently, a similar system was used for steam reforming CH₄ for effectively converting CH₄ and greatly improved the fuel cell efficiency at the reduced temperature.³⁰⁹ These processes provide an alternative approach for CH₄/CO₂ utilization and achieve fuel/chemical feedstock production.

Fig. 17 (a) Schematic preparation procedures of NiCo/PBM catalyst selectively loading on BZCYYb in the porous nickel cermet anode, (b) the role of PBM nanocatalyst and Ni in cermet, (c) electrochemical performance of PCFCs fed with 50 ppm H₂S–CH₄/CO₂ fuel using different fuel electrodes and (d) the exhaust gas compositions of FC-NiCo/PBM during continuous 36 h testing.³⁰⁶ Reproduced with permission.

3.2.1.2 Hydrogen co-generation/separation. Different from the case of power and chemical cogeneration in SEMRs during the dehydrogenation process where the H₂ is used to produce electricity, the penetrated H₂ can utilize/compact directly or be catalytically converted to other chemicals. However, additional energy to promote the dehydrogenation and hydrogenation reaction in the form of electricity or heat energy is required. An additional benefit of such an operation is close to 100% product since only proton conduction takes place in the electrolyte considering the hybrid O²⁻/H⁺/hole(electronic) conduction of the widely used BaCeO₃-based proton conductors. So under the typical operational conditions, the former cases can be considered as a hybrid oxidative and non-oxidative dehydrogenation process. If the cathode side is the pure hydrogen process, the overall process for the latter case can be considered as a pure non-oxidative process.
3.2.1.2.1 Chemicals and hydrogen co-generation. In 2018, Ding first reported the electrochemical non-oxidative deprotonation of ethane using BZCYYb H-SEMRs with Ni-BZCYYb anode and PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) cathode catalysts (Fig. 18a and b).³¹⁰ They demonstrated that under an electrochemical overpotential of 140 mV, a constant current density of 1000 mA cm⁻² should be obtained for ethane dehydrogenation reaction under SOEC mode (Fig. 18c and d). Nearly 100% ethylene selectivity and a hydrogen production rate of 0.448 mol cm⁻² per day could be achieved at 400 °C. Moreover, they further demonstrated that compared with a conventional industrial ethane steam cracker, such an electrochemical dehydrogenation process can save 65% of process energy (Fig. 18e) and reduce 72% of carbon footprint (Fig. 18f). Those values can be further improved and the process can be a net gain energy process if renewable electricity and heat are used and the heating value of produced hydrogen is taken into account, suggesting a disruptive approach for petrochemical manufacturing, shifting the paradigm from thermal chemical practice to a clean electrochemical energy regime with renewable energy. Similar energy saving and process efficiency were also reported by Xie using H-SEMRs but with different electrode components.³¹¹ One of the differences is that an active dehydrogenation catalyst Pt/Ga-ZSM5 was used to improve the ethane conversion rate to 40% in a single pass conversion, while only 3.9% ethane was dehydrogenated in Ding's report.³¹⁰ The C₂H₄ yield can be improved to over 50% with the applied temperature of 700 °C. Furthermore, a steam electrolysis treatment was proposed to regenerate the catalyst by oxidizing the deposited carbon, such an additional operation alleviates catalytic degradation by 74% compared with the in-treated system. To further improve the ethane conversion and ethylene selectivity, the same group developed an active exsolved metal–oxide Ni_xCu_1−x–Nb_1.33(Ti_0.8Mn_0.2)_0.67O_4−δ hybrid catalyst and applied CO₂-assisted H₂ oxidation process in the cathode.⁵⁴ The highest ethane conversion of 75.2% and ∼100% ethylene selectivity at an applied voltage of 0.8 V in this electrochemical reactor were obtained, accompanied by the syngas production in the cathode. This demonstrated that the high-temperature electrochemical dehydrogenation process using SEMRs integrates the advances of electrochemistry and thermal catalysis would be a promising method to replace the classic thermochemical process.

Fig. 18 Ethane dehydrogenation using H-SEMRs. (a) Schematic of applied reactors and cell components, (b) a cross-sectional SEM image of an actual electrochemical cell, (c) the corresponding voltage and (d) durability test at a current density of 1 A cm⁻² as a function of time, and a comparison of the (e) process energies and (f) carbon footprint for ethylene production from ethane based on electrochemical and industrial steam cracker processes.³¹⁰ Reproduced with permission.

3.2.1.2.2 Hydrogen separation. SEMRs allow electrochemical supply of one of the active species/intermediates (H⁺, O₂ in the form of O²⁻, CO₂ in the form of CO₃²⁻…) from one electrode to another electrode to finish the electrochemical reactions. When there are no reactants that will react with those active species/intermediates, a gas evolution reaction will take place. With a continuous gas evolution reaction, the gas will be concentrated and even compacted at the sweep side; the whole process can be considered as a gas separation process. With more and more attention paid to future sustainable energy carriers such as H₂, research interests increased considerably in recent years. SEMRs are also used as membrane reactors for highly purified hydrogen generation from low-concentration H₂, steam, NH₃, CH₄, and biogas.^15,274,312 For example, Zhan reported that the separation rates of highly purified H₂ at 0.51 V and 500 °C was 3.3 and 2.4 mL cm⁻² min⁻¹ out of the simulated renewable hydrogen blended in natural gas pipelines with 10% H₂–90% N₂ and 10% H₂–90% CH₄, respectively, using a 10 μm thick BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte sandwiched by two Ni-BZCY electrodes.³¹³ The total Faraday efficiency surpassed 96%, suggesting the feasibility of such technology for the on-site purification of hydrogen. Moreover, when the reactor integrates other processes like electrochemical hydrogen compression, it can achieve both super high hydrogen removal/purification efficiency and overall energy efficiency. Rebollo first reported that a dense mixed ionically and electronically conductive BaCe_0.65Zr_0.20Y_0.15O_3−δ (BCZ20Y15) and Ce_0.85M_0.15O_2−δ (M = Y and Gd, hereafter referred to as YDC15 and GDC15) all-ceramic composite membrane was applied as a hydrogen separation membrane at temperatures higher than 500 °C.³¹² In such composite materials, BCZ20Y15 is considered an H⁺ conductor, and Ce_0.85M_0.15O_2−δ is a typical O²⁻ conductor while presenting mixed O²⁻ and e⁻ conducting behavior under hydrogen reduction conditions, which is frequently used as a composite electrolyte material in SOFCs, so it still can be considered as SEMRs. When membranes with a 50 : 50 volume ratio, BCZ20Y15–GDC15, at a thickness of 610 μm were exposed to both 3% hydrated Ar (sweep side) and H₂ (feed side) using Pt as a catalyst, the hydrogen separation rate increased with the applied temperature and reached values up to 0.27 mL min⁻¹ cm⁻² at 755 °C, one of the highest H₂ fluxes obtained for bulk mixed protonic–electronic membranes. When the temperature was further increased to 1040 °C, a temperature high enough for thermal splitting of H₂O, the flux remarkably increased to 1.75 mL min⁻¹ cm⁻² and 2.40 mL min⁻¹ cm⁻² when both sides of the membrane (700 μm thick) or only the sweep side was hydrated, respectively, which is the highest value of H₂ fluxes based on bulk mixed protonic–electronic membranes. It is interesting to see that both H₂ flux from the feed side in the form of bulk H⁺ transportation across the membrane and H₂ production through the water splitting reaction in the sweep side co-contributed to the superior H₂ flow in such a composite membrane. They also showed impressive CO₂ and sulfur tolerance under the practical hydrogen separation conditions.³¹²

3.2.2 Hydrogenation.
3.2.2.1 CO₂ hydrogenation to methane. Compared with the extremely endothermic H₂/CO formation reaction in the electrochemical process, the production of CH₄ in the reactor is a less endothermic process, which enables improved efficiency. Moreover, the easy storage capability and higher energy density of CH₄ fuel are widely pursued.^86,118 On the other hand, as discussed in the previous paragraph, temperature plays a crucial role in the in situ methanation process. H-SOECs, therefore, offer several key benefits to achieve high-efficiency direct CH₄ production: (1) better balance between the thermodynamics of the exothermic Sabatier reaction and the chemical kinetics of CO₂ conversion because of the much higher proton conductivity below 600 °C than its oxygen ionic conductor counterpart; (2) physical separation of CO₂ and H₂O allows gases at opposing electrodes, which favors a Le Chatelier's principle-free process, facilitating higher CH₄ production. In addition, the principle of proton conductor-based methanation devices is distinct from that of the O-SOECs. Only the electrolysis splitting of H₂O at the anode is required, which produces O₂ and H⁺. The former is released as a gas directly at the anode, and the latter passes through the proton conducting electrolyte and reacts with CO₂ at the cathode before or after the formation of H₂ to form the targeted hydrocarbon fuel (CH₄ in this case). The separate supply of H₂O and CO₂ can effectively improve the CH₄ yield by reducing the reverse conversion according to Le Chatelier's principle.¹¹⁷ The research on the development of proton conductors, the compatible electrode materials, and their fabrication and applications in ceramic fuel cell/electrolysis cells for power and chemical fuel production has made considerable progress in recent years,^{20,21,23,47,171,267,271,276,292,314–317} which favored the research on the CO₂ hydrogenation to CH₄ efficiency. For example, Xie reported synthesis of CH₄ by the electrochemical reduction of CO₂ in an SOEC based on the BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3−δ electrolyte of 60 μm in thickness and Fe/FeO_x composite cathode.³¹⁷ The application of a proton conductor enabled the cell's operation at 614 °C even though a largely applied over-potential of 2.6 V was required to reach a current density of 1.5 A cm⁻², and increased CH₄ contents in the outlet gas were observed. The production of CH₄ and CO reaches 0.07 and 3.25 mL min⁻¹ cm⁻², respectively, under the above operational conditions with a CO₂ conversion rate of 65%. The CH₄ yield was double that through the control process of F–T synthesis under similar conditions. In addition, the authors also observed that the electrochemical reduction of CO₂ is more efficient than the reverse water gas shift reaction (CO₂ + H₂ → CO + H₂O) as follows under the same geometry and compositions.

Under favorable thermodynamic conditions, the challenge to achieve higher CH₄ production requires the development of an active electrode and catalyst to realize higher H₂ yield efficiency and subsequently improved CO₂ hydrogenation process.^{20,21,171,318} With the quick evolution of new advanced electrode materials and cell fabrication technology and the understanding of the reaction kinetic process, remarkable CH₄ yield improvement has been achieved. It was reported that PCECs using BaZr_0.8Y_0.2O_3−δ (BZY) as the electrolyte and SrEu₂Fe_1.8Co_0.2O_7−δ as the anode displayed an exceptional current density of 1.23 A cm⁻² at 1.5 V and 100 h of smooth operation was achieved at 550 °C.³¹⁸ The presence of CO₂ in the cathode interestingly promoted the electrode reaction by accelerating the proton transportation and simultaneously shifting the equilibrium to boost the formation of synthetic hydrocarbons. Moreover, both the formation of CO and CH₄ increased with the increase of electrolysis density. A combined experimental and in situ spectroscopy study confirmed that the yielded proton in the anode continuously replenished to the cathode to react with the intermediate formed by adsorption of CO₂ on the electrode surface and finally yield CO and CH₄. An impressive study by Duan and O'Hayre reported that a button PCEC with BZCYYb electrolyte and highly active triple conductive electrode BZCFY showed a CH₄ yield of 7.5% at 500 °C.²⁰ Further combination of PCEC with a methanation fixed bed (300 °C) using CeO₂ with 10 wt% Ni can reach a CO₂ conversion of 79.7% and CH₄ selectivity of 99.1% at atmospheric pressure, i.e., CH₄ yield of 79%. Moreover, with careful control of the operational conditions, the hole conduction of proton conductor which is detrimental to the electrical efficiency of the electrolysis process is effectively suppressed, and a high faradaic efficiency of up to 98% has been achieved. Both the performance metrics were higher than those of the previous reports (by 2–3 times), and the Faraday efficiency and electricity to hydrogen efficiency were the highest in the open literature. Another breakthrough study by Li et al.²⁶⁷ reported selectively producing CH₄ (up to 95%) using Ir–ceria-based catalysts at 400 °C in PCECs for CO₂ reduction that operates at low overpotential and ambient pressure. Moreover, dedicated control of Ir speciation can be achieved by tuning the Ir–O hybridization through adjusting the Ir cluster size on ceria-based oxides. The Ir speciation in the form of either an ionic state or metallic state with a distinct catalyst surface chemical environment can selectively produce either CO or CH₄, significantly demonstrating the techno-economic feasibility of the process. Furthermore, the authors also demonstrated that such a strategy is also workable on other functional oxides like PBM-BZY and when in tandem with electrode hydrogenation of C₂H₆ to produce C₂H₄ and CH₄. This work highlighted the importance of precise control of the chemical environments of the surfaces of the catalyst but also pointed out the roles of intermediate temperature PCFCs with the combined thermal–electrochemical process for upgrading CO₂ for valuable synthetic fuels. More recent work also evaluated the one-step yield to 34.6% at 1 A cm⁻² and 450 °C using a designed PCEC unit-cell stack on a large scale (active area of 5 cm²) that enables the efficient utilization of reactants (Fig. 19 and b).¹¹⁷ With the increased applied current density, both the H₂ yield and CH₄ yield increased (Fig. 19c–e). Moreover, they found that with exhaust H₂ recycling, a critical yield of 71.2% could be achieved because of the increased CO₂ conversion and reduced hole conduction (Fig. 19f). In addition, the direct comparison of the electrochemical conversion of H₂O–CO₂ and thermochemical conversion of CO₂–H₂ under similar conditions confirmed a higher yield for the former process, suggesting that the effect of EPOC effect is also suitable for the CO₂ methanation reaction. Therefore, continuous work can focus on the development of active catalysts to achieve acceptable rates and selectivity. Recently, Chen deposited a CeO₂ thin film on the surface of Ni-BZCYYb.⁸⁶ It is found that the decorated CeO₂ modulates the adsorption of CO₂ and facilitates proton transfer for the hydrogenation process, leading to accelerated CH₄ production. Experimentally, an improvement of more than 3 times CH₄ selectively (absolute value 17%) at 550 °C and 1205 mA cm⁻², i.e., 0.2 mL min⁻¹ cm⁻² under typical PCEC conditions, was obtained compared with unmodified cells. The addition of the CeO₂ layer modified CO₂/CO adsorption, and promoted the formation of surface C–H by accelerating the migration of H⁺ toward the electrode surface, then improving CH₄ synthesis. This work highlighted the importance of SOFC reactors by combining heat, solar–thermal, and thermo-electrochemical processes.

Fig. 19 (a) Schematic illustration of the possible electrochemical and chemical reactions during electrolysis and upgrading CO₂–H₂O into CH₄ in the PCEC (b), (c–e) one pass CO₂ conversion, CH₄ selectivity, and CH₄ yields at 400–550 °C, and (f) with H₂ recycling at 450 °C.¹¹⁷ Reproduced with permission.

Different from the above cases, Duan reported the selective production of CO instead of CH₄ by the CO₂ hydrogenation process,³⁰⁸ whereas most of the work in the literature has reported mixed yields of CO and CH₄.^20,117,267 In their work, novel exsolved NiFe alloy nanoparticles socketed on an Sr₂Fe_1.4Mo_0.5O_6−δ negative electrode instead of a classic Ni-based cermet electrode were applied. It is found that selectivity to CO was ∼100% over a wide range of operating temperatures (400–600 °C) and applied potentials/current densities. Further investigation of the CO₂ reduction mechanism based on the in situ diffuse reflectance infrared spectroscopy (DRIFTS) showed that the designed electrode inhibited the formation of formic species, which therefore leads to selective production of CO under such operational conditions. Such an exception again demonstrates the crucial rule of tuning product selectivity by engineering the negative electrode. Further improving the yields of CH₄ through the CO₂ hydrogenation process is still highly desirable. Pressurized SEMRs based on proton conductors under optimized reaction conditions, like total pressure, H₂/CO₂ ratio, and current density would be a potential solution.³¹⁵

3.2.2.2 Electrocatalytic NH₃ synthesis. Ammonia (NH₃) is one of the largest-volume industrial chemicals produced in the world as an agricultural fertilizer and chemical compound, which is also considered an energy carrier in the future hydrogen economy, Hydrogen 2.0, because of carbon-free characteristics and well-equipped infrastructure. However, its synthesis relies on the Haber–Bosch (H–B) process, where high temperatures (400–600 °C) and high pressures (20–40 MPa) with an iron-based catalyst are required. The H–B process is a low temperature-favored exothermic reaction, while high temperature is needed to cleave the strong N≡N bond.^5,319 Moreover, this process inevitably produces huge CO₂ emissions (1% of global value) both for hydrogen production and the direct synthesis process.³²⁰ In contrast to thermal synthesis, electrosynthesis showed multiple benefits:^8,321 operation under mild conditions since e⁻ reduction enables a much-reduced temperature and pressure; much reduced decentralized plants; eliminating the hydrogen facility by using water as a hydrogen source; free from carbon sources and energy by integrating renewable power sources.^322,323

Versatile electrolytes can be used for electrochemical NH₃ synthesis.³²¹ Among these, solid-state proton conductor efficiently operated at 400–700 °C showed the most promise since both suitable temperature and active solid-state proton promote the kinetics of NH₃ and are free from subsequent gas purification as well as the utilization of water as a hydrogen source. The major challenges of electrochemical NH₃ synthesis are related to the compromise of activity (current density) and selectivity (NH₃). Another challenge to the wide commercialization of this process is the limited NH₃ synthesis rate, with the majority of the experimental studies reporting rates lower than 10⁻⁸ mol cm⁻² s⁻¹,³²⁴ which is one order of magnitude lower than that of the suggested value of 10⁻⁷ mol cm⁻² s⁻¹ plus a faradaic efficiency of up to 50%, as suggested by Giddey.³²⁰ Therefore, active and selective cell components are highly desirable.

The first experimentally electrochemical NH₃ synthesis on a solid-state proton conductor (SrCe_0.95Yb_0.05O₃) was demonstrated by Marnellos in 1998.²⁶⁶ In their work, H₂ and N₂ were supplied separately to the chamber of the end-closed tubular cell with Pd as the electrode catalyst. Under the polarization voltage conditions, they reported that 80% H⁺ which is the reaction product of H₂ oxidation (H₂ − 2e⁻ → 2H⁺) in the anode was converted to NH₃ through the following reaction N₂ + 6H⁺ + 6e⁻ → 2NH₃, and the overall conversion rate was limited by the proton supply in the anode to cathode. Although the yield of this work is quite low, it demonstrated and offered an alternative route to permit operation at atmospheric pressures and temperatures, eliminating the thermodynamic requirement on conventional catalytic reactors. The same group also demonstrated the use of a single chamber cell configuration to synthesize NH₃.³²³ Improved performance was further reported through the deployment of an anode supporting cell structure. Wang, therefore, developed Ni-cermet anode-supported PCFCs with perovskite oxide Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−α as the cathode.³²⁵ A dense BaCe_0.85Y_0.15O_3−α (BCY15) electrolyte was prepared by the modified spin coating method on the anode layer. Under a typical H₂/N₂ atmosphere, a maximum rate of ammonia formation of 4.1 × 10⁻⁹ mol s⁻¹ cm⁻² was obtained under a direct current of 1 mA at 530 °C, which is double the value of the previous work, suggesting the promise in improving the NH₃ yield by that potential method. A similar method was also employed by Vasileiou to reduce the ohmic resistance of proton electrolytes in a single chamber reactor using Rh film as the cathode to improve the NH₃ efficiency.³²⁶ Due to the limitation of proton conductivity at the applied temperature range, Tao et al. proposed to use ceria-based composite electrolytes,³²⁷ like ceria–Ca₃(PO₄)₂–K₃PO₄ (ref. 328) and carbonate-oxide^329–331 to replace oxide proton conductors because of the improved ionic conductivity at the intermediate temperature range which favors both the cleavage of the triple bond of the N₂ molecule and NH₃ stability. The hybrid ionic conduction of composite electrolytes also enabled distinct operational modes, proton or oxygen ionic conduction, with input electrical energy using different raw gases, including H₂/N₂, H₂O/N₂,³³¹ natural gas,³²⁸ and even with wet air^332,333 based on different catalysts. It is found that the applied temperature, voltage/current, and catalysts influenced the final N₂ conversion rate and NH₃ yield. In 2007, Liu's group first demonstrated the synthesis of NH₃ from N₂ and natural gas with a ceria–Ca₃(PO₄)₂–K₃PO₄ composite electrolyte.³²⁸ Natural gas was used as a proton source because of its wide availability. Ag–Pd was used as the symmetric electrode catalyst. It was found that the NH₃ product rate first increased with the applied cell potential, up to 1 V, and then it remained unchanged above 1.0 V. NH₃ yield rate also increased first with the applied temperature, and reached the maximum value of 6.95 × 10⁻⁹ mol s⁻¹ cm⁻² at 650 °C and then reduced with further increase of temperature. Another critical issue for NH₃ synthesis is related to the electrode activity, especially for the N₂ activation. Efforts were also made to develop active catalysts to improve NH₃ efficiency. Fe has been used as an industrial catalyst in the H–B process, therefore, its catalytic activity was also tested in electrochemical systems.³²² It was found that an increase of 80% reaction rate was obtained when protons were electrochemically “pumped” to the Fe electrode compared with that under OCV conditions. Ru is a well-known catalyst for NH₃ synthesis at relatively low temperatures because of the high activity for the dissociation of the N–N triple bond. However, due to its precious metal nature, effective utilization of Ru is required. Kosaka therefore applied the exsolution method to deposit 2–5 nm nanoparticles that are highly dispersed on the La_0.3Sr_0.6TiO₃ perovskite oxide surface.³³⁴ An improved NH₃ formation rate was observed in this system. A recent study employed BZCYYb as a proton electrolyte, BZCFY as an air electrode, Ni-based cermet as a cathode, and a novel Ru–(BaO)₂(CaO)(Al₂O₃) NH₃ catalytic functional layer;³³⁵ a record NH₃ yield of 1.2 × 10⁻⁸ mol cm⁻² s⁻¹ was obtained and it can be further improved to 2.1 × 10⁻⁶ mol cm⁻² s⁻¹ with the application of a pressurized system (12.5 bar) and the reactor maintained exceptional stability during more than 160 h testing. More interestingly, a hybrid approach to cyclic operation between NH₃-fueled power generation and electric power-driven NH₃ synthesis was demonstrated in this work, which may find suitable application in a future hydrogen economic society. To this end, Li recently synthesized Ru/La_0.25Ce_0.75O_2−x through hydrothermal treatment with abundant hydroxyl groups as active catalysts for NH₃ production integrated with H-SEMRs.³³⁸ Both the computational and experimental results demonstrated that Ce³⁺–OH/Ru sites facilitate both the triple bond decoupling and single bond N–H formation at 400 °C and 1 bar of N₂, which resulted in an NH₃ rate of 2.92 mol h⁻¹ m⁻² and a maximum Faraday efficiency of 25%. The former is about two orders of magnitude higher than the literature value. A more general integration of co-producing NH₃ and other chemicals, like alkane dehydrogenation or natural gas reforming process on one device and in one step, is desirable, which will benefit the efficiency and sustainability of such coupling technology. To this end, as can be seen from Table 3, the NH₃ yield rate is still lower than that of the benchmarking value, 4.35–8.75 × 10⁻⁷ mol s⁻¹ cm⁻² for industrial application,³²⁰ the faradaic efficiency should also be much improved. Novel N₂ activation or ionic conducting electrolyte materials are expected.³³⁶

Table 3 SEMR material systems and performance for NH₃ electrochemical generation^a

Anode	Electrolyte	Cathode	Catalyst	Temp. ( C)	Faradaic efficiency	NH3 rate, mol cm^−2 s^−1 × 10^9	Cathode reactant	References
a SSC: Sm0.5Sr0.5CoO3−δ; CGCO: Ce0.8Gd0.18Ca0.02O2−δ; PNFCO: Pr0.6Ba0.4Fe0.8Cu0.2O3; LSFC: La0.6Sr0.4Fe0.8Cu0.2O3−δ; CGO: Ce0.8Gd0.2O2; BZCYYb: BaZr0.1Ce0.7Y0.1Yb0.1O3−x; BSCF: Ba0.5Sr0.5Co0.8Fe0.2O3; BCTYZ: Ba0.95Ce0.6Tb0.1Y0.2Zr0.1O3.
Pd	SrCe0.95Yb0.05O3	Pd	—	570	78%	H2 pump rate dependent	N2	266
NiO-BCY	BaCe0.85Y0.15O3−α	BSCF	—	530	58%	4.1	N2	325
Ni	BaCe0.2Zr0.7Y0.1O2	Rh	—	600	6.2	13	N2	326
Ag–Pd	YDC-Ca3(PO4)2–K3PO4	Ag–Pd	—	650	—	6.95	N2	328
Ag–Pd	LiAlO2–(Li/Na/K)2CO3	Ag	CoFe2O4	400	—%	0.2	N2	329
			Co3Mo3N	450/0.8V		0.33	N^2	337
Ni-SDC	SDC-(Li/Na/K)2CO3	LSFC-Ce0.8Sm0.2O2−δ	—	450	—	5.39	N2	330
SSC-CGCO	(Li,Na,K)2CO3-CGCO	CoFe2O4-CGCO	—	400/0.6 V	—	0.65	N2 + H2O	331
LSFC-CGCO		SSC-CGCO	—	400/1.4 V	0.1%	0.05	N2 + H2O	333
PNFCO-CGO	CGO–(Li,Na,K)(2)CO3	PNFCO-CGO	—	400/1.4 V	1%	1.07	Wet air	332
La0.3Sr0.6Ti0.6Ru0.4O3	BaCe0.9Y0.1O3	Pt	—	500	—	3.8 × 10^−3	N2	334
BZCFY	BZCYYb	Ni-BZCYYb	Ru–B2CA	400	—	12	N2	335
Ag–Pd	BaCe0.7Zr0.2Sm0.1O3	Ag–Pd	—	500	50%	2.7	N2	338
Ni-BZCYYb	BZCYYb	PrBa0.5Sr0.5CoFeO5+δ (PBSCF)	Ru/La0.25Ce0.75O2−x	400/0.6 V	25%	81	N2	339
Ag–Pt or LSCF	BaZr0.8Y0.2O3−δ	Ag–Pt or LSCF		500/0.8 V	0.5%	0.085	N2	340
Pt	BaCe0.9Y0.1O3	K, Al-modified Fe-BCY	—	650/1.5 V	0.1%	0.6	N2–H2	341
NiO-BCTYZ	BCTYZ	Fe@MXene	—	650	8.4%	8.24	N2	342
La0.33Sr0.67Cr0.33Fe0.67O3	GDC	La0.33Sr0.67Cr0.33Fe0.52Ru0.15O3	—	500	0.25%	0.473	N2 + H2O	343
Pt	La5.5WO11.25−δ	Ru	—	350	43.8%	3.78	N2	319

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3.3 SEMRs with hybrid ionic conductors

High temperature ionic conductors generally showed higher single ionic transfer numbers than the low-temperature polymer-based electrolyte. Indeed, most recently employed fluorite-type or perovskite-type oxides were supposed to be either pure proton or oxygen ion conductors, rather than dual-ion conductors, even though in 2009, Liu experimentally demonstrated that the Yb doped conventional BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCYYb) was hybrid oxygen ionic and proton conductors and it demonstrated exceptional activity and anti-coke deposition capability with hydrocarbon fuel even with the Ni-based cermet electrode.¹⁶⁹ Such outstanding results demonstrated the advantages of mixed ionic conduction over single ionic conductors. In addition, because of the non-conflicting oxygen ion and proton diffusion channels, i.e., oxygen ion transport through oxygen vacancy, while protons move with the lattice oxygen, each ionic conductivity is an addition to total ionic conductivity, therefore, hybrid ionic conductors show higher ionic conductivity over single ionic conductors³⁴⁴ and avoids the use of a fuel gas humidification system at the anode side.³⁴⁵ Besides the hybrid oxygen ionic and proton conductor, composite electrolytes based on ceria and salt also showed hybrid proton, oxygen ionic, and carbonate conductivity under a typical fuel cell atmosphere, which have also resulted in different SEMR applications in the literature.

3.3.1 Mixed H⁺/O²⁻ conductors. With regard to the mixed ionic conductor development, Fop reported that the undoped hexagonal perovskite Ba₇Nb₄MoO₂₀ showed pure ionic conduction with high proton and oxide ion conductivity over a range of p_O2,³⁴⁶ in which oxygen ionic conductivity reaches 10^−2.5 S cm⁻¹ at 800 °C in dried air, and proton conductivity reaches a similar value at 510 °C in air + H₂O, and the total 10^−2.5 S cm⁻¹ at 450 °C under an air + H₂O (p_H2O ∼ 0.021 atm) atmosphere and slightly increased with the applied temperature. The proton conduction was also elucidated by the isotope effect. This material also showed improved ionic conductivity stability over the classic BaCeO₃-based perovskite oxide proton conductors. Another study reported that the hybrid ionic conductivity of the conventional BZCYYb can be further improved by simply introducing B-site cation deficiency.³⁴⁷ Both the proton and oxygen conductivity of the designed BZCYYb electrolyte is improved to a large extent of 2.5 times up to 4.6 × 10⁻² S cm⁻¹ at 700 °C and by 6.3 times up to 1.2 × 10⁻² S cm⁻¹ at 900 °C compared to the BZCYYb electrolyte. With the improved ionic conductivity, the fuel cell performance is increased by 23.5% under the same operational conditions (650 °C) and the cell exhibited no observable performance degradation during the 300 h test.

The co-ionic character, mainly in typical proton ceramics, has been successfully explored for SEMR applications.^23,47,348 One typical example is used for methane coupling for the direct production of aromatics,^47,302 where the proton conduction is used to extract H⁺ from methane to shift the balance toward the production of CH* and subsequently to realize non-oxidative methane de-hydro-aromatization in the presence of the catalyst, and the minor oxygen ionic conduction was fully used to distribute oxide ions along the reactor length to activate the coke and reactivate the catalyst for the dehydrogenation reaction. Such unique co-ionic conduction enables an extremely high carbon efficiency, improving the techno-economic process viability. Another exceptional case of a used mixed ionic conductor is the dual-chamber water splitting to produce green hydrogen in “hybrid SOECs” (Fig. 20a–c).⁵⁶ When combined with highly active electrode materials, Ni as the anode catalyst and NdBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ as the cathode catalyst, the resultant SOEC gave a significantly improved current density of 3.16 A cm⁻² at 1.3 V and 750 °C in 10% humidified hydrogen/air atmospheres over single H- or O-SOEC reported in the literature (Fig. 20d and e) and showed much-reduced overpotential than that of the PEM electrolysis operated at the low temperature (Fig. 20f). Such a hybrid SOEC represents a promising alternative option for cost-effective and highly efficient green hydrogen generation. To this end, mixed ionic conduction brings many advantages, which showed impressive electrochemical performance and could find interesting applications in SEMRs. However, since the operational conditions and atmosphere are different from that of a single ionic conduction mode, both the suitable electrode and cell/stack material system should be developed and optimized to maximize electrical efficiency and product selectivity.

Fig. 20 Schematic diagrams of the working principles of (a) O-SOEC, (b) H-SOEC, (c) hybrid-SOEC, and (d–f) electrochemical performance and comparison with the literature work and other electrolysis systems.⁵⁶ Reproduced with permission.

3.3.2 Multi-ionic (O²⁻, H⁺, carbonate) conductor. Another hybrid ionic conductor is the ceria–salt composite, with ceria–carbonate as a typical example. In such composites, not only oxygen ionic conduction through ceria takes place, but also, proton conduction through the salt, and the interface between ceria and salt, carbonate ion/OH⁻ under a given atmosphere as well as cations co-transport through the constructed electrolyte membrane. In some cases, the salts are present as molten phases. However, the overall electrolyte membrane is solid state. Amorphous or molten carbonate salts are present in the pores of the ceria matrix with the help of capillary forces. Because of the multi-ionic conduction properties, they have been widely used as SEMRs with different purposes, like the high-efficiency inter-conversion between electricity and chemicals in the form of the fuel cell or electrolysis cell at a much-reduced temperature,^349,350 and electrochemical NH₃ synthesis either based on oxygen or proton conduction mode,^332,351 as well as CO₂ separation with permeation selectivity close to unity.^352,353 The CO₂ separation capability will be detailed in the next section. Regarding the NH₃ synthesis, in 2014, Tao first reported and demonstrated the direct synthesis of NH₃ from wet air using a single chamber SEMRs with Ce_0.8Gd_0.2O_2−δ (CGO)–(Li,Na,K)₂CO₃ composite electrolyte and a perovskite oxide Pr_0.6Ba_0.4Fe_0.8Cu_0.2O_3−δ symmetric electrode.^332,351 An ammonia formation rate of 1.07 × 10⁻¹⁰ mol s⁻¹ cm⁻² was realized at 400 °C and an applied voltage of 1.4 V, which was just slightly lower than that with wet N₂. Based on the high ionic conductivity of the Ce_0.8Gd_0.2O_2−δ (CGO)–(Li,Na,K)₂CO₃ composite electrolyte, the electrochemical process can be carried out at a much lower temperature, 400 °C, which avoids deep oxidation of NH₃ by O₂ in a single chamber configuration. It should be noted that the competing reaction, water splitting reaction, also took place, which led to quite lower Faradaic efficiency. However, the direct use of steam instead of H₂ and air to replace N₂ simplifies the overall process and improves the total efficiency compared with the classic Haber–Bosch process when operating at 430–480 °C. Further improvement of the reaction selectivity with active electrode materials could one day surpass the current technology.

Generally, when the solid electrolyte is a mono-ion conductor, an external circuit is required to separate gases of a high degree of purity. For the oxide–carbonate composite, the hybrid CO₃²⁻ and O²⁻ conduction enables highly selective CO₂ separation without an external circuit, the electrochemically active species are transported through the membrane under a gradient of electrochemical potential/or active substance partial pressure difference, only CO₂ is permeated through the electrolyte in such a pure ceria–carbonate composite membrane, which significantly reduces the cost and complexity of further treatment process. As demonstrated by Wade,^354,355 opposing ionic currents of carbonate and oxide ions are presented under the separation conditions on the YSZ–carbonate and ceria–carbonate composite membrane while the membrane with the Al₂O₃ matrix does not result in strong CO₂ permeability or selectivity. Thereby, its selectivity to CO₂ is exclusive, which has excellent compatibility with the high-temperature process stream where membrane-operated elevated temperatures in a continuous fashion are needed. When such a ceria–carbonate was used as a membrane for CO₂ separation, under the gas atmosphere of 4.8% H₂–CO₂–N₂/He, the CO₂ permeation rate reached 1.84 mL min⁻¹ cm⁻² at 700 °C when the carbonate content is 50% of the total volume.³⁵² Such an exceptional value is at least one order of magnitude higher than that of the literature value. It was found that the CO₂ flux rate is linearly increased with the increase of H₂ partial pressure of the feed side that provides increased oxide-ion flux because of increased oxygen chemical gradient across the membrane. Another Bi_1.5Y_0.3Sm_0.2O₃/molten Li/Na/K₂CO₃ (43.5/31.5/25 mol%) composite material gave a CO₂ permeation flux of 6.60 × 10⁻² mL cm⁻² min⁻¹ at 650 °C.³⁵⁶ These results demonstrated the promise of using multi-ionic conductive oxide-carbonate SEMRs for high-flux membrane separation.

Another interesting application of SEMRs for membrane separation is targeting chemical synthesis. With the extraction of CO₂ from the reaction zone, the thermodynamic equilibrium of typical reactions may be shifted, or a higher reactant concentration of reactant will be realized which is beneficial to the chemical reaction kinetics. For example, Dong reported highly purified H₂ using a catalyst-free ceramic–carbonate dual-phase membrane reactor.³⁵⁷ The gasifier syngas with the composition of H₂, CO, CO₂, and H₂O passes through the tubular ceria–carbonate composite membrane reactor since the membrane serves as a CO₂ permeation membrane. When CO₂ is removed from the mixed gas the water gas shift reaction H₂O + CO → H₂ + CO₂ is right-shifted, producing a higher H₂ partial pressure without any catalyst. When operated at 900 °C, the SEMRs gave a CO₂ flux rate of 2.7 × 10⁻³ mol m⁻² s⁻¹, and CO conversion and CO₂ recovery of 26.1% and 18.7%, respectively, which are much higher than the conventional fixed bed reactor under the identical conditions. Such catalyst-free WGS SEMRs could result in a substantial reduction in the capital cost and remarkable simplification of highly purified H₂ production. Such composite electrolyte-based SEMRs are also used in hydrogen production with carbon dioxide capture during steam reforming of the methane process,³⁵⁸ which shares a similar principle to the above case.

To this end, the CO₂ separation functionality of the ceria–carbonate membrane is also used in direct carbon SOFCs (DCFCs). Generally, high performance is mainly contributed by the electrochemical oxidation of CO, which is produced by the oxidation reaction of carbon fuel with the in situ produced CO₂, instead of carbon direct oxidation due to the limited solid–solid interfaces. To intensify CO production and remove CO₂, the inert carrier gas is frequently used, which, however, reduces the CO utilization efficiency, resulting in low carbon to electricity conversion efficiency. To overcome such a compromise, Shao et al.²³⁸ applied a ceria–carbon composite membrane onto the U-shaped DC-SOFCs, both of which served as an anode sealant and CO₂ separation membrane. With such a composite membrane separator, sufficient CO₂ partial pressure is ensured which facilitates the carbon reforming reaction to produce CO, the latter remains in an anode chamber for electrochemical oxidation. The CO₂ pressure gradient between the anode chamber and the atmospheric environment could force CO₂ permeation/removal from the anode chamber. With the addition of a ceria–carbonate composite membrane, an attractive peak power density of 279.3 mW cm⁻² was achieved for the DC-SOFCs at 850 °C, and a small stack composed of two cells can be operated continuously for 200 min, which is much better than that without the ceria–carbonate membrane. Further work could focus on the fabrication, design, and up-scaling of such promising technology.

4. Advancing SEMRs with multiple architectures

Different from the conventional two-chamber SEMRs (in the disc configuration), frequently in SOFCs, where the reactants are separated by the solid electrolytes that are dense enough to overcome the gas cross-issue, reactants can be co-fed into a chamber and the fuel and oxidant are selectively oxidized and reduced at the anode and cathode. The solid electrolyte disk is suspended in a flow of the reacting mixture and serves as a catalyst supporter. Such a specific reactor configuration is called a single-chamber solid oxide fuel cell (SC-SOFC).^{49,50,186,359} In 2000, Hibino first reported running SC-SOFCs with propane and air at 500 °C.¹⁸⁶ An OCV of 0.9 V and peak power density of 403–101 mW cm⁻² at 500–350 °C were obtained. Such a promising electrochemical performance stimulated further development of SC-SOFCs. In 2005, a thermally self-sustaining micro SC-SOFC stack was demonstrated for portable applications.⁴⁹ Similar fuel cell performances were also obtained at the stack level, but the self-sustaining character, low temperature operation, and hydrocarbon fuel direct usage make SC-SOFCs suitable for potential application to replace other energy conversion technologies. It should be noted that in the initial study, SC-SOFCs were mainly developed for energy conversion, while in 2011, Shao further combined SC-SOFCs with the downstream reforming catalyst to construct a reactor for the co-generation of power and syngas with zero waste gas emission.⁵⁰ The same group also compared the electrochemical performances of the CH₄–O₂ gas mixture (2 : 1) co-fed SC-SOFCs with two different modes and with or without catalysts (Fig. 21): one mixed gas flowed instantaneously to the anode and cathode (Fig. 21a), another mixed gas first to the cathode then to the anode chamber (Fig. 21b). Both the OCV and peak power density of the second mode were higher than those of the first mode, whose peak power density was 2 times higher.¹⁹⁷ When the GdNi/Al₂O₃ catalyst was further added to the gas before the anode chamber (Fig. 21c), 2.6 times higher peak power density was achieved and such a cell can be stably operated at 700 °C for 10 h. With a further addition of the catalyst layer at the outlet of the anode chamber (Fig. 21d), synthesis gas with a H₂ : CO ratio close to 2 was obtained, suggesting a significant advantage for practical applications.

Fig. 21 Different SEMR design based on SC-SOFCs with/without catalyst components in tubular structures: (a) all flow-through mode, (b) first cathode chamber then anode chamber, (c) similar to mode b but with catalyst at the inlet of anode chamber and (d) similar to mode c with additional catalyst at the outlet of the anode chamber.¹⁹⁷ Reproduced with permission.

In the SC-SOFCs, the electrolyte layer acts as the ionic conductor, not a gas separator, so a dense electrolyte layer may not be needed while can still effectively extract energy from chemicals through SOFC functionality. Therefore, in 2013, Guo proposed a all-porous SOFC concept to serve as a bridging technology between dual-chamber SOFCs and SC-SOFCs.^48,360 Interestingly, such a porous SOFC not only effectively solves the deactivation issue caused by carbon deposition when using hydrocarbons in conventional dual chamber SOFCs, but also enables higher fuel cell electrical efficiency and overcomes the inflammability issue over SC-SOFCs. By carefully controlling the distribution of gaseous O₂ through the cathode chamber partial pressure and the flowing rate at the anode side, all porous SOFCs showed a maximum OCV of 0.855 V and the proof of concept cells operated in a methane-containing atmosphere showed very stable power density without carbon deposition for 2000 h.⁴⁸ An additional advantage for such novel SOFCs is the fast-start procedure by applying oxygen-rich conditions at the anode, which may promote the SOFCs for portable or mobile applications. In their later work, an anode supported by all porous SOFCs was further developed to replace the electrolyte one with low electrochemical performance. Even though the OCV values are reduced due to a large oxygen partial pressure in the anode chamber, the peak power density of the former one improved to 214 mW cm⁻² at 750 °C, which is over 14 times than that observed with the latter case. No visible carbon deposition or anode microstructure change was observed at the anode after operation in CH₄ for 220 h.³⁶⁰

Similar to SC-SOFCs and all porous SOFCs, sealant-free direct flame SOFCs (DF-SOFCs) using combustible gases, liquids, and solids were also proposed for combined power and heat co-generation.^361–363 In this cell configuration, the anode side is directly exposed to a fuel-rich flame, while the cathode is open to ambient air. The flame serves as the fuel since the insufficient combustion or utilization of chemicals that are partially reformed by air, and heat suppliers enables thermal sustenance of the DF-SOFCs. The initial DF-SOFCs gave an inferior electrochemical performance while, by the slight modification of the anode by creating serially connected cells, the DF-SOFCs showed an OCV over 0.8 V and a peak power density of 318 mW cm⁻².³⁶¹ Moreover, even with Ni-cermet electrodes, the coke issue is hardly observed in DF-SOFCs since the fuels in the flame are homogeneously converted to syngas before reaching the anode, which shows a clear advantage over conventional dual chamber SOFCs.

The disc structural reactor involves mature fabrication technology, and optimal conduction/mass transfer, but requires dedicated sealing and the active area is limited. Differently, the tubular and hollow SEMRs are less sensitive to sealing issues, especially at elevated temperatures, while they suffer from current collection and low power density problems. Thus, further development of strategies to obtain and deploy ion-conducting materials and active electrodes with new configurations/structure design as well as topological optimization and multiscale modeling would be a great advance for the application of SEMRs. To this end, Li et al.³⁶⁴ used commercially available materials to build a ceramic fuel cell having an evolved ‘micro–monolithic’ geometry with droplet-shaped channels (Fig. 22a). Such a unique structure enabled the deliberate spatial separation of the electronic electrochemically active area from the electronically conducting mechanical support, therefore, remarkably reducing the gaseous fuel's diffusional length (Fig. 22b and c). Such a novel device configuration could use low calorific value gases (LCVGs) like methane with concentrations as low as 10% and create an area-specific atmosphere that suppresses coking in the electronic electrochemically active region. When operated with 10% LCVGs, such a design reactor achieved an impressive power density of 1.77 W cm⁻² at 750 °C (Fig. 22d). Long-term stability up to 500 h was also demonstrated at 700 °C and a constant voltage of 0.7 V when fueled with a CH₄–CO₂ mixture (1 : 1 ratio) fuel and ambient air as the oxidant, respectively.

Fig. 22 Novel SEMR (SOFC) design: (a) the micro–monolithic anode support incorporating 6 sub-channels with droplet shapes, (b) full structure simulation, (c) utilizing LCVGs with reactions at triple-phase boundaries, and (d) power density/voltage–current density characteristics with various LCVGs fed as fuels. With permission (open access) from ref. 364.

5. Process intensified SEMRs

SEMRs operated at high temperatures with redox electrical potential are devices that are integrated thermal and electrical processes, which have already shown high potential over the individual process.^365,366 Besides, when integrated with other possible energies, like solar or more general photo energy, or thermoelectric for a single component reaction or system level energy integration, on one hand, the electrochemical reaction kinetics will be further improved, on the other hand, such SEMRs will be operated by the external energy to realize the energy conversion/storage in chemicals or power, which represents the most interesting options for process identification and modularization.^{180,367–369} With the quick deployment of renewable energy sources, more attention is given to light energy harvesting and conversion since solar energy could be considered as the final source of all kinds of energy with sustainable characteristics. These have stimulated considerable efforts and achieved fundamental progress in recent years.

5.1 Photo-intensified solid oxide cell process

In 2013, Ye reported and theoretically demonstrated that by intentionally replacing a liquid or polymeric solid electrolyte in conventional photo-electrochemical cells (PENs) with a solid ionic conductor, the operating temperature could be well extended from the current 60–80 °C to a much higher range.³⁷⁰ Moreover, the results of this work contrast sharply with the conventional view: solar-to hydrogen efficiency decreases with increased temperature. Instead, it reaches a maximum of 17% at 723 K with a non-precious electro-catalyst-based SOEC integrated single-junction light absorber (band-gap of 2.0 eV). Such a case not only demonstrates the feasibility of solid oxide photo-electrochemistry for the dissociation of water to H₂, but also proves the potential higher efficiency because of its operating isothermal nature. Another interesting phenomenon of the SEMRs with semiconductor-based electrodes (SrTiO₃ with Pt current collector) showed a cell voltage in the 200 mV range at ca. 400 °C upon UV light, which suggests the direct photo energy storage in SEMRs in the form of chemical energy. Moreover, such a cell voltage remains even after switching off the light.^371,372 The time-dependent stoichiometry of oxygen in semiconductor materials (SrTiO₃,³⁷¹ TiO₂,^371,372 CeO₂ (ref. 373) as shown in Fig. 23a) upon UV light is ascribed to the changed cell voltage.^374,375 When a photovoltaic cell (Sr-doped LaCrO₃ thin film on a SrTiO₃ single crystal) is in tandem contact with a solid oxide electrochemical cell (porous LaSrFeO₃/YSZ/porous LaSrFeO₃), a photo-voltage as high as 900 mV at 400 °C was obtained,³⁷⁶ which enables a current of ≈1 mA cm⁻² to pump oxygen from low to high partial pressures, i.e., to convert radiation energy to chemical energy. All these cases show the feasibility of achieving high-temperature photo-electrochemical water splitting.

Fig. 23 Process-intensified SEMRs using different external energy sources: (a) light-induced cell voltage increase in solid oxide photo-electrochemical cells,³⁷³ (b) thermoelectric SOFCs,³⁷⁷ (c) SEMRs intensified by H₂ energy storage in the anode⁵¹ and (d) plasma intensified SEMRs for N₂ activation and NH₃ synthesis.³⁷⁸ Reproduced with permission from individual reference-related publishers.

Light not only induces the bulk oxygen stoichiometry change with time but also leads to the change of the chemical environment of the grain boundary of polycrystalline ceramics, leading to the change of the conductivity of ceramic materials. For grain boundary conductivity, it is considered as the main limitation of the low ionic conductivity of most ceramic oxide ionic conductors. In 2022, Defferriere and colleagues⁵² found that the grain boundary conductivity of a 3GDC thin film increased approximately 3.5 times at 250 °C and the conductance activation energy was reduced from 1.12 to 0.68 eV under illumination. After experimentally excluding the contribution of heat and electronic conductivity, photo-generated electrons that decreased the potential barrier heights associated with space charge zones depleted in charge carriers between adjacent grains was contributed; in other words, the phenomenon of light-induced reduction in the grain boundary space charge potential occurred, reducing the grain boundary resistance.

Recent work reported that the hydroxyl radical yielded from reduced TiO_x powder under high temperature infrared light excitation,⁵³ a typical condition of SOFCs, can help the removal of deposited carbon at the SOFC anode. Moreover, SOFC electrochemical performance was also improved by 7 times with simulated biogas fuel (70% CH₄+ 30% CO₂) at 850 °C. Such work provided a novel method to solve the commonly observed carbon deposition issue in hydrocarbon--fueled SOFCs, promoting wide application.³⁷⁹ To this end, Su and Hu recently reported a combined thermal and photo-promoted activation of C–H and C=O bonds of CO₂/C₂H₆ drying reforming gas in a novel low temperature carbonate super-structured SOFCs with an additional functional layer of Pt/black TiO₂ catalyst at 550 °C.³⁸⁰ Compared with the regular thermal catalytic anode process, the hybrid catalytic anode process induced by light illumination improved both the conversion rate (25%) of CO₂/C₂H₆ and the yield of CO/H₂, which improved SOFC peak power density; a record-high PPD of 168 mW cm⁻² was achieved at 550 °C, 17% higher than that operated in the dark. Furthermore, the cell exhibited excellent stability without coking and delamination during ∼50 h operation. The joint thermo–photo catalysis was demonstrated to be a unique chemical process for SOFCs in the aspects of improved efficiency and operational stability.

5.2 Integrated H-SEMRs by the palladium membrane

As aforementioned, PCECs have exhibited the most promising characteristics to replace the traditional O-SOFCs for energy conversion at reduced temperatures. With the evolution of SOFC fabrication technology and novel electrode material development especially the triple conducting cathode materials,^171,272,276 PCFCs have shown impressive electrochemical performance at temperatures of 500–600 °C, such as 500 mW cm⁻² at 500 °C. Further modification of the interface properties of PCECs improves the electrochemical efficiency.^173,272 A distinguished example of further improvement of the electrochemical performance of PCFCs was the one called “hydrogen membrane fuel cells (HMFCs)”.^381,382 In 2005, the authors smartly deposited a 0.7 μm BaCe_0.8Y_0.2O₃ layer on a Pd film (40 μm in thickness, a exceptional proton transportation and storage metal) by pulsed laser deposition. When integrated with a common perovskite oxide cathode, such a Pd membrane integrated PCECs gave peak power densities of 0.9–1.4 W cm⁻² at the temperature range of 400–600 °C, and the corresponding total area specific polarization resistance was about 0.34 and 0.2 Ω cm², in which electrode polarization resistance occupied about two-thirds of the cell total resistance but still much lower than that of the general case with the same cathode materials. The promoted cathode reaction in such HMFCs was proven by Jeong et al.³⁸³ in 2021. They found that, compared with the general Ni-based porous anodic/electrolyte interface, the blocking effect of the Pd membrane and its metal/oxide interface resulted in an accumulation of O²⁻ at the interface, which acquires extra protons to compensate for the negative charge from the accumulated O²⁻, subsequently resulting in higher proton conductivity at the interface than that in identical PCFC membranes; with the application of overpotential, such an accumulation behavior was further improved, triggering proton pumping to the perovskite oxide cathode side, therefore improving the cathode kinetics and delivering exceptional electrochemical performance at the reduced temperature which is hard to be accessed in normal case.

5.3 Thermoelectric SOFCs

In 2012, Huang proposed and demonstrated a concept of thermoelectric SOFCs. The basic concept is illustrated in Fig. 23b.^377,384 An elongated thermoelectric p-type Na_1−xCu_xCo₂O₄ was used as both a cathode for SOFCs and a thermoelectric generator. Due to the elongated design, the area close to the electrolyte is the hot end (T1), whose temperature is higher than that (T2) of the cold end (Fig. 23b). For such a thermoelectric material, the temperature gradient drives charge carriers from the hot end to the cold end and therefore, appending a positive thermoelectric voltage to the SOFC because they share the same electrical field direction. Therefore, the whole cell voltage consists of two parts: one from the conventional SOFC and other from the thermoelectric generator (Fig. 23b). The total voltage (V_total) of the thermoelectric SOFCs is expressed as V_total = V_cell + ΔV_themo. It is found that the additional voltage (ΔV_themo) was 13.9 mV for NaCo₂O₄ and 11.1 mV for Na_0.7Cu_0.3Co₂O₄ at 800 °C, respectively, while it was only 0.1 mV for the SOFCs with a typical Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ cathode, demonstrating the successful process integration and energy intensification.

5.4 Intensified conversion by energy storage

Another interesting example of SEMR intensification is energy storage. Fuel cells are characterized by continuous energy conversion as long as the fuel is provided. However, their power output quickly decreases to zero if the fuel supply is interrupted. The discontinuous power supply may not be desirable for niche applications such as miniature autonomous systems and military technologies that require operational characteristics for short periods.⁵¹ Auxiliary energy storage devices like lithium-ion batteries may be coupled for such a purpose. However, the total weight and volume of the system will increase. In this context, storing chemical energy in the fuel cell materials would therefore be a great advantage.

Van Overmeere proposed and demonstrated that vanadium oxide would be a good choice.⁵¹ Because of its change in oxidation state, vanadium oxide is a potential candidate for fuel oxidation. Moreover, hydrogen can be first inserted into the lattice of VO₂ and V₂O₅ and the nonstoichiometric V₆O₁₃ and V₃O₇,³⁸⁵ leaving a wide H/V ratio in the range of 0.3–1.9 depending on the oxide and the applied conditions with sufficient hydrogen fuel. When integrated into a silicon wafer supported free-standing miniaturized SOFC with YSZ (80–95 nm in thickness) and Pt cathode, the deposited vanadium oxide (VOx) and Pt/VOx anode material-based SOFCs showed inferior performance over the pure Pt-based electrode (Fig. 23c). Moreover, compared to the potential decrease (to 0 V) in 15 s for the Pt anode cells when fuel was intentionally interrupted, the VOx and VOx/Pt-based cells maintained 32–210 s, depending on the vanadium oxide film thickness and the applied current density (Fig. 23c). In other words, the vanadium oxide anode-based fuel cells supply electricity for a 2–14 times longer period after the hydrogen is turned off. Both the change of the vanadium valence state and the hydrogen storage capability are ascribed to the extended power supply period, which may contribute to the ultra-miniaturization of power sources for mobile energy in the future (Fig. 23c). Similarly, Guan showed that the metal hydride could also serve such a purpose for uninterrupted electricity generation.³⁸⁶ With the metal hydride anode, fuel can be oxidized to produce electricity in the fuel cell mode (continuous fuel supply), while the same device would be operated in battery mode, the stored H₂ in metal hydride decomposes to metal and H, which continuously serves as fuel for a certain time. Taking TiH₂ as an example, the TiH₂–air battery shows a continuous 96 h discharge at 10 mA current density, and a flat discharge potential of 0.85 V over 90 h, which is quite impressive for practical applications. Moreover, stable cycling operation between the fuel cell (H₂ injection) and battery mode (Ar injection into the anode) was demonstrated for 75 h with each mode being operated for 15 min, showing promise for load leveling in the electricity grid.

5.5 Plasma-activated SEMRs for N₂ conversion

Electrochemical NH₃ synthesis is a promising approach for N₂ fixation. However, in Section 3.2.2.2, we saw that both ammonia formation rate and faradaic efficiency are lower than that of economic targeted values, >10⁻⁷ mol cm⁻² s⁻¹ and 50%. The low rate of N₂ triple bond cleavage is a primary challenge. In this regard, Sharma proposed a plasma-activated H-SEMR (Ni–BaCe_0.2Zr_0.7Y_0.1O_3−δ/BaCe_0.2Zr_0.7Y_0.1O_3−δ/Pt) for NH₃ synthesis (Fig. 23d).³⁷⁸ In the SEMR anode, water is electrochemically splited into O₂ and H⁺. The latter is transported to the cathode chamber to react with the plasma-activated nitrogen to form NH₃. A maximum NH₃ production rate of 26.8 nmol s⁻¹ cm⁻² with an exceptional Faraday efficiency of 88% at 500 °C. Moreover, such plasma-assisted SEMRs proved superior to the individual pure plasma and plasma catalysis approaches. The much-improved N₂ activation and the interaction of the N activation intermediate with the catalyst co-contributed to the outstanding N₂ reduction and NH₃ synthesis reaction. By changing the SEMRs from a proton conducting oxide to an oxygen ionic conducting oxide, the same group by the proof of principle also demonstrated that high faradaic efficiencies up to 93% were achieved for NO production at 650 °C, and NO concentration was 1000 times greater than the reaction equilibrium concentration at the same temperature and pressure.¹⁶² These two studies showed the huge scope of SEMRs on integration of plasma energy sources to intensify the targeting reaction.

5.6 Single-step hydrogen generation, separation, and compression

SEMRs simultaneously enable chemical reaction, product separation and compression in one step, therefore holding promise for process intensification. As you can see in the above case, both H₂ flux and water splitting reaction contributed to the super high H₂ production rate at an extremely high temperature. Serra proposed the use of a BaZrO₃-based proton-conducting electrolyte membrane for producing high-purity H₂ from CH₄, NH₃, and biogas steam reforming reactions.^15,274 The designed tubular H-SEMRs with a Ni particle catalyst enabled the production of a nearly pure stream of CO₂ and in situ electrochemically compressed H₂ up to 50 bar, which enables effective carbon capture and 99% hydrogen element utilization from reactants (H₂O and CH₄) (Fig. 24c–e).²⁷⁴ Moreover, the thermal energy was well managed by coupling thermal-chemical processes, i.e., endothermic reforming reactions with heat from electrochemical gas separation and compression by the galvanic operation of the electrochemical cell (Fig. 24a and b). Therefore, a thermally balanced operation regime is also achieved by coupling several thermo-chemical processes with total energy efficiency higher than 78% in a small-scale hydrogen plant according to the simulation result. Besides the H₂O splitting reaction, CH₄, NH₃, and biogas also demonstrated as are sources of H₂ energy. Clark demonstrated similar results using NH₃ and biogas as sources.¹⁵ Moreover, they developed a new nickel-based glass–ceramic composite interconnect to scale up such SEMRs at the single cell level to 36-cell stack level, which allows good balance of the endothermic reaction without producing temperature gradients and entropic effects that lead to the drop of efficiency. Such a counter-flowing stream balanced the heat flow and maintained stable operating conditions, enabling 99% hydrogen recovery and up to 140 bar hydrogen compression, suggesting a high degree of process intensification. Such progress highlights the potential of the practical application of the thermochemical device for efficient hydrogen production, also the realization of carbon neutrality with 100% CO₂ capture and utilization.

Fig. 24 (a) Schematic illustration of H-SEMRs integrated with an electro-chemical H₂ compression process, (b) energy balance and system micro-integration for operation at 800 °C for a feed inlet of one mole CH₄, (c) tabular SEMRs with Ni catalyst loaded inside the ceramic tube (insets are SEM cross-section images of the overall cell and electrode reactions), (d) H₂ production rate versus current density compared with the theoretical value and (d) conversion, yield of CO₂ and CO versus hydrogen recovery at 800 °C, 10 bar, S/C = 2.5. Steam pressure on the hydrogen side inlet was 0–2.0 bar. Reproduced with permission from ref. 274.

6. Electrochemical promotion of catalysis (EPOC) in SEMRs

As can be seen from the above discussion, significant research efforts have been made in the application of SEMRs to different processes and reactions by combining heterogeneous catalysis and solid-state electrochemistry at elevated temperatures. Under the typical conditions, SEMR allows the electrochemical supply of one of the reactive, O²⁻/H⁺, or mixed conductive ions by a faradaic operation, where the reaction rate is in general proportional to the applied current/ionic transportation rate. Nevertheless, the catalytic reaction rate, in many cases, exceeded the rate of the electrochemical supply of ionic species (non-faradaic operation), even by several orders of magnitude, which is the combinative effect of electric field (faradaic process) and catalytic activity (non-faradaic process). In those cases, the electrochemically supplied ionic species could behave as a catalytic promoter to significantly improve the reaction rate and/or product selectivity.^27,40 Such a phenomenon is called the EPOC or NEMCA effect, which was first discovered by Stoukides and Vayenas in 1981.³⁸⁷ They found that the C₂H₄O selectivity and yield of ethylene oxidation on polycrystalline silver can be affected significantly by electrochemical operational modes. Under the open circuit conditions, the Ag catalyst acts as a normal catalyst on the YSZ electrolyte, like a fixed bed reactor. While under the applied external voltage that pumped O²⁻ to the catalyst, C₂H₄O's selectivity and yield increased considerably. In particular, the increase in the rate of C₂H₄O production can exceed the rate of O²⁻-pumping by a factor of 400. The opposite effect was also observed upon inversion of the voltage polarity. Since the discovery of the EPOC effect, more than 100 reaction systems have been demonstrated.^{40,116,388–394} The EPOC effect can be quantitatively expressed by the use of two dimensionless parameters,^40,389Λ = Δr/(I/nF) and ρ = r/r₀, where F is Faraday's constant, ∼96 485 C mol⁻¹, n is the charge of the transported ion, r is the catalytic reaction rate obtained under closed circuit conditions, r₀ is the open-circuit catalytic rate (i.e., at I = 0), Δr = r − r₀ and I/nF is the ionic flux through the electrolyte based on the Faraday law. If Λ = 1, the effect is faradaic, where the reaction rate equals the rate of ion transport rate. In many cases, a strong non-faradaic effect was observed with Λ values ≫1, even as high as 3 × 10⁵, and ρ values as high as 150 (Table 4). The observed change in the catalytic rate is non-faradaic since the products formed surpass considerably the amount of ions supplied to the reaction.

Table 4 Case studies of the EPOC effect on SEMRs for different reactions

Reaction	Reactor	Λ (times)	ρ	References
C2H4 oxidation	Ag/ZrO2(Y2O3)/Ag	—	400	387
H2O + CH4 for H2	Pt-YSZ/YSZ/Au, single chamber	3–4	—	359
CH4 oxidation	Pd–Co3O4/YSZ	80	—	395
RWGS	Pt/YSZ/Pt	10	—	396
CO2 hydrogenation	Ni–Ri on carbon fiber/YSZ/Au	1600	3	397
	Tubular Pt/YSZ/Ru	—	2	116
	Ru–Co3O4/BZY/Au	10	2	398
	Pt, Pd, Ag/YSZ/Pt, Pd, Ag	6000	100–3 × 10^5	393
	Pt/YSZ/Au	74 000	26	388
	Ru–Co/BZY/Au	60	—	391
	Ru/YSZ/Au	5	1.4	392
	Ru/BZY/Au	500	—	390
Ammonia synthesis	Fe/CaIn0.1Zr0.9O3−α/Ag	13	6	399
C2H4 + NOx to N2	Pt/β′′-alumina/Au	—	15	400
NO + CO/C3H6	Pt/β′′-alumina/Au	—	—	401
NO + CO	Pd/YSZ/Au	700	1.5	402
NO reduction	Ni-YSZ/YSZ/LSMC-GDC/Pd-GDC	—	100–1000	165
NOx reduction	La0.8Sr0.2Co0.9Ru0.1O3−δ/YSZ/Pt	30	—	403
Ethylene oxidation	Pt	10^5	74 000	404

---

EPOC effect is universally observed in both single-chamber and two-chamber configurations, which are well suitable for SEMR operation, either in fuel cell mode or electrolysis mode. For example, Caravaca et al.³⁵⁹ experimentally found that the H₂ formation rate in single-chamber steam electrolysis cells (Pt-YSZ/YSZ/Au) varied significantly under methane partial oxidation, steam electrolysis, and methane auto-thermal reforming conditions, respectively. The highest H₂ production rates were achieved under the auto-thermal reforming conditions and with an application of negative currents over the Pt catalyst-working electrode at 600 °C. It was demonstrated that the application of the negative currents modified the work function of the Pt catalyst and altered its catalytic properties, which allowed an additional H₂ production based on the intrinsic EPOC effect, therefore, overcoming the limitation by Faraday's law and reducing the cost of Zagoraios et al.³⁹⁵ reported the electrochemical promotion of methane oxidation effect on monodispersed Pd/Co₃O₄ catalysts with YSZ solid-state electrolytes. It was found that the catalytic rate increased by 2.5-fold and faradaic efficiency values increased by 8000%. In addition, they confirmed that the EPOC effects enhance the reaction rate at temperatures lower than those observed in previous studies. Another interesting case is its application in reverse water gas shift reaction (RWGS) based on SEMRs with proton conductors.³⁹⁶ Under such H₂/CO₂ conditions, hydrogen was electrochemically supplied as H⁺ other than as molecular H₂. Under closed circuit operation, Λ value up to 10 for CO formation was achieved, which should be ascribed to direct electrocatalysis together with the enhancement of the open circuit reaction rate. More attention to the EPOC effect is given to CO₂ hydrogenation with SEMR both on oxygen-ion conducting electrolytes^116,397 and on proton-conducting electrolytes.^{116,390–392,398} In the thermochemical catalytic process, the rate-determining process is generally ascribed to either the dissociation of the C–O bond or the hydrogenation of the intermediates, while in the O-SEMRs, the CO₂ electrolysis was suggested to go through a CO₃²⁻ intermediate, which is promoted by the applied anodic bias.^81,82 For example, Chatzilias intentionally adopted a low temperature (340–400 °C) O-SOFC to the electrochemical promoting reactor for CO₂ hydrogenation (Fig. 25a).¹¹⁶ At the first glance, O²⁻ conducting from the cathode moving to H₂/CO₂ may be detrimental to hydrogenation. Even with the EPOC effect, the low temperature operation can only result in quite low fuel cell performance. Indeed, the peak power density could only reach 0.45 mW cm⁻² at 400 °C (Fig. 25b), while rate enhancement ratios of up to 3 (at 440 °C) and faradaic efficiency values of up to 1600 were found at lower temperatures (<360 °C) with H₂ and CO₂ partial pressure of 7 kPa and 1 kPa, and the selectivity of CH₄ reduced with the increased temperature (Fig. 25c). In contrast, the faradaic efficiency values observed for CO formation were between −20 and −10, with the corresponding rate enhancement ratios being close to 0.5 (Fig. 25c). If sufficient power (as a fuel cell) could be produced for the intensification of the EPOC effect, it can be applied for the valorization of CO₂ to value products.

Fig. 25 (a) Schematic representation of the tubular SOFC reactor with reactions at each electrode under OCV and short-circuit conditions, (b) power generation characteristics, and (c) temperature dependence under steady-state conditions on the Λ and ρ. P_H2 = 7 kPa, P_CO2 = 1 kPa, FT = 200 cm³ min⁻¹. Reproduced with permission from ref. 116.

In PCECs, a CO₃²⁻ intermediate was observed in the fuel electrode,³¹⁸via in situ Raman and diffuse reflectance infrared Fourier transform spectroscopy characterization. To further reduce CO₂, the continuously replenished protons play a critical role in CO₂ conversion and CH₄/CO selectivity.^390,398 Kalaitzidou prepared a Ru-deposited BZY proton conductor as the model catalyst for the hydrogenation of CO₂ at temperatures of 300–450 °C and atmospheric pressure.³⁹⁰ It was observed that the rate and the selectivity to CH₄ are enormously enhanced by the removal of a proton from the catalyst surface to the proton-conducting support. The EPOC effect parameter Λ for CH₄ reached 500 and depended strongly on the thickness of the porous Ru catalyst film. Such an enormous improvement in the CH₄ yield may suggest the practical application of such a reaction of both environmental and industrial importance. Moreover, it has been found that the catalytic activity of ammonia synthesis and NO_x reduction can be promoted by the EPOC effect.³⁹⁹ The catalytic activity of industrially commercialized ammonia synthesis catalysts (BASF S6-10RED) was enhanced by up to 1300%, or higher, by depositing on a CaIn_0.1Zr_0.9O_3−α proton conductor with electrochemical supply of protons to the catalyst surface at 400 °C and 50 atmospheric pressures according to Yiokai's report.³⁹⁹ Furthermore, the NH₃ production rate is 6 times larger than the proton pumping rate, which indicates the EPOC effect. This work also showed the first demonstration of the scale-up of an SEMR with 24 electrically connected catalyst pellets, which highlights the important role of the EPOC effect in promoting the wide application of SEMRs for electrochemical NH₃ synthesis. Besides, the literature reveals EPOC effects in the reduction of nitric oxide (NO_x) to N₂ under lean conditions¹⁶⁸ with or without reducing agents using β′′-alumina^400–402 or zirconia^{165,402,403,405} based SEMRs. The selectivity and electrochemical promotion reduction of NO_x significantly reduce the system complexity without compromising the conversion efficiency, suggesting the wide application in SEMR technology.

As can be seen from the above discussion, EPOC could have a significant impact on catalytic research in energy and environmental fields with multiple benefits. (1) Unlike gas phase or solid-state promotors, the modification in EPOC is electrochemical, which is much more easily controlled, in particular, when one of the reactants in the reactor could act both as a power generator for the EPOC effect and the reactant for the catalytic reaction, resulting in much-reduced system complexity. Moreover, the energy consumption for the EPOC effect is much lower than the stoichiometry required.^39,116 From a practical viewpoint, SEMRs consume very little electrical energy to operate and product generation is not limited by Faraday's law. (2) With the combined faradaic and EPOC effects, the overall efficiency of SEMRs could be significantly increased, which could facilitate the temperature reduction to the range of interest.³⁵⁹ Taking the methanation reaction as a typical example, the current H₂O/CO₂ electrolysis studies are performed at a temperature above 600 °C to ensure sufficient kinetics, while the methanation reaction is preferred at 300–500 °C. If the EPOC effect is coupled, the possibility of dynamic modification of the catalyst activity could favor such a temperature reduction, which could lead to a on-site electrolysis and methanation process. In addition, sharing the same benefits of the current temperature reduction in SOFCs, the stability, lifespan as well as system cost will be guaranteed because of the absence of thermal stresses. Under the reduced temperature, the economic potential of cogeneration to a large number of reactions could be expected, promoting the practical application of those cogeneration processes. (3) EPOC allows for the controlled enhancement of the catalytic activity/selectivity by potential or small current application between a catalyst (working electrode) supported on a solid electrolyte and a counter electrode.^57,308,390 A typical example with a very high NEMCA effect is the production of CO₂ from ethylene and O₂ on the Pt catalyst using the YSZ electrolyte and temperatures in the range of 260–450 °C where a potentiostat–galvanostatic operation was employed.²⁷ Kalaitzidou et al.³⁹⁰ also observed threefold enhancement in the catalytic rate of CH₄ formation with concomitant 50% suppression of the CO formation rate when protons were removed from the catalysts to the electrolyte supporter, which suggests that the EPOC effect parallelly regulated the methanation reaction and CO yield reaction (water–gas shift reaction) are parallel route (Fig. 26b). Such ionic-mediated electrochemical processes could potentially be harnessed to promote the activity of other reactions, such as the electrochemical ammonia synthesis where hydrogen evolution reaction occurs in parallel.

Fig. 26 (A) Schematic of the basic principle of EPOC when ions (oxide ions or cations) are moved to the catalyst of the working electrode⁴⁰⁸ and (B) CH₄ and CO production rates when CO₂ hydrogenation is carried out over a range of negative and positive polarization.³⁹⁰ Reproduced with permission from ref. 390 and 408.

Different mechanisms have been proposed to explain the EPOC effect in SEMRs: (i) With the applied current or direct potential, ion migration from the solid electrolyte (spillover) to the catalyst surface or removal from the catalyst surface changes the work function or electronic structure of the electrode materials, which leads to the dramatic change of catalytic properties in a reversible manner.^{359,398,404,406} In the context of SOECs, it is believed that catalytic activity is enhanced due to the promotion of the work function of catalytic surfaces generated by oxygen ion pumping to/from the electrolyte onto the catalyst surface.⁴⁰⁷ Unlike structural promoters that improve the dispersion and the chemical stability of the active catalyst phase, electronic promoters directly modify the catalytic activity of the catalyst itself (Fig. 26a). By controlling the surface work function, one can control the adsorption, and surface diffusion properties for intermediates and partially discharged promoters. Such an EPOC effect has been well demonstrated in the initial discovery of oxygen ions on an Ag catalyst for the ethylene oxidation reaction.^117,387

(ii) Reactant activation includes improved reactant chemical adsorption and/or charge transfer process.^401,405 Under the specific electrical field conditions, the ion migrated to the triple phase boundary, which formed an effective electrochemical double layer,³⁹⁴ altering the chemisorptive properties of the catalyst. The reactant may be polarized and induced with the improved chemical adsorption and then is activated subsequently resulting in improved charge transfer between the electrode and reactant.

(iii) The ionic species from the fuel/oxidant for power generation also serves as a sacrificial promoter. In other words, these ionic species/promoters can be destroyed as easily as they are generated, just by varying the polarization of the catalytic surface.³⁹⁵ Hence, the EPOC phenomenon allows in situ control and optimizes the number of promoter species supplied to the catalyst/electrode, which has very important implications for practical purposes over other controlled technologies.

(iv) Intensified metal–support interaction.³⁹⁵ By carefully comparing the additional effect of Co₃O₄ to Pd supported on YSZ (over Pd/YSZ film), a 2.5-fold increase of the catalytic rate and a faradaic efficiency of 80 was obtained. Therefore, metal–support interactions significantly improve the EPOC effect, in which Co₃O₄ behaved as a “bridge” to allow for the O^δ− promoter ions to reach the metal nanoparticles, showing a much improved EPOC effect.

Finally, the maturity of the EPOC effect which has resulted from more than 40 years of intensive and extensive research because of the multiple benefits should offer the opportunity for many attractive applications. However, application of the EPOC effect in SEMRs to produce targeted chemicals requires further R&D activities. Future work may focus on a clear perception of the governing reactions, and reaction kinetics thereof, choice of electrode–electrolyte–catalyst combination, and finally, optimization of the operating conditions.

7. Summary and perspectives

Solid electrolyte membrane reactors (SEMRs) provide a promising alternative to conventional thermal catalysis because of the well-controlled reaction rate, avoidance of chemical balance, catalytic selectivity as well as possible EPOC effect. This review summarizes the applications of high temperature solid membranes for energy (among thermal, electrical, and chemical) conversion and storage, chemical generation, and environmental protection, with a focus on (reversible) conversion of small chemical molecules such as water, H₂, CO, CO₂, CH₄, NH₃, and NO_x through (de)oxygenates, (de)hydrogenation in form of oxidative or non-oxidative processes. Currently, the application of oxygen ion conductors for SEMRs is still the most important field of research and applications, even though proton conducting membranes offer new possibilities for the implementation of electrochemical reactors. In both cases, a combined effect such as cogeneration of electricity and chemicals, detoxification, and electrical energy generation, “achieving two things through a single action”, could be well designed and deployed. Detailed case studies of the application of versatile ionic conductors, like O²⁻, H⁺, CO₃²⁻, Na⁺ and their hybrid conductors, their multi-functional application in SEMRs as well as the corresponding key assembly units (e.g., electrode materials and microstructure membranes and catalysts), and electrochemical performances like conversion, selectivity, and yields were presented. High temperature SEMRs showed remarkable applicability for different targeted reactions because of the coupled thermal and electrochemical process, as well as diverse cell configurations, such as two chamber-based (panel and tubular), single chamber-based, porous electrolyte-based, non-symmetric and symmetric SEMRs. Among these, electrolysis and co-electrolysis of H₂O in the form of steam in SEMRs and CO₂ are considered a cornerstone for the future chemical industry and sustainable energy systems, enabling efficient conversion and long-term storage of electrical energy and production of sustainable fuels and chemicals from renewable sources. Electrochemical selective oxidation or coupling of an alkane, their conversion to an alkene or arene/aromatics, and NH₃ synthesis in both O- and H-SEMRs are of interest for both fundamental research regarding elementary catalytic steps and practical applications for improved conversion efficiency and large-scale deployment. In addition, concepts/strategies of process intensification/integration of SEMRs with other effective and renewable energy forms such as photo, thermoelectric, and plasma energy also showed improvement in energy efficiency, reaction selectivity and accelerated targeting chemical activation and conversion. Another benefit in SEMRs is the non-faradaic process/effect on the existing reactions, i.e., EPOC effect, which dramatically alters the conversion rate and selectivity of expected reactions over the typical faradaic reaction, which has been widely used in both studies and influence the rates of catalytic reactions. With the EPOC effect, it is expected to extend the measurements to lower temperatures of general interest and help elucidate the reaction mechanism that could not be obtained otherwise. In other words, SEMRs provide a vast opportunity to address important challenges beyond other technologies' capabilities and have therefore become an emergent technology/device to promote electrification of the current chemical/catalytic industry.

However, the progress of SEMRs has not been sufficient to promote these processes on a larger scale, i.e., very limited cases with stack-level or large-scale cell applications.^15,117,399 It should be noted that the massive commercialization of SEMRs relies on their capital and operating costs, lifespan, and reliability, while the operating cost is partly dependent on the system's output performance and operational stability. For example, the electrochemical output of a chemical and power co-generation reactor is much lower than that of the state-of-the-art SOFC technology, which is also integral to other functions of SEMRs. The conversion rate, selectivity and yield of CO₂ reduction to hydrocarbons is still not well controlled and understood.³⁰⁸ Increasing reactor durability and decreasing capital costs are the main challenges to achieving market penetration. These essential technologies are currently at very low technology readiness levels (TRLs), among which the high-temperature CO₂ electrolyzers using solid oxide electrolytes is approaching a mature level (TRL 8) and have been demonstrated with over 1000 h of stability.⁴⁰⁹ The durability and capital cost reduction of SEMRs highly rely on developing reliable reactor materials and robust devices. Leaning to experience from SOFCs, for example, we can build electrolyte-supported instead of anode-supported cell configuration based on the state-of-the-art cell material which remarkably improves the cell/reactor durability. With more industrial exploration or demonstration, combining the intrinsic high energy efficiency, and high reaction kinetics, it is expected that the capital cost will be much reduced, making them compete with other commercial processes and promoting the SEMR process on a larger scale.

There are also a variety of fundamental issues for the performance and degradation of SEMRs, including SEMR itself (key components' chemical, thermal, and mechanical, properties and their associated compatibility), the interaction between the reactant and impurity, and lifespan requirement of at least 10–20 years.⁶² Finding the degradation factors and developing means for reviving degraded performance would offer great value in improving SEMRs' commercial viability. Progress in materials science and solid-state ionics could be used for SEMRs with multiple purposes with minor modifications. There is also a significant gap between laboratory-scale research and industrial processes. In general, the industry has been reluctant to adopt the SEMR process because of limited ionic fluxes.¹¹⁷ In addition, the SEMRs should be scaled up into larger flat cells or stacks and, even pilot scale, to validate stable and high-efficiency operation. Moreover, the operation under application-sound conditions is necessary, for instance, by testing at higher total reactor pressure or pulsed current/voltage conditions which is matched with the nature of intermittent renewable energy, as well as grid-scale application. Another challenge is that there is a lack of techno-economic studies and bottom-up cost analysis of SEMR systems to replace the current chemical industrial plants. The techno-economic aspects should be carefully evaluated to meet further industrial needs.

Regarding the EPOC or NEMCA effect in SEMRs, many classic reactions including CO₂ hydrogenation and hydrocarbon fuel oxidation have been demonstrated with an impressively improved effect. However, no consensus mechanism for EPOC has been reached. Detailed primitive reaction step investigation with advanced electrochemical and photo-spectroscopy characterization techniques is preferred. The good news is that the increasing number of publications/literature dealing with practical aspects of NEMCA application address questions related to pilot plant scale tests, and reactor designs but are also relevant to catalyst and electrolyte manufacturing technology.⁴¹⁰

Finally, SEMRs generally provide one or more reactants for continuous chemical synthesis, therefore, a combination of SEMRs with catalysts of targeted catalytic reactions in appropriate electrode–electrolyte–catalyst structure design/geometry,^15,47 and a clear perception of the governing reactions and reaction kinetics is required. High energy efficiency,¹⁵ reaction conversion rate/yield, and system stability²⁰ can be expected through optimization of the working conditions. Learning from commercial chloralkali processes, electrolysis of alumina, and organic chemical synthesis like acrylonitrile dimerization and fluorination of ethers will be important for the scale-up of electrochemically driven energy conversion and storage, chemical synthesis and environmental treatment in SEMRs.

Data availability

No primary research results, software, or code have been included and no new data were generated or analyzed as part of this review.

Author contributions

L. F.: Conceptualization, Writing – original draft, Resources, Funding acquisition, Writing – review & editing, Supervision; W. L., Q. F., Q. H., and Y. F.: Writing – review & editing; T.-W. C. and P. D. L.: Resources, Writing – review & editing, Supervision; all authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Abbreviations

6Sc1CeSZ	Sc0.06Ce0.01Zr0.93O2
ASR	Area-specific resistance
BSCF	(Ba,Sr)(Co,Fe)O3−δ
BZCFY	BaCo0.4Fe0.4Zr0.1Y0.1O3−δ
BZCY	BaZr0.7Ce0.2Y0.1O3−d
BZCYYb	BaZr0.1Ce0.7Y0.1Yb0.1O3−δ
BZY	BaZr0.8Y0.2O3−δ
C2	Hydrocarbon with two carbon atoms
DCFC	Direct carbon fuel cell
De-NOx	Removal of nitric oxide
DF-SOFCs	Direct flame solid oxide fuel cells
DRIFTS	In situ diffuse reflectance infrared spectroscopy
EPOC	Electrochemical promotion of catalysis
FAE	Fuel-assisted electrolysis
F–T	Fischer–Tropsch process
GDC or CGO	Gd doped ceria, normally Gd0.1Ce0.9O2−δ
H-SEMRs	Hydrogen ionic conductive SEMRs
j 0	Exchange current density
LSCF	(La,Sr)(Co,Fe)O3−δ
LSGM	(La,Sr)(Ga,Mg2)O3−δ
LSM	(La,Sr)MnO3−δ
LSMC	La0.8Sr0.2Mn0.9Cu0.1O3
NASA	National aeronautics and space administration
NEMCA	Non-faradaic electrochemical modification of catalytic activity
NOx	NO or NO2
OCM	Oxidative coupling of methane
OCV	Open circuit voltage
O-SEMRs	Oxygen ionic conductive SEMRs
O-SOEC	Oxygen ionic conductive SOEC
O-SOFCs	Oxygen ionic conductive SOFCs
PBM	PrBaMn2O5+δ
PCEC	Proton ceramic electrolysis cell
PCFCs	Proton ceramic fuel cell
PEM	Proton exchange membrane
PENs	Photo-electrochemical cells
RSOCs	Reversible solid oxide cells
RWSR	Reverse water shift reaction
SC-SOFCs	Single-chamber solid oxide fuel cells
ScSZ	Scandia stabilized zirconia
SDC	Sm doped ceria, normally Sm0.2Ce0.8O2−δ
SEMRs	Solid electrolyte membrane reactors
SFM	Sr2Fe1.5Mo0.5O6−δ
SOCs	Solid oxide cells
SOECs	Solid oxide electrolysis cells
SOFCs	Solid oxide fuel cells
STEM	Scanning transmission electron microscope
TPO-CO2	Temperature-programmed oxidation using CO2
TRL	Technology readiness levels
XPS	X-ray photoelectron spectroscopy
YSZ	Yttrium-stabilized zirconia

Acknowledgements

L. Fan is thankful for the financial support from the National Natural Science Foundation of China (22378268), the Guangdong Basic and Applied Basic Research Foundation (2024A1515012212), and the Shenzhen University-National Taipei University of Technology Joint Research Program (2023011).

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