Electricity-to-ammonia interconversion in protonic ceramic cells: advances, challenges and perspectives

Abstract

NH₃ is an attractive alternative fuel to hydrogen and methane, offering advantages such as easy compression at room temperature, straightforward storage and transportation, high volumetric energy density, and carbon-free nature. However, conventional NH₃ synthesis requires high temperatures and pressures, resulting in substantial energy consumption and increased equipment and maintenance costs. Protonic ceramic cells (PCCs), as a cutting-edge energy conversion technology, can realize NH₃ synthesis at moderate pressures and low-to-intermediate temperatures by utilizing surplus renewable electricity generated by wind and solar power. Additionally, PCCs can be employed to convert NH₃ into electricity to meet instantaneous demand, providing a means to address the seasonal and intermittent nature of renewable energy sources. Despite their potential, the commercial application of electricity-to-NH₃ interconversion in PCCs faces several challenges, primarily related to insufficient performance and durability. This review systematically explores the mechanisms and challenges of electricity-to-NH₃ interconversion in PCCs, highlights recent advancements in NH₃ synthesis using PCCs and direct NH₃-fueled proton ceramic fuel cells (DA-PCFCs), and discusses perspectives for realizing high-efficiency electricity-to-NH₃ interconversion. This review aims to establish a scientific foundation for efficient electricity-to-NH₃ interconversion via PCCs and provides critical insights for designing high-performance and durable PCC components.

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Mingzhuang Liang

Mingzhuang Liang obtained his PhD in Chemical Engineering from Nanjing Tech University, China, under the supervision of Prof. Zongping Shao. He is currently a Postdoctoral Fellow at Seoul National University. His research interests are mainly focused on the development of key materials for applications in solid oxide cell.

Jinwook Kim

Jinwook Kim is currently a Postdoctoral Fellow at the School of Materials Science and Engineering, at Northwestern University, USA. He received his PhD in Materials Science and Engineering from Korea Advanced Institute of Science and Technology, Korea, in 2023. His research interest focuses on the fabrication and characterization of novel solid oxide fuel cell electrodes for efficient energy storage and conversion.

Tae Ho Shin

Tae Ho Shin is a Director and Principal researcher for Division of Carbon Neutrality & Materials Digitalization at Korea Institute of Ceramic Engineering and Technology (KICET). He received his PhD at Kyushu University, Japan. He worked as Research Fellow in the School of Chemistry at the University of St Andrews, and he then joined KICET, Korea (2015). His research involves developing new oxide electrodes for electrochemical devices such as solid oxide fuel cells and electrolysis cells. An area of particular interest is crystal structure analysis for understanding and predicting the design of materials with targeted electrochemical properties.

Zongping Shao

Zongping Shao is a John Curtin Distinguished Professor at Curtin University, Australia. He obtained his PhD from Dalian Institute of Chemical Physics, China, in 2000. He worked as a visiting scholar at the Institute Researches Sur La Catalyse, CNRS, France, and then a postdoctoral fellow at California Institute of Technology, USA, from 2000 to 2005. His research interests include mixed conducting membranes, solid oxide electrochemical cells, electrocatalysis, advanced energy storage devices including lithium/sodium-(ion) batteries, metal–air batteries, supercapacitors, and solar cells. He has been recognized as a Highly Cited Researcher by Clarivate Analytics since 2017.

WooChul Jung

WooChul Jung is an Associate Professor at Seoul National University, Korea. He received his PhD degree at MIT and served as a postdoctoral fellow at Caltech. The main goal of his research activities is to understand the reactions that occur at the interfaces between ionic solids (oxides in particular) and gases and thereby to improve the reaction kinetics for applications in chemical and electrochemical catalysis, such as, solid oxide fuel cells, electrolyzers, and hydrocarbon reformers. He has been developing model oxide structures with well-defined interface geometries and analyzing true surface properties and reaction characteristics.

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Broader context

The conventional Haber–Bosch process for NH₃ synthesis necessitates energy-intensive high-temperature and high-pressure conditions, leading to substantial energy consumption and significant operational costs. Proton ceramic cells (PCCs) offer a compelling solution by enabling the electrochemical synthesis of NH₃ from renewable electricity at intermediate temperatures and pressures. Furthermore, PCCs can efficiently convert the synthesized NH₃ back into electricity to meet instant demand, addressing the intermittency inherent in renewable energy sources. While PCCs exhibit immense potential, achieving efficient bidirectional electrochemical interconversion between electricity and NH₃ remains a critical challenge for their practical application. This review comprehensively examines the mechanisms and advantages of this electrochemical interconversion in PCCs, with a particular focus on recent advancements and critical challenges in NH₃ synthesis utilizing PCCs and direct NH₃-fueled proton ceramic fuel cells (DA-PCFCs). The review concludes with a discussion of the future prospects and critical research directions towards realizing efficient electrochemical interconversion between electricity and NH₃ using PCC technology. This review aims to establish a scientific foundation for efficient electricity-to-NH₃ interconversion via PCCs and provides critical insights for designing high-performance and durable PCC components.

1. Introduction

The rapid expansion of the global economy has intensified the use of traditional fossil fuels, leading to significant environmental pollution and heightened greenhouse gas emissions. To ensure environmental protection and sustainable development, there is an urgent need to identify clean energy alternatives to fossil fuels and to advance energy conversion technologies. Recent decades have seen extensive exploration of renewable energy sources, including wind, solar, and tidal energy. However, the intermittent nature of these sources poses challenges in meeting immediate energy demands.^1–3 Hydrogen energy, an efficient and clean secondary energy source, shows promise as a key component of future energy systems due to its sole combustion byproduct being water.^4–7 Despite its advantages, H₂ presents challenges for storage and transportation due to its small molecular size, propensity to leak, and the difficulty of liquefaction under normal temperatures and pressures, all of which contribute to increased costs.^8–10 Consequently, it is crucial to develop effective H₂ storage and transport methods. Among the various options, NH₃ has emerged as a promising H₂ carrier, offering a potential solution to these challenges.^11,12

NH₃ is a nitrogenous compound comprising one nitrogen atom and three hydrogen atoms. It can be readily liquefied under ambient conditions, offering significant economic and safety benefits for storage and transportation. With an exceptional volumetric energy density of 12.9 MJ L⁻¹, NH₃ significantly outperforms liquid H₂ (8.6 MJ L⁻¹), making it an attractive option for energy storage.^13,14 Moreover, global NH₃ production exceeds 170 million tons annually, supported by a well-established, cost-effective production and supply chain. The mature technology for NH₃ synthesis, combined with its ideal combustion products being primarily N₂ and water without CO₂ emissions, underscores its potential to mitigate global climate change. These characteristics position NH₃ as a promising clean fuel and H₂ carrier. The Haber–Bosch process, the current dominant method for NH₃ synthesis, requires high temperatures (400–500 °C) and pressures (150–350 atm) to drive the reaction between N₂ and H₂ over an iron-based catalyst.^15,16 However, this process has significant drawbacks, including substantial energy consumption (1–2% of global energy consumption from steam-methane reforming), considerable CO₂ emissions (∼1.8% of global CO₂ emissions), and the high equipment and maintenance costs associated with such extreme operating conditions. Moreover, the H₂ required for this process is typically derived from methane, further leading to substantial carbon emissions.^17,18 Consequently, alternative NH₃ synthesis methods, including electrochemical approaches, plasma techniques, and solid nitride cycling, have attracted considerable interest.^18–20 Among these, electrochemical approaches, with their potential for seamless integration with renewable energy systems, are the most promising technologies for sustainable NH₃ production. Based on the operating temperature, electrochemical NH₃ synthesis devices can be categorized into low-temperature (<100 °C), intermediate-temperature (200–600 °C), and high-temperature (>700 °C) systems.²¹ Compared to other low- and intermediate-temperature devices such as alkaline water electrolyzers and proton exchange membrane electrolyzers, solid oxide electrolysis cells (SOECs) typically require fewer noble metal catalysts, exhibit faster reaction rates, and achieve higher energy efficiency.^22–24

Solid oxide cells (SOCs), encompassing both SOECs and solid oxide fuel cells (SOFCs), are an emerging technology for energy conversion and storage.^25–27 These cells offer the versatility to switch between electrolysis cell and fuel cell modes, facilitating the efficient interconversion between electrical and chemical energy.^28–30 This reversible-mode capability positions SOCs as a promising technology for enhancing the flexibility, stability, and sustainability of energy systems, marking them a key focus for future energy research. SOCs are categorized based on the type of charge carrier in their electrolytes: oxygen ion conducting SOCs (O-SOCs) and protonic ceramic cells (PCCs).^31,32 PCCs typically operate at intermediate to low temperatures (<600 °C) due to the relatively low activation energy required for proton transport compared to oxygen ion transport.³³ PCCs can function in two modes: protonic ceramic electrolysis cells (PCECs) and protonic ceramic fuel cells (PCFCs). In PCEC mode, excess renewable electricity can be utilized for NH₃ synthesis and storage.^34,35 Conversely, PCFCs can convert stored NH₃ back into electricity to meet immediate energy demands.^36,37 Despite the potential of PCCs, challenges remain in the electricity-to-NH₃ interconversion, particularly related to catalyst stability and efficiency, material and manufacturing costs, and the optimization of reaction conditions.^38–41 Overcoming these challenges is crucial for advancing the practical application of PCCs in future energy systems.

Numerous studies have previously been published, focusing on the development of electrolytes, electrode catalysts, and operating conditions for NH₃ synthesis in PCCs and direct NH₃-fueled proton ceramic fuel cells (DA-PCFCs). This review begins by outlining the fundamental mechanisms involved in electricity-to-NH₃ interconversion in PCCs, followed by a detailed analysis of the challenges associated with achieving such conversions. Subsequently, the advancements and achievements in electricity-to-NH₃ interconversion using PCCs over the past few decades are summarized. Finally, the review concludes with a forward-looking perspective on the future applications of PCCs in electricity-to-NH₃ interconversion, highlighting the critical role they could play in shaping next-generation sustainable energy systems. By providing an in-depth examination of the state of the art, the objective of this review is to serve as a comprehensive guide for researchers engaged in the advancement of proton-conducting ceramic technologies, addressing both the current obstacles and potential breakthroughs that could drive innovation in clean energy conversion, which is critical for accelerating the transition to a sustainable and carbon-neutral global energy landscape.

2. Mechanisms of the electricity-to-NH₃ interconversion in PCCs

2.1. NH₃ synthesis in PCECs

PCECs have been employed for NH₃ synthesis through various electrochemical processes, with the goal of achieving reactors that offer high energy efficiency and minimal greenhouse gas emissions.^38,42,43 Currently, three main types of PCEC reactors are utilized for NH₃ synthesis, each based on different reactions: the hydrogen oxidation reaction (HOR), methane-steam reforming reaction (MSRR), and water oxidation reaction (WOR), as illustrated in Fig. 1. In the following sections, we will examine the operating mechanisms of these reactors and discuss the challenges in NH₃ synthesis by PCECs.

Fig. 1 The PCC reactors for NH₃ synthesis via (a) HOR, (b) MSRR and (c) WOR. (d)–(f) The potential mechanisms of the NRR.

In Reactor 1, H₂ is supplied to the anode, while N₂ is introduced at the cathode (Fig. 1a). Under an applied voltage, H₂ molecules dissociate into protons and electrons (H₂ → 2H⁺ + 2e⁻). The generated protons then migrate across the electrolyte to the cathode and combine with adsorbed nitrogen to produce NH₃ (6H⁺ + N₂ + 6e⁻ → 2NH₃).^44,45 This approach offers significant advantages over the traditional high-pressure NH₃ synthesis methods, as it operates under moderate conditions and directly converts electrical energy into chemical energy. However, a primary challenge lies in the need for a reliable H₂ source. Consequently, Reactor 1 must be coupled with H₂ production systems, such as methane reforming or water electrolysis, which significantly increases the overall system cost.

Reactor 2, a more integrated approach, utilizes renewable electricity to reform methane and water steam into H₂ and CO₂ on the anode side (Fig. 1b).^46,47 The protons from H₂ dissociation then migrate across the electrolyte to the cathode, combining with adsorbed nitrogen to form NH₃. Unlike Reactor 1, Reactor 2 eliminates the need for additional H₂ production equipment, greatly simplifying the overall system. By combining the endothermic reforming reaction with the exothermic nitrogen reduction, Reactor 2 achieves a thermoneutral state, enhancing energy efficiency. Additionally, the generated CO₂ can be further reformed with methane to produce CO, reducing carbon emissions. However, the anode's exposure to methane, steam, and CO₂ necessitates a stable and highly active electrode material to ensure optimal performance and longevity. Fortunately, numerous methane steam reforming anode catalysts developed for SOFCs can be adapted for use in Reactor 2.^48–50

Recent advancements in PCECs have enabled efficient H₂ production through water electrolysis. Therefore, researchers have increasingly explored PCECs for a more environmentally friendly approach to NH₃ synthesis (Fig. 1c).^51,52 In Reactor 3, the WOR at the anode is driven by renewable electricity, leading to the generation of protons and oxygen. These protons migrate through the electrolyte to the cathode, where they react with absorbed nitrogen to form NH₃.^52,53 This process, utilizing only water and N₂, offers a carbon-free pathway for NH₃ production. To ensure optimal performance and longevity in the high-steam environment required for Reactor 3, the development of advanced anode materials is crucial. Fortunately, some anode materials already used in PCEC water electrolysis for H₂ production, such as PrBa_0.5Ca_0.5Co₂O_5+δ (PBCC), Ce_0.2Ba_0.2Sr_0.2La_0.2Ca_0.2CoO_3−δ (CBSLCC), and BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY), are also well-suited for this NH₃ production process.^54–56

In all the three pioneering NH₃ synthesis reactors, despite varying anode reactions, the cathode, where the nitrogen reduction reaction (NRR) takes place, shares a striking resemblance. Currently, there are three main NRR mechanisms, as illustrated in Fig. 1d–f. In Fig. 1d, the NRR within the PCEC reactor proceeds via a dissociative mechanism. Initially, N₂ molecules are adsorbed onto the catalyst surface, undergoing subsequent activation. Subsequently, hydrogen protons from the anode and electrons combine with the activated nitrogen in a hydrogenation process to form NH₃. Finally, the NH₃ molecules desorb from the catalyst surface.^57,58 Owing to the substantial energy barrier for N₂ dissociation on the catalyst surface, this step is generally regarded as the rate-determining step (RDS) in NH₃ synthesis. Consequently, promoting N₂ dissociation can effectively enhance NH₃ production rate.^59,60Fig. 1e and f depict the NRR associative mechanism, further categorized into alternating and distal hydrogenation pathways based on distinct hydrogenation sequences.⁶¹ In Fig. 1e, a N₂ molecule initially adsorbs onto the catalyst surface. Hydrogenation occurs preferentially at the distal nitrogen atom of the adsorbed N₂ molecule. This stepwise hydrogenation process culminates in the formation of an NH₃ molecule, which then desorbs from the catalyst surface. Subsequent hydrogenation of the remaining nitrogen atom completes the catalytic cycle, yielding a second NH₃ molecule that desorbs from the catalyst surface.²¹ Conversely, in Fig. 1f, both N atoms of the adsorbed N₂ molecule experience sequential hydrogenation. When the distal nitrogen atom reaches the NH₃ state and desorbs, the proximal nitrogen atom continues to be hydrogenated until it also forms NH₃ and desorbs.⁶² Consequently, for the associative mechanism, enhancing adsorbed N₂ activation significantly augments NH₃ synthesis rates via NRR.^63,64 Unlike ambient temperature processes, the elevated operating temperature of PCECs (300–600 °C) promotes the dissociation of the strong N≡N bond, a critical step in NH₃ synthesis. To elevate the electricity-to-NH₃ conversion rate, developing cathodes with exceptional NRR activity is paramount. The PCEC reactor's cathode typically consists of metal nanoparticles supported on an oxide, which is conducive to the NRR process.^38,65,66 The catalytic activity of catalysts for the NRR in PCECs is generally influenced by their electronic structure, crystal structure, and surface morphology.^67,68 Moreover, different preparation methods can significantly affect the catalyst's particle size, dispersion, and specific surface area, thus impacting its catalytic performance.

2.2. Direct NH₃ utilization in PCFCs

PCFCs exhibit excellent fuel flexibility, enabling the utilization of electrochemically produced NH₃ for power generation. In DA-PCFCs, water steam is generated at the cathode, preventing dilution of the fuel gas at the anode, thus enhancing fuel utilization.^69–71 Additionally, the electrolyte effectively separates oxygen ions from NH₃, preventing the formation of NO_x.^72,73

In DA-PCFCs, the NH₃ molecule is initially adsorbed onto the anode surface and dissociates into H₂ and N₂. Subsequently, the electrochemical process involves the oxidation of H₂ at the anode, generating protons. These protons are conducted through the electrolyte to the cathode, where they participate in the reduction of oxygen to form H₂O (Fig. 2a).⁷⁴ An effective anode catalyst enhances NH₃ decomposition by providing a greater number of active sites for adsorption and subsequent decomposition.^75,76 An ideal anode catalyst should possess high NH₃ adsorption capacity, low N–H bond dissociation energy, and excellent resistance to poisoning.^77,78 To date, a diverse array of anode catalysts has been developed, encompassing noble metal-based catalysts (Pt, Pd, Ru, etc.), transition metal oxide catalysts (Ni, Co, Fe, etc.), and metal-ceramic catalysts (Ni–BZCY, Ni–BZCYYb, Sr₂CoMo_0.8Ni_0.2O_6−δetc.).^79–82 The catalytic activity of these catalysts is influenced by a complex interplay of factors, including their electronic structure, surface properties, and the nature of their interactions with the support material.

Fig. 2 (a) Schematic illustration of the operating mechanism of the DA-PCFC. (b) The mechanism of NH₃ decomposition on the anode side.

NH₃ decomposition at the anode surface is a complex process involving the following steps:^83,84

NH₃ (g) + * ↔ NH₃*

NH₃* + * → NH₂* + H*

NH₂* + * → NH* + H*

NH* + * → N* + H*

2H* → H₂ (g) + 2*

2N* → N₂ (g) + 2*

wherein, the reaction site for NH₃ decomposition is denoted by “*”. The nitrogen atoms in NH₃ molecules are initially adsorbed at the active sites of the PCFC anode. Subsequently, the N–H bonds undergo cleavage, leading to the release of H₂ molecules. Finally, following recombination of adjacent nitrogen atoms into nitrogen molecules, the newly formed molecules desorb from the surface (Fig. 2b). The kinetics of NH₃ decomposition on metal sites (M) is strongly influenced by the strength of the bond formed between the metal and nitrogen atoms (M–N). While a stronger M–N bond promotes the breaking of N–H bonds, it also hinders the release of N₂ molecules from the catalyst surface. Conversely, a weaker M–N bond facilitates the release of N₂, it impedes the breaking of N–H bonds.^85,86 Consequently, optimizing the M–N bond strength is essential for enhancing NH₃ decomposition. The RDS for NH₃ decomposition is generally considered to be N–H bond cleavage on noble metals such as Ru and Pd due to their strong H₂ adsorption capabilities, which facilitate the formation of stable metal–H bonds and inhibit further N–H bond cleavage.⁸⁷ For non-noble metals like Fe, Co, and Ni, the N₂ desorption is considered to be the RDS. This is attributed to the relatively strong interaction between N₂ and these metals, making it difficult for N₂ molecules to desorb from the catalyst surface. However, the electronic structure of the catalyst and the properties of the support can influence the nature of the active sites and thus alter the RDS.^83,85 Therefore, the RDS of NH₃ decomposition is dependent on the nature of the catalyst.

3. Challenges of electricity-to-NH₃ interconversion in PCCs

3.1. NH₃ synthesis in PCECs

Research and development of PCEC technology for NH₃ synthesis is currently in its nascent stages, with significant potential for future advancements. However, achieving high faradaic efficiency (FE) for NH₃ synthesis remains a critical challenge for the advancement of PCEC technology, primarily due to the competitive relationship between the hydrogen evolution reaction (HER) and the NRR.⁸⁸ This competition is influenced by various factors, including the applied potential, local reactant availability (H⁺/N₂), and the characteristics of the PCEC cathode, which determine the binding of reactants and electron transfer.⁸⁹ Although higher temperatures can promote N₂ dissociation and adsorption, they can also induce NH₃ decomposition, leading to a decrease in FE. Thus, precise control of the reaction temperature is essential.^67,68 In the reactor depicted in Fig. 1a, the NH₃ yield can be significantly enhanced by adjusting the partial pressures of H₂ and N₂.^90,91 Additionally, numerous strategies have been reported to modify the electronic structure and increase the active sites of NRR catalysts, including tailoring the size and morphology, elemental doping, and introducing defects, which can enhance the FE of NH₃ production in PCECs.^22,92,93 Although reported NH₃ synthesis rates (10⁻¹³ to 10⁻⁸ mol cm⁻² s⁻¹) surpass those of low-temperature electrochemical methods (<100 °C), the low FE remains a hurdle for practical implementation. Therefore, rational design of both the cathode and reaction conditions is imperative for advancing PCEC NH₃ synthesis technology.

3.2. Direct NH₃ utilization in PCFCs

Despite the significant advantages of DA-PCFCs, their practical implementation is currently hindered by insufficient stability and suboptimal low-temperature performance.

3.2.1. Limited power densities at low temperatures. At present, DA-PCFCs exhibit limited power densities at low temperatures (<500 °C), primarily stemming from the increased ohmic resistance of the electrolyte, sluggish oxygen reduction kinetics at the cathode, and poor anode performance in NH₃ decomposition at low temperatures. Novel thin-film electrolyte fabrication techniques, such as pulsed laser deposition, tape-casting and spin coating, enable the preparation of thin electrolyte layers (<10 μm), significantly reducing the ohmic resistance.^94–96 The sluggish oxygen reduction kinetics leads to a drastic increase in the polarization resistance of the cathode at decreased temperatures, thus degrading the performance of DA-PCFCs. Advanced triple-conducting cathodes, such as BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ, BaCo_0.8Ta_0.2O_3−δ, and PrBa_0.8Ca_0.2Co₂O_5+δ, have demonstrated excellent electrochemical activity in H₂-fueled PCFCs and show promise as potential cathode materials for DA-PCFCs.^54,97,98 The poor NH₃ decomposition performance of most of the DA-PCFC anodes or anode catalytic layers at low temperatures severely compromises the open-circuit voltage and power density of DA-PCFCs.^99,100

3.2.2. Poor durability. When DA-PCFCs operate at low temperatures, the Ni-based anode, due to its insufficient activity, cannot completely decompose NH₃ into N₂ and H₂, leading to the nitridation of Ni particles to form nickel nitride (NH₃ + 3Ni → Ni₃N + 1.5H₂).^101,102 However, Ni₃N is unstable under H₂ conditions and can be reduced back to Ni.¹¹ This process leads to microstructural changes within the DA-PCFC anode, specifically at the interface with the electrolyte. These changes increase the interfacial polarization resistance, hindering efficient charge transfer at the interface, thereby causing cell degradation and even electrolyte cracking. To address this issue, researchers have proposed adding additional NH₃ crackers or an anode catalyst layer. This can decompose most of the NH₃ before it reaches the Ni-based ceramic anode of the DA-PCFCs, effectively reducing the contact between the Ni-based ceramic anode and high-concentration NH₃, thereby improving the operational stability of the DA-PCFCs.^103–106 Furthermore, NH₃ decomposition is an endothermic process. Non-uniform temperature distribution during NH₃ decomposition can induce thermal stresses within the cell, resulting in severe performance degradation, which is particularly pronounced in cell stacks.¹⁰¹ Additionally, the mismatch in thermal expansion coefficients (TECs) among different components makes DA-PCFCs susceptible to performance degradation during rapid thermal cycling.¹⁰⁷ In 2021, Shao et al. introduced a negative thermal expansion compensation strategy and incorporated the negative thermal expansion oxide Y₂W₃O₁₂ (TEC = −7 × 10⁻⁶ K⁻¹) into the oxide SrNb_0.1Co_0.9O_3−δ (TEC = 20.5 × 10⁻⁶ K⁻¹) to successfully develop a SOFC cathode SrWO₄–Sr_x(Y_y(Nb_0.1Co_0.9)_1−y)O_3−δ with a low TEC (12.9 × 10⁻⁶ K⁻¹), effectively improving the thermal cycling stability of SOFCs.¹⁰⁸ Recently, Chen et al. impregnated Ru_0.95Cu_0.05 into the anode skeleton of DA-PCFCs to prepare Ru_0.95Cu_0.05Ni_x–Ni–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (RCN–Ni–BZCYYb) anode catalysts, significantly improving the thermal cycling stability of DA-PCFCs.⁹⁹ Therefore, for DA-PCFCs supported by Ni-based ceramic anodes, developing catalysts with high catalytic activity for NH₃ decomposition, either as an anode catalytic layer or for additional NH₃ crackers, is of profound significance for improving the stability of DA-PCFCs. Additionally, developing electrodes that are compatible with the electrolytes’ TEC is equally important for improving the thermal cycling stability of DA-PCFCs.

4. Recent advances

4.1. NH₃ synthesis in PCECs

Recent advances in PCEC NH₃ synthesis research can be grouped into three main approaches: hydrogen-based, methane-based, and water-based synthesis. Each approach presents unique advantages and challenges, and the focus of research varies based on the materials used. Hydrogen-based synthesis has garnered the most attention due to its simple system and relatively established pathways.¹⁰⁹ Researchers are mainly focusing on optimizing the NRR catalyst to enhance overall efficiency and NH₃ selectivity. Methane-based synthesis is less explored but presents a unique advantage in reducing the complexity of the reactor system.¹¹⁰ Since methane can be directly reformed to produce H₂, it eliminates the need for separate reformers or other processing units, which can reduce both the size of the overall system and streamline the synthesis process by decreasing the number of required steps. This streamlined process has the potential to lower costs and simplify industrial-scale implementation, making it a promising alternative for future research. However, not only the NRR but also the methane-steam reforming reaction (MSRR) should be considered, and the selection of materials are limited due to carbon coking.^48,111,112 Water-based synthesis represents the ultimate goal for a fully sustainable NH₃ production method.¹¹³ This approach offers a sustainable pathway for NH₃ synthesis by utilizing water as the source of protons and nitrogen from the air, eliminating carbon emissions and relying solely on abundant resources. However, water-based synthesis currently faces significant challenges, particularly in terms of energy consumption and reaction efficiency. Despite these obstacles, the potential environmental benefits have spurred considerable research activity, and advancements in catalyst and cell design are steadily addressing the technical hurdles.

4.1.1. NH₃ synthesis via the hydrogen oxidation reaction (HOR). NH₃ synthesis using the HOR in PCECs has been explored since the early stages of PCEC research. Initially, metal electrodes such as Pd were used to facilitate NH₃ synthesis due to their proven electrochemical activity.¹¹⁴ Early study with Pd electrodes by Marnellos et al. in 1998, demonstrated the feasibility of PCEC for NH₃ production, laying the groundwork for further development.

As PCEC research progressed, the development of highly active triple-conducting oxides (TCO) significantly advanced the field.^5,98 These materials, which can conduct protons, oxygen ions, and electrons or holes, have been widely adopted to increase cell performance from wide active reaction sites. Therefore, several studies were conducted by using TCO material as the PCEC electrode for NH₃ production.¹¹⁵ However, in the case of NH₃ production, simply achieving high current densities is not sufficient; NH₃ selectivity is equally critical. To improve selectivity for NH₃ synthesis, many studies have introduced catalysts like Ru, which is known for its high NRR activity and selectivity.^65,116–119 Ru-based catalysts, especially when used in exsolution techniques, allow for efficient catalyst utilization even in small quantities. However, the high cost of precious metals such as Ru presents a challenge, even when used in small amounts, as it significantly raises the overall cost of the electrode. In an effort to overcome this limitation, researchers have explored the use of alternative materials, including transition metals like Fe, Co, and Ni.^{38,39,44,66,68,115,120,121} These metals have shown promise as catalysts for NH₃ synthesis, offering a cost-effective solution while maintaining reasonable activity.

More recently, perovskite electrodes have garnered attention for their potential in NH₃ synthesis.^22,67,68 The La_0.9Bi_0.1FeO_3−δ (LBiF) perovskite was prepared by Chen et al. and was used as a cathode in a PCEC to realize NH₃ synthesis (Fig. 3a and b).⁶⁷ The introduction of Bi dopants resulted in the formation of new Bi³⁺/Bi²⁺/Bi⁰ redox electron pairs, which facilitated electron transfer and thereby improved the electrical conductivity of the material (Fig. 3c). Furthermore, Bi significantly boosted the NRR activity and effectively suppressed the HER. When LBiF was used as the NRR electrocatalyst, the faradaic efficiency (FE) for NH₃ synthesis reached a maximum value of 2.03% at 650 °C and 0.4 V, with an NH₃ production rate of 4.47 × 10⁻⁹ mol cm⁻² s⁻¹, surpassing that of LaFeO_3−δ (LF), La_0.9FeO_3−δ (L_0.9F), and LBi (Fig. 3d and e). In their other research, an NRR electrocatalyst, LaCu_0.1Fe_0.9O_3−δ (LCuF), for NH₃ synthesis in a PCEC was fabricated by Cu doping (Fig. 3f).⁶⁸ The synergistic effect of Cu and Fe facilitates electron transfer through the Fe–O^2−δ–Cu pathway, enriching Fe sites with Fe⁴⁺ and Cu sites with Cu⁺. This enhanced electron transfer kinetics boosts NRR performance. At an operating temperature of 650 °C and a cell voltage of 0.4 V, the PCEC with the LCuF electrode achieved a FE of 2.8% and an NH₃ production rate of 5.12 × 10⁻⁹ mol cm⁻² s⁻¹ (Fig. 3g and h). Furthermore, they prepared Sr_0.9Ti_0.6Fe_0.4O_3−δ (S_0.9TF) via A-site defect engineering. The abundant oxygen vacancies, Ti³⁺ species, and exsolved Fe active particles enhanced N₂ adsorption and activation, improving NRR activity. At 650 °C and 0.6 V, PCEC with S_0.9TF exhibited a maximum NH₃ production rate of 6.84 × 10⁻⁹ mol cm⁻² s⁻¹ and a FE of 2.79%. Through a simple B-doping method, they synthesized a synergistic mixed catalyst, Sr(Ti_0.6Fe_0.4)_0.8B_0.2O_3−δ, composed of Sr₃B₂O₆ (SB) and Sr_1−yTi_0.6Fe_0.4O_3−δ (S_1−yTF).¹²² SB acts as a proton acceptor, effectively suppressing the HER and promoting proton-coupled electron transfer for NH₃ synthesis. Furthermore, the grain boundaries between S_1−yTF and SB introduce more defects, significantly enhancing the NH₃ production rate and FE. In these reports, the NH₃ content was quantified using the indophenol blue method, which detects NH₃ by forming a blue-colored complex with phenol and hypochlorite under alkaline conditions, measured spectrophotometrically to determine the NH₃ concentration.

Fig. 3 (a) Schematic of NH₃ synthesis in PCEC with the LBiF electrode. (b) Rietveld refined XRD pattern of LBiF. (c) Electrical conductivities of LF and LBiF in 10% H₂ + 90% N₂. (d) The FE values and (e) NH₃ production rate in PCEC with the LBiF electrode. Reproduced with permission.⁶⁷ Copyright 2024, Elsevier. (f) Schematic diagram of the NH₃ synthesis device. (g) The FE values and (h) NH₃ production rate in PCEC with the LCuF electrode. Reproduced with permission.⁶⁸ Copyright 2024, Royal Society of Chemistry.

While most research has focused on NRR electrodes and catalysts, recent studies have also investigated the role of electrolytes in enhancing NH₃ synthesis. An ideal PCEC electrolyte should exhibit high proton conductivity. In the NH₃ synthesis process, H₂ is supplied electrochemically in the form of protons.¹²³ Therefore, the H₂ supply rate is limited by the maximum proton flux. In the early stages of PCEC NH₃ production research, Ma et al. synthesized cubic perovskite BaCe_0.85Y_0.15O_3−δ (BCY15) samples via a microemulsion method, exhibiting nearly pure proton conductivity in wet H₂. Cavity ring-down spectroscopy was employed to quantify NH₃ production in a symmetric Ag–Pd electrode cell with a BCY15 electrolyte. By measuring the ring-down time of a laser beam in an optical cavity, shortened by NH₃ absorption, this technique enables the detection of exceedingly low NH₃ concentrations. Under an applied current of 0.75 mA and a temperature of 500 °C, a maximum NH₃ synthesis rate of 2.1 × 10⁻⁹ mol s⁻¹ cm⁻² was achieved.¹²⁴ Other Ba-based electrolytes, such as BaZr_0.8Y_0.2O_3−δ, BaCe_0.65Zr_0.2Er_0.15O_3−δ, BaCe_0.2Zr_0.7Y_0.1O_3−δ, and BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ, have been extensively studied.^{109,123,125,126} More recently, Wang et al. developed cells using La_5.5WO_11.25−δ (LWO) electrolyte instead of the traditional Ba-rich materials (Fig. 4a).⁴⁵ LWO electrolyte possesses high stability under CO₂ conditions and unique conductivity properties.^127,128 This high proton conduction ensures that activated protons can efficiently participate in the NRR, leading to enhanced NH₃ production without compromising the FE. Consequently, the LWO electrochemical membrane reactor demonstrated superior performance across a wide temperature range. Quantitative analysis of NH₃ production was conducted using the indophenol blue method. Its FE and NH₃ production rate surpassed those of most other electrochemical NH₃ synthesis systems, including those operating at room temperature with Ru-based catalysts and those at elevated temperatures above 500 °C (Fig. 4b). Furthermore, the reactor demonstrated excellent stability, maintaining a consistent NH₃ production rate and FE over long-term testing at 350 °C (Fig. 4c). This durability, along with its enhanced performance, makes LWO a highly promising material for use in PCEC NH₃ synthesis.

Fig. 4 (a) Cross-section SEM image of the LWO membrane reactor. (b) Comparison NH₃ production rate and FE of the LWO membrane reactor with other reference data. (c) Long-term stability of the LWO membrane reactor at 2500 μA cm⁻² and 350 °C. Reproduced with permission.⁴⁵ Copyright 2023, Cell Press.

4.1.2. NH₃ synthesis via the methane-steam reforming reaction (MSRR). While the ideal method for NH₃ synthesis involves using H₂ or water to minimize CO₂ emissions, the reality is that the dominant origin of H₂ production is still achieved through steam methane reforming. This H₂ is then typically used in the Haber–Bosch process for NH₃ production. Given this, synthesizing NH₃ directly from methane reforming presents a practical eco-friendly approach in the short to medium term until green H₂ production becomes more cost-competitive and widespread. Methane reforming-based NH₃ synthesis could therefore serve as a practical bridge toward green NH₃ production.

The study by Stoukides et al. presents an approach that combines methane reforming reaction and NH₃ synthesis in a protonic ceramic membrane reactor.¹²⁹Fig. 5a shows the integrated cell used for NH₃ synthesis via methane reforming reaction. The system combines the vanadium nitride–iron (VN–Fe) cathode and Ni–BaZr_0.7Ce_0.2Y_0.1O_3−δ (Ni–BZCY72) anode with a BaZr_0.8Ce_0.1Y_0.1O_3−δ (BZCY81) electrolyte, facilitating both NH₃ synthesis and methane reforming reactions. This integration simplifies the process by combining methane reforming and NH₃ synthesis in a single reactor unit. Fig. 5b–g present the results of the study, displaying CH₄ conversion, CO₂ selectivity, H₂ production rate, NH₃ production rate, FE for NH₃ production, and current–voltage characteristics at various operating temperature (550 °C, 600 °C, 650 °C). A maximum NH₃ synthesis rate of 68 mmol NH₃ m⁻² h⁻¹ with a FE of 5.5% was achieved at 600 °C, as determined by the indophenol blue method.

Fig. 5 (a) Schematic and SEM images of the VN–Fe (cathode)/BZCY81 (electrolyte)/Ni–BZCY72 (anode) cell for NH₃ synthesis using methane and steam. Experimental results on (b) CH₄ conversion, (c) CO₂ selectivity, (d) H₂ generated rate, (e) NH₃ synthesis rate, (f) FE to NH₃, and (g) cell voltage versus open-circuit voltage as a function of current. Reproduced with permission.¹²⁹ Copyright 2020, Cell Press.

Ding et al. developed Ru/La_0.25Ce_0.75O_2−x catalysts via hydrothermal treatment, introducing hydroxyl groups and inducing electronic restructuring for PCEC NH₃ synthesis. In situ generated Ce³⁺–OH/Ru sites facilitated N≡N bond cleavage and N–H bond formation, significantly enhancing the NRR process.¹¹⁰Fig. 6a illustrates the catalyst synthesis method using a hydrothermal process, which integrates Ru into the La_0.25Ce_0.75O_2−δ (LDCRu) support to create a catalyst with optimized particle size and surface hydroxyl groups. These modifications enable better NRR activity by creating multiple active sites for NH₃ production. Fig. 6b and c provide cross-sectional images of the constructed PCEC using different materials for each purpose. Fig. 6b shows the Ni–BZCYYb based anode support cell used to facilitate the electrochemical reactions with H₂ fuel, while Fig. 6c displays an electrolyte support configuration. Both setups integrate LDCRu catalysts in the cathode to drive the NH₃ synthesis reactions. Fig. 6d shows the current density as a function of applied bias voltage for different cathodes: PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_3−δ/Ru (P/Ru), P/LDCRu-D (dry), and P/LDCRu-W (wet). It is evident that the wet-prepared P/LDCRu-W catalyst outperforms the others in terms of current density, indicating the importance of surface hydroxyl groups in facilitating proton transport and NH₃ production. Fig. 6e presents the NH₃ production rate, which was determined using the Nessler reagent colorimetric method. The P/LDCRu-W loaded cell significantly enhances the production rate compared to the P/LDCRu-D loaded cell and the conventional Ru catalyst loaded cell. This finding indicates that the surface hydroxyl groups, which are likely formed during the wet preparation process, play a critical role in facilitating the NRR and enhancing its efficiency. Fig. 6f demonstrates the operational stability of the Ni–BZCYYb/P/LDCRu-W system, where the current density remained stable over 550 h at 400 °C. The P/LDCRu-W cathode demonstrated excellent performance, achieving the highest NH₃ production rate of 0.452 mol m⁻² h⁻¹ at an applied voltage of 1.5 V and the highest FE of 36.5% at a lower voltage of 0.6 V (Fig. 6g). They also conducted a similar experiment using C₂H₆ (ethane) as the hydrogen source at the anode. In this case, the highest NH₃ production rate was 0.840 mol h⁻¹ m⁻² at 1.2 V, while the highest FE of 35.0% was achieved at 0.6 V (Fig. 6h). Moreover, the cell with the P/LDCRu-W cathode showed stable FE for NH₃ production under different hydrocarbon gas feeds and operating voltages (Fig. 6i). The results show that the system maintains stable NH₃ production across different conditions, confirming the effectiveness of the Ru/LDC catalyst.

Fig. 6 (a) Schematic of the gel preparation process for the Ru/LDC catalyst. (b) and (c) SEM of two types of PCC for NH₃ production. (d)–(f) Current density, NH₃ production rate, and long term stability of anode support type cells fueled with H₂. (g) FE and production rate of NH₃ with electrolyte support cells with a mixture of CO₂ and CH₄ as fuel. (h) FE and production rate of NH₃ with electrolyte support cells with C₂H₆ as the fuel. (i) Long-term stability of electrolyte support type cell with various fuels. Reproduced with permission.¹¹⁰ Copyright 2022, Elsevier.

Overall, methane reforming coupled with optimized catalysts constitutes a significant advancement towards efficient and scalable NH₃ synthesis via PCEC. While challenges remain, such as improving long-term stability and addressing carbon coking, the advancements highlighted in these studies pave the way for future breakthroughs in NH₃ production.

4.1.3. NH₃ synthesis via the water oxidation reaction (WOR). Steam-based NH₃ synthesis using PCECs focuses on using steam at the anode for the WOR and nitrogen at the cathode for the NRR. This method has the advantage of being carbon-free and relies on abundant feedstocks. However, research in this specific configuration remains limited compared to others, such as hydrogen-based approaches. One of the main challenges of steam-based NH₃ synthesis lies in the thermodynamic inefficiency at lower operating temperatures.³⁵ While converting H₂ and N₂ to NH₃ is more efficient at typical operating temperatures (300–500 °C), the use of steam as a feedstock introduces an energy consumption step due to the endothermic nature of the steam electrolysis reaction. Although less energy-intensive than the traditional Haber–Bosch process, steam-based NH₃ synthesis in PCECs requires more energy compared to alternative PCEC configurations, especially as more energy is required for water electrolysis. Additionally, optimizing not just one electrode, but both the WOR and NRR electrodes as well as the electrolyte simultaneously is crucial for improving the efficiency of steam-based NH₃ synthesis. If either of the electrodes is not optimized, the overall reaction kinetics will be sluggish, leading to reduced performance. Beyond electrode optimization, addressing the durability of the electrolyte and minimizing electronic leakage are also critical. In steam-based PCECs, NH₃ production rates can be lower due to electronic leakage in the electrolyte.¹³⁰ This leakage diminishes FE and limits NH₃ production. Therefore, optimizing all system components, including both electrodes and the electrolyte, is essential to fully realize the potential of steam-based NH₃ synthesis. Despite these challenges, efforts to optimize this technology have shown promise, and several strategies have been explored to enhance the overall performance. Yoo et al. attempted NH₃ synthesis using the WOR by introducing both metal electrodes and perovskite-based electrodes (LSCF) into a BaZr_0.8Y_0.2O_3−δ electrolyte.¹²⁶ In their study, an electrolyte-supported cell configuration was employed for the electrochemical synthesis of NH₃ at atmospheric pressure. The feed gas consisted of 3% steam and Ar, supplied to the cell at the operating temperatures ranging from 47 °C to 600 °C. Their results demonstrated that the NH₃ production efficiency varied significantly depending on the catalyst material. Specifically, the Pt electrocatalyst produced less than 10⁻¹² mol cm⁻² s⁻¹ of NH₃, while LSCF electrocatalysts showed much higher production rates of 8.5 × 10⁻¹¹ mol cm⁻² s⁻¹ at 550 °C under 0.8 V. These findings highlight the significantly lower efficiency of steam-based NH₃ synthesis compared to hydrogen-based approaches, necessitating further innovation in material development and system design. To address these challenges, researchers have explored external strategies that can further enhance NH₃ synthesis efficiency in PCECs.

One promising approach is the use of external catalysts integrated into the system to improve reaction kinetics and stability. Sullivan et al., introduced an external Ru-based catalyst to enhance NH₃ production in a PCEC.¹¹³Fig. 7a–c show a schematic of the PCC with the external catalyst layer integrated into the system. The Ru-based catalyst with high dispersion and uniformity is positioned to facilitate the NH₃ synthesis process. In Fig. 7d, the FE of the cell shows nearly 100% for H₂ between 500 and 1000 mA cm⁻². In Fig. 7e, as the driving current density increases from 2000 to 5000 mA cm⁻², the NH₃ synthesis rate normalized to the mass of Ru was approximately 2.5 × 10⁻⁸ mol cm⁻² s⁻¹ under a current of 5000 mA cm⁻². The NH₃ generation rate was quantified using a Drager tube method, enabling direct gas-phase estimation of NH₃ concentration, albeit with lower precision compared to spectroscopic techniques. Despite these improvements, the resulting NH₃ production rates, when normalized to the mass of Ru catalyst, are still comparatively lower than those achieved with other Ru-based catalysts reported in the literature. To address this limitation, the researchers pressurized the NH₃ synthesis reactor up to 12.5 bar by introducing additional hydrogen. This pressurization led to a dramatic increase in NH₃ synthesis rates, reaching as high as 2.1 × 10⁻⁶ mol cm⁻² s⁻¹, which is 100 times higher than the rates achieved under ambient pressure (Fig. 7f). The increased pressure not only enhances reaction kinetics but also shifts the equilibrium to favor NH₃ synthesis, demonstrating the potential of high-pressure conditions for scaling up this technology. A unique aspect of Sullivan et al.'s hybrid approach is its capability for cyclic operation, directly enabling transitions between using NH₃ to generate power and using power to synthesize NH₃. Fig. 7g showcased the system's versatility in switching seamlessly between energy generation and NH₃ production modes, making it a promising candidate for practical applications in sustainable energy systems.

Fig. 7 (a) Schematic for NH₃ production and the power generation process using the Ru–B₂CA catalyst. (b) SEM and (c) TEM image of the Ru–B₂CA catalyst. (d) FE and voltage as a function of the current density of the cell. (e) NH₃ synthesis rate as a function of the current density of the cell. (f) NH₃ synthesis rate as a function of operation pressure. (g) Reversible NH₃ synthesis/NH₃ fuel cell operation result. Reproduced with permission.¹¹³ Copyright 2021, Springer Nature.

Another innovative approach is plasma-assisted NH₃ synthesis. Plasma technology provides a powerful method to activate N₂ by breaking its triple bond, which is one of the most energy-intensive steps in the NRR. In a study by Sanden et al., a radio frequency (RF) plasma source was applied externally to the PCEC system to aid in N₂ dissociation.¹⁷ The plasma pre-activated N₂ molecules before they reached the cathode, reducing the activation energy required for the NRR. Fig. 8a presents a schematic of the plasma-electrocatalysis setup, showing how the RF plasma is applied to activate nitrogen molecules prior to the electrochemical reaction. The generated NH₃ was diluted with He and then quantified by mass spectrometry. Fig. 8b shows the resulting NH₃ concentration and production rate as a function of the current density, demonstrating the significantly increased NH₃ production rates with plasma activation. Fig. 8c highlights the FE, illustrating that the plasma-assisted process effectively lowers the activation energy for the NRR and improves selectivity. At lower current densities, the plasma-assisted system achieves a FE of up to 88%, though this efficiency decreases as current density increases due to the competing HER reaction. These additional techniques offer new pathways for overcoming the inherent limitations of steam-based NH₃ synthesis via the WOR. By addressing the challenges of electrode performance, nitrogen activation, and overall system efficiency, these innovative approaches bring NH₃ synthesis in PCECs closer to being a competitive alternative to traditional methods.

Fig. 8 (a) Schematic representation of the hybrid plasma electrochemical reactor setup. (b) NH₃ production rate as a function of current density and (c) faradaic efficiency of NH₃ production with/without 80 W plasma power. Reproduced with permission.¹⁷ Copyright 2020, American Chemical Society.

4.2. Direct NH₃ utilization in PCFCs

Unlike NH₃ synthesis, NH₃ decomposition is an endothermic reaction (2NH₃ → 3H₂ + N₂, ΔH = +45.9 kJ mol⁻¹). By reducing the flow rate of the reactant gas mixture, the residence time of NH₃ molecules on the catalyst surface is extended, allowing for more opportunities for reaction. Consequently, elevated operating temperatures and decreased flow rates can promote NH₃ decomposition.^102,131 The overall performance of DA-PCFCs is strongly dependent on the rate at which NH₃ is decomposed at the anode, highlighting the crucial role of anode catalysis in these systems. The NH₃ decomposition rate increases with NH₃ partial pressure and decreases with H₂ partial pressure, which can be described by the Temkin–Pyzhev equation:^132,133

Additionally, the materials and thicknesses of the anode, electrolyte, and cathode significantly impact the overall performance of DA-PCFCs.^4,134,135 Therefore, the recent research on DA-PCFCs will be described from the aspects of anodes, electrolytes, and cathodes.

4.2.1. Anodes. In DA-PCFCs, NH₃ molecules are initially adsorbed onto the anode surface, where they undergo decomposition to form N₂ and H₂. Subsequently, the generated H₂ undergoes electrochemical oxidation.^136,137 Consequently, the anode's catalytic activity for NH₃ decomposition is paramount to the cell's performance. Common anode materials in DA-PCFCs primarily consist of Ni-based ceramics and Pd-based materials, including Ni–BaZr_0.1Ce_0.7Y_0.2O_3−δ (Ni–BZCY), Ni–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (Ni–BZCYYb), and Pd.

Among single-metal catalysts, Ru-based catalysts exhibit the high activity and stability for NH₃ decomposition, as depicted in Fig. 9a.^138,139 Masel et al. identified N₂ desorption as the rate-determining step (RDS) in NH₃ decomposition over Fe, Co, and Ni surfaces, while the cleavage of N–H bonds is the RDS on Rh, Ir, Pd, Pt, and Cu surfaces. They also experimentally determined the following activity order: Ru > Ni > Rh > Co > Ir > Fe ≫ Pt > Cr > Pd > Cu ≫ Te, Se, Pb.¹⁴⁰ Although Ru offers superior catalytic activity to Ni, its high cost and scarcity have hindered its widespread application. Therefore, Ni was extensively investigated as a potential catalyst for NH₃ decomposition. Furthermore, the excellent electronic conductivity of Ni is beneficial for enhancing the NH₃ decomposition process (Fig. 9b).¹⁴¹ Therefore, Ni has been the focus of extensive research as an NH₃ decomposition catalyst.

Fig. 9 (a) Relationship of theoretical NH₃ decomposition volcano curve and experimental turnover frequencies (TOFs) of various transition-metal catalysts at 850 K obtained by microkinetic modeling and the nitrogen binding energies (Q_N(0)). Reproduced with permission.¹³⁸ Copyright 2021, American Chemical Society. (b) The relationship diagram between anodic polarization conductivity of various metals at 1000 °C and heat of oxide generation function. Reproduced with permission.¹⁴¹ Copyright 2024, Wiley-VCH.

In 2015, Yang et al. introduced Ni–BaCe_0.75Y_0.25O_3−δ (Ni–BCY25) as a promising anode for DA-PCFCs.¹⁴² Notably, the NH₃ decomposition rate of Ni–BCY25 at temperatures ranging from 400 to 700 °C significantly surpassed those of Ni–8mol% Y₂O₃–ZrO₂ and Ni–Ce_0.90Gd_0.10O_1.95 (Fig. 10a). They further investigated the impact of steam content on NH₃ decomposition over the Ni–BCY25 anode, as depicted in Fig. 10b. Introducing 0.8% steam led to a significant decline in the NH₃ decomposition rate. This reduction primarily stems from steam adsorption onto the BCY25 surface. The resulting species, including hydroxide groups and protons, can potentially occupy active reaction sites at the interface between the nickel catalyst and the BCY25 support, resulting in a significant water poisoning effect.^143,144 When operating on NH₃ fuel, a promising peak power density (PPD) of 216 mW cm⁻² was attained at 650 °C in a single cell designated as Ni–BCY25|BaCe_0.9Y_0.1O_3−δ|Sm_0.5Sr_0.5CoO_3−δ (Fig. 10c). Miyazaki et al. assessed the suitability of Ni–Ba(Zr,Y)O_3−δ (Ni–BZY) as DA-PCFC anodes by comparing the NH₃ decomposition activity of Ni–BZY10, Ni–BZY20, Ni–BCY10, and Ni–YSZ.¹⁴⁵ As depicted in Fig. 10d, Ni–BZY10 and Ni–BZY20 catalysts exhibited superior NH₃ decomposition performance compared to Ni–YSZ, suggesting that proton conductors play a more significant role in the decomposition process. Among the catalysts evaluated for NH₃ decomposition under varying H₂ partial pressures, Ni–BZY20 exhibited the highest NH₃ conversion activity and resistance to hydrogen inhibition. Additionally, a DA-PCFC comprising a Ni–BZY20 anode, a BZY20 electrolyte, and a Pt cathode achieved a PPD of 125 mW cm⁻² at 700 °C. The study by Haile et al. using Ni–BZCYYb4411 as the anode in a DA-PCFC revealed a strong correlation between NH₃ conversion rate and open circuit voltage.¹⁴⁶ While NH₃ conversion rates above 85% were achieved at temperatures above 550 °C (Fig. 10e), a substantial decrease to 40% was observed at 450 °C, likely explaining the sharp OCV reduction at this temperature (Fig. 10f). A further observation was the rapid decrease in OCV with increasing NH₃ flow rate, suggesting that higher flow rates hinder NH₃ decomposition and consequently affect the cell performance. The Ni component within the Ni-based ceramic anode of DA-PCFCs is susceptible to the formation of Ni₃N when exposed to NH₃. However, Ni₃N can be readily reduced by hydrogen, leading to structural degradation of the anode and compromising the long-term stability of DA-PCFCs.^11,105,147 The strong chemisorption of nitrogen atoms onto Ni-based catalysts, while promoting NH₃ decomposition, can also hinder N₂ desorption, resulting in a poisoning effect and limiting the overall NH₃ decomposition rate.^141,148

Fig. 10 (a) NH₃ conversion on several Ni-based anodes. (b) NH₃ conversion on Ni–BCY25 in dry and wet gases. (c) I–V–P curves of a cell with the structure of Ni–BCY25|BCY10|SSC. Reproduced with permission.¹⁴² Copyright 2015, American Chemical Society. (d) NH₃ decomposition rate as a function of partial pressure of hydrogen on several Ni-based anodes at 500 °C. Reproduced with permission.¹⁴⁵ Copyright 2017, Elsevier. (e) NH₃ conversion rate of the Ni–BCZYYb4411 anode on a PCFC. (f) OCV as functions of NH₃ flow rate from 450 to 600 °C. Reproduced with permission.¹⁴⁶ Copyright 2024, American Chemical Society.

The catalytic activity of Ni-based ceramic anodes for NH₃ decomposition can be enhanced by the in situ exsolution of B-site cations. Shao et al. proposed a Pd-doped perovskite BZCYYb (BZCYYbPd) and evaluated its NH₃ decomposition activity as a DA-PCFC anode (Fig. 11a–e).¹⁴⁹ As indicated in Fig. 11b and c, Pd metal nanoparticles exsolved from the BZCYYbPd lattice, creating B-site deficient structures that facilitate proton conduction.¹⁵⁰Fig. 10d demonstrates that Ni–BZCYYbPd exhibits superior NH₃ decomposition activity compared with Ni–BZCYYb, primarily attributed to the beneficial effects of Pd nanoparticles and B-site defect structures. Therefore, the Ni–BZCYYbPd anode enabled the DA-PCFC to achieve a PPD of 724 mW cm⁻² at 650 °C (Fig. 11e). They also developed a Ru and Fe co-doped PCFC anode material, Ni–Ba(Zr_0.1Ce_0.7Y_0.1Yb_0.1)_0.94Ru_0.03Fe_0.03O_3−δ.¹³⁵ Secondary oxidation–reduction treatment led to the formation of more RuFe nanoparticles on the surface, enhancing its catalytic activity for NH₃ decomposition. Consequently, the electrochemical performance of the DA-PCFC with this anode was significantly improved. Recently, Ciucci et al. developed a novel DA-PCFC anode denoted as Ni–Ba(Zr_0.1Ce_0.7Y_0.1Yb_0.1)_0.95Ni_0.05O_3−δ (BZCYYbNi), which significantly improved the electrochemical performance of DA-PCFCs.¹⁵¹ Ni nanoparticles exsolved from the BZCYYbNi perovskite structure create numerous active sites for NH₃ decomposition, significantly boosting the catalytic activity of the Ni–BZCYYbNi anode (Fig. 11f). A DA-PCFC employing the Ni–BZCYYbNi ceramic anode achieved a PPD of 523 mW cm⁻² at 650 °C, as illustrated in Fig. 11g. The Ni–BZCYYbNi anode offers a more cost-effective alternative to Pd-based anodes for DA-PCFCs due to the lower cost of Ni.

Fig. 11 (a) The operating mechanism of DA-PCFCs with the Ni–BZCYYbPd anode and NH₃ decomposition pathway on the Ni–BZCYYbPd anode. (b) XRD patterns and (c) HR-TEM images of BZCYYbPd powder before and after H₂ reduction at 650 °C. (d) NH₃ conversion rates of Ni–BZCYYb and Ni–BZCYYbPd. (e) I–V–P curves of the DA-PCFC. Reproduced with permission.¹⁴⁹ Copyright 2021, Wiley-VCH. (f) Schematic of DA-PCFCs: (left) Ni–BZCYYb anode and BZCYYb electrolyte; (right) Ni–BZCYYbNi anode and BZCYYbNi electrolyte. (g) I–V–P curves of a DA-PCFC with the Ni–BZCYYbNi anode. Reproduced with permission.¹⁵¹ Copyright 2022, Wiley-VCH.

Incorporating high catalytic activity species (Ru, Fe, etc.) into Ni-based anodes can significantly enhance DA-PCFCs’ performance and operating stability. Chen et al. successfully developed high-performance DA-PCFCs by decorating Ni–BZCYYb anodes with Fe, achieving a PPD of 1.257 W cm⁻² at 650 °C.¹⁵² Fe modification alters the adsorption strength of NH₃ and the barrier for N₂ associative desorption, contributing to improved performance and stability. In their another study, they infiltrated Ru_0.95Cu_0.05 into the Ni–BZCYYb anode, resulting in the formation of Ru_0.95Cu_0.05Ni_x (RCN) catalysts through an in situ reaction between Ru_0.95Cu_0.05 nanoparticles and Ni grains (Fig. 12a and b).⁹⁹ As shown in Fig. 12c, the RCN–BZCYYb anode exhibited the highest NH₃ decomposition rate among the evaluated anodes (∼98% at 550 °C). Consequently, DA-PCFCs with RCN-catalyzed Ni–BZCYYb anodes demonstrated a high PPD of 732 mW cm⁻² and enhanced stability. Recently, Shim et al. treated a Ni–BZCYYb anode with a Pd catalyst using atomic layer deposition (ALD).¹⁵³ This process formed a Pd catalytic layer on the Ni–BZCYYb surface and introduced some Pd atoms into the anode's interior (Fig. 12d). This approach effectively improved NH₃ decomposition rates, inhibited Ni₃N formation, and enhanced DA-PCFC performance and durability (Fig. 12e).

Fig. 12 (a) Schematic illustration of NH₃ decomposition on the RCN nanoparticle-modified Ni–BZCYYb anode. (b) STEM-EDX mapping results of the RCN–Ni–BZCYYb anode. (c) NH₃ decomposition rates of Ni–BZCYYb and Ru_xCu_1−x–Ni/BZCYYb (x = 0–1) anodes. Reproduced with permission.¹⁵² Copyright 2022, Royal Society of Chemistry. (d) Bright-field, BZCYYb, NiO, and Pd TEM-EDS mapping images. (e) Schematic illustration of the nitridation of Ni in bare Ni–BZCYYb (left) and Pd-treated Ni–BZCYYb (right) anodes of DA-PCFCs. Reproduced with permission.¹⁵³ Copyright 2023, Wiley-VCH.

4.2.2. Electrolytes. In DA-PCFCs, hydrogen protons, generated through NH₃ decomposition and H₂ oxidation, traverse the electrolyte to react with oxygen species. Consequently, an exceptionally thin-film electrolyte can significantly reduce ohmic resistance and enhance proton conduction, leading to improved DA-PCFC performance at lower temperatures. BaCeO_3−δ based oxides are often employed as the electrolytes of DA-PCFCs, such as BaCe_0.8Gd_0.2O_2.9 (BCG),^154,155 BaCe_0.9Nd_0.1O_3−δ (BCN),¹⁵⁶ BaCe_0.8Gd_0.19Pr_0.01O_3−δ (BCGP),¹⁵⁷ BaCe_0.8Y_0.2O_3−δ (BCY),^158,159 BaCe_0.8Sm_0.2O_3−δ (BCS),¹⁶⁰ BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)¹⁶¹ and BaZr_0.1Ce_0.7Y_0.1YbO_3−δ (BZCYYb).^99,152 Various techniques exist for fabricating thin-film electrolytes for DA-PCFCs, including wet colloidal spraying,¹⁶² modified suspension spraying,¹⁶⁰ radio frequency (RF) sputtering deposition¹⁶³ and tape casting.⁹⁹ Maffei et al. successfully deposited a 15 μm thin-film BCY electrolyte onto a NiO-BCY anode using wet colloidal spraying.¹⁶² Habazaki et al. employed RF sputtering to fabricate a 1 μm-thick BZCY electrolyte film on a Pd solid anode, achieving a PPD of 0.58 W cm⁻² at 600 °C.¹⁶⁴ However, the high cost of Pd may impede its widespread adoption. Recently, Duan et al. introduced ultrasonic spray coating for thin-film electrolyte fabrication, controlling electrolyte thickness at 3 μm, thereby minimizing proton transport resistance across grain boundaries and effectively reducing the cells’ ohmic impedance (Fig. 13a).¹⁶⁵ Co-tape casting and co-sintering, a technology enabling precise control of electrolyte thickness, has also gained attention in DA-PCFCs. Liu et al. introduced the detailed co-tape casting and co-sintering process (Fig. 13b) and fabricated DA-PCFCs with ∼10 nm electrolyte.⁹⁹ These DA-PCFCs showed high performance and remarkable durability, which indicates that the co-tape casting and co-sintering technology has significant promise for DA-PCFC development.

Fig. 13 (a) The ultrasonic spray coating process for preparing thin-film electrolytes and SEM images of the single cell. Reproduced with permission.¹⁶⁵ Copyright 2023, Nature Portfolio. (b) Fabrication process of the single cell using a tape casting technique. Reproduced with permission.¹⁵² Copyright 2022, Royal Society of Chemistry.

4.2.3. Cathodes. In DA-PCFCs, cathodes exhibiting superior triple conducting behavior (e⁻/O²⁻/H⁺) can substantially enhance the cell performance. Given that the distinction between H₂-fueled PCFC lies in the fuel gas, H₂-fueled PCFC cathodes can also be utilized in DA-PCFCs. Consequently, we will outline some recent research on H₂-fueled PCFC cathodes.

O’Hayre et al. demonstrated a high-performance PCFC cathode by infiltrating the triple conductor BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY) into a BaZr_0.3Ce_0.6Y_0.1O_3−δ (BZCY63) framework (Fig. 14a).⁹⁷ The high oxygen vacancy concentration and proton conductivity of BCFZY contribute to its superior catalytic activity,¹⁶⁶ enabled the corresponding PCFC to achieve a PPD of 0.648 W cm⁻² at 600 °C (Fig. 14b) and maintain stable operation for over 1100 h at 500 °C (Fig. 14c). Some high-performance BCFZY derivatives were further developed, such as Ba_0.9Co_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ,¹⁶⁷ BaCo_0.7Fe_0.1Zr_0.1Y_0.1O_3−δ,¹⁶⁸ Ba(Co_0.4Fe_0.4Zr_0.1Y_0.1)_0.95Ni_0.05O_3−δ,¹⁶⁹ BCFZY–NiO,¹⁷⁰ Ba(Co_0.4Fe_0.4Zr_0.1Y_0.1)_0.95Mg_0.05O_3−δ³² and Ba_0.95(Co_0.4Fe_0.4Zr_0.1Y_0.1)_0.95Ni_0.05O_3−δ,¹⁷¹ which can also be applied to DA-PCFCs.

Fig. 14 (a) Process for preparing BCFZY infiltrated BCZY63 composite. (b) I–V–P and (c) stability of the cells under H₂/air. Reproduced with permission.⁹⁷ Copyright 2015, American Association for the Advancement of Science.

Shao et al. developed a PCFC triple-conducting nanocomposite cathode BaCo_0.7(Ce_0.8Y_0.2)_0.3O₃ (BCCY), comprising BaCe_0.8Y_0.2O₃ (P-BCCY) as a proton conductor, BaCo_0.9(Ce_0.8Y_0.2)_0.1O₃ (M-BCCY) as an oxygen-ion conductor, and BaCoO_3−δ (BC) with high electronic conductivity (Fig. 15a–c).¹⁷² This synergistic combination endowed BCCY with excellent catalytic activity. When employed as a PCFC cathode, BCCY achieved a high PPD of 508 mW cm⁻² at 550 °C and demonstrated an operational stability exceeding 800 h (Fig. 15d and e).

Fig. 15 (a) Refined XRD profiles of the BCCY sample. (b) STEM image and (c) STEM-EDX result of BCCY. (d) I–V–P curves and (e) durability of PCFCs with the BCCY cathode. Reproduced with permission.¹⁷² Copyright 2019, Cell Press.

4.2.4. External NH₃ cracking reactor and anode catalytic layer. While Ni catalysts demonstrate reasonable catalytic activity for NH₃ decomposition, prolonged exposure to high-concentration NH₃ under operating conditions can lead to Ni phase coarsening, agglomeration, and nitridation, ultimately causing significant cell degradation. To mitigate this issue, Sullivan et al. coupled an NH₃-cracking reactor containing the Ru–(BaO)₂(CaO)(Al₂O₃) (Ru–B₂CA) catalyst to DA-PCFCs.¹¹³ The Ru–B₂CA catalyst, supported on refractory insulating fibers, exhibits high catalytic activity for NH₃ decomposition, enabling a PPD of 0.65 W cm⁻² and stability of 1250 h for DA-PCFCs at 600 °C.

Incorporating an anode catalytic layer (ACL) can also effectively enhance both the performance and durability of DA-PCFCs. Liu et al. fabricated tubular DA-PCFCs featuring an iron catalytic layer on the Ni–BZCYYb anode (Fig. 16a).¹⁷³ These DA-PCFCs, utilizing a highly active Fe catalyst for NH₃ decomposition, achieved a record PPD of 1.078 W cm⁻² at 700 °C for tubular DA-PCFCs. By decomposing most of the NH₃ into H₂ and N₂, the Fe catalytic layer minimizes direct contact between NH₃ and the Ni–BZCYYb anode, thus enhancing the stability of the tubular DA-PCFCs. The high catalytic activity of Fe for NH₃ decomposition has also been demonstrated in other studies.^106,174 Chen et al. prepared a material designated as Fe–CeO_x (Fe : Ce = 6 : 4) via a wet chemical method and incorporated it as a catalytic layer on the anode side of DA-PCFCs (Fig. 16b).¹⁷⁵ This composite material not only maintains a high NH₃ decomposition rate but also suppresses the formation of iron nitride, ensuring the long-term operational stability of DA-PCFCs.

Fig. 16 (a) Schematic representation of a tubular cell utilizing NH₃ as the fuel.¹⁷³ (b) Schematic diagram of the synthesis process and the crystal structure of the Fe–CeO_x (Fe : Ce = 6 : 4) catalyst. Reproduced with permission.¹⁷⁵ Copyright 2022, Elsevier.

Beyond simple metal catalytic layers, perovskite oxides present an alternative for the DA-PCFC catalytic layer, enhancing cell performance and stability. In 2023, Chen et al. developed the reduced Sr₂Fe_1.35Mo_0.45Cu_0.2O_6−δ (r-SFMC) anode catalytic layer, which demonstrates high NH₃ adsorption and decomposition capabilities (Fig. 17a and b).¹⁷⁶ The tubular Ni–BZCYYb anode-supported DA-PCFCs equipped with an r-SFMC ACL achieved a remarkable PPD of 1.02 W cm⁻² and exhibited outstanding stability over 200 h at 650 °C, marking a significant advancement in DA-PCFC technology (Fig. 17c). More recently, Shao et al. substituted 5, 10, 15 and 20% Ru into Pr_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (PSCF), respectively, and found reduced-Pr_0.6Sr_0.4(Co_0.2Fe_0.8)_0.85Ru_0.15O_3−δ (r-PSCFR15) exhibits the optimal NH₃ decomposition activity among these catalysts (Fig. 17d).¹⁰⁰ During the reduction process, CoFe (CF) and CoFeRu (CFR) alloys form on the PSCF and PSCFR15 surfaces, respectively. The CFR alloy promotes the N₂ desorption process in the NH₃ decomposition reaction, thereby boosting NH₃ decomposition efficiency (Fig. 17e and f). Under the influence of the r-PSCFR15 catalytic layer, the DA-PCFC exhibited a significant improvement in cell performance, reaching 625 mW cm⁻² at 650 °C. Additionally, the DA-PCFC demonstrated robust durability, maintaining stable performance over 340 h (Fig. 17g). After the operational stability test, the Ni particles in the anode of the PCFC with r-PSCFR15 ACL were noticeably smaller compared to those on the anodes without ACL (Fig. 17h and i), which indicates that the r-PSCFR15 effectively prevented the Ni sintering in the Ni-based anode.

Fig. 17 (a) NH₃ temperature-programmed desorption profiles of r-SFMC and Ni–BZCYYb. (b) NH₃ catalytic activities of r-SFMC, Ni–BZCYYb, and their mixture. (c) PPDs of DA-PCFCs with and without r-SFMC ACL fueled by wet H₂ and dry NH₃. Reproduced with permission.¹⁷⁶ Copyright 2023, Wiley-VCH. (d) NH₃ decomposition rates of r-PSCF, r-PSCFR05, r-PSCFR10, r-PSCFR15, r-PSCFR20, Ni–BZCYYb, and Ni–BZCYYb|r-PSCFR15. E_NH3 and E_N2 calculated for (e) CF(011) and (f) CFR(011). (g) The operational stability of DA-PCFCs under NH₃ fuel conditions. The SEM images of Ni–BZCYYb anodes of DA-PCFCs (h) without ACL and (i) with r-PSCFR15 ACL after the stability test. Reproduced with permission.¹⁰⁰ Copyright 2024, Wiley-VCH.

5. Perspectives for electricity-to-NH₃ interconversion in PCCs

5.1. NH₃ synthesis in PCECs

The low FE and NH₃ production rate in PCEC NH₃ synthesis are primarily attributed to the competitive HER.⁸⁸ To enhance these metrics, future research should prioritize the development of catalysts with high NRR activity. Optimizing the electronic structure, crystal structure, and surface morphology of catalysts can effectively suppress the HER and promote the NRR. Exploring novel catalytic materials, such as nitride catalysts, is also crucial. DFT calculations and advanced in situ characterization techniques can aid in identifying the active sites for the HER and NRR, significantly benefiting the development of NH₃ synthesis catalysts. Moreover, optimizing electrolyte design and NH₃ synthesis operating conditions is of paramount importance.

5.1.1. Investigations of nitride-based electrodes. Nitride-based catalysts, with their unique electronic and structural properties, have garnered significant interest in the field of NH₃ synthesis.^177–180 Their nitrogen vacancies can enhance the reactivity and selectivity of the NRR, making them promising alternatives to precious metal catalysts. In addition to lower cost, nitrides offer considerable potential for NH₃ synthesis. A comprehensive understanding of nitride behavior under PCEC operating conditions is crucial for their successful implementation. This necessitates further investigation into their long-term stability and performance under high temperatures, mixed N₂/H₂ environments, and electrical biases. Examining the long-term stability of nitride electrodes under operating conditions is a crucial area for future research.

5.1.2. Identification of NRR and HER active sites. Currently, the FE of PCEC NH₃ synthesis remains low, primarily due to the competitive relationship between the HER and the NRR at the PCEC cathode. A deeper understanding of catalyst properties can enable better optimization of NRR activity and NH₃ selectivity. If the active sites for NRR and HER can be identified through density functional theory (DFT) calculations and in situ characteristics (in situ XPS or Raman), catalysts can be optimized to increase NRR active sites and reduce HER active sites, thereby maximizing the FE of PCEC NH₃ synthesis.^181,182 However, PCEC NH₃ synthesis technology is still in its nascent stage, resulting in a scarcity of reports on NRR active sites at the atomic scale. Additionally, screening for the optimal catalyst model within a given catalyst system through DFT calculations, followed by experimental validation, could significantly expedite progress in this field.

5.1.3. Investigations of electrolyte materials and structures. In PCECs, the choice of electrolyte plays a crucial role in determining the overall performance of the cell, as electronic leakage in protonic conducting electrolytes can significantly reduce FE. Promising proton conducting electrolytes such as BZCYYb4411 and BZCYYb7111 exhibit mixed ionic-electronic conductivity at elevated temperatures.^5,56 At 600 °C, the electronic transport number of BZCYYb7111 is approximately 0.1, indicating that a portion of the applied current is lost through electronic leakage rather than being utilized for electrochemical reactions. This electronic leakage can significantly reduce FE, particularly in PCEC modes (including electrochemical NH₃ synthesis). Strategies to mitigate this issue and enhance overall efficiency include optimizing electrolyte composition, reducing operating temperatures, employing moderate current densities, and enhancing the catalytic activity of electrodes.

5.1.4. Operation condition optimization. NH₃ synthesis in PCECs can benefit from optimizing operating conditions, especially regarding pressure and temperature. Studies, such as those involving Ru-based external catalysts, have shown significant increases in NH₃ production rates at higher pressures.¹¹³ Operating PCECs under elevated pressures can shift reaction equilibria favorably toward NH₃ synthesis. Therefore, exploring advanced materials capable of withstanding high-pressure environments and conducting systematic studies on temperature-pressure combinations could lead to enhanced NH₃ production rates while ensuring long-term stability.

5.2. Direct NH₃ utilization in PCFCs

5.2.1. NH₃ decomposition rate at low temperatures. In DA-PCFCs, NH₃ undergoes initial decomposition into H₂ and N₂ on anodes, with the generated H₂ subsequently participating in power generation. Although elevated temperatures facilitate NH₃ decomposition, they also increase energy consumption and system maintenance costs in DA-PCFCs. Furthermore, lower temperatures can result in incomplete NH₃ decomposition, leading to issues such as Ni nitridation in the Ni-based cermet anode and poor stability of DA-PCFCs. This necessitates the development of catalysts with enhanced NH₃ decomposition activity at lower temperatures. Fortunately, numerous catalysts have been developed in the field of NH₃ thermal decomposition, some of which demonstrate promising activity at lower temperatures and can be employed in external NH₃ cracking reactors for DA-PCFCs.^183,184 In particular, the unique structural and performance characteristics of high-entropy alloys (HEAs) have made them a focal point of research in catalysis. Recently, Wang et al. have demonstrated the potential of CoMoFeNiCu HEA for NH₃ decomposition.⁸⁴ While there have been limited reports on the application of high-entropy alloys as anodes or anode catalytic layers in DA-PCFCs, their potential in this field is promising. Furthermore, tailoring the catalyst morphology, particularly through the design of nanostructured catalysts or single-atom catalysts, enables complete NH₃ conversion at lower temperatures.^185,186 This may allow the electrochemical performance of DA-PCFCs to approach that of H₂-fueled PCFCs. Finally, DFT calculations can provide insights into the kinetics of NH₃ adsorption, the initial N–H bond cleavage, and N₂ desorption on the catalyst surface, thereby elucidating the rate-limiting step in the overall NH₃ decomposition process. This knowledge can guide the rational design and development of new catalytic materials that exhibit high activity for NH₃ decomposition at lower temperatures, enabling more efficient and sustainable processes.

5.2.2. Design of anode catalytic layer. Adding a catalytic layer with high NH₃ decomposition activity to the anode of DA-PCFCs can promote NH₃ decomposition, reduce direct contact between the anode and high-concentration NH₃, and thereby enhance the overall performance and stability of DA-PCFCs. Current reports on DA-PCFC anode catalytic layers consistently demonstrate performance improvements not only for DA-PCFCs but also for H₂ fueled PCFCs, suggesting that the catalyst layers may participate in the HOR on the anode side.^{100,173–176} Consequently, in addition to seeking high NH₃ decomposition activity, it is crucial to consider the TEC matching and chemical compatibility between the catalyst layer material and the Ni-based cermet anode. Mismatched thermal expansion behavior can lead to delamination between the catalytic layer and anode during temperature cycling, severely impacting the electrochemical performance and stability of DA-PCFCs.

5.2.3. DA-PCFC stack design. Currently, research on DA-PCFCs is primarily focused on button cells, with no reports on kilowatt-level power output DA-PCFC stacks. Future research on DA-PCFCs should not only focus on developing anodes or catalysts with excellent NH₃ decomposition activity but also on the design, fabrication, and analysis of DA-PCFC stacks. In recent years, extensive research has been conducted on NH₃-fueled oxygen-ion conducting SOFCs (NH₃–O–SOFCs) and H₂-fueled PCFC stacks, which may provide valuable insights for the design of DA-PCFC stacks. Kishimoto et al. developed an NH₃–O–SOFC stack using 30 planar anode-supported cells, achieving a PPD of up to 1 kW and durability of 1000 h at 750 °C.¹⁸⁷ During the stack stability test, an energy conversion efficiency of 57% was attained at a PPD of 700 W. Song et al. proposed a two-step sintering method for fabricating large PCFC cells with excellent mechanical properties, optimizing the anode/electrolyte interface.¹⁸⁸ The stack using these large PCFC cells demonstrated stable operation for over 350 h at 600 °C. Furthermore, studies on modeling, thermodynamics, kinetics, and cost analysis of NH₃–O–SOFC and PCFC stacks can provide valuable guidance for DA-PCFC stack research. However, NH₃ can compromise pipeline integrity, causing embrittlement and corrosion that may lead to leaks. This also remains a critical area for future research.

6. Conclusions

PCCs represent a promising energy conversion technology, enabling both the storage of excessive renewable electricity via NH₃ synthesis and on-demand electricity generation from NH₃. This review comprehensively introduces the reaction mechanisms of electricity-to-NH₃ interconversion in PCCs, analyzes the challenges of this technology and summarizes the research advancements in this field. Moreover, this review emphasizes the need for researchers to explore not only novel catalytic electrodes and electrolyte materials, but also to optimize operating conditions and identify active sites for the NRR and HER in order to achieve efficient NH₃ production in PCECs. For DA-PCFCs, the development of anode electrodes or anode catalytic layers with high catalytic activity for NH₃, as well as the fabrication of DA-PCFC stacks, should be the primary focus of future research. This review aims to encapsulate the progress in PCC-based electricity-to-NH₃ interconversion technology and provide insights for future research in this field.

Author contributions

Mingzhuang Liang: conceptualization, visualization, and writing – original draft. Jinwook Kim: investigation and writing – review & editing. Xiaomin Xu: writing – review & editing. Hainan Sun: writing – review & editing. Yufei Song: writing – review & editing. SungHyun Jeon: writing – review & editing. Tae Ho Shin: supervision and writing – review & editing. Zongping Shao: supervision and writing – review & editing. WooChul Jung: supervision, writing – review & editing, and project administration.

Data availability

The data supporting the findings of this study are available in the manuscript, and additional data are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Ceramic Strategic Technology R&D program through the Korea Institute of Ceramic Engineering & Technology (KICET) (grant NTIS no. 240002182). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021M3H4A3A02086497). We appreciate the support from Research Institute of Advanced Materials.

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Footnote

† These authors contributed equally to this work.

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