Cerium oxide-based electrolytes for low- and intermediate-temperature solid oxide fuel cells: state of the art, challenges and future prospects

Abstract

Solid oxide fuel cells (SOFCs) are important, efficient, and environmentally friendly energy conversion devices that also serve as solid oxide electrolysers, producing hydrogen and oxygen by reversing chemical reactions. Research and development of electrode and electrolyte materials is still very much needed for their efficient working in low (≤650 °C) and intermediate (650–850 °C) temperature regimes. The present article reviews undoped and doped ceria-based electrolytes in light of processing parameters such as synthesis methods, sintering time, temperature and different doping strategies. The article focuses primarily on the various factors that affect the conductivity of ceria-based electrolytes. Different approaches to enhance the conductivity and improve the cell parameters have also been discussed. Conclusion, challenges and direction for further research are also provided at the end of this article.

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Paramvir Kaur

Dr Paramvir Kaur received an MSc (Honours School) degree in Physics from Panjab University, Chandigarh, in 2014. She was awarded a PhD degree in 2022 by the Department of Physics and Materials Science, Thapar Institute of Engineering and Technology (Deemed to be University), Patiala, Punjab, India. She has worked as a Project Associate on a DST-funded Hydrogen and Fuel Cell-2018 project. Her research focuses on developing perovskite-structured oxides for solid oxide fuel cells. Her work is published in reputed SCI journals such as Ionics and Ceramics International. She has also authored a chapter in a book published by the Academic Press.

K. Singh

Dr Kulvir Singh is a Professor at the Department of Physics and Materials Science, Thapar Institute of Engineering and Technology (Deemed to be University), Patiala, Punjab, India. He received his PhD degree from HPU Shimla in 1995 and later worked as a guest scientist at Forschungszentrum Jülich, Germany. His current research interests are developing ceramic materials for solid oxide fuel cells, batteries, and biomaterials. He also specialises in the development of glasses and glass ceramics. As a reviewer for several high-impact journals, he has authored over 260 research papers, with more than 8790 citations and an h-index of 45.

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1. Introduction

Electricity and water are the basic amenities for the sustainable development of society. However, with the depletion of resources and the increasing population, there is a need to produce clean energy with less carbon and unprocessed waste. The most sought-after way to reduce carbon footprints and meet global energy demands is by using renewable, cleaner, greener energy sources. Hydrogen-based energy sources are an alternative to conventional fuels, such as natural gas, coal, fossil fuels, etc. There are many different sources of hydrogen production based on which they are colour-coded, as shown in Fig. 1.^1,2 Green hydrogen is produced from renewable energy sources and produces zero greenhouse gas emissions. Energy conversion devices, i.e., solid oxide fuel cells (SOFCs), utilise green hydrogen as a fuel, and solid oxide electrolysis cells (SOECs) produce green hydrogen.^3,4

Fig. 1 Sources of hydrogen production and technological applications of green hydrogen.

SOFCs are efficient electrochemical energy conversion devices that produce electricity through a chemical reaction between the fuel (H₂) and air. With fuel flexibility, SOFCs are good candidates for electricity production, having the highest efficiency compared to other fuel cells.^5–8 Additionally, SOFCs are also being used for water production in space missions. Thus, SOFCs could be a future energy source and a strategic source of water production. A single unit of SOFCs consists of two electrodes (anode and cathode) and an electrolyte, having an output of only ∼1 volt (V) and power density <2 W cm⁻². Therefore, multiple single cells are connected using (metallic or ceramic) interconnects to enhance overall electricity production. The design of SOFCs also incorporates an additional component, such as a glass sealant, in the case of the planar configuration.^9,10

SOFCs are multipurpose devices that can also be used as electrolysers, i.e., SOECs, to produce hydrogen and oxygen by reversing the reactions to split water into its constituent gases.^11–14 The diagrammatic illustration of SOFCs versus SOECs is shown in Fig. 2(a) and (b). In SOFCs, air or oxygen is fed to the cathode and is converted into oxide ions. These oxide ions travel through the electrolyte towards the anode, where they react with the fuel (H₂) to form water (H₂O).¹⁵ The reactions that occur at the electrodes are given as follows:

Cathode: O₂ + 4e⁻ → 2O²⁻ (1)

Anode: 2H₂ + 2O²⁻ → 2H₂O + 4e⁻ (2)

Overall: 2H₂ + O₂ → 2H₂O (3)

Fig. 2 A schematic illustration of the working of (a) SOFC and (b) SOEC.

On the other hand, in the case of SOECs, steam (H₂O) is fed to the cathode and electricity is applied. At the cathode–electrolyte interface, H₂O decomposes into H₂ and O²⁻. H₂ gas travels through the pores of the cathode to its surface, where it is collected. The O²⁻ ions travel through the electrolyte towards the anode, and at the anode–electrolyte interface, O²⁻ gets oxidized into O₂ gas. The reactions occurring at SOEC electrodes are as follows:¹⁶

Cathode: H₂O + 2e⁻ → H₂ + O²⁻ (4)

Anode: O²⁻ → 1/2O₂ + 2e⁻ (5)

Overall: H₂O_cathode → H_{2 cathode} + 1/2O_{2 anode} (6)

Therefore, the same device can also produce hydrogen and oxygen. The hydrogen produced from SOECs that use electricity from renewable sources is known as green hydrogen and can be stored in any form: solid, liquid or gas. However, solid-state storage provides many benefits, such as higher volumetric energy density than other storage methods.

In addition to direct biogas and many other fuels, carbon monoxide can also be used in SOFCs. It is converted into less dangerous carbon dioxide during the operation of SOFCs. In fact, oxygen could also be used for many applications, particularly in the medical field, due to its purity, using both these operations (SOFCs and SOECs).^17,18 It is a fact that once a technology is developed in one device, others can be modified slightly and used for various purposes. So much research is going on to develop this technology in parallel to other devices, such as batteries, supercapacitors, etc., for use in different applications.¹⁹ Many excellent reviews are available on materials development for various components of SOFC technology.^7,20–25

The present article focuses mainly on developing electrolyte materials since they are integral to SOFCs and SOECs. An electrolyte is a medium through which ions are conducted from one electrode to another, and the dispersion of anions and cations at an applied potential is responsible for current generation. In fact, the overall device resistance also depends on the ionic flow through the electrolyte. Different fuel cells are also named after the type of electrolyte used (molten carbonate fuel cells, alkaline fuel cells, polymer electrolyte membrane fuel cells, direct methanol fuel cells, phosphoric acid fuel cells, solid oxide fuel cells and reversible fuel cells). Solid electrolytes offer many advantages over aqueous electrolytes and can be used in many applications.²⁶ Even the temperature categorisation of SOFCs is based on the operating temperature of the electrolyte, i.e., high-temperature (HT-SOFC): 850–1000 °C, intermediate-temperature (IT-SOFC): 650–850 °C and low-temperature (LT-SOFC): <650 °C.

Ceramic materials are predominantly used for SOFCs or SOECs as electrodes and electrolytes. Ceramic materials with high chemical stability, good mechanical strength, and low activation energy (E_a) for ionic conduction are required to qualify as electrolyte materials. Materials with good relative density (∼>95%) preclude open porosity and avoid reactant crossover from the anode to the cathode and vice versa. The material requirements for different SOFC components are clearly discussed in the literature.^20,27–36 As mentioned earlier, the conducting properties of electrolyte materials are of utmost importance. Therefore, in the present article, the primary focus is to analyse the conductivity of electrolyte materials and correlate other properties with conductivity.

2. Electrolyte materials for SOFCs

Since the inception of SOFCs, many materials such as fluorite-structured ZrO₂, CeO₂, Bi₂O₃-based oxides, perovskite-structured LaGaO₃, SrTiO₃, Bi₄V₂O₁₁, La₂Mo₂O₉, Ba₂In₂O₅ oxides, brownmillerite and pyrochlore-structured materials have been investigated and developed for use as electrolytes for SOFCs.^20,37–45 SOFC electrolytes could be oxygen, proton or dual (oxygen and proton) conducting.

Eight mol% yttria stabilised zirconia (8-YSZ) is a widely used oxide ion electrolyte material in HT-SOFCs since it is reported to have an ionic conductivity of 0.1 S cm⁻¹ at 1000 °C. It also exhibits good mechanical properties without compromising the stability of the cell and the durability of its performance.^30,46 However, in IT- and LT-SOFCs, the ionic conductivity of 8-YSZ decreases drastically to ∼7.2 × 10⁻⁴ S cm⁻¹ at 500 °C.⁴⁷ This issue of poor ionic conduction can be resolved by reducing the thickness of the electrolyte or by finding a substitute for YSZ.^48–50 Other than YSZ, scandia stabilised zirconia (ScSZ) is also known to exhibit high ionic conductivity. The close ionic radii of Zr⁴⁺ and Sc³⁺ decrease the association enthalpy of the defects, resulting in higher conductivity for ScSZ, i.e., 3 × 10⁻¹ S cm⁻¹ at 1000 °C, than that of YSZ. However, the high cost and rarity of scandia limit the use of ScSZ, and the reaction of zirconia with strontium or lanthanum present in the cathode limits their use as electrolytes.^51,52

Compared to YSZ and ScSZ, bismuth-based fluorite-structured materials (δ-Bi₂O₃) exhibit the highest oxygen ionic conductivity. This is due to a high concentration of oxygen vacancies and easy anion mobility in these electrolytes. However, the ionic conduction decreases significantly in these oxides when the operating temperature is lowered. A large dopant concentration is used to stabilise these oxides. Gd, La, Cu, Ti, Al, Ga, etc., are commonly used dopants.^{43,44,53–55} The dopant ionic radius and polarizability affect the conductivity and stability of these bismuth oxide-based electrolytes. γ-Bi₄V₂O₁₁ doped with transition metals, i.e., BIMEVOX, demonstrates high ionic conductivity and higher stability at 600 °C. It exhibits three polymorphs (α, β, and γ) with respect to temperature, and the higher temperature γ-phase has a higher oxygen vacancy disorder in the O–V polyhedra. The major drawback of Bi₄V₂O₁₁ is the slow phase transformation (δ → α) occurring between 500 and 600 °C, which decreases its conductivity and limits its usage in LT- and IT-SOFCs. Additionally, Bi₂O₃ decomposes into metallic Bi in reducing environments, has poor mechanical strength and a high coefficient of thermal expansion (CTE) and is chemically reactive with other SOFC components, thus limiting its usage as electrolyte.^52,56–60

Perovskite-based oxides are another popular electrolyte material choice with high SOFC performance. At 500 °C, the oxide ion conductivity of Sr_0.55Na_0.45SiO_2.755 is > 10⁻² S cm⁻¹, having an activation energy of 0.3 eV. A SOFC with Sr_0.55Na_0.45SiO_2.755 electrolyte has high power densities of 431 and 213 mW cm⁻² at 600 and 500 °C, respectively. It is a promising SOFC electrolyte candidate with activation energy that is much lower than that of other commonly used oxygen ion conductors and high power density.⁶¹ Mo-substituted SrFeO₃ achieved power densities between 0.24 and 1.12 W cm⁻² across 600–800 °C. A SOFC single cell employing a SrFe_0.93Mo_0.07O_3−δ cathode exhibited excellent operational stability over 270 h at 700 °C.⁶² Lanthanum-based oxides such as LaAlO₃, LaSrO₃, LaInO₃, LaScO₃, LaYO₃, etc., exhibit minimum electronic conduction and high stability at lower operating temperatures. Adding Co, Ni, or Bi in small quantities at the gallium site and co-doping at the La and Ga sites enhances ionic conductivity.^63–67 However, phase instability, gallium volatilisation at high temperatures, and chemical incompatibility with nickel limit the use of these materials.^20,68

Fluorite-structured electrolytes offer many advantages over traditionally used materials; therefore, they are widely used as SOFC electrolyte materials. The following sub-section discusses ceria-based electrolytes in detail.

2.1. Ceria-based electrolytes

Cerium dioxide (CeO₂) with different dopants is used in many applications.^69–73 As mentioned in the introduction section, the primary focus of the present review is on the various parameters affecting the conductivity of CeO₂ and discussing its applicability as an electrolyte for SOFCs. CeO₂, commonly known as ceria, has a fluorite structure (AX₂) with A as the cation and X as the anion. Structurally, each Ce⁴⁺ cation is coordinated to eight O²⁻ anions. Each O²⁻ anion is located at the tetrahedral interstices of four Ce²⁺ cations.⁷⁴ The electronic configuration of cerium is [Xe] 4f²6s² with two common valence states, cerium(III) and cerium(IV). A detailed overview of the structure of CeO₂ is given by Sun et al. and shown in Fig. 3.⁶⁹ The fluorite structure with the space group Fm3m is stable over a wide temperature range (room temperature-2400 °C). Unlike ZrO₂, Bi₂O₃, etc., electrolytes, the phase transition does not occur up to their melting point. In some electrolytes, it can be stabilised using different dopants and their concentration. The task is stabilising the high conducting phase, such as γ-phase in Bi₄V₂O₁₁ and the cubic phase of ZrO₂, to use as electrolytes in SOFC technology. In the case of CeO₂, the cubic phase already exists with unoccupied octahedral sites necessary for oxide ion diffusion. The cationic or anionic vacancies can be further increased by introducing appropriate dopants and their concentrations. These vacancies are required since ionic conduction occurs via the vacancy diffusion mechanism.

Fig. 3 Schematic representation of the effect of doping on the crystal structure of CeO₂. (a) The undoped CeO₂ structure. (b) and (c) Ce⁴⁺ replacement by lower valence cations (Aⁿ⁺: n = 3, 2, 1) leads to the formation of disordered oxide ion vacancies (V^˙˙_O), and (d) vacancy clustering. The higher number of vacancy clusters lowers the ionic mobility, which decreases the ionic conductivity.

Pure CeO₂ ceramic is a poor oxide ion conductor (∼0.24 × 10⁻³ S cm⁻¹ at 700 °C) since it lacks sufficient vacant sites for oxide ion transport, as shown in Fig. 3(a).^75–77 At lower temperatures, the reduction of Ce⁴⁺ to Ce³⁺ leads to the generation of more oxygen vacancies and electrons. These electrons act as polarons as they are localised to Ce³⁺; therefore, electron hopping between Ce³⁺ and Ce⁴⁺ leads to n-type conductivity, which reduces the open circuit voltage (OCV) to a large extent. This causes the electrolyte to be a mixed ionic-electronic conductor rather than a pure ionic conductor. Also, the larger ionic radius of Ce³⁺ (r_ionic = 1.01 Å) than Ce⁴⁺ (r_ionic = 0.87 Å) obstructs the migration of O²⁻ ions, thereby reducing the ionic conductivity in this temperature range.^78,79 Additionally, the electrolyte should be a purely ionic conductor; therefore, the presence of electrons in the electrolyte hinders its performance and poses a risk of short-circuiting. Unwanted reactions at the electrode–electrolyte interface lead to the degradation of the electrolyte, reducing the device's efficiency.⁸⁰

The ionic radii of the dopant cation significantly affect the ionic conductivity of the electrolyte. Doping with larger cation sizes can block the migration of vacancies, leading to higher E_a of ionic conduction. Thus, a better strategy is to have the dopant and host cation size comparable for better ionic conductivity. Additionally, to sufficiently increase the ionic conductivity of CeO₂, doping of lower valence cations is done to increase the oxygen vacancy concentration.⁸¹ When a lower valence cation (aliovalent) replaces a higher valence cation, oxide ion vacancies are introduced to maintain overall charge neutrality. This is when the fluorite structure of Ce⁴⁺ becomes distorted. It allows transportation of the oxide ions on thermal activation through the lattice by hopping from one crystal lattice site to its neighbouring vacant site, as shown in Fig. 3(b) and (c). Therefore, aliovalent doping is better than isovalent doping for enhancing oxide ion conductivity in CeO₂ due to defect engineering. Replacement by a dopant of different ionic radii (aliovalent) creates point defects (vacancies) in the system. These vacancies expedite the ionic conduction in CeO₂. Dopants cause unit cell expansion and contraction via altering the average size of cations and the lengthening or contracting of the cation–oxygen bond length, which are determined by the host's and dopant's oxidation states and their ionic radius. The unit cell expands or contracts due to both factors working together.²² A smaller ionic radius causes a compressive strain responsible for charge neutrality. Therefore, aliovalent doping in CeO₂ causes a local lattice strain, which facilitates the formation and migration of oxygen vacancies.

Aliovalent-doped ceria has received significant attention due to its superior ionic conductivity between 500 and 800 °C in air.^82,83 Sr²⁺ and Ca²⁺ doping improve the oxide ion conductivity of ceria-based electrolytes significantly. Sr²⁺ doped CeO₂, Ce_0.925Sr_0.075O_2−δ has a relative density, conductivity and power density of 97%, 6.46 × 10⁻³ S cm⁻¹ and 89 mW cm⁻², respectively, at 600 °C. Here, Sr²⁺ doping serves a dual purpose. It creates oxygen vacancies in CeO₂ and acts as a sintering aid.⁸⁴ Doping of Ce_0.8Sm_0.2O_2−δ with Bi³⁺, Mg²⁺ and Li⁺ results in a higher conductivity of 0.089 × 10⁻³, 0.39 × 10⁻³, and 0.13 × 10⁻³ S cm⁻¹ and power density of 716, 929 and 1097 mW cm⁻² in comparison to undoped Ce_0.8Sm_0.2O_2−δ, having a conductivity and power density of 0.11 × 10⁻³ S cm⁻¹ and 281.5 mW cm⁻², respectively. The higher conductivity and power density in the doped samples were attributed to the small amount of impurities in the ceria lattice.⁸⁵ Generally, alkaline earth dopants are cost-effective, readily available and eco-friendly. The small amount of these oxides works as sintering aids when used as dopants in CeO₂ electrolytes. However, their solid solubility is limited compared to rare earth dopants, leading to secondary phase formations. Additionally, larger ionic radii of some of the alkaline earth dopants cause the lattice to expand and decrease the ionic mobility.⁸⁶

Apart from alkaline earth dopants, there is extensive literature on doping ceria with rare-earth elements such as La³⁺, Nd³⁺, Gd³⁺, Y³⁺, Sm³⁺, Eu³⁺, etc., since they exhibit higher bulk ionic conductivities than other elements.^27,87–90 Gd-doped ceria (GDC) has been a popular electrolyte material since it has higher conductivity (10⁻² S cm⁻¹) than YSZ (10⁻³ S cm⁻¹) at 500 °C. This is because it reduces the reduction of Ce, which restricts electronic conduction. Also, the ionic radius of Ce⁴⁺ (r_ionic = 0.87 Å) is greater than that of Zr⁴⁺ (r_ionic = 0.72 Å); therefore, CeO₂ has a more open fluorite structure than YSZ. It leads to an increase in the Ce–O bond lengths and a decrease in the E_a, which is responsible for the increase in the ionic conductivity of CeO₂.^20,91–93 Similarly, the ionic radii of Gd³⁺ (r_ionic = 0.938 Å) and Sm³⁺ (r_ionic = 0.958 Å) are closer to that of Ce⁴⁺ (r_ionic = 0.87 Å); therefore, these dopants show maximum ionic conductivity with minimum lattice distortion due to size difference. The differences in ionic sizes of the dopants influence the localisation of oxygen vacancies by defect association, formation of secondary phases, etc., all of which modify grain and grain-boundary conductivity. The conductivity of divalent ions is less than that of trivalent ions because of the larger ionic radius mismatch between divalent ions and cerium ions (Ce⁴⁺).^20,94,95

Doping can be done only up to a specific concentration of the dopant. It happens for several reasons, such as vacancy clustering, vacancy repulsion and a rampant increase in the solid solubility limit. As reported in the literature, the ionic conductivity of doped ceria can reach a maximum only up to a certain extent of dopant concentration. After this limit, there is an increase in the defect interactions that gives rise to the clustering of vacancies, after which the mobile ions reduce, as shown in Fig. 3(d). Larger clusters require more atoms that need to be displaced for ionic motion. This requires larger E_a, and there is a decrease in the ionic conductivity.^96,97 The solid solubility limits of some commonly used dopants are given in Fig. 4. The maximum solubility limit is observed where the ionic radii of dopants are close to that of Ce⁴⁺, after which there is formation of secondary phases which are usually segregated along the grain boundaries, hindering the movement of mobile ions.^98–100

Fig. 4 Solid solubility limits of common dopants used in CeO₂ synthesised using the solid-state reaction and wet-chemical routes.^98–100

The following thermodynamic relations are responsible for governing the solubility limit of the dopants at equilibrium:

ΔG = ΔH − TΔS (7)

where G, H, T, S, R, and pO₂ refer to Gibbs free energy, enthalpy, temperature and entropy, gas constant and oxygen partial pressure, respectively. Δ refers to the changes in the thermodynamic quantities, and δ represents the oxygen non-stoichiometry. The slope and intercept of ln pO₂versus 1/T for a constant δ give ΔH and ΔS as a function of δ. The Gibbs free energy must be negative for a dopant to be completely soluble. This is because entropy always increases with temperature. Increased atomic mobility at elevated temperatures facilitates dopant diffusion and incorporation into thermodynamically favourable lattice sites, promoting equilibrium solubility. In contrast, low temperatures suppress diffusion kinetics, limiting dopant activation and incorporation into the crystal structure.^101–103

On the whole, the dopant type and concentration affect the electrolyte performance. Higher and lower ionic radius dopants introduce cationic and oxide ion vacancies, respectively. Dopants only up to a certain limit can be introduced in the system since higher concentrations lead to the formation of secondary phases that disrupt the conductivity of the electrolyte. Several factors affect the ionic conductivity of the electrolytes, many of which are discussed in the following sections.

3. Role of processing parameters in conductivity and other properties

The selection of starting materials is essential for better homogeneity of a mixture. It is imperative to understand the starting chemicals used for different synthesis methods. Generally, oxides and carbides are used as starting materials for the solid-state reaction and nitrates are used for the sol–gel (chemical wet) method. Chemicals used for synthesis can vary in purity depending on the studied property. For example, point defects (oxide vacancies) are introduced into the system for better ionic conduction. Raw materials with lower purity can provide the necessary defects in this case. Apart from the selection of raw materials, microstructural uniformity, particle size, grain growth, grain size and shape, density, sintering time, and temperature are also important for different synthesis methods.¹⁰⁴

For a homogeneous mixture, it is necessary to consider the grain size of the composition, which varies from one synthesis method to another. In powder methods, the mixing/ball milling of powders is done before sintering. The difference in grain size may be attributed to the particles' composition, which introduces defects and internal stress to their structure.¹⁰⁵ The effect of calcining composition, Li_6.4Fe_0.2La₃Zr₂O₁₂, using the solid-state and sol–gel methods was studied by Paulus et al.¹⁰⁶ A mixture of the cubic and tetragonal structures was observed in the solid-state reaction method due to the inhomogeneous distribution of iron in the sample. However, samples calcined using the sol–gel method yielded homogeneous, single-phase cubic structures. Calcination by sol–gel also decreased the sintering time by 2 hours. The sample exhibited a total ionic conductivity of 1.82 × 10⁻³ S cm⁻¹ at 25 °C, the highest for garnet-related materials. Therefore, the exact composition synthesised using different techniques yields different results.

Control over the grain size is essential for the dielectric response of CeO₂. There are several ways to control the grain size: ball-milling, varying sintering temperature and doping.¹⁰⁷ Smaller grains have a more intense dielectric response. Due to the greater concentration of grain boundaries in smaller grains, the effect of grain size is primarily caused by increased surface stress.^108,109 Cationic diffusion governs the grain boundary mobility through an interstitial mechanism that is influenced by oxygen vacancies.¹¹⁰ Densification occurs by grain boundary diffusion rather than lattice diffusion. Sintering is dependent on the reduction of the excess surface energy, in which the following equation gives the flux of atoms (J) along a grain boundary:

J = MC∇μ (10)

where M is the mobility of the atoms along the grain boundary, C is the vacancy concentration, and ∇μ is the gradient in the chemical potential between the atoms in the materials and the neck of adjacent particles.^111,112 Dopants that increase any of these parameters lead to lower sintering temperatures.

Processing parameters play an important role in modifying various properties of ceramic materials.¹¹³ The dopant composition, the thickness of the electrolyte, the microstructure, the sintering temperature, etc., affect the cell's overall performance as illustrated in Fig. 5. The shape and size of grains potentially affect the interaction of dopant-oxygen vacancies, affecting ion migration and ionic conductivity. The microstructure can have regularly or irregularly arranged grains of different shapes, such as hexagonal, polygonal, globular, etc. Finer grain sizes lead to more grain boundaries necessary for ionic conduction. As reported in the literature, CeO₂ generally deviates from the stoichiometric composition by releasing gaseous oxygen; this process could also be monitored from the morphology of the samples (bubbles on the surface).^114,115 Surface modifications (synthesis techniques and sintering temperature) can significantly enhance the ionic conductivity of CeO₂ without using dopants. Hydrogen treatment leads to the formation of chemical defects in CeO₂, facilitating the reduction of Ce⁴⁺ to Ce³⁺. Surface-modified CeO_2−δ shows a conductivity and power density of 0.1 S cm⁻¹ and 660 mW cm⁻² at 550 °C.¹¹⁶

Fig. 5 Factors that increase the ionic conductivity of CeO₂-based electrolytes.

Synthesis methods play an essential role in the densification of the microstructure, affecting the conducting properties of SOFCs.¹¹⁷ Different methods have been used to synthesise doped CeO₂, such as co-precipitation, hydrothermal, sol–gel and solution combustion.^117–123 A comparative diagrammatic representation of the high- and low-temperature synthesis methods is given in Fig. 6. Synthesis of calcium-doped ceria electrolyte using two different synthesis methods, the solid-state reaction and sol–gel, resulted in varying microstructures. The sol–gel method resulted in well-defined homogeneous grains compared with the solid-state reaction method. Furthermore, the samples synthesised using sol–gel exhibited better conductivity than those synthesised by the solid-state reaction method. Therefore, the low-temperature synthesis method yielded better results.¹²⁴

Fig. 6 Comparative representation of the high- and low-temperature synthesis methods.

Sol–gel, co-precipitation, hydrothermal, etc., are some low-temperature synthesis methods. Sol–gel is based on the polymerisation of molecular precursors. It involves the preparation of inorganic polymers or ceramics from a solution through a transformation from liquid precursors into a sol and then into a network structure called a gel. This high-purity process leads to a homogenised composition.^125,126 Smaller crystallite size, the low energy band gap value, and the homogeneous distribution of Sm³⁺ in the ceria lattice, with its high ionic mobility, accounted for the high conductivity of (CeO₂)_0.96Sm_0.04 solid electrolyte synthesised by the sol–gel process.¹²⁷

CeO₂-based electrolytes exhibit rapid grain growth when sintered at high temperatures. This leads to poor mechanical stability, partial reduction of Ce⁴⁺ to Ce³⁺, undesirable chemical expansion, high energy cost, phase diffusion and chemical interaction between the components, thus affecting the electrochemical performance of the cell.^49,128 Therefore, there has been a shift in the synthesis of CeO₂ towards lower-temperature methods. The literature reports far more publications on the low-temperature synthesis of CeO₂ electrolytes than the high-temperature methods. Low-temperature methods are generally wet chemical methods. They differ from powder methods in that the starting materials, in this case, are synthetically derived. This reduces the additional step of removing the impurities. Synthetically derived materials have uniform grain sizes and high purity without any agglomerates.

Accardo et al. reported that the conductivity of 20 mol% of GDC is 1.9 × 10⁻² to 5.5 × 10⁻² S cm⁻¹ at 600 °C and 800 °C, respectively. However, doping of Bi³⁺ increased the conductivity of GDC to 3.1 × 10⁻²-1.7 × 10⁻¹ S cm⁻¹ between 600 and 800 °C. The dense microstructure of the GDC pellets is composed of highly packed spherical grains due to the decrease in particle size (∼30 and 28 nm).^117,129 In another case, GDC powders were prepared using three complexing agents/fuels (ethylene glycol, glycerol, and tartaric acid) with sinterability strongly dependent on the microstructure, which in turn depended on the processing route.¹³⁰ Low-temperature methods usually result in crystallite and grain sizes in the nanometer and micrometre range, respectively. Smaller crystallite sizes favour better crystallinity with densities >90%. Single-phase Ce_0.8Sm_0.2O_1.9 synthesised by sol–gel with gelatin as a polymerising agent exhibited a dense, regular polygonal microstructure with a density, crystallite and particle size of > 97%, 12 nm and 15 μm, respectively. The conductivity of the samples was in the order of 10⁻³ S cm⁻¹, and the E_a was ∼0.94 eV at 600 °C.¹³¹

In the case of doped ceria electrolytes, high ionic conductivity is essentially due to low E_a, which, in turn, is related to an optimal balance between elastic and electronic coulombic defect interactions. The Arrhenius curve is instrumental in identifying the E_a based on the temperature range. As shown in Fig. 7, the Arrhenius curve is fitted separately with two lines, one for the high-temperature range and the other for the low-temperature range. The low-temperature line is slightly curved since the vacancies are temperature-dependent. In the low-temperature range, the E_a is the sum of enthalpies, association (ΔH_a) and migration (ΔH_m) for conduction, while for the high-temperature range, it is equal to only the ΔH_m. This is because most oxygen vacancies are free to migrate in the high-temperature region. It is worth noting that for electrolytes to work in the low-temperature region, ΔH_a should be a minimum.^132–135

Fig. 7 Representative Arrhenius curve. The dotted line represents the change in the type of conduction.

Other processes, such as microwave sintering, can further enhance densification. In a particular case, GDC nanoparticles with lithium as an additional dopant were used as a liquid-phase sintering aid during synthesis by the co-precipitation method. Furthermore, microwave sintering resulted in a decrease in the sintering temperature to 600 °C. The grain sizes of the Li-doped samples exhibited a grain size of 150 nm compared to pristine GDC (30 nm). At 600 °C, Li-doped samples showed an ionic conductivity of 1 × 10⁻² S cm⁻¹ in air with an E_a of 0.53 eV.¹³⁶ As discussed before, the partial electronic conductivity at high temperatures is related to the reduction of Ce⁴⁺ to Ce³⁺. The electrons thus created participate in the conduction process through the small polaron hopping mechanism. The decrease in the OCV due to this partial electronic conductivity also lowers the cell's overall efficiency and might lead to an electrical short-circuit. This is a paramount concern when synthesising doped ceria electrolytes at high temperatures. Therefore, many researchers are synthesising doped ceria electrolytes at low temperatures. Out of the various synthesis methods listed in Table 1, sol–gel and auto-combustion methods result in highly dense (≥95%) electrolytes with good conductivity values (∼10⁻¹ S cm⁻¹).

Table 1 Comparative analysis of different ceria-based electrolytes

Composition	Synthesis method	ρ rel (%)	σ (S cm^−1)	E a (eV)	References
Ce0.8Gd0.2O1.95	Combustion	—	3.34 × 10^−3 (680 °C)	—	105
Ce0.9Gd0.1O1.95	Sol–gel combustion	95	1.67 × 10^−2 (600 °C)	0.81	92
Ce0.8Ca0.2O1.95	Combustion	—	6.90 × 10^−4 (680 °C)	—	105
Ce0.9Dy0.1O2−δ	Solid state reaction	—	1 × 10^−1.91 (650 °C)	0.71	115
Ce0.8Nd0.2O1.9	Molten salt	92	0.15 × 10^−2 (600 °C)	0.87	142
Ce0.83Er0.17O2	Sol–gel	—	1.57 × 10^−3 (700 °C)	—	143
Ce0.8Yb0.2O1.9	Solid state reaction	94	1.30 × 10^−2 (800 °C)	1.07	144
Ce0.9Pr0.1O2	EDTA citrate	—	1.21 × 10^−2 (700 °C)	1.28	109
Ce0.9La0.1O2−δ	Auto-combustion	—	1.01 × 10^−2 (750 °C)	0.70	145
Ce0.8La0.2O2−δ	Co-precipitation	—	0.81 × 10^−2 (650 °C)	0.86	165
Ce0.8Zr0.2O2−δ	Co-precipitation	—	0.15 × 10^−2 (650 °C)	0.88	158
Ce0.7Sm0.3O1.85	Solid state reaction	97	1.6 × 10^−3 (500 °C)	0.92	146
Ce0.8Sm0.2O1.95	Combustion	—	8.32 × 10^−3 (680 °C)	—	105
Ce0.8Sm0.2O2−δ	Sucrose-pectin modified sol–gel	95	1.0 × 10^−2 (600 °C)	0.89	134
Ce0.8Sm0.2O1.9	Solid state reaction	97	5 × 10^−2 (800 °C)	1.03	147
Ce0.95Sm0.05O1.95	Auto-combustion	93	3.26 × 10^−3 (500 °C)	0.87	148
Ce0.96Sm0.04O1.92	Sol–gel	—	0.17 × 10^−2 (500 °C)	—	127
Ce0.8Gd0.10Pr0.10O1.90	Citric acid nitrate combustion	94	5.1 × 10^−2 (750 °C)	0.58	135
Ce0.8Y0.18La0.02O2−δ	Sol–gel	97	5.7 × 10^−2 (800 °C)	0.87	150
Ce0.6Zr0.2La0.2O2−δ	Co-precipitation	—	0.32 × 10^−2 (650 °C)	0.87	165
Ce0.8Nd0.10Mg0.10O2−δ	Sol–gel	98	26.81 × 10^−2 (450 °C)	0.74	152
Ce0.8Sm0.05Mg0.15O2−δ	Solid state reaction	96	1.18 × 10^−2 (600 °C)	0.60	153
Ce0.8Er0.1Gd0.1O2	Sol–gel	—	2 × 10^−3 (700 °C)	—	165
Ce0.875Gd0.1Sr0.025O1.925	Sol–gel combustion	96	1.20 × 10^−2 (600 °C)	0.96	154
Ce0.65Sm0.2Bi0.15O1.825	Co-precipitation	94	5.6 × 10^−2 (800 °C)	0.83	155
Ce0.80Ba0.10Ga0.10O3−δ	Co-precipitation	—	7.1 × 10^−2 (650 °C)	0.46	156
Ce0.76La0.08Pr0.08Sm0.08O2−δ	Sol–gel auto combustion	98	1.4 × 10^−2 (500 °C)	0.76	151
Ce0.80Sm0.10Er0.05Ba0.05O2−δ	Solid state reaction	89	1.86 × 10^−2 (800 °C)	—	157
Ce0.76Pr0.08Sm0.08Gd0.08O2−δ	Sol–gel auto combustion	95	1.86 × 10^−2 (600 °C)	0.56	158
Ce0.76Pr0.08Sm0.08Gd0.08O2−δ	Microwave-assisted sol–gel auto-combustion route	98	3.47 × 10^−2 (600 °C)	0.69	159
Ce0.82La0.06Sm0.06Gd0.06O2−δ	Sol–gel	91	3.8 × 10^−2 (600 °C)	0.59	160
Ce0.8Sm0.2O1.9. (CuO)1.0	Combustion	96	1.46 × 10^−2 (600 °C)	0.59	149
[Ce0.82La0.06Sm0.06Gd0.06O2−δ-(Li– Na)2CO3]	Sol–gel	86	4.2 × 10^−1 (600 °C)	1.51	161
[Ce0.76La0.08Pr0.08Sm0.08O2−δ-(Li–Na)2CO3]	Solid state reaction	87	4.6 × 10^−1 (600 °C)	1.03	169

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Sometimes, liquid phase formation occurs at high temperatures, where a liquid phase is formed during the sintering process that coexists with the solid particles. The liquid during the liquid-phase sintering eliminates the solid–vapour interface. With further sintering, the driving force for the densification results from the decreasing liquid–vapour pore surface area.¹³⁷ Sintering aids such as transition metals (cobalt, copper, lithium and iron) are used to increase the mobility of the atoms along the grain boundaries, promoting liquid phase sintering.^111,138,139 A comparison of the sintering aids (Li, Co, Cu, and Fe) used in samarium-doped CeO₂ (SDC) concluded that CuO was the best, as it lowered the sintering temperature between 750 and 1100 °C. CuO also has better relative density and maximum shrinkage rate than other electrolytes. Cobalt-sintered SDC had a more prominent grain size, while FeO had better conductivity than the others.¹⁴⁰ The impact of different dopants (Li, Co, Fe and Mg) on the sintering temperature of Ce_0.9Pr_0.1O₂ showed that the dopants had a different impact on the sintering temperature. Li and Co reduced the sintering temperature, Fe increased it, and Mg had little effect on the sintering temperature. The reduction in the sintering temperature resulted in rapid densification due to the formation of a liquid phase in the grain boundaries of the sample. This is attributed to the diffusion of the liquid phase under capillary action and the rearrangement of grains during the sintering process.⁴⁹ Ca doping also acts as a suitable sintering aid, and doping with an appropriate ratio of Gd/Ca also enhances conductivity. Ce_0.8Gd_0.12Ca_0.06O_1.87 exhibits a relative density of >95% when sintered at 1400 °C, denser than the ceria sintered at 1600 °C. This lowering of sintering temperature is due to grain refining, the modified gel-casting synthesis route, and Ca as a sintering aid. The samples had a conductivity and an E_a of 0.082 S cm⁻¹ and 0.786 eV, respectively, at 800 °C. The conductivity increases with increasing Ca content due to the majority of oxygen vacancies being fluid.¹⁴¹

Each cell component must be chemically and thermally stable. All the SOFC components must have good chemical compatibility; there should be no or negligible mismatch. If this mismatch persists, it will lead to cell degradation. Also, the components must be stable in both oxidising and reducing oxygen partial pressure atmospheres. During SOFC fabrication, the long duration of operation often leads to mechanical and thermal stress. However, the addition of dopants improves the mechanical strength and leads to an increase in CTE. Therefore, research is needed to balance these properties to avoid any mismatch during operation. The mismatch in CTE among various SOFC components leads to interface delamination; in fact, multiple SOFC components form different interfaces, as shown in our recent review article.¹⁰ Also, the increase in the oxygen vacancies contributes to weakening the binding energy, which increases CTE. In the planar design of SOFCs, electrolytes form interfaces with the cathode, anode and sealants. If the CTE largely differs in every component, a SOFC device will not form. However, with the proper selection of dopants, CTE can be tailored accordingly. In the case of anode-supported SOFCs, the CTE difference between electrolyte and the anode substrate leads to a significant thermoelastic bending of the cells that causes cell fracture in stack assembling.^161–163 La-doped CeO₂ exhibited a CTE of 13.4 × 10⁻⁶ per °C without compromising the conductivity (0.81 × 10⁻² S cm⁻¹ at 650 °C).¹⁶⁴ However, tri-doped ceria, Ce_0.76Pr_0.08Sm_0.08Gd_0.08O_2−δ, exhibits comparable CTE (13.25 × 10⁻⁶ per °C) between 30 and 800 °C.¹⁵⁸

Mechanical properties must be checked and verified thoroughly for any fuel cell design under all conditions, i.e., operation, start-up, and shutdown. Wettability, joint strength, fracture toughness, elastic modulus, and hardness are some pointers to be checked for SOFC materials. The change in the crystal structure affects the elastic modulus and stiffness of SOFC materials.^165–168 The correlation between electrical and mechanical properties of the La_0.90Sr_0.10Ga_0.8Mg_0.2O_3−δ and La_0.85Sr_0.15Ga_0.8Mg_0.2O_3−δ ceramics showed that significant segregation of secondary phases occurs at the grain boundaries. This led to a substantial drop in the hardness and grain boundary conductivity, which decreased the total ionic conductivity.¹⁷⁰ An increase in sintering temperature facilitates enhanced grain growth, reducing the grain boundary area per unit volume. Consequently, these grains are less resistant to localised plastic deformation and exhibit poorer hardness.¹⁷¹

Enhancement of strength over that of zirconia would lower the likelihood of fracture and enhance ionic conductivity at the same thickness, which would lower ohmic loss. Materials such as ceria would enable highly dependable and energy-efficient SOFCs.¹⁷² It has been observed that ceria-based materials exhibit slightly lower hardness and toughness values compared with commonly used electrolyte materials such as 8-YSZ (12.83 GPa and 2.73 MPa√m), as shown in Table 2.^173–176 The mechanical strength can be improved without compromising the electrical conductivity by reduction-annealing the sample. In such a case, a compressed surface layer is formed by the chemical expansion that occurs during contact reduction of its surface.¹⁷⁷ Doping CeO₂ enhances the fracture toughness of the solid solution by promoting a greater extent of stress-induced tetragonal to monoclinic phase transformation.¹⁷⁸

Table 2 A comparison of hardness and fracture toughness of some of the ceria-based electrolytes with those of 8-YSZ

Composition	Hardness (GPa)	Fracture toughness (MPa√m)
8-YSZ	12.83	2.73
GDC	5.46	1.16
Ce0.8Y0.2O2	7.9	2.16
Ce0.8Sm0.2O2	8.2	2.3
Ce0.8Sm0.1Y0.1O2	8.34	2.28
Gd0.1Ce0.89Ge0.01O0.01	10.85	3.18
Ce0.9Sm0.1O1.95	5.37	2.95

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4. Approaches to improve conductivity and cell parameters

The reduction of Ce⁴⁺ to Ce³⁺ under low oxygen partial pressures leads to high electronic conductivity; therefore, they show low OCV and low efficiency due to the leakage current. Thus, there are different ways through which these problems could be solved. Some of the suggestions to solve the above issues related to ceria-based electrolytes are given in the following points:

(1) Nanomaterials and thin films can effectively improve the properties of ceria-based electrolytes. It is generally observed that compared to conventional single crystalline and polycrystalline materials, the ionic conductivities of nanocrystalline materials such as cubic zirconia, ceria and titania are much higher. It is reported that CeO₂ nanoparticles under high pressure exhibit an ionic conductivity of 3 × 10⁻³ S cm⁻¹ at 300 °C under 5 GPa for 6 mol% SDC nanoparticles.^179,180 Apart from nanoparticles, GDC thin film electrolytes have a higher power density than YSZ electrolytes. This is due to the higher conductivity of the GDC electrolyte against that of the YSZ electrolyte and the expansion of the electrochemical reaction zone. The improved ionic conductivity increases the oxygen transport rate at the electrolyte–electrode interface, which reduces the electrode overpotential. The surface area availability increases significantly in nanoparticles compared to bulk materials.^181–183

(2) Nanocomposite electrolyte materials have the potential for use in LT-SOFCs due to their high ionic conductivity at low temperatures and low cost. Fundamentally, a typical nanocomposite (CeO_2−δ/CeO₂, GDC/CoFe₂O₄, YSZ/SrTiO₃, etc.) consists of a core–shell type structure on a nano-scale. It has a core (ceria) and a salt (carbonate or another oxide) that develops a shell layer covering the core. The functionality of nanocomposites is determined by the interfaces between the constituent phases, which lead to fast ionic transport at the interfaces. Different ceria-based nanocomposites, such as ceria-carbonate, ceria-halide, ceria-sulphate, ceria-hydroxide, ceria-alumina and ceria-oxide, have been of significant interest for use as SOFC electrolyte materials.^{116,184–186} Generally, ionic conductivity and power density of ceria-carbonate nanocomposites have been reported to be >0.1 S cm⁻¹ at 300 °C and ∼1000 mW cm⁻² between 450 and 500 °C, respectively.¹⁸⁷ At slightly higher temperatures (600–650 °C), the protonic conductivity of Sm_0.2Ce_0.8O₂–Na₂CO₃ is reported to be ∼0.044 S cm⁻¹ with a power density of 281.5 mW cm⁻². These results are much higher than those of single-phase oxide proton-conducting electrolytes.¹⁸⁸ Shah et al. observed that a 10% coating of Na₂CO₃ on GDC generates appreciable O²⁻ vacancies compared to GDC.¹⁸⁰ This led to high power density (968 mW cm⁻²) and high ionic conductivity (0.2 S cm⁻¹ at 520 °C) with sufficient OCV (1.013 V). It was due to the formation of a composite core–shell heterostructure between GDC and amorphous Na₂CO₃. The formation of a junction suppressed the electronic conduction while enhancing the ionic transportation through the electrolyte membrane. A binary composite, (Li/Na)₂CO₃ – SDC, exhibited a high conductivity and cell performance of 0.31 S cm⁻¹ and 617 mW cm⁻², respectively, at 600 °C since the interface acted as a superionic highway, which enabled the transportation of ions.¹⁸⁹ Composites with SDC reach power densities as high as 640 and 760 mW cm⁻² at low temperatures of 500 and 550 °C, respectively.¹⁹⁰ In another study, the addition of a Ni_0.8Co_0.15Al_0.05Li membrane into Ce_0.8Sm_0.2O_2−δ-Na₂CO₃ electrolyte eliminated the polarisation between different interfaces and resulted in a high-power density of 1072 mW cm⁻² at 550 °C.¹⁹¹ Compared to pure NaFeO₂, composites such as CeO₂-coated NaFeO₂ have a power output of 727 mW cm⁻² and an OCV of 1.06 V at 550 °C. This is due to the hetero-interfaces between NaFeO₂ and CeO₂ that provide a fast oxide ion conducting path and to CeO₂ that creates more oxygen vacancies for protonic transportation. Ionic conduction through the interface is much easier than structural bulk conduction. Principally, point defects in ceria-based materials are responsible for originating ionic charge carriers. Therefore, in LT-SOFCs, interface conduction should be promoted as a new approach for ion-conducting electrolytes.^192,193

(3) Another solution is to make use of electron-blocking layers. A BaCeO₃–Ni-based composite anode achieves higher OCV due to the reaction of diffused Ba ions with doped ceria electrolyte during the sintering process, forming an electron-blocking interlayer. This interlayer eliminates the problem of internal short-circuiting in doped ceria electrolytes. Apart from BaCeO₃, ZrO₂, Bi₂O₃, and SrCeO₃ are used as electron-blocking layers.^79,194 Another way to solve the short-circuiting problem is to use the bi- and tri-layer electrolyte strategy. The bi-layer electrolyte strategy uses a pure oxygen ion-conducting YSZ layer to block electron conduction. On the other hand, the tri-layer electrolyte system consists of a GDC layer on the anode side, a YSZ electron-blocking layer in the middle and a second GDC buffer layer on the cathode side.¹⁹⁵ In a tri-layer electrolyte system, the first dense GDC electrolyte is fabricated by co-sintering a thin, screen-printed GDC layer with an anode support (NiO-8YSZ substrate and a NiO-GDC anode). In contrast, the two electrolyte layers are deposited via physical vapour deposition. For a tri-doped electrolyte system, the electrolyte resistance is only 0.01 Ω cm⁻² with a power density of 1.2 W cm⁻² at 650 °C.¹⁹⁵ However, problems with low ionic conductivity, low chemical and mechanical stability, undesirable solid solutions, and thermal mismatch of the multilayer electrolytes pose issues in utilising the electron-blocking layer method.¹⁹⁴

(4) A new approach was proposed wherein surface doping of Al³⁺ into Ce⁴⁺ created surface defects and surface O²⁻ vacancies at the interface of CeAlO₂. This approach enhanced the ionic conductivity (0.19 S cm⁻¹) and power density (1020 mW cm⁻²) at 520 °C. It was observed that surface doping required band alignment between CeO₂ and CeAlO₂ due to the difference in the Fermi level. This established a space charge region constituting a built-in field, enhancing charge transportation and minimising e-conduction.¹⁹⁶ Co-doping and tri-doping have effectively been observed to reduce the sintering temperature and enhance the conductivity of ceria-based electrolytes, as observed from the data listed in Table 1. They generate higher oxygen vacancies than single-cation doping, resulting in higher diffusion due to minimum distortion. Also, ceria-based composites have high conductivities.

Based on the solutions mentioned above, it can be concluded that ceria-based materials show enhanced properties when used in doped, composites, bilayered and heterostructured composite forms. Nanocomposites, electron-blocking layered structures and surface doping in CeO₂ resulted in very high power densities of >1000 mW cm⁻², thereby increasing the long-term stability of these oxides.

5. Conclusion and future direction for research

Solid oxide fuel/electrolyser cells (SOFCs/SOECs) are versatile and multipurpose devices that produce electricity, water, hydrogen and oxygen for different applications. The electrolyte is an essential and integral part of an SOFC. The present article discusses the different aspects that affect the ionic conductivity of ceria-based electrolytes for LT- and IT-SOFCs to make them cost-effective and increase their commercial viability. Gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC) are very good and suitable ionic conducting electrolyte materials for LT- and IT-SOFCs due to the comparable ionic radii of Gd³⁺ (r_ionic = 0.938 Å) and Sm³⁺ (r_ionic = 0.958 Å) to Ce⁴⁺ (r_ionic = 0.87 Å). The solid solubility limit of both dopants is the highest compared to that of other dopants. However, a high concentration of dopants leads to secondary phase formations and clustering of oxygen vacancies, reducing ionic conductivity.

Different approaches can be used to increase the solid solubility of the dopants, optimise the microstructure and prevent the reduction of Ce⁴⁺ to Ce³⁺. Surface engineering, chemical routes and thin film technology can be used to address these issues. Apart from the processing methods, different mechanisms such as utilising multi-dopants, nanocomposites, electron-blocking layers, and CeO_2−δ/CeO₂ composites improve the conductivity of ceria-based electrolytes significantly. The densification of the electrolytes can be achieved by using sintering aids, particularly the addition of small amounts of alkali and alkaline earth metal oxides during processing. Alkali metal oxides increase the liquid phase sintering and wettability of CeO₂, while alkaline earth metal oxides avoid the reduction of cerium. As a future prospect, double dopants (alkali and alkaline earth oxides) could be beneficial to achieve high density with a reduced chance of cerium reduction in doped CeO₂ electrolyte. Additionally, studies on small quantities of nano-size sintering aids and sintering of composites, i.e. two suitable ionic conducting materials (doped CeO₂-bismuth vanadate, doped CeO₂–LaAlO₃, etc.) at appropriate temperatures and durations can be synthesised to obtain highly dense electrolytes for LT- and IT-SOFCs. Using double-layered electrolytes such as GDC-YSZ can also be a good approach to reduce interface-related issues with improved conductivity. Proton-conducting electrolytes could also be explored as electrolytes for intermediate temperature SOFCs/SOECs.

Data availability

The datasets used to support the findings of this study are available in the referenced publications.

Author contributions

Paramvir Kaur – conceptualization; data curation; formal analysis; visualization; writing-original draft. K. Singh – conceptualization; funding acquisition; supervision; validation; visualization; writing-review & editing.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors acknowledge the funding received from the Department of Science and Technology (DST), Government of India, through the Hydrogen and Fuel Cell (HFC)-2018 scheme [Project no. DST/TMD/HFC/2k18/123].

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