Comparative Electrochemical Performance of Solid Oxide Fuel Cells: Hydrogen vs. Ammonia Fuels—A Mini Review

Abstract

Solid oxide fuel cells (SOFCs) have garnered significant attention as a promising technology for clean and efficient power generation due to their ability to utilise renewable fuels such as hydrogen and ammonia. As carbon-free energy carriers, hydrogen and ammonia are expected to play a pivotal role in achieving net-zero emissions. However, a critical research question remains: how does the electrochemical performance of SOFCs compare when fuelled by hydrogen vs. ammonia, and what are the implications for their practical application in power generation? This mini-review paper is premised on the hypothesis that while hydrogen-fuelled SOFCs currently demonstrate superior stability and performance at low and high temperatures, ammonia-fuelled SOFCs offer unique advantages, such as higher electrical efficiencies and improved fuel utilisation. These benefits make ammonia a viable alternative fuel source for SOFCs, particularly at elevated temperatures. To address this, the mini-review paper provides a comprehensive comparative analysis of the electrochemical performance of SOFCs under direct hydrogen and ammonia fuels, focusing on key parameters such as open-circuit voltage (OCV), power density, electrochemical impedance spectroscopy, fuel utilisation, stability, and electrical efficiency. Recent advances in electrode materials, electrolytes, fabrication techniques, and cell structures are also highlighted. Through an extensive literature survey, it is found that hydrogen-fuelled SOFCs exhibit higher stability and are less affected by temperature cycling. In contrast, ammonia-fuelled SOFCs achieve higher OCVs (by 7%) and power densities (1880 mW/cm² vs. 1330 mW/cm² for hydrogen) at 650 °C, along with 6% higher electrical efficiency. Despite these advantages, ammonia-fuelled SOFCs face challenges such as NO_x emissions, nitride formation, environmental impact, and OCV stabilisation, which are discussed alongside potential solutions. This mini review aims to provide insights into the future direction of SOFC research, emphasising the need for further exploration of ammonia as a sustainable fuel alternative.

1. Introduction

Renewable energy has been a topic of discussion for decades; researchers have been exploring ways to store and generate energy efficiently. There are various renewable energy sources, the most common being wind power, solar energy, hydroelectric energy, and bio-energy [1]. While these sources have significantly reduced reliance on fossil fuels, they are not without limitations. Intermittency, energy storage challenges, and geographical constraints often hinder their widespread adoption and efficiency [2]. In contrast, fuel cells present a promising alternative for power generation, offering excellent efficiency and low carbon emissions [3]. Fuel cells are electrochemical cells that use the chemical energy of a fuel to produce clean electricity. They depend on a continuous fuel supply and an oxidant to the cell’s electrodes, enabling sustained energy production. Among the various types of fuel cells—such as alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), polymer electrolyte membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) SOFCs have emerged as an up-and-coming technology due to their high efficiency, fuel flexibility, and ability to operate at high temperatures [4].

The global urgency to decarbonize energy systems and mitigate climate change has intensified the search for sustainable energy solutions. Currently, fossil fuels dominate the global energy mix and are the primary contributors to greenhouse gas emissions, driving global warming and environmental degradation [5]. Renewable energy sources, while cleaner, face inherent limitations. For instance, solar and wind energy are intermittent and require large-scale energy storage systems to ensure a stable power supply, which remains a significant technological and economic challenge [6]. Hydroelectric power, though reliable, is limited by geographical and environmental constraints, while bioenergy raises concerns about land use and competition with food production [7]. In this context, fuel cells, specifically SOFCs, offer a compelling alternative. They can operate on various fuels, including hydrogen (H₂) and ammonia (NH₃), both of which are considered key players in the transition to a low-carbon economy [8]. Hydrogen produces only water when used in fuel cells, making it an ideal candidate for decarbonization. However, its low energy density and challenges related to storage and transportation have limited its widespread adoption [9]. Ammonia, on the other hand, has recently gained attention as a viable alternative. It has a higher energy density than hydrogen, can be easily liquefied for storage and transport, and does not produce carbon emissions when used in SOFCs [10]. Moreover, ammonia can be synthesised using renewable energy sources, making it a sustainable fuel option. Ammonia and hydrogen, as SOFC fuels, differ in their environmental impact. For instance, hydrogen undergoes an electrochemical oxidation reaction with fast kinetics, producing only water vapour as a by-product, with no environmental consequences. On the other hand, ammonia can decompose into hydrogen and nitrogen before H₂ electrochemical oxidation. Alternatively, it can follow direct NH₃ electro-oxidation to generate NO_x, and H₂O as by-products [11]. Thus, the product stream may be composed of unconverted ammonia due to incomplete cracking reaction, especially at low temperatures, and NO_x gases, which harm the environment. For example, NO_x is the precursor for harmful pollutants, including ground-level ozone and aerosols [12,13]. However, optimising anode materials and operating conditions can improve NH₃ conversion levels and mitigate NO_x emissions. The comparison of SOFC performance using these two fuels is essential, as it could pave the way for more efficient and practical energy systems.

Fuel cells, including SOFCs, contribute to decarbonization by enabling the use of clean fuels and reducing reliance on fossil fuels. Unlike combustion-based power generation, fuel cells operate through electrochemical reactions, which are more efficient and produce fewer pollutants [14]. SOFCs, in particular, are highly efficient as they can utilise waste heat for cogeneration, further enhancing their overall energy efficiency [15]. Additionally, their ability to operate on a wide range of fuels, including hydrocarbons, hydrogen, and ammonia, makes them versatile and adaptable to different energy scenarios. This flexibility is crucial for transitioning to a sustainable energy future, as it allows the implementation of renewable fuels into existing energy infrastructures. Despite their advantages, fuel cells face challenges that must be addressed to truly understand their full potential. High operating temperatures, material degradation, and high costs are some of the key limitations of SOFCs [16]. However, ongoing research is focused on developing new materials, improving cell durability, and reducing manufacturing costs. For instance, advancements in ceramic materials and nanostructured electrodes have shown promise in enhancing the performance and longevity of SOFCs. Furthermore, innovative theoretical frameworks and computational models are being developed to optimise fuel cell design and operation, bridging the gap between previously unrelated research areas such as materials science, electrochemistry, and thermodynamics [17].

This mini-review paper aims to comprehensively compare the electrochemical performance of SOFCs fuelled by ammonia and hydrogen, highlighting their respective advantages, challenges, and potential applications for power generation and contributing to the global effort to combat climate change by advancing clean energy technologies. By examining key performance metrics such as open-circuit voltage (OCV), power density, fuel utilisation, stability, electrical efficiency, and electrochemical impedance spectroscopy (EIS), this study provides insights into the most effective strategies for decarbonising energy sectors and advancing clean energy technologies to combat climate change. This review addresses the fundamentals of SOFCs, including oxygen-conducting (O-SOFC) and hydrogen-conducting (H-SOFC) systems, while discussing the functions of electrolytes and electrodes and providing examples of commonly used materials. Moreover, the influence of operating temperature and cell/stack load would affect the round-trip efficiency for ammonia and hydrogen-fuelled SOFCs differently. Furthermore, the paper highlights the role of SOFCs in addressing the limitations of current renewable energy sources and proposes directions for future research. For instance, developing low-temperature SOFCs and exploring alternative fuels such as biogas and synthetic fuels could open new pathways for sustainable energy production [18,19]. Finally, this mini review offers future perspectives and advances in the design, fabrication, and synthesis of SOFC components and their potential application for direct ammonia and hydrogen SOFCs.

2. Solid Oxide Fuel Cells

2.1. O-SOFCs and H-SOFCs

SOFC is a promising technology for power generation due to its high efficiency, low CO₂ emissions, long lifetime, and fuel flexibility. A typical SOFC consists of a cathode, anode, and electrolyte, as shown in Figure 1, where fuel would enter the anode, and air would be fed to the cathode. The differences lie between an oxygen ion-conducting electrolyte-based SOFC (O-SOFC) and a proton-conducting electrolyte-based SOFC (H-SOFC). In O-SOFC, the oxygen ions transport from the cathode to the anode to react with hydrogen and produce water. However, in H-SOFC, hydrogen ions transport from the anode to the cathode via the electrolyte to react with oxygen ions to produce water vapour. The electrons are generated through a hydrogen oxidation reaction and then transferred from the anode to the cathode to produce electricity, as shown in Figure 1 and

Table 1. The set of reactions taking place at the anode and cathode chambers of an O-SOFC and H-SOFC fuelled with hydrogen and ammonia fuels.

	Anode		Cathode
	Hydrogen	Ammonia
O-SOFC	H2 + O2− → H2O + 2e−	2NH3 → N2 + 3H2 H2 + O2− → H2O + 2e− or 2NH3 + 5O2− → 3H2O + 2NO + 10e− or 2NH3 + 3O2− → N2 + 3H2O + 6e−	½ O2 + 2e− → O2−
H-SOFC	H2 → 2H+ + 2e−	2NH3 → N2 + 3H2 H2 → 2H+ + 2e−	½ O2 + 2H+ + 2e− → H2O

.

The process of generating electricity in H-SOFCs and O-SOFCs, when fuelled with different fuels, such as hydrogen and ammonia gas, differs at the anode. For instance, the anode of O-SOFCs fuelled with hydrogen gas in Figure 1A results in hydrogen gas reacting with oxygen ions to produce water and electrons. Meanwhile, depending on the anode’s material, multiple other mechanisms could occur in ammonia-fuelled O-SOFCs, as shown in Figure 1C. In another mechanism, ammonia is directly oxidised into water and nitrogen oxide or nitrogen gas, producing electrons. Despite being fuelled with ammonia or hydrogen gas, SOFCs will reduce oxygen gas into oxygen ions in the cathode chamber. Since the electrolyte in an H-SOFC is proton-conducting, the process differs from that of an O-SOFC, as shown in Figure 1B,D. When hydrogen gas is used, the hydrogen ions from the gas will be transported through the proton-conducting electrolyte to the cathode, and electrons will travel through the load to the cathode, where they will react with oxygen to generate water vapour and electricity. Ammonia-fuelled H-SOFCs undergo ammonia decomposition similar to O-SOFCs, but the hydrogen gas from the decomposition reaction produces protons and electrons, which will be transported to the cathode to react with oxygen and produce water and electricity. Despite O-SOFC being commonly used instead of H-SOFC, there are a lot of drawbacks associated with it, such as NO_x emissions, the complexity of fabrication, and high operating temperatures, which usually result in higher thermo-mechanical stresses. H-SOFCs exhibit higher ionic conductivity at intermediate temperatures and are more effective at decomposing ammonia due to the nature of proton-conducting electrolytes that facilitate the breakdown of ammonia. Meanwhile, O-SOFCs are less susceptible to hydrogen poisoning since they rely on oxygen–ion conduction, which is less affected by impurities like hydrogen sulphide in the fuel. Furthermore, O-SOFCs have more stability in a variety of fuel environments, including hydrocarbons, as they can handle impurities better than H-SOFCs [20,21,22].

2.2. Direct and Indirect SOFC Under Ammonia Fuel

The structure of an SOFC involves the electrolyte placed between the anode and cathode. The oxygen from the air supplied to the cathode (positive electrode) is reduced to oxygen ions. These oxygen ions are then transported through the electrolyte to the anode (negative electrode), where they react with the fuel (hydrogen or ammonia, as shown in Equations (2)–(4). The electrons released in this process travel through the electric circuit to the cathode for oxygen reduction and generate electric energy. In the case of ammonia SOFCs, ammonia will flow into the anode chamber, where it decomposes to produce nitrogen and hydrogen (Equation (1)). The hydrogen and nitrogen released in this reaction can oxidise with oxygen ions to produce water (Equations (2) and (3)) and nitrous oxide (Equation (3)). Ammonia can also directly oxide to produce water and nitrogen (Equation (4)).

$$2\mathrm{N}{\mathrm{H}}_3\mathrm{ }\to \mathrm{ }{\mathrm{N}}_2\mathrm{ }+\mathrm{ }3{\mathrm{H}}_2$$

(1)

$${\mathrm{H}}_2+{\mathrm{O}}^{2-}\to \mathrm{ }{\mathrm{H}}_2\mathrm{O}\mathrm{ }+\mathrm{ }2{\mathrm{e}}^{-}$$

(2)

$${2\mathrm{N}\mathrm{H}}_3+{5\mathrm{O}}^{2-}\to {3\mathrm{H}}_2\mathrm{O}+2\mathrm{N}\mathrm{O}+{10\mathrm{e}}^{-}$$

(3)

$${2\mathrm{N}\mathrm{H}}_3+{3\mathrm{O}}^{2-}\to {{\mathrm{N}}_2+3\mathrm{H}}_2\mathrm{O}+{6\mathrm{e}}^{-}$$

(4)

In the context of ammonia being used as a fuel, there are two options for the cracking of ammonia, either externally or internally [11]. External decomposition consists of the standard anode, electrode, and cathode setup shown in Figure 2, but an ammonia decomposition reactor is placed outside next to the SOFC system. The decomposition reaction would occur in the external reactor, meaning that ammonia would be cracked into nitrogen and hydrogen before being delivered to the SOFC [23].

For external ammonia decomposition, another option is to thermally integrate the ammonia cracking reactor with the SOFC stack, so the Joule heating produced in the SOFC is used for the endothermic ammonia cracking reaction, as shown in Figure 3b. Direct decomposition is schematically shown in Figure 3a, where the ammonia cracking occurs in the anode using the Joule heat in the electrochemical reaction. Ammonia would flow through the fuel feed and decompose into nitrogen and hydrogen (Equation (1)). The hydrogen would react with the oxygen ions flowing from the cathode through the electrolyte to produce water (Equation (2)), and the electrons would go through the electric circuit. The direct decomposition mode does not require a reactor, as the cracking occurs in the cracking chamber from the heat generated in the Joule heating. However, it does require a multifunctional electrode that provides sufficient catalytic activity in ammonia decomposition and electrochemical reactions. This mini-review will focus on direct ammonia- and hydrogen-fuelled SOFCs.

3. SOFC Parameters Under Ammonia and Hydrogen Fuels

Catalysts in SOFCs enhance the rates of reactions at the anode and cathode compartment, as they improve the rate of NH₃ cracking and electrochemical reactions and reduce NO_x formation. The most common types of catalysts used in SOFC are perovskite-type catalysts, like (La, Sr)CoO3-δ (LSM) and (La, Sr)(Co, Fe)O3-δ (LSCF), usually supported on ceramic materials, such as zirconia [24,25]. Perovskites are among the most used catalysts as they possess high reactivity in oxygen reduction reactions, have mixed ionic and electronic conductivity, and are capable of self-assembling under operating conditions [26,27]. Noble metals are a notable example of catalysts as they speed up ammonia decomposition reactions [28,29]. Self-assembly refers to when molecules position themselves into an organised structure without external support [30]. The variety of materials being tested as catalysts for SOFC’s is dependent on the characteristics discussed in this section, which will cover the essential parameters for evaluating the performance of the materials, such as open-circuit voltage, power density, fuel consumption, fuel utilisation, stability, electrical efficiency, and electrochemical impedance spectroscopy.

3.1. Open-Circuit Voltage (OCV)

OCV is the maximum potential difference that a SOFC can produce when no current flows through the cell. It is influenced by various factors, such as the type of fuel used, operating temperature, and electrode materials. OCV is an important parameter for evaluating the performance of SOFCs, as it also aids in identifying degradation mechanisms [31]. Multiple studies have investigated the OCV of SOFCs using different combinations of fuels, electrolytes, and operating conditions. A study by Corigliano et al. [31] presented an overview of the SOFC energy systems for stationary power generation applications, and they investigated the effects of fuelling, impurities, and hybrid systems on the SOFC OCV. They highlighted the importance of fuel processing and impurity removal to maintain a high OCV and a stable operation of SOFCs. These studies demonstrated that the OCV of SOFCs is a complex function of many factors and highlighted that it can be used to indicate cell performance and degradation.

The Nernst equation is the most fundamental in the thermodynamics of SOFCs; it provides an expression to relate the voltage or electromotive force (EMF) of an electrochemical cell to the standard cell potential and the concentrations of chemical species involved in cell reactions [32]. Equation (5) shows the Nernst equations for hydrogen or ammonia-fed SOFCs [33].

$$E_{Nernst}=E^0+{\displaystyle \frac{RT}{nF}}\mathrm{l}\mathrm{n}\left({\displaystyle \frac{P_{H_2}{P}_{O_2}^{\frac{1}{2}}}{P_{H_{2}O}}}\right)$$

(5)

where E_Nernst is the Nernst potential or cell voltage;

E⁰ is the standard cell potential;

R is the universal gas constant;

T is the absolute temperature;

n is the number of electrons transferred in the cell reaction (n = 2 for hydrogen and n = 3 for ammonia);

and F is Faraday’s constant.

$P_{H_2},P_{O_2},P_{H_{2}O}$ are the partial pressures of hydrogen, oxygen, and water, respectively.

Ammonia SOFCs operate on a similar principle as hydrogen SOFCs, but the reaction mechanism is different. For both fuels, the Nernst equation allows predicting cell voltage under non-standard conditions while considering the gases’ real temperature and partial pressures [32]. The Nernst equation is crucial in understanding the operation of SOFCs, as it helps predict the effects of operating parameters. A common mistake in the fuel cell modelling literature is incorrectly using the cell/stack current in Nernst’s equation when calculating the output voltage. This mistake comes from misunderstanding the difference between reversible and irreversible potentials in the cell. Hence, it is crucial to consider these factors when analysing the OCV in the context of the Nernst equation and thermodynamics [34].

The OCV is seen as a fundamental concept in electrochemistry and is closely tied to thermodynamics; it is related to the Gibbs free energy of the overall reaction. The Nernst equation states that the overall reaction’s Gibbs free energy is equivalent to the maximum electrical work achievable, which defines the OCV. However, the OCV measurement does not reflect the thermodynamic equilibrium, as non-equilibrium phenomena such as corrosion affect the OCV value. Moreover, it is important to note that OCV models should be thermodynamically consistent [35]. Figure 4 compares the OCV of a SOFC when hydrogen and ammonia fuels are used [36]. As temperature increases for a cell fuelled with hydrogen, the Nernst factor $\left({\displaystyle \frac{RT}{nF}}\right)$ increases and H₂O vapour pressure enhances due to faster kinetics and evaporation, which all lowers the Nernst potential demonstrated in Figure 4. The ammonia decomposition efficiency depends strongly on temperature. At low temperatures (450–600 °C), there is incomplete NH₃ decomposition that leads to low H₂ yield and residual NH₃. Furthermore, the anode gas contains a mix of ammonia, hydrogen, and nitrogen gases, reducing adequate hydrogen partial pressure, which causes OCV fluctuations due to an unstable H₂ supply. Ammonia decomposition becomes efficient at higher temperatures (650–850 °C), producing ample H₂. Nitrogen gas acts as an inert diluent, which lowers the partial pressure of H₂O and shifts the Nernst potential upward, counteracting the temperature-driven decline seen in H₂. The Nernst equation, a relation between the OCV and the cell characteristics, is employed to correctly interpret the measured OCV value. In short, the OCV is a crucial parameter that requires a solid grasp of thermodynamics to understand and predict the behaviour of SOFCs.

3.2. Power Density

Power density is the ratio of the electrical power output to the active area of a SOFC. It reflects the ability to convert chemical energy into electrical energy; hence, it is a vital parameter in determining the performance of SOFCs. Power density depends on various factors, such as fuel composition, current density, materials and design of the cell, and operating temperature [37]. It can be calculated as per Equation (6) below:

$$P=V\times i$$

(6)

where P is power density in W/cm², V is the cell voltage in Volts, and i is the current density in A/cm².

DA-SOFCs face some challenges that can negatively impact power density, such as fuel composition, where ammonia contains a lower hydrogen concentration due to the diluting effect of nitrogen, nitrogen poisoning, and NO_x formation. Oh et al. [38] focused on the power density of thin-film (TF) SOFCs with ammonia as fuel and air as the oxidant. The cell consisted of a Ni-GDC anode, an anode functional layer (AFL), a YSZ electrolyte, and an LSCF-GDC cathode. They reported the highest power density recorded for DA-SOFCs of 1330 mW/cm² at 650 °C. Meanwhile, their hydrogen fuel SOFC system produced an 1880 mW/cm² power density at 650 °C as shown in Figure 5. The significant power density difference is attributed to the reduced ammonia conversion to hydrogen and nitrogen at low temperatures. Another study by Ma et al. [39] investigated the performance of a Ni-BaCe_0.8Gd_0.2O_2.9 (BCGO)|BCGO|La_0.5Sr_0.5CoO_3−δ (LSCO)–BCGO cell under liquefied ammonia and hydrogen fuel separately. The power density of the cell was recorded under current density, demonstrating the cell operating on a maximum power density of 371 mW/cm² and 355 mW/cm² at 700 °C under hydrogen and ammonia, respectively. The cells also demonstrated similar I-V slopes with OCVs of 0.996 V and 0.985 V under hydrogen and ammonia fuel, respectively. The minimal difference in maximum power density and OCV demonstrates the practical usage of ammonia as a substitute fuel.

Fuerte et al. [40] conducted a comparative study on the performance of ammonia and hydrogen as fuels for SOFCs. They observed OCVs of 0.99V and 1.06V for ammonia and hydrogen, respectively, at 900 °C. The higher OCV for hydrogen can be attributed to its higher electrochemical potential and lower overpotentials than ammonia. The study also found that hydrogen fuels exhibited a higher power density (104 mW/cm²) than ammonia (88 mW/cm²). This can be explained by the efficient electrochemical conversion of hydrogen, which generally has faster reaction kinetics than ammonia. However, ammonia SOFCs demonstrated a higher resistance (2.77 Ω cm²) compared to hydrogen (2.37 Ω cm²) at 900 °C, likely due to the additional energy required to decompose ammonia into nitrogen and hydrogen before participating in electrochemical reactions. Despite these differences, ammonia shows potential as a viable fuel for SOFCs, especially considering its ease of storage and transportation and the absence of significant drawbacks in its use, aside from the need for more extensive research.

3.3. Fuel Utilisation

Fuel utilisation in SOFCs is a critical factor influencing the efficiency and performance of the cell. It is defined as the ratio of the fuel consumed by the electrochemical reaction to the total fuel supplied to the anode side of the SOFC. High fuel utilisation improves the electrical efficiency and power density of the SOFC but could also increase the risk of NO_x formation, fuel starvation, and thermal stress [41]. Therefore, finding optimal fuel utilisation for several types of fuels and operating conditions is an arduous task for SOFC researchers and engineers. The equation for fuel utilisation (U_f) is given as follows:

$$U_f={\displaystyle \frac{FuelConsumedbytheelectrochemicalreaction}{Totalfuelsuppliedtotheanode}}$$

(7)

This equation measures how effectively the SOFC converts the supplied fuel into electricity.

Fuel utilisation in SOFCs depends on numerous factors, such as operating temperature, pressure, fuel composition, cell geometry, and current density. Some studies have investigated the effects of these factors on the performance and durability of direct SOFCs. For example, Selvam et al. [42] compared the performances of the dead-end anode (DEA) SOFC setup, which refers to a technique that evaluates the relative efficiency of a set of decision-making units (DMUs) based on their input and output, and the standard SOFC setup and reported a decrease in fuel consumption by 18.7% achieved by the complete process of anode off-gas recirculation. In addition, the net system efficiency of the DEA configuration was 12.7% higher than the standard setup. Yi and Kim [43] studied the effects of fuel utilisation on SOFC/gas turbine combined power generation systems. They observed a 76% peak efficiency for SOFCs as Gas Turbine Combined Cycle (GTCC) Power Plants, and the optimal fuel utilisation corresponding to peak efficiency was 60%. They also observed a voltage drop of 2.3% when there was a 10% fuel utilisation change. Another study by Kishimoto et al. [44] obtained fuel and air utilisation of 80% and 50%, respectively. Li et al. [45] reported a comparative study on the performance of a SOFC/gas turbine (GT) system under different fuels and studied the effect of fuel utilisation on system efficiency. The efficiency and fuel utilisation of hydrogen and ammonia is presented in Figure 6 [45]. The figure shows that although both fuels experience a similar rate of efficiency increase, the ammonia-fuelled system efficiency is significantly higher compared to hydrogen, with peaks of approximately 61% and 55%, respectively.

Luo et al. [46] studied the fuel utilisation of tubular DA-SOFCs and H₂-fed SOFCs using Ni-YSZ|ScSZ|LSM-ScSZ at different voltages at a temperature of 800 °C. They reported increased fuel utilisation for ammonia and hydrogen fuels with decreasing operating voltage. The highest fuel utilisation of H₂-fuelled SOFCs did not exceed 56%; however, it was almost twice that of direct NH₃-fed SOFCs, which was attributed to the lower flux of hydrogen as well as the improved electrochemical performance for H₂-fed SOFC. Jantakananuruk and co-workers [47] investigated the performance of tubular Ni-YSZ|YSZ|GDC-LSCF cells under ammonia and hydrogen atmospheres in a temperature range of 700–900 °C. Ammonia-fed SOFCs achieved a fuel utilisation of 85%, identical to that of H₂-fuelled SOFCs at 900 °C. However, at a lower temperature of 700 °C, the fuel utilisation for DA-SOFCs (81%) was slightly lower than that of H₂-fed SOFCs (84%), which can be ascribed to the lower conversion of ammonia to hydrogen and nitrogen at lower temperatures.

3.4. Stability

SOFCs are exceptional for power generation; however, one of the biggest challenges is achieving long-term stability. Good stability means slow degradation at harsh operating conditions (i.e., 0.2%/1000 h) [4]. Multiple factors affect SOFC degradation. Generally, the YSZ electrolyte demonstrates high conductivity and stability under harsh operating conditions, prolonged exposure at 1000 °C leads to multiple microstructural changes that would degrade the electrolyte conductivity and overall SOFC performance. The main degradation mechanisms in the electrolyte include phase transition, impurities, dopant diffusion, and mechanical failures. When an electrolyte layer undergoes a phase transition, zirconia from YSZ transforms from cubic to tetragonal, and eventually, cell degradation occurs. Another factor that causes degradation from the electrolyte is the chemical interactions that may occur between the electrolyte and electrodes as they form insulating secondary phases.

The cathode degradation mechanism involves poisoning, microstructural deformation, and chemical and thermal strains that are often related to the operating environment (e.g., contaminants in the air or fuel stream) and thermal cycling. If the poisoning occurs chemically, it will lead to a decline in electrical properties and obstruct the gas pathways. Anode degradation can occur from electrode catalyst poisoning, thermal stress in the fuel electrode, or impurities in the fuel, such as sulphur in hydrogen fuel, despite hydrogen being generally clean [23]. Materials used in anodes, such as iron and nickel, influence their stability and degradation. Iron in the anode causes nitridation at low temperatures (600 °C) when nitrogen is present, typically from ammonia fuel. The effect of nickel in the anode depends on the support used. For instance, when an alumina support is used, nickel coarsening occurs after a long-term operation [14]. Meanwhile, when nickel is on ceria support, the coarsening stabilises with time and decreases catalytic activity. Ni-based anodes commonly experience microstructural changes such as Ni coarsening, Ni nitridation, Ni migration, and Ni depletion. Ni coarsening significantly impacts SOFCs, reducing current density and the long-term durability of the cell. For instance, the coarsening of nickel causes the triple-phase boundary area, which is where electrochemical reactions occur, to decrease. Furthermore, it would lead to Ni migration due to evaporation and diffusion.

Part of a study by Okanishi et al. [48] included a 1000 h stability test for ammonia-fuelled direct SOFCs. The SOFC stack was anode-supported NiO-YSZ|ZrO₂-based material|perovskite-type oxide material and was tested at 770 °C with the change in power and voltage monitored when fuelled with ammonia and hydrogen fuel as shown in Figure 7. The stack voltage decreased slightly over time when fuelled with ammonia but was relatively stable when hydrogen was used. Furthermore, the direct H₂-fueled SOFC stack maintained a power of at least 235 W during the operation.

3.5. Electrical Efficiency

For SOFCs, electrical efficiency is a critical performance metric that usually ranges between 45 and 55%, indicating the amount of energy in the fuel converted directly into electrical energy [49]. This efficiency is significantly higher than that of gas turbine or steam turbine power generators (approximately 35%) [50,51].

The electrical efficiency of SOFCs is defined as below [44]:

$$\eta ={\displaystyle \frac{{Power}_{net}}{F_{fuel}\times LHV}}={\displaystyle \frac{n\times F\times V\times U_f}{LHV}}$$

(8)

where LHV is the low heating value of the fuel, V is the average voltage, $F_{fuel}$ is the molar flow rate of fuel, U_f is the fuel utilisation, and n is the number of electrons involved in the electrochemical reaction.

The high operating temperature of SOFCs contributes to their high electrical efficiency [52]. SOFCs operate at elevated temperatures, eliminating the need for costly platinum group metal catalysts essential for lower-temperature fuel cells. Moreover, the thermal energy from SOFCs can be further utilised in combined cycle systems, achieving efficiencies of 65% or more efficiencies. This is due to the ability of SOFCs to utilise the heat generated during the electrochemical reaction. In summary, the high electrical efficiency of SOFCs and their ability to operate at elevated temperatures and utilise heat in combined cycle plants make them a promising technology for efficient power generation. Bąkała et al. [53] conducted a study comparing the electrical efficiencies in an SOFC stack when fuelled by ammonia and hydrogen. The ammonia-fuelled SOFC stack system achieved an electrical efficiency of 55%, higher than that of hydrogen SOFCs, which achieved an electrical efficiency of 47.6% at 680 °C. The literature reported that DA-SOFCs exhibit higher electrical efficiency than SOFCs fed with H₂ and equivalent N₂/H₂ mixture fuels [46,47,54]. For example, Luo et al. [46] examined the electrochemical performance of Ni-YSZ|ScSZ|LSM-ScSZ SOFCs under ammonia and hydrogen atmospheres at various temperatures of 600–800 °C. The findings are plotted in Figure 8, and it was revealed that, under a voltage of 0.7 V and U_f = 50%, the DA-SOFC exhibited an electrical efficiency of 32%, which is slightly higher than that of H₂-fed SOFCs (27%). When the U_f is increased to 90%, the electrical efficiency is 57% for direct NH₃-fed SOFCs, notably greater than that of H₂-fuelled SOFCs (49%). The study showed that at equal fuel utilisations, the electrical efficiency of ammonia-fuelled SOFCs was 1.2 times higher than that of the hydrogen-fuelled SOFCs due to the lower gas flow rate of ammonia SOFCs.

Jantakananuruk and colleagues [47] have reported that Ni-YSZ|YSZ|GDC-LSCF SOFCs fed with ammonia had better efficiency compared to SOFCs operated under H₂ and equivalent N₂/H₂ mixture fuels at different temperatures (700–900 °C). Particularly, DA-SOFCs presented an electrical efficiency of 50% at 900 °C, which is higher than that of SOFCs operated under H₂ (44%) and N₂/H₂ mixture (44%) atmospheres. Cinti et al. [54] compared the electrical efficiency of a short stack based on four planar Ni-YSZ|YSZ|LSM cells fuelled with NH₃ and the equivalent N₂/H₂ mixture at 750 °C. It was found that the DA-SOFC achieved a higher electrical efficiency compared to the N₂/H₂ mixture in the U_f range of 60–80%. For instance, at U_f = 80%, the DA-SOFCs showed an efficiency of about 37%, whereas the N₂/H₂–fed SOFCs had nearly 34% efficiency.

3.6. Electrochemical Impedance Spectroscopy

EIS is an essential electro-analytical technique for investigating the electrochemical properties of materials. It can provide information on conductivity, dielectric constant, ohmic resistance, and polarisation resistance of cells and interconnects. It measures the impedance of an electrochemical system (e.g., SOFC) and analyses the circuits representing the electrical properties of the system. The impedance of a system would then be recognised as the ratio of the potential to the current at time t [55]. Ionic and electronic resistance are essential parameters to determine the efficiency of a material as a component in an SOFC; both parameters contribute to the ohmic resistance of a cell, meaning that ionic and electronic resistance can be measured using EIS [56].

As shown in

Table 2. Potential and current input and outputs of potentiostatic EIS and galvanostatic EIS where E(t) and i(t) are the potential and current at time t, respectively, E₀ and i₀ are the amplitude of the potential and current signal, respectively, and ω is the radial frequency [56].

	Potentiostatic EIS	Galvanostatic EIS
Input	E(t) = E0 = sin(ωt)  (9)	i(t) = i0sin(ωt)  (10)
Output	i(t) = i0sin(ωt − ϕ)  (11)	E(t) = E0sin(ωt − ϕ)  (12)

, there are two methods for analysing the system using EIS; the first approach is called the potentiostatic EIS, where a sinusoidal potential input (Equation (9)) is applied and the sinusoidal current (Equation (10)) generated is measured. Another method, galvanostatic EIS, uses a sinusoidal current input (Equation (11)) and measures the generated sinusoidal potential (Equation (12)).

The widely used plots are Bode and Nyquist plots to visualise electrochemical impedance spectroscopy. A Bode plot is a combination of two plots, where the x-axis is a logarithmic scale of frequency as the frequency varies with two y-axes, the logarithm of impedance and the phase shift. Meanwhile, in a Nyquist plot, the imaginary impedance is plotted against the real impedance to observe the total and ohmic resistance in the system. The advantage of these plots is that they can be plotted simultaneously for effective comparison. For instance, a study by Oh et al. [38] observed the efficiency of hydrogen, ammonia, and a mixture of the gases in multiple cells, one of them being the thin-film (TF)-Ni/YSZ direct ammonia SOFC with the EIS analysis shown in Figure 9. By analysing the Nyquist plot in Figure 9A, the plot shows the total cell resistances when fuelled with hydrogen and ammonia individually were 0.4 Ω cm² and 0.8 Ω cm², which indicates that the presence of ammonia fuel increases the total cell resistance by double, compared to pure hydrogen. Further analysing the plot, the ohmic resistance is the high-frequency intercept, indicating that the device had an ohmic resistance of approximately 0.25 Ω cm² and 0.15 Ω cm² for ammonia and hydrogen, respectively. The polarisation resistance is the diameter of the plot, the difference between the total and ohmic resistance. Therefore, the polarisation resistances are approximately 0.55 Ω cm² and 0.25 Ω cm² for ammonia and hydrogen, respectively. Figure 9B shows the Bode plot of the cell with the impedance increasing in the low-frequency region before decreasing, showing that the supply of hydrogen from the ammonia decomposition was inefficient for the cell. The area-specific cell resistance (ASR) plotted against temperature in Figure 9C shows that the polarisation resistance of the ammonia-fuelled cell was significantly higher than hydrogen. The higher resistance for ammonia suggests that optimising its decomposition process or improving the catalyst could enhance its performance. Still, the cell produced the same ohmic resistance when fuelled with ammonia and hydrogen.

A more detailed view of the regions in the graph can be found from EIS by analysing the impedance at various frequencies depending on the region. For instance, EIS can focus on the activation polarisation region by observing the impedance at low frequencies, and by measuring the charge transfer resistance, a better understanding of the kinetics of surface reactions and electrochemical process efficiencies can be obtained. Meanwhile, the real impedance in the ohmic polarisation region (intermediate frequencies) provides insight into the ohmic resistance. At high current densities, the concentration overpotentials are significant; hence, the region can reflect voltage drops due to reactant depletions near the cathodes by analysing the impedance at higher frequencies.

In addition to the performance metrics reviewed above, the environmental impacts of hydrogen and ammonia as fuels are critical considerations. Hydrogen, when used in SOFCs, produces only water as a byproduct, making it a zero-emission fuel at the point of use. However, the environmental impact of hydrogen depends on its production method [57]. Green hydrogen, produced via water electrolysis using renewable energy, has minimal environmental impact, whereas grey or blue hydrogen, derived from fossil fuels with or without carbon capture, respectively, can still contribute to greenhouse gas emissions [57]. Ammonia, on the other hand, is a carbon-free fuel that can be synthesised using renewable energy, but its production is currently dominated by the energy-intensive Habor–Bosch process, which relies heavily on fossil fuels [58]. Regardless, ammonia combustion or decomposition can produce low NO_x emissions, making ammonia a promising low-carbon fuel alternative. Both fuels offer pathways to decarbonisation, but their overall environmental benefits depend on sustainable production methods and efficient utilisation in SOFC systems. A summary of the parameters reviewed in this section is provided in

Table 3. Comparative summary of the parameters of SOFC under hydrogen and ammonia fuels.

	H2	NH3
OCV	1.13 V at 500 °C	1.04 V at 500 °C
	1.125 V at 600 °C	1.12 V at 600 °C
	1.08 V at 800 °C	1.17 V at 800 °C
Maximum Power Density	1880 mW/cm2 at 650 °C	1330 mW/cm2 at 650 °C
Fuel Utilisation	55.5% efficiency	61.5% efficiency
Stability (after 1000 h)	12.5–11.5 V	10.5–9.5 V
	250–225 W	200–175 W
Electrical Efficiency	49%	57%
EIS (total cell resistance)	0.4 Ω cm2	0.8 Ω cm2

.

4. Strategies and Advances for SOFC Cell Materials Fabrication

Various methods have been adopted to synthesise and/or fabricate the anode, cathode, and electrolyte, given in Table 4. Each technique has its benefits and drawbacks. The most common synthesis techniques have been ball milling, solid-state reaction (SSR), template-based synthesis, and wet chemical methods such as sol–gel, co-precipitation, hydrothermal, solvothermal, and combustion methods. Ball milling is a mechanochemical method that grinds the powder material to small particle sizes by colliding with ceramic media. The technique consists of heavy ball grinding with a large-scale starting material as a powder mixture in the ball mill; it consists of fracturing, grinding, high-speed plastic deformation, cold welding, thermal shock, and intimate mixing [59]. The process is simple and straightforward, with many benefits, such as low cost and the ability to use wet and dry materials. However, with those benefits come drawbacks such as long milling time, increased cleaning time, loud noise, the need to be taken under well-controlled temperature, and the possibility of producing contaminants.

Another common fabrication method is tape casting. It is mainly employed to produce thin ceramic layers, chip carrier substances, sensor supports, and dielectrics for capacitors. The fabrication process of an anode, for example, involves a powdered mixture of a ratio of anode powder, electrolyte powder, and starch powder [60]. The powdered mixture is then mixed with a solvent and dispersant before being placed in a ball mill with zirconia media for approximately 2 h. Afterward, a binder, plasticiser, de-foamer, and surfactant are mixed with the solution before being placed back in the ball mill for another few hours. Once the ball milling step is complete and no pores or air bubbles are present, the slurry can be cast on a polyethylene terephthalate (PET) film using a tape casting machine.

Table 4. Deposition rate/thickness of film from some of the deposition techniques.

Deposition Technique	Thickness of Film (μm)	Ref.
Pulsed laser deposition	60–150 μm	[37,61]
Spin coating (colloidal)	30–100 nm	[62]
Dip coating (colloidal)	25–200 μm	[63]
Screen printing (colloidal)	10–200 μm	[64]
Electrophoretic deposition (EPD)	1–200 μm	[61]
Tape casting	10–1500 μm	[64]
Dry pressing	1–100 μm	[63]

Table 4. Deposition rate/thickness of film from some of the deposition techniques.

Deposition Technique	Thickness of Film (μm)	Ref.
Pulsed laser deposition	60–150 μm	[37,61]
Spin coating (colloidal)	30–100 nm	[62]
Dip coating (colloidal)	25–200 μm	[63]
Screen printing (colloidal)	10–200 μm	[64]
Electrophoretic deposition (EPD)	1–200 μm	[61]
Tape casting	10–1500 μm	[64]
Dry pressing	1–100 μm	[63]

4.1. Anode

The anode’s role in the SOFC is to initiate the fuel oxidation reaction and conduct electrons to the circuit. In ammonia SOFCs, a process known as cracking occurs at the anode. The process occurs when ammonia cracks into hydrogen and nitrogen gas, which is facilitated by the high operating temperatures of SOFCs and the catalytic properties of the Nickel contained in the anode [65]. To be a promising anode, a material should have high electrical conductivity and porosity. For example, Ni-YSZ is a common material adopted as an anode; however, Ni-based ceramic anodes bring several drawbacks, such as an increase in the curvature of the sample during sintering and the presence of impurities that may inhibit the functionality of the anode and evaporation of nickel under high steam concentration [66]. In another example with NiO-GDC as an anode, a study found that the material demonstrated an excess in electronic conductivity, which led to an increased chance of reduction in the performance of the SOFC.

Table 5. Table outlining anode materials or systems with their respective characteristics: operating temperature (T), open-circuit voltage (OCV), and power density.

Anode Material	Fuel	T (°C)	OCV (V)	Power Density (mW/cm2)	Ref.
Ni-YSZ	H2	700	1.08	160	[30,67]
LSCM	H2	750	-	270	[68]
LDC	H2	800	-	1017	[68]
SCF	H2	800	-	634	[68]
Ni-YSZ	NH3	650	1.13	911	[38]
Ni-GDC	NH3	650	1.11	1330	[38]
NiO-YSZ	NH3	750	1.07	299	[69]
Ni-SDC	NH3	700	0.83	250	[69]

outlines different anode materials with their respective characteristics used for hydrogen- and ammonia-fuelled SOFCs.

Mixed ionic and electronic conductor (MIEC) materials combine ionic and physical properties and can transport ions and electrons simultaneously [70]. These materials are characterised by their ionic and electronic conductivity. They are widely used in various applications such as oxygen storage, electrochromic windows, metal oxidation, and oxygen separation membranes. Some methods based on MIEC membranes can decrease energy consumption and production costs by around 60% and 35%, respectively [71]. In SOFCs, MIEC materials play a significant role as a cathode or oxygen separation membrane. The MIEC, as an oxygen separation membrane, would go through semi-permeation, where O₂ would be separated from a mixed gas feed via the selective diffusion of oxygen ions. MIEC material performance as a cathode showed an increase in electrochemical performance, making it a great material to be used in SOFCs [72]. Other examples include titanates such as SrTiO₂ doped with yttrium or lanthanum, neodymium and praseodymium doped A-site deficient lanthanides, and barium titanate doped with iron as well as perovskites based on chromites such as strontium-doped lanthanum chromite doped with transition metals [73].

4.2. Cathode

The cathode component of SOFCs is where oxygen reduction (ORR) occurs. The cathode should have high electronic conductivity, low costs, a thermal expansion coefficient (TEC) like the anode and electrolyte, and sufficient porosity for oxygen diffusion [66]. Materials commonly used as cathodes for SOFCs are ceramic perovskite-based materials. Their total net charge includes A and B cations; the A-site cations are coordinated to 12 oxide ions and are a mixture of elements like La, Sr, Ba, etc. On the other hand, B-site cations coordinate with six oxide ions or are a mixture of metals such as Fe, Co, Ni, etc. Compared to A-site metals, B-site cations exhibit a higher valency and lower ionic radius. The characteristics of varied materials used as cathodes are compiled in

Table 6. List of some cathode materials and their respective characteristics.

Cathode Material	Fuel	Structure	T (°C)	Rp (Ω cm2)	OCV (V)	Power Density (mW/cm2)	σe (S/cm)	Ref.
LSMN6382	-	cubic	800	0.039	–	–	70	[74]
LSMN7373	-	cubic	800	0.155	1.12 V at 500 °C	-	42	[74,75]
NBCCFN-GDC	H2	tetragonal	750	0.049	-	882.2	-	[76]
LSCF	H2	-	750	1.36	953	710	275	[77,78]
BSCF	NH3	-	650	-	0.768	1190	-	[79]
MNMO (0.4)-YSZ	NH3	-	600	0.52	1.1	202	~5	[80]

.

Common techniques for fabricating cathode layers are electrostatic spray deposition (ESD), pulsed laser deposition, inkjet printing, and screen printing. Lanthanum Strontium Cobalt Ferrite (LSCF) has shown impressive results as a cathode for SOFCs. Rehman et al. [78] used urea-assisted ultrasonic spray infiltration and deemed it viable for fabricating LSCF cathodes. The process combined the ultrasonic spray technique with the urea-assisted infiltration, and the infiltration was scaled up to the required cathode area. With the characterisation of the LSCF cathode, Figure 10 shows the formation of a nanolayer cathode after several infiltration cycles.

Another study by Lim et al. [81] fabricated LSCF using the flashlight sintering method, which involves a Xenon arc lamp generating a flashlight in the visible wavelength range between 300 and 950 nm; the light would then be absorbed and converted into thermal energy, leading to a sharp increase in surface temperature. The method has many advantages, such as simple process conditions and quick process time. The study observed the cathode’s chemical element diffusion and microstructure cross-section using SEM and Energy-Dispersive Spectrometry (EDS) analysis.

4.3. Electrolytes

Electrolytes are one of the key components of SOFCs, transporting oxygen ions from the cathode to the anode. For a material to be best suited for electrolytes, it should exhibit good oxygen ionic conductivity, as a low ionic conductivity implies that the material will not be able to transport oxygen ions efficiently. The materials commonly used are YSZ, doped ceria, ScSZ, and perovskite structured LaGaO3. A study by Han et al. [82], fabricated YSZ electrolytes by tape casting, tape calendaring, and gel casting. The results showed four different grain sizes in the electrolytes: 0.1–0.4, 0.3–1.5, 1–5, and 8–15 μm, as different sintering processes took place. Once the conductivities of the different grain sizes were measured, the results showed that there were no significant differences at elevated temperatures. However, the smaller grain-sized electrolytes displayed higher conductivity at lower temperatures than larger ones.

For a material to be considered a suitable electrolyte, it should have good minimal electronic conductivity to prevent current leakage and good thermodynamic and chemical stability at different temperatures and oxygen partial pressures [66]. Another factor is dopant concentration, which is responsible for the composition of crystal structures. The electrolyte thickness plays a crucial role in SOFC performance. Reducing the electrolyte thickness can significantly enhance power density and reduce operating temperature, which is important for improving commercial viability [83]. Reducing the electrolyte thickness can also lower the ohmic resistance of the cell. However, it also causes a slight rise in the oxygen partial pressure at the anode because of minor gas leakage through the thinner electrolyte. Moreover, the thickness of the electrolyte is associated with the ohmic losses in electrolyte-supported cells (ESCs), which are higher than those of other electrode-supported fuel cells [84]. These ohmic losses are primarily due to the electrolyte thickness used in ESCs. Therefore, minimising the thickness of the electrolyte while maintaining mechanical integrity has been a focus of recent research.

Table 7. List of characteristics of some electrolyte materials.

Electrolyte	Fuel	Temp. (°C)	Thickness (μm)	Ionic Conductivity (S/cm)	OCV (V)	Power Density (mW/cm2)	Ref.
YSZ	H2	600	~0.8	1.8 × 10−2 at 700 °C	1.09	446	[85,86]
GDC	-	500	5	4.3 × 10−2 at 700 °C	1.05	-	[85,87]
LSGM	H2	750	-	20 S/cm at 600 °C	-	508	[88]
ScSZ	H2	800	170	5.3 × 10−2 at 700 °C	1.05	202	[60,85,89]
BCY	H2	1000	600	0.476 at 550 °C	1.05	405	[71]
YSZ	NH3	850	30	-	1.03	526	[90]
SDC/NCAL	NH3	550	-	-	0.87 at 450 °C	755	[91]

lists the characteristics of electrolyte materials used for hydrogen- and ammonia-fuelled SOFCs.

5. H₂ and NH₃ Fuel SOFC: Operating Characteristics

As SOFCs allow the use of different fuels, such as ammonia, hydrogen, and hydrocarbons, further research on the fuels used is required to optimise the performance of SOFCs. The two most common fuels used in SOFCs are hydrogen and methane fuels. Hydrogen is viewed as a clean and environmentally friendly energy source; as it is carbon-free, it can also be produced and stored. It is a prosperous fuel due to its immense energy content and availability. The fuel is eco-friendly, an alternative to fossil-based fuels. It is one of the most energy-dense fuels, carrying almost three times more gravimetric energy density than gasoline. A significant advantage of hydrogen is that it can be used as a form of energy storage since it can be produced via electrolysis when there is excess energy and later used when needed, allowing a balanced grid and providing power during peak demand times [92].

With hydrogen revealing several challenges, ammonia presents benefits that allow it to be considered an alternative fuel and storage solution for renewable energy, as it is a suitable hydrogen carrier and carbon-free. It offers advantages such as the ability to be easily liquefied, with a volumetric energy density higher than that of liquid hydrogen; in addition, it is safer to handle as it is less flammable and does not emit greenhouse gases. Liquid ammonia is cost-efficient and can be stored at moderate pressures and temperatures in a tank with a specific energy about two times higher than that filled with gaseous hydrogen [14]. Furthermore, ammonia decomposes into hydrogen and nitrogen, allowing hydrogen fuel use in SOFCs. A significant advantage of ammonia fuel for SOFCs is the use of ammonia crackers redundant. However, a common challenge in using ammonia is the safety risks associated with its high reactivity and risk of explosion. Other challenges such as the inconsistency of OCVs for ammonia-fuelled SOFCs, the thermal shocks during shutdowns that can degrade the performance of SOFCs over time, emission of NO_x, and nitride formation from anodes containing nickel [14].

5.1. Operating Temperature

Operating temperature is an essential parameter for SOFC systems, as it influences the cell’s efficiency. The main objective of a lot of researchers is to find a fuel and cell component combination that presents impressive results at the lowest temperature possible [93]. Reducing temperature is essential as it reduces the possibility of cell failure and thermal fatigue and lowers startup/shutdown time. While elevated temperatures (above 800 °C) have been traditional, the intermediate range (around 500–700 °C) is gaining attention. Lower temperatures tend to reduce material degradation and extend cell lifetimes. In comparison, higher temperatures lead to a rise in thermo-mechanical challenges due to the difference in thermal expansion coefficients. When it comes to electrical efficiency and cathode optimisation, operating temperature affects them directly. Higher temperatures enhance ionic conductivity but elevate the thermal stresses. Meanwhile, lower temperatures increase activation and ohmic losses [38].

For direct ammonia SOFCs, ammonia is thermally cracked on the anode before proceeding with the electrochemical oxidation of hydrogen [94]. This process heavily depends on temperature, and the conversion of ammonia cracking approaching 100% depends on sufficient contact time with the anode at normal operating temperatures of SOFCs. However, the conversion rate decreases at lower temperatures, leading to lower fuel utilisation and power density [95]. On the other hand, hydrogen-fed SOFCs have a more direct process occurring at the anode where hydrogen gas is split into protons and electrons. This process is less sensitive to temperature changes compared to the ammonia cracking process in ammonia-fuelled SOFCs.

Li et al. [45] directly compared the effects of operating temperature on SOFC efficiency under different fuels, it can be observed in Figure 11 that cells would perform better at 1075 K (802 °C) due to the increasing voltage of the cell. In addition, the cell fuelled with ammonia produced the highest efficiency percentage, with hydrogen coming in second compared to other fuels. In terms of electrical efficiency and cathode optimisation, operating temperature has a direct effect [94]. Higher temperatures enhance ionic conductivity but elevate the thermal losses. Meanwhile, lower temperatures reduce activation losses but increase ohmic losses. Therefore, balancing these factors is imperative for optimising the SOFC’s performance.

Meng et al. [79] measured the cell performance of nickel-based anode-supported SOFCs at three different temperatures: 650 °C, 600 °C, and 550 °C. Despite ammonia SOFCs producing a higher efficiency than hydrogen SOFCs,

Table 8. Maximum power density of nickel-based anode-supported SOFC when powered by ammonia and hydrogen at different operating temperatures [79].

	Maximum Power Density (mW/cm2)
Temperature (°C)	Ammonia	Hydrogen
650	1190	1872
600	434	1357
550	167	748

shows that when hydrogen was used as a fuel, the maximum power densities decreased from 1872 mW/cm² at 650 °C to 748 mW/cm² at 550 °C. When the cell was fuelled with ammonia, the maximum power density decreased from 1190 mW/cm² at 650 °C to 167 mW/cm² at 550 °C.

5.2. Round-Trip Efficiency

The round-trip efficiency of ammonia as a renewable energy transportation medium has been studied extensively [96]. The study explores three potential applications of ammonia fuel, measuring the round-trip efficiency of ammonia as (1) a high-quality hydrogen carrier for fuel cell vehicles (PEMFCs), (2) a hydrogen carrier for stationary fuel cells (SOFCs), and (3) a direct fuel for internal combustion engines and gas turbines. Renewable energy can be converted into hydrogen and then into ammonia, moving it to places with low renewable energy availability and changing the ammonia back to hydrogen for local use. Figure 12 shows that the ammonia-fuelled SOFC offers the best round-trip efficiency.

When investigating round-trip efficiencies between hydrogen and ammonia SOFCs, a study by Wang et al. [97] found that hydrogen-based systems achieved 42.6% efficiency, while ammonia-based systems reached 38.6%. This difference stems from fundamental disparities in their energy conversion processes. Hydrogen undergoes fewer energy-intensive conversion steps than ammonia, as hydrogen only goes through electrolysis and fuel cell operation (where hydrogen recombines with oxygen to generate power). Electrochemically, ammonia’s oxidation is less efficient due to lower reaction kinetics and the nitrogen dilution of the fuel stream. Hydrogen, by contrast, reacts more readily, minimising activation losses while maximising electrical output.

5.3. SOFC Stack

SOFC designs are classified into one of two design structures: planar and tubular [98]. Although both designs consist of the fuel and air electrodes, solid electrolyte, interconnect, and sealant, their configurations are vastly different. Planar SOFCs are usually used for single cells and stacks; they come with advantages such as high power density performance, low manufacturing costs, and the ease of fabricating flat components in SOFCs. The planar design, demonstrated in Figure 13a, usually has a simple and cheap fabrication process. It consists of a flat plate compact assembly of electrolytes and electrodes. On the other hand, the tubular SOFC, shown in Figure 13b, consists of an array of electrolytes and electrodes stacked above each other in a specific length and diameter. Unlike the planar design, it is best suited for stationary power plant systems as the structure is suitable for large-scale applications. Tubular SOFCs exhibit higher resistance to thermal cycling, which refers to the repeated heating and cooling cycles that occur during operation, allowing for efficient heat transfer and minimal thermal stress [4]. On the other hand, planar SOFCs are more susceptible to thermal stress during cycling, and their flat geometry can lead to the cracking and delamination of layers under temperature fluctuations. Thus, tubular SOFCs exhibit higher solid thermo-cycling performance than planar SOFCs.

Button cells are the ideal type of SOFCs that are used for evaluating parameters at a small scale with a quick turnaround. Still, whether the observations gathered from button cells would apply to the commercial-size SOFC stacks remains unclear. SOFC stacks typically consist of 10 planar cells of a large surface area stacked on each other in between interconnects. They are used to investigate the power generation performance and long-term durability of SOFC systems. Interconnects are crucial for cell stacking as they need to ensure sufficient electrical conductivity and no fuel mixing between the anode and cathode chambers. There are two types of interconnects in fuel cells: ceramic and metallic. Ceramic interconnects show lower electrical conductivity and are stable in an oxidising atmosphere. Meanwhile, metallic interconnects are more cost-efficient and show higher electronic conductivity. The inner and outer interconnects differ depending on the SOFC structure (planar or tubular) [99]. For tubular SOFCs in Figure 13b, the inner interconnects connect the anode of one cell to the cathode of the adjacent cell and vice versa for the outer interconnect. On the other hand, the inner interconnects in Figure 13a connect the anodes of both adjacent cells, while the outer interconnect connects the cathodes of each cell.

A study by Cinti et al. [54] analysed the potential of a SOFC stack of four anode-supported planar cells using hydrogen fuel, ammonia fuel, and a mixture of both. The results showed that regardless of temperature, the OCV of the ammonia/hydrogen mixture was always higher than hydrogen, as shown in

Table 9. OCV of SOFC stacks fuelled with hydrogen, ammonia, and a hydrogen/ammonia mixture at temperatures of 700 °C, 750 °C, and 800 °C, respectively.

	700 °C	750 °C	800 °C
H2	4.675 V	4.625 V	4.580 V
N2/H2	4.688 V	4.648 V	4.599 V
NH3	4.69 V	4.645 V	4.58 V

. Stacks fuelled with ammonia fuel showed approximately equal, if not slightly higher, OCV; however, it was significantly lower than other fuels at 800 °C. In terms of system efficiency, it increased by 22% when the stack system was fed with ammonia compared to the hydrogen and ammonia mixture. Quach et al. [100] performed a parametric study of a high-performance ammonia-fed Ni-YSZ|YSZ|LSCF SOFC 80-cell stack standalone system. The stack’s anode, cathode, and electrolyte thicknesses were 315 µm, 17.5 µm, and 10 µm, respectively. They computed the system efficiency under various operating conditions, such as current density, fuel utilisation, and stack in/out temperature. They found that the system efficiency was around 54% at 800 °C, strongly influenced by fuel utilisation and current density but not by temperature gradients.

6. Conclusions and Future Directions

This review paper explores the electrochemical performance of SOFCs under direct hydrogen and ammonia fuels, highlighting the advantages and challenges of each fuel. Hydrogen-fuelled SOFCs have been extensively studied and exhibited higher stability, especially with temperature cycling. They offer higher OCV and greater fuel utilisation. However, hydrogen storage and transportation present significant challenges due to its low volumetric energy density. In contrast, ammonia-fuelled SOFCs represent an emerging technology with the potential to address some of the limitations associated with hydrogen fuel, as studies have shown that it has exhibited higher power densities, improved electrical and round-trip efficiency, and greater efficiency in stacked systems. Nevertheless, ammonia-fuelled SOFCs face specific challenges, such as NO_x emissions, nitride formation, and fluctuations in OCV.

Looking ahead, several research gaps must be addressed. First, there is a need for efficient catalysts to enhance ammonia cracking at lower temperatures and improve hydrogen reaction kinetics at reduced hydrogen partial pressures. Additionally, strategies to minimise NO_x emissions during ammonia decomposition require further exploration. Future studies should focus on developing alternative anode materials with enhanced resistance to degradation in ammonia environments to mitigate issues such as nitride formation and nickel coarsening. This could involve investigating novel electrode materials with tailored compositions, microstructures, and surface properties. Furthermore, systematic studies comparing the longer-term durability and performance of hydrogen- and ammonia-fuelled SOFCs under real-world operating conditions are essential to identify optimal use cases for each fuel. In conclusion, while SOFC technology faces several challenges, it also presents numerous opportunities for research and development. With continued innovation and advancement, SOFCs could become a cleaner and more efficient power generator for the future.