1. Introduction

Globally, power generation is managed in a centralized manner using high-efficiency large-scale facilities. This system has the advantage of efficient management; however, several critical problems have led to questions regarding the need for further scale expansions. These problems include the difficulty of accurately predicting and meeting rapidly increasing power demands, high losses owing to long-distance transmission, and the challenges of coping with sudden load increases[1].

The global power industry is shifting from a government-led and centralized structure to a privately led and autonomous market. Consequently, the power supply structure is also changing from centralized to distributed power generation, aiming to address the aforementioned problems[2].

In distributed power generation, renewable energy is used to power small-scale facilities located close to where the energy is required. This approach aims to address issues such as the social conflicts and cost increases associated with large-scale power plants and transmission lines. However, the increasing use of variable renewable energy sources such as solar and wind power has led to instability in power systems, making it difficult to ensure stable supply and demand. These sources are difficult to predict owing to their reliance on weather conditions and to limitations in adjusting their output power. As a result, there is a corresponding growing demand to maintain the current laws and market systems based on a centralized demand-supply system. Currently, electricity supply and demand are determined in the market the day before; however, with the increasing volatility of renewable energy sources, there are concerns regarding supply and demand instability[3].

The intermittent nature of renewable energy technology, which relies on external conditions or geological factors, represents a limitation to its widespread adoption and use. Therefore, fuel cell power generation technology, which can produce electricity continuously and has little environmental impact, is gaining increased attention[4].

A fuel cell is an electrochemical device that produces electrical energy directly from the chemical energy of a fuel by using an electrochemical reaction to convert the fuel into electricity. It has a higher energy conversion efficiency than an internal combustion engine and can significantly reduce pollutant and greenhouse gas emissions. Fuel cells are especially effective as distributed power sources in urban areas owing to their high energy density. Fuel cells can be classified into several types based on operating temperatures and electrolyte types, such as proton exchange membrane fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells (SOFCs)[5]. SOFCs are fuel cells that operate at high temperatures (600~800 °C) by using a ceramic electrolyte with ion conductivity. They have the highest electric conversion efficiency, ranging from 45~60%. SOFCs can use H₂ and other hydrocarbon fuels such as city gas and C₃H₈, as well as biomass and biogas, either partially reformed or directly without reforming. However, most fuel cells currently operating in Korea generate power by using city gas as fuel, resulting in the emission of CO₂ as an exhaust gas by-product during operation. Therefore, it is necessary to link these fuel cells to carbon capture, utilization, and sequestration technology for their storage and utilization and the use of carbon-free fuel[6].

SOFCs operating on H₂ as fuel emit only water as a by-product while producing heat and electricity; thus, they do not emit harmful substances such as fine dusts or greenhouse gases. However, to effectively use H₂ fuels in fuel cells, it is necessary to overcome the limitations concerning H₂ storage and transportation. Currently, H₂ is typically stored through either liquefaction or compression in high-pressure tanks, requiring a significant initial financial investment and offering a limited storage space per unit volume. In addition, it is expensive to establish an efficient system for H₂ transportation and long-distance transportation remains difficult. To address these issues, research on H₂ carriers has been conducted to convert H₂ into other compound forms, aiming to increase the storage capacity per volume and reduce the economic costs of storage or transportation[7,8].

One major limitation of current fuel cells is their reliance on H₂ as fuel. Although H₂ is an ideal fuel for the future, producing and storing it remains a challenge. 96% of H₂ is produced by reforming hydrocarbons; this consumes 20~30% of the hydrocarbons and increases production costs. SOFCs can directly reform hydrocarbons such as biomass, biogas, and syngas without external reforming, making it easy to set up a simple system[9]. However, the internal reforming of hydrocarbons at the anode can lead to carbon deposition. This carbon can potentially cover the active points of the anode and damage it. This is more likely to occur with fuels with a carbon number of C2 or higher[10]. In this context, SOFCs have the potential to contribute to distributed power generation by improving the performance of unit cells through the development of SOFC anodes using hydrocarbons as fuel.

In theory, SOFCs can directly use dry hydrocarbons as fuel without the need for a pre-conversion process[11]. Figure 1 shows the complex reaction sequence of an entire cell.

The solid electrolyte present in an SOFC enables the direct oxidation of the hydrocarbon fuel through oxide ion conduction, significantly improving the feasibility of SOFC technology. Although the current best-performing Ni-based ceramic anodes for SOFCs have exceptional catalytic activity and electronic conductivity for fuel oxidation, one drawback to this approach is that the use of Ni as a catalyst for carbon formation can result in carbon accumulation when exposed directly to dry hydrocarbons[12].

This paper provides an overview of the current state of research on the utilization of hydrocarbons in SOFC anodes. It evaluates the previous work performed on selecting appropriate materials and designing the structure of the anode and identifies the barriers that must be overcome to achieve commercialization. This paper also presents a literature survey and discusses the research trends for fuel-flexible anode architectures in SOFCs while highlighting the key focus areas.

2. Materials

2.1. Perovskite materials

Perovskite has an ABO₃ crystal structure and is a promising anode material for SOFCs. It has high electrical conductivity, stability, and the ability to operate at elevated temperatures. These properties have made perovskite anodes sought after for their superior power output, fuel utilization, and long-term stability[13,14]. Perovskite anodes are also important for SOFCs fueled by hydrocarbon sources such as natural gas and C₃H₈. The significant electrocatalytic activity of perovskite for the oxidation of hydrocarbons makes it a suitable choice as an anode material for SOFCs. The perovskite anode serves as a site for the oxidation of hydrocarbon fuels, releasing electrons that can generate electricity within the SOFC[15].

The use of perovskite anodes in SOFCs with hydrocarbon fuels offers several advantages, including high electrocatalytic activity for hydrocarbon oxidation, high fuel utilization and power density, low production costs (making it a low-cost alternative to other anode materials), stability at high temperatures (critical for high-temperature SOFC operation), tolerance to carbon deposition (a common problem in hydrocarbon- fueled SOFCs), and high oxygen-ion conductivity (enabling low-temperature operation and reducing the need for high-temperature materials)[15]. Consequently, researchers have focused on improving the performance and stability of perovskites by synthesizing and characterizing different perovskites for hydrocarbon fuels.

In 2003, S. Tao & J. T. Irvine[16] demonstrated the use of La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ in a perovskite SOFC anode with an electrochemical performance comparable to that of an Ni/YSZ cermet. The electrode polarization resistance of this anode approached 0.2 Ω⋅cm² at 900 °C in 97% H₂/3% H₂O. It was also found to exhibit exceptional performance for the oxidation of CH₄ without the need for excessive steam. The anode demonstrated stability under both fuel and air conditions and maintained a consistent electrode performance when exposed to CH₄. Recent studies have focused on the effects of A- and B-site doping on perovskite anode materials for hydrocarbon-fueled SOFCs. The inclusion of cations with lower coordination numbers such as Mn, Co, Fe, and Ni improves the catalytic activity of perovskite materials. In high-temperature-reducing environments, these cations can generate oxygen vacancies, leading to improved electrical conductivity and more efficient perovskite anode performance. P. Vernoux et al.[17] examined La_0.8Sr_0.2Cr_0.97V_0.03O₃ and found that it showed low activity for gradual CH₄ steam reforming at 800 °C, with no carbon deposition after 100 h of operation. In 2010, N. Danilovic et al.[18] investigated La_0.75Sr_0.25Cr_0.5X_0.5O_3-δ (X = Co, Fe, Ti, Mn) as an anode material and found that the activity depended on the substituent element X, with the highest activity for X = Co, followed by Mn, Fe and Ti. However, the Co-containing catalyst was unstable under reducing conditions. In 2011, McIntosh et al.[19] investigated the activity and reaction mechanisms of anodes based on La_0.75Sr_0.25Cr_1-xMn_xO_3-δ (LSCM) and found that a low oxygen vacancy concentration and the presence of reducible B-site cations favored the complete oxidation of CH₄. The Pd-doped LSCM exhibited exceptional performance, as shown in Figure 2.

In 2016, C. Aliotta et al.[20] studied intermediate-temperature anode SOFCs using La_1-xSr_xCr_1-yFe_yO_3-δ perovskite-type oxide powders synthesized via a solution combustion synthesis. The results showed that regardless of the CH₄ concentration, complete and partial CH₄ oxidation took place in the temperature range of 450~1000 °C, as shown in Figure 3.

In 2013, Y. Liu et al.[21] synthesized La_0.75Sr_0.25Cr_0.5-xFe_xMn_0.5O_3-δ (LSCFMx) by using iron as a dopant to replace chromium in a liquid phase method. X-ray diffraction patterns indicated that the perovskite- like LSCFM0.15 and LSCFM0.2 were stable in pure H₂ and showed comparable performances as catalytic materials. In 2017, F. Liu et al.[22] revealed that the incorporation of an Sm_0.2Ce_0.8O_2-δ layer between the anode and electrolyte improved the performance of the La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ (LSFMo) anode for hydrocarbon fuel utilization at intermediate temperatures. The LSFMo showed stable stability after several power cycle tests, as shown in Figure 4.

The material exhibited resistance to carbon deposition when exposed to CH₄ fuel, but showed inadequate catalytic performance for the steam reforming of methane (SRM) at 800 °C. Perovskite anodes based on La-substituted SrTiO₃ (LST) have also attracted considerable interest owing to their exceptional electronic conductivity in reducing environments, impressive dimensional and chemical stability during redox cycling, and remarkable ability to withstand sulfur and coke[23]. In 2012, Y.B. Yoo et al.[24] evaluated the performance of an LST (La_0.2Sr_0.8TiO₃) perovskite as an anode. They compared its performance with those of two other anodes, Ni-(Gd_0.2Ce_0.8O_2-δ) (i.e., Ni-gadoliniadoped ceria (Ni-GDC)) and Ni-impregnated (La_0.2Sr_0.8Ti_0.98Co_0.02O₃)- GDC, by testing two single cells with an (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ) (LSGM) electrolyte. The single cells were operated for 30 h while using H₂ and CH₄ as fuels.

Recently, there has been increasing interest in exploring alternative materials for use as anodes in SOFCs operating on hydrocarbon fuels. These materials include La_0.33Sr_0.67Ti_1-xMxO₃ (M = Feⁿ⁺, Mnⁿ⁺, or Sc) and Sr₂MMoO₆ (M = Mg, Fe, or Co). These materials have shown promising stability and catalytic activity under certain conditions; however, they also have limitations, such as low electron conductivities.

Double perovskite materials such as La₂NiMnO₆ and PrBaMn₂O_5+δ are being actively studied as promising anode materials for SOFCs using hydrocarbon fuels, owing to their high catalytic properties[25,26]. Double perovskites have a unique structure consisting of two different transition metal cations at the B-site of the perovskite structure. Double perovskites have been found to have high electronic conductivities and good chemical stabilities, making them suitable as anode materials in SOFCs. In 2015, S. Sengodan et al.[25] obtained metal nano-catalysts by doping PrBaMn₂O_5+δ (a double perovskite oxide) with Co and Ni and using a dissolution method. The fuel cells using these anodes showed a performance of approximately two times that of a fuel cell without the dissolution method (PrBaMn_1.7Co_0.3O_5+δ = 1.15 W⋅cm^-2 @800 °C, H₂ fuel standard). Additionally, fuel cells with exsolved nanocatalysts showed excellent resistance to hydrocarbons and could operate for 200 h with high performance (0.331 W⋅cm^-2), even with C₃H₈ fuel. In 2018, M. K. Rath et al.[27] conducted a study on the synthesis of Ba₂LnMoO₆ (Ln = Pr or Nd), which exhibits a cubic double- perovskite structure after reduction in 5% H₂/Ar. The conductivities of Ba₂PrMoO₆ and Ba₂NdMoO₆ were measured as 348.5 S⋅cm^-1 and 146 S⋅cm^-1, respectively, when tested with humidified CH₄ at 800 °C. However, although Ba₂LnMoO₆ exhibited superior catalytic activity for the H₂ oxidation reaction relative to the CH₄ oxidation reaction, carbon deposition during the CH₄ oxidation tended to block the fuel pathway. In 2006, Y.-H. Huang et al.[28] investigated the use of double perovskite materials, specifically Sr₂Fe_1.5Mo_0.5O_6-d (SFMO), as anode materials in natural gas-fueled SOFCs. They found that SFMO has a high electrical conductivity, chemical stability, and electrochemical activity, making it a promising anode material. In 2010, P. Zhang et al.[29] investigated the use of Sr₂CoMoO₆ as anode material. They found that this material exhibited a maximum density of 634 mW/cm² at 800 °C when fueled with CH₄. Additionally, the authors discovered that Sr₂CoMoO₆ has an extremely high electrochemical activity within 400~500 °C owing to the loss of oxygen, making it an excellent mixed oxide-ion/electronic conductor. This allows it to oxidize H₂ and CH₄ effectively. However, they noted that the oxygen vacancy formation, which can lead to anode degradation, needed to be addressed. In 2022, N. Yu et al.[30] investigated a new anode system with the potential to simultaneously produce electricity and chemicals from direct hydrocarbon-fueled SOFCs. The C₃H₈-fueled SOFC used Sr₂Fe_1.4Co_0.1Mo_0.5O_6-δ (SFMCo) as the anode. During the reduction process, the SFMCo transformed into a composite nanomaterial comprising Co-Fe bimetallic catalytic nanocatalysts on the SFMCo, resulting in excellent electrochemical performance and stability.

The in-situ exsolution method for producing catalytic materials is also an alternative to traditional chemical deposition and impregnation methods. This facilitates the production of widely dispersed nanoparticles in a condensed timeframe, resulting in a greater number of active sites and an increased specific surface area for the catalytic processes. Additionally, this method can prevent grain coarsening by utilizing the strong interactions between the matrix and nanoparticles. In 2018, K.-Y Lai et al.[31] investigated La_0.3Sr_0.7Cr_0.3Fe_0.6Co_0.1O_3-δ for use in SOFCs. As shown in Figure 6, the material allowed for the exsolution of Co-Fe nanoparticles on the oxide surface at intermediate temperatures, increasing the power density of the anode. The self-regeneration process could also drive carbon deposition and reactivate the catalysts without high-temperature processes. The dispersed Co-Fe nanoparticles ensured long-term stability, carbon deposition, and H₂S tolerance.

In 2015, D.E. Fowler et al.[32] published findings on La_0.33Sr_0.67 Cr_1-x-yFe_xRu_yO_3-δ which has both Fe and Ru as replacement ions. The study showed that these two ions worked together to further reduce the resistance of the anode to polarization, making the electrochemical H₂ oxidation the limiting factor and resulting in anodes with significantly reduced impedance. The formation of Ru nanoparticles on the anode surface appeared to promote H₂ dissociation. In 2021, M. Qin et. al.[33] published results on the use of Ru/Nb co-doped (Pr_0.5Sr_0.5)_0.9Fe_0.8 Ru_0.1Nb_0.1O_3-δ (PSFRN) as an anode material in SOFCs. As shown in Figure 7, after treatment with wet H₂ at 900 °C for 2 h, the PSFRN partially transformed into a Ruddlesden-Popper (RP)-layered perovskite known as PrSrFe_0.8Ru_0.1Nb_0.1O_4+δ (RP-PSFRN) and exsolved Fe_0.7Ru_0.3 alloy-FeOx oxide (FRA@FO) core-shell nanoparticles on the reduced PSFRN substrate surface. With the addition of the in-situ exsolved FRA@FO core-shell nanoparticles, the RP-PSFRN anode showed remarkable catalytic activity for H₂ and hydrocarbon fuels. The SOFC achieved a power density of 0.683 W⋅cm^-2 with wet H₂ and 0.537 W⋅cm^-2 with C₃H₈ as fuel at 800 °C. The fuel cell also demonstrated stable performance under 15 A⋅cm^-2 when using C₃H₈ as fuel, indicating its resistance to carbon deposition and coking.

Other studies have focused on modifying the surfaces of perovskites to increase their reactivity towards hydrocarbons. In 2022, M. Wu et al.[34] investigated the variables governing the exsolution and catalytic conversion of hydrocarbons and proposed a strategy for the exsolution of complex oxides on perovskite surfaces as a hierarchical electrode/ catalyst for fuel conversion. As shown in Figure 8, the authors used La_0.8Ce_0.2FeO₃ (LCF) as a host oxide to release ceria in air and further incorporated Pr³⁺ (La_0.7Pr_0.1Ce_0.2FeO₃, LPCF) or Gd³⁺ (La_0.7Gd_0.1Ce_0.2FeO₃, LGCF) to facilitate the formation of PrO_x- and Gd₂O₃-doped ceria (i.e., GDC). During the exsolution process, Fe₂O₃ segregation occurred, enhancing the fuel oxidation. The LPCF infiltrated with 1 wt% Ni showed the best performance in the H₂ fuel at 800 °C. However, under hydrocarbon fuel conditions, both the LCF and LGCF anodes performed well when the Ni was infiltrated to aid deprotonation.

Although perovskite anodes have many advantages when used in SOFCs operating on hydrocarbon fuels, several disadvantages have been identified, such as their poor electronic conductivity, poor chemical stability, high sintering temperature, and carbon deposition. Researchers are actively developing new approaches to overcome these disadvantages and improve the performance of perovskite anodes.

In addition, researchers have explored ways to mitigate the exsolution effect by doping perovskites with various elements such as Ni, Ni-Co, and Ni-Cr; this helps to reduce the concentration of oxygen vacancies and inhibit the formation of oxygen-deficient phases[35].

Overall, perovskite anodes are promising alternatives for SOFCs operating on hydrocarbon fuels owing to their high electrocatalytic activity, low cost, and high thermal stability. Ongoing research is focusing on ways to further improve the performance of perovskite anodes and overcome their disadvantages. Further research is needed to fully understand these phenomena and develop strategies for mitigating their negative effects.

2.2. High-entropy alloys

A new area of study in materials science has emerged owing to the exceptional properties of high-entropy alloys (HEAs). These materials were first proposed by K.-H. Huang & J. Yeh et al. in 1996[36] and are characterized by a composition of at least five elements in equimolar proportions. Examples include the CrMnFeCoNi system developed by B. Cantor et al. in 2004[37] and high-entropy CuCoNiCrAlxFe alloy produced by J.W. Yeh et al. in 2004[38]. HEAs differ from conventional multicomponent alloys in that they exhibit single-phase solid solutions in which the constituent elements are uniformly and randomly dispersed. In contrast, conventional multicomponent alloys typically exhibit multiple phases. HEAs have been shown to have remarkable properties, such as high strength while retaining plasticity, low thermal conductivity, slow diffusion of elements, and mobility at the grain boundaries. The crystal lattice structure of HEAs is significantly distorted, increasing their hardness, and hindering the movement of dislocations. The slow diffusion of elements can lead to the formation of nanosized particle structures, contributing to the development of local stresses and an increase in the migration energy barrier. The combination of the lattice distortion and component mixing in HEAs leads to a “cocktail” effect where the properties of the material differ significantly from those of the individual components. Consequently, HEAs have great potential for applications in various industries.

SOFCs are suitable for use with hydrocarbons owing to their high operating temperatures and all-solid-state design, making them ideal for stationary applications and auxiliary power units in vehicles[39]. When using CH₄ as a fuel, SOFCs typically operate on syngas. Syngas is a mixture of carbon monoxide and hydrogen obtained from an external, indirect internal, or internal steam-reforming process. However, the catalysts used in modern steam-reforming processes, such as platinum (Pt) and rhodium (Rh), are expensive. To reduce this cost, the absorption energy of the catalyst alloys can be adjusted by using different compositions and incorporating lower-cost materials such as diluents[40]. The direct use of hydrocarbons can increase the efficiency of fuel cells by reducing the losses owing to external reformers. However, this is not feasible with commonly used Ni-based cermets, for the following reasons[40]: (1) hydrocarbon deposits on the Ni surface, (2) dissolution in the bulk of the Ni particles, and (3) precipitation into fibers, causing Ni losses as Ni atoms are detached from the surface and leading to reduced conductivity. Additionally, the growth of carbon fibers in the SOFC leads to mechanical stress and fracture[39]. The application of a highly catalytically active layer to a Ni-based anode has been shown to be an effective approach to improving the long-term stability and performance of hydrocarbon-fueled SOFCs. Another approach is the development of hydrocarbon-compatible SOFC anodes. Various ceramic anodes are suitable for hydrocarbon-fueled SOFCs, including perovskites with different compositions (chromite, titanate, vanadate, manganate, molybdate, cerate/zirconate, etc.), double perovskite, spinel and tantalate structures, pyrochlore, spinel, and rutile structures, ceria with fluorite structures and doped ceria, Ni-, Co-, and Cu-based cermets with added oxygen ions (YSZ, ScSZ, doped CeO₂) or proton conductors (cerate-zirconate), and Ni-Cu, Ni-Fe, Co-Cu, Fe-Cu, and Ni-Mo bimetallic cermets. HEAs containing more than five transition metals with mixed surface sites (such as near-isoelectric and non-isoelectric alloys) are promising materials for SOFC cermet anodes[41,42].

In 2022, K. X. Lee[43] reported the use of HEAs as anodes and reforming catalysts in SOFCs. The HEAs were synthesized on Gd-doped CeO₂ (GDC) and their ability to perform SRM was tested. The experiments were conducted in a fixed-bed catalytic reactor and evaluated under typical SOFC operating conditions including 700~800 °C, a pressure of 1 atm, gas hourly space velocity of 45,000 h^-1, and steam-to-carbon ratio of 2. Figure 9 shows the results. The HEA/GDC catalyst performed worse than conventional Ni cermet anodes at 700 °C but had good stability with no evidence of carbon deposition. Furthermore, when assessed as an anode in an SOFC, the HEA/GDC exhibited exceptional electrochemical stability and current densities comparable to those of Ni electrodes. They were free of the carbon deposited on and inside the electrodes.

In 2023, D. Chen et al.[44] investigated the use of FeCoNiCu_XCe_0.8Sm_0.2O₂ (SDC) materials. They were found to have high conductivity (comparable to that of Ni-based anodes) and showed excellent catalytic activity for H₂, CH₄, and CO₂. In tests, LSGM electrolyte- supported single cells with FeCoNiCuAl-SDC as the anode showed maximum power densities of 779 mW⋅cm^-2 when using H₂ as the fuel gas and 526 mW⋅cm^-2 when using CH₄ at a temperature of 850 °C. A relaxation time analysis showed that the main rate-limiting step for the FeCoNiCuX-SDC anode was the adsorption and dissociation of the fuel gas. The high-entropy FeCoNiCuX alloys with different valence states generated additional active sites, resulting in a synergistic effect for H₂, CH₄, and CO₂ catalysis.

The use of HEAs as anodes in SOFCs is an active area of research and much remains to be learned regarding their potential benefits and limitations. Although they show promise as potential high-performance anode materials, further developments and optimization are necessary to fully realize their potential in SOFCs.

2.3. Proton-conducting ceramics

Proton-conducting ceramics are another type of SOFC anode material. They have attracted attention owing to their high proton conductivity, which allows for the efficient transfer of protons from fuel to oxygen during the electrochemical reactions in the SOFC. These materials have the potential to improve the overall performance of SOFCs, including their power density and efficiency[45]. Proton-conducting ceramics have several advantages over traditional SOFC anode materials such as Ni-based ceramics. They are more resistant to degradation and have lower production costs, making them promising candidates for large-scale deployment. In addition, they have higher fuel utilization efficiency, i.e., they can convert more fuel into electricity.

The most advanced proton-conducting oxides are ABO₃ perovskites, which are classified into five compositional groups: yttrium-doped barium zirconate, yttrium-doped barium cerate (BCY), yttrium- and cerium- doped barium zirconate, and yttrium-, ytterbium-, and cerium- doped barium zirconate (BCZYYb)[46]. These oxides have a comparatively low activation energy for proton conduction (0.4~0.6 eV), resulting in exceptional proton conductivity (10^-3–10^-2 S⋅cm^-1) at moderate operating temperatures (400~600 °C)[47,48].

Both proton-conducting SOFCs (P-SOFCs) and oxide-conducting SOFCs (O-SOFCs) directly convert CH₄ and other hydrocarbon fuels into electricity[49]. However, the coking of the Ni-YSZ anode can block the active sites at the three-phase boundary, causing serious performance degradations; this has hindered the commercialization of SOFCs. In 2011, L. Yang et al.[49] reported that the terminal voltage of an O-SOFC at a current density of 500 mA⋅cm^-2 dropped rapidly to almost zero after only 30 min of operation when fed with dry C₃H₈. As a result, interest is growing in the development of alternative direct hydrocarbon fuel cells such as P-SOFCs. In 2003, W. G. Coors et al.[50] validated the concept of a direct CH₄ P-SOFC with an Ni anode. Although the current density was low, the P-SOFC exhibited excellent resistance to coking. Y. Feng et al.[51,52] demonstrated the use of C₃H₈ as a fuel to enhance the performance of P-SOFCs. These findings have led to increased interest in the development of direct hydrocarbon fuel cells, particularly P-SOFCs, as a potential solution to the coking problem associated with O-SOFCs. Duan et al.[53] used solid-state reactive sintering to fabricate P-SOFCs with NiO-BaCe_0.6Z_0.3Y_0.1O_3-δ and CuO designed on the anodes to improve the performance and demonstrated a direct CH₄ P-SOFC with stable operation for 1400 h at 500 °C and excellent performance with CH₄. They also found that an Ni/BaZr_0.8Y_0.2O_3–δ anode fueled with different fuels including H₂, CH₄, and C₃H₈ showed degradation rates of less than 1.5% per 1000 h for most fuels at 500~600 °C[54]. In 2017, S. Liu et al.[55] reported the use of a double-layered perovskite (Pr_0.4Sr_0.6)₃(Fe_0.85Mo_0.15)₂O₇ with an in situ exsolution of Co-Fe alloy nanoparticles. They showed high C₂H₆ partial dehydrogenation activity and improved electrocatalytic activity. The P-SOFC also exhibited good stability and excellent coking resistance, as demonstrated by stable power output during a 100 h stability test showing a maximum output power density of 348.84 mW⋅cm^-2 in C₂H₆ at 750 °C (and with a high C₂H₄ selectivity of over 91%). This full set of durability measurements demonstrated that the direct hydrocarbon P-SOFCs were very stable. In addition to power generation, in 2016, B. Hua et al.[56] developed a new process for the coproduction of electricity and CO-concentrated synthesis gas via the selective oxidation of H₂ in a high-performance P-SOFC during a CH₄-CO₂ dry reforming process. This approach was accomplished by integrating an added functional layer comprising a composite material Ni_0.8Co_0.2-La_0.2Ce_0.8O_1.9 into the anode support to form a layered SOFC configuration. The bimetallic nanoparticles embedded in this layer displayed exceptional activity in the in-situ dry reforming process and significantly improved the resistance to CO₂ and internal reforming efficiency. The layered SOFC demonstrated a high CO₂ conversion efficiency and galvanostatic stability for up to 100 h in a CH₄-CO₂ feed stream at 1 A⋅cm^-2. The heat energy generated by the electrochemical oxidation of H₂ was sufficient to compensate for the heat required by the highly endothermic dry reforming reaction, resulting in the thermal self-sufficiency of the entire process. This new process has the potential to play an important role in future CO₂ conversion and utilization efforts. In 2010, X.-Z. Fu et al.[57] synthesized BaCe_0.85Y_0.15O_3-δ (i.e., BCY) nanopowders of approximately 20 nm using a combustion method. These nanopowders were approximately 100 times larger in specific volume than the sintered BCY. Two layers composed of the precursor powder and a starch mixture were pressed and then co-sintered at a high temperature to easily fabricate a bilayer proton-conducting film with a thick porous BCY substrate and integrally supported dense BCY thin film. A porous BCY layer matrix was impregnated with Pt as an anode catalyst for the dehydrogenation of C₂H₆ to C₂H₄. The co-generation of a high selectivity for C₂H₄ was demonstrated using a BCY thin film electrolyte and Pt electrode, achieving an C₂H₆ conversion rate of 36.7% and an electrical energy output of 216 mW⋅ cm^-2 at 700 °C. Moreover, increasing the current density resulted in a higher C₂H₆ conversion and C₂H₄ selectivity, as shown in Figure 10.

Proton-conducting oxides have also been used to decorate O-SOFC anodes and improve their durability. In 2015, M. Li et al.[58] studied Ni-GDC, which suffered from a low carbon deposition resistance in direct CH₄ SOFCs. They showed that the impregnation of Ni-GDCs with proton-conducting perovskites such as BCY and BaCe_0.9Yb_0.1O_3-δ (BCYb) not only improved their initial polarization performance, but most importantly inhibited carbon deposition and formation, resulting in a significant improvement in stability in wet CH₄ (3% H₂O in CH₄). In wet CH₄, in the BCY + Ni-GDC and BCYb + Ni-GDC anodes, the cell voltage remained constant at 0.62 to 0.65 V when tested for 48 h under the same conditions. In 2021, L. Wang[59] synthesized a La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ-CeO₂ (LSFM)-CeO₂ composite by impregnating CeO₂ into a porous La_0.5Sr_0.5Fe_0.9Mo_0.1O_3-δ perovskite as an anode. The maximum power density was 190 mW⋅cm^-2 in C₂H₆ at 750 °C, notably higher than those obtained from a single cell using only the LSFM as the anode. The fuel cell also showed a high C₂H₄ selectivity and C₂H₆ yield of 93.4% and 37.1%, respectively, at 750 °C. In addition, the single cell using the LSFM-CeO₂ composite as the anode demonstrated no significant performance degradation or carbon deposition during 22 h of continuous operation fueled by C₂H₆, as shown in Figure 11. The exceptional electrochemical performance of this composite material was attributed to several factors, including enhanced electronic and ionic conductivities, an abundance of active sites, and a broad three-phase interface existing in the resulting composite anode.

Proton-conducting ceramics are promising as anodes for SOFCs owing to their high proton conductivity and potential for improved performance and stability. Further research is needed to address their challenges (e.g., low electrical conductivity and low oxygen vacancy concentrations) to fully realize the potential of these materials.

3. Structures

3.1. Anode on-cell catalyst reforming layer

An anode on-cell catalytic reforming layer (OCRL) is a promising design for direct CH₄ SOFCs. The effectiveness of reforming catalysts depends on their active components, substrates, and catalyst promoters, which together affect their catalytic activity, sintering resistance, and coking resistance during CH₄ reforming[60]. Considerable research has been conducted on reforming catalyst materials with a focus on their catalytic activity, sintering resistance, and coking resistance. In the context of its CH₄ reforming activity and exceptional carbon growth resistance, an OCRL requires a matched coefficient of thermal expansion (CTE) and chemical properties compatible with the Ni-based anode. A reforming catalyst material usually consists of an active component, substrate, and catalyst promoter. The composition and size of the active ingredient, the properties and surface area of the substrate, and interactions between the active component and substrate are the main factors affecting the catalytic activity, resistance to sintering, and resistance to coking of CH₄-reforming catalysts[61,62]. Despite the significant progress made in the research on OCRL materials about the reforming activity, mechanical and chemical compatibility with Ni-based anodes, and coking resistance, there are still challenges remain to be overcome. Below, we provide an overview of the effects of different configurations on the properties of OCRL materials, and a list of the most commonly used OCRL materials, as well as guidance for the future development of OCRL materials. The active component in OCRL materials is typically a metal catalyst for the reforming reaction. At SOFC operating temperatures (typically above 750 °C), carbon deposition and catalyst sintering are inevitable owing to the endothermic nature of CH₄ reforming reactions, including the dry reforming of methane and SRM. These high operating temperatures are necessary to maintain the high efficiency of SOFCs; therefore, the thermal stability of the catalysts in OCRL materials is essential to preventing catalyst deactivation owing to the carbon deposition and sintering. The most extensively studied catalysts for CH₄ reforming are classified into noble and non-noble Group VIII metals[63]. Noble metals such as Rh, Ru, Ir, Pd, and Pt generally have excellent reforming catalytic activity and good coking resistance. The excellent anti-coking performance of these catalysts is due to their small particle size and high dispersion in the substrate[64].

In 2005, Z. Zhan et al.[65] proposed using Ru-CeO₂ as an OCRL material for the Ni-based anodes of SOFCs. This substance allowed for the direct internal reforming (DIR) of isooctane, resulting in a maximum power density of 300~600 mW⋅cm^-2 at 670~770 °C without the formation of coke. This breakthrough was a major advancement in the progress of direct hydrocarbon SOFCs. However, the connection between Ru-CeO₂ OCRL and Ni-based ceramic anodes may degrade after several thermal cycles. In 2011, W. Wang et al.[66] synthesized and tested Ru-Al₂O₃ composites with different Ru contents to determine their suitability as anode catalyst functional layers in SOFCs operating on CH₄ as fuel. The results showed that 3–7 wt% Ru-Al₂O₃ composites exhibited high catalytic activity for both CH₄ partial oxidation and CO₂/H₂O reforming reactions. Among these composites, the 3 wt% catalyst exhibited remarkable operational stability and thermo- mechanical compatibility with the Ni-YSZ anode. A cell with a 3 wt% Ru-Al₂O₃ catalyst functional layer achieved maximum power densities of 1006, 952, and 929 mW⋅cm^-2 at 850 °C when operating on CH₄-O₂, CH₄-H₂O, and CH₄-CO₂ mixed gases, respectively. These are similar to those achieved with H₂ fuel, as shown in Figure 12.

Noble metal catalysts are highly effective but expensive and scarce, leading to interest in non-noble transition metal catalysts. However, with the latter catalysts, carbon deposition remains an issue. To achieve an extended activation time, it is essential to maintain high dispersion, a small surface area, and a small particle size for the non-noble metal catalysts. In 2009, W. Wang et al.[67] investigated a high-activity Ni/Al₂O₃ catalyst layer for a Ni-ScSZ cermet anode operating on CH₄ fuel. It showed high activity for partial oxidation, steam, and CO₂ reforming of CH₄ to syngas, comparable to Ru-CeO₂ and significantly higher than that of an Ni-ScSZ cermet anode. The introduction of the Ni/Al₂O₃ catalyst layer more than doubled the maximum power density of 382 mW⋅cm^-2 at 850 °C. The Ni/Al₂O₃ catalyst layer acted as a gas diffusion barrier, enhancing the O²⁻ to CH₄ ratio, preventing coke formation, and improving the operating stability without coke deposition. In 2020, Z. Lyu et al.[68] investigated the DIR of biogas in an SOFC as a promising approach, but faced challenges owing to carbon deposition. This study investigated modifications to conventional Ni-YSZ anodes using Ce_0.9Gd_0.1O_2-δ (i.e., a GDC) infiltration and a Ni-GDC reforming catalytic layer as shown in Figure 13. It shows the results, i.e., slight decrease in the cell performance under humidified H₂. However, the Ni-GDC catalytic layer modification provides exceptional performance when using simulated biogas with improved stability under different operating currents, despite the carbon deposition. The GDC infiltration only benefits the cell performance at very low or high CO₂/CH₄ ratios.

Recently, new bimetallic catalysts have shown improved reforming performance, with higher activity and superior coking resistance than mono-metallic catalysts. The carbon deposition is reduced by the metal interactions and doped metal[69]. In 2010, C. Jin et al.[70] developed a Cu_1.3Mn_1.7O₄ spinel oxide as an anode internal reforming layer for Ni-SDC anode-supported SOFCs operating directly on CH₄ fuel. The in-situ reduction by the CH₄ produced a highly dispersed nano-sized Cu metal. The cell with a Cu_1.3Mn_1.7O₄ internal reforming layer resulted in maximum power densities of 242 and 311 mW⋅cm^-2 at 650 and 700 °C, respectively, with improved cell performance stability. In 2008, X.-F. Ye et al.[71] found that a Cu-CeO₂ catalyst layer added to the anode surface enhanced the C₂H₆O fuel performance. They also investigated the effects of the catalyst synthesis conditions on the composite anode performance w. Single cells with this anode achieved a maximum power density of 566 mW⋅cm^-2 at 800 °C using C₂H₆O vapor, along with a long-term performance of up to 80 h without carbon deposition.

The efficacy of transition metal catalysts is determined by the active component and by the support material, especially for Ni-based catalysts[72]. In 2018, T. Wei et al.[73] used an LaMnO₃-based perovskite doped with 20 mol% Ni-doped La_0.9Mn_0.8Ni_0.2O₃ (LMN) and exsolved (R-LMN) to prepare oxide substrate-supported catalysts for CO₂ dry reforming of CH₄. As shown in Figure 14, the Ni nanoparticles dissolved in the R-LMN were identical in size, uniformly distributed, partially inserted, and strongly attached to the substrate. As a result, stable nanosized Ni particles and the LMN substrate were achieved, providing additional oxygen vacancies for CO₂ activation and leading to stable conversion and selectivity, as well as strong resistance to carbon deposition.

Overall, OCRLs are promising alternatives to traditional Ni-based anodes in SOFCs. They provide several advantages, including an improved resistance to carbon deposition and Ni coarsening, and enable the efficient reforming of hydrocarbon fuels. Consequently, OCRLs have the potential to increase the efficiency and durability of SOFCs, making them a viable option for clean energy production.

3.2. Anode functional layer

Anode functional layers (AFLs) are a specific type of anode material used in SOFCs to enhance the electrochemical reactions at the anode, ultimately resulting in improved SOFC performance[74]. These layers can be made from a variety of materials, including ceramics, metals, and alloys, and can be designed to have specific properties, such as high electrical conductivity, high surface areas, and good catalytic activity. One advantage of AFLs is that they can improve the fuel utilization efficiency of SOFCs by promoting electrochemical reactions for converting fuel into a usable form. This can lead to a higher power density and improved overall performance. Additionally, these materials can improve the stability and durability of SOFCs by reducing the risk of degradation over time. However, some challenges are associated with the use of AFLs in SOFCs. They are sensitive to the operating conditions of the SOFC, such as its temperature and fuel composition, which can affect their performance and stability. Another challenge is that they can be expensive to produce, limiting their widespread adoption.

Another effective approach for utilizing hydrocarbons as fuel for SOFCs is to assemble additional functional layers with high catalytic activity toward hydrocarbon fuels. Various methods can be used for this purpose, including impregnation and spray casting. The additional functional layer can create active sites for steam reforming of the fuel before the anode. In 2016, P. Li et al.[75] added a specific Mo content to a Ni-Ce_0.8Sm_0.2O_1.9 (i.e., SDC) anode and investigated the performance of the additive. The addition of Mo significantly increases the catalytic activity of methanol pyrolysis and improves carbon resistance. Figure 15 shows that the cell with the composite anode can achieve a maximum power density of 680 mW⋅cm^-2 at a temperature of 700 °C. This study also showed that the stability of the cells was strongly influenced by the amount of Mo added.

In 2005, Z. Zhan et al.[63] reported that the addition of a Ru-CeO₂ layer to an anode substrate allows for efficient reforming of the fuel prior to its delivery to the cell system. Cells using C₈H₁₈ as fuel achieved a maximum power density of 0.6 W⋅cm^-2 at 770 °C, demonstrating excellent cell performance and chemical stability. However, there are still obstacles to overcome in the development of low-cost and easily fabricated commercial anodes that are tolerant to carbon. In 2016, B. Hua et al.[76] studied a Ni-Cu-Fe alloy catalyst layer for a CH₄ fuel on an anode substrate. They found that the catalyst layer could significantly improve the steam-reforming performance of the CH₄ and alleviate the degree of carbon deposition. The cell life with the catalyst layer was lasted for 48 h without degradation, as shown in Figure 16.

Compounds such as TiO₂, commonly known as oxides, are widely used as reforming layers to effectively convert hydrocarbon fuels. In a reducing atmosphere, the abundant oxygen vacancies on the surfaces of these oxides can enhance the adsorption of steam and reforming of hydrocarbon fuels. In 2014, Z. Wang et al.[77] examined NiTiO₃ catalysts for CH₄ steam reforming and showed excellent durability when using CH₄ and C₃H₈ as fuel. However, the introduction of a modification layer has a disadvantage in that the catalyst and anode layers are incompatible, which may cause delamination during thermal cycling. In addition, owing to the low electrical conductivity of the catalyst layer, current collection is a significant challenge. To solve these problems, internal reforming, in which the anode substrate is impregnated with functional materials, has been adopted. In 2015, Y.-F. Sun et al.[78] developed a composite anode of La_0.4Sr_0.5Ba_0.1TiO₃- La_0.4Ce_0.6O_1.8 (LDC)–YSZ via impregnation. The cell produced a stable current density of 0.23 A⋅cm^-2 at 0.65 V at 850 °C without any degradation. As shown in Figure 17, the addition of LDC facilitated the oxidation of the CH₄ formed on the anode material and the removal of carbon deposits.

4. Summary and perspective

The aim of this study was to provide a thorough review of the latest developments in the field of carbon-resistant anode materials for use in hydrocarbon fuels. This review aimed to stimulate further research and development in this field. This paper outlined and grouped different anode materials and configurations investigated and documented in the scientific literature, such as Ni-based anodes, alloy-based anodes, perovskite anodes, and steam-reforming functional layers. The critical properties of SOFCs using these anodes, such as their efficiency, catalytic activity, and long-term durability, were also assessed.

However, SOFCs using carbon-resistant anode materials with hydrocarbon fuels are still far from meeting the criteria for commercialization, owing to significant technical difficulties. To overcome these challenges and achieve more effective anodes, we suggest several areas for future research.

4.1. Materials

Ceramic and metallic materials that offer resistance to carbon have been extensively studied and some progress has been made. However, the progress to date has not been sufficient to achieve optimum catalytic performance for hydrocarbon fuels. To make significant progress in this area, priority should be given to developing new materials or exploring new crystal structures that can offer better performance.

Initially, research should focus on doping perovskite or double perovskite structures with B-sites or on improving the catalytic activity of the electrodes by in-situ methods. Perovskite materials have a crystalline structure that makes them promising candidates for use in catalytic converters. Doping them with B-sites can improve their performance by modifying their electronic and chemical properties. In-situ methods involve the modification of the surface of the catalyst during use, which can lead to improved catalytic activity.

Second, HEAs should be further studied to identify the links between the different metal combinations that can improve the performance of SOFC catalysts. HEAs are promising materials owing to their high entropy and complex crystal structures, which offer better performance than conventional alloys. Research in this area can help identify the best combinations of metals and supports for superior catalytic activity.

Finally, proton-conducting ceramic materials should be studied to identify structures with improved sinterability and stability of the catalytic activity, as proton-conducting materials are limited. Proton-conducting ceramics are promising materials for catalytic converters because they offer high stability and can be used at high temperatures. Researchers can identify materials with better performances and stability by studying the structures and properties of proton-conducting ceramics.

4.2. Structures

In addition to improving the properties and performance of ceramic materials, it is critical to optimize the anode structure to achieve the correct porosity. This optimization not only enables the formation of vapor-reformed functional materials on the anode substrate, but also regulates the gas diffusion to provide the correct reactants and facilitates the removal of reaction products from the active sites within the anode. Therefore, further research should design and develop optimal anode structures to improve the catalytic performance of SOFCs.

Anode OCRLs are among the most promising designs for direct methane SOFCs. However, OCRLs face certain obstacles during the mass transfer of fuel gas. Therefore, it is necessary to determine the optimum points and control the porosities and microstructures of the OCRL catalysts and supports. Porosity is an essential factor for regulating gas diffusion and enables the formation of vapor-reformed functional materials on anode substrates. The microstructures of the OCRL catalysts and supports also play critical roles in determining the catalytic activity and coking resistance.

AFLs can improve the electrochemical and catalytic performance of SOFCs using different catalyst layers. It is necessary to determine the optimum point and control the sintering properties of electrodes and electrolytes with high catalytic performance. The sintering properties of the electrodes and electrolytes determine the porosities, microstructures, and surface areas of the anode functional layers, thereby affecting the catalytic performance of SOFCs. By optimizing the sintering properties of the electrodes and electrolytes, researchers can increase the catalytic activity and stability of AFLs, thereby improving the performance of SOFCs.

In conclusion, the development of anode materials with high carbon resistance is a critical area of research and development, with significant implications for global energy needs. With continued efforts to develop innovative anode materials and optimize anode structures, SOFCs using flexible fuels are expected to become practical in the near future.