Anode-Supported SOFCs with a Bi₂O₃-Doped NiO–ScSZ Anode and ScSZ Electrolyte: Low-Temperature Co-Sintering and High Performance

Abstract

In this study, a novel anode-supported solid oxide fuel cell (SOFC) comprising a Bi₂O₃-doped NiO-ScSZ anode and an ScSZ electrolyte was successfully fabricated via a low-temperature co-sintering process at 1300 °C. The incorporation of 3 wt% Bi₂O₃ effectively promoted the sintering of both the anode support and electrolyte layer, resulting in a dense, gas-tight electrolyte and a mechanically robust porous anode support. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed the formation of phase-pure, highly crystalline ScSZ with an optimized microstructure. Electrochemical performance measurements demonstrated that the fabricated cells achieved excellent power density, reaching a peak value of 0.861 W cm⁻² at 800 °C under humidified hydrogen fuel conditions. The cells maintained stable performance under dry methane operation, with a maximum power density of 0.624 W cm⁻² at 800 °C, indicating resistance to carbon deposition. Gas chromatographic analyses further revealed that the Bi₂O₃-doped NiO-ScSZ anode facilitated earlier and more stable electrochemical oxidation of methane-derived species compared with the conventional NiO-YSZ system, even under conditions of an elevated methane partial pressure. These findings demonstrate that Bi₂O₃ co-doping, combined with low-temperature co-sintering, provides an effective approach for fabricating high-performance intermediate-temperature SOFCs with enhanced structural integrity and electrochemical stability. The developed methodology presents a promising pathway toward achieving cost-effective and durable SOFC technologies.

1. Introduction

Solid oxide fuel cells (SOFCs) are recognized as one of the most promising electrochemical energy conversion technologies owing to their high efficiency, broad fuel flexibility, and environmental compatibility [1,2]. Unlike polymer electrolyte fuel cells (PEFCs) or molten carbonate fuel cells (MCFCs), SOFCs operate at elevated temperatures (typically 600–1000 °C), enabling the direct utilization of a wide variety of fuels, including hydrogen, natural gas, and hydrocarbons, without extensive external reforming [1,3,4,5,6]. The high operating temperature also permits the use of cost-effective, non-precious metal catalysts, offering significant economic advantages [7]. However, these high temperatures impose stringent requirements on material stability and mechanical robustness, which have motivated intensive efforts to reduce the operating temperature into the intermediate range (600–800 °C) [8]. Achieving this reduction necessitates the careful optimization of the electrolyte and electrode materials regarding composition, microstructure, and interfacial properties [2,9,10,11].

Among candidate electrolyte materials for intermediate-temperature SOFCs, scandia-stabilized zirconia (ScSZ) has attracted considerable attention due to its superior oxygen ion conductivity, which surpasses the conventional yttria-stabilized zirconia (YSZ) under comparable conditions [12,13]. This enhanced conductivity is attributed to the stabilized cubic fluorite structure and the high mobility of oxygen vacancies introduced by scandia doping [14]. Nevertheless, ScSZ faces several critical challenges that limit its practical application. First, ScSZ typically requires high sintering temperatures (above 1400 °C) to achieve full densification, leading to increased fabrication costs and higher energy consumption [13]. Second, ScSZ may suffer from phase instability at intermediate temperatures, where undesirable tetragonal or rhombohedral phases can emerge and degrade long-term conductivity [15]. Furthermore, mismatches in thermal expansion coefficients and shrinkage behaviors between the electrolyte and anode materials, such as NiO-ScSZ composites, often cause interfacial compatibility issues during co-sintering [16].

To address these challenges, several strategies have been explored, including co-doping with additional oxides and the incorporation of sintering aids [17]. In addition, emerging fabrication technologies, such as 3D printing, have shown potential in enabling low-temperature and structurally controlled ceramic layers [18]. Among them, bismuth oxide (Bi₂O₃) has received significant attention owing to its exceptionally high ionic conductivity (~0.1 S/cm at 750 °C) and low melting point (~825 °C) [9,14]. Particularly in its δ-phase, Bi₂O₃ acts as an effective sintering promoter, substantially lowering the densification temperature of ceramic electrolytes. Studies have demonstrated that even small amounts of Bi₂O₃ doping can stabilize the desired cubic phase of ScSZ, promote grain growth, and enhance microstructural densification, enabling low-temperature processing without sacrificing electrochemical performance. Moreover, the simultaneous Bi₂O₃ doping of both electrolyte and anode materials has been shown to improve interfacial adhesion, reduce porosity, and facilitate enhanced ionic transport across the electrode/electrolyte interface. However, careful control of the Bi₂O₃ content is critical, as excessive doping may lead to phase segregation or unwanted electronic conduction pathways, adversely affecting cell performance [9,13].

Previous research has shown that the addition of Bi₂O₃ to Sc-stabilized zirconia (ScSZ) electrolytes can promote cubic phase stabilization and enhance sintering at lower temperatures, thereby improving ionic conductivity and reducing the required sintering temperature [19]. In this study, Bi₂O₃ was introduced into both the anode and electrolyte layers at a concentration of 3 wt% to facilitate low-temperature co-sintering. For the electrolyte, a comparative evaluation of 1 wt%, 3 wt%, and 5 wt% Bi₂O₃ was conducted. While 1 wt% offered limited densification, and 5 wt% caused abnormal grain growth and possible secondary phases, 3 wt% resulted in a dense and uniform microstructure without structural degradation. For the anode support, Bi₂O₃ levels above 3 wt% led to excessive sintering and mechanical collapse during co-firing. Therefore, 3 wt% was chosen as the optimal concentration for both layers to ensure structural integrity, enhanced sintering, and high electrochemical performance. We successfully achieved co-sintering at a relatively low temperature of 1300 °C by introducing 3 wt% Bi₂O₃ into both the anode and electrolyte layers, effectively overcoming the limitations associated with conventional high-temperature sintering processes [13,20]. The resulting half-cells demonstrated excellent microstructural integrity and high electrochemical performance under both hydrogen and methane atmospheres, achieving stable operation and notable power densities [21,22]. These results highlight the potential of Bi₂O₃ co-doping as an effective strategy for enabling cost-effective, high-performance intermediate-temperature SOFCs, offering a viable pathway toward the commercial realization of SOFC technologies.

2. Materials and Methods

2.1. Cell Fabrication

The Bi₂O₃ powder used in this study was synthesized via the thermal decomposition of bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, ≥99.9%, Aladdin, Riverside, CA, USA). The precursor was dissolved in deionized water, dried at 120 °C, and then calcined at 600 °C for 4 h in air to obtain phase-pure yellow Bi₂O₃.

Building upon our previous methodology [23], we adapted the process to fabricate Bi₂O₃-doped NiO-ScSZ substrates. Figure 1 illustrates the fabrication process of the anode-supported SOFC, including the preparation of the NiO-ScSZ-Bi₂O₃ anode, spray-coating the Bi₂O₃-modified ScSZ electrolyte, and the deposition of the LSM-based cathode. The anode support was fabricated via a dry-pressing method using a ternary mixture of NiO, ScSZ, and Bi₂O₃, with a mass ratio of 56:44:3. Commercial NiO (99.9%, Kojundo Chemical Lab, Nerima, Japan), 11 mol% Sc₂O₃-stabilized ZrO₂ (ScSZ, Japan Fine Ceramics Co., Ltd., Sendai, Japan), and Bi₂O₃ powders were weighed accordingly and thoroughly mixed with 10 wt% starch as the pore-forming agent in ethanol. The mixture was transferred into a ball mill jar and subjected to 24 h of planetary ball milling to ensure uniform dispersion of the powders. After milling, the slurry was dried at 80 °C, sieved through a 100-mesh screen, and granulated. Subsequently, 4 wt% of polyvinyl butyral (PVB) dissolved in ethanol was added as a temporary binder, and the mixture was further hand-ground until the ethanol fully evaporated. The resulting granulated powder was pressed into 24 mm diameter green disks under a uniaxial pressure of 220 MPa. The green bodies were then pre-sintered at 1000 °C for 2 h in air to obtain mechanically robust and porous NiO–ScSZ–Bi₂O₃ substrates [24].

The ScSZ–Bi₂O₃ composite electrolyte was deposited directly onto the pre-sintered anode substrates using a spray-coating technique [17,25]. The electrolyte slurry was formulated by dispersing ScSZ powder and 3 wt% Bi₂O₃ in anhydrous ethanol with 3 wt% PVB and 1 wt% ethyl cellulose as binders. The initial dispersion was carried out in a beaker using magnetic stirring for 30 min, followed by ultrasonic treatment for another 30 min to improve the suspension stability and minimize particle agglomeration. The optimized slurry exhibited good flowability and homogeneity, which are essential for uniform spraying. Prior to spraying, the surface of the anode substrate was cleaned with ethanol and fixed in position. The slurry was loaded into a pneumatic spray gun operated at 0.2 MPa, and multiple thin coats were applied, with drying at 80 °C between cycles to build up sufficient thickness. After the final coating layer was dried, the electrolyte-coated anode support was co-sintered at 1300 °C for 4 h in air to achieve a dense and gas-tight electrolyte membrane. The diameter of the half-cell after sintering was 20 mm.

For the cathode material, lanthanum strontium manganite (LSM-20, 99%, Sigma-Aldrich^®, St. Louis, MO, USA) was selected. The LSM powder was manually ground using an agate mortar and thoroughly mixed with a predetermined ratio of pore-forming agents and organic binders to prepare a printable slurry. The resulting cathode slurry was uniformly deposited onto the sintered electrolyte surface using the doctor blade technique. The sample was then dried at 80 °C and subsequently sintered in air at 1200 °C for 2 h to complete the cell fabrication. The effective cathode area was defined as 0.78 cm².

2.2. Cell Testing and Characterization

The phase composition of the prepared SOFC half-cells was analyzed using X-ray diffraction (XRD, Rigaku SmartLab 9 kW, Akishima, Japan) with Cu Kα radiation. Scans were conducted in the 2θ range of 20–80°, with a step size of 0.01°. The microstructural and morphological features of the SOFC single cells were observed using a field-emission scanning electron microscope (FE-SEM, JSM-7000F, JEOL Ltd., Akishima, Japan), focusing on the cross-section of the electrolyte layer and the porosity distribution in the anode. In addition, energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the materials’ elemental compositions.

The porosity of the NiO-ScSZ-Bi₂O₃ anode was evaluated using the Archimedes method to assess structural variations before and after reduction. Three types of samples were tested: (i) the pre-sintered anode without an electrolyte coating, (ii) the sample sintered in air at 1300 °C for 4 h, and (iii) the reduced Ni-ScSZ-Bi₂O₃ support obtained by reduction in H₂ at 800 °C for 2 h. Each sample was weighed under three conditions: dry weight (W_d), suspended in water (W_s), and immersed weight after boiling in deionized water (W_i) to eliminate trapped air. The porosity (P) was calculated using the following equation:

$$P={\displaystyle \frac{W_s-W_d}{W_s-W_i}}\times 100\%$$

(1)

All measurements were conducted at room temperature using deionized water, and each test condition was repeated at least three times to ensure reproducibility and accuracy.

As illustrated in Figure 2, mass flow controllers (MFCs) were used to regulate the gas flow rates to both the anode and cathode sides of the SOFC. During periods when fuel gases were not introduced, argon was supplied as a protective gas to prevent the oxidation or contamination of the cell components. The anode exhaust gases could either be directly vented or directed to a gas chromatograph (GC) for compositional analysis.

For cell assembly, a single cell was sealed onto an alumina tube using glass rings and a ceramic sealant. Platinum mesh (1 × 1 cm) was employed as the current collector on both the anode and cathode sides to ensure good electrical contact. The mesh was carefully placed over the central region of the SOFC to cover the active reaction area fully. The cell was heated at a rate of 5 °C/min to 800 °C and held for 1 h to reach thermal equilibrium. The electrochemical performance was tested under both humidified hydrogen (~3 vol% H₂O; flow rate: 50 mL/min) and dry methane atmospheres. Ambient air was supplied to the cathode. Current–voltage (I-V) curves and power density outputs were recorded using a programmable direct current (DC) electronic load. The open-circuit voltage (OCV) was monitored throughout the heating process and gas switching.

Dry methane (CH₄, 99.999%) was directly introduced at three concentrations (3.85%, 5.6%, and 10.6%) to investigate the anode’s carbon deposition tolerance, with the total gas flow rate fixed at 50 mL/min. These concentrations were achieved by adjusting the dilution ratio with inert gas to simulate different hydrocarbon fuel conditions. The anode tail gas was analyzed using an online gas chromatograph (Agilent GC 7890A, Santa Clara, CA, USA). Calibration was performed using two certified reference gases, and the total measurement error was controlled within ±2%. The carbon deposition rate was calculated using the carbon balance method, while the water production was estimated based on hydrogen and oxygen consumption.

3. Results and Discussion

3.1. Material Characterizations

XRD analysis was conducted to investigate the phase composition and crystallinity of the electrolyte and anode support layers, as shown in Figure 3. The diffraction patterns of the electrolyte (Sample A) and the anode support (Sample B) are compared alongside the standard reference patterns for NiO (PDF#04-011-2340), Bi₂O₃ (PDF#97-017-3834), and ScSZ (PDF#00-051-1604 and PDF#97-009-2133).

Sample A primarily exhibited an ScSZ structure, as evidenced by major diffraction peaks located at approximately 30.2°, 35.1°, 50.4°, and 59.9°, which corresponded to the standard reflections of cubic ScSZ (PDF#04-008-1973). Additionally, a slight peak shift and minor broadening compared with the ideal cubic positions indicated a rhombohedral distortion, which is consistent with the structural model of ScSZ described in PDF#00-051-1604. This indicates that the sample retained a highly symmetric structure, intermediate between cubic and rhombohedral phases, characteristic of ScSZ with a high scandia content. Although no distinct diffraction peaks corresponding to δ-Bi₂O₃ were observed, a slight intensity perturbation near 27.5° indicated the possible presence of a minor amount of a segregated Bi₂O₃ phase. It can be inferred that while most of the Bi₂O₃ was incorporated into the ScSZ lattice, a small fraction may have existed as isolated nanocrystalline or amorphous domains below the detection limit. The sharpness and symmetry of the major peaks, combined with a low background intensity, confirm the excellent crystallinity. Full-width at half-maximum (FWHM) analysis of the (111) peak at ~30.2° yielded an estimated crystallite size of approximately 45–55 nm, indicating high crystallinity.

Sample B showed dominant crystalline phases identified as NiO and ScSZ. The diffraction peaks at approximately 37.2°, 43.3°, and 62.8° corresponded to the 101, 012, and 110/104 reflections from the NiO phase, while additional peaks near 30.2°, 35.1°, and 50.4° matched those of cubic ScSZ. The possibility of minor rhombohedral distortion, similar to that observed in the electrolyte, cannot be completely excluded. No distinct Bi₂O₃-related peaks were detected in Sample B, although the potential existence of an amorphous Bi₂O₃ domain remains plausible. FWHM analysis of the major NiO peak at ~43.3° indicated an estimated crystallite size of approximately 40–50 nm. The clear and sharp diffraction peaks, coupled with a low and stable background, further demonstrated the high crystallinity and low defect density of the anode support.

The rhombohedral distortion in the anode layer appeared less pronounced compared with the electrolyte layer, as no evident peak shift or broadening beyond the cubic ScSZ references was observed. This may be attributed to the presence of NiO and the different local chemical environment in the anode layer, which may have inhibited the degree of rhombohedral distortion in the ScSZ phase.

While the XRD results confirm high crystallinity and phase purity, microstructural features such as porosity, grain connectivity, and interfacial adhesion cannot be inferred from XRD and are evaluated using SEM analysis.

ScSZ samples with 0, 1, 3, and 5 wt% Bi₂O₃ were prepared and sintered at high temperatures to obtain dense structures to investigate the phase evolution induced by Bi₂O₃ doping. The X-ray diffraction (Cu-Kα; 2θ = 20–80°) results are shown in Figure 4. All samples exhibited major reflections at 2θ ≈ 29.6°, 30.6°, 35.2°, 50.1°, and 51.3°, corresponding to the hexagonal phase reflections of ScSZ (PDF#97-008-6704), including the 006, 152, 154, 158, and 740 planes.

No tetragonal (t-ZrO₂; 2θ ≈ 30.3°) or cubic (δ-Bi₂O₃; 2θ ≈ 27.9°) peaks were observed, indicating that up to 5 wt% Bi₂O₃ was fully incorporated into the ScSZ lattice without forming detectable secondary phases. This suggests that Bi³⁺ ions were successfully doped into the ScSZ crystal structure, stabilized by the cooperative substitution of Sc³⁺ and Bi³⁺, which suppressed phase separation.

As shown by comparison with the orange dashed line, a slight peak shift toward lower angles and a broader full-width at half-maximum (FWHM) was observed with an increasing Bi₂O₃ content, implying lattice expansion due to Bi³⁺ substitution. The expansion was attributed to the significantly larger ionic radius of Bi³⁺ (1.17 Å) compared with Zr⁴⁺ (0.84 Å), leading to local strain and crystal swelling.

Two major effects may arise from the observed lattice expansion:

(1)

Enhanced ionic transport, as enlarged oxygen pathways may facilitate oxygen ion migration and thereby improve conductivity;

(2)

Compromised structural stability since excessive expansion could induce internal stress and increase the risk of microcracking during thermal cycling.

3.2. SOFC Microstructure

The influence of the Bi₂O₃ doping content on the sintering behavior of ScSZ electrolytes was examined using cross-sectional SEM images (Figure 5). These images illustrate the microstructural evolution of samples sintered at 1300 °C for 4 h under varying doping conditions.

The undoped sample (Figure 5a) exhibited insufficient densification, evidenced by a high density of interconnected pores across the cross-section. Nonetheless, distinct grain boundaries were still visible. Upon the addition of 1 wt% Bi₂O₃ (Figure 5b), the grain boundaries became more well-defined, and the overall porosity was reduced, although a small number of isolated pores remained.

At a doping level of 3 wt% Bi₂O₃ (Figure 5c), the sample exhibited a well-balanced microstructure characterized by homogeneous grain growth and significantly reduced porosity, with no large pores detected. This suggests that 3 wt% Bi₂O₃ facilitated optimal densification and enhanced the electrolyte’s structural integrity.

In contrast, when the Bi₂O₃ content was increased to 5 wt% (Figure 5d), abnormal grain growth and heterogeneous particle distribution became apparent. The high-magnification image (×5000) reveals many residual pores dispersed throughout the cross-section, indicating that excessive Bi₂O₃ hindered effective densification.

This deterioration was likely due to the low melting point of Bi₂O₃, which, when present in excess, may lead to uneven liquid-phase formation and the entrapment of volatile species during sintering. The presence of Bi-rich glassy phases along grain boundaries, as reported in a previous study [26], supports the interpretation that Bi₂O₃ can promote liquid-phase sintering. However, excessive Bi₂O₃ may result in microstructural instability, residual porosity, or pore clustering, ultimately compromising densification and uniformity.

EDS mapping and spot analysis confirmed the presence of Bi in all doped electrolyte samples. However, the Bi signals were marked with asterisks (*), indicating that the detected concentrations were close to the instrument’s detection limit and subject to quantification uncertainty. While these results verify the presence of Bi, the exact chemical state and influence of residual Bi-containing species on ionic conductivity remain to be clarified through further investigation.

Figure 6 presents cross-sectional SEM images of the single cell incorporating 3 wt% Bi₂O₃-doped ScSZ electrolyte. In the annotated Figure 6a, the layered structure of the anode-supported SOFC is clearly distinguished, including the porous cathode, dense electrolyte, and porous anode support. The interfaces between layers appear well bonded, with no visible delamination or interfacial defects, indicating good structural integration.

Figure 6b presents a high-magnification SEM image of the electrolyte layer doped with 3 wt% Bi₂O₃. The microstructure exhibits high densification, with well-defined grain boundaries and the absence of interconnected pores. These features confirm the enhanced sintering behavior promoted by Bi₂O₃ addition. Such a dense and structurally uniform layer is critical for preventing gas crossover and achieving a high open-circuit voltage (OCV) in SOFC operation.

Figure 6c presents the porous microstructure of the anode layer doped with Bi₂O₃. A well-developed network of interconnected pores and aggregated particles is observed, indicating that the introduction of Bi₂O₃ did not disrupt the original anode architecture. This preserved porosity facilitated gas diffusion and provided a high surface area, which was favorable for the formation of triple-phase boundaries (TPBs) during fuel cell operation.

Figure 6d depicts the cathode layer, which was fabricated without Bi₂O₃ doping. The cathode maintained a loose, highly porous microstructure with small, uniformly distributed grains. This morphology offered a sufficient surface area and gas pathways to facilitate the oxygen reduction reaction (ORR), demonstrating that the undoped cathode retained excellent catalytic performance. These results confirm that selective doping of Bi₂O₃ into the electrolyte layer improves sintering and densification without compromising the porous structure and functionality of the electrodes.

The porosity was quantified using the Archimedes method to provide further insights into the anode microstructure. Unlike previous studies, in which pre-sintered NiO–ScSZ anodes without Bi₂O₃ doping tended to fracture due to insufficient mechanical integrity, the present sample doped with 3 wt% Bi₂O₃ remained intact, enabling porosity assessment at the pre-sintering stage. This improvement highlights the role of Bi₂O₃ in enhancing low-temperature sintering and mechanical robustness. The pre-sintered NiO-ScSZ support exhibited a porosity of approximately 58.0%, indicating a well-connected porous structure. Subsequent sintering at 1300 °C resulted in a reduced porosity of 44.7%, suggesting significant densification. Upon reduction at 800 °C in hydrogen for 2 h, the porosity increased to approximately 50.0%, attributed to the volumetric shrinkage during the NiO-to-Ni transformation. These results are consistent with the SEM observations in Figure 6c and confirm the effectiveness of the adopted fabrication strategy. The preservation and partial recovery of porosity upon reduction is beneficial for fuel diffusion and TPB formation, which are both critical for high electrochemical performance.

3.3. SOFC Electrochemical Performance

Figure 7 presents the current–voltage (I–V) and current density–power density (I–P) characteristics of the fabricated anode-supported SOFC at various operating temperatures under a humidified hydrogen (~3 vol% H₂O) atmosphere. Before electrochemical measurements, the sealing integrity of the single cell was assessed using a soap bubble flow meter. The gas flow rates at the anode and cathode outlets were nearly identical, confirming excellent sealing performance and electrolyte gas tightness. During the operation, humidified hydrogen and air were continuously supplied to the anode and cathode sides, respectively, with the flow rates kept constant throughout the experiment, ensuring the reliability of the electrochemical data.

The measured OCVs were approximately 1.05 V at 800 °C, 1.03 V at 750 °C, and 1.00 V at 700 °C, indicating the sufficient densification and impermeability of the electrolyte layer. The measured OCV at 800 °C was slightly lower than the theoretical value of ~1.10 V for a system with 3 vol% H₂O. This deviation can be attributed to hydrogen dilution by water vapor. As the operating temperature increased, both the current density and peak power density significantly improved. The SOFC achieved peak power densities of 0.861 W/cm² at 800 °C, 0.643 W/cm² at 750 °C, and 0.552 W/cm² at 700 °C.

The performance enhancement at elevated temperatures can be mainly attributed to the increased ionic conductivity of the YSZ electrolyte, reduced ohmic resistance, lower activation overpotential at the electrode interfaces, and accelerated electrochemical reaction kinetics. The I–V profiles exhibited a typical monotonic voltage decrease with increasing current density, reflecting the combined effects of activation, ohmic, and concentration polarizations. At 700 °C, a steeper voltage drop was observed, suggesting that activation polarization was likely the dominant factor under low-temperature operation due to the limited electrocatalytic activity of the LSM cathode. In contrast, the voltage decrease at 800 °C was much more gradual, indicating improved charge transfer characteristics and enhanced electrode reaction kinetics.

This observation underscores the critical importance of maintaining a high operating temperature to minimize polarization loss and optimize cell performance.

Electrochemical impedance spectroscopy (EIS) measurements were not included in this study due to persistent technical issues during frequency sweeps. Future work will incorporate EIS analysis to further distinguish the contributions of ohmic resistance and electrode polarization.

The electrochemical performance of the SOFC under a dry methane atmosphere (CH₄ concentration: 16.7 vol%) was further evaluated, as shown in Figure 8. The initial OCVs under methane were recorded at approximately 1.22 V at 800 °C, 1.20 V at 750 °C, and 1.17 at 700 °C, which were slightly higher than those obtained under a hydrogen atmosphere, consistent with the higher theoretical potential of methane oxidation. Moreover, the stability of the OCVs during the initial open-circuit operation confirmed the gas tightness of the electrolyte, even under hydrocarbon conditions.

Despite the high initial OCVs, a more pronounced voltage decline with increasing current density was observed during methane operation, particularly at lower temperatures. This behavior was primarily attributed to the sluggish electrochemical oxidation kinetics of methane compared with hydrogen, as well as the increased anode polarization resistance associated with limited fuel oxidation rates. The maximum power densities achieved under a methane atmosphere were 0.624 W/cm² at 800 °C, 0.477 W/cm² at 750 °C, and 0.386 W/cm² at 700 °C, all lower than those measured under hydrogen operation. The performance degradation was likely due to the combined effects of slower methane oxidation, local fuel depletion at high current densities, and potential carbon deposition on the anode surface, which could block active sites and increase polarization losses.

Overall, the fabricated SOFC demonstrated a strong temperature-dependent performance under both humidified hydrogen and dry methane atmospheres, with a significantly enhanced output at elevated temperatures. Nevertheless, even at 800 °C, the power output under methane remained inferior to that under hydrogen, highlighting the necessity for further optimization of the anode microstructure, catalytic activity, and carbon tolerance to achieve efficient and stable direct hydrocarbon-fueled SOFC operation.

3.4. Gas Analysis

Methane undergoes thermal decomposition at elevated temperatures, including at the TPB on the anode side. The decomposition products—primarily carbon and hydrogen—can subsequently react with oxygen ions (O²⁻) transported from the cathode. In addition, methane may adsorb onto the surface of the anode catalyst and directly participate in electrochemical oxidation reactions with O²⁻ ions, thereby contributing to the overall fuel oxidation process.

The gas analysis at the anode outlet revealed the presence of six major components: CH₄, CO, CO₂, H₂, H₂O, and deposited carbon. These species were involved in a network of electrochemical and thermochemical reactions that collectively defined the local reaction environment and fuel conversion behavior.

The reaction mechanisms of methane reforming and oxidation were previously detailed in [23] and are briefly summarized in

Table 1. Summary of electrochemical and chemical reactions at the anode.

No.	Reaction Equation	Reaction Type	Description
(2)	${\mathrm{C}\mathrm{H}}_4+4{\mathrm{O}}^{2-}\to {\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}$	Electrochemical	Complete oxidation
(3)	${\mathrm{C}\mathrm{H}}_4+{\mathrm{O}}^{2-}\to \mathrm{C}\mathrm{O}+2{\mathrm{H}}_2+2{\mathrm{e}}^{-}$	Electrochemical	Partial oxidation
(4)	$\mathrm{C}{\mathrm{H}}_4+2{\mathrm{O}}^{2-}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$	Electrochemical	Partial oxidation
(5)	$\mathrm{C}{\mathrm{H}}_4+2{\mathrm{O}}^{2-}\to \mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2+4{\mathrm{e}}^{-}$	Electrochemical	Partial oxidation
(6)	${\mathrm{C}\mathrm{H}}_4+3{\mathrm{O}}^{2-}\to \mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}$	Electrochemical	Partial oxidation
(7)	$\mathrm{C}{\mathrm{H}}_4+3{\mathrm{O}}^{2-}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}$	Electrochemical	Partial oxidation
(8)	${\mathrm{H}}_2+{\mathrm{O}}^{2-}\to {\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$	Electrochemical	Hydrogen oxidation
(9)	$\mathrm{C}\mathrm{O}+{\mathrm{O}}^{2-}\to \mathrm{C}{\mathrm{O}}_2+2{\mathrm{e}}^{-}$	Electrochemical	CO oxidation
(10)	$\mathrm{C}+{\mathrm{O}}^{2-}\to \mathrm{C}\mathrm{O}+2{\mathrm{e}}^{-}$	Electrochemical	Carbon oxidation
(11)	$\mathrm{C}{\mathrm{H}}_4\to \mathrm{C}+2{\mathrm{H}}_2$	Chemical (thermal)	Thermal decomposition
(12)	$\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2$	Chemical (thermal)	Steam reforming
(13)	$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$	Chemical (thermal)	Water–gas shift
(14)	$\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{O}}_2\to 2\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2$	Chemical (thermal)	Dry reforming
(15)	$2\mathrm{C}\mathrm{O}\to \mathrm{C}+\mathrm{C}{\mathrm{O}}_2$	Chemical (thermal)	Boudouard reaction
(16)	$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2$	Chemical (thermal)	Carbon gasification

.

The gas evolution behaviors of NiO-ScSZ/ScSZ/LSM anode-supported SOFCs were systematically investigated under varying methane concentrations (3.85%, 5.6%, and 10.6%) at 800 °C to assist in evaluating the anode performance, particularly its oxidation activity and carbon tolerance. The results were compared with those of a previously reported NiO–YSZ-based SOFC to clarify the effect of anode composition on electrochemical reaction initiation, methane-reforming behavior, and carbon deposition suppression.

In the NiO-YSZ/YSZ/LSM system under 3.85% methane, gas chromatographic analysis showed that at low current densities (<0.192 A·cm⁻²), the anode exhaust predominantly contained CH₄, CO, and H₂, with no detectable CO₂ or H₂O. This indicates that methane decomposition was the dominant process in this regime, with negligible electrochemical oxidation. As the current density increased to approximately 0.192 A·cm⁻², H₂O appeared, signaling the onset of hydrogen electrochemical oxidation. CO₂ formation was only observed above 0.256 A·cm⁻², reflecting the delayed initiation of CO and carbon oxidation. This late activation of oxidation pathways implies insufficient early oxygen ion transport, which can lead to the accumulation of solid carbon and subsequent anode degradation under direct hydrocarbon operation [23,27,28].

In contrast, the Bi₂O₃-doped NiO-ScSZ/ScSZ/LSM cell operating under 3.85% methane exhibited significantly improved oxidation behavior, as shown in Figure 9. CO₂ formation initiated at approximately 0.128 A·cm⁻², accompanied by earlier H₂O generation and quicker CH₄ consumption with increasing current density. This earlier activation of oxidation reactions suggests that the improved oxide ion conductivity of the ScSZ electrolyte, together with the optimized anode microstructure, enhanced the oxygen ion flux to the anode, thus enabling the prompt oxidation of reforming intermediates even under mild operating conditions. The ability to rapidly initiate CO and carbon oxidation is critical for minimizing the risk of carbon deposition, ensuring stable anode operation during hydrocarbon fuel utilization.

At a 5.6% methane concentration, as shown in Figure 10, the gas evolution behavior exhibited noticeable changes. The actual onset of CO₂ formation was delayed to approximately 0.256 A·cm⁻². Although minor CO₂ signals were detected at lower current densities, these were considered instrumental artifacts. The elevated methane partial pressure increased the concentrations of H₂ and CO in the anode environment, intensifying the local reducing atmosphere and thereby suppressing the early electrochemical oxidation of intermediate species. Consequently, a higher external current load was required to supply sufficient oxygen ions to overcome the local chemical environment and drive CO and carbon oxidation reactions. Nevertheless, even under these conditions, the NiO-ScSZ/ScSZ/LSM cell maintained stable CH₄ consumption and showed gradual CO₂ and H₂O generation as the current increased, confirming the robust anode reaction kinetics.

At a 10.6% methane concentration (Figure 11), the suppression of early oxidation reactions became even more pronounced. The onset of substantial CO₂ production shifted further to approximately 0.445 A·cm⁻². Under such high methane partial pressure, the strongly reducing environment significantly inhibited the early stages of oxygen-ion-involved oxidation reactions. Despite these harsher conditions, the Bi₂O₃-doped NiO-ScSZ/ScSZ/LSM cell exhibited steady CH₄ conversion and continued the generation of CO and H₂, indicating effective reforming activity. Moreover, the emergence of H₂O at relatively low current densities suggested that hydrogen oxidation could proceed even before the full activation of CO or carbon oxidation pathways, reflecting the high electrochemical activity supported by the enhanced oxide ion transport properties of ScSZ.

Overall, the gas analysis results demonstrate that the Bi₂O₃-doped NiO–ScSZ/ScSZ/LSM anode, combined with the ScSZ electrolyte, effectively promoted earlier and more stable electrochemical oxidation of methane-derived species compared with the conventional NiO-YSZ anode structure. Even as the methane concentration increased and the local anode environment became increasingly reducing, the ScSZ-based system retained strong oxidation activity and maintained fuel conversion without signs of serious carbon accumulation. These characteristics confirm the superior stability and carbon tolerance of the developed anode, supporting its excellent performance after low-temperature co-sintering. The gas evolution behavior under dry methane atmospheres thus provides important auxiliary evidence for the enhanced anode microstructural and electrochemical properties achieved in this work.

4. Conclusions

In this study, a novel anode-supported SOFC configuration employing Bi₂O₃-doped NiO-ScSZ as the anode and ScSZ as the electrolyte was successfully fabricated via a low-temperature co-sintering process at 1300 °C. The introduction of 3 wt% Bi₂O₃ effectively enhanced the sintering ability of both the anode and electrolyte layers, enabling the formation of dense and gas-tight ScSZ electrolytes and mechanically robust porous anode supports without compromising structural integrity.

Comprehensive material characterizations confirmed that the electrolyte had a highly dense microstructure with well-preserved cubic/rhombohedral ScSZ phases, while the anode support maintained a favorable porous framework suitable for gas transport and electrochemical activity. The optimized microstructure contributed to excellent electrochemical performance, achieving peak power densities of 0.861 W cm⁻² at 800 °C under hydrogen operation and maintaining open-circuit voltages close to the theoretical values, indicating superior electrolyte densification and gas-sealing quality.

Under dry methane operation, the cells demonstrated stable output with maximum power densities reaching 0.624 W cm⁻² at 800 °C. Although the performance under methane was lower than that under hydrogen, the cell exhibited good tolerance to carbon deposition, as further evidenced by systematic gas analysis. The Bi₂O₃-doped NiO-ScSZ/ScSZ/LSM cell promoted earlier and more stable electrochemical oxidation of methane-derived species compared with conventional NiO-YSZ systems, even under increasingly reducing environments at higher methane concentrations. This behavior reflects the enhanced oxygen ion transport capability and improved anode structural stability enabled by Bi₂O₃ addition.

Overall, the results highlight that Bi₂O₃ co-doping combined with optimized low-temperature co-sintering is an effective strategy to achieve high-performance anode-supported SOFCs with ScSZ electrolytes. The proposed fabrication approach reduces the sintering temperature and processing cost while also improving cell performance and stability, providing valuable insights into the development of cost-effective and durable intermediate-temperature SOFC technologies.