Structural Robustness Engineering for NiFe Metal-Supported Solid Oxide Fuel Cells

Abstract

The chromium-free oxide precursor strategy effectively avoids chromium volatilization and electrode contamination in metal-supported solid oxide fuel cells (MS-SOFCs), while enabling high-temperature co-sintering in air to simplify the fabrication process. However, the drastic microstructural coarsening, dimensional shrinkage, and thermal expansion mismatch with adjacent components of such substrates during high-temperature sintering, reduction, and thermal cycling collectively contribute to the interfacial instability and structural degradation of MS-SOFCs. Herein, we address these issues by incorporating a small amount of Gd_0.1Ce_0.9O_1.95 (GDC) to the NiO-Fe₂O₃ (NFO) substrate. The incorporation of GDC significantly enhances the sintering compatibility and reduction stability of the MS-SOFCs, alleviating the stress-induced warping and distortion. Moreover, the GDC phase has a pinning effect to suppressing the coarsening of the substrates during high-temperature sintering and reduction processes, enhancing mechanical integrity and structural robustness of the single cell. With 15 wt% GDC incorporated into the NiFe substrate, the corresponding MS-SOFC with GDC electrolyte film achieves a peak power density of 0.56 W cm⁻² at 600 °C, along with markedly improved structural integrity and operational reliability. This work demonstrates a viable pathway for designing heterophase-engineered supports with matched thermomechanical properties, offering promising prospects for enhancing the durability of MS-SOFCs.

1. Introduction

Among various fuel cells for electrochemical energy conversion, solid oxide fuel cells (SOFCs) enables the direct transformation of chemical energy into electricity with a high efficiency, low emissions, broad fuel flexibility, and low cost [1,2,3,4,5]. Among various structural configurations, metal-supported SOFCs (MS-SOFCs) have emerged as a highly attractive candidate for applications such as auxiliary power units and range extenders in transportation, portable or emergency power supplies, distributed combined heat and power systems requiring flexible start-up and shutdown, and military applications, owing to their cost-effectiveness, rapid start-up capability, simplified fabrication process, and enhanced mechanical strength [6,7,8,9].

At present, ferritic stainless steels are widely employed as structural support materials in MS-SOFCs [10,11,12,13]. However, this material system faces intrinsic challenges under high-temperature operating conditions. Ferritic stainless steels typically contain 10.5–26% chromium, which tends to diffuse rapidly in humid and elevated-temperature environments, leading to the formation of a Cr-rich oxide layer on the surface. Although the bulk conductivity of this oxide scale remains within an acceptable range, its continuous growth increases interfacial resistance, thereby elevating the area-specific resistance (ASR) of the cell [14,15]. In addition, volatile chromium species generated at high temperatures can migrate to the cathode surface or the cathode–electrolyte interface, where they block active sites for the oxygen reduction reaction (ORR) and significantly deteriorate the electrochemical performance [16,17,18,19]. Furthermore, to mitigate oxidation of the steel support during fabrication, dense electrolyte films are typically deposited using low-temperature techniques such as plasma spraying, pulsed laser deposition, and physical vapor deposition [20]. However, these techniques are often associated with high production costs and present challenges for large-scale fabrication and process reliability.

To address these challenges, a promising strategy has been proposed involving the use of NiO-Fe₂O₃ oxide precursor to construct Cr-free support substrates, which intrinsically eliminate chromium volatilization and contamination issues at the material level [21,22,23,24,25]. Moreover, such substrates can be directly sintered at high-temperatures in air without relying on low-temperature deposition techniques for electrolyte films, significantly simplifying the fabrication process and reducing the production costs. During the reduction stage of the substrates, the NiO-Fe₂O₃ precursor is in situ transformed to a porous NiFe alloy with porosity of 30–50%, which ensures sufficient fuel gas permeability. This approach offers excellent structural tunability and holds great promise for practical applications.

Despite its notable structural and processing advantages, this oxide precursor strategy still faces critical challenges related to coupled stress in practical applications. On the one hand, during high-temperature co-sintering, significant mismatch in sintering shrinkage between the NiO-Fe₂O₃ precursor and the adjacent electrolyte and functional layers can lead to stress concentration at the interfaces. Subsequent reduction of the substrate, which involves volumetric shrinkage due to metal formation, further exacerbates the interfacial compressive stress induced by dimensional mismatch [26,27]. On the other hand, thermal expansion coefficient (TEC) differences among cell components can introduce thermal mismatch stress during operation [28,29]. These multi-origin coupled stresses—arising from sintering behavior, phase transformation, and thermomechanical responses—not only compromise interfacial adhesion strength but may also initiate crack propagation and structural degradation, posing significant barriers to the long-term stability and reliability of the cell.

To mitigate the multi-origin coupled stresses arising from mismatched dimensional evolution and to enhance the structural stability of NiFe substrates, researchers have explored the incorporation of more thermally stable and less reducible secondary phases at the material design level. Wang et al. introduced 0.5 wt% MgO into NiFe alloys, which significantly suppressed both sintering and reduction-induced shrinkage of the substrate, thereby enhancing its overall thermal stability [30]. Lu et al. increased the MgO content to 10 wt%, reducing the reduction shrinkage rate of the NiFe substrate from 12.64% to only 0.48% [31]. In addition, Liu et al. demonstrated that incorporating 15 wt% Y₂O₃-stabilized ZrO₂ (YSZ) into NiFe alloys effectively suppressed reduction-induced structural collapse while largely maintaining the electrical conductivity, leading to improved geometric flatness and operational stability of MS-SOFCs [32]. However, most of these shrinkage control strategies have been developed for MS-SOFCs employing YSZ electrolyte. In contrast, although Gd_0.1Ce_0.9O_1.95 (GDC) has been widely recognized as a promising electrolyte for low-to-intermediate temperature MS-SOFCs (500–600 °C) due to its high oxygen ion conductivity and outstanding redox stability, its roles in regulating the shrinkage behavior, interfacial compatibility, and structural robustness in metal substrates remain underexplored.

Herein, a small amount of GDC phase ranging from 5 to 15 wt% was introduced to systematically investigate its effect on the structural characteristics of the NiO-Fe₂O₃ substrate and the electrochemical performance of single cells. The results reveal that the incorporation of GDC effectively mitigates the structural warping and stress concentration caused by dimensional mismatch during high-temperature sintering and reduction processes. Additionally, the GDC phase exerts a pinning effect that suppresses the coarsening of substrates, thereby significantly improving the mechanical strength and structural integrity of the single cells. Moreover, the GDC phase also contributes to the dimensional stability and thermal compatibility of the substrate under reducing atmospheres. With the incorporation of 15 wt% GDC, the single cell achieved a peak power density of 0.56 W cm⁻² at 600 °C, along with markedly enhanced structural stability and operational reliability. This work presents an effective strategy for constructing metal supports with matched thermomechanical properties through heterophase design, offering a promising potential for enhancing the long-term stability of MS-SOFCs.

2. Results and Discussion

Figure 1 presents the phase analysis and surface morphologies of NiFeO_x-GDC (NFO-GDC) substrates sintered at 1400 °C, which were designated as NFO-0GDC, NFO-5GDC, NFO-10GDC, and NFO-15GDC corresponding to GDC weight fractions of 0 wt%, 5 wt%, 10 wt%, and 15 wt%, respectively. X-ray diffraction (XRD) analysis reveals that the NFO-0GDC sample exhibits distinct diffraction peaks corresponding to NiO and NiFe₂O₄ phases (Figure 1a). The formation of NiFe₂O₄ is as a result of the reaction between NiO and Fe₂O₃, which is thermodynamically driven due to lowered system’s Gibbs free energy [33]. Since some diffraction peaks of NiO and NiFe₂O₄ overlap, the higher relative intensity between the peaks at 43.4° and 30.3° compared to that of the pristine NiFe₂O₄ indicates additional contribution from NiO, further confirming the coexistence of both phases. With the incorporation of GDC, characteristic diffraction peaks corresponding to NiO, NiFe₂O₄, and GDC are simultaneously observed in the NFO-GDC samples without additional phases, and the intensity of the GDC peaks progressively increases with increasing the GDC content as expected (Figure 1a). Scanning electron microscopy (SEM) images demonstrate a marked evolution in surface morphology as the GDC content increases, i.e., the initially smooth substrate surface transforms into a more textured structure enriched with fine GDC particulates, accompanied by a notable reduction in the size of NFO grains (Figure 1b–f). Specifically, the average particle size of NFO decreases from 6.3 ± 1.0 μm to 2.1 ± 0.3 μm as the GDC content increases from 0 wt% to 15 wt% (Figure 1b). The decrease in grain size of NFO is attributed to a pinning effect exerted by the GDC phase during sintering, which impedes the migration of grain boundary and thereby suppresses excessive grain growth [34].

Further optical photos illustrate the surface morphology evolution of the half-cells after sintering (Figure 1g). In the absence of GDC, the surface of NFO-0GDC half-cell exhibits numerous pits, indicative of pronounced structural inhomogeneity during sintering. Upon incorporation of 5 wt% GDC, only a few pits are observed, while the NFO-10GDC and NFO-15GDC cells exhibit uniformly flat and pit-free surfaces, highlighting the positive role of GDC in enhancing the surface uniformity during sintering. During high-temperature sintering, the NFO substrate tends to be densified rapidly. Without effective structural regulation to provide both skeletal support and shrinkage accommodation, local sintering non-uniformity may occur, leading to uneven shrinkage across the substrate. This non-uniform shrinkage can induce localized structural collapse, ultimately resulting in surface pits in the final product [35]. In addition, sintering shrinkage measurements reveal a slight increase in linear shrinkage with increasing the GDC content—measured at 11.2%, 11.5%, 11.8%, and 11.9% for 0, 5, 10, and 15 wt% GDC, respectively (Table S1). These shrinkage values are close to that of the anode layer (12.9%) with a higher GDC content of 50 wt%. After sintering, the surfaces of half-cells become more flat with the incorporated GDC phase in the substrate.

The sintered NFO-GDC substrates were reduced in an H₂ atmosphere at 600 °C for 2 h, and their phase analysis and morphologies are illustrated in Figure 2. The reduced NFO-0GDC, NFO-5GDC, NFO-10GDC, and NFO-15GDC substrates are hereafter referred to as NF-0GDC, NF-5GDC, NF-10GDC, and NF-15GDC, respectively. During the reduction process, NiO is reduced to metallic Ni, while NiFe₂O₄ is reduced to NiFe₂. These reduced species then undergo solid-state interdiffusion and alloying, ultimately forming a NiFe alloy, as evidenced by the XRD results (Figure 2a) [36,37]. Meanwhile, the GDC phase remains structurally stable under reducing conditions, contributing to the overall stability of the composite substrate (Figure 2a). With increasing the GDC content, the diffraction peaks of the NiFe alloy progressively shift toward lower angles, whereas those of GDC shift toward higher angles, suggesting strong interactions between NiFe and GDC. These interactions lead to lattice expansion in NiFe and contraction in GDC (Figure 2a), arising from the lattice mismatch that induces strain accommodation and subtle structural adjustments [38]. In addition, limited cation interdiffusion and oxygen vacancy formation under reducing conditions may further contribute to the observed lattice changes [39]. Rietveld refinement further corroborates this observation: the lattice parameter of the NiFe phase increases from 2.533 Å in NF-0GDC to 2.558 Å in NF-15GDC, while that of GDC decreases correspondingly, confirming a lattice-tuning mechanism driven by the phase interaction between the two components (Figure S1 and Table S2).

Microstructural analysis reveals that the NF-0GDC substrate without GDC exhibits relatively large surface grains with an average size of 7.1 ± 1.4 μm, accompanied by pronounced porosity in localized regions (Figure 2b,g). The porous morphology arises primarily from the Kirkendall effect, induced by the mismatch in diffusion rates between Ni and Fe during the alloying process between NiFe₂ and Ni [40]. As the diffusion coefficient of Fe in NiFe alloys is substantially higher than that of Ni, Fe atoms diffuse outward from the NiFe₂ grains faster than Ni atoms diffuse inward from the Ni grains. This imbalance leads to vacancy accumulation and void formation within the original NiFe₂ grains. Energy-dispersive X-ray spectroscopy (EDS) analysis further supports this mechanism: compared to the dense region (point 1, Fe:Ni = 0.69), the porous region is markedly enriched with Fe (point 2, Fe:Ni = 1.51) (Figure 2f). With increasing the GDC content, a large number of dense, fine particles appear on the substrate surface in addition to the original porous NiFe phase. EDS analysis confirms that these newly formed particles correspond to GDC, as indicated by the arrows, while the original porous phase is identified as NiFe (Figure 2c–e and Figure S2). Concurrently, the NiFe grain size decreases significantly, with the average grain size reduces to 1.3 ± 0.3 μm in the NF-15GDC substrate (Figure 2g). This trend suggests that GDC particles also exert a pinning effect during the reduction process. Their high thermal stability allows them to act as barriers for the migration of grain boundaries of NFO, impeding atomic diffusion and effectively suppressing grain coarsening—a phenomenon particularly pronounced at a high GDC content.

The incorporation of GDC phase plays a critical role in regulating the reduction-induced shrinkage behavior of NiFe-based substrates. After reduction, the NF-0GDC substrate exhibits pronounced warping, with a shrinkage rate as high as 4.07% (Figure 3a,b). As the GDC content increases, the diameter of reduced substrates progressively enlarges, and the extent of shrinkage is significantly suppressed (Figure 3a,b). Specifically, NF-5GDC shows slight warping, while NF-10GDC and NF-15GDC remain nearly flat with shrinkage rates reduced to 0.69% and 0.13%, respectively. This remarkable improvement in dimensional stability mitigates the mechanical mismatch between the substrate, the Ni-GDC anode, and the GDC electrolyte, as during the reduction process, both the anode and the electrolyte exhibit negligible shrinkage (Table S3). Consequently, in the NF-GDC substrates with a GDC content less than 5 wt%, compressive stress accumulation at the interface may induce bending or even fracture of the thin-film electrolyte. In contrast, as the GDC content in the substrate reaches 10 wt% or 15 wt%, the dimensional change becomes well matched with that of the adjoining component, effectively relieving the interfacial stress. This enhanced compatibility significantly increases the success rate of in situ reduction in terms of structural integrity of the cell.

The incorporation of GDC significantly modulates the physical properties of the NiFe-based substrates in multiple aspects, with the change in TEC being particularly critical. As the GDC content increases, TEC of the NF-GDC substrates gradually decreases from 13.3 × 10⁻⁶ K⁻¹ for NF-0GDC to 13.0 × 10⁻⁶ K⁻¹ for NF-5GDC, 12.6 × 10⁻⁶ K⁻¹ for NF-10GDC, and eventually to a minimum of 12.1 × 10⁻⁶ K⁻¹ for NF-15GDC (Figure 3c and Figure S3). This downward trend is attributed to the intrinsically lower TEC of GDC (11.6 × 10⁻⁶ K⁻¹) and the synergistic lattice distortion caused by NiFe lattice expansion and GDC lattice contraction (Figure S3). GDC incorporation thus improves thermal compatibility with the electrolyte, effectively relieving thermal stress and enhancing structural stability. In addition to the thermomechanical properties, GDC also influences the electrical conductivity of NiFe substrates to some extent. As shown in Figure 3d, with increasing the GDC content, the electrical conductivity at 600 °C decreases from 2001 S cm⁻¹ for NF-0GDC to 1076 S cm⁻¹ for NF-15GDC. This reduction is primarily attributed to the interruption of the electronic conduction pathways of NiFe by the ion-conductive GDC phase. Nevertheless, due to the relatively low GDC amount (15 wt% is equivalent to 15.95 vol%), the overall impact on the electrical conductivity of the substrates is limited. Moreover, NF-15GDC substrate exhibits a lower conduction activation energy than Ni-GDC substrate (2.71 ± 0.27 vs. 3.57 ± 0.88 kJ mol⁻¹) (Figure 3d), underscoring the superior electronic transport properties of the NF-GDC substrates. GDC incorporation also significantly enhances the mechanical strength of the NiFe substrates. As shown in Figure 3e, the flexural strength increases progressively with increasing the GDC content, from 35.9 MPa for NF-0GDC to 38.4, 41.0, and 53.8 MPa for NF-5GDC, NF-10GDC, and NF-15GDC, respectively. This improvement is partly attributed to the higher intrinsic flexural strength of GDC compared to the NiFe substrate (40.3 vs. 35.9 MPa), which contributes to the overall mechanical reinforcement of the composite substrate. Furthermore, the reduced size of NiFe alloy grains in NF-GDC contributes to the grain-boundary strengthening effect and thus NF-15GDC achieves a flexural strength notably higher than GDC (53.8 vs. 40.3 MPa).

Figure 4 presents the electrochemical performance and stability of NF-GDC supported single cells. During the reduction process of the substrates, GDC incorporation plays a crucial role in maintaining structural stability and preserving open-circuit voltage (OCV) of the single cells. Notably, NF-0GDC and NF-5GDC cells exhibit a gradual decline in OCV during reduction, accompanied by the formation of microcracks on the electrolyte surface and thereby gas leakage. In particular, the initial OCV of NF-0GDC cell is 0.81 V, decreases gradually up to 23 min, followed by a rapid drop, indicating the severe structural failure of the single cell. In contrast, the NF-10GDC and NF-15GDC cells maintain a stable OCV of approximately 0.82 V throughout the reduction process, suggesting enhanced mechanical integrity and improved resistance to crack formation due to thermal stress. Repeatability tests confirm this trend for both NF-0GDC and NF-15GDC cells (Figure S4). It is worth noting that structural rupture results in serious gas leakage, and therefore performance testing is typically limited to intact cells. Although majority of the NF-0GDC cells were fractured during the reduction process, one single cell survived, providing a reference for subsequent performance and stability comparison. As shown in Figure 4b, the OCVs of NF-0GDC, NF-10GDC, and NF-15GDC cells are all close to 0.82 V at 600 °C, consistent with the theoretical OCV for GDC-based electrolyte. At 600 °C, the peak power densities (PPDs) of these cells are 0.60, 0.55, and 0.56 W cm⁻², respectively, indicating comparable overall performance. Electrochemical impedance spectroscopy (EIS) analysis reveals that the ohmic resistance (R_Ω) for all three cells is approximately 0.1 Ω cm², while the electrode polarization resistance (R_p) shows a slight variation—0.15, 0.18, and 0.17 Ω cm² for NF-0GDC, NF-10GDC, and NF-15GDC, respectively—but remains within a narrow range. R_Ω includes contributions from the bulk resistances of the anode, substrate, electrolyte, and cathode, as well as interfacial and current collector contact resistances [41]. For the NF-0GDC cell, the lack of OCV stabilization likely causes the irregular impedance responses at low frequency, yet the high- and medium-frequency arcs remain clear, allowing reliable extraction of R_Ω and R_p. In addition, although GDC incorporation slightly reduces the electrical conductivity of the substrate (Figure 3d), it does not significantly increase the total ohmic resistance of the cells, suggesting that its impact on the overall electrochemical performance is relatively minor.

Figure 4d shows the galvanostatic stability profiles of NF-0GDC, NF-10GDC, and NF-15GDC cells at a constant current density of 0.25 A cm⁻². The only NF-0GDC cell that successfully completed the initial performance test exhibits poor operational stability with rapid voltage decay after approximately 18 h of operation. This degradation is likely attributed to cracking of the GDC electrolyte film under mechanical stress and subsequent cell failure. In contrast, both NF-10GDC and NF-15GDC cells maintain steady voltage output under identical conditions without noticeable fluctuations, demonstrating significantly improved durability.

Overall, the degradation of NF-0GDC cells can be primarily ascribed to drastic mismatches between the substrate and the electrolyte in terms of sintering behavior, reduction shrinkage, and TEC. These mismatches result in the accumulation of residual and thermal mismatch stresses during high-temperature processing and operation. With increasing the GDC content, the structural compatibility between the NiFe substrate and the GDC electrolyte is markedly enhanced in NF-10GDC and NF-15GDC cells, effectively mitigating stress concentration and crack propagation. Moreover, a certain degree of sintering shrinkage of the metallic substrate continues during cell operation. In cells exhibiting warping, tensile stresses are prone to concentrate at curved regions, increasing the risk of electrolyte film fracture. A schematic illustration is shown in Figure 5, which further highlights the role of GDC in establishing structural compatibility and enhancing the mechanical stability of the cell architecture.

3. Materials and Methods

3.1. Preparation and Characterization of Metal Substrates

NiO (99.0%, Aladdin, Shanghai, China), Fe₂O₃ (99.0%, Aladdin, Shanghai, China), tapioca starch, and polyvinyl butyral (PVB) powder were weighed and mixed at a mass ratio of 50:56:5:1.25. Subsequently, GDC powder (AGC Seimi Chemical Corporation, Tokyo, Japan) was added to the mixture at mass fractions of 0 wt%, 5 wt%, 10 wt%, and 15 wt%, respectively. The resulting powder mixtures were dispersed in anhydrous ethanol and subjected to ball milling for 48 h to ensure homogeneous mixing of all components. After drying, the resulting powders were uniaxially pressed into circular pellets with a diameter of 18 mm under a pressure of 140 MPa, followed by pre-calcination at 900 °C to obtain sufficient mechanical strength. In this way, NFO-GDC composite substrates with different GDC contents were obtained, and the corresponding reduced samples were obtained as NF-GDC substrates.

To elucidate the phase evolution behavior of the substrates induced by the reduction process, the crystal structures before and after reduction were characterized by XRD (Rigaku ULTIMA III, Tokyo, Japan) using a Cu Kα radiation source over a 2θ range of 20–80°. The surface morphology of the substrates was examined by SEM (Carl Zeiss SUPRA 55, Oberkochen, Germany), and elemental distribution was analyzed using EDS. To assess the mechanical strength, 1 g of the premixed powder was pressed into bar-shaped samples (30 × 8 × 1.3 mm³) under a pressure of 140 MPa, sintered at 1400 °C for 2 h, and subsequently reduced in H₂ atmosphere. The reduced specimens were subjected to three-point bending test using a universal testing machine (CMT 4104, SENS, Shenzhen, China). The test data were linearly fitted to generate the corresponding strength–displacement curves. The metal-supported specimens used for electrical conductivity measurement had dimensions of 25.2 × 6.4 × 1.1 mm³. As shown in Figure S5, silver wires were attached to both ends of the sample and fixed with silver paste, followed by drying at 150 °C. The specimens were then connected to a Keithley 2400 source meter, and their resistance was measured over the temperature range of 200–800 °C using the DC four-terminal method. The reduction induced shrinkage rate was evaluated by comparing the sample diameters before and after reduction. For TEC measurements, the supports and electrolytes were separately pressed under a pressure of 140 MPa, followed by heating at 2 °C min⁻¹ to 1400 °C and sintering for 2 h. The sintered cylindrical specimens exhibited a diameter of approximately 5.4 mm and a thickness of approximately 2.0 mm. TECs of the reduced substrates and electrolyte were measured using a dilatometer (DIL402C/4/G, Selb, Germany) under argon atmosphere, in a temperature range of 50–750 °C and a heating rate of 5 °C min⁻¹.

3.2. Fabrication of Single Cells

MS-SOFC single cells were fabricated with an integrated structure of 470 μm thick NFO-GDC substrate|9 μm thick NiO-GDC anode|9 μm thick GDC electrolyte film|30 μm thick PrBa_0.8Ca_0.2Co₂O_5+δ-Gd_0.2Ce_0.8O_1.9 (PBCC-GDC20) cathode. The anode layers were fabricated on the pre-calcined substrate via a combination of spin coating and co-sintering processes. Initially, a NiO-GDC anode slurry was spin-coated onto the NFO-GDC substrate, followed by heat-treatment at 600 °C for 2 h to remove the organic components. Subsequently, a GDC electrolyte slurry was deposited by spin coating, followed by co-sintering at 1400 °C for 2 h. PBCC-GDC20 composite cathode with an area of 0.22 cm² was screen-printed onto the GDC electrolyte, followed by drying at 150 °C for 2 h without further sintering at high temperatures. The composite cathode powder was synthesized via a one-pot method and calcined at 750 °C for 3 h. The detailed fabrication procedures have been thoroughly described in our previous studies [37,42,43,44,45,46].

3.3. Electrochemical Performance Testing

Platinum paste (Gwent Electronic Materials, Pontypool, UK) was uniformly applied to both the cathode and substrate of the single cell, followed by placement of platinum mesh as the current collector. Silver wires were affixed to the platinum mesh using silver paste and dried at 150 °C for 1 h. The assembled single cell was then sealed into an alumina tube using a ceramic sealant (Aremco Products, Inc., Valley Cottage, NY, USA), cured at room temperature for 12 h, and subsequently transferred to a programmable high-temperature furnace for further processing. The cell was heated to 600 °C at a rate of 5 °C min⁻¹, with 50 mL min⁻¹ N₂ introduced to the anode side as a protective gas. Upon reaching the target testing temperature, the anode atmosphere was switched to H₂ for in situ reduction over a period of 2 h, while the cathode side was exposed to ambient air. EIS and I–V curves of the single cells were measured using an electrochemical workstation (Gamry Interface 1000E, Warminster, PA, USA). EIS measurements were conducted under a DC bias of 0.7 V over a frequency range of 100 kHz to 0.1 Hz with an AC perturbation amplitude of 10 mV. Galvanostatic stability tests were carried out at 600 °C under a constant current density of 0.25 A cm⁻², monitored using another electrochemical workstation (Arbin BT-2000, College Station, TX, USA).

4. Conclusions

This study systematically elucidates the synergistic role of GDC incorporation in tuning the structural, thermal, and mechanical properties of NiFe-based substrates. GDC incorporation significantly enhances dimensional shrinkage compatibility between the substrates and the adjacent components during high-temperature sintering and reduction processes, alleviating stress-induced warping of the single cells. In addition, GDC phase plays a pinning role in inhibiting grain growth, promoting a fine-grained microstructure and markedly strengthening the mechanical robustness of the substrate. The intrinsically low TEC of GDC further mitigates the interfacial stress accumulation and enhancing long-term structural integrity. Moreover, the high structural stability of GDC is also beneficial to maintain the integrity of electrolyte film by preventing the progressive shrinkage during extended operation. With the incorporation of 15 wt% GDC, the single cell delivers a peak power density of 0.56 W cm⁻² at 600 °C, along with excellent structural and operational stability. Overall, this work demonstrates the effectiveness of heterophase-enhancement strategy for the design and fabrication of MS-SOFCs with high durability and extended service life.