Compositionally graded Ni–YDC thin-film anodes via oblique angle sputtering for low-temperature SOFC

Abstract

Low-temperature thin-film solid oxide fuel cells (TF-SOFCs) require the simultaneous enhancement of interfacial electrochemical activity and anode microstructural stability; however, single-composition nickel (Ni)-based cermet anodes inherently suffer from a trade-off between performance and durability. In this study, a compositionally graded Ni-yttria-doped ceria (YDC) thin-film anode is fabricated using an asymmetric glancing-angle co-sputtering process that compensates for the deposition-rate imbalance between metallic Ni and ceramic YDC. Single-layer Ni–YDC anodes with varying Ni content were evaluated at 500 °C, revealing that increasing Ni content improves initial peak power density but accelerates short-term degradation due to Ni coarsening and pore collapse. Microstructural and electrochemical analyses confirm rapid triple-phase boundary (TPB) loss and increased polarization resistance in Ni-rich electrodes, while Ni-lean compositions exhibit superior structural stability at the expense of initial performance. To decouple electrochemical activation from structural stabilization, two-layer compositionally graded anodes, combining a highly active functional layer and a structurally stable layer fabricated at distinct nickel sputtering powers of 150 W and 90 W, respectively, were designed with controlled stacking sequences and thickness ratios at a fixed total thickness of 1.5 µm. All graded anodes exhibit higher initial performance than the single-layer baseline with lower Ni content and markedly improved durability compared to the high-Ni-content reference. The optimized configuration achieves a low initial polarization area-specific resistance of 0.046 Ω cm², approaching the catalytic activity of the high-Ni-content single layer, while maintaining a 6 hour post-operation resistance lower than the low-Ni-content reference (0.114 vs. 0.147 Ω cm²). The device demonstrates a degradation rate of 36.7% after 6 hours, comparable to the low-Ni baseline (34.1%) despite significantly higher initial performance. These results demonstrate concurrent gains in initial performance and stability through the combination of two graded layers.