Deciphering Ni Content-Induced Microstructural Evolution in Ni-YSZ Cermets for Direct Ammonia SOFC Performance

The Ni-YSZ anode of a direct ammonia solid oxide fuel cell (DA-SOFC) with three different Ni contents is investigated and compared through microstructural analysis and electrochemical measurements of the cell. This work reveals that increasing the Ni loading by adjusting the initial NiO/YSZ weight ratio from 50:50, 60:40, and further to 65:35 improves the peak power density to 122, 313.4, and 383.9 mW/cm², respectively, at 800 °C. This can be explained by the diminishing trend in polarization resistance, particularly the charge transfer resistance (R_H), with increasing Ni content. The three dimensional (3D) microstructure of the three anodes is characterized using X-ray micro-computed tomography (micro-CT), and each phase's volume fraction and associated particle Feret diameter, directional percolation, tortuosity factors, triple-phase boundary (TPB) density, effective ionic and electronic conductivities, surface areas, and interfacial contact areas per unit volume are quantified. These parameters are then correlated to the electrochemical performance of the three anode compositions to understand the impact of microstructural variation. The Ni and YSZ networks are almost entirely connected, accounting for average percolation percentages of >98.7% for all anode compositions. It is noteworthy that no evident correlation is found between total TPB density and polarization resistance, suggesting that other transport-related descriptors, including tortuosity and percolation, are at play. The NiO/YSZ (65:35) SOFC showed promising stability over 96 h at 800 °C with a low voltage degradation of 5.06%, which is ascribed to Ni coarsening, as confirmed by post-test micro-CT analysis.