Analyzing the degradation mechanism of solid oxide fuel cell during different time periods

Improving the lifetime and robustness of solid oxide fuel cells (SOFCs) is crucial for their commercial viability. This study aims to elucidate the degradation mechanism of SOFCs during galvanostatic tests and understand the internal and external factors influencing the cell degradation. Three nominally identical cells underwent galvanostatic testing for different durations to illustrate the evolution mechanism of SOFCs degradation at different stages. The electrochemical impedance spectroscopy (EIS) measurements under the open circuit voltage (OCV) and the direct current (DC) bias at different periods were carefully analyzed. By combining distribution of relaxation times (DRT) with equivalent circuit model (ECM) fitting, the contribution of each electrode process to cell performance degradation during galvanostatic testing was quantified. The polarization impedance, primarily associated with the charge transfer reactions in the anode, exhibited continuous increase under DC bias, contributing more than 80 % to the increase overall polarization impedance. Conversely, the cathode, related to O2 dissociation and diffusion process, contributed 7.2 %. The rapid degradation in the early stage was attributed to the degeneration of the anode microstructure, resulting from the formation of numerous isolated micron Ni particles and a subsequent decrease in three phase boundary (TPB) density. The anode microstructure after galvanostatic testing was characterized, revealing a novel Ni migration evolution mechanism. This mechanism involves the rapid agglomeration of Ni crystal particles at the beginning test, subsequent diffusion as microparticles within 25 h, and eventual coarsening into dispersive agglomerates over time. The proposed mechanism of anodic microstructure evolution offers insights for optimizing anodic structures and lays the foundation for enhancing the long-term stability of SOFCs.