Performance degradation of solid oxide fuel cells analyzed by evolution of electrode processes under polarization

The electrochemical impedance spectroscopy (EIS) is one of the most promising methods to unfold the complex electrode processes in solid oxide fuel cells (SOFCs). In this study, evolution of different electrode processes during galvanostatic operation of a single cell is analyzed based on deconvolution of recorded EIS data. The overall operation can be roughly divided into two stages, i.e. a rapid and nonlinear degradation stage, and a slow and quasi-linear degradation stage. Based on the analysis of EIS measured under open circuit voltage (OCV), it is found that the R-p decreases in the first stage while gradually increases in the second stage, which is mainly due to changes in anodic gas-phase diffusion and cathodic O-2 surface exchange and bulk diffusion processes. The effect of DC electrical current (polarization) on different electrode processes is emphasized and analyzed in detail. Accordingly, we speculate that the dominant degradation mechanism is closely related to the operating current, i.e. performance degradation in small current region is dominated by the deterioration of O-2 surface exchange kinetics at the cathode, while performance degradation in moderate current range is dominated by the weakening of charge transfer reactions at the anode.