Microstructure changes in the catalyst layers of PEM fuel cells induced by load cycling: Part II. Simulation and understanding

The microstructure of catalyst layers (CLs) is a naturally random medium and changes in it greatly affect the performance of polymer electrolyte membrane fuel cells. In this paper, the mechanical analysis method, developed in Part I for understanding the mechanism of microstructure changes, is further extended to describe CLs as random three-phase microstructures. Microstructure reconstruction is accomplished using statistical information from experimental images of practical CLs. In the microscopically complex reconstructed microstructure, mechanical analysis is performed in order to understand the mechanism of changes caused by the cycling of start-up and shutdown during operation. Numerical simulation shows that, although different reconstructed microstructures have different changes, there have in common the competition between crack initiations in phases and delamination between different phases in the CLs. This competition plays an important role in microstructure change and results in performance degradation, indicated by the decrease in connection length among different solid components in the CLs after certain duty cycles.