Revealing inter-cell water-gas-coupled transfer characteristics and degradation-driven evolution within large-scale fuel cell stacks

Abstract

Hydrophobicity decay is a critical driving force behind the degradation of proton exchange membrane fuel cells. However, the cross-scale linkage between microscopic material-level hydrophobic degradation and macroscopic stack-level water-gas transfer behaviors remains largely obscure, as well as the further inducing consequences. Bridging the gap, this study reveals the inter-cell water-gas-coupled transfer characteristics and degradation-driven evolution within large-scale fuel cell stacks. Applying the micro-current excitation method on a 60-cell stack, the evolution of membrane electrode assembly (MEA) material states is recognized after the 1000-h durability test. Except for typical catalyst degradation, an anomalous degradation feature is captured, wherein hydrophobicity degradation facilitates internal water accumulation and reduces ohmic resistance, consequently leading to the performance rise in ohmic polarization region. The degradation inconsistency within stack is found to be correlated with flow maldistribution. At the inter-cell level, cells with lower flow rates suffer from more severe degradation due to water accumulation and potential air starvation, which is especially pronounced in end cells. At the intra-cell in-plane level, the hydrophobicity degradation of MEA at the air downstream region is also more serious. For stacks assembled with MEAs with different hydrophobic states driven by degradation, the internal water accumulation difference and inter-cell coupling effect combine to cause gas redistribution. The evolution of gas distribution with humidity clearly indicates that the tendency of water accumulation within degraded cells increases the flow resistance and decreases the flow rate under high humidity conditions. Consequently, degraded MEAs suffer a greater risk of water flooding and significant deterioration in dynamic response performance, which forms a vicious cycle of accelerated degradation. This study enriches the cross-scale degradation mechanism theory of fuel cell stacks and holds significant values for consistency guarantee and lifetime extension.