Data-Driven Quantitative Microscopy to Explain Ionomer-Carbon Ratio and Humidity Effects on Cathode Structure-Performance Degradation in Proton Exchange Membrane Fuel Cells

This study establishes quantitative correlations between catalyst-layer (CL) microstructure, electrochemical performance, and degradation in proton exchange membrane fuel cells (PEMFCs) as a function of ionomer-to-carbon (I/C) ratio and relative humidity (RH). Conventional cathode catalyst layers were analyzed using high-resolution SEM, epoxy-free TEM, and STEM-EDS mapping, combined with automated deep-learning and Python-based image quantification. Structural descriptors - including ionomer coverage, connectivity, porosity, and platinum particle size and redistribution - were correlated with in-situ performance and accelerated stress test data. Increasing I/C ratio enhanced ionomer connectivity and proton transport under dry conditions but promoted water-mediated Pt dissolution, migration, and particle growth under humid operation. High RH amplified Pt ion mobility and membrane-side Pt band formation, leading to significant electrochemically active surface area loss. Principal component analysis (PCA) revealed two competing degradation modes: catalyst transport–driven degradation governed by ionomer connectivity and humidity, and ionomer-related structural densification dominating under dry conditions. The findings highlight a critical trade-off - enhanced ionomer connectivity improves short-term performance yet accelerates long-term degradation. This data-driven quantitative microscopy framework provides mechanistic insight and practical guidance for optimizing CL architecture and humidity management to achieve durable, high-performance PEMFC electrodes.