Gradient engineering in proton exchange membrane fuel cell cathodes: an electrochemical study of charge transfer, mass transport, and Pt utilization

This study addresses a key research gap in proton exchange membrane fuel cell development by first establishing a pre-optimized non-graded catalyst layer as a reference, enabling a clearer understanding of performance improvements achieved through structural optimization. The reference catalyst layer was tuned for ionomer content and distribution, providing a high-performing baseline. Building on this, we systematically introduced through-plane gradients in Pt/C loading, ionomer-to-carbon ratio, and ionomer equivalent weight, both individually and in combination. Electrochemical impedance spectroscopy was used to unravel the underlying transport and kinetic effects. The fully graded catalyst showed a 32% performance increase at 0.6 V (humid conditions) and a 17% gain at 0.3 V (dry conditions) compared to the pre-optimized reference. These gains result from improved catalyst utilization near the membrane, enhanced gas diffusion and water management near the gas diffusion layer, and balanced ionic conductivity across the catalyst layer. The findings highlight the critical importance of combining a robust baseline optimization with rational gradient design, offering a comprehensive path to improve performance while minimizing precious metal usage. While structural factors are known to influence catalyst layer performance, this study focuses specifically on electrochemical behavior to provide detailed insights into compositional gradient effects.