Structure-property-performance parametric investigation of the effect of cathode ionomer loading on RH-sensitivity of low Pt loading proton exchange membrane fuel cells

Cathode ionomer loading is traditionally optimised to maximise proton exchange membrane fuel cell performance, yet the mechanistic origin of its humidity sensitivity remains unresolved. Here, a structure-property-performance investigation was conducted to determine and quantify how cathode ionomer loading (10-50 wt%) governs the humidity-dependent behaviour of low-platinum (0.10 mgPt.cm −2 ) catalyst layers. The results identify platinum accessibility as a mechanistically relevant descriptor linking catalyst layer microstructure, ionomer loading and electrochemical performance. Increasing ionomer loading enhances protonic connectivity and platinum utilisation but progressively restricts oxygen transport through pore filling and thicker ionomer films, producing an optimum balance near 30 wt% under well-humidified operation. Under dry conditions, however, the optimum shifts to approximately 40 wt% owing to improved retention of protonic connectivity. The catalyst layer exhibiting the highest peak performance also displays the greatest humidity sensitivity, revealing a fundamental trade-off between maximum performance and operational robustness. These findings demonstrate that no universal optimum ionomer loading exists and that catalyst layers should instead be tailored to their intended humidification environment. Furthermore, the results suggest that catalyst support microstructure and ionomer loading should be co-optimised to maximise platinum accessibility and improve humidity tolerance. The resulting design framework provides practical guidance for application-specific low-platinum PEMFC cathodes.