• At a 10 wt.% ionomer/Ir ratio, long-side-chain (LSC) ionomer achieves 2.141 V at 2.00 A/cm², outperforming short-side-chain (SSC) ionomer (2.208 V).
• MEA with LSC ionomer shows better electrochemical performance than SSC at the same I/Ir ratio, especially at high current density.
• LSC ionomer produces larger average pore size and porosity, improving mass-transfer properties and reducing ohmic and mass transfer resistances.
• Optimizing ionomer side-chain structure and content is a cost-effective strategy to enhance PEM water electrolysis performance.