Porous Iridium Oxide Inverse Opal Catalysts Enable Efficient PEM Water Electrolysis

Green hydrogen produced by proton exchange membrane water electrolysis (PEM-WE), has gained significant attention as a future energy carrier and as a feedstock for the chemical industry. Reducing the use of scarce iridium in PEM-WE anodes is a critical requirement. In this work, porous iridium-based inverse opal structures (IrO_x-IO) of varying pore sizes are introduced as novel unsupported bulk anode catalysts and their superior performance compared to commercial alternatives is demonstrated. The influence of porosity and surface area on the electrochemical performance is systematically investigated and categorized using voltage breakdown analysis and equivalent circuit modeling. Efficient IrO_x-IO operation requires balancing surface area and pore size, enabling high performance up to 13 A cm^{- 2} with iridium utilizations below 0.1 g_Ir/kW at 70% efficiency. The findings advance our understanding of unsupported bulk catalysts and, more importantly, expand the range of viable anode materials by clarifying how catalyst morphology influences electrode reactivity.