Free-Standing Catalyst Layers Based on Porous Polymer Composites with Tailored Mass Transport Properties for AEM Water Electrolysis

Anion-exchange membrane water electrolysis (AEMWE) for green hydrogen (H2) production is hindered by safety-critical H2 crossover. Current gas management strategies often overlook the role of catalyst layer (CL) porosity, primarily because conventional ink coating and drying preparation creates dense CLs. Here, we introduce free-standing, fully conductive polymer composite CLs with defined pore structures and high porosities (∼50–75%), which are fabricated through scalable film casting and nonsolvent phase separation of electrode pastes (matrix polymer, carbon filler, nickel catalyst, and solvent). These free-standing CLs can be simply cold-pressed into membrane electrode assemblies (MEAs), enabling high current density AEMWE operation (>3 A/cm2). Crucially, gas–liquid transport within these CLs can be engineered through their pore structure to mitigate H2 crossover. A free-standing cathode CL with good bubble-releasing characteristics reduced H2 crossover at 0.75 A/cm2 to 1% in O2, compared to 2.5% for a CL with suppressed bubble release and 2.3% for a conventional Ni felt.