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 (H 2 ) production is hindered by safety-critical H 2 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/cm 2 ). Crucially, gas–liquid transport within these CLs can be engineered through their pore structure to mitigate H 2 crossover. A free-standing cathode CL with good bubble-releasing characteristics reduced H 2 crossover at 0.75 A/cm 2 to 1% in O 2, compared to 2.5% for a CL with suppressed bubble release and 2.3% for a conventional Ni felt.