Surface topology engineering of cathode catalyst layers for bubble management in cathode-fed PEM water electrolysis

Proton exchange membrane water electrolysis (PEMWE) is a promising technology for regenerative oxygen generation in long-duration space missions and lunar-resource-utilization systems, where mass, volume, reliability, and auxiliary fluid-management requirements are severely constrained. Cathode-fed PEMWE offers a simplified architecture for such oxygen generation systems by supplying water from the hydrogen side and transporting it across the membrane to the oxygen-evolving anode, thereby reducing liquid-water discharge and downstream separation on the oxygen side. However, this configuration shifts the key water-management bottleneck to the cathode-membrane electrode assembly (MEA) interface, where retained H₂ bubbles can block water access, disrupt liquid continuity, and restrict transmembrane water supply. Here, we develop a cathode surface topology engineering (STE) strategy to regulate gas-liquid-solid contact within the MEA interface. The patterned cathode topology weakens bubble contact-line pinning, limits lateral bubble spreading, and promotes earlier H₂ bubble departure while preserving water-access pathways. Bubble-release analysis, transparent-cell visualization, electrochemical stability testing, and impedance measurements reveal that effective STE requires an appropriate morphology window rather than maximized roughness or deformation. Experimental validation under terrestrial gravity conditions (1g) shows that a moderately developed topology enables stable cathode-fed operation, reaching 1.813 V at 1.0 A cm⁻² after 250 min and an average degradation rate of 0.21 mV h⁻¹ during 200 h operation at 0.5 A cm⁻². This work establishes cathode-interface topology as a template-assisted MEA-level strategy for passive two-phase management in lightweight PEMWE oxygen generation systems.