Resolving Swelling by Swelling: Water-Mediated Interface Regulation for Ultralow-Pt Membrane Electrode Assemblies

Anisotropic swelling of ion-exchange membranes (IEMs)-a phenomenon that is both physically inevitable and operationally detrimental-poses disproportionately severe interfacial instability challenges in ultralow-Pt membrane electrode assemblies (MEAs). Here, we report a counterintuitive yet effective strategy, termed resolving swelling by swelling, in which the catalyst layer/IEM (CL/IEM) interface is constructed directly on a fully preswollen membrane. By exploiting the dynamic interplay between interchain hydrogen bonding and intrachain elastic contraction inherent to the membrane network, this approach harnesses controlled swelling to drive the in situ optimization of a three-dimensional interpenetrating CL/IEM interface. The resulting interface exhibits reinforced interfacial stability under operating conditions, a water-mediated active area resistant to ionomer poisoning, and enhanced gas and ion transport pathways. MEAs fabricated via this strategy deliver a peak power density of 1.22 W cm^-2 under H₂/Air conditions at an ultralow cathode Pt loading of 0.05 mg_Pt cm^-2, representing a 1.68-fold improvement over conventionally fabricated proton exchange membrane fuel cells, alongside a record-breaking cathode Pt utization of 24.4 kW g_Pt ^-1. These MEAs sustain excellent electrochemical performance under low-humidity operating conditions, long-term durability and 10 000 wet-dry cycles. The broad applicability of this strategy is further validated by its successful extension to anion exchange membrane fuel cells.