Catalyst Layers Subjected to Sequential Cation Exchange and Thermal Annealing for Efficient and Durable Proton Exchange Water Electrolysis

The nonuniform spatial distribution of perfluorosulfonic acid (PFSA) ionomers in anode catalyst layers (ACLs) critically limits the performance and lifespan of proton exchange membrane water electrolyzers (PEMWEs) by decreasing utilization of the IrO₂ catalyst and impeding interfacial proton/electron conduction. In this study, a protocol is introduced for sequential cation exchange and thermal annealing (SCETA) to simultaneously optimize the chain rearrangement and spatial distribution of PFSA ionomers in ACLs. We used integrated characterization (including small-angle X-ray scattering, transmission electron microscopy, and liquid-phase atomic force microscopy) to demonstrate that SCETA facilitates the dissociation of PFSA ionomers from oversized aggregates while increasing the binding affinity of these ionomers to IrO₂ catalyst particles. This synergistic restructuring enables IrO₂ aggregates to be encapsulated by a homogeneous, conformal ultrathin PFSA film. These structural modifications establish continuous proton and electron conduction pathways while preserving the requisite porosity for gas and water transport. A treated ACL had a 43% lower proton transfer resistance (3.0 mΩ cm²) and a 49% higher electrical conductivity (0.91 S m^–1) than a conventional ACL (5.3 mΩ cm² and 0.61 S m^–1, respectively). A membrane electrode assembly (MEA) with the treated ACL achieved a current density of 3.5 A cm^–2 at 1.9 V (a 29.6% increase over that of a conventional MEA), surpassing the U.S. Department of Energy 2025 technical target, and a decay rate of 7.0 μV h^–1 at 1.5 A cm^–2 over 2000 h of operation. The proposed treatment is a scalable and cost-effective solution for manufacturing MEAs for highly efficient PEMWE systems with long-term operation.