Microenvironment Engineering as a Design Principle for Suppressing Catalyst Metal Loss.

Stability losses in platinum electrocatalysts arise from dissolution, redeposition, and restructuring processes that are strongly shaped by the catalyst microenvironment. Yet the degree to which ionomer chemistry and support porosity jointly govern the true platinum mass balance remains unresolved. Here, by integrating operando ICP-MS with an ion‑exchange approach that quantifies platinum species retained within the ionomer, we distinguish apparent dissolution from actual metal loss across model polycrystalline platinum and supported nanoparticles. We find that direct ionomer-platinum contact enhances intrinsic dissolution while simultaneously trapping dissolved platinum ions, leading to a pronounced mismatch between measured and actual platinum loss. Extending this framework to porous and nonporous carbon supports reveals that pore-confinement fundamentally alters degradation pathways, promoting local redeposition and suppressing net metal loss. Our results show that managing ionomer access and nanoscale transport within the catalyst microenvironment is a key practical factor in improving electrocatalyst durability.