Ionomer-dependent interfacial water structure and carbon oxidation susceptibility in PEMFC catalyst layers

Carbon oxidation remains a major durability challenge in proton exchange membrane fuel cells (PEMFCs), yet how ionomer chemistry influences this process across multi-scale catalyst-layer architectures is not fully understood. Here, we establish a multi-scale framework linking pore-scale water retention with local interfacial hydration, identifying the steady-state interfacial water configuration as a key spectroscopic indicator associated with carbon oxidation susceptibility in PEMFC catalyst layers. By comparing short-side-chain (SSC) and long-side-chain (LSC) ionomers, we show that ionomer architecture regulates the local oxidative environment at the electrode interface. SSC ionomers foster a strongly hydrogen-bonded, pre-activated interfacial water network featuring intramolecular O–H bond weakening, which favors the initial stages of carbon oxidation. As a result, SSC-based electrodes exhibit pronounced voltage loss and up to 22% catalyst-layer thinning, whereas LSC ionomers largely preserve structural integrity. In-situ Raman spectroscopy, pore-scale structural analysis, and density functional theory calculations consistently demonstrate that sulfonic acid rich interfacial environments reorganize the hydration layer and facilitate C–O bond formation on the carbon support. These findings indicate that carbon oxidation is strongly influenced by steady-state interfacial hydration together with pore-scale water retention rather than bulk water content alone, establishing interfacial hydration engineering as a promising strategy for durable PEMFC catalyst layers.