Engineering Ionomer–Substrate Interface to Improve Thin-Film Proton Conductivity in Proton Exchange Membrane Fuel Cells

Abstract

Interfacial confinement-induced weak proton conduction in sub-μm-thick ionomer films impacts the performance of electrochemical devices. In such films, a fraction of ionomer chains align parallel next to the substrates. The ionomer−substrate interaction pins the chains to the substrate, limiting proton conductivity across the film. We addressed this interfacial chainpinning and ion transport limitation by covalently immobilizing (3aminopropyl)triethoxysilane (APTES) on SiO 2 before depositing Nafion films (∼60−700 nm thick) on top of it. The silane layer reduced the ionomer chain density next to the substrate (scanning electron microscopy with energy dispersive X-ray spectroscopy) and disrupted the chain packing and orientation within Nafion films (grazing incidence small-angle X-ray scattering, ellipsometry). These changes, especially the suppression of surface-parallel lamella, effectively minimized the substrate-pinning of Nafion chains, reduced film stiffness, narrowed down the poorly proton-conducting region next to the substrate (confocal laser scanning microscopy), and improved both in-plane (∼2 times) and out-of-plane (about an order of magnitude) proton conductivity of Nafion thin films (electrochemical impedance spectroscopy). This work pointed toward a simple but effective surface engineering approach to improve thin-film proton conductivity as well as inform and guide the design of better ionomer−catalyst interfaces for H-fuel cells and other electrochemical devices.