Decoupling Interfacial Proton Conductivity in Ionomer Thin Films on Pt and Carbon Electrodes

Resolving proton transport at specific polymer-substrate interfaces is essential for understanding ultrathin ionomer films, yet conventional impedance spectroscopy under N₂ typically yields only a single semicircle, inhibiting the clear discussion on substrate-dependent conductivity. Here, we demonstrate for the first time that combining extended low-frequency measurements under nitrogen environment with systematic modulation of interdigitated electrode (IDE) pad length enables experimental separation of multiple interfacial contributions in 54 ± 4 nm thick Nafion films. Nafion was deposited on IDE substrates containing SiO₂ gaps and embedded Pt or carbon pads. Varying the pad length (20-100 μm) shifted the RC time constants associated with each interface, allowing the high-frequency (first, R₁) and low-frequency (second, R₂) resistance components to be distinguished under conditions where they previously appeared merged. After geometric normalization, the extracted proton conductivities became independent of IDE geometry. The high-frequency component corresponded to Nafion/SiO₂ transport, whereas the low-frequency component reflected Nafion/Pt and Nafion/carbon transport. Even when not visually resolved on Nyquist plot, the second component yielded consistent conductivity values across pad lengths, establishing an intrinsic interfacial origin. The interface-specific conductivities associated with SiO₂, carbon and Pt supports were of the same order of magnitude, with only modest differences (within a factor of about two) among them, while remaining highly reproducible across IDE geometries. This methodology provides the first geometry-independent framework for quantifying proton transport at individual interfaces in supported ionomer films under an inert atmosphere.