Interface-driven enhancement of Nafion thin-film conductivity via controlled aminothiol modification

Abstract

The performance of electrochemical devices is critically influenced by the interfacial confinement of ionomer chains at the catalyst interface. In sub-μm-thick ionomer films at the catalyst interface, confinement of water and ionomer chains limits proton conductivity, hindering electrochemical reactions at the cathode and compromising fuel cell performance. Here, we investigated the effect of interfacial engineering on Nafion thin-film conductivity by covalently modifying gold (Au) electrodes with cysteamine (Cys), an aminothiol linker. Nafion films (∼60 nm thick) spin-coated on Cys-modified Au exhibited up to 4-fold increase in in-plane proton conductivity (σ_IP) at 85% RH compared to films on bare Au, with insignificant changes in water uptake. Molecular dynamics (MD) simulations revealed that the Cys modification of Au induced a more surface-parallel orientation of the Nafion backbone near the substrate. This promoted highly ordered –SO₃H groups and in-plane proton-conducting pathways while slightly lifting Nafion chains away from the hard wall-like Au surface. Attaching to the –NH₂ terminal of Cys, but hanging slightly up from Au, rendered a milder confinement to Nafion chains and surrounding water molecules. Consequently, the in-plane self-diffusion coefficients of water and hydronium ions increased, and the storage modulus of the films decreased, supporting more water-ionomer mobility and a milder confinement. While the more surface-parallel nature of ionomer chains with lower confinement on Cys improved in-plane proton conductivity of Nafion films, the out-of-plane conductivity did not change significantly for the same reason. These findings demonstrated the importance of controlling interfacial chemistry and linker design to gain control over ionomer chain orientation, confinement, and directional ionic conductivity, offering a promising strategy to enhance ion transport in thin ionomer films for next-generation electrodes and electrochemical devices.