Effect of Substrate Surface Charges on Proton Conduction of Ultrathin Nafion Films

A potential approach to enhance the suppressed proton conductivity of nanoscale ultrathin Nafion films is to adjust the ionomer structure via regulating the catalyst-ionomer interaction. To understand the interaction between substrate surface charges and Nafion molecules, self-assembled ultrathin films (∼20 nm) were prepared on the SiO₂ model substrates, which were modified with silane coupling agents to carry either negative (COO^-) or positive (NH₃⁺) charges. Specifically, the surface energy, phase separation, and proton conductivity were investigated by contact angle measurements, atomic force microscopy, and microelectrodes to illuminate the relationship between the substrate surface charge, thin-film nanostructure, and proton conduction. Compared to electrically neutral substrates, ultrathin films formed faster on the negatively charged substrate with an 83% increase in proton conductivity but formed more slowly on the positively charged substrate, with proton conductivity decreased by 35% at 50 °C. The surface charges interact with sulfonic acid groups of Nafion molecules to alter molecular orientation, resulting in different surface energies and phase separation, which are responsible for proton conductivity variation.