High-Resolution Ion-Flux Imaging of Proton Transport Through Graphene|Nafion Membranes

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This “intrinsic proton conductivity” of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. To clarify the nature of proton transmission through graphene, local ion-flux imaging is performed herein on graphene|Nafion membranes using an “electrochemical ion (proton) pump cell” mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes etc.), and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, most of the graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at only ≈20 – 60 localized sites across a ≈0.003 mm2 area of membrane (>5000 measurements, total). When localized proton transport occurs, it can be a highly dynamic process, with new transmission sites “opening” and a small number of sites “closing” under an applied electric field, on the seconds timescale. Applying a simple equivalent circuit model of ion-transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer-scale, implying that rare atomic defects are responsible for proton conductance through monolayer graphene. Overall, this work reinforces SECCM as a premier tool for the structure−property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, e.g., for diagnosing failure mechanisms in protective surface coatings.