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

Abstract

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. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) 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, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20–60 localized sites across a ≈0.003 mm2 area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites “opening” and a small number of sites “closing” under an applied electric field on the seconds time scale. 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. 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, for example in diagnosing the failure mechanisms of protective surface coatings.

Deconstructing proton transport through atomically thin monolayer CVD graphene membranes

“…The presence of defects and tears in CVD graphene transferred to Nafion is also reported by Bentley et al , where they show enhanced proton conductance. 44 Here, we deconstruct the influence of wrinkles and tears by leveraging the well-defined porous polymer support such as polycarbonate track etched (PCTE with ∼200 nm cylindrical straight channel pores) membranes to support graphene and use interfacial polymerization (IP) to seal the large tears/defects (Fig. 4A).…”

“…A recent study by Bentley et al probing Naonjgraphene membranes via scanning electrochemical cell microscopy (SECCM) in areas that are free from visible large tears and pinholes suggests proton transport occurs primarily via few localized defects in CVD graphene. 44 We note that the 2-3 orders of magnitude differences between proton conductance values measured over micronscale and centimeter-scale devices may also result from the differences in quality of CVD graphene used in the different studies and other experimental procedures, 28,29,34,35,39 but the typical cation selective nature of defects in CVD graphene emerges as a common theme. 28,29,38 To the best of our knowledge, no reports exist on proton transport through small and large-scale devices for the same CVD graphene along with insights on the contribution from sub-nanometer scale defects (specically for large-area membranes), and we aim to bridge this gap in the literature to rationally advance the practical applications of 2D membranes.…”

“…A recent study by Bentley et al probing Nafion|graphene membranes via scanning electrochemical cell microscopy (SECCM) in areas that are free from visible large tears and pinholes suggests proton transport occurs primarily via few localized defects in CVD graphene. 44…”

The new era of high throughput nanoelectrochemistry

“…222 While a major focus of SECCM is on electron transfer processes, a new local electrochemical ion (proton) pump mode of SECCM has been developed to image proton transfer across membranes. 219 In this mode, a double-barrel probe, is used so that the meniscus cell can be landed on the membrane irrespective of its ion transfer characteristics. The driving force for ion transport is a potential applied between the SECCM probe and a counter electrode on the trans-membrane side.…”

Stochastic Local Breakdown of Oxide Film on Ni from Identical-Location Imaging: One Single Site at a Time

“…To gain insight into the breakdown of the oxide film at electrochemical interfaces, we use scanning electrochemical cell microscopy (SECCM), which can study ion transport through thin-film-covered interfaces. , Correlative multimicroscopy with SECCM is powerful in revealing the structural and mechanistic insight in various electrochemical systems, including batteries, − electrocatalysis, − and corrosion. − Herein, we use polycrystalline Ni covered by native NiO film (∼2 nm by X-ray photoelectron spectroscopy (XPS), Figure S1) as a model system to study the breakdown of metal oxide. SECCM allows the NiO film breakdown at single sites, significantly simplifying the analysis.…”