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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. 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.
Supplementary materials
Title
Supporting Information
Description
Assessment of graphene as a working electrode (Figure S1); schematic of local proton conduction mechanisms (Figure S2); movie captions; co-located quasi-topographical maps, collected synchronously with electrochemical data in SECCM (Figures S3 and S4); estimation of overpotential, capacitance and RC time constant (Figure S5); equivalent circuit model of ion-transport through a nanopore (Figure S6); summary of reported areal conductivity values of graphene membranes (Table S1); electron microscopy images of SECCM probes (Figure S7) and; XPS characterization of the Nafion and graphene|Nafion membranes (Figure S8 and Table S2)
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Title
Movie S1
Description
Spatially-resolved (51 × 51 pixels over a 100 μm × 100 μm area) voltammetric (scan rate = 0.1 V/s, 1 cycle) movie, visualizing local proton transmission through a graphene|Nafion membrane, corresponding to Figure 2 in the main text.
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Title
Movie S2
Description
Spatially-resolved (51 × 51 pixels over a 100 μm × 100 μm area) voltammetric (scan rate = 0.2 V/s, 2 cycles) movie, visualizing local proton transmission through a graphene|Nafion membrane, corresponding to Figure 3 in the main text.
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Title
Movie S3
Description
Spatially-resolved (51 × 51 pixels over a 125 μm × 70 μm area) amperometric (pulse time = 10 seconds) movie, visualizing local proton transmission through a “damaged area” of a graphene|Nafion membrane, corresponding to Figure 4a-i in the main text.
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Title
Movie S4
Description
Spatially-resolved (51 × 51 pixels over a 125 μm × 70 μm area) amperometric (pulse time = 10 seconds) movie, visualizing local proton transmission through a more pristine area of a graphene|Nafion membrane, corresponding to Figure 4a-ii in the main text.
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