Broad deployment of redox flow batteries is hindered, in part, due to the lack of highly selective ion-exchange membranes with high proton conductivity. We report a post-functionalization strategy for polybenzimidazole copolymer (PBI-co) membranes by a reacting mixture of concentrated sulfuric and phosphoric acids that led to improved membrane selectivity while retaining high proton conductivity. Fourier-transform infrared spectroscopy confirmed successful sulfonation and protonation of benzimidazole functionalities. PBI-co exhibits a very high room-temperature proton conductivity of 0.138 S cm(-1), as measured by electrochemical impedance spectroscopy. PBI-co also demonstrates electromigration crossover mitigation under applied current densities up to 500 mA cm(-2), and voltage pulses up to 1.4 V, while also inhibiting diffusion-driven vanadium crossover for over 20 days. Single-cell vanadium flow battery testing on post-functionalized PBI-co further confirms state-of-the-art battery performance with an similar to 99.5% Coulombic efficiency at 100 mA cm(-2) and an area-specific resistance of similar to 20 m Omega cm(2) lower than Nafion-212 membrane.

Energy storage and conversion devices require an ion-exchange membrane with high transmission of charge-balancing ions and separation of anode and cathode electrolytes/gases. This ensures optimum device performance. Most conventional membranes suffer huge cross-permeation resulting in low energy efficiency and material degradation. This work investigated hydrogen permeability and proton transmission through membrane electrode assemblies (MEAs) containing a monolayer of hexagonal boron nitride and single-layer and bi-layer graphene in a gas-phase small-scale cell and a liquid cell. We found that the hydrogen crossover flux through MEAs with 2D materials was inhibited by at least a factor of 5 compared to the one without. Single-layer graphene and boron nitride enabled high proton transmission, but bi-layer graphene inhibited proton conduction. Defect visualization of 2D materials revealed few atomic-scale defects in graphene. These findings suggest that a monolayer of 2D material may provide good selectivity for energy conversion and storage devices by blocking species crossover while allowing high proton transmission.

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.

We report near-zero crossover for vanadium cross-permeation through single-layer graphene immobilized at the interface of two Nafion (R) polymer electrolyte membranes. Vanadium ion diffusion and migration, including proton mobility through membrane composites, were studied with and without graphene under diffusion and migration conditions. Single-layer graphene was found to effectively inhibit vanadium ion diffusion and migration under specific conditions. The single-layer graphene composites also enabled remarkable ion transmission selectivity improvements over pure Nafion (R) membranes, with proton transport being four orders of magnitude faster than vanadium ion transport. Resistivity values of 0. 02 +/- 0.005 Omega cm(2) for proton and 223 +/- 4 Omega cm(2) for vanadium ion through single atomic layer graphene are reported. This high selectivity may have significant impact on flow battery applications or for other electrochemical devices where proton conductivity is required, and transport of other species is detrimental. Our results emphasize that crossover may be essentially completely eliminated in some cases, enabling for greatly improved operational viability. (C) 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Graphene and related two-dimensional (2D) materials are being intensively studied for use in membrane separators. The ultrathin nature of 2D materials combined with the opportunities they afford for intentional creation of nanometer-sized pores having controllable size and chemical character could allow for membrane separations with high flux and also high selectivity. Proton transport through graphene is especially interesting because it may occur through the intact pristine graphene rather than through defects / pores. Other cations, for example lithium or sodium, may pass through graphene but that transport is through to occur through defects / pores. This presentation will focus on selective cation transport through graphene layers that are embedded between Nafion membranes. Ion transmission through graphene is measured using a four-electrode cell in which two Luggin capillaries sense the potential difference that develops across the membrane in response to an ion current that is driven by a pair of platinum wire drive electrodes. Proton transmission in this cell configuration occurs more than 100 times faster than transport of any other ion. This difference is thought to reflect the different mechanisms by which protons and other cations pass though graphene. The lecture will discuss defect characterization in CVD graphene films, and will consider the role of defects, which are in fact nanopores, in allowing for selective transport of ions through graphene.

Fifty atomic layer deposition (ALD) cycles of trimethylaluminum and water were applied to single-layer graphene on copper and graphene on Nafion membranes that result in alumina coatings that fully block photoelectron emission from the underlying substrate materials (copper and Nafion, respectively). This finding is consistent with relatively uninhibited alumina ALD nucleation and growth to a thickness of similar to 5 nm and further suggests that the alumina layer is continuous with no uncoated regions exposed. The ALD-derived alumina coatings are good barriers to large ions, e.g., from aqueous ferric chloride, but they have a relatively modest effect on proton transmission through graphene as measured in electrochemical hydrogen pump cell experiments. Proton currents of similar to 0.5 A cm(-2) were obtained through Nafion/graphene/ALD alumina/Nafion at a modest bias voltage of 150 mV, which reflects a diminishment by less than half relative to the value for membranes without the 50 cycles of ALD alumina applied to the graphene. Proton transmission through the ALD alumina layer is thought to occur via hydrated alumina surfaces within the relatively porous ALD alumina layer on the graphene surface and perhaps also at grain boundaries formed in the alumina during the ALD process.

Recent reports from several laboratories indicate that proton transmission through graphene and other 2D materials in contact with proton-conducting electrolytes can occur rapidly with a much lower activation energy than expected from a wide range of computational studies on the proton / graphene interaction energy, and a much higher selectivity relative to transmission of other ions than would be expected based only on ion size. It would be desirable to be able to control transport through the graphene, and many approaches have been tried to rationally control transport rates through graphene via chemical treatments applied to the graphene. One such approach involves use of atomic layer deposition (ALD), in which single layers of inorganic materials, often oxides, are coated in a self-limiting way onto a surface by a series of pulsed reaction steps delivered in series. A common ALD implementation involves coatings of aluminum oxide, or alumina, created by sequential dosing of a surface with trimethyl aluminum and water vapor. We report here on use of the ALD method to coat single-layer CVD graphene supported on copper foil or on Nafion membranes with layers of alumina. Coatings resulting from 50 ALD cycles are found to be thick enough and homogeneous enough to block photoelectron emission from the underlying substrates (copper or Nafion) when the coatings are analyzed by X-ray photoelectron spectroscopy. Electrochemical hydrogen pump experiments were then performed to assess the degree to which ALD treatment of the graphene on Nafion, delivered prior to preparing Nafion | graphene | Nafion sandwich membranes, could change the rate of proton transmission through the graphene that is embedded within the Nafion. As little as two ALD treatment cycles are sufficient to reduce proton transmission currents through graphene by approximately half relative to graphene that is not treated by ALD. Surprisingly, fifty ALD cycles has almost the same effect as two cycles. This finding indicates that that ALD alumina coating is a relatively good proton conductor which is thought to reflect the action of transport pathways on the hydrated surface of the porous ALD alumina layer. The insensitivity of proton transmission to the number of ALD cycles applied suggests that the function of the ALD is to change the transport rates at interfaces, perhaps by creating a new ALD alumina / Nafion interface that protons must traverse in order to pass through the membrane.

Our research group has recently demonstrated the ability to probe within composite membranes formed by sandwiching single layer graphene (SLG) between two Nafion layers (Nafion 211, 25 µm thickness). These graphene-sandwich membranes have shown high selectivity for proton transport across electrochemical cells [1,2]. Using a microscope equipped with an oil-immersion objective, the graphene layer is readily detected in confocal Raman microscopy depth-profiles (z-direction profiles) of the composite membranes. Spectra are sensitive to defects and stress within the buried graphene layer. This presentation will discuss the use of confocal Raman microscopy for structural characterization and spatial profiling in the development of composite polymer electrolyte membranes that contain a graphene layer as a barrier to ion transport. In a second application area, infrared spectroscopy is being adapted to investigate properties of Nafion ionomer particles in dilute dispersions and during transformation to gel and solid phases through the use of time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Since the properties of dispersing fluids can affect the mechanical strength of solution-cast Nafion thin films [3], it is of interest to gain insights into ionomer properties in the dispersion and during phase changes. A well-resolved peak associated with vibrational motions of the Nafion side chain -SO3 -end group is shown to be sensitive to gel-phase development. The persistence of hydration water in the vicinity of the -SO3 -group helps maintain a constant local refractive index that limits optical distortion in the band as the ionomer assembles into the phase-separated framework structure of the solid membrane. ATR FTIR measurements predict trends in evaporative water loss from the dispersion and entry into the gel-phase consistent with expectations of gravimetric measurements and earlier studies [3] of Nafion ionomer dispersion properties. 1. Bukola, S., et al. J. Am. Chem. Soc. 2018, 140, 1743. 2. Bukola, S., et al., ACS Appl. Nano Mater. 2019, 2, 964. 3. Kim, Y.S., et al., Macromolecules 2015, 48, 2161.

Confocal Raman microscopy is being adapted to probe interfaces within bipolar membranes and ionomer-graphene composites of interest for use in fuel cell and related electrochemical devices [1-2]. Although versatile and adaptable to in-situ measurements, confocal Raman microscopy depth-profiling has pitfalls when applied to bulk membranes. Optical aberration and sensitivity to light scattering from sample regions outside the probe beam focus limit depth resolution. With these constraints in mind, initial studies focused on detecting single layer graphene (SLG) as a barrier between two Nafion membranes (Nafion 211, 25 µm thickness). These graphene-sandwich membranes have shown high selectivity for proton transport across electrochemical cells [2]. Using a microscope equipped with oil-immersion optics to provide refractive index matching, the graphene layer was easily detected in confocal Raman microscopy depth-profiles. Spectra were sensitive to defects and stress within the buried graphene layer. Building on the ability to probe membrane-supported SLG, SLG adsorbed to a glass microscope coverslip (SLG/SiO2) was adapted as a platform for confocal aperture alignment and probe volume characterization [1]. Using SLG/SiO2to estimate the limiting axial spatial resolution, the junction separating the AEM/CEM (anion exchange membrane / cation exchange membrane) phases of bipolar polymer membrane (Fumasep FBM) was probed and shown to be consistent with roughness at the boundary on the order of a few micrometers (~3 µm). Continuing efforts are aimed at developing 2-dimensional (x-, y-direction) Raman spatial maps of buried polymer membrane interfaces and model AEM/CEM interfaces for high spatial resolution (< 10 nm) neutron reflectometry depth profiling. 1. Korzeniewski, C.; Kitt, J.P.; Bukola, S.; Creager, S.E.; Minteer, S.D.; Harris, J.M. ”Single layer graphene for estimation of axial spatial resolution in confocal Raman microscopy depth profiling” Anal. Chem.2019, 91, 1049 (DOI:10.1021/acs.analchem.8b04390). 2. Bukola, S.; Liang, Y.; Korzeniewski, C.; Harris, J.M.; Creager, S.E. “Selective proton / deuteron transport through Nafion | graphene | Nafion sandwich structures at very high current density” J. Am. Chem. Soc. 2018, 140, 1743. (DOI: 10.1021/jacs.7b10853)

Proton transmission through single-layer CVD graphene in graphene/proton-exchange-membrane (PEM) sandwich structures is found to be more than 100 times faster than for any other cation. Ion transmission rates were measured for protons and a series of other cations including Li+, Na+, K+, Rb+, Cs+, and NH4+ using a four-electrode method in which two platinum electrodes drive ionic current through the membrane and two reference electrodes installed in Luggin capillaries sense the transmembrane potential difference induced by the forced ion flow. Characterization studies including confocal Raman microscopy and X-ray photoelectron spectroscopy for graphene on Nafion, and defect visualization by etching through defects for graphene on copper, are also reported. All findings are consistent with a defect-based mechanism for transmission through graphene of all cations except protons, which likely follow a different mechanism, perhaps involving high-rate transmission through sites at which transmission of other ions is forbidden. Electrochemical impedance spectroscopy (EIS) was also used to study ion transmission rates through graphene in PEM sandwich structures. EIS gave much lower resistances for ion transmission through graphene than were obtained using the four-electrode method. This latter finding is thought to reflect a capacitive coupling of mobile ions with/through graphene at the high frequencies (up to 100 kHz) used in the EIS measurement. Near-steady-state dc methods are thus necessary to evaluate true ion transmission rates through graphene.

The relative rates of transmembrane proton and hydrogen gas transmission are of high importance in most PEM-based electrochemical energy conversion devices. Membrane separators that simultaneously have high rates of proton transmission and low rates of gas transmission are highly desired but this property combination is difficult to achieve because most modifications that could give higher proton transmission rates through a membrane, tend also to give higher gas crossover rates. This presentation will present results indicating that one monolayer of CVD single-layer graphene embedded between two polyelectrolyte membranes can give this desired membrane property combination. Proton transmission occurs through single-layer graphene in a PEM sandwich structure with area-specific resistance values less than 40 mΩ cm2, which is less than the resistance of most PEM membranes. Hydrogen crossover rates in PEM / graphene / PEM sandwich structures were measured by a limiting current method and found to be reduced by more than eight times relative to values in similar membranes without graphene. The lecture will present data on these points and discuss the mechanism(s) by which graphene might provide this desirable set of membrane properties.

Transmission rates for protons and deuterons across single-layer graphene embedded in Nation vertical bar graphene vertical bar Nation sandwich structures are measured as a function of temperature in electrochemical hydrogen pump cells. Rates of ion transmission through graphene are obtained in the form of area-normalized ion-transfer resistances, and are interpreted in terms of ion-exchange current densities and standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step is then used to provide rate constants for the fundamental microscopic step of ion (proton or deuteron) transmission across graphene. Application of this rate model to interpret variable-temperature data on proton and deuteron transmission rates provides values for the activation energy and pre-exponential factor for the fundamental ion transmission step across graphene. Activation energies obtained from the Arrhenius plots for proton and deuteron transmission are as follows; for proton, E-act = 48 +/- 2 kJ/mole (0.50 0.02 eV) and for deuteron, E-act = 53 +/- 5 kJ/mole (0.55 +/- 0.05 eV). The difference between these two values of approximately 5 kJ/mole is in good agreement with the expected difference in vibrational zero-point energies for O-H and O-D bonds, albeit with some uncertainty given the uncertainties in the activation energy values. Pre-exponential frequency factor values of 8.3 +/- 0.4 x 10(13) s(-1) and is 4.7 +/- 0.5 x 10(13) s(-1) were obtained for proton and deuteron transmission respectively across graphene. These pre-factor values are both quite large, on the order of the values predicted from the Eyring - Polanyi equation with a transmission coefficient near one. The ratio of 1.8 for the rate pre-factors (H/D) is in reasonable agreement with the value of 1.3 for the ratio of bond vibrational frequencies for O-H and O-D stretching, respectively. Taken together, these data support a model in which proton and deuteron transmission across graphene are largely adiabatic processes for which the differences in transmission rate at room temperature are due largely to differences in activation energies. (C) 2018 Elsevier Ltd. All rights reserved.

High-rate proton transmission through single-layer graphene has been reported by several research groups but the mechanism by which it occurs is still not clear. Low activation energies for proton transmission have been experimentally obtained but are difficult to reconcile with computational modeling studies that predict high activation energies and low transmission rates for protons passing through pristine single-layer graphene at ambient temperatures. It seems likely that a reaction coordinate with an activated complex structure involving proton passage through graphene is involved, though the structural details of such a complex are still not clear. This lecture will review some of our recent experimental results on proton transmission rates through CVD single-layer graphene at variable temperature, with special attention to a kinetics model whereby experimentally-obtained proton-transfer resistances are interpreted as proton-transfer resistances that are linked to heterogeneous proton—transfer rate constants, in a manner that is formally similar to the Butler-Volmer model used to treat electron-transfer rate data in the low overpotential limit. The heterogeneous proton-transfer rate constants are then interpreted using a pre-equilibrium model for protons approaching graphene, coupled to a first-order rate constant for proton transmission through graphene from the pre-equilibrium complex. This first-order proton-transmission rate constant has fundamental significance and may be interpreted in terms of an activation energy and frequency factor for proton transmission. In applying such an analysis to our proton transmission rate data, we obtain a proton-transmission frequency factor that is within a factor of two of the vibrational frequency of the O-H bond in water, suggesting that O-H bond vibration is involved in the rate-determining step of proton transmission.

Proton transmission rates across single-layer CVD graphene embedded in sandwich structures with proton-exchange membranes made from the perfluorosulfonic acid (PFSA) ionomer Nafion were measured and found to occur more than 100 times larger than for any other cation. Measurements were made for protons and a series of other cations including Li+, Na+, K+, Rb+, Cs+ and NH4 + using a four-electrode method in which two platinum electrodes drive ionic current through the membrane and two reference electrodes installed in Luggin capillaries sense the transmembrane potential difference induced by the forced ion flow. Proton transmission rates across graphene were between 150 and 350 times larger than for any of the other cations studied. Characterization studies of graphene on Nafion using confocal Raman microscopy and X-ray photoelectron spectroscopy, and defect counting for graphene on copper by chemical etching through graphene on copper will also be reported. Findings are consistent with transport through physical defect structures for all cations except protons. Proton transmission occurs at very high rates and may involve sites that are more widely distributed and that have much higher proton selectivity than the sites responsible for transmission of other ions. Proton transmission rates across graphene were also measured using electrochemical hydrogen pump cells at variable temperature, which provides an activation energy for proton transmission across graphene. Rate constants for proton transmission are interpreted using a charge-transfer resistance model that allows for interpretation of ionic currents in terms of a rate constant for interfacial proton-transfer. Anrhenius analysis of the rate constants gives values for the activation energy and frequency factor for the interfacial proton-transfer reaction.

This talk will cover a wide range of recent studies involving proton transmission across CVD graphene layers incorporated into proton-exchange-membrane (PEM) sandwich structures. Many low-temperature electrochemical systems including fuel cells and water electrolysis cells involve proton transmission across proton-exchange membranes integrated with electrodes. A key to optimizing such systems is the accomplishment of selective proton transmission at high rates, while simultaneously inhibiting crossover transmission of other molecular species such as water, hydrogen / oxygen, methanol, and other species. Single-layer CVD graphene has been shown to accomplish this. Proton transmission across graphene in Nafion | graphene | Nafion sandwich structures has been shown to occur with an area-normalized resistance of less than 0.05 ohm cm2 at room temperature, with high selectivity relative to other ions. The graphene layer is nearly defect free and should be an excellent barrier to other species besides protons. The talk will include studies of proton transmission rates using hydrogen pump cells, water electrolysis using PEM cells, and related work. Graphene characterization studies using confocal Raman spectroscopy and X-ray photoelectron spectroscopy, and graphene defect characterization using chemical etching of graphene on copper, will also be covered. Effects of the graphene structure (single vs double layer) and the nature of the 2D material (graphene vs. hexagonal boron nitride) on proton transmission rates, and the rates for other ions besides proton, will also be discussed.

Ion current densities near 1 A cm-2 at modest bias voltages (<200 mV) are reported for proton and deuteron transmission across single-layer graphene in polyelectrolyte-membrane (PEM)-style hydrogen pump cells. The graphene is sandwiched between two Nafion membranes and covers the entire area between two platinum-carbon electrodes, such that proton transfer is forced to occur through the graphene layer. Raman spectroscopy confirms that buried graphene layers are single-layer and relatively free of defects following the hot-press procedure used to make the sandwich structures. Area-normalized ion conductance values of approximately 29 and 2.1 S cm-2 are obtained for proton and deuteron transport, respectively, through single-layer graphene, following correction for contributions to series resistance from Nafion resistance, contact resistance, etc. These ion conductance values are several hundred to several thousand times larger than in previous reports on similar phenomena. A ratio of proton to deuteron conductance of 14 to 1 is obtained, in good agreement with but slightly larger than those in prior reports on related cells. Potassium ion transfer rates were also measured and are attenuated by a factor of many thousands by graphene, whereas proton transfer is attenuated by graphene by only a small amount. Rates for hydrogen and deuterium ion exchange across graphene were analyzed using a model whereby each hexagonal graphene hollow site is assumed to transmit ions with a specific per-site ion-transfer self-exchange rate constant. Rate constant values of approximately 2500 s-1 for proton transfer and 180 s-1 for deuteron transfer per site through graphene are reported.

Single layer graphene (SLG), with its angstrom-scale thickness and strong Raman scattering cross section, was adapted for measurement of the axial ( Z-direction) probe beam profile in confocal Raman microscopy depth-profiling experiments. SLG adsorbed to a glass microscope coverslip (SLG/SiO2) served as a platform for the estimation of axial spatial resolution. Profiles were measured by stepping the confocal probe volume through the SLG/SiO2 interface while measuring Raman scattering from the sample. Using a high numerical aperture (1.4 NA) oil immersion objective, axial profiles were derived from the graphene 2D vibrational mode and fit to a Lorentzian instrument response function (IRF). Subsequently, the Z-direction spatial resolution in depth-profiling studies of polymer interfaces was estimated through convolution of the Lorentzian IRF with a step function representing the ideal junction separating the phases of interest. In the study of a bipolar polymer membrane, confocal Raman depth profiles of the AEM/CEM (anion exchange membrane/cation exchange membrane) interface show that the transition region is broader than the limiting response and are consistent with roughness at the boundary on the order of a few micrometers. Using ClO4- as a Raman active mobile ion probe, application of self-modeling curve resolution (SMCR) to spectral data sets within a profile showed ClO4- ions track the spatial distribution of the AEM phase. Finally, in measurements on a liquid-solid interface formed between 1-octanol and a polydimethylsiloxane (PDMS) membrane, the IRF derived from fitting the experimental profile was slightly narrower than those obtained from profiling SLG, indicating the potential to use polymer-liquid interfaces formed from widely available materials and reagents for estimation of axial spatial resolution in confocal Raman depth-profiling.

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures. Single-layer graphene prepared by chemical vapor deposition onto copper foil was sandwiched between Nafion membranes by a series of hot-pressing steps coupled with chemical etching to remove the copper. Proton current densities in excess of 1 A / cm2 were observed for proton transfer across the graphene at cell bias voltages less than 200 mV. Confocal Raman spectroscopy measurements of the Nafion / graphene sandwich structures are consistent with single-layer graphene having very few structural defects embedded between Nafion membranes. Ion-transfer rates for lithium, sodium, potassium, and other cations were also measured and found to be much lower than rates for proton transfer, in most cases by many orders of magnitude. Two-layer and few-layer (3-5 layer) graphene sandwich structures were also studied and found to give proton currents much lower than single-layer graphene. Proton currents are described in terms of an area-normalized resistance following correction for all other contributions to series resistance in the cells. Resistances are then analyzed in terms of a rate constant for proton self-exchange across graphene. Figure 1

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton and deuteron transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures in proton and deuteron form respectively. Ion currents up to 1 A / cm2 were obtained for proton transfer across graphene, and were diminished by a factor of 14 for otherwise identical cells pumping deuterium ions. Proton / deuterium pumping rates were nearly identical to each other for cells having no graphene. Arrhenius analysis of variable temperature studies of deuterium pumping across Nafion | graphene | Nafion membranes yielded an activation energy of approximately 0.5 +/- 0.1 eV for deuterium transfer across graphene. Figure 1

A simple and convenient small-scale fuel-cell test platform was created from a commercial compression fitting and graphite rod current collectors and used to conduct diagnostic tests on disk-shaped membrane-electrode assemblies (MEAs) fabricated from Nafion membranes and Nafion-impregnated platinum-on-carbon-cloth anodes and cathodes. A key advantage of this test platform is that it requires very little material, perhaps just a few milligrams, to conduct a fuel-cell test on a supported catalyst. Electrochemically-active surface area (ECSA) values for supported platinum on carbon-cloth electrodes were obtained by in-situ (in the fuel cell) and ex-situ (in liquid electrolyte) cyclic voltammetry on similarly-prepared electrodes, and values obtained by these methods were compared with each other to estimate the fraction of platinum catalyst contacted by the Nafion ionomer in the fuel-cell cathode. Polarization curves were acquired under controlled-potential conditions using slow-scan cyclic and sampled-current voltammetry and potential-step amperometry methods with conventional electroanalytical instrumentation. Tests performed using this platform are complementary to rotating disk electrode (RDE) voltammetry tests which also allow for catalyst testing on small amounts of material, albeit in the presence of liquid electrolyte, and are commonly used for initial screening of new fuel-cell catalysts. They are also complementary to conventional fuel-cell testing that is commonly performed on MEAs having active areas more than 100 times larger than that in the present cells.