Deuteron Transport through Nafion|Graphene|Nafion

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Selective proton / deuteron transport through Nafion | graphene | Nafion sandwich structures at very high current density Saheed Bukola, Ying Liang, Carol Korzeniewski, Joel Harris, and Stephen Creager J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10853 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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Journal of the American Chemical Society

Selective proton / deuteron transport through Nafion | graphene | Nafion sandwich structures at very high current density Saheed Bukola@, Ying Liang#, Carol Korzeniewski#, Joel Harris$, and Stephen Creager@* @ Department of Chemistry, Clemson University, Clemson SC 29634 # Department of Chemistry, Texas Tech University, Lubbock TX 79409 $ Department of Chemistry, University of Utah, Salt Lake City UT 84112

KEYWORDS Graphene, 2D materials, proton transfer, isotopic sieving, hydrogen pumping, polyelectrolyte membrane (PEM) cells

ACS Paragon Plus Environment

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ABSTRACT

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 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.


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INTRODUCTION Pristine graphene and related 2D materials have been described as near-perfect barrier materials.1-3 Even in single-layer form, pristine graphene will completely block transport of nearly all atomic, ionic and molecular species at ambient temperature, including gas-phase helium.4 The blocking is thought to reflect strongly repulsive interactions between electron clouds in graphene and in any potential permeant species. Selective ion / molecule transport through graphene is of very high interest but is generally thought to require sub-nanometer to nanometer-sized defect structures at which repulsive forces between graphene and permeants are smaller than in pristine graphene and might be tailored to achieve selective transport.5 Much work has been done over the past decade to demonstrate and understand selective transport through defect sites in a wide variety of 2D materials, especially graphene.6-25 A 2014 report from Hu and co-workers challenged this conventional wisdom by suggesting that proton transfer could occur directly though pristine single layers of 2D materials including hexagonal boron nitride, graphene, and molybdenum disulfide.26 Their report was surprising because until it appeared, it was thought that the high activation energy and low probability expected for proton desolvation and transmission through graphene and related 2D materials would prevent transmission from occurring at high rates, at least at ambient conditions.27 A follow-on study from Lozada-Hidalgo and co-workers in 2016 reported the equally surprising result of a strong isotopic selectivity in transport of hydrogen ions across graphene, with proton transport favored by a factor of approximately 11 over deuteron transport.28 This finding is thought to reflect a difference in zero-point energy for solvated protons vs. deuterons, as is traditionally invoked to explain H/D kinetic isotope effects.29 Nearly all the work reported in refs


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was accomplished using microfabricated structures having sizes less than 50

micrometers. A 2017 report from Lozada-Hidalgo and co-workers extended their prior work by showing that hydrogen / deuterium separation could be accomplished at larger scale by electrochemical pumping using macroscopically-sized cells having active areas of several square centimeters.30 In these cells a large-area graphene layer initially created by chemical vapor deposition (CVD) onto copper foil was positioned on one side of a Nafion membrane. Following removal of the copper by oxidative etching, a hydrogen-evolution catalyst, usually platinum or palladium, was then coated onto the other side of the graphene. Selective transport of protons over deuterons was again demonstrated with selectivity similar to the prior work. Further scale-up from this level should be straightforward and could enable significant new technologies for hydrogen isotope separation. The early studies of Hu and Lozada-Hidalgo and co-workers reported an area-normalized room-temperature graphene proton conductance of approximately 3 mS cm-2. This proton conductance value is high relative to the expectation of near-zero ionic conductance through graphene at room temperature but it is much lower than the area-normalized proton conductance of a typical Nafion membrane. For example, a 50-micrometer-thick Nafion membrane (e.g. Nafion 212) having an ionic conductivity of approximately 60 mS cm-1 (appropriate for a fullyhydrated membrane at near-ambient temperature)31-32 will provide an area-normalized proton conductance of approximately 12 S cm-2 which is approximately 4000 times larger than the value first reported by Hu and co-workers for graphene proton conductance.26 The 2017 study from Lozada-Hidalgo and co-workers reported a larger area-normalized graphene proton conductance of approximately 90 mS cm-2 (estimated from the published data using a current density of


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approximately 140 mA cm-2 in a cell at a bias voltage of approximately 1.6 V; Figure 2d in reference 30.) This proton conductance value is substantially increased from the earlier work but it is still well below the conductance expected for a Nafion membrane. These relatively low areanormalized conductance values for proton transfer across graphene would have significant consequences for a practical cell. For example, a cell passing 100 mA cm-2 of current would experience an ohmic loss of just 8 mV from a 50-micrometer-thick Nafion membrane having an area-normalized conductance of 12 S cm-2 but a much larger loss of approximately 1.1 V from a graphene layer having an area-normalized proton conductance of 90 mS cm-2. It is unclear at this time whether these relatively low area-normalized proton conductance values for graphene are a fundamental limitation of proton transfer across graphene, or whether they could be improved. This question is important from a fundamental perspective in seeking mechanistic explanations for how protons and related ions are transferred across 2D materials, and also from a practical perspective in thinking about possible technological applications of graphene in protonconducting devices. We report here on a study of proton and deuteron transport through graphene layers embedded between two Nafion membranes in polyelectrolyte membrane (PEM)-style electrochemical cells configured for very efficient hydrogen evolution and hydrogen pumping. Hydrogen pump cells are easy to prepare in general but special care is needed to obtain high activity.33-34 We utilized a recently-described35 miniature test cell that is small enough to allow for use of modestly-sized electrodes and membranes while still allowing for testing under conditions favoring high catalyst activity and high current density with minimal losses from gas concentration polarization, electrode kinetics, and other series resistance contributions such as membrane and electrical lead and contact resistance. Cells pumping hydrogen and deuterium gas with and without graphene


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layers were compared to study relative rates of proton and deuteron transport through the graphene. Area-normalized graphene ionic conductance values of approximately 30 S cm-2 for protons and 2.1 S cm-2 for deuterons (or, equivalently, area-normalized resistances of 34 and 466 mΩ cm2 respectively) were obtained following correction for the contributions to cell resistance from the Nafion ionomer and the other series electronic resistances in the cells. The proton to deuteron conductance ratio from this work is 14:1, in good agreement with the value of 11:1 reported by Lozada-Hidalgo and co-workers.28,

30

Proton-transfer conductance values through

graphene were analyzed as charge-transfer resistances in a Butler-Volmer-like model to provide rate constants for proton and deuteron exchange through graphene hexagonal hollow sites.

(a)

graphene Copper

Nafion Nafion Electrodes hot Press

Nafion First Nafion hot press

Second Nafion hot press

(b) DC source

Copper APS etch

electrons

Nafion

H2

H+

H2

D2

D+

D2

anode

Nafion Nafion cathode

Figure 1. (a) Fabrication of a Nafion | graphene | Nafion sandwich structure from CVD graphene on copper foil and two Nafion 211 membranes. Photomicrograph of the completed Nafion | graphene | Nafion membrane-electrode assembly (MEA) shows the location of anode and cathode, separated by the Nafion | graphene | Nafion sandwich. (b) Illustration of selective proton pumping through a Nafion | graphene | Nafion sandwich membrane-electrode assembly.


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Protons are pumped between the two platinum-coated carbon cloth electrodes (gray) through the Nafion and the graphene. The drawing is not to scale; in reality, the Nafion | graphene | Nafion membrane is very thin relative to the electrode diameters and the graphene layer extends several millimeters beyond the electrode edges, to avoid contributions from proton current around the graphene edges.

RESULTS AND DISCUSSION Making the Nafion | Graphene | Nafion sandwich structure MEAs. Figure 1 illustrates the process used to make the Nafion | graphene | Nafion sandwich structures and then attach platinum-on-carbon-cloth electrodes onto them to make the membrane-electrode assemblies (MEAs). Similar approaches to transferring CVD graphene onto polymer and solid substrates have been described by many others.36-53 The procedures for incorporating ionomer into electrodes and for attaching electrodes onto the Nafion membrane are similar to long-standing protocols for making PEM fuel-cell MEAs.54-55 Further details of the MEA fabrication process are provided in the Methods section and in the Supplemental Materials. Raman spectroscopy can provide important information on graphene and is a very useful tool for characterizing the Nafion | graphene | Nafion structures from this work.56-59 Figure 2 presents Raman spectra from a Nafion | graphene | Nafion sandwich structure (in sodium ion form) acquired after formation by hot-pressing but before application of electrodes. The spectra were acquired using a confocal Raman microscope operating with an oil-immersion objective and near diffraction limited probe volume (ca. 600 nm beam waist in the detection region). Details of the spectrometer system are provided in the Supplemental Materials. Spectrum (a) was recorded


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from a Nafion 211 membrane without graphene and shows all the expected Nafion features, which fall below 1400 cm-1 and outside the frequency range for the graphene 2D and G bands.[60] The remaining spectra in Figure 2 comprise a series recorded while the probe volume was stepped towards the Nafion | graphene | Nafion interfacial region (b-d). The topmost spectrum showing the strongest graphene peaks has all the features expected for single-layer graphene. The bands at 1584 and 2660 cm-1 are the G and 2D peaks respectively, and their positions are as expected for single-layer graphene.56-59 The FWHM width of the 2D peak in the topmost spectrum is 31 cm-1 and the I(G)/I(2D) ratio in this spectrum is approximately 0.20, both of which are also as expected for single-layer graphene.60-62 The lack of a large D-peak near 1350 cm-1 in any of the spectra indicates a relative lack of graphene defects having edge-plane character that could induce out-of-plane bond vibrations58, 63-65, though it is difficult to be very definitive on this point because Nafion also exhibits Raman peaks in this wavelength range. In fact, spectrum (d) where Nafion peaks are small relative to graphene shows a small peak at 1334 cm-1 that might indicate the presence of some graphene defects having out-of-plane bond vibration character. Figure S-5 in the supplementary material considers this spectral region in more detail for spectra acquired at several different locations on a Nafion | graphene | Nafion sample. In nearly all cases the spectra near 1350 cm-1 are indistinguishable from Nafion alone, which suggests that graphene defects correlated with D-band spectral features are very rare. The portions of the Raman spectra in Figure 2 between 1500 and 3000 cm-1 are very similar to the spectrum provided by the vendor for the CVD graphene on copper materials that was used to fabricate the Nafion | graphene | Nafion structures.66 Taken together, these results strongly indicate that graphene layers in the sandwich structures are single layer and relatively free of


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defects, and that they survive the Nafion hot-pressing procedure intact without any significant creation of additional defects.

2660 cm-1 (2D)

20 rel cts Naf-211 | Graphene | Naf-211

Intensity (arbitrary units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1584 cm-1 (G) 1334 cm-1 (d) x4 (c) x4 (b) 732 cm-1

500

Nafion 211

1000

1500

2000

(a)

2500

3000

Wavenumbers (cm-1) Figure 2. Confocal Raman spectra of a Nafion 211 sample (a) and a Nafion | graphene | Nafion structure recorded while stepping the probe volume toward the graphene (b-d). The interrogated region is far from the membrane center (b) and progressively nearer to the membrane center (c,d). Spectrum (d) was acquired with the interrogation region at an optimal


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position for high sensitivity detection of the graphene layer. Spectra of the sandwich structures were recorded with the laser power into the microscope objective attenuated to 2.8 mW.

Electrochemical hydrogen / deuterium evolution in asymmetric cells. The miniature test cells used in this work are convenient for studying the hydrogen and deuterium evolution reactions (HER and DER respectively) at platinum-on-carbon electrodes. The relatively small electrode size and low absolute cell currents are well matched to tests involving small amounts of valuable materials using simple instrumentation.35 Figure 3 presents several current-voltage (IV) curves for HER and DER at MEAs containing only Nafion, without graphene, in an asymmetric cell configuration with humidified hydrogen (or deuterium) gas flowing at one electrode (the anode) and humidified argon gas at the other electrode (the cathode). This type of hydrogen pump cell is useful for studying HER / DER at one electrode in the absence of concentration or kinetic polarization limits at the other electrode. The curve shapes in Figure 3 are as expected for a cell operating in this mode, with a near-zero baseline current at bias potentials positive of zero volts and significant rising reductive currents at potentials negative of zero volts applied to the cathode vs the anode. The curves at left are for three independently-prepared MEAs, each of which was made from two Nafion 211 membranes bonded together without graphene then used to make a MEA. The three curves are nearly identical which serves to illustrate the very high degree of reproducibility that is achievable for MEA fabrication and testing with this cell. A similar MEA prepared using a single Nafion 212 membrane, having the same nominal thickness as two Nafion 211 membranes bonded together, shows nearly identical behavior to that in Figure 3(a), indicating that the process for hot-pressing two Nafion 211 membranes together produces a structure that is nearly indistinguishable from a single membrane of the same thickness.


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The two IV curves in Figure 3(b) compare the behavior of MEAs run using hydrogen gas (red) and deuterium gas (blue). The membrane used for the HER experiment was pre-treated in an aqueous sulfuric acid solution to ensure that it was fully in the proton form, as was the case for all HER experiments reported here, whereas the membrane used for DER experiments was pretreated in a solution of deuterated sulfuric acid in D2O to ensure that it was fully in the deuteron form. The two curves are nearly identical, with the DER curve showing slightly lower currents than the HER curve for similar overpotentials. This behavior is consistent with the Nafion ionic conductivity being quite similar in the proton and deuteron forms, as is expected for a wellhydrated membrane in which the mobile ions are hydrated protons and deuterons (e.g. H3O+ and D3O+) which have masses that are not greatly different from each other.

0

0

(a)

(b)

-2

Current density (mA cm )

-2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-200 -400

MEA-1 MEA-2 MEA-3

-600 -800 -1000

-200 -400 HER DER

-600 -800 -1000

-0.2

-0.1

0.0

0.1

0.2

0.3

-0.2

Potential (V)

-0.1

0.0 0.1 Potential (V)

0.2

0.3

Figure 3. (a) Hydrogen evolution for three identically-prepared MEAs made from two Nafion 211 membranes bonded together without graphene. The similarity of these three IV curves demonstrates the high degree of reproducibility that may be achieved. (b) Hydrogen evolution (HER) vs deuterium evolution (DER) at two identically-prepared MEAs, one prepared for


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hydrogen evolution and the other for deuterium evolution. The similarity of these two IV curves shows that, in the absence of graphene, hydrogen and deuterium evolution voltammograms are very similar to each other. Other conditions and parameters as described in the Methods section.

Figure 4 presents side-by-side comparisons of HER (a) and DER (b) in MEAs with and without a graphene layer (blue and red) sandwiched between the two Nafion membranes in the MEA. The graphene layer has almost no effect in the HER cell but it has a relatively large effect in the DER cell, with DER currents diminished by a factor of approximately three when the graphene layer is included. This result is fully consistent with the findings reported by Hu and co-workers and Lozada-Hidalgo and co-workers in their related experiments.28, 30 A significant difference in the present work is that the HER and DER reactions both occur with much higher current densities and at much lower applied biases in the cells studied here compared with the cells studied by Lozada-Hidalgo. For example, in reference

30

, Lozada-Hidalgo and co-workers

report a proton current density of approximately 140 mA cm-2 at an applied bias of approximately -1.6 V, whereas we see a proton current density of approximately 900 mA cm-2 at an applied bias of just -0.15 V. This large difference in proton current density could reflect differences in the way the cells are prepared. For example, the cells reported here have the graphene layer embedded between two Nafion membranes whereas the cells studied by LozadaHidalgo have a palladium-decorated graphene layer positioned on one side of a Nafion membrane. The cells reported here also use catalyst layers and gas flow fields that are optimized for hydrogen oxidation / proton reduction reactions, which helps to minimize kinetic and transport losses associated with the electrode reactions.


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200

200

(a) HER

(b)DER

0

0

-2


-2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-200 -400 MEA- w/o graphene MEA- with 1L graphene

-600 -800

-200 -400

MEA- w/o graphene MEA- with 1L graphene

-600 -800

-1000

-1000 -0.2

-0.1

0.0 0.1 Potential (V)

0.2

0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Potential (V)

Figure 4. (a) Hydrogen evolution reaction (HER) for two similarly-prepared MEAs made from two Nafion 211 membranes bonded together, one without graphene and the other with a graphene layer between the Nafion membranes. (b) Deuterium evolution reaction (DER) for two separate similarly-prepared MEAs. These data show clearly that a graphene layer in Nafion has little effect on hydrogen evolution (a) but a significant effect on deuterium evolution (b).

Electrochemical hydrogen / deuterium pumping in symmetric cells. The HER / DER cells are useful for observing general trends associated with proton and deuteron transport through graphene but they have limitations for quantifying that transport. For example, the baseline current at positive potentials can sometimes vary from zero, as in the red curves in Figure 3 (b) and 4 (a), which is presumably due to small amounts of hydrogen (or deuterium) gas that become trapped in the cathode or in the gas-diffusion electrode during voltammetric scanning. Also, the rising portions of the HER / DER curves contain contributions from both the membrane /


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graphene ionic resistance and the proton / deuteron ionic resistance and reduction kinetics at the platinum catalyst sites. Deconvolution of these and other factors affecting the current is necessary to focus just on the effects of graphene on proton transmission through the cell. A better cell configuration for quantifying ion-transport rates is the symmetric hydrogen / deuterium pump cell, in which the electrodes on both sides of the cell are identical and both sides of the cell are bathed with humidified hydrogen or deuterium gas. Current-voltage curves for such a cell are expected to be symmetric about the zero-bias position and will exhibit a linear response that is easily interpreted as a simple resistance. By comparing resistances for different MEA architectures and accounting for the various contributions to series resistance, it is a simple matter to obtain an effective area-normalized resistance for transport of protons / deuterons through graphene, in the absence of limitations from any other transport or kinetic phenomena.

60 H-Pump (a)

60

D-Pump (b)

40 Current (mA)

40 Current (mA)

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20 0 -20 Hpum p-Electronic resistance MEA-(N-211 single) MEA-(N-211 double) MEA-(N-211 double with graphene)

-40 -60 -0.10

-0.05

0.00 Potential (V)

0.05

0.10

20 0 -20

D pump-Electronic resistance MEA- (N-211 single) MEA- (N-211 double) MEA- (N-211 double with graphene)

-40 -0.10

-0.05

0.00

0.05

0.10

Potential (V)

Figure 5. (a) Hydrogen pumping and (b) deuterium pumping in two sets of MEAs, one set in proton form (a) and the other in deuterium form (b), having various combinations of Nafion and


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graphene. Cathode and anode are both 3/16 inch diameter carbon cloth disks (area = 0.178 cm2) having a platinum loading of approximately 0.3 mgPt cm-2. Further details are given in the footnotes of Table 1, and in the Methods section.

Figure 5 presents sets of IV curves for hydrogen (a) and deuterium (b) pumping for cells in different membrane configurations with and without graphene. For each cell the green line is an IV curve obtained for just the two graphite rod current collectors connected to each other through two gas diffusion layers. The resistances obtained from the slopes of these lines must be subtracted from the overall cell resistance to obtain the resistance due just to the MEA. The other lines correspond to one and two Nafion 211 membranes without graphene, and to two Nafion 211 membranes with graphene sandwiched between them. As expected, the slopes of the IV curves systematically decrease as more Nafion membrane layers are included and when a graphene layer is included. The effect of graphene is greatest for the deuterium pump cell, again as expected from the results with HER and DER cells shown in Figures 3 and 4. Table 1 summarizes these results and presents calculations of area-normalized resistance and conductance for the different cells, with the resistance in the absence of any membranes subtracted. Once this correction is made, the expected trend of doubling the resistance for two Nafion membranes vs one is obtained, and the additional resistance from the graphene layer is clearly seen to be much larger in the deuterium pump cell compared with the hydrogen pump cells. The MEA area resistance is obtained by simply multiplying the membrane resistance by the electrode area. By making a further subtraction of the membrane resistance from the MEA resistance, it is possible to obtain the area-normalized resistance of just the graphene layer, for proton vs. deuteron transport. These values are reported in Table 1, along with values for the


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graphene area conductance which is simply the reciprocal of the area-normalized resistance. The ratio of proton to deuteron conductance is approximately 14 to 1, consistent with but slightly larger than the ratio reported by Lozada-Hidalgo.28, 30 Table 1. Cell resistances from hydrogen / deuterium pump experiments Resistance,

Corrected Resistance(b)

MEA area resistancec

Graphene area resistanced

Graphene area conductance

Ω

Ω

mΩ cm2

mΩ cm2

S cm-2

Zero Nafion 211

1.266

--

--

--

--

One Nafion 211

1.530

0.264

47

--

--

Two Nafion 211

1.828

0.562

100

--

--

2.018

0.752

134

34

29

Zero Nafion 211

1.266

--

--

--

--

One Nafion 211

1.863

0.598

106

--

--

Two Nafion 211

2.398

1.133

202

--

--

5.019

3.754

668

467

2.1

Sample

Proton forma

Two Nafion 211 + graphene Deuteron forma

Two Nafion 211 + graphene

(a) For the cell in proton form, the MEA has been pre-treated with a solution of sulfuric acid in water and has hydrogen gas fed to both electrodes. For the cell in deuterium form the MEA was treated with a solution of deuterated sulfuric acid in D2O, and has deuterium gas fed to both electrodes.


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(b) Corrected resistance is the cell resistance following subtraction of the “zero Nafion 211” resistance. (c) MEA area resistance is the cell resistance multiplied by the active electrode area, which for all cells in this table was 0.178 cm2. (d) Graphene area resistance is given by the difference between the MEA area resistance for cells with two Nafion 211 membranes, with and without and embedded graphene layer between the Nafion layers. (e) Graphene area conductance is the reciprocal of the graphene area resistance.

Hydrogen vs. potassium ion transport through graphene. It is instructive to consider the possibility that other ions besides protons / deuterons could be made to pass through graphene. Conventional wisdom suggests and prior measurements have shown that atoms as small as helium are unable to pass through pristine graphene,4 so it is expected that all other ions except proton and its isotopes should be blocked by pristine graphene because they all possess an electron cloud that will interact strongly with graphene to prevent transport. On the other hand, significant ion currents through graphene have been observed by many groups, mostly associated with defect structures of various kinds.11,

22, 67-69

Studies of ion transport through the present

Nafion | graphene | Nafion structures are important for gaining understanding of the cause of the very large proton and deuterium ion current densities reported in Figure 5. Ion transport through graphene is easily tested using a modified version of the present cells in which the graphite rod current collectors are used as contacts for silver foil electrodes that are


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pre-coated with a thin layer of silver chloride by brief anodization in a chloride-containing solution. These electrodes are then assembled into cells using porous filter papers wetted with various chloride-containing aqueous electrolytes positioned between the silver / silver chloride electrodes. Nafion membranes with and without graphene are converted into ionic forms that match the electrolyte and are also included in the cell between the electrolyte-wetted papers. Figure 6 shows IV curves obtained for two sets of cells prepared in this way, one containing an HCl electrolyte ((a), left) and the other containing a KCl electrolyte ((b), right). The current densities in both of these cells are much smaller than in the hydrogen pump cells and the IV curves are not fully linear. This behavior probably reflects the fact that the electrode reactions now involve silver oxidation / reduction at heterogeneous Ag / AgCl interfaces with involvement of chloride ions from solution and with ion transport occurring in solution. Even so, the effect of graphene on ion transport is very different in the two cells, with proton transport being nearly identical through Nafion vs. a Nafion | graphene | Nafion separator (Figure 6 (a)), whereas potassium ion transport through Nafion is rapid without graphene present in the Nafion but is almost completely shut off for the Nafion | graphene | Nafion separator (red vs green, Figure 6 (b)). The results from Figure 6 are summarized in Table 2 which presents resistances for cells containing protons and potassium ions, with and without graphene in the Nafion membrane. Potassium ion current through Nafion is attenuated by graphene by a factor of approximately 5400 (84 vs. 450,000 Ω cm2), whereas proton current through Nafion is attenuated by a factor of just 2.0 (59 vs. 30 Ω cm2) by graphene. The graphene layer is almost completely blocking towards potassium but is highly permeable towards protons.


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0.02

(a) Current (mA)

0.02

Current (mA)

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0.00

(b)

0.00

-0.02

-0.02

KCl Paper

HCl Paper





KCl Paper + Nafion -211



KCl Paper + Nafion -211 + graphene

HCl Paper + Nafion -211



HCl Paper + Nafion -211 + graphene

-0.04 -0.010

-0.005 0.000 0.005 Potential (V)

-0.04 -0.010

0.010

-0.005

0.000

0.005

0.010

Potential (V)

Figure 6. Current-voltage curves for proton pumping (a) and potassium ion pumping (b) from aqueous solutions through various Nafion membranes. Electrodes are silver-chloride-coated silver metal disks contacting aqueous solutions of 0.1M KCl and 0.1M HCl respectively. Currents for potassium ion pumping are greatly diminished when graphene is present in the Nafion, whereas currents for proton pumping are almost unaffected by graphene.

Table 2. Cell resistances from potassium / proton transport experiments Corrected Resistance

MEA area resistance

Graphene area resistance

Graphene area conductance

Ω

Ω

Ω cm2

Ω cm2

mS cm-2

459

--

--

--

--

Resistance Sample

Proton transport 0.1M HCl Paper


19


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0.1M HCl Paper + Nafion 211

519

60

30

--

--

0.1M HCl Paper + Nafion 211+ graphene

578

119

59

29

34

0.1M KCl Paper

451

--

--

0.1M KCl Paper + Nafion 211

621

170

84

0.1M KCl Paper + Nafion 211+ graphene

0.9 x106

0.9 x106

4.5 x105

4.5 x105

2.2x10-3

Potassium ion transport

It is significant that the area-normalized conductances for both proton and deuteron transport through graphene obtained in this work are quite high, much higher than in previous reports. As noted earlier, this fact could reflect a reduction in transport and kinetic losses in our cells compared with other cells, perhaps associated with the high catalytic activity afforded by Nafionimpregnated platinum-carbon electrodes and the rapid transport of hydrogen and deuterium gas in flow channels coupled to gas-diffusion layers adjacent to the electrodes. It may also reflect an improved interfacial contact between Nafion and graphene afforded by the hot-press method compared with other methods for making this contact. Intimate contact between graphene and the protogenic groups in Nafion would presumably strongly favor proton (deuteron) transport from one Nafion layer to the other through the graphene layer. It will be interesting to learn more about the interfacial structure of Nafion | graphene interfaces, particularly regarding the variation in ion concentration with distance (i.e. the double-layer structure) near the interface, and the possibility of graphene being charged. In any case, the fact that high ion currents are possible for


20

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hydrogen isotope transport through graphene is important both fundamentally for understanding how such transport is possible, and also practically because it suggests that graphene-based ion filtering may be possible in devices intended for operation at quite high current densities, for example PEM fuel cells and water electrolysis cells. Electrochemical isotope separation and production may be possible at the large scales needed for many technological applications.

Figure 7. SEM micrographs of a graphene-on-copper sample subjected to a 5 second etch using 0.1 M FeCl3 in water at various magnifications (a) 2 mm (b) 50 μm (c) 40 μm scale bars

The Role of Defects. Structural defects must always be considered in mass transport studies involving graphene-based materials, and this is particularly so in transport studies on macroscopic CVD graphene samples of relatively large area.9,

11-12, 14-15, 18, 22, 24-25, 38, 67-72

We

tested our single-layer graphene samples on copper for relatively large-scale structural defects using a recently-described method involving brief chemical etching of copper through the graphene barrier layer, followed by SEM imaging to visualize the resulting etch pits.69 This method is reported to be sensitive to defects as small as 20 nm in size. Results of this


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characterization are presented in Figure 7. Low-magnification SEM imaging reveals the grain structure of the underlying copper and a circular region where a 0.1 M ferric chloride etchant solution droplet was placed. The treated region is clearly different from the non-treated region, and upon closer inspection the treated region is seen to contain many discrete etch pits that form underneath individual defect sites. Some etch pits are isolated suggesting single defect sites, and others are larger and irregularly shaped, suggesting larger defects and/or clusters of defects. The complete absence of any very large etch pits indicates that the graphene sample is free of rips, tears, cracks or any other type of macroscopic defect that would expose a large area of copper. The presence of these etch pits clearly proves that the CVD graphene is not an ideal barrier, however the remarkable thing about them is not that they exist, but rather how few in number they are. Analysis of the images in Figure 7 indicates an average defect density of approximately 110 defects in a 40x40 micrometer square, or approximately 7x106 defects per cm2. (Details of the image analysis are provided in the Supplemental Materials.) If each defect was a 20 nm diameter disk, the fractional area occupied by disks would be approximately 2 x 10-5, or 0.002 %. The actual defect area is probably larger than this because some etch pits counted as one defect are relatively large and probably are caused by more than one defect or by defects larger than 20 nm in size. A more definitive treatment will require more detailed knowledge of the nature, the amount, and the spatial distribution of graphene defects and is beyond the scope of this report. This defect analysis is generally consistent with our observation that the ion resistance for potassium ion transport through a Nafion sandwich structure containing a graphene layer is much larger than that in the absence of the graphene layer. It seems likely that the potassium ion current passing through graphene in the Nafion | graphene |


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Nafion sandwich structures may be passing through the same kinds of defects that gave rise to the etch pits in Figure 7. The proton currents we observed through Nafion | graphene | Nafion sandwich structures are much larger than the potassium currents and are almost certainly too large to be explained solely by the defect sites considered above. Proton transfer must be occurring in the regions in Figure 7 that are blocking towards FeCl3 and towards potassium chloride. Two different explanations for such large proton currents through graphene have been proposed. In the first explanation, proton transfer could occur directly through pristine graphene, as discussed in the introduction, by passage through the small spaces in the carbon hexagons of the graphene structure. This explanation was initially put forth by Hu and co-workers26 to explain the large proton currents they observed through graphene flakes in microfabricated small cells. This explanation is attractive even though it contradicts many studies predicting that activation energies for proton transfer through graphene would be far too high for proton transfer to occur at measurable rates and ambient temperatures.27 A second explanation is that proton transfer could be occurring at defect sites that are rare and that are very permeable to protons but not to potassium ions. A similar explanation was recently put forth by Achtyl and co-workers to explain their observation that graphene had no discernable effect on rates of silanol group protonation / deprotonation on the opposite side of graphene supported on silica and in contact with aqueous acid solutions.73 If defects of this type were responsible for the results reported here, they would have to be rare enough to be nearly undetectable, yet also active enough to support an average proton current density up to 1 A cm-2. As an example, considering just area ratios, if proton transfer was occurring through defects that occupied just one percent of the graphene surface area, each of


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those defects would have to support a current density of 100 A cm-2 to achieve an overall average current density of 1 A cm-2. An electrochemical model. It is instructive to consider how the current densities we observe could be accommodated in a rate model that considers proton transfer across graphene at specific sites, with specific rate constants. Such a model could prove useful in understanding how proton transfer occurs, i.e. what parts are controlled by activation energetics, dynamical motion along a reaction coordinate, proton tunneling, etc. We present here such a model for the case of proton / deuteron transfer occurring directly through the hexagonal sites in pristine graphene. A similar model could be easily developed for other proton-transfer sites. All that would be needed is an average area per site to allow for estimation of the flux at each site. Considering transport through the hexagonal carbon sites of graphene, from the known C-C bond distance of 1.42 Å in graphene,74 each hexagonal site has an area of approximately 5.24 Å2 or 5.24x10-16 cm2. Assuming the proton flux is evenly distributed among graphene hexagonal sites, the area-normalized proton-transfer resistance of 34 mΩ cm2 corresponds to a per-site proton-transfer resistance of approximately 6.5x1013 Ω, or 65 teraohm. This resistance may in turn be considered as a charge-transfer resistance for proton transfer at each site in the context of Butler-Volmer theory. In such a case, the following relationship between the charge-transfer resistance and the rate constant for proton exchange at a single site across the graphene layer applies;

=

=


24

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In this expression, RCT is the charge-transfer resistance for a single site, io is the proton exchange current per site, e is charge on a proton, and krxn is the first-order rate constant for proton transfer at a single site. Solving this expression for krxn gives a value of approximately 2500 s-1. Application of the same analysis to the area-normalized resistance of 467 mΩ cm2 for deuteron transfer through graphene yields values of 891 teraohm for the per-site resistance and 180 s-1 for the per-site deuterium exchange rate constant. The analysis given above is certainly very simplistic and is not intended to capture all the nuances of proton / deuteron transfer across graphene, which to be sure is a complex topic that continues to be the subject of much attention from both experimental and theoretical / computational perspectives. The analysis is provided in the spirit of demonstrating that the very high current densities we have observed for proton transfer across graphene are reasonable and may be interpreted in the context of conventional chemical kinetics models, suitably adapted to account for the activation energies and transmission probabilities for proton transfer across graphene in a Nafion | graphene | Nafion sandwich.

CONCLUSIONS Very high rate transmission of protons and deuterons through single-layer CVD graphene is demonstrated in Nafion | graphene | Nafion sandwich structures. Proton transmission is favored over deuteron transmission by a factor of approximately 14, in good agreement but slightly higher than in prior related work.

Area-normalized conductances for proton and deuteron

transport through graphene are much higher than in previous works which probably reflects a combination of improved cell design and fabrication, and improved interfacial contact between


25


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Nafion and graphene. Prospects are good for creation of graphene-based ion filters for use in PEM-style devices, e.g. fuel cells and electrolysis cells, intended for operation at high current density.

METHODS Materials. Nafion was obtained from The Fuel Cell Store and CVD graphene was obtained from ACS Material. Other materials used to assemble cells, including platinized carbon cloth electrodes and gas diffusion electrodes were obtained from The Fuel Cell Store. Part numbers are indicated below. Hydrogen (Ultra high purity-200) and argon (Ultra high purity-300) gases were provided from large cylinders connected to gas lines, and deuterium gas (Research grade) was provided from a smaller type 35 cylinder that was dedicated to this process and located close to the cells being tested. Gas streams were humidified by bubbling through water for hydrogen and argon, and through D2O for deuterium. Gas flow rates were set using rotameters and were generally held at 20 standard cubic centimeters per minute (sccm) or less. In the case of hydrogen, this gas flow rate corresponds to at least ten times the stoichiometric amount needed to support the cell currents. Nafion | Graphene | Nafion sandwich fabrication. Membranes having single-layer graphene positioned between two Nafion membranes were fabricated by a three-step process as illustrated in Figure 1. First, a small piece of copper-supported graphene, approximately 2 cm square, was cut from a larger piece and placed on top of a Nafion 211 disk having a thickness of approximately 25 micrometers and cut to a diameter of ¾ inch (1.91 cm). This assembly was placed in a Carver hot press and pressed at 140 oC for 2 minutes. Next, the Nafion | graphene |


26

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copper assembly was placed in a 0.3M solution of ammonium persulfate in water and allowed to react until the copper layer was fully oxidized, as indicated by visual inspection. After a brief water rinse, a second Nafion 211 disk was placed on top of the graphene layer, and the assembly was again hot-pressed using the same conditions as for the initial treatment. Raman microscopy / spectroscopy. Raman spectra were acquired at the University of Utah using a Raman microscope system that has been previously described in detail.75 In brief, the system uses a 638 nm laser source that is focused to a 600 nm diameter beam from which the Raman signal is collected. Samples were ion-exchanged into the sodium ion form prior to Raman characterization. Just prior to spectral measurements, to eliminate background fluorescence, membrane samples were hydrated by brief (ca. 120 s) immersion in 0.5 M NaCl containing 0.3 % H2O2 followed by rinsing in deionized water. Both solutions were at 60 °C. After removal from water, the membrane was set on a Kimwipe tissue to remove surface water droplets before placing the sample on a glass coverslip positioned on the microscope stage. A glass microscope slide was placed on top of the membrane to hold it firmly against the coverslip and maintain constant hydration. Further details on Raman measurements are provided in the Supplementary Materials. MEA fabrication. Membrane-electrode assembly fabrication followed previously described procedures for the miniaturized PEM test cell. In brief, for each MEA, two disk-shaped carbon cloth electrodes were cut, one for the anode and one for the cathode. For HER / DER cells, anodes were 5/16 in diameter and were cut from a larger carbon cloth electrode that was coated with 4 mg / cm2 of platinum black (Fuel Cell Store, part number 1610008), and cathodes were 3/32 in diameter and were cut from platinized carbon cloth containing 0.03 mg / cm2 platinum on carbon black (Fuel Cell Store part number 1610000). Additional Nafion ionomer was added to


27


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each electrode from a Nafion suspension (5 wt. % Nafion® solution); for the anodes, 3.5 μL microliter of Nafion suspension was used, and for the cathodes, 0.5 μL microliters was used. For the symmetric hydrogen / deuterium pump cells, identical electrodes of 3/16 inch diameter (area = 0.178 cm2) with a platinum loading of 0.3 mgPt cm-2 (Fuel Cell Store part number 1610002) were used. Following drying, the electrodes were assembled onto a Nafion disk, with or without embedded graphene, with catalyst-coated sides facing towards the Nafion, and when appropriate with the smaller cathode disk placed approximately in the center of the larger anode disk. This assembly was then hot-pressed at 140 C for 5 minutes. Cell assembly and testing. Hydrogen pump experiments were performed using a recentlydescribed miniaturized PEM fuel cell fixture. In brief, the cell consists of a modified fluoropolymer compression fitting suitable for holding ¾ inch tubes and adapted to allow for insertion of two flat-ended graphite rods that serve as current collectors. Each graphite rod has two holes drilled in it, one to allow for gas inlet and another for gas exit. The rods make electrical contact with the MEA electrodes via gas diffusion layer (GDL) disks (AvCarb GradeP50T Carbon Fiber Paper, Fuel Cell store, part number 590242) that are interposed between them. Hydrogen pump experiments were performed in two modes, asymmetric and symmetric. In asymmetric mode, the cell was configured with humidified hydrogen or deuterium gas at the anode and humidified argon at the cathode, whereas in symmetric mode both electrodes were bathed in humidified hydrogen or deuterium gas. Cell testing was done at 30 oC with gases humidified at 30 oC with H2O for hydrogen and D2O for deuterium. Current-voltage curves were acquired using a CH Instruments model 1140B potentiostat in linear sweep or cyclic voltammetry mode. Scan rates were 5 mV s-1 for all asymmetric cell experiments and 1 mV s-1 for all symmetric H/D-pump experiments; in both cases currents were independent of scan rate.


28

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Silver / silver chloride cells. Ion-pump experiments in aqueous solution were conducted using the same Swage-style cell that was used for hydrogen pump experiments, except that the graphite rod current collectors were modified with disks of silver metal foil (5/16 in diameter) that had been briefly anodized in a hydrochloric acid solution. Following anodization, the electrodes were placed in contact with Millipore glass fiber paper that were soaked in the chloridecontaining electrolyte (either 0.1 M HCl or 0.1 M KCl) in which they were intended to be used. Cell assembly consisted of placing two layers of electrolyte-soaked filter paper between the Ag/AgCl disks and pressing them together. Cells were studied with and without a Nafion membrane (Nafion membranes were converted into the appropriate cation form by ion exchange prior to use) placed between the electrolyte-soaked papers. Nafion membranes were studied with and without an embedded graphene layer. Cell characterization consisted of current-voltage curve acquisition for a bias range of +/- 10 mV, at a potential scan rate of 1 mV s-1. Graphene-on-copper defect visualization.

Defect visualization for graphene-on-copper

samples was accomplished following Kidambi69 by briefly rinsing the samples with water then placing a small droplet of 0.1 M FeCl3 in water on top of the sample. After five seconds the sample was immediately immersed in water to interrupt etching. Samples were then dried and promptly subjected to SEM imaging using a Hitachi model S-3400N Variable Pressure Scanning Electron Microscope.

ASSOCIATED CONTENT


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The following file is available free of charge. Supplemental materials file contains detailed descriptions of test cells, MEA fabrication protocols, Raman microscopy / spectroscopy, and defect counting from electron micrographs. File type is PDF.

AUTHOR INFORMATION Corresponding Author * Stephen Creager, [email protected]

ACKNOWLEDGMENT Financial support for this work was provided by the Savannah River National Laboratory (SRNL)'s Laboratory Directed Research and Development program. SRNL is managed and operated by Savannah River Nuclear Solutions, LLC under Contract No. DE-AC09-08SR22470 with the United States Government. The authors also gratefully acknowledge the Office of Science, US Department of Energy, through Grant DE-SC0018151 for financial support of the work, and additional support for Raman microscopy at the University of Utah, through Grant DE-FG03-93ER14333.


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TOC graphic;

e-

H2 D2

H+ D+

H2 D2


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Deuteron Transport through Nafion|Graphene|Nafion

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