Single-Layer Graphene Sandwiched between Proton-Exchange

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Single-Layer Graphene Sandwiched between ProtonExchange Membranes for Selective Proton Transmission Saheed Bukola, Kyle Beard, Carol Korzeniewski, Joel M. Harris, and Stephen E. Creager ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02270 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Nano Materials

Single-Layer Graphene Sandwiched between Proton-Exchange Membranes for Selective Proton Transmission Saheed Bukola‡, Kyle Beard‡, Carol Korzeniewski#,$, Joel M. Harris$ and Stephen E. Creager‡*

‡ Department

of Chemistry, Clemson University, Clemson SC 29634, United States

# Department

of Chemistry, Texas Tech University, Lubbock, TX 79409, United States

$

Department of Chemistry, University of Utah, Salt Lake City, UT 84112, United States

*

Corresponding author; email [email protected]; phone 864-656-4995.

KEYWORDS Graphene, 2D material, proton exchange membrane, Nafion®, ion transmission, nanopore

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ABSTRACT

Proton transmission through single-layer CVD graphene in graphene / proton-exchangemembrane (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.


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Graphene and related 2D materials are being intensively studied as selective barrier materials for applications in gas separation and purification, water desalination, ultrafiltration, and other areas.1-13 These materials are atomically thin so their transport selectivity comes from the selectivity of active sites at which permeant transmission occurs, which is in contrast to conventional polymer-based ultrafiltration membrane separators for which selectivity usually comes from the relative permeability of a thin bulk polymer layer to permeants. Polymer-based ultrafiltration and related membranes, e.g. membranes for reverse osmosis water desalination, typically exhibit low flux especially for the larger polymer thicknesses (larger than atomic thickness) that are usually required to achieve high selectivity. Much higher flux is desired and should be possible with high selectivity using 2D materials for which selectivity is achieved via selective transport at active sites. Ion transmission across 2D materials such as graphene, particularly when driven by a potential difference, is particularly important and several studies have reported results on this topic.7, 14-25 Studies are often performed using free-standing graphene that is suspended across a nanopore, for example in a nanofabricated silicon nitride layer or a track-etched polycarbonate membrane, or at the tip of an electrolyte-filled nanopipette. Transmembrane ionic transport is driven by electrical polarization across the graphene layer via electrolyte solutions that are in contact with each side of the graphene layer. Garaj and co-workers reported in 2010 that a significant ionic conductance was present across graphene membranes separating aqueous solutions of alkali metal chlorides, e.g. XCl where X = Li, Na, K, Rb and Cs.25 Conductance values between 25 and 70 pS were reported for graphene layers suspended across 200 x 200 nm square holes in silicon nitride and separating 1 M salt solutions. (These conductance values correspond to areanormalized graphene ionic resistances of 3-8 ohm cm2.) Ion transmission is thought to occur at 3 ACS Paragon Plus Environment


small defect sites in the graphene with selectivity caused by a combination of electrostatic and ion hydration energy effects, with ion size being a secondary factor.22, 26 Subsequent reports noted a high selectivity for cation vs. anion transport,15 and current-voltage curve shapes that are sometimes linear and sometimes non-linear.15, 17, 19, 21 Ion conductance across graphene is significantly increased if the graphene is treated to intentionally create defect sites, e.g. using plasmas, ozone, electron or ion beams, or other means. 18 Proton transmission is a special case for several reasons. 19, 27-36 Protons in water exist in a highly solvated form with a hydration energy that is more than double that of other simple monocations.37 Proton transport in bulk water occurs by a combination of vehicle diffusion and structure diffusion, also known as proton hopping,38-39 which is in contrast to other aqueous ions for which vehicle diffusion is the sole means of transport. Proton size depends strongly on solvation, e.g. hydrated protons may be described as H3O+, H5O2+, H9O4+, and other forms, and of course the desolvated proton H+ is a nuclear particle of very small size. Proton transmission through pristine graphene was thought from many computational studies to require activation energies well above 1 eV,31-32, 36, 40-41 which would render transmission rates quite low at room temperature. It was thus quite surprising when high proton transmission rates through nominally pristine graphene were initially reported in 2014.27 Several subsequent experimental studies have confirmed this initial observation,19, 28, 30, 33 and have noted that activation energies for proton transmission through graphene are well below 1 eV with proton transmission favored over deuteron transmission by a factor of at least ten. 28-30, 34 The precise mechanism by which high-rate proton transmission occurs through graphene is still uncertain. 36 Possibilities include direct transmission through pristine graphene and transmission at defect sites that rare enough to


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be not easily detected33 but active enough to support a high ion flux with high selectivity for protons vs deuterons.34 Proton transmission across graphene has been studied in electrochemical hydrogen pump cells42-43 in which graphene is associated with a proton-conducting polymer electrolyte membrane (PEM) such as Nafion and proton flux is provided by hydrogen electrodes on either side of the membrane.27-30, 34 This cell configuration is convenient for studying proton transmission but is not so convenient for studying other ions. We recently reported on a preliminary study of potassium ion transmission through graphene using a PEM material exchanged into potassium form, in a cell where the hydrogen electrodes were replaced by silver/silver chloride electrodes.30 Potassium ion transmission across graphene was found to be more than 10,000 times slower than that for protons, but this result is a bit uncertain because the silver / silver chloride electrodes exhibit relatively slow electrode kinetics, they are subject to variation due to concentration polarization of chloride, and they respond strongly to trace amounts of other ions that can form insoluble precipitates with silver or chloride. So far this result has not been followed up by a more comprehensive study focusing on transmission of different ions through graphene in PEM / graphene composites. A study examining transmission of a wide range of ions across graphene, all studied under similar conditions using similar methods, would be valuable because it would reveal the selectivity of graphene for ion transport without complications from the different methods and cell configurations used to measure transmission rates for different ions. We report here on a study of cation transport through graphene embedded in Nafion | graphene | Nafion sandwich structures, using a four-electrode electrochemical method in which a pair of platinum electrodes drives ions through graphene and a pair of reference electrodes in Luggin 5 ACS Paragon Plus Environment


capillaries senses the transmembrane potential difference induced by the forced ion flow. This method does not rely on specific electrode reactions occurring at the PEM surfaces so it is suitable for use with any membrane and any electrolyte. Electrolyte concentration was kept relatively low to ensure that ion currents through Nafion and graphene are dominated by cation transport. The key finding from the work is that proton transmission rates through graphene are 150 - 350 times larger than rates for any other cation besides proton. We also report on a wide range of graphene characterization studies including confocal Raman microscopy and X-ray photoelectron spectroscopy (XPS) for graphene on Nafion, and defect visualization by chemical etching through graphene defects of copper supports for graphene on copper. All findings are consistent with a very high proton transmission rate through graphene by a mechanism that is likely different from that which applies to other cations that exhibit much lower transport rates. We also report here on a parallel study using electrochemical impedance spectroscopy (EIS) to study ion transmission rates through graphene embedded in Nafion membranes. Graphene was found to have a relatively small effect on the high-frequency resistance of Nafion | graphene membranes in all ionic forms, which is in contrast to results obtained using the four-electrode cell. This finding is thought to reflect a capacitive coupling of mobile ions in Nafion with the graphene at the high frequencies used in the EIS measurement, rather than a high rate of true ion transmission through graphene. Near-steady-state methods are thus necessary to evaluate the true rates of ion transmission through graphene.


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RESULTS AND DISCUSSION The principal aim of this work is to examine transmission rates for protons and other cations through single-layer graphene in Nafion | graphene | Nafion sandwich structures, for the purpose of learning about mechanisms of ion transmission through graphene. Measurements must be made by the same method for all cations to provide the most valid comparison. We report on tests using two methods, which are illustrated in the two cell designs shown in Figure 1. For the cell in Figure 1A, ion transmission is measured using a four-electrode Devanathan-Stachurski (D-S) cell with two Pt wire drive electrodes and two Luggin-capillary reference electrodes. Current-voltage curves are acquired using slow-scan linear sweep voltammetry in which ion current flow through the membrane is driven by the drive electrodes and the ion transmission resistance is measured by the reference electrodes. The I-V curve slopes are indicative of membrane resistance. For the cell in Figure 1B, carbon cloth electrodes are affixed to the membrane to create a membrane-electrode assembly (MEA) that is then analyzed using

Ref

Pt wire

(A)

Pt wire

Ref

(B)

MEA

Figure 1. Illustration of cells used to measure ion transmission rates through graphene in membranes. (A) Four-electrode Devanathan-Stachurski (D-S) cell used to measure iontransmission rates. The Pt wire drive electrodes force current flow through the membrane, and the sensing reference electrodes detect the potential difference across the membrane caused by the resistance to ion transmission. (B) Membrane-electrode assembly with graphite rod current collectors for study of membrane resistance by electrochemical impedance spectroscopy (EIS).



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electrochemical impedance spectroscopy (EIS) to determine membrane resistance. Further details on cell design are given in the Methods and the Supporting Materials.

CVD graphene Copper

Nafion Nafion

Nafion First Nafion hot press

Copper Ammonium Persulfate etch Nafion

Second Nafion hot press

5/8 in

1 in

Figure 2. Illustration of the steps used for fabricating Nafion | graphene | Nafion sandwich structures for use with the D-S dcell. The photograph at bottom right is of a completed membrane, and the illustration as bottom left shows the active area in the D-S cell, which is a 5/8 inch diameter disk.

Graphene sandwich structures with Nafion membranes for use with the four-electrode D-S cell were made by a method similar to that used in our recent previous work, but adapted to create membranes without electrodes attached to them. In brief, a sample of single-layer CVD graphene on copper was hot-pressed onto a Nafion membrane, then the resulting structure was 8 ACS Paragon Plus Environment

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subjected to an oxidative etch with ammonium persulfate to remove the copper, then a second Nafion membrane was attached in a second hot-press step. These steps are illustrated in Figure 2. The resulting membrane is a 1-inch diameter disk, with graphene embedded over the entire area, between the Nafion membranes. Further details are provided in the Methods section.

A

+

H

20

Current (mA)

50 0 -50 0.1M HCl 0.1M HCl + Nafion-211 0.1M HCl + Nafion-211 + graphene PET

-100

-0.10 -0.05 0.00 0.05 Potential (V) 40

C

30

Na

Current (mA)

100

75

Current (mA)

0.1M NaCl 0.1M NaCl + Nafion-211 0.1M NaCl + Nafion-211+ graphene

-30

-0.05 0.00 0.05 Potential (V)

0.1M LiCl 0.1M LiCl +Nafion-211 0.1M LiCl +Nafion-211+graphene

0.10

D

50 25 0

+

H (N-211+1L graphene) + Li (N-211+1L grapehene) + Na (N-211+1L graphene) + K (N-211+1L graphene) + Rb (N-211+1L graphene) + Cs (N-211+1L graphene) + NH4(N-211+1L graphene)

-25 -50 -75

-40 -0.10

-10

100

-10 -20

0

-0.10 -0.05 0.00 0.05 Potential (V)

+

0

+

Li

10

0.10

10

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-20

20 Current (mA)

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


-100

0.10

-0.10 -0.05 0.00 0.05 Potential (V)

0.10

Figure 3. Current-voltage curves in D-S cells without Nafion, with Nafion, and with Nafion + graphene. A, HCl electrolyte; B, LiCl electrolyte; C. NaCl electrolyte. D, curves for Nafion + graphene in a series of chloride electrolytes with different cations. Electrolyte concentration is 0.1 M.

Figure 3 presents current-voltage (I-V) curves acquired using the Devanathan-Stachurski (D-S) cell with Nafion membrane separators with and without embedded graphene, for several



representative electrolytes. A more complete data set including other cations is given in Figures S2 and S3 in the Supporting Information. Electrolyte concentration is 0.1 M in all cases which is low enough to avoid significant anion partitioning into the membranes (i.e. Donnan failure) and ensure that the ionic current in Nafion is carried principally by cations. 44-46 The I-V curves are all highly linear near zero bias, which is consistent with current limitation by ohmic factors and not by electrochemical kinetics or transport factors such as diffusion that often exhibit highly nonlinear dependence of current on bias voltage. A closer examination of the curves acquired in HCl, LiCl and NaCl electrolyte (Figure 2A, B and C) reveals that in all three cases, the cell resistance shows a measurable but relatively small increase when the Nafion membrane is put in place, compared to the cell with no membrane in place. This finding is consistent with the Nafion membrane having a slightly lower ionic conductivity than the equivalent volume of liquid electrolyte that it replaces. The most important finding from Figure 3 comes from a comparison of results for Nafion membranes containing embedded graphene in HCl electrolyte with those in the other electrolytes. The resistance to ion transmission of the graphene-containing membrane in HCl electrolyte (Figure 3A) is nearly the same as that of a graphene-free membrane. This fact is indicated by the very small change in slope of the I-V curve for the membrane with graphene, compared to the membrane without graphene (blue vs. green). In contrast, the resistances of graphene-containing membranes in lithium and sodium form (Figures 3B and 3C) are both dramatically increased compared with graphene-free membranes (again, green vs blue curves). The graphene layer presents a much more resistive barrier to lithium and sodium ion transmission compared with proton transmission. Figure 3D compares the current-voltage curves for Nafion – graphene membranes across all the electrolytes used in this study, which include 10 ACS Paragon Plus Environment

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HCl, LiCl, NaCl, KCl, RbCl, CsCl, and NH4Cl. The clear trend in these data is that graphene ionic resistance is small for transmission of protons and much larger for all other cations. Singlelayer graphene is a good through-plane conductor of protons but a poor conductor of all other cations. Table 1. Resistance values measured in Devanathan-Stachurski (D-S) cells with no membrane, Nafion membrane, and Nafion membrane with embedded graphene.

Solution only in DS cell Cations H+ Li

+

Na+ +

K

(Ω)

(Ω)

(Ω)

(Ω cm2)

1.08

1.32

1.64

0.6

4.7

6.7

118

220

6.0

7.7

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90

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106

117

+

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48

119

140

5.6

28

56

110

--

--

--

Rb Cs

Solution with Nafion Graphene Solution and area with Nafion graphene resistance

NH4

+

PET

7

5x10

The data in Figure 3 are conveniently summarized in Table 1, which presents Nafion membrane resistance values with and without graphene for all electrolytes studied. A simple subtraction of the membrane resistance without graphene from that with graphene reveals the resistance of the graphene layer alone, which is easily converted to an area-normalized resistance by multiplying resistance by the active area of 1.96 cm2. Graphene areal resistance to ion 11 ACS Paragon Plus Environment


transmission for a series of different cations is given in the far right column of Table 1. The trend is as expected from Figure 3; proton transmission through graphene occurs with quite low resistance whereas resistance to transmission of all other cations is much higher, by factors of between 150 and 350. Resistance values among the other ions besides proton show no clear trend other than that they are all much higher than for proton. We note as an aside that a control experiment was run using a polyethylene terephthalate (PET) disk in place of Nafion®, to test for leaks in the cell. The cell resistance measured with PET in place was approximately 107 times larger than with any of the Nafion® membranes in place. The measured cell resistances certainly reflect ion transmission through the Nafion ® membranes and not through cell leaks. A similar control was run with a PET ring having a 5/8 inch diameter hole in it, equal in size to the active area of the membrane mount in the D-S cell, mounted on top of the Nafion® membrane. This experiment was run to test whether there was any contribution to ionic current from regions outside of the 5/8-inch- diameter active area that is defined by the membrane mount in the D-S cell. Current-voltage curves were identical in cells with and without the PET ring in place, indicating that the vast majority, probably all, of the observed current comes from ion passage through the active region delineated by the membrane mount in the D-S cell. Electrochemical measurements provide insight into ion transmission rates through graphene but they are less informative about the nature of active sites where ion transmission might occur. We have therefore used a wide range of techniques to characterize the graphene layers used in this work, to seek insight into the nature of the sites at which ion transmission occurs. Raman spectroscopy is perhaps the most useful diagnostic technique for identifying and studying defect sites in graphene.47 The Raman D-band near 1350 cm-1 in graphene is strongly diagnostic of 12 ACS Paragon Plus Environment

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defect sites having edge character, including atom-vacancy pores as well as graphene crystal grain edge boundaries48-50. The absence of a D-band is commonly taken to mean that a sample is free of such defects. The Raman G and 2D bands near 1580 and 2650 cm -1 respectively have characteristic peak positions, peak widths and intensity ratios that are diagnostic of multiple graphene layers,51 doping effects (e.g. Fermi level)52-53 and mechanical strain effects.54-56

0.25 rel. cts.

TEA+ x6

d

aman Sccattering Ra

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


x3

c

b

2D

G

750

1250

1750

a

2250

2750

Wavenumber W b (cm ( -11) Figure 4. Raman spectra of single-layer graphene on a glass microscope coverslip and in Nafion – graphene – Nafion sandwich structures. a, graphene transferred onto BK-7 glass using the method recently described by Korzeniewski and co-workers in Reference 59; b, Nafion membrane without graphene; c, Nafion – graphene – Nafion in sodium ion form; d, Nafion – graphene – Nafion with one side in sodium form and the other in tetraethylammonium (TEA) ion form.



Variation of these spectral features with spatial position in samples thus reports on the localized presence of defects having edge character, and on local variations in doping and mechanical strain.55, 57 Figure 4 presents a collection of representative Raman spectra acquired for graphene layers on a glass microscope coverslip and within graphene / Nafion sandwich structures. Table S1 presents a summary of the graphene peaks from these and other similar spectra that are included as Figures S4 – S9 in the Supporting Information. The spectra were recorded using a confocal Raman microscope operating with oil-immersion optics to localize the detection volume on graphene at the buried polymer-polymer interface.30 The microscope probe beam diameter at the focus (approximately 600 nm), together with the collection aperture, defined a confocal probe volume with 90 % collection efficiency within a depth along the z-dimension of about 1200 nm.58-59 After positioning a sample against a coverslip on the microscope stage, the probe was manually translated through the glass coverslip and Nafion membrane, toward the graphene layer, and adjusted to achieve a high intensity for the G and 2D peaks. Figures 4a and 4b are for graphene on a glass microscope coverslip, and a Nafion 112 membrane without graphene. The graphene spectrum on glass shows no evidence of a D-peak, indicating a lack of graphene defects having edge character at least in the graphene region that was sampled in this spectrum. The Nafion-only spectrum shows the expected peaks for Nafion PFSA ionomer60 including a pair of peaks at 1299 and 1371 cm-1. These Nafion peaks overlap with the position expected for the graphene D-peak, so spectra of graphene / Nafion sandwiches must be carefully examined to judge the presence / absence of a D-peak. The spectra in Figure 4c and 4d are both for graphene / Nafion sandwich structures, in one case with Nafion in the sodium ion form (Figure 4c) and in another case with Nafion in sodium form on one side of the 14 ACS Paragon Plus Environment

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graphene and tetraethylammonium (TEA) ion form on the other side. These spectra both show the expected graphene G and 2D peaks along with peaks from Nafion. The relative sizes of the Nafion and graphene peaks vary, which probably reflects small variations in the exact position of the graphene layer within the detection volume and differences in polymer swelling and hydration among the samples. Average peak widths are approximately 17 cm-1 and 33 cm-1 for the G-peak and 2D-peak respectively, and intensity ratios I(2D)/I(G) vary between 4 and 8 for the different spectra. These spectral signatures are consistent with the presence of single-layer graphene with minimal edge-like defects in the Nafion sandwich structures. A close examination of the data summary in Table S1 reveals some small but significant variation among the spectra. For example the spectrum for NGN TEA2 shows evidence of a Dpeak at 1328 cm-1 (it is the only spectrum that shows clear evidence of a D-peak that could be distinguished from the Nafion peaks) and that spectrum also shows a red-shifted G peak (to 1580 cm-1), a blue-shifted 2D peak (to 2655 cm-1) and a relatively low I(2D)/I(G) ratio of 4. The spectrum of sample NGN TEA4 is also somewhat different from the other spectra in terms of the peak positions and widths for the G and 2D peaks. Localized variation in spectral parameters for supported graphene layers has been previously reported55, 57 and attributed to localized variations in electronic doping and / or mechanical strain. It seems likely that the variations in spectra that we have observed might also indicate some localized doping and/or strain, and some edge-type defects, in the samples. A further study with Raman spectra optimized for imaging would be required to definitively address this point and is planned for future work. X-ray photoelectron spectroscopy (XPS) is a useful analytical tool for probing the elemental composition of the very near-surface region of samples. Figure 5 shows XPS data for Nafion membranes with and without a graphene overlayer. The expected elements of carbon, fluorine, 15 ACS Paragon Plus Environment


oxygen and sulfur are present in both survey spectra in Figure 5A and B. A close examination of the C 1s region (Figure 5 C and D) shows some particularly interesting results. The Nafion-only spectrum shows a large carbon peak with a high binding energy (BE) of 291.2 eV and a smaller peak with a low BE of 284.5 eV. The peak at high BE is from carbon bonded to one or more fluorine atoms (usually two or three fluorine atoms) and is characteristic of Nafion.61 The smaller peak at 284.5 eV is probably due to adventitious carbon, e.g. adsorbed hydrocarbons from the ambient environment. The presence of such carbon on samples that have been exposed to ambient conditions and not subsequently cleaned in-situ, e.g. by sputtering, is quite common. The C 1s spectrum of the sample with the graphene overlayer is very different from that without graphene. It still has the high BE peak for Nafion but a second quite prominent carbon peak is now also present, again at 284.5 eV. For this sample, the peak at 284.5 eV is almost certainly from carbon in graphene. Carbon atoms from aliphatic and aromatic hydrocarbon molecules, and from graphite, all show XPS binding energies near 285 eV so it is difficult to be sure from XPS data alone whether this carbon peak is due to graphene or to some other carbon source, however it seems likely that it is from graphene in this case since the samples are known from Raman spectroscopy studies to contain graphene on the Nafion surface. In this regard we note that Raman spectra from this work (e.g. Figure 4) show no significant evidence of hydrocarbon contamination. A more detailed analysis of the XPS data, including a table of element intensity values with and without graphene and an XPS imaging analysis that directly compares regions on the same sample with and without graphene, is included in the Supporting Information.


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B

F1s (688.5 eV)

A

F1s (689.8 eV)

â

(291.2 eV)

FkLL

C1s O1s (535.4 eV) (169.2 eV) S2p

1200 1000 800 600 400 200 Binding energy (eV)

0

C1s

D

C1s (291.2 eV)

Nafion

Ò

membrane

C1s (284.5 eV)

300

295 290 285 280 Binding energy (eV)

FkLL

275

C1s (284.02 eV) O1s (532.e eV) S2p (170.2 eV)

1200 1000 800 600 400 200 Binding energy (eV)

Intensity (a.u.)

C

Nafion + graphene

Intensity (a.u.)

Intensity (a.u.)

Nafion membrane

Intensity (a.u.)

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


C1s (291.6 eV)

300

0

C1s

C1s (284.5 eV)

Nafion + graphene

295 290 285 Binding energy (eV)

280

Figure 5. X-ray photoelectron survey spectra (A and B) and carbon-only spectra (C and D) for Nafion samples without a graphene overlayer (A and C) and with a graphene overlayer (B and D).



Kidambi and co-workers recently reported on a very useful method for characterizing defect sites in CVD graphene layers on copper, based on the fact that solution-phase chemical etchants such as ferric ion can pass through graphene at defect sites 62 to create etch pits in the copper that are localized underneath the graphene defect sites, and that may be subsequently visualized by SEM after the etchant solution is removed. Figure 6 shows SEM images of a graphene-on-copper sample from our commercial supplier (ACS Materials) that was briefly treated with a ferric chloride etchant solution prior to SEM imaging. Etch pits are clearly visible across the entire sample surface with a relatively even distribution. Higher magnification images show that nearly all etch pits are either points or short lines, often with flat walls and edges. Etch pit widths are usually one micrometer or less. Regarding the spatial distribution of etch pits, we note that there are quite large regions of sample surface, many micrometers in extent, where there are no etch

Figure 6. SEM images of CVD single-layer graphene on Cu substrate subjected to a 5 s etch using 0.1 M FeCl3 solution prior to imaging. Images are acquired at various magnifications. Scale bar is as follows; (A) 2mm, (B) 50 μm, (C) 40 μm, (D) 20 μm, (E) 10 μm and (F) 5 μm.


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pits. These regions likely consist of nearly pristine-free graphene that is lacking any kind of site at which ferric ions could pass. The graphene defect sites that allow for transmission of ferric ions to etch copper are in our view very likely to be the same sites that allow for transmission of all the cations in our samples except for proton. The much lower resistance to proton transmission compared to transmission of other ions is difficult to explain via a mechanism that relies only on these defect sites. The defect sites would need to have only a modest selectivity (less than a factor of three) across the entire series of alkali metal cations, yet show extremely high selectivity for protons over all of those other cations. This selectivity pattern seems unlikely in any kind of simple structural defect that could be imagined in graphene. An alternative explanation is that protons are transmitted across the entire area of graphene, not just at the sites responsible for etching / passage of other ions besides protons. The nature of the sites at which proton transmission occurs, with very high selectivity for protons relative to other ions, is still uncertain, but the present results suggest that the sites at which protons are transmitted are probably not the same as the sites at which other ions are transmitted, and that they probably exist over much more of the sample surface than just the portion that is occupied by the defect sites that appear in etching visualization experiments. This idea is the key finding of this paper. Finally, ion transport rates through ionomer membranes are often measured using an electrochemical impedance spectroscopy (EIS) method whereby the ionomer membrane is sandwiched between two electronically-conductive electrodes.44, 63-75 The cell configuration is similar to that used in our prior works,30, 34, 76 and is shown in Figure 1B and in a photograph in the Supporting Materials. The membrane is considered as a simple resistor representing ion motion in the membrane in series with a pair of capacitors that represent the electrode / 19 ACS Paragon Plus Environment


membrane contacts. EIS experiments over a range of frequencies yield Nyquist plots of the real vs. imaginary components of impedance, from which a high-frequency limiting resistance may be obtained by extrapolation to the real impedance axis at infinite frequency. This method has previously been used to measure through-plane ionic resistance and conductivity of ionomeric membranes without graphene. We have sought to extend the EIS method as a complement to the LSV method described earlier in this paper to measure resistance of membranes in various ionic forms, with and without graphene, to judge the effect of graphene on ionic resistance.

50

50 40

40

30

30

+

H + Li + Na + K

20 10 0

B

A

-Zimag (W)

-Zimag (W)

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+

H + Li + Na + K

20 10 0

0

10

20 30 Zreal (W)

40

50

0

10

20 30 Zreal (W)

40

50

Figure 7. EIS spectra for Nafion membranes without graphene (A) and with graphene (B) embedded in the Nafion. AC amplitude of 50 mV was used during data acquisition between 1 kHz and 100 kHz at 0V DC voltage. Figure 7 presents Nyquist plots obtained using this cell for Nafion membranes in proton, lithium, sodium and potassium form, with and without embedded graphene. Plots at left are for Nafion only, and plots at right are for Nafion containing graphene. The high-frequency intercepts in these plots are indicative of the ohmic resistance between the two carbon cloth electrodes, which includes both the Nafion membrane resistance and the graphene resistance. High-


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frequency intercepts for both sets of plots are reported in Table 2. Following the method used earlier in studies with the D-S cell, a simple subtraction of the membrane resistance without graphene from that with graphene gives the graphene contribution to ionic resistance. Values for the area-normalized graphene ionic resistance for H+, Li+, Na+ and K+ are listed in the far right column of Table 2. Surprisingly, the graphene ionic resistances for all four ions are quite low, below 1 ohm cm2, which is in sharp contrast to the results obtained with the D-S cell, for which graphene ionic resistance is low for protons but much higher, between 90 and 220 ohm cm2, for Li+, Na+ and K+. It appears from the EIS measurements that graphene is highly permeable to many different cation types, not just protons. This is a quite large discrepancy that cannot be easily explained by minor differences in the two measurements. Table 2. High-frequency resistance values for Nafion membrane with and without embedded graphene, in various ionic forms.

No Graphene

Cations H+ Li

+

Na K+

+

With Graphene

MEA HighMEA Highfrequency MEA frequency resistance conductivity resistance

Graphene area resistance

(Ω)

(mS cm -1)

(Ω)

(Ω cm2)

0.74

46

1.61

0.15

4.3

6.9

6.5

0.40

3.8

7.8

5

0.22

28

1.0

32

0.73

We believe that the large difference between high-frequency EIS and near-steady-state LSV measurements of graphene resistance to ion transmission reflects a capacitive coupling between 21 ACS Paragon Plus Environment


the graphene and the electrolytes at the two faces of the graphene sheet. In the measurements using LSV, ions must pass completely through the graphene for relatively long periods of time (typically tens of seconds) to support the near-steady-state currents that are observed in the slowscan LSV experiments. In contrast, EIS measurements were made at frequencies as high as 100 kHz such that cations are constantly and rapidly changing their direction of motion. The graphene / ionomer interfaces could support transient accumulation and depletion of ions as the applied potential increases during a sinusoidal cycle but these accumulation / depletion regions would reverse as the applied potential was reversed. This situation, which is illustrated in Figure 8, could be modeled as a pair of capacitors in series corresponding to the interfacial capacitances of the two graphene / Nafion interfaces. At high frequency, the impedance of the two capacitors will trend towards zero, and the measured impedance will then be just that of the ionomer membrane without graphene. The EIS technique works well for measuring the ionic resistance of ionomer membranes for which the entire membrane volume is appropriately considered to be a simple ionic conductor, but in cases were the membrane has a secondary filtering layer embedded in it, this simple equivalent circuit is no longer appropriate. This situation explains the apparently low graphene ionic resistance when measured by EIS, but the much higher ionic resistance when measured by LSV.


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Figure 8. Illustration of ion motion near a Nafion | graphene | Nafion interface subjected to a transient AC polarization.



CONCLUSIONS

Proton transmission through single-layer graphene in Nafion | graphene | Nafion sandwich structures occurs more than 100 times faster than for any other cation. Confocal Raman spectroscopy and X-ray photoelectron spectroscopy confirm the presence of single-layer graphene on Nafion and in Nafion sandwich structures and suggest that the graphene is relatively free of defects, albeit with some localized evidence for defects having graphene edge character as indicated by the occasional presence of a D-band in Raman spectra acquired at small spots. Defect visualization by chemical etching of copper through CDV graphene on copper indicates that some defect sites are present, albeit in relatively small numbers, and which chemical etchants could traverse the graphene sheet. All other cations besides proton are thought to be transmitted through the graphene at the same defect sites that are visualized by the chemical etching technique. Proton transmission may also occur at these defects but it seems likely that protons are also transmitted elsewhere at the graphene surface at sites, the nature of which is still unknown, at which transmission of other cations is forbidden.


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METHODS

Materials. Single-layer chemical-vapor-deposition (CVD) graphene on copper foil samples were purchased from ACS Materials, LLC. Nafion® 211 membrane and CeTech carbon cloth with a microporous layer (W1S1009) were purchased from Fuel Cell Store. Hydrochloric acid was purchased from Alfa Aesar. Lithium chloride (from Alfa Aesar), sodium chloride (from Mallinckrodt Chemicals), rubidium chloride (from Beantown Chemical), cesium chloride (from Beantown Chemical), and ammonium chloride (from Acros Organics) were used as purchased. Deionized (DI) water was used for all experiments. Membrane pretreatment and ion-exchange process. Nafion membranes were pretreated prior to use to convert them into the desired cationic forms. For the proton form, the membrane was immersed in 0.1 M sulfuric acid at 80 oC for 1 h and then boiled in DI H2O for 1 hr. Thereafter, the membrane was soaked in 0.1 M HCl for 24 h, then rinsed in DI H 2O copiously to remove any impurities and remaining HCl solution. The membrane was then air-dried at ambient temperature. For conversion of membranes to other cationic forms, the Nafion®-211 membranes initially in acid form were soaked in 0.1 M XCl electrolyte (X = Li, Na, K, Rb, Cs, and ammonium) solution. The electrolyte solution was replaced with fresh solution at least three times, and the pH of the rinse solution was monitored until it was no longer acidic. Thereafter, the membrane was further soaked in the XCl electrolyte that had been preheated to 80 oC for 1 h and then left in the solution for at least 48 h. This was done to ensure complete ion-exchange of the membrane to the desired cation form and to improve the membrane’s water uptake and expansion of its ion cluster that would facilitate faster ion transport. Finally, the membranes were rinsed thoroughly in DI H2O and allowed to dry at ambient temperature.



Nafion | graphene | Nafion membrane fabrication for four-electrode Devanathan-Stachurski (DS) cell measurements. Sandwich structures containing single-layer graphene positioned between Nafion membranes were fabricated following our previously published method,30, 34, 76 adapted for the D-S cell. First, two one-inch-diameter Nafion membrane disks were cut from a Nafion 211 membrane sample in the desired ionic form using an arch punch. Next, a one-inch-diameter sample of CVD graphene on copper foil was cut and hot pressed (at 140 oC, 600 lbf, for 2 min) onto one of the Nafion membrane disks. The Nafion and graphene-on-Cu-foil were placed between two sheets of PTFE-coated fiberglass during hot-pressing to avoid direct contact with the hot-press plates. The Cu was then removed by chemical etching using a 0.3 M aqueous ammonium peroxydisulfate solution, leaving the graphene on one side of the membrane. The sample was then thoroughly rinsed with deionized water and allowed to dry under ambient conditions. Next, the second Nafion disk was hot-pressed onto the graphene side of the first disk, using the hot-press conditions noted above. After cooling, the sample was then ready to use in electrochemical or other characterization experiments. Nafion | graphene | Nafion membrane fabrication for Electrochemical Impedance (EIS) cell measurements. Membrane samples for EIS characterization were prepared similarly to those described above but with the following differences; (1) the membrane diameter was 0.75 inch (19.1 mm) instead of 1 inch; (2) the graphene layer was not a 1 inch disk, but a 1.5 x 1.5 cm square; and (3) the membrane was modified in a final hot-press step with two carbon cloth electrodes that are 3⁄16 inch diameter (0.48 cm) disks. The carbon cloth disks were positioned directly across from one another and in a region of the sample where there was graphene covering the entire area where the carbon cloth disks overlap. The carbon cloth electrodes have no platinum in them, so they form a purely capacitive contact with the Nafion. The MEA active area is ca. 0.178 cm 2. Prior 26 ACS Paragon Plus Environment

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to impedance measurement, the MEA in different cationic forms (H +, Li+, Na+, K+, Rb+, Cs+ and NH4+) with and without single-layer graphene were soaked in DI H2O to ensure that they are in wet form prior to being studied in cells. Electrochemical measurements with four-electrode Devanathan-Stachurski (D-S) cell. The D-S cell configuration is a four-electrode cell consisting of two platinum wire drive electrodes that drive current flow through a membrane, and two reference electrodes in Luggin capillaries the tips of which are positioned very close to the opposing surfaces of the membrane. This cell configuration is commonly used to study hydrogen permeation through metal samples 77 but it may also be used to study ion transmission through membranes, as long as the membrane ion transmission rates are significantly different from the ion transmission rates through the liquid electrolyte. The cell used in this work was fabricated by Adams and Chittenden Scientific Glass (model 949838). It consists of two electrolyte chambers (approximate volume 50 mL) separated by a membrane held in a membrane mount. The membrane size is 1 inch diameter disk but the active area is a 5/8 inch (1.98 cm2) diameter disk. Each compartment of the cell was filled with 50 mL of an electrolyte solution (0.1 M HCl and 0.1 M XCl where X + = Li+, Na+, K+, Rb+, Cs+ and NH4+). Homemade Ag/AgCl electrodes filled with a saturated KCl solution were used as the reference electrodes. All of the four electrodes were connected to the Galvanostat / Potentiostat Solartron Instrument (Model No: 1280B). The reference electrode leads were connected to the two reference electrodes, and the working and counter electrode leads were connected to the two drive electrodes. Ion transmission measurements were performed in potentiostatic mode using linear sweep voltammetry (LSV) at 5 mV s -1 to measure the through-plane resistance to ion transport through membranes with and without single-layer graphene. Membrane resistance was obtained from the slope of the current-voltage curves. 27 ACS Paragon Plus Environment


Electrochemical measurements with two-electrode Electrochemical Impedance Spectroscopy (EIS) cell. The cell used in this work is similar to that used in our recently reported work on hydrogen pump cells with Nafion / graphene membranes. For the present work, the cell consists of two 5/8 inch diameter graphene rod current collectors fitted into ¾ inch outer diameter PTFE sleeves which are then fitted into a ¾ inch diameter swage-style compression fitting. The MEA in appropriate ionic form with carbon electrodes on each side is placed in the center of the cell and the cell is then assembled by pressing the graphite rods against the two sides of the membrane. EIS measurements were conducted in a two-electrode mode configuration using a Solartron 1287 electrochemical interface and Solartron 1260 impedance/gain-phase analyzer in the highfrequency range from 1 kHz to 100 kHz. AC amplitude voltage of 50 mV was applied at DC potential of zero volts to ensure accurate measurement. Each measurement takes approximately two minutes. Membrane resistance was taken as the high-frequency intercept on the real axis of a Nyquist plot. Raman spectroscopy/microscopy measurement. Raman spectra were acquired using a confocal Raman microscope system at the University of Utah that has been previously described in detail. 5859

In brief, the excitation source was a Kr+ laser (Coherent, Santa Clara, CA, USA) operating at

647.1 nm and 3 mW power. The confocal probe volume, defined by the excitation beam focus (ca. 600 nm diameter) together with the collection aperture, was within a depth along the z-dimension of 1200 nm (90 % collection efficiency).58-59 Samples containing Nafion 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 % H 2O2 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 28 ACS Paragon Plus Environment

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remove surface water droplets before placing the sample on a glass coverslip (BK-7 glass, No. 1.5 thickness) 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. In some cases, one side of a sample was ion-exchanged into a tetraethylammonium (TEA) form. This exchange had no significant effect on the spectra other than to cause appearance of some Raman peaks for TEA. X-ray photoelectron spectroscopy (XPS). XPS characterization of graphene on Nafion samples was performed using a PHI 5000 VersaProbe III (Ulvac PHI Inc.), equipped with a monochromatic, micro-focused Al Kα X-ray source operating at 25 W, under a vacuum chamber pressure of 1 × 10-8 Pa. The micro-focused raster X-ray beam was scanned across the sample surface. The survey scans were collected at fixed analyzer pass energy of 112 eV and quantified empirically with the sensitivity factors provided by Ulvac PHI Inc. For XPS spectroscopy of localized regions of the sample, the X-ray probe beam diameter was 100 micrometers. CVD graphene defect visualization and counting. The CVD graphene on Cu samples were subjected to a defect visualization protocol as described by Kidambi and co-workers.62 In brief, small pieces of graphene samples were rinsed in DI H 2O and then a small drop of 0.1 M ferric chloride etchant solution was placed on the sample surface. The samples were rinsed in DI H 2O after a period of 5 sec and were dried at ambient conditions. The samples were then examined under SEM imaging using a Hitachi model S-3400N Variable Pressure Scanning Electron Microscope. The etch pits observed in the SEM are indicative of sites in the graphene that allow etchant solution to pass through the graphene to etch the copper.



ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supporting Information; contains additional IV curves for electrolytes than were not included in Figure 2; additional Raman spectra that were not included in Figure 3; electron microscopy imaging data for Nafion with and without graphene; additional XPS data that were not included in Figure 4 ( PDF). Raman spectra; ACSII data files for Raman spectra in the Supporting Information file. (MS Excel). Funding Sources We gratefully acknowledge the Office of Science, U.S. Department of Energy, through Grant DE-SC0018151 for SEC, for financial support of the work, and additional support for Raman microscopy at the University of Utah, through Grant DE-FG03-93ER14333. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Technical assistance from Mr. George Wetzel with XPS and electron microscopy measurements, and support for XPS acquisition from the Clemson University Office of the Vice President of Research, are gratefully acknowledged.


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