Measuring the Capacitance at Few- and Many-Layered Graphene

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Measuring the Capacitance at Few- and Many-Layered Graphene Electrodes in Aqueous Acidic Solutions Alison J. Downard, Anna K. Farquhar, and Paula Brooksby J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12493 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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Measuring the Capacitance at Few- and Many-Layered Graphene Electrodes in Aqueous Acidic Solutions Alison J. Downard, Anna K. Farquhar, Paula A. Brooksby* MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand ABSTRACT The differential capacitance of 1-2 layer and 6-7 layered graphene was measured in aqueous 0.01, 0.1, 1.0 and 3 M perchloric and sulfuric acid solutions. The total measured capacitance was evaluated for approx. ± 500 mV either side of the potential of zero charge to observe the contribution from the quantum capacitance and shielding effects on the measured capacitance. The experimental results were compared to recent theoretical evaluations of similar electrode – electrolyte interfaces for supercapacitor applications. At 6-7 layered graphene electrodes the measured differential capacitance was dependent of the solution and electrical double layer structures, and although the 1-2 layered electrode showed far fewer differences upon changing solution conditions, it was not strictly independent. The concept of shielding effects within the graphene electrode and a dielectric capacitance as proposed by theory would account for these observations.

INTRODUCTION Graphene (G) has a limited density of states (DOS) at the Dirac point and hence a limited number of charged carriers which ultimately gives rise to a quantity termed the quantum capacitance (CQ) of graphene; by contrast, metal electrodes have an infinite DOS and therefore infinite capacitance. Therefore, at an electrode – solution interface in which the electrode is graphene, the total capacitance of the system is not solely derived from the solution side as is the case at metal electrodes. The electrolyte solution adjacent to an electrode has a structure typically containing the Stern, Helmholtz and diffuse layers. Each of these layers has its own capacity contributions to the system, and often collectively termed the double layer capacitance (CDL). In aqueous solutions, the CQ is smaller than the CDL over an approximate 500 mV potential window around the potential of zero charge (pzc) region corresponding to the Dirac point. Since CQ and CDL are in series, the smaller capacitance will have the largest contribution to the total interfacial capacitance for a given electrode potential. At electrode potentials outside this ~500 mV window the CDL is calculated to be smaller than CQ and the total interfacial capacitance presumably responds similar to that found at metal electrode – solution interfaces in the absence of any chemical reactions. Recent theoretical studies of single- and few-layered graphene electrodes show there is a complex relationship between the DOS, the mobile ions in the adjacent electrolyte, and the resulting capacity of the interface 1-3. It has been calculated that with few-layered graphene electrodes, an electric field is generated that gives rise to a shielding effect inside the electrode. The shielding causes a dielectric response within the graphene layers and adds a third capacity component termed the dielectric capacitance (Cdiel) that is non-quantum in nature. The impact on the total capacitance from the shielding depends on the number of graphene layers, n. The total capacitance was described by Jiang 1 now as, Equation (1)

1 1 1 1 = + + ‫ܥ‬௧௢௧௔௟ ‫ܥ‬ொ ‫ܥ‬ௗ௜௘௟ ‫ܥ‬஽௅ Analysis of the potential drop across the graphene electrode – solution interface with a fixed surface charge density of 9 µC.cm-2 showed Cdiel became comparable to CQ when n ≈ 3 while the double layer remains essentially constant (determined using a Pt model electrode). When n > 4, the total capacitance of the interface is no longer dominated by the quantum component and Ctotal converges to ~6 µF.cm-2 1. Goto et al. used electrochemical and theoretical calculations to study the capacitance of single- and few- layered electrodes adjacent to ionic liquid solutions 2. Similar results to Jiang et al. were observed when n > ~ 4, and shielding within the electrodes was explored as the origin of the capacitance behavior with voltage. The models of Goto 2, Hwang 3 and Jiang 1 suggest the electrolyte has very little influence on CQ. Solution- or liquid-gated field effect transistors (FETs) made with single- and few-layered graphene electrodes are an emerging area of research for sensing of electroinactive biomolecules.4-9 Graphene has a high conductance and low noise, and the source – drain current – voltage profile around the charge neutrality (Dirac) point is very sensitive to small changes in the solution ionic strength, pH and presence or absence of adsorbed molecular species on the graphene electrode. This is clear evidence that solution and interfacial conditions near graphene do indeed influence its DOS and consequently the conductance. Thus, the expectation that interfacial capacitance at graphene is independent of the adjacent solution properties requires more insight from experimental measurements. At a metal electrode, the ionic strength and identity of ions in the adjacent electrolyte dictates the magnitude of the capacitance for all practical electrode voltages. Molecules that adsorb onto the electrode add a complicating factor in any study,

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thus the choice of electrolyte with which to begin investigating double layer capacitance at graphene should mitigate against that possibility. Improving the energy storage capacity of graphene based systems requires a greater understanding of the double layer, quantum, and dielectric behaviors of electrolyte species in the double layer. In this study, we investigate electrode – solution interactions at 1-2 (few) and 6-7 (many) layered graphene electrodes to gain insight into the relationships between capacitance, potential, and electrolyte conditions. We aim to add to the body of experimental knowledge to compare with the theoretical propositions described in recent literature. We use few and many layered-graphene sheets to illustrate their behavioral similarities and differences with the view towards understanding and improving energy storage device architectures. Thus, voltammetry and impedance spectroscopy were used to evaluate and measure the differential capacitance for 1:1 and 1:2 perchloric and sulfuric acid solutions and concentrations from 0.01 to 3 M. EXPERIMENTAL METHODS Chemicals. High purity grade HClO4 (Merck) and H2SO4 (J.T. Baker), ultra-high purity methane, hydrogen and argon (BOC, New Zealand), copper foil (25 µm thick, 99.999%, Alfa Aesar), (NH4)2S2O8 (Merck) were all used as received. Water was milli-Q with > 18 MΩ.cm. CVD Growth of Graphene and Etching from Copper. The procedure for the atmospheric pressure (AP) CVD growth of the graphene electrodes was derived from Wu et al.10 Prior to growth, copper foils were cleaned in dilute nitric acid solution, then Milli-Q water, and dried with nitrogen. The foil was cut into approximately 1 × 1 cm2 pieces and placed in a furnace within a silicon glass tube. The furnace was first heated to 120 °C for 20 min, under 300 sccm Ar. The furnace was then ramped to 1050 °C under 300 sccm Ar + 15 sccm H2 atmosphere and held at this temperature for 30 min. The furnace was further stepped to 1060 °C under 200 sccm Ar + 50 sccm H2 and held at this temperature for 1 h. After 1 h, Ar was increased to 500 sccm, H2 decreased to 30 sccm and methane at 5 sccm was introduced for 6 min. Following this growth phase, the furnace temperature was reduced to 980 °C, Ar and methane gases were stopped and H2 reduced to 20 sccm for 10 min. After 10 min the furnace was cooled to 600 °C while maintaining a gas flow of Ar at 300 sccm + H2 at 10 sccm. The samples were then pulled out of the heating zone of the furnace, but remained within the furnace tubing. The copper was etched using 0.1 M ammonium persulfate solution for 20 min to release the graphene from one side. The foil was then float-

ed on a fresh 0.1 M ammonium persulfate solution overnight, leaving a single sheet of graphene. This was then collected in a watch glass and rinsed five times with water by pipetting out the old solution and replacing with water. This growth protocol gave typically 1-2 layered graphene sheets. The growth protocol for 6-7 layered graphene is given in reference 11. The number of graphene layers was determined by the reduction in transparency between 800 and 600 nm of the graphene sheet mounted onto a quartz slide (Cary 50 UV−vis Spectrometer) 12. The quality of the graphene sheets were assessed using Raman spectroscopy as described elsewhere and as expected there was a small D band in the spectrum indicative of some defective structure in the sheets 11, 13. Electrochemistry. Electrochemical measurements were made using an Eco Chemie Autolab potentiostat running NOVA software. The electrochemical cell was set up with the graphene electrode placed horizontally between an insulated metal base plate and a glass solution cell with a hole in the bottom. A complete description of the mounting method for graphene and the cell was given previously with the exception for the reference electrode being housed external to the main cell via a Luggin capillary 11. All solutions were sparged with N2 for 30 min prior to measurement at 20 ± 2 °C. RESULTS AND DISCUSSION Preparing graphene sheets on copper substrates using the APCVD method generates multilayered graphene with defects throughout the layers. While these defects are beneficial for improved capacitance and thus highly desirable, they are irreproducible. Once these sheets are manipulated into electrode devices, there are local variations in topography and hence the measured quantum capacitance 14. In our experience, within a single batch of samples, the capacitance – potential behaviors have gross similarities when examined using electrochemical impedance spectroscopy (EIS), however, between batches the shapes of these plots can be variable as illustrated in Figure (S1) (supporting information). Thus, the approach taken in this study is to observe the relative measured changes from each graphene electrode under varying conditions of applied potential and electrolyte, rather than the absolute differences between each electrode. The electrode area was measured by optical imaging and software evaluation of the epoxy edges using ImageJ. We have previously established that solution ingress to the backside of the electrode occurs, thus, both sides of the graphene sheet are contacting the electrolyte 13. Hence all figures are given with respect to double the observable measured electrode area.

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Potential (SCE) / V Figure 1. Initial cyclic voltammograms of 1-2 layered G in (top, black) 0.01, 0.1, 1.0 and 3.0 M perchloric, and (bottom, red) 0.01, 0.1 and 1.0 M sulfuric acids. Insets are magnified regions truncating the HER current and for clarity the noisy 3.0 M perchloric trace was omitted. The traces keys: (solid) 0.01, (dash-dot) 0.1, (dash) 1.0, and (dotted) 3.0 M. Scan rate = 20 mV.s-1.

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The electrolytes chosen for this study are perchloric and sulfuric acids at 0.01, 0.1, and 1 M concentrations; perchloric acid was additionally investigated at 3 M concentration, which equates to the same ionic strength as the most concentrated sulfuric acid solution. Perchloric acid was an initial acid choice because the polyatomic anion is not known to interact strongly with surfaces and is a commonly selected anion for studies at metal – solution interfaces. In addition, since graphene is a targeted new material for supercapacitor applications, a more practical anion would be sulfate or hydroxide. Thus, sulfuric acid was also selected for analysis as it provides an opportunity to examine how an asymmetrical electrolyte with a different chemical identity effects the capacitance while solution pH conditions were kept relatively similar. Cyclic voltammograms of graphene electrodes were measured prior to, and following, all electrochemical testing to confirm no significant changes to the sheet structure occurred after impedance measurements. The graphene electrodes were fabricated having an average of either 1-2 or 6-7 layered sheets. Representative initial voltammograms for few- and manylayered graphene electrodes in the acid solutions are shown in Figures (1) and (2), respectively. The perchloric acid traces (black) have a slightly larger integral capacitance compared to the equivalent concentration of sulfuric acid (red) between 0.2 and 0.2 V based on the area between the curves. Note however, these electrolyte types are different (1:1 and 1:2) and thus their ionic strengths are dissimilar. Perchloric and sulfuric acid solutions with the same ionic strength show only small capacity differences within experimental error. Theory predicts the quantum and dielectric capacitances should be insensitive to solution conditions. Certainly, for

Current density / µA.cm

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Potential (SCE) / V Figure 2. Initial cyclic voltammograms of 6-7 layered G in (top, black) 0.01, 0.1, 1.0 and 3.0 M perchloric, and (bottom, red) 0.01, 0.1 and 1.0 M sulfuric acids. Insets are magnified regions truncating the HER current and for clarity the noisy 3.0 M perchloric trace was omitted. The traces keys: (solid) 0.01, (dash-dot) 0.1, (dash) 1.0, and (dotted) 3.0 M. Scan rate = 20 mV.s-1. large changes in the acid concentrations from 0.01 to 3 M there is little change to the current density measured in the plateau region of the voltammograms in each family of graphs. Note however, the inset plots for few- and many-graphene electrodes show an increased overall current density for greater numbers of graphene layers, and the integral capacitance has almost doubled for a 4 to 5-fold increase in the number of G layers as predicted by theory. Therefore, increasing the number of layers equates to an increased areal integral capacitance. A second notable effect of increasing concentration is the onset for hydrogen evolution reaction (HER) in perchloric acid is significantly shifted to more positive potentials at fewlayered graphene electrodes only.11 This effect was not as significant for sulfuric acid solutions or with many-layered graphene electrodes. Lastly, at high perchloric acid concentration (3 M) the stability of the voltammogram was poor. This behavior was observed regardless of the initial direction of scanning, the number of scans, or the history of the electrode. To obtain differential capacitance values EIS measurements were collected at 100 Hz at 50 potential steps between 0.2 and -0.6 V in perchloric acid and 0.4 and -0.4 V in sulfuric acid solutions 11, 15. EIS measurements were started at positive potentials and stepped to towards negative potentials. Differential capacitance values were extracted directly using Equation (2) where C, f and Z” are the capacitance, frequency, and imaginary component of the measured complex impedance. However, the HER is a Faradic process and therefore capacitance values extracted directly are likely flawed for E < -0.3 V.

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-0.6 -0.4 -0.2 0.0 0.2 -0.4 -0.2 0.0 Potential (SCE) / V

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Figure 3. Plots showing the differential capacitance against applied potential for 2 samples; (top) 1-2 LG and, (bottom) 6-7 LG. Measurements were made in perchloric and sulfuric acid solutions. (black) 0.01 M; (red) 0.1 M; (blue) 1 M; (green) 3M acid solutions. The differential capacitance curves in Figure (3) are for the same 1-2 and 6-7 layered graphene electrodes shown in Figures (1) and (2). Figure (4) summarizes the capacitance and potential minimum (potential of zero charge; pzc) behavior against ionic strength for five newly prepared electrodes. In general from Figure (3), the plots from few-layered graphene are more symmetrical with better defined pzc and steeper slopes either side of pzc compared to the many- layered graphene plots. The sulfuric acid solutions typically gave complex pzc with either a small local hump or broadened minima in most cases. From Figure (4), the differential capacitance values slightly increase with increasing ionic strength; the capacitance of the many-layered graphene electrodes (dashed lines) are higher than the 1-2 layered G electrodes (solid lines); and on average the sulfuric acid solutions gave slightly greater capacitance values than the same electrode in perchloric acid solutions at the same ionic strength. Regardless of the number of G layers, the pzc in perchloric acid was always less than that in sulfuric acid solutions at high ionic strength by as much as 300 mV, but can be at comparable potentials in very low ionic strength solutions.

Figure 4. Plots of (top) capacitance and (bottom) pzc against ionic strength for five different samples. Black = perchloric acid; red = sulfuric acid; solid lines = 1-2 LG; dashed lines = 6-7 layer G.

The pzc minima and the slopes of the plots either side (Figure (3)) correspond to the electron- and hole-doped G regions provide insight into the DOS and doping level of FLG. The slopes of the plots for 1-2 layered G are approximately -3.1 and 4.5 µF.cm-2.V-1, and for the 6-7 layered G, -1.5 and 3.5 µF.cm-2.V-1. These values are similar (-1.2 and 3.3 µF.cm-2.V1 ) to those shown by Ruoff et al. 16 for single layer G with the number of defects (nd) between 0.1 to 0.7 × 10-11 cm-2 or when the dopant charge carrier density (ne) is up to 4 × 1013 cm-2. The slopes decreases with increasing number of G layers has also been observed by Ruoff et al. 15 and approaches that observed with HOPG – electrolyte layers. The negative pzc shift with increasing ionic strength (Figure 4) indicates increasingly positive charge doping of FLG.16 The more positive pzc values for the sulfuric than perchloric acid suggests anion types and their solution structure near the electrode do impact the measured capacitance. Doping by noncovalent factors such as an adjacent substrate, electrolyte, adatoms, and redox species is well-documented 14, 17-20. The apparent spread of pzc values for low ionic strength sulfate solutions may in part be due to the complex minima of the plots. It is clear the identity and structure of the electrolyte solution adjacent to G does affect the measured capacitance, even for 1-2 G layers. Thus, the notion that quantum capacitance is entirely unresponsive to the nature of the G electrode environment appears questionable. Indeed, in FET devices, and

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The Journal of Physical Chemistry studies related to sensing based on these devices, the environment has a measurable impact on the G conductance. The implication being is can therefore impact the measured capacitance. To begin to understand this complex interface, comparing the experimentally measured differential capacitance between 1-2 and 6-7 layered graphene under the same conditions of applied potential and electrolyte strength is a method of directly observing the effects of screening on capacitance. Figure (3) illustrates well the effect of increasing the number of G layers and thus increasing screening within the FLG electrode: with fewer G layers the total capacitance is lower, and aside from the pzc minima shift with electrolyte type, the shape and behavior of the capacitance-potential plots with changing electrolyte ionic strength are similar for both acid types. This implies the identity of the ions are less important than the electrostatic effects in the electrical double layer – FLG interface. Williams et al. 21 used molecular dynamics to study the G – electrolyte interface. In their model, the π electron cloud of G, the electrolyte ions, and the water were each allowed to be polarizable. Commonly, the solvent is the only polarizable component of such models. Their results showed G polarization plays a significant role in defining the structure of the interface, and a complex relationship exists between ion valence, hydration, and adsorption. For example, in 1 M concentrations, Li+, Na+, and K+ ions strongly contact adsorb onto graphene with loss of some water molecules from their hydration shell. By contrast, anions such as Cl- contact adsorb but with their hydration shell intact. Our understanding of the Helmholtz plane in relation to double layer capacitance and ion adsorption is largely derived from the study of metallic electrodes and other types of graphitic surfaces and should be revisited in the context of G and multi-layered G surfaces. Increasing number of G layers to 6-7 increases the total measured differential capacitance. There is also greater asymmetry of the capacitance – potential plots compared to 1 – 2 G layers. The CQ contributions no longer dominate the capacitance behavior with this many G layers.22 Hence, the solution structure adjacent the graphene is important and the total measured differential capacitance and appear to have at least two distinct origins. At 1-2 layered G the solution structure and ionic adsorption may indirectly affect the DOS in G because there is little shielding, whereas at 6-7 layered G with greater internal shielding, these electrodes may behave more like traditional metal surfaces in which the solution structure greatly affects the Helmholtz capacitance. CONCLUSION In this study a differential capacitance comparison was made of few (1-2) and many (6-7) layered G electrodes in perchloric and sulfuric acid electrolytes, and then contrasted against that predicted from theoretical studies. The cyclic voltammetry shows the integral capacitance from perchlorate solutions is sensitive to the ionic strength and the number of layers, whereas sulfate solutions showed much less variability. At high ionic strength the perchlorate system gave pronounced current instabilities during the voltammetry measurements, no instabilities were observed for similar ionic strength sulfate solutions. Both acid types gave greater integral capacitance values for 6-7 layered G compared to 1-2 layered G electrodes.

The impedance spectroscopy results show differential capacitances at both 1-2 and 6-7 layered G surfaces are a function not only of the quantum capacitance of G but also the solution structure in the electrical double layer. The sulfate containing electrolyte gave greater capacitances compared to perchlorate for similar ionic strength solutions. Lastly, the 6-7 layered G electrodes gave larger capacitances compared to 1-2 layered G electrodes irrespective of electrolyte type. Although sheet number and electrolyte choices impact capacitance values, the gains were not significant within the variables measured in this study. The theoretical convergence of the dielectric and quantum capacitances at the fourth graphene layer, and the capacitive behaviour either side, at three and five layered graphene, would be instructive to examine. Of note is the assumption that the dielectric, quantum, and solution capacities are in series and this has yet to be confirmed. Experimentally and physically the quantum and dielectric capacitances may be better described as in parallel and this should be tested in future efforts. These results, combined with recent theory and the FET studies, are all indicating the capacitance derived from G-based electrodes depends on the condition of the surface and the adjacent solution structure. Thus, in any practical supercapacitor device there will be tradeoff between total weight and the number of G layers and understanding the capacitance limitations on graphene materials will be critically important. The results presented here corroborate the theoretical predictions and allows us to begin a more in depth investigation of the structure – capacitance relationships in graphene-based energy storage devices. Of interest is a recent theoretical account that alludes to strong ion adsorption at carbon surfaces being an important factor on the capacitance behavior. 23 A feature of which may be responsible for the dissimilar shaped potential – capacitance plots in Figure (3) and the distinctly difference pzc minimum potential for perchlorate and sulfate solutions.

SUPPORTING INFORMATION Capacitance of duplicate FLG samples against applied electrode potential.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Tel: 64 (3) 364 2453

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This project was supported by the Royal Society of New Zealand Marsden Fund (UOC1307).

ABBREVIATIONS APCVD, atmospheric pressure chemical vapor deposition; C, capacitance; CV, cyclic voltammetry; diel, dielectric; DOS, density of states; diel, dielectric; DL, double layer; EIS, electrochemical impedance spectroscopy; FET, field effect transistor; FLG, few-layered graphene; G, graphene; LG, layered graphene; Q, quantum;

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