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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

The Capacitance of Basal Plane and Edge-Oriented Highly-Ordered Pyrolytic Graphite: Specific Ion Effects Pawin Iamprasertkun, Wisit Hirunpinyopas, Ashok Keerthi, Bin Wang, Radha Boya, Mark A. Bissett, and Robert A.W. Dryfe J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03523 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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The Journal of Physical Chemistry Letters

The Capacitance of Basal Plane and Edge-oriented Highly-ordered Pyrolytic Graphite: Specific Ion Effects Pawin Iamprasertkun†,§, Wisit Hirunpinyopas†,§, Ashok Keerthi⊥, §, Bin Wang†, Radha Boya⊥, §,Mark A. Bissett‡,§, Robert A.W. Dryfe†,§,*

†School

of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL,

United ‡School

Kingdom. of Materials, University of Manchester, Oxford Road, Manchester M13 9PL,

United Kingdom. ⊥School

of Physics and Astronomy, University of Manchester, Oxford Road, Manchester

M13 9PL, United Kingdom.

ACS Paragon Plus Environment

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§National

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Graphene Institute, University of Manchester, Oxford Road, M13 9PL, United

Kingdom.

AUTHOR INFORMATION Corresponding Author *R.A.W. Dryfe: e-mail, [email protected]. Fax: (+44) 161-275-4598


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Abstract

Carbon materials are ubiquitous in energy storage, however, many of the fundamental electrochemical properties of carbons are still not fully understood. In this work, we studied the capacitance of highly ordered pyrolytic graphite (HOPG), with the aim of investigating specific ion effects seen in the capacitance of the basal plane and edgeoriented planes of the material. A series of alkali metal cations, from Li+, Na+, K+, Rb+ and Cs+ with chloride as the counter-ion, were used at a fixed electrolyte concentration. The basal plane capacitance at a fixed potential relative to the potential of zero charge was found to increase from 4.72 µF cm−2 to 9.39 µF cm−2 proceeding down Group 1. In contrast, the edge-orientated samples display capacitance ca. 100 times higher than the basal plane, attributed to pseudocapacitance processes associated with the presence of oxygen groups, and largely independent of cation identity. This work improves understanding of capacitive properties of carbonaceous materials, leading to their continued development for use in energy storage.


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TOC GRAPHIC

In recent years, electrical double layer capacitors (EDLCs) or supercapacitors have attracted much interest due to their fast charge/discharge rates (a few seconds, power density of 10 kW kg-1) compared to those of batteries (~ 0.1 kW kg-1)1. EDLCs possess a low specific energy because the charge is typically stored via a physisorption process2. Under application of voltage to the cell, solvated ions of opposing charges migrate to the outer Helmholtz plane to maintain local charge neutrality. Hence, the Helmholtz capacitance is limited by the surface area of the electrode materials2. In order to increase the Helmholtz capacitance to achieve higher energy densities, carbonaceous materials − e.g. activated carbon3, so-called carbon “ onions ” 4, carbon nanotubes5, and graphene6, 7 – have shown promise due to their low mass, high specific


4

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surface area, high electrical conductivity, low charge separation length (less than 1 nm)8. Among these, the properties of graphene have been much vaunted because of its high theoretical specific surface area (~2,600 m2 g−1)6, high theoretical capacitance (reported to be 550 F g−1 on a gravimetric basis, although this value assumes an areal capacitance of 21 µF cm−2)7, high electrical conductivity (~2 × 103 S cm−1) and charge carrier mobility9, which is relevant for functionality in the EDLC context. To further develop graphene and associated composite materials for high performance supercapacitors, a greater understanding of the capacitive properties is essential. Based on its atomic structure10, the capacitive contribution of graphene-type materials can arise from two distinct components: (1) the basal plane region, consisting of monolayer carbon atoms arranged in a hexagonal lattice with sp2 hybridization; and (2) the edge planes, i.e. the monoatomic steps at the end of the carbon plane for the case of monolayer graphene, which can be terminated by hydrogen or oxygen atoms leading to pseudocapacitance in the latter case11, 12.


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In order to investigate the capacitive contribution of the basal and edge planes, Yuan et. al, investigated the electrochemistry of basal plane and edge-oriented graphene prepared by chemical vapor deposition (CVD) in 0.1 M KCl(aq). It was reported that the edge-oriented capacitance is ~1.0 × 105 µF cm−2, which is much higher than the value reported for the basal plane (4 µF cm−2)12. This is because mechanical cutting can create structural defects with so-called “dangling bonds”, which are unstable in air hence often become terminated by oxygen-containing groups leading to the large pseudocapacitance13. Yang et. al., fabricated vertically-oriented graphene by a plasma-based CVD method. They increased the basal plane region by controlling the growth conditions. They observed that as the basal:edge aspect ratio increased from 1 to 2.5, the volumetric capacitance of the sample decreased from 10.3 F cm−3 to 2.38 F cm−3, indicating that the edge-oriented regions possess a higher surface charge density than the basal plane14. This was further supported in work reported by Zhan et.

al., who performed joint density functional theory (JDFT) level calculations on basal plane and edge-oriented graphene. This work predicted that both zigzag and armchair


6

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edges provide higher capacitance than the basal plane (by a factor of 4 to 5) due to the influence of dielectric screening and the quantum capacitance at the edge regions15. Highly-ordered pyrolytic graphite (HOPG), which is the bulk material formed from multiple graphene sheets, is of interest from the perspective of understanding its capacitance properties. Additionally, it is instructive to make comparison with HOPG and graphene-based materials, given the intrinsic difficulties associated with accurate measurement of edge areas of graphene samples16. HOPG is suitable for fundamental studies of capacitance because it provides a smooth surface without defects on the basal plane and can be simply cleaved by “scotch” tape to obtain a fresh basal plane1619.

Work in this direction can be traced back to the seminal studies of Randin and

Yeager, who reported a potential-dependent capacitance of the stress-annealed HOPG basal plane, with a minimum between 2.6 µF cm−2 and 4.0 µF cm−2 in aqueous solution (at the potential zero charge, PZC)18. This value is in reasonable agreement with that subsequently reported by Zou et. al. (minimum capacitance between 4.3 µF cm−2 and 6.0 µF cm−2), albeit in highly concentrated solutions16. The capacitance of edge oriented HOPG was also studied by Randin and Yeager: it was found that the edge-orientation


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displayed higher capacitance (about 50 to 70 µF cm−2) than the basal plane18. Rice and McCreery reported that the edge-oriented capacitance of HOPG could increase to 150 µF cm−2, depending on the preparation mode of the edge surface20. However, the effect of ion identity, on both basal and edge facets, has not been explored to the best of our knowledge. In this work, we use HOPG as a model “well-defined carbon” to investigate the relationship between capacitance and cationic sizes at the basal and edge-oriented planes. The cation was varied from Li+, Na+, K+, Rb+, Cs+, with chloride as the anion, at a fixed electrolyte concentration of 0.5 M. The basal plane capacitance is found to be dependent on the cation size, while the capacitance of the edge-oriented samples is dominated by pseudo-faradaic reactions. This study of the influence of cationic size on basal plane and edge-oriented HOPG capacitance should provide fundamental principles, which will aid the understanding of capacitive properties and development of carbon materials, including graphene and its composites. Commercial supercapacitor devices are generally based on activated carbons, which are complex, porous materials possessing a compromise between sp2 type carbons (imparting conductivity) and sp3


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environments (providing a pseudo-capacitive component)21. It is difficult to draw general conclusions form studies with such materials, given the complexity of activated carbon, hence the use of HOPG in this work.

To understand the physical properties of the HOPG electrode, it is necessary to characterize the electrode at its basal and edge-orientations, as shown in Figure 1. The Raman spectra in Figure 1a display dominant G peaks on both basal plane and edgeoriented HOPG at ~1580 cm-1, referred to as the E2g stretching mode, associated with the sp2 carbon atoms22. The D and 2D2 peaks are absent on the basal plane due to the low density of defects23. On the other hand, the D and 2D2 peaks located at ~1350 cm−1 and ~2900 cm−1, respectively, are found on the edge-oriented sample: these correspond to the lattice discontinuities created at the edge, which effectively create a defect23. Apart from Raman spectra, the dynamic water/air contact angle (WCA) was performed in a high humidity chamber to minimize the evaporation of water as shown in Figure 1b. It can be clearly seen that the basal plane is moderately hydrophilic with a WCA of 69.2◦ (at t=0) and maintains similar wettability after 600 seconds exposure to


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the liquid (WCA of 69.3◦), which is in good agreement with previous reports24-27. On the edge plane, in contrast to the observations of similar edge wettability reported by Wei and Jia, who reported WCA of 56.2 ± 8.424, the edge-oriented plane was found to be considerably more hydrophobic with a WCA of 146.08◦ at t=0. This is attributed to the increased roughness of the edge plane compared to the basal plane: air pockets are left between the water droplet and graphite surface28-29, which represent the Cassie-Baxter state30. It is noted that, the WCA value, reported by W ł och et. al.31 on the basis of molecular dynamics simulation of the edge atoms, indicated edge hydrophobicity, although the specific WCA was predicted to depend on surface termination. Polishing the sample to decrease the surface roughness decreases the WCA, (see the time dependent WCA after the sample polishing in Figure S1), however it is also likely to change the oxygen content of the surface. After the droplet on the edge-oriented plane was exposed to air, the WCA decreased to 138.7◦ at t=600 s owing to the presence of surface oxygen species such as carbonyl, carboxylic, hydroxyl, which lower the WCA24, 31, 32


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Figure 1: Physical properties of the basal plane and edge-oriented HOPG (a) Raman spectra, and (b) time dependence of the water contact angle, (c) C1s X-ray photoelectron spectra of the basal plane, (d) C1s X-ray photoelectron spectra of the edge-oriented sample (The inset figure displays the atomic concentration in each of the components), and (e) O1s X-ray photoelectron spectra of both regions.


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To define the surface chemical composition at the basal plane and edgeoriented HOPG, the XPS survey spectra of both areas were recorded: they display predominant C1s peaks at 284.5 eV33, and a minor O1s peak at ~532 eV34 (See Figure S2). As can be seen in the inset of Figure S2, the basal plane shows a higher carbon concentration, of > 99.8 atom%, than the edge-orientation (97.4 atom%). The small number of oxygen species (less than 0.2 atom%) found on the basal plane is likely due to rapid moisture and airborne organic contamination16. The edge-orientated plane shows a higher degree of oxidation, containing 2.6 atom% oxygen. This is consistent with the termination of the graphene edges with oxygen functionality on exposure to ambient conditions35-36. The C1s spectra of the basal plane in Figure 1c can be divided into two components: namely sp2 and the π-π* shakeup of sp2 carbon, at binding energies of 284.5 eV and 291.4 eV, respectively34, 37. The basal plane exhibits the ideal “honeycomb” structure with minimal defects, hence its high percentage (97.26 atom%) of sp2 carbon. In contrast, the edge-oriented sample displays a significant amount of carbon atoms in sp3 and C-O environments, 9.47 atom% and 4.78 atom%, respectively, due to edge termination by hydrogen and oxygen species (See supporting information).


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Under ambient conditions, the active carbon atoms will react with oxygen or moisture to form a C-O bond or C-OH bond (see Figure 1d)22, 38, 39. The O1s peak located at 532.8 eV therefore indicates that the carbon atoms at the edge are more reactive than the basal plane due to the immobilized free radicals (“dangling” bonds), which can react with the ambient oxygen to form C-O or C-OH22,

39.

The conclusions drawn from the

XPS data are in good agreement with Raman, WCA and SEM (see Figures S2 and S3) analysis.

To

understand

the

capacitive

properties,

the

capacitance

at

the

electrode/electrolyte interface of semi-metallic materials can be divided into three components: (1) Helmholtz capacitance (CH), which arises from the compact double layer capacitance, (2) diffuse layer capacitance (Cdiff), and (3) space charge capacitance (CSC), which arises within graphite due to the limited number of charge carriers16, 18. Therefore, the total measured capacitance (CT) of HOPG can be defined by Equation (1), assuming the individual components are independent of one another. Under conditions of high ionic strength, the diffuse layer capacitance is commonly large


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when compared with CH and CSC. In this work, 6 M LiCl was used as an electrolyte16, 27 to investigate the capacitance-potential curve, pzc, and the space charge capacitance of HOPG16, 27 as can be seen in Figure 2. (Note that lower concentrations were used for comparison between electrolytes because of the lower solubility of some of the salts,

vide infra.) The capacitance-potential curves of HOPG in Figure 2 exhibit an almost symmetrical shape on the positive and negative branches, with the data suggesting a stronger adsorption of the chloride ion than the lithium. Previous experimental data on HOPG has also suggested that anion interactions with basal plane graphite may be stronger16,

27;

in contrast molecular dynamics40 and density functional theory41

simulations have reached opposing predictions about cation vs. anion adsorption energies42, although this discrepancy may be related to the failure to include ioninduced polarization in the former case. The pzc is located at 0.1 V vs. Ag/AgCl, which provides a minimum capacitance of 4.85 ± 0.14 µF cm−2. This value is in reasonable agreement with the minimum capacitance previously reported for a carbon aerogel in aqueous electrolyte43, for basal plane HOPG in this electrolyte (4.4 µF cm−2, 100 Hz)16 ,


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and slightly exceeds the value reported for a more dilute aqueous electrolyte (~3 µF cm−2 in 0.9 M NaF at 1000 Hz and 25 ◦C18).

1 C total



1 1 1   C SC C H C diff

(1)

Figure 2: Capacitance-potential response of basal plane HOPG in 6 M LiCl over the potential window from −0.3 V to 0.4 V vs. Ag/AgCl.

One approach to estimate CH is to use the value measured at a polycrystalline Pt electrode with the same electrolyte, found to be 12.4 (± 2.8) µF cm−2: the minimum CSC (7.5 ± 0.3 µF cm−2) can thus be calculated from Equation (1) if the Cdiff term is neglected


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at these high electrolyte conditions. It can be concluded that both CH and CSC contribute approximately equally to the capacitance of basal plane HOPG. On the other hand, the edge-oriented sample showed a much higher capacitance at 0.0 V vs. Ag/AgCl (439.9 ± 10.0 µF cm−2) at the same electrolyte concentration, which can be attributed to surface Faradaic charge storage, associated with the oxygen species39,

44

(Figure S5). The

capacitance, in contrast with the basal plane response, decreases when the potential is adjusted to more positive or negative values due to the depletion of the pseudocapacitive effect. In order to study the effect of various alkali metal ions on the basal and edgeoriented capacitance of HOPG, electrochemical impedance spectroscopy (EIS) was performed in HCl, LiCl, NaCl, KCl, RbCl, and CsCl at a fixed concentration of 0.5 M. For reference, the hydrated ionic sizes are given in Table 1. To investigate the impact of cation size on the capacitance, frequencies from 100 kHz to 1 Hz were used at a potential of −0.1 V vs. pzc (0.0 V vs. Ag/AgCl). Moreover, the capacitive behavior of the chloride anion was also studied by applying the same frequency range at +0.1 V (i.e. symmetrically with respect to the pzc of the system as shown in Figure S5). Note that a


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shift of the pzc to more positive potentials was seen in the presence of Cs+, indicating stronger adsorption of this ion on the HOPG surface. Further details on the experimental setup are given in the Supporting Information.

Table 1: the crystalline diameter (rx), the molar Gibbs free energies of ions hydration (∆Ghyd)45, and hydrated diameter (dH) of the ions used46, 47.

Hydrated ions

H+/H2n+1On+

Li+

Na+

K+

Rb+

Cs+

Cl-

dx (nm)

0.030

0.080

0.100

0.160

0.180

0.210

0.190

∆Ghyd (kJ mol-

−1050

−475

−365

−295

−275

−250

−340

0.790

0.764

0.716

0.662

0.658

0.658

0.662

1)

dH (nm)

Overall, it can be clearly seen that the capacitance of the basal plane (See Figure 3a) is related to the ionic size, and increases for ions of smaller hydrated diameter. Proceeding down the group from Li+ to Cs+, increases the basal plane capacitance from 4.7 (± 0.4) µF cm-2 to 9.4 (± 0.6) µF cm-2.

In addition, the H+


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adsorption shows the lowest capacitance of about 4.0 (± 0.3) µF cm-2. This is attributed to the high hydration energy of the proton, which results in a large hydration shell. Extracting CH from the basal plane capacitance which, in the simplest interpretation, can be related to ion size via Equation (2)48:

CH 

 0 r S d

(2)

where ε0 is the permittivity of the vacuum, εr is the relative permittivity of the electrolyte, S is the exposed area of the electrode, and d is charge separation, which can be approximated by the distance from electrode plane to the center of adsorbed ion49. As expected, in the negative branch of the C vs. (E) data for the electrolytes, the Cl− counter ions display a similar range of capacitance, between 5.0 and 5.8( ± 0.8) µF cm−2. In contrast, the capacitance for the edge-oriented samples, shown in Figure 3b, is two orders of magnitude higher than that of the basal plane. This is because capacitance at the edges are dominated by the pseudo-faradaic process from the oxygen functionalities,44 such


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as the reduction of quinone moieties to hydroquinone (see Equation (3))50, the presence of which was confirmed by the earlier surface analysis data.

(3)

Moreover, the capacitance of the edge is largely independent of cationic size. The capacitance of alkali metal cations with chloride anions lie in the range of 430.1 ± 9.9 µF cm−2 to 443.6 ± 13.8 µF cm−2. Note, the CV responses at 50 mV s−1 for the basal plane and edge-oriented HOPG in each electrolyte are shown in Figure S12. Further details on the studies of oxygen contribution to the basal plane and edge-oriented HOPG are given in the Supporting information.


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Figure 3: Areal capacitance as a function of the cationic size for the indicated metal chloride electrolytes at a fixed electrolyte concentration of 0.5 M. (a) basal plane and (b) edge-oriented HOPG.


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In addition to the capacitance, the Nyquist plot of the basal plane HOPG displays a straight line, parallel to the vertical axis, indicating ideal EDLC behavior (See Figure S13a). The Nyquist plot of the edge-oriented HOPG shows a deviation from this purely capacitive response owing to the surface roughness at the edge position51 as well as the pseudocapacitive component52 (see Figure S13b). Likewise, the Bode plot in Figure 4a of the phase angle for each electrolyte approaches −90° at frequencies below 100 Hz for the basal plane, denoting ideal EDLC behavior over the frequency range 1 to 100 Hz53. In general, the relaxation time constant (τ0) is the minimum time required to deliver the stored charge, which can be calculated as τ0=1/f0, where the f0 denotes the characteristic frequency where the phase angle reaches −45°

54.

While decreasing the

size of the hydrated cation, it is noticeable that the f0 of the basal plane HOPG is shifted to higher frequencies, i.e. a shorter characteristic time is associated with relaxation of the stored charge. On the contrary, the edge-oriented HOPG exhibits values of f0 of about 150 to 200 Hz, which are largely independent of ion identity as shown in Figure 4b. The edge-oriented HOPG displays a lower frequency limiting phase angle than the basal plans, of −82°, due to the diffusional limits of pseudocapacitance53, 55. Moreover,


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the real part of the complex capacitance at both basal and edge planes was then calculated according to Equation (4)56.

C

 Z' 2fZA

where Z’ is the real part of the impedance, and Z is equal to

(4)

(Z ' )2  (Z ' ' )2 .

The complex capacitance values with respect to frequency are shown in the insets of Figure 4a and Figure 4b. At the basal plane, it is clear that the capacitance is increased when the hydrated ionic size is smaller. By contrast, with the edge plane the capacitance is again shown to be independent of the hydrated ionic size. The H+ shows the highest capacitance due to its participation in the surface faradaic process from quinone to hydroquinone50, which is consistent with Figure 3. The calculated relaxation time constants of both the basal plane and edge-oriented HOPG are shown in Figure 4c. The maximum frequency with which the ions can respond to the oscillatory electrode potential should be related to their transport rate in solution, hence the relaxation times


22

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would be expected to show a correlation with ionic diffusion coefficients (see Figure 4d). H+ gives the lowest time constant, about 0.025 ms, reflecting its anomalously high diffusion coefficient in aqueous solution (9.31 × 10-5 cm2 s-1)57, whereas the K+, Rb+, and Cs+ show equivalent relaxation times (0.11 µs), while Li+ and Na+ show slightly higher values (0.15 ms and 0.13 ms, respectively.) All parameters calculated from the basal plane and edge-oriented HOPG capacitance can be found in Tables S1 and S2.


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Figure 4: the analysis of electrochemical impedance spectroscopy at the basal plane and edge-oriented HOPG (a) Bode plot of basal plane, (b) Bode plot of edge-oriented graphite (inset is the real part of the complex capacitance with respect to frequency), (c) plot of relaxation time constant as a function of cation sizes and (d) plot of reciprocal of relaxation time constant (τ0-1) observed for each of the cations on basal plane HOPG as a function of the ionic diffusion coefficient.

In summary, HOPG was used as a model for graphene electrodes to study the effect of ionic sizes on the capacitance. In this work, we provide the first demonstration of the relationship between cation identity and capacitance on both basal plane and edge-oriented HOPG. It is found that the capacitance of the basal plane is increased from 4.72 ± 0.37 µF cm−2 to 9.39 ± 0.62 µF cm−2 when the hydrated ionic radius falls from 0.310 nm (Li+) to 0.118 nm (Cs+), because of the shorter charge separation distance. By contrast, the edge-oriented samples are largely insensitive to cation size/identity. The major capacitive contribution of the edge-oriented samples arises from the pseudo-faradaic processes associated with the oxygen functionalities, which


24

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exhibits an effective capacitance of 430.1 ± 9.9 µF cm−2. Furthermore, the smaller ions exhibit a faster relaxation time on the basal plane, whereas the edge-oriented samples show deviation from the ideal EDLC response, resulting in higher relaxation times. These quantifications of capacitive properties at the basal plane and edge-oriented graphite are crucial for designing high-performance supercapacitors from graphene and other carbonaceous materials.

ACKNOWLEDGMENT P.I. would like to thank the Ministry of Science and Technology, Royal Thai Government for the award of a scholarship. R.A.W.D. acknowledges funding from EPSRC (grant reference: EP/R023034/1). Supporting Information.

The Experimental section including the chemical and material, electrode preparation,

electrochemical

evaluation,

Investigation

Helmholtz

capacitance,

characterization, XPS deconvolution, and O2 plasma treatment process. The Result


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section including the water/air contact angle of edge-oriented HOPG after polishing, XPS survey spectra, SEM images, voltammetry and capacitance-potential curves of fresh, polishing and plasma exposed edge-oriented HOPG, calculated CH in each of the electrolytes, CV curves in different electrolytes, Nyquist plot, the summary of all parameters of the basal plane and the edge-oriented HOPG.

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