Physisorbed Hydroquinone on Activated Charcoal as a

Publication Date (Web): April 26, 2015. Copyright © 2015 American ... *E-mail [email protected]. ... Fax: 91-755-6692392. ..... Received 9 February ...
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Physisorbed Hydroquinone on Activated Charcoal as a Supercapacitor: An Application of Proton-Coupled Electron Transfer Chanderpratap Singh and Amit Paul* Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, MP 462066, India S Supporting Information *

ABSTRACT: High surface area carbon materials have high double layer capacitances because of their enhanced internal surface area and hence are attractive materials for supercapacitor applications. In this work, we demonstrate that utilizing a simple shaking experiment, hydroquinone can be physisorbed inside the pores of activated charcoal, and the material can be used as a supercapacitor having a highest specific capacitance of ∼200 F/g in 1 M H2SO4. Nearly 40% of the specific capacitances were pseudocapacitance in nature because of the observed reversible redox chemistry of hydroquinone/benzoquinone couples, where hydroquinone underwent proton-coupled electron transfer (2H+/2e−) to form benzoquinone. The redox chemistry of hydroquinone/quinone is chemically irreversible, but the same chemistry has been found to be chemically reversible, and fast electron transfer kinetics at the electrode surface was observed in this study, presumably because of proton-coupled electron transfers that were catalyzed by oxide sites present on the activated charcoal. Due to the observed reversible electrochemistry, the material also showed excellent capacitance retention in the long term cyclic tests. The approach presented in this study conceptually brings a new dimension to improve the chemistry of energy storage systems by simultaneous introduction of physisorption and proton-coupled electron transfer.

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INTRODUCTION In the current era, renewable energy research has received new momentum because of the global energy crisis. In this regard, electrochemical supercapacitors are emerging as devices of prime importance because of their applications such as shortterm power boosts, emergency power supplies and peak power assistances for batteries.1 Traditionally, supercapacitors are electrical double layer capacitor (EDLC) in which charge storage arises due to the ion adsorption at the electrode− electrolyte interface. High specific capacitances were achieved by coating the electrodes with high surface area materials (e.g., graphene2−4), which provided the platform for enhanced ion adsorption near the electrode surface.1 Specific capacitance values in the range of 100−150 F/g was observed for graphenebased materials.2,5,6 Introducing nitrogen doping in graphene nanomaterials, specific capacitance of 400 F/g was also achieved.7 In spite of all these efforts, researchers are now in consensus that the charge storage capacity cannot be increased significantly utilizing only the double layer formation. The alternative way to increase the specific capacitances is incorporation of pseudocapacitances where electron/charge transfer occurs across the electrode−electrolyte interface. Metal oxides such as RuO2, MnO2, and MoO3 are examples of pseudocapacitors wherein redox transitions at the metal centers existing in different oxidation states provide charge storage capabilities.8−11 Hybrid materials such as graphene-metal oxide or graphene oxide-metal oxides also have been explored for these applications where metal oxides were covalently attached on graphene/graphene oxide.12−15 Specific capacitance of ∼350 © XXXX American Chemical Society

F/g was observed for hybrid graphene-metal oxides nanomaterials.12,13 However, loss of conductivity upon covalent attachment of metal oxide on graphene remained an issue which inhibited fast charging of supercapacitors.13 Santamaria and co-workers have shown a new conceptual pathway to increase the specific capacitances where hydroquinone was added in the electrolyte, which increased the pseudocapacitance significantly due to electron transfer between redox molecules present in the electrolyte and electrode. They reported the highest specific capacitance of ∼700 F/g.16 However, a 65% drop in specific capacitance in the long term cyclic test was observed because of the irreversible electrochemistry of hydroquinone.16 It is important to emphasize that, for the development of supercapacitor technology, introduction of new conceptual pathways are also crucial. Meyer and co-workers were first to demonstrate that utilizing oxide sites of electrode surface, the electrochemistry of catechol can be made reversible utilizing proton-coupled electron transfer between oxide sites of electrode and catechol which otherwise shows irreversible electrochemistry.17 Since then, several reports appeared in the literature discussing the mechanism of observed reversible electrochemistry of catechol or hydroquinone on oxide surfaces.18−22 Recently, Cronin and co-workers demonstrated that the hydroquinone (HQ)/ benzoquinone (BQ) redox couple can efficiently decouple an oxygen evolution reaction Received: February 9, 2015 Revised: April 25, 2015

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Electrochemical Measurements. Electrochemical experiments were carried out on CH Instruments, Austin, TX, bipotentiostat (Model CHI 760D) and potentiostat (Model CHI 620E). A regular three electrode cell was used in which saturated calomel electrode (SCE), Pt wire, and HQ-AC coated Pt foil were used as reference, counter, and working electrode, respectively. Cyclic voltammetry (CV) experiments were performed in a potential window of 0.6 V in different pH solutions at various scan rates (5, 10, 20, 50, and 100 mV/s). CV experiments were also performed by using glassy carbon electrode as a working electrode. Understanding the electron transfer kinetics was one of the aspects of this study (vide infra). Hence, before collecting the CVs, uncompensated solution resistances (Ru) were determined at a potential where faradaic processes do not occur using iRu compensation tool of CH software.26 All the CVs shown in this manuscript were collected at a Ru value of 0.2 Ω. Complete iRu correction caused too much oscillation in the voltammograms and was hence avoided.27 Long-term cyclic performance experiments by cyclic voltammetry were performed without iRu compensation correction. Galvanostatic charge/discharge was performed in a potential window of 0.6 V at various pH solutions at variable current densities (1, 2, 5, and 10 A/g). Electrochemical impedance spectroscopy (EIS) experiments were acquired at a potential of 0 V versus SCE with amplitude of 10 mV over a frequency range of 100 kHz to 0.01 Hz. All the experiments were performed at 25 °C at ambient condition. The formal potentials (E0′) for the redox couples have been determined from the cyclic voltammograms (CV) by using eq 1, that is, the average of anodic (Epa) and cathodic peak potential (Epc). Capacitances were calculated for CV experiments using eq 2, where C, I, E, and ν represent capacitance, current, potential window, and scan rate of CV, respectively. First, the numerator (∫ I·dE) of this equation was calculated by integrating the area of cyclic voltammograms utilizing Origin software. Subsequently, capacitances were calculated by dividing the integrated area of voltammograms by scan rate and two times potential window of cyclic voltammetry according to eq 2. Finally, specific capacitances were calculated by dividing calculated capacitances by the weight of HQ-AC deposited on a single electrode. For CV experiments, the total specific capacitance had two contributions. The first contribution was due to double layer formation, and the other contribution was due to redox reaction, which is also known as pseudocapacitance. CH Instruments software allows drawing a background line under the redox peaks and provides the charges under the redox peaks. For both anodic and cathodic peaks, the charges were calculated. Summations of charges for two peaks were considered as faradaic charges, which are mathematically equivalent to (∫ I·dE)/ν. After that, pseudocapacitance contributions were calculated by dividing the charge by twice according to eq 2. Finally, pseudocapacitances in F/g were calculated by dividing calculated pseudocapacitances by the weight of HQ-AC deposited on a single electrode. The total specific capacitance is the summation of double layer and pseudocapacitance contribution. Hence, by subtracting the pseudocapacitances from the total specific capacitances, the double layer capacitances in F/g were calculated. Capacitances from galvanostatic charge/discharge were calculated by using eq 3, where t is the time taken to complete the charging/ discharging process and I is the applied current density. Similar to CV experiments, specific capacitances by galvanostatic

from the hydrogen evolution reaction during electrolytic water splitting.23 In this study, we demonstrate a new conceptual pathway for supercapacitor application, where (HQ) was physisorbed on activated charcoal by simple shaking of a solution containing these two chemicals. This material showed high specific capacitances due to significant pseudocapacitance contribution from HQ redox chemistry with excellent capacitance retention in the cyclic test. These results have been explained by fast proton-coupled electron transfer (PCET)24,25 between HQ and oxide sites present on activated charcoal. Furthermore, the chemistry has been extended in a wide range of pH (0.30−7.2) for capacitor application.

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EXPERIMENTAL SECTION Chemicals and Electrodes. Activated charcoal (AC; 100 mesh particle size, powder), HQ (99.5%), N-methyl-2pyrrolidone (NMP), poly(vinylidene fluoride) (PVDF), sulfuric acid (H2SO4; 98%), phosphoric acid (H3PO4; 85 wt %), sodium dihydrogen phosphate (NaH 2 PO 4 ; 99%), and potassium bromide were purchased from Sigma-Aldrich. Acetic acid (CH3COOH; 100%) and disodium hydrogen phosphate anhydrous (Na2HPO4; 98%) were purchased from Merck. Carbon black (100%), and sodium acetate anhydrous (CH3COONa; 98%) were purchased from Alfa-Aesar and SDFCL, India, respectively. All the chemicals were used in this study without any further purification. Glassy carbon (3 mm diameter) and saturated calomel electrodes were purchased from CH Instruments Inc., TX, U.S.A. Platinum (Pt) foil and Pt wire electrodes were purchased from Alfa Aesar, U.S.A. MilliQ water was used throughout the study. Preparation of Active Material (Hydroquinone Physisorbed on Activated Charcoal (HQ-AC)). For the preparation of HQ adsorbed on activated charcoal material, in a 250 mL flask, hydroquinone (HQ; 50 mM, 275 mg in 50 mL water) was taken and mixed well using sonication for 5 min. After that, 0.5 g of charcoal was added to the solution. Then, the flask was allowed to shake at 90 rpm at a constant temperature (50 °C) inside the shaker for 24 h. After that, the material was filtered by sintered glass funnel and dried at 60 °C for 24 h in an oven. This prepared material will be termed as HQ-AC throughout the manuscript. In order to maximize the adsorption, experiments were performed using various concentrations of HQ and at various temperatures as well. Electrochemical studies indicated that the protocol mentioned here provided maximum HQ adsorption on activated charcoal (AC). The adsorption of HQ on AC is a physisorption process that was confirmed by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and infrared spectroscopy (IR) studies (vide infra). Electrode Fabrication. For electrochemical measurements, Pt foils were coated with HQ-AC. For fabrication purpose, first Pt foils were cleaned in acetone by sonication for half an hour and dried in air. After that, 10 mL NMP, 15 mg PVDF binder, 75 mg HQ-AC, and 10 mg of carbon black were added. The solution mixture was stirred for 6 h to enhance the homogeneity of the solution. Finally, 0.75 mg of active material was deposited on the electrode in a 1 cm2 area by taking 100 μL of aliquot through drop casting. After that, electrodes were dried at 100 °C for 8 h in a hot air oven. The binder prohibits the materials to dissolve in electrolytes from the electrode and carbon black improves the conductivity of the film.1 B

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mV/s. Half-peak width versus log (scan rate) has been plotted in Figure S10(a). Half-peak widths remained almost constant at higher scan rates. Figure S10(b) shows a plot of anodic peak potential (Ep) versus log(scan rate). As the scan rate increased, the peak potential shifted anodically with a linear variation. The slope of Ep versus log(ν) was 34 mV/dec. These results are similar to the earlier reports and an ECEC (“E” represents electron transfer at the electrode surface and “C” represents a homogeneous chemical reaction) mechanism was proposed for the chemical irreversibility of HQ. According to this mechanism, (Ep − Ep/2) is expected to be 47 mV and the slope for Ep versus log(ν) is expected to be 30 mV/dec.27,29−33 The same experiment was also performed in higher pH solutions, and the formal potentials (E0′) in different pH solutions were determined using eq 1. E0′ values determined from electrochemical experiments were plotted against pH in Figure 1b. The plot is known as “Pourbaix diagram”.34 Figure 1b demonstrates that, as the pH increased, the E0′ started to shift toward the cathodic direction. For a proton-coupled electron transfer (PCET) reaction, E0′ values are shifted toward the cathodic direction with increasing pH according to eq 4, where m is the number of protons transferred and n is the number of electrons transferred.24,25 According to eq 4, for a 2H+/2e− PCET reaction, the slope of the Pourbaix diagram should be 59 mV. The experimentally found slope was 56 mV/ pH, and hence, these results confirm that the HQ/BQ redox reaction is a PCET reaction.

charge/discharge were calculated by dividing capacitances by the weight of HQ-AC deposited on a single electrode. E0′ =

C= C=

Epa + Epc 2

(1)

∫ I · dE 2Ev I ·t 2·E

(2) (3)

Thermal Gravimetric Study (TGA). The TGA experiments were performed by PerkinElmer TGA 4000 instrument in a temperature range from 30° to 350 °C at a scan rate of 5 °C/min with a N2 flow rate of 20 mL/min. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC) experiments were performed by PerkinElmer DSC 6000. HQ-AC samples were enclosed in an aluminum pen and performed in the temperature range of 30° to 250 °C at a scan rate of 5 °C/min with a N2 flow rate of 20 mL/min. Infrared Spectroscopy (IR) Study. Infrared spectroscopy was performed using a PerkinElmer Spectrum BX spectrophotometer. AC and HQ-AC samples were grinded with potassium bromide and pelletted. IR spectra were taken in a wavelength range from 500 to 4000 cm−1. Scanning Electron Microscopy (SEM). The activated charcoal and HQ-AC samples were dried in vacuum for 6 h. After drying, samples were directly sprinkled on conductive carbon tape and coated with gold by sputter coating for 2 min. The scanning electron microscopy (SEM) experiments were performed by Carl ZEISS (ultraplus) FE-SEM at a working voltage of 3.6 kV. Brunauer−Emmett−Teller (BET) Gas Adsorption. BET gas adsorption measurements were performed by using Belsorp-max (BEL Japan) automatic volumetric adsorption instrument. All the glasswares used were of ultrapure research grade (99.99%). Other Characterizations. Thicknesses of the films were determined by Dektak XT surface profiler (Bruker). Elemental analysis was performed by Elementar Analysensysteme GmbH, Germany.

E0′ ∝ −

0.059m pH n

(4)

Characterization of HQ-AC and AC by DSC, TGA, and IR spectroscopy. Hydroquinone was adsorbed on AC using a simple shaking methodology as described in the Experimental Section. HQ-AC was first characterized by TGA. Figure 2 shows that a total 8 wt % mass loss occurred in a temperature range of 120 to 240 °C, which can be attributed to loss of HQ from AC. The material was further characterized by DSC to identify the nature of interactions between HQ and AC. The result represented in Figure S1 shows that the HQ desorbed from HQ-AC at a temperature of 172 °C, which is also the melting temperature of HQ. DSC results suggest that heat was released during the desorption process, that is, the reaction was exothermic. Change in enthalpy for HQ desorption (ΔHdes) from HQ-AC was found to be −7.27 kJ/mol or −0.569 kJ/g (molecular weight of HQ-AC was determined using elemental analysis results, Table S5). Since desorption of HQ was an exothermic process, adsorption of HQ (ΔHads) on AC must be an endothermic process. Hence, the change in entropy for HQ adsorption (ΔSads) on AC should be positive to make the process thermodynamically feasible, that is, a negative ΔGads (change in Gibbs free energy for adsorption process). A detailed thermodynamic study by Srivastava and co-workers on catechol and resorcinol adsorption on granulated activated carbon suggests that indeed the adsorption process was entropically favored and enthalpically unfavored, that is, a positive ΔHads. They proposed an increased randomness at the solid-solution interface during the adsorption process with some structural change in the adsorbates, and the charcoal was responsible for a positive ΔSads value.35 IR spectra of AC and HQ-AC were taken. CC stretching frequency at 1650 cm−1 was not shifted upon HQ adsorption (Figure S2). The small enthalpy value and IR result confirm that the HQ adsorption on AC was a physisorption process. Elemental analyses were

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RESULTS AND DISCUSSION Electrochemistry of Hydroquinone (HQ). A cyclic voltammogram (CV) for HQ/(BQ) redox reaction (Scheme 1) on a glassy carbon electrode at a scan rate of 5 mV/s in 1 M H2SO4 (pH = 0.30) has been shown in Figure 1a. The result represented in Figure 1a demonstrates that the electrochemistry of HQ/BQ is chemically irreversible in a cyclic voltammetry time scale since the coulometric charge ratio under the anodic (QA) and cathodic (QC) peak was 1.35.28 The half-peak width (Ep − Ep/2) for the anodic peak was 42 mV at 5 Scheme 1. Chemical Equation Showing the Overall Redox Chemistry of Hydroquinone

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Figure 1. (A) Cyclic voltammogram has been shown for 10 mM HQ on a glassy carbon electrode in 1 M H2SO4 at a scan rate of 5 mV/s. (B) Pourbaix diagram (E0′ vs pH) for HQ/BQ has been shown. Blue diamonds are experimentally measured E0′ and the blue straight line is used to determine the slope of the plot.

HQ-AC had less surface area but had higher pore size compared to AC, a structural transition during HQ adsorption can be attributed to the increased pore size of HQ. It can be further proposed that the structural transition during HQ adsorption on AC made the process entropically favorable and thermodynamically feasible. The higher pore size of HQ-AC compared to AC has been further confirmed by SEM results (vide supra) and consistent with electrochemical results as well (vide infra). Electrochemical Performances of HQ-AC. Electrochemistry of HQ-AC was performed by depositing the material on a Pt electrode. The thickness of the film found to be 3.2 ± 0.1 μm. CVs for HQ-AC in 1 M H2SO4 (pH = 0.3) electrolyte solution have been shown in Figure 4a and the results are summarized in Table 1. The coulometric charges under the anodic (QA) and cathodic (QC) peaks were calculated using the methodology described in the Experimental Section. The results revealed that the QA/QC was close to 1 and peak separation between anodic and cathodic peaks (ΔEP) was only 6 mV at 5 mV/s scan rate. The coulometric charge ratio indicates that the redox chemistry was chemically reversible, at least in the CV time scale (Table 1).28 This finding is significant since electrochemistry of HQ/BQ was found to be chemically irreversible on a glassy carbon electrode (Figure 1a). On the other hand, ΔEP values are an indicator of electron transfer kinetics or electrochemical reversibility.28,37−40 For an electrochemically reversible reaction, that is, electron transfer event to the electrode is faster than the scan rate of voltammetry, ΔEP should be ideally close to 0 mV for a surface bound redox species.28,37−40 A ΔEP value of only 6 mV for HQ-AC in 1 M H2SO4 indicates that the electron transfer kinetics was very fast and it can be concluded that the electrochemical reversibility was achieved at a slow scan rate. ΔEP increased with increasing scan rate which indicates that the electron transfer process to the electrode started to become quasireversible with increasing scan rate, that is, electron transfer rate was becoming slower than the scan rate of voltammetry (Table 1).37 ΔEP was only 54 mV at a scan rate of 100 mV/s, which further confirmed rapid electron transfer kinetics between HQ and electrode. It is important to also highlight that the coulometric charge ratios were close to 1 at all scan rates, which indicates that chemical reversibility was maintained (Table 1). Full width half peak maximum (fwhm) is ideally expected to be 90.6/n mV (i.e., 45.3 mV in this study, since n = 2) for a surface bound species at a slow scan rate for an electrochemically reversible process.28 Fwhm for both cathodic and anodic peak were broader than the ideal value, which was expected considering the inhomogenity

Figure 2. TGA trace has been shown for HQ-AC.

also performed for AC and HQ-AC. The materials had ∼21% oxygen and 76−77% carbon contents (Table S5). Characterization of HQ-AC and AC by SEM and BET. SEM images of AC and HQ-AC have been shown in Figure 3.

Figure 3. SEM images of (a) AC and (b) HQ-AC.

A comparison between the images clearly indicates that the morphology of HQ-AC was different compared to AC, and HQ-AC was more porous in nature. Electrochemical experiments were carried out on a Pt electrode, where coated materials also had PVDF binder and carbon black (vide supra). Hence, SEM images of both materials in the presence of PVDF and carbon black were also taken, and the images have been shown in Figure S9. A comparison between Figures 3 and S9 reveal that the morphologies for both materials did not change due to the presence of PVDF and carbon black. BET N2 adsorption experiments were performed for AC and HQ-AC and the nitrogen sorption isotherms are shown in Figure S8. The BET surface areas were found to be 936 and 320 mt2/g for AC and HQ-AC. The decrease of HQ-AC surface area compared to AC can be attributed to HQ adsorption on AC. Insets of Figure S8 show the pore size distributions calculated using nonlocal density function theory (NLDFT) for both materials.36 The average pore sizes were 1.75 and 2.9 nm for AC and HQ-AC. These observations suggest that even though D

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Figure 4. Electrochemical performance of HQ-AC in 1 M H2SO4 (pH 0.30). (a) Cyclic voltammograms at scan rates 5 (green trace), 10 (blue), 20 (red), and 50 (black) mV/s. (b) Specific capacitance dependence on scan rates of voltammetry. (c) Galvanostatic charge/discharge at current densities 1 (black), 2 (red), 5 (blue), and 10 (green) A/g. (d) EIS results collected at 0 V vs SCE reference electrode (inset: high frequency region) over frequency range 0.01−105 Hz at an amplitude of 0.01 V. (e) Long-term cyclic stability performance at a scan rate of 20 mV/s in cyclic voltammetry (results shown for every 50 cycles).

(Table 1).37 The coulometric charge calculation from the CV of HQ-AC at 5 mV/s scan rate indicated that at least 5.5 wt % of HQ-AC was HQ. This value is lower than the results obtained from TGA (8 wt %), which is expected since electrochemical results provide a lower end of adsorption because all the redox molecules may not respond during electrochemistry, especially in a thick film. The comparison of TGA and electrochemical results suggests that ∼69% HQ of HQ-AC was accessible in electrochemistry. Utilizing the CVs of Figure 4a and eq 2, specific capacitances for HQ-AC in 1 M H2SO4 electrolyte solutions were calculated and listed in Table 2. The highest specific capacitance value of 207 F/g was observed at 5 mV/s. Specific capacitance values decreased with increasing scan rates but the decrease was

Table 1. Anodic and Cathodic Charge Ratio (QA/QC), Full Width Half Peak Maxima (FWHM), and Anodic and Cathodic Peak Separation (ΔEP) at Different Scan Rates of Cyclic Voltammetry for HQ-AC in 1 M H2SO4 Solution scan rate (mV/s)

anodic (QA)/cathodic charge (QC)

fwhm (anodic/ cathodic; mV)

ΔEP (mV)

5 10 20 50 100

1.00 1.00 1.00 1.00 1.00

87/108 89/110 90/111 93/117 102/131

6 11 17 33 54

of the film, where the repulsive interaction between the oxidized sites of AC and HQ can cause the peak broadening

Table 2. Specific Capacitance, Pseudocapacitance, and Double Layer Capacitance Calculated from Cyclic Voltammetry for HQAC in 1 M H2SO4, and Specific Capacitance for HQ-AC Calculated at Different Current Densities from Galvanostatic Charge− Discharge Experiments scan rate (mV/s)

specific capacitance (F/g)

pseudocapacitance (F/g)

double layer capacitance (F/g)

current density (A/g)

specific capacitance (F/g)

5 10 20 50 100

207 201 196 188 185

82 78 75 72 70

125 123 120 116 115

1 2 5 10

175 162 154 142

E

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Figure 5. Cyclic voltammograms have been shown for HQ-AC at scan rates 5 (green), 10 (blue), 20 (red), and 50 (black) mV/s of cyclic voltammetry in (a) 1 M phosphate buffer (H3PO4/NaH2PO4 (pH 2.2)), (b) 1 M acetate buffer (CH3COOH/CH3COONa (pH 4.8)), and (c) 1 M phosphate buffer (NaH2PO4/Na2HPO4 (pH 7.2)).

minimal and a specific capacitance value of 185 F/g was observed at 100 mV/s (Table 2, Figure 4b). Similar results were obtained by galvanostatic charge−discharge experiments (Table 2, Figure 4c). Nyquist plot of EIS study has been shown in Figure 4d and the inset shows the high frequency region. Nyquist plots are a plot of imaginary component of the impedance (Z″) against the real component (Z′). A sharp vertical rise of the spectra along the Z″ axis was observed in the low frequency region. This impedance rise along the Z″ axis in the low frequency range is known as the Warbung impedance or constant phase element in EIS which arises due to ion transport/mass transport inside the material.2,28,41 Impedance due to mass transport drops out at high frequencies since the time scale is too short so that mass transfer cannot manifest.28,41 Hence, the starting frequency of Warbung impedance or constant phase element is an indicator for efficiency of mass transport inside the material. A higher starting frequency indicates a better mass transport, whereas a lower starting frequency indicates an inferior mass transport.28,41 The starting frequency for mass transport has been shown by an arrow in Figure 4d, and the value was found to be 31.6 Hz for HQ-AC in 1 M H2SO4, which indicates excellent ion transport inside the material due to its porosity. Figure 4e represents the long-term stability of HQ-AC to perform as a supercapacitor. The capacitance retention was 82% after 1000 cycles. Long-term cyclic stability test was also performed by galvanostatic charge−discharge experiment at 1 A/g current density (Figure S3) and 82% capacitance retention was observed in this experiment. These results are significant and highlights that HQ electrochemistry was not merely chemically reversible during one electrochemical cycling, but was stable for 1000 cycles. The total specific capacitance of HQ-AC had two contributions and those were double layer capacitance and pseudocapacitance. At each scan rate of CV, the double layer capacitance and pseudocapacitance have been calculated and summarized in Table 2. The results demonstrate that nearly

40% of the specific capacitance was pseudocapacitance in nature. Experiments were also performed for AC to test the material as a supercapacitor and the observed highest specific capacitance was only 60 F/g (Figure S7). Comparison of this value with Table 2 reveals that the specific capacitance of HQAC was nearly three times higher compared to AC. This improvement can be partly attributed to pseudocapacitance due to HQ/BQ redox reaction and also because of the improvement of double layer capacitance. The improvement of double layer capacitance was due to a better morphology of HQ-AC compared to AC that allowed more ions to diffuse inside the pore of the material, which was evident by SEM images (Figure 3). The AC impedance results show that for HQ-AC and AC in 1 M H2SO4, the starting frequency for mass transport were 31.6 and 1 Hz, respectively (Figures 4d and S7c). These results also support that the mass transport inside the pores of HQ-AC was better than AC. BET results showed that the average pore size for AC and HQ-AC were 1.75 and 2.9 nm, respectively (Figure S8). According to the definition of the IUPAC, pore diameter less than 2 nm is called micropore, while macropores are greater than 50 nm and mesopores are in between.6 According to this definition, AC was microporous, while HQ-AC was mesoporous in nature. Mesopores of materials facilitate better ion transport inside the materials,6 and hence, higher double layer capacitance for HQ-AC was observed compared to AC. Inspired by these electrochemical results, electrochemical experiments were performed for HQ-AC in 1 M phosphate buffer (pH 2.2 (H3PO4/NaH2PO4)), 1 M acetate buffer (pH 4.8 (CH3COOH/CH3COONa) and 1 M phosphate buffer (pH 7.2 (NaH2PO4/Na2HPO4)) also. CVs in each solution are shown in Figure 5 and results are summarized in Table S1, S2 and S3. In every case, CV results revealed that the HQ redox chemistry was chemically reversible although peak separation (ΔEp) started to increase with increasing pH which was probably due to slower proton-coupled electron transfer between HQ and the electrode. Specific capacitance, double F

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molecules provided all the oxygen functional groups are proton accepting functional groups such as carbonyl, epoxide, and so on. The oxygen content in AC was 7× larger than this ideal value, and hence, it is possible for all HQ molecules to form hydrogen bonds with oxygen functional groups of AC. Gonzalez and co-workers studied electrochemistry of HQ in dimethyl sulfoxide (DMSO) solvent, and they have shown evidence for HQ forming hydrogen bonds with DMSO.29 Since DMSO is a weak base and HQ is a weak proton donor, hence, formal proton transfer from HQ to DMSO is unlikely. They proposed that, due to hydrogen bond formation between DMSO and HQ, dissociation energy of the O−H bonds decreases and makes the proton transfers from HQ to DMSO feasible during a proton-coupled electron transfer. A similar mechanism can operate in this study. We propose that HQ presumably can form a hydrogen bond with carbonyl functionality of AC (Scheme 2), which can weaken the O−H

layer capacitance, and pseudocapacitance in these electrolyte solutions at 5 mV/s scan rate of CV are plotted in Figure 6 and

Figure 6. Total capacitance, double layer capacitance, and pseudocapacitances calculated from cyclic voltammograms at 5 mV/ s scan rate for HQ-AC in different pH electrolyte solutions.

Scheme 2. Proposed Overall Mechanism of HQ Redox Chemistry on Electrode Surface when HQ is Physisorbed on ACa

summarized in Table S4. The results revealed that the specific capacitance systematically started to decrease with increasing pH. Pseudocapacitance also decreased systematically with increasing pH, whereas double layer capacitance significantly decreased between pH 0.3 and 2.2 and to some extent remained same in the pH range 2.2 to 7.2. The decrease of pseudocapacitance with increasing pH was presumably due to slower proton-coupled electron transfer at higher pH. The drop in double layer capacitance between pH 0.3 to 2.2 can be attributed to the fact that the proton (H+) has much better mobility compared to other cations such as Na+.28 AC impedance results show that the starting frequency for mass transfer region in 1 M H2SO4 (pH 0.30) and 1 M phosphate buffer (pH 2.2) were 31.6 and 6.8 Hz, respectively (Figures 4d and S4c), and hence supports the explanation of mobility of cations (vide supra).28,41 The performance of HQ-AC as a supercapacitor was tested in different pH solutions by long-term cyclic test. Capacitance retentions after 1000 cycles were 88, 86, and 84% in pH 2.2, 4.8, and 7.2, respectively (Figures S4d, S5d, and S6d). Indeed, these results are significant in demonstrating long-term chemical stability of HQ during electrochemical cycling in a range of pH. Mechanism for Electrochemical Reversibility of HQAC. Meyer and co-workers reported the mechanism for catechol oxidation on an activated glassy carbon electrode surface (i.e., oxide containing glassy carbon electrode) where catechol was diffusing in solution.17 A glassy carbon electrode was activated by applying high positive potentials for different time periods, and hence, they were able to prepare electrodes at different stages of oxidation. They showed at the initial stage of electrode activation, only 1H+/1e− transfer was selectively catalyzed by oxides on electrode surface which was confirmed by two peaks in CVs. However, at a fully activated electrode, second proton-coupled electron transfer was also catalyzed and eventually overtook the first proton-coupled electron transfer. Hence, at a fully activated electrode, only one peak for 2H+/2e− was observed. In this study, also, for HQ/BQ transformation, a single peak in CV was observed, and the CVs were reversible, too. AC contains oxides on the surface, and these oxides can form hydrogen bonds with HQ. The oxygen percentage in AC was 21 wt % (Table S5) and 8 wt % HQ was physisorbed on AC according to the TGA result. A simple calculation shows that in an ideal scenario ∼3 wt % oxygen functionality on AC could be enough to form hydrogen bonds, with all HQ

a

The equal arrows represent observed reversible electrochemistry.

dissociation energy of HQ. Due to this hydrogen bond formation, proton transfer from HQ to AC becomes feasible during the oxidation step of proton-coupled electron transfer, even though carbonyl groups have weak basicity. In a similar fashion, during the reduction process of proton-coupled electron transfer, the protons are transferred back from AC to BQ and regenerate HQ. The process of proton transfer can be termed as proton shuttling between HQ and AC, which also helped to achieve chemical reversibility. The overall oxidation reaction involved 2e− transfer to the electrode and 2H+ transfer to the oxides of AC from HQ (Scheme 2). PCET reactions could follow stepwise mechanisms, such as PT-ET (proton transfer followed by electron transfer), ET-PT (electron transfer followed by proton transfer), or a concerted electron−proton transfer (EPT) mechanism.25,31 The current study cannot distinguish between stepwise and concerted mechanisms. A future study directing toward mechanistic understanding can shed more light on this issue. It is also important to emphasize that this study indicate that the removal of HQ oxidation product (BQ) is not required for redox cycling to continue, rather the presence of redox reaction product (BQ) in the proximity of electrode surface helps to provide chemical reversibility.

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CONCLUSIONS In conclusion, based on a simple shaking experiment, hydroquinone was physisorbed on activated charcoal, and the material has been utilized for supercapacitor application. First, the most important feature of this study is that the material showed reversible electrochemistry for HQ, which was due to PCET reaction between HQ and oxide sites on the AC surface, which improved the specific capacitance of activated charcoal. G

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Second, electrochemical results indicated that a fast protoncoupled electron transfer occurred on the electrode surface which was catalyzed by the oxide sites of activated charcoal. This result is significant since fast electron transfer is the key for rapid charging of a capacitor. The PCET reaction also provided chemical reversibility and the work highlights the importance of proton-coupled electron transfer in the supercapacitor field. Third, these results demonstrate without any involvement of chemical treatment, pseudocapacitance can be incorporated through physisorption. Fourthly, this work represents an extremely cheap material for capacitor application, and most significantly, this work brings out the fascinating electrochemistry of hydroquinone in the field of energy storage. Finally, it is also important to mention that the electrochemical experiments were performed at a relatively high mass loading (0.75 mg) of active material, and yet, hydroquinone continued to demonstrate its interesting electrochemical properties.

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ASSOCIATED CONTENT

S Supporting Information *

DSC, IR, BET, SEM, elemental analysis, and additional electrochemical results. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01322.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Ph.: 91-755-6692375. Fax: 91-7556692392. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.P. acknowledges financial support from the Department of Atomic Energy (DAE), India (Grant No: 2012/20/34/9/ BRNS), Department of Science and Technology (DST), India (Grant No: SB/FT/CS-165/2012), and IISER Bhopal. C.S. acknowledges Council for Scientific and Industrial Research (CSIR), India, for providing fellowship. We sincerely thank Dr. Deepak Chopra for critically reviewing the manuscript. We also thank Mr. Sujoy Bandyopadhyay for helping us in BET analysis.

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