Electrochemically Activated Reduced Graphene Oxide Used as Solid

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Electrochemically Activated Reduced Graphene Oxide Used as SolidState Symmetric Supercapacitor: An X‑ray Absorption Spectroscopic Investigation Han-Wei Chang,†,‡ Ying-Rui Lu,†,‡,§ Jeng-Lung Chen,‡ Chi-Liang Chen,‡ Jin-Ming Chen,‡ Yu-Chen Tsai,*,∥ Wu Ching Chou,⊥ and Chung-Li Dong*,† †

Department of Physics, Tamkang University, Tamsui 25137, Taiwan National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan § Program for Science and Technology of Accelerator Light Source, National Chiao Tung University, Hsinchu 30010, Taiwan ∥ Department of Chemical Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan ⊥ Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan ‡

ABSTRACT: A solid-state symmetric supercapacitor is successfully fabricated by assembling the sulfuric acid− poly(vinyl alcohol) (H2SO4−PVA) gel electrolyte between the two pieces of electrochemically activated reduced graphene oxide (aRGO) electrodes. The electrochemical and electronic properties of reduced graphene oxide (RGO) and aRGO were characterized by ex situ X-ray absorption spectroscopy (XAS). The aRGO exhibits better electrochemical supercapacitive performances than RGO, owing to the conjunction of the electrochemical double-layer capacitance (EDLC) and the pseudocapacitance. The specific capacitance increases with the oxygen-containing functional groups content in the electrochemically activated aRGO than RGO, increasing the pseudocapacitive contribution. The aRGO20//aRGO20 solid-state symmetric supercapacitors (SSC) exhibit an energy density of 4.7 Wh kg−1 at a power density of 402 W kg−1 and 4 Wh kg−1 at a power density of 1989 W kg−1, which is competitive with the commercially available supercapacitors. To elucidate the atomic and electronic structures of the RGO and aRGO electrodes in the charge/discharge process, ex situ XAS at the C and O K-edge were performed. Both RGO and aRGO electrodes exhibit a reversible energy shift of the O K-edge, owing to the reversible redox pseudocapacitance close to the surface of graphene-based materials. However, the absorption edge of O K-edge for aRGO electrodes shifts more significantly than that for RGO electrodes during the charge/discharge process. The results indicate that aRGO electrodes exhibit rapid and more active redox reactions for pseudocapacitance than the RGO electrodes. The results herein suggest potential use in high-performance symmetric supercapacitors.

1. INTRODUCTION The electrochemical capacitor (supercapacitor) is a promising energy storage devices on account of the high power density, great cycling stability, swift charge/discharge rate, compactness, and high energy efficiency.1−3 Two charge-storage mechanisms for supercapacitors are electrochemical double-layer capacitance (EDLC) and pseudocapacitance. EDLC includes nonFaradaic ion adsorption/desorption near the electrode− electrolyte interface. Pseudocapacitance involves fast and reversible electrochemical Faradaic redox reactions.4,5 Electrode materials importantly affect supercapacitor performance. In recent years, carbon-based nanomaterials, such as activated carbon, carbon nanotube (CNT), and graphene, have been widely studied as electrode materials for use in supercapacitors. These forms of carbon typically exhibit good electrical conductivity, a large specific surface area, and excellent chemical stability and so are ideal for use in rapid energy conversion and © 2016 American Chemical Society

storage devices. Two-dimensional (2D) graphene has been widely utilized in powerful nanomaterials for supercapacitors owing to its unique electrical and mechanical properties. The theoretical capacitance of graphene-based materials is calculated to be 550 F g−1, which exceeds most reported values for other carbon-based nanomaterials.6 Graphene conventionally tends to undergo intrinsic restacking and agglomeration during its preparation, which reduces its specific surface area and electrolyte ionic accessibility, lowering its capacitance below its theoretical value. Therefore, increasing the effective surface area to improve supercapacitor performance is essential. Another approach to advance the supercapacitor performance of graphene-based electrodes is the introduction of heteroatomReceived: May 16, 2016 Revised: September 11, 2016 Published: September 12, 2016 22134

DOI: 10.1021/acs.jpcc.6b04936 J. Phys. Chem. C 2016, 120, 22134−22141

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this work, the electrochemically activated reduced graphene oxide (aRGO) was prepared by electrochemical activation treatments. Reduced graphene oxide (RGO) and aRGO as supercapacitors were investigated in detail during charge/ discharge processes using ex situ XAS and electrochemical measurements, to elucidate charge insertion/extraction, local electronic structure, and the effect caused by oxygen-containing functional groups. The excellent supercapacitor performance of aRGO is attributable to the formation of additional oxygencontaining functional groups by electrochemical activation, significantly enhancing Faradaic redox at the interface of electrode−electrolyte.

containing functional groups (nitrogen- and oxygen-containing functional groups), which expand the area of active sites for the Faradaic redox reaction, increasing pseudocapacitance.7−9 Graphene-based materials with a large effective surface area and heteroatom-containing functional groups exhibit both EDLC and pseudocapacitance. Intensive research has focused on increasing effective surface area and fast Faradaic redox reactions (pseudocapacitance), corresponding to the Faradaic redox of oxygen-containing functional groups via activation treatments. The surface properties of graphene-based materials can be changed using different activation treatments. The activation steps were conducted to create active sites on the surface to introduce heteroatom-containing functional groups, but these steps also caused some damage or structural changes to the frameworks of graphene-based materials, significantly improving their capacitive properties. Ning et al. synthesized porous nitrogen-doped graphene/carbon electrode materials via chemical activation with potassium hydroxide (KOH). The prepared electrode materials have high specific surface areas and high heteroatom contents. As expected, they exhibit a markedly enhanced specific capacitance of 405 F g−1 at a low current density of 0.2 A g−1 and 249 F g−1 at a high current density of 10 A g−1. The high capacitance is mainly attributable to the conjunction of EDLC and pseudocapacitance. The redox reactions of heteroatom-containing functional groups are responsible for most of the pseudocapacitance.10 Fan et al. prepared a graphene-incorporating carbon composite by the simple hydrothermal reaction of glucosamine and an activation reaction with KOH. The graphene-incorporating carbon composite exhibited an enhanced specific capacitance of 300 F g−1 at 0.1 A g−1 in 6 M KOH, suggesting that the material can be used more efficiently in future applications owing to the large surface area of the carbon that is obtained by KOH activation and the pseudocapacitance (associated with the redox reactions of oxygen-containing functional groups).11 Sun et al. investigated activated graphene aerogel (aGA) that was activated using urea and phosphoric acid (H3PO4). The aGA performed excellently in energy storage, with a high energy density, high specific capacitance (204 F g−1 at 0.2 A g−1), increased rate capability (69% retention from 0.2 to 30 A g−1), decreased equivalent series resistance (∼3.8 mΩ) and lessened time constant (∼0.73 s). The aGA was chemically activated using urea and H3PO4 to generate nanoporous and further increase the pseudocapacitance contribution owing to oxygen containing functional groups and large specific surface area. Hence, the capacitive performance of aGA was improved by increasing the ion-accessible surface area and the rate of electrolyte diffusion.12 Various activation processes have been used in studies of supercapacitors to modify the surface properties of graphene-based nanomaterials to optimize their capacitive performance. Additionally, the capacitive performance characteristics of a supercapacitor evidently depend strongly on the variations in local atomic and electronic environments of the particular element under working conditions. X-ray absorption spectroscopy (XAS) is an extremely useful approach for studying the local structural and chemical environments of the particular element. It deals with core electrons excitation to the unoccupied electronic states so is element-selective and sensitive to electronic/atomic environment of the active element. Thus, XAS has been employed to study the electronic and atomic structures of energy relevant materials and has been utilized to observe changes in charge state under operational conditions.13−15 In

2. EXPERIMENTAL SECTION 2.1. Reagents. Graphite powder, urea, and ascorbic acid (AA) were obtained from Sigma−Aldrich. All chemicals were of analytic level and used without additional purification. All solutions were prepared by a water purification system (MilliQ, USA). 2.2. Preparation of RGO and aRGO. Graphene oxide (GO) was fabricated by a modified Hummers method.16 First, 10 mg of GO was ultrasonically dispersed in 10 mL of deionized water for approximately 15 min to form the GO solution. GO can be converted to RGO under mild conditions using AA.17 Following this conversion, AA (100 mg) was added into a GO dispersion (0.1 mg mL−1) for 24 h. The aRGO was prepared following a procedure similar to that described in the literature.18 The RGO-modified glassy carbon working electrode (GCE, diameter ∼3 mm) was submerged in nitric acid (HNO3) (0.2 M), and potential scanning was conducted at a scan rate of 50 mV s−1 from potential 1 to 2 V for different numbers of electrochemical activation cycles (10, 20, 40, and 60 cycles); the composites thus obtained were called aRGOx (x = 10, 20, 40, and 60), respectively. The resultant materials were rinsed using deionized water several times for residual reagent removal and subsequently characterized. 2.3. Fabrication of Solid-State Symmetric Supercapacitors. The solid-state symmetric supercapacitor was prepared by a two-electrode system using sulfuric acid− poly(vinyl alcohol) (H 2 SO 4−PVA) gel electrolyte and aRGO20. The gel electrolyte was made by mixing PVA (1 g) and H2SO4 (1 g) in deionized water (10 mL) at 85 °C and stirring to form a clear solution. Then the two aRGO20 electrodes were submerged in the gel electrolyte for about 30 min at room temperature. Two electrodes were assembled faceto-face and left overnight under ambient conditions until the electrolyte solidified. 2.4. Characterization. The morphology was characterized by using a Field emission scanning electron microscopy (FESEM, JEOL, JSM-7410F, Japan). The chemical composition was determined by X-ray photoelectron spectroscopy (XPS, PHI-5000 Versaprobe, ULVAC-PHI, Japan). Electrochemical measurements, cyclic voltammetry (CV), and galvanostatic charge/discharge were obtained using an electrochemical analyzer (Autolab, model PGSTAT30, Eco Chemie, Netherlands). The conventional three-electrode system was comprised of as-made samples that were coated on the GCE, a platinum wire as a counter electrode, and an Ag/AgCl (3 M KCl) reference electrode in 1 M H2SO4. The solid-state symmetric supercapacitor was formed by assembling H2SO4− PVA between the two aRGO20-modified screen-printed carbon electrodes (SPCE, 3 mm diameter) and the performance was examined by conventional two-electrode system. The mass 22135

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Figure 1. FESEM images of (a) RGO and (b) aRGO20.

Figure 2. XPS spectra of (a) RGO and (b−e) aRGO with different electrochemical activation cycles (aRGO10, aRGO20, aRGO40, and aRGO60). (f) Table shows the ratio of the peak intensity of CO to C−O of RGO and aRGO.

3. RESULTS AND DISCUSSION

loading of the as-made samples that were coated on the electrodes was determined using a microbalance. To understand better the energy storage mechanisms and the performance characteristics of supercapacitors during charging− discharging, the modifications of the local electronic structures and chemical states of the prepared electrodes (RGO and aRGO20) upon the charge/discharge under three applied potentials (0.0, + 0.8, and 0.0 V (r0.0 V)), were determined by ex situ XAS. Synchrotron XAS spectra were obtained at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan, operated at an energy of 1.5 GeV with a storage current ∼300 mA. To examine the amount of functional groups in the aRGO, XAS was carried out with sample drain current mode, rather than fluorescence yield mode, in which a saturation effect generally distorts the XAS signal. XAS at C and O K-edges were made at beamline BL20A.

The surface morphologies of RGO and aRGO20 were examined by FESEM. Figure 1 compares the FESEM images of RGO and aRGO20. The morphology of RGO is similar to that of electrochemically activated aRGO20. Both RGO and aRGO20 composites have a uniform wrinkled 2D sheet-shaped structure, which shortens and facilitates the transport of electrolyte ions. Additionally, RGO and aRGO20 are expected to have different oxygen-containing functional groups. The one with the more oxygen functional groups may exhibit a higher effective surface area and so perform better as a supercapacitor. To determine the amount and effect of oxygen-containing functional groups in aRGO20 after activation, RGO and aRGO20 were analyzed using XPS and XAS. XPS is a useful tool for characterizing the variation in the surface functional groups of carbon nanomaterials. Figure 2 22136

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functional groups that correspond to C−O π* (hydroxyl or ether), CO π* (carbonyl groups), and OC−O π* (anhydride, lactone, or carboxylic acids).23,24 The presence of C−C π* and C−C σ* reveals that the RGO and aRGO have a similar long-range order to that in the electronic structure of pristine graphene, indicating that both samples are favorable to capacitive performance. Comparing the C K-edge XAS spectra of RGO and aRGO reveals that the latter exhibits much more intense features (especially B3a, but also B3a′ and B3a″) as the number of activation cycles increases. This result indicates that structural defects in aRGO have a significantly greater effect than those in RGO and the enhancement of the bonding of the carbonyl groups upon electrochemical activation gives rise to a strong interaction between oxygen-containing functional groups and RGO. Figure 3b displays the O K-edge of RGO and aRGO following different numbers of cycles of electrochemical activation. The distinctive strucutres in the O K-edge of RGO and aRGO at approximately 530.0−550.0 eV correspond to different oxygen-containing functional groups. Feature A3b at 532.8 eV is associated with electron excitations from O 1s core level to CO π* states of carbonyl groups that are bonded to an aromatic ring at the RGO edge sites. Feature B3b at 540.0−545.0 eV is attributable to the transitions of the O−H σ*, C−O σ* and CO σ* symmetry states from carbonyl, carboxylic acid, and hydroxyl groups that are bonded to an aromatic ring at RGO basal sites.25 The aRGO exhibited a much less intense σ* transition than RGO. Clearly, the intensity of feature B3b gradually declines as the number of activation cycles increases whereas the intensity of feature A3b increases slightly (inset in Figure 3b). The increase in intensity of feature A3b is attributed to CO π* groups at the RGO edge sites following HNO3 activation, while the decrease in the intensity of feature B3b implies the removal of oxygencontaining functional groups σ* symmetry in RGO. The results demonstrate that the π*/σ* peak intensity ratio increases upon activation treatment, suggesting that the carbonyl groups are π*-bonded to RGO edge sites, probably because of the formation of ring ethers as furans, pyrans, and ketones in the form of benzoquinones.26 Additionally, the literature indicates that the electrochemical performance and capacitive performance of edge plane carbon materials are much greater than those of basal plane carbon materials.27,28 Therefore, introducing more carbonyl groups at aRGO edge sites with HNO3 activation, resulting in a strong redox reaction, has potential to improve capacitive performance, based on XAS evidence. This claim is consistent with the XPS results. The specific capacitance, working voltage, electrochemical cycling and stability determine the energy that can be stored in the electrochemical capacitors. Therefore, the atomic and electronic structures of RGO and aRGO in the working potential window are of great importance. The role of each constituent element in the electrode during electrochemical reaction needs to be understood before a supercapacitor with high electrochemical stability and reversibility can be designed. Thus, to reveal the electronic structural changes of graphenebased materials and the electrolyte interfacial phenomenon during the charge/discharge, ex situ XAS was conducted. Parts a and b of Figure 4 display the O and C K-edge ex situ XAS spectra of RGO and aRGO20 electrodes during the charge/ discharge process with three applied potentials in the order 0.0, + 0.8, and 0.0 V (r0.0 V). Notably, the spectral profile in Figure 4a changes with the bias potential applied to the electrode. In the charge process, feature B4a is shifted to lower energy and

presents the XPS of RGO and aRGO with different electrochemical activation cycles (aRGO10, aRGO20, aRGO40 and aRGO60). In parts a−e of Figure 2, the XPS spectra of C 1s are fitted by four main peaks, which are originated from C−C (sp2), C−C (sp3), C−O (epoxy and alkoxy), and CO groups, at 284.4, 285.0, 286.1, and 288.4 eV, respectively.8 The table in Figure 2f presents the ratio of the XPS fitted peak intensities of CO to C−O for RGO and aRGO. These observations suggest the presence of oxygencontaining functional groups in the form of sp3-hybridized carbons throughout the basal plane and the edge of the graphene-based materials. Electrochemical activation increases the ratio of the peak intensity of CO to C−O, demonstrating that aRGO contains more carbonyl groups than RGO, revealing that most additional carbonyl groups are bonded to the surface of RGO during electrochemical activation. The carbonyl groups on aRGO can result in charge insertion/extraction without ion exchange, which is effective in promoting the capacitance. The preparation of aRGO in this way greatly improves its capacitance as the carbonyl groups add pseudocapacitance.19−21 The aRGO was prepared by electrochemically activating RGO, giving rise to modification of the surface chemical environment of RGO. As a surface sensitive and element-selective approach, soft XAS can be utilized to elucidate the change in the electronic structure of aRGO upon HNO3 activation. Parts a and b of Figure 3 present the XAS at C K-edge and O K-edge,

Figure 3. XAS spectra of (a) C K-edge, and (b) O K-edge of RGO, aRGO10, aRGO20, aRGO40, and aRGO60. Insets in parts a and b magnify spectra in energy ranges 285−290 and 528−535 eV, respectively.

respectively, of RGO and aRGO with different electrochemical activation cycles (aRGO10, aRGO20, aRGO40 and aRGO60). The C K-edge has several distinct features (A3a, B3a, and C3a) (Figure 3a). Features A3a and C3a at 284.6 and 290.9−291.9 eV are associated with the transitions from C 1s to the unoccupied π* and σ* orbitals, respectively, reflecting the bonding perpendicular to the ring plane and within the ring plane. A weak and nearly unresolved feature (A3a′) at 283 eV on the lowenergy side of peak A3a is associated with quinone-like species that broaden the C−C π* peak.22 The inset in Figure 3a magnifies the area in the energy range 285.5−290 eV. Clearly, three peaks B3a′ (286.3 eV), B3a (287.5 eV), and B3a″ (289.1 eV) are associated with chemically functionalized carbon atoms and/or defects in the graphene, which are attributable to interlayer states or transitions from the oxygen-containing 22137

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in the charge/discharge process, implying that the aRGO electrode may enhance the combined effect of the EDLC and pseudocapacitive contributions, improving the capacitive characteristics since most of the carbonyl groups are at the aRGO edge sites. Figure 4b compares the C K-edge ex situ XAS spectra of the RGO and aRGO20 electrodes. The presence and absence of the weak and nearly unresolved feature A4b′ at 283 eV in the spectra of aRGO20 that are obtained in the charged and discharged states, respectively, are observed, suggesting that the aRGO has more quinone groups on account of the HNO3 activation than does RGO, and so a fast and reversible Faradaic quinone/hydroquinone-like redox reaction may occur at the aRGO electrode−electrolyte interface. Relevant results further suggest that aRGO electrodes are more active than RGO electrodes, and so may exhibit better supercapacitive performance. To examine the supercapacitive performance of RGO, aRGO10, and aRGO20, CV measurements were made. Figure 5a compares the CV curves of RGO, aRGO10, and aRGO20 electrodes that were performedwith a potential window of 0 to 0.8 V against Ag/AgCl in 1 M H2SO4 solution at a scan rate of 50 mV s−1. The CV curves of all the graphene-based materials exhibit quasi-rectangular and these materials all yielded a pair of evident redox peaks. The appearance of humps in the CV profile revealed both EDLC and pseudocapacitive contributions to the total capacitance.29 The CV curve of aRGO has a much larger area than that of RGO, suggesting that it exhibits a higher specific capacitance. The current density gradually increases with the number of electrochemical activation cycles, indicating that provides an additional pseudocapacitance contribution in aRGO. To elucidate in detail the capacitive behaviors of RGO and aRGO, a galvanostatic charge/discharge analysis was carried out. Figure 5b compares the galvanostatic charges/ discharges of RGO, aRGO10, and aRGO20 electrodes at a

Figure 4. Ex situ XAS spectra of (a) O K-edge, and (b) C K-edge of RGO and aRGO20 electrodes during charge/discharge process under three applied potentials in the order 0.0, + 0.8, and 0.0 V (r0.0 V).

the intensity of feature A4a declines, which phenomena are attributable to the removal of CO groups from the edge sites of graphene-based materials. In the discharge process, feature B4b is shifted back to higher energy. An increase in the intensity of feature A4a was observed, revealing that CO groups were attached to the edge sites of the graphene-based materials. The shift in feature B4a and the variations in intensity of the XAS spectra at the O K-edge indicate favorable structural and chemical reversibility during the charge/discharge process, suggesting that the oxygen-containing functional groups provide pseudocapacitance through Faradaic reactions that occur near the surface of the graphene-based materials: >C− OH ↔ CO + H+ + e−, − COOH ↔ − COO + H+ + e−, and >CO + e− ↔ >C−O−.3 Notably, the shift of feature B4a in the aRGO electrodes is larger than that of the RGO electrodes

Figure 5. (a) CV (at 50 mV s−1), (b) galvanostatic charge/discharge (at 2 A g−1), (c) EIS, and (d) cyclic stability (at 40 A g−1) of RGO, aRGO10, and aRGO20 electrodes with a potential window of 0 to 0.8 V against Ag/AgCl in 1 M H2SO4 solution. (e) CV of aRGO20 electrodes at scan rates from 5 to 600 mV s−1 in 1 M H2SO4 solution. (f) Galvanostatic charge/discharge of aRGO20 electrodes at current densities from 0.5 to 40 A g−1 in 1 M H2SO4 solution. 22138

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The Journal of Physical Chemistry C current density of 2 A g−1 with a potential window of 0 to 0.8 V against Ag/AgCl in 1 M H2SO4 solution. The charge curves of RGO and aRGO were almost symmetrical and triangular with a slight curvature, perhaps because of the combination of the contributions to EDLC and pseudocapacitance by the presence of oxygen-containing groups at RGO and aRGO.11 This finding is in accord with the CV curves. The specific capacitances of the RGO and aRGO electrode are evaluated using the following formula. C = (IΔt)/(ΔV m), where C, I, Δt, ΔV, and m denote specific capacitance (F g−1), charge/discharge current (A), discharge time (s), potential change during discharge (V), and the mass of the active materials in the electrode (g). Both aRGO electrodes exhibited much higher specific capacitance than the RGO electrodes at the same current density because the aRGO electrodes have more oxygen-containing functional groups, and therefore a significantly higher pseudocapacitance. The specific capacitances of RGO, aRGO10, and aRGO20 are determined to be 161, 225, and 298 F g−1 at 2 A g−1, respectively. This specific capacitance increases with the number of electrochemical activation cycles, suggesting that the increase is mainly attributable to the additional pseudocapacitance that is provided by the large number carbonyl and quinone groups. Figure 5c compares the electrochemical impedance spectra of RGO, aRGO10, and aRGO20 electrodes in 1 M H2SO4 solution. All electrodes show a straight line at low frequencies and exhibits favorable capacitive property. The equivalent series resistance (ESR) that is derived from the intercept of the real axis comprises the electrolyte/electrode interface resistance, the intrinsic resistance of electrode, and the contact resistance between current collector and electrode. At high frequencies, ESR increases from 6 to 7 Ω cm2 with the number of activation cycles. The small ESR results in a low voltage drop and high-rate capability owing to the wrinkled 2D sheet-shaped architecture, suggesting that RGO and aRGO are in efficient contact with the electrolyte, favoring the rapid transfer of electrons and ions.30 The slightly higher resistance of both aRGO electrodes reveals CO vibrations in carbonyl groups following the activation of RGO, which provide more pseudocapacitance from surface redox reactions. The long-term cycling performance of RGO and aRGO electrodes was assessed with galvanostatic charge/ discharge at a high current density. Figure 5d compares the charge/discharge processes of RGO, aRGO10, and aRGO20 electrodes over 1000 cycles at a current density of 40 A g−1 from 0 to 0.8 V in 1 M H2SO4 solution. Approximately 95− 99% of the specific capacitance of all electrodes remained over 1000 cycles, indicating permanence of electrochemical performance for all graphene-based electrodes. Figure 5e plots the CV curve of aRGO20 electrodes at a series of scan rates from 5 to 600 mV s−1 in 1 M H2SO4 solution. The current response of aRGO20 rises as the scan rate increases, demonstrating favorable capacitive property. The superior CV characteristic at such a high scan rate suggests a swift current response to reverse voltage at both ends of potential window. The almost straight sides of the rectangle reflect a low equivalent series resistance for the electrode and the rapid electrolyte ions diffusion between the graphene-based materials and the electrolyte. Figure 5f plots the galvanostatic cycling of aRGO20 electrodes, obtained at a series of current densities from 0.5 to 40 A g−1 in 1 M H2SO4 solution. The aRGO20 has a specific capacitance of 512 F g−1 at 0.5 A g−1 because aRGO20 has a uniform wrinkled 2D sheet-shaped structure and

differences in the oxygen-containing functional groups enable rapid and effective electron transport. To evaluate further the electrochemical performance of the aRGO20 electrodes for practical use, an aRGO20//aRGO20 solid-state symmetric supercapacitors (SSC) device and H2SO4−PVA gel electrolyte, were fabricated. Figure 6a

Figure 6. (a) CV, (b) galvanostatic charge/discharge, (c) specific capacitance against current density, and (d) Ragone plot (power density vs energy density) of the aRGO20//aRGO20 solid-state SSC device.

compares the CV curves of aRGO20//aRGO20 solid-state SSC devices that were recorded at scan rates of 10, 20, 50, and 100 mV s−1. The galvanostatic cycling were made at current densities from 1 to 5 A g−1, as displayed in Figure 6, parts b and c. The CV curves are nearly rectangular, and the current density gradually increases as the scan rate raises, revealing the ideally capacitive and rapid charge/discharge behavior. Specific capacitances of 53, 51, and 46 F g−1 were obtained at current densities of 1, 2, and 5 A g−1, respectively. Additionally, Figure 6d presents the Ragone plot of the aRGO20//aRGO20 solidstate SSC device that was derived from the results of galvanostatic charge/discharge. The energy density (E) and the power density (P) were derived from the formula, E = 0.5 C ΔV2 and P = EΔt−1. The aRGO20//aRGO20 solid-state SSC device exhibited an energy density of 4.7 Wh kg−1 (4 Wh kg−1) at a power density of 402 W kg−1 (1989 W kg−1). The aRGO20//aRGO20 solid-state SSC device was used to power the light-emitting-diode (LED). Parts a and b of Figure 7 plot the CV curves and galvanostatic charge/discharge of a single aRGO20//aRGO20 solid-state SSC device and three aRGO20//aRGO20 solid-state SSC devices in series. The three aRGO20//aRGO20 solid-state SSC devices connected in series exhibited an output voltage of 2.4 V. Furthermore, the LED can be turned on using a device that comprises three aRGO20//aRGO20 solid-state SSC devices connected in series, indicating that such as-made electrode materials have commercial potential for use in supercapacitor devices. The 22139

DOI: 10.1021/acs.jpcc.6b04936 J. Phys. Chem. C 2016, 120, 22134−22141

Article

The Journal of Physical Chemistry C

Figure 7. (a) CV (at 50 mV s−1) and (b) galvanostatic charge/discharge (at 5 A g−1) of a single aRGO20//aRGO20 solid-state SSC device and three aRGO20//aRGO20 solid-state SSC devices in series by assembling the H2SO4−PVA gel electrolyte. (c) Photograph of LED lit using three aRGO20//aRGO20 solid-state SSC devices in series. Nitrogen-Doped Reduced Graphene Oxide in Acidic and Alkaline Electrolytes. J. Power Sources 2013, 227, 300−308. (5) Peng, Y. Y.; Liu, Y. M.; Chang, J. K.; Wu, C. H.; Ger, M. D.; Pu, N. W.; Chang, C. L. A Facile Approach to Produce Holey Graphene and Its Application in supercapacitors. Carbon 2015, 81, 347−356. (6) Liu, C.; Yu, Z.; Neff, D.; Zhamu, A.; Jang, B. Z. Graphene-Based Supercapacitor with An Ultrahigh Energy Density. Nano Lett. 2010, 10, 4863−4868. (7) Kerisit, S.; Schwenzer, B.; Vijayakumar, M. Effects of OxygenContaining Functional Groups on Supercapacitor Performance. J. Phys. Chem. Lett. 2014, 5, 2330−2334. (8) Khandelwal, M.; Kumar, A. One-Step Chemically Controlled wet Synthesis of Graphene Nanoribbons from Graphene Oxide for High Performance Supercapacitor Applications. J. Mater. Chem. A 2015, 3, 22975−22988. (9) Lai, L.; Yang, H.; Wang, L.; Teh, B. K.; Zhong, J.; Chou, H.; Chen, L.; Chen, W.; Shen, Z.; Ruoff, R. S.; et al. Preparation of Supercapacitor Electrodes through Selection of Graphene Surface Functionalities. ACS Nano 2012, 6, 5941−5951. (10) Ning, X.; Zhong, W.; Li, S.; Wang, Y.; Yang, W. High Performance Nitrogen-Doped Porous Graphene/Carbon Frameworks for Supercapacitors. J. Mater. Chem. A 2014, 2, 8859−8867. (11) Fan, X.; Yu, C.; Yang, J.; Ling, Z.; Qiu, J. Hydrothermal Synthesis and Activation of Graphene-Incorporated Nitrogen-Rich Carbon Composite for High-Performance Supercapacitors. Carbon 2014, 70, 130−141. (12) Sun, X.; Cheng, P.; Wang, H.; Xu, H.; Dang, L.; Liu, Z.; Lei, Z. Activation of Graphene Aerogel with Phosphoric Acid for Enhanced Electrocapacitive Performance. Carbon 2015, 92, 1−10. (13) Cui, Y.; Abouimrane, A.; Lu, J.; Bolin, T.; Ren, Y.; Weng, W.; Sun, C.; Maroni, V. A.; Heald, S. M.; Amine, K. (De)Lithiation Mechanism of Li/SeSx (x = 0−7) Batteries Determined by in Situ Synchrotron X-ray Diffraction and X-ray Absorption Spectroscopy. J. Am. Chem. Soc. 2013, 135, 8047−8056. (14) Ogasawara, Y.; Hibino, M.; Kobayashi, H.; Kudo, T.; Asakura, D.; Nanba, Y.; Hosono, E.; Nagamura, N.; Kitada, Y.; Honma, I.; et al. Charge/Discharge Mechanism of A New Co-Doped Li2O Cathode Material for A Rechargeable Sealed Lithium-Peroxide Battery Analyzed by X-ray Absorption Spectroscopy. J. Power Sources 2015, 287, 220− 225. (15) Yogi, C.; Takamatsu, D.; Yamanaka, K.; Arai, H.; Uchimoto, Y.; Kojima, K.; Watanabe, I.; Ohta, T.; Ogumi, Z. Soft X-ray Absorption Spectroscopic Studies with Different Probing Depths: Effect of An Electrolyte Additive on Electrode Surfaces. J. Power Sources 2014, 248, 994−999. (16) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (17) Zhang, J.; Yang, H.; Shen, G.; Cheng, P.; Zhang, J.; Guo, S. Reduction of Graphene Oxide Vial-Ascorbic Acid. Chem. Commun. 2010, 46, 1112−1114.

performances of the device are believed to be able to be further improved using a large-area manufacturing technique.

4. CONCLUSION The supercapacitive properties of RGO and aRGO electrodes were determined by electrochemical and ex situ XAS. The aRGO electrodes exhibit differences of changes in the oxygencontaining functional groups following electrochemical HNO3 activation, which provide additional pseudocapacitance in the charge/discharge cycling owing to the occurrence of redox reactions. The results also reveal the effectiveness of ex situ XAS in shedding light on the mechanisms that are associated with charge storage in aRGO electrodes. Accordingly, aRGO20// aRGO20 solid-state SSC devices were also prepared and used to drive an LED. On the basis of these findings, aRGO is a highly promising candidate for use in supercapacitor devices.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +886-4-22857257. Fax: +886-4-22854734. E-mail: [email protected] (Y.-C.T.). *Telephone: +886-2-26215656 x3152. Fax: +886-2-26209917. E-mail address: [email protected] (C.-L.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology (MoST) of Taiwan, for financial support under Contracts MoST 104-2112-M-032-008-MY3 and 104-2923-M032-001-MY3.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b04936 J. Phys. Chem. C 2016, 120, 22134−22141

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DOI: 10.1021/acs.jpcc.6b04936 J. Phys. Chem. C 2016, 120, 22134−22141