Improving energy density of supercapacitors by ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Template Synthesis of 2D Carbon Nanosheets: Improving Energy Density of Supercapacitors by Dual Redox Additives Anthraquinone2-sulfonic Acid Sodium and KI Xiao Na Sun, Dong Xu, Wei Hu, and Xiang Ying Chen* School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China S Supporting Information *

ABSTRACT: How to massively produce 2D carbon materials remains an interesting issue. In this work, by a template carbonization approach, 2D carbon nanosheets possessing large surface area of 1229 m2 g−1 and high pore volume of 1.66 cm3 g−1 have been synthesized, using sodium stearate as the carbon source and magnesium powder as a hard template. More importantly, dual redox additives KI and anthraquinone-2-sulfonic acid sodium (AQS) are developed for remarkably improving the capacitances; neutral KNO3 electrolyte can synchronously extend the operating voltage window. It is revealed that KI and AQS undergo redox reactions at the positive and negative electrodes, respectively; the operating voltage window can be prolonged up to 1.8 V, larger than the limitation of water (1.23 V). Introducing dual redox additives KI and AQS into KNO3 electrolyte results in a high energy density of 33.81 Wh kg−1. The present strategy that utilizes dual redox additives occurring at the positive and negative electrodes respectively has provided us a simple but effective avenue for highly improving the energy density of supercapacitors. KEYWORDS: Carbon nanosheets, Dual redox additives, KNO3 electrolyte, Energy density

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INTRODUCTION As energy-storage devices, supercapacitors have attracted a huge amount of attention due to their high power densities, long cycling lives, and high rate.1−4 However, the energy densities of supercapacitors are lower than those of batteries and fuel cells,5,6 and this severely handicaps their application domain. Therefore, in recent years, a lot of research work has been aimed at improving the energy densities of supercapacitors.7,8 It has been demonstrated that nanoporous carbon materials, in particular the case of 2D carbon nanosheets with large surface area and high pore volume, are crucial to the realization of high-energy/-power supercapacitors.9−11 Besides, based on the equation for the energy density of a supercapacitor, E = 1/2CV2, it is clear that the energy density is proportional to the capacitance (C) and the square of the operating voltage window (V). As for as the enhancement of capacitance is concerned, an interesting and alternative approach has been developed to enhance the energy densities of supercapacitors by simply introducing a small amount of redox additives into the electrolyte. In general, the redox additives can be primarily categorized into inorganic (KI, KBr, © 2017 American Chemical Society

VOSO4, K4Fe(CN)6, CuSO4, FeSO4, etc.) and organic species (hydroquinone, p-phenylenediamine, methylene blue, indigo carmine, etc.).12,13 Moreover, these redox additives have been proven to significantly improve the specific capacitances and energy densities of supercapacitors via reversible redox reactions at the electrode/electrolyte interface, but it is worth noting that these redox additives can only undergo reversible redox reactions at either the positive or negative electrode. For instance, the halogens, iodide and bromide ions, bring the occurrence of redox reactions just on the positive electrode. In addition, K4Fe(CN)6 and hydroquinone make the redox reactions happen only at the positive electrode.14 Analogously, VOSO4 is suitable for the negative electrode, whereas it is not effective at the positive electrode. For acquiring higher energy density, dual redox additives, showing excellent redox active behaviors at the positive and negative electrodes, respectively, have drawn intensive interest, since the total capacitance of a Received: March 11, 2017 Revised: May 18, 2017 Published: June 5, 2017 5972

DOI: 10.1021/acssuschemeng.7b00759 ACS Sustainable Chem. Eng. 2017, 5, 5972−5981

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ACS Sustainable Chemistry & Engineering

pressure of 20 MPa, serving as the current collector. Finally, the prepared electrodes were dried at 110 °C for 12 h in a vacuum oven. Preparation of Electrolyte. A series of mixed electrolytes were prepared by adding different contents of AQS (2.5, 5, 10 mmol L−1) and KI (10 mmol L−1) in 1 mol L−1 KNO3 solution under magnetic stirring at room temperature, and the corresponding resultant electrolytes were named AQS-2.5/5/10 and KI-10. The mixed electrolyte consisting of AQS and KI with different mole ratios were also prepared, and the electrolyte was named AQS:KI-2:1/1:1/1:2. As for the pristine sample without any redox additive, we refer to it as Carbon-blank. Characterizations and Measurement Techniques. The structure characterization and measurement techniques are given in the Supporting Information. Parameters Calculations. The detailed calculation equations employed in this work are illustrated in the Supporting Information.

supercapacitor is determined by both electrodes. To achieve the above objective, Frackowiak et al. used KI aqueous solution as electrolyte for the positive electrode and VOSO4 solution as electrolyte for the negative one.15 Likewise, Fan et al. introduced KI/VOSO4 redox additives into the PVA-H2SO4 gel electrolyte to increase the energy density of a single device.16 To date, the intrinsic redox mechanism synchronously occurring both at the negative and positive electrodes especially toward the dual redox additives is still unclear. Therefore, further exploring and understanding this kind of redox mechanism is interesting for the practical application of supercapacitors. Moreover, the energy densities of supercapacitors can also be improved through enhancing the operating voltage. As we know, the operating voltage of a supercapacitor mainly relies on the applied electrolyte, and it cannot exceed 1.0 V because of the thermodynamic decomposition of water at 1.23 V and low overpotential for hydrogen evolution when using traditional acidic or alkali electrolyte in supercapacitor system. Comparatively, organic electrolytes can provide higher operating voltage (up to 2.5−3.0 V). However, there are also some disadvantages for organic electrolytes in terms of safety concerns, low power delivery, and high viscosity. Additionally, their fabrication must be handled in an atmosphere free of water and oxygen, which limits their commercial applications.1 On the basis of the above analysis, we therefore selected a neutral electrolyte (KNO3) that is expected to achieve a broad operating voltage window (up to 1.5−2.0 V) to further increase the energy density.17,18 In this study, 2D carbon materials are first prepared by the template carbonization method, using sodium stearate as the carbon source and magnesium powder as a hard template. Subsequently, we proposed the dual redox additives KI and AQS, which were added into the neutral electrolyte of KNO3, and it is thus expected that the capacitance and the resultant energy density could be markedly improved by the synchronous dual redox reactions respectively occurring at the positive and negative electrodes. Furthermore, the KNO3 electrolyte provides a broad operating voltage window of 1.8 V, which could further improve the energy density of supercapacitor.

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RESULTS AND DISCUSSION Physical Characterizations. The microstructure and morphology of the Carbon-blank sample were examined by FESEM and HRTEM techniques. Figure 1a exhibits that the

Figure 1. Carbon-blank sample: (a, b) FESEM images; (c, d) HTREM images as well as the SAED pattern.

Carbon-blank sample has agaric-like morphology with a size of several hundreds of micrometers. Furthermore, interconnected and ultrathin carbon nanosheets can be observed in the highmagnification SEM image in Figure 1b. The microstructure of the Carbon-blank sample is also confirmed by HRTEM technique. As depicted in Figure 1c, the Carbon-blank sample shows irregular wrinkled structure with curvature of the carbon nanosheets. Besides, Figure 1d reveals that the Carbon-blank sample has no obvious ordered lattice fringe, implying its amorphous structure. The SAED pattern (the inset of Figure 1d) further evidences the amorphous structure of the Carbonblank sample. As a consequence, the present work has provided one quite simple but effective template carbonization method to produce 2D carbon nanosheets, which probably possesses the potential application in supercapacitors. XRD and Raman spectroscopy have been employed to investigate the phases and crystallinities of the Carbon-blank sample in present work. Figure 2a presents a broad peak located at ca. 23.1°, corresponding to the graphitic (002) planes, which shows the amorphous feature of the Carbonblank sample.19 The result is further illustrated by the Raman spectrum showed in Figure 2b. The three characteristic peaks in

EXPERIMENTAL SECTION

All chemicals (analytical-grade) were purchased from Sinopharm Chemical Reagent Co., China, and used as received without further purification. In this work, magnesium powder is silver gray and its particle size is in the 4−30 mesh range. More details of magnesium powder are presented in Table S1. All electrochemical measurements were made under ambient conditions. Synthesis of 2D Carbon Materials. In a typical procedure, sodium stearate and magnesium powder with a mass ratio of 3:1 were first ground uniformly and then transferred into a porcelain boat. After flushing with N2 flow for 30 min, the mixture was further heated in a horizontal tube furnace up to 800 °C at a rate of 5 °C min−1 and maintained for 2 h under N2 flow. The resultant products were purified with diluted hydrochloric acid to remove the inorganic impurities, followed by washing with adequate deionized water until the filtrate was neutral. Finally, the powder was dried under vacuum at 110 °C for 12 h to achieve the Carbon-blank sample. Preparation of Electrodes. Typically, the carbon sample, polytetrafluoroethylene binder, and graphite (conducting agent) were mechanically mixed using ethanol as a solvent at a weight ratio of 80:5:15 to form homogeneous slurry. Then, the slurry was subsequently pressed onto a nickel form (1 cm × 1 cm) under a 5973


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Figure 2. Carbon-blank sample: (a) XRD pattern; (b) Raman spectrum; (c) N2 adsorption−desorption isotherm; (d) pore size distribution curve.

the Raman spectra are the so-called D, G, and 2D bands, which locate at ca. 1341.5, 1574.4, and 2683.0 cm−1, respectively. In general, the D band is associated with the disordered in the carbon sample, and the G band is related to the ordered graphitic structure of carbon. The 2D band indicates the stacking order of the graphitic structure along the c axis.20−22 Besides, the ID/IG rate can be used to evaluate the ordered degree of carbon. From the result of Figure 2b, what we can find is that the Carbon-blank sample possesses a disordered structure and low degree of graphitization. The results are in good accordance with the above HTREM images and XRD result. Nitrogen adsorption−desorption analysis and the pore size distribution can prove the developed porous structure. As shown in Figure 2c, the Carbon-blank sample displays a typical IV isotherm with a pronounced hysteresis loop in the mediumand high-pressure regions. It reveals that the Carbon-blank sample is basically made up of abundant mesoporous together with a small part of micropores. As a result, the Carbon-blank sample possesses a large specific surface area of 1229 m2 g−1, and the pore volume is 1.66 cm3 g−1. In addition, the pore size distribution for the Carbon-blank sample is depicted in Figure 2d. The Carbon-blank sample has a relatively small fraction of micropores with the pore size of 1.30 nm, and the pore radius is mainly distributed in 2−20 nm corresponding to the mesopores. The average pore width is 6.83 nm. The existence of multiple pore structures is beneficial for obtaining prominent electrochemical performance, since micropores are related to charge storage and mesopores are related to the fast ion diffusions.23 On the basis of the above structural analysis of the carbon material, we propose the formation mechanism of such unique agaric-like microstructures of the Carbon-blank sample, as shown in Figure 3. First, sodium stearate and magnesium powder are directly carbonized at high temperatures; to be specific, the sodium stearate slowly melts and covers the outside surface of magnesium powder when the heating temperature overtakes its melting point (205 °C). Then, as the temperatures rises, especially beyond the magnesium powder’s melting point (648 °C), the molten sodium stearate reacts with magnesium

Figure 3. Schematic of the preparation of nanoporous carbon materials.

powder to produce the carbon materials and MgO.24,25 During this process, the gaseous products stemmed from thermal decomposition of sodium stearate probably enable the formation of micropores, and the produced MgO can serve as hard template, which was subsequently removed with HCl aqueous solution. The mesopores can thus be formed in the carbon materials.26 Finally, the 2D porous carbon nanosheets were obtained, which can be well-observed by FESEM and HTREM images, as displayed in Figure 3. Design of Dual Redox Additives. As is well-known, the total capacitance of a supercapacitor hinges on positive and negative electrodes. Therefore, in this work, KI and AQS were selected as dual redox additives to improve the capacitance at the both electrodes. The study of the feasible charging mechanisms is depicted in Figure 4. Figure 4a shows the GCD profiles of the KI-10 sample and its working potentials of positive and negative electrodes. It can be seen that the potential of the positive electrode varies narrowly from 0.28 to 0.62 V versus SCE, revealing that the iodide shows a superior redox active behavior at the positive electrode.14,27 The corresponding possible redox reaction of KI at the electrode/ electrolyte interface is shown in Figure 4b,13,28,29 and the potential of the negative electrode has changed linearly in the range of 0.28 to −1.02 V versus SCE electrode, indicating a double-layer charging mechanism with inert K+.14 In a word, from the GCD profiles, the twisted curve is observed at the positive electrode, while linear one appears at the negative 5974


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from the KI at the positive electrode and AQS at the negative electrode. It is therefore expected that the energy density of supercapacitor could be substantially improved by the pseudocapacitance which originates from the reversible redox reaction processes of KI and AQS at both electrodes. Electrochemical Measurements. Measurements Conducted in a Three-Electrode System. The electrochemical behaviors of the Carbon-blank sample have been studied by a three-electrode system using 1 mol L−1 KNO3 as electrolyte, and the CV and GCD results are displayed in Figure S1a,b. In general, the total specific capacitance consists of the electric double layer capacitance (EDLC) and pseudocapacitance.37 As can be seen from Figure S1a, the CV curves of the Carbonblank sample show quasi-rectangular shapes without obvious reduction−oxidation peaks, implying that the EDLC dominates most of the total capacitive contribution. Besides, the GCD curves also display nearly ideal isosceles triangle shapes, also indicating its excellent EDLC feature.38 In contrast, the presence of pseudocapacitance may be attributed to the Faradaic reaction of oxygen-containing functional groups on carbon materials surface.39,40 Specifically, this Faradaic reaction takes place at the electrode surface, in which the oxygencontaining functional groups on the surface of carbon materials store and release an electron. As a redox additive for supercapacitor that can increase the pseudocapacitance by the redox reactions occurring at the negative electrode, AQS is still scarcely reported to date. Therefore, some electrochemical experiments have been first implemented for estimating the electrochemical behaviors of AQS in this study. Figure 5a shows the comparative CV curves of the Carbon-blank and AQS-2.5/5/10 samples at a scan rate of 30 mV s−1; what we can find is that the AQS-2.5/5/10 samples have a couple of obvious redox peaks at negative potential region, demonstrating the existence of pseudocapacitance due to the redox reaction of AQS in electrolyte. In detail, during the charging process, the quinine structure on the AQS is reduced into phenolic hydroxyl group by gaining 2 electrons and 2 protons at the electrode/electrolyte interface.31−36,41 Moreover, with the exception of the GCD curve of

Figure 4. Redox reaction mechanisms of KI and AQS occurring at the positive and negative electrodes in KNO3 electrolyte, respectively.

electrode, suggesting that the redox reaction of KI occurs at the positive electrode. Similarly, for the AQS-10 sample, as depicted in Figure 4c, when the GCD cycles vary from 0 to 1.8 V, the resulting potential windows are −0.11 to 1.08 V (vs SCE) for the positive electrode and −0.15 to −0.66 V (vs SCE) for the negative electrode. Moreover, the negative electrode displays a nonlinear behavior compared to the positive electrode, indicating that the redox reaction of AQS occurs at the negative electrode.30,31 The corresponding probable redox reaction of AQS is shown in Figure 4d,31−36 which involves two-proton and two-electron transfer. On the basis of the above results, we assumed that the dual redox additives composed of KI and AQS still can offer the occurrence of redox reactions at both electrodes respectively and simultaneously in neutral electrolyte of KNO3, as shown in Figure 4e. It can be seen that both electrodes of the AQS:KI-1:1 sample show nonlinear charging/discharging curves and obvious platforms, demonstrating that it is possible to undergo reversible redox reactions

Figure 5. Carbon-blank and AQS-2.5/5/10 samples when measured in a three-electrode system: (a) CV cures at 30 mV s−1; (b) GCD cures at 2 A g−1; (c) specific capacitances calculated from GCD curves; (d) Nyquist plots. 5975


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Figure 6. AQS:KI-2:1/1:1/1:2 samples measured in a three-electrode system: (a) CV cures at 30 mV s−1; (b) GCD cures at 3 A g−1; (c) CV curves of the AQS:KI-1:1 at different scan rates; (d) liner relationships between anodic and cathodic peak currents and scan rates.

AQS:KI-1:1 sample is larger than those of other samples, implying that dual redox additives composed of KI and AQS with a mole ratio of 1:1 can provide a higher capacitance. The above results are in good accordance with the GCD curves shown in Figure 6b. The GCD curves of AQS:KI-2:1/1:1/1:2 samples at a current density of 3 A g−1 exhibit two couples of distinct redox plateaus, indicating the synchronous occurrence of reversible redox reactions of KI and AQS in the charge/ discharge process. Moreover, the AQS:KI-1:1 sample exhibits a longer discharge time, suggesting it possesses larger capacitance. Through the above analysis, we find that the AQS:KI-1:1 sample possesses the best electrochemical behavior. In order to better understand the kinetic behavior of the AQS:KI-1:1 sample, the relationships between the peak current (ip) and the scan rate (v) have been quantitatively analyzed by the linear fitting, judging whether the redox reaction is diffusioncontrolled or surface-controlled.43,44 Consequently, a series of CV profiles concerning the AQS:KI-1:1 sample are recorded at different scan rates (Figure 6c). As a result, the peak current (ip) and the square root of the scan rate (v1/2) takes on a good linearity, displaying R2 = 0.9982 and 0.9994 for O1 and R1 peaks, respectively (Figure 6d). It is thus revealed that the redox reaction of AQS is quasi-reversible and diffusioncontrolled.44 Furthermore, the redox-peak currents and the peak potential separation increase with the enhancement of the scan rate, further proving the diffusion-controlled reaction process. It is interesting to note that the O2 and R2 peak currents (ip) also display an almost linear relationship (R2 = 0.9958 of the O2 and R2 = 0.9986 of the R2) with the square root of the scan rate (v) (Figure 6d), also indicating the diffusion-controlled redox process for KI. Moreover, the kinetics of a single redox additive, AQS or KI, also has diffusion-controlled features, as shown in Figure S2. To sum up, for the cases of dual redox additives AQS and KI system, both of redox reactions involved in KNO3 electrolyte correspond to diffusion-controlled mechanisms.

the Carbon-blank, all the GCD curves display a pair of obvious redox platforms in Figure 5b, further indicating the occurrence of the redox reactions of AQS, and this result is in agreement with the CV curves of the AQS-2.5/5/10 samples. However, the values of specific capacitances calculated from the GCD curves are also shown in Figure 5c. It can clearly be seen that the specific capacitances of the AQS-2.5/5/10 samples are much higher than those of Carbon-blank sample and that the capacitances elevate with the increase of AQS concentration. Even so, we set the AQS concentrations as 2.5, 5, and 10 mmol L−1 due to the limited solubility of AQS in 1 mol L−1 KNO3 solution. These phenomena are consistent with the CV and GCD curves presented in Figure 5a,b mentioned above. Typical Nyquist plots of the Carbon-blank and AQS-2.5/5/10 samples are depicted in Figure 5d. It can be found that each of the samples shows a semicircle at high frequency and an almost straight line at the low frequency region, but it is worth noting that the AQS-10 sample exhibits not only a lower inner resistance (Ri) counted from the point of intersecting with the x-axis in the high-frequency region but also a smaller chargetransfer resistance (Rct) derived from the span of the semicircle along the x-axis from the high- to low-frequency regions.42 These results indicate the AQS-10 sample possesses better electrochemical impedance performances with higher ionic conductivity and more rapid charges/ions transfer. Because of the fact that the AQS-10 sample has the best electrochemical performances among those samples, in this section we chose AQS-10 as redox additive for the negative electrode. Bearing this in mind, dual redox additives containing both AQS and KI with different mole ratio are added to KNO3 electrolyte for studying the electrochemical behavior. As Figure 6a shows, the CV curves of the AQS:KI-2:1/1:1/1:2 samples at 30 mV s−1 display two pairs of obvious and almost symmetric redox peaks at a wide potential range. Compared with the CV curve of the AQS sample, that of the AQS:KI sample shows a pair of new redox peaks at a positive potential range of 0.2−0.6 V, which can be ascribed to the redox reactions of KI.13,28,29 Besides, it is observed that the area of the CV curve of the 5976


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Figure 7. Carbon-blank, AQS-10, KI-10, and AQS:KI-1:1 samples measured in a two-electrode system: (a) CV curves at 30 mV s−1; (b) GCD cures at 2 A g−1; (c) specific capacitances calculated from GCD curves; (d) CV curves of the AQS:KI-1:1 sample at operating voltage windows at 30 mV s−1; (e, f) GCD potential profiles for the positive electrode, negative electrode, and total two-electrode cell in electrolytes of Carbon-blank and AQS:KI-1:1 at 2 A g−1, respectively.

Measurements Conducted in a Two-Electrode System. It is generally recognized that the electrochemical performances of supercapacitors measured in the two-electrode system are more accurate and valuable in contrast with those in three-electrode system. Consequently, the CV curves of the Carbon-blank, AQS-10, KI-10, and AQS:KI-1:1 samples were measured in the two-electrode system at 30 mV s, designating the operating voltage window of 0−1.8 V, as depicted in Figure 7a. It can be found that the CV curve of the Carbon-blank sample is an ideal rectangular profile, apparently revealing its typical electric double-layer capacitive performance. Compared to either AQS10 or KI-10 sample with a couple of redox peaks, there are two pairs of redox peaks for the AQS:KI-1:1 sample, implying the simultaneous presence of redox reactions of AQS and KI. Furthermore, the CV curve of the AQS:KI-1:1 sample encloses more area than that of the other samples, which is ascribed to the increase of pseudocapacitance arising from the redox reactions of dual redox additives (AQS and KI). Figure 7b exhibits the comparisons GCD curves of the Carbon-blank, AQS-10, KI-10, and AQS:KI-1:1 samples at a current density of 2 A g−1. It is clear that the AQS:KI-1:1 sample shows a more nonlinear charge/discharge (redox plateau) curve and longer discharge time compared to those of the Carbon-blank, AQS10, and KI-10 samples. This result further demonstrates that the present dual redox additives AQS and KI really contribute for the improved capacitance in terms of their redox behaviors. Specific capacitances calculated from GCD curves at the operating voltage window of 0−1.8 V are presented in Figure

7c. Among these samples, the AQS:KI-1:1 sample shows the maximum specific capacitance of 75 F g−1 at 2 A g−1, while those of the AQS-10 and KI-10 samples are 26 and 43 F g−1, respectively. Interestingly, the specific capacitance of the AQS:KI-1:1 sample is higher than the sum of the capacities of the AQS-10 and KI-10 samples. This phenomenon can be attributed to the occurrence of synergistic effect between AQS and KI, which will be discussed in later section of this work. Overall, it is clearly found that the dual redox additives can significantly increase the capacitance of supercapacitor compared with the single redox additive. Moreover, CV of the AQS:KI-1:1 sample at different operating voltage windows from 1.4 to 2.2 V was carried out at a scan rate of 30 mV s−1, as shown in Figure 7d. There is no predominant evolution tail observed even at high operating voltage of 1.8 V. It implies that neutral electrolyte of KNO3 possesses broad operating voltage, which is therefore anticipated to further improve the energy density. In order to further understand the redox behavior of dual redox additives occurring at positive or negative electrode in the energy storage process, the GCD curves (black lines) of the Carbon-blank and AQS:KI-1:1 samples were measured for the positive electrode (red line) and the negative electrode (blue line), respectively. For the Carbon-blank sample, as shown in Figure 7e, the shape of the GCD curves for each electrode is nearly isosceles (triangular). The potential of the positive electrode changes in the range of 0.01 to 0.93 V (vs SCE), and that of the negative electrode varies from 0.01 to −0.82 V (vs 5977


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On the basis of the redox behaviors of the KI and AQS redox additives mentioned above, we next further demonstrated the electrochemical reaction mechanisms and synergistic effect of dual redox additives KI and AQS more intuitively, and the resulting diagrammatic sketch is given in Figure 8. The CV curves of single redox additive of the KI-10 and AQS-10 samples are presented in order to compare the redox reactions for the AQS:KI-1:1 sample, as shown in Figure 8a−c. For the KI-10 and AQS-10 samples, the redox processes can be proved by the CV curves. For the AQS:KI-1:1 sample, two pairs of obvious redox peaks almost overlap with the peaks of individual KI and AQS, respectively, demonstrating that the redox reactions of KI and AQS occur synchronously. Second, in order to illustrate the synergistic effect between AQS and KI more intuitively and clearly, we present the specific capacitance data of the AQS-10, KI-10, and AQS:KI-1:1 samples as a bar chart, shown in Figure 8d. Note that the AQS:KI-1:1 sample has the maximum specific capacitance compared to those of other samples (AQS-10 and KI-10). Furthermore, its specific capacitance is much higher than the theoretical data (the direct sum of the specific capacitances of the AQS-10 and KI-10 sample), as displayed in Figure 8e. The additional specific capacitance of the AQS:KI-1:1 sample (dark cyan bar in Figure 8d) is ascribed to the synergistic effect of dual redox additives (AQS and KI).16,46,47 Besides, it can be found that the total specific capacitance contribution of the AQS:KI-1:1 sample should be attributed to the total contribution of EDLCs from carbon materials and the pseudocapacitance from the redox reactions of the dual redox additives. Superior energy densities and power densities are expected for supercapacitors in real applications. Therefore, Ragone plots of the Carbon-blank, KI-10, AQS-10, and AQS:KI-1:1 samples in operating voltage window of 0−1.8 V are displayed in Figure 9a. It is apparent that the energy densities of supercapacitors with redox additives are higher than that of supercapacitor without redox additive. As to the details, the AQS:KI-1:1 sample possesses the maximum energy density of

SCE). All in all, the positive and negative electrodes of the Carbon-blank sample all show a typical EDLCs feature.38,45 However, for the AQS:KI-1:1 sample, the shape of the GCD curves changes significantly (Figure 7f), that is, the overall potential profiles show some distortions compared to that of the Carbon-blank sample. More specifically, for the positive electrode, it is similar to that of the KI-10 sample (Figure 4a) in that positive electrode reveals a nonlinear pseudocapacitive behavior, revealing the appearance of redox reactions. However, for the negative electrode, a pair of redox plateaus between −0.6 and −0.9 V (vs SCE) are observed, which is similar to that of the AQS-10 sample (Figure 4c). This result indicates that AQS undergoes redox reactions at the negative electrode. Furthermore, all of the above results are consistent with the CV curves (see below, Figure 8a−c) in the three-electrode system.

Figure 8. CV curves of the KI-10, AQS-10, and AQS:KI-1:1 samples measured in a three-electrode system (a−c) and specific capacitances calculated from GCD curves when measured in a two-electrode system (d).

Figure 9. (a) Ragone plots of the Carbon-blank, KI-10, AQS-10 and AQS:KI-1:1 samples; (b) SD curves of the Carbon-blank and AQS:KI-1:1 samples; (c) cycling stability as well as the energy efficiency of the AQS:KI-1:1 sample measured at 10 A g−1. 5978


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ACS Sustainable Chemistry & Engineering 33.81 Wh kg−1 at a power density of 1000 W kg−1. Even at a high power density of 10 000 W kg−1, the AQS:KI-1:1 sample still has an energy density of 17.33 Wh kg−1, while the energy density for the Carbon-blank sample is 9.40 Wh kg−1 at 1000 W kg−1. The energy density of the AQS:KI-1:1 sample increases by 3.6 times compared to that of the Carbon-blank sample. Only introducing either AQS or KI into the KNO3 electrolyte, the energy density is 11.55 and 19.35 Wh kg−1 at the same power density (1000 W kg−1), respectively. Moreover, the energy density and power density of AQS:KI-1:1 sample are much higher than those of other supercapacitors in previous literatures (as shown in Figure 9a).28,29,48−50 The better electrochemical performance of the AQS:KI-1:1 sample is due to the pseudocapacitive contribution derived from the synchronous dual redox reactions occurring at the positive and negative electrodes, respectively. That is, the redox reactions related to the KI at the positive electrode and the AQS at the negative electrode synchronously occur. Furthermore, the KNO3 electrolyte provides a broad operating voltage window of 1.8 V, which can further improve the energy density of supercapacitor. As a whole, it can be inferred that the dual redox additives and wide operating voltage window are a valid approach for enhancing the performances of supercapacitors. In addition, self-discharge (SD) is an important parameter for the supercapacitors; therefore, the supercapacitors of the Carbon-blank and AQS:KI-1:1 samples were charged to 1.8 V and relaxed for 10 h with the open circuit voltage recorded in this study. These results are shown in Figure 9b. Beginning at about 4 h, a rapid SD process is undergone for the both samples, and the SD process then displays a much lower discharge rate. Finally, the voltages are stable at 1.15 and 1.36 V after 10 h for the Carbon-blank and AQS:KI-1:1 samples, respectively. It is obviously inferred that the AQS:KI-1:1 sample presents a lower SD rate, indicating a more stable performance. In contrast, the floating test was also applied for the supercapacitors (Figure S3), further revealing the superior stability of the AQS:KI-1:1 sample. These results indicate that the introduction of dual redox additive (AQS and KI) is an acceptable method for the supercapacitors in practical applications. Except for the SD, the excellent cyclic durability and energy efficiency are another two important problems for supercapacitors. Figure 9c exhibits the cycle stability of the AQS:KI1:1 sample at a current density of 10 A g−1 for 5000 cycles. It is observed that the retention of specific capacitance for the AQS:KI-1:1 sample still can reach up to 95.2% after 5000 cycle, implying that it has a long cyclic durability. Moreover, the CV and GCD curves of the AQS:KI-1:1 sample before/after 5000 cycles (in Figure S4) can further confirm the high specific capacitance retention. However, what we find is that the energy efficiency of the AQS:KI-1:1 sample is 59.8% after 5000 cycles, and it is only slightly decreased compared to that from before 5000 cycles (60.2%). These results indicate that the AQS:KI1:1 sample possesses good electrochemical performance. In addition, the excellent electrochemical behaviors of the AQS:KI-1:1 sample also can be derived from the frequencydependent capacitance of supercapacitors (Figure S5).

behaviors of KI and AQS at both electrodes as well as their synergistic effect are well-illustrated by electrochemical techniques. The main scientific advantages in the present work are as follows. (1) We provide a general but highly facile template carbonization method for producing large surface area, high pore volume, and unique agaric-like 2D carbon materials. (2) The dual redox additives KI and AQS have been incorporated into the KNO3 electrolyte, providing a simple but effective approach to enhance the capacitive performance of carbon-based supercapacitors. (3) For the cases of dual redox additives AQS and KI, the redox reactions related to KI occur at the positive electrode, and those of AQS occur at the negative electrode, respectively and synchronously. (4) The redox reactions of dual redox additives AQS and KI are diffusioncontrolled process. (5) Using neutral KNO3 electrolyte can extend the operating voltage window to 1.8 V for carbon-based supercapacitors. The dual redox additives (KI and AQS) improve the performance of both the electrodes by an interesting and facile method for a high-performance energy storage device.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00759. CV and GCD curves of the Carbon-blank samples, CV curves and linear relationship between current densities and scan rates of the AQS-10, KI-10 samples when measured in a three-electrode system, and CV and GCD curves of AQS:KI-1:1 sample before/after 10 000 cycles when measured in a two-electrode system (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiang Ying Chen: 0000-0002-0433-4759 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the financial supports from National Natural Science Foundation of China (51602003).

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REFERENCES

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CONCLUSIONS We first presented a template carbonization method to prepare unique 2D carbon materials. Next, introducing dual redox additives KI and AQS into KNO3 electrolyte has substantially improved the capacitive performance. Besides, the redox 5979


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