Template Synthesis of Nitrogen-Doped Carbon Nanosheets for High

Research Article pubs.acs.org/journal/ascecg

Template Synthesis of Nitrogen-Doped Carbon Nanosheets for HighPerformance Supercapacitors Improved by Redox Additives Wei Hu,† Dong Xu,† Xiao Na Sun,† Zheng Hui Xiao,† Xiang Ying Chen,*,† and Zhong Jie Zhang*,‡ †

School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China ‡ College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei, Anhui 230039, P. R. China S Supporting Information *

ABSTRACT: In this work, nitrogen-doped sheetlike carbon materials have been synthesized by a template carbonization method using 1,5diphenylcarbazide and MgCl2·6H2O as carbon source and template, respectively. The method presents a carbon sample with amorphous characteristics as well as high surface area and large pore volume. More importantly, introducing the redox additives 4-hydroxybenzoic acid (HBA), 3,4-dihydroxybenzoic acid (DHBA), and 3,4,5-trihydroxybenzoic acid (THBA) with functional hydroxyl groups into 1 mol L−1 H2SO4 has largely improved the capacitances as well as the energy density. As expected, the supercapacitor with the redox additive HBA exhibits higher capacitances, with an increase of 1.57 times compared with the conventional H2SO4 electrolyte. Besides, compared with the supercapacitor without any redox additive, the redox additive DHBA produces a large improvement of capacitances, increasing by 3.18 times. In addition, the redox reactions of HBA and DHBA are reversible, while that of THBA is irreversible. Moreover, HBA with a hydroxyl group can release/gain a proton/electron, and DHBA, which owns a pair of hydroxyl groups, can release two protons/electrons. Moreover, both of the redox processes of HBA and DHBA are controlled by a diffusion mechanism. KEYWORDS: Template, Porous carbon, Redox additive, Hydroxyl group, Supercapacitor

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INTRODUCTION As one of the most promising energy storage devices, supercapacitors have drawn much attention due to their fast charge−discharge rates, low costs, long cycling lives, and environmental friendliness.1−3 There are three main types of supercapacitor distinguished by the electrode active materials: metal oxide, conducting polymer, and carbon-based.4−7 Among them, carbon-based supercapacitors have been the alternative one, because their advantages depend on the carbon materials involved.6 However, compared with batteries, carbon-based supercapacitors display lower energy density, which is related to the charge storage mechanism of the electric double-layer capacitor. Some literature has reported that energy density can be acquired according to the formula E = 1/2CV2, where C is related to the total capacitances and, at the same time, V corresponds to the operational potential of the cell.8,9 In order to promote energy density, adding additional capacitances has been regarded as a useful way to enhance the total capacitance.10 Interestingly, adding additional capacitances can be mainly classified into two aspects: (1) heteroatom doping, such as nitrogen/sulfur doping, and (2) introducing redox additives into the electrolyte.11−13 The method of nitrogen doping has been often reported, while redox additives have been rarely mentioned until now. © 2017 American Chemical Society

As we know, the redox additives can be classified into two categories: inorganic and organic. However, the inorganic ones are limited quantitatively, which quite restrict their application areas. By contrast, the quantities of organic ones are much broader and more promising. Up to now, many redox additives have been employed to promote electrochemical performances, such as p-phenylenediamine (PPD),14,15 p-benzenediol (PB),16,17 indigo carmine,18 methylene blue,19 and rutin,20 which can enormously enhance the storage property of a supercapacitor. Notably, most of these organic redox additives possess amino and/or hydroxyl functional groups, which have redox reactions in alkaline/acidic electrolyte. Moreover, carboxyl functional groups are typical electron-withdrawing groups. Hence, it is interesting to research the electrochemical behavior when the hydroxyl and carboxyl functional groups coexist. As a consequence, we employed 4-hydroxybenzoic acid (HBA), 3,4-dihydroxybenzoic acid (DHBA), and 3,4,5trihydroxybenzoic acid (THBA) as redox additives, owing to their characteristic functional groups of hydroxyl and carboxyl Received: April 17, 2017 Revised: August 8, 2017 Published: August 30, 2017 8630

DOI: 10.1021/acssuschemeng.7b01189 ACS Sustainable Chem. Eng. 2017, 5, 8630−8640

Research Article

ACS Sustainable Chemistry & Engineering

the unit cell size, corresponding to the amorphous feature.21,22 Moreover, the Raman technique was also employed to further study the crystallographic structure of the C-blank sample. Figure 1b exhibits the Raman spectrum of the C-blank sample at the wavenumber range of 0−4000 cm−1, and it can be seen that the D band and G band center at ∼1353.0 and ∼1587.1 cm−1, respectively. Generally, D band refers to the sp3 hybridized carbon. In other words, the D band corresponds to the chaotic and defective structure in the carbon sample, while the G band is related to the degree of graphitization. To further study the graphitization degree of porous carbon material, the ID/IG (the intensity ratio between the D band and G band) was employed. As shown in Figure 1b, the ID/IG ratio is 1.06, indeed revealing the low graphitization degree. Besides, from Figure 1b it can be also seen that there is an inconspicuous peak situated at ∼2706.0 cm −1 , which corresponds to the 2D band, which manifests that the Cblank sample possesses a low graphitization degree.23−25 The surface characteristics of the obtained materials were analyzed by nitrogen adsorption−desorption. The textural properties of the C-blank sample is demonstrated in Table 2. Figure 1c,d shows a type IV adsorption−desorption isotherm of the C-blank sample with a hysteresis loop, implying that the samples exhibit mesopores.26 The Brunauer−Emmett−Teller (BET) surface area and pore volume of the C-blank sample are 607 m2 g−1 and 1.32 cm3 g−1, respectively. The pore size distribution curve shows the peaks centered at 1.82 and 4.30 nm and an average pore diameter of 8.69 nm. The existence of a multiple pore structure is beneficial to obtain prominent electrochemical performance, for micropores and mesopores play important roles in charge storages and fast ion diffusions, respectively.27−29 The morphology of the C-blank sample was investigated by scanning electron microscopy (FESEM) and transmission electron microscopy (HRTEM). The FESEM image of the C-blank sample (Figure 2a) shows a curly surface, where ultrathin layers can be observed in the whole sample. Besides, as shown in the inset of Figure 2a, we can gain a high productivity (2.69 g of products come from 6.00 g of 1,5diphenylcarbazide). HRTEM characterization further confirms the results mentioned above. As presented in Figure 2b,c, the C-blank sample displays an obviously wrinkled and 2D sheetlike structure. Besides, in Figure 2d, the HRTEM image of the C-blank sample implies the amorphous and porous features, which are further demonstrated by the dim diffraction rings of the SAED pattern (the inset of Figure 2d), corresponding to the XRD and Raman results mentioned above. On the basis of the structural analysis of the carbon material mentioned above, we further proposed a schematic illustration for producing amorphous 2D carbon structures when utilizing 1,5-diphenylcarbazide and MgCl2·6H2O as carbon/nitrogen precursor and template, which is vividly depicted in Figure 3. 1,5-Diphenylcarbazide could decompose into carbon matrix together with some gases at the elevated carbonization temperature. In particular, the fresh produced gases containing NH3, N2, etc. would treat the carbon matrix, primarily giving rise to the occurrence of nitrogen-doped carbon materials. This kind of organic molecule jointly composed of carbon and nitrogen species also has been widely employed to produce nitrogen-doped carbon materials in recent years.30,31 Noteworthily, both of them have increased the nitrogen incorporation during the carbonization process.

groups, which can release/gain protons and electrons in the electrolyte. In this work, we chose a simple but efficient protocol of salt template carbonization method for compounding porous carbon materials, using 1,5-diphenylcarbazide and magnesium chloride hexahydrate as carbon source and template, respectively. Next, HBA/DHBA/THBA, regarded as efficient redox additives, were added into H2SO4 electrolyte to improve the electrochemical performance of the supercapacitor.

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EXPERIMENTAL SECTION

All the chemicals were purchased from Sinopharm Chemical Reagent (Shanghai, China) Co. Ltd. and used without further purification. Synthesis Procedure for Porous Carbon Materials. Porous carbon materials were synthesized as follows: First, 1,5-diphenylcarbazide and magnesium chloride hexahydrate (MgCl2·6H2O) were mixed at a mass ratio of 1:3 and then pulverized into powders. The powders were put into a porcelain boat, heated up to 800 °C at a rate of 4 °C min−1 surrounded by N2 flow, and kept there for 2 h. After reducing the temperature to the room temperature, the carbonized products were pulverized into powders and dissolved in 1 mol L−1 HCl solution to remove inclusions. Then, the products were washed with deionized water until the colature became neutral. Finally, the products were dried overnight at 110 °C. Eventually, the C-blank sample was obtained. Preparation Procedure for Mixed Electrolytes. A series of mixed electrolytes have been obtained via adjusting the molar quantities of HBA/DHBA/THBA. Concretely, the mixed electrolytes were prepared by introducing different molar masses (0.001, 0.0015, and 0.002 mol) of HBA/DHBA/THBA into 100 mL of H2SO4 aqueous electrolyte (1 mol L−1), in which the concentrations of HBA/DHBA/THBA were 0.01, 0.015, and 0.02 mol L−1, respectively. The obtained HBA/DHBA/THBA-based redox-active electrolytes were denoted as C-HBA-10/15/20, C-DHBA-10/15/20, and CTHBA-10/15/20, as shown in Table 1.

Table 1. Summary of Mixed Electrolytes Created by Adding HBA/DHBA/THBA/mHBA into 1 mol L−1 H2SO4 Solution, Respectively sample C-blank C-HBA-10 C-HBA-15 C-HBA-20 C-DHBA-10 C-DHBA-15 C-DHBA-20 C-THBA-10 C-THBA-15 C-THBA-20 C-mHBA-20

HBA (mmol L−1)

DHBA (mmol L−1)

THBA (mmol L−1)

mHBA (mmol L−1)

10 15 20 10 15 20 10 15 20 20

Characterizations and Measurement Techniques. The structure characterization and measurement techniques are given in the Supporting Information (SI). Parameters Calculations. The detailed equations employed for calculations in this work are illustrated in the SI.

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RESULTS AND DISCUSSION Physicochemical Characterization. Figure 1a demonstrates the XRD pattern of the C-blank sample, and the XRD diffraction peak is located at 21.2° in the spectrogram. Besides, the XRD diffraction peak is lower than the 2θ standard value of graphite (26.6°), which reflects an increase in the d-spacing and 8631


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Figure 1. C-blank sample: (a) XRD pattern, (b) Raman spectrum, (c) N2 adsorption−desorption isotherm, (d) cumulative pore volume and pore size distribution curves.

energies of 398.3, 400.1, and 401.4 eV, which are related to pyridine (N-6, ∼16.38 atom %), pyrrolic (N-5, ∼25.69 atom %), and quaternary (N-Q, ∼57.93 atom %) nitrogen,1,31,35−37 respectively. Hulicova et al. reported that nitrogen situated at the edge of graphene layers could produce an additional capacitive effect.38 Hence, the high content of nitrogen can have a contribution to the electrochemical performance of carbon materials. Besides, Figure 4c displays that three peaks of the O 1s spectrum are located at 531.4, 532.3, and 533.2 eV, which are associated with O-1 (∼41.23 atom %), O-2 (∼30.98 atom %), and O-3 (∼27.79 atom %) bonded to the carbon surface, respectively.35,36,39,40 Moreover, the C 1s spectrum exhibits four peaks in Figure 4d: carbon atoms connected with each other by sp2 hybridization in rings at 284.7 eV (C-1), carbon atoms sp3 combined with each other in rings at 285.5 eV (C-2), carbon atoms singly bound to oxygen in phenol at 286.5 eV (C-3), and carbon atoms sp2 linked to oxygen or carbon atoms sp3 linked to nitrogen at 287.5 eV (C-4),35,41 respectively. At the same time, the CV curve of the C-blank sample is shown in Figure S2 (SI). A pair of broad, reversible humps can be seen, confirming the coeffects of electrical double-layer capacitances and additional capacitances due to the presence of nitrogen and oxygen moieties on carbon materials. In general, rich nitrogen and oxygen functional groups can not only considerably contribute to additional capacitances but also improve the wettability of carbon materials in aqueous electrolyte.36−40 In order to further study the electrochemical performance of the C-blank sample, we employed series measurements of a three-electrode system using the conventional 1 mol L−1 H2SO4 aqueous solution as electrolyte, and the corresponding results are shown in Figure S3 (SI). Figure S3a (SI) illustrates the cyclic voltammetry (CV) curves at different scan rates from 5 to 100 mV s−1 as well as the potential window ranges from 0 to 1 V. The CV curves show the rectangular shapes in 1 mol L−1 H2SO4 electrolyte, and their capacitances primarily come from the electrical double-layer capacitor (EDLC).41 Besides,

Table 2. Characteristic Surface Areas and Pore Structures of the C-Blank Samplea SBET (m2 g−1)

pore volume (cm3 g−1)

sample

ST

Smicro

VT

Vmicro

dav (nm)

C-blank

607

45

1.32

0.015

8.69

a

SBET = BET surface areas. ST = total BET surface areas. Smicro = the SBET of the micropores. VT = total pore volume. Vmicro = the VT of the micropores. dav = average pore width.

Interestingly, we also employed commercially available MgCl2·6H2O as an efficient template largely to form pore structures within carbon materials. As we know, the melting point of MgCl2·6H2O is 117 °C; it thereby decomposes into MgO substance as well as some gases at the elevated temperature up to 415 °C under N2 flow.32,33 In particular, as clearly shown in Figure 3, the freshly produced MgO has been well-confirmed by XRD. Obviously, the MgO substance can serve as a hard template for the formation of mesopores, while the other gases function as a soft template for the production of micropores. In other words, the porous inorganic salt MgCl2·6H2O actually acts as a multiple template, which is evidently favorable for the preparation of a hierarchical pore structure of various kind of carbon materials.33 Moreover, the XPS technique was employed to gain a deeper investigation of the obtained samples with regard to the chemical compositions and electronic states of various elements. As we know, the species of nitrogen functional groups comprise pyridine nitrogen, pyridone/pyrrolic nitrogen, and quaternary nitrogen.34 Meanwhile, the types of oxygen functional groups consist of quinines, ketone and aldehyde type groups, ether and phenol type groups, and chemisorbed oxygen and/or water bonded to the carbon surface,35,36 respectively. Besides, Figure 4a demonstrates that the C/O/N contents are quantitatively calculated to be 78.81, 12.73, and 8.46 atom %, respectively. Obviously, as shown in Figure 4b, the N 1s spectrum can be divided into three peaks located at binding 8632


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Figure 2. C-blank sample: (a) FESEM image and (inset) mass of the product from each experiment and (b−d) HTREM images as well as (d, inset) the SAED pattern and enlarged HRTEM image.

Figure 3. Schematic illustration of the production process for porous carbon materials, using MgCl2·6H2O as template.

capacitances of the C-blank sample are 114 F g−1 at 1 A g−1 and still remains 75 F g−1 at a current density of 10 A g−1. Besides, cycling stability, regarded as a significant factor, has also been employed to distinguish the practicality of the supercapacitor, which was evaluated by 5000 charge/discharge processes at 10 A g−1, and the results are displayed in Figure S3d (SI). The retention of specific capacitances of the C-blank sample still

the galvanostatic charge−discharge (GCD) is also employed to evaluate the electrochemical capacitances of materials. The GCD profiles of the C-blank sample at the current densities of 1−10 A g−1 shown in Figure S3b (SI) demonstrated shapes of almost an isosceles triangle, implying the feature of an EDLC. In addition, specific capacitances of the C-blank sample are displayed in Figure S3c (SI). It is observable that the specific 8633


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Figure 4. C-blank sample: (a) XPS survey (the inset is the summary of carbon/nitrogen/oxygen contents), (b) N 1s spectrum, (c) O 1s spectrum, and (d) C 1s spectrum.

Figure 5. C-blank and C-HBA/DHBA/THBA-20 samples measured in a three-electrode system: (a) CV at 10 mV s−1, (b) GCD at 2 A g−1, (c) specific capacitances calculated from GCD curves, and (d) cycling stability of C-blank and C-HBA/DHBA-20 samples measured at 10 A g−1.

remain 94.9% after 5000 charging/discharging cycles, indicating its excellent cycling stability. In addition, HBA/DHBA/THBA as redox additives were added into 1 mol L−1 H2SO4 electrolyte in order to promote the electrochemical performance of the supercapacitor, and the CV and GCD curves of HBA/DHBA/THBA with different concentrations are displayed in Figure S4−S6 (SI). As shown in

Figure S4a,c,e (SI), the CV curves of the C-HBA-10/15/20 samples were measured in a three-electrode system at different scan rates from 5 to 100 mV s−1. It is obvious that the CV curves exhibit distorted, protruding, broad rectangular shapes in 1 mol L−1 H2SO4 electrolyte and a pair of distinct peaks centers at 0.35 and 0.50 V, respectively. In addition, GCD curves at the current densities of 1−10 A g−1 are exhibited in Figure S4b,d,f 8634


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blank sample and C-HBA/DHBA-20 samples were further measured by 5000 charging/discharging cycles at the current density of 10 A g−1, and the corresponding results are displayed in Figure 5d. Observably, the C-blank sample and the C-HBA/ DHBA-20 samples possess noticeable electrochemical stability with high retention of capacitances (94.9%, 85.2%, and 89.9% of the primary specific capacitances after 5000 cycles, respectively). Moreover, the specific capacitances of the CHBA/DHBA-20 samples have been compared with the specific capacitances of some other redox additives, which are listed in Table 3. As shown in Table 3, introducing DHBA into H2SO4

(SI). It can be clearly observed that the discharge time of the supercapacitor based on the redox additive of HBA is longer with the increased concentration of HBA in H2SO4; that is to say, adding HBA with the concentration of 20 mmol L−1 is the most appropriate condition to improve the specific capacitances comparing with those of 10 and 15 mmol L−1. The conclusion is also applied for DHBA and THBA, which are shown in Figures S5 and S6 (SI). Besides, it demonstrates the obvious charge/discharge potential platforms, confirming the presence of Faradaic reactions derived from the gain or loss of electrons and protons. In addition, DHBA, regarded as a redox additive, was added into 1 mol L−1 H2SO4 electrolyte, and the corresponding CV and GCD curves at different concentrations are demonstrated in Figure S5 (SI). As shown in Figure S5a,c,e (SI), CV curves of the C-DHBA-10/15/20 samples were measured in a three-electrode system at different scan rates from 5 to 100 mV s−1. Clearly, the C-DHBA-10/15/ 20 samples reveal two pairs of obvious and reversible redox peaks, which are attributed to the redox reactions that occurred at the electrolyte/electrode interface. Besides, GCD curves at the current densities of 1−10 A g−1 are exhibited in Figure S5b,d,f (SI). Two obvious charge/discharge potential platforms are observed, in agreement with the reversible redox peaks of CV curves. In addition, THBA was also used as redox additive and compared with HBA and DHBA. In Figure S6a,c,e (SI), CV curves of the C-THBA-10/15/20 samples were measured in a three-electrode system at different scan rates from 5 to 100 mV s−1. It is obvious that the CV curves exhibit distorted, protruding, broad rectangular shapes in 1 mol L−1 H2SO4 which is mainly due to Faradaic reactions, and notably, the CV curves have an obvious oxidation peak, but no reduction peak exists. The oxidation peak is attributed to the oxidation process of the phenolic hydroxyl group; nevertheless, the reaction is irreversible.42 Meanwhile, GCD curves at the current densities of 1−10 A g−1 are demonstrated in Figure S6b,d,f (SI). We can find out that there is an obvious charge potential platform while the no potential platform of discharge curve exists. On the basis of the information mentioned above, we can conclude that the redox additives HBA/DHBA/THBA possess possibilities to promote the electrochemical performances of a supercapacitor. In order to discuss the influence of redox additives that possess different quantities of hydroxyl on the electrochemical properties, we carried out a comparison of HBA/DHBA/THBA, as exhibited in Figure 5. In detail, Figure 5a presents the CV curves of the C-blank sample and C-HBA/ DHBA/THBA-20 samples at the scan rate of 10 mV s−1 in the potential range of 0−1 V. Notably, the C-DHBA-20 sample possesses the largest integral area among these samples, indicating the largest specific capacitances of the C-DHBA-20 sample. As seem from the GCD curves at 2 A g−1 shown in Figure 5b, the C-DHBA-20 sample has the longest discharge time as compared to other samples, implying the prominent discharge ability of the sample for electrode materials, which is well-according with the results of CV measurements. In addition, the specific capacitances of the C-blank sample and C-HBA/DHBA/THBA-20 samples at different current densities are displayed in Figure 5c. As a result, the specific capacitances of all samples decrease with the increase of current density, which mainly result from the increasing limitation of ion diffusion.43 Furthermore, it is clearly shown that the specific capacitances of the C-HBA-20 sample are much higher than those of other samples. Besides, the cycling stabilities of the C-

Table 3. Comparison of the Increased Specific Capacitances upon Introducing Redox Additives Cs (F g‑1) redox additive

pristine

with additive

current density (A g−1)

increase fold

ref.

rutin pyrocatechol pyrocatechol violet Fe3+/Fe2+ HBA

66 66 254

120 368 483

2 2 2

1.8 5.6 1.9

20 20 44

379 106

1062 166

2 2

2.8 1.6

DHBA

106

337

2

3.2

45 present work present work

can achieve large capacitances of up to 337 F g−1, which are about 3.2-fold that of H2SO4. Notably, the present data are better than most of other redox additives for supercapacitors. With respect to the electrochemical impedance spectroscopy (EIS) of the C-blank, C-HBA-20, and C-DHBA-20 samples, they were also employed to investigate the chemical and physical processes occurring at the electrode and are demonstrated in Figure S10 (SI). Moreover, we also studied the electrochemical characteristics and charge-storage kinetics of the C-HBA-20 and C-DHBA-20 samples in a three-electrode cell configuration. According to the CV curves at various scan rates, we can infer the relationships between the peak current (ip) and the scan rate (v) (redox reactions are diffusion-controlled or surface-controlled).46 Hence, parts a and c of Figure 6 exhibit the CV curves of the C-HBA-20 and C-DHBA-20 samples at scan rates from 5 to 50 mV s−1, respectively, and the corresponding relationships between ip and v are shown in parts b and d of Figure 6, respectively. As demonstrated in Figure 6b, linear relationships are observed between the peak current (ip) and the scan rate (v1/2), with R2O1 = 0.9945 and R2R1 = 0.9955 for O1 and R1 peaks, respectively, indicating the diffusion-limited redox reaction of HBA.47,48 On the other hand, Figure 6d shows the ip vs v1/2 plot, giving a linear relationship with R2O2 = 0.9977 and R2R2 = 0.9980 for anodic and cathodic peaks in Figure 6c, respectively. Likewise, it indicates that the redox reaction of DHBA is diffusion-limited. With regard to the detailed results mentioned above, we can find out that a certain dosage of redox additives (HBA, DHBA, and THBA) can produce Faradaic capacitances at the electrode and electrolyte interface. Consider the fact that HBA, DHBA, and THBA can release protons and electrons into the H2SO4 electrolyte, resulting in increased total capacitances. Moreover, the reversible reaction mechanisms of HBA and DHBA are illustrated in Figure 7I. As seen from the structure of HBA in Figure 7I(1), it contains a phenolic hydroxyl located at position 8635


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Figure 6. C-HBA-20 and C-DHBA-20 samples measured in a three-electrode system: (a, c) CV curves at different scan rates and (b, d) variation of anodic and cathodic peak current with scan rate.

Figure 7. (I) The electrochemical reaction mechanisms of redox additives HBA/DHBA. For the C-blank and C-HBA/DHBA-20 samples, (II) CV curves at 10 mV s−1, (III) discharge curves at 2 A g−1, (IV) specific capacitances calculated from discharge curves, and (V) the electrochemical reaction mechanism of electrolyte redox additive THBA.

during the positive scan. On the contrary, the aromatic ketone group can be reduced into a phenolic hydroxyl group after getting a proton and electron during the reverse scan.

a, which can produce a reversible redox reaction. That is to say, the hydroxyl group located at position a can be oxidized into an aromatic ketone group by loss of a proton and an electron 8636


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Figure 8. C-blank and C-HBA/DHBA-20 samples measured in a two-electrode system: (a) CV curves at 10 mV s−1, (b) GCD curves at 1 A g−1, (c) specific capacitances calculated from GCD curves, and (d) cycling stability measured at 10 A g−1 before/after 5000 cycles.

electrochemical reaction mechanism of redox additive THBA is displayed in Figure 7V. Each process gains/losses a proton and an electron, with no peak on the reverse scan, which is wellmatched with the results of CV curves, indicating an irreversible process, which corresponds to the direct electrochemical oxidation of THBA.42,50 To further explore the contribution of the phenolic hydroxyl groups to capacitances, we have also studied 3-hydroxybenzoic acid (the C-mHBA-20 sample) in this work. As shown in Figure S7a (SI), the CV curve of the C-mHBA-20 sample was measured in a three-electrode system at different scan rates from 5 to 100 mV s−1. It is obvious that the CV curves exhibit distorted, protruding, broad rectangular shapes as well as a pair of redox peaks. Besides, GCD curves at different current densities are also demonstrated in Figure S7b (SI). The GCD curves exhibit obvious charge/discharge platforms, implying the redox reaction of the hydroxyl group connected with the benzene ring, and this corresponds well to the redox peaks of CV curves. In order to visually observe the contribution of phenolic hydroxyl groups to capacitance, the GCD curves and specific capacitances of C-mHBA/HBA/DHBA-20 samples are provided in Figure S7c,d (SI). As displayed in Figure S7d, the C-mHBA-20 sample delivers the maximal specific capacitances of 137 F g−1 at the current density of 2 A g−1 compared with those of the C-HBA/DHBA-20 samples (166 and 337 F g−1, respectively). Noticeably, the specific capacitances of the CDHBA-20 sample are larger than the total capacitances of the C-mHBA-20 and C-HBA-20 samples, which may be due to synergistic effects of the hydroxyl groups at positions a and b. It is generally recognized that the two-electrode system plays an importantly practical role in estimating the electrochemical performance of a supercapacitor. Hence, in order to further investigate the performance of a supercapacitor using HBA and DHBA as redox additives, the two-electrode configuration is applied for measurements, and the corresponding results are provided in Figure 8. Figure 8a presents the CV curves of the C-blank, C-HBA-20, and C-DHBA-20 samples at the scan rate of 10 mV s−1 (the detailed results are exhibited in Figure S8,

In addition, the structure of DHBA is shown in Figure 7I(2), which contains an hydroxyl group located at both positions a and b. During the reversible redox reaction, the hydroxyl groups situated at positions a and b can be oxidized into quinone by loss of two protons and two electrons, and in contrast, the quinone groups can be reduced into phenolic hydroxyl groups after receiving two protons and two electrons.49 Additional capacitances are caused by redox reactions, which result in a rising total capacitance. As seen from the CV curves shown in Figure 7II, the C-HBA-20 sample exhibits a pair of distinct peaks that is related to the reversible redox reaction of HBA. Moreover, the C-DHBA-20 sample delivers two pairs of obvious redox peaks, which are the product of the redox reaction of DHBA. In addition, on the basis of the integral area of the C-blank sample, the C-HBA-20 sample demonstrates an extra part attributed to the capacitances produced by the redox reaction of the phenolic hydroxyl group located at position a. Besides, in terms of the integral area of the C-HBA-20 sample, the C-DHBA-20 sample exhibits an additional area that corresponds to the capacitances produced by the redox reaction of the phenolic hydroxyl group situated at position b. In addition, as shown in Figure 7III, the discharge time of the HBA/DHBA-20 sample is longer than that of the C-blank sample, suggesting the contribution for capacitances caused by the redox reactions of the phenolic hydroxyl groups located at positions a and b, respectively. As shown in Figure 7IV, the specific capacitances of the C-blank and C-HBA/DHBA-20 samples are 106, 166, and 337 F g−1, respectively. According to the C-blank sample, the specific capacitances of the C-HBA-20 sample have an increase of 60 F g−1, which is related to the redox reaction of the phenolic hydroxyl group located in position a. Moreover, the specific capacitances of the C-DHBA20 sample have a growth of 172 F g−1 compared with that of the C-HBA-20 sample, which is connected with the effects of the phenolic hydroxyl group located in position b. However, it is noticeable that the additive THBA is not the same as HBA and DHBA, due to the irreversibility of the reaction. The 8637


ACS Sustainable Chemistry & Engineering SI). Clearly, the CV curve of the C-blank sample is a quasirectangular shape. Similarly, the absence of redox peaks derived from the Faraday reaction also can be observed from the CV curves of C-HBA-20 and C-DHBA-20 samples. However, the integral area of the C-DHBA-20 sample is larger than those of the C-blank and C-HBA-20 samples, respectively. That is to say, the C-DHBA-20 sample carried out in 1 mol L−1 H2SO4 electrolyte has higher capacitances compared with the C-blank and C-HBA-20 samples. Besides, GCD curves of the C-blank, C-HBA-20, and C-DHBA-20 samples are shown in Figure 8b. Obviously, the discharging time of the C-DHBA-20 sample is much longer than that of the C-blank and C-HBA-20 samples, which corresponds to the results of the CV curves. In addition, the specific capacitances of the C-blank, C-HBA-20, and CDHBA-20 samples calculated from GCD curves are revealed in Figure 8c. Evidently, the C-DHBA-20 sample possesses the largest specific capacitances at the same current density and excellent rate capability among all samples. Besides, cycling stability, regarded as a significant factor, has also been employed to distinguish the practicality of supercapacitors and was evaluated here by 5000 continuous charge/ discharge processes under a current density of 10 A g−1, and the results are displayed in Figure 8d. Apparently, the C-DHBA-20 sample displays a higher cycling stability (91% of primary specific capacitances after 5000 cycles) than the C-HBA-20 sample (87% of primary specific capacitances after 5000 cycles) (the CV curves of the samples before/after 5000 cycles at 20 mV s−1 are displayed in Figure S9, SI). Hence, the C-DHBA-20 sample exhibits excellent electrochemical performance, which is vastly motivating its employment in a supercapacitor. However, there are obvious decreases of cycling stabilities for C-HBA-20 and C-DHBA-20 samples compared with the C-blank sample. The reduced specific capacitances are mainly due to the increasing aggregation of HBA and DHBA in the pores of the electrode active material.51 In terms of the practical application, energy density and power density have been regarded as vital and significant elements of supercapacitors, and the Ragone plots of the Cblank, C-HBA-20, and C-DHBA-20 samples are displayed in Figure 9. As demonstrated in Figure9, the C-HBA/DHBA-20

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CONCLUSION

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ASSOCIATED CONTENT

In this work, we displayed a simple and efficient approach for producing hierarchical carbon materials. In addition, HBA, DHBA and THBA were employed as effective redox additives to modulate the capacitive performance. Thus, considering the above experimental results, we can draw the following conclusions: (1) The template carbonization method is quite simple but effective for producing hierarchical carbon structures with high porosities, which are expected to deliver superior capacitive performance. Especially, the component MgCl2· 6H2O is commercially available and inexpensive, making it possible for large-scale production of templated carbon materials. It thereby decomposes into MgO substance as well as some gases at the elevated temperature, at which MgO can serve as a hard template for formation of mesopores and the other gases as soft template for production of micropores. In a words, the porous inorganic salt MgCl2·6H2O actually acts as a multifunctional template, which is evidently favorable for the preparation of hierarchical pore structures of various kinds of carbon materials. (2) Compared with the supercapacitor with traditional electrolyte, the supercapacitor with H2SO4−HBA/ DHBA electrolyte possesses higher capacitances as well as energy density. The redox additives HBA and DHBA are chosen mainly because of their functional hydroxyl groups. In terms of HBA and DHBA, the specific capacitances are proportional to the number of hydroxyl groups embedded in the benzene ring. Besides, the redox reactions of HBA and DHBA introduced into H2SO4 are reversible. (3) Compared with the redox reactions of HBA and DHBA, the redox reaction of THBA added into H2SO4 is irreversible. Moreover, each process gains and loses a proton and an electron, leading to higher capacitances than traditional electrolyte and lower capacitances compared with those of HBA and DHBA, respectively. In summary, the effects of the different quantities of phenolic hydroxyl groups have been explicated in detail for modulating the performance of supercapacitors, which could act as useful manual for the future research about phenolic hydroxyl groups in the application of supercapacitors.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01189. The methods for detailed structural characterization and electrochemical measurement; schematic representation of the two-electrode system; the CV and GCD curves of the C-blank, C-HBA/DHBA-10/15/20 samples measured in a three-electrode system; The CV and GCD curves of the C-mHBA-20 sample at different scan rates and current densities, respectively; GCD curves and specific capacitances of the C-mHBA/HBA/DHBA-20 samples; the CV and GCD curves of the C-blank, CHBA/DHBA-20 samples measured in a two-electrode system; the CV curves of the samples before/after 5000 cycles at 20 mV s−1 in a two-electrode system; electrochemical impedance spectroscopy of C-blank, CHBA-20, and C-DHBA-20 samples, as well as the detailed analysis of impedance plots (PDF)

Figure 9. Ragone plots of specific energy versus specific power for the C-blank and C-HBA/DHBA-20 samples.

samples possess a larger energy density of 10.5/14.7 Wh kg−1 with a power density of 1.0 kW kg−1 compared with that of the C-blank sample (5.92 Wh kg−1 at 1.0 kW kg−1). The results are better than those of some other supercapacitors based on a mixed electrolyte in the literature.52−55 8638


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AUTHOR INFORMATION

Corresponding Authors

*X.Y.C. e-mail: [email protected]. *Z.J.Z. 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 National Natural Science Foundation of China (21101052). Z.J.Z. was financially supported by the National Natural Science Foundation of China (51602003), the Natural Science Foundation of Anhui Province (1508085QE104), the University Scientific Research Project from Department of Education of Anhui Province (KJ2016A039), and the Startup Foundation for Doctors of Anhui University (J01003211).

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Template Synthesis of Nitrogen-Doped Carbon Nanosheets for High

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