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sulfur co-doped reduced graphene oxide aerogels for high-performance supercapacitors with ionic liquid electrolyte
Materials Chemistry and Physics 238 (2019) 121932
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Synthesis of nitrogen/sulfur co-doped reduced graphene oxide aerogels for high-performance supercapacitors with ionic liquid electrolyte Yujuan Chen, Li Sun, Zhaoen Liu, Yuyang Jiang, Kelei Zhuo * Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� N/S co-doped graphene aerogels were prepared by a mild method. � The N/S-rGA-1 has high sulfur content and porous network structure. � The N/S-rGA-1 delivers a high energy density up to 75 Wh kg 1. � The N/S-rGA-1 exhibits impressive electrochemical performance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene Nitrogen and sulfur co-doping Ionic liquid Trithiocyanuric acid Supercapacitor
Graphene with a unique two-dimensional lamellar structure is considered an attractive carbon-based electrode material for supercapacitors. However, the actual electrochemical performance of graphene-based super capacitors is far below the theoretical level. In this work, reduced graphene oxide aerogels co-doped with different amounts of nitrogen and sulfur (N/S-rGAs) were prepared via a one-pot hydrothermal approach using trithiocyanuric acid as a doping agent. Characterization of the materials showed that the optimal N/S-rGA had a high sulfur content (4.1 at%) and an interconnected porous network structure that provided accessible diffusion channels for electrolyte ions, thus leading to low ion diffusion resistance. The optimal N/S-rGA displayed high electrical conductivity of 11.5 S cm 1 and a specific capacitance of 180.5 F g 1 at 1 A g 1. This N/S-rGA delivered a high energy density of 75 Wh kg 1 at a power density of 0.9 kW kg 1 in an ionic liquid (1-ethyl-3-methyl imidazolium tetrafluoroborate, EMIMBF4) electrolyte, and its energy density was still 33 Wh kg 1 when its power density was 15 kW kg 1. Consequently, N/S-rGAs are promising electrode material for high-performance ionic liquid-based supercapacitors.
1. Introduction Supercapacitors offer great potential as energy storage devices because of their higher safety, longer cycle life (105 cycles), faster
charge/discharge capability (within seconds), and higher power density (10 kW kg 1) than those of traditional energy storage devices [1–3]. In this regard, supercapacitors are attractive candidates for high-power equipment and grid energy storage systems for clean renewable
* Corresponding author. E-mail address: [email protected] (K. Zhuo). https://doi.org/10.1016/j.matchemphys.2019.121932 Received 4 April 2019; Received in revised form 26 July 2019; Accepted 1 August 2019 Available online 3 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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energy. However, the energy density of supercapacitors is usually rela tively low [4], which limits the practical applications of most super capacitors. Therefore, there is an urgent need to enhance the energy density of supercapacitors to expand their practical application range. At present, the most common method used to increase the energy density of supercapacitors to meet practical demands is to develop highperformance electrode materials. Carbon nanomaterials such as gra phene, carbon nanotubes, carbon fiber, and carbon derivatives [5,6] are often used as electrode materials for supercapacitors. In particular, graphene is considered a rising star of carbon nanomaterials [7,8], and is seen as a promising carbon-based electrode material for high-performance electrochemical supercapacitors because of its large theoretical specific surface area, excellent electronic conductivity, high mechanical stability, and high theoretical gravimetric specific capaci tance (526 F g 1) [9,10]. However, due to the strong π-π conjugation and van der Waals forces between graphene sheets, the inevitable agglom eration of graphene fabricated by electrochemical or chemical reduction severely suppresses the emergence of its favorable intrinsic properties [11]. In consequence, the electrochemical performance of current graphene-based supercapacitors has not reached their theoretical level. To eliminate the innate deficiencies of graphene, many researchers have been devoted to changing its external morphology and internal structure to inhibit the overlap of graphene sheets. Three-dimensional graphene materials such as graphene hydrogels, graphene aerogels (GAs), and graphene nanomesh possess hierarchical porous structure, and thus not only exhibit a larger accessible specific surface area, but also contain more convenient channels for the trans port of electrolyte ions compared with the case for two-dimensional graphene [12]. Doping graphene with heteroatoms can modify its in ternal electronic structure and bond polarization, thus increasing its surface wettability and electrical conductivity [13–15]. Heteroatom doping can also lead to pseudocapacitance, which enhances the elec trochemical performance of graphene [10]. Among doping elements, nitrogen has received considerable attention. Many researchers believe that including nitrogen atoms as electron donors can effectively improve the electrochemical performance of supercapacitors [16–18]. Further more, the introduction of sulfur can also induce lattice distortion and tailor the charge density distribution of graphene materials. Because sulfur atoms have a specific valence electronic structure and large size, sulfur doping can raise the capacitance of graphene-based supercapacitors. Recently, various methods have been developed to prepare heteroatom-doped graphene materials. For instance, sulfur and nitrogen co-doped mesoporous graphene with high energy density and capaci tance retention was synthesized by chemical vapor deposition and calcination at 800 � C [19]. Chen et al. [20] synthesized nitrogen and sulfur co-doped nanoporous carbon with a high sulfur content (3.36%) that displayed a specific capacitance of 73 F g 1 at a current density of 1 A g 1. Huang et al. [21] annealed graphene oxide-encapsulated ami no-modified silica nanoparticles at 1000 � C to fabricate a sulfur-doped three-dimensional porous reduced graphene oxide containing 2% sul fur. This material exhibited good electrochemical performance and excellent cycling stability. However, the sulfur-doped carbon materials with high sulfur content reported to date are prepared by complex procedures or at high temperature. Therefore, it remains important to develop simple and mild methods to prepare heteroatom-doped gra phene materials for use in high-performance supercapacitors. The electrolyte is another important factor affecting the energy density of supercapacitors because energy density is proportional to the square of the operating voltage of the electrolyte. At present, aqueous electrolytes with high ionic conductivity and safety have been widely used. However, the low decomposition voltage of water (1.23 V) limits the application of aqueous electrolytes [22]. As a result, supercapacitors based on aqueous electrolytes have low energy density. Organic elec trolytes possess a relatively wide operating voltage range and are favored by many supercapacitor researchers. However, most organic
electrolytes use acetonitrile or propylene carbonate as the solvent. The flammability, toxicity, and environmentally hazardous nature of these solvents are inevitable problems [23]. Ionic liquids (ILs) are composed of organic cations and anions that form molten salts at room tempera ture and are considered promising electrolytes for supercapacitors because of their nonvolatile nature, non-flammability, low vapor pres sure, and wide electrochemical stability window. Trithiocyanuric acid (TTCA), which has a high sulfur content, is an excellent heteroatom-containing precursor for photocatalysts, lith ium–sulfur batteries, and oxygen reduction catalysts [24–28]. In this work, nitrogen and sulfur co-doped reduced graphene oxide aerogels (N/S-rGAs) with high sulfur content are synthesized from TTCA and graphene oxide (GO) by a mild and effective one-pot hydrothermal approach. The electrochemical performance of the N/S-rGAs in IL electrolyte is investigated. 2. Experimental 2.1. Preparation of materials GO was prepared by oxidation of graphite powder according to the modified Hummers method [15,29,30]. A schematic of the preparation process of N/S-rGAs is shown in Scheme 1; experimental and charac terization details have been described in the Supporting Information. 2.2. Electrode preparation and electrochemical measurements Full details of electrode preparation and electrochemical testing methods are described in the Supporting Information. The gravimetric specific capacitances of the active material in the electrode (Cs) and cell (Ccell) were calculated from discharge curves using the following equa tions [15,31]: Cs ¼
2IΔt mΔU
Ccell ¼
1 Cs 4
(1) (2)
where I is the constant discharge current (A), Δt is the discharge time (s), ΔU is the discharge voltage with the IR drop removed (V), and m is the mass of active material in the electrode (g). The gravimetric energy density (E), power density (P), and coulombic efficiency (ƞ) of the electrode material in each cell were evaluated using the following equations: E¼
1 CsΔU 2 28:8
P ¼ 3600
η¼
E Δt
td tc
(3) (4) (5)
where td and tc represent the discharge time and charge time (s), respectively. 3. Results and discussion 3.1. Morphology characterization of materials The microstructural features of the undoped reduced graphene oxide aerogel (rGA) and N/S-rGAs were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as presented in Fig. 1. Fig. 1a shows that rGA has a porous block structure that was formed by the overlap and agglomeration of graphene sheets. N/S-rGA-1 (Fig. 1b) possesses a continuous cotton-like porous structure 2
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Scheme 1. Schematic illustration of the synthesis procedure of N/S-rGAs.
with good uniformity. Compared with inert graphene, GO possesses numerous oxygen-containing functional groups, such as carboxyl, hy droxyl, and epoxy, which endows it with higher chemical reactivity than that of graphene. During the hydrothermal reaction, sulfhydryl groups of TTCA will undergo esterification reactions with carboxyl groups of GO, and dehydrate with the hydroxyl groups to form C–S–C. In addition, as a nucleophile, sulfhydryl groups can also react with epoxy groups of GO to form water molecules [32,33]. Thus, the porous structure was
generated through the formation of hydrogen bonds between oxygen-containing functional groups on the GO surface and the sulfhy dryl groups of TTCA [12]. The TEM image of rGA showed that was an opaque sheet (Fig. 1a). In contrast, N/S-rGA-1 (Fig. 1c) possesses a thin and crumpled texture without severe aggregation. The difference in morphology between rGA and N/S-rGA-1 may be caused by the defect structure formed by the addition of heteroatoms into the graphene lat tice [34]. The unique structure of N/S-rGA-1 can provide more
Fig. 1. SEM images of (a) rGA and (b) N/S-rGA-1. TEM images of (c) rGA and (d) N/S-rGA-1. 3
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adsorption sites and transmission channels than in the case of rGA, which can improve its accessibility for electrolyte ions during the charging and discharging processes. It is believed that an interconnected porous structure is conducive to energy storage, giving rise to improved electrochemical properties [35,36]. SEM and TEM images of N/S-rGA-0.5 and N/S-rGA-2 indicated that these materials also possessed porous structures, but their morphologies were uneven because of the agglomeration and collapse of graphene layers (see Fig. S1). To explore the chemical composition of the materials and bonding mode of heteroatoms, the materials were evaluated using X-ray photo electron spectroscopy (XPS). In the XPS survey spectrum of N/S-rGA-1 (Fig. 2a), five peaks located at 284.8, 400, 533, 228, and 164 eV cor responding to C1s, N1s, O1s, S2s, and S2p, respectively [37], were observed, which confirmed that graphene was successfully doped with nitrogen and sulfur atoms. The atomic percentages of C, N, O, and S in the samples determined by XPS are summarized in Table S1. The sulfur content of N/S-rGA-1 was much higher than that of sulfur-doped gra phene materials reported previously [38,39]. The C1s spectra of GO and rGA are displayed in Fig. S2. Compared with those for GO, the C–O and C¼O peak intensities in the C1s spectrum of rGA were obviously weakened, indicating that oxygen-containing groups on the GO sheets were partly removed during the synthesis procedure [40]. The C1s spectrum of N/S-rGA-1 (Fig. 2b) was deconvoluted into four peaks, corresponding to C–C/C¼C (284.6 eV), C–S/C–O/C–N (285.1 eV), C¼O (286.2 eV), and O–C¼O (289.1 eV) [41,42]. Fig. 2c shows the high-resolution N1s spectrum of N/S-rGA-1, which was deconvoluted into two peaks centered at 400.2 and 401.8 eV that were assigned to pyrrolic-N and graphitic-N [43], respectively. Pyrrolic-N plays a major role in improving the surface wettability of graphene [44]. In addition, the existence of graphitic-N can change the electron density of the car bon network, resulting in increased electron conductivity and enhanced electrochemical performance [45]. The high-resolution S2p spectrum of N/S-rGA-1 (Fig. 2d) was deconvoluted into three peaks located at 164.0, 165.1, and 168.9 eV, which were consistent with C–S–C and C-SOx-C states (sulfonate or sulfone) [12,46]. The formation of thiophene-S
structure was attributed to spin–orbit coupling between sulfur atoms and their neighboring carbon atoms [47–49]. The XPS data for N/S-rGA-0.5 and N/S-rGA-2 are displayed in Fig. S3. All the spectra confirmed that the N/S-rGAs were doped with both nitrogen and sulfur. X-ray diffraction (XRD) is a suitable research method to obtain the crystal structure of carbon materials. Fig. 3a presents the XRD patterns of the GO, rGA, and N/S-rGAs. The peak located at about 2θ ¼ 9.8� was the characteristic diffraction peak of GO, which corresponds to the (001) crystal plane of graphite [50]. The corresponding interlayer spacing (d) was 0.9 nm, indicating that graphite was converted to GO. After hy drothermal reaction, this diffraction peak disappeared completely; instead, rGA displayed a relatively narrow peak at 25.1� . This result indicates that GO was reduced to rGA during hydrothermal treatment and rGA has a relatively high content of graphitic structure [51]. The N/S-rGAs displayed broad weak diffraction peaks centered at 2θ ¼ 24.5� and approximately 43.5� , which were indexed to the (002) and (100) planes, respectively. The XRD patterns indicate that the heteroatom doping decreased the crystallinity of graphene. Additionally, d of rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2 calculated by the Bragg equation were 0.35, 0.37, 0.38, and 0.37 nm, respectively. Thus, d of N/S-rGA-1 was the largest. The diameter of a sulfur atom (0.2 nm) is larger than that of a carbon atom (0.15 nm), so it is logical that d of graphene increases after sulfur atoms are added into the carbon skeleton [52]. The increase of d can facilitate the transfer of both electrons and electrolyte ions in the graphene-based materials. Raman spectra of the materials are shown in Fig. 3b. All the spectra exhibit two peaks situated at around 1352 and 1590 cm 1, corre sponding to D and G bands, respectively. The D band arises from the disordered carbon structure and crystal defects, including vacancies, amorphous carbon species, and heteroatoms, and the G band originates from the stretching vibration of in-plane sp2 hybridized carbon atoms in graphene [53,54]. The intensity ratio of D to G bands (ID/IG) can reflect the structural characteristics of graphene [42]. The ID/IG value for N/S-rGA-1 of 1.25 was higher than that of rGA (1.08), revealing that heteroatom doping destroyed the structure of the carbon rings and increased the defect density. The increase of ID/IG confirmed the
Fig. 2. (a) XPS survey spectrum of N/S-rGA-1. (b) C1s, (c) N1s, and (d) S2p high-resolution spectra and fitting peaks for N/S-rGA-1. 4
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Fig. 3. (a) XRD patterns of GO, rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2. (b) Raman spectra of rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2.
existence of amorphous carbon structure in N/S-rGA-1. Moreover, the presence of a relatively weak 2D band at ~2700 cm 1 indicated the existence of few graphene nanosheet layers in the samples [55]. Nitrogen adsorption/desorption isotherms and pore size distribu tions of rGA and N/S-rGA-1 are shown in Fig. 4. The specific surface area and pore volume for each sample determined from the isotherms are listed in Table S1. Fig. 4a reveals that the samples displayed type-IV isotherms. N/S-rGA-1 exhibited a type-H3 hysteresis loop at high rela tive pressure (P/P0) from 0.85 to 1.0, indicating that this sample con tains narrow mesoporous and macroporous structures [56]. The specific surface area of N/S-rGA-1 was smaller than that of rGA, which is attributed to higher microporosity of rGA than that of N/S-rGA-1. Hu et al. [57] pointed out that specific surface area is not the only factor that contributes to the specific capacitance of electrode materials; that is, the electrochemical properties of electrode materials are determined by both pore volume and specific surface area. Although rGA has a larger surface area than that of N/S-rGA-1, most of the specific surface area in rGA may not be utilized effectively during electrochemical cycling because micropores are not conducive to the diffusion of large ions present in IL electrolytes, especially under high loading rate [58]. The pore size distributions of the samples are displayed in Fig. 4b. The pore size distribution of N/S-rGA-1 had a broad peak at 2.7 nm and most pores were 2–4 nm in size, which indicates that N/S-rGA-1 has a large number of mesopores. In addition, N/S-rGA-1 contains some macro pores that could be suitable for ion storage. In contrast, the pore size distribution of rGA had a high peak at 1.5 nm, revealing the presence of abundant micropores. N/S-rGA-1 had a larger mesopore volume than that of rGO, and thus more effective specific surface are to provide adsorption sites for electrons and electrolyte ions. Furthermore, the larger pore width of N/S-rGA-1 than that of rGA can provide open transport channels to ensure the fast passage of the large ions of IL electrolytes.
3.2. Electrochemical performance of the aerogels To further study the electrochemical performance of the aerogels, they were used as electrode materials in symmetric coin cells. Cyclic voltammetry (CV) curves of the samples measured at 20 mV s 1 are displayed in Fig. 5a. The CV curve of rGA possesses a symmetrical approximately rectangular shape, indicating pure electrochemical double-layer capacitance behavior. The CV curves of N/S-rGAs are also approximately rectangular, suggesting that the main energy storage mechanism of N/S-rGAs is electrical double-layer capacitance. The CV curves of N/S-rGAs have a larger area than those of rGA, proving that doping with sulfur and nitrogen increases capacitance performance. The CV curve of the cell containing N/S-rGA-1 possessed the largest area of the samples, illustrating that it has outstanding capacitive properties. The excellent capacitance performance of N/S-rGA-1 is caused by its large layer spacing and effective pore volume that is favorable for the migration of electrolyte ions between the electrode surface and electrolyte. The galvanostatic charge–discharge (GCD) curves of the materials measured at a current density of 2 A g 1 displayed nearly symmetrical triangular shapes (Fig. 5b), suggesting ideal capacitor behavior [59]. The charge–discharge time for N/S-rGA-1 was the longest of the samples and it showed the highest capacitance. Notably, the materials exhibited small ohmic IR drops (rGA ¼ 0.268 V, N/S-rGA-0.5 ¼ 0.073 V, N/S-rGA-1 ¼ 0.072 V, and N/S-rGA-2 ¼ 0.081 V), which would ensure low equivalent series resistances. The specific capacitance of N/S-rGA-1calculated from its discharge curve was 175.8 F g 1 at 2 A g 1, which was larger than those of N/S-rGA-0.5 (135.2 F g 1), N/S-rGA-2 (147.3 F g 1), and rGA (53.7 F g 1). The poor capacitance performance of rGA is caused by the high resistance of its microporous structure to ion migration. Electrochemical tests of the aerogels are compared with those of commercial reduced graphene oxide (C-rGO) in Fig. S4. The electrochemical properties of rGA were similar to those of C-rGO; N/S-rGA-1 demonstrated the best electrochemical performance
Fig. 4. (a) Nitrogen adsorption/desorption isotherms of rGA and N/S-rGA-1. (b) Pore size distributions of rGA and N/S-rGA-1. Inset is the pore size distribution of rGA and N/S-rGA-1 in the range from 2 to 4 nm. 5
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Fig. 5. Electrochemical performance of rGA and N/S-rGAs. (a) CV curves measured at 20 mV s 10 A g 1, and (d) Nyquist plots. Inset is a magnified view of the high frequency range.
because of its effective mesoporous structure. Fig. 5c shows the change of the specific capacitance of the aerogels with increasing current density. The introduction of heteroatoms into rGA greatly increased its specific capacitance. For a given material, specific capacitance decreased with rising current density from 1 to 10 A g 1. N/S-rGA-1 delivered the highest specific capacitance of 180.5 F g 1 at 1 A g 1, which was much larger than that of rGA (72.9 F g 1). Even at a high current density of 10 A g 1, N/S-rGA-1 still reached a specific capacitance of 133.4 F g 1, whereas that of rGA was only 20.9 F g 1. The capacitance retention of N/S-rGA-1 was 74%, illustrating that N/S-rGA-1 has excellent capacitance property and rate performance. Electronic conductivity is an important factor when evaluating the electrochemical properties of a material. Electronic conductivities of the materials were determined to explain their electrochemical properties. The electrical conductivities of rGA, N/S-rGA-0.5, N/S-rGA-1, and N/SrGA-2 were 0.5, 3.9, 11.5, and 8.0 S cm 1, respectively, indicating that N/S-rGA-1 possessed the highest electrical conductivity of the materials. The effective improvement of conductivity is caused by the presence of nitrogen and sulfur atoms, which change the electronic structure of graphene, and thus improve its electrochemical properties. Electro chemical impedance spectroscopy (EIS) measurements were conducted on rGA and N/S-rGAs to investigate the interaction of the electrode materials with the electrolyte. Nyquist plots of the symmetric super capacitors are shown in Fig. 5d. The intercept of the plots with the x-axis was ascribed to the combined resistance (Rs), which is determined by the internal resistance of ions and electrons, as well as the contact resistance of the electrode material with the separator [60,61]. The Rs values for rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2 were 2.78, 5.59, 1.72, and 6.42 Ω, illustrating that N/S-rGA-1 had the lowest Rs of the mate rials. In addition, the incomplete semicircles at the high frequency re gion represent the Faradaic charge transfer resistance (Rct) at the electrode/electrolyte interface. The Rct values determined for rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2 were 14.05, 1.26, 1.06, and 3.06 Ω, respectively, indicating that N/S-rGA-1 has the lowest Rct of the samples. The near-vertical lines in the low frequency region indicate the
1
, (b) GCD curves measured at 2 A g
1
, (c) rate capability from 1 to
perfect capacitive performance of the electrodes. The slope of the ver tical line was the steepest for N/S-rGA-1, meaning that it displays better capacitance characteristics than the other materials. Because doping with nitrogen and sulfur changes the morphology and electron structure of graphene, the N/S-co-doped materials exhibited relatively low Rct and fast ion diffusion, which led to excellent capacitive properties. The equivalent circuit model used to fit the experimental EIS data and the values of equivalent circuit parameters of the as-prepared electrodes were displayed in the Supporting information (Fig. S5 and Table S3). The above results revealed that the specific capacitance of N/S-rGA-1 was the largest of the materials. Here, we use N/S-rGA-1 as an example to embody the optimal electrochemical performance of the aerogels. The CV profiles of N/S-rGA-1 (Fig. 6a) measured at different scan rates in the specific electrochemical window of 0–3.5 V shows quasi-rectangular shapes, suggesting relatively good capacitive characteristics. The CV curves retained their shape even at a high scan rate of 200 mV s 1, confirming that N/S-rGA-1 exhibited good capacitance behavior. Fig. 6b presents the GCD curves of N/S-rGA-1 at various current densities. The specific capacitances calculated for N/S-rGA-1 were 180.5, 175.8, 169.1, 156.5, and 133.4 F g 1 at 1, 2, 3, 5, and 10 A g 1, respectively, corresponding to slight IR drops of 0.042, 0.072, 0.171, 0.178, and 0.366 V, respectively. Comparison of the specific capacitance of N/SrGA-1 with those of other reported materials (Table S2) revealed that N/S-rGA-1 displayed superior electrochemical performance to that of other materials. Ragone plot of a cell with N/S-rGA-1 as the electrode material is shown in Fig. 6c. The plot indicates that the energy density of N/S-rGA-1 is 75 Wh kg 1 at a power density of 0.9 kW kg 1. When the power density was raised to 15 kW kg 1, the energy density of N/S-rGA-1 was still as high as 33 Wh kg 1. The energy density of N/S-rGA-1 is higher than those reported for some other carbon-based electrode materials [18,62–68] (Table S2). For example, the energy density of N-rGO in BMIMBF4 is 55 Wh kg 1 [18] and that for diamine/triamine function alized graphene in BMIMBF4 is 51 Wh kg 1 [69]. The cycling stability of N/S-rGA-1 in a voltage window of 3.5 V at a current density of 5 A g 1 was also investigated (Fig. 6d). After 5000 charge and discharge cycles, 6
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Fig. 6. Electrochemical performance of N/S-rGA-1. (a) CV curves, (b) GCD curves, (c) Ragone plot of this work compared with other literature data, and (d) the cycling stability of N/S-rGA-1 at 5A g 1.
80% of the initial capacitance was still retained, revealing the good cycling stability of N/S-rGA-1. In addition, the coulombic efficiency of N/S-rGA-1 was almost 94%. The excellent electrochemical properties of N/S-rGA-1 were ascribed to the following reasons [8,41]: (1) the co-doped heteroatoms enhanced the charge storage capacity of graphene by producing more defects, which increased the content of active sites; (2) the introduction of sulfur and nitrogen influenced the spin density and charge distribution of carbon atoms of graphene, and thus enlarged the effective activation region of the material surface; (3) heteroatom doping increased the surface polarity of the material because of the difference in electro negativity between carbon atoms and doped heteroatoms, thus improving surface wettability; (4) the mesoporous structure resulting from heteroatom doping provided short and passable ion transfer pathways, which were beneficial for enhancing the accessibility of electrolyte ions to the electrode/electrolyte interface; (5) heteroatom doping altered the electronic structure of graphene to enhance the conductivity of the materials.
Acknowledgments Financial support from the National Natural Science Foundation of China (Nos. 21873026, 21573058, and 21303044) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121932. References [1] Y. Gao, Y. Wan, B. Wei, Z. Xia, Capacitive enhancement mechanisms and design principles of high-performance graphene oxide-based all-solid-state supercapacitors, Adv. Funct. Mater. 28 (2018) 1706721. [2] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, A review of electrolyte materials and compositions for electrochemical supercapacitors, Chem. Soc. Rev. 44 (2015) 7484–7539. [3] A.C. Forse, C. Merlet, J.M. Griffin, C.P. Grey, New perspectives on the charging mechanisms of supercapacitors, J. Am. Chem. Soc. 138 (2016) 5731–5744. [4] C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-based supercapacitor with an ultrahigh energy density, Nano Lett. 10 (2010) 4863–4868. [5] P.C. Gao, W.Y. Tsai, B. Daffos, P.L. Taberna, C.R. P� erez, Y. Gogotsi, P. Simon, F. Favier, Graphene-like carbide derived carbon for high-power supercapacitors, Nano Energy 12 (2015) 197–206. [6] J. Kalupson, D. Ma, C.A. Randall, R. Rajagopalan, K. Adu, Ultrahigh-power flexible electrochemical capacitors using binder-free single-walled carbon nanotube electrodes and hydrogel membranes, J. Phys. Chem. C 118 (2014) 2943–2952. � [7] E. Sest, G. Dra�zi�c, B. Genorio, et al., Graphene nanoplatelets as an anticorrosion additive for solar absorber coatings, Sol. Energy Mater. Sol. Cells 176 (2018) 19–29. [8] P.V. Javier, S. Elisa, C. David, L. Luis, Functionalized graphene nanoplateletnanofluids for solar thermal collectors, Sol. Energy Mater. Sol. Cells 185 (2018) 205–209. [9] Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya, L.C. Qin, Graphene and carbon nanotube composite electrodes for supercapacitors with ultra-high energy density, Phys. Chem. Chem. Phys. 13 (2011) 17615–17624. [10] W. Zhang, Z. Chen, X. Guo, K. Jin, Y. Wang, L. Li, Y. Zhang, Z. Wang, L. Sun, T. Zhang, N/S co-doped three-dimensional graphene hydrogel for high performance supercapacitor, Electrochim. Acta 278 (2018) 51–60. [11] Y. Xu, C.Y. Chen, Z. Zhao, Z. Lin, C. Lee, X. Xu, C. Wang, Y. Huang, M.I. Shakir, X. Duan, Solution processable holey graphene oxide and its derived macrostructures for high-performance supercapacitors, Nano Lett. 15 (2015) 4605.
4. Conclusion N/S-rGA-1 with high sulfur content was prepared via a mild one-pot hydrothermal method using TTCA as a source of both nitrogen and sulfur. N/S-rGA-1 possessed a hierarchical porous structure and large pore volume, thus providing numerous effective adsorption sites for electrolyte ions. The N/S-rGA-1 electrode exhibited a high specific capacitance of 180.5 F g 1 at a current density of 1 A g 1 and good capacitance retention from 1 to 10 A g 1. More importantly, N/S-rGA-1 delivered an energy density as high as 75 Wh kg 1 in EMIMBF4 elec trolyte with a potential window of 3.5 V as well as stable cycling per formance. The favorable electrochemical performance of N/S-rGA-1 may be ascribed to its short ion diffusion pathways, numerous surface active sites, and high conductivity resulting from the incorporation of a high content of sulfur into the carbon skeleton. The impressive elec trochemical performance of N/S-rGA-1 makes it promising for highperformance supercapacitors.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of nitrogen/sulfur co-doped reduced graphene oxide aerogels for high-performance supercapacitors with ionic liquid electrolyte Yujuan Chen, Li Sun, Zhaoen Liu, Yuyang Jiang, Kelei Zhuo * Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� N/S co-doped graphene aerogels were prepared by a mild method. � The N/S-rGA-1 has high sulfur content and porous network structure. � The N/S-rGA-1 delivers a high energy density up to 75 Wh kg 1. � The N/S-rGA-1 exhibits impressive electrochemical performance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene Nitrogen and sulfur co-doping Ionic liquid Trithiocyanuric acid Supercapacitor
Graphene with a unique two-dimensional lamellar structure is considered an attractive carbon-based electrode material for supercapacitors. However, the actual electrochemical performance of graphene-based super capacitors is far below the theoretical level. In this work, reduced graphene oxide aerogels co-doped with different amounts of nitrogen and sulfur (N/S-rGAs) were prepared via a one-pot hydrothermal approach using trithiocyanuric acid as a doping agent. Characterization of the materials showed that the optimal N/S-rGA had a high sulfur content (4.1 at%) and an interconnected porous network structure that provided accessible diffusion channels for electrolyte ions, thus leading to low ion diffusion resistance. The optimal N/S-rGA displayed high electrical conductivity of 11.5 S cm 1 and a specific capacitance of 180.5 F g 1 at 1 A g 1. This N/S-rGA delivered a high energy density of 75 Wh kg 1 at a power density of 0.9 kW kg 1 in an ionic liquid (1-ethyl-3-methyl imidazolium tetrafluoroborate, EMIMBF4) electrolyte, and its energy density was still 33 Wh kg 1 when its power density was 15 kW kg 1. Consequently, N/S-rGAs are promising electrode material for high-performance ionic liquid-based supercapacitors.
1. Introduction Supercapacitors offer great potential as energy storage devices because of their higher safety, longer cycle life (105 cycles), faster
charge/discharge capability (within seconds), and higher power density (10 kW kg 1) than those of traditional energy storage devices [1–3]. In this regard, supercapacitors are attractive candidates for high-power equipment and grid energy storage systems for clean renewable
* Corresponding author. E-mail address: [email protected] (K. Zhuo). https://doi.org/10.1016/j.matchemphys.2019.121932 Received 4 April 2019; Received in revised form 26 July 2019; Accepted 1 August 2019 Available online 3 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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energy. However, the energy density of supercapacitors is usually rela tively low [4], which limits the practical applications of most super capacitors. Therefore, there is an urgent need to enhance the energy density of supercapacitors to expand their practical application range. At present, the most common method used to increase the energy density of supercapacitors to meet practical demands is to develop highperformance electrode materials. Carbon nanomaterials such as gra phene, carbon nanotubes, carbon fiber, and carbon derivatives [5,6] are often used as electrode materials for supercapacitors. In particular, graphene is considered a rising star of carbon nanomaterials [7,8], and is seen as a promising carbon-based electrode material for high-performance electrochemical supercapacitors because of its large theoretical specific surface area, excellent electronic conductivity, high mechanical stability, and high theoretical gravimetric specific capaci tance (526 F g 1) [9,10]. However, due to the strong π-π conjugation and van der Waals forces between graphene sheets, the inevitable agglom eration of graphene fabricated by electrochemical or chemical reduction severely suppresses the emergence of its favorable intrinsic properties [11]. In consequence, the electrochemical performance of current graphene-based supercapacitors has not reached their theoretical level. To eliminate the innate deficiencies of graphene, many researchers have been devoted to changing its external morphology and internal structure to inhibit the overlap of graphene sheets. Three-dimensional graphene materials such as graphene hydrogels, graphene aerogels (GAs), and graphene nanomesh possess hierarchical porous structure, and thus not only exhibit a larger accessible specific surface area, but also contain more convenient channels for the trans port of electrolyte ions compared with the case for two-dimensional graphene [12]. Doping graphene with heteroatoms can modify its in ternal electronic structure and bond polarization, thus increasing its surface wettability and electrical conductivity [13–15]. Heteroatom doping can also lead to pseudocapacitance, which enhances the elec trochemical performance of graphene [10]. Among doping elements, nitrogen has received considerable attention. Many researchers believe that including nitrogen atoms as electron donors can effectively improve the electrochemical performance of supercapacitors [16–18]. Further more, the introduction of sulfur can also induce lattice distortion and tailor the charge density distribution of graphene materials. Because sulfur atoms have a specific valence electronic structure and large size, sulfur doping can raise the capacitance of graphene-based supercapacitors. Recently, various methods have been developed to prepare heteroatom-doped graphene materials. For instance, sulfur and nitrogen co-doped mesoporous graphene with high energy density and capaci tance retention was synthesized by chemical vapor deposition and calcination at 800 � C [19]. Chen et al. [20] synthesized nitrogen and sulfur co-doped nanoporous carbon with a high sulfur content (3.36%) that displayed a specific capacitance of 73 F g 1 at a current density of 1 A g 1. Huang et al. [21] annealed graphene oxide-encapsulated ami no-modified silica nanoparticles at 1000 � C to fabricate a sulfur-doped three-dimensional porous reduced graphene oxide containing 2% sul fur. This material exhibited good electrochemical performance and excellent cycling stability. However, the sulfur-doped carbon materials with high sulfur content reported to date are prepared by complex procedures or at high temperature. Therefore, it remains important to develop simple and mild methods to prepare heteroatom-doped gra phene materials for use in high-performance supercapacitors. The electrolyte is another important factor affecting the energy density of supercapacitors because energy density is proportional to the square of the operating voltage of the electrolyte. At present, aqueous electrolytes with high ionic conductivity and safety have been widely used. However, the low decomposition voltage of water (1.23 V) limits the application of aqueous electrolytes [22]. As a result, supercapacitors based on aqueous electrolytes have low energy density. Organic elec trolytes possess a relatively wide operating voltage range and are favored by many supercapacitor researchers. However, most organic
electrolytes use acetonitrile or propylene carbonate as the solvent. The flammability, toxicity, and environmentally hazardous nature of these solvents are inevitable problems [23]. Ionic liquids (ILs) are composed of organic cations and anions that form molten salts at room tempera ture and are considered promising electrolytes for supercapacitors because of their nonvolatile nature, non-flammability, low vapor pres sure, and wide electrochemical stability window. Trithiocyanuric acid (TTCA), which has a high sulfur content, is an excellent heteroatom-containing precursor for photocatalysts, lith ium–sulfur batteries, and oxygen reduction catalysts [24–28]. In this work, nitrogen and sulfur co-doped reduced graphene oxide aerogels (N/S-rGAs) with high sulfur content are synthesized from TTCA and graphene oxide (GO) by a mild and effective one-pot hydrothermal approach. The electrochemical performance of the N/S-rGAs in IL electrolyte is investigated. 2. Experimental 2.1. Preparation of materials GO was prepared by oxidation of graphite powder according to the modified Hummers method [15,29,30]. A schematic of the preparation process of N/S-rGAs is shown in Scheme 1; experimental and charac terization details have been described in the Supporting Information. 2.2. Electrode preparation and electrochemical measurements Full details of electrode preparation and electrochemical testing methods are described in the Supporting Information. The gravimetric specific capacitances of the active material in the electrode (Cs) and cell (Ccell) were calculated from discharge curves using the following equa tions [15,31]: Cs ¼
2IΔt mΔU
Ccell ¼
1 Cs 4
(1) (2)
where I is the constant discharge current (A), Δt is the discharge time (s), ΔU is the discharge voltage with the IR drop removed (V), and m is the mass of active material in the electrode (g). The gravimetric energy density (E), power density (P), and coulombic efficiency (ƞ) of the electrode material in each cell were evaluated using the following equations: E¼
1 CsΔU 2 28:8
P ¼ 3600
η¼
E Δt
td tc
(3) (4) (5)
where td and tc represent the discharge time and charge time (s), respectively. 3. Results and discussion 3.1. Morphology characterization of materials The microstructural features of the undoped reduced graphene oxide aerogel (rGA) and N/S-rGAs were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as presented in Fig. 1. Fig. 1a shows that rGA has a porous block structure that was formed by the overlap and agglomeration of graphene sheets. N/S-rGA-1 (Fig. 1b) possesses a continuous cotton-like porous structure 2
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Scheme 1. Schematic illustration of the synthesis procedure of N/S-rGAs.
with good uniformity. Compared with inert graphene, GO possesses numerous oxygen-containing functional groups, such as carboxyl, hy droxyl, and epoxy, which endows it with higher chemical reactivity than that of graphene. During the hydrothermal reaction, sulfhydryl groups of TTCA will undergo esterification reactions with carboxyl groups of GO, and dehydrate with the hydroxyl groups to form C–S–C. In addition, as a nucleophile, sulfhydryl groups can also react with epoxy groups of GO to form water molecules [32,33]. Thus, the porous structure was
generated through the formation of hydrogen bonds between oxygen-containing functional groups on the GO surface and the sulfhy dryl groups of TTCA [12]. The TEM image of rGA showed that was an opaque sheet (Fig. 1a). In contrast, N/S-rGA-1 (Fig. 1c) possesses a thin and crumpled texture without severe aggregation. The difference in morphology between rGA and N/S-rGA-1 may be caused by the defect structure formed by the addition of heteroatoms into the graphene lat tice [34]. The unique structure of N/S-rGA-1 can provide more
Fig. 1. SEM images of (a) rGA and (b) N/S-rGA-1. TEM images of (c) rGA and (d) N/S-rGA-1. 3
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adsorption sites and transmission channels than in the case of rGA, which can improve its accessibility for electrolyte ions during the charging and discharging processes. It is believed that an interconnected porous structure is conducive to energy storage, giving rise to improved electrochemical properties [35,36]. SEM and TEM images of N/S-rGA-0.5 and N/S-rGA-2 indicated that these materials also possessed porous structures, but their morphologies were uneven because of the agglomeration and collapse of graphene layers (see Fig. S1). To explore the chemical composition of the materials and bonding mode of heteroatoms, the materials were evaluated using X-ray photo electron spectroscopy (XPS). In the XPS survey spectrum of N/S-rGA-1 (Fig. 2a), five peaks located at 284.8, 400, 533, 228, and 164 eV cor responding to C1s, N1s, O1s, S2s, and S2p, respectively [37], were observed, which confirmed that graphene was successfully doped with nitrogen and sulfur atoms. The atomic percentages of C, N, O, and S in the samples determined by XPS are summarized in Table S1. The sulfur content of N/S-rGA-1 was much higher than that of sulfur-doped gra phene materials reported previously [38,39]. The C1s spectra of GO and rGA are displayed in Fig. S2. Compared with those for GO, the C–O and C¼O peak intensities in the C1s spectrum of rGA were obviously weakened, indicating that oxygen-containing groups on the GO sheets were partly removed during the synthesis procedure [40]. The C1s spectrum of N/S-rGA-1 (Fig. 2b) was deconvoluted into four peaks, corresponding to C–C/C¼C (284.6 eV), C–S/C–O/C–N (285.1 eV), C¼O (286.2 eV), and O–C¼O (289.1 eV) [41,42]. Fig. 2c shows the high-resolution N1s spectrum of N/S-rGA-1, which was deconvoluted into two peaks centered at 400.2 and 401.8 eV that were assigned to pyrrolic-N and graphitic-N [43], respectively. Pyrrolic-N plays a major role in improving the surface wettability of graphene [44]. In addition, the existence of graphitic-N can change the electron density of the car bon network, resulting in increased electron conductivity and enhanced electrochemical performance [45]. The high-resolution S2p spectrum of N/S-rGA-1 (Fig. 2d) was deconvoluted into three peaks located at 164.0, 165.1, and 168.9 eV, which were consistent with C–S–C and C-SOx-C states (sulfonate or sulfone) [12,46]. The formation of thiophene-S
structure was attributed to spin–orbit coupling between sulfur atoms and their neighboring carbon atoms [47–49]. The XPS data for N/S-rGA-0.5 and N/S-rGA-2 are displayed in Fig. S3. All the spectra confirmed that the N/S-rGAs were doped with both nitrogen and sulfur. X-ray diffraction (XRD) is a suitable research method to obtain the crystal structure of carbon materials. Fig. 3a presents the XRD patterns of the GO, rGA, and N/S-rGAs. The peak located at about 2θ ¼ 9.8� was the characteristic diffraction peak of GO, which corresponds to the (001) crystal plane of graphite [50]. The corresponding interlayer spacing (d) was 0.9 nm, indicating that graphite was converted to GO. After hy drothermal reaction, this diffraction peak disappeared completely; instead, rGA displayed a relatively narrow peak at 25.1� . This result indicates that GO was reduced to rGA during hydrothermal treatment and rGA has a relatively high content of graphitic structure [51]. The N/S-rGAs displayed broad weak diffraction peaks centered at 2θ ¼ 24.5� and approximately 43.5� , which were indexed to the (002) and (100) planes, respectively. The XRD patterns indicate that the heteroatom doping decreased the crystallinity of graphene. Additionally, d of rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2 calculated by the Bragg equation were 0.35, 0.37, 0.38, and 0.37 nm, respectively. Thus, d of N/S-rGA-1 was the largest. The diameter of a sulfur atom (0.2 nm) is larger than that of a carbon atom (0.15 nm), so it is logical that d of graphene increases after sulfur atoms are added into the carbon skeleton [52]. The increase of d can facilitate the transfer of both electrons and electrolyte ions in the graphene-based materials. Raman spectra of the materials are shown in Fig. 3b. All the spectra exhibit two peaks situated at around 1352 and 1590 cm 1, corre sponding to D and G bands, respectively. The D band arises from the disordered carbon structure and crystal defects, including vacancies, amorphous carbon species, and heteroatoms, and the G band originates from the stretching vibration of in-plane sp2 hybridized carbon atoms in graphene [53,54]. The intensity ratio of D to G bands (ID/IG) can reflect the structural characteristics of graphene [42]. The ID/IG value for N/S-rGA-1 of 1.25 was higher than that of rGA (1.08), revealing that heteroatom doping destroyed the structure of the carbon rings and increased the defect density. The increase of ID/IG confirmed the
Fig. 2. (a) XPS survey spectrum of N/S-rGA-1. (b) C1s, (c) N1s, and (d) S2p high-resolution spectra and fitting peaks for N/S-rGA-1. 4
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Fig. 3. (a) XRD patterns of GO, rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2. (b) Raman spectra of rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2.
existence of amorphous carbon structure in N/S-rGA-1. Moreover, the presence of a relatively weak 2D band at ~2700 cm 1 indicated the existence of few graphene nanosheet layers in the samples [55]. Nitrogen adsorption/desorption isotherms and pore size distribu tions of rGA and N/S-rGA-1 are shown in Fig. 4. The specific surface area and pore volume for each sample determined from the isotherms are listed in Table S1. Fig. 4a reveals that the samples displayed type-IV isotherms. N/S-rGA-1 exhibited a type-H3 hysteresis loop at high rela tive pressure (P/P0) from 0.85 to 1.0, indicating that this sample con tains narrow mesoporous and macroporous structures [56]. The specific surface area of N/S-rGA-1 was smaller than that of rGA, which is attributed to higher microporosity of rGA than that of N/S-rGA-1. Hu et al. [57] pointed out that specific surface area is not the only factor that contributes to the specific capacitance of electrode materials; that is, the electrochemical properties of electrode materials are determined by both pore volume and specific surface area. Although rGA has a larger surface area than that of N/S-rGA-1, most of the specific surface area in rGA may not be utilized effectively during electrochemical cycling because micropores are not conducive to the diffusion of large ions present in IL electrolytes, especially under high loading rate [58]. The pore size distributions of the samples are displayed in Fig. 4b. The pore size distribution of N/S-rGA-1 had a broad peak at 2.7 nm and most pores were 2–4 nm in size, which indicates that N/S-rGA-1 has a large number of mesopores. In addition, N/S-rGA-1 contains some macro pores that could be suitable for ion storage. In contrast, the pore size distribution of rGA had a high peak at 1.5 nm, revealing the presence of abundant micropores. N/S-rGA-1 had a larger mesopore volume than that of rGO, and thus more effective specific surface are to provide adsorption sites for electrons and electrolyte ions. Furthermore, the larger pore width of N/S-rGA-1 than that of rGA can provide open transport channels to ensure the fast passage of the large ions of IL electrolytes.
3.2. Electrochemical performance of the aerogels To further study the electrochemical performance of the aerogels, they were used as electrode materials in symmetric coin cells. Cyclic voltammetry (CV) curves of the samples measured at 20 mV s 1 are displayed in Fig. 5a. The CV curve of rGA possesses a symmetrical approximately rectangular shape, indicating pure electrochemical double-layer capacitance behavior. The CV curves of N/S-rGAs are also approximately rectangular, suggesting that the main energy storage mechanism of N/S-rGAs is electrical double-layer capacitance. The CV curves of N/S-rGAs have a larger area than those of rGA, proving that doping with sulfur and nitrogen increases capacitance performance. The CV curve of the cell containing N/S-rGA-1 possessed the largest area of the samples, illustrating that it has outstanding capacitive properties. The excellent capacitance performance of N/S-rGA-1 is caused by its large layer spacing and effective pore volume that is favorable for the migration of electrolyte ions between the electrode surface and electrolyte. The galvanostatic charge–discharge (GCD) curves of the materials measured at a current density of 2 A g 1 displayed nearly symmetrical triangular shapes (Fig. 5b), suggesting ideal capacitor behavior [59]. The charge–discharge time for N/S-rGA-1 was the longest of the samples and it showed the highest capacitance. Notably, the materials exhibited small ohmic IR drops (rGA ¼ 0.268 V, N/S-rGA-0.5 ¼ 0.073 V, N/S-rGA-1 ¼ 0.072 V, and N/S-rGA-2 ¼ 0.081 V), which would ensure low equivalent series resistances. The specific capacitance of N/S-rGA-1calculated from its discharge curve was 175.8 F g 1 at 2 A g 1, which was larger than those of N/S-rGA-0.5 (135.2 F g 1), N/S-rGA-2 (147.3 F g 1), and rGA (53.7 F g 1). The poor capacitance performance of rGA is caused by the high resistance of its microporous structure to ion migration. Electrochemical tests of the aerogels are compared with those of commercial reduced graphene oxide (C-rGO) in Fig. S4. The electrochemical properties of rGA were similar to those of C-rGO; N/S-rGA-1 demonstrated the best electrochemical performance
Fig. 4. (a) Nitrogen adsorption/desorption isotherms of rGA and N/S-rGA-1. (b) Pore size distributions of rGA and N/S-rGA-1. Inset is the pore size distribution of rGA and N/S-rGA-1 in the range from 2 to 4 nm. 5
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Fig. 5. Electrochemical performance of rGA and N/S-rGAs. (a) CV curves measured at 20 mV s 10 A g 1, and (d) Nyquist plots. Inset is a magnified view of the high frequency range.
because of its effective mesoporous structure. Fig. 5c shows the change of the specific capacitance of the aerogels with increasing current density. The introduction of heteroatoms into rGA greatly increased its specific capacitance. For a given material, specific capacitance decreased with rising current density from 1 to 10 A g 1. N/S-rGA-1 delivered the highest specific capacitance of 180.5 F g 1 at 1 A g 1, which was much larger than that of rGA (72.9 F g 1). Even at a high current density of 10 A g 1, N/S-rGA-1 still reached a specific capacitance of 133.4 F g 1, whereas that of rGA was only 20.9 F g 1. The capacitance retention of N/S-rGA-1 was 74%, illustrating that N/S-rGA-1 has excellent capacitance property and rate performance. Electronic conductivity is an important factor when evaluating the electrochemical properties of a material. Electronic conductivities of the materials were determined to explain their electrochemical properties. The electrical conductivities of rGA, N/S-rGA-0.5, N/S-rGA-1, and N/SrGA-2 were 0.5, 3.9, 11.5, and 8.0 S cm 1, respectively, indicating that N/S-rGA-1 possessed the highest electrical conductivity of the materials. The effective improvement of conductivity is caused by the presence of nitrogen and sulfur atoms, which change the electronic structure of graphene, and thus improve its electrochemical properties. Electro chemical impedance spectroscopy (EIS) measurements were conducted on rGA and N/S-rGAs to investigate the interaction of the electrode materials with the electrolyte. Nyquist plots of the symmetric super capacitors are shown in Fig. 5d. The intercept of the plots with the x-axis was ascribed to the combined resistance (Rs), which is determined by the internal resistance of ions and electrons, as well as the contact resistance of the electrode material with the separator [60,61]. The Rs values for rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2 were 2.78, 5.59, 1.72, and 6.42 Ω, illustrating that N/S-rGA-1 had the lowest Rs of the mate rials. In addition, the incomplete semicircles at the high frequency re gion represent the Faradaic charge transfer resistance (Rct) at the electrode/electrolyte interface. The Rct values determined for rGA, N/S-rGA-0.5, N/S-rGA-1, and N/S-rGA-2 were 14.05, 1.26, 1.06, and 3.06 Ω, respectively, indicating that N/S-rGA-1 has the lowest Rct of the samples. The near-vertical lines in the low frequency region indicate the
1
, (b) GCD curves measured at 2 A g
1
, (c) rate capability from 1 to
perfect capacitive performance of the electrodes. The slope of the ver tical line was the steepest for N/S-rGA-1, meaning that it displays better capacitance characteristics than the other materials. Because doping with nitrogen and sulfur changes the morphology and electron structure of graphene, the N/S-co-doped materials exhibited relatively low Rct and fast ion diffusion, which led to excellent capacitive properties. The equivalent circuit model used to fit the experimental EIS data and the values of equivalent circuit parameters of the as-prepared electrodes were displayed in the Supporting information (Fig. S5 and Table S3). The above results revealed that the specific capacitance of N/S-rGA-1 was the largest of the materials. Here, we use N/S-rGA-1 as an example to embody the optimal electrochemical performance of the aerogels. The CV profiles of N/S-rGA-1 (Fig. 6a) measured at different scan rates in the specific electrochemical window of 0–3.5 V shows quasi-rectangular shapes, suggesting relatively good capacitive characteristics. The CV curves retained their shape even at a high scan rate of 200 mV s 1, confirming that N/S-rGA-1 exhibited good capacitance behavior. Fig. 6b presents the GCD curves of N/S-rGA-1 at various current densities. The specific capacitances calculated for N/S-rGA-1 were 180.5, 175.8, 169.1, 156.5, and 133.4 F g 1 at 1, 2, 3, 5, and 10 A g 1, respectively, corresponding to slight IR drops of 0.042, 0.072, 0.171, 0.178, and 0.366 V, respectively. Comparison of the specific capacitance of N/SrGA-1 with those of other reported materials (Table S2) revealed that N/S-rGA-1 displayed superior electrochemical performance to that of other materials. Ragone plot of a cell with N/S-rGA-1 as the electrode material is shown in Fig. 6c. The plot indicates that the energy density of N/S-rGA-1 is 75 Wh kg 1 at a power density of 0.9 kW kg 1. When the power density was raised to 15 kW kg 1, the energy density of N/S-rGA-1 was still as high as 33 Wh kg 1. The energy density of N/S-rGA-1 is higher than those reported for some other carbon-based electrode materials [18,62–68] (Table S2). For example, the energy density of N-rGO in BMIMBF4 is 55 Wh kg 1 [18] and that for diamine/triamine function alized graphene in BMIMBF4 is 51 Wh kg 1 [69]. The cycling stability of N/S-rGA-1 in a voltage window of 3.5 V at a current density of 5 A g 1 was also investigated (Fig. 6d). After 5000 charge and discharge cycles, 6
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Fig. 6. Electrochemical performance of N/S-rGA-1. (a) CV curves, (b) GCD curves, (c) Ragone plot of this work compared with other literature data, and (d) the cycling stability of N/S-rGA-1 at 5A g 1.
80% of the initial capacitance was still retained, revealing the good cycling stability of N/S-rGA-1. In addition, the coulombic efficiency of N/S-rGA-1 was almost 94%. The excellent electrochemical properties of N/S-rGA-1 were ascribed to the following reasons [8,41]: (1) the co-doped heteroatoms enhanced the charge storage capacity of graphene by producing more defects, which increased the content of active sites; (2) the introduction of sulfur and nitrogen influenced the spin density and charge distribution of carbon atoms of graphene, and thus enlarged the effective activation region of the material surface; (3) heteroatom doping increased the surface polarity of the material because of the difference in electro negativity between carbon atoms and doped heteroatoms, thus improving surface wettability; (4) the mesoporous structure resulting from heteroatom doping provided short and passable ion transfer pathways, which were beneficial for enhancing the accessibility of electrolyte ions to the electrode/electrolyte interface; (5) heteroatom doping altered the electronic structure of graphene to enhance the conductivity of the materials.
Acknowledgments Financial support from the National Natural Science Foundation of China (Nos. 21873026, 21573058, and 21303044) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121932. References [1] Y. Gao, Y. Wan, B. Wei, Z. Xia, Capacitive enhancement mechanisms and design principles of high-performance graphene oxide-based all-solid-state supercapacitors, Adv. Funct. Mater. 28 (2018) 1706721. [2] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, A review of electrolyte materials and compositions for electrochemical supercapacitors, Chem. Soc. Rev. 44 (2015) 7484–7539. [3] A.C. Forse, C. Merlet, J.M. Griffin, C.P. Grey, New perspectives on the charging mechanisms of supercapacitors, J. Am. Chem. Soc. 138 (2016) 5731–5744. [4] C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-based supercapacitor with an ultrahigh energy density, Nano Lett. 10 (2010) 4863–4868. [5] P.C. Gao, W.Y. Tsai, B. Daffos, P.L. Taberna, C.R. P� erez, Y. Gogotsi, P. Simon, F. Favier, Graphene-like carbide derived carbon for high-power supercapacitors, Nano Energy 12 (2015) 197–206. [6] J. Kalupson, D. Ma, C.A. Randall, R. Rajagopalan, K. Adu, Ultrahigh-power flexible electrochemical capacitors using binder-free single-walled carbon nanotube electrodes and hydrogel membranes, J. Phys. Chem. C 118 (2014) 2943–2952. � [7] E. Sest, G. Dra�zi�c, B. Genorio, et al., Graphene nanoplatelets as an anticorrosion additive for solar absorber coatings, Sol. Energy Mater. Sol. Cells 176 (2018) 19–29. [8] P.V. Javier, S. Elisa, C. David, L. Luis, Functionalized graphene nanoplateletnanofluids for solar thermal collectors, Sol. Energy Mater. Sol. Cells 185 (2018) 205–209. [9] Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya, L.C. Qin, Graphene and carbon nanotube composite electrodes for supercapacitors with ultra-high energy density, Phys. Chem. Chem. Phys. 13 (2011) 17615–17624. [10] W. Zhang, Z. Chen, X. Guo, K. Jin, Y. Wang, L. Li, Y. Zhang, Z. Wang, L. Sun, T. Zhang, N/S co-doped three-dimensional graphene hydrogel for high performance supercapacitor, Electrochim. Acta 278 (2018) 51–60. [11] Y. Xu, C.Y. Chen, Z. Zhao, Z. Lin, C. Lee, X. Xu, C. Wang, Y. Huang, M.I. Shakir, X. Duan, Solution processable holey graphene oxide and its derived macrostructures for high-performance supercapacitors, Nano Lett. 15 (2015) 4605.
4. Conclusion N/S-rGA-1 with high sulfur content was prepared via a mild one-pot hydrothermal method using TTCA as a source of both nitrogen and sulfur. N/S-rGA-1 possessed a hierarchical porous structure and large pore volume, thus providing numerous effective adsorption sites for electrolyte ions. The N/S-rGA-1 electrode exhibited a high specific capacitance of 180.5 F g 1 at a current density of 1 A g 1 and good capacitance retention from 1 to 10 A g 1. More importantly, N/S-rGA-1 delivered an energy density as high as 75 Wh kg 1 in EMIMBF4 elec trolyte with a potential window of 3.5 V as well as stable cycling per formance. The favorable electrochemical performance of N/S-rGA-1 may be ascribed to its short ion diffusion pathways, numerous surface active sites, and high conductivity resulting from the incorporation of a high content of sulfur into the carbon skeleton. The impressive elec trochemical performance of N/S-rGA-1 makes it promising for highperformance supercapacitors.
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