Nitrogen, sulfur and phosphorus tri-doped holey graphene oxide as a novel electrode material for application in supercapacitor

Journal of Alloys and Compounds 815 (2020) 152328

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Nitrogen, sulfur and phosphorus tri-doped holey graphene oxide as a novel electrode material for application in supercapacitor Jilei Liu, Yirong Zhu, Xianhong Chen*, Wenjie Yi College of Metallurgy and Material Engineering, Hunan University of Technology, Zhuzhou, 412007, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 May 2019 Received in revised form 5 September 2019 Accepted 17 September 2019 Available online 18 September 2019

Heteroatom doped graphene-based materials have been illustrated to be a superior approach to improve the performance of electrode materials for supercapacitors. In this work, the graphene oxide (GO) was prepared by a modified Hummers’ method employing expanded graphite as precursor, and further etched by hydrothermal method to obtain holey graphene oxide (denoted as HHGO). The resulting HHGO was utilized to synthesize a novel nitrogen(N), sulfur(S) and phosphorus (P) tri-doped holey GO (N, S, PHHGO) material by a hydrothermal approach utilizing ammonium dihydrogen phosphate and L-cysteine as N, S and P sources and used as supercapacitor electrode materials for the first time. The electrochemical results reveal that the N, S, P-HHGO delivers a high gravimetric capacitance of 295 F g
1 at 1 A g
1 in 2 M KOH aqueous electrolyte, outstanding rate capability with 71.2% of capacity retention rate from 1 to 20 A g
1 and excellent cycling stability with 93.5% of initial capacity retention at 3 A g
1 above 10000 cycles. The outstanding electrochemical properties of N, S, P-HHGO can be attributed to a superior pore-size distribution and the introduction of N, S and P heteroatoms, and shows great potential application in supercapacitors as well as other energy storage devices. © 2019 Elsevier B.V. All rights reserved.

Keywords: Holey graphene oxide N, S, P tri-doping Electrode materials Electro-chemistry Supercapacitors

1. Introduction With the further depletion of fossil fuels, energy demand is getting more urgent [1,2]. As one of the very important energy storage devices, supercapacitors have aroused keen interest because of their high power density, rapid charge-discharge process, length of cycling time and high reliability [3,4]. Generally, a high-performance supercapacitors electrode demands high electrical conductivity, high ion-accessible surface area, high ionic transport rate and high electrochemical stability. Unfortunately, the low energy density of supercapacitors hinders their widespread application. In order to solve this problem, various efforts have been carried out to improve the properties of electrode materials because the performance of supercapacitors is mainly determined by electrode materials [5]. The electrode materials of supercapacitors chiefly include carbon materials, transition metal oxides and conductive polymer materials [6e8]. Among them, carbon materials not only own high conductivity, big specific surface area, outstanding cycling stability, relatively low cost as well as facile

* Corresponding author. E-mail address: [email protected] (X. Chen). https://doi.org/10.1016/j.jallcom.2019.152328 0925-8388/© 2019 Elsevier B.V. All rights reserved.

preparation technology, but also are environmentally friendly, and play key roles in electrode materials for supercapacitors [5,6]. This is the reason why commercial supercapacitors mainly apply activated carbon to the active electrode materials. However, its relatively low specific capacitance makes it unable to meet the needs of scientific and technological development [9]. Therefore, the development of supercapacitor electrode materials with remarkable properties has become an expectation. Compared to activated carbon, graphene, the first one-atomthick two-dimensional carbon material, has become a promising material on account of its unique structure and superior physicochemical properties [10e13]. Especially, graphene displays high surface area, pore size distribution, and better surface exposure to electrolytes, has been considered to be an ideal component of the supercapacitors electrode materials [8,11,14,15]. Although graphene-based symmetric supercapacitor devices exhibit a large electrical double layer capacitance, the capacitances of capacitors are generally in the range of 100e200 F g
1 using aqueous and organic electrolytes [10], which is exceedingly below the theoretical electrochemical double-layer capacitance of 550 F g
1 [16]. The main reason is that graphene sheets tend to aggregate in the process of fabrication due to Van Der Waals interactions, which greatly reduces the specific surface area, induces inferior ions transport

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capabilities, and hinders a substantial number of active sites inaccessible to reactants especially at high charging rates [17e19]. Therefore, the stacking of graphene nanosheets represents a limitation for development of supercapacitors. Research effort mainly highlights the increase of specific capacitance without sacrificing the cycle life. As for graphene-based materials, there are two strategies to alleviate the disadvantage: design porous graphene-based electrode materials and introduce heteroatoms into graphene. Actually, 3D porous graphene with high surface area and high conductivity is hard to be synthesized [20]. On the other hand, it has been proven to be an effective approach that functionalization of heteroatoms can alter the intrinsic chemical and electrical properties of carbon [21]. Doping graphene with heteroatoms (N, B, S, P, etc.) is able to introduce active sites with a minimal change of the conjugation length. Recently, some studies have shown that singly N-doping [3,17,19,22e25] or P-doping [26] or S-doping [27], dual N, S-doping [4,14,28,29] or N, P-doping [5,30] or N, B-doping [31] of graphene can enhance the wettability and the accessibility of the active surface area to the electrolyte solution as well as the binding with ions. Moreover, surface modifcation of graphene-based materials can provide additional surface redox pseudocapacitance which occurs on or near the electrode surface through accommodation/ relaxation of charges on the electrochemical active sites with Faradaic charge transfer [32,33]. Along this line, ternary heteroatoms (N, S, P)-doped graphene [34,35] and N, P, O-doped graphene have been investigated [36]. For example, Zhao et al. [36] prepared an NePeO co-doped free standing 3D graphene by one-pot red phosphorus-assisted “cutting-thin” technology. The as-prepared NePeO co-doped free standing 3D graphene exhibited enhanced gravimetric capacitances (413 F g
1 in 6 M KOH aqueous electrolyte) in comparison with other carbon materials. The results of density functional theory calculations demonstrate that NePeO codoping may remarkably improve charge delocalization and electrochemical activity, leading to increasing capacitance for supercapacitor applications. The work strongly suggests that tri-doping can effectively improve graphene properties utilized as the supercapacitor electrode material for their various structure and bonding when compared to single or dual atom doped materials [4]. Although great progress has been made in heteroatoms doped with graphene-based electrode materials, how to effectively develop multi-doped graphene-based electrode materials for supercapacitors is still a challenge in practical applications. As far as we are concerned, the application of N, S, P tri-doped holey graphene oxide (N, S, P-HHGO) in supercapacitors has not been reported. In the current work, a novel N, S, P tri-doped holey graphene oxide (N, S, P-HHGO) was prepared and utilized as an electrode material of supercapacitors for the first time. In a typical process, the graphene oxide (GO) was prepared by using expanded graphite as a precursor with a modified Hummers’ method, and then three different routes were used to synthesize porous reduced graphene oxide, including chemical activation, reflux and hydrothermal methods (denoted as AHGO, RHGO and HHGO, respectively). The FESEM result reveals that HHGO has more surface folds and more porous structures, and electrochemical test shows that HHGO possesses better electrochemical performances compared with AHGO and RHGO. Therefore, the HHGO was further selected for doping of heteroatoms to fabricate the N,P-doped holey graphene oxide (N, P-HHGO), N,S-doped holey graphene oxide (N, S-HHGO) and N,S,P tri-doped holey graphene oxide (N, S, P-HHGO) by a hydrothermal method. Compared with N, P-HHGO and N, S-HHGO, the N, S, P-HHGO exhibits an enhanced electrochemical performance. The gravimetric specific capacitance of N, S, P-HHGO is up to 295 F g
1 at 1 A g
1, which is higher than that of N, P-HHGO (230 F g
1) and N, S-HHGO (260 F g
1). Moreover, the N, S, P-HHGO

displays an outstanding cycling stability of 93.5% of initial capacity retention at a current density of 3 A g
1 over 10000 cycles. The superior electrochemical performance of N, S, P-HHGO can be attributed to a superior pore-size distribution and the introduction of N, S and P heteroatoms. Therefore, the N, S, P-HHGO electrode material is likely to be a potential candidate in the application of other fields. 2. Experimental procedures 2.1. Materials preparation GO was prepared from natural graphite based on a modified Hummers' method [37]. Firstly, the expanded graphite was prepared through hydrothermal method [38]. Typically, 3 g of nature graphite, 45 mL of concentrated sulfuric acid, 9 mL of phosphoric acid and 6 g of potassium permanganate mixed well in a three-neck flask at 0  C, stirred for 30 m, and then the temperature was adjusted to 35  C. After stirring for another 90 m, the mixture became a thick paste, and then 100 mL of distilled water was added dropwise to the viscous mixture. The mixture was further maintained at 35  C and stirred for another 8 h. After being cooled to ambient temperature, the mixture was poured into ice water (500 mL) with 10 mL of H2O2 (30 wt%). The as-obtained mixture was centrifuged at 3500 rpm and the solid precipitate was collected by centrifugation, washed with HCl (5 wt%) and deionized water several times, respectively. The obtained product was dried at 60  C for 24 h and grounded into powder. The expanded graphite was obtained after the powder product was put into a muffle furnace at 400  C for 90 s. Secondly, GO was synthesized by using the prepared expanded graphite as a precursor according to a modified Hummers’ method. The holey graphene oxide was prepared by a hydrothermal method according to the literature [39]. Typically, 5 mL of H2O2 (30 wt%) and 50 mL of GO dispersion (2 mg/mL) were well dispersed in an Teflon-lined stainless steel autoclave and heated in an oven at 100  C for 2 h. The as-prepared product was collected by centrifugation, washed with deionized water for three to five times to remove impurities and further freeze-dried, the final product (HHGO) was obtained. The activated holey graphene oxide (AHGO) was prepared by chemical activation method referring to the literature [40]. Typically, 5 mL of 7 M KOH aqueous solution and 50 mL of GO dispersion (2 mg/mL) mixed adequately and ultrasonicated for 2 h. Then, the as-obtained mixture solution was dried at 60  C for 20 h and further heated at 180  C for 1 h in a vacuum oven. Finally, the mixture was washed with distilled water for three to five times and then freezedried to obtain the product. As a contrast, the reflux holey graphene oxide (RHGO) was prepared by reflux synthetic method. In a typical process, 5 mL of hydrogen peroxide and 50 mL of GO dispersion (2 mg/mL) were well dispersed in a three-neck flask and heated at 100  C for 2 h and the as-prepared mixture was further collected by centrifugation, washed with deionized water for three to five times and freezedried to obtain the RHGO. Nitrogen, sulfur and phosphorus tri-doped holey graphene oxide (N, S, P-HHGO) was prepared by a hydrothermal synthesis route employing HHGO dispersion as a precursor and ammonium dihydrogen phosphate (ADP) and L-cysteine as dopants. In this process, 60 mL of HHGO (1 mg/mL), 66 mg of ADP and 60 mg of L-cysteine were well dispersed in a Teflon-lined stainless steel autoclave and heated in an oven at 140  C for 2 h. After being cooled to room temperature, the product was collected by centrifugation, washed with deionized water for three to five times, and then freeze-dried for one day to obtain the N, S, P-HHGO. N, P co-doped holey GO (N,

J. Liu et al. / Journal of Alloys and Compounds 815 (2020) 152328

P-HHGO) and N, S co-doped holey GO (N, S-HHGO) was prepared by aforesaid similar synthesis routes, and the difference is that the dopants of ammonium dihydrogen phosphate (ADP) and L-cysteine were used, respectively. 2.2. Materials characterization The X-ray diffraction (XRD) patterns of the samples were achieved on a Rigaku D/max 2550 VBþ. The chemical structure of the samples was examined by utilizing the Fourier Transform Infrared Spectrophotometer (FTIR, AVTATAR, 370). Field Emission Scanning Electron Microscopy (FESEM) imaging, Energy Dispersive Spectroscopy (EDS) and the corresponding elemental mapping were carried out using a MIRA3 Field Emission Scanning Electron Microscopy. Transmission electron microscopy and high resolution transmission electron microscopy were performed employing a JEM-2100F instrument to characterize the morphology and microstructure of the samples. Raman spectra were obtained using a Lab RAM Raman system and the 532 nm line of an argon ion laser was utilized as the excitation source. Surface compositions of the samples were investigated by an ESCALab250 X-ray Photo-electron Spectroscopy (XPS). The Brunauere-Emmette-Teller (BET, BELSORPMINIII) specific surface area was gained from the nitrogen adsorption-desorption isotherm recorded at 77 K and the pore-size distribution was explored by utilizing the Barrette-JoynereHalenda (BJH) model.

Fig. 1. XRD patterns of the prepared samples.

2.3. Electrochemical measurements The working electrodes were prepared by blending the asobtained electroactive materials, polyvinylidene difluoride (PVDF) binder, and carbon black in a weight ratio of 80:10:10, and then the as-obtained mixture was pressed onto current collectors under 10 MPa for 10 s. The average mass of the active material in the fabricated electrode is about 2 mg cm
2. The electrochemical properties of the prepared samples were determined in a threeelectrode system, which is composed of the working electrode, platinum counter electrode and Hg/HgO reference electrode. Cyclic voltammetry (CV), galvanostatic current charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out on a CHI760E electrochemical workstation in 2 M KOH aqueous electrolyte. EIS measurements were performed by employing an AC voltage with 5 mV amplitude in a frequency range from 0.1 Hz to 100 kHz at open circuit potential. Fig. 2. FT-IR spectra of the prepared samples.

3. Results and discussion The synthetic process of N, S, P-HHGO is shown in Scheme 1.

Scheme 1. The synthetic process of the N, S, P-HHGO sample.

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Simply, GO sheet was etched by H2O2 under hydrothermal condition, and the as-prepared HHGO sheet was further doped with N, S, and P heteroatoms by a hydrothermal reaction using ADP and Lcysteine as dopants. By the two-step hydrothermal process the asresulted N, S, P-HHGO was successfully obtained. XRD patterns of the samples are exhibited in Fig. 1. The characteristic diffraction peak of GO at 2q ¼ 10.1 can be ascribed to the (001) crystalline plane [41], indicating the GO has been successfully

prepared from nature graphite. Compared with GO, the diffraction peak of AHGO and RHGO at 2q ¼ 10.1 disappears and exhibits a new peak at 2q ¼ 24.5 which is attributed to (002) plane of graphite [42], implying a relatively larger interlayer spacing [43]. This is probably due to the introduction of heteroatoms such as N, S as well as P and the existence of some remained oxygen functional groups between the layers such as eCOOH. Unlike AHGO and RHGO, the diffraction peak of HHGO shifts to 2q ¼ 11.7, which

Fig. 3. (a) XPS survey spectrum, high-resolution (b) C 1s spectrum, (c) N 1s spectrum, (d) S 2p spectrum, (e) P 2p spectrum and (f) O 1s spectrum of the N, S, P-HHGO sample.

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demonstrates the presence of some oxygen-containing groups in the HHGO matrix [4,22]. Meanwhile, the peak at 2q ¼ 24.5 becomes weaker and broader which reveals that the order restacking of the HHGO sheets increases [14,32]. After doping HHGO with heteroatoms, the peak at 2q ¼ 11.7 is disappeared for N, S, P-HHGO, N, P-HHGO and N, S-HHGO, implying that the HHGO can be partly reduced during the hydrothermal reaction [22]. Simultaneously, an obvious peak at 2q ¼ 24.5 can be found, indicating graphitic structure gets more disorder [34]. And another small peak at 2q ¼ 43.1 can be observed, which is corresponding to (100) inplane hexagonal atom arrangement of graphene [42], illustrating

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abundant defect sites and disorders of graphene after doping heteroatoms [14]. Additionally, the N, S, P-HHGO presents a weaker and broader peak at 2q ¼ 24.5 compared with N, P-HHGO or N, SHHGO, indicating the defects increase in N, S, P-HHGO [4], which may be attributed to the additional introduction of S or P atom into HHGO sheets. The FT-IR spectra of samples are displayed in Fig. 2. As for GO, the characteristic peaks at 3437, 1726, 1632, 1395, and 1058 cm
1 can be ascribed to eOH stretching vibration, C]O stretching vibration, eOH bending vibration from hydroxyl groups, and CeO stretching vibrations, respectively [43,44]. By comparing three

Fig. 4. FESEM images of (a) EG, (b) GO, (c) RHGO, (d) AHGO, (e) HHGO, (f) N, P-HHGO, (g) N, S-HHGO and (h) N, S, P-HHGO, respectively. (i) FESEM image along with the EDS elemental mapping and (j) EDS spectrum of N, S, P-HHGO.

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different kinds of porous graphene oxide (RHGO, AHGO and HHGO) with GO, no remarkable changes of their peaks have been observed. As for the N, S, P-HHGO, the peak at about 3437 cm
1 can be attributed to overlapping of the stretching vibration of -O-H and NeH moieties, and the peaks at 1576 and 1161 cm
1 can be associated with the C]C (aromatic rings), CeO, CeN, and CeS linkages, respectively [45]. Besides, the peaks at 1161 and 1078 cm
1 can be ascribed to the P]O and PeOeC, respectively [46,47]. These results demonstrate that N, S and P have been successfully introduced into HHGO. The XPS measurements further verified the successful doping of N, P and S into the graphitic matrix of the graphene sheets as shown in Fig. 3. The XPS survey spectrum of the N, S, P-HHGO in Fig. 3a shows O 1s, N 1s, C 1s, S 1s and P 1s signals acquired at various binding energies of 532.0, 399.5, 284.5, 164.0 and 136.9 eV, respectively [35,48], which implies the presence of N, S and P species within the HHGO matrix. The chemical composition of N, S, P-HHGO was also confirmed by element analysis, and the relative content of C, N, O, S and P is found to be 81.91, 2.94, 12.28, 2.2 and 0.67 at.%, respectively. The O signal at 532.02 eV is probably ascribed to moisture, atmospheric O2 adsorbed on N, S, P-HHGO, as well as the residual oxygencontaining groups [35]. The high resolution spectra can provide more detailed information about the bonding configurations and chemical environments for the tri-doped graphene. In detail, the high resolution spectrum of the C 1s (Fig. 3b) can be fitted into three sub-peaks, which reflects the species of C]C/CeS (284.6 eV), CeO/CeP/CeN (285.6 eV) and CeOeC (286.9 eV) [5,14,49]. The presence of CeP, CeN and CeS peaks further indicate that three heteroatom species are successfully doped into the HHGO sheets, which are in agreement with the above results of FT-IR spectra.

Similarly, the N 1s peak (Fig. 3c) could be deconvoluted into three different signals with binding energies of 398.2, 400.0 and 401.4 eV that correspond to pyridinic N, pyrrolic N and graphitic N, respectively [14]. The S 2p peak was further examined, as shown in Fig. 3d. It can be fitted with three peaks, corresponding to CeSeC species of S 2p3/2 at 163.9 eV, S 2p1/2 at 164.8 eV and SOx species at 168.2 eV, respectively [4]. Fig. 3e shows high resolution P 2p peak can be fitted with two peaks located at 134.0 and 134.9 eV, which implies PeO binding and PeC, respectively [35,50]. The O 1S (Fig. 3f) shows two peaks at 531.7 and 533.5 eV, corresponding to the PeO and CeO bonds, respectively [49], which confirms again that the P have been successful doped into HHGO sheets. From the XPS results, we could successfully speculate that N, P and S were covalently bonded with C in graphene lattice. The morphological and structural features of the samples were investigated by FESEM, EDS and corresponding elemental mapping, TEM and HRTEM. The expanded graphite (Fig. 4a) displays a wormlike structure, and the GO (Fig. 4b) reveals a layered structure with slightly wrinkled surface. As shown in Fig. 4c and d, the wrinkles of AHGO and RHGO are more obvious than those of graphene oxide, which may be caused by the introduction of pore-forming agents. Different from AHGO or RHGO, the HHGO (Fig. 4e) presents a layered structure with more obvious wrinkles on the sheets. The morphologies of N, P-HHGO, N, S-HHGO and N, S, P-HHGO were observed in Fig. 4feh, in which N, S-HHGO and N, S, P-HHGO demonstrate a similar lamellar interconnected network structure while N, P-HHGO displays a multi-layer spongy-like structure. This structural difference may be attributed to the use of different dopants. Among these samples, these curly N, S, P-HHGO sheets are connected in disorder to form a network structure, which can easily form a three-dimensional porous structure. Deformation in

Fig. 5. (aec) TEM images of N, S, P-HHGO with different magnifications. (d) HRTEM image of the N, S, P-HHGO sample.

J. Liu et al. / Journal of Alloys and Compounds 815 (2020) 152328

Fig. 6. Raman spectra of the GO, HHGO and N, S, P-HHGO samples.

structure may be ascribed to the decomposition of L-cysteine and ADP into NH3, H2S, and PH3 [29,36]. NH3 gas can be decomposed into free radicals such as NH2, NH and H radicals at 140  C, and these radicals are able to react with the carbon of holey graphene oxide, resulting in the gasification of carbon in the form of methane HCN and so on [51]. Elemental mapping images of N, S, P-HHGO sample (Fig. 4i) reveals that N, S and P are homogeneously doped into HHGO. EDS spectrum of N, S, P-HHGO sample is presented in Fig. 4j, and atomic content of C, N, O, S and P are about 80.95, 3.06, 12.83, 2.51 and 0.65 at.%, respectively. The results are consistent with XPS investigation. TEM and HRTEM were further carried out to investigate the microstructure of the as-synthesized N, S, P-HHGO, and a typical transparent two-dimensional nanosheet-like structure is exhibited in Fig. 5aed. They are not a typical smooth graphene sheet, but are covered by disordered wrinkles, which may be caused by the introduction of the heteroatoms (N, S and P) into HHGO. Besides, a highly curved structure can be observed through HRTEM (Fig. 5d), which plays a significant role in increasing the specific surface area of N, S, P-HHGO.

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The structural information about GO, HHGO and N, S, P-HHGO was further obtained from Raman spectra as shown in Fig. 6. Obviously, the three samples display D-band characeristic peak (disordered sp3 hybridized carbon) at around 1325 cm
1 and Gband characeristic peak (the E2g mode of the sp2 carbon domains) at around 1582 cm
1 [4,35]. In general, the D-band is related to the defect sites or edges of graphene and the G band is ascribed to the band stretching of sp2 hybridized carbon network [52]. Therefore, the intense ratio of D band to G band (ID/IG) is utilized to investigate the degree of defect [35]. It can be watched from Fig. 6 that the ID/IG value of N, S, P-HHGO, HHGO and GO are 1.33, 1.29 and 1.26, respectively. The ID/IG value of GO is the lowest. The results imply that N, S, P-HHGO possess more defects (or active sites) than HHGO and GO after the tridoping of N, S and P. This could be because doping of N, S and P induce the electron density and the spin density of carbon atoms in GO sheets changed and further improve activity of GO, which was substantiated in the electrochemical performance analysis part later. The specific surface area and pore-size distribution of N, S, PHHGO sample was performed using BET (Fig. 7). The adsorption/ desorption curve can be attributed to a type-IV isotherm with a H3 hysteresis loop [53], implying the existence of mesoporous structure of N, S, P-HHGO. The specific surface area of the N, S, P-HHGO is 26.70 m2 g
1 from Fig. 7a by computation. Besides, the mesoporous structure was also confirmed by the Barrette-Joynere-Halenda (BJH) pore-size distribution data (Fig. 7b). The pore size of the N, S, P-HHGO sample is mainly concentrated in the range of 1e5 nm and next 20e60 nm. Intensive research demonstrate that carbon materials with high specific surface areas and suitable mesopores (especially 2e5 nm) are beneficial to enhance the properties of supercapacitors by providing abundant electroactive sites and shorting diffusion channel for charge transports [9,44]. Besides, rich mesopores usually result in superior rate capability because of their improved reaction kinetics, while a certain number of macropores are useful for ion reservation, leading to the enhanced electrochemical capacitive performances. Therefore, such porous structure with rational pore-size distribution is considered to be very beneficial for application in supercapacitors. The electrochemical properties of the prepared samples were explored by CV, GCD and EIS. Herein, the fabricated working electrode, platinum counter electrode and Hg/HgO reference electrode constituted a three-electrode system, which was utilized in 2 M KOH aqueous solution. Fig. 8a compares the CV curves of the

Fig. 7. (a)Nitrogen adsorption/desorption isotherm and (b) the pore size distribution of the N, S, P-HHGO sample.

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Fig. 8. (a) CV curves of the prepared electrodes at a scan rate of 100 mV s
1, (b) galvanostatic charge-discharge curves of the prepared electrodes at a current density of 1 A g
1, (c) the specific capacitance change of the prepared electrodes as a function of current density.

prepared electrodes at a scan rate of 100 mV s
1, and all the curves show similar rectangular shapes, obviously indicating remarkable electrical double layer property. It can be clearly observed that the capacitance response of the HHGO electrode is larger than that of the RHGO and AHGO electrodes. Moreover, we can see that the N, S, P-HHGO electrode presents the largest capacitance response among these test electrodes, which may be attributed to introduction of N, S and P heteroatoms, resulting in the improvement of

electrochemical properties. Fig. 8b presents the comparison of GCD curves of these prepared electrodes at a current density of 1 A g-1. The curves exhibit the similar triangular for the prepared samples, implying the electric double-layer property. The specific capacitances of these prepared electrodes can be calculated by the following equation [54]: Cm ¼ (I  Dt)/(m  DV), where I (A) denotes the current of discharge, Dt is the discharge time, m is the mass of active electrode and DV is the discharge potential range. The mass specific capacitances of the prepared GO, RHGO, HHGO, AHGO, N, P-HHGO, N, S-HHGO and N, S, P-HHGO electrodes are 107, 134, 172, 157, 230, 260 and 295 F g
1 at 1 A g
1, respectively. From these calculated values, we know that the HHGO electrode exhibits the larger specific capacitance (172 F g
1) compared with the RHGO (134 F g
1) and AHGO (157 F g
1) electrodes, which may be attributed to the richer winkles on the HHGO sheet. Additionally, the N, S, P-HHGO electrode presents a larger specific capacitance of 295 F g
1 than that of the N, P-HHGO (230 F g
1) and N, S-HHGO (260 F g
1) electrodes, which can be ascribed to the synergistic effect of N, S and P-doping. The obtained specific capacitance of the N, S, P-HHGO electrode is comparable to that of hierarchically structured P-incorporating nanoporous carbons (265.43 F g
1 in organic electrolyte) and heteroatom-doped carbon dots and nanoporous carbons (298.43 F g
1 in 1 M KOH aqueous electrolyte) reported in the literature [55,56]. Fig. 8c exhibits the specific capacitance vs current density plot for the prepared electrodes as evaluated from GCD curves. The results reveal that the specific capacitance decreases with the increase of current density, which is attributed to the slow diffusion of ions in the electrolyte and the inadequate utilization of the active material at higher current [57]. Fig. 9a exhibits the CV curves of the N, S, P-HHGO electrode at scanning rates of 5e200 mV s
1, which shows similar rectangular shapes, clearly implying superior electrical double layer property. Furthermore, the GCD curves of the N, S, P-HHGO electrode at different current densities (1e20 A g
1) are presented in Fig. 9b. The specific capacitance of the N, S, P-HHGO electrode is calculated to be 295, 254, 234, 225, 216 and 210 F g
1 at current densities of 1, 2, 3, 5, 10 and 20 A g
1, respectively. In addition, the capacity retention rate of N, S, P-HHGO electrode at current densities of 10 and 20 A g
1 (compared with 1 A g
1) are 73.2 and 71.2%, respectively. The outstanding rate performance may be caused by the superior pore-size distribution with abundant mesopores and a certain number of macropores and the modification of nitrogen, sulfur and phosphorus heteroatoms. Moreover, in order to better understand the remarkably improved capacitance of the N, S, PHHGO electrode, we have made a further analysis based on Dunn's work [58]. It is reported that the percentage of pseudo-capacitive and capacitive contribution can be determined by the following equation [59]: i (V) ¼ k1v þ k2v1/2, Where k1 and k2 are adjustable values and could be determined from the slope and the y-axis intercept of linear equation through plotting i/v1/2 versus v1/2. Then, the percentage of pseudo-capacitive contribution can be calculated, and the capacitive contribution ratio of N, S, P-HHGO electrode is 39.4% at 5 mV s
1 as shown in Fig. 9c (inner blue area for capacitive contribution). Besides, Fig. 9d presents the calculated contribution ratio of the capacitive capacity at different scan rates ranging from 5 to 200 mV s
1. Obviously, the capacitive contribution ratio is growing gradually with the increasing scan rates and the N, S, PHHGO represents a high pseudo-capacitive contribution ratio (42.8% of total specific capacitance) even at 200 mV s
1. This pseudocapacitance may be ascribed to the rich surface sites which play a key role for the improved electrochemical performance [60], and thus it still exhibits a high specific capacitance even at a relatively low specific surface area. The cycle stability of the electrode materials is very important for practical energy storage applications. The capacitance retention of the N, S, P-HHGO electrode

J. Liu et al. / Journal of Alloys and Compounds 815 (2020) 152328

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Fig. 9. (a) CV curves of the N, S, P-HHGO electrode at various scan rates, (b) galvanostatic charge-discharge curves of the N, S, P-HHGO electrode at various current densities. (c) CV curves of the N, S, P-HHGO electrode at 5 mV s
1 and the shadowed areas represent the capacitive contribution. (d) The capacitive contribution of N, S, P-HHGO at different scan rates.

investigated at a current density of 3 A g
1 for 10000 cycles is displayed in Fig. 10. The GCD curves of the N, S, P-HHGO electrode from the 9991th to the 10000th cycle are presented in the inset Fig. 10, which reveals only minor changes. After 10000 cycles, the retention of specific capacitance is 93.5%, and the superior cycling stability can be attributed to a porous structure of the N, S, P-HHGO electrode. Thus, the as-synthesized N, S, P-HHGO is a superior candidate electrode material for supercapacitors that can provide high specific capacitance, excellent rate performance and ultralong cycle stability. EIS measurements were conducted to explore the dynamical characteristics of electrodes in a frequency range of 100 KHz to 0.01 Hz, and the Nyquist plots of the as-prepared electrodes were compared in Fig. 11. Obviously, the intersections on the real axis of seven curves are similar (lower than 0.6 U), implying that the low internal resistances (Rs) of these samples. The smallest semicircle observed for N, S, P-HHGO implies the lower charge transfer resistance (Rct). The Rct value of the N, S, P-HHGO electrode is smaller than those of the N, P-HHGO and N, S-HHGO, illustrating that the N, S, P-HHGO enhances the interfacial charge transfer. This low resistance can also be attributed to a decrease in the polarity of the carbon surface caused by the introduction of N, S and P heteroatoms.

4. Conclusions In summary, the N, S, P tri-doped holey GO has been successfully prepared using a hydrothermal method and employed as electrode materials of supercapacitor for the first time. The GO was synthesized by a modified Hummers’ method using expanded graphite as a precursor, and then the AHGO, RHGO and HHGO were further prepared by etching GO utilizing three different routes. Finally, N, S and P doped holey GO was prepared by a simple hydrothermal method. The N, S, P-HHGO was systematically characterized by XRD, FT-IR, XPS, FESEM, EDS, Elemental mapping, TEM, HRTEM, Raman, BET and tested by CV, GCD and EIS. The results suggest that porous structure of N, S, P-HHGO and the introduction of N, S and P are also confirmed. Furthermore, the electrochemical results demonstrate that the N, S, P-HHGO electrode possesses a high gravimetric capacitance (295 F g
1 at 1 A g
1), exceptional rate capability (71.2% of capacity retention rate from 1 to 20 A g
1) and superior cycling stability (93.5% of initial capacity retention at 3 A g
1 over 10000 cycles). The superior electrochemical performances of N, S, P-HHGO can be ascribed to a porous structure with a certain number of micropores and macropores, and abundant mesopores as well as the introduction of nitrogen, sulfur and phosphorus heteroatoms. Therefore, the novel N, S, P-HHGO is

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[4]

[5]

[6] [7]

[8] [9]

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[11] [12] Fig. 10. The cycling stability of the N, S, P-HHGO electrode at a current density of 3 A g
1 for 10000 cycles (Inset: charge-discharge curves for the last 10 cycles (9991th to 10000th)).

[13]

[14]

[15] [16] [17]

[18]

[19]

[20]

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[22]

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Fig. 11. Nyquist impedance plots of the prepared electrodes.

[24]

anticipated to be highly potential electrode material for application in supercapacitors as well as other energy storage devices.

[25]

[26]

Acknowledgements [27]

The work is financially supported by National Natural Science Foundation of China (21601057), and Project funded by Hunan Provincial Natural Science Foundation of China (2019JJ40069).

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