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P co-doped graphene for high-performance supercapacitors
Journal of Power Sources 365 (2017) 380e388
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Facile and controllable synthesis of N/P co-doped graphene for highperformance supercapacitors Kaisheng Xia a, *, Zhiyuan Huang a, Lin Zheng a, Bo Han a, Qiang Gao b, Chenggang Zhou a, **, Hongquan Wang b, Jinping Wu a a b
Sustainable Energy Laboratory, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, 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/P co-doped graphene is controllably prepared by one-step activation. Increased oxygen functionalities on the precursor can facilitate N and P doping. High volumetric capacitance and excellent rate and cycle performance are observed. Enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) is obtained.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2017 Received in revised form 29 August 2017 Accepted 3 September 2017
Improving the energy density of carbon-based supercapacitors is one of the most urgent demands for developing high-power energy supplies, which in general requires delicate engineering of the carbon composition and textures. By pre-functionalization of graphene nanosheets and successive one-step (NH4)3PO4 activation, we prepared a type of nitrogen and phosphorus co-doped graphene (NPG) with high specific surface areas, hierarchical pore structures as well as tunable N and P contents. The asobtained NPG shows high specific capacitances of 219 F g1 (123 F cm3) at 0.25 A g1 and 175 F g1 (98 F cm3) at 10 A g1, respectively. Accordingly, the NPG-based symmetrical supercapacitor device, working at a potential window of 1.3 V, could deliver an enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) at a power density of 162 W kg1 (91 W L1), which still retains 6.7 Wh kg1 at 6.5 kW kg1. In particular, under a current density of 5 A g1, the device endows an 86% capacitance retention of initial after 20,000 cycles, displaying superior cycle stability. Our results imply the feasibility of NPG as a promising candidate for high-performance supercapacitors. © 2017 Elsevier B.V. All rights reserved.
Keywords: Supercapacitors Graphene nanosheets Heteroatom doping (NH4)3PO4 activation Texture
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Xia), [email protected] (C. Zhou). http://dx.doi.org/10.1016/j.jpowsour.2017.09.008 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Supercapacitors have attracted great attention due to their high power density, superior rate capability and long operation life, where the electrical double layer capacitor (EDLC) represents an important branch which electrostatically store charges through reversible ion adsorption/desorption at the electrolyte/electrode
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interfaces [1e4]. EDLCs can be quickly charged and do not lose their storage capabilities over hundreds of thousands cycles [5,6]. However, the energy density of EDLCs is generally low due to the limited surface where ions reside. Improving the energy density without sacrificing the power density and cyclic stability is a great challenge for EDLCs. Given that the energy density of an EDLC is proportional to the specific capacitance (SC) and the square of the operating voltage of the capacitor cell [7,8], it is of essential importance for an EDLC to simultaneously possess high specific capacitance and wide operating voltage in order to achieve satisfactory energy storage ability. Carbon materials are the most widely used electrode materials for EDLCs. For the carbon electrodes, surface area has been considered to be the key concern that governs their SC. However, in most cases, the SC of carbon-based supercapacitors is less than 250 F g1 even with a very large specific surface area (more than 2500 m2 g1) [9,10]. This is because of the fact that most of the micropores in carbon material, which are the major contributor to SC, are inaccessible by the electrolyte or wetting deficiencies of electrolytes [11]. Recently, numerous efforts have been made to improve the SC of carbon-based EDLCs, in which nitrogen doping has been proven to be an efficient method because of the conductivity enhancement and additional pseudocapacitance endowed by N inclusion [12e18]. Significantly, Huang and coworkers recently prepared an N-doped mesoporous carbon which owns a SC of as high as 855 F g1 in aqueous electrolytes, demonstrating the important role of N doping in capacitance increment [10]. On the other aspect, phosphorus doping of carbon materials could markedly increase their SCs because of the highly improved wettability brought by the P-containing groups in aqueous electrolyte [19e26]. In particular, P doping can widen the operating cell voltage due to the enhanced electro-oxidation resistance of carbon electrodes induced by interfacial phosphorus groups [27], which consequently increases the energy density of the carbon-based EDLCs significantly. For example, the P-doped carbons prepared by Hulicova-Jurcakova et al. [19] can operate steadily over a wide cell voltage of 1.3 V and deliver a high energy density of 16.3 Wh kg1 at a power density of 33 W kg1. Therefore, one can rationally realize that co-doping with N and P would effectively increase the energy storage performance of porous carbon electrodes. In fact, several efforts have demonstrated the potential viability of N and P co-doped (N/P co-doped) porous carbons as high-performance supercapacitors [28e34]. Graphene has been cognized as a promising supercapacitor materials, whose supercapacitive performance can be naturally expected to be enhanced through N/P co-doping. Lei et al. [35] prepared N/P co-doped graphene via a two-step procedure involving the hydrothermally-derived N-containing graphene aerogel followed by phosphoric acid activation. The product exhibited high specific capacitance (204 F g1 at 0.2 A g1), excellent cycling stability (92% capacitance retention after 10000 cycles), and enhanced energy density (7.1 Wh kg1 at 100 W kg1). Alternatively, Qiu and co-workers [36,37] synthesized N/P co-doped graphene by a simple thermal annealing of exfoliated graphene oxide in the presence of ammonium phosphate ((NH4)3PO4). The electrode shows a specific capacitance of 165 F g1 at 0.1 A g1 and a retention ratio of >80% at 2 A g1. Unlike the two-step process where N and P doping were carried out in stages [35], the (NH4)3PO4 activation provides a simple yet efficient method to simultaneously render N and P sources for N/P co-doping of graphene, which can be easily scaled-up. However, the relatively low specific surface area (152 m2 g1) of the co-doped graphene [37] constraints its SC performance, which may originate from the relatively low O contents (C/O ~ 11) of the graphene oxide precursors.
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In principle, for doping heteroatoms, specifically N and P in this case, the oxygen-containing groups (carboxyl, carbonyl, and epoxy groups, etc) attached on the surface of graphene are the most critical factor, which determines the porosity and specific surface area of the resultant derivatives [38e40]. However, conventional thermal exfoliation of graphene oxides normally eliminates a high percentage of O-containing groups, which of course will lead to lower co-doping efficiency, resulting in only moderately improved SC performances underneath expectation. Herein, we employed our previously reported ultra-fast thermal exfoliation technique [41] to prepare functionalized graphene nanosheets (FGN) with a high oxygen content (C/O ~ 5.7) to facilitate the N/P co-doping. Using this type of FGN as precursor, (NH4)3PO4 activation was sequentially conducted which enables high specific surface areas of up to 320 m2 g1 in the resultant N/P co-doped graphene (NPG). In particular, by adjusting the (NH4)3PO4/FGN mass ratio, the N and P contents can be delicately tuned. Under optimal conditions, the asprepared NPG promises to show high specific capacitances of 219 F g1 (123 F cm3) at 0.25 A g1 and 175 F g1 (98 F cm3) at 10 A g1, respectively. More importantly, the as-assembled symmetrical supercapacitor delivers an enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) and superior cycle stability (86% capacitance retention after 20,000 cycles) within a potential window of 1.3 V. 2. Experimental section 2.1. Sample preparation The starting materials and chemicals were analytical grade and without any further purification. Graphite oxide (GO) was synthesized by the modified Hummer's method [42]. The detailed synthesis procedures of GO are shown in the Supplementary data. To prepare FGN, the dried GO was firstly ground into fine powder. Then, it was put into a long quartz tube that is sealed at one end and open at the other end. Subsequently, the powder was flushed with nitrogen (N2) for 30 min, then the quartz tube was quickly inserted into a tube furnace preheated to 800 C and held in the furnace for 60 s. After that the FGN was obtained. The NPGs were prepared by thermal annealing of FGN at high temperature in the presence of (NH4)3PO4 under an inert atmosphere. Firstly, 0.15 g FGN powder was physically mixed with different amount of (NH4)3PO4 (0, 0.15, 0.30, 0.45 g). The mixture was then transferred to a quartz boat and thermally annealed at 800 C for 2 h under N2 atmosphere at a temperature ramp rate of 10 C min1. The product was collected and washed with hot water to remove any impurities and then dried at 100 C overnight. According to the amount of (NH4)3PO4 introduced during the synthesis, the obtained representative samples in this study are denoted briefly as TRG, 1NPG, 2NPG and 3NPG, respectively. To investigate the roles played by oxygen-containing groups on the surface of FGN, we thermally treated TRG with (NH4)3PO4 (mass ratio ¼ 0.5) at the same conditions, and the obtained sample was named NPTG for comparative purpose. 2.2. Materials characterization The microscopic feature and morphology of the samples were observed by field scanning electron microscopy (FE-SEM, Hitch SU8010) with an energy dispersive spectrum (EDS) analyzing system and transmission electron microscopy (TEM, Philips CM12 TEM/STEM) operated at 120 kV. Powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded on a Bruker AXS D8-FOCUS diffractometer using monochromatized Cu-Ka radiation (40 kV, 20 mA; l ¼ 0.1540598 nm). Surface areas and pore distributions of the samples were carried out by N2 adsorption-
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desorption analysis at 77 K using a Micromeritics ASAP 2020 HD88 system. All the samples were degassed at 200 C for 6 h before adsorption experiments. The specific surface area (SSA) was calculated from the adsorption data in the relative pressure (P/P0) range between 0.05 and 0.35 using Brunauer-Emmett-Teller (BET) method. Total pore volume (TPV) was measured at a relative pressure of 0.995. The pore size distribution was analyzed by Barrett-Joyner-Halenda (BJH) method using the data of the adsorption branch. X-ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo.VG Corporation) was used in the surface analyses of the samples using a monochromatized Al Ka X-ray source (1486.71 eV). The static contact angles of de-ionized (DI) water for the samples were carried out using a JC2000C contact-angle system (POWEREACH, China), and average values were measured from three different points on the same surface.
All electrochemical measurements were performed using an electrochemical workstation Bio-logic VMP3 in 6 M KOH electrolyte at 25 C. The electrode materials were prepared by mixing the NPGs, acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio of 80: 10: 10 with ethanol. And then the pastes were pressed into Ni foam and then dried overnight at 80 C. The dried electrode was then pressed at 10 MPa for 1 min. Approximately, 1.5 mg of active materials was loaded in each electrode. In a threeelectrode system, a Pt wire and an Hg/HgO electrode were used as counter and reference electrodes, respectively. As for the symmetric two-electrode supercapacitor, we used two 2NPG electrodes with the same mass loading. To determine the density of 2NPG electrode, we mixed the 2NPG powder with PTFE to prepare disks. After pressing under 10 MPa for 5 min as shown in Fig. S1, the compression density of the electrode was calculated to be 0.56 g cm3 through division of the disk mass by the disk volume. The electrochemical measurements were carried out by cyclic voltammetry (CV), galvanostatic chargeedischarge (GCD) and electrochemical impedance spectroscopy (EIS) methods. The CV and GCD were recorded in the 1.0 to 0 V potential window. EIS measurements were carried out in the frequency range of 100 kHz to 10 mHz. The methods for calculating the specific capacitance, energy density and power density of the electrode are provided in the Supplementary data. 3. Results and discussion The schematic diagram for the preparation of NPGs is illustrated in Fig. 1. During the first thermal exfoliation process, the functional groups on graphite oxide (GO) partially decomposed and produced gases that exfoliate GO layers into many curved graphene nanosheets, which may stack together to form highly porous structure. Meanwhile, numerous oxygen-containing groups (such as carboxyl, carbonyl, hydroxyl, and epoxy groups) were still preserved after exfoliation due to the very short heating time. The abundant oxygen functional groups are crucial for the following (NH4)3PO4 activation because the oxygen functionalities provide the main active sites except for structural defects including vacancies and edges. Upon activation, the (NH4)3PO4 gradually decomposed to ammonia (NH3) and phosphoric acid, which subsequently reacted with the active domains on the surface of FGN. As a result, the active carbon atoms would covalently bond with the N atoms from NH3 and the P atoms from phosphate anion, thus leading to N and P incorporation into the graphene materials [43,44]. By increasing the oxygen contents of the graphene oxide, the reaction between (NH4)3PO4 and graphene oxide can be enhanced. Moreover, the chemical reaction can be further tuned by controlling the initial
Fig. 1. Schematic diagram of the preparation process of NPGs.
2.3. Electrochemical measurements
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(NH4)3PO4/FGN mixing ratio. Finally, N/P co-doped graphene materials with high surface areas and tunable N and P contents were obtained. The morphological features of the obtained products were first investigated by SEM and TEM. As shown in Fig. 2a and b, the 2NPG shows a two-dimensional sheet-like wrinkled structure, similar to that of FGN and other NPGs (Fig. S2). Besides, a large number of macropores formed by the restacking of curved graphene sheets were well preserved after thermal treatment with (NH4)3PO4. The EDS measurement (Fig. 2c) indicates the presence of C, O and P elements in 2NPG, confirming the successful incorporation of P into graphene. The XRD patterns of different samples are displayed in Fig. 2d. The pristine GO has a sharp diffraction peak around 11
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indicative of numerous oxygen functional groups between the graphene sheets, which is consistent with our previous report [41]. Upon either thermal annealing or (NH4)3PO4 treatment, the products exhibit only one broad and low intensity peak in the range of 20e30 . It suggests that the GO was exfoliated into many singleand several-layer nanosheets and the exfoliated structure is well retained after the doping process. The pore structures of these samples were next analyzed using N2 adsorption/desorption measurement at 77 K. It is seen that all samples have type IV/II isotherms (Fig. 2e), which is indicative of the presence of mesopores and macropores [45e47]. Moreover, an H3 type hysteresis loop at P/P0 above 0.5 is observed in these samples, indicating a strong capillary condensation in slit-shaped pores. The pore size
Fig. 2. (a) SEM image, (b) TEM image and (c) EDS spectrum of 2NPG; (d) XRD patterns, (e) nitrogen adsorption/desorption isotherms and (e) pore size distribution curves of all the samples.
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Table 1 Specific surface areas (SSAs), total pore volumes (TPVs) and elemental compositions of different samples. Samples
SSAs (m2 g1)
TPVs (cm3 g1)
C (at. %)
N (at. %)
O (at. %)
P (at. %)
FGN TRG 1NPG 2NPG 3NPG NPTG
432 434 320 284 288 308
1.45 1.43 0.97 0.86 0.67 1.18
85.0 e 90.9 88.0 87.0 89.4
e e 2.5 3.0 3.7 1.3
15.0 e 6.0 7.8 8.0 8.7
e e 0.6 1.2 1.3 0.6
distribution curves (Fig. 2f) display a pronounced peak centered at ~2.5 nm and a broad distribution of pores in the range of 5e50 nm, which confirm the coexistence of mesopores and macropores in these samples. As summarized in Table 1, it is found that the SSAs and TPVs of the samples decreased after (NH4)3PO4 treatment. We suspect that the reaction with (NH4)3PO4 under high temperature might facilitate the restacking and overlapping of FGN or TRG. Even so, the SSAs of NPGs (284e320 m2 g1) are much higher than that of the reported N/P-TRGO [37] prepared using the same (NH4)3PO4 treatment. Hence, the prepared NPGs are of great interest for supercapacitors, taking into account their favorable macropores and mesopores for ion transport and large surface areas for charge accommodation. The XPS analyses were carried out to quantify the elemental atom ratios and to get the detailed bonding information in the samples. The survey spectra of the elements present in all samples are shown in Fig. 3a, and detailed data are listed in Table 1. The FGN
possesses a high surface O content of 15.0 at. %, demonstrating abundant surface oxygen functional groups on the surface of FGN. In addition, the peaks occurred at ~400 and 134 eV, which can be assigned to N 1s and P 2p energy levels, respectively, are observed for NPGs and NPTG. The results clearly show the effective incorporation of N and P into the FGN material after thermal annealing with (NH4)3PO4. Moreover, the contents of heteroatoms increase with the amount of (NH4)3PO4, and the highest doping contents of 3.7 at. % N and 1.3 at. % P was found in 3NPG. In order to verify the key role of oxygen functional groups on FGN for the N and P codoping process, we prepared the NPTG by thermally annealing of TRG with (NH4)3PO4. Because the additional thermal reduction can significantly eliminate the oxygen functionalities of FGN, the resultant NPTG has much lower N (1.3 at. %) and P contents (0.6 at. %) (Table 1). Thus, our results prove that high-concentrated oxygen functionalities can facilitate the N and P doping into the exfoliated graphene nanosheets. Furthermore, the XPS results for N and P bonding in the 2NPG are shown in Fig. 3b and c. There are four types of N atoms in 2NPG, which are differentiated based on their binding energies: pyridinicN (N-6, 398.6 eV), pyrrolic-N (N-5, 400.0 eV), quaternary-N (N-Q, 401.2 eV), and pyridine-N-oxide (N-X, 403 eV) [48,49]. Considering that the thermal decomposition of (NH4)3PO4 releases NH3, the nitrogen-containing functional groups are believed to be formed during the reaction between NH3 and oxygen functionalities present on FGN upon heating [39,44]. At initial stages of heating (<500 C), NH3 reacts with side functional groups of FGN such as carboxylic acid (-COOH) or hydroxyl species (-OH) to form amides (-CONH2), lactams (-CO-NH-C-) and imides (-CO-NH-CO-). On
Fig. 3. (a) XPS spectra of NPGs and NPTG; (b) XPS spectra for N 1s of 2NPG; (c) XPS spectra for P 2p of 2NPG; (d) Contact angles of FGN, 2NPG and TRG.
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further heating at 500 to 800 C, these species undergo dehydration or decarbonylation reactions to form more stable N components such as pyrrolic-N and pyridinic-N. Moreover, quaternary-N is formed due to the reorganization of unsaturated carbon and N species [50]. According to the literature [51], N-5 and N-6 are considered to be responsible for the enhanced capacitance due to the lowered bandgap and the improved semiconducting properties of carbon, whereas N-Q can effectively improve electronic conductivity and electron transfer, thus leading to an enhanced capacitance especially at high rates. The P2p spectrum (Fig. 3c) of 2NPG also contains four peaks. The peak at 134.1 eV is associated to species in which P is not directly bonded to C such as C-O-PO3 and (C-O)3PO. The peak at 133.5eV and 132.8eV is characteristics of C-P bonding such as C2ePO2 or CPO3 groups, and the peak at 132.3eV can be assigned to C3PO groups [23,27]. These P-containing functional groups can effectively promote the surface wettability of NPGs in the aqueous electrolyte, which allows electrolyte to permeate in smaller pores, leading to higher surface area available for the formation of EDLC [23]. The improved wettability was further proved by the contact angle measurements as shown in Fig. 3d, in which the contact angles were measured to be 58 , 44 and 99 for FGN, 2NPG and TRG, respectively. Owing to the presence of plentiful N- and P-containing groups, 2NPG shows the smallest contact angle. In contrast, TRG displays the largest contact angle due to its rather clean surface. The electrochemical performance of the different samples for potential application as supercapacitor electrodes were evaluated in a three-electrode system. Fig. 4a shows the CV curves of the
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different samples in 6 M KOH at a scan rate of 50 mV s1. It can be seen that the FGN electrode shows a distorted rectangular shape, revealing the existence of pseudocapacitance associated with the surface oxygen-containing functional groups [52]. In contrast, the NPGs and TRG exhibit nearly rectangular CV curves over the range from 1.0 to 0 V, indicating their excellent EDLC behaviors. Furthermore, no obvious polarization degradation was found in the CV curves of the NPGs even at a large scan rate of 200 mV s1 (Fig. S3). It is interesting to note that the N and P functionalities on NPGs do not cause distortions in their CV curves. The possible reason may be that the pseudocapacitance induced by N-containing groups is inconspicuous in alkaline electrolyte such as 6 M KOH [44]. Moreover, the P-containing groups cannot generate pseudocapacitance but effectively promote the surface wettability of carbon electrode in the aqueous electrolyte, and thus increase its electrochemically active surface area [20,23]. The ideal capacitive behaviors of NPGs electrodes were also reflected in their triangular chargeedischarge profiles (Fig. 4b). The SCs of these samples calculated from discharge curve at different current densities are plotted in Fig. 4c. As expected, the SC values of NPGs are definitely higher than that of either NPTG or TRG, showing the importance of both heteroatom doping and high SSA for carbon-based EDLCs. Although the FGN has a high SC of 202 F g1 at 0.25 A g1, it rapidly decreases to 144 F g1 at 10 A g1. Among the prepared N/P codoped graphene materials, 2NPG shows the highest SC of 219 F g1 at 0.25 A g1, which is larger than that of dual-doped GA (204 F g1, 0.2 A g1) [35] and N/P-TRGO (165 F g1, 0.1 A g1) [37]. As for the 2NPG, it remains a high value of 175 F g1 at 10 A g1,
Fig. 4. (a) CV curves of different samples in 6 M KOH with scan rate of 50 mV s1; (b) GCD curves of different samples under currents density of 5 A g1; (c) SC dependence of different samples on constant current density; (d) The Nyquist impedance plots for the different samples.
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demonstrating its good rate capability as supercapacitor electrodes. However, thermal treatment with excessive (NH4)3PO4 led to a slight capacitance decrease because of the sharply decreased pore volume of 3NPG despite of its higher N and P contents. It highlights that a well-developed pore network is indispensable for promoting the kinetic diffusion of electrolyte in the interior of electrode. Besides, we estimated the volumetric SC of 2NPG by compressing the 2NPG powders into disks. The volumetric capacitance of 2NPG is calculated to be 123 F cm3, which is higher than O/N co-doped graphene foams (106 F cm3) [53], but is still lower than the potassium humate-based graphite oxide (144 F cm3) [54] and holey graphene (216 F cm3) [55]. Fig. 4d shows the Nyquist impedance plots of different samples. It is seen that FGN exhibits a more inclined curve at low frequency region, implying a higher contribution of pseudocapacitance. The other samples display vertical lines at low frequency region which indicates very good capacitive behaviors. The radius of semicircle in the high frequency region reflects the impedance on electrode/ electrolyte interface [56]. It is found that the 3NPG electrode has the smallest semicircle, indicating the lowest charge transfer resistance in 3NPG. Comparatively, the FGN electrode exhibits the highest charge transfer resistance. The intercept at high frequency side represents the internal resistance of electrode. Consequently, the NPGs electrodes also represent the smallest internal resistance. Therefore, the EIS analysis shows that the charge transfer resistance and internal resistance of graphene nanosheets can be significantly reduced after N and P dual doping.
For practical application of NPGs, we constructed a symmetric two-electrode (2NPG//2NPG) supercapacitor cell in 6 M KOH aqueous electrolyte. Firstly, the operating potential window of the 2NPG//2NPG symmetric capacitor was tested in a wider range (Fig. 5a), which demonstrates that a high cell voltage of 1.5 V can be reached. The widen potential window of the supercapacitor has been ascribed to the enhanced electro-oxidation resistance of carbon electrodes with the aid of P-containing functional groups [26]. An approximate rectangular CV curve within 0e1.3 V can be observed at a scan rate of 5 mV s1 and the shape is well maintained at the scan rate of 200 mV s1 (Fig. S4). The GCD measurements with a potential range from 0 to 1.3 V were carried out at current densities from 0.25 A g1 to 10 A g1 (Fig. 5b), the result indicates the ideal capacitive property and good rate capability of the cell. As shown in Fig. 5c, the galvanostatic cycling at 5 A g1 of the cell was performed, and we found that it can be operated stably at potential window of 1.3 V for 20000 cycles with a capacitance retention up to 85.6%. It demonstrates the robustness and stable capacitive performance of the device. Furthermore, the energy density and power density of the device were calculated to evaluate its application potential. The results show that the 2NPG-based supercapacitor exhibits a high energy density of 8.17 Wh kg1 with a power density of 162 W kg1. Accordingly, the maximum volumetric energy density reaches a prominent value of 4.6 Wh L1 with a power density of 91 W L1. Even at a high power density of 6500 W kg1 (3640 W L1), the energy density can still retain 6.68 Wh kg1 (3.7 Wh L1). This value is higher than N-doped CNFs
Fig. 5. (a) CV curves of 2NPG with different potential windows at 100 mV s1; (b) The GCD curves of symmetric supercapacitors based on 2NPG at different current densities within 0e1.3 V; (c) Cycling performance of 2NPG based symmetric capacitors at current density of 5 A g1 with inset showing the first 15 cycles of chargeedischarge curves; (d) Cameracaptured photographs of LED indicator driven by two series-connected 2NPG supercapacitors.
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[57], P-functionalized CNTs [25], N/P co-doped GAs [35] and many other N/P co-doped carbons recently reported, as shown in Table S1. Nevertheless, our energy density is still lower than that of OMFLC-N [10] and 3D-HPG [58], which have ultrahigh SCs (400e800 F g1), far above our values (175e219 F g1). Thus, further work is needed to improve the SC of the N/P co-doped graphene. In order to check the practical feasibility of our supercapacitor, we assembled two series-connected cells based on the 2NPG electrode. As demonstrated in Fig. 5d, after charging at 1 A g1 with the voltage up to 1.3 V, the supercapacitor can efficiently light a LED indicator (2.0 V, 20 mA) for about 5 min, indicating the promising practical application of N, P co-doped graphene electrode. 4. Conclusion In summary, NPG materials were synthesized by prefunctionalization of graphene nanosheets and successive one-step (NH4)3PO4 activation. Our results demonstrate that a delicately tailored oxygen content (C/O ~ 5.7) on the surface of FGN precursor can effectively promote the (NH4)3PO4 activation, thus enabling high specific surface areas of up to 320 m2 g1 and efficient N/P codoping as high as 3.7 at. % for N and 1.3 at. % for P in the NPGs. Furthermore, by adjusting the (NH4)3PO4/FGN mass ratio, the N and P contents of the NPGs can be effectively tuned. The optimal 2NPG shows a high specific capacitance of 219 F g1 (123 F cm3) at 0.25 A g1 and 175 F g1 (98 F cm3) at 10 A g1, respectively. Significantly, the 2NPG-based symmetrical supercapacitor device, working at a potential window of 1.3 V, could deliver an enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) at a power density of 162 W kg1 (91 W L1), which still retains 6.7 Wh kg1 at 6.5 kW kg1. In particular, under a current density of 5 A g1, the device endows an 86% capacitance retention of initial after 20,000 cycles, displaying superior cycle stability. These results support that NPG is a promising candidate for further development of highperformance supercapacitors. Acknowledgments The authors gratefully acknowledge financial support from Natural Science Foundation of Hubei Province (No. 2015CFB187), National Natural Science Foundation of China (No. 21303170), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGL150414, CUGL140413 and CUG120115). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.09.008. References [1] P. Simon, Y. Gogotsi, B. Dunn, Materials science. Where do batteries end and supercapacitors begin? Science 343 (2014) 1210e1211. [2] Y. Huang, J. Liang, Y. Chen, An overview of the applications of graphene-based materials in supercapacitors, Small 8 (2012) 1805e1834. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater 7 (2008) 845e854. [4] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater 22 (2010) 28e62. [5] S.W. Lee, B.M. Gallant, H.R. Byon, P.T. Hammond, Y. Shao-Horn, Nanostructured carbon-based electrodes: bridging the gap between thin-film lithium-ion batteries and electrochemical capacitors, Energy Environ. Sci. 4 (2011) 1972e1985. [6] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 8 (2009) 2520e2531. [7] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater 4 (2014) 1e43.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile and controllable synthesis of N/P co-doped graphene for highperformance supercapacitors Kaisheng Xia a, *, Zhiyuan Huang a, Lin Zheng a, Bo Han a, Qiang Gao b, Chenggang Zhou a, **, Hongquan Wang b, Jinping Wu a a b
Sustainable Energy Laboratory, Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, 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/P co-doped graphene is controllably prepared by one-step activation. Increased oxygen functionalities on the precursor can facilitate N and P doping. High volumetric capacitance and excellent rate and cycle performance are observed. Enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) is obtained.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2017 Received in revised form 29 August 2017 Accepted 3 September 2017
Improving the energy density of carbon-based supercapacitors is one of the most urgent demands for developing high-power energy supplies, which in general requires delicate engineering of the carbon composition and textures. By pre-functionalization of graphene nanosheets and successive one-step (NH4)3PO4 activation, we prepared a type of nitrogen and phosphorus co-doped graphene (NPG) with high specific surface areas, hierarchical pore structures as well as tunable N and P contents. The asobtained NPG shows high specific capacitances of 219 F g1 (123 F cm3) at 0.25 A g1 and 175 F g1 (98 F cm3) at 10 A g1, respectively. Accordingly, the NPG-based symmetrical supercapacitor device, working at a potential window of 1.3 V, could deliver an enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) at a power density of 162 W kg1 (91 W L1), which still retains 6.7 Wh kg1 at 6.5 kW kg1. In particular, under a current density of 5 A g1, the device endows an 86% capacitance retention of initial after 20,000 cycles, displaying superior cycle stability. Our results imply the feasibility of NPG as a promising candidate for high-performance supercapacitors. © 2017 Elsevier B.V. All rights reserved.
Keywords: Supercapacitors Graphene nanosheets Heteroatom doping (NH4)3PO4 activation Texture
1. Introduction
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Xia), [email protected] (C. Zhou). http://dx.doi.org/10.1016/j.jpowsour.2017.09.008 0378-7753/© 2017 Elsevier B.V. All rights reserved.
Supercapacitors have attracted great attention due to their high power density, superior rate capability and long operation life, where the electrical double layer capacitor (EDLC) represents an important branch which electrostatically store charges through reversible ion adsorption/desorption at the electrolyte/electrode
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interfaces [1e4]. EDLCs can be quickly charged and do not lose their storage capabilities over hundreds of thousands cycles [5,6]. However, the energy density of EDLCs is generally low due to the limited surface where ions reside. Improving the energy density without sacrificing the power density and cyclic stability is a great challenge for EDLCs. Given that the energy density of an EDLC is proportional to the specific capacitance (SC) and the square of the operating voltage of the capacitor cell [7,8], it is of essential importance for an EDLC to simultaneously possess high specific capacitance and wide operating voltage in order to achieve satisfactory energy storage ability. Carbon materials are the most widely used electrode materials for EDLCs. For the carbon electrodes, surface area has been considered to be the key concern that governs their SC. However, in most cases, the SC of carbon-based supercapacitors is less than 250 F g1 even with a very large specific surface area (more than 2500 m2 g1) [9,10]. This is because of the fact that most of the micropores in carbon material, which are the major contributor to SC, are inaccessible by the electrolyte or wetting deficiencies of electrolytes [11]. Recently, numerous efforts have been made to improve the SC of carbon-based EDLCs, in which nitrogen doping has been proven to be an efficient method because of the conductivity enhancement and additional pseudocapacitance endowed by N inclusion [12e18]. Significantly, Huang and coworkers recently prepared an N-doped mesoporous carbon which owns a SC of as high as 855 F g1 in aqueous electrolytes, demonstrating the important role of N doping in capacitance increment [10]. On the other aspect, phosphorus doping of carbon materials could markedly increase their SCs because of the highly improved wettability brought by the P-containing groups in aqueous electrolyte [19e26]. In particular, P doping can widen the operating cell voltage due to the enhanced electro-oxidation resistance of carbon electrodes induced by interfacial phosphorus groups [27], which consequently increases the energy density of the carbon-based EDLCs significantly. For example, the P-doped carbons prepared by Hulicova-Jurcakova et al. [19] can operate steadily over a wide cell voltage of 1.3 V and deliver a high energy density of 16.3 Wh kg1 at a power density of 33 W kg1. Therefore, one can rationally realize that co-doping with N and P would effectively increase the energy storage performance of porous carbon electrodes. In fact, several efforts have demonstrated the potential viability of N and P co-doped (N/P co-doped) porous carbons as high-performance supercapacitors [28e34]. Graphene has been cognized as a promising supercapacitor materials, whose supercapacitive performance can be naturally expected to be enhanced through N/P co-doping. Lei et al. [35] prepared N/P co-doped graphene via a two-step procedure involving the hydrothermally-derived N-containing graphene aerogel followed by phosphoric acid activation. The product exhibited high specific capacitance (204 F g1 at 0.2 A g1), excellent cycling stability (92% capacitance retention after 10000 cycles), and enhanced energy density (7.1 Wh kg1 at 100 W kg1). Alternatively, Qiu and co-workers [36,37] synthesized N/P co-doped graphene by a simple thermal annealing of exfoliated graphene oxide in the presence of ammonium phosphate ((NH4)3PO4). The electrode shows a specific capacitance of 165 F g1 at 0.1 A g1 and a retention ratio of >80% at 2 A g1. Unlike the two-step process where N and P doping were carried out in stages [35], the (NH4)3PO4 activation provides a simple yet efficient method to simultaneously render N and P sources for N/P co-doping of graphene, which can be easily scaled-up. However, the relatively low specific surface area (152 m2 g1) of the co-doped graphene [37] constraints its SC performance, which may originate from the relatively low O contents (C/O ~ 11) of the graphene oxide precursors.
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In principle, for doping heteroatoms, specifically N and P in this case, the oxygen-containing groups (carboxyl, carbonyl, and epoxy groups, etc) attached on the surface of graphene are the most critical factor, which determines the porosity and specific surface area of the resultant derivatives [38e40]. However, conventional thermal exfoliation of graphene oxides normally eliminates a high percentage of O-containing groups, which of course will lead to lower co-doping efficiency, resulting in only moderately improved SC performances underneath expectation. Herein, we employed our previously reported ultra-fast thermal exfoliation technique [41] to prepare functionalized graphene nanosheets (FGN) with a high oxygen content (C/O ~ 5.7) to facilitate the N/P co-doping. Using this type of FGN as precursor, (NH4)3PO4 activation was sequentially conducted which enables high specific surface areas of up to 320 m2 g1 in the resultant N/P co-doped graphene (NPG). In particular, by adjusting the (NH4)3PO4/FGN mass ratio, the N and P contents can be delicately tuned. Under optimal conditions, the asprepared NPG promises to show high specific capacitances of 219 F g1 (123 F cm3) at 0.25 A g1 and 175 F g1 (98 F cm3) at 10 A g1, respectively. More importantly, the as-assembled symmetrical supercapacitor delivers an enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) and superior cycle stability (86% capacitance retention after 20,000 cycles) within a potential window of 1.3 V. 2. Experimental section 2.1. Sample preparation The starting materials and chemicals were analytical grade and without any further purification. Graphite oxide (GO) was synthesized by the modified Hummer's method [42]. The detailed synthesis procedures of GO are shown in the Supplementary data. To prepare FGN, the dried GO was firstly ground into fine powder. Then, it was put into a long quartz tube that is sealed at one end and open at the other end. Subsequently, the powder was flushed with nitrogen (N2) for 30 min, then the quartz tube was quickly inserted into a tube furnace preheated to 800 C and held in the furnace for 60 s. After that the FGN was obtained. The NPGs were prepared by thermal annealing of FGN at high temperature in the presence of (NH4)3PO4 under an inert atmosphere. Firstly, 0.15 g FGN powder was physically mixed with different amount of (NH4)3PO4 (0, 0.15, 0.30, 0.45 g). The mixture was then transferred to a quartz boat and thermally annealed at 800 C for 2 h under N2 atmosphere at a temperature ramp rate of 10 C min1. The product was collected and washed with hot water to remove any impurities and then dried at 100 C overnight. According to the amount of (NH4)3PO4 introduced during the synthesis, the obtained representative samples in this study are denoted briefly as TRG, 1NPG, 2NPG and 3NPG, respectively. To investigate the roles played by oxygen-containing groups on the surface of FGN, we thermally treated TRG with (NH4)3PO4 (mass ratio ¼ 0.5) at the same conditions, and the obtained sample was named NPTG for comparative purpose. 2.2. Materials characterization The microscopic feature and morphology of the samples were observed by field scanning electron microscopy (FE-SEM, Hitch SU8010) with an energy dispersive spectrum (EDS) analyzing system and transmission electron microscopy (TEM, Philips CM12 TEM/STEM) operated at 120 kV. Powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded on a Bruker AXS D8-FOCUS diffractometer using monochromatized Cu-Ka radiation (40 kV, 20 mA; l ¼ 0.1540598 nm). Surface areas and pore distributions of the samples were carried out by N2 adsorption-
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desorption analysis at 77 K using a Micromeritics ASAP 2020 HD88 system. All the samples were degassed at 200 C for 6 h before adsorption experiments. The specific surface area (SSA) was calculated from the adsorption data in the relative pressure (P/P0) range between 0.05 and 0.35 using Brunauer-Emmett-Teller (BET) method. Total pore volume (TPV) was measured at a relative pressure of 0.995. The pore size distribution was analyzed by Barrett-Joyner-Halenda (BJH) method using the data of the adsorption branch. X-ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo.VG Corporation) was used in the surface analyses of the samples using a monochromatized Al Ka X-ray source (1486.71 eV). The static contact angles of de-ionized (DI) water for the samples were carried out using a JC2000C contact-angle system (POWEREACH, China), and average values were measured from three different points on the same surface.
All electrochemical measurements were performed using an electrochemical workstation Bio-logic VMP3 in 6 M KOH electrolyte at 25 C. The electrode materials were prepared by mixing the NPGs, acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio of 80: 10: 10 with ethanol. And then the pastes were pressed into Ni foam and then dried overnight at 80 C. The dried electrode was then pressed at 10 MPa for 1 min. Approximately, 1.5 mg of active materials was loaded in each electrode. In a threeelectrode system, a Pt wire and an Hg/HgO electrode were used as counter and reference electrodes, respectively. As for the symmetric two-electrode supercapacitor, we used two 2NPG electrodes with the same mass loading. To determine the density of 2NPG electrode, we mixed the 2NPG powder with PTFE to prepare disks. After pressing under 10 MPa for 5 min as shown in Fig. S1, the compression density of the electrode was calculated to be 0.56 g cm3 through division of the disk mass by the disk volume. The electrochemical measurements were carried out by cyclic voltammetry (CV), galvanostatic chargeedischarge (GCD) and electrochemical impedance spectroscopy (EIS) methods. The CV and GCD were recorded in the 1.0 to 0 V potential window. EIS measurements were carried out in the frequency range of 100 kHz to 10 mHz. The methods for calculating the specific capacitance, energy density and power density of the electrode are provided in the Supplementary data. 3. Results and discussion The schematic diagram for the preparation of NPGs is illustrated in Fig. 1. During the first thermal exfoliation process, the functional groups on graphite oxide (GO) partially decomposed and produced gases that exfoliate GO layers into many curved graphene nanosheets, which may stack together to form highly porous structure. Meanwhile, numerous oxygen-containing groups (such as carboxyl, carbonyl, hydroxyl, and epoxy groups) were still preserved after exfoliation due to the very short heating time. The abundant oxygen functional groups are crucial for the following (NH4)3PO4 activation because the oxygen functionalities provide the main active sites except for structural defects including vacancies and edges. Upon activation, the (NH4)3PO4 gradually decomposed to ammonia (NH3) and phosphoric acid, which subsequently reacted with the active domains on the surface of FGN. As a result, the active carbon atoms would covalently bond with the N atoms from NH3 and the P atoms from phosphate anion, thus leading to N and P incorporation into the graphene materials [43,44]. By increasing the oxygen contents of the graphene oxide, the reaction between (NH4)3PO4 and graphene oxide can be enhanced. Moreover, the chemical reaction can be further tuned by controlling the initial
Fig. 1. Schematic diagram of the preparation process of NPGs.
2.3. Electrochemical measurements
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(NH4)3PO4/FGN mixing ratio. Finally, N/P co-doped graphene materials with high surface areas and tunable N and P contents were obtained. The morphological features of the obtained products were first investigated by SEM and TEM. As shown in Fig. 2a and b, the 2NPG shows a two-dimensional sheet-like wrinkled structure, similar to that of FGN and other NPGs (Fig. S2). Besides, a large number of macropores formed by the restacking of curved graphene sheets were well preserved after thermal treatment with (NH4)3PO4. The EDS measurement (Fig. 2c) indicates the presence of C, O and P elements in 2NPG, confirming the successful incorporation of P into graphene. The XRD patterns of different samples are displayed in Fig. 2d. The pristine GO has a sharp diffraction peak around 11
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indicative of numerous oxygen functional groups between the graphene sheets, which is consistent with our previous report [41]. Upon either thermal annealing or (NH4)3PO4 treatment, the products exhibit only one broad and low intensity peak in the range of 20e30 . It suggests that the GO was exfoliated into many singleand several-layer nanosheets and the exfoliated structure is well retained after the doping process. The pore structures of these samples were next analyzed using N2 adsorption/desorption measurement at 77 K. It is seen that all samples have type IV/II isotherms (Fig. 2e), which is indicative of the presence of mesopores and macropores [45e47]. Moreover, an H3 type hysteresis loop at P/P0 above 0.5 is observed in these samples, indicating a strong capillary condensation in slit-shaped pores. The pore size
Fig. 2. (a) SEM image, (b) TEM image and (c) EDS spectrum of 2NPG; (d) XRD patterns, (e) nitrogen adsorption/desorption isotherms and (e) pore size distribution curves of all the samples.
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Table 1 Specific surface areas (SSAs), total pore volumes (TPVs) and elemental compositions of different samples. Samples
SSAs (m2 g1)
TPVs (cm3 g1)
C (at. %)
N (at. %)
O (at. %)
P (at. %)
FGN TRG 1NPG 2NPG 3NPG NPTG
432 434 320 284 288 308
1.45 1.43 0.97 0.86 0.67 1.18
85.0 e 90.9 88.0 87.0 89.4
e e 2.5 3.0 3.7 1.3
15.0 e 6.0 7.8 8.0 8.7
e e 0.6 1.2 1.3 0.6
distribution curves (Fig. 2f) display a pronounced peak centered at ~2.5 nm and a broad distribution of pores in the range of 5e50 nm, which confirm the coexistence of mesopores and macropores in these samples. As summarized in Table 1, it is found that the SSAs and TPVs of the samples decreased after (NH4)3PO4 treatment. We suspect that the reaction with (NH4)3PO4 under high temperature might facilitate the restacking and overlapping of FGN or TRG. Even so, the SSAs of NPGs (284e320 m2 g1) are much higher than that of the reported N/P-TRGO [37] prepared using the same (NH4)3PO4 treatment. Hence, the prepared NPGs are of great interest for supercapacitors, taking into account their favorable macropores and mesopores for ion transport and large surface areas for charge accommodation. The XPS analyses were carried out to quantify the elemental atom ratios and to get the detailed bonding information in the samples. The survey spectra of the elements present in all samples are shown in Fig. 3a, and detailed data are listed in Table 1. The FGN
possesses a high surface O content of 15.0 at. %, demonstrating abundant surface oxygen functional groups on the surface of FGN. In addition, the peaks occurred at ~400 and 134 eV, which can be assigned to N 1s and P 2p energy levels, respectively, are observed for NPGs and NPTG. The results clearly show the effective incorporation of N and P into the FGN material after thermal annealing with (NH4)3PO4. Moreover, the contents of heteroatoms increase with the amount of (NH4)3PO4, and the highest doping contents of 3.7 at. % N and 1.3 at. % P was found in 3NPG. In order to verify the key role of oxygen functional groups on FGN for the N and P codoping process, we prepared the NPTG by thermally annealing of TRG with (NH4)3PO4. Because the additional thermal reduction can significantly eliminate the oxygen functionalities of FGN, the resultant NPTG has much lower N (1.3 at. %) and P contents (0.6 at. %) (Table 1). Thus, our results prove that high-concentrated oxygen functionalities can facilitate the N and P doping into the exfoliated graphene nanosheets. Furthermore, the XPS results for N and P bonding in the 2NPG are shown in Fig. 3b and c. There are four types of N atoms in 2NPG, which are differentiated based on their binding energies: pyridinicN (N-6, 398.6 eV), pyrrolic-N (N-5, 400.0 eV), quaternary-N (N-Q, 401.2 eV), and pyridine-N-oxide (N-X, 403 eV) [48,49]. Considering that the thermal decomposition of (NH4)3PO4 releases NH3, the nitrogen-containing functional groups are believed to be formed during the reaction between NH3 and oxygen functionalities present on FGN upon heating [39,44]. At initial stages of heating (<500 C), NH3 reacts with side functional groups of FGN such as carboxylic acid (-COOH) or hydroxyl species (-OH) to form amides (-CONH2), lactams (-CO-NH-C-) and imides (-CO-NH-CO-). On
Fig. 3. (a) XPS spectra of NPGs and NPTG; (b) XPS spectra for N 1s of 2NPG; (c) XPS spectra for P 2p of 2NPG; (d) Contact angles of FGN, 2NPG and TRG.
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further heating at 500 to 800 C, these species undergo dehydration or decarbonylation reactions to form more stable N components such as pyrrolic-N and pyridinic-N. Moreover, quaternary-N is formed due to the reorganization of unsaturated carbon and N species [50]. According to the literature [51], N-5 and N-6 are considered to be responsible for the enhanced capacitance due to the lowered bandgap and the improved semiconducting properties of carbon, whereas N-Q can effectively improve electronic conductivity and electron transfer, thus leading to an enhanced capacitance especially at high rates. The P2p spectrum (Fig. 3c) of 2NPG also contains four peaks. The peak at 134.1 eV is associated to species in which P is not directly bonded to C such as C-O-PO3 and (C-O)3PO. The peak at 133.5eV and 132.8eV is characteristics of C-P bonding such as C2ePO2 or CPO3 groups, and the peak at 132.3eV can be assigned to C3PO groups [23,27]. These P-containing functional groups can effectively promote the surface wettability of NPGs in the aqueous electrolyte, which allows electrolyte to permeate in smaller pores, leading to higher surface area available for the formation of EDLC [23]. The improved wettability was further proved by the contact angle measurements as shown in Fig. 3d, in which the contact angles were measured to be 58 , 44 and 99 for FGN, 2NPG and TRG, respectively. Owing to the presence of plentiful N- and P-containing groups, 2NPG shows the smallest contact angle. In contrast, TRG displays the largest contact angle due to its rather clean surface. The electrochemical performance of the different samples for potential application as supercapacitor electrodes were evaluated in a three-electrode system. Fig. 4a shows the CV curves of the
385
different samples in 6 M KOH at a scan rate of 50 mV s1. It can be seen that the FGN electrode shows a distorted rectangular shape, revealing the existence of pseudocapacitance associated with the surface oxygen-containing functional groups [52]. In contrast, the NPGs and TRG exhibit nearly rectangular CV curves over the range from 1.0 to 0 V, indicating their excellent EDLC behaviors. Furthermore, no obvious polarization degradation was found in the CV curves of the NPGs even at a large scan rate of 200 mV s1 (Fig. S3). It is interesting to note that the N and P functionalities on NPGs do not cause distortions in their CV curves. The possible reason may be that the pseudocapacitance induced by N-containing groups is inconspicuous in alkaline electrolyte such as 6 M KOH [44]. Moreover, the P-containing groups cannot generate pseudocapacitance but effectively promote the surface wettability of carbon electrode in the aqueous electrolyte, and thus increase its electrochemically active surface area [20,23]. The ideal capacitive behaviors of NPGs electrodes were also reflected in their triangular chargeedischarge profiles (Fig. 4b). The SCs of these samples calculated from discharge curve at different current densities are plotted in Fig. 4c. As expected, the SC values of NPGs are definitely higher than that of either NPTG or TRG, showing the importance of both heteroatom doping and high SSA for carbon-based EDLCs. Although the FGN has a high SC of 202 F g1 at 0.25 A g1, it rapidly decreases to 144 F g1 at 10 A g1. Among the prepared N/P codoped graphene materials, 2NPG shows the highest SC of 219 F g1 at 0.25 A g1, which is larger than that of dual-doped GA (204 F g1, 0.2 A g1) [35] and N/P-TRGO (165 F g1, 0.1 A g1) [37]. As for the 2NPG, it remains a high value of 175 F g1 at 10 A g1,
Fig. 4. (a) CV curves of different samples in 6 M KOH with scan rate of 50 mV s1; (b) GCD curves of different samples under currents density of 5 A g1; (c) SC dependence of different samples on constant current density; (d) The Nyquist impedance plots for the different samples.
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demonstrating its good rate capability as supercapacitor electrodes. However, thermal treatment with excessive (NH4)3PO4 led to a slight capacitance decrease because of the sharply decreased pore volume of 3NPG despite of its higher N and P contents. It highlights that a well-developed pore network is indispensable for promoting the kinetic diffusion of electrolyte in the interior of electrode. Besides, we estimated the volumetric SC of 2NPG by compressing the 2NPG powders into disks. The volumetric capacitance of 2NPG is calculated to be 123 F cm3, which is higher than O/N co-doped graphene foams (106 F cm3) [53], but is still lower than the potassium humate-based graphite oxide (144 F cm3) [54] and holey graphene (216 F cm3) [55]. Fig. 4d shows the Nyquist impedance plots of different samples. It is seen that FGN exhibits a more inclined curve at low frequency region, implying a higher contribution of pseudocapacitance. The other samples display vertical lines at low frequency region which indicates very good capacitive behaviors. The radius of semicircle in the high frequency region reflects the impedance on electrode/ electrolyte interface [56]. It is found that the 3NPG electrode has the smallest semicircle, indicating the lowest charge transfer resistance in 3NPG. Comparatively, the FGN electrode exhibits the highest charge transfer resistance. The intercept at high frequency side represents the internal resistance of electrode. Consequently, the NPGs electrodes also represent the smallest internal resistance. Therefore, the EIS analysis shows that the charge transfer resistance and internal resistance of graphene nanosheets can be significantly reduced after N and P dual doping.
For practical application of NPGs, we constructed a symmetric two-electrode (2NPG//2NPG) supercapacitor cell in 6 M KOH aqueous electrolyte. Firstly, the operating potential window of the 2NPG//2NPG symmetric capacitor was tested in a wider range (Fig. 5a), which demonstrates that a high cell voltage of 1.5 V can be reached. The widen potential window of the supercapacitor has been ascribed to the enhanced electro-oxidation resistance of carbon electrodes with the aid of P-containing functional groups [26]. An approximate rectangular CV curve within 0e1.3 V can be observed at a scan rate of 5 mV s1 and the shape is well maintained at the scan rate of 200 mV s1 (Fig. S4). The GCD measurements with a potential range from 0 to 1.3 V were carried out at current densities from 0.25 A g1 to 10 A g1 (Fig. 5b), the result indicates the ideal capacitive property and good rate capability of the cell. As shown in Fig. 5c, the galvanostatic cycling at 5 A g1 of the cell was performed, and we found that it can be operated stably at potential window of 1.3 V for 20000 cycles with a capacitance retention up to 85.6%. It demonstrates the robustness and stable capacitive performance of the device. Furthermore, the energy density and power density of the device were calculated to evaluate its application potential. The results show that the 2NPG-based supercapacitor exhibits a high energy density of 8.17 Wh kg1 with a power density of 162 W kg1. Accordingly, the maximum volumetric energy density reaches a prominent value of 4.6 Wh L1 with a power density of 91 W L1. Even at a high power density of 6500 W kg1 (3640 W L1), the energy density can still retain 6.68 Wh kg1 (3.7 Wh L1). This value is higher than N-doped CNFs
Fig. 5. (a) CV curves of 2NPG with different potential windows at 100 mV s1; (b) The GCD curves of symmetric supercapacitors based on 2NPG at different current densities within 0e1.3 V; (c) Cycling performance of 2NPG based symmetric capacitors at current density of 5 A g1 with inset showing the first 15 cycles of chargeedischarge curves; (d) Cameracaptured photographs of LED indicator driven by two series-connected 2NPG supercapacitors.
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[57], P-functionalized CNTs [25], N/P co-doped GAs [35] and many other N/P co-doped carbons recently reported, as shown in Table S1. Nevertheless, our energy density is still lower than that of OMFLC-N [10] and 3D-HPG [58], which have ultrahigh SCs (400e800 F g1), far above our values (175e219 F g1). Thus, further work is needed to improve the SC of the N/P co-doped graphene. In order to check the practical feasibility of our supercapacitor, we assembled two series-connected cells based on the 2NPG electrode. As demonstrated in Fig. 5d, after charging at 1 A g1 with the voltage up to 1.3 V, the supercapacitor can efficiently light a LED indicator (2.0 V, 20 mA) for about 5 min, indicating the promising practical application of N, P co-doped graphene electrode. 4. Conclusion In summary, NPG materials were synthesized by prefunctionalization of graphene nanosheets and successive one-step (NH4)3PO4 activation. Our results demonstrate that a delicately tailored oxygen content (C/O ~ 5.7) on the surface of FGN precursor can effectively promote the (NH4)3PO4 activation, thus enabling high specific surface areas of up to 320 m2 g1 and efficient N/P codoping as high as 3.7 at. % for N and 1.3 at. % for P in the NPGs. Furthermore, by adjusting the (NH4)3PO4/FGN mass ratio, the N and P contents of the NPGs can be effectively tuned. The optimal 2NPG shows a high specific capacitance of 219 F g1 (123 F cm3) at 0.25 A g1 and 175 F g1 (98 F cm3) at 10 A g1, respectively. Significantly, the 2NPG-based symmetrical supercapacitor device, working at a potential window of 1.3 V, could deliver an enhanced energy density of 8.2 Wh kg1 (4.6 Wh L1) at a power density of 162 W kg1 (91 W L1), which still retains 6.7 Wh kg1 at 6.5 kW kg1. In particular, under a current density of 5 A g1, the device endows an 86% capacitance retention of initial after 20,000 cycles, displaying superior cycle stability. These results support that NPG is a promising candidate for further development of highperformance supercapacitors. Acknowledgments The authors gratefully acknowledge financial support from Natural Science Foundation of Hubei Province (No. 2015CFB187), National Natural Science Foundation of China (No. 21303170), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGL150414, CUGL140413 and CUG120115). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.09.008. References [1] P. Simon, Y. Gogotsi, B. Dunn, Materials science. Where do batteries end and supercapacitors begin? Science 343 (2014) 1210e1211. [2] Y. Huang, J. Liang, Y. Chen, An overview of the applications of graphene-based materials in supercapacitors, Small 8 (2012) 1805e1834. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater 7 (2008) 845e854. [4] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater 22 (2010) 28e62. [5] S.W. Lee, B.M. Gallant, H.R. Byon, P.T. Hammond, Y. Shao-Horn, Nanostructured carbon-based electrodes: bridging the gap between thin-film lithium-ion batteries and electrochemical capacitors, Energy Environ. Sci. 4 (2011) 1972e1985. [6] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 8 (2009) 2520e2531. [7] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent advances in design and fabrication of electrochemical supercapacitors with high energy densities, Adv. Energy Mater 4 (2014) 1e43.
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