P Codoped Porous Carbon-Coated Graphene Nanohybrid as a

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N/P codoped porous carbon-coated graphene nanohybrid as a high performance electrode for supercapacitors Lin Zheng, Kaisheng Xia, Bo Han, Chenggang Zhou, Qiang Gao, Hongquan Wang, Song Pu, and Jinping Wu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01552 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Applied Nano Materials

N/P codoped porous carbon-coated graphene nanohybrid as a high performance electrode for supercapacitors Lin Zheng,a Kaisheng Xia,*a,b Bo Han,b Chenggang Zhou,* a,b Qiang Gao,b Hongquan Wang,a Song Pu,a and Jinping Wu b a Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty

of Materials Science and Chemistry, China University of Geosciences ， 388 Lumo Road, Wuhan 430074, P. R. China b

Faculty of Material Science and Chemistry, China University of Geosciences, 388

Lumo Road, Wuhan 430074, P. R. China

*Corresponding authors E-mail address: [email protected] (K. Xia), [email protected] (C. Zhou) Tel.: +86 027 67883049; fax: +86 027 67883431.

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Abstract Carbon nanomaterials have shown great promise for supercapacitors but are usually limited to their unsatisfactory energy densities. To address this issue, it requires rational design and tuning of the carbon composition, texture and microstructure. Herein, we present a nanohybrid strategy for the preparation of nitrogen/phosphorus codoped porous carbon-coated graphene (KNPG) by conjunction of carbonization and activation of phytic acid on the graphene oxide in the presence of ethylenediamine. The assynthesized KNPG is endowed with a unique three-dimensional (3D) nanohybrid architecture consisting of graphene layers sandwiched by porous carbon nanosheets, a hierarchically micro/mesoporous structure, a high specific surface area (up to 596 m2g-1) and an efficient N/P co-doping (3.6 at. % for N and 0.3 at. % for P). As a supercapacitor electrode, the KNPG shows a gravimetric capacitance of 201 F g-1 (200 F cm-3) at 0.5 A g-1, and an excellent rate capability with a capacitance retention ratio of 75% at 20 A g-1. Moreover, the obtained symmetric supercapacitor in 6 M KOH delivers a high gravimetric energy density of 9.10 Wh kg-1, a large volumetric energy density of 9.07 Wh L-1, and a superior cycle stability of 84.6 % retention after 20,000 cycles. The present study opens up new opportunities to couple graphene and N/P co-doped carbon for high-performance supercapacitors. Keywords: nitrogen/phosphorus co-doping, graphene, nanohybrid, carbon electrodes, supercapacitors, energy density


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1. Introduction Supercapacitors have been regarded as a promising high power supply because of their ultrahigh power density, long cycling life and ultrafast charge/discharge rate.1, 2 Among the available electrode candidates for supercapacitors, carbon materials afford several advantages including natural abundance, high conductivity and tunable textural properties, and have thus been widely investigated in recent years. However, the limited energy storage density of carbon-based supercapacitors has severely narrowing their range of applications.3 According to the equation of energy density E=1/2 CV2, where C is the specific capacitance and V the cell potential, the energy density is highly depending on the capacitance of the electrode for a given electrolyte.4-6 Therefore, improving the specific capacitances of carbon electrode materials without sacrificing their rate capability will absolutely promote applications of carbon-based supercapacitors. Graphene has attracted extensive attention as supercapacitor electrodes owing to its favorable electrical properties, large theoretical surface area and outstanding mechanical flexibility.7-9 Unluckily, the prepared graphene normally exhibits a very limited accessible surface area resulting from severe agglomeration of graphene sheets, which makes its real performance far inferior to our expectation.5, 10, 11 Recent studies indicated that improved supercapacitive properties can be realized by combining graphene with porous carbons to form a carbon-carbon nanocomposite.12-14 Various graphene-based composite materials including ordered mesoporous carbon/graphene14, 15, hierarchical porous carbon/graphene,17, 18 and sandwiched porous carbon/graphene19,



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have been fabricated by different pathways. The results demonstrated that the

incorporation of porous carbons could effectively restrain the restacking of individual graphene layers. Moreover, by coupling the advantages of the two kinds of carbon materials, these hybrid architectures could possess both high conductive and mechanically stable network and large ion-accessible surface area for supercapacitors. Although impressive progress has been made, the enhancements of specific capacitance are still limited due to EDLC 21, 22 mechanism in these carbon electrodes. Additionally, their power characteristics are restrained originating from randomly dispersion of graphene in the composite and weak interaction between porous carbon and graphene.23-25 Apart from the above carbon-carbon hybrid strategy, heteroatoms (such as N, B, P, and S) doping is another effective method for improving the electrochemical performance of carbon electrode.26-40 N doping can increase electrical conductivity of carbon frameworks and introduce additional pseudocapacitance for supercapacitors.20, 35-37

However, the N-rich carbons normally show poor rate and cycling performance

because of dissolution or deactivation of redox active surface moieties.38 On the other aspect, P doping can improve the wettability and electrochemical stability of carbons, and thus render the electrodes superior rate capability and cyclability39,40. Despite these advantages, the enhancement of specific capacitance in the reported P-doped carbons is unconspicuous.25 Particularly, N and P co-doping is believed to induce a synergistic effect between the two elements, and thus improve the supercapacitive properties of Ndoped or P-doped carbons.41 Considerable efforts have been devoted in recent years to 4 ACS Paragon Plus Environment

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developing N/P co-doped graphene,42-44 N/P co-doped porous carbon,45-47 N/P codoped carbon sphere,41, 48 and N/P co-doped carbon nanofiber49 as high-performance supercapacitor electrodes. Nevertheless, fabricating of N/P codoped carbons that simultaneously deliver high capacitance and high power density remains a substantial challenge. With respect to the above considerations, one can envision that carbon materials with hierarchical carbon-carbon hybrid nanostructures and N/P co-doping features would combine the merits of EDLCs and pseudo-capacitors, resulting in high performance energy storage. To the best of our knowledge, such hybrid material has rarely been explored as an electrode for supercapacitor.50, 51 Herein, we engineered an N/P co-doped porous carbon-coated graphene (KNPG) nanohybrid by conjunction of carbonization and activation of phytic acid (PA) on the surface of graphene oxide (GO) in the presence of ethylenediamine (EDA). The N/P co-doped carbon film with welldeveloped porosity and good wettability coating on graphene layers could offer plentiful active sites for charge storage. Furthermore, the graphene layers play as flexible and conductive supporter for N/P co-doped carbon film, enabling hierarchical pore channels for efficient electrolyte ion diffusion and conductive networks for rapid electron conduction. The obtained KNPG exhibits high volumetric and gravimetric capacitances of 200 F cm-3 and 201 F g-1 at 0.5 A g-1, respectively, and a superior rate capability with a retention ratio of 75% at 20 A g-1. Significantly, the as-assembled symmetric supercapacitor delivers an enhanced charge storage density of 9.10 Wh kg-1 (9.07 Wh L-1) and a superior cycle stability of 84.6 % retention after 20,000 cycles. 5 ACS Paragon Plus Environment


2. Experimental section 2.1. Sample preparation

In our experiment, GO aqueous suspension (2 mg mL-1) was obtained by the modified Hummer’s method.52 To prepare NPG, 1.4 mL of EDA was added to 25 ml GO aqueous suspension and mixed under stirring for 6 h at 75 ℃. Subsequently, 3.9 mL of PA (50 wt% aqueous solution) was added dropwise and stirred for another 6 h at room temperature. After evaporation, the residue was heated at 100 ℃ for 6 h and then precarbonized at 160 ℃ for another 6 h. It is noted that the pre-carbonization process is effective for increasing the yield of the final products. Finally, the remnant solid was ground and pyrolyzed at 900 ℃ for 3 h at the ramp rate of 5℃ min-1 in N2 atmosphere to get NPG. Details in the determination of optimum EDA/GO ratio can be found in supplementary Data. To prepare KNPG, the NPG was mixed with KOH at a NPG/KOH mass ratio of 0.5, and then heated at 600 ℃ for 4 h under N2. After washing with dilute hydrochloric acid and deionized water, the solid residue was vacuum dried at 65 °C for overnight. For comparison purposes, NG was prepared under the same conditions as used for NPG except that no PA was used, RGO was obtained by directly heating of GO at 900 ℃ for 3 h under N2, and PC was obtained by preheated of PA at 100 ℃ for 6 h and 160 ℃ for another 6 h and then calcined at 900 ℃ for 3 h under N2, respectively. 2.2. Materials characterization

The morphology measurements were performed by Hitch SU8010 field scanning electron microscopy (SEM) and Philips CM12 transmission electron microscopy 6 ACS Paragon Plus Environment

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(TEM). The X-ray diffraction (XRD) patterns of the samples were recorded on the AXS D8-FOCUS XRD diffractometer

42.

Fourier transform infrared spectroscopy (FTIR)

was examined on the Nicolet6700 FT-IR spectra (Thermo Fisher Scientific, USA; 4000 to 500 cm−1). Nitrogen adsorption-desorption isotherms were collected on an ASAP 2020 HD88 system (Micromeritics). The calculation methods of total pore volume (VP), specific surface area (SBET) and overall pore size distribution have been reported in our previous works.

52

Raman spectra (Horiba Jobin-Yvon, LabRAM HR800) were

measured using 532 nm laser excitation. The JC2000C contact-angle system (POWEREACH) was used to measure the contact angles for the as-prepared carbons. 52

The ESCALAB 250 spectrometer was employed to collect the X-ray photoelectron

spectra (XPS). 2.3. Electrochemical measurements

Bio-logic VMP3 electrochemical workstation was used to collect cyclic voltammetry (CV) curves, galvanostatic charge–discharge (GCD) profiles and electrochemical impedance spectroscopy (EIS) in 6 M KOH electrolyte. The working electrodes were prepared by mixing active material, polytetrafluoroethylene (PTFE) and Super P in a mass ratio of 85:10:5 to obtain homogeneous slurry, then coated onto a Ni foam. After drying at 80 ºC for overnight, the electrode was pressed. The methods to assemble the three-electrode and two-electrode systems can be found in our previous work.

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A

detailed descriptions of the calculation equations for gravimetric specific capacitance (Cg), volumetric specific capacitance (Cv), energy density (E) and power density (P) were provided in supplementary data. 7 ACS Paragon Plus Environment


3. Results and discussion

Fig. 1. Schematic for the preparation process of KNPG.

The strategy to prepare the KNPG nanohybrid is schematically shown in Fig. 1. Firstly, the EDA, which has two electron-donating amine groups on a carbon aliphatic spacer, was introduced into the GO suspension to functionalize it. Then the amine on EDA reacts with carboxylic acid species and formed amide-like structure (GO-NH2).53, 54 At a high addition amount of EDA (e.g., EDA/GO mass ratio >10), the product of GONH2 mainly contains the tail conformation,55 where only one end of the diamines reacts with one epoxy or carboxyl group in the GO sheet, leaving another amine unreacted. To determine the optimum EDA amount, different EDA/GO ratios were examined in our work. When the EDA/GO ratio is higher than 25, the increase of N content in the resultant carbons is not obvious. Taking into consideration of the effects of EDA/GO ratio on the structural and electrochemical properties of the NPGs, the EDA/GO ratio of 25 was selected as the suitable condition. More details can be found in Section Ⅱ of the Supplementary Information. It is noted that the EDA serves as both an N source but a crosslinking agent between graphene and PA. The PA, which contains six phosphate 8 ACS Paragon Plus Environment

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groups, was used as the P source. Upon adding of PA, a GO-NH2-PA complex is subsequent formed via the chemical reaction of phosphate groups and amine groups. After a thermal pyrolysis process, it can be readily converted into an N/P co-doped porous carbon-coated graphene nanohybrid (NPG). A further chemical activation can effectively increase the surface area and porosity, leading to the formation of KNPG.

Fig. 2. SEM images of PC (a), RGO (c), NPG (d) and KNPG (e). TEM images of PC (b) and KNPG (f). 9 ACS Paragon Plus Environment


The morphological features of the prepared carbons can be obtained in the SEM and TEM images. It is observed that PC is composed of interconnected carbon sheets (Fig. 2a), which displays a highly-developed micro-mesoporous structure with pore diameter in the range of 1~3 nm (Fig. 2b). RGO shows a large and thick planar structure (Fig. 2c), which is due to the restacking of graphene layers during the thermal reduction process. In contrast, NPG and KNPG exhibit a similar flower-like structure (Fig. 2d and e) with each petal a curly and wrinkly sheet morphology. The change of morphological features in these carbons should be related to the incorporation of porous carbons, which could prevent the restacking of graphene layers. A great number of micrometer-sized pores (0.5~5 μm) can be observed between the carbon sheets in the NPG and KNPG. The high-resolution TEM image (Fig. 2f and Fig. S2) confirms that the KNPG has a hybrid configuration with a layer of porous carbon (pore size around 2 nm) tightly coated on the graphene sheets. Thus, we can conclude that the GO precursor acts as a substrate for the assembling of EDA and PA successively in the synthesis process, while the EDA and PA can also serve as a bridge to interconnect the adjacent carbon nanosheets, leading to a three-dimensional (3D) architecture.


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Fig. 3. (a) XRD patterns, (b) FTIR spectra, (c) nitrogen adsorption/desorption isotherms and (d) pore size distributions of the prepared samples.

Fig. 3a shows the XRD patterns of different samples. A characteristic diffraction peak of GO was observed at 2θ = 11.1°, which is associated with the abundant oxygencontaining groups existing between the graphene layers. After thermal reduction, the basal reflection peak of GO disappeared due to the exfoliation of graphene sheets, and a broad band at around 24.5° was found for the RGO due to an increased interlayer distance in it. Moreover, no obvious diffraction peaks was observed for PC, NPG and KNPG, indicating their amorphous nature. In order to examine the defective nature of these samples, Raman spectra were next carried out. As seen in Fig. S3, the carbons have two peaks at 1350 cm−1and 1590 cm−1, characteristic of D and G bands, respectively.

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Generally, the relative intensity of the D and G band (ID/IG) is 11 ACS Paragon Plus Environment


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proportional to the number of defect sites inside. The calculated ID/IG value of PC is as high as 1.31, suggesting the highest level of defects in PC. In contrast, the ID/IG value decreases to 1.02 for KNPG due to its nanohybrid architecture consisting of graphene and N/P co-doped carbon.56,57 The FT-IR spectra shown in Fig. 3b suggest that all samples exhibits two characteristic bands at 1617 and 1067 cm-1, which is ascribed to from the C=C and C-C stretching vibrations, respectively. Moreover, three peaks at 1715, 1411 and 1230 cm-1 corresponding to carboxyl (-COOH), hydroxyl (-OH), and alkoxy (C-O-C) are observed in the spectrum of GO, indicating abundant oxygen functionalities on the GO surface. In comparison, the intensity of these peaks is greatly suppressed in other samples. Significantly, the occurrence of a new peak at 1382 cm-1 in the NG, NPG and KNPG reveals the formation of C-N bond, which confirms the successful doping of N atoms into those samples.

Table 1. The textural properties of the different carbons. Samples

SBET

VP

Pore size

(m2 g-1)

(cm3 g-1)

(nm)

RGO

338

0.23

2.7

NG

787

0.52

2.6

PC

2321

1.58

2.7

NPG

384

0.44

4.5

KNPG

596

0.55

3.7

The textural properties of the prepared products are shown in Fig. 3c and 3d. The calculated specific surface area, total pore volume and average pore size calculated 12 ACS Paragon Plus Environment

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from isotherms are given in Table 1. It is seen that RGO has a type I isotherm, which is indicative of the presence of a dominant micropores. Its specific surface area and pore volume are 338 m2 g-1 and 0.23 cm3 g-1, respectively. The PC, which is obtained by direct calcination of PA, exhibits a type I/II isotherm44 with prominent N2 adsorption below P/P0 of 0.1. It implies that the PC has a large amount of micropores and small mesopores, in consistent with the pore size distribution by NL-DFT method, in which three main peaks centered at 0.7, 1.3 and 2.8 nm, respectively, are seen (Fig. 3d). An ultrahigh surface area of 2321 m2g-1 and large pore volume of 1.58 cm3g-1 is found in the PC. After deposition of N or N/P containing moieties, the NG and NPG show similar type IV isotherms with higher N2 adsorption capacity relative to the RGO. It is noted that the NPG has a much lower surface area than NG (384 and 787 m2g-1, respectively). The possible reason is that the strong interaction between EDA and PA may lead to a densification of the hybrid structure. After KOH activation of NPG, the resultant KNPG shows remarkably enhanced specific surface area of 596 m2g-1 and pore volume of 0.55 cm3g-1, demonstrating the effective introduction of micropores by KOH activation.



Fig. 4. XPS analysis of KNPG. (a) C 1s, (b) N 1s (c) O 1s and (d) P 2p spectra.

Elemental compositions and chemical bonds of the carbon materials were determined by the XPS analyses. The elemental survey spectra present in NPG and KNPG are shown in Fig. S4, which shows O 1s peak at 530.3eV, N 1s peak at 400.2 eV, C 1s peak at 284.5 eV and P 2p peak at 133.7 eV. For KNPG, the atomic contents of C, O, N and P are calculated to be 76.9, 19.2, 3.6 and 0.3 at%, respectively. Using the same method as adopted in this work, the PC was reported to contain a higher P content of 2.9 at%.58 Moreover, the P content of NPG is calculated to be 1.4 at% as listed in Table S3. The markedly decreased P content in KNPG may be attributed to the KOH activation which can consume the P functionalities in NPG. The C 1s spectra (Fig. 4a) are curve-fitted into three configurations at ~284.6 eV, ~285.8 eV, and ~287.3 eV that correspond to C=C, C-C, and –C=O/C=N, respectively. The O 1s spectra (Fig. 4c) can be deconvoluted into quinone groups at ~530.3 eV, C=O/P=O groups at ~531.6 14 ACS Paragon Plus Environment

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eV, C-O/C-O-P groups at ~532.6 eV, and O-H groups at ~533.6 eV. There are four N configurations in KNPG based on the binding energies (Fig. 4b): pyridine-type N (~398.7 eV), pyrrolic-type N (~400.3 eV), quaternary-type N (~401.8 eV), and pyridine-N-oxide (~403.1 eV).42, 59 It is seen that the pyridine and pyrrolic N, which are reported to be responsible for improved supercapacitive performance of carbon electrodes,60 represent the main N binding forms in the KNPG. In addition, the P2p spectrum (Fig. 4d) contains two peaks. The peak at ~132.6 eV is associated to pyrophosphate ([P2O7]4-), and the peak at ~ 134.3 eV can be assigned to metaphosphate ([PO3]-).42 Data from XPS analysis can definitely confirm the successfully doping of N and P into the KNPG.

Fig. 5. Electrochemical performances of the selected samples in 6 M KOH: (a) CV curves at a scan rate of 100 mV s-1; (b) galvanostatic charge/discharge curves of 15 ACS Paragon Plus Environment


KNPG at different current densities; (c) specific capacitances dependence on current density; (d) gravimetric and volumetric capacitance of the KNPG.

The electrochemical performance of these samples as supercapacitor electrodes were firstly evaluated in a three-electrode system. The cyclic voltammetry (CV) curves obtained at a scan rate of 100 mV s-1 in 6 M KOH electrolyte are shown in Fig. 5a, where we can see near rectangle-shaped curves for these samples. It is interesting that no obvious redox peaks associated with N, P and O functionalities were observed even at very low scan rates (Fig. 5a and Fig. S5). The possible reason may be that pseudocapacitances induced by the present N-, and O-containing groups are inconspicuous in 6 M KOH.61 Taking into account of the very low P content, the Pcontaining groups is hard to provide pronounced pseudocapacitance for the KNPG. Nevertheless, it may help to improve the surface wettability of KNPG electrode, and thus increase its surface area accessible to electrolyte ions. As illustrated in Fig. S6, the contact angle decreases from 51o of RGO and 66o of NG to 46o of KNPG, suggesting the surface wettability of the carbons increased after P doping. The excellent supercapacitive behavior of the KNPG electrodes was also reflected in its symmetrical triangular charge–discharge profiles without voltage drop (Fig. 5b). The specific capacitances of various carbon electrodes determined from discharge curves ranging from 0.5 to 20 A g-1 are plotted in Fig. 5c. The highest specific capacitances of various electrodes at 0.5 A g-1 follow the degressive order: KNPG (201 F g-1) > NPG (115 F g1)

> PC (112 F g-1) > NG (99 F g-1) > RGO (63 F g-1). The RGO shows the lowest 16 ACS Paragon Plus Environment

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specific capacitance among the samples, which is probably due to its low specific surface area and porosity. Increased capacitance is observed for NG upon N doping. Even so, the values of NG are somewhat below our expectation in view of its distinctly higher surface area and heteroatom doping nature. It is noted that NPG has almost the same capacitances as the PC at all current densities, although the NPG (384 m2g-1) has a much lower specific surface area than PC (2321 m2g-1). The poor supercapacitive performance of PC can be ascribed to its high microporosity and high level of defects as indicated by the Raman analysis. Another important reason lies in the low conductivity of PC, which will be discussed later in the EIS part. Moreover, the superior property of NPG can be attributed to the beneficial roles of N/P co-doping and hierarchical pore structure in it. After activation of NPG, due to the much higher surface area and more open pore network, a huge boost of specific capacitance is found in the KNPG. In comparison with NG (without P doping, 787 m2g-1), the KNPG (with 0.3% P doping, 596 m2g-1) shows much higher specific capacitance, indicating the positive effect of P doping on capacitance enhancement. When the current density is increased to 20 A g-1, the KNPG still maintains impressive capacitance of 150 F g-1 with a retention ratio of 75%, indicating its superior rate capability. Such high capacitance retention ratios as a function of current density are also obtained in NPG (70%) and PC (68%). It indicates that the P doing is advantageous to improve the rate capability of carbon electrode due to its more hydrophilic surface and more efficient ion diffusion enabled by P doping.

42

Besides, we calculated the volumetric capacitance of KNPG.

Owing to its high density of 0.997 g cm-3, the volumetric capacitance of KNPG can 17 ACS Paragon Plus Environment


achieve 200 F cm-3 at 0.5 A g−1 (Fig. 5d), which is among the highest reported in the literature (Fig. S7 and Table S4). The cyclic stabilities of the different samples were further evaluated for up to 2000 charge–discharge cycles at a constant current density of 5 A g-1 (Fig. S8). The KNPG exhibits outstanding cycle performance with only 2% degradation after 2000 cycles. The cyclic stability of the KNPG via CV measurement is also shown in Fig. S9. EIS analysis was conducted to further understand the various supercapacitive performances in these electrodes. As shown in Fig. S10, all of the plots feature vertical curves in low-frequency region. It demonstrates the excellent capacitive behaviors in these samples.52 The intercept at high frequency region indicates the electrode resistance (RΩ), which follows the order: PC < KNPG < RGO < NG < NPG. The low RΩ values of PC, KNPG and RGO indicate their good electrical conductivity. The radius of semicircle in the high frequency region reflects the charge transfer resistance (Rct), which increases in the following order: KNPG < NPG < PC < NG < RGO. The Rct mainly depends on the wettability between the electrode and electrolyte, the surface area and conductivity of electrode.62 Thus, the KNPG possesses the lowest Rct due to its hydrophilic surface endowed by N/P co-doping and high specific surface area upon KOH activation. Besides, the projected length of the 45º portion in medium frequency presents the Warburg resistance (Rw) and is associated with the frequency-dependent of ion diffusion from electrolyte into the electrode. Clearly, the KNPG shows the smallest Rw, which suggests a highly efficient ion diffusion in KNPG. In order to further investigate the application potential of the KNPG, we assembled 18 ACS Paragon Plus Environment

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a symmetric KNPG//KNPG supercapacitor device using 6 M KOH electrolyte. The previous reports have demonstrated that P-doped carbon electrodes displayed a wider potential window due to the blockage of active oxidation sites by phosphate groups at positive potential and the adsorption of hydrogen at negative potential.42, 63 According to the equation of energy density, the increased potential window will definitely enhance the energy density of the supercapacitor. Therefore, the potential window of the supercapacitor was operated in an enlarged range. Based on the CV curves of the KNPG//KNPG supercapacitor in different operation voltages at the scan rate of 50 mV s-1 (Fig. 6a), and the GCD curves at the current density of 1 A g-1 (Fig. 6c), a high cell voltage of 1.5 V can be reached in this supercapacitor. In contrast, the maximum potential window of NG//NG supercapacitor is only 1.3 V (Fig. 6b), demonstrating that P doping can improve the electrochemical stability of carbon electrodes. Fig. S11 displays the rate performance of the KNPG//KNPG symmetric supercapacitor in the operation voltages of 1.4 V. It shows a high gravimetric of 134 F g-1 and a high volumetric capacitances of 134 F cm-3 at 0.5 A g-1. As shown in Fig. 6d, the galvanostatic cycling at a moderate current density of 5 A g-1 was performed, and the result shows that the device can be operated stably at 1.4 V for 20,000 cycles with a retention up to 84.6 % of its initial capacitance. Fig. 6e exhibits the Ragone plot of KNPG based symmetric supercapacitor compared with the previously reported carbon electrodes. Benefited from the wide operating voltage of 1.4 V, the KNPG//KNPG symmetric supercapacitor delivers a maximum energy density of 9.10 Wh kg-1 at a power density of 350 W kg-1, and still maintains 5.94 Wh kg-1 at 13,000 W kg-1, which 19 ACS Paragon Plus Environment


is higher than those of previously reported carbon electrodes in aqueous electrolytes (as shown in Table S5). Notably, the maximum volumetric energy density of KNPG based supercapacitor is calculated to be 9.07 Wh L-1. As far as we know, this value is among the highest volumetric energy density reported for the carbon-based symmetric supercapacitors (Fig. 6f, Table S6). A long cycle life is also crucial performance criteria in practical production applications.

Fig. 6. CV curves of KNPG//KNPG (a) and NG//NG (b) symmetric supercapacitors in different operation voltages at 50 mV s-1; (c) GCD curves of symmetric 20 ACS Paragon Plus Environment

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supercapacitor in different operation voltages at 1 A g-1; (d) Cycle performance of KNPG//KNPG symmetric supercapacitor at 5 A g-1, the inset shows the GCD curves for the selected cycles; (e) Comparison of symmetric supercapacitor performances based on KNPG and other reported carbon materials in Ragone plots;16, 20, 26-27, 64-79 (f) Gravimetric and volumetric energy density of KNPG and other carbon-based symmetric supercapacitors.16, 68, 70, 73, 77-78, 80-84

4. Conclusion In summary, we have developed an effective approach for the fabrication of N/P codoped porous carbon-coated graphene nanohybrid by conjunction of carbonization and activation of phytic acid on the surface of graphene oxide in the presence of ethylenediamine. The as-synthesized KNPG material shows a unique 3D nanohybrid architecture consisting of graphene layers sandwiched by porous carbon nanosheets, a hierarchically micro/mesoporous structure, a high specific surface area and an efficient N/P co-doping feature. Owing to the combined merits of EDLC and pseudocapacitance for efficient charge storage, the well-developed hierarchical porosity and good wettability for efficient ion diffusion, and the beneficial 3D graphene-based skeleton for rapid electron conduction, the KNPG electrode exhibits high capacitance, outstanding

rate

capability,

superior

cycle

stability,

and

enhanced

gravimetric/volumetric energy storage density at high power condition. Our work opens up new opportunities to couple N/P co-doped carbon and graphene for improved charge storage. 21 ACS Paragon Plus Environment


Acknowledgments The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21773217), National Key R&D Program of China (No. 2018YFF0215404), Wuhan Science & Technology Project (No. 2018010401011276), Natural Science Foundation of Hubei Province (No. 2015CFB187), and the Open Fund of the Guangdong Provincial Key Laboratory of Advance Energy Storage Materials (No. AESM201815). The authors also thank Prof. Wanjun Lu (State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan) and Prof. Zhen Li (Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan) for the Raman measurements.

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We developed a nano-hybrid strategy to fabricate nitrogen/phosphorus co-doped porous carbon-coated graphene (KNPG) towards high-energy density electrodes for supercapacitors.


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P Codoped Porous Carbon-Coated Graphene Nanohybrid as a

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