P Codoped Carbon with

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Facile synthesis of hierarchically porous N/P co-doped carbon with simultaneously high-level heteroatom-doping and moderate porosity for high-performance supercapacitor electrodes Ying Zhang, Qi Sun, Kaisheng Xia, Bo Han, Chenggang Zhou, Qiang Gao, Hongquan Wang, Song Pu, and Jinping Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05024 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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ACS Sustainable Chemistry & Engineering

Facile synthesis of hierarchically porous N/P co-doped carbon with simultaneously high-level heteroatom-doping and moderate porosity for high-performance supercapacitor electrodes Ying Zhang+, Qi Sun+, Kaisheng Xia*, Bo Han, Chenggang Zhou*, Qiang Gao, Hongquan Wang, Song Pu, and Jinping Wu 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

*Corresponding authors E-mail address: [email protected] (K. Xia), [email protected] (C. Zhou) + These

two authors contributed equally to this work.

Tel.: +86 027 67883049; fax: +86 027 67883431.

1

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Abstract Multiple heteroatoms doping represents an effective strategy for improving the supercapacitive performance of carbon electrodes due to its combined merits of pseudocapacitance and double layer capacitance. However, a green and efficient approach for generating heteroatoms co-doped carbons which simultaneously possess high-level heteroatom-doping and moderate porosity remains a big challenge. Here, we put forward a CaCO3-assistant technique for the fabrication of nitrogen/phosphorus codoped hierarchical porous carbons (NPHCs). The as-prepared 1NPHC-850 integrates the structural characteristics of high-level heteroatom-doping (8.72 at. % for N, 4.44 at. % for P and 10.24 at. % for O), large surface area (up to 414 m2g-1) and triple micromeso-macro pore structure. It exhibits a high specific capacitance of 212 F g-1 at 0.5 A g-1, and an excellent rate performance with a capacitance ratio of 75% at 20 A g-1. Moreover, the 1NPHC-850-based symmetrical supercapacitor device could achieve a high energy density of 10.61 Wh kg-1in aqueous electrolyte, and an ultra-long cycling life (capacitance retention of 86.3 % after 10,000 cycles). Our work not only offers a facile strategy to produce advanced multiple heteroatom-doped carbon materials, but also provides reference for rational regulation of chemical composition and pore structure in pursuit of better carbon electrodes for supercapacitors. Keywords:

Hierarchical

porous

carbon,

Green

and

efficient

Nitrogen/phosphorus co-doping, Supercapacitors, Electrode materials

2


method,

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Introduction Carbon-based supercapacitors

1, 2

have attracted extensive attention due to their

high-power density, safe reliability, long cycle life, wide working temperature and environmental friendliness. Various types of carbon materials, mainly including activated carbon

3, 4,

porous carbon

5, 6,

carbon nanotube

7, 8,

graphene

9, 10,

etc., have

been investigated for supercapacitor electrodes 11. Generally, the specific capacitance, rate capability and cycling stability of carbon electrodes are the key factors to evaluate their suitability for supercapacitor application. Over the past decade, significant advancements towards improved specific capacitance have been made by either increasing the surface areas of carbon materials or promoting their microporosity 5, 12. Although an ultra-high surface area can be obtained for carbon materials by postactivation

13,

the deteriorated electrical conductivity and increased structural defects

may restrict their supercapacitive characteristics 14. Meanwhile, a highly-developed microporosity may cause unfavorable electrolyte ion diffusion inside the electrodes at high current density, leading to poor rate capability. More importantly, the enhancement in specific capacitances is still unsatisfactory due to the limitation of electric double-layer capacitor (EDLC) mechanism. Therefore, further breakthrough in increasing capacitance of carbon electrodes is essential for meeting the rising demand for high energy density supercapacitors. In recent years, many studies have demonstrated that the electrochemical performance of carbon electrodes can be significantly improved through doping using heteroatoms such as N, B, P, and S 15, 16. N-doping is an effective method to improve 3



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the electrical properties and specific capacitances of carbon electrodes

17, 18.

Nevertheless, N-rich carbons generally exhibit poor rate and cycling stability 16, 19. Pdoping can improve the hydrophilicity and stability of the carbon materials thus 20, 21.

improve their rate and cycling performance

The promotion of electrochemical

capacitance in the developed P-doped carbons is yet limited

8, 22.

Given the

complementary effects between N and P doping, one can envision that N and P codoping would effectively further improve the supercapacitive properties of carbons. Some recent works have recently been devoted to developing N/P co-doped carbon materials for supercapacitor application

23-28.

In spite of the significant achievements

reported in the literatures, further enhancement is still expected. To fully realize the advantages of pseudo-capacitors and EDLCs, an ideal N/P codoped carbon electrode requires abundant and accessible N and P species for prominent pseudocapacitance, high specific surface area for effective ion adsorption, and hierarchical pore structure for fast diffusion of electrolyte ions

29, 30.

Relatively high heteroatom content can be

obtained by direct pyrolysis of mixed heteroatom-containing precursors under moderate conditions, but the resultant co-doped carbon materials exhibits rather low 26, 31.

surface areas and almost non-pore structures

Employing post-activation such as

KOH activation can remarkably increase the specific surface area of the doped carbons 32,

however, the harsh treatment often leads to limited concentration of heteroatoms and

unfavorably high microporosity in them

12.

Moreover, the additional chemical

activation is relatively complex, time- and energy-consuming. Thus, the exploration of simple, efficient and scalable approaches to produce N/P co-doped carbons with 4


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simultaneously high heteroatom contents, large surface areas and hierarchical pore structures is still challenging and pursued 33-35. Here, we developed a green and efficient strategy to fabricate N/P co-doped hierarchical porous carbons (NPHCs), in which CaCO3 nanoparticles (50 nm) serves as both a hard template and an inner-activator

36, 37,

widespread glucose as the carbon

source, cheap melamine as the nitrogen source, and biomass of phytic acid as the ecofriendly phosphorus source. Unlike the KOH-activated carbons with limited heteroatom contents and high microporosities 11, the obtained NPHCs are characterized by highlevel heteroatom-doping of N, P and O, large specific surface area and hierarchically micro-meso-macroporous structure. Benefiting from the synergistic effect of abundant heteroatomic functionalities and multilevel porosity, the NPHCs electrodes are endowed with high specific capacitance, outstanding rate performance and excellent cycling stability. Notably, the as-assembled 1NPHC-based symmetrical supercapacitor shows a high energy density of 10.61 Wh kg-1 at a power density of 400 W kg-1 in aqueous electrolyte, which still retains 7.95 Wh kg-1 at 16 kW kg-1. This attempt wound not only give a facile strategy to produce advanced multiple heteroatom-doped carbon materials, but also provide reference for rational regulation of chemical composition and pore structure for obtaining better supercapacitors.

Experimental section Sample preparation The analytical grade chemicals were purchased from Sinopharm Chemical Reagent 5



Beijing Co. Ltd. In a typical synthesis, 1 g of melamine and 47.2 mg of glucose were completely dissolved in 250 ml of 25 oC deionized water (Fig. S1a). Then, 1 ml of 50% phytic acid solution was added dropwise to the above solution under stirring, and a white precipitate was immediately formed (Fig. S1b). It is noted that the glucose can not only serve as the carbon source but also increase the yield of final product, which can be found in Table S1. Details in measurement of optimal phytic acid amount can be observed in Fig. S2. Subsequently, 1 g of CaCO3 powder (50 nm, Shanxi Hengxin Nano Mater. Co., Ltd) was added (Fig. S1c), and the mixture was stirred overnight. In addition, the good water dispersive of CaCO3 powder can be seen from Fig. S3. After filtration and drying at 65 oC for 12 h, the obtained white solid was transferred to a corundum boat and placed in the center of tube furnace. Later, the solid was precarbonized 38, 39 at 160 oC for 6 h and thermally annealed at 500 oC for 1 h and 850 oC for 1.5 h at a heating rate of 2 oC min-1 under N2 atmosphere, respectively. The precarbonization process is necessary to obtain higher yield of final product (Table S1). After cooling to room temperature, the collected product was washed with 2 M HCl and deionized water until neutral, and then dried under vacuum at 60 oC for 12 h. The final product was named as xNPHC-y, where x represents the mass ratio of CaCO3/ melamine, and y represents the final carbonization temperature. For comparison purposes, the NPC-850 was prepared in the same manner as 1NPHC-850 except that no CaCO3 was used. More information on control samples (NC-850, PC-850, NHC850, and PHC-850) can be found in the Supporting Information (Section I and Fig. S4 in Section II). 6


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Materials characterization Transmission electron microscopy (TEM, Tecnai G20) and scanning electron microscopy (SEM, Hitachi SU8010) were used to observe the microscopic feature of the samples. High-angle annular dark-field transmission electron microscopy (HAADF-TEM) and elemental mapping analysis were carried out on a Titan G260-300 electron microscope operating at 200 kV. Bruker AXS D8-FOCUS diffractometer was applied to record the powder X-ray diffraction (XRD) patterns of the samples. Nicolet6700 Fourier transform infrared (FT-IR) spectroscopy (Thermo Fisher Scientific) was used to examine the FT-IR spectra and analysis the surface functional groups. Micromeritics Tristar II 3020 and ASAP 2460 systems were used to analysis the N2 adsorption-desorption isotherms at relative pressure above 0.1 and below 0.1, respectively. All the samples were outgassed at 120 oC for 6 h before adsorption analysis. The surface areas and pores distributions were calculated by Brunner-EmmetTeller (BET) method and Barrett-Joyner-Halenda (BJH) method respectively. The micropore surface areas and micropore distributions were determined by t-plot method and density functional theory (DFT), respectively. The surface analyses of the samples were measured by using X-ray photoelectron spectroscopy (XPS, ESCA-LAB250). Electrochemical measurements An ethanol suspension of active material, Super P and polytetrafluoroethylene (PTFE) in a weight ratio of 80:10:10 was made by sonication using an ultrasound for 1 h. And then the pastes were pressed into a 1 ×2 cm portion of Ni foam current collector and then dried at 65 oC overnight. Subsequently, the dried electrode (mass loading of 7



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2.0 mg/cm2) was pressed at 10 MPa. In a three-electrode configuration, the working electrodes were tested with a Pt wire counter electrodes and Hg/HgO reference electrodes in 6 M KOH electrolyte. The two-electrode symmetrical supercapacitor was assembled to further analog the actual device behavior of the carbon material. As for the symmetric two-electrode (1NPHC-850//1NPHC-850) supercapacitor, two identical electrodes with the same mass loading were separated by glass fibers membrane using 1 M Na2SO4 aqueous solution as an electrolyte, and then assembled into a columnar cell (as shown in the Fig. S5). All electrochemical measurements were carried out on a Bio-logic VMP3 electrochemical workstation. The electrochemical performance of the electrode materials was characterized by electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) methods. The calculations of the specific capacitance, power density and energy density of the electrode are shown in the Supporting Information.

Results and discussion The synthesis procedure of NPHCs is schematically illustrated in Fig. 1. Melamine can serve not only as nitrogen source for NPHCs, but also as skeletons to form supramolecular structure. This is because each melamine molecule possesses nine hydrogen bond sites and several unpaired electrons, which can be used as H-bond acceptor 27, 40. The phytic acid, which contains six phosphate groups, acts as phosphorus source and pH regulator for supramolecular crosslinking

41.

When the melamine,

glucose and phytic acid are simultaneously dissolved in water, they will react with each 8


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other to form a crosslinked melamine-glucose-phytic acid (MGP) supramolecular structure due to H-bond and acid-base reactions. After the following nano-CaCO3 dispersion process, a homogeneous mixture of MGP and CaCO3 can be obtained. During the carbonization stage, the MGP supramolecular can be easily transformed into an N/P co-doped porous carbon skeleton and the nano-CaCO3 would decompose into CaO and CO2, in which the CaO nanoparticles acting as a hard template for mesopores and the CO2 gas playing as the inner-activator for micropores and small mesopores, respectively. Upon further acid etching, the residue template can be readily removed and thus the NPHCs with hierarchical pore structures can be successfully prepared.

Fig. 1. Schematic illustration of the synthesis of NPHCs. The morphological feature and pore geometry of the NPHCs were examined by SEM and TEM measurements and the results are displayed in Fig. 2. As seen in Fig. 2a and b, the 1NPHC-850 shows a three-dimensional (3D) beehive-like morphology 9



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constructed by hierarchically porous carbon skeletons. The SEM images illustrated that there are two types of pores in 1NPHC-850: macropores with a diameter of approximately a few hundreds of nanometers which stem from the gas escaping during thermal pyrolysis and mesopores with a diameter of approximately 20~40 nm which originate from the removing of residual CaO template. More information regarding the pore texture of 1NPHC-850 can be found in Fig. 2c. Except for the mesopores (around 30 nm) is further confirmed by the TEM image, a large number of micropores smaller than 2 nm are clearly observed, which is due to the activation effect of CO2

42, 43.

To

highlight the unique role of nano-CaCO3 template, we also synthesized NPC-850 without using nano-CaCO3 template for comparison. Unlike the hierarchical porous 1NPHC-850, the SEM image (Fig. 2d) of NPC-850 demonstrate that it exhibits a lamellar morphology with smooth surface and almost nonporous structure. Moreover, the TEM elemental mapping (Fig. 2e) and SEM elemental mapping results (Fig. S6) confirmed the existence and evenly distribution of elements (C, N, O and P) in 1NPHC850. It is seen that the contents of P and O element is higher than that of N, which is may associated with the fact that phosphate groups mainly exist on the surface of the 1NPHC-850 while the nitrogen starring exists in the carbon skeleton 27.

10


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Fig. 2. (a, b) SEM images of 1NPHC-850 at different magnifications, (c) TEM image of 1NPHC-850, (d) SEM image of NPC-850, and (e) TEM image and the corresponding elemental mappings of 1NPHC-850. 11



The XRD patterns of the products are shown in Fig. 3a. It is found that the NPC850, 0.5NPHC-850 and 1NPHC-850 have similar diffraction patterns with a weak and broad peak near the 2θ of 25°, which is associated with the carbon (002) crystal face. The XRD results indicate the amorphous nature in these samples and the successful removal of residual CaO. FT-IR spectra has been used to analyze the surface functional groups in these samples. As displayed in Fig. 3b, the samples exhibit several characteristic bands at ca. 3433 cm-1 and 1401 cm-1 due to stretching vibration peak from O-H, 2923 and 2856 cm-1 owing to the stretching vibration peak from C-H, 1625 cm-1 corresponding to stretching vibration peak of carbonyl C=O/C=N, 1140 cm-1 attributing to P=O/P=OOH bending vibration, and 1038 cm-1 assigning to C-O stretching vibration 28. Moreover, the Raman analysis was performed to examine the structural defects in these doped carbons. As illustrated in Fig. S7, the calculated ID/IG values are 1.18 for NPC-850, 1.14 for 0.5NPHC-850 and 1.14 for 1NPHC-850, respectively. The results show that the CaCO3 activation helps to decrease the level of defects in NPC-850. Fig. 3c and 3d describe the N2 sorption isotherms and BJH pore size distribution (PSD) curves, respectively. The type II sorption isotherm of NPC-850 shows the existence of only macropores which is also confirmed by its PSD curve. As illustrated in Fig. S8, the 0.5NPHC-850 and 1NPHC-850 show dramatic increase in N2 adsorption at relative pressure (P/P0) below 0.1, implying the occurrence of a large number of micropores as a result of CO2 activation. In addition, they exhibit type IV isotherms with a typical hysteresis loop in the P/P0 range of 0.4-1.0, indicating the presence of abundant mesopores. The PSD curves determined by BJH method clearly 12


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show that 0.5NPHC-850 and 1NPHC-850 have two distinct pore distributions at 3~5 nm and 20~30 nm, respectively, in consistent with the previous SEM and TEM images. The former small mesopores should ascribe to activation effect of the in-situ formed CO2; the latter large mesopores should come from the removal of hard template. Moreover, the 1NPHC-850 displays enhanced adsorption quantity in the tested P/P0 range in relative to 0.5NPHC-850, which suggests increasing the amount of CaCO3 can further increase the micropores and mesopores. Actually, we also prepared 2NPHC850 sample at CaCO3/melamine ratio of 2, however, we found that the increased addition of CaCO3 would lead to a high carbon burn-off greater than 90%. More detailed textural properties of these carbons can be found in Table 1. The NPC-850 possesses a low BET surface area (SBET) of 62 m2 g-1 and a small total pore volume (VP) of 0.26 cm3 g-1. After adding of CaCO3 as both a template and an activator, the SBET and VP can be increased to 414 m2 g-1 and 0.34 cm3 g-1 for 1NPHC-850, respectively. The obtained highest SBET and VP in 1NPHC-850 is higher than those of some reported literatures, such as N/P co-doped graphene (50 m2 g-1 and 0.04 cm3 g-1) 28, N/P co-doped carbon nanowires (258 m2 g-1 and 0.204 cm3 g-1)

44,

and N/P co-doped microporous

carbons (353 m2 g-1 and 0.256 cm3 g-1) 33. More information on the micropore analysis can be found in Fig. S8. The above results clearly demonstrate the key role of CaCO3 nanoparticles in realizing the hierarchically micro-meso-macroporous structure in the N/P co-doped carbon matrix.

13



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Fig. 3. (a) XRD patterns, (b) FT-IR spectra, (c) N2 sorption isotherms, and (d) pore size distribution curves of NPC-850, 0.5NPHC-850 and 1NPHC-850.

Table 1 Textural parameters and chemical composition of NPHCs. Samples

SBET a (m2 g-1)

SM b (m2 g-1)

VP (cm3 g-1)

VM c (cm3 g-1)

Pore size (nm)

NPC-850 62 5 0.26 0.01 15.7 0.5NPHC-850 350 80 0.30 0.09 4.1 1NPHC-850 414 86 0.34 0.11 3.8 1NPHC-750 aS BET: Specific surface area calculated by BET method. b S Micropore surface area determined by t-plot method. M: c V Micropore volume determined by t-plot method. M:

Atomic composition (at. %) 54.92 76.64 69.10

12.50 8.72 11.64

20.72 10.24 14.42

The heteroatom content and the chemical state of nitrogen, oxygen and phosphorous species of the synthesized carbons were measured by XPS analysis. The survey spectrum (Fig. 4a) of each sample shows a predominant O 1s peak at 532 eV, N 1s peak at 400 eV, C 1s peak at 284.5 eV, and weaker P 2p peak at 133.3 eV 14


21, 45,

11.86 4.44 4.84

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respectively. The contents of C, N, O and P elements in different samples are listed in Table 1. It is seen that the quantities of the heteroatoms decreases after introducing of CaCO3 nanoparticles, which is probably due to the reaction between CO2 gas and active C atoms and heteroatom functionalities under high temperature. Additionally, raising the calcination temperature from 750 to 850 oC further reduces the contents of N and O in the resultant products. A rather high doping contents of 8.72 at. % N, 4.44 at. % P and 10.24 at. % O was obtained in 1NPHC-850. In general, a high heteroatom doping level and large specific surface area are mutually exclusive and hard to implement simultaneously for doped carbon materials. Interestingly, our results suggests that the template-assistant strategy developed in this work represents a promising technique to resolve this puzzle and is particularly valuable for synthesis of co-doped carbons. There are two main advantages of our technique: the first is that the one-step carbonization/activation method can effectively reduce the loss of heterogeneous elements during the high temperature treatment; and the second is that the nano-CaCO3 serves as both an efficient pore-forming agent and a mild inner-activator.

Fig. 4. (a) XPS survey spectra of NPC-850, 1NPHC-850 and 1NPHC-750 and (b–e) 15



high-resolution C 1s, O 1s, N 1s and P 2p XPS spectra of 1NPHC-850.

The high-resolution XPS spectra of C1s (Fig. 4b, S9b and S10b) show a very similar asymmetric peak shape. Four peaks (C-C, C-P, C-N and C-O) centered at 284.5, 285.5 eV 286.5 eV and 287.2 eV, respectively, were obtained after deconvoluting the C 1s spectra 46. The main peak at 284.6 eV is related to the graphitic carbon which indicating most of the C atoms are arranged in a conjugated honeycomb lattice. The presence of the C-P and C-N bonding definitely confirm that P and N atoms have been successfully doped into the lattice of carbon. The O1s spectrums (Fig. 4c, S9c and S10c) of the samples can be divided into three different components such as the quinone-type C=O and non-bridged oxygen of P=O at 530.1 eV (O-I), the single bonded oxygen of C-O,CN and C-O-P at 531.6 eV (O-II), and the single bonded oxygen of O-H at 533.1 eV (OIII) 47. As illustrated in Table S2, the 1NPHC-750 and 1NPHC-850 have much higher O-I content compared with NPC-850. The possible reason is that the CO2 activation could promote the formation of quinone-type C=O and non-bridged oxygen of P=O, which has been observed in our previous report 46 and will be beneficial for capacitance enhancement 49, 50. The deconvolution of N 1s spectra in Fig. 4d, S9d and S10d show four types of N species in these N/P co-doped carbons, including the N types can be classified into pyridinic-N (N-6, 398.4 eV), pyrrolic-N (N-5, 400 eV), quaternary-N (N-Q, 401.4 eV) and pyridinic-N-oxide (N-X, 403 eV) 24, 51. It should be noted that the peak at about 401.0 eV can correspond to be quaternary-N (“edge-N” and “bulk-likeN”) 33,52. It is found that the N-6 and N-5 account for the majority of N species in the 16


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three samples (Table S2). The high-resolution P 2p spectrums (Fig. 4e, S9e and S10e) reveal three types of chemical bonding: C3-PO, C-PO3/C2-PO2 and C-O-PO3 bonding at 132.6 eV, 133.5 eV and 134.2 eV, respectively 44, 46. Previous studies demonstrated that the formation of C3-PO and C-O-PO3 bonds could supply additional electrons to the electron-deficient carbon atoms and be advantageous for its hydrophilic property 46, 49.

Fig. 5. Electrochemical performances of NPC-850, 0.5NPHC-850 and 1NPHC-850 electrodes: (a) CV curves at a scan rate of 50 mV s-1, (b) galvanostatic charge/discharge curves at a current density of 5 A g-1, (c) the specific capacitance as a function of current density (0.5-20 A g-1), (d) Nyquist impedance plots.

The electrochemical performance of the electrodes were firstly tested using a threeelectrode configuration with an aqueous KOH electrolyte. Fig. 5a shows the CV curves of NPC-850, 0.5NPHC-850 and 1NPHC-850 electrodes at a scan rate of 50 mV s-1. It 17



is seen that NPC-850 displays a distorted rectangular shape, revealing the presence of pseudocapacitance related to redox reactions of the different functionalities in it. As illustrated in Table 1, the total content of N, O and P atoms in NPC-850 reaches up to 45.1 at%, providing proof of the existence of pseudocapacitance. In comparison, the 0.5NPHC-850 and 1NPHC-850 exhibit near-rectangular CV curves with a slight distortion. It indicates their excellent supercapacitive performance and the combination of electric double-layer capacitance and pseudocapacitance. Galvanostatic charge– discharge (GCD) measurement is deemed as a more accurate technique for capacitance 44, 53,

hence, GCD was carried out at different current densities ranging from 0.5 to 20

A g-1 for the selected samples. As shown in Fig. 5b, the three electrodes show similar linear and symmetric charge-discharge profiles, but different charge-discharging duration, indicating their distinct electrochemical capacitances. The specific capacitances of three electrodes calculated from discharge curve at different current densities are plotted in Fig. 5c. Although the NPC-850 has an ultrahigh heteroatoms content, its specific capacitance is calculated to be merely 137 F g-1 at 0.5 A g-1. In contrast, remarkably enhanced capacitances of 192 and 212 F g-1 at the same current density are found for 0.5NPHC-850 and 1NPHC-850, respectively. This is due to the fact that 0.5NPHC-850 and 1NPHC-850 possess obviously increased surface areas and tuned pore structures, which can afford rich ion-accessible active sites for both interfacial charge accumulation and heteroatom-related redox reactions, thus leading to a large capacitance. It is noting that, at 20 A g-1, all the samples maintain high capacitance retentions up to 75% (Fig. 5c), indicating their excellent rate capability as 18


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supercapacitor electrodes. It should be associated with the effective P doping in them which may render the carbon surface more hydrophilic and enable rapid ion diffusion inside the electrode

24.

The effect of carbon loading on the rate capability of the

electrode is shown in Fig. S11, in which a capacitance retention of 60 % can be obtained when the carbon loading was increased to 4 mg/cm2. More information can be found in the Nyquist impedance plots (Fig. 5d), in which nearly vertical curves in the lowfrequency region demonstrate the ideal capacitive behavior of these samples. In comparison with NPC-850 and 0.5NPHC-850, the 1NPHC-850 shows the smallest intersect with the horizontal axis at high frequency indicating lowest internal resistance and smallest semicircle at high frequency manifesting smallest charge transfer resistance. The charge transfer resistance for 1NPHC-850 is estimated to be 0.38 Ω, which is among the smallest values compared to the state-of-the-art carbon-based supercapacitors

54.

Thus, the EIS results can further explain the best electrochemical

performance of 1NPHC-850. As a comparison, we also prepared 1NPHC-750 and 1NPHC-800 samples at different temperatures (750 and 800 oC). However, lowering the temperature will reduce the capacitance and rate performance of the materials (Fig. S12), so we choose 850 oC as the optimal carbonization temperature.

19



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Fig. 6. (a) CV curves of 1NPHC-850 tested in varied potential windows at 20 mV s-1; (b) GCD curves of 1NPHC-850 based symmetric supercapacitor within 0-1.6 V; (c) Cyclability of 1NPHC-850 based symmetric capacitor at 5 A g-1 with inset revealing the first 15 GCD curves; (d) Ragone plots of the symmetric capacitor.

We have constructed a symmetric two-electrode (1NPHC-850//1NPHC-850) supercapacitor cell in 1 M Na2SO4 aqueous electrolyte to evaluate the practical application of NPHCs. As illustrated in Fig. S5, the diameter of the 1NPHC-850 circular electrode is about 15 mm. Firstly, the potential window of the 1NPHC850//1NPHC-850 symmetric capacitor was operated in a wider range. It evidences that a high cell voltage of 1.6 V can be attained in 1 M Na2SO4 (Fig. 6a and S13). The widened potential window of the supercapacitor can be attributed to the increased electro-oxidation resistance of carbon electrodes after P-doping 20


49, 55.

The maximum

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potential window of the symmetric capacitor is only 1.3 V in 6 M KOH (Fig. S13). The GCD curves within 0 and 1.6 V were measured from 0.5 to 20 A g-1 (Fig. 6b), the result indicates the ideal capacitive property of the cell. The cycling performance of the cell at a current density of 5 A g-1 was also tested. As shown in Fig. 6c, it can be run steady at 1.6 V for 10,000 cycles with an 86.3% retention. The excellent cycling stability of 1NPHC-850 can also be realized in alkaline electrolyte (Fig. S14). In addition, the power and energy density of the device were further calculated. The results reveal that the 1NPHC-850-based supercapacitor possesses a high energy density of 10.61 Wh kg-1 at 400 W kg-1. Even at a high power density of 16 kW kg-1, the energy density can still retain 7.95 Wh kg-1 (Fig. 6d). This value is higher than those of previously reported hierarchically porous carbon 56, N-doped hierarchical porous carbon 57, N/P co-doped graphene aerogels

58,

P-doped CNTs

8

and many other N/P co-doped carbons, as

illustrated in Table S3. Nevertheless, it is still lower than that P-doped threedimensional hierarchical porous carbons 22 and N-doped ordered mesoporous few-layer carbon 59. Thus, the SC of the N/P co-doped hierarchical porous carbon need further improvement.

Conclusions In summary, a unique N/P co-doped hierarchical porous carbon (NPHC) has been prepared by a facile and scalable CaCO3-assistant technique, in which nano-CaCO3 serves as both a hard template for mesopores and an inner-activator for micropores and small mesopores. The as-prepared NPHC integrates the structural features of high-level 21



heteroatom-doping of N, P and O, large specific surface area and hierarchically micromeso-macroporous structure. Due to the synergistic advantages of pseudocapacitance and EDLC for sufficient charge storage, the good wettability and 3D interconnected hierarchical pore channel for efficient ion diffusion and the low resistance for rapid electron conduction, the NPHC electrode is endowed with high specific capacitance, outstanding rate performance and excellent cycling stability. The as-assembled symmetrical supercapacitor device could deliver an enhanced energy density of 10.61 Wh kg-1 at a power density of 400 W kg-1 in aqueous electrolyte, which still retains 7.95 Wh kg-1 at 16 kW kg-1. Our results imply that NPHC is a promising candidate for high energy density supercapacitor electrodes. Moreover, this fabrication approach is versatile and readily scalable and could be used to produce other multiple heteroatomdoped carbon materials on a large scale.

Associated Content Supporting Information Preparation of control samples, Calculation methods of specific capacitance, energy density and power density, Rate capability , Digital photos, TEM images, SEM image, Elemental mappings, Raman spectra, Nitrogen adsorption isotherms, Micropore size analysis, XPS spectra, Electrochemical performance, Comparison of yield, Elemental compositions and Comparison of literature of different samples.

Acknowledgments 22


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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. Limin Guo at Huazhong University of Science and Technology and Prof. Huanwen Wang at China University of Geosciences for the nitrogen adsorption-desorption measurements.

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A green and efficient technique was developed for preparing hierarchically porous N/P co-doped carbons with simultaneously high-level heteroatom-doping and well-developed porosity as high-performance supercapacitor electrodes. 49x24mm (600 x 600 DPI)


P Codoped Carbon with

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