Phosphorus-doped 3D hierarchical porous carbon for high-performance supercapacitors: A balanced strategy for pore structure and chemical composition

Accepted Manuscript Phosphorus-doped 3D hierarchical porous carbon for high-performance supercapacitors: A balanced strategy for pore structure and chemical composition Wang Yang, Wu Yang, Lina Kong, Ailing Song, Xiujuan Qin, Guangjie Shao PII:

S0008-6223(17)31164-8

DOI:

10.1016/j.carbon.2017.11.050

Reference:

CARBON 12580

To appear in:

Carbon

Received Date: 27 September 2017 Revised Date:

17 November 2017

Accepted Date: 18 November 2017

Please cite this article as: W. Yang, W. Yang, L. Kong, A. Song, X. Qin, G. Shao, Phosphorus-doped 3D hierarchical porous carbon for high-performance supercapacitors: A balanced strategy for pore structure and chemical composition, Carbon (2017), doi: 10.1016/j.carbon.2017.11.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Phosphorus-doped 3D hierarchical porous carbon for high-performance supercapacitors: A balanced strategy for pore structure and chemical composition

Guangjie Shaoa,b* a

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Wang Yanga,b, Wu Yanga,b, Lina Konga,b, Ailing Songa,b, Xiujuan Qina,b,*,

State key Laboratory of Metastable Materials Science and Technology, Yanshan

Hebei Key Laboratory of Applied Chemistry, College of Environmental and

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b

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University, Qinhuangdao 066004, China

Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

Abstract

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Phosphorus-doped three-dimensional hierarchical porous carbons (P-3DHPCs) have been synthesized by direct pyrolysis of mixture containing glucose, manganese nitrate and sodium hypophosphite without any hard templates. Glucose and sodium

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hypophosphite are used as carbon and phosphorus source in the facile template-free

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strategy, respectively. The P-3DHPCs not only possess favorable hierarchical pore structure which is beneficial to ion adsorption and transportation, but also acquire effective heteroatoms doping, further improving the capacitive performance. More importantly, the amount of sodium hypophosphite plays a critical role in textural properties and phosphorus content of P-3DHPCs. The results demonstrate that

*

Corresponding author: Tel.: 0086-335-8061569; fax: 0086-335-8059878. E-mail address: [email protected] (G. Shao); [email protected] (X. Qin). 1

ACCEPTED MANUSCRIPT P-3DHPC-0.2 shows the best electrochemical performance compared to the other samples. High specific capacitance (367 F g-1 at 0.3 A g-1) is obtained in 6 M KOH, and the capacitance still maintains 319 F g-1 when tested at 20 A g-1 (ca.88%

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capacitance retention). Moreover, the P-3DHPC-0.2 also possesses good cycling stability with only a loss of 3.5% after 10000 cycles at 3 A g-1. The facile preparation method and good electrochemical performance render P-3DHPCs to be a promising

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candidate for supercapacitor application.

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Keywords: Hierarchical porous carbon; Phosphorus doping; Supercapacitors 1. Introduction

Supercapacitors, including electrochemical doublelayer capacitors (EDLCs) and pseudocapacitors, have been considered as promising alternative or complement for

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high-power apparatuses due to their high specific power and long cycle life[1-3]. It is well known that carbon materials are the most common electrode material for EDLCs ascribed to their high specific surface area and good electrical conductivity. Various

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carbon materials, such as activated carbon (AC), mesoporous carbon and graphene

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have been extensively reported as electrode material for EDLCs[4,5]. Among these carbon materials, AC have been conventionally used as electrode material for EDLCs, owing to their large specific surface area. However, the specific capacitance of AC is limited because of its tortuous micropores, which is unfavorable for effective adsorption and transport of electrolyte ion. Accordingly, carbon materials with multiple pore size distribution (micro-, meso-, and macro-pores) are necessary to obtain good energy storage characteristics[6,7]. In the hierarchical pore structure,

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ACCEPTED MANUSCRIPT micropores and mesopores furnish effective surface area for ion storage, while additional mesopores and macroporous provide a fast pathway for ion transportation. Thus, carbon materials with hierarchical pore structure would exhibit great

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advantages compared with other carbon materials[8,9]. Nevertheless, the preparation of carbon materials with mesopores was constantly involved in complicated and expensive template synthesis routes[10-12], which have seriously restricted their

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practical applications. Therefore, facile and economical methods should be explored

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to introduce mesopores and macroporous into carbon materials.

In addition to a favorable pore structure, heteroatoms doping is another effective approach to improve the capacitive performance of the carbon materials for supercapacitors[13]. To date, heteroatoms such as, boron, nitrogen, oxygen, sulfur and

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phosphorus have been incorporated into porous carbon materials[14-18]. The heteroatomic functional groups can affect the electron-donor characteristics of carbon materials, thus improving the capacitive behavior of supercapacitors. Furthermore,

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heteroatoms can also be incorporated in either dual-doped or multi-doped manner,

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which can influence the electrochemical properties of carbon materials attributed to their synergistic effect[19-22]. In the recent years, many investigations about nitrogen-doped carbon materials have been implemented as electrode materials for supercapacitors[23-26]. The phosphorus-doped carbon materials have attracted considerable attention especially for oxygen reduction reaction catalysis[27-31]. However, only a few literatures have been reported on phosphorus-doped carbon materials for supercapacitors[32-34]. Hence, further research is needed to apply

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ACCEPTED MANUSCRIPT phosphorus-doped carbon to supercapacitors and preliminarily identify the relationships between physico-chemical property and phosphorus doping. In addition, the preparation of these existing heteroatom-doped carbon materials generally

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requires complex and time-consuming synthesis routes[35-37]. Consequently, it is highly desirable to develop a facile strategy to effectively prepare heteroatom-doped carbon materials, which is crucial for widespread implementation.

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Herein, we designed and synthesized phosphorus-doped three-dimensional

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hierarchical porous carbons (P-3DHPCs) through a simple template-free method. Sodium hypophosphite was introducted into the mixtures of glucose and manganese nitrate in the synthesis process, and the interaction between these three components at high temperature yielded the phosphorus species introduced into the carbon matrix.

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Quite intriguingly, the amount of sodium hypophosphite not only affects the doping content of phosphorus, but also influences the formation of pore structure in carbonization process. The prepared P-3DHPCs present favorable pore structure

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(micro-, meso-, and macro-pores), leading to rapid ion transport and effective

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adsorption. Futhermore, the effective heteroatom doping synergistically improve the physicochemical properties and the capacitive behavior. As a result, the P-3DHPC-0.2 exhibits good electrochemical performance when employed as electrode material for supercapacitor. We believe that this attempt not only provides a facile strategy for the construction of advanced heteroatom-doped carbon materials, but also gives new insights on the simultaneous regulation of pore structure and chemical composition for achieving high performance supercapacitors.

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ACCEPTED MANUSCRIPT 2. Experimental 2.1. Chemicals In this experiment, all chemicals were used as purchased without any further

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processing. Deionized water was employed throughout the experiment. 2.2. Preparation of phosphorus-doped three-dimensional hierarchical porous carbon The

phosphorus-doped

three-dimensional

hierarchical

porous

carbons

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(P-3DHPCs) were prepared via a preliminary heating process followed by

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carbonization at high temperature. Typically, 1 g glucose and 2 g manganese nitrate (50%) were dispersed in 5 mL deionized water, and 0.2 g sodium hypophosphite was dispersed in another 5 mL deionized water. Subsequently, the latter was added into the former mixture with vigorous stirring to form homogeneous solution. Then the

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solution was put into oven and heated at 120 oC for 12 hours to obtain the brown powder. Afterwards, the powder was annealed in tube furnace under nitrogen at desired temperature(750 oC) for 1 h with a heating rate of 5 oC min-1. The obtained

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intermediate products were then washed with 15wt.% HCl solution to remove the

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manganese oxide. Finally, the samples were filtered, washed with abundant distilled water, and dried at 80 oC overnight. By varying the amount of sodium hypophosphite (0.1-0.5 g), the P-3DHPCs with different content of phosphorus were also prepared by the same method. The carbon samples were labeled as P-3DHPC-x, where the x refers to the weight of sodium hypophosphite. As control sample, the hierarchical porous carbon without sodium hypophosphite has also been prepared as the same process and was named as HPC.

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ACCEPTED MANUSCRIPT 2.3. Material characterization The structure of as-prepared materials were measured by X-ray diffraction (XRD) on a Rigaku Smart Lab X-ray diffractometer operated at 40 kV with Cu Kα radiation

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at a scan rate of 5° min-1. The morphologies of all the samples were analyzed by scanning electron microscopy (FESEM, Hitachi S-4800, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100F). The Raman spectra was

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recorded on a Horiva (Xplora Plus) at an excitation wavelength of 532 nm and the

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Raman laser power on the sample is 0.1 mW. Nitrogen adsorption and desorption isotherms were characterized by Micromeritics V-Sorb 2800P analyzer at 77 K. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The total pore volume was determined from the adsorbed amount at a relative pressure (p/p0) of

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0.99. Pore size distribution (PSD) in the micropore range (<2 nm) was obtained by the Saito-Foley (SF) method and pore size distribution in the larger range was obtained by the Barrette-Joynere-Halenda (BJH) method. Elemental analysis was analyzed by

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X-ray photoelectron spectroscopy (XPS) with a Kratos XSAM-800 spectrometer with

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monochromatized Al Kα radiation. The electric conductivity of the carbon samples was measured by ST2722-SZ Semiconductor powder resistivity instrument under 7 MPa.

2.4. Electrochemical measurement The electrochemical performance was tested in 6 M KOH electrolyte. Hg/HgO electrode and active carbon electrode were used as the reference electrode and the counter electrode, respectively. To prepare the working electrode, P-3DHPC,

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ACCEPTED MANUSCRIPT acetylene black and PTFE were mixed at a weight ratio of 80:15:5 to form the homogeneous slurry. Then, the slurry was continuously spread on current collectors of 1 cm2 nickel foam, and dried at 80 oC for 12 h. The total mass loading of each

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electrode was approximately 2 mg. The fabricated electrodes were pressed at 4 MPa for 30 s, and immersed in 6 M KOH electrolyte for 24 h. The specific capacitance of the working electrodes in three-electrode system was obtained from the galvanostatic

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discharge process via C = I∆t (m∆V ) , where C is the specific capacitance (F g-1), I

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(A) refers the discharge current, ∆t (s) is the discharge time, m (g) is the mass of the active material for working electrode and ∆V (V) is the potential change excluding the IR drop during the discharge process.

To further simulate the actual device behaviour of the carbon material, the

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two-electrode symmetrical supercapacitor was assembled in 6 M KOH. The mass of active material was approximately 4 mg cm-2. The specific capacitance of the total cell (Ccell) was obtained from the two-electrode galvanostatic discharge process via

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Ccell=I∆t/m∆V, where I (A) is the discharge current, ∆t (s) is the discharge time, m (g)

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is the total mass of the active material for two electrodes and ∆V (V) is the cell voltage excluding the IR drop during the discharge process. The specific energy was calculated using the equation E = Ccell∆V2/2, where E (W h kg-1) is the specific energy, Ccell (F g-1) is the specific capacitance of the total cell and ∆V (V) is the cell voltage excluding the IR drop. The specific power was obtained by the formula P = E/∆t, where P (W kg-1) is the specific power, E is the specific energy and ∆t (h) is the discharge time.

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ACCEPTED MANUSCRIPT The galvanostatic charge-discharge measurement was recorded on a NEWARE auto-cycler. The potential window was -1.0 to 0 V in 6 M KOH for the three-electrode system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)

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measurements were conducted on a CHI660E electrochemical workstation (Chenhua, Shanghai, China). The frequency range for the EIS measurement was from 1 mHz to 1 MHz with a perturbation amplitude of 5 mV.

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The complex capacitance C(ω) versus frequency can be defined as following:

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C(ω) = C′(ω) - jC″(ω), the real part of the capacitance C′(ω) = (−Z″(ω))/(ω|Z(ω)|2), and imaginary part of the capacitance C″(ω) = (Z′(ω))/(ω|Z(ω)|2). Where Z(ω) is the complex impedance, Z'(ω) and Z″(ω) represent the real and imaginary parts of Z(ω), respectively. The angular frequency can be obtained by ω = 2πf.

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3. Results and discussion

Fig. 1. Schematic representation of the fabrication process of P-3DHPCs.

The general strategy for fabricating P-3DHPCs is illustrated in Fig. 1. In a typical synthesis, glucose as carbon source was thoroughly soaked with aqueous solution

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ACCEPTED MANUSCRIPT containing manganese nitrate and sodium hypophosphite at a relatively low temperature (120

o

C), acquiring a uniform distribution of Mn2+ and sodium

hypophosphite within the precursor architecture. Then, the resulting powders were

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followed by thermal annealing in a nitrogen atmosphere. In this process, the Mn2+ could be transformed to manganese oxide and imbedded in the carbon framework at elevated temperatures, which would serve as the mesoporous template. Meanwhile,

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the negative ions containing phosphorus and nitrogen would react during the heating,

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which constantly releases abundant gases such as NO, NO2, PH3 and CO2 etc[17, 38]. These gaseous phases would generate homogenous exfoliation on the intermediate product and led to the formation of macropores. Simultaneously, they would overcome the van der waals attraction between the carbon layers, and efficiently

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intercalate into the carbon lattice, then part of O and P elements would be doped to the carbon matrix. Micropores may be caused by the decomposition of glucose and the activation effect of the gases during the the carbonization process. Finally, manganese

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oxide nanoparticles are removed after washing the intermediate products by HCl

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solution, then the mesopores formed. More importantly, the content of doped phosphorus and pore structure of the carbon material could be controlled by the amount of the sodium hypophosphite. Fig. 2(a-d) depicts the SEM images of the precursors before calcining. It is

observed that all the precursors have a similar bulk structure. However, with the increase of the sodium hypophosphite content, the surface of the precursors gradually become rough, which is probably due to the interaction between sodium

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ACCEPTED MANUSCRIPT hypophosphite and manganese nitrate. After carbonized at high temperature, the intermediate products wrapped with numerous manganese oxide nanoparticles are obtained (Fig. 2(e-h)). Interestingly, the addition of sodium hypophosphite affects the

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final morphology of the carbon materials. When the amount of sodium hypophosphite is relatively low (0.1 g and 0.2 g), the samples show significant macropores; as the amount is increased to 0.3 g, the macroporous structure is less noticeable; continuing

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to increase to 0.5 g, there is basically no macroporous structure, and only a large

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amount of manganese oxide nanoparticles are observed to adhere closely to the carbon sample. These results may be attributed to the decomposition of nitrates during the carbonation process[38]. Fig. 2(i) presents the SEM image of the P-3DHPC-0.2. It is clearly that, after the acid washing, the P-3DHPC-0.2 exhibits a three-dimensional

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interconnected structure, which is composed of numerous mesopores and macropores. The corresponding energy dispersive X-ray spectroscopy (EDS) mapping images of C,

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O and P elements indicate the homogeneous distribution of doped heteroatoms. The

Fig. 2. (a-d) SEM image of the precursor of the samples containing different amount

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ACCEPTED MANUSCRIPT of sodium hypophosphite; (a) 0.1; (b) 0.2; (c) 0.3; (d) 0.5. (e-h) SEM image of the samples containing different amount of sodium hypophosphite after carbonization; (e) 0.1; (f) 0.2; (g) 0.3; (h) 0.5. (i) SEM image of P-3DHPC-0.2 and corresponding EDS

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mapping images of carbon; oxygen and phosphorus.

three-dimensional hierarchical pore structure accompanied by the uniform phosphorus synergistically

conduce

that

the

P-3DHPC-0.2

possess

sufficient

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doping,

pseudocapacitive active sites and favorable ion transport channel, thus enhancing the

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capability of electrical energy storage for supercapacitor[39,40].

The pore structure of the carbon samples were further observed by TEM (Fig. 3). Similarly, all the samples show a uniform mesopores owing to the manganese oxide nanoparticles. In addition, with the increase of sodium hypophosphite content, the

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macroporous structures gradually become unconspicuous and even disappear, which is consistent with the SEM images. Fig. 3(d-f) demonstrate the TEM image of

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P-3DHPC-0.2 at different magnifications. The sample shows good

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Fig. 3. (a-c) TEM image of the carbon samples after carbonization and acid washing; (a) P-3DHPC-0.1; (b) P-3DHPC-0.3; (c) P-3DHPC-0.5; (d-f) TEM image of the

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P-3DHPC-0.2 at different magnifications.

three-dimensional network structure and hierarchical pore distribution, which is

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advantageous for ion transport and effective energy storage[41]. To explore the formation mechanism of carbon samples, XRD spectrum of the

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precursors, intermediate products without acid washing, and the final carbon samples are shown in Fig 4(a-c). XRD patterns of the precursors (Fig. 4(a)) present that, when the amount of sodium hypophosphite reaches 0.2 g, a relatively weak diffraction peak of NaNO3 appears. Clearly, as the sodium hypophosphite content increased, the diffraction peak gradually enhanced, corresponding to PDF#36-1474. The intermediate products without acid washing were obtained after carbonized at high temperature, and the XRD spectrum is shown in Fig. 4(b). We can see that all of them 12

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Fig. 4. (a) XRD patterns of the precursors containing different amount of sodium

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hypophosphite; (b) XRD patterns of samples containing different amount of sodium hypophosphite after carbonization; (c) XRD patterns of the P-3DHPCs; (d) Raman

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spectra of the P-3DHPCs.

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display an evident diffraction peak of manganese oxide (PDF#06-0592). Specially, as the sodium hypophosphite content increased (such as 0.5 g), some other diffraction peaks appears, which are probably due to the complicated manganese compound formed in the carbonization process. The final carbon samples were acquired after the acid washing. XRD patterns (Fig. 4(c)) of the samples exhibit that the manganese oxide and manganese compound were completely removed. Furthermore, the typical diffraction peak at 24°and an inconspicuous peak at 43°represent the (002) and

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ACCEPTED MANUSCRIPT (100) planes of graphite respectively, indicating the amorphous structure of the carbon samples[42]. Fig. 4(d) displays Raman spectra of the carbon samples. It is noted that all of them show a typical D (defect) band at around 1350 cm-1 and G (graphite) band

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at around 1580cm-1. The G-band is assigned to the E2g symmetric vibration of sp2-bonded carbon atoms, while the D band can be attributed to the disordered or defective structures in carbon materials[43]. The intensity ratio of ID/IG are 1.34, 1.38,

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1.46, 1.39 and 1.36 for HPC, P-3DHPC-0.1, P-3DHPC-0.2, P-3DHPC-0.3 and

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P-3DHPC-0.5, respectively. The poor graphitization degree of the samples might be caused by relatively low preparation temperature 750 C. The ID/IG of P-3DHPCs are all larger than HPC, this phenomenon can be ascribed to the phosphorus doping, which generates defects in the carbon framework[16]. With the increase of sodium

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hypophosphite amount, the graphitization degree increases first and then decreases, which may be resulted from the combination of phosphorus doping content and pore characteristics.

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Fig. 5 presents N2 adsorption-desorption isotherms and the pore size distributions

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of the carbon samples. According to IUPAC classification[44], all the isotherms belong to a mixed type corresponding to porous carbon with hierarchical pore size distribution. The significant uptake at very low relative pressure is accordance with type Ⅰ(a), indicating the presence of some micropores generated from the carbonization process. With the increase of the relative pressure, the accretion of nitrogen adsorption accompanied by hysteresis belongs to type Ⅳ(a), implying the existence of some mesopores. At high relative pressure, the curve shows unrestricted

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adsorption up to high p/p0, which matches well with type Ⅱ and can be indicative of a

Fig. 5. (a) Nitrogen adsorption-desorption isotherm; (b) pore size distribution of the

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P-3DHPCs.

macroporous size distribution. The nitrogen adsorption-desorption isotherm further confirms hierarchical pore structure of the carbon samples. Fig. 5(b) exhibits

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corresponding pore size distribution. As can be seen from the figure, as the amount of sodium hypophosphite increases, the size of the micropores become slightly larger, while there is no significant change about the mesoporous size. The textural

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parameters of the carbon samples are presented in Table 1. With the increase of

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sodium hypophosphite amount, the BET specific surface area (SSA) of the samples are 815, 940, 805 and 757 m2 g-1, respectively. Apparently, most of the specific surface area are contributed from mesopores and the average pore diameter are 10-13 nm. These results are in good agreement with SEM and TEM images. As for the P-3DHPC-0.2, the size of micropores matches well with the diameters of solvated electrolyte ions, which provide effective sites for charge storage; the mesopores and

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ACCEPTED MANUSCRIPT macropores can facilitate the transport of electrolyte ion[7,41]. Therefore, the P-3DHPC-0.2 is suitable for supercapacitor application.

P-3DHPCs. Vt (cm3

Vmicro

g-1)

g-1)

g-1)

(cm3 g-1)

HPC

810

759

2.73

0.33

P-3DHPC-0.1

815

721

2.33

0.30

P-3DHPC-0.2

940

879

3.11

0.35

P-3DHPC-0.3

805

637

2.15

0.31

P-3DHPC-0.5

751

620

1.94

0.28

Sample

XPS (at.%)

dp (nm)

13.5

C

P

O

96.1

0

3.9

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Smeso(m2

11.4

95

0.8

4.2

13.2

94

1.12

4.88

10.7

93.7

1.37

4.93

10.3

93.3

1.5

5.2

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SBET (m2

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Table 1. BET surface area, porosity parameters and chemical composition of

SBET - BET surface area, Smeso - BJH Adsorption cumulative surface area, Vt - total pore volume, dp - Total adsorption average

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pore width. Vmicro - SF micropore volume.

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Chemical composition of the carbon samples were analysed in detail by X-ray photoelectron spectroscopy (XPS) in Fig. 6 and Fourier-transform infrared (FTIR) in

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Fig. S6. From Fig. 6(a), the survey spectrum of each sample shows a predominant C 1s peak at 284.5 eV, O 1s peak at 532 eV and relatively weak P 2p peak at 133.3

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eV[17]. With the increase of sodium hypophosphite amount, the percentage composition of phosphorus and oxygen increase gradually (Table 1). Fig. 6 (b) illustrates the high-resolution XPS spectrum of C 1s. The C 1s spectra of the samples can be deconvoluted into three peaks (C-C, C-P, and C-O/C=O) centered at 284.5,

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285.5 eV and 286.2eV, respectively[19]. With the increase of sodium hypophosphite amount, the C-C peak becomes more broad and the intensity of the other two peaks

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(C-P and C-O/C=O) enhance gradually. It can be inferred that the carbon tends to be disordered due to phosphorus doping, which is consistent with Raman results (Fig.

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4(d)). In Fig. 6(c), the O 1s spectra can be deconvoluted into five peaks located at 530.9, 532.1, 533, 533.6 and 534.2 eV, which correspond to the quinone-type oxygen O

(

O

), carbonyl groups (C=O), bridge-bonded oxygen (-O- in C-O-H, C-O-C and

C-O-P), ester groups (O-C=O) and chemisorbed oxygen or water (COOH/O2/H2O), respectively[22]. With the increase of sodium hypophosphite amount, the C=O peak slightly become narrow and the intensity of the other peaks become weak. The variation of C-O-P cannot be clearly obtained, because the peak C-O-C, C-O-H and 17

ACCEPTED MANUSCRIPT C-O-P are coincident. The oxygen groups especially the quinone-type oxygen and C=O groups are electrochemically active, which can improve the electrochemical performance of the carbon materials. The high resolution spectrum of P 2p (Fig. 6(d))

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can be fitted into two peaks: a low binding energy peak centered at 132.6 eV (P-C) and a high binding energy peak centered at 134.2 eV (P-O), respectively[17,45]. As the amount of sodium hypophosphite increases, the intensity of P-O peak becomes

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weak gradually, and accordingly the P-C peak enhances slightly. This result illustrates

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that the more sodium hypophosphite in the precursor, the more doped phosphorus atoms into the carbon framework. Based on XPS data, the P and O atoms have been successfully incorporated into the carbon framework. The few but measurable quantity of phosphorus (0.8-1.5 at%) accompanying with moderate oxygen (4.2-5.2

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at%) in the carbon matrix is considered to be beneficial for the capacitive improvement. The possible advantages are as following: (i) improvement of wettability: the P and O doping produced the heteroatom functional groups on the

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carbon surface, which is favorable for electrolyte soaked into the interior of material;

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(ii) pseudocapacitance contribution: some functional groups such as quinone-type oxygen, C=O, P=O are redox active and can provide electroactive sites during a pseudocapacitive process for the enhanced capacitances[13,46]; (iii) improvement of heterostructure in carbon matrix: C-P can change the charge and spin densities due to lower electronegativity of P than that of carbon, and generate the structural defects because of bigger size of P atoms than that of carbon[13,47].the heteroatoms can yield more defects in the carbon framework, which can act as active sites in the

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ACCEPTED MANUSCRIPT electrochemical process; iv) the co-doped effects: the P, O dual-doped can modify the electron-donor properties and thus affects the electrochemical properties attributed to

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their synergistic effect.

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Fig. 7. Electrochemical performance of the carbon samples tested by three-electrode system in 6 M KOH. (a) CV curves of HPC and P-3DHPC-0.2 at the scan rate of 10

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mV s-1; (b) The galvanostatic charge-discharge profiles of HPC and P-3DHPC-0.2 at the current density of 1 A g-1; (c) Rate performance of HPC and P-3DHPC-0.2 at different current densities from 0.3 A g-1 to 20 A g-1; (d) CV curves of P-3DHPC-0.2 at different scan rates from 5 mV s-1 to 200 mV s-1; (e) The galvanostatic charge-discharge profiles of P-3DHPC-0.2 at different current densities from 0.3 A g-1 to 10 A g-1; (f-i) Rate performance (f), CV curves at 10 mV s-1 (g), galvanostatic charge-discharge profiles at 1 A g-1 (h), Cycle stability after 10000 cycles at 3 A g-1 (i) 19

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To probe the electrochemical performance of the carbon samples, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were measured by three-electrode

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system in 6 M KOH. Fig. 7(a) presents the CV curves of the HPC (without sodium hypophosphite) and P-3DHPC-0.2 at the scan rate of 10 mV s-1. Clearly, the

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P-3DHPC-0.2 shows a slightly distorted rectangular CV curve with a wide reversible hump, suggesting its electric double-layer capacitor (EDLC) performance along with

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the pseudocapacitance characteristics[40]. The HPC exhibits better-symmetric and rectangular CV curves, which can be ascribed to the absence of phosphorus doping. The GCD curves of the two samples at 1 A g-1 (Fig. 7(b)) demonstrate that the P-3DHPC-0.2 shows more non-linear characteristics than the HPC, further revealing

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the pseudo-capacitive feature generated from the doping elements[22], which is consistent with the CV result. Moreover, Fig. 7(c) exhibits the rate performance of the

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two samples with different current densities from 0.3 A g-1 to 20 A g-1. The specific capacitance calculated from the GCD curve is 367 F g-1 at 0.3 A g-1, and retains 319 F

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g-1 at 20 A g-1 (the retention is as high as 88%). CV curves of P-3DHPC-0.2 at different scan rates from 5 mV s-1 to 200 mV s-1 are presented in Fig. 7(d), we can see that the slightly distorted rectangular have no obvious change even at high scan rate, demonstrating its good rate performance. The same conclusion can be obtained from the GCD results of the P-3DHPC-0.2 tested at different current densities from 0.3 A g-1 to 10 A g-1 (Fig. 7(e)). Such a good capacitance and rate performance is derived from the synergistic effect of hierarchical porous structure and phosphorus doping: the 20

ACCEPTED MANUSCRIPT mesopores not only provide effective sites for charge storage, but also facilitate ion penetration and transport; the macropores serve as a bridge connecting the electrolyte with the internal channels of the material; the phosphorus doping into the carbon

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framework contribute pseudocapacitance and the functional groups can also improve the wettability of the carbon-based electrode, which is advantageous to enhance the capacitance behavior[7,48,49]. Furthermore, the rate performance of the P-3DHPCs

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with different sodium hypophosphite (0.1~0.5 g) were tested from 0.3 A g-1 to 20 A g-1,

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as shown in Fig. 7(f). With the increase of sodium hypophosphite amount, the capacitance retention are 89.1%, 83.4%, 84.9% for the sample P-3DHPC-0.1, P-3DHPC-0.3 and P-3DHPC-0.5, respectively. The rate performance of P-3DHPC-0.1 is better than P-3DHPC-0.3 and P-3DHPC-0.5, which indicates that the existence of

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macropores is advantageous to improve the rate performance of capacitor[3]. The rate performance slightly improve when the amount of sodium hypophosphite reaches 0.5 g, which is probably due to the enhancement of electric conductivity with the increase

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of phosphorus doping content (Fig. S1). The variation trend of the rate performance is

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determined by the pore structure and conductivity collectively. Fig. 7(g-h) present the CV curves and GCD patterns of the P-3DHPCs, and the P-3DHPC-0.2 delivers the best capability. The sample also shows a good cycling performance at a current density of 3 A g-1 in Fig. 7(i). The specific capacitance retention after 10000 cycles is 96.5%. This result may be related to the variation of the doping content and pore structure. The P-3DHPC-0.2 sample possesses appropriate pore characteristics and moderate heteroatom doping, thus exhibiting the best electrochemical performance.

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ACCEPTED MANUSCRIPT The electrochemical impedance spectroscopy (EIS) measurement was conducted to explore the resistance and capacitive property of the P-3DHPCs (Fig. 8). The Nyquist plots (Fig. 8(a)) show that there is no obvious semicircle in correlation with

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the charge-transfer resistance[50]. The approximate vertical line at low frequency

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region further demonstrates the good capacitive characteristics of the carbon

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Fig. 8. The electrochemical impedance spectroscopy of the P-3DHPCs. (a) Nyquist plots of P-3DHPCs; (b) Bode diagrams of phase versus frequency; (c) The normalized real part capacitance versus frequency; (d) The normalized imaginary part capacitance versus frequency.

samples[51]. Among them, the P-3DHPC-0.2 presents the most vertical line, that is, it delivers the best capacitive characteristics. The corresponding equivalent circuit for

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ACCEPTED MANUSCRIPT the samples have also been modeled, as shown in Fig. S4. The model fitting parameters are provided in Table S1. The equivalent series resistance (ESR) can be obtained by the intercept of the Nyquist plot with the real axis, which include the

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electrolyte resistance, the intrinsic resistance of the carbon material, and the contact resistance of the current collector/electrode interface[50]. It should be noted that the constant phase component CPE2-P is regarding to the deviation in slope of the

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vertical line in the low frequency region[51]. The CPE2-P of the P-3DHPC-0.2

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sample is most close to 1, indicating its good capacitive behaviour. From the bode phase diagrams (Fig. 8(b)), it can be seen that the phase angle of all the samples are close to 90°, which is consistent with the Nyquist plots, implying that the P-3DHPCs exhibit good capacitive characteristic. Fig. 8(c) and Fig. 8(d) reveal the evolution of

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normalized real part of the capacitance C′ and imaginary part of the capacitance C′′ versus frequency, respectively. Fig. 8(c) illustrates that the normalized C′ decreases with the increase of frequency. When C′=0.5, the corresponding frequency (f0) and

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relaxation time constant (τo=1/2πf0) are obtained. The τo refers to the consumed time

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of the electrode being charged and discharged reversibly[51]. The f0 of the P-3DHPC-0.2 and P-3DHPC-0.5 are 0.682 and 0.786 Hz, higher than P-3DHPC-0.1 (0.596 Hz), P-3DHPC-0.3 (0.558 Hz), and the τo of the samples are 0.27, 0.23, 0.28 and 0.2 s with the increase of sodium hypophosphite amount. As shown in Fig. 8(d), the normalized C′′ reaches a peak value, which represents that the component converts to purely capacitive nature[52]. Apparently, the P-3DHPC-0.2 and

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ACCEPTED MANUSCRIPT P-3DHPC-0.5 are more favorable for ion diffusion than the other two samples

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especially at higher charge-discharge rates.

Fig. 9. Ragone plots of P-3DHPC-0.2 and commercial AC in 6 M KOH electrolyte.

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To further simulate the actual device behaviour of P-3DHPC-0.2, the electrochemical performance was measured in a symmetrical two-electrode

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configuration in 6 M KOH electrolyte, as shown in Fig.9 and Fig.S7. Fig. 9 provides a comparison of the Ragone plots of P-3DHPC-0.2 and commercial AC. The

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P-3DHPC-0.2 exhibits specific energy of 8.5-11.11 Wh Kg-1, which delivers good performance compared with the commercial AC (3.2-6.8 Wh Kg-1). Generally, the superior electrochemical performance is

closely related to the favorable

microstructure with connectivity of pores in the 3D level, hierarchical porous size distribution and definite phosphorus content.

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ACCEPTED MANUSCRIPT Table

2.

Comparison

of

the

performance

of

P-3DHPC-0.2

with

other

heteroatom-doped porous carbon materials.

NPC-P2 PZCC hpGAs SP-AG P-3DHPC-0.2

Specific capacitance (F g-1)

Ref.

6 M KOH

273 at 0.5 A g-1

237 at 5 A g-1

16

6 M KOH

253 at 1 A g-1

238 at 20 A g-1

20

0.5 M K2SO4

375 at 5 mV s-1

250 at 100 mV s-1

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6 M KOH

190 at 1 A g-1

133 at 10 A g-1

34

1M H2SO4

352 at 1 A g-1

299 at 28 A g-1

46

1M H2SO4

438 at 10 mV s-1

381 at 500 mV s-1

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6 M KOH

367 at 0.3 A g-1

319 at 20 A g-1

This work

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PSDPC-800

Specific capacitance (F g-1)

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P2-3DOMC

electrolyte

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Sample

Doping element and content P (3.18 wt%) S(0.64 at%) P(0.49 at%) P (1.96 wt%) N (2.93 wt%) S(0.4 at%) P(1.3 at%) P (4.6 at%) S (5.8 at%) P (4.6 at%) O(4.88 at%) P(1.12%)

The specific capacitance was calculated from 3-electrode system.

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The above characterization results demonstrate P-3DHPCs with various phosphorus contents and pore structures have been successfully fabricated by a simple template-free method. The glucose containing manganese nitrate and sodium

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hypophosphite as the precursor. In the carbonization process, the formation of gases including NO2, NO, etc. led to the three-dimensional network structure containing

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abundant macropores. Meanwhile, the Mn2+ was transformed to manganese oxide nanoparticles and embed in the carbon network, which serves as the mesoporous template. More importantly, the amount of sodium hypophosphite playes an important role. On the one hand, its amount directly affects the content of doped phosphorus, and more addition of sodium hypophosphite signify more phosphorus doping. On the other hand, it also influences the formation of pore structure in the carbonization

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ACCEPTED MANUSCRIPT process. As evidenced by XRD and SEM results, when the amount of sodium hypophosphite increases to 0.5 g, the diffraction peak of NaNO3 was significantly enhanced, thus the decomposition of NO3- into gases probably to be weakened

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somewhat. Consequently, the three-dimensional network structure cannot be formed very well. The optimal amount of sodium hypophosphite was 0.2 g, which was most appropriate electrode material for supercapacitors compared to the other samples. The

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good capacitive behavior of the P-3DHPC-0.2 can be attributed to the following two

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features: i) suitable pore structure led to rapid ion transport and effective ion-accessible surface area; ii) effective heteroatom doping synergistically introduce pseudocapacitance and improve the physicochemical properties of carbon materials. Therefore, the P-3DHPC-0.2 prepared in this work exhibits better capacitive behavior

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than other reported phosphorus-doped carbon materials (Table 2). These results should enhance the comprehension of the role of combining advantageous pore structure with the doping effect in the electrochemical energy storage application.

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4. Conclusions

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In summary, phosphorus-doped three-dimensional hierarchical porous carbons (P-3DHPCs) were prepared by a facile template-free method. Generally, the pore structure and content of doping elements (phosphorus and oxygen) all depend on the amount of sodium hypophosphite. The P-3DHPC-0.2 shows the best physicochemical properties and electrochemical performance: hierarchical pore structure (micro-, meso-, and macro-pores) and appropriate pore size benefit the ion adsorption and transportation; suitable content of doping heteroatoms (O 4.88 at%, P 1.12 at%)

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ACCEPTED MANUSCRIPT provide more electrochemical activity sites. Therefore, the synergistic effects of the physical structure and chemical modification enable the P-3DHPC-0.2 with good electrochemical performance in 6 M KOH electrolyte, significantly improving the

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capacitance and rate performance of heteroatom-doped carbon materials in supercapacitors application. Acknowledgments

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We are grateful for the financial support from the National Natural Science

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Foundation of China (No. 51674221) and the Postgraduate Innovation Project of

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Hebei Province (No. CXZZBS2017058).

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