Carbon wrapped CoP hollow spheres for high performance hybrid supercapacitor

Journal Pre-proof Carbon wrapped CoP hollow spheres for high performance hybrid supercapacitor Xiaojie Zhang, Shujin Hou, Zibiao Ding, Guang Zhu, Haoran Tang, Yuancheng Hou, Ting Lu, Likun Pan PII:

S0925-8388(19)34824-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.153578

Reference:

JALCOM 153578

To appear in:

Journal of Alloys and Compounds

Received Date: 24 October 2019 Revised Date:

27 December 2019

Accepted Date: 27 December 2019

Please cite this article as: X. Zhang, S. Hou, Z. Ding, G. Zhu, H. Tang, Y. Hou, T. Lu, L. Pan, Carbon wrapped CoP hollow spheres for high performance hybrid supercapacitor, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153578. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Xiaojie Zhang: Zhang: Conceptualization, Methodology, Shujin Hou: Software, Validation. Zibiao Ding. Ding.: Data curation, Writing- Original draft preparation. Guang Zhu: Zhu Supervision.

Tang: Software, Visualization, Haoran Tang Yuancheng Hou: Hou Investigation. Ting Lu: Lu: Writing- Reviewing and Editing, Likun Pan: Project administration.

Carbon wrapped CoP hollow spheres for high performance hybrid supercapacitor

Xiaojie Zhang,a* Shujin Hou,b Zibiao Ding,b Guang Zhu,c Haoran Tang, a Yuancheng Hou, a Ting Lu,b Likun Panb*

a

National & Local Joint Engineering Research Center for Deep Utilization

Technology of Rock-salt Resource, Faculty of Chemical Engineering, Huaiyin Institute of Technology, Huaian 223003, China b

Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Electronic

Science, East China Normal University, Shanghai 200062, China c

Anhui Key Laboratory of Spin Electron and Nanomaterials, Suzhou University,

Anhui Suzhou, 234000, China *

Corresponding author: Tel: 86 21 62234132; Fax: 86 21 62234321;

E-mail: [email protected] (Xiaojie Zhang), [email protected] (Likun Pan)

1

Abstract Carbon coated CoP hollow spheres (CoP/C) were prepared from cobalt-based MOFs via simple calcining and subsequent phosphorization process. Meanwhile, N-doped carbon shells were also synthesized by a low-cost hard-templating method. Both of the obtained CoP/C and N-doped carbon shells deliver remarkable specific capacitances and excellent capacitance retentions when used as electrodes for supercapacitor. Remarkably, the asymmetric supercapacitor device based on the CoP/C and N-doped carbon shells delivers a high energy density of 16.14 Wh kg-1 at a power density of 700 W kg-1 and outstanding cycling stability (99.5% capacitance retention after 5000 cycles at a current density of 7 A g-1). CoP/C should be a promising electrode material for next-generation supercapacitor application.

Keywords CoP/C; Hollow spheres; Hybrid supercapacitor; Excellent electrochemical performance

2

1.

Introduction Supercapacitor is an emerging energy storage technology which has gained rapidly increasing

attention for supplementing existing battery technology [1-3]. As a component of supercapacitor, electrode material is one of the important factors affecting its performance. Currently, the most common used method is to design the structure, chemical composition and substructure unit of nanomaterials to optimize their electrochemical performances. The rational design of complicated nano architecture can endow materials unique properties to meet the needs of existing and emerging technologies [4]. Metal oxides or metal hydroxides with higher specific capacity than carbon materials, in the past decade have attracted much attention due to their high specific capacity, environmental friendliness, natural abundance and low-cost, but most of these materials is p-type semiconductor with low electrochemical reactivity, which is not beneficial to energy storage in term of rapid charge transfer within electrodes [5-7]. Therefore, it is necessary to develop other kinds of electrode materials [8-10]. It is worth nothing that the transition metal phosphides are typically n-type semiconductors with metal-like properties and ultra-fast electronic conductivity, which have been widely used in electrocatalysis and lithium ion batteries [11-13]. Among various metal phosphides, the Ni-based phosphide exhibits a high specific capacity but their rate and cycling performances are poor, which should be caused by the structure instability during the rapid charging and discharging process [14, 15], while the Co-based phosphide has a lower specific capacitance, but its stability is excellent, which makes it a strong candidate for battery-type supercapacitor electrode materials. Chen et al. [16] synthesized Co2P with shape-controlled nanostructures, and found that higher specific capacitance could be obtained when the content of phosphorus in the material is higher. However, the achieved energy storage capability is still to be improved for practical application [17]. Therefore, how to improve both the stability and specific capacity of metal phosphide electrode materials has become an urgent issue to be addressed. Generally, the reasonable design of porous structure and the introduction of carbon skeleton with high conductivity can effectively increase the active sites, shorten the ion diffusion path and reduce the charge resistance in the electrochemical reaction process [18, 19], which should be helpful in improving the energy storage capability and stability of metal phosphide. Recently, the preparation of transition metal phosphide electrode with regular pore structure 3

using MOFs as precursor or template has aroused much interest [20]. Tian et al. [21] reported that the Ni2P nanoparticles derived from MOFs showed ultra-low overpotential and high current density for electrocatalytic hydrogen production. However, the currently reported MOFs and their derivatives are mainly consisting of solid spherical particles with particle sizes ranging from hundreds of nanometers to several micrometers. Their large particle size is not beneficial to the diffusion of ions, resulting in the loss of internal active sites for ion accommodation [22-24]. Therefore, some strategies are proposed to prepare MOFs derivatives with hollow structure by optimizing the annealing conditions, such as heating rate and atmosphere conditions, etc. [25-27], because the hollow structure with small volume density is more accessible for the electrolyte to infiltrate. However, the past treatment is often difficult to control and the formation mechanism is still unclear, which limit the large-scale application of these methods [28, 29]. If MOFs with hollow structure can be synthesized first and then subjected to high temperature heat treatment, transition metal phosphatides with hollow structure can be obtained more simply and controllably. Herein, we first controllably synthesized the hollow structure of Co-MOFs by Ostwald curing process, and then prepared hollow structure of porous carbon wrapped CoP (CoP/C) after subsequent high temperature annealing treatment. When CoP/C was used as anode and hollow N-doped carbon shell (HNCS) as cathode, the assembled battery-type hybrid supercapacitor exhibits excellent electrochemical performances with high energy density, power density and excellent long cycle stability. 2.

Experimental

2.1 Preparation of Co-MOFs with hollow structure Co(NO3)2·6H2O (21, 42, 84 mg) with 1, 3, 5-benzenetricarboxylic acid (BTC) (7, 14, 28 mg, ) were dissolved in ethylene glycol (30 mL) and absolute ethyl alcohol (20 mL) mixed solutions. And then the resultant solutions were added in a 100 mL solvothermal kettle and reacted at 150 for 24 h. After cooling down to room temperature, the products were collected with centrifugal separation and washed 4 times with anhydrous ethanol, and then dried at 60

for 24 h. The

Co-MOFs products were named as CM-1, CM-2 and CM-4 according to the mass of Co(NO3)2·6H2O and BTC. 2.2 Synthesis of CoP/C with hollow structure The prepared Co-MOFs and commercial NaH2PO2·H2O were put in the corundum crucible with 4

the mass ratio of 1:10, which was placed in the tubular furnace. The product was obtained via annealing at 500

for 3 h with a heating rate of 5

min-1 under N2 atmosphere and named as

CoP/C. 2.3 Preparation of polystyrene (PS) spheres and dopamine-coated PS spheres A certain amount of pretreated styrene, 0.5 g PVP and 90 mL H2O were added in a 250 mL three necked bottle. After stirring for 10 min (900 r min-1) flowing N2 for 10 min, 30 ml K2S2O8 (1 g) solution was added slowly and the solution was heated in a water bath at 70

for 15 h. Finally,

the PS with uniform size was obtained. The dopamine-coated PS spheres were prepared as following: 180 mg PS spheres were put into 200 mL Tris-Cl buffer solution (pH=8.5) under 30 min ultrasonic dispersion. Then 240 mg dopamine was added and stirred for 12 h in dark. At last, the products were centrifuged and washed by water four times and dried at 60

for 24 h, which was named as PDA-PS.

2.4 Synthesis of N-doped hollow carbon shells The PDA-PS was annealed at 700

for 2 h with a heating rate of 2

min-1 under N2

atmosphere to obtain the N-doped hollow carbon shell gained, named as HNCS. 2.5 Materials Characterization The surface morphology, structure and chemical composition of the samples were characterized by field emission scanning electron microscopy (FESEM, JSM-7001-F type, JEOL, Japan) with energy dispersive X-ray spectrum (EDX, Bruker quantax400), transmission electron microscope (TEM, JEM 2010 JEOL, Japan), X-ray diffraction (XRD, D/Max-2500/PC, Rigaku Corporation, Japan, Cu Kα radiation) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI photoelectron spectrometer with Al Kα radiation), respectively. The specific surface area and pore size distribution of the samples were measured by N2 isotherm adsorption and desorption curves by V-Sorb 2800P analyser. 2.6 Electrochemical tests The electrodes were prepared by mixing the samples, Super-P and polyvinylidene fluoride (PVDF) with a mass ratio of 80:10:10 in n-methy1-2-pyrrolidone solvent, and coating the obtained slurry on the graphite paper, which was subsequently dried at 80

for 24 h. The electrochemical

performances of CoP/C were measured in a three-electrode mode with a saturated calomel reference electrode and a platinum counter electrode. The hybrid supercapacitor (HSC) was 5

assembled by CoP/C anode and HNCS cathode and measured in a two-electrode mode. The electrolyte was 2 M KOH solution. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were carried out using an electrochemical workstation (Autolab, M204). The specific capacitance (Cs, F g-1) of electrode materials was calculated according to the following equation: Cs = I × ∆t/(∆V × m)

(1)

where I (A) represents the discharge current, ∆t (s) corresponds to the discharge time, ∆V (V) is the potential window, and m (g) is the mass of active materials. The specific capacitance Cm (F g-1) of HSC was calculated by the following equation: Cm = I × ∆t/(∆V × M)

(2)

where M (g) is the total mass of the active materials in the two electrodes of HSC. The energy density E (Wh kg-1) and power density Pav (W kg-1) were calculated as follows:

3.

E = 0.5 × Cm × ∆V2/3.6

(3)

Pav = 3600 × E/∆t

(4)

Results and Discussion Fig. 1 reveals the schematic illustration of the synthesis of hollow Co-MOFs and CoP/C. At

First, Co-MOFs were synthesized by using BTC as organic ligand and Co2+ as metal ion. The hollow structure of Co-MOFs (CM-1), the core and shell structure of Co-MOFs (CM-2) and the solid structure of Co-MOFs (CM-4) could be obtained by Ostwald curing process through changing the reactant concentration during solvothermal reaction. The mechanism of this process can be explained as follows: Co2+ coordinates with BTC firstly to form particles of with uneven size. Due to the high surface energy of small particles, the surrounding mother liquor diffuses to the surface of large particles with low concentration, causing small particles to gradually dissolve and then deposit on the surface of stable large particles. Because the internal structure of formed spherical MOFs is not stable, small particles are deposited on the wall of MOFs after dissolution and hollow structure of Co-MOFs is obtained. If the concentration of reactants in the solution increases, the dissolution and redeposition of small particles can be effectively inhibited. Therefore, the core inside the Co-MOFs gradually enlarges and eventually grows to a solid MOFs. Finally, the hollow structure Co-MOFs were carbonized and phosphatized at a high temperature to prepare CoP/C composites. 6

Fig. 1 Schematic illustration of the synthesis of hollow Co-MOFs and CoP/C.

Fig. 2 Schematic of the synthesis process of HNCS. Fig. 2 shows the schematic of the preparation of hollow HNCS. Firstly, the PDA-PS composite material was obtained via the self-polymerization of dopamine on the surface of PS sphere with regular size. Then, during high temperature annealing, PS was gradually decomposed into volatile gas and taken away by N2 airflow. Because dopamine itself contains rich nitrogen content, the N-doped hollow carbon shell was obtained after carbonization of PDA at high temperature. Notably, N-doping of carbon materials is beneficial to improving the conductivity and good hydrophilicity and can facilitate Faraday reaction on the surface to contribute pseudo-capacitance [30, 31].

7

(a)

(b)

(c)

(e)

(d)

(f)

(g)

Fig.3 (a, b) SEM and (c) TEM images of Co-MOFs. (d, e) SEM images, (f) TEM image, and (g) EDX elemental mapping of CoP/C. 8

SEM and TEM were used to characterize the structure and elemental distribution of prepared Co-MOFs and CoP/C. As shown in Fig. 3 (a) and (b), the Co-MOFs presents a spherical structure with a smooth surface and a size of about 800 nm~1 mm. Fig. 3 (c) shows the TEM image of CM-1. It can be seen that the Co-MOFs displays a hollow structure. Fig. 3 (d) and (e) show SEM images of CoP/C. It can be found that after carbonization and phosphorization at high temperature, the smooth surface becomes rough and larger particles are distributed, which should be CoP particles formed by Co2+ in MOFs diffusing to the external surface at high temperature and reacting with PH3. Fig. 3(f) shows that CoP/C displays the hollow structure, which is consistent with SEM image. Fig. 3 (g) shows the elemental distribution of CoP/C characterized by EDX. It can be found that the elements Co, C and P are uniformly distributed in the sample, indicating the successful synthesis of CoP/C via high temperature carbonization of Co-MOFs and subsequent phosphorization [32].

Fig 4 (a) SEM images of PS spheres. (b) and (c) Low- and high-magnification SEM images of PDA-PS composite. (d) SEM image of the HNCS. SEM was used to characterize the morphology and structural characteristic of HNCS. As seen from Fig. 4 (a), the prepared PS is regular and spherical with a smooth surface and a size 9

distribution of 300 ~350 nm. Fig.4 (b) and (c) show the SEM images of PDA-PS. The surface of smooth PS spheres becomes rough and uneven after being coated, while the particle size of the spheres slightly increases, proving that dopamine was successfully coated on the surface of PS spheres. The PDA-PS particles show obvious aggregation owing to the polymerization of dopamine. After high-temperature carbonization in N2, the PS spheres was decomposed, and PDA shells were transformed into N-doped hollow carbon shells. As shown in Fig. 4 (d), HNCS is aggregated together and displays a honeycomb-like structure.

Fig. 5 XPS spectra of CoP/C: (a) survey spectrum, (b-d) high-resolution (b) Co 2p, (c) P 2p and (d) C 1s spectra. Fig. 5(a) shows the XPS survey spectrum of CoP/C, demonstrating that Co, P, O and C elements exist in the sample. From the high-resolution Co 2p spectrum (Fig. 5 (b)), the peak located at 782.5eV corresponds to Co 2p3/2, while the other main peak at 798.4eV is attributed to Co 2p1/2. In addition, two weak peaks at 786.5 and 803.2 eV represent the vibrational excitation state of high spin Co2+ [33, 34]. The high-resolution P 2p spectrum (Fig. 5 (c)) shows two peaks at binding energies of 129.9 and 134.4 eV, which are characteristic of P 2p [6]. The high resolution 10

C1s spectrum(Fig. 5(d)) can be deconvoluted into three peaks at 284.8, 285.6 and 289.0 eV, which correspond to C-C, C-O-C and -C=O, respectively [35, 36]. . According to the XPS analysis, the content of CoP in CoP/C sample is about 66.34%.

Fig.6 (a) Raman spectrum and (b) XPS survey spectrum of HNCS. (c) and (d) High-resolution XPS N 1s and C 1s spectra of HNCS. Fig. 6(a) shows the Raman spectrum of HNCS, two conspicuous peaks located at about 1344 and 1587 cm-1, corresponding to D and G bands of carbon materials. The D band represents the structural disorder and defects of graphite, while the G band is attributed to the sp2 bond structure of carbon. The defect degree of carbon materials can be determined via the intensity ratio of the two bands (ID/IG), and the ID/IG value is calculated to be about 0.92, indicating that HNCS possesses abundant disorders and defects, which is beneficial to the electrochemical performance by providing more active sites for ion accommodation [37]. Fig. 6(b)-(d) show the XPS spectra of the HNCS. The survey spectrum of HNCS demonstrates the presence of C, N and O elements, as shown in Fig. 6(b). The high-resolution N1s spectrum (Fig. 6(c)) can be deconvoluted into pyridinic N (398.4 eV), pyrrolic N (399.8 eV), graphitic N (401.0 eV) and oxidized N (402.5 eV) [38], respectively. Meanwhile, the high-resolution C 1s spectrum can split into three peaks at 11

800

(a)

Ads Des

600 500 400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

1.6 1.4

(b)

0.8

V dV(d)

1.2

0.6

1.0 0.8

0.4

0.6 0.2

0.4

dV(d) (cc/nm/g)

Volume@STP (cc)

700

Cumulative Pore Volume (cc/g)

284.8 eV, 285.6 eV and 287.3 eV, respectively, which correspond to C-C, C=N and C-N [39].

0.2 0.0

0.0 4

8

12

16

20

24

28

Pore Diameter (nm)

Fig.7 Nitrogen adsorption-desorption isotherm (a) and pore size distribution (b) of HNCS. Fig.7 (a) displays the nitrogen adsorption-desorption curve of HNCS, which presents a typical type IV isotherm with a hysteresis loop. The BET specific surface area and pore volume of HNCS are 623 m2 g-1 and 1.266 cm3 g-1, respectively. Moreover, Fig. 7(b) shows the pore size distribution of HNCS and the peak is mainly distributed in the range of 4~5 nm, showing a typical mesoporous structure. Large specific surface area and pore volume with reasonable pore size distribution is beneficial for the rapid diffusion of electrolyte ions and significantly increase effective active sites in electrode materials for ion accommodation.

12

Fig.8 Electrochemical performances of CoP/C measured in a three-electrode system in 2 M KOH electrolyte: (a) Nyquist plots (Inset shows the corresponding magnified plot in the high-frequency region and the corresponding equivalent circuit), (b) CV curves at different scan rates, (c) GCD curves at various current densities, (d) specific capacitance vs. current density. A series of electrochemical tests (EIS, CV, and GCD) of CoP/C were performed using an electrochemical workstation in a three-electrode system in 2M KOH electrolyte, and the results are shown in Fig. 8. The EIS spectrum of CoP/C was tested in a frequency range of 0.01 Hz to 100 kHz, as shown in Fig. 8(a). The inset of Fig. 8a exhibits magnified plot in the high frequency region, and it can be seen that the CoP/C displays a very small Rct value (0.2 Ω ), indicating its facilitated charge transfer ability owing to carbon-coated layer. Fig. 8(b) displays the CV curves of CoP/C at various scan rate from 2 to 50 mV s-1, demonstrating the obvious redox peaks for Co2+/Co3+ conversion, which is the typical feature of pseudo-capacitor [18]. And the reaction can be described as following: CoP+2OH-↔CoP(OH)2+2e-

(2)

The current intensity in CV curves increases with the increasing scan rate and maintain similar shape, indicating good reversibility during high-rate charge-discharge processes. 13

Fig. 8(c) presents the GCD curves of CoP/C electrode at different current densities, which show asymmetric contour due to its remarkable pseudo-capacitive behavior during the charge/discharge process. The specific capacitances were calculated according to the discharge curves of GCD are about 302.9, 243.2, 193.2, 155.6, 124.6, 98.7, 57.4 and 25.5 F g-1, at the current densities of 1, 2, 3, 4, 5, 6, 8 and 10 A g-1, respectively, as shown in Fig. 8(d). The high specific capacitance of CoP/C is mainly attributed to its carbon coated structure with rich defects to provide fast ion diffusion and electron transport.

Fig. 9 Electrochemical performances of HNCS: (a) Nyquist plots (Inset shows the corresponding magnified plot in the high-frequency region and the corresponding equivalent circuit), (b) CV curves at various scan rates, (c) GCD curves at various current densities, (d) specific capacitance vs. current density. Fig. 9 (a) shows the Nyquist plots of HNCS, which consist of a slope line in the low frequency region and a small semicircle in the high frequency region (related to the charge transfer resistance, Rct). The Rct value of HNCS is fitted to be about 0.155 Ω, indicating that the HNCS has a good electron conductive ability, which is beneficial to reducing the charge transfer between electrode material and electrolyte [9]. The CV profiles of HNCS were tested at various scan rate in 14

the range of -1 and 0 V, Fig. 9(b) shows the CV curves of HNCS displaying good mirror symmetry without obvious redox peaks, which is the typical characteristic of electric double layer capacitor (EDLC). Fig. 9(c) exhibits the GCD curves of HNCS at various current densities, which display triangular symmetry owing to its remarkable EDLC behavior.

The specific capacitances were

calculated from discharge curves and their values are 431.6, 306.2, 204.8, 149.5, 114.1 and 88.6 F g-1 at the current densities of 1, 2, 4, 6, 8 and 10 A g-1, respectively. When the current density increases to 10 times higher than initial value, a specific capacitance retention of 20.5% could still be maintained owing to the existence of micropores which exhibit high rapid current response capability and good charge transfer stability at high-rate charge and discharge process.

Fig. 10 (a) CV curves of the CoP/C and HNCS electrodes at 20 mV s-1 measured in a 15

three-electrode system. (b) CV curves and (c) GCD curves of the HNCS//HNCS symmetric device collected in different voltage windows. (d) CV curves at different scan rates and (e) GCD curves of HNCS//CoP/C asymmetric supercapacitor at different current densities. (f) Specific capacitances vs. current density. To validate the practical applications of CoP/C, a two-electrode asymmetric supercapacitor was constructed using CoP/C as cathode electrode and HNCS as anode. It can be seen from Fig. 10 (a), the voltage windows of HNCS and CoP/C electrodes in the three-electrode system tests are -1.0~0 V and -0.3~0.5 V, respectively. In order to obtain the maximum specific capacitance, the electrode material mass ratio in the hybrid supercapacitor can be calculated according to the equation: m+/m- = (C- × ∆V-)/(C+ × ∆V+)

(6)

where m+, C+ and ∆V+ represent the active material mass, specific capacitance and voltage range of the cathode; m-, C- and ∆V- represent the active material mass, specific capacitance and voltage range of the anode. Fig. 10 (b) and (c) exhibit the CV and GCD curves of HNCS//HNCS symmetric supercapacitor with different voltage windows. The symmetric supercapacitor can be operated at 1.8 V without an obvious increase in anodic current from the CV and GCD curves. According to the CV curves at various potential windows (Fig. 10(d)), the asymmetric supercapacitor can be operated at 1.4 V without obvious anodic peak current. The GCD profiles (Fig. 10€) of asymmetric supercapacitor show similar symmetrical linear curves and a low IR drop, indicating excellent electrochemical reversibility and low internal series resistance. Fig. 10(f) shows the specific capacitances at different current densities, demonstrating the specific capacitances of 59.3, 46.2, 34.7, 29.7, 24.1, 22.3 and 15.2 F g-1 at a current density of 1, 2, 3, 4, 6, 8 and 10 A g-1, respectively. To explore the long-cycling stability of HNCS//CoP/C asymmetric supercapacitor, the cycling performance was tested at a current density of 7 A g-1, and the results are displayed in Fig. 11(a). The specific capacitance retention of the asymmetric supercapacitor can reach up to 99.5% after 5000 cycles, demonstrating the excellent cycling stability. The excellent performance is attributed to the high specific surface area and novel porous structure with carbon-coated layer of CoP/C, which can provide more active sites for ion accommodation and facilitate ion and electron transfer. Fig. 11 (b) shows a Ragone plot of the corresponding power and energy densities of the 16

HNCS //CoP/C asymmetric supercapacitor. It can be found that a high energy density of 16.14 Wh kg-1 at a high power density of about 700W kg-1 is achieved. And the asymmetric supercapacitor still remains a value of 4.14 Wh kg-1 at a very high-power density of 5600W kg-1. The assembled asymmetric supercapacitor can provide both high power density and ultra-high energy density, confirming that the as-prepared CoP/C is very promising as an electrode material for high-power supercapacitors.

Fig.11 (a) Cycling performance of the HNCS//CoP/C asymmetric supercapacitor at a current density of 7 A g-1. (b) Ragone plots of the HNCS//CoP/C asymmetric supercapacitor. 4.

Conclusion In summary, carbon coated CoP hollow sphere hybrid material was prepared from the hollow

structure of Co-MOFs via high-temperature carbonization and phosphorization process. When used as electrode material for supercapacitor, CoP/C displays a high specific capacitance of 302.9 F g-1 at a current density of 1 A g-1 and good rate capacity due to its excellent electronic conductivity and special hollow structure. Moreover, the assembled HNCS//CoP/C asymmetric supercapacitor exhibits a high energy density of 16.14 Wh kg-1 at a power density of 700 W kg-1 and the capacitance retention can reach up to 99.5% after charge/discharge for 5000 cycles at a current density of 7 A g-1. The CoP/C electrode material should be promisingly applied in energy storage equipment and electronic devices. The present strategy offers an effective and low-cost method to prepare hollow and carbon coated metal phosphide for high-performance supercapacitor electrodes.

Acknowledgements Financial support from National & Local Joint Engineering Research Center for 17

Deep Utilization Technology of Rock-salt Resource (No. SF201802), Provincial Natural Science Foundation of Anhui (1908085ME120), Primary Research and Development Program of Anhui Province (201904a05020087), and Key Discipline of Material Science and Engineering of Suzhou University (2017XJZDXK3) is gratefully acknowledged.

References [1] S. Zhou, X. Kong, B. Zheng, F. Huo, M. Strømme, C. Xu, Cellulose Nanofiber @ Conductive Metal–Organic Frameworks for High-Performance Flexible Supercapacitors, ACS Nano 13 (2019) 9578-9586. [2] J. Wen, S. Li, T. Chen, B. Li, L. Xiong, Y. Guo, G. Fang, Porous nanosheet network architecture of CoP@Ni(OH)2 composites for high performance supercapacitors, Electrochim. Acta 258 (2017) 266-273. [3] X. Zhang, Q. Fu, H. Huang, L. Wei, X. Guo, Silver-Quantum-Dot-Modified MoO3 and MnO2 Paper-Like Freestanding Films for Flexible Solid-State Asymmetric Supercapacitors, Small 15(13) (2019) 1805235. [4] Y. Li, W. Ou-Yang, X. Xu, M. Wang, S. Hou, T. Lu, Y. Yao, L. Pan, Micro-/mesoporous carbon nanofibers embedded with ordered carbon for flexible supercapacitors, Electrochim. Acta 271 (2018) 591-598. [5] T. Lu, Y. Zhang, H. Li, L. Pan, Y. Li, Z. Sun, Electrochemical behaviors of graphene–ZnO and graphene–SnO2 composite films for supercapacitors, Electrochim. Acta 55(13) (2010) 4170-4173. [6] Y. Zhang, H. Li, L. Pan, T. Lu, Z. Sun, Capacitive behavior of graphene–ZnO composite film for supercapacitors, J. Electroanal. Chem. 634(1) (2009) 68-71. [7] X. Li, L. Shi, L. Li, C. Dong, C.-z. Li, S. Shuang, Recent Advances in Carbon Nanodots: Properties and Applications in Cancer Diagnosis and Treatment, J. Analy. Test. 3(1) (2019) 37-49. [8] X.H. Xia, J.P. Tu, Y.Q. Zhang, Y.J. Mai, X.L. Wang, C.D. Gu, X.B. Zhao, Three-Dimentional Porous Nano-Ni/Co(OH)2 Nanoflake Composite Film: A Pseudocapacitive Material with Superior Performance, J. Phys. Chem. C 115(45) (2011) 22662-22668. [9] Y. Lu, J.-k. Liu, X.-y. Liu, S. Huang, T.-q. Wang, X.-l. Wang, C.-d. Gu, J.-p. Tu, S.X. Mao, Facile synthesis of Ni-coated Ni2P for supercapacitor applications, CrystEngComm 15(35) (2013) 7071-7079. [10] X. Zhang, X. Gao, Z. Wu, M. Zhu, Q. Jiang, S. Zhou, Y. Huang, Z. Rao, High-capacitance supercapacitor based on nitrogen-doped porous carbons-sandwiched graphene hybrid frameworks, Ionics 25(12) (2019) 6017-6023. [11] A. Panneerselvam, M.A. Malik, M. Afzaal, P. O'Brien, M. Helliwell, The Chemical Vapor Deposition of Nickel Phosphide or Selenide Thin Films from a Single Precursor, J. Am. Chem. Soc. 130(8) (2008) 2420-2421. [12] D. Yang, J. Zhu, X. Rui, H. Tan, R. Cai, H.E. Hoster, D.Y.W. Yu, H.H. Hng, Q. Yan, Synthesis of Cobalt Phosphides and Their Application as Anodes for Lithium Ion Batteries, ACS Appl. Mater. Inter. 5(3) (2013) 1093-1099. [13] Y. Lu, J.-P. Tu, Q.-Q. Xiong, J.-Y. Xiang, Y.-J. Mai, J. Zhang, Y.-Q. Qiao, X.-L. Wang, C.-D. Gu, S.X. Mao, Controllable Synthesis of a Monophase Nickel Phosphide/Carbon (Ni5P4/C) Composite Electrode via 18

Wet-Chemistry and a Solid-State Reaction for the Anode in Lithium Secondary Batteries, Adv. Fun. Mater. 22(18) (2012) 3927-3935. [14] H. Liang, C. Xia, Q. Jiang, A.N. Gandi, U. Schwingenschlögl, H.N. Alshareef, Low temperature synthesis of ternary metal phosphides using plasma for asymmetric supercapacitors, Nano Energy 35 (2017) 331-340. [15] X. Wang, Y.V. Kolen'ko, L. Liu, Direct solvothermal phosphorization of nickel foam to fabricate integrated Ni2P-nanorods/Ni electrodes for efficient electrocatalytic hydrogen evolution, Chem. Commun. 51(31) (2015) 6738-6741. [16] X. Chen, M. Cheng, D. Chen, R. Wang, Shape-Controlled Synthesis of Co2P Nanostructures and Their Application in Supercapacitors, ACS Appl. Mater. Inter. 8(6) (2016) 3892-3900. [17] G.Q. Zhang, B. Li, M.C. Liu, S.K. Yuan, L.Y. Niu, Liquid Phase Synthesis of CoP Nanoparticles with High Electrical Conductivity for Advanced Energy Storage, J. Nanomater. (2017) 9728591. [18] S. Hou, X. Xu, M. Wang, Y. Xu, T. Lu, Y. Yao, L. Pan, Carbon-incorporated Janus-type Ni2P/Ni hollow spheres for high performance hybrid supercapacitors, J. Mater. Chem. A 5(36) (2017) 19054-19061. [19] L. Zhang, H.B. Wu, X.W. Lou, Metal–Organic-Frameworks-Derived General Formation of Hollow Structures with High Complexity, J. Am. Chem. Soc. 135(29) (2013) 10664-10672. [20] D.P. Dubal, K. Jayaramulu, J. Sunil, Š. Kment, P. Gomez-Romero, C. Narayana, R. Zbořil, R.A. Fischer, Metal–Organic Framework (MOF) Derived Electrodes with Robust and Fast Lithium Storage for Li-Ion Hybrid Capacitors, Adv. Fun. Mater. 29(19) (2019) 1900532. [21] T. Tian, L. Ai, J. Jiang, Metal–organic framework-derived nickel phosphides as efficient electrocatalysts toward sustainable hydrogen generation from water splitting, RSC Adv. 5(14) (2015) 10290-10295. [22] B.Y. Guan, X.Y. Yu, H.B. Wu, X.W. Lou, Complex Nanostructures from Materials based on Metal–Organic Frameworks for Electrochemical Energy Storage and Conversion, Adv. Mater. 29(47) (2017) 1703614. [23] X. Xu, Y. Liu, M. Wang, C. Zhu, T. Lu, R. Zhao, L. Pan, Hierarchical hybrids with microporous carbon spheres decorated three-dimensional graphene frameworks for capacitive applications in supercapacitor and deionization, Electrochim. Acta 193 (2016) 88-95. [24] J. Li, D. Yan, X. Zhang, S. Hou, T. Lu, Y. Yao, L. Pan, ZnS nanoparticles decorated on nitrogen-doped porous carbon polyhedra: a promising anode material for lithium-ion and sodium-ion batteries, J. Mater. Chem. A 5(38) (2017) 20428-20438. [25] X. Zhang, W. Ou-Yang, G. Zhu, T. Lu, L. Pan, Shuttle-like carbon-coated FeP derived from metal-organic frameworks for lithium-ion batteries with superior rate capability and long-life cycling performance, Carbon 143 (2019) 116-124. [26] X.Y. Gao, G. Zhu, X.J. Zhang, T. Hu, Porous carbon materials derived from in situ construction of metal-organic frameworks for high-performance sodium ions batteries, Micropor. Mesopor. Mater. 273 (2019) 156-162. [27] P. Liu, R. Bao, D. Fang, J. Yi, L. Li, A facile synthesis of CNTs/Cu2O-CuO heterostructure composites by spray pyrolysis and its visible light responding photocatalytic properties, Adv. Powder Technol. 29(9) (2018) 2027-2034. [28] L. Zhou, Z. Zhuang, H. Zhao, M. Lin, D. Zhao, L. Mai, Intricate Hollow Structures: Controlled Synthesis and Applications in Energy Storage and Conversion, Adv. Mater. 29(20) (2017) 1602914. [29] L. Yu, H. Hu, H.B. Wu, X.W. Lou, Complex Hollow Nanostructures: Synthesis and Energy-Related Applications, Adv. Mater. 29(15) (2017) 1604563. 19

[30] Y. Liu, T. Chen, T. Lu, Z. Sun, D.H.C. Chua, L. Pan, Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization, Electrochim. Acta 158 (2015) 403-409. [31] D. Gueon, J.H. Moon, Nitrogen-Doped Carbon Nanotube Spherical Particles for Supercapacitor Applications: Emulsion-Assisted Compact Packing and Capacitance Enhancement, ACS Appl. Mater. Inter. 7(36) (2015) 20083-20089. [32] Y. Du, H. Li, Construction of 3D nanoarchitectural porous carbon supported carbon nanotubes@CoP with enhanced lithium ions storage performance, Chem. Phys. Lett. 732 (2019) 136633. [33] T. Zhai, L. Wan, S. Sun, Q. Chen, J. Sun, Q. Xia, H. Xia, Phosphate Ion Functionalized Co3O4 Ultrathin

Nanosheets

with

Greatly

Improved

Surface

Reactivity

for

High

Performance

Pseudocapacitors, Adv. Mater. 29(7) (2017) 1604167. [34] Y.M. Hu, M.C. Liu, Q.Q. Yang, L.B. Kong, L. Kang, Facile synthesis of high electrical conductive CoP via solid-state synthetic routes for supercapacitors, J. Energy Chem. 26(1) (2017) 49-55. [35] X. Ge, Z. Li, L. Yin, Metal-organic frameworks derived porous core/shellCoP@C polyhedrons anchored on 3D reduced graphene oxide networks as anode for sodium-ion battery, Nano Energy 32 (2017) 117-124. [36] J. Díaz, G. Paolicelli, S. Ferrer, F. Comin, Separation of the ${\mathit{sp}}^{3}$ and ${\mathit{sp}}^{2}$ components in the C1s photoemission spectra of amorphous carbon films, Phys. Rev. B 54(11) (1996) 8064-8069. [37] Y. Liu, X. Xu, M. Wang, T. Lu, Z. Sun, L. Pan, Nitrogen-doped carbon nanorods with excellent capacitive deionization ability, J. Mater. Chem. A 3(33) (2015) 17304-17311. [38] Y. Liu, X. Xu, T. Lu, Z. Sun, D.H.C. Chua, L. Pan, Nitrogen-doped electrospun reduced graphene oxide–carbon nanofiber composite for capacitive deionization, RSC Adv. 5(43) (2015) 34117-34124. [39] Y. Li, G. Zhu, H. Huang, M. Xu, T. Lu, L. Pan, A N, S dual doping strategy via electrospinning to prepare hierarchically porous carbon polyhedra embedded carbon nanofibers for flexible supercapacitors, J. Mater. Chem. A 7(15) (2019) 9040-9050.

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Fig. 10 (a) CV curves of the CoP/C and HNCS electrodes at 20 mV s-1 measured in a three-electrode system. (b) CV curves and (c) GCD curves of the HNCS//HNCS symmetric device collected in different voltage windows. (d) CV curves at different scan rates and (e) GCD curves of HNCS//CoP/C asymmetric supercapacitor at different current densities. (f) Specific capacitances vs. current density.

Research Highlights Carbon coated CoP hollow spheres were synthesized from cobalt-based metal organic frameworks. Carbon coated CoP hollow spheres were used as electrode materials for supercapacitor. Carbon coated CoP hollow spheres electrode materials display l remarkable specific capacitances and excellent capacitance retentions for supercapacitor.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: