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Ni2P2O7 hollow nanospheres for high-performance supercapacitors
Journal Pre-proofs Porous CoP/Ni2P2O7 hollow nanospheres for high-performance supercapaci‐ tors JiaWei Xu, HeLiagn Fan, XueYu Tao, LiTong Guo, ZhangSheng Liu PII: DOI: Reference:
S0167-577X(20)30272-X https://doi.org/10.1016/j.matlet.2020.127567 MLBLUE 127567
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
29 January 2020 19 February 2020 25 February 2020
Please cite this article as: J. Xu, H. Fan, X. Tao, L. Guo, Z. Liu, Porous CoP/Ni2P2O7 hollow nanospheres for high-performance supercapacitors, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127567
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.
© 2020 Published by Elsevier B.V.
Porous CoP/Ni2P2O7 hollow nanospheres for high-performance supercapacitors JiaWei Xu, HeLiagn Fan, XueYu Tao, LiTong Guo, ZhangSheng Liu *School of Material Science and Engineering, China University of Mining and Technology, XuZhou，221116, China Abstract In this paper, porous CoP/Ni2P2O7 hollow nanospheres have been synthesized via a two-step method. The obtained nanocomposites display superior specific capacitance (1998F g−1 at a current density of 1A g−1), excellent rate capability and commendable cycle stability (capacity retention of 90% after 1000 cycles), which can be a promising electrode material for supercapacitor. Keywords: Energy storage and conversion; Nanocomposites; Supercapacitors; CoP/Ni2P2O7 1.
Introduction Recently, metal pyrophosphates (M2P2O7, M=Mn2+, Ni2+ and Co2+, etc.) have been investigated as
a new kind of pseudocapacitive material [1-3]. Among them, Ni2P2O7 attracts more and more attention from researchers due to its nontoxicity, cost-effectiveness and high theoretical capacitance. Up to date, many Ni2P2O7 materials with different morphologies have been reported. For example, Wei et al [4] reported Ni2P2O7 hexagonal tablets, whose specific capacitance was 557.7F g-1 at a current density of 1.2A g−1. Zhou et al [5] prepared Ni2P2O7 nanowires by chemical precipitation method, which delivered 772.5F g-1 of specific capacitance at 1A g−1. Senthilkumar et al [6] claimed Ni2P2O7 nanoparticles with a maximum capacitance of 1863F g-1. Nonetheless, limited by the low conductivity, Ni2P2O7 suffers poor cycling stability and unsatisfactory rate capability, which extremely restrict its practical
* Corresponding author (Tel.:+86 516 83591979; fax:+86 51683591870. E-mail: [email protected])
1
applications. Combining Ni2P2O7 with other high conductive active materials may be a good solution to improve the electrochemical performance. Recently, cobalt phosphide (CoP) has been reported as a high conductive active material, in which Co combined by metallic bonds can provide lots of free electrons [7]. In view of the advantage, it is expected that CoP/Ni2P2O7 composites will possess the enhanced electrochemical performance comparing with pure Ni2P2O7. Herein, we have developed CoP/Ni2P2O7 hollow nanospheres via a two-step process of co-precipitation and annealing method. These hollow nanospheres are porous and thin-walled in structure, facilitating the ion diffraction. The composites exhibit superior electrochemical performance, which can be an ideal electrode material for supercapacitor applications. 2.
Experimental Ni2P2O7 was prepared by mixing 2mmol Ni(CH3COO)2·4H2O and 2mmol Na2HPO4·12H2O in
250ml deionized water. After stirred overnight, the precipitate was washed and dried, which was then annealed at 300 °C for 3 h in a nitrogen atmosphere. To fabricate CoP/Ni2P2O7 hollow nanospheres, 0.3mmol of Ni2P2O7 was added into 20 mL ethanol solution containing 0.09mmol CoCl2·6H2O and 0.27mmol NH4HCO3. After stirred for 5h, the obtained precursor was washed and dried. Then, 100mg of the precursor was placed at the downstream side of the furnace next to 20mg NaH2PO2, which was annealed at 300 °C for 3 h in a nitrogen atmosphere. The samples were characterized by X-ray diffraction (XRD, Bruker D8 Advance), X-ray photoelectron spectroscopy (XPS: Thermo ESCALAB250, USA), scanning electron microscopy (SEM, FEI Quanta-250) and transmission electron microscopy (TEM, JEOL-2010). 3.
Results and discussion 2
Fig 1 (a) XRD patterns of samples; XPS spectra of (b) Ni 2p, (c) Co 2p and (d) P 2p for CoP/Ni2P2O7 The phase compositions of the samples were examined by XRD. Similar with the previous reports [5,6], Ni2P2O7 is poor crystallization with weak and broad peaks(Fig 1a), which correspond to the monoclinic phase Ni2P2O7 (PDF#39-0710). After Ni2P2O7 is combined with CoP, there is no obvious change in the XRD pattern, which may result from high dispersity and poor crystallization of CoP. Fig 1b shows the Ni 2p XPS spectrum of CoP/Ni2P2O7. The peaks located at 861.6 eV and 879.6eV correspond to the satellite peaks, while those at 856.2eV and 873.9eV can be assigned to Ni 2p3/2 and Ni 2p1/2. The spin energy separation of 17.7eV indicates that Ni exists in divalent oxidation state. Differently, the Co 2p spectrum can be de-convoluted into two shake-up satellites (790.4 and 805.6 eV) and two spin-orbit doublets (Fig 1c). The peaks at 797.0 and 778.5 eV corresponding to Co 2p1/2 and Co2p3/2 are ascribed to CoP [8], while those at 801.1 and 785.4 eV are related to the chemical states of Co2+, which may arise from superficial oxidation of CoP because of air contact [9]. Besides, in P 2p XPS spectrum (Fig 1d), the peak at 133.2 eV is ascribed to P-O and P=O for Ni2P2O7, while that at 130.3eV is very close to the binding energy of P 2p in CoP. Based on the above results, it is concluded 3
that CoP/Ni2P2O7 has been successfully synthesized.
Fig 2 FESEM image of (a) Ni2P2O7 and (b) CoP/Ni2P2O7; (c) TEM and (d) EDX mapping images of CoP/Ni2P2O7 The microstructures of the samples were characterized by SEM and TEM. As shown in Fig 2a, Ni2P2O7 consists of lots of nanoparticles with high agglomeration, whose sizes vary from 20–50 nm. Interestingly, CoP/Ni2P2O7 composites display a mass of hollow nanospheres, which can be revealed by partially broken nanospheres (Fig 2b). TEM image also verifies hollow spherical nanostructures from the sharp contrast between the center and the edge (Fig 2c). Further observation reveals that these hollow nanospheres are porous and thin-walled with ~3nm of the shell thichness. Besides, the lattice fringe of nanosphere is indistinguishable (inset of Fig. 2c), indicating its poor crystallization nature, which is consistent with XRD results. The possible formation process of hollow nanospheres is proposed as follows: initially, when Ni2P2O7 particles were added into CoCl2·6H2O-contained ethanol 4
solution, they would transform into Ni2P2O7·6H2O by obtaining crystal water; then, the Co species was attached to their surfaces with the help of NH4HCO3, forming a quasi-core-shell structure; finally, porous hollow nanospheres could be formed due to dehydration of core materials and surface energy minimization during the annealing treatment. Fig 2d presents the element mapping of CoP/Ni2P2O7 hollow nanospheres. The Co element shows similar profile to Ni and P, indicating that CoP is uniformly dispersed in CoP/Ni2P2O7, which may help to high conductivity of the composites.
Fig. 3 (a) Comparative CV curves, (c) Comparative GCD curves and (f) Nyquist plots of as-prepared samples; (b) CV curves, (d) GCD curves and (e) Cycling performance of CoP/Ni2P2O7 The electrochemical performance of as-prepared samples was investigated in a three-electrode cell with 2M KOH as an electrolyte. Fig 3a depicts the cyclic voltammograms (CV) curves of Ni2P2O7 and CoP/Ni2P2O7 at the scan rate of 10mV/s. Both samples show a well-defined pair of redox peaks, suggesting their pseudocapacitive behavior. Based on the previous reports, the corresponding faradaic reactions can be described by Eqs (1) and (2) [5]: Ni2P2O7+ 2OH− ↔ Ni2P2O7(OH)2+2 e− 5
(1)
CoP + 2OH− ↔ Co (OH)2 +P+2e−
(2)
Comparatively, CoP/Ni2P2O7 presents a higher anodic and cathodic current density with a larger area enclosed in the CV, meaning that it can store more charges than Ni2P2O7. Fig 3b shows the CV curves of CoP/Ni2P2O7 at different scan rates. With increasing the scan rates from 5 to 100mV/s, all the CV curves maintain the same shape, verifying superior kinetic reversibility. Besides, there is a good linear relationship between the peak current and the scan rate (inset of Fig. 3b), indicating that adsorption and desorption of ions dominate the rate-controlling process [10]. Fig 3c shows the galvanostatic charge/discharge (GCD) curves of Ni2P2O7 and CoP/Ni2P2O7 at the current density of 1A g-1. Obviously, the discharge time of CoP/Ni2P2O7 is much longer than that of Ni2P2O7. The specific capacitance of CoP/Ni2P2O7 is calculated as 1998 F g−1, which surpasses the reported M2P2O7 materials [2,4,5,6]. Fig 3d presents the GCD curves of CoP/Ni2P2O7 at various current densities. The symmetric curve profiles manifest high coulombic efficiency and good reversibility of redox reactions. The specific capacitance of CoP/Ni2P2O7 electrode is calculated as 1998F g−1，1880F g−1, 1698F g−1, 1640F g−1, 1660F g−1, 1440F g−1, 1290F g−1 and 1200F g−1 at a current density of 1, 2, 3, 4, 5, 10, 15 and 20A g−1, respectively. Even at current density of 20A g−1, CoP/Ni2P2O7 electrode still offers 60% of the capacitance retention, suggesting commendable rate capability. Besides, the long-term cycling stability of CoP/Ni2P2O7 electrode was studied at the current density of 3A g-1. After 1000 cycles, the composite electrode can maintain 90% of initial capacitance, suggesting excellent electrochemical stability (Fig 3e). Fig 3f offers the electrochemical impedance spectroscopies (EIS) of Ni2P2O7 and CoP/Ni2P2O7. After fitted by Z-view software using the equivalent circuit model (inset of Fig. 3f), the equivalent series resistance (Rs) values of Ni2P2O7 and CoP/Ni2P2O7 are determined as 0.95 Ω and 6
0.29 Ω, respectively. A small Rs value means a faster charge transfer process in the composites, which can be attributed to the introduction of CoP. In short, the above results display CoP/Ni2P2O7 composites possess excellent electrochemical performance, which can be ascribed to the following reasons: (1) The porous hollow spherical structure facilitates the contact between electrolyte and the electrode material, which is conducive to the ions diffusion. (2) The poor crystallinity endows the composites with abundant active sites, supporting the fast redox reaction [11]. (3) The high electrical conductivity of CoP in the composites improves the electron transfer. 4. Conclusion In summary, porous CoP/Ni2P2O7 hollow nanospheres have been successfully fabricated by a two-step method. The electrochemical results show that the composites possess high specific capacitance, good rate performance and commendable cycle stability, which are ascribed to unique porous hollow nanostructure and the introduction of high conductive CoP. The composites can be a promising electrode material for supercapacitor applications. Acknowledgement This work was supported by Fundamental Research Funds for the Central Universities (2018XKQYMS21). References [1] H. Pang, Z.Z. Yan, W.Q. Wang, Y.Y. Wei, X.X. Li, J. Li, J. Chen, J.S. Zhang, H.H. Zheng, Template-free controlled fabrication of NH4MnPO4 · H2O and Mn2P2O7 micro-nanostructures and study of their electrochemical properties, Int. J. Electrochem. Sci. 7(2012)12340-12353. [2] L.R. Hou, L. Lian, D.K. Li, J.D. Lin, G. Pan, L.H. Zhang, X.G. Zhang, Q.G. Zhang, C.Z. Yuan, 7
Facile synthesis of Co2P2O7 nanorods as a promising pseudocapacitive material towards high-performance electrochemical capacitors, RSC Adv. 3(2013)21558-21562. [3] Z. Khan, B. Senthilkumar, S. Lim, R. Shanker, Y. Kim, H. Ko, Redox-additiveenhanced high capacitance supercapacitors based on Co2P2O7 nanosheets, Adv. Mater. Interfaces, 4(2017) 1700059 [4] C.Z Wei, C Cheng, S.S Wang, Y.Z Xu, J.D Wang, H Pang, Sodium-doped mesoporous Ni2P2O7 hexagonal tablets for high performance flexible all-solid-state hybrid supercapacitors, Chem. Asian. J. 10 (2015)1731-1737. [5] Y J Zhou, C Y Liu, X Li, L L Sun, D Y Wu, J Z Li, P W Huo, H Q Wang, Chemical precipitation synthesis of porous Ni2P2O7 nanowires for supercapacitor, J Alloy Comp, Volume 790(2019)36-41 [6] B. Senthilkumar, Z. Khan, S.Y. Park, K. Kim, H. Ko, Y.S. Kim, Highly porous graphitic carbon and Ni2P2O7 for a high performance aqueous hybrid supercapacitor, J.Mater.Chem. A3(2015)21553– 21561. [7] 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 Net Gas Chem, 26(2017)49–55 [8] H C Chen, S P Jiang, B H Xu, C H Huang, Y Z Hu, Y L Qin, M X He, H J Cao, Sea-urchin-like nickel–cobalt
phosphide/phosphate
composites
as
advanced
battery
materials
for
hybrid
supercapacitors, J. Mater. Chem. A, 7(2019)6241-6249 [9] Q Liu, J Q Tian, W Cui, P Jiang, N Y Cheng, A M Asiri, X P Sun, Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution, Angew. Chem. Int. Ed. 53(2014)6710-6714 [10] Y N Shen, K Zhang, B H Chen, F Yang, K B Xu, X H Lu, Enhancing the electrochemical performance of nickel cobalt sulfides hollow 8
nanospheres by structural modulation for asymmetric supercapacitors, J Colloid Interf Sci, 557(2019)135-143 [11] N Li, Z J Xu, Y F Liu, Z H Hu, Fructose 1,6-bisphosphate trisodium salt as a new organic phosphorus source for synthesis of nanoporous amorphous nickel phosphate microspheres electrode materials in supercapacitors, J Alloy Comp, 789(2019)613-621
Highlights • Porous CoP/Ni2P2O7 hollow nanospheres were prepared. • The composites possess excellent specific capacitance and rate capability. • The materials exhibit high cycling stability.
Conflict of interest
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted。
JiaWei Xu
Author contributions JiaWei Xu: Resources, Original Draft. HeLiang Fan: Methodology, XueYu Tao: Investigation, LiTong Guo: Methodology, Investigation, 9
ZhangSheng Liu: Methodology, Investigation, Writing - Review & Editing, Supervision,
10
S0167-577X(20)30272-X https://doi.org/10.1016/j.matlet.2020.127567 MLBLUE 127567
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
29 January 2020 19 February 2020 25 February 2020
Please cite this article as: J. Xu, H. Fan, X. Tao, L. Guo, Z. Liu, Porous CoP/Ni2P2O7 hollow nanospheres for high-performance supercapacitors, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127567
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.
© 2020 Published by Elsevier B.V.
Porous CoP/Ni2P2O7 hollow nanospheres for high-performance supercapacitors JiaWei Xu, HeLiagn Fan, XueYu Tao, LiTong Guo, ZhangSheng Liu *School of Material Science and Engineering, China University of Mining and Technology, XuZhou，221116, China Abstract In this paper, porous CoP/Ni2P2O7 hollow nanospheres have been synthesized via a two-step method. The obtained nanocomposites display superior specific capacitance (1998F g−1 at a current density of 1A g−1), excellent rate capability and commendable cycle stability (capacity retention of 90% after 1000 cycles), which can be a promising electrode material for supercapacitor. Keywords: Energy storage and conversion; Nanocomposites; Supercapacitors; CoP/Ni2P2O7 1.
Introduction Recently, metal pyrophosphates (M2P2O7, M=Mn2+, Ni2+ and Co2+, etc.) have been investigated as
a new kind of pseudocapacitive material [1-3]. Among them, Ni2P2O7 attracts more and more attention from researchers due to its nontoxicity, cost-effectiveness and high theoretical capacitance. Up to date, many Ni2P2O7 materials with different morphologies have been reported. For example, Wei et al [4] reported Ni2P2O7 hexagonal tablets, whose specific capacitance was 557.7F g-1 at a current density of 1.2A g−1. Zhou et al [5] prepared Ni2P2O7 nanowires by chemical precipitation method, which delivered 772.5F g-1 of specific capacitance at 1A g−1. Senthilkumar et al [6] claimed Ni2P2O7 nanoparticles with a maximum capacitance of 1863F g-1. Nonetheless, limited by the low conductivity, Ni2P2O7 suffers poor cycling stability and unsatisfactory rate capability, which extremely restrict its practical
* Corresponding author (Tel.:+86 516 83591979; fax:+86 51683591870. E-mail: [email protected])
1
applications. Combining Ni2P2O7 with other high conductive active materials may be a good solution to improve the electrochemical performance. Recently, cobalt phosphide (CoP) has been reported as a high conductive active material, in which Co combined by metallic bonds can provide lots of free electrons [7]. In view of the advantage, it is expected that CoP/Ni2P2O7 composites will possess the enhanced electrochemical performance comparing with pure Ni2P2O7. Herein, we have developed CoP/Ni2P2O7 hollow nanospheres via a two-step process of co-precipitation and annealing method. These hollow nanospheres are porous and thin-walled in structure, facilitating the ion diffraction. The composites exhibit superior electrochemical performance, which can be an ideal electrode material for supercapacitor applications. 2.
Experimental Ni2P2O7 was prepared by mixing 2mmol Ni(CH3COO)2·4H2O and 2mmol Na2HPO4·12H2O in
250ml deionized water. After stirred overnight, the precipitate was washed and dried, which was then annealed at 300 °C for 3 h in a nitrogen atmosphere. To fabricate CoP/Ni2P2O7 hollow nanospheres, 0.3mmol of Ni2P2O7 was added into 20 mL ethanol solution containing 0.09mmol CoCl2·6H2O and 0.27mmol NH4HCO3. After stirred for 5h, the obtained precursor was washed and dried. Then, 100mg of the precursor was placed at the downstream side of the furnace next to 20mg NaH2PO2, which was annealed at 300 °C for 3 h in a nitrogen atmosphere. The samples were characterized by X-ray diffraction (XRD, Bruker D8 Advance), X-ray photoelectron spectroscopy (XPS: Thermo ESCALAB250, USA), scanning electron microscopy (SEM, FEI Quanta-250) and transmission electron microscopy (TEM, JEOL-2010). 3.
Results and discussion 2
Fig 1 (a) XRD patterns of samples; XPS spectra of (b) Ni 2p, (c) Co 2p and (d) P 2p for CoP/Ni2P2O7 The phase compositions of the samples were examined by XRD. Similar with the previous reports [5,6], Ni2P2O7 is poor crystallization with weak and broad peaks(Fig 1a), which correspond to the monoclinic phase Ni2P2O7 (PDF#39-0710). After Ni2P2O7 is combined with CoP, there is no obvious change in the XRD pattern, which may result from high dispersity and poor crystallization of CoP. Fig 1b shows the Ni 2p XPS spectrum of CoP/Ni2P2O7. The peaks located at 861.6 eV and 879.6eV correspond to the satellite peaks, while those at 856.2eV and 873.9eV can be assigned to Ni 2p3/2 and Ni 2p1/2. The spin energy separation of 17.7eV indicates that Ni exists in divalent oxidation state. Differently, the Co 2p spectrum can be de-convoluted into two shake-up satellites (790.4 and 805.6 eV) and two spin-orbit doublets (Fig 1c). The peaks at 797.0 and 778.5 eV corresponding to Co 2p1/2 and Co2p3/2 are ascribed to CoP [8], while those at 801.1 and 785.4 eV are related to the chemical states of Co2+, which may arise from superficial oxidation of CoP because of air contact [9]. Besides, in P 2p XPS spectrum (Fig 1d), the peak at 133.2 eV is ascribed to P-O and P=O for Ni2P2O7, while that at 130.3eV is very close to the binding energy of P 2p in CoP. Based on the above results, it is concluded 3
that CoP/Ni2P2O7 has been successfully synthesized.
Fig 2 FESEM image of (a) Ni2P2O7 and (b) CoP/Ni2P2O7; (c) TEM and (d) EDX mapping images of CoP/Ni2P2O7 The microstructures of the samples were characterized by SEM and TEM. As shown in Fig 2a, Ni2P2O7 consists of lots of nanoparticles with high agglomeration, whose sizes vary from 20–50 nm. Interestingly, CoP/Ni2P2O7 composites display a mass of hollow nanospheres, which can be revealed by partially broken nanospheres (Fig 2b). TEM image also verifies hollow spherical nanostructures from the sharp contrast between the center and the edge (Fig 2c). Further observation reveals that these hollow nanospheres are porous and thin-walled with ~3nm of the shell thichness. Besides, the lattice fringe of nanosphere is indistinguishable (inset of Fig. 2c), indicating its poor crystallization nature, which is consistent with XRD results. The possible formation process of hollow nanospheres is proposed as follows: initially, when Ni2P2O7 particles were added into CoCl2·6H2O-contained ethanol 4
solution, they would transform into Ni2P2O7·6H2O by obtaining crystal water; then, the Co species was attached to their surfaces with the help of NH4HCO3, forming a quasi-core-shell structure; finally, porous hollow nanospheres could be formed due to dehydration of core materials and surface energy minimization during the annealing treatment. Fig 2d presents the element mapping of CoP/Ni2P2O7 hollow nanospheres. The Co element shows similar profile to Ni and P, indicating that CoP is uniformly dispersed in CoP/Ni2P2O7, which may help to high conductivity of the composites.
Fig. 3 (a) Comparative CV curves, (c) Comparative GCD curves and (f) Nyquist plots of as-prepared samples; (b) CV curves, (d) GCD curves and (e) Cycling performance of CoP/Ni2P2O7 The electrochemical performance of as-prepared samples was investigated in a three-electrode cell with 2M KOH as an electrolyte. Fig 3a depicts the cyclic voltammograms (CV) curves of Ni2P2O7 and CoP/Ni2P2O7 at the scan rate of 10mV/s. Both samples show a well-defined pair of redox peaks, suggesting their pseudocapacitive behavior. Based on the previous reports, the corresponding faradaic reactions can be described by Eqs (1) and (2) [5]: Ni2P2O7+ 2OH− ↔ Ni2P2O7(OH)2+2 e− 5
(1)
CoP + 2OH− ↔ Co (OH)2 +P+2e−
(2)
Comparatively, CoP/Ni2P2O7 presents a higher anodic and cathodic current density with a larger area enclosed in the CV, meaning that it can store more charges than Ni2P2O7. Fig 3b shows the CV curves of CoP/Ni2P2O7 at different scan rates. With increasing the scan rates from 5 to 100mV/s, all the CV curves maintain the same shape, verifying superior kinetic reversibility. Besides, there is a good linear relationship between the peak current and the scan rate (inset of Fig. 3b), indicating that adsorption and desorption of ions dominate the rate-controlling process [10]. Fig 3c shows the galvanostatic charge/discharge (GCD) curves of Ni2P2O7 and CoP/Ni2P2O7 at the current density of 1A g-1. Obviously, the discharge time of CoP/Ni2P2O7 is much longer than that of Ni2P2O7. The specific capacitance of CoP/Ni2P2O7 is calculated as 1998 F g−1, which surpasses the reported M2P2O7 materials [2,4,5,6]. Fig 3d presents the GCD curves of CoP/Ni2P2O7 at various current densities. The symmetric curve profiles manifest high coulombic efficiency and good reversibility of redox reactions. The specific capacitance of CoP/Ni2P2O7 electrode is calculated as 1998F g−1，1880F g−1, 1698F g−1, 1640F g−1, 1660F g−1, 1440F g−1, 1290F g−1 and 1200F g−1 at a current density of 1, 2, 3, 4, 5, 10, 15 and 20A g−1, respectively. Even at current density of 20A g−1, CoP/Ni2P2O7 electrode still offers 60% of the capacitance retention, suggesting commendable rate capability. Besides, the long-term cycling stability of CoP/Ni2P2O7 electrode was studied at the current density of 3A g-1. After 1000 cycles, the composite electrode can maintain 90% of initial capacitance, suggesting excellent electrochemical stability (Fig 3e). Fig 3f offers the electrochemical impedance spectroscopies (EIS) of Ni2P2O7 and CoP/Ni2P2O7. After fitted by Z-view software using the equivalent circuit model (inset of Fig. 3f), the equivalent series resistance (Rs) values of Ni2P2O7 and CoP/Ni2P2O7 are determined as 0.95 Ω and 6
0.29 Ω, respectively. A small Rs value means a faster charge transfer process in the composites, which can be attributed to the introduction of CoP. In short, the above results display CoP/Ni2P2O7 composites possess excellent electrochemical performance, which can be ascribed to the following reasons: (1) The porous hollow spherical structure facilitates the contact between electrolyte and the electrode material, which is conducive to the ions diffusion. (2) The poor crystallinity endows the composites with abundant active sites, supporting the fast redox reaction [11]. (3) The high electrical conductivity of CoP in the composites improves the electron transfer. 4. Conclusion In summary, porous CoP/Ni2P2O7 hollow nanospheres have been successfully fabricated by a two-step method. The electrochemical results show that the composites possess high specific capacitance, good rate performance and commendable cycle stability, which are ascribed to unique porous hollow nanostructure and the introduction of high conductive CoP. The composites can be a promising electrode material for supercapacitor applications. Acknowledgement This work was supported by Fundamental Research Funds for the Central Universities (2018XKQYMS21). References [1] H. Pang, Z.Z. Yan, W.Q. Wang, Y.Y. Wei, X.X. Li, J. Li, J. Chen, J.S. Zhang, H.H. Zheng, Template-free controlled fabrication of NH4MnPO4 · H2O and Mn2P2O7 micro-nanostructures and study of their electrochemical properties, Int. J. Electrochem. Sci. 7(2012)12340-12353. [2] L.R. Hou, L. Lian, D.K. Li, J.D. Lin, G. Pan, L.H. Zhang, X.G. Zhang, Q.G. Zhang, C.Z. Yuan, 7
Facile synthesis of Co2P2O7 nanorods as a promising pseudocapacitive material towards high-performance electrochemical capacitors, RSC Adv. 3(2013)21558-21562. [3] Z. Khan, B. Senthilkumar, S. Lim, R. Shanker, Y. Kim, H. Ko, Redox-additiveenhanced high capacitance supercapacitors based on Co2P2O7 nanosheets, Adv. Mater. Interfaces, 4(2017) 1700059 [4] C.Z Wei, C Cheng, S.S Wang, Y.Z Xu, J.D Wang, H Pang, Sodium-doped mesoporous Ni2P2O7 hexagonal tablets for high performance flexible all-solid-state hybrid supercapacitors, Chem. Asian. J. 10 (2015)1731-1737. [5] Y J Zhou, C Y Liu, X Li, L L Sun, D Y Wu, J Z Li, P W Huo, H Q Wang, Chemical precipitation synthesis of porous Ni2P2O7 nanowires for supercapacitor, J Alloy Comp, Volume 790(2019)36-41 [6] B. Senthilkumar, Z. Khan, S.Y. Park, K. Kim, H. Ko, Y.S. Kim, Highly porous graphitic carbon and Ni2P2O7 for a high performance aqueous hybrid supercapacitor, J.Mater.Chem. A3(2015)21553– 21561. [7] 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 Net Gas Chem, 26(2017)49–55 [8] H C Chen, S P Jiang, B H Xu, C H Huang, Y Z Hu, Y L Qin, M X He, H J Cao, Sea-urchin-like nickel–cobalt
phosphide/phosphate
composites
as
advanced
battery
materials
for
hybrid
supercapacitors, J. Mater. Chem. A, 7(2019)6241-6249 [9] Q Liu, J Q Tian, W Cui, P Jiang, N Y Cheng, A M Asiri, X P Sun, Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution, Angew. Chem. Int. Ed. 53(2014)6710-6714 [10] Y N Shen, K Zhang, B H Chen, F Yang, K B Xu, X H Lu, Enhancing the electrochemical performance of nickel cobalt sulfides hollow 8
nanospheres by structural modulation for asymmetric supercapacitors, J Colloid Interf Sci, 557(2019)135-143 [11] N Li, Z J Xu, Y F Liu, Z H Hu, Fructose 1,6-bisphosphate trisodium salt as a new organic phosphorus source for synthesis of nanoporous amorphous nickel phosphate microspheres electrode materials in supercapacitors, J Alloy Comp, 789(2019)613-621
Highlights • Porous CoP/Ni2P2O7 hollow nanospheres were prepared. • The composites possess excellent specific capacitance and rate capability. • The materials exhibit high cycling stability.
Conflict of interest
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted。
JiaWei Xu
Author contributions JiaWei Xu: Resources, Original Draft. HeLiang Fan: Methodology, XueYu Tao: Investigation, LiTong Guo: Methodology, Investigation, 9
ZhangSheng Liu: Methodology, Investigation, Writing - Review & Editing, Supervision,
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