Fabrication of Metallic NickelâCobalt Phosphide Hollow Microspheres

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Fabrication of Metallic Nickel-Cobalt Phosphide Hollow Microspheres for High-Rate Supercapacitors Miao Gao, Wei-Kang Wang, Xing Zhang, Jun Jiang, and Han-Qing Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07716 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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The Journal of Physical Chemistry

Fabrication of Metallic Nickel-Cobalt Phosphide Hollow Microspheres for HighRate Supercapacitors

Miao Gao,† Wei-Kang Wang,† Xing Zhang, Jun Jiang, Han-Qing Yu CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied Chemistry, University of Science & Technology of China, Hefei, 230026, China

†These

authors contributed equally to this work.

*Corresponding author: Dr. Jun Jiang, Fax: +86-551-63602449; E-mail: [email protected] Prof. Han-Qing Yu, Fax: +86-551-63601592; E-mail: [email protected]

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ABSTRACT Electrode materials for high-rate supercapacitors are greatly desired. Among actively studied electrode materials, phosphide compounds with metallic features are recognized as such promising supercapacitor materials. In this work, considering the benefits of multimetal redox centers and superior electrical conductivity of ternary nickel-cobalt metal phosphides for improving the supercapacitor performance, urchin-like nickel-cobalt phosphide hollow spheres were prepared with mild hydrothermal method followed by phosphorization. The formation process was explored using time-dependent experiments. The prepared material exhibited an excellent capacity of 761 C g-1 at the current density of 1 A g-1 with 693 C g-1 remaining even at 20 A g-1, which exhibited a high-rate capability with approximately 91.1% retention of its initial capacity. Moreover, the asymmetric supercapacitor assembled with the prepared NiCoP hollow sphere and activated carbon exhibited a maximum energy density of 35.6 W h kg-1 and maximum power density of 8387 W kg-1. Such a high performance of the as-prepared material was originated from the combination of superior electrical conductivity, synergistic effect in Ni and Co and effective urchin-like hollow structure. Hence, the as-prepared ternary nickel-cobalt metal phosphide could be applied as an effective supercapacitor electrode material with a high power density and long-term stability for practical applications.

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INTRODUCTION

Supercapacitors, one of effective and practical technologies for energy storage, have attracted worldwide interests.1-2 The capacitance value of a pseudocapacitor, especially that of intercalation pseudocapacitor is usually much larger than double layer capacitance, thus appealing more and more widespread explorations.3 However, for practical applications of pseudocapacitors, exploration of high-rate performance and stable electrode materials is the main challenge. Currently, although various materials were synthesized and used for pseudocapacitor applications, effective electrode materials that possess high capacitance considering the charge storage mechanism were still desired. To date, RuO2 is regarded as an excellent electrode material with high theoretical capacitance value. Nevertheless, RuO2 is improper for practical application due to its high cost and low reserve.4 Considering this issue, much effort has been made to substitute RuO2 with noble-metal-free materials, including metal oxides,5-7 metal hydroxides,8,9 and metal sulfides.10,11 In addition, among many preparative methods aiming at improving performance, it is a useful strategy to transform the materials of use from a usual consecutively bulk structure to a nanostructured hollow structure,12 and the hollow phase has more active sites.13 Some hollow structure materials including Co3O4,14 NiO,15 and NiCo2S413 have shown excellent pseudocapacity performance. Nevertheless, transition metal phosphides with the superior electrical conductivity and 3



metalloid characteristics16,17 are more kinetically favorable for rapid electron transport compared with these metal oxides or hydroxides, and this feature is essential for high-rate and high-power density supercapacitors.18 Additionally, those hollow materials assembled from low-dimensional (1D) subunits, might possess additional advantages in comparison with the traditional hollow structure made up with 0-D nanoparticles, but it is more challenging to be synthesized. Recently, phosphides have gradually become a developing type of material for efficient supercapacitor electrode because of rich valences as well as superior electrical conductivity.16 In particular, ternary metal phosphides could offer a higher electrochemical activity than that of mono-metal phosphides.19-21 The configuration of transition ternary metal phosphide structures reported previously is usually simple and intricate hollow structure of ternary metal phosphides is more difficult to synthesize than mono-metal oxides and phosphides. Here, a novel urchin-like ternary nickel-cobalt phosphide hollow sphere was successfully prepared using an easy and facile hydrothermal method followed by phosphorization. Obtained product was characterized and their formation mechanism was also explored. Furthermore, the capacity and stability of NiCoP as a supercapacitor electrode material were evaluated using a suite of electrochemical measurements, and the mechanisms for its high-rate performance was elucidated. As a result, an efficient and costeffective supercapacitor electrode material became available.

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EXPERIMENTAL SECTION Preparation of Nickel-Cobalt Carbonate Hydroxide (NiCoCH) Precursor. Ni(NO3)2·6H2O (0.437 g), Co(NO3)2·6H2O (0.436 g) and urea (0.72 g) were mixed and dissolved in deionized water (17.0 mL) and ethanol (3.0 mL). Such preceding solution was under 20-min magnetic stirring before put into a stainless steel autoclave with Teflon-lined (50 mL), which was heated to 120 oC and kept at this temperature for 12 h in an oven. Preparation of NiCoP. NiCoP was synthesized in a corundum crucible at the tube furnace.22 NaH2PO2 powder was put in upward side of the corundum crucible, while NiCoCH at opposite end. The mass ratio of NiCoCH and NaH2PO2 was 1:10. After passing over argon for 30 min, the samples were heated to 300 oC at a heating rate of 2 oC min-1 and maintained at this temperature for 2 h in a static Ar ambience. Finally, black NiCoP powders were obtained after the furnace cooled down. Preparation of Ni5P4 and CoP. For Ni5P4, Ni(NO3)2·6H2O (0.437 g) and Co(NO3)2·6H2O (0.436 g) were replaced with an equal amount of Ni(NO3)2·6H2O (0.873 g) to prepare Ni-precursor. Co-precursor was similarly prepared with Co(NO3)2·6H2O. Then, the phosphorization was performed, just like the synthesis of NiCoP. Physicochemical Characterization. In the supporting information. Electrochemical

Measurements.

The

electrochemical

performance

for

supercapacitor was determined by the three-electrode system with a CHI 660C electrochemical workstation (Chenhua Instrument Co., China). The working electrode was prepared by mixing NiCoP with polyvinylidene fluoride (PVDF) and acetylene black 5



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according to a mass ratio of 8:1:1 in N-methyl-2-pyrrolidinone. After blending, these mixtures were coated onto a clean nickel foam with effective geometric area to be 1×1 cm2 and then dried under a vacuum for 12 h at 60 oC. The mass load for NiCoP was calculated to be around 1.0 mg cm-2. Reference electrode was a saturated Ag/AgCl and counter electrode was a Pt wire. Electrochemical impedance spectroscopy (EIS) was tested in a frequency range from 1 Hz to 100 kHz. The electrolyte was 1.0 M KOH. The asymmetric supercapacitor was packaged with an active carbon (AC) for negative electrode and NiCoP for positive electrode. NiCoP and AC were mixed with polyvinylidene fluoride (PVDF) and acetylene black in N-methyl-2-pyrrolidinone respectively. Next, they were separately coated onto a clean Ni foam and then dried under a vacuum for 12 h at 60 °C. Separated by one piece of cellulose paper separator, these two electrodes were pressed together and tested 1.0 M KOH. The current densities here were calculated on account of the total weight of materials in the positive and negative electrodes. The mass ratio between the positive (NiCoP) and negative electrode materials (AC) is calculated according to the following equation: m+/m- = C-V-/C+V+. The calculated mass ratio of NiCoP and AC was 1:7 (Figure S11), and that mass load for NiCoP and AC was 1.0 mg and 7.0 mg, respectively. In three-electrode system, the specific capacity C was obtained from the integration of the galvanostatic discharge curves on account of the equation (1):23,24

C

2im  Vdt V

6


(1)

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whereas the specific capacity (Ccell), energy density E and power density P were calculated on account of the equation (2-4) for two-electrode system:23,24 Ccell 

2iM  Vdt V

(2)

E  iM  Vdt

(3)

E t

(4)

P

where im (A g-1) and iM (A g-1) are the current density on account of the effective mass loading of products on the working electrode and the total mass loading on two electrodes, respectively, and t (s) and V (V) are the discharge time and potential, respectively.

RESULTS AND DISCUSSION

Characterization and Growth Mechanism. The urchin-like ternary NiCoP hollow spheres were successfully converted from hydrothermally formed NiCoCH precursor as shown in Figure 1a. Compared to the colloidal method25 and hydrothermal method for preparing phosphides, our method could synthesize materials with complex morphology. Figure 1b shows the power XRD patterns of NiCoCH and NiCoP. The NiCoCH retained the orthorhombic carbonate-hydroxide structure and was identified to be NixCo2xCO3(OH)2

compared with Co2CO3(OH)2 (JCPDS No. 48-0083). To further examine the

Ni/Co ratio in the precursor, ICP-AES analysis was also performed and the atom Ni/Co ratio was found to be approximately 1:1. Moreover, the power XRD patterns clearly 7



suggest that NiCoCH was converted into hexagonal NiCoP (JCPDS No. 71-2336) during phosphorization process, further confirming the effectiveness of the NaH2PO2 treatment for converting carbonated hydroxides to phosphides. The SEM images for NiCoCH (Figure 1c and S1a) reveal that NiCoCH was composed of microspheres assembled with individual nanorods. In addition, the TEM image (Figure 1d) verifies its hollow configuration, which was completely kept after phosphorization. The NiCoP exhibited hierarchical structures with nanorods on hollow microspheres (Figure 1e and Figure S1b,). Such a highly open structure could combine the advantages of 1D nanoarchitecture26 and hollow structure, which is found to shorten the ion diffusion length as well as offer plenty of specific surface areas for charge storage for a supercapacitor.27 In Figure 1g, the EDS mapping further confirmed the existence of P, Co and Ni, which were uniformly distributed over the entire sphere. The investigation of formation mechanism for the urchin-like NiCoCH hollow microspheres were carried out via the time-dependent experiments. The SEM and TEM images (Figure 2) of the products were gained at a series of reaction duration. Both nanosheets and nanoparticles were formed in solution at the initial reaction stage (0.5 h) (Figure 2a and d). After 1-h reaction (Figure 2b and e), the nanosheets and nanoparticles gathered to generate microspheres. The microspheres contained a large portion of small nanosheets on the surface. After hydrothermal treatments for 2h, the well-defined microspheres with hierarchical shells started forming (Figure 2c and f). The template effect of urea could be one of reasons for the formation of the urchin-like structure. The gas 8


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bubble templating mechanism of urea has been widely recognized28 and our experimental results show that the urchin-like structure could not be formed with little urea (Figure S2a). Also, the amount of broken spheres increased when excess urea was present (Figure S2b). The TEM images of an individual nanorod are shown in Figure 3a (inset) and b. In HRTEM image of NiCoP (Figure 3c), crystalline NiCoP nanoparticles were surrounded by amorphous shell, suggesting that the NiCoP was partially oxidized to form phosphates.29 This could be used to explain the peak of PO43- detected by XPS (Figure 3f). The lattice fringes with an interplanar lattice spacing of 0.22 nm matched well with the (111) atomic planes of NiCoP phase. The polycrystalline nature of NiCoP was further evidenced via its corresponding SAED pattern (inset in Figure 3b). Such an amorphous incorporated in crystalline component was conducive to protecting the NiCoP crystalline from suffering damage in process of charge and discharge.30 For comparison, mono-metal phosphides were also synthesized with a similar way and the XRD patterns of Ni5P4, CoP, Ni-precursor and Co-precursor were illustrated in Figure S3.31,32 In the phosphorization process, the effect of different amounts of NaH2PO2 on products was examined. The resulting SEM images and XRD patterns (Figure S4 and S5) show that the presence of excess NaH2PO2 was not advantageous to keeping morphology because of the high reaction rate, while NiCoCH could not be completely converted into NiCoP when no sufficient NaH2PO2 was present. The surface chemical states and compositions for NiCoP were explored by the XPS analysis. In Figure 3d, two peaks at 855.7 and 861.6 eV were related to Ni-O and the 9



satellite peak, respectively, in the Ni 2p3/2 spectrum of NiCoCH. As for the spectrum of NiCoP, peaks at 852.7, 856.4 and 860.7 eV were assigned to the Ni-P, Ni-POx and its satellite peak, respectively.19,20,29 Likewise, a new peak emerged at 778.3 eV in the Co 2p3/2 spectrum (Figure 3e) for NiCoP in comparison with that of NiCoCH because of the formation of Co-P. The peak at 782.3 eV could be associated with Co-POx.19,20 The peak at 129.0 eV agreed well with P 2p from NiCoP in P 2p spectrum (Figure 3f),19,20,29 while at 133.5 eV might be attributed to PO43- or P2O5 caused by the oxidation of phosphide particles, consistent with the HRTEM result.33 The stoichiometric formula for nickel-cobalt phosphide was NiCoP1.3 on account of XPS results, near that of NiCoP. Electrochemical Properties. To evaluate the electrochemical properties of NiCoP, galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) tests were carried out. NiCoCH, Ni5P4, CoP and nickel foam electrodes were also measured following the same condition. The CV curves for NiCoP, NiCoCH, Ni5P4 and CoP electrodes in Figure 4a were tested at a scan rate of 5 mV s−1 in a potential range of 0 to +0.5 V vs. Ag/AgCl. Characteristic reduction and oxidation peaks in each curve were ascribed to the different redox reactions during GCD process.18,34 The CV integral area was in the order of CoP < Ni5P4 < NiCoCH < NiCoP, which demonstrated the best electrochemical performance of the NiCoP electrode in comparison with CoP, Ni5P4 and NiCoCH, indicating an important role of phosphorization treatment in electrochemical performance. The GCD curves of the NiCoP, NiCoCH, Ni5P4, CoP and Ni foam in Figure 4b were depicted. Clearly, the nonlinear GCD curves of samples at 1 A g-1 illuminated their faradaic behaviors.4 The 10


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calculated capacity of NiCoP (761 C g-1) was better than that of NiCoCH (288 C g-1), Ni5P4 (247 C g-1) and CoP (182 C g-1), consistent with the CV results. This finding also supported the synergistic effect in Co and Ni for NiCoP. Moreover, nickel foam alone exhibited a limited capacity activity (21 C g-1), indicating that the good capacity performance was attributed to NiCoP, rather than the Ni foam. Figure 4c displays the GCD curves of NiCoP measured from 1 to 20 A g-1, which were approximately symmetric at any current densities, indicating that NiCoP had reversible redox reactions and fine electrochemical capacity characteristics.35 NiCoP electrode delivered capacity values of 761, 760, 757, 753, 752, 751, 733 and 693 C g-1 at current densities of 1, 2, 3, 5, 8, 10, 15 and 20 A g-1, respectively. The calculated specific capacity and coulombic efficiency were summarized in Figure 4d. It is worth noting that when the current density increased from 1 to 20 A g-1, approximately 91.1% of the capacity was retained, and such a high-rate capacity performance made difference in practical application. The coulombic efficiencies of NiCoP were all above 89% at different current densities. Its good capacity performance is better than that of many previously reported Ni, Co-based materials (see Table S1 in Supporting Information for comparison). Figure 5a displays the CV curves for NiCoP electrode from 5 to 100 mV s-1. Its CV integral area at different scan rates increased gradually with the increasing scan rate. In addition, the reductive peak shifted to more negative potential and the oxidative peak to more positive potential with a higher scan rate, caused by the increased internal diffusion resistance with higher scan rates.36 Even at high scan rates the redox peaks were still 11



obvious, suggesting the good capacitive behavior. With the above CV curves, separate contributions of diffusion and capacitive processes were estimated. According to the equation i = aνb, the diffusion-controlled process was involved in the charge storage when the b values equal to 0.5, and that capacitive process occurs when the b values equal to 1. After calculated on account of the anodic and cathodic currents of the CV curves at different scan rates, the b values were plotted against the potential in Figure 5b, which were near 0.5 close to the peak potentials, illustrating that at these potentials diffusion-controlled process was mainly involved. The separate contributions between capacitive process and diffusive process were further estimated by adopting the Trasatti's method. In this method, the diffusion law q (v) = qc + k × v-1/2 was applied to estimate that total charge storage of NiCoP, where qc is the charge during capacitive process and during diffusion process its charge is expressed by k × v-1/2. When v → ∞, q (v) → qc, indicating that the charge storage was totally dominated by capacitive process in this condition. The calculated capacitive charge was 129.3 C g-1 by plotting capacity as a function of v-1/2 (Figure 5c). Figure 5d shows the detailed contribution of capacitive process and diffusive process at different scan rates. The diffusion capacity contributed approximately 77% of the total capacity at a low scan rate of 10 mV s-1, while the diffusion capacity accounted for 45% at 100 mV s-1. All specific capacity of NiCoP, NiCoCH, Ni5P4 and CoP (Figure S6) decreased with the increasing current density. This was because the redox transition can’t be sustained entirely with the inner active sites that at higher current densities.4 Importantly, all specific capacities of NiCoP were larger than that of NiCoCH, Ni5P4 and CoP electrodes under any 12


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current densities. In Figure 6a, the cycling life for NiCoP electrode was evaluated by 5000-cycle repetitive GCD test at 10 A g-1, and the product exhibited a good cycling stability with 80.0% capacity retention. Furthermore, the morphology of the NiCoP electrode after cycling test didn’t change substantially compared with that before cycling (Figure S8a) despite the partial destruction on the sphere surface. Moreover, the XPS spectra of NiCoP electrode after 5000 cycles (Figure S8b-d) were similar to those in a fresh electrode except the peaks for Ni-P and Co-P became weaker because of surface oxidation during charging and discharging process. Surface oxidation of NiCoP is beneficial for its stability via protecting it from further corrosion. The electrode kinetics of NiCoP and NiCoCH were investigated by EIS, and the same equivalent circuits were fitted. The electron transfer kinetics of the redox reaction at the electrode interface were governed by the electron transfer resistance (Rct), which was reflected by the arc diameter of the Nyquist plots. As shown in Figure 6b, the NiCoP electrode exhibited smaller arc, indicating its lower electron transfer resistance than that of NiCoCH. The equivalent series resistances (Rs) of NiCoP and NiCoCH were estimated to be 1.0 and 1.1 Ω, respectively, confirming a higher conductivity of NiCoP than that of NiCoCH. These results agree well with their GCD and CV behaviors. BET surface-areas of NiCoCH and NiCoP were 106.7 and 96.1 m2 g-1, respectively (Figure 6c). These values were close, but the specific capacity of NiCoP was more than two times higher than that of NiCoCH. Thus, the synthesized NiCoP had more active sites than NiCoCH with a similar level of specific surface area. 13



Practical Asymmetric Supercapacitor. The asymmetric supercapacitor was packaged for practical application in order to further assess the NiCoP electrode (Figure 7a), and the electrochemical properties of AC were measured (Figure S10). To gain proper operating potential window, CV curves (Figure 7b) at 50 mV s-1 with different potential windows were measured. Evolution of oxygen occurred clearly when the potential of 1.5 V was extended. Thus, 1.4 V was applied as the operating potential. As shown in Figure 7c, CV curves were tested from 10 to 50 mV s-1, and their current densities increased with larger scan rates, similar to these in the three-electrode system. The quasi-lined GCD curves at different current densities of the asymmetric supercapacitor (Figure 7d) confirmed the combination of electric double layer and faradic characteristics of the NiCoP//AC supercapacitor.37 Calculated from the GCD curves at different current densities, the specific capacities and coulombic efficiencies were shown in Figure 7e. Their specific capacities for NiCoP//AC supercapacitor at 2, 3, 5, 8 and 10 A g-1 were 183, 171, 159, 155 and 149 C g-1, respectively. Furthermore, high coulombic efficiencies with all above 80% at various current densities were exhibited for this device. Figure 7f shows the Ragone plots of the asymmetric supercapacitor, and the energy density of 35.6 W h kg-1 was obtained with the power density of 1823 W kg -1 and still reached 28.9 W h kg-1 even when the power density increased to 8387 W kg -1, which were comparable to those of Ni, Cobased electrode materials.14,24,29,38,39 The above results and analyses show that the high performance of NiCoP for supercapacitor is attributed to these unique factors. Firstly, the superior electrical 14


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conductivity of the transition metal phosphide could facilitate electron transfer at the electrode/electrolyte interfaces and within the electrode, endowing its high-rate and highpower performance.29,40 Secondly, the synergistic effect in Co and Ni promoted its electrochemical performance.37,41 The NiCoP offered plentiful active sites for charge storage reactions.29 In addition, the specific structure consisting of nanorods and hollow structure could promote electrolyte diffusion.

CONCLUSIONS

The urchin-like ternary nickel-cobalt phosphide hollow spheres were successfully prepared via a facile hydrothermal method followed by phosphorization and used as electrode material for supercapacitor. The NiCoP electrode displayed a remarkable capacity of 761 C g-1 at a current density of 1 A g-1, which was nearly 2.6 times higher than that of NiCoCH (288 C g-1). Also, the capacity for NiCoP was larger than its mono-metal phosphides. More importantly, the obtained NiCoP showed excellent high-rate performance as it retained a high specific capacity of 693 C g-1 even at 20 A g-1, approximately 91.1% of its capacity at 1 A g-1. The NiCoP//AC asymmetric supercapacitor exhibited a high energy density of 35.6 W h kg-1 and a high power density of 8387 W kg-1 in aqueous electrolytes. These results demonstrate that the urchin-like NiCoP is a hopeful material for supercapacitor electrode.

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SUPPORTING INFORMATION AVAILABLE SEM images of NiCoCH and NiCoP spheres (Figure S1), SEM images of the products obtained with different amounts of urea (Figure S2), XRD patterns of Ni-precursor, Coprecursor, Ni5P4 and CoP (Figure S3), SEM images of the products gained with different dosage of NaH2PO2 (Figure S4), XRD patterns for materials gained with different dosage of NaH2PO2 (Figure S5), specific capacity of NiCoCH, Ni5P4 and CoP electrodes (Figure S6), volume and areal capacities of NiCoP, NiCoCH, Ni5P4 and CoP (Figure S7), SEM image and XPS spectra of NiCoP after 5000 cycles (Figure S8), volume and areal capacities of NiCoP//AC hybrid supercapacitors (Figure S9), electrochemical performance of AC (Figure S10), CVs of AC and NiCoP electrodes (Figure S11), and cycling stability for NiCoP at 10 A g-1 for 10000 cycles (Figure S12). This information is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21590812, 21707135, 51538011 and 51821006) and the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Ministry of Education of China for supporting this work.

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Figure Legends

Figure 1. (a) Schematic illustration of the synthetic routes of the urchin-like NiCoP hollow spheres. (b) Power XRD patterns of NiCoCH and NiCoP. (c-d) SEM image and TEM image of NiCoCH. (e-f) SEM image and TEM image of NiCoP. (g) HAADF-STEM image and corresponding EDS mapping images of NiCoP. Figure 2. SEM (a-c) and TEM (d-f) images of products obtained after different reaction durations: (a, d) 0.5 h, (b, e) 1 h, and (c, f) 2 h. Figure 3. (a, b) TEM image of nanorods on the surface of spheres, and (c) the corresponding high-resolution TEM images of the area marked in (b). The inset of (b) is the SAED pattern. (d) Ni 2p3/2 spectra, and (e) Co 2p3/2 spectra of NiCoCH and NiCoP. (f) P 2p spectrum of NiCoP. Figure 4. (a) CV curves at a scan rate of 5 mV s-1, and (b) GCD curves of NiCoP, NiCoCH, Ni5P4, CoP and Ni foam electrodes at a current density of 1 A g-1. (c) GCD curves of NiCoP electrode at different current densities. (d) Specific capacities and coulombic efficiencies at different current densities calculated by the GCD curves in (c). Figure 5. (a) CV curves of NiCoP electrode at different scan rates. (b) Plot of b-values calculated from the cathodic and anodic currents of the CV curves at different scans rates, (c) the plot of capacity (q) against v-1/2, and (d) the contribution of capacitive and diffusion-controlled processes towards the total capacity of NiCoP at different scan rates. Figure 6. (a) Cycling stability of NiCoP electrode at a current density of 10 A g-1. (b) Nyquist plots of NiCoP and NiCoCH electrodes. (c) N2 sorption isothermals and the corresponding pore size distribution curve (insert) of NiCoP and NiCoCH electrodes. Figure 7. (a) Schematic diagram of NiCoP//AC asymmetric supercapacitor device. (b) CV curves measured at different potential range. (c) CV curves measured at different scan rates. (d) GCD curves at different current densities. (e) Specific capacities 24


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and coulombic efficiencies at different current densities calculated by the GCD curves in (d). (d) Ragone plot of the device.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Table of Contents

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Biography of Jun Jiang

Dr. Jun Jiang obtained his Ph.D. from Department of Chemistry at University of Science and Technology of China (USTC), China in 2015. Then, he moved to the CAS Key Laboratory of Urban Pollutant Conversion in the same university as a postdoctoral fellow. Early in 2018, Dr. Jiang was offered a faculty position and started to work in the School of Metallurgy and Environment at Central South University as an Associate Professor. His current research work focuses on the synthesis of functional materials and their applications for energy conversion and storage, pollution control and environmental remediation.

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Fabrication of Metallic NickelâCobalt Phosphide Hollow Microspheres

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