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DOI: 10.1039/C4RA05486E (Paper) RSC Adv., 2014, 4, 41772-41777

Nickel vanadate and nickel oxide nanohybrid on nickel foam as pseudocapacitive electrodes for electrochemical capacitors

Wei-Bin Zhanga, Ling-Bin Kong*ab, Xue-Jing Maa, Yong-Chun Luob and Long Kangb
aState Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou, 730050, P. R. China. E-mail: konglb@lut.cn; Fax: +86-931-2976578; Tel: +86-931-2976579
bSchool of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, 730050, P. R. China

Received 9th June 2014 , Accepted 11th August 2014

First published on 11th August 2014

Abstract

A novel self-supported electrode of nickel vanadate and nickel oxide nanohybrid on nickel foam with excellent pseudocapacitive properties was synthesized using a combination of a hydrothermal strategy and subsequent annealing treatment. The porous nanostructure not only provides a larger surface area for faradic reactions, but also allows the rapid transportation of electrolyte ions for improving rate capability. The electrode demonstrates outstanding capacitance, satisfying rate capability and good cycling stability, showing the coupling effects of nickel vanadate and nickel oxide. In this case, the electrode has an energy density of 46 W h kg−1 at a power density of 101 W kg−1, demonstrating the importance and great potential of nickel vanadate in the development of energy storage systems.

1 Introduction

Most of electrochemical capacitors (ECs), also named supercapacitors, use carbon or carbon-based materials as electrode materials or electrodes directly and store energy by a charge separation at the electrode/electrolyte interface.1 Only 5–10% or even less of the capacitance is due to surface redox reactions on carbon-based materials, and this class of ECs are named electrochemical double layer capacitors. The capacity of such ECs is limited to about 250 F g−1 in an aqueous electrolyte for high surface area carbon-based materials.2 To overcome this limitation, new types of ECs named pseudocapacitors have been developed.3,4 In this case, electrodes consist of metal oxides, hydroxides or inorganic salts, such as manganese dioxide,5–8 cobalt hydroxide9–12 or nickel molbdate,13–15 and their high capacitance is due to the combination of minor double layer capacitance and major faradaic surface reactions. A tremendous amount of work has been developed using ruthenium dioxide electrodes,16–18 owing to their perfect pseudocapacitive properties. Moreover, the main conclusions about the others pseudocapacitive materials are the lack of capacitance and electronic conductivity compared to ruthenium dioxide, which severely limits the energy density of the electrodes.

Recently, we reported a new class of pseudocapacitive materials, namely, metal vanadates, such as nickel vanadate, which demonstrated quite interesting pseudocapacitive properties in an alkaline electrolyte.19 Detailed studies on such pseudocapacitive materials indicate that an element having a low oxidation state plays a significant role in the generation of pseudocapacitance, while an element having a high oxidation state has a minor influence on the pseudocapacitance. Normally, the molar ratio between the nickel (low oxidation state) ion and vanadate (high oxidation state) acid radical in nickel vanadate is 3[thin space (1/6-em)]:[thin space (1/6-em)]2, which indicates that there is stronger redoxomorphism based on the nickel ion, possessing higher pseudocapacitance.20 It is a pity that such type of pseudocapacitive materials also suffer from a lack of electronic conductivity. Despite all this, the good news is that there is a higher electronic conductivity on nickel oxide (0.01–0.32 S cm−1)21 compared to manganese dioxide (10−5–10−6 S cm−1),22 tricobalt tetroxide (10−4–10−2 S cm−1),23 or vanadium pentoxide (10−4–10−2 S cm−1).24 All of these inspire us to develop a higher capacitance and higher electronic conductivity composite, and nickel vanadate and nickel oxide composite is exploited.

In fact, a nickel vanadate and nickel oxide nanohybrid on nickel foam as a pseudocapacitive electrode exhibits excellent pseudocapacitive properties and good charge transfer. The nanohybrid couples the advantages of both the electronic conductivity of nickel oxide and the high capacitances of nickel vanadate, showing attractive electrochemical properties as electrode materials for pseudocapacitors. The findings also demonstrate the importance and great potential of metal vanadates in the development of energy storage systems.

2 Experimental

2.1 Synthesis of the electrode

Analytical grade Ni(NO3)2·6H2O, CO(NH2)2, NiCl2·6H2O and Na3VO4·12H2O were purchased from Sinopharm® Chemical Reagent Co. Ltd.

The nanohybrid electrode was prepared by a three-step process. First of all, the precursor was synthesized on nickel foam by a hydrothermal process. For this purpose, 0.63 mmol of Ni(NO3)2·6H2O and 3.12 mmol of CO(NH2)2 were dissolved in 50 mL of deionized water, and then the obtained homogeneous solution was transferred into a Teflon-lined stainless steel autoclave containing a piece of clean nickel foam (1 cm × 1 cm × 0.01 cm) at 120 °C for 4 h growth. Secondly, the nickel foam with the as-grown precursor was annealed at 300 °C for 4 h to obtain the porous NiO. Finally, 1.69 mmol of NiCl2·6H2O and 1.12 mmol of Na3VO4·12H2O were dissolved in 50 mL of deionized water, and then the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave containing the substrate of process two at 100 °C for 3 h. After that, the samples were dried at 60 °C for 8 h to obtain the Ni3(VO4)2&NiO nanohybrid electrode.

In addition, a series of samples with different reactant molar ratios were also prepared to discuss their performance as electrodes. For this purpose, Ni(NO3)2·6H2O was set at a constant value and the quantity of NiCl2·6H2O gradually increased, and the reactants molar ratios were obtained by calculations.

2.2 Structural characterization

The crystallite structure was determined by X-ray diffraction (XRD) using a Rigaku® D/MAX 2400 diffractometer with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 60 mA. The microstructure and morphology were characterized by a field emission JEOL® JSM-6701F scanning electron microscope (SEM) and JEOL® JEM-2010 transmission electron microscope (TEM). The porous properties, including the BET surface area and pore size distribution, were investigated volumetrically by ASAP® 2020 nitrogen adsorption/desorption experiments.

2.3 Electrochemical measurements

Electrochemical measurements were carried out using a Chenhua® CHI660C electrochemical working station in a three-electrode cell at room temperature. A platinum electrode and a saturated calomel electrode (SCE) served as the counter and reference electrodes, respectively. The electrolyte was a 2 mol L−1 aqueous solution of KOH.

3 Results and discussion

3.1 Structure characterization

The crystallographic phase was determined by XRD. Fig. 1a shows the XRD patterns of Ni3(VO4)2&NiO, Ni3(VO4)2, and NiO. In order to reduce the strong impact of the nickel foam substrate on the XRD peak signals, the Ni3(VO4)2&NiO powder was scratched from the nickel foam for XRD analysis. As revealed in Fig. 1a, NiO peaks of (101), (012), (110), (113) and (202) are observed in the XRD pattern, in accordance with those reported (JCPDF card no. 44-1159), revealing the existence of a spinel NiO phase. Ni3(VO4)2 peaks of (122), (042) and (442) were also observed in the XRD pattern, in accordance with those reported (JCPDF card no. 74-1485), revealing the existence of a Ni3(VO4)2 phase. Moreover, the poor crystallization of the Ni3(VO4)2&NiO was evidenced by the significant reduced intensity of the diffraction peaks in the composite pattern, which is favorable for the composite to exhibit a high capacitive performance since a material having poor crystallinity may results in more transportation channels than a highly crystalline one.25 In addition, energy dispersive spectroscopy (EDS) microanalysis shown in Fig. 1b reveals that the nanohybrid consists of Ni, V and O, which suggests that the composite was made of NiO and Ni3(VO4)2, which is consistent with the XRD analysis.
image file: c4ra05486e-f1.tif
Fig. 1 (a) XRD patterns of Ni3(VO4)2&NiO, Ni3(VO4)2, and NiO; (b) EDS pattern of Ni3(VO4)2&NiO.

The morphology and nanostructure are represented by SEM and TEM. Fig. 2a and b show the SEM images of NiO and Ni3(VO4)2, respectively. Both exhibit two-dimensional flake-like morphology, but the size of the NiO nanoflakes are larger than that of the Ni3(VO4)2 nanoflakes, which indicates that the NiO nanoflakes are suitable to serve as the forerunner skeleton for the construction of the Ni3(VO4)2&NiO nanohybrid. Fig. 2c and d show the SEM images of the Ni3(VO4)2&NiO nanohybrid. The Ni3(VO4)2 nanoflakes are grown staggered or inlaid around the NiO nanoflakes, forming a three-dimensional nanohybrid and leaving large numbers of pores or ditches between the nanoflakes, which can be further confirmed by the result of the TEM image shown in Fig. 2e. The porous structure of the nanohybrid not only provides a larger surface area for faradic reactions, but also allows rapid transportation of electrolyte ions, which is helpful for improving the rate capability.26 The selected area electron diffraction (SAED) image shown in Fig. 2f reveals a polycrystal of nanohybrid, which means that there is a larger proportion of crystal boundary than single crystal, signifying easy proliferation and migration of matter.27


image file: c4ra05486e-f2.tif
Fig. 2 (a) SEM image of NiO; (b), SEM image of Ni3(VO4)2; (c and d) SEM images of Ni3(VO4)2&NiO; (e) TEM image of Ni3(VO4)2&NiO; (f) SAED pattern of Ni3(VO4)2&NiO; (g) nitrogen adsorption/desorption isotherms; (h) pore size distribution; (i) high-resolution TEM image of Ni3(VO4)2&NiO.

The porous characteristics of the Ni3(VO4)2&NiO nanohybrid were further investigated by nitrogen adsorption/desorption experiments. Fig. 2g and h show the nitrogen adsorption/desorption isotherms and corresponding BJH pore size distributions. As seen from Fig. 2g, the existence of the hysteresis loops indicates the porous structure of the Ni3(VO4)2&NiO nanohybrid. In Fig. 2h, the Ni3(VO4)2&NiO nanohybrid exhibits a pore-size distribution at about 2 nm, suggesting a porous structure, which is favorable to exhibit a high capacitive performance. The above description of morphology and structure is also consistent with the high-resolution TEM image shown in Fig. 2i.

3.2 Pseudocapacitive properties

Fig. 3a shows the cyclic voltammetry curves of the Ni3(VO4)2&NiO nanohybrid electrode at different scan rates within a potential range of −0.2–0.6 V. A pair of redox peaks are visible in each of the curves, suggesting that the measured capacitance is mainly based on the redox mechanism, which corresponds to the reversible reactions of Ni2+/Ni3+. The faradaic reactions corresponding to the redox peaks are as follows:15
 
Ni2+ ↔ Ni3+ + e− (1)
image file: c4ra05486e-f3.tif
Fig. 3 (a) Cyclic voltammetry curves of Ni3(VO4)2&NiO; (b) charge/discharge curves of Ni3(VO4)2&NiO; (c) specific capacitances of Ni3(VO4)2&NiO at controlled current densities; (d) cycling performance of Ni3(VO4)2&NiO.

The peak current increases linearly with increments in the scan rate, which indicates that the kinetics of the interfacial faradaic redox reactions and the rates of electronic and ionic transport are rapid enough at the present scan rates.26 The cyclic voltammetry curves retain their original shape with increasing scan rates, which demonstrates the favourable electron and ionic conduction.

To understand the rate capability and calculate the specific capacitance of the Ni3(VO4)2&NiO nanohybrid electrode, the charge/discharge measurements performed at various current densities are shown in Fig. 3b. The specific capacitance of the Ni3(VO4)2&NiO nanohybrid electrode was calculated from the discharge time Δt, and discharge current I according to eqn (2) shown below:2

 
image file: c4ra05486e-t1.tif(2)

The specific capacitance shown in Fig. 3c are 2068 F g−1, 1973 F g−1, 1835 F g−1, 1690 F g−1 and 1540 F g−1 correspond to the discharge current density of 0.5 A g−1, 1 A g−1, 2 A g−1, 4 A g−1 and 8 A g−1, respectively. The specific capacitance gradually decreased at higher current density due to the incremental voltage drop and insufficient active material involved in the redox reaction at a higher current density. However, the as-fabricated Ni3(VO4)2&NiO possesses a good rate capability. Even if at a high current density of 8 A g−1, nearly 74.4% of the initial capacitance value remains.

The cycle stability of the Ni3(VO4)2&NiO nanohybrid electrode is demonstrated in Fig. 3d. The specific capacitance gradually decreased with an increase in cycle number and 75.0% of the initial specific capacitance remained after 5000 cycles. However, the capacitance loss mainly occurred during the first 1500 cycles, which indicates that the faradaic reactions behave irreversibly or induce a degradation of the microstructure at the beginning of the cycle test.27 Subsequently, a slow loss was observed from 1500 to 5000 cycles, showing a good stability.

3.3 Coupling effects of the nanohybrid

To investigate the coupling effects of the Ni3(VO4)2 and NiO, a series of Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratio dependent experiments were performed.

Fig. 4a shows the specific capacitances of the Ni3(VO4)2&NiO nanohybrid with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios at different current densities, which indicates that the high capacitance of Ni3(VO4)2 noticeably improves the capacitances of the Ni3(VO4)2&NiO. Obviously, the composite with the Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]3 shows the highest capacitance and the best rate capability, which is caused by the coupling effects. Thus, the Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]3 is considered as the optimum ratio for the synthesis, and the preceding part of the results is also at this condition.


image file: c4ra05486e-f4.tif
Fig. 4 (a) Specific capacitances of Ni3(VO4)2&NiO with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios at different current densities; (b) Nyquist plots of impedance spectra of Ni3(VO4)2&NiO with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios; (c) cyclic voltammetry curves of Ni3(VO4)2&NiO with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios; (d) cycling performance of Ni3(VO4)2&NiO with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios.

Fig. 4b shows the Nyquist plots of impedance spectra of Ni3(VO4)2&NiO with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios. All composite electrodes show semi-arcs in the high-frequency region followed by a linear component in the low-frequency region. Such semi-arcs are attributed to the charge transfer resistance. The impedance can be expressed as a function of ω:28,29

 
image file: c4ra05486e-t2.tif(3)

The internal resistance R0, which includes the total resistances of the ionic resistance of electrolyte, intrinsic resistance of active materials and contact resistance at the active material/current collector interface, can be obtained from the intercept of the plots on the real axis. The semi-arc corresponds to the pseudo charge transfer resistance Rct. In addition, the calculated R0 and Rct of the four electrodes are listed in Table 1. The Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]3 shows a smaller charger transfer resistance than monolithic NiO or Ni3(VO4)2, which is beneficial to the charge storage process. As semiconductors, the band gap of NiO and Ni3(VO4)2 are different, according to band theory, impurity bands are introduced after these oxides are mixed together in situ, thus improving the electron conductivity of the composite.30 At lower frequencies, the straight line represents the Warburg impedance of the electrolyte ions in host materials. Compared with NiO and Ni3(VO4)2, the Ni3(VO4)2&NiO nanohybrid electrode shows a lower Warburg impedance due to its effective mesoporous structure, which facilitates the diffusion of electrolyte ions in the composite.

Table 1 R0 and Rct of Ni3(VO4)2&NiO with different Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO molar ratios
Ni3(VO4)2[thin space (1/6-em)]:[thin space (1/6-em)]NiO 10[thin space (1/6-em)]:[thin space (1/6-em)]0 9[thin space (1/6-em)]:[thin space (1/6-em)]1 7[thin space (1/6-em)]:[thin space (1/6-em)]3 5[thin space (1/6-em)]:[thin space (1/6-em)]5 3[thin space (1/6-em)]:[thin space (1/6-em)]7 1[thin space (1/6-em)]:[thin space (1/6-em)]9 0[thin space (1/6-em)]:[thin space (1/6-em)]10
R0 (Ω) 1.08 1.89 1.86 1.93 1.83 2.01 1.84
Rct (Ω) 0.21 0.16 0.09 0.11 0.11 1.10 0.11

The coupling effects are also exhibited with the capacitance characteristics and the cycling stability (75.0% for Ni3(VO4)2&NiO versus 66.7% for Ni3(VO4)2 or 50.8% for NiO) shown in Fig. 4c and d.

3.4 Energy density and power density

The energy density E and power density P are calculated from the following equations:31,32
 
image file: c4ra05486e-t3.tif(4)
 
image file: c4ra05486e-t4.tif(5)

Fig. 5 shows the Ragone plots of the NiO electrode, Ni3(VO4)2 electrode and Ni3(VO4)2&NiO nanohybrid electrode at their optimal conditions, respectively. The composite electrode exhibits a significantly higher energy density than the NiO electrode and Ni3(VO4)2 electrode at the same power density. The composite electrode has an energy density of 46 W h kg−1 at a power density of 101 W kg−1. The composite electrode has a higher specific capacitance and smaller charge transfer resistance than the NiO electrode, Ni3(VO4)2 electrode, and the other similar materials such as GO-NiO33 (12.8 W h kg−1 at a power density of 158.9 W kg−1) and Co3O4–Ni3(VO4)2 (ref. 20) (31.2 W h kg−1 at a power density of 102.1 W kg−1). Therefore, the composite electrode shows a higher power density than the NiO electrode or Ni3(VO4)2 electrode.


image file: c4ra05486e-f5.tif
Fig. 5 Ragone plots of NiO, Ni3(VO4)2 and the Ni3(VO4)2&NiO nanohybrid electrode at their optimum conditions.

4 Conclusion

In summary, a self-supported electrode constructed of nickel vanadate and nickel oxide nanohybrid on nickel foam with excellent pseudocapacitive properties was synthesized using a facile method. The porous nanostructure not only provides a larger surface area for faradic reactions, but also allows rapid transportation of electrolyte ions for improving the rate capability. The electrode manifests outstanding capacitance, satisfying rate capability and good cycling stability, showing the coupling effects of nickel vanadate and nickel oxide. This work puts forward a new synthetic strategy and confirms the feasibility of such advanced electrodes in energy storage systems.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51362018, 21163010), the Key Project of Chinese Ministry of Education (no. 212183), and the Natural Science Funds for Distinguished Young Scholars of Gansu Province (no. 1111RJDA012).

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