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Influence of metallic oxide on the morphology and enhanced supercapacitive performance of NiMoO4 electrode material
Inorganic Chemistry Communications 112 (2020) 107697
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Influence of metallic oxide on the morphology and enhanced supercapacitive performance of NiMoO4 electrode material
T
Yong Zhanga,b, , Cui-rong Changa, Xiao-dong Jiaa, Yang Caoa, Ji Yana, He-wei Luoa, Hai-li Gaoa, , Yi Rua, Han-xin Meia, Ai-qin Zhanga, Ke-zheng Gaoa, Li-zhen Wanga ⁎
a b
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Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Pine needle-like Ramotan fruit-like Coral cluster-like NiMoO4 composite
Transition metal oxide modified NiMoO4 supercapacitor electrode materials were prepared by a simple solvothermal method. The effects of MnMoO4, ZnMoO4 and CoMoO4 on the structure, morphology and electrochemical performance of NiMoO4 materials were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), Raman, laser particle size analyzer, and electrochemical measurements. The results indicate that the metal oxides have significant influence on the morphology of NiMoO4. The NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials present pine needle-like, micron rod-like, ramotan fruit-like and coral cluster-like can be prepared by using three different transition metal oxides with the same experimental parameters. At the current density of 1 A g−1, the discharge specific capacitance is 620, 430, 556 and 740 F g−1, respectively. After 2000 charge and discharge cycles, the capacitance retention rate is as high as 99.4, 60.1, 96.0 and 89.1% with current density of 10 A g−1, respectively. Compared with the other three morphologies, the coral cluster-like NiMoO4/CoMoO4 composite structure provides more space and redox active sites, which could improve electrolyte diffusion efficiency and enhance electron transport.
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Corresponding authors at: Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China (Y. Zhang). E-mail addresses: [email protected] (Y. Zhang), [email protected] (H.-l. Gao).
https://doi.org/10.1016/j.inoche.2019.107697 Received 10 September 2019; Received in revised form 30 October 2019; Accepted 21 November 2019 Available online 09 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107697
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1. Introduction
Compared with the traditional single oxide, bimetallic oxide has multiple oxidation. Due to the synergistic effect between oxides, these materials can usually obtain higher specific capacitance, which can then be used in high-performance electrode materials for supercapacitor [10,11]. NiMoO4, due to its advantages of low cost, abundant resources, environmental protection, no pollution and high activity, has become a research hotspot of supercapacitor electrode materials. In addition, MnMoO4, ZnMoO4 and CoMoO4 materials have attracted much attention because of their multi-oxidation state and higher theoretical capacity [12]. Although these materials have been improved significantly in recent years, as electrode materials of supercapacitors, their low electrical conductivity and small surface area usually showed poor rate performance, small capacitance and limited cycle stability in the process of redox reaction [13,14], which largely restrict their further industrial application in pesudocapacitors. Therefore, the design of a new composite electrode material, which has a synergistic effect between different electrode materials and good electrochemical properties, has aroused great interest of the vast number of scientific research workers. Therefore, in order to meet the demand of high-performance supercapacitors, researchers have adopted various strategies to improve the structure and morphology of existing materials, some of which include composite composition, hierarchical structure, core-shell
With the wide application of electronic products, researchers urgently need to seek an electrochemical energy storage device with high energy and power density, convenience and economic sustainability [1–3]. Compared with traditional batteries, supercapacitors (SCs), as a new type of energy storage device, has become a research hotspot at home and abroad because of its advantages of high power density, stable cycle performance, fast charging/discharging speed, and long cycle life [4,5]. According to the energy storage mechanism of electrode materials, supercapacitors can be divided into electric double-layer capacitors (EDLCs) and pseudo-capacitors [6,7]. Because of their fast and reversible redox reaction, pseudo-capacitors have higher characteristic capacitance and energy density than EDLCs. The EDLCs use carbon as electrode material, and forms opposite double-layer charge at electrode/electrolyte interface to achieve the purpose of energy storage. Pseudo-capacitors store charges mainly by highly reversible redox reactions between electrodes and electrolytes interface using active materials. Electrode material is an important part of supercapacitors and usually determines the properties of supercapacitors. The commonly used electrode materials mainly include metal sulfide [8], metal selenide, metal hydroxide, metal oxide, conjugated polymer, etc [9].
Fig. 1. SEM images of (a, b, c) NiMoO4, (d, e, f) NiMoO4/MnMoO4, (g, h, i) NiMoO4/ZnMoO4 and (j, k, l) NiMoO4/CoMoO4. 2
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Fig. 2. (a) XRD patterns and (b) Raman spectras of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4.
structure and material doping, showing enhanced electrochemical properties. For instance, Senthilkumar et al. [15] designed and synthesized CoMoO4-NiMoO4·xH2O composite material with good electrochemical performance by chemical coprecipitation method. The composite showed higher specific capacitance than CoMoO4, better rate capability than NiMoO4·xH2O, and the largest specific capacitance is 1039 F g−1 at 2.5 mA cm−2 current density. Wei et al. [16] synthesized hierarchical C@NiMoO4 core–shell composite material by two-step hydrothermal method. The results showed that C@NiMoO4 exhibited good rate characteristics and cycle stability, the specific electrochemical is 268.8 F g−1 at 1 A g−1, and the capacity retention rate is 88.4% after 2000 charge–discharge cycles. Feng et al. [17] synthesized
N-doping of graphene/NiMoO4 with heterostructure for supercapacitors. The as-prepared samples exhibited the best performance with a specific capacitance of 1913 F g−1 at 1 A g−1. The NG/ NiMoO4//AC device exhibits a maximum energy density of 22.2 Wh kg−1 at the power density of 400 W kg−1, and a high capacitance of 62.5 F g−1 at 0.5 A g−1. Saravanakumar et al. [18] prepared MnMoO4 nanoparticles with different morphologies by adjusting pH values (7, 9, 11), and studied their electrochemical properties. With the increase of pH value of the precursor solution, the products gradually form uniformly distributed nanorods from irregular nanoparticles, and then to irregularly nanoplates. The MnMoO4 nanorods formed under the optimum conditions (pH = 9) are relatively uniform, and the first
Fig. 3. Laser diffraction based particle size distribution of (a) NiMoO4, (b) NiMoO4/MnMoO4, (c) NiMoO4/ZnMoO4, and (d) NiMoO4/CoMoO4. 3
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Fig. 4. CVcurves of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 at scan rates of (a) 5, (b) 10, (c) 15 and (d) 20 mV s−1.
discharge specific capacity is 697.4 F g−1 at 0.5 A g−1 current density. A large number of experimental results show that the special morphology structure has excellent physical and chemical properties, such as low density, large effective specific surface area and good mass permeability. It has more electrochemical active sites and faster redox reaction, which are conducive to improving the charge transfer capacity and electrochemical properties of materials [19]. However, these synthesis schemes sometimes involve several experimental stages, environmentally unfriendly solvents and surfactants. In addition, as far as we know, there are few literatures on the modification of NiMoO4 by metal oxides, and there is no report on such a simple synthesis scheme of coral cluster-like NiMoO4/CoMoO4 composite. This special coral cluster-like morphology can provide stable and effective ions and electrons transport pathways. Based on the above considerations, NiMoO4 morphology was controlled by doping transition metal oxides MnMoO4, ZnMoO4 and CoMoO4 to synthesize electrode materials with special morphology and structure, and the influence of morphology on the electrochemical properties of NiMoO4 was further studied. The results indicate that the metal oxides have significant influence on the morphology of NiMoO4 composite. The NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials with pine needle shape, micron rod shape, ramotan fruit shape and coral shape can be prepared by using three different transition metal oxides with the same experimental parameters. Compared with the other three morphologies, the NiMoO4/CoMoO4 composite with coral cluster-like structure provides more space and redox active sites, and showing better capacitance characteristics.
2. Experimental section 2.1. Materials and chemicals The Ni foams matrix (Changsha Lyrun New Material Co., Ltd., Changsha, China) was pretreated by degreasing in acetone, etching in HCl for 5 min, then rinsed with deioned water, and dried before use. All reagents used in the experiments were of analytical grade and purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD, and used without further purification. 2.2. Material preparation In typical synthesis, 1 mmol NiSO4·6H2O, 2 mmol Na2MoO4·2H2O and 6 mmol urea are completely dissolved in 60 mL ethanol and water mixture solution, then 1 mmol Mn(CH3COO)2·4H2O, Zn (CH3COO)2·2H2O and Co(NO3)2·6H2O were added to the above three mixed solutions respectively. After that, the treated Ni foam was put into the bottom of the reactor, and the obtained uniform mixed solution was transferred to the 100 mL stainless steel autoclave and maintained at 150 °C for 6 h. Finally, the NiMoO4 precursor was calcined at 400 °C for 2 h, and the final NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 composite materials are obtained. In order to compare the changes of morphology and electrochemical properties after modification, pure NiMoO4 electrode materials were prepared by the same method. According to the experimental conditions and relevant references [20], the possible formation mechanism of NiMoO4,
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Fig. 5. GCD curves of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 at different current densities of (a) 1, (b) 4, (c) 7 and (d) 10 A g−1.
Fig. 6. (a) Discharge specific capacitance vs. different current density, and (b) Cycling stability of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/ CoMoO4.
MnMoO4, ZnMoO4 and CoMoO4 can be expressed by Eqs. (1)–(4).
NiSO4 + Na2 MoO4
NiMoO4 + Na2 SO4
2.3. Structure characterizations (1)
Mn (CH3 COO ) 2 + Na2 MoO4
MnMoO4 + 2CH3 COONa
(2)
Zn (CH3 COO) 2 + Na2 MoO4
ZnMoO4 + 2CH3 COONa
(3)
Co (NO3) 2 + Na2 MoO4
CoMoO4 + 2NaNO3
The morphology and shape of the as-synthesized samples were characterized by JEOL JSM-6490LV scanning electron microscope (SEM). The crystallinity and structural properties were investigated in the 2θ range of 10–80° on an X-ray diffraction (XRD) (Bruker D8 Advance; Bruker Corp, Billerica) using Cu Kα X-ray radiation. Raman spectra of samples were performed by using a Raman spectrometer—Lab RAM (LabRAM system, Dilor, Lille) with excitation wavelength and laser power of 532 nm and 50 mW, respectively. A
(4) 5
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Fig. 7. (a) EIS plots and (b) the enlarged EIS plots of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 electrodes at the high frequency region.
Microtrac S3500 laser particle sizer was used for the particle size and distribution measurements of the as-prepared samples.
interface resistance, thus improving the electrochemical reaction activity significantly. Fig. 2a shows the XRD patterns of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials, respectively. The NiMoO4/MnMoO4 diffraction peaks at 2θ = 26.6° and 41.7° correspond to the planes of (2 2 0) and (2 2 2), which are in accordance with the standard data (JCPDS No. 45-0142). For NiMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 samples, the patterns are in good accordance with α-NiMoO4 (JCPDS No. 33-0948). All the primary diffraction peaks around 14.3°, 25.3°, 28.8°, 32.6°, 43.9° and 47.4° correspond to the crystal planes of (1 1 0), ( −1 1 2), (2 2 0), (0 2 2), (3 3 0) and ( −2 0 4) with monoclinic structure [13,24], which means that the monoclinic phase structure of NiMoO4 has not been changed by doping either Zn2+ or Co2+ ions. The peaks of NiMoO4/CoMoO4 are generally less intense, indicating the poor crystallinity [25]. It is well known that the low crystallinity is conducive to electrons transfer [15,26]. In order to further identify the structure of NiMoO4 and its composites, Raman spectroscopy was carried out. It can be seen from Fig. 2b that there are characteristic peaks of NiMoO4 with a strong peak at 960 cm−1, a medium-strong peak at 704 cm−1, and several weak peaks at 261, 377, 813, 876 and 911 cm−1, among which the peaks at 876, 813 and 911 cm−1 indicate the bending vibration of the MoeOeMo bond and the stretching vibrations of the Mo]O group [27,28]. The observed peaks at 704 cm−1, 377 cm−1, 261 cm−1 can be attributed to the symmetrical stretching vibrations of the terminal NieOeMo bond, the deformation vibrations of the terminal MoeOeMo bond and the bending vibrations of the terminal MoeO bond, respectively [29–31]. The peaks at 351 cm−1, 823 cm−1, 886 cm−1 and 932 cm−1 correspond to the characteristic peaks of MnMoO4 [18]. The peaks at 915 cm−1 and 869 cm−1 correspond to the symmetric and asymmetric stretching vibrations of MoO4 in ZnMoO4 [32]. Several weak peaks at 876 cm−1, 815 cm−1 and 367 cm−1 are attributed to the stretching vibration of MoeOeCo bond [26]. All of characteristic peaks suggest the successful synthesis of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ ZnMoO4 and NiMoO4/CoMoO4 composites. Fig. 3 shows the particle size distribution of NiMoO4, NiMoO4/ MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 composite materials, respectively. As shown, all the samples showed an obvious threepeak distribution. Fig. 3a shows that the particle size of NiMoO4 is 2.75 μm and 15.56 μm for D50 and D90, respectively. Fig. 3b shows that the particle size of NiMoO4/MnMoO4 sample is 6.54 μm and 11.0 μm for D50 and D90, respectively. Fig. 3c shows that the particle size of NiMoO4/ZnMoO4 sample is 6.54 μm and 15.56 μm for D50 and D90, respectively. And Fig. 3d shows that the particle size of NiMoO4/ CoMoO4 sample is 2.31 μm and 15.56 μm for D50 and D90, respectively. It can be seen from the figure that the particle size of this sample is the
2.4. Electrochemical measurements Electrochemical measurements (galvanostatic charge/discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) were measured in 3 mol·L−1 KOH aqueous solution at room temperature on a CHI 660E electrochemical workstation (ChenHua Instruments Co., Ltd., Shanghai, China). Among them, the GCD characteristics of the samples were investigated at various current densities of 1, 4, 7 and 10 A g−1 between 0 and 0.5 V vs. Hg/HgO. CV test recorded at scann rates of 5, 10, 15, 20 and 25 mV s−1, respectively. The EIS measurement is operated in the frequency range of 0.01 ~ 100 KHz with potential amplitude of 5 mV. The three-electrode system consisted of Pt plate as the counter electrode, Hg/HgO as the reference electrode, and the products prepared on Ni foam as the working electrodes, respectively. The specific capacitance (Cm, F g−1) is calculated from the charge-discharge curve according to the following equation (5):
Cm =
C i× t = m m× u
(5)
where i (A), m (g), △t (s) and △u (V) represent the discharge current, the mass of active electrode material, the total discharge time and the potential window, respectively. 3. Results and discussion Fig. 1 shows the SEM images of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 composite materials, respectively. As can be seen from Fig. 1a–c, NiMoO4 shows mixed characteristics of nanospheres and pine needles with an average length of about 8 μm. Pine needle can not only provide more active points for redox reaction of active substances, but also increase the contact surface area between active substances and electrolytes, thereby improving their capacitance performance [21]. NiMoO4/MnMoO4 (Fig. 1d–f) contains a mixed morphology of microrods and nanospheres, and the length of the microrod is about 8 μm. The morphology of NiMoO4/ZnMoO4 (Fig. 1g–i) shows the mixed characteristics of pine needle and ramotan fruit-like. The diameter of ramotan fruit is about 4 μm. NiMoO4/CoMoO4 (Fig. 1j–l) contains only one coral cluster-like morphology, the length of which is about 6 μm. This coral cluster-like forms a highly porous structure that provides sufficient accessible channels for ion diffusion within the electrode [22,23], which is conducive to accelerating the redox reaction speed and reducing the 6
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smallest and the most evenly distributed. More uniform particle size distribution and smaller particle size are beneficial to increase ion diffusion rate and specific surface area [33], thus improving the performance of supercapacitors. CV curves was used to study the rate performance of the electrode materials at different scanning rates. Fig. 4a–d shows the CV curves of NiMoO4 and its composites in potential range from 0–0.5 V vs. Hg/HgO with scanning rate of 5, 10, 15 and 20 mV s−1. A pair of redox peaks on the CV curve can be clearly observed, indicating that the electrode material has typical pseudocapacitance characteristics. The redox peaks mainly come from the Faraday current of Ni2+/Ni3+ redox reaction [34]. The shape of CV curves has no obvious deformation with the increase of scanning rate, which indicates that electrolyte ions can diffuse and migrate rapidly, indicating that it has good capacitance performance and high rate characteristics. The current density increases with the increase of scanning rate, which indicates that the symmetrical supercapacitor has a good I-V response [35]. Generally speaking, the area enclosed by CV curve is proportional to the specific capacitance [36]. It can be seen that the CV curve area of NiMoO4/ CoMoO4 material is the largest at all scanning rates, and is much larger than that of other materials with the increase of scanning rate. This shows that NiMoO4/CoMoO4 has larger charge storage capacity, and the improvement of its performance is mainly attributed to the synergistic effect of NiMoO4 and CoMoO4. Fig. 5a-d shows the GCD curves of NiMoO4 and its composite materials at current densities of 1, 4, 7 and 10 A g−1, respectively. The shape of GCD curves of all electrode materials is similar, and the charge-discharge curves are almost symmetrical, indicating good electrochemical reversibility [30,37]. In addition, each charge-discharge curve has an obvious charge-discharge platform, and the plateau potential of the discharge curve is consistent with the reduction peak of CV curve, indicating that the electrode material is rich in redox reaction and has good electrochemical performance [38]. The relationship between discharge specific capacity and current density is shown in Fig. 6a. The specific capacitance of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 is calculated to be 620, 430, 556 and 740 F g−1 at current density of 1 A g−1 using Eq. (5). And at a current density of 10 A g−1, the specific capacitance of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/ CoMoO4 is calculated to be 380, 268, 326 and 474 F g−1, which is about 61.3%, 62.3%, 58.6% and 64.1% of the specific capacitance at 1 A g−1, indicating NiMoO4/CoMoO4 has excellent capacitance and rate capability performance, which is consistent with the conclusions of CV curve analysis. The result shows much higher than those of previously reported studies, such as NiMoO4·H2O (specific capacitance of 680 F g−1 at 1 A g−1) [39], 1D NiMoO4·nH2O nanorods (specific capacitance of 161 F g−1 at 5 A g−1) [40], MnMoO4/CoMoO4 (specific capacitance of 187.1 F g−1 at 1 A g−1) [41], and Mn1.5Co0.75O/NF (specific capacitance of 668.4 F g−1 at 1 A g−1) [42]. It is shown that with the increase of current density, the discharge specific capacitance decreases steadily. This is due to the slow movement of OH− ions at low current density (1 A g−1), which can contact more electrolytes, thus promote the electrochemical redox reaction of active material [43]. Fig. 6b shows the cycling stability of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 at a current density of 10 A g−1 for 2000 cycles. It can be seen from the cycling performance curves that the specific capacitance of the electrode material increases first and then decreases. This may be due to the increase of wettability between electrode materials and electrolytes, and enhancing the electrochemical activity of the electrode material [44,45]. Furthermore, we can conclude that the specific capacitance are in the order NiMoO4/ CoMoO4 > NiMoO4 > NiMoO4/ZnMoO4 > NiMoO4/MnMoO4, and the capacitance of NiMoO4/CoMoO4 is larger than other samples. Although the specific capacitance decreases with cycling, still 89.1% retention of the initial capacitance remains after 2000 cycles, indicating that the NiMoO4/CoMoO4 electrode has a long cycle life. The excellent
cycle stability and rate performance of NiMoO4/CoMoO4 electrode may be due to its unique coral cluster-like structure, which has high mechanical strength strain adjustment ability, and the synergistic effect between bimetallic ions, so that it will not be damaged during the cycle [46]. In order to further study the kinetic characteristics of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 electrodes, we carried out the EIS measurements. Fig. 7a shows the Nyquist plots of these electrodes recorded in an open circuit potential at a frequency range of 0.01–100 KHz with a perturbation of 5 mV. These impedance spectrums are similar and composed of one semicircle in the high to medium frequency region, and a sloped lines in the low frequency region. In the high frequency intercept of the real axis is equivalent series resistance (Rs), which includes the bulk resistance of electrolyte, contact resistance of electrolyte/electrode, and the inherent resistance of the electroactive material, while the diameter of the semicircle corresponds to the interfacial charge transfer resistance (Rct) [32,47], and the smaller the Rct, the stronger the transmission ability of the electron is. The Warburg resistance (Zw), represented by the sloped line at the low frequency region of the real axis, reflects electrolyte diffusion to electrode surface and a steeper slope usually represents faster ion diffusion [13,26,48]. Besides, according to the enlarged Nyquist plots (Fig. 7b), NiMoO4/CoMoO4 electrode displays a smaller Rs and Rct than other samples, demonstrating its lower internal resistance and charge transfer resistance, which is due to special coral structures accelerate the ion and electron transfer in some extent [13]. Moreover, a more ideal vertical line can be observed for NiMoO4/CoMoO4 curve among the four samples, indicating higher electrolyte and proton diffusion [49], thus resulting in a better supercapacitive behavior. 4. Conclusions In summary, transition metal oxide MnMoO4, ZnMoO4 and CoMoO4 modified NiMoO4 electrode materials have been prepared by a simple solvothermal method. The metal oxides have significant influence on the morphology of NiMoO4 composite. The NiMoO4, NiMoO4/ MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials with pine needle shape, micron rod shape, ramotan fruit shape and coral shape can be prepared by using three different transition metal oxides with the same experimental parameters. At the current density of 1 A g−1, the discharge specific capacitance is 620, 430, 556 and 740 F g−1, respectively. After 2000 charge and discharge cycles, the capacitance retention rate is as high as 99.4, 60.1, 96.0 and 89.1% with current density of 10 A g−1, respectively. Compared with the other three forms, the NiMoO4/CoMoO4 composite with coral cluster-like structure have larger space and redox active sites, so they have excellent electrochemical performance, and make the composite as a promising material for supercapacitors. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work is supported by the Key Scientific Research Project of the Higher Education Institutions of Henan Province of China (Grant No. 20A530001), and the National Natural Science Foundation of China (Grant No. 21503193, U1404201). References [1] L. Wang, H. Gao, H. Fang, S. Wang, J. Sun, Effect of methanol on the electrochemical behaviour and surface conductivity of niobium carbide-modified stainless steel for DMFC bipolar plate, Int. J. Hydrogen Energy 41 (2016) 14864–14871. [2] H. Gao, L. Wang, Y. Zhang, A. Zhang, Y. Song, Tartaric acid assisted synthesis of
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Influence of metallic oxide on the morphology and enhanced supercapacitive performance of NiMoO4 electrode material
T
Yong Zhanga,b, , Cui-rong Changa, Xiao-dong Jiaa, Yang Caoa, Ji Yana, He-wei Luoa, Hai-li Gaoa, , Yi Rua, Han-xin Meia, Ai-qin Zhanga, Ke-zheng Gaoa, Li-zhen Wanga ⁎
a b
⁎
Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Pine needle-like Ramotan fruit-like Coral cluster-like NiMoO4 composite
Transition metal oxide modified NiMoO4 supercapacitor electrode materials were prepared by a simple solvothermal method. The effects of MnMoO4, ZnMoO4 and CoMoO4 on the structure, morphology and electrochemical performance of NiMoO4 materials were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), Raman, laser particle size analyzer, and electrochemical measurements. The results indicate that the metal oxides have significant influence on the morphology of NiMoO4. The NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials present pine needle-like, micron rod-like, ramotan fruit-like and coral cluster-like can be prepared by using three different transition metal oxides with the same experimental parameters. At the current density of 1 A g−1, the discharge specific capacitance is 620, 430, 556 and 740 F g−1, respectively. After 2000 charge and discharge cycles, the capacitance retention rate is as high as 99.4, 60.1, 96.0 and 89.1% with current density of 10 A g−1, respectively. Compared with the other three morphologies, the coral cluster-like NiMoO4/CoMoO4 composite structure provides more space and redox active sites, which could improve electrolyte diffusion efficiency and enhance electron transport.
⁎
Corresponding authors at: Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China (Y. Zhang). E-mail addresses: [email protected] (Y. Zhang), [email protected] (H.-l. Gao).
https://doi.org/10.1016/j.inoche.2019.107697 Received 10 September 2019; Received in revised form 30 October 2019; Accepted 21 November 2019 Available online 09 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
Inorganic Chemistry Communications 112 (2020) 107697
Y. Zhang, et al.
1. Introduction
Compared with the traditional single oxide, bimetallic oxide has multiple oxidation. Due to the synergistic effect between oxides, these materials can usually obtain higher specific capacitance, which can then be used in high-performance electrode materials for supercapacitor [10,11]. NiMoO4, due to its advantages of low cost, abundant resources, environmental protection, no pollution and high activity, has become a research hotspot of supercapacitor electrode materials. In addition, MnMoO4, ZnMoO4 and CoMoO4 materials have attracted much attention because of their multi-oxidation state and higher theoretical capacity [12]. Although these materials have been improved significantly in recent years, as electrode materials of supercapacitors, their low electrical conductivity and small surface area usually showed poor rate performance, small capacitance and limited cycle stability in the process of redox reaction [13,14], which largely restrict their further industrial application in pesudocapacitors. Therefore, the design of a new composite electrode material, which has a synergistic effect between different electrode materials and good electrochemical properties, has aroused great interest of the vast number of scientific research workers. Therefore, in order to meet the demand of high-performance supercapacitors, researchers have adopted various strategies to improve the structure and morphology of existing materials, some of which include composite composition, hierarchical structure, core-shell
With the wide application of electronic products, researchers urgently need to seek an electrochemical energy storage device with high energy and power density, convenience and economic sustainability [1–3]. Compared with traditional batteries, supercapacitors (SCs), as a new type of energy storage device, has become a research hotspot at home and abroad because of its advantages of high power density, stable cycle performance, fast charging/discharging speed, and long cycle life [4,5]. According to the energy storage mechanism of electrode materials, supercapacitors can be divided into electric double-layer capacitors (EDLCs) and pseudo-capacitors [6,7]. Because of their fast and reversible redox reaction, pseudo-capacitors have higher characteristic capacitance and energy density than EDLCs. The EDLCs use carbon as electrode material, and forms opposite double-layer charge at electrode/electrolyte interface to achieve the purpose of energy storage. Pseudo-capacitors store charges mainly by highly reversible redox reactions between electrodes and electrolytes interface using active materials. Electrode material is an important part of supercapacitors and usually determines the properties of supercapacitors. The commonly used electrode materials mainly include metal sulfide [8], metal selenide, metal hydroxide, metal oxide, conjugated polymer, etc [9].
Fig. 1. SEM images of (a, b, c) NiMoO4, (d, e, f) NiMoO4/MnMoO4, (g, h, i) NiMoO4/ZnMoO4 and (j, k, l) NiMoO4/CoMoO4. 2
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Fig. 2. (a) XRD patterns and (b) Raman spectras of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4.
structure and material doping, showing enhanced electrochemical properties. For instance, Senthilkumar et al. [15] designed and synthesized CoMoO4-NiMoO4·xH2O composite material with good electrochemical performance by chemical coprecipitation method. The composite showed higher specific capacitance than CoMoO4, better rate capability than NiMoO4·xH2O, and the largest specific capacitance is 1039 F g−1 at 2.5 mA cm−2 current density. Wei et al. [16] synthesized hierarchical C@NiMoO4 core–shell composite material by two-step hydrothermal method. The results showed that C@NiMoO4 exhibited good rate characteristics and cycle stability, the specific electrochemical is 268.8 F g−1 at 1 A g−1, and the capacity retention rate is 88.4% after 2000 charge–discharge cycles. Feng et al. [17] synthesized
N-doping of graphene/NiMoO4 with heterostructure for supercapacitors. The as-prepared samples exhibited the best performance with a specific capacitance of 1913 F g−1 at 1 A g−1. The NG/ NiMoO4//AC device exhibits a maximum energy density of 22.2 Wh kg−1 at the power density of 400 W kg−1, and a high capacitance of 62.5 F g−1 at 0.5 A g−1. Saravanakumar et al. [18] prepared MnMoO4 nanoparticles with different morphologies by adjusting pH values (7, 9, 11), and studied their electrochemical properties. With the increase of pH value of the precursor solution, the products gradually form uniformly distributed nanorods from irregular nanoparticles, and then to irregularly nanoplates. The MnMoO4 nanorods formed under the optimum conditions (pH = 9) are relatively uniform, and the first
Fig. 3. Laser diffraction based particle size distribution of (a) NiMoO4, (b) NiMoO4/MnMoO4, (c) NiMoO4/ZnMoO4, and (d) NiMoO4/CoMoO4. 3
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Fig. 4. CVcurves of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 at scan rates of (a) 5, (b) 10, (c) 15 and (d) 20 mV s−1.
discharge specific capacity is 697.4 F g−1 at 0.5 A g−1 current density. A large number of experimental results show that the special morphology structure has excellent physical and chemical properties, such as low density, large effective specific surface area and good mass permeability. It has more electrochemical active sites and faster redox reaction, which are conducive to improving the charge transfer capacity and electrochemical properties of materials [19]. However, these synthesis schemes sometimes involve several experimental stages, environmentally unfriendly solvents and surfactants. In addition, as far as we know, there are few literatures on the modification of NiMoO4 by metal oxides, and there is no report on such a simple synthesis scheme of coral cluster-like NiMoO4/CoMoO4 composite. This special coral cluster-like morphology can provide stable and effective ions and electrons transport pathways. Based on the above considerations, NiMoO4 morphology was controlled by doping transition metal oxides MnMoO4, ZnMoO4 and CoMoO4 to synthesize electrode materials with special morphology and structure, and the influence of morphology on the electrochemical properties of NiMoO4 was further studied. The results indicate that the metal oxides have significant influence on the morphology of NiMoO4 composite. The NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials with pine needle shape, micron rod shape, ramotan fruit shape and coral shape can be prepared by using three different transition metal oxides with the same experimental parameters. Compared with the other three morphologies, the NiMoO4/CoMoO4 composite with coral cluster-like structure provides more space and redox active sites, and showing better capacitance characteristics.
2. Experimental section 2.1. Materials and chemicals The Ni foams matrix (Changsha Lyrun New Material Co., Ltd., Changsha, China) was pretreated by degreasing in acetone, etching in HCl for 5 min, then rinsed with deioned water, and dried before use. All reagents used in the experiments were of analytical grade and purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD, and used without further purification. 2.2. Material preparation In typical synthesis, 1 mmol NiSO4·6H2O, 2 mmol Na2MoO4·2H2O and 6 mmol urea are completely dissolved in 60 mL ethanol and water mixture solution, then 1 mmol Mn(CH3COO)2·4H2O, Zn (CH3COO)2·2H2O and Co(NO3)2·6H2O were added to the above three mixed solutions respectively. After that, the treated Ni foam was put into the bottom of the reactor, and the obtained uniform mixed solution was transferred to the 100 mL stainless steel autoclave and maintained at 150 °C for 6 h. Finally, the NiMoO4 precursor was calcined at 400 °C for 2 h, and the final NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 composite materials are obtained. In order to compare the changes of morphology and electrochemical properties after modification, pure NiMoO4 electrode materials were prepared by the same method. According to the experimental conditions and relevant references [20], the possible formation mechanism of NiMoO4,
4
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Fig. 5. GCD curves of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 at different current densities of (a) 1, (b) 4, (c) 7 and (d) 10 A g−1.
Fig. 6. (a) Discharge specific capacitance vs. different current density, and (b) Cycling stability of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/ CoMoO4.
MnMoO4, ZnMoO4 and CoMoO4 can be expressed by Eqs. (1)–(4).
NiSO4 + Na2 MoO4
NiMoO4 + Na2 SO4
2.3. Structure characterizations (1)
Mn (CH3 COO ) 2 + Na2 MoO4
MnMoO4 + 2CH3 COONa
(2)
Zn (CH3 COO) 2 + Na2 MoO4
ZnMoO4 + 2CH3 COONa
(3)
Co (NO3) 2 + Na2 MoO4
CoMoO4 + 2NaNO3
The morphology and shape of the as-synthesized samples were characterized by JEOL JSM-6490LV scanning electron microscope (SEM). The crystallinity and structural properties were investigated in the 2θ range of 10–80° on an X-ray diffraction (XRD) (Bruker D8 Advance; Bruker Corp, Billerica) using Cu Kα X-ray radiation. Raman spectra of samples were performed by using a Raman spectrometer—Lab RAM (LabRAM system, Dilor, Lille) with excitation wavelength and laser power of 532 nm and 50 mW, respectively. A
(4) 5
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Fig. 7. (a) EIS plots and (b) the enlarged EIS plots of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 electrodes at the high frequency region.
Microtrac S3500 laser particle sizer was used for the particle size and distribution measurements of the as-prepared samples.
interface resistance, thus improving the electrochemical reaction activity significantly. Fig. 2a shows the XRD patterns of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials, respectively. The NiMoO4/MnMoO4 diffraction peaks at 2θ = 26.6° and 41.7° correspond to the planes of (2 2 0) and (2 2 2), which are in accordance with the standard data (JCPDS No. 45-0142). For NiMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 samples, the patterns are in good accordance with α-NiMoO4 (JCPDS No. 33-0948). All the primary diffraction peaks around 14.3°, 25.3°, 28.8°, 32.6°, 43.9° and 47.4° correspond to the crystal planes of (1 1 0), ( −1 1 2), (2 2 0), (0 2 2), (3 3 0) and ( −2 0 4) with monoclinic structure [13,24], which means that the monoclinic phase structure of NiMoO4 has not been changed by doping either Zn2+ or Co2+ ions. The peaks of NiMoO4/CoMoO4 are generally less intense, indicating the poor crystallinity [25]. It is well known that the low crystallinity is conducive to electrons transfer [15,26]. In order to further identify the structure of NiMoO4 and its composites, Raman spectroscopy was carried out. It can be seen from Fig. 2b that there are characteristic peaks of NiMoO4 with a strong peak at 960 cm−1, a medium-strong peak at 704 cm−1, and several weak peaks at 261, 377, 813, 876 and 911 cm−1, among which the peaks at 876, 813 and 911 cm−1 indicate the bending vibration of the MoeOeMo bond and the stretching vibrations of the Mo]O group [27,28]. The observed peaks at 704 cm−1, 377 cm−1, 261 cm−1 can be attributed to the symmetrical stretching vibrations of the terminal NieOeMo bond, the deformation vibrations of the terminal MoeOeMo bond and the bending vibrations of the terminal MoeO bond, respectively [29–31]. The peaks at 351 cm−1, 823 cm−1, 886 cm−1 and 932 cm−1 correspond to the characteristic peaks of MnMoO4 [18]. The peaks at 915 cm−1 and 869 cm−1 correspond to the symmetric and asymmetric stretching vibrations of MoO4 in ZnMoO4 [32]. Several weak peaks at 876 cm−1, 815 cm−1 and 367 cm−1 are attributed to the stretching vibration of MoeOeCo bond [26]. All of characteristic peaks suggest the successful synthesis of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ ZnMoO4 and NiMoO4/CoMoO4 composites. Fig. 3 shows the particle size distribution of NiMoO4, NiMoO4/ MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 composite materials, respectively. As shown, all the samples showed an obvious threepeak distribution. Fig. 3a shows that the particle size of NiMoO4 is 2.75 μm and 15.56 μm for D50 and D90, respectively. Fig. 3b shows that the particle size of NiMoO4/MnMoO4 sample is 6.54 μm and 11.0 μm for D50 and D90, respectively. Fig. 3c shows that the particle size of NiMoO4/ZnMoO4 sample is 6.54 μm and 15.56 μm for D50 and D90, respectively. And Fig. 3d shows that the particle size of NiMoO4/ CoMoO4 sample is 2.31 μm and 15.56 μm for D50 and D90, respectively. It can be seen from the figure that the particle size of this sample is the
2.4. Electrochemical measurements Electrochemical measurements (galvanostatic charge/discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)) were measured in 3 mol·L−1 KOH aqueous solution at room temperature on a CHI 660E electrochemical workstation (ChenHua Instruments Co., Ltd., Shanghai, China). Among them, the GCD characteristics of the samples were investigated at various current densities of 1, 4, 7 and 10 A g−1 between 0 and 0.5 V vs. Hg/HgO. CV test recorded at scann rates of 5, 10, 15, 20 and 25 mV s−1, respectively. The EIS measurement is operated in the frequency range of 0.01 ~ 100 KHz with potential amplitude of 5 mV. The three-electrode system consisted of Pt plate as the counter electrode, Hg/HgO as the reference electrode, and the products prepared on Ni foam as the working electrodes, respectively. The specific capacitance (Cm, F g−1) is calculated from the charge-discharge curve according to the following equation (5):
Cm =
C i× t = m m× u
(5)
where i (A), m (g), △t (s) and △u (V) represent the discharge current, the mass of active electrode material, the total discharge time and the potential window, respectively. 3. Results and discussion Fig. 1 shows the SEM images of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 composite materials, respectively. As can be seen from Fig. 1a–c, NiMoO4 shows mixed characteristics of nanospheres and pine needles with an average length of about 8 μm. Pine needle can not only provide more active points for redox reaction of active substances, but also increase the contact surface area between active substances and electrolytes, thereby improving their capacitance performance [21]. NiMoO4/MnMoO4 (Fig. 1d–f) contains a mixed morphology of microrods and nanospheres, and the length of the microrod is about 8 μm. The morphology of NiMoO4/ZnMoO4 (Fig. 1g–i) shows the mixed characteristics of pine needle and ramotan fruit-like. The diameter of ramotan fruit is about 4 μm. NiMoO4/CoMoO4 (Fig. 1j–l) contains only one coral cluster-like morphology, the length of which is about 6 μm. This coral cluster-like forms a highly porous structure that provides sufficient accessible channels for ion diffusion within the electrode [22,23], which is conducive to accelerating the redox reaction speed and reducing the 6
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smallest and the most evenly distributed. More uniform particle size distribution and smaller particle size are beneficial to increase ion diffusion rate and specific surface area [33], thus improving the performance of supercapacitors. CV curves was used to study the rate performance of the electrode materials at different scanning rates. Fig. 4a–d shows the CV curves of NiMoO4 and its composites in potential range from 0–0.5 V vs. Hg/HgO with scanning rate of 5, 10, 15 and 20 mV s−1. A pair of redox peaks on the CV curve can be clearly observed, indicating that the electrode material has typical pseudocapacitance characteristics. The redox peaks mainly come from the Faraday current of Ni2+/Ni3+ redox reaction [34]. The shape of CV curves has no obvious deformation with the increase of scanning rate, which indicates that electrolyte ions can diffuse and migrate rapidly, indicating that it has good capacitance performance and high rate characteristics. The current density increases with the increase of scanning rate, which indicates that the symmetrical supercapacitor has a good I-V response [35]. Generally speaking, the area enclosed by CV curve is proportional to the specific capacitance [36]. It can be seen that the CV curve area of NiMoO4/ CoMoO4 material is the largest at all scanning rates, and is much larger than that of other materials with the increase of scanning rate. This shows that NiMoO4/CoMoO4 has larger charge storage capacity, and the improvement of its performance is mainly attributed to the synergistic effect of NiMoO4 and CoMoO4. Fig. 5a-d shows the GCD curves of NiMoO4 and its composite materials at current densities of 1, 4, 7 and 10 A g−1, respectively. The shape of GCD curves of all electrode materials is similar, and the charge-discharge curves are almost symmetrical, indicating good electrochemical reversibility [30,37]. In addition, each charge-discharge curve has an obvious charge-discharge platform, and the plateau potential of the discharge curve is consistent with the reduction peak of CV curve, indicating that the electrode material is rich in redox reaction and has good electrochemical performance [38]. The relationship between discharge specific capacity and current density is shown in Fig. 6a. The specific capacitance of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 is calculated to be 620, 430, 556 and 740 F g−1 at current density of 1 A g−1 using Eq. (5). And at a current density of 10 A g−1, the specific capacitance of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/ CoMoO4 is calculated to be 380, 268, 326 and 474 F g−1, which is about 61.3%, 62.3%, 58.6% and 64.1% of the specific capacitance at 1 A g−1, indicating NiMoO4/CoMoO4 has excellent capacitance and rate capability performance, which is consistent with the conclusions of CV curve analysis. The result shows much higher than those of previously reported studies, such as NiMoO4·H2O (specific capacitance of 680 F g−1 at 1 A g−1) [39], 1D NiMoO4·nH2O nanorods (specific capacitance of 161 F g−1 at 5 A g−1) [40], MnMoO4/CoMoO4 (specific capacitance of 187.1 F g−1 at 1 A g−1) [41], and Mn1.5Co0.75O/NF (specific capacitance of 668.4 F g−1 at 1 A g−1) [42]. It is shown that with the increase of current density, the discharge specific capacitance decreases steadily. This is due to the slow movement of OH− ions at low current density (1 A g−1), which can contact more electrolytes, thus promote the electrochemical redox reaction of active material [43]. Fig. 6b shows the cycling stability of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 at a current density of 10 A g−1 for 2000 cycles. It can be seen from the cycling performance curves that the specific capacitance of the electrode material increases first and then decreases. This may be due to the increase of wettability between electrode materials and electrolytes, and enhancing the electrochemical activity of the electrode material [44,45]. Furthermore, we can conclude that the specific capacitance are in the order NiMoO4/ CoMoO4 > NiMoO4 > NiMoO4/ZnMoO4 > NiMoO4/MnMoO4, and the capacitance of NiMoO4/CoMoO4 is larger than other samples. Although the specific capacitance decreases with cycling, still 89.1% retention of the initial capacitance remains after 2000 cycles, indicating that the NiMoO4/CoMoO4 electrode has a long cycle life. The excellent
cycle stability and rate performance of NiMoO4/CoMoO4 electrode may be due to its unique coral cluster-like structure, which has high mechanical strength strain adjustment ability, and the synergistic effect between bimetallic ions, so that it will not be damaged during the cycle [46]. In order to further study the kinetic characteristics of NiMoO4, NiMoO4/MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 electrodes, we carried out the EIS measurements. Fig. 7a shows the Nyquist plots of these electrodes recorded in an open circuit potential at a frequency range of 0.01–100 KHz with a perturbation of 5 mV. These impedance spectrums are similar and composed of one semicircle in the high to medium frequency region, and a sloped lines in the low frequency region. In the high frequency intercept of the real axis is equivalent series resistance (Rs), which includes the bulk resistance of electrolyte, contact resistance of electrolyte/electrode, and the inherent resistance of the electroactive material, while the diameter of the semicircle corresponds to the interfacial charge transfer resistance (Rct) [32,47], and the smaller the Rct, the stronger the transmission ability of the electron is. The Warburg resistance (Zw), represented by the sloped line at the low frequency region of the real axis, reflects electrolyte diffusion to electrode surface and a steeper slope usually represents faster ion diffusion [13,26,48]. Besides, according to the enlarged Nyquist plots (Fig. 7b), NiMoO4/CoMoO4 electrode displays a smaller Rs and Rct than other samples, demonstrating its lower internal resistance and charge transfer resistance, which is due to special coral structures accelerate the ion and electron transfer in some extent [13]. Moreover, a more ideal vertical line can be observed for NiMoO4/CoMoO4 curve among the four samples, indicating higher electrolyte and proton diffusion [49], thus resulting in a better supercapacitive behavior. 4. Conclusions In summary, transition metal oxide MnMoO4, ZnMoO4 and CoMoO4 modified NiMoO4 electrode materials have been prepared by a simple solvothermal method. The metal oxides have significant influence on the morphology of NiMoO4 composite. The NiMoO4, NiMoO4/ MnMoO4, NiMoO4/ZnMoO4 and NiMoO4/CoMoO4 materials with pine needle shape, micron rod shape, ramotan fruit shape and coral shape can be prepared by using three different transition metal oxides with the same experimental parameters. At the current density of 1 A g−1, the discharge specific capacitance is 620, 430, 556 and 740 F g−1, respectively. After 2000 charge and discharge cycles, the capacitance retention rate is as high as 99.4, 60.1, 96.0 and 89.1% with current density of 10 A g−1, respectively. Compared with the other three forms, the NiMoO4/CoMoO4 composite with coral cluster-like structure have larger space and redox active sites, so they have excellent electrochemical performance, and make the composite as a promising material for supercapacitors. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work is supported by the Key Scientific Research Project of the Higher Education Institutions of Henan Province of China (Grant No. 20A530001), and the National Natural Science Foundation of China (Grant No. 21503193, U1404201). References [1] L. Wang, H. Gao, H. Fang, S. Wang, J. Sun, Effect of methanol on the electrochemical behaviour and surface conductivity of niobium carbide-modified stainless steel for DMFC bipolar plate, Int. J. Hydrogen Energy 41 (2016) 14864–14871. [2] H. Gao, L. Wang, Y. Zhang, A. Zhang, Y. Song, Tartaric acid assisted synthesis of
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