Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor

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Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor Xiaodan Zhang a, Luchao Yue a, Shuaiguo Zhang b, Yu Feng c, Lulu An a, Miao Wang a, Jie Mi a,* a

Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, China b College of Chemical and Material Engineering, Henan University of Urban Construction, Pingdingshan, 467036, PR China c College of Textile Engineering, Taiyuan University of Technology, Jinzhong, 030600, Shanxi, China

highlights
NixCo1-xMoO4 materials are prepared using a time-saving approach.
The Ni0.5Co0.5MoO4 material presents a 3D chrysanthemum structure.
The Ni0.5Co0.5MoO4 electrode exhibits excellent electrochemical performance.
We analyze the capacitance storage mechanism of the electrode material.
The asymmetric supercapacitor (Ni0.5Co0.5MoO4//C sphere) exhibits high energy.

article info

abstract

Article history:

Herein, nickel-doped cobalt molybdate (NixCo1-xMoO4, x ¼ 0, 0.1, 0.3, 0.4, 0.5, 0.6) with

Received 15 October 2019

nanorods structure are successfully prepared through a facile co-precipitation approach.

Received in revised form

The molar ratio of nickel and cobalt would affect the morphologies. The Ni0.5Co0.5MoO4

5 January 2020

electrode exhibits satisfactory specific capacity and rate performance (325.9 C/g at 0.5 A/g,

Accepted 19 January 2020

260 C/g at 10 A/g). Moreover, the asymmetric supercapacitor (ASC) device is assembled

Available online xxx

using Ni0.5Co0.5MoO4 and carbon spheres as positive and negative electrodes, respectively. The ASC device possesses high energy density of 31.57 Wh/kg at the power density of

Keywords:

400 W/kg, and even maintains high energy density of 13.56 Wh/kg at the power density of

NixCo1-xMoO4

8000 W/kg. Meantime, the ASC device possesses excellent capacitance retention rate and

Co-precipitation

coulomb efficiency after 5000 charge-discharge cycles. And a red light-emitted diode is

Chrysanthemum structure

illumed using two ASC devices in series.

Carbon sphere

© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Superior cycle stability Supercapacitors

* Corresponding author. E-mail address: [email protected] (J. Mi). https://doi.org/10.1016/j.ijhydene.2020.01.127 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Introduction Electrode materials are used as a key factor to improve the energy storage performance of supercapacitor devices, which have attracted extensive attention owing to long-cycle life, high power density and rapid charge-discharge ability [1e5]. The semiconductor metal oxides are promising electrode materials due to the unique physical and chemical properties and enormous application in various filed [6e10]. However, many issues are need to be addressed for the high energy density of supercapacitors using these electrodes. According to E ¼ CV2/2, the energy density of the supercapacitor can be improved by increasing the operating potential window and elevating the capacitance of the electrode [11,12]. It is well known that the organic electrolyte can obtain a high voltage window of around 3 V, but its flammability and poisonousness limit its wide application [13e15]. On the other hand, the potential window of nontoxic aqueous electrolyte is typically low. Hence, a promising method is to construct asymmetric supercapacitor for promoting the operating potential window of aqueous supercapacitor. In addition, boosting the capacitance of electrode is another effective strategy to enhance the energy density of supercapacitors. Nowadays, various molybdates (Co3O4 [16], NiMoO4 [17,18], CoMoO4 [19,20], CuMoO4 [21], Bi2Mo2O9 [22]) and tungstates (NiWO4 [23], CuWO4 [24] and ZnWO4 [25]) exhibit high specific capacitance when used as electrode materials. Among them, CoMoO4 has attracted extensive attention because of abundant sources, excellent rate performance and superior cycle stability [26]. Actually, the specific capacitance of pure CoMoO4 material can’t satisfy the requirement of high energy storage devices. In the past few years, different approaches have been applied to improve the specific capacitance of CoMoO4-based electrode materials. For example, Li et al. [19] prepared hierarchical CoMoO4@Co3O4/OMEP nanocomposites via the two-step hydrothermal approach, which achieved 7.13 F/m2 at 0.6 A/g. Besides, CoMoO4@Ni(OH)2 coreshell nanotube was fabricated through the multi-step synthesis path. This material exhibited splendid specific capacitance of 1246 F/g at 1 A/g [27]. Similarly, CoMoO4@Ni(OH)2 core-shell nanoflowers had a high capacitance of 8.55 F/cm2 at 2 mA/cm2 [28]. However, it is difficult to fabricate these materials in mass production because of time-consuming and low output. In contrast, many researchers now improve the electrochemical performance of electrode materials through different metal ions doping [29]. Generally, heteroatom doping would introduce abundant redox reaction and superior conductivity, which are benefit for excellent electrochemical performance [30,31]. For example, Mohammad S. et al. [32] prepared dual Ni/Co-MOF nano-composites. Thanks to the synergy between different metals, Ni/Co-MOF obtained higher specific capacitance than Co-MOF and Ni-MOF. Furthermore, Co-doped Ni@Ni3S2 composite possessed a distinctly increased volumetric capacity (703 C/cm3) than bare Ni@Ni3S2 (105 C/cm3) at 0.25 A/cm3 [33]. Theoretically, nickel atoms can easily enter the crystal lattice of CoMoO4 to form a random solid solution due to the approximate ionic radius (Co2þ ¼ 0.074, Ni2þ ¼ 0.069 nm) and electronegativity

(Co ¼ 1.88, Ni ¼ 1.91) [30]. Therefore, the electrochemical performances and physical properties of CoMoO4 might be significantly improved by nickel doping. Here, we fabricated nickel-doped CoMoO4 (NixCo1-xMoO4) material with the nanorods structure through one-pot coprecipitation method. NixCo1-xMoO4 material exhibited smaller nanorod diameter and larger surface area than the pure CoMoO4 material. The structure changed from disorganized distribution to ordered distribution of nanorods due to nickel doping, which was a rare occurrence. We explained possible causes and determined the optimal amount of nickel doping. Meantime, nickel doping improved the intrinsic conductivity of CoMoO4, which lead to superior electrochemical performances. Furthermore, asymmetric supercapacitor was assembled with Ni0.5Co0.5MoO4 as positive electrode and carbon spheres as negative electrode. The ASC device displayed high power density, outstanding energy density and cycle stability by adjusting the voltage window and the mass ratio (m-/mþ). Moreover, two ASC devices in series could light up a red light-emitting diode for about 10 min.

Experiments All chemical reagents used in the experiments were analytical grade. Cobalt chloride (CoCl2$6H2O), Nickel chloride (NiCl2$6H2O) and Sodium molybdate (Na2MoO4$2H2O) were purchased from the Aladdin industrial corporation.

Synthesis of NixCo1-xMoO4 The NixCo1-xMoO4 material was fabricated by coprecipitation method. First, 5 mmol CoCl2$6H2O and NiCl2$6H2O (with different mole ratios of 0:1, 1:9, 3:7, 4:6, 5:5, 6:4) were dissolved into 40 ml deionized water and stirred at 70  C to form a homogeneous mixture solution (solution A). Then, 40 mL deionized water containing 5 mmol Na2MoO4$2H2O was added to solution A and stirred for 2.5 h. Subsequently, the precursor was collected by centralization and rinsed with deionized water and ethanol three times. In the end, the product was obtained after drying at 80  C for 12 h and annealing at 350  C for 2 h. The obtained samples were named as CMO and NCMO-X (X ¼ 1e5) corresponding to pure CoMoO4 and NixCo1-xMoO4 (x ¼ 0.1, 0.3, 0.4, 0.5, 0.6), respectively.

Synthesis of carbon sphere The carbon sphere was fabricated by hydrothermal method. First, 5 g glucose was dissolved in 50 mL deionized water and stirred for 1 h. Then, the solution was transferred to 100 mL Teflon-lined stainless steel autoclave and heated at 180  C for 24 h. The precursor was obtained through centralization six times with deionized water and dried at 60  C overnight. Subsequently, the precursor and KOH were mixed with a mass ratio of 3:1 and annealing at 700  C for 2 h in nitrogen atmosphere. The final products were obtained after washed with 1 mol/L hydrochloric acid for 2 h and rinsed with deionized water until neutral.

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Characterization The phase and crystallographic structure were identified using X-ray diffraction (XRD, DX-2700, l ¼ 1.5418  A) with CuKa radiation and Raman spectroscopy (Horiba Scientific LabRAM HR Evolution) with a 532 nm excitation laser. The chemical composition and chemical bond states were investigated by X-ray photoelectron spectroscope (XPS, Escalab 250Xi) and Energy-Dispersive X-Ray Spectroscopy (EDS). The morphologies of samples were detected through scanning electron microscope (FSEM, FEI Inspect F50), transmission electron microscope (TEM, FEI Tecnai G2 F20 at 200 kV). The Brunauer-Emmett-Teller (BET, JW-BK122W) surface areas were calculated through N2 adsorption-desorption isotherms. The pore volumes and pore size were calculated with the Barrett-Joyner-Halenda (BJH) model.

Electrochemical measurements The electrochemical performance was studied using the three-electrode system with 2 M KOH aqueous electrolyte. The working electrode (1 cm  1 cm) was fabricated by pressing homogeneous slurry of active material, acetylene black and polytetrafluoroethylene (PTFE) on Ni foam with a mass ratio of 8:1:1 and drying in a vacuum oven at 80  C for 12 h. The mass loading of active material was controlled at 1.5e2.0 mg/cm2. The Hg/HgO and Pt plate (1 cm  1 cm) was utilized as reference electrode and counter electrode，respectively. The cycle voltammetry (CV) and Galvanostatic charge/discharge (GCD) curves were recorded on the CHI660E electrochemical workstation. The specific capacity (Cs, C/g) was calculated from the following equation: Cs ¼ I  Dt=m ZV Cs ¼

IðVÞdV

(1)  mn

(2)

V0

where, I is the current (A), Dt is discharge time (s), m is the mass of active materials (g), n is scan rate (mV/s). Besides, the asymmetric supercapacitor was fabricated using polyethylene (PE) as separator, Ni0.5Co0.5MoO4 as positive electrode and carbon spheres as negative electrode. The mass of the positive electrode and the negative electrode was balanced by the following formula: mþ  Cþ  DVþ ¼ m  C  DV

(3)

Furthermore, energy density (E, Wh/kg) and power density (P, W/kg) were calculated according to the following equations (where C is specific capacitance (F/g) of ASC device):  E ¼ CV2 7:2

(4)

P ¼ 3:6  E=Dt

(5)

Results and discussion The crystal phase of the resultant samples is identified by Xray diffraction (XRD). As shown in Fig. 1a, the diffraction peaks

3

of all samples can be indexed to the monoclinic phase CoMoO4 (JCPDS 21e0868) [26]. It is worth noted that the diffraction peaks slightly shift to high angles when nickel atoms are introduced into CoMoO4, which might be ascribed to the smaller radius of heteroatoms than the host atoms [34]. The average crystallite size is determined to be 21.9, 7.1, 11.1, 11.9, 11.7 and 11.8 nm for pure CoMoO4 and NixCo1-xMoO4 (x ¼ 0.1, 0.3, 0.4, 0.5 and 0.6) by Scherrer formula. It is obvious that the crystallite size becomes small for NixCo1-xMoO4 materials. These results indicate that the Ni2þ has been successfully doped into the CoMoO4 lattice [30,35]. It has been proved that the small crystallite size and poor crystallization are favorable for good capacitive performance [26]. Thus, NixCo1-xMoO4 will show better electrochemical performance than pure CoMoO4. The structure changes are obtained by the Raman spectra and results are shown in Fig. 1c and d. The peaks located at 936 cm1, 880 cm1, 818 cm1 and 350 cm1 can be directed to the characteristic peaks for a-CoMoO4 [36]. It is worthy to note that the Raman spectrum of Ni0.5Co0.5MoO4 mostly matches with that of CoMoO4, accompanying by a slight redshift phenomenon. In addition, the peaks become broader arising from heteroatom doping [37]. These results further confirm the formation of CoMoO4 and the successful incorporation of nickel into CoMoO4 lattice. The morphologies with different nickel-cobalt molar ratios are shown to investigate the effect of nickel on the morphologies. As shown in Fig. 2a, pure CoMoO4 presents rod-like morphology with the diameter of about 200 nm. The rod structure is maintained in the NixCo1-xMoO4 (x ¼ 0.1, 0.3, 0.4, 0.5, 0.6). Furthermore, the diameter of the NixCo1-xMoO4 nanorods becomes smaller and Ni0.5Co0.5MoO4 displays chrysanthemum structure. The chrysanthemum structure is beneficial to charge transport and contact between electrolyte and electrode [38]. The change in morphologies and growth mechanism for all samples might be explained as follows and shown in Scheme 1. First, Co2þ and MoO2 4 might be combined randomly, when there is no Ni2þ. Then, Ni2þ and Co2þ are combined with MoO2 4 competitively when introducing nickel ions. As the amount of nickel ions increases, the competitive effect is more obvious. Nickel and cobalt ions are successively arranged to combine with molybdate ions when the molar ratio of nickel and cobalt ions is 1. These promote the orderly growth of the nanorods to form a chrysanthemum structure. Subsequently, the competitive effect is weakened with the further increases of nickel ions, leading to the destruction of the chrysanthemum structure. The specific surface area of electrode materials is also an important parameter to electrochemical performance. Generally, large surface area can provide more active sites and facilitate transport of electrolyte ions. As shown in Fig. 3 and Fig. S1, the isothermal adsorption-desorption curves of all samples can be classified as typical IV-type with the H3 hysteresis loop, demonstrating the resultant samples are mesoporous material [10]. Furthermore, Ni0.5Co0.5MoO4 exhibits largest specific surface among all samples (Table S1). Thus, we can reasonably reveal that nickel doping is beneficial to increase the surface area. XPS is employed to investigate the elemental composition and valence state. In Fig. 4a, the XPS signals of elements Ni 2p, Co 2p, Mo 3 d, O 1s and C 1s can be observed in the full survey

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Fig. 1 e (a) XRD patterns of the CNMO samples (b) Partial magnification of XRD pattern in (a); (c) Raman spectra of the CNMO sample (d) Partial magnification of Raman spectra in (c).

Fig. 2 e SEM images of (a) pure CoMoO4, (b)e(e) NixCo1-xMoO4 (x ¼ 0.1, 0.3, 0.4, 0.5, 0.6, respectively).

spectrum of Ni0.5Co0.5MoO4, which indicates that nickel atoms are successfully introduced into CoMoO4. Fig. 4b presents the high-resolution spectrum of Co 2p. The existence of Co2þ can be evidenced by two spin-orbit doublets with two shakeup satellite peaks (denoted at as “Sat.“) at 781.2 eV (Co 2p3/2) and 797.3 eV (Co 2p1/2) [39]. In the spectrum of Ni 2p (Fig. 4c), the two main peaks located at 856.3 eV and 873.9 eV can

correspond to Ni 2p3/2 and Ni 2p1/2, respectively [40]. The bond between the two main peaks can be separated by 17.6 eV, indicating the existence of the Ni2þ oxidation state [41]. The high-resolution spectrum of Mo 3 d displays two main peaks at 232.3 and 235.4 eV, which can be ascribed to Mo 3d5/2 and Mo 3d3/2, respectively. And the energy gap of two peaks is 3.1 eV, which indicates the existence of Mo6þ oxidation state

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Scheme 1 e The growth mechanism for formation of NixCo1-xMoO4.

Fig. 3 e (a), (c) Nitrogen adsorption-desorption isotherms; (b), (d) Pore-size distribution curves for pure CoMoO4 and Ni0.5Co0.5MoO4.

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Fig. 4 e (a) XPS survey spectrum of the sample Ni0.5Co0.5MoO4; High-resolution XPS spectrum of (b) Co 2p, (c) Ni 2p, (d) Mo 3 d, (e) O 1s for the Ni0.5Co0.5MoO4.

[41,42]. Fig. 4e exhibits the O 1s spectrum with two peaks at 530.6 eV and 532.3 eV. These peaks can be indexed to Co/NieO oxidation state and oxygen ions in low coordination on the surface, respectively [20,41]. Furthermore, the obtained nickel-cobalt ratio can be found to be 0.36:0.5, according to the results of XPS analysis. Moreover, we study Ni0.5Co0.5MoO4 in detail because of the best electrochemical performance as discussed below. In Fig. 5a, Ni0.5Co0.5MoO4 takes on 3D chrysanthemum structure with nanorod diameter of about 100 nm. In Fig. 5d, many mesopores are uniformly distributed on the nanorods, which is beneficial to the high electrochemical performance. Furthermore, the HR-TEM image is employed to investigate the lattice fringes of the Ni0.5Co0.5MoO4 material. In Fig. 5e, the characteristic spacing of 0.36 nm corresponds well to the (002)

lattice planes of CoMoO4, which is consistent with XRD results. It is worth noting that the lattice spacing is slightly larger than the typical value, attributing to the lattice distortion of nickel doping. The selected area electron diffraction (SAED, Fig. S2) confirm the sing-crystalline nature of the obtained samples [43]. The EDS result reveals the existence of Ni, Co, Mo and O in the Ni0.5Co0.5MoO4 material, further indicating the successful introduction of nickel. Moreover, the elemental mapping (Fig. 5g) reveals the homogeneous distribution of Ni, Co, Mo, O on the surface of Ni0.5Co0.5MoO4 material. The obtained molar ratio between nickel and cobalt is 0.3:0.5, which is close to the result given by XPS. CV curves, GCD curves and EIS spectra are recorded in the three-electrode system using aqueous 2 M KOH electrolyte to investigate the electrochemical performances. In Fig. 6a, the

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Fig. 5 e (aeb) SEM images; (ced) TEM images of Ni0.5Co0.5MoO4; (e) HR-TEM; (f) EDS; (g) EDX elemental mapping spectrums of Ni0.5Co0.5MoO4.

CV curves of all samples exhibit a pair of redox peaks, indicating the pseudo-capacitive behavior. The possible reactions can be described as equations (7) and (8) [38]. Among all samples, the area enclosed by CV curves of the Ni0.5Co0.5MoO4 electrode is largest at 20 mV/s, proving the highest specific capacity. Furthermore, the CV curve of Ni foam is also tested to confirm the capacity influence of Ni foam substrate. Obviously, the CV-integrated area of nickel foam is negligible, indicating little capacity contribution from nickel foam. Additionally, the GCD curves also demonstrate that the Ni0.5Co0.5MoO4 electrode possesses higher specific capacity than other samples. Therefore, the electrochemical properties of Ni0.5Co0.5MoO4 are systematically tested. In Fig. 6c, the shape of CV curves is similar with the increase of scan rate from 5 to 100 mV/s, which demonstrates this material possesses excellent rate performance. Additionally, it is clearly found that anode peaks disappear and cathode peaks shifts to lower potential at high scan rate, which might be ascribed to the polarization effect at high scan rate [44]. In Fig. 6d, the GCD curves are symmetric, indicating this material has high coulomb efficiency and excellent reversibility [30]. The detailed specific capacity of all electrode at different current densities and scan rate are calculated and corresponding results are shown in Fig. 6e and Fig. S3. Ni0.5Co0.5MoO4 electrode achieves specific capacity of 325.9 C/g at 0.5 A/g, while the pure CoMoO4 electrode and NixCo1-xMoO4 (x ¼ 0.1, 0.3, 0.4, 0.6) electrode only have specific capacity of 114.0 C/g, 133.4 C/g, 200.0 C/g, 232.4 C/g and 286.7 C/g, respectively. In addition, the

Ni0.5Co0.5MoO4 electrode achieves capacity retention of 79.8% when the current density increases from 0.5 to 10 A/g, which is significantly higher than that of the other samples. Such an excellent rate performance might be attributed to the increased electrical conductivity. In addition, the cycle stability is tested for all samples at 5 A/g. The capacity retentions maintain above 81% (Fig. 6f and Fig. S4) for all samples after 3000 cycles, indicating good cycle stability. Moreover, Fig. S5 exhibits the morphology of Ni0.5Co0.5MoO4 before and after 3000 cycles. The rod structures are difficult to distinguish due to the presence of PTFE and acetylene black. However, the Ni0.5Co0.5MoO4 electrode maintains a uniform structure with a slightly roughened surface after 3000 cycles. The result highlights the structure durability of the electrode material. 

3½Nix Co1x ðOHÞ3  4 3Nix Co1x OOH þ 3H2 O þ 3e

(6)

CoOOH þ OH 4CoO2 þ H2 O þ e

(7)

EIS is also performed to investigate the electronic conductivity of all samples and results are shown in Fig. 7a. As we all know, the EIS curve consists of the semicircle of high frequency region and the straight line of the low frequency region. Generally, the slop of straight line represents the Warburg resistance form ion diffusion and transport. Obviously, the slope in the low frequency region of Ni0.5Co0.5MoO4 is larger than other samples, certifying the lower ion diffusion impedance in the electrochemical process. The diameter of semicircle in high frequency represents the charge transport

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Fig. 6 e (a) CV curves (20 mV/s) and (b) GCD curves (0.5 A/g) of NixCo1-xMoO4(x ¼ 0, 0.1, 0.3, 0.6, 0.5, 0.6); (c) CV curves and (d) GCD curves of Ni0.5Co0.5MoO4; (e) Specific capacity versus current densities of all samples; (f) Cycling stability of pure CoMoO4 and Ni0.5Co0.5MoO4 at current density of 5 A/g.

resistance (Rct) associated with the faradaic reactions, while the intercept of semicircle with real axis represents the ionic conductivity of the electrolyte and the electronic conductivity between the electrode and the current collector resistance (Rs).The specific value are calculated by an equivalent circuit (insert in Fig. S6) and shown in Table S2. Obviously, the Ni0.5Co0.5MoO4 electrode possesses the smallest resistance value. This is beneficial for the improvement of electrochemical performance, which promotes the effective diffusion of electrolyte ions in redox reaction [30]. Fig. 7b show the Bode plot to assess the frequency-dependent capacitor behavior. The phases at the low frequent are determined to 80.5 , 78 , 75.9 , 78.1 , 75.4 and 70.1 for NixCo1-xMoO4 (x ¼ 0, 0.1,

0.3, 0.4, 0.5 and 0.6), respectively. The large phase angle indicates good capacitive performance [45,46]. Fig. 7c and d exhibit the graph between the complex capacitance and frequency. The complex capacitance can be calculated by formula (8), (9): 0

00

ZðuÞ ¼ Z ðuÞ þ jZ ðuÞ

0

CðuÞ ¼ C ðuÞ  jC00 ðuÞ

  . 0  u  ZðuÞj2 C ðuÞ ¼  Z00 u

  . 00  u  ZðuÞj2 C ðuÞ ¼ Z0 u

(8) (9)

where u is the angular frequency (2pG), |Z(u)| is the impedance modulus (U). The relaxation frequency (Gr) is the corresponding frequency value where C00 takes the maximum value in graph of C00 -log G. And the relaxation time (tr) is defined as 1/Gr.

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Fig. 7 e EIS measurements of the NixCo1-xMoO4 (x ¼ 0, 0.1, 0.3, 0.4, 0.5, 0.6) electrodes: (a) Nyquist plots, and the evolution of (b) the phase angle, (c) the real capacitance and (d) the imaginary capacitance versus frequencies.

The relaxation time is determined to 6.36, 4.47, 3.02, 2.85, 2.54 and 26.58 s for NixCo1-xMoO4 (x ¼ 0, 0.1, 0.3, 0.4, 0.5 and 0.6), respectively. The Ni0.5Co0.5MoO4 electrode possesses the smallest tr, indicating the fastest ion diffusion capability and the best frequency response performance [47]. The largest relaxation time of NCMO-5 may be ascribed to agglomeration of nanorod. Based on the above analysis, nickel doping has a great influence on the electrochemical performances of the electrode materials. The enhanced electrochemical behaviors can be ascribed to the following two aspects. First, the synergistic

effect of different metal atoms reduces charge transfer resistance and improves electrical conductivity. Secondly, nickel doping leads to ordered growth of nanorods and large surface area. To better understand the charge storage kinetics of Ni0.5Co0.5MoO4 electrode, the two different charge storage mechanisms are further studied by the Power’s law. Generally, the charge storage mechanism consists of two components: diffusion-controlled processes and capacitivecontrolled processes (including pseudo-capacitance and double layer capacitance) [48]. To distinguish them, the

Fig. 8 e (a) Plots between log (i) versus log (v); (b) Variation of b-value with applied potentials; (c) Normalize contribution of capacitive capacities at different scan rates for Ni0.5Co0.5MoO4.

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Fig. 9 e (a) CV curves of the ACS device with different working-voltage window at scan rate of 20 mV/s; (b) Specific capacitance of different mass ratio(m-/mþ, total 4.5 mg); (c) CV curves of Ni0.5Co0.5MoO4//C sphere ASC at different scan rates from 10 to 100 mV/s; (d) GCD curves of Ni0.5Co0.5MoO4//AC ASC at different current densities from 0.5 to 10 A/g; (e) EIS curves before and after 5000 cycles; (f) Cycling stability of Ni0.5Co0.5MoO4//C sphere ASC at current density of 1 A/g.

dependence of the CV current response can be expressed as equation (10) [49,50]. i ¼ avb

(10)

where, i is current and v is scan rate, a and b are the adjustable parameters. The b value can be obtained by the slope of log i vs log v and it is often used to analyze the type of charge storage. When b is 0.5, it can be inferred that the storage mechanism of electrode material is diffusion-controlled processes. When b is 1, it corresponds to capacitive-controlled processes [38]. As can be seen from Fig. 8a and b, the b value of Ni0.5Co0.5MoO4 is close to 1 at various voltages, indicating that capacitive-

controlled is domination [31,48]. Based on the previous study and the result of CV curves, it is very meaningful to calculate the capacitive contribution rate. It can be described by the following equations [48,51]: iðVÞ ¼ k1 v þ k2 v1=2

(11)

 i v1=2 ¼ k1 v1=2 þ k2

(12) 1/2

are capacitive-controlled contribution where k1v and k2v and diffusion-controlled contribution, respectively. As can be seen from Fig. 8c, the capacitive contribution rate varies with scan rate, which is determined to be 82.49%, 85.16%, 86.55%,

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Fig. 10 e Ragone plot of the Ni0.5Co0.5MoO4//C sphere ASC.

87.20% and 92.72% of the total charge storage, respectively. Obviously, the capacitive contribution rate gradually increases as the scan rate increases. Furthermore, the high capacitive contribution rate is beneficial to improve the rate performance and cycle stability of the material [31]. The aqueous asymmetric supercapacitors (ASC) are assembled to study the practical application of Ni0.5Co0.5MoO4 material. Generally, the working-voltage window is fixed for aqueous asymmetric supercapacitor device. Therefore, the energy density of ASC is mainly dependent on the cell capacitance, which is subject to the one with low specific capacitance, namely, the negative electrode. Considering that the poor electrochemical performance of active carbon (102 F/ g at 0.5 A/g, Fig. S8), aqueous ASC is fabricated using Ni0.5Co0.5MoO4 as positive electrode and carbon sphere as negative electrode (214 F/g at 0.5 A/g), respectively. To get the optimal voltage windows of the ASC device, a series of CV measurements are recorded with different voltage windows at 20 mV/ s. There is no serious polarization reaction with potential window up to 1.6 V. Therefore, the electrochemical performance of asymmetric supercapacitor should be evaluated under the 0e1.6 V. And the specific capacitance with different mass ratio (m-/mþ, the total mass is 4.5 mg) are recorded to obtain the optimal mass ratio. It is obvious that the optimal mass ratio is confirmed to 1.5e2.0, which is consistent with theoretical value (1.625). Therefore, the electrochemical performance of the ASC device are systematically tested with mass ratio of 1.5e2.0 under 0e1.6 V. Fig. 9c describes the CV curves of the ASC device at different scan rates from 10 to 100 mV/s. The CV curves of the ASC device are quasi-rectangular, which suggests that the ASC device has a typical EDLC behavior [42]. In addition, the redox peaks in CV curves may be ascribed to the pseudocapacitance caused by chemical reactions [52]. Fig. 9d describes the GCD curves of the ASC device at various current density (0.5e10 A/g). Based on the discharge times, the specific capacitance of the ASC device can achieve 88.78 F/g at 0.5 A/g and retain 38.13 F/g at 10 A/g. Furthermore, the GCD curves are

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symmetrical, demonstrating that this device possesses high reversibility. Meantime, a slight platform is observed due to the redox reaction of the metal oxide, which agrees with the CV results [44]. The relaxation time (calculated from Fig. S9) is used to further investigate the electrochemical performance of the ASC device. The relaxation time is determined to be 11 s for the ASC device. The small relaxation time indicates that the device has fast reversible charge and discharge performance [53]. Fig. 9e shows the EIS curves before and after 5000 cycles. The specific value is calculated by an equivalent circuit (Fig. S10) and the fitting result was shown in Table S3. The value of charge transfer resistance (Rct) and ionic and electronic conductivity (Rs) only increase a little after 5000 cycles. Fig. 9f exhibits the cycling performance of the ASC device at 1 A/g. The ASC device maintains a high capacitance retention of 115% and the excellent coulomb efficiency of 100% after 5000 charge-discharge cycles. These results indicates that the ASC device possesses excellent cycle stability and reversibility. To further evaluate the electrochemical performance of the ASC device, energy density and power density are calculated using equations (5) and (6). Fig. 10 presents the Ragone plot between energy density and power density. The Ni0.5Co0.5MoO4//C sphere ASC device exhibits the maximum energy density of 31.57 Wh/kg and power density of 8000 W/kg. This value is superior to the Ni0.5Co0.5MoO4//AC device and many previous reported values [54e59]. Furthermore, a red light-emitted diode can be illuminated using two ASC devices in series.

Conclusions In summary, the NixCo1-xMoO4 nanorods are fabricated through co-precipitation method. The original disordered structures of the rods gradually become ordered, owing to nickel doping. Among them, Ni0.5Co0.5MoO4 exhibits chrysanthemum and excellent electrochemical performance. The specific capacity of Ni0.5Co0.5MoO4 electrode can achieve 325.9 C/g at 0.5 A/g and 260 C/g at 10 A/g. Moreover, the assembled asymmetric supercapacitor (Ni0.5Co0.5MoO4//C sphere) achieves maximum energy density of 31.57 Wh/kg and power density of 8000 W/kg by adjusting the voltage window and the mass ratio (m-/mþ). Furthermore, the ASC device possesses satisfactory cycle stability and reversibility. In addition, a red light-emitted diode can be illuminated for about 10 min with two devices in series.

Acknowledgments This work was financial supported by the Major projects of Shan Xi province [MC2015-04].

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.127.

Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127

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Please cite this article as: Zhang X et al., Nickel-doped cobalt molybdate nanorods with excellent cycle stability for aqueous asymmetric supercapacitor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.127