Supercapacitive performance of CoMoO4 with oxygen vacancy porous nanosheet

Journal Pre-proof Supercapacitive performance of CoMoO4 with oxygen vacancy porous nanosheet

Pengxi Li, Chaohui Ruan, Jing Xu, Yibing Xie PII:

S0013-4686(19)32206-6

DOI:

https://doi.org/10.1016/j.electacta.2019.135334

Reference:

EA 135334

To appear in:

Electrochimica Acta

Received Date:

17 August 2019

Accepted Date:

18 November 2019

Please cite this article as: Pengxi Li, Chaohui Ruan, Jing Xu, Yibing Xie, Supercapacitive performance of CoMoO4 with oxygen vacancy porous nanosheet, Electrochimica Acta (2019), https://doi.org/10.1016/j.electacta.2019.135334

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Journal Pre-proof Supercapacitive performance of CoMoO4 with oxygen vacancy porous nanosheet Pengxi Li, Chaohui Ruan, Jing Xu, Yibing Xie*, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

ABSTRACT: CoMoO4 with oxygen vacancy (CoMoO4-x) porous nanosheet is prepared by hydrothermal synthesis and hydrogenation reduction method. The CoMoO4-x porous nanosheet has a higher specific surface area together with a more diverse pore size distribution than CoMoO4 nanosheet. The density functional theory calculation result exhibits that CoMoO4-x has lower energy band gap (0.041 eV) than CoMoO4(1.683 eV). The electronic density of states plots reveals that CoMoO4-x has more electron state distribution at Fermi energy level than CoMoO4. CoMoO4-x porous sheet achieves higher specific capacitance (1989 g-1) than CoMoO4 (917 F g-1) at 2 mA cm-2. It also achieves superior capacitance retention rate (91.3%) than CoMoO4 (83.9%) with the current density increasing from 2 to 20 mA cm-2. The introduction of oxygen vacancy can increase the carrier density, accelerate the electron transfer, promote the electrical conductivity, and accordingly strengthen the redox reactivity of CoMoO4-x. An asymmetric supercapacitor is also constructed by using CoMoO4-x as positive electrode and activated carbon as negative electrode. It achieves high energy density of 62.3 W h kg-1 at a power density of 800 W kg-1, together with a good cyclic life. Both experimental measurement and theorical calculation are applied to prove the promotive role of oxygen vacancy in supercapacitive performance of CoMoO4-x.

Keywords: Oxygen vacancy; CoMoO4; Porous nanosheet; Supercapacitive performance; Theorical calculation

* Corresponding author. E-mail address: [email protected] (Y. Xie) 1

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Journal Pre-proof 1. Introduction Due to the continuous growth of energy demand, electrochemical energy storage is still a hot research field. Among the numerous new energy sources and storage devices, supercapacitors are favored by researchers on account of the safety, reliability, long service life, fast charging and discharging speed [1, 2]. Electrode material, as an important part of supercapacitors, is a key factor affecting and limiting its development [3-6]. It is necessary to research progressive electrode materials which own excellent electrochemical features, including specific capacitance, high rate performance, long cycling stability[7-9]. Metal oxides, especially transition metal oxides, have the advantages of high theoretical capacitance, low price and easy preparation[10, 11]. Thus, they could play the role of outstanding electrode materials. Moreover, transition metal oxides can provide more active sites for reversible faradaic reactions and chemical substance exchanges, rather than just ion exchanges between current collectors, which contribute greatly to energy storage capacitance[1214]. The redox electrolyte materials also contribute to the pseudo-capacitance performance [15-17]. Liu et. al. have used ingenious methods to prepare an hierarchical shell-core NiO nanospines@carbon electrode material with good capacitive properties[18]. When the current density was 2 A g-1, specific capacitance was 1161 F g-1. As the current density reached 10 A g-1, specific capacitance could remain at 839 F g-1. Ma et. al. have prepared hierarchical ZnCo2O4@MnO2 core-shell nanotube arrays upon the surface of Ni foam matrix by a two-step hydrothermal approach[19]. The specific capacitance could achieve to 1981 F g-1 with current density at 5 A g-1. As the current density reached 40 A g-1, the specific capacitance could remain 1611 F g-1. In addition, Fe2O3[20], Co3O4[21], CoFe2O4[22] and NiCo2O4[23] have similarly been surveyed as electrode materials for supercapacitor. Transition metal molybdate is a candidate for a new type of electrode material for supercapacitors on account of its impressive electronic conductivity, such as NiMoO4[24], CoMoO4[25], ZnMoO4[26], FeMoO4[27]. Although their conductivity is better than that of other bimetallic oxides, they still fall short of expectations. The slow electrochemical kinetics also make them unfavorable. For solving this problem, researchers are putting a lot of efforts to modify them to improve their electrochemical properties. Many efforts have focused on forming nanocomposite structures or combining with other conductive substrates[28, 29]. Chen et. al. has prepared core-shell NiMoO4@Ni-Co-S nanorods on nickel foam by hydrothermal and electrodeposition methods. With 3

Journal Pre-proof the current density at 5 mA cm-2, specific capacitance for NiMoO4@Ni-Co-S nanorods was 1892 F g-1. Going through 6000 cycles, the capacity retention rate remained at 91.7% [30]. Kumar et. al. have used a flexible method to synthesize honeycomb-structured CoMoO4-MnO2 nanocomposites on graphene foam[31]. With the current density at 3 mA cm-2, specific capacitance for CoMoO4MnO2 reached 8.01 F cm-2. For CoMoO4, the hybrid structure can improve the electrochemical properties to some extent. However, this improvement only relies on other active materials, and does not fundamentally solve the intrinsic problem and improve the performance. Fortunately, introducing oxygen vacancies into the surface is a wise choice and method[32]. The hydrogenation reduction process can make metal oxides produce oxygen vacancy and reduce their electrical resistance[33]. After the formation of oxygen vacancy, oxygen vacancy has a positive charge, which can increase the adsorption intensity for OH-[34]. The newly generated oxygen vacancy promotes the electronic conductivity and enhance redox reaction kinetics. More importantly, the oxygen vacancies also serve as active sites to improve the capacitance[35]. Herein, CoMoO4 nanosheet was firstly synthesized via hydrothermal method. The CoMoO4-x porous nanosheet containing oxygen vacancy was obtained via the following hydrogenation process during which the Mo6+ was partially reduced to Mo4+. CoMoO4-x and activated carbon are separately employed as positive and negative materials to assemble asymmetric supercapacitor (ASC).

2. Experiment section Before sample preparation, 1 M HCl was used to conduct ultrasonic cleaning on Ni foam (5×1×0.15 cm3) matrix for 5 minutes. Thus the oxide layer on the surface was removed. Subsequently, Ni foam matrix was cleaned using anhydrous ethanol and distilled water repeatedly. Fig. 1 exhibits a schematic diagram of preparation of CoMoO4 porous nanosheet, which was obtained employing a simple synthetic means including two-step procedures. Firstly, CoMoO4 nanosheet generated upon Ni foam matrix was obtained by hydrothermal means and heat-treated in Ar atmosphere in the first step. Then, CoMoO4 nanosheet was reduced in mixture of Ar and H2 atmosphere to obtain CoMoO4 porous sheet containing oxygen vacancy (CoMoO4-x). Thus, the CoMoO4-x porous nanosheet was acquired on Ni foam matrix.

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Fig. 1. Schematic for the preparation process of CoMoO4 and CoMoO4-x. 2.1. Preparation of CoMoO4 nanosheet The CoMoO4 nanosheet was synthesized via a simple hydrothermal procedure and post-annealed method. Adding the appropriate amounts of Co(NO3)2·6H2O (0.291 g), Na2MoO4·2H2O (0.242 g), CO(NH2)2(0.24 g) and NH4F(0.074g) into a mixture solution of 25 ml water and 5 ml ethanol and stirring were done until complete dissolution. The mixed solution containing Ni foam matrix was transferred to the Teflon-lined stainless-steel. The autoclave was heated to 160 °C and maintained for 12 h. The gained sample is cleaned and ultrasonated to remove impurities attached to the surface. Finally, CoMoO4 nanosheet was gained after calcining in Ar atmosphere at 450 °C for 2 h. And the heating rate was 5 °C min-1. The mass loading of CoMoO4 nanosheet on Ni foam matrix was ~1.5 mg cm-2. 2.2. Preparation of CoMoO4-x porous nanosheet The CoMoO4-x porous nanosheet containing oxygen vacancy was synthesized via a hydrogenation reduction treatment. Simply, the as-obtained CoMoO4 nanosheet was calcined in H2/Ar (10 vol% H2 and 90 vol% Ar) atmosphere at 400 °C for 1 h. The flow rate of mixture gas is 100 sccm. The resulted CoMoO4 nanosheet was converted into CoMoO4-x porous sheet. The mass loading of CoMoO4-x nanosheet on Ni foam matrix was ~1.4 mg cm-2. 2.3. Construction of all solid-state asymmetric supercapacitor CoMoO4-x and activated carbon are separately employed as positive and negative materials to assemble asymmetric supercapacitor (ASC). The separator is non-woven fabric and the electrolyte is polyvinyl alcohol/potassium hydroxide (PVA/KOH) gel electrolyte. Simply, the PVA/KOH gel electrolyte was prepared according the subsequent procedure. Firstly, PVA (1 g) was added to 5

Journal Pre-proof distilled water (10 mL) and stirred at 80 °C with 2 h until PVA was completely dissolved, resulting in a diaphanous solution with uniform and low viscosity. Then, 5 mL of 6 M KOH solution was added to the above solution slowly and stirred, resulting in a transparent gel with uniform and moderate viscosity. Lastly, a suitable amount of PVA-KOH gel electrolyte was coated upon the electrode, dried at room temperature to remove the residual water in the electrolyte. The positive together with negative electrodes were superimposed upon each other using a non-woven cloth coated with electrolyte as a separator. The electrodes were sealed with an ultra-thin plastic film. A sandwich-shaped all-solid asymmetric supercapacitor was obtained and named CoMoO4-x//AC supercapacitor. The mass of activated carbon on the negative electrode is obtained by the charge balance formula: Q+m+=Q-m-

(1)

in which, Q+ and Q- respectively denote the charge on positive and negative electrodes, m+ and mrespectively represent the mass of samples on positive and negative electrodes. C+(ΔV)+ m+= C-(ΔV)- m-

(2)

in which, C+ and C- respectively represent the specific capacitances of positive and negative electrode materials in three electrodes test equipment, (ΔV)+ and (ΔV)- respectively represent potential windows of positive and negative electrode materials in three electrodes test equipment. 2.4. Theoretical calculation The CASTEP module of Materials Studio together with Density Functional Theory (DFT) were employed for theoretical simulation of electrode materials [36]. OTFG ultrasoft played the role of pseudopotential in the theoretical calculation. In addition, the treatment of all electronic energies of the exchange-correlation was based on the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The cut-off energy of plane wave basis set was 381 eV. The convergence criteria for the total energy calculation and geometric optimization step were set as follows: (ⅰ)The self-consistent field (SCF) energy tolerance was 2.0×10-6 eV per atom. (ⅱ) The maximum force tolerance was 0.05 eV Å-1. (ⅲ) The maximum displacement tolerance was 2×10-3 Å. 2.5. Characterization and Electrochemical measurement The crystal structure samples were determined by X-ray diffraction (XRD, D8 ADVANCE, Cu 6

Journal Pre-proof Kα radiation, 40 kV, 200 mA). The morphology of samples was detected using scanning electron microscopy (SEM, JEOL JSM-6700). The X-ray photoelectron spectroscopy spectrometer (XPS, ESCALAB 250 X-ray photoelectron spectrometer, Al Kα radiation) was used to analyse the chemical state of the samples. The surface area together with pore size distribution of samples were confirmed employing N2 adsorption/desorption tests (Micromeritics ASAP 2010, 77 K). Before testing, the sample was dried overnight at 200°C for dehydration in vacuum condition. BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) strategies were separately employed to confirm surface area together with pore size distribution of samples. The electrochemical properties of the samples was investigated via cyclic voltammetry (CV)， galvanostatic charge-discharge (GCD), as well as electrochemical impedance spectroscopy (EIS) methods. The testing procedures were carried out on a CHI760C electrochemical workstation using a conventional three-electrode test equipment. The reference electrode was saturated calomel electrode and the counter electrode was Pt foil electrode. The working electrode was our freshly synthesized sample. The aqueous electrolyte for measurement was 6 M KOH solution. Moreover, the rate capability performance together with cycling life were surveyed employing LAND CT2001A battery equipment.

3. Results and discussion 3.1. Morphology and structure characterization The crystal structure of electrode materials is determined by XRD analysis. The CoMoO4 and CoMoO4-x powders were extracted from nickel foamed substrate to avoid the strong peak effect of metallic nickel. As shown in Fig. 2, both of the CoMoO4 and CoMoO4-x show characteristic diffraction peaks of CoMoO4 phase (PDF#21-0868). The introduction of oxygen vacancy reduced the diffraction peak of CoMoO4-x and the crystallinity of the crystal[37]. The XRD curves of the two samples proved the successful preparation of CoMoO4.

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Fig. 2. XRD curves for CoMoO4 and CoMoO4-x. SEM was used to investigate the morphology and structure of CoMoO4 and CoMoO4-x. Fig. 3a and 3c demonstrated the SEM images of CoMoO4 in various magnification. The CoMoO4 nanosheets were grown well upon Ni foam matrix and interlaced together, forming a criss-crossed morphology. Fig. 3b and 3d exhibit the SEM images of CoMoO4-x in various magnification. After hydrogenation reduction, many pores were generated on CoMoO4-x nanosheet. Moreover, CoMoO4x

nanosheet became thinner and farther apart. The pores created on the nanosheet and the enlarged

interlayer spacing provide easier transport channels for ions and electrons, enabling more electrochemical reactions. The specific surface area together with pore diameter distribution of nanomaterials also have momentous effects on the electrochemical performance of samples. Fig 3e and 3f show the adsorption and desorption curves of N2 for CoMoO4 and CoMoO4-x and both of them are typical IV isotherms curves. The BET specific surface area for CoMoO4 is 36 m2 g-1 and while CoMoO4-x is 61 m2 g-1. Supposedly, increase of specific surface area may be caused by the pores on CoMoO4 nanosheet and the enlarged interlayer spacing. The pore size distribution curves of CoMoO4 and CoMoO4-x are exhibited in the insets of Fig. 3e and 3f, respectively. For CoMoO4, it presents two well-located peaks at 38.2 nm and 40.2 nm. For CoMoO4-x, it owns four well-located peaks, including 30.5 nm, 39.5 nm, 43.4 nm and 67.2 nm. Therefore, CoMoO4-x own more diverse pores structure than CoMoO4 after hydrogenation reduction. For CoMoO4-x, higher specific surface area together with more abundant pores can accelerate ion and electron transfer and promote 8

Journal Pre-proof electrochemical reaction. All of these are beneficial to intensify the electrochemical performance of CoMoO4-x.

Fig. 3. (a and c): SEM images for CoMoO4; (b and d) SEM images for CoMoO4-x; (e and f): N2 adsorption-desorption isotherm for CoMoO4 and CoMoO4-x (the inset shows the pore size distribution). Aiming at better understanding the element composition and valence state of electrode materials, we carried out XPS analysis on them. Fig. 4a shows the XPS spectra of CoMoO4 and CoMoO4-x, both of which contain the peaks of Co 2p, Mo 3d and O 1s. The high resolution spectrum for Co 2p of CoMoO4 and CoMoO4-x both display four peaks which are exhibited in Fig. 4b. One of the main peaks (781.2 eV) coupled with satellite peak (786.9 eV) correspond to Co 2p3/2[38]. Another main peak (796.9 eV) together with satellite peak (803.1 eV) correspond to Co 2p1/2[39]. Both of main peaks with satellite peaks verify the existence of Co2+ in both CoMoO4 and CoMoO4-x electrode

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Journal Pre-proof materials[40]. The high resolution spectrum of Mo 3d of CoMoO4 and CoMoO4-x are shown in Fig. 4c. For CoMoO4, the two peaks (232.3 eV, 235.5 eV) respectively represent Mo 3d5/2 and Mo 3d3/2 [41]. The two peaks with a gap of 3.3 eV reveal a Mo6+ oxidation state in CoMoO4[42]. For CoMoO4-x, there are four distinct peaks. The three peaks at 236.9 eV, 235.3 eV, 232. eV is in accordance with the Mo6+ [43, 44]. Moreover, the peak located at 233.8 eV is confirmed to be the 3d3/2 peak of Mo4+ [43]. Thus, during the hydrogenation process, Mo6+ was partially reduced to Mo4+. The high resolution spectrum of O 1s of CoMoO4 and CoMoO4-x demonstrate two peaks (Fig. 4d). The peaks defined at 530.2 eV represents metal-oxygen bonds. And the peak defined at 531.7 eV corresponds to oxygen defects[45]. The relative magnitudes of M-O and oxygen vacancy for the two oxides indicate CoMoO4-x owns more oxygen defects after hydrogenation than CoMoO4. Moreover, according to the results of XPS, the calculated stoichiometry based the on cobalt, molybdenum, and oxygen XPS peak areas of CoMoO4-x was CoMoO3.523.

Fig. 4. (a): Full XPS spectra; (b): Co 2p XPS spectra; (c): Mo 3d XPS spectra(c); (d): O 1s XPS spectra for CoMoO4 and CoMoO4-x. 3.2. Electrochemical properties The CV curves at 5 mV s-1 of CoMoO4 and CoMoO4-x are tested with the potential ranging from 10

Journal Pre-proof 0 to 0.45 V (Fig. 5a). It is obvious that CoMoO4-x has a broader redox peak as well as a larger geometric area than those of CoMoO4. The amount of redox species and/or the accessibility of the redox sites may be higher. Also, there is even a larger capacitive current before the onset of the redox activity. The oxygen vacancies upon the surface of sample could not only improve the adsorption intensity of OH-, but speed up the redox reaction speed. Besides, the GCD curves at 2 mA cm-2 of CoMoO4 and CoMoO4-x are tested from 0 to 0.45 V (Fig. 5b). The charging and discharge times of CoMoO4-x are longer than those of CoMoO4, which proves once again that CoMoO4-x has a higher specific capacitance. In order to get rid of the influence of Ni foam substrate on material properties, pure Ni foam was also tested as a contrast experiment. The Ni foam respectively shows a negligible area and a negligible discharge time in the CV and GCD curves. This indicates that it has little contribution to specific capacitance. Introducing oxygen vacancy into CoMoO4 can increase carrier density and redox reaction activity on the surface of CoMoO4. At the same time, oxygen vacancy can also act as the reactive site to improve its capacitive properties. The improved electrochemical performance of the CoMoO4-x sample is not only related to its higher surface area, increased porosity/interlayer spacing, but more oxygen vacancies are automatically available at the electrode/electrolyte interface as a result of the enhanced morphological properties.

Fig. 5. CV curves at 5 mV s-1(a), GCD curves at 2 mA cm-2(b) for CoMoO4 and CoMoO4-x. Aiming at further investigating the electrochemical performances of CoMoO4 and CoMoO4-x, the respective detailed CV and GCD curves of them were also tested. Fig. 6a and 6c respectively show CV curves of CoMoO4 and CoMoO4-x at the sweep rates (5~50 mV s-1). At the same sweep rate, CoMoO4-x presents a larger response current and a broader redox peak than CoMoO4. This indicates that CoMoO4-x has a higher specific capacitance and a more adequate electrochemical reaction. Fig. 6b and 6d respectively display the GCD curves of CoMoO4 and CoMoO4-x at different current 11

Journal Pre-proof densities (2~20 cm-2). Consequently, the area-specific capacitance (Cs, F cm-2) together with massspecific capacitance (Cm, F g-1) were acquired by making use of the discharge curves using the formulas below, respectively: CS= IΔt/SΔV

(3)

Cm= IΔt/mΔV

(4)

in which, I is the constant discharge current (A), Δt is the discharge time (s), S is the geometric surface area (cm2), ΔV is the potential window (V), m is the mass for sample (g) Fig. 6e and 6f respectively show the corresponding relation diagrams of area-specific capacitance and mass-specific capacitance. It is appreciated that CoMoO4-x has better capacitance performance than CoMoO4. With increasing current density (2~20 mA cm-2), area- and mass-specific capacitances of CoMoO4-x decrease from 2.74 F cm-2 to 2.54 F cm-2 and from 1989 F g-1 to 1816 F g-1, respectively. However, area- and mass-specific capacitances for CoMoO4 decrease from 1.33 F cm-2 to 1.15 F cm-2 and from 917 F g-1 to 770 F g-1, respectively. And it was surprising that the mass-specific capacitance retention for CoMoO4-x was 91.3% with current densities increasing (2~20 mA cm-2). It was better than that of CoMoO4 (83.9%). When oxygen vacancy is introduced into CoMoO4, the adsorption capacity of OH- for CoMoO4-x becomes larger and the electrochemical reaction becomes more sufficient and faster. Meanwhile, the oxygen vacancy as carrier could enhance the electronic conductivity of CoMoO4-x, as well as the charge transfer ability. Moreover, the higher specific surface area coupled with more abundant pores of CoMoO4-x can accelerate ion and electron transfer and promote electrochemical reaction Therefore, CoMoO4-x shows superior specific capacitance and better capacitance retention.

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Fig. 6. CV curves for CoMoO4 (a) and CoMoO4-x (c); GCD curves for CoMoO4 (b) and CoMoO4-x (d); Areal specific capacitances (e) and mass specific capacitances (f) for CoMoO4 and CoMoO4-x. Meanwhile, the cyclic stability and rate performance are also important criteria for evaluating the performance of electrode materials in supercapacitor. The cyclic life for CoMoO4 and CoMoO4-x at 20 mA cm-2 are tested for 5000 cycles (Fig. 7a). It is clear that the specific capacitance of CoMoO4-x can remain at 98.8% after 5000 cycles. And the specific capacitance of CoMoO4 remains at 92.5%. This result reveals that both of them have good cyclic stability. The introduction of oxygen vacancy still guarantees the stability of crystal structure, as well as the mechanical and cycling stability of samples. The rate performance curves of CoMoO4 and CoMoO4-x are shown in Fig. 7b, which were 13

Journal Pre-proof tests for 100 cycles at every current density. As the current density increases, the specific capacitances of both CoMoO4 and CoMoO4-x reduce in a step-like manner. The specific capacitance of CoMoO4-x could recover to 97.8% of the initial capacitance as the current densities return to 2 mA cm-2 again. While the specific capacitance for CoMoO4 could recover to 86.8% of the initial capacitance. CoMoO4 itself has a considerable rate performance and it could be further enhanced via the introduction of oxygen vacancy. This is mainly attributed to that the oxygen vacancy can improve the conductivity of CoMoO4-x.

Fig. 7. (a) Cycling stability for CoMoO4 and CoMoO4-x; (b) Rate capability for CoMoO4 and CoMoO4-x. The electrochemical interface properties of CoMoO4 and CoMoO4-x electrode materials were tested by EIS with the frequency from 10-2 to 105 Hz at the open circuit potential. The Nyquist curves of CoMoO4 and CoMoO4-x electrodes are exhibited in Fig. 8a. Both of them were fitted based on the equivalent circuit in the illustration. In high frequency region, they both show a semi-arc shape. While in low frequency region, they both show a linear shape. The fitting values of equivalent circuit parameters for CoMoO4 and CoMoO4-x are displayed in Table 1. The Ro stands for ohm resistance and its value could be read according to the intercept of the curve with horizontal axis. Therefore, Ro values of CoMoO4 and CoMoO4-x are 0.6119 Ω and 0.4608 Ω, respectively, preliminarily proving that CoMoO4-x has better electronic conductivity. The Rct represents the charge transfer resistance, and its value corresponds to the diameter of the semi-arc. The inset of Fig. 8a reveals that the semi-arc of CoMoO4-x is smaller than that of CoMoO4. The Rct values of CoMoO4 and CoMoO4-x are 2.5915 Ω and 0.8581 Ω respectively, which reveals that CoMoO4-x has better charge transfer ability. CPE is used to compensate for the non-homogeneity of electrode materials, which illustrates the deviation of non-ideal capacitor. CPE is defined by CPET and CPEP. 14

Journal Pre-proof The value of CPET is related to the interfacial capacitance at the electrode/electrolyte interface. The CPET value for CoMoO4 and CoMoO4-x is 5.21×10-2 and 1.89×10-4, respectively, which indicated the improvement in capacitance on introduction of oxygen vacancy. The CPEP represents the exponent of a constant phase element (0~1). The CPEP value of CoMoO4-x (0.9308) is larger than that of CoMoO4 (0.7324). It indicates that the capacitance of CoMoO4-x is more ideal. WR represents Warburg impedance, corresponding to ionic diffusion resistance. The WR value of CoMoO4-x (1.7866 Ω) is smaller than that of CoMoO4 (3.6192 Ω). The WT represents for the diffusion time constant and WP is a fractional exponent between 0 and 1. As a result, the WT value of CoMoO4-x (0.6331) is smaller than that of CoMoO4 (1.3703). The Wp value of CoMoO4-x (0.4354) is higher than that of CoMoO4 (0.3851). This results show that CoMoO4-x has a smaller diffusion resistance and a faster diffusion rate. Accordingly, the results of SEM and BET indicated that CoMoO4-x own higher specific surface area together with more abundant pores, which could accelerate ion and electron transfer and promote electrochemical reaction. The Bode phase angle curves of CoMoO4 and CoMoO4-x electrodes are exhibited in Fig. 8b. Obviously, the phase angle at the tail of CoMoO4 is ~59.7°. While it is ~76.4° for CoMoO4-x. This further proves that CoMoO4-x electrode possesses the more ideal capacitor performance [46]. Thus, the results of impedance data could convincingly attest that CoMoO4-x possess better electrochemical properties than CoMoO4. The fascinating electrochemical properties are primarily ascribed to the introduction of oxygen vacancy. As a carrier, oxygen vacancy could strengthen the conductivity of sample, as well as lower the internal resistance. At the same time, when the oxygen vacancy is introduced, the adsorption of the electrode material for OH- is enhanced, and the electrochemical reaction is more rapid. This indicate that the CoMoO4-x owns better capacitive performance, which is consistent with the previous electrochemical data of CV and GCD.

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Fig. 8. (a) Nyquist plots for CoMoO4 and CoMoO4-x; (b) Bode angle plots for CoMoO4 and CoMoO4-x.

Table 1 Fitting values of the equivalent circuit elements for CoMoO4 and CoMoO4-x. Electrode material

CPE Ro（Ω）

Wo

CPET

CPEP

Rct（Ω）

WR（Ω） WT

WP

CoMoO4-x

0.4608

5.21×10-2

0.9308

0.8581

1.7866

0.6331

0.4354

CoMoO4

0.6119

1.89×10-4

0.7324

2.5915

3.6192

1.3703

0.3851

Table 2 display the electrochemical properties of CoMoO4-x electrode compared with similar electrode materials that have been reported. Compared with other electrode materials, CoMoO4-x presents a higher specific capacitance. At the same time, it also shows a proud cyclic stability after a long-time cycling test. The CoMoO4-x porous nanosheets demonstrates excellent electrochemical properties for the following reasons:(ⅰ )The introduction of oxygen vacancy in nanosheets can increase the carrier density, thus improving the electro-conductivity and accelerating electron transfer. (ⅱ) Oxygen vacancy could serve as the active site to absorb more OH- and accelerate the kinetics of surface reactions, which is illustrated in Fig. 9. (ⅲ ) After hydrogenation reduction, CoMoO4-x porous nanosheets possess higher specific surface area, coupled with more diverse pores.

Table 2 Comparison of specific capacitance and cycling stability for CoMoO4-x with related electrode materials 16

Journal Pre-proof Electrode material

Electrolyte

Specific capacitance (F g-1)

Cycle life

Reference

[email protected]

3 M KOH

1405 F g-1 at 1 A g-1

92% (1000th) at 10 A g-1

[47]

NiMoO4/CoMoO4

1 M KOH

1445 F g-1 at 1 A g-1

78.8%(3000th) at 10 A g-1

[48]

CoMoO4@Ni(OH)2

1 M KOH

1812 F g-1 at 2 mA cm-2

87.4%(5000th) at 50 mA cm-2

[49]

graphene@CoMoO4

2 M KOH

1225 F g-1 at 1 A g-1

91.3%(3000th) at 1 A g-1

[50]

CoMoO4@C@MnO2

3 M KOH

1824 F g-1 at 3 A g-1

86% (5000th) at 3 A g-1

[51]

Co3O4/CoMoO4

2 M KOH

1902 F g-1 at 1 A g-1

99%(5000th) at 5 A g-1

[52]

CoMoO4/CuO

3 M KOH

1176 F g-1 at 1 mV s-1

95.1%(5000th) at 2.5 A g-1

[53]

CoMoO4-x

6 M KOH

1989 F g-1 at 2 mA cm-2

98.8%(5000th) at 20 mA cm-2

This work

Fig. 9. Schematic for oxygen vacancy serving as the active site to absorb more OH3.3. DFT theoretical calculation In order to understand theoretically the improvement of electrochemical properties by oxygen vacancy, the energy band structure together with density of states (DOS) for CoMoO4 and CoMoO4x

were investigated by DFT calculation. Fig. 10 show the crystal structures for CoMoO4 and

CoMoO4-x before and after introducing oxygen vacancy. After hydrogenation, an oxygen atom in CoMoO4 crystal was reduced to form an oxygen vacancy.

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Fig. 10. Crystal structures for CoMoO4 and CoMoO4-x before and after introducing oxygen vacancy For exploring the relationship of electronic conductivity with electrochemical properties, the band structure and DOS of CoMoO4 and CoMoO4-x are both calculated and analyzed. Fig. 11a exhibited the energy band structure of CoMoO4. According to the breadth from the top value for valence band to the bottom value for conduction band, the band gap for CoMoO4 is 1.683 eV. The energy band structure for CoMoO4-x is shown in Fig. 11b, of which the band gap is merely 0.041 eV. Fortunately, the band gap for CoMoO4-x is smaller than that of CoMoO4, indicating that it has better electronic conductivity. The density of states (DOS) of CoMoO4 and CoMoO4-x are shown in Fig. 11c. By comparing their DOS, it can be seen CoMoO4-x has more electron state distribution near Fermi energy level. This result proves that CoMoO4-x owns a better electronic conductivity. Fig. 11d and 11e show the differential charge density of CoMoO4 and CoMoO4-x respectively. The blue color represents electrons depletion. The red and yellow color represent electrons collection. It can show the charge changes of each atom before and after oxygen vacancy introduced. After hydrogenation, the positions of the reduced oxygen atoms change from red to yellow, indicating that fewer electrons were gained. Thus, the oxygen vacancy could strengthen the adsorption intensity for OH-. And it could improve the kinetics for redox reactions. All the calculated results agree with the electrochemical test results. This could provide theoretical support for the electrochemical test results. The results of DFT calculation show that CoMoO4-x has higher electronic conductivity and electrochemical activity, which is conducive to improving its electrochemical performance.

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Fig. 11. Band structure for CoMoO4 (a) and CoMoO4-x (b); DOS for CoMoO4 and CoMoO4-x(c); differential charge density for CoMoO4 (d) and CoMoO4-x (e). 3.4. Electrochemical application An all solid-state asymmetric supercapacitor was constructed for investigating practical application of CoMoO4-x as an electrode material. The CV and GCD curves for AC were tested in 6 M KOH solution, which were separately exhibited in Fig. 12a and 12b. With the current density at 1 A g-1, its specific capacitance achieved 258 F g-1. The schematic diagram of CoMoO4-x//AC supercapacitor is displayed in Fig. 12c. Among them, CoMoO4-x is used for positive electrode, while AC acts as negative electrode. Fig. 12d exhibited the CV curves for CoMoO4-x (0~0.45 V) and AC 19

Journal Pre-proof (-1~0 V) at the same scan rate (5 mV s-1). The voltage window expansion (1.3~1.7 V) curves of CV (5 mV s-1) and GCD (1 A g-1) were respectively demonstrated in Fig. 12e and 12f. When the voltage reaches 1.7 V, polarization phenomenon appears in the CV curve. Therefore, 1.6 V was chosen as the voltage window in subsequent electrochemical measurements.

Fig. 12. CV curves (a) and GCD curves (b) for AC; (c) A schematic diagram for CoMoO4-x//AC supercapacitor; (d) CV curves for AC (-1~to 0 V) and CoMoO4-x (0~0.45 V); (e) CV curves for CoMoO4-x//AC supercapacitor from 1.3 V to 1.7 V at 5 mV s-1; (f) GCD curves for CoMoO4-x//AC supercapacitor from 1.3 V to 1.7 V at 1 A g-1 Fig. 13a displays the CV curves of CoMoO4-x//AC supercapacitor. And Fig. 13d shows the GCD 20

Journal Pre-proof curves. As the current densities increased (1~10 A g-1), the specific capacitance for CoMoO4-x//AC supercapacitor only reduced from 175.3 F g-1 to 128.1 F g-1. It reveals that it has a good rate capability. Moreover, the energy density, E, together with power density, P, for CoMoO4-x//AC supercapacitor were obtained according to the formulas below: E (W h kg-1) =Cm(ΔV)2/2

(5)

P (W kg-1) =I(ΔV)/2m

(6)

in which, Cm represented mass specific capacitance (Cm, F g-1); ΔV represented voltage window (V); I represented discharge current, m represented mass of cathode and anode active materials (g). Moreover, Fig. 13c demonstrates the Ragone plot of CoMoO4-x//AC compared with some previous reports. Surprisingly, the energy density of CoMoO4-x//AC can reach 62.3 W h kg-1 at a power density of 800 W kg-1. It is superior to those in previous reports, such as CoMoO4@NiCo2S4//AC (60.2 W h kg-1)[29], CoMoO4@NiCo-LDH//AC (59.5 W h kg-1)[54], CoMoO4@MnO2//AC (54 W h kg-1)[55], CoMoO4@NiMoO4//AC (28.7 W h kg-1)[56], graphene/CoMoO4//AC (26.8 W h kg-1)[50], Co3O4/CoMoO4//AC (45.2 W h kg-1)[52]. After 5000 cycles at 5 A g-1, the specific capacitance for CoMoO4-x//AC supercapacitor can be maintained at 91% (Fig. 13d). Fig. 13e and 13f show optical photographs of small electric devices driven by CoMoO4-x//AC supercapacitor. What's exciting, it could illume a green light-emitting-diode (LED) and drive a small electric fan. The above results show that it has potential value of application for energy storage.

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Fig. 13. (a) CV curves for CoMoO4-x//AC supercapacitor; (b) GCD curves for CoMoO4-x//AC supercapacitor; (c) Ragone plots for CoMoO4-x//AC supercapacitor in comparison with references; (d) Cycling stability for CoMoO4-x//AC supercapacitor; optical photographs for CoMoO4-x//AC supercapacitor powering green LED (e) and electric fan (f).

4. Conclusions CoMoO4-x porous nanosheet containing oxygen vacancy is prepared by hydrothermal and hydrogen reduction methods. The electrochemical results show that CoMoO4-x porous sheet exhibits better electrochemical properties than CoMoO4 nanosheet. The specific capacitance of CoMoO4-x porous sheet achieves to 1989 F g-1 at 2 mA cm-2. It is more than twice of CoMoO4 (917 F g-1). Going through 5000 cycling tests, the specific capacitance for CoMoO4-x could remain at 98.8% of the initial capacitance. The introduction of oxygen vacancy can improve carrier density and charge 22

Journal Pre-proof transfer capacitance. The surface oxygen vacancies could strengthen the adsorption intensity for OH- and promote the redox reaction speed. Meanwhile, the density functional theory calculation results keep up with the electrochemical test results. Moreover, the CoMoO4-x//AC asymmetric supercapacitor demonstrates an eminent energy density of 62.3 W h kg-1 at a power density of 800 W kg-1. So, oxygen vacancy is a meaningful method to improve electrochemical properties and CoMoO4-x can act as a promising electrode material.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21373047), Fundamental Research Funds for the Central Universities (2242018K41024), Graduate Innovation Program of Jiangsu Province (KYCX18_0080), as well as the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] L. Guardia, L. Suarez, N. Querejeta, V. Vretenar, P. Kotrusz, V. Skakalova, T.A. Centeno, Biomass waste-carbon/reduced graphene oxide composite electrodes for enhanced supercapacitors, Electrochim. Acta, 298 (2019) 910-917. [2] M. Guo, Y. Zhou, H. Sun, G. Zhang, Y. Wang, Interconnected polypyrrole nanostructure for highperformance all-solid-state flexible supercapacitor, Electrochim. Acta, 298 (2019) 918-923. [3] Z. Zhao, Y. Xie, Electrochemical supercapacitor performance of boron and nitrogen co-doped porous carbon nanowires, J. Power Sources, 400 (2018) 264-276. [4] Y. Xie, Electrochemical Performance of Transition Metal-coordinated Polypyrrole: A Mini Review, Chem. Rec., 19 (2019) 1-16. [5] Z. Zhao, Y. Xie, L. Lu, Electrochemical performance of polyaniline-derivated nitrogen-doped carbon nanowires, Electrochim. Acta, 283 (2018) 1618-1631. [6] Y. Xie, Y. Zhou, Enhanced capacitive performance of activated carbon paper electrode material, J. Mater. Res., 34 (2019) 2472-2481.

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Declaration of Interest Statement

The authors declare that they have no conflict of interest in the manuscript entitled “Supercapacitive performance of CoMoO4 with oxygen vacancy porous nanosheet”.