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Facile Synthesis of Co3O4 Nanowires Grown on Nickel Foam with High Electrochemical Capacitance.

Byline: Bo Ren and Meiqing Fan

Summary: Co3O4 nanowire arrays freely standing on nickel foam were prepared by a hydrothermal method. The detailed microstructure and morphology of Co3O4 nanowire were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) etc. The results indicated that the Co3O4 nanowires have diameters of around 80 nm and composed of many nanoparticles with diameters of about 20 nm. The electrochemical results show that Co3O4 nanowires demonstrate enhanced specic capacitance (616 F g-1) and electrochemical reversibility, and the specific capacitance of nanowires only decreased 19% after 3000 charge-discharge cycles

Keywords: Cobalt oxide; Nanowires; Supercapacitor; Nickel foam.

Introduction

Supercapacitors, namely electrochemical capacitors, have attracted much more attention due to their higher power density, longer cycle life and fast charge/discharge process compared to the commercial chemical batteries [1-3]. The electrode material is one of the important factors to affect the performances of supercapacitors. The electrode materials can be classified into three types: carbon, metal oxides, and conducting polymers. Many efforts have been made to research metal oxides, due to their much higher specific surface area and electrical conductivity. RuO2 have been used as pseudocapacitive electrode materials once upon a time, however the high cost limit their commercialization. Therefore, the development of electrode materials with low cost and high performance has attracted considerable attention. Transition metal oxides such as NiO, MnO2, Co3O4 and Fe2O3 have been widely studied [4-6].

Among them, Co3O4 are reported to be promising electrode materials due to their great reversibility, high theoretical specific (3560 F g-1), high redox activity and quite ordered structures [7]. For instance, Li et al synthesized Co3O4 thin film by a chemical bath deposition, which showed a large specic capacitance of 227 F g-1 at 0.2A g-1, and when the specific current increased to 1.4A g-1 the specific capacitance only decreased 33% [8]; Zhang et al prepared porous Co3O4 nanoflake array film grown on nickel foam by a hydrothermal synthesis. The results showed that the specific capacity was 210, 289 and 351 F g-1 at 2 A g-1 tested at 5 AdegC, 25 AdegC and 60 AdegC, respectively, and the remaining specific capacity is 187,342 and 124 F g-1 tested at 5 AdegC, 25 AdegC and 60 AdegC. After 4000 cycles at 2 A g-1[9]; Liao et al synthesized novel flower-like porous Co3O4 hierarchical microspheres by a facile hydrothermal strategy and template-free method.

The results demonstrated that Co3O4 exhibited a large specific capacitance of 541.9 F g-1 and 483.8 F g-1 at 5 -1mV s and 1 A g-1, respectively [10]. Furthermore, after a 2000 cycles test, the specific capacity reduced to 89.5%. In addition to all these achievements, many researches have made a great contribution in the study of Co3O4 materials, but the results are still unsatisfying. Therefore, how to improve the specific capacitance is still a challenge. In this paper, Co3O4 nanowires grown on nickel foam were prepared by hydrothermal method. The results indicated that the nanowires showed a larger specific surface area and good electrochemical performance, and that was a promising electrode material.

Experimental

All the reagents were analytical grade and were used without further purification. Co3O4 nanowire arrays supported on nickel foam were prepared via a hydrothermal method [11-14].The process of the preparation of Co3O4 nanowire can be described briefly. 2g Co(NO3)2*6H2O, 1g Hexa decyl trimethyl ammonium Bromide (CTAB), 6 ml water and 30 ml absolute methanol were mixed together under vigorous magnetic stirring. The obtained solution was then transferred into a 40 ml Teflon-lined stainless steel autoclaves. A piece of nickel foam (1 cmx1 cm) was added to the solution, and then the autoclave was put in an oven at 180 AdegC for 24 h to allow the growth of Co3O4 nanowires. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-IIIB diffractometer using Cu K[alpha] radiation (I>>=1.5406 A). The morphology of the samples was inspected with a field-emission scanning electron microscope (SEM, Philips XL 30).

Transmission electron microscopy (TEM) and images were obtained from a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. The electrochemical properties of the products were investigated under a three-electrode electrochemical cell. The nickel foam supported Co3O4 nanowires were used as the working electrode, platinum foil acted as counter electrode, and a saturated calomel electrode (SCE) were used as reference electrodes. KOH (6.0 M) aqueous solution was used as the electrolyte. Cyclic voltammetry (CV) tests were measured between 0 V and 0.5 V (vs. SCE) at scan rates of 5, 10, and 20 mV s-1. Galvanostatic charge/discharge curves were done in the potential range of 0-0.5 V (vs.SCE) at current densities of 5, 10, and 20 mA*cm-2, and EIS measurements were also carried out in thefrequency range from 100 kHz to 0.05 Hz.

Results and Discussion

Fig. 1 shows the XRD patterns of Co3O4 nanowire. The main peaks at 2I, values of 19.00Adeg, 31.14Adeg, 36.58Adeg, 38.45Adeg, 59.30Adegand 65.20Adegbelong to the crystal planes of (111), (220), (311), (222), (511) and (440), which indicates that pure Co3O4 (JCPDS card No. 42-1467) formed. While the peaks at 2I, values of 44.68Adegand 55.57Adegcorrespond to Ni substrate (JCPDS no. 01-1258). Fig. 2 (a) (b) shows SEM image of Co3O4 nanowire, it can be seen that Co3O4 nanowire grow on nickel foam with diameters range from 80 to 100 nm. The morphologies of Co3O4 nanowire are further characterized by TEM (Fig. 2 (c)). It indicates that the Co3O4 nanowires are composed of nanoparticles with diameters of about 20 nm. The lattice fringe in Fig. 2 (d) with interplanar spacing of 0.29 nm is assigned to the (220) planes of the Co3O4 crystal [15]. A possible mechanism of Co3O4 is suggested as follows:

CH3OH + H2OaCH3OH2+ + OH-

Co(NO3)2aCo2+ + NO3-

Co2+ + OH-aCo(OH)2

Co(OH)2 + O2aCo3O4 + H2O

Hydroxyl ions is produced from methanol, and cobalt ions (Co2+) from the Co(NO3)2 solution react with hydroxyl ions to form Co(OH)2. Co3O4 is generated after calcined.

Fig. 3 (A) shows CV curves of Co3O4 nanowire at different scan rates of 5, 10 and 20 mV *s-1. Two pairs of redox peaks are observed, which attributed to the transition of Co(II)/Co(III) and Co(III)/Co(IV). The possible redox reactions can be described as follows: [16-17]

Co3O4 + OH- + H2Oa3CoOOH + e

CoOOH + OH-aCoO2 + H2O + e-

with the increase of the scan rate, the anodic peaks shifted toward positive potential and cathodic peaks shifted toward negative, indicating the quasi-reversible feature of the redox couples [18-19]. Fig.3 (B) indicates galvanostatic charge/discharge curves of Co3O4 nanowire at various current density of 5, 10 and 20 mA*cm-2. The specific capacitance can be caculated according to the following equation [20].

Cm = I x It/IV x m

where Cm is the specific capacitance of the electrode(F g-1), I is the charge/discharge current (A), It is the discharge time (s), IV is the potential drop during discharge, and m is the mass of active electrode materials. According to Eq (1), the specific capacitance values of the Co3O4 nanowires are calculated to be 616, 540 and 480 F g-1 at the current density of 5, 10 and 20 mA*cm-2, respectively. The large values of capacitance can be attributed to the large specific surface area of the nanowires [21]. With the increasing of the current density, the specific capacitance values decreased. This indicates that the insertion and extrusion of OH- is slow when the current density is lower, and more active surface area can be provided for Faradaic reactions.

Fig.3 (C) shows the electrochemical impedance spectras (EIS) of Co3O4 nanowire. The equivalent circuit in accordance with the Nyquist plot is presented in Fig.3 (C) (upper right inset). The value of Rs (the solution resistance of the electrochemical system) can be read from the intersection With the X axis, and then the Rs of Co3O4 nanowire is 0.65a|. The value of Rct (Faradaic interfacial charge transfer resistance [22]) can be read from the semicircle of EIS, so the Rct of Co3O4 nanowire is 0.1a|. Fig.3 (D) indicates cycling performance of Co3O4 nanowire at constant current of 5 mA*cm-2. It can be seen that the specific capacitance only decreased 19% after 3000 test cycles, which demonstrates that the Co3O4 nanowire has a stronger stability, and it is appropriate for long time capacitor applications in KOH solution.

Conclusions

Co3O4 nanowire arrays freely standing on nickel foam were prepared by a hydrothermal method. The results indicated that the Co3O4 nanowires have diameters of around 80 nm and composed of many nanoparticles with diameters of about 20 nm. The electrochemical properties suggest that Co3O4 nanowire has good electrochemical reversibility and displays superior capacitive performance with large capacitance (616 F g-1), as well as excellent cycling stability after 3000 cycles.

Acknowledgements

This work was supported by The Education Department of Jilin Province, "13th Five-Year" science and technology research project JJKH20180541KJ.

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Author:Ren, Bo; Fan, Meiqing
Publication:Journal of the Chemical Society of Pakistan
Article Type:Technical report
Date:Dec 31, 2018
Words:2291
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