Electrochemical supercapacitive performance of spray deposited Co3O4 thin film nanostructures
Graphical abstract
Introduction
In day to day life, the problems of energy utilization and environmental concerns along with human development are facing challenges. Today, the world’s massive energy demands are mainly accomplished by the conventional and environmentally unfavorable fossil fuels. To replace this, pursuit of the renewable and clean energy sources, including hydrogen storage and supercapacitors have become important [1], [2], [3]. Supercapacitors have appealed significant attention due to their high power density; fast charge/discharge rate and good cycle stability against galvanostatic charge/discharge [4], [5], [6], [7], [8], [9].
A balanced surface area and porous surface are highly desirable for electrode materials to be utilised in electrochemical supercapacitors, which is provided by metal oxides [10]. Out of the various metal oxides, ruthenium oxide is the most favorable electrode material with outstanding specific capacitance but its cost is huge. Cobalt oxide is one of the most studied oxides because of its use in various applications, such as solar selective absorbers, pigment for glasses and ceramics [11] and catalyst for oxygen evolution and reduction reaction [12]. It is also widely used as an electrochromic material, sensors, and electrochemical anodes [13], supercapacitors [14], [15], [16]. Cobalt oxide exists in three forms, CoO, Co2O3 and Co3O4, but Co3O4 is excessively reported having large applications due to its strong chemical stability and better electrochemical properties. Co3O4 in thin film form can be deposited by employing different deposition techniques such as sputtering [17], hydrothermal [18], coprecipitation [5], [19], chemical route [20], chemical vapor deposition [21], sol-gel [22] and spray pyrolysis [23], [24]. Amongst these, spray pyrolysis has numerous benefits including low cost, flexibility, ability for depositing porous and nanostructure thin films and appropriate for large area deposition.
Kandalkar and coworkers [20] have prepared cobalt oxide films on a copper substrate at room temperature. These films showed a maximum specific capacitance of 165 Fg−1 in 1 M aqueous KOH electrolyte at sweep rate of 10 mVs−1. Amin-Chalhoub et al. [21] have processed low reflective CoO coatings by CVD. A fractal cauliflower like morphology was observed. Ambare and colleagues [23] have studied spray deposited ruthenium incorporated cobalt oxide electrodes. The double-pseudo-capacitive behavior with specific capacitance of 628 Fg−1 at the scan rate 1 mVs−1 in 1 M KOH has been observed. The spray deposition of cobalt oxide nanowires on stainless steel substrates is also reported in the literature [25]. From these and other [5], [12], [13], [14], [15], [16] studies, it is well understood that Co3O4 is the most beneficial material for the supercapacitor uses. Therefore in current investigation, it was decided to deposit good quality Co3O4 nanostructures for high-performance supercapacitors by chemical spray pyrolysis using mixed aqueous/organic solvent.
In the current report, the effect of substrate temperature on various physical and electrochemical properties of Co3O4 thin films is studied. It is perceived that film thickness increases with increase in substrate temperature reaches maximum at 350 °C and decreases thereafter. XRD study confirmed the cubic phase of Co3O4. Porous morphology is seen from SEM. The as deposited Co3O4 at 350 °C saw minimum electrical resistivity of 2.08 × 103 Ω cm. The films are highly capacitive with maximum specific capacitance of 425 Fg−1 at scan rate of 5 mVs−1. The noteworthy cycle stability of 92.56% has been observed at a current density of 1 Ag−1 after 1000 charge/discharge cycles.
Section snippets
Deposition of FTO substrates
The 2 M stannic chloride with 20 wt% doping of ammonium fluoride was used for deposition of fluorine doped tin oxide (FTO) glass substrates. For all depositions, 10 cc from stock solution is mixed with 10 cc of propan-2-ol to create the 20 cc final spraying solution. This 20 cc solution is sprayed on preheated substrates at 475 °C. These FTO substrates have the sheet resistance of 8–10 Ωcm−2.
Deposition of Co3O4 thin films
A mixture of 0.05 M cobaltous chloride (CoCl2.6H2O) in a 1:1 (Propan-2-ol + Water) was used as precursor
Film formation
The Co3O4 nanostructures were deposited at various substrate temperatures. The as deposited Co3O4 thin films appear blackish in color. The possible reaction mechanism can be written as:3CoCl2 + 2H2O +O2 → Co3O4↓ + 3Cl2↑ + 2H2↑
The similar type of reaction was also reported earlier by Shinde and coworkers [24] for Co3O4. Film thickness of spray deposited cobalt oxide thin films is computed by gravimetric weight difference method using sensitive precision mass microbalance (LC = 0.01 mg). Fig. 1 shows the
Conclusions
Co3O4 nanostructures are successfully deposited with computerized spray pyrolysis. The XRD study endows cubic Co3O4 phase with polycrystalline nature and crystallinity enhances with rise in substrate temperature. SEM images show porous morphology. Optical band gap energies are in the range 1.40-1.45 eV and 1.72-1.97 eV for lower energy region and higher energy regions respectively. Cyclic voltammetry and EIS study shows the Co3O4 with improved electrochemical performance. The maximum specific
Acknowledgements
Dr. A. A. Yadav is grateful to the Science and Engineering Research Board, Department of Science and Technology, New Delhi, India for the financial assistance through the Project under the SERC Fast Track Scheme for Young Scientist (File No. SB/FTP/PS- 068/2013).
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