Electrochemically synthesized MnO2-based mixed oxides for high performance redox supercapacitors

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Abstract

Nanostructured and microporous nickel–manganese oxide (NMO) and cobalt–manganese oxide (CMO) were deposited by potentiodynamic method onto inexpensive stainless steel substrate. A scan rate of 200 mV s−1 was used for the deposition. The binary oxide deposited electrodes were characterized by cyclic voltammetry (CV) and charge/discharge cycling in 1 M Na2SO4 electrolyte for redox supercapacitor applications. Maximum specific capacitance (SC) values of 621 and 498 F g−1 were obtained with NMO and CMO electrodes, respectively, at a CV scan rate of 10 mV s−1. SC values of 377 and 307 F g−1 were obtained with NMO and CMO electrodes, respectively, at a high CV scan rate of 200 mV s−1, indicating high power characteristics of the binary oxides. The results were more interesting from charge/discharge cycling, where SC values of 685 and 560 F g−1 were obtained with NMO and CMO electrodes, respectively, at a current density of 2 mA cm−2. These values of electrical parameters are much higher than those obtained with just MnO2. Long cycle-life and excellent stability of the materials were also demonstrated.

Introduction

Supercapacitors have an ever expanding array of applications since portable electronics continue to gain in popularity. Supercapacitors or electrochemical capacitors (ECCs) take advantage of the charge stored in the electrochemical double layer and provide extremely high specific capacitance of more than 1000 F g−1. These devices have applications in computer power back-up, power electronics, electric vehicles and space flight technology. However, power and energy demands of these applications vary significantly. Supercapacitors, also referred to as ultracapacitors, can play an important role in future electrical vehicles (EVs) by taking the peak power demand from the battery and also by helping to reduce the fuel consumption of hybrid vehicles. This led to an extensive research in the area of ECCs [1], [2] and the development of EVs being powered by a combination of a rechargeable battery and an ECC. Two basic types of electrochemical capacitors can be realized using different charge-storage mechanisms [1], [2]: (i) electrical double-layer capacitors (EDLCs), which utilize the capacitance arising from charge separation at an electrode/electrolyte interface, and (ii) redox supercapacitors or ultracapacitors, which utilize the charge-transfer pseudocapacitance arising from reversible Faradaic reactions occurring at the electrode surface [1], [2], [3], [4].

The electrodes of electrochemical redox supercapacitors consist of electroactive materials with several oxidation states. These types of capacitors are under extensive investigation in the recent times because of high capacitive and high energy characteristics [1], [2], [3], [4]. Since the pseudocapacitance comes from the reversible redox transitions of the electroactive materials, transition metal oxides [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and conducting polymers [19], [20] with various oxidation states are considered to be promising materials for the applications of redox supercapacitors. Recently, hydrous RuO2 has been extensively studied [5], [6] as an active electrode material for supercapacitors because it possesses capacitance as high as 720 F g−1 in aqueous acidic electrolytes. Although RuO2 gives high specific capacitance, it has disadvantages of high cost and toxic nature. In view of finding an inexpensive alternate to RuO2, manganese oxide prepared by both chemical and electrochemical routes was found to possess capacitive characteristics with acceptable values of specific capacitance [7], [8], [9], [10], [11]. Electrochemical deposition kinetics of MnO2 on platinized Ti and Pb substrates were studied by Rodrigues et al. [21]. In a recent report [11], we have shown that nanostructured MnO2 can be synthesized by a potentiodynamic method at high scan rate. We have also shown that the potentiodynamically deposited MnO2 gives high specific capacitance (SC) values. Nickel oxide synthesized by sol–gel method [16] or electrochemical method [17] was found to give a pseudocapacitance value of 50–65 F g−1. Sol–gel-derived cobalt oxide was also suggested [18] for supercapacitor applications. However, the operating potential of these oxides in various electrolytes is below 0.6 V [16], [17], [18]. Because the energy density is proportional to the square of the potential difference, a large potential window is considered to be an important factor for supercapacitors.

The aim of the present study is to potentiodynamically deposit mixed oxides based on MnO2. Nickel–manganese oxide (NMO) and cobalt–manganese oxide (CMO) were synthesized by potentiodynamic method at a scan rate of 200 mV s−1 and characterized for supercapacitor applications. Material characterization was carried out by transmission electron microscopy (TEM), atomic force microscopy (AFM), cyclic voltammetry (CV) and charge/discharge cycling. Excellent values of electrical parameters and excellent stability of the materials were demonstrated.

Section snippets

Experimental

All the chemicals required for the deposition and Na2SO4 were purchased from Aldrich. All the solutions were made by using double-distilled water. The NMO and CMO have been deposited onto a commercially pure stainless steel (SS) (grade 304) foil of 0.2 mm thickness by potentiodynamic deposition. The scan rate used for the deposition was 200 mV s−1. Before deposition, a 1 × 10 cm SS foil was polished with emery paper (grit number 120) to a rough finish, washed free of emery particles and then air

Results and discussion

Electrochemical deposition techniques are known to have better control over the properties of the deposited material than the chemical synthesis. The morphology of the deposited material can also be controlled in electrochemical deposition method. X-ray diffraction patterns of NMO and CMO were recorded (not shown here), which shows the formation of amorphous oxides on SS in the present method. Fig. 1 shows the TEM image of NMO prepared in the present study. A nanostructured and highly porous

Conclusions

Nanostructured, microporous and amorphous nickel–manganese oxides and cobalt–manganese oxides were deposited onto an SS electrode by potentiodynamic method at a high scan rate. Maximum SC values as high as 621 and 498 F g−1 were obtained with NMO and NMO electrodes, respectively, at a CV scan rate of 10 mV s−1. High power density and excellent stability of the materials was demonstrated. The pseudocapacitance depends on the water content of the oxide [22]. With increasing water content, the ionic

Acknowledgements

The present work was supported by Japan Science and Technology (JST) through “Core Research for Evolutional Science and Technology (CREST)” under the project “Development of advanced nanostructured materials for energy conversion and storage”. The authors also thank Dr. Daisuke Terada for helping in TEM observations.

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