Effects of electrochemical-deposition method and microstructure on the capacitive characteristics of nano-sized manganese oxide
Introduction
Electrochemical capacitors have been recognized as a key component in high power applications due to their ability to deliver pulsed power with high power density in a short duration. The need of high power sources is vital for the development of next-generation high performance micro-electro-mechanical systems, hybrid vehicles, portable devices, etc. [1], [2], [3], [4]. The supercapacitors have high power, very long cyclic lives and high efficiency because they store energy in the form of charges at the electrode–electrolyte interface without the involvement of irreversible chemical reactions as in the case of batteries. Based upon their charge-storage mechanisms, supercapacitors can be classified into two types of capacitors: (i) the electric double-layer capacitors (EDLCs), in which the capacitance arises from the charge separation at an electrode–electrolyte interface and (ii) the Faradic pseudo-capacitors where the capacitance arises from the redox reactions of the electrochemically active material. High surface-area carbon materials such as activated carbon come under the category of EDLCs. However, their low average specific-capacitance value of nearly 200 F g−1 in aqueous electrolyte medium is a drawback [5], [6]. Therefore, a significant attention has been given to develop redox-supercapacitor materials, such as metal oxides [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25] and conducting polymers, due to their excellent electrochemical characteristics.
Among several metal oxides (e.g. oxides of Ru, Mn, Fe, Co, Ni, Cr, In, Sn, Mo, V, etc.), the largest specific capacitance of 720 F g−1 and excellent reversibility have been obtained with ruthenium oxide so far [7]. However, the high cost and toxic nature of ruthenium oxide inhibited its commercial use. On the other hand, manganese oxide materials have shown excellent supercapacitive characteristics with good values of specific capacitance as compared to EDLCs and are in abundance as well as non-toxic. Therefore, a lot of efforts have been devoted to study its supercapacitive characteristics of manganese oxide [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. The manganese-oxide-based materials have the potential to be a low cost with high power for supercapacitors in household applications if the optimized conditions for electrochemical and power properties are well achieved. Although the capacitive properties of manganese oxide, prepared by various methods of anodic deposition in aqueous media [9], [10], [11], [12], [13], [14], [15], [16], [17], have been found to be quite different, a clear relationship between the morphologies and the specific capacitance of the manganese oxides prepared by different methods has not yet established except for some preliminary reports [10], [15]. Therefore, in the present work, the effects of electrochemical deposition condition onto the microstructure as well as the specific capacitance of manganese oxide are examined, and the optimum experimental conditions for the optimum power–energy characteristics are established.
Section snippets
Experimental
A research grade stainless-steel (SS) sheet (grade 304, 0.2 mm thick) was obtained from the Nilaco® Corporation. The SS sheet was cut into the given size (1 cm× 10 cm). The obtained SS strip was polished with emery paper to a rough finish, washed free of emery particles and then air-dried. H2SO4, MnSO4·5H2O and Na2SO4 were purchased from Aldrich. An electrochemical cell was assembled in three-electrode configuration in which the SS strip, a platinum (Pt) plate and a saturated-calomel electrode
Electrochemical deposition of the manganese oxide film
The deposition methods are shown in Fig. 1(a)–(c), respectively. The pH value of the electrolyte solution, i.e. 0.15 M H2SO4 + 2 M MnSO4·5H2O was 1.2 ± 0.1. The galvanostatic depositions were performed in the range of 1.25–5 mA as shown in Fig. 1(a). The potentiostatic deposition was carried out at various potentials, as shown in Fig. 1(b). The potentiodynamic deposition was performed at various scan rates, as shown in Fig. 1(c). From the potential–pH diagram of Mn [26], the anodic deposition of MnO2
Conclusions
The manganese oxide nano-structure deposited electrochemically showed high specific capacitance and good reversibility. In particular, the potentiodynamic deposition at high scan rates gives the optimum result. It was found out that both the microstructure and the porous characteristics play vital role in determining the capacitive characteristics of the manganese oxide deposit. The highest specific capacitance of 410 F g−1 and the specific energy of 86 Wh kg−1 (at the specific power of 54 kW kg−1)
Acknowledgements
The present work was supported by the Japan Science and Technology (JST) Agency through the Core Research for Evolutionary Science and Technology (CREST) program under the project “Development of advanced nanostructured materials for energy conversion and storage.”
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