Effect of thermal treatment on the structural characteristics and electrochemical properties of amorphous Mn oxide prepared by an ethanol-based precipitation method
Highlights
► Novel precipitation method to synthesize nanostructured Mn oxide, using ethanol as solvent. ► Effect of ethanol on crytallinity of a nanostructured Mn oxide was investigated. ► Crystallinity of Mn oxide can be controlled by adequate thermal treatment. ► Relationship among thermal treatment, crystallinity, and electrochemical performance was investigated. ► Optimum condition between degree of amorphous and electrochemical activity was found.
Introduction
Supercapacitors have attracted considerable interest for use as possible auxiliary electric energy storage devices. Supercapacitors based on a metal oxide have been extensively investigated due to their high-power-energy-density and excellent reversibility based on pseudocapacitance effects [1], [2]. A capacitor containing amorphous hydrous ruthenium oxide prepared by a sol-gel process was reported to have a specific capacitance value higher than 720 F/g [3]. However, extensive efforts have been devoted to identifying alternative metal species with high electrochemical activities, because of the high cost of ruthenium oxide.
Manganese (Mn) oxide represents one of the most promising electrode materials for use in supercapacitors due to its high electrochemical activity, natural abundance, and environmentally benign nature [4], [5], [6]. Among the various forms of Mn oxide, nanostructured Mn oxide is known to show the most promising performance. As a result, a number of methods have been proposed for the synthesis of nanostructured Mn oxide, including electrodeposition [7], [8], co-precipitation [9], [10], a sol-gel process [11], and hydrothermal synthesis techniques [12]. Although these methods all can be used to produce Mn oxide species with a high electrochemical performance, they are somewhat complicated, time-consuming or limited to applications related to substrate materials. In addition, it is not clear how amorphous characteristics of Mn oxides can be formed.
For the production of nanostructured amorphous Mn oxides, the use of organic solvents during the precipitation step has been reported previously [13]. Here, we report on the effect of alkyl chains of organic solvents on the formation of nanostructured amorphous Mn oxide by changing the thermal treatment temperatures. The alkyl chain of ethanol played an important role in the formation of nanostructured amorphous Mn oxide and in preventing MnO6 octaheron transformations from the transition to a highly crystalline structure as well. Furthermore, optimal structural characteristics were observed for the maximum electrochemical activities.
Section snippets
Experimental
Nanostructured Mn oxide was prepared by a precipitation method performed in ethanol. A calculated amount of Mn precursor (MnCl2∙2H2O, Sigma–Aldrich) was dissolved in ethanol. A 1 M solution of NaOH (Sigma–Aldrich) was added drop-wise into the metal salt solution, until the pH of the solution reached 10. The solution containing the Mn precipitates was stirred to permit the preparation to undergo aging. The mixture was filtered, and the precipitate exhaustively washed with D. I. water. The
Results and discussion
Fig. 1 shows TGA/DSC profiles for as-synthesized Mn oxide produced by precipitation in ethanol. A sharp weight loss corresponding to an endothermic reaction was detected at around 100 °C. This is due to the continuous dehydration of adsorbed and lattice water. An exothermic peak was detected at around 200 °C, which is attributed to the decomposition of organic species on the surface. Within the temperature range of 200–500 °C, the rate of weight loss was decreased slightly, due to the further
Conclusions
In summary, we prepared nanostructured Mn oxide by a facile precipitation method performed in ethanol for use as an electrode material in supercapacitors. Organic species arise from the ethanol induced formation of amorphous hydrous Mn oxide, as evidenced by thermal and elemental analyses. These organic species permit nanostructured Mn oxide to maintain its structure even after a high temperature treatment at 300 °C. Capacitance activity could be increased by an appropriate thermal treatment
Acknowledgment
This work was supported by WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-10013).
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