Recent progress on manganese dioxide based supercapacitors
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Feiyu Kanga) Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen City, Guangdong Province, 518055 China; and Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084 China
Baohua Li and Hongda Du Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen City, Guangdong Province, 518055 China (Received 19 February 2010; accepted 8 April 2010)
The increasing worldwide interest in MnO2 for supercapacitor applications is based on anticipation that MnO2-based high-voltage aqueous supercapacitors will ultimately serve as a safe and low-cost alternative to state-of-the-art commercial organic-based electrochemical double-layer capacitors or RuO2-based acid systems. In this paper, the physicochemical features, synthesis methods, and charge storage mechanism of MnO2 as well as the current status of MnO2-based supercapacitors are summarized and discussed in detail. The future opportunities and challenges related to MnO2-based supercapacitors have also been proposed.
I. INTRODUCTION
Energy is always a priority issue for human beings. Humans need nutrition from food to maintain life and heat to drive away cold. In the 21st century, as the end of the fossil fuel era is getting close, pollution due to combustion of fossil fuel, which has triggered the biggest environmental issue—global warming—has become a worldwide problem. The renewable and clean energy sources as well as efficient use of energy are highly necessary to make our economy, environment, society, and human species sustainable. Energy storage, an intermediate step to energy, creates a new approach to use energy versatilely, cleanly, and efficiently. Hence, more concerns and interests have been paid to energy storage devices. Supercapacitor is a kind of power-type energy storage device that can provide a high power density (higher than a few kW/kg) and a moderate energy density. It is also characterized by environmental friendliness, high safety, and good efficiency and can be operated in a wide temperature range with a nearly infinitely long cycle life. Therefore, supercapacitors have been applied in and are showing potential application in communications, transportation, consumer electronics, aviation, and related technologies.1–5 Normally, a supercapacitor consists of a pair of ideally polarizable electrodes, which are immersed in an electroa)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0211 J. Mater. Res., Vol. 25, No. 8, Aug 2010
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lyte and physically separated by a porous membrane through which ions can diffuse but electrons cannot. If electrochemically active materials of both electrodes are identical, it will be a symmetric supercapacitor; otherwise, it is asymmetric. The energy stored in such devices is expressed by E = ½1/2 CV2, where C is capacitance and V is operating voltage across the electrodes. The capacitance of
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