A high-energy-density micro supercapacitor of asymmetric MnO 2 ecarbon configuration by using micro-fabrication technologies Caiwei Shen a, b , Xiaohong Wang a, b, * , Siwei Li a, b , Jian’gan Wang a, c , Wenfeng Zhang a, c , Feiyu Kang a, c a Tsinghua National Laboratory for Information Science and Technology, PR China b Institute of Microelectronics, Tsinghua University, Beijing 100084, PR China c Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China highlights < A micro supercapacitor of asymmetric MnO 2 ecarbon configuration is presented. < Micro electrodes with different active materials are separated in a 3D structure. < The device has an extended working voltage window and a large specific capacitance. < It outperforms symmetric devices using either MnO 2 or activated carbon. article info Article history: Received 1 September 2012 Accepted 25 October 2012 Available online 9 February 2013 Keywords: Micro supercapacitor Energy storage device Asymmetric electrodes Manganese dioxideeactivated carbon configuration abstract This paper demonstrates an asymmetric micro supercapacitor with superior overall performance that combines the advantages of both MnO 2 positive electrode and carbon negative electrode for energy storage on a chip. Nano-structured MnO 2 with rough surface is synthesized as active material for micro electrode. A self-supporting composite containing MnO 2 and another containing nanoporous activated carbon (AC) are separated in an interdigital structure by using micro-electro-mechanical systems (MEMS) fabrication technologies. Measurements of the prototype show that the asymmetric micro supercapacitor has well-performed capacitive behavior, and its working voltage range is extended from 1 V to 1.5 V in aqueous electrolyte due to the MnO 2 eAC configuration. Calculations prove that it stores much higher energy densities than symmetric ones using either MnO 2 or AC. Moreover, this work provides an attractive approach to achieve various asymmetric micro energy storage systems on chips. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The latest technology trends have shown growing interests in developing independent power supplies for wireless sensor net- works, portable electronic devices and other self-powered micro systems, which particularly requires energy storage devices that can offer adequate energy and power with a lifetime matching that of the system being powered. Considering the demand of minia- turization and reducing the complexity of the whole system, designing energy storage elements that are miniaturized or even integrated with other parts is of great significance [1]. Although micro batteries have been studied as one of the solutions for those applications, they suffer from fundamental problems including relatively low charge/discharge rates and limited cycle lives (hundreds to thousands of cycles), which are caused by the volu- metric electrochemical reactions they are based on [1,2]. Supercapacitor, as one of the energy storage devices used in diverse power systems, offers energy densities and power densities between those of batteries and conventional capacitors [3e5]. It finds applications when fast charge/discharge rates and long cycle lives (thousands to millions of cycles), as well as adequate energy densities are needed. Two kinds of supercapacitors are dis- tinguished depending on the charge storage mechanisms. One is based on the electrical double layer (EDL) effect which stores static charges on the interface of high-surface-area electrodes and ion- containing electrolytes. Its electrode materials are represented by various nanoporous carbons. The other kind mainly relies on fast and reversible redox reactions that happen at the surface of active materials, and is known as pseudo capacitance. Transition metal oxides and electrically conducting polymers are examples of pseudo-capacitive electrode materials. The charge storage of EDL capacitors is fast but offers relatively low capacitance, while that of * Corresponding author. Institute of Microelectronics, Tsinghua University, Bei- jing 100084, PR China. Tel.: þ86 10 62798432; fax: þ86 10 62771130. E-mail address: wxh-ime@tsinghua.edu.cn (X. Wang). Contents lists available at SciVerse ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2012.10.101 Journal of Power Sources 234 (2013) 302e309