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Developing high energy density supercapacitors is of great importance to the transportation, consumer electronics, and micro-grid energy storage sectors. Recently, the development of high voltage organic electrolyte based supercapacitor devices has been gaining much attention. Among them, there is an on-going intense interest in investigating high capacity lithium ion storage anode materials in hybrid supercapacitors. However, developing high capacity cathode materials for high voltage organic electrolyte supercapacitor devices is rarely investigated. The low electrical double layer capacitances of carbon cathode electrodes, which are widely used in current supercapacitor devices, are often the limiting bottleneck. In this contribution, we investigated the electrochemical energy storage behavior of a polyaniline (PANI)-single wall carbon nanotube (SWCNT) composite material in an organic electrolyte as a supercapacitor cathode. The PANI-SWCNT composite exhibits a high specific capacitance of 503 F/g, of which 58.8% of the total capacitance is attributed to the pseudocapacitive and electrical double layer energy storage. The cycling stability of the PANI-SWCNT composite could be further improved by polydopamine (PDA) modification. The PDA with strong adhesion properties is able to prevent mechanical degradation. The PDA modified PANI-SWCNT shows excellent stability with only 5% degradation after 2000 cycles.

I. INTRODUCTION

Supercapacitors are important energy storage devices that can provide transient, but high power output for various machines and devices.1,2 Electric double layer capacitors (EDLCs) have already been utilized in various applications, such as backup power banks, electricity leveling units, consumer electronics and heavy duty machines.3 However, the current energy density of commercial EDLC devices can only reach around 10 W h/kg, which makes it hard to penetrate into a larger application market, such as hybrid electrical vehicles. As a result, developing high energy density supercapacitor devices is a topic that has attracted considerable attention. The evolution of supercapacitor devices from EDLCs to higher energy density supercapacitors can be summarized into two directions.4 The first direction is to fabricate aqueous electrolyte based asymmetric supercapacitor devices using pseudocapacitive materials and EDLC carbon materials. The second direction is to fabricate organic electrolyte based hybrid supercapacitors using lithium ion battery like energy storage materials Contributing Editor: Edward M. Sabolsky a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2015.342 J. Mater. Res., Vol. 30, No. 23, Dec 14, 2015

and EDLC carbon materials [so called lithium ion capacitor (LIC) or hybrid capacitor]. Extensive research efforts have been exerted in the first direction. However, due to water decomposition in aqueous electrolyte systems, the operation voltages of asymmetric supercapacitors are limited, which det