CHOI ET AL . VOL. 7 ’ NO. 3 ’ 2453–2460 ’ 2013 www.acsnano.org 2453 February 11, 2013 C 2013 American Chemical Society Enhanced Pseudocapacitance of Ionic Liquid/Cobalt Hydroxide Nanohybrids Bong Gill Choi, †,3 MinHo Yang, †,‡,3 Sung Chul Jung, † Kyoung G. Lee, # Jin-Gyu Kim, § HoSeok Park, ) Tae Jung Park, z Sang Bok Lee, ‡,^ Young-Kyu Han, †,Â, * and Yun Suk Huh †,0, * † Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea, ‡ Graduate School of Nanoscience and Technology (WCU), Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea, # Center for Nanobio Integration & Convergence Engineering, National Nanofab Center, Daejeon 305-806, Republic of Korea, § Division of Electron Microscopic Research, Korea Basic Science Institute, Daejeon 305-806, Republic of Korea, ) Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1 Seochon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea, z Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Republic of Korea, ^ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States, Â Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea, and 0 Department of Biological Engineering, College of Engineering, Inha University, Incheon 402-751, Republic of Korea. 3 B.G.C. and M.Y. contributed equally to this work. S upercapacitors, which can store elec- trical energy using faradaic and/or nonfaradaic reactions, are becoming attractive energy storage systems because of their desirable properties, including high power density (10 times higher than batteries), long cyclic lifetime (>100 000 cycles), rapid charging (within seconds), and low maintenance cost. 1À3 For super- capacitors to truly become an important energy system in the future, however, their energy densities need to be increased. 4 Although carbon-based electrical double layer capacitors (EDLCs) have already been studied intensively, their charge storage mechanism ; which uses the physical ad- sorption of ions ; has limited their specific capacitances. Thus, the development of nanostructured pseudocapacitive materials for supercapacitors is one of the most important tasks because, in many cases, the energy density arising from redox reac- tions caused by the adsorption/desorp- tion process of oxidants and reductants at the electrode surface is greater than that of EDLCs. 5À7 In particular, layered metal hydroxides (e.g. , Co(OH) 2 and Ni- (OH) 2 ) have drawn immense attention as alternative capacitive materials to the state-of-the-art amorphous RuO 2 be- cause of their high theoretical capacitance, unique electrochemical properties, low cost, and environmentally benign nature. 8À11 Most works on such hydroxide-based super- capacitors have concentrated on control- ling material geometries to achieve a large surface area, which is related to high specific capacitance; these have included 0D nano- particles, 12 1D nanowires, 13 2D platelets, 14À16 and 3D mesoscopic or macroscopic * Address correspondence to ykhan@kbsi.re.kr, yunsuk.huh@inha.ac.kr. Received for review December 13, 2012 and accepted February 11, 2013. Published online 10.1021/nn305750s ABSTRACT Development of nanostructured materials with enhanced redox reaction capabilities is important for achieving high energy and power densities in energy storage systems. Here, we demonstrate that the nanohybridization of ionic liquids (ILs, 1-butyl-3- methylimidazolium tetrafluoroborate) and cobalt hydroxide (Co(OH) 2 ) through ionothermal synthesis leads to a rapid and reversible redox reaction. The as-synthesized IL-Co(OH) 2 has a favorable, tailored morphology with a large surface area of 400.4 m 2 /g and a mesopore size of 4.8 nm. In particular, the IL-Co(OH) 2 -based electrode exhibits improvement in electrochemical characteristics compared with bare Co(OH) 2 , showing a high specific capacitance of 859 F/g at 1 A/g, high-rate capability (∼95% retention at 30 A/g), and excellent cycling performance (∼96% retention over 1000 cycles). AC impedance analysis demonstrates that the introduction of ILs on Co(OH) 2 facilitates ion transport and charge transfer: IL-Co(OH) 2 shows a higher ion diffusion coefficient (1.06 Â 10 À11 cm 2 /s) and lower charge transfer resistance (1.53 Ω) than those of bare Co(OH) 2 (2.55 Â 10 À12 cm 2 /s and 2.59 Ω). Our density functional theory (DFT) calculations reveal that the IL molecules, consisting of anion and cation groups, enable easier hydrogen desorption/adsorption process, that is, a more favorable redox reaction on the Co(OH) 2 surface. KEYWORDS: supercapacitor . redox reaction . ionic liquid . Co(OH) 2 . nanohybrid ARTICLE