8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P1.16Fri 911:10-14:00Computer simulation study on room-temperature ionicliquid/graphene supercapacitorYounJoon Jung, 1 Youngseon Shim, 1 and Hyung Kim 21 Seoul National University, Department of Chemistry, 1 Gwanak-ro, Gwanak-gu 151-747,Seoul, South Korea2 Carnegie Mellon University, Pittsburgh, United States of AmericaEnergy density of supercapacitors based on graphene electrodes is studied via molecular dynamics(MD) computer simulations. Two electrolytes of different types, pure 1-ethyl-3- methylimidazoliumtetrafluoroborate (EMI + BF 4 - ) and an 1.1 M solution of EMI + BF 4 - in acetonitrile,are considered as a prototypical room-temperature ionic liquid (RTIL) and organic electrolyte, respectively.Structure of ions near the electrode surface varies significantly with its charge density,especially in pure RTIL. Specific capacitance normalized to the electrode surface area is foundto be higher in RTIL than in acetonitrile solution. This is due to strong screening of the electrodecharge by RTIL ions in the former. The difference in size and shape between cations andanions and the resulting difference in their local charge distribution as counter-ions near the electrifiedgraphene surface yield cathode-anode asymmetry in the electrode potential in RTIL. As aconsequence, specific capacitance of the positively-charged electrode is higher than that of thenegatively-charged electrode. A similar degree of disparity in electrode capacitance is also foundin acetonitrile solution because of its non-vanishing potential at zero charge. Despite high viscosityand low ion diffusivity of EMI + BF 4 -, its overall conductivity is comparable to that of theacetonitrile solution thanks to its large number of charge carriers. The present study thus suggeststhat as a supercapacitor electrolyte, RTILs are comparable in power density to organic electrolytes,while the former yield considerably better energy density than the latter at a given cell voltage.16