▎ 摘　　要

Energy density of supercapacitors based on a single-sheet graphene electrode is studied via molecular dynamics (MD) computer simulations. Two electrolytes of different types, pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4-) and an 1.1 M solution of EMI+BF4- 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 found to be higher in EMI+BF4- than in acetonitrile solution by 55-60%. This is due to strong screening of the electrode charge by RTIL ions in the former. The RTIL screening behavior is found to be rather insensitive to temperature T. As a result, the capacitance of supercapacitors based on pure EMI+BF4- decreases by less than 5% as T increases from 350 to 450 K. The difference in size and shape between cations and anions and the resulting difference in their local charge distribution as counterions near the electrified graphene surface yield cathode anode asymmetry in the electrode potential in RTIL. As a consequence, specific capacitance of the positively charged electrode is higher than that of the negatively charged electrode by more than 10%. A similar degree of disparity in electrode capacitance is also found in acetonitrile solution because of its nonvanishing potential at zero charge. Despite high viscosity and low ion diffusivity of EMI+BF4-, its overall conductivity is comparable to that of the acetonitrile solution thanks to its large number of charge carriers. The present study thus suggests that 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.