Abstract:
Currently, supercapacitors are being highlighted for their high-power densities and cycle lifetimes, which enable them to charge and discharge faster than batteries and for a longer period of time. However, the energy density of supercapacitors falls short of that of batteries. By exceeding the operating voltages of standard aqueous and organic electrolytes, room temperature ionic liquids (ILs) may be one approach to increase energy density. Unfortunately, because of their intrinsically high viscosities, ILs have very low conductivities and must be used with caution. Organic solvent dilution has been proven to increase device rate performance even in the most viscous ionic liquids used in electrochemical applications. But the dilution of ILs with water is still under research. The competing effects of both ionic and electronic transport, as well as porosity, make it difficult to thoroughly analyze the effect on devices. This work utilizes cyclic voltammetry, galvanostatic charge-discharge, and impedance spectroscopy in a model system to gain more insight into how dilution with water influences the properties and interfacial behavior of one IL ([Bmim]+Cl-) with reduced graphene oxide (rGO) electrode. The surface properties and porosity of rGO are characterized by FTIR, XRD, FESEM, and EDX spectroscopy. However, a maximum in both ionic conductivity and intrinsic capacitance is observed at a concentration of 0.5 M [Bmim]+Cl-/H2O, suggesting that previously seen changes in bulk properties with dilution are also accompanied by a change in the capacitive behavior of the interface. It is speculated that this maximum in intrinsic capacitance is attributed to both the increased dissociation of ions, as well as the tendency of solvent molecules to screen the electrostatic interactions between ions. Further measurements in devices with rGO electrodes indicate that dilution drastically improves the rate performance of IL-based devices but was unable to exceed an operating potential of 2.4 volts, indicating that impurities are a major issue in this two-component system. A specific capacitance of 324 F g-1 with an energy density of 47 Wh kg-1 was found in 1 A g-1 current density. The supercapacitor with diluted IL and rGO electrodes also showed a 97 % coulombic efficiency with a capacity retention of 92 % even after the 10000 cycles at 10 A g-1.
