Electrochemical Capacitors Based on Graphene Oxide Sheets Using Different Aqueous Electrolytes

Graphene oxide (GO)-based electrochemical capacitors have been fabricated and investigated in 1 M Li2SO4 and Na2SO4 aqueous electrolytes. The GO sheets were derived from natural graphite powders and subsequently coated over carbon paper forming a composite electrode. The GO sheets have highly oxidized planes and edges, occupied by oxygen functionalities including carboxyl, carbonyl, and ether groups. The GO-based capacitor displays specific capacitances of 238.0 and 98.8 F/g at 0.5 mA/cm(2) in Li2SO4 and Na2SO4 electrolytes, respectively. The electrochemically active areas for Li and Na ions are calculated to be 452.8 and 219.3 m(2)/g at the first discharge cycle, respectively. The staking layer of the hydrated Li molecules forms dual layers, whereas the hydrated Na molecules tend toward a monolayer adsorption on the oxidized sheets. On the basis of the Randles plot, the apparent diffusion coefficient of Li+ is calculated to be 3.1 x 10(-15) cm(2)/s, which is about three times higher than that of Na+ in the GO-based electrodes. Compared with the Na2SO4 electrolyte, the GO-based capacitor in Li2SO4 exhibits high stable capacitance, low inner resistance, and high diffusivity. This originates from the smaller ionic size and the lower hydration number, thus facilitating the performance of the capacitor.