Enhancing the Rate Capability of Hybrid Supercapacitors Based on Redox Active Electrolytes through High-Surface Area Reduced Graphene Oxide

Abstract

Electrochemical double layer capacitors, which are the most widespread type of supercapacitors, have inherent energy limitations due to the low energetic of electrostatic charge storage. Increasing the energy density of supercapacitors is therefore a major challenge to open up wider application market, with targeted energy values set nowadays at 20-30 Wh/kg. Ongoing energy improvement efforts focus on the following routes: i) materials and composites overcoming capacitance values of traditional nanoporous carbons; ii) innovative electrolytes; iii) hybrid supercapacitor configurations. A particular example of such work is the combination of redox active electrolytes and carbon materials in a hybrid configuration, which provides affordable aqueous-based systems allowing for more than doubling the maximum energy output. However, energy gains come at the expense of worsening power capability and cycle life, which are the main advantages of supercapacitors over batteries. To alleviate the drawbacks of hybrid supercapacitors based on redox active electrolytes, we have developed an asymmetric system, in which a nanoporous activated carbon electrode is substituted by high-surface-area reduced graphene oxide (rGO). Importantly, rGO with surface area about 1000 m2 g-1 is made by a simple process route not involving any activation step. Using high-surface-area rGO has led to a dramatic improvement in the rate capability of hybrid devices whereas the high surface area of rGO provided for effective charge balance. The high conductivity of rGO has allowed cycling hybrid cells up to a high current density of 6.5 A/g (considering the total active mass of both electrodes) without any significant capacitance fading. Significant energy improvements can further be expected by exploring different redox active electrolytes and graphene materials with even higher specific surface area.