Electrochemically Active Phosphotungstic Acid Assisted Prevention of Graphene Restacking for High-Capacitance Supercapacitors

Abstract

Direct reduction of graphene oxide usually leads to the agglomeration of the as-generated graphene sheets, thus suppressing the surface exposed for energy storage. Herein, graphene oxide was reduced by a one-pot hydrothermal process in the presence of an electrochemically active phosphotungstic acid to produce three-dimensional porous phosphotungstic acid/reduced graphene oxide composites. Phosphotungstic acid molecules were found to be uniformly anchored on the surface of reduced graphene oxide sheets through the electrostatic interaction to prevent the reduced graphene oxide sheets from restacking. Meanwhile, phosphotungstic acid has the capability of undergoing fast reversible multi-electron redox reactions and hence providing the pseudocapacitance. As expected, phosphotungstic acid/reduced graphene oxide composites display the specific capacitance as high as 456.7 F g−1 at 5 mV s−1 and 363.8 F g−1 at 0.5 A g−1, which are much larger than those obtained in reduced graphene oxide (162.4 F g−1 at 5 mV s−1 and 190.6 F g−1 at 0.5 A g−1). The retention also maintains 82.9% of initial specific capacitance after 1000 charge/discharge cycles. Such excellent electrochemical performance comes from the three-dimensional porous architecture and synergistic interaction between the pseudocapacitive phosphotungstic acid and electric double-layer capacitive graphene.