Asymmetric Supercapacitors Based on Reduced Graphene Oxide with Different Polyoxometalates as Positive and Negative Electrodes

Abstract

Nanofabrication using a "bottom-up" approach of hybrid electrode materials into a well-defined architecture is essential for next-generation miniaturized energy storage devices. This paper describes the design and fabrication of reduced graphene oxide (rGO)/polyoxometalate (POM)-based hybrid electrode materials and their successful exploitation for asymmetric supercapacitors. First, redox active nanoclusters of POMs [phosphomolybdic acid (PMo₁₂ ) and phosphotungstic acid (PW₁₂ )] were uniformly decorated on the surface of rGO nanosheets to take full advantage of both charge-storing mechanisms (faradaic from POMs and electric double layer from rGO). The as-synthesized rGO-PMo₁₂ and rGO-PW₁₂ hybrid electrodes exhibited impressive electrochemical performances with specific capacitances of 299 (269 mF cm^-2 ) and 370 F g^-1 (369 mF cm^-2 ) in 1 m H₂ SO₄ as electrolyte at 5 mA cm^-2 . An asymmetric supercapacitor was then fabricated using rGO-PMo₁₂ as the positive and rGO-PW₁₂ as the negative electrode. This rGO-PMo₁₂ ∥rGO-PW₁₂ asymmetric cell could be successfully cycled in a wide voltage window up to 1.6 V and hence exhibited an excellent energy density of 39 Wh kg^-1 (1.3 mWh cm^-3 ) at a power density of 658 W kg^-1 (23 mW cm^-3 ).

Keywords: asymmetric supercapacitors; carbon; hybrid materials; polyoxometalates; reduced graphene oxide.