Large performance improvement of carbon-based supercapacitors using dual-redox additives phosphotungstic acid and potassium ferricyanide

In this work, highly porous and hierarchical carbon materials with a BET surface area of 2085 m2 g−1 and pore volume of 2.80 cm3 g−1 were produced by a simple but effective template carbonization approach using ZnO as the hard template. Furthermore, to significantly improve the energy density of the supercapacitors, dual-redox additives of phosphotungstic acid (PTA) and potassium ferricyanide (KFC) have been developed based on their integrative effect. The sample prepared by mixing PTA and KFC at equal concentrations of 15 mmol L−1 in 1 mol L−1 H2SO4 delivers a large capacitance of 77 F g−1, as well as an optimum energy density of 21.1 W h kg−1, which are much larger than those obtained with PTA or KFC alone. In addition, introducing PTA, KFC or their mixture can result in diffusion-controlled redox processes. Moreover, the redox reaction of [Fe(CN)6]3-/[Fe(CN)6]4- occurs on the positive electrode, and opposite PTA species with various chemical valences are on the negative electrode. The dual-redox additives of PTA and KFC are effective, integrative and easy to operate at ambient conditions, and thus, they are anticipated to exert a crucial role in remarkably elevating the energy density of supercapacitors.