Incorporating redox additives in sodium hydroxide electrolyte for energy storage device with the nickel cobalt molybdenum oxide active material

Abstract

Abstract To enhance the redox reactions and improve the energy storage ability, redox additives have been widely incorporated in the electrolyte to assemble the battery supercapacitor hybrid (BSH). However, due to the possible generation of numerous redox reactions for the metal oxide with abundant redox states, it is limited to apply the redox additives in the multiple metal oxide system for producing more redox reactions and promoting the energy storage. In this study, two redox additives of p-phenylenediamine (PPD) and K3[Fe(CN)6] are incorporated in KOH electrolyte for fabricating BSH with the nickel cobalt molybdenum oxide active material. The addition of PPD in the electrolyte causes the reduction of the specific capacitance (CF) for the BSH, while the greatly enhanced CF value is obtained for the BSH applying K3[Fe(CN)6] as the redox additive in the electrolyte. With the optimized K3[Fe(CN)6] concentration in the electrolyte, the BSH shows the CF value of 3.13 F/cm2 at 10 mV/s along with the maximum energy density of 48.0 Wh/kg at 756.0 W/kg. The Coulombic efficiency higher than 95% and capacitance retention of 75% are obtained for the optimized BSH. This work firstly achieves the opposite results for the BSH with different sorts of redox additives in electrolyte. It is suggested that the ion size and the compatibility between the ions in the electrolyte and the pores in the active material are important to decide the capability for energy storage.