Cobalt hexacyanoferrate/MnO2 nanocomposite for asymmetrical supercapacitors with enhanced electrochemical performance and its charge storage mechanism

Cobalt hexacyanoferrate (CoHCF) possessing high capacity as well as remarkable cycling stability is generally considered to store charge via the electrochemical reaction of Fe3+/Fe2+ redox couple and Co ions are electrochemically inactive because of the low decomposition voltage of water. In this work, we report an efficient approach to fabricate the MnO2 nanosheet (MnO2 NS) coated CoHCF with enhanced electrochemical performance. More importantly, the capacity contribution from the Co3+/Co2+ redox couple is verified in neutral aqueous electrolytes, leading to a new understanding of the charge storage mechanism of CoHCF. The CoHCF/MnO2 nanocomposite exhibits a specific capacitance of 385 F g−1 (1 A g−1) in 0.5 M Na2SO4 aqueous electrolyte. It is revealed that CoHCF/MnO2 undergoes the reversible electrochemical reactions of Co3+/Co2+, Fe3+/Fe2+ and Mn4+/Mn3+ redox couples to store charges. Furthermore, an asymmetrical supercapacitor is fabricated with utilizing CoHCF/MnO2 as the cathode and activated carbon as the anode. The device operates at a high cell voltage of 2.0 V and delivers an energy density of 37.6 Wh kg−1 at a power density of 1.1 kW kg−1. Excellent cycling stability is exhibited that 86% of the initial discharge capacitance is maintained after 5000 charge-discharge cycles.