Hybrid energy storage of battery-type nickel hydroxide and supercapacitor-type graphene: redox additive and charge storage mechanism

Abstract

Herein, hybrid energy storages (HESs) of battery-type Ni(OH)₂ and supercapacitor-type electrochemically reduced graphene oxide (ERGO) were fabricated using potassium ferricyanide (K₃Fe(CN)₆) as a redox additive in KOH electrolyte for high specific energy and power applications. The as-fabricated HES of Ni(OH)₂//ERGO in a single coin cell (CR2016) size in 4 mM K₃[Fe(CN)]₆ in 1 M KOH provides a wide working voltage up to 1.6 V and exhibits a maximum specific energy of 85 W h kg⁻¹ at the specific power of 726 W kg⁻¹ with a high capacity retention over 88% after 10 000 cycles, while the HES in 1 M KOH provides a lower maximum specific energy of 61 W h kg⁻¹. A Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox couple has a great electrochemical reversibility in nature since Fe(CN)₆³⁻ can obtain electrons from Ni(OH)₂ through the reduction process and Fe(CN)₆⁴⁻ can donate electrons to NiOOH for the oxidation process. The HES reported herein may be practically used for high energy applications.