Ultrahigh capacitive performance from both Co(OH)₂/graphene electrode and K₃Fe(CN)₆ electrolyte

Pseudocapacitance is commonly associated to the reversible redox reactions from electrode materials, but the enhancement in pseudocapacitance that only relies on electrode materials is limited. Here, we explore the possibility of enhancing pseudocapacitance through both Co(OH)₂/graphene nanosheets (GNS) electrode and K₃Fe(CN)₆ electrolyte. With a good conductivity and favoring electron transfer, GNS are hybridized with Co(OH)₂ to improve the pseudocapacitance of Co(OH)₂, including enhancing its rate capability and electrochemical stability. Adding K₃Fe(CN)₆ into KOH electrolyte further enhances the pseudocapacitance via both directly contributing pseudocapacitance to Co(OH)₂/GNS and promoting the electron gain and loss of Co ions. This novel Co(OH)₂/GNS-K₃Fe(CN)₆/KOH electrode system shows an ultrahigh specific capacitance of 7514 Fg⁻¹ at 16 Ag⁻¹ in mixed 1 M KOH and 0.08 M K₃Fe(CN)₆, more than 100% coulombic efficiency, and long-term cycling stability (the capacitance retention is 75% after 20000 continuous charge-discharge cycles in mixed 1 M KOH and 0.04 M K₃Fe(CN)₆.