Towards bi-functional all-solid-state supercapacitor based on nickel hydroxide-reduced graphene oxide composite electrodes

Highlights: • The operation window of Ni(OH)₂-rGO based, all-solid-state supercapacitor is expanded to 3 V with lithium-ion GPE. • Faradaic reactions are proposed for the Ni(OH)₂-rGO based supercapacitor after activation. • Bi-functionality of the activated Ni(OH)₂-rGO supercapacitor is demonstrated to be controllable. Supercapacitors are promising in increasing energy recycle efficiency when employed with regenerative braking systems in electric vehicles, because of their high power density and long cycle life. However, the low energy density and small operation window of supercapacitors are limiting their applications. Nickel hydroxide-reduced graphene oxide (Ni(OH)₂-rGO) composite electrodes exhibit high electrochemical performance in aqueous alkaline electrolyte. Replacing the aqueous electrolyte with a lithium-ion gel polymer electrolyte (GPE), an all-solid-state supercapacitor with widened operation window is demonstrated. After activation, the Ni(OH)₂-rGO supercapacitor can successfully operate up to 3 V with battery-like behavior, resulting in a high specific capacity of 85 mAh g⁻¹, specific energy of 36.1 Wh kg⁻¹, and specific power of 77.01 W kg⁻¹. By lowering the upper-limit potential to 2.5 V, the supercapacitor exhibits capacitor-like behavior with a specific capacitance of 6.7 F g⁻¹, specific energy of 12.4 Wh kg⁻¹, and specific power of 6.8 kW kg⁻¹. Further reducing the upper potential limit to 2 V leads to stable operation of the capacitor for over 15,000 cycles with coulombic efficiency of over 95% under the current density of 1.54 A g⁻¹. Our work demonstrates that the Ni(OH)₂-rGO supercapacitor with the lithium-ion GPE can work bi-functionally either as a battery-like or a capacitor-like device depending on the operation potential range.