Nickel hexacyanoferrate on graphene sheets for high-performance asymmetric supercapacitors in neutral aqueous electrolyte

Highlights: • The PRGO-NiHCF possesses many mesopores and small nanoparticals for efficient charge storage. • The specific capacitance of PRGO-NiHCF is 294 F g⁻¹ at 2 mV s⁻¹, higher than that of pure NiHCF (176 F g⁻¹). • The SGC//PRGO-NiHCF asymmetric supercapacitor delivers a high energy density of 25.4 Wh kg⁻¹. -- Abstract: Nickel hexacyanoferrate (NiHCF) with rigid open framework is promising for energy storage devices, yet still suffer from inferior conductivity and low energy density. Herein, NiHCF supported on diallyldimethylammonium chloride (PDDA) modified graphene (PRGO-NiHCF) was successfully synthesized via a facile co-precipitation method. The size of NiHCF nanoparticals decreased from 50 nm to 20 nm with the introduction of PDDA-induced reduced graphene oxide. The unique structure can not only reversibly accommodate hydrated alkali cations with 3D open-framework, but also facilitate the electrons transportation through the graphene basal plane. Consequently, the as-obtained PRGO-NiHCF exhibits higher specific capacitances of 294 F g⁻¹ at 2 mV s⁻¹ compared to pure NiHCF (176 F g⁻¹). Moreover, under the combination of PRGO-NiHCF positive electrode and the interconnected sandwiched graphene carbon (SGC) negative electrode, the as-assembled asymmetric supercapacitor delivers a wide operation potential of 1.8 V and a high energy density of 25.4 Wh kg⁻¹.