A High-Energy Aqueous Sodium-Ion Capacitor with Nickel Hexacyanoferrate and Graphene Electrodes

Sodium-ion capacitors have received much attention compared to lithium-based systems, owing to the improved safety and earth abundancy. Here, we assembled an aqueous sodium-ion capacitor by using nickel hexacyanoferrate and graphene as positive and negative electrodes, respectively, in 1 M Na2SO4 electrolyte. The fabricated capacitor can work in a wide potential window from 0 to 2 V, giving an energy density of 39.35 Whkg(-1) with better capacitance retention of about 91%, even after 2000 cycles. Besides, the cost-effective precursors as well as environmentally friendly and earth-abundant electrolytes with high safety will ensure that the fabricated sodium-ion capacitor system is suitable for next-generation energy storage applications.