Carbon with ultrahigh capacitance when graphene paper meets K₃Fe(CN)₆

For the first time, we have shown a novel supercapacitor system with a graphene paper electrode in the redox-electrolyte of K₃Fe(CN)₆ based on the system-level design principle. By combining electric double-layer capacitance and pseudocapacitance, the specific capacitance could be increased 5-fold compared with the conventional electrode-electrolyte system. This large improvement is attributed to the additional redox reactions on the graphene paper electrode via the constituent ions of K₃Fe(CN)₆ in the redox-electrolyte, during which the discharge process is highly enhanced. More importantly, the potential interval could reach as high as 1.6 V beyond the limited operating voltage of water (∼1.23 V). After 5000 continuous cycles, 94% of the initial capacitance was retained. The newly designed novel supercapacitor system is binder-free, conducting additive-free and can deliver higher capacitance. This designed graphene-paper-electrode/redox-electrolyte system can provide a versatile strategy for high-capacitance supercapacitor systems.