Salts-assistant synthesis of g-CN/Prussian-blue analogue/nickel foam with hierarchical structures as binder-free electrodes for supercapacitors.

The exploitation of high-performance electrode materials is significant to develop supercapacitors with satisfied energy and power output properties. In this study, a g-CN/Prussian-blue analogue (PBA)/Nickel foam (NF) with hierarchical micro/nano structures was developed by a simple salts-directed self-assembly approach. In this synthetic strategy, NF acted as both 3D macroporous conductive substrate and Ni source for PBA formation. Moreover, the incidental salt in molten salt-synthesized g-CN nanosheets could regulate the combination mode between g-CN and PBA to generate interactive networks of g-CN nanosheets-covered PBA nano-protuberances on NF surfaces, which further expended the electrode/electrolyte interfaces. Based on the merits from this unique hierarchical structure and the synergy effect of PBA and g-CN, the optimized g-CN/PBA/NF electrode exhibited a maximum areal capacitance of 3366 mF cm at current of 2 mA cm, as well as 2118 mF cm even under large current of 20 mA cm. The solid-state asymmetric supercapacitor using g-CN/PBA/NF electrode possessed an extended working potential window of 1.8 V, prominent energy density of 0.195 mWh cm and power density of 27.06 mW cm. Compared to the device with pure NiFe-PBA electrode, a better cyclic stability with capacitance retention rate of 80% after 5000 cycles was also achieved due to the protective effect of g-CN shells on the etching of PBA nano-protuberances in electrolyte. This work not only builds a promising electrode material for supercapacitors, but also provide an effective way to apply molten salt-synthesized g-CN nanosheet without purification.