Salts-assistant synthesis of g-C3N4/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-C3N4/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-C3N4 nanosheets could regulate the combination mode between g-C3N4 and PBA to generate interactive networks of g-C3N4 nanosheets-covered PBA nanoprotuberances 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-C3N4, the optimized g-C3N4/ PBA/NF electrode exhibited a maximum areal capacitance of 3366 mF cm-2 at current of 2 mA cm-2, as well as 2118 mF cm-2 even under large current of 20 mA cm-2. The solid-state asymmetric supercapacitor using g-C3N4/ PBA/NF electrode possessed an extended working potential window of 1.8 V, prominent energy density of 0.195 mWh cm-2 and power density of 27.06 mW cm-2. 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-C3N4 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-C3N4 nanosheet without purification.