Construction of fly ash-based CoFeSi/NiCo-LDH heterostructure for hybrid supercapacitors

Transition metal silicates are potential supercapacitor electrode materials due to their natural abundance, excellent theoretical specific capacitance and superior structural stability. However, the inferior conductivity and slow reaction kinetics lead to a limited actual electrochemical performance of transition metal silicates. Herein, using fly ash-based silicates as the silicon source, the heterostructure CoFeSi/NiCo-LDH was successfully synthesized through a two-step solvothermal method. The heterostructure of CoFeSi/NiCo-LDH promotes charge transfer at the heterointerface by reconstructing the electric field at the heterointerface. Moreover, the strong redox activity of the NiCo-LDH component introduces abundant active sites and enhances the internal energy storage capability of the composite, thereby significantly improving its electrochemical performance. In addition, the ratio of ethanol to water in the second solvothermal reaction was optimized to further boost performance. The optimized CoFeSi/NiCo-LDH-20 displays a high specific capacitance of 1419 F g^-1 at 1 A g^-1, excellent rate performance (70 % capacitance retention when the current density was increased 16-fold) and a superior cycle life (89.35 % capacitance retention after 10,000 cycles). The assembled hybrid supercapacitor device (CoFeSi/NiCo-LDH-20//AC) presents an energy density of 26.6 Wh kg^-1 at a power density of 400 W kg^-1 as well as an excellent cycle life with a 95.7 % capacitance retention after 10,000 cycles. This study provides a new idea for preparing high-performance electrode materials using raw fly ash.