Ni-foam supported Ni(OH)₂ sheets decorated by MXene quantum dots for high performance supercapacitors

The MXene quantum dots (MQDs) were prepared by microwave-sonication exfoliation, and further loaded on Ni(OH)₂ sheets using Ni form as substrate by one-pot solvothermal synthesis. Three different exfoliation mediums (ethanol-EtOH, N,N-dimethylformamide-DMF, dimethyl sulfoxide-DMSO) were changed to investigate their influences on the structure of MQDs and MQDs-Ni(OH)₂ composites. The DMF medium is better than the other two due to its good reducibility and small polarity, and the obtained MDQ_f from DMF exfoliation has less oxidation and higher quantum yield, while the prepared MQD_f-Ni(OH)₂ composites possessed large Ni(OH)₂ sheet, high Ni³⁺ ratio and high N doping content. All these material changes caused by DMF can promote the charge-discharge process during the supercapacitor application, resulting in the best electrochemical performance of 1660 F/g at 1 A/g using MQD_f-Ni(OH)₂ composites as electrodes. Moreover, the asymmetric supercapacitors using MQD_f-Ni(OH)₂ composites and active carbon as electrodes possessed a maximum specific capacitance of 98 F/g, and a maximum energy density of 30.6 Wh/kg (with a power density of 750 W/kg).