Facile preparation of carbon, nitrogen co-doped NiMoO4·MoO2 heterostructures derived from polyoxometalate with ultrahigh energy density for zinc-ion capacitors

Aqueous zinc ion hybrid supercapacitors (ZHSC) have attracted great attention in recent years. However, their electrochemical performance is prohibited by the low energy density and poor cycling stability, mainly due to the difficulty in effectively activating the ionic and electronic conductivity of the cathode material. In this work, we innovatively propose a low-cost, precisely customized strategy for doping trace amounts of carbon and nitrogen elements into metal oxides to improve electrochemical performance. The carbon and nitrogen-doped NiMoO4·MoO2/NC heterostructures derived from polyoxometalates maintain high electrochemical activity while achieving high ionic conductivity. Trace amounts of carbon and nitrogen elements can significantly improve electronic conductivity and charge storage capacity. Due to the synergistic contribution, the composite structure with multi-redox sites exhibits a high specific capacity value of 364.96 F/g at 1 A/g in 1 M ZnSO4. A ZHSC was assembled with NiMoO4·MoO2/NC as the cathode and zinc sheet as the anode. After 10000 cycles, the capacitance retention rate still reached over 80 %. In addition, its energy density and power density can also reach 102 Wh Kg-1@4140 W Kg-1, which is higher than that of MoO2-based traditional supercapacitors. This strategy fully demonstrates the great practicality and development prospects of polyoxometalate derivatives for ZHSC.