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

Aqueous zinc ion hybrid supercapacitors (ZHSCs) have attracted great attention in recent years. However, their electrochemical performance is impeded by their 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^-1 at 1 A g^-1 in 1 M ZnSO₄. A ZHSC was assembled with NiMoO₄·MoO₂/NC as the cathode and a zinc sheet as the anode. After 10 000 cycles, the capacitance retention rate still reached over 80%. In addition, its energy density and power density can also reach 102 W h kg^-1@4140 W kg^-1, which is higher than those of MoO₂-based traditional supercapacitors. This strategy fully demonstrates the great practicality and development prospects of polyoxometalate derivatives for ZHSCs.