Composite capillary carbon tube Nb₁₈W₁₆O₉₃as advanced anode material for aqueous ion capacitors

Niobium-tungsten bimetal oxides have received wide attention due to their excellent lattice properties. In this work, Nb₁₈W₁₆O₉₃(NbWO) with a tetragonal tungsten bronze structure was synthesized by simple hydrothermal method. NbWO was modified to provide high specific surface area via combining with hollow carbon nanotubes. Meanwhile, NbWO grows along the tube wall of carbon nanotubes, thus buffering the volume effect of NbWO particles. Also, the migration distance of Li-ion is effectively shortened, as well as the improved ion transfer efficiency and the reaction kinetics. In addition, carbon tube can enhance conductivity of NbWO, contributing to outstanding charge storage capacity and rate energy. Precisely, NbWO@C as electrode possesses large specific capacity (249.6 F g^-1at 0.5 A g^-1) and good rate performance (55.9% capacity retention from 0.5 to 2 A g^-1). The aqueous Li-ion capacitor presents the advantages of high safety, low cost and good environmental friendliness. An asymmetric aqueous capacitor AC//NbWO@C, based on 'water-in-salt' electrolyte with high concentration lithium acetate, exhibits a large energy density of 43.2 Wh kg^-1and a power density of 9 kW kg^-1. Generally, NbWO@C as anode materials shows superior application perspective.