Novel Dual-Ion Hybrid Supercapacitor Based on a NiCo₂O₄ Nanowire Cathode and MoO₂-C Nanofilm Anode

Cobalt/nickel-based compounds have been extensively used as cathode (positive electrode) materials in alkaline electrolyte for hybrid supercapacitors (HSCs). In these HSCs, however, the anodes (negative electrodes) are almost carbon-based materials that exhibit limited capacitance, leading to relatively low energy density of the device. Herein, we report a novel dual-ion HSC concept, that is, utilizing anion and cation in the electrolyte, respectively, by the two electrodes for charge storage, to promote the device's performance. Based on this, it is possible to exploit cation-consumed metal oxide as a capacitive anode to couple with a cobalt/nickel oxide cathode. As a demonstration, a 1.8 V MoO₂-C/LiOH electrolyte/NiCo₂O₄ HSC device is established. In such a design, NiCo₂O₄ cathode and MoO₂-C anode react with OH^- and Li⁺, respectively, to store energy. With the benefits from enhanced kinetics in NiCo₂O₄ nanowire array (direct electron transport pathway and sufficient electrolyte/ion penetration) and increased stability and electrical conductivity in carbon-encapsulated MoO₂ nanofilm, our device delivers a high capacitance (94.9 F g^-1), high energy density and power density (41.8 Wh kg^-1 and 19922.2 W kg^-1), long cycling stability >3000 times, and good rate capability (∼3.3 s charging/discharging with 43.6% capacitance retention). The dual-ion charge storage concept will stimulate great interest in the design of high-performing all-oxide hybrid electric energy storage systems.