Face centered cubic Mo ₂ C superconductive thin film was epitaxially grown on a sapphire substrate controlled by the substrate orientation.

Although CoO is a promising electrode material for supercapacitors due to its high theoretical capacitance, the practical applications still suffering from inferior electrochemical activity owing to its low electrical conductivity, poor structural stability and inefficient nanostructure. Herein, we report a novel Cu⁰/Cu⁺ co-doped CoO composite with adjustable metallic Cu⁰ and ion Cu⁺ via a facile strategy. Through interior (Cu⁺) and exterior (Cu⁰) decoration of CoO, the electrochemical performance of CoO electrode has been significantly improved due to both the beneficial flower-like nanostructure and the synergetic effect of Cu⁰/Cu⁺ co-doping, which results in a significantly enhanced specific capacitance (695 F g⁻¹ at 1 A g⁻¹) and high cyclic stability (93.4% retention over 10,000 cycles) than pristine CoO. Furthermore, this co-doping strategy is also applicable to other transition metal oxide (NiO) with enhanced electrochemical performance. In addition, an asymmetric hybrid supercapacitor was assembled using the Cu⁰/Cu⁺ co-doped CoO electrode and active carbon, which delivers a remarkable maximal energy density (35 Wh kg⁻¹), exceptional power density (16 kW kg⁻¹) and ultralong cycle life (91.5% retention over 10,000 cycles). Theoretical calculations further verify that the co-doping of Cu⁰/Cu⁺ can tune the electronic structure of CoO and improve the conductivity and electron transport. This study demonstrates a facile and favorable strategy to enhance the electrochemical performance of transition metal oxide electrode materials.