Novel rhombus CoO-nanocapsule CuO heterohybrids for efficient photocatalytic water splitting and electrochemical energy storage applications.

The most appealing and prominent approach for improving energy storage and conversion performance is the development of heterojunction interfaces with efficient and unique metal oxide nanostructures. Rhombus CoO, nanocapsule CuO, and their heterojunction composites were synthesized using a single-step hydrothermal process. The resulting heterojunction CoO-CuO nanocomposite outperformed the pristine CoO and CuO nanostructures for the electrochemical supercapacitor and water splitting performances. The composite showed 2.4 and 1.3 times higher specific capacitance than the associated pristine CuO and CoO nanostructures, while its capacitance was 395 F g at a current density of 0.5 A g. In addition, long-term GCD results with more than 90% stability and significant capacity retention at higher scan rates revealed the unaffected structures interfaced during the electrochemical reactions. The composite photoelectrode demonstrated more than 20% of photocurrent response with light illumination than the dark condition in water splitting. CoO-CuO heterostructured composite electrode showed a 0.16 mA/cm photocurrent density, which is 3.2 and 1.7 times higher than the pristine CuO and CoO electrodes, respectively. This performance was attributed to its unique structural composition, high reactive sites, strong ion diffusion, and fast electron accessibility. Electron microscopic and spectroscopic techniques confirmed the properties of the electrodes as well as their morphological properties. Overall, the heterojunction interface with novel rhombus and capsule structured architectures showed good electrochemical performance, suggesting their energy storage and conversion applications.