Hierarchical nanosheet-based CoMoO4–NiMoO4 nanotubes for applications in asymmetric supercapacitors and the oxygen evolution reaction

Abstract

Hollow structures with hierarchical architecture and multi-composition have attracted extensive interest because of their fascinating physicochemical properties as well as wide applications. Herein we report the designed synthesis of hierarchical nanosheet-based CoMoO₄–NiMoO₄ nanotubes by a hydrothermal treatment and a subsequent calcination method. The walls of the hierarchical nanotubes are composed of interconnected NiMoO₄ nanosheets and CoMoO₄ nanoparticles anchored on the surface of the nanosheets. The diameter of the hollow interior, the thickness of NiMoO₄ nanosheets and the diameter of CoMoO₄ nanoparticles are around 180 nm, less than 6 nm and 3 nm, respectively, providing the hierarchical nanotubes with a high surface area and a large pore volume. When evaluated as electrodes for pseudocapacitors, the hierarchical nanotubes with an appropriate amount of CoMoO₄ show a high specific capacitance of 1079 F g⁻¹ at a current density of 5 A g⁻¹ and excellent stability with 98.4% capacitance retention after 1000 cycles. Furthermore, an asymmetric capacitor, consisting of active carbon and hierarchical nanosheet-based CoMoO₄–NiMoO₄ nanotubes as negative and positive electrodes, respectively, delivers an energy density of 33 W h kg⁻¹ at a power density of 375 W kg⁻¹, and 16.3 W h kg⁻¹ even at a high power density of 6000 W kg⁻¹. The supercapacitive properties are much higher than those of single-phase NiMoO₄ nanotubes, and most of the other metal molybdates and metal oxides reported previously. Besides, the hierarchical nanotubes also exhibit much better electrocatalytic activity for the oxygen evolution reaction than single-phase NiMoO₄ nanotubes, and most of the other metal molybdates and metal oxides. Our results demonstrate the importance of rational design of complex hollow structures with enhanced properties for a wide range of practical applications.