Binary transition metal oxides (BTMOs) possess higher reversible capacity, better structural stability and electronic conductivity, and have been widely studied to be novel electrode materials for supercapacitors. In this review, we present an extensive description of BTMO materials and the most commonly used synthetic methods. Furthermore, we review several notable BTMOs and their composites in application of supercapacitors. With the increasing attention for energy storage, more and more exciting results about BTMO materials will be reported in the future.

Hierarchical mesoporous spinel NiCo2O4 was synthesized by a facile hydrothermal method assisted by polyvinylpyrrolidone (PVP) and a post annealing treatment. The synthesized hierarchical mesoporous NiCo2O4 presents a hierarchical mesoporous structure with diameters of 5.0 and 25 nm, respectively. Compared to conventional flower-like NiCo2O4, the hierarchical mesoporous structured NiCo2O4 exhibits excellent supercapacitor performance. The specific capacitance can reach 1619.1 F g−1 at a current density of 2.0 A g−1. When the current density is increased to 10.0 A g−1, a specific capacitance of 571.4 F g−1 can be obtained. Furthermore, the hierarchical mesoporous structured NiCo2O4 presents excellent stability. The outstanding electrochemical performance of the hierarchical mesoporous NiCo2O4 reveals its potential to be a promising material for use in supercapacitors, and also inspires continued research on binary metal oxides as energy transformation materials.

A facile two-step method is developed for large-scale preparation of graphene-based three-dimensional hierarchical sandwich-type architecture (graphene/carbon nanotubes (CNTs)/Mn2O3) for high performance supercapacitor. The synthesis involves a chemical vapor deposition (CVD) method to fabricate sponge-like three-dimensional (3D) graphene/CNTs and an electrodeposition process to deposit Mn2O3 nano-sheets on the surface of 3D graphene/CNTs. With the novel sandwich-type composite as an electrode, the measurements indicate the synergy effects of Mn2O3 and 3D carbonous materials make the electrode present a high specific capacitance. The composite electrode also presents a high reversible capacity, excellent cycle performance and rate capability. It could be concluded that the composite of Mn2O3 with 3D graphene/CNTs displays excellent synergy effects of transition-metal oxide and carbonous materials. This work also inspires in-depth research for the application of 3D graphene-based composites for high performance supercapacitors.

Nanostructured Co3O4 materials attracted significant attention due to their exceptional electrochemical (pseudo-capacitive) properties. However, rigorous preparation conditions are needed to control the size (especially nanosize), morphology and size distribution of the products obtained by conventional methods. Herein, we describe a novel one step shape-controlled synthesis of uniform Co3O4 nanocubes with a size of 50 nm with the existence of mesoporous carbon nanorods (meso-CNRs). In this synthesis process, meso-CNRs not only act as a heat receiver to directly obtain Co3O4 eliminating the high-temperature post-calcination, but also control the morphology of the resulting Co3O4 to form nanocubes with uniform distribution. More strikingly, mesoporous Co3O4 nanocubes are obtained by further thermal treatment. The structure and morphology of the samples were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. A possible formation mechanism of mesoporous Co3O4 nanocubes is proposed here. Electrochemical tests have revealed that the prepared mesoporous Co3O4 nanocubes demonstrate a remarkable performance in supercapacitor applications due to the porous structure, which endows fast ion and electron transfer.

Binary transition metal oxides (BTMOs) possess higher reversible capacity, better structural stability and electronic conductivity, and have been widely studied to be novel electrode materials for supercapacitors. In this review, we present an extensive description of BTMO materials and the most commonly used synthetic methods. Furthermore, we review several notable BTMOs and their composites in application of supercapacitors. With the increasing attention for energy storage, more and more exciting results about BTMO materials will be reported in the future.