Hierarchically nanostructured transition metal oxides for supercapacitors

Highly efficient, clean, and sustainable electrochemical energy storage technologies have been investigated extensively to counter the shortage of fossil fuels and increasingly prominent environmental problems. Supercapacitors (SCs) have received wide attention as critical devices for electrochemical energy storage because of their rapid charging-discharging capability and long life cycle. Various transition metal oxides (TMOs), such as MnO2, NiO, Co3O4, and CuO, have been extensively studied as electrode materials for SCs. Compared with carbon and conducting polymers, TMO materials can achieve higher specific capacitance. For further improvement of electrochemical performance, hierarchically nanostructured TMO materials have become a hot research area for electrode materials in SCs. The hierarchical nanostructure can not only offer abundant accessible electroactive sites for redox reactions but also shorten the ion diffusion pathway. In this review, we provide an overall summary and evaluation of the recent progress of hierarchically nanostructured TMOs for SCs, including synthesis methods, compositions, structures, and electrochemical performances. Both single-phase TMOs and the composites based on TMOs are summarized. Furthermore, we also prospect the developing foreground of this field. In this view, the important directions mainly include: the nanocomposites of TMOs materials with conductive materials; the cobalt-based materials and the nickel-based materials; the improvement of the volume energy density, the asymmetric SCs, and the flexible all-solid-state SCs.