Nanoarchitectured transition metal oxides and their composites for supercapacitors

Supercapacitors have acquired a considerable scientific and technological position in the energy storage field owing to their compelling power capability, good energy density, excellent cycling stability, and ideal safety. The supercapacitor is the burgeoning candidate to cope with the ever-growing need for green and renewable energy. High-performance supercapacitors are realized by nanostructured electrode designs, which provide ameliorated surface area for abundant electrode-electrolyte interaction, ease of electron transfer and movement, and short ion-diffusion pathways that lead to increased performance. In this regard, transition metal oxide (TMO)-based electroactive materials are of significant interest owing to the remarkable combination of structural, mechanical, electrical, and electrochemical properties. Besides their high specific capacitance and energy density due to rich redox chemistry, highly reversible and fast charge-discharge processes, low cost due to abundance, and environment-friendliness make them the most promising materials for next-generation supercapacitors. But poor electrical conductivity and rate capability, inferior cycling life, and low power density are some of the major challenges that need to be addressed. Therefore, various nanostructures of pristine TMOs and their composites with other materials with complementary characteristics have been fabricated and investigated to realize supercapacitors with improved performance. This review summarizes all such reported pristine TMOs with different nanostructured dimensions namely, 3D, 2D, 1D, and 0D, and their composite structures for their application as electrode materials in supercapacitors. Design of different pristine and composite nanostructures, synthesis strategies, comprehensive structure-dependent electrochemical properties, present challenges, and future perspectives are reviewed.