Nanostructured Metal Oxides in Supercapacitors: Structure–Property–Performance Relationships

Metal oxide transition materials that exhibit pseudo-capacitive performance have been extensively studied due to their ability to be easily modified with respect to both structure and redox activity. Although many metal oxide electrochemical capacitor systems have exhibited very high capacitance values (as much as double or even triple those of activated carbon), these values can be misleading as they do not account for the actual kinetic limitations and degradation mechanisms associated with such materials. This review will examine how three primary aspects (i.e., composition, crystallographic arrangement, and defect/dimensionality) affect charge storage in metal-oxide based supercapacitors. In addition, the review will highlight the need to distinguish clearly among surface redox pseudocapacitance, intercalation behavior, and "battery" type reactions so as to provide accurate classification of material behavior. Additionally, the effects of using 0D-3D nanostructures, oxygen vacancies, bimetallic mixed-valent states, and multimetal oxides on the structural-property relationship of metal-oxide supercapacitors will be discussed from a structure-property-performance perspective. Furthermore, device level factors (e.g., mass loading, electrode geometry and/or microstructure, and experimental configuration) will be evaluated. Finally, by identifying common misconceptions and pitfalls related to material selection and characterization issues, the review will provide practical recommendations for the design of metal-oxide supercapacitors that are scalable, long-lived, and accurately compared to other capacitor systems.