Porous spinel-type transition metal oxide nanostructures as emergent electrocatalysts for oxygen reduction reactions

Porous spinel-type transition metal oxide (PS-TMO) nanocatalysts comprising two kinds of metal (denoted as A(x)B(3-x)O(4), where A, B = Co, Ni, Zn, Mn, Fe, V, Sm, Li, and Zn) have emerged as promising electrocatalysts for oxygen reduction reactions (ORRs) in energy conversion and storage systems (ECSS). This is due to the unique catalytic merits of PS-TMOs (such as p-type conductivity, optical transparency, semiconductivity, multiple valence states of their oxides, and rich active sites) and porous morphologies with great surface area, low density, abundant transportation paths for intermediate species, maximized atom utilization and quick charge mobility. In addition, PS-TMOs nanocatalysts are easily prepared in high yield from Earth-abundant and inexpensive metal precursors that meet sustainability requirements and practical applications. Owing to the continued developments in the rational synthesis of PS-TMOs nanocatalysts for ORRs, it is utterly imperative to provide timely updates and highlight new advances in this research area. This review emphasizes recent research advances in engineering the morphologies and compositions of PS-TMOs nanocatalysts in addition to their mechanisms, to decipher their structure-activity relationships. Also, the ORR mechanisms and fundamentals are discussed, along with the current barriers and future outlook for developing the next generation of PS-TMOs nanocatalysts for large-scale ECSS.

Automated summary

Porous spinel-type transition metal oxide (PS-TMO) nanocatalysts, composed of two kinds of metal, have emerged as promising electrocatalysts for oxygen reduction reactions (ORRs) in energy conversion and storage systems (ECSS). The unique catalytic merits and porous morphologies of PS-TMOs, combined with their easy preparation from Earth-abundant and inexpensive metal precursors, make them a highly attractive option for sustainable and practical applications. This review focuses on recent advances in engineering the morphologies and compositions of PS-TMOs nanocatalysts, as well as their structure-activity relationships and potential for future large-scale ECSS.