Electron spin modulation engineering in oxygen-involved electrocatalysis

Electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reduction (OER) are regarded as the key reactions via the sustainable system (fuel cell and water splitting), respectively. In OER, the transition from singlet oxygen species to triplet oxygen molecules is involved, meanwhile the ORR involves the transition from triplet oxygen molecules to singlet oxygen species. However, in these processes, the number of unpaired electrons is not conserved, which is not thermodynamically favorable and creates an additional energy barrier. Fortunately, regulating the electrocatalysis by spin-state modulation enables a unique effect on the catalytic performance, but the current understanding on spin-state engineering for electro-catalyzing ORR and OER is still insufficient. Herein, this review summarized the in-spin engineering for the state-of-the-art ORR and OER electrocatalysts. It began by introducing engineering of spin-state to egfilling for ORR and OER process, and then moved to spin polarization and spin-pinning effect for OER process. Various designed strategies focusing on how to regulate the spin-state of the active center have been summarized up. The connectivity of the structures of typical ORR (e.g. metal-nitrogen-carbon) and OER (e.g. design strategies oxides, metal organic frameworks) catalysts depending on the spin level is also discussed. Finally, we present the outlook from the aspects of template catalysts, characterization methods, regulation strategies, theoretical calculations, which will further expand the possibility of better electrocatalytic performance through spin-state modulation. This review concluded some open suggestions and prospects, which are worthy of the community's future work.