The two-electron reduction of dioxygen with two protons produces hydrogen peroxide, which is directly used as a liquid fuel in hydrogen peroxide fuel cells, whereas the four-electron reduction of dioxygen is combined with the two-electron oxidation of hydrogen in hydrogen fuel cells. Platinum (Pt)-based nanocomposites are the most efficient commercial electrocatalysts for the oxygen reduction reaction (ORR). However, the poor stability, scarcity and high cost of these Pt-based oxygen electrocatalysts are major barriers for the large-scale implementation of fuel cell technologies. Replacing noble metal-based electrocatalysts with highly efficient and inexpensive earth-abundant metal-based oxygen electrocatalysts has been of critical importance for practical applications. To develop efficient catalysts for the two-electron and four-electron reduction of dioxygen, it is crucially important to clarify the catalytic mechanisms of two-electron/two-proton versus four-electron/four-proton reduction of dioxygen with earth-abundant metal complexes. This review focused on the factors that control the two-electron/two-proton versus four-electron/four-proton reduction of dioxygen by electron donors (one electron reductants) such as ferrocene, catalyzed by earth-abundant metal complexes such as iron, cobalt, copper, manganese and nickel complexes in the homogeneous phase, by detecting catalytic intermediates, which determine the catalytic pathways of the two-electron versus four-electron reduction of dioxygen. The electrocatalytic two-electron or/and four-electron reduction of dioxygen with earth-abundant metal complexes and metal oxides has also been discussed in relation with the homogeneous catalysis.
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The authors gratefully acknowledge the contributions of their collaborators and co-workers in the cited references, and support by a SENTAN project (to S.F.) from Japan Science and Technology Agency (JST), Japan, JSPS KAKENHI (No. 16H02268 to S.F.), and the NRF of Korea through CRI (NRF-2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353 to W.N.) and Basic Science Research Pro-
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- Proton-Coupled Electron Transfer
- earth-abundant metals
- oxygen reduction reaction
- reaction mechanisms