Mechanisms of Two-Electron versus Four-Electron Reduction of Dioxygen Catalyzed by Earth-Abundant Metal Complexes

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.