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.