Alkali-metal cations steer the product selectivity of O₂ reduction on M-N₄ sites

The oxygen reduction reaction (ORR) to either H₂O₂ or H₂O generation is important to meet diverse application demands. The product selectivity of the ORR is strongly correlated with the nature of the catalyst. We report herein that alkali-metal cations (AM⁺) can steer the product selectivity of the ORR catalysed on a molecular model catalyst with Co-N₄ sites. The electron-transfer number of the ORR increases with Li⁺ ≈ Na⁺ < K⁺ < Rb⁺ < Cs⁺. A series of electrochemical measurements reveal the 2e^-+2e^- ORR pathway in large AM⁺ electrolytes at neutral pH. In situ electrochemical scanning tunneling microscopy resolves the formation of high-contrast species in the cobalt octaethylporphine (CoOEP) monolayer on Au(111) in large AM⁺ electrolytes when the ORR occurs. The high-contrast species is assigned to the HO₂ ^-, as the 2e^- ORR product, adsorbed on CoOEP. Combined electrochemical scanning tunneling microscopy, electrochemical measurements and theoretical calculations reveal that large AM⁺ can stabilize HO₂ ^- on CoOEP and promote its further reduction, which accounts for the AM⁺-dependent selectivity of the ORR. Revealing the unrecognized effect of AM⁺ on ORR selectivity opens up new avenues for modulating the distribution of ORR products by adjusting the electrolyte composition.