Alkali metal cations steer ORR selectivity on M-N4 sites

Oxygen reduction reaction (ORR) is one of the most widely investigated cathodic processes owing to its important role in fuel cells and the synthesis of green chemicals. Both 2e^- reduction producing H₂O₂ and 4e^- reduction producing H₂O by ORR are valuable. A recent study published in National Science Review finds that, in addition to the catalyst itself, AM⁺ in the electrolyte can significantly affect the reaction selectivity.

Researchers from the Chinese Academy of Sciences have found that AM⁺ direct oxygen reduction pathways on Co-N₄ sites. Specifically, the electron transfer number of ORR increases with cation size, following the trend: Li⁺ ≈ Na⁺ < K⁺ < Rb⁺ < Cs⁺. In small AM⁺ electrolytes, O₂ is mainly reduced to H₂O₂. In large AM⁺ electrolytes, the 2e^- reduction product is further reduced to H₂O, completing a 4e^- transfer via the 2e^-+2e^- pathway.

Using in situ electrochemical scanning tunneling microscopy (EC-STM), the research team in situ investigates the catalytic conversion of O₂ on CoOEP, showing the adsorbed HO₂^-, as the 2e^- ORR product, on CoOEP in large AM⁺ electrolytes during ORR. These species are not observed in small AM⁺ electrolytes. Furthermore, it is shown that the electron transfer number of ORR correlates positively with the surface coverage of HO₂^-. In conclusion, large AM⁺ stabilizes HO₂^- and promotes its further reduction, shifting the main product of ORR from hydrogen peroxide to water. They further validated the AM⁺ effect in a practical catalyst, COF-366-Co, demonstrating its broad applicability.

This study reveals that AM⁺ alter the ORR pathway at Co-N₄ sites by tuning the stability of reaction intermediates, and proposes a promising strategy to control ORR selectivity via electrolyte composition rather than catalyst replacement.