In Situ Probing and Phosphate-Assisted Recovery of Proton Transfer to Eliminate H₂O₂ Formation in the Oxygen Reduction Reaction

Single-atom Fe-N-C is a promising candidate for the oxygen reduction reaction (ORR) at the cathode of proton-exchange membrane fuel cells (PEMFCs). However, under strongly acidic conditions, Fe-N-C suffers from severe oxidation from Fenton reactions caused by trace amounts of dissolved Fe and a 2-electron (2e) ORR product of H₂O₂. Herein, we demonstrate a facile and general strategy to nearly eliminate the 2e path of the ORR by introducing phosphates. We discover that bubbling O₂ into water introduces an inherent problem in breaking the hydrogen-bond network and thus hindering proton transfer, resulting in a decreased 4e ORR selectivity. Introducing phosphates is found to recover the hydrogen-bond network to eliminate the 2e path. This strategy works well for Fe-N-C, commercial Pt/C, and even carbon catalysts with a dominant 2e selectivity, resulting in negligible H₂O₂ production and better stability in both the rotating ring-disk electrode system and flow cell. Our work provides deep insight into the ORR mechanism and a useful strategy to lower the cost and lengthen the lifetime of PEMFCs by using nonnoble metal electrocatalysts as cathodes.