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.