Acidic and Alkaline pH Controlled Oxygen Reduction Reaction Pathway over Co‐N4C Catalyst

Enhanced oxygen reduction reaction (ORR) kinetics and selectivity are crucial to advance energy technologies like fuel cells and metal-air batteries. Single-atom catalysts (SACs) with M-N₄/C structure have been recognized to be highly effective for ORR. However, the lack of a comprehensive understanding of the mechanistic differences in the activity under acidic and alkaline environments is limiting the full potential of the energy devices. Here, a porous SAC is synthesized where a cobalt atom is coordinated with doped nitrogen in a graphene framework (pCo-N₄C). The resulting pCo-N₄C catalyst demonstrates a direct 4e^- ORR process and exhibits kinetics comparable to the state-of-the-art (Pt/C) catalyst. Its higher activity in an acidic electrolyte is attributed to the tuned porosity-induced hydrophobicity. However, the pCo-N₄C catalyst displays a difference in ORR activity in 0.1 m HClO₄ and 0.1 m KOH, with onset potentials of 0.82 V and 0.91 V versus RHE, respectively. This notable activity difference in acidic and alkaline media is due to the protonation of coordinated nitrogen, restricted proton coupled electron transfer (PCET) at the electrode/electrolyte interface. The effect of pH over the catalytic activity is further verified by Ab-initio molecular dynamics (AIMD) simulations using density functional theory (DFT) calculations.