  M-N-C single-atom catalysts (SACs) have emerged as a potential substitute for
the costly platinum-group catalysts in oxygen reduction reaction (ORR).
However, several critical aspects of M-N-C SACs in ORR remain poorly
understood, including their pH-dependent activity, selectivity for 2- or
4-electron transfer pathways, and the identification of the rate-determining
steps. Herein, analyzing >100 M-N-C structures and >2000 sets of energetics, we
unveil a pH-dependent evolution in ORR activity volcanos from a single-peak in
alkaline media to a double-peak in acids. We found that this pH-dependent
behavior in M-N-C catalysts fundamentally stems from their moderate dipole
moments and polarizability for O* and HOO* adsorbates, as well as unique
scaling relations among ORR adsorbates. To validate our theoretical discovery,
we synthesized a series of molecular M-N-C catalysts, each characterized by
well-defined atomic coordination environments. Impressively, the experiments
matched our theoretical predictions on kinetic current, Tafel slope, and
turnover frequency in both acidic and alkaline environments. These new insights
also refine the famous Sabatier principle by emphasizing the need to avoid an
"acid trap" while designing M-N-C catalysts for ORR or any other pH-dependent
electrochemical applications.
