Insights into the pH-dependent Behavior of N‑Doped Carbons for the Oxygen Reduction Reaction by First-Principles Calculations

The oxygen reduction reaction (ORR) is one of the important cathodic reactions because of its central role in important applications, such as fuel cells. Nitrogen (N)-doped carbons have been demonstrated to be one of the most promising and affordable materials as an ORR catalyst. However, their catalytic performance under acidic conditions is about two orders inferior than that under basic conditions, which is too low to be significant. Such an unexpected pH-dependent behavior has not been adequately explained and is still under debate. In this work, we investigate this pH-dependent behavior by using first-principles density functional theory (DFT) calculations. With consideration of the solvation effect and applied voltage, our simulation results show switching of active sites from pyridinic N to graphitic N that explains the changes in reaction rates from acidic to alkaline conditions. These observations not only well explain the existing experiment but also provide guidance for designing more efficient carbon-based catalysts for the ORR in an acidic medium.