Exploring the Inner- and Outer-Sphere Mechanistic Pathways of ORR on M–N–Cs with Pyrrolic MN4 Motifs

Abstract

Metal- and nitrogen-doped carbon materials (M–N–Cs) have emerged as promising alternatives to costly platinum-group metals (PGMs) for the oxygen reduction reaction (ORR) in renewable energy applications. Notably, there is increasing experimental and theoretical evidence supporting pyrrolic MN4 coordination over pyridinic MN4 in these materials, which aligns closer to the MN4 geometries found in nature. This study utilizes density functional theory (DFT) to elucidate the ability of each metal to catalyze various ORR mechanisms at the pyrrolic MN4 sites. Among the M–N–Cs of first–row transition metals, pyrrolic CrN4 and FeN4 exhibit exceptional 4e-ORR activity, promoting both inner- and outer-sphere mechanisms and H2O2 dissociation. Pyrrolic CoN4 is also promising for 2e-ORR catalysis due to its effective outer-sphere electron-transfer capabilities. These findings offer valuable insights for designing sustainable electrocatalysts to exploit the full potential of renewable energy sources, advancing the path toward carbon neutrality.