The Tetrapyrollic Motif in Nitrogen Doped Carbons and M‐N‐C Electrocatalysts as Active Site in the Outer‐Sphere Mechanism of the Alkaline Oxygen Reduction Reaction

Development and fundamental understanding of precious‐group‐metal‐free electrocatalysts is hampered by limitations in the quantification of the intrinsic activity of different catalytic sites and understanding the different reaction mechanisms. Comparing isomorphic nitrogen‐doped carbons, Zn‐N‐Cs and Fe‐N‐Cs with the common tetrapyrrolic motif, a catalyst‐independent outer‐sphere rate‐determining step in the alkaline oxygen reduction reaction is observed. Density functional theory (DFT) simulations on tetrapyrrolic model structures indicate the highest occupied molecular orbital (HOMO) level as a good descriptor for the catalytic activity. Contour plots suggest that the electron transfer occurs directly from the tetrapyrrolic coordination site, rather than from the metal center. Metal‐free tetrapyrrolic N4 sites are discovered to be highly active oxygen reduction reaction (ORR) active sites in alkaline that reach turnover frequencies (TOF) of 0.33 and 1.84 s−1 at 0.80 and 0.75 VRHE in the order of magnitude of tetrapyrrolic Fe–N4 sites in the acidic ORR. While Zn‐coordination lowers the HOMO level and therefore the catalytic activity, Fe‐coordination lifts the HOMO level resulting in TOF values of 0.4 and 4 s−1 for tetrapyrrolic Fe–N4 sites at 0.90 and 0.85 VRHE, respectively. At higher mass activities, the peroxide reduction becomes rate‐limiting, where highest peroxide production rates are observed for the nitrogen‐doped carbon. By complexation of tetrapyrrolic nitrogen doped carbons with iron ions and decomplexation of the corresponding Fe‐N‐C catalysts, herein intrinsic activities of tetrapyrrolic active sites are determined. It turns out that the activity trend follows the highest occupied molecular orbital (HOMO) energy of the materials pointing to an outer sphere mechanism at the rate‐determining step for oxygen reduction reaction (ORR) in alkaline electrolyte.