Toward a unified pH-performance picture of active sites in nitrogen-doped carbon materials

Nitrogen-doped carbon materials (NCMs) are an important class of energy materials, which are widely recognized for their good electrocatalytic performance, particularly in the oxygen reduction reaction (ORR). NCMs exhibit the pH-dependent behavior: their electrocatalytic performance changes drastically as the pH of the reaction environment is varied; however, their ORR activity origins at different pH are still in dispute. The underlying interfacial reaction mechanisms and the pH-dependent activity origins for NCMs need to be clarified to put an end to the long-standing controversy. In this work we successfully prepared a set of model catalysts whose active sites were carefully controlled and determined. By developing the spectroelectrochemistry-aided catalyst layer voltammetry method, we revealed the roles of different active sites for the ORR and their interfacial electron-transfer mechanisms in NCMs at different pH. The experimental and theoretical evidence clearly showed that nitrogen-doping sites and their neighboring carbon atoms exhibited no significant ORR activity under strong acidic conditions. In comparison, the 1,3-cyclopentadiene-like defects (i.e., the edge pentagonal defects) proved to be the main pH-universal active sites. Their unique electronic structure and electrostatic effect engendered the effective adsorption and activation of O 2 molecules under acidic, alkaline and neutral conditions, particularly with the ability to directly dissociate O 2 in acidic media. Finally, a unified pH-performance picture for the various active sites in NCMs across a broad pH range was presented to provide deeper insights into the origin of the pH-universal ORR. • Dioxygen-reduction activity origins of nitrogen-doped carbons are determined. • Spectroelectrochemistry-aided catalyst layer voltammetry is developed. • A unified pH-performance diagram for nitrogen-doped carbons is presented. • A series of model catalysts for specific active sites is prepared. • Interfacial electron-transfer mechanisms over a wide pH range are proposed.