Beyond the N-doping controversy: Molecularly deciphering the active origins in metal-free carbon for electrocatalytic oxygen reduction reaction

The intriguing catalytic performance of non-metallic nitrogen-doped carbon materials in the oxygen reduction reaction (ORR) has garnered considerable interest in elucidating their active site structure. The existing studies, however, present conflicting conclusions regarding the contributions of various N species to ORR activity. In this work, we designed and synthesized a series of molecules featuring well-defined structures and single N species, such as pyridinic N, graphitic N+ (positively charged), and pyrrolic N, to serve as model catalysts. This approach aimed to eliminate the interference caused by the coexistence of multiple N species within carbon network, thereby providing a more precise understanding of their individual contributions to ORR activity. Our experimental results revealed that pyridinic N elevated onset potential, and graphitic N+ enhanced current density in ORR. Both pyridinic N and graphitic N+ can promote the transition of the ORR process from the 2e⁻ to the 4e⁻. Theoretical calculations further indicated that neutral graphitic N is more active than graphitic N+. Additionally, dynamic interconversion between graphitic N and graphitic N+ (e.g., via [4+1]e⁻, [2+1]e⁻, or [4–1]e⁻ pathways) during catalysis may adversely affect the intrinsic activity of graphitic N in practical applications. These findings provide insights into mechanism understanding in depth and design guidance for carbon-based catalysts for ORR.