Uncovering pH-dependency and interfacial electron-transfer mechanisms of oxygen reduction reaction sites in nitrogen-doped carbon materials

Nitrogen-doped carbon materials (NCMs) are an important class of energy catalytic materials, which are also widely recognized for their excellent electrocatalytic performance, particularly in O2 reduction reaction (ORR). However, many NCMs face the pH-dependency problem in the ORR in that their electrocatalytic performance often changes drastically as the pH of the reaction environment is varied. The underlying interfacial reaction mechanisms and the pH-dependent activity origins for NCMs need to be clarified to put an on end to the long-standing controversy over these issues. In this study, we successfully prepared a set of NCM model catalysts to determine the active sites through a series of careful control experiments. By applying the spectroelectrochemistry-aided catalyst layer voltammetry study method in combination with density functional theory (DFT) calculations, we deeply revealed the roles of different active sites and their interfacial electron-transfer mechanisms in NCMs. Experimental results clearly show that both pyridinic-N and graphitic-N sites exhibited no significant ORR activity under strong acidic conditions. The 1,3-cyclopentadiene-like defects (i.e., edge pentagonal defects) were considered as the main pH-universal active site, due to their unique electronic structure and electrostatic effects, enabling effective adsorption and activation of O2 molecules under acidic, alkaline, and neutral conditions, particularly with the ability to directly dissociate O2 in acidic media. Finally, a pH-performance diagram for the various active sites in NCMs across a broad pH range was presented to understand the chemistry of pH-dependent ORR active sites in NCMs.