Unveiling the roles of multiple active sites during oxygen reduction reaction in Cr2O3@Cr-N-C composite catalyst

The fundamental understanding of oxygen reduction reaction (ORR) mechanism facilitates the targeted design of high-performance electrocatalysts. Herein, we successfully synthesized a composite catalyst Cr2O3@Cr-N-C, and systemically unveiled the roles of different active sites during ORR process. Cr2O3@Cr-N-C exhibits comparable ORR activity to the state-of-the-art catalysts and is promising to be an ideal candidate to substitute noble metal-based catalysts, which catalyzes ORR via a 2 e− + 2 e− pathway by two kinds of active sites. Type I active sites (pyrrolic-N sites, graphitic-N sites, and Cr-N sites) catalyze oxygen into hydrogen peroxide and then the as-generated hydrogen peroxide is further reduced into water by type II active sites (pyridinic-N sites). Benefiting from the combined effect of the two types of active sites, Cr2O3@Cr-N-C demonstrates a near 4 e− ORR pathway and exhibits remarkable ORR activity. The decoration of Cr2O3 nanoparticles takes effect on decreasing ORR Tafel slope (accelerating electron transfer) at lower overpotential. The insights into the multielectron ORR process on Cr2O3@Cr-N-C offer rational approaches for the design and synthesis of non-noble metal doped carbon materials as efficient and stable ORR catalysts.