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.