A Defect-Driven Metal-free Electrocatalyst for Oxygen Reduction in Acidic Electrolyte

Newly N-S-C coordination-structured active sites, originating from the integrity of edged thiophene S, graphitic N, and pentagon defects, were reconstructed by N-modified S defects in carbon aerogel with a 3D hierarchical macro-meso-microporous structure. This metal-free material exhibited outstanding oxygen reduction reaction (ORR) activity (e.g., half-wave potentials of 0.76 V in 0.5 M H2SO4 and 0.1 M HClO4; 0.85 V in 0.1 M KOH) with good stability and high current density in both acidic and alkaline electrolytes. The experimental and computational results reveal that the pentagon S defect is essential for the ORR in acidic electrolytes because it provides a remarkable increase in reactivity. One graphitic-type N in the meta-position of the pentagon defect further significantly improves the reactivity as a result of locally precise control of the electronic structure, thus forming highly active sites for ORR in acid.