De novo-design of highly exposed Co−N−C single-atom catalyst for oxygen reduction reaction

The nitrogen-coordinated metal single-atom catalysts (M−N−C SACs) with an ultra-high metal loading synthetized by direct high-temperature pyrolysis have been widely reported. However, most of metal single atoms in these catalysts were buried in the carbon matrix, resulting in a low metal utilization and inaccessibility for adsorption of reactants during the catalytic process. Herein, we reported a facile synthesis based on the hard-soft acid-base (HSAB) theory to fabricate Co single-atom catalysts with highly exposed metal atoms ligated to the external pyridinic-N sites of a nitrogen-doped carbon support. Benefiting from the highly accessible Co active sites, the prepared Co−N−C SAC exhibited a superior oxygen reduction reactivity comparable to that of the commercial Pt/C catalyst, showing a high turnover frequency (TOF) of 0.93 e⁻ s⁻¹ site⁻¹ at 0.85 V vs. RHE, far exceeding those of some representative SACs with a ultra-high metal content. This work provides a rational strategy to design and prepare M−N−C single-atom catalysts featured with high site-accessibility and site-density.