Axial chlorine-induced asymmetric cobalt single-atom coordination fields for boosting oxygen reduction reaction

Abstract

The development of oxygen reduction reaction (ORR) catalysts with high activity, stability, and economic applicability plays a decisive role in reducing expenses and enhancing the discharge performance of seawater-based zinc-air batteries (SWZABs). Co- and Fe-based single-atom catalysts (M-N4-C) with metal-N4 structure offer advantages of well-defined active structure and high active site utilizations. However, the oxygen electrocatalytic performance of M-N4-C remains a formidable challenge due to the highly stable centrosymmetric electronic structure. To overcome the dilemma, we develop a Co-N4Cl-C with axial coordination of Cl atoms. The axial coordination drags the Co atoms out of the Co-N4 centrosymmetric configuration. This alters the electronic configuration of Co single-atom sites, resulting in a valence state change from +1.83 to +0.67 and forming a localized negative charge environment. These alternations enhance the electronic orbital overlap between Co single-atom sites and oxygen species, promote the rapid evolution of *OOH intermediates, and inhibit the adsorption of toxic Cl- ions, ensuring the ORR kinetics and stability. Co-N4Cl-C exhibits a high oxygen reduction onset potential of 1.05 mV and a half-wave potential of 0.88 mV vs. the reversible hydrogen electrode. The SWZAB, featuring a Co-N4Cl-C catalyst cathode, Zn anode, and NaCl electrolyte supplemented with KOH, reaches a discharge voltage platform of 1.27 V and a peak power density of 179 mW·cm-2, even at 10 a current density of mA·cm-2. This study sheds important light on advancing single-atom catalysts with superior ORR performance and economic viability.