Regulating electron transfer over asymmetric low-spin Co(II) for highly selective electrocatalysis

Abstract

Modulating the steric-electronic configuration of metal-organic centers is key for tuning the activity and selectivity of heterogeneous reactions, especially multi-electron transfer reactions. Here, three different asymmetric metal-organic complexes with unique steric-electronic structures are immobilized on nanocarbon for an electron-transfer-controlled oxygen reduction reaction. The strong-field ligand-induced low-spin ( LS ) Co II creates a necessary steric configuration for regulating reaction selectivity through ligand's proton transfer ability, for which acidic diamine ligands facilitate a four-electron transfer (94%), whereas basic ligands drive a highly selective two-electron route (97%). The steric-electronic regulation of the reaction selectivity at catalytic sites is characterized using X-ray absorption spectroscopy, reaction kinetic path analysis, and density functional theory calculation. Our results indicate that an LS state of Co II with asymmetric coordination is necessary to form a unique “flytrap” structure to promote O 2 capture for the subsequent proton-coupled electron transfer, which is regulated by the Brønsted acidity of coordinating ligands. • A series of asymmetric metal-organic complexes are immobilized on nanocarbons • Advanced characterizations are used to explore their structures at the atomic level • Selectivity of ORR is regulated by changing the ligand field strength of the active site • Operando X-ray spectroscopy and computational analysis confirm the reaction mechanism The majority of energy conversion processes such as hydrogen peroxide production, nitrogen fixation, and CO 2 reduction generally involve multi-electron transfer, resulting in multiple by-products and requiring tedious purification process. The development of highly selective electrocatalysts can not only reduce the energy consumption in the production process but also benefit to promote their large-scale application. Here, we report an asymmetric ligand design of molecular “Venus flytrap” at atomic Co II catalysts on a heterogeneous carbon surface for oxygen electrocatalysis with controlled selectivity. Our XAFS and DFT calculations show that the strong-field ligand-induced LS Co II centers offer inimitable steric-electronic advantages in capturing O 2 and the capability to regulate reaction pathways through an H-bonding interaction between the ligands and peroxo intermediate. The proposed concept completes the picture of designer single-atom catalyst by covering the often overlooked exterior coordination sites, and is expected to be generally applicable to other heterogeneous catalyst systems. Steric-electronic configuration regulation plays a key role in achieving highly selective electrocatalysis. As a proof of concept, a series of asymmetric metal-organic complexes were immobilized on a conductive substrate for oxygen reduction reaction (ORR). The strong-field ligand-induced low-spin cobalt sites exhibit an excellent selectivity for four-electron ORR, whereas the medium-field ligand linked Co sites display high efficiency for the two-electron reduction of oxygen into hydrogen peroxide.