Molecular modification of planar four-coordinated cobalt active site for the electrochemical reduction of carbon dioxide: a density functional theory study

Abstract

Single atomic catalysts (SACs) with planar metal-N₄ sites are promising electrocatalysts for CO₂ reduction reaction (CO₂RR) in which the coordination environment plays a crucial role in intrinsic catalytic activity. Cobalt porphyrin is one of the most intensely studied model compound towards CO₂ reduction photocatalysts and electrocatalysts because of the central metal located in a well-defined planar four-coordinated (Co–N₄) environment. Appropriate structural adjustment of tetraaza-macrocyclic ligands has been demonstrated to improve their thermodynamic reactivity towards CO₂ reduction. Herein, we theoretically reveal that such approach enables to tune the electronic structure around active metallic sites due to the changed Co–N₄ local structures with broken D_4h symmetry and different conjugated ligands from density functional theory (DFT) calculations upon a series of model compounds. They contain cobalt corrole (Co2), cobalt octahydroporphyrin (Co3), and cobalt 1,5,9,13-tetraazacyclohexadecane (Co4), referring to the cobalt porphyrin (Co1) as the benchmark. In addition, the replacement of N atom(s) in Co–N₄ sites in Co1 using O and S heteroatoms to form cobalt 21-oxaporphyrin (Co5), cobalt 21,23-dioxaporphyrin (Co6), 21,22-dioxaporphyrin (Co7), cobalt 21-thiaporphyrin (Co8), cobalt 21,23-dithiaporphyrin (Co9), and cobalt 21,22-dithiaporphyrin (Co10) has been evaluated for the rational design and synthesis of high efficiency Co–N–C (cobalt-based) catalysts made up of single metallic atoms coordinated by nonmetallic ligands. In particular, Co3 and Co8 show the lower limiting potentials for CO product as −0.61 and −0.58 eV, respectively, compared with that of Co1. Co2 and Co5 have promising activity for the CH₃OH product with the limiting potentials of −1.30 and −1.04 eV and for CH₄ product with the limiting potentials of −1.37 and −1.04 eV. This study not only provides a comprehensive view of Co-porphyrinoids for CO₂RR but also presents a theoretical screening path for designing and searching efficient molecular catalysts toward electrochemical reactions.