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

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 D4h 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. A systematic theoretical investigation on the carbon dioxide reduction reaction over ten single cobalt-containing compounds was performed to screen the excellent catalyst candidates from the thermodynamics perspective.