New Insight into the Electronic Effect for Cu Porphyrin Catalysts in Electrocatalytic of CO₂ into CH₄

Perturbation of the copper (Cu) active site by electron manipulation is a crucial factor in determining the activity and selectivity of electrochemical carbon dioxide (CO2) reduction reaction (e-CO2RR) in Cu-based molecular catalysts. However, much ambiguity is present concerning their electronic structure-function relationships. Here, three molecular Cu-based porphyrin catalysts with different electron densities at the Cu active site, Cu tetrakis(4-methoxyphenyl)porphyrin (CuT(OMe)PP), Cu tetraphenylporphyrin (CuTHPP), and Cu tetrakis(4-bromophenyl)porphyrin (CuTBrPP), are prepared. Although all three catalysts exhibit e-CO2RR activity and the same reaction pathway, their performance is significantly affected by the electronic structure of the Cu site. Theoretical and experimental investigations verify that the conjugated effect of OCH3 and Br groups lowers the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbitals (LUMO) gap of CuT(OMe)PP and CuTBrPP, promoting faster electron transfer between Cu and CO2, thereby improving their e-CO2RR activity. Moreover, the high inductive effect of Br group reduces the electron density of Cu active site of CuTBrPP, facilitating the hydrolysis of the bound H2O and thus creating a preferable local microenvironment, further enhancing the catalytic performance. This work provides new insights into the relationships between the substituent group characteristics with e-CO2RR performance and is highly instructive for the design of efficient Cu-based e-CO2RR electrocatalysts.