Electron effect regulation: A study on the influence of electron-donating and withdrawing group modification on the performance of metal-coordinated catalysts for electrochemical carbon dioxide reduction

Electron effect regulation is a crucial factor influencing the activity and selectivity of Cu-based coordination compound catalysts in the electrochemical carbon dioxide reduction reaction (CO₂RR). Despite significant progress, the structure-activity relationship and the underlying regulatory mechanisms warrant further in-depth investigation. In this study, three types of Cu-[ONNO] tetradentate coordination molecular catalysts with varying electron densities, namely Cu-N₂O₂, methoxy-modified Cu-N₂O₂ (Cu-EDG-N₂O₂), and nitro-modified Cu-N₂O₂ (Cu-EWG-N₂O₂), were prepared using a substituent regulation strategy. The prepared catalyst's micromorphology and structural characteristics were analyzed using various characterization methods. Systematic electrocatalytic CO₂RR experiments were conducted to evaluate the performance of these catalysts. Compared to the unmodified Cu-N₂O₂, the Cu-EDG-N₂O₂ catalyst exhibited superior reduction performance for CH₄ and C₂H₄ products. At an applied potential of -1.7 V vs. the reversible hydrogen electrode, the Faradaic efficiencies for CH₄ and C₂H₄ of Cu-EDG-N₂O₂ were 37.8 ± 2.2 % and 25.0 ± 0.5 %, respectively. In contrast, the Cu-EWG-N₂O₂ catalyst demonstrated higher activity towards the production of H₂ as a by-product. The effects of electronic properties of substitutions on catalyst performance were revealed by combining experimental characterization and theoretical simulation. The results showed that the conjugation effect of the -OCH₃ group facilitates faster electron transfer between Cu and CO₂, thereby enhancing CO₂RR activity. Additionally, the introduction of different substituents modulates the local microenvironment around the Cu active centers, significantly influencing the catalytic performance. This study provides valuable theoretical and experimental insights into the design of efficient Cu-N₂O₂-type metal coordination electrocatalysts for CO₂RR processes.