Elucidating the Donor/Acceptor Regulatory Mechanism for CO₂ Electroreduction

Catalysts with Cu─N_x active sites have shown promising performance in the electrochemical CO₂ reduction reaction (CO₂RR), but it remains elusive to systematically modulate their CO₂RR activity. Herein, a series of substituted copper phthalocyanines (CuPc-R) with various electron-withdrawing/donating groups are designed to study the relationship between the CO₂RR activity and the electron density at the Cu─N₄ center. Notably, a positive shift of +0.27 V for the reduction potential of phthalocyanine ligands is observed by changing substituents from electron-donating to stronger electron-withdrawing effect (correlated to increasing Hammett constant), owing to the decreasing electron density of phthalocyanines center. Their CO₂RR-to-CH₄ activity displays approximately a linear relationship with the Hammett constant, while CuPc-Cl_4, with the strongest electron-withdrawing substituent, shows the maximum FE_CH4 of 73.7% and related j_CH4 of -147.4 mA cm^-2. The DFT calculations and in situ spectroscopic characterizations illustrate that the decreased electron density of the Cu─N₄ center via strong electron-withdrawing groups for CuPc-R contributes to the reduced hydricity for restricted HER activity, thus promoting the CH₄ selectivity.