Electrocatalytic reduction of CO2 by copper (II) cyclam derivatives

Abstract

This study investigates Cu(II) complexes of cyclam, propylene cross-bridged cyclam (PCB-cyclam), and propylene crossbridged cyclam diacetate (PCB-TE2A) as homogeneous electrocatalysts for CO2 reduction in comparison with Ni(II)- cyclam. It is found that Cu(II)-cyclam can catalyze CO2 reduction at the potential close to its thermodynamic value (0.75 V vs. Ag/AgCl) in tris-HCl buffer (pH 8.45) on a glassy carbon electrode. Cu(II)-cyclam, however, suffers from severe demetalation due to the insufficient stability of Cu(I)-cyclam. Cu(II)-PCB-cyclam and Cu(II)-PCB-TE2A are revealed to exhibit much less demetalation behavior, but poor CO2 reduction activities as well. The inferior electrocatalytic ability of Cu(II)-PCB-cyclam is ascribed to its redox potential that is too high for CO2 reduction, and that of Cu(II)-PCB-TE2A to the steric hindrance preventing facile contact with CO2 molecules. This study suggests that in addition to the redox potential and chemical stability, the stereochemical aspect has to be considered in designing efficient electrocatalysts for CO2 reduction.