Electrochemical CO₂ Reduction Catalyzed by Copper Molecular Complexes: The Influence of Ligand Structure

Inexpensive and earth abundant copper complexes are popular choices to catalyze the electrochemical conversion of carbon dioxide (CO₂) into value-added products. Cu complexes have been applied in homogeneous catalysis dissolved in solutions or in heterogeneous catalysis immobilized on electrode surfaces. The aim of this review is to examine correlations between the ligand structures of the Cu complexes on the CO₂ reduction product selectivity in both homogeneous and heterogeneous applications. This review covers nine main categories of Cu complexes grouped by their ligand structures, enabling the comparisons among small and large complexes, mononuclear or dinuclear complexes, and complexes with different geometry/number of ligand binding sites. In homogeneous catalytic systems, small mononuclear Cu complexes tend to favor carbon monoxide or formate production, whereas dinuclear complexes based on a disulfanediyl ligand produced oxalate through carbon–carbon coupling. In heterogeneous catalysis, macrocyclic ligand-containing Cu complexes and dinuclear complexes were able to produce higher carbon products of ethanol and ethylene. However, establishing robust correlations between molecular geometry, coordination numbers or the denticity of the ligands and the CO₂ reduction pathways are difficult, because tetrahedral complexes were mostly applied in homogeneous systems while square planar complexes in heterogeneous systems. A third of the examined Cu complexes formed Cu complex-derived catalysts on the electrode surface, which may affect product selectivity. Despite the complex behavior of CO₂ reduction systems based on Cu complex catalysts, molecular structure–property correlations uncovered in this comprehensive review are expected to help researchers design new complexes tailored for CO₂ reduction realizing high selectivity in both homogeneous and heterogeneous systems. The design of new complexes should be guided by combinatorial studies to discover robust correlations in the multidimensional space of molecular structure–electrochemical CO₂ reduction conditions–product selectivity and efficiency of electrochemical CO₂ reduction.