A study of the mechanism for the electrocatalysis of carbon dioxide reduction by nickel and cobalt square planar complexes in solution

Cyclic voltammetry and controlled potential electrolysis have been used to investigate the mechanism by which some square planar complexes of nickel and cobalt with macrocyclic ligands catalyse the cathodic reduction of carbon dioxide in acetonitrile + water mixtures. The complexes can have a high turnover number and, in terms of a reduction in overpotential, the complexes can also be effective catalysts; the presence of the catalyst may reduce the overpotential by more than 0.5 V. On the other hand, the maximum current density which has been achieved for these catalysed reductions is less encouraging, although the rate of the catalytic cycle is enhanced by the presence of water. It has been confirmed that the product of the reduction of carbon dioxide is carbon monoxide, although bicarbonate is also formed due to the generation of base. The influence of added proton donor, the choice of base electrolyte cation and the presence of a heterocyclic compound able to act as a ligand to the octahedral sites of the complexes have also been investigated.