Acceleration of the reduction of carbon dioxide in the presence of multivalent cations

The accelerating effect of various multivalent cations, the halogen anions and the acidity of the solution on the rate of the electrochemical reduction of CO2 on a Cu (88)–Sn (6)–Pb (6) alloy cathode was studied. In 1.5 mol L−1 HCl containing various cations, at −0.65 V vs. Ag/AgCl the rate increases with the increase of the surface charge of the cation of the supporting electrolyte in the order Na+ < Mg2+ < Ca2+ < Ba2+ < Al3+ < Zr4+ < Nd3+ < La3+. In La3+ containing electrolyte the rate was two-times higher than that in the case of Na+ at the same potential. The acceleration effect was attributed to the participation of the radical anion (CO2−) in the rate determining step. The effect of the cation was somewhat higher at pH > 4, but less pronounced at quite negative potentials (−1.7 V). This was attributed to the change of the rate-determining step of the reduction at high overpotentials. In strongly acidic solution at −0.65 V the halogen anion increases the rate in the order Cl− < Br− < I−, while the increase of the [H3O+] from 0.1 to 2 mol L−1 resulted to an increase in the rate by 53%. The main products of the reduction were CH3OH, CH3CHO, HCOOH and CO. The % current efficiency (% CE) of CH3OH and HCOOH displayed maxima (∼35 and 28%) at −0.6 and −0.65 V respectively. The %CE of CH3CHO was continuously increased with increasing negative potential from −0.55 to −1.2 V, while that of CO followed the reverse trend. In the presence of Zr4+ a relatively high %CE of CH3CHO (17.6%) was obtained. The main conclusion of this work is that the rate of CO2 reduction can be increased at low overpotentials and the distribution of the products can be controlled simply by varying the composition of the electrolyte.