ELECTROCATALYTIC PROCESS OF CO SELECTIVITY IN ELECTROCHEMICAL REDUCTION OF CO2 AT METAL-ELECTRODES IN AQUEOUS-MEDIA

Electrochemical reduction of CO2 at metal electrodes yields CO, HCOO-, CH4, C2H4, and alcohols in aqueous media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCCO- ones (Pb, Hg, In, Sn, Cd, Tl). Foreign atom modified electrodes (the coverage is virtually unity) showed variable product selectivity between CO and HCCO-, which depends upon the combination of modifier tom and substrate electrode. Critical investigation of foreign atom modified electrodes derived a series of CO selectivity, as Au > Ag > Cu > Zn >> Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 reduction are well correlated with the heat of fusion of metals and the potential of H-2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 reduction, and is rationalized in terms of stabilization of intermediate species CO2.- at the electrode surface. CO is produced from stably adsorbed CO2.-, and HCOO- is formed from free or weakly adsorbed CO2.-.