Electrolytic Reduction of Carbon Dioxide at Mercury Electrode in Aqueous Solution

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Abstract
Cathodic reduction of carbon dioxide at a mercury pool electrode was investigated in aqueous solutions of NaHCO3, NaH2PO4–Na2HPO4, NaCl, NaClO4, Na2SO4, LiHCO3, and KHCO3 as well as their combinations. The reaction product was confirmed to be HCOO− by a qualitative spot test. HCOO− was quantitatively analyzed by a permanganate titration. The partial current densities of HCOO− formation (ic) and H2 evolution (iH) were calculated. In the present paper, relationships between electrode behavior and solution properties, some of which have been qualitatively described in previous papers, are quantitatively clarified as follows. The electrode potential Ec for constant ic (0.5 mA/cm2) remains constant irrespective of pH and anions in the pH region of 2.4 to 7.8, whereas the potential EH for constant iH (0.5 mA/cm2) varies linearly with a slope of 125 mV/pH. The present experimental results verify Eyring et al.’s presumption that in a relatively high overvoltage region (region 2) the cathodic reduction of CO2 at Hg proceeds with the charge transfer to CO2 as the rate determining step. CO2 reduction and H2 evolution are concluded to proceed independently at the Hg electrode. The dependence of Ec on electrolyte concentration is interpreted in terms of the diffuse double layer theory. The effect of cation species on Ec also is discussed.