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Electrochemical reduction of CO2 with high current density was studied in a CO2-methanol medium. The mole fraction of CO2 in this medium varied from 0.7% to 94% with changing the pressure of the system from 1 to 60 atm. Carbon dioxide was reduced to CO, CH4, C2H4, and methyl formate at a Cu electrode. A methyl group and a formyl group of methyl formate are derived from methanol and CO2, respectively. Methyl formate production in this system corresponds to formic acid formation in aqueous systems. A Tafel plot obtained at 40 atm (the mole fraction of CO2 is 33%) indicated that the reduction of CO2 to CO was no longer limited by mass transfer of CO2. Total current density and current efficiency of CO2 reduction at -2.3 V were 436 mA cm-2 and 87%, respectively, at 40 atm. The studied pressure range, 0-60 atm, was classified into three regions with boundaries at 20 and 40 atm; 20 atm was the point above which the mass transfer of CO2 is sufficiently high for the reaction under the current density of 200 mA cm-2, and 40 atm was the point at which the significant change occurs in the property of CO2-methanol medium. Reduction of CO2 to CO and methyl formate proceeded even at 60 atm, at which the mole fraction of CO2 is 94%.

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