Electrochemical Reduction of Carbon Dioxide to Methane at High Current Densities

There has been considerable effort spent over the last several years directed towards promoting the electrochemical reduction of CO 2 to CH 4 at both high Faradaic efficiencies and current densities. Much recent work has been reported on CO 2 reduction at Ru electrodes(l) in CO 2 saturated aqueous Na2SO 4, where Faradaic efficiencies up to 42 % have been reported(2) at current densities of 0.11mA/cm 2. Ru was sys-tematically chosen because of its previously demonstrated catalytic activity at lower tem-peratures (~I00 ~ for promoting the gas phase conversion of CO 2 to CH4(3). Other workers(4) have recently reported dramatically higher per-formance for the electrochemical reduction of CO 2 to CH 4 with Faradaic efficiencies up to 65% in CO 2 saturated 0.5M KHCO 3 and current densi-ties up to 10mA/cm 2, using high purity Cu cath-odes(5). These results can in part be expected related to the electrolyte in which these elec-trolyses were performed. It is of interest to note that CO 2 reduction to formic acid at amal-gamated Cu electrodes in aqueous Na2SO 4 has been shown(6) to occur with 18 % Faradaic effi-ciency and increase to 80 % upon addition of NaHCO 3 to the electrolyte. We wish to report recent work performed in our laboratory on the electrochemical reduction of CO 2 to CH 4 using 1/8 " thick cold-rolled Cu (ASTM B370) cathodes, prepared from a material widely used in the building industry for roof-ing flashings. This material had a nominal purity of 99.9 % with major impurities being Ag, Pb, As, Sb, Se, Te, Ni and Bi. All electro-lyses were performed in a glass H-cell with separation between anolyte and catholyte com-partments being achieved by Nafion 417 (0.017in, hydrogen ion form, equivalent weight ii00). The 0.5M KHCO 3 (pH 9.2) electrolyte used in this cell was initially pre-electrolyzed at 20~A/cm 2 for 16-20 hours using a Cu cathode and platinized platinum anode prior to its intro-duction into the H-cell. The Cu working elec-trode used in the H-cell possessed a total front surface area of 0.57cm 2. The remainder of the Cu electrode assembly was isolated from