Highly Selective Reduction of CO₂ to Methane Induced by Subzero Depression of the Electrode Surface Temperature

In the CO₂ reduction reaction, achieving single product selectivity with comprehensive understanding of the corresponding reaction mechanism is considered a holy grail. Among numerous reactor and electrode surface parameters, temperature has surprisingly seldom been considered as a variable for selectivity regulation due to the limited range of control in aqueous electrolytes. Here we employed a reactor design for electrode surface cooling to subzero temperatures with minimal impact on the bulk electrolyte temperature. Distinct reactivity patterns were observed when the electrode was surface-cooled, compared to those of experiments when the entire reactor was chilled. Much improved methane single product selectivity was achieved (80%, comparable to the best efficiencies reported thus far) at subzero-cooled electrodes. Moreover, significantly lower overpotentials (ca. 300 mV at identical current density) were maintained with surface cooling because the temperature of the electrolyte bath was not affected. In situ IR spectroscopy revealed that the electrode surface-adsorbed CO species was the chemical intermediate in the methane-producing pathway.