Influence of Water Molecules on CO Reduction at the Pt Electrocatalyst in the Membrane Electrode Assembly System.

CO electroreduction using a Pt catalyst in an aqueous solution system is known to produce only H. Recently, a remarkable result has been reported that CH can be obtained by reducing CO using a membrane electrode assembly (MEA) containing a Pt catalyst. A big difference that exists between the two systems is the number of water molecules. Therefore, this study investigated the influence of water molecules on the CO-reduction process at the Pt electrocatalyst in the MEA system. As a result, cyclic voltammetry indicated that adsorbed CO (CO) was formed by CO reduction in the MEA system more preferably than the aqueous solution system. In detail, the ratio of CO at the atop sites (linear CO, CO) on Pt, which participates in the CH generation reaction, to the total CO formed by the CO reduction became higher as the lower relative humidity (RH) at 50 °C in the MEA system. Cyclic voltammetry combined with in-line mass spectrometry revealed that the amount of CO and CH generated by the CO reduction reached their maximums at 63.1% RH. CH production by the extremely low-overpotential CO reduction was significantly achieved under all the RH conditions. Consequently, the Faradaic efficiency of the CH production at 63.1% RH was improved by 1.35 times compared to that at 100% RH. These results would be mainly obtained based on the HO-involved chemical equilibrium of the reactions for the CO and CH formation. Overall, the present study experimentally clarified that the formation of CO (particularly CO) and the following CH from the CO reduction at the Pt electrocatalyst in the MEA system was facilitated by appropriately controlling the water-molecule content.