Influence of Water Molecules on CO₂ Reduction at the Pt Electrocatalyst in the Membrane Electrode Assembly System

CO 2 electroreduction using a Pt catalyst in an aqueous solution system is known to produce only H 2 . Recently, a remarkable result has been reported that CH 4 can be obtained by reducing CO 2 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 2 -reduction process at the Pt electrocatalyst in the MEA system. As a result, cyclic voltammetry indicated that adsorbed CO (CO ads ) was formed by CO 2 reduction in the MEA system more preferably than the aqueous solution system. In detail, the ratio of CO ads at the atop sites (linear CO, CO L ) on Pt, which participates in the CH 4 generation reaction, to the total CO ads formed by the CO 2 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 L and CH 4 generated by the CO 2 reduction reached their maximums at 63.1% RH. CH 4 production by the extremely low-overpotential CO 2 reduction was significantly achieved under all the RH conditions. Consequently, the Faradaic efficiency of the CH 4 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 H 2 O-involved chemical equilibrium of the reactions for the CO ads and CH 4 formation. Overall, the present study experimentally clarified that the formation of CO ads (particularly CO L ) and the following CH 4 from the CO 2 reduction at the Pt electrocatalyst in the MEA system was facilitated by appropriately controlling the water-molecule content.