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

CO₂ electroreduction using a Pt catalyst in an aqueous solution system is known to produce only H₂. Recently, a remarkable result has been reported that CH₄ can be obtained by reducing CO₂ 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 CO₂-reduction process at the Pt electrocatalyst in the MEA system. As a result, cyclic voltammetry indicated that adsorbed CO (CO_ads) was formed by CO₂ 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 CH₄ generation reaction, to the total CO_ads formed by the CO₂ 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 CH₄ generated by the CO₂ reduction reached their maximums at 63.1% RH. CH₄ production by the extremely low-overpotential CO₂ reduction was significantly achieved under all the RH conditions. Consequently, the Faradaic efficiency of the CH₄ 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 H₂O-involved chemical equilibrium of the reactions for the CO_ads and CH₄ formation. Overall, the present study experimentally clarified that the formation of CO_ads (particularly CO_L) and the following CH₄ from the CO₂ reduction at the Pt electrocatalyst in the MEA system was facilitated by appropriately controlling the water-molecule content.