Suppressing H₂ Evolution and Promoting Selective CO₂ Electroreduction to CO at Low Overpotentials by Alloying Au with Pd

CO₂ electroreduction is a promising technology to produce chemicals and fuels from renewable resources. Polycrystalline and nanostructured metals have been tested extensively while less effort has been spent on understanding the performance of bimetallic alloys. In this work, we study compositionally variant, smooth Au-Pd thin film alloys to discard any morphological or mesoscopic effect on the electrocatalytic performance. We find that the onset potential of CO formation exhibits a strong dependence on the Pd content of the alloys. Strikingly, palladium, a hydrogen evolution catalyst with reasonable exchange current density, suppresses hydrogen evolution when alloyed with gold in the presence of CO₂. Cyclic voltammetry, in situ surface enhanced infrared absorption spectroscopy, and potential-dependent online product analysis strongly suggest that by alloying Au with Pd a significant increase in the surface coverage of adsorbed CO occurs with increasing Pd content at low overpotentials (e.g., approximately -0.35 V vs RHE). Such an increase in CO coverage suppresses H₂ evolution due to the lack of vacant active sites. Moreover, the overall increase in the binding energy with the CO₂ intermediates gained with the addition of Pd increases the CO production at low overpotentials, where polycrystalline Au suffers from poor CO₂ adsorption and poor selectivity for CO production. These results show that promising CO₂ reduction electrode materials (e.g., Au) can be alloyed not only to tune the catalyst's activity but also to deliberately decrease the availability of surface sites for competitive H₂ evolution.