Coverage of capping ligands determining the selectivity of multi-carbon products and morphological evolution of Cu nanocatalysts in electrochemical reduction of CO2

Abstract

Identifying the active sites of Cu nanoparticles that convert CO₂ to multi-carbon (C₂₊) materials has remained elusive. It is caused by the reconstruction of Cu nanoparticles during electrochemical CO₂ reduction and the unrevealed effect of capping ligands covering the Cu surface. We show that the C₂₊ selectivity and morphological evolution of Cu nanoparticles largely depend on the density of the capping ligand, tetradecylphosphonate (TDP). Ultraviolet-ozone pre-treatment of Cu nanoparticles reduced the density of TDP. Desorption of the remaining ligands was expedited within 3 h of the CO₂ reduction process, and the concurrent aggregation of NPs formed bare Cu clusters. The increased C₂₊ selectivity of >50% faradaic efficiency revealed that the existing grain boundaries of Cu clusters promoted CO dimerization to produce C₂₊ materials. Despite similar cluster formation, Cu nanoparticles without ultraviolet-ozone treatment showed poor C₂₊ selectivity, suggesting that the remaining TDP, half of the original concentration, passivated the grain boundaries. Further the morphological transformation of Cu did not increase C₂₊ selectivity even after 20 h of the reaction.