Size effects and active sites of Cu nanoparticle catalysts for CO2 electroreduction

Abstract

Electrochemical conversion of CO2 into useful chemicals represents a promising approach to mitigate our pressing environment and energy problems. To date, only Cu electrodes, of all metals examined experimentally, have been shown to be capable of producing significant quantities of hydrocarbons with adequate Faradaic efficiency from CO2 electroreduction. In this work, we use density functional theory calculations to study size-dependent changes on the adsorption of reaction intermediates and reaction free energy for CO2 reduction on Cu nanoparticles ranging from 13 to 561 atoms. We found that the adsorbate-induced surface charge perturbation on Cu nanoparticles becomes more local as the Cu nanoparticles size increases, and the extent of charge perturbations on 309 atom Cu appears to be very similar in Cu(1 1 1) surface. For two-electron products (CO, HCOOH and H2), Cu147 appears near the top in the volcano plots, implying that Cu147 has a near optimal binding energy of the key intermediates to produce two-electron products. To identify the active sites for the CO2 reduction, we compared the activity of different reaction sites, namely, facet, edge and corner sites of Cu nanoparticles, Cu(1 1 1), Cu(1 0 0), Cu(2 1 1) and hexagonal Cu nanowire. We find that the under-coordinated sites are more active than the low-index surfaces. The presented size-activity and structure-activity relationships will provide useful insight for the design better nanostructured Cu catalysts for CO2 electroreduction.