Molecular Insights into the Key Role of Alkali Metal Cations for CO2 Activation and Initial CO2 Electroreduction at Cu/Electrolyte Interfaces

The crucial roles and essential influences of alkali metal cations for CO2 activation and initial CO2 electroreduction toward CO at Cu(100)/electrolyte interfaces accompanied by the competing HER have been systematically studied using DFT calculations with the explicitly solvated model. The present studies show that the quasi-specifically adsorbed alkali metal cations can be formed, which help with CO2 activation and electroreduction by changing the interfacial microenvironment. The monotonously decreasing barriers for CO2 activation are obtained following the increasing size of cations of Liδ+ < Naδ+ < Rbδ+ < Csδ+. It is worth noting that stronger adsorbed cations lead to more positive overpotentials, by which the origin of the differences of promotional effects on CO2 activation ability can be revealed. The hydronium ion can play a crucial role in switching initial CO2 electroreduction pathways via COOH, which may be attributed to electrostatic attractions between proton and O atom with greater electronegativity via H bond. The promotional effect of cations on initial CO2 electroreduction can be attributed to stabilization of the chemisorbed CO2 through the noncovalent electrostatic interactions. The cations can facilitate CO2 electroreduction by inhibiting HER following the increasing size of cations of Liδ+ < Naδ+ < Rbδ+ < Csδ+. The prohibited HER is ascribed to an increasing kinetic blockage effect of quasi-specifically adsorbed cations and increasing electrostatic repulsive interactions between alkali metal cations and protons. A reasonable polynomial relationship between proton-Cu distances and cation charge implies that the increasing repulsive interactions can lead to more and more inhibition of HER. Our present studies will provide molecular insights into design of efficient electrochemical interfaces for CO2 electroreduction.