Direct Electrooxidation of Ethylene to Ethylene Glycol over 90% Faradaic Efficiency Enabled by Cl^– Modification of the Pd Surface

Direct electrochemical ethylene-to-ethylene glycol (C₂H₄-to-EG) conversion can potentially reduce the consumption of fossil fuels and the emission of carbon dioxide (CO₂) compared with the traditional thermo-catalytic approach. Palladium (Pd) prepared by electrodeposition is represented as a promising electrocatalyst; however, it exhibits low Ethylene glycol (EG) current density (<4 mA cm^-2), Faradaic efficiency (<60%), and productivity (<10 μmol h^-1), hindering practical applications. Herein, we report a nanodendrite palladium catalyst supported on a large-area gas diffusion electrode. This catalyst gives high EG current density (12 mA cm^-2) and productivity (227 μmol h^-1) but low Faradaic efficiency (65%). With further Cl^- ions modification, Faradaic efficiency increased to a record-high value of 92%, and EG current density (18 mA cm^-2) and productivity (∼340 μmol h^-1) were also promoted. Experimental data suggest that the strong electron-withdrawing feature of Cl^- reduces the oxidation ability of in situ generated Pd-OH species, inhibiting EG overoxidation to glycol aldehyde. Meanwhile, Cl^- alters EG adsorption configuration─from parallel and dual-site coordination to vertical and single-site coordination─over the Pd surface, thus preventing C-C bond cleavage of EG to CO₂. In addition, Cl^- adsorption facilitates the generation of Pd-OH active species to improve catalytic activity. This work demonstrates the great potential of surface ion modification for improving activity and selectivity in direct electrochemical C₂H₄-to-EG conversion, which may have implications for diverse value-added chemicals electrosynthesis.