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.