Elemental Trade-Off in the Selective Electro-Oxidation of Ethylene Glycol on Palladium-Silver/Nickel Electrodes

The synthesis and properties of PdAg electrodes coated on Ni foam and their application in the selective electro-oxidation of ethylene glycol to glycolate are studied. This reaction is a route to glycolic acid, which is a key component of biodegradable packaging. Using a combination of cyclic voltammetry, energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis, it is found that a 3:1 Pd:Ag ratio gives optimal results. It is shown that the oxidation of ethylene glycol on palladium occurs between 0.3 and 1.2 V versus a reversible hydrogen electrode (RHE), and depends on the presence of a Pd(0) active site. Electrochemical impedance spectroscopy experiments show that the charge-transfer resistance (R_CT) follows the same trend as EGO_Pd activity, with the 3:1 Pd:Ag electrode having the lowest R_CT. Electrolysis with this electrode at 0.705 V versus RHE, where Pd is reduced, results in glycolate production with no overoxidation to formate or oxalate. We then move to a flow setup operating under industrial conditions, and show that the Pd-Ni electrode yields >80% Faradaic efficiency to glycolate for over 140 h. Long-term electrode deactivation can be overcome in this system by a periodic self-refresh cycle.