Unraveling Side Reactions in Paired CO2 Electrolysis at Operando Conditions: A Case Study of Ethylene Glycol Oxidation

Replacing the oxygen evolution reaction (OER) in CO2 electrolysis with an energetically and economically favorable alternative is very promising. Yet, understanding paired organic oxidation in the environment for CO2 reduction is particularly challenging, as monitoring multiple side reactions is problematic. Herein, we examined the oxidation of ethylene glycol (EG), one of the simplest polyols, as a model reaction on a series of nickel oxyhydroxide model catalysts (β-NiMxOOH, M = Ni, Co, Fe, and Cu). Using in situ techniques, including surface-enhanced infrared absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS), together with various ex situ approaches, we obtained the potential-resolved and quantitative information on various side reactions comprising the OER, overoxidation to CO/CO2, catalyst dissolution, and CO2 evolution from electrolyte decarbonation. Many factors including impurity cations, pH, and potential can substantially influence the product distribution and side reactions. Such influences are nearly identical for both the electrocatalytic and chemical-electrochemical oxidation pathways. The optimized system can achieve stable and high Faradaic efficiencies of formate (∼100%), glycolaldehyde (∼86%), and glycolate (∼66%), respectively. Importantly, paired electrolysis can easily suffer from higher energy consumption than the conventional counterpart, provided side reactions are unregulated. Yet the modulated one consumed 21.1% less energy even when product separation was considered. This work reveals the unique side reactions in paired CO2 electrolysis, opening up opportunities for designing efficient systems for real-life applications.