The Role of Glyoxal as an Intermediate in the Electrochemical CO2 Reduction Reaction on Copper

The C2 product formation mechanism in the electrochemical reduction reaction of CO2 (CO2RR) is still poorly understood. This work aims to analyze the copper-catalyzed electroreduction of aqueous glyoxal to understand its role as a potential reaction intermediate during CO2RR. Multiple reaction pathways are observed during glyoxal reduction, including its electroreduction to ethanol and ethylene glycol, disproportionation to glycolate and formate, and further coupling toward the formation of C4 compounds and graphitic carbon. A significantly high ethylene glycol to ethanol ratio indicates that glyoxal may not be the main intermediate toward ethanol production in CO2RR on Cu, contradicting previous hypotheses. Density functional theory calculations show that the hydration of aldehyde functional groups can shift the ethylene glycol vs ethanol selectivity, in which the former is preferred when the carbonyl groups remain unhydrated. A CO2- to-glycolate pathway is also possible as a consequence of the base-catalyzed internal Cannizzaro disproportionation of glyoxal. Finally, C-C coupling during glyoxal reduction may open up a CO2RR pathway toward C4 products such as tetroses and 1,4-butanediol that have not been previously observed in electrochemical CO2RR. The formation of graphitic carbon also suggests that the carbon deposits usually observed during CO2RR may originate from glyoxal-derived C-C coupling. Our findings offer valuable insights onto the glyoxal pathway of CO2RR and the various multicarbon products that result from the further conversion of glyoxal.