Quantitative Assessment for the Disproportionation Reaction of HCHO Formed during CO and CO₂ Electrolysis (Supporting Information)

The electrochemical reduction of carbon dioxide (CO₂ electrolysis) is attracting much attention as a promising approach to address environmental challenges. Among the various products of CO₂ electrolysis, formaldehyde (HCHO) stands out for its potential as a precursor for longer-chain compounds. However, reports on HCHO production are scarce, and comprehensive discussions are still lacking. A major obstacle to deeper analysis is the inherent instability of HCHO, which tends to undergo disproportionation reactions, transforming it into different substances. In this study, we propose a quantitative indicator to evaluate the progression level of the disproportionation reaction of HCHO. Utilizing this indicator, we explore how the progression of the disproportionation reaction is influenced by changes in the electrolyte’s pH and catalyst layer thickness in the electrochemical reduction of carbon monoxide (CO) – a key intermediate in CO₂ electrolysis. Our results showed that the impact of the disproportionation reaction increases with higher pH values and greater thickness of the catalyst layer. Moreover, the insights obtained from CO electrolysis are effective in managing the generation of HCHO during CO₂ electrolysis.