Revealing transport kinetics for efficient electrochemical conversion of captured CO2 in amine solutions

Abstract

Direct electrochemical conversion of captured CO2 in amine solutions offers a promising route to upgrade dilute CO2 into valuable chemicals, bypassing the energy-intensive stripping step. However, this reaction is obscured by the complex equilibrium among carbamate, bicarbonate, dissolved CO2, and protonated ammonium in CO2-loaded amine solutions and suffers low selectivity due to the competing reduction of protonated ammonium. Here, we elucidate the reaction mechanism and reveal the mass transport as the governing factor for electrochemical conversion of captured CO2 in monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA), which are the representatives of primary, secondary, and tertiary amines with the same functionality. Bicarbonate-derived CO2, rather than carbamate, is identified as the reactive species for CO generation across all amines. TEA is found to be the optimal amine, offering the highest CO selectivity (80%) and stability with heterogenized cobalt phthalocyanine as catalyst. This is attributed to the significantly hindered mass transport of both reactive bicarbonate and protonated ammonium in TEA than in the other two amines, with protonated TEA exhibiting particularly sluggish diffusion. These findings pave the way for the rational design of amine systems for efficiently converting captured CO2 through mass transport manipulation.