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

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 from 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 the 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.