Direct Electrochemical Conversion of Amine-Captured CO2 into High Concentrated C1 Products
Authors: Won Da Hye , Langie Kezia , Tak Kyungjae , Lee Ung 
Published: December 2024

Electrochemical CO2 conversion (eCO2R) into value-added products presents a promising strategy for mitigating global warming. However, this technology faces a significant challenge: supplying high-purity CO2 and purifying the product requires a substantial amount of thermal energy, as most systems typically use highly concentrated CO2 as a reactant. Herein, we suggested a new concept of system that bypasses energy-intensive conditioning by directly converting amine-captured CO2 into highly concentrated syngas and formic acid, using a novel amine-type CO2 absorbent, triethylamine (TREA). Our experiments show that the CO2 absorption rate (>84%) of TREA is comparable to conventional alkanolamine, with the majority of CO2 captured as bicarbonate‒a form preferred for direct electrochemical use due to its easy desorption. To efficiently convert TREA-captured CO2 into C1 products, we optimized the system configuration based on the membrane electrode assembly (MEA) electrolyzer, and bipolar membrane applied MEA emerged as optimal. This system requires different mass transport approaches since the reactant is delivered in liquid phase, not as gaseous CO2. We developed a hydrophilic coral-Ag/carbon electrode that provides CO production active sites and facilitates interaction with TREA-captured CO2. This electrode achieved 70% CO Faradaic efficiency (FE) and 30% H2 FE at 20 mA cm-2, with selectivity varying by current density in the TREA-captured CO2 conversion system. Moreover, we discovered that high-purity syngas production is possible due to the re-capturing of unreacted CO2 in TREA, eliminating the need for post-production purification. Based on the experimental results and rigorous process modeling, we reveal that this system can produce high pressure syngas at a reasonable cost with negligible CO2 emissions.

To enhance system feasibility, we also developed TREA-captured CO2 conversion to produce highly concentrated formic acid, which exists primarily as formate (HCOO-) in aqueous solutions. In conventional eCO2R systems, converting formate to formic acid involves energy- and cost-intensive steps, including acidification and purification to separate formic acid from electrolyte precursors. Our method directly converts TREA-captured CO2 to formate using a Sn-Cu alloy catalyst, simplifying the purification of formate to formic acid in two-steps. Protonated TREA chemically stabilizes formate through ionic interaction, allowing for the stabilization of highly concentrated formate in the TREA solution. The Sn-Cu alloy achieved maximum 60% of formate FE at 50 mA cm-2 with 28% of H2 FE and 14% of CO, with selectivity varying by current density. During long-term operation at a constant current density condition of 100 mA cm-2, the Sn-Cu alloy produced 2.6 M formate in the TREA-captured CO2 system, recording 94% conversion captured CO2. The purification process of formate into formic acid does not require high temperatures (not exceeding 150°C) or harsh chemicals by utilizing amine switching method, highlighting our system's environmental and practical benefits. These demonstrate the practical feasibility and minimal global warming impact of the TREA-captured CO2 conversion system, compared to other advanced Carbon Capture and Utilization strategies.

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