Electrochemically Mediated CO₂ Capture Using Aqueous Cu(II)/Cu(I) Imidazole Complexes

A major goal of developing electrochemical CO₂ capture technologies is to minimize the energy demand. One strategy for decreasing energy demands of electrochemical capture technologies is increasing the ratio of CO₂ molecules captured per transferred electron. Here, we examined an electrochemical capture approach that has the potential to capture up to two CO₂ molecules per electron, which is higher than many existing approaches. We used the Cu(II)/Cu(I) redox couple to control the aqueous availability of a CO₂ sorbent, 1,2-dimethylimidazole (Me²Im, by transitioning between Cu(Me₂Im)_4(aq)²⁺ and Cu- (Me₂Im)_2(aq)⁺. As expected from equilibrium calculations, a solution containing reduced Cu(I) had a greater CO₂ capacity than the oxidized Cu(II) state. In a bench-scale test, the energy demand for CO₂ capture was 27 ± 6 kJe/mol C, despite operating at 7-11% energy efficiency due to a high experimentally-set cell voltage. We estimate that under market-ready concentration conditions and the same low energy efficiency, the energy demand will be approximately 65 ± 14 kJe/mol C, although it can only remove 60% of the CO₂ from coal power plant flue gas (P_CO2 = 0.15 atm) at equilibrium. To address this issue, we used an equilibrium model of the relevant chemical reactions to identify how altering the substituent groups on imidazole will influence the CO₂ capture capacity and energy demand.