Electrochemically induced carbon dioxide capture from air with an aqueous fluoflavine sorbent

Direct air capture (DAC) holds great potential in mitigating climate change. A promising approach is redox sorbent-based electrochemically induced DAC, which operates at ambient conditions with significantly lower energy costs while utilizing clean electricity. However, the oxygen sensitivity of redox sorbents remains a major challenge. Thus, the feasibility of achieving electrochemically induced DAC with redox sorbents remains uncertain. Despite the vast difference in partial pressures between carbon dioxide (CO2) and molecular oxygen (O2) in air, ribulose-1,5-bisphosphate in plant leaves can preferentially react with CO2 in the sunlight, thus achieving the net CO2 removal. Motivated by the nature-based DAC solution, we hereby developed an aqueous-soluble oxygen-tolerant fluoflavine disulfonate (FFDS) sorbent for DAC. Electrochemical characterizations, spectroscopic studies, along with theoretical calculations, reveal that FFDS undergoes proton-coupled electron transfer and exhibits a redox potential higher than the two-electron transfer oxygen reduction potential. When exposed to simulated flue gas containing 10% O2, the FFDS sorbent demonstrates reversible CO2 capture and release with an average CO2/e molar ratio of 0.88. Implemented for both indoor and outdoor DAC in Singapore, FFDS sorbent shows stable cycling performance over 40 days with a three-day DAC under the normal discharge-charge mode, and exhibits an enhanced CO2 capture capacity and a Coulombic efficiency exceeding 99% with an accelerated overnight DAC under the deep discharge-charge mode. Doubling the concentration of FFDS leads to a twofold increase in CO2 capture/release capacity, regardless of whether the three-day DAC is conducted indoors or outdoors. The energy cost for CO2 release and sorbent regeneration ranges from 58 to 190 kJ mol–1 CO2, depending on the applied current densities and discharge-charge modes.