Direct air capture of CO2 in a hybrid electrochemical flow cell

CO2 capture based on a pH swing driven electrically through the reversible proton-coupled electron transfer of organic molecules can be powered entirely by clean electricity. A major technical challenge is the reversible chemical oxidation of the reduced organics by atmospheric O2, penalizing energy efficiency and capture capacity. We report the development of a hybrid phenazine flow cell system that employs a pH-swing facilitated direct air capture (DAC) process, utilizing redox-active cyclic poly(phenazine sulfide) fabricated solid electrodes. The system maintains a separation between the air and the O2-sensitive reduced phenazine, enabling stable and effective CO2 capture from gas mixtures containing O2. This hybrid flow cell demonstrated significant oxygen compatibility, exhibiting a coulombic efficiency of 99% and requiring only 73 kJ molCO2-1 for simulated flue gas and 126 kJ molCO2-1 for DAC. The hybrid cell design strategy of isolating vulnerable species offers an efficient pathway for DAC and may be broadly applicable to avoiding undesirable side reactions in other electrochemical devices.