Direct air capture of CO2 in an electrochemical hybrid flow cell with a spatially isolated phenazine electrode

CO2 capture based on a pH swing driven electrically through the reversible proton-coupled electron transfer of organic molecules could be powered entirely by clean electricity. A major technical challenge is the reversible chemical oxidation of the reduced organics by atmospheric O2, which can lower energy efficiency and capture capacity. Here we report the development of a hybrid phenazine flow cell system that uses a pH-swing 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 flow cell demonstrated substantial oxygen compatibility, exhibiting a coulombic efficiency of 99% and requiring only 73 kJ mol−1 CO2 for simulated flue gas and 104 kJ mol−1 CO2 for DAC. The 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.