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

Abstract

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. Electrochemical CO2 capture is hindered by the oxidation of redox-active organic molecules by O2, affecting energy efficiency and capacity. Here the authors develop a flow cell in which the O2-sensitive components are isolated from O2, achieving 99% coulombic efficiency with low energy requirements.