Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing

We demonstrate a carbon capture system based on pH swing cycles driven through proton-coupled electron transfer of sodium (3,3′-(phenazine-2,3-diylbis(oxy))bis(propane-1-sulfonate)) (DSPZ) molecules. Electrochemical reduction of DSPZ causes an increase of hydroxide concentration, which absorbs CO 2 ; subsequent electrochemical oxidation of the reduced DSPZ consumes the hydroxide, causing CO 2 outgassing. The measured electrical work of separating CO 2 from a binary mixture with N 2 , at CO 2 inlet partial pressures ranging from 0.1 to 0.5 bar, and releasing to a pure CO 2 exit stream at 1.0 bar, was measured for electrical current densities of 20–150 mA cm −2 . The work for separating CO 2 from a 0.1 bar inlet and concentrating into a 1 bar exit is 61.3 kJ mol CO2 −1 at a current density of 20 mA cm −2 . Depending on the initial composition of the electrolyte, the molar cycle work for capture from 0.4 mbar extrapolates to 121–237 kJ mol CO2 −1 at 20 mA cm −2 . We also introduce an electrochemical rebalancing method that extends cell lifetime by recovering the initial electrolyte composition after it is perturbed by side reactions. We discuss the implications of these results for future low-energy electrochemical carbon capture devices. This work demonstrates a safe and scalable electrochemical CO 2 separation method that allows promisingly low (62 kJ/mol CO2 ) energetic cost at a high current density, and it can be used for direct air capture when a suitable molecule is used.