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 CO2; subsequent electrochemical oxidation of the reduced DSPZ consumes the hydroxide, causing CO2 outgassing. The measured electrical work of separating CO2 from a binary mixture with N2, at CO2 inlet partial pressures ranging from 0.1 to 0.5 bar, and releasing to a pure CO2 exit stream at 1.0 bar, was measured for electrical current densities of 20 to 150 mA/ cm2. The work for separating CO2 from a 0.1 bar inlet and concentrating into 1 bar exit is 61.3 kJ/molCO2 at a current density of 20 mA/cm2 and extrapolates to 57.1 kJ/molCO2 in the low-current-density limit. At this limit, the cycle work for capture from 0.4 mbar extrapolates to 108-212 kJ/ molCO2 depending on the initial composition of the electrolyte. 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.