Optimization of an electrochemical direct air capture process with decreased CO₂ desorption pressure and addition of background electrolyte

An electrochemical process based on pH-swing has been proposed recently to regenerate spent alkaline absorbent from direct air capture (DAC). In this work, we experimentally investigated and theoretically simulated two optimization strategies to further reduce the energy consumption of such novel electrochemical process. First, partial vacuum was applied to the gas phase during CO₂ desorption to increase the gas production rate. The energy consumption of the electrochemical cell decreased by 12 to 15% when the CO₂ partial pressure in the gas phase was reduced from 0.9 to 0.3 atm. Second, phosphate and sulphate were tested as background electrolyte to the alkaline absorbent, reducing the energy consumption by minimizing the ohmic losses in the electrochemical cell. The optimal concentration for phosphate was 0.1 M, while the CO₂ production rate was limited by either the total carbon feeding rate or the high acidifying solution pH at higher concentrations of phosphate. Moreover, due to the low pKa and high molar conductivity of sulphate compared to phosphate, sulphate addition showed an even lower energy consumption than phosphate addition. Finally, the lowest experimental energy consumption was 247 kJ mol⁻¹ CO₂ achieved with CO₂ partial pressure of 0.3 atm and 0.1 M of sulphate addition at current density of 150 A m⁻² while our mathematical model predicted a theoretical minimum energy consumption of 138 kJ mol⁻¹ under the same condition. Overall, the investigated optimization strategies advanced the development of an energy-efficient electricity-driven process for direct air capture.