Process integration of pH swing electrolysis and phase-change absorbents for energy-efficient CO₂ capture

CO₂ capture is widely acknowledged as an essential tool for enabling the transition beyond fossil-based chemical and energy systems. While current industrial deployments mainly target emission sources with high CO₂ partial pressures, there is a growing necessity and economic justification for capturing CO₂ from dilute streams, such as ambient air. Many CO₂ separation methods rely on thermal energy to release CO₂ and regenerate the sorbent. In contrast, systems that use electrochemical regeneration have recently gained traction, particularly for sources with low CO₂ partial pressure. These systems benefit from applying regeneration energy directly to the CO₂-binding sorbent, avoiding energy losses associated with heating solvents or solid supports. This work presents a novel electrochemical CO₂ capture process centered on a pH swing electrolysis system, which alkalizes and acidifies aqueous solutions. While the base scrubs CO₂ from the feed gas, the acid releases CO₂ from the solvent and regenerates the sorbent capacity. In the conventional cell configuration, CO₂ release and sorbent regeneration are carried out within the electrochemical cell, which introduces inefficiencies and operational challenges. Additionally, H₂₎-splitting imposes a high energy burden. To overcome these limitations, two process intensification strategies are considered. At the unit operation scale, the electrochemical cell is modified by integrating a hydrogen-depolarized anode (HDA), which reduces open-circuit voltage (OCV) and overpotentials. At the broader process scale, a precipitating agent is added to the absorbent solution (phase-change absorbent). Through this approach, CO₂ is captured in the form of solid HCO₃^- or CO₃^₂-, decoupling CO₂ release from electrochemical regeneration of the sorbent. The resulting process consists of three main unit operations: gas-liquid reactive precipitation, reactive desorption and HDA-assisted pH swing electrolysis. This thesis investigates the operability and second-law efficiency of the proposed system for capturing CO₂ from flue gas (₂₎_₂ = 15 vol%) and air (₂₎_₂ = 0. 04 vol%). The process modifications primarily impact gas-liquid reactive precipitation and HDA-assisted pH swing electrolysis. Both plant units demonstrate stationary operability under experimental conditions. In addition, a spray tower designed and set up for gas-liquid reactive precipitation exhibits no issues with scaling or clogging, despite recirculating slurry. Second-law efficiency analyses indicate the process holds promise for direct air capture (DAC). In contrast, the separation of CO₂ from flue gases exhibits only inferior performance. The Faraday efficiency is identified as the dominant driver of molar electrolysis work. Unfavorable pK values of the bicarbonate-precipitating bis (iminoguanidine) system and cross-membrane migration of H^+ are identified as major barriers for process competitiveness. Moreover, operational current densities below 0. 1 A~cm^-2 suggest that non-ohmic cell resistances dominate the energy consumption. The findings highlight the critical role of buffer systems in the electrolyte for enhancing process performance.