DESIGN, DEVELOPMENT, AND SCALE-UP OF AN ELECTROCHEMICALLY-DRIVEN CARBON DIOXIDE SEPARATOR USING ANION EXCHANGE MEMBRANES

Abstract

Carbon capture from concentrated point sources and dilute ambient air followed by utilization or sequestration have been identified as crucial strategies for mitigating climate change. Research on carbon capture has predominantly focused on point sources as these technologies benefit from the high CO2 concentrations making separations more feasible and efficient. However, when faced with the dilute atmospheric concentrations of CO2, conventional gas separation technologies struggle to efficiently capture CO2. In this thesis, we propose an electrochemical device for separating CO2 from ambient air by harnessing the alkaline environment surrounding an anion exchange membrane (AEM). The initial design of the electrochemically-driven CO2 separator (EDCS) was inspired by the CO2 purging mechanism observed in hydroxide exchange membrane fuel cells in the presence of ambient air, and proof-of-concept performance was established. A model was developed to explain experimental observations and identify methods to improve CO2 removal performance. Systematic changes to the cathode catalyst layer, gas diffusion layer, and flow field in a 25 cm2 single cell resulted in over a 90% reduction in CO2 mass transport resistance and the ability to remove over 99% of CO2 from ambient air in a single pass. The impact of operating parameters, such as inlet CO2 concentration, current density, anode flow, cathode flow, and temperature, on EDCS performance metrics (CO2 removal fraction, CO2 flux, and electron efficiency) are investigated in depth. Additionally, the versatility of the EDCS was demonstrated for a range of dilute CO2 applications from direct air capture to life support applications for spacecraft and submarines. Operating regimes were identified based on the constraints of the system and scaled to compare to incumbent technologies. A shorted AEM was introduced to the EDCS to provide a through-membrane electronic pathway, eliminating the need for bipolar plates or current collectors and opening a wide range of flow architectures for scaling the EDCS. Indirect current control was achieved by controlling the hydrogen flow rate and partial pressure through dilution. A multi-cell stack prototype using the shorted AEM was designed, fabricated, and tested. The 2-cell and 4-cell stacks achieved comparable CO2 removal, flux, and electron efficiency to the 25 cm2 single cells when scaled for active area. Finally, the manufacturing and operating costs of the EDCS were calculated in a techno-economic analysis to validate the feasibility of scale-up in direct air capture applications. In this dissertation, the EDCS was taken from proof-of-concept to a scaled prototype capable of efficiently removing CO2 from ambient air with the potential to be used in a wide range of CO2 separation applications. Future work will need to focus on identifying and mitigating electrochemical degradation. Also, the EDCS could greatly benefit from the development of selective or reinforced membranes and tunable hydrogen diffusion barriers to improve indirect current control with shorted AEMs.