(280f) Investigating the Effects of Ion Exchange Membrane Properties on the Performance of a Hydrogen-Powered Electrochemically-Driven CO­2 Separator

The increasing need to reduce carbon dioxide emissions may be insufficient to meet the Paris Climate Agreement which demands new avenues for carbon capture. Particularly, direct air capture will become vital to meet these capture demands. Although traditional carbon capture technology has focused on adsorbent/absorption technology, they pose prohibitively large energy and capital costs. An alternative CO₂ capture approach has been demonstrated by exploiting the alkaline environment generated by anion exchange membrane (AEM) hydrogen fuel cells. Here, hydroxides are electrochemically generated at the cathode which chemically scrubs CO₂ out of the air, producing (bi)-carbonates that transport across the AEM and accumulate at the anode. There, the local pH environment lowers until CO₂ generation is thermodynamically favorable resulting in a concentrated CO₂ stream. From the stoichiometry of the proposed acid-base mechanism, the EDCS has a theoretical upper limit of 1 CO­­₂: e^- (mol captured/mol consumed), typified as 100% electron efficiency. Previous work on the electrochemically-driven CO₂ separator (EDCS) for DAC applications with 400 ppm CO₂ focused on understanding and improving the gas transport of CO₂ to the cathode. However, there is still a need to understand the effect of ion transport on performance. As seen in Figure 1, the intersection between the dashed and solid lines indicates an optimum for high electron efficiencies and CO₂ removal. At current densities near this optimum, the back-diffusion of carbonates is hypothesized to reduce the capture rate of CO­­₂ and negatively affect electron efficiency. The membrane resistance can be increased to improve performance as illustrated in Figure 1. Although membrane resistance increases with increasing membrane thickness, thicker membranes would result in higher capital costs. Thus an alternative approach is to decrease the ion exchange capacity of the membrane for a given membrane thickness. PAP-TP-85 has been previously employed for the EDCS, and it is known that at high temperatures (>80 oC), the ion exchange sites of the polymer are unstable in their hydroxide form but stable in their halide form. Therefore, by heating PAP-TP-85 at various hydroxide-to-chloride ratios, a method for selectively degrading ion exchange sites in hydroxide form has been developed. The membranes are characterized after degradation by employing potentiometric titration to determine IEC and EIS to measure membrane resistance. The impact of membrane resistance is examined in the EDCS on a range of IECs that have been prepared using the PAP-TP-85 membrane. Preliminary evidence shows improvement in electron efficiency at higher membrane resistance in the lower current range. This may suggest that diffusion plays a more important role at the lower currents when hydroxides are limiting in the CO₂ capture process. To meet the $100/tCO₂ requirement, it is crucial to optimize energy efficiencies and CO­­­₂ flux for lower operating and capital costs.