CO2-Selective Membrane for Enhancing H2 Utilization in Solid Oxide Fuel Cells

In order to utilize the H₂ remaining in the anode exhaust of a solid oxide fuel cell (SOFC), a CO₂-selective membrane was developed to remove CO₂ from the anode exhaust and recycle the concentrated H₂ back to the SOFC. As the cooled exhaust still has an average temperature of 120ºC, it is challenging for polymeric membranes to maintain both high CO₂/H₂ selectivity and CO₂ permeance. Previously, facilitated transport membranes were developed using tetramethylammonium hydroxide (TMAOH) as the mobile carrier to selectively remove CO₂. However, TMAOH has been reported to degrade via a nucleophilic substitution mechanism, and a possible higher temperature of the anode exhaust in practice may exacerbate the degradation.

Herein, more stable fluoride- and hydroxide-containing species were investigated for their potentials of replacing TMAOH. The thermal stabilities of the quaternaryammonium-containing compounds were characterized by TGA. Moreover, quantitative results from NMR showed that tetramethylammonium fluoride (TMAF) was 14 times more stable than TMAOH at 130ºC. Next, membranes with the optimal composition containing TMAF were synthesized, and their transport performances at different temperatures and feed CO₂ partial pressures were measured, respectively. The membrane showed a CO₂ permeance of 108 GPU and a CO₂/H₂ selectivity of 106 at 120ºC, and it was stable for 120 h. The TMAF-containing membrane also displayed an improved stability of about 4 times better than the TMAOH-containing membrane at 130ºC. To date, scale-up of 21ʺ-wide membranes with a total length of 720 ft has been demonstrated, and the membranes exhibited consistent transport results as the lab-scale samples. Additionally, a techno-economic analysis showed that, when the H₂ recovery of the membrane process was set at 99%, the CO₂ removal cost was calculated to be $62.9/tonne. Lastly, a CO₂ removal of 76.9% could be obtained when the air sweep was replaced by vacuum on the permeate side.