CO2 capture, sequestration, and utilization is a critical path to mitigate the CO2 emissions to the environment. An important strategy is to decarbonize fossil fuels and produce H2 and CO 2 at high temperatures. The resulting H2/CO2 mixture must be separated to obtain H2 for usage and CO2 for sequestration or utilization. Membrane technology has inherent advantages for gas separation because of its high energy-efficiency, small footprint, and ease of scale up. However, cost-effective materials with superior H2/CO2 separation properties at 150 °C or above are lacking. This dissertation addresses this gap via the development of novel membrane materials and the structural optimization of industrial thin film composite (TFC) membranes.

In membrane material studies, we firstly demonstrated that H2/CO 2 separation performance can be significantly enhanced by chemical cross-linking of solid polybenzimidazole (PBI) films in terephthaloyl chloride solution. Our second study illustrated that PBI doped with polyprotic acids such as H3PO4 and H2SO4 exhibits ultrahigh H2/CO2 selectivity. The acids cross-link PBI chains, decrease free volume and drastically improve H2/CO2 diffusivity selectivity. Third, we investigated mixed matrix materials (MMMs) containing PBI and palladium (Pd) nanoparticles, which have a strong affinity towards H2 and thus exhibit extremely high H2/CO2 solubility selectivity. The loading of Pd nanoparticles dramatically increases H2 solubility and H2 /CO2 solubility selectivity, resulting in a significant increase in H2 permeability and H2/CO2 selectivity at temperatures of 100--200 °C.

This dissertation also focuses on membrane fabrication using the developed novel materials as well as membrane structure optimization. Industrial membranes are often TFC membranes with a thin selective layer and a porous support which should have negligible resistance to gas transport. To optimize membrane structures, we systematically studied the geometric restriction of support on gas permeance using an integrated experimental and modeling approach. Track-etched polycarbonate nanofiltration membranes and real industrial poly(ether sulfone) microporous supports were used as model supports in the demonstration. Our study confirms that high-flux TFC membranes require porous supports with high porosities and small pores. Finally, high-performance TFC membranes were fabricated for H2 purification and CO2 capture based on our novel materials and the optimized membrane structure. Our TFC membranes exhibited great H 2/CO2 separation performance at temperature as high as 230°C with H2 permeance of ca. 400 gas permeation unit and H2/CO 2 selectivity of ca. 90. Such performance is superior to state-of-the-art H2 purification membranes, demonstrating their potential application in industry.