(349c) Thin-Film Composite Membranes By Engineering Nanoconfinement Effects in Selective Polymer Layers

Post-combustion CO₂ capture from fossil fuel-fired power plants is of great importance in reducing the global CO₂ concentration worldwide. Recently, membrane-based CO₂ capture has drawn keen attention due to its potentially high energy efficiency, small footprint, and simple operation. Polymers are the most studied membrane materials due to their good processability and relatively low cost. Traditionally, a polymer material of interest has been cast into a freestanding bulk film with a thickness of >50 µm, and its intrinsic gas transport properties have been evaluated in terms of permeability (a pressure and thickness-normalized flux, Barrer unit, and selectivity (a permeability ratio of a certain gas pair). Permeability has been a useful figure of merit to estimate the potential of developed membrane materials for the next steps such as scale-up and modularization. Based on this guidance, a tremendous number of materials have been examined to overcome the trade-off relationship between permeability and selectivity in polymeric membranes. Indeed, over the decades, these efforts have contributed to redefining the upper bounds for CO₂ capture membrane materials in 2008 and 2019 since the first report in 1991. However, the ultimate productivity of a membrane should be gauged by permeance (a pressure-normalized flux or a permeability divided by thickness, GPU unit, since this indicates the actual gas transport rate of a certain membrane system. For post-combustion CO₂ capture, membranes with a high CO₂ permeance (>1,000 GPU) and a moderate CO₂/N₂ selectivity (>30) are expected to meet the commercial interest. Unlike the permeability, theoretically, the permeance strongly depends on the membrane thickness in inverse proportion. In this regard, thin-film composite (TFC) membrane is a promising membrane configuration to maximize the CO₂ permeance, which is composed of (1) a porous substrate for mechanical robustness at the bottom, (2) an intermediate layer to prevent pore clogging or defects, often called gutter layer, and (3) a thin selective layer (<1 µm) that offers separation abilities at the top. We envisioned that there are significant but undisclosed transitions when converting bulk materials into thin films to fabricate TFC membranes, which may limit the further enhancement in their CO₂ permeance. To this end, four commercially available polyether BCPs were selected in this study to study the potential structural changes depending on membrane thickness; Pebax 2533 (PA12-PTMO), Pebax 1657 (PA6-PEO), Pebax 1074 (PA12-PEO), and Polyactive (PBT-PEO), where PA6 = polyamide 6, PA12 = polyamide 12, PEO = poly(ethylene oxide), PTMO = poly(tetramethylene oxide), and PBT = poly(butylene terephthalate), respectively. These polymers have been acknowledged as a promising membrane material for post-combustion CO₂ capture. For decades, tremendous research efforts have pioneered advanced membrane materials for CO₂ capture. However, despite these fascinating findings, the currently available commercial membranes still rely on traditional polymers such as polysulfone and cellulose acetate. This highlights the urgent need for an in-depth study on membrane engineering – how to well convert the existing materials into thin membranes. In this work, we have demonstrated the rational control of confinement effects in commercially available polymers offers a new degree of freedom to push the limits of current CO₂ capture membranes. Our concept was able to develop high-performance TFC membranes for post-combustion CO₂ capture by not only disclosing the structural changes in thin films induced by confinement effects but also providing universal applicability. Hence, we propose that engineering confinement effects in developed membrane materials will pave the way for a paradigm shift from the routine strategy for seeking novel materials to the more real-world relevant research for the industrial deployment of TFC membranes. Beyond the CO₂ capture, these efforts are expected to contribute to developing next-generation TFC membranes for other challenging separation applications such as hydrogen purification, olefin/paraffin separation, desalination, crude oil fractionation, and flow batteries.