Poly(ionic liquid)/Ionic Liquid Ion-Gels with High “Free” Ionic Liquid Content: Platform Membrane Materials for CO2/Light Gas Separations

The recycling or sequestration of carbon dioxide (CO2) from the waste gas of fossil-fuel power plants is widely acknowledged as one of the most realistic strategies for delaying or avoiding the severest environmental, economic, political, and social consequences that will result from global climate change and ocean acidification. For context, in 2013 coal and natural gas power plants accounted for roughly 31% of total U.S. CO2 emissions. Recycling or sequestering this CO2 would reduce U.S. emissions by ca. 1800 million metric tons-easily meeting the U.S.'s currently stated CO2 reduction targets of ca. 17% relative to 2005 levels by 2020. This situation is similar for many developed and developing nations, many of which officially target a 20% reduction relative to 1990 baseline levels by 2020. To make CO2 recycling or sequestration processes technologically and economically viable, the CO2 must first be separated from the rest of the waste gas mixture-which is comprised mostly of nitrogen gas and water (ca. 85%). Of the many potential separation technologies available, membrane technology is particularly attractive due to its low energy operating cost, low maintenance, smaller equipment footprint, and relatively facile retrofit integration with existing power plant designs. From a techno-economic standpoint, the separation of CO2 from flue gas requires membranes that can process extremely high amounts of CO2 over a short time period, a property defined as the membrane "permeance". In contrast, the membrane's CO2/N2 selectivity has only a minor effect on the overall cost of some separation processes once a threshold permeability selectivity of ca. 20 is reached. Given the above criteria, the critical properties when developing membrane materials for postcombustion CO2 separation are CO2 permeability (i.e., the rate of CO2 transport normalized to the material thickness), a reasonable CO2/N2 selectivity (≥20), and the ability to be processed into defect-free thin-films (ca. 100-nm-thick active layer). Traditional polymeric membrane materials are limited by a trade-off between permeability and selectivity empirically described by the "Robeson upper bound"-placing the desired membrane properties beyond reach. Therefore, the investigation of advanced and composite materials that can overcome the limitations of traditional polymeric materials is the focus of significant academic and industrial research. In particular, there has been substantial work on ionic-liquid (IL)-based materials due to their gas transport properties. This review provides an overview of our collaborative work on developing poly(ionic liquid)/ionic liquid (PIL/IL) ion-gel membrane technology. We detail developmental work on the preparation of PIL/IL composites and describe how this chemical technology was adapted to allow the roll-to-roll processing and preparation of membranes with defect-free active layers ca. 100 nm thick, CO2 permeances of over 6000 GPU, and CO2/N2 selectivity of ≥20-properties with the potential to reduce the cost of CO2 removal from coal-fired power plant flue gas to ca. $15 per ton of CO2 captured. Additionally, we examine the materials developments that have produced advanced PIL/IL composite membranes. These advancements include cross-linked PIL/IL blends, step-growth PIL/IL networks with facilitated transport groups, and PIL/IL composites with microporous additives for CO2/CH4 separations.