Puscalau, Constantin
(2025)
Metal-organic frameworks/polymer mixed matrix membranes for CO2 gas capture and separation in Li-air batteries.
PhD thesis, University of Nottingham.
Abstract
Climate change and energy consumption share a direct relationship, a strong and increasingly alarming link caused by the use of fossil fuels. It is necessary to change consumption patterns and to introduce new technologies that can replace fossil fuel sources with renewable and very low carbon emission ones, which otherwise would continue to have a negative impact on our climate, as increasing quantities of harmful gases are expelled into the atmosphere. The need for energy storage capabilities has increased dramatically with the expanded use of renewable sources and electrochemical energy storage is a key technology in the world’s transition to more sustainable energy systems, with the potential to support a wide range of services needed for the transition.
The specific energy storage capacity of state-of-the-art lithium-ion batteries is insufficient to meet the requirements of an ever-growing energy demand, due to their limited theoretical capacity of 387 Wh kg-1. The global challenge lies in the development of novel energy storage and conversion systems that possess greater energy density and efficiency. Lithium-air batteries (LABs) have attracted much interest due to their high power and energy density (11,140 Wh kg-1), potentially increasing the cost-effectiveness, as the battery relies on readily available oxygen from atmospheric air to operate.
Nevertheless, LABs are not yet widely commercialized due to components reactivity with ambient air gas contaminants, such as CO2, which can form Li2CO3 upon battery entry and cause parasitic reactions, severely impeding an efficient electrochemical performance. A solution to this problem can be achieved by introducing a gas separation membrane, which can selectively capture and separate CO2 from air. Despite advancements achieved by utilizing pure polymer membranes, which suffer from permeability/selectivity trade-off, much improvement of the gas separation efficiency can be done through carefully selecting the membrane material, in turn promoting better performing LABs. An effective strategy to enhance the CO2 separation efficiency is the use of mixed matrix membranes comprised of metal-organic frameworks (MOFs). MOFs are materials that consist of metal ions and organic ligands; the high porosity and extensive surface areas are well-suited for CO2 capture. The ability to customize the structure and functional groups of MOFs in order to selectively adsorb CO2 over other gases and increase the capacity for adsorption make them well-suited sorbents.
This work investigates MOF-based mixed matrix membranes (MMMs) for CO2 gas capture/separation efficiency and electrochemical performance of LABs using MMMs. Membranes were fabricated by incorporating micro- or nanoscale MOF fillers (namely UTSA-120, Mg-CPO-27 and HKUST-1) into a polydimethylsiloxane (PDMS) polymeric matrix at various loadings (2 – 50 wt%).
Structure stability and morphology of fillers upon membrane inclusion, thermogravimetric adsorption capacity, single gas CO2 and dry air permeability tests, together with electrochemical charge/discharge studies of coin cells were evaluated.
Results show that upon PDMS inclusion, all MOFs remain structurally stable and adsorption/desorption tests conducted on resulting membranes showed an increased uptake of CO2 gas scaling with MOF wt% loading over 4 adsorption-desorption cycles, proving that the MMMs can be regenerated and reused.
Overall, MOF introduction into the polymeric matrix displayed a significant enhancement in diffusional selective properties of the resulting composites, in comparison with pure PDMS matrix, along with improved CO2/dry air selectivity.
Preliminary electrochemical testing of Li-air batteries with PDMS/HKUST-1 10% implementation as gas purifying material exhibited an improvement of the cell life and deliverable energy boost in terms of areal capacity. Despite promising results, further studies are needed in order to optimize the electrochemical performance of LABs and enhance CO2 capture/separation efficiency of membranes.
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