(Invited) Next Generation Ion Exchange Membranes for Electrochemical Polymer Membrane Processes

Abstract

The ion exchange membrane is a key component in many electrochemical membrane processes such as fuel cells, flow batteries and electrolysers. Typically, these ion exchange membranes are assembled in stacks and allow the transport of the charge carrying component, i.e. a cation (e.g. proton) or an anion, while retaining the other species and electrolytes preventing their crossover to the other side of the cell. Simultaneously electrons travel through an external circuit powering a device or to store energy.Conventional ion exchange membranes have two major problems: They are based on expensive materials (e.g. Nafion®e. PFSA; perfluorosulfonic acid) or on environmentally harmful chemicals and chemical reactions. Although due to its molecular structure and composition, PFSA membranes show good performances, the major limitation of PFSA membranes is the very high material costs often contributing for more than 35% to the total stack costs [1, 2].One of the major challenges of ion exchange membrane development is the tradeoff between high ion transport rates through the membrane while simultaneously preventing electrolyte crossover [3]. This talk will first present a comprehensive overview of required membrane characteristics and an extensive benchmark study of state-of-the-art performances of ion exchange membranes in different electro-membrane processes. Following on this, the challenges in ion exchange membrane development will be addressed and most importantly two new routes for the development of next generation ion exchange membranes will be presented and their characteristics will be compared to those of a series of extensively benchmarked commercially available ion exchange membranes.The first approach, electrospinning is an effective, versatile method to produce cheap ion exchange membranes [3-6]: Multiple polymers can be employed simultaneously during spinning and this is combined with high degrees of interchain entanglement. This results in an interconnected network of ionic pathways that promote high ionic conductivities confined in a matrix of an inert polymer that guarantees high rejections towards electrolytes to prevent crossover (Figure 1a). Moreover, it is a simple technique that can be easily adapted to large scale production.The second approach uses liquid crystalline (LC) polymers to make ion exchange membranes [7]. This approach has the potential to offer true molecular selectivity and a high degree of flexibility to actually tune this selectivity. LC polymer materials self-organize into structures with well defined isoporosity (Figure 1b). Subsequent template removal or chemical bond cleavage with an acid or base results in the formation of molecular pores. The pores of these materials can be functionalized and depending on the functionality, selectivity can be introduced. Depending on the bulkiness of the functional group also pore sizes can be smaller or bigger. Crosslinking of the formed structures allows control over the swelling of the material and with that reduces crossover. In this way one can rely on both charge-charge interactions as well on size sieving to separate species. The major challenge is the formation of organized structures over larger length scales and the identification of structure-property relationships and with that control over the membrane separation performance.Design principles of both newly developed membrane types are discussed, the membranes are extensively characterized and their performance in electrochemical processes is compared to that of conventional ion exchange membranes. The talk is concluded with a future outlook on the perspectives of ion exchanhe membrane development. T. Cho, et al., Energy Technol. 1 (2013) 596–608. https://doi.org/10.1002/ente.201300108.Lin, et al., J. Electrochem. Soc. 163 (2016) A5049–A5056. https://doi.org/10.1149/2.0071601jes.A. Hugo, et al., Journal of Membrane Science 566 (2018) 406. 10.1016/j.memsci.2018.09.006.Woo Park, et al., J. Membr. Sci. 541 (2017) 85–92. https://doi.org/10.1016/j.memsci.2017.06.086.Choi, et al., Macromolecules. 41 (2008) 4569–4572. https://doi.org/10.1021/ma800551w.J.B. Ballengee, P.N. Pintauro, Macromolecules. 44 (2011) 7307–7314. https://doi.org/10.1021/ma201684j.Kloos, et al., Journal of Membrane Science 620 (2021) 118849. https://doi.org/10.1016/j.memsci.2020.118849 Figure 1