(Invited) Cross-Linked Radiation-Grafted Anion-Exchange Membranes for Energy Conversion Systems: When to Cross-Link?

Abstract

Anion-exchange membranes (AEM) are being developed for use in a wide range of electrochemical energy technologies including fuel cells (AEMFC), H2-generating electrolysers (AEM-WE), CO2 reduction cells (CO2ER), and reverse electrodialysis (RED). An interesting class of AEM is radiation-grafted types (RG-AEM), which can exhibit very high conductivities and favourable in situ water transport characteristics. Hence, RG-AEM have shown significant promise in application in AEMFCs, producing high performances and promising durabilities [Energy Environ. Sci., 12, 1575 (2019) and Nature Commun., 11, 3561 (2020)]. An Achilles heel with RG-AEM types is that they can swell excessively in water and have large dimensional changes between the dehydrated and hydrated states. This limits the ion-exchange capacities (IEC) that can be used: excessive IECs in RG-AEM will cause excessive swelling and poorer robustness. This clearly indicates that additional crosslinking is needed. As Kohl et al. have highlighted, optimised crosslinking can lead to production of high-IEC AEMs that are both robust enough to be < 20 µm in thickness and also low swelling [e.g. J. Electrochem. Soc., 166, F637 (2019)], allowing truly world-leading AEMFC performances.RG-AEMs are also being used as a screening platform for down-selecting different (cationic) head-group chemistries for use in RED cells (a salinity gradient power technology) [active UK EPSRC grant EP/R044163/1], where different head-groups may lead to different AEM characteristics such as: in-cell resistance (when in contact with aqueous electrolytes), permselectivity, and fouling characteristics (when real world waters such as industrial brines, seawater and freshwater are used). It was evident very early on in these studies that RG-AEMs (desirably) exhibit extremely low resistances but also (undesirably) very low permselectivities when un-crosslinked (less than the required 90%+). Our work on RG-type cation-exchange membranes [Sustainable Energy Fuels, 3, 1682 (2019)] clearly shows that introduction of crosslinking can improve permselectivity.Crosslinking always involves a compromise, where its introduction can improve a membrane characteristic (e.g. reduced swelling or improved permselectivity) but also leads to lower conductivities or poorer transport of chemical species through the membranes. Hence, crosslinking types and levels need to be carefully controlled. With RG-AEMs (made by electron-beam activation (peroxidation) of inert polymer films, followed by grafting of monomers and post-graft amination), we have a choice of introducing crosslinking at various stages. The figure below summarises the two different approaches to crosslinking that will be discussed in the presentation: adding a divinyl-type crosslinker into the grafting mixture or adding a diamine-type crosslinker into the amination step.This presentation will present the results from an initial investigation of these two crosslinking approaches. As well as crosslinking in the amination stage being practically easier and more repeatable (we used N,N,N',N'-tetramethyl-1,6-hexane diamine (TMHDA) as a model diamine, being readily commercially available) compared to adding divinylbenzene (DVB – a model commercially available divinyl crosslinking agent) in the grafting step, we will show that such diamine crosslinking also enhances desirable properties (such as permselectivity) with less sacrifice of conductivity (less increases in resistance) compared to the use of DVB crosslinking. Figure 1