A well-established method for the synthesis of anion exchange membranes (AEMs) is via radiation induced graft co-polymerisation (RG) of commercially available polymer films.¹ These AEMs are in the process of being applied to the field of CO₂ Electrolysis. This process is being carried out as part of an EU consortium² that aims to develop electrolysis devices enabling CO₂ to be selectively converted into the high value chemicals CO, ethanol or ethylene.

Contrary to our experience synthesising and optimising RG membranes for alkaline fuel cells and electrolysers,^3,4 and more recently reverse electrodialysis stacks⁵, less is known regarding their optimisation for CO₂ electrolysis. Four key parameters have been identified: base film chemistry, film thickness, functional groups, and degree of crosslinking.

Six different AEMs were produced via the RG method. Vinylbenzyl chloride (VBC) monomer was grafted onto electron-beamed (in air) poly(ethylene-co-tetrafluoroethylene) commercial (ETFE) films that were either 25 or 50 μm thick and the resulting membranes aminated with either trimethylamine (TMA), N-methylpyrrolidine (MPY), or N-methylpiperidine (MPIP) yielding the corresponding quaternary ammonium functionalised RG-AEMs. We down-selected MPIP-functionalised AEMs synthesised from the thinner 25 μm base film as they showed the best performance / Faradaic efficiency balance for CO₂/CO reduction (comparable to commercial Sustanion AEM).

Traditionally, amine functionalisation is carried out using high concentrations of amine solution (to 50% v/v).⁶ Whilst providing 100% conversion of grafted CH₂-Cl groups to CH₂-NR₃⁺ groups, this is a brute force approach with little stoichiometric control. Synthetic strategies targeting finer (less wasteful) stoichiometric control have been developed, these methods allow for the implication of multiple functional chemistries in a single step whilst using only a small excess of amine reagents. However, AEMs synthesised via such stoichiometric methods can display substantially different hydration properties to those produced via the traditional large excess routes, despite evidence indicating that they have near identical chemistries. This mystery will be discussed further in this presentation.

References

1. Wang et al. An optimised synthesis of high-performance radiation-grafted anion-exchange membranes, Green Chem., 19, 831 (2017).
2. Select CO2, https://selectco2.eu/index.php/en/
3. R. Varcoe et al. Anion-exchange membranes in electrochemical energy systems, Energy Environ. Sci., 7, 3135 (2014).
4. Aggarwal, Isoindolinium Groups as Stable Anion Conductors for Anion-Exchange Membrane Fuel Cells and Electrolyzers, DOI: 10.1021/acsmaterialsau.2c00002
5. R. Willson et al. Radiation-grafted cation-exchange membranes: an initial ex situ feasibility study into their potential use in reverse electrodialysis, Sustainable Energy Fuels, 3, 1682 (2019).
6. L. Gonçalves Biancolli, et al. ETFE-based anion-exchange membrane ionomer powders for alkaline membrane fuel cells: a first performance comparison of head-group chemistry, J. Mater. Chem. A, 6, 24330 (2018).