Flow synthesis of organic monomers for fabrication of anion exchange membranes with enhanced properties in fuel cells and electrolysers

With the continuous climate crisis and rising atmospheric CO2 levels, renewable sources of electricity and chemical feedstocks are of interest to the chemical community. Fuel cell and electrolyser technologies offer sustainable, and scalable solutions if issues involving cost, the use of rare-metal catalysts, selectivity, performance, and durability are addressed. CO2 electroreduction (CO2ER) offers synthetic access to valuable commodity chemicals such as CO, formic acid, and ethene from CO2. Anion exchange fuel cells and electrolysers offer improved reaction kinetics, and reduced fuel crossover compared to proton exchange fuel cells and electrolysers, and the alkaline environment allows for the use of non-precious metal electrocatalysts. The anion exchange membranes (AEMs) in these devices must exhibit high conductivity and permselectivity, alongside high chemical, mechanical, and thermal durability. AEMs in this work are synthesised via radiation-grafting (RG) of commercial films and radical polymerisation of vinyl arenes. This work aimed to synthesise vinyl monomers that, when included in AEMs, improve permselectivity and stability. Further, it aimed to investigate degradation pathways in AEMs through small molecule models. RG quaternary ammonium (QA) AEMs containing thiophene groups were desired due to reports of increased Faradaic efficiency (FE) and permselectivity against formate when they are present. Chapter 2 presents the optimisation of the continuous flow synthesis of 2 vinylthiophene, for 37 g at 93% yield, via in situ Grignard reagent formation, a Grignard reaction, and a Peterson olefination. This synthetic method was then applied to various vinyl arenes, showing tolerance towards an ester functional group and high yields (78% – 93%) for low and medium polarity vinyl arenes. Thiophene-containing QA AEMs were synthesised and assessed in CO2ER to CO where they showed improved FE for CO at higher current densities, including an improvement of 38% FE at 300 mA cm-2. However, for formate, a similar FE compared to non-thiophene membranes was observed. A small molecule analogue of poly(styrene) was targeted for analysis of degradation reaction pathways affecting AEMs by both MS and NMR. After various routes were trialled, key intermediates were synthesised via a base-catalysed aldol condensation with 75% yield for 0.36 g of the key intermediates. The subsequent hydrogenation was incomplete and requires further optimisation. The synthesis of a chloroalkyl styrene ‘spacer’ monomer, designed to increase AEM chemical stability by avoiding having an ammonium group in a benzylic position and preventing a chain scission mechanism, was optimised using in situ Grignard reagent formation and copper catalysed Grignard coupling and was deemed unsuitable for continuous flow due to inability to control formation of a side product dimer. Both CuCl.LiCl and CuCl2.LiCl catalysts were trialled. A different route utilising a Duff aldehyde synthesis was also deemed unsuitable for continuous flow due to immiscible solvents and reactants.

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