Advanced anion exchange membranes for water electrolysis

One key strategy to slow down the global warming and mitigate harmful environmental effects comprises replacing fossil carbon-based energy sources with renewable carbon-free resources. A promising approach for this so-called decarbonisation is to utilise hydrogen as energy vector thanks to its numerous advantages, including the high energy density per mass unit and the production of water as the only by-product of combustion. Anion exchange membrane water electrolysis (AEM-WE) now appears as the election technology for green hydrogen production, combining the use of cheaper materials for the fabrication of cell components (e.g. bipolar plates and electrocatalysts) with the high-performing working conditions of the solid-state electrolyte-based technologies, that include higher current density, the possibility to work with a discontinuous electrical supply and diluted alkaline supporting electrolyte solution, as well as a compact cell design. However, AEM-WE early-stage development limits such a technology to the small-scale production and the preparation of cost-effective, durable and highly-performing anion exchange membranes (AEMs) still represents the main goal for its industrialisation. The need for effective hydroxyl ion transport and reduced hydrogen permeability as well as superior chemical and mechanical stability make the design and preparation of the ideal membrane extremely challenging. A widespread and straightforward strategy to prepare polymers for AEM production relies on the post-modification and functionalisation of commercially available copolymers, including poly(olefin)s, fluoropolymers and aromatic-based engineering plastics (e.g. poly(sulfone)s, poly(ether)s, poly(benzoimidazole)s). An overview of the most recent advances in polymeric AEMs for AEM-WE as well as the state-of-the-art on such a technology are parts of a comprehensive introduction reported in Chapter 1. Starting from the approach based on the post-modification of polymers currently present on the market, this work was mainly focussed on the preparation, by different approaches, of advanced and innovative AEMs that in any case were derived from the post-functionalisation of commercially available and relatively cheap styrene-butadiene block copolymers, selected also for their inherently ability to micro-phase separate at the nanoscale, phenomenon that is expected to drive the formation of nanostructured morphologies even in the post-modified copolymers. In Chapter 2, the synthesis in solution of vinyl benzyl chloride (VBC)-grafted copolymers via either free or controlled radical techniques by starting from commercially available styrene-butadiene block copolymers is reported. Films derived from VBC-grafted copolymers were aminated with trimethylamine (TMA) and their thermal, mechanical, water uptake and ex-situ electrochemical properties (i.e. ion exchange capability, conductivity and hydrogen crossover) were investigated. The effects on AEM thermal, mechanical and ex-situ electrochemical properties of the different structural parameters of the graft copolymer precursors, including the amount of VBC, the inclusion in the grafts of styrene units distributed according to either a random or a block architecture and the increase in styrene molar content in the starting polymeric material were pointed out. One of the most relevant findings was that graft copolymers, deriving from the styrene-butadiene starting material with a higher content of styrene (named SB), showed the best balance of mechanical and electrochemical performance, thanks to the increased mechanical robustness, due to the larger amount of styrene, that compensated for the higher water uptake at higher molar content of VBC (or functionalisation degree FD). On the basis of this result, in Chapter 3, a SB-based graft copolymer with an intermediate FD of 15 mole% was selected to be aminated with different mono-amines and mono-/poly-amine mixtures and the effects of the quaternisation step, especially on the mechanical and ex-situ electrochemical properties of the AEMs derived therefrom, was highlighted. In Chapter 4, SB-based graft copolymers with a higher FD (2232 mole%) were used to be blended with a commercially available engineering plastics, i.e. poly(p-phenylene oxide) (PPO), with the aim of counterbalancing the effects of the larger water uptakes with the use of a hydrophobic and rigid polymer. Main findings demonstrated that small amounts of PPO (35 wt%) were sufficient to significantly improve the mechanical resistance and reduce water uptake, without negatively impacting too much on the electrochemical performance of the AEM. In Chapter 5, the SB-based graft copolymers were functionalised with poly(ethylene glycol) monomethyl ether with 8 oxyethylenic units on average (mPEG8). The incorporation of mPEG8 side chains had a more marked effect in terms of overall hydrophilicity in membranes at higher FDs, for which resulted in higher WU and comparable conductivity, but at the expense of mechanical stability. In Chapter 6 selected SB-based AEMs membranes were subjected to different and complementary analyses, including atomic force microscopy (AFM), differential scanning calorimetry (DSC) and fluorescence recovery after photobleaching (FRAP) aimed at elucidating the possible implications of the AEM chemistry and structure on the water content and transport in a confined region. The higher FD was found to result in high-performing membranes in real working (in-situ) AEM-WE conditions, with enhanced water and ion transport properties. The increase in FD from 13, to 15 up to 20 mole% led to a more stable potential and longer durability from 15 to 70 up to 95 days, respectively. Moreover, SB-based membranes containing 27 mole% showed the highest in-situ conductivity, being, to the best of our knowledge, one of the most promising among those reported in literature for AEMs operating in as close as possible AEM-WE working conditions. A preliminary study of the AEM performance, when used for in-situ AEM-FC tests, was also carried out to respond to the great challenge to find a polymer chemistry and material suitable for both the applications. Finally, for all the classes of developed AEMs, structure-property-performance correlations, potentially useful to draw guidelines for the design of next generation AEMs, were highlighted.