• polymeric membranes
• proton exchange membranes
• PVDF
• redox flow battery
• SPEEK

In recent years there has been a great deal of interest in energy storage systems due to the growing demand for energy and increasingly looming environmental issues. Energy storage systems can be used in combination with intermittent renewable sources, such as solar and wind power. This provides a reduction in fossil fuels and consequently in greenhouse gas emissions. Redox flow batteries (RFBs) have the feature of modulating separately energy capacity and power. Thanks to this characteristic, they are considered attractive systems for energy storage. The membrane is a critical component for RFBs as it influences the charge-discharge performance and the durability of the battery. Ideally, membranes for RFBs should have a high ion exchange capacity to ensure good conductivity, a low crossover of redox-active species to avoid system efficiency losses, high chemical stability and mechanical robustness, and at the same time to be cost-effective to achieve a device with adequate power density. Nafion proton exchange membrane (PEM) is currently regarded as the benchmark for RFBs due to its high ionic conductivity as well as good thermo-mechanical stability in chemically aggressive environments. However, the use of Nafion in large-scale RFBs is limited mainly by its high cost. Therefore, there is a growing interest in developing novel, alternative, cost-effective PEMs that satisfy the application requirements and possibly outperform the benchmark in terms of mechanical, chemical, and electrochemical properties. The development of new effective membranes requires choosing appropriate materials and optimizing synthetic procedures in order to obtain systems with well-controlled morphology and structure. Nowadays, the researchers' efforts are focusing on the use of alternative and highly efficient synthetic strategies, shifting the focus to (co)polymers and blends obtained via preparation methods that allow to combine the advantages of different materials in an additive or synergistic way, in order to prepare membranes that are equally or even better performing than the Nafion benchmark. Both poly(vinylidene fluoride) (PVDF) and poly(ether ether ketone) (PEEK) are among the most widely investigated commercially available specialty polymers as starting materials to be converted into polymer precursors for the preparation of alternative PEMs for vanadium RFBs, that is the most mature technology in the field of RFBs.
Starting from this rationale, in this work, we report the preparation and characterisation of PVDF and PEEK-based PEMs that, to the best of our knowledge, are, for the first time, employed for semi-organic RFBs based on 9,10-anthraquinone-2,7-disulfonic acid disodium salt (AQDS)/bromine, an innovative technology developed by the Green Energy Storage (GES) company, partner of the present research project.
Regarding fluorine-based polymer precursors, PVDF was used as macroinitiator for the grafting of styrene monomer by atom transfer radical polymerization (ATRP). The obtained graft copolymers were then subjected to a sulfonation reaction to incorporate sulfonic acid functional groups (-SO3H) on the polystyrene side chains. Different copolymers with tuned mole content of styrene and sulfonated styrene were synthesized and used to prepare PEMs by solution casting and their chemical, thermal and mechanical properties were investigated. The electrochemical performance was also evaluated through ex-situ tests (ion exchange capacity and crossover of redox active species) and in-situ tests in a AQDS/bromine based single cell configuration. Finally, selected membranes in the hydrated state were subjected to different and complementary analyses, including differential scanning calorimetry (DSC) and solution nuclear magnetic resonance spectroscopy (NMR) aimed at elucidating the possible implications of the state of water inside the PEMs and its interactions with ionic functional groups. One of the most relevant findings was that graft copolymers, containing a low amount of sulfonated styrene (6 mol%) or low degree of sulfonation (35 mol%) allowed to obtain promising results in AQDS/bromine RFB, comparable to that of Nafion 115, in terms of voltage, coulombic and energy efficiencies. DSC and NMR studies combined with ex-situ tests allowed speculations on the possible differences in ion transport between Nafion 115 and the synthesized fluorinated membranes.
Regarding fluorine-free polymer precursors, PEEK was functionalized by sulfonation reaction, in order to introduce sulfonic acid functional groups on the polymer backbone (SPEEK). SPEEK with diverse degree of sulfonation (DS) was used alone or blended with PVDF or poly(ether sulfone) (PES) for the preparation of PEMs by solution casting. By varying the DS of SPEEK and its amount in the blends with PVDF or PES, it was possible to modulate the ion-exchange capacity and water uptake of the final membranes. The obtained PEMs were extensively characterised to evaluate their chemical, thermal and mechanical properties. Moreover, water uptake, ion exchange capacity and crossover of redox active species were also evaluated as well as in-situ performance by an AQDS/bromine single-cell tests, with the ultimate aim to establish possible correlations between the chemical composition and the property-performance of the PEMs. On selected membranes DSC and water transport analysis were also performed, to evaluate the state and interactions of water with the functional groups within PEMs. Only membranes with DS < 71 mol% were found to withstand cell conditions without occurrence of chemical degradation phenomena. In single-cell tests, membranes with low DS (56 and 60 mol%) and their blends with PVDF showed the best results, comparable or better than Nafion 115, suggesting that low DS and blending with PVDF (instead of PES) are favourable for preparation of membranes for AQDS/bromine RFBs.
For the two classes of developed PEMs, structure-property-performance correlations, potentially useful to draw guidelines for the design of next generation PEMs, were highlighted.