France, George and Lin, Hungyen and Gupta, Gaurav and Buscher, Monika and Dawson, Richard and Harper, Richard (2026) Characterising water properties of perfluorinated sulfonic acid membranes using terahertz time-domain spectroscopy. PhD thesis, Lancaster University.
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Abstract
Perfluorinated sulfonic acid ionomers (PFSAs) are synthetic polymers used in electrochemical devices such as fuel cells and electrolysers due to their low operating temperatures, high proton conductivity, chemical/mechanical stability and low reactant crossover. Membrane performance is highly dependent upon water content due to the role of water in proton conduction mechanisms such as the Grotthus hopping and vehicle mechanisms. Terahertz timedomain spectroscopy (THz-TDS) is an emerging technique which has previously shown sensitivity to the reorientation dynamics of water molecules within thick Nafion 117 PFSAs at a hydrated state and ambient conditions. It is challenging to make comparisons and utilise this information due to the different environments in which additional sample measurements, complimentary experiments and other techniques are performed. This approach is also limited to thicker membranes and not suitable to more industrially relevant thin membranes. This work therefore aims to address these issues and asses the techniques applicability for studying additional water properties within PFSAs. Therefore, in this work a humidity-controlled environment was developed in which THz-TDS measurements could be acquired to extract both water uptake and water states. This work found that the relative proportion of bulk water which is associated with proton conduction increases with higher water contents and relative humidities. Differences in these states between membranes have been observed providing additional material insight which could be utilised for membrane optimisation. Utilising this controlled environment, this work also demonstrates that THz-TDS has the ability to contactlessly study transient water diffusivity and hygral swelling of PFSAs.