Smith, Keenan; (2023) Structure–Property Informed Development of Nascent Composite Proton Exchange Membranes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Ion transport is a critical element of energy conversion devices, especially of protons in the membrane electrode assembly (MEA) of fuel cells (FC). Despite a high performing commercial standard, a lack of ionomer progress over the past half century has hampered attempts to utilise proton exchange membrane (PEM) technology in a sustainable hydrogen economy. Gaining a fundamental understanding of the structure-property relationship is at the heart of overcoming this hurdle. This thesis details a multi-pronged approach to reduce ohmic losses in FCs with both ‘top-down’ optimisation of ionomer formulation and fabrication, as well as ‘bottom-up’ investigation into molecular structure and dynamics. Ultrasonic spray printing (USP) was used to prepare composite membranes of Nafion codeposited with polytriazine imide structured graphitic carbon nitride (PTI). Crystalline, amphoteric, and hygroscopic properties were found to enhance water management and proton conductivity of these membranes, whilst reducing fuel crossover, significantly enhancing FC performance. Dynamic processes of protons and water within the ionomer domains were probed with quasi-elastic neutron scattering at a range of instrument resolutions to reveal mechanisms occurring at pico- to nano- second timescales. Thin film investigation of composites at varying relative humidity and temperature elucidated PTI and graphene oxide induced structural tuning of polymer phase separation in unprecedented resolution. A Langmuir-Blodgett approach was separately developed to obtain single layer PTI sheets supported on Nafion. The procedure was optimised to obtain a dense <2 nm thick film which mitigated species crossover and resulted in considerably greater long-term stability. This reduction of crossover and increased stability has potential to solve a major issue of FC progress, reduction of ionomer thickness without deleterious consequences. Fundamental understanding of structure-property relationships revealed in this work provide a new level of insight and substantiate new material advances that mark a step change in potential PEM FC technology for the future.

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