Structure-Property Based Design of Novel Composite Proton Exchange Membranes

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EPSRC : Keenan Smith : EP/L015749/1Sustainable energy generation and storage devices are critical to mitigate the continued impacts of climate change on the environment and society. Proton exchange membranes (PEM) are vital components of energy storage and conversion devices, such as fuel cells (FC), electrolysers and redox flow batteries, conducting protons between electrodes, whilst minimizing the transport of reactant molecules and electrolytic anions. Despite over half a century of development in this area, perfluorinated sulfonic acid (PFSA) membranes remain the industry standard. Composite PFSA PEMs with complementary components exhibit enhanced performance and alleviate the major issues of water management, low-temperature operation and fuel crossover, currently hindering the success of these devices. This placement focusses on the two main concepts of my PhD, through novel fabrication techniques and complementary characterisation tools to provide a complete structural understanding.Crystalline polytriazine imide (PTI), is a novel 2D material, being studied at UCL. It exhibits 3.88 Å pores with protruding piperidine -NH groups and favourable crystallographic stacking that results in 'aquaporin-like' water transport. We are pioneering the use of ultrasonic spray printing (USP) to produce composite polymer films with homogenously distributed PTI and graphene oxide (GO) throughout the PFSA framework. PTI's properties provide composite PEMs that outperform conventional polymers, as well as GO composite PEMs. The mechanism by which PTI and GO additives impart their beneficial properties, in composite PEMS, will be revealed by fundamental study of water uptake, swelling ratio, phase separation, free volume, and elastic modulus in thin film (<500 nm) samples at U. Calgary. Single-, double- and tri-layer 2D materials, such as graphene and hexagonal boron nitride, have been speculated as 'perfect' fuel cell membranes, with the highest theoretical proton conductivity, due to the selective permeation of protons through the dense electron clouds of regularly arranged atomic lattices. In addition to the specific properties addressed previously, this suggests that PTI has significant potential as a thin ion-sieving layer providing unimpeded proton transport. However, difficulty in obtaining a large-scale layer of PTI restricts the use of its remarkable through-plane transport properties for application in FCs. U. Waterloo has expertise in nanomaterial processing and have used this to develop films of densely tiled 2D material monolayers at low cost and complexity using innovative approaches. Thus, the role these materials were hailed to provide for FC application can be realised by integrating nano-thick films of PTI into PEMs by use of a simple, scaleable approach. In addition to the interfacial layering procedure, Prof. Pope's group have also established a route to effectively incorporate ionic liquids (IL) into graphene oxide (GO) lamellae for electrode applications. This surfactant driven assembly provides a comprehensive route to incorporate IL, which can adopt the proton mediator role of water, into a mechanically robust framework of graphene lamellae. This presents a system that has potential to provide anhydrous and high-temperature proton conduction, overcoming the issues of low-humidity and high-temperature performance that currently hinder the success of PFSA PEMs. The material and device advances, proposed here, that tap into the 'wonder' properties of 2D materials have the potential to provide high power density FCs with greater efficiency and durability. In conjunction with a structural and mechanistic understanding of these novel materials, the barrier to commercially viable systems will be circumvented, resulting in new materials infiltrating the green energy market and surpassing the established technologies.