Date of Award
Doctor of Philosophy
Renewable energy applications (i.e. fuel cells, flow batteries, electrolyzers) have been at the forefront of green energy and environmental research over the past couple of decades and the research associated with them has skyrocketed due to changes in funding and incentives. The extensive research over the years have resulted in higher efficiency and longer lasting devices for renewable energy applications, but there is still a major bottleneck that all these devices share; the ion-exchange membrane (IEM). The development of polymer ion-exchange membranes has been very beneficial for these devices as they allow for higher working temperatures and increase the longevity and efficiency of said devices. IEM research can be summed up into two major types of membranes; proton- and anion-exchanging. Of these materials, proton-exchanging membrane (PEM) are well established and studied due to how long they have been manufactured and the ease of manufacturing. There has been a variety of different PEMs developed and tested, but none have been commercialized as heavily or used as universally as Nafion® (developed by DuPont in the 1960s) although it still suffers from setbacks like its high cost, low working temperatures and its low tolerance for fuel impurities. On the other hand, anion-exchange membranes (AEM) have become popular in this field of study as they boast a non-acidic substitute as well as more efficient oxygen reduction reactions allowing for operation without the use of expensive catalysts. AEMs are first in line to replace commercial PEMs like Nafion®, the major bottleneck being their ionic conductivities. Pairing the structural characteristics of PEMs with the efficient and more cost effective AEMs we sought out to design and synthesize new IEMs to compete with current commercial membranes. By using ring opening metatheses polymerization (ROMP) we have designed and developed numerous hydrocarbon polynorbornene derivative membranes with the intention of incorporating amino-phosphine ion exchange groups (IEG) to compete with current IEMs in both efficiency and cost with the major application of fuel cells and flow batteries in mind. We also performed different modifications to the initial membranes such as crosslinking and alkyl chain addition to increase the mechanical strength and mitigate the degradation of the membranes. Using results gathered from developing polynorbornene IEMs, we pivoted to another multitude of membranes, this time focusing on the PEM capabilities of fluorinated polymers instead of their hydrocarbon alternatives for use in redox flow batteries with the main goal of decreasing electrolyte crossover, therefore increasing the longevity of the devices. Several new IEMs were designed as composite membranes of Nafion® and aromatic organic IEGs and synthesized to compete with the current commercial IEMs while testing the effect of different aromatic IEGs on the salt permeability and mechanical strength of the membrane. Synthesis of a stable IEM with good electrolyte crossover and conductivity properties was achieved by combining a grafted Nafion® backbone with 2-phenylbenzimidazole side chains containing a long hydrocarbon chain to facilitate hydrophobicity and increase mechanical strength. These composite membranes take advantage of the imidazole’s highly stable chemical backbone and proton exchanging properties allowing it to withstand highly acidic and oxidative environments as well as relying on benzimidazoles tight packing to reduce electrolyte permeability throughout the membrane.
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