Towards Hydrogen Storing Systems for Vehicular Applications

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Little, Vanessa Renee
Heterogeneous Catalysis , Hydrogen Storage
The rising environmental and financial consequences of using fossil fuels as an energy source and energy carrier are a global concern. Described herein are two hydrogen-storing technologies, each of which was envisioned as a potential solution to said consequences: hydrogen-storing polymethylpyridylsiloxanes for use as an alternative energy carrier to fossil fuels; and thermally regenerative fuel cell systems to supplement or supplant vehicular alternators. A thermally regenerative fuel cell (TRFC) system is being developed to convert waste heat from an internal combustion engine (ICE) system into electricity that can be used to power auxiliary vehicular components. The TRFC system will comprise a dehydrogenation reactor and a fuel cell positioned relative to the ICE system such that the two components are held at 200 °C and 100 °C, respectively. 1-Phenyl-1-propanol has been identified as an optimal hydrogen storing liquid (XH2) that will selectively dehydrogenate over a heterogeneous catalyst to give a dehydrogenated liquid (propiophenone, X) and H2. The heterogeneous catalyst that currently provides the best selectivity (99.65%) for X at 200 °C is Pd/SiO2. A selectivity of ≥ 99.9% was desired to obtain the longest possible operational lifetime for the working fluids XH2/X. To increase the selectivity for X from 99.65% to ≥ 99.9%, size and shape specific Pd nanoparticles were synthesized. Pd nanocubes (20 nm) provided the best selectivity for X at 99.26%. It was concluded that a reproducible selectivity for X of ≥ 99.9% was not currently obtainable, and that a selectivity for X no greater than 99 % should be assumed when calculating the working fluids’ operational lifetime. Hydrogen-storing polymethylpyridylsiloxanes were proposed as energy carrier alternatives to fossil fuels. Polymethylpyridylsiloxanes were considered, in part, due to the expansive liquid ranges of siloxane polymers [-40 ˚C to 250 ˚C]; this would allow the polymethylpyridylsiloxanes to be stored and pumped into vehicles using existing refueling infrastructure. Polymethylpyridylsiloxanes, and analogs thereof, however, were not successfully synthesized and reversibly hydrogenated: either the desired product(s) could not be synthesized, isolated, and/or purified; or, hydrogenation resulted in product decomposition. It was concluded, therefore, that implementing polymethylpyridylsiloxanes as hydrogen-storing liquids is not viable.
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