Catalyzed Hydrogen Release from BH- and BNH-based Hydrogen Storage Materials
Creators: name: Mostajeran, Mehdi
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2017
Subjects: Chemical hydrogen storage
Ammine metal borohydrides

description: In order to reduce our ties to fossil-based energy and mitigate the undeniable impacts of climate change on the environment, remarkable efforts have been directed over the last 4 decades toward developing renewable energy sources such as solar, wind, geothermal, etc. For transportation applications biofuels, electricity and hydrogen all offer potential solutions although current usage is still largely linked to fossil fuels (bio-based ethanol-gasoline mixtures, power generation for battery recharging, and steam reforming for hydrogen production). While hydrogen offers the greatest potential in terms of energy density, its poor volumetric density (0.01 MJ/L at RT) requires costly compression and pressurized storage. When future technology finally allows for efficient hydrogen release from water splitting, we need to have optimal solutions in place for hydrogen storage. One promising solution is chemical hydrogen storage in which thermolysis of a chemical precursor affords a controlled hydrogen release that can then be reversed in an off-board regeneration step. With a focus on maximum gravimetric hydrogen storage, various BNH compounds have been shown to be promising chemical hydrogen storage precursors. In this Thesis we summarize the state of the art in B-N-H hydrogen storage compounds (Chapter 1) and then investigate several new chemical hydrogen storage solutions with a focus on portable power generation. In the first project (Chapter 2) we sought to prepare a robust, base-metal borohydride hydrolysis catalyst for use in a custom hydrogen generator designed to use the reaction heat to help separate the borate spent fuel. Active ‘reverse opal’ layered double hydroxide (LDH) catalysts were prepared and tested. While the classical Ni-Mg-Al LDH released 3.4 equiv. of hydrogen at 50 °C in 150 minutes, the polystyrene templated Ni-Mg-Al catalyst released 4 equiv. of hydrogen with a higher initial rate under the same reaction conditions. The long-term objective of this project was to test these catalysts in fuel cells for underground mine forklifts with our industry collaborator (Kingston Process Metallurgy Inc.). In the next three chapters, the synthesis and hydrogen release properties of ammine metal borohydrides [M(BH4)m(NH3)n, AMBs] were investigated. As promising hydrogen storage materials with high hydrogen content (10-15 wt%), AMBs can access lower hydrogen release temperatures resulting from the combination of protic (N-Hδ+) and hydridic (B-Hδ-) hydrogens. While AMBs also do not suffer from diborane formation that plagues thermolysis of metal borohydrides, hydrogen release is often accompanied by small concentrations of ammonia that deactivate the fuel cell catalyst. Our objective for this work was to identify base metal catalysts that could suppress ammonia formation by further reducing the energy barrier to H2 release. In Chapter 3 our studies of the solution synthesis of AMB materials (Y, La, Zn, etc.) in coordinating solvents such as tetrahydrofuran (thf) and diethyl ether revealed the unexpected formation of ammonia-borane (H3NBH3, AB). It was shown that while the amounts of produced AB correlate with the Zhang electronegativity for the s- and p-block metals, ionic radius is a stronger determining factor for the transition metals. It was also observed that reducible metals such as Ti and V produce large amounts of AB while Zn produced the least. This knowledge was then used in Chapter 4 to prepare pure samples of the Y and La complexes, M(BH4)3(NH3)4 that were characterized by thermal analysis (TGA-MS), powder X-ray diffraction, FT-IR and 11B and 1H MAS NMR spectroscopy. Furthermore, a series of base-metal nanoparticle catalysts, prepared using a novel route from MCl2 and liquid hexylamine-borane, was shown to suppress ammonia formation from these Y and La AMBs. Immobilizing 5 wt.% of Co NPs on Y(BH4)3(NH3)4 and 5 wt.% of Fe NPs on La(BH4)3(NH3)4 resulted in reduction of ammonia release by three- and fourfold, respectively. In Chapter 5 the attempted solution synthesis of Zn(BH4)2(NH3)2 revealed complications due to preferred formation of MIZn(BH4)3 [instead of Zn(BH4)2] from the reaction of ZnCl2 and MIBH4 (MI= Li, Na, K). As a result, the mixed-metal AMB, KZn(BH4)3(NH3)n was prepared and characterized. Although the effects of both heterogeneous and homogeneous catalysts were not as pronounced as those for Y and La, using 5 wt.% FeNPs resulted in fourfold reduction in the amount of released ammonia which led to a purer hydrogen stream (98.9 mol%) compared to the uncatalyzed thermolysis (97.0 mol%). Finally, in Chapter 6 our results are considered vs. the current state of the art and suggestions are made for further investigations.