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Study of dehydrogenation properties of metallic borohydrides for hydrogen storage

Abstract : This thesis work is dedicated to the exploration of metal borohydride compounds and their derivatives as a family of materials that can potentially enable solid-state storage of hydrogen, with high gravimetric capacity (>10wt%) and moderate desorption temperatures (<150°C). The candidate materials are ammine metal borohydrides, AxM(BH4)m+x(NH3)n where A is alkali metal, M any other metal (for example Zn, Al, Mg) , m is the oxidation state of M, n is the number of neutral ammonia adducts and x=0-2. These materials contain positively (N-Hδ+) and negatively (B-Hδ-) charged hydrogens in their structure, upon heating the electrostatic attraction between these oppositely charged hydrogens, their combination and release as a hydrogen molecule is the main dehydrogenation process. The major challenges preventing the application of these materials are the lack of straight-forward synthesis process to obtain desired composition, precise control of dehydrogenation properties based on the chemical formula and the rehydrogenation of spent fuel.High electronegativity metal borohydrides are stabilized by combining them with alkali metal borohydrides. By adhering to the same logic, we explored the potential of ammonium cation [NH4]+ as pseudo-alkali cation that can also participate in decomposition of Zn(BH4)2 and increase its theoretical hydrogen capacity. So, we synthesized ammonium zinc borohydride (NH4)Zn(BH4)3 for the first time. However, the compound was not stable at room temperature and its thermal decomposition released a lot of diborane and borazine gases.In literature, the typical synthesis process involves salt metathesis reaction using ball-milling or solvothermal approaches to obtain metal borohydride, followed by exposure to ammonia gas. To bypass this multi-step approach we developed liquid ammonia synthesis process that directly enables the formation of AZn(BH4)3(NH3)2. The technique allowed us to synthesize LiZn(BH4)3(NH3)2 and KZn(BH4)3(NH3)2 for the first time. By combining the known compounds from literature Zn(BH4)2(NH3)2 and NaZn(BH4)3(NH3)2, we obtained a unique set of compounds that have zinc cation sharing its coordination sphere with two borohydride and two ammonia molecules. We determined that monometallic and bimetallic compounds have different dehydrogenation reactions. The presence of an alkali cation with lower polarizing power than zinc cation in the structure is more detrimental to the purity of released hydrogen. Use of zinc chloride precursors results in chloride anion substitution of borohydride sites in AZn(BH4)3(NH3)2 which also increases ammonia impurity. Chloride anion substitution is eliminated by using ZnF2 precursor which also enables the filtration of the main phase from ammonia insoluble alkali fluoride byproduct. Hydrogen capacity and the purity can be improved by adding ammonia borane (NH3BH3). ZnCl2―8NaZn(BH4)3(NH3)2―20NH3BH3 mixture releases 10 wt% pure hydrogen, which is highest performance for Zn-B-N-H system. This performance comes at the cost of thermal stability which lower the dehydrogenation temperature too much making the mixture essentially unstable at room temperature. Therefore, we explored Mg-B-N-H system instead which have higher thermal stability. We observed that magnesium metal (or magnesium hydride) in liquid ammonia can react with ammonia borane to form magnesium amidoboranes Mg(NH2BH3)2(NH3)3 type compounds. This reaction even under 67 bar hydrogen pressure cannot be forced to form borohydride phases. Since magnesium amidoboranes is a type destabilized ammonia borane we continued our efforts for synthesis of Mg(BH4)2(NH3)6 phase. Synthesis of these compounds with MgF2 precursors were not possible; instead, the reaction of MgH2 with L·BH3 (L= triethylamine) in liquid ammonia was successful. The hydrogen storage properties of amorphous Li2Mg(BH4)4(NH3)2 was the most attractive with at least 10 wt% hydrogen capacity and high hydrogen purity.
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Parviz Hajiyev. Study of dehydrogenation properties of metallic borohydrides for hydrogen storage. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes [2020-..], 2020. English. ⟨NNT : 2020GRALY003⟩. ⟨tel-02939130⟩

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