High capacity metal and mixed metal borohydride ammoniates for hydrogen energy storage applications

This thesis explores the promising class of metal borohydride ammoniates and mixed metal borohydride ammoniates (MBA and MMBAs) for hydrogen storage applications. Although possessing high hydrogen (H2) gravimetric capacities (7 ≤ wt.% ≤ 18) and low dehydrogenation temperature (< 200 ◦C), the H2 release is often coupled with the release of ammonia (NH3). Suppressing the NH3 release is vital for applications requiring high purity H2. In order to direct work to explore new MBA/MMBAs that preferentially release H2, the gas evolution characteristics were compared with a variety of properties of known MBA/MMBAs. It is observed that a decreasing ionic radii / increasing charge density of the metal cation of the MBA/MMBA results in preferential H2 release. If the metal cation has an ionic radii < 0.61 Å and/or charge density value ≥ 200 C mm-3 , the MBA/MMBA is shown to release H2 preferentially in comparison to NH3. The atom interaction analysis conducted on the B-H bond length and intramolecular B-N distance of MBAs with a range of NH3 ligands (Ce, La, Mn and Y) indicated that a lower number of NH3 ligands results in shorter B-H bond lengths, B-N distances and a preference to release H2. It is suggested that the shorter B-H bond length leads to a more negatively charge hydridic atom (Hδ- ) in the BH4 unit, which is more attracted to the protic atom (Hδ+) in the NH3 ligand and a shorter B-N distance allows longer duration of the Hδ+ and Hδ- being in close proximity. Both would aid the dehydrogenation reaction; H2NHδ+... δ-HBH3. The rationale of studying MBA/MMBAs with a high electronegative and/or charge density metal cation came from the analysis of known materials. Novel MBA/MMBA composites (MClx.nNH3 - M′BH4) with high electronegativity (χp ≥ 1.88) and charge density (≥ 491 C mm-3) metal cations (M = Co, Ni, Mo and W) were synthesised using mechanochemistry. 4.21 wt.% of H2 was released exothermically with a peak Tdehyd of 166 ◦C for WCl6.8NH3-LiBH4 (1:6). NiCl2.6NH3-LiBH4 (1:2), however, releases H2 endothermically at low temperatures (ca. 106 ◦C). This is the first for any known MBA and indicates a possibility of reversible (de)hydrogenation. NH3 coordinating metal chlorides (CoCl2, CrCl3 and NiCl2) with highly electronegative metal cations have greatly improved the H2 storage characteristics of LiBH4.NH3. The peak Tdehyd has been reduced from ca. 450 ◦C for LiBH4.NH3 to 55 ◦C for LiBH4.NH3-CoCl2 (3:1). In addition, NH3 release can be completely suppressed from 32.41 wt.% for LiBH4.NH3 down to 0 wt.% for LiBH4.NH3-NiCl2 (2:1). This work has identified that varying the properties (electronegativity and charge density) of the metal cation within MBA/MMBAs and LiBH4.NH3-MClx composites can influence the gas evolution characteristics and enable tuning of the material to a specific application

High capacity metal and mixed metal borohydride ammoniates for hydrogen energy storage applications

This thesis explores the promising class of metal borohydride ammoniates and mixed metal borohydride ammoniates (MBA and MMBAs) for hydrogen storage applications. Although possessing high hydrogen (H2) gravimetric capacities (7 ≤ wt.% ≤ 18) and low dehydrogenation temperature (< 200 ◦C), the H2 release is often coupled with the release of ammonia (NH3). Suppressing the NH3 release is vital for applications requiring high purity H2. In order to direct work to explore new MBA/MMBAs that preferentially release H2, the gas evolution characteristics were compared with a variety of properties of known MBA/MMBAs. It is observed that a decreasing ionic radii / increasing charge density of the metal cation of the MBA/MMBA results in preferential H2 release. If the metal cation has an ionic radii < 0.61 Å and/or charge density value ≥ 200 C mm-3 , the MBA/MMBA is shown to release H2 preferentially in comparison to NH3. The atom interaction analysis conducted on the B-H bond length and intramolecular B-N distance of MBAs with a range of NH3 ligands (Ce, La, Mn and Y) indicated that a lower number of NH3 ligands results in shorter B-H bond lengths, B-N distances and a preference to release H2. It is suggested that the shorter B-H bond length leads to a more negatively charge hydridic atom (Hδ- ) in the BH4 unit, which is more attracted to the protic atom (Hδ+) in the NH3 ligand and a shorter B-N distance allows longer duration of the Hδ+ and Hδ- being in close proximity. Both would aid the dehydrogenation reaction; H2NHδ+... δ-HBH3. The rationale of studying MBA/MMBAs with a high electronegative and/or charge density metal cation came from the analysis of known materials. Novel MBA/MMBA composites (MClx.nNH3 - M′BH4) with high electronegativity (χp ≥ 1.88) and charge density (≥ 491 C mm-3) metal cations (M = Co, Ni, Mo and W) were synthesised using mechanochemistry. 4.21 wt.% of H2 was released exothermically with a peak Tdehyd of 166 ◦C for WCl6.8NH3-LiBH4 (1:6). NiCl2.6NH3-LiBH4 (1:2), however, releases H2 endothermically at low temperatures (ca. 106 ◦C). This is the first for any known MBA and indicates a possibility of reversible (de)hydrogenation. NH3 coordinating metal chlorides (CoCl2, CrCl3 and NiCl2) with highly electronegative metal cations have greatly improved the H2 storage characteristics of LiBH4.NH3. The peak Tdehyd has been reduced from ca. 450 ◦C for LiBH4.NH3 to 55 ◦C for LiBH4.NH3-CoCl2 (3:1). In addition, NH3 release can be completely suppressed from 32.41 wt.% for LiBH4.NH3 down to 0 wt.% for LiBH4.NH3-NiCl2 (2:1). This work has identified that varying the properties (electronegativity and charge density) of the metal cation within MBA/MMBAs and LiBH4.NH3-MClx composites can influence the gas evolution characteristics and enable tuning of the material to a specific application

Alpine compressional tectonics in the Southern Alps. Relationships with the N-Apennines

The southern Alps orogenic development has been produced by non-cylindrical accretion of arcuated thrust belts. The western-northern sector of the chain is occupied by the Orobic Arc, an Eoalpine to Mesoalpine structural system including the Grigna and Presolana overthrusts. The radiometric age of the dykes cutting the overthrusting contacts suggests Late Cretaceous as the age of the principal tectonic emplacement. Apart from the Dinaridic influence in the eastern sector, the remaining parts of the Southern Alps were deformed mainly by the Oligo-Miocene, Miocene and Pio-Pleistocene events. The Chattian to Burdigalian tectonic phase (mainly Aquitanian in age) produced the backthrust system during the Gonfolite l.s. sedimentation. This WNW-trending compressions] belt predominates in the Po Plain subsurface between the Piedmont and Lombardy regions. Prominent tectonic structures belonging to this system also outcrop in eastern Lombardy. The structures of \uabDinaric phase\ubb in the Dolomites may be referred mainly to this event; in this picture the Mt. Parei conglomerates could be an equivalent of the Lombardian Gonfolite. The Middle-Late Miocene compressional events produced widespread deformation in the central and eastern regions of the Southern Alps with ENE-trending prevailing structures, including the NNE trending thrusts of the Giudicarie region. The Messinian to Plo-Pleistocene compressional structures mainly affected the eastern regions of the Southern Alps along the southernmost border in the Friuli external arc. This last belt is probably kinematically linked to the buried frontal Apennine chain. A comparison between the neoalpine tectonic accretion of the Southern Alps and the Northern Apennines is attempted in the frame of the geodynamic evolution of the Western Mediterranean domains