• Login
• Admin

Prosser, J.L. (2024) High capacity metal and mixed metal borohydride ammoniates for hydrogen energy storage applications. PhD thesis, University of Nottingham.

PDF (Corrections) (Thesis - as examined) - Repository staff only until 18 July 2026. Subsequently available to Repository staff only - Requires a PDF viewer such as GSview, Xpdf or Adobe Acrobat Reader Available under Licence Creative Commons Attribution. Download (6MB)

Abstract

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.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Walker, G.S. Grant, D.M. Manickam, K.
Keywords:	Metal borohydride ammoniates; Hydrogen storage
Subjects:	T Technology > TP Chemical technology
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID:	77490
Depositing User:	Prosser, Jacob
Date Deposited:	19 Jul 2024 10:24
Last Modified:	19 Jul 2024 10:24
URI:	https://eprints.nottingham.ac.uk/id/eprint/77490

Actions (Archive Staff Only)

Edit View