Li, Ping
(2013)
Phase Behavior and Reaction Mechanism Study of Hydrogen Storage in Sodium Borohydride.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Complex hydrides are promising hydrogen storage materials and have received significant attention due to their high hydrogen storage capacity. NaBH4 has high gravimetric (10.6 wt%) and volumetric (115 kg of H2 per cubic meter) hydrogen content. The NaBH4 crystal structure, bulk and surface properties were determined from first-principles density functional theory (DFT) calculations. The surface of the NaBH4 crystal is hygroscopic and adsorption of water vapor results in deliquescence. Interactions between H2O and NaBH4(100), which is the lowest energy surface, were investigated using van der Waals-corrected DFT. The results indicate that the adsorption energy per water molecule is roughly independent of water coverage, which suggests that a thick film of water can form on the surface of NaBH4 without enthalpy penalty, and explains the hygroscopic nature of NaBH4 at a molecular level. The initial stage of deliquescence was simulated via ab initio molecular dynamics (AIMD) at finite temperature and the first four significant steps of deliquescence were identified. These are the firstly atomically detailed studies of the hygroscopicity and deliquescence of NaBH4.
Hydrolysis of NaBH4 is of importance for niche hydrogen storage applications, such as portable power generation. Hydrolysis of NaBH4 produces hydrogen gas and hydrated sodium metaborate, NaBO2•nHO, when n depends on the hydrolysis conditions. One of the major challenges in practical use of NaBH4 hydrolysis is the minimization of the degree of hydration of NaBO2•nHO. Accordingly, DFT calculations on sodium metaborate with variying degrees ofhydration were carried out and the vibrational frequencies of solids for n=1/3 and n=2 have been calculated. The calculated frequencies are used to identify the vibrational modes observed in experiments and characterize the possible hydrolysis products. The most likely elementary reaction steps for aqueous NaBH4 hydrolysis have been identified from DFT calculations and the corresponding energy barriers computed via the climbing image nudged elastic band (cNEB) method. The literature contains conflicting reports on the existence of BH4-x(OH)x- (x=1,2,3) intermediates, with some reports claiming to have observed x=2 and x=3 species, and other researchers claiming the only observe x=4 and trace amounts of x=1. Calculations reported in this work confirm the existence of intermediates for x=1,2,3. A new reaction pathway has been identified that involve intramolecular rearrangement of BH3-OH2 and BH2OH-OH2 to produce H2. Reactions pathways involving two hydroxyborates were identified. It was found that extensive proton shuttling and solution rearrangement are naturally involved in the individual reaction steps for NaBH4 hydrolysis. The cNEB calculations reveal that reactions between BH4-x(OH)x- (x=0,1,2,3) and H2O have similar energy barriers to the initial step; therefore, the concept of a rate-limiting step cannot be applied to NaBH4 hydrolysis.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
25 September 2013 |
| Date Type: |
Publication |
| Defense Date: |
3 June 2013 |
| Approval Date: |
25 September 2013 |
| Submission Date: |
5 June 2013 |
| Access Restriction: |
5 year -- Restrict access to University of Pittsburgh for a period of 5 years. |
| Number of Pages: |
113 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Chemical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
hygroscopic, adsorption, hydrolysis, proton transfer |
| Date Deposited: |
25 Sep 2013 13:14 |
| Last Modified: |
25 Sep 2018 05:15 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/18878 |
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