Date of Award

2010

Document Type

Campus Access Dissertation

Department

Chemical Engineering

First Advisor

Michael A. Matthews

Abstract

One of the major challenges for fuel cells and the hydrogen economy is technology to store and release hydrogen on demand. Chemical hydrides such as sodium borohydride (NaBH4), release hydrogen via exothermic reaction with water as follows: NaBH4 + (2+x) H2O → 4H2 + NaBO2·xH2O + heat. The gravimetric and volumetric capacities of NaBH4 combined with its relatively low cost make it a prominent near-term candidate for hydrogen storage. In addition to hydrogen, hydrated sodium metaborate (NaBO2·xH2O), is formed as a byproduct. The term x represents the excess water of hydration in the sodium borate crystal. A value of x greater than zero corresponds to unused water, which incurs a gravimetric and volumetric penalty on the storage system. Thus, minimizing "x" is crucial for maximum hydrogen storage efficiency.

Our novel approach is to expose solid chemical hydrides to steam or water vapor rather than liquid water. We have demonstrated essentially 100% hydrogen yield without the need for a catalyst, excess water of solution, or addition of caustic. In order to develop a hydrogen storage system based on this method, it is crucial to understand the mechanism of the steam/solid reaction and the formation of the borate byproduct. Therefore, this dissertation investigates the mechanism and physical phenomena associated with the water vapor hydrolysis pathway. A key finding of this work is that the solid NaBH4 powder absorbs water from the vapor phase until it is completely dissolved, resulting in a viscous solution where reaction to release hydrogen can then occur. This crucial step in the reaction sequence is deliquescence, which depends on temperature and relative humidity. Water uptake was monitored as a function of conditions using dynamic vapor sorption, and increasing temperature resulted in an increased water uptake rate between 25 and 80oC. Furthermore, dehydration of the hydrated sodium metaborate byproduct has been characterized from 25 to 400oC. Three stable phases (x=2, 1/3 and 0) and their transition temperatures were observed using Raman spectroscopy, TGA/DSC and XRD. In-situ Raman spectroscopy of the NaBH4 reaction was also explored to probe the reaction pathway. Knowledge of the reaction mechanism and the effect of reaction conditions on the pathway is required for the successful design a hydrogen storage system based on water vapor hydrolysis of sodium borohydride.

Rights

© 2010, Amy Mathis Beaird

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