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A First Principles Study of Mass Transport in the Dehydrogenation of Lithium Amides and Lithium Alanates

  • Author(s): Rolih, Biljana
  • Advisor(s): Ozolins, Vidvuds
  • et al.
Abstract

The pursuit of competitive alternatives to energy derived from the combustion of fossil fuels, has led to a great variety of new technologies. Exceptional develop- ments in electrochemical storage and production promise to lead to clean burning passenger vehicles. The high chemical density of a hydrogen fuel cell enables it to meet current standards for driving range and weight required of vehicles, making it an excellent candidate for universal application in the automotive industry. One of the biggest obstacles the fuel cell industry has yet to overcome is the means of practical hydrogen storage. Solid state metal hydrides are a class of materials that show potential for both economic and practical hydrogen storage. The search for the ideal metal hydride is defined by thermodynamic and kinetic constraints, since the requirements for a viable system are a rapid release of hydrogen in the temperature range of -40C, to 80C.

First-principles density functional theory is an excellent method for gaining insight into the kinetics and thermodynamics of metal hydride solid state reactions. In the work presented here, density functional theory is used to explore formation energies, concentrations and migration barriers of metal hydrides. In particular, the following systems were analyzed:

* Li - N - H It is well known that the reactive hydride composite LiNH2 + LiH reversibly releases a large amount of hydrogen gas, with more favorable thermodynamics than LiNH2 alone. Kinetics of mass transport during the dehydrogenation of LiNH2 + LiH are investigated. A model is developed for determining activation energies of native defects in bulk crystals. In order to establish whether mass transport is the rate-limiting step in the dehydrogenation reaction, results are compared to experimental values.

* Li - Al - H Kinetics of mass transport during the dehydrogenation of the metal hydride LiAlH2 are investigated. It is known that LiAlH4 endothermi- cally decomposes via a two step reaction. The kinetics of both steps in the reactions are studied. Results are compared to experiments in order to de- termine whether mass transport is the rate-limiting process in the reactions.

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