Graduation Year

2007

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Chemical Engineering

Major Professor

John T. Wolan, Ph.D.

Keywords

Hydrogen desorption, Lithium borohydride, Magnesium hydride, Dehydrogenation, Mechano-chemical process, Dopants

Abstract

The demands on Hydrogen fuel based technologies is ever increasing for substitution or replacing fossil fuel due to superior energy sustainability, national security and reduced greenhouse gas emissions. Currently, the polymer based proton exchange membrane fuel cell (PEMFC), is strongly considered for on-board hydrogen storage vehicles due to low temperature operation, efficiency and low environmental impact. However, the realization of PEMFC vehicles must overcome the portable hydrogen storage barrier. DOE and FreedomCAR technical hydrogen storage targets for the case of solid state hydrides are: (1) volumetric hydrogen density > 0.045 kgH2/L, (2) gravimetric hydrogen density > 6.0 wt%, (3) operating temperature < 150 degrees C, (4) lifetimes of 1000 cycles, and (5) a fast rate of H2 absorption and desorption. To meet these targets, we have focused on lithium borohydride systems; an alkali metal complex hydride with a high theoretical hydrogen capacity of 18 wt.%.

It has been shown by Vajo et al. that adding MgH2, improves the cycling capacity of LiBH4. The pressure-composition-isotherms of the destabilized LiBH4 + MgH2 system show an extended plateau pressure around 4-5 bars at 350 degrees C with a good cyclic stability. The mentioned destabilizing mechanism was successfully utilized to synthesize the complex hydride mixture LiBH4 + 1/2MgH2 + Xmol% ZnCl2 catalyst (X=2, 4, 6, 8 and 10) by ball milling process. The added ZnCl2 exhibited some mild catalytic activity which resulted in a decomposition temperature reduction to 270 degrees C. X-ray powder diffraction profiles exhibit LiCl peaks whose intensity increases proportionately with increasing ZnCl2 indicating an interaction between catalyst and hydride system, possibly affecting the total weight percent of desorbed hydrogen.

Thermal gravimetric analysis profiles for MgH2 + 5mol% nanoNi and LiBH4 + ZnCl2 + 3mol% nanoNi indicate that small concentrations of nano-nickel acts as an effective catalyst that reduces the mixture desorption temperature to around 225 degrees C and 88 degrees C, respectively. Future work will be focused on thermodynamic equilibrium studies (PCT) on the destabilized complex hydrides.

Download

Share

COinS