Nanocrystalline Metal/Metal Hydrides for Fuel Cell Applications

Abstract

Formation of nanocrystalline films by low temperature deposition of metallic films was investigated with special emphasis for hydrogen storage. Large grain boundary volume associated with nanocrystalline films was shown to be favorable for hydrogen absorption and hydride formation in zirconium and titanium metals.

Nanocrystalline films of titanium and zirconium were deposited on porous alumina and silicon substrates via low temperature deposition (-50oC). Low substrate temperature was obtained by circulating nitrogen gas through the substrate holder. Nitrogen gas was cooled by passing through a coil that was submerged in liquid nitrogen. The films were charged with hydrogen via molecular and ionic charging. The films were characterized for microstructure, crystallinity, hydrogen capacity, and kinetics of hydrogen absorption and desorption using several techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy, secondary ion mass spectrometry (SIMS), and electrical resistance measurements. High resolution TEM was used to determine the volume fraction of amorphous region and grain size. The shift in the X-ray diffraction peaks with lattice parameter changes resulting from hydrogen incorporation in the lattice and the increase in half-peak-width with smaller grain size were determined. X-ray mapping, line scan, and energy dispersive spectrometry (EDS) to identify different elements present in the films were used along with secondary electron imaging in the SEM to determine morphology and composition. Optical microscopy was also employed at higher magnification to examine the morphology of the hydrides from the surface of the films. The relative hydrogen capacity in the hydrogen charged films compared to that in the films without hydrogen charging was determined by SIMS depth profiling analysis. Electrical resistance measurements as a function of temperature were instrumental in identifying the presence of hydrogen in the films. Annealing with varying temperature was carried out to determine the kinetics of desorption.

The results show that the films deposited at low temperature were much smaller in grain size than those deposited at room temperature. Hydrogen concentration was much greater in the films deposited at low temperature, and also, hydride concentration was much higher in the hydrogen plasma treated (ionic charged) films deposited at low temperature. The results clearly show that low temperature physical vapor deposition of nancrystalline films is a favorable method for zirconium and titanium alloys.