Nano-Columnar Mg Thin Films Created by Plasma Sputter Deposition for Improved Hydrogen Kinetics and Storage Properties

摘要

The hydrogen storage properties of nanostructured Mg and MgH_2 thin films were studied as created by Ar and Ar+H_2 plasma sputter deposition. Columnar structures with typical widths of ～120 nm are observed with their long columnar axis extending throughout the thickness of the films. Applying substrate bias voltages during deposition results in narrower columns. A concomitant reduction in hydrogen desorption temperature from 400℃ to 360℃ is observed. Capping the Mg films with a ～100 nm thin Pd layer leads to significantly reduced hydrogen desorption temperatures of ～200℃ induced by the catalytic activity of the Pd cap layer. Also, hydrogen permeation of the films is strongly improved. The rate-determining factor is found to be the dissociation of the hydrogen molecules. Optimum hydrogen loading conditions of the Pd/Mg films were obtained just above ～200℃ at hydrogen pressures of 0.25-1.0 MPa, resulting in hydrogen storage capacities in the range of 4-7 wt%.