Hydrogen solubility and diffusion in amorphous and crystalline metal hydride thin film electrodes

Many transition metals and alloys are capable of reversibly absorbing large amount of hydrogen. This feature made them very attractive as hydrogen storage and sensing materials. This thesis focused on the hydrogen diffusion and solubility in La-Ni and Pd thin films with different structures.; La-Ni and Pd Thin films were fabricated by physical vapor deposition in ultrahigh vacuum. The structure of the thin films is characterized using RHEED, XRD, and TEM. Thin films were also examined by EPMA, and XPS depth profiling for composition, thickness, impurity level, and uniformity. The reversible hydrogen storage capacity and hydrogen diffusion coefficient in these films were measured by electrochemical cycling and electrochemical stripping methods. The hydride forming kinetics during hydrogen charge/discharge cycling was further studied using the quartz crystal microbalance (QCMB) technique.; The hydrogen diffusion coefficient D{dollar}sb{lcub}rm H{rcub}{dollar} in Pd thin films at temperatures ranging from 280 to 330 K was found to be two to three orders of magnitude smaller than that in the Pd bulk. The D{dollar}sb{lcub}rm H{rcub}{dollar} vs. 1/T plots demonstrate that the D{dollar}sb{lcub}rm H{rcub}{dollar} of thin films with different thickness (220A to 1400A) follow the Arrhenius behavior D{dollar}sb{lcub}rm H{rcub}{dollar} = D{dollar}sb0{dollar} exp({dollar}-{dollar}E{dollar}sb{lcub}rm a{rcub}{dollar}/RT) with approximately the same activation energy E{dollar}sb{lcub}rm a{rcub}{dollar} and a decreasing prefactor D{dollar}sb0{dollar} as the film thickness decreases.; The maximum hydrogen discharge capacity of amorphous LaNi{dollar}sb{lcub}rm X{rcub}{dollar} (2 {dollar}<{dollar} x {dollar}<{dollar} 5) thin films was found to be about 160 mAh/g (H/M {dollar}sim{dollar} 0.5). These films can also cycled for more than 500 cycles before their discharge capacity decreases to half of the maximum value. The hydrogen storage capacity and cycle life of these films was determined to be 3 times better than that reported in the literature. The chemical potential vs. the H/M curves demonstrate that the hydrogen is most likely stored in two tetrahedral sites: La-Ni{dollar}sb3{dollar} and La{dollar}sb2{dollar}-Ni{dollar}sb2{dollar}, instead of just the La-Ni{dollar}sb3{dollar} site found in LaNi{dollar}sb5{dollar} crystalline material. The H/M ratio can also be calculated semi-quantitatively using the Harris model with two types of sites.; The maximum H/M of crystalline LaNi{dollar}sb5{dollar} thin films is about 0.6. A plateau was observed in the potential vs. discharge time curve for crystalline LaNi{dollar}sb5{dollar} films as thin as 600A. However, there is no plateau for amorphous films. The plateau, observed during the first several charge/discharge cycles of the 600 A-thick crystalline film, disappeared after further cycling. This observation indicates a possible phase transformation from crystalline to amorphous.; Amorphous LaNi{dollar}sb5{dollar} thin films have also been deposited simultaneously onto AT and BT-cut quartz crystals to study the hydride forming mechanism during charge/discharge cycling. The quartz crystal microbalance setup operated successfully under 6 M KOH solutions. The resonant frequency changes due to the mass loading and stress in the films upon the absorption of hydrogen can be separated by AT-cut and BT-cut crystals. Film oxidation and reduction has been observed during discharge and charge periods. Further studies are required to elucidate the oxide reduction mechanism.