Hydrogen Absorption in Nanocrystalline Cubic Ti-Based Compounds

Hydrogen absorption in a series of compounds having the same crystallographic structure but different chemical compositions was studied. To this end, high-energy ball milling was used to prepare cubic nanocrystalline Ti-based compounds of general formula Ti-Ru-M, with M = Rh, Ru, Pd, V, Ag, Al, Cr, Fe, and Ni. After extensive milling, the most important phase (> 95 wt %) has a simple cubic structure (CsCl). Hydrogen absorption in these compounds was assessed by potential step and current efficiency measurements, as well as by in situ electrochemical X-ray diffraction experiments. In the case of potential step experiments, severe degradation of nanocrystalline Ti-Ru-V, Ti-Ru-Cr, and Ti-Ru-Ni electrodes was observed. This is because these measurements were obtained after a succession of hydrogen absorption/hydrogen desorption cycles, that emphasize the mechanism responsible for the degradation of the electrodes. This is not the case with current efficiency measurements, and an estimation of the amount of H atom absorbed was obtained for all electrode materials. It was shown that as much as 0.7 H atom per unit cell can be absorbed for M = V. In situ electrochemical X-ray diffraction patterns were recorded while the electrodes were negatively polarized. The change of the lattice parameter, Delta a, and the change of the volume of the unit cell, Delta V, were measured as a function of M. For M belonging to the fifth period, it was shown that the nature of M does not have any marked influence on the stability of the electrode, all nanocrystalline Ti-Ru-M materials absorbing only small amount of hydrogen. This assertion does not hold true in the case of M coming from the fourth period. In that case, the amount of hydrogen absorbed is correlated to the unit cell volume of the material.