Icosahedral cluster formation in Ni-based hydrogen separation amorphous membranes and the effect of hydrogenation-a...

The demand for hydrogen is increasing due to commercialization of fuel cells. Palladium (Pd)-based crystalline membranes have been used for separation of hydrogen from a mixture of gases in coal-based power generation process. However, very high cost of Pd has prompted to explore inexpensive alternative alloys. Amorphous Ni-Nb-Zr alloy membranes are promising cheaper alternatives which exhibit comparable hydrogen permeability to Pd membranes at nominal temperature of ~ 400 °C. Constant exposure to high temperature and hydrogen pressure may lead to changes in the local atomic structure and possible devitrification of membrane. It is critical to understand short-range order of these membranes in order to improve their hydrogen permeability and durability. Icosahedral clusters are the building blocks of amorphous material and hydrogen is expected to interact with them in various different ways. The density functional theory-based molecular dynamics (DFT-MD) approach is the best suited approach to study the local atomic structures for (Ni_0.6Nb_0.4)₉₀Zr₁₀ and (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous membranes with the help of nearest neighbor distances and icosahedral cluster analysis. It can help predict the behavior of the membrane under extreme operating conditions. Three types of icosahedra (so called Ni-centered, Zr-centered, and Nb-centered) were identified in six different compositions in these amorphous alloys. Evolution of these icosahedra with temperature and in the presence of hydrogen gave an insight into the local structure of the membrane. Zr plays an important role in the formation of icosahedra. Hydrogen atoms interact with the icosahedra in three different ways. It is observed that H atoms did not show tendency to enter Ni-centered icosahedra leading to easier hydrogen diffusion outside the icosahedra. Hence, the more the number of Ni-centered icosahedra, the better the permeation properties of the alloy.