Computational Workflow for Investigating Hydrogen Permeation in Novel Hydrogen Storage Materials

The United States Department of Energy (DOE) has set ambitious targets for hydrogen storage materials for onboard light-duty cars which are to be achieved by 2027. One of the major problems in solid hydrogen storage materials is the sluggish uptake/release kinetics. Much attention has been focused on understanding kinetics. Hydrogen solid-state diffusion is a rate-controlling step in the majority of metal hydrides. Here we discuss computational workflow which can be used to estimate hydrogen diffusivity. A detailed study of hydrogen concentration, hydrogen neighbors is performed on nickel hydride (NiH) fcc materials to understand their effect on H diffusion. The nudged elastic band (NEB) method is used to determine hydrogen diffusion barrier with various hydrogen concentrations in presence of hydrogen neighbors. The energy barriers for hydrogen hopping were calculated for few million different configurations with various local chemical environments. Two paths for H hopping from one octahedral site to a vacant neighbor octahedral site are identified: one path is straight, and the other is curved via tetrahedral site. The curved path shows diffusion faster than the straight path. This study demonstrates that H diffusion is faster at higher hydrogen concentration, as the concomitant volume expansion lowers the energy barrier.