  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 dis-cuss computational workflow
which can be used to estimate hydrogen diffusivity. A de-tailed study of
hydrogen concentration, hydrogen neighbors is performed on nickel hy-dride
(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 hydro-gen 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 H diffusion is faster at higher hydrogen concentration, as the
concomitant volume expansion lowers the energy barrier.
