  Expanding pollution and rapid consumption of natural reservoirs (gas, oil,
and coal) led humankind to explore alternative energy fuels like hydrogen fuel.
Solid-state hydrogen storage is most desirable because of its usefulness in the
onboard vehicle. In this work, we explored the yttrium decorated ultra porous,
two-dimensional holey-graphyne for hydrogen storage. Using the first principles
DFT simulations, we predict that yttrium doped holey graphyne can adsorb up to
seven hydrogen molecules per yttrium atom resulting in a gravimetric hydrogen
weight percentage of 9.34, higher than the target of 6.5 wt% set by the US
Department of Energy (DoE). The average binding energy per H$_2$ and desorption
temperature come out to be -0.34 eV and ~ 438 K, respectively. Yttrium atom is
bonded strongly on HGY sheet due to charge transfer from Y 4d orbital to C 2p
orbital whereas the adsorption of H$_2$ molecule on Y is due to Kubas-type
interactions involving charge donation from H 1s orbital to Y 3d orbital and
back donation with net charge gain by H 1s orbital. Furthermore, sufficient
energy barriers for metal atom diffusion have been found to prevent the
clustering of transition metal (yttrium) on the HGY sheet. The stability of the
system at higher temperatures is analyzed using Ab-initio molecular dynamics
(AIMD) method and the system is found to be stable at room and the highest
desorption temperature. Stability of the system at higher temperatures,
presence of adequate diffusion energy barrier to prevent metal-metal
clustering, high gravimetric wt% of H$_2$ uptake with suitable binding energy,
and desorption temperature signifies that Y-doped HGY is a promising material
to fabricate high capacity hydrogen storage devices.
