  By applying density functional theory (DFT) and ab-initio molecular dynamics
(AIMD) simulations, we predict the ultrahigh hydrogen storage capacity of K and
Ca decorated single-layer biphenylene sheet (BPS). We have kept various alkali
and alkali earth metals, including Na, Be, Mg, K, Ca, at different sites of BPS
and found that K and Ca atoms prefer to bind individually on the BPS instead of
forming clusters. It was found that 2x2x1 supercell of biphenylene sheet can
adsorb eight K, or eight Ca atoms, and each K or Ca atom can adsorb 5 H$_2$,
leading to 11.90 % or 11.63 % of hydrogen uptake, respectively, which is
significantly higher than the DOE-US demands of 6.5 %. The average adsorption
energy of H$_2$ for K and Ca decorated BPS is -0.24 eV and -0.33 eV,
respectively, in the suitable range for reversible H$_2$ storage. Hydrogen
molecules get polarized in the vicinity of ionized metal atoms hence get
attached to the metal atoms through electrostatic and van der Waals
interactions. We have estimated the desorption temperatures of H$_2$ and found
that the adsorbed H$_2$ can be utilized for reversible use. We have found that
a sufficient energy barrier of 2.52 eV exists for the movement of Ca atoms,
calculated using the climbing-image nudged elastic band (CI-NEB) method. This
energy barrier can prevent the clustering issue of Ca atoms. The solidity of K
and Ca decorated BPS structures were investigated using AIMD simulations.

Materials Science