Hydrogen storage and optoelectronic properties of lithium-based Li2BH6 (B = Pt, Pd, Ni) free-lead perovskite hydrides: A first-principles investigation

Hydride perovskites have emerged as promising materials for solid-state hydrogen storage due to their structural flexibility and high hydrogen content. In this work, we investigate the effect of substituting Pt with Pd and Ni in the well-known cubic perovskite ${\mathrm{Li}}_2{\mathrm{PtH}}_6$, forming a series of ${\mathrm{Li}}_2{\mathrm{BH}}_6$ (B = Pt, Pd, Ni) perovskite-type hydrides. Using first-principles density functional theory calculations, we examine their structural, thermodynamic, dynamic, mechanical, hydrogen storage, and optoelectronic properties. Our results confirm that all three compounds are structurally stable and satisfy thermodynamic, dynamic, and mechanical stability criteria. They exhibit high volumetric hydrogen capacities exceeding the DOE target of 40 g H${}_2$/L, with gravimetric capacities increasing from 2.8 wt% in ${\mathrm{Li}}_2{\mathrm{PtH}}_6$ to 7.7 wt% in ${\mathrm{Li}}_2{\mathrm{NiH}}_6$. Moreover, the hydrogen desorption temperatures decrease significantly, from 816 K in ${\mathrm{Li}}_2{\mathrm{PtH}}_6$ to 467 K in ${\mathrm{Li}}_2{\mathrm{NiH}}_6$ and 424 K in ${\mathrm{Li}}_2{\mathrm{PdH}}_6$, indicating enhanced release performance suitable for moderate-temperature polymer electrolyte membrane (PEM) fuel cell hydrogen storage. Electronic structure analysis reveals that all compounds are wide-bandgap semiconductors, making them promising for optoelectronic and energy conversion applications. Optical properties show strong absorption ( $10^6$ cm⁻¹) across the UV–Vis spectrum, particularly in ${\mathrm{Li}}_2{\mathrm{PdH}}_6$ and ${\mathrm{Li}}_2{\mathrm{NiH}}_6$, with absorption edges shifting toward the visible region. These findings position ${\mathrm{Li}}_2{\mathrm{BH}}_6$ perovskites as lead-free, mechanically robust candidates with promising dual functionality for hydrogen storage and optoelectronic applications.