Enhancing hydrogen storage aspects and energy conversion efficiency in BaSiH6 via A@Ba-site substitution (A = Be, Mg, Ca, Sr, and Ra): first-principles and AIMD simulations.

Hydrogen is widely acknowledged as a highly effective potential solution to meet the steadily increasing demand for clean and environmentally friendly energy, where hydridosilicates emerge as an efficient hydrogen storage material. Here, the various physical aspects of BaSiH6 via A@Ba-site substitution (A = Be/Mg/Ca/Sr/Ra) are investigated using first-principles calculations. The stability of these systems was confirmed through formation enthalpy, with values ranging from -30.1 kJ mol-1 H2 (Be@Ba) to -49.1 kJ mol-1 H2 (Sr@Ba), where Ca@Ba (-42.4 kJ mol-1 H2) and the pristine system (-40.9 kJ mol-1 H2) have almost ideal values (-40.9 kJ mol-1 H2) for practical reversibility, indicating tunable thermodynamic stability for all structures. The estimated desorption temperatures (derived from formation enthalpies) for the pristine (292 K) and Mg@Ba/Ca@Ba/Ra@Ba-doped systems (235/303/250 K) fall within the desirable operating window of 233-333 K, which provides a comparative thermodynamic assessment of hydrogen-release behavior rather than exact decomposition temperatures. We observe that the hydrogen storage capacity (C wt%) is a direct function of the dopant's molar mass, with lighter elements significantly boosting C wt% to 5.64/5.26/4.92/4.12/3.53/2.8% for Be/Mg/Ca/Sr/Ba(pristine)/Ra-doped systems, while maintaining volumetric capacities of 64.98/65.58/65.35/62.09/57.91/57.16 gH2 per L. The structural integrity of all structures was rigorously validated through ab initio Molecular Dynamics Simulations at 300 K and mechanical stability checks via Born criteria. Electronic structure analysis reveals that the pristine material exhibits a bandgap (E g) of 2.69 eV, while Be@Ba significantly reduces it to 0.58 eV, while other dopants maintain moderate gaps in the range of 1.85-2.63 eV. Alongside this, the work function exhibits a non-monotonic enhancement from 5.71 eV (pristine) to 6.11 (Be@Ba), 6.36 (Mg@Ba), 6.38 (Sr@Ba), and 6.25 eV (Ra@Ba), while CA@Ba attains the maximum value of 6.45 eV, indicating that surface dipole strengthening is governed by an optimal interplay of dopant size. Ultimately, thermoelectric performance is significantly enhanced upon doping, with improved electrical conductivity and reduced lattice thermal conductivity, resulting in a maximum figure of merit of 0.75/0.85 at 800 K for the Ra@Ba/Ca@Ba-doped structures.