First-principles investigation of structural, optoelectronic, mechanical, thermodynamic and hydrogen storage properties of Si-based XCsSiH6 (X = K, Rb) hydrides

Abstract

First-principles density functional theory (DFT) calculations are employed to investigate the structural, optoelectronic, mechanical, thermodynamic, and hydrogen storage properties of XCsSiH6 (X = K, Rb). The hydrogen atoms form discrete SiH6 octahedra stabilized by alkali metal cations, confirming a stable cubic symmetry upon structural optimization. Both compounds exhibit both thermal and dynamical stability. The calculated gravimetric hydrogen storage capacities are 2.94wt% for KCsSiH6 and 2.40wt% for RbCsSiH6. Electronic band structure analysis indicates indirect band gaps of 3.14 eV (KCsSiH6) and 3.17 eV (RbCsSiH6), with hydrogen contributing mainly to the valence band maximum (VBM) and Cs/Rb/K atoms to the conduction band minimum (CBM). Optical property analyses of the dielectric response, absorption, and reflectivity show pronounced ultraviolet activity, suggesting suitability for optoelectronic and UV-filtering applications. Both hydrides are found to be brittle yet elastically isotropic, with KCsSiH6 being slightly less stiff than RbCsSiH6. Comprehensive thermodynamic analysis demonstrates favorable variations in entropy, heat capacity, and Gibbs free energy up to 800 K, indicating the pronounced thermal stability of the investigated systems. Overall, XCsSiH6 hydrides appear to be stable semiconductors with moderate hydrogen storage capacity and desirable optical properties, suitable for various energy and electronic applications.

1. Introduction

The growing demand for clean, renewable, and sustainable energy worldwide has positioned hydrogen (H2) as one of the most promising energy carriers. Its appeal lies in several key advantages: clean combustion that produces only water, a high gravimetric energy density of 120 MJ/kg, and the potential for large-scale production from renewable sources such as solar and wind energy [1–2]. As global efforts intensify toward decarbonization and energy security, hydrogen is expected to play a pivotal role in future energy systems, particularly in transportation, industrial processes, and grid-scale storage [3–4]. Its versatility further enhances its value, as hydrogen can serve as a feedstock for the chemical industry, a fuel for fuel-cell vehicles (FCVs), and a medium for storing excess renewable electricity through power-to-gas technologies [5–6]. Moreover, hydrogen enables sector coupling by effectively linking heating, mobility, and electricity systems for more integrated and efficient energy use [7]. The International Energy Agency (IEA) projects that, with continued advances in electrolysis and storage technologies, hydrogen could supply up to 18% of global final energy demand by 2050 [4]. Reflecting this potential, many countries across the Middle East, Asia, and Europe are actively investing in green hydrogen production and developing hydrogen roadmaps.

Despite this encouraging outlook, large-scale adoption of hydrogen faces several challenges. High production costs, limited infrastructure, and—most critically—technological barriers in hydrogen storage and transportation remain major obstacles [8–9]. To address these limitations, extensive research has focused on materials-based storage methods, which provide portable, efficient, and safe alternatives suitable for both stationary and mobile applications [10–12].