First-Principles Study of Structural, Electronic, Elastic, and Thermoelectric Properties of XMoH3 (X = Na, K, Rb) for Sustainable Hydrogen Storage Applications

The transition toward a sustainable hydrogen economy requires the development of advanced materials capable of efficient hydrogen storage and energy conversion. In this work, we present a comprehensive first-principles investigation of the structural, electronic, elastic, and thermoelectric properties of cubic perovskite hydrides XMoH3 (X = Na, K, and Rb) using the density functional theory within the generalized gradient approximation combined with the Boltzmann transport theory. The calculated gravimetric hydrogen storage capacities are 2.48 wt%, 2.19 wt%, and 1.64 wt% for NaMoH3, KMoH3, and RbMoH3, respectively, indicating moderate storage potential. Elastic analysis confirms mechanical stability and reveals predominantly brittle-to-intermediate behavior with mixed bonding characteristics. Electronic band structures and density of states demonstrate metallic conductivity, driven mainly by Mo-d orbital contributions near the Fermi level, which may facilitate charge transport and hydrogen mobility. Thermoelectric analysis shows temperature-dependent electrical and thermal conductivities, with KMoH3 and NaMoH3 exhibiting relatively higher power factors at elevated temperatures, although the overall figure of merit (ZT < 0.3) remains below the threshold for high-performance thermoelectric applications. Despite these limitations, the combined properties of structural stability, metallic conductivity, and moderate hydrogen storage capacity highlight the potential of XMoH3 compounds as multifunctional materials for integrated hydrogen storage and thermal energy recovery systems. This study provides fundamental insights into the design of perovskite hydrides and underscores their relevance as tunable platforms for future sustainable energy technologies.