A critical review of Rb-based cubic hydride perovskites for hydrogen storage: Insights from first-principles studies

Hydrogen is a clean and sustainable energy carrier, but safe and efficient storage remains a major challenge for a hydrogen-based economy. Perovskite-type hydrides (ABH₃) are promising solid-state storage materials due to their structural flexibility, favourable thermodynamics, and tunable properties. This review systematically analyses the structural, electronic, optical, mechanical, and hydrogen-storage characteristics of perovskite hydrides, with a particular focus on RbSH₃ and RbSeH₃ as representative case studies derived from recent first-principles (DFT) investigations. From a computational standpoint, the most critical properties for hydrogen storage are the gravimetric hydrogen capacity, mechanical ductility (to accommodate volumetric changes during cycling), and metallic electronic conductivity (which promotes rapid charge transfer and favourable kinetics). The DFT data reveal that RbSH₃ and RbSeH₃ exhibit promising combinations of these attributes: calculated gravimetric capacities of 2.51 wt% and 1.81 wt%, respectively, mechanical ductility (Poisson's ratios of 0.33–0.37 and B/G > 1.75), and metallic behaviour arising from a finite density of states at the Fermi level. However, these values fall considerably short of the U.S. Department of Energy targets for on-board vehicular storage (5.5 wt%), and their optical absorption in the UV–visible range—though potentially useful for photo-assisted systems—remains speculative pending experimental validation. Critically, no experimental synthesis, cycling-stability assessment, or hydrogen-absorption/desorption kinetics has yet been reported for RbSH₃ or RbSeH₃, representing the most significant barrier to practical application. Moreover, rubidium is a relatively scarce and costly alkali metal compared with sodium or potassium, and the synthesis of these hydride perovskites requires high-pressure hydrogenation (50–200 bar) and strictly controlled inert atmospheres, posing additional economic and scalability challenges. Consequently, while RbSH₃ and RbSeH₃ emerge as computationally promising candidates—particularly for stationary storage and photo-assisted systems where weight constraints are relaxed and functional benefits may offset capacity limitations—their real-world viability hinges entirely on future experimental synthesis, kinetic characterisation, and cycling-stability tests. This review consolidates current computational understanding, identifies critical research gaps, and provides guidance for designing next-generation perovskite-based hydrogen storage materials, with an emphasis on bridging the theory–experiment divide.