A critical review of hydrogen storage: toward the nanoconfinement of complex hydrides from the synthesis and characterization perspectives

To meet the growing global energy demand and keep our planet healthy, more than 10 terawatts of carbon-neutral energy will be required by 2050. H₂, which has an energy density of 33.33 kW h kg⁻¹, has been identified as a renewable and clean energy carrier to meet this energy demand and as a substitute for fossil fuels. H₂ storage is crucial for harnessing H₂ energy to its fullest potential and realizing the H₂ economy. Although compression and liquefaction are established H₂ storage techniques, safety concerns, energy consumption (up to 18 and 40% of H₂'s LHV for compression and liquefaction, respectively), and boil-off losses of up to 3% per day in liquefaction remain the main limitations. Researchers currently are exploring safe, compact, and efficient solid-state H₂ storage methods. Complex hydrides such as LiBH₄, NaBH₄, LiAlH₄, and NaAlH₄, which are formed by the coordination of complex anions such as [BH₄]⁻ and [AlH₄]⁻ stabilized by metal cations such as Na⁺, Li⁺, Mg²⁺, and Ca²⁺, are a class of solid-state H₂ storage materials with promising storage capacities. In principle, most of them are capable of meeting the ultimate volumetric (0.05 kg H₂ per L) and gravimetric (6.5 wt%) storage capacity goals set by the U.S. DoE. However, they suffer from unfavorable thermodynamics-T_des (150–600 °C), high desorption kinetic barrier-E_ades (50–275 kJ mol⁻¹), and limited reversibility. One intriguing approach to address these limitations is nanoconfinement in suitable host materials, benefiting from the synergetic effects of nanosizing, immobilization, destabilization, and, sometimes, catalysis for scaffolds that mutually induce catalytic effects. In this review, major H₂ storage techniques are briefly discussed. Developments in the nanoconfinement of complex hydrides, host materials, synthetic methods, characterizations, and advances in improving kinetics, thermodynamics, and reversibility via nanoconfinement are discussed. This paves the way for the use of hydrides in practical H₂ economy technologies, and contributes to the advancement of clean energy solutions.