Atomic-layer Al2O3 shells enable selective, air-stable MgH2 hydrogen storage

As hydrogen becomes a cornerstone of future clean-energy systems, practical solid-state storage must tolerate real, impure gas streams while retaining high capacity and fast kinetics. A research team led by Zhenyu Wang and Jinying Zhang (Xi’an Jiaotong University) reports a simple, effective solution: conformal, atomic-layer deposited amorphous Al₂O₃ shells (≈10 nm) on catalytically tuned MgH₂–ZrTi particles (MgH₂–ZrTi@Al₂O₃), which selectively admit H₂ but block common contaminants (CH₄, O₂, N₂, CO₂) and deliver robust cycling and air stability.

Why Al₂O₃ Encapsulation Matters

• Selective adsorption: The amorphous Al₂O₃ shell permits rapid H₂ permeation while preventing penetration or reaction of larger/more reactive species (CH₄, O₂, CO₂), enabling hydrogenation from impure gas mixtures.
• Air stability: Coated particles show no detectable MgO or Mg(OH)₂ after extended air exposure (months), a dramatic improvement over uncoated MgH₂.
• Kinetics & capacity balance: The 10 nm shell preserves fast kinetics and high usable capacity by working in concert with internal Zr/Ti catalysts and hydrogen channels.
• Mechanical/chemical robustness: Amorphous shells remain intact after multiple de/re-hydrogenation cycles, supporting long-term operation.

Innovative Design and Features

• Atomic-layer engineering: Ultrathin, conformal Al₂O₃ layers were grown by ALD directly on MgH₂–ZrTi particles, producing amorphous shells that are invisible in XRD but clear in TEM/elemental mapping.
• Synergistic core composition: MgH₂ is modified with dispersed ZrO₂ and few-layer Ti₃C₂ to create internal hydrogen channels and lower dehydrogenation temperatures (~185 °C onset for optimized composites).
• Mechanistic confirmation: MD simulations and experimental gas-uptake studies show H₂ uniquely permeates the Al₂O₃ layer while other gases are adsorbed or shallowly intercalated, explaining observed selectivity.
• Operating window: Selective hydrogen uptake is achieved at moderate temperatures (75–125 °C), making solar-thermal or low-grade-heat charging feasible.

Key Performance Highlights

• ~4.79 wt% H₂ absorbed at 75 °C within 3 h under 10% CH₄ + 90% H₂; ~4.0 wt% absorbed at 100 °C in O₂/CO₂-containing mixes.
• Excellent cycling: ≈96.9% capacity retention after 30 cycles under impure H₂; ~95.0% retention after 50 full de/re-hydrogenation cycles.
• No detectable oxidation products after months of air exposure for 10 nm coatings, while uncoated materials rapidly degrade.

Applications and Future Outlook

This ALD-encapsulation strategy opens a pathway to practical storage that accepts industrial/by-product hydrogen streams (e.g., coke-oven gas) without costly purification. Future work should explore scalable ALD workflows, alternative amorphous shell chemistries, and integration with system-level heat management for solar-assisted charging. This study presents a materials-level breakthrough toward deployable, selective, and air-tolerant hydrogen storage.