High-entropy hydrides (HEHs) have emerged as a transformative class of materials for hydrogen storage, leveraging the synergistic effects of multiple principal elements to achieve unprecedented thermodynamic and kinetic properties. Recent studies have demonstrated that HEHs such as (TiZrNbHfTa)H2 exhibit hydrogen storage capacities exceeding 2.5 wt% at ambient temperatures, with desorption enthalpies ranging from 30 to 50 kJ/mol H2, making them ideal for near-room-temperature applications. Advanced computational modeling, including density functional theory (DFT) and machine learning algorithms, has revealed that the configurational entropy in these systems stabilizes the hydride phase, reducing the energy barriers for hydrogen absorption and desorption. Experimental validation using in-situ neutron diffraction and X-ray absorption spectroscopy has confirmed reversible hydrogen cycling over 1,000 cycles with less than 10% capacity loss, highlighting their robustness.
The tunability of HEHs allows for precise engineering of their hydrogen storage properties by adjusting elemental compositions and atomic arrangements. For instance, substituting Cr into (TiZrNbHfTa)H2 has been shown to lower the desorption temperature by 20°C while maintaining a capacity of 2.3 wt%. Similarly, introducing rare-earth elements like La or Ce into HEHs enhances hydrogen diffusion kinetics, achieving absorption rates of 0.8 wt%/min at 25°C. High-throughput synthesis techniques have identified optimal compositions such as (TiVNbCrMn)H2 with a gravimetric density of 2.7 wt% and volumetric density of 100 g/L, surpassing traditional metal hydrides like MgH2 (7.6 wt%, but requiring >300°C for desorption). These advancements underscore the potential of HEHs to bridge the gap between high-capacity and low-temperature hydrogen storage systems.
Mechanical alloying and additive manufacturing have revolutionized the synthesis of HEHs, enabling scalable production with tailored microstructures. Cryomilling of (TiZrNbHfTa) powders followed by hydrogenation at 200°C yields nanostructured HEHs with grain sizes below 10 nm, which exhibit enhanced hydrogen diffusion pathways and reduced hysteresis losses (<5 kJ/mol H2). Additive manufacturing techniques such as selective laser melting have produced porous HEH architectures with surface areas exceeding 500 m²/g, facilitating rapid hydrogen uptake (<1 minute to reach 90% capacity) at moderate pressures (10 bar). These innovations address critical challenges in material processing and integration into practical storage systems.
The integration of HEHs into hybrid storage systems has further expanded their applicability. Combining HEHs with carbon-based scaffolds or metal-organic frameworks (MOFs) has resulted in composite materials like (TiZrNbHfTa)H2@Graphene with capacities up to 3.0 wt% and improved thermal conductivity (>50 W/m·K), enabling efficient heat management during cycling. Similarly, embedding HEH nanoparticles into MOF-808 matrices has achieved record-breaking volumetric densities of 120 g/L while maintaining cyclability over 500 cycles. These hybrid systems demonstrate the versatility of HEHs in addressing both technical and economic barriers to widespread adoption.
Despite these advancements, challenges remain in optimizing cost-effectiveness and environmental sustainability. Life cycle assessments reveal that HEH production currently requires energy inputs of ~50 kWh/kg H2 stored, primarily due to the use of rare metals like Hf and Ta. However, ongoing research into earth-abundant alternatives such as Fe-based HEHs ((FeCrMnNiAl)H2) shows promise, achieving capacities of 1.8 wt% with significantly lower production costs (<$20/kg). Coupled with renewable energy-driven synthesis methods, these developments position HEHs as a cornerstone technology for realizing a sustainable hydrogen economy.
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