The fossil fuels have propelled the essential progress of human society, but at the same time constrain the further improvement of living standards^[1−3]. Under the increasing global demand for primary energy sources such as coal, oil, and natural gas, environmental issues like smog, greenhouse gas emissions, and global warming have consequently emerged. Currently, it is imperative to vigorously develop renewable energy sources such as solar, wind, biomass, and hydrogen energy^[4−7]. Driven by the global energy transition and carbon neutrality goals, hydrogen energy acts as a clean, high-calorific, and abundant new energy source, which offers distinctive advantages and provides an effective solution to current energy and environmental challenges. Both academia and industry have conducted extensive research on the large-scale application of hydrogen energy, which is expected to contribute to sustainable industrial and social development worldwide in the future^[8−10]. However, the extremely low density (STP: 0.0899 kg/m³) and flammable, explosive nature of hydrogen makes efficient storage a major bottleneck for its large-scale safe application, posing serious challenges to the joint development of the hydrogen energy chain, including production, storage, and utilization^[11]. Hydrogen storage methods can be categorized into gaseous^[3], liquid^[12] and solid-state storage^[13]. Conventional high-pressure gaseous storage (35–70 MPa) is limited by low gravimetric H₂ density and safety concerns, while cryogenic liquid H₂ storage requires cooling temperatures below −253 °C (20 K), which faces severe insulation requirements for storage tanks and high energy consumption during operation. In contrast, solid-state chemical hydrogen storage technology features high volumetric H₂ density, moderate storage/release conditions, and superior safety and economy, making it a prominent research hotspot in the H₂ energy industry chain.

Solid-state chemical hydrogen storage materials can be classified into metal hydrides (e.g., MgH₂, TiFeH₂, LaNi₅H₆), light metal complex hydrides (e.g., LiAlH₄, NaAlH₄, NaBH₄, LiBH₄) and chemical hydrides (e.g., NH₃BH₃)^[14]. Meanwhile, MgH₂ exhibits the notable advantages as follows: (1) a prominent volumetric hydrogen storage density of 110 kg/m³, over 1000 times than that of traditional gaseous under ambient conditions, significantly exceeding the cryogenic liquid H₂ storage of 70.8 kg/m^{3 [15]}; (2) a high gravimetric hydrogen storage density of up to 7.6%, far exceeding the 1.0%–2.0% of high-pressure gaseous storage and 1.5 times that of cryogenic liquid hydrogen storage (5.5%)^[16], which outnumbers five times that of rare-earth-based hydrogen storage alloys like LaNi₅ (1.4%)^[17]; (3) eminent cycling reversibility and vast potential for modification^[9]; (4) abundant reserves (Mg accounts for 2.3% of the Earth's crust, ranking 8th among elements, over 85% of the global supply produced in China) and low cost (approximately 20￥/kg), causing it one of the most promising candidates for solid-state hydrogen storage materials^[18].

Nevertheless, Mg-based H₂ storage materials still face several challenges in practical applications that need to be addressed:

(1) The kinetic rate during hydrogenation/dehydrogenation is extremely slow due to the low diffusion coefficients of H atoms in MgH₂ and Mg, which are only 4×10⁻¹³ and 1.58×10⁻¹⁶ m²/s, respectively.

(2) The operating temperatures for H₂ absorption and desorption of MgH₂ are relatively high (above 300 °C), which is mainly due to the strong stability of the Mg−H bond, requiring an adsorption energy barrier of 1−10 kJ/mol to be overcome during the hydriding process and a corresponding dissociation energy barrier as high as 432 kJ/mol during the dehydriding process.

(3) Metallic Mg is highly active and easily forms an oxide layer in air, which hinders further surface hydrogenation and may pose fire risks, while maintaining an oxygen-free environment is particularly difficult and costly.

Due to these unfavorable factors, the large-scale application of MgH₂ has been suspended, and how to rationally modify MgH₂ to overcome these shortcomings has become a major research focus for enhancing its H₂ storage performance. Currently, the major modification methods for MgH₂ include alloying, nanostructuring^[11,19−21], and catalytic doping with various materials^[9,22−23], including transition metals and their oxides, carbon-based materials, and metal-organic frameworks^[24]. This article summarizes the recent research progress on Mg-based H₂ storage materials and discusses the primary modification methods and relevant reaction mechanisms, while offering a prospective outlook for their future development.

Many scholars regard alloying as a traditional modification method, which involves adding metallic elements (such as transition metals Ni, Co and Mn) into Mg-based materials in designated proportions to produce multielement alloys. The successful introduction of metals can create new solid-solution phases and reduce their thermal stability as expected. A large number of transition-metal-centered active sites provide additional reaction pathways for hydrogen absorption and release, thereby effectively enhancing the H₂ storage performance of the prepared materials^[25].

Related studies on the alloying modification of Mg-based H₂ storage materials can be traced back to the pioneering work of Reilly et al.^[26] in 1968. They discovered that the intermetallic compound Mg₂Ni, synthesized by alloying Ni with Mg, could significantly decrease the hydrogenation temperature to 300 °C (2 MPa), highlighting the pivotal role of transition metal elements in manipulating the H₂ storage properties of Mg-based materials. Further investigations by Selvam et al.^[27] and Stucki et al.^[28] found that the Ni content in the alloy had a dominant influence on material performance, and the material almost lost its hydrogenation capability when excess Ni formed the MgNi₂ phase, owing to the enhanced metallic bonding and metastable structural stability. In contrast, a proper amount of Ni doping could produce Mg₂Ni, which effectively weakened the Mg−H bond and reduced the reaction activation energy, thereby facilitating the hydriding/dehydriding process of Mg₂NiH₄. Based on this mechanism, Yang et al.^[29] introduced eight transition metals (Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn) into Mg₂Ni via planetary ball milling. The results showed that Cr, Co and Mn markedly reduced the reaction plateau pressure, while Ti and Cu significantly lowered the H₂ absorption temperature, increasing the hydriding capacity of the modified material to 3.0%–3.5% and accelerating its kinetics. Shao et al.^[30] reported that Mg₂Cu (formed by Cu alloying) underwent disproportionation during H₂ absorption, producing MgH₂ and MgCu₂, which resulted in poor cycling reversibility. Zlotea et al.^[31] compared the H₂ absorption behaviors of Mg₅Ga₂ and Mg₆Pd, and revealed that both samples experienced partial disproportionation reactions but exhibited different microstructural evolutions: Mg₅Ga₂ formed elongated polycrystalline particles, while Mg₆Pd generated pyramid-shaped particles. Yajima et al.^[32], Polanski et al.^[33], Gennari et al.^[34], Kyoi et al.^[35] investigated element doping including Fe, Ti and La, and their work demonstrated clear improvements in the H₂ storage properties of Mg-based alloys. Ding et al.^[25] prepared Mg-2Ni alloy via casting and compared it with a sample after ultrasonication (Mg-2Ni UST). The experimental results confirmed that the latter showed a remarkable enhancement in hydriding capacity and kinetic performance. Moreover, the DSC curve of Mg-2Ni UST showed that its decomposition peak temperature decreased from 392.99 to 345.6 °C, and the activation energy dropped from 101.93 to 88.65 kJ/mol. These improvements were attributed to the increase in grain boundary density and crystal refinement induced by ultrasonic treatment, which alleviated the hysteresis effect of the Mg-Mg₂Ni structure and accelerated the formation and decomposition kinetics of metal hydrides. Liu et al.^[36] synthesized MgH₂-15% LaNi_4.5X_0.5 (X = Ni, Fe, Co, Al) through melting, wet ball milling, and dry ball milling. Among them, LaNi_4.5X_0.5 showed the most effective modification, reducing the dehydrogenation temperature of MgH₂ from 380 to 214 °C and lowering the hydrogenation pressure from 3 to 1 MPa. This improvement was attributed to the weakening of the Mg−H bond strength induced by LaNi_4.5X_0.5, thereby reducing the activation energy of the samples. Liu et al.^[37] prepared a combined LiBH₄/Mg₁₁CeNi hydride system by ball milling a mixture of LiBH₄ and Mg₁₁CeNi hydride at a molar ratio of 17:2.5. This system could release 6.6% H₂ within 280−425 °C. Specifically, 4.0% H₂ was released from the decomposition of MgH₂ and Mg₂NiH₄ at 280−350 °C, while the remaining H₂ was obtained from the decomposition of LiBH₄ catalyzed by Mg₂Ni, CeH₂ and newly formed Mg at 350−425 °C. Moreover, the dehydrogenated sample could be partially regenerated at 400 °C and 9 MPa, achieving a reversible H₂ storage capacity of 4.9%, indicating that hydrogenated Mg-RE-Ni ternary alloys may effectively lower the dehydrogenation temperature of LiBH₄ and improve its reversibility. Yang et al.^[38] produced a three-phase composite of MgH₂-Bi₂Ti₂O₇-Bi₂Mg₃ by ball milling hollow spherical Bi₂Ti₂O₇ with MgH₂. During ball milling, the novel Bi₂Mg₃ alloy appeared, showing a clear promotive effect on hydriding performance. The dehydrogenation onset temperature was only 194.3 °C, much lower than that of pure MgH₂ (335 °C), and the rapid hydrogen absorption capability was demonstrated by absorbing 5.4% H₂ within 60 s at 200 °C. After 50 cycles, the capacity of MgH₂-Bi₂Ti₂O₇-Bi₂Mg₃ remained at 5.2% with a retention rate of 98.1%. Further density functional theory (DFT) calculations indicated that the appearance of Bi₂Mg₃ caused the Mg−H bond length to expand from 1.71 to 1.792, and also reduced the dissociation energy barrier from 1.21 to 0.47 eV. Yoon et al.^[39] employed DFT calculations to reveal the multiscale effects of various elements (e.g., P, As, Sb, Bi, S, Se) on the H₂ storage performance of Mg-Mg₂Ni alloys. The results indicated that Ba, Ca, Y and La preferentially occupy Mg sites, while Ag, Cu and P tend to occupy Ni sites in Mg-rich alloys. Meanwhile, the volume change (ΔV) during hydrogenation of pure Mg to $\alpha $-MgH₂ and γ-MgH₂ resulted in volume expansions of 31.8% and 29.4%, respectively, which agreed with experimental data of 31.4% and 29.1%. Alkali metals (K, Rb, Cs) and alkaline earth metals (Ba, Ca, Sr) markedly reduced ΔV, while transition metals (Co, Ni, etc.) significantly increased it. Further electronic structure analysis showed that Mg₂NiH₄ is a semiconductor with a DOS bandgap of 1.53 eV, where the d-orbital of Ni strongly hybridized with the s-orbital of H, dominating the density of states near the Fermi level. Notably, the introduction of elements such as Sb and P depleted the bandgap and weakened the metal−H bond, whereas Ba and Ru enhanced electron localization and bond strength.

As a result, alloying modification has been proven to be an effective strategy for enhancing the H₂ storage/release performance of MgH₂, and its mechanism primarily involves introducing multi-metal components to generate more active sites, thereby improving the related thermodynamic and kinetic properties. Simultaneously, the adsorption and dissociation of H atoms are promoted through electronic interactions between hydrogen and metal atoms. However, this method has two distinct limitations: (1) irreversible metal bond breakage during metal doping, leading to structural instability and cycling degradation after multiple H₂ absorption/desorption cycles; (2) the use of high-atomic-weight metals inevitably reduces the theoretical gravimetric H₂ storage density of the prepared materials. Due to these constraints, pure alloying modification usually fails to meet practical application requirements. A more promising strategy is to integrate alloying with other modification approaches, particularly catalytic material combination, which can retain the benefits of alloy-derived active sites, further lower reaction energy barriers, and avoid excessive capacity loss. For example, combining nanocatalytic materials with alloying can not only maintain high H₂ storage capacity but also significantly improve cycling stability, enabling comprehensive optimization of the H₂ storage performance of the prepared materials.

The nanostructuring approach involves physical or chemical treatments applied to the prepared material or the reduction of its internal particle size to the nanoscale (below 100 nm)^[40]. This strategy can significantly weaken the strength of Mg−H bonds by introducing defects and altering reaction pathways^[41], thereby requiring an in-depth understanding of the rate-determining steps during H₂ absorption/desorption to improve material performance. Meanwhile, the hydrogenation process of Mg (to MgH₂) can be summarized in four steps: (1) physical adsorption of H₂ on the surface of the metal hydrides; (2) dissociation of hydrogen into H atoms; (3) surface penetration and bulk diffusion of H atoms; (4) nucleation and growth of MgH₂. Correspondingly, the dehydrogenation of MgH₂ consists of three stages: (1) dissociation of H atoms from the MgH₂ lattice; (2) diffusion of H atoms to the particle surface (through the MgH₂/Mg phase interface); (3) recombination of H atoms into H₂ followed by the desorption process^[42−44].

During dehydrogenation, the molar volume of MgH₂ and nano-MgH₂ follows a quantitative relationship with the corresponding partial pressure, as shown in Equation (1):where p represents the equilibrium partial pressure during dehydrogenation, V denotes the molar volume (m³/mol) of the particle, r is the particle radius (m), and γ represents the surface energy (J/m²)^[45−46].

This expression demonstrates that when the particle size decreases to the nanoscale, the noticeable increase in specific surface area exerts a dual effect on the reaction process. On the one hand, the larger specific surface area and shorter interparticle distance can enhance reactant contact and accelerate the diffusion of H atoms, thereby significantly improving kinetic rates. On the other hand, the accumulated free energy at particle surfaces and interfaces can reduce the reaction activation energy barrier, thus endowing the material with higher chemical activity^[47−48].

In 2005, Wagemans et al.^[49] investigated the impact of MgH₂ grain size on its thermodynamic properties through theoretical calculations, and the resulting geometric structure is shown in Figure 1. The authors concluded that the dehydrogenation temperature of the material dropped markedly when the particle size fell below 1.3 nm (each cluster containing fewer than 19 Mg atoms), highlighting the crucial role of nanoscale effects in regulating the H₂ storage performance of the designed materials.

Cheung et al.^[50] conducted a related simulation study and found that the thermodynamic properties were notably improved during the dehydrogenation process when the MgH₂ particle size reached 2 nm, which may provide theoretical evidence for the application of nanoscale technology. Dong et al.^[51] further explored the dehydrogenation process of MgH₂ and discovered a unique “burst effect”. Theoretical calculations showed that both the reaction energy barrier and Mg−H bond strength gradually decreased as the dehydriding reaction progressed, resulting in an accelerated dehydrogenation rate and indicating that precise control of nanoparticle size is of vital importance for promoting both the kinetic rate and thermodynamic stability of MgH₂ materials.

Nevertheless, two key issues still need to be addressed: (1) developing preparation methods capable of precisely controlling nanoscale dimensions, and (2) designing effective surface modification strategies to inhibit nanoparticle agglomeration.

In recent years, nanoscale technology has achieved significant progress in improving the H₂ storage and release performance of MgH₂, which can be essentially classified into three preparation methods: mechanical alloying (ball milling), nanoconfinement, and electrochemical reduction. Among them, mechanical ball milling is a conventional method for producing nano-MgH₂ due to its operational convenience and low cost. This method can introduce numerous defects into the samples and reduce particle size through strong metal-metal interactions generated by mechanical energy, thereby effectively shortening the H₂ diffusion distance and accelerating reaction kinetics. Zhang et al.^[52] prepared solid-state H₂ storage materials MgH₂+fl-TiO₂@C and MgH₂+fl-TiO₂ via ball milling. The catalytic precursors were first obtained as white titanium glycerolate powder by hydrothermal synthesis, followed by annealing at 450 °C in air and Ar atmospheres to produce black fl-TiO₂@C and white fl-TiO₂, respectively. As shown in the DSC curves (Figure 2), the initial dehydrogenation temperatures for MgH₂, MgH₂+fl-TiO₂, and MgH₂+fl-TiO₂@C were 294.6, 199.2 and 180.3 °C, respectively, and the corresponding peak temperatures appeared at 362.6, 245.4 and 233.0 °C, respectively. Notably, MgH₂+fl-TiO₂ absorbed 6.0% H₂ in 40 min at 150 °C and 8 min at 250 °C, with a dehydrogenation reaction activation energy of 77.72 kJ/mol.

Chen et al.^[53] used Co(NO₃)₂·6H₂O, Ti(OC₄H₉)₄, C₆H₈O₇·H₂O, and ethanol to carry out a solvothermal reaction in a Teflon-lined autoclave at 190 °C, followed by calcination at 700 °C under Ar atmosphere to obtain the catalytic material Co/TiO₂. The final ball-milled products MgH₂-Co/TiO₂, MgH₂-TiO₂ and MgH₂-Co exhibited dehydrogenation peak temperatures of 235.2, 288.4 and 329.4 °C, respectively, in DSC tests, which were all significantly lower than 367.4 °C for ball-milled MgH₂, confirming the excellent effect of metallic Co and TiO₂ in reducing dehydrogenation temperature. Importantly, MgH₂-Co/TiO₂ started its dehydriding process at 190 °C and released 6.2% H₂ within 15 min at 250 °C.

Under a 6 MPa H₂ supply pressure, MgH₂-Co/TiO₂ absorbed 6.07%, 5.56% and 4.24% H₂ within 10 min at 165, 130 and 100 °C, respectively, and 4.0% H₂ after 10 h at 50 °C.

For nanoconfinement research, Nielsen et al.^[54] employed chemical impregnation to load magnesium dibutyl (MgBu₂) precursor into aerogel nanochannels, and then prepared confined MgH₂ nanoparticles after the hydrogenation process. The obtained nanoconfined structure reduced the dehydrogenation temperature to below 200 °C, which increased the dehydrogenation rate by a factor of five compared with pure MgH₂, further confirming the effectiveness of nanoconfinement. Li et al.^[55] successfully prepared diameter-regulable Mg nanowires by precisely manipulating Ar flow during vapor deposition, and found that the nanowire diameter decreased with lower Ar flow, resulting in a clear improvement in hydrogen storage performance. The prepared sample released 3.28% H₂ in 30 min at 200 °C and achieved a maximum capacity of 6.77% at 300 °C, while maintaining excellent capacity stability after 50 absorption-desorption cycles. Compared with traditional bulk materials, the outstanding performance of the as-prepared nanowires likely originated from their high specific surface area, which significantly increased active sites and provided more efficient H₂ diffusion channels. Xia et al.^[56] investigated the combined nanoconfinement effect with graphene (GR) addition on MgH₂. As shown in Figure 3(a)–(d), DFT calculations revealed that (C₄H₉)₂Mg had the lowest adsorption energy among (C₄H₉)₂Mg, C₆H₁₂, Mg and MgH₂ during the assembly of MgBu₂ on graphene. By precisely regulating the MgBu₂/GR ratio, a series of H₂ storage materials including MHGH-20, MHGH-40, MHGH-60, MHGH-70 and MHGH-75 were synthesized with different MgH₂ loading amounts, and MHGH-75 showed the best H₂ absorption/desorption performance (Figure 4(a)–(d)).

Subsequently, Ni nanoparticles were self-assembled on MgH₂ nanoparticle surfaces via a solvothermal method, and the resulting Ni-MHGH-75 displayed superior H₂ storage properties, absorbing 2.3% H₂ within 60 min at 50 °C under 3 MPa and releasing 5.4% H₂ in 30 min at 250 °C, with 99.2% capacity retention after 100 cycles.

In electrochemical reduction for nanostructuring, Aguey-Zinsou et al.^[57] obtained nano-sized Mg particles by reducing magnesium salts (Mg²⁺) in tetrahydrofuran (THF), followed by hydrogenation at 60 °C under 2 MPa to synthesize MgH₂. The resulting product achieved complete dehydrogenation at just 85 °C, a remarkable decrease compared with conventional bulk MgH₂ (> 350 °C for dehydriding).

It can be concluded from the above investigations that nanostructuring technology plays a vital role in modifying Mg-based H₂ storage materials, as it can overcome the limitations of traditional approaches and balance the thermodynamic stability and kinetic performance of nanomaterials. However, the preparation process introduces additional excess energy that may reduce the stability of MgH₂ and cause unexpected agglomeration and grain growth. As a result, the existing synthesis routes remain relatively complex, and continuous efforts are still required to enable large-scale practical applications in three aspects: fundamental research, engineering implementation, and commercialization. Future research should focus on precise nanostructure control and scalable preparation to advance practical applications.

Catalytic material modification is a crucial strategy for effectively improving the performance of Mg-based H₂ storage materials. It essentially involves combining MgH₂ with designated functional materials in adjustable proportions to promote the desired H₂ storage characteristics. The conventional catalytic materials include transition metals, transition metal oxides, and carbon-based materials, while recent research hotspots have also explored novel catalysts such as MOFs and MXenes. The corresponding reaction mechanisms can generally be divided into four categories:

(1) Hydrogen pump effect: Theoretically, doping with transition metals (e.g., Nb, Ni) may destabilize the material and favor dehydrogenation. In this process, H atoms tend to form hydride clusters with the surrounding transition metals, which facilitates rapid hydrogen transfer during reversible hydrogenation/dehydrogenation and thus enhances absorption/desorption kinetics.

(2) Spillover effect: Hydrogen molecules first dissociate into H atoms at the active sites on the catalyst surface, and these atoms then migrate to the MgH₂ interface via surface diffusion.

(3) Electron transfer: Catalytic materials (e.g., transition metal oxides, MXenes) can alter the electron cloud distribution of Mg–H bonds through electronic interactions. When electrons transfer from the catalytic material to MgH₂, the Mg–H bonds may be weakened and the hydrogen dissociation energy barrier reduced.

(4) Channeling effect: Catalysts with well-defined pore structures or unique interfacial properties (e.g., porous MOFs) can serve as diffusion pathways for H atoms, enabling efficient hydrogen transfer between the catalyst and MgH₂.

Although MgH₂ has attracted worldwide attention due to its high hydrogen storage density, strong modification potential, and inherent safety, its thermodynamic and kinetic properties still require significant improvement. In recent years, many scholars have made substantial contributions to enhancing the hydrogen storage performance of MgH₂, and the major research outcomes can be summarized as follows:

Alloying method: Manipulating the composition and ratio of metals can effectively improve the thermodynamic and kinetic properties of Mg-based materials. However, the introduction of metals usually reduces their hydrogen storage capacity, and the breaking of metal bonds during hydrogenation/dehydrogenation may compromise cycling stability. Thus, selecting suitable alloying elements to balance high storage capacity with stable cycling performance remains a major challenge.

Nanostructuring method: Nanostructured materials, with their high surface areas, porosity, and abundant defects, can significantly accelerate hydrogen absorption/desorption kinetics and lower the activation energy of MgH₂. Nanoconfinement, which restricts materials within frameworks with fixed cavities, can effectively suppress nanoparticle growth while preserving desirable thermodynamic and kinetic properties. Nevertheless, several challenges persist: ball milling often induces agglomeration and deteriorates storage performance; chemical synthesis lacks controllability and may produce unwanted oxidized products; other synthetic methods are too complex for large-scale application. Therefore, developing robust nanostructuring strategies is essential to maintain high performance and prevent particle coarsening.

Catalytic material doping: Introducing catalysts can trigger mechanisms such as spillover, hydrogen pump, channeling, and electron transfer, thereby enhancing hydrogenation/dehydrogenation activity. The synergistic use of carbon-based materials and transition metals has shown impressive improvements in kinetics, activation energy reduction, and cycling stability. This demonstrates that catalytic doping can effectively accelerate reaction kinetics while maintaining hydrogen storage density and retention. However, many existing catalysts still fall short of the comprehensive performance requirements, highlighting the need for novel catalysts with abundant active sites and superior thermal/kinetic characteristics.

Although each of these strategies can independently enhance the hydrogen storage performance of MgH₂, a single approach is insufficient to achieve the ideal effect. Recent studies have revealed more pronounced benefits from synergistic strategies, such as alloying combined with nanostructuring or catalytic doping coupled with nanoconfinement.

For large-scale application and development of Mg-based hydrogen storage materials, future efforts should focus on the following directions:

Systematic design of novel materials: Leveraging AI-driven big data for catalyst design and exploring emerging modifiers such as MXenes and MOFs, which can provide both catalytic activity and nanoconfinement to enhance cycling stability.

In-depth mechanistic study: Integrating theoretical models (e.g., finite element analysis, kinetics, DFT) with experiments to understand hydrogen adsorption, diffusion, and reaction mechanisms. Kinetic rate-limiting models can guide the rational design of efficient multi-component systems (e.g., HEAs), with experimental validation used to refine theoretical frameworks.

Improvement of preparation techniques: Applying ultrasonic treatment for uniform dispersion of metal/catalyst mixtures, adopting solvothermal methods or aging processes to facilitate lattice incorporation, and combining solvothermal synthesis with ball milling for synergistic modification.

In summary, these strategies can bridge the gap between laboratory research and industrial application, ultimately accelerating the deployment of high-performance Mg-based hydrogen storage materials.