Synthesis of NaAlH4/Al composites and their applications in hydrogen storage

In solid-state hydrogen storage in light metal hydrides, nanoconfinement and the use of catalysts represent promising solutions to overcoming limitations such as poor reversibility and slow kinetics. In this work, the morphology and hydrogen desorption kinetics of NaAlH₄ melt-infiltrated into a previously developed Ti-based doped porous Al scaffold is analysed. Small-angle X-ray scattering and scanning electron microscopy analysis of low NaAlH₄ loading in the porous Al scaffold has revealed that mesopores and small macropores are filled first, leaving the larger macropores/voids empty. Temperature-programmed desorption experiments have shown that NaAlH₄-infiltrated porous Al scaffolds show a higher relative H₂ release, with respect to NaAlH₄ + TiCl₃, in the temperature range 148–220 °C, with the temperature of H₂ desorption trending to bulk NaAlH₄ with increasing scaffold loading. The Ti-based catalytic effect is reproduced when the dopant is present in the scaffold. Further work is required to increase the mesoporous volume in order to enhance the nanoconfinement effect.     

Direct synthesis of nanocrystalline titanium dioxide/carbon composite and its catalytic effect on NaAlH4 for hydrogen storage

Nanocrystalline titanium dioxide/carbon composite (TiO₂/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)₄) and resol as precursors. The prepared TiO₂/C composite was mainly in the anatase structure with an average particle size under 20 nm, which was then introduced in NaAlH₄ as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO₂/C and TiO₂ can obviously improve the kinetics of NaAlH₄, while NaAlH₄ with 3 mol% TiO₂/C exhibits better cycling stability than NaAlH₄ with 3 mol%TiO₂. The hydrogen storage capacity of NaAlH₄ with TiO₂/C remains stable after 5th cycle, and about 94% of initial hydrogen is released, while the capacity of NaAlH₄ with TiO₂ decreases continuously during cycling, and only 88% of initial hydrogen is released after 10th cycle. Furthermore, NaAlH₄ with 3 mol%TiO₂/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload 4.16 and 1.63wt% hydrogen at 50 and 30 bar hydrogen pressures, respectively.     

Synergistic effects played by CMK-3 and NbF5 co-additives on de/re-hydrogenation performances of NaAlH4

In the present work, a strategy for simultaneously reducing the thermal stability of NaAlH₄ and enhancing its dehydrogenation kinetics was suggested by means of synergistic effects from co-additives of mesoporous carbon material CMK-3 and NbF₅. The ball milled NaAlH₄ + 10 wt% (NbF₅ + CMK-3) (NbF₅: CMK-3 = 1:1 in weight ratio) composite can liberate hydrogen at an onset temperature of 358 K, which was drastically decreased by 93 K from that of pristine NaAlH₄. By means of Kissinger's method, the activation energy of NaAlH₄ + 10 wt% (NbF₅ + CMK-3) can be identified as 99.2 kJ mol^?1, which was greatly reduced from that of pristine NaAlH₄ (121 kJ mol^?1). Investigations on the dehydrogenation process revealed that CMK-3 was beneficial to reducing the particle size of NaAlH₄ during ball milling, while NbF₅ was actively involved in the decomposition of NaAlH₄ and yielded some Nb-relevant intermediate phases NbH_0.89 during the heating process. The modified dehydrogenation pathway of NaAlH₄ also results in the destabilization of dehydrogenation by 2.13 kJ mol^?1 H₂ from that of pristine NaAlH₄. During the hydrogenation process, the NbH_0.89 and the mesoporous carbon material CMK-3 played synergistic roles in improving the dehydrogenation performance of NaAlH₄.     

Dehydrogenation and low-pressure hydrogenation properties of NaAlH4 confined in mesoporous carbon black for hydrogen storage

For hydrogen to be successfully used as an energy carrier in a new renewable energy driven economy, more efficient hydrogen storage technologies have to be found. Solid-state hydrogen storage in complex metal hydrides, such as sodium alanate (NaAlH₄), is a well-researched candidate for this application. A series of NaAlH₄/mesoporous carbon black composites, with high NaAlH₄ content (50–90 wt%), prepared via ball milling have demonstrated significantly lower dehydrogenation temperatures with intense dehydrogenation starting at ∼373 K compared to bulk alanate's ≥ 456 K. Dehydrogenation/hydrogenation cycling experiments have demonstrated partial hydrogenation at 6 MPa H₂ and 423 K. The cycling experiments combined with temperature-programmed dehydrogenation and powder X-ray diffraction have given insight into the fundamental processes driving the H₂ release and uptake in the NaAlH₄/carbon composites. It is established that most of the hydrogenation behavior can be attributed to the Na₃AlH₆ ↔ NaH transition.     

Improved hydrogen sorption performance of NbF5-catalysed NaAlH4

The effect of NbF₅ on the hydrogen sorption performance of NaAlH₄ has been investigated. It was found that the dehydrogenation/hydrogenation properties of NaAlH₄ were significantly enhanced by mechanically milling with 3 mol% NbF₅. Differential scanning calorimetry results indicate that the ball-milled NaAlH₄-0.03NbF₅ sample lowered the completion temperature for the first two steps dehydrogenation by 71 °C compared to the pristine NaAlH₄ sample. Isothermal hydrogen sorption measurements also revealed a significant enhancement in terms of the sorption rate and capacity, in particular, at reduced operation temperatures. The apparent activation energy for the first-step and the second-step dehydrogenation of the NaAlH₄-0.03NbF₅ sample is estimated to be 88.2 kJ/mol and 102.9 kJ/mol, respectively, by using Kissinger’s approach, which is much lower than for pristine NaAlH₄, indicating the reduced kinetic barrier. The rehydrogenation kinetics of NaAlH₄ was also improved with 3 mol% NbF₅ doping, absorbing ∼1.7 wt% hydrogen at 150 °C for 2 h under ∼5.5 MPa hydrogen pressure. In contrast, no hydrogen was absorbed by the pristine NaAlH₄ sample under the same conditions. The formation of Na₃AlH₆ was detected by X-ray diffraction on the rehydrogenated NaAlH₄-0.03NbF₅ sample. Furthermore, the structural changes in the NbF₅-doped NaAlH₄ sample after ball milling and the hydrogen sorption were carefully examined, and the active species and mechanism of catalysis in NbF₅-doped NaAlH₄ are discussed.     

Carbon nanostructures as catalyst for improving the hydrogen storage behavior of sodium aluminum hydride

The present paper reports the catalytic effect of carbon nanomaterials, particularly carbon nanotubes (CNTs) and graphitic nanofibres (GNFs) with two different structure morphology, namely planar GNFs (PGNFs) and helical GNFs (HGNFs) as the catalyst for improving the dehydrogenation and rehydrogenation behavior of sodium aluminum hydride (NaAlH₄). It has been observed that HGNFs posses superior catalytic activity than other carbon nanoforms in improving the desorption kinetics and decreasing the desorption temperature of NaAlH₄. Temperature programmed desorption (TPD) reveals that HGNFs admixed NaAlH₄ undergo hydrogen desorption at a much lower temperature than PGNFs and CNTs (SWCNTs and MWCNTs) admixed NaAlH₄. Thus for the heating rate of 2 °C/min, the peak desorption temperature corresponds to initial step decomposition of NaAlH₄ admixed with 2 wt.% HGNFs and 2 wt.% PGNFs has been lowered to 143.6 °C and 152.6 °C, respectively (for pristine NaAlH₄, it is ∼170 °C). In addition to the enhancement in desorption kinetics, the HGNFs admixed NaAlH₄ undergoes fast rehydrogenation at the moderate condition. Microstructural investigation reveals that the HGNFs were present on the surface of NaAlH₄ grains, whereas CNTs were tunneled into the grains of NaAlH₄ suggesting a distinct catalytic behavior of different carbon nanovariants.     

Improvement of the hydrogen storage kinetics of NaAlH4 with nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) by melt infiltration

Nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) with 10wt% TiO₂ were synthesized through an easy one-step method using phenolic resols, titania nanoparticles and Pluronic F127 as organic carbon sources, inorganic precursors and surfactant, respectively. The results show that the as-prepared Ti-CSs composite is spherical shape with a diameter ranging from 0.3 to 2 μm, and rutile TiO₂ nanoparticles are distributed on the surface of the carbon spheres. Then the kinetics of NaAlH₄ was improved through depositing it on the surface of as-prepared Ti-CSs by melt infiltration. The results show that NaAlH₄ with Ti-CSs exhibits better hydrogen desorption kinetics than TiF₃ or nanocrystalline TiO₂ catalysted-NaAlH₄, and it starts to release hydrogen at about 40 °C and releases about 25% of the hydrogen content during heating to 60 °C. The results from SEM and XPS show that hydrogen storage properties of NaAlH₄ were considerably improved due to the formation of special structure during melt infiltration and the nanocrystalline TiO₂ and/or amorphous phase Ti–Al clusters near the subsurface sites, which succeed in combining catalyst addition (TiO₂ nanoparticles) and nanoconfinement to improve the kinetics of NaAlH₄.     

Catalytic effect of Gd2O3 and Nd2O3 on hydrogen desorption behavior of NaAlH4

This study investigated the effect of Nd₂O₃ and Gd₂O₃ as catalyst on hydrogen desorption behavior of NaAlH₄. Pressure-content-temperature (PCT) equipment measurement proved that both two oxides enhanced the dehydrogenation kinetics distinctly and increasing Nd₂O₃ and Gd₂O₃ from 0.5 mol% to 5 mol% caused a similar effect trend that the dehydrogenation amount and average dehydrogenation rate increased firstly and then decreased under the same conditions. 1 mol% Gd₂O₃–NaAlH₄ presented the largest hydrogen desorption amount of 5.94 wt% while 1 mol% Nd₂O₃–NaAlH₄ exerted the fastest dehydrogenation rate. Scanning Electron microscopy (SEM) analysis revealed that Gd₂O₃–NaAlH₄ samples displayed uniform surface morphology that was bulky, uneven and flocculent. The difference of Nd₂O₃–NaAlH₄ was that with the increasing of Nd₂O₃ content, the particles turned more and more big. Compared to dehydrogenation behavior, this phenomenon demonstrated that small particles structure were beneficial to hydrogen desorption. Besides, the further study found that different catalysts and addition amounts had different effects on the microstructure of NaAlH_4.     

Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO₂, VCl₃, HfCl₄) on the hydrogen desorption/absorption of NaAlH₄. The HfCl₄ doped NaAlH₄ showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl₃ or TiO₂ desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH₄ doped with TiO₂ reduced to 140 °C. On the contrary, in the case of NaAlH₄ doped with HfCl₄ or VCl₃, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH₄ better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO₂, while the amount of restored hydrogen was lower for HfCl₄ and VCl₃ doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO₂ doped samples. In contrast, there was evidence of Al–V alloy in the VCl₃ doped sample and Al–Hf alloy in the HfCl₄ doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH₄ showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Co-effects of Tm2O3 and porous silica on reversible hydrogen storage in NaAlH4

The co-effects of lanthanide oxide Tm₂O₃ and porous silica on the hydrogen storage properties of sodium alanate are investigated. NaAlH₄-Tm₂O₃ (10 wt%) and NaAlH₄-Tm₂O₃ (10 wt%)-porous SiO₂ (10 wt%) are prepared by the ball milling method, and their hydrogen desorption/re-absorption capacities are compared. Dehydrogenation process was performed at 150 °C under vacuum and rehydrogenation was performed at 150 °C for 4 h under ∼9 MPa in highly pure hydrogen. The results show that Tm₂O₃ has a catalytic effect on the hydrogen desorption and re-absorption of NaAlH₄. The hydrogen desorption capacity of Tm₂O₃ single-doped NaAlH₄ is 4.6 wt%, higher than that of undoped NaAlH₄ (4.3 wt%). During the dehydrogenation process, NaAlH₄ is completely decomposed and no intermediate product Na₃AlH₆ is detected. The addition of porous silica improves the dehydrogenation performance of NaAlH₄. Tm₂O₃ and porous silica co-doped NaAlH₄ could release a maximum hydrogen amount of 4.7 wt%, higher than that of undoped NaAlH₄ and Tm₂O₃ single-doped NaAlH₄. Moreover, porous silica improves the reversibility of hydrogen storage in NaAlH₄.     

NaAlH4 confined in ordered mesoporous carbon

In this paper we performed a comprehensive investigation of the structural and sorption properties of a 40 wt. % NaAlH₄ confined in a ordered mesoporous carbon (OMC, i.e. CMK-3) by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), ²³Na{¹H} and ²⁷Al{¹H} solid-state magic angle spinning-nuclear magnetic resonance (MAS-NMR).     

Mesoporous nano-Co3O4: A potential negative electrode material for alkaline secondary battery

A potential negative electrode material (mesoporous nano-Co₃O₄) is synthesized via a simple thermal decomposition of precursor Co(OH)₂ hexagonal nanosheets in the air. The structure and morphology of the samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the nano-Co₃O₄ is present in mesoporous hexagonal nanoparticles. The average size of holes is about 5-15 nm. The electrochemical performances of mesoporous nano-Co₃O₄ as the active starting negative electrode material for alkaline secondary battery are investigated by galvanostatic charge-discharge and cyclic voltammetry (CV) technique. The results demonstrate that the prepared mesoporous nano-Co₃O₄ electrode displays excellent electrochemical performance. The discharge capacity of the mesoporous nano-Co₃O₄ electrode can reach 436.5 mAh g⁻¹ and retain about 351.5 mAh g⁻¹ after 100 cycles at discharge current of 100 mA g⁻¹. A properly electrochemical reaction mechanism of mesoporous nano-Co₃O₄ electrode is also constructed in detail.     

Improvement in desorption kinetics of NaAlH4 catalyzed with TiO2 nanopowder

NaAlH₄ has been homogeneously mixed with micron- and nano-sized TiO₂ powders by high-energy ball milling and their sorption properties have been investigated during hydrogen absorption/desorption cycles. NaAlH₄ with TiO₂ nanopowder exhibits as good desorption kinetics as NaAlH₄ with TiCl₃, whereas poor desorption kinetics is observed with micron-sized TiO₂ powder. NaAlH₄ with TiO₂ nanopowder also provides improved cyclic property compared to NaAlH₄ with TiCl₃ in terms of both desorption rate and hydrogen capacity. X-ray diffraction analysis shows that micron-sized TiO₂ remains stable with NaAlH₄ after milling, although thermodynamic calculation predicts that TiO₂ reacts with NaAlH₄.     

In situ neutron diffraction of NaAlD4/carbon black composites during decomposition/deuteration cycles and the effect of carbon on phase segregation

The influence on the decomposition and reforming of the hydrogen storage material NaAlH₄ by adding relatively low amounts of mesoporous carbon black is investigated with in situ diffraction. A 60:40 NaAlH₄/carbon black composite is prepared via ball milling and characterised ex situ via X-ray diffraction, gas adsorption, temperature-programmed decomposition, and dehydrogenation/hydrogenation cycling methods. The prepared composite is deuterated, and the crystalline phase composition is determined with in situ neutron powder diffraction method during multiple decomposition/deuteration cycles. Changes in the crystalline phase composition start slightly below the melting temperature of the pristine alanate, whereas the release of deuterium starts at considerably lower temperatures. The decomposition of Na₃AlD₆ to NaD is almost completely reversible at the applied low deuterium pressures of ≥2 MPa. Thus, the strong effect of even low concentrations of a mesoporous carbon black on the capability to store H₂ reversibly is showcased and analysed in-depth.     

Hydriding-dehydriding kinetics and the microstructure of La- and Sm-doped NaAlH4 prepared via direct synthesis method

By directly introducing LaCl₃, La₃Al₁₁, SmCl₃, SmAl₃ into NaAlH₄ system using one-step synthesis method, the effects of these additives on NaAlH₄ were systematically investigated with regard to hydriding and dehydriding properties. Results showed that the materials doped with aluminide exhibit similar kinetics to the chloride-doped NaAlH₄. The apparent activation energy E_a of doped NaAlH₄ were calculated to be 86.4-93.0 kJ/mol and 96.1-99.3 kJ/mol for the first and second dehydrogenation step respectively by using Kissinger’s approach, much lower than those of pristine NaAlH₄. A reversible hydrogen capacity of 4.8 wt% can be achieved for the La₃Al₁₁- and SmAl₃-doped NaAlH₄, which is 10-20% higher than chloride-doped NaAlH₄. Investigations on the phase evolvement and microstructure in the cycling in LaCl₃- and La₃Al₁₁-doped NaAlH₄ clearly demonstrate that La species is presented as the form of La-Al nanoclusters in the materials. The combination of hydrogen storage properties and the microstructures unequivocally reveal that the in situ formed rare-earth-Al species play a crucial rule in catalyzing the chloride-doped NaAlH₄.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

In-situ synchrotron X-ray diffraction study on the dehydrogenation behavior of NaAlH4 modified by multi-walled carbon nanotubes

The effect of multi-walled carbon nanotubes (MWCNTs) addition on the dehydrogenation behavior of NaAlH₄ (sodium alanate) is investigated using high-pressure thermal gravimetric analysis (HPTGA) and in-situ synchrotron X-ray diffraction (in-situ synchrotron XRD) technique. The HPTGA results show that the addition of MWCNTs facilitates dehydrogenation of NaAlH₄ by lowering the first-step dehydrogenation temperature to 80 °C. In-situ synchrotron XRD analysis demonstrates that the dehydrogenation pathway can be modified by the addition of MWCNTs which resulting in an enhanced hydrogen desorption rate and reduced desorption temperature.     

Synthesis and hydriding/dehydriding properties of nanosized sodium alanates prepared by reactive ball-milling

TiF₃-doped NaH/Al mixture was hydrogenated into Na₃AlH₆ and NaAlH₄ complex hydrides by reactive ball-milling at room temperature through the optimization of milling duration and hydrogen pressure. The analysis of the preparation of NaAlH₄ samples during reactive ball-milling process has been performed by XRD and TG/DSC. It has been found that Na₃AlH₆ was formed under 0.5 MPa hydrogen pressure and 30 h milling duration, while NaAlH₄ was formed under 0.8 MPa hydrogen pressure and 45 h milling duration. The process of preparing NaAlH₄ by ball-milling was found accomplished via two reaction steps, namely: (1) NaH + Al + H₂ → Na₃AlH₆ and (2) Na₃AlH₆ + Al + H₂ → NaAlH₄. As the hydrogen pressure and milling duration increase, the synthetic yield of NaAlH₄ and its corresponding dehydriding capacity are both increased. With increased hydrogen pressure (0.8-3 MPa) and milling duration (45-60 h), the cell volume of Na₃AlH₆ decreases while that of NaAlH₄ increases gradually. The abundance of Na₃AlH₆ phase decreases from 57.76 (x = 0.8, y = 45) to 8.69 wt.% (x = 3, y = 60), and the abundance of NaAlH₄ phase increases from 20.63 (x = 0.8, y = 45) to 86.50 wt.% (x = 3, y = 60). All the samples prepared in this way have fairly good activation behavior and fast hydriding/dehydriding reaction kinetics, which are capable of absorbing 4.26 wt.% hydrogen at 120 °C and desorbing 4.12 wt.% hydrogen at 150 °C, respectively. The improvement of hydriding/dehydriding properties is ascribed to the favorable microstructure and ultrafine particle features of nanosized NaAlH₄ formed during ball-milling at the optimum synthetic condition.     

Electronic and dehydrogenation properties of TiB2 cluster-doped NaAlH4 (101) surface: A first-principle approach

The crystal structures, electronic and dehydrogenation properties of TiB₂ cluster-doped NaAlH₄ (101) surface have been investigated by the first-principles density functional theory method. In the TiB₂ cluster-doped NaAlH₄ (101) surface, a Ti-centered TiB₂–Al₂H₈–AlH₅–AlH₃ complex is observed, and the AlH₃ and (AlH₅)²⁻ units in the TiB₂–Al₂H₈–AlH₅–AlH₃ favor the first-step decomposition reaction of NaAlH₄. The calculated electronic properties show that B–Ti bonds are stronger than B–Al and Ti–H bonds, which demonstrates that TiB₂ does not change its configuration in catalyzing the decomposition reaction of NaAlH₄. The results of hydrogen desorption energies imply that the import of TiB₂ makes the strength of Al–H bonds decreases. Therefore, the removal of H atoms, especially the removal of H atoms in the Ti–H–Al bonds is easier in the TiB₂ cluster-doped NaAlH₄ than in pure NaAlH₄.