NEXAFS study of 2LiF–MgB2 composite

Synthesis of lithium borohydridofluorides may pave a new way to pursue improved hydrogen storage properties of LiBH₄ as reversible hydrogen storage materials that fit the fuel cell application. The main products of the hydrogen absorption by 2LiF–MgB₂ composite are MgF₂ and LiBH₄. In addition to them, LiBH_4−xF_x compounds might be present during hydrogen absorption–desorptions and play important role on their kinetics and reversibility.     

Improved hydrogen storage performance of the LiNH2–MgH2–LiBH4 system by addition of ZrCo hydride

Significant improvements in the hydrogen absorption/desorption properties of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ composite have been achieved by adding 3wt% ZrCo hydride. The composite can absorb 5.3wt% hydrogen under 7.0 MPa hydrogen pressure in 10 min and desorb 3.75wt% hydrogen under 0.1 MPa H₂ pressure in 60 min at 150 °C, compared with 2.75wt% and 1.67wt% hydrogen under the same hydrogenation/dehydrogenation conditions without the ZrCo hydride addition, respectively. TPD measurements showed that the dehydrogenation temperature of the ZrCo hydride-doped sample was decreased about 10 °C compared to that of the pristine sample. It is concluded that both the homogeneous distribution of ZrCo particles in the matrix observed by SEM and EDS and the destabilized N–H bonds detected by IR spectrum are the main reasons for the improvement of H-cycling kinetics of the 2LiNH₂–1.1MgH₂–0.1LiBH₄ system.     

Enhanced reversible hydrogen storage performance of NbCl5 doped 2LiH–MgB2 composite

2LiBH₄ + MgH₂ system is considered as an attractive candidate for reversible hydrogen storage with high capacity and favorable thermodynamics. However, its reaction kinetics has to be further improved for the practical application. In this work, we investigated the effect of NbCl₅ additive on the de/hydrogenation kinetics and microstructure refinement in 2LiH–MgB₂ composite systematically. The hydrogenation and dehydrogenation kinetics of 2LiH–MgB₂ composite can be significantly enhanced with the increase of NbCl₅ content. The 3 mol% NbCl₅ doped 2LiH–MgB₂ composite exhibits the superior reversible hydrogen storage performance, which requires 50 min to uptake 9.0 wt% H₂ at 350 °C and release 8.5 wt% H₂ at 400 °C, respectively. In contrast, the undoped 2LiH–MgB₂ sample uptakes 6.2 wt% H₂ and releases 3.1 wt% H₂ under identical measurement conditions. Moreover, the 3 mol% NbCl₅ doped 2LiH–MgB₂ composite can release more than 9.0 wt% H₂ within 300 min at 400 °C without obvious degradation of capacity over the first 10 cycles. Microstructure analyses clearly indicate that NbCl₅ additive first reacts with LiH to form Nb and LiCl during ball-milling process, and then NbH is formed after the first hydrogenation and stabilized upon further de/hydrogenation cycling. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system through facilitating hydrogen diffusion rapidly as well as prevent the particles from further growth in the subsequent hydrogenation and dehydrogenation processes.     

Thermodynamic and kinetic studies of NaBH4 regeneration by NaBO2–Mg–H2 ternary system at isothermal condition

NaBH₄ is a candidate for H₂ storage in solid phase. NaBH₄ hydrolysis readily produces H₂ gas and NaBO₂ which can regenerate NaBH₄ with pressurized hydrogen and the aid of a reducing agent like Magnesium above 500 °C. This paper deals with the NaBH₄ thermochemical regeneration from the NaBO₂–Mg–H₂ ternary system at isothermal temperatures between 558 and 634 °C and H₂ pressure in the range 2–31 bar. A simplified Langmuir adsorption model has been applied for the interpretation of the in-situ H₂ pressure variations. The applied model is zero-dimensional but provides a reasonable approach to identify the rate determining step and acquire relevant thermodynamic and kinetic parameters such as equilibrium constant (K_eq), Gibbs free energy (Δ_rG⁰) and reaction rate coefficients (k). The study provides an apparent activation energy and Gibbs free energy of this process of 29.2 kJ/mol and −76.9 kJ/mol, respectively.     

Rehydrogenation and cycle studies of LiBH4–CaH2 composite

Rehydrogenation behavior of 6LiBH₄ + CaH₂ composite with NbF₅ has been studied between 350 and 500 °C after dehydrogenation at 450 °C. The composite exhibits the best rehydrogenation feature at 450 °C in terms of the overall rehydrogenation rate and the amount of absorbed hydrogen. It is found that about 9 wt% hydrogen is absorbed at 450 °C for 12 h. Up to 10 dehydrogenation–hydrogenation cycles have been carried out for the composite. It is demonstrated that 6LiBH₄ + CaH₂ with 15 wt% NbF₅ maintains a reversible hydrogen storage capacity of about 6 wt% at 450 °C after a slight degradation between the 1st and 5th cycles. The addition of NbF₅ seems to improve the cycle properties by retarding microstructural coarsening during cycles.     

Catalytic effect of MgFe2O4 on the hydrogen storage properties of Na3AlH6–LiBH4 composite system

The effect of MgFe₂O₄ on the hydrogen storage properties of the composite Na₃AlH₆4LiBH₄ was studied for the first time, where it was found that MgFe₂O₄ addition decreased the onset desorption temperature of Na₃AlH₆4LiBH₄. Hydrogen (～9.5 wt%) was released in three stages and the dehydrogenation temperatures were reduced to 80 °C, 350 °C, and 430 °C for the first, second, and third stage, respectively. The absorption kinetics of Na₃AlH₆4LiBH₄ was also significantly improved due to the catalytic effect of MgFe₂O₄. Using Kissinger analysis, the apparent activation energies of decomposition of the Li₃AlH₆ and NaBH₄ stages in Na₃AlH₆4LiBH₄-10 wt% MgFe₂O₄ were calculated to be 72 and 141 kJ/mol, respectively. These values were considerably lower than the corresponding values for the undoped composite. X-ray diffraction analysis revealed the formation of new products such as MgO and Fe during the heating process. Our results suggest that MgFe₂O₄ enhanced the hydrogen storage properties of Na₃AlH₆4LiBH₄ through the formation of active species, such as MgO and Fe.     

Enhanced hydrogen uptake/release in 2LiH–MgB2 composite with titanium additives

The influence of different titanium additives on hydrogen sorption in LiH–MgB₂ system has been investigated. For all the composites LiH–MgB₂–X (X = TiF₄, TiO₂, TiN, and TiC), prepared by ball-milling in molar ratios 2:1:0.1, five hydrogen uptake/release cycles were performed. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and attenuated total reflection infrared spectroscopy (ATR-IR) have been used to characterize crystal phases developed during the hydrogen absorption–desorption cycles.     

The catalyst-free hydrolysis behaviors of NaBH4–NH3BH3 composites

NaBH₄–NH₃BH₃ composites were prepared by high-energy ball-milling processes for hydrogen generation through hydrolysis. After ball milling, there were no new phases found in the XRD patterns of NaBH₄–NH₃BH₃ composites. The experimental results demonstrate that when the molar ratios of NaBH₄–NH₃BH₃ composites range from 1:4 to 2:1, these composites can release above 90% hydrogen in 30 min at 70 °C. Comparing with neat NaBH₄ or NH₃BH₃, the hydrolysis properties of these composites are greatly enhanced. And the hydrolysis reaction mechanism is turned out to be more explicit as Na₂B₄O₅(OH)₄·8H₂O appears in the hydrolysis products. Since the preparation processes of these composites are simple and cost-effective and the hydrolysis of these composites achieves efficient hydrogen release, it is promising that this kind of composites can be applied in hydrogen generation.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Ni–Fe–B catalysts for NaBH4 hydrolysis

A series of Ni–Fe–B catalysts with different Fe/(Fe + Ni) molar ratios, used for the hydrolysis of NaBH₄, were prepared by chemical reduction of NiCl₂ and FeCl₃ mixed solution with NaBH₄. The measurements revealed that the catalysts with the molar ratio of Fe/(Fe + Ni) (30%) exhibited the highest catalytic activity, and the optimal reduction temperature is 348 K. In addition, the effects of the concentration of NaBH₄, NaOH and the hydrolytic temperature of NaBH₄ were discussed in detail. The results show that the reaction rate of hydrolysis first rises up and then goes down subsequently with the increase of NaBH₄ concentration, as well as the concentration of NaOH. The activation energy of the hydrolysis for Ni–Fe–B catalysts is fitted to 57 kJ/mol. The maximum value of hydrogen generation is 2910 ml/(min g) at 298 K.     

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.     

Enhanced hydrogen desorption from the Co-catalyzed LiBH4–Mg(BH4)2 eutectic composite

Co-based catalyst can significantly improve the dehydrogenation kinetics of the eutectic composite of LiBH₄–Mg(BH₄)₂ (1/1 M ratio). The onset hydrogen desorption temperature of the composite is at about 155 °C, which is ca. 245, 110 or 27 °C lower than that of LiBH₄, Mg(BH₄)₂ or pristine LiBH₄–Mg(BH₄)₂, respectively. Upon holding the samples at 270 °C, the Co catalyzed composite can release hydrogen at a rate 1.6 times faster than that of the pristine one. Electron Paramagnetic Resonance (EPR) characterization evidenced that Co was in a reduced state of Co⁺ which may serve as the functional species in catalyzing the dehydrogenation of the composite.     

Electroless-deposited Co–P catalysts for hydrogen generation from alkaline NaBH4 solution

Cobalt–phosphorus (Co–P) catalysts, which were electroless deposited on Cu sheet, have been investigated for hydrogen generation from alkaline NaBH₄ solution. The microstructures of the as-prepared Co–P catalysts and their catalytic activities for hydrolysis of NaBH₄ are analyzed in relation to pH value, NaH₂PO₂ concentration, and the deposition time. Experimental results show that the Co–P catalyst formed in the bath solution with pH value of 12.5, NaH₂PO₂ concentration of 0.8 M, and the deposition time no more than 6 min presents the highest hydrogen generation rate of 1846 mL min⁻¹ g⁻¹. Furthermore, the as-prepared catalyst also shows good cycling capability and the corresponding activation energy is calculated to be 48.1 kJ mol⁻¹. The favorable catalytic performance of the electroless-deposited Co–P catalysts indicates their potential application for quick hydrogen generation from hydrolysis of NaBH₄ solution.     

Effect of SWNTs on the reversible hydrogen storage properties of LiBH4–MgH2 composite

Single-walled carbon nanotubes (SWNTs) were mechanically milled with LiBH₄/MgH₂ mixture, and examined with respect to its effect on the reversible dehydrogenation properties of the Li–Mg–B–H system. Experimental results show that the addition of SWNTs results in an enhanced dehydriding rate and improved cyclic stability of the LiBH₄/MgH₂ composite. For example, the LiBH₄/MgH₂ composite with 10 wt% purified SWNTs additive can release nearly 10 wt% hydrogen within 20 min at 450 °C, with an average dehydriding rate over 2 times faster than that of the neat LiBH₄/MgH₂ sample. Based on the results of phase analysis and a series of designed experiments, the mechanism underlying the observed property improvement was discussed.     

Sorption behavior of the MgH2–Mg2FeH6 hydride storage system synthesized by mechanical milling followed by sintering

The hydrogen sorption behavior of the Mg₂FeH₆–MgH₂ hydride system is investigated via in-situ synchrotron and laboratory powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), particle size distribution (PSD) and volumetric techniques. The Mg₂FeH₆–MgH₂ hydride system is obtained by mechanical milling in argon atmosphere followed by sintering at high temperature and hydrogen pressure. In-situ SR-PXD results show that upon hydriding MgH₂ is a precursor for Mg₂FeH₆ formation and remained as hydrided phase in the obtained material. Diffusion constraints preclude the further formation of Mg₂FeH₆. Upon dehydriding, our results suggest that MgH₂ and Mg₂FeH₆ decompose independently in a narrow temperature range between 275 and 300 °C. Moreover, the decomposition behavior of both hydrides in the Mg₂FeH₆–MgH₂ hydride mixture is influenced by each other via dual synergetic-destabilizing effects. The final hydriding/dehydriding products and therefore the kinetic behavior of the Mg₂FeH₆–MgH₂ hydride system exhibits a strong dependence on the temperature and pressure conditions.     

Revision of the NaBO2–H2O phase diagram for optimized yield in the H2 generation through NaBH4 hydrolysis

The binary phase diagram NaBO₂–H₂O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH₄ hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO₂·4H₂O (Na[B(OH)₄]·2H₂O), NaBO₂·2H₂O (Na[B(OH)₄]), NaBO₂·2/3H₂O (Na₃[B₃O₄(OH)₄]), NaBO₂·1/3H₂O (Na₃[B₃O₅(OH)₂]) and NaBO₂ (Na₃[B₃O₆]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from −10 to 80 °C have been determined, confirming literature data at low temperature (20–50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO₂·4H₂O, occurring at −7 °C, has been determined by DSC. The Liquid–Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO₂·2/3H₂O has been estimated at 131.6 °C. This knowledge is paramount in the field of hydrogen storage through NaBH₄ hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO₂–H₂O binary phase diagrams and its consequence related to hydrogen storage through NaBH₄ hydrolysis.     

Light-weight NaNH2–NaBH4 hydrogen storage material synthesized via liquid phase ball milling

As a light-weight and low-cost hydrogen storage composite, NaNH₂–NaBH₄ (molar ratio of 2:1) was prepared by a liquid phase ball milling (LPBM) method under the co-protection of argon and cyclohexane. The structure evolution and the thermal decomposition performance of the as-prepared sample were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermo gravimetric-differential thermal analysis (TG-DTA), respectively. It is found that the interaction of NaNH₂ with NaBH₄ is enhanced by LPBM, thus causes a preferred orientation for the crystal structure of NaNH₂, and the red-shifts of the N–H stretching vibration and the B–H stretching vibration. In addition, the as-prepared NaNH₂–NaBH₄ (2/1) can achieve a low activation energy of 76.4 kJ mol⁻¹ during the main decomposition stage, which is only 47.9% of that of the one synthesized via a solid state ball milling (SSBM) method, and is very close to that of the Co–B catalyst promoted one. This indicates the LPBM method is an efficient way to get high-performance NaNH₂–NaBH₄, whose thermal decomposition kinetics can be greatly improved without any catalyst.     

Study on the hydrogen storage properties and reaction mechanism of NaAlH4–MgH2–LiBH4 ternary-hydride system

In this paper, we report the hydrogen storage properties and reaction mechanism of NaAlH₄–MgH₂–LiBH₄ (1:1:1) ternary-hydride system prepared by ball milling. It was found that during ball milling, the NaAlH₄/MgH₂/LiBH₄ combination converted readily to the mixture of LiAlH₄/MgH₂/NaBH₄ and there is a mutual destabilization among the hydrides. Three major dehydrogenation steps were observed in the system, which corresponds to the decomposition of LiAlH₄, MgH₂, and NaBH₄, respectively. The onset dehydrogenation temperature of MgH₂ in this system is observed at around 275 °C, which is over 55 °C lower from that of as-milled MgH₂. Meanwhile, NaBH₄-relevant decomposition showed significant improvement, starts to release hydrogen at 370 °C, which is reduced by about 110 °C compared to the as-milled NaBH₄. The second and third steps decomposition enthalpy of the system were determined by differential scanning calorimetry measurements and the enthalpies were changed to be 61 and 100 kJ mol⁻¹ H₂ respectively, which are smaller than that of MgH₂ and NaBH₄ alone. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂ and NaBH₄ in the composite was reduced to 96.85 and 111.74 kJ mol⁻¹ respectively. It is believed that the enhancement of the dehydrogenation properties was attributed to the formation of intermediate compounds, including Li–Mg, Mg–Al, and Mg–Al–B alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway.     

Performance evaluation of hydrogen generation system with electroless-deposited Co–P/Ni foam catalyst for NaBH4 hydrolysis

In this work, the performance of a hydrogen generation system with an electroless-deposited Co–P/Ni foam catalyst for NaBH₄ hydrolysis was evaluated. The performance of a hydrogen generator using a combination of Co/γ-Al₂O₃ and Co–P/Ni foam catalysts was also evaluated in order to address the shortcomings with the individual catalysts. The generator had high conversion efficiency, fast response characteristics, and strong catalyst durability. Hydrogen generation tests were performed to investigate the effect of the composition of the NaBH₄ solution on the hydrogen generation properties. The generator's conversion efficiency decreased with an increase in the amount of solute dissolved in NaBH₄ solution because of the accumulation of precipitates on the catalyst, and NaOH concentration had a greater effect on the hydrogen generation properties than did NaBH₄ concentration. According to these results, the hydrogen generation system with the Co–P/Ni foam catalyst is suitable as a hydrogen supplier for proton exchange membrane fuel cells owing to the strong durability and inexpensive cost of the catalyst.