Hydrogen sorption properties of MgH2/NaBH4 composites

The hydrogen sorption properties of magnesium hydride–sodium borohydride composites prepared by means of high-energy ball milling under Ar atmosphere were investigated. Mutual influence of milling time and the content of NaBH₄ were studied. Microstructural and morphological analyses were carried out using X-ray Diffraction (XRD), laser scattering measurements and Scanning Electron Microscopy (SEM), while kinetic analysis and cycling were performed in a Sievert's volumetric apparatus. It has been shown that low content of NaBH₄ and short milling time are beneficial for hydrogen sorption kinetics.     

Reversible hydrogen storage in a 3NaBH4/YF3 composite

A 3NaBH₄/YF₃ hydrogen storage composite was prepared through ball milling and its hydrogen sorption properties were investigated. It is shown that NaBH₄ does not react with YF₃ during ball milling. The dehydrogenation of the composite starts at 423 °C, which is about 100 °C lower than the dehydrogenation temperature of pure NaBH₄, with a mass loss of 4.12 wt%. Pressure–Composition–Temperature tests reveal that the composite has reversible hydrogen sorption performance in the temperature range from 350 °C to 413 °C and under quite low hydrogenation plateau pressures (<1 MPa). Its maximum hydrogen storage capacity can reach up to 3.52 wt%. The dehydrogenated composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. Based on the Pressure–Composition–Temperature analyses, the hydrogenation enthalpy of the composite is determined to be −46.05 kJ/mol H₂, while the dehydrogenation enthalpy is 176.76 kJ/mol H₂. The mechanism of reversible hydrogen sorption in the composite involves the decomposition and regeneration of NaBH₄ through the reaction with YF₃. Therefore, the addition of the YF₃ to NaBH₄ as a reagent forms a reversible hydrogen storage composite.     

Effects of alkaline treatment of hydrogen storage alloy on electrocatalytic activity for NaBH4 oxidation

Activation of the MmNi_4.03Co_0.42Mn_0.31Al_0.24 hydrogen storage alloy electrode is performed by immersing the electrode in a solution containing 6.0 mol dm⁻³ NaOH and 0.1 mol dm⁻³ NaBH_4. The effects of activation on the electrocatalytic activity of the electrode for NaBH₄ oxidation are investigated by cyclic voltammetry and chronoamperometry. Immersion activation greatly improves the electrocatalytic activity of the alloy electrode. Hydrogen was absorbed in the alloy during the immersion activation treatment and its electrooxidation is responsible for the high initial oxidation current. The stabilized current mainly results from the direct oxidation starting from the borohydride species. The effects of activation on structure and surface chemistry of the alloy are also discussed.     

Enhanced hydrogen storage performance of LiAlH4-MgH2-TiF3 composite

The mutual destabilization of LiAlH₄ and MgH₂ in the reactive hydride composite LiAlH₄-MgH₂ is attributed to the formation of intermediate compounds, including Li-Mg and Mg-Al alloys, upon dehydrogenation. TiF₃ was doped into the composite for promoting this interaction and thus enhancing the hydrogen sorption properties. Experimental analysis on the LiAlH₄-MgH₂-TiF₃ composite was performed via temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), isothermal sorption, pressure-composition isotherms (PCI), and powder X-ray diffraction (XRD). For LiAlH₄-MgH₂-TiF₃ composite (mole ratio 1:1:0.05), the dehydrogenation temperature range starts from about 60 °C, which is 100 °C lower than for LiAlH₄-MgH₂. At 300 °C, the LiAlH₄-MgH₂-TiF₃ composite can desorb 2.48 wt% hydrogen in 10 min during its second stage dehydrogenation, corresponding to the decomposition of MgH₂. In contrast, 20 min was required for the LiAlH₄-MgH₂ sample to release so much hydrogen capacity under the same conditions. The hydrogen absorption properties of the LiAlH₄-MgH₂-TiF₃ composite were also improved significantly as compared to the LiAlH₄-MgH₂ composite. A hydrogen absorption capacity of 2.68 wt% under 300 °C and 20 atm H₂ pressure was reached after 5 min in the LiAlH₄-MgH₂-TiF₃ composite, which is larger than that of LiAlH₄-MgH₂ (1.75 wt%). XRD results show that the MgH₂ and LiH were reformed after rehydrogenation.     

Effect of LiBH4 on hydrogen storage property of MgH2

The effect of lithium borohydride (LiBH₄) on the hydriding/dehydriding kinetics and thermodynamics of magnesium hydride (MgH₂) was investigated. It was found that LiBH₄ played both positive and negative effects on the hydrogen sorption of MgH₂. With 10 mol.% LiBH₄ content, MgH₂–10 mol.% LiBH₄ had superior hydrogen absorption/desorption properties, which could absorb 6.8 wt.% H within 1300 s at 200 °C under 3 MPa H₂ and completed desorption within 740 s at 350 °C. However, with the increasing amount of LiBH₄, the hydrogenation/dehydrogenation kinetics deteriorated, and the starting desorption temperature increased and the hysteresis of the pressure-composition isotherm (PCI) became larger. Our results showed that the positive effect of LiBH₄ was mainly attributed to the more uniform powder mixture with smaller particle size, while the negative effect of LiBH₄ might be caused by the H–H exchange between LiBH₄ and MgH₂.     

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.     

Solution combustion synthesized cobalt oxide catalyst precursor for NaBH4 hydrolysis

In this paper we report the solution combustion synthesis of cobalt oxide nanofoam from solutions of cobalt nitrate and glycine and subsequent use as an effective catalyst precursor for NaBH₄ hydrolysis. The catalytic activity results show that the hydrogen generation rate (HGR) at room temperature was much higher for the solution combustion synthesized material than for commercial Co₃O₄ nanopowder, though their specific surface areas were comparable (∼26–32 m²/g). Using a 0.6 wt.% aqueous solution of NaBH₄ at 20 °C and a 5 wt.% catalyst precursor loading, a HGR of 1.93 L min⁻¹ g_cat⁻¹ was achieved for solution combustion synthesized Co₃O₄. In contrast, at the same conditions, for commercial Co₃O₄ and elemental Co powders HGRs of 0.98 and 0.49 L min⁻¹ g_cat⁻¹ were achieved respectively. This type of synthesis is amenable to many complex metal oxide catalysts as well, such as LiCoO₂, which have also been shown to be good catalyst precursors for hydrolysis of NaBH₄.     

First-principles study on the synergistic effects of Ti and F co-doping on the structure and dehydrogenation properties of NaBH4

The synergistic effects of Ti and F co-doping on the structure and dehydrogenation properties of NaBH₄ are investigated by using density functional theory calculations. The results show that Ti is more likely to substitute Na, while F tends to replace the H in the BH₄ unit. It is found that Ti and F co-doped NaBH₄ systems are more stable than Ti-doped NaBH₄ system. The results of hydrogen desorption energies imply that the co-doped Ti and F decrease the strength of B–H bonds. In addition, the hydrogen desorption energies decrease as increasing the concentration of F atoms. The dehydrogenation reaction of Ti and F co-doped NaBH₄ is more likely to form TiB₂, B, NaF, and H₂.     

Simple and fast fabrication of polymer template-Ru composite as a catalyst for hydrogen generation from alkaline NaBH4 solution

Polymer template-Ru composite (Ru/IR-120) catalyst was prepared using a simple and fast method for generating hydrogen from an aqueous alkaline NaBH₄ solution. The hydrogen generation rate was determined as a function of solution temperature, NaBH₄ concentration, and NaOH (a base-stabilizer) concentration. The maximum hydrogen generation rate reached 132 ml min⁻¹ g⁻¹ catalyst at 298 K, using a Ru/IR-120 catalyst that contained only 1 wt.% Ru. The catalyst exhibits a quick response and good durability during the hydrolysis of alkaline NaBH₄ solution. The activation energy for the hydrogen generation reaction was determined to be 49.72 kJ mol⁻¹.     

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.     

Thermochemical recycling of hydrolyzed NaBH4. Part II: Systematical study of parameters dependencies

This paper focuses on the yields of both main product NaBH₄ and byproduct MgH₂ of the thermochemical process. The influence of parameters such as i) the isothermal reaction temperature in the range 480 °C–660 °C, ii) the stoichiometric ratio of solid reactants NaBO₂:Mg prepared from 1:2 to 1:8, iii) H₂ pressure supplied from 2 to 31 bars and iv) the reaction time kept at isotherm from 0 to 16 h have been systematically investigated. The yields are estimated by in-situ and ex-situ evaluations. Two temperature regimes for MgH₂ and NaBH₄ formation are recognized from 370 °C to 450 °C and above 500 °C respectively. With regard to NaBH₄ regeneration, temperature is the most important factor that positively accelerates the apparent reaction rate between 500 °C and 650 °C providing a sufficient H₂ pressure. To efficiently obtain high NaBH₄ yield mixtures with molar stoichiometric ratio between solid reactants not less than 1:4 is suggested. Experimental results also reveal that at 12 bars of H₂ pressure high NaBH₄ yield is obtained. Hence, more efficient way to improve mass transfer of solid reactants (e.g. advance reactor enhances mobility of reactants) rather than increasing H₂ pressures is advised. Under optimized condition, 100% conversion of NaBO₂ can be achieved within 1.5 h.     

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.     

New insights into the mechanism of H2 generation through NaBH4 hydrolysis on Co-based nanocatalysts studied by differential reaction calorimetry

To our knowledge, the present study is the first investigation by liquid-phase calorimetry of the mechanism of hydrogen generation by hydrolysis of sodium borohydride catalyzed by Co₂B nanoparticles generated in situ. The differential reaction calorimeter was coupled with a volumetric hydrogen measurement, allowing a simultaneous thermodynamic and kinetic study of the reaction. At the end of the reaction, the catalyst was characterized ex situ by TEM, XRD, magnetism, N₂ adsorption, TGA–DTA, and the liquid hydrolysis products were analyzed by Wet-STEM and ¹¹B-NMR. The in situ preparation method made it possible to form nanoparticles (<12 nm) of Co₂B which are the active phase for the hydrolysis reaction. In semi-batch conditions, the Co₂B catalyst formed in situ is subsequently reduced by each borohydride addition and oxidized at the end of the hydrolysis reaction by OH⁻ in the presence of metaborate. A coating of the nanoparticles has been observed by calorimetry and physico-chemical characterization, corresponding to the formation of a 2–3 nm layer of cobalt oxide or hydroxide species.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Hydrogen generation from Al/NaBH4 hydrolysis promoted by Li-NiCl2 additives

On-demand hydrogen generation from solid-state Al/NaBH₄ hydrolysis activated by Li-NiCl₂ additives are elaborated in the present paper. Hydrogen generation amount and rate can be regulated by changing Al/NaBH₄ weight ratio, Li and NiCl₂ amount, hydrolytic temperature, etc. The optimized Al−10 wt.% Li−15 wt.% NiCl₂/NaBH₄ mixture (weight ratio of 1:1) yields 1778 ml hydrogen/1 g mixture with 100% efficiency within 50 min at 323 K. The improved hydrolytic performance comes from the effect of Li-NiCl₂ additives, which decrease aluminum particle size in the milling process and produce the catalytic promoter BNi₂/Al(OH)₃ in the hydrolytic process. Compared with the conventional reaction of Al and NaBH₄ in water, there is an interaction of Al/NaBH₄ hydrolysis which improves the hydrolytic kinetics of Al/NaBH₄ via the catalytic effect of hydrolysis by-products Al(OH)₃, BNi₂, and NaBO₂. The Al/NaBH₄ mixture may be applied as a portable hydrogen generation material. Our experimental data lay a foundation for designing practical hydrogen generators.     

Effect of ball milling time on the hydrogen storage properties of TiF3-doped LiAlH4

In the present work, the catalytic effect of TiF₃ on the dehydrogenation properties of LiAlH₄ has been investigated. Decomposition of LiAlH₄ occurs during ball milling in the presence of 4 mol% TiF₃. Different ball milling times have been used, from 0.5 h to 18 h. With ball milling time increasing, the crystallite sizes of LiAlH₄ get smaller (from 69 nm to 43 nm) and the dehydrogenation temperature becomes lower (from 80 °C to 60 °C). Half an hour ball milling makes the initial dehydrogenation temperature of doped LiAlH₄ reduce to 80 °C, which is 70 °C lower than as-received LiAlH₄. About 5.0 wt.% H₂ can be released from TiF₃-doped LiAlH₄ after 18 h ball milling in the range of 60 °C–145 °C (heating rate 2 °C min⁻¹). TiF₃ probably reacts with LiAlH₄ to form the catalyst, TiAl₃. The mechanochemical and thermochemical reactions have been clarified. However, the rehydrogenation of LiAlH₄/Li₃AlH₆ can not be realized under 95 bar H₂ in the presence of TiF₃ because of their thermodynamic properties.     

Synergistic hydrogen generation from AlLi alloy and solid-state NaBH4 activated by CoCl2 in water for portable fuel cell

Solid-state AlLi/NaBH₄ mixture activated by CoCl₂ salt is fabricated for hydrogen generation via a milling process, providing uniform dispersion of AlLi alloy and CoCl₂ salt among pulverized NaBH₄ particles in order to improve NaBH₄ hydrolysis through the contact of NaBH₄ with active catalytic sites. The active catalytic sites come from Co₂B loaded in Al(OH)₃ (Bayerite) or LiAl₂(OH)₇ hydrate, generated from the reaction of CoCl₂, AlLi alloy, and NaBH₄ in water. The results show that the gravimetric hydrogen storage capacity is as high as 6.4 wt.% and an efficiency of above 90% in 30-min hydrolysis at 323 K could be achieved using the limited amount of water. The hydrogen generation amount and rate could be regulated by changing the composition, mixing style, mixture/water weight ratio, and hydrolysis temperature. The relative mechanism is explored.     

Preparation and catalytic activity of PVP-protected Au/Ni bimetallic nanoparticles for hydrogen generation from hydrolysis of basic NaBH4 solution

Poly(N-vinyl-2-pyrrolidone)(PVP)-protected Au/Ni bimetallic nanoparticles (BNPs) were prepared in one-vessel via chemical reduction of the corresponding ions with dropwise addition of NaBH₄, and their catalytic activity in the hydrogen generation from hydrolysis of a basic NaBH₄ solution was examined. The structure, particle size, and chemical composition of the resultant BNPs were characterized by Ultraviolet–visible spectrophotometry (UV–Vis), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HR-TEM). The effects of processing parameters such as metal composition, metal ion concentration, and mole ratio of PVP to metal ion on the hydrolysis of a basic NaBH₄ solution were studied in detail. The results indicated that as-prepared Au/Ni BNPs showed a higher catalytic activity than corresponding monometallic NPs (MNPs) in the hydrogen generation from the hydrolysis reaction of a basic NaBH₄ solution. Among all the MNPs and BNPs, Au/Ni BNPs with the atomic ratio of 50/50 exhibited the highest catalytic activity, showing a hydrogen generation rate as high as 2597 mL-H₂ min⁻¹ g-catalyst⁻¹ at 30 °C, which can be ascribed to the presence of negatively charged Au atoms and positively charged Ni atoms. Based on the kinetic study of the hydrogen generation from the hydrolysis reaction of a basic NaBH₄ solution over the PVP-protected Au/Ni BNPs, the corresponding apparent activation energy was determined as 30.3 kJ/mol for the BNPs with the atomic ratio of 50/50.