Long-term stability of sodium borohydrides for hydrogen generation

As a source of the high purity hydrogen, sodium and potassium borohydrides are investigated in terms of long-term stability in the form of the concentrated solutions, heterogeneous mixtures and in the solid state corresponding to NaBH₄ or KBH₄ crystal hydrates. In order to improve their stability during the long-term storage sodium and potassium hydroxides were added to the initial borohydride compositions. The effect of temperature, concentration of the borohydride and the alkaline solution, and the nature of the cation in the alkaline solution on the rate of borohydride hydrolysis was investigated. The differential technique developed for evaluation of the rate of borohydride hydrolysis was successfully applied for the determination of the long-term stability of the water-alkaline solutions containing NaBH₄(KBH₄)·5H₂O with 1–10 wt.% of NaOH or KOH at 30 °C and 50 °C.     

Preparation and dehydrogenation properties of two cobalt-based ammine borohydrides: CoCl3·3NH3/3LiBH4 and CoCl2·3NH3/2LiBH4

Two new cobalt-based ammine borohydrides were prepared via ball milling of LiBH₄ and CoCl_n·3NH₃ (n = 3, 2) with molar ratios of 3:1 and 2:1, respectively. X-ray diffraction (XRD) results revealed the as-prepared composites having amorphous state. Thermogravimetric analysis-mass spectrometry (TG-MS) measurements showed that the two composites mainly release H₂, concurrent with the evolution of a small amount of NH₃. Further results showed that the excessive addition of LiBH₄ can suppress the liberation of NH₃, resulting in the release of H₂ with a high purity (>99 mol.%). By combination with the temperature-programmed-desorption (TPD) results, the CoCl₃·3NH₃/4LiBH₄ and CoCl₂·3NH₃/3LiBH₄composites can release 7.3 wt.% (4.2 wt.% including LiCl) and 4.2 wt.% (2.0 wt.% including LiCl) pure hydrogen, respectively, in the temperature range of 25–300 °C. Isothermal dehydrogenation results reveal that CoCl₃·3NH₃/3LiBH₄ shows favorable dehydrogenation rate at low temperatures, releasing about 5.2 wt.% (2.9 wt.% including LiCl) of hydrogen within 45 min at 80 °C.     

Effects of electroless deposition conditions on microstructures of cobalt–phosphorous catalysts and their hydrogen generation properties in alkaline sodium borohydride solution

Cobalt–phosphorous (Co–P) catalysts with a high hydrogen generation rate in alkaline sodium borohydride (NaBH₄) solution are developed by electroless deposition. The microstructures of the Co–P catalysts and their catalytic activities for hydrolysis of NaBH₄ are analyzed as a function of the electroless deposition conditions such as the pH and temperature of the Co–P bath. The electroless-deposited Co–P catalysts are composed of nano-crystalline Co and amorphous Co–P. The size of the nano-crystalline Co particles dispersed in amorphous Co–P matrix depends largely on the electroless deposition conditions. Moreover, Co–P catalysts with finer crystalline Co exhibit a higher hydrogen generation rate. In particular, the Co–P catalysts formed in a pH 12.5 bath at 60–70 °C exhibit the best hydrogen generation rate of 3300 ml min⁻¹ g⁻¹-catalyst in 1 wt.% NaOH + 10 wt.% NaBH₄ solution at 30 °C, which is 60 times faster than that obtained with a Co catalyst.     

Synthesis of ammine dual-metal (V,Mg) borohydrides with enhanced dehydrogenation properties

The penta-ammine vanadium (III) borohydride, i.e. V(BH₄)₃·5NH₃, was successfully synthesized via ball-milling of VCl₃·5NH₃ and LiBH₄ in a molar ratio of 1:3. This compound was shown to release 11.5 wt% hydrogen with a H₂-purity of 85 mol% by 350 °C. To improve the dehydrogenation purity of V(BH₄)₃·5NH₃, Mg(BH₄)₂ with various molar ratios was mixed with V(BH₄)₃·5NH₃ to synthesize expected ammine metal-mixed borohydrides, among which the formed VMg(BH₄)₅·5NH₃ was indexed to be a monoclinic unit cell with lattice parameters of a = 19.611 Å, b = 14.468 Å, c = 6.261 Å, β = 93.678° and V = 1772.75 Å³. Dehydrogenation results revealed that the Mg(BH₄)₂ modified V(BH₄)₃·5NH₃ system presents significantly enhanced dehydrogenation purity. For example, in the case of V(BH₄)₃·5NH₃/2Mg(BH₄)₂ sample, 12.4 wt% pure hydrogen can be released upon heating to 300 °C. Further investigation on the dehydrogenation mechanism of the VMg(BH₄)₅·5NH₃ system by isotope tagging revealed that the interactions of homo-polar BH units also participated throughout the dehydrogenation process (onset at 75 °C) as complementary to the prime combination of BH···HN.     

Carbon-supported cobalt catalyst for hydrogen generation from alkaline sodium borohydride solution

Low cost transition metal catalysts with high performance are attractive for the development of on-board hydrogen generation systems by catalytic hydrolysis of sodium borohydride (NaBH₄) in fuel cell fields. In this study, hydrogen production from alkaline NaBH₄ via hydrolysis process over carbon-supported cobalt catalysts was studied. The catalytic activity of the supported cobalt catalyst was found to be highly dependent on the calcination temperatures. The hydrogen generation rate increases with calcination temperatures in the range of 200–400 °C, but a high calcination temperature above 500 °C led to markedly decreased activity. X-ray diffraction patterns reveal that the catalysts experience phase transition from amorphous Co–B to crystalline cobalt hydroxide with increase in calcination temperatures. The reaction performance is also dependent on the concentration of NaBH₄, and the hydrogen generation rate increases for lower NaBH₄ concentrations and decreases after reaching a maximum at 10 wt.% of NaBH₄.     

Enhanced hydrogen storage properties of LiBH4 modified by NbF5

In this work, the hydriding–dehydriding properties of the LiBH₄–NbF₅ mixtures were investigated. It was found that the dehydrogenation and reversibility properties of LiBH₄ were significantly improved by NbF₅. Temperature-programed dehydrogenation (TPD) showed that 5LiBH₄–NbF₅ sample started releasing hydrogen from as low as 60 °C, and 4 wt.% hydrogen could be obtained below 255 °C. Meanwhile, ∼7 wt.% H₂ could be reached at 400 °C in 20LiBH₄–NbF₅ sample, whereas pristine LiBH₄ only released ∼0.7 wt.% H₂. In addition, reversibility measurement demonstrated that over 4.4 wt.% H₂ could still be released even during the fifth dehydrogenation in 20LiBH₄–NbF₅ sample. The experimental results suggested that a new borohydride possibly formed during ball milling the LiBH₄–NbF₅ mixtures might be the source of the active effect of NbF₅ on LiBH₄.     

Investigation of sodium borohydride production process: “Ulexite mineral as a boron source”

Currently, the energy requirements of the entire world are mostly provided by hydrocarbon-based fossil fuels, such as coal, fuel oil, or natural gas. Because of environmental pollution, decrease in energy sources, and difficulties in storing electricity, more attention is dedicated to new sources of energy, such as hydrogen. Presently, sodium borohydride (NaBH₄) appears to be an excellent hydrogen-storage medium due to its high theoretical hydrogen yield by weight, 10.6%. The main aim of the present study is to investigate NaBH₄ production from ulexite mineral (NaCaB₅O₉·8H₂O). The experimental investigation consists of four steps, such as (1) Characterization of NaCaB₅O₉·8H₂O by X-ray diffraction, differential thermal and thermogravimetric analysis, scanning electron microscopy, and attenuated total reflectance of Fourier-transform infrared spectroscopy; (2) Preparation of ulexite–borosilicate glass (NaCaB₅O₉·SiO₂); (3) Synthesis of NaBH₄ from ulexite–borosilicate glass; and (4) Separation of NaBH₄ from the reaction mixture. NaBH₄ can thus be produced by heating ulexite mineral form of borosilicate glass with metallic sodium under 3-atm. hydrogen pressure at 450–500 °C for 4 h.     

Hydrogen storage properties of Li–Ca–N–H system with different molar ratios of LiNH2/CaH2

In this work, dehydrogenation and rehydrogenation of three LiNH₂/CaH₂ samples with LiNH₂/CaH₂ molar ratio of 2/1, 3/1 and 4/1 were systematically investigated. Remarkable differences were observed in the temperature dependence of hydrogen desorption and subsequent absorption. LiNH₂/CaH₂ in a molar ratio of 2/1 transforms to ternary imide Li₂Ca(NH)₂ after desorbing about 4.5 wt.% H₂ at 350 °C. And it has a reversible hydrogen storage capacity of 2.7 wt.% at 200 °C. As for the LiNH₂/CaH₂ mixture in a molar ratio of 4/1, it transforms to a new compound with a composition of Li₄CaN₄H₆ after being dehydrogenated at 350 °C. The rehydrogenation of both LiCa(NH)₂ and Li₄CaN₄H₆ gives LiNH₂, LiH and the solid solution of 2CaNH–Ca(NH₂)₂.     

The reactions in LiBH4-NaNH2 hydrogen storage system

Stepwise reactions were observed in the ball milling and heating process of the LiBH₄-NaNH₂ system by means of X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FT-IR). During the ball milling process, two concurrent reactions take place in the mixture: 3LiBH₄ + 4NaNH₂ → Li₃Na(NH₂)₄ + 3NaBH₄ and LiBH₄ + NaNH₂ → LiNH₂ + NaBH₄. The heating process from 50 °C to 400 °C is mainly the concurrent reactions of Li₃Na(NH₂)₄ + 3LiBH₄ → 2Li₃BN₂ + NaBH₄ + 8H₂ and 2LiNH₂ + LiBH₄ → Li₃BN₂H₈ → Li₃BN₂ + 4H₂, where the intermediate phases Li₃Na(NH₂)₄ and LiNH₂ serve as the reagents to decompose LiBH₄. The merged equations for the mechanochemical and the heating reactions below 400 °C can be denoted as 3LiBH₄ + 2NaNH₂ → Li₃BN₂ + 2NaBH₄ + 4H₂. The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H₂, which agrees well with the theoretical capacity (5.5 wt.% H₂) of the merged equation and thus demonstrates the suggested pathway. The subsequent step consists of the decompositions of NaBH₄ and Li₃Na(NH₂)₄ within the temperature range of 400 °C-600 °C. The apparent activation energies of the two steps are 114.8 and 123.5 kJ/mol, respectively. They are all lower than that of our previously obtained bulk LiBH₄.     

Improved dehydrogenation properties of the combined Mg(BH4)2·6NH3–nNH3BH3 system

A combined strategy via mixing Mg(BH₄)₂·6NH₃ with ammonia borane (AB) is employed to improve the dehydrogenation properties of Mg(BH₄)₂·6NH₃. The combined system shows a mutual dehydrogenation improvement in terms of dehydrogenation temperature and hydrogen purity compared to the individual components. A further improved hydrogen liberation from the Mg(BH₄)₂·6NH₃–6AB is achieved with the assistance of ZnCl₂, which plays a crucial role in stabilizing the NH₃ groups and promoting the recombination of NH^δ+?HB^δ−. Specifically, the Mg(BH₄)₂·6NH₃–6AB/ZnCl₂ (with a mole ratio of 1:0.5) composite is shown to release over 7 wt.% high-pure hydrogen (>99 mol%) at 95 °C within 10 min, thereby making the combined system a promising candidate for solid hydrogen storage.     

Thermochemical production of sodium borohydride from sodium metaborate in a scaled-up reactor

Sodium borohydride (NaBH₄) is a safe and practical hydrogen storage material for on-board hydrogen production. However, a significant obstacle in its practical use on-board hydrogen production system is its high cost. Hence, the reproduction of NaBH₄ from byproducts that precipitate after hydrolysis is an important strategy to make its use more cost effective. In this work, we focused on the optimization of thermochemical NaBH₄ reproduction reaction in a large-scaled reactor (∼100 ml), and we investigated the effects of the reaction temperature (400–600 °C) and H₂ pressure (30–60 bar) on the NaBH₄ conversion yield using Mg as a reducing agent. The conversion yield of NaBO₂ to NaBH₄ increased with an increase in H₂ pressure to 55 bar and then decreased slightly at 60 bar. The yield increased with an increase in the reactor temperature from 400 to 600 °C. The maximum yield was 69% at 55 bar and 600 °C using homogenized reactants by ball-milling for 1 h under an Ar atmosphere. Though Ca as a reducing agent makes the thermochemical reproduction reaction more favorable, the NaBH₄ yield was low after 1 h of production at 55 bar and 600 °C. This result may be due to the fact that Ca is not as effective as Mg in catalyzing the conversion of hydrogen gas to protide (H⁻), which can substitute oxygen actively in NaBO₂.     

NH3BH3/LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation

The significantly enhanced dehydrogenation performance of binary complex system, NH₃BH₃/LiBH₄·NH₃, were achieved through a chemical modification of LiH to form ternary composites of x (LiH–NH₃BH₃)/LiBH₄·NH₃. Among the studied composites, 3LiH–3NH₃BH₃/LiBH₄·NH₃ released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 °C with fast kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile byproducts, were released from 3NH₃BH₃/LiBH₄·NH₃ at the same conditions. Further investigations revealed that the hydrogen emission from x (LiH–NH₃BH₃)/LiBH₄·NH₃ composites is based on the combination mechanism of H^δ+ and H^δ− through the interaction between LiH–NH₃BH₃ and NH₃ group in LiBH₄·NH₃, in which the controllable protic hydrogen source from the stabilized NH₃ group played a crucial role in providing optimal stoichiometric ratio of H^δ+ and H^δ−, and thus leading to the significant improvement of dehydrogenation capacity and purity.     

Hydrogen release from a mixture of NaBH4 and Mg(OH)2

Hydrogen generating reaction between sodium borohydride, NaBH₄, and magnesium hydroxide, Mg(OH)₂ (brucite), was studied. Reaction rate was found to depend on the degree of reactants homogenization and/or their particle size. Kinetic of the reaction was studied in isothermal approach in the temperature range of 240–360 °C. It is shown that the reaction obeys 2D diffusion mechanism and its activation energy is 155.9 kJ/mol. Powder XRD analysis and Raman spectroscopy reveal that mechanically activated mixture of NaBH₄ and Mg(OH)₂ reacts yielding MgO as the only crystalline phase in the temperature range of 240–318 °C. At higher temperatures a new crystalline tetragonal phase of as yet undetermined composition is developed.     

Hydrogen generation from sodium borohydride solution using a ruthenium supported on graphite catalyst

The catalyst with high activity and durability plays a crucial role in the hydrogen generation systems for the portable fuel cell generators. In the present study, a ruthenium supported on graphite catalyst (Ru/G) for hydrogen generation from sodium borohydride (NaBH₄) solution is prepared by a modified impregnation method. This is done by surface pretreatment with NH₂ functionalization via silanization, followed by adsorption of Ru (III) ion onto the surface, and then reduced by a reducing agent. The obtained catalyst is characterized by transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Very uniform Ru nanoparticles with sizes of about 10 nm are chemically bonded on the graphite surface. The hydrolysis kinetics measurements show that the concentrations of NaBH₄ and NaOH all exert considerable influence on the catalytic activity of Ru/G catalyst towards the hydrolysis reaction of NaBH₄. A hydrogen generation rate of 32.3 L min⁻¹ g⁻¹ (Ru) in a 10 wt.% NaBH₄ + 5 wt.% NaOH solution has been achieved, which is comparable to other noble catalysts that have been reported.     

Investigation of the anhydrous molten Na–B–O–H system and the concept: Electrolytic hydriding of sodium boron oxide species

Although sodium borohydride (NaBH₄) can act as an excellent hydrogen storage material, its cost renders it impractical for automotive applications. In this paper the concept of electrolytic production of NaBH₄ from sodium metaborate (NaBO₂) is introduced following a literature review of NaBH₄ synthesis. By deduction, we assert that only by employing dense solid oxide ion electrolytes and a molten salt solution containing the two constituents would such a process be possible. We investigated the molten anhydrous Na–B–O–H system by pressure differential thermal analysis (PDTA), X-ray diffraction (XRD) and gas evolution analysis (GEA) using the starting reagents sodium hydride (NaH), NaBO₂ and NaBH₄. We found that molten NaBH₄ is not stable with NaBO₂ above 600 °C due to the formation of sodium orthoborate (Na₄B₂O₅), hydrogen and boron. However, the quasi-reciprocal ternary system, (4/5)NaH–NaBO₂–(1/5)NaBH₄–(2/5)Na₄B₂O₅, that was discovered, proves that molten Na₄B₂O₅ is miscible and stable with molten NaBH₄ to at least 650 °C under the hydrogen pressures used in this study. As well, the compound Na₆B₂O₅H₂ was discovered and a substantial portion of the anhydrous Na–B–O–H phase diagram has been experimentally deduced. There is a large ionic liquid composition domain within the system that would allow for the electrolytic hydriding of sodium boron oxide species to be tested.     

Combination of two H-enriched B–N based hydrides towards improved dehydrogenation properties

A new hydrogen storage system NaZn(BH₄)₃?2NH₃-nNH₃BH₃ (n = 1–5) was synthesized via a simple ball milling of NaZn(BH₄)₃?2NH₃ and NH₃BH₃ (AB) with a molar ratio from 1 to 5. Dehydrogenation results revealed that NaZn(BH₄)₃?2NH₃-nAB (n = 1–5) showed a mutual dehydrogenation improvement in terms of significant decrease in the dehydrogenation temperature and preferable suppression of the simultaneous evolution of by-products (i.e. NH₃, B₂H₆ and borazine) compared to the unitary compounds (NaZn(BH₄)₃?2NH₃ and AB). Specially, the NaZn(BH₄)₃?2NH₃-4AB sample is shown to reach the maximum hydrogen purity (99.1 mol %) and favorable dehydrogenation properties rapidly releasing 11.6 wt. % of hydrogen with a peak maximum temperature of 85 °C upon heating to 250 °C. Isothermal dehydrogenation results revealed that 9.6 wt. % hydrogen was liberated from NaZn(BH₄)₃?2NH₃-4AB within 80 min at 90 °C. High-resolution in-situ XRD and Fourier transform infrared (FT-IR) measurements indicated that the significant improvements on the dehydrogenation properties in NaZn(BH₄)₃?2NH₃-4AB can be attributed to the interaction between the NH₃ group from NaZn(BH₄)₃?2NH₃ and AB in the mixture, resulting a more activated H^δ+···^−δH combination. The research on the reversibility of the spent fuels of NaZn(BH₄)₃?2NH₃-4AB showed that regeneration could be partly achieved by reacting them with hydrazine in liquid ammonia. These aforementioned favorable dehydrogenation properties demonstrate the potential of the combined systems to be used as solid hydrogen storage material.     

Nanoconfinement of LiBH4·NH3 towards enhanced hydrogen generation

Successful synthesis of LiBH₄·NH₃ confined in nanoporous silicon dioxide (LiBH₄·NH₃@SiO₂) was achieved via a new “ammonia-deliquescence” method, which avoids the involvement of any solvents during the process of synthesis. Compared to the pure LiBH₄·NH₃, the confined LiBH₄·NH₃@SiO₂ exhibited significantly improved dehydrogenation properties, which not only suppressed the emission of NH₃, but also decreased the onset dehydrogenation temperature to 60 °C, thus leading to an enhanced conversion of NH₃ to H₂. In the temperature range of 60–300 °C, the mole ratio of H₂ release for the confined LiBH₄·NH₃@SiO₂ is 85 mol % of the total gas evolved, compared to 2.66 mol % for the pristine LiBH₄·NH₃. Isothermal dehydrogenation results showed that the LiBH₄·NH₃@SiO₂ is able to release about 1.26, 2.09, and 2.35 equiv. of hydrogen, at 150 °C, 200 °C, and 250 °C, respectively. From analysis of the Fourier transform infrared, Raman, and nuclear magnetic resonance spectra of the confined LiBH₄·NH₃@SiO₂ sample heated to various temperatures, as well as its dehydrogenation product under NH₃ atmosphere, it is proposed that the improved dehydrogenation of LiBH₄·NH₃@SiO₂ is mainly attributable to two crucial factors resulting from the nanoconfinement: (1) stabilization of the NH₃ in the nanopores of SiO₂, and (2) enhanced combination of LiBH₄ and NH₃ groups, leading to fast dehydrogenation at low temperature.     

Study of by-product of NaBH4 hydrolysis and its behavior at a room temperature

We report here the observation of phase transitions within mixtures of NaBH₄ and various organic additives exposed to open air at a room temperature 22(±1) °C and air humidity 35(±5)%. During the exposure the phase composition of materials was monitored by X-ray diffraction. Also, we found that new crystal phases appear during the exposure. Initially the phase, which was identified as the orthorhombic phase with unit cell parameters a = 10.48 Å, b = 6.91 Å and c = 12.25 Å and Pbca space group, was formed. It has been ascertained that orthorhombic phase is NaBH₄·2H₂O – hydrated sodium boron hydride. Upon prolonged exposure this phase then finally transformed to the known crystalline tincalconite (Na₂B₄O₅(OH)₄·3H₂O). We explained the observed phase transitions as hydrolysis of NaBH₄ and hydration of by-product due to absorption of air humidity. It was ascertained that kinetics of processes depends on kind of organic additives.     

The enhanced hydrogen storage performance of (Mg–B–N–H)-doped Mg(NH2)–2LiH system

Doping Mg(NH₂)₂–2LiH by Mg₂(BH₄)₂(NH₂)₂ compound exhibits enhanced hydrogen de/re-hydrogenation performance. The peak width in temperature-programmed desorption (TPD) profile for the Mg(NH₂)₂–2LiH–0.1Mg₂(BH₄)₂(NH₂)₂ was remarkably shrunk by 60 °C from that of pristine Mg(NH₂)₂–2LiH, and the peak temperature was lowered by 12 °C from the latter. Its isothermal dehydrogenation rate was greatly improved by five times from the latter at 200 °C. XRD, FTIR and NMR analyses revealed that a series of reactions occurred in the dehydrogenation of the composite. The prior interaction between LiH and Mg–B–N–H yielded intermediate LiBH₄, which subsequently reacted with Mg(NH₂)₂ and LiH in molar ratio of 1:6:9 to form Li₂Mg₂(NH)₃ and Li₄BN₃H₁₀ phases. The observed 6Mg(NH₂)₂–9LiH–LiBH₄ combination dominated the hydrogen release and soak in the composite system, and enhanced the kinetics of the system.