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.     

Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

Ammonia borane (AB, NH₃BH₃) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H₂ yield (11.5 wt% overall H₂ yield, 2.23 H₂ equivalent) at 85 °C, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH₃ (20–30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.     

Mechanistic insight into the promoting effect of magnesium nickel hydride on the dehydrogenation of ammonia borane

In this paper, we report an in-depth study of the post-milled 4AB/Mg₂NiH₄ sample, with a special focus on the promoting mechanism of Mg₂NiH₄ on the dehydrogenation of AB. A combination of X-ray diffraction (XRD), Fourier transformation infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) characterizations, together with selective isotopic labelling and other designed experiments, revealed that AB and Mg₂NiH₄ react with each other from the starting phase of the dehydrogenation process, which eventually results in the formation of MgNiBNH complexes. On the other hand, it was found that the reaction between AB and Mg₂NiH₄ cannot proceed directly, but requires phase transition of normal AB to its mobile phase AB* to occur first. Hence, the promoting mechanism of Mg₂NiH₄ on the dehydrogenation of AB is attributed to its promoting effect on phase transition of normal AB to AB* under mild conditions and in particular its chemical modification of AB with Mg and Ni.     

High and rapid hydrogen release from thermolysis of ammonia borane near PEM fuel cell operating temperatures

Ammonia borane (NH₃BH₃, AB), containing 19.6 wt % hydrogen, is a promising hydrogen storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles. Our experiments demonstrate the highest H₂ yield (∼14 wt %, 2.15 H₂ equivalent) values obtained by neat AB thermolysis near PEM FC operating temperatures, along with rapid kinetics, without the use of either catalyst or additives. It was also found that only trace amount of ammonia (<10 ppm) is produced during dehydrogenation reaction and spent AB products are polyborazylene-like species, which can be efficiently regenerated using currently demonstrated methods. The results indicate that our proposed method is the most promising one available in the literature to-date for hydrogen storage, and could be used in PEM FC based vehicle applications.     

Enhanced ammonia dehydrogenation over Ru/La(x)-Al2O3 (x = 0–50 mol%): Structural and electronic effects of La doping

Ru (1.0 wt% loaded)-based catalysts supported on La(x)-Al₂O₃ (x = 0, 1, 5, 10, and 50 mol%) were synthesized and characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) measurement, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and temperature programmed reduction (TPR). The as-prepared La(x)-Al₂O₃ materials were found to have increased amounts of the LaAlO₃ phase as the La doping level (x) increased from 0 to 50 mol%. In addition to metal-to-support interactions between Ru and Al₂O₃, the newly formed LaAlO₃ phase in the Ru catalysts was proposed to interact strongly with Ru active sites based on the XRD, H₂-TPR and XPS results. The Ru/La(x)-Al₂O₃ catalysts were active for the dehydrogenation of ammonia, and among them, the Ru/La(10)-Al₂O₃ and Ru/La(50)-Al₂O₃ (or Ru/LaAlO₃) catalysts exhibited superior performance with >96% conversions of ammonia at 550 °C. When an increased Ru content (2.0 wt%) was impregnated onto La(10)-Al₂O₃, the dehydrogenation activity was significantly improved with nearly 100% conversion (>95%) of ammonia at 500 °C. This catalyst further displayed an enhanced thermal stability towards ammonia decomposition with the GHSV_NH3 of 10,000 mL/g_cat h at 550 °C for >120 h. The incorporated element La is thought to play an important role in enhancing metal-support interaction, ultimately facilitating ammonia dehydrogenation even at low temperatures.     

Beyond Haber-Bosch: The renaissance of the Claude process

Ammonia may be one of the energy carriers in the hydrogen economy. Although research has mostly focused on electrochemical ammonia synthesis, this however remains a scientific challenge. In the current article, we discuss the feasibility of single-pass thermochemical ammonia synthesis as an alternative to the high-temperature, high-pressure Haber-Bosch synthesis loop. We provide an overview of recently developed low temperature ammonia synthesis catalysts, as well as an overview of solid ammonia sorbents. We show that the low temperature, low pressure single-pass ammonia synthesis process can produce ammonia at a lower cost than the Haber-Bosch synthesis loop for small-scale ammonia synthesis (<40 t-NH₃ d^?1).     

LiBH4·NH3BH3: A new lithium borohydride ammonia borane compound with a novel structure and favorable hydrogen storage properties

Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio results in the formation of a new complex, LiBH₄·NH₃BH₃. Its structure was successfully determined using combined X-ray diffraction and first-principles calculations. LiBH₄·NH₃BH₃ was carefully studied in terms of its decomposition behavior and reversible dehydrogenation property, particularly in comparison with the component phases. In parallel to the property examination, X-ray diffraction and Fourier transformation infrared spectroscopy techniques were employed to monitor the phase evolution and bonding structure changes in the reaction process. Our study found that LiBH₄·NH₃BH₃ first disproportionates into (LiBH₄)₂·NH₃BH₃ and NH₃BH₃, and the resulting mixture exhibits a three-step decomposition behavior upon heating to 450 °C, totally yielding ∼15.7 wt% hydrogen. Interestingly, it was found that h-BN was formed at such a moderate temperature. And owing to the in situ formation of h-BN, LiBH₄·NH₃BH₃ exhibits significantly improved reversible dehydrogenation properties in comparison with the LiBH₄ phase.     

Li2(NH2BH3)(BH4)/LiNH2BH3: The first metal amidoborane borohydride complex with inseparable amidoborane precursor for hydrogen storage

A novel lithium amidoborane borohydride complex, Li₂(NH₂BH₃)(BH₄), was synthesized using mechanochemical method and its crystal structure was successfully determined by a combination of X-ray diffraction (XRD) analysis and first-principles calculations. Interestingly, this compound does not exist as a pure phase, but requires almost equivalent amount of amorphous LiAB as a stabilizing agent. In this paper, we report a careful study of the structure, property, and dehydrogenation mechanism of the 1:1 Li₂(NH₂BH₃)(BH₄)/LiAB composite. This composite can release ∼8 wt% H₂ at 100 °C via a two-step dehydrogenation process, with dehydrogenation kinetics better than the parenting phases. The composite and its dehydrogenation products were characterized by the combined XRD, Fourier transformation infrared (FTIR) spectroscopy, and solid-state ¹¹B MAS NMR techniques. Selective deuterium labeling was performed to elucidate a reaction sequence for the hydrogen release by analyzing the released gases.     

Freeze-dried ammonia borane-polyethylene oxide composites: Phase behaviour and hydrogen release

A solid-state hydrogen storage material comprising ammonia borane (AB) and polyethylene oxide (PEO) has been produced by freeze-drying from aqueous solutions from 0% to 100% AB by mass. The phase mixing behaviour of AB and PEO has been investigated using X-ray diffraction which shows that a new ‘intermediate’ crystalline phase exists, different from both AB and PEO, as observed in our previous work (Nathanson et al., 2015). It is suggested that hydrogen bonding interactions between the ethereal oxygen atom (–O–) in the PEO backbone and the protic hydrogen atoms attached to the nitrogen atom (N–H) of AB molecules promote the formation of a reaction intermediate, leading to lowered hydrogen release temperatures in the composites, compared to neat AB. PEO also acts to significantly reduce the foaming of AB during hydrogen release. A temperature-composition phase diagram has been produced for the AB-PEO system to show the relationship between phase mixing and hydrogen release.     

High and rapid hydrogen release from thermolysis of ammonia borane near PEM fuel cell operating temperatures: Effect of quartz wool

Ammonia borane (NH₃BH₃, AB), containing 19.6 wt% hydrogen, is a promising hydrogen storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles. We recently demonstrated that using quartz wool, the highest H₂ yield (2.1–2.3H₂ equivalent) values were obtained by neat AB thermolysis near PEM FC operating temperatures, along with rapid kinetics, without the use of either catalyst or chemical additives. It was found that quartz wool minimizes sample expansion and facilitates the production of diamoniate of diborane (DADB), which is a key intermediate for the release of hydrogen from AB. It was also found that only trace amount of ammonia (<10 ppm) is produced during dehydrogenation reaction and spent AB products are found to be polyborazylene-like species, which can be efficiently regenerated using currently demonstrated methods. The results indicate that our proposed method is the most promising one available in the literature to-date for hydrogen storage, and could be used in PEM FC based vehicle applications.     

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.     

Zn-MOF assisted dehydrogenation of ammonia borane: Enhanced kinetics and clean hydrogen generation

We report controllable and enhanced hydrogen release kinetics at reduced temperatures in ammonia borane (AB) catalyzed by Zn-MOF-74. AB is loaded into the unsaturated Zn-metal coordinated one-dimensional hexagonal open nanopores of MOF-74 (ABMOF) via solution infiltration. The ABMOF system provides clean hydrogen by suppressing the release of detrimental volatile byproducts such as ammonia, borazine and diborane. These byproducts prevent the direct use of AB as a hydrogen source for polymer electrolyte membrane fuel cell applications. The H₂ release temperature, kinetics, and byproduct generation are dependent on the amount of AB loading. We show that nanoconfinement of AB and its interaction with the active Zn-metal centers in MOF are important in promoting efficient and clean hydrogen generation.     

Catalytic dehydrogenation of ammonia borane in non-aqueous medium

Dehydrogenation of Ammonia Borane (NH₃BH₃, AB) catalyzed by transition metal heterogeneous catalysts was carried out in non-aqueous solution at temperatures below the standard polymer electrolyte membrane (PEM) fuel cell operating conditions. The introduction of a catalytic amount (∼2 mol%) of platinum to a solution of AB in 2-methoxyethyl ether (0.02–0.33 M) resulted in a rapid evolution of H₂ gas at room temperature. At 70 °C, the rate of platinum catalyzed hydrogen release from AB was the dehydrogenation rate which was 0.04 g s⁻¹ H₂ kW⁻¹.     

Enabling easy and efficient hydrogen release below 80 °C from NaBH4 with multi-hydroxyl xylitol

NaBH₄ has been widely considered as a low-cost hydrogen storage material with high gravimetric hydrogen capacity of about 10 wt%. However, its strong thermodynamic stability severely hinders the application of NaBH₄ to obtain hydrogen. In this study, multi-hydroxyl xylitol, for the first time, was chosen as a protic hydrogen carrier to destabilize and react with NaBH₄. The NaBH₄-xylitol composites were easily prepared through hand-milling, which leads to the fast liberation of hydrogen from moderate temperatures lower than 80 °C with more than 3.6 wt% hydrogen desorption capacity. Besides, over 80% of hydrogen could be efficiently released in NaBH₄-xylitol composite. The dehydrogenation process is proved to be completely impurity-free and controllable by altering the NaBH₄/xylitol ratios and operating temperatures. The reaction between NaBH₄ and xylitol is investigated to be similar to an alcoholysis procedure, however no observable liquid phase takes place before and after the dehydrogenation of NaBH₄-xylitol composite, showing a great potential to produce hydrogen under low temperatures through solid-state carriers.     

Noncatalytic hydrothermolysis of ammonia borane

Hydrolysis of ammonia borane (AB) is attractive as a chemical method for hydrogen storage. The use of catalysts is, however, usually required. In the present paper, two new methods for releasing hydrogen from AB and water are investigated which do not involve any catalyst. One method is based on combustion of AB mixtures with nanoscale aluminum powder and gelled water. It is shown experimentally that these mixtures, upon ignition, exhibit self-sustained combustion with hydrogen release from both AB and water. The other method involves external heating of aqueous AB solutions to temperatures 120 °C or higher, under argon pressure to avoid water boiling. To clarify the reaction mechanism, isotopic experiments using D₂O instead of H₂O were conducted. It is shown that heating AB/D₂O solution to temperatures 117–170 °C releases 3 equiv. of hydrogen per mole AB, where 2–2.1 equiv. originate from AB and 0.9–1 equiv. from water. The prospects of both methods for hydrogen storage are discussed.     

Hydrogen storage in amine boranes: Ionic liquid supported thermal dehydrogenation of ethylene diamine bisborane

A variety of ionic liquids has been tested for its catalytic effect on the dehydrogenation of ethylene diamine bisborane (EDB). The catalytic activity of ionic liquids, such as 1-butyl-2,3-dimethylimidazolium chloride ([BMMIM]Cl), 1-butyl-2,3-dimethylimidazolium acetate ([BMMIM][OAc]), 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]) and 1-butyl-3-methylimidazolium methylsulfonate ([BMIM][OMs]) is compared and the mixture [BMMIM]Cl/EDB was investigated. This system is able to deliver about 6.5 wt% of hydrogen at 140 °C competing with conventional hydrogen storage pressure tanks. The correlation between polarity of the ILs and hydrogen yield was investigated and the suitability for hydrogen storage systems is evaluated and discussed.     

Study of the decomposition of a 0.62LiBH4–0.38NaBH4 mixture

This work highlights the dehydrogenation mechanisms of a 0.62LiBH₄–0.38NaBH₄ mixture in the range of 25–650 °C in flowing Ar. The dehydrogenation starts from 287 °C followed by two decomposition steps at 488 °C and 540 °C. These peak temperatures are in the range of 470 °C (for pure LiBH₄)–580 °C (for pure NaBH₄) due to different Pauling electronegativity values for Li⁺ (0.98) and Na⁺ (0.93) that affects the stability and decomposition temperatures. The 1^st step of dehydrogenation is accompanied with precipitation of LiH, Li₂B₁₂H₁₂ and B in between 287 and 520 °C; whilst the 2^nd step of dehydrogenation is mainly accompanied by the precipitation of Na and B when temperature is higher than 520 °C. The total amount of H₂ released is 10.8 wt.% that exceeds the estimated amount (8.9 wt.%), indicating less metal dodecaborate (than that for pure LiBH₄) is formed during the decomposition.