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.     

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.     

Effect of maleic acid on onset temperature and H2 release kinetics for thermal dehydrogenation of ammonia borane

Ammonia borane (AB, NH₃BH₃) has received great attention as an attractive hydrogen storage candidate because it has high hydrogen contents and releases hydrogen under mild operating conditions. Despite the favorable properties, AB thermolysis has several drawbacks such as long induction period, slow kinetics, and relatively high onset temperature, compared to hydrolysis approach. In this study, hydrogen release properties from AB were investigated in the addition of maleic acid (C₄H₄O₄, MA). Using thermogravimetric analysis, temperature programmed reaction with mass spectrometry, and FTIR analyses, the solid and gaseous products generated by thermolysis of the AB-MA mixture were characterized to understand the reaction mechanism. It was found that with the addition of MA, hydrogen yield and release kinetics were enhanced, while the onset temperature reduced significantly to ~60 °C. It is likely that the hydrolysis between O–H bonds in MA and B–H bonds in AB was initiated, and the heat released from the hydrolysis triggers the thermolysis of AB. It was also confirmed that a combination of the two additives (MA and boric acid) enables a further increase of H₂ yield while the onset temperature remains at ~60 °C. Our results suggest that MA is a promising additive to improve AB dehydrogenation.     

Room temperature hydrolytic dehydrogenation of ammonia borane catalyzed by Co nanoparticles

Amorphous and well dispersed Co nanoparticles (less than 10 nm) have been in situ synthesized in aqueous solution at room temperature. The as-synthesized Co nanoparticles possess high catalytic activity (1116 L mol⁻¹ min⁻¹) and excellent recycling property for the hydrogen generation from aqueous solution of ammonia borane under ambient atmosphere at room temperature. The present low-cost catalyst, high hydrogen generation rate and mild reaction conditions (at room temperature in aqueous solution) represent a promising step toward the development of ammonia borane as a viable on-board hydrogen-storage and supply material.     

Mechanistic studies of ammonia borane dehydrogenation

Ammonia borane (NH₃BH₃, AB) has received extensive attention as a potential hydrogen storage medium, however hydrogen release mechanisms from AB are not well understood. AB follows different reaction routes if the dehydrogenation occurs in solvent or solid state, but a comparative study for AB dehydrogenation in these two states is not available. In this work, a detailed study of AB dehydrogenation mechanism in diglyme and solid state is presented, and a comprehensive reaction network for both cases is proposed. The experimental and DFT results suggest that two main reaction pathways occur; one involves cyclization of monomers which results in faster dehydrogenation at lower temperature, while the other involves propagation to acyclic intermediates which requires higher temperature to carry out the cyclization step. AB dehydrogenation in solid state was experimentally found to be initiated by B–N bond cleavage and not by direct dehydrogenation, which agrees with high level CCSD(T)/MP2 calculations reported previously. It was found that diglyme plays a significant role in hindering B–N bond cleavage of AB which facilitates the cyclization pathway. In solid state, experiments including labeled AB (ND₃BH₃) mapped out the source of hydrogen (from hydridic or protonic ends), and a clear difference in the degree of dehydrogenation from the two ends is demonstrated.     

Study of kinetics and thermal decomposition of ammonia borane in presence of silicon nanoparticles

Ammonia borane (AB, NH₃BH₃) is a promising material by virtue of its high gravimetric hydrogen storage capacity of 19.6 wt%. Hydrogen release from AB initiates at around 100 °C and as such is compatible to meet the present-day requirements of a PEM fuel cell. The thermal decomposition of AB is a complex process involving several reactions. Major issues include poor reaction kinetics, leading to delayed commencement of hydrogen generation i.e. long induction period, and the small amount of hydrogen released at optimal temperature. In the current paper the thermal decomposition of AB is studied at different temperatures. Further the effect of Si nanoparticles on the induction period and kinetics as well as the gas release reaction is studied in detail using different characterization techniques. It was found that the induction period reduced and the amount of gas released increased as a result of Si nanoparticle addition. This was facilitated by a reduction in the activation energy of decomposition and improved kinetics with the addition of silicon nanoparticles.     

Mo remarkably enhances catalytic activity of Cu@MoCo core-shell nanoparticles for hydrolytic dehydrogenation of ammonia borane

Ammonia borane (AB) has been identified as one of the most promising candidates for chemical hydrogen storage. However, the practical application of AB for hydrogen production is hindered by the need of efficient and inexpensive catalysts. For the first time, we report that the incorporation of Mo into Cu@Co core-shell structure can significantly improve the catalytic efficiency of hydrogen generation from the hydrolysis of AB. The Cu_0.81@Mo_0.09Co_0.10 core-shell catalyst displays high catalytic activity towards the hydrolysis dehydrogenation of AB with a turnover frequency (TOF) value of 49.6 molH₂ mol_cat^?1 min^?1, which is higher than most of Cu-based catalysts ever reported, and even comparable to those of noble-metal based catalysts. The excellent catalytic performance is attributed to the multi-elements co-deposition effect and electrons transfer effect of Cu, Mo and Co in the tri-metallic core-shell NPs.     

Metal-free rapid dehydrogenation kinetics and better regeneration yield of ammonia borane

Ammonia borane (AB), one of the promising hydrogen storage materials, is receiving more attention because of its high hydrogen content. However, to act as a sustainable material for hydrogen storage, regeneration from products obtained on dehydrogenation is essential. Regeneration of AB reported in literature involves the use of expensive reagents with high toxicity and environmental impact. In this work, we attempted to aid the dehydrogenation/regeneration process of pure ammonia borane as well as products obtained on thermolysis of AB, using acetone as the dehydrogenation/digestion reagent. During dehydrogenation, a total of 1.7 equivalent of hydrogen was obtained through a thermal and chemical reaction with the resultant formation of tri and tetrasubstituted borate ester. In the current work, a simple method for regeneration of AB is being demonstrated from the dehydrogenated products, which are borate esters, in this case. Using this method, an overall regeneration yield, i.e., from starting material AB→ dehydrogenated products→ regenerated material AB is observed to be 68%. The conversion yield of regenerated AB, starting from dehydrogenated products, was found to be 83%, and that into hydrazine borane (HzB) to be 15% (which can also act as hydrogen source).     

One-step synthesis of graphene supported Ru nanoparticles as efficient catalysts for hydrolytic dehydrogenation of ammonia borane

Ru nanoparticles supported on graphene have been synthesized via a one-step procedure using methylamine borane as reducing agent. Compared with NaBH₄ and ammonia borane, the as-prepared Ru/graphene NPs reduced by methylamine borane exhibit superior catalytic activity towards the hydrolytic dehydrogenation of ammonia borane. Additionally, the Ru/graphene NPs exhibit higher catalytic activity than its graphene free counterparts, and retain 72% of their initial catalytic activity after 4 reaction cycles. A kinetic study shows that the catalytic hydrolysis of ammonia borane is first order with respect to Ru concentration, the turnover frequency is 100 mol H₂ min⁻¹ (mol Ru)⁻¹. The activation energy for the hydrolysis of ammonia borane in the presence of Ru/graphene NPs has been measured to be 11.7 kJ/mol, which is the lowest value ever reported for the catalytic hydrolytic dehydrogenation of ammonia borane.     

Enhanced dehydrogenation of ammonia borane by reaction with alkaline earth metal chlorides

The study of ammonia borane (AB) with controllable dehydrogenations is an active research topic for solid-state hydrogen storage materials. The present work shows that tuning the reactivity of both B–H and N–H bonds in AB by alkaline earth metal chlorides not only results in a significantly decrease in the onset dehydrogenation temperature to 40 °C but also suppresses undesirable volatile by-products due to the incorporation of alkaline earth metal chlorides in the AB dehydrogenation process. These results provide further insights into the promotion of hydrogen release from amidoboranes and related borohydride ammine complexes.     

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.     

Solvent-free synthesis of Rh/meso-Al2O3 via mechanochemistry for hydrolytic dehydrogenation of ammonia borane

An effective strategy synthesis of Rh/meso-Al₂O₃ catalysts was demonstrated by mechanochemistry for hydrolytic dehydrogenation of ammonia borane (AB). These catalysts are characterized systematically by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the turnover frequency (TOF) and activation energy (E_a) are 246.8 mol_H2·mol_Rh^?1·min^?1 and 47.9 kJ mol^?1 for hydrolytic dehydrogenation of at 298 K catalyzed by Rh/Al₂O₃-CTAB-400, obviously higher than those previously reported catalysts. Furthermore, catalyst Rh/Al₂O₃-CTAB-400 can be recycled by simple centrifugal separation and the catalytic activity is still well maintained after five cycles. In addition, a plausible mechanism for hydrolytic dehydrogenation of AB has also been proposed. This mechanochemical synthesis method exhibits great application prospects for the preparation of heterogeneous catalysts.     

Magnetically recyclable Fe-Ni alloy catalyzed dehydrogenation of ammonia borane in aqueous solution under ambient atmosphere

In this work, we report a very simple method to in situ prepare the Fe_1−xNi_x (x = 0, 0.3, 0.4, 0.5, 0.7 and 1) nano-alloys as the catalysts for H₂ generation from the aqueous NH₃BH₃ solution under ambient atmosphere at room temperature. The prepared nano-alloys possess Pt-like high catalytic activity, especially for the specimen of Fe_0.5Ni_0.5, with which the hydrolysis of NH₃BH₃ would totally complete in only 2.2 min. Moreover, these catalysts can be easily magnetically separated for recycle purpose, and can almost keep the same high activity even after 5 times of recycle under ambient atmosphere. Such alloy catalysts are expected to be useful for fuel cells, metal-air batteries and electrochemical sensors. Moreover, the concepts behind these preliminary results present a wide range of possibilities for the further development of synthesis of air and water-stable magnetic nano-alloys.     

Hydrolysis of solid ammonia borane

Ammonia borane NH₃BH₃ is a promising hydrogen storage material by virtue of a theoretical gravimetric hydrogen storage capacity (GHSC) of 19.5 wt%. However, stored hydrogen has to be effectively released, one way of recovering this hydrogen being the metal-catalyzed hydrolysis. The present study focuses on CoCl₂-catalyzed hydrolysis of NH₃BH₃ with the concern of improving the effective GHSC of the system NH₃BH₃-H₂O. For that, NH₃BH₃ is stored as a solid and H₂O is provided in stoichiometric amount. By this way, an effective GHSC of 7.8 wt% has been reached at 25 °C. To our knowledge, it is the highest value ever reported. Besides, one of the highest hydrogen generation rates (HGRs, 21 ml(H₂) min⁻¹) has been found. In parallel, the increases of the water amount and temperature have been studied and the reaction kinetics has been determined. Finally, it has been observed that some NH₃ release, what is detrimental for a fuel cell. To summarize, high performances in terms of GHSCs and HGRs can be reached with NH₃BH₃ and since research devoted to this boron hydride is at the beginning we may be confident in making it viable in a near future.     

Low-cost CuFeCo@MIL-101 as an efficient catalyst for catalytic hydrolysis of ammonia borane

Trimetallic nanoparticles of non-noble Cu–Fe–Co with different molar ratios were successfully immobilized in the metal-organic frameworks (MIL-101) via an easy impregnation–reduction process. XRD, TEM, XPS, ICP-MS and BET methods were used to characterize the catalyst. Comparing to their bimetallic counterparts, Cu₆Fe_0.8Co_3.2@MIL-101 demonstrates the best catalytic performance for dehydrogenation of ammonia borane by hydrolysis at 298 K Cu₆Fe_0.8Co_3.2@MIL-101 shows a total turnover frequency (TOF) value of 23.2 mol_H2 mol_catalyst⁻¹ min⁻¹ and an activation energy value of 37.1 kJ mol⁻¹. The enhancement of catalytic activity was attributed to a synergistic effect among copper, cobalt and iron nanoparticles supported on MIL-101. In addition, the catalyst still exhibits good stability and magnetic recyclability after seven cycles. The low-cost catalyst has good prospect for application in the field of hydrogen storage.     

Monodisperse PtCu alloy nanoparticles as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Pt-M alloy nanoparticles (NPs) with well-defined size and compositions exhibit dramatically catalytic performance in chemical reactions. In this work, monodisperse PtCu NPs with controlled size and compositions were synthesized by the co-reduction method in the presence of oleylamine. These NPs have excellent catalytic activities in the hydrolytic dehydrogenation of ammonia borane (AB) and their activities were composition dependent. Among the different-composition PtCu NPs, the Cu₅₀Pt₅₀ NPs exhibit the highest catalytic activity with an initial turnover frequency of 102.5 mol_(hydrogen)·mol_(catalyst)^?1·min^?1 and an apparent activation energy of 36 kJ·mol^?1, which demonstrate the validity of partly replacing Pt by a first-row transition metal on constructing high performance heterogeneous nanocatalysts for the hydrolytic dehydrogenation of AB.     

Catalytic hydrolysis of ammonia borane via magnetically recyclable copper iron nanoparticles for chemical hydrogen storage

In this work, a series of new Cu_1−xFe_x alloy nanoparticles (NPs) have been successfully in situ synthesized by a very simple method and used as catalysts for hydrogen generation from the aqueous solution of ammonia borane (AB) under ambient atmosphere at room temperature. The prepared nanoalloys exhibit excellent catalytic activity, especially for Cu_0.33Fe_0.67 sample outperform the activity of monometallic counterparts, and even of Cu@Fe core–shell NPs. By using an external magnet, these catalysts can be readily separated from the solution for recycle purpose, and can keep the high activity even after 8 times of recycle under ambient atmosphere. The hydrolysis activation energy for the Cu_0.33Fe_0.67 alloy NPs was measured to be approximately 43.2 kJ/mol, which is lower than most of the reported activation energy values for the same reaction using many different catalysts except for some noble-metal containing catalysts, indicating the superior catalytic performance of Cu_0.33Fe_0.67 nanocatalysts.     

Thermal decomposition of alkaline-earth metal hydride and ammonia borane composites

We demonstrate a method to improve the promising hydrogen storage capabilities of ammonia borane by making composites with alkaline-earth metal hydrides using ball-milling technique. The ball-milling for the mixtures of alkaline-earth metal hydride (MgH₂ or CaH₂) and ammonia borane (AB) yields a destabilization compared with the ingredient of the mixture, showing the hydrogen capacity of 8.7 and 5.8 mass% at easily accessible dehydrogenation peak temperatures of 78 and 72 °C, respectively, without the unwanted by-product borazine. Through detailed analyses on the dehydrogenation performance of the composite at various ratios in the hydride and AB, we proposed a different chemical activation mechanism from that in the LiH/AB and NaH/AB systems reported in a previous literature.