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.     

Ceria-based quantum dots/nanorods supported metallic cobalt for efficient photocatalytic hydrogen production

The concentration of photogenerated electrons on support surface has a significant impact on the photocatalytic performance of the corresponding catalyst. Herein, the CeO₂-based homojunction support consisted of quantum dots/nanorods (QDs/NRs) was fabricated by two-step calcination with assistance of KCI and NaCl. Based on CeO₂-QDs/NRs support, the Co-based catalyst exhibited excellent catalytic performance for photocatalytic hydrogen evolution from Ammonia Borane (NH₃BH₃). The catalysts exhibited the highest activity with TOF 96.15 min⁻¹ under optimized conditions, which was significantly improved compared Co/CeO₂ NRs (68.5 min⁻¹). Detailed structure characterizations revealed that the QDs with size range from 2 to 5 nm grow on the surface of NRs, which had capacity to transfer more photogenerated electrons from the bulk to surface compared with pristine CeO₂ NRs. Meanwhile, work function was upshifted from CeO₂ NRs to CeO₂-QDs/NRs. The synergy of two factors drove more electrons transfer from CeO₂-QDs/NRs to active metal Co, accelerating the adsorption and activation of NH₃BH₃. In addition, the forming mechanization QDs by inducing the morphological evolution of CeO₂ nanoparticles was also investigated. This work not only provides efficient photocatalyst for H₂ evolution from NH₃BH₃ but also provides new insights into the design and preparation efficient QDs-based homojunction catalyst.     

Applied ultrasound assisted research on synthesis and in-situ hydrolysis of ammonia borane for hydrogen energy

Ultrasound-assisted research on both synthesis and in-situ hydrolysis of ammonia borane (NH₃BH₃) for hydrogen energy application was experimentally investigated in this study. The salt metathesis reaction between sodium borohydride (NaBH₄) and ammonia sulfate ((NH₄)₂SO₄) in the presence of organic solvent, tetrahydrofuran (THF), was performed to NH₃BH₃ synthesis by application of ultrasound. Specifically, the effect of molar ratio (0.96–1.06), solvent volume (100–300 ml), time (40–160 min) and temperature (20–60 °C) on yield was evaluated to optimize the reaction conditions. The effect of drying process was also assessed under specified conditions, and chemical structures of synthesized samples were compared with commercial supplied NH₃BH₃. The present investigation provided data on the crystal, and chemical properties of lab-made NH₃BH₃ and results confirmed the promoting effect of ultrasound on synthesis with excellent yield-96 wt. %. Moreover this, hydrogen deposition characteristics of concentrated lab-made NH₃BH₃ solutions (5–25 wt.%) were assisted by ultrasound to evaluate hydrogen by in-situ reaction in the presence of active metals (Co²⁺-, Ni²⁺-, and Cu²⁺-).     

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.     

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.     

Large-scale decomposition of green ammonia for pure hydrogen production

It is acknowledged that Hydrogen has a decisive role to play in insuring a reliable and efficient penetration of renewable electricity in the energy mix. Nonetheless, building a sustainable Hydrogen Economy is faced with numerous challenges across the value chain. Namely, large-scale production and storage are still open issues that need to be addressed. A promising solution is to store renewable H₂ in the form of green ammonia often referred to as Power-to-Ammonia. Thus unlocking all available infrastructure for ammonia to effectively store and export hydrogen in bulk. In this value chain, the missing link is ammonia cracking to recover back hydrogen at high purities. The present work explores a technical solution to recover hydrogen from ammonia at large-scale. Through an exhaustive technoeconomic analysis, we have demonstrated the feasibility of large-scale production of pure H₂ from ammonia. The designed Ammonia-to-H₂ plant operates at a thermal efficiency of 68.5% to produce 200 MTPD of pure hydrogen at 250 bar. Furthermore, this study has established a final estimation of the Levelized Cost of Hydrogen (LCOH) from green ammonia. It was revealed that LCOH is mostly dependent on green ammonia cost, which in turn varies with renewable electricity cost.     

Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation

In this report, graphene oxide (GO) nanosheets decorated with ultrafine Pd nanoparticles (Pd NPs) have been successfully fabricated through a reaction between [Pd₂(μ-CO)₂Cl₄]²⁻ and water in the presence of GO nanosheets without any surfactant or other reductant. The as-synthesized small Pd NPs with average diameter of about 4.4 nm were well-dispersed on the surface of GO nanosheets. The Pd/GO nanocomposites show remarkable catalytic activity toward the hydrogenation of p-nitrophenol at room temperature. The kinetic apparent rate constant (k_app) could reach about 34.3 × 10⁻³ s⁻¹. Furthermore, the as-prepared Pd/GO nanocomposites could also be used as an efficient and stable catalyst for hydrogen production from hydrolytic dehydrogenation of ammonia borane (AB). The catalytic activity is much higher than the conventional Pd/C catalysts.     

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.     

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.     

Boron- and nitrogen-based chemical hydrogen storage materials

Boron- and nitrogen-based chemical hydrides are expected to be potential hydrogen carriers for PEM fuel cells because of their high hydrogen contents. Significant efforts have been devoted to decrease their dehydrogenation and hydrogenation temperatures and enhance the reaction kinetics. This article presents an overview of the boron- and nitrogen-based compounds as hydrogen storage materials.     

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.     

Enhanced dehydrogenation performance of ammonia borane con-catalyzed by novel TiO2(B) nanoparticles and bio-derived carbon with well-organized pores

A novel TiO₂(B) confined in porous bio-derived carbon has been prepared for dehydrogenated catalyzation of NH₃BH₃. The microstructural characterizations of as-prepared samples show that the nanoconfinement in well-organized micro/mesopores of carbon can avoid the aggregations of TiO₂(B) nanoparticles and NH₃BH₃. The dehydrogenation measurement demonstrates the dehydrogenated thermodynamic and dynamic properties of NH₃BH₃ could be improved under the con-catalyzation of TiO₂(B) and porous carbon. The results suggest that both TiO₂(B) and porous bio-derived C are promising catalysts. Additionally, it also provides a high-value solution for the disposal of agricultural wastes.     

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.     

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.     

A review on the catalysts used for hydrogen production from ammonia borane

Boron compounds have recently attracted attention in hydrogen production since they contain many hydrogen atoms. Among these compounds, ammonia borane, which has high hydrogen density (in weight basis), can be used to produce hydrogen through a hydrolysis reaction. However, since the ammonia borane solution is highly resistant to hydrolysis under ambient conditions, there is a need for active and stable catalysts to accelerate the reaction. In this review paper, unsupported and carbon-based supported metal catalysts used for hydrogen production through the hydrolysis of ammonia borane are presented. Noble metal catalysts (Ru, Rh, Pd, Pt and their binary and ternary alloys) and non-noble metal catalysts (Co, Ni, Fe, Cu and their binary and ternary alloys) were examined. The activation energy of reaction and turnover frequency (TOF) values were compared for these catalysts. Among the unsupported catalysts, it was concluded that the multi-metal catalyst systems (binary, ternary and quaternary) have higher catalytic activity than a single use of the same metals. In addition, the comparison showed that the supported catalysts are more resistant to catalytic cycles and suitable for long-term use. It was observed that CNT supported Rh (TOF = 706 mol H₂ mol cat⁻¹ min⁻¹) and graphene supported Ru (TOF = 600 mol H₂ mol cat⁻¹ min⁻¹) catalysts are the most active catalysts for the hydrogen generation from the ammonia borane at room temperature.     

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.     

Calcium hydrazinidoborane: Synthesis,characterization, and promises for hydrogen storage

Viewing calcium hydrazinidoborane Ca(N₂H₃BH₃)₂ (9.3 wt% H) as a potential hydrogen storage material, we long sought to synthesize it by solid-solid reaction of calcium hydride CaH₂ and hydrazine borane N₂H₄BH₃. However, it was elusive because of unsuitable experimental conditions. In situ synchrotron thermodiffraction helped us to identify the key role played by the temperature in the formation of the new phase. From 45 °C, new diffraction peaks appear, and the DSC analysis shows an exothermic signal. Thermal activation is thus required to make solid-state CaH₂ react with melted (liquid-state) N₂H₄BH₃. The XRD pattern can be indexed using a mixture of two phases: (i) unreacted CaH₂ as a minor phase (29 wt%) and (ii) the hitherto elusive Ca(N₂H₃BH₃)₂ (71 wt%). The as-formed Ca(N₂H₃BH₃)₂ crystallizes in a monoclinic Ic (No. 9) unit cell where the intermolecular interactions form chains (layers) along the a axis, resulting in intra-chain and inter-chain Ca⋅⋅⋅Ca distances as short as 4.39 and 7.04 Å respectively. Beyond 90 °C, Ca(N₂H₃BH₃)₂ decomposes, as evidenced by the diffraction peaks fading, an exothermic signal revealed by DSC, a weight loss (5.3 wt% at 200 °C) observed by TGA, and a gas release (H₂, and some N₂, NH₃, N₂H₄) monitored by MS. The as-formed thermolytic residue is amorphous and of complex polymeric composition. These results and the next challenges, are discussed herein.     

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).     

Amorphous Ni/C nanocomposites from tandem plasma reaction for hydrogen evolution

There is a growing interest in the nanostructure of metal nanoparticles encapsulated in carbon shells (M/C) to act as cost-effective and high-performance electrocatalysts for hydrogen production. Despite tremendous efforts, large-scale and controllable production of highly-active M/C nanocomposites for electrocatalysis remains a great challenge. Herein, we report a tunable and general strategy tandem plasma reaction (TPR) for manufacturing a series of carbon encapsulated Ni/C nanocomposites with different compositions. Benefiting from the synergistic effect between the conductive carbon layer and Ni nanoparticles, the electrocatalytic performance can be adjusted to an optimal state. A volcano shape dependence between electrocatalytic activity and the percentage of carbon is presented experimentally. With the simplicity and intelligence of the approach, the strategy based on tandem plasma reaction presents great potential to the preparation of extendable metal/carbon nanocomposites for versatile applications.     

Theoretical exploration of the mechanisms on the iron complexes catalyzed ammonia borane dehydrogenation and polyaminoborane formation

Density functional theory study was carried out to provide mechanistic insights into the bifunctional iron complex [Fe(DCPE)(PhNCH₂)₂ (DCPE = 1,2-bis(dicyclohexylphosphino)ethane)] catalyzed AB dehydrogenation and polyaminoborane formation with the detailed mechanistic pathways. Computational results imply a favorable scenario that complies with a proton transfer pathway between the metal center and ligand, which could further lead to a kinetically feasible transition state with low energy barrier. Subsequent polyaminoborane formation involves the generation of nucleophiles, and the NH₂BH₂ moiety, which is featured with a nitrogen lone pair, plays a crucial role in the chain polyaminoborane formation. Natural bond orbital analysis (NBO), and energy decomposition analysis (EDA) were utilized to study the orbital interactions and intramolecular weak interactions were revealed by the independent gradient model based on Hirshfeld partition method (IGMH).