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.     

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.     

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.     

Improved low-temperature hydrogen generation from NH3BH3 and TiO2 composites pretreated with water

Ammonia borane (AB, NH₃BH₃) is nontoxic easily transportable solid hydride with high stability in air. In this work we demonstrate that simple mixing of AB with TiO₂ (anatase) allows for hydrogen gas to be generated at temperatures as low as 80 °C. No losses of hydrogen have been observed during preparation of hydride-containing composites. It was shown that the adsorption of water vapor on TiO₂ and the increase of TiO₂ loading considerably accelerated the rate of AB decomposition. The experimentally observed formation of B–O chemical bonds and the elevated heat emission suggest strong interaction of AB with the adsorbed water species on TiO₂ surface. It has been found that this interaction proceeds at a higher rate comparing with binary AB/H₂O systems. The heat being released in the process is thought to contribute to overcoming the activation barrier in the dehydrogenation of ammonia borane to produce hydrogen gas.     

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.     

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.     

Transport characteristics of dehydrogenated ammonia borane and sodium borohydride spent fuels

Ammonia borane (AB) and sodium borohydride (SBH) are candidate materials for on-board hydrogen storage that can be dehydrogenated upon demand. The rheological properties of the dehydrogenated by-products are important to quantify their removal and transportability from the hydrogen storage system. This paper presents visco-elastic property (elastic stiffness and viscous damping) measurements of the spent fuels obtained from AB hydrolysis, hydrothermolysis and thermolysis; and SBH hydrolysis. Smaller stiffness and larger mobility (or smaller viscous damping) indicate better transportability of the spent fuel. In addition, flow property (dynamic angle of repose and avalanching time) measurements for the hydrolysis spent fuels of AB and SBH are also presented. Comparing with the SBH hydrolysis spent fuel, the AB hydrolysis spent fuel had a lower stiffness and larger mobility, as well as lower angles of dynamic repose and avalanche power peaks, indicating that it is more transportable. Among all the investigated AB spent fuels in the present study, those obtained from its neat thermolysis (at 120 °C) and hydrothermolysis (40–70 wt%) were found to be the most transportable, followed by-products of its hydrolysis and ionic solvent-aided thermolysis, respectively.     

Efficient and highly rapid hydrogen release from ball-milled 3NH3BH3/MMgH3 (M = Na,K, Rb) mixtures at low temperatures

The stoichiometric reactions of ammonia borane (NH₃BH₃, AB) and selected alkali or alkaline-earth metal hydrides produce metal amidoboranes, which possess dehydrogenation property advantages over their parent AB. However, the losses of hydrogen capacity and chemical energy in the preparation process make metal amidoboranes less energy-effective for hydrogen storage application. In the present study, by combining the M⁺–Mg²⁺ double cations remarkably lowers the reactivity of the alkali metal hydrides toward AB. As a result, the starting Mg-based ternary hydrides MMgH₃ (M = Na, K, Rb) and AB phases are largely stable in the mechanical milling process, but transform to the corresponding mixed-cation amidoboranes in the subsequent heating process. Importantly, when the post-milled 3AB/MMgH₃ mixtures are isothermally heated at above 60 °C using water bath, the formation and decomposition processes of the mixed-cation amidoboranes can be favorably combined, giving rise to rapid and efficient dehydrogenation performances at the mild temperatures (60–80 °C). The results acquired may provide a generalized reactions coupling strategy for designing and synthesis other potentially efficient hydrogen storage system.     

Improved hydrogen desorption properties of ammonia borane by Ni-modified metal-organic frameworks

Ammonia borane (AB) has attracted intensive study because of its low molecular weight and abnormally high gravimetric hydrogen capacity. However, the slow kinetics, irreversibility, and formation of volatile materials (borazine and ammonia) of AB limit its practical application. In this paper, new strategies by doping AB in metal-organic framework MIL-101 (denoted as AB/MIL-101) or in Ni modified MIL-101 (denoted as AB/Ni@MIL-101) are developed for hydrogen storage. In AB/MIL-101 samples, dehydrogenation did not present any induction period and undesirable by-product borazine, and decomposition thermodynamics and kinetics are improved. For AB/Ni@MIL-101, the peak temperature of AB dehydrogenation was shifted to 75 °C, which is the first report of such a big decrease (40 °C) in the decomposition temperature of AB. Furthermore, borazine and ammonia emissions that are harmful for proton exchange membrane fuel cells, were not detected. The interaction between AB and MIL-101 is discussed based on both theoretical calculations and experiments. Results show that Cr-N and B-O bonds have generated in AB/MIL-101 nanocomposites, and the decomposition mechanism of AB has changed.     

Effects of nano additives on hydrogen storage behavior of the multinary complex hydride LiBH4/LiNH2/MgH2

Multinary complex hydrides comprised of borohydrides, amides and metal hydrides have been synthesized using the solid state mechano-chemical process. After the optimization of the system, it was found that LiBH₄/LiNH₂/MgH₂ exhibits potential reversible hydrogen storage behavior (>6 wt.%) at temperatures of 125–175 °C. To further improve the hydrogen performance of the system, various nano additives namely, nickel, cobalt, iron, copper, and manganese were investigated. It was observed that some of these additives (Co, Ni) lowered the hydrogen release temperature at least 75–100 °C in the major hydrogen decomposition step. While other additives acted as catalysts and increased the rate at which hydrogen was released. Combinatorial addition of selected materials were also investigated and found to have both a positive effect on kinetics and reduction in hydrogen desorption temperature.     

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.     

Thermal coupling of a high temperature PEM fuel cell with a complex hydride tank

Sodium alanate doped with cerium catalyst has been proven to have fast kinetics for hydrogen ab- and de-sorption as well as a high gravimetric storage density around 5 wt%. The kinetics of hydrogen sorption can be improved by preparing the alanate as nanocrystalline material. However, the second decomposition step, i.e. the decomposition of the hexahydride to sodium hydride and aluminium which refers to 1.8 wt% hydrogen is supposed to happen above 110 °C. The discharge of the material is thus limited by the level of heat supplied to the hydride storage tank. Therefore, we evaluated the possibilities of a thermal coupling of a high temperature PEM fuel cell operating at 160–200 °C. The starting temperatures and temperature hold-times before starting fuel cell operation, the heat transfer characteristics of the hydride storage tanks, system temperature, fuel cell electrical power (including efficiency) as well as alanate kinetics were varied by system modelling with gPROMS^®. The kinetics of the hydride decomposition was found to have a major influence on the performance of the system. A cumulative output of 0.8 kWh was reached in a test run.     

Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles

2LiBH₄/MgH₂ system is a representative and promising reactive hydride composite for hydrogen storage. However, the high desorption temperature and sluggish desorption kinetics hamper its practical application. In our present report, we successfully introduce CoNiB nanoparticles as catalysts to improve the dehydrogenation performances of the 2LiBH₄/MgH₂ composite. The sample with CoNiB additives shows a significant desorption property. Temperature programmed desorption (TPD) measurement demonstrates that the peak decomposition temperatures of MgH₂ and LiBH₄ are lowered to be 315 °C and 417 °C for the CoNiB-doped 2LiBH₄/MgH₂. Isothermal dehydrogenation analysis demonstrates that approximately 10.2 wt% hydrogen can be released within 360 min at 400 °C. In addition, this study gives a preliminary evidence for understanding the CoNiB catalytic mechanism of 2LiBH₄/MgH₂     

3D simulation of hydrogen production by ammonia decomposition in a catalytic membrane reactor

Ammonia decomposition in an integrated Catalytic Membrane Reactor for hydrogen production was studied by numerical simulation. The process is based on anhydrous NH₃ thermal dissociation inside a small size reactor (30 cm³), filled by a Ni/Al₂O₃ catalyst. The reaction is promoted by the presence of seven Pd coated tubular membranes about 203 mm long, with an outer diameter of 1.98 mm, which shift the NH₃ decomposition towards the products by removing hydrogen from the reaction area. The system fluid-dynamics was implemented into a 2D and 3D geometrical model. Ammonia cracking reaction over the Ni/Al₂O₃ catalyst was simulated using the Temkin-Pyzhev equation.Introductory 2D simulations were first carried out for a hypothetic system without membranes. Because of reactor axial symmetry, different operative pressures, temperatures and input flows were evaluated. These introductory results showed an excellent ammonia conversion at 550 °C and 0.2 MPa for an input flow of 1.1 mg/s, with a residual NH₃ of only a few ppm. 3D simulations were then carried out for the system with membranes. Hydrogen adsorption throughout the membranes has been modeled using the Sievert’s law for the dissociative hydrogen flux. Several runs have been carried out at 1 MPa changing the temperature between 500 °C and 600 °C to point out the conditions for which the permeated hydrogen flux is the highest. With temperatures higher than 550 °C we obtained an almost complete ammonia conversion already before the membrane area. The working temperature of 550 °C resulted to be the most suitable for the reactor geometry. A good matching between membrane permeation and ammonia decomposition was obtained for an NH₃ input flow rate of 2.8 mg/s. Ammonia reaction shift due to the presence of H₂ permeable membranes in the reactor significantly fostered the dissociation: for the 550 °C case we obtained a conversion rate improvement of almost 18%.     

Improved hydrogen desorption in lithium alanate by addition of SWCNT-metallic catalyst composite

A LiAlH₄/single walled carbon nanotube (SWCNT) composite system was prepared by mechanical milling and its hydrogen storage properties investigated. The SWCNT - metallic particle addition resulted in both a decreased decomposition temperature and enhanced desorption kinetics compared to pure LiAlH₄. The decomposition temperature of the 5 wt.% SWCNT-added LiAlH₄ sample was reduced to 80 °C and 130 °C for the first and second stage, respectively, compared with 150 °C and 180 °C for as-received LiAlH₄. In terms of the desorption kinetics, the 5 wt.% SWCNT-added LiAlH₄ sample released about 4.0 wt.% hydrogen at 90 °C after 40 min dehydrogenation, while the as-milled LiAlH₄ sample released less than 0.3 wt.% hydrogen for the same temperature and time. Differential scanning calorimetry measurements indicate that enthalpies of decomposition in LiAlH₄ decrease with added SWCNTs. The apparent activation energy for hydrogen desorption was decreased from 116 kJ/mol for as-received LiAlH₄ to 61 kJ/mol by the addition of 5 wt.% SWCNTs. It is believed that the significant improvement in dehydrogenation behaviour of SWCNT-added LiAlH₄ is due to the combined influence of the SWCNT structure itself and the catalytic role of the metallic particles contained in the SWCNTs. In addition, the different effects of the SWCNTs and the metallic catalysts contained in the SWCNTs were also investigated, and the possible mechanism is discussed.     

Chemical kinetics of Ru-catalyzed ammonia borane hydrolysis

Ammonia borane (AB) is a candidate material for on-board hydrogen storage, and hydrolysis is one of the potential processes by which the hydrogen may be released. This paper presents hydrogen generation measurements from the hydrolysis of dilute AB aqueous solutions catalyzed by ruthenium supported on carbon. Reaction kinetics necessary for the design of hydrolysis reactors were derived from the measurements. The hydrolysis had reaction orders greater than zero but less than unity in the temperature range from 16 °C to 55 °C. A Langmuir–Hinshelwood kinetic model was adopted to interpret the data with parameters determined by a non-linear conjugate-gradient minimization algorithm. The ruthenium-catalyzed AB hydrolysis was found to have activation energy of 76 ± 0.1 kJ mol⁻¹ and adsorption energy of −42.3 ± 0.33 kJ mol⁻¹. The observed hydrogen release rates were 843 ml H₂ min⁻¹ (g catalyst)⁻¹ and 8327 ml H₂ min⁻¹ (g catalyst)⁻¹ at 25 °C and 55 °C, respectively. The hydrogen release from AB catalyzed by ruthenium supported on carbon is significantly faster than that catalyzed by cobalt supported on alumina. Finally, the kinetic rate of hydrogen release by AB hydrolysis is much faster than that of hydrogen release by base-stabilized sodium borohydride hydrolysis.     

Promoted hydrogen generation from ammonia borane aqueous solution using cobalt–molybdenum–boron/nickel foam catalyst

Ammonia borane (AB) is an intriguing molecular crystal with extremely high hydrogen density. In the present study, by using a modified electroless plating method, we prepare a robust supported cobalt–molybdenum–boron (Co–Mo–B)/nickel (Ni) foam catalyst that can effectively promote the hydrogen release from AB aqueous solution at ambient temperatures. The catalytic activity of the catalyst towards the hydrolysis reaction of AB can be further improved by appropriate calcination treatment. In an effort to understand the effect of calcination treatment on the catalytic activity of the catalyst, combined structural/phase analyses of the series of catalyst samples have been carried out. Using the catalyst that is calcined at optimized condition, a detailed study of the catalytic hydrolysis kinetics of AB is carried out. It is found that the hydrolysis of AB in the presence of Co–Mo–B/Ni foam catalyst follows first-order kinetics with respect to AB concentration and catalyst amount, respectively. The apparent activation energy of the catalyzed hydrolysis reaction is determined to be 44.3 kJ mol⁻¹, which compares favorably with the literature results for using other non-noble transition metal catalysts.     

Tube-wall catalytic membrane reactor for hydrogen production by low-temperature ammonia decomposition

Ammonia has attracted great interest as a chemical hydrogen carrier. However, ammonia decomposition is limited kinetically rather than thermodynamically below 400 °C. We developed a tube-wall catalytic membrane reactor that could decompose ammonia with high conversion even at temperatures below 400 °C. The reactor had excellent heat transfer characteristics, and thus nearly 100% conversion for an NH3 feed of 10 mL/min at 375 °C was achieved with a 2-μm-thick palladium composite membrane, and hydrogen removal from the decomposition side resulted in a large kinetic acceleration.     

Preparation of Ru–Cs catalyst and its application on hydrogen production by ammonia decomposition

Ammonia can be a hydrogen source for many applications including fuel cells. Using Ru or Cs–Ru as the catalyst, hydrogen is generated from ammonia by decomposition reaction. These catalysts are deposited on carbon powder by either chemical reduction or precipitation method in this study. Different carbon powder pre-treatment solutions and catalyst deposition conditions are evaluated. Nitric acid pre-treatment followed by precipitation at pH of 6 produces the highest catalyst loading from solution with given concentration of catalyst precursor. Hydrogen generation rate is measured at different catalyst compositions, ammonia inlet flow rates, decomposition temperatures, amount of catalyst packing, and ratio of Cs/Ru. The optimal condition for the ammonia decomposition reaction is Cs/Ru weight ratio at 3, ammonia inlet flow at 6 ml min⁻¹, reaction temperature at 400 °C. At this condition, the ammonia conversion rate reaches 90% and hydrogen generation rate reaches 29.8 mmol/min-g_cat.     

An experimental study of neat and ionic liquid-aided ammonia borane thermolysis

This paper reports hydrogen (H₂) yield and reaction rate measurements of ammonia borane (AB) thermolysis in the neat form as well as facilitated by the presence of an ionic solvent, 1-butyl-3-methylimidazolium chloride (bmimCl). The measurements were conducted at various temperatures between 85 and 120 °C under quasi-isothermal conditions. The details of fast hydrogen evolution at the initial stage of the thermolysis process were captured for the first time. The presence of bmimCl led to significant increases in both the rate and the amount of hydrogen released, compared to the corresponding quantities at identical temperatures for neat AB thermolysis. Measurements reported in the literature are in qualitative agreement with this observation but lack the time resolution necessary for the quantitative comparisons. At 120 °C, the measured gravimetric H₂ storage capacity from the neat AB thermolysis was 9.9 wt% (material base) and that from the AB/bmimCl mixture (80/20 wt%) thermolysis was 11.2 wt%. Also, the reaction rate of the thermolysis of AB/bmimCl mixture (80/20 wt%) was twice as fast as that of the neat AB thermolysis at this temperature. In the bmimCl (20 wt%) aided AB thermolysis, a significant increase in the H₂ yield occurred at temperatures over 107 °C.