Enhanced hydrogen generation by hydrolysis of LiBH4 doped with multiwalled carbon nanotubes for micro proton exchange membrane fuel cell application

LiBH₄ has a high hydrogen storage capacity and could potentially serve as a superior hydrogen storage material. However, during the hydrolysis process for hydrogen generation, the agglomeration of the hydrolysis product of LiBH₄ limits its full utilization. In order to completely release the stoichiometric amount of H₂ from LiBH₄ hydrolysis, multiwalled carbon nanotubes (MWCNTs) were doped with LiBH₄ by mechanical milling. The results show that MWCNT carried LiBH₄ can slowly react with water vapor at room temperature which is 25 °C lower than the reaction temperature of neat LiBH₄. Agglomeration can be avoided when the addition of MWCNTs exceeds 7 wt.%, which results in a complete hydrolysis process. The total hydrogen capacity is 7.5 wt.%. The enhanced hydrolysis of LiBH₄ can be attributed to the MWCNTs which increased the contact areas between LiBH₄ and water and created gas channels for hydrogen diffusion. The performance of a micro proton exchange membrane fuel cell connected to MWCNT-doped LiBH₄ powder packed-bed reactor was examined. The result demonstrates that doping with MWCNTs enhanced the hydrogen generation of LiBH₄ hydrolysis. MWCNT-doped LiBH₄ can be applied as hydrogen source of fuel cells.     

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.     

Hydrogen generation from hydrolysis of NH3BH3/MH (M = Li,Na) binary hydrides

Hydrogen release from hydrolysis of LiNH₂BH₃, NaNH₂BH₃, LiH–NH₃BH₃ and NaH–NH₃BH₃ respectively was investigated in this paper. It is shown experimentally that LiNH₂BH₃ and NaNH₂BH₃ hydrolysis can release 3 equiv. of hydrogen at 25 °C. Hydrolysis of LiNH₂BH₃ or NaNH₂BH₃ exhibits greatly improved kinetics in comparison with neat NH₃BH₃ hydrolysis. The electronic and structural changes from NH₃BH₃ to [NH₂BH₃]⁻ play a crucial role in the improvements. The mechanism of LiNH₂BH₃ and NaNH₂BH₃ hydrolysis is the combination of H⁺ and OH⁻ ions of water with the polar ions of LiNH₂BH₃ and NaNH₂BH₃. The process of LiH–NH₃BH₃ and NaH–NH₃BH₃ hydrolysis comprises two steps: LiH or NaH first reacts with water and then the generated heat initiates thermohydrolysis of NH₃BH₃. LiH or NaH hydrolysis is prior to the reaction of LiH or NaH with NH₃BH₃. Our results show a novel strategy to promote hydrogen release kinetics of LiNH₂BH₃ and NaNH₂BH₃. Furthermore, our results also present a novel noncatalytic method for hydrogen release from NH₃BH₃ by co-hydrolyzing it with other highly exothermic hydrides.     

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.     

A physico-chemical study of an NH3BH3-based reactive composition for hydrogen generation

In order to reduce the use of fossil energies, the development of new technologies, such as those concerning fuel cells, is required. However, fuel cells currently involve issues of storing and generating hydrogen. Borohydride materials, like ammonia borane (NH₃BH₃), seem to present an interesting solution to these problems. In fact, NH₃BH₃ contains 19.6 wt.% of hydrogen, of which a high percentage can easily be released by moderate heating. Understanding and controlling the behaviour of ammonia borane would allow the development of a safe, lightweight and compact hydrogen storage system. The final purpose would consist in having a device that is able to be integrated in nomad applications such as GSM, PDA; or in thermal accumulators. The present study reports on thermal decompositions of ammonia borane doped with various percentages of NH₄NO₃. Differential scanning calorimetry (DSC) permitted an understanding of the thermal behaviour of the material, and the detection of released hydrogen was examined by evolved gas analysis on a thermo-gravimetric analyser (TGA) coupled to a mass spectrometer (MS). Finally, in order to avoid fuel cell malfunctioning due to pollutant gases, an identification of the decomposition products was carried out.     

Hydrogen release by thermolysis of ammonia borane NH3BH3 and then hydrolysis of its by-product [BNHx]

Ammonia borane (AB) is one of the most attractive hydrides owing to its high hydrogen density (19.5 wt%). Stored hydrogen can be released by thermolysis or catalyzed hydrolysis, both routes having advantages and issues. The present study has envisaged for the first time the combination of thermolysis and hydrolysis, AB being first thermolyzed and then the solid by-product believed to be polyaminoborane [NH₂BH₂]_n (PAB) being hydrolyzed. Herein we report that: (i) the combination is feasible, (ii) PAB hydrolyzes in the presence of a metal catalyst (Ru) at 40 °C, (iii) a total of 3 equiv. H₂ is released from AB and PAB-H₂O, (iv) high hydrogen generation rates can be obtained through hydrolysis, and (v) the by-products stemming from the PAB hydrolysis are ammonium borates. The following reactions may be proposed: AB → PAB + H₂ and PAB + xH₂O → 2H₂ + ammonium borates. All of these aspects as well as the advantages and issues of the combination of AB thermolysis and PAB hydrolysis are discussed.     

Pt and Ru dispersed on LiCoO2 for hydrogen generation from sodium borohydride solutions

Nano-sized platinum and ruthenium dispersed on the surface LiCoO₂ as catalysts for borohydride hydrolysis are prepared by microwave-assisted polyol process. The catalysts are characterized by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Very uniform Pt and Ru nanoparticles with sizes of <10 nm are dispersed on the surface of LiCoO₂. XRD patterns show that the Pt/LiCoO₂ and Ru/LiCoO₂ catalysts only display the characteristic diffraction peaks of a LiCoO₂ crystal structure. Results obtained from XPS analysis reveal that the Pt/LiCoO₂ and Ru/LiCoO₂ catalysts contain mostly Pt(0) and Ru(0), with traces of Pt(IV) and Ru(IV), respectively. The hydrogen generation rates using low noble metal loading catalysts, 1 wt.% Pt/LiCoO₂ and 1 wt.% Ru/LiCoO₂, are very high. The hydrogen generation rate using Ru/LiCoO₂ as a catalyst is slightly higher compared with that of Pt/LiCoO₂.     

Direct NaBH4/H2O2 fuel cells

A fuel cell (FC) using liquid fuel and oxidizer is under investigation. H₂O₂ is used in this FC directly at the cathode. Either of two types of reactant, namely a gas-phase hydrogen or an aqueous NaBH₄ solution, are utilized as fuel at the anode. Experiments demonstrate that the direct utilization of H₂O₂ and NaBH₄ at the electrodes results in >30% higher voltage output compared to the ordinary H₂/O₂ FC. Further, the use of this combination of all liquid fuels, provides numerous advantages (ease of storage, reduced pumping requirements, simplified heat removal, etc.) from an operational point of view. This design is inherently compact compared to other cells that use gas phase reactants. Further, regeneration is possible using an electrical input, e.g. from power lines or a solar panel. While the peroxide-based FC is ideally suited for applications such as space power where air is not available and a high energy density fuel is essential, other distributed and mobile power uses are of interest.     

rGO supported PdNi-CeO2 nanocomposite as an efficient catalyst for hydrogen evolution from the hydrolysis of NH3BH3

The hydrogen economy is a proposed system that utilizes hydrogen to deliver energy. For the realization of this concept, how to safely, controllably and reversibly store and release hydrogen are critical problems which must be resolved. Metal alloys combined with suitable support materials are widely applied to various catalytic reactions. Here palladium nickel bimetallic nanoparticles doped with cerium oxide on a reduced graphene oxide (rGO) support were prepared by combining metal ion precursors and graphene oxide in a one-pot co-reduction approach. The as-received catalysts were characterized by XRD, TEM, SEM, XPS and ICP-OES, and the results revealed that PdNi-CeO₂ nanoparticles were uniform dispersal on rGO. The as-synthesized PdNi-CeO₂/rGO had been adopted as a heterogeneous catalyst for the hydrogen evolution from the hydrolysis of ammonia borane (NH₃BH₃, AB) at room temperature. Kinetically, the hydrogen-release rate was first-order with the increased concentration of catalysts. The optimized catalyst of Pd_0.8Ni_0.2-CeO₂/rGO with the CeO₂ content of 13.9 mol% exhibited an excellent activity with a turnover frequency value of 30.5 mol H₂ (mol catalyst)^?1 min^?1 at 298 K, and a low apparent activation energy (E_a) of 37.78 kJ mol^?1. The robust catalytic performance of the Pd_0.8Ni_0.2-CeO₂/rGO is attributed to the uniform controlled nanoparticle size, the synergic effect between the nanoparticles bimetallic properties, and the effective charge transfer interactions between the metal and support.     

Hydrogen generation from solid state NaBH4 by using FeCl3 catalyst for portable proton exchange membrane fuel cell applications

Being a boron-based compound, sodium borohydride, NaBH₄, is a convenient hydrogen storage material for applications like unmanned air vehicles. There are several concerns behind commercialization of hydrogen gas generator by NaBH₄ hydrolysis systems. This study aims to contribute to the solution of the problems of NaBH₄ hydrolysis system in three main ways. First, the usage of solid state NaBH₄ enables to increase the durability and the gravimetric H₂ storage capacity of the system in order to meet US DOE targets. Second, solid NaBH₄ usage decreases the system's weight since it does not require a separate fuel storage tank, which is very important for portable, on demand applications. Finally, the system's cost is decreased by using an accessible and effective non-precious catalyst such as ferric chloride, FeCl₃. The maximum hydrogen generation rate obtained was 2.6 L/min and the yield was 2 L H₂/g NaBH₄ with an efficiency of 76% at its most promising condition. Moreover, the novel solid NaBH₄ hydrogen gas generator developed in the present work was integrated into a proton exchange membrane fuel cell and tested at the optimum operating conditions.     

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.     

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.     

Hydrogen generation from solid NaBH4 with catalytic solution for planar air-breathing proton exchange membrane fuel cells

In the pursuit of the development of alternative mobile power sources with high energy densities, this study elucidated a new hydrogen generation approach from solid NaBH₄ using a new catalyst, sodium hydrogen carbonate (NaHCO₃), which was placed in a small and compact cartridge. A planar air-breathing PEMFC system fitted with the cartridge has been investigated for testing hydrogen generation from NaBH₄ and NaHCO₃. NaHCO₃ allowed the hydrogen cartridge to control hydrogen generation and to improve the power density, fuel efficiency, energy efficiency, and cell response. The cell performance of solid NaBH₄ air-breathing PEMFC system strongly depended on the operating conditions: the feeding rates and concentrations of catalytic solutions for NaBH₄ hydrolysis. In various concentrations (5 - 12 wt %) of NaHCO₃ aqueous solutions, 10 wt % NaHCO₃ aqueous solution exhibited the highest maximum power density of 128 mW cm⁻² at 0.7 V, which was estimated to be a Faradic efficiency of 78.4% and an energy efficiency of 46.3%. The data illustrated that NaHCO₃ was an effective catalyst for hydrogen generation with the solid NaBH₄, which is considered as a hydrogen carrier for air-breathing micro PEMFCs operated without auxiliary hydrogen controller or devices.     

Aqueous hydrazine borane N2H4BH3 and nickel-based catalyst: An effective couple for the release of hydrogen in near-ambient conditions

Nickel-based bimetallic catalysts were screened using the sodium borohydride NaBH₄ hydrolysis and the aqueous hydrazine borane N₂H₄BH₃ dehydrogenation. A total of 22 bimetallic catalysts were synthesized according to an easy process while focusing on metals like Fe, Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt and Au. In the end, the bimetallic candidate Ni_87.5Pt_12.5 showed to be the most active and the most selective for the dehydrogenation of N₂H₄BH₃. At 70?°C, it is able to decompose N₂H₄BH₃ into 5.8 equivalents of H₂+N₂ in less than 12?min such as: N₂H₄BH₃?+?3H₂O?→?0.95 N₂?+?0.1 NH₃?+?B(OH)₃?+?4.85H₂. Durability and stability tests were also performed. In our conditions, Ni_87.5Pt_12.5 was found to suffer from small loss of performance because of an electronic evolution of the catalytic surface leading to modified sorption properties of the catalytic sites. Our main results are reported and discussed herein.     

Hydrogen generation from hydrolysis of concentrated NaBH4 solutions under adiabatic conditions

In this study, catalytic hydrolysis of aqueous solutions of NaBH₄ under isothermal and adiabatic conditions was investigated. A finely dispersed cobalt powder based on titanium oxide was used as a model catalyst. It was determined that catalytic activity of this catalyst practically did not change after 20 cycles. It was shown that the activation energy, determined by the rate of hydrogen generation, depends on NaBH₄ concentrations. We believe that this effect is associated with sorption/desorption processes. If to conduct hydrolysis under conditions close to adiabatic, the time of hydrolysis is significantly reduced. As united solution of equations of kinetics and energy conversation shows, the data of experiments in a thermally insulated reactor can be reasonably predicted.     

Hydrogen generation by hydrolysis of NaBH4

The hydrolysis of sodium-borohydride (SBH) to produce hydrogen has been studied at various temperatures using salts of nickel (II) or iron (III) as catalyst. Excess of water has been added to a mixture of solid SBH and catalyst to start hydrolysis reaction and the evolved hydrogen measured as a function of time. After a sudden peak a constant hydrogen flow was observed when Ni is used as catalyst. The activation energy has been evaluated from the dependence of the reaction time and of the hydrogen flow on the inverse of temperature. If Ni is substituted by a Fe based catalyst, after the initial increase, a different shape is observed in the hydrogen flow: it reaches a maximum and then monotonously decrease to zero. The different shape has been related to the different activity of the catalyst. The reaction activation energy was evaluated to be 73 KJ/mol.     

Fully-integrated micro PEM fuel cell system with NaBH4 hydrogen generator

A fully-integrated micro PEM fuel cell system with a NaBH₄ hydrogen generator was developed. The micro fuel cell system contained a micro PEM fuel cell and a NaBH₄ hydrogen generator. The hydrogen generator comprised a NaBH₄ reacting chamber and a hydrogen separating chamber. Photosensitive glass wafers were used to fabricate a lightweight and corrosion-resistant micro fuel cell and hydrogen generator. All of the BOP such as a NaBH₄ cartridge, a micropump, and an auxiliary battery were fully integrated. In order to generate stable power output, a hybrid power management operating with a micro fuel cell and battery was designed. The integrated performance of the micro PEM fuel cell with NaBH₄ hydrogen generator was evaluated under various operating conditions. The hybrid power output was stably provided by the micro PEM fuel cell and auxiliary battery. The maximum power output and specific energy density of the micro PEM fuel cell system were 250 mW and 111.2 W h/kg, respectively.     

Fast hydrogen generation from NaBH4 hydrolysis catalyzed by carbon aerogels supported cobalt nanoparticles

Carbon aerogels (CAs) with oxygen-rich functional groups and high surface area are synthesized by hydrothermal treatment of glucose in the presence of boric acid, and are used as the support for loading cobalt catalysts (CAs/Co). Cobalt nanoparticles distribute uniformly on the surface of ACs, creating highly dispersed catalytic active sites for hydrolysis of alkaline sodium borohydride solution. A rapid hydrogen generation rate of 11.22 L min⁻¹ g_(cobalt)⁻¹ is achieved at 25 °C by hydrolysis of 1 wt% NaBH₄ solution containing 10 wt% NaOH and 20 mg the CAs/Co catalyst with a cobalt loading of 18.71 wt%. Furthermore, various influences are systematically investigated to reveal the hydrolysis kinetics characteristics. The activation energy is found to be 38.4 kJ mol⁻¹. Furthermore, the CAs/Co catalyst can be reusable and its activity almost remains unchanged after recycling, indicating its promising applications in fuel cell.