Stability of alkaline aqueous solutions of sodium borohydride

Experimental results regarding long-term stability of the alkaline-water borohydride solutions for hydrogen generation are presented. The influence of the concentration of sodium borohydride and sodium hydroxide on the rate of borohydride hydrolysis is analyzed at various temperatures, such as 25 °C, 40 °C, and 80 °C, and various concentrations of NaOH. The rate of hydrolysis decreases with the increase of the water to sodium borohydride mole ratio. For diluted solutions at H₂O/NaBH₄ >30, the rate of hydrolysis and hydrogen generation at a given temperature remains constant. At room temperature in 1.0 N NaOH, the degree of hydrolysis is 0.01% NaBH₄/h that meets the stability requirements for the borohydride solutions during the long-term storage.     

Hydrogen generation from aqueous acid-catalyzed hydrolysis of sodium borohydride

In this study, the hydrogen feed from both Ru-catalyzed and organic acid-catalyzed hydrolysis of NaBH₄ was studied in terms of hydrogen generation rate and integrated PEMFC performance. Hydrogen feed generated from the conventional Ru-catalyzed hydrolysis of NaBH₄ caused a drastic loss of PEMFC performance. It was found that the presence of sodium ion in hydrogen feed was a main factor that increased the interfacial resistance of fuel cell and, consequently, reduced the performance. Acid-catalyzed hydrolysis with powder form of NaBH₄ was adopted in order to minimize the detrimental effect of sodium ion. The hydrogen feed from acid-catalyzed hydrolysis was quite dry so that even water vapor, the carrier of sodium ion, was not detected after condensation of hydrogen feed. It was confirmed by the several experiments that the hydrogen release rate can be controlled by varying the injection rate and concentration of aqueous acid. Various organic acids were employed in the production of hydrogen and found that acidity, acid type and chemical structure are also important factors on hydrolysis of NaBH₄. The performance from the integrated acid-catalyzed hydrogen generation system with PEMFC was quite stable and no significant loss was observed contrary to that from the integrated Ru-catalyzed hydrogen generation system–PEMFC test. This result also clarified that the detrimental effect of sodium ion could be removed by minimizing the water vapor in this manner. Based on the experiment of acid-catalyzed hydrolysis, a small-scale hydrogen-generating device was designed and fabricated, from which hydrogen release was controlled by the acid concentration and injection rate of aqueous acid solution.     

Hydrolysis of sodium borohydride in concentrated aqueous solution

Sodium borohydride is being commercialized to provide hydrogen storage for portable fuel cells. Prior kinetic studies have focused on catalytic hydrolysis of dilute aqueous solutions at room temperature. This work reports on a new NMR method for studying the kinetics of non-catalyzed sodium borohydride hydrolysis in highly concentrated solutions. The effects of initial NaBH₄ concentration, temperature and pH on conversion are studied. It is found that higher initial NaBH₄ concentration and higher temperature both improve the reaction rate. The reaction rate is slowed down with increasing pH of basic solutions and is accelerated with decreasing pH of acidic solutions. In addition, temperature effect seems to be more important than that of the acidic pH on the reaction rate.     

Synergistic hydrogen generation from aluminum, aluminum alloys and sodium borohydride in aqueous solutions

A new method to produce high purity hydrogen using reactions of aluminum and sodium borohydride with aqueous alkaline solutions is described. This process mainly consumes water and aluminum (or its alloys) which are cheaper raw materials than the borohydride. As a consequence, this process could be competitive for in situ production of hydrogen. Moreover, a synergistic effect has been observed in hydrogen production rates and yields combining aluminum or aluminum alloys with sodium borohydride in aqueous solutions. Good results have been obtained for powders of Al, Al/Si and Al/Co alloys. The development of this idea could improve yields and reduce costs in power units based on fuel cells which use borohydride as raw material for hydrogen production.     

Electrocatalytic oxidation of sodium borohydride on metal ad-atom modified Au(111) single crystal electrodes in alkaline solution

The electrochemical oxidation of borohydride was investigated by using various ad-atom modified Au(111) electrodes in alkaline media in comparison to Au(111) single crystal, polycrystalline Au, Pt and Zn electrodes. The catalytic activity of gold towards borohydride oxidation has tended to increase in more alkaline media as reflected in the oxidation peak in the concentration range of NaOH (0.01-2.0 M) studied. Additional shift on the oxidation peak potential of borohydride on Pt and Zn ad-atom modified Au(111) electrodes was observed for both ad-atom modified electrodes to more negative potentials compared to that of bare electrodes, respectively. The ad-atom modified Au(111) electrodes surfaces do not only provide a superior electrical contact, but also accelerates electron transfer as proven by the increase in peak current and positive shift in the peak potential.     

Structure stability of metal-organic framework MIL-53 (Al) in aqueous solutions

The structural stability of MIL-53 (Al) in different pH aqueous solutions from room temperature to 100 °C has been investigated. Experimental results show MIL-53 (Al) is stable and highly resistant to hydrolysis in neutral and acidic solutions. It can retain its crystallinity and permanent porosity without structural collapse. The good structure stability of MIL-53 (Al) to aqueous solutions is quite unusual among the MOFs. The nitrogen adsorption for the soaked frameworks show a typical type I isotherm. In basic aqueous solution, MIL-53 (Al) undergoes structure transformation.     

Investigation of carbon-supported Au hollow nanospheres as electrocatalyst for electrooxidation of sodium borohydride

A carbon-supported Au hollow nanospheres composite electrocatalyst was prepared in aqueous solution with Co nanoparticles as sacrificial templates at room temperature. The electrocatalyst was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersion X-ray (EDX) spectra and electrochemical tests. The results showed that carbon-supported Au hollow nanospheres were porous, and composed of discrete Au nanoparticles with the crystallite size of about 6 nm. For comparison, the electrocatalytic performances of both carbon-supported Au hollow nanospheres and carbon-supported Au solid nanoparticles on borohydride electrooxidation in fuel cells were investigated by cyclic voltammetry, chronoamperometry, and chronopotentiometry. The fundamental electrochemical test results indicated that the electrocatalytic activity of carbon-supported Au hollow nanospheres is much better than that of carbon-supported Au solid nanoparticles.     

Hydrogen production by aluminum corrosion in aqueous hydrochloric acid solution promoted by sodium molybdate dihydrate

Recycled aluminum alloys corrosion system was studied for hydrogen production in 1.4 M HCl solution at low temperature, 333 K. Molybdate ions were used as corrosion promoter. Commercial Na₂MoO₄·2H₂O was tested, varying its concentration (0–0.036 M) in order to analyze the promoter effect in H₂ production. Moreover, Na₂MoO₄·2H₂O particles were synthesized via sonochemical method and by ultrasonic method. Later, synthesized promoters were tested in order to analyze the effect of their physicochemical properties in H₂ production. Gas composition produced during the reaction was analyzed by gas chromatography. Results showed that reaction rate was notorious affected by molybdate ions concentration. The highest hydrogen volume and the greatest reaction rate obtained was with the use of 1050 aluminum foil alloy with very low promotor concentration (0.004 M), becoming, therefore, a low-cost process for high purity hydrogen production.     

Physical and electrochemical characteristics of supercapacitors based on carbide derived carbon electrodes in aqueous electrolytes

FIB-SEM, XPS and gas adsorption methods have been used for the characterisation of physical properties of microporous carbide derived carbon electrodes prepared from Mo₂C at 600 °C (noted as CDC-Mo₂C). Cyclic voltammetry, constant current charge/discharge, and electrochemical impedance spectroscopy have been applied to establish the electrochemical characteristics for supercapacitors consisting of the 1 M Na₂SO₄, KOH, tetraethyl ammonium iodide or 6 M KOH aqueous electrolyte and CDC-Mo₂C electrodes. The N₂ sorption values obtained have been correlated with electrochemical characteristics for supercapacitors in various aqueous electrolytes. The maximum gravimetric energy, E_max, and gravimetric power, P_max, for supercapacitors (taking into consideration the active material weight) have been obtained at cell voltage 0.9 V for 6 M KOH aqueous supercapacitor (E_max = 5.7 Wh kg⁻¹ and P_max = 43 kW kg⁻¹). For 1 M TEAI based SC somewhat higher E_max (6.2 Wh kg⁻¹) and comparatively low P_max (7.0 kW kg⁻¹) have been calculated.     

A semi-global reaction rate model based on experimental data for the self-hydrolysis kinetics of aqueous sodium borohydride

This work studied the self-hydrolysis kinetics of aqueous sodium borohydride (NaBH₄) for hydrogen generation and storage purposes. Two semi-global rate expressions of sodium borohydride and hydrogen ion consumption were derived from an extensive series of batch process experiments where the following parameters were systematically varied: solution temperature (298 K–348 K), NaBH₄ concentration (0.5 wt% to 25.0 wt%), and sodium hydroxide (NaOH) concentration (0.0 wt% to 4.0 wt%). Transient hydrogen generation rates and transient solution pH were measured during the hydrolysis experiments. Given initial conditions (temperature, NaBH₄ concentration, and H⁺ concentration), the two coupled semi-global rate equations can be integrated to obtain the transient time history of H₂ generation (or NaBH₄ consumption) and solution pH (or H⁺ concentration). Comparing analytical results of transient hydrogen generation rate and transient solution pH with experimental data, good agreement was reached for many conditions, especially for elevated solution pH values, levels at which NaBH₄ solutions are used practically.     

Generation of hydrogen from sodium borohydride at low temperature using metal halides additive

A series of MnCl₂ impregnated NaBH₄ composite mixture was synthesized by facile solution method. The weight percent of MnCl₂ varied from 10 wt% to 50 wt% in sodium borohydride during synthesis of the material. The effects of other additives were also studied by considering CaCl₂, and ZnCl₂. The synthesized materials were characterized by using X-ray diffraction (XRD), TGA, FE-SEM, Raman spectroscopy, and Fourier Transform Infra-red spectroscopy (FTIR). The generation of hydrogen was monitored by using a Gas Chromatograph. The study suggested that the formation of a homogenous composite mixture and spherical structure of MnCl₂/NaBH₄ composite material. The optimum loading of additive was found to be 20 wt% MnCl₂ for the generation of hydrogen. The addition of additive assists in lowering the thermal decomposition temperature of sodium borohydride at 373 K. The order of thermolysis performance by addition of various additives was as follows: 20CaCl₂/NaBH₄ > 20MnCl₂/NaBH₄ > 20ZnCl₂/NaBH₄. However, the FTIR spectra and the thermolysis study suggested that the decomposition of NaBH₄ was incomplete at 373 K even by the addition of additives.     

Photoelectrochemical characterisation of indium nitride and tin nitride in aqueous solution

Indium nitride (InN) and tin nitride (SnN_x) films were produced with reactive d.c. magnetron sputtering technique. The thin film semiconductors were optically and photoelectrochemically characterised and the energetic positions of the two semiconductors’ band edges were determined with respect to the normal hydrogen electrode. The sputtered InN thin film showed an indirect bandgap of 1.4 eV and a direct bandgap of 1.8 eV. The optical spectra of SnN_x indicated a bandgap energy of approximately 1.4 eV. All nitride films showed n-type photoresponse in KI (aq) electrolyte at an irradiation intensity of 1000 W/m². The photoelectrochemical characterisation indicated that InN and SnN_x with a bias of about 400 mV or less can be used for photo-oxidation of water.     

Microscopic study on the solidification of aqueous solutions using laser interferometry

Solidification of an aqueous solution was studied using a laser interferometry technique combined with an optical microscope. In order to measure the concentration distribution in an aqueous NaCl solution near an ice crystal and the three-dimensional shape of the ice crystal in a known temperature field, a directional solidification stage was used. This was composed of low- and high-temperature blocks and a moving bed. In our experiment, ice crystals showed four shapes: flat, treelike, swordlike, and needlelike. As a result of the interferometry experiment, it was observed that the concentration increases in the bulk solution for thick samples, swordlike ice crystals have an asymmetric triangular cross section, and treelike crystals have a flat and low top. We also studied ice formation response to injection of a high-concentration solution. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 353–364, 1998     

Optimizing the surfactant for the aqueous processing of LiFePO4 composite electrodes

Aqueous processing would reduce the costs associated with the making of the composite electrode. To achieve the incorporation and the dispersion of the carbon black (CB) conductive agent in aqueous slurries, a surfactant is needed. In this paper, three surfactants are compared, an anionic one, the sodium dodecyle sulphate (SDS), a non-ionic one, the isooctylphenylether of polyoxyethylene called commercially Triton X-100 and a cationic one, the hexadecyltrimethylammonium bromide (CTAB), by using rheology and laser granulometry measurements on electrode slurries on one hand, and SEM observations, porosity and adhesion measurements and electrochemical testing on composite electrodes on the other hand. Ionic surfactants were found to be not suitable because a corrosion of the aluminium current collector occurred. The utilization of Triton X-100 favoured a more homogeneous CB distribution, resulted in a better electronic wiring of the active material particles and higher rate behavior of the electrode. Optimal electrochemical performances are obtained for an optimal surfactant concentration which depends on the BET surface area of the CB powder.     

A new type of high energy asymmetric capacitor with nanoporous carbon electrodes in aqueous electrolyte

A new type of low cost and high energy asymmetric capacitor based on only activated carbons for both electrodes has been developed in a safe and environment friendly aqueous electrolyte. In such electrolyte, the charges are stored in the electrical double-layer and through fast faradaic charge transfer processes. By taking profit of different redox reactions occurring in the positive and negative ranges of potential, it is possible to optimize the capacitor either by balancing the mass of the electrodes or by using different optimized carbons for the positive and negative electrodes. The best results are obtained in the latter case, by utilizing different pseudo-faradaic properties of carbons in order to increase the capacitance and to shift the potentials of water decomposition and destructive oxidation of activated carbon to more negative and positive values, respectively. After an additional adjustment of potentials by mass-balancing the two electrodes, the electrochemical capacitor can be reversibly charged/discharged at 1.6 V in aqueous medium, with energy densities close to the values obtained with electrical double-layer capacitors working in organic electrolytes, while avoiding their disadvantages.     

Advances in the implementation of PVD-based techniques for the preparation of metal catalysts for the hydrolysis of sodium borohydride

Sodium borohydride constitutes a safe alternative for the storage of hydrogen with a high gravimetric content. Catalytic hydrolysis of sodium borohydride permits on-demand hydrogen generation for multiple applications. In this field, the rational design of efficient metal catalysts deposited on structured supports is highly desirable. For most reactions, chemical methods are the most commonly used methods for the preparation of supported metal catalysts. Physical vapour deposition techniques are emerging as an alternative for the preparation of catalytic materials because of their multiple advantages. They permit the one-step deposition of catalysts on structured supports with controlled microstructure and composition, avoiding the multi-step procedures and the generation of hazardous by-products associated with chemical routes.In this short review, we will describe the available literature on the application of physical vapour deposition techniques for the preparation of supported metal catalysts for the hydrolysis of sodium borohydride. The effects of the deposition parameters on the properties of the catalytic materials will be discussed, and strategies for further improvement will be proposed. Here, we also present our new results on the study of nanoporous Pt catalysts that are prepared through the chemical dealloying of magnetron-sputtered Pt–Cu thin films for the hydrolysis of sodium borohydride. We discuss the capabilities of the technique to tune the microstructure from columnar to closed porous microstructures, which, coupled with dealloying, produces more active supported catalysts with lower noble metal loading. At the end, we briefly mention the application of PVD for the preparation of supported catalysts for the hydrolysis of ammonia borane, another hydrogen generating reaction of high interest nowadays.     

Minimizing water utilization in hydrolysis of sodium borohydride: The role of sodium metaborate hydrates

Hydrogen can be obtained from hydrolysis of chemical hydrides, making this an interesting option for hydrogen storage. The gravimetric and volumetric energy densities attainable from hydrolysis of chemical hydrides depend in large measure on the amount of water required for the process. The steam hydrolysis of sodium borohydride produces hydrated metaborate byproducts in which the degree of hydration of the metaborate is closely tied to the amount of water in the reaction. Experimental characterization of these sodium metaborate byproducts indicates that the primary byproduct of steam hydrolysis is _NaBO2·2_H2O${\mathrm{NaBO}}_2·2{\mathrm{H}}_2\mathrm{O}$ under a variety of reaction conditions. A group contribution method is used to estimate the heats and free energies of formation of a variety of hydrated metaborates. This information will be useful in establishing the preferred (stable) hydration state of the byproducts, and in distinguishing these from metastable products.     

Hydrogen generation from the hydrolysis of sodium borohydride with new pyridinium dicationic salts containing transition metal complexes

Pyridinium based dicationic ionic salts [C₆(mpy)₂] containing transition metal halide anions [NiCl₄]²⁻ and [CoCl₄]²⁻ were synthesized and characterized by ¹H NMR, ¹³C NMR, LRMS-MS & TGA. The catalytic activities of dicationic ionic liquids were studied for the first time in the hydrolysis reaction of sodium borohydride. The reported work includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (Ea = 56.36 kJ/mol) and effects of the amount of catalyst, amount of substrate and temperature on the rate for the catalytic hydrolysis of NaBH₄. The activation energy of the catalyst dicationic tetrachloronickelate (II) anion for hydrogen production is comparable to that of other metal halide catalysts.     

Improving the direct borohydride fuel cell performance with thiourea as the additive in the sodium borohydride solution

In this study, the effects of the additive thiourea (TU) have been investigated under steady state/steady-flow and uniform state/uniform-flow systems with the aim of minimizing the anodic hydrogen evolution on Pd in order to increase the performance of a direct borohydride fuel cell. The fuel cell has consisted of Pd/C anode, Pt/C cathode and Na⁺ form Nafion membrane as the electrolyte. There has been a small improvement in peak power density and fuel utilization ratio by addition of TU (1.6 × 10⁻³ M) into the sodium borohydride solution; the peak power densities of 14.4 and 15.1 mW cm⁻², and fuel utilization ratios of 21.6% and 23.2% have been obtained without and with TU, respectively.     

Synthesis of ruthenium nanoparticles deposited on graphene-like transition metal carbide as an effective catalyst for the hydrolysis of sodium borohydride

The graphene-like transition metal carbide (Ti₃C₂X₂(X = OH, F)) which was synthesized from etching the layered Ti₃AlC₂ material was applied as a carrier for depositing Ru nanoparticles (Ru/Ti₃C₂X₂). The as-prepared nanocomposites were characterized by SEM, TEM, XRD, XPS and FTIR. During the hydrolysis process, Ru nanoparticles were uniformly generated on the surface of the carrier and acted as catalysts for the hydrogen generation from hydrolysis of NaBH₄ at room temperature. It was found that the catalyst Ru/Ti₃C₂X₂ exhibited excellent catalytic activity toward the hydrolysis of sodium borohydride with a hydrogen generation rate of 59.04 L H₂/g_Ru•min and an activation energy of 22.1 kJ/mol.