Development of a compact hydrogen generator from sodium borohydride

Chemical hydrides can be a simple and safe hydrogen vector for polymer fuel cells. In particular the catalytic hydrolysis of sodium borohydride (NaBH₄) is here envisaged to produce on-demand hydrogen to be supplied to a small solid polymer fuel cell in a portable energy generator.     

Apparatus for the production of hydrogen from sodium borohydride in alkaline solution

Extended application of hydrogen as energy carrier demands an economical, safe and reliable technology for storage. In particular, chemical hydrides appear as capable and promising to overcome the issues related to hydrogen safety and handling and to be considered competitive with respect to conventional fuels.     

New reactor design for catalytic sodium borohydride hydrolysis

Hydrogen supply to a fuel cell for portable consumer products requires a simple and safe technology for its storage and on-demand production. Hydrogen production by hydrolysis of sodium borohydride solution in the presence of metal catalyst could be a promising and feasible method.     

Hydrogen production through NaBH4 hydrolysis over supported Ru catalysts: An insight on the effect of the support and the ruthenium precursor

The NaBH₄ hydrolysis over Ru catalysts on different supports was here investigated. It was found that the support has a strong effect on the catalytic activity which was found to be in the order: Ru/activated carbon >> Ru/CeO₂ > Ru/TiO₂ > Ru/γ-Al₂O₃. On the most active system, namely Ru/activated carbon, the effect of the Ru loading and the metal precursor (RuCl₃ and Ru(NO)(NO₃)₃) was studied in more detail. Tests carried out with and without temperature control demonstrated that, for each precursor, the activity increases on increasing the Ru content. Moreover samples prepared by RuCl₃ were more active than the corresponding ones prepared by Ru(NO)(NO₃)₃. Characterization data, namely CO chemisorption and TEM analysis, point out that the Ru particles size is strongly affected from the Ru precursor used, Ru(NO)(NO₃)₃ resulting in very small Ru particles (diameter < 1 nm), RuCl₃ leading to Ru nanoparticles with average diameter of 2.4 nm. This latter size was found to be appropriate for the NaBH₄ hydrolysis reaction, which is proposed to be a structure sensitive reaction.     

Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: A review

Hydrogen generation from the hydrolysis of sodium borohydride (NaBH₄) solution has drawn much attention since early 2000s, due to its high theoretical hydrogen storage capacity (10.8 wt%) and potentially safe operation. However, hydrolysis of NaBH₄ for hydrogen generation is a complex process, which is influenced by factors such as catalyst performance, NaBH₄ concentration, stabilizer concentration, reaction temperature, complex kinetics and excess water requirement. All of these limit the hydrogen storage capacities of NaBH₄, whose practical application, however, has not yet reached a scientific and technical maturity. Despite extensive efforts, the kinetics of NaBH₄ hydrolysis reaction is not fully understood. Therefore, better understanding of the kinetics of hydrolysis reaction and development of a reliable kinetic model is a field of great importance in the study of NaBH₄ based hydrogen generation system. This review summarizes in detail the extensive literature on kinetics of hydrolysis of aqueous NaBH₄ solution.     

Sustainable hydrogen production by catalytic hydrolysis of alkaline sodium borohydride solution using recyclable Co-Co2B and Ni-Ni3B nanocomposites

In this article, we report Co-Co₂B and Ni-Ni₃B nanocomposites as catalyst for hydrogen generation from alkaline sodium borohydride. Kinetic studies of the hydrolysis of sodium borohydride with Co-Co₂B and Ni-Ni₃B nanocomposites reveal that the concentration of NaBH₄ has no effect on the rate of hydrogen generation. Hydrolysis was found to be first order with respect to the concentration of catalyst. The catalytic activity of Co-Co₂B was found to be much higher than that of Ni-Ni₃B as inferred from the activation energies 35.245 KJ/mol and 55.810 kJ/mol, respectively. Co-Co₂B nanocomposites were found to be more magnetic than Ni-Ni₃B. These catalysts showed superior recyclability with almost the similar catalytic activities for several hydrolytic cycles supporting the principles of sustainability. Co-Co₂B catalyst showed hydrogen generation rate of about 4300 mL/min/g which is comparable to most of the reported good catalysts till date.     

Kinetics of Ru-catalyzed sodium borohydride hydrolysis

Chemical hydrides have been identified as a potential medium for on-board hydrogen storage, one of the most challenging technical barriers to the prospective transition from gasoline to hydrogen-powered vehicles. Systematic study of the feasibility of the sodium borohydride systems, and chemical-hydride systems more generally, requires detailed kinetic studies of the reaction for use in reactor modeling and system-level experiments. This work reports an experimental study of the kinetics of sodium borohydride hydrolysis with a Ru-on-carbon catalyst and a Langmuir-Hinshelwood kinetic model developed based on experimental data. The model assumes that the reaction consists of two important steps: the equilibrated adsorption of sodium borohydride on the surface of the catalyst and the reaction of the adsorbed species. The model successfully captures both the reaction's zero-order behavior at low temperatures and the first-order behavior at higher temperatures. Reaction rate constants at different temperatures are determined from the experimental data, and the activation energy is found to be 66.9 kJ mol⁻¹ from an Arrhenius plot.     

Long-term stability of sodium borohydrides for hydrogen generation

As a source of the high purity hydrogen, sodium and potassium borohydrides are investigated in terms of long-term stability in the form of the concentrated solutions, heterogeneous mixtures and in the solid state corresponding to NaBH₄ or KBH₄ crystal hydrates. In order to improve their stability during the long-term storage sodium and potassium hydroxides were added to the initial borohydride compositions. The effect of temperature, concentration of the borohydride and the alkaline solution, and the nature of the cation in the alkaline solution on the rate of borohydride hydrolysis was investigated. The differential technique developed for evaluation of the rate of borohydride hydrolysis was successfully applied for the determination of the long-term stability of the water-alkaline solutions containing NaBH₄(KBH₄)·5H₂O with 1–10 wt.% of NaOH or KOH at 30 °C and 50 °C.     

All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications

A portable electric power plant is developed using a NaBH₄ (sodium borohydride)-based proton exchange membrane fuel cell stack. The power plant consists of a NaBH₄-based hydrogen generator, a fuel cell stack, a DC-DC converter, a micro-processed controller and a data monitoring device. The hydrogen generator can produce 5.9 L/min pure hydrogen gas using catalytic hydrolysis of 20 wt% NaBH₄ to feed a 500-W scale fuel cell stack. Thus, the Co/γ-Al₂O₃ and Co-P/Ni foam catalysts in the hydrogen generator play significant roles in promoting hydrogen production rates that are as fast as necessary by enhancing the slow response that is intrinsic to using only Co-P/Ni foam catalysts. Moreover, different hydrogen production rates can easily be achieved during the operation by controlling NaBH₄ solution rates using a fuel pump so that the hydrogen storage efficiency can be improved by supplying required hydrogen gas in accordance with load demands. The specific energy density of the electric power plant was measured 211 Wh/kg. Therefore, the power plant described here can be a power source for mobile applications, such as cars and UAVs, as well as a stationary power supplier when electric energy is required.     

Partial oxidation of ethanol in a membrane reactor for high purity hydrogen production

Partial oxidation of ethanol was performed in a dense Pd–Ag membrane reactor over Rh/Al₂O₃ catalyst in order to produce a pure or, at least, CO_x-free hydrogen stream for supplying a PEM fuel cell. The membrane reactor performances have been evaluated in terms of ethanol conversion, hydrogen yield, CO_x-free hydrogen recovery and gas selectivity working at 450 °C, GHSV ∼ 1300 h⁻¹, O₂:C₂H₅OH feed molar ratio varying between 0.33:1 and 0.62:1 and in a reaction pressure range from 1.0 to 3.0 bar. As a result, complete ethanol conversion was achieved in all the experimental tests. A small amount of C₂H₄ and C₂H₄O formation was observed during reaction. At low pressure and feed molar ratio, H₂ and CO are mainly produced, while at stronger operating conditions CH₄, CO₂ and H₂O are prevalent compounds. However, in all the experimental tests no carbon formation was detected. As best results of this work, complete ethanol conversion and more than 40.0% CO_x-free hydrogen recovery were achieved.     

Aluminum chloride for accelerating hydrogen generation from sodium borohydride

The present research paper reports preliminary results about the utilization of anhydrous aluminum chloride (AlCl₃) for accelerating hydrogen generation through hydrolysis of aqueous solution of sodium borohydride (NaBH₄) at 80 °C. To the best of our knowledge, AlCl₃ has never been considered for that reaction although many transition metal salts had already been assessed. AlCl₃ reactivity was compared to those of AlCl₃·6H₂O, AlF₃, CoCl₂, RuCl₃ and NiCl₂. With AlCl₃ and a NaBH₄ solution having a gravimetric hydrogen storage capacity (GHSC) of 2.9 wt.%, almost 100% hydrogen was generated in few seconds, i.e., with a hydrogen generation rate (HGR) of 354 L min⁻¹ g⁻¹(Al). This HGR is one of the highest rates ever reported. Higher HGRs were obtained by mixing AlCl₃ with CoCl2, RuCl₃ or NiCl₂. For example, the system RuCl₃:AlCl₃ (50:50 mass proportion) showed a HGR > 1000 L min⁻¹ g⁻¹(Ru:Al). The hydrolysis by-products (once dried) were identified (by XRD, IR and elemental analysis) as being Al(OH)₃, NaCl and Na₂B(OH)₄Cl and it was observed that even in situ formed Al(OH)₃ has catalytic abilities with HGRs of 5 L min⁻¹ g⁻¹(Al). All of these preliminary results are discussed, which concludes that AlCl₃ has a potential as accelerator for single-use NaBH₄-based storage system.     

Cobalt (II) salts,performing materials for generating hydrogen from sodium borohydride

Hydrogen generation through sodium borohydride (NaBH₄) hydrolysis has attracted much attention. This reaction has to be catalyzed by metal-based materials. We studied the catalytic potential of cobalt (II) and (III) salts. Some of them have never been studied, and compared to e.g. cobalt nanoparticles or powder, and cobalt borides. CoCl₂ showed the best performance. In our opinion, CoCl₂ should not be dismissed from the large number of catalysts. One could conceive portable applications using CoCl₂; this is briefly discussed. CoCl₂ was compared to both commercial cobalt boride and in-situ formed (through our hydrolysis conditions) cobalt boride. Their hydrogen generation rates were 86.3, 1.0 and 1.6 L min⁻¹ g⁻¹(Co), respectively. The hydrogen generation rate of CoCl₂ is one of the highest ones reported so far. It is assumed that cobalt boride surface evolves during the reaction and depends on the hydrolysis medium features. Further studies are required to fully explain the complex reaction mechanisms.     

Fluorinated cobalt for catalyzing hydrogen generation from sodium borohydride

The present paper reports preliminary results relating to a search for durable cobalt-based catalyst intended to catalyze the hydrolysis of sodium borohydride (NaBH₄). Fluorination of Co [Suda S, Sun YM, Liu BH, Zhou Y, Morimitsu S, Arai K, et al. Catalytic generation of hydrogen by applying fluorinated-metal hydrides as catalysts. Appl Phys A 2001; 72: 209–12.] has attracted our attention whereas the fluorination of Co boride has never been envisaged so far. Our first objective was to compare the reactivity of fluorinated Co with that of Co boride. We focused our attention on the formation of Co boride from fluorinated Co. Our second objective was to show the fluorination effect on the reactivity of Co. Our third objective was to find an efficient, durable Co catalyst. It was observed a limited stabilization of the Co surface by virtue of the fluorination, which made the formation of surface Co boride more difficult while the catalytic activity was unaltered. The fluorination did not affect the number of surface active sites. Nevertheless, it did not prevent the formation of Co boride. The fluorination of Co boride was inefficient. Hence, fluorination is a way to gain in stabilization of the catalytic surface but it is quite inefficient to hinder the boride formation. Accordingly, it did not permit to compare the reactivity of Co boride with that of Co.     

Hydrogen generation from catalytic hydrolysis of sodium borohydride solution using supported amorphous alloy catalysts (Ni–Co–P/γ-Al2O3)

The supported amorphous alloy catalysts Ni–Co–P/γ-Al₂O₃ were synthesized by electroless plating for hydrogen generation from catalytic hydrolysis of sodium borohydride solution. The influences of deposition time, pH, NaBH₄ concentration and the Co/Ni atomic ratio on the hydrogen generation rate were investigated in this paper. The reported work also includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (E_a = 52.05 kJ mol⁻¹). Energy dispersive X-ray spectrometer (EDS), field emission scanning electron Microscope (SEM), inductively coupled plasma-atomic emission spectrometer (ICP-AES) and X-ray diffraction (XRD), nitrogen adsorption–desorption isotherm were used to characterize surface element composition, morphology and structure of the amorphous alloy.