FR2892111A1 - MATERIAL FOR GENERATING HYDROGEN BY HYDROLYSIS OF BOROHYDRIDE, PROCESS FOR PRODUCING SUCH MATERIAL, AND FUEL CELL USING SUCH MATERIAL - Google Patents

Abstract

This material for generating hydrogen is obtained by hydrolysis of a borohydride.This borohydride is in powder form, the constituent grains of which are partially or completely covered by particles of catalytic material.

Description

MATERIAL FOR GENERATING HYDROGEN BY HYDROLYSIS OF A BOROHYDRIDE,

  PROCESS FOR PRODUCING SUCH A MATERIAL AND FUEL CELL USING SUCH MATERIAL FIELD OF THE INVENTION The present invention relates to the production of material for generating hydrogen, in particular for generating hydrogen gas. In known manner, such a production can be carried out by a hydrolysis reaction of a borohydride. This invention finds application in particular as a fuel for PEMFC proton exchange membrane fuel cells (Proton Exchange Membrane Fuel Cells O. More particularly in the context of this application, the present invention relates to fuel cells intended for the electrical power supply of portable electric or electronic apparatus, that is to say devices requiring a low electrical power.This invention may however find application to supply hydrogen fuel cells of higher powers. TECHNICAL It is known that the fuel cell represents a non-polluting and alternative source of energy for the combustion of hydrocarbons, in particular for automobiles.In the case of portable applications, one of the main advantages of fuel cells, compared to the usual batteries such as a lithium battery, is based on the fo energy density (power ratio and autonomy on the mass used) of their fuels.

  A fuel cell is a battery in which electricity is generated by the oxidation of a reducing fuel, for example hydrogen, to an electrode, coupled with the reduction of an oxidant, such as the oxygen of the fuel. air, on the other electrode. It is possible to accelerate the oxidation reaction of hydrogen by means of a catalyst which generally contains a metal element. However, in the case of fuel cells using hydrogen, an extremely flammable product, one of the main obstacles is the production and / or storage of this fuel. This is why technological choices are currently focused on liquid fuels directly oxidized at the cell's anode or to fuels, liquid or solid, capable of generating hydrogen on demand, that is to say which means that the amount of hydrogen generated equals the amount of hydrogen consumed by the cell.

  One of the known hydrogen production methods consists of hydrolyzing borohydrides, in solid or liquid form, such as sodium borohydride or tetrahydroborate NaBH4, dissolved in solution and brought into contact with a solid state catalytic material. The hydrolysis reaction of sodium borohydride is effected under certain conditions according to the following reaction scheme: M (BH 4) n + 2nH 2 O M [B (OH) 4] n + 4nH 2 where M is an alkaline or alkaline earth element and n is a positive integer equal to the number of valence electrons of the element M. This element M can for example be sodium (Na), in which case the number n is equal to 1, which leads to the following reaction scheme: NaBH 4 + 2H 2 O ---> NaB (OH) 4 + 4H 2. Nevertheless, the element M could consist of potassium (K), lithium (Li) or another suitable element.

  This hydrolysis reaction of a borohydride has the advantage of involving reagents and residues which are harmless, that is to say non-toxic and non-polluting, contrary, for example, to the reactions involved in the direct methanol fuel cells of DMFC type (for Direct Methanol Fuel Cell) or formic acid type FAFC (for Formic Acid Fuel Cell), which respectively use methanol or formic acid as fuel. According to this reaction scheme, the commonly used sodium borohydride generates four moles of hydrogen by reacting with two moles of water.

  In general, a solution of sodium borohydride is used to facilitate the reaction because the hydrolysis requires the use of catalysts, acid or oxidant.

  This solution is brought into contact with a catalytic material; typically it passes over a catalytic bed, in order to generate hydrogen. Flow control of the borohydride solution controls the reaction. Nevertheless, it has been found that such a mode of generating hydrogen has a rather low yield, the yield being defined here as the ratio of the mass of hydrogen generated to the fuel mass used. Indeed, the use of a borohydride solution has a major disadvantage for the application to fuel cells, in particular that intended for portable devices: the solubility in water of sodium borohydride and its product is relatively low .

  Thus, about 3.8 moles of water are required to solubilize one mole of sodium borohydride (NaBH4) and 13.3 moles of water to solubilize one mole of the reaction product (NaBO2). The amount of water to be used to obtain either a solution of sodium borohydride or a solution in which the reaction product is soluble is much greater than the amount of water required for the simple hydrolysis reaction.

  As a result, the actual hydrogen genesis efficiency is lower than the theoretical yield (10.8%). It would thus be about 7% if the solution solubilized only sodium borohydride (NaBH4), but it is of the order of only 3% because the solution solubilizes sodium borohydride (NaBH4) and the reaction product (NaBO2). ). There is therefore currently no hydrogen generator based sodium borohydride solution, capable of working with yields greater than 4%.

  In addition, it should be noted that borohydride solutions are not stable over time: they hydrolyze by generating hydrogen H 2, which proves troublesome for storage.

  In order to approximate the theoretical yield and raise the energy density of the fuel, it is therefore preferable to use only the amount of stoichiometric water required for the hydrogen genesis reaction. For this, it has been envisaged to use borohydrides in solid form, in particular in the divided state, that is to say in powder form. The hydrogen is then generated by adding solid borohydride to an aqueous solution preferably containing a catalytic material.

  However, it is difficult to control such a reaction and hence the hydrogen flow rate generated. Indeed, this type of heterogeneous reaction, that is to say involving reagents in the liquid state and in the solid state, is difficult to control because it produces diffusion mechanisms, in this case the diffusion of the water towards the borohydride. However, such diffusion mechanisms strongly depend on the porosity of the reactive medium, which itself varies during the progress of the reaction due to the formation of more or less compact by-products.

  However, it is recalled that the battery systems must be optimized in terms of weight. It is therefore necessary to optimize the production of hydrogen by optimally controlling the reaction. SUMMARY OF THE INVENTION The object of the present invention is to provide a hydrogen genesis material not having the disadvantages of the prior art. The subject of the present invention is therefore a material for generating hydrogen by hydrolysis of a borohydride making it possible to better control the reaction kinetics, and therefore the flow rate of hydrogen generated, and in addition to limiting the formation of a passivating layer. . According to the invention, the borohydride is in the form of powder, the grains of which are covered in whole or in part by particles of catalytic material. In other words, the catalyst covers all or part of the borohydride grains.

  Such a material, taking into account the activation mode of the solid borohydride, makes it possible to generate hydrogen by operating a heterogeneous solid-liquid reaction that is easily controllable, regardless of its degree of advancement. Indeed, the catalytic material covers all or part of the surface of the solid, so that the activation of the reaction requires no catalyst transport by the hydrolyzing phase. According to a first embodiment of the invention, the borohydride may be a borohydride of an alkaline or alkaline-earth element. Such an alkaline or alkaline earth element does not pollute, is nontoxic and can lead to good reaction yields. Advantageously, this element may be chosen from the group comprising sodium (Na), lithium (Li) and magnesium (Mg). These three elements have a high hydrogen capacity. Na BH 4 sodium tetrahydroborate is furthermore the most advantageous since it is easy to produce and inexpensive. According to an advantageous embodiment of the invention, the borohydride powder may have a specific surface area of between 0.5 and 100 m 2 g -1, and preferably between 1 and 10 m 2 g -1.

  A specific surface thus defined can lead to high reaction yields. The specific surface of the powdery borohydride is an essential parameter governing the intrinsic reactivity of the compound. The specific surface area depends on the median diameter of the particles and their density and is characteristic of the division of non-porous materials such as borohydrides. In practice, the borohydride may be based on tetrahydroborate ion (BH4-). The choice of this ion to achieve the material object of the invention maximizes the amount of hydrogen produced.

  According to another embodiment of the invention, the particles of catalytic material may be constituted by a noble or noble metal or an alloy or a salt or an oxide of at least one noble or non-noble metal. These materials have the advantage of effectively catalyzing the hydrolysis reactions of borohydrides. In practice, the metal may be selected from the group consisting of ruthenium (Ru), platinum (Pt), palladium (Pd), cobalt (Co), nickel (Ni), iron (Fe), gold (Au), silver (Ag), manganese (Mn), rhenium (Re), rhodium (Rh), titanium (Ti), vanadium (V) and cerium (Ce). The alloy may be advantageously PtRu.

  Advantageously, the salt may be a boride or an organometallic salt of at least one noble or non-noble metal, for example cobalt boride or nickel boride, both of which are good catalysts. When one or more of these metals are used to form the particles of catalytic material, it is possible to achieve good yields of hydrogen genesis reaction by hydrolysis of the material object of the invention.

  Cobalt and nickel do not lead to deposits of metal particles but to the formation of highly active corresponding borides as they are divided at the nanoscale.

  By way of example, the particles of catalytic material may have a median diameter of between 1 and 100 nm, and preferably between 3 and 5 nm. Such nanoparticles make it possible to catalyze the hydrolysis reaction of the material so as to obtain optimum yield.

  Advantageously, the powder, when its constituent grains are covered by the particles of catalytic material substantially uniformly, may comprise between 0.5 and 10% by weight of said particles, and preferably between 1 and 5% by weight of said particles. Such a proportion makes it possible to carry out a hydrolysis reaction of the material having a high reaction yield.

  Furthermore, the present invention also relates to a process for producing a material for generating hydrogen by hydrolysis of a borohydride. According to the invention, this process comprises the steps of: producing particles of catalytic material by reducing a salt or an alloy or an oxide of at least one metal, noble or non-noble; depositing these particles on the surface of the borohydride grains by a homogeneous or heterogeneous phase reaction of a dilute solution of a precursor of this catalytic material on a solution or suspension comprising the borohydride grains.

  A material manufactured according to such a method makes it possible to carry out a high efficiency hydrogen genesis reaction.

  By way of example, this process may comprise the steps of: suspending dry sodium borohydride in dry tetrahydrofuran; vigorously agitating said suspension; bringing said suspension to a temperature of about 40 C; adding dropwise solution to said suspension a solution containing ruthenium acetylacetonate; maintain vigorous agitation for about 14 hours; cooling the resulting solution to room temperature; filtering said solution under an argon sweep; drying the solid filtrate at a temperature of about 60 C and a reduced pressure to about 102 mbar.

  The present invention also relates to a fuel cell comprising an electrolyte, anode and a cathode whose oxidant is oxygen (02) and the hydrogen reductant (H2) produced by hydrolysis. According to the invention, this cell comprises a device capable of carrying out a hydrolysis reaction of a material which is the subject of the invention as previously described.

  The invention will become more clearly apparent from the description of the following particular embodiments. The object of the invention is however not limited to these particular embodiments and other embodiments of the invention are possible.

  EMBODIMENTS OF THE INVENTION The invention is illustrated below by means of a practical example of a process which is the subject of the invention, making it possible to manufacture the material which is also the subject of the invention, that is to say a borohydride in powder form, the grains of which are covered in whole or in part by particles of catalytic material. In addition, the hydrolysis reaction of said material is detailed, although it is based on a known reaction scheme.

  Preparation of sodium borohydride powder partially coated with 30 ruthenium nanoparticles

  In this particular embodiment according to the method which is the subject of the invention, 5 g of dry sodium borohydride, ie 1.32 × 10 4 mol, are suspended in 150 ml of dry tetrahydrofuran of formula C 4 H 80. The sodium borohydride of formula NaBH4 is said to be dry because it is previously dehydrated at 150 ° C. under primary vacuum for 4 hours. Thus, according to a particular embodiment of the invention, the borohydride is a borohydride of an alkaline element and based on tetrahydroborate ion (BH4). A material made from these compounds achieves a high yield for the hydrogen genesis reaction. It is also possible, without departing from the scope of this invention, to use a borohydride of an alkaline earth element, such as magnesium (Mg); such a borohydride could for example consist of Mg (BH4) 2.

  The use of this sodium borohydride (NaBH 4) does not require any particular safety measure, since this compound is harmless, that is to say, it does not pollute and is nontoxic. This is one of the advantages of borohydrides of alkaline elements or alkaline earth elements, besides the fact that they can lead to good reaction yields. The sodium borohydride (NaBH4) employed herein has, in accordance with one embodiment of the present invention, a specific surface area of 1.8 m 2 g -1, i.e. between 0.5 and 100. m2.g-1. Such a specific surface area can lead to high reaction yields. The surface area of the powdery borohydride is an important parameter governing the intrinsic reactivity of the compound. It corresponds to the surface developed by all the grains. Thus, the higher the specific surface area, the better the chemical yield of the hydrolysis reaction. Of course, the specific surface area depends on the median diameter of the particles and their density and is characteristic of the division of non-porous materials such as borohydrides.

  One method of determining the surface area of a powder may be to use the BET method (Brunauer, Emmett and Teller). It is a method of gas adsorption which consists in fixing a gas (nitrogen or krypton) at low temperature in monolayer on the surface of each grain.

  Thus, the sodium borohydride powder (NaBH4) used in the example described has a specific surface area of 1.8 m2.g-1.

  As already stated, the sodium borohydride powder (NaBH4) is thus suspended in 150 ml of dry tetrahydrofuran, that is to say unsolubilized. The use of this solvent makes it possible to subsequently solubilize the ruthenium salt.

  The suspension thus obtained is subjected to vigorous stirring and brought to 40 C.

  In practice, 50 ml of a solution containing 143.2 mg, ie 0.36 mmol of ruthenium acetylacetonate, of formula Ru [C 5 H 70 2] 3 are added dropwise to this suspension. The choice of this catalytic material is thus determined according to an embodiment of the invention, wherein the particles of catalytic material are made of a noble metal.

  However, it is also possible, without departing from the scope of the invention, to use an alloy or a salt, such as a boride or an organometallic salt, of a noble metal, but also of a non-noble metal. . For example, it is possible to implement an alloy of the PtRu type. In practice, the metal could also be selected from the group consisting of platinum, ruthenium, palladium, cobalt, nickel, iron, gold, silver, manganese, rhenium, rhodium, titanium , vanadium and cerium (Ce). It is also preferable that their dispersion be nanometric, in order to achieve a high yield.

  As can be seen, there are in this group of noble metals and non-noble metals. Thus, when one or more of these metals is (are) used to form the particles of catalytic material, it is possible to achieve good yields of hydrogen genesis reaction by hydrolysis of the material object of the invention.

  It should be noted here that cobalt and nickel do not lead to deposition of metal particles on the surface of the borohydride powder, but to the formation of the corresponding borides, namely CoB and NiB. These borides can be very active because they are divided at the nanoscale and therefore have a large surface area.

  The stirring and the temperature to which the suspension is subjected, now put in solution with ruthenium acetylacetonate, are maintained for 14 hours. The solution is then cooled to room temperature and then filtered under an argon sweep so as to avoid hydrolysis of the borohydride by the humidity of the air. The solid filtrate thus obtained is then dried under a reduced pressure up to 102 mbar and at a temperature of 60 C.

  Finally, a slightly gray sodium borohydride powder is obtained, which is an example of the material according to the present invention. Its slightly gray color indicates the presence of particles of the catalytic material, in this case ruthenium metal.

  The elemental analysis indeed confirms the presence of 0.94% by weight of ruthenium metal, which covers the grains of sodium borohydride in a substantially uniform manner and in general in a partial manner. Such a weight percentage is in accordance with the characteristic of the invention according to which the powder, when its grains are covered by the particles of catalytic material, may comprise between 0.5 and 10% by weight of said particles. Such a proportion makes it possible to carry out a reaction with a high hydrogen yield of the hydrolysis of the material which is the subject of the invention.

  In addition, according to another characteristic of the invention, the ruthenium is in the form of particles of about 5 nm median diameter; this median diameter is therefore between 1 and 100 nm. This is why these particles are called nanoparticles. They make it possible to catalyze the hydrolysis reaction of the material which is the subject of the invention so as to obtain an optimal yield in the genesis of hydrogen.

  Moreover, the specific surface of the sample seems unchanged; in a known manner, it is evaluated after determining the median diameter of the grains by X-ray diffractometry. This can be explained by the small size of the particles of catalytic material (5 nm), which is negligible compared to the size of the grains of the powder which these cover, and by the insignificant variation of the density of the powder.

  Hydrolysis reaction of the material of the present invention The 5 g of ruthenium-coated borohydride powder are then introduced into a test tube. This powder may also be in the form of pellets obtained by compression. This test tube is surmounted by a rubber septum comprising a syringe to add the hydrolyzing solution, and a tube for evacuating the gradually generated gaseous hydrogen. For the hydrolysis reaction, 8.8 ml of water are gradually added to the test tube. Since it is the flow of water that controls the reaction, according to one of the advantages provided by this invention, the hydrogen flow rate produced stabilizes around 10 ml / min. Finally, 12 liters of gaseous dihydrogen (measured in the Standard Conditions of Temperature and Pressure, ie at atmospheric pressure and ambient temperature) are produced.

  Therefore, the overall yield of this hydrolysis reaction is 7.2% by weight, which is much higher than the yields available with the materials of the prior art for which the catalytic material is dissociated from the powder. of borohydride. The yield of the reaction is calculated by relating the mass of hydrogen collected on the total mass of reactants, water and sodium borohydride, reacted.

  Since it is likely that the hydrogen is generated at the point of triple contact between the borohydride, the catalytic material and the water, it is understood that the yield of the hydrolysis reaction strongly depends on the specific surface of the hydrolysed powder. Indeed, the specific surface characterizes the division of the powder and the larger it is, the smaller the grains, so the more the number of triple contact points is high. In addition, since the particles of catalytic material cover all or part of the grain surface of the borohydride, the activation of the hydrolysis reaction does not require any catalyst transport by the hydrolyzing phase.

  A material in accordance with the present invention thus makes it possible to generate hydrogen by a hydrolysis reaction whose overall yield is very high and whose control is easily achieved when the hydrolyzing solution is added. Given the mode of activation of the solid borohydride made possible by the present invention, the hydrolysis reaction, although heterogeneous (solid / liquid), is easily controllable, regardless of its degree of advancement. It is thus possible to avoid any uncontrolled runaway of this reaction and, in the context of the application to a fuel cell, to avoid the risk of ignition of the hydrogen which would be dangerously accumulated because produced in quantities not immediately consumed.

  In addition, a material according to the present invention makes it possible to limit the formation of a passivating layer, that is to say a layer constituted by by-products of the hydrolysis reaction. In fact, the metaborates formed by contact between water, the catalyst and sodium tetrahydroborate are distributed on the surface of the grains, and thus do not constitute a continuous and blocking layer.

  The material - object of the invention therefore makes it possible to manufacture a hydrogen generator, of very high energy density, for supplying fuel to a cell which is the subject of the invention. Such a battery conventionally comprises an electrolyte, an anode and a cathode. It involves oxygen gas (O 2) as oxidizing agent and as reducing agent, hydrogen gas (H 2) produced by hydrolysis of the material according to the present invention. This battery can in particular power devices of low power (1 W to 100 W).

  According to the hypotheses and observations made, the parameters influencing the final yield of the hydrolysis reaction are therefore numerous and include: the nature of the borohydride used; the atomic number of the element combined with this borohydride; the specific surface of the material; the nature and the median diameter of the particles of catalytic material; The mass percentage and the surface distribution of the particles of catalytic material relative to the material produced object of the invention.

  But mainly, a parameter controls the kinetics of this hydrolysis reaction of the sodium borohydride powder, namely the flow of hydrolyzing solution, or at least the molar amount of water reacted. 15

Claims (12)

  1. Material for generating hydrogen by hydrolysis of a borohydride, characterized in that said borohydride is in powder form, the constituent grains of which are covered in whole or in part by particles of catalytic material.   2. Material for generating hydrogen by hydrolysis according to claim 1, characterized in that the borohydride is a borohydride of an alkaline element or an alkaline earth element.   3. Material for generating hydrogen by hydrolysis according to claim 2, characterized in that said element is selected from the group comprising sodium (Na), lithium (Li) and magnesium (Mg).   4. Material for generating hydrogen by hydrolysis according to one of the preceding claims, characterized in that the borohydride powder has a specific surface area between 0.5 and 100 m2.g-1, and preferably between 1 and 10 m2.g-1. 20   5. Material for generating hydrogen by hydrolysis according to one of the preceding claims, characterized in that the borohydride is based on tetrahydroborate ion (BH4). 25   6. Material for generating hydrogen by hydrolysis according to one of the preceding claims, characterized in that the particles of catalytic material are constituted by a noble or noble metal, an alloy or a salt or an oxide of less a metal, noble or not noble. 30   7. Material for generating hydrogen by hydrolysis according to claim 6, characterized in that the metal is selected from the group comprising platinum, ruthenium, palladium, cobalt, nickel, iron, gold, silver, manganese, rhenium, rhodium, titanium, vanadium and cerium.   8. Material for generating hydrogen by hydrolysis according to claim 6, characterized in that the salt is a boride or an organometallic salt, at least one metal, noble or non-noble.   9. Material for generating hydrogen by hydrolysis according to claim 8, characterized in that the salt is constituted by cobalt boride or by nickel boride.   10. Material for generating hydrogen by hydrolysis according to one of the preceding claims, characterized in that the borohydride powder, when its constituent grains are covered by the particles of catalytic material substantially uniformly, comprises between 0.5 and 10% by weight of the particles of catalytic material, and preferably between 1 and 5% by weight of said particles.   11. A process for producing a material for generating hydrogen by hydrolysis of a borohydride, characterized in that it comprises the steps of: producing particles of catalytic material by reducing a salt or a alloy or an oxide of at least one metal, noble or non-noble; depositing said particles on the surface of the constitutive grains of said borohydride by reaction in a homogeneous or heterogeneous phase of a dilute solution of a precursor of said catalytic material on a solution or a suspension comprising the borohydride grains.   Fuel cell comprising an electrolyte, anode and a cathode, the oxidant of which is oxygen (O 2) and the hydrogen reductant (H 2) produced by hydrolysis, characterized in that it comprises a generator hydrogen capable of hydrolyzing a material according to one of claims 1 to 10.