Field of the invention

The present invention refers to the field of the controlled production of hydrogen from compounds containing combined hydrogen, in particular to devices for the generation of hydrogen

State of the art

The use of metal borohydrides, in particular of sodium borohydride (NaBH ₄ , sodium tetrahydroborate) for the production of hydrogen gas by aqueous hydrolysis is a well-known process which has been widely investigated. A recent scientific article describes exhaustively the state of the art of the catalyzed generation of hydrogen from aqueous solutions of sodium borohydride, of the catalysts employed and of the uses of the hydrogen gas produced U. B. Demirci et al. Fuel Cells 2010, 10 (No. 3), 335). In addition to hydrogen gas, the hydrolysis reaction (1 ) yields sodium metaborate (NaBO ₂ ), which is a recyclable product with many industrial applications.

NaBH ₄₍ aq) + 2H ₂ O → NaBO ₂₍ aq) + 4H ₂ † + heat (300 kJ) (1 )

Reaction (1 ) is a spontaneous and exothermic process that, for practical uses, has to be accelerated by means of suitable catalysts, generally based on finely dispersed transition metals. The catalysts of reaction (1 ) include noble metal salts (Pt, Rh, Ir, Ru), non-noble metal salts (Mn, Fe, Co, Ni, Cu), metal borides of Co or Ni, metal in the 0 oxidation state either as nano- or micro-structured powders or supported on metal oxides or porous carbons. A perusal of the recent literature (U. B. Demirci et al. Fuel Cells 2010, 10 (No. 3), 335) shows how cobalt boride (CoB), cobalt-cobalt boride (Co-CoB) and nickel-cobalt boride (Ni-CoB) combine an excellent catalytic activity, up to 11 L H ₂ min ^"1 g ^"1 (H. B. Dai et al. J. Power Sources 2007, 177, 17; Wu et al. Mat. Letters 2005, 59, 1748) with a low cost and the possibility of being separated from the reaction mixture by magnetic attraction. As shown in reaction (1 ), the hydrolysis of one mole of NaBH ₄ yields theoretically four moles of hydrogen and consumes two moles of water which are responsible for the production of two moles of hydrogen. It is therefore correct to state that the hydrogen of reaction (1 ) is generated by the NaBH -H ₂ O system. In real conditions, reaction (1 ) consumes more than two moles of water per mole of NaBH ₄ due to the formation of a hydrated salt of sodium metaborate whose solubility (28 g in 100 g of H ₂ 0 at 25 °C) is lower than that of NaBH ₄ (55 g in 100 g of H ₂ O). Accordingly, to avoid that NaB0 ₂ , precipitating in the solution, may deactivate the catalyst of reaction (1 ), with consequent reduction of the hydrogen production, it is appropriate to use an initial NaBH4 concentration lower than 16 g in 100 g of water. For practical applications of the NaBH ₄ -H ₂ O system to generate hydrogen in a controlled manner, one has to take into account also the stability of the NaBH solutions with time given the thermodynamic spontaneity of reaction (1 ). To this purpose are commonly employed alkali metal hydroxides, generally sodium or potassium hydroxide (NaOH o KOH). Indeed, the NaBH4 solutions are more stable in alkaline environment with a half-life time depending on the pH value and temperature (Eq. 2) (V. G. Minkina et al. Russ. J. Appl. Chem. 2008, 81, 380). log(fi/ ₂ ) = pH -(0.034 T-1 .92) where T = K (2)

Recent studies (B. H. Liu et al. Thermochim. Acta 2008, 471, 103) have demonstrated that an optimum stabilization of the NaBH ₄ -H ₂ 0-NaOH system is achieved dissolving 150 g of NaBH ₄ (3.9 moles) and 100 g of NaOH (ca. 2.5 moles) in ca. 750 ml_ of water. In the light of what said above, the Gravimetric Hydrogen Storage Capacity (GHSC) of aqueous solutions of NaBH ₄ cannot be much higher than 3 wt%, a value which is largely inferior to what recommended by the U.S. Department of Energy (DOE) for the use of NaBH ₄ as material for the onboard hydrogen generation for automotive applications. Such a GHSC value is, however, acceptable to feed power generators based on fuel cell stacks up to some hundred watts. Recently, it has been announced the commercialization of portable power generators fuelled with hydrogen generated by hydrolysis of aqueous solutions of NaBH and capable of supplying powers up to 50 W (Hydropak by Horizon, www.horizonfuelcell.com).

It is evident that the generation of the hydrogen gas required to feed a fuel cell stack must take place in a reactor where the NaBH -H ₂ O-NaOH system reacts on a suitable catalyst. Several types of such reactors, either static or dynamic, are known. In some static devices, the catalyst is introduced into the vessel containing the NaBH ₄ solution as powders, pellets or it is supported on inert porous materials such as honeycomb monolyths (Y. Kojima et al. J. Power Sources 2004, 33, 1845; http://www.fractalcarbon.com). The static systems exhibit generally low efficiency due to various reasons, among which there is the difficulty of catalyst separation from the exhausts, the catalyst leaching from the support, the de-activation of the catalyst occasioned by the precipitation of sodium metaborate and, finally, mass transport phenomena. Higher efficiency seems to be shown by the dynamic systems based on the flow of the NaBH ₄ -NaOH solution inside a tubular reactor containing an appropriate catalyst. In an attempt of separating the catalyst from the NaBH ₄ solution and avoiding the contamination of the exhausts by the catalyst have been used filters (US 6.534.033). Obviously, this technology requires appropriate dimensions of the catalyst with potential activity losses. In some scientific papers (S. C. Amendola et al. Int. J. Hydrogen Energy 2000, 25, 969; S. C. Amendola et al. J. Power Sources 2000, 85, 186) and patents (US 2003/0037487, US 2005/0268555, US 6932847, WO 03/004145) is described the use of a peristaltic pump that forces the NaBH solution to pass through a reactor containing a Ru-based catalyst supported on ion-exchange resins. Such devices are not free of Ru-resin degradation due to the high pH values of the NaBH -H ₂ O- NaOH system as well as to catalyst physical leaching occasioned by the turbulence generated by the hydrogen bubbles and by the locally high pressure of the hydrogen gas that forms not only on the catalytic layer, comprising nano- and micro particles, but also inside the layer itself. The reactor described in the papers and patents reported above produces a maximum hydrogen flow of ca. 200 mL min ^"1 gcatalyst ^"1 and is part of the Millennium Cell and Horizon Fuel Cell technology applied to the Hydropak generators (www.millenniumcell.com; www.horizonfuelcell.com) with a nominal maximum power of 50 W.

A further method to increase the physical stability of the catalyst during the hydrogen generation has been realized with the use of permanent magnets externally positioned to a tube inside which a ferromagnetic catalyst, preferably based on Fe-Pt-Rh, is immobilized by action of the external magnetic field, during the flow of an alkaline solution of NaBH ₄ (EP 496014A1 ; A. Pozio et al. Int. J. Hydrogen Energy 2008, 33, 51 ; A. Pozio et al. Int. J. Hydrogen Energy 2009, 34, 4555). Such a technology has allowed the ERRE DUE srl company (http://www.erreduegas.it/) to develop and commercialize device for hydrogen generation capable of supplying a maximum flow of 300 ml_ min ^"1 .

The hydrogen gas produced upon hydrolysis of aqueous solutions of NaBH ₄ is extremely pure, devoid of carbon oxides and naturally humid, hence appropriate for its utilization in fuel cells with a polymeric electrolyte of the type known with the acronym PEMFC (Polymer Electrolyte Membrane Fuel Cell). It is generally agreed that the PEMFCs contain a solid electrolyte constituted by a polymeric cation- exchange membrane. There is no whatsoever restriction to use the hydrogen gas produced by aqueous NaBH ₄ hydrolysis in fuel cells where the electrolyte is an anion-exchange membrane, known with the acronym AEFCs (Alkaline Electrolyte Fuel Cells), and the oxygen reduction at the cathode produces hydroxyl ions (OH ) that migrate to the anode instead oxide ions that remain at the anode to combine with the protons formed at the anode side as occurs in a PEMFC.

As one may realize reading what reported above, despite several studies and improvements achieved in this field, it is still necessary to develop new devices for the generation of hydrogen containing stable catalysts with time, able to provide higher gas flows, as compared to those obtainable at present, by virtue of a strict control of the internal temperature of the reactor and, at the same time, easier to handle and capable of higher yields as compared to the known devices, while warranting safe performance.

Summary of the invention

In this invention is described a hydrogen generator comprising a reactor that, utilizing known catalysts, allows for the hydrolysis of aqueous alkaline solutions of alkaline metal or alkaline-earth metal borohydrides. The reactor is contained in a gastight container which allows for the release of the hydrogen produced and the recycling of the borohydride solution in any external tank with respect to the reactor.

Description of the figures

Figure 1 shows schematically in horizontal and vertical sections (A, B and C), and in a perspective view (D) of a hydrogen generator of the invention.

Figure 2 shows schematically an apparatus for the generation of hydrogen containing a hydrogen generator of the invention.

Figure 3 shows schematically an apparatus containing a hydrogen generator of the invention coupled to a fuel cell stack.

Figure 4 shows the hydrogen evolution with time in the experimental conditions of example 1.

Figure 5 shows the hydrogen evolution with time in the experimental conditions of example 2.

Figura 6 shows the power output with time supplied by a PEMFC stack (nominal power 100 W) fed with the hydrogen gas produced in the experimental conditions of example 1.

Detailed description of the invention

The present invention allows one to overcome the problems of the known hydrogen generators mentioned above thanks to a hydrogen generator comprising a reactor for the catalyzed hydrolysis of aqueous alkaline solutions of alkaline or alkaline-earth metal borohydrides, contained in a gastight container that allows for the discharge of the hydrogen produced and the recycling of the borohydride solution in any tank external to the reactor.

The improvements provided by the generator of the invention, besides the possibility of recycling the solution due to the contained described above, are originated also by the reactor where the hydrolysis reaction of the aqueous solution of the borohydrides, eventually stabilized by strong bases occurs. The temperature inside the reactor is strictly controlled by means of a cooling system of the solution that goes into the reactor.

As shown in Figure 1 , the generator of the invention is constituted by a hollow container 7 provided with two holes 12 disposed in mutual correspondence on the two opposite surfaces of the container.

The container may have any shape (parallelepiped, cubic, cylindrical, etc.), preferably it will have a cylindrical shape and, in this case, the said holes 12 will be positioned on the cylinder bases in mutual correspondence. On the container surface turned to the top is then placed a manifold 6 that allows the hydrogen produced to get out.

Inside the container 7 is positioned the reactor 11 , constituted by a hollow body of such dimensions to be placed into the container through the holes 12, leaning out from the container towards the outside with both its ends.

Preferably, also the reactor 11 will have a cylindrical shape and will be placed in such a way that its external surface lays on the internal surface of the container 7 opposite to the manifold 6, while suitable gaskets, placed along the edges of the said holes 12 will ensure a watertight seal between container 7 and reactor 11. The two opposite sides of the reactor are closed by two caps 2 one of which presents two holes to allow the introduction of the feeding pipe 1 of the reactor 11 and of the pipe 5 for the discharge of the solution.

The feeding pipe 1 lengthens inside the reactor, preferably tangent to the internal surface, up to reach the opposite side of the reactor, while the discharge pipe 5, given its function, passes through the reactor only for a short length.

Inside the container 7 are then positioned the supporting surfaces 8, one end of which is firmly fixed to the internal surface of the container 7, while the other end is fixed to the external surface of the reactor 11.

In view of the layout of the reactor, as described, it is evident that there will be an empty space left inside the container 7 whose volume will depend on the relative dimensions of the container 7 and of the reactor placed internally.

Inside the reactor 11 is placed an extractable guide 4 supporting one or more permanent magnets with either circular or quadrangular shape and with various thicknesses and having an external surface resistant to solutions of strong bases (for example, NdFeB magnets coated with a Ni-Cu alloy supplied by Supermagnete company). On the magnet surface is anchored a desired amount of catalyst.

In the portion of the surface of reactor 11 turned to the top there are the holes 10 and above them is placed the aerosol abatement system 3 constituted by a surface parallel to the reactor surface and fixed to the cylinder upper surface by suitable supports 13. The aerosol abatement system 7 serves to carry out a first separation of the gas from the solution drops and, at the same time, allow for the recovery of the latter that are conveyed inside the reactor through the holes 9 placed along the base of the surfaces 8.

In advance, a desired amount of catalyst is anchored to the magnets by magnetic attraction, which is easily realized by bringing the magnets close to the catalyst or rolling the extractable magnet-holding guide 4 on a plane covered by the catalyst. The latter can be used in various forms and morphologies, preferably powders. The pulling out of the magnet-holding guide can be made either mechanically or magnetically; in either case one may add more catalyst or a different catalyst. The catalyst can be removed from the catalytic block by the plain immersion into a diluted aqueous solution of any strong acids (HCI, H ₂ S0 ₄ ).

A huge variety of ferromagnetic catalysts can be effectively used in the reactor of the invention: cobalt or nickel powders, cobalt or nickel Raney, alloyed Co-Ni Raney, cobalt or nickel nanoclusters, cobalt or nickel wires, nano- or micro- structured aggregates of nickel with nickel borides, nano- or micro-structured aggregates of cobalt with cobalt borides, mixed aggregates of cobalt and nickel with cobalt borides (U. B. Demirci et al. Fuel Cells 2010, 10 (No. 3), 3359). In the specific case of the reactor of the invention are preferably employed catalysts based on cobalt borides of the type Co-Co _x B (x = 1 , 2, 3) due to their high catalytic activity, the excellent resistance to strong bases and to chemical poisoning, the low activation temperature and the elevated resistance to passivation by NaB0 ₂ (H. B. Dai et al. J. Power Sources 2007, 177, 17; Wu et al. Mat. Letters 2005, 59, 1748; (U. B. Demirci et al. Fuel Cells 2010, 10 (No. 3), 335).

The operation of the device of the invention is extremely simple.

The solution of a metal borohydride, stabilized by a strong base, is pumped from an external tank (see Figure 2) into the reactor through the feeding pipe 1 that drives the solution to the opposite side with respect to the exit, increasing the contact time between reactive solution and catalyst anchored to magnets. Thus, the solution goes through the entire axis of the reactor coming into contact with the catalyst supported on the magnets 4 where hydrogen is evolved. In view of the magnets disposition, the force lines of the magnetic field generated by the permanent magnets are parallel to the reactor axis as well as to the flow of the NaBH4-NaOH solution. Such a solution allows for an effective anchoring of the catalyst inside the reactor, thus inhibiting the risk of leaching of the catalyst inside the feeding/discharging circuit of the device. Further on, such a solution allows for the recovery of the catalyst itself by the magnets close to the way out of the reactor, in the case that the evolving hydrogen might physically remove some catalyst. The catalyzed guide is easily removed through the cap 2 for the regeneration or substitution of the catalyst.

The hydrogen gas produced is discharged out of the reactor through the holes 10 and meets the aerosol abatement system for a first separation of the gas (that is stored in the container 7) from the solution. The recovered solution goes back to the reactor collected by the surfaces 8 through the holes 9 as said above.

The hydrogen produced goes out of the container through the manifold 6 and is conveyed to the purification system (see Figure 2), constituted by a material that is able to remove the humidity (silica gel, molecular sieves, calcium chloride). Finally, the hydrogen is available to the end user that may be represented by a fuel cell stack (as shown in Figure 3 for example) or by a combustion apparatus. The partially exhausted solution gets out of the reactor through the discharge pipe 5 and is conveyed to be recycled to the storage tank of the metal borohydrides (Figure 2).

Therefore, as said above, the hydrogen generator of the invention is equipped with an its own gastight container inside which the aerosols are abated and recycled. Such a solution frees the generator (an in particular the reactor contained inside the generator) from the tank containing the feeding solutions. The tank may have any volume in function of the desired endurance and of the application.

The exothermic reaction (1 ) occurs prevalently in the reactor 11 , hence in an environment whose volume is largely smaller than that of the tank containing the borohydride solution. This fact and the high average contact time (this depends on the solution flow controlled by the pump) between solution and catalyst allow one to adjust the NaBH ₄ hydrolysis temperature, preferably between 10 °C and 80 °C, with consequent control of the catalytic activity, hence of the intensity of the hydrogen flow and of the solubility of the sodium metaborate produced upon hydrolysis of NaBH ₄ . The latter point is of vital importance to increase the efficiency of the catalytic system because the precipitation of sodium metaborate onto the catalyst may significantly decrease its performance (U. B. Demirci et al. Fuel Cells 2010, 10 (No. 3), 335). In the reactor of the invention one may therefore use high NaBH ₄ concentrations up t reach the optimal one of 15 wt% (W. Ye at al. J. Power Sources 2007, 164, 544; B. H. Liu et al. Thermochim. Acta 2008, 471 , 103).

In conclusion, the generator of the invention exhibits several advantages over the known devices, in particular one may notice the following remarkable improvements:

a) the hydrogen production capacity expressed as mL H ₂ min ^"1 gcat ^"

b) the catalyst stability with time

c) the operations of catalyst substitution

d) the cost of the catalyst

e) the possibility to shut off the hydrogen production on demand

f) the possibility of using concentrated NaBH solutions (up to 15 wt%) without damaging the catalytic system.

In addition, the hydrogen generator of the invention, being connected to a heat exchanger through which flows the feeding solution before it enters the reactor, allows for an accurate control of the temperature, hence of the amount of hydrogen produced.

Any types of heat exchanger can be employed to control the internal temperature of the reactor when operating. For example, as heat exchanger, one may use a radiator with tubes made of a metal resistant to strong bases (stainless steel, copper), in contact with a heat dissipator (Figure 2) cooled by means of an axial or centrifugal fan electronically controlled by a thermocouple or any other temperature sensor positioned inside the reactor.

The hydrogen production is immediately stopped by switching off the pump (Figure 2) which completely drains the reactor. A characteristic of the generator of the invention is just the possibility to interrupt the hydrogen production by switching off the centrifugal internal pump, thus causing the complete, passive draining of the reactor. In such a way, there is a constant control of the reactor activity and hydrogen can be generated on demand by the electronics of the system (Figure 3). This characteristic, together with the fact that the catalyst does not leach out of the reactor to contaminate the storage tank, ensure a high degree of operational safety even when the control electronics is malfunctioning or the pump fails.

One has also to consider that the hydrogen produced in the reactor is stored in the container of the reactor itself. The container, appropriately measured, allows for an easy moving and connection of the generator to any apparatuses that might require its application.

For the construction of the reactor body one may use any non-ferromagnetic materials, metallic or polymeric, resistant to strong bases. For the construction of the generator of the invention one may use any common materials, metallic or polymeric, resistant to strong bases.

In examples 1-3 is described the production of hydrogen gas with the generator of the invention and, by comparison, the production obtainable with a static reactor in comparable experimental conditions.

In example 4 is described the production of electrical energy with a PEMFC stack (nominal power 100 W) fed with the hydrogen produced in the experimental conditions of example 1.

Example 1

Into the external fuel tank of a generator of the invention are introduced 10 L of an aqueous solution containing NaBH ₄ (4 M, 1520 g) and NaOH (0.2 M, 80 g) which is pumped into the generator by a pump. The temperature of the NaBH ₄ -NaOH solution in the tank is ca. 29 °C. The reactor contains 1 g of a C0-C0 ₂ B catalyst, prepared as described in Phys. Chem. Chem. Phys. 2009, 1 1 , 770, dispersed onto nine circular permanent magnets (NdFeB coated with Ni-Cu alloy) each of which with a diameter of 2.5 cm. The hydrogen gas produced is forced to pass through a cartridge filled with molecular sieves to reduce the humidity of the gas before it enters a flow-meter (Bronkhorst High-Tec B. V.). Immediately after switching on the pump (200 - 400 mL min ^"1 ), the hydrolysis reaction (1 ) starts occurring in the reactor. Just after 10 min, the device starts producing a hydrogen flow of ca. 1 100 and 1200 mL min ^"1 at an internal temperature between 36 and 37 °C. This temperature interval is kept constant by means of the external cooling system shown in Figure 2. As shown in the diagram reported in Figure 4, the hydrogen flow remains constant for more ca. 50 h, in agreement with a zero-order kinetics in NaBH ₄ concentration of the catalyzed hydrolysis of NaBH (Y. Kojima et al. Int. J. Hydrogen Energy 2002, 27, 1029; S.- C. Amendola et al. J. Power Sources 2000, 85, 186; A. Levy et al. Ind. Eng. Chem. 1960, 52, 21 1 ) as well as an overall conversion of the NaBH -H ₂ 0 system into H ₂ higher than 85%.

Once removed the exhausted solution (selective formation of NaBO ₂ as shown by an ¹¹ B{ ¹ H} NMR analysis), a new identical solution of NaBH ₄ -H ₂ O-NaOH is introduced into the storage tank and the immersion pump is again switched on. The hydrogen flow is identical to that observed previously. Identical results are obtained repeating the experiment four times without changing the catalyst.

Example 2

Into the external tank of a generator of the invention (Figure 2) are introduced 10 L of an aqueous solution containing NaBH (4 M, 1520 g) and NaOH (0.2 M, 80 g) which is pumped into the generator as described in example 1 . The temperature of the solution is 29 °C. The reactor contains 2 g of a Co-Co ₂ B catalyst, prepared as described in Phys. Chem. Chem. Phys. 2009, 1 1 , 770, dispersed onto nine circular permanent magnets (NdFeB coated with Ni-Cu alloy) with a diameter of 2.5 cm. The hydrogen gas produced is forced to pass through a cartridge filled with molecular sieves to reduce the humidity of the gas before it enters a flow-meter (Bronkhorst High-Tec B. V.). Immediately after switching on the immersion pump (200 - 400 mL min ^"1 ), the hydrolysis reaction (1 ) starts occurring in the reactor. After less than 10 min, the device starts producing a hydrogen flow between ca. 2050 and 2150 mL min ^"1 at an internal temperature between 37 and 39 °C. This temperature interval is kept constant by means of the cooling system shown in Figure 2. As shown in the diagram reported in Figure 5, the hydrogen flow remains constant for ca. 25 h, in agreement with a zero-order kinetics of the catalyzed hydrolysis of NaBH ₄ (Y. Kojima et al. Int. J. Hydrogen Energy 2002, 27, 1029; S.- C. Amendola et al. J. Power Sources 2000, 85, 186; A. Levy et al. Ind. Eng. Chem. 1960, 52, 21 1 ) as well as an overall conversion of the NaBH ₄ -H ₂ 0 system into H ₂ higher than 90%. Once removed the exhausted solution (selective formation of NaBO ₂ as shown by an ¹ B{ ¹ H} NMR analysis), a new identical solution of NaBH ₄ - H ₂ O-NaOH is introduced into the storage tank and the immersion pump is again switched on. The hydrogen flow is identical to that observed previously. Identical results are obtained repeating the experiment four times without changing the catalyst inside the reactor.

Example 3 (comparative example)

Into a gastight container containing five liters of an aqueous NaBH ₄ solution (4 M, 760 g) and NaOH (0.2 M, 40 g) at a temperature of 30 °C is introduced 1 g of a Co-Co2B catalyst, prepared as described in Phys. Chem. Chem. Phys. 2009, 1 1 , 770, anchored to a permanent magnet (NdFeB coated with Ni-Cu alloy) with cylindrical shape (1 cm diameter, 4 cm height). Reaction (1 ) starts immediately after the introduction of the catalyst. The hydrogen produced is forced to pass through a cartridge filled with molecular sieves before entering the flow-meter (Bronkhorst High-Tec B. V.). The initial hydrogen flow is ca. 750 ml_ min-1 and after 60 min it increases to more than 10 L min ^"1 while the temperature of the solution reaches gradually the boiling point. The hydrogen flow decreases rapidly until it comes to an end after 120 min. The overall conversion of the NaBH -H2O system into H ₂ is equal to ca. 77 %.

Example 4

A commercial PEMFC stack with self-breathing cathodes and a nominal power of 100 W is fed with the hydrogen produced in the experimental conditions of example 1 (1 100 mL min ^"1 ). The stack performance is evaluated by means of a Scribner Associates 850e (USA) instrument. Figure 6 shows a galvanostatic diagram for a 6 A current.