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International Journal of Hydrogen Energy 33 (2008) 51 – 56www.elsevier.com/locate/ijhydene

Apparatus for the production of hydrogen from sodium borohydridein alkaline solution

A. Pozioa,∗, M. De Francescoa, G. Monteleonea, R. Oronzioa, S. Gallia,C. D’Angelob, M. Marruccib

aENEA CR Casaccia, Via Anguillarese, 301 - 000123 Rome, ItalybErredue s.r.l., Via G.Gozzano, 3 - 57121 Livorno, Italy

Received 7 February 2007; received in revised form 25 June 2007; accepted 31 August 2007Available online 10 October 2007

Abstract

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

In the present work the production of hydrogen from the sodium borohydride (NaBH4) reaction has been studied by experimental benchtests. Promising results on the NaBH4 hydrolysis, catalyst performance and stability have allowed practical application of this technology in abrand new commercial device for small laboratory production of hydrogen gas.

A hydrogen generator prototype was manufactured and tested by Erredue srl (Italy). Results confirm an effective operation of the deviceand stable behaviour of the hydrolysis catalyst material. Experimental results and modeling approach are shown here.

The reduction of sodium borohydride production cost and the new recycling methods for its reaction product appear as the critical issues,and R&D targets to solve them for a widespread energy application.� 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Sodium borohydride; Hydrogen; Hydrolysis; Magnetic reactor

1. Introduction

The main obstacles to the widespread use of hydrogen as anenergy carrier are related to the unsolved problems concern-ing its storage, transport distribution and to the choice of thebest system to produce energy [1,2]. Hydrogen can be storedin different ways: compressed [3], liquefied gas [4] or ab-sorbed on activated carbon and carbon nanotubes [5–9] and al-loys [10–16]. Hydrogen can also be stored in chemical hydride[17–23] such as NH3 [17], NH2–NH2 [18], ammonia borane[19], LiH [20], NaAlH4 [21,22], NaH [23] and sodium boro-hydride (NaBH4) [24–28]. Among these, attention has beenpaid to the hydrolysis of NaBH4 because of its large theoretical

∗ Corresponding author. Fax: +39 06 3048 6357.E-mail address: alfonso.pozio@casaccia.enea.it (A. Pozio).

0360-3199/$ - see front matter � 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2007.08.024

hydrogen content (10.9%). In this case, hydrogen is confinedin the chemical bonds of boron and alkaline metal formingborohydride.

NaBH4 is stable when compared with other chemical hy-drides and easy to handle. At room temperature only a smallpercentage of the theoretical amount of hydrogen is releasedby the hydrolysis reaction of NaBH4 and water, with reactionenhanced by a catalyst. Through reaction with water, borohy-dride produces hydrogen that can feed directly the anode ofa fuel cell, thus forming a simple electrical energy generator.An NaBH4 solution (35 wt% of sodium borohydride solutionis equivalent to 7.7 wt% of hydrogen) is chemically stable inbasic environment, not flammable, not explosive, and producesneither pollution nor by-products capable of causing the green-house effect. Finally, it appears as the only technology for stor-ing hydrogen in liquid (aqueous) form at a temperature verysuitable for a possible substitution of the current car fuel filling

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station. On the other hand, the critical issues to be solved are(1) the current high cost of the borohydrides, (2) developmentof a borate recycling process in order to reduce production cost,(3) improvement of the catalyst both in the reaction kineticsand in the chemical stability.

The sodium borohydride is a crystalline white substance,thermally stable and hygroscopic that decomposes by hydrol-ysis according to the following exothermic reaction:

NaBH4 + 2H2O → NaBO2 + 4 H2. (1)

The rate of decomposition of the aqueous borohydride solutionsis expressed by its half-life (time needed for the hydrolysis of50 wt% of borohydride) as a function of pH and temperature(T ) [29]:

log(t1/2) = pH − (0.034 T − 1.92), (2)

where t1/2 is indicated in minutes and T in Kelvin degrees.The decomposition rate equation shows that the reaction can becontrolled by varying the acidity and/or temperature. The totalhydrolysis of 1 g of borohydride releases 2.374 l of hydrogenin standard conditions (STP), with a kinetics that in a shorttime decelerates with increasing pH caused by the formationof metaborate basic salt. Suitably stable aqueous solutions forhydrogen generation are in the concentration range of 1–30 wt%of NaBH4, with NaOH or KOH concentrations higher than5 wt%.

James [30] reported that suitable metal salts, such as nickeland cobalt, and noble metals, such as ruthenium, platinum,rhodium, iridium, etc., are effective catalytic accelerators ofreaction (1). Amendola et al. [31] described the preparation ofa metallic ruthenium catalyst supported on anionic resin (IRA-400). The latter should facilitate the catch of the borohydrideion (BH−

4 ) near the ruthenium, increasing the contact time andconsequently the hydrolysis kinetics, which is in fact high. Thesame authors [31] specified that the catalyst is contained in areactor equipped with filters at the top and the base in order tokeep the particles with a diameter of about 25 �m diameter, andthe solution of NaBH4/NaOH/H2O is fluxed inside by meansof a pump through which it is possible to regulate the speed ofhydrogen generation rate.

Suda [32] prepared two catalysts containing nickel powderor nickel–magnesium alloy (25 �m diameter) separately pre-treated both with a hydrofluoric acid/sodium fluoride solutionin order to “fluoride” the surface. The borohydride hydrolysiskinetics is slow at the beginning, then it accelerates and after6 h of work it reaches worth values smaller than the ones foundby Amendola et al. [31]. However, there are no long-term data[32], no information about the chemical stability of this systemand the tests were carried out in the absence of flux we have.

Kojima et al. [33] described the preparation of the“metal/metal oxide” catalysts Pt/TiO2, Pt/CoO and Pt/LiCoO2.The first catalyst gives a good borohydride-to-hydrogen con-version, but the results refer to a non-stabilized NaOH solution.In these conditions the test is altered in that, even without thecatalyst, after 1 h the conversion reaches 7 wt% at 20.23 ◦C.The tests for the Pt/CoO and Pt/LiCoO2 catalysts with an

NaBH4/NaOH/H2O solution flowing through a tubular reactorhave an exceptionally high kinetics of H2 generation.

Nevertheless, these types of catalysts are chemically insta-ble and react directly with borohydride, thus generating hydro-gen and being converted to slightly divided cobalt borides andmetallic cobalt with a very high surface area. The reported highkinetics [33] is therefore the result of the main redox reactionsof the same catalysts. Furthermore, at pH values higher than13, cobalt borides and metallic cobalt tend to dissolve in thesolution and disperse in a device continuously supplied withthe NaBH4 solution.

More recently, Zhang et al. [34,35] assembled a packed-bed reactor using a commercial 3 wt% Ru on carbon catalyst.In their conclusion the authors stated that because most hy-drolysis reactions are exothermic the reaction heat must beremoved from the system continuously to prevent thermal run-away. Moreover, in a flow system the continuous removal ofthe reaction products is also crucial and requires the reactionproducts to be either liquids or solids that are highly solublein the solvent used in the reaction. The challenge is to simul-taneously satisfy all the three requirements in order to achievea reliable engineering system.

The present work aims especially at solving the issue for thecontrol of the hydrogen production kinetics, the chemical sta-bility of the catalyst and the heat removing problem. An H2generator device should have the following three fundamentalcharacteristic: (1) fast kinetics of hydrogen generation, (2) con-trol of the hydrogen volume produced in the unit of time and(3) a high chemical stability in time.

2. Experimental

2.1. Reactor and catalyst design

The new concept reactor, based on a proprietary design[36], is constituted by a magnetic containment, as depicted inFig. 1. It consists essentially of a magnetic field comprising apart of the line of the power circuit of the NaBH4/NaOH/H2Osolution. The power lines of the magnetic field are parallel to

Fig. 1. Magnetic containment reactor design.

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Fig. 2. Illustrative scheme of the prototype system.

each other. The homogeneous density of the lines carries outtheir action with a transversal geometry compared to the line ofthe circuit, crossing its entire volume for a well-defined length.The magnetic containment device uses permanent trade mag-nets with a work temperature of at least 100 ◦C. The ENEApowder catalyst inserted in the magnetic reactor consists of acore magnetic support with an average granulometry of 10 �mwhich was activated with Ru 5 wt% on the surface with a pro-prietary method described elsewhere [36]. The catalytic mate-rial has iron-magnetic properties, so that the intensity of themagnetic field applied out of the circuit is suitable to firmlymaintaining it along the circuit length without any leakage dueto the entrainment of the power solution or to the turbulencecaused by the hydrogen gas bubbles on its surface.

The chemical stability tests were fulfilled using a coupleof faced permanent magnets of type Neodymium–Iron–Boronwith a magnetic field, of about 1–5 KGauss, which had a effecton a section of a closed circuit (] 1.6 cm) and 5 cm length.Along the reactor, 0.5 g of the catalytic material in powderform was placed on the walls of the reaction area (Fig. 1).A peristaltic pump Gislon Minipuls fluxed at 1 ml/min, in theclosed circuit, 1 l of an NaBH4 20 wt%/NaOH 10 wt% solutionat room temperature for 6 days.

The freshly prepared and the recovered ENEA catalysts weresubjected to a static kinetics test in order to verify the hydrogenrate production. In this test, 30 ml of an aqueous solution ofNaBH4 20 wt%/NaOH 10 wt% was paced in a flask of 100 mlequipped with a filter lid and a small glass tube that conveys thegenerated gas into a graduated container full of water, upsidedown and with the mouth dipped into the water held in a thirdcontainer. The solution was previously thermostated at 35 ◦Cand then 0.25 g of catalyst was added measuring the hydrogenvolume produced.

The ENEA catalyst was compared with a 5 wt% Ru catalystplaced on two commercial resins IRA-400 and A27 (Amberlite)following the method described elsewhere [27,28].

2.2. Prototype scale-up

Based on the laboratory tests a prototype apparatus was as-sembled and tested. The prototype was scaled up for a produc-tion of 150 ml/ min with a maximum pressure of 4.5 bar [37].A scheme of the experimental system is presented in Fig. 2.

An adequate volume of (8%) borohydride solution was addedinto a tank (volume ca. 1 l). During operation, a fuel pump di-rects fuel (sodium borohydride solution) from the tank into acatalyst reactor and recycles it into the reactor until the pres-sure reaches a set-point value. The recycling of borohydridesolution allows an increase in system efficiency and a practicalmethod for the heat removal.

The borohydride magnetic reactor is made of two concentriccylinders: in the internal tube, containing supported catalyst,the borohydride solution enters the bottom of the column andreacts with catalyst bed to generate H2. The generated H2 risesto the top of the column and passes through a silica gel drierto remove residual drops of solution.

The fuel solution generates hydrogen gas and soluble sodiummetaborate: this one remained in solution, and to avoid its pre-cipitation there is a second tank with water that is added witha pump when the level of solution decreases. The hydrogendry flow rate was monitored by a mass flow meter (F-201CBronkhorst Hi-TEC) while the sodium borohydride hydrolysisexothermic reaction followed using two thermocouples: one onthe external surface of reactor and the other in the internal tubeof reactor.

All signals were acquired by a 34970 A Agilent HP systemand then logged on a computer through an HP VEE software.

3. Result and discussion

3.1. Catalyst

The diagram in Fig. 3 shows the volume of hydrogen pro-duced by the two different conventional catalysts and our newone. In all three cases a linear trend is observed, evidencinga zero-order kinetics, where the slope represents the borohy-dride hydrolysis rate. The highest kinetic was observed for theENEA magnetic catalyst (7.1 cm3 of H2/s per gram of cata-lyst). Among the two anionic resin/Ru catalyst, the better per-formance was performed by the IRA-400/Ru 5 wt% (5.6 cm3

of H2/s per gram of catalyst) confirming the results of Amen-dola et al. [27,28].

The three catalysts were subjected to a long hydrogen gener-ation period (6 days), as described in the experimental section.Then the recovered catalyst underwent the static kinetics testagain. The generated hydrogen of the ENEA catalyst was un-changed, thus evidencing for a stable kinetics. On the opposite,both the anionic resin/Ru catalysts were not stable at the highpH values considered. The resin, at first present in small balls,eventually tends to dissolve and the ruthenium detaches fromthe resin due to the tensions produced at the interface duringthe hydrogen development. Most importantly, the same ruthe-nium dissolves due to the strong corrosive alkaline power [38].

3.2. Pressure and temperature profiles during hydrogengeneration

The prototype was scaled-up to evaluate the effects of tem-perature and pressure on the hydrogen generation and theeffects of NaBH4 concentration.

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00 250

Hyd

roge

n ge

nera

ted/

cc.

600

500

400

300

200

100

50 100 150

Time/sec

200 300 350 400 450

Fig. 3. Volume of H2 generated by 0.25 g catalyst consisting of A-27/Ru 5% (�), IRA-400/Ru 5% (�), ENEA (©), in 20 wt% NaBH4, 10 wt% NaOH,70 wt% H2O solution as a function of time (t = 35 ◦C).

Table 1Prototype characteristic and operative conditions

H2 flow (ml/min) 100Pressure set-point (bar) 3.5Temperature (◦C) 30–70Solution volume (L) 1.175Volume H2 attempted (L) 223H2 purity 99.999%

Typically, the reactor was started at room temperature andoperates at variable pressure even if the pressure of the hydro-gen delivered from the system is constant. The experimentalconditions tested are summarized in Table 1.

The reactor loading, production phases and operation phaseswere discontinuous and activated by the pump. Through theuse of a control system the pressure of the generator is mon-itored and managed: when the pressure decreases under theset-point value, the solution is pumped in the reactor and theH2 reaction starts with the increase in pressure: when the pres-sure reaches the set-point, the pump stops and the solution isunloaded to the reservoir. During the analysis of the reactor’soperation two phases of the hydrogen production were charac-terized: the “active phase”, where the hydrogen production isdue to the contact between flowing solution and catalyst; andthe “passive phase”, where even if the reactor is empty thereis a further production of hydrogen due to the small quan-tity of solution remained adsorbed on the catalyst. The systembehaviour is shown in Fig. 4 with a typical reactor operationcycle. The active (I–II) and passive (II–IV) phases and the tem-perature effect on hydrogen generation are a function of theheat produced during the exothermic reaction. During the ac-tive phase the temperature decreases due to the cooling effectof the fed recycled borohydride solution, and the heat producedby the hydrolysis is emptied to the external environment. In thepassive phase, the reactor starts to empty. Initially, the tempera-ture increases again until the complete emptying of the reactor.When the reactor is empty, the heat produced by the reaction is

accumulated because the system is not able to exchange it withthe external environment and then the temperature increases upto a maximum that also corresponds to the maximum condi-tions of pressure.

In Fig. 5 is plotted the H2 generation as a function of timewith an 8 wt% NaBH4 solution. The average H2 flow was 0.16±0.03 L/min during a working time of 15 h. Even if the hydrogenproduction does not depend on the NaBH4 concentration (seenext paragraph), it lowers slightly due to the increase in borateconcentration and the related NaBH4 mass transfer.

3.3. Mathematical model

A simple mathematical model describing the process of cat-alytic hydrolysis of the NaBH4 solution was verified with ex-perimental data [39]. In this model the prototype can be assim-ilated to a continuous stirred tank reactor (CSTR) where thegenerator consists of a reactor, a gas–liquid separator, a tankand a pump for recycling solution (Fig. 6). From the mass bal-ance of the reactor and recycling, the following equation of theCSTR model is defined:

v ∗ dcAdt = F in − F out ± r ∗ V , (3)

where CA is the concentration of NaBH4 in the solution, V isthe volume of the reactor and r is the reaction rate. Consideringthe operating conditions and introducing the time of residencein the reactor, �, defined as ratio between the volume of thereactor V and Fin, it is possible to write

cA = cA0 − r ∗ � = c0

A − K ∗ CnA ∗ �. (4)

Eq. (4) allows to calculate the concentration of the reacting A(NaBH4) as a function of time. The experimental data showed adecreasing linear profile of the concentration of NaBH4 solutionin the time, confirming the validity of the used model (Eq. 3)and the zero-order of the reaction of hydrolysis (n = 0), as thereaction rate does not depend on the concentration (CA), but

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45

46

47

48

49

50

0

Tem

pera

ture

/°C

0

1

2

3

4

5

I

II

II

IV

Pre

ssur

e/ba

r

350300250

Time/sec

20015010050

III

Fig. 4. Temperature (− ◦ −) and pressure (—) vs. time during the reactor phases.

0

20

40

60

80

0 100 600

Hyd

roge

n pr

oduc

ted/

L

160

140

120

100

time/min

200 300 400 500 700 800 900

Fig. 5. H2 generated by an 8 wt% NaBH4 solution as a function of time.

Fig. 6. Scheme of the CSTR reactor.

0

20

40

60

80

0 100

effi

cenc

y %

UNLOAD

LOAD

Hydrogen Volume/L

100

25 50 75 125 150 175

Fig. 7. Hydrogen production efficiency.

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only on the kinetics constant (K). The value of K calculatedfrom the data is 0.0002 s.

The efficiency of the process in the operative conditions ofTable 1 was evaluated in terms of the total volumes of H2 gen-erated in a working cycle with respect to the theoretically ex-pected value. The amount of NaBH4 in the solution was 94 g,corresponding to an expected volume of 223 L (STP) of hydro-gen. Fig. 7 shows a linear trend for the efficiency until 60%and then it reaches an asymptotic value due to the exhaustionof the solution.

4. Conclusion

A hydrogen production apparatus from sodium borohydrideas well as its chemical storage is presented. A magnetic con-tainment reactor and a specific catalyst for hydrogen generationfrom alkaline borohydride were developed. R&D at the ENEAlaboratories produced a catalyst for this technology that is ableto allow high kinetics rate and enhanced time stability with re-spect to other known materials. Starting from this know-how,a brand new prototype was scaled-up and tested, showing veryinteresting performances in terms of flow rate and efficiency.The system demonstrated to be cost competitive with smallpolymeric electrolysers and its commercialization as a hydro-gen generator for laboratory applications has been initialized.

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