International Journal of Hydrogen Energy 31 (2006) 961–965www.elsevier.com/locate/ijhydene

Catalytic hydrolysis of sodium borohydride in an auto-thermalfixed-bed reactor

Q. Zhang∗, G. Smith,Y. Wu, R. MohringMillennium Cell Inc., One industrial Way West, Eatontown, NJ 07724, USA

Received 21 November 2004; received in revised form 7 June 2005Available online 18 August 2005

Abstract

Catalytic hydrolysis of sodium borohydride is a promising technology for hydrogen storage. In this study, we report theperformance of Millennium Cell Hydrogen on DemandTM (HODTM) hydrogen storage system using a supported catalyst.Dynamic reactor startup and response characteristics were measured using a fixed-bed catalyst reactor at various hydrogendemand conditions. The catalyst reactor was operated without need of fuel preheating. Catalyst durability of more than 700 hwas demonstrated with a sodium borohydride conversion over 95% at a liquid feed hourly space velocity (LHSV) of about15 h−1 and under a system pressure of 483 kPa.� 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen storage; Sodium borohydride; Catalyst durability; Heterogeneous catalysis

1. Introduction

Hydrogen storage is one of the most important issues andpotentially biggest roadblock for the implementation of ahydrogen economy [1]. Of all known hydrogen storage ma-terials to date, chemical hydrides, particularly sodium boro-hydride (NaBH4), offers the most promise for meeting re-cently announced FreedomCAR goals in the gravimetric andvolumetric hydrogen storage densities [1]. For example, anaqueous solution of 20 or 30 wt% NaBH4 offers a hydrogenstorage density of 4.2 and 6.5 wt%, respectively. Hydrol-ysis of sodium borohydride produces hydrogen from bothNaBH4 and water with released heat sufficient to sustain anauto-thermal reactor operation (Eq. (1)).

NaBH4 + 4H2O = NaB(OH)4 + 4H↑2 + 300 kJ. (1)

∗ Corresponding author. Tel.: +1 732 544 5721;fax: +1 732 542 2846.

E-mail address: zhang@millenniumcell.com (Q. Zhang).

0360-3199/$30.00 � 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2005.07.008

Sodium borohydride based hydrogen storage system of-fers several distinct advantages: hydrogen can be stored inliquid (NaBH4 solutions) form under ambient conditionsthat makes it possible for potential utilization of current liq-uid fuel based infrastructure for fuel delivery; high intrinsicenergy density of the fuel; safety for hydrogen storage thatdoes not require high pressure or extremely low temperaturecondition; fast rate of hydrolysis reaction and auto-thermaloperation that offers possibility for delivery hydrogen ondemand and high system energy density for hydrogen stor-age. In practice, hydrogen is produced on demand by feed-ing NaBH4 solution over a supported solid-phase catalystin a packed-bed reactor. The by-product sodium metabo-rate, NaB(OH)4 (or NaBO2 ·2H2O), is removed and can beused as raw material for regeneration to NaBH4 [2]. Withinthe fuel system, NaBH4 can be stored as either a stabilizedaqueous solution or as a solid. The effectiveness of the hy-drolysis catalyst is the key for rapid and controllable hydro-gen release from NaBH4.

One of the pioneer works on the catalyzed NaBH4 hy-drolysis by Brown and co-workers [2] examined a number of

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metal salts and established relative rates for hydrogen gen-eration using these homogeneous catalysts. In their studies,aqueous solutions of the metal salts were added to aque-ous solutions of NaBH4 at 25 ◦C. Ruthenium and rhodiumsalts showed the best catalytic activity. Chloride salts ofmanganese, iron, cobalt, nickel and copper [2,3] were alsodemonstrated to be active for catalytic hydrolysis of sodiumborohydride to produce hydrogen gas at room temperature.Although these homogeneous catalysts are active, the metalions dissolved in aqueous solution cause serious contami-nation concerns for discharged fuel recycling or disposal.There were also reports using pellets of sodium borohy-dride containing 3–7% of CoCl2 [2,4] or tablets of an alkalimetal borohydride (zinc metal, sodium chloride) [5] additionto water for rapidly production of hydrogen gas. However,once a pellet is dropped into water, the reaction cannot bestopped easily when hydrogen is no longer needed. The ap-proach of using metal salts containing sodium borohydridepellets also causes problem for discharge fuel recycle thatrequires additional process steps for metal ions separation.

In order to make sodium borohydride hydrolysis a com-mercially viable approach for on-demand hydrogen gener-ation, the desired catalyst has to offer superior activity anddurability for fast hydrogen generation on demands and canbe easily separated from reaction media. In this regard, het-erogeneous catalysts offer distinct advantages of ease sep-aration of catalyst from discharge fuel and for controllablegeneration and delivery of hydrogen on demand. There area few reports on heterogeneous catalysts for sodium boro-hydride hydrolysis reaction under near ambient conditions.In these studies, cobalt and nickel metals or a hydrogen-absorbing alloy such as Mg2Ni [6], metal oxide supportedPt, Pd, Rh, Ru, Ir, Os, Au, Ag catalysts [7,8] as well as nickelboride [9] were reported to be active for catalytic hydroly-sis of sodium borohydride in an aqueous alkaline solutionat near room temperature conditions. None of these cata-lysts have been proven to be commercially viable, particu-larly under typical hydrogen fuel cell application conditions.Neither the stability (durability) nor the performances ofthese catalysts were known under elevated temperature andpressure conditions. Typically, the sodium borohydride hy-drolysis reactor is operated under pressures up to 1100 kPato achieve the desired system energy storage density. Highoperating pressure is useful in minimizing the volume ofballast tank that is used in the hydrogen storage system tosupply hydrogen to fuel cell during initial reactor startupand reducing overall balance of plant (BOP) of a sodiumborohydride based hydrogen storage system.

In this study, we reported durability and reactor perfor-mance of a supported catalyst for hydrogen generation fromstabilized sodium borohydride fuel solution in response tovarious hydrogen-load conditions. Dynamic startup and re-sponse characteristics of a Millennium Cell HODTM hy-drogen storage system using a packed bed catalyst reactorwere measured under various hydrogen demand conditions.Durability of the supported catalyst used in the storage sys-

tem was evaluated at a liquid fuel feed hourly space veloc-ity (LHSV) of about 15 h−1 and at a system pressure of483 kPa. Direction for further advancement in sodium boro-hydride based hydrogen storage technology is discussed.

2. Experimental

2.1. Catalyst

The catalyst was supported on a suitable metallic sub-strate according to methods taught in US patent 6,683,025B2 [10]. A BMR06 catalyst (Millennium Cell Inc.) was se-lected for testing. An aqueous solution of 20 wt% sodiumborohydride and 3 wt% sodium hydroxide (as a fuel stabi-lizer) was used as fuel for reactor performance and catalystdurability evaluations.

2.2. Reactor performance measurements

Reactor performance was measured in a test bench with apacked bed tubular reactor (2.14 cm i.d. by 17.8 cm length)under various fuel flow conditions. In operation, a fuel pumpdirects fuel (sodium borohydride solution) from a tank ofsodium borohydride solution into a catalyst reactor. Thefuel flow rate was monitored by using a scale and a timer.Upon contacting the catalyst bed, the fuel solution gener-ates hydrogen gas and sodium metaborate (in solution). Thehydrogen and metaborate solution were then separated ina gas–liquid separator. The humidified hydrogen was thencooled down to room temperature through a heat exchangerto condense water vapor in hydrogen stream and then agas–liquid separator to remove condensed liquid water. Thehydrogen stream then passes through a silica drier to removeremaining water vapor. The flow rate of dry hydrogen wasmonitored with a mass flow meter and an on-line computer.Typically, reactor was started at room temperature and oper-ated at a constant pressure of 483 kPa. Since sodium borohy-dride hydrolysis reaction is an exothermic reaction, no fuelpreheating, or other reactor heating elements are required tostart or to sustain the hydrolysis reaction, reactor can thusbe operated auto-thermally to deliver hydrogen over a widerange of hydrogen or fuel flow rates.

Reactor startup profile, measured at a constant fuel flowrate of 20 g/min starting from room temperature at 483 kPa,is an important indicator of startup characteristics of aNaBH4 based hydrogen storage system. Dynamic reactorresponses to hydrogen demands were also measured at var-ious fuel flow rates. Catalyst durability data were measuredat a liquid fuel hourly space velocity of 15 h−1 in a flowreactor operated in a semi-continuous mode. Typically, thereactor was started at ambient conditions at a constant liq-uid fuel space velocity and then operated continuously at483 kPa for 6–8 h before the reactor shutdown. Followingshutdown, reactor was flushed with water to remove residualfuel in reactor. Without water flushing, catalysts could be

Q. Zhang et al. / International Journal of Hydrogen Energy 31 (2006) 961–965 963

Table 1Experimental conditions for evaluation of reactor performance

Performance metrics Operating conditions

Reactor startup Fuel flow rate: 20 g/minStart at room temperature and 483 kPa

Reactor response Various fuel flows(throughput) Steady-state operation at each flow rate

483 kPa

Catalyst durability Liquid fuel space velocity: 15 h−1

Multi-cycles of startup and shutdown,accumulative steady-state operationat 483 kPa

foiled by metaborate and potentially leads to slow re-startupor even damage of catalysts as a result of crystallizationof metaborate inside support structure. Accumulated op-erating hours were recorded against sodium borohydrideconversion and rates of hydrogen generation. The multiplestartup and shutdown cycles allow one to simulate practicaloperation of a hydrogen generator. Detailed test conditionsfor reactor performance evaluations are given in Table 1.

3. Results and discussion

3.1. Reactor startup profiles

One of the advantages of sodium borohydride based hy-drogen storage system is that reactor can be started fromambient conditions without fuel preheating and the reactorcan be operated auto-thermally without need of additionalheating sources. Unlike hydrocarbon fuel reformer, wherethe reformer has to be preheated either by additional heat-ing elements or by combustion of fuel to start and sustainhydrogen generation reaction. Furthermore, hydrogen withthe desired moisture content generated from sodium boro-hydride based system does not contain CO or other contam-inants and can be directly fed to PEM fuel cell. To ensurean instantaneous response to hydrogen demand, a bufferingor ballast tank is usually incorporated in the hydrogen stor-age system to supply hydrogen to fuel cell during reactorstart up and to hold hydrogen generated from residual fuelinside reactor after reactor shutdown. Fast reactor startup isvery important in minimizing size of hydrogen ballast tank(maximizing the overall system hydrogen storage density)needed to delivery hydrogen instantaneously to fuel cell inresponding to hydrogen demands. To assess the reactor per-formance, the rate of reaction temperature increase during acool startup was measured after fuel pump was turned on ata constant fuel flow rate of 20 g/min at 483 kPa. The risingrate of reaction temperature directly reflects speed of reac-tor startup and provides critical information about activity

0

2

4

6

8

10

190 210 230 250

Time elapsed from fuel pump on, s

Rat

e of

tem

pera

ture

incr

ease

, °C

/s After 13h

After 265h

Fig. 1. Startup temperature profiles at different stages of durabilitytests with a fuel flow rate of 20 g/min.

of the catalyst as well as performance of the catalyst reac-tor. Fast reactor startup indicates high catalyst activity andoffers possibilities in reducing balance of plant of NaBH4based hydrogen storage system, thus increasing system en-ergy density. As shown in Fig. 1, the reactor temperatureincreased rapidly from ambient temperature to maximumsteady-state temperature. The maximum rate of temperatureincrease reached as high as 8 ◦C/s during the startup period.The profiles of measured increase in reaction temperatureafter 13 h is very close to that after 265 h operation, that in-dicates a negligible change in catalyst activity after 265 hon-stream operation or good catalyst durability. It is worthto note that reactor startup time depends on intrinsic kinet-ics of catalyzed hydrolysis reaction, mass transfer rate (orfuel feed rate), heat transfer rates and hydrodynamic condi-tions. Reactor startup time can be shortened by optimizingthese factors so that the rate of reaction can be maximizedand the heat loss during reactor startup can be minimized.

3.2. Catalyst durability

Catalyst durability is crucial for sodium borohydridebased hydrogen application. Poor durability not only createsproblems for discharge fuel recycle due to metal contami-nation but also lead to large reactor size needed to achievethe desired fuel conversion, and results in low overall sys-tem hydrogen storage density. Caustic compound such asNaOH is often added into fuel solution at a concentrationlevels 3–5 wt% to achieve desired fuel stability for hydrogenstorage. Thus, the catalytic hydrolysis of chemical hydrideis a truly multiphase reaction in hot caustic environment.Hot caustic reaction media coupled with possibilities offormation of large bulky sodium metaborate by-productconstitutes significant challenges for chemical and mechan-ical stability of the catalyst. In this study, catalyst durabilitywas measured at a steady-state operation of reactor undera liquid fuel hourly space velocity (LHSV) of 15 h−1 at483 kPa. A fuel conversion over 92% was achieved overmore than 700 h time-on-stream operation (Fig. 2). The

964 Q. Zhang et al. / International Journal of Hydrogen Energy 31 (2006) 961–965

0102030405060708090

100

0 100 200 300 400 500 600 700

Time on stream, h

Fue

l Con

vers

ion,

%

80

100

120

140

160

180

200

Rea

ctor

thro

ughp

ut,

SLP

M H

2/L

_rea

ctor

Fig. 2. Time on stream profiles of a reactor packed with a Millen-nium Cell BMR06 catalyst. Operating pressure: 483 kPa; Liquidfuel hourly space velocity: 15 h−1.

start up profiles measured at different period of durabilitytests (Fig. 1) with conversion versus time-on-stream profiles(Fig. 2) clearly demonstrate good durability of MillenniumCell BMR06 catalyst for sodium borohydride hydrolysis. AtLHSV of 15 h−1, the reactor throughput is about 120 SLPMH2 per liter of reactor volume (Fig. 2). The reaction temper-ature measured with three thermocouples evenly embeddedinside catalyst bed indicate a reaction temperature of about145 ◦C at 483 kPa under auto- thermal operation conditions.After 700 h of operation and periodic startup–shutdown cy-cles, the BMR06 catalysts reactor still yielded satisfactoryfuel conversions. Operating reactor with higher fuel con-centration than 20 wt% NaBH4 potentially leads to higherhydrogen storage density. However, solubility limitation ofmetaborate product constitutes a great challenge for reactoroperation. Additional works for heat and water managementhave to be done to address issue of metaborate precipitationand to prevent reactor from clogging.

3.3. Reactor operating window

For practical applications, robustness of a hydrogen stor-age system in responding to aggressive rapid swing powerloads of fuel cell or hydrogen demands depends on capabil-ity or operating window of a hydrogen generation reactor inwhich the desired fuel conversion is achieved under a widerange of hydrogen flow rates or reactor throughputs. The de-sired (ideally complete) fuel conversion has to be deliveredthrough hydrogen generation reactor with minimum reactorsize under various fuel flow rates. In this study, dynamicreactor responses to hydrogen demands were measured atdifferent time-on-stream periods of catalyst durability tests.The reactor was operated auto-thermally over a wide fuelflow rates with a fuel conversion over 80% (Fig. 3). It is veryimportant to note that a high reactor throughput (above 500SLPM H2 per liter reactor volume) was achieved at a fuelconversion over 80% (Fig. 3). Nearly complete fuel con-

0

20

40

60

80

100

0 100 200 300 400 500 600

Reactor throughput, SLPM/L_reactor

Fue

l con

vers

ion,

%

After 265hAfter 501hAfter 707h

Fig. 3. Reactor performance during different stages of durabilitytests at various hydrogen demands.

version was achieved at a reactor throughput less than 100SLPM/L. Slowly decrease in fuel conversion was observedwith increase in reactor throughput after different hours ofoperation. Although high fuel conversion was achieved ata fuel space velocity of 15 h−1 over 700 h (Fig. 2), lowerfuel conversions at high fuel space velocity (or high reactorthroughput) were evident from hydrogen response profilesmeasured at different history of catalyst and under varioushigh fuel flow conditions (Fig. 3). Complete fuel conversionis highly desirable and might be achieved by optimizingcatalyst and reactor design as well as hydrodynamic condi-tions inside reactor. Nevertheless, these experimental resultsclearly demonstrated that the supported HODTM catalystoffers a wide operating window for hydrogen storage andhydrogen delivery on-demand.

3.4. A dynamic response of a HODTM system

The wide operating window of the catalyst reactor makesit possible for a sodium borohydride based hydrogen stor-age system to rapidly respond to aggressive and rapidlychanging hydrogen demands in various fuel cell applica-tions and to achieve high system energy storage density.To further evaluate robustness of sodium borohydride basedhydrogen storage system, dynamic response of a proto-type hydrogen storage system was tested against a com-pressed hydrogen system. Fig. 4 shows a dynamic responseof a Millennium Cell HODTM system with a BMR06 cata-lyst reactor to an aggressive hydrogen demand from a fuelcell simulator. Millennium Cell’s sodium borohydride basedHODTM system showed a comparable response profiles tothat of a compressed hydrogen system. The fact that load-following profile of a sodium borohydride based Hydrogenon DemandTM storage system closely superseded that ofcompressed hydrogen demonstrated that the catalyst reactoroffered a fast start–stop characteristics with wide operatingwindow and fast response to hydrogen flow ranging from afew SLPM to 800 SLPM. Consistent with startup (Fig. 1)

Q. Zhang et al. / International Journal of Hydrogen Energy 31 (2006) 961–965 965

0100200300400500600700800900

0 50 100 150 200 250 300Time elapsed, s

H2

outp

ut, S

LPM

Sodium Borohydride SystemCompressed Hydrogen System

Fig. 4. Dynamic load-following profiles of a HODTM hydrogenstorage system.

and operating window (Fig. 3) measurements, dynamic load-following profiles revealed the robustness of sodium boro-hydride based hydrogen storage system for hydrogen stor-age and delivery on demand. Sodium borohydride based fuelcan be stored in liquid or solid form under ambient condi-tions without need of high pressure or extremely low tem-perature to achieve required energy density. Hydrogen canbe generated and delivered on demand with rapid responseto hydrogen or power loads. Overall system hydrogen stor-age density depends on catalysts activity and durability, re-actor throughput and system simplicity as well as operatingconditions. Hydrogen storage system can be controlled us-ing system pressure as control parameter and hydrogen canbe delivered on demands. The ballast tank is typically usedto delivery hydrogen to fuel cell during reactor startup andto hold hydrogen after system shutdown. When hydrogen isconsumed, the pressure drop occurs that triggers fuel pumpcontrols to response to hydrogen demands. As discussed ear-lier, modest high operating pressure could lead to reductionin ballast size and minimize water loss through heat man-agement. Further improvements in system energy density ofsodium borohydride based hydrogen storage system mightbe achieved through improvement in catalyst activity anddurability, reactor design and process integration so that re-actor throughput can be maximized with reductions in sizesof reactor and other balance of plant components in a hy-drogen storage system.

4. Conclusion

A supported catalyst was demonstrated for hydrogen gen-eration from catalytic hydrolysis of sodium borohydride fuel

solutions in a flow reactor. Desired fuel conversion (> 95%)

was achieved over a 700 h operation at a liquid fuel spacevelocity of 15 h−1 and 483 kPa. The catalyst reactor offersfast startup and can be operated auto-thermally over a widerange of fuel flow conditions. Dynamic load-following ca-pability of a hydrogen-on-demand system was also demon-strated using the supported catalyst with a closely matchedload-following profile to that of a compressed hydrogen tanksystem.

Acknowledgements

We would like to thank Ian Eason, Robert Molter, JosephPosaidlik and Ibrihim M. Qureshi for assistance in experi-mental measurements.

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