热力学研究 (2)

A durable ruthenium catalyst for the NaBH 4hydrolysisY.C.Zou,Y.M.Huang *,X.Li,H.L.LiuKey Laboratory of Specially Functional Polymeric Materials and Related Technology of the Ministry of Education,Department of Chemistry,East China University of Science and Technology,Shanghai 200237,PR Chinaa r t i c l e i n f oArticle history:Received 22September 2010Received in revised form 4January 2011Accepted 7January 2011Available online 2February 2011Keywords:Hydrogen generation Sodium borohydride Supported Ru catalyst Durability of catalysta b s t r a c tRu-active carbon (Ru/C)catalysts are prepared by impregnation reduction method for hydrogen generation via hydrolysis of alkaline sodium borohydride (NaBH 4)solution.The corresponding activity and durability of the prepared catalysts are tested in an immobile bed reactor.The variation of hydrogen generation rate with the increasing of flux and concentration of NaBH 4solution is measured.The durability of the catalysts prepared under various reductive pH values and reductants is tested by using different concentra-tions of NaBH 4solution (10&15wt%).It is found that the durability of catalyst in 15wt%NaBH 4solution is longer than that in 10wt%NaBH 4solution.The deactivation of Ru/C catalysts is considered as the comprehensive effect of three factors:the loss of Ru,the deposition of byproducts on the catalyst surface and the aggregation of Ru particles.Copyright ª2011,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rightsreserved.1.IntroductionPeople are very interested in hydrogen because it is considered to be an ideal power source in the future.It can provide both heat energy through combustion and electricity via fuel cells.Fuel cell is an energy-conversion device which can directly convert the chemical energy into electric energy at high effi-ciency [1].Among various fuel cells,the proton exchange membrane fuel cell (PEMFC)is considered as the best choice for vehicle,portable power and small-scale stationary appli-cations due to its high power density,low operating temper-ature,high efficiency,fast response,low system weight as well as zero emission [2].However,the inconvenience of the storage and transportation of the fuel of PEMFC,hydrogen,hinders the application of PEMFC dramatically.Chemical hydrides are widely used in the field of fuel cells.The unique properties of chemical hydrides have attracted increasing interest and are being recognized for their poten-tial applications in hydrogen pared with othertechnologies including gas compression or liquefaction,solid-state materials and so on,these hydrides can store hydrogen at much milder ambient,room temperature and relatively low pressure [3,4].Among these hydrides,NaBH 4has become the most prominent one of its high stability,nonflammability,nontoxicity in nature and high hydrogen content [5e 7].Cento et al.[8]studied the hydrolysis of sodium borohydride (SBH)to produce hydrogen at various tempera-tures.Wee et al.[9]introduced some research achievements on the use of H 2generated from the NaBH 4hydrolysis for PEMFCs.The hydrolysis of NaBH 4in the presence of a suitable catalyst could be carried out according to the reaction (1)[10].Various catalysts have been developed for the catalytic hydrolysis of NaBH 4solution to generate pure hydrogen,such as Pt-based catalyst [11],Ru-based catalyst [12e 16],Pt e Ru alloy catalyst [17],Pd-based catalyst [18],Co-based catalyst [19,20],Ni-based catalyst [21],etc.The studies proved that the catalysts containing Ru showed significant advantages in liberating H 2from NaBH 4.*Corresponding author .Tel.:þ862164250924;fax:þ862164252921.E-mail address:huangym@ (Y.M.Huang).A v a i l a b l e a t w w w.s c i e n c e d i r e c t.c o mj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /h ei n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)4315e 43220360-3199/$e see front matter Copyright ª2011,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.ijhydene.2011.01.027NaBH 4þðx þ2ÞH 2O !CatalystNaBO 2$x H 2O þ4H 2[(1)Amendola et al.[22]investigated ruthenium based catalyst supported on ion exchange resin beads for hydrogen genera-tion from hydrolysis of NaBH 4.Shang et al.[23]studied the kinetic of NaBH 4hydrolysis with carbon-supported Ru and found that the hydrolysis rate of NaBH 4with Ru catalyst was the zero order with respect to the concentration of NaBH 4when water was sufficient.Hung et al.[24]hold the view that the reaction order with the NaBH 4concentration depends on the temperature,which is a zero order at the low temperature and the first order at the high temperature.Shang and Chen [25]found that the hydrogen generation rate of concentrated alkaline NaBH 4solutions was affected not only by NaBH 4concentration,NaOH concentration but also by the byproduct (sodium metaborate).So far,the study of catalysts for hydrolysis of NaBH 4has been focusing on intermittent reaction performance,while the durability and the deactivation mechanism of catalyst in continuous and semi-continuous reaction process have been rarely investigated.In this work,supported Ru catalysts were prepared with different reductants and reductive pHs firstly.Both the durability of the catalysts and the hydrogen gener-ation rate were tested by pumping the different concentra-tions of NaBH 4solution (10&15wt%)into an immobile bed reactor.Furthermore,the deactivation mechanism of Ru/C catalysts is discussed via EDS,XRD and TEM.2.Material and methods2.1.Preparation of catalystThe Ru/C catalysts were prepared by impregnation method in which the analytical reagent grade ruthenium trichloride RuCl 3was used as the metal precursor and the spherical active carbon as the carrier.Synthesis procedure of Ru/C catalyst was summarized as below.40ml 0.06mol/L RuCl 3solution was prepared and its pH value was adjusted to 3.0.A weighed amount of the spherical active carbon was added to the above solution and dipped in the solution for 24h at the room temperature.It could be observed that the color of this solu-tion turned from red-brown to colorless.Subsequently,the 5wt%NaOH was used to adjust pH value of the mixture to 10e 11,and NaBH 4or HCHO was dropwise added to the mixture as the reductant.After that,the catalyst was filtered and washed repeatedly with deionized water until no Cl Àcould be detected.Finally,the sample was dried at 60 C for 6h in a vacuum system and then the Ru/C catalyst was obtained.The above method was applied to prepare three cata-lysts with different reductive pH and reductants,which were named as Ru/C e NaBH 4-pH10,Ru/C e HCHO-pH10and Ru/C e HCHO-pH11respectively.The surface area of the carrier was determined by BET (Micrometrics ASAP 2010)and the pore diameter distribution was obtained by BJH.The phases and lattice parameters of the catalysts were characterized by X-ray diffraction (XRD)with Cu K a radiation.The scan range was from 10 to 90 ,and the scan rate was 4 /min in step of 0.02 .The accelerating voltage and current were 40kV and 100mA.The surface morphologyof the catalysts was observed by a transmission electron microscope (TEM,JEM-2010).The energy dispersive spectrum (EDS)was used to analyze the atomic composition of the catalysts.2.2.Durability determination of catalystThe catalytic durability of the catalysts in the hydrolysis of NaBH 4was determined by measuring the conversion of sodium borohydride.Fig.1shows the experimental installation.NaBH 4solution was placed in a plastic tank and pumped into the reactor by a feed pump,and the rate of hydrogen generation was controlled by adjusting the flux of reaction solution.There was a gas e liquid separator equipped with the reactor,the hydrogen produced from the hydrolysis of sodium borohydride overflowed from the upper part of the gas e liquid separator,while the solution containing the sodium metabo-rate and unreacted sodium borohydride flowed into the effluent liquid tank.The load of catalyst in the reactor was about 2g.The experimental procedure was summarized as below.The feed pump was started at 9:00am and the reaction was stopped at 9:00pm everyday,and the part of effluent liquid was collected at a given interval,then the NaBH 4concentration in the collected liquid was determined by the indirect iodimetry.Correspondingly,the conversion of NaBH 4was calculated by comparing with the concentration of NaBH 4before reaction.The initial conversion of NaBH 4solution was about 98%.The reaction didn’t stop until the conversion of NaBH 4solution decrease to 50%(the time is named as active half-life of catalyst).The indirect iodimetry was performed as following steps:the excess KIO 3solution was added into quantitative NaBH 4solution and the redox reactionoccurred.Fig.1e Experimental installation of catalyst durability:1.reactor;2.gas e liquid separator;3.container for NaBH 4;4.feed pump;5.effluent liquid tank.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)4315e 432243163NaBH4þ4KIO3/3NaBO2þ4KIþ6H2O(2)The solution was acidified with H2SO4and then the remaining KIO3reacted with excess KI,which generated I2.KIO3þ5KIþ3H2SO4/3I2þ3K2SO4þ3H2O(3)At last,Na2S2O3standard solution was used to titrate the amount of I2and the starch as the indicator.I2þ2Na2S2O3/2NaIþNa2S4O6(4)Specific experimental steps were described as follows.2.00ml NaBH4solution was taken in50ml volumetricflask with the pipette and then diluted to the scale with1mol/L NaOH solution.5.00ml sample solution was moved to the iodineflask and then20.00ml standard potassium iodate solution was added.After they mixed fully,appropriate KI solution and sulfuric acid solution were added.At last, Na2S2O3standard solution was used to titrate the solution and the starch was added to the solution as the indicator. Continue to drip standard Na2S2O3solution until the color of the solution turned from dark blue to colorless.The concen-tration of sodium borohydride can be determined by the following formula:C NaBH4¼ðV1C1ÀV2C2=6ÞÂ154(5)C1d the concentration of KIO3standard solution,mol/L.C2d the concentration of Na2S2O3standard solution,mol/L. V1d the volume of KIO3standard solution,ml.V2d the volume of Na2S2O3standard solution,ml.The durability of three catalysts prepared in this work was tested in10wt%NaBH4solution,and then the effects of two preparation conditions on the durability of catalysts were investigated.The durability of catalysts in15wt%NaBH4 solution was also tested in order to investigate the effect of NaBH4concentration.2.3.Generation of hydrogenAccording to the results of durability determination,the catalyst which had the longest durability was selected to test the hydrogen generation rate by hydrolysis of NaBH4.Fig.2shows the experimental installation which consists of feed pump,reactor, gas e liquid separator and the alkali gas absorber.The alkali gas absorber is connected to the top outlet of the gas e liquid sepa-rator to absorb the alkali mist produced from the hydrolysis of NaBH4solution.The hydrogen generation rate was measured with a stopwatch and a recording rotameter at differentflow rates of NaBH4and different NaBH4concentrations.The NaBH4flow rate was0.5,1.0,1.5and2.0ml/min,while the concentration of NaBH4was10wt%,15wt%and20wt%,respectively.3.Results and discussion3.1.Characterization of Ru/C catalystsAdsorption e desorption isotherm of the carrier is displayed in Fig.3.It presents that the branches of adsorption and desorption are almost superposition,and the hysteresis loop is of a small area which elucidates that the support has both micro-pore and mid-pore.Pore diameter distribution of the support is shown in Fig.4,where we can see that pore diam-eter of the support is mainly about2nm which is consistent with the results of adsorption experiment.The specific surface area of the carrier measured by N2adsorption is 976m2/g.Fig.5(2e4)displays the XRD patterns of Ru/C e NaBH4-pH10, Ru/C e HCHO-pH10and Ru/C e HCHO-pH11catalysts,respec-tively.The diffraction peak at2q¼20e30 is that of carbon and the diffraction peaks around44 and78 are characteristic ones of Ru,which indicates that Ru is loaded on the carrier successfully.According to Scherrer formula,the catalyst particle size calculated from three curves is4.27,4.60and5.47nm,respectively.The curve1is the XRD pattern of Ru/C e NaBH4-pH10catalyst reacted in10wt%NaBH4solution for 216h,and the size of it is4.78nm which is bigger than that of the original one(4.27nm),which elucidates that the metal particles on catalysts have agglomerated during the course ofreaction. Fig.2e Experimental installation of testing hydrogen generation rate:1.reactor;2.alkali gas absorber;3. container for NaBH4;4.feed pump;5.gas e liquidseparator.Fig.3e Nitrogen adsorption e desorption isotherms of support.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y36(2011)4315e43224317Fig.6presents the quantitative chemical composition of Ru/C catalysts demonstrated by EDS.Fig.6(a),(c)and (e)represent the catalysts before reaction,while Fig.6(b),(d)and (f)represent the catalysts after reaction in 10wt%NaBH paring Fig.6(a)with Fig.6(b)about Ru/C e NaBH 4-pH10catalyst,we can see that the content of Ru in the catalyst decreases from 0.89at%to 0.67at%before and after reaction,which reveals that the loading of the metal ruthenium decreases during the reaction.Furthermore,the Na atoms appear in Fig.6(b)but not in Fig.6(a),which indicates that the reaction byproducts or the reactant NaBH 4deposited on the catalyst surface.The presence of sediment will cover the ac-tive positions of the catalysts,which is one of the reasons for deactivation of catalysts.Fig.6(c)and Fig.6(d)provide the chemical composition of Ru/C e HCHO-pH10catalyst before and after reaction.The loading of Ru in the catalyst decreases from 1.79at%to 0.89at%during the reaction.The amount of loaded Ru on Ru/C e HCHO-pH10catalysts is more than that on Ru/C e NaBH 4-pH10catalysts,but the load intensity between the metal particles and the carrier is weaker,which leads to much more loss of Ru during the reaction.Similarly,the Na atoms can be observed in Fig.6(d)but not in Fig.6(c).Fig.6(e)and (f)provide the difference of Ru/C e HCHO-pH11catalystbefore and after reaction.The loading of Ru is 1.87at%and 0.75at%,respectively,which are close to the loading of Ru in Ru/C e HCHO-pH10catalyst.From the comparison among (a),(c)and (e),it can be found that the loading of Ru on catalysts prepared with the reductant HCHO is more than that with the reductant NaBH 4.However,the effect of pH on the loading of Ru is less obvious.Fig.7presents TEM micrographs of Ru/C e NaBH 4-pH10catalyst.Fig.7(a)displays that the original catalyst is covered by uniform small metal particles.Fig.7(b)shows the morphology of catalyst reacted in 10wt%NaBH 4solution for 216h.From Fig.7,it can be seen that the small metal particles on catalysts tend to aggregate together,which leads to the increase of particle size and the decrease of active specific surface area.The same conclusion has been drawn in Fig.5that the Ru particle size in Ru/C e NaBH 4-pH10catalyst changed from 4.27nm to 4.78nm before and after reaction.It is obvious that the catalyst particles with higher active specific surface area are less stable in thermodynamics and have lower melting point.The hydrolysis of sodium borohydride is exothermic reaction.The temperature of 10wt%NaBH 4solution hydrolyzed with 2g Ru/C e NaBH 4-pH10catalyst is about 75 C,while the temperature inside the catalyst is likely to be higher.The large amount of heat released in NaBH 4hydrolysis promotes the fusion of these small particles.The melting particles then move along the carrier surface,merge mutually and finally lead to the formation of stable macro-particle.This is the possible reason for the recognizable reduction of catalytic activity in the present study.3.2.Durability of Ru/C catalysts 3.2.1.Effect of reductive pHFig.8illustrates the durability of catalysts prepared under the condition of different pH and HCHO as the reductant in 10wt%NaBH 4solution,in which the content of NaOH is 4wt%.As shown in Fig.8,the initial conversion rate of the NaBH 4solution of Ru/C e HCHO-pH10is higher than that of Ru/C e HCHO-pH11.As the reaction proceeds,the activities of the two catalysts show a marked decline.After 132h reaction,the conversion rate of NaBH 4of Ru/C e HCHO-pH11decreases to 51.89%and the catalyst nearly reaches its active paratively,the active half-life of Ru/C e HCHO-pH10is prolonged to 192h.The difference of active half-life between two catalysts should be attributed to the different reductive pH values which have important influence on the metal particle size.It can be confirmed by Fig.5that the Ru particle size of Ru/C e HCHO-pH10catalyst calculated via XRD is 4.60nm and that of Ru/C e HCHO-pH11catalyst is 5.47nm.The catalysts with small particle size will have big active specific surface area,which makes the catalysts have high activity.The above results suggest that the catalyst prepared under the pH value of 10is more suitable to catalyze the hydrolysis of NaBH 4solution.3.2.2.Effect of reductantsFig.9depicts the durability of catalysts prepared with different reductants in 10wt%NaBH 4solution,in which the content of NaOH is 4wt%.It is found that the initial conver-sion rate of 10wt%NaBH 4of Ru/C e HCHO-pH10catalyst is higher than that of Ru/C e NaBH 4-pH10catalyst.Reasonably,Fig.4e Pore diameter distribution ofsupport.Fig.5e XRD patterns of catalysts.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)4315e 43224318the loading of Ru in Ru/C e HCHO-pH10is much more than that in Ru/C e NaBH 4-pH10from Fig.6(a)and (c).However,with the reaction going on,the deactivation rate of Ru/C e NaBH 4-pH10catalyst becomes slower and slower.The conversion rates of two catalysts reach the same value after ter,the conversion rate of NaBH 4of Ru/C e NaBH 4-pH10catalyst becomes higher than that of Ru/C e HCHO-pH10catalyst.The active half-life of Ru/C e NaBH 4-pH10catalyst andRu/C e HCHO-pH10catalyst are 216h and 192h,respectively.The above results are consistent with the analysis of EDS in Fig.6in which the catalyst Ru/C e HCHO-pH10loads more Ru but with a faster loss of it during the course of reaction.3.2.3.Effect of NaBH 4concentrationRu/C e NaBH 4-pH10catalyst and Ru/C e HCHO-pH10catalyst were chosen to test the durability in 15wt%NaBH 4in ordertoFig.6e EDS spectra of catalysts:(a)Ru/C e NaBH 4-pH10before reaction;(b)Ru/C e NaBH 4-pH10after reaction;(c)Ru/C e HCHO-pH10before reaction;(d)Ru/C e HCHO-pH10after reaction;(e)Ru/C e HCHO-pH11before reaction;(f)Ru/C e HCHO-pH11after reaction.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)4315e 43224319demonstrate the effect of NaBH 4concentration on the dura-bility of the catalysts.The content of NaOH in 15wt%NaBH 4was also 4wt%.Fig.10reflects the effect of NaBH 4concentration on the durability of catalysts.As shown in Fig.10(a),both 10wt%andFig.7e TEM of catalysts:(a)Ru/C e NaBH 4-pH10before reaction;(b)Ru/C e NaBH 4-pH10afterreaction.Fig.8e Durability of catalysts prepared under different pH in 10wt%NaBH 4.Fig.9e Durability of catalysts prepared with different reductant in 10wt%NaBH 4.Fig.10e Durability of catalysts in different concentration of NaBH 4solution:(a)Ru/C e NaBH 4-pH10in 10wt%and 15wt %NaBH 4solution;(b)Ru/C e HCHO-pH10in 10wt%and 15wt%NaBH 4solution.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)4315e 4322432015wt%NaBH 4solution catalyzed with Ru/C e NaBH 4-pH10catalyst have the high initial conversion of 96%.As the reaction proceeds,the activity of catalyst in 10wt%NaBH 4solution decreases obviously,while that hardly changes at the first 150hin 15wt%NaBH 4solution.After 156h,the conversion rate of 10wt%NaBH 4is 74.85%,but that of 15wt%NaBH 4is still as high as 95.38%under the action of catalyst.When the reaction is going on for 216h,the conversion rate of 10wt%NaBH 4reduces to 50.31%and the catalyst reaches its half-life.At the same time,the conversion rate of 15wt%NaBH 4is 92.20%,which reflects that the durability of the catalyst in 15wt%NaBH 4is much longer than that in 10wt%NaBH 4.Fig.10(b)provides the durability of Ru/C e HCHO-pH10catalyst in 10wt%and 15wt%NaBH 4solution,which shows the similar result.The above results are contrary to our expectations,which may be attributed to the cooperation of the viscosity of hydro-lysis product and the formation of crystallite.As reported,the phase composition after NaBH 4hydrolysis reaction was different when the concentration of NaBH 4was between 0e 12.3wt%and 12.3e 25.9wt%[26].On the one hand,the hydrolysis product of the latter begins to form microcrystal of NaB(OH)4$2H 2O that may enhance the viscosity of the hydrolysis product macroscopically.The viscosity of the initial hydrolysis product for 10wt%NaBH 4and 15wt%NaBH 4are 4.68and 10.42Pa $s,respectively.As the same concentration of NaBH 4solution,the viscosity of the hydrolysis product at initial reaction stages is apparently larger than that at latter reaction stages.On the other hand,crystallite will deposit on the surface of catalyst and grow up,which covers the active center of catalyst and leads to the deactivation of catalyst.We believe that the increase of viscosity would prevent the crystallite from subsiding,which benefits the maintenance of the catalyst activity.Therefore,the conversion rate of NaBH 4at the initial reaction stages declines slowly and the recession of it speeds up gradually along with the reaction time.However,it is worth mentioning that the depo-sition of crystallite will dominate the deactivation of catalysts when the concentration of NaBH 4is high enough.3.3.Hydrogen generationFig.11shows the hydrogen generation rate in different concentrations of NaBH 4solution with Ru/C e NaBH 4-pH10catalyst.It is shown that the variation of the concentration and flow rate of NaBH 4solution hardly affects the response time of the paring the curves of each figure separately,we can easily find that the hydrogen generation rate increases in proportion to the flow rate of NaBH paring the curves at the same flow rate in three figures,such as the flow rate at 0.5ml/min,we find that the hydrogen generation rate of 10wt%NaBH 4is about 55ml/min/g,while that of 15wt%and 20wt%are about 85ml/min/g and 110ml/min/g,respectively.Obviously,the hydrogen generation rate is proportional to the concentration of NaBH 4at the same flux,which indicates that the catalyst has the high sensitivity and satisfactory perfor-mance.The hydrogen generation rate can reach 376ml/min/g with 20wt%NaBH 4when the flow rate is 2.0ml/min.4.ConclusionsIn this work,three catalysts,Ru/C e NaBH 4-pH10,Ru/C e HCHO-pH10and Ru/C e HCHO-pH11,were prepared under the condition of different reductants and different reductive pH value for hydrogen generation by hydrolysis of NaBH4Fig.11e Hydrogen generation rate in differentconcentration of NaBH 4with Ru/C e NaBH 4-pH10:(a)Hydrogen generation rate in 10wt%NaBH 4;(b)Hydrogen generation rate in 15wt%NaBH 4;(c)Hydrogen generation rate in 20wt%NaBH 4.i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 36(2011)4315e 43224321solution.It is found that both the reductant and the reductive pH greatly affect the performance of the catalysts.The dura-bility of Ru/C e NaBH4-pH10catalyst in10wt%NaBH4solution is about216h,which is longer than that of other two catalysts. Furthermore,the durability of catalyst in15wt%NaBH4 solution is longer than that in10wt%NaBH4solution.When Ru/C e NaBH4-pH10catalyst reacts in15wt%NaBH4solution for264h,the conversion of15wt%NaBH4is still78.40%.The hydrogen generation rate was carefully tested.When the Ru/C e NaBH4-pH10catalyst with0.89at%Ru is applied to the hydrolysis of20wt%NaBH4solution,the hydrogen generation rate can reach376ml/min/g with theflux of2.0ml/min.According to the durability of catalysts as well as other characterization results,the deactivation mechanism of Ru/C catalysts is summarized as below:loss of the active compo-nent Ru,deposition of byproducts on the catalyst surface and agglomeration of Ru particles.AcknowledgmentsThis work was supported by the Fundamental Research Funds for the Central Universities(WK1013001)and the Program for Changjiang Scholars and Innovative Research Team in University of China(gs2)(Grant IRT0721).r e f e r e n c e s[1]Krishnan P,Hsueh KL,Yim SD.Catalysts for the hydrolysis ofaqueous borohydride solutions to produce hydrogen for PEM fuel cells.Appl Catal B Environ2007;77:206e14.[2]Patel N,Fernandes R,Miotello A.Hydrogen generation byhydrolysis of NaBH4with efficient Co e P e B catalyst:a kinetic study.J Power Sources2009;188:411e20.[3]Kong VCY,Kirk DW,Foulkes FR,Hinatsu JT.Development ofhydrogen storage for fuel cell generators II:utilization ofcalcium hydride and lithium hydride.Int J Hydrogen Energy 2003;28:205e14.[4]Sifer N,Gardner K.An analysis of hydrogen production fromammonia hydride hydrogen generators for use in militaryfuel cell environments.J Power Sources2004;132:135e8. 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