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1、RECENT PROGRESS IN BORON-AND NITROGEN-BASEDCHEMICAL HYDROGEN STORAGEZHANG-HUI LU and QIANG XU*National Institute of Advanced Industrial Science and Technology(AIST)Ikeda,Osaka 563-8577,Japan,and Graduate School of EngineeringKobe University,Nada Ku,Kobe,Hyogo 657-8501,Japan*q.xuaist.go.jpReceived 27
2、 November 2011;Accepted 25 December 2011;Published 6 April 2012Boron-and nitrogen-based chemical hydrogen storage materials,such as metal borohydrides,ammonia borane,hydrazine borane,metal-nitrogen-hydrogen systems,ammonia,and hydrazine,have been extensively investigated in the past years.A variety
3、ofmethods have been developed to decrease the reaction temperature and enhance the reaction kinetics of these systems.This featurearticle is to serve as an up to date account of the recent progress in chemical hydrogen storage with the boron-and nitrogen-basedmaterials.Keywords:Hydrogen storage;boro
4、n-based chemical hydrides;nitrogen-based chemical hydrides;dehydrogenation.1.IntroductionThere is an increasing and impending demand for sufficientenergy supply along with the continuously growing popu-lation and the rising standards of living in the world.Hydrogen has attracted considerable attenti
5、on as a globallyaccepted clean energy carrier.The use of hydrogen fuel cellsin portable electronic devices or vehicles requires lightweighthydrogen storage or on-board hydrogen generation.Cur-rently,the search of safe and efficient hydrogen storagematerials is one of the most difficult challenges fo
6、r thetransformation to hydrogen-powered society as a long-termsolution for a secure energy future.1Vehicular applications and off-board uses enforce scien-tific efforts to discover hydrogen storage materials that canstore and release hydrogen in a safe and efficient way.Forvehicular applications,the
7、 U.S.Departments of Energy(DOE)has set storage targets;the gravimetric(volumetric)system targets for near-ambient temperature(?40C?85C)and moderate pressure(100bar)are 6.0wt.%(45g/L)forthe year 2010 and 9.0wt.%(81g/L)for 2015.2In order tomeet the targets set by the U.S.DOE,different storagesolutions
8、 have been developed and a large number of litera-tures on hydrogen storage materials,such as metal hydrides,1on-board reforming of hydrocarbon into hydrogen,3metalorganic frameworks,4,5and organic hydrides,6have beenreported.However,big challenges still remain.Chemical storage materials with high h
9、ydrogen contentsare highly promising as hydrogen sources for fuel cells.Among them,boron-and nitrogen-based compounds,such asmetal borohydrides,7ammonia borane,8hydrazine bor-ane,9,10metal-nitrogen-hydrogen systems,11ammonia,12andhydrazine,13,14have been extensively investigated in the pastyears.Thi
10、s feature article is to serve as an up to date accountof the recent progress in chemical hydrogen storage withthese boron-and nitrogen-based compounds.2.Boron-based Chemical Hydrides2.1.Metal borohydridesMetal borohydrides have received considerable researchinterest as potential hydrogen storage mat
11、erials owing to theirhigher gravimetric and volumetric hydrogen capacity.7,15?17Lithium borohydride(LiBH4,LB)is a stable white solid witha hydrogen content of 18.5wt.%and reacts slowly with waterto produce H2but relatively stable in dry air.Hydrogendesorption from LB is highly endothermic and the hi
12、ghtemperature(400C)was required for the dehydrogenationof pure LB.18A number of approaches have been adopted todestabilize LB for hydrogen storage.Ball milling LB 2LiNH2gave a composite with a chemical compositionof Li3BN2H8,which is capable of releasing more than10wt.%of H2in the temperature range
13、of 250?350CFunctional Materials LettersVol.5,No.1(2012)1230001(9 pages)World Scientific Publishing CompanyDOI:10.1142/S17936047123000101230001-1Review(see Table 1).19,20More than 9wt.%of H2can be releasedfrom the LB 2LiNH2mixture at ca.180C in the presenceof 2.6mol%of Co catalyst.21In particular,it
14、was reportedthat 17.8wt.%of H2can be released from the Co-catalyzedlithium borohydride ammoniate,LiNH343BH4,in thetemperature range of 135C?250C(Table 1),22which isthe highest amount of H2emission in the temperature rangeever reported for hydrogen storage materials.A very prom-ising concept of desta
15、bilization of LB using MgH2asdestabilizing additive was suggested by Vajo et al.23Theaddition of LiAlH4to LB combined with metal halides as acatalyst was reported to have a significant enhancement in thedehydrogenation kinetics.24Recently,it was found that thenanoscale LB exhibited the ultralow onse
16、t temperature forreleasing H2(at?32C).25An alternative way to generatehydrogen from LB was via hydrolysis.Kojima et al.foundthat the gravimetric and volumetric hydrogen densitiesincreased,followed by a decrease,with increasing H2O/LBratio.26However,during the hydrolysis process for hydrogengeneratio
17、n,the agglomeration of the hydrolysis product ofLB limits its full utilization.27Recently,Weng et al.proposedthat full hydrolysis of LB can be achieved by the addition ofMWCNTs and the hydrogen capacity reached 7.5wt.%intotal.28Sodium borohydride(NaBH4,SB)is stable in dry air andstores 10.8wt.%of hy
18、drogen.The thermolysis of SB is notconceivable because it is achieved at too high temperatures(500C).Recent efforts have been devoted to incorporatingadditives,such as MgH2,29LiAlH4,30CaH2and CaBH42,tothermodynamically destabilize SB toward an lowered tem-perature for hydrogen release.31In the absen
19、ce of an alkalinesolution,SB spontaneously reacts slowly with water and itshydrogen generation rate is low.However,the hydrolysis ofSB can effectively liberate H2at room temperature in thepresence of a catalyst via the following reaction:NaBH4 2H2O!NaBO2 4H2:1Since Schlesinger et al.discovered that
20、SB hydrolysis reac-tion can be significantly accelerated by the addition of acid,32a great variety of catalysts have been examined to acceleratethe SB hydrolysis at room temperature.15Kojima et al.reported that by using Pt-LiCoO2catalyst the hydrogengeneration rates were high compared with those usi
21、ng othermetal and metal oxide.33Krishnan et pared thehydrogen release kinetics using IRA-400 anion resin dis-persed Pt,Ru catalysts and LiCoO2-supported Pt,Ru andPtRu catalysts,and they reported the most effective catalystwas 10wt.%PtRu-LiCoO2.34In addition,some non-nobletransition elements also exh
22、ibited high activities.15Cobaltand nickel borides were the most investigated catalysts.Itwas reported that the catalytic performance of Co3O4for thehydrolysis of SB was at par with the noble metal-basedcatalysts at low concentration of SB and far superior to noblemetal-based catalysts at higher conc
23、entration.35Despiteconsiderable efforts in research and development,the U.S.Department of Energy(DOE)recommended a no-go for SBfor on-board automotive hydrogen storage because the aqu-eous solution of SB does not meet DOE criteria in terms ofstorage capacity,spent fuel recycling and cost.36However,N
24、aBH4may still have a potential for portable applications.37The main challenge today is improve catalyst durability.15Magnesium borohydride(MgBH42)has a hydrogencontent of 14.9wt.%.It decomposed to hydrogen at tem-peratures above 340C.38Recently,Soloveichik et al.pro-posedafour-stagepathwayforthether
25、maldecompositionwithformation of intermediate magnesium polyboranes and ulti-mately ends with formation of MgB2.39Other borohydrides,7such as CaBH42and ZnBH42,40,41also decomposed toTable 1.Summary of typical chemical hydrogen storage materials/systems.Dehydrogenation reactionsMass.%Temp.(C)Ref.2LiB
26、H4!2LiH 2B 3H213.6200?50018LiBH4 2LiNH2!Li3BN2 4H211.9150?35019,202LiBH4 MgH2!2LiH MgB2 4H211.5270?44023NaBH4 2H2O!NaBO2 4H210.92515,29nNH3BH3!NH2BH2n nH2!NHBHn 2nH212.970?20048LiNH2BH3!LiNBH 2H210.975?9566NH3BH3 2H2O!NH4BO2 3H29.0258,77N2H4BH3 3H2O!BOH3 N2 5H29.75010LiNH2 2LiH!Li2NH LiH H2!Li3N 2H2
27、10.5150?45011MgNH22 2LiH!Li2MgNH2 2H25.6100?250117,1212NH3!N2 3H217.86001314NH3 3LiBH4!3LiNH343BH4!Li3BN2 2BN 12H217.8135?25022N2H4!N2 2H212.04?8013N2H4?H2O!H2O N2 2H28.025107,181Z.-H.Lu&Q.Xu1230001-2hydrogen,buteitherathighdecompositiontemperatureorwithunwanted side products(e.g.B2H6).Recently,an i
28、nterestingcorrelation between the thermodynamic stability of metalborohydrides and the electronegativity of metal in MBH4n(M Li,Na,K,Mg,Ca,Al,Zn,Zr,etc.;n 1?4)wasproposed,42and it was found that the dehydriding temperatureof metal borohydrides decreased with increasing electro-negativity of metal.Mo
29、re recently,mixed alkali metal bor-ohydrides,such as MLim?nBH4m(M Zn,Al,Zr),LiKBH42andLiCaBH43,cameintopicture,whichcontaindual cations.43?45The mixed alkali metal borohydrides offerthe possibility to adjust the decomposition temperature by anappropriate combination of cations.2.2.Ammonia boraneAmmo
30、nia borane NH3BH3;AB has an impressive gravi-metric capacity of 19.6wt.%H2,possessing potential capa-bility of meeting the DOE targets.There are considerablereports on the hydrogen release from the thermal decompo-sition of AB,which occured in three sequential steps around110,150,and 500C,with 6.5wt
31、.%H2released in eachstep(Table 1).46?48The thermolysis temperature can belowered in an organic solution or ionic liquid.49,50Since thepioneering work by Autrey and coworkers on the dehy-drogenation of AB confined in the mesoporous scaffoldSBA-15,51various research groups have reported that loadingAB
32、 in a solid scaffold such as CMK-352and MOFs53,54canlead to a decrease in the hydrogen release temperature and toan increase in the purity of the hydrogen released.It was alsoreported that transition metal additives and catalysts orchemical promoters can significantly improve the kinetics ofthermoly
33、sis of AB.55?62Recently,it has been demonstratedthat the polymeric spent fuel from AB dehydrogenation canbe converted back to AB by treatment with hydrazine N2H4in liquid ammonia(NH3)at 40C in a sealed pressurevessel.63It should be noted that ammonium borohydride(NH4BH4),a compound closely related t
34、o AB,which is asolid with a highest content of thermodynamically andkinetically accessible hydrogen,has been thermodynamicallyand structurally investigated.64On the other hand,chemical compositional modificationwas found to be an effective way in the alteration of dehy-drogenation thermodynamics.Met
35、al amidorborane(MAB)isanewclassofcompoundswithrarelyobservedNH2BH3?-units,which was formed by replacing one of thehydrogen atoms bonded with N in AB with alkali or alkalineearth element.65LiAB,66?68NaAB,66KAB,69MgAB,70CaAB,67,71SrAB72and YAB,73have been investigated byvarious groups.Thermal decompos
36、ition of AB usually required hightemperature and the reaction was relatively difficult tocontrol.In contrast,the catalytic hydrolysis provides a moreconvenient method for hydrogen generation from AB.8,74?76Considerable efforts have been dedicated to the hydrogengenerationinthehydrolysisofABatanambie
37、nttemperature.8,77?96The reaction can be briefly expressed asfollows:NH3BH3 2H2O!NH4BO2 3H2:2We first reported that transition metals such as Pt,Rh and Pdexhibited high catalytic activity to the dissociation and hy-drolysis of AB at room temperature.77The carbon supportedPt catalysts showed the high
38、est activities and the reaction wascompleted in less than 2min(Pt AB 0:018).Noble metalsRu,Rh,Pd,Pt and Au supported on Al2O3,C and SiO2werealso investigated for hydrolysis of AB and Pt/Al2O3wasfound to be the most active.80Recently,zeolite-stabilized Rhnanoclusters were found to have good catalytic
39、 activity forthis reaction.85To lower the cost of the catalysts,non-noblemetals such as Fe,Co,Ni and Cu have been extensivelyinvestigated in the past several years.79,81,82,84,87,89,92Sup-ported Co,Ni and Cu catalysts were the most active at roomtemperature,with which hydrogen was released with anal
40、most stoichiometric amount from aqueous AB,whereassupported Fe is catalytically inactive for this reaction.79Interestingly,amorphous Fe NPs have recently been found tohave Pt-like catalytic activity,with which the hydrolysisreaction can be completed within 8min(Fe AB 0:12).81The amorphous Co and Ni
41、NPs were also found to havemuch better catalytic activity than their crystalline counter-parts.84,91Bimetallic catalysts usually show enhanced cata-lytic performance in comparison to their monometalliccounterparts.83,86,90,97For example,core-shell structured Au-NiSiO2nanospheres showed higher cataly
42、tic and betterdurability in the hydrolysis of AB compared with mono-metallic AuSiO2and NiSiO2.86Recently,AuCocore-shell structured NPs have been synthesized through aone-step seeding-growth pathway with AB as the reducingagent under mild conditions.90Unexpectedly,the resultantmagnetically recyclable
43、 AuCo NPs exhibited excellentcatalytic and long-term stability towards hydrolytic dehy-drogenation of aqueous AB at room temperature,comparedto the alloy and monometallic counterparts(Fig.1).Non-noble metal-based core-shell CuM(M Fe,Co,Ni)NPsalso show excellent catalytic performances for hydrolyticd
44、ehydrogenation of AB.97Notably,the convenience of performing AB hydrolysisreaction makes it be applicable and has already been exten-sively used as a test(model)reaction for examining thecatalytic activity of new nanomaterials.87,90In addition to the hydrolysis of AB,methanolsis of ABhas been report
45、ed to generate hydrogen at room temperatureover various catalysts,such as RuCl3,RhCl3,CoCl2,NiCl2,Recent Progress in Boron-and Nitrogen-Based Chemical Hydrogen Storage1230001-3Pd/C,Raney Ni,Co-Co2B and Ni-Ni3B.98,99The hydrogencapacity of the methanolysis system except catalysts wascalculated to be
46、about 3.9wt.%,which is lower than thatfrom the hydrolytic system(8.9wt.%).2.3.Hydrazine boraneHydrazine-borane(N2H4BH3,HB)hasagravimetrichydrogen storage capacity of 15.4wt.%(4H,3H?),rela-tively easily prepared by mixing sodium borohydride andhydrazine hemisulfate at room temperature N2H52SO42NaBH4!
47、2N2H4BH3 Na2SO4 2H2.100Another routefor the synthesis of HB was found by Gunderloy.101Exper-imental spectroscopy and DFT calculation were performed tounderstand the structure of HB.102,103The thermal decomposition of solid HB was first reportedby Ricker and Goubeau in 1961,100and further investigati
48、onson the reaction kinetics were published by Zhigach et al.in1973.104It melted at 65C,at which temperature thermaldecomposition commenced.104The hydrogen release wasstudied at various temperatures ranging from 65C to 150C,and it was found that HB released more H2at 140C than150C.9At 140C,a total of
49、 about 6.5wt.%H2was releasedwithin 16h.The hydrogen storage capabilities of HB can besignificantly improved by adding an equal amount of lithiumhydride(LiH),which yields a mixture containing 14.8wt.%H2that releases more than 11wt.%H2at 150C in less thanan hour(Fig.2).9The hydrogen release mechanism
50、wasexplored by using theoretical calculation,and the energybarrier for H2-loss of the HB was predicted to be 38.2kcalmol?1,1.4kcal mol?1higher than that of 36.8kcal mol?1forAmmonia borane.105Recently,metal catalyzed hydrolysis of HB under air atroom temperature was reported by Karahan et al.,and onl