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substantial/大量的实质的/impact/影响/oncarbonemissions,withmorethan
132millionvehicles/工具/intheUnitedStatesalone.2
Batteryandfuel-celltechnologiesarestrongcandidates/候选者/to
replacegasoline/汽油/anddiesel/柴油/engines.Inparticular,hydrogen
isanattractiveenergycarrierbecauseitiscarbon-free,
abundantly/丰富的/availablefromwater,andhasanexceptionalmass
energydensity.3Unfortunately,hydrogenisanextremely
volatile/挥发性的/gasunderambient/周围的/conditions,resultingina
volumetricenergydensitythatismuchtoolowforpractica
applications.Foron-boarduse,hydrogenmustbecompressed
toveryhighpressuresorstoredcryogenically/低温/,bothofwhich
costenergyandsubstantial/大量的实质的/lyincreasevehicleweight.Thegoal
thereforeistodesignlow-cost,light-weightmaterialsthatcan
reversibly/可逆的/andrapidlystorehydrogennearambient/周围的/conditions
atadensityequaltoorgreaterthanliquidhydrogen.TheUS
DepartmentofEnergy2010targetsforahydrogenstorage
systemare:
acapacityof45gH2perL,arefuelling/补充燃料/timeof
10minorless,alifetimeof1000refuelling/补充燃料/cycles,andanability
tooperatewithinthetemperaturerange_30to501C.4,5Itis
importanttonotethatthesetargetsarefortheentirestorage
system,suchthattheperformanceofastoragematerialmust
beevenhigherinordertoaccountforthestoragecontainer
and,ifnecessary,temperatureregulating/校正/apparatus/设备/.
Hydrogenbindstosurfacesbyweakdispersive/分散的/interactions
(physisorption)orthroughstrongerchemicalassociations
((chemisorption).Physisorptioncorrelateswithsurfacearea,
withgreatergasuptakefavoredbyhighersurfacearea.Thus,
materialswithlargesurfaceareasandlowdensities,suchas
metal–organicframeworksandcertainactivatedcarbons,are
attractiveforhydrogenstorageapplications.Inthetemperature
regimedesiredforautomotiveapplications,however,
dispersive/分散的/forcescannotfacilitatesubstantial/大量的实质的/hydrogen
uptake.Themodularnatureofmetal–organicframeworks
allowsforthefacile,orderedincorporationofnewfunctionalities
toenhancethehydrogenstorageproperties.Here,
wereviewthecurrentstateofhydrogenstorageinmetal–
organicframeworks,focusingonstrategiesforimproving
thestoragecapacityofthesecompounds.Thedesignof
newframeworksdependsonadetailedchemicalunderstanding
oftheinteractionofhydrogenatsiteswithinthestructure.
Wethereforealsodiscussbrieflysomespectroscopictools
thatareavailabletointerrogatehydrogenbindinginthese
systems.
H2adsorptioninmetal–organicframeworks
Excessversustotaluptake
Mostarticlesdealingwithhydrogenstorageinmetal–organic
frameworksreporttheH2uptakecapacityatapressureof
ca.1bar,whereexcessandtotaladsorptionvaluesarenearly
identical.However,sincepressuresofupto100barare
deemedsafeforautomotiveapplications,measurementsat
higherpressures,wherethesetwoquantitiescandiffer
considerably,havebecomecommon.Excessadsorptionrefers
totheamountofH2takenupbeyondwhatwouldbe
contained,underidenticalconditions,withinafreevolume
equivalenttothetotalporevolumeofthesample.Thus,
thisquantityapproximatestheamountofH2adsorbed
onthesurfaceswithinthematerial.Sincetheefficiencyof
packingandcompressinggasmoleculeswithintheconfinesof
theporesofamicroporoussolidislessthanthatachievedina
freevolume,theexcessadsorptionwillreachamaximumat
somepressure(typically20–40bar)andthendecrease.Despite
thedecrease,measurementsatpressuresabovethemaximum
inexcessadsorptionareofvalueforassessingthecompressibility
ofH2withinthematerialandevaluatingthetotal
1295
oneshouldbecarefultoemployanaccurateintermolecularH2
potentialenergyfunction18andtoensurethatthecomparison
dataareforanauthenticsample.19Thesestudiesindicatethe
presenceofjustvanderWaals-typeinteractionsbetweenH2
andmostframeworks,consistentwiththeapproximate
correlationofH2uptakeat77Kwithsurfaceareaandthe
verylowstoragecapacitiesobservedat298K.Indeed,with
justtwoelectrons,H2formsextremelyweakvanderWaals
bonds,resultinginisostericheatsofadsorptionthatare
typicallyintherange4–7kJmol_1.
Partialcharges,eitherpositiveornegative,onthemetal–
organicframeworksurfacecanprovideameansofstrengthening
thebindingofH2throughdipole–induceddipole
interactions.6,20Onlyafewcomputationalstudieshavedealt
withframeworksexhibitingsuchheterogeneoussurface
potentials.Thesehavefocusedmainlyonthechiefexperimental
strategyadopted,thatofutilizingframeworkswithexposed
metalcationsitesonthesurface.Anaddedcomplication
inperformingcalculationsonframeworksbearingopen
transitionmetalcoordinationsitesstemsfromthefactthat
thesemetalssometimeshaveopen-shellelectronconfigurations,
forwhichassignmentofthespinstatecanbedifficult.
Forinstance,therelativelystrongmetal–H2interactionswithin
Mn3[(Mn4Cl)3(BTT)8]2(H3BTT=benzene-1,3,5-tris(1Htetrazole)),
whichexhibitsanisostericheatofadsorptionof
10.1kJmol_1atzerocoverage,21havebeenattributed
variouslytoaspin-statechangeuponbinding22ortoa
classicalCoulombicattraction.23Understandingmetal–H2
interactionsofthistypeisinstrumentaltothedesignof
improvedstoragematerials,andthedevelopmentofcomputational
approachesthatcanreliablyhandleinteractions
withopen-shellmetalionswouldpresentanimportantstep
forward.
Clearly,increasingtheH2bindingenergywithinmetal–
organicframeworksisthemostimportantchallengefor
creatinghydrogenadsorbentsthatoperateat298K.Recent
workhasaddressedthisissueandpredictedoptimal
parametersforhydrogenstorageinmicroporousmaterials.
First,Langmuirisothermswereemployedtoderiveequations
thatallowthecalculationofanoptimaladsorptionenthalpy,
DHopt,foragivenadsorptiontemperature.24Accordingtothis
model,whichcanbereducedtotheempiricalequation
_DHopt/RT=6.1,amicroporousadsorbentoperating
between1.5and100barat298Kwouldideallyhavean
adsorptionenthalpyof13.6kJmol_1overtheentireH2
uptakecurve.Similarly,themodelallowsonetocalculate
theoptimaloperatingtemperatureforanadsorbentwitha
givenenthalpyofadsorption.Forinstance,itpredictsthata
typicalmetal–organicframeworkwithanaverageadsorption
enthalpyof6kJmol_1wouldfunctionoptimallyata
temperatureof131K.
Theaforementionedmodelhasrecentlybeenadjusted
throughintroductionofanentropy–enthalpycorrelation
term.25WhereasDSadshadpreviouslybeenassumedtobe
constantandequalto_8R,thenewmodelarguesthat
Langmuiradsorptionisinfactgovernedbyapositive
correlationbetweenentropyandenthalpy.Takingthis
empiricalcorrelationintoaccountsuggeststhatamaterial
operatingbetween1.5and30barat298KrequiresaDHoptof
22–25kJmol_1,whichissignificantlyhigherthanthat
obtainedwiththepreviousmodel.Thus,forpressuresranging
upto100bar,onewouldliketocreatenewmetal–organic
frameworksfeaturingsurfaceswithaDHoptofca.
20kJmol_1,representinganenhancementbyafactorof
3or4oversimplephysisorption.
Asexpected,inamicroporousmaterialwherephysisorption
andweakvanderWaalsforcesdominatetheadsorption
picture,thestoragedensityisalsogreatlydependentonthe
sizeofthepore.Calculationsonidealizedhomogeneous
materials,suchasgraphiticcarbonsandcarbonnanotubes,
predictthatmicroporousmaterialswith7A˚-wideporeswill
exhibitmaximalH2uptakeatroomtemperature.Ineffect,
a7A˚-wideslit-shapedporemaximizesthevanderWaals
potentialbyallowingexactlyonelayerofH2moleculesto
adsorbonopposingsurfaces,withnospaceleftinbetween.
Notably,at77Kalayersandwichedinbetweenthesetwo
opposingsurfacemonolayersbecomesfavorable,andtheideal
poresizeformaximumvolumetricH2uptakeat100baris
predictedtobe10A˚,regardlessofwhetheraslitshapeor
cylindricalporeshapeisconsidered.26
Finally,anidealhydrogenstoragematerialwouldbestable
toanypotentialimpuritiesthatmightcommonlybepresentin
H2gas(e.g.,H2S,carbon–sulfurcompounds,CO,CO2,N2,
H2O,andhydrocarbons),andtoaccidentalexposuretothe
atmosphere.Indeed,metal–organicframeworksexhibiting
someofthebestperformancecharacteristics,suchas
Zn4O(BDC)3andMn3[(Mn4Cl)3(BTT)8]2,areknownto
decomposeinair,19,21,27,28whichwouldneedtobeaccounted
forinthedesignofastoragesystem.However,byproducing
frameworksfeaturingstrongm