Hydrogen Clathrates
Gupta, Anshul; Baron, Gino V.; Perreault, Patrice; Lenaerts, Silvia; Ciocarlan, Radu George;
Cool, Pegie; Mileo, Paulo G.M.; Rogge, Sven; Van Speybroeck, Veronique; Watson, Geert;
Van Der Voort, Pascal; Houlleberghs, Maarten; Breynaert, Eric; Martens, Johan; Denayer, Joeri F.M.
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Energy Storage Materials
DOI:
10.1016/j.ensm.2021.05.044
Publication date:
2021
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CC BY-NC-ND
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Citation for published version (APA):
Gupta, A., Baron, G. V., Perreault, P., Lenaerts, S., Ciocarlan, R. G., Cool, P., ... Denayer, J. F. M. (2021).
Hydrogen Clathrates: Next Generation Hydrogen Storage Materials. Energy Storage Materials, 41, 69-107.
https://doi.org/10.1016/j.ensm.2021.05.044
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Energy Storage Materials
journalhomepage:www.elsevier.com/locate/ensm
Hydrogen Clathrates: Next Generation Hydrogen Storage Materials
Anshul Gupta
1,2, Gino V. Baron
1, Patrice Perreault
3,4, Silvia Lenaerts
5, Radu-George Ciocarlan
6, Pegie Cool
6, Paulo G.M. Mileo
7, Sven Rogge
7, Veronique Van Speybroeck
7, Geert Watson
8, Pascal Van Der Voort
8, Maarten Houlleberghs
9, Eric Breynaert
9, Johan Martens
9,
Joeri F.M. Denayer
1,∗1Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050, Brussel, Belgium
2Department of Metallurgical and Materials Engineering, National Institute of Technology, Srinagar, India-190006
3University of Antwerp, Faculty of Science, Institute for Environment and Sustainable Development (IMDO), Groenenborgerlaan 171, 2020 Antwerpen, Belgium
4University Of Antwerp, Blue App, Middelheimlaan 1, 2020 Antwerpen, Belgium
5University of Antwerp, Department of Bioscience Engineering, Sustainable Energy Air and Water Technology, Groenenborgerlaan 171, 2020 Antwerpen, Belgium
6Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium
7Center for Molecular Modeling (CMM), Ghent University, Zwijnaarde, Belgium
8Center for Ordered Materials, Organometallics and Catalysis, Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
9Centre for Surface Chemistry and Catalysis, NMRCORE - NMR - XRAY - EM Platform for COnvergence Research, Department of Microbial and Molecular systems (M2S) – KU Leuven, B-3001 Leuven
a r t i c le i n f o
Keywords:
Hydrogen storage Clathrates Hydrogen Hydrates Raman Spectroscopy NMR
a b s t r a ct
Extensiveresearchhasbeencarriedonthemolecularadsorptioninhighsurfaceareamaterialssuchascarbona- ceousmaterialsandMOFsaswellasatomicbondedhydrogeninmetalsandalloys.Clathratesstandamongthe onestoberecentlysuggestedforhydrogenstorage.Although,thesimulationspredictlowercapacitythanthe expectedbytheDOEnorms,theadditionalbenefitsofclathratessuchaslowproductionandoperationalcost, fullyreversiblereaction,environmentallybenignnature,lowriskofflammabilitymakethemoneofthemost promisingmaterialstobeexploredinthenextdecade.Theinherentabilitytotailorthepropertiesofclathrates usingtechniquessuchasadditionofpromotermolecules,useofporoussupportsandformationofnovelreverse micellesmorphologyprovideimmensescopecustomisationandgrowth.Asrapidlyevolvingmaterials,clathrates promisetogetascloseaspossibleinthesearchof“holygrail” ofhydrogenstorage.Thisreviewaimstoprovide theaudiencewiththebackgroundofthecurrentdevelopmentsinthesolid-statehydrogenstoragematerials,with aspecialfocusonthehydrogenclathrates.Thein-depthanalysisofthehydrogenclathrateswillbeprovidedbe- ginningfromtheirdiscovery,variousadditivesutilisedtoenhancetheirthermodynamicandkineticproperties, challengesinthecharacterisationofhydrogeninclathrates,theoreticaldevelopmentstojustifytheexperimental findingsandtheupscalingopportunitiespresentedbythissystem.Thereviewwillpresentstateoftheartinthe fieldandalsoprovideaglobalpictureforthepathforward.
1. Introduction
Protectionoftheenvironmentisofparamountimportanceforsus- tainabledevelopmentandgrowthofanation.Thedevelopmentofalter- nateenergysources,whichcanaddressseriousissueslikeglobalwarm- ing,havegainedgraveurgency.Hydrogen,withitshighfuelefficiency (141.7MJ/kg)andenvironmentfriendly nature(byproductof com- bustioniswater),isthemostpromisingalternativetofossilfuel-based
Abbreviations:SDS,Sodiumdodecylsulfate;THF,Tetrahydrofuran;i-PA,iso-propylamine;n-PA,n-propylamine;LH2,Liquidhydrogen;LNG,LiquifiedNatural Gas;t-BuNH2,Tert-Butylamine;t-BA,TertiaryButylAlcohol;PRD,Pyrrolidine;TBAB,Tetra-n-butylammoniumbromide;HCFC,Hydrochlorofluorocarbon;TBACL, Tetrabutylammoniumchloride;DTAC,Dodecyltrimethylammoniumchloride;THT,Tetrahydrothiophene;CP,Cyclopentadienide;DXL,D-Xylaricacid;HRXRD,High ResolutionX-RayDiffraction.
∗Correspondingauthor.
E-mailaddress:[email protected](J.F.M.Denayer).
energysources[1].However,duetoitslowdensity(0.08g/L)andhighly flammablenature(ataslowas4%concentrationinair),safeandeffi- cientstorageisachallenge[1].USDepartmentofEnergyhasspecified storagerequirementsof~5.5%hydrogenbyweightforacommercial vehicleon-boardstoragesystem,withamicablepressure(5-12bar)and temperature(<85°C)conditions[2].Inthisregard,storageofhydrogen inmaterialsisanattractivealternativetotheconventionalhigh-pressure cylinders.Importantclassesofmaterialsdevelopedforhydrogenstorage
https://doi.org/10.1016/j.ensm.2021.05.044
Received18March2021;Receivedinrevisedform27May2021;Accepted28May2021 Availableonline8June2021
2405-8297/© 2021TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense
include:(i)metalsandalloys(intermetallics),(ii)carbonaceousmateri- als,(iii)metalorganicframeworks(MOFs),(iv)zeolites&(v)clathrates.
Multiplestudieshavebeencarriedouttoinvestigatethehydrogenstor- agecharacteristicsofthesematerials.However,noneofthematerials hasbeenfoundtobesuitableforcommercialuseinanon-boardvehic- ularapplication.
Hydrogenstorageinclathratesdrewattentionofthescientistinthe beginningof21stcentury.Aftertheirdiscoveryin1810bySirHumphry Davy[3], clathrateswere proposed ashydrogen storage materialsin 2002byMaoetal.[4].Theyshowedsuccessfulformationofwater-based hydrogenclathrateswithclassicsIIstructureat249K,underapressure of250MPa;ithadhydrogenstoragecapacityof5.3wt.%.Thiswasfol- lowedbydetailedinvestigationsofoccupationofhydrogeninhydrate cages,interactionofH2withwatermolecules,electronicenvironment aroundwatermolecules[5].Advancedcharacterisationtoolshavebeen usedtoconfirmthepresenceofhydrogen;in-situRamanspectroscopy tostudythemodesofvibrationofH2molecules,synchrotronradiation underhighpressuretodeterminetheprecisestructureand1HNMRto determinethecageoccupancy[6].Thispaperdiscussesindetailthehy- drogenstoragecharacteristicsoftheclathrateswithanin-depthanalysis oftheirthermodynamicandkineticproperties,challengesinthechar- acterisationofhydrogeninclathratesandtheupscalingtechniquesso astobringoutthekeyissueslimitingthegrowthoftheclathratesasthe nextrevolutionaryon-boardhydrogenstoragematerial.
2. Hydrogenstoragematerials:Abriefoverview
Interaction of hydrogen with materials can be divided into two broadcategories;physisorptionandchemisorption.Duringphysisorp- tion,molecularhydrogeninteractswiththehostmaterialusingweak Vander Waals’ forces, apart from electrostatic and orbital interac- tions[7,8].Theproblemarisesastheseinteractionsarequiteweakand hydrogenmoleculepossessnodipolemovementorchargeandisonly weaklypolarizing[7].Theadsorptionisfavouredatlowtemperatures (77K)anddecreasesuponincreasingtemperature.Atthesametime, poresizeformsanimportantparametergoverningtheamountofgas adsorbed.Ithasbeenfoundthatenthalpyofadsorptionincreaseswith decreasingporesize[9].Innarrowpores,thepotentialsofneighbour- ingatomsoverlapandinteractionstrengthincreases[10].Asaresult, withdecreasingporesize,theamountofadsorptionincreasesandusu- allyTypeIisotherm (IUPACnomenclature)isfollowedbyphysisorp- tionbasedmaterialsat77K[11].Theadsorptionincreasesmonotoni- callywithpressureandsaturatesatacertainpressure.Chemisorption, ontheotherhandtakesplacebydissociationofmolecularhydrogen intoatomicform.Activationenergybarrierhastobe crossedforthe chemisorptiontotakeplace. Atomichydrogencan furtherdiffuseto thebulkofthematerialandformeitherdisorderedsolidsolutionor acompound.Thisisaccompaniedbyaphasetransformationresulting inchangeincrystalstructureorlatticeparameter.Atypicalsequence ofstepsduringhydrogenadsorptioninmagnesiumisprovidedinthe Figure1.
Aplethoraofmaterialsexist forsolidstatehydrogenstorage[13]. Thesecanbeclassifiedbasedontheformofhydrogenstored:themate- rialswhichstorehydrogeninmolecularformfallunderthecategoryof physisorptionviz.themolecularhydrogenisphysicallyadsorbedonthe surfaceofthematerial[14].Materialssuchascarbonaceous,MOFsand zeolitesfallunderthiscategory.Ontheotherhand,thematerialswhich absorbhydrogeninatomicformareclassifiedunderchemisorption[15]. Metalhydridesandcomplexhydridesareexamplesoffewsuchmate- rials.Metalhydridesusuallyforminterstitialhydridesinwhichatomic hydrogenoccupiesinterstitialvoidsinthecrystalandcomplexhydrides formionicorcovalentbondwithhydrogen.Animportantsetofcriteria whichgovernstheabilityofthematerialtobeutilisedinon-boardap- plicationhavebeenformulatedbyDepartmentofEnergy(DOE),United States[16].Thesecriterianotonlyfocusontheoverallhydrogenstorage capacityofsystem(5.5wt.%),butalsotakeintoaccounttherateoftank
filling(1.65wt.%H2perminute),temperature(~85°C)andpressureof storageandrelease(5-12bar), theabilityofthematerialtoundergo multiplecycleswithoutdegradation(>1000cycles)amongotherfac- tors.However,nomaterialhasbeenabletomeetthesetargetstillnow.
Recentlydevelopedmaterialscloselymeetingthesetargetshavebeen showninFigure2.
2.1. Carbonaceousmaterials
Amongthephysisorptionbasedmaterials,carbonbasedmaterials haveattractedthelargestattention[24,25]. Quiteafewtypesofcar- bonmaterialshavebeenextensivelystudiedsuchasactivatedcarbon, microporoustemplatedcarbon,carbonnanotubesandnanofibersand graphene[26–31].Amongthese,activated carbons areamorphousin nature andexhibitextremely largesurfacearea(3000 m2/g forAX- 21)andporosity[32].Thispropertymakes themidealforabsorption ofhydrogenwithcapacityreportedupto5wt.%at77kand20bar[32]. Severalcomputationalmodelshavebeendevelopedtooptimisethepore structureandsurfacepropertiesofactivatedcarbonforenhancedhydro- genstorage[33,34].Zuetteletal.[35]havedevelopedacomputational modelwhichsuggeststhatdensityofhydrogenonthesurfaceofcarbon materialisexpectedtobe2.28wt.%m2/g.Thiscorroborateswellwith themodeldevelopedbyexperimentallycomparingthespecificsurface area(SSA)andthehydrogenstoragecapacity[36].
Toenhancetheambienttemperaturehydrogenstorage,dopingwith metalnanoparticlecatalystssuchasNi,Pd,Pt,VandCohasbeencar- riedout[37].Ptdopedsuperactivatedcarbonshows1.2wt.%hydrogen absorptionat298Kand100bar[38].Theincreasehasbeenattributed to‘hydrogenspillover’mechanism[39].Thisreferstothedissociationof hydrogenmoleculesatopthecatalyst,diffusionofhydrogenatomsto- wardsthecarbonaceoussupportandadsorptionatsurfacesites,which wereinaccessibleotherwise[39].Thepresenceofsurfaceoxygengroups hasfoundtofurtherenhancetheeffectofmetalcatalyst[40].Ithasbeen reportedthatPd-dopedreducedgrapheneoxideabsorbs3wt.%hydro- genat298Kand40bar[41].Surfaceoxygenfunctionalgroupshave been foundtoact secondarysurfacefor thehydrogen spillover[42]. Otherelementssuchasboronandnitrogenhavealsoshowntohave beneficial effectonthehydrogenabsorptionin carbonmaterials[43–
46].Inarecentstudy,borondopedactivatedcarbonhavebeenirra- diatedbyneutronstooptimisetheporesizeandstructure[47].They haveshown~9wt.%hydrogenstoragecapacityat80Kand100bar.
Inarecentstudy,celluloseacetatederivedactivatedcarbon(SSA=3800 m2/g)havebeenreportedtostore~8.9wt.%hydrogenat77Kand 30bar[17].However,theirhydrogenstoragecapacityatroomtemper- ature(298K)remainslow(~ 1.2wt.%at30bar),withheatofadsorp- tionbeing10kJ/mol17.Turningaroundthedangerouswastetouseful materials,microporousporouscarbonsderivedfromcigarettebutthave beenreportedtoshow11.2wt.%hydrogenabsorptionat77Kand40 bar[48].Theyexhibitultra-highsurfacearea(4300m2/g)andporevol- umeof2.09cm3/g.Thisisthehighestreportedcapacitytilldatefor thecarbonaceousmaterialsatcryogenictemperatures.However,repro- ducibilityandaccuracyofresultsareacauseofconcernforthecarbon materialsandmultipleroundrobintestshavebeenconductedtoverify thesame[49–51].
Incontrast toactivated carbons,thetemplated carbonsrepresent materials withnarrowrange ofpore size,givingbetter controlover structure.Zeolitetemplatecarbon,withhighsurfacearea(3200m2/g) andfineporesize(0.5-0.9nm),haveshownhydrogenstoragecapac- ityupto6.9wt.%at77Kand20barwithheatofadsorptionof4-8 kJ/mol[52].Carbonnanotubes,especiallysinglewallcarbonnanotubes (SWNTs),showmoderatehydrogenabsorptioncapacityof~2wt.%at 77K[53].Multipleattemptshavebeenmadetoenhancethehydrogen storagecapacitybydecoratingthesurfacewithelementssuchasalu- minium,lithium,calcium andnickel[54].Doping withNinanoparti- cleshasshownenhancementinthehydrogenstoragecapacityofmulti- walledcarbonnanotube(MWNTs),with~2.27wt.%hydrogenstoredat
Figure1. Interactionofhydrogenwithmaterialinvarioussteps:(i)physisorption,(ii)chemisorption,(iii)solidsolutionand(iv)compoundformation[12].
Figure2. Anoverviewofthehydrogenstoragematerials,fromtheperspectiveofDOErequirements[16].Arepresentativematerialofeachcategoryhasbeentaken toplotthedata.Theseare:(a)ActivatedCarbon(CelluloseDerivedActivatedCarbon)[17],(b)MOF(MOF-5)[18],(c)ComplexHydrides(LiBH4/MgH2)[19],(d) Nanoconfinement(Mg@CMK)[20],(e)ComplexHydride(NaAlH4@TiO2&Carbon)[21],(f)MgH2nanopartciles[22]and(g)Mgnanowires[23].DOEtargetsrefer tomaterialbasedtargets(anestimatekeepinginmindtheweightofauxiliaries).
298Kand80bar[55].AtomicoxygenirradiatedMWCNTshaveshown 1.6wt.%hydrogenstoragecapacityat100kPaand298K[56].Although computationalstudieshavepredictedhydrogenstoragecapacityashigh as19wt.%in3-Dassembliesofcarbonnanotubes,experimentalvali- dationoftheseresultshavenotyetbeenachieved[57].Whilecarbon materialshavebeenstudiedfromtheearlystagesofresearch,several othermaterialshavegainedattentionintherecentpastforhydrogen storagesuchasMOFs(MetalOrganicFrameworks)[58,59],COFs(Co- valentOrganicFrameworks)[60,61],zeolites[62]andPIMs(Polymers withintrinsicMicroporosity)[63].
2.2. MetalOrganicFrameworks(MOFs)
MOFsincludingMOF-5,IRMOF-6andIRMOF-8werefirstreported forhydrogenstoragebyRosietal[64].Theyreported4.5wt.%at77 Kand0.07MPaforMOF-5.Poresizehasbeenoneoftheimportant parametersfortuningthehydrogenstoragecapacityinMOFs.Apore sizeclosetothekineticdimeterofhydrogenmolecule(2.89Ǻ)issug- gested;itwouldenhancetheinteractionofhydrogenwiththeframe- workandstrengthentheVanderWalls forces.Thisphenomenonhas beenobservedatlowpressures(1bar)andcryogenictemperatures(77 K);e.g.MIL-53(1.66wt.%,poressize:6.8Ǻ)andIRMOF-3(1.42wt.%, poresize:9.6Ǻ).
Ina recentstudy, nearly half a millionMOFs were screened us- ingGCMC(GrandCanonicalMonteCarlo)calculationsandexceptional hydrogenstoragecapacitywasreportedforthethreeMOFs:NU-100, UMCM-9andSNU-7065. Capacityof 13.9wt.%(NU-100),11.3wt.%
(UMCM-9)and10.6wt.%(SNU-70)wasobservedat77Kand100bar.
ThisexceededthecapacityforthebenchmarkMOFsi.e.7.8wt.%for MOF-5and9.1wt.%fortheIRMOF-20,undersimilarconditions[18]. Apartfromthegravimetricstoragecapacity,volumetricstoragecapac- ityisalsovitalinMOFs,astheirdensityislowestamongthecrystalline materials(0.61g/cm3forMOF-5)[66,67].AnotherMOFwithcomposi- tionNi2(m-dobdc)hasshownimpressivevolumetriccapacityof23g/l (equivalentto2wt.%H)fortheabsorptionat−75°Canddesorptionat 25°C,thehighestreportedinthistemperaturerange[68].Theyalsore- portedagravimetrichydrogenstoragecapacityof1.2wt.%at110bar and298K68.InconformingwithtankdesignrecommendationsofDOE, NU-125(BETarea=1870m2/cm3)wasreportedtohave49g/lvolu- metriccapacityand8.5wt.%gravimetriccapacity,withabsorptionat 100barand77Kanddesorptionat5bar and160K[69].Theyhave alsoobservedanexceptiontothewellknowChahin’srule,whichstates thatforevery500m2/gsurfacearea,anexcessadsorptionof1wt.%is observed[18,69].Theruleisapplicableonlyuptosurfaceareaof3000 m2/g,whereasafterthatapproximately0.5wt.%excessadsorptionis observedforeveryadditional1000m2/g69.Manyotherstrategiessuch asintroductionofmetalnanoparticlesinthepores[70],restrictingthe poresizeusingcatenation[71],nanoconfinement[72,73]andincorpo- rationofguestmetalions[74,75]havebeenutilisedtoenhancethehy- drogenstoragecapacityofMOFs.
2.3. Zeolites
Zeolitesrepresentmicroporousmaterialswithhighsurfaceareaand porosity,buthavebeenunabletoachievepromisingresultswithregard tohydrogenstorage[62,76].ForNa-Yzeolite,hydrogenabsorptionca- pacityof1.81wt.%wasobservedat77Kand1.5MPa[77].Around 2.55wt.%hydrogenabsorptionwasreportedat77Kand40barNa- X[62].Computationalstudieshaveshownthattheupperlimitforhy- drogenabsorptioninzeolitesliesintherangeof2.65to2.68wt.%[78]. AmongthePIMs,triptycenebasedpolymershaveshownhydrogenca- pacityupto2.7wt.%at1MPaand77K[79].Asimilarclassofmaterials, HypercrosslinkedPolymers(HCPs),haveshownhighercapacityof3.68 wt.%at15MPaand77K[80].Amongtheemergingsetofmaterials, COFshavefaredbetterthanHCPsandPIMsintermsofgravimetrichy- drogenstoragecapacities[60].COF-102and103haveshown7.2wt.%
Table1
Brief timeline of the progress in the development of magnesium-basedhydrogenstoragematerials.
Year Material Wt.% H T (°C) Ref 1968 Mg 2NiH 4 3.6 325 [85]
1996 La 2Mg 17–LaNi 5 3.5 250 [127]
2000 MgH 2–Ge 7.6 350 [128]
2001 (Mg 0.68Ca 0.32)Ni 2 1.4 40 [129]
2004 MgH 2–Si 5 20 [130]
2007 Mg nanowires 3 100 [23]
2013 MgH 2–C 5 270 [131]
2014 𝛾-MgH 2(1.99 wt%) 5 300 [132]
2018 Ni-Mg-C 6 200 [120]
2019 Mg-Glass-Zr 2Pd 2 50 [121]
2020 MgH 2nanoparticles 6.7 30 [22]
and7.3wt.%hydrogen storagecapacity, respectivelyat77 Kand9 MPa[61,81].Tongetal.[60]havecarriedoutsimilarscreeningexercise asAhmedetal.[65]onCOFsandhavetheoreticallycalculatedvolumet- ricstoragecapacityof56.1g/Landgravimetriccapacityof10.3wt.%
inahypothetical3-Dbor-GCOF.Although,noneoftheabovematerials hasbeenabletomeetthecriteriaforpracticalhydrogenstoragemedia, importantadvancementshavebeenmadein thecontextofhydrogen adsorptioninmicroporousmaterialswhichbringusonestepcloserto theDOEtargets[82].
2.4. MetalHydrides
As statedearlier,metalhydridesdistinguishthemselves fromthe abovematerialswithregardtothepresenceofhydrogenintheatomic form.Typically,metals(suchasMg,Pd)andintermetalliccompounds (suchasLaNi5)constitutethetypicalexamplesofsuchmaterials.Among metals,Mgisoneofthemostattractivematerialduetoitseasyavail- abilityandhighhydrogenstoragecapacity(7.6wt.%),whichexceeds theultimateDOEtargets(6.5wt.%)[83].However,initsnativeformit suffersfromhightemperaturesofabsorption(>300°C),sluggishkinetics andsensitivitytooxidationuponexposuretoair[84].Someofthestrate- giesused toimprovethehydrogenabsorption characteristicsof Mg- basedmaterialsincludealloying(Mg2Ni[85],Mg2Fe[86],Mg3Cd[87], Mg0.95In0.05[88], Mg3Ag[89], Mg2Si[90], Mg5Ga2 [91], Mg3LaNi0.1 [92]),addition of catalysts (such as B[93], Co[94], Cu[95], Fe[96], Ge[97], Gd[98]), formation of metastable phases[99], hybrids[100–
103], nanocomposites[104–108] and nanostructured materials (pre- paredusingballmilling[109],chemicalvapourdeposition[23],hybrid combustion[110],chemicalsynthesis[111,112]andmeltspinning[113–
115]) and nanoconfinement[116–119] (to keep the particles in the nanosizedregimebyrestrictingtheirgrowthusingascaffold).Abrief timelineof thedevelopments is givenin Table1. Someof themost promisingresultshavebeenseenwhenthenanocompositeshavebeen preparedbycombiningthehighsurfaceoramorphousmaterialswith nanoscalemagnesium.Forexample,polymerembeddedMgnanocrys- talshavebeensynthesisedwithoutadditionofheavymetalcatalystsand show~6wt.%hydrogenstorageat200°Cand35barwithin60min104. Hard carbonspherewrappedNi-Mgcompositehaveshown 3.5wt.%
hydrogen absorptionat75°Cand30 barwithin 200min120.Magne- siumnanocrystalshavebeenalsobeenballmilledwithmetallicglass and Zr2Pdalloy to give~2 wt.% hydrogenabsorption at 50°C and 10barwithin10min121.MgH2nanoparticleshavealsobeeninvesti- gatedforhydrogenstorageandhaveshownpromisingproperties[122]. Ofrecentinteresthasbeen introductionofgraphene inthecompos- ites.MgH2nanoparticlesongrapheneweresynthesisedbyXiaandco- workers,inwhichgrapheneprovidedthenecessarystructuralsupport aswellasactedasabarriertopreventthegrowthofnanoparticles[123]. MgnanocrystalsencapsulatednanocompositehasbeenproposedbyCho andco-workers,consistingofatomicallythinlayerofreducedgraphene oxide(rGO)[108].Thiscompositeshows6.5wt.%hydrogenabsorption
at200°Cand15barin120min.ArecentpublicationbyZhangetal.[22]
hasshown6.7wt.%hydrogenat30°Cunderapressureof30barinul- trafineMgH2nanoparticles(4-5nm).Althoughthistechniquedoesnot ensureindustrialscale-upproduction,therehasstillbeencontinuedin- terestintheMgbasedhydrogenstoragematerialsandrecentworkshave shownsomeconsiderableprogressinreachingtowardsDOEgoals[83]. Fordetails,thereaderisreferredtothededicatedreviewarticlesonthe magnesium-basedhydrogenstoragematerials[83,124–126].
Apartfrommetals,intermetalliccompoundsalsoexhibitgoodhydro- genabsorptioncharacteristics,partlyduetothelargenumberofinter- stitialsitespresentintheircrystalstructure[133].Forexample,LaNi5, anintermetalliccompoundbetweenLaandNi,readilyreactswithhy- drogenunderambienttemperatureandpressureconditions,witharea- sonablyquickreaction[134].Althoughtheamountofhydrogenstored byvolumeinthismaterialisalmostequaltothethatofliquidhydrogen (71g/L),thepresenceofheavyelementsuchasLamakesthegravimet- ricstoragecapacitydismal(~1.4wt.%at25°Cand2bar)[135,136].
Anotherimportantintermetallic compoundwhich hasbeenof lotof intertestis TiFe, withthecurrent focuson improvingtheactivation characteristicsandenhancingkinetics[137–139].Severalreviewarti- clesgivedetailedinsightintothedevelopmentsintheintermetallicsfor hydrogenstoragestorage[140–142].
2.5. ComplexHydrides
Complexhydridesformwhentheionichydrides(alkalimetalhy- drides such asNaH andLiH) react with covalenthydrides (such as BH3 andAlH3),leadingtoformation ofmetalsalts(such asNaAlH4 andNaBH4).Hydrogeniscovalentlybondedtothecentralatominthe anionicunit[143],consistingofalanates[AlH4]−1,amides[NH2]−1or borohydrides[BH4]−1.Theyareeasiertohandlethancovalentorionic hydridesandoffermuchmorestability[144].Complexhydridesarerel- ativelynewamongchemisorptionbasedmaterialsandhaveattracted muchattentioninthepastdecade[145,146].Thisispartlyduetotheir hightheoreticalhydrogenstoragecapacityintherangeof7-18wt.%.
However,duetotheirstronginteractionwithhydrogen(heatofadsorp- tionintherange40-100kJ/mol),issuessuchirreversibility,hightem- peratureofdesorption(>400°C),sluggishkineticsanddifficultsynthe- sisprotocolshavelimitedtheirsuccessfuluse[147].Someoftheearliest studieswhichindicatedtheiruseasaattractivehydrogenstoragemate- rialproposedTiCl3asacatalystforNaAlH4[148].Thiswasfollowedby useofvariousotheradditivessuchasScandNb(transitionmetals),Ce andSm(rare-earthmetals)andcarbonaceousmaterials[149,150].Few otherstudiesalsofocussedontheuseofnano-catalystssuchasnano-TiN (5.4wt.%Hat130°C)[151]andnano-CeB6(4.9wt.%Hwithin20min at180°C)[152].
Someprogresshasalsobeenmadeinovercomingthesebottlenecks byusingthepreviouslysuggestedtechniquesformetalhydrides.Among these,nanoconfinementofcomplexhydridesinporousmaterials,cou- pledwithcatalyticadditive,hasbeenoneofthemostrapidlyexplored methods[153–156].Earliestdevelopmentsinthisareawerereportedby Bladeetal.[156],inwhichassmallas2-10nmNaAlH4particleswere confinedincarbonfibers.Thisledtosharpdecreaseinhydrogendes- orptiontemperatureto70°C.Atthesametime,anotherstudybyZheng etal.[153]reportedthespaceconfinedNaAlH4inorderedmesoporous silicaandwereabletoachievehydrogenationinthetemperaturerange of125-150°C.NaAlH4,confinedinnanocrystallineTiO2embeddedin carbonmatrix,hasshown4.5wt.%hydrogenabsorptionat50°Cand 100bar[157]. Thehybridwas abletoreleasecompletehydrogen at temperaturesaslowas63°C.Undersimilarstrategy,Tinanoparticles (3-5nm)wereincorporatedinamorphouscarbon(nano-Ti@C)toen- hancethehydrogenstorageinNaAlH421.Thenano-Ti@C-NaAlH4hy- bridshowed5wt.%hydrogenabsorptionin3minat100°Cand120bar.
Thehydrogencanbereleasedat140°Cwithin60min.Thehybridstays stabletill100cyclesofhydrogenabsorption/desorption.However,mul- tiplelacunaeneedtobeovercomebeforeitcanfulfiltherequirementsof
apracticalstoragematerial.Thereaderisreferredtothereviewarticles oncomplexhydridesfordetails[143–146,158–160].
2.6. Core-shellandhollowstructures
Novelstructuresbeingexploredforhydrogenstorageincludecore- shellstructures[161],hollow nanostructures[162,163] andhierarchi- calstructures[164].Hollownanospheres,ascomparedtonanospheres, havetheadvantagesofhighsurfacearea,greaterstructuralstabilityand betterchemicalinertness[162].Nitrogen-dopedhollowcarbonspheres haveshownimprovedperformanceatambienttemperature,with2.25 wt.%hydrogenabsorptionat80bar.Hollowglassmicrosphereshave alsobeenexploredforhydrogenstorage[165].Amongthemetalhollow spheres,multi-modehydrogenabsorptionphilosophyhasbeendemon- strated by Shervani etal.[166],further carried forwardby Gupta et al.[167,168]andAmaladasseetal.[169].Intheirwork,existenceofhy- drogeninmultipleformsi.e.freegaseousform,weaklyadsorbedform, solidsolutionformaswellascompoundformhasbeendemonstratedin palladiumandnickelhollowspheres.Althoughthemaximumhydrogen absorptioniswellbelowtheDOEnorms,itformspromisingtechnique forfurtherdevelopmentofhydrogenstoragematerials.
A comparison of hydrogen storage capacity with temperature is shownschematicallyinFigure3forselectedsetofmaterials.Itisevi- dentfromthefigurethatsignificantstrideshavebeenmadeintheef- fortstowardsdevelopmentofhydrogenstoragematerials.Ononehand, materialssuchasMOFs,activatedcarbonshaveshownpromisinghydro- genstoragecapacityatlowtemperatures(77K),complexhydridesand nano-confinedstructureshavebeenabletoshowconsiderableimprove- mentinperformancenearroomtemperature(asshowninFigure2and Figure3).Hybridsseemtobemostpromisingamongthemwithchar- acteristicsclosesttotheDOErequirements.However,foramaterialto bepracticallyviable,ithastobeeasilyamenabletoscaleupproduc- tionandprovidelowcost.Advancedmaterialssuchasthehybridsare nonethelessexcellentintermsoflaboratoryuse,buttheirsensitivityto theatmosphere,difficultsynthesisprocess,harmfuleffectstotheen- vironmentandsometimeshighercostactasdeterrenttotheirusein thepracticalapplications.Itisthereforeimperativethateffortsarein- vestedinanalternativesetofmaterialswhichcanovercometheselim- itations,withoutcompromisingtheunderlyingperformancecharacter- istics.Anemergingandpromisingclassofmaterialsinthisregardare hydrogenhydrates.Theyarenotonlyenvironmentfriendly,butalsolow incostandeasytosynthesise.Clathratecompoundsaredifferentfrom eitherMOForZeolitesintermsoftheirflexibilityintuningthestruc- ture.Parameterssuchastemperature,pressure,concentrationofguest molecules,amongothers,canbemodifiedtoachievethedesiredproper- ties.Abriefintroductiontothestructureandpropertiesofclathratesis giveninnextsection,followedbytheirhydrogenstorageproperties.Due totherapidlygrowinginterestinthefield,scientificliteratureisbeing continuouslyreviewed,withaspecialfocusonthethermodynamicand kineticaspectsofthehydrogenclathrates[3,6,170–174].However,the currentreviewnotonlycoverstheessentialscientificaspectsofthehy- drogenclathrates,butcorrelatesthemwiththetheoreticalandtechno- economicalaspectsofthefield.Distinctively,thisreviewalsotakesa sharplookattheaccuracyofhydrogencharacterisationinclathrates, whichisoneofthebiggestchallengeprevailinginthesolid-statehydro- genstorage.
3. Clathrates:OriginandStructure
Clathrates are a class of inclusion compounds, where the guest molecules residein the cagesformed bythe host lattice. Clathrates areconventionally classified in twocategories based on theinterac- tionoftheguestspecieswiththehoststructure:clathratehydratesand semi-clathratehydrates.ClathratehydratesexhibitweakvanderWaal’s forces ofattractionbetweentheguestmoleculesandthehost.Semi- clathratehydrates,inadditiontovanderWaal’sinteraction,showpar-
Figure3. Acomparisonofhydrogenstoragecapacityofthevariousmaterialswithincreasingtemperature.Thematerialspresentedaboveare(i)AC[17],(ii) MOF[65],(iii)CNT[175],(iv)MOF[68],(v)MgH222,(vi)TiO2@C,NaAlH421,(vi)Ni@C-Mg[120],(vii)Mg@rGO[108]and(viii)MgH2@CMK-320.
tialhydrogenbondingbetweentheguestspeciesandthehostframe- work.
Abriefoutlineofthestructuresformedbyhydrogenclathratesin giveninTable1[3].PurehydrogenhydratesformsIIstructure,with a smalldodecahedron cage(512) and large hexakaidecahedroncage (51264).Otherstructures of hydratesinclude sIhavingsimilar small cage and large tetrakaidecahedron cage (51262) and sH with same smallercageandirregulardodecahedron(435663)alongwithicosahe- dron(51268)aslargecages.Aspertheearlierstudiesandtheoretical estimates[4,176],ithasbeenobserved thatthenumberof hydrogen moleculesthatcanbeaccommodatedinahydratedependsonthetype ofcagepresent;512cagescanaccommodateupto2molecules,51264 canaccommodateupto4molecules,435663cageupto1,51262upto2 and51268canaccommodateupto5molecules.Consideringtheabove estimates,hydrogenstoragecapacitiesof sI,sIIandsHhydratescan reachupto6.33wt.%,4wt.%and4.67wt.%,respectively.Oneofthe importantrolesplayedbypromotersorsecondaryguestmoleculesin thehydratesisthestabilisationofthesIorsHstructurewithascopeof increasingthehydrogenstoragecapacity.
Indetermining whichstructure is thermodynamicallystable,itis importantto take intoaccount themolecular diameterof theguest molecule.Forexample,smallmoleculessuchasmethaneandpropane formsIstructure(Figure4),whereasthelargermoleculessuchasTHF stabilizethesIIstructure.ThelargecagesofsIIstructureactaspre- ferredenclosures forincorporationof these molecules.Further, very smallmoleculessuchH2alsoformsIIstructureastheyhavebeenpro- posedtooccupysmallcages(16suchcages),whichexceedthetotal numberlargecages(8suchcages)insIIstructure.
4. Discoveryofhydrogeninclathrates:Thesagaof Tetrahydrofuran(THF)
InapathbreakingstudybyMaoetal.[4]in2002,itwasproclaimed thatpurehydrogenhydratescouldstoreupto5wt.%hydrogenunder pressureof200MPaat243K.Inspiteofthehigh-pressureconditions
reportedinthisstudy,itopenedupthenewvistaforsearchfortheholy grailofhydrogenstorage.Soonafter,Florusseetal.[178]usedawater solublereagent(THF)asanadditivetobringdowntheclathrateforma- tionpressurebytwoordersofmagnitudeto5MPaat279.6K.How- ever,thisledtoasignificantdropinthehydrogenstoragecapacityto1 wt.%,withsingleH2moleculeoccupancyinsmallcagesandcomplete absenceinlargecages.Florusseetal.[178]suggestedthatTHFmolecules occupythelargecages,whichconstraintsthepresenceofhydrogenin thesecagesandlimitstheextentofincrementpossible.However,inan anotherremarkablestudy,Leeetal.[179]reiteratedtheuseofTHFas anadditiveandshowedsignificantlyhighhydrogenstoragecapacityof upto4wt.%at12MPaand270K.ThiswasattributedbyLeeetal.[179]
tothe‘tuningeffect’,whichallowedhydrogenstoragecapacitytobein- creasedbycontrollingtheamountofTHFinthehydrate.Theyobserved thatupondecreasingtheamountofTHFto0.15mol%,thehydrogen occupancycanbeincreasedinthelargecages,withco-existenceTHF andH2.TheybasedtheirpostulateonhighpressureRamanspectraat differentTHFconcentrations,andadditionallyconfirmedbyNMR(un- derhighpressure).Also,incontrasttoMaoetal.[4],theyconfirmed thepresenceoftwoH2moleculesinthesmallcages.Aschematicshow- ingtimelineofmajoreventsassociatedwiththeTHFbasedhydrogen clathratesisshownFigure5.
Withaspikeininterestinthefield,multipleattemptsweremadeto repeatthepromisingresultsofLeeetal.[179].However,onlyfewre- portedthefeasibilityofthetuningeffect,withmostsuggestingittobe plainlyuntenable[180–184].Strobeletal.[180]attributedthisaspectto thecompleterelianceonRamanspectratoquantifythehydrogenstor- agecapacity.Tocircumventthisissue,theyusedvolumetricgasuptake measurementandobservedmaximum 1wt.%hydrogenuptake, irre- spectiveoftheconcentrationofTHF.Ramanspectrawereinaccordance withthatofFlorusse etal.[178],withmaximumoneH2moleculein smallcages.Toconfirmtheobservations,Strobeletal.[185]usedanal- ternateapproachinwhichconcentrationofTHFwasfixedat5.6mol%, whereasthepressureofhydrogenwasincreasedto150MPa.Thiswould indirectlybesimilartotheapproachofLeeetal.[179],whereinlieu
Figure4. (a)Methanehydrateunitcellform- ing sI structure and (b) methane molecule enclosed in a large tetrakaidecahedron cage (drawnusingCrystalMaker[177]).
Figure5.Importantmilestonesintheresearch pathofTHFpromotedhydrogenclathrates.
ofreducingtheconcentrationofTHF,thefugacityofhydrogenwasin- creased.Theyfoundthatat150MPaand250K,purehydrogenhydrates showedpresenceofhydrogeninbothsmallandlargecages,whereas thesamewasnottruefortheTHFbasedhydrates,asshowninFigure6 (i).Inthelattercase,hydrogenonlyoccupiedsmallcagesandthelarger onesseemedtobefullyoccupiedbyTHF.Also,Strobeletal.[185]found discrepancyintheapproachofusingtheintegratedareaoftheRaman peakstoquantifyhydrogenmoleculespresentineachcage.Theyob- servedthatpeakareadifferedwhenthespectrawascollectedat77K (0.1MPa)and90K(150MPa).Theyascribedthisanomalytothechange inpolarizationofthehydrogenmoleculeswithinthecages,asthein- ternucleardistancesevengobelowthevaluesobservedforthesolidhy- drogen[185].Theyconcludedthatthereisneedforamoresophisticated techniquetoaccuratelycharacterisetheamountofhydrogenpresentin thecages.Inlight ofthesefindings,Nishikawa etal.[186]performed in-situRamanspectroscopymeasurementsonTHFpromoted(lessthan stoichiometricconcentration)hydrogenhydratesat74.3MPaand265 K.Theyhavereportedthathydrogenmolecules(withuptodoubleoc- cupancy)arepresentinlargecagesalongwithTHF.Theoccupancyin smallcagescouldnotbedistinguishedinthisstudyduetooverlapwith fluidphasehydrogen.Thisledtofurthersupportforthe‘tuningeffect’.
However,themeasurementsystemproposedbyNishikawaetal.[186]
couldnotaccuratelyquantifytheamountofTHFaddedandthuspro- videdonlyqualitativecomparisonacrossearlierworks.
ToinvestigatetheeffectofTHFonthehydrogenstoragecapacityof theclathrates,Liuetal.[188]carriedoutabinitiomoleculardynamic simulations.Theyshowedthatsmallcagesareoccupiedbyasinglehy-
drogenmoleculewhileinthelargecagesoneTHFmoleculeislikelyto coexistwithonehydrogenmolecule.Itwasalsoobservedthatthein- teractionofaTHFmoleculewiththehostwaterframeworkleadstoa slightdistortionoftheclathratenetwork,wherelargecagesshrinkand smallcagesexpand.However,thiseffectisdiminishedasthehydrogen occupancyinthecagesincreases.Theyhavepredictedahydrogenstor- agecapacityonTHFpromotedclathratesintherangeof1.6to3.8wt.%, dependingontheconcentrationoftheTHFpromotor[188].Thisisinac- cordancewiththeexperimentalstudiesofLeeetal.[179]andSugahara etal.[187].Inananotherstudy,Papadimitriouetal.[189]carriedout MonteCarlosimulationsonbinaryTHF+H2hydratesassumingthatthe concentrationofTHFwouldnotinfluencethehydrate’shydrogenstor- agecapacity,inaccordancewithexperimentalobservationsofStrobelet al.[180].Theircalculateduptakeswereverysimilartoexperimentalre- sults,especiallyfromNMRspectroscopy.Theseresultsagreedwiththe previousobservationbyStrobeletal.[180]showingsingleoccupancy ofhydrogenmoleculesinsmallcagesandshoweduptakesverycloseto themaximumassumptionofsingleH2occupanciesinthelarge(THF- occupied)andsmallclathratecages.
The assignment of peaks in Raman spectra for the hydrogen moleculesinlargecageshasbeensubjectedtodebateinliterature.To studythisaspectindetail,Strobeletal.[190]havepreparedsimplehy- drogenhydratesunderapressureof200MPaat250Kandcarriedout ex-situRamanspectrameasurementat77K(afterquenchinginliquid nitrogen). Allthepeakswereobserved tobered-shiftedwithrespect tothefreehydrogen. Highestfrequency Ramanpeakswereassigned tothequadrupleoccupiedhydrogen,middlerangefrequencytotriply
Figure6.(i)Ramanspectraof(a)fluidhydrogen,(b)THF(5.6mol%)+H2 hydrate,(c)pureH2hydrate(adaptedwithpermissionfromStrobeletal.[185], Copyright2007ElsevierLtd).(ii)Ramanspectraofnon-stoichiometricconcentrationofguestmolecules(THF,acetone,CHONE,MCH)measuredex-situat77Kand 0.1MPa(reprintedwithpermissionfromSugaharaetal.[187],Copyright2010AmericanChemicalSociety).
occupiedhydrogen,lowerrangefrequencytothedoublyoccupiedhy- drogen.Thisisinlinewiththeargumentthatasdensityofmolecules inacageincreases,greaterrepulsionbetweenpotentialsurfacestakes place,thusincreasingthevibrationalfrequencyofhydrogenmolecules.
Thisassignmentofpeaksisalsoinaccordancewiththeobservationby Lokshinetal.[191]carriedoutusinghighpressureNeutrondiffraction.
However,Gianassietal.[192]haveproposed adifferentpeakassign- mentundersimilarconditions.Theyhaveassignedthepeakstotriple, doubleandsingleoccupationintheorderofdecreasingfrequency.Their assignmentwasbasedontheintegratedintensitiesofRamanpeaksaris- ingduetopresenceofhydrogenmoleculesinsmallandlargecage.A possiblereasonforthediscrepancyinassignmentofpeaksinliterature canbeduetochangeinRamanscatteringcross-sectionofthehydrogen moleculeswithlocalenvironment.Itisevidentfromthefrequencyshift, observedformoleculespackedinacage,thatthepolarizabilityofhydro- genmoleculesisalsosubjecttochange[190].Atthesametime,athigh hydrogenconcentration,theinteractionofthemoleculeswiththeelec- tromagneticradiationincreases,asisevidentfromthelargerbreadthof thehighfrequencybands.Tounequivocallyexplainthephenomenon, detailedquantificationusingothertechniquessuchasNMRcrosscali- brationisneeded[193].Onthewhole,therestillseemstoagreementto thequalitativeassignmentofRamanbands,withincreasingintensityof Ramanpeaksdemonstratinghigheroccupancyofthecages.
ThesubjectofRamanbandassignmenthasalsobeencomputation- allydealt byPlattner& Meuwly[194]. TheyhaveusedatomisticMD simulationstocalculatethefrequencyshiftsforhydrogenclathratehy- dratesandstudytheeffectoftemperatureandpressureonthespectra.
Ithasbeenobserved thatoptimising thestructuresbefore frequency calculationsleadstooverestimationoffrequencyshifts.Theiroberved frequencyshiftsliewithintherangeofexperimentalpeakmaximaob- servedat4120cm−1(−35)and4148cm−1(−7)underpressureof2000 barat99K.Considerationofquantumdynamicaleffectsleadstoshift infrequencyfortwoH2in512cagesby47cm−1(MP2calculations)at 50K.Theshiftinfrequencyforlargecagesissmallandislessaffected byincreaseintheH2 molecules,whichisalsoinagreementwiththe experimentalresults.Thereasonforthishasbeenattributedtothedis- tancebetweenthehydrogenatominamoleculeandneighbouringoxy- genatominwatermolecule.Insmallercagesthesedistancesdecrease
withincreaseindensityofH2,whereastheseareleastaffectedinthe largercages.Theoveralldifferencesarelesspronouncedathighertem- peraturethanatlowertemperatures.Computationsperformedonperi- odicsystemshelptocapturetheeffectoflatticevibrationsofclathrate molecules.Inotherwords,periodicsystemareabletoquantifytheef- fectofmoleculesatlargedistances.FrequencyfortwoH2moleculesin smallcage512reducesby17cm−1at150Kandincreasesbythesame amountat50Kdueto1000barpressure.Thisconfirmsthatfrequency notonlydependentonthenumberofmoleculesoccupyingacage,but alsoonpressure(moresensitiveathigherpressures).
Inmajorityoftheabovemeasurements,thehydrateswereprepared separatelyinhighpressurecellsandquenchedinliquidnitrogenafter releaseofpressure,forsubsequentex-situcharacterisationusingRaman spectroscopyorX-raydiffraction.Ithasbeensuspectedthatthepeaks observedinRamanspectroscopyduringex-situmeasurementsaredue totheformationofhydratesuponquenchingandnotaspertheforma- tionconditionsproposed[195].Tobringlighttothisanomaly,Grimet al.[195]haveproposedamethodof‘hydrateseeding’inwhichapre- formedhydrateismixedwiththepowderedice,beforebeginningthe processofhydrateformation.Thehydratehadalowerconcentrationof promoter(0.55mol%THF)andwaskeptunderhydrogenpressurea shortertime(~4hours),ascomparedtoareferencehydratewith5.6 mol%THFandhydrateformationtimeof3-4days.Bothsampleswere quenchedinliquidnitrogenfor20minuponreleaseofpressureforex- situcharacterisation.Itwasobservedthatthe‘hydrateseeded’sample showsformationofhydratesatparwiththenon-hydrateseededsample, despitehavinglowerconcentrationofTHF(0.5mol%).ThisledGrimet al.[195]toconcludethatliquidnitrogenquenchingmightleadtounin- tendedgrowthofhydrate,especiallyinthenon-stoichiometricsamples whereasomepartoftheicewasalwaysleftwithoutconversiontohy- drate.Hence,the‘tuningeffect’proposedearliermightbeduetopure hydrogenhydrateformationduringliquidnitrogenquenchingseeded bythesurroundinghydrates,insteadofTHFpromotedhydrates.This wasunliketheproposedmechanismbyLeeetal.[179]andSugaharaet al.[196].
Animportantdifferenceinalltheabovestudieswasthemethodof preparationofhydrates,whichmighthaveresultedindiscrepancyin theobservedresults,asobservedbySugaharaetal.[196].Accordingto
theauthors,therateofhydrateformationwasoneofthekeyfactors hamperingtherealisationof‘tuningeffect’.Todemonstratethis,Suga- haraetal.[196]usediceandsolidTHFunderpressurisedhydrogengas environmentat77Kandslowlyheatedthemixture.Theyobserved3.6 wt.%H2storagecapacityupondecreasingtheTHFconcentrationto0.5 mol%at70MPaand255K.SolidTHFstartsmeltingat164.7Kandits highmobilityprovidesrapidrateofhydrateformation.Thiswasinline withthe‘tuningeffect’andconfirmedtheobservationsofLeeetal[179]. ThistheorywasalsosupportedbyLokshinetal.[191],wheretherate ofhydrateformationupondirectreactionwithicewas100timesfaster thanthatwithwater, inthetemperaturerangeof 77-273K.Onthe basisofthisevidence,Sugaharaetal.[196]proposedthatduringhy- drateformation,anintermediatemetastablestructureisformedwhich favoursencapsulationofhydrogenmoleculesinthelargecages.How- ever,this‘tuningeffect’isnotobservedwhenthepressurisedhydrogen isincontactwiththeaqueoussolutionofTHF,aswasthecaseinthe experimentscarried outby Hashimotoetal[182]. Arecentstudyby Zhongetal.[197]hasshownthatitmaybepossibletotraptwohydro- genmoleculesinthesmallcagesofTHFpromotedhydrates,leadingto hydrogenstoragecapacityof3.08wt.%undermoderateconditionsof 3.8MPaand273K.However,thehydrateformationwaslimitedtothe surfacelayeroflessthan0.5mminthickness,beyondwhichthehy- drogenoccupancydecreasedtosinglemoleculeinatimeframeof24 hours.ControversysurroundingthehydrogenstoragecapacityofTHF promotedhydrogenhydrateshasrefusedtodiedown,withfewrecent studiesstillnotfullyconvincedofthehighcapacityachievedusingthe
‘tuningeffect’[198–200].
5. SecondaryGuestMoleculesPromotedHydrogenClathrates
Asseenin theprevious section,binary hydrates consistof asec- ondaryguestmoleculealongwithhydrogenwhichstabilisesthestruc- turedue toitshigher affinityforthehost watermolecules.Detailed studycarriedoutbyBelosludovetal.[176]suggestedasetofcriteriafor selectionof‘ideal’promotermoleculeforformationofbinaryhydrate withlowpressureandhighhydrogenstoragecapacity.Therequisite criteriastatethat:(a)thesizeofthepromotershouldbesimilartothe cavitysizeofthesmaller(512)cage,(b)itshouldbeabletoformboth s-Iands-IIhydratesinpureformand(c)amongtheabovetwostruc- tures,s-Ishouldbemorestable[176].Manyotherpromotershavebeen exploitedfortheadvantageofhydrogenstoragepropertiesinclathrates andformaimportantfoundationforthefuturedevelopments inthe field.
5.1. Hydrocarbons
Amongthegaseousadditivesorco-guests,hydrocarbonshavere- centlybeenexploredassecondaryguestsinhydrogenclathrates.Hydro- carbons,atthesametime,alsoprovideanadditionalenergymedium, asopposedtohydrophobicpromoterssuchTHFwhichprovidenoaddi- tionaladvantage.Purehydrocarbonssuchasmethaneformgashydrates underfarmildconditionsthanpurehydrogenhydrates.Forexample, propaneformshydrates undermildpressure(0.2to0.5MPa)inthe temperaturerangeof274.15to278.15K[201].Methanehasbeenthe mostsoughtafterhydrocarbonwithlowestweightpenaltyonthehydro- genstoragecapacity[202].Moreover,ithasbeenpostulatedthatsince methanehasasmallersizethanTHF,hydrogenmaybeaccommodated inthelargecagesalongwithmethaneandthusenhancingthehydrogen storagecapacity.EarlyinvestigationintotheH2-CH4-H2Osystemwere carriedoutbySkibaetal.[203],usingagasmixtureof40%H2and 60%CH4 indirectcontactwithicetoformhydratesat20MPaand 259K.However,theyfailedtoobserveanyevidenceofhydrogeninthe formedhydrates;methaneoccupiedbothsmallandlargecagesforming puremethanehydrates.Followingthis,Skibaetal.[204]reportedfor- mationofpropanepromotedhydrogenhydratesat24barand259K, withsingleoccupancyofhydrogenmoleculeinsIIstructure.Thisledto
manytheoreticalandexperimentalstudiestoinvestigatetheadditionof hydrocarbonaspromoterinhydrogenhydrates[205,206].
5.1.1. Methane
Matsumotoetal.[202]haveshownthatwhenaddedinconcentra- tionof5mol%(gasphase),methanereducesthepressureofhydrogen clathrateformationto50MPaat263K.However,underthesecondi- tions,sIstructureisformedwithsinglehydrogenmoleculeoccupying thesmallcageandtwohydrogenmoleculesoccupyinglargecage.The hydrateundergoesphasetransformationuponfurtherincreaseinpres- sureto70MPa.ThenewphasewithsIIstructurenucleatesattheendof 5h,takingaslongas24hoursforthetransformationtocomplete[202]. Theplotshowingchangeinstructurewithpressureandcompositionof methaneinthegasmixtureisshowninFigure7(i).Ithasbeenproposed thatformationofmetastablesIstructureactsasseedforthenucleation ofthesII.ThestructuresIIshowssingleoccupancyofsmallcagesand quartetoccupancyforlargecagesat263Kand70MPa,asconfirmed byin-situRamanspectroscopy[202].Since,itwasnotpossibletoesti- matethehydrogenstoragecapacityfromtheexperimentaldata,ther- modynamicmodellingusingderWaalsandPlatteeuw(vdWP)theory wasused.ItwasfoundthatstructuresIstores0.02wt.%H2,whereas thestructuresIIstores0.31wt.%H2202.Thehydrogenstoragecapacity ofthesehydratesremainedfarbelowthepracticalrequirements.
Toexploretheadditionalpossibilityofincreasingthecapacityand theoreticallyestimatetheamountofhydrogenthatcan beentrapped inthesehydrates,computationalmodelweredevelopedbyBelosludov etal.[176].Theystudiedthermodynamicstabilityandcageoccupancy of thebinaryCH4+H2hydrateswithboths-Iands-IIstructureusing vanderWaalsandPlatteeuw(vdW-P)theoryundervarioustempera- turesandpressures.Ithasbeenobservedthat~6%gasphaseconcen- trationofmethanecan stabilises-Istructureat250Kwithhydrogen storagecapacityof1.75wt.%,butunderextremepressureconditionsof 200MPa.However,structures-IIisstableat250Kand70MPa,with ahigherhydrogenstoragecapacityof 2.6wt.%. Whilethehydrogen storagecapacityobtainedbyexperimentationweresmaller,thisfuelled furtherinterestintheinvestigationofhydrocarbon-basedhydratepro- moters.ThestabilityofCH4+H2hydrateshasbeenevaluatedbyZhang etal.[207]usingmoleculardynamicssimulations.Theyhavereported co-occupancyofhydrogenandmethaneinthelargecages,withupto threehydrogenmoleculespresent.
5.1.2. PropaneandButane
Propelledbyinterestincomputationalandexperimentalstudiesin methane promoted hydrogenhydrates, higher hydrocarbons suchas propaneandbutanewereexaminedaspromotermolecules.Veluswamy et al.[208] have reported thataddition of 9.5mol%propane tohy- drogenbringsdowntheequilibriumpressureto1.53MPaat274.2K (Figure7(ii)).However,sluggishkineticsremainsadrawbackwithap- proximately16hoursrequiredat8.5MPaforclathrateformation.This canbedrasticallyreducedto30minbyadditionofsmallamounts(500 ppm)ofSDStothepropaneandhydrogenmixture[209].Inspiteofthe benefit,theirhydrogenstoragecapacityofstillremainslowat0.032 wt.%[208].Koh etal.[210] haveshown promisingresults byexperi- mentally investigatingpropane andisobutane asguestmolecules.As thestructures-IIconsistof16smallcagesvisavis8largecages,asmall changeinthehydrogenoccupancyinsmallcagefromsingletodouble, leadstosignificantincrease(~1-2wt.%)inoverallhydrogenstorage capacityofhydrate.Keepingthisinview,sIIhydratestructurewithsto- ichiometric(5.56mol%)andnon-stoichiometric(1mol%)amountsof propane warecompared intermsoftheirhydrogenoccupancy.They havereportedthatabove30MPaat243K,stoichiometricamountof propanehelpstoexpandthelatticeby~1%(3%byvolume),ascon- firmedbysynchrotronhighresolutionXRD.Thisleadstohydrogenstor- agecapacityof2.29wt.%forsIIhydrate[210].Thisassumptionhasbeen verifiedbyGCMCsimulationsusingdifferentpromoterssuchasTHFor
Figure7. (i)Hydratestructuresatdifferentpressureandcompositionofmethane(inH2+CH4gasmixture)at263Kandt=72h(circlesrepresentsIhydrate, squaressIIhydrate,diamondssimplemethanesIhydrateandtrianglesnohydrateformation,reprintedwithpermissionfromMatsumotoetal.[202],Copyright2014 AmericanChemicalSociety)(ii)Phaseequilibriumplotsofpropanepromotedhydrogengashydrates.Similarplotsforpurepropaneandpurehydrogenhydrates havebeenoverlaidforclarity(adaptedwithpermissionfromVeluswamyetal.[208],Copyright2015AmericanChemicalSociety).
iso-butane,andtheresultsremainconsistentaslongasthelatticeex- pansionofsimilarmagnitudeisobserved[210].
Thisstudyhighlightstheimportanceofcagedimensionsintuning thehydrogenstoragecapacityoftheclathratehydrates.Ramanspec- troscopyfurtherconfirmsthatpropanemoleculesoccupylargecages, thusleavingthesmallcagestobefilledwithtwohydrogenmolecules.
However,whentheconcentrationofpropanewasdilutedto0.5or1 mol%,itwasfoundthatuptofourhydrogenmoleculescouldbeac- commodatedinthelargercages,withsingleoccupancyinsmallcage.
Thisincreasesthehydrogenstoragecapacityto3.84wt.%[210].How- ever,inboth thecases, theoccupancyofhydrogenin smallcagesis dependentontemperatureofmeasurement,sinceRamanandHRXRD measurementswerecarriedoutex-situat77 K.Kohetal.[210]have observedthatupongradualincreaseintemperature,doubleoccupancy insmallcagestartstodiminishat90K,withsinglehydrogenmolecule frequencydisappearingat160K.Similartrendhasbeenobservedfor thedeuteriuminthelargecages(simplehydrogenhydrates),fromfour H2moleculesat80Ktoloweringdowntotwoat160K.[211]
Figure8and27
Inlinewiththeexperimentalstudieshighlightingtheimportanceof dimensionof thecageontheclathrateformation,Lietal.[213]have usedhighlevelquantumchemicalmethodsforinvestigatingthesame.
Theyhavefoundthatthebondenergybetweenhydrogenmoleculesin aclusterisasensitivefunctionofthesidelengthofhydratecages.The bondenergydecreasessharplywithincreaseinthecagedimensionto 2.84or2.86Ǻ forsmallcage(2.82Ǻ withouthydrogen),suggestinga possibilityofincorporationofupto4ormorehydrogenmolecules.This leadstoahypothesisforstabilityofhydrogenmoleculesuponincor- porationofpromotermolecules.Promotermolecules(suchasTHFor TBAB)occupythelargecagesandinturnhelpinincreasingtheedge lengthoftheneighbouringsmallcages.Oneoftheimportantrequire- mentsofestimationofhydrogenstorageinclathratesbysimulationsis theaccuratedeterminationoftheinteractionenergies.Thisaspectbe- comesevenmoresignificantforweakinteractionssuchasthosebetween thehydrogenmoleculesinacluster.Foraccurateresults,Lietal.[213]
haveusedCCSD(T)leveltocalculatetheseinteractions.Atthesame time,toaccuratelycarryoutQMcalculations,largebasissethavebeen usedbyLietal.[213]whichincludepolarization,diffusionandfloating functions.Animportantworkhighlightingtheimportanceoflatticepa- rameteronthehydrogenstoragecapacityofsIIstructurehasbeendone
byPapadimitriouetal.[214].UsingGCMCsimulations,theyhavefound thatincreaseinlatticeparameterof3%couldleadtoincreaseinthehy- drogenstoragecapacityby13%,forthemeasurementsinthepressure rangeof100-300MPa.However,thiseffectisseenpronouncedonlyin thelargecages,withthesmallercagesremainunaffectedbythechanges inlatticeparameter.Atthesame,thenumberofsmallcages(singlyoc- cupiedhydrogen)andlargecages(with2-4hydrogen)decreasedwith increaseinlatticeparameter[214].Boththesestudiesfurthersupport theexperimentalobservationsreportedbyKohetal.[210].
5.1.3. MixedHydrocarbons
Toexplorefurtherscopeofadditionofhydrocarbontothehydrogen clathrate,methaneandpropanemixturehasbeenusedasapromoterby Ahnetal.[212].Theyobservedhydrogenstoragecapacityof1.78wt.%
undermoderateconditions(9MPaand250K).Animportantaspectof this workhasbeenthedirectreaction ofpowderedicewithmixture of hydrogen,methaneandpropanegas.Whereas,thereactionof the hydrogenwithpre-formedhydrates (ofmethaneandpropane)under similarconditions,failedtoproducethedesiredresults.Thecomposi- tionofthegasmixturealsohadadirectimpactonthetypeofstructure andsubsequentlythehydrogenstoragecapacityofthehydrates.Higher concentrationofmethane(90%)inthefeedgasproducedsIIstructure withhighercapacity(1.7wt.%H2),whereaslowerconcentration(70%) leadstotheformationofsIstructure(0.67wt.%H2)[212].Ex-situRa- manspectroscopycarriedoutat77Khasrevealedadoubleoccupancy ofsmallcagesaswellaspossibilityofdoubletotripleoccupancyfor largecages.Thisisthefirstdirectevidenceofdoublehydrogenoccu- pancyinsmallcagesfoundusingRamanspectroscopy,althoughNMR spectroscopyhasprovidedsuchevidenceinthepast[179].Theblueshift observedforthedoubleoccupancyinthesmallcageshasalsobeencon- firmedbycomputationaltechniques[194]. Ithasbeensuggestedthat duetothesmallsizeofmethaneascomparedtootherhydrocarbons,it occupiessmallcagesofhydratestructuresandisfurthersubstitutedby hydrogeninasocalled‘replacementreaction’[212].Ahnetal.[212]also observedthatdirectlyformedclathrateshad~0.5%largeredgelength ascomparedtoindirectlyformedones,thusconfirmingthepreviously stipulatedroleofthecagedimensionuponthehydrogenoccupancyby Kohetal.[210]andLietal.[213].
Figure8. (i)H–HvibronregioninRamanspectraofthe(A)sIhydrates(CasesIandII)and(B)sIIhydrates(CasesIIIandIV)showingthedirectevidenceofmultiple H2occupancies.(ii)Aschematicshowingthetransformationoficephasetothehydratestructureincaseofdirectreactionoficewiththegasmixture.(Reprinted withpermissionfromAhnetal.[212],Copyright2020ElsevierLtd).
Figure9. MolecularstructuresofTHF,acetone,i-PAandn-PA(bluecirclerep- resentsthehydrophilicgroupintheorganiccompound).ReprintedfromPark etal.[217]withpermissionfromthePCCPOwnerSocieties.
5.2. AlkylAmines
Alkylaminescomeunderarareclassofadditiveswhichtypically formsemi clathrates,withdirectional hydrogenbonding instoichio- metrichydrates.However,ithasbeenobservedthattheymayundergo structural transformationuponthe incorporation assecondary guest moleculessuchasmethane[215,216].Toinvestigatethisindetail,Park etal.[217]haveemployediso-propylamine(i-PA)andn-propylamine (n-PA)asguestinhydrogenclathratesandexaminedtheirbehaviourat concentrationrangingfrom1to13.3mol%.Ithasbeenobservedthat i-PAclathrateswithconcentrationbelow5.6mol%displayedsIIstruc- ture,withpossible‘tuningeffect’observedinthesehydrates.Itwasseen that1mol%i-PAshoweddoubleoccupancyofsmallcagesandtriple occupancyoflargecages(confirmedusingex-situRamanspectra),un- likethehigherconcentration(i-PA=5.6mol%)hydrates.Thisamounts tohydrogenstoragecapacityof3.08wt.%at40MPaand243K.How- ever,similarphenomenonwasnotobservedinanotherisomerofpropyl amine(n-PA),withsingleoccupancyofhydrogenmoleculesreportedir- respectiveofconcentrationofn-PAmolecules.Crystalstructureofn-PA clathrateswasindexedtobe monoclinic,withnotransformationob- servedwithchangesintheadditionalparameters(suchasconcentra- tion,temperatureandpressure).Parketal.[217]haveascribedthisto thepresenceofhydrophilicgroupinthemiddleoftheringasini-PA,un- likeattheendoftheringpresentinn-PA.Thesameisalsoapplicableto additivessuchasTHFandacetone,whichhaveshownfavourablehydro- genabsorption.ThisisshowninFigure9,highlightingthehydrophilic group.Thiswassignificantasfewstudieshavebeenreportedwhichex-
aminethestructuraltransformationinthesesystemsandthisopensnew vistasforexplorationofsuitablehydrogenstoragematerial[218].
5.3. InertGases(ArandN2)
Diatomicgaseshavealsobeenutilisedaspromotersforthehydro- genclathrates.Amanoetal.[219]haveinvestigatedhydrogenhydrates promotedbyArgasusingin-situaswellasex-situRamanspectroscopy measurements.Theyhaveobservedhydrogenoccupancyinlargecages aswellassmallcagesat27MPaand276.1K,with37mole%H2present intheAr-H2gasmixture.Thesameresultswereobtainedbyex-situRa- manspectroscopymeasurementswithuptofourH2moleculesobserved inlargecagesandonemoleculeinsmallcages.Anotherideaofusing N2asaguestintheclathratewasfirstfloatedbyLuetal.[220]in2012.
Theyreportedformationofclathratesundermildconditionsof243K and15MPa,withhydrogenreportedtobe presentinbothsmalland largecages.Althoughtheycouldnotproducehomogenousanduniform samplesintheirexperiments,hydrogenoccupancyresultswereclose totheearlierreports onpurehydrogenhydrates,withtwohydrogen moleculespresentin smallcageandfourhydrogenmoleculesin the largecage[220].
Tostudythepotentialofnitrogenasaguestinhydrogenclathrate hydrates,Liuetal.[221]haveusedabinitioandstandardmoleculardy- namicscalculationsintheirwork.TheyobservedthatreplacingN2with H2isthermodynamicallyfavourableinsmallcages,whereasthelarge cagesfavourcoexistenceofthebothmolecules.Thisreflectstheclose similaritywiththeexperimentalobservationsofParketal[222].Liuet al.[221]observedpresenceof2L-1Sasstableconfigurationfromther- modynamicscalculations, whereas 3L-2Sconfiguration wasfavoured underdynamicconditions,at243Kand15MPa.Thisleadstotheo- reticalhydrogenstoragecapacityof2.5wt.%forthefirstcase(thermo- dynamicallyfavourable)and4.4wt.%forthesecondcase(dynamically favourable)[221].
5.4. MixedPromoters(InertGasandSolid/Liquid)
Parketal.[222]proposedadditionofsolidadditives(hydrateform- ing THF& non-hydrate forming PRD)tothe existingN2 as a guest moleculeat35MPaand243K.Theadditionof1mol%THFwithN2
showedimprovedhydrogenstoragecapacityof2.24wt.%,withdou- bleandquartetoccupancyinsmallandlargecages.Thiswasattributed tothe‘exchangemechanism’betweennitrogenandhydrogen,inwhich hydrogenreplacesthelargerdiameternitrogenmolecule,thusallowing forreductioninthecagedimensions.SynchrotronXRDdatashowedthe latticeparameterdecreaseduponincreasingtheamountofhydrogenin thecages,confirmingthestipulated‘exchangemechanism’.Also,itwas observedthat‘exchangemechanism’wasdominantinsmallcages,with largecagespreferringco-existenceofN2andH2 moleculesforstabil- ity[222].Thiswasevidentfromhigherhydrogenoccupancyseeninthe largecagesforthePRD+N2sampleswhenthepartialpressureofhydro- genwasreducedto15MPa,withremaininggasbeingN2(totalpressure of35MPa)[222].
5.5. GreenhouseGases(SF6andCO2)
Consideringthefactthatalmostallthestudiesdonepreviouslyhad formation>10MPa,Leeetal.[223]suggestedtheuseofSF6asapro- motermolecule,whichformshydratesevenunderatmospherepressure below273K.Thiswassignificantasitposedadoubleadvantage:bring- ingdownthehydrogenhydrateformationpressureaswellascaptur- inganextremelypotentgreenhousegas,SF6,withimpactontheatmo- spheregreaterthan23900timesthanCO2.MolecularsizeofSF6isbig enough(~6Ǻ)tooccupythelargecagesof thesIIstructure,leaving thesmallcagesemptyforhydrogen.Theirstudyrevealsformationof metastablestructure,undertheformationconditionsof0.5MPaand 263K,withsinglehydrogenmoleculepresentinthesmallandlarge cages[223].However,thepresenceinlargecageislostwiththepassage oftime,coupledwithreductioninthenumberofsmallcagesoccupied.
TheintensityofRamanbandshasbeenobservedoveraperiodof6days withsignificantdropseenintheinitial2-3days.Itisexpectedthathy- drogenmoleculesdiffusethroughthelargecagesuntiltheyfindtheyan emptysmallcage.Althoughitdoesnotrepresentasignificantdevelop- mentwithrespecttothehydrogenstoragecapacity(0.014wt.%),itre- flectsimportantdevelopmentinexplorationofnewpromotermolecules withanabilitytoco-hostthehydrogeninthelargecagesnearambient temperatureandpressure[223].
InordertostudytheoccupationofhydrogeninsIclathrates,Grimet al.[224]usedsIhydrateformers(CO2andCH4)assecondarygueststo hydrogenhydrateswithsIstructure(258Kand70MPa).Thishasbeen possiblebycarefullycontrollingtheconditionstobepresentinthesI phaseregionofthephasediagram.Theyhaveobserved,inaccordance withthesIIstructures,hydrogenstartsfillingwithsmallcages.How- ever,presenceofuptotwohydrogenmoleculesinlargecageshasbeen observedusingRamanspectroscopyalongwithCH4/CO2at70MPa,as showninFigure10(ii).Uponincreaseofpressureto130MPa,noextra peaksappearinthespectrum.Thisisunlikethebehaviourobservedin thesIIstructure,wheretheoccupancyoffourhydrogenisobservedat 140MPa.MonteCarlosimulationstudiesonthehydrogenoccupancyin sIstructureshavereportedthat51262largecagescanstoreuptothree molecules,which isin closeagreementtotheexperimentalobserva- tions[225].Theoreticalestimatesshowthathydrogenstoragecapacity ofthesestructuresiscloseto3.2wt.%,whichiscomparabletotheTHF promotedhydrogenclathrates.
5.6. Tertiaryalcohol
Tothrow more light on the phenomenon observed by Prasad et al.[226],Tanabeetal.[227]studiedphaseequilibriumforthe(t-BA)- H2O-H2system.Theyhaveobservedthatintheabsenceofhydrogen, t-BAoccupieslargecagesofthesVIhydratestructureandnostructural phasechangesareobservedupto112MPaatafixedconcentrationof9.3 mol%t-BA.However,whenhydrogenisaddedtothesystemat25.3MPa and274K,ittransformstosIIstructure.Thetransformationpressurere- ducesto2.35MPaand267Kiftheconcentrationoft-BAisreducedto
5.6mol%,asshowninFigure10[227].Notably,thestoichiometriccon- centrationhasalowertransformationpressurethennon-stoichiometric one. AsseenfromtheRamanSpectra,nohydrogenoccupancyisob- servedbelowthetransformationpressureinthesmall4454cagesofsVI hydrate.InthesIIstructure,hydrogenwasseentooccupysmallcages.
This showsthatthesmallercagesofsVIstructurearenotfavourable fortheoccupationofhydrogen.ThisledTanabeetal.[227]topropose thatpressurealonecannotbeadrivingforceforthestructuraltrans- formationandpresenceofsmallerguestmoleculessuchasmethaneor hydrogenwasessentialforit.
5.7. Organiccompounds
InexaminingthestabilityofsVIstructureforthehydrogenencap- sulation,Prasadetal.[226]studiedthehydrogenoccupancyintheH2- H2O-(t-BuNH2)system.Ithasbeenobservedthatpuret-BuNH2hydrate (5.86 mol%)formsasVIstructureat 250K,withlatticeparameter of 18.81 Ǻ [228]. However, atransformation tosIIis seen undera hydrogenpressure of13.8MPaat250 K.Prasadetal.[226] claimed thatconsideringthesizeofhydrogenmolecule(2.72Ǻ)withthesizeof thesmaller4454cage(5.8Ǻ),itwasexpectedthathydrogenmolecules wouldbesmallenoughtofitintothesecages.Thisjustificationisbased on thepreviousstudyofCH4 +(t-BuNH2)hydrates,wheremethane (4.36Ǻ)wascloseinsizetothesmallercavityof4454cage,butdidnot stabilizesVIstructure.ThisledtophasetransformationfromsVItosII wherethesmall512cageswereoccupiedbymethanemolecules[229]. However,thesimilarphenomenonwasobservedinthecaseofH2+(t- BuNH2)hydrates,whereevidencesupportedthetransformationtosII structureandoccupancyofhydrogeninsmallercages.Prasadetal.[226]
perceivedthistobepressureinducedstructuraltransformation,inspite oftheavailabilityoftheappropriatecagesizes.Prasadetal.[226]fur- thermeasuredthehydrogenstoragecapacityofthehydrateusingvol- umetricgasreleasemeasurements,andtheyfound0.7wt.%H2present inthehydrate.Thiswastoolowtobepracticallyutilised.
ConsideringthetheoreticalhydrogenstoragepotentialofsHstruc- tures of clathrates to be higher than 5.6 wt.%, Strobel et al.[230]
have usedthewell-known sHhydrateformerslikemethyltert-butyl ether(MTBE),methylcyclohexane(MCH),2,2,3-trimethylbutane(2,2,3- TMB),and1,1-dimethylcyclohexane(1,1-DMCH)asco-gueststoform hydrogenhydratesat150MPaand275K.TheformationofsHstruc- turewasconfirmedbyXRDmeasurementswithpeaksindexedtospace grouphexagonalcrystalsystem(p6/mmm)havinglatticeparametera= 12.203Ǻ andc= 9.894Ǻ.Amongtheco-guestsused, MTBEshowed highestrateofhydrateformationduetoitsenhancedice-wettingproper- tiesandlargesurfacearea.Itwasproposedthatanypromotermolecule withsizelarge enoughtostabilisetheicosahedral 51268 large cages couldleadtotheformationofthesHstructureundersuitablethermo- dynamicconditions.Duetothesimilarsizeof512and435663cagesin thesHstructure,theRamanbandsof thehydrogeninthetwocages overlappedwitheachother,thuscreatingdifficultyinidentificationin eachcage.BycomparingwiththeearlierstudiesofTHF/H2hydrates, itwasconcludedthatH2ispresentinboththesmallercagesofthesH hydrates[180].Variousotherliquidhydrocarbonpromoterswereused fortheformationofsHstructureinthesubsequentstudiesbyDurateet al.[231,232],withpredictedhydrogenstoragecapacitiesreachingupto 1.4wt.%.
InabidtoenhancethehydrogenstoragepropertiesofsHhydrates, Kohetal.[233]appliedastrategytoencapsulatehydrogeninlargecages alongwithawatersolubleco-former.Atthesametime,theyalsoinves- tigatedthe‘tuningeffect’proposedbyLeeetal.[179]toincreasethe occupancyof hydrogeninthese cages.Forthis purpose,theyuseda watersolublesHco-former(1-methylpiperidine),whichhadshownsu- periorperformanceearlierinthecaseofmethanehydrates[234].They observedthatuptoaconcentrationof2mol%1-methylpiperidine,sH hydratesareformedwithsingleoccupancyofhydrogenbothsmalland
