Supersaturated-Zincate-Solutions

TECHNICALPAPERSELECTROCHEMICALSCIENCEANDTECHNOLOGYSupersaturatedZincateSolutionsAStructuralStudyCatherineDebiemme-Chouvy~andJacquesVedel*Laboratoired'Electrochimie(unitdassoci~eauCNRS),EcoleNationaIeSupdrieuredeChimiedeParis,75005Paris,FranceMarie-ClaireBellissent-FunelLaboratoireLdonBrillouin,CENSaclay,91191Gif-sur-YvetteCedex,FranceRobertCortesL.U.R.E.,Universitdd'Orsay,91405Orsay,Fra~ice4ABSTRACTUndersaturatedandsupersaturatedzincatesolutionshavebeenpreparedelectrochemicallyandchemically,andstud-iedusingnuclearmagneticresonance(NMR)spectrometryandextendedx-rayabsorptionfinestructure(EXAFS).67ZnNMRspectroscopyshowsonlyoneresonanceline,theareaofwhichfirstincreaseswiththeZnconcentrationuntilsaturaotionandthenremainsconstantforhigherconcentrations.EXAFSshowsfouroxygenneighborsatadistanceof1.979Aofeachzincatom.Identicalresultswereobtainedwithundersaturatedandsaturatedzincatesolutions.NoZn...Zncorrelationwasfoundindicatingthereisnopolymericspeciesinthesupersaturatedsolutions.Itisproposedthatinzincatesolution(KOH8M)thesupersaturationisduetothepresenceofoxodihydroxozincate(ZnO(OH)~-).IntroductionTheanodicoxidationofzincelectrodesinconcentratedpotassiumhydroxidesolutionsthatoccursduringthedis-chargeofalkalinezincbatteriesleadstoelectrolyteswithaZn(II)contentmuchhigherthanthatobtainedwhenasolidZnOisequilibratedwithKOHsolutionsofthesamecon-centrationatthesametemperature.ITheZn(II)excessslowlyprecipitatesasZnOfromthesesupersaturatedsolu-tions,disturbingtheworkingofthebatteries.Thesesolu-tionsalsocanbepreparedbyaslowadditionofwatertoamoreconcentratedZn(II)solutionmadebydissolvingZnOinhotandveryconcentratedKOHsolutions.2Itisgenerallybelievedthatthepredominantzincatespe-ciesintheundersaturatedsolutionistetrahydroxozincateZn(OH)~.SupersaturationisattributedtothepresenceinthesolutionsofZn(II)speciesformedatthetimeofthedissolutionandthatafterwardremaindissolvedinametastablestate.Severalhypotheseshavebeenproposedconcerningtheoriginandnatureofthesespeciesthatleadtodifferentdecompositionreactions.1.ThespeciesareformedbyfixationofaprotononthetetrahydroxozincateZn(OH)~-.3-5WecallthemtypeIspe-cies.Theformationreactionsare[Zn(OH)4]2-+H§--~[Zn(OH)3(H~O)]-[i][Zn(OH)~(H20)]-+H§--~[Zn(OH)2(H20)z][2]withthefollowingdecompositionreactions[Zn(OH)2(H20)2]-+ZnO+3H20[3]*ElectrochemicalSocietyActiveMember.aPresentaddress:Universit~VersaillesSaint-Quentin-en-Yve-lines,Laboratoired'EleetrochimieetChimiedeSolidesInor-ganiques,78035VersaillesCedex,France.J.Electrochem.Soc.,Vol.142,No.5,May19959TheElectrochemicalSociety,Inc.[Zn(OH)3(H20)]--~ZnO+OH-+2H20[4]2.Thespeciesarepolymericandtheyareformedbycon-densationoftwo(ormore)tetrahydroxozincateanions.2'6TheyarecalledtypeIIspecies2[Zn(OH)4]2--~[(OH)3--Zn--O--Zn--(OH)3]4-+I-I20[5]andtheydecomposeasfollows[(OH)3--Zn--O--Zn--(OH)3]4--+2ZnO+4OH+H20[6]3.ThespeciesareformedbyhydrationofdioxozincateZnO~-7(typeIIIspecies)[ZnQ]2+H20--~[ZnO(OI-I)2]2-[7][ZnO(OH)2]~-+H20--~[Zn(OH)4]2-[8]anddecomposefollowingreactionslikeZnO~-+H20-~ZnO+2OH-[9][ZnO(OH)2]2--~ZnO+2OH-[10]Inapreviouspaper7westudiedthedecompositionkinet-icsofsupersaturatedzineatesolutionandshowedthat1.TheratioofprecipitatedZnO/releasedOH-remainsunchangedforallsupersaturatedsolutionsandequalto2whichisalsotheratioofdissolvedZnO/OH-neededtodissolveitobservedduringtheformationofundersatu-ratedsolutionsbyequilibrationatroomtemperatureandduringtheformationofsupersaturatedsolutionsbychem-icalorelectrochemicalrouteChemicalroutezno+2OH---~[ZnO2(H20)~]2-+(1-x)H~O[11]1359Downloaded05Mar2011to202.197.63.201.RedistributionsubjecttoECSlicenseorcopyright;see9TheElectrochemicalSociety,Inc.ElectrochemicalrouteZn+2OH-+xH~O--+[ZnO2(H20)~]2-+H21[12]withx=0orlor22.Thedecompositionoccursfollowingsimultaneousho-mogeneousandheterogeneousreaction;thedecompositionisanautocatalyticreaction.TheZnOformedactsasasourceofnucleationcenters.3.Thesereactionsareofthesecondordervs.thespeciescausingthesupersaturation.WeconcludedthatonlytypeIIandtypeIIIcompoundscouldbeconsideredasspeciescausingthesupersaturation,reactions3and4leadingobviouslytodifferentvaluesoftheZnO/OH-ratio.Nothinginthekineticstudyenabledustodecidebetweenthosetwopossibilities.Theanswermaycomefromstructuralstudies.Previ-ously,thiskindofstudyhasbeenperformedessentiallybyRamanandinfrared(IR)spectroscopy,3'5-13andproton5'~'15and67ZnNMRspectroscopy3~Concerningtheundersatu-ratedsolutions,mostauthorshaveconcludedthat[Zn(OH)4]2-speciespresentsatetrahedralsymmetry(sym-metrygroupTd).Thestudiesonsupersaturatedsolutionsarelessnumerous.Someauthorsreportabreakintheevo-lutionofacharacteristicpropertywhentheZn(II)concen-trationpassesthesaturationvaluebutsomeoftheseobser-vationsarecontroversial.Suchisthecase,forexample,forthoseofBriggsetal.~whoreportedslopechangesinthevariationoftheprotonchemicalshiftevolutionfor[Zn(II)]concentrationsof1.25and3.10Min10MKOHsolution,resultswhichwerenotobservedbyNewmanandBlom-gren'5andbyVanDoorneandDirkse.~However,itisgen-erall