P

Parallelplateandframecell481●PPPackedbedelectrode—Astaticthree-dimensionalelectrodeconsistingofarestrainedbedofelectroni-callyconductingparticlesincontinuousintimatecon-tact.PackedBedElectrodes(PBEs)presenthighelec-troactiveareaperunitelectrodevolumeandmoderatelyhighmasstransportcharacteristics(thelimitingcur-rentataPBEmayexceedtimestheoneobservedatatwo-dimensionalelectrodeofthesamevolume).Manyconfigurationdesignsarepossibleforcellsus-ingPBE,dependingupon:theelectrolyteflowcondi-tions(trickleorflooded);theelectricalcurrentfeed-ers/collectorsconnectionstothebed(monopolarorbipolar);therelativedirectionsofcurrentandelectrolyteflows(parallelorperpendicular);thecounterelectrodetype(two-orthree-dimensional);theelectrodegeome-try(cylindrical,rectangular,orprofiled,wherethecross-sectionareaand/ortheparticles’sizesarechangedalongthecell).PBEcanbeusedforthedestructionoforganiccom-pounds,regenerationofhexavalentchromium,produc-tionofhydrogenperoxide,buttheirmajorapplicationisinmetalremovalfromdilutesolutions.Inthissitua-tion,thechoiceofsmall-sizedparticlesistemptingfortheincreaseofprocessefficiency;nevertheless,acom-promisemustbeconsideredsinceveryfineparticlesalsodecreasethebedpermeabilitycausingproblemstotheelectrolyteflow.Toimprovetheporosityandalsotheioniccon-ductivity,itisusefultoformulatethebedbymixingtheelectronicallyconductingparticles(graphiteormetallicgranules)withionicconductingones(forexample,ionicresinbeads).Inextremelyadverseoperativeconditions,highcurrentdensityandlowbedvoidance,thePBEmaybecompletelypluggedwithmetalintheirinterstices.Torecoverthedepositedmetal,thepackedbedcanbere-movedfromthecellbyvacuumingorsubjecttodissolu-tioninasmallvolumeofelectrolyteforobtainingacon-centratesolution.TheprincipaldrawbacksofPBEare:possibilityofchanneling;unevendistributionoftheelectricfields;needforelectricalcurrentsnearthelimitingvaluetoachievelargeconversionyield;efficiencyloweringwiththeincreaseinbedthickness.Refs.:[i]WalshFC()Afirstcourseinelectrochemicalengineering.TheElectrochemicalConsultancy,Romsey,Hants;[ii]PletcherD,WalshFC()Industrialelectrochemistry,ndedn.ChapmanandHall,Lon-don;[iii]RajeshwarK,IbanezJC()Environmentalelectrochemistry,fundamentalsandapplicationsinpollutionabatement.AcademicPress,SanDiegoLMAPalladiumelectrodematerialsParaffinimpregnatedgraphiteelectrode(PIGE)elec-trodepreparedfromgraphiterodsbyimpregnationwithmeltedparaffinundervacuum.Theseelectrodesarenotpermeatedbyaqueoussolutionsandcanbeusedforso-lutionstudies,aswellasforimmobilizingmicroparticlesandmicrodropletstostudytheirelectrochemistry.Seealsocarbon,voltammetryofimmobilizedmicropar-ticles.Ref.:[i]ScholzF,SchröderU,GulaboskiR()Electrochemistryofimmobilizedparticlesanddroplets.Springer,BerlinFSParallelcircuitcircuitParallelplateandframecell—Thiselectrolysiscelltypehasbeendevelopedsincethes,andreliablecellsfromlaboratoryuptoindustrialproductionscaleindividedorundividedformarecommerciallyavail-able.Parallelplateandframecellscompriseinterelec-trodegapsoftomm.Theycanbeusedasmonopolar(A)orbipolarelectrode(B)arrangement,seefigure.Intype(A),eachelectrodeisconnected,andbothfacesoftheelectrodehavethesamepolarity,whileintype(B),i.e.,bipolarconnections,onlytheendplatesarecontacted,andtheotherelectrodesarepolarizedintheelectricfield,sothattwofacesoftheelectrodes,there-fore,havedifferentpolarities.Monopolarconnectedcellsrequirealowvoltageandahighcurrent(cellvoltagepluscellcurrentmultipliedbythenumberofcells)tobesuppliedbyaDCrectifier.Thecurrentpaththroughtheelectrodesisratherlong,andelectrodeswithahighParallelplateandframecell—Figure482ParsonsandZobelplot●Pconductivityhavetobeused.Incaseofabipolarconnec-tion,thecellcurrentplusthecellvoltagemultipliedbythenumberofcellshastobesupplied,therefore,alowercurrentbutahighvoltageisrequired.Less-conductingelectrodematerialscanbeused,sincethecurrentpaththroughtheelectrodesissmall.Ref.:[i]SteckhanE()Organicelectrochemistry.In:Ullmann’sency-clopediaofindustrialchemistry,thedn.Wiley-VCHMHerParsonsandZobelplot—Inseveraltheoriesfortheelectricdoublelayerintheabsenceofspecificad-sorption,theinterfacialcapacityCperunitareacanformallybedecomposedintotwocapacitiesinseries,oneofwhichistheGouy–Chapman(Gouy,Chap-man)capacityCGC:C=CH+CGC.Thecapac-ityCHisassumedtobeindependentoftheelectrolyteconcentrations,andhasbeencalledtheinner-layer,theHelmholtz,orSternlayercapacitybyvariousauthors.IntheearlyworkbyStern,CHwasattributedtoanin-nersolventlayerontheelectrodesurface,intowhichtheionscannotpenetrate;morerecenttheoriesaccountforanextendedboundaryregion.InaParsonsandZobelplot,CHisdeterminedbyplottingexperimentalvaluesforCvs.CGC.Specificadsorptionresultsinsignif-icantdeviationsfromastraightline,whichinvalidatesthisprocedure.Ref.:[i]ParsonsR,ZobelFGR()JElectroanalChem:WSPartialcurrent(density)—ThepartialcurrentconcepthasbeenintroducedbyErdey-GrúzandVolmer[i],i.e.,theyconsideredanodicpartialcurrent(Ia)andcathodicpartialcurrent(Ic)inordertointerpretthecurrent(I)–potential(E)function(