introduction to fuel cells - fritz haber...
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IntroductionIntroduction to to Fuel CellsFuel Cells
FHI-SeminarDecember 14, 2004
Timo Jacob
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ContentsContents• Motivation• Overview over different FC types• Net reactions and ideal potentials (Nernst Equation)• Actual performance and energy losses• Atomistic view of electrodes
– Double layer models– Potentials– Capacity model– Charge distribution
• Problems for fuel cell modeling:– Electrode / Interface– Membrane– H2 storage
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First Fuel CellFirst Fuel Cell
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In 1839, William Grove, a British jurist and amateur physicist, In 1839, William Grove, a British jurist and amateur physicist, first first discovered the principle of the fuel cell. Grove utilized four cdiscovered the principle of the fuel cell. Grove utilized four cells, ells, each containing hydrogen and oxygen, to produce electric power each containing hydrogen and oxygen, to produce electric power which was then used to split the water in the smaller upper cellwhich was then used to split the water in the smaller upper cell into into hydrogen and oxygenhydrogen and oxygen
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Why Fuel Cells?Why Fuel Cells?
• Limitation offossile energyresources
• IncreasingCO2 emission
• Global warming• Transport problems• .....
Pros – Cons (Ideal Situation)Pros – Cons (Ideal Situation)
+• Direct energy conversion (no combustion)• Low emissions• No moving part in the energy converter• Quiet• High availability of lower temperature units• Siting ability• Fuel flexibility• Remote/unattended operation• Small size
—• High market entry cost, production cost• Unfamiliar technology to the power industry• Almost no infrastructure• Still at level of development
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Working principleWorking principle
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Fuel Cell TypesFuel Cell Types
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Fuel Cell typesFuel Cell types
ηη 5050--65%65% 5050--8080%% 3535--45%45% 4545--60%60% 5050--60%60%
ApplicationsApplications Transport, Transport, spacespace, , shipsships
transporttransport,,carscars, , spacespace,,houseshouses,,shipsships,mobile ,mobile applappl..
100 MW 100 MW plantsplants5050--500 kW 500 kW block block heating heating plantsplants
100 MW100 MW plantsplants5050--500 kW 500 kW blockblock heating heating plantsplants
100 MW100 MW plantsplants5050--500 kW 500 kW blockblock heating heating plantsplants
Status quoStatus quo 5050--100 kW, 100 kW, can be boughtcan be bought, , expensiveexpensive
20 kW, high 20 kW, high efficiencyefficiency
100 kW 100 kW prototypeprototype
25 kW25 kW prototypeprototype
Fuel Cell OverviewFuel Cell Overview
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Electrode catalysis:•Make more efficient•Reduce overpotential
Electrolyte:•Less expensive•More efficient•Wide operation temperature range
H2 storage:•Carbon Nanotubes•Metal Hydrides
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Pros – Cons (specific)Pros – Cons (specific)•• PEMFC:PEMFC: + Solid electrolyte excellent
resistance to gas crossover– Low CO tolerance (~ppm level)– low temperature
+ working temperature ~80Cshort startup time
– unefficient to use rejected heat for cogeneration of additional power
+ can work at high current densities compared to other cells
– difficult heat and water management limit the operating power densities
CF2 CF2 CF CF2
O CF2 CF O CF2 CF2 SO3
x
y
CF3
z
H
NafionNafion®®
Pros – Cons (specific)Pros – Cons (specific)•• AFCAFC:: + Excellent performance on H2 and
O2 compared to other cells due toactive O2 electrode kinetics(70% efficiency)
+ relatively low temperatures (~80C)
– very clean fuel required, since small contaminations dissociate alkaline base,
– CO2 poisoning
+ wide range of possible electrocatalysts
– significant pressure differenceacross the membrane is required
– large Pt quantity is needed because of harsh conditions
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ISSISSKOHKOH
in matrixin matrixPtPtPtPt
Pros – Cons (specific)Pros – Cons (specific)•• PAFC:PAFC:
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+ CO2 (of reformed fuel gas stream) resistent electrolyte
– lower performance due to slowcathode reaction (37-42%)
+ low working temperatures (~200C)
+ water boiling point is not limiting
– fuel from external hydrocarbon reformation
+ less complex fuel conversion(no membrane and attendentpressure drop)
– high cost catalysts– harsh conditions– CO poisoning
(water gas shift reaction required)
HH33POPO44 PtPtPtPt
TimesquareTimesquare
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Pros – Cons (specific)Pros – Cons (specific)• MCFC : + higher temperatures (~650C)
less sensitive reactions and lessexpensive materials
+ Ni as catalyst
– very corrosive and mobile electrolyte
– source of CO2 is required at cathode to form carbonate ions
+ reforming within the cell+ CO can directly be used as fuel+ heat exhaust can be used with external gas turbine (η~80%)
– low sulfur tolerance– high temperatures
(~650C)– stainless steel as cell hardware– complex working procedure
LiLi22COCO33KK22COCO33
NiNiNiNi
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Pros – Cons (specific)Pros – Cons (specific)• SOFC + ceramic construction
no hardware corrosionno gas crossover due to absence of liquid
+ fast kinetics
– incoming air has to preheated– high temperature (~1000C)
thermal expansion mismatchsealing between cells is difficult
+ fast kinetics+ CO is directly usable+ no CO2 is required at cathode
(as in MCFC)
– constraints on material collection– difficult fabricate process– high electrical resistivity in the
electrolyte low performance+ high temperature (~1000C)
fuel can be reformed within the cell
– high cost catalysts
Anode: NiAnode: NiCathode: LaMnOCathode: LaMnO33Electrolyte: YSZ Electrolyte: YSZ
ZrOZrO22[Y[Y22OO33]]
Ideal PerformanceIdeal Performance
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Nernst EquationNernst EquationAA++((solutionsolution) + e) + e–– (metal) ½ A(metal) ½ A22 (g)(g)
Electrochemical Electrochemical potential:potential:
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Nernst EquationNernst Equation
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FC reactions and FC reactions and their corresponding their corresponding Nernst equationsNernst equations
Nernst EquationNernst Equation
Temperature Temperature dependence of Hdependence of H22/O/O22Ideal PotentialIdeal Potential
Ideal voltage (for oxidation of hydrogen) as a function of cell Ideal voltage (for oxidation of hydrogen) as a function of cell temperaturetemperature
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ActualActual PerformancePerformance
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Actual PerformanceActual Performance
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••Concentration Polarization Concentration Polarization reactions reduce educt concentrations reactions reduce educt concentrations ηηconcconc
••Activation Polarization Activation Polarization Rates of electrochemical reactions Rates of electrochemical reactions ηηactact
••Ohmic Polarization Ohmic Polarization resistance of ion flow resistance of ion flow ηηohmohm
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Concentration PolarizationConcentration PolarizationRate of mass transport to an electrode surface can be described Rate of mass transport to an electrode surface can be described by Fick’s first law of by Fick’s first law of diffusion:diffusion:
((DD: diffusion coefficient, : diffusion coefficient, ccii: concentration, : concentration, δδ: thickness of diffusion layer: thickness of diffusion layer
The limiting current (The limiting current (iiLL) is a measure of the maximum rate at which reactants can be ) is a measure of the maximum rate at which reactants can be supplied to an electrode and occurs at supplied to an electrode and occurs at ccsurfacesurface=0:=0:
Nernst equation for Nernst equation for reactant speciesreactant species at equilibrium isat equilibrium is
When current flows When current flows ccsurfacesurface is less than is less than ccbulkbulk andand
Potential difference produced by a concentration change:Potential difference produced by a concentration change:
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Activation PolarizationActivation PolarizationThe activation polarization is customarily expressed by a semiThe activation polarization is customarily expressed by a semi--empirical equation, called Tafel equation:empirical equation, called Tafel equation:
Here Here αα is the electron transfer coefficient of the reaction at the is the electron transfer coefficient of the reaction at the electrode, and electrode, and ii00 is the exchange current density.is the exchange current density.
The usual form is given byThe usual form is given bywhere a = (where a = (−−2.32.3RTRT//ααννF)F)··loglogii00 and b= and b= 2.32.3RTRT//ανανF.F.
B is called the Tafel slopeB is called the Tafel slope
Actual PerformanceActual Performance
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Sum of electrode polarization:Sum of electrode polarization: ηηanodeanode= = ηηact,aact,a+ + ηηconc,aconc,a
ηηcathodecathode= = ηηact,cact,c+ + ηηconc,cconc,c
The effect is to shift the potential of an electrode (The effect is to shift the potential of an electrode (ΦΦelectrodeelectrode):):
ΦΦ’’cathodecathode= = ΦΦcathodecathode −− | | ηηcathodecathode||ΦΦ’’anodeanode= = ΦΦanodeanode + | + | ηηanodeanode||
ΦΦcellcell= = ΦΦcathodecathode − | − | ηηcathodecathode| − (| − (ΦΦanodeanode + | + | ηηanodeanode|) − I|) − I··RR
ΦΦcellcell==∆∆ΦΦ − | − | ηηcathodecathode| − | | − | ηηanodeanode| − I·R| − I·R
ΦΦcellcell= − | = − | ηηcathodecathode| − | | − | ηηanodeanode| − I·R| − I·RCurrent flow reduces cell voltage due to electrode and ohmic polCurrent flow reduces cell voltage due to electrode and ohmic polarizationarization
Goal: Minimize the polarizationGoal: Minimize the polarization
Electric Double Electric Double Layer Layer
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Atomistic View of ElectrodeAtomistic View of Electrode
Steps Steps of of electrochemical reactionelectrochemical reaction::
1.1. Transport of Transport of reactants reactants to to electrodeelectrode
2.2. Adsorption of Adsorption of reactants reactants at at electrode electrode ((maybe dissociative adsmaybe dissociative ads.).)e.g. He.g. H22 HH2,2,adsads; H; H2,2,adsads 2H2Hadsads; H; H22 HH++ + A+ A––
3.3. Electron transfer through phase boundaryElectron transfer through phase boundarye.g. He.g. H++ + e+ e–– HHadsads
4.4. FollowFollow--reactions reactions at at electrode electrode surface and desorptionsurface and desorptione.g. 2He.g. 2Hads ads HH22(g)(g)
5.5. Removing reaction products Removing reaction products to to solutionsolution
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Electron TransferElectron Transfer
EEFF: : FermiFermi--level level of metalof metal
aa: Tunnel distance: Tunnel distance
ΦΦ: : Work function Work function of metalof metal
EAEA: : Electron affinityElectron affinity
EA EA > > ΦΦ for reductionfor reduction
ΦΦ––EAEA should be small
MetalMetal ElectrolyteElectrolyte
should be small
Open Open circuitcircuit: : Reaction takes place until equilibrium Reaction takes place until equilibrium at at electrodeelectrode
equilibriumequilibrium potential potential φφ0
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Opposite charge distribution Opposite charge distribution in metal and in metal and electrolyte0 electrolyte
Double-LayerDouble-Layer
MetalMetal Electrolyte Electrolyte ((solutionsolution))xx
solutionsolution ElectroneutralElectroneutral!!Potential Potential is is a a result result of of the charge distributionthe charge distribution
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••EquilibriumEquilibrium: : dynamic equilibrium dynamic equilibrium of of charge transfer charge transfer in in both both directionsdirections of of phase boundaryphase boundary
••Electrochemical process only if coupled with reductionElectrochemical process only if coupled with reduction ((oxidationoxidation) ) at at opposite opposite electrodeelectrode
••openopen circuitcircuit: EMK = : EMK = φφ00 of of galvanic cellgalvanic cell
PSZ-Potential of Zero ChargePSZ-Potential of Zero Charge
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Negative Negative charging currentcharging current
PositivePositivecharging currentcharging current
At a At a certain electrode certain electrode potential (potential of potential (potential of zerozero chargecharge)) the the electrode electrode surface surface charge vanishescharge vanishes: : q q = 0, = 0, σσ = 0= 0
This can be measured polarographicallyThis can be measured polarographicallymeasuring measuring II for for different different φφ
II
Potential ConventionPotential Convention
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ψ (outer potential)
χφ (inner potential)
Remove electrolyte without changing the charge distribution Remove electrolyte without changing the charge distribution of of the the metal (surface)metal (surface)
11
22
33
Region 1: Test Region 1: Test charge feels Coulomb charge feels Coulomb potential at far potential at far distancesdistances
Region 2: Surface Region 2: Surface charge is not charge is not a point a point chargecharge
Region 3: Test Region 3: Test charge feels charge feels surface surface chargescharges
vacuumvacuumEE
Detailed view of Double-LayerDetailed view of Double-LayerPhysisorptionPhysisorption::-- electrostatic interaction between electrostatic interaction between
chargescharges-- chargecharge--dipol interactiondipol interaction-- polarisation polarisation of of molecules molecules and and ionsions-- dispersion forcesdispersion forces, , vdW forcesvdW forces-- Attachment Attachment of of opposite chargedopposite charged
ions ions and and moleculemolecule--dipolsdipols
ChemisorptionChemisorption::-- chemical interaction betweenchemical interaction between
particles particles and metaland metal
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Detailed view of Double-LayerDetailed view of Double-Layer
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Inner Helmholtz Plane:Inner Helmholtz Plane:plane of plane of adsorbed species adsorbed species ((direct electrode contactdirect electrode contact))
Inner Helmholtz Inner Helmholtz LayerLayer::Area Area of of reactions with desolvated reactions with desolvated particlesparticles
Outer Outer Helmholtz Plane:Helmholtz Plane:plane of plane of closest possible solvatedclosest possible solvatedparticlesparticles
Outer Hemholtz LayerOuter Hemholtz Layer::Area Area of of redoxreaction between redoxreaction between solvated particles solvated particles
Condensator Model of Double-Layero.H.p. model
Condensator Model of Double-Layero.H.p. model
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Charge Charge density density on metal surface:on metal surface:
equivalent charge density equivalent charge density in o.H.p.in o.H.p. electric electric double double layerlayer
Integral double Integral double layer capacity layer capacity (relative to (relative to psz levelpsz level):):
Differential double Differential double layer capacitylayer capacity::
Measurement Measurement of differential of differential capacity with capacity with DC DC methodsmethods, , electrocapillarelectrocapillar--experimentsexperiments
Condensator Model of Double-Layero.H.p. model
Condensator Model of Double-Layero.H.p. model
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Double Double layer capacitylayer capacity
Electrolyte resistanceElectrolyte resistance
Capacity Capacity ofofreference electrodereference electrode((usually neglectedusually neglected))
Measurement by Measurement by potential drop potential drop methodmethod::
Solving DiffSolving Diff. . EqEq. . forfor q,q, then integrate over then integrate over t:t:
EvaluateEvaluate I:I:
Models for electric double layerModels for electric double layer
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Helmholtz (1853)Helmholtz (1853)static static double double layerlayer
GouyGouy (1910) and (1910) and Chapman Chapman (1913)(1913)diffuse double diffuse double layerlayer
Stern (1924) Stern (1924) staticstaticand diffuse double and diffuse double layer layer ((combinationcombination))
New New model model also also includeinclude::
••Specific adsorption Specific adsorption at at electrodeelectrode
••IonIon--metal, metal, moleculemolecule--metal metal interactioninteraction and and catalytic effectscatalytic effects
••Structure Structure of of conduction bandsconduction bands
••Overlap between valence Overlap between valence band and band and conduction conduction band at band at phase boundaryphase boundary
••Dielectric filling Dielectric filling of double of double layerlayer
••Deformation of Deformation of conduction conduction bandband
Stern ModelStern ModelDouble Double layer without adsorption layer without adsorption of of ions ions ((chargescharges) ) within the within the i.H.l.i.H.l.
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o.H.l.: o.H.l.: static static double double layerlayer, , free free of of chargescharges
Poisson equation with conditionsPoisson equation with conditions::
ZetaZeta--Potential at Potential at the boundary betweenthe boundary betweenstatic static Helmholtz Helmholtz layer layer and diffuse and diffuse layerlayer
DistanceDistance--dependencydependency of potential:of potential:
Diffuse double Diffuse double layerlayer:: Electrolyte beyond the Electrolyte beyond the o.H.l. (o.H.l. (bulk solutionbulk solution))
Poisson equationPoisson equation::
Stern ModelStern ModelDiffuse double Diffuse double layerlayer
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ConditionsConditions::BoltzmannBoltzmann--Ansatz Ansatz for the charge for the charge distribution within the electrolytedistribution within the electrolyte Insert into Poisson equationInsert into Poisson equation::
IfIf small concentrations small concentrations Taylor Taylor expansionexpansion
Solution of Solution of the the partial differential partial differential equationequation::
Stern ModelStern ModelDiffuse double Diffuse double layerlayer
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ConditionsConditions::
1. Condition:1. Condition:
2. Condition:2. Condition:
DebyeDebye--distance: distance: κκ--11
center center of of charge charge in in the the diffuse double diffuse double layerlayer
For x = For x = κκ--11::
ChargeCharge--densitydensity
Stern ModelStern ModelDiffuse double Diffuse double layerlayer
Small Small κκ broadbroad double double layer extends into electrolyte regionlayer extends into electrolyte regionςς of of the the o.H.p.o.H.p. can be replaced by the electrode can be replaced by the electrode potential potential
ElectricElectric FieldField::
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Continous transition Continous transition atat xx = = aa::
Charge Charge density as density as a a function function of of the electrolyte concentration the electrolyte concentration and and electrode electrode potential:potential:
Stern ModelStern ModelDiffuse double Diffuse double layerlayer
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Low concentrationsLow concentrations:: PotentialPotential--drop drop within the within the o.H.l. o.H.l. is almost zerois almost zero
High High concentrationsconcentrations::
PotentialPotential--drop drop within the within the o.H.l. o.H.l. increases with increasing concentrationsincreases with increasing concentrations, , linearisation linearisation of of Poisson equation may be criticalPoisson equation may be critical!!
Charge of Charge of the the diffuse diffuse layer layer
Capacity Capacity of of static static double double layerlayer: : Capacity Capacity of diffuse double of diffuse double layerlayer: :
Plate condensator Plate condensator at distance at distance κκ
Problems Problems for Fuel Cell for Fuel Cell ModelingModeling
ElectrodesElectrodes / / InterfacesInterfaces
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Current problems for modeling ICurrent problems for modeling I•• MorphologyMorphology ofof the catalystthe catalyst
In order toIn order to provideprovide aa large surface area for simultaneous catalysislarge surface area for simultaneous catalysisPtPt nanoparticlesnanoparticles are used insteadare used instead of (semiof (semi--) infinite systems) infinite systemsHoweverHowever::–– Nanoparticles have structuralNanoparticles have structural andand
electronical propertieselectronical properties,, which might which might strongly dependstrongly depend onon sizesize andand shape shape (e.g. Quantum(e.g. Quantum Size EffectsSize Effects))
–– Nanoparticles combineNanoparticles combine aa varietyvariety of of differentdifferent functional groupsfunctional groups: different: differentsurfacessurfaces,, stepssteps,, kinkskinks,, tipstips,, vacanciesvacancies, ...),, ...),havinghaving differentdifferent propertiesproperties
Eb=0.49eV Eb=1.10eV
Current problems for modeling IICurrent problems for modeling II
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•• InfluenceInfluence ofof the supportthe support– Pt nanoparticles are attached to
larger Carbon particles (~50nm),which will change their structures.Smaller particles less deformation, close to spherical structureLarger particles strong deformation, ellipsoidal shape
– Pt nanoparticles are assumed to be 2-5nm in diameter.However, due to relatively low diffusion barriers agglomeration may occur to create larger particles. (again having modified properties)
– Supporting material:Pd particles on Au have ~10 timeshigher reactivity than Pd particles on Cu
Current problems for modeling IIICurrent problems for modeling III
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•Bulk region should have structural properties (density, distribution, …) of bulk Nafion, otherwise system need to be expanded
•Water layer forms between electrode and membrane(will be different in presence of E-field)
after 2ns
Current problems for modeling IVCurrent problems for modeling IV•• ReactiveReactive EnvironmentEnvironment
–– CathodeCathode isis surroundedsurrounded byby a a varietyvariety of different of different compoundscompounds::•• HH22O of O of thethe electrolyteelectrolyte, and , and reactionreaction productproduct•• HH33OO++ as as protonproton carriercarrier ((surroundedsurrounded byby waterwater moleculesmolecules))•• OO2 2 gas as gas as reactionreaction componentcomponent•• Different Different reactionreaction intermediatesintermediates
(e.g. O, H, OH, OOH, HOOH,...)(e.g. O, H, OH, OOH, HOOH,...)•• ImpuritiesImpurities in in thethe fuelfuel (e.g. CO, (e.g. CO, NoNoxx, ...), ...)
TheThe environmentenvironment mightmight influenceinfluence thethe structurestructure, , stabilitystability, and , and compositioncomposition of of thethe Pt Pt nanoparticlesnanoparticles ((oror certaincertain functionalfunctionalgroupsgroups).).ThisThis maymay varyvary forfor different different TT and and pp conditionsconditions
44
Current problems for modeling VCurrent problems for modeling V
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•• SolvationSolvation effectseffects duedue to solvent in to solvent in thethe electrolyteelectrolyte–– In In casecase of of PEMPEM--FCsFCs thethe electrolyteelectrolyte isis hydratedhydrated–– At At thethe cathodecathode waterwater isis generatedgenerated
TheThe electrodeelectrode isis surroundedsurrounded byby waterwatermoleculesmolecules, , whichwhich influenceinfluence adsorptionadsorptionenergiesenergies and and structuresstructures..ThisThis mightmight changechange thethe reactionreactionmechanismmechanism, , butbut also also stabilizestabilize orordestabilizedestabilize certaincertain structuresstructures(e.g. (e.g. electrodeelectrode, , adsorbatesadsorbates) )
SolvationSolvation cancan bebe treatedtreated byby a a twotwo-- ororthreethree--shellshell modelmodel
Current problems for modeling VICurrent problems for modeling VI
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•• ElectrodeElectrode potentialpotential–– BesidesBesides TT and and pp, , thethe electrodeelectrode potential potential φφ isis anotheranother parameterparameter, , whichwhich
influencesinfluences structuresstructures and energeticsand energeticsExampleExample: Au(100) : Au(100) surfacesurface in 0.01M HClOin 0.01M HClO44--solutionsolutionφφ<0.25V quasi<0.25V quasi--hexagonal hexagonal reconstructionreconstructionφφ>0.25V >0.25V unreconstructedunreconstructed surfacesurface (H. (H. IbachIbach et al., Surf. et al., Surf. SciSci. 375, 107 . 375, 107
(1997))(1997))
–– Distribution of negative and positiveDistribution of negative and positivechargescharges withinwithin thethe solutionsolution changeschanges
–– CounterCounter chargescharges will will bebe on on thethe surfacesurfaceof of thethe metal metal electrodeelectrode((wholewhole interfaceinterface isis electroneutralelectroneutral))
electricelectric double double layerlayer establishesestablishespotential drop (potential drop (strongstrong EE--fieldfield) on ) on thethe electrodeelectrode surfacesurface
Current problems for modeling VIICurrent problems for modeling VII
•• All All previousprevious problemsproblems deal deal withwith thethe system in system in equilibriumequilibrium..HoweverHowever, , underunder steadysteady statestate conditionsconditions structuresstructures, , compositionscompositions, and , and reactionreaction mechanismsmechanisms mightmight bebe significantlysignificantly different?different?
UseUse kinetickinetic simulationssimulations to to studystudy exactlyexactly thesethese influencesinfluences..
HoweverHowever, in order to , in order to getget reliablereliable resultsresults all all significantsignificant processesprocesses shouldshouldbebe studiedstudied and and thethe correspondingcorresponding parametersparameters extractedextracted: :
AdsorptionsAdsorptions, , desorptionsdesorptions, , diffusionsdiffusions, , reactionsreactions, …, …
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ElectrolyteElectrolyte
48
Ions exert dielectric friction on H+ mobility
Sensitive parameter for conductivity and
permeation
Each level has implications on
membrane performance
Width of water domains affected by electrostatic repulsions and polymer
elasticity
Coarseness of the surface affects viscous movement of the ion
Connectivity of water domains determines
transport
Mechanical properties of the matrix
water
polymer
Stochastic & Analytical Modeling
Nafion Microstructure
Atomistic & Coarse GrainSimulations
Nanosegregation
5 nm
3.5 nm 1 nm
Atomistic Simulations
Interface analysis
Hydrated Nafion is a multiscale
heterogenousmaterial
SO3
Nanostructure & Dynamics of Polyelectrolyte MembranesNanostructure & Dynamics of Polyelectrolyte Membranes
Rational approach to the study of PEM: experiments and simulation are complementary
Backbone flexibilityBackbone hydrophobicityEquivalent weightCharge localizationSide chain flexibilitySide chain polarityConfinement 49
Blockiness of polymerSide chain lengthAcidity of ionomerCounter ion Water contentTemperatureArchitecture of side chain
Microphase segregationWater diffusionMechanical propertiesWater’s electroosmotic dragMaximum water uptakeWater evaporationExtent of ionomer solvationMechanism of gas diffusion
Counter ion migrationHydronium transportPatchiness of the water/PE
interfaceInterfacial free volumeDegree of crystallinity
T of glass transition(s)Water percolation
Membrane structureMembrane structureNafion 117CF2 CF2 CF CF2
O CF2 CF O CF2 CF2 SO3
x
y
CF3
z
M
50
=
ALTERNATED POLYMER
SMALL PATCHES
=
BLOCKY POLYMER
BIG SEGREGATED PATCHES
Effect of polymer sequenceBlockiness affects the extent of nanophase segregation (is better for the blocky polymer) and water-polymer interface heterogeneity.
We compute a water percolation threshold that is in agreement with the one inferred from conductivity
The percolation of the hydrophilic phase is necessary for proton conductivity. 0
Edmondson et al.Solid State Ionics
(2002) 152: 355-361
Effect of water content
All the ions of the polymer contact the water nanophase. Water domain is far from spherical
Non perco. percolatedNon perco.
Water diffusion is ~25% faster in the more segregated structure. Experimental results between blocky and random.
Vehicular diffusion of hydronium is comparable for both sequences (no exp). Activation energies of water diffusion in
agreement with experiment.
Membrane characteristicsMembrane characteristics
Characteristics of water mobility in Characteristics of water mobility in
(a) Pure water, (b) neutral and ionized nafion, (c) (a) Pure water, (b) neutral and ionized nafion, (c) hydrophobic slabs, (d) hydrophilic slabs, (e) neutral and hydrophobic slabs, (d) hydrophilic slabs, (e) neutral and
ionized ionized homogeneous solutionhomogeneous solution of nafion fragmentsof nafion fragments
(a) (b) (c) (d) (e)
51
52
HH22 storagestorage
Doped Carbon-Based MaterialsDoped Carbon-Based Materials
MD/MC MD/MC studiesstudies::
3
4
5
6
7
8
6 7 8 9 10 11 12
Interlayer distance (Å)
Hyd
roge
n up
taki
ng m
ass%
57
67
77
37
47
27
Hydrogen density (kg/m
3)
hydrogen density
mass% 50 bar
100 bar
100 bar 50 bar
target line
3.5
4.5
5.5
6.5
7.5
6 7 8 9 10 11 12
Inter-tube distance (Å)
Hyd
roge
n up
taki
ng m
ass%
62
72
52
32
42
Hydrogen density (kg/m
3)
hydrogen density
mass%
50 bar
100 bar
100 bar
50 bar
target line
53
W. Deng, W. A. GoddardW. Deng, W. A. Goddard
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