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)

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22

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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

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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

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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)

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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

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W. Deng, W. A. GoddardW. Deng, W. A. Goddard