Lalitha Stanford Presentation Feb2010[1]

“Bridging the gap between theory and experiment: which theoretical

Leveraging Simulations

g g g p y papproaches are best suited to solve real problems in nanotechnology and biology?”

Leveraging Simulations to Gain Insights into

Polymer Electrolyte Membrane F l C llFuel Cells

Stanford University

Dr. Lalitha SubramanianSr. Director and Fellow

24th February, 2010

Accelrys Inc.

Outline

• PEM FC overview

• Rational PEM DesignRational PEM Design– Morphology of perfluorosulfonic acid

(i.e., Nafion®) membranes– Further PEM studies

• Proton transport mechanism• Chemical/mechanical durability• Alternate membrane materials

• Rational Electrocatalyst Design– High Throughput Screening

• Combines both experimental and• Combines both experimental and simulation/modeling insight

© 2008 Accelrys, Inc. 2

Fuel Cell powered cars is a reality –all major manufacturers committed to FC vehicles and develop PEMFC stacks

• HONDA – FCX concept (on lease in the US i 2008)US since 2008)

• Toyota – FCHV (on lease from 2005/2006) and Fine-X concept

• General Motors – Chevrolet Equinox (planned lease from fall 2007)

• Peugeot-Citroen – GENERAC stack, London Taxi conceptLondon Taxi concept

• Nissan – X-Trail FCV• …


Major FC related business/public initiatives

• USA DOE Hydrogen program (www.hydrogen.energy.gov)

• FreedomCAR USA• FreedomCAR, USA (www1.eere.energy.gov/vehiclesandfuels)

• California Fuel Cell Partnership (www.cafcp.org)

• Japan Hydrogen and Fuel Cell Demonstration Project (JHFC) (www.jhfc.jp)

• Clean Energy Partnership (Germany) (CEP) (www.cep-berlin.de)

• Icelandic New Energy (INE) (www.newenergy.is)

• EU Research Framework programsEU Research Framework programs


Fuel Cell TechnologyPresents Challenges

Challenges

• Fuel/Hydrogen Storage

Presents Challenges

• Catalyst optimization

• Electrode reactions

• PEM optimization

• Data Flow Management


PEM Fuel Cells Challenges

• Fuel/Hydrogen Storage− Material selection/optimizationMaterial selection/optimization

• Catalyst optimization− Selectivity, activity, stability− Varied feedstocks, promotersVaried feedstocks, promoters

• Electrode reactions− Hydrogen Evolution/Oxygen

Reduction ReactionReduction Reaction− Degradation (dissolution,

oxidation, poisoning)

• PEM optimization

d

yelectricitOHHO 222 22

PEM optimization− Microstructure, hydration− Proton transport properties− Mechanical/chemical durability


Anode:

Cathode:

eHH 442 2

OHeHO 22 244 • Data Flow Management

– Stack Assembly Engineering

Rational Proton Exchange Membrane Design


Alternate Energy Industry Requirements

• There is a pressing requirement to develop polymer electrolyte membranes (PEM)– conduct protons at low levels of

hydration, – do not degrade upon prolonged

operation at elevated temperature, – and offer selective ionic and

molecular transport. • To optimize the chemistry of membranes

f t t t i f d t lfor proton transport requires fundamental understanding of – proton transport,

Rational design of the next generation of– mechanical properties– chemical degradation

Rational design of the next generation of polymer membranes is needed


Challenges in PEM Design

• Cost– The cost of fuel cell power systems must be reduced before they can be

competitive with conventional technologiescompetitive with conventional technologies

• Durability and Reliability– Match durability and reliability of current automotive engines [i.e., 5,000-

hour lifespan (150 000 miles)] and the ability to function over the fullhour lifespan (150,000 miles)] and the ability to function over the full range of vehicle operating conditions (40°C to 80°C). For stationary applications, more than 40,000 hours of reliable operation in a temperature at -35°C to 40°C will be required for market acceptancetemperature at -35 C to 40 C will be required for market acceptance

• System Size– The size and weight of current fuel cell systems must be further reduced

t t th k i i t f t bilto meet the packaging requirements for automobiles

• Air, Thermal, and Water Management

• Improved Heat Recovery Systems


• Improved Heat Recovery Systems

Wish list for PEM Material

• A good performance at a temperature of 120 ºC without the need to pressurize, i.e(RH) ≤ 40%. At this temperature, about 50 ( ) pppm CO can be tolerated without air bleed

• Conductivity σ = 0.1 -1 cm-1

• Hydrogen oxygen gas permeability <• Hydrogen-oxygen gas permeability < 1x10−12 (mol cm)/(cm2 s kPa))

• Limited swelling in water

• Mechanical properties better than Nafion®

• A chemical stability similar or superior to Nafion, i.e., a durability of around 40,000 h (≤ 1 μV/h)

• A cost target of ≤ $10/kW at 500,000 stacks/y (for automotive application)


• …

Membrane Reliability - A Multiscale Problem

• The task is challenging because

th i t f th b i l– the environment of the membrane is complex

– the pore network morphology is dynamic p p gy y

– and the membrane dynamics takes place on much longer scales compared to proton transferlonger scales compared to proton transfer


Molecular & Mesoscale Simulation

• Two major areas of fundamental* investigation

– Characterizing the chemical features that affect performance– the chemical nature of protonation sites– local concentration of protons – and local level of hydration

– Characterizing the underlying polymer morphologyU d t di t di t ib ti l ti h d ti it– Understanding water distribution, percolation, hence conductivity

– Polymer structure-hydrated morphology relationships

* Excluding transport, CFD and FEM type models


Morphology in hydrated perfluorosulfonic acid membranes

• Morphology of Nafion at the nanoscale?

• SAXS SANS:• SAXS, SANS: – Nanophase segregation into hydrophilic and hydrophobic domains, – Debate over the shape and structure of the ionic clusters: spherical, ellipsoid, or

lamellar?

• Observations of the surface morphology via TEM and AFM– Three-phase model consisting of spherical water clusters surrounded by sulfonic

acid interfaces.– Also observed the coalescence and growth of ionic clusters with an increasingAlso observed the coalescence and growth of ionic clusters with an increasing

water content using AFM.

• Use mesoscale modeling to compare and contrast with exp. Observations

Wescott, Qi, Subramanian and Capehart, J. Chem. Phys. 124, 134702 (2006) collaboration between Accelrys and General Motors


(2006) – collaboration between Accelrys and General Motors

Models of hydrated perfluorosulfonic acid membranes

Gierke’s et al Cluster – Network Model Yeo and Eisenberg’s Model

Yeager and Steck’s Model Starkweather bilayer-> Litt model ->Haubold model


Multiscale Approach

TIMEwater3.5 nm

Diffraction TEM, AFM

ConductivityModulus

Experiment

h

FINITEmin

ANALYTICAL MODELS

PERCOLATION THEORY

water

5 nm

s

s

MESODYN(field method)

ELEMENTANALYSIS

•The task is challenging because

ps

ns MOLECULARDYNAMICS

F= M A

(field method) Statistics

Flory-Huggins Modeling

–the environment of the membrane is complex

–the pore network morphology is dynamic

fsQUANTUM

MECHANICSH=E

ELECTRONS => ATOMS => BEADS => GRIDS => PARAMETERSAtomic potential

y ggParameters –and the membrane dynamics

takes place on much longer scales compared to proton transfer

© 2008 Accelrys, Inc. 15DISTANCE

1 A 1 nm 100nm micron mm

ELECTRONS > ATOMS > BEADS > GRIDS > PARAMETERS

Increasing Length and Time Scales

Atomistic Mesoscopic

Length nm 100’s of nm (or more)

Units atoms Beads representing many atomsg y

Time ns as much as milli-seconds

Dynamics F=ma Diffusion, hydrodynamics


Coarse graining strategy

CF2 CF2 CF CF2x y Hydrophobic fluorocarbon backbone

O CF2 CF O CF2 CF2 SO3

CF3

zM Nafion 117 (EW=1100)

y=1, z=1, x=7

Hydrophilic sidechain with Sulfonic groups

SF

W234 atoms ~ 3 beads

S: Side chain ~ 306Å3

F: 4 -[CF2-CF2]- monomers ~ 325Å3

W: 10 water molecules ~ 315Å3F, S and W Beads


W: 10 water molecules 315Å

Mesoscale Parameterization

Interaction Flory-Huggins

mixing parameterbetween beads(Mesodyn Input)

: solubility parameters

for each bead(MD)

energybetween F, S

and W(Mesodyn)( y p )

RTV JIref

JI

2)(

( )

VEcohI /

( y )

/0 vIJIJ

=9 8 =15 7 =0 7FS=9.8, Fw=15.7, WS=0.7

F beadsat experimental

density of Nafion

S beadsat experimental

density of N fi 117

W beadsat experimental density of water


density of Nafion 117, 2.05g/cm3

F = 13.9 (MPa)½

Nafion 117, 2.05g/cm3

S = 21.1 (MPa)½

density of water 1g/cm3

W = 47.9(Mpa)½

Three-phase Morphology

30nm

H2OYeager and Steck’s Model

SA

: water / sulfonic group

FC

Water clusters (~4nm) surrounded by sulfonic phase

Embedded in a hydrophobic PTFE matrix

with =8, or 20% water: water / sulfonic group

Embedded in a hydrophobic PTFE matrix Consistent with Yeager-Steck[1] three-phase model and Xue’s observation[2]

Order Parameter: (metric for degreeof phase separation)

V III drr

VP 22 )(1

• Water cluster/fluorocarbon degree of


[1] J. Electrochem. Soc. 128, 1880 (1981)[2] J. Membr. Sci. 45, 261 (1989)

• Water cluster/fluorocarbon degree ofPhase separation increases with increasing water volume fraction

Three-phase Morphology at =8

W30nm

W F SWater clusters (~4nm) surrounded by sulfonic phase embedded in a hydrophobic PTFE matrixC i t t ith Y St k[1] thConsistent with Yeager-Steck[1] three

phase model and Xue’s observation[2]

[1] J. Membr. Sci. 45, 261 (1989)


[1] J. Membr. Sci. 45, 261 (1989)[2] J. Elctrochem. Soc. 128, 1880 (1981)

Percolation of water domains/ Percolation for conductivity

%62 %208

• Simulated morphology consistent with the structural i f ti i f d f ll

Volume fraction of water

%6,2 %20,8information inferred from small-angle scattering• Simulated morphology at low gywater content produces spherical hydrophilic domains of reverse micelles - similar to

%11,4 %33,16of reverse micelles similar to model of Gierke• Simulated morphology at hi h t t thigher water content –domains deform into elliptical and barbell shapes – similar to


three-phase model of Yeager and Steck

Compare with Experimental Diffraction Data

SANSSIMULATEDScattering Curves at Different Water Content

=8

ensi

ty

=2

Ionomer k

1

2

SANS

I2

0.01 0.10.01 0.1 Q (A-1)Q (A-1)

Inte peak

0.05 0.1 0.2 0.5

0.2

0.5I

=16

tens

ity

=4

1.5

2

3

I6

0.01 0.10.01 0.1Q (A-1)

Int

Q (A-1)0.05 0.1 0.2 0.5

1


Ionomer peak: associated with hydrophilic domains

2

,( ) ( ) exp( ) exp( )i j i j

i jI q S q F F iq r iq r

Rational Electrocatalyst Design


Overview

• Oxygen reduction reaction (ORR) is acritical performance limiting step inProton Exchange Membrane FuelCells (PEMFC).

• ORR is catalyzed by the cathodewhich must satisfy the followingy grequirements:– Fast ORR kinetics– Stability against oxidation and

contamination– Compatibility with other PEMFC

componentsL t i l d f t t– Low materials and manufacture cost

OHeHO 22 244


Structure and Composition

Bulk and surface defects Alloying Clustering

Defect decoration Surface segregation Skin formation


Cathode material optimization:

• Increasing number of skin alloys and core/shell nanoparticles are being

Key property:

Surface stoichiometry ≠ Nominal stoichiometryreported to have ORR activity superior to that of pure Pt:

PtNi PtCo PtY PtPd

Understanding surface phase diagram under relevant conditions is critical

• PtNi, PtCo, PtY, PtPd• PtAu• Pt double layers• …

• Durability of these systems is a main challenge:• Re-alloyingRe alloying• Leaching and dissolution• Coalescence • Detachment from support


Shuo Chen et al Am. Chem. Soc./ 2008, 130, 13818

Adsorption and activation energies: ORR

EE

E0=E(O2+*)

ETS=E(O*-O*)

Reaction coordinate

E1=E(O2*)E2=2E(O*)

E1=E(O2*)

Reaction coordinate

Ediss=E2-E1 Eads1=E1-E0Ea=ETS-E1 Eads2=E2-E0E =E E


More reaction steps need to be added for electro-reduction

Eads1=E1-E0

Summary

• Ab initio High Throughput Approach offers valuable insight into factors defining catalytic activity of materials.

I t tl it ll t dd i lt l th bl f f d• Importantly it allows to address simultaneously the problems of surface and chemical reactivity.

• Our approach streamlines calculations of descriptors such as d-band centre position, atomic fraction of solute atoms near the surface and electron workposition, atomic fraction of solute atoms near the surface and electron work function.

• This opens the possibility of in silico cathode material optimisation complimentary to the experiment.


Acknowledgements

• Dr. James Wescott

• Dr. Patricia Gestoso-SoutoDr. Patricia Gestoso Souto

• Dr. Jacob Gavartin


Thank You

For more information contact …

Dr. Lalitha Subramanian SSr. Director and FellowAccelrys, [email protected](858)799-5340(858)799 5340


Lalitha Stanford Presentation Feb2010[1]

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