“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• …
© 2008 Accelrys, Inc. 3
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
© 2008 Accelrys, Inc. 4
Fuel Cell TechnologyPresents Challenges
Challenges
• Fuel/Hydrogen Storage
Presents Challenges
• Catalyst optimization
• Electrode reactions
• PEM optimization
• Data Flow Management
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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
© 2008 Accelrys, Inc. 6
Anode:
Cathode:
eHH 442 2
OHeHO 22 244 • Data Flow Management
– Stack Assembly Engineering
Rational Proton Exchange Membrane Design
© 2008 Accelrys, Inc. 7
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
© 2008 Accelrys, Inc. 8
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
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• 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)
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• …
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
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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
© 2008 Accelrys, Inc. 12
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
© 2008 Accelrys, Inc. 13
(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
© 2008 Accelrys, Inc. 14
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
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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
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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
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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
© 2008 Accelrys, Inc. 19
[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)
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[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
© 2008 Accelrys, Inc. 21
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
© 2008 Accelrys, Inc. 22
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
© 2008 Accelrys, Inc. 23
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
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Structure and Composition
Bulk and surface defects Alloying Clustering
Defect decoration Surface segregation Skin formation
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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
© 2008 Accelrys, Inc. 26
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
© 2008 Accelrys, Inc. 27
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.
© 2008 Accelrys, Inc. 28
Acknowledgements
• Dr. James Wescott
• Dr. Patricia Gestoso-SoutoDr. Patricia Gestoso Souto
• Dr. Jacob Gavartin
© 2008 Accelrys, Inc. 29
Thank You
For more information contact …
Dr. Lalitha Subramanian SSr. Director and FellowAccelrys, [email protected](858)799-5340(858)799 5340
© 2008 Accelrys, Inc. 30