Leveraging Simulations to Gain Insights into Polymer Electrolyte Membrane

Size: px

Start display at page:

Download "Leveraging Simulations to Gain Insights into Polymer Electrolyte Membrane"

Annis Bridges
5 years ago
Views:

1 Bridging g the gap between theory and experiment: which theoretical approaches are best suited to solve real problems in nanotechnology and biology? Leveraging Simulations to Gain Insights into Polymer Electrolyte Membrane Fuel Cells Stanford University 24 th February, 2010 Dr. Lalitha Subramanian Sr. Director and Fellow Accelrys Inc.

2 Outline PEM FC overview Rational 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 simulation/modeling insight 2008 Accelrys, Inc. 2

3 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 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 concept Nissan X-Trail FCV 2008 Accelrys, Inc. 3

4 Major FC related business/public initiatives USA DOE Hydrogen program ( FreedomCAR, USA (www1.eere.energy.gov/vehiclesandfuels) California Fuel Cell Partnership ( Japan Hydrogen and Fuel Cell Demonstration Project (JHFC) ( Clean Energy Partnership (Germany) (CEP) ( Icelandic New Energy (INE) ( EU Research Framework programs 2008 Accelrys, Inc. 4

5 Challenges Fuel Cell Technology Presents Challenges Fuel/Hydrogen Storage Catalyst optimization Electrode reactions PEM optimization Data Flow Management 2008 Accelrys, Inc. 5

PEM Fuel Cells Challenges Fuel/Hydrogen Storage Material selection/optimization Catalyst optimization Selectivity, activity, stability Varied feedstocks, promoters Electrode reactions Hydrogen

6 PEM Fuel Cells Challenges Fuel/Hydrogen Storage Material selection/optimization Catalyst optimization Selectivity, activity, stability Varied feedstocks, promoters Electrode reactions Hydrogen Evolution/Oxygen Reduction Reaction Degradation (dissolution, oxidation, poisoning) O 2H 2 H 2O electricity 2 2 Anode: Cathode: 2H 2 4H 4e O2 4H 4e 2H 2O PEM optimization Microstructure, hydration Proton transport properties Mechanical/chemical durability Data Flow Management Stack Assembly Engineering 2008 Accelrys, Inc. 6

7 Rational Proton Exchange Membrane Design 2008 Accelrys, Inc. 7

8 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 for proton transport requires fundamental understanding of proton transport, mechanical properties chemical degradation Rational design of the next generation of polymer membranes is needed 2008 Accelrys, Inc. 8

9 Challenges in PEM Design Cost The cost of fuel cell power systems must be reduced before they can be competitive with conventional technologies Durability and Reliability Match durability and reliability of current automotive engines [i.e., 5,000- hour lifespan (150, 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 acceptance System Size The size and weight of current fuel cell systems must be further reduced to meet the packaging requirements for automobiles Air, Thermal, and Water Management Improved Heat Recovery Systems 2008 Accelrys, Inc. 9

10 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 ppm CO can be tolerated without air bleed Conductivity σ = cm -1 Hydrogen-oxygen oxygen gas permeability < 1x10 12 (mol cm)/(cm 2 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) 2008 Accelrys, Inc. 10

11 Membrane Reliability - A Multiscale Problem The task is challenging because the environment of the membrane is complex the pore network morphology is dynamic and the membrane dynamics takes place on much longer scales compared to proton transfer 2008 Accelrys, Inc. 11

12 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 morphology Understanding di water distribution, ib ti percolation, hence conductivity it Polymer structure-hydrated morphology relationships * Excluding transport, CFD and FEM type models 2008 Accelrys, Inc. 12

13 Morphology in hydrated perfluorosulfonic acid membranes Morphology of Nafion at the nanoscale? 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 increasing water content using AFM. Use mesoscale modeling to compare and contrast with exp. Observations Wescott, Qi, Subramanian and Capehart, J. Chem. Phys. 124, (2006) collaboration between Accelrys and General Motors 2008 Accelrys, Inc. 13

14 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

$5 nm QUANTUM MECHANICS H =E MOLECULAR DYNAMICS F= M A Diffraction TEM, AFM water 5 nm MESODYN (field method) Flory-Huggins Parameters FINITE ELEMENT$ ANALYSIS Statistics Conductivity Modulus Experiment ANALYTICAL MODELS PERCOLATION THEORY The task is challenging because the environment of the membrane

ANALYSIS Statistics Conductivity Modulus Experiment ANALYTICAL MODELS PERCOLATION THEORY The task is challenging because the environment of the membrane

15 Multiscale Approach TIME h min s s ns ps fs Modeling 3.5 nm QUANTUM MECHANICS H =E MOLECULAR DYNAMICS F= M A Diffraction TEM, AFM water 5 nm MESODYN (field method) Flory-Huggins Parameters FINITE ELEMENT ANALYSIS Statistics Conductivity Modulus Experiment ANALYTICAL MODELS PERCOLATION THEORY The task is challenging because the environment of the membrane is complex the pore network morphology is dynamic Atomic potential ELECTRONS =>ATOMS => BEADS => GRIDS => PARAMETERS and the membrane dynamics takes place on much longer scales compared to proton transfer 1 A 1 nm 100nm micron mm 2008 Accelrys, Inc. DISTANCE 15

16 Increasing Length and Time Scales Atomistic Mesoscopic Length nm 100 s of nm (or more) Units atoms Beads representing many atoms Time ns as much as milli-seconds Dynamics F=ma Diffusion, hydrodynamics 2008 Accelrys, Inc. 16

17 Coarse graining strategy CF 2 CF 2 CF CF 2 x y Hydrophobic fluorocarbon backbone O CF 2 CF zo CF 2 CF 2 SO 3 CF 3 Hydrophilic sidechain with Sulfonic groups M Nafion 117 (EW=1100) y=1, z=1, x=7 234 atoms ~ 3 beads F S W F, S and W Beads S: Side chain ~ 306Å 3 F: 4 -[CF2-CF2]- monomers ~ 325Å 3 W: 10 water molecules ~ 315Å Accelrys, Inc. 17

V Flory-Huggins mixing parameter between

density of Nafion 117, 2.05g/cm 3 S = 21.

18 Mesoscale Parameterization : solubility parameters for each bead (MD) I E coh / V Flory-Huggins mixing parameter between beads (Mesodyn Input) I J V ref ( I J RT 2 ) Interaction energy between F, S and W (Mesodyn) 0 / IJ FS =9.8, Fw =15.7, WS =0.7 IJv F beads at experimental density of Nafion 117, 2.05g/cm 3 F = 13.9 (MPa) ½ S beads at experimental density of Nafion 117, 2.05g/cm 3 S = 21.1 (MPa) ½ W beads at experimental density of water 1g/cm 3 W = 47.9(Mpa) ½ 2008 Accelrys, Inc. 18

Three-phase Morphology 30nm H 2 O Yeager and Steck s Model SA FC : water / sulfonic group with =8, or 20% water Water clusters (~4nm) surrounded by sulfonic phase Embedded in a hydrophobic PTFE

19 Three-phase Morphology 30nm H 2 O Yeager and Steck s Model SA FC : water / sulfonic group with =8, or 20% water Water clusters (~4nm) surrounded by sulfonic phase Embedded in a hydrophobic PTFE matrix Consistent with Yeager-Steck [1] three-phase model and Xue s observation [2] Order Parameter: (metric for degree PI of phase separation) 1 V V I I 2 2 ( r) dr [1] J. Electrochem. Soc. 128, 1880 (1981) [2] J. Membr. Sci. 45, 261 (1989) Water cluster/fluorocarbon degree of Phase separation increases with increasing water volume fraction 2008 Accelrys, Inc. 19

20 Three-phase Morphology at =8 W F S 30nm Water clusters (~4nm) surrounded by sulfonic phase embedded in a hydrophobic PTFE matrix Consistent C i t t with Yeager-Steck [1] three phase model and Xue s observation [2] [1] J. Membr. Sci. 45, 261 (1989) [2] J. Elctrochem. Soc. 128, 1880 (1981) 2008 Accelrys, Inc. 20

Percolation of water domains/ Percolation for

the structural information inferred from smallangle

content produces spherical hydrophilic domains of

Simulated morphology at higher h water content t

$Volume fraction of water 2, 6% 8, 20 % 4, 11% 16,$

21 Percolation of water domains/ Percolation for conductivity Simulated morphology consistent with the structural information inferred from smallangle scattering Simulated morphology at low water content produces spherical hydrophilic domains of reverse micelles - similar to model of Gierke Simulated morphology at higher h water content t domains deform into elliptical and barbell shapes similar to three-phase model of Yeager and Steck Volume fraction of water 2, 6% 8, 20 % 4, 11% 16, 33% 2008 Accelrys, Inc. 21

22 Compare with Experimental Diffraction Data Inte ensity Scattering Curves at Different Water Content SIMULATED =2 Ionomer peak =8 2 1 I 0.5 SANS Int tensity Q (A -1 ) = Q (A -1 ) =16 I Q (A -1 ) Q (A -1 ) I q S q FF iq r iq r 2 ( ) ( ) i j exp( i)exp( j) i, j Ionomer peak: associated with hydrophilic domains 2008 Accelrys, Inc. 22

23 Rational Electrocatalyst Design 2008 Accelrys, Inc. 23

Overview Oxygen reduction reaction (ORR) is a critical

requirements: Fast ORR kinetics Stability against oxidation and

24 Overview Oxygen reduction reaction (ORR) is a critical performance limiting step in Proton Exchange Membrane Fuel Cells (PEMFC). ORR is catalyzed by the cathode which must satisfy the following requirements: Fast ORR kinetics Stability against oxidation and contamination Compatibility with other PEMFC components Low materials and manufacture cost O2 4H 4e 2H 2O 2008 Accelrys, Inc. 24

25 Structure and Composition Bulk and surface defects Defect decoration 2008 Accelrys, Inc. Alloying Surface segregation Clustering Skin formation 25

Cathode material optimization: Increasing number of skin alloys and core/shell

PtCo, PtY, PtPd PtAu Pt double layers Durability of these systems is a main challenge:

Surface stoichiometry Nominal stoichiometry Understanding surface phase diagram under

26 Cathode material optimization: Increasing number of skin alloys and core/shell nanoparticles are being reported to have ORR activity superior to that of pure Pt: PtNi, PtCo, PtY, PtPd PtAu Pt double layers Durability of these systems is a main challenge: Re-alloying Leaching and dissolution Coalescence Detachment from support Key property: Surface stoichiometry Nominal stoichiometry Understanding surface phase diagram under relevant conditions is critical Shuo Chen et al Am. Chem. Soc./ 2008, 130, Accelrys, Inc. 26

27 Adsorption and activation energies: ORR E E 0 =E(O 2 +*) E TS =E(O*-O*) E 1 =E(O 2 *) E 2 =2E(O*) Reaction coordinate E diss =E 2 -E 1 E ads1 =E 1 -E 0 More reaction steps need to be added for electro-reduction E a =E TS -E 1 E ads2 =E 2 -E 0 E ads1 =E 1 -E Accelrys, Inc. 27

28 Summary Ab initio High Throughput Approach offers valuable insight into factors defining catalytic activity of materials. Importantly tl it allows to address simultaneously l 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 work function. This opens the possibility of in silico cathode material optimisation complimentary to the experiment Accelrys, Inc. 28

29 Acknowledgements Dr. James Wescott Dr. Patricia Gestoso-Souto Dr. Jacob Gavartin 2008 Accelrys, Inc. 29

30 Thank You For more information contact Dr. Lalitha Subramanian Sr. Director and Fellow Accelrys, Inc. (858) Accelrys, Inc. 30

August 27, International ANSYS Conference Nick Reynolds, Ph.D. Director, Materials Pre-Sales, US, Accelrys

August 27, International ANSYS Conference Nick Reynolds, Ph.D. Director, Materials Pre-Sales, US, Accelrys Multiscale modeling and simulation developing solutions which link the atomistic, mesoscale, and engineering scales August 27, 2008 2008 International ANSYS Conference Nick Reynolds, Ph.D. Director, Materials