b@g - sklwut.whut.edu.cnsklwut.whut.edu.cn/xshy/201603/P020160602404757338919.pdf · Finite element...
Transcript of b@g - sklwut.whut.edu.cnsklwut.whut.edu.cn/xshy/201603/P020160602404757338919.pdf · Finite element...
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2016-05-21
Institute of New Energy (INE)
() China University of Petroleum, Beijing (CUPB)
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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PEM properties
Dself x109 m2/s =5 =10 =15 =22
H2 2.57 2.79 3.80 5.51
O2 0.15 0.40 0.41 0.71
Pore volume
=5 0.24 ml/g
=10 0.29 ml/g
=15 0.37 ml/g
=22 0.48 ml/g
Equilibrated Nafion membrane
Water channel
Gas diffusivity in PEM
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Modeling of Nafion membrane
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Microstructure of Nafion membrane
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Degradation reactions of PEM
Reaction constants
Simulation of PEM degradation reactions using Kinetic Monte Carlo
Degradation reactions of Nafion:
-CF2CF2-COOH + OH -> -CF2COOH + CO2 + HF (main chain unzipping)
-CF2CF2SO3H + OH-> -CF2COOH + CO2 + HF (side chain scission)
-CF2CF2SO3H + OH-> -CF2COOH + CO2 + HF (ether cleavage)
Working scheme of modeling
Experimental validation: Gravimetry, HPLC-MS, Ion
chromatography, FTIR & 19F NMR
Model PEM degradation in atomic details
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Kinetic Monte Carlo (KMC) method:
KMC algorithm is used to simulate degradation processes.
The algorithm follows:
1. Set the time t at t = 0
2. Construct a list of all possible events. Suppose that there are N events in total with the rate constants r1, , rN in units of events per unit of time. It is assumed that these events are independent.
3. Select one of the events k with a probability pk = rk/ri4. Execute this event and advance the time for the next event (k+1) by
tk+1 = tk+ |ln(f)|/ ri, where f is an uniformly distributed random number between 0 and 1.
5. Repeat this process, starting from step 2.
Important factors: Reaction definition & Reaction rates
Modeling of PEM chemical degradation
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Degradation of Nafion Main chain unzipping
Polymer Degradation and Stability 94 (2009) 14361447
Reaction #1
-CF2CF2-COOH + OH -> -CF2COOH + CO2 + HF
Reaction rate r1 = k1[-COOH][OH] (k1 = 1)
Reaction #2
-CF2CFO-COOH + OH -> -CF2COOH + CO2 + HF
Reaction rate r2 = k2[-COOH][OH] (k2 = 0.01)
Obtaining reaction rates:
1. DFT estimation of reaction enthalpy
2. Fitting relevant experimental data
Need to predefine
Carboxylation rate
Need to predefine
Carboxylation rate
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Degradation of Nafion Side chain scission
ECS Transactions, 16 (2) 235-255 (2008)
Reaction #3
-CF2CF2SO3H + OH-> -CF2COOH + CO2 + HF
Reaction rate r3 = k3[-SO3H][OH] (k3 = 0.01)
Mostly take place in Fentons test of gas phase (dry conditions)
Obtaining reaction rates:
1. DFT estimation of reaction enthalpy
2. Fitting relevant experimental data
Need to predefine -SO3-
deprotonation rate
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Degradation of Nafion ether cleavage
Cause backbone & side chain scission
Reaction #4
-CF2CF2SO3H + OH-> -CF2COOH + CO2 + HF
Reaction rate r4 = k4[-CFO-][OH] (k4 = 0.002)
Macromolecules 2007, 40, 8695-8707
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Snapshot of degraded Nafion
Main chain unzipping Side chain scission
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Simulation validation - Ion chromatography
Simulation results
Experimental results
Validate exhaust components by Ion chromatography
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Simulation validation - HPLC-MS
Validate side-chain products by
HPLC-MS
CF3-COOH SO3H-CF2-COOH
Nafion side chain
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Simulation validation - FTIR
Validate backbone composition by FTIR
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Simulation validation - 19F NMR
Validate Nafion composition by 19F NMR
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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Development of MDM and MEAPLS
MEA Model(global chemical
degradation model
from T5 using
Kinetic model from
T1)
Sorption Model
(nature of water
domains T1)
Hierarchical
Fiber Bundle
Model (micro-crack
initialization from
T1)
Finite element fracture
model
(a new fracture
mechanics model
combined with standard
FEM model and FEM
Fatigue model from T2)
Molecular simulation
(mechanical properties of fibers and fundamental
degradation mechanisms from
T3)
MDM
MEAPLS
FEM
(in situ stress
from T2)
critical crack sizecrack density
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Molecular simulation of mechanical degradation
Molecular simulation:
1. Model Nafion membrane (20-50 nm)
2. Extract mechanical property of Nafion fiber unit (2-5nm)- Mechanical degradation
3. Model chemical degradation process of Nafion membrane - Chemical degradation
4. Calculate mechanical property of Nafion fiber unit upon chemical degradation Coupling Chemical/mechanical degradation
5. Explore structural change of Nafion under chemical and mechanical degradation
Membrane
degradation
Mechanical stress
Chemical dissolution
RH cycling
Structural
analysis
+
+
Gas
permeation
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Analysis of PEM mechanical failure:
Crazes in polymers are regions of highly localized deformation, which require the presence of hydrostatic tension.
Crazes nucleate in regions with stress concentrations such as crack-tips, pinholes, and surface defects, is sufficiently large.
Two categories of craze growth : (1) craze tip advance, or crack formation, and (2) craze widening, which corresponds to fibril formation and deformation. As the fibrils deform and elongate in the latter case, crazes grow in width and eventually the fibrils break down, resulting in separation of the material behind the crack-tip, similar to crack propagation.
Craze growth can be considered as a precursor to crack propagation.
Fibril elongation leads to localized plastic deformations and slowing the crack propagation by bridging the crack plane. When the fibrils are strong enough to hold the crack, craze formation occurs
Modeling Fatigue Failure of PEM
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Calculating PEM mechanical properties
Mech Time-Depend Mater (2008) 12: 205220
The stress-strain behavior of cross-linked ionic polymeric networks was investigated using molecular dynamics simulations.
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Reference of micro-crack simulation in polymer membrane
Crazing of entangled polymer chains
Growth, microstructure, and failure of crazes in glassy polymers, J. Rottler and M. O. Robbins, Phys Rev E, 68, 011801 (2003).Jamming under tension in polymer crazes, J. Rottler and M. O. Robbins, Phys Rev Lett, 89, 195501 (2002).Cracks and crazes: On calculating the macroscopic fracture energy of glassy polymers from molecular simulations, J. Rottler, S. Barsky, M. O. Robbins, Phys Rev Lett, 89, 148304 (2002)
Tensile pull on adhesive polymer chains
Large-scale simulation of adhesion dynamics for end-grafted polymers, S. W. Sides, G. S. Grest, M. J. Stevens, Macromolecules, 35, 566-573 (2002)Effect of end-tethered polymers on surface adhesion of glassy polymers, S. W. Sides, G. S. Grest, M. J. Stevens, S. J. Plimpton, Journal of Polymer Science, Part B (Polymer Physics), 42, 199-208 (2004)
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Computation procedures
Parallel working scheme
System Ionomers Dimension cluster Required
cores
Running
time
Purpose
Small 45 10 nm lattice 32 5.5 h/ns Testing
Large 1600 30 nm parallel 240 21.9h/ns Formal
Simulation steps:
1. Randomize initial membrane
2. pre-stabilize membrane
3. Stabilize membrane
4. Anneal membrane
5. Equilibrate membrane
1 2
3
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Structure of small Nafion system
PAGE 24
Initial Nafion membrane with
=6
Dimension = 10 nm
Water domain
Nafion framework
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J. Polym. Res. 2006, 13, 379.
TEM Analysis of PEM
H+ sites are replaced by metal ions to
increase the image contrast.
Dark regions are microscopic water
cavities in Nafion membrane.
Gray regions are polymer backbone.
Hongwei Zhang et al., Int. J. Hyd. Energy, 37 (2012) 4657-4664
J.Memb. Sci. 356 (2010) 4451
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6 8 10 12 14 16 18
Dimension 10.10 10.28 10.46 10.63 10.79 10.95 11.10
Swelling - 1.8% 3.6% 5.2% 6.8% 8.4% 10%
=8
=16
Observation:
Reorganization of Nafion
framework caused by water
uptake
Effect of RH cycling
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Morphology change under RH cycling
=6 (initial) =12 (equilibrated) =6 (equilibrated)
Dimension [nm] =6 =12 =6
Cycle 1 10.07 10.63 10.17
Cycle 2 10.17 10.63 10.21
Cycle 3 10.21 10.62 10.25
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Simulation of stress-induced mechanical degradation
A structure-based modeling approach:
1. Resemble the working scheme of fiber bundle model
2. Crucial to develop breakable bond of ionomer bundles
3. Capable of describe the fracture process
(microvoid initialization craze elongation fibril breakage crack propagation)
4. TEM images of crack morphology of membrane samples 2016
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Scheme of fracture process
Microvoidformation
Craze formation
Craze elongation
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Scheme of fracture process
Fibril recombination
Fibril breakage
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TEM images of Crack Morphologies
(50nm)
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Molecular simulation of chemical degradation
25%50%75%
Weight loss
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Molecular simulation of membrane strain
Begin(50% weight loss)
End (strain 200%)
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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Building unit of carbon black
The energetically the most stable finite two-dimensional structure
Hexagonal graphite sheets
Structural property
Core-shell structure
Shell unit d = 3.7 nm
Core unit d = 0.7 nm
Carbon density = 2 g/cm
Particle size = 20 nm
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Morphology of carbon black
Slice view Surface view
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Porosity of carbon black
Slice view of porous carbon (dmean=2 nm ) Slice view of porous carbon (dmean=4 nm )
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Porosity of carbon black
Reproduce of targeted PSD in carbon particles
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Example of carbon degradation
Simulation of degradation of carbon black primary particles (20 nm) in catalyst layer of fuel cell
50%
weight loss
Experimental validation
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Background
Synthesis:
1. Pyrolysis in NH3
Create micropores
Dope Nitrogen at carbon surface
2. Catalytic sites hosted only in micropores (
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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Research strategy
Non-noble metal catalyst
Carbon support Nitrogen source Metal precursor
Mainly FeMolecules can be as simple as NH3
Nanoparticles with complex structure, composition, chemical and physical properties
The most important factor for the synthesis of non-noble metal catalysts (needs extensive study)
Desire to keep them standard and simple
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Carbon models
Graphitic
building
unit
d=2.7 nm d=3.2 nm d=3.7 nm
Graphitic
levelCarbon 44 (2006) 753761
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Formation of active site
Non-porous carbon
Active site formation in created micropore of carbon,
particularly between graphitic gaps, during pyrolysis
Need to be studied
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Oxidation of carbon
Original
carbon
Oxidized
carbon (50%
weight loss)
Carbon Vol. 36, No. 4, pp. 433 441, 1998
modelingexperiment
Formation of
micropores
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Porosity of oxidized carbon
Carbon Vol. 36, No. 4, 433-441, 1998
Growth of porosity in oxidized carbon
Applied materials & interfaces
VOL. 1, NO. 8, 16231639, 2009
Modeling
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Scale up active sites to
molecular level (nm)
Structure of active site
-3.901 eV
One possible structure of
Fe-N active sites from DFTPotential analysis in molecular simulation
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Modeling of active site formation
Initial state Equilibrated state
Observation:
1. Two types of active sites formed in carbon pores
2. Type I active sites is slightly less than Type II at Fe loading of 1 wt%
Fe loading 1 wt%
Graphene layers
Fe atoms (green bead)
Type I active site (-3.9 eV)
Type II active site (-1.9 eV)
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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Example of Pt degradation
Simulation of Carbon black supported Pt
catalyst in fuel cell
Pt degradation processes for
modeling
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Pt degradation: migration and coalescence
This process involves motion of Pt particles and coalescence where they meet on the carbon support
A sequence of STEM HAADF images showing the coalescence of Pt nanoparticles after (a) 45 s, (b) 115 s and (c) 165 s with the beam left on in between images.
This phenomenon is observed in simulation, but with a less pronounced effect
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Pt degradation: molecular snapshots
Pt on carbon black (Pt/C=1) at 300 K
Original Pt particles (1~3 nm)
Pt coalescence
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
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Challenges of FCVs for commercializationCost reduction, High power density, Durability, Sub-zero startup
Catalyst Layers (CLs) : Key componentReduction of Pt as a result of achieving high Pt utilization
Motivation
Scale / mm10-3 10-2 10-1 100 101
FabricationMaterial Structure Properties
Performance(Pt Utilization)
CL design for high Pt utilization
Size complexity in CLs
10-4
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0.0001
0.001
0.01
0.1
0 20 40 60 80 100 120
Relative Humidity / %
Ion
om
er
Con
cuctivity /
Scm
-1
0.0001
0.001
0.01
0.1
0 20 40 60 80 100 120
Relative Humidity / %
Ion
om
er
Co
ncu
ctivity /
Scm
-1
Ref: Iden et al., J. Electrocem. Soc., 156, B1078 (2009)
Motivation
Graphitized Ketjen
Black (without Pt)
Graphitized Ketjen
Black (without Pt)
Graphitized Ketjen
Black (with Pt)
Ketjen Black
(without Pt)
Ketjen Black and Graphitized Ketjen Black have similar macro-scopic structure, but different micro-scopic structure and surface properties.
How carbon supports and Pt particles affect structures and transport properties???? ???
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Effect of Water Contents (Bare GC)A
tom
Num
ber
1000
040
0
50
0
400
0
F
S
O (water)
O(H3O
+)
Z Direction Distance (Perpendicular to Carbon Sheet) / nm
Graphite sheet
0 1 2 3 4-1
510
2215
l = 22 l = 5
Z
Y
X
Y
(A)
(B)
A) Water adsorption on the surface of Nafion , not in the vicinity of GC.
B) Nafion thickness is independent of water contents. (No swelling)
Water Back bone Side chain GC
Water content: l
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5.6 nm
1.8 nm
Morphology of Nafion and Water
l = 22 (High water content)Thickness in x-direction: 2 [nm]
Y
Z
Nafion is attached to the GC via back bones. Sulfonic acid groups are mainly oriented away from the GC. Surface of Nafion film is hydrophilic.
Water Hydronium ion Back boneSide chain GC
Sulfonic acid group
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GC w/ 6 Pt, 12 Oligomers
Water occupied free carbon surface with increasing water contents in the case with Pt clusters.
Nafion and water coverage was independent of water contents in the case without Pt clusters.
GC w/o Pt, 12 Oligomers
Nafion & Water Coverage on Carbon
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Water H3O+ Back bone Side chain PtGC w/ 6 Pt clusters, 12 Oligomers
(a)(b)
(c)(d)
(e)
(f)
(a) (b)
(c) (d) (e) (f)
Area covered with water or
hydronium ions
Nafion adsorption on Pt clusters Small water coverage
Morphology of Nafion on Pt
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Nafion and Water Coverage on Pt
Water H3O+ Back
bone Side chain Pt
22 15 10 5
Water contents l
High Nafion coverage. Active sites slightly increase with water contents.
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A) Primary adsorption on sulfonic acid groups and Pt surface
B) Secondary adsorption and capillary condensation on Nafion
(A)(B)
Water H3O+ Back bone Side chain Pt
Water Adsorption on GC with Pt
l = 22 (High water content) l = 5 (Low water content)
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Morphology of water Adsorbed on the surface of Nafion film Affected by carbon support properties Affected by Pt
Morphology of Nafion ionomer Thin film (different from bulk
membrane) No swelling No penetration into primary pores Affected by Pt
Carbon
Ionomer(back bone)Ionomer(Side chain)
Water Secondary pore
Primary pore
H+
Transport of H+
Water clusters adsorbed on the surface of Nafion film mainly contribute to H+
transport.
How Relevant to CLs?
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0.0001
0.001
0.01
0.1
0 20 40 60 80 100 120
Relative Humidity / %
Ion
om
er
Con
cuctivity /
Scm
-1
0.0001
0.001
0.01
0.1
0 20 40 60 80 100 120
Relative Humidity / %
Ion
om
er
Co
ncu
ctivity /
Scm
-1
Answer to the First Question
Graphitized Ketjen
Black (without Pt)
Graphitized Ketjen
Black (without Pt)
Graphitized Ketjen
Black (with Pt)
Ketjen Black
(without Pt)
Different carbon support
Water morphology
Proton conductivity
Presence of Pt
Ionomer morphology
Water morphology
Proton conductivity
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NRC
1.
Ballard
2.
Toyota
3.
Nissan
2016
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CAD+FEM
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2016