PEM Fuel Cell Durability - Energy.gov · by H2O (Pt mobility?) Active Pt Catalytic Surface Area...
Transcript of PEM Fuel Cell Durability - Energy.gov · by H2O (Pt mobility?) Active Pt Catalytic Surface Area...
Fuel Cell Program
2005 DOE Hydrogen Program ReviewWednesday May 25, 2005
Rod BorupLos Alamos National Laboratory
PEM Fuel Cell Durability
John Davey, David Wood, Fernando Garzon, Michael Inbody, Dennis Guidry
Project #: FC40
This presentation does not contain any proprietary or confidential information.
Fuel Cell Program
Overview
Timeline2001: Project started as Fuel Cell Stack Durability on Gasoline Reformate2004: Changed focus to concentration on PEM H2Durability
Barriers• Durability (Barrier P)• Electrode Performance (Barrier Q)• Stack Material & Manf. Cost (Barrier O)
DOE Technical Targets (2010)
• Durability 5000 hours• Precious metal loading (0.2 g/rated kW)• Survivability (includes thermal cycling and
realistic driving cycles)
Budget• FY04: $900 k• FY05: $950 k
Fuel Cell Program
Technical Objectives:Quantify and Improve PEM Fuel Cell Durability
• Define degradation mechanisms• Design materials with improved durability
• Identify and quantify factors that limit PEMFC Durability• Measure property changes in fuel cell components during life testing
• Life testing of materials• Examine testing conditions, esp. drive cycle
• Membrane-electrode durability• Electrocatalyst activity and stability• Electrocatalyst and GDL carbon corrosion• Gas diffusion media hydrophobicity• Bipolar plate materials and corrosion products
• Develop and apply methods for accelerated and off-line testing• Improve durability
Fuel Cell Program
Approach to Durability Studies• Fuel Cell MEA Durability Testing and Study
• Constant roltage/current/power and power cycling (drive cycle)• 5 cm2, 50 cm2 and full-size active area (200 cm2) stack• VIR / cell impedance• catalyst active area• effluent water analysis
• in situ and post-characterization of membranes, catalysts, GDLs• SEM / XRF / XRD / TEM / ICP-MS / neutron scattering / H2 adsorption
• Develop and test with off-line and accelerated testing techniques• Potential sweep methods• Environmental component testing and characterization (GDL)• in situ XRD• Component interfacial durability property measurements• Membrane thinning and analysis
Fuel Cell Program
DOE Review MeetingReviewer Comments
FY2004 Reviewer Comments (Scores of 4 Outstanding and 1 Good)• The team seems to have the big picture well understood, which
consequently elevated its capability to do this analytical focused project
Strengths• Important work about durability• Excellent project execution with comprehensive set of tasks
Weaknesses• Did not show any collaboration outcome even with LANL group
FY2003 Reviewer comments:• The durability objective of this project is very important and I hope it will be actively addressed.
• I especially like the proposal of operating the system in a duty cycle operating mode.
• Introduction of drive cycle dynamics and start-up for next year is a plus …
• Need more fundamental work.
Fuel Cell Program
Interactions/Collaborations• Fuel Cell Materials
– MEAs (3M, Gore, LANL)– GDLs (Spectracorp, Toray, SGL, ETEK)– Catalysts (ETEK, SMP)
• Supporting measurements/interactionsAugustine Scientific (Chris Rulison – Contact angle)Oak Ridge National Laboratory (Karren More– TEM / SEM/EDS)Celanese Ticona (Rong Chen - SEM)LANL - NMT Division (Dave Wayne ICP/MS, Laser Ablation)LANL - LANSCE (Jaroslaw Majewski, Eric Watkins - Neutron Reflectivity)UNM (Plamen Atanassov – ICP/MS, Ion chromatography, Washburn Adsorption)NREL (Tony Markel – Fuel Cell Vehicle Drive Cycle)
• Stack: Teledyne Energy Systems
Fuel Cell Program
Electrocatalyst DurabilityDurability testing shows loss of active Pt surface area
0
5
10
15
20
25
120284 120384 120484 120584 120684 120784 120884 120984 121084 121184 121284
Time / sec
Pow
er /
Wat
t
Command PowerRead Power
• Drive Cycle testing shows faster degradation than steady-state testing
• Greater Pt Surface Area Loss• Larger Pt Particle
0
5
10
15
20
25
30
0 100 200 300 400 500 600 700 800 900 1000Time (hr)
True
Pt E
lect
roca
taly
tic
Surf
ace
Are
a (m
2 /g P
t)
Average
Linear (Average)
CV Set Points:100 mV/sCell Temp. = 80°CAnode/Cath. Humid. Temp. = 105/80°C
Average Rate of Loss of True Pt Electrochemically Active Surface Area = 71.4 cm2/g-Pt/hr
Catalyst Surface Area DuringConstant Power Operation Drive cycle ‘controls’ power
• Catalyst surface area decreases during testing• Pt Particles increase in size
Fuel cells in automotive applications cycle in power
Fuel Cell Program
Current Density (amps cm-2)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Volta
ge (v
olts
)
0.0
0.2
0.4
0.6
0.8
1.0
prior to CV cyclingafter 300 cyclesafter 900 cyclesafter 1200 cyclesafter 1500 cycles
Electrocatalyst DurabilityPotential Cycling Measurements
Polarization Curves For Fuel CellCycled between 0.1V - 1.2V
Voltage (mV)0 200 400 600 800 1000
Cur
rent
(mA
)
-200
-150
-100
-50
0
50
100
prior to CV cyclingafter 300 cyclesafter 600 cyclesafter 900 cyclesafter 1200 cyclesafter 1500 cycles
Number of CV Cycles0 200 400 600 800 1000 1200 1400 1600%
Initi
al P
t Cat
alyt
ic S
urfa
ce A
rea
50
60
70
80
90
100
0.1V - 1.2V
Cycling:10 mV/sec 100 mV/sec
Active Pt Catalytic Surface Area
Characterization CVsAfter Cycling between 0.1V - 1.2V
Cell 80 °CH2 226% RHAir 100% RH
• Potential cycling:• Simulates drive cycle• Accelerated catalyst aging
Variables Examined:Potential RangeTemperatureCycles vs. Time (Scan Rate)Relative HumidityCatalyst Loading
Fuel Cell Program
Potential & Temperature Effect on Catalyst Growth
Number of CV Cycles0 200 400 600 800 1000 1200 1400 1600%
of I
nitia
l Pt C
atal
ytic
Sur
face
Are
a
0
20
40
60
80
100
0.1V - 1.2V0.1V - 1.2V0.1V - 1.2V0.1V - 1.0V0.1V - .96V0.1V - .96V
% of Initial Active Pt Surface AreaAfter 1500 CV Cycles
0 20 40 60 80 100
Cat
hode
Pt G
rain
Siz
e (n
m)
2
3
4
5
6
7
8
90.4V - 0.96V0.1V - 0.96V0.1V - 1.2V0.1V - 1.0V
Correlation Between Pt Surface Area and Pt Particle Size
Cell 80 °CH2 226% RHAir 100% RH
Active Pt Catalytic Surface Areavs. Cycling Voltage Range
50%
60%
70%
80%
90%
100%
0 100 200 300 400
Time Above 0.9V (min)
% In
itial
Sur
face
Are
a 10 mV/sec50 mV/sec
Comparison of Scan Rates: Time
Cycling from 0.1V - 0.96V
• Peak potential has large effect on particle growth
• Direct relation between XRD particle size and surface area
• Pt Sintering correlates with # of cycles
• Time at peak potential has lower correlation
Comparison of Scan Rates: Cycles
50%
60%
70%
80%
90%
100%
0 300 600 900 1200 1500
Number of Potential Cycles
% o
f Ini
tial S
urfa
ce A
rea 10 mV/sec
50 mV/sec
Cycling from 0.1V - 0.96V
Fuel Cell Program
Temperature Effect on Catalyst Growth
0
1
2
3
4
5
6
Initial 60 80 100 120
Cycling Temperature / oC
Pt P
artic
le S
ize
/ nm
50%
60%
70%
80%
90%
100%
60 80 100 120
Cycling Temperature / oC
% P
erfo
rman
ce Initial300 Cycles600 Cycles900 Cycles1200 Cycles1500 Cycles
Pt particle size Normalized performance decaybased on polarization curve current density at standard conditions: comparison at 0.65 V, 80 oC, 100 % RH
50%
60%
70%
80%
90%
100%
0 500 1000 1500 2000# of Potential Cycles
% In
itial
Act
ive
Surf
ace
Are
a
60 C80 C100 C120 C
After 1500 cycles
Increasing temperature leads to increased catalyst sintering
Fuel Cell Program
Humidity Effect on Catalyst Growth
Number of CV Cyles0 200 400 600 800 1000 1200 1400 1600
% o
f Ini
tial S
urfa
ce A
rea
50
60
70
80
90
100
10% RH50% RH100% RH
0
0.5
1
1.5
2
2.5
3
3.5
10 50 100
Cycling Relative Humidity / %
Pt P
artic
le S
ize
/ nm
0
0.2
0.4
0.6
0.8
1
1.2
10 50 100
Cycling Humidity / RH%
% P
erfo
rman
ce Initial300 Cycles600 Cycles900 Cycles1200 Cycles1500 Cycles
Normalized performance decaybased on polarization curve current density at standard conditions: comparison at 0.65 V, 80 oC, 100 % RH
Pt particle size After 1500 cycles to 1.0 V
• Pt particle growth rate increases with humidity
• Growth mechanism enhanced by H2O (Pt mobility?)
Active Pt Catalytic Surface Area
(Cell Temp 80 °C, Cycled from 0.1V - 0.96V)
Fuel Cell Program
X-ray scattering provides Pt particle size and size distribution
• X-ray peak widths• Larger particles show narrower diffraction peaks• Least square minimization to Pt peaks
Pt Cathode Catalyst Aging Simulated particle size distribution:- continuous line log-normal type profile (coalescence growth mechanism)-- dashed line for an Oswald Ripening profile Ascarelli, Contini, and Giorgi J. Appl. Phys., Vol. 91, No. 7, 1 April 2002
OswaldRipening
LogNormal
• Catalysts have non-monotonic size distributions
– Log normal distribution typical for synthesized particles (J. App. Physics V47 5 1976)– Oswald Ripening if catalyst grows by atom migration from small crystals to large ones– Coalescence mechanism for particle-particle growth
Crystal size • 38 Å average (distribution)• 51 Å average (volume)Particle size distribution- redCumulative distribution-blueAqua –log normal Brown –sample log normal
Pt-Cathode Cycled 0.1-1.0 V All Samples show Log Normal Distribution(up to 1.5V cycling)
Suggests catalysts grow by particle coalescence for all testing and conditions
Fuel Cell Program
Electrode Carbon Corrosion
0
1
2
3
4
5
6
7
8
0.96 V 1.0 V 1.2 V 1.5 V
Cycling Potential
XRD
Pt /
Car
bon
Rat
io
0
1
2
3
4
5
6
7
8
10% RH 50% RH 100% RH
Cycling Relative Humidity
Pt P
artic
le S
ize
/ nm
Carbon corrosion measured by monitoring XRD Pt/C ratios
0
1
2
3
4
5
6
7
8
60 C 80 C 100 C 120 C
Cycling Temperature
XRD
Pt/C
Rat
io
Potential Effect Relative Humidity Effect
Temperature Effect
80 oC 80 oC100 % RH
100 % RH
0.1 – 0.96 V
0.1 – 0.96 V• Carbon corrosion occurs at:
• High potentials (> 1.0 V)• Low Humidity
• Temperature not observed to have an effect at 100 % RH and 1.0 V
Fuel Cell Program
Electrocatalyst Size GrowthXRD analysis of electrocatalysts
0123456789
Condit
ioned
60 C
0.4-0
.9680
C 0.
4-.96
100 C
0.4-0
.96
120 C
0.4-0
.9680
C - 0
.11 - .
96
80 C
- 0.11
- 1.0
80 C
- 0.11
- 1.2
80 C
- 0.1-
1.2
80 C
- 0.1-
1.5
0.1 - 0
.96 10
mV/se
c
0.1 - 0
.96 50
mV/se
c10
% RH
50% R
H0.2
mg P
t 0.1-
0.96
0.2 m
g Pt 0
.4-0.9
6
Pt P
artic
le S
ize
• Pt particle growth on cathode occurs for steady-state, enhanced with cycling• No growth in anodes Pt crystallites• No Oswald ripening mechanism during cycling (potential cycling up to 1.5 V)• Particle growth increases with temperature• Particle growth increases with humidification• Pt loadings (0.2 mg/cm2 to 0.4 mg/cm2) did not effect the Pt sintering• Carbon corrosion is present at high potentials, and low humidification
•Cycling increases Pt particle growth rate over steady-state operation
•# cycles has larger effect on catalyst sintering than duration at high potential
•Catalyst grows by particle coalescence regardless of cycling conditions
600 cycles
1500 cycles
in situ XRD Measurements
Fuel Cell Program
non-destructive particle size analysisFramed MEA for non-destructive testing in XRDUtilizes 0.022” Nafion to separate electrocatalyst layers
Design for potential controlled in situXRD testing, possible ‘real-time’synchrotron particle size analysis
Nafion .022”Used to separate catalyst layer signals during XRD MEA
Incident x-raysDiffracted x-rays
Fuel Cell Program
Loss of Hydrophobicity in Gas Diffusion Layer (GDL)
3 µm10k×
60 µm500×
Graphitized Fiber (~10 µm)Sintered PTFE Nanoparticles
(200-500 nm dia.)
1 µm30k×
MEA (3-Layer)
Aggressive Operating Environment (O2, H2, e- Passage, Voltage, H2O(l), low Nafion® pH, Electro-oxidation, Electro-reduction, Time)
From Membrane → SO4-2, SO3
-2, F-
Catalyst → Pt (Co, Cr, etc.)From C-Support → Na+, Ca+2, Zn+2, Fe+3 (10-100 ppm), CO2 (CO3
-2)Others (Electrocatalytic Byproducts, etc.)
Attacking Species on PTFE and Graphite
Composite Microstructure
Typical Bilayer GDL Courtesy SGL Carbon and Celanese Ticona
Fuel Cell Program
Comparison of Hydrophobicity MeasurementsBulk Material vs. Single Fiber
Fiber (10.74 ±0.10 µm dia.)
Wilhelmy Plate Contact Angle for Bulk Plain TGP-H 120 was 121.8 ± 0.3°, which
is ~41° higher than for a single fiber
Why?
108
112
116
120
124
128
132
0 2 4 6 8 10Time (min)
Th
eta
(d
eg
ree
s)
Toray TGP-H 060 (10 wt% PTFE), Slope = -0.385 deg/min
Toray TGP-H 060 (Plain), Slope = -0.957 deg/min
Dynamic Sessile-Drop θ for Toray TGP-H 060
Wilhelmy θwas 80.3 ±
0.2° for single Toray fiber
Fuel Cell Program
GDL Hydrophobicity Aging
UntreatedToray Fiber
80.3o
Toray TGP-H090 17.2 wt% FEP
94.6o
Toray TGP-H-06017.0 wt% FEP
83.2o
Toray TGP-H-09016.7 wt% FEP
88.4o
80oC 80oC
460 hr 680 hrN2 Air
116.0
118.0
120.0
122.0
124.0
126.0
128.0
130.0
132.0
0.0 200.0 400.0 600.0 Time (seconds)
Con
tact
Ang
le (d
egre
es)
B1 Unaged #1 Toray TGP-H 090B2 Unaged #2 Toray TGP-H 090B3 460 hr aged in 60C DI water N2 SpargeB4 460 hr aged in 80C DI water N2 SpargeB5 680 hr aged in 60C DI water Air SpargeB6 680 hr aged in 80C DI water with Air Sparge
SessileDrop Spreading Contact Angle
Single Fiber
o Unaged #1 Toray TGP-H 090o Unaged #2 Toray TGP-H 090o 460 hr aged in 60oC DI Water with N2 Toray TGP-H 060o 460 hr aged in 80oC DI Water with N2 Toray TGP-H 090o 680 hr aged in 60oC DI Water with Air Toray TGP-H 060o 680 hr aged in 80oC DI Water with Air Toray TGP-H 060
Loss of hydrophobicity increases with temperature and oxidation
Fuel Cell Program
Decrease in Hydrophobicity of FEP-treated Toray GDL
N2 (60°C) Loss = −0.020 deg/hrN2 (80°C) Loss = −0.024 deg/hr
90
100
110
120
130
140
150
0 100 200 300 400 500 600 700Time (hr)
Con
tact
Ang
le (d
eg)
N2 at 60°C N2 at 80°C Air at 60°C Air at 80°C
Nitrogen Atmosphere
Air AtmosphereAir (60°C) Loss = −0.032 deg/hrAir (80°C) Loss = −0.029 deg/hr
Toray TGP-H (17 wt% FEP)
020406080
100120140160180
InitialMS1
FinalMS1
InitialMS2
FinalMS2
InitialNMS1
FinalNMS1
InitialNMS2
FinalNMS2
Con
tact
Ang
le • Hydrophobicity loss on microporouslayer
• No loss in hydrophobicity of non-microporous layer
0.30
0.40
0.50
0.60
0.70
0.80
0.90
40 50 60 70 80 90 100 110Anode/Cathode Humidifier T (°C)
Cur
rent
Den
sity
(A/c
m2)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Hig
h-Fr
eque
ncy
Res
ista
nce
(mΩ
*cm
2 )
Fresh GDLs
1006-hr-Aged GDLs
Fresh GDLs
1006-hr-Aged GDLs
Cell Temp. = 80°CCell Voltage = 0.702Anode/Cathode Gas Pressure = 15/15
HRF
CurrentDensity
MS = Microporous layerNMS = non-microporous layer
Fuel Cell Program
SPEAR (Surface Profile Analysis Reflectometer )
Substrate
50Å layerθi θf
ki koutQz
Qz =4π sin(θ)/λ10 -7
10 -5
10 -3
10 -1
0.00 0.10 0.20 0.30
Ref
lect
ivit
y
Neutron Reflectometry
SPEAR:Film thicknessRoughnessCoverage
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
0.005 0.015 0.025 0.035 0.045 0.055 0.065 0.075 0.085 0.095 0.105
Qz (Angstrom-1)R
efle
ctiv
ity
1100 EW Raw Data1100 EW Error950 EW Raw Data950 EW Error5 wt% Solution Spin Coated
33 vol% Nafion (1:2) with Ethanol.
δfilm,NS3 = 2π/∆Qz = 61.5 nm
δfilm,NF4 = 2π/∆Qz = 69.8 nm
Thin-Film Nafion/Si(Comparison between 1100 and 950 EW)
Nafion 1100EW / 950EW Scattering Length Densities (SLDs) = 4.23x10-6 / 3.95x10-6
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
0.005 0.015 0.025 0.035 0.045 0.055 0.065
Qz (Angstrom-1)
Ref
lect
ivity
Bare Glassy C Raw DataBare Glassy C ErrorNafion 1100 on Glassy C Raw DataNafion 1100 on Glassy C Error
5 wt% Solution Spin Coated 33 vol% Nafion (1:2) with
Ethanol
δfilm,NGC1 = 2π/∆Qz = 62.1 nm
Thin-Film Nafion1100EW/Glassy-C
Calculated Scattering Length Density (SLD) for Glassy C = 5.10x10-6, Surface Roughness = 79.7 ÅGlassy Carbon (~ only) acceptable surface to study PEM Fuel cell polymer/carbon interactions
Fuel Cell Program
Membrane Thinning
Groove
Mag: 10x15
Post-characterization of membrane• Membrane degradation exemplified by cross-over, and hole formation
• Post analysis shows membrane ‘thins’• Peroxide formation a key to
membrane degradation
01020304050607080
0 200 400 600 800 1000
Time / hr
Cro
ss-o
ver C
urre
nt /
mA
• Examining membrane properties:• H form of membrane compressed in flowfield exposed to inert gas and temperature.
• No backing layers, catalyst layer, proton conduction, etc.
H2 Cross-over vs. Testing Time
0
0.2
0.4
0.6
0.8
1
1.2
80 C dry 120 C dry 80 C Humid 120 C Humid
Rel
ativ
e M
embr
ane
Thic
knes
s CompressedUncompressed
Regardless of compression, membrane showed thinning with exposure to dry gases
Fuel Cell Program
Future Plans• Catalyst Durability / Characterization
– in situ XRD analysis of Pt particle growth period– Modeling of particle growth to correlate growth conditions– Pt equilibrium diagram with PEM fuel cell conditions with F-, sulfates, etc.– Carbon bonding interaction with Pt - develop stable Pt/C catalysts– Pt alloys with higher stability– Examine non-carbon electrocatalyst supports for durability
• Carbon Corrosion– Further examine carbon corrosion in electrocatalyst layers and GDL materials
• Component Interfacial Durability Property Measurements– GDL material interfacial contact with the MEA catalyst layer– examine Nafion / PTFE degradation and carbon bonding via neutron scattering
• Membrane Degradation– examine conditions leading to membrane thinning– examine conditions leading to membrane failure
Fuel Cell Program
Publications and PresentationsMicrostructural Changes of Membrane Electrode Assemblies during PEFC Durability Testing at High Humidity Conditions, Xie et al., Journal of The Electrochemical Society, 152 5 A1011-A1020 2005
Durability Study of Polymer Electrolyte Fuel Cells at High Humidity Conditions, Xie et al., Journal of The Electrochemical Society, 152 A104-A113 2005
Effects of Long-Term PEMFC Operation on Gas Diffusion Layer and Membrane Electrode Assembly Physical Properties, Wood et al., 206th Meeting of The Electrochemical Society, Honolulu, Hawaii, October 5th, 2004
Long-Term Performance Characterization of Proton Exchange Membrane Fuel Cells, Wood et al., 206th Meeting of The Electrochemical Society, Honolulu, Hawaii, October 5th, 2004
PEM FUEL CELL DURABILITY, Borup et al., FY 2004 DOE EERE Hydrogen Program Annual Report
DURABILITY ISSUES OF THE PEMFC GDL and MEA UNDER STEADY-STATE AND DRIVE-CYCLE OPERATING CONDITIONS, Wood et al., 2004 Fuel Cell Seminar, San Antonio Texas, Nov. 1-5
PEM Electrocatalyst Durability Measurements, Borup et al., To be presented at the Electrochemical Society, June 12 – 17 2005, Las Vegas NV
PEM Electrocatalyst Durability Measurements, Davey et al., To be presented at the Fuel Cell Seminar, 2005, Palm Springs, CA, Nov. 14 -18, 2005
MASS-TRANSPORT PHENOMENA AND LONG-TERM PERFORMANCE LIMITATIONS IN H2-AIR PEMFC DURABILITY TESTING, Wood et al., To be presented at the Fuel Cell Seminar, 2005, Palm Springs, CA, Nov. 14 - 18, 2005
Fuel Cell Program
Hydrogen Safety
The most significant hydrogen hazard associated with this project is:
Hydrogen leak in the hydrogen supply coupled with ignition leading to a significant hydrogen fire.
Fuel Cell Program
Hydrogen Safety
Our approach to deal with this hazard is:
Hydrogen and carbon monoxide room sensors are electrically and computer interlocked with the test stand power and the gas supplies.
H2 sets off the H2 sensors (set at 10% of LFL)H2 also sets off the CO sensors, (set at 30 ppm)
Limits H2 far from the flammable or explosive limit
Work has been reviewed through Los Alamos National Lab’s safety programs:Hazard Control Plan (HCP) - Hazard based safety reviewIntegrated Work Document (IWD) - Task based safety reviewIntegrated Safety Management (ISM)