DECODEDECODEDECODEDECODE

Progress in the Understanding of PEFC Degradation Progress in the Understanding of PEFC Degradation related to Liquid Water interactions related to Liquid Water interactions

K. Andreas Friedrich,K. Andreas Friedrich, GeGermanrman

Aerospace Aerospace CenterCenter

(DLR), Institute of Technical Thermodynamics(DLR), Institute of Technical Thermodynamics

DECODEDECODEDECODEDECODEOutlineOutline

••

Introduction to the issue of degradation in PEFC fuel cellsIntroduction to the issue of degradation in PEFC fuel cells

••

Short description of DECODE ProjectShort description of DECODE Project

••

Selected examples of degradation and mitigation results Selected examples of degradation and mitigation results of:of:

••

Catalytic layer and membraneCatalytic layer and membrane

••

Gas diffusion layers (GDL)Gas diffusion layers (GDL)

••

Bipolar PlatesBipolar Plates

••

ModelingModeling

DECODEDECODEDECODEDECODEDevelopmentDevelopment und Research und Research ChallengesChallenges forfor Polymer Polymer ElectrolyteElectrolyte FuelFuel CellsCells

CostsCostsFor high volume production noble metal cost will dominate the syFor high volume production noble metal cost will dominate the system stem cost. Therefore, loading of <0.2gcost. Therefore, loading of <0.2gPtPt

/kW with power density >0.9 W/cm/kW with power density >0.9 W/cm22

(stack size) at >0.60V (heat removal) required.(stack size) at >0.60V (heat removal) required.

DurabilityDurability5500 h operation with only 50 to 75 mV voltage loss at 0.6 A cm5500 h operation with only 50 to 75 mV voltage loss at 0.6 A cm--22

300,000 load cycles with humidity changes300,000 load cycles with humidity changes 30,000 start / stop cycles 30,000 start / stop cycles ––

transient potentials of > 1.5 V transient potentials of > 1.5 V

Water managementWater managementcritical operation with compromise of high conductivity and formcritical operation with compromise of high conductivity and formation of ation of liquid water liquid water ~2500 starts at subzero temperatures with transformation from ic~2500 starts at subzero temperatures with transformation from ice to e to liquid waterliquid water

dimension changes of membranedimension changes of membrane

DECODEDECODEDECODEDECODEPolymer Polymer ElectrolyteElectrolyte Fuel Fuel CellsCells –– State of State of thethe ArtArt

Source: Ballard

2009

Function

of Operation

DECODEDECODEDECODEDECODEEU Project EU Project ““Understanding of Degradation Mechanisms to Improve Components and Design of PEFC”

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ElectrodeMembrane

Bipolar Plates

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Gas diffusion layers

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ObjectiveObjective of DECODE: of DECODE: Assessment of the different degradation processes with Assessment of the different degradation processes with focus on focus on liquid waterliquid water interactionsinteractions

Flooding may lead to severe

degradation of catalysts, gas

diffusion layers and bipolar plates

DECODEDECODEDECODEDECODEKey Degradation Key Degradation MechanismsMechanisms

Edge Edge failurefailureFiber Fiber puncturepunctureMembrane Membrane wear/tearwear/tear

duedue

to to shrinkageshrinkage

expansionexpansion

DamageDamage

byby

abrasionabrasion

oror

contactcontact

pressurepressure

HigherHigher

temperaturetemperature

exarcebatesexarcebates

degradationdegradationFreezingFreezing

leadsleads

to to volumevolume

expansionexpansion

of of waterwater

Catalyst

corrosion

(support, Pt dissolution)Catalyst

deactivation

and surface

loss

(Pt sintering)

Membrane thinning, pin

hole formation(Functionality

impaired

by

contaminants)

Bipolar Bipolar plateplate

corrosioncorrosion

Mechanical

Thermal

(Electro)chemical

Levers for mitigation: Operating Conditions - Design - Materials

inte

ract

ion

inte

ract

ion

DECODEDECODEDECODEDECODEMechanical + Chemical Stress: Edge FailureMechanical + Chemical Stress: Edge Failure

Degradation issues: Gas X-over on the edges leads to chemical degradation

Mechanical shear stress during dynamic operation (membrane expansion/shrinkage) Membrane exposed to GDL fiber puncture

Suitable edge/gasket designs will avoid these failure modes

Simple gasket design:

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CurrentCurrent DensityDensity Distribution Distribution withwith a Membrane a Membrane PinholePinhole

OCV Current

Density Fluctuations

Current

Density

Distribution at under load

(approx. 100 mA/cm2)

DECODEDECODEDECODEDECODECycling Conditions / DryCycling Conditions / Dry--Wet Transitions of the Wet Transitions of the CellCellTesting of Testing of AquivionAquivionTMTM

((SolexisSolexis) based MEA) based MEA--

Durability tests: Durability tests: Dynamic cycles protocol:Dynamic cycles protocol:40 s at 0.12 A/cm40 s at 0.12 A/cm²²20 s at 0.6 A/cm20 s at 0.6 A/cm²²Stationary protocol:Stationary protocol:fixed j = 0.6 A/cmfixed j = 0.6 A/cm²²

--

Conditioning in Conditioning in stationary conditions:stationary conditions:6060°°C, 1.5 bars,C, 1.5 bars,0/100%RH0/100%RH

i = 0.6 A/cmi = 0.6 A/cm²²

0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500t (s)

DECODEDECODEDECODEDECODECycling Conditions / DryCycling Conditions / Dry--Wet Transitions of the CellWet Transitions of the CellEnhanced durability by Enhanced durability by preventing edge failurepreventing edge failure

DDynamicynamic

cycles with the cycles with the AquivionAquivion

membrane during ~1000 hours membrane during ~1000 hours (60000 cycles) before membrane failure in the active zone(60000 cycles) before membrane failure in the active zone

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Polarization curves test 130-A41 (E79-03S edge protected)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,2 0,4 0,6 0,8 1 1,2

Current Density [A/cm²]

Volta

ge [V

]130-A41 BOL 130-A41 hour 100130-A41 hour 200 130-A41 hour 300130-A41 hour 400 130-A41 hour 500130-A41 hour 600 130-A41 hour 700130-A41 hour 800 130-A41 hour 900

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Comparison Cycling and Stationary OperationComparison Cycling and Stationary OperationDECODEDECODEDECODEDECODE

200

300

400

500

600

700

800

900

1000

0 200 400 600 800 1000 1200i (mA/cm²)

U (m

V)

MAqPP618 80°C 40/60%RH

MAqPP618 80°C 40/60% 245h

MAqPP619 80°C 40/60%RH

MAqPP619 80°C 40/60%RH 306hCycling stationary

•• OCV still not affected for this durationOCV still not affected for this duration••

Important performance degradation from low to Important performance degradation from low to

high current densities with an acceleration for the high current densities with an acceleration for the cycling operation compared to the stationary test cycling operation compared to the stationary test due to electrode degradationdue to electrode degradation

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0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

0 200 400 600 800 1000 1200

Time (Hour)

Volta

ge (V

)

N117N115NR212NR211

Computer update

Air compressor shutdown

Potential decay trend of individual cells for 1000 hours

•

Significant degradation occurred for NR211 cell during 600 to 800hours;

•

Significant degradation occurred for NR212 cell during 800 to 1000hours.

•

Devastating degradation for NR211 cell during 800 to 1000hours.

Effect of membrane thickness on Effect of membrane thickness on degradationdegradation

HGF

DECODEDECODEDECODEDECODEStructure of MEA before and after OCV Structure of MEA before and after OCV ageingageing

Micro-structure Change of the MEA cross section before and after OCV degradation

before after

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(a) TEM of as-received MEA(b) TEM of anode catalyst after degradation. (c) TEM of cathode catalyst after degradation

Structure of Catalyst before and after OCV ageingStructure of Catalyst before and after OCV ageing

DECODEDECODEDECODEDECODEStabilized Stabilized AquivionAquivionTMTM

MembraneMembrane

0.0

1.0

2.0

3.0

4.0

5.0

0 50 100 150 200 250 300 OCV duration (hrs)

Hyd

roge

n C

ross

over

(mA

/cm

²) 70 °C 50 % RH std

70 °C 50 % RH stab

90 °C 50 % RH std

90 °C 50 % RH stab

standardgrades

stabilized grades

Polarisation curves E79-03S + LT250EW (25cm² cell)0.6 V constant (=~1A/cm²)

100% reactant humidification - 75°C - 2.5 Bar abs

0,40

0,50

0,60

0,70

0,80

0,90

1,00

0,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40

Current Density (A/cm²)

Cel

l Vol

tage

(V)

BOL Hour 1000 Hour 2000 Hour 3000 Hour 4000

OCV at 75 °C

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C-F (backbone)

C-S (Sulfonicacid group) C-O-C (ether

side chain)

C-S (Sulfonicacid group)

After 600h of cycling operation

No evidence for loss of any functional groups whereas already observed in other studies Apparent promising chemical stability of the Aquivion membrane But no OCV and < 600 hours other analyses after > 1000 hours to be discussed

Vibrational modes visible with Raman spectroscopy

Spectroscopic Characterization of Stabilized Spectroscopic Characterization of Stabilized AquivionAquivionTMTM MembranesMembranes

Confocal Raman microscopy (Chalmers University)

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nm

nA

Conductive Atomic Force Microscopy (DLR)Conductive Atomic Force Microscopy (DLR)

large conductive areas composed of small conductive channel featlarge conductive areas composed of small conductive channel features ures

with sizes limited by the spatial resolution of the tip. with sizes limited by the spatial resolution of the tip. No changes upon fuel cell operation detected so far. No changes upon fuel cell operation detected so far.

SSuperioruperior water retention capabilities than water retention capabilities than NafionNafion 112 under similar 112 under similar conditionsconditions

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Conductivity

measurements

image

AFM Characterization AFM Characterization of Stabilized Stabilized AquivionAquivionTMTM

MembranesMembranes

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Partial decomposition of PTFE identified by XPS PTFE decomposition mainly on the anode Decrease of hydrophobicity Changed water balance Reversible loss of performance

Decomposition of Polymer in Electrode and GDL

DECODEDECODEDECODEDECODEForceForce -- Separation Separation CurveCurve as as measuredmeasured byby AFM AFM ((„„HarmoniXHarmoniX““--Mode Mode VeedoVeedo))

Dissipation Energy

x/nm

Peak Force

Adhesion

Force

x/nm

F/nN

StiffnessΔF/ Δx

0

Surface

DECODEDECODEDECODEDECODEAFM measurement of Gas Diffusion Layer (GDL)AFM measurement of Gas Diffusion Layer (GDL)

before operation

Z-Range: 768 meVZ-Range: 768 meVZ-Range: 768 meV

Anode

1 μm

after operationCathodeafter operation

Overall decrease in dissipation area -

at cathode more than at anode

less PTFE at cathode

GDL with Micro Porous Coating at ambient conditions / GDL with Micro Porous Coating at ambient conditions / Dissipation distribution after 12 h operationDissipation distribution after 12 h operation

DECODEDECODEDECODEDECODEExperimental Ageing of BPP: 5 Stacks testedExperimental Ageing of BPP: 5 Stacks testedDurability run of DECODE Stacks @ 600 mA/cm2

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 200 400 600 800 1000 1200

time in h

cell

volta

ge in

V

DECODE6 UaveSS904L at CEA

DECODE4 UaveSS904L at ZSW

DECODE3 UaveSS316L at DANA

DECODE6 UaveSS316L with Au at DANA

DECODE8UaveSS316L with Au at ZSW

DECODEDECODEDECODEDECODETaskTask 5.2: Long term testing Task 5.3 experimental 5.2: Long term testing Task 5.3 experimental determination of ageing BPP: corrosion analysisdetermination of ageing BPP: corrosion analysis

Cell

4 Cell

5

Deg

rada

tion

Rat

e in

V

/h

No obvious correlation between contamination from metallic ions and degradation within a Stack

0

20

40

DECODEDECODEDECODEDECODETaskTask 5.2: Long term testing Task 5.3 experimental 5.2: Long term testing Task 5.3 experimental determination of ageing BPP: corrosion analysisdetermination of ageing BPP: corrosion analysis

Deg

rada

tion

Rat

e in

V

/h

0

20

40

Correlation between contamination from water ions and degradation within a Stack?

DECODEDECODEDECODEDECODEComparison of Corrosion and CFD Comparison of Corrosion and CFD modelingmodeling ResultsResults

Relative humidity in the middle of the channel

Areas (channel and landing) with a few traces of corrosion which has in the simulation a lower velocity of the gas.Areas with stronger traces of corrosion which has no different velocity in the simulation.

DECODE3 Cathode 4

Inlet

Outlet

Inlet

Outlet

DECODEDECODEDECODEDECODEModeling of Liquid Water in Channels Modeling of Liquid Water in Channels Volume Of Fluid Numerical method (JRC)Volume Of Fluid Numerical method (JRC)

VOF method: developed in the 1980sa volume fraction

indicator C is used to determine the location of the indicator C is used to determine the location of the

interfaceinterface

Cair

= 1 Cwater

= 0

Cair

= 0 Cwater

= 1

Cair

= 0.08 Cwater

= 0.92

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DECODEDECODEDECODEDECODEDECODEDECODEDECODEDECODEModellingModelling of Liquid Water in Channels of Liquid Water in Channels GeometryGeometry

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• Influence of the bipolar plate contact angle:

• Initial conditions:

WP 5: Simulation WP 5: Simulation –– Results (JSR)Results (JSR)

110°

85°

65°

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DECODEDECODEDECODEDECODEInfluencesInfluences of (liquid) Water on Degradationof (liquid) Water on Degradation

••

Liquid Liquid waterwater

leadlead

to to enhancedenhanced

catalystcatalyst

layerlayer

degradationdegradation••

WetWet--drydry

cyclescycles

leadlead

to to mechanicalmechanical

stress on stress on membranesmembranes

••

Liquid Liquid waterwater

playsplays

a a rolerole

in in lossloss

of of hydrophobicityhydrophobicity

of of GDLsGDLs and and otherother

porousporous

mediamedia

••

Liquid Liquid waterwater

isis

importantimportant

forfor

corrosioncorrosion

of of biploarbiploar

platesplates

Acknowledgement:

Partners of DECODE:

Acknowledgement:

Partners of NRC-HGF PEFC Durability Project:Riny Yuan, Shengsheng Zhang, Cheng Huang, Shuai Ban, Jun Shen, Haijiang Wang, Institute for Fuel CellInnovation, National Research Council, Canada

R. Hiesgen, University of Applied Sciences Esslingen, talk on Wednesday at 9:45

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DECODEDECODEDECODEDECODEFiber Penetration Fiber Penetration intointo CatalyticCatalytic LayerLayer and Membraneand Membrane

DECODEDECODEDECODEDECODEComparison of normal and fast GDL DegradationComparison of normal and fast GDL Degradation

Dissipation

GDL 1 BC25:

after 650 h decrease at cathode

small increase at anode

GDL 2 DC35:

after 12 h large decrease at anode

and larger decrease at cathode

GDL 3 BC25: after 1 h cooking with H2

O2

Adhesion GDL 1: after 650 h decrease at cathode twice of anodeGDL 2: after 12 h smaller decrease at anode than at cathodehole in membrane leads to very large degradation and failure

GDL 2

Before Anode Cathode

Force/nN

GDL 1

14

16

18

20

22

24

26

28

H2

O2

GDL 3

E/meV

Before Cathode

GDL 1

GDL 2

Anode

4

5

6

7

8

H2

O2

GDL 3

DECODEDECODEDECODEDECODE„„HarmoniXHarmoniX““--Mode for Tapping Mode AFMMode for Tapping Mode AFM

http://coen.boisestate.edu/bknowlton/research/resources/files/DS82_HarmoniX_Nanoscale_Material_Property_Mapping.pdf

Force-distance-curve reconstructed by Fourier synthesis from a pulse applied by the tip at every image point

Evaluation of adhesion force, phase shift, stiffness, maximal force, and dissipation energy

„HarmoniX“-Mode (Multimode, Veeco Instr.) AFM tip for sensing higher

harmonic vibrations

DECODEDECODEDECODEDECODETechnical results / Technical results / TaskTask 3.3 3.3 -- ExEx--situ characterization (DLR)situ characterization (DLR)••

Progress & ResultsProgress & Results--

Conductive Atomic Force Microscopy (DLR)Conductive Atomic Force Microscopy (DLR)First characterization of First characterization of AquivionAquivion

membranemembrane

Topography of Topography of AquivionAquivion

E79E79--03S indicates some 03S indicates some nanoscalenanoscale

roughness and some occasional roughness and some occasional holes. holes. In general topography images indicate a smooth polymer surfaceIn general topography images indicate a smooth polymer surface

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Topography

nm

µ