Electrode Degradation in PEM Fuel Cells in Model Systems ... · Electrode Degradation in PEM Fuel...

Electrode Degradation in PEM Fuel Cells in Model Systems and PEMFC

Testing

F.A. de Bruijn V.A. Dam

G.J.M. Janssen R.C. Makkus

Presented at the 16th ECS Meeting Fall 2009, 4-9 October 2009, Vienna, Austria

ECN-L--09- 174 December 2009

El t d D d ti i PEM F l C ll i M d l S t d PEMFC T tiElectrode Degradation in PEM Fuel Cells in Model Systems and PEMFC TestingFrank de Bruijn1,2, Van Anh Dam2, Gaby Janssen1, Robert Makkus1

1 Energy research Centre of the Netherlands ECN , Petten, NL2 Inorganic Chemistry and Catalysis Group Eindhoven University of Technology Eindhoven NLInorganic Chemistry and Catalysis Group, Eindhoven University of Technology, Eindhoven, NL

216th ECS Meeting Fall 2009 Vienna, Austria

Fuel Cell Vehicles are on the road!

Toyota FCHV: driving range of 780 km (EPA)due to high fuel efficiency (> 80 mpge)

Honda Clarityhigh fuel efficiency (74 mpge)For lease for governments January 2007 first F-Cell >100,000 km and

2000 h without significant performance loss

216th ECS Meeting Fall 2009 Vienna Austria2

2000 h without significant performance loss

Field Trials

Coast to Coast USA, Simplon Pass, ,4500 km

p ,2000 m altitude, - 9 °C


Fuel Cell Busses

• 30 busses in 10 citiesV i li t diti• Various climate conditions

• Various options hydrogen production• Availability fuel cell busses higher than

diesel busses1 8 illi k d 116 000 h f• 1.8 million km and >116,000 h ofoperation by all Citaro busses by end February 2007


Improvements needed on short and long term

Short term: before large scale market introduction can take off

Cost Reduction

Improvement of Life time and Durability

L t f th i t d d f f ll k t t tiLong term: further improvements needed for full market penetration

Increase in Operating Temperature

Operation at Reduced Water Content


DoE analysis of the technology status versus the TargetsDoE analysis of the technology status versus the Targets

300 5000Durability (under cycling conditions)Fuel cell system costs *

150

200

250 $/kW

3000

4000 hours

$110 $

0

50

100

150

1000

2000$30

$110 $94

02002 2006 2007 2015

target 02003 2006 2015 target

* for 500,000 vehicles produced per yearCost reducing strategies could have a major impact on durability


J. Milliken et al., Hydrogen Technologies for the Developing World Forum, Moscow 2008

D E l i f F l C ll St k tDoE analysis of Fuel Cell Stack costs

Carlsson Fuel Cell Seminar 2005

Membranes: using thinner membranes leads to reduced materials costs but to increase of gas

membrane

seal

bos

assembly

Carlsson , Fuel Cell Seminar 2005 cross over, as well as to increased risk of damagingduring MEA manufacturing

cooling plate

electrodeGDL

biplar plate

Platinum: lowering its loading is viable,but how stable are these low loading electrodes?

Carlsson , Fuel Cell Seminar 2005


Durability: capability to work reliably under real life conditions

Freeze – thaw cycles

Load cycles

Cooling problems


g p

PEMFC degradation: literature reviewPEMFC degradation: literature review

Analyzing published long term PEMFC operation, we concluded that fuel cell degradation varies strongly with conditions:

F.A. de Bruijn et al. Fuel Cells, 8 (2008) 3

fuel cell degradation varies strongly with conditions:

Ideal conditions: RH = 100%; T = 75 °C; no cycling Materials: membranes ~ 125 μm; no ultralow Pt loadings

voltage decay: 1-2 μV.hr-1

lifetime: > 10,000 hrs

Voltage decay generally increases when: thin membranes are usedRH < 100%, but strongly depends on flow field design

lt li i li d i ll h OCV i i l d dvoltage cycling is applied, especially when OCV is includedT > 75 °Cfreeze/thaw cycles are applied


PEMFC degradationg

SEM cross section of PEMFC MEA

GDL Loss of hydrophobicity

Pt/C catalyst Carbon corrosion; Platinum sintering;

Platinum dissolution; Catalyst fouling

electrolyte Chemical degradation:Membrane thinning; pinhole formation;I h b t l iIon exchange by metal ions


D bili i f li iDurability issues for transport applications

•Load (voltage cycling) accelerates platinum dissolution/coarsening

• Start-up/shut down can lead to extremely high cathode potentials (up to 1.4 V)Carbon corrosion / platinum dissolution

Hi h ti t t & t t d l d t b d d ti• High operating temperatures & unsaturated gases can lead to membrane degradation

• System must operate in wide window of conditions: –30 and + 40 ºC ambient

• High purity requirements lead to high costs for hydrogen production and infrastructure Contaminants in hydrogen and air can lead to catalyst and membrane poisoning

When developing components for more cost effective fuel cell systems, these must meet the durability requirements to enable 5000 hrs of operation !!


Stability should be a selection criterion from the beginning!!!

For development of more stable and robust PEMFCs we need:

In situ Ex situ

Accelerated tests which are still representative for conditions in PEMFC applications

Model studies on components and materials at well-defined conditions

Characterization tools that can discriminate between various contributions

Advanced techniques for mechanistic studies

Quantitative data on degradation Understanding of degradationQuantitative data on degradation

Life-time predictionsPost test information

Understanding of degradation

Mitigation strategiesImproved materials


Content of this Work

In situMEA testing under stressing conditionsMEA testing under stressing conditions

MEA potenial and load cyclingMEA freeze/thaw cycles

Characterization by polarisation curves, EIS, CV, chronoamparometryPost test analysis (TEM,SEM)

Ex situPotentiostatic studies on Platinum, and Carbon Platinum/Carbon electrodes

Characterization by Quartz Crystal Microbalance, CVPost test analysis TEM


In Situ Accelerated Stress TestsIn Situ Accelerated Stress TestsQualify materials:

• Potential cycling (N2/H2) → catalyst• OCV hold → membrane• OCV hold → membrane• Potential hold ( ≥1.2 V) → carbon support

Qualify MEAs → membrane, catalyst, support, ionomer phase, GDLLoad or potential (H2/air) cycling :

• Variations in potential Pt dissolution, Pt particle growth, carbon corrosion• Exposure to H2/air(O2) membrane/ionomer chemical degradation• Humidity variation shrinkage/swelling of membrane/ionomer• Fuel starvation carbon corrosion

and coupling effects!and coupling effects!Freeze/Thaw cycling includes• Ice formation electrode delamination, GDL damage


AST i h b k d f ll l lAST with break-down of cell voltage losses1.5

reversible heat

),,(log

)()()()(

2

0

0

jPRfpOmijbRjE

jjjEjV

mpPtm

hf

transportORRcell

−⎟⎠⎞⎜

⎝⎛−⋅−=

−−−= Ω ηηη0.9

1.2

cell

/ V

oxygen reduction reaction

)(1i

j transportORRair Δ+Δ+Δ=Δ Ω ηηηη 0.3

0.6V c

ohmic

transport),,(log 2

1

jPRfiibRj mp

m

mhf ΔΔ++Δ⋅=

1i

0.00 0.2 0.4 0.6 0.8 1 1.2

current density / A cm-2

),(log)( 2

1

2 jRfiibRjj p

m

mhfO Δ++Δ⋅=Δη Transport losses depend on Rp and Pm,

contributions difficult to separate

Transport loss in pure O2 dominated by Rp


Rp can be measured by EIS

• H2/O2,

• Under load

Electrochemical Impedance Spectroscopy

0.10• Under load• High frequency

real axis intercept forOhmic resistanceN i t th d 0.05(Ω

.cm

2 )

Rhf+Rpol

• Nyquist arc: cathode 0.05

-Im Z

45o

Rhf

0.000.00 0.05 0.10 0.15

Re Z (Ω.cm2)

45o

Linear part: (Cdl from cyclic voltammetry) ⎟⎠⎞

⎜⎝⎛−+=

4exp)( πi

CωR

RωZdl

phf


Fast voltage cycles load profileFast voltage cycles – load profileH2/Air,80oC, 80% RH, ambient pressurePotential cycling between 0.7 and 0.9 V IR-corrected (or OCV ) (30 s hold at each)F ll h t i ti ft 0 1000 5000 10000 30000 l

1.0 100

Full characterization after 0, 1000, 5000, 10000, 30000 cycles

0 4

0.6

0.8

m2 ) a

n V c

ell (

V)

70

80

90

0.0

0.2

0.4

J (A

/cm

50

60

70

0 00 200 400 600 800 1000 1200

time / h

50

MEA1: Hispec 9100, MEA2: Hispec 9100,


N:C=0.7, NRE211CS N:C=0.55, NRE212CS

Fast voltage cyclingFast voltage cyclingMEA 1 MEA 2

Membrane damage


Fast voltage cycling

Low ionomer content:

NRE211CS: damage

Low ionomer content:Rp decreases

Green MEA 1Red MEA 2

NRE211CS: damageNRE212CS: stable


Fast voltage cycling: break down of ΔVair @ 0 4 A cm-2Fast voltage cycling: break down of ΔVair @ 0.4 A cm-2

Loss ECSA & activityAfter 30000 fast cycles

• slight decrease of ohmic losses and increase of kinetic lossesMEA 1 MEA 2 of kinetic losses

•change in in oxygen gain large contribution; depends on ionomer content

•substantial decrease of permeability (P )

MEA 1 MEA 210000 c ECSA 61% 55%activity 54% 57%30000 cECSA 95% 73% substantial decrease of permeability (Pm)

suspected

MEA1 MEA2

ECSA 95% 73%activity 87%

MEA1 MEA2


Post test analysis TEM cathode catalystPost test analysis – TEM cathode catalyst

Pt particle coarsening

0 x 10000 x 30000 x

20%

30%

rtic

les

01000030000

Pt particle coarsening

d 0 2.4 nm - SA loss 0%

d10000 3.8 nm - 38%10%

frac

tion

of p

a


d30000 5.8 nm - 55% 0%1 2 3 4 5 6 7 8 9 10

particle size (nm)

Post test analysis – SEM MEA cross section

After 30000 voltage cycles: membrane thinning observed for NRE211CS (↓ 15 μm)membrane thinning observed for NRE211CS (↓ 15 μm)no significant electrode thinning significant Pt deposition in NRE211CS near cathodesome Pt deposition in NRE212CS near anode

→ ECSA loss is combined effect of Pt particle growth and dissolution


MEA 1 (NRE211CS) MEA 2 (NRE212CS)

Load on/off cycles: break down of ΔVair @ 0 4 A cm-2Load on/off cycles: break down of ΔVair @ 0.4 A cm-2

Hispec 4000 N:C=0.7G.J.M. Janssen et al., J. Power Sources 191 (2009) 501

After 5 x 10

cycles / 600 hr 674 mV

Hi 9100 N C 0 7

y

at 80oC674 mV

Hispec 9100 N:C=0.55Hispec 9100 N:C=0.7

683 mV 679 mV


In situ AST

• Freeze/thaw experiments– -20 oC/ 65 oC20 C/ 65 C– no water removal– gas supply shut off

• Start /StopStart /Stop – 5 oC/ 65 oC– gas supply shut off

• 58 cycles, each 4th cycle 0.8

1

1.2

V)y , ycharacterisation

0 2

0.4

0.6

Cel

l vol

tage

(V

0

0.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Current density (A/cm2)


Fresh S/S end F/T end

F/T & S/S cyclingF/T & S/S cycling

0.9

1.2

cm2

80%

100%

g-1

0.3

0.6

Rp

/ O

hm c

20%

40%

60%

ECSA

/ m

2 g

0 08

0.00 20 40 60

# cycles

0%0 10 20 30 40 50 60

# cycles

Green S/SRed F/T

6

8

10

mA

cm

-2

0.04

0.06

0.08

Ohm

cm

2

0

2

4

0 20 40 60

i x / m

0.00

0.02

0 10 20 30 40 50 60

Rhf

/ O


0 20 40 60# cycles

0 10 20 30 40 50 60# cycles

0.20

Break down of ΔVair @ 0.4 A cm-2

0.10

0.15

η/ V

0 0

0.00

0.05

Δη

Δη ORR

Δη airΔη O2Δη ΩF/T

Sli ht h i -0.050 10 20 30 40 50 60

# cycles0.20

Slight changes in transport losses only

0 05

0.10

0.15

Δη

/ VΔη airΔη O2

Δη Ω

S/S

-0.05

0.00

0.05

0 10 20 30 40 50 60

Δη ORR

Δη Ω


0 10 20 30 40 50 60

# cycles

Conclusions in situ tests • Potential and load cycles at 80oC with breakdown of cell losses show

• Ohmic losses: constant although membrane damage could be observedOhmic losses: constant although membrane damage could be observed (thinning, increased H2 cross-over) for 25 μm membranes

• Kinetic losses increase due to decreased mass activity (Pt growth and dissolution)

• Transport losses dominate cell voltage loss and are strongly affected by catalyst layer composition

• Protonic resistance electrode stable or decreasing, depending on ionomer content

• Oxygen permeability seems strongly reduced (increased hydrophilicity?)• No significant carbon corrosion observed (potentials ≤ OCV)

• F/T cycles and S/S show:• No membrane damage• Slight increase in mass transport losses


• Stability of Pt, C, and Pt/C at operating conditions of fuel cell (elevated T, E)

Ex Situ Electrochemical study of platinum and carbon stability

• Real time monitoring of catalyst loss (Pt & C) by Quartz Crystal Microbalance (QCM) and Cyclic Voltammetry

Potentiostat

CE RE

PC

CE RE

• Δ(frequency) = - K Δ(electrode mass)

• High accuracy ~ 0 25 ng/cm2

QCM

QCM probe (WE)

• High accuracy ~ 0.25 ng/cm2

• Diversity of electrode materials

1M HClO


QCM probe (WE) 1M HClO4

Pt thin film mass change at 0.85V & 80°Cg

3

1

2

30.85 V

0.5 mA

(I) (II)

from the slope,

m (μ

g)

0

1E vs. RHE

0.6 0.8 1.0 1.2 1.4

Buildingoxide

Dissolvingoxide

the net platinum dissolution rate canbe calculated

-2

-1 layer layer

tox

t (min)0 100 200 300 400 500

-3ox


• During dissolution, first an oxide layer is f d hi h i f th di l d d di

100

80oC

Pt thin film dissolution

formed, which is further dissolved depending on T & E.

• The oxide layer is still continuously formed e (μ

g/h)

0 1

1

10 80 C

60oCy y

during dissolution.

• At E ≤ 1.15V, 80 ° C , log. of dissolution rate linearly depends on E (0 55 times / 0 1 V)

diss

. rat

e

0 001

0.01

0.1 60oC

linearly depends on E (0.55 times / 0.1 V).

• At E > 1.15 V and 80°C, the dissolution rate decreases due to passivating Pt oxide layer E vs RHE (V)

0.8 1.0 1.2 1.4 1.60.0001

0.001

• Dissolution rate increases 103 times when temperature increases from 60 to 80°C.

E vs. RHE (V)


Dam and De Bruijn, J. Electrochem. Soc. 154 (2007) B494

Carbon corrosion at 60°C (Vulcan XC72R)Carbon corrosion at 60°C (Vulcan XC72R)

μ g) 30

A)

0.00420oC Sample 1 fresh60oC Sample 1 after 20h, 1.05V

e ar

ea u

nit (μ

10

20 1.15 V

e ar

ea u

nit (

A

0.002

p60oC Sample 2 after 20h, 1.15V20oC Sample 2 fresh

er e

lect

rode

0

10

1.05 V

0.95 V

er e

lect

rode

-0.002

0.000

t (h)0 10 20 30 40 50

m p

e

-10

E (V)0.2 0.4 0.6 0.8 1.0

I pe

t (h)

• At 0.95 & 1.05V, mass increase corresponds to water uptake

• At E = 1.15V, mass decrease can be observed → CO2 formation (OLEMS)

E (V)


• Quinone/hydroquinone production increases with the potential.

St bilit f C b f ti f t t (V l XC72R)Stability of Carbon as function of temperature (Vulcan XC72R)

0.61 15 V Increase in temperature accelerates

2 ) 0.2

0.4

60oC

1.15 V carbon corrosion

m (μ

g/cm

2

0.0

m

-0.4

-0.2

80oC

0 10 20 30 40 50-0.6

80 C C


t (h) V.A.T. Dam et al., Fuel Cells 9 (2009) 453

Platinum/Carbon catalysts


Pt/C h t 1 05V & 80°CPt/C mass change at 1.05V & 80°C

2

At 80°C, a complex picture is observed which could be caused by:

• platinum oxidation and dissolution

0

platinum oxidation and dissolution

• carbon oxidation and corrosion

• combinations of these processesm (μ

g)

-4

-2

-8

-6

t (h)0 20 40 60

8

How can we discriminate between these processes??


pAt indicated time intervals, cyclic voltammograms are recorded

Pt/C dissolution at 1.05V & 80°CPt/C dissolution at 1.05V & 80 C0.004

freshafter 2hafter 23.5hafter 48h

A First cyclic voltammograms taken at various intervals during exposure to

0.002after 48hafter 68hafter 92h

during exposure to 1.05 V and 80°C.

D id f d d i

I (A) 0.000

Deep oxides are formed duringexposure to 1.05 V-0.002

0.2 0.4 0.6 0.8 1.0 1.2-0.004

1.05 V & 80oC


E (V)

Pt/C dissolution at 1.05V & 80°Ct/C d sso ut o at 05 & 80 C

0.004freshafter 2hft 23 5h

B Second cyclic voltammograms t k t i i t l

0.002

after 23.5hafter 48hafter 68hafter 92h

taken at various intervals during exposure to 1.05 V and 80°C.

I (A

)

0.000 23 – 92 h Pt surface area continuously decreases during potential hold

-0.002 Platinum oxide reduction potential shifts first to slightly more positive b h i l

0.2 0.4 0.6 0.8 1.0 1.2-0.004

1.05 V & 80oC but then to more negative values


E (V)

Pt/C dissolution at 1 05V & 80°CPt/C dissolution at 1.05V & 80°C)

0.78

0.79

m2 )

12

16

8

10Bmaximum average

particle size

Eox

.red.

peak

(V)

0.76

0.77

rface

are

a (c

m

8

12

x.re

d.pe

ak (m

C)

6

8

0 20 40 60 80 1000.74

0.75

0 20 40 60 80 100

Pt s

ur

0

4 QO

x

2

4

Pt surface area Charge of ox.red. peak

From the second CVs:• In the first 23 hours, averaged Pt particle size increases, then decreases.

t (h)t (h)

0 20 40 60 80 100

In the first 23 hours, averaged Pt particle size increases, then decreases.

• Pt surface area decreases with time.

It is very unlikely that Pt redeposition occurs at E = 1.05 V


Surface area decrease can be attributed to shrinking of particles

Pt/C dissolution at 1.05V & 80°C

16 16 2

area

(cm

2 )

8

12

d P

t (μ g

)

8

12

μ g) -2

0

Pt s

urfa

ce a

4

8

Dis

solv

ed0

4 m (μ

-6

-4 4.8 μg4.9 μg

t (h)0 20 40 60 80 100

0 -4

t (h)0 20 40 60

-8

6

• Pt loss determined from cyclic voltammetry data (assuming spherical particles) is in agreement with QCM mass loss in same time span

t (h)


• Pt dissolution rate ~ 0.14μg/h or 0.01μg/h.cm2

Electrodeafter 118 hours at 1.05 V

Fresh electrode


El t dElectrodeafter 118 hours at 1.05 V

Fresh electrode


Fresh electrode

Particle size distribution in Pt/C ink fresh and after 118 hrs, 1.05 V, 80 ºC

50

60 60After 118 hrs, 1.05 V, 80 ºCFresh

20

30

40

50

mbe

r of p

artic

les

20

30

40

50

mbe

r of p

artic

les

0

10

1 2 3 4 5 6 7

particle size (nm)

num

0

10

1 2 3 4 5 6 7

particle size (nm)nu

mp ( )

Average particle size: 4.0 ± 1.2 nm

particle size (nm)

Average particle size: 3.0 ± 1.4 nm


V.A.T. Dam et al., Fuel Cells 9 (2009) 453

Pt surface area lossPt surface area loss60 °C /1.05 V

80°C /1.05 V

80°C /1.05 V

80°C /1.15 V

80°C /1.15 V

80°C /1.15 V

80 °C /1.25 V

Rate of Pt areachange

(cm2 Pt/ cm2

-0.0016 -0.00849 -0.0072 -0.30044 -0.014 -0.015 -0.0779

Ptfresh.hr-1)

Total Loss of Ptsurface areaobserved (as % of

40% in262hours

71% in92 hours

83% in118hours

90% in 6hours

24% in17 hours

30% in20 hours

85% in 6hours

observed (as % oforiginal area)

hours hours

Compensating for decrease of average particle size:


Compensating for decrease of average particle size:85% loss of platinum

Conclusions ex situ tests

Potentiostatic hold at 1.05 V and higher leads to considerable loss of Platinum surface area. Platinum dissolution is dominant mechanism.

Note: high potentiostatic potential and high electrolyte volume prevents redeposition

Potentiostatic hold at 1.15 V and higher leads to carbon corrosion. Increase in temperature accelerated corrosion. Quinone formation preceeds corrosion, and takes place at lower potentials as well.

Decreasing temperature can prevent loss of platinum by dissolution by orders of magnitude


Overall conclusions

• Ex situ electrochemical model studies explain in situ MEA observations:• Under conditions applied during potential cycling platinum can dissolve• Under conditions applied during potential cycling platinum can dissolve,

leading to a large loss of electrochemical surface area• Potentials during potential cycling are too low for carbon corrosion, which

takes place at 1.15 V and higherF ti f i i ht l i i d h d hili it f

• Automotive durability targets are not met with present generation components:

• Formation of quinone groups might explain increased hydrophilicity of electrodes

• Electrode properties change considerably • Membranes contribute less to voltage decay, but still a concern


Acknowledgements

The authors wish to thank - E F Sitters A J Grootjes and M de la Rie (ECN) for MEA preparation andE.F. Sitters, A.J. Grootjes and M. de la Rie (ECN) for MEA preparation and testing. - K. Jayasayee and S. Celebi (TU/e) for electrochemical tests and TEM characterization

A Pf (JRC IE) f TEM l i- A. Pfrang (JRC-IE) for TEM analysis- M. Roos (ECN) for SEM analysis

This work was part of the Dutch EOS-LT PEMLIFE project, contract no.EOSLT01029, and EOS-LT Consortium PEMFC, contract nos. EOSLT 06005 d EOSLT 07005 t d b th D t h Mi i t f E i Aff i06005 and EOSLT 07005 supported by the Dutch Ministry of Economic Affairs.


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