Membrane Durability in PEM Fuel Cells

Chemical Degradation

Vishal Mittal and Sathya Motupally

UTC Power

South Windsor, CT 06074

High Temperature Membrane Working Group Meeting

June 9, 2008

1

MEMBRANE DURABILITY

Outline

Membrane Durability

Chemical degradation mechanism

Impact of HT and low RH

Impact of cell components

2

MEMBRANE DURABILITY

Overview of mechanism HH2

H2O2Anode

Cathode H2O2

RadicalRadical Attack of pAttack of poolymerlymerHH22OO22 formaformattionion foformationrmation weak sitesweak sites

MaMatteerriiaall LLoocalicalizzed stressed stress CrossoverCrossover prproperoperttiesies prpromotes cromotes cracks,acks, fafailuilurree degrdegradeade fissurfissureses occuroccurss

HH22 OOOO 2

H2 + O2 (…)Æ OH• (& OOH• )+ polymer-CF3 Æ F-

Measured in reactant effluent (FER) D. E. Curtin, R.D. Losenberg, T.J. Henry, P.C. Tangeman, M.E. Tisack J. Power Sources 131 41-48 (2004).

3

MEMBRANE DURABILITY

Direct mechanism of OH generation on partially covered Pt

Pt

H+aq

e–

OOH

Pt

•OH

Pt

O

At low potential peroxide

formation takes place

O2

H2O2

Pt

•OH

Pt

Pt

O2

H+ aq

e –

Pt

High potentials This peroxide can not be detected because it decomposes at Pt with OH generation

Ea=0.0eV (@0V), Ea=0.36eV (@1V) − +O2 O2ads + e + H → OOHads H2O2ads →OHads OH•+ Ea=0.0eV

Ea=0.54eV OOHads + e− + H + → H2O2+•OOH → O OHads ads Ea=0.0eV (@0V), Ea=0.47eV (@1V)

Peroxide and OH radical formation take place by ORR if oxygen molecule bonds with one Pt site Adsorbed peroxide decomposes with formation OH radicals at any potential if peroxide molecule bonds with one Pt site. S. Burlatsky et al., Gordon Conference, July 2006, Bryant University, RI, USA

S. Motupally, Crete Degradation Workshop September 2007, Crete, Greece. 4

MEMBRANE DURABILITY

TEM analysis of degraded MEA

After OCV, 90 C, 30 % RH, ~100 hrs

~ 5 nm particles at ~300 nm spacing observed in BF TEM

S. Motupally, Crete Degradation Workshop September 2007, Crete, Greece.

5

1

MEMBRANE DURABILITY

ln F

ER

/ x1

0-6 m

ol h

-1 c

m-2

Effect of operating conditions

2

FER

/ x1

0-6 m

ol h

-1 c

m -2

Temperature

H2-O2, OCV At 30% RH

ln (FER) (1/T)∝ 2.70 2.80 2.90 3.00 3.10

T-1 / x 10-3 K-1

Relative Humidity

H2-O2, OCV At 90oC

FER -RH∝

0

-1

-2

-3

1

0

0 20 40 60 80 100

RH / %

“FER” = Total FER = Anode FER + Cathode FER Paik et al. 207th ECS Meeting, Abstract # 771, Quebec City, May 15 (2005) 6

MEMBRANE DURABILITY

Model simulations at low RH

FER

, mic

rom

ol/c

m2-

hr

0.60

0.50

0.40

0.30

0.20

0.10

0.00

Simulation: H2-O2 90 C OCV

0 20 40 60 80 100 Relative Humidity, %

Lower gas cross-over High gas cross-overHigher gas mole fraction Lower gas mole fraction in flow-field

S. Burlatsky et al., Gordon Conference, July 2006, Bryant University, RI, USA a

7

MEMBRANE DURABILITY

Effect of load

Hold allows more %F loss (less localized); Reinforced allows more %F loss

1. Reinforced membrane

17.8 18.3

4

20

7

Hold (Load) Hold (OCV) RH-Load Cycling

Cast

Cast

Reinf111

Reinf

(TFCP)

tolerates higher %Acc F

% A

cc F

luor

ide

at F

ailu

re

20loss.

2. Hold allows higher cumulative F loss at failure than Cycling.

10

0

C. Paik et al. 207th ECS Meeting, Abstract # 771, Quebec City, May 15 (2005) 8

MEMBRANE DURABILITY

Impact of catalyst stability 0

10 20 30 40 50 60

Pt PtCo PtIrCo

% E

CA

lost

OC

V , V

1

10

100

1000

50 100 150 200 250 300

TIME, hrs

µ

mol

e F-

em

itted

0.01

0.1

1

10

100

FER

, µm

ol/h

r

Pt

PtCo

PtIrCo

1.1

1

0.9

0.8

0.7

0.6

Accelerated Membrane Degradation 100C, 25% RH, H2/O2, OCV, 150 kPa

Pt PtIrCo

PtCo

0 50 100 150 200 250 300

time, hrs

FER : Pt > PtCo > PtIrCo Membrane durability Consistent with Pt stability

L. Protsailo, “Advances in Materials for Proton Exchange Membrane Fuel cell Systems”, Asilomar Conference Grounds, Pacific Grove, CA 2005 9

Tcell 120 0 c ycles Tcell 120 600 cycles#1 Tcell 120 1000 cy cles#1 Tcell 120 1400 cy cles#1 Tcell 120 1800 cy cles#1 Tcell 120 2200 cy cles#1

Tcell 120-0 cycles Tcell 120-200 cycles#2 Tcell 120-600 cycles Tcell 120-1000 cycles Tcell 120-1400 cycles Tcell 120-1800 cycles Tcell 120-2800 cycles

0.0.0011

0.0.11

11

MEMBRANE DURABILITY

Impact on ionomer durability – 10 % H2 in N2, low utilization – Electrode Ionic resistance

changes with time

– PtIrCo cathode prevents ionomer poisoning

H2H2 PP uummpp EE LLEECCTTRROODDEE RERE SSIISSTTANCEANCE

PtPt/C/C

Pt EMPA map after cycling

Pt PtCo PtIrCo HH22 PP uummpp ELEL ECECTTRROODDE RE REESSIISTSTAANNCCEE CC uurrvveess

PtPtIIrrCCoo/C/C

11

EELL

EECC

TTRROO

DDEE

R REE

SSIISS

TTAN

CE

AN

CE

,,

= -= -= -= -= -= -

Vo

ltage

,V Tcell=120-0 c ycles Tcell=120-600 cycles#1 Tcell=120-1000 cy cles#1 Tcell=120-1400 cy cles#1 Tcell=120-1800 cy cles#1 Tcell=120-2200 cy cles#1

Vo

ltage

, V

=======

Tcell=120-0 cycles Tcell=120-200 cycles#2 Tcell=120-600 cycles Tcell=120-1000 cycles Tcell=120-1400 cycles Tcell=120-1800 cycles Tcell=120-2800 cycles

0.0.11

0.0.0101

00 110000 202000 330000 404000 550000 0.00.00011 CuCurrrreenntt DeDe nnssiittyy,, mmAA//ccmm 22 00 110000 220000 330000 404000 550000 606000

CuCurrrreenntt DeDe nnssii ttyy,, mm AA//ccmm22

L. Protsailo, FC20, 2006 DOE Hydrogen Program Review, May 13-16, 2006, Arlington, VA 10

0.0.000011

BPSH

N112

MEMBRANE DURABILITY

Hydrocarbon membranes

PPEEM19M1988--BPBPSHSH CC yycclliinngg RReessuullttss -- OO CCP DP Deeccaayy && HyHyddrrogogenen xx --ovoverer CC uurrrreenntt 10100 C, 250 C, 25%% RH, 15RH, 150 k0 kPPa, 0.5Sa, 0.5SLM H2 [ALM H2 [Anonodede], 1.0 S], 1.0 SLLM OM O22 [Ca[Catthhodeode]]

1.1.22 88

0.20.2

BPSH

N112 CCrroo

sso

ssov

everr c

ucu

rrrreenntt

aatt 0 0

..44 V V

,,m

A/c

mA

/cm

2m

2

Load cycle protocol:

• 100oC, 25%RH

• 0.5 SLPM H2/1.0 SLPM O2

• 1min @ 1V, 1min @ 0.4V

11 66

0.0.88

OC

OC

VV,, V V

44

22

0.0.66

0.0.44

00

00 5500 110000 151500 220000 252500 330000 353500 440000

TTiimmee, ho, hoururss [Cy[Cy cclliinng prg protocotocolol: 1 m: 1 miin @n @ 11VV,, 11mmiinn @@ 00 ..44 VV]]

EMPA post test analysis: 20µm 60µm

• BPSH retained its thickness in load cycle test

L. Protsailo, FC20, 2006 DOE Hydrogen Program Review, May 13-16, 2006, Arlington, VA 11

MEMBRANE DURABILITY

Impact of seal/membrane interface

Impact of seal design and materials on lifetime under accelerated conditions (same membrane and catalyst)

Data taken by Toshiba Fuel Cell Power Systems (H. Chizawa, A. Maekawa, and T. Aoki)

S. Motupally, “Fuel Cells Durability & Performance”, Knowledge Foundation, Miami, FL 2006

12

MEMBRANE DURABILITY

Summary HT accelerates chemical degradation of membranes. Mechanical

degradation coupled chemical attack key for membrane durability.

At very low RH, chemical degradation rates decrease.

Impact of Pt dissolution on performance and membrane durability could be key for design of advanced HTM membranes.

Hydrocarbon membranes have low chemical degradation rates under OCV conditions. Improvement in mechanical properties key.

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