Characterization of PEMFC Membrane Durability

Robert B. Moore and Kenneth A. MauritzThe University of Southern Mississippi

DuPont Fuel Cells SubcontractMay-16-2006

Project ID #FCP 19This presentation does not contain any proprietary or confidential information

2

Overview

• Start – Oct. 1, 2003• Finish – Sept. 30, 2006• Percent complete – 75%

• Barriers addressed– Membrane Durability– Membrane Cost

• Targets– Durability: 5000 hrs.

(transportation); 40,000 hrs. stationary

• Total project funding– DOE share– $200K– Contractor share– $50K

• Funding received in FY05• $100K• Funding for FY06• $50K

Timeline

Budget

Barriers

• DuPont Fuel Cells

Partners

3

Objectives

OverallTo produce lifetime improvements of low-temperature Proton Exchange Membranes (PEM) by seeking technologies that prevent premature membrane failure.

2005To develop characterization methods (based on control studies of 2004) to evaluate the chemical and physical effects of chemical degradation on membrane properties.

2006To extend the developed methods in the characterization of “used” membrane electrode assemblies subjected to accelerated-degradation operating conditions (i.e., post-mortem analysis).

4

Approach• Development and use of advanced characterization methods for

membranes subjected to accelerated chemical degradation (85% complete).

– Morphology• ESEM, TEM, AFM• SAXS, WAXD

– Chemical composition/structure• 19F SSNMR, FTIR• EDX

– Mechanical/electrical properties• Tensile testing• DMA (counterion effects)• Dielectric spectroscopy

– Transport properties• H+ conductivity• Water diffusion (PFGSE NMR) and solvent swelling• Ionic mobility and gas transport

• Correlate structure/property information with control virgin characteristics to elucidate degradation mechanisms (45% complete). Provide input to DuPont for the stimulation of membrane improvements.

5

Technical Accomplishments/ Progress/Results

• Identification of the molecular and morphological origins of dynamic relaxations in PFSA membranes. Changes in the relaxations foundto be characteristic of degradation.

• Discovery of the Tg of H+-form Nafion® at ca. -20 oC. Operation at temperatures above the α-relaxation (ca. 100 oC) can cause morphological changes over time.

• Peroxide degradation using Fenton’s reagent has the tendency to produce large voids (bubbles) in the membrane that may be linkedto mechanical failure (cracks, pinholes) in FC operations.

• Dielectric spectroscopy is found to be particularly sensitive tomolecular-level changes that occur in the membrane upon degradation.

• Broad-based characterization suggests that degradation affects the PFSA molecular weight distribution.

Variable Temperature Studies of Morphological and Molecular

Relaxations in Nafion®

200

400

600

800

1000

1200

1.52.0

2.53.0

3.5

50

100

150

200

250

300

Inte

nsity

(arb

. uni

ts)

q (1/nm)

Tempe

rature

(o C)

VT SAXS VT SS-NMR

TMA+

TEA+

TBA+

TPA+

Na+

Temperature (oC) -100 0 100 200 300

Tan δ

0

1αβ

αβ

DMA

Page, K.A.; Cable K.M.; Moore, R.B. Macromolecules 2005, 38, 6472.

ppm-180-160-140-120-100-80-60-40-20

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Correlation of DMA, SAXS, and NMR Results for TMA+ Nafion®

Temperature (oC)

50 100 150 200 250 3000.05

0.10

0.15

0.20

0.25

0.30

Nor

mal

ized

Inte

nsity

0.5

0.6

0.7

0.8

0.9

1.0

Spin

Diff

usio

n Ti

me

(ms)

15

20

25

30

35

40

45

50

Stor

age

Mod

ulus

(MPa

)

1e+3

1e+4

1e+5

1e+6

1e+7

1e+8

1e+9

1e+10ta

n δ

8

DMA of Partially-Neutralized Nafion®

(Variable H+/TBA+ Compositions)

Temperature (oC)-100 -50 0 50 100 150 200

Tan

Del

ta

0.0

0.1

0.2

0.3

0.4100% TBA+

75% TBA+

50% TBA+

35% TBA+

15% TBA+

10% TBA+

0% TBA+

Percent TBA+ Content0 20 40 60 80 100

Tem

pera

ture

(o C)

-40

-20

0

20

40

60

80

100

120

140

Alpha Relaxation Beta Relaxation

β-relaxation systematically shifts to lower temperatures with decreasing TBA+

content.

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VFT Behavior of the β-Relaxation in H+-form Nafion® Using Dielectric

Spectroscopy

3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

-5-4

-3-2

-10

1

0.01

0.1

1

10

100

1e-11e+0

1e+11e+2

1e+31e+4

1e+51e+6 -40

-20

020

40

Per

mitt

ivity

"

Frequency (Hz)

Temperature (o C)

log τ m

ax[s

ec]

1/Temperature [10-3K-1]

( ) exp( )V

BTT T0

⎛ ⎞τ = τ ⎜ ⎟−⎝ ⎠

Vogel-Fulcher-Tamman Fit

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Current Understanding of the Molecular Dynamics of Nafion®

The β-relaxation is attributed to the onset of segmental motions of the polymer chains within a static electrostatic network. This relaxation may be assigned to the genuine Tg of Nafion®.

The α-relaxation is attributed to the onset of long-range mobility of chains/sidechains as a result of destabilization of the electrostatic network (i.e., chain motions within a dynamic electrostatic network).

The chain motions at the α-relaxation also involve a significant contribution from the ion-hopping processes.

Overall Conclusion: Due to the dominance of the electrostatic network, the Tg for Nafion® is usually not the principle relaxation process. At temperatures above the α–relaxation, significant morphological reorganization may occur.

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Degradation of Nafion NR 112Scanning Electron Microscopy

• As-received samples are smooth

• Degraded samples are rough– Ruptured

bubbles (gas origin?)

– Crack in bubble (mechanical failure nuclei?)

Fenton’s DegradationAs Received

200 μm 100 μm

100 μm100 μmCro

ss-s

ectio

n vi

ewTo

p Vi

ew

12

Degraded Nafion SamplesScanning Electron Microscopy

NR-111 NRE-211 NRE-211-CS

NR-112 NRE-212 NRE-212-CS

Scale bar = 500 μm Conclusion: Bubble formation not prevented by CS.

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The α relaxation of the virgin membranes are all quite similar.

With degradation, the α relaxation shifts to higher temperatures.

Portions of the CS membranes are less susceptible to this behavior; however, other regions are greatly affected – this indicates some heterogeneity in the CS membranes.

DMA of As-received and Degraded Membranes

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Effects of Chemical Degradation on Tensile Properties of NRE 212

Degradation Time (hr)

AR 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45

Stra

in (m

m/m

m)

0

1

2

3

4

5

Strain (mm/mm)

0 1 2 3 4

Stre

ss (M

Pa)

0

10

20

30

40

As degradation time increases a general trend of decreasing strain at break is observed.

A loss of toughness through chemical degradation may be attributed to a change in the molecular weight distribution.

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Methanol and Water Uptake After Chemical Degradation

MembraneSolubility (MeOH Boil, 1hr)

VAR 112 Stable

VNR 112

VNRE 212

VNRE 212CS

DAR 112 Stable, weak film

DNR 112Partially dissolved after 1 hr

DNRE 212

DNRE 212CS

Disintegrated into gel-like particles after 1 hr

Completely dissolved upon boiling

AR = As-received (Extruded)

V = Virgin

D = Degraded

CS = Chemically Stable

VAR 112DAR 112

VNR 112DNR 112

VNRE 212DNRE 212

VNRE 212CS

DNRE 212CS

Sol

vent

Upt

ake

(mol

sol

vent

/mol

sul

fona

te)

0

10

20

30

40

50

60

methanol water

Significant increase in methanol and water uptake from extruded to solution cast membranes

Slight increase in methanol and water uptake upon degradation in solution cast films, except CS membranes

Stability of degraded films toward dissolution in boiling methanol suggests crosslinking during degradation

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Dielectric Spectra: Signature of Molecular Motions

Relaxation Peaks:– 2 β peaks at lower

temp. Major and minor β

peak components change with moisture

– α peak at higher temp.

– d.c. conductivity at high T and low f

Perm

ittiv

ity”

Frequency (Hz)

0.5

1.0

1.5

2.0

Per

mitt

ivity

minor β peak

major β peak

d.c. conductivity and/or α peak

Per

mitt

ivity

”

10 -310 -1

101103

105107

Frequency [Hz]

10-4

10-2

100

102

104

Perm

ittiv

ity

-200-100

0100

200

Temperature [°C]

α

β

Frequency (Hz) Temperature (oC)

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Degraded vs. Non-degradedNRE 212 Samples

CS vs. non-CS β peak - no difference in fmax

Degraded CS vs. degraded non-CS βpeak

-Upward shift in fmaxw/ degradation

Commensurate with a broader distribution of smaller chain lengths upon degradation

0.01

0.1

1

10

100

0.001 0.1 10 1000 105 107

212 CS-Cleaned

212 CS-Degraded

212-Cleaned

212-Degraded

ε''

f (Hz)

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Future Work• Expand chemical structure analysis (before and after

degradation) using high temperature 19F SSNMR.– Technique found to yield extremely high resolution structural

information in as-received/as-degraded membranes.• Complete method development with dielectric

spectroscopy – correlation with molecular level information from spectroscopy

• Correlate our solid-state characterization data with DuPont’s solution-state data for elucidation of a refined mechanism fo chemical degradation.

• Begin processing history – performance study for continuing projects.

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Summary•Beta relaxation assigned to the Tg of PFSIs, while the higher temperature alpha relaxation attributed to the onset of longrange chain motions in a dynamic network.•SEM shows chemical degradation which produces bubbles that become cracked – mechanical failure nuclei• Bubble formation not prevented by CS• DMA displays increase in alpha relaxation temperature with chemical degradation• Strain at break decreases with degradation time• Stability of degraded films in methanol suggest possible crosslinking• Dielectric spectroscopy (DS) is diagnostic of chemical degradation•Degradation broadens distribution - modes less distinct – shift toward faster motions

• Interpretation: broadened molecular weight distribution of shorter chain lengths.

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Publications• Phillips, A.K.; Moore, R.B. “Mechanical and Transport Property Modifications of

Perfluorosulfonate Ionomer Membranes Prepared with Mixed Organic and Inorganic Counterions,” J. Polym. Sci., Part B: Polym. Phys. 2006, in press.

• Page, K.A.; Cable K.M.; Moore, R.B. “SAXS Analysis of the Thermal Relaxation of Anisotropic Morphologies in Oriented Nafion® Membranes,” Macromolecules2006, in press.

• Page, K.A.; Cable K.M.; Moore, R.B. “Molecular Origins of the Thermal Transitions and Dynamic Mechanical Relaxations in Perfluorosulfonate Ionomers,”Macromolecules 2005, 38, 6472.

• Mauritz, K.A.; Moore, R.B. “State of Understanding of Nafion,” Chemical Reviews2004, 104(10), 4535.

• Page, K.A.; Moore, R.B. “Influence of Electrostatic Interactions on Chain Dynamics and Morphological Development in Perfluorosulfonate IonomerMembranes,” Mater. Res. Soc. Symp. Proc. 2005, 856E, BB6.4.1. Trophy Award

• Phillips, A.K.; Moore, R.B. “Morphological Manipulation and Plasticization of the Electrostatic Network in Perfluorosulfonate Ionomers,” Mater. Res. Soc. Symp. Proc. 2005, 856E, BB8.11.1.

• Osborn, S.J.; Moore, R.B. “Effects of Humidity and Partial Neutralization on the Mechanical Properties of Perfluorosulfonate Ionomer Membranes,” Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 2005, 46(2), 597.

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Presentations• “Probing the Molecular and Morphological Origins of Dynamic Behavior in

Nanostructured Membrane Systems,” Department of Chemistry, Virginia Tech, Blacksburg, VA, March, 23, 2006.

• “Probing the Glass Transition of Perfluorosulfonate Ionomers,” Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH, February 17, 2006.

• “Molecular and Morphological Origins of the Mechanical Behavior of Nanostructured Perfluorosulfonate Ionomers,” DuPont Fuel Cells, Wilmington, DE, September 27, 2005.

• “Correlations between Spectroscopic, Morphological, and Dynamic Mechanical Information to Determine the Molecular Origins of Thermal Relaxations in Perfluorosulfonate Ionomer Membranes,” Department of Chemistry, University of North Carolina, Chapel Hill, NC, September 21, 2005.

• “Alteration of membrane properties of Perfluorosulfonate Ionomers using Solution and Melt Procesing Procedures,” 2005 Fall ACS Meeting, Washington, DC, September 1, 2005.