Processing-Performance Relationships for Perfluorosulfonate Ionomer … · 2008-09-10 · Introduction to the structure and morphology of perfluorosulfonate ionomers (PFSI’s) Ionic

Processing-Performance Relationships for Perfluorosulfonate Ionomer Membranes

ProcessingProcessing--Performance Relationships for Performance Relationships for Perfluorosulfonate Ionomer MembranesPerfluorosulfonate Ionomer Membranes

Robert B. MooreShawn Osborn, Gilles Divoux, and Jong Keun Park

Virginia TechDepartment of Chemistry

[email protected]

Moore Research Group

Topics to be DiscussedTopics to be DiscussedTopics to be Discussed

Introduction to the structure and morphology of perfluorosulfonate ionomers (PFSI’s)

Ionic domains (clusters)Backbone crystallinity

Processing of PFSI membranes and MEAs– Solution-Processing considerations– Melt-Processing considerations

Impact of processing and morphological manipulation on fuel cell performance.General conclusions and acknowledgements


Complex Morphology of Perfluorosulfonate Ionomers

Complex Morphology of Complex Morphology of Perfluorosulfonate IonomersPerfluorosulfonate Ionomers

Mauritz, K.A.; Moore, R.B. “The State of Understanding of Nafion,” Chem. Rev. 2004, 104(10), 4535.

SCF2

CF2

OCF

CF2O

CF O

O

O

CF3

CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2CF2

CF2

S

CF2CF2

OCF

CF2O

CF

O

O O

CF3

CF2

CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2

CF2 CF2

S CF2

CF2 O

CF CF2

O CF

O

O

O

CF3CF2

CF2CF2

CF2

CF2CF2

CF2CF2

CF2

CF2CF2

SCF2

CF2O

CFCF2

OCF

OO

O

CF3

CF2

CF2CF2

CF2CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2CF2

CF2

CF2

S

CF2 CF2

O CF

CF2 O

CF

O

OO

CF3

CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2CF2

CF2CF2S

CF2

CF2

O

CF

CF2

O

CF

OOO

CF3

CF2 CF2

CF2 CF2

CF2

CF2

CF2

CF2

CF2

M+

M+

M+

M+

M+

M+S

CF2CF2

OCF

CF2O

CF O

O

O

CF3

CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2CF2

CF2

S

CF2CF2

OCF

CF2O

CF

O

O O

CF3

CF2

CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2

CF2 CF2

S CF2

CF2 O

CF CF2

O CF

O

O

O

CF3CF2

CF2CF2

CF2

CF2CF2

CF2CF2

CF2

CF2CF2

SCF2

CF2O

CFCF2

OCF

OO

O

CF3

CF2

CF2CF2

CF2CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2CF2

CF2

CF2

S

CF2 CF2

O CF

CF2 O

CF

O

OO

CF3

CF2

CF2

CF2

CF2

CF2

CF2CF2

CF2CF2

CF2CF2S

CF2

CF2

O

CF

CF2

O

CF

OOO

CF3

CF2 CF2

CF2 CF2

CF2

CF2

CF2

CF2

CF2

M+

M+

M+

M+

M+

M+

SO3-SO3-

SO3-

SO3-

SO3-SO3-

SO3-

SO3-SO3-

SO3-

SO3-

SO3-

SO3-

SO3-

SO3-SO3- SO3-

SO3-

SO3-SO3-

SO3-

SO3-

1.0 nm4.0 nm

5.0 nm

Cluster-Network Model

Hsu, W.Y.; Gierke, T.D. J. Memb. Sci. 1983, 13, 307.

Nafion®

Rubatat, L.; Rollet, A.-L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050.

n = 6.6 for 1100 EWca. 15 mol% sidechains

Rod-like Aggregates

(CF2-CF2) (CF2-CF)

O

CF2

CF3CF

OCF2

CF2

n

SO3X


Morphological Models of PFSAsMorphological Models of PFSAsMorphological Models of PFSAs

Lc

(c)(b)

Li

Lc

Lc

Li(a)

Lc

(c)(b)

Li

Lc

Lc

Li(a)

Fringed-MicelleModel

LamellarModel

Semi-CrystallineRod-like Model

Litt, M. H. Polym. Prepr. 1997, 38, 80.Kim, M-H.; Glinka, C.J.; Grot, S.A.; Grot, W.G. Macromolecules 2006, 39, 4775.

Gebel, G.; Diat, O. Fuel Cells 2005, 5, (2), 261.

Crystalline order and proximity to the ionic domains vary widelyCrystalline order and proximity to the ionic domains vary widely among current models.among current models.

Synchrotron SAXS/WAXD experiments at the APS (Argonne Nat. Lab) Synchrotron SAXS/WAXD experiments at the APS (Argonne Nat. Lab) are focused on are focused on the crystalline character of PFSA membranes following various ththe crystalline character of PFSA membranes following various thermal and processing ermal and processing treatments.treatments.


New Morphological Model for PFSIs (Klaus Schmidt-Rohr and Q. Chen, Nature Materials, 2007)

New Morphological Model for PFSIsNew Morphological Model for PFSIs ((Klaus SchmidtKlaus Schmidt--Rohr and Q. Chen, Rohr and Q. Chen, Nature MaterialsNature Materials, 2007, 2007))

Extended-chain crystallites are randomly-distributed and locally-parallel to the rod-like, inverted micelle clusters. Crystallites may template and “stabilize” the development of the polar domains.

q-1.0


Processing Effects on the Scattering (SAXS) Behavior of Nafion®

Processing Effects on the Scattering Processing Effects on the Scattering (SAXS) Behavior of Nafion(SAXS) Behavior of Nafion®®

q (Å-1)

0.01 0.1

Inte

nsity

(cm

-1)

1

10

AR 117 CS H+Solution-Processed H+NRE 212CS H+

q-2.8

q-1.6

q-1.0


SAXS of Nafion Precursors (Various EW)

SAXS of Nafion Precursors SAXS of Nafion Precursors (Various EW)(Various EW)

q(A-1)

0.01 0.1

Inte

nsity

(a.u

.)

0.1

1

10

100

1000

1000EW 1100EW 1200EW 1500EW

Samples Low q Slope

1000 EW -1.304

1100 EW -1.304

1200 EW -1.386

1500 EW -1.303


Manipulation of PFSI Properties by Variable Processing Procedures

Manipulation of PFSI Properties by Manipulation of PFSI Properties by Variable Processing ProceduresVariable Processing Procedures

Commercial processing of Nafion® involves 1) the melt- processing of a non-ionic, sulfonyl fluoride precursor followed by solid-state hydrolysis/neutralization, or 2) dispersion-casting of the ionized polymer. This yields the “As-Received State.”

Solution-ProcessingMelt-Processing

Both procedures are used in the state-of-the-art MEA and thin film production processes for PEM fuel cells.


PEM Processing in Fuel Cell SystemsPEM Processing in Fuel Cell SystemsPEM Processing in Fuel Cell SystemsXie, J.; Garzon, F.; Zawodzinski, T.; Smith, W. J. Electrochem. Soc. 2004, 151(7), A1084.

State-of-the-art MEA Preparation

Melt-Processing of NafionMoore, R.B.; Cable, K.M.; Croley, T.L J. Membrane Sci. 1992, 75, 7 .

Melt-Processable NafionMoore, R.B.; Cable, K.M.; Croley, T.L J. Membrane Sci. 1992, 75, 7 .

Solution-Processing of NafionMoore, R.B.; Martin, C.R. Macromolecules 1988, 21, 1334 .

(typically 50 oC)

Recast Nafion

New 1 and 2 mil membranes are now solvent cast.

Controlled PEM processing in MEA fabrication is of fundamental importance!


Solution-Processing of Nafion® MembranesSolutionSolution--Processing of NafionProcessing of Nafion®® MembranesMembranes

Solvent Evaporation

Substrate

Substrate

@ Low Temp

Continuous Networkof Chain

Entanglements@ High Temp

Discreet ParticleCharacteristics

Nafion®

in “solution”

Nafion®

Aggregates in typical dispersions

Poor film-formation

Good film-formation

Amorphous film – brittle, mud-crackedPoor H+ conductivity (0.06 S/cm @ 80oC)Siroma, Z. et al., Electrochem. Comm. 2002, 4, 143.

Semi-crystalline film – pliableHigh H+ conductivity (0.1 S/cm @ 25oC)

Moore, R.B.; Martin, C.R. Macromolecules 1988, 21, 1334.

Casting temperature affects both O2 diffusion and solubility in the catalyst layer.Lee, K.; Ishihara, A.; Mitsushima, S.; Kamiya, N.; Ota, K. J. Electrochem. Soc. 2004, 151(4), A639.


Recast vs. Solution Processed PFSI’sRecast vs. Solution Processed PFSIRecast vs. Solution Processed PFSI’’ss

Recast Nafion

Solution-processed Nafion

EtOH/Water

EtOH

PrOH

BuOH

DMF DMSO

EG




Effect of Casting Temperature on Gas Transport Properties

Effect of Casting Temperature on Gas Effect of Casting Temperature on Gas Transport PropertiesTransport Properties


Melt-Processing PFSI’s (Overcoming the Barriers to Flow)

MeltMelt--Processing PFSIProcessing PFSI’’ss (Overcoming the Barriers to Flow)(Overcoming the Barriers to Flow)

CH3

CH2CH2

CH2

N+CH2

CH2

CH2

CH3

CH2

CH2CH2

CH3

CH2

CH2

CH2

CH3

Na+

Sodium

TetrabutylammoniumTBA+

Large counterions weaken dipole-dipole interactions and destabilize the electrostatic network.

Strength of dipole-dipole interactions decreases with increasing cation size.

PFSI crystallinity may be eliminated by the processing procedure.

m Solution-processing at low temp.m Rapid cooling from the melt.

Organic ions may effectively plasticize the PFSI.

CF2 CF2 CF2 CF

O CF2 CF CF3

O CF2 CF2 SO3H

n

n = 6.6

Nafion® 1100

Temperature (oC)

-50 0 50 100 150 200 250 300 350

tan δ

0

Precursor

TBA+

H+Na+

Temperature (oC)

-50 0 50 100 150 200 250 300 350

tan δ

0

Precursor

TBA+

H+Na+

Moore, R.B.; Cable, K.M.; Croley, T.L J. Membrane Sci. 1992, 75, 7.


Optical Microscopy of Melt-Pressed PFSI Films (viewed with λ-plate filter)

Optical Microscopy of MeltOptical Microscopy of Melt--Pressed Pressed PFSI Films PFSI Films (viewed with(viewed with λλ--plate filter)plate filter)

200 oC

Na+ TBA+


Solvent Stability of Nafion®

Membranes in Boiling Methanol Solvent Stability of NafionSolvent Stability of Nafion®®

Membranes in Boiling MethanolMembranes in Boiling Methanol

AR 112Extruded

(Left)

vs.

NRE 212Cast

(Right)


Solution-Processing of Nafion® MembranesSolutionSolution--Processing of NafionProcessing of Nafion®® MembranesMembranes

Solvent Evaporation

Substrate

Substrate

@ Low Temp

Continuous Networkof Chain

Entanglements@ High Temp

Discreet ParticleCharacteristics

Nafion®

in “solution”

Nafion®

Aggregates in typical dispersions

Poor film-formation

Good film-formation

Amorphous film – brittle, mud-crackedPoor H+ conductivity (0.06 S/cm @ 80oC)Siroma, Z. et al., Electrochem. Comm. 2002, 4, 143.

Semi-crystalline film – pliableHigh H+ conductivity (0.1 S/cm @ 25oC)




Effect of Casting Temperature on Membrane Solubility and Tensile Properties

Effect of Casting Temperature on Membrane Effect of Casting Temperature on Membrane Solubility and Tensile PropertiesSolubility and Tensile Properties

Temperature (C)

100 125 150 180

% S

olu

bili

ty

0

2

4

6

8

10

12

14

16

18

SP 112 H+-form

Temperature (C)

100 125 150 180

% S

olu

bili

ty

0

2

4

6

8

10

12

14

16

18

SP 112 H+-form

Solubility in 50:50 Ethanol/Water, Solubility in 50:50 Ethanol/Water, 30 min. sonication30 min. sonication

Weak membranesWeak membranes

Stable membranesStable membranes

Membranes cast from dispersions prepared from NRE 212CSMembranes cast from dispersions prepared from NRE 212CS

Strain (mm/mm)

0.0 0.5 1.0 1.5 2.0

Stre

ss (

MP

a)

0

10

20

30SP 112 H+-form 100CSP 112 H+-form 125CSP 112 H+-form 150CSP 112 H+-form 180C

Tensile PropertiesTensile Properties


Solvent Stability of Thermally-Annealed Dispersion Cast Nafion® NRE 212CS Membranes

Solvent Stability of ThermallySolvent Stability of Thermally--Annealed Annealed Dispersion Cast NafionDispersion Cast Nafion®® NRE 212CS MembranesNRE 212CS Membranes

Disintegrates in 5 min.





Stable

Stable

Disintegrates < 1 min.

Stable

Stability in Stirred MeOH (60 °C) Na+-form Films

Stable2.351Cast from DMSO at 180 oCSP 2 mil

Stable0.415Annealed 200 oCNRE 212CS


Disintegrates in 20 min.0.724Annealed 150 oCNRE 212CS



Stable1.915Annealed 175 oCwith DMSONRE 212CS

Disintegrates < 1 min.7.343As-receivedNRE 212CS

Stable1.317As-receivedAR 112

Stability in Stirred MeOH (60 oC)

% Soluble in 50:50 EtOH/Water

Water Uptake, λConditionSample






Stable

Stable

Disintegrates < 1 min.

Stable

Stability in Stirred MeOH (60 °C) Na+-form Films

Stable2.351Cast from DMSO at 180 oCSP 2 mil






Stable1.915Annealed 175 oCwith DMSONRE 212CS

Disintegrates < 1 min.7.343As-receivedNRE 212CS

Stable1.317As-receivedAR 112

Stability in Stirred MeOH (60 oC)

% Soluble in 50:50 EtOH/Water

Water Uptake, λConditionSampleMembranes in Boiling MeOH

As-received NRE 212CS membranes tend to disintegrate in boiling MeOH.For H+-form samples, annealing at elevated temperatures improves solvent

stability. Annealing has no effect on the Na+-form samples.


Fenton’s Test on Annealed NRE 212CSFentonFenton’’s Test on Annealed NRE 212CSs Test on Annealed NRE 212CS

H+-form samples progressively darken with annealing temperature and time.For the H+ samples, this thermal degradation apparently enhances susceptibility to chemical attack.Chemical degradation significantly increases above critical annealing temperatures for both H+-form (175 °C) and Na+-form (275 °C)

SamplesAvg F- (ppm) after 75 h % wt loss

H+ 212CS Unannealed 5.10 0.31H+ 212CS 100 °C 1hr 5.42 0.40H+ 212CS 125 °C 1hr 8.34 0.72H+ 212CS 150 °C 1hr 11.44 1.99H+ 212CS 175 °C 1hr 18.08 3.90H+ 212CS 200 °C 1hr 28.67 12.06H+ 212CS 225 °C 1hr 27.44H+ 212CS 250 °C 1hr 22.61


Refinement of Ionic Domain and Crystalline Order by Thermal Treatment

Refinement of Ionic Domain and Refinement of Ionic Domain and Crystalline Order by Thermal TreatmentCrystalline Order by Thermal Treatment

Crystalline Peak

IonomerPeak

Thermal annealing above Tα also significantly improves the order within the crystalline domains.


Effect of Membrane Thermal Treatment on PEMFC Performance

Effect of Membrane Thermal Treatment on Effect of Membrane Thermal Treatment on PEMFC PerformancePEMFC Performance

Current Density (mA/cm2)

0 500 1000 1500 2000 2500 3000 3500

Pow

er D

ensi

ty (m

W/c

m2 )

0

200

400

600

800

1000

1200

AR112AR112 --> TMA+ --> H+

AR112 --> TMA+ --> Annealed at 300oC/30mn -->H+

AR112 = as-received (extruded) 2 mil 1100 EW Nafion® membranes; 5 cm2 cell; Pt loading = 0.5mg/cm2 on anode and cathode (decal method); H2 /O2 stoich = 1.5/2.0; Tcell = 60 oC, T anode humidifier = 70 oC (100% RH), T cathode humidifier = 55 oC (80% RH).


0 500 1000 1500 2000 2500 3000 3500

Pote

ntia

l (V)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

AR112AR112 --> TMA+ --> H+

AR112 --> TMA+ -->Annealed at 300oC/30mn -->H+

Thermal annealing above Tα significantly improves the order within the ionic domains, which consequently improves PEMFC performance.


Relative Humidity (% RH)20 30 40 50 60 70 80 90 100

Prot

on C

ondu

ctiv

ity (m

S/cm

)

1

10

100

1000

NR 117 as receivedNRE 117--> TMA+ annealed at 200oCNRE 117--> TMA+ annealed at 225oC

Proton Conductivity of Annealed TMA+-form Nafion®

Proton Conductivity of Annealed Proton Conductivity of Annealed TMATMA++--form Nafionform Nafion®®

80 oC


Effect of Mixed Counterion Compositions During Casting

Effect of Mixed Counterion Compositions Effect of Mixed Counterion Compositions During CastingDuring Casting

Solution processed Nafion® membranes (180 oC) 2 mil thick 1100EW with mixed TBA+ and Na+ counterions, reacidified in 4M Sulfuric acid in Methanol; 5cm2 cell; Pt loading=0.5 mg/cm2 on anode and cathode (decal method); H2 /O2 stoich.=1.5/2; Tcell =60oC, T anode humidifier =70oC (100% RH), T cathode humidifier =55oC (80% RH)

Solution-processing above Tα (TBA+-form) improves PEMFC performance, relative to casting below Tα (Na+-form).

Temperature (ºC)

-100 0 100 200 300

Tan

Del

ta

0.0

0.2

0.4

0.6

0.8100% Na+

75/25 Na+/TBA+

50/50 Na+/TBA+

25/75 Na+/TBA+

100% TBA+

As CastAs CastSolution-processing

temperature

23

Current density (mA/cm2)

0 1000 2000 3000 4000 5000

Cel

l Vol

tage

(V)

0.0

0.2

0.4

0.6

0.8

1.0

Res

ista

nce

(Ω*c

m2 )

0.0

0.1

0.2

0.3

0.4

0.5

0% TBA+ 100% Na+

25% TBA+ 75% Na+

50% TBA+ 50% Na+

75% TBA+ 25% Na+

100% TBA+ 0% Na+

Performance After RePerformance After Re--acidificationacidification


Effect of Mixed Counterion Compositions During Casting Effect of Mixed Counterion Effect of Mixed Counterion

Compositions During CastingCompositions During Casting


0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Cel

l Vol

tage

(V)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pow

er D

ensi

ty (m

W/c

m2 )

0

200

400

600

800

1000

1200

1400

1600

18000% TBA+ 100% Na+

25% TBA+ 75% Na+

50% TBA+ 50% Na+

75 %TBA+ 25% Na+

100% TBA+ 0% Na+

Temperature (ºC)

-150 -100 -50 0 50 100 1500.0

0.1

0.2

0.3

0.4

0.5

0.6

H+ reacidified from 100% Na+

H+ reacidified from 75/25 Na+/TBA+



H+ reacidified from 100% TBA+

After Acid TreatmentAfter Acid Treatment

tan δ

Solution processed Nafion® membranes (180 oC) 2 mil thick 1100EW with mixed TBA+ and Na+ counterions, reacidified in 4M Sulfuric acid in Methanol; 5cm2 cell; Pt loading=0.5 mg/cm2 on anode and cathode (decal method); H2 /O2 stoich.=1.5/2; Tcell =60oC, T anode humidifier =70oC (100% RH), T cathode humidifier =55oC (80% RH)


General ConclusionsGeneral ConclusionsGeneral Conclusions

The morphology of PFSIs is quite complex, yet offers a wealth ofopportunities for enhanced properties through controlled morphological development.Morphological manipulation is required for a more in-depth understanding of morphology-property relationships.

o If we limit our studies to “As-Received” Nafion, then we are destined to observe only a predetermined behavior!

o Unique processing conditions may be used to offer morphology as a tailored variable.

More attention must be paid to the specific role of crystallinity in affecting PFSI membrane performance and durability.The morphology-property relationships learned by studies of PFSIs may certainly be extended the other ionomeric membrane systems.

o Processing parameters can influence ionic domain development for enhanced properties.o Can the physical effects of crystallinity (e.g., the nature of physical cross-links) be

emulated in alternative hydrocarbon membranes?

AcknowledgmentsAcknowledgmentsAcknowledgments

Funding and Support:

Department of EnergyNational Science Foundation (CBET - 0756439)

DuPont Fuel Cells Nissan Motors

American Society for Engineering EducationNational Defense Science and Engineering Graduate Fellowship

Advanced Photon Source (ANL)National Synchrotron Light Source (BNL)

Effect of Thermal Annealing in Various Counterions

Relative Humidity (% RH)20 30 40 50 60 70 80 90 100

Prot

on C

ondu

ctiv

ity (m

S/cm

)

1

10

100

1000NRE 212 CS as received

NRE 212 CS annealed in its Na+ form at 325oC for 1 hour and reacidified

NR117 CS annealed in its Cs+ form at 275oC for 30 min and reacidified

Nafion SP TBA+ form 190oC in DMSO and reacidified

Time (minutes)0 2000 4000 6000 8000 10000 12000

Prot

on C

ondu

ctiv

ity (S

/cm

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Time (hours)

0 25 50 75 100 125 150 175 200

T=80oC RH=95% T=80oC RH=30% (1)T=80oC RH=30% (2)

Documents

Processing-Performance Relationships for Perfluorosulfonate Ionomer … · 2008-09-10 · Introduction to the structure and morphology of perfluorosulfonate ionomers (PFSI’s) Ionic