2006 DOE Hydrogen Program Review Presentation

Advanced Materials for Proton Exchange Membranes

James E. McGrath, Ph.D. NAE

University Distinguished Professor of ChemistryMacromolecules and Interfaces Institute

and Department of ChemistryVirginia Tech

Blacksburg, VA 24061Jmcgrath@vt.edu

This presentation does not contain any proprietary or confidential information

McGrath Group 2006

Front Row (L to R): Dr. W. Harrison (Ph.D. 2004), Prof. Y.J. Kim (Sung KyunKwan Univ, Korea),J.E. McGrath, A. Badami, H. Wang.

Back Rows (L to R): M. Paul, X. Yu, A. Roy, E. Morazzani (undergrad), A. Cleaton (undergrad) N. Arnett,M. Hill, Dr. Z. Zhang, R. Hopp, O. Lane, Y. Chen, H.S. Lee, Y. Li, Dr. G. Fan,Dr. M. Sankir, J. Yang, S. Takamuku (visiting scientist)

3

McGrath Research Group: past and present

May 2006” DOE Contract on “High Temperature, Low Relative Humidity, Polymer-type Membranes Based on Disulfonated Poly(arylene ether) Block and Random Copolymers Optionally Incorporating Protonic Conducting Layered Water Insoluble Zirconium Fillers”

Acknowledgements

4

• Research Progress Objectives– Approach and Strategy– Partially fluorinated systems– Multiblocks– Crosslinking– Blends– Film Casting

OUTLINE

5

OBJECTIVESOBJECTIVES

• Design, identify, and develop the knowledge base to enable proton exchange membrane films and related materials to be utilized in automotive fuel cell applications, particularly for H2/Air systems at 120oC/low RH.

Objectives

6

• Thermally, hydrolytically, and oxidatively stable aromatic ionomers with high Tg, ductility, and controlled hydrophilicity are required

• Synthesis–Linear and crosslinked statistical

hydrophobic/hydrophilic copolymers–Linear multiblock hydrophobic/hydrophilic

copolymers (current & future)

Research Approach/Hypothesis

M.A. Hickner, H. Ghassemi, Y.S. Kim and J.E. McGrath, Alternate Polymer Systems for Proton Exchange Membranes, Chemical Reviews (2004), 104(10), 4587-4611.

7

Proton Exchange Membrane (PEM) Requirements

O2

Pt supportedon carbon withpolymer matrix

H2 H+e-

H2O

AnodeElectrode

CathodeElectrode

5 μm

e-

150 μm

H2O

H2O

H2OH2O

H2O

Platinum (3-5nm)

Carbon Black (0.72μm)PEM

O2

Pt supportedon carbon withpolymer matrix

H2 H+e-

H2O

AnodeElectrode

CathodeElectrode

5 μm

e-

150 μm

H2O

H2O

H2OH2O

H2O

Platinum (3-5nm)

Carbon Black (0.72μm)PEMPEM

• CRITICAL PEM PROPERTIES– high protonic conductivity,

even at low RH– low electronic conductivity– low permeability to fuel and

oxidants– low water transport - diffusion and

electro-osmotic drag– oxidative and hydrolytic stability

under acidic conditions, for thousands of hours!

– Good dry and wet mechanical properties at ambient and higher temperatures

– Cost effective and able to be fabricated into robust membrane electrode assemblies (MEAs)

M. Hickner, H. Ghassemi, Y. Kim ,B. Einsla and J E McGrath, Chem Reviews. 2004,104,4587-4611

Membrane Electrode Assembly (MEA)

The benchmark is Nafion™for the membrane and electrodes

8

O O SO2 co O O SO2

SO3HSO3H

Hydrophobic Hydrophilic

n x1-x

Acidification TreatmentMethod 1: 1.5M H2SO4, 30°C, 24hrs, then deionized H2O, 30°C, 24hrs.Method 2: 0.5M H2SO4, boil, 2hrs, then boiled deionized H2O, 2hrs.

Acronym: BPSH-xx-MxBi Phenyl Sulfone: H Form (BPSH)xx= molar fraction of disulfonic acid unit, e.g., 30, 40, etc.Mx: Acidification method, e.g., M1, etc.

F. Wang, M. Hickner, Y.S. Kim, T. Zawodzinski and J.E. McGrath, “Synthesis and Characterization of Sulfonated Poly(arylene ether sulfone) Random (Statistical) Copolymers Via Direct Polymerization: Candidates for New Proton Exchange Membranes,” Journal of Membrane Science, 197 (2002), 231-242.

Sulfonated Poly(arylene ether sulfone)s (BPSH)

9

Advantages of Direct Polymerization

High yields from 40MM lb/year precursorsPrecise control of ionic concentration during synthesisWell-defined ion conductor location; morphology controlHigh H+ conductivityEnhanced stability due to deactivated position of -SO3HCompatible with additives for >100ºC studies Very high molecular weight copolymers possible

S ClClO

O

NaO3S

SO3Na

SDCDPS

n

10

S

O

O

ClClS

O

O

ClCl

NaO3S SO3Na

OH Ar OH

CH3

CH3

CF3

CF3

O S

O

O

O

SO3HHO3S

Ar O S

O

O

O Ar

+

140 oC / 4 h 190 oC / 24 h

+

K2CO3

NMP / Toluene

Ar =

n 1-n

H2SO4

x

Hydrophilic Hydrophobic

Harrison, W.L.; Wang, F.; Mecham, J.B.; Bhanu, V.A.; Hill, M.; Kim, Y.S.; McGrath, J.E. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2264-2276.

K2CO3TolueneDMAc

Acidification

140 ºC/4 h165 ºC/48 h

Copolymer Synthesis by Nucleophilic Aromatic Substitution

• Monomer and copolymers are scaleable to multi-kilogram quantities

11

Water Uptake of BPSH Random Copolymers is Dependent on Morphology

(a) Control; (b) 40% ionic content; (c) 50% ionic content; (d) 60% ionic content

(a) (b)

(c) (d)

12

0

1

10

100

1000

0 10 20 30 40 50 60 70 80 90 100 110

Relative Humidity (%RH)

Con

duct

ivity

(mS/

cm)

BPSH 35 conditioned at 70% RH 1-19-04

Nafion 112 conditioned at 70% RH 1-8-04

Comparing Four Electrode Conductivity Comparing Four Electrode Conductivity of BPSH 35 120of BPSH 35 120ooC, 500 C, 500 sccmsccm HH22, 230 , 230 kPakPa

13

Comparison of Stress-Strain Behavior of BPSH-40 and Nafion 117*

Strain (%)

0 50 100 150 200

Stre

ss (k

gf/c

m2 )

0

100

200

300

400

500

600

700

1: BPSH-40 (dry)2: BPSH-40 (wet)3: Nafion 117 (dry)4: Nafion 117 (wet)

1

2

3

4

The wet state of BPSH-40 and Nafion 117 had 22% and 16%, total water,respectively.

Colleagues at UTC (Dr. Protsailo/Dr. Haug) have shown BPSH outperformsthe benchmark membrane as judged

by open circuit voltage (OCV) in H2/O2 accelerated tests

15

N112 and BPSH35-1mil at 120°C and 50%RH

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700

Current Density (mA/cm2)

Volta

ge (V

)

1 N112 @120C & 20PSIG2 N112 @ 120C & 7.2PSIG3 BPSH35 @ 120°C & 20psig4 BPSH35 @ 120°C & 7.2psig

At cell temperature of 80°C: Anode bottle=105°CCathode bottle=90°C

At cell temperatures of 100°C and 120°C: Anode bottle=100°Cathode bottle=100°C

32

4

1

16

AGING STUDIES (no failures)

•Boiling water for 10,000 hours•High pressure (2 atm) steam at 120oC for 1000 hours•MEA H2/air fuel cell, 80oC, 1500 hours•MEA, DMFC, 80oc, 3000 hours

17

Comparisons of Stress-Strain Curves of BPSH35 Samples with Different Molecular Weights

0.00E+00

1.00E+01

2.00E+01

3.00E+01

4.00E+01

5.00E+01

6.00E+01

7.00E+01

0 20 40 60 80 100Strain(%)

Stre

ss(M

pa) control

20K30K40K50K

The tensile testing was performed using a universal Instronmachine at room temperature (23oC) and 40% R.H, with a crosshead displacement speed of 5mm/min.

18

Chemical Composition, IEC, Molecular Weight, and Water Uptake are Not Changed After a 700 hr Fuel Cell Test at 80oC

IEC (meq/g)Life test Disulfo-nation by 1H NMR (%)

Calc. Experi-mental

Before 36 1.4 0.7 34

after 34 1.4 0.8 37 (33)a

1.5

IV(dL/g)

Water uptake (wt.%)

1H NMR spectroscopy of BPSH-35 before and after life test

No major chemical degradation of BPSH membrane was found

8 .5 8 .0 7 .5 7 .0 P P M

After life test

PPM7.07.58.08.5

O O SO2 co O O SO2

HO3S SO3H1-x x

1 2 3 4 5 6 7 8

9

8 .5 8 .0 7 .5 7 .0 6 .5 P P M

PPM6.57.07.58.08.5

7

9

4

8

2,6 1,3,5

Before life test

a after life test and recast

In cooperation with Dr. Y.S. Kim from LANL

19

States of Absorbed Water in a Hydrophilic Polymer

Temperature (oC)

-50 0 50 100 150

Tg depression due to the plasticiazation of nonfreezing water

melting of freezable bound water

melting of free water

At Least Three States of Water1. Non-freezable, tightly-bound water

plasticises polymer – lowers Tgwater that is strongly bound to sulfonic acid groups

2. Freezable, loosely-bound waterbroad melting behavior from -20 to 20°C

water that weakly bound to the polymer chain

3. Free watersharp melting point at 0°C

water that has the same phase transitions as bulk water

Nakamura, K.; Hatakeyama, T.; Hatakeyama, H. Polymer 1983, 24, 871. Kim, Y. S.; Dong, L.; Hickner, M. A.; Glass, T. E.; Webb, V.; McGrath, J. E. Macromolecules 2003, 36(17), 6281

DSC Thermogram

End

o

20

Why We Are Interested in States of Water in PEM?

Previous studies and current research show• Only the bound non freezing water causes the depression in glass

transition temperature .• Presence of free water facilitates the transport process .• Methanol permeability and conductivity tends to depend on free water

and loosely bound water.• Electro-osmotic drag increases with increase in free water.• Activation energy for proton transport and methanol transport decreases

with increasing amounts of free water. • Operation of fuel cells under freezing conditions is likely a function of the

states of water in the membrane

Kim, Y. S.; Dong, L.; Hickner, M. A.; Glass, T. E.; Webb, V.; McGrath, J. E. Macromolecules 2003, 36(17), 6281

21

Optimization of the membrane-electrode interface utilizing a partially fluorinated poly (arylene ether) membrane (6F-35) afforded stable long-term performance (3000 hours) with cell resistance that actually decreased under DMFC conditions.

Performance loss after 3000 h of the life test for 6F-35 was only 60 mA/cm2 and the methanol leaking current was reduced more than 50%.

6F-35

Time (h)

0 500 1000 1500 2000 2500 3000

Cur

rnet

Den

sity

(A/c

m2 )

0.05

0.10

0.15

0.20

0.25

HFR

( Ω c

m2 )

0.0

0.1

0.2

}unrecovered loss ~ 60 mA/cm2

Nafion 1135

Time (h)

0 500 1000 1500 2000 2500 3000

Cur

rnet

Den

sity

(A/c

m2 )

0.05

0.10

0.15

0.20

0.25H

FR ( Ω

cm

2 )

0.0

0.1

0.2

} unrecovered loss ~ 44 mA/cm2

Methanol leaking current: 50 mA/cm2

Methanol leaking current: 120 mA/cm2

Membrane thickness: 58 micron

Membrane thickness: 90 micron

Long-Term DMFC Performance Studies at LANL (Y.S. Kim) on Partially Fluorinated BPSH(6FSH-35) showed Excellent Stability for 3000 Hours

@ 80oC for Portable Power

22

In situ Determination of Proton conductivity as a Function of RH and Temperature Using ESPEC Humidity

Oven and Solartron Impedance Analyzer

Membrane conditioning

80°C, 20%RH for 16 hr

23

CN

O OSO

OO

SO3H

SO3H

Ox 1-x

SO

OO

Strategy to Achieve the Goal

Controlled Amount of

Fluorinated andNon-Fluorinated

Monomer

Controlled Amount of

Fluorinated andNon-Fluorinated

Monomer

Fixed at 35 or 45 mol percent

CF3

CF3HO OH

HO OH

CopolymerPAEB-XX*

IV** (dL/g)

PAEB35 1.21.11.21.51.4

6F25CN356F50CN35 6F75CN35

6F100CN35

*XX: Target Degree of Disulfonation**IV measurements were done using 0.05 M LiBr

Synthesized CopolymersPAEB35 : 35 mol % disulfonation 6F25CN35 : 25 mol % 6F+ 75 mol % BP6F50CN35 : 50 mol % 6F+ 50 mol % BP6F75CN35 : 75 mol % 6F+ 25 mol % BP6F100CN35 : 100 mol % 6F

Synthesized CopolymersPAEB35 : 35 mol % disulfonation 6F25CN35 : 25 mol % 6F+ 75 mol % BP6F50CN35 : 50 mol % 6F+ 50 mol % BP6F75CN35 : 75 mol % 6F+ 25 mol % BP6F100CN35 : 100 mol % 6F

24

• Micro phase-separated morphology can be precisely controlled by synthesis

• Enhanced mechanical strength, water uptake, and proton conductivity are expected

Multiblock Copolymers with Hydrophilic-Hydrophobic Blocks

A. Noshay and J.E. McGrath, "Block Copolymers: Overview and Critical Survey," 520 pages, Academic Press, New York, January 1977, p.91.

An Example of an Ideal Cocontinuous Film Morphology in Block Copolymers

25

Synthesis of High MW Multiblock Copolymers Employing Mixed Solvent

Systems

~~~~~~~~(Poly arylene ether sulfone)m~~~~~~(Polyimide)n~~~~~~~~~~~

+

NO

O

OO

O

O SO

OO NN

O

OO

O

O SO

OO ON

O

OO

O

n

O SO

OO O S

O

OO

NH2

KO3S

SO3K

KO3S

SO3KH2N

m

1) Benzoic Acid2) NMP (80 ºC 4hr)3) m-Cresol4) Extra NMP5) 180 ºC 12hr6) Isoquinoline7) 180 ºC 12hr

26

Why We Are Interested in Block Copolymers as PEMs?

0.1

1

10

100

1000

10 30 50 70 90 110

Relative Humidity %

Prot

on C

ondu

ctiv

ity (m

S/cm

)

Nafion117

BPSH100-PI100 (20:20)

Random (HQSH30)

Measured at 80°C

27

100 nm100 nm100 nm 100 nm

BPSH 15 – PI 15

400 nm

AFM Images of Multiblock Copolymers

28

Multiblock Perfluorinated Hydrophobic-Hydrophilic Copolymer Synthesis

MO O S

O

O

SO3M

MO3S

O O S

O

OO OM

0.75 0.25

F

F F

F F

F

F

F

F O O

CF3

CF3

F

F F

F F

F

F

F

F+n

O O S

O

O

SO3M

MO3S

O O S

O

OO O

F

F F

F F

F

F

F

O OCF3

CF3

F

F F

F F

F

F

F

0.75 0.25

n

K2CO3, NMP

105~115oC, 4~5 days

29

Proton Conductivity vs. RH for 6FB6, a 15,000-9,000 gm/mole Multiblock Copolymer

0.1

1

10

100

1000

20 40 60 80 100

Relative Humidity (%)

Prot

on C

ondu

ctiv

ity(m

S/cm

) 6FB6(15:9)Nafion117

Target IEC: 1.25

Water uptake: 42%

Proton conductivity in liquid water: 0.09 S/cm

30

Synthesis of 6FK-BPSH Multiblock Copolymers

CO

F O CCF3

O

CF3

CO

Fn

S

O*MO O O OM

O m

SO3MMO3S- +

-+

-+

- +

S

O*O O

O m

SO3MMO3S- +-+

* CO

O CCF3

O

CF3

*

n

Not Isolated

Add dropwise at 160 oCThen 180 oC

provides good proton conductivity and low permeability

Hydrophilic Hydrophobic

offers excellent thermal and mechanical properties

, in NMP, two days

31

Comparison of Conductivity vs. RH for 6FK-BPSH Multiblock(4:4)K and Nafion 117

0.0001

0.001

0.01

0.1

1

20 40 60 80 100 120

Relative Humidity (%)

Prot

on c

ondu

ctiv

ity (m

S/cm

)

6FK-BPSH (4:4)kNafion 117

80 oC

32

Why We Are Interested in Crosslinked PEMs ?

• Proton conductivity increases with increase in IEC or degree of disulfonation .

• But water uptake also increases at the same time causing the membrane to lose its dimensional stability .

• Introduction of crosslink within the membrane may reduce water uptake without losing conductivity .

33

Synthesis Route of 4-Phthalonitrile Containing BPS60-15k Copolymer

+M-O O SO

OO

OSOO

O

SO3-M+

+M-O3S

NO2

CNCN

4-nitrophthalonitrileK2CO3, DMAC60oC, 24 hrs

+M-O

x

1-xn

O O SO

OO

OSOO

O

SO3-M+

+M-O3S

O

x

1-xn

NC

NC

NC

NC

34

DSC Thermogram of a Phth-BPS-60 Film Shows Curing Exotherm Starting from 300°C

-0.29

-0.28

-0.27

-0.26

-0.25

-0.24

Hea

t Flo

w (W

/g)

180 200 220 240 260 280 300 320 340

Temperature (°C)Exo Down Universal V4.1D TA

Curing exotherm

5°C/minute in nitrogen atmosphere

35

Film Casting of BPSH Polymers

BPSH-35 Films

36

CONCLUSIONS•Hydrogen/air PEMFC’s based on alternative membranes have good mechanical properties and the potential to provide satisfactory fuel cell performance below 100oC.

•Conductivity at 120oC and low RH is challenging and may require long sequenced block copolymers to generate co-continuous hydrophilic-hydrophobic morphologies and/or organic-inorganic composite systems.•Membrane MW is a critical parameter defining PEM durability.

•Tailored membrane/electrode interfaces are important with Nafion electrodes.