Polyphenylene Sulfonic Acid:a new PEM

Sergio Granados-Focil andMorton H. Litt

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106

Outline of Talk

•Introduction and rationale

•Materials syntheses

•Polymer charcterization

•Effect of comonomers

•Conclusions and future work

PEM desired properties

• High proton conductivity with low sensitivity to relative humidity. Conductivity should be a minimum of 0.05 S/cm at operating conditions.

• Fuels should have essentially no permeation through the PEMs.

• High chemical, dimensional, and mechanical stability during the preparation and under the working conditions of the fuel cell. Membrane must be water insoluble in its final form and show little or no swelling.

•Can be directly cast on electrode as PEM in MEA processing for low power micro-fuel cells.

CF2 CF2 CF2 CF

O

CF2

C

O CF2CF2

CF3F

SO3H

n mNafion ®

The most widely used polymer electrolyte for the manufacturing of fuel cells.

Main limitations:

•Structural instability at temperatures above 100ºC

•Low conductivity at low relative humidity: if high temperatures are needed, must work at elevated pressure.

•High methanol permeability, limiting its use in Direct Methanol Fuel Cells

Materials alternative to Nafion®, molecular design

Use rigid-rod liquid crystalline polymers in which a few bulky or angled comonomer units can separate chains over their whole length, creating permanent pores lined with SO3H groups. The controlled architecture of these materials allows them to hold water very strongly.

(c). Copolymers with angled, and/or rigid comonomers

SO3H

SO3H

SO3H

SO3HH2OH2O

H2OH2O H2O

SO3H

SO3H

SO3H

SO3HSO3H

SO3H

SO3H

SO3H

H2OH2O

SO3H

SO3H

SO3H SO3H

SO3H SO3H

SO 3H

SO 3H

H2OH2OH2OH2O

H2O

H2O

(a). Homopolymer or copolymer with linear, small comonomer

(b).Copolymers with linear,

bulky comonomer

x

ArNN

O

O

O

O

SO3H

HO3S

NN

O

O

O

Oy

z

Ar= Bulky or angled comonomer

Thermally stable up to 300ºC

Maximum conductivity ~ 0.8 S/cm (100% RH, 100ºC)

Conductivity remains above 10-

3S/cm even at 15% R.H.

Imide groups slowly hydrolyze in acidic environments

Y. Zhang, M.H. Litt, ACS Polymer preprints Aug. 1999, 40(2), 480

Main GoalUse the same molecular design approach that worked for the polyimides to obtain hydrolytically stable polymer electrolytes able to absorb and retain water over a wide range of relative humidity and temperature.

R:

HO3S

SO3H n m

R: Bulky or angled comonomerSpecific tasks:

1.- Design and optimization of an efficient polymerization approach.

2.- Structural characterization of the obtained materials.

3.- Evaluation of water absorption, proton conductivity and thermalstability of the polymer membranes, and comparison with existing PEMs.

Design and optimization of thepolymerization approach

Ullman coupling (P.E. Fanta, Chem. Rev. 64, 613 (1964))

IMe

SO3R

MeMe

SO3R

RO3S

Cu200ºC

R:

yield 88%

H

yield 12%Sulfonic acidester

Free acid

Diazotization (Courtot Ch., Lin C.C. Bull. Soc. Chim. Fr. 1931, [4] 49, 1047)

HO3S

SO3H

NH2H2N

4,4'-diamino-2,2'-biphenyldisulfonic acid 4,4'-diiodo-2,2'-biphenyl disulfonic acid

1)NaNO2

2)HCl3)KI

II

SO3H

HO3S

75% yield

Design and optimization of thepolymerization approach

Sulfonic acid protection

NaO3S

SO3Na n

NaO3S

SO3Na

II

Cu (activated)NMP ( dry )

N2 atmosphere

RO3S

SO3R

IICu (activated)NMP ( dry )

N2 atmosphere140º C

XHydrolysis

or cation exchange

HO3S

SO3H n

Cationexchange

n

RO3S

SO3R

RIntrinsic Visc. (dl/g) Synthesis at 150ºC

RIntrinsic Visc.(dl/g)Synthesis at 150ºC

0.09

0.24

0.12

0.12

0.07

0.07

NH

C9H19 HN N

HN

H3C

H3C

CH3

Design and optimization of the polymerization approach

Previously studied monomer Sulfonic acidmeta to the iodine

4,4´diamino-2,2´biphenyldisulfonic acid 4,4´diiodo-2,2´biphenyldisulfonic acid

I I

HO3S

SO3H

H2N NH2

HO3S

SO3H

1)NaNO2

2)HCl3)KI

A new monomer in which the halogen atom, as well as its position relative to the sulfonic acid, has been changed, couples more efficiently to yield higher molecular weight polymers than the previous monomer did.

Newly synthesized monomer

4,4´dibromobiphenyl 4,4´dibromo-3,3´biphenyldisulfonic acid

H2SO4(98%)180ºC, 45min

Br Br

HO3S

SO3H

Br Br

Sulfonic acidortho to the bromine

Design and optimization of the polymerization approach

R

Intrinsic Visc. ( dl/g) DiBrBS

Intrinsic Visc. ( dl/g) DiIPS

0.07

0.08

0.31

0.2

0.61

0.16

NH

C9H19

Br Br

RO3S

SO3R

I I

RO3S

SO3R

CH2 NCH3

CH3

CH3

protection

BrBr

SO3Na

NaO3S

Cu (activated)NMP (dry)

N2 atmosphere140ºC

Cu (activated)NMP ( dry )

N2 atmosphere140º C

X 1. Hydrolysisor

cation exchange

2. Cationexchange

BrBr

SO3R

RO3S n

SO3R

RO3S

n

SO3Na

NaO3Sn

SO3H

HO3S

Sulfonic acid

Structural characterizationNMR analysis

7 .2 07 .4 07 .6 07 .8 08 .0 08 .2 08 .4 0

9 8 .9 1 0 7 .1 1 0 0 .0 7 .8 3 .9

7 .2 07 .4 07 .6 07 .8 08 .0 08 .2 08 .4 0

9 8 .2 1 0 0 .0 1 0 1 .1

BrBr

SO3H

HO3S

ab

c

abc

ab

cn

SO3H

HO3S

ab

c

Structural characterizationFTIR analysis

3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

1211 1094

1447

ν OH C-CAr

SOO

ν as ν sy

n

SO3H

HO3S

3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600

10711222

3484 1451ν OH

C-CAr

ν as ν sy

BrBr

SO3H

HO3S

SOO

SOO S

OO

Evaluation of conductivity and water absorption of polymer membranes

Conductivity as a function of Relative Humidity and Temperature

*Sone Y., Ekdunge P. and Simonson D., J. Electrochem. Soc. 1996, 143 (4) 1254.

0.0081

0.0040

0.0023

Conductivity DBBS-Pol(S/cm) 15%R.H

0.00008

0.00008

0.00008

Conductivity Nafion ®*(S/cm)15%RH

0.048

0.025

0.010

Conductivity DBBS-Pol(S/cm) 35%R.H.

0.004

0.004

0.004

Conductivity Nafion ®*(S/cm)35%RH

0.01

0.01

0.01

Conductivity Nafion ®*(S/cm)50%RH

0.270

0.140

0.080

Conductivity DBBS-Pol(S/cm) 75%R.H.

0.030.04550

0.030.08675

0.030.01925

Conductivity DBBS-Pol(S/cm) 50%R.H.

Conductivity Nafion ®*(S/cm)

Temp (ºC)

0 .0 0 0 0 1

0 .0 0 0 1

0 .0 0 1

0 .0 1

0 .1

1

1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

R H (% )

Con

duct

ivity

(S/c

m)

2 5 C5 0 C7 5 Cn a fio n

Determination of Methanol Permeability in Polymer Electrolytes

RESULTS FOR DBBS POLYMER at 125 C

ca. 1 x 10-72.8 x 10-70.0035392

ca. 1 x 10-72.5 x 10-70.0020248

Diffusion Coefficient

cm2/s

Sorption Coefficient

moles of methanol per (cm3 film x torr)

Sorptiong methanol

per g polymer

Methanol Pressure

(torr)

Estimated Crossover current for a 50 micron thick film: = 1.1 mA/cm2 (1:1 Methanol:Water Feed)

Water Sorption Results suggest water will be preferentially sorbed over methanol

Evaluation of conductivity and water absorption of polymer membranes

Water absorption at room temperature

Nafion ®#DBBS-Pol

3.45.850

Molecules of water absorbed per sulfonic acid (λ)

R.H. (%)

7.6

4.8

5.675

2.735

79.8245.8332.5∆z

5.543∆y

532∆x

75%50%35%Relative Humidity

Change in dimensions (%)

Dimensional change from 20% R.H.

x z

y

75.61°C94.07%

155.08°C84.70%

267.08°C78.50%

335.19°C62.99%

497.90°C43.29%

0.0

0.2

0.4

0.6

0.8

Der

iv. W

eigh

t (%

/°C

)

40

60

80

100

Wei

ght (

%)

0 100 200 300 400 500 600

Temperature (°C)

Sample: DBBSPOGA1Size: 5.7340 mgMethod: SERGIOComment: DBBSPOL GA TECH 2ND BATCH COMPLETELY ACID FORM

TGAFile: C:...\My Documents\tgas\DBBSPOGA.004Operator: SERGIORun Date: 17-Apr-03 16:56

Universal V3.1E TA Instruments

Thermal stability

FTIR analysis shows no degradation after 4 hours at 250ºC

PEM special requirements

• High proton conductivity with low sensitivity to relative humidity. Conductivity should be a minimum of 0.05 S/cm at operating conditions.

• Fuels should have essentially no permeation through the PEMs.

• High chemical, dimensional, and mechanical stability during the preparation and under the working conditions of the fuel cell. Membrane must be water insoluble in its final form and show little or no swelling.

•Can be directly cast on electrode as PEM in MEA processing for low power micro-fuel cells.

Conductivity of the homopolymer is 0.27S/cmat 75% RH and 75ºC

Preliminary measurements show essentially no permeation of Methanol

The newly synthesized polymers are stable up to 250 C.

Membranes are soluble in water and swell significantly.

Films can be cast from water and a variety of polar organic solvents

Intrinsic Conductivity Comparison

PEM Conductivities as a function of λ.

λ, Water molecules per acid group

0 2 4 6 8 10 12 14

Con

duct

ivity

, S/c

m.

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

RF5 Aq. Cond.Naf. Aq. Cond.

RF5Nafion

PSSAPSSA Aq. Cond.

Membrane stabilization

Incorporation of cross-linkable biphenyl groups.

Incorporation of bulky tert-butyl benzene groups.

n m

SO2

HO3S

SO3H

HO3S

Active site for crosslinking reaction

n m

SO2

HO3S

SO3H

HO3S

CH3CH3H3C

Copolymers containing from 5 to 20% of biphenyl sulfone groups crosslink after 20 minutes at 200ºC.

5% of t-butyl benzene sulfone renders the copolymer water insoluble

Proton conductivity of water insoluble polymer membranes

0.00001

0.0001

0.001

0.01

0.1

1

10 20 30 40 50 60 70 80 90 100 110

Relative humidity (%)

Con

duct

ivity

(S/c

m)

Nafion (R)

Crosslinked copolymer containing 20%biphenyl copolymer containing 5% tbutyl

parent polymer

copolymer containing 25%tbutyl

polymer with 5% biphenyl

What we have learned from the bulky and crosslinked copolymers

• Both the bulky and crosslinked copolymers have much lower conductivity than the homopolymer. The dependence of conductivity on relative humidity suggests that the membrane structure de-swells rapidly as humidity drops until it approaches its built-in free volume. It then holds the remaining water tightly.

• The use of larger bulky and/or crosslinkable comonomers should increase the built-in free volume, improving water retention and thus the low humidity membrane conductivity.

Conclusions• A new poly (phenylene sulfonic acid) has been obtained

using the copper catalyzed coupling of dibromo aromatic sulfonates in high molecular weight to cast free standing films.

• The polyphenylene sulfonic acids have conductivities at 25oC and 15% relative humidity that range from 0.8 to 2.3 mS/cm. Conductivity rises rapidly with temperature.

• The newly synthesized materials are chemically stable up to 250ºC.

• Copolymers containing bulky or crosslinkable groups have much lower conductivity than the homopolymer, but the data imply that bulkier comonomers could generate materials with high conductivities at low humidities.

• Further work needs to be done to increase the molecular weight and improve the dimensional stability of the materials without sacrificing proton conductivity.

Future work • Further increase the polymer molecular weight by optimizing the

Ullman coupling conditions and/or using alternative coupling methods

• Optimize the reaction conditions to obtain copolymers containingbulky, crosslinkable groups.

R'

Cu (activated)NMP (dry)

N2 atmosphere

BrBr

RO3S

SO3R

+ R' BrBr

Deprotection

R: CH2 NCH3

CH3

CH3

or

tetra alkyl ammoium counterion

R'n

m

HO3S

SO3H

R'n

m

RO3S

SO3R

Bulky "crosslinkabl e" c omonomer

Alternative bulkycounterion

SO3HHO3S

NH3C CH3

Future work

Characterization:

a. Analysis: X-Ray to confirm structureb. Optimization of crosslinking conditions.

c. Mechanical properties of new polymers as a function of composition and environment (water/MeOH in liquid and gaseous state).

d. Water content and conductivity as a function of RH and temperature for all new copolymers.

.

Acknowledgments

• DARPA for the support of the project• Drs. Jesse Wainright and Robert Savinell

for their advice and help in learning to measure conductivity.