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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.