Hickner superacid Norway 2013

Water Binding Interactions in Superacid
Proton Exchange Membranes
Michael A. Hickner, Associate Professor
Department of Materials Science and Engineering
The Pennsylvania State University
310 Steidle Building
[email protected]
NTNU
Trondheim, Norway
3 - 4 October 2013
Pennsylvania State University
State College, Pennsylvania
Population of town: 50,000
Students in University: 40,000
Total: 90,000 inhabitants
Acknowledgements
Dr. Guang Chen – Ph.D. Chemistry Chinese Academy of Sciences, Changchun  Dr. Shudipto Dishari –
Ph.D. CHE Singapore National University  Dr. Dongyang Chen – Ph.D. CHE Sun Yat-sen University,
China  Dr. Brandon Calitree – Ph.D. Chemistry SUNY Buffalo  Dr. Ian Sines – Ph.D. Chemistry Penn
State  Dr. Min Zhang – Ph.D. Chemistry Chinese Academy of Sciences, Changchun  Dr. Nanwen Li –
Ph.D. Chemistry Chinese Academy of Sciences, Changchun  Dr. Geoff Geise – Ph.D CHE UT Austin
 He Xie – B.S. MATSE Tsinghua U.  Stephanie Petrina – B.S. MATSE Virginia Tech  David Jones –
B.S. Engineering Physics CWRU  Alfonso Mendoza – B.S. Ceramic Engineering Rutgers, M.S. MATSE
PSU  Sarah Black – B. S. Chemistry Virginia Tech  Brian Chaloux (NRL) – B.S. Chemistry CWRU 
Melanie Disabb-Miller - B. S. MATSE Florida, M.S. MATSE Northwestern  Lizhu Wang – B. S.
Chemistry Jilin U., China, M.S. Chemistry Marquette U.  Zhaoyan Ai – B.S. Chemistry Hong Kong
Polytech  Sean Nunez – B.S. Chemistry SUNY Buffalo, M.S. Chemistry PSU  Changwoo Nam – B.S.
CHE Pukyong National University, M.S. CHE Seoul National University  Douglas Kushner – B.S.
MATSE PSU  John Nese – PSU MATSE  Will Salem – PSU MATSE
Water-absorbing versus nonaqueous ion-conducting polymers
For batteries – polymer dynamics
mediates ion transport
O
O
C
C
For fuel cells – water mediates ion
transport
OCH2CH2 O
m
n
SO3Li
Colby, Runt, et al., Chem. Mater. 2006.
Hickner, Materials Today 2010.
Hickner, J. Polym. Sci. B. Polym. Phys. 2012.
Structure-property paradigm for sulfonated polymers
-
-
-
-
-
-
-
-
-
-
Nafion, 0.91 meq g-1
Kreuer, J. Membrane Sci. 2001.
SDAPP3, 1.8 meq g-1
Fujimoto, et al. Macromolecules 2005.
Hickner, et al. Polymer 2006.
What is happening inside the “pores”?
•  Add water to get larger pores.
•  Permittivity inside pores approaches liquid water.
Small Pores
- low conductivity
- low permeability
Large Pores
- high conductivity
- high permeability
Paddison, et al., J. Electrochem. Soc. 2000.
Kreuer, et al., Chem. Rev. 2004.
Block copolymers facilitate control of the
physical structure of the ionic domains
CH2 CF2
n
CF2 CF
my
CF3
z
z'
x
x'
y
SO3H
SO3H
L. Rubatat, Z. Shi, O. Diat, S. Holdcroft,
B. J. Frisken, Macromolecules 2006, 39, 720-730.
Balsara, et al., Nano Lett. 2007, 7, 3547.
Synthesis of multifunctional block copolymers
+- + +
- -
++- +-
Disabb-Miller, M. L., Z. D. Johnson, M. A. Hickner, “Ion Motion in Anion and ProtonConducting Triblock Copolymers,” Macromolecules 2013, 46(3), 949–956.
Block copolymer assemblies
to form ionic channels
+- +- +- +Hexyl chains
Flexible endblock segments
+- +- +- +Perfluoro chains
Stiff endblock segments
Order in flexible PHMA-b-PS-b-PHMA leads to
high proton conductivity
PHMA
PFMA
Moore, Saito, Hickner, J. Mater. Chem. 2010.
Macromolecules 2010.
10
Connectivity and acidity
Nafion – perfluoro sulfonate
PHMA/PFMA – aryl sulfonate
acidity
connectivity
Need superacid polymers for high
conductivity at low relative humidity
Poly(styrene) superacid membranes
Post-functionalization with Suzuki-Miyaura coupling
Chang, Brunello, Fuller, Hawley, Kim, Disabb-Miller, Hickner, Jang, Bae, “Aromatic Sidechain
Ionomers: Acidity, Hydration, and Proton Conductivity,” Macromolecules 2011, 44(21), 8458.
Poly(sulfone) superacid membranes
Superacid monomers
Xu, K., H. Oh, M. A. Hickner, Q. Wang, “Highly Conductive Aromatic Ionomers with
Perfluorosulfonic Acid Side Chains for Elevated Temperature Fuel Cells,” Macromolecules
2011, 44 (12), 4605–4609.
Superacid monomers
Similar conductivity to PFSAs was achieved with CF2CF2SO3H
moieties on aromatic backbones.
Xu, Oh, Wang Hickner, Macromolecules 2011.
Poly(sulfone) superacid membranes
Much smaller morphological
size in PSU-based
membranes, but similar
conductivity to Nafion.
Morphology is important, but
superacid groups are more
important in these samples
for high conductivity at low
RH.
Chang, Y., G. F. Brunello, J. Fuller, M. L. Disabb-Miller, M. E. Hawley, Y. S. Kim, M. A.
Hickner, S. S. Jang, C. Bae, “Acidity Effects in Poly(sulfone)–based Proton Exchange
Membranes,” Polym. Chem. 2013, 4, 272–281.
Phase separation and contrast for PS and PSU
PS
PSU
•  PSU is more polar than PS. May cause more intermixing of sulfonates
and backbone.
•  X-ray contrast in PSU may not be high enough between hydrophilic
and hydrophobic domains due to the presence of sulfur in both regions.
Hydrated domain contrast by D2O SANS
•  No SANS contrast in dry materials.
•  ~4-6 nm interdomain spacing in hydrated PSU and PS backbones with
both aromatic acid and superacid groups.
Ionic transport is determined by water motion
Ion hopping - hydrogen bond reorientation
Hydrophobic region
+
+
Hydrophilic region
Ion diffusion - break hydrogen bonds to move
+
+
Falk, M. Can. J. Chem. 1980, 58, 1495.
Moilanen, et al. Langmuir 2008, 24, 3690.
O-D stretching frequency is a signature
of the hydrogen bonding network
dynamics.
Redshift – stronger H-bonding
Blueshift – weaker H-bonding
How does hydrogen bonding affect OD stretch?
Hydrogen bond to sulfonate group
(weak H-bond high frequency)
Hydrogen bond to water
(strong H-bond low frequency)
Strength of hydrogen bonds:
O—H...:N (29 kJ/mol)
bond strength
O—H...:O (21 kJ/mol)
υ∝
mass
N—H...:N (13 kJ/mol)
N—H...:O (8 kJ/mol)
O—H to carbonyl (14 kJ/mol)
OD stretch peak.
Intensity normalized.
High frequency
shoulder
Falk and Fayer interpreted the broad peak (2553- 2590 cm-1 for OD) as
OD groups of water which form hydrogen bonds either with water or
sulfonate groups.
They also interpreted the high frequency shoulder (2724 cm-1) as an
indication that these water molecules were experiencing a nonpolar
environment and were probably in contact with the fluorocarbon
backbone.
Hydration in superacid polymers
Increased acidity of the superacid
weakens the H-bond between
sulfonate and water causing a red
shift in the O-D stretch
Hydration in superacid polymers
•  S1 superacid consistently
shows more molecules of
water association due its
strong ionicity.
•  S1 water molecules are less
strongly bound to the
superacid – longer H-bonds
from acid to water gives
higher OD frequency.
Superacid versus aromatic acid hydration
H
e- withdrawing
stronger acid
CF2
H
H
O
S
CF2
O
H
O
O
H
O
O
D
O
More associated water to
stabilize the high ionicity of
stronger acid
H
H
weaker water-SO3- H-bond
Blue shift in OD stretch
No strong e- withdrawing
weaker acid
H
O
H
O
S
O
O
H
Less associated water due to
lower ionicity of weaker acid
O
D
O
H
H
stronger water-SO3- H-bond due
to e- density on SO3Red shift in OD stretch
Block copolymer superacids
Shift to lower frequency with more hydration.
Different behavior at 100 % RH.
Water binding in block copolymer superacids
•  Block copolymer has
similar or lower
frequency OD stretch
compared to random
copolymer below 70 %
RH.
•  Shift to higher
frequency at 100 %
RH in the blocks – may
be indication of
swelling.
Speculative Model
Dry block copolymer
O
CF2
CF2
O S O
OH
O
Water molecules
have low
interactions with
closely-spaced
acid groups.
O
CF2
CF2
O S O
OH
H
O
H
O
H
H
Swollen block copolymer
O
CF2
CF2
O S O
OH
O
Polymer swells
and increased
headgroupwater contacts.
O
CF2
CF2
O
O S
O
H
H
O
H
H
H OH
O
H
Water is bound more strongly in sulfonated CEMs
1
AEM
CEM 1.1 meq
g-1
AEM 0.7 meq g-1
Absorbance (Normalized)
Absorbance (Normalized)
CEM
0
2700
2600
2500
2400
-1
Wavenumber (cm )
OD peak freq. in liquid water = 2509 cm-1
1
CEM 2.5 meq g-1
AEM 2.4 meq g-1
0
2700
2600
2500
-1
Wavenumber (cm )
2400
Center of mass of OD peak shows less waterpolymer interactions in AEMs
0.03
0.02
0.01
0.00
sulf PSf 2
AEM 2.4
Absorbance (arb. units)
Absorbance (arb. units)
0.4
2
R =0.995
AEM 0.7
R =0.998
0.3
0.2
2650
2600
2550
2500
2450
2400
2700
2650
-1
2600
2550
2500
2560 0.7 mmol/g 2531 2.0 mmol/g 2550 1.6 mmol/g 2522 2450
2.5 mmol/g 2546 2.4 mmol/g 2519 2400
-1
Wavenumber (cm )
2
R =0.998
0.04
2
R =0.992
CEM 2.5
Liquid water OD-COM = 2509 cm-1
Absorbance (arb. units)
Absorbance (arb. units)
0.10
0.05
0.00
0.03
0.02
0.01
0.00
2700
2650
2600
2550
2500
-1
Wavenumber (cm )
OD-­‐COM (cm-1)
0.1
Wavenumber (cm )
CEM 1.1
QA PSf 1.1 mmol/g 0.0
2700
OD-­‐COM (cm-­‐1) 2450
2400
2700
2650
2600
2550
2500
-1
Wavenumber (cm )
2450
2400
Ammonium functionalized
poly(sulfone) show an OD peak
closer to that of bulk water. May
be the origin of their higher water
flux.
Conclusions
•  Ionic domain morphology in proton exchange membranes is
important.
o  Controlled by the chemical composition and processing of the
polymer.
o  One route to high conductivity polymers at high hydration is by
employing block copolymers.
•  Water dynamics plays a key role in how ions and small
molecules move through hydrated membranes.
o  We need to pay attention to what is happening in the watery
domains in order to understand the material properties.
•  Superacids provide a route to high conductivity proton
exchange membranes at low relative humidity.
•  We can understand how superacids are working in proton
exchange membranes by measuring water-polymer
interactions.
Energy
Energy
OD energy surface
r (bond length)
r (bond length)
Water/Polymer interaction for sulfonated
and aminated polysulfone
-1
Flux (L micron m h bar )
8
-2
-1
6
4
2
CEM
AEM
0
R:
100
CEM
AEM
AEM have higher flux and lower rejection
than CEMs…Why?
Need to investigate what water is doing in
each of these systems.
NaCl rejection (%)
80
60
40
20
0
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
-1
IEC (mmol g )
Hibbs, M. R., M. A. Hickner, T. M. Alam, S. K. McIntyre, C. H. Fujimoto, C. J. Cornelius, “Transport Properties
of Hydroxide and Proton Conducting Membranes,” Chem. Mater. 2008, 20(7), 2566-2573.