Progress Towards New Heterocycle-Containing Proton Exchange Membranes
February 7, 2008
Corinne Lipscomb
Mahanthappa Group
2
Hydrogen Fuel Cells
http://www.inhabitat.com/2007/05/25/shanghai-to-build-hydrogen-fuel-cell-infrastructure/Hoffman, P. Tomorrow's Energy. The MIT Press: Cambridge, Massachusetts, 2001.
The H-Racer
Completely Hydrogen-Powered Car
Are being researched for: Automobiles Cell phones Portable electronics NASA’s continued use
Have been used in: NASA missions since Gemini Concept vehicles Toys
3
The Hydrogen Fuel Cell
Anode : H2 2H+ + 2e-
Cathode : O2 + 4e- 2O2-
External circuit for electrons
Oxygen ions and protons form H2O
Palladium Catalyst
Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5-39.
http://blog.wired.com/cars/2007/05/index.html
Proton Exchange (Polymer Electrolyte) Membranes
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Ideal Material
High proton conductivity
Low electron conductivity
Low permeability to fuel and oxidant
Low water permeability
Chemically, thermally, and mechanically stable
Inexpensive
Ability to be fashioned into Membrane Electrode Assemblies (MEAs)
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The Industry Standard
Nafion - 1964 E. I. du Pont de Nemours and Co.
Connolly, D. J.; Gresham, W. F. (E.I. Du Pont de Nemours and Company). USA. U. S. Pat. 3,282,875, 1966.
CF2
CF2
CF
CF2
O
CF2
CF
O
CF2
CF2
SO3H
CF3
x y n
6
Synthesis of Nafion
Free radical initiated
Pressure to control gaseous monomer
Connolly, D. J.; Gresham, W. F. (E.I. Du Pont de Nemours and Company). USA. U. S. Pat. 3,282,875, 1966.
F2C CF2
F2C
FC
O
F2C F
C F2C
CF2
FCF3
FF3C
FN NF , 80C, 800 psi N2
F2C
CF2
CFCF2
O
F2C F
CO
CF2
F2C
SO2F
CF3
x y n
F2C
CF2
CFCF2
O
F2C F
CO
CF2
F2C
SO3H
CF3
x y n
OCF2
F2C
SO2F
CF3
1. NaOH/MeOH reflux, 4 hrs2. HCl
EW =Mass of Polymer
Moles of SO3H
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Advantages of Nafion
Stable material Selective ion permeability
Compatible with current fuel cell technology
High proton conductivity under aqueous conditions ~ 0.1 S/cm
Deluca, N. W.; Elabd, Y. A. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2201-2225.Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75-83.
Conductivity
Typically measured inSeimens/cm (S/cm)
1 S = 1 -1
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Disadvantages of Nafion
Low conductivity at low water content Permeable to MeOH (Direct Methanol Fuel Cell)
Poor mechanical strength at high temperatures
Processability and fabrication issues
DOE goal - 0.1 S/cm at 120C and 50% relative humidity
Nafion cannot meet this goal.
Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. Rev. 2004, 104, 4587-4611.Deluca, N. W.; Elabd, Y. A. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2201-2225.
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Proton Conduction Mechanisms
Structural diffusion or proton “hopping” Grotthus mechanism in H2O Rearrangement of hydrogen bonds simultaneously Protons transferred quickly
Vehicular diffusion Proton carried by one molecule Diffusion Protons transferred slowly
HO
H
H
H
O
H
HO
H
H H
O
H
Kreuer, K. D. Solid State Ionics 1997, 94, 55-62.
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Water Mimics
Mineral acids - H2SO4, H3PO4
Heterocycles
High boiling
Immobilization possible
N
NHN
N
HN
N
HN
N
N
HN
N
HN
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Tg - Glass Transition Temperature
Amorphous solids - glasses, polymers, etc.
Below Tg - ‘solid-like’ behavior material becomes rigid upon cooling
Above Tg - ‘liquid-like’ behavior material softens upon heating
Some variability - depends on the heating/cooling rate
Above Tg segmental mobility increases significantly important in conductivity
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Polymer Molecular Weights
Mn - Number Average Molecular Weight
PDI - Polydispersity Index the breadth of the distribution
GPC - Gel Permeation Chromatography
290
295
300
305
310
315
320
325
330
335
10 12 14 16 18 20
Retention Volume (mL)
Refr
acti
ve I
nd
ex
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Nomenclature
O
O
N
N NH
Heterocycle-Spacer length
Tz6
Heterocycle-Spacer length-Polymer
SiO n
O
N
N
HIm5Si
Small Molecules Polymer
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Conducting Heterocycle Solvents
EW = 740 g/mol
Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, M.; Maier, J. Electrochim. Acta 1998, 43, 1281-1288.
O O
O
SO3H
O O
O
SO3H
NHN
NHN
Conductivity
Conductivity
O O
O
SO3H
H2O HighConductivities
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Impedance Spectroscopy
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Experimental Apply sinusoidal potential Measure current response Usually done at high frequencies
Impedance
Real term - Resistance Change in amplitude
Imaginary term - Capacitance Phase shift At high freq. goes to zero
Conductivity
Resistance =(Resistivity) Length
Area
= Conductivity =1
(Resistivity)
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Proton Conductivity
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Imidazole doped Pyrazole doped
Heterocycles can conduct protons like water.
Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, M.; Maier, J. Electrochim. Acta 1998, 43, 1281-1288.
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Imidazole Immobilization
Herz, H. G.; Kreuer, K. D.; Maier, J.; Scharfenberger, G.; et al. Electrochim. Acta 2003, 48, 2165-2171.
O(CH2)nOHCl
NaH, HO(CH2)nOH
O(CH2)n-1
O
OHN
N(CH2)n-1O
HN
N(CH2)n-1
x
O
O
NH3,
DMF, rt
DMSO, Cl Cl
OO
DCM, -60C
MeOH,0C
AIBN, 100C, 3hrs
ex.
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Free Radical Polymerization
N N
NC
CNCN
N22
O
NH
N(CH2)n-1
CN
O
NH
N(CH2)n-1
NC
O
NH
N(CH2)n-1
O
NH
N(CH2)n-1
x
Monomer
Initiator 1/2DP =
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Proton Conductivities
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Herz, H. G.; Kreuer, K. D.; Maier, J.; Scharfenberger, G.; et al. Electrochim. Acta 2003, 48, 2165-2171.
Immobilized imidazoles can conduct protons.
O
x
N
HN
O
x
N
HN
10b (Tg = 32°C) 12 atom spacer
10a (Tg = 51°C) 6 atom spacer
Both polymers stable to >200°C
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Poly(siloxane) Backbones
HS OH
O H2N
H2N
4M aq. HCl
reflux, 40 hrs
N
NH
HS
Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.
Si
OSi O
Si
OSiO
O
SiO
Si
OSi BuLi
THF, rt
SiO
SiO
x y
V4 D3
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Anionic ROP Mechanism
THF
V4 D3
Si
OSiO
Si
OSi O
O
SiO
Si
OSi
Bu Li
BuSi
OSi
OSi
OSi
O-
+LiBuSi
OSi
OSi
OSi
OSi
OSi
OSi
O-
+Li
Me3SiClSiO
SiO
x y
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Benzimidazole Poly(siloxanes)
Copolymer [V4]/[D3]
Feed ratio
Mol% X
Vinyl siloxane
Mn (GPC)
Final Comp.
PDI
Final Comp.
Bz5Si - 5 13300 2.2
Bz5Si 0.5 16 15800 1.4
Bz5Si 1.125 33 10600 1.4
Bz5Si 3 57 10800 1.3
Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.
N
NH
HS
SiO
SiO
x y
AIBN, 66C, 40 hrsSi
OSi
Ox y
S
N
HN
THF
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Tg and Heterocycle Content
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Tg rises with heterocycle content for poly(siloxanes).
Thermally stable to ~200ºC
Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.
SiO
SiO
x y
S
N
HN
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H+ Conductivity vs. BzIm Content
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At lower temperatures - conductivity depends on Tg
At higher temperatures - conductivity depends on heterocycle content
For conductivity to be unaffected by segmental mobility: T > Tg + 50ºC
Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.
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PEO Backbones
Persson, J. C.; Jannasch, P. Chem. Mater. 2006, 18, 3096-3102.
HOO
Hn
1. NaH2.
OO
Melt, 100C
HOO
OO
H
O
O
y x y
N
NH
HS
1.
2. AIBN
MeOH64¼C
HOO
OO
H
O
OS
y x y
S
N
NH
N
HN
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Stability and Physical Properties
Copolymer % BzIm Mn (g/mol)
GPC
PDI
Sample 1 18 3770 1.12
Sample 2 46 5880 1.03
Sample 3 55 6850 1.03
Sample 4 65 8420 1.03
Sample 5 86 16078 1.07
Persson, J. C.; Jannasch, P. Chem. Mater. 2006, 18, 3096-3102.
T g ( C )
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mol% benzimidazole-grafted AGE units
Tg rises with heterocycle content for multiple polymers.
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Conductivity and Mass Fraction
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Absolute Proton Conductivity Intrinsic Proton Conductivity
Persson, J. C.; Jannasch, P. Chem. Mater. 2006, 18, 3096-3102.
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IncreasingBenzimidazole
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Sample 1Sample 2Sample 3Sample 4Sample 5Bz8PEO
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Triazole Tethered Polyacrylates
OH
Cl
O DCM, TEA O
O
Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.
Tz6
HMTz6
O
O
N3 O
O
1. CuSO4, NaAscorbate, H2O / BuOH, rt
2. 0.1 M NaOH/MeOH
O
O
O
O
N
N NH
N
N NOH
OO
OH
ONaHO
HO
NaAscorbate =
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Triazole Tethered Polyacrylates
n = 6-8
X, Y, and Z controlled with feed ratios
Tz6
HMTz6
PEG
Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.
O
O
O
O
N
N NH
N
N NOH
AIBN, 60C
DMSO
O O
N N
NH
O O
N N
N
OH
x y
OO
O
O O
O
z
n
n
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Conductivity with Acid Doping
Doping with TFA increases conductivity significantly
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Sample Mol % TFA Tg (°C)
1 130 -27
2 100 -21
3 80 -25
4 50 -25
5 20 -26
6 0 -25
Samples had 28 mol% PEG and30 mol% HMTz6 compared to Tz6
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Sample 2Sample 3Sample 1Sample 4Sample 5Sample 6
Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.
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Proton Conductivity & Tg
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Sample Mol % PEG Tg (ºC)
A 52 -43
B 30 -29
C 22 -19
D 13 -3
E 0 16
As Tg goes down - goes up
Samples had the same Mol % HMTz6 compared to Tz6
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Sample ASample BSample CSample DSample E
Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.
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Intrinsic Conductivity & Tg
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As more PEG added, mass fraction of heterocycle goes down
Decreasing mass fraction ofthe heterocycle: decreases Tg
decreases conductivity
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Sample ASample BSample CSample DSample E
Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.
Decreasing Tg: increases conductivity
Mass fraction of heterocycle and Tg are interconnected.
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Imidazole Polysiloxanes
N
HN
1. NaOH/H2O2. PhCH2Cl
N
N
Bn
H H
O
N
N
Bn
OH
1. NaH, 0C, 1 hr2.1. NaH, 0C, 3 hrs
2. OOTosBr
N
N
ON
N
O O
Bn Bn
0C, overnight stir overnight, rt3. 40C, 8 hrs
40°C H2O, 150°C
Im5 Im8
Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.
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Imidazole Polysiloxanes
70% cyclictrimers
Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.
PartialHydrolysis
Im5N
NO
1. H2PtCl6 / MeSiHCl22. EtOH / NEt3
N
NO Si
EtO OEt
H+/H2O[THF/Toluene]
*Si
O*
n
O
N
N
Bn Bn
Bn
SiO n
O
N
N
Bn
SiO n
O
N
N
H
KOH/ 18-crown-6Pd-C / H2, 50CDidodecyldimethyl-ammonium bromide 120C, 150 hrs
Same polymerization carried out with Im8
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Triazole Tethered Polysiloxanes
Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.
BrHO
1. NaH/DMF, 0C, 30 minO
Br
H TMS
PdCl2(PPh3)2
CuI, Et3N
TMS
N3 O
O
1
2
CuSO4, tBuOH/H2O
1 or 2
N N
N OR
O
NaAscorbate, (Bu4NF for 2) THF, 24 hrs
2.
rt, 2 hrs
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Triazole Tethered Polysiloxanes
Tz2Si
FBz2Si
Tz8Si
Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.
SiO
Me
H n
N N
NPOM
Toluene, Pt0, 50¼C2. 0.1 M NaOH/MeOH, NEt3
1.
SiO
Me
n
N
NH
CF3
SiO
Me
n
N N
NH
SiO
Me
n
O
N N
NH
N
NH
CF3
ON
N
N
POM
2. 0.1 M NaOH/MeOH, NEt3
Toluene, Pt0, 50¼C
1.
1.
Toluene, Pt0, 50¼C
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Conductivity & Tether Length
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Tz2SiTg = 19°C
43% Het.
Im5SiTg = 41°C
36% Het.
Im8SiTg = 7°C
28% Het.
Tz8SiTg = -5°C
28% Het. QuickTime™ and aTIFF (Uncompressed) decompressor
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Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.
SiO
Me
n
O
O
N
NH
SiO
Me
n
O
N
HN
SiO
Me
n
O
N NNH
SiO
Me
n
N NNH
Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.
Different heterocycles needdifferent tether lengths.
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Conductivity and pKa
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N
HN
N N
HNCF3
pKa ~ 14
Tz2Si higher than FBz2SiDespite having: the same pKa
the same tether length Tg factored out
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Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.
Same pKas different conductivities
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Conductivity and pKa
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N
HNN
N
HN
pKa = 13.6 pKa = 18.6
Despite having: Tg factored out the same tether length the same mass fraction a higher pKa
Higher pKa has a higher conductivity
Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.
Tz8Si Im8Si
Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.
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Conductivity and pKa
Tg masks pKa effects
Mass fraction more dominant than pKa
Different heterocycles form the aggregates necessary for proton conduction under different conditions.
Each polymer system shouldbe optimized separately.
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Conclusions
Tg is the most dominant effect
Mass fraction of the heterocycle also dominant
Tether length and pKa are concerns
Immobilization decreases vehicular diffusion allowing for structural diffusion
Polymers shown - ~ 10-6 - 10-3 S/cm
Nafion - ~ 10-1 S/cm
Heterocycle-containing polymers present a new routetowards non-aqueous proton conduction at high temperatures
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Future Directions
Optimization of tether length, mass fraction,Tg, and acid doping Block copolymers - systematic control over morphology and mechanical properties Living polymerization techniques - systematic control over PDI and molecular weight
Protocols for evaluation of PEMs TGA for thermal stability, DSC for Tg, & GPC for polydispersity index and molecular weight EIS for proton conductivity
Protocols for hydrogen fuel cells Rheology for mechanical stability Membrane Electrode Assemblies
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Acknowledgements
Professor Mahesh Mahanthappa
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