Progress Towards New Heterocycle-Containing Proton Exchange Membranes February 7, 2008 Corinne Lipscomb Mahanthappa Group.

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

4

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)

5

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

7

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

8

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.

9

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.

10

Water Mimics

Mineral acids - H2SO4, H3PO4

Heterocycles

High boiling

Immobilization possible

N

NHN

N

HN

N

HN

N

N

HN

N

HN

11

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

12

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

13

Nomenclature

O

O

N

N NH

Heterocycle-Spacer length

Tz6

Heterocycle-Spacer length-Polymer

SiO n

O

N

N

HIm5Si

Small Molecules Polymer

14

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

15

Impedance Spectroscopy

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.



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)

16

Proton Conductivity





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.

17

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.

18

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 =

19

Proton Conductivities



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

20

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

21

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

22

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


N

NH

HS

SiO

SiO

x y

AIBN, 66C, 40 hrsSi

OSi

Ox y

S

N

HN

THF

23

Tg and Heterocycle Content



Tg rises with heterocycle content for poly(siloxanes).

Thermally stable to ~200ºC


SiO

SiO

x y

S

N

HN

24

H+ Conductivity vs. BzIm Content



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


25

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

26

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


T g ( C )



mol% benzimidazole-grafted AGE units

Tg rises with heterocycle content for multiple polymers.

27

Conductivity and Mass Fraction





Absolute Proton Conductivity Intrinsic Proton Conductivity






IncreasingBenzimidazole

ContentQuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

Sample 1Sample 2Sample 3Sample 4Sample 5Bz8PEO

28

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 =

29

Triazole Tethered Polyacrylates

n = 6-8

X, Y, and Z controlled with feed ratios

Tz6

HMTz6

PEG


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

30

Conductivity with Acid Doping

Doping with TFA increases conductivity significantly



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



Sample 2Sample 3Sample 1Sample 4Sample 5Sample 6


31

Proton Conductivity & Tg



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



Sample ASample BSample CSample DSample E


32

Intrinsic Conductivity & Tg



As more PEG added, mass fraction of heterocycle goes down

Decreasing mass fraction ofthe heterocycle: decreases Tg

decreases conductivity



Sample ASample BSample CSample DSample E


Decreasing Tg: increases conductivity

Mass fraction of heterocycle and Tg are interconnected.

33

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.

34

Imidazole Polysiloxanes

70% cyclictrimers


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

35

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

36

Triazole Tethered Polysiloxanes

Tz2Si

FBz2Si

Tz8Si


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

37

Conductivity & Tether Length



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

are needed to see this picture.1000/K


SiO

Me

n

O

O

N

NH

SiO

Me

n

O

N

HN

SiO

Me

n

O

N NNH

SiO

Me

n

N NNH


Different heterocycles needdifferent tether lengths.

38

Conductivity and pKa



N

HN

N N

HNCF3

pKa ~ 14

Tz2Si higher than FBz2SiDespite having: the same pKa

the same tether length Tg factored out


are needed to see this picture.T-Tg


Same pKas different conductivities

39




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


Tz8Si Im8Si


40


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.

41

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

42

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

43

Acknowledgements

Professor Mahesh Mahanthappa

The Mahanthappa Group• Erin Henninger• Joan Widin• Chris Bates• David Bunck

Practice Talk Antendees

• Mary Beth Anzovino• Matt Bierman• Maren Buck• Matt Christianson• Jeremy Higgins• Beth Landis• Katie Partridge• Dr. Dino Ress

Helpful Conversations• Beth Landis

Progress Towards New Heterocycle-Containing Proton Exchange Membranes February 7, 2008 Corinne Lipscomb Mahanthappa Group.

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