325 steevn

Simon Fraser University

2

Dept. of Chemistry (Physics#)

Simon Fraser University

Burnaby, British Columbia

Canada

Model Polymers for Fuel Cell Membranes.

E.M.W. Tsang, A. Yang, Z. Shi,T. Weissbach, R. Narimana,# B. Frisken,#

S. Holdcroft*

Funding:Dec, 2013,ICAER 2013

Proton Exchange Membrane Fuel Cell (PEMFC)

Cathode Reaction

O2

+ 4e

O2

H2O

+ 4H + - 2H 2O

H2

Anode Reaction

2H2

4H ++ 4e -

Proton Exchange

Membrane (PEM)

ElectrodeCatalyst

H+

e-

e-

(Hydrogen)

Fuel Cells – Stacks

gasket

electrodebacking

MEA

flow field

Bi-polar plate

Automotive: 80,000 W

~350-400 MEAs

750 Bipolar plates

Structure of Nafion (PFSI)

CF2 CF2 CF CF2

OCF2CF

CF3

O(CF2)2SO3H

x y

z

1 micron (1/1000 mm)

Disadvantages• Very expensive

• High H2,O2, N2 & methanol crossover

• Humidification necessary

• Failure at high temperature ( >100 0C)

• Catalyst poisoning

• High electro-osmotic drag

Advantages• High proton conductivity

• Efficient even at low operating temp

• Good mechanical properties

• High durability

• Good flexibility at low temp

Perfluorinated vs Hydrocarbon PEMs

6

There is a need to develop alternative advanced membranes based on aromatic hydrocarbons

Disadvantages• Very expensive

• High H2,O2, N2 & methanol crossover

• Humidification necessary

• Failure at high temperature ( >100 0C)

• Catalyst poisoning

• High electro-osmotic drag

Advantages• High proton conductivity

• Efficient even at low operating temp

• Good mechanical properties

• High durability

• Good flexibility at low temp

Perfluorinated vs Hydrocarbon PEMs

7

F2C CF2

H2C CH2

C CH2 CH2 CH2

SO3H

x y

z

SO3H

CF2CF

SO3H

CF2CF

m n

R

ETFE-g-PSSA

BAM

S-SEBS

CH2CH

SO3H

CH2CH CH2CH2 CH3CH2

CH2CH3

CH2CH

SO3H

CH2CH

Nafion

CF2CF2x

CFCF2y

OCF2CF

CF3

OCF2CF2SO3Hz

Examples of

PEMs

O O C

O

nHO3SS-PEEK

Potential Polymer Architectures for PEM Materials

Microphase Separation in Block Copolymers

F.S. Bates and G. H. Fredrickson, Physics Today, Feb. 1999.

Block

Copolymers

Graft

Polymers

Chain Transfer Radical Polymerization Macroinitiator

x CF2=CH2 + y CF2=CF-CF3

CuX / bpy

Sulfonation

R-XCH2CF2 CF2CFx y R'-X

CF3

ATRP

CH2CF2 CF2CF CH2CH

CF3

n

x y nClSO3H or CH3COOSO3H CH2CF2 CF2CF CH2CH

SO3H

CF3

x y nX X

• Chain Transfer Radical Emulsion Polymerization

• Atom Transfer Radical Polymerization

• Sulfonation

Synthesis of Novel Fluoropolymer-block-

Ionic Polymers

HFP VDF20%HFP80%VDF

10

Synthesis of Fluorous-Ionic Graft

Copolymer

P(VDF-co-CTFE)-g-SPS

P(VDF-co-CTFE)-g-SPS

P(VDF-co-CTFE)Macroinitiator

P(VDF-co-CTFE)-g-PS

CF2

CH2 CF

2CFx y+

Cl

Emulsion Polymerization

n

CuCl/ bpy

ATRP

CH3COOSO3H

Sulfonation

Na2S

2O

5+ K

2S

2O

8

* CH2CF2 CF2CFyx

*

Cl

* CH2CF2 CF2CFyx

Cl

CF2CFz

*

CH2CHn

Cl

* CH2CF2 CF2CFyx

Cl

CF2CFz

*

CH2CHn

Cl

SO3H

2.6mol%CTFE 97.4mol%VDF

Membrane Morphology

Perforated Lamellar Morphology:- “Ionic” channels width = 8 – 15 nm

CF2CFCH2CF2

CF3

CH2CH

SO3H

x' y' n'

H

100 nm

CH2CF2 CF2CFx y

CF2CF

CH2CH

z

n

SO3H

Cl

Disordered cluster-network Morphology:-Ionic cluster size = 2 – 3 nmNote: Nafion cluster size = 5 – 10 nm

Graft copolymer:Diblock copolymer:

11

100 nm

IEC (mmol/g)

0.0 0.5 1.0 1.5 2.0 2.5

H2O

]/[S

O3

- ])

0

30

60

90

120

150

180

IEC (mmol/g)

0.0 0.5 1.0 1.5 2.0 2.5

Pro

ton C

onductivity (

S/c

m)

0.00

0.02

0.04

0.06

0.08

0.10

Diblock vs Graft Membrane

Diblocks (long-range channels):- Greater water uptake Higher proton conductivity and mobility- Excessive water swelling mechanical instability and limited attainable IEC.

Grafts (small ionic clusters):- Less water swelling Lower proton mobility- Maintain good mechanical property and high proton concentrations

IEC (mmol/g)

0.0 0.5 1.0 1.5 2.0 2.5

eff x

103

(cm

2 s

-1 V

-1)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

[H+

] (M)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Graft copolymer:

100 nm

Diblock copolymer:

100 nm

dissolves

6-17% PS, Fully Sulfonated:

45% PS, Partially Sulfonated:

35% PS, Partially Sulfonated:

100 nm

E

B

100 nm

100 nm

H

100 nm

J50% PS, Partially Sulfonated:

Proton Conductivity :

IEC (mmol/g)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Pro

ton

Co

nd

ucti

vit

y (

S/c

m)

0.00

0.02

0.04

0.06

0.08

0.10

0.126-17% PS, fully sulfonated

35% PS, partially sulfonated

45% PS, partially sulfonated

50% PS, partially sulfonated

Nafion 117

13

- Fully sulfonated : continuous increase in

proton conductivity with IEC

- Partially sulfonated: initial increase followed

by drop in proton conductivity at high IEC.

6-17% PS, Fully Sulfonated

35% PS, p. Sulfonated

45% PS, p. Sulfonated

50% PS, p. Sulfonated

(IONIC PURITY):

Fluorinated polymer Partially Sulfonated polystyrene

& water

SANS: Contrast Variation Effect

Rubatat, Holdcroft, Diat, Shi. Frisken100 nm

Conclusions

• Model fluorous-ionic diblock copolymers with different block ratios have been synthesized to investigate structure-property relationships in PEMs.

• Water sorption, proton conductivity, proton mobility, anisotropy, etc, depend strongly on the membrane morphology….and on the degree of sulfonation within an “ionic” channel.

• Ionic purity of the “ionic channel” is critical.

• The graft structure allows for very high IEC without dissolution – promising for low RH conductivity.

100 nm

G

IEC = 0.89 mmol/g100 nm

K

IEC = 0.68 mmol/g100 nm

C

IEC = 0.70 mmol/g500 nm500 nm500 nm

T.J. Peckham, S. Holdcroft. Adv. Mater., 22 (2010) 4667–4690

Yossef Elabd and Michael Hickner“Block Copolymers for Fuel Cells”

Macromolecules, 2011, 44 (1), pp 1–11