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Rochester Institute of Technology

RIT Scholar Works

Theses

Thesis/Dissertation Collections

2006

Polyvinylidene fluoride composites as an option for

proton exchange membranes in fuel cells

Rohini P. Sajanpawar

Follow this and additional works at:

http://scholarworks.rit.edu/theses

This Thesis is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contactritscholarworks@rit.edu.

Recommended Citation

(2)

POL YVINYLIDENE FLUORIDE COMPOSITES

AS AN

OPTION FOR PROTON

EXCHANGE MEMBRANES IN FUEL CELLS

ROHINI P. SAJANPAWAR

June

,

2006

The

s

is

Submitted in partial fulfillment

of requirements for the degree of Master of Science in M

ate

rials

Science and Engineering

Approved:

Thomas W. Smith

Dr. Thomas W. Smith (Advisor)

Department

of

Materials Science

and

Engineering

Accepted:

K.5. V. Santhanam

Dr. K.S. V. Santhanam

(Director)

Department

of

Materials

Science and Engineering

(3)

Copyright Release Form

POL YVINYLIDENE FLUORIDE COMPOSITES AS AN OPTION FOR PROTON

EXCHANGE MEMBRANES IN FUEL CELLS

I,

Rohini

P.

Sajanpawar,

grant

permission to the Wall

ace

Memorial Library

of

the Roche

ster

Institute of Technology, to reproduce this thesis in whole

or in

part. Any

reproduction

will not be

for commercial

use

or

profit.

D a t e :

(4)

ACKNOWLEDGEMENTS

I would like to

truly

thank Dr. Thomas W.

Smith,

as my research advisor, for his efforts and

direction

throughout my education at RIT. His incredible wealth of

knowledge,

dedication and

encouragement made this thesispossible. Dr. Smithmadetheorganic chemistryclass,

laboratory

sessions andtheresearchgroupmeetings extremely enjoyable andinteractivewhilelearning.

I would also like to sincerely thank the members of my committee, Dr. Massoud

Miri,

Dr.

Timothy

Fuller(General Motors Global AlternativePropulsion

Center,

NY). IthankDr. Miri for

helping

me use his

laboratory

to fabricate several melt processed films. I particularly appreciate

Dr. Fullerfor

helping

us withconductivitymeasurements aswell asgivingme valuable guidance

and suggestionsto conductthisresearchwork.

Iwould like to expressmy gratitudetowards our Materials Scienceand Engg. Department Head

Dr. K.S.V

Santhanam,

for encouraging me throughout my MS at RIT. I would also like to

acknowledge everyone in the

Chemistry

and Materials Science Dept.

Specifically,

Mr. Tom

Allstonand Matt fortheir assistance in using theDSC

facilities,

and all the stockroom personnel

forconstanthelp.

I owe a particularword ofthanks to our secretarial staff, Brenda

Mastrangelo,

Ann Gottorff for

theirassistance

during

the entireperiod ofmy MS.

My

specialthanks to the entireteam at Xerox

Corporation,

NY,

where I did my

Co-op;

fortheir

excellentguidance in everythingas well as in my thesis. Ithankallmyresearch group members,

seniors, peers,

juniors,

roommatesand allmy friends for making my stayatRIT somuchfunand

enjoyable.

Nothing

canbeaccomplishedwithout

family

support. Iwouldliketo extendmy specialgratitude
(5)

Shrirang, Pallavi, Vaishali,

Aditya and

Vedika;

fortheir

love,

support and encouragement from

thousands of miles away.

Many

thanks are due to my fiance Rohit for his tremendous

love,

support and guidance. Much

appreciation is also due to him for

being

with me in the laboratories till

long

hours,

and also

helping

me assemblethis thesis. Their support, encouragement and patience enable metopursue
(6)

ABSTRACT

This thesis details approaches for the preparation of composite membranes derived from poly

(vinylidene

fluoride), PVDF,

or copolymers thereof. It was expected that these membranes

would withstandtheharsh thermaloxidative conditions ofa practicalfuel cell and might exhibit

high proton mobility. Sulfonatedcarbon blackwas used as the proton conducting componentin

the membrane. The advantages and the disadvantages ofthe materials used for fabrication of

proton exchange membranes were reviewed. This thesis provides procedures for thepreparation

ofcomposites ofPVDF with sulfonated carbon black

(CB)

whichmight be employed as proton

exchangemembranes. Thin films from all compositions were prepared

by

compressionmolding

or solution casting. All composites and films were

thermally

analyzed

by

differential scanning

calorimetry. These studies indicated different crystallization behaviorwith different loadings of

carbon black. Solution cast films containing

PVDF,

CB and Nafion 1000 were also prepared

andcharacterized

by

differential scanning calorimetry.

The electrical conductance of all solution cast films was evaluated at GM Fuel Cell

Activities,

Honeoye

Falls,

NY. Methods developed for fabricationof membranes were shown towork well.

The membrane materials were however found to be electrically conductive. It is suggestedthat

electrically conductive PVDF composites with sulfonated carbon black might have utility as

electrodes. The processes and procedures employedto make the PVDF/sulfonated carbon black

composites studied in this thesis might also beadapted to prepare PVDF composites with other

(7)

TABLE OF

CONTENTS

CHAPTER 1: Introduction 11

CHAPTER 2: Literature Review 13

2.1 FuelCells16

13

2.1.1

History

13

2.1.2 Fuel Cell

Technology

Development'"""

14

2.2 Generalprinciples and

functionality

offuelcells 16

2.3 Types ofFuel Cells 19

2.3.1 Alkaline fuel Cell

(AFC)

20

2.3.2 Proton Exchange Membranes Fuel Cells

(PEMFC)

21

2.3.3 Direct Methanol Fuel Cell

(DMFC)

23

2.3.4 Phosphoric Acid Fuel Cell

(PAFC)

24

2.3.5 MoltenCarbonate Fuel Cell

(MCFC)

25

2.3.6 Solid Oxide Fuel Cell

(SOFC)

26

2.4

Protonically

Conductive Ion Exchange Membranes 28

2.4.1 Structure ofthePolymer Electrolyte Membrane 28

2.4.2 Polymer Electrolyte Membrane 29

2.5 Various Polyperfluorinated Copolymers 32

2.6 Polyvinylidene Fluoride

(PVDF)

anditscopolymersasprotonexchange materials 35

2.7 SulfonationofPolymers 39

2.8 Other

Commercially

Available Proton Exchange Membranes 41

2.9 Alternative MaterialstoNafion 42

2.10 Organic/Inorganic Composite Membranes 44

CHAPTER 3: Experimental 46

3.1 Materials 46

3.2 Composites Preparation 46

3.2.1 Process # 1: PreparationofPVDFandPVDFcopolymer composites from

FeCl3

gel46

3.2.2 Process # 2: PreparationofPVDF/HFP

(Kynar2801)

Composites from

FeCl3

gel. 48 3.2.3 Process # 3: PreparationofPVDF/HFPcopolymercomposites fromDMF solution. 49

3.2.3.1 PreparationofPVDF-HFPcopolymer/CB-SO"3H Composites 49

3.2.3.2 PreparationofPVDF-HFP

/Nafion/CBSO~3

H Composites 50

3.3 Composites Membrane Fabrication 51

(8)

3.3.2.1 Puddle

Casting

52

3.3.2.2 Draw-Down Technique 53

3.4 Characterization 54

3.4.1 Differential

Scanning Calorimetry (DSC)

54

3.4.2 Electrical

Resistivity

Measurements 55

CHAPTER 4: ResultsandDiscussions 56

4.1 Proton Exchange Membranes 56

4.2 PreparationofPVDF Composites 57

4.3 Differential

Scanning Calorimetry

(DSC)

66

4.4 Electrical

Resistivity

Measurements 84

CHAPTER 5: Conclusions 88

CHAPTER 6: Future Considerations 90

(9)

LIST OF

FIGURES

Figure 2.1 William R. Grove (181

1-1896)

demonstratedin 1839thatelectricitycanbegenerated fromhydrogenand oxygen incontact withplatinumstrips,which extendintodilutesulfuric acid

functioning

as adiaphragm.21 13

Figure 2.2 Membraneelectrodeassemblyandelectrochemistry for H2/airandmethanolfuel

cells35

17

Figure 2.3 Alkali Fuel Cell

(AFC)

20

Figure 2.4 Schematic

drawing

ofhydrogen/oxygenfuelcell anditsreactionsbased onthe

protonexchange membranefuelcell

(PEMFC)41

21

Figure 2.5 Schematic

drawing

ofthe operatingprinciples offuel cellutilizingmethanol asthe

fuel,

i.e. adirectmethanolfuelcell(DMFC)41 23

Figure2.6 Phosphoric Acid Fuel Cell(PAFC)41

24

Figure2.7 MoltenCarbonate Fuel Cell(MCFC)41

25

Figure 2.8 Solid Oxide Fuel Cell(SOFC)41

26

Figure 2.9 Chemical structure of membranematerial;Nafion

by

DuPont 28

Figure 2.10 Schematic of microphase separationinahydrated ionomer 31

Figure2.11 Chemical structureofNafion.xandyrepresent molar compositions anddonot

imply

a sequence length 33

Figure 2.12 ProposedstructureofDow XUSperfluorinated copolymer 33

Figure 2.13 Structureof

PVF2

36

Figure 3.1 Pellet die 51

Figure 3.2 Carver Press 51

Figure3.3 Puddle castingon a cellotape 52

Figure 3.4 Multimeter 55

Figure 4.1 GelationofKYNAR 301 inMethanolic

FeCl3

.H2O 61
(10)

Figure 4.3 SolutionCast Film 65

Figure 4.4 DSC thermogramofthe second run(heat-cool curve)of plain Kynar 201 powder

representingtheTA Instrumentsoftware calculatedmelting

temperature,

crystallization

temperature andtheglasstransition temperature 66

Figure 4.5 DSC overlayplots ofKynar

201, 2801,

301 and7201 powders 68

Figure4.6 DSC overlay ofKynar 201 plainpolymer,its black compositepowder with

sulfonated carbonblack

by

Process #

1,

andits melt pressedfilm 69

Figure4.8 DSC overlaysofKynar 2801 plain whitepowder,itsblack compositepowder with

sulfonated carbonblack

by

Process # 1 andthemelt pressedfilm fabricated fromthecomposite

powder 72

Figure4.9 DSCoverlays ofKynar 2801 plainpolymer,its black composite powder with

sulfonated carbonblack

by

Process# 2 andthemeltpressed film fabricated fromthecomposite

powder 74

Figure 4.10

(a)

DSCthermogramofKynar 2801 Melt Pressed film from Process # 1 76

Figure4.10

(b)

DSC thermogramofKynar 2801 Melt Pressed film from Process # 2 76

Figure 4.1 1 DSCoverlaysofPVDF Kynar2801 solution castfilmswithvaryingproportions

ofsulfonatedcarbonblack

(CB)

in it using Process# 3 78

Figure 4.12 DSCoverlaysofPVDF Kynar2801 solution castfilm from

DMF,

and also

Kynar 2801 solution castfilm from DMF containing Nafion

1000,

by

Process #3 80

Figure 4. 13 DSCoverlaysofPVDF Kynar2801 solutioncastfilm from

DMF,

and alsoof

films containing Nafion

1000,

as well as films containing both Nafion 1000and sulfonated

carbon black

by

using Process # 3 82

Figure 4.14 DSC overlays ofPVDF Kynar 2801 solutioncastfilm containing Nafion 1000

plus sulfonatedcarbonblackandthe film containing thehighestcarbonblack

loading

in

Process# 3 84

Figure4.15 Electrical

Resistivity

Measurementsforsolution castfilmsofPVDFKynar 2801

impregnatedwith sulfonated carbonblack

(CB)

(Process#

3)

85

Figure 4.16

(a)

[PC-1][1/1.2]

PVDF Kynar2801 solution castfilm

having

thehighest

loading

of sulfonated carbonblack,

(b) [PC-3] [1/0.4]

PVDF Kynar2801 film

having

low

loading

of

carbonblack,

(c)

[PNC-1]

PVDFKynar2801 filmcontainingbothNafionandsulfonated
(11)

LIST OF TABLES

Table 1 Thedifferent Fuel Cellsthathave beenrealizedandarecurrentlyinuse and development37'38'39'40'41

20

Table 2 Physical Properties ofPVDF 37

Table 3 XRD CrystalliteSize 60

Table 4 EnthalpiesofKynargrades ofPVDF polymer(Referenceto

Fig

4.5)

68

Table 5 EnthalpiesofKynar 201 andits composite materials(Ref. to

Fig

4.6)

69

Table 6 Enthalpies ofKynar 7201 andits composite materials(Ref. to

Fig

4.7)

71

Table 7 Enthalpies ofKynar 2801 andits composite materials

by

Process # 1 (Ref. to

Fig 4.8)

72

Table 8 Enthalpies ofKynar 2801 andits compositematerials

by

Process# 2 (Ref. to

Fig 4.9)

74

Table 9 EnthalpiesofPVDF Kynar 2801/sulfonatedcarbon black

(CB)

solution castfilms

(Ref to

Fig

4.

11)

79

Table 10 Enthalpies ofPVDF Kynar 2801solution castfilm from DMF andKynar 2801

solution castfilm from DMF containing Nafion 1000 (Ref. to

Fig 4.12)

81

Table 1 1 EnthalpiesofPVDF Kynar 2801 solution castfilm from DMF andKynar2801

solution cast film from DMF containing only Nafionas well as films containing both Nafionand

sulfonatedcarbonblack (Ref. to

Fig 4.13)

83
(12)

CHAPTER 1: Introduction

Polymer electrolyte fuel cell systems include several components such as end plates that

encapsulate the current collectors, bipolar plates, gas diffusion

layers,

catalysts and the proton

exchange membrane (PEM). Themembrane electrode assembly

(MEA)

,

including

the PEMand

catalyst electrodes,is the essenceofthe fuel cell and isresponsible fortheprotontransport from

the anodeto the cathode,

thereby

directly

creating electricity fromchemical energy. PEMs must

have a combination ofhigh proton conductivity and good mechanical, thermal, and chemical

stability. High proton conductivity in organic polymers is generally realized

by incorporating

monomers

bearing

sulfonic acid

functionality

or

by

post-sulfonationof a preformed

polymer.1

The level of research and development of new proton conductors and the availability of new

PEM compositions has increased

dramatically

over the past decade 2-3'4'5,6-7 This activity has

been driven

by

the demand for fuel cell technology, particularly forvehicles andportable power

applications. These applicationsimposerather strictrequirementson proton conductors, such as

stable performanceinthe temperature rangebetween 80 and200Cwith aconductivitynot lower

than 0.1 S cm"2, chemical, mechanical, and thermal stability, and

impermeability

to gases,

methanol,and charge carriers otherthanprotons.

Some of these demands are met

by

the best currently available fluorosulfonic acid polymer

electrolyte membranes such as

Nafion9, Aciplex10, Flemion11,

and

Dow12

membranes. These

materials are essentially strong polymer acids. When exposed to water,

they

hydrate and
(13)

polymer and free mobile protons in the aqueous solution.

Thus,

the free protons move through

thehydrogenbondednetwork of watermolecules insidethepolymer.13

Perfluorinated polymer electrolytes such as Nafion have been demonstrated to be excellent

protonexchange membranes.

However,

thehigh costofthe materialisabarriertothe largescale

use of fluorosulfonic acid polymers in PEMFC14'15'16'17. Low proton conductivity at high

temperatures above 100 Candlowrelative

humidity

isalso aproblem.

In

1997,

itwas reportedthatPVDF grafted withpolystyrene andsubsequently sulfonated was an

option forthe fabrication of proton conducting membranes for polymer fuel cells. This thesis

provides three different procedures for the preparation of PVDF composites with sulfonated

(14)

CHAPTER 2: Literature Review

2.1 Fuel Cells16

2.1.1 History19

"As early as

1839,

Sir William Grove (often referred to as the "Father of the Fuel

Cell")

discoveredthatit maybe possibleto generate electricity

by

reversingthe electrolysis of

water.20

It was not until 1889 that two researchers, Charles Langer and

Ludwig

Mond,

coined the term

"fuel cell"

as

they

were

trying

to engineer the first practical fuel cell using air and coal

gas.21

While furtherattempts were madein theearly 1900s to

develop

fuel cells that could convertcoal

orcarboninto electricity , theadventofthe internal combustion engine

temporarily

quashedany

hopes offurtherdevelopmentofthe

fledgling

technology."

02

H?

Figure 2.1 William R. Grove(1811-1896)demonstratedin 1839thatelectricitycanbegeneratedfrom

hydrogenand oxygenincontact with platinumstrips,which extendinto dilutesulfuric acid

functioning

as a
(15)

"Francis Bacon developed what was perhaps the first successful fuel celldevice in

1932,

with a

hydrogen-oxygencell using alkaline electrolytes and nickel electrodes inexpensive alternatives

to the catalysts used

by

Mond and

Langer".22

Bacon and company faced substantial number of

technical

hurdles,

until

1959,

when

they

first demonstrated a practical five-kilowatt fuel cell

system.

Incidentally,

in that same year,

Harry

Karl

Ihrig

demonstrated a 20-horsepower fuel

cellpoweredtractor,which is currently

well-known.23

The late 1950's saw NASA starting to build a compact electricity generator for use on space

missions. Hundreds of research contracts

involving

fuel cell

technology

soon got

funding

from

the

NASA,

in an attempt to explore several promising approaches to the construction of a

practical power generation

system.24

The firstmajor success ofthis

big

research attempt was to

be found in the Gemini series ofearth-orbiting missions, inwhich ion-exchange membrane fuel

24 cells developed

by

the General Electricwereused.

Inmore recent years federal agencies and several industrial manufacturers,

including

majorauto

makers,havemade ongoing investments inresearch forthe development offuelcell technology.

In the near future it is expected that traditional power sources can be replaced

by

fuel cell

devices ranging from micro fuel cells for cell phones to stationary power sources and

high-powered fuelcellsforvehiculartransport.

2.1.2 Fuel Cell

Technology

Development

Over the last

decade,

the trend towards increased

flexibility

in electricity generation, and the

increase ofthe world's population have led to anincreased interest in the development of more

powerful and widely distributed power generation capabilities. It is expected that the

(16)

the overall efficiency due to the possibility ofthe co-generation of electricity and heat. The

distributionofheat is easier and more efficientin smaller systems, where production ofheat and

itsusage areincloser proximity.

Thedevelopment offuelcell

technology

hasbeen influenced

by

the

increasing

concern about the

environmental impactof

burning

offossil fuelstoproduce electricityandto propel vehicles.

Hydrogen fuel cells may

help

to reduce our dependence on fossil fuels and diminish

C02

emissions into the atmosphere. Hydrogen fuel cells are more efficient than the internal

combustionengines.

Using

pure

hydrogen,

fuelcellsonlyproducewater, thuseliminating

locally

all carbonaceous emissionsemittedin theproductionofelectricity from fossil fuels. Theshare of

renewable energy fromwind, waterand sun willincrease further but thesesources are not suited

to deliver the high power densities or to cover the bulk ofthe power electrical demand due to

their irregular availability. As population

increases,

non-renewable energy sources, such as

petroleum,willdiminishandtheharmful greenhouse gases that

they

emitmay depletethehealth

of humankind.

Therefore,

renewable energy sources based on hydrogen are of extreme
(17)

2.2

General

principles and

functionality

offuel cells

A fuel cell is a device that

directly

converts the chemical energy of reactants (a fuel and an

oxidant) into low voltage d.c electricity. Like the familiar flashlight (zinc-manganese

dioxide)

primary cell and the lead-acid (lead-lead

dioxide)

rechargeable

(secondary)

battery,

a fuel cell

effects this conversion via electrochemical reactions. Ifviewed

merely as an energy conversion

device,

a fuel cell performs the same function as a galvanic cell

(primary battery)

or a

discharging

storage

battery (secondary

battery).

However,

the design and

engineering of a fuel

cell are quite different from those ofbatteries. A primary galvanic cell, for example, must be

discarded when its fuel supply (e.g. the zinc casing) is exhausted, or when the products of

reaction (zinc oxychlorides or oxides) are produced in such a quantity that it cannot operate

further.

Similarly,

a secondary storage

battery

must be regenerated

by

periodic recharging

by

voltagerehearsal, normally over several

hours,

sothat the reaction products are reconverted into

reactants. In the case ofa fuel cell,

however,

the reactants are stored outside the reaction areas

(the electrodes), which

ideally

are invariant in composition. The reactants are fed to the

electrodesonlywhenpowergenerationisrequired.

In spite ofthe complexities involved in the construction and operation of a practical fuel cell

system, theprincipleof operation of a fuelcellis readily grasped.As showninthe Figure2.2,33 a

fuel cell consists ofa oxidant electrode

(anode)

and an reducing electrode

(cathode),

separated

by

an

ion-conducting

electrolyte. The electrodes are connected electrically through a load (such

as an electric motor)

by

a metallic external circuit. In the metallic part of the circuit, electric

currentis transported

by

theflow ofelectrons, whereas inthe electrolyte it is transported

by

the
(18)

alkaline electrolytes. In

high-temperature

fuel

cells, the corresponding ionic carriers

may be the

carbonate ion

(C032)

in molten carbon electrolytes, and the oxide ion

(O2)

in the case ofthe

solid oxide system.

Anode

Electrode

Ho

p/WS

Cathode

Electrode

Platinum

(3-5nm)

Pt supported

on carbon with

polymer matrix

Carbon Black

(0.72|ain)

Electrochemistry

Anode:

2H2

MH++4e

Anode Reaction

Cathode: 02+4H++4e

2H20

CH3OH

+

H20-^CO2

+ 6H+ +

6e-Cell

reaction 2H,+0,~2 v2 ?2H,0

Energy

Source

H2

from

hydrocarbon

fuel

02

from

air

*

Product: H,0

Cathode Reaction

3/2

02

+ 6H*

4e-=*3H20

Overall Reaction

CH3OH

+

H20

+

3/2 02~^

C02

+

3H20

Pt Catalyst

2Pt-H

2Pt-H

-*2Pt+2H++e

H2+2Pt~^Pt-H

(19)

Figure 2.2 displays a simplified schematic diagram ofa

hydrogen-oxygen

fuel cell employing

an acid electrolyte. At the anode,

incoming

hydrogen gas is oxidized with an electrocatalytic

material thatis usually based ontheprecious metal platinum.

Thus,

the oxidation of

H2

produces

protons and electrons. In view ofthe fact that the electrolyte is a non-electronic conductor, the

electrons flow away from the anode via the metallic external circuit. Whereas at the cathode,

oxygen gas is reduced and reacts with migrating hydrogen ions from the electrolyte and

incoming

electrons from the external circuitto producewater. There mightbe a situation where,

depending

on the operatingtemperature ofthe cell, theproduct water may enter the electrolyte,

thereby

diluting

it and

increasing

its volume, or be lostthoughthe cathode as vapor.

Depending

on the circumstances, careful water management ofthe electrolyte mayormay notbe necessary

to remove product water. In all fuel cells with liquid electrolytes operating below the

boiling

point ofwater, an electrolyte circulation system

incorporating

an external evaporator may be

necessary. Foracid fuelcells employing a solidpolymerelectrolyte operating inthis temperature

range, the liquidwater productmay be rejected

directly

from the semi-solid gel electrolyte into

thecathode gas chamber.

The overall reaction that takes place in the fuel cell is the sum of the anode and cathode

reactions; in thepresent case, the combination ofhydrogen with oxygen to produce water. This

overall reaction maybe viewed as the coldcombustion ofhydrogenwith oxygen, inthat it takes

placeat a muchlowertemperature than theconventional combustion ofthetwo gases. Insteadof

thewhole ofthe energyofthe reaction

being

releasedas

heat,

as would be the case if hydrogen

burned with oxygen, part ofthe free energy of reaction is released

directly

as electrical energy.

The difference between this available free energy and the heat of reaction is producedas heat at

(20)

from

gaseous hydrogenandoxygenat 1 atmpressure at25C is 1.229V.35

In

theory

the fuelcell

shown in Figure 2.2 should betherefore capable ofgenerating d.c electrical energy at 1.3 V. In

practice,

however,

on account ofelectrode polarization and other

irreversibilities,

under net flow

ofcurrentthe terminalvoltagewillbe lowerthan this idealvalue.

In any event, itcanbe seenthatas

long

as hydrogenand oxygen arefedto the fuelcell, theflow

ofelectric currentwillbesustained

by

electronicflowintheexternal circuitandionic flow inthe

electrolyte.

By

electrically connecting amultiplicity of unit cells either in series or inparallel, it

is possible to form afuel cell

battery

ofany desiredvoltageand current output.

By

the use of an

efficient inverter itwill alsobepossibleto convertthisd.c electricityto a.c

electricity.35

2.3 Typesof Fuel

Cells

Fuel cells are usually classified

by

the electrolyte employed in the cell. An exception to this

classificationis theDMFC (Direct Methanol Fuel

Cell)

which is afuel cell inwhichmethanolis

directly

fed to the anode. The electrolyte ofthis cell is not

determining

forthe class. A second

grouping can be done

by

looking

at the operating temperature for each ofthe fuel cells. There

are, thus, low-temperature and high temperature fuel cells. Low-temperature fuel cells are the

Alkaline FuelCell

(AFC),

thePolymerElectrolyte Fuel Cell

(PEMFC),

theDirect Methanol Fuel

Cell and the Phosphoric Acid Fuel Cell (PAFC). The high-temperature fuel cells operate at

temperatures in the range of 600-1000C and two different types have been

developed,

the

Molten Carbonate Fuel Cell

(MCFC)

and the Solid Oxide Fuel Cell (SOFC). All types are

presented in this section in order of

increasing

operating temperature. An overview ofthe fuel
(21)

Table 1 The different Fuel Cellsthathavebeenrealized and arecurrentlyinuse anddevelopment37 ,38,39,40,41 AFC (Alkaline) PEMFC (PolymerElectrolyte Membrane) DMK (Direct Methanol) PAFC (Phosphoric Acid) mctc (Molten Carbonate) sore (Solid Oxide)

Operatingkmp.CO <100 60-120 60-120 160-220 600-80D 800-1000 lowtemperature

(500-600)possible

Anodereaction Hj1 20H"

-.

2H20+2e

H2 +2e CHjOH)HjO-. H2 .2rf(2e HjlCQj2 . HjOtCO,12e

H,t02

H20t2e

Cathodereaction 40i+H20t2e

-20H

hO,+2H'

t2e 3/2Q2+6rfi6e

3HjO ^02t2H* )2e HjO hOj+COj^e CO,2 W02t2e Applications Transportation Space Military Energystorage systems

Combinedheatand

powerfor decentralisedsta

tionarypower systems

Combined heatand powertor stationary

decentralisedsystem*andtor transportation (trains,boats,..)

Realised Power Smallplants

5-150kW modular Smallplants 5-250 kW modular Smallplant) 5kW

Small- medium

sized planb

50kW-llMW

Smallpower plants

10O.kW-2MW

Smallpower plants

100-250kW

Charge Carrier inthe

Electrolyte

Off H* H* H* CO,1 O2

2.3.1 Alkalinefuel Cell

(AFC)

Electron Alkali Fuel Cell

vw

Oxygen Anode Eteclro^ Ca,h0<le

Figure2.3 AlkaliFuel Cell(AFC)41

Operationof alkalifuel cells is basedon compressedhydrogenandoxygen and uses a solution of

potassiumhydroxide in waterasthe electrolyte. Insidealkali cells theoperating temperatures are

(22)

were an optimal power source for spacecraft. NASA selected alkali fuel cells for the Space

Shuttle

fleet,

as well as the Apollo program ofthe

1960's,

mainly because ofpower generating

efficiencies that approach 70 percent. While it is an advantage to use alkali fuel cells in

spacecraft, theiruse forvehiculartransport on earthis limited

by

the presence of

C02

in air. Air

drawn into the cathode would introduce

C02

into the system

thereby

forming

solid alkali

carbonates. Carbonates can be destructive to the electrolyte and

thereby

the cell performance

canrapidlydecrease.42

2.3.2 Proton Exchange Membranes Fuel Cells

(PEMFC)

porous porous Cathode Electrolyte Anode

Figure2.4 Schematicdrawingofhydrogen/oxygenfuelcell anditsreactionsbasedontheproton exchange

membranefuelcell

(PEMFC)41

Withtheexception oftheadvanced aerospace alkaline fuelcell,

having

comparableperformance,

the Proton Exchange Membrane Fuel Cell43 offers an order of magnitude higher power

density

than any other fuel cell system. For the preparation ofanode and cathode, thin sheet ofporous

graphitized paper, which has been previously wet-proofed with

Teflon,

is applied with a small
(23)

oz/kW. Platinum requirements can be expected to practically to be reduced to 0.035 oz/kW or

about$2/kWwiththeimprovements inproton exchange membrane performance.

Typically

the proton exchange membrane operates at 70-85C. Its low operating temperature

provides instant start-up and requires no thermal shielding to protect personnel. At room

temperature, about 50% ofmaximum power is

immediately

available Under normal conditions

full operatingpower is available within about 3 minutes. The possibility oflowercost than any

otherfuelcell systemis offered

by

recent advances inperformance anddesign.

Ballardhas achieved astack-onlypower

density

ofover 5.4kW/ft3 intheir5kWproductionfuel

cell stacks. Targets to approach power densities 14.2 kW/ft.3 are certainly feasible. An

upcoming system operating on hydrogen and air at 45 psia, inclusive of fuel and oxidant

controls, cooling, and product water removal will provide 1.25 kW/ft3 and40 W/lb. Performance

is improved

by

pressurizing theair which is truewith all fuel cells. In everyapplication, there is

boundtobe atrade-offbetweenthe energyand financialcost associated with compressing airto

higher pressures and the enhanced performance

thereby

obtained. For a large number of
(24)

2.3.3 Direct Methanol Fuel Cell

(DMFC)

F.tfn-nttldroit!

V

Anode CHjOH*H,0

i CO,+ST+6H

Platinisedcarbon-'

eleclrodes ~ .

Polymerelectrolyte membrane(PEM)

Figure 2.5 Schematicdrawingoftheoperatingprinciples offuelcellutilizingmethanol asthefuel,i.e.a directmethanolfuelcell

(DMFC)41

In direct methanol fuel cells, methanol, is the

fuel,

and complicated catalytic reforming is not

needed, as methanol is

directly

fed intothe fuel cell. Asmethanol isa

liquid,

it does not needto

be stored at high pressures.

Hence,

storage of methanol is not difficult as compared to that of

hydrogen.

However,

due to high permeation of methanol through the most efficient

PEMs,

efficiency is low. Though DMFCcan have the limitation ofproducing less power,

they

can still

store a great deal ofenergy in a small space. This means

they

have the ability to produce small

amount of power over a longer duration.

Hence,

they

are well suited to power applications in
(25)

2.3.4Phosphoric Acid Fuel Cell

(PAFC)

PhosphoricAcidand PEMFuelCells

Electron-*

fiwv

|

AAA,

t

I

Load

Hydrogen\%

: i*

Figure 2.6 Phosphoric Acid Fuel Cell(PAFC)41

Interms of system development and commercializationactivities,the PhosphoricAcidFuel Cell

isthemost establishedfuel celltechnology. Ithas beenunderdevelopmentfor 20 years or more.

Liquid phosphoric acid is used as the electrolyte in the phosphoric acid fuel cell. A Teflon

bonded silicone carbide matrix contains the phosphoric acid. The acid is kept in place through

the capillaryaction provided

by

the small pore structureofthematrix. Addition of acid may be

required after many hours of operation as some acid may be entrained in the fuel or oxidant

streams. Both the oxidizing

(anode)

and reducing

(cathode)

sides ofthe electrolyte use platinum

catalyzed, porous carbon electrodes. Electrical efficiencies are seen in the range from 36% to

42% [HHV2 (Higher

Heating Value)

Phosphoric acid fuel cell power plant designs show].

Pressurized reactants are used to operate the higher efficiency designs. More components are

required

by

the higher efficiency pressurized design and this

invariably

means at higher cost.

Thoughthe majorityofthethermal energyis supplied at ~150C, a portion ofthe thermal energy

can be supplied at temperatures of~ 250C to ~ 300C. The phosphoric acid fuel cell has a

(26)

Phosphoric

acid fuelcells havetodeal withone

issue,

that

is,

ifthe sourceofits hydrogen fuel is

reformed gasoline, sulfurmustbe removed from the fuel enteringthe cell or it will damage the

electrode catalyst.

2.3.5 MoltenCarbonateFuel Cell

(MCFC)

MollenCarbonate

Fuel Cell

<0*>%'<k

Figure 2.7 Molten Carbonate Fuel Cell(MCFC)41

Electrolyte used in a Molten Carbonate Fuel Cell is a molten carbonate salt mixture, usually

lithium carbonate and potassium carbonate. A ceramic matrixhas the electrolyte suspendedin it.

Here,

nickel-chromium alloy isused asananode, and lithium-dopednickel oxide as a

cathode.42

Operating

temperatures ofthecell rangefrom 600-800C.

Hydrogen canbeextracted

by

froma varietyof

fuels,

employingeitheraninternal oran external

reformer. High-temperature fuel cells are also less prone to carbon monoxide "poisoning" than

lower temperature fuel cells making coal-based fuels more attractive for this type of fuel cell.

Catalysts made of nickelwork fine with molten carbonate fuel cells and is much less expensive

thanplatinum.

Up

to 60 percent efficiency is exhibited

by

molten carbonate fuel cells, and this

canriseto 80percent ifthewaste heat isutilizedforcogeneration.

Presently,

demonstrationunits

haveproduced upto 2 megawatts

(MW),

but designs exist forunits of50 to 100 MW capacity.
(27)

to solid oxide cells. One isthe complexityofworkingwith aliquid electrolyte ratherthan a solid

andthe other stems fromthe chemical reaction inside amolten carbonate cell. As carbonate ions

from the electrolyte get used up in the reactions at the anode, it becomes necessary to

compensate

themby

injecting

carbondioxideatthecathode.42

Also,

theelectrolyte usedinmolten carbonatefuel cellsis

highly

corrosive, restrainingsome ofit

potential applications.

2.3. 6 Solid Oxide Fuel Cell

(SOFC)

SoldOxide Fuel Cell

Electron ^

"<<i

wv

Load

Hydrogen |0

<#*>Wo T~~r9

Water

Cathode Beckotyle

Figure 2.8 Solid Oxide Fuel Cell(SOFC)41

To reduce the corrosion considerations and to eliminate the electrolyte management problems

associated with the liquid electrolyte fuel cells, the Solid Oxide Fuel Cell uses aceramic,

solid-phase electrolyte. Solidceramic isused as an electrolyte inthesolidoxidefuelcell. Dense

yttria-stabilized zirconia is the preferred electrolyte material. The solid oxide fuel cell is a solid state

deviceandshares certain properties andfabricationtechniques with semi-conductordevices. The

anode is a porous nickel/zirconia cermet while the cathode is magnesium-doped lanthanum

manganate. The solid oxide fuel cell has demonstrated 0.6V/cell at about 232 A/ft2 in

development cells and small stacks. Lifetimes in excess of30,000 hours for single cells have

beendemonstrated as have a numberofheat/cool cycles. Fuel is deliveredto electric efficiencies

in the range of45% (HHV- Higher

(28)

solid oxide fuel cells. Argonne National

Laboratories

propose that pressurized

systems could

yield fuel efficiencies of60% (HHV). The solid oxide fuel cell offers the possibility ofinternal

reforming because ofits high operating temperature. As in the molten carbonate fuel

cell, CO

does not act as a poison and canbeused

directly

as a fuel.

Especially

for sulfur, the solid oxide

fuel cell is also the most tolerant ofany fuel cell type. Several orders of magnitude more sulfur

can be tolerated

by

it as compared to the other fuel cells. Significant start-up time is required

witha 1830F

(1000C)

operatingtemperature inthe solidoxidefuelcell.

Depending

on the size ofthepower plant it is estimated that the fuel to electricity efficiency of

solid oxide fuel cells range from 50-70%. In addition, these efficiencies hold from about

15%-100% power, making the cells ideal for applications in which a wide range ofloads is found.

They

can readily operate on hydrocarbon fuels such as coal gas, gasoline, diesel

fuel,

jet

fuel,

alcohol, and natural gas because most solid oxide fuel cells utilize both hydrogen and carbon

monoxide fuel insidethe cell.45

For two major reasons the efficiency ofthe solid oxide fuel cell used in combined heat and

power applicationswillbe higherthan thepolymerelectrolyte fuelcells. "Thefirstreasonisthat

thehydrocarbon fuel isreformedinto hydrogenandcarbon monoxidefuel

largely

inside thecell.

This results in some ofthe high temperature waste heat

being

recycled back into the fuel. The

second reason is that air compression is not

required."

This results in a higher amount of net

electricity

being

produced and quieter operation especially on smaller systems. The solid oxide

fuelcell maynotbepractical forsizes muchbelow 1,000 wattsor when portable applications are

(29)

2.4

Protonically

Conductive Ion

Exchange

Membranes

Membranes withhigh protonic conductivity are potentially useful as separators and electrolytes

inelectrochemical cells such as fuel cells.

Among

the firstproton-conductingmembranes usedin

fuel cells were sulfonated, crosslinked

polystyrenes.47

Moreover perfluorosulfonic acid polymer

films have been extensively studied as proton-conducting

membranes48

for use in fuel cells.

Nafion

is, however,

averyexpensivematerial, andother materials are

being

sought.

2.4.1 Structure ofthePolymerElectrolyte Membrane

- CFZ- CF -

CF2-I O

I

CF2

I CF

-CF3

I O

I

CF2

I

CF2

I

S03 FT

Figure2.9 Chemicalstructureof membranematerial;NafionbyDuPont

The polymer electrolyte membrane is a solid, organic polymer, usually poly [perfluorosulfonic

acid]."Atypicalmembranematerial, suchas

Nafion,

consists ofthreeregions:

(1)

The

Teflon-like,

fluorocarbon

backbone,

hundredsofrepeating- CF2-CF- CF2-units in

length,

(2)

The sidechains, -O-CF2-CF- O-CF2-CF2-,which connectthemolecularbackboneto

the third region,

(3)The ionclusters consistingofsulfonic acid

ions,

SO3"

(30)

The negative

ions, S03",

arepermanently attachedto the side chain and cannotmove.

However,

when the membrane becomes hydrated

by

absorbing water, the hydrogen ions become mobile.

Ion movement occurs

by

protons, bondedto watermolecules,

hopping

through

S03

site within

the membrane. Because of this mechanism, the solid hydrated electrolyte is an excellent

conductorofhydrogenions."

2.4.2 Polymer Electrolyte Membrane

In the presence ofwater, an ordinary electrolyte is a substance that dissociates into positively

charged

ions,

thereby

making thewater solutionelectrically conducting. Theelectrolyte whichis

usually referred to as a membrane in a polymer electrolyte membrane fuel cell is a type of

plastic, orapolymer. The appearanceofthe electrolytemay vary

depending

uponthemaker, but the most established membrane, Nafion produced

by

DuPont,

bears a resemblance to the

plastic wrap, varying in thickness from 50 to 175 microns. To put this in perspective, consider

that a piece of normal writing paper has a thickness of about 25 microns. Thus polymer

electrolyte membranes have thicknesses comparable to that of 2 to 7 pieces of paper. The

membrane in an operating fuel cell is well humidified so that the electrolyte looks like a moist

pieceofthickplastic wrap.

Themembranereadily absorbswater andthe negative ions arecovalently bondedto thepolymer within the structure,

thereby

making the polymer electrolyte membranes a somewhat unusual

electrolyte. Positive ionsarefreeto carrypositive charge through themembrane and arethe only

mobile ions contained within the membrane. The term proton exchange membranes in polymer

electrolyte membrane fuel cells, hence originates from these positive ions which are hydrogen

ions,

or protons. In the fuel cell operation, it is essential to have the movementofthe hydrogen
(31)

this movement ofionic charge within the fuel cell, the circuit defined

by

cell, wires, and load

remains open,and no current wouldflow.

Polymerelectrolytemembranes are relatively strong, stable substances because theirstructure is

based on a

Teflon

backbone. A polymer electrolyte membrane, although thin, is an effective

gas separator. Itcan

keep

thehydrogen fuel separatefromtheoxidantair, afeature veryessential

to the operationof a fuel cell.

Although,

as ionic conductors, polymer electrolyte membranes do

not conduct electrons. Anotherfeatureessential to fuelcell operation isthat the organicnature of

the polymer electrolyte membrane structure makes them electronic insulators. Asthe membrane

does not permit the passage ofthe electrons, the electrons produced at one side ofthe cellmust

travel, throughanexternalwire, to the other side ofthe cell tocompletethe circuit. Theelectrons

provide electrical power to run acar or a powerplant as it is in theirroute through the circuitry

externalto thefuel cell.45

In proton conducting polymers, the phase segregation occurs

during

solvent

casting.50 The

formationoftwo phases: an ion-richphase (i.e. ionclusters,) and an ion-poorphaseis aresult of

the aggregation ofions occurring due to the electrostatic interaction between ion pairs. Figure

2.10 shows the concept ofmicrophase separation inaPEM. The polymer separates into regions

ofion clusters andnonionic clusters. Itis understood thatprotonsmigrate fromone ionic cluster

to the other through water channels that connect ionic clusters. Gierke et

al.51

determined the

ionicnanostructure ofNafion

by X-ray

analysis andsuggestedion clusters approximately 5nm
(32)

Ionic Cluster

-10nm

Semicrystalhne Hydrophobic Region

Figure 2.10 Schematicof microphase separationinahydrated ionomer

There are two different transport mechanisms forprotons in PEMs. The first is the Grotthuss

mechanism, where the proton hops from one ion rich site to another.52

The second is

electroosmotic

drag,

where protons diffuse through the membrane, attached to water as

hydronium

ions,

H3O . Proton transport is also dependent on polymer morphology or ionic

nanostructure.54

A few examples ofPEMs include

Nafion,

sulfonated polystyrene, sulfonated

poly [bis

(3-methylphenoxy)]

phosphazene and sulfonatedpoly (ether ketone).

Wewillbe reporting thepreparation and characterization ofproton-conducting membranes with

poly (vinylidene

fluoride), PVDF,

films as matrices. The preparation involves homogeneous

mixingofPVDF and sulfonated carbon black to form composite materials to be fabricated into

films. The concentration of proton conducting species are varied

by forming

composites of

different loadings ofsulfonatedcarbonblack.

Inthepresentinvestigationthe melting behaviourandthe crystallinityofthematrix material and

(33)

evaluate the effect ofconcentration ofsulfonated carbon black on the crystallinity and on the

microstructure ofthe membrane. The matrix polymer is a non-porous film ofPVDF however

composite membranes were expectedto beprotonconductingwhenhydrated.

2.5

Various Polyperfluorinated

Copolymers

The current state-of-the-art proton exchange membrane is typified

by

Nafion,

a DuPont

product that was developed in the late 1960s primarily as a permselective separator in

chlor-alkali

electrolyzers.55'56

The poly (perfluorosulfonic acid) structure of Nafion imparts

exceptional oxidative and chemical stability, which is also important in fuel cell applications.

Nafion consists ofa fluorocarbonpolymer

backbone,

similarto

Teflon,

to which sulfonic acid

groupshavebeen chemically bonded. The acidmolecules are fixedto thepolymerand cannotbe

leached out, but the protons on these acid groups are free to migrate through the electrolyte.

Lifetimetests inthe fuel cell environment ofthe Nafion copolymer has indicated a lifetime of

over 50,000 hours at 80 C at 100

%RH,

which has led researchers and the world to have

increased confidence in the PEM fuel cell for alternative energy

sources.57

A large amount of

literature and research in the fuel cell research population has been devoted to

Nafion,

including

published papers, reviews and

books.58'59'60

Furthermore,

composites ofNafion with

inert PTFE matrices like Gore membranes orinorganic additives like heteropolyacidshave been

shown to improve the physical and electrochemical characteristics.

Reinforcing

the Nafion

membranes with Teflon or Gore-Tex fabric has produced thinner films that have increased

(34)

-f'^-^iff-^-t

OCF-. CF 0(ChiV.SO,II

"

I

CF,

Figure 2.11 Chemicalstructure ofNafion.xandyrepresent molar compositions anddonotimplya

sequencelength.

In

theory,

ion content can be varied

by

changing the ratio ofthe two components (x and y in

Figure 2.1 1). Nafionhas been commerciallyavailable in

900,

1

100,

1200,

and other equivalent

weights. The equivalentweight ismeasured

by defining

the grams of polymer per mole offixed

S03

groups.

However,

Nafion 1 100 EW inthicknessesof

2, 5, 7,

and 10 mil (1 mil equals 25.4

|um) (Nafion

112, 115, 117,

and

1110)

seems tobe the onlygrades ofNafionthat arecurrently

widely available. This equivalent weight provides high protonic conductivity and moderate

swelling in water, which seems to suit most current applications and research efforts. Modest

retention of a semicrystalline morphology at this composition is no doubt important for

mechanical strength. The thinner membranes are generally applied to hydrogen air applications

to minimize ohmic

losses,

while thickermembranes are employed for directmethanol fuel cells

(DMFCs)

toreduce methanol crossover. 26

CFo

CF;

-CF

CF-CF2

CF2-S03H

(35)

Literature observed in the patent material for both Dow XUS (Figure

2.12)

and Nafion

suggests that perfluorinated copolymers are synthesized in the sulfonyl fluoride form and then

converted into the acid form after processing into membranes.62

The synthesis ofthe copolymer

canbe generalizedinto fourdifferent steps.

1. Reaction of tetrafluoroethylene with

S03

(fuming

sulfuric acid) to form the cyclic

sulfone.

2. Condensation of the products with sodium carbonate followed

by

free radical

copolymerization with

tetrafluoroethylene,

which forms an insoluble semicrystalline but

meltextrudable resin.

3. Alkaline hydrolysis oftheextrudedfilmtoproducea perfluorosulfonic copolymer

4. Counterionexchange of sodiumtoformtheprotonformofthe salt copolymer.

Other perfluorosulfonate cation exchange membranes with similar structures have also been

developed

by

the Asahi Chemical

Company

(Aciplex)

and the Asahi Glass

Company

(Flemion).63

The Dow Chemical

Company

also developed a material with a shorter side chain

than those ofNafion andthe otherperfluorosulfonates, which isno longer available. The length

oftheperfluorosulfonic acid side chain andthevalues forthe equivalentweightmay bevariedto

someextent.

There are three specific drawbacks to Nafion. The first is the cost to the consumer, which is

about $700-$800per square meter, the second is its performance at temperatures above 100 C

where it dehumidifies and the third is the methanol permeability.

By

increasing

the fuel cell

temperatures, reaction kinetics and carbon monoxide poisoning can be improved. The high cost

(36)

process. The specialty co-monomers that are needed in such a copolymer are, in general, the

main reasonforthehighcost.58

All of these polyperfluorosulfonic acid membranes are expensive and suffer from the same

shortcomings as

Nafion,

namely low conductivity at low water content, relatively low

mechanical strength athigher

temperature,

and moderate glasstransitiontemperatures.

2.6 Polyvinylidene Fluoride

(PVDF)

and its copolymers as proton exchange

materials

In the

PEMFC,

ion exchange membranes play a vital

role64'65

and the development of cheaper

and better proton conducting polymer electrolyte membranes than those available

today66'67

is

urgently needed, because the presently used perfluorosulfonic acid membranes are excessively

costly due to the fluorine chemistry involved. A good proton conducting membrane should

combine chemical, mechanical and thermal stability along with favorable electrochemical

properties such ashigh ionic conductivityandgood oxygen reductionkinetics.

Several newtypes ofmembranes have been developed inrecent years

by

polymerizationof new

types of polymers or

by doping

ofexistingpolymers

by

suitable acid constituents. ' ' '

' ' In

1997,

it was reported that PVDF grafted polystyrene and subsequently sulfonated might be an

alternative to perfluorosulfonic acid polymers for the fabrication of proton-conducting

(37)

PVDF,

is anon-reactive and semicrystallinethermoplastic

fluoropolymer.

H

H

Figure 2.13 StructureofPVF2

It is very expensive relativeto commoditypolymers;however it is an order ofmagnitude lower

in cost than Nafion. The use of PVDF is generally reserved for applications requiring the

highestpurity, strength, and resistance to solvents, acids,bases andheat. It is available as piping

products, sheet and plate. It can be injection molded and welded and is commonly used in the

chemical, semiconductor and medical industries. PVDF is a ferroelectric polymer, exhibiting

efficient piezoelectric and pyroelectric properties. These characteristics make it useful in sensor

and

battery

applications.

Owing

to its important pyro and piezoelectric properties, PVDF has

been widely studied since 1970. It is known to exist in four crystalline forms a

(monoclinic),

P

(orthorhombic,

piezoelectricphase), y

(monoclinic)

and5 or ap. The ap is a apolarform ofthe a
(38)

Table2 PhysicalPropertiesofPVDF

Property

Metric English

Melting

point 134-169 AC 273-336 AF

Density

1.78 g/cm3

1 1 1 lb/ftJ

Thermal

Conductivity

0.18 Wm"1 K"1

Coefficientof

Expansion

0.18A10"bK'

0.10A10"6AoF"'

YieldStrength 15-35 MPa 2.2-5.0 kpsi

ElongationatRupture 200%-750%

Modulus of

Elasticity

350-1 100 MPa 50-160 kpsi

Volume

Resistivity

>lx!0l2CA-m

KYNAR

301 poly

(vinylidene)

fluoride is the homopolymerof

1,

1-di-fluoro-ethene, and is a

tough, semicrystalline engineering thermoplastic that offers a unique balance of properties.

When exposed toharsh thermal, chemical, and

ionizing

environments, it has the

distinguishing

stability of

fluoropolymers,

while the alternating

CH2

and

CF2

groups along the polymer chain

provide a unique polarity thatinfluences its solubilityand electric properties. Standardmethods

of extrusion and injection/compression molding readily melt-process KYNAR resins.

KYNAR

PVDF canbe dissolved inpolar solvents such as organic esters and amines at elevated

temperatures. This selective solubility is a benefit in the preparation of corrosion resistant

(39)

Characteristics

ofKYNARPVDFinclude:

Mechanical

strength, toughness, high abrasion resistance, high thermal stability, high dielectric

strength, high purity, and melt processibility. It is resistant to most chemicals and solvents,

resistanttoultraviolet and nuclearradiation, resistant toweathering, and alsoresistantto fungi. It

has low permeability to most gases and

liquids,

low flame and smoke characteristics. Available

versions of

KYNAR

PVDF arerigid andflexible.

Chemical Process

Industry

Applications:

KYNAR PVDF components are

broadly

used in the high purity semiconductor market (low

extractible values),pulp and paper

industry (chemically

resistantto halogens andacids), nuclear

waste processing (radiation and hot acid applications), and the general chemical processing

industry

(chemical and temperature applications). In addition,

KYNAR

fluoropolymers have

also met specifications forthe food and pharmaceutical processing industries.

KYNAR

PVDF

is fabricated into broad range of components including:

Pipes,

fitting

and valves pump

assemblies, sheet and stockshapes, films

tubing

architectural.

Lithium Ion Batteries:

Inthe

battery

industry,

KYNAR PVDF homopolymersand copolymers have gainedsuccess as

binders forcathodes and anodes in lithium-ion technology, and as

battery

separators in

lithium-ionpolymertechnology.

"Arkema istheworld's

leading

producer ofPVDF". Theirparticularbrand ofhighpurity,

battery

grade PVDF resins are

KYNAR

and Kynar FlexA. Arkema has been working closely with

(40)

design thinner,

smaller lithium-ion

batteries

and has been with lithium

technology

from the

beginning.

2.7

Sulfonation

of

Polymers

For many years sulfonated polymers have been

investigated

intensively

because oftheir current

and potential applications in many areas.73

One ofthe most importantapplications is theiruseas

an ion selective separator in electrolysis or electrodialysis

systems.68

Many

studies were also

concerned with water-selective pervaporation membranes

containing sulfonated polymers for

separation of aqueous organic

mixtures.74

The reverse osmosis as well as gas permeation

properties of sulfonated polymers such as sulfonatedpoly (phenylene oxide)75'76'77 or sulfonated

polysulfone were also studied. Some work was concerned with the preparation of rigid-rod

molecular composites via ionic interactions between a polyelectrolyte host and a reinforcing

rigid-rodpolymer. ' The compatibilizing effect of suchpolymers was also mentionedto lead

to a real miscibility improvedofinsoluble polymers. 81'82 These sulfonatedpolymers have been

also used as dopant forwater-soluble

polyaniline.83

One more recent and promising application

is the ion-conductive membranes for batteries or fuel cells. ' ' For

instance,

perfluorosulfonated ionomer

(Nafion)

membranes have beenused for thispurpose dueto their

efficientproton conduction

(10"'

S cm"1)inthe

fully

hydratedprotonic

form)

and

long

lifetime.88

However,

the high cost of this ionomer is a major drawback for the development of this

technology. The most common wayto modifyaromatic polymers forapplication as aPEM isto

employ electrophilic aromatic sulfonation. Aromatic polymers are easily sulfonated using

concentrated sulfuric acid,

fuming

sulfuric acid, chlorosulfonic acid, or sulfur trioxide or

complexes thereof. Postmodification reactions are usuallyrestricteddue to their lack ofprecise

(41)

degradation

ofthepolymerbackbone.

Regardless,

this areaofPEM synthesis hasreceived much

attention and may be the source of emerging products such as sulfonated Victrex

PEEK,

poly(etherether

ketone).89'90

Sulfonated polymers are usually prepared either

by

sulfonation of a polymer in solution or

by

post-modification of a polymer film. Sulfonated poly (arylene ether sulfone)s, synthesized

by

attaching sulfonic acid groups in polymer modification reactions have been investigated

intensively

sincethepioneering work of

Noshay

and

Robeson,

whodeveloped a mild sulfonation

procedure for the commercially available bisphenol A-based poly(ether

sulfone).91 Different

sulfonating agents have been employed for this polymer modification, such as chlorosulfonic

acid ' and a sulfurtrioxide-triethylphosphate complex.

Sulfonation is an electrophilic substitution reaction; therefore, its application depends on the

substituents present on the aromatic ring.

Electron-donating

substituents will favor reaction,

whereas electron-withdrawing substituents will not.

Additionally,

the sulfonic acid group is

usuallyrestrictedto the activated position on the aromaticring. For the case ofthe bisphenol

A-basedsystems,nomorethanone sulfonic acidgroupper repeat unitcouldbe

achieved.93

For achieving a number of objects sulfonation of carbon black can also be desirable. Ion

exchange materials canbeprepared from sulfonated carbonblackwhereinadvantage is obtained

by

the inert chemical and thermal properties ofthe carbon black

being

combined with the high

chemical reactivity and versatility of sulfonic acid groups on the surface there over.

Moreover,

such sulfonated carbon blacks are especially useful, because of the hydrophilic character

imparted

by

sulfonic acidgroupsto carbonblacks usedas fillers andpigments inpaperand other

cellulosic products. Thesesulfonated carbon blacks are also especially useful as fillers in resins

(42)

2.8 Other

Commercially

Available

Proton Exchange

Membranes

Nearly

all of the commercially available membranes are based on perfluorosulfonic acid

polymer, such as Nafion. Nafion also has the largest

body

ofliterature devoted to its study

because ofits demonstrated industrial

importance

and availability. Nafion composite systems

also have already become significant in both industrial and academic research. Commercial

alternativesto the state-of-the-artNafionmembranes are

few,

butsome doexist.

Ballard Advanced

Materials,

Dias Analytical and W. L. Gore & Associates have all produced

commercial PEMs. W. L. Gore and Associates produce both membranes and MEAs for

commercial

resale.95

The GORE-SELECT membranes are reportedly based on Nafion

membranesbut the membranes are supportedwith porous GORE-TEX material making them

thinner and mechanically

stronger.96

Since GORE-TEX and Nafion are both fluorinated

polymers, the compatibility of the two materials is probably relatively

good.97

However,

Nafion swells in aqueous environments and GORE-TEX does not. This could cause

delamination ofthe Nafion from the GORE-TEX and increase the resistance ofthe ions of

the composite material. Because the Nafion material is used in only minimal quantities, the

cost and thickness of the membranes decreases.

However,

thinner membranes have higher

permeabilityandincreased fuelcrossover.

Therefore,

asthemembranes reachthicknesses asthin

as 20 micrometers, fuel crossover results in low open circuit voltage and low fuel efficiency.

Furthermore,

GORE-SELECT membranes and PREVIEA MEAs use Nafion as the poly

electrolytemembrane, whichis only morphologicallystable attemperatures lessthan 100 C. At

lowrelative

humidity,

andtemperaturesinexcess of100C thesemembranes are plagued

by

the
(43)

Dias

Analytic

has produced sulfonated membranes from the well known Kraton-G

styrene-b-ethylene/butylene-b-styrene copolymers.98

Thesehydrocarbon backbonepolymers are ofinterest

for low cost, low temperature and low current

density

arrangements. Dais membranes have

proven to have similar conductivities at a given water-sulfonate ratio, but higher water uptake

than that ofNafion for a given ion exchange capacity. The targeted area of use for these

membranes is for low cost applications that are used atroomtemperatureenvironments for short

periods oftimesince oxidativestability is limited.99

Nowadays,

a mass market for PEM fuel cells is emerging, sparking interest in lower-cost

membranes. In thepast year, earlycommercial versions ofPEM cells have begun to showup in

portable andemergency powerunits. For example, to replace small internal-combustion engines

in hand-held appliances, Ballard Power Systems in

Burnaby,

B.C.,

is

field-testing

a 250-kw

stationarysystemfor factoriesanda 1.2-kw

system.100

Anumber of new partnerships reflecttheintensifiedeffortto

develop

new membranes:

To commercialize a lower-cost membrane based on Dow's Index ethylene-styrene

interpolymer

(ESI)

Dais-Analytic,

Odessa,

Fla.,

is

teaming

with Dow

Plastics, Midland,

Mich.,.

Apolybenzimidazole (PBI)-basedmembrane for fuel cells thatoperates atrelatively high

(150

C)

temperatures is

being

developed

by

Celanese Ventures

GmbH, Frankfurt,

Germany.

To generate two new membrane alternatives cell maker Ballard is working with the

British parent of Victrex

USA, Greenville,

S.C. One of them is based on sulfonated

polyaryletherketone(avariant of

PEEK)

resin supplied

by

Victrex."

2.9 Alternative Materials to Nafion

The presentlyavailable state-of-the-art membrane materialsfor PEM fuelcells are perfluorinated

sulfonic acid

(PFSA)

functionalized polymers such as DuPont's Nafion and W. L. Gore's
(44)

gleaned from experimental studies has recently been reviewed

by

several

anthnrc 101,102,103,104,105,106,107 T, _ , .

aurnors. ihese electrolytes are two-phase systems, containing water

dispersed

as a second phase in aprincipally amorphouspolymeric (e.g.

fluorocarbon,

aromatic)

primary phase.

A '"

The water solvates the polymeric acidic groups and promotes proton

mobilityviaboth structural diffusion110'11' (i.e.

Grotthuss-type

"hopping"

oftheprotons through

the

hydrogen-bonded

network ofwater molecules) and vehicular motion (i.e. coupled

proton-water transport ofhydronium ions).112 In both transport mechanisms, the presence of water is

critical in the formation ofhydratedprotons (i.e. as

Zundel,

H 502

+,

or

Eigen,

H 904+, cations)

and mobility of the protons. The hydration requirement of conventional PEMs results in a

problematic operatingtemperature limitedto the

boiling

point ofwater(i.e. T= 100C at 1 atm).

Several different approaches have been used in attempts to improve proton conduction in

PEMs.96'113

These include the use of alternative fluids (e.g. phosphoric acid and

polybenzimidazole,114'115 phosphonic

acid,116

and

imidazole117'118)

to replace the function of

water in the membrane, the addition ofinorganic

particles97

(e.g. silica,

heteropolyacids)

into

polymeric conductors that purportedly allow proton conduction along the inorganic surface or

maintain the water content ofthe membrane

by

adding an additional hydrophilic component,

and, as alluded to above, the preparation of various alternative proton-conducting polymers.

Along

Figure

Figure 4.7DSC overlays of Kynar 7201 Plain Polymer, and its melt pressed film with sulfonated carbonblack by Process* 1.

References

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