Rochester Institute of Technology
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Theses
Thesis/Dissertation Collections
2006
Polyvinylidene fluoride composites as an option for
proton exchange membranes in fuel cells
Rohini P. Sajanpawar
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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
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 :
ACKNOWLEDGEMENTS
I would like to
truly
thank Dr. Thomas W.Smith,
as my research advisor, for his efforts anddirection
throughout my education at RIT. His incredible wealth ofknowledge,
dedication andencouragement 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 AlternativePropulsionCenter,
NY). IthankDr. Miri forhelping
me use hislaboratory
to fabricate several melt processed films. I particularly appreciateDr. Fullerfor
helping
us withconductivitymeasurements aswell asgivingme valuable guidanceand 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 toacknowledge everyone in the
Chemistry
and Materials Science Dept.Specifically,
Mr. TomAllstonand Matt fortheir assistance in using theDSC
facilities,
and all the stockroom personnelforconstanthelp.
I owe a particularword ofthanks to our secretarial staff, Brenda
Mastrangelo,
Ann Gottorff fortheirassistance
during
the entireperiod ofmy MS.My
specialthanks to the entireteam at XeroxCorporation,
NY,
where I did myCo-op;
fortheirexcellentguidance in everythingas well as in my thesis. Ithankallmyresearch group members,
seniors, peers,
juniors,
roommatesand allmy friends for making my stayatRIT somuchfunandenjoyable.
Nothing
canbeaccomplishedwithoutfamily
support. Iwouldliketo extendmy specialgratitudeShrirang, Pallavi, Vaishali,
Aditya andVedika;
fortheirlove,
support and encouragement fromthousands of miles away.
Many
thanks are due to my fiance Rohit for his tremendouslove,
support and guidance. Muchappreciation is also due to him for
being
with me in the laboratories tilllong
hours,
and alsohelping
me assemblethis thesis. Their support, encouragement and patience enable metopursueABSTRACT
This thesis details approaches for the preparation of composite membranes derived from poly
(vinylidene
fluoride), PVDF,
or copolymers thereof. It was expected that these membraneswould 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 protonexchangemembranes. Thin films from all compositions were prepared
by
compressionmoldingor solution casting. All composites and films were
thermally
analyzedby
differential scanningcalorimetry. These studies indicated different crystallization behaviorwith different loadings of
carbon black. Solution cast films containing
PVDF,
CB and Nafion 1000 were also preparedandcharacterized
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
TABLE OF
CONTENTS
CHAPTER 1: Introduction 11
CHAPTER 2: Literature Review 13
2.1 FuelCells16
13
2.1.1
History
132.1.2 Fuel Cell
Technology
Development'"""14
2.2 Generalprinciples and
functionality
offuelcells 162.3 Types ofFuel Cells 19
2.3.1 Alkaline fuel Cell
(AFC)
202.3.2 Proton Exchange Membranes Fuel Cells
(PEMFC)
212.3.3 Direct Methanol Fuel Cell
(DMFC)
232.3.4 Phosphoric Acid Fuel Cell
(PAFC)
242.3.5 MoltenCarbonate Fuel Cell
(MCFC)
252.3.6 Solid Oxide Fuel Cell
(SOFC)
262.4
Protonically
Conductive Ion Exchange Membranes 282.4.1 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 352.7 SulfonationofPolymers 39
2.8 Other
Commercially
Available Proton Exchange Membranes 412.9 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
gel463.2.2 Process # 2: PreparationofPVDF/HFP
(Kynar2801)
Composites fromFeCl3
gel. 48 3.2.3 Process # 3: PreparationofPVDF/HFPcopolymercomposites fromDMF solution. 493.2.3.1 PreparationofPVDF-HFPcopolymer/CB-SO"3H Composites 49
3.2.3.2 PreparationofPVDF-HFP
/Nafion/CBSO~3
H Composites 503.3 Composites Membrane Fabrication 51
3.3.2.1 Puddle
Casting
523.3.2.2 Draw-Down Technique 53
3.4 Characterization 54
3.4.1 Differential
Scanning Calorimetry (DSC)
543.4.2 Electrical
Resistivity
Measurements 55CHAPTER 4: ResultsandDiscussions 56
4.1 Proton Exchange Membranes 56
4.2 PreparationofPVDF Composites 57
4.3 Differential
Scanning Calorimetry
(DSC)
664.4 Electrical
Resistivity
Measurements 84CHAPTER 5: Conclusions 88
CHAPTER 6: Future Considerations 90
LIST OF
FIGURES
Figure 2.1 William R. Grove (181
1-1896)
demonstratedin 1839thatelectricitycanbegenerated fromhydrogenand oxygen incontact withplatinumstrips,which extendintodilutesulfuric acidfunctioning
as adiaphragm.21 13Figure 2.2 Membraneelectrodeassemblyandelectrochemistry for H2/airandmethanolfuel
cells35
17
Figure 2.3 Alkali Fuel Cell
(AFC)
20Figure 2.4 Schematic
drawing
ofhydrogen/oxygenfuelcell anditsreactionsbased ontheprotonexchange membranefuelcell
(PEMFC)41
21
Figure 2.5 Schematic
drawing
ofthe operatingprinciples offuel cellutilizingmethanol asthefuel,
i.e. adirectmethanolfuelcell(DMFC)41 23Figure2.6 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 28Figure 2.10 Schematic of microphase separationinahydrated ionomer 31
Figure2.11 Chemical structureofNafion.xandyrepresent molar compositions anddonot
imply
a sequence length 33Figure 2.12 ProposedstructureofDow XUSperfluorinated copolymer 33
Figure 2.13 Structureof
PVF2
36Figure 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 61Figure 4.3 SolutionCast Film 65
Figure 4.4 DSC thermogramofthe second run(heat-cool curve)of plain Kynar 201 powder
representingtheTA Instrumentsoftware calculatedmelting
temperature,
crystallizationtemperature andtheglasstransition temperature 66
Figure 4.5 DSC overlayplots ofKynar
201, 2801,
301 and7201 powders 68Figure4.6 DSC overlay ofKynar 201 plainpolymer,its black compositepowder with
sulfonated carbonblack
by
Process #1,
andits melt pressedfilm 69Figure4.8 DSC overlaysofKynar 2801 plain whitepowder,itsblack compositepowder with
sulfonated carbonblack
by
Process # 1 andthemelt pressedfilm fabricated fromthecompositepowder 72
Figure4.9 DSCoverlays ofKynar 2801 plainpolymer,its black composite powder with
sulfonated carbonblack
by
Process# 2 andthemeltpressed film fabricated fromthecompositepowder 74
Figure 4.10
(a)
DSCthermogramofKynar 2801 Melt Pressed film from Process # 1 76Figure4.10
(b)
DSC thermogramofKynar 2801 Melt Pressed film from Process # 2 76Figure 4.1 1 DSCoverlaysofPVDF Kynar2801 solution castfilmswithvaryingproportions
ofsulfonatedcarbonblack
(CB)
in it using Process# 3 78Figure 4.12 DSCoverlaysofPVDF Kynar2801 solution castfilm from
DMF,
and alsoKynar 2801 solution castfilm from DMF containing Nafion
1000,
by
Process #3 80Figure 4. 13 DSCoverlaysofPVDF Kynar2801 solutioncastfilm from
DMF,
and alsooffilms containing Nafion
1000,
as well as films containing both Nafion 1000and sulfonatedcarbon black
by
using Process # 3 82Figure 4.14 DSC overlays ofPVDF Kynar 2801 solutioncastfilm containing Nafion 1000
plus sulfonatedcarbonblackandthe film containing thehighestcarbonblack
loading
inProcess# 3 84
Figure4.15 Electrical
Resistivity
Measurementsforsolution castfilmsofPVDFKynar 2801impregnatedwith sulfonated carbonblack
(CB)
(Process#3)
85Figure 4.16
(a)
[PC-1][1/1.2]
PVDF Kynar2801 solution castfilmhaving
thehighestloading
of sulfonated carbonblack,
(b) [PC-3] [1/0.4]
PVDF Kynar2801 filmhaving
lowloading
ofcarbonblack,
(c)
[PNC-1]
PVDFKynar2801 filmcontainingbothNafionandsulfonatedLIST 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)
68Table 5 EnthalpiesofKynar 201 andits composite materials(Ref. to
Fig
4.6)
69Table 6 Enthalpies ofKynar 7201 andits composite materials(Ref. to
Fig
4.7)
71Table 7 Enthalpies ofKynar 2801 andits composite materials
by
Process # 1 (Ref. toFig 4.8)
72
Table 8 Enthalpies ofKynar 2801 andits compositematerials
by
Process# 2 (Ref. toFig 4.9)
74
Table 9 EnthalpiesofPVDF Kynar 2801/sulfonatedcarbon black
(CB)
solution castfilms(Ref to
Fig
4.11)
79Table 10 Enthalpies ofPVDF Kynar 2801solution castfilm from DMF andKynar 2801
solution castfilm from DMF containing Nafion 1000 (Ref. to
Fig 4.12)
81Table 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)
83CHAPTER 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 protonexchange membrane (PEM). Themembrane electrode assembly
(MEA)
,
including
the PEMandcatalyst electrodes,is the essenceofthe fuel cell and isresponsible fortheprotontransport from
the anodeto the cathode,
thereby
directly
creating electricity fromchemical energy. PEMs musthave 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 acidfunctionality
orby
post-sulfonationof a preformedpolymer.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 hasbeen driven
by
the demand for fuel cell technology, particularly forvehicles andportable powerapplications. 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 polymerelectrolyte membranes such as
Nafion9, Aciplex10, Flemion11,
andDow12
membranes. These
materials are essentially strong polymer acids. When exposed to water,
they
hydrate andpolymer and free mobile protons in the aqueous solution.
Thus,
the free protons move throughthehydrogenbondednetwork of watermolecules insidethepolymer.13
Perfluorinated polymer electrolytes such as Nafion have been demonstrated to be excellent
protonexchange membranes.
However,
thehigh costofthe materialisabarriertothe largescaleuse 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 anoption forthe fabrication of proton conducting membranes for polymer fuel cells. This thesis
provides three different procedures for the preparation of PVDF composites with sulfonated
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 FuelCell")
discoveredthatit maybe possibleto generate electricity
by
reversingthe electrolysis ofwater.20
It was not until 1889 that two researchers, Charles Langer and
Ludwig
Mond,
coined the term"fuel cell"
as
they
weretrying
to engineer the first practical fuel cell using air and coalgas.21
While furtherattempts were madein theearly 1900s to
develop
fuel cells that could convertcoalorcarboninto electricity , theadventofthe internal combustion engine
temporarily
quashedanyhopes 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"Francis Bacon developed what was perhaps the first successful fuel celldevice in
1932,
with ahydrogen-oxygencell using alkaline electrolytes and nickel electrodes inexpensive alternatives
to the catalysts used
by
Mond andLanger".22
Bacon and company faced substantial number of
technical
hurdles,
until1959,
whenthey
first demonstrated a practical five-kilowatt fuel cellsystem.
Incidentally,
in that same year,Harry
KarlIhrig
demonstrated a 20-horsepower fuelcellpoweredtractor,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 celltechnology
soon gotfunding
fromthe
NASA,
in an attempt to explore several promising approaches to the construction of apractical power generation
system.24
The firstmajor success ofthis
big
research attempt was tobe 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
majorautomakers,havemade ongoing investments inresearch forthe development offuelcell technology.
In the near future it is expected that traditional power sources can be replaced
by
fuel celldevices ranging from micro fuel cells for cell phones to stationary power sources and
high-powered fuelcellsforvehiculartransport.
2.1.2 Fuel Cell
Technology
DevelopmentOver the last
decade,
the trend towards increasedflexibility
in electricity generation, and theincrease 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
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 influencedby
theincreasing
concern about theenvironmental impactof
burning
offossil fuelstoproduce electricityandto propel vehicles.Hydrogen fuel cells may
help
to reduce our dependence on fossil fuels and diminishC02
emissions into the atmosphere. Hydrogen fuel cells are more efficient than the internal
combustionengines.
Using
purehydrogen,
fuelcellsonlyproducewater, thuseliminatinglocally
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 aspetroleum,willdiminishandtheharmful greenhouse gases that
they
emitmay depletethehealthof humankind.
Therefore,
renewable energy sources based on hydrogen are of extreme2.2
General
principles andfunctionality
offuel cellsA fuel cell is a device that
directly
converts the chemical energy of reactants (a fuel and anoxidant) 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 celleffects 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 adischarging
storagebattery (secondary
battery).However,
the design andengineering 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 storagebattery
must be regeneratedby
periodic rechargingby
voltagerehearsal, normally over several
hours,
sothat the reaction products are reconverted intoreactants. 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 theelectrodesonlywhenpowergenerationisrequired.
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),
separatedby
anion-conducting
electrolyte. The electrodes are connected electrically through a load (suchas an electric motor)
by
a metallic external circuit. In the metallic part of the circuit, electriccurrentis transported
by
theflow ofelectrons, whereas inthe electrolyte it is transportedby
thealkaline electrolytes. In
high-temperature
fuelcells, the corresponding ionic carriers
may be the
carbonate ion
(C032)
in molten carbon electrolytes, and the oxide ion(O2)
in the case ofthesolid 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:
2H2MH++4e
Anode Reaction
Cathode: 02+4H++4e
2H20CH3OH
+H20-^CO2
+ 6H+ +6e-Cell
reaction 2H,+0,~2 v2 ?2H,0Energy
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++eH2+2Pt~^Pt-H
Figure 2.2 displays a simplified schematic diagram ofa
hydrogen-oxygen
fuel cell employingan acid electrolyte. At the anode,
incoming
hydrogen gas is oxidized with an electrocatalyticmaterial thatis usually based ontheprecious metal platinum.
Thus,
the oxidation ofH2
producesprotons 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 andincreasing
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 benecessary. Foracid fuelcells employing a solidpolymerelectrolyte operating inthis temperature
range, the liquidwater productmay be rejected
directly
from the semi-solid gel electrolyte intothecathode 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
releasedasheat,
as would be the case if hydrogenburned 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
from
gaseous hydrogenandoxygenat 1 atmpressure at25C is 1.229V.35In
theory
the fuelcellshown in Figure 2.2 should betherefore capable ofgenerating d.c electrical energy at 1.3 V. In
practice,
however,
on account ofelectrode polarization and otherirreversibilities,
under net flowofcurrentthe terminalvoltagewillbe lowerthan this idealvalue.
In any event, itcanbe seenthatas
long
as hydrogenand oxygen arefedto the fuelcell, theflowofelectric currentwillbesustained
by
electronicflowintheexternal circuitandionic flow intheelectrolyte.
By
electrically connecting amultiplicity of unit cells either in series or inparallel, itis possible to form afuel cell
battery
ofany desiredvoltageand current output.By
the use of anefficient 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 thisclassificationis theDMFC (Direct Methanol Fuel
Cell)
which is afuel cell inwhichmethanolisdirectly
fed to the anode. The electrolyte ofthis cell is notdetermining
forthe class. A secondgrouping can be done
by
looking
at the operating temperature for each ofthe fuel cells. Thereare, thus, low-temperature and high temperature fuel cells. Low-temperature fuel cells are the
Alkaline FuelCell
(AFC),
thePolymerElectrolyte Fuel Cell(PEMFC),
theDirect Methanol FuelCell 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,
theMolten Carbonate Fuel Cell
(MCFC)
and the Solid Oxide Fuel Cell (SOFC). All types arepresented in this section in order of
increasing
operating temperature. An overview ofthe fuelTable 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<leFigure2.3 AlkaliFuel Cell(AFC)41
Operationof alkalifuel cells is basedon compressedhydrogenandoxygen and uses a solution of
potassiumhydroxide in waterasthe electrolyte. Insidealkali cells theoperating temperatures are
were an optimal power source for spacecraft. NASA selected alkali fuel cells for the Space
Shuttle
fleet,
as well as the Apollo program ofthe1960's,
mainly because ofpower generatingefficiencies that approach 70 percent. While it is an advantage to use alkali fuel cells in
spacecraft, theiruse forvehiculartransport on earthis limited
by
the presence ofC02
in air. Airdrawn into the cathode would introduce
C02
into the systemthereby
forming
solid alkalicarbonates. Carbonates can be destructive to the electrolyte and
thereby
the cell performancecanrapidlydecrease.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 smalloz/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 temperatureprovides instant start-up and requires no thermal shielding to protect personnel. At room
temperature, about 50% ofmaximum power is
immediately
available Under normal conditionsfull 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 intheir5kWproductionfuelcell 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 isboundtobe atrade-offbetweenthe energyand financialcost associated with compressing airto
higher pressures and the enhanced performance
thereby
obtained. For a large number of2.3.3 Direct Methanol Fuel Cell
(DMFC)
F.tfn-nttldroit!
V
Anode CHjOH*H,0i 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 notneeded, as methanol is
directly
fed intothe fuel cell. Asmethanol isaliquid,
it does not needtobe stored at high pressures.
Hence,
storage of methanol is not difficult as compared to that ofhydrogen.
However,
due to high permeation of methanol through the most efficientPEMs,
efficiency is low. Though DMFCcan have the limitation ofproducing less power,
they
can stillstore a great deal ofenergy in a small space. This means
they
have the ability to produce smallamount of power over a longer duration.
Hence,
they
are well suited to power applications in2.3.4Phosphoric Acid Fuel Cell
(PAFC)
PhosphoricAcidand PEMFuelCells
Electron-*
fiwv
|
AAA,
t
I
LoadHydrogen\%
: 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 berequired 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 platinumcatalyzed, 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 thisinvariably
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
Phosphoric
acid fuelcells havetodeal withoneissue,
thatis,
ifthe sourceofits hydrogen fuel isreformed 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 acathode.42
Operating
temperatures ofthecell rangefrom 600-800C.Hydrogen canbeextracted
by
froma varietyoffuels,
employingeitheraninternal oran externalreformer. 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 exhibitedby
molten carbonate fuel cells, and thiscanriseto 80percent ifthewaste heat isutilizedforcogeneration.
Presently,
demonstrationunitshaveproduced upto 2 megawatts
(MW),
but designs exist forunits of50 to 100 MW capacity.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.42Also,
theelectrolyte usedinmolten carbonatefuel cellsishighly
corrosive, restrainingsome ofitpotential 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
solid oxide fuel cells. Argonne National
Laboratories
propose that pressurizedsystems 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 oxidefuel 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 requiredwitha 1830F
(1000C)
operatingtemperature inthe solidoxidefuelcell.Depending
on the size ofthepower plant it is estimated that the fuel to electricity efficiency ofsolid 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, dieselfuel,
jetfuel,
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. Thesecond 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 oxidefuelcell maynotbepractical forsizes muchbelow 1,000 wattsor when portable applications are
2.4
Protonically
Conductive IonExchange
MembranesMembranes withhigh protonic conductivity are potentially useful as separators and electrolytes
inelectrochemical cells such as fuel cells.
Among
the firstproton-conductingmembranes usedinfuel 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 arebeing
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)
TheTeflon-like,
fluorocarbonbackbone,
hundredsofrepeating- CF2-CF- CF2-units inlength,
(2)
The sidechains, -O-CF2-CF- O-CF2-CF2-,which connectthemolecularbackbonetothe third region,
(3)The ionclusters consistingofsulfonic acid
ions,
SO3"
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
throughS03
site withinthe 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 whichisusually 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 producedby
DuPont,
bears a resemblance to theplastic 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 unusualelectrolyte. 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 hydrogenthis movement ofionic charge within the fuel cell, the circuit defined
by
cell, wires, and loadremains 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 veryessentialto the operationof a fuel cell.
Although,
as ionic conductors, polymer electrolyte membranes donot 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
solventcasting.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 5nmIonic 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 ashydronium
ions,
H3O . Proton transport is also dependent on polymer morphology or ionicnanostructure.54
A few examples ofPEMs include
Nafion,
sulfonated polystyrene, sulfonatedpoly [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 homogeneousmixingofPVDF and sulfonated carbon black to form composite materials to be fabricated into
films. The concentration of proton conducting species are varied
by forming
composites ofdifferent loadings ofsulfonatedcarbonblack.
Inthepresentinvestigationthe melting behaviourandthe crystallinityofthematrix material and
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 DuPontproduct 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,
similartoTeflon,
to which sulfonic acidgroupshavebeen 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 haveincreased 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 andbooks.58'59'60
Furthermore,
composites ofNafion withinert PTFE matrices like Gore membranes orinorganic additives like heteropolyacidshave been
shown to improve the physical and electrochemical characteristics.
Reinforcing
the Nafionmembranes with Teflon or Gore-Tex fabric has produced thinner films that have increased
-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 variedby
changing the ratio ofthe two components (x and y inFigure 2.1 1). Nafionhas been commerciallyavailable in
900,
1100,
1200,
and other equivalentweights. The equivalentweight ismeasured
by defining
the grams of polymer per mole offixedS03
groups.However,
Nafion 1 100 EW inthicknessesof2, 5, 7,
and 10 mil (1 mil equals 25.4|um) (Nafion
112, 115, 117,
and1110)
seems tobe the onlygrades ofNafionthat arecurrentlywidely 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. 26CFo
CF;
-CFCF-CF2
CF2-S03H
Literature observed in the patent material for both Dow XUS (Figure
2.12)
and Nafionsuggests 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 cyclicsulfone.
2. Condensation of the products with sodium carbonate followed
by
free radicalcopolymerization with
tetrafluoroethylene,
which forms an insoluble semicrystalline butmeltextrudable 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 ChemicalCompany
(Aciplex)
and the Asahi GlassCompany
(Flemion).63
The Dow Chemical
Company
also developed a material with a shorter side chainthan 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 celltemperatures, reaction kinetics and carbon monoxide poisoning can be improved. The high cost
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 lowmechanical strength athigher
temperature,
and moderate glasstransitiontemperatures.2.6 Polyvinylidene Fluoride
(PVDF)
and its copolymers as proton exchangematerials
In the
PEMFC,
ion exchange membranes play a vitalrole64'65
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 newtypes of polymers or
by doping
ofexistingpolymersby
suitable acid constituents. ' ' '' ' In
1997,
it was reported that PVDF grafted polystyrene and subsequently sulfonated might be analternative to perfluorosulfonic acid polymers for the fabrication of proton-conducting
PVDF,
is anon-reactive and semicrystallinethermoplasticfluoropolymer.
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 hasbeen 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 aTable2 PhysicalPropertiesofPVDF
Property
Metric EnglishMelting
point 134-169 AC 273-336 AFDensity
1.78 g/cm31 1 1 lb/ftJ
Thermal
Conductivity
0.18 Wm"1 K"1Coefficientof
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 kpsiVolume
Resistivity
>lx!0l2CA-mKYNAR
301 poly
(vinylidene)
fluoride is the homopolymerof1,
1-di-fluoro-ethene, and is atough, semicrystalline engineering thermoplastic that offers a unique balance of properties.
When exposed toharsh thermal, chemical, and
ionizing
environments, it has thedistinguishing
stability of
fluoropolymers,
while the alternatingCH2
andCF2
groups along the polymer chainprovide 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
Characteristics
ofKYNARPVDFinclude:Mechanical
strength, toughness, high abrasion resistance, high thermal stability, high dielectricstrength, 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. Availableversions of
KYNAR
PVDF arerigid andflexible.
Chemical Process
Industry
Applications:KYNAR PVDF components are
broadly
used in the high purity semiconductor market (lowextractible values),pulp and paper
industry (chemically
resistantto halogens andacids), nuclearwaste 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 pumpassemblies, sheet and stockshapes, films
tubing
architectural.Lithium Ion Batteries:
Inthe
battery
industry,
KYNAR PVDF homopolymersand copolymers have gainedsuccess asbinders forcathodes and anodes in lithium-ion technology, and as
battery
separators inlithium-ionpolymertechnology.
"Arkema istheworld's
leading
producer ofPVDF". Theirparticularbrand ofhighpurity,battery
grade PVDF resins are
KYNAR
and Kynar FlexA. Arkema has been working closely with
design thinner,
smaller lithium-ionbatteries
and has been with lithiumtechnology
from thebeginning.
2.7
Sulfonation
ofPolymers
For many years sulfonated polymers have been
investigated
intensively
because oftheir currentand 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 alsoconcerned 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 theirefficientproton conduction
(10"'
S cm"1)inthe
fully
hydratedprotonicform)
andlong
lifetime.88However,
the high cost of this ionomer is a major drawback for the development of thistechnology. 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 orcomplexes thereof. Postmodification reactions are usuallyrestricteddue to their lack ofprecise
degradation
ofthepolymerbackbone.Regardless,
this areaofPEM synthesis hasreceived muchattention 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 orby
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 ofNoshay
andRobeson,
whodeveloped a mild sulfonationprocedure 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 isusuallyrestrictedto 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 blackbeing
combined with the highchemical 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 othercellulosic products. Thesesulfonated carbon blacks are also especially useful as fillers in resins
2.8 Other
Commercially
AvailableProton Exchange
MembranesNearly
all of the commercially available membranes are based on perfluorosulfonic acidpolymer, such as Nafion. Nafion also has the largest
body
ofliterature devoted to its studybecause ofits demonstrated industrial
importance
and availability. Nafion composite systemsalso 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 producedcommercial 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 higherpermeabilityandincreased fuelcrossover.
Therefore,
asthemembranes reachthicknesses asthinas 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 polyelectrolytemembrane, whichis only morphologicallystable attemperatures lessthan 100 C. At
lowrelative
humidity,
andtemperaturesinexcess of100C thesemembranes are plaguedby
theDias
Analytic
has produced sulfonated membranes from the well known Kraton-Gstyrene-b-ethylene/butylene-b-styrene copolymers.98
Thesehydrocarbon backbonepolymers are ofinterest
for low cost, low temperature and low current
density
arrangements. Dais membranes haveproven 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-costmembranes. 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.,
isfield-testing
a 250-kwstationarysystemfor 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.,
isteaming
with DowPlastics, Midland,
Mich.,.
Apolybenzimidazole (PBI)-basedmembrane for fuel cells thatoperates atrelatively high
(150
C)
temperatures isbeing
developedby
Celanese VenturesGmbH, 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 sulfonatedpolyaryletherketone(avariant of
PEEK)
resin suppliedby
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'sgleaned from experimental studies has recently been reviewed
by
severalanthnrc 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. coupledproton-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+,
orEigen,
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 ofwater in the membrane, the addition ofinorganic
particles97
(e.g. silica,
heteropolyacids)
intopolymeric 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.