Effects of Freezing and Thawing on the Structures of Porous Gas Diffusion Media in PEM Fuel Cells

Rochester Institute of Technology

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2007

Effects of Freezing and Thawing on the Structures

of Porous Gas Diffusion Media in PEM Fuel Cells

Joaquin A. Pelaez

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Recommended Citation

Effects of Freezing and Thawing on the

Structures of Porous Gas Diffusion Media in

Approved By:

PEM Fuel Cells

by

Joaquin A. Pelaez

A Thesis Submitted in

Partial Fulfillment of the

Requirement for the

MASTER OF SCIENCE

IN

MECHANICAL ENGINEERING

Dr. Satish G. Kandlikar

Department of Mechanical Engineering

Dr. Steven Day

Department of Mechanical Eningeering

Dr. Jeffrey Kozak

Department of Mechanical Engineering

Dr. Edward C. Hensel

Department Head of Mechanical Engineering

DEPARTMENT OF MECHANICAL ENGINEERING

THESIS REPRODUCTION PERMISSION STATEMENT

Permission Granted

Title of Thesis: Effects of Freezing and Thawing on the Structures of Porous Gas

Diffusion Media in PEM Fuel Cells

I

, Joaquin

A.

Pelaez, hereby grant permission to the Wallace Library of the Rochester

Institute of Technology to reproduce my thesis in whole or in part

.

Any reproduction will

not be for commercial use or profit.

Abstract

Fuel

cells are a

very

promising

technology

for

transportation applications

in

the

future.

Many

companiesare

performing

research

in

ordertomakethe

implementation

of

fuel

cell-_{powered vehiclesmore}

feasible.

One

issue

_thatneedsto

be

_addressed

is

_the

fact

that

fuel

cellvehicles will

be

used

in

sub-freezing

climates.

Vehicles

undergo

frequent

shut-down and

startup

events, and as_such,

freezing

and

thawing

effects on

fuel

cell

components

become important

whenthevehicle

is

shut off and

left standing in

cold

climates.

When

shut_off,

fuel

cellswill maintainwater

in

themembrane electrode

assembly

(MEA)

and gas

diffusion layer

(GDL)

unlesscertain

purging

protocols are

followed. Excessive purging

will

lead

tomembrane

dryout

and

increased

systemcosts.

Understanding

theeffectsofrepeated

freeze-thaw

cycling

onthe

GDL is

critical

in

developing

effective

purging

techniques.

When

thecell

is

subjectedto

sub-freezing

temperatures,

thewater

remaining in

thesemediawill

freeze. This

freezing

could

have

a

detrimental impact

onthepore _structure,

fiber

integrity,

and

binder

effectiveness

in

the

GDL,

thereby

decreasing

the electrochemicalactive surface area oftheelectrolytes and

hurting

the overall performance ofthecell.

This

thesispresents a numericalsimulationto

highlight

the

damage

caused

by

freezing,

followed

by

an experimental

study

toobserve

theseeffects

in

a

GDL

under a compressed state torepresentactual

fuel

cell

operating

conditions.

This study

validatesthe

damage incurred

through

freeze-thaw cycling

and

confirmstheneed

for

developing

cost-effective

purging

protocols.

Another

finding

of

this

study

is

theusefulness of electrical resistancemeasurementtechniques

in

identifying

Table

of

Contents

Abstract iii

TableofContents iv

ListofFigures vii

ListofTables xi

Nomenclature xii

SymbolsandAbbreviations xii

Greek xiii

Subscripts xiii

1. Introduction 1

2. Literature Review 4

2. 1.

Freezing

structural effectsin Porous Media 5

2.2. Waterand

freezing

behavior in PEMfuelcells 9

2.3. PEM

Freezing

Performanceend-effects 14

3. Objectives

3.1. Research Needsinstudying GDL freezeeffects 20

3.2. Objectivesofthepresent work 20

3.3. Approach 21

4. Analytical

Modeling

24

4.1. Model 24

4.2. Material Properties 27

4.3. Results 31

4.4. Discussion 34

5.1. Objectivesoftheexperimental_study 36

5.2. Test Section 37

5.3. GDL Samples 40

5.4. Test Section Compression Fixture 41

5.5. Air

Supply Loop

43

5.6. Water

Supply Loop

44

5.7. Electrical System 45

5.8. High-Speed Videosystem 48

5.9. Experimental Parametersand

Uncertainty

50

5.9.1.

Uncertainty

inElectricalResistance 50

5.9.2.

Uncertainty

in Differential Pressure Measurements 51

5.9.3.

Uncertainty

in Droplet Departure Measurements 52

6. Experimental Procedure 53

6.1. Thermal

Cycling

53

6.2. Waterand_{Air supply Procedures} 54

6.3. Performance

Testing

61

6.3.1. GDL Through-planepressure

drop

6 1

6.3.2. Electrical Resistance 62

6.3.3. Droplet Behaviorobservation 64

6.3.4. Beginning- andEnd-of-live Surfaceobservation 64

7. Data ReductionandAnalysis 66

7. 1. Electrical Resistance 66

7.2. Through-PlaneAir Flow Measurement 72

7.3. ImageAnalysis: Contact Angle 73

7.4. Image Analysis: Visual Damage Inspection 79

8. ResultsandDiscussion 82

8.3. Contact AnglesandTeflon Content 86

8.4. Visible Damage Mechanisms 87

9. Conclusions 107

References 109

Future Work 112

List

of

Figures

Figure 1.1 - PEM fuel

cellcross section 2

Figure2.1-_{Sample Matrix}used

by

Salmonet.al

[16]

5

Figure 2.2- Percolation Clusters from Salmon_et.al

[16]

6

Figure 2.3- Hori's

micro-crackstress model

[15]

7

Figure 2.4- Frost

depthandheaveovera6-mo. Period

[14]

8

Figure2.5- Frost

heave inporous material

[10]

9

Figure 2.6- Location

oficelensformation

[10]

10

Figure3.1-_{PEMreactantgas path 23}

Figure 3.2- PEM

current path 23

Figure4.1-_{LocationofGDLmodeled region 24}

Figure 4.2- Model

areas 25

Figure 4.3

-Meshedareas 30

Figure 4.4- Nodes

andtemperatureapplication 32

Figure 4.5

-Nodaldisplacements 32

Figure 4.6

-Stress iny-direction 33

Figure 4.7

-Von MisesStresses intheGDL 33

Figure5.1-_Complete

testing

loop

37

Figure 5.2- Flattened

electrode

tip

usedinresistance measurement probes 38

Figure 5.3- Electrode

bottomsfixedtochannel plates 39

Figure 5.4

-Electrodetipson channel plates 40

Figure 5.5- Compression

Figure 5.7- Air

loop

schematic:

(3)

_electrodes,

(9)

air_tank,

(10)

air_regulator,

(11)

air 43

rotameter,

(12)

airtestsection

inlet, (13)

testsection air/water outlet

Figure 5.8

-Water supplyloop:

(1)

syringe_pump,

(2)

testsection water

inlet,

(3)

44

electrodes,

(4)

testsection wateroutlet,

(5)

t-fitting,

(6)

pressure

transducer,

(7)

bleedvalve,

(8)

bleedtube exit,

(15)

computer

Figure 5.9- Four-point

methodfor measuringelectrical resistance ofGDLsamples 45

during

freeze

testing

Figure5.10-_Electrodeand wire solder connections 47

Figure5.11-_Mountedtestfixturewithwire connections 48

Figure5.12- High-speed_camera

setup 49

Figure5.13-_Video

lighting

setup 50

Figure 6.1 - Electrode

placementinchannels 63

Figure7.1-_{Combined Through-Plane/In-Plane [A &}

C]

68

Figure 7.2- Short In-Plane [B &

C]

68

Figure 7.3

-Long

In-Plane[D &

C]

69

Figure 7.4- Simple Through-Plane[A &

B]

70

Figure 7.5- Combined Through-Plane/In-Plane [A &

D]

70

Figure 7.6- Simple In-Plane [B &

D]

71

Figure 7.7

-Airflowresistance plot 73

Figure 7.8- Droplet

measurements 75

Figure 7.9- Contact

angle plot 78

Figure7.10-_{Departure diameter}plot 78

Figure7.11-Videotestmatrix 79

Image 7.12- Keyence_{confocal microscope and stage}for GDLobservation 80

Figure 8.2

-AOO Before Thermal

Cycling,

450x 88

Figure 8.3

-AOO After Thermal

Cycling,

450x 88

Figure8.4

-A07 Before Thermal

Cycling,

450x ₈₉

Figure8.5- A07

After Thermal

Cycling,

450x 89

Figure8.6

-A15 Before Thermal

Cycling,

450x 90

Figure 8.7

-A15 After Thermal

Cycling,

450x 90

Figure8.8- MOO Before Thermal

Cycling,

450x _{9 1}

Figure8.9-_{MOO After Thermal}

Cycling,

_{450x 91}

Figure8.10-_{M07 Before Thermal}

Cycling,

450x 92

Figure8.11-_{M07After Thermal}

Cycling,

450x 92

Figure 8.12- M10 Before Thermal

Cycling, 450x,

#1 93

Figure 8.13

-M10 After Thermal

Cycling,

450x,

#1 93

Figure8. 14

-M10 Before Thermal

Cycling, 450x,

#2 94

Figure 8.15- M10 After Thermal

Cycling, 450x,

#2 94

Figure 8.16

-M15 Before Thermal

Cycling,

450x 95

Figure 8. 1 7

-Ml 5 After Thermal

Cycling,

450x 95

Figure 8. 1 8

-ZOO Before Thermal

Cycling, 450x,

#1 96

Figure 8. 1 9- ZOO After Thermal

Cycling, 450x,

#1 96

Figure 8.20- ZOO Before Thermal

Cycling, 450x,

#2 97

Figure 8.2 1- ZOO After Thermal

Cycling, 450x,

#2 97

Figure 8.22

-Z07 Before Thermal

Cycling, 450x,

#1 98

Figure 8.23

-Z07 After Thermal

Cycling, 450x,

#1 98

Figure 8.24- Z07 Before Thermal

Cycling, 450x,

#2 99

Figure 8.25

-Z07 After Thermal

Cycling, 450x,

#2 99

Figure 8.26- Z07 Before Thermal

Cycling, 450x,

#3 101

Figure 8.27- Z07 After Thermal

Cycling, 450x,

#3 101

Figure 8.28- Z10 Before Thermal

Figure 8.30- Line

fracture in GDLafter removal 1 03

Figure8.3 1

-Linefractureafter_removal,focalplane#2 104

Figure 8.32- GDL

gas channelimpression 105

Figure 8.33- GDL

List

of

Tables

Table 4.1- Water

thermalproperties 28

Table 4.2

-Structural PropertiesofGDLand water 29

Table 6.1 - GD L

weights with water 5 5

Table 7.1- Resistance

Measurements 66

Table 7.2- Electrical

curvefit data 72

Table7.3- Pressure_measurementdata 72

Table 7.4- Droplet

Angle Data 76

Table7.5- Statistical

Nomenclature

Symbols

and

Abbreviations

e

-Charge

onone electron

F

-Charge

on one mole of electrons

GDL

-

Gas

Diffusion Layer

GDM

-

Gas Diffusion Media

I

-

Current

MEA

-Membrane Electrolyte

Assembly

MPL

-Micro-porous

layer

N

-Avogadro's

number n

-Sample

size

OCV

-Open Circuit Voltage

Pe

-Electrical

power outputof

fuel

cell

PEM

-Proton Exchange Membrane

or

Polymer Electrolyte Membrane

PEFC

-Polymer Electrolyte Fuel Cell

PEMFC

-PEM Fuel

Cell

R

-Resistance

s

-Sample

standard

deviation

V

-Voltage

Vc

-

Fuel

cell operational voltage

x

Greek

A

-Stoichiometric

ratio

\i

-Population

mean

Subscripts

a

1

-

Introduction

Fuel

_cells, andpolymer-electrolytemembrane

fuel

cells

(PEMFCs)

in particular,

are a

very promising

technology

for

transportationapplications

in

the

future.

They

offertheadvantage

ofzero

local

_emissions, as a

hydrogen

fuel is

the

only

required

input,

and clean water

is

the

only

product ofthechemicalreaction

occurring

during

operation.

Additionally,

PEM

fuel

cells are

low-temperature

fuel

cellsince

they

operateat about

80C. This

allows

for

shorter

warm-up

times than thatrequired

for

a

high-temperature fuel

cell.

For

_{these reasons,}

many

companies are

performing

research

in

ordertounderstandthe

fundamental underlying issues

and makethe

implementation

of

fuel

cell-_{poweredvehicles more}

feasible.

A

basic PEM fuel

cell unit

itself

consists of no

moving

_parts,which

is

a greatadvantage

overcurrentpetroleum-_{or diesel-}

fueled internal

_{combustion engines.}

Each

individual

_cell

in

_a

stack

is

composedof several_parts, as shown

in Figure 1.1.

The

basic

PEM

fuel

cell unit

is

shownonthe

left-half

of

Figure 1.1. On

eitherside ofthe

cell, a

bipolar

plate

is

used

for

several purposes.

First,

the

bipolar

plate

has

gas channels

machined,

stamped,

ormolded

into

their

faces. These

channelsareonthe order of

1mm

in

their

4

*

area

hydraulic

diameter,

Dh

(Dh

=

),

or smaller.

These

gaschannelsserve to

carry

the perimeter

reactant gasestoall points oftheactive surface and remove productwater

from

thecell.

Inward

ofthe

bipolar

plates arethegas

diffusion layers (GDLs). These GDLs

allowthereactant gasesto

reachthecatalyst sites andtoallow productwaterto travel

from

thecatalyst sitesto thegas

channels

for

removal.

The

catalyst

layer,

typically

platinum,

is bonded

toeitherside ofthe

positively-chargedprotons

(Hydrogen

ions)

to

diffuse

from

theanode-sideto thecathode-side

reaction sites.

\|/ \|/Nj/y_{v\|/ y}

Air-Side Bipolar Plate" "

Membrane "CatalystLayers "GasDiffusion Layers

Hydrogen-SideBipolar Plate

*' .n.

cz

rz

LZ

V V _y V

1

.77

Figure

1.1-

PEM fuel

_{cell cross section}

The fuel

cell operates

using

thereverseelectrolysis process.

Oxygen

or air

is

delivered

to

thecathode _side, and

hydrogen

is delivered

to theanode side.

On

the

hydrogen

side,the

molecules are split

into

positively-charged protonsand negatively-chargedelectrons.

The

protons, as

previously

_mentioned,

diffuse

throughthemembrane andreducetheoxygen

moleculesto

form

water.

To

completetheelectrical circuit,

however,

the electrons musttravel

through anexternal

load

connectedto theendsofthe

fuel

cell stack.

This

chemical reaction

providestheelectricalpowerofthe

fuel

cell.

The

right

half

of

Figure 1.1

showsthatwater

is

produced atthecatalyst sites

between

the

GDL

andthemembrane.

From

here,

thiswater must [image:16.532.39.466.94.401.2]

Unfortunately,

thiswaterpresents operational and

durability

concerns

in PEM

fuel

cells.

Fuel

cells

currently

employed

in

static power applications are sometimes mounted

outdoors,

and

in

cold weatherapplications,

damage

to the

MEA

can

be

caused

by

freezing during

periods of

fuel

cellshutdown.

This is because

waterremains

in

the

fuel

cell

if

not

properly

purged.

If

fuel

cells are

implemented into

productionvehicleson a

large

_{scale, these}powerplants

may

potentially

sufferthesame

fate.

Four

aspects of

fuel

cell_{overpotentials,}or

losses,

are as

follows:

activation

losses,

fuel

crossover and

internal

_currents,ohmic

losses,

and masstransport

losses.

The

twoareasthat the

GDL

plays an

important

rolearetheohmic

losses

andthe masstransport

losses. Ohmic losses

are relatedto theelectrical resistance of

many

ofthe

fuel

cellcomponents.

The

membrane must

adequately

conduct_{the protons,}andtherest ofthe

fuel

cell's electrical circuit must

properly

conducttheelectrons.

The

catalyst sites must make proper contactwiththemicro-porous

layer

(MPL)

andthe

GDL,

which

in

turn,

must make contactwiththe

current-collecting bipolar

plates.

The

masstransport

in

the

GDL

pertainsto

how easily

thereactant gases reachthecatalyst sites

and

how easily

water can

be

removed

from

thecatalyst sites

into

thegas channels.

The

focus

ofthepresentwork

is

to

study

the

freezing

effects on a

GDL. This

damage is

expectedtomanifest

itself in

thesurface ofthe

GDL

through the

hydrophobic

propertiesor

physical _{structure, the}pore structurethrough

changing

air_{transport properties,}orthroughthe

structurethat

impacts

electrical properties.

A

detailed

literature

review

is

presentednext,

2

-

Literature

Review

A

great

deal

ofresearch

has been

performed as to the

ability

of

fuel

cells to start

up in

cold climates

[1-6]

and on the effect of

freeze-thaw cycling

on the performance of

PEM fuel

cells

[6-13],

but

there

is

little

published work ontheprecise effects that the

freeze-thaw

cycling

has

onthepore and

fiber

structureofthegas

diffusion

mediathemselves.

Previous

research

details

the

freezing

and

thawing

effects ofconcrete and soil

[14-17],

as

these two materialsare

commonly

present

in

sub-freezing

climates, are exposed to

liquid

water,

and

have

the

ability

to

carry

the

liquid

water

using capillary forces.

There

are

generally

two

different

phenomenathatareusedto

describe

the

freezing

effects

in

porous media.

The

first is

a simple

9%

volumetric expansion asthe

liquid

water

freezes

and

forms ice.

This

theory

is

appropriate

for

water,

but degradation

effects

have been

shown

for

fluids

that contract upon

freezing,

such as

benzene

and nitrobenzene

[17].

The

theory

that

is

used

in

this case

is frost

heave,

in

which subcooled

liquid

water

diffuses

toward

ice

lenses,

causing

these

lenses

to grow.

Frost

heave

is

dependent

on the presence of

liquid

waterand the

ability

ofthe porous material to allowwaterto

diffuse

throughout themedia.

This

chapter will

presentthe

different

theories ofporousstructure

degradation,

and review publishedwork onthe

previous research that

has

been done

on the

fuel

cell system and the effects of

freeze-thaw

2.1

-Freezing

Structural

effects

in Porous Media

To

more

fully

understandthe

failure

phenomena

in

theporous materials

in

a polymer

electrolyte membrane

fuel

cell

(PEMFC),

it

is beneficial

tounderstandthe

freeze/thaw

phenomena

in

otherporous materials.

This

section providesanoverview of

basic

freezing

behavior

due

toavolume

increase

aswater

freezes,

aswellas a more advanced concept called

frost

heave. These

phenomenaare

discussed in

referencetoporous materials such as soil and

cement,aswellaswithrespectto

PEM

fuel

cells.

Salmon

et.al

[16]

explores

degradation in

porous material as a result of a volume

expansion as

liquid

water changes

its

phase to

form ice.

To

model this volumetric _change, a

porous medium was modeled as a matrix or

lattice.

Each ny location in

the matrix

is

representativeofa poreat

location

row and column

j,

and with size given

by

the valueofny.

() 0>) (c)

0 78 0.99 0 58 0.45 0.91 0 78 099 0.58

B3

0.91 0.78 0.990.58 086 0.91

0.19 0.67 0 81 0 12 0.70 ^80670.8lR!l^w! 0.31 0.67 0.81 0.27 0.94

0.84 0.75 0.620 980.72 0.84075El098

d

D.S3^P!^S|^M

0.84 0.7S 0.09 0.98 0.78

035083O.S0 0.09 0.36 0.46 0.83 0.87 0.39 0.72

0.60081 0.77 0.16 0.91 waai <ku[*e uuj n n n II II

0.84 0.95 0.80 0.51 0.99

Figure 2.1

-Sample Matrix

used

by

Salmon

et.al

[16]

Figure 2. 1

shows the

5x5

sample matrix, andeach cell

(or pore)

is

assigned a

different n^

size

value.

The fluid

transport method

is

called the

invasion

percolation _{movement, the}

basis

of

which

is

capillary

action.

The basic

principle

is

that thesmaller pores will

be filled first due

to

said

capillary forces.

All

pores adjacent to a

filled

pore willthen

be

filled

with _priorityon the

smallest_pores,

just

as

in

the

first iteration

or time step.

When

a_satisfactory cluster of pores

is

simulation,

each pore size value

is

increased based

on a simple algorithm.

As

the simulation

is

repeated, thepore sizes evolve.

D=l n=2 n=3

Figure 2.2

-

Percolation

Clusters from Salmon

et.al

[16]

Figure 2.2

shows an

interesting

behavior

of

fluid invasion using

a

200x200

matrix.

At

successive

iterations

of_{the simulation, the}

invasion

clusters

do

not repeat.

This behavior is

referred to as "self-avoiding" and

is due

to the simulated

increase in

size of each pore

in

the

freeze/thaw

cycle ofthe previous cluster.

This

theme

is

common

for

other

commonly

accepted

pore growth algorithms aswell.

Hori

[15]

expands upon the simple volume expansion principle and elaborates on the

topic of

frost damage

to porous

brittle

material.

Unlike Salmon

et.al

[16],

Hori

explains this

phenomena

in

terms of

large

pores

further expanding

and smaller pores

contracting

upon

freezing.

When

the water

frozen in

the pores

thaws,

it

can

flow

through the enlarged pores,

thereby

further

increasing

their size and

amplifying

the

irreversible damage.

Hori

models the

cracked solid

tt

pore structure

^1

_L--^ Applied stress ^..L___i_^_a,macrostress

u

&

')

(

/

-JLn

. ;

i

isolatedcrackin infinile domain

Figure 2.3

-Hori's

micro-crack stress model

[15]

The

stresses

due

to the

increased

volume of

ice

can cause the micro-cracks _{to grow,}

thereby

causing

a

deformation

anda

loss

of stiffness.

This

model examines

how

the

degradation

affects

the

deformation in

the structure ofa cave.

The

three major

factors

that must

be

taken

into

consideration when

determining

the extent of the

damage

to the porous structure are

(1)

the

temperature to which the material

is

cooled,

(2)

the number ofthermal cycles the material

is

subjected

to,

and

(3)

the external

loads

applied to the material.

A

certain

cooling

temperature

must

be

reached

before

any

damage is

sustained_{to the material,} and the

damage

will

be

more

severe as more extreme

cooling

temperatures are attained.

Additionally,

the number ofthermal

cycles will

increase

the extent ofthe

damage.

However,

each subsequent cycle will

have

a

decreasing

damage

contribution until additional water

is

introduced

to the system.

The final

factor

contributing

to the extent of

damage due

to

freeze

andthaw

cycling is

theexternal

load(s)

appliedto the media.

Hori

statesthat a

free-standing

medium will sustain more

damage

than a

medium subjected to compressive

forces.

However,

as the compressive

forces

exceed a certain

threshold,

they

can contributeto

irreversible

shrinkage orcompressionofthemedia asaresultof

While

asimplevolumechange

is

a convenient

model,

another phenomenonthatexists

in

porous materials

in

freezing

climates

is

called

frost heave.

Hermansson

and

Guthrie

[14]

provide

a

very

detailed

description

of

how frost heave

occurs

in

brittle

materials.

While

soil

is

used as

the media

in

whichto

study frost

heave,

thisconcept can

conceivably

be

appliedto the

study

of

ice

formation

in

fuel

cells.

Frost

heave is

the

foremost

contributorto the

degradation

of paved

surfaces

in

cold climates.

<#* <*> cj=

dp < & o> # of> _{6> <} cf> _& #

Date

|

-FrostDepth ?_{Frost Heave}

|

Figure 2.4

-Frost

depth

and

heave

over a

6-mo. Period

[14]

Figure

2.4

illustrates

the

frost depth

and magnitude of

frost heave

over an

approximately

6-month period.

While

frost heave

appears

during

periods of

sub-freezing

temperatures,

it has

consequences

in both

the cold and warm periods.

When

the soil

is

subjected to

sub-freezing

temperatures and

frost

heave

is

_{present, the} surface ofthe pavement can

become

cracked and

otherwise marred.

In

warm_periods, whenthe soil

is

no

longer

freezing,

the

damage incurred

by

the

frost heave

can

lead

to a

depreciated

load-carrying

capacity.

The

existence and magnitude of

frost heave itself depends

on several

factors,

such as the

depth

ofthe

frost,

the availability of

available.

Supercooled liquid

water

may

stillexist

in

the media attemperatures

below freezing.

Due

topressure gradients

in

_{the pores, these}

films

of water can

diffuse

through the media and

travel towards the

ice

crystals as

they

are

forming.

These

crystals proceed to

form ice

lenses,

which are

typically

parallel to the surface of the media.

These

lenses

create a substantial

expansion

force,

or

heave,

on_{the media,} andthat

is

wherethemajor

damage

is incurred.

2.2

-

Water

and

freezing

behavior

in PEM fuel

cells

He

and

Mench

[10]

discuss how frost heave

can

be found

directly

in fuel

cells.

There

are

severalarticlesthat

discuss

theend effects ofthermal

cycling in

an operational

fuel

_cell, and

thereare othersthat

discuss how

water

freezes

and

behaves in

porousmedia, thisarticle

combines theseprinciplesto

demonstrate

how

and wherewater

freezes in

a

PEM

fuel

cell.

The

phenomenonthat

is

explored

is frost

heave,

and

it

can occur

in

porous media even when

it is

saturatedwitha

fluid

thatcontractsupon

freezing.

,

-Ice Lens -,v_

Figure 2.5

-Frost

heave

in

porous material

[10]

The

model presented

by

He

and

Mench

is

aone-dimensional model.

The diffusion

mediaand

temperature,

capillary

forces,

and

freezing

temperature.

Due

to the

fact

that

GDLs

are

typically

constructed

from

carbon_paper,

Toray

carbon paper

is

used

for

theexamination of

freezing.

Perhaps

the most

important

contributionto the

understanding

ofthisphenomenon

is

their

proposal

for

the

location

ofthe

ice lens formation.

,Diffusion Media

Catalystlayer

' '

/ / / /

////,

Hf^^///A

V

Row

Channel

A A A A ^ A

/

/

/ A Row

////A

Channel

////A

1 a^^^^t .^-^-^. ₁

Bectrolyte

Figure 2.6

-

Location

of

ice lens formation

[10]

He

and

Mench

proposethat the

ice lenses

aremost

likely

to

form

wherethecatalyst

layer

meets

themembrane

(3),

wherethe catalyst

layer

meetsthe

diffusion

media

(2),

andwherethe

diffusion

media meetsthechannel

(1),

ascan

be

seen

in Figure 2.6. He

and

Mench's

model

simulation

is

still

in

progress.

Lee

and

Merida

[11]

have

also presentedresearch

very

pertinentto thecurrent work.

They

are

using

ex-situ

testing

methodsto

determine

the

damage

mechanisms

in

a

GDL

sample

from

several_sources,

freezing being

one ofthem.

They

prepared a

GDM

samplesimilarto that

employed

in

a

working fuel

cell.

A Teflon coating

was _added,as wasamicro-porous

layer. The

GDM

samples were placed

in

atest

fixture

and_compressed,andthe samples'

strain responsesas

a

function

of cycle number wererecorded.

permeability,

surfacecontact_{angle, porosity,} and water vapor

diffusion.

The

electrical

resistivity

measurements werecalculatedthroughthe

4-point measuring

method.

The

surface contact angle

was measured

by

using

the

sessile-drop

method and

measuring

the contact angleofa

1 5mL

droplet.

The porosity

was measured

using

a

mercury porosity-measuring

device.

Air

diffusion

was calculated as a

function

ofthe airflowthrough the

GDL

as a

function

of pressure.

The

Darcy

coefficients werecalculated.

Finally,

water vapor

diffusion

was measured

using

a custom

dual-chamber

device. The

twochambers wereevacuated of

any

_moisture,and moist air was

introduced into

one chamber.

The

moisture content oftheair ontheother chamberas a

function

oftimegives a measureofthewater vapor

diffusion

through the

GDL.

Lee

and

Merida

experiencedsome

interesting

results.

The

in-plane

electrical _resistivity,

bending

stiffness,plate-sidecontact_angle,catalyst-side contact_{angle, porosity,}and water vapor

diffusion

all exhibitednochange

before

and afterthe

freeze

cycling.

The only

aspectsthat

exhibited

any

change werethe

in-plane

andthrough-planeair permeability.

The

author attributes

thesechangesto a

loss

ofthemicro-porous

layer (MPL). This layer

is

important

tothecurrent

collection

function

ofthe

GDL.

While

thisresearch

is

similarto thatcontained

herein,

thereare somethingsthatshould

be done differently.

First,

the

GLD

samples were

freestanding

when

they

were

frozen. This

does

not

accurately

representthe conditions

in

a

fuel

cell.

Second,

themethodof

measuring

resistivity

that theauthor used

only

measures

in-plane

resistivity.

The

combined

in-plane

and

through-plane

resistivity

are

both

important,

andas_such, should

be

measured.

Benziger, Nehlsen,

et.al

[18]

investigate how

thewater produced

in

the catalyst

layer

flows

and

behaves

in

relationto the

GDL.

They

first

describe how

the

GDL behaves in

regards

andthiswater willremain

in

thepores and

hinder

gastransport.

However,

if

the

GDL

is

hydrophobic,

water will

be

prevented

from entering

themedium unless a requisite pressure

is

reached

in

orderto

convectively force

waterthrough themedium.

A

GDL-appropriate Carbon

paper sheet

is

naturally

hydrophobic,

andas

Teflon

is

addedto the

GDL,

the

hydrophobicity

will

increase.

Additionally,

GDLs have

acounter-current_ability,

in

thatair and water currents

flow

in

opposite

directions

simultaneously.

To

determine

thevoid

fraction

in

_{the medium, the}authors

used

Kerosene,

which

freely

wicks

into

thesamples.

As far

asthe

hydro

staticpressure

is

concerned, theauthorsnoticeda

very

interesting

phenomenon.

The

waterpressure

head

was

increased,

but

no

flow

throughthemedium was noticed until

5200 Pa. Above

_{that point,}water

would

flow. Once

waterwas

flowing

through the

GDL,

thepressure was

decreased

tomeasure

thepressure/flow correlation.

Even

whenthepressure

head

was

below 5200

Pa,

water would

still

flow. Another

relevantobservationthe authors made

is

that thepressure

head

requiredto

force

waterthroughthe

GDL

samples

decreased from

thevirgin samplesto theusedsamples.

The

authors also statethatthecarbon cloth

has

poresas

large

as

250

microns,whereasthecarbon

paper

has

pores of

only 50

microns.

Those

pores,

however,

are

irregular

and

do

not offer a

direct

paththrough themedium.

As

mentioned

in

several porous mediapapers, thewater willtake a

percolation paththroughthe

GDL,

but

here,

theauthorsstatethat thewater willtravel through

the

largest

pores _only,asthe

Teflon

keeps

thesmallerpores

from

taking

on water.

In

order

for

thewatertotravelthroughthe

GDL,

apressure

is

requiredasaforementioned.

This

pressure

is

derived

either

from

thevolumetric constraints atthewater generationsites

(catalyst

layer)

or

through

being

absorbed

into

the

PEM,

causing it

toswell.

Their

work

in

regardsto

measuring

In summary, up

to

60

percentofthevolume

in

a

GDL

can

be filled

with_water,

but

only

a

fraction

ofthatspace

is

used

for

watertransport.

The

GDLs

need

irregular

pore

distributions

so

astoallow

larger

pores

for

watertransport

away

form

thecatalyst and smaller ones

for

gas

transport to thecatalyst sites.

Nishida

et.al

[19]

investigate

watergeneration and

freezing

behavior

in

an operational

5-cm2

fuel

cell.

They

aimtopromote waterremoval

from

thecathode

GDL

because

thecondensed

water

in

the

GDL

will

inhibit

gas

flows

to thereaction sites.

To study

water generationand

distribution,

they

ranthe cell at an environmentalchamber

temperatureof

30

C

and

subsequently decreased

the temperature to

10

C.

Upon making

this

change, thesize and numberofthe

droplets

increased.

This

suggeststhatastemperature

is

decreased,

theamountofwater condensed

in

the

GDL is increased.

Similarly,

thesize ofthe

droplets

produced

from

the

GDL

surfacewill

increase in

the

downstream direction. This is

attributedtothe

fact

that the

humidity

ofthegases

increases

in

this

direction,

as well asthe

fact

thattheamountofcondensedwater

is increased.

They

alsostudied watertransport throughthe

GDL.

They

determined

that thewater

droplets

are

formed

at ornearthe catalyst

layer

and are

transported to the

GDL

surface

by

capillary

motion.

Another

phenomenonthat

Nishida

et.alwantedto

study

wastheremovalofwater

from

the

GDL

volume.

An

improvement

they

madetoacurrent

GDL

wastoput

very

small and

shallow grooves

in

the

GDL

surface.

They

determined

thatwiththis

improvement,

the

liquid

waterthat

is

produced

quickly

migrates to thegrooves and will

be

removed much moreeasily.

In

testing,

a cell withoutthegrooves

in

the

GDL

wassetto aprescribed current

density

and

producedcould not

be

_removed,andthe

flooding

inhibited

reactant gases

from

reaching

the

catalyst surface.

Nishida

et.al also

looked

at

freezing

phenomena.

However, they

observed

how

water

froze from

cold

start-up

asopposedtopost-shut-down

freezing

of a

fuel

cell.

They

notedthat

waterproduced

in

a cold cellwouldnot

freeze

immediately

due

to the

heat from

the chemical

reactionthat

is

occurring.

2.3

-

PEM

Freezing

Performance

end-effects

While it is important

to understand themechanisms of

degradation in

porous _structures,

the end effect must also

be

examined.

In

the

following

investigations,

experiments are run

in

order to

determine

(a)

the effects ofthe

freeze/thaw cycling

onthe

fuel

cell system as a _whole,

and

(b)

theeffectsthat

adhering

toa water-purgeprotocol can

have

onthe

life

ofa

fuel

cell.

Yan

et.al

[6]

studied the cold start

behavior

of

PEMFCs

and the effects of

low

environmentaltemperatures ontheperformanceof an

operating fuel

cell.

Custom

catalyst

ink is

formulated

and applied to carbon papertomake the electrodes.

The MEAs

are thenassembled

using Nafion

and

hot-pressing

the electrodes on each side.

To

benchmark

the performance

characteristics, polarization curves weretaken

immediately

before,

during,

and after operationat

the

desired

sub-zero temperatures.

The

polarization curves are then compared to

look for

irreversible

damage

caused

by

thecycling.

Tests

are conductedto

determine

the

PEM fuel

cell's

starting

characteristics

from

_{-5, -10,}

and -15

C.

The

cell was

first

runateither roomtemperature or

80

C.

After allowing

the

fuel

(cathode), (b)

they

were purgedwith

dry

nitrogen

(anode)

and air

(cathode),

or

(c)

they

werenot

purged.

For

restartattempts,

dry

gases at roomtemperaturewere

fed

to thecells.

Successful

starts wereperformed

from

-5

C

giventhat thecells were purged.

However,

purging did

not

necessarily

guarantee that the cells would

successfully

start

from

temperatures

lower

than -5

C

because

the water produced

during

the chemical reaction would

freeze

and

hinder

thegas

flow.

Additionally,

whenthe

feed

gaseswere_pre-heated,thecellstarted

up

much

more easily.

However,

it

couldnotstartat

low stoichiometry

ortemperature.

To

gauge performance of the

fuel

cell at

different

temperatures,

the environmental

chamber was used to change the ambient temperature around the

fuel

cell.

The

temperatures

were varied

from

-15

C

to

80

C

at

different

intervals.

As

the chamber temperature was

changed

from

-15 to

25

C,

performance

increases

were noticed.

However,

performance

degradation

occurred

during

chambertemperaturechanges

from 25

to

80

C.

Yan

et.al attribute

this to the

fact

that the membrane

loses

proton

conductivity

above

25

C.

The

best fuel

cell

performance,

therefore,

wasrealizedat roomtemperature.

Water

can exist

in

three

forms in

the

GDL

and on the membrane

(free

water,

bound

water, and

bound-non-freezing

water).

The

term "bound"

is

used to refer to the presence of

chemical

interactions

with protons present

in

themembrane.

The

bound

non-freezing

_water, as

the name _suggests, will not

freeze.

The bound

water

freezes

typically

attemperatures

between

260 K

and

213 K.

The

free

water

freezes

at about

273

K,

or

0C.

The

freezing

water causes

several sources of

decay

whenthecell

is

exposedto the

low

temperatures.

First,

ice

can

form in

thecell and

hinder

or

block

the transportofgases acrossthe

GDLs.

Second,

de-lamination

ofthe

Cho

et.al

(2003)

[7]

conducted similar

freeze/thaw

research on an operational

fuel

cell.

They

utilized a

very

thorough test procedure to examine

precisely

the sources of

degradation

when a

PEMFC is

thermal cycled.

Catalyst

ink

was created and applied to the

electrodes,

and

the

MEA

was assembled

in

a

very

similar manner to

Yan

et.al

[6].

Cho

attributes the

degradation

of

PEMs

to the volume change caused

by

water's liquid-solid-

liquid

phase and

density

changes

in

water when

it

is

freeze

cycled.

During

the

testing,

theenvironmental chamber

and

fuel

cellwere

brought

to thecell's

80C operating

temperature.

After

thecell operationwas

ceased, thechamberand

fuel

cellsystem were

brought

to-10C, sustained atthat temperature

for

one

hour,

and

brought back up

to

80C.

During

the

testing,

thecurrentand voltage

behavior

of

the cell was monitored.

Other

parameters examined were the current

density

at

0.6

_volts, pore

size

distribution,

electrochemical active surface _area, and proton

conductivity

of the polymer

electrolyte.

Despite

thermal

freeze/thaw

cycling, the

OCV

ofthe

fuel

cell, as wellas theproton

conductivity

ofthe polymer _membrane, were

fairly

constant and seemed

independent

of the

number ofthermal cycles that the systemunderwent.

The

current

density, however,

decreased

about

9.3%

after

only four

cycles.

The

pore size was

similarly

altered, as the

cycling increased

the average pore size

by

over

66%. The

electrochemical active surface area and catalyst site

utilization were also reduced.

The decreases in

performance of _{the cell,}

therefore,

can

be

attributedtoan

increase in both

theohmicoverpotential andchargetransferresistance.

Cho

et.al

claim that since membrane's proton _conductivity remains unchanged, the contact resistance

is

the culprit

for

the

increase in

ohmicresistance.

Cho

et.al

[8]

then published a _similarly titled article

in

which

they

discussed

the

80

C

cycle

testing

wasusedasthecontrol

data. Two

separatetest groups were_{then run,} and

in

each_group, a

different fuel

cell

water-purging

method was

implemented.

During

the

first

experimentalrun conducted

by

Cho

_et.al,

dry

gases wererunthrough the

cell until therelative

humidity

ofthe expelled gases reached a preset value.

In

these

tests,

the

OCV (open

circuit _voltage, zero

load condition)

stayed

fairly

_constant, and the current

density

declined

very slightly (about 0.06%

percycle).

This

gas purge method provedto

be

an effective

methodtoreduce

degradation

ofthe cell.

However,

the

disadvantage

of

using

gasestopurgethe

cell

is

that

it

took around

twenty

minutes to reach the

desired

relative

humidity

value ofthe

outlet gas streams.

During

the second experimental

test,

antifreeze solutions were used as a purge method.

The

antifreeze was run through the cell via the gas

feed lines for

a

few

seconds.

Since

the

antifreeze can cause the membrane electrode

assembly

_{to swell,} methanol and ethylene glycol

solutions were chosen

in

order to minimize the swelling.

Thermal cycling using

this purge method yielded negative

degradation

_rates, which

imply

that the performance

is slightly

increased

through antifreeze solution purging.

Moreover,

both

of the experimental

water-purging

methods provedto

be

effective,

but

antifreeze

purging

might

be

themore viable option

toprevent significant

degradation in

portable

PEMFC

applications.

In

order to

further

examine the type and

degree

of physical

damage

to the membrane

electrolyte assemblies through thermal cycling,

Guo

and

Qi

[9]

performed three

different

treatmentcases

for

the

MEAs.

In

all such _{cases, the}

MEAs

are thermal cycled

from 20C

to

-30C.

The

author used

30

minutes

for

thetransient

heating

and

cooling

stages and six

hours

of

The

first

treatment case consisted of

examining

freestanding

MEAs. The

assemblies that

were

only

exposedto therelative

humidity

in

the ambient air sustained

significantly less damage

than the

MEAs

thatwere

fully

hydrated. The

treatment ofthe

latter

group

of

MEAs

consisted of

soaking in

water at

80C for

_{ten minutes,}

wiping

to remove superficial _water, and then

introduction

to the thermal cycling.

These

freestanding,

hydrated

MEAs

exhibited

delaminated

and

detached

catalyst_material,which could

lead

to

increased

contact overpotential.

In

both

additionaltreatment groups, the

MEAs

were assembled

in

a single-cell.

The

cell

was_operated, andthe reactant streams werecut abruptly.

After undergoing

_{thermal cycling, the}

MEAs

weretested.

However,

priorto the

freezing,

onetreatment

group

underwenta

dry

reactant

gas

purging

procedure

followed

by

a

dry

nitrogen purge

for

fifteen-minutes

and _one-minute,

respectively.

The MEA

that

did

notundergo a

dehydration

step

sustained significant

damage

in

the

form

of

fractured

catalyst material.

The MEA

that was

dehydrated,

on the other

hand,

exhibited no apparent

damage. This illustrates

that

if

water

is

removed

before

thecell

is

allowed

to

freeze,

less

damage

will occur.

While

damage

was sustained

by

the

MEA

in

the operational

fuel

cell,

both

ofthese testcases appearedto

be

in

a

better

statethanthe

freestanding

MEA. This

is

attributed to the

clamping assembly force in

the operational

GDL

and the

increased hydration

levels

in

the

freestanding

samples.

Despite sustaining

cracks and

damage

to the

MEAs,

the operational

fuel

cell exhibited no

major short-term performance

degradation. The

authorconjectures,

however,

that the

lifespan

of

the cell will still

be negatively

impacted

by

the

damage. Guo

and

Qi,

therefore,

similarly

concludethat

decreasing

or

eliminating

thewater content

in

the

MEA

priorto

allowing

a

Several

researchersat

Ballard Power

Systems

(St-Pierre, Roberts,

_et.al,

(2005) [13])

presenttheirrecent workwith

PEM

and

DM

(direct

methanol) fuel

cells and

how dry-gas

purging

can

be

applied.

This

allowsthewaterto

be drawn

away

by

convection andto evaporate

into

the

dry

gases.

This

will

help

prevent

ice formation in

thecellwhen

it

is frozen.

For

the

testing,

the

fuel

cellwas allowedtocooltoroomtemperature

before

purging

took

place, and after

freezing,

it

was allowedto thawpriorto the

introduction

of reactant-gas and

coolant streams.

The

authorsmake an

important

point as

far

asthe

timing

ofthepurge.

When

purge gases were

fed

through_{the cell, the}channels would

dry

out

first,

followed

by

the

diffusion

media, and

subsequently

themembrane.

Therefore,

if

purge gaseswere

fed for

too

long

of a

duration,

themembrane would

dry

out and

negatively

affect performanceuponstartup.

Since

membrane

conductivity

would

decline,

thecurrent provided at

startup

would

be

limited

and

therefore therateof

heating

the

fuel

cellwould

be

sub-optimal.

However,

the authors still saw

3

-

Objectives

3.1

-

Research

needs

in studying GDL

freeze

effects

From

the

literature survey

presented

in

theprevious_chapter, several observations can

be

made.

First,

the

freezing

does

have

a significant negative

impact

onthe

durability

and

performanceofa

fuel

cell

operating in

sub-zero environmental conditions.

Second,

the

free

standing

porous materials

like GDLs

will sustain more

damage

than those testedunder a

compressedstate.

Additionally,

purging

with a

dry

gas or antifreeze

is

_effective,

but purging

timesandmethodsneedto

be carefully

evaluatedtoavoid

excessively

drying

themembrane.

Finally,

theprecisemechanismof

freeze

damage

in

the

GDL

is

not

known

in

the

GDL

matrix.

3.2

-

Objectives

of

the

present work

In

orderto

better

understandthese

freezing

damage

mechanisms

in

the

PEMFC

GDL,

the

objectivesofthepresent workaretocreate ananalytical model andanex-situtest

fixture

of a

PEM

fuel

cell

GDL in

orderto:

Develop

aphysical modelto

identify

the

forces

caused

by freezing

water

in

the

GDL's

capillary

pores

Experimentally

determine

the

freeze damage in

termsof

o

GDL's

electricalresistance

in

acombinedthrough- and

in-plane

direction

o

GDL's

through-planeresistanceto air

flow

o

Surface hydrophobic

properties ofthe

GDL

Develop

an

understanding

ofthe

GDL

damage

mechanisms

by

studying

_theresults

from

thenumericalandexperimentalstudies

These

objectiveswill

ultimately

aid

in

determining

the

degradation

ofa

working fuel

cell

due

to the

losses

thatarerepresented.

First,

the gas

permeability

measurement should give an

idea

of

how

masstransport

losses

willcorrespondto thermalcycling.

If

thestructure ofthe

GDL

is

changedthroughan

increased in

thepore_sizes, gas

permeability

should

increase,

and mass

transportmightthereforeoccur more

readily

and reducethe

fuel

cell

inefficiency.

Another

area

of concern

in fuel

cells

is

theohmicoverpotential.

The

electrical

conductivity

ofthe

GDL is

very

important

to theefficient operation

fuel

cell.

This

will

be

measured as a

function

ofthermal

cycling.

The

results

from

this

testing

should

be

able togive

insight

asto theeffects ofthermal

cycling

onthe ohmic

losses

andmasstransport

losses.

3.3

-

Approach

These

objectiveswill

be

accomplishedthroughthe

following

engineering

techniques

(in

theirrespective

order)

An ANSYS

model will

be

createdto

determine

thestresseswhen awater-saturated

GDL

is frozen

A

test

fixture

will

be

devised

thatallows

for

in-plane,

through-plane,

and combined

through- _and

in-plane

_{electricalresistance}measurements

The

pressure-flow rate characteristicsofthe

GDL in

thethrough-plane

direction

will

be

A high-speed

camera will

be

usedtorecord

droplet departure

events sothe

diameters

and

contactangles can

be

measured

A

microscopewill

be

usedto take

highly-detailed

picturesofthe

GDL

both before

and

afterthe

cycling

An

additionalexplanationoftheelectricalmeasurements

is in

order

for

clarity.

In

the

fuel

cell, asaforementioned, theelectricalcurrentmust

be

ableto travel

from

thecatalyst _sites,

through the

MPL

and

GDL,

and

ultimately

to the

bipolar

plateswith as

little

resistance as

possible.

However,

thepathtaken

by

theelectricalcurrent

is

not

necessarily

a

strictly

through-plane one.

The

greenarrows

in Figure 3.1

representtheapproximate

pathway

of air

diffusion

from

thegas channels.

The

electrochemical activesurface_area, while

existing

underthe

lands

of

the

GDL,

willalso exist underthechannelsthemselves.

Since

the

bipolar

plates serve as current

collectors, theelectrical current willneedto travelon_pathways, such as shown

in

thenext

figure. The

red arrows

in Figure 3.2

representthepathsthat theelectrical current

may

needto

take to travel

from

the electrochemical active surfacesto the

lands

ofthe

bipolar

plates.

It

can

therefore

be

observedthat

both

thethrough-planeand

in-plane

electrical resistances are of

Bipolar Plate

Gas

channel

Land Region

GDL

Catalyst

Layer

Membrane

/A\

'notdrawn toscale

Figure

3.1-

PEM

_{reactantgaspath}

* _Bipolar

riate

Gas

channel

~~

Land Region

\^f/J

*

oUL

^__ Catalyst Layer

* _iviemDrane

*not drawn toscale

Figure 3.2

-

PEM

4

-

Analytical

Modeling

There

are severalpapersthat

detail

analytical modelsof

freezing

in

pores

in

soil or

concrete

[14

-17],

but

none

detail

how

thiswould

apply

to a carbon paper-type structure.

The

finite-element

modeling

software

ANSYS

will

be

employedto addressthis.

4.1

-

Model

The

modelwill consist of aportion of a

GDL

cross-section.

The

entiresectionwill

be

190

pm

thick,

which

is

theapproximate thicknessof a

GDL.

The

sectionwaschosento

be 210

pm wide

for

modeling.

In

addition, threepores were added.

Literature

on gas

diffusion layer

structurestatesthatpores can

be

ontheorder of

10-30mm

in diameter. As

_{such, the}widths of all

thepores

in

themodel are chosento

be 30

pm

diameter,

which

is

onthe

high-end

oftherange

but is

still areasonable size.

^a

Modeled Region

Bipolar Plate

section

GDLsection

Cathode Catalyst

section

Figure

4. 1

-Location

of

GDL

modeled region

layer.

It is

nottheentirewidth ofthe gaschannel,

but

rather

just

a small portionthatwill

be

modeled.

Figure 4.2

-Model

areas

Figure

4.2

showsthe areas

in

thecross-sectional model.

Area A2

(purple)

is

the

GDL

material.

Areas

Al, A4,

and

A3 (light

blue,

darker

blue,

andred,

respectively)

arethepores

in

the sample.

Pore

Al

is

fully

enclosed

by

the

GDL. Pore A4 is

modeled atthe

bottom

ofthe

GDL

cross _section, and

is

assumedto

be bounded

by

the

GDL

onthree sides andthecatalyst

layer

onthe

bottom. Pore A4 is located

toward theupper surface ofthe

GDL

and

is

exposedto

thegas channel.

Pore A4

is

bounded

by

the

GDL

onthe_{three remaining sides,}

but

is free

to

expand upwards

into

the channel region.

Pores Al

and

A4

are modeledas

30

x

30

pm square

pores,whereas

A3 is

modeled as

130

pm

deep. The

top

ofthe

GDL

is

takento

be

the surface

inside

thegas

flow

_channel, andthe

lower

region

containing

pore

A4

is

the surface againstthe

In

this simulation, thepores are modeled as

being

filled

with water.

The

GDL's

top

and

bottom

surfaces willthen

be

exposedto atemperatureof

249 Kelvin

(about

-1

1F

and-24C).

The

water'sthermalexpansioncoefficientwill

be

usedtomodelthewater's volume changeupon

freezing.

This

expansionwill put a stress onthepores modeled

in

the

GDL

structure.

The

model

does

nottake

into

accountthe

possibility

ofsub-cooled

liquid

water

being

present

in

the

pores.

The

simulationwill

be

acoupledthermal-structuralmodel.

To

perform

this,

thesystem

must

be

modeled as athermalelement, saved

in

athermal environment, andthenconvertedtoa

structural element and saved

in

astructuralenvironment.

The

thermalproblem will

be

_solved,

and aftertheresults are

found,

thestructural model will

be

solved

using

the temperature

distribution found in

thethermalanalysis.

These

analysestogetherwill give astress

distribution

in

the

GDL

as a resultof

freezing.

The

freezing

will

be

modeled as

starting

atthe

top

and

bottom boundaries

ofthe

GDL

(gas

channeland catalyst

layer,

respectively).

It

is

assumedthat theupperpore

is filled

with

water,which willnot exit

into

the air channel.

This

assumption

is based

onthe

fact

thatwater

willremain

in

certainpores

in

the

GDL

structure and

is held

in

with surface tension

forces. The

freezing

front

willtravel

from

theupperand

lower boundaries

towards thecenter ofthe

GDL.

Again,

thisensuresthat the

liquid

water

is

not evacuated

into

the channel

in

themodel.

The

desired

output ofthismodel

is

the

distribution

ofthe

Von Mises

stresses

in

the

GDL. This

will

allow a conjectureto

be

made astowhether

GDL damage is

expected

due

tothepresence of

water

during

a

freezing

event.

This

output will

be

comparedtoand verified againstthe

4.2

-

Material Properties

As

therearetwoseparateanalysesthatmust

be incorporated into

this

mode,

both

thermal

and structural properties ofthesematerials must

be

in

put

into ANSYS. For

the

Thermal

analysis,

only

threepropertiesareneeded: thethermal

conductivity

[20],

the specific

heat

[21],

andthe

density

[20].

However,

the

GDL

is

acarbon papermaterial,and as_{such, the}

fiber

orientation

in

the

GDL

is

notunidirectionaloreven

in

aprescribed

direction. As

microscopic

images

have

shown, the

GDL fibers

are

in every

direction,

not

only in

the

x-y

(in-plane)

direction,

but

also

in

thez-direction

(through-plane). Due

to

this,

the

GDL

must

be

modeled as

an

isotropic

material with

bulk

propertiestaken

from

themanufacturer's website.

The

manufacturer'swebsite

does

not

list

aspecific

heat,

so

Matweb

was usedto

locate

asimilar

material withthisvalue

listed.

The GDL

carbon material

is

simulated as

having

isotropic

properties as

follows:

a

specific

heat

of

710

J/kg.K,

athermal

conductivity

of

21

W/m.K,

and a

density

of

440

kg/m3.

The

thermal

conductivity

andthe

density

are givenonthemanufacturer's website

(Toray)

[20],

andthe specific

heat

wastaken

from Matweb

asaspecific

heat

of a comparablecarbon cloth

(Thornel Carbon Fiber VCB-20 Carbon

Cloth)

[21]. This

material was chosen

due

to the

carboncomposition of_{the cloth,}which

is

assumedto

be

similarto thegraphite/carbon

composition ofthe

GDL.

The

thermalproperties of water must also

be

tabulated

in

ANSYS. The

relevant

properties arethermal

conductivity

and enthalpy.

The

temperature-dependent properties of water

are shown

in

Table

4. 1. The

nomenclature

Kxx

and

ENTH

are

ANSYS'

notation

for isotropic

In

additionto_{the thermal properties, the} structural properties of

both

materials must

be

input.

There

is

no

data

onthemodulus of

elasticity

(Young's

Modulus)

given

by

Toray,

but it

does

list

a

bending

modulus.

To

determine

an appropriate value

for Young's modulus,

data

obtained

in

the

Introduction

to

Composites

course was referenced.

A T300

fiber

and

5208

matrix composite material

has

an axialmodulus of

132 GPa

and atransversemodulusof

10.8

GPa. Since

thismaterial'stransversemodulus

is in

the

vicinity

of

Toray'

s

10 GPa

flexural

modulus,

Toray'

s

figure

will

be

used

for Young's

modulus.

The Posson's

ratiowill

be

taken

from

the

T3 00/5208

compositematerial

data.

Conductivity (Kxx)

for Water at243.1 5 K

at273.1 5 K

at305 K

at330 K

at353.15 K

EnthalDV

(ENTH)

for Water

0.5690 W/m.K W/m.K W/m.K W/m.K W/m.K 0.5691 0.6200 0.6500 0.6699

at243.1 5 K

at273.1 5 K

at305 K

at330 K

at353.15 K

0

J/kg

J/kg

J/kg

J/kq

0.1 1.34E+05 2.38E+05 3.35E+05

GDL

Ex _{1.00E+10 Pa}

Prxy

0.24 ~

Water

Ex

1.00E+10

Pa

Prxy

0.24 ~

Temp

Temp

Strain,

CTE

K C E

243.15 -30 0.0276

253.15 -20 0.0281

263.15

-10 0.0287

273.15 0 0.0294

277.128 3.978 0

283.15 10 9.00E-05

293.15 20 0.000587

303.15 30 0.00144

Table 4.2

-Structural Properties

of

GDL

and water

Table 4.2

shows allthe

input

structural

data

for

the

GDL

andthewater.

The

thermal

strain

data for

thewaterwas calculated as

follows; first,

thevolume

for

1kg

of waterwas

calculated

from

thewater's

density,

p.

m[kg]

V[m3]

=

Plkg/m*]

Assuming

thatthisvolume

is

a_cube, eachedge

length

ofthe cube

is

calculated.

l-MV

From

thisedge

length, /,

the

linear

_{strain, e,}

is

calculated.

61

E=

In

Equation 4. 1

Equation 4.2

Equation 4.3

In

the

Equation

4.3,

l0

is

the"original"edge

length

atthereferencetemperature.

This

temperature

is

takenas

277.128 K (3.978

C)

because

thewater's

density

is

greatest andthe

the

instantaneous

coefficientsofthermalexpansion required

for

the stress analysis.

It

is

also

important

tonotethat

Modulus

of

Elasticity

andPoisson's

Ratio

values are noted

for

thewater.

These

values are

irrelevant in

the scope of

calculating

stresses

in

the

GDL. These

give stress

distributions in

the

ice,

which

is

of

little importance.

Due

to

this,

thesame values

for

the

composite

GDL

were used

for

thewater.

Before any further

testing

could

be

_{conducted, the}model

had

to

be

meshed.

Figure

4.3

shows

how

themesh

has been

constructed.

The

entire sample wasmeshed

using

a

ANSYS'

meshtooland aglobal meshsizeof

4. The

meshwasthenrefinedateach

internal

boundary

of

thepores.

The

refinement wasperformed

using

ANSYS'

"refine

mesh"

feature in

themeshtool.

4.3

-

Results

The

thermalanalysis must

be

conducted priorto

performing

thestructural analysis.

Initially,

thethermalanalysis was conducted as atransientanalysis.

The

GDL

and water system

wasgiven some

initial

temperatureof

294 Kelvin. The

upper and

lower

edges were exposedtoa

freezing

temperature

(249

Kelvin). The

transientanalysis showed

that,

giventhe

high

thermal

conductivity

ofthematerial andthesize ofthemodel

(190mm

x

210

mm), temperature gradients

in

themodeled sectionwould

be

small andthe temperaturewouldreach

steady

state

very

rapidly.

Due

to

this,

it

was

deemed

that

performing

atransientanalysiswas

fairly

_unnecessary,as

the

GDL

reaches

its steady

statetemperature

distribution

so quickly.

The

thermalanalysiswould

be

conducted

by

applying

a

freezing

temperature atthe

top

and

bottom

surfaces.

Figure

4.4

showsthenodes at each ofthemeshed_points,aswellasthenodesatthe

top

and

bottom

where

the

freezing

temperaturewas applied.

After

the steady-stateproblem was_solved,auniform

distribution

of

244 Kelvin

was achieved.

This

data