Design and Fabrication of a

Direct Methanol Proton Exchange Membrane

Fuel CeIl and Test Station

by

Aarnir M. Sadiq

A thesis subrnitted to the Department of Chemistry and Chernical Engineering

in conformity with the requirements for the degree of

Master of Engineering

Royal Military College of Canada

Kingston, ON

Canada

January, 2000

Copyrighto Aarnir M. Sadiq, 2000

National Libraiy If of Canada Bibliothèque nationale du Canada

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Abstract

The objectives of this work were the design, fabrication and testing of a direct methanol

proton exchange membrane fuel ce11 and test system. Design and development issues associated

with the test bed and data acquisition system represent a significmt part of the work.

Two different fuel cells were developed for this study. The first was a low pressure/low

temperature transparent acrylic cell. This was used to observe the two-phase flow behavior in the

anode flow channel. The second ce11 was a high pressurehigh temperature graphite cell. This

was used to study the effect of anode flow channel depth on performance and operational

characteristics. The latter ce11 was operated at temperatures of 70 and 95 OC and at gauge

pressures of 3 10 kPa (anode) and 345 kPa (cathode). The fuel used was one molar ( m o n )

aqueous methanol solution and the oxidmt was ultra high purity oxygen.

Two types of flow maldistributions were observed using the acrylic cell. These included

channel flow maldistributions and exit manifold flow maldistributions. In both cases, product

gas coalesced into larger gas bubbles and blocked the movement of fluid through the flow field.

This prevented fuel from reaching the catalyst layer of the electrode and resulted in concentration

polarization, lirniting the ce11 to low current densities.

Both the shallow and deep channels of the graphite cell resulted in significantly lower

peak power densities as compared with the medium depth channel. Methanol crossover and fuel

maldistribution characteristics of flow were identified as major 1 irniting factors to the

performance of the cell. Analysis of the cathode exhaust confirmed the presence of methanol

crossover and products of methanol oxidation which suggested that ce11 performance was limited

by rnixed potentials.

The graphite ce11 was operated for more than three hundred hours. Maximum current

densities of 1 10 mA/cm2 were attained at 0.4 V using the medium depth flow channel at 95 OC.

The ce11 produced a peak power density of 45 mW/cm2. It was concluded that channel depth, or

equivalent channel diameter, has a significant effect on the performance of the fuel cell.

Acknowledgments

First, and foremost, it is with the help of God that this Master thesis project was possible

and accomplished.

1 am lucky to have parents who, from the start, have done everything to motivate, support,

and guide me to set goals and strive to achieve them. Both have stressed the importance of hard

work, education, regimented discipline, and constant and never-ending improvement.

1 cannot express enough gratitude toward Dr. R. Weir, Dean of Graduate Studies.

Dr. Weir always made himself available to any cadet, whether it was at 4:00 hrs or 16:OO hrs. It

was with his guidance and support that 1 passed through a difficult junction of my career.

Dr. Wiederick, from my first year at RMC, had always encouraged me to look over the

horizon. He taught me to identify goals and how to plan my career. 1 learned from his

supervisory styIe and will miss his insights. This project could not have been completed without

support from Dr. Brant Peppley. Although there were many ups and downs during the project, he

made sure there was never a du11 moment. 1 am also grateful for having Ela Halliop for use of

her acrylic cell. It is with her setup that I was able to observe the two-phase flow characteristics

of the direct methanol PEM systems.

My friends Pat Ianniciello, Sarah Meharg, and Eric Allistrop (Mr. Canada, 1999) were

pil las of support and sanity. Thanks guys- 1 would also like to thank Dr. Roberge and the

technicd support staff, who had a hand in getting the fuel ce11 to be.

This project is dedicated to the memory of:

My grandfather: Major M. Sadiq, Royal Corps. of Engineers (RPE)

My mentor: Dr. Harvey Wiederick

Table of Contents Page

TitlePage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

. . Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i l

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tableofcontents vi

ListofTables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nomenclature xii

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 1 O Introduction 1

1 . 1 . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 2 Direct Methanol Theory - 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. The Electrochernicd Reaction - 7

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Thermodynamics - 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. CellLosses 9

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Temperature Effect 13

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Anode 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Pure Platinum Catalyst 14

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Bifunctional Alloy Catalyst 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Methanol Concentration 18

2.3. Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Cathode Catalyst 19

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Proton Exchange Membrane - 2 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Crossover -21

77 . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Effects of Channel Flow on Cet1 Performance -- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1. Anode Stream 23

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - 5 2 Cathode Stream 25

2 Dimensional Analysis of Anode Stream Flow Characteristics .............. 25

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 3 = Test Station - 2 8 ................................................ 3.1. System Overview - 2 8

.................................................. 3.2. System Tubing - 3 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Pressure and Flow Components - 3 1

3.3.1. Pressure Control ........................................... - 3 1

3.3.2. Cell Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 3 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Sampling Components - 3 3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Electrical System Design - 3 4 ............................................ 3.4.1. The Load Bank - 3 5

3.5. Data Acquisition and instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.5.1. Measurement and Control Device Specifications . . . . . . . . . . . . . . . . . . 36 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2. Data Acquisition f rogram - 4 2

Chapter 4 . Fuel Cell Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -44

. . . . . . . . . . . . . . . . . . . . . 4.1. Graphite Fuel Cell Design and Fabrication Overview 44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Membrane Electrode Assembly (MEA) -45

. . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Graphite Plates and Flow Channel Design .. - 4 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Flow Field Requirements 49 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Flow Field Configuration -49

4.3.3. Channel Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 5 2 4.3.4. Machining of Graphite Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Current Collectors 54 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Clamping system - 5 5

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Cell Ports - 5 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Fuel Cell Sealing - 5 8

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1. MEMraphite Plate Interface 59 4.7.2 . PodGraphite Plate Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.8. Acrylic Fuel Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 6 3

Chapter 5 . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 6 4

5.2. Cell Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 6 4

5.3. Start-up and MEA Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 6 6

5.4. Shut-down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.5. Baseline Internal Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 6 8

5.6. Experimental Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

vii

.................................. 5.7. Measurement of Ce11 Performance -69 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8. Expenmental Design - 7 2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1. Baseline Conditions - 7 4 .................... 5.8.2. Expenmental Accuracy and Error Estimation - 7 4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9. Calculation of Dimensionless Groups 75

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 6 . Results and Discussion - 7 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. System Response and Controllability 78

........................ 6.1.1. Pressure Differential Effects on the MEA 81 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Channel Flow Characteristics - 8 2

............................................ 6.2.1. Gas Production - 8 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Channel Flow Maldistribution 83

. . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3. Exit Manifold Flow Maldistribution - 8 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Interna1 Ce11 Resistance 85

............................................ 6.4. Fuel Ce11 Performance -88 .................................. 6.4.1. Effect of Ce11 Temperature - 9 1

.................................... 6.4.2. Effect of Channel Depth - 9 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3. Open Circuit Voltage 99

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Exhaust Stream Analysis 100

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 7 = Conclusions and Recommendations 103 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Conclusions 103

. . . . . . . . . . . . 7.2. Recommended Modifications for the Test Station and the Ce11 104 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1. Anode Pressure Control 104

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2. Electrical System 105 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3. Product Collection and Analysis .. 105

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.4. Anode Flow Field Design 105

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A $ample Calculations and Error Analysis 110

viii

List of Tables Page

Table 1.1 . Operational Characteristics and TechnoIogical Status of H2/air and direct methanol ........................................................... PEMfueIcells 5

. . . . . . . . . . Table 1 -2 . A mass cornparison between the H@r and direct methanol PEM cells - 6 . . . Table 2.1 . Surnrnary of DMFC thermodynamic properties at standard conditions (298 K) - 9

............... Table 2.2 . Variables used for the dimensional anaiysis of the anode channel 26

Table 3.1 . Resistor values used for the fuel ce11 polarization curves and their estimated

accuracies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Table 3.2 - Specifications of instrumentation used to regulate and monitor the test station

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . parameters and fluid strearns - 3 9 ........ Table 3-3 - Accuracies for the Fiuke multimeter are given for voltage and resistance 40

. . . . . . . . . . . Table 4.1 - Direct methanol PEM fuel ce11 electrode and catdyst specifications - 4 7 . . . . . . Table 4.2 - Specifications for Poco graphite plates AXF-SQC with resin impregnation -48

Table 4.3 - CNC system and tool specifications used to cut the graphite plate flow channels . ..................................................................... 53

. . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 4.4 - Material specifications for the clarnping system 57 Table 5.1 - Coding levels for temperature and channel depth . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Table 5.2 - The experimentai run order is shown with coded channel depth and temperature . . 73 Table 5.3 - Set-points for the controlled parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Table 5-4 - Surnrnary of the independent dimensionless numbers used to describe the anode flow

. . . . . . . . field are given with respect to fuel flow rate, temperature, and channel depth 77 Table 6.1 - Average pressure and preheat temperature for the experimental study . These averages

are calculated from every datum taken in the experiment . . . . . . . . . . . . . . . . . . . . . . . . 81

Table 6.2 - Component weight percent of the cathode and anode liquid and vapor samples . . . 100

List of Figures Page

. . . . . . . . . . . . . . Figure 2.1 . The effects of varying the concentration of methanol in the feed I l . . . . . . . . . . . . . . . . . . . . . . . . . . . . Figure 2.2 . Effects of increasing the operating temperature 13

. . . . . . . . . . . . . . . . . . . . . . Figure 3.1 . Picture of the Direct Methanol PEM Fuel Ce11 test bed 29 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Figure 3.2 . Schematic diagram of the electncal circuit 34

. . . . . . . . . . . Figure 3.3 . Schematic diagram of the direct methanol PEM fuel ceil test station 38 Figure 3.4 . A fuel cell current density versus time graph shows the resolution of the multimeter .

..................................................................... 41 Figure 3.5 - impact of background voltage noise on current density measurements is shown at

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . open circuit conditions - 4 2 . . . . . . . . . . . . . . . . Figure 4.1 - A component illustration of the h e l ce11 as it is broken apart 46

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Figure 4.2 - Picture of the MEA 47

Figure 4.3 (a, b) - Designs A and B are isometric views of the negatives for two flow fields considered for the fuel ce11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Figure 4.4 . The cathode assembly is shown in its operational orientation, tilted at a forty-five degreeangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Figure 4.5 . Schematic of the clarnping system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -56

Figure 4.6 . Schematic of an aiternate graphite plate design . . . . . . . . . . . . . . . . . . . . . . . . . . . - 6 0 . . . . . . . . . . . . . . . . . . . . Figure 4.7 . Cross-section of the sealing system used for the cet1 ports 62

Figure 4.8 -The flow field of the acrylic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Figure 5.1 . Picture of the assembled fuel ce11 connected to the test bed . . . . . . . . . . . . . . . . . -66 Figure 5.2 . Graph of methanol and oxygen flow rates used for the expenments . . . . . . . . . . . - 7 1

Figure 5.3 (a. b) . Plot of the Euler number (Eu) as a function of flow rate (current density) and temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Figure 6.1 (a. b) . Transient responsr for a change in load resistors frorn short circuit to 50. 000 Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Figure 6.2 - Transient response from a 4.94 Q load to a IO . 15 n load is shown for experiment 2, repeat3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Figure 6.3 - Steady-state voltage and power density from the repeat experimental runs two, five . eight and nine are given for each load condition (medium channel. high temperature) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Figure 6.4 . Cell internal resistance for experiment 2 (medium channel. high temperature) . . - 8 6 . . . . . . . . . . . . . . . . . . . . . Figure 6.5 . Ce11 interna1 resistance for experiments 1.2. 3.5. and 6 86

Figure 6.6 . A cornparison among the first experimental run. the last run. and repeat runs of experirnenttwo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Figure 6.7 - Polaïization and power density plots for experiment 1, shallow channel at 95 OC.

Figure 6.8 - Polarization and power density plots for experiment 2, medium channel depth at 95°C. ................................................................ 89

Figure 6.9 - Polarization and power density plots for expenment 3, deep channel depth at 9S°C.

Figure 6.10 - Polarization and power density plots for expenment 5, medium channel depth at

70°C. ................................................................ 90

Figure 6.1 1 - Polarization and power density plots for experiment 6, deep channel depth at 70°C. ..................................................................... 90

Figure 6.12 - A comparison of the effect of temperature on the performance of the fuel ce11 using .............................................. the medium depth channel. .92

Figure 6.13 - A comparison of the effect of temperature on the performance of the fuel cell using thedeepchannel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Figure 6.14 - A comparison of polarization curves from experiments 1 to 3. . . . . . . . . . . . . . . .96 Figure 6.15 - A comparison of the effect of flow field depth on the performance of the fuel ce11

................................. using the low temperature conditions, 70°C. -97

Nomenclature

cross-sectional area (m') confidence interval, student's t-test

channel depth (mm)

conduit equivalent diarneter (m)

conduit wall roughness (m) electrons standard electrode potential (V) reversible ce11 potential (V)

Euler number (dimensionless) reversible ce11 efficiency (dimensionless) potential efficiency (dimensionless)

Faraday constant (96,487 C equivalent-') compressive force (N) Froude number (dimensionless) gravity (m s-') standard Gibb's free energy change (I mol-') standard change in enthalpy (1 moT1) current density (mA cm-')

current (A) exchange current density (mA cm")

conduit length (m) molarity (mol L-l) active surface site on a catalyst reaction layer dimension for m a s used in the dimensional analysis number of equivalents involved in a reaction overvoltage (V) overvoltage due to activation (V)

overvoltage due to ohmic resistance (V)

anodic polarization (V) cathodic polarization (V)

pressure (Pa) pressure drop (Pa)

wetted channel perimeter (m) density (kg m") power density (mW cm-')

xii

fiow rate (cm3 min") channel radius (mm) gas constant, (8.3 14 J mol-' k') resistance of the circuit ( S Z ) interna1 resistance of the ce11 (Q) resistance of the load (Q) resistance of the shunt (Q) resistance of the ce11 and system (Q) Reynolds number (dimensionless) standard change in entropy (J mol-' K") standard deviation of x

temperature (K, unless otherwise specified) dimension for time (s) student's t distribution statistic viscosity (Pa s) potentiai (V) atomic volume (cm3 g') fluid velocity (m sA') degrees of freedom, student's t-test channel width (mm) specific energy ( k W h kg') Weber number (dimensionless) surface tension (kg s-') calculation error for x

association parameter for water

Chapter 1 - Introduction

1.1. Background

There are five families of fuel cells (FC). These are phosphoric acid fuel cells, proton

exchange membrane (PEM) fuel cells, alkaline fuel cells, niolten carbonate fuel cells, and solid

oxide fuel cells; each have their own advantages and disadvantages. Significant progress has

been made with hydrogen PEM fuel cells in the last decade. However, this technology suffers

from high costs, concems arising from the extraction of hydrogen from base fuels, and the need

to store hydrogen fuel. To be used commercially, fuel cells must meet criteria including fast

startup, high power density, high fuel efficiency, easy and safe handling, long life-span and low

cost. None of the ce11 types yet satisQ dl these requirements (Raissi et al., 1997).

For hydrogen to be used as a fuel in fuel cells, it is likely to be extracted (reformed) from

higher rnolecular weight hydrocarbon hels (such as gasoline and diesel) or alcohols. The

extraction process from higher molecular weight hydrocarbon fuels has several disadvantages.

For example, these fiels require high processing temperatures as well as additional processing

and gas purification steps (Dams et al., 1997). The hydrocarbons must also be purified of

catdyst attacking impurities (such as sulphur) prior to any catalytic extraction process. This

makes the application of such fuels lirnited. Consequently, there is interest in developing lower

alcohols such as methanol and ethanol as the main feedstock for hydrogen fuel ce11 applications.

Methanol is of interest since it is one of the easiest fuels to reform at low temperatures

(250°C) when compared with higher molecular weight hydrocarbon fuels (Ahrned et al., 1997).

It is a renewable resource with a theoretical energy density of 6.37 kWh/kg. It can be produced

from a variety of sources such as biomass. Methanol is becoming the fuel of choice for PEM

fuel ce11 research and hydrogen extraction.

There are several types of fuel processing systems that c m be used to extract hydrogen

from methanol. Al1 types yield varying hydrogen gas concentrations in the exit Stream. The

hydrogen purity ranges from 45% for partial oxidation, 75% for stearn reforming. to near 100%

when using palladium membrane reactors (Dams et ai., 1997). The product stream also includes

carbon monoxide (CO) which poisons the platinum (Pt) electro-catalyst on the electrodes. As a

result, product stream cleaning steps (such as preferentid oxidation or water-gas shift reactions)

are required to lower the CO concentration beiow toxic levels (10 ppm) (Dams et al., 1997).

These steps add systern complexity and reduce system efficiency.

A system where methanol may be directly fed into the fuel ce11 without needing the

reforming and CO reduction steps would be advantageous. Such systems are known as direct

methanol PEM fuel cells (direct rnethanol cells) and will be the focus of this thesis.

Direct methanol fuel cells bypass the external reforming steps required to extract

hydrogen from methanol. The cells are directly fed Iiquid or vapor phase methanol into the

anode. The fuel is a dilute aqueous methanol solution with a concentration of up to four molar.

The platinum-based anode catalyst chemisorbs water and methanol, and extracts protons from the

fuel. Although considered zero emission devices, direct methanol PEM cells ernit much less CO2

than interna1 combustion (IC) engines.

The ~af ion" membrane used in the ce11 is porous to both the methanol fuel and the

diffusing protons. As a result unreacted methanol crosses over into the cathode charnber. This

process is referred to as methanol crossover and is a major source of ce11 performance los. The

permeating methanol oxidizes on the cathode catalyst and results in a reduction in ce11

performance through mixed potentids. Water perrneating through the membrane may flood the

cathode catdyst Iayer and prevent oxygen from diffusing ont0 the catalyst.

Gas management is another issue plaguing direct methanol cells. The two phase flow

encountered on the anode side may inhibit mass transport. Any carbon dioxide (CO,) bubbles

forrned may become entrapped on the electrode surface or block the ceIl channels. This would

prevent fresh fuel from accessing the active sites in that area. Water may flood the cathode

reaction layer and prevent the oxidant from diffusing to the active catalyst sites (Scott et al.,

1997).

This thesis will deal with liquid fed direct methanol cells. Some of the advantages of a

Iiquid phase fuel is efficient heat removal and thermal control through the circulating liquid. As

vvell, the liquid fuel provides constant membrane humidification and prevents membrane dry-out

at elevated temperatures (Halpert et al., 1997).

Individual fuel cells cm be connected in series to form fuel ce11 stacks to increase system

voItage at constant current densities. DMFC stacks have been reported to give as much as

200 &cm2 at OSV, or 100 mw/cm2 (Lamy and Léger, 1997). The catalyst loading required for

DMFCs, as compared with hydrogen fuel cells, is relatively high. Further research is needed to

reduce the Pt loading and methanol crossover before the performance of DMFC stacks can

compete with hydrogen ce11 stacks.

Although the HJair PEM cells require energy intensive reformers, presently the overail

system efficiencies and fuel utilizations of hydrogen cells are higher than those for direct

methanol cells. Using performance data dated from 1997, direct methanol cells are capable of

approximately one-sixth the power densities achieved with hydrogen cells. With better crossover

limiting membranes, improved methanol oxidation resistant cathodic electrodes and fuel

recycling systems, the efficiency of methanol cells is expected to decrease the performance

difference with hydrogen cells. However, even with these improvements, it is not yet expected

that the life expectancy of methano1 cells will match that of the hydrogen cells. A summary of

this comparison is given in Table 1.1.

Preidel et ai. (1998) performed a system mass comparison between hydrogen cells and

direct metbanol cells for transport vehicles. It was assumed that both systems had similar system

efficiencies of 40 %. Both systems required 55 kg of methanol h e l with a refueling range of 400

km, and both were capable of 20 kW at 65 km/hr. They further assumed the power density of the

DMFC stack was one-fifth that of HJair stacks. Consequently, the DMFC stack required five

times the active catalyst surface area as the hydrogen stack. The stack mass for the H,/air system

was approximated to be 20 kg and 100 kg for the methanol stack.

Table 1.1 - Operational Characteristics and Technological Status of HJair and

direct methanol PEM fuel cells.

Cornparison

Operating Temperature (OC)

Choice of Fuel

1 Oxidant

1 Fuel efficiency (%)

1 Lifetime (continuous years)

1 Projected Capital Costs ($/kW)

HJO* PEM FC

60- 100'

1. hydrogen 2. reforrned hydro- carbons or alcohols

O?, air

45

A 6

Direct Methanol PEM FC

80- 140

methanol

O?, air

N/A

30

> 1.2

>200

Schmidt et al., 1994

The direct methanol system made up its mass losses when the penpherai equipment of the system

was taken into account. Without the need for reformers, high pressure stearn lines, pumps,

thermal and fuel management systems, it was estimated that the direct methanol system would

require 100 kg of peripherd equipment. However, the peripheral reforming equipment needed

by the hydrogen system had a mass of 180 kg. When the two systems were compared with their

peripherd equipment, both had a total system mass of 255 kg. A summary of this data is

presented in Table 1.2

Table 1.2 - A mass cornparison between the HJair and direct rnethanol PEM cells. The final mass of each system (assurning sirnilar system reliability) was similar (Preidel et al., 1998).

1 Required Mass 1 H J O l (kg) 1 Direct Methanol (kg)

Al1 else being equal, direct methanol may be an alternative to hydrogen cells. Further

catalyst and membrane development is required to increase the efficiency of direct methanol

PEM fuel cells so that this technology may compete with hydrogen cells. Hereafter, direct

methanol PEM fuel cells wiII be referred to as direct rnethanol fuel ce11 (DMFC), the methanol

cell, or fuel ce11 (FC), unless otherwise specified,

1.2. Aim

The aim of this thesis was to design and fabricate a direct methanol proton exchange

membrane fuel ce11 (DMPEMFC) system and accompanying test station, as well as to determine

the effects of channel depth and temperature on the performance of the cell. To this end, an

overview of direct methanol fuel ce11 theory is given in chapter 2. The design and fabrication of

the test station and fuel ce11 are detailed in chapters 3 and 4, and the experimental procedures

used are described in chapter 5. The experimental results are discussed in chapter 6.

Conclusions and recommendations are given in chapter 7.

Fuel

Stack

Peripherals

Total (kg)

55

20

180

255

Chapter 2 - Direct Methanol Theory

There are several similarities between the hydrogen and direct methanol electro-oxidation

technologies. The electrodes containing the anode and cathode catalysts are situated on either

side of a proton conducting solid polyrner membrane (a solid electrolyte). Together this is called

the membrane electrode assernbly (MEA) and it is the platform from which the direct electro-

oxidation of methanol occurs. The catalyst (typically platinum based) in the anodic electrode

facilitates the extraction of hydrogen ions and electrons from the fuel. The extracted protons

rnigrate through the PEM electrolyte to the oxidizing electrode, and liberated electrons from the

anode are transported through an external circuit where work is performed on a load. The

electrons then combine with the diffusing hydrogen ions in an oxygen (O2) rich Stream to

produce water at the cathode catalyst.

2.1. The Electrochemical Reaction

The concept of direct oxidation is currently limited to simple species (specifically

rnethanol) due to the lack of suitable electro-catdysts. Methanol is the only fuel with substantial

electro-activity which can be directly oxidized to CO, and water at low temperatures above

60 OC. However, this process suffers from several electrochernical losses (parasitic and

Fxadaic) which result in poor overall conversion efficiencies (Ravikumar and Shukla, 1996;

Xiaoming et al., 1997).

Direct oxidation of methanol is significantly slower than hydrogen oxidation in a H2/air

fuel cell. Direct rnethanol Fuel cells have about one-third the power densities of hydrogen PEM

ceus with current catalysts (Scott et al., 1997). Two electrons are liberated during hydrogen

oxidation, whereas, six electrons must be liberated from the fuel for complete methanol

oxidation. This results in slower oxidation kinetics for the methanol cell. The effective rate of

reaction per unit volume is increased by increasing the active surface area of the electrode. Thus,

metal catalysts are supported on high area carriers such as activated carbon (Berger, 1968).

The anodic process is given by Equation 2.1 as follows,

CH,OH + H,O -+ CO, + 6 H' aq + 6e- (2-1)

The anode potentia., E,', is 0.016 V where aqueous methanol reacts to f o m CO2, H' ions, and

electrons on the anode catalyst (Lamy and Léger, 1997). The six protons are transported through

the PEM to the cathode for the electro-reduction of oxygen. The cathodic reaction is represented

by Equation 2.2,

The electrode potential of the cathode, v, is 1.229 V, SHE. Assurning the reaction goes to

completion, one mole of reacted methanol combines with one-and-one-haif moles of oxygen to

produce one mole CO, and two moles water. The electro-oxidation of methanol is surnrnarized

by Equation 2.3,

This gives an overall DMFC reversible ce11 potential, EP, of 1.2 1 V, SHE, as compared with the

reversible potential of 1.23 V for hydrogen fuel cells (Scott et al., 1997).

2.1.1. Thermodynamics

A fuel cell converts Gibb's free energy change (AG) of an electrochemical reaction to

electricai energy. This is given by Equation 2.4,

AG = -nFE r (2.4)

where Er is the reversible ce11 potentiai, rz is the number of electrons specific to the reaction, and

F is the Faraday constant. fn operational conditions the cell potential is less due to polarization

effects. A summary of the thermodynarnic properties for the oxidation of methanol in DMFCs is

presented in Table 2.1.

Table 2.1 - Sumrnary of DMFC thermodynamic properties at standard conditions (298 K). These values assume the oxidation reactions are complete (100% utilization) (Scott et al., t 997; Lamy and Léger, 1997).

Thermodynarnic Property

AG'

AH'

2.1.2. Ce11 Losses

-

Value

-702 W/mole

-726 kT/mol

W (specific ener-v)

r,,,, (reversible)

fuel

I

6.1 kWhr/kg '

97 %

Fuel cells do not operate reversibly, even at low current densities. The terminal voltage

of the ce11 is always less than the reversible potential and decreases as current is increased. This

occurs since energy is used to overcome various resistances to current flow (Berger, 1968). The

as0 Er"

mass of the fuel

-0.08 1 kJ/mol K

1.214 V

terminal voltage of the ce11 is given by the ce11 reversible potential minus ce11 losses. This is

given by Equation 2.5,

where V is the terminal voltage, q, is the anodic polarization, qR is the ohmic loss in the

electrolyte, q. is the cathodic polarization, and %-,,,,, is polarization caused by rnethanol

crossover. 17, and 17, depend on activation energy effects, rnass transfer effects and ohmic effects

in the electrolyte or near the porous electrode (Berger, 1968). Ce11 polarization is the difference

between Er and V. The theoretical work iost is given by Equation 2.6,

Loss of work = nF(E - V) r

Ce11 polarization is sometimes expressed as a voltage efficiency, E,,, given by Equation 2.7. r , is

typically around 40 % at a working emf of 0.5 V and at 100 rnA./cm2.

There are three types of polarization. These are ohmic, activation, and concentration

polarizations. Ohrnic loss occurs within the electrolyte and electrodes of the cell. It occurs due

to the voltage gradients which drive charged ions through the electrolyte or electrons through the

electrode material. In Figure 2.1, the region of ohmic loss can be seen on the 0.5 M polarization

curve between 25 mNcrn2 and 200 mNcm2. Ohmic resistance is the resistance to mass transport

of ions or electrons being driven by this potential gradient (Berger, 1968).

(60°C, 20 PSlG 02) I -

2.0 M METHANOL _ I

-

- 4.0 hl METHANOL

- I

0.5 M METHANOL

CURRENT OENSlTY (rn~,,~cm~ 1

Figure 2.1 - The effects of varying the concentration of methanol in the feed. For each system, an optimum depends on system conditions. Both catalyst activity and methanol crossover effects can be seen. This figure was taken from Surampudi ( 1993), Figure 10.

Activation polarization occurs due to the irreversibility of the electrochemical reactions

under a current drain. As soon as a ce11 in equilibrium is connected to an extemal load, current

flows and the voltage gradient of the electrode double layer re-equilibrates such that V < Er. The

magnitude of the voltage change at the electrode depends on the rate of the electrochernicaI

reaction at equilibrium, or the exchange current density (Berger, 1968). The open circuit voltage

is reduced from EeqO as a result of rnixed potentials at the cathode caused by parallel reactions and

reaction intermediates. At the onset of activation polarization the rate of change of voltage with

current is large and levels off at higher current density. This region is clearly visible at current

densities under 25 &cm' in Figure 2.1.

Concentration polarization is the voltage loss on current drain due to the resistance to

mass transport of the reacting species. At high current densities, when a reactant is consumed

rapidly by the electrochemical reaction, there is a concentration gradient across the ce11 flow

channels. The driving force for mass transfer is the concentration gradient. As the

electrochemical reaction proceeds, the concentration of methanol at the electrode surface

decreases. This results in a decrease in ce11 potential, as compared to the potential if the bulk

methanol concentration were available. The mass transfer effect appears as a change in double

layer voltage. Concentration polarization can be seen on the 0.5 M polarization curve given in

Figure 2. i at current densities more than 200 rnA/cm2.

0.8 1 I 1 I 8 i

! (WRU, 2M CH30H 20 PSIG 02)

0.7 - a

Figure 2.2 - Effects of increasing the operating temperature. An increase in performance of the fuel ce1 occurs as the operating temperature increases. This is limited by the high temperature properties of the membrane. This figure was taken frorn Surampudi ( 1993), fig 9-

2.1-3. Temperature Effect

The effect of operating temperature on DMFC performance is shown in Figure 2.2. An

increase in ce11 performance is realized with an increase in temperature over the range from 30 OC

to 90 OC. At a potential of 0.55 V, the curent densities are 10 and 140 m ~ / c m ' at tempentures

of 30 and 90 OC, respectively. This temperature performance trend is analogous to what has been

observed in H,/air fuel cells (Lamy and Léger, 1997).

2.2. Anode

2.2.1. Pure Platinum Catalyst

Ce11 reaction kinetics are further slowed down as a result of active site blockage by

reaction intermediates, parallel processes, or impurities. Oxidation of the intermediates to CO,

requires the adsorption of oxygen containing species, such as OH' and H 2 0 . However, on a pure

platinum (Pt) catalyst the adsorption of these species does not occur substantially at potentials

under ce11 open circuit voltage (OCV). Consequently, on its own, Pt is not a good methanol

oxidation electro-catalyst (Lamy and Léger, 1997). Chan et al. (1998) described the mechanism

of electro-oxidation of methanol on a Pt anode catalyst, as given by Equations 2.8 to 2.14.

Pt + CH,OH + Pt - (CH,OH), (2-8)

Pt - (CH,OH), -+ Pt - (CH,O), + H' + e' (2-9)

Pt - (CH#), -t Pt - ( C H 2 0 ) , + H' + e- (2.10)

Pt - ( - O ) , + Pt - (CHO),, + H' + e - (2. l l )

Pt - (CHO), + Pt - (CO),, + H' + e- (2.12)

M + &O + - ( - O ) , (2.13)

P t - (CO) ,+ M - ( H 2 0 ) , + Pr+ M + C 0 + 2 H t + 2 e - (2.14)

Equation 2.14 is likely a more complicated reaction than is shown here. A more complete system

of equations is given by Sriramulu et al. ( 1999). This mechanism involves the adsorption and

oxidation of methanol on the Pt surface by a series of chemisorption reactions. Equation 2.12

shows that adsorbed CO is eventually formed on the Pt surface. This poisons the surface by

reducing the number of available active Pt sites for the electro-oxidation reaction. The adsorbed

CO then reacts with activated water to yield CO2 and two protons. The oxidation of CO to CO2

represents the slow step in the mechanism (Chan et al., 1998).

2.2. Z.I. CO Poisoning in Mydrogen Fuel Celk

Many substances cm inhibit the catdytic activity of active surfaces. Poisoning occurs

when substances strongly adsorb on the catdyst and reduce the number of active sites available

for the less strongly adsorbed reactants or reaction intermediates. As the surface is filled, the

energy of adsorption increases and inhibits further adsorption of reactant molecules (Berger,

1968).

The CO poisoning reactions in a hydrogen ce11 are as follows (Rodrigues et al., 1997),

CO causes a decrease in fuel ce11 performance by strongly chemisorbing on the electro-catalyst

surface and decreasing its activity (Raissi et al., 1997). It is the active Pt catalyst sites,

responsible for chernisorption and dissociation, that are blocked when CO is present in the anode

Stream (Rodrigues et al., 1997).

Schmidt et al. (1994) report that the voltage decrease due to CO poisoning becomes

significant at CO concentrations above 20 ppm in H2 gas. Over time, as more active Pt sites

adsorb CO, fuel ce11 performance decreases significantly until a steady-state is established. At

steady-state the concentration of Pt=CO sites relative to free Pt sites become constant and further

performance degradation is limited.

2.2.1.2. Poisoning by Reaction Intermediates and By- Products

Strong cornpetitive adsorption of reaction intermediates on the catalyst and products of

parallel processes also result in slow rates of methanol oxidation on current electro-catalysts

(Chan et al., 1998). Reaction intermediates and products of parallel processes will be referred to

as impurities for this discussion. In general, such processes may give rise to two effects. First,

like the CO poisoning effect, an impurity may chernisorb strongly on the catalyst and block off

the surface. Second, the impurity may produce an impurity current resulting in mixed potentials.

If more than one reaction occurs on the catalyst, the total current is the sum of the individual

currents. The resulting potentiai is then the intermediate between that which would exist if one

reaction were canying the current and that which would exist if the other reaction were

generating ail the current; hence the term "mixed potentid" (Berger, 1968). Actually it is the

current which is mixed, whereas the potentiai of the electrode double layer only has a single

value.

2.2.2. Bifunctionai Alloy Catalyst

The mechanism of methanol oxidation commences with the progressive dehydrogenating

of rnethanol on the Pt surface to forrn adsorbed CO or other carbon-oxygen surface species

(Equations 2.8 to 2.14). Alloying Pt with mthenium (Ru) increases the activity of the anode

catalyst for methano1 oxidation reactions over a broad range of temperatures (as compared with

pure Pt catalysts). The PkRu, alloy creates a bifunctional catalyst where Ru catalyzes the

hydroxyl fonning reaction at lower potentials by forming a surface oxide in the potential range

for methanol oxidation. This improves the reaction kinetics and helps oxidize methanol more

efficiently than the Pt mono-cataiyst.

Due to the high concentration of water in the reactant solution, the coverage of the active

surface by adsorbed water is likely to be complete. Consequently, MA. HZO may be considered as

the active site with respect to chemisorption from solution (M" represents an active surface site

on the catalyst reaction layer) (Berger, 1968)- In such aqueous solutions water must first be

displaced frorn the catalyst surface sites before the adsorption of fuel (given by Equation 2.8) can

proceed. The water displacement reaction is given by Equation 2.17.

were R is the reducing agent. In the direct methanol fuel ce11 system using a Pt catalyst (as in this

project), this becomes:

CH,OH + Pt[H,O] + Pt[CH,OH] + H,O (2.17b)

The latter steps of the methanol oxidation mechanism require water to be adsorbed ont0

the catalyst. Ru present in the catalyst promotes this water displacement reaction (which

produces oxygen species on Ru sites). Ru adsorbs OH- species more strongly at lower potentids

(0.2 V) than Pt and it favors the electro-oxidation of methanol adsorption residues to CO,

(Equations 2.13 and 2.14) (Schmidt et al., 1994). Water is absorbed on the catalyst at this later

stage as follows,

P t + H , O + P t - O H + H ' + e - (2-18)

R u + H , O + R u - O H + H ' + e - (2 .19)

where reaction 2.18 occurs at a higher potential than reaction 2.19. Water from the aqueous

solution is consumed at a rate of one mole of water per mole of methanol (Argyropoulos et al.,

1999a). Arico et al. (1996) report that at temperatures more than 95 OC a considerable portion of

Ru sites c m chernisorb OH- groups as the overpotentid slightly increases. The Ru surface also

reduces the peak potential for CO adsorbate oxidation to CO, by 200 mV, as compared with the

pure Pt catalyst. This makes the catalyst more resistant to the effects of CO poisoning (Lamy and

Léger, 1997; Scott et al., 1997; and Schmidt et al., 1994). Product CO? is also less tightly bound

to the Pt sites, making the sites more available for reaction. The overall reaction for CO

oxidation and the regeneration of active sites is given by Equation 2.20,

Chan et al. (1998) and Lamy and Léger (1997) report that a 1: 1 Pt:Ru surface concentration

(Pt&Ru,, 1 : 1 d o ) is the most effective Pt:Ru combination for methanol oxidation.

However, Gottesfeld ( 1994) reports that even with the Pt:Ru catalyst, the turnover

frequency of the oxidation of methanol is still Iess than one reaction per Pt site per second. In

comparison, he reports that the turnover frequency of a HJair Pt catdyst is about 10 reactions per

Pt site per second. There is an order of magnitude difference between the two activities which

indicates that new cataiysts must be developed that will increase the reaction kinetics of direct

methanol fuel ce11 cataiysts to levels comparable to the hydrogen cell.

2.2.3. Methanol Concentration

The PtRu catalyst provides additionai oxygen species which react with surface CO to

form CO,. Higher methanol concentration aliows more fuel to diffuse through the aqueous

solution to the catalyst surface. This increases the exchange current density of the reaction and

results in a decrease in concentration polarization. Concentration polarization occurs at high

current densities where the performance of the anode is limited by the mass transport of methanol

through the aqueous solution rather than the activity of the catalyst (Narayanan et al., 1996). In

Figure 2.1, the region of the 0.5 M polarization curve above 150 mA/crn2 where the curve drops

suddenly illustrates the concentration polarization effect. The ce11 voltage decreases as mass

transport limits the amount of methanol reaching the catalyst surface. For fuel concentrations of

2 M and 4 M, concentration polarization is not observed until higher current densities.

2.3. Cathode

As oxidant passes through the cathode it diffuses through the carbon cloth to the catalyst

layer where it is consumed. in addition to kinetic losses at the anode, cell performance can be

affected by depolarization losses at the cathode. The discussion on the anode catalyst may at first

suggest that higher current densities, at a given polarization, may be obtained by using high

methanol concentrations with the Pt:Ru catalyst. However, any potential gains from increases in

fuel concentration at the anode rnay be offset by potential losses at the cathode due to methanol

crossover and the formation of rnixed potentiais (Scott et al., 1997).

2.3.1. Cathode Catalyst

Methanol crossover occurs when unreacted methanol permeates from the anode across

the proton exchange membrane, adsorbs ont0 the cathode catalyst, reacts with 0, and creates

mixed potentials. Since the cathode Pt catalyst also provides a surface for methanol oxidation, a

reaction s i rnik to the anodic reaction takes place at the cathode. The perrneated methanol is

oxidized on the cathodic electrode, producing CO, and H,O. However, in this case no electrons

are liberated ir:to the extemal circuit and, thus, no worlc cm be done (Halpert et al., 1997). This

parailel reaction leads to parasitic loss of fuel and depolarized cell voltages which decrease ce11

efficiency. The degree to which crossed-over methanol is oxidized at the cathode catalyst is

uncertain. However, Argyropoulos et al. (1999a) c l a h that a portion of the methanol remains

unreacted and leaves the cell with the condensate and exhaust gases.

The effect of methanol crossover c m be seen in the 4 M methanol solution, Figure 2.1.

The entire polarization curve for this solution shows larger polarization losses than curves for

lower fuel concentrations. Even with methanol crossover, a cathode catalyst tolerant to methanol

would allow the use of higher methanol concentrations at the anode without promoting

depolarization effects at the cathode. Crossed-over methanol could possibly be recovered and

recycled back into the anode feed. This would increase overall ce11 fuel utilization. By

improving the activity of the methanol oxidation catalyst and reducing the activity of the cathode

catalyst to methanol oxidation, a higher fuel concentration may be used to promote methanol

oxidation without penalizing the performance through mixed potentials caused by the cathode.

Reeve et al. (1 998) report that cathode catdysts based on transition metal sulfides may

decrease voltage losses at the cathode. An increase of 100 mV was reported by using a RhRu,S,

cathode catalyst instead of a Pt catalyst (this was at a ce11 current density of 100 mA/crnL using an

aqueous solution of two molar methanol). Other catalyst alloy combinations have been reported

with varying degrees of improvement over the current Pt catalysts (Bett et al., 1998; Lamy and

Léger, 1995).

2.4. Proton Exchange Membrane

Membranes used in direct methanol Fuel cells must be chemically stable, have high

proton conductivity, low methanol and water permeability, and be able to operate at high

temperatures for long durations. The most cornrnon PEM electrolyte is ~afion", a poly-

perfluorosulfonic acid materid made by Dupont. Sulfonic acid ionomers rely on water to solvate

the protons generated by the ionization of the sulfonic acid groups (Savinell, 1993). These

membranes are widely used in H Jair and direct methanol fuel cells systems due to their high

ionic conductivities. However, they are prone to methanol crossover since they have not been

specifically designed for direct methanol applications.

2.4.1. Crossover

Methanol and water permeate across the membrane from the anode to the cathode.

Permeation of water o r methanol through a ~ a f i o n @ membrane occurs under the dnving forces of

concentration gradients, pressure gradients, and electro-osmosis (Scott et aI., 1997; Xiaoming et

al., 1997). As water crosses-over. it blocks the reaction sites on the cathode catdyst and prevents

oxidant from reaching them. The number of water molecules dragged across the membrane with

each H' ion depend on the membrane and systern temperature. The water drag coefficient in a

Nafion@ 117 membrane was found to be dependent on temperature, but independent of current

density up to 600 mA/cm2. O n average, 2.5 water molecules are dragged with each proton

(Argyropoulos et d., 1999a; Xiaoming et al., 1997).

Methanol crossover through a Nafion@' membrane can be inhibited by reducing the

concentration of the aqueous rnethanol fuel or by creating a reverse pressure differential between

the anode and cathode (the cathode at a higher pressure) (Argyropoulos et al., 1999b). In Figure

2.1, the polarization curve for the four molar methanol solution is lower than the polarization

curves for both 0.5 M and 2.0 M solutions for most of the current density range. Xiaoming et al.

( 1997) report that for specific anode conditions the methanol crossover rate through the Nafion"

1 17 membrane can nearly be eliminated at ce11 temperatures of 100 OC. However, this is done

with a 50 % reduction in power density (they do not discuss how this was accomplished).

The electrochemicd consumption of methanol on an anode catdyst and the crossover rate

are both functions of concentration, current density, and temperature. Methanol crossover losses

are most significant with high methanol concentrations and low current densities. Ravikumar

and Shukla (1996) report that crossover affects ce11 performance for fuel concentrations over two

molar. At 70 "C (and an oxidant pressure of four atmospheres) anodic polarization was reported

to decrease with methanol concentration up to 2.5 M methanol. The opposite effect was

observed for the cathodic electrode. As methanol concentration was increased frorn 0.5 to 2.5 M,

the open-circuit potential of the cathode decreased by 50 mV. This suggests that increases in fuel

concentration Iead to opposing effects at the anode and cathode.

The effects of mixed polarization from methanol crossover have also been reported to

increase with temperature more than 70 OC when using a ~afiion* 1 17 membrane. Ravikumar

and Shukla (1996) report that the ce11 rnay be operated at 70 OC and 2.5 M methanol without

significant performance degradation. However, at 95 "C they found that methanol crossover

began affecting the cathodic electrode at a fuel concentration of two molar methanol. This

suggests that the performance degradation associated with methanol crossover increases with

higher operating temperatures when using the ~ a f i o n @ 117 membrane. Such an increase may be

attributed to an increase in cathodic catalytic activity with increases in temperature (Ravikumar

and Shukla, 1996).

Thus, the two main areas which require improvement to reduce ce11 mixed potentials are

the membrane and cathode catdyst. A membrane is required that will significantly reduce or

eliminate fluid crossover, and 3 cathodic catalyst is required that will be tolerant to the presence

of methanol.

2.5. Effects of Channel Flow on Ce11 Performance

As the electrochemical reaction proceeds, reactant diffuses to the surface of the electrode

to maintain the current. This creates a concentration gradient. If the reactant is charged it can go

to the electrode surface by diffusing under the influence of a concentration gradient, migrating

under the influence of a potentid gradient, or it cm be brought in by bulk movement of the fluid

(Berger, 1968). As reactant is consumed, the surface concentration of methanol at the electrode

decreases compared to the bulk supply. This reduced concentration affects the rate of reaction

and ce11 polarization. In contrast, the concentration of reaction products increases at the electrode

until a sufficient driving force is established to give a balance of mass transfer away from the

electrode. Any increase in product concentration may also affect ce11 polarization if the reverse

electrochernical reaction (product to reactant) occurs at a significant rate (Berger, 1968). The

presence of product near the electrode surface can also dilute the concentration of fuel.

Consequently, flow distribution in the ce11 must promote peneuation of the reactant to the

catdytic surface and it must promote the quick removal of products away from the diffusion and

reaction layers.

2.5.1. Anode Stream

There are two types of flow maldistribution that may inhibit the performance of the

DMFC anode. The first is within the channel, and the second is manifold induced. In the first

case, a uniform supply of aqueous methanol fuel must reach al1 exposed areas of the MEA for the

reaction to proceed at high current densities and high efficiencies. The flow of methanol fuel to

the anode catalyst is induced by the anode reaction and the electro-osmotic transport of water and

methanol (Argyropoulos et al., 1999). However, product gas accumulation in the anode flow

channels may lead to regions where the flow of liquid is restricted. This rnay block off entire

sections of the ce11 and cause decreases in ce11 voltage (Argyropoulos et al., 1999). Manifold

induced flow maldistribution occurs at inlet and outlet ports where different fluid strearns

flowing through the inlet manifold, plate passages, and outlet manifold, experience different total

flow lengths (Arbgyropoulos et ai., 1999~).

Gas management in the anode is criticd to the performance of liquid fed cells. At the

anode, the main product of the electro-oxidation reaction is carbon dioxide. It is released into the

flow strearn as a gas. This creates a two-phase flow in the anode chamber. With higher current

densities, large amounts of product COz in the flow channels can reduce the flow area of the

aqueous fuel and, thus, penetration of the fuel to the cataiyst layer (Ara"yropoulos et al., 1999). A

large product gas residence time inside the chamber c m result in gas entrapment within the gas

diffusion layer of the electrode. This may block micro-channels of the electrode and restrict

reactants from reaching the anode catalyst, resulting in concentration polarization.

COz and aqueous methanol solution move counter-currently in the catalyst layer, the gas

diffusion layer, and the carbon cloth backing layer of the electrode. This two-phase flow

situation can create transport problems with gas bubbles moving against a liquid flow. To avoid

this, the flow of CO, and liquid fuel should be isohted into discrete paths for the two phases such

that one does not inhibit the flow of the other (Argyropoulos et al., 1999). Teflon cm be added

to the carbon cloth or gas diffusion layer to make the surfaces hydrophobie and create regions for

free gas movement. This promotes the situation where the two phases flow in discrete paths.

Argyropoulos et al. (1999) found that an electrode with a 20 wt% Teflon content on the carbon

cloth backing improved ce11 performance up to crirrent densities of 160 mA/cm2. However, they

fùrther report that Teflon rnay increase the electrical resistance of the carbon cloth.

Ce11 pressure drop also has a significant effect on fluid flow. Argyropoulos et al. (1999b)

state that pressure drop on the anode side is less affected by the temperature gradient than by

volumetric flow rate and current density. increasing the flow rate increases ce11 channel friction

losses, while increasing current density reduces overall losses since it leads to the production of

larger quantities of gas. Understanding variability in pressure drop as a function of system

parameters can help avoid problems such as anode side gas management difficulties from

insufficient gas removal and cathode side water flooding as is discussed next (Argyropoulos et

al., 1999a).

2.5.2. Cathode Stream

The flow characteristics of the air or O2 cathode strearn differ from the anode. The

porous oxygen electrode c m become saturated with water at current densities more than

100 mNcm2. Water droplets, produced as a product of the overall reaction or crossed-over from

the anode, can cause flooding of the efectrode structure (Argyropoulos et al. 1999). This may

also block the micro-channels of the electrode and impede the oxidant from reaching the catalyst.

Higher oxidant flow rates help remove vapor from the micro-channels; hence, the flow rnust be

sufficient to prevent water from flooding the cathode (Argyropoulos et al., 1999b).

2.6. Dimensional Analysis of Anode Stream Flow Characteristics

Many important engineering problems cannot be solved completely by theoretical or

mathematical methods due to their complexity. Problems of this type are especially common in

fluid-flow, heat-flow and operations involving diffusion. One method of attacking a problem for

which the mathematical equations would be intractably complex is to use dimensional analysis

(McCabe and Smith, 1967).

The Buckingham pi theorem was used to analyze the performance data of the direct

methano1 fuel ce11 by defining dimensionless groups that describe two-phase incompressible flow

in the anode Stream. It was assumed that only small diameter bubbles were forrned in the anode

Stream. Ten variables were used to identiQ the conduit flow. These are listed in Table 2.2. This

system consisted of ten variables, three core variables, and seven independent dimensionless

groups (n,) (Welty et al., 1984; Brodkey and Hershey, 1988). Depending on the core variables

chosen, different parameters were obtained. The system was partially described by the Euler

number, the inverse of Reynolds number (based on the conduit length), inverse of the Weber

number, inverse of the Froude number, and the relative roughness factor when L, u, and p were

chosen as core variables.

Table 2.2 - Variables used for the dimensional analysis of the anode channel. Ten variables were identified to describe the anode flow system. The variables, symbols, SI units, and dimensions used in the dimensional analysis are listed. Seven dimensionless parameters were obtained to describe this two-phase conduit flow system. M., L and t represent the dimensions of mass, length and time in this section,

Conduit Equivalent Diarneter m L

Conduit Length m L

Variable

FIuid Velocity

Pressure drop

Density (liquid phase)

Viscosity (Iiquid phase)

1 Conduit Wall Roughness 1 m L

u m s-' L t-'

AP kg m-' $2 M. L - 1 t-'

P kg rn-' M. L-j

Ci kg m" S.' M. L-' te'

Density (gas phase)

1 Surface tension I o kg s-' M. t-'

Po. kg ,Tl'J M. L-3

Gravi ty D O m s" L t-'

Another solution (with D,, u, and p as core variables) resulted in the system being partially

described by the inverse of Reynolds number (based on the equivalent diarneter of the conduit),

the Euler number, and the relative roughness factor. The seven dimensionless groups found to

describe ce11 flow characteristics (from the latter set of core variables) were found to be:

1. inverse of Reynolds number (Re),

2. the Euler number (Eu),

3. relative roughness factor (e),

4. the inverse of length to diarneter ratio,

5. dimensionless parameter similar to the invers'

replacing L),

e Froude number (Fr) (with D

6. parameter similar to the inverse of the Weber number (We) (with D replacing L),

7. two-phase fluid density ratio,

The analysis assurned the fluid Stream was at steady-state. No reactions are considered.

Chapter 3 - Test Station

The design and construction of a direct methanol PEM fuel ce11 and the test station are

discussed in the next two chapters. Chapter 3 focuses on the requirements, design and

fabrication of the test station.

3.1. System Overview

The test station was designed to control, measure, and record the desired operating

conditions and performance characteristics of the fuel cell. It was constructed to provide

flexibility for a wide range of operating conditions and experimental objectives. The design

provided the following capabilities and features:

1. temperatures up to 200°C and gauge pressures up to 690 kPa (100 psig)

2. chernical tolerance to dilute aqueous methanol fuel

3. relatively small fuel and oxidant system resident times

4. abiIity to visually inspect key fuel delivery points

5. electrical insulation of the fuel ce11 from the test station and environment

6 . safe operation

7. constant operating conditions

8. gas chromatopph analysis of the fuel supply and anode exhaust

These capabilities will be discussed in the following sections. A picture of the test station is

given as Figure 3.1.

HPLC puiip Mcdiaiial Flask Oven

Figure 3.1 - Picturc of the Direct Methanol PEM Fuel Cell test bed. The teiiipcrüture coiitmlled oven is sliowii on the right side and the saniple collection glasswiirc ports can be seen in the center. Tlic HPLC puinp and ineiliaiiol siorage vesse1 are sliown on tlie left. The ceIl bypüss caii he seen as a 'u' coiniiig out of the oven.

3.2. System Tubing

The bulk of the tubinp and wetted hardware of the test station was made of annealed 304

stainless steel ASTM A269. Care was taken to exclude brass fittings, especially upstrearn of the

anode, to avoid contarninating the catalyst with copper ions. Berger (1968) claims that cornmon

metal ions having five or more d-electrons act as poisons for Pt catalysts. Copper (CU") ions fa11

in this category and, thus, alloys containing copper were avoided upstream of the catalysts.

Based on the minimum fuel flow rate of 0.7 ml/rnin, 3.18 mm (one eight inch) tubing was

setected to minimize system volume and fuel residence time in the inlet anode strearn. This

provided fast response to changes in operating conditions. Since the oxidant was a gas with

higher flow rates than the anode strearn (up to 500 sccm), system response was fast enough using

6.350 mm tubinp instead of the smaller diarneter tubing.

Heating tape was used to preheat the fuel and oxidant prior to entering the oven. The

temperature was controlled using potentiometers. Because of the high thermal conductivity of

the tubing, heating tape was used as close to the oven inlet as possible. This configuration

ensured that the anode and cathode streams reached the set-point operating temperature pnor to

entering the flow channels. A coi1 of tubing was used inside the oven to equilibrate the inIet feed

Stream with the temperature of the oven.

In key areas of the system it was necessary to visually inspect the fiow strearns. FEP

fluoropolymer translucent tubing (3.175 mm) was used in these regions. The tubing was

chernically inert to the reactants and products of the system. It was rated to 205 OC and over

1370 kPa. The tubing was used downstream and upstream of the fuel ce11 anode chamber.

Upstream, the translucent tubing was used to venfy that the fuel remained in liquid state and that

no degassing occurred during the preheating process. Downstream of the anode, the translucent

tubing allowed visual inspection to confirm the presence or absence of anode gaseous product.

This was useful during ce11 purging to clear the anode of large arnounts of gaseous product. As

well, FEP tubing was used in al1 inlet and exit ports of the fuel ce11 to electrically isolate the ce11

from the test station.

3.3. Pressure and Flow Components

Pressure control, flow control, and exit stream sarnpling devices were critical components

in the test station. This section will deai with the setup of these devices.

3.3.1. Pressure Control

Cathodic pressure was controlled using a gas regulator and a back-pressure metering

valve. The system was designed to accommodate both air and oxygen cathode streams. Both the

equiprnent and data acquisition system (DAS) were setup such that cathode flow and pressure

remained undisturbed during mid-experiment oxidant or tank changes. The oxidant tanks were

placed away from the oven, directly across from the oxidant and fuel back pressure valves and

pressure gauges. This setup allowed the operator to steadily increase system pressure under

controlled conditions. Needle valves, which were located on the exit streams of the anode and

cathode, maintained back-pressure in the cell.

During pre-expenmental runs it was obsewed that the pressurized aqueous methanol

solution degassed when heated. Modifications were made to the feed reservoir to degas the fuel

solution prior to being pumped into the system. Helium gas was bubbled through the fuel storage

tank using a frit and the fuel storage vesse1 was preheated to between 25 and 27 OC to promote

degassing before the fuel entered the inlet stream. A magnetic stirrer was used in the storage

tank to prornote circulation of the helium in the heated fuel. Fuel degassing was no longer

observed after these modifications.

Since the HPLC reciprocating purnp rnaintained a constant fuel flow rate to the anode,

there was a tendency for pressure build-up as gaseous product was produced in the anode

charnber. Constant adjustments to the anode back pressure valve were needed to maintain anode

pressure. At times the systern experïenced pressures above the set-point in excess of 35 kPa.

Since too large an anode/cathode pressure difference would physically deform or rupture the

MEA, a proportionai pressure relief valve was used upstrearn of the anode. As the anode

pressure exceeded the set-point, the relief valve opened proportionally. The higher the pressures

exceeded the set-point, the more the valve opened to relieve the pressures. The valve was set

approximately 70 kPa (10 psi) higher than the required anode pressures, or 35 kPa higher than

the cathode pressure. The relief valve was activated on numerous occasions during anode

runaway pressures.

A pulse dampener was used on the anode inlet strearn. Pump pulses were undesirable due

to possible MEA damage and reduced ability for the system to reach or maintain steady-state.

However, even though the HPLC pump was designed to minimize pulses, small pulses caused 2

noticeable effect on the ce11 interna1 resistance.

3.3.2. Ce11 Bypass

A fuel ceIl bypass was designed to redirect fuel flow from the anode inlet directly to the

anode outlet Stream. This was done for several reasons. First, it allowed the anode fuel systern

to reach steady-state conditions pnor to bnnging the fuel cell online. If the rnethanol

concentration needed to be changed, the bypass allowed the heater and darnping systems to be

purged of the previous fuel concentrations. It dso allowed the fuel to be preheated to the desired

temperature prior to being fed into the cell. In case the pump (or the system) needed to be purged

of trapped gas, the bypass dlowed the fuel ce11 to maintain pressure while the system was

purged. As a result, significant experimental time was saved by not having to shut down and

restart the ce11 each time system purging was required. Finally, during the course of the

experiment or between experiments, the bypass allowed the ce11 to be purged with helium

without affecting the fuel supply system. This allowed the ce11 to be shutdown, purged, and

restarted quickly.

3.3.3. Sarnpling Components

A sampling system was designed to collect product gas and liquid sarnples for analysis

over the course of the experiments. It was designed to allow the presence of product CO, and

water to be verified. Both anode and cathode exit strearns were sarnpled at atmospheric pressure.

Glass liquid/gas knockout cylinders were used for both anode and cathode exit streams. The

gaseous phase was sampled in 5 ml sarnpling ports equipped with septa. The liquid phase was

collected in sealed vials also equipped with septa. Using syringes, both phases could then be

sarnpled and manually injected into the GC for analysis.

Prior to each expriment, the sarnpling system was purged with helium for five minutes

to ensure the removal of products from previous mns and atmospheric gases. This procedure,

however, diluted the product concentration in the coIlection ports and made it difficult to detect

Iower concentration components. The collection ports are visible in Figure 3.1.

3.4. Electrical System Design

The test station was designed to provide varying electrical loads for the fuel cet1 so that

ce11 terminal voltage and current could be measured. The ce11 intemd resistance (h,,,), and

system load (RI,,,) and shunt (Rh,,,) resistors were connected as a series circuit. A schematic of

the circuit is shown in Figure 3.2. The resistance (R,,,,,) depicts the resistance of the entire

circuit including wiring and connectors. The rnilli-ohmmeter (R,) reads the fuel ce11 in parailel

with the system.

Figure 3.2 - Schematic diagram of the electrical circuit shows the load resistor (Rloail), the shunt res:stor (&,,,,), and the circuit wire resistance (R,,,,,). Al1 are connected in series with the fuel ce11 interna1 resistance (R,,,). Ce11 voltage and shunt voltage are both read by the multimeter through the rnultiplexor. RceIl is read by the milli-ohrnmeter. The milli-ohrnmeter (RT) will read RceIl in parallel with ail the other system resistance.

3.4.1. The Load Bank

A set of load resistors was used to provide loads for the system. Table 3.1 shows the

values of the load resistors used and their experimental accuracies.

Table 3.1 - Resistor vaiues used for the fuel cell polarization curves and their estimated accuracies. Total circuit resistance excludes the ce11 intemal resistance.

Resistor (n) Resistor Error l (9%)

Open circuit

1 short circuit 1

Shunt (Q) Total Circuit Resisbnce (Q)

Open circuit

50.2 k

254.8

106.5

24.49

10.28

5 -09

2.26

1.28

0.74

0.45

0.27

0.25

- -

Total Circuit Error (%)

The switch shown in Figure 3.2 represents the alligator clips used to connect the resistors to the

circuit. The clips were connected together to short the cell. During shon circuit the resistance of

the wires (not including the shunt, load resistor, or fuel cell) varied from 166 d to 22 1 mQ.

The average resistance was (200 t 28) ma. The alligator clips were the major source of this

variance. The quaiity of the connection varied each time the clips were attached. This effect was

35

significant only with load resistance below 2 0. The overall base-line circuit resistance (RCiKuk)

was 0.25 Q 1 O. 10.

3.5, Data Acquisition and Instmmentation

The instrumentation controlled or monitored the following pararneters,

1. ce11 oven temperature

2. anode fuel flow rate

3. anode pressure

4. anode oven inlet temperature (preheat)

5. cathode pressure

6 . cathode oven inlet temperature (preheat)

7. cathode oxidant flow rate

8. interna1 ce11 resistance

9. ce11 voltage

10. ce11 current

The data acquisition program read and recorded al1 but the first four pararneters.

3.5.1, Measurernent and Control Device Specifications

The test station maintained constant operating conditions by using a data acquisition

computer and an array of metenng and control devices, as seen in Figure 3.3. in order from the

fuel and oxidant sources to the oven, the device array included a flow metering device, a pressure

uansducer, a pressure indicator, and a RTD. These are labeled in Figure 3.3 as FT/FI, PT, PI,

and TT respectively. The RTDs measured temperatures dong the heated section of the tubing.

The fuel ce11 was maintained at a predetermined operating temperature using an oven and

a temperature controller. The flow rate of the fuel was regulated using a reciprocating piston

pump. These devices operated independently of other devices and used manuaily entered set

points. Table 3.2 surnrnarizes the devices used to regulate and monitor system pararneters (their

accuracies are shown where applicable).

RTDs, pressure transducers, the fuel pump, and the oxidant flow meter were al1 calibrated

within their experimental operating ranges at room temperature, unless otherwise specified. The

pressure transducers were calibrated to 553 kPa (80 psi& using an analogue pressure gauge. The

RTDs were calibrated between O and 98 OC using a heated water tank and a themorneter. Linear

regression was used to calculate the operating coefficients and intercepts of al1 the above devices.

The HPLC pump was calibrated at 3 10 kPa (45 psig) using an intemal calibration prograrn

supplied by the manufacturer.

Al1 parameters, except for system temperatures, were recorded as voltages. The outputs

of the transducers and RTDs were O to 5 V (dc) and O to 100 respectively. Ce11 voltage was

read directly and the current was read as a voltage across a current shunt. Signals were read by a

Fluke digital multimeter (Model 8840A) through a sixteen channel mu1 tiptexer. The multiplexer

was equipped with a digital input decoder, and each channel was equipped with a reed relay. The

multirneter converted the analogue signal into a digital signal, which was readable by a

Multifunction IEEE-488 card (Model PCC 848 A / ' by Avantech). The accuracy of the FIuke

rnultimeter is given in Table 3.3.

Table 3.2 - Specifications of instrumentation used to regdate and monitor the test

station parameters and fluid streams. Flow meter readings of up to ten sccm were given errors of I 6 sccm due to gas expansion caused by heating of the cathode

inlet. Al1 devices were calibrated and perfomed to specifications during the mns.

Device used Accuracy Notes I Moni tored Parameter

Pressure ( AnodeKat hode)

a) Omega Thin film pressure transducer, 0-690 kPa b) Winters Analogue gauge, 0-690 kPa

+ 0.4 % of full scale

Methanol resistant, read by the data acquisition system Analogue reading 2 1.0 % of

full scaie

Temperature (inlet strearns)

Omega Resistance Thermal Device (RTD) Probe

Methanol resistant, read by the data acquisition system

Oven set point temperature

Omega Autotune P D controller Model CN76000

Manually set

ISCO Inc: HPLC reciprocating pump Mode1 2350

1 1 % at 22 OC, 2.00 ml/min, 13.8 MPa

Manually set and caiibrated at 0.700 d m i n , 3 10 kPa and 22 OC.

pp

Sierra: Top-Trak flow meter Series 820

Cathode oxidant flow rate

c 2 % of full scale

Range: O to 250 sccm Read by data acquisition system

Ce11 intemal resistance

Hewlett Packard milli-ohmmeter Model 4328A AC 1000 Hz

i 2 % of full scale

Range: O to 30 rnn 4-terminal measurement Read by data acquisition system

Fuel and product Stream composition

Hewlett Packard Gas C hromatograph (GC) and integrator Model 5890A Poropak-N column

- -

Detectable components: O,, CO1, CO, methanol, and water

Table 3 3 - Accuracies for the Fluke multimeter are given for voltage and

resistance (within 90 days of a meter calibration). The multimeter was set to read at a normal speed with automatic ranging. The accuracy of the Fluke multimeter musc be added to the device accuracies given in Table 3.2. The input resistance of the multimeter is 2 100,000 MQ. These specifications were obtained from the

product manual.

1 Fluke Multimeter (Mode1 8840 A) Accuracy 1 -

DC Voltage (200 mV range)

A QuickBasicmf program displayed the digital signal on the DAS computer screen and recorded

it to a data file.

Current from the cell could not be directly read through the multiplexer. It was first

converted into a voltage using a shunt and then read by the muhimeter through the multiplexer.

The voltage was then mathematically converted into a current for data recording. Two shunts

were used for the voltage to current conversion. A 5.0 Cl shunt was used for resistors ranging

from 102.1 Q to 50 kL2 to open circuit. A 50 m a shunt was also used for the rest of the Ioad

resistors ranging from short circuit to 25 Q, as shown by Table 3.1. At higher resistance and

lower current, the larger shunt was required to provide a larger voltage signal such that the

multimeter could read the current with satisfactory resolution, as dictated by Ohm's law.

The resolution of the multimeter for the 200 mV range was +- 0.001 mV plus 0.003% of

the reading (or 5% digits). The DAS recorded as low as 0.002 mA, or 0.000 1 rnNcm', based on

a resolution of + 0.01 mV when using a 5.0 Q shunt for low currents. With the 0.050 $2 shunt

i 0.007 % + 4 counts of reading l DC Voltage ( 2 V range)

Resistance (Q)

I 0.004 % + 3 counts of reading

I 0.004 % + 3 counts of reading

O 2 4 6 8 10 12

Tirne (minutes)

Figure 3.4 - A fuel ce11 current density versus time graph shows the resolution of the muItimeter. The data is taken from a preliminary experiment during unsteady-state conditions; hence, the upward trend seen in the graph should be disregarded. The resolution of the Fluke multimeter can be seen by the stepping pattern created (a 50 rnn shunt is used). The stepping occurs at 10 pA since the active area of the fuel ce11 is 19.05 cmZ. At higher resistance this effect is amplified.

the resolution was I 0.02 mA, or t 0.00 1 rn~fcm'. The precision of the multimeter current

reading can be seen in Figure 3.4 where the resolution of the reading can be seen to be 10 PA,

using a 35 Q load resistor. This gave a precision of 20 PA, also seen in Figure 3.4.

In summary, the resolution of the multimeter using the 5.0 R shunt was limited by the

number of digits recorded by the DAS program, whereas, the resolution of the multimeter using

the 0.05 SZ shunt was limited by the accuracy of the Fluke multimeter. System noise became

pronounced as the current approached zero at open circuit. Figure 3.5 illustrates this for the

50 mSZ shunt. Background noise is evident along with the resolution of the system.

O 5 1 O 15 20 25

Time (minutes)

Figure 3 5 - Impact of background voltage noise on current density measurements is shown at open circuit conditions. System resolution is shown by the distinct steps in the readings. Theoretically, ceil current should approach zero at open circuit. Using a 50.0 WZ resistor the background noise is reduced substantially.

35.2, Data Acquisition Program

The program used for the DAS was written in Microsoft QuickBasic" versi on 1.1. It

cycled through the monitored parameters via a multiplexer. Al1 data were time stamped,

displayed on a user interface screen, and stored as a data file. The program was designed to be

user friendly and utilized function keys to change the program parameters. The program

allowed the input of the following system variabIes,

1. load resistance

2. shunt type

3. oxidant type (oxygen or air)

4. oven temperature

5. file narne

6. oxidant and fuel stoichiometric ratios

7. graphing pararneters.

Using the collected data and specified stoichiornetric ratios, the prograrn calculated the required

oxidant and fuel flow rates, current and power densities and the elapsed time. The program also

recorded time starnped user cornrnents.

The program stored up to two hours of ce11 voltage and current information in the DAS

RAM for plotting purposes as data was read from the test station. Voltage or current plots were

updated on-screen with respect to time. The time domain of the screen plot was modifiable from

five minutes to two hours and the plot range was modifiable to suit expenmental conditions.

This allowed the user to examine data over a long enough period to determine whether steady-

state had been achieved. The graphic display also served as a diagnostic tool for system

troubleshooting. The program included alarrns for several parameters. An audio and visual

darm was activated if anode pressure varied more than 5 % from the set point (3 10 kPa).

The method used for data sampling was easily modifiable through the prograrn. The

number of parameter readings the prograrn averaged for each data point was a system variable

defined by the user. During an average time period of three seconds, the program was set to take

six readings per data point from the multiplexer. Al1 prograrn features were integrated into a

graphic user interface.

Chapter 4 - Fuel Ce11 Apparatus

The direct methmol PEM fuel cells used in this project were designed and fabricated with

the intent of initiating direct methanol PEM fuel cell research at RMC. The objectives were to:

1. verify that the direct methanol fuel ce11 was capable of sustained power

production

2. identify the main reaction products in the anode exhaust

3. confirm that the results were reproducible

4. demonstrate that system performance was comparable to that reported in the

published literature

5. provide recornmendations for future work-

This chapter will first discuss the design and fabrication of the high temperaturehigh pressure

graphite fuel ce11 and then the low temperatureflow pressure ricrylic fuel ce11 will be descnbed.

4.1. Graphite Fuel Cell Design and Fabrication Overview

The design of the high temperaturehigh pressure fuel ce11 revolved ciround a number of

project objectives inc tuding,

1. in-house manufacturing

2. manufacturing accuracy and precision (where required)

3. the use of off-the-shelf components as much as possible

4. simple and reproducible design

5. quick and repeatable ce11 assembly and disassembly with minimal ce11 damage

6. reliable operation and sealing

The main components of a direct methanol PEM fuel ce11 are as follows:

1. MEA

2. graphite flow distribution and current collection plates

3. current collectors

4. clarnping system

5. fuel and oxidant inletIoutlet ports

6. seding system

Several concepts for the fuel ce11 design were developed on IdeasrM, a computer aided design

(CAD) program. The final design is shown in Figure 4.1. The following sections will discuss

the design and manufacturing of the different components. The fuel ce11 (excluding the clarnping

system and fluid ports) will be referred to as the cell.

4.2. Membrane Electrode Assembly (MEA)

Since qudity control was important, a commercial E-TEK hc. MEA was used. The

MEA was mechanically assernbled (not hot pressed) and had a thickness of (0.88 + 0.05) mm. It

was cornposed of a 100.0 cm' Dupont ~ a f i o n @ 1 17 poly-pertluorosolfonic acid membrane

(0.177 mm dry thickness), and two 30.25 cm' ELAT carbon cloth electrodes centered on the

membrane. Excess ~ a f i o n " 1 17 was used in the MEA as a sealing gasket. Table 4.1 summarizes

specifications of the electrodes. The MEA is shown in Figure 4.2.

\-, Excess Nation membrane 'gasket' area

- Electrode

Figure 4 3 - Picture of the MEA. The 30.25 cm' anodic electrode is centered on a 100.0 cm' Dupont Nafion" 117 poly- perfluorosulfonic acid membrane. The catalyst is impregnated on a Vulcan XC-72 support and applied to a single-sided carbon cloth. The excess Nafion 117 in the MEA is used as a gasket to seal the cell. The MEA was fabricated by E-TEK Inc.

Table 4.1 - Direct methanol PEM fuel ce11 electrode and catalyst specifications. The electrodes were manufactured and supplied by E-TEK Corp.

Specifica tions 1 Anode

Catal yst

Loading 1 2.0 mg/cm'

~eo tne t r i c Surface Area 1 25.0 cm'

Cathode -.

Pt Black

4.0 mg/cm2

25.0 cm'

The Pt:Ru ( 1: 1 d o ) anode catalyst was deposited on Vulcan XC-72 carbon, which was

mixed with Tefion emulsion into a paste, and then deposited on carbon cloth. The oxygen

electrode was impregnated with a Pt black catalyst. Both electrodes were cornrnercially

manufactured.

4.3. Graphite Plates and Flow Channel Design

The ce11 was designed to rninirnize ceil interna1 resi stan ce (R,,,,). It was necessary to

a good electrical conductor for the flow chaniiel plates. Graphite (supplied by Poco Graphite

Inc.) met this criterion. Specifications of the graphite plates are given in Table 4.2.

Table 4.2 - Specifications for Poco graphite plates AXF-5QC with resin impregnation.

The graphite plates were fabricated with a zinc phosphate resin which prevented fluid

from penetrating the pore structure of the graphite during storage and operation. The resin

AXF-SQC Graphite Specifications

limited the maximum operating temperature of the plate to 200 O C . The electrical resistivity of

the plates was 250 pR.cm and their compressive strength was much higher than the required

1 MPa. Because of the small particle diameter of five microns, the plates were able to be

Dimensions (cm)

Particle size (microns)

Total porosity

Density (g/cm3)

Compressive strength (MPa)

EIectrical resistivity ( p i k r n )

10.16 x 10.16 x 0.95

5

negligible

3.12

2 10

250

machined to high tolerances. It was possible to mil1 three millimeter deep channels separated by

0.25 mm wide ribs (lands).

4.3.1. Flow Field Requirernents

Several flow channel configurations were considered for the cell. To simplify futare

modeling it was decided to use a flow field design that would approximate plug flow behavior.

Criteria considered for designing the flow fields were as follows:

1. reactant/electrode contact

2. reactant flow and distribution

3. product removal

4. graphite plate/electrode contact

5. wetted MEA surface area

The minimum rib thickness required to support the required 1 MPa (gauge) clarnping

pressure was 0.075 cm for a channel depth of up to 0.500 cm.

4.3.2. Flow Field Configuration

Vertical channels were designed to allow removal of CO, bubbles from the anode flow

field using buoyancy and entrainment. It was expected that shallower channels would facilitate

product removal from the chambers due to greater fluid velocity and Reynolds number. The

increase in fuel flow was expected to dislodge the bubbles into the fluid strearn as the Reynolds

number increased.

Since the anode produced paseous product, the anode flow field was designed such that

fuel would enter from the bottom and exit the top of the ce11 dong with gaseous product. Since

liquid product formed at the cathode, oxidant entered from the top of the ce11 and exited from the

bottom. This promoted liquid in the cathode to drain with the help of grüvity and flow from the

gaseous Stream.

Figures 4.3 (a) and 4.3 (b) show two flow field designs which were considered.

2.38 1 mm (3/32 inch) wide channels were used in both designs. This first design was rejected

for two reasons. First, it would be more difficult to mode1 than the second design. Secondly, the

horizontal portions of the channels would trap product bas and liquid effluent. The second

design, Figure 4.3 (b), addressed these concerns. The manifolds have a number of discrete

openings in the sidewalls (known as laterals) through which the fluid enters the flow fields. The

buoyancy of CO2 in aqueous solution and the upward flow of fuel was expected to remove

product CO2 into the exhaust manifolds. Water was expected to percolate down from the

electrode into the exhaust manifold on the cathode. An increase in fluid velocity in the exhaust

manifolds was expected to carry away the products into the ce11 outlets. This channel design was

selected. The 'L' shape of the inlet and exhaust manifolds was designed to adapt this fiow field

configuration to the other designed and manufactured fuel ceil components that were based on

designs similar to Figure 4.3 (a). It also provided a volume for exiting product to coalesce and

exit. Of the total 25 cm' channe1 cross-sectionai area, the total wetted surface area of the flow

field was 19.05 -c 0.01 cm'. This excludes the cross-sectionai area of the channel ribs. The

channel design was sirnilar to the one employed by Argyropoulos et al. ( 1999). The cross-

sectional area of each of the manifold channels was 1.65 * 0.0 1 cm'.

Figure 4.3 (a, b) - Designs A and B are isometric views of the negatives for two flow fields considered for the fuel cell. The isometric view was chosen to represent the flow channels such that three diiiiensional properties of the channels could better be viewed. The inlet and exhaust iiianifolds for both designs are parallel to the Iiorizontül plane iiiid the flow channels in both cases are al1 vertical. Both the inlet and exit ports of the designs are shown as cylinders beneatti the flow fields. Design B was chosen so that i t could be riiodeled as a plug flow reactor. Design B was expected to facilitate [lie renioval of liquid and gaseous products. The original design of the fuel cell accoini~iodated the port placeiiient shown in design A. The ports of design B were created to inatch the original design.

4.3.3. Channel Depth

A single cathode graphite plate was fabricated with a 1.19 1 mm channel depth. Three

anodic plates were fabricated with channel depths of 1.19 1 mm, 2.38 1 mm, and 4.763 mm

(shallow, medium, and deep). For a given flow rate of fuel this would result in a ratio of

Reynolds number of approximately 2.2: 1.6: 1 .O, respectively.

4.3.4. Machining of Graphite Plates

Accuracy and precision were required for the design and rnilling of the flow fields. If the

anode and cathode flow channels did not line up, an increase in ce11 internd resistance and a

reduction in the utilized active area of the MEA would occur. As a result, a Computer Aided

Design and Computer Aided Machining (CADKAM) system was employed for the milling of

the plates. Severai flow field designs were developed using the ldeasrM CAD program.

The design of the graphite plate flow channels was translated to a CAM system for

production. The CAM system used a three axis Computerized Numerical Control (CNC)

Matsuura MC-5ûûV machining center with a Fanuc controller. This system was capable of

milling to an accuracy of 0.005 mm. A vacuum unit on the cutting table was used to collect

graphite dust produced from the graphite milling process. This ensured that graphite dust did not

darnage the rnilling machinery and that it did not become airborne.

Operational specifications used for the milling process are listed in Table 4.3. A PutnarnB

2.38 1 mm (3/32 inch) bail-nose Mitee-Mite end-mil1 was used to cut filleted flow channels in the

plate. This avoided sharp corners on the floor of the flow channel and helped disperse the

compressive forces created by the clarnping system. A rnilling bit speed of 3500 r.p.m. and a

feed rate of 200 mmlmin was used to cut the channels. A shallow cutting depth of 0.298 mm

was used to reduce plate chatter and tool flexing. As a result, several passes of the cutting tool

were required for each flow channel depth. Approximately twenty minutes were required to cut a

1.19 1 mm (3/64 inch) depth channel. The CNC instruction codes for the cathode plate were

'mirrored' to mil1 the opposing anode plates.

Table 4.3 - CNC system and tool specifications used to cut the graphite plate flow

channels.

CNC Machine and End-Mill Specifications

Cutting feed rate (cm/min)

1 Cutter speed (rpm) 1 - - - pp --

Cutting depth (mm)

1 CNC Machine accuracy (mm) 1 End rnill 2.38 1 mm (3/32 inch)

ball-nose

Regular length two flute

1 Flat rearning mil1 1 3.175mrn(1/8inch)

Each plate was rearned with two 3.175 m (one-eighth inch) aligning holes, shown in

Figure 4.4. The holes were primarily designed to accommodate steel aligning pins during ce11

assembly. However, during the manufacturing process the holes were used to precisely place the

graphite plates on the milling bed. This ensured that the cathode and anode plates were perfect

rnirror images and, if necessary, that the plates could be re-cut accurately.

Figure 4.4 - The cathode assembly is shown in its operational orientation, tilted at a forty-five degree angle. The top right aiigning pin is shown inserted in the assembly as is the case during ce11 assembly or disassembly. The inlet and exit channel ports can be seen in the flow design. In this case, the top port is the oxidant inlet and the bottom port is the outlet for the products and unreacted oxidant. The area of the current collecter covers the entire back-side of the graphite plate to rninirnize component electncal resistance and its tabs are used to connect the ceIl to external circuits.

4.4. Current Collectors

Although the graphite plates were good current conductors, it was difficult to wire them

to collect current and read voltages without complicating the design and requiring additional

miIIing. Instead, busplate current collectors were used as ce11 electncal connections. Material

for the current collectors was selected on the basis of electrical conductivity, resistance to

forrning oxidized passive layers, and corrosion resistance to aqueous methanol. Resistance to

forming passivating surface Iayers was needed to minirnize ce11 intemal resistance. As a result,

molybdenum (Mo) foi1 was chosen for the current collectors.

Good electrical contact between the current collectors and graphite plates was of

paramount importance to minirnize ce11 intemd resistance. Maximum surface area contact

between the graphite plates and busplate was achieved by using the design shown in Figure 4.4.

The current collectors (cut from a single piece of molybdenum) were in continuous contact with

the back-face of the graphite plates. Two holes were drilled in the current collectors to

accommodate the inlet and outlet ports. The holes were ptaced such that the current collectors

were not in contact with the ports. Each busplate was designed with two electncal connection

tabs to connect the ce11 to the electrical circuit. Devices to measure ce11 voltage and intemal

resistance were connected to these ports as well.

4.5. Clamping system

A fuel ce11 clarnping system, capable of delivering greater compressive pressure than the

ce11 interna1 fluid pressure, was required to sed the cell. The clarnping system was designed to

provide constant and even pressure over the area of the graphite plates and MEA to ensure good

ce11 sealing. This, in turn, provided good electrical contact between the graphite pIate/MEA

interface and the graphite plates/current collecter interfaces. The clamping system also

electrically insulated the cell from the surroundings. The through-bolt construction also provided

aiignment for the graphite plates during assembly and disassembly.

The clarnping system included two

GPO-3 glass polyester laminates fiberglass

boards, two 606 i -T6 alurninum plates and Fibcrgiass board

eight 6.53 mm hardened steel bolts, shown C d port housing

Figure 4.5 - Schematic of the clamping system. The clarnping system of the fuel cell consists of two GPO-3 fiberglass (glass polyester laminates) boards, two 606 1-T6 aluminum plates and eight 6.53 mm hardened steel bolts. The system provides system rigidity and distributes clamp pressure across the plate surface/electrode interface.

in Figure 4.5. Specifications for each

component are given in Table 4.4. This

setup ensured an even pressure distribution

across the graphite plate and MEA. The

fiberglass boards served three functions.

First, they electricaily insulated the ce11 from

the remaining clarnping system and the

environment, they provided ngid support for

the graphite plates, and they housed the inlet

and outlet fluid ports.

The aluminum back-plates

distributed the clarnping forces evenly across the fiberglass boards and graphite plates. They also

provided support and housing for the bolts. Overall, the clamping system was capable of

delivering more than 1 .O MPa (gauge) to seai the cell. The bolts had 7.9 threaddcm (20.0

threadslinch) and were 5.60 cm long.

Table 4.4 - Material specifications for the clarnping system.

Clamping System Specifications

Aluminum plates ' Dimensions (cm) Tensile Strength (MPa)

Fiberglass boards Dimensions (cm)

Electricai Resistance (Q) Compressive S trength (MPa)

Tensile Strength (MPa) Maximum Temperature (OC) 1

-- - - - - - - --

Type 606 1 -T6, T65 1 ~lurninum' 13.0 x 13.0 x 0.6

3 I O

4.6, Ce11 Ports

Steei bolts Diameter (cm)

Length (cm) Threading (threadkm)

Swagelok Cajon VCO fittings were rnodified to adapt the O-ring face seals to Swagelok

fittings (Quick-connects). Threaded tubing was welded ont0 the glands to provide a good seal

between the cell gland and test station tubing. The inner diameter of the ports was 4.57 mm.

This helped in heating the fuel to oven temperature by increasing residence time in the ce11 inlet

ports.

The fibreglass plates were counterbored to accept the VCO fitting such that the sealing

face was flush with the surface. The O-ring face seai and the busplate spacing provided electricd

insulation between the gland and the graphite plate.

Hardened ailen-head cap screw 0.64

5.60 20.0

' Oberg and Jones, 1992. § Metal Handbook, 1985.

4.7. Fuel Ce11 Sealing

The fuel ce11 was designed to withstand pressures up to 480 kPa. This was lirnited by the

graphite/MEA seal at the set clarnping pressures and the compressible strength of the membrane.

The fuel ce11 was designed to withstand temperatures up to 120 OC. Beyond this point the

Buna-N O-rings and fiberglass board would degrade. The highest temperature the ce11 was tested

at was 1 I O OC.

Sealing of the fuel cet1 was important for three reasons. It was needed to ensure that the

measured oxidant and fuel flows were actually flowing through the ceIl, to prevent products from

leaking, and to maintain system pressure. Factors considered in the design and selection of

sedant materials were,

1. thermal stability

2, mechanical strength

3. chemicai resistivity to methanol

4. fomability

5. maximum allowable membrane compression

6. good electrode and graphite plate contact

Critical areas which required a good seal were the MENgraphite plate interface and the ce11 exit

ports.

4-7.1- MENGraphite Plate Interface

Good sealing between the MEA and the graphite plate was difficult to obtain. The

sealing medium had to be pliable and had to take up plate surface imperfections and variations in

membrane thickness during dry and wet operational conditions. It had to be electrically

insulating, chemically resistant to an acid medium and capable of withstanding temperatures

above 100 OC.

Severai sealing materials were considered. These included 1.59 mm thick Buna-N sheets

and 0.40 mm thick Nitrile bound Blue-Guard 3000 gaskets (made by Garlock hc.). Buna-N and

nitrile elastomers both have excellent chernical resistivity to methanol solvents (Parker Seal Inc.,

1992). However, due to the thicknesses that these materiais were available in, the plates would

have required additionai milling to provide good MEA/plate contact (shown in Figure 4.6). This

would have cornplicated the fabrication process.

A (O. 10 t 0.01) mm thick Teflon" P T E (polytetnfluoroethylene) sheet made by Dupont

was the alternative gasket material. This sheet acted as a spacer between the membrane and the

plate. This allowed the ce11 to be clarnped with sufficient sealing pressure without over

compressing and damaging the MEA region. Additionai milling was not required for this design.

Therefore, Teflon was selected as the MEmraphite interface sealant.

Teflon is chemically inert to most industrial chernicals and solvents, including methanol,

and it is a good electrical insulator. With a high viscosity of 0.02 Paos at 25 OC (ASTM E70), a

lateral tensile strength of 900 kPa and an elongation of 1 12 % (using ASTM method D-882), the

gasket flowed and deformed to allow for surface imperfections of the graphite plate. This aIso

compensated for variations in the thickness of the membrane dunng sealing and heating

4.7.2. PodGraphite Plate Interface

Two designs were considered to sed the fluid port and graphite plate interface. The

pnmary design used 3.18 mm thick Buna-N septa for the seal. Grooves were drilled in the back-

face of the cathode graphite plate to house the septa. The septum rested against the flat sealing

face of the port and the plate. However, this design increased graphite plate milling tirne and

reduced the maximum allowable depths of the plate flow channels.

The second design employed a Buna-N O-ring crush seal system between the ce11 ports

and the anode graphite plate. An O-ring groove was machined into the sealing face of the port

fitting. The busplate provided clearance between the graphite plate and the port face. The

clearance housed the O-ring and electrically insulated the ce11 from the port. Figure 4.7 shows the

cross-sectional view of this design.

Both designs were rated to 121 OC. The O-ring design had advantages over the septum

design since milling of each plate back-tàce was not required. This design allowed deeper fiow

channels to be milled and it simplified the manufacturing and assembly process.

Figure 4.7 - Cross-section of the sealing systein used for the ceIl ports. Section A of the anode üsseinbly ihrough the ce11 port shows the coiiiponent interfaces with the crush seal O-ring design. The inethanol inlet portlgraphiie plate interface is shown. The face of the port fiiting is flush wiih the fiberglass board and is not in direct contact with the graphite plate, the curient collcctor, or the aluminuin outer plate. The drüwing is iiot to scale.

4.8. Acrylic Fuel Cell

An acrylic fuel ce11 was

developed as part of a separate project

which primarily investigated MEA

fabrication. Setup and construction of

the acrylic fuel cell, including the

sealing mechanism, were similar to the

high temperature, high pressure

graphite ceIl described previously. Figure 4.8 - The flow field of the acrylic cell.

Experiments on the acrylic ce11 were performed using the same E-TEK MEA used in the

graphite cell. The clear flow plates of the acrylic fuel ce11 (as shown in Figure 4.8) allowed the

two-phase fluid flow in the anode and cathode chambers to be observed during ce11 operation.

Due to the materials used in its construction, the acrylic ce11 was limited to operate at near

ambient pressure and temperatures below 70 "C.

The flow field of the acrylic ce11 differed from the graphite cell. The 25 cm2 flow

configuration was based on a waffle design instead of a plug flow design. Titanium mesh

screens were used as current collectors between the MEA and the tlow channels.

Chapter 5 - Experimental

5.1 Overview

Ce11 internai resistance, performance repeatability, and MEA reusability depended on the

method of assembly. A procedure was developed to reduce MEA damage, promote good

MENelectrode contact, and ensure a good seal for each assembly.

Aligning pins were used during ce11 assembly and disassembly to align the MEA, the

Teflon gaskets, and the flow plates such that the ce11 was repeatedly assembled as intended.

Without the aligning pins, the gaskets and MEA shifted during assembly. This reduced the cell

active area and prevented proper sealing. During ce11 disassembly, aligning pins were used to

keep the anode and cathode assemblies intact (see Figure 4.4). Only the component which

required replacement or modification (MEA or gaskets) was dismantled or adjusted- The

following sections discuss ce11 assembly, start-up, and disassembly procedures.

5.2. Ce11 Assembly

The following are the steps required to assemble the cell. Both anode and cathode were

assembled using step 1.

1. Stainless steel aligning pins were used to align and hold the alurninum clarnping

plate, the fiberglass board and the current collector together. Ce11 ports were

added and water soaked O-rings were placed on the sealing faces of the inlet and

outlet ports. The graphite plate was then slid into place on the aiigning pins. The

aligning pins in the cathode assembly were pushed through the graphite plate such

that they protruded 3 mm frorn the face of the plate. In the case of the anode

plate, the pins were depressed about 3 mm from the surface of the graphite plate

to provide clearance for the cathode pins.

2. The face of the graphite plate was soaked in water and the Teflon gasket (pre-

punched with aligning holes) was digned and hand flattened. This created a

water tight seal. The Teflon gasket face was wetted and the MEA (aiso punched

with aiigning holes), anode side down, was slid through the aligning pins to rest

on the gasket and MEA flow channels.

3. Aligning holes on the anode graphite plate were then digned and slid ont0 the

protruding aligning pins in the cathode plate. Bolts were inserted through the

assembly and hand tightened.

4. A torque wrench was used to evenly tighten the eight clamp bolts up to 8.5 Nm

(75 in-lb,). The clamping system was tightened in increments of 2.8 Nm

(25 in-lb,) using a C ~ S S C ~ O S S pattern. This delivered an estimated compressive

pressure of (800 I 40) kPa (approximately 1 15 psig) to the cell. Aligning pins of

the cathode and anode were removed prior to tightening the clamp to its

maximum pressure.

A load ce11 was used to determine the compressive forces delivered to the ce11 by the clamping

bolts. From this the pressure exerted on the MEA was cdculated. It was assumed that the

compressive forces were evenly distributed dong the surface area of the graphite plates

(103 cm'). Sarnple calculations are given in Appendix A. Step three ensured the proper

alignment of the gasket and MEA on the graphite plate flow field. This tightening procedure

ensured that the ce11 was tightened flat with an even pressure distribution across the MEA.

Figure 5.1 - Picture of the assembled fuel ce11 connected to the test bed. The ceIl is inside the oven. The milli-ohrnmeter and electrical wires can be seen connected to the current collectors. Teflon tubing and the Quick- connects, connecting the ceIl to the test bed, are clearly visible. The cathode stream is located on the fat side. A blue Garlock gasket matenal was used to insulate the ce11 exit ports from the clamping system when required.

Finally, electrical connections were attached to the current collecter tabs, and ceIl ports were

connected to the test station Quick-connects. Figure 5.1 shows the ce11 assembled and

connected to the test station using the Quick-connects.

5.3. Start-Up and MEA Conditioning

Prior to beginning experimentation, the new MEA was conditioned at open circuit

voltage for 32 hours by passing one molar methanol fuel through the anode and UHP oxygen

through the cathode at 70 OC and 70 kPa (gauge). The fuel mixture was prepared using High

Pressure Liquid Chromatography (KPLC) grade methanol and distilled water. Ultra-high-purity

(UHP) grade oxygen was used as the oxidant. Minimum flow rates of 0.7 mljmin of aqueous

methanol solution and 20 sccm (gas) for 0, were used. This procedure was required to hydrate

the ~ a f i o n " membrane (Shukla et al., 1998).

After the initiai conditioning procedure, the ce11 was reconditioned for a minimum of one

hour for every subsequent ce11 start-up. Conditions used were the same as the initial conditioning

procedure. Ce11 Ieakage during reconditioning was normal. Upon the completion of the

reconditioning procedure the ce11 was Ieak checked using drops of a 50 % isopropanol and water

solution. Normaily, the ce11 re-sealed itself by the end of the reconditioning procedure.

Upon confirmation of proper ce11 sealing, fuel flow, oven temperature and system

pressures were brought up to the desired operating conditions. Anode and cathode pressures

were ramped up to operating conditions in increments of 35 kPa. Caution was taken to ensure

the pressure difference between the anode and cathode never exceeded 70 kPa. It took

approximately fifteen minutes to bring the system to the set-point conditions.

5.4. Shut-down

The ce11 was shut-down by purging the anode and cathode charnbers with high veIocity

gas (oxygen and helium for the cathode and anode respectively). Following is the procedure used

to shut-down the cell:

1. shut off oven heater and open oven door

2. shut off pump

3. open anode exhaust valve gradually

4. simultaneous1y open cathode exhaust valve gradually

5. switch fuel-mix to bypass

6. connect the heliurn Iine to the anode exhaust and purge anode of fuel-mix

7. simultaneously purge cathode with oxidant

The anode and cathode were depressurized simuttaneously. Their pressure difference was kept

within 40 kPa. To purge the anode, the helium line was connected such that high velocity helium

67

flowed from the exit port, down through the anode, and out the inlet port exhaust nipple. This

drained the anode chamber. The cathode was purged with oxidant with a flow of several liters

per minute. Both the anode and cathode were purged for ten minutes at approxirnately 140 kPa

(gauge). This procedure cleared the ceil chambers of fuel, oxidant, and products.

5.5. Baseline Interna1 Resistance

The baseline interna1 resistance of the ce11 is a measure of the simple electrical resistance

of the cell. The measurement is taken from the tabs of the anode current collector to the tabs of

the cathode current collector (excluding the MEA and its sealing materials) using the four

terminal AC Hewlett Packard rnilli-ohmmeter. It is a measure of the total eiectrical resistance of

ce11 components and electrical contacts and it is a function of the assembiy of the cell. The

baseline resistance increases if the ce11 is not assembled properly. This establishes the minimum

electrical resistance for the ce11 excluding the effects of the electrochemical reaction and the ionic

resistance of the MEA. The ce11 was assembled as per section 5.1, excluding the Teflon gaskets

and MEA. The ce11 was not connected to the test station.

5.6. Experimental Objectives

The goals of the study were to characterize the performance of the fuel ce11 at standard

operating conditions and to study the effect of anode channel depth on ce11 performance at two

temperatures. The following data were recorded for each experiment:

1. fuel ce11 operating temperature

2. anode and cathode pressure

3. anode and cathode flow rate

4. methanol concentration

5. current

6 . voltage

7. intemal resistance

8. composition of the exit stream of the anode and cathode

5.7. Measurement of Cell Performance

This study consisted of several experiments. Each experiment (dso referred to as

experimental mn) was done for a set of operating conditions where the temperature and channel

depth were varied. For each experiment the ce11 was operated at different load conditions, each

of which resulted in a single data point on its polarization curve (made up of a voltage and a

current). Each data point was the average of several points collected at that load condition, either

consecutively, or over the course of the experiment.

The MEA was initially conditioned at 70 OC for thirty hours as described in section 5.3.

Once the initial conditioning procedure was cornplete, the ce11 was shut-down for a penod of

fifteen hours prior to cornrnencing the polarization experiments. Each experiment began with the

fuel ce11 at roorn temperature and at open circuit conditions. The ce11 was re-conditioned for two

hours, as detailed in section 5.3, and brought to steady-state open voltage operating conditions

prior to commencing the data collection.

Steady-state operating conditions were defined as follows:

1. the attainment and maintenance of operating parameters to within 3 % of their

set-points over ten minutes (with the exception of the anodic pressure)

2. the attainrnent and maintenance of the anodic pressure to within 5 % of the

set-point for more than ten minutes

3. the maintenance of a steady open circuit voltage to within one millivolt for ten

minutes

4. stable interna1 resistance below 20 rr&l

Steady-state was usually attained within forty-five minutes of the completion of the

reconditioning procedure. The following are procedures used during the experimentd phase of

this project.

The fuel ce11 was maintained at steady-state conditions for a minimum of four minutes for

each load condition. Steady-state was declared achieved when the voltage varied by less than

two percent for four minutes. Stoichiometric ratios of 15 and IO0 times for the fuel and oxidant,

respectively, were maintained with respect to ce11 current density. A high stoic hiornetric oxidant

ratio was maintained on the cathode to minimize the effects of concentration polarization and to

prevent the accumulation of water on the electrode. The experimentai was designed to ver* the

effects of channel depth on the anode side; for this reason, it was desirable to minimize losses at

the cathode. A high stoichiometric ratio was used on the anode side to reduce bubble

accumulation in the anode chamber. A graph of flow rates as a function of current density is

shown as Figure 5.2. Sarnple calculations used to tabulate the graph are given in Appendix A.

O 20 40 60 80 100 120 140

Current Density (mA/crri?)

Figure 5.2 - Graph of methanol and oxygen flow rates used for the expenments. One molar methanol and WHP oxygen Flow rates are given at stoichiometric ratios of 15x and l û û x respectively. The flow rate of oxygen is given at ambient temperature and at cathodic pressures of 345 kPa, as read by the cathodic flow meter.

The load resistors used were: 50.0 kS2, 102.1 Q, 24.3 1 Q , 10.15 Q, 4.94 Q, 2.09 Q,

1 -30 Q,0.490 Q, 0.198 Q, 0.0 16 Q, and short-circuit. The order of the loads was randomized

for each experiment. For loads over 100 SZ, a 5.00 Q shunt was used to increase the voltage

signal. Otherwise a 0.05 Q shunt was used to minimize circuit resistance. A minimum of two

load conditions were repeated at least twice for each run. The open circuit voltage was taken

three times, during the start, the middle, and the end of the experirnent.

If build-up of CO1 was suspected in the channek, the anode flow rate was increased to

5.00 ml/rnin and system pressure was reduced by 70 kPa for one minute. The ceII was brought

back to steady-state operating and voltage conditions before the experiment resumed. For current

densities more than 90 m~/cm' , the cathode was purged by increasing oxidant flow for twenty

seconds between loads to flush the charnber of excess product water. This was done as required.

For each experirnent the product gas from the anode and cathode Stream was collected

and sealed in a gas sampling port. Product liquid from the anode and cathode streams was

collected and sealed in 5.0 ml glass viais. These samples were analyzed later using gas

c hromatograp hy (GC) .

The following are the procedures used between experiments,

1. the system w u depressurized and the anode and cathode compartments were

purged for ten minutes with helium and UHP oxygen, respectively. Methanol

flow was turned off and the ce11 bypass was engaged.

2. the ce11 was held at open circuit conditions for a minimum of one half hour prior

to the next experirnent

3. the cet1 was cooled to beIow 70 OC

4. if necessq, the ceil was disassembled to change the flow field plates

5. the ce11 was shut-down if the ceil was taken off-line overnight

5.8. Experimental Design

Al1 experiments for the Direct Methano1 PEM Fuel Ce11 system used a single E-TEK

MEA. Two variables were investigated: temperature and flow channel depth. The temperature

was coded as a high and low value; and the channel depth was coded into a high, medium, and

low value. Coding for the factors is shown in Table 5.1.

The study was designed with nine experimentai mm, including three replicate runs. The

expenmental runs were partially randomized in the order given by Table 5.2. The three replicate

runs were performed using the high temperature and medium channel depth conditions. Due to

the possibility of MEA damage during ceIl assembly and disassembly, the expenmental runs

were grouped such that the number of times the ce11 was taken apart was rninirnized.

Table 5.1 - Coding levels for temperature and channel depth.

Factor

Temperature

Channel Depth

Level

High

Low

Shallow

Medium

Deep

Coding

Table 5.2 - The experimental run order is shown with coded channel depth and temperature. The order was designed to rninimize ce11 assembly and disassembly

to reduce MEA damage. Repeat runs of run experiment two are identified with

l ~ Run Experiment Nurnber

Channel Depth Temperature

5.8.1. Baseline Conditions

The set-point for each of the controlled parameters is given in Table 5.3.

Table 5.3 - Set-points for the controlled parameters.

- - - - --

Experirnental Conditions

Cell Temperature

Orientation

Anode Methanol concentration

Pressure S toichiometric ratio

Flow rate (minimum) Preheat

Channel depth

Cathode: Oxidant Pressure

Stoichiometric ratio Flow rate (minimum)

Preheat Channel depth

variable orees 45 de,

1 .O molar 3 10 kPa

15 0.700 d m i n

30 OC variable

UHP Oxygen 345 kPa

100 10 sccm

50 OC 1.19 mm

The fuel was fed from the bottom port of the ce11 and products exited through the top of

the mode. The oxidant was fed from the top of the ce11 and the cathodic products were collected

from the bottom port, resutting in an anodekathode countercurrent flow. Anodic pressure was

maintained below that of the cathodic pressure.

5.8.2. Experimental Accuracy and Error Estimation

The data acquisition system was set to take averages for each raw data point recorded.

Each of the current and voltage measurements were the average of six readings. Al1 other

rneasurements were the average of three readings. Approximately two readings were taken per

second for each input. If the high and low readings for each average differed by more than

0.05 %, the program reinitialized the rnultimeter, re-read and re-averaged the measurements. The

program was set for a maximum of three repeats.

The pure error variance for the experimental rneasurements was deterrnined using the

repeat run data of experiment two. Data for each load resistor were averaged and their standard

deviation was calculated. The calculated percent variance for each data point on the polarization

curve was applied to d l other experimentd runs. The calculated error took into account MEA

deterioration over the course of the experiments, variability in ceil assembly, and drift in the

electronic system zero.

5.9. Calculation of Dimensionless Groups

The values of five dimensionless nurnbers: the Reynold (Re), Euler (Eu), Froude (Fr),

Weber (We), and the UD ratio for the set experimental conditions are summarized in TabIe 5.4.

Sarnple calculations are provided in Appendix A.

Reynolds number varies Iinearty with flow rate and equivalent diarneter. Since flow rate

was set as a function of current density, Re varied linearly with current density. For al1

experimental conditions the anode flow is strictly Iarninar. The Reynolds number remaked

below a value of 2.0 for the entire study. Re, Fr, We and the length to diameter ratio al1 decrease

with an increase in channel depth. The Euler number, which describes the dynamic pressure of

the channels (momentum generation to convective momentum transfer in the channels) does not

continue to increase with increasing channel depth. Figure 5.3 (a) shows that Eu reaches a

minimum with the medium depth channel. This result is expected since the medium channel

has a square cross-sectional area (2.38 1 mm x 2.38 1 mm) which maximizes the surface area to

volume ratio, or gives the minimum hydraulic diameter. The inverse of the Euler number (Eu-'),

given by Figure 5.3 (b), reaches a maximum at the medium flow channel throughout the range

of flow and temperatures.

A dimensional analysis is useful in examining ce11 performance by analyzing how the

dimensionless numbers Vary with ce11 performance. However, this study did not result in

sufficient data to build an empirical mode1 to perform such an analysis. For such an analysis the

study would have to incorporate several temperatures and several channel depths to provide

more than thee points on a plot such as Re vs. temperature or Re vs. channel depth.

Euler's Number l l E u

70°c c j (3

mShalbw Medium U D e e p

Figure 5.3 (a,b) - Plot of the Euler number (Eu) as a function of flow rate (curent density) and temperature. The Euler number reaches a minimum value with the medium channel flow depth at 70 OC and 95 OC throughout the flow range. Figure 5.3 (a) is a plot of the Euler number vs. flow rate and channel depth. Figure 5.3 (b) is a plot of the inverse of the Euler number vs. flow rate and channel depth. The maxima for 1/Eu have been calculated to occur at the medium channel depth (2.38 1 mm) for both temperatures.

Table 5.4 - Summary of the independent dimensionless iiumbers used to describe the anode flow fjeld are given with respect to fuel flow rate, temperature, and channel depth. The flow rate is given for the entire cell. The flow rate of eüch channel is 1/16 of the total flow rate into the cell.

Temperature Flow Rate (OC ) (cm3/m i n)

Channel Depth (mm)

Reynolds Euler Froude Weber Number xlo3 Number ~ 1 0 " Number x106 Numbcr x10'

'Cr

xlo3 (m) L/D ratio

Chapter 6 - Results and Discussion

6.1. System Response and Controllability

It took between three and fifteen minutes for the ce11 to achieve steady-state conditions

following a change in load. The transient response c m be seen in Figures 6.1 (a) and (b). The

load is changed from short circuit conditions to a 50,000 Q load resistor. Steady-state voltage

was achieved within ten minutes of a change in load. A similar response is noticeable in Figure

6.2 where the load resistor is changed from 4.94 Q to 10.15 a,

Data at steady-state were collected for a minimum of three minutes. The last

two-and-a-half minutes of data (approximately five data points) were averaged to give a single

point on the polarization curve for each load condition. Steady-state points are shown as points

from 6.5 minutes to 9 minutes in Figures 6.1 and 6.2. Points for the repeat runs of expenment

two (two, five, eight, and nine) are given in Figure 6.3. The averaged polarization and power

density points are given as curves with error bars.

Of the operating parameters, the anode pressure was the most difficult to regulate.

Constant adjustments were required to control the anode back-pressure valve to keep the anode

pressure near the set-point. Pressure variation of the anode in cornparison to the cathode c m be

seen in Figures 6.1 (a, b) and 6.2. The average gauge anode pressure for al1 the experimental

runs is 3 10 I 30 kPa (45 t 4 psig). Whereas, the average gauge pressure of the cathode for al1

the experimental runs is 343 + 9 kPa (50 + 1 psig). This translates to a relative error of I 10 %

for the anode pressure as compared with a relative error of + 3 % for the cathode pressure about

its set-point.

. . 250

O 2 4 6 8 10 12 14

Time (minutes)

-Voltage +Anode Pressure -&-Cathode Pressure

Time (minutes)

+Voltage +Anode Pressure -2-,Cathode Pressure

Figure 6.1 (a,b) - Transient response for a change in load resistors from short circuit to 50,000 Q. Cathode back-pressure is relatively constant as compared with the anode pressure. Voltage error bars are large and have not been included (refer to subsequent plots for voItage errors). Steady- state is considered for the final two-and-a-half minutes of these data points, as shown by Figure 6.1 (b). Only these data and repeat data for the 50,000 n condition were averaged to obtain a single point on the polarization curve.

665 ' I I , T r T ; 8 . . k 2 0 0

O 2 4 6 8 10

Time (minutes)

&Voiîage +Anode Pressure .*Cathode Pressure

Figure 6 3 - Transient response from a 4.94 $2 load to a 10.15 L2 load is shown for experiment 2, repeat 3. The pIot only voltage after the change in load resistor (not the voltage at 4.94 Q). Steady-state was achieved after six minutes from the time of resistor change. Anode pressure fluctuations about the set point are clearly noticeable about the set-point (shown as a grey dashed Iine).

O 2 O 40 6 0 8 0 1 00 120

Current Density (niAlcm ')

0 Exp. 2, mV i Repeat 1. mV Repeat 2. mV 1 Repeat 3. mV

O Exp 2. mW /cm2 Repeat 1 . mW /cm2 Repeat 2. rnW /cm2 1 Repeat 3. mW/cm2

Figure 6.3 - Steady-state voItage and power density from the repeat experimenta1 nins two, five, eight and nine are given for each load condition (medium channel, high temperature). Each datum on the pIot is the average of five points over two-and-a-half minutes. Both the polarization curve and the power density curve are shown. The average polarization and power density curve, with the resulting standard deviations, is &en as Figure 6.8.

80

Table 6.1 gives a list of the averages and standard deviations for pressures and temperatures

during the experiments. Sarnple caiculations are given in Appendix A.

Table 6.1 - Average pressure and preheat temperature for the expenmentai study. These averages are calculated from every datum taken in the experiment.

6 . 1 1 Pressure Differential Effects on the MEA

Experiment four (shallow channel depth at 70 OC) could not be completed because the

anode pressure could not be controtled. As a result, the ce11 could not be run at steady-state long

enough for data points to be collected. After inspection of the test station and the cell, it was

concluded the MEA was swelling into and blocking the shallow anode channels. This was

caused by the 35 kPa pressure difference between the anode and cathode charnbers. The swelling

was enough to reduce the flow of fuel in the anode channels drastically and resulted in a pressure

increase independent of the setting of the anode back pressure valve.

Just before experiment four, experiment one (shallow channel depth at 95 OC) was

cornpleted successfully. It is possible that the pressure difference and higher temperature during

rhis experiment caused the MEA to deform. The sarne MEA was used in the remaining

experiments with deeper channels (experiment six, three, and the repeat mns of experiment two)

without evidence of any problems.

Swelling of the MEA into the flow channels of the anode, when subjected to a pressure

differential would decrease the equivalent diameter of the flow channel used in the dimensional

analysis. The degree the MEA swells and decreases the De, of the anode is not known.

However, if it is enough to block fluid Wow in the shallow channel, it must also reduce D,, of

deeper channels by at least as much.

6.2. Channel Flow Characteristics

The formation and flow characteristics of gas bubbles in the anode chamber were

videotaped using the transparent acrylic cell.

6.2.1. Gas Production

It was observed that CO2 was not uniforrnly produced at the surface of the gas diffusion

layer. Several point sources of bubble generation were observed d o n g the exposed portion of the

MEA in the flow channels. A similar situation was observed behind the channel ribs, but on a

lirnited scaie. This phenornenon can be attributed to the structure of the gas diffusion electrode

and the influence of the hydrophobic regions created by the ~ e f l o n ' " on the carbon cloth

(Argyropoulos et al., 1999). It is probable that the complex paths connecting the electrode

surface and the catdyst layer were blocked or flooded with the liquid phase and left only a few

hydrophobic open channels to serve as CO, removal paths. Before exiting the electrode, the gas

may have been accumulating in the diffusion or reaction layers as bubbles and inhibited liquid

reactants from reaching the catalyst layer. Argyropoulos et al. ( 1999) report that accurnulating

gas may rnove dong the vertical plane of the electrode until it reaches an open channel, which

leads it away from the reaction site to the surface. This is consistent with the observations of this

work.

It was also observed that the rate of bubble generation on the electrode surface increased

with current density. At current densities above 80 mNcm2 additional point sources of gas

evolution were visible on the electrode sudace. This observation is consistent with

Argyropoulos et al. (1999) who report that the number of gas evolution sites on the anode

electrode surface multiplies as current density or cathode pressure increase. They further state

that these increases lead to an increase in the pressure at the reaction layer, which is the driving

force for gas removal from the electrode layers. The effect of pressure on the evolution of gas

could not be studied due to the limitations of the acrylic cell.

6.2.2. Channe1 Flow Maldistribution

By using the acrylic test cell, it was observed that as gas bubbles reached the electrode

surface they grew in size before dislodging from the porous surface and exiting through the flow

channel. Bubbles would dislodge frorn the surface as they reached critical size, which seemed to

Vary depending on the location of the bubble (such as its proxirnity to the side wdls of the

graphite channels).

The type of gas flow varied from fine dispersed bubbles to larger bubbles to slugs of gas

formed from the coalescing of smaller bubbles into larger gas pockets. Larger bubbles could

span the width of the flow channel and were observed to restrict the flow of liquid to dong the

wall of the channel. In this case, gas would fiow slowly dong the core of the channel. As

reported by Argyropoulos et al. (1999), such a situation rnay eventually starve sections of the ce11

of reactants for the electrochemical reaction. Bubble coalescing and c hannel blockage occurred

predorninately at high current densities or low liquid flow rates. Argyropoulos et al. (in press)

report that as they increased the fuel flow rate, the size of bubbles in the channels decreased and

more readily exited the flow fields.

It is critical that large gas bubbles are not allowed to tom in the channels and that fresh

aqueous fuel is fed continuously to the MEA. Any evolved gas on the anode electrode surface

must be quickly dislodged away from the site of gas evolution. This prevents gas slugs from

fonning and clogging the flow channels. Argyropoulos et al. (1999) state that gas residence time

on the electrode c m be reduced with a proper channel configuration and high fluid velocity.

6.2.3, Exit Manifold Flow Maldistribution

The behavior of fluid in the exhaust manifold was also observed with the acrylic fuel cell.

Once product CO2 dislodged from the electrode, it was transported up the flow field by buoyancy

and entrainment. However, as gas cleared the fIow field it had the tendency to collect in the

exhaust manifold before exiting the cell. Argyropoulos et al. ( 1999) rcported a sirnilar effect

with their ceIl as well. As slugs formed in the exhaust manifold of their cell, the slugs could only

exit through the small opening of the outlet port by cornpressing or by breaking into smaller

bubbles. This happened as the fuel from below forced the bubbles through the ports.

The problem was partially addressed in the acrylic ce11 by increasing the anode flow and

by reshaping the exhaust manifold. Argyropoulos et al. (1999b) state that higher liquid flow

resulted in finer bubbles and helped cornpress the gas through the exit manifold as it exited. The

cross-sectional area of the anode exhaust was increased on the acrylic ce11 and reconfigured to

direct gas slugs toward the exit port. This approach was also used by Argyropoulos et al. ( 1999)

who reported significant improvements in gas removal.

6.3. Interna1 Ce11 Resistance

Baseline internal resistance for a dry ce11 without the MEA or sealing material was

8.0 + 1.2 d. This resistance includes the resistance of the graphite plates, the current collectors

and their interfaces, it does not include the resistance of the MEA. At startup ce11 internal

resistance (the resistance of the baseline plus the MEA) vai-ied from 40 r d to 200 m a . The

internal resistance decreased considerably as the system approached steady-state conditions. The

cell, at steady-state with a conditioned MEA, had a total internal resistance of between L 1 d

and 2 1 ma. Sarnple calculations for the ce11 internal resistance and the associated errors are

given in Appendix A.

Ce11 internal resistance for experiment two (medium channel, high temperature) is given

in Figure 6.4. The plot contains large errors ranging from 25 % to 60 % which makes it difficult

to identiQ trends, however, ce11 resistance clearly does not Vary significantly with current

density. Figure 6.5 further illustrates this for the other experimental runs. Although a significant

trend between the cell resistance of different experimentai runs is not visible in this plot, a

progressive increase in ce11 resistance is noticeable between the first and last runs of the study.

This is more clearly shown in Figure 6.6 which contains resistance plots of the first experiment

performed (experiment five) and the plots of experiment two and its repeats (including the last

experiment performed). Expenment five has a lower average internai resistance over the current

density range than does the last experimentai run (experiment two, repeat three). The internal

resistance of other runs progressively falls within these two expenments suggesting that ce11

internai resistance may have increased with time. This may be attnbuted to MEA degradation

40 6 0 80

Current Density (rn~/crn')

Figure 6.4 - Cell intemal resistance for expetiment 2 (medium channeI, high temperature). This is the average of al1 repeat runs For experiment two. A significant interna1 resistance trend is not visible with current density. This is the case with al1 experimental runs.

O f O 2 0 40 6 0 80 100 120

Current Density ( r n ~ l c r n ~ )

'7 - Exp 1 (Run 3). Shatl(95C) CExp 2 (Run 2). Med (95C) A Exp 3 (Run 5). Deep (95 C)

0 Exp 5 (Run 11, Med (70 C ) @Exp 6 (Run 4). Deep (70 C )

Figure 6 5 - Cell intemal resistance for experiments 1, 2, 3, 5, and 6. Because of the large ettors, a significant trend is not noticeable between experiments. Expenment one, run number three (shaIIow channel at 95 OC) is written as Exp 1 (Run 3), Shall (95 C).

caused by, for example, ce11 assembly and disassembly between experiments or by changes in

membrane structure over time.

The repeat runs of experiment two resulted in similar ce11 resistances over the current

density range; although, repeat 1 (nin number 5) seerns to have a higher ce11 intemal resistance

than repeats two and three (Figure 6.6). This is inconsistent with the observations discussed

above but may be explained by variances associated with ceIl assembIy since the quality of

contact between the graphite plates and the electrodes vaned each time the celi was assembled.

Nevertheless, due to the large errors associated with the data, determining any conclusive trends

with ce11 resistance is di fficult.

C urrent Densiîy ( m ~ / c m * )

a Exp 2 (Run 2) ~ E x p 2. Repeat 1 (Run 5) ~ E x p 2. Repeat 2 (Run 8)

O Exp 2. Repeat 3 (Run 9) Exp 5 (Run 1 )

Figure 6.6 - A comparison among the first experimental run, the last run, and repeat runs of experiment two. Although a significant trend is not present due to the large errors, a trend showing a general progressive increase in ce11 resistance between the first and last runs may be noticeable.

6.4. Fuel Cell Performance

Figures 6.7 to 6.11 show the average polarization and power density plots for

experiments 1, 2,3,5, and 6 with the respective error bars. As discussed earlier, data for

experiment 4 were not obtainable. The curves shown for the polarization and power density

plots are third order and second order models, respectively. The curves were approximated by

MS Excel and have been included only to show general trends for clarity. They do not serve any

purpose in the analysis.

Figure 6.8 is a plot of average polarization and power density data for experiment two

(including the repeat runs) with standard deviations. The percent errors calculated from

experiment two and its repeats (obtained from the replicate analysis) were applied to al1 data

points for other experimental runs. Sample calculations are provided in Appendix A.

The effects of activation, ohmic, and concentration polarization are present in the

polarization curves. An activation ovewoltage at low current densities and the effect of

concentration polarization may be visible at current densities of 70 m ~ / c m ' and higher.

O 20 4 O 6 0 80 1 O0 120

Current Density (mA/cm 2,

A Voltage 0 Power Density

Figure 6.7 - PoIarization and power density plots for experiment 1, shallow channel at 95 OC.

1 ; O

O 20 40 6 0 8 0 100 1 20

Cunent Density (mA/cm 2,

A Voltage r Power Density

Figure 6.8 - Polarization and power density plots for experiment 2, medium channel depth at 95 "C. The standard deviations, calcuiated from the

700

600 - > E - 500

FI> m - - 5 400

300

200

repeat mns, are given as x and y axis error bars.

r

O 20 4 0 6 0 8 C 1 O0 120

Current Density (rnAicrn 2,

r Voltage Power Density

Figure 6.9 - Polarization and power density plots for experirnent 3, deep channel depth at 95 "C.

Note that in most of the plots a relatively straight line can be drawn through data points above

30 mA/cm2. As a result, the presence of concentration polarization may not be visible. The

relatively linear portion between the end regions is a combination of al1 the effects.

Current Density (rnA/cm 2,

Voitage Power Densiiy

Figure 6.10 - Polarization and power density plots for experiment 5, medium channel depth at 70°C.

O 20 40 6 O 80 100 120

Current Densiîy (mA/cm 2,

A Voitage Power Density

Figure 6.11 - Polarization and power density plots for experiment 6, deep channel depth at 70 OC.

6.4.1. Effect of Ce11 Temperature

Figures 6.12 to 6.15 compare ce11 performance with respect to channel depth and

temperature. By using ce11 intemal resistance data from Figure 6.5, the IR free voltage has been

calculated. Consequently, the plots mainly compare ce11 activation and concentration

polarization losses with temperature and channel depth. Sample calculations used to plot these

data are given in Appendix A.

The medium depth flow channel at 95 OC resulted in the least polarization losses over the

entire current density range as compared with the deep and shallow channels, as seen in Figure

6.14. This may be correlated with the Euler number (Figure 5.3); however, additional data

points are required at different channel depths and ce11 temperatures to identify trends. Results

from experiment two, Figure 6.8, were chosen as a benchmark for discussing ce11 performance.

Figures 6.12 and 6.13 compare the performance of the fuel ceII with respect to

temperature. Although Atico et al. (1996) report that the activity of the Pt: Ru ( 1 : 1 a/o) anode

catalyst rises significantly at ceIl temperatures of 95 "C, Figure 6.12 does not show an

improvement in ce11 performance between 70 "C and 95 'C for the medium channel.

Ravikumar and Shukla (1996) found that at 95 O C the rate of methanol crossover increases and

begins to affect the activity of the cathode significantly. This would limit any performance

gains realized at the anode due to temperature increases. With an increase in temperature,

activities for both the anode and cathode catalysts rise. Any gains realized by the anode catalyst

are diminished or eliminated by mixed potentials created at the catliode.

Current Dençity (mA/cm ')

0 9 5 C (Exp 2) a70 C (Exp 5)

Current Density (mAjcm *)

O 95 C (Exp 2) a 70 C (Exp 5)

Figure 6.12 - A cornparison of the effect of temperature on the performance of the fuel ce11 using the medium depth channel. This plot is IR corrected.

Current Density (mAlcm ')

O 95 C (Exp 3) CI 70 C (Exp 6 )

O 2 O 4 0 6 O 8 0 1 O0 1 20

Current Density (mAlcm ')

a 95 C (Exp 3) a70 C (Exp 6)

Figure 6.13 - A cornparison of the effect of temperature on the performance of the fueI ce11 using the deep channel. This plot is IR corrected.

Xiaoming et al. (1997) state that water drag coefficient across the membrane increases

with higher temperatures independent of current density. For a constant stoichiometric ratio

expenment, higher current density, the higher oxidant flow can remove crossed-over water

continuously. However, at low current density, when the oxidant flow is reduced, permeated

watet may flood the cathode and block the active catalyst sites (Xiaoming et al., 1997). Thus,

anode performance improvements with higher cell temperatures may be offset by the flux of

methanol and water into the cathode chamber.

In cornparison, Figure 6.13 shows a significant increase in the deep channel

overpotential at 70 "C when compared with ce11 overpotential at 95 'C. in fact, experiment six

showed the worst performance with a peak power density of approximately 20 mW/cm2, less

than half the power density for experiment 2 or 5. This poor cell performance was possibly a

combination of low catalyst activation at the lower temperature and poor channel flow

characteristics of the deep channel.

Since fuel velocity and Reynolds number are small for the deep channel as compared

with the shallower channels, the effect of forced convective mass trrinsfer decreases and mass

transport of fuel to the diffusion layer is dictated by diffusion through the aqueous solution.

Since the diffusivity of methanol in water is proportional to temperature, as given by the Wilke

and Chang equation (Welty et al., 1984), at 70 OC the diffusivity of methanol in aqueous

solution is two-thirds the diffusivity of methanol in aqueous solution at 95 'C (0.045 cm2/s at

70 OC as compared with 0.067 cm2/s at 95 OC). A sample calculation is s h o w in Appendix A.

As current density nses, mass transport limitations from the slow diffusion of methanol to the

electrode double layer cause an increase in concentration polarization. Mass transfer limitations

become pronounced just before the slope of the polarization curve begins to decrease at high

current density (as illustrated in Figure 2.1) or just before the peak in the power density curve.

However, Figures 6.12 and 6.13 do not show such a slope decrease in the polarization curve at

higher current density nor do they clearly depict a peak in power density. Consequently, for the

ce11 current density range, the ce11 does not seem to be affected significantly by concentration

polax-ization at ei ther temperatures.

6.4.2. Effect of Channel Depth

Figures 6.14 and 6.15 compare ce11 performance with respect to channel depth. A

significant difference in ce11 performance is not present between the shallow and medium depth

channels. However, at current densities of more than 75 mA/cm2, deep channel ce11

performance is less than that of the other channels. Since the deep channel has a low flow

velocity and low Reynolds number when compared with shallower channel depths, at higher

current density mass transport of methanol fuel from the bulk solution is IikeIy hindered due to

the buildup of products at the electrode surface. As gas accumulates, the electrode surface area

wetted by fuel decreases. The ce11 is starved of fuel.

This effect is likely compounded by the buildup of large amounts of gas bubbles within

the flow channels. Due to low fluid velocity, bubbles most likely coalesce into slugs and block

off methanol flow to sections of the cell. It was observed that only small arnounts of product gas

were present in the anode exhaust of the deep channel ce11 at a low current density. However,

pockets of gas were expelled in the exhaust at current densities more than 80 mA/cm2.

Generally, gas was expelled from the deep channel ce11 as large pockets in discrete intervaIs

spaced one to two minutes apart.

C m n t Density ( m N m 2,

a SMlow Chamel (Exp 1) O Mediun Chamel (Eg 2) A Deep Charnel (Exp 3)

Shallow Channel (Exp 1) 0 Medium Channel (Exp 2) A Deep Channel (Exp 3)

Figure 6.14 - A cornpanson of polarization curves from expenments 1 to 3. This plot is IR corrected. The performance of three different channel depths at 95 OC are shonn. Activation polarization is the dominant effect at low current density. The medium channel depth resulted in the highest potentials over the entire current density range.

O 20 40 60 80 100 120

Curent Density (miVcm 7

O Medi un Chamel (Ew 5) A Deep Chamel (E>q, 6)

Figure 6.15 - A comparison of the effect of flow field depth on the performance of the fueI ce11 using the low temperature conditions, 70°C. This plot is IR corrected.

When using the shailower channels, gas was not observed to be expelled as pockets o r in discrete

intervais. This suggests that gas residence time is higher in the deeper channel ce11 and that

pockets are formed in the exhaust manifolds. This is likely the reason that ce11 performance was

significantly lower than the shallower channels at 95 OC and that the ce11 did not achieve a

current density (which was calculated based on a fixed catalyst active surface area of 19.1 cm'

regardless of the volume of gas present in the channels) above 90 rn~lcm'.

Figure 6.15 shows ce11 performance with respect to channel depth at 70 OC. Lower

current densities were again observed for the deeper channels than for the shallower channels.

This is likely due to gas accumulation in the channels. As weH, the slope of the polarization

curve is also more negative for the deep channel ce11 than it is for the medium channel ce11 at

current densities greater than 30 r n ~ k m ' . Since Figure 6.15 was adjusted to compensate for ce11

internai resistance and since the characteristic curve identiwing the onset of significant

concentration polarization effects is absent, the steeper dope for the deep channel ce11 may be

attributed to an increase in activation polarization. The difference in deep channel ce11

performance at a lower temperature may suggest that the effect of channel depth becomes more

critical at Iower temperatures with lower catalyst activation. Unfortunately, experiment four

(shallow channei at 70 OC) could not be completed to verify this.

As shown in Figure 6.14, a significant difference between shdlow channel ce11

performance and performance of the other cells was not identified. This is, at least in part, due to

insufficient data collected for the shallow channel cell. However, it is possible that ce11 pressure

drop affected bubble volume in the channels. Using the Hagen-Poiseuille equation the pressure

drop (at 70 OC and the minimum fuel flow rate of 0.7 m l h i n ) in the shailow channel (160 Pa)

was estirnated to be six times larger than the pressure drop in the medium channel(26 Pa) and 22

times larger than in the deep channel (7 Pa) (refer to Appendix A for sarnple calculations). It is

clear that the drop in pressure for al1 three flow channels are small when compared to the overall

ce11 pressure.

performance.

and exhaust t

As a result, it is not expected that pressure drop will have much of an effect on cell

The higher fluid velocity in the shallow channel, however, would help disperse

ie gas (Argyropoulos et al. 1999). The insignificant change in ce11 performance

between the shallow channel and the deeper channels given in Figure 6.14 indicates that these

two effects may be counterbalancing one another. Unfortunately, lack of data at 70 OC prevents

any meaningful conclusions to be drawn.

6.4.3. Open Circuit Voltage

A significant difference was not found between ce11 open circuit voltage when comparing

experiments done at different channel depths and at different temperatures. The average open

circuit voltage for the expenments was 680 mV. When compared with the reversible ce11

potential of the direct methanol cell, this results in an open circuit efficiency of 55 %. Even at

open circuit the ce11 is subject to methanol crossover and, hence, mixed potentials at the cathode.

Mixed potentials reduce the ce11 terminal voltage to below the reversible ce11 potential even

before activation, ohmic, or concentration polarizations take effect. As a result, the open circuit

voltage efficiency indicates the extent the ce11 is being subject to mixed potentials and, hence,

methanol crossover. The higher the crossover, lower the efficiency,. This suggests that the ce11

was subject to a significant arnount of methanol crossover and, as a result, there should be

evidence of the crossover and mixed potentials in the cathode exhaust samples. This is discussed

in the next section.

6.5. Exhaust Stream Analysis

Table 6.2 gives the results of the analysis of the liquid and gas samples of the cathode and

anode. The samples were taken from experiment two, repeat two.

Table 6.2 - Component weight percent of the cathode and anode liquid and vapor sarnples- A GC with a TCD was used to analyze samples from repeat two of

experiment two. The 1 M methanol feed was analyzed to provide a cornparison for the anode and cathode streams.

S trearn I

Cathode Liquid 98.7

1 Gas 1 9.3

Anode Liquid 97.6

Gas 35.1 -. . -

1 Feed Liquid 1 96.5

Methanol

Weight %

CO,

13.8

Other

The concentration of methanol in the anode exhaust was 0.6 molar, as compared with

1 .O rnolar for the inlet. Some of this decrease in methanol concentration can be attributed to

methanol vapor leaving in the anode gas exhaust with the product CO-. The remaining rnethanol

is expected to have reacted on the anode or crossed-over into the cathode chamber. Crossed-over

methanol was found to be present in the cathode exhaust, both in gaseous and liquid samples.

A retatively large arnount of water was present in the cathode sarnple. Part of this water

content was due to the methanol oxidation reaction. However, it is expected that the most of it

was from anode crossover.

CO2 was found in both anode and cathode gas samples. This is evidence that part of the

crossed-over methanol oxidized on the cathode Pt catalyst and that the ce11 was subject to mixed

potentials. It is also possible that some CO, crossed over from the anode to the cathode. This

may be caused by pressure created in the anode catalyst layer frorn gas evolution.

Oxygen was found in the anode gas Stream. One of the sources of this oxygen may have

been dissolved oxygen introduced during sampling and injection or, due to the negative

anodekathode pressure differential, it may have crossed-over from the cathode. However, since

the fuel was degassed and since oxygen was not found in the gas chromatogram of the fuel

sarnple, the latter seems a more Iikely source for the oxygen in the anode gas. It is also possible

that atmospheric oxygen was introduced into the sampling vesse1 throush leaks in the gas

collection apparatus or that it rnay have been introduced into the gas syringe used to inject

samples into the GC. Further investigation is required to determine the source of the oxygen.

The Iiquid sarnples of the cathode were a Iight shade of transparent yellow, as compared

with a clear anode exhaust. Since none of the above components wouid give the solution such a

color, it was suspected that other organic compounds may have been present in the exhaust. A Pt

catalyst combined with fuel in an oxygen Rch environment can result in many different possible

paths for reactions of crossed-over species. Chromatographs of the cathode and anode samples

had a peak at approximately 29.5 minutes (the peak in the anode liquid sarnple was small

compared with the others at less than 0.1%). It is plausible that this peak represents a product or

products of incomplete methanol oxidation occumng at both the anode and cathode.

Three additional procedures were used to identify any unknown components in the liquid

cathode sarnple. These included nuclear rnagnetic resonance (NMR), infra-red (IR)

spectroscopy, and ultraviolet visible (UV-vis) spectroscopy. However, none of these procedures

conclusively identified the presence of additiond compounds.

Chapter 7 - Conclusions and Recommendations

7.1. Conclusions

A DMFC test station was designed, fabricated and tested. It operated within design

parameters throughout the study. As well. a DMFC with an idealized flow field configuration

machined in graphite plates was designed, fabricated and tested. The design of the ce11 allowed

for quick and repeatable assembly and disassembly (with Iittle MEA damage). The sealing

system operated acceptably.

An acrylic fuel ce11 was used to observe the two-phase flow characteristics inherent with

liquid fed direct rnethanol cells. This helped with the understanding of possible flow

maldistributions occurring in the high temperaturehigh pressure fuel cell. Two kinds of flow

maldistribution were observed. These were channel flow maldistribution and exit manifold flow

maidistribution. in both cases, product gas coalesced into larger gas bubbles and blocked the

movement of fluid through the flow field. This prevented fuel from reaching the catalyst layer of

the electrode.

The effect of channel depth and temperature on the performance of direct methanol cells

was studied. It was found that any gains realized by the anode catalyst due to increased anode

catalyst activity from higher ce11 temperatures were offset by increased methanol crossover

leading to mixed potentids. Thus. ce11 performance at higher temperatures was limited by ce11

depolarization loss at the cathode.

It was found that anode channel depth signi ficantly affected ce11 performance, especially

at a high current density and low temperatures of 70 OC. The medium anode channel provided

the highest ce11 perf'ormance with a peak power density of 45 mW/cm2.

The anode and cathode exhaust streams were analyzed. Methano1 and CO, were detected

in the cathode exhaust strearn by gas chromatography. This indicated the presence of methanol

crossover and ce11 mixed potentials. Discotoration of the cathode exhaust indicates the presence

of other cornpounds. However, the analysis methods used did not concIusively identify any

compounds to be present.

The performance of the DMFC of this work was compared with the published

performance results for DMFCs (Argropoulos et al, 1999a; Scott et al., 1998). The performance

of the ce11 in this study is in the same range as the published performance. However, differences

in IR-Iosses due to the interna1 resistance of the ceil in this study resulted in clearly inferior

performance when compared with the other celIs.

7.2. Recommended Mdifications for the Test Station and the Cell

The following recommendations address some difficulties encountered in operating the

DMFC and test system. They apply to four general areas:

1. anode pressure control

2. electrical system wiring and connectors

3. product coliection and analysis

4. anode flow field design

7.2.1. Anode Pressure Control

A major improvement in the test station would be the addition of a liquid pressure

controller on the anode exit strearn to replace the anode-back pressure valve. During operation

the fuel ce11 system was prone to runaway anode pressures. Although lirnited by the proportional

pressure relief valve, if the anode pressure increased by more than 35 kPa, system steady-state

conditions were affected. Five to fifteen minutes were required before steady-state could be

reestablished after such system disturbances.

7.2.2. Electrical System

To reduce the minimum external circuit resistance several modifications should be

implemented, such as smaller shunts. For example, a 10 r d 2 shunt would provide 1 mV output

for 100 rnA. However, such small shunts were not readily available. Much better control of the

circuit can be achieved directly by using a bipolar power supply.

7.2.3. Product Collection and Analysis

Because of the low product volume produced from the single ce11 and the large volume of

the exit Stream tubing, analyzing the liquid and gaseous product Stream for specific ceIl

conditions without requiring the ce11 to operate at steady-state for long periods was difficult.

Product collection would be improved and system residence time reduced by using 1.588 mm

(one-sixteenth inch) tubing for the exhaust, shorter tubing, and smaller sampling ports.

7.2.4. Anode Flow Field Design

Three design changes are recommended for the geometry of the anode flow fields. These

1. increase the volume of the exhriust manifold to accornrnodate gas slugs

2. reconfigure the exhaust manifold to channel gas to the exit (Argyropoulos et al.,

1999).

3. widen the channel width and reduce the depth, keeping D, constant. This will

keep Eu and Re constant but will increase the exposed active surface area of the

MEA and ohmic resistance due to lateral current flow in both electrodes.

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Appendix A - Sample Calcuiations and Error Analysis

1, Dimensionai Analysis

l = n - r i = 1 0 - 3 = 7 i - number of independent dimensionless groups

There are seven dimensionless groups n - number of variables

r - rank of matrix (core variables)

Choose the core variables as D, u, and p. The independent dimensionless groups become,

Solving for the second dimensionless number using the Buckingham pi theorem (Brodkey and

Hershey, 1988),

M., L and t represent the dimensions of rnass, Iength and time in this section. Solving this system of equations gives, d = - I , e = - l , f = - 1 This gives the inverse of Reynolds number,

R, - = ~ - ' u - ' p - ' ~

2. Sarnple calculation of Reynolds number

The equivdent diameter for the flow fields is calculated using the following equation (Welty et

al., 1984),

D, will be calculated for the medium channel, 2.38 1 mm depth. A diagram of a channel is given

below.

equivalent diameter (m) cross-sectional area of flow (m') wetted channel penmeter (m) channel radius ( 1.19 1 + 0.00 1 mm)

= channel width (2.38 1 i 0.00 1 mm) = channel depth (2.38 1 & 0.00 1 mm) = d - 1.191 inin= 1.191 10.001 nim

= w + 2 i + m ~ 2 . 3 8 1 +2(2.381 - 1.191) + ~~(1 .191) = 8.502 x 10e3 m = Ci, + 2 q +O,

= 0.00 1 + 0.002 + 0.00 1

= 0.004 x 10 -~ m = (8.502 + 0.004) x 10-~ m

= cross-sectional area of flow (m') = wi + %IC$ = (2.381 x 1.191) + !ha(l.l9l) ' = 2.8346 + 2.2263

= 5.061 x 104 m2

Calculate D,

Calculate the fluid velocity through the flow channel,

Q = (0.7 I I .O%)ml/min

Reynolds number using the medium depth channel7.0 nVmin, and 70 OC. Viscosity and density were approximated at 70 OC using Hysis and arc: assurned as constants.

p = 963.96 I: 1 % kg/m3 p =0.04231 I 1% Pas

3. Sarnple caiculation of the Euler number, L = length of flow channels L = 0.0476 + 0.000 1 m

Calculation of pressure drop in the medium flow channel, 70 OC. Eu = 8 1 8 1 0 + 3 0 5 0 v = ( 1 -44 + 0.01) 10-4 ds

P = 963.96 I 1 % kg/m3

4. Sarnple calculation of the Froude number,

(1.44x10~m / s)' Fr =

(238 lx1 0-'rn)(9.806m / s' )

5. Sample calculation of the Weber number,

We = ~ u ' p

c ~ e r r r i o r i

o,,~,,, was estimated using CRC Chernical Handbook (70 OC),

a,,,, = 0.05kg 1 s' t 5.0%

6- Sample calculation of errors for the polarization curve. The polarization curves are made up of averages of nw data, Each voltage, current

density, and interna1 resistance point is the average of repeat measurements. The error calculation for each point is composed of the standard deviation and reading errors of a specific load, The measurernent procedure is given in chapter 5.

The errors shown on the polarization curves were taken from repeat experiments. Experiment #2, high temperature and medium channel depth, was repeated three tirnes. The data was averaged at each load condition over the four experimentai runs. The standard deviation was

taken for each load condition from the four experiments. The following are the formulas for the average (2) and the standard deviation (s,) cdculations.

Table Al takes the average and standard deviation of the raw data from experiment #2 and its three repeats for an open circuit condition.

Table A l - Raw data points for the high temperature, medium depth channel at open circuit are given. The average and standard deviation was cdculated for this data. A sirnilar procedure was used to calculate each point on the polarization curves and their errors, as shown in chapter 6.

1 Experiment

Experimcnt #2

Voltage (mV)

679.585

679.970 68 1 .O75 68 1 -343 68 1.558

Experiment

Experiment #2 Repeat 1

Experiment #2 Repeat 3

1 Average Voltage

1 Standard Deviation

Voltage (mV)

690.265 686.008 684.643 684.408

684.408

Applying the student's t-test (95% confidence interval):

(N - number of data points, m - number of replicate runs)

Using Spiegel( 1993,

This was added to the multimeter reading error to give the total error. The multimeter reading error for the 2 V range is t 0.004 % + 3 counts (Table 3.3). Reading error = a (0.004 % + 3 )

= k ((689.7 14 x 0.00004) + 0.003) mV = * 0.03 1 mV

Total error = t29.0.0305(~X) + Reading error = 15.4 16 + 0.03 1 rnV = 15.447 mV

V = (690 I 16) mV

The percent standard deviation was then calculated for this load condition from the repeat mns, % Error = Totd Error / Reading

= 16 mV / 690 mV = 2.3 95

This percent error was then applied to al1 the averaged open circuit voltages for al1 the experiments. A similar method was used to calculate ce11 current density and total intemal resistance read by the multimeter (R,).

7. Sample calculation of Power Density

Since power density was rnathernatically calculated frorn voltage and current density, its error was derived from the errors of voltage and current density,

Data for a 10.15 resistor for experirnent 2, i = (3.433 t 0.127) m ~ l c m '

V = (665.03 1 + 23.6 10) mV pd = Vi

= (3.433 r n ~ k m ' ) x (665.03 1 mV) / 1000 = 2.283 m ~ l c m '

op = (2.283 mWlcm2) x [(23.610/665.03 1) + (0.127/3.133)] = 0.166 m ~ l c m '

p, = (2.28 I 0.17) rnW/cm2

8. Sample calculation of ce11 internai resistance error. This calculation is done for experiment two using a 10.15 $2 load resistor. The system circuit is given below.

RT = Total resistance read by the milli-ohrnrneter = (0.0 1729 -C 0.00307)

R,,,, = Interna1 resistance o f the fuel ce11 i = (3.433 + 0.127) mA/cm2

1 = (65.392 3.453) mA V = (665.03 1 + 23.6 10) mV

R,,= resistance of the system circuit (shunt, circuit, load)

Cdculate IR,,

O , = 0.244~ 1 O-~V

IR, = 1-13 1 + o . ~ ~ ~ x I o - ~ v

R,,,, is caiculated as follows,

43 7c Error = -

173 % Error = 25%

9. Sarnple calculation of ce11 voltage when corrected for ce11 intemal resistance. This calculation is done for expenment two using a 10.15 Q load resistor.

Rte,, = ( 17.3 t 4.3) d V =(665.031 123.610) mV i = (3 -433 -c 0.127) mNcm2 1 = (65.392 I 2.453) mA

10. Sample calculation of the oxidant flow rate (used to tabulate Figure 5.2),

Qo2 = 02276cm3 1 min at STP

0% = 0.0085

Qo2 = 0228 f 0.009cm3 1 min at STP

Calculate the number o f moles of oxygen required at 65.392 mA,

no: = l.Ol7xl0-~rnol / min

O , ? = 4.015x10-' mol 1 min

no2 = (1.02f 0 . 0 4 ) ~ lom5 mol / min

Calculate the flow rate of oxygen at lOOx stoichiometric ratio and at the conditions of the oxygen flow meter,

P = 345 kPa = 3.4 atm

T =295.15 K

no' = (1.02~ l ~ - ~ moi 1 min)x(100)

no2 = (1.02 + 0.04)~ 10') mol 1 min

a% =(727xl0-'~/ min)

O,% = 0.29xl0-~ L 1 min

Q,: = (7.27 + 0.29) x 1 O" L 1 min

1 1. Sample caiculation of the mass difisivity of dilute rnethanol (MeOH) through water at

70 OC using the Wilke and Chang equation, Welty et al. ( 1984).

Constants given in Welty et al. ( 1984): Va - atornic volume (cm3/g)

- association parameter for water

Va,,, = 14.8crn3/gmol Va,,, =3.7cm3/gmol

V, ,,, = 7.4 cm3/g mol MB = 18g/mol

@me, = 2.26 Pwater = 412 x IO6 P a s T = 343 K

Va (M~OH) = Va ici + Va c ~ i + Va ( O )

= 14.8 + 4(3.7) + 7.4 cm3/g mol = 37 cm3/g mol

g mol

12. Sample calculation of the compressive force delivered to the MEA.

The force from each bolt (Fm,,) in the clamping system was measured using a load cell. Each bolt was tightened to roughly 8.5 Nm using a torque wrench. The surface area of the graphite plates (minus the wetted area of the flow channels) was used to calculate the total pressure

exerted on the MEA. It was assumed that the compressive forces were evenly distributed across

the surface area of the graphite plates. n,,, = number of bolts

= 8

F,,,, = compressive force = 840 + 35 kN

A, = effective graphite surface area

= 0.0084 rn' + 1%

P = clarnping pressure (kN/m2)