custom microporous layers for polymer electrolyte membrane ... · pranay shrestha master of applied...

Custom Microporous Layers for Polymer Electrolyte Membrane Fuel Cells

by

Pranay Shrestha

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Sciences

Department of Mechanical and Industrial Engineering

University of Toronto

© Copyright by Pranay Shrestha 2018

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Custom Microporous Layers for Polymer Electrolyte Membrane Fuel Cells

Pranay Shrestha

Master of Applied Sciences

Department of Mechanical and Industrial Engineering

University of Toronto

2018

Abstract

Custom microporous layers (MPLs) were fabricated and designed to enhance fuel cell performance

and water management at two specific operating conditions, operation without anode

humidification and operation at high current densities. The two studies were aimed at addressing

two challenges of water management, i.e., membrane dehydration and cathode liquid water

flooding. The fuel cell performance and impedance measurements were paired with synchrotron

X-ray visualization to quantify cell potential, membrane and mass transport resistances, and liquid

water within gas diffusion layers (GDLs). The application of hydrophilic MPLs decreased the

membrane resistance, and increased liquid water retention at the catalyst layer-MPL interface.

MPLs with spatially graded PTFE content reduced the cathode GDL liquid water accumulation

and oxygen transport resistance. Both MPLs led to increased fuel cell performance. The knowledge

gained from this thesis can inform the design of next-generation fuel cell materials that further

improve cell performance at desired operating conditions.

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Acknowledgements

I would like to thank my supervisor, Prof. Aimy Bazylak for providing me with this wonderful

opportunity. You have been a great role model and a continuous source of support, encouragement,

and guidance for my growth, in career and in life. For that, I am forever grateful! You have taught

me invaluable lessons in clarity, scientific thought, and communication. Thank you for pushing

me to be the best version of myself.

Thank you to all my colleagues. You all make this workplace fun and exciting! Thank you David

for your tireless help and support during the thesis writing process! Thank you James, Stephane,

Rupak, Robin, Jongmin, Faraz, Nan, Chung, Dan, Hang, Mike, Jason, Andrew, Nico, Svenja, Eric,

Manoj, Hisan, Jack, Bonnie for your amazing discussions and interactions. I cannot thank each of

you enough for making my experience here so wonderful and memorable. All the laughs, intense

work talk, and lunches have been etched in my memory!

A huge thanks my family! I am where I am today with the help of your love and belief. The roots

of value that you planted in me has guided me through the tough times and has helped me stay

grounded. Mummy, you have planted in us the motivation to pursue our dreams. Daddy, you have

been a pillar of strength and character that we all aspire to be. Apu dijju, your wisdom and

optimism has supported us throughout. Aki dijju, your immense love and support for us is

unparalleled. Pranav, you have been the stronghold of my intellectual and moral foundation. Sosna,

you have been the sunshine of my days; Your love, care, and support has made every second worth

the while! Thank you all for the unflinching love and support! Thank you to all family and friends!

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Thank you to all my teachers, in life and academia. You have spurred my enthusiasm and curiosity,

and have taught me invaluable lessons.

Finally, thank you reader for your precious time! I hope you have a thought-provoking, informative,

and enjoyable read!

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Table of Contents

Acknowledgements........................................................................................................................iii

Table of Contents.............................................................................................................................v

List of Tables...................................................................................................................................ix

List of Figures..................................................................................................................................x

Abbreviations................................................................................................................................xiv

Nomenclature.................................................................................................................................xv

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

1.1 Preamble ........................................................................................................................... 1

1.2 Motivation ........................................................................................................................ 2

1.3 Objectives ......................................................................................................................... 3

1.4 Contributions .................................................................................................................... 4

1.4.1 Conference paper (accompanied by oral presentation) ............................................. 4

1.5 Co-authorship ................................................................................................................... 4

1.6 Organization of thesis....................................................................................................... 6

Chapter 2 Background and literature review ............................................................................. 8

2.1 Chapter introduction ......................................................................................................... 8

2.2 Polymer electrolyte membrane fuel cells ......................................................................... 8

2.3 Basic components ............................................................................................................. 9

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2.3.1 Membrane ................................................................................................................. 9

2.3.2 Catalyst layers ......................................................................................................... 11

2.3.3 Gas diffusion layers (GDLs) ................................................................................... 11

2.3.4 Current collector/ flow field.................................................................................... 13

2.4 Water balance within the fuel cell .................................................................................. 14

2.4.1 Hydrophilic MPLs for preserving membrane hydration at low gas humidification 15

2.4.2 Functionally graded GDLs for reduced cathode liquid water flooding at high current

densities16

2.5 Fuel cell diagnostic tools ................................................................................................ 18

2.5.1 Polarization curve and fuel cell overpotentials ....................................................... 19

2.5.2 Electrochemical impedance spectroscopy .............................................................. 21

2.5.3 Synchrotron X-ray visualization ............................................................................. 22

2.6 Chapter summary ........................................................................................................... 24

Chapter 3 Methodology ........................................................................................................... 26

3.1 Chapter introduction ....................................................................................................... 26

3.2 Fuel cell testing .............................................................................................................. 26

3.2.1 Fuel cell hardware and operating conditions .......................................................... 26

3.2.2 Fuel cell impedance measurements ........................................................................ 28

3.3 Synchrotron X-ray visualization .................................................................................... 31

3.4 Chapter summary ........................................................................................................... 36

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Chapter 4 Hydrophilic microporous layer coatings for polymer electrolyte membrane fuel cells

operating without anode humidification ....................................................................................... 38

4.1 Chapter abstract .............................................................................................................. 38


4.3 Chapter-specific methodology ....................................................................................... 39

4.3.1 Hydrophilic microporous layer coatings ................................................................. 40

4.3.2 Microstructure characterization .............................................................................. 42

4.3.3 Fuel cell testing ....................................................................................................... 43


4.4 Results and discussion .................................................................................................... 44

4.4.1 Need for reducing membrane resistance at low anode inlet RH ............................. 45

4.4.2 Hydrophilic coatings for reducing membrane resistance ........................................ 48

4.4.3 Interfacial liquid water retention ............................................................................. 50

4.4.4 Cathode GDL liquid water accumulation and oxygen transport resistances .......... 55

4.5 Chapter summary ........................................................................................................... 60

4.6 Appendix A: Model fit parameters for EIS equivalent circuit ....................................... 62

Chapter 5 Microporous layers with graded polytetrafluoroethylene (PTFE) for enhanced liquid

water removal in polymer electrolyte membrane fuel cell gas diffusion layers ........................... 63

5.1 Chapter abstract .............................................................................................................. 63


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5.3 Chapter-specific methodology ....................................................................................... 65

5.3.1 MPL fabrication ...................................................................................................... 65

5.3.2 Wavelength dispersive spectroscopy ...................................................................... 67

5.3.3 Fuel cell testing ....................................................................................................... 69

5.3.4 Synchrotron X-ray radiography .............................................................................. 69

5.4 Results and discussion .................................................................................................... 71

5.4.1 Relative PTFE concentration within MPLs ............................................................ 72

5.4.2 Fuel cell polarization curves ................................................................................... 75


5.4.4 Oxygen transport resistance .................................................................................... 81

5.4.5 Membrane dehydration ........................................................................................... 83

5.4.6 Designed threshold capillary gradient for enhanced liquid water removal ............ 85

5.5 Chapter summary ........................................................................................................... 87

5.6 Appendix B: Model fit parameters for EIS equivalent circuit (Chapter 5) .................... 88

5.7 Appendix C: Characteristic time of water diffusion in the membrane (Chapter 5) ....... 89

Chapter 6 Conclusions and future work .................................................................................. 91

6.1 Conclusions .................................................................................................................... 91

6.2 Future Work ................................................................................................................... 94

References......................................................................................................................................96

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List of Tables

Table 4-1. Details of the GDL materials used in the study presented in this chapter. The base

material for all the GDLs was SGL 25 BC. .................................................................................. 41

Table 4-2. Details of tests performed to analyze repeatability of performance results ................ 44

Table 4-3. Model parameters (averaged over three tests) for equivalent circuit used to fit Nyquist

plots in Figure 4-7, along with standard deviation among the three tests..................................... 62

Table 5-1. Model parameters for equivalent circuit used to fit Nyquist plots in Chapter 5, along

with standard error. ....................................................................................................................... 88

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List of Figures

Figure 2-1. Schematic of a typical polymer electrolyte membrane (PEM) fuel cell .................... 13

Figure 2-2. Sample fuel cell polarization curve. A, B, and C represent regions dominated by

activation, ohmic, and concentration overpotentials, respectively ............................................... 21

Figure 3-1. Schematic of the fuel cell test setup. .......................................................................... 28

Figure 3-2. Equivalent circuit used to fit Nyquist plots obtained from electrochemical impedance

spectroscopy. ................................................................................................................................. 29

Figure 3-3. a) Sample radiograph showing the components of the fuel cell. The axes in red show

the x- and y-(through-plane) directions (z-direction goes into the plane of the figure). The

boundaries of anode and cathode GDLs are shown using green solid lines. The boundaries of the

catalyst coated membrane are shown with dotted red lines. b) Sample processed image with the

color of each pixel corresponding to normalized liquid water thickness. The MEA regions under

the channels and the ribs are shown with solid and dotted lined white boxes respectively. The white

scale bars in the lower right corner of a) and b) are equivalent to 1 mm. Sample profile of

normalized liquid water thickness along the through-plane position (y-direction) for regions under

c) the channels and d) the ribs. The liquid water profiles were normalized by the distance within

the GDL traversed by the beam. ................................................................................................... 35

Figure 4-1. Schematic of the procedure of applying hydrophilic MPL coating on SGL 25 BC GDL.

....................................................................................................................................................... 42

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Figure 4-2. The effect of reducing the inlet humidification on the ohmic resistance of a fuel cell

with the bi-layer GDL for varied a) anode and b) cathode inlet RH. Anode and cathode inlet RH

are presented in the legend with A as anode and C as cathode. .................................................... 47

Figure 4-3. The effect of hydrophilic MPL coating on a) ohmic resistance, and b) cell voltage and

power density for fuel cells with the bi-layer and tri-layer GDLs. The inlet relative humidity was

maintained at 0 % for the anode and 100 % for the cathode. ....................................................... 49

Figure 4-4. Through-plane liquid water thickness profiles (normalized by the distance within the

GDL traversed by the beam) under a) channels and b) ribs for MEAs with the bi-layer and tri-layer

GDL. The inlet relative humidity was maintained at 0 % for the anode and 100 % for the cathode.

....................................................................................................................................................... 53

Figure 4-5. Average liquid water saturation at the a) anode and b) cathode MPL-catalyst layer

interfaces (13 µm or 2 pixels were averaged in y-direction in the GDL) of fuel cells with the bi-

layer GDL and the tri-layer GDL. All the fuel cell tests were conducted with inlet relative humidity

of 0% at the anode and 100% at the cathode. ............................................................................... 54

Figure 4-6. Liquid water saturation profiles along the through-plane position (y-direction) under

a) channels and b) ribs for cathode bi-layer and tri-layer GDLs. The inlet relative humidity was

maintained at 0 % for the anode and 100 % for the cathode. The porosity profiles of the GDLs are

shown in the secondary vertical axes. ........................................................................................... 57

Figure 4-7. Nyquist plots obtained from electrochemical impedance spectroscopy performed at

current density steps of 1.0, 1.5, and 2.0 A/cm2 for a) the bi-layer GDL (including a scaled-up inset)

and (b) tri-layer GDL. The inlet relative humidity was maintained at 0 % for the anode and 100 %

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for the cathode. The Nyquist plots were fit using an equivalent circuit model (shown in Figure 3-2).

c) The mass transport resistance and d) the diffusive time for oxygen obtained using the EIS model

fit, averaged for three tests. ........................................................................................................... 59

Figure 5-1. GDLs with microporous layers fabricated with varying PTFE content: a) 10 wt.%

PTFE, b) 20 wt.% PTFE, and c) graded 20-10 wt.% PTFE ......................................................... 67

Figure 5-2. Sample liquid water thickness distribution (normalized by the length of the GDL along

the beam path) for fuel cells containing GDLs with a) 10%, b) 20%, and c) graded (20-10%) PTFE

at 1.0 A/cm2. The black scale bar at the bottom is equivalent to 1 mm. White dashed lines

represents anode ribs and denote rib locations (ribs). The y-positions of the flow fields, GDLs, and

CCM are shown beside each processed image. The axes for each image is shown using white

arrows. The white dotted lines in each image represent 25 pixels (in the through-plane direction)

within the cathode GDL ................................................................................................................ 71

Figure 5-3. Cross-sectional fluorine elemental maps for GDLs with a) 10 wt.% PTFE, b) 20 wt.%

PTFE, and c) graded 20-10 wt.% PTFE. The color bar represents measured fluorine intensity in

counts per second. The locations of bulk MPL and substrate edge (denoted by S) are shown in

boxes above each image. d) Averaged through-plane PTFE distribution (normalized by the average

PTFE concentration of 20 wt.%). Position along y-direction was normalized using bulk MPL

thickness. ....................................................................................................................................... 74

Figure 5-4. Cell voltage from tests on fuel cells containing GDLs with 10 wt.%, 20 wt.%, and

graded (20-10 wt.%) PTFE in MPLs. Steady-state cell potential values are shown with markers

and transient data are presented with solid lines. .......................................................................... 76

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Figure 5-5. Temporal profile of average liquid water (normalized by the distance within the GDL

traversed by the beam) within cathode GDL (25 pixels) .............................................................. 78

Figure 5-6. Through-plane liquid water thickness (normalized by the distance within the GDL

traversed by the beam) for cathode GDL regions above ribs and channels for fuel cell operated at

current densities of a) 1.0 A/cm2 and b) 1.5 A/cm2. ..................................................................... 80


current density steps of a) 1.0 and b) 1.5 A/cm2. c) Mass transport resistance and d) diffusive time

calculated using EIS equivalent circuit. ........................................................................................ 83

Figure 5-8. Ohmic resistance of fuel cells containing MPLs with 10 wt.%, 20 wt.%, and graded

(20-10 wt.%) PTFE. ...................................................................................................................... 85

Figure 5-9. Characteristic time of diffusion in the membrane obtained from EIS model fit ........ 90

xiv

Abbreviations

Technical abbreviations

CCM Catalyst coated membrane

Cryo-SEM Cryo-scanning electron microscopy

EIS Electrochemical impedance spectroscopy

ICE Internal combustion engine

GDL Gas diffusion layer

MEA Membrane electrode assembly

Micro-CT Micro-computed tomography

MPL Microporous layer

PEM Polymer electrolyte membrane

RH Relative humidity

Wt. Weight

Chemical abbreviations

CO2 Carbon dioxide

e- Electron

H+ Hydrogen ion (proton)

H2 Hydrogen molecule

H2O Water molecule

O2 Oxygen molecule

SO3- Sulphonic acid side chain (in Nafion®)

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Nomenclature

Variables

𝐴 Active area of the catalyst layer [cm2]

𝐶A Anode electric double layer capacitance [F/cm2]

𝐶𝐶 Cathode electric double layer capacitance [F/cm2]

𝐷𝑒𝑓𝑓 Effective diffusion coefficient [m2/s]

𝐸°° Maximum theoretical potential of the fuel cell [V]

𝐸°° Nernst open circuit voltage [V]

𝐹 Faraday constant [sA/mol]

𝐼 X-ray irradiance transmitted through sample

𝐼0 Incident X-ray irradiance

𝑗 Unit imaginary number, √−1

𝐿𝑦 Gas diffusion layer (GDL) thickness [m]

𝐿𝑧 Length of the GDL parallel to the beam path [cm] (𝐿𝑧 = 0.80 𝑐𝑚)

𝑛 Number of electrons participating in reaction

�̇� Molar flow rate [mol/s]

𝑁𝑡 Number of frames over time

𝑁𝑥 Number of pixels in x-direction

𝑁𝑦 Number of pixels in y-direction

𝑃c Threshold capillary pressure [N/m2]

𝑃nw Pressure of non-wetting fluid [N/m2]

𝑃w Pressure of wetting fluid [N/m2]

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𝑟 Radius of throat [m]

𝑅A Anode activation resistance [Ωcm2]

𝑅P Cathode charge transport resistance [Ωcm2]

𝑅mt Mass transport resistance of oxygen within the cathode [Ωcm2]

𝑅Ω Ohmic resistance [Ωcm2]

𝑠𝑤(𝑦) Liquid water saturation profile in the through-plane y-direction

�̅�𝑤,𝑎𝑣𝑒 Average liquid water saturation in regions of interest

𝑡 Distance of propagation of beam within material [cm]

𝑡𝑤,𝑛 Normalized liquid water thickness [cm/cmGDL]

𝑡�̅�,𝑛(𝑦) Normalized liquid water thickness profile, averaged in x-direction and time

[cm/cmGDL]

𝑡�̅�,𝑛,𝑡(𝑡) Temporal profile of normalized liquid water thickness profile, averaged in x-

and y- directions [cm/cmGDL]

𝑣 Stoichiometric coefficient

𝑍𝐴 Impedance of the anode electrochemical reaction [Ωcm2]

𝑍𝐶 Cathode impedance [Ωcm2]

𝑍𝑇𝑜𝑡 Total impedance of the equivalent circuit [Ωcm2]

𝑍𝑊 Cathode Warburg impedance [Ωcm2]

𝑍Ω Ohmic impedance [Ωcm2]

Greek characters

Δ𝐻 Change in molar enthalpy of a reaction [J/mol]

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Δ𝐺 Change in molar Gibbs free energy of a reaction [J/mol]

𝜀(𝑦) Porosity profile of GDL in the through-plane y-direction

𝜃 Contact angle between the liquid-air and liquid -solid interfaces [rad]

𝜇 Attenuation coefficient [cm-1]

𝜎 Interfacial tension between the non-wetting and wetting fluids [N/m]

𝜔 Frequency of the AC signal [rad/s]

Subscripts

𝑤 Relating to liquid water

𝑅𝑒𝑓 Relating to reference image (dry-state image)

𝑊𝑒𝑡 Relating to wet-state image obtained during fuel cell tests

1

Chapter 1 Introduction

1.1 Preamble

There is a pressing need to reduce the unprecedented rate of global warming facing the world.

Among many other detrimental effects, global warming leads to sea-level rise due to melting

glaciers and polar ice caps, habitat loss, and increased vulnerability to extreme climates [1].

Increases in atmospheric carbon dioxide concentrations, largely due to anthropogenic emissions,

is a major contributor to climate change [2]. Climate change mitigation can be addressed by

transitioning to renewable energy sources and zero-emission technologies. Polymer electrolyte

membrane (PEM) fuel cells are promising devices that complement renewable energy

infrastructure by providing on-demand electrical power with zero local emissions. PEM fuel cells

are particularly attractive in the transportation sector (currently accounting for 14 % of total

anthropogenic CO2 emissions [3]) as an alternative to internal combustion engines (ICEs). PEM

fuel cells have higher efficiencies of 0.4-0.6 (state of the art energy efficiency of 0.6 [4]) compared

to the efficiency of 0.2-0.3 for ICEs. Accounting for the higher efficiencies of fuel cells, PEM fuel

cells reduce greenhouse gas emissions by 40% if the hydrogen is obtained from natural gas and by

85% is the hydrogen is renewably produced from water [5]. The cost of PEM fuel cells needs to

be competitive with incumbent and future technologies for widespread adoption. The fuel cell

stack cost for an 80-kW system was $53/kW in 2015, which is higher than the target set by the US

Department of Energy ($40/kW by 2020) [6]. Effective water management strategies improve fuel

cell power output and efficiency and help alleviate some of these challenges facing fuel cell

technologies [7-19].

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In this chapter, I provide a brief overview of fuel cell technology and its current standpoint. Then

I present the main motivation and objectives of the thesis, followed by the key contributions of the

research. Finally, an outline of the thesis is presented.

1.2 Motivation

Effective water management within the fuel cell is crucial for achieving high cell performance and

reducing fuel cell cost. Effective water management requires a balance between two competing

phenomena. Sufficient water is needed to hydrate the membrane to maintain high protonic

conductivity. To achieve the necessary protonic conductivity, external humidifiers are

conventionally employed to humidify the reactant gases. However, the accumulation of excess

liquid water inhibits the efficient transport of reactant gases to the catalyst layer reaction sites [20].

In this thesis, custom microporous layers were fabricated and employed to address two separate

issues of water management within PEM fuel cells; both approaches aimed to reduce fuel cell cost

in separate ways.

First, the issue of membrane dehydration at low gas humidification was investigated using

hydrophilic MPLs. Hydrophilic MPLs provide promising performance improvements under low

humidity conditions. Achieving high fuel cell performance at low humidification removes or

reduces the reliance on external humidifiers. It is advantageous to remove the external humidifiers

to help reduce the fuel cell system cost, size and parasitic power demands [21-23]. The direct cost

reduction of removing a humidifier from an 80-kW fuel cell system is estimated to be $1/kW [24],

which by itself covers around 8% of the total cost reductions required to meet the target of $40/kW

3

by 2020. In addition, the decreased parasitic power demand increases the power output of the fuel

cell and further reduces the cost of the system.

Second, the issue of excess liquid water accumulation at the cathode, commonly referred to as

cathode flooding, was addressed with the use of functionally graded MPLs. Reducing cathode

flooding can greatly benefit the high current density performance of the fuel cell by facilitating

more efficient transport of oxygen within the cathode gas diffusion layer (GDL). Improving the

high current density performance of a PEM fuel cell is a robust strategy to increase the power

density of the fuel cell [24]. The cost of a fuel cell stack has the highest sensitivity to power density.

An increase in power density of around 50% (from 9.8 W/cm2) is expected to yield around 70 %

of the cost reduction required to meet the 2020 target (of $40/kW) [24]. In addition, increasing

power density decreases the required stack size to achieve a target power output. Reducing stack

size causes a decrease in the amount of fuel cell components (such as platinum catalyst, membrane,

and gas diffusion layers) required, which further reduces the cost.

1.3 Objectives

Custom microporous layers (MPLs) were fabricated and designed to tailor water management and

enhance fuel cell performance at two specific fuel cell operating conditions (corresponding to two

studies), i.e. operation without anode humidification in Chapter 4 and operation at high current

densities in Chapter 5. The two studies were aimed at addressing two separate challenges of water

management, i.e., membrane dehydration in Chapter 4 and cathode liquid water flooding in

Chapter 5. The objective of this thesis is to understand the effect of these custom MPLs on fuel

cell impedances (membrane resistance and oxygen transport resistance) and liquid water

4

distributions within the gas diffusion layers (GDLs). In Chapter 4, I investigate the effect of the

application of a hydrophilic MPL coating on membrane hydration and liquid water distribution

within GDLs during fuel cell operation without external anode humidification. In Chapter 5, I

investigate the impact of incorporating spatially graded polytetrafluoroethylene (PTFE) in MPLs

on oxygen transport resistance and GDL liquid water distributions, particularly at high current

densities. The knowledge gained from these two studies will help inform the design of next

generation materials that are better suited for the specific operating conditions.

1.4 Contributions

The research presented in this thesis has led to the following first-authored contributions:

1.4.1 Conference paper (accompanied by oral presentation)

1. Shrestha, P., Banerjee, R., Lee, J., Bazylak, A. (2017, August 21-23). “Hydrophilic

microporous layer coatings for polymer electrolyte membrane fuel cells.” Paper presented

at the International Conference on Fluid Flow, Heat and Mass Transfer, Toronto, ON.

1.5 Co-authorship

The fuel cell research conducted at the synchrotron facility in Aimy Bazylak’s research group

requires intensive planning, organization, and collaborative team work. The fuel cell experiments

need to be conducted in parallel with synchrotron X-ray imaging. Being a part of a large dynamic

team, I made contributions to the setup of fuel cell experiments and image acquisition for a variety

5

of research projects of my peers in the lab. The following co-authored publications resulted from

the collaborative work conducted at the synchrotron facility:

1. Lee, J., Banerjee, R., George, M.G., Muirhead, D., Shrestha, P., Liu, H., Ge, N., Chevalier,

S., Bazylak, A. (2017) “Multiwall carbon nanotube-based microporous layers for polymer

electrolyte membrane fuel cells.” Journal of the Electrochemical Society, 164(12), F1149-

F1157.

2. George, M.G., Liu, H., Muirhead, D., Banerjee, R., Ge., N., Shrestha, P., Lee, J., Chevalier,

S., Hinebaugh, J., Messerschmidt, M., Zeis, R., Scholta, J., Bazylak, A. (2017)

“Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion

Layers Part 3: Mass Transport Resistance and Liquid Water Accumulation at Limiting

Current Density with in operando Synchrotron X-ray Radiography.” Journal of the

Electrochemical Society, 164(7), F714-F721.

3. Liu, H., George, M.G., Banerjee, R., Ge, N., Lee, J., Muirhead, D., Shrestha, P., Chevalier,

S., Hinebaugh, J., Zeis, R., Messerschmidt, M., Scholta, J., Bazylak, A. (2017)

“Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion

Layers: Part 2 – Steady State Liquid Water Distributions with in Operando Synchrotron X-

ray Radiography.” Journal of the Electrochemical Society, 164(7), F704-F713.

4. Chevalier, S., Ge, N., Lee, J., George, M.G., Liu, H., Shrestha, P., Muirhead, D., Lavielle,

N., Hatton, B.D., Bazylak, A. (2017) “Novel electrospun gas diffusion layers for polymer

electrolyte membrane fuel cells: Part II. In operando synchrotron imaging for microscale

liquid water transport characterization.” Journal of Power Sources, 352, 281-290.

5. Banerjee, R., Ge, N., Lee, J., George, M.G., Chevalier, S., Liu, H., Shrestha, P., Muirhead,

D., Bazylak, A. (2017) “Transient liquid water distributions in polymer electrolyte

6

membrane fuel cell gas diffusion layers observed through in-operando synchrotron X-ray

radiography” Journal of the Electrochemical Society, 164(2), F154-F162.

6. Chevalier, S., Ge, N., George, M.G., Lee, J., Banerjee, R., Liu, H., Shrestha, P., Muirhead,

D., Hinebaugh, J., Tabuchi, Y., Kotaka, T., Bazylak, A. (2017) “Synchrotron X-ray

radiography as a highly precise and accurate method for measuring the spatial distribution

of liquid water in operating PEM fuel cells.” Journal of the Electrochemical Society, 164(2),

F107-F114.

1.6 Organization of thesis

The thesis is organized in 6 chapters. This chapter provides a brief overview of fuel cell technology,

a summary of the main motivation and objectives of the thesis, and the thesis contributions.

Chapter 2 presents the background and literature review for the thesis. The basic electrochemical

principles and components of a PEM fuel cell are presented, along with the fundamentals on fuel

cell performance and impedance measurements. The water balance challenges within the fuel cell

are discussed. Chapter 3 presents the general methodology used for the studies in this thesis,

including characterization of fuel cell performance and impedances and liquid water within the

fuel cell using synchrotron radiography. The thesis can be divided into two studies that are

described in detail in Chapter 4 and Chapter 5. Chapter 4 presents the effects that the application

of hydrophilic MPL coatings had on membrane hydration and liquid water distribution within the

GDLs during fuel cell operation without anode humidification. Chapter 5 presents the effects of

spatially graded polytetrafluoroethylene (PTFE) within PEM fuel cell MPLs on the oxygen

7

transport resistance and GDL liquid water distribution. Finally, Chapter 6 presents the conclusions

of the thesis and recommended future work.

8

Chapter 2 Background and literature review

2.1 Chapter introduction

This chapter presents the background and the literature review for this thesis. This chapter first

introduces the fundamental electrochemical principles of polymer electrolyte membrane (PEM)

fuel cells in Section 2.2. In Section 2.3, the basic components within a PEM fuel cell are then

discussed. In Section 2.4, the background and the significance of proper water management within

the fuel cell is discussed. Within this section, two specific water management strategies are

discussed in detail. The first strategy is the use of hydrophilic microporous layers (MPLs) to

prevent membrane dehydration at low gas humidification, presented in Section 2.4.1. In Section

2.4.2, the use of functionally graded GDLs to reduce cathode liquid water flooding at high current

densities is presented. The research gaps that need to be addressed require an understanding of

water management and fuel cell performance within fuel cells with novel GDLs. Unique insights

on water management can be provided using a combination of liquid water visualization and

measurements of membrane resistance and oxygen transport resistance. Section 2.5 presents the

literature review on the relevant diagnostic tools, namely polarization curve, electrochemical

impedance spectroscopy, and liquid water visualization using synchrotron X-ray radiography.

2.2 Polymer electrolyte membrane fuel cells

Figure 2-1 shows a schematic of a typical polymer electrolyte membrane (PEM) fuel cell. In this

device, hydrogen gas, H2, is supplied to the anode compartment, while oxygen gas, O2, (usually in

air) is supplied to the cathode compartment. The hydrogen and oxygen gases react

electrochemically on the anode and cathode respectively, as follows,

9

Hydrogen oxidation reaction at anode: 𝐻2 → 2𝐻+ + 2𝑒− Eq. 2-1

Oxygen reduction reaction at cathode: 1

2𝑂2 + 2𝑒− + 2𝐻+ → 𝐻2𝑂 + 𝐻𝑒𝑎𝑡 Eq. 2-2

When hydrogen is oxidized, electrons and protons are released. The electrons conduct through an

external circuit and the protons conduct through the membrane. These ions bond with the supplied

oxygen in the cathode compartment to generate water and heat [25].

2.3 Basic components

As shown in Figure 2-1, a typical PEM fuel cell has a multi-layered architecture. This architecture

consists of the following components: membrane, catalyst layers, gas diffusion layers, and current

collectors/ flow fields. The following sub-sections present a background of each component.

2.3.1 Membrane

The central membrane in a PEM fuel cell is a solid polymer electrolyte that is typically composed

of a perfluorinated ionomer. This ionomer consists of a chemically stable polytetrafluoroethylene

(PTFE) matrix with fixed side chains of sulphonic acid groups (SO3-). One of the most commonly

used membranes is a Nafion® type membrane, for example Nafion® 212, which has a thickness of

50 µm. The membrane serves two main functions. The first is to prevent mixing of the anode and

cathode reactant gases, to ensure that the anode and cathode reactions occur within their own

10

separate compartments. The second function is to transport protons from the anode to the cathode,

while blocking the flow of electrons.

It has been demonstrated in the literature that the geometrical and transport properties of Nafion®

type membranes (e.g., thickness, diffusivity of oxygen and water, and proton conductivity) are

very sensitive to the amount of absorbed water within the membrane [26,27]. It has been

hypothesized [27] that when the membrane is not sufficiently hydrated, the sulphonic groups exist

as isolated clusters in the polymer matrix. As the membrane becomes more hydrated, the water

molecules form a weak bond to the sulphonic side chains and form inverted micelles. With higher

membrane hydration, the micelle clusters grow and may form interconnections between each other.

Thus, with higher membrane hydration, the network of sulphonic acid groups becomes more

interconnected and facilitates more efficient transport of protons. Hence, the membrane needs to

be sufficiently hydrated to conduct protons effectively [27].

The transfer of protons through the membrane creates a driving force for water flux from the anode

to the cathode, due to the polar attraction of water molecules to the protons. This water flux, termed

electro-osmotic drag, is always in the direction from the anode to the cathode and is proportional

to the current density of the fuel cell. In addition to electro-osmotic drag, water transport in the

membrane occurs in the form of diffusion (driven by concentration gradient across the membrane),

hydraulic permeation (driven by gas pressure gradient across the membrane), and thermo-osmosis

(temperature driven flow) [25].

11

2.3.2 Catalyst layers

The catalyst layers are placed on both sides of the membrane, i.e., at the anode and the cathode.

The catalyst layer is a porous structure composed of a support material (typically carbon), an

ionomer (typically Nafion®), and catalyst particles (typically platinum). The catalyst layers serve

as reaction sites for the half-cell reactions described in Eq. 2-1 and Eq. 2-2.

2.3.3 Gas diffusion layers (GDLs)

The gas diffusion layer (GDL) is a porous, electrically-conductive layer that is placed between the

catalyst layer and the flow field. The assembly of the anode and cathode GDLs and the catalyst

coated membrane form what is known as the membrane electrode assembly (MEA). The GDL

serves as a means to transport gases, water, electrons, and heat to and from the catalyst layers and

the flow fields. The solid matrix of the GDL is meant to serve as an efficient means to transport

electrons and heat, and to help maintain the structural integrity of the CCM. A connected network

of GDL pores form pathways to transport reactant gases from the flow fields to the catalyst layers.

The pores also serve as pathways for water (in vapor and/ or liquid form) to and from the catalyst

layer.

The two-phase transport of water and reactant gases within the GDL is an important consideration

in GDL design and modeling. Water is generated as a byproduct of the oxygen reduction reaction

at the cathode reaction sites and is usually supplied along with reactant gases using external

humidifiers. Since the connected pores of the GDLs are pathways for water and reactant gases,

12

liquid water accumulation within these pores impedes the transport of the gases from the flow field

to the reaction sites. Liquid water accumulation decreases the effective porosity (volume fraction

of open pore space) of the GDL and increases the tortuosity of open pore space. Liquid water

accumulation is typically more prominent at the cathode GDL due to liquid water generation at

the cathode and electro-osmotic drag of water from the anode to the cathode [28]. This excess

liquid water accumulation at the cathode GDL, also known as cathode flooding, impedes fuel cell

performance by increasing oxygen transport resistance. Visualizing the accumulation and transport

behavior of liquid within the GDLs provides valuable insight for modeling and designing next-

generation GDLs.

The gas diffusion layer is typically composed of the following components:

• Gas diffusion layer substrate: The gas diffusion substrate is traditionally composed of

porous carbon fiber paper, graphite felt, or carbon cloth [29]. The carbon fiber substrate

provides structural integrity to the gas diffusion layer.

• Microporous layer (MPL): The MPL is an electrically and thermally conductive porous

layer, consisting of micron to sub-micron level pores [30,31], placed between the fuel cell

macro-porous gas diffusion substrate and catalyst layers. Conventionally, the MPL

comprises of carbon black and hydrophobic polytetrafluoroethylene (PTFE) binder. The

MPL serves to reduce contact resistance, reduce damage to the catalyst layer from the

fibers, and decrease liquid water accumulation (particularly at the cathode) [32].

13

2.3.4 Current collector/ flow field

The flow field plates are composed of a solid, electrically and thermally conductive material such

as graphite. On these plates, a set of channels are fabricated. These channels provide a means for

the reactants and byproducts to travel in and out of the fuel cell. The space between adjacent

channels is commonly known as ribs. The ribs provide a continuous pathway for the electrons to

travel to and from the catalyst layers, help control the fuel cell temperature, and provide structural

support to the MEA. The electrons conducting through the flow fields often conduct through

current collectors. This layer provides a means to connect the fuel cell to the external circuit.

Figure 2-1. Schematic of a typical polymer electrolyte membrane (PEM) fuel cell

14

2.4 Water balance within the fuel cell

Water is introduced into the fuel cell by two possible means. First, water is generated as a by-

product of the oxygen reduction reaction within the cathode catalyst layer (Eq. 2-2). The rate of

water generation is proportional to the current density of the fuel cell, as given by Faraday’s Law,

shown as

�̇� = 𝑣 ∙𝑖 ∙ 𝐴

𝑛 ∙ 𝐹

Eq. 2-3

where �̇� is the molar flow rate of an arbitrary generated species [mol/s], 𝑣 is the stoichiometric

coefficient of the corresponding arbitrary species, 𝑖 is the current density [A/cm2], 𝐴 is the active

area of the catalyst layer [cm2], 𝑛 is the number of electrons participating in the reaction, and 𝐹 is

Faraday constant [As/mol], which represents electrical charge per mole of electrons. The second

source of water is from the supplied reactant gases. For fuel cell systems, this humidity is achieved

through use of external humidifiers.

To achieve high fuel cell power output and efficiency, effective water management is crucial.

Although sufficient water is needed to hydrate the ionomer within the membrane and catalyst

layers to promote high protonic conductivity, an excess amount of water can lead to an

accumulation of liquid water within the catalyst layers and GDLs, which can hinder the transport

of reactant gases to the catalyst layer reaction sites [20]. As such, GDLs have been tailored to

address specific water management challenges within the PEM fuel cell [7-13,15-19]. The

following sub-sections describe two specific strategies in detail:

15

2.4.1 Hydrophilic MPLs for preserving membrane hydration at low gas humidification

Sufficient water is needed to hydrate the membrane to maintain high protonic conductivity. To

achieve this, external humidifiers are conventionally employed to humidify the reactant gases. It

is advantageous to remove the external humidifiers to help reduce the fuel cell system cost, size

and parasitic power demands [33,34]. The direct cost reduction of removing a humidifier from an

80-kW fuel cell system is estimated to be $1/kW [24], which by itself covers around 8% of the

total cost reductions required to meet the target of $40/kW by 2020. In addition, the decreased

parasitic power demand increases power output of the fuel cell and further reduces the cost of the

system. However, fuel cells typically suffer from performance losses (due to increased ohmic

resistance) upon removing (or reducing) external humidification of inlet gases [21-23]. For

example, the net power output of fuel cell systems (tested with commercial MEAs) decreased by

up to 17 % when external humidification was removed [34].

Recent studies have shown that hydrophilic MPLs improve the fuel cell performance under low

inlet humidification [7-13]. For instance, Kitahara et al. [7-10] fabricated multi-layered MPLs and

controlled the hydrophilicity of the cathode MPLs. They demonstrated that a 5-μm thick

hydrophilic MPL, situated between the catalyst layer and the adjacent hydrophobic MPL,

enhanced the fuel cell performance under low cathode humidification. They hypothesized that the

hydrophilic layer helped preserve the hydration state of the membrane and the adjacent

hydrophobic layer served as a barrier for water removal by dry air. Tanuma et al. [11-13] showed

that the fuel cell performance of a hydrophilic MPL, which consisted of an ionomer (Flemion®)

and vapor grown carbon fiber, was less susceptible to changes in gas pressure and inlet

16

humidification under high temperature operating conditions. Their findings suggest that the

addition of a hydrophilic MPL helps preserve water content within the membrane during low

humidity conditions. Ahn et al. [14] demonstrated that a cathode GDL substrate coated with a

hydrophilic MPL, which consisted of an ionomer (Nafion®) and carbon black, enhanced the fuel

cell performance at both the fully humidified and non-humidified conditions. Despite the

demonstrated performance improvement, the effect of the hydrophilic MPLs on the liquid water

distribution in the GDLs still needs to be better understood. The liquid water within the GDL

affects the fuel cell performance by influencing the membrane hydration state and the oxygen

transport resistance within the fuel cell. Visualizing liquid water within the GDLs can thus provide

insights on how the accumulated liquid water influences the fuel cell performance. The insights

gained from this liquid water information could be applied to design next-generation GDLs for

fuel cells that operate without external humidification.

2.4.2 Functionally graded GDLs for reduced cathode liquid water flooding at high current

densities

The fuel cell performance is often hindered at high current densities (typically ≥ 1.5 A/cm2)

because of excess liquid water accumulation, or flooding, within the cathode. This accumulation

of excess liquid water causes increased oxygen transport resistances, and leads to reduced oxygen

concentration at the cathode catalyst layer, which impedes the oxygen reduction reaction. The use

of a microporous layer (MPL) has been demonstrated to be an effective means to reduce the

prevalence of cathode flooding [35-41]. Several mechanisms have been proposed to explain the

improved water management capabilities of an MPL, such as increased rate of water diffusion

17

from the cathode to the anode [35-37], increased rate of water vapor diffusion from the cathode

catalyst layer to the cathode flow field, and enhanced capillary-driven transport of liquid water

from the cathode catalyst layer to the cathode flow field [38-41]. Several groups have demonstrated

that capillary-driven transport is the dominant mode of transport within MPLs when the gases are

fully humidified [38-41]. However, the capillary-driven transport within the MPL has the potential

to be further tailored with the use of functionally graded properties [15-19], since most commercial

MPLs are functionally homogenous.

In the case of functionally graded GDLs, Zhan et al. [15] simulated liquid water transport within

GDLs with a positive gradient in porosity (in the direction from the catalyst layer to the flow-field).

They reported that the porosity gradient favored liquid water discharge from the GDL and led to

reduced residual liquid water within the GDLs. Kong et al. [16] proposed bi-layer GDLs with

double backing layers for enhanced water removal. The GDL towards the exit had higher porosity

and lower hydrophobicity than the GDL towards the catalyst layer. Using numerical simulations,

the authors showed that these double GDLs enhanced liquid water removal capabilities of the GDL

compared to uniform GDLs. Wang et al. [17] used bi-layer GDL substrates (with two substrate

backing layers) with graded PTFE loading (with less PTFE content towards the flow-field exit).

The authors experimentally demonstrated that bi-layer GDLs with PTFE-graded substrates led to

higher fuel cell performance (higher limiting currents) and less voltage fluctuations, compared to

non-graded GDLs. They used numerical simulations to show that the PTFE gradient led to reduced

overall saturation within the GDLs. Tang et al. [18] created MPLs with graded porosity using

ammonium chloride pore-formers. They demonstrated that fuel cells with graded MPLs performed

better than conventional homogenous MPLs, particularly at high current densities. They

18

hypothesized that graded porosity within MPLs was beneficial in expelling liquid water more

effectively from the catalyst layer to carbon paper (compared to an MPL with uniform porosity).

These studies demonstrate the potential of functionally graded porous materials to achieve

enhanced capillary-driven removal of liquid water. However, the use of PTFE, a standard

component of the MPL, to create functionally graded MPLs has not yet been demonstrated. In

addition, liquid water within these functionally graded GDLs has not been quantified

experimentally during fuel cell operation. Liquid water distributions within GDL materials would

provide valuable insights into the effect of novel GDL designs on fuel cell performance.

2.5 Fuel cell diagnostic tools

Several electrochemical, physical, and chemical diagnostic tools are available for PEM fuel cell

research; for example: polarization curve measurement, current interruption, electrochemical

impedance spectroscopy, cyclic and linear sweep voltammetry, pressure drop measurement, gas

chromatography, neutron imaging, synchrotron X-ray imaging, and current and temperature

mapping [42,43]. To address research gap, we need to understand the effect of novel MPLs on fuel

cell water management and cell performance. Liquid water quantification, in combination with

measurements of membrane resistance and oxygen transport resistance, provides valuable insights

into these water management properties. The liquid water can be effectively quantified in an

opaque, operating fuel cells through use of synchrotron X-ray radiography, whereas the membrane

and oxygen transport resistances can be effectively quantified using electrochemical impedance

spectroscopy (EIS). Polarization curve is the standard tool for quantifying fuel cell performance.

A combination of these three diagnostic tools provide a powerful means to understand the water

19

management and cell performance characteristic within fuel cells with these novel MPLs. The

following sub-sections describe the basic principles of the three relevant fuel cell diagnostic tools:

2.5.1 Polarization curve and fuel cell overpotentials

A schematic of a typical fuel cell polarization curve is shown in Figure 2-2. A polarization curve

is the standard electrochemical tool to characterize the fuel cell performance [42], whereby the

voltage losses (also called polarizations or overpotentials) incurred at a given current density can

be quantified. The maximum theoretical cell potential, 𝐸°° [V], known as the enthalpy potential,

is given as

𝐸°° = −Δ𝐻

𝑛𝐹

Eq. 2-4

where Δ𝐻 represents the change in molar enthalpy of the reaction [J/mol]. However, not all of the

energy contained within the reactants and byproducts is usable because of the entropy change from

the reaction. As such, the enthalpy potentials is reduced to a maximum possible reversible voltage,

𝐸° [V], described as

𝐸° = −Δ𝐺

𝑛𝐹

Eq. 2-5

where Δ𝐺 represents the change in molar Gibbs free energy of the reaction [J/mol]. A departure

(or decrease) in 𝐸° occurs primarily when reactants travel across the membrane and interact with

the catalyst layer in the opposite fuel cell compartment; for example, hydrogen crossing over and

reacting within the cathode. Due to this departure from 𝐸°, the maximum realized cell potential is

at open circuit voltage (OCV) condition, when no current is drawn from the fuel cell, as shown in

20

Figure 2-2. When current is drawn from the fuel cell, the following overpotentials cause a decrease

in the cell potential:

1. Activation or kinetic overpotential at the electrodes: This overpotential occurs required to

overcome the activation potential of the reactions at the catalyst layer reaction sites. To

drive a reaction forward from the equilibrium state, a minimum amount of energy is

required to overcome a transition energy state. This transition energy is reduced, but not

be eliminated, with the use of catalysts. Region A shown in Figure 2-2 is dominated by this

overpotential.

2. Ohmic polarization: The ohmic polarization incorporates the losses incurred due to

resistance to the motion of protons within the membrane and catalyst layers, and the motion

of electrons within the fuel cell components (GDLs, flow fields, and connecting wires) and

component interfaces (contact resistance). The near-linear region B shown in Figure 2-2 is

dominated by ohmic overpotentials.

3. Concentration overpotential: The concentration overpotential accounts for the voltage

losses incurred from decreased reactant concentrations at the surface of the anode and

cathode reaction sites. This decrease in reactant concentration can occur due to gas phase

transport limitations of the reactants, and increased saturation of liquid water (that impedes

gas transport). This overpotential tends to dominate at higher current densities (region C

shown in Figure 2-2), because higher reactant fluxes are required to sustain the reactions.

Under these conditions the reactant transport is further impeded if the pores of the diffusion

layers are blocked by liquid water [25].

21

Figure 2-2. Sample fuel cell polarization curve. A, B, and C represent regions dominated by

activation, ohmic, and concentration overpotentials, respectively

2.5.2 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a powerful electrochemical diagnostic tool that

can be used to characterize and resolve a wide range of sources of polarization losses [42]. EIS has

been widely employed in PEM fuel cells to isolate the sources of electrical losses caused due to

individual electrochemical and transport phenomena [44-53]. In EIS, the cell voltage or current is

perturbed at a given amplitude and frequency and the response of the fuel cell is measured. This

perturbation can be provided at a range of frequencies to capture and isolate the impedance from

22

physical phenomena occurring at a wide range of time scales. Several authors [44,45,48-51,54]

have interpreted the resulting Nyquist plots obtained from EIS by developing an equivalent circuit

of the fuel cell; this process is known as equivalent circuit modeling. For instance, several authors

[44,48,53] have used the high frequency resistance obtained from EIS (at frequencies of ~1-5 kHz)

to quantify the ohmic resistance of the fuel cell. The ohmic resistance is composed of membrane

resistance, and electrical resistance within the fuel cell components and component interfaces

(contact resistances) [46,47,51,55]. At low frequencies (~0.1 Hz to 32 Hz), the oxygen transport

resistances, especially those within the cathode, can be captured [44,45,48-50].The frequencies

between the high and low frequencies often capture anode and cathode electrochemical kinetics.

In this thesis, EIS was used to quantify the ohmic and oxygen transport resistances, in conjunction

with liquid water measurements (obtained from synchrotron X-ray radiography, discussed in

Section 2.5.3) to understand the physical phenomena governing fuel cell performance.

2.5.3 Synchrotron X-ray visualization

Visualizing liquid water within the GDL during fuel cell operation provides valuable information

for the design of novel materials. However, the opacity of fuel cell materials presents challenges

for direct real-time imaging of liquid water within the fuel cell. Synchrotron X-ray radiography is

a powerful tool for real-time visualization of liquid water within the opaque fuel cell with high

spatial and temporal resolutions [56-64]. At a synchrotron facility, electrons are accelerated to

near-light speeds and circulated within a large circular ring. Dipole magnets are used to change

the direction of electrons, the process of which emits electromagnetic radiation (light) with high

photon flux (intensity). The high intensity light in the form of X-rays has the capability to penetrate

23

materials that are otherwise opaque; for instance, the high photon flux also allows for high

temporal resolutions to be obtained, which is ideal for in situ and in operando fuel cell experiments.

Furthermore, the X-rays can be tuned to specific wavelengths using a monochromator [65], which

increases the precision of measured thicknesses of attenuated material along the beam path [60,66].

Several researchers have used synchrotron X-ray radiography to visualize and quantify liquid

water within GDLs during fuel cell operation [56-64]. As such, the effect of particular GDL

parameters on liquid water distributions and transport can be examined. For instance, Lee et al.

[64] investigated the liquid water distributions in GDLs with and without MPLs. They

demonstrated that the presence of the MPL prevented the agglomeration of large liquid water

clusters at the catalyst layer-MPL interface, which led to for improving oxygen transport within

the GDL. Alrwashdeh et al. [57] investigated the water transport dynamics within MPLs with

perforations, and demonstrated that these perforations act as preferential pathways for liquid water.

Antonacci et al. [67] used a combination of electrochemical impedance spectroscopy and

synchrotron X-ray radiography to investigate the role of MPL thickness on the fuel cell

performance. By quantifying the ohmic resistance, mass transport resistance, and the liquid water

within the GDL, they concluded that the MPL thickness could be tailored to balance the mass

transport and membrane resistances. Furthermore, the quantified liquid water information can also

be combined with the porosity profile of the material [63,68-70] to quantify the liquid water

saturation within the GDL, which can provide useful information for GDL modeling and design

[71-73].

24

2.6 Chapter summary

In this chapter, the background and literature review for the thesis was presented. The basic

electrochemical principles of the fuel cell were presented, followed by a description of each of the

basic fuel cell components. The background and significance of water balance within the fuel cell

was discussed, with focus on two specific strategies. Section 2.4.1 presented the literature review

on the use of hydrophilic microporous layers (MPLs) to help preserve the membrane hydration

state at low gas humidification. Section 2.4.2 presents the literature review on the use of

functionally graded GDLs to reduce cathode liquid water flooding at high current densities. From

this literature survey, the following main research gaps were identified:

1. Hydrophilic MPLs have been demonstrated to improve fuel cell performance under low

humidity operation. However, there remains a need to understand how the hydrophilic

MPLs affect the liquid water distribution within the GDLs, as the liquid water influences

membrane hydration state and oxygen transport resistance within the fuel cell. The insights

gained from this liquid water information could be applied to design next-generation GDLs

for fuel cells that operate without external humidification.

2. Although functionally graded porous materials have been shown to enhance liquid water

removal, the use of PTFE, a standard component of the MPL, to create functionally graded

MPLs has not been demonstrated yet. Additionally, the effect that these graded materials

have on the liquid water distribution has not been quantified experimentally.

The research gaps that need to be addressed require an understanding of water management and

fuel cell performance within fuel cells with novel GDLs. Unique insights on water management

25

can be provided using a combination of liquid water visualization and measurements of membrane

resistance and oxygen transport resistance. A literature review was conducted to examine the

relevant diagnostic tools, namely polarization curve, electrochemical impedance spectroscopy, and

liquid water visualization using synchrotron X-ray radiography.

26

Chapter 3 Methodology


The two studies presented in this thesis are organized into two separate chapters, specifically

Chapter 4 and Chapter 5. The common methodologies between both studies are presented in this

chapter, whereas chapter-specific methodologies are discussed in their respective chapters. First,

the methodology used for fuel cell testing is presented in Section 3.2. In this section, a sub-section

(Section 3.2.1) is devoted to describing the fuel cell hardware and operating conditions. A second

sub-section (Section 3.2.2) presents the procedure to measure the fuel cell impedances using

electrochemical impedance spectroscopy (EIS). Finally, Section 3.3 presents the procedure to

quantify the amount of liquid water accumulated within the fuel cell, through use of synchrotron

X-ray radiography.

3.2 Fuel cell testing

This section describes the methodology employed for fuel cell testing. The details on the hardware

and operating conditions used fuel cell testing is described in Section 3.2.1. Section 3.2.2 presents

the procedure for quantifying fuel cell impedances using equivalent circuit model fitting of EIS

measurements.

3.2.1 Fuel cell hardware and operating conditions

A custom miniature fuel cell, shown in Figure 3-1, was used to simultaneously measure electrical

output (e.g., current, potential) and impedances and liquid water accumulation within the fuel cell.

27

The anode and cathode flow fields consisted of parallel channels and ribs, with dimensions of 0.5

mm for rib width, channel width, and channel depth. The fuel cell had an active area of 0.68 cm2

(0.85 cm × 0.80 cm). The MEA consisted of a commercial catalyst coated membrane (CCM) that

was sandwiched between the anode and cathode gas diffusion layers (GDLs). The CCM comprised

of a Nafion® HP membrane coated with Pt/C catalyst layers (Ion Power) with platinum loadings

of 0.30 mg/cm2 on each side. Each GDL were compressed to the desired thickness controlled using

rigid polyethylene naphthalate (PEN) spacers. The fuel cell MEAs were compressed through use

of 6 M4 bolts that were each tightened to a torque of 20 lbf-in. The fuel cell was conditioned with

ten voltage cycles between a cell potential of 0.90 V to 0.30 V at a step of 0.10 V. Each cell

potential was held for 3 minutes. For the conditioning, hydrogen (Grade 5.0) and air (Grade 0.1)

were supplied at flow rates of 1 L/min and relative humidity of 90%. By the tenth cycle, the

polarization curve was found to be consistent between successive cycles.

The fuel cell was controlled using a fuel cell test stand (Scribner 850e, Scribner Associates Inc.)

equipped with a potentiostat (885 Fuel Cell Potentiostat, Scribner Associates Inc.). Hydrogen and

air were supplied at inlet volume flow rates of 1 slpm. High gas flow rates of 1 L/min (1.47

L/(min.cm2)) were used to prevent liquid water accumulation within the channels, and to maintain

uniform gas concentrations along the length of the flow channels. Subsequent to the conditioning

described above, the current density was maintained at incremental constant steps for 15 minutes

each. This 15-min period facilitated steady state conditions for the water distributions and cell

voltages, as shown by Banerjee et al. [61]. The cell temperature was measured using a T-type

thermocouple (5SRTC-TT-T-30-36, OMEGA Engineering Inc.), embedded within the cathode

flow field under the active area. The cell temperature was maintained at 60 °C by circulating heated

28

water through the end plates of the fuel cell, using a water bath (Isotemp™ 4100R20, Fisher

Scientific Co.).

Figure 3-1. Schematic of the fuel cell test setup.

3.2.2 Fuel cell impedance measurements

At the end of each constant current step, electrochemical impedance spectroscopy (EIS) was

performed between the frequencies of 0.1 Hz and 10 kHz with an amplitude of 10 % of the DC

current, using the fuel cell test stand. Figure 3-2 shows the equivalent circuit used to analyze the

fuel cell impedances [44,48,51,52,74].

29

Figure 3-2. Equivalent circuit used to fit Nyquist plots obtained from electrochemical impedance

spectroscopy.

The total impedance of the equivalent circuit, 𝑍𝑇𝑜𝑡, can be described as

𝑍𝑇𝑜𝑡 = 𝑍𝐴 + 𝑍𝐶 + 𝑍Ω , Eq. 3-1

where 𝑍𝐴 is the impedance of the electrochemical reaction at the anode, 𝑍𝐶 is the cathode

impedance, and 𝑍Ω is the ohmic impedance. The ohmic impedance (𝑍Ω ) is equivalent to the ohmic

resistance of the fuel cell (𝑅Ω ) as

𝑍𝛺 = 𝑅𝛺 Eq. 3-2

The ohmic resistance consists of membrane resistance, the electrical resistance within the fuel cell

components, and the contact resistances between material interfaces [46,47,52,55]. The electrical

and contact resistances of the fuel cell configurations were assumed to be constant, due to the

implementation of identical test setups (i.e., identical MEAs, PEN spacers, and fuel cell). However,

the variation in the measured ohmic resistance was attributed to changes in the membrane

resistance, caused by differences in the hydration state of the membrane. The dominance of

30

membrane resistance in the ohmic resistance has also been shown in the literature [55]. The ohmic

resistance, 𝑅Ω, was quantified as the high frequency resistance (i.e., real component of the EIS

impedance at 5 kHz).

The anode electrochemical impedance can be described as

𝑍𝐴 = (1

𝑅𝐴+ 𝑗 ∙ 𝜔 ∙ 𝐶𝐴)

−1

, Eq. 3-3

where 𝑅𝐴 is the anode activation resistance, 𝐶𝐴 is the anode electric double layer capacitance, 𝜔 is

the frequency of the AC signal in rad/s, and 𝑗 is the unit imaginary number, √−1.

The cathode electrochemical impedance can be described as

𝑍𝐶 = (𝑗 ∙ 𝜔 ∙ 𝐶𝐶 +1

𝑅𝑃 + 𝑍𝑊)

−1

, Eq. 3-4

where 𝐶𝐶 is the cathode electric double layer capacitance, 𝑅𝑃 is the charge transport resistance

related to the electrochemical kinetics, and 𝑍𝑊 is the Warburg impedance. The Warburg

impedance is described as

𝑍𝑊 =𝑅𝑚𝑡 tanh(√𝑗 ∙ 𝜔 ∙ 𝜏)

√𝑗 ∙ 𝜔 ∙ 𝜏,

Eq. 3-5

where 𝑅𝑚𝑡 is the mass transport resistance of oxygen at the cathode, and 𝜏 is the diffusive time,

which can be expressed as

31

𝜏 =𝐿𝑦

2

𝐷𝑒𝑓𝑓,

Eq. 3-6

where 𝐿𝑦 is the GDL thickness and 𝐷𝑒𝑓𝑓 is the effective diffusion coefficient. In porous media,

the effective diffusion coefficient is proportional to the bulk diffusion coefficient and the porosity,

and inversely proportional to the tortuosity [75]. The equivalent circuit model was fit to the

measured impedance data using ZView software (Scribner Associates Inc.), where the weight of

each data point was normalized by its magnitude to obtain the fit.

3.3 Synchrotron X-ray visualization

The custom fuel cell was visualized during operation using synchrotron X-ray radiography at the

Biomedical Imaging and Therapy Bending Magnet (05B1-1) beamline facility at the Canadian

Light Source (CLS) in Saskatoon, Canada [76]. A monochromatic collimated X-ray beam with

photon energy of 24 keV was used to visualize the cell in the plane perpendicular to the z-direction

(where z-direction was into the plane of the figure in Figure 3-3). A 10 µm-thick scintillator (AA40,

P43, Hamamatsu Photonics K.K.) was used to convert the transmitted X-ray irradiance into visible

light. The converted light was detected by a digital scientific complementary metal-oxide-

semiconductor (sCMOS) camera (ORCA-Flash4.0, Hamamatsu Photonics K.K.). The radiographs

had pixel and temporal resolutions of 6.5μm/pixel and 3 s per frame, respectively.

The image intensities in the raw radiographs were processed to obtain liquid water thickness values.

The raw radiographs were corrected for background camera noise, decay in the beam intensity

over time, and unwanted translation of the cell during cell operation. The background camera noise

was characterized from an image of the fuel cell (called dry-state image) that was captured without

32

the application of the X-ray beam. The background intensities of this dry-state image were

subtracted from the intensities in the test images (called wet-state images) to correct for

background noise. The decay in beam intensity was characterized by tracking the average intensity

of a region unaffected by liquid water (such as the region above the anode channels). The change

in the intensity of this region over time provided a quantification of the decay in beam intensity

and was used to correct all the test images. Unwanted translation or movement was detected using

negative values in processed images at material interfaces that were free of liquid water (such as

the interface between the anode rib and channel of the flow field). Each test image was translated

to minimize this detected movement, if such a movement was detected. For a more detailed

explanation of these image correction procedures, the reader is referred to the publications by Ge

et al. [66] and Hinebaugh et al. [62]. A sample image of the corrected radiograph is presented in

Figure 3-3 (a). The image is an average of 100 frames (equivalent to 5 minutes) at the end of the

2.0 A/cm2 current density step. The axes in red show the x- and y-(through-plane) directions (z-

direction is into the plane of the figure) used in this thesis. The boundaries of anode and cathode

GDLs are shown in green solid lines. The boundaries of the catalyst coated membrane are shown

with dotted red lines.

The corrected images were normalized with respect to a reference “dry-state” image to obtain the

liquid water thickness. This reference image was obtained by averaging 5 frames at the end of

open circuit voltage conditions at the start of each test. The transmitted irradiance from the

reference sample (i.e., each pixel of the dry-state image) can be described using the Beer-Lambert

Law as

33

𝐼𝑅𝑒𝑓 = 𝐼0 exp(− ∑(𝜇 ∙ 𝑡)𝑅𝑒𝑓), Eq. 3-7

where 𝐼𝑅𝑒𝑓 and 𝐼0 are the transmitted and incident irradiances of the reference sample respectively,

𝜇 is the attenuation coefficient of the material in cm-1 and 𝑡 is propagation distance of the beam

within the material in cm.

Similarly, the transmitted irradiance from the test sample can be described using the Beer-Lambert

Law as

𝐼𝑊𝑒𝑡 = 𝐼0 exp(− ∑(𝜇 ∙ 𝑡)𝐼𝑊𝑒𝑡), Eq. 3-8

where 𝐼𝑊𝑒𝑡 and 𝐼0 are the transmitted and incident irradiances of the reference sample respectively.

Assuming that the reference sample was dry at open circuit voltage conditions and that the

corrected images were free of unwanted movement, the corrected transmitted irradiance of the test

sample can be described in terms of liquid water (added to the system) as

𝐼𝑊𝑒𝑡 = 𝐼0 exp(− ∑(𝜇 ∙ 𝑡)𝑅𝑒𝑓 − 𝜇𝑤 ∙ 𝑡𝑤), Eq. 3-9

where 𝜇𝑤 is the attenuation coefficient of water in cm-1 and 𝑡𝑤 is thickness of water along the

beam path [cm].

Combining Eq. 3-7 and Eq. 3-9, and dividing by the GDL length along the beam path (𝐿𝑧 =

0.80𝑐𝑚), we obtain an expression for the normalized liquid water thickness [cm/cmGDL] in the test

sample for each pixel location as a function of x-, y- directions and time, as

34

𝑡𝑤,𝑛(𝑥, 𝑦, 𝑡) = −1

𝜇𝑤 ∙ 𝐿𝑧ln

𝐼𝑊𝑒𝑡(𝑥, 𝑦, 𝑡)

𝐼𝑅𝑒𝑓(𝑥, 𝑦).

Eq. 3-10

A sample image of the processed radiograph is shown in Figure 3-3 (b), where the value of each

pixel corresponds to normalized liquid water thickness, 𝑡𝑤,𝑛(𝑥, 𝑦). The MEA regions under the

channels and the ribs are shown with solid and dotted lined white boxes respectively.

The normalized water thickness values were averaged along x-direction and time to obtain y-axis

profiles (at the constant current steps tested), as

𝑡�̅�,𝑛(𝑦) =1

𝑁𝑡 ∙ 𝑁𝑥∑ ∑ 𝑡𝑤,𝑛(𝑥𝑖, 𝑦, 𝑡𝑘)

𝑁𝑥

𝑖=1

𝑁𝑡

𝑘=1

,

Eq. 3-11

where 𝑡�̅�,𝑛 is the average normalized liquid water thickness profile along the y-direction, 𝑁𝑡 is the

total number of frames averaged over time, 𝑁𝑥 is the number of pixels averaged in the x-direction.

The GDL regions under the flow field channels and ribs were averaged and presented separately,

as shown in Figure 3-3 (c) and (d), to capture the inhomogeneity in the water distribution under

the two regions [69]. 𝑁𝑥 was 567 and 584 for the regions under 7 central ribs and 8 channels

respectively. At the end of each constant current test 100 frames, equivalent to 5 mins, were

averaged over time. The measurement uncertainty was quantified with a coverage factor of 3,

corresponding to a confidence interval greater than 99 %, as detailed by Chevalier et al. [60].

To analyze the evolution and accumulation of liquid water over time, the temporal profile of the

averaged normalized liquid water, 𝑡�̅�,𝑛(𝑡), was calculated for the regions of interest (within the

cathode GDL in Section 5.4.3), as

35

𝑡�̅�,𝑛,𝑡(𝑡) =1

𝑁𝑦 ∙ 𝑁𝑥∑ ∑ 𝑡𝑤,𝑛(𝑥𝑖, 𝑦𝑙, 𝑡)

𝑁𝑥

𝑖=1

𝑁𝑦

𝑙=1

,

Eq. 3-12

where 𝑁𝑦 is the total number of frames averaged in the y-direction.

Figure 3-3. a) Sample radiograph showing the components of the fuel cell. The axes in red show

the x- and y-(through-plane) directions (z-direction goes into the plane of the figure). The

boundaries of anode and cathode GDLs are shown using green solid lines. The boundaries of the

catalyst coated membrane are shown with dotted red lines. b) Sample processed image with the

color of each pixel corresponding to normalized liquid water thickness. The MEA regions under

the channels and the ribs are shown with solid and dotted lined white boxes respectively. The white

scale bars in the lower right corner of a) and b) are equivalent to 1 mm. Sample profile of

36

normalized liquid water thickness along the through-plane position (y-direction) for regions under

c) the channels and d) the ribs. The liquid water profiles were normalized by the distance within

the GDL traversed by the beam.

Liquid water saturation profile in the y-direction, 𝑠𝑤(𝑦), was calculated by dividing the normalized

liquid water thickness profile within the GDL by the GDL porosity profile as

𝑠𝑤(𝑦) =𝑡�̅�,𝑛(𝑦)

𝜀(𝑦).

Eq. 3-13

The liquid water saturation profile along the y-direction was further averaged in regions of interest

(as defined in the Section 4.5.3), as

�̅�𝑤,𝑎𝑣𝑒 =1

𝑁𝑦∑ 𝑠𝑤(𝑦𝑗)

𝑁𝑦

𝑗=1

,

Eq. 3-14

where 𝑁𝑦 is the number of pixels averaged in the region of interest, and �̅�𝑤,𝑎𝑣𝑒 is the liquid water

saturation averaged along the x- and y- directions, and time. The uncertainty in the average was

calculated as the root mean square of the uncertainties in the liquid water saturation profile values

in the region of interest.

3.4 Chapter summary

This chapter presented the common methodology employed by the two studies presented in

Chapter 4 and Chapter 5. This common methodology includes the experimental setup, the fuel cell

37

design, and the operating conditions used in this thesis, along with the experimental techniques to

diagnose the fuel cell performance; namely, electrochemical impedance spectroscopy (EIS) and

synchrotron X-ray radiography. The EIS data was used to quantify the ohmic and oxygen transport

resistances. Synchrotron X-ray radiography was used to visualize the liquid water distribution and

to help understand the impact of the presence of liquid water on the fuel cell performance.

38

Chapter 4 Hydrophilic microporous layer coatings for polymer electrolyte

membrane fuel cells operating without anode humidification

4.1 Chapter abstract

For the study presented in this chapter, a hydrophilic microporous layer (MPL) coating was applied

to a commercial hydrophobic bi-layer gas diffusion layer (GDL). We investigated the effect of the

hydrophilic MPL coating on membrane hydration and liquid water distribution within the GDLs

during fuel cell operation without anode humidification, using fuel cell performance monitoring

and simultaneous synchrotron X-ray visualization. The application of the hydrophilic coating was

found to enhance performance of the fuel cell. Specifically, the application of the hydrophilic MPL

coating led to an increase in cell potential of up to 14 % (0.07 V at 1.5 A/cm2) and a decrease in

fuel cell ionic resistance, while resulting in a marginal change in the pore structure of the GDL.

The decrease in ionic resistance was attributed to improved membrane hydration. This

improvement in membrane hydration was hypothesized to be caused by the increase in liquid water

retention at the catalyst layer-MPL interfaces. At high current densities, the application of the

hydrophilic MPL coating led to increased liquid water accumulation within the cathode GDL,

which subsequently led to increased oxygen transport resistance. This chapter demonstrates that

the wettability of the transport layers in a fuel cell can be tailored to enhance fuel cell performance

for a desired range of operating conditions by balancing membrane hydration and oxygen transport.

39


External humidifiers are conventionally employed in a fuel cell system to humidify the reactant

gases and hydrate the membrane to ensure high protonic conductivity [21]. However, it is

advantageous to remove the external humidifiers from a fuel cell system to simplify the fuel cell

system and to reduce its cost, size and parasitic power demands [33,34]. Hydrophilic MPLs

provide promising performance improvements under low humidity conditions. However, the

influence of hydrophilic MPLs on in-operando liquid water distributions in the GDL needs to be

better understood, to inform the design of next-generation GDLs suited for fuel cell operation

without humidification.

In this chapter, we investigate the effect of the application of a hydrophilic MPL coating on

membrane hydration and liquid water distribution within GDLs during fuel cell operation without

external anode humidification. We monitor the electrical output and impedances of the fuel cell

while simultaneously visualizing the GDL liquid water distribution via synchrotron X-ray

radiography. We will focus our discussion on tests conducted with an inlet relative humidity of 0 %

for the anode and 100 % for the cathode.

4.3 Chapter-specific methodology

In this chapter, custom hydrophilic microporous layer coatings were applied to commercial

hydrophobic GDLs. Porosities of the GDLs were characterized using X-ray micro-computed

tomography (micro-CT). Fuel cells were assembled with custom-made and commercial GDLs.

The fuel cell electrical output (cell potential, power density) and impedances and the GDL liquid

water distributions of the two fuel cell configurations were compared and evaluated.

40

4.3.1 Hydrophilic microporous layer coatings

Custom hydrophilic MPL coatings were applied onto commercial hydrophobic Sigracet (SGL)

25 BC GDLs (Sigracet® GmbH), as illustrated in Figure 4-1. The hydrophilic MPL was coated on

the free surface of the hydrophobic MPL of the SGL 25 BC GDL and was positioned towards the

catalyst layer in an MEA. Table 4-1 provides the specifications of the GDL materials used in the

study presented in this chapter. The unmodified SGL 25 BC GDL, named bi-layer GDL in this

chapter, consisted of a hydrophobic carbon fiber substrate with a hydrophobic coated MPL. This

bi-layer GDL was used as a reference. The in-house modified GDL, named tri-layer GDL, was

composed of an SGL 25 BC GDL that was coated with the custom hydrophilic MPL slurry. For

each of the experiments, the same GDL material was used on both the anode and the cathode.

The slurry for the hydrophilic coating was composed of carbon black (Vulcan® XC-72R, Fuel Cell

Store), deionized water, a surfactant (Triton™ X-100, Sigma-Aldrich), and a dispersion of

perfluorosulphonic acid ionomer (10 wt.% of ionomer in water, Nafion® dispersion D1021,

IonPower) in the ratio of 1: 10: 0.2: 0.2 by weight. The surfactant was added to the deionized water

and stirred for 20 minutes using a magnetic stirrer. The carbon black was added to the mixture in

three equal batches by weight. The resulting slurry was stirred and sonicated continuously for 30

minutes at 50 % amplitude (equivalent to 90 µm tip displacement amplitude) using a Q125

sonicator with a 1/8 inch-diameter #4422 probe tip (Q Sonica, LLC). The ionomer dispersion was

added to the slurry and the resulting mixture was stirred for 25 minutes (using a magnetic stirrer)

and sonicated continuously for 30 minutes at 50 % amplitude.

41

Table 4-1. Details of the GDL materials used in the study presented in this chapter. The base

material for all the GDLs was SGL 25 BC.

GDL name Layers within GDL Custom fabrication details

Bi-layer GDL Hydrophobic MPL

Hydrophobic substrate

Unmodified commercial SGL 25 BC

Tri-layer GDL Hydrophilic MPL coating

Hydrophobic MPL

Hydrophobic substrate

SGL 25 BC coated with hydrophilic MPL

coating, consisting of carbon black and

Nafion® (2 wt.% of carbon black)

The slurry was manually applied with a film thickness of 5μm, on the MPL side of SGL 25 BC

GDL, using an adjustable micrometer film applicator (Microm II, Paul N. Gardner Company, Inc.).

The slurry was dried with a controlled relative humidity (RH) of 98 % at ambient temperature in

an environmental chamber (Tenney C-EVO Temperature / Humidity Test Chamber, Thermal

Product Solutions) for 3.5 days. The high RH within the environmental chamber slowed the rate

of water evaporation from the MPL compared to ambient conditions, thereby reducing the drying

stresses within the MPL (to avoid the formation of mud cracks within the MPL). Subsequently,

the MPL was heated to 250 °C for 1 hour and treated with a 1-hour sintering process at 350 °C in

a convection oven (DHG9000JB, MTI Corporation). The resulting MPL had a carbon black to

ionomer (solid) weight ratio of 1 : 0.02. It should be noted that a hydrophilic material exhibits a

water contact angle (angle between the liquid-vapour and solid-liquid interfaces) less than 90°,

and a hydrophobic material exhibits a water contact angle greater than 90°. Binder-free (untreated)

carbon is hydrophilic with a contact angle of ~ 23° [14], and the ionomer has a hydrophilic receding

42

contact angle of 25° [77]. The combination of the two materials rendered the custom MPL coating

hydrophilic, in comparison to the hydrophobic commercial MPL in SGL 25 BC. The apparent

contact angle of a porous surface is influenced by surface roughness [78]. The average surface

roughness (i.e. arithmetic average of surface height deviations from the mean height) for the

SGL 25 BC MPL was 8 ±2 µm [79,80]. The contact angles for the custom MPL surfaces were not

measured, since the apparent contact angle measured at the surface of custom MPLs is expected

to be influenced by the surface roughness in addition to the contact angles of the MPL constituents.

Figure 4-1. Schematic of the procedure of applying hydrophilic MPL coating on SGL 25 BC

GDL.

4.3.2 Microstructure characterization

The porosity profiles of the compressed GDL materials were characterized using X-ray micro-

computed tomography (micro-CT). The GDLs were compressed in a custom compression device

to simulate the compression under the ribs of the fuel cell [81]. The compressed GDLs were

scanned using a desktop micro-CT scanner (SkyScan 1172, Bruker Corporation) to obtain two-

dimensional (2D) radiographic projections. The 2D projections of the GDLs were reconstructed

43

into three-dimensional (3D) images using NRecon software (Bruker Corporation). The

reconstructed images were segmented into trinary images that consisted of void, MPL, and fiber

voxels using an in-house segmentation software. The porosity profiles of the GDLs, 𝜀(𝑦), along

the thickness of the GDLs were computed using the microstructural information obtained from the

segmented images. For more details regarding the procedure for calculating the porosity profiles,

please refer to the work by Banerjee et al. [81].

4.3.3 Fuel cell testing

This section details the fuel cell hardware, operating conditions and repeatability tests that were

specific to the study in this chapter. For a detailed description of the fuel cell test setup and the

applied procedure for the fuel cell tests, the reader is referred to Section 3.2 in Chapter 3.

4.3.3.1 Fuel cell hardware and operating conditions

The following fuel cell hardware and operating conditions were specific to the study in this Chapter:

1. The GDLs were compressed to 72-74 % of their original thickness using rigid polyethylene

naphthalate (PEN) spacers.

2. Hydrogen and air were supplied at inlet volume flow rates of 1 slpm, back pressure of

100 kPa (gauge), and inlet relative humidity of 0 % (anode) and 100 % (cathode)

3. The fuel cell current density was maintained at constant steps of 0.25, 0.50, 1.0, 1.5, and

2.0 A/cm2 for 15 minutes each.

44

4.3.3.2 Repeatability tests

Each fuel cell test was repeated three times to analyze the reproducibility of the performance

results. The three sets of repeatability tests were named Test 1, 2, and 3 in this chapter. The

variability in the fuel cell performance between different material batches and fuel cell builds were

tested, as detailed in Table 4-2. The standard deviation among the three tests was calculated and

reported as error bars in the results section (Section 4.5). Test 1 was conducted with synchrotron

X-ray visualization.

Table 4-2. Details of tests performed to analyze repeatability of performance results

Repeatability test Material batch Fuel cell build

Test 1 Batch I Build A

Test 2 Batch I Build B

Test 3 Batch II Build C


For the methodology on synchrotron X-ray radiography, the reader is referred to Section 3.3 of

Chapter 3.

4.4 Results and discussion

This section presents the results and discussion of the study presented within this chapter. First,

the membrane resistance at varied operating inlet RH was examined to emphasize the need for

45

reducing membrane resistance at low anode inlet RH. Fuel cell performance monitoring was

coupled with synchrotron visualization to study the effect of the application of hydrophilic MPL

coatings on membrane resistance and liquid water retention (at the catalyst layer-MPL interfaces).

Finally, cathode GDL liquid water accumulation and oxygen transport resistances at high current

densities were examined.

4.4.1 Need for reducing membrane resistance at low anode inlet RH

This sub-section demonstrates the need for reducing the membrane resistance of the PEM fuel cell

operated without anode humidification. Figure 4-2 depicts the ohmic resistances of a fuel cell

containing the hydrophobic bi-layer GDL for a range of operating inlet RH and current densities.

It was observed that the ohmic resistance of this fuel cell configuration was more sensitive to

changes in the anode humidity than changes in the cathode humidity. For example, at 1.0 A/cm2,

the ohmic resistance increased by 0.062 Ωcm2 (101 %) when the anode was operated without

humidification, compared to the test with humidification (100 % RH at anode and cathode), as

shown in Figure 4-2 (a). Comparatively, the increase in ohmic resistance between the tests with

and without cathode humidification (100 % and 0 % RH at cathode respectively) was 0.022 Ωcm2

(35 %) as shown in Figure 4-2 (b). This result indicates that the membrane was more susceptible

to dry-out when the anode gas was not humidified, as compared to when the cathode gas was not

humidified. This occurs because water can only enter a non-humidified anode compartment

through diffusion or thermo-osmosis across the membrane (pressure driven flow is negligible since

46

anode and cathode gases are pressurized to the same level). On the other hand, a non-humidified

cathode compartment has two additional means of water entry (both of which increase with current

density), namely electro-osmotic drag and water generation from oxygen reduction reaction. As

such, there is a need to reduce the elevated ohmic resistances of the fuel cell operated without

anode humidification. In the following sections, we will investigate the effect of the application of

a hydrophilic MPL coating on membrane hydration and GDL liquid water distribution. We will

focus our discussion on tests conducted with an inlet relative humidity of 0 % for the anode and

100 % for the cathode.

47

Figure 4-2. The effect of reducing the inlet humidification on the ohmic resistance of a fuel cell

with the bi-layer GDL for varied a) anode and b) cathode inlet RH. Anode and cathode inlet RH

are presented in the legend with A as anode and C as cathode.

0.0 0.5 1.0 1.5 2.00.05

0.10

0.15

0.20

A0% C100% RH

A50% C100% RH

A100% C100% RH

Oh

mic

Re

sis

tance

(c

m2)

Current Density (A/cm2)

Decreasing anode inlet RH

0.0 0.5 1.0 1.5 2.00.05

0.10

0.15

0.20

A100% C0% RH

A100% C50% RH

A100% C100% RH

Oh

mic

Re

sis

tance

(c

m2)


Decreasing cathode inlet RH

a)

b)

48

4.4.2 Hydrophilic coatings for reducing membrane resistance

The effect of the application of hydrophilic MPL coating on ohmic resistance, cell voltage, and

power density are shown in Figure 4-3. Figure 4-3 (a) presents ohmic resistance of fuel cells

containing the bi-layer and tri-layer GDLs at each of the tested current densities. Since the two

fuel cell configurations had identical test setups (i.e. identical MEAs, PEN spacers, and fuel cell),

the changes in the measured ohmic resistance is dominated by the changes in the membrane

resistance. For both materials, the ohmic resistance was highest at the lowest current density and

decreased with increasing current density. The high ohmic resistances were attributed to poor

membrane hydration caused by the influence of the dry inlet anode gas. With increasing current

density, the rate of electrochemical water generation at the cathode increased, which facilitated an

increased uptake of water by the membrane. This led to better hydration of the membrane and a

decrease in ionic resistance with increasing current density. Comparing the results between the

two materials showed that the application of a hydrophilic MPL coating (in the tri-layer GDL) led

to a decrease in the membrane resistance. At current densities greater than 0.25 A/cm2, ohmic

resistances for the fuel cell containing the tri-layer GDL decreased by up to 19% (or 0.020 Ωcm2,

at 1.5 A/cm2) relative to the fuel cell containing the bi-layer GDL. The cause of the decrease in

ohmic resistance was hypothesized to be improved liquid water retention at the MPL-catalyst layer

interface and was investigated further in Section 4.5.3.

49

Figure 4-3. The effect of hydrophilic MPL coating on a) ohmic resistance, and b) cell voltage and

power density for fuel cells with the bi-layer and tri-layer GDLs. The inlet relative humidity was

maintained at 0 % for the anode and 100 % for the cathode.

Figure 4-3 (b) presents plots of steady-state cell potential (left vertical axis) and power density

(right vertical axis) of the fuel cell at the current densities tested with dry anode inlet gas. Reduction

in membrane resistance at a particular current density contributes to an increase in voltage and

0.0 0.5 1.0 1.5 2.00.05

0.10

0.15

0.20 Bi-layer GDL

Tri-layer GDL

Ohm

ic R

esis

tance (c

m2)


0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

Bi-layer GDL

Tri-layer GDL

Ce

ll V

olta

ge

(V

)


0.0

0.2

0.4

0.6

0.8

1.0

Po

we

r D

en

sity (

W/c

m2)

a)

b)

50

power output of the fuel cell. At all the current densities, the steady-state voltage output and power

density of the fuel cell containing the tri-layer GDL increased) relative to the fuel cell containing

the bi-layer GDL, by a maximum increase of to 14% (or 0.07 V and 0.10 W/cm2, at 1.5 A/cm2).

Between the current densities of 1.5 and 2.0 A/cm2, a relatively steep decrease in cell potential was

measured for the fuel cell with the tri-layer GDL. The associated increase in overpotential was

dominated by the increased oxygen transport resistance, caused by increased water accumulation

within the cathode tri-layer GDL. This effect is further investigated in Section 4.5.4. It should be

noted that the large error bars in cell potential (±0.05 V) and power density (±0.10 W/cm2), at a

current density of 2.0 A/cm2, were attributed to the variation in concentration overpotential (due

to oxygen transport losses) between Batches I and II of the tri-layer GDL. The current density of

2.0 A/cm2 was close to the threshold that marked the onset of the high concentration overpotential,

(caused by liquid water flooding at cathode tri-layer GDL). This onset of notable concentration

overpotential occurred before and after 2.0 A/cm2 for fuel cells with Batch I and II respectively.

This caused the variation in performance observed at the 2.0 A/cm2 current density step (for the

tri-layer GDL) and led to the high error bars.

4.4.3 Interfacial liquid water retention

This sub-section presents liquid water results obtained from synchrotron X-ray visualization

conducted during fuel cell operation (Test 1). The through-plane liquid water thickness profiles

(normalized with respect to the GDL thickness along the beam path) for the current densities of

1.0, 1.5, and 2.0 A/cm2 are presented in Figure 4-4. The MEA regions under the channels and the

51

ribs, as shown in Figure 4-4 (a) and (b) respectively. The MEA region includes the anode GDL,

the CCM, and the cathode GDL. Liquid water in the MEA regions under the flow-field channels

and ribs are shown in Figure 4-4 a) and b), respectively. An increase in average liquid water

thickness levels was observed with increasing current density. This was attributed to the increase

in the rate of electrochemical water generation with increasing current density. At high current

densities (≥ 1.5 A/cm2), the measured liquid water under the flow field ribs was higher than the

water under the flow field channels. This heterogeneity in the liquid water profiles is expected due

to the local condensation at the hydrophilic graphite ribs [69]. Comparing the results between the

two materials showed that the MEA with the tri-layer GDL had higher average water thickness

levels than the MEA with the bi-layer GDL. The increased liquid water thickness levels,

specifically at the catalyst layer-MPL interfaces, were investigated for all the current densities in

Figure 4-5.

52

0 50 100 150 200 250 300 350

0.0

0.2

0.4

0.0

0.2

0.4

0.0

0.2

0.4

Bi-layer GDL Tri-layer GDL

Through-plane position (µm)

2.0 A/cm2

Norm

aliz

ed liq

uid

wate

r th

ickness (

)

1.5 A/cm2

MPL MPL

CCM Cathode GDL

1.0 A/cm2

Anode GDL

0 50 100 150 200 250 300 350

0.0

0.2

0.4

0.0

0.2

0.4

0.0

0.2

0.4



2.0 A/cm2

Norm

aliz

ed liq

uid

wate

r th

ickness (

)

1.5 A/cm2

MPL MPL

CCM Cathode GDL

1.0 A/cm2

Anode GDL

a)

b)

53

Figure 4-4. Through-plane liquid water thickness profiles (normalized by the distance within the

GDL traversed by the beam) under a) channels and b) ribs for MEAs with the bi-layer and tri-layer

GDL. The inlet relative humidity was maintained at 0 % for the anode and 100 % for the cathode.

The liquid water saturation values averaged at the anode and cathode catalyst layer-MPL interfaces

are presented in Figure 4-5 (a) and (b) respectively. The catalyst layer-MPL interface could not be

isolated as a pixel-wide region since a sharp infinitesimally thin catalyst layer-MPL interface is

not realized in practical fuel cell assemblies [82]. For this study, the catalyst layer-MPL region

was approximated as a 2 pixel-wide (or 13 µm-wide) region at the microporous layer adjacent to

the catalyst layer. Since the hydrophilic MPL coating was thin (< 1 pixel), this 13 µm-wide region

included regions of the hydrophilic MPL coating and the hydrophobic MPL. Average liquid water

saturation values were calculated as described earlier (in Section 3.3). The fractional liquid water

saturation levels at the catalyst layer-MPL interfaces increased by up to 0.14 (i.e., a 97% and 120%

increase for the cathode and anode interfaces, respectively, at 1.5 A/cm2) for the fuel cell

containing the tri-layer GDL, relative to the fuel cell containing the bi-layer GDL. The application

of a hydrophilic MPL coating led to an increase in liquid water retention at the catalyst layer-MPL

interfaces. This increase in interfacial liquid water retention was hypothesized to improve the

hydration state of the membrane. The improved membrane hydration led to decreased membrane

resistance, as shown in Figure 4-3 a). This subsequently led to increased cell voltage and power

density, shown in Figure 4-3 b).

54

Figure 4-5. Average liquid water saturation at the a) anode and b) cathode MPL-catalyst layer

interfaces (13 µm or 2 pixels were averaged in y-direction in the GDL) of fuel cells with the bi-

layer GDL and the tri-layer GDL. All the fuel cell tests were conducted with inlet relative humidity

of 0% at the anode and 100% at the cathode.

0.25 0.5 1.0 1.5 2.00.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Ave

rag

e liq

uid

wa

ter

satu

ration

( )


Bi-layer GDL

Tri-layer GDL

At anode catalyst layer-MPL interface

0.25 0.5 1.0 1.5 2.00.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Ave

rag

e liq

uid

wa

ter

satu

ration

( )


Bi-layer GDL

Tri-layer GDL

At cathode catalyst layer-MPL interface

a)

b)

55

The increased liquid water retention at the catalyst layer-MPL interface of the tri-layer GDL can

be explained using capillary condensation (condensation due to reduction of vapor pressure within

capillaries) [83] and invasion percolation theory [84]. According to the Kelvin equation [83], a

hydrophilic pore (i.e. a pore with lower contact angle), compared to a hydrophobic pore, favors

capillary condensation by the reduction of vapor pressure. Liquid water is expected to accumulate

within the pores and cracks of the hydrophilic MPL coating due to increased capillary

condensation and liquid percolation from the catalyst layer. This liquid water is expected to

encounter a high capillary barrier (a sharp increase in threshold capillary pressure) imposed by the

adjacent hydrophobic MPL [32,85], since the pores of the hydrophobic MPL exhibit a high

breakthrough pressure for liquid water invasion. In addition, the ionomer within the custom MPL

coating has a hydrophilic receding contact angle [77] and would increase the adhesion to liquid

water [86]. As a result, liquid water accumulated within the hydrophilic MPL coating at the

catalyst layer-MPL interfaces.

4.4.4 Cathode GDL liquid water accumulation and oxygen transport resistances

Figure 4-6 presents the liquid water saturation (left vertical axis) and the porosity (right vertical

axis) profiles along the through-plane position (y-direction) within the cathode bi-layer and tri-

layer GDLs. The porosity of the tri-layer GDL was measured to be 8.7% lower than the porosity

of the bi-layer GDL at the catalyst layer-GDL interface (221 µm to 234 µm in Figure 4-6). This

marginal variation in porosity was attributed to the presence of the hydrophilic MPL coating. The

two GDLs were compressed to the same thickness and the effect of the hydrophilic MPL coating

on the resulting GDL pore structure was minor.

56

The liquid water saturation for the regions under the channels and ribs were averaged and presented

separately in Figure 4-6 (a) and (b), respectively. The liquid saturation profiles were calculated as

described previously, in Section 3.3. The application of the hydrophilic MPL coating led to an

increase in liquid water saturation at the cathode GDL, especially under the channels with a relative

increase in the average saturation values of up to 0.15 (or 550 % at 2.0 A/cm2). The highest

recorded increase in liquid water saturation was 0.32 at the through plane position of 300 μm in

the regions under the channels, at the current density of 2.0 A/cm2. As previous studies have

reported [67,69], liquid water accumulation at the cathode GDL is expected to increase oxygen

transport resistance (discussed further).

57

Figure 4-6. Liquid water saturation profiles along the through-plane position (y-direction) under

a) channels and b) ribs for cathode bi-layer and tri-layer GDLs. The inlet relative humidity was

225 250 275 300 325 350 375

0.0

0.2

0.4

0.6

0.0

0.2

0.4

0.6

2.0 A/cm2


1.5 A/cm2

CCM

Flow-field

channels

Cathode GDLMPL

Liq

uid

wate

r satu

ration (

)


0.0

0.2

0.4

0.6

0.8

1.0

Poro

sity p

rofile

( )

225 250 275 300 325 350 375

0.0

0.2

0.4

0.6

0.0

0.2

0.4

0.6

2.0 A/cm2


1.5 A/cm2

CCM

Flow-field

ribs

Cathode GDLMPL

Liq

uid

wate

r satu

ration (

)


0.0

0.2

0.4

0.6

0.8

1.0

Poro

sity p

rofile

( )

b)

a)

58

maintained at 0 % for the anode and 100 % for the cathode. The porosity profiles of the GDLs are

shown in the secondary vertical axes.

Figure 4-7 (a) and (b) show sample Nyquist plots obtained from EIS for the bi-layer and tri-layer

GDLs, at current densities of 1.0, 1.5, and 2.0 A/cm2. The sample plots show the experimental data

and model fit for Test 1 of each material configuration. The Nyquist plots were fit using an

equivalent circuit model, as described previously in Section 3.2.2. The model fit parameters from

the three tests (Tests 1, 2, and 3) were averaged are presented in Table 4-3 in Section 4.7 (Appendix

A), along with standard deviation among each test.

Figure 4-7 (c) and (d) show the cathode mass transport resistance (Eq. 3-5) and diffusive time (Eq.

3-6), respectively. The oxygen transport resistance was observed to increase with current density,

as the rate of water generation increased within the fuel cell. A relatively steep increase in the

average values of oxygen transport resistance and diffusive time was noted for the tri-layer GDL

configuration (in comparison to the bi-layer GDL), when the current density increased from

1.5 A/cm2 to 2.0 A/cm2. At 2.0 A/cm2, the tri-layer GDL had a significantly higher mass transport

resistance (by 0.47 Ωcm2 or 280 %) and diffusive time for oxygen (by 0.044 s or 190 %),

compared to the bi-layer GDL configuration. The application of a hydrophilic MPL coating led to

an increase in oxygen transport resistance at high current densities, particularly at 2.0 A/cm2. As

discussed previously in Section 4.5.2, the large error bars at 2.0 A/cm2 for the tri-layer GDL were

attributed to the variation in the concentration overpotentials between Batches I and II. It should

be noted that these tests were conducted with dry hydrogen at the anode inlet. When the anode and

59

cathode gases were fully humidified (100% RH at the inlet), a further increase in oxygen transport

resistance was observed at the high current density of 2.0 A/cm2 (not shown) for the fuel cell with

the tri-layer GDL, compared to the fuel cell with the bi-layer GDL (commercial material). To

ensure high cell performance, it is important to consider the desired range of operating conditions.


current density steps of 1.0, 1.5, and 2.0 A/cm2 for a) the bi-layer GDL (including a scaled-up inset)

and (b) tri-layer GDL. The inlet relative humidity was maintained at 0 % for the anode and 100 %

for the cathode. The Nyquist plots were fit using an equivalent circuit model (shown in Figure 3-2).

c) The mass transport resistance and d) the diffusive time for oxygen obtained using the EIS model

fit, averaged for three tests.

0.0

0.1

0.2

0.3

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

-Z''

(.c

m2)

1.0 A/cm2

1.5 A/cm2

2.0 A/cm2

Model

Bi-layer GDL

Tri-layer GDL

1.0 A/cm2

1.5 A/cm2

2.0 A/cm2

Model

-Z''

(.c

m2)

Z' (.cm2)

0.1 0.2 0.30.00

0.02

0.04

0.06

0.08

-Z''

(.c

m2)

Z' (.cm2)

0.0

0.2

0.4

0.6

0.8

1.0

1.0 1.5 2.00.00

0.02

0.04

0.06

0.08

0.10Mass tra

nsport

resis

tance (

.cm

2)

Bi-layer GDL

Tri-layer GDL

Diffu

siv

e tim

e (

s)


SGL 25 BC

Tri-layer GDL

a) c)

b) d)

60

The increase in the cathode mass transport resistance and oxygen diffusive time was attributed to

the increase in liquid water saturation within the cathode GDL (with the application of the

hydrophilic MPL coating). The higher liquid water saturation at the cathode GDL was expected to

reduce the effective porosity and increase the tortuosity of open pore space within the GDL. This

would impede the transport of oxygen from the flow field to the reaction sites and decrease the

effective diffusion coefficient of oxygen within the cathode GDL.

In the study presented within this chapter, the benefits of membrane hydration provided by the

applied hydrophilic MPL coating outweighed the losses incurred due to blockage of oxygen

transport pathways. In general, the wettability should be considered while designing MPLs and

GDLs to enhance fuel cell performance, especially during operation without external

humidification.

4.5 Chapter summary

In this chapter, a hydrophilic MPL coating was applied to a commercial hydrophobic GDL to

investigate membrane hydration and liquid water distribution within the GDLs during fuel cell

operation without external anode humidification. We monitored the electrical output and

impedances of the fuel cell while simultaneously visualizing the GDL liquid water distribution via

synchrotron X-ray radiography. The application of the hydrophilic MPL coating led to a decrease

in the membrane resistance and an increase in cell potential (by up to 14 % or 0.07 V) and power

output (by up to 14 % or 0.10 W/cm2). Simultaneously, an increase in liquid water retention at the

catalyst layer-MPL interfaces of up to 0.14 (increase in average saturation) was measured. The

decrease in the membrane resistance was attributed to the increase in membrane hydration. This

61

improvement in membrane hydration was hypothesized to be caused by the increase in liquid water

retention at the catalyst layer-MPL interfaces. At high current densities (particularly at 2.0 A/cm2),

the application of the hydrophilic MPL coating led to increases in liquid water accumulation at the

cathode GDL and subsequently increased oxygen transport resistances (by up to 0.47 Ωcm2 or

280 %).

This chapter demonstrates that the wettability of the transport layers in a fuel cell can be tailored

to balance membrane hydration and oxygen transport to enhance fuel cell performance for desired

operating conditions. In the study presented within this chapter, the benefits of membrane

hydration provided by the applied hydrophilic MPL coating outweighed the losses incurred due to

blockage of oxygen transport pathways. In general, the wettability should be considered while

designing MPLs and GDLs to enhance fuel cell performance, especially during operation without

external humidification.

62

4.6 Appendix A: Model fit parameters for EIS equivalent circuit

This appendix presents supplementary information for Section 4.5.4 in Chapter 4. Table 4-3

presents the model fit parameters that were averaged over three tests and obtained from fitting

Nyquist plots in Figure 4-7 of Chapter 4, along with the standard deviation among the three tests.

Table 4-3. Model parameters (averaged over three tests) for equivalent circuit used to fit Nyquist

plots in Figure 4-7, along with standard deviation among the three tests.

Current

Density

[A/cm2]

GDL

Average values

𝑹𝑨

[Ωcm2]

𝑪𝑨

[F/cm2]

𝑹𝜴

[Ωcm2]

𝑪𝑪

[F/cm2]

𝑹𝒑

[Ωcm2]

𝑹𝒎𝒕

[Ωcm2]

𝝉

[s]

1.0 Bi-layer 0.115 0.0103 0.124 0.0038 0.032 0.048 0.022

Tri-layer 0.084 0.0176 0.103 0.0039 0.035 0.075 0.028

1.5 Bi-layer 0.095 0.0143 0.105 0.0044 0.025 0.073 0.021

Tri-layer 0.057 0.0389 0.085 0.0043 0.027 0.162 0.033

2.0 Bi-layer 0.079 0.0211 0.098 0.0045 0.022 0.116 0.024

Tri-layer 0.124 0.0389 0.082 0.0075 0.019 0.439 0.068

Standard deviation values

Δ𝑹𝑨

[Ωcm2]

Δ𝑪𝑨

[F/cm2]

Δ𝑹𝜴

[Ωcm2]

Δ𝑪𝑪

[F/cm2]

Δ𝑹𝒑

[Ωcm2]

Δ𝑹𝒎𝒕

[Ωcm2]

Δ𝝉

[s]

1.0 Bi-layer 0.018 0.0012 0.002 0.0006 0.009 0.010 0.005

Tri-layer 0.019 0.0046 0.001 0.0005 0.005 0.003 0.001

1.5 Bi-layer 0.023 0.0020 0.002 0.0006 0.006 0.007 0.002

Tri-layer 0.018 0.0173 0.002 0.0004 0.005 0.041 0.003

2.0 Bi-layer 0.022 0.0038 0.002 0.0006 0.005 0.009 0.003

Tri-layer 0.153 0.0321 0.005 0.0054 0.012 0.267 0.033

63

Chapter 5 Microporous layers with graded polytetrafluoroethylene (PTFE)

for enhanced liquid water removal in polymer electrolyte membrane fuel

cell gas diffusion layers

5.1 Chapter abstract

This chapter presents the effects of spatially graded polytetrafluoroethylene (PTFE) within PEM

fuel cell MPLs on the oxygen transport resistance and GDL liquid water distribution. A negative

gradient in PTFE (from the catalyst layer to the substrate) was achieved within the custom-made

MPLs. MPLs with graded and uniform PTFE were tested in fuel cells. The electrical output (cell

potential and impedances) of the fuel cells was characterized, along with in operando liquid water

visualization within the cathode GDL via synchrotron X-ray radiography. The MPL with graded

PTFE content led to decreased liquid water accumulation within the cathode GDL substrate and a

subsequent decrease in oxygen transport resistance at high current densities (≥ 1.0 A/cm2). It was

found that the negative spatial gradient in PTFE content within this MPL creates a positive spatial

gradient in porosity and a negative spatial gradient in hydrophobicity (or contact angle). This

resulted in a negative spatial gradient in the threshold capillary pressure, which helped facilitate

capillary-driven removal of liquid water. Furthermore, membrane dehydration at high current

densities (≥ 1.0 A/cm2) led to membrane shrinkage, which is hypothesized to be caused by

increased local temperatures at the catalyst layers. The results presented in Chapter 5 demonstrated

that an understanding of liquid water transport within the GDLs and its effect on fuel cell

impedance can inform the design of novel GDLs with specialized transport properties.

64


Improving the high current density performance of a polymer electrolyte membrane (PEM) fuel

cell can help reduce the system cost and size. An effective way to achieve this is by mitigating

liquid water flooding within the cathode gas diffusion layer (GDL) through use of a microporous

layer (MPL). Microporous layers (MPLs) have been shown to reduce liquid water flooding within

GDLs. Several mechanisms have been proposed to explain the improved water management,

including increased rate of water diffusion from the cathode to the anode [35-37], increased rate

of water vapor diffusion from the cathode catalyst layer to the cathode flow field, and enhanced

capillary-driven transport of liquid water from the cathode catalyst layer to the cathode flow field

[38-41]. Studies have demonstrated that functionally graded porous materials have the potential to

further enhance capillary-driven removal of liquid water [15-19]. However, the use of

polytetrafluoroethylene (PTFE) to create functionally graded MPLs has not yet been demonstrated.

Furthermore, liquid water within these functionally graded GDLs has not been quantified

experimentally during fuel cell operation. Liquid water distributions within GDL materials would

provide valuable insights into the effect of novel GDL designs on fuel cell performance.

In this chapter, I investigate the effect that spatially graded PTFE content within an MPL has on

oxygen transport resistance and GDL liquid water distributions. Custom MPLs were fabricated

with uniform and graded PTFE content and assembled into fuel cells. During fuel cell operation,

the current density, cell potential, and impedance were monitored, while simultaneously

visualizing the GDL liquid water distribution via synchrotron X-ray radiography.

65

5.3 Chapter-specific methodology

This section discusses the methodology that was applied to fabricate the custom MPLs, the

experimental approach to verify the PTFE distributions within the MPLs, and the experimental

testing procedure to measure the performance (cell potential, current density and impedance) and

the liquid water distributions within an operating fuel cell.

5.3.1 MPL fabrication

Custom microporous layers (MPLs) were fabricated with uniform and spatially varied PTFE

content. Two MPL slurries were fabricated; one with 10 wt.% PTFE, and the other with 20 wt.%

PTFE. Both the MPL slurries consisted of carbon black (Vulcan® XC-72R, Fuel Cell Store), PTFE

emulsion (60 wt. % PTFE solid content in water, EQ-Lib-PTFE, MTI Corporation), deionized

water, and surfactant (Triton™ X-100, Sigma-Aldrich). These ingredients were combined in ratios

of 1: 0.17: 10: 0.2 (carbon black: PTFE emulsion: dionized water: surfactant) by weight of carbon

black for the 10 wt.% PTFE MPL, and 1: 0.33: 10: 0.2 by weight of carbon black for the 20 wt.%

PTFE MPL. To fabricate the MPLs, the following procedure was applied:

1) The surfactant was added to deionized water in a beaker, and stirred under ambient

conditions for 15 minutes using a magnetic stirrer.

2) Carbon black was added to the mixture in 3 equal batches (by weight), while the mixture

was stirred continuously. Each batch was added to the mixture after 10 minutes of stirring.

3) The resulting MPL slurry was stirred for an additional 15 minutes, and then sonicated

(Q125 sonicator, Qsonica Sonicators LLC.) for 30 minutes at 50% amplitude (equivalent

to a tip displacement amplitude of 90 µm) in pulse mode (with 5 seconds on and 1 second

off). In this study, a 1/8 inch-diameter #4422 sonicator probe tip was used.

66

4) The PTFE emulsion was then added to the slurry, which was mixed for 10 minutes using

a magnetic stirrer, and then sonicated for 30 minutes at 50 % amplitude in pulse mode (with

5 seconds on and 1 second off).

5) The resulting slurry was coated onto a commercially available GDL substrate, Sigracet

(SGL) 25 BA (Sigracet® GmbH.) using an adjustable micrometer thin film applicator

(Microm II, Paul N. Gardner Company, Inc.). The SGL 25 BA is a carbon paper substrate

with a thickness of 190 µm with 5 % PTFE by weight.

6) The thin film applicator was sequentially set to two thickness settings during coating, i.e.

50 µm and 100 µm above the substrate. The slurry was coated 5 times at the 50 µm

thickness setting and then 5 times at the 100 µm thickness setting. Between each coating,

the MPL was dried under ambient conditions for 10 minutes. Five coating layers were

applied at each thickness setting to obtain a dried MPL thickness that approached the

coated thickness.

7) Following the coating procedure, the MPLs were heated to 250 °C (for drying) and 270 °C

(for removing surfactant) for 1 hour each and treated with a 1-hour sintering process at

350 °C in a convection oven (DHG9000JB, MTI Corporation). The resulting MPLs

consisted of only carbon black and PTFE (constituents that remained after the sintering

process).

Figure 5-1 schematically illustrates the three GDLs fabricated with varying PTFE content. The

MPLs with uniform PTFE (either 10 wt.% and 20 wt.% PTFE) were fabricated by sequentially

coating the GDL substrate with the MPL slurry with the corresponding PTFE. The MPL with

67

graded PTFE was fabricated. The MPL with graded PTFE was fabricated by initially coating the

GDL substrate with the 10 wt.% PTFE MPL slurry at a thickness setting of 50 µm, and then coating

the resulting MPL with the 20 wt.% PTFE MPL slurry at the thickness setting of 100 µm. This

MPL also followed the same coating procedure, i.e. 5 coatings at each thickness setting.

Figure 5-1. GDLs with microporous layers fabricated with varying PTFE content: a) 10 wt.%

PTFE, b) 20 wt.% PTFE, and c) graded 20-10 wt.% PTFE

5.3.2 Wavelength dispersive spectroscopy

Wavelength dispersive spectroscopy (WDS) was used to quantify the spatial distributions of PTFE

content within the custom MPLs. The concentration of PTFE was proportional to the measured

intensity of fluorine using WDS. Fluorine is a suitable element to detect the presence of PTFE in

GDLs, since fluorine is highly concentrated in the PTFE polymer chains while absent within the

carbon matrix of the GDL [87].

To prepare the GDL samples for the WDS, 10 mm (width) by 3 mm (height) GDLs were mounted

vertically in a spring. The GDL samples were then placed in a circular mold where epoxy resin (4

parts resin and 1 part hardener; EpoThin 2 Resin system, Buehler) was poured and cured for 24

hours. The hardened epoxy immobilized the GDLs during the remaining preparation and imaging

68

steps. It should be noted that the epoxy did not contain fluorine and did not affect the measurement

of fluorine intensity during elemental mapping. Once the mold was cured, the GDLs within the

resin were ground using silicon carbide sand paper (with ISO grit designations of P280, P800,

P1200, and P2500, used successively), and polished by using oil-based diamond suspensions of

9 µm and 1 µm. The polished epoxy was then carbon-coated to increase the surface electrical

conductivity, which inhibited electrical charging of the samples and prevented thermal damage

during the imaging step.

The elemental maps were conducted using an electron probe microanalyzer (EPMA) (JXA8230

SuperProbe, JEOL USA Inc.) at a magnification of 250x. The size of each image was 518 pixels

by 384 pixels, and each pixel corresponded to a coverage area of 0.9×0.9 µm2. An exposure time

of 5 ms was used at each pixel location, and each pixel location was scanned three times to increase

the accumulated signal intensity. To subtract the background noise levels, a standard sample of

pure carbon (without any fluorine content) was scanned prior to the GDL scans. The PTFE

concentrations for all the MPLs were normalized relative to the average concentration of PTFE

within a reference MPL (MPL with 20 wt% PTFE). Through-plane profiles of relative PTFE

concentrations were calculated along the thickness of the MPL to examine the distribution of PTFE

within the MPLs.

69

5.3.3 Fuel cell testing

For a detailed description of the fuel cell test setup and the applied procedure for the fuel cell tests,

the reader is referred to Sections 3.2 in Chapter 3. However, the following test procedure and

operating conditions that were specific to the study in this Chapter are as follows:

1. For a given experiment, the same GDL material was used on both the anode and cathode.

The GDLs were compressed to 75% ± 3% of their original thickness using rigid 250 µm-

thick polyethylene naphthalate (PEN) spacers. The fuel cell MEAs were compressed

through use of 6 M4 bolts that were each tightened to a torque of 20 lbf-in.

2. The anode and cathode gases were fully humidified (dew point temperature of 60 °C and

cell temperature of 60 °C), and were supplied without external back pressure (atmospheric

pressure at the fuel cell outlets).

The fuel cell current density was maintained at constant steps of 0.25, 0.50, 1.0, and 1.5 A/cm2 for

15 minutes each. The current was ramped between each constant current step at the rate of

0.001 A/s.

5.3.4 Synchrotron X-ray radiography

For the detailed methodology of synchrotron X-ray radiographic imaging, the reader is referred to

Section 3.3 in Chapter 3. The details of synchrotron X-ray radiography that were specific to this

chapter are presented in this sub-section.

70

Figure 5-2 shows a set of liquid water thickness distributions (normalized by the length of the GDL

along the beam path) for fuel cells with MPLs containing a) 10 wt.%, b) 20 wt.%, and c) graded

(20-10 wt.%) PTFE. The color of each pixel represents the normalized liquid water thickness

averaged for 100 frames (equivalent to 5 minutes) at the end of the 1.0 A/cm2 current density step.

The central region of Figure 5-2, indicated by the pink band to the right of the figure, represents

the catalyst coated membrane (CCM). The CCM is sandwiched by the anode (top) and the cathode

(bottom) GDLs. These GDLs were sandwiched by the anode and cathode flow-field ribs (the anode

rib locations are depicted as white dashed lines at the top image). The region in each image outlined

with the white dotted lines contains 25 pixels (163 µm) in the through-plane (y-direction) of the

cathode GDL. This 25-pixel region was used to obtain the temporal profile of the averaged

normalized liquid water described by Eq. 3-12. This 25-pixel region was selected since it was

unaffected by membrane shrinkage close to the catalyst layer-MPL interface. Membrane shrinkage

will be examined and discussed separately in Section 5.4.3. The normalized liquid water thickness

values within the cathode GDL above the flow field ribs and channels were averaged in the in-

plane direction (x-direction) to obtain the through-plane water thickness distributions (Eq. 3-11).

71

Figure 5-2. Sample liquid water thickness distribution (normalized by the length of the GDL along

the beam path) for fuel cells containing GDLs with a) 10%, b) 20%, and c) graded (20-10%) PTFE

at 1.0 A/cm2. The black scale bar at the bottom is equivalent to 1 mm. White dashed lines

represents anode ribs and denote rib locations (ribs). The y-positions of the flow fields, GDLs, and

CCM are shown beside each processed image. The axes for each image is shown using white

arrows. The white dotted lines in each image represent 25 pixels (in the through-plane direction)

within the cathode GDL

5.4 Results and discussion

This section presents the results and discussion of this chapter. First, the relative PTFE

concentrations within the custom-made MPLs are quantified using WDS scans. The three custom-

72

made GDLs were assembled into fuel cells and the effect of graded PTFE (within MPLs) on cell

potential was investigated. To explain these performance results, synchrotron X-ray radiography

was used to obtain information on liquid water distributions within the cathode GDL and

membrane movement. The oxygen transport resistances within the fuel cell were then quantified

and were explained using the GDL liquid water distributions. Then, Section 5.4.6 presents a

discussion on how a gradient in threshold capillary pressures enhances liquid water removal

towards the exit. Finally, membrane shrinkage is explained using a discussion on membrane

dehydration.

5.4.1 Relative PTFE concentration within MPLs

To verify the fabrication process, the distribution of PTFE content within the three custom-MPLs

were quantified and examined. This was accomplished through use of wavelength dispersive

spectroscopy (WDS), whereby the fluorine concentration distribution was measured along the

MPL thickness. Fluorine was specifically targeted in this study due to its abundance in PTFE, and

non-existence in carbon. The cross-sectional elemental maps of fluorine for the three custom MPLs

are shown in Figure 5-3 a), b), and c). Regions of high fluorine concentrations (shown in red) in

Figure 5-3 a), b), and c) indicate regions of PTFE accumulation near the GDL substrate-MPL

interface. Rofaiel et al. [87,88] reported that commercial carbon fiber paper substrates exhibit

peaks in PTFE concentration (corresponding to PTFE agglomerations) near the substrate surface

edges. In this study, these regions of high PTFE concentration within the substrate were used to

define the substrate edge and hence mark the boundary of the bulk MPL.

73

The PTFE concentrations for all MPLs were normalized relative to the average concentration of

PTFE within a reference MPL (i.e. 20 wt% PTFE MPL), since 20 wt.% PTFE is a commonly

available configuration. One-dimensional through-plane distributions of the relative PTFE

concentration were obtained for each MPL, as shown in Figure 5-3 d). The PTFE distributions

within the 10 wt.% and 20 wt.% PTFE MPLs are shown to be uniform within the bulk MPL. In

the graded MPL, the relative PTFE concentration was comparable to the 20 wt.% PTFE MPL in

the through plane positions from 0.0 to 0.2. This relative PTFE concertation was comparable to

the 10 wt.% PTFE MPL in the through-plane positions from 0.4 to 1.0. A transition region between

the two concentration levels was found to exist in the through-plane positions from 0.2 to 0.4,

whereby the PTFE concentration decreased from 20 wt.% to 10 wt.% PTFE (negative gradient in

PTFE content). These results showed that MPLs with uniform and graded PTFE were successfully

fabricated.

74

Figure 5-3. Cross-sectional fluorine elemental maps for GDLs with a) 10 wt.% PTFE, b) 20 wt.%

PTFE, and c) graded 20-10 wt.% PTFE. The color bar represents measured fluorine intensity in

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.5

1.0

1.5

2.0

Su

bstr

ate

Re

lative

PT

FE

co

nce

ntr

atio

n (

)

Normalized MPL position ( )

PTFE 10%

PTFE 20%

PTFE 20-10%

Bu

lk M

PL

d)

75

counts per second. The locations of bulk MPL and substrate edge (denoted by S) are shown in

boxes above each image. d) Averaged through-plane PTFE distribution (normalized by the average

PTFE concentration of 20 wt.%). Position along y-direction was normalized using bulk MPL

thickness.

5.4.2 Fuel cell polarization curves

The three custom-made GDLs were assembled into fuel cells and the effect that the graded PTFE

(within MPLs) had on the cell potential was investigated. Figure 5-4 shows the cell potentials of

the fuel cells containing MPLs with 10 wt.%, 20 wt.% and graded (20-10 wt.%) PTFE. The steady-

state cell potentials at current densities of 0.25, 0.50, 1.0, and 1.5 A/cm2 are shown with markers,

and the intermediate data (obtained during current density ramps) are presented with solid lines.

At current densities less than 1.0 A/cm2, the steady-state cell potentials for all three materials were

comparable to one another, and were within 2.3% of each other. However, at current densities

greater than 1.0 A/cm2, the cell potential for each of the three fuel cell materials began to deviate

more significantly. For instance, fuel cell with the 10 wt.% PTFE MPL incurred higher voltage

losses and the cell potential reached zero volts before reaching the 1.5 A/cm2 current step. The fuel

cell with the graded PTFE MPL provided the highest cell potential of 0.15 V at a current density

of 1.5 A/cm2. For comparison, the fuel cell with the 20 wt.% PTFE MPL obtained a cell potential

of 0.06 V at a current density of 1.5 A/cm2. The variation in cell potential for each of the MPL

materials is attributed to the variation in the liquid water distribution within the three cathode

GDLs. The amount of accumulated liquid water is investigated in detail in Section 5.4.3.

76

Figure 5-4. Cell voltage from tests on fuel cells containing GDLs with 10 wt.%, 20 wt.%, and

graded (20-10 wt.%) PTFE in MPLs. Steady-state cell potential values are shown with markers

and transient data are presented with solid lines.


This sub-section presents the liquid water distributions obtained from synchrotron X-ray

radiography. Figure 5-5 shows the temporal distribution of the average liquid water thickness that

normalized by the length of the GDL along the beam path. This normalized liquid water thickness

is arithmetically averaged within the boxed region (white dotted line box just below the center in

each image) in the cathode GDL, shown in Figure 5-2. At low current densities (less than

0.25 A/cm2), the rate of water generation within the cathode catalyst layer was too low to cause

any significant liquid water accumulation, as observed by normalized liquid water thicknesses that

were below 0.01. However, at current densities above 0.25 A/cm2, the normalized liquid water

0.0 0.5 1.0 1.50.0

0.2

0.4

0.6

0.8

1.0

PTFE 10%

PTFE 20%

PTFE 20-10%

Ce

ll p

ote

ntia

l (V

)

Current density (A/cm2)

77

thickness within the cathode GDL began to increase. Of the three samples, the GDL with 10 wt.%

PTFE in the MPL had the highest average normalized liquid water thicknesses (at current densities

≥ 0.5 A/cm2), with average normalized liquid water thickness values that were approximately 0.04

higher than that of the other samples. The higher liquid water thickness was due to the greater

hydrophilicity of the MPL within the GDL material, which caused the material to be more prone

to retaining liquid water. The higher liquid water thicknesses contributed to cathode GDL flooding,

and led to fuel cell failure (cell potential reached zero volts) before reaching the current density of

1.5 A/cm2. Among the two remaining materials, the GDL with 20 wt.% PTFE MPL had

approximately 20% more liquid water than the GDL with graded MPL, especially at the highest

current density of 1.5 A/cm2. The liquid water profiles along the through-plane direction are

further investigated for the current densities of 1.0 A/cm2 and 1.5 A/cm2.

78

Figure 5-5. Temporal profile of average liquid water (normalized by the distance within the GDL

traversed by the beam) within cathode GDL (25 pixels)

Figure 5-6 shows the through-plane distributions of normalized liquid water within the cathode

GDLs above the ribs and channels. At the current density of 1.0 A/cm2, the GDL with the 10 wt.%

PTFE MPL had higher overall liquid water within the substrate (also seen in Figure 5-5) above the

ribs and the channels, compared to the other GDLs. Following a similar trend as shown in Figure

5-5, the GDL with graded PTFE MPL had the lowest liquid water accumulation within the GDL

substrate above the ribs and the channels (for both the current densities of 1.0 A/cm2 and

1.5 A/cm2). For example, the normalized liquid water thickness peak within the substrate core (at

0 1000 2000 3000 4000 5000 60000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Ave

rag

e n

orm

aliz

ed

liq

uid

wa

ter

with

in

ca

tho

de

GD

L (

cm

/cm

GD

L)

Time (s)

PTFE 10%

PTFE 20%

PTFE 20-10%

Current Density

0.0

0.5

1.0

1.5

Cu

rre

nt

De

nsity (

A/c

m2)

79

160 μm) for this GDL decreased by 0.09 at 1.0 A/cm2 and by 0.12 at 1.5 A/cm2 (compared to the

normalized liquid water thickness peaks within the other materials). As previous studies have

reported [67,69], liquid water accumulation at the cathode GDL is expected to influence oxygen

transport resistance. The implications of liquid water within the GDL on oxygen transport

resistance and cell potential is discussed in further detail in Section 5.4.4. Furthermore, the

mechanism by which graded PTFE in the MPLs enhanced liquid water transport is discussed in

more detail in Section 5.4.6.

As seen in Figure 5-6, negative liquid water thicknesses were detected in the processed radiographs

of test images. However, it should be noted that these processed images are compared against a

“dry-state” reference image. Therefore, any measurement of negative values provides meaningful

information regarding the physical interfacial movement in the test images (with respect to the

dry-state image). For example, at a given pixel location, when a material in the reference dry-state

image is replaced by a less attenuating material during the test, the resulting processed value at

that pixel location becomes negative. This phenomenon is observed at the catalyst layer-MPL

interface during membrane shrinkage, when the highly attenuating Pt-loaded catalyst layer is

replaced (in its initial location) by the less attenuating carbon-based MPL [44]. In effect, we are

able to detect membrane shrinkage by quantifying and examining negative values of normalized

liquid water thickness. In Figure 5-2 and Figure 5-6, negative values were observed at the catalyst

layer-MPL interfaces for all the three materials. This provides evidence of membrane shrinkage

for all the three fuel cell configurations at high current densities (≥ 1.0 A/cm2).

80

Figure 5-6. Through-plane liquid water thickness (normalized by the distance within the GDL

traversed by the beam) for cathode GDL regions above ribs and channels for fuel cell operated at

current densities of a) 1.0 A/cm2 and b) 1.5 A/cm2.

25 50 75 100 125 150 175 200 225

-0.2

0.0

0.2

25 50 75 100 125 150 175 200 225

-0.2

0.0

0.2

0.4

0.6

Above channels


Substrate

Above ribs

CCM Flow-field

MPL

Norm

aliz

ed

liq

uid

wate

r th

ickne

ss (

cm

/cm

GD

L)

PTFE 10% PTFE 20% PTFE 20-10%a)

25 50 75 100 125 150 175 200 225

-0.2

0.0

0.2

25 50 75 100 125 150 175 200 225

-0.2

0.0

0.2

0.4

0.6

Above channels


Substrate

Above ribs

CCM Flow-field

MPL

Norm

aliz

ed

liq

uid

wate

r th

ickne

ss (

cm

/cm

GD

L)

PTFE 10% PTFE 20% PTFE 20-10%b)

81

5.4.4 Oxygen transport resistance

Figure 5-7 a) and b) show sample Nyquist plots obtained at current densities of 1.0 A/cm2 and

1.5 A/cm2, respectively. In the figure, the markers represent the experimental data and the lines

represent the model fit for each material configuration. To interpret the Nyquist plots, an

equivalent circuit model was used, as described in Section 3.2.2. The model fit parameters and

their standard errors are presented in Table 5-1 in Section 5.6 (Appendix B). Using this approach,

the high frequency resistance was used to quantify the ohmic resistance within the fuel cell, and to

investigate membrane hydration (in Section 5.4.5). The secondary low frequency arc (frequencies

above 0.6 Hz) was used to quantify the characteristic time for water diffusion in the membrane

and to further discuss the membrane hydration state in Section 5.7 (Appendix C). Finally, the

oxygen transport resistance and the diffusive time were quantified using the low frequency

response between frequencies 0.6 Hz and 32 Hz [45].

Figure 5-7 c) and d) show the oxygen mass transport resistance and diffusive time, which are

obtained from the Warburg impedance (Eq. 3-5) in the equivalent circuit model that was discussed

in Section 3.2.2. The oxygen transport resistance and diffusive time increased significantly within

the fuel cells with 10 wt.% and 20 wt.% MPLs, as they approached high current densities

(≥ 1.0 A/cm2). More specifically, when the current density was increased from 0.5 A/cm2 to

1.0 A/cm2, the oxygen transport resistance for the fuel cell with 10 wt.% PTFE MPL increased by

130% or by 0.15 Ω·cm2; the diffusive time for this fuel cell increased by 0.02 s. For the fuel cell

with 20 wt.% PTFE MPL, similar increases were observed at higher current densities (from

1.0 A/cm2 to 1.5 A/cm2), where the oxygen transport resistance increased by 440%, or 0.49 Ω·cm2,

82

and the diffusive time increased by 0.04 s. In comparison, the fuel cell with graded PTFE in the

MPL had lower oxygen transport resistance and diffusive time at high current densities. The

reduced oxygen transport resistance and diffusive time for the fuel cell with the graded PTFE MPL

was hypothesized to be caused by reduced liquid water accumulation within the cathode GDL

(seen in Figure 5-5 and Figure 5-6). High liquid water accumulation within the cathode GDL

(cathode flooding) significantly increases the oxygen transport resistance within the fuel cell,

especially at high current densities (when a higher flux of oxygen is required to sustain the cathode

reactions) [67,69]. Furthermore, an increase in diffusive time can be attributed to the decrease in

effective diffusion coefficient of oxygen within the GDL (due to a decrease in effective porosity

of the GDL and an increase in tortuosity of open pore space, as shown in Eq. 3-6). As such, an

increased oxygen transport resistance would yield increased oxygen concentration polarization

losses, and decreased fuel cell potential (as seen in Figure 5-4). Hence, the use of graded PTFE

within MPLs was shown to decrease the oxygen transport resistance and improve fuel cell

performance at high current densities, by reducing the liquid water accumulation within the

cathode GDL.

83


current density steps of a) 1.0 and b) 1.5 A/cm2. c) Mass transport resistance and d) diffusive time

calculated using EIS equivalent circuit.

5.4.5 Membrane dehydration

Figure 5-8 shows the ohmic resistance of the fuel cell configurations. The measured ohmic

resistance accounts for the ionic resistance within the membrane, electrical resistance within the

fuel cell components (i.e., GDL, graphite flow fields, current collectors, and current carrying leads

from the experimental setup), and the electrical contact resistance at the interface between of each

fuel cell component. Since the three fuel cell configurations had identical test setups (i.e. identical

0.0

0.1

0.2

0.3

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

-Z''

(.c

m2)

PTFE 10%

PTFE 20%

PTFE 20-10%

Model

a) 1.0 A/cm2

b) 1.5 A/cm2

-Z''

(.c

m2)

Z' (.cm2)

0.0

0.2

0.4

0.6

0.0 0.5 1.0 1.50.00

0.02

0.04

Ma

ss tra

nsp

ort

re

sis

tan

ce

(

.cm

2)

PTFE 10%

PTFE 20%

PTFE 20-10%

c)

d)

Diffu

siv

e tim

e (

s)


84

MEAs, PEN spacers, and fuel cell), the changes in the measured ohmic resistance within a test is

dominated by the changes in the membrane resistance. As shown in Figure 5-8, the ohmic

resistance increased with increasing current density. This change in ohmic resistance is attributed

to the change in the membrane resistance. The membrane resistance is a function of the local

temperature and the membrane water content [89,90]. As the current density increases, the heat

generation from fuel cell inefficiencies increases and causes the temperature at the catalyst layers

to increase. Increased temperatures on their own would increase the membrane ionic conductivity,

by increasing the proton mobility, dilating the membrane pores, and increasing the membrane

water uptake. However, an increase in membrane resistance with current density was observed,

the effect of decreasing membrane water content was more dominant in influencing the membrane

resistance. Increased local temperatures at the membrane lead to increased rates of water

desorption from the membrane and evaporation [91]. This causes the membrane to dehydrate, and

leads to increased ionic resistance and membrane shrinkage; which is consistent with the observed

membrane shrinkage in Figure 5-2 and Figure 5-6. It is to be noted that the MPLs used in this study

were approximately 100 μm thick. Thick MPLs act as thermal barriers that further increase local

temperatures at the catalyst layer. The use of 100 μm-thick MPLs have been previously reported

[67] to lead to increased ohmic resistance and membrane dehydration. An increase in membrane

dehydration and membrane shrinkage with increasing current density (from 1.0 A/cm2 to

1.5 A/cm2) was also observed by quantifying the characteristic time for water diffusion within the

membrane (obtained from the secondary low frequency arc in Nyquist plots), further detailed in

Section 5.7 (Appendix C).

85

Figure 5-8. Ohmic resistance of fuel cells containing MPLs with 10 wt.%, 20 wt.%, and graded

(20-10 wt.%) PTFE.

5.4.6 Designed threshold capillary gradient for enhanced liquid water removal

The threshold capillary pressure, 𝑃𝑐, for the invasion of a non-wetting fluid (i.e., liquid water in

the case of a hydrophobic porous material) into a pore is defined as

𝑃𝑐 = 𝑃𝑛𝑤 − 𝑃𝑤 = −2𝜎 ∙ cos (𝜃)

𝑟

Eq. 5-1

where 𝑃𝑛𝑤 is the pressure of the non-wetting fluid (liquid water), 𝑃𝑤 is the pressure of the wetting

fluid (air), 𝜎 is the interfacial tension between the non-wetting and wetting fluids, 𝜃 is the contact

angle (between the liquid-air interface and the liquid-solid interface), and 𝑟 is the radius of the

throat being invaded.

0.0 0.5 1.0 1.50.10

0.15

0.20

0.25

PTFE 10%

PTFE 20%

PTFE 20-10%

Oh

mic

re

sis

tan

ce

(c

m2)


86

From Eq. 5-1, an increase in throat radius or a decrease in contact angle decreases the pressure

required for liquid water to invade into a pore (i.e., the threshold capillary pressure of the pore).

As such, a pore with lower threshold capillary pressure will be preferentially invaded compared to

a connected pore with a higher threshold capillary pressure.

PTFE within the MPL influences the contact angle and mean pore diameter of the MPL. PTFE

constricts the pores within the MPL by filling the existing pore spaces. An increase in PTFE

content within the MPL increases the hydrophobicity (by increasing the fraction of hydrophobic

pores) and decreases the MPL porosity (by decreasing the mean pore diameter) [31]. A negative

spatial gradient in PTFE content (from the catalyst layer to the flow field) creates a positive spatial

gradient in porosity and a negative spatial gradient in hydrophobicity (or contact angle). From, Eq.

5-1, this negative spatial gradient in PTFE creates a negative spatial gradient in the threshold

capillary pressure within the pores from the catalyst layer to the flow field. This negative gradient

in threshold capillary pressure favors liquid water flow in the through-plane direction (from the

catalyst layer to the substrate) rather than in the in-plane direction (assuming isotropic in-plane

characteristics). This reduces the accumulation of liquid water within the porous material by

increasing the mobility of the liquid water towards the exit (flow field).

This study experimentally showed that a capillary pressure gradient can be designed within the

porous layers of a fuel cell to enhance capillary-driven removal of liquid water and effectively

reduce the liquid water accumulation within the porous layers. Furthermore, the decreased liquid

water accumulation within cathode GDLs decreased oxygen transport resistance and increased fuel

cell performance. Hence, this study demonstrates that an understanding of liquid water transport

87

within the GDLs and its effect on fuel cell impedance can inform the design of novel GDLs with

specialized transport properties.

5.5 Chapter summary

This chapter presented the effects of spatially graded PTFE within MPLs on the oxygen transport

resistance and GDL liquid water distribution. GDLs with graded and uniform PTFE within the

MPLs were assembled into fuel cells to measure the cell potential, current density, and impedances

of the fuel cell, and the liquid water distribution within the cathode GDL via synchrotron X-ray

radiography. The PTFE gradient within the MPL led to decreased liquid water accumulation within

the cathode GDL substrate. This led to a decrease in oxygen transport resistance at high current

densities (≥ 1.0 A/cm2). A negative spatial gradient in PTFE content (from the catalyst layer to

the flow field) creates a positive spatial gradient in porosity and a negative spatial gradient in

hydrophobicity (or contact angle). This results in a negative spatial gradient in the threshold

capillary pressure that promotes capillary-driven removal of liquid water. Furthermore, membrane

dehydration at high current densities ( ≥ 1.0 A/cm2) led to membrane shrinkage, which was

hypothesized to be caused by increased local temperatures at the catalyst layers. The study within

this chapter demonstrates that an understanding of liquid water transport within the GDLs and its

effect on fuel cell impedance can inform the design of novel GDLs with specialized transport

properties.

88

5.6 Appendix B: Model fit parameters for EIS equivalent circuit (Chapter 5)

This appendix shows supplementary information for Sections 5.4.4 and 5.4.5 (Chapter 5). Table

5-1 presents the model fit parameters and standard error from fitting Nyquist plots in Figure 5-7

Table 5-1. Model parameters for equivalent circuit used to fit Nyquist plots in Chapter 5, along

with standard error.

PTFE

Content

[wt.%]

i

[A/cm2]

Average values

𝑹𝑨

[Ω.cm2]

𝑪𝑨

[F/cm2]

𝑪𝑪

[F/cm2]

𝑹𝒑

[Ω.cm2]

𝑹𝒎𝒕

[Ω.cm2]

𝝉

[s]

𝑹𝜴

[Ω.cm2]

𝑹𝒎𝒆𝒎

[Ω.cm2]

𝑪𝒎𝒆𝒎

[F/cm2]

𝝉𝒎𝒆𝒎

[s]

20-10

0.25 0.171 0.0400 0.0022 0.042 0.146 0.003 0.139 N/A N/A N/A

0.50 0.115 0.0399 0.0024 0.038 0.109 0.002 0.142 N/A N/A N/A

1.0 0.153 0.0498 0.0023 0.038 0.133 0.003 0.160 0.052 47.574 2.492

1.5 0.301 0.0454 0.0022 0.050 0.172 0.005 0.196 0.104 10.618 1.100

20

0.25 0.177 0.0399 0.0017 0.058 0.124 0.003 0.135 N/A N/A N/A

0.50 0.142 0.0381 0.0018 0.056 0.099 0.003 0.137 N/A N/A N/A

1.0 0.239 0.0355 0.0018 0.057 0.114 0.003 0.151 N/A N/A N/A

1.5 0.077 0.0369 0.0014 0.048 0.609 0.044 0.192 0.160 5.415 0.867

10

0.25 0.197 0.0295 0.0019 0.050 0.119 0.003 0.134 N/A N/A N/A

0.50 0.132 0.0412 0.0020 0.047 0.113 0.002 0.137 N/A N/A N/A

1.0 0.110 0.0177 0.0021 0.045 0.263 0.023 0.157 0.105 19.897 2.097

PTFE

Content

[wt.%]

i

[A/cm2]

Standard error

Δ𝑹𝑨

[Ω.cm2]

Δ𝑪𝑨

[F/cm2]

Δ𝑪𝑪

[F/cm2]

Δ𝑹𝒑

[Ω.cm2]

Δ𝑹𝒎𝒕

[Ω.cm2]

Δ𝝉

[s]

Δ𝑹𝜴

[Ω.cm2]

Δ𝑹𝒎𝒆𝒎

[Ω.cm2]

Δ𝑪𝒎𝒆𝒎

[F/cm2]

Δ𝝉𝒎𝒆𝒎

[s]

20-10

0.25 0.033 0.0144 0.0002 0.008 0.034 0.001 N/A N/A N/A N/A

0.50 0.023 0.0145 0.0002 0.006 0.023 0.000 N/A N/A N/A N/A

1.0 0.013 0.0084 0.0002 0.004 0.014 0.000 N/A 0.024 10.369 0.245

1.5 0.012 0.0039 0.0001 0.004 0.013 0.000 N/A 0.012 1.335 0.016

20

0.25 0.027 0.0119 0.0001 0.007 0.027 0.001 N/A N/A N/A N/A

0.50 0.019 0.0097 0.0011 0.005 0.019 0.001 N/A N/A N/A N/A

1.0 0.011 0.0035 0.0001 0.004 0.011 0.000 N/A N/A N/A N/A

1.5 0.018 0.0065 0.0001 0.006 0.021 0.003 N/A 0.023 1.056 0.024

10

0.25 0.029 0.0085 0.0002 0.008 0.030 0.001 N/A N/A N/A N/A

0.50 0.020 0.0116 0.0001 0.006 0.020 0.000 N/A N/A N/A N/A

1.0 0.012 0.0013 0.0001 0.004 0.014 0.002 N/A 0.024 2.572 0.061

89

5.7 Appendix C: Characteristic time of water diffusion in the membrane (Chapter 5)

This sub-section describes the methodology and results for quantifying the characteristic time of

liquid water diffusion in the membrane, which provides insights on membrane hydration and

shrinkage from Chapter 5. The low frequency capacitive arc in the Nyquist plots (at frequencies

from 0.1 Hz to 0.6 Hz in Figure 5-7 of Chapter 5) can be attributed to the low rate of diffusion of

water through the membrane [44,55]. To capture this capacitive arc, a membrane impedance

element was added to the equivalent circuit in Figure 3-2. The equivalent circuit equation in Eq.

3-1 (from Section 3.2.2) was described with an additional impedance (impedance of liquid water

transport in the membrane, 𝑍mem) as

𝑍𝑇𝑜𝑡 = 𝑍𝐴 + 𝑍𝐶 + 𝑍Ω + 𝑍mem, Eq. 5-2

where 𝑍mem is the impedance of liquid water transport in the membrane that is expressed as

𝑍𝑚𝑒𝑚 = (1

𝑅𝑚𝑒𝑚+ 𝑗 ∙ 𝜔 ∙ 𝐶𝑚𝑒𝑚)

−1

, Eq. 5-3

where 𝑅𝑚𝑒𝑚 is the resistance and 𝐶𝑚𝑒𝑚 is the capacitance of liquid water diffusion in the

membrane. The characteristic time of liquid water diffusion in the membrane, 𝜏𝑚𝑒𝑚 is expressed

as

𝜏𝑚𝑒𝑚 = 𝑅𝑚𝑒𝑚 ∙ 𝐶𝑚𝑒𝑚, Eq. 5-4

𝜏𝑚𝑒𝑚 decreases with membrane shrinkage and with decreasing membrane hydration [44,55].

𝜏𝑚𝑒𝑚 was quantified for the fuel cell current densities that exhibited the low frequency capacitive

arc in the Nyquist plots, as shown in Figure 5-9. As can be seen from Figure 5-9, a decrease in

𝜏𝑚𝑒𝑚 is observed with increasing current density (from 1.0 A/cm2 to 1.5 A/cm2). This decrease in

90

𝜏𝑚𝑒𝑚 indicated membrane dehydration and shrinkage. These results align with the results

presented in Section 5.4.5 of Chapter 5. It is to be noted that the 𝜏𝑚𝑒𝑚 could not be quantified for

all the current densities since the low frequency arc (above 0.6 Hz) was not observed in the Nyquist

plots for all the current densities.

Figure 5-9. Characteristic time of diffusion in the membrane obtained from EIS model fit

0.0 0.5 1.0 1.50.0

0.5

1.0

1.5

2.0

2.5

3.0

PTFE 10%

PTFE 20%

PTFE 20-10%

Cha

racte

ristic tim

e o

f liq

uid

wate

r

diffu

sio

n in

mem

bra

ne

, m

em (

s)


91

Chapter 6 Conclusions and future work

6.1 Conclusions

In this thesis, custom microporous layers (MPLs) were fabricated and designed to tailor water

management and enhance fuel cell performance at two specific fuel cell operating conditions, i.e.

operation without anode humidification (Chapter 4) and operation at high current densities

(Chapter 5). The two studies were aimed to address two separate challenges of water management,

i.e., membrane dehydration in Chapter 4 and cathode liquid water flooding in Chapter 5. The

objective of this thesis was to understand the effect of these custom MPLs on fuel cell impedances

(membrane resistance and oxygen transport resistance) and liquid water distributions within the

GDLs. The custom MPLs were tested in fuel cells to monitor the fuel cell performance (cell voltage)

and impedances and liquid water distributions within the gas diffusion layers (GDLs).

In Chapter 4, a hydrophilic MPL coating was applied to a commercial hydrophobic GDL to

investigate membrane hydration and GDL liquid water distributions during fuel cell operation

without external anode humidification. The electrical output and impedances of the fuel cell were

monitored with concurrent visualization of the GDL liquid water distribution via synchrotron X-

ray radiography. The following were discovered for fuel cell operation without external

humidification:

• The application of the hydrophilic MPL coating led to a decrease in the membrane

resistance and an increase in cell potential (by up to 14 % or 0.07 V) and power output (by

up to 14 % or 0.10 W/cm2).

92

• A simultaneous increase in liquid water retention at the catalyst layer-MPL interfaces of

up to 0.14 (increase in average saturation) was measured. The decrease in the membrane

resistance was attributed to the increase in membrane hydration. This improvement in

membrane hydration was hypothesized to be caused by the increase in liquid water

retention at the catalyst layer-MPL interfaces.

• At high current densities (particularly at 2.0 A/cm2), the application of the hydrophilic

MPL coating led to increases in liquid water accumulation at the cathode GDL and

subsequently increased oxygen transport resistances (by up to 0.47 Ωcm2 or 280 %).

In Chapter 4, the benefits of membrane hydration provided by the applied hydrophilic MPL coating

outweighed the losses incurred due to blockage of oxygen transport pathways. Chapter 4

demonstrated that the wettability of the transport layers in a fuel cell can be tailored to balance

membrane hydration and oxygen transport to enhance fuel cell performance for desired operating

conditions.

In Chapter 5, a microporous layer (MPL) was fabricated with a negative spatial gradient in PTFE

content (from the catalyst layer to the substrate). The effects of the spatially graded PTFE within

MPLs on the oxygen transport resistance and GDL liquid water distribution was investigated.

GDLs with graded and uniform PTFE within the MPLs were assembled into fuel cells to monitor

fuel cell performance (cell potential, current density, and cell impedances) and to visualize the

liquid water distribution within the cathode GDL via synchrotron X-ray radiography. The

following key contributions were identified from Chapter 5:

93

• The PTFE gradient within the MPL led to decreased liquid water accumulation within the

cathode GDL substrate. This led to a decrease in oxygen transport resistance at high current

densities (≥ 1.0 A/cm2).

• A negative spatial gradient in PTFE content (from the catalyst layer to the flow field)

creates a positive spatial gradient in porosity and a negative spatial gradient in

hydrophobicity (or contact angle). This results in a negative spatial gradient in the threshold

capillary pressure that promotes capillary-driven removal of liquid water.

• Membrane dehydration at high current densities (≥ 1.0 A/cm2) led to membrane shrinkage,

which was hypothesized to be caused by increased local temperatures at the catalyst layers.

Chapter 5 demonstrated that the design of novel GDL materials with specialized water transport

properties can be informed and enhanced with an understanding of liquid water transport within

the GDLs and its effect on fuel cell impedance.

The research presented in this thesis demonstrates that novel MPL designs can be used to tailor

water management within the fuel cell. This water management was effectively characterized

using a combination of liquid water visualization and fuel cell performance and impedance

measurements. The knowledge gained from the work can be used to design next-generation fuel

cell materials that further improve fuel cell performance at desired operating conditions. Improved

fuel cell performance through these water management strategies help reduce the fuel cell cost and

thus increase its viability for widespread adoption.

94

6.2 Future Work

The following future work is recommended based on the insights gained from the study:

• GDL surface wettability optimization for desired operating conditions: GDL surface

wettability influences membrane hydration and oxygen transport resistance by affecting

the water balance within the fuel cell. Membrane resistance was shown to dominate at

lower current densities, while oxygen transport resistance dominated at higher current

densities. The wettability of the GDL surface can be tuned to optimize cell performance at

desired range of operating conditions (or current densities). This optimization study can be

performed by following a methodology similar to Chapter 4, where GDLs with a range of

surface hydrophilicity can be fabricated and tested using electrochemical impedance

spectroscopy and synchrotron X-ray radiography. The measured liquid water distributions,

along with measured membrane and oxygen transport resistances, could be analyzed to

find an optimal wettability for the operating conditions.

• Numerical simulations: The use of numerical simulations is recommended, to aid the

design of novel GDL materials. For instance, a pore network modeling software, such as

OpenPNM [92,93], can be used to simulate liquid transport within functionally graded

GDLs. A range of gradients of threshold capillary pressure (created by varying pore/ throat

diameter and/or contact angle) can be created in stochastically generated GDL materials.

The invasion of liquid water into these GDL materials can be simulated to provide insight

on the liquid water distributions within these GDLs. The experimentally quantified liquid

water should be used to validate the simulations.

• Design GDLs with functional variability in 2D or 3D: Chapter 5 demonstrated the

enhanced liquid water removal capabilities of GDLs with functional gradients in 1D

95

(through plane direction). This design approach should be extended to 2D or 3D. As such,

these designs can be used to modify the 2D or 3D liquid water distributions and transport

properties within these materials. For example, liquid water tends to accumulate near flow

field ribs, creating a heterogeneous water profile in 2D (observed at high current densities,

in Figure 4-4 and Figure 5-6). A 2D threshold capillary pressure gradient can be designed

in the GDL at the GDL- rib interface to reduce this liquid water accumulation near the flow

field ribs. This would increase in-plane conductivity of oxygen within the GDL and thus

increase fuel cell performance by decreasing oxygen transport resistance.

96

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