Tailored Electrospun Gas Diffusion Layers for Polymer Electrolyte

Membrane Fuel Cells: Design and Durability

by

Manojkumar Balakrishnan

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Department of Mechanical and Industrial Engineering

University of Toronto

© Copyright by Manojkumar Balakrishnan 2019

ii

Tailored Electrospun Gas Diffusion Layers for Polymer Electrolyte

Membrane Fuel cells: Design and Durability

Manojkumar Balakrishnan

Master of Applied Science

Department of Mechanical and Industrial Engineering

University of Toronto

2019

Abstract

The polymer electrolyte membrane (PEM) fuel cell is a versatile alternative for the decarbonization

of the transportation sector. However, high cost and limited durability of materials hinder

widespread adoption. This thesis comprises two studies that aim to address both cost and durability

of PEM fuel cells via the design of tailored gas diffusion layers (GDLs).

First, electrospinning is presented as a platform to fabricate and design tailored GDLs with pore

size gradients for the improved high current performance of PEM fuel cells. Specifically, the novel

graded GDL was found to reduce ohmic resistance and improve mass transport performance. Next,

the durability of electrospun GDLs was investigated via an ex situ accelerated degradation

procedure. The degraded GDLs exhibited reduced hydrophobicity due to loss of surface groups

and reduced electrical conductivity due to carbon degradation. This thesis offers insight into

designing next generation, durable GDLs with tailored structures for PEM fuel cells.

iii

Acknowledgments

Prof. Bazylak, thank you for your unwavering support and invaluable guidance. I have grown

immensely as a scientist, engineer, and person due to your mentorship. You have provided me

with opportunities that I didn’t know were possible. Thank you.

Prof. Hatton, thank you for opening your lab to me. Your support, advice, and insight has greatly

improved my work.

Prof. Schulz, I am very grateful to have had the opportunity to work with you during your stay in

Toronto. Your advice and guidance have had a great impact on my work.

To Pranay, Eric, Nan, Chung, Hang, Robin, Kieran, David, Hisan, Pascal, Kevin, Jason L.,

Jason C., Dan, and Andrew – I would like to thank all of you for your friendship. The sometimes

long and grueling hours of grad school were never a problem due to your comradery. I cannot wait

to see the brilliant work that you will all inevitably do in the future.

To Eric, Pranay, and Sosna – Thank you for making momos with me on Fridays, having brunch

with me on Saturdays, and forcing me to grill and eat vegetables on Sundays.

To Nico, László, Kerstin, and Prof. Zeis – thank you for my making my stay in Germany such a

wonderful, rewarding, and productive experience. I will never forget my time spent in Ulm.

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To the M&S group chat – I always enjoyed catching up on the 99 unread messages accumulated

through the day discussing various important current events and news stories – sorry – I meant

basketball, because ball is life.

To all the staff and faculty members at the University of Toronto who I had the chance to work

with – thank you for opening your doors to an excited kid who wanted to try everything.

There are many more people I could still acknowledge, but I would like to finish by thanking my

family. Amma, Appa, Dhanush, and Grandma – you provide with undying support in every single

thing I do. You make it possible for me to pursue my dreams. I can always rely on you for a push

in the right direction, or just simply a loving talk to take my mind off things. Thank you.

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Table of Contents

Acknowledgments.......................................................................................................................... iii

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

List of Tables ...................................................................................................................................x

List of Figures ................................................................................................................................ xi

List of Appendices ....................................................................................................................... xiv

Abbreviations and Nomenclature ..................................................................................................xv

CHAPTER 1 – Introduction.............................................................................................................1

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

1.2 Motivation and Objectives ...................................................................................................2

1.3 Contributions........................................................................................................................4

1.4 Co-authorship .......................................................................................................................5

1.5 Thesis Organization .............................................................................................................5

CHAPTER 2 – Background .............................................................................................................7

2.1 The PEM Fuel Cell – An Introduction .................................................................................7

2.2 PEM Fuel Components ......................................................................................................10

2.2.1 Gas Diffusion Layer ...............................................................................................10

2.2.1.1 Conventional Gas Diffusion Layers ........................................................11

2.2.1.2 Electrospun Gas Diffusion Layers ...........................................................14

2.2.2 Catalyst Coated Membrane ....................................................................................15

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2.2.2.1 Polymer Electrolyte Membrane ......................................................... 15

2.2.2.2 Catalyst Layer ..........................................................................................16

2.3 Fuel Cell Performance .......................................................................................................16

2.3.1 Region I - Thermodynamic and OCV losses .........................................................18

2.3.2 Region II – Activation Losses................................................................................19

2.3.3 Region III – Ohmic Losses ....................................................................................19

2.3.4 Region IV – Mass Transport Losses ......................................................................20

2.4 Transport Mechanisms within the PEM Fuel Cell .............................................................20

2.4.1 Electronic and Ionic Transport...............................................................................21

2.4.2 Heat Transport .......................................................................................................22

2.4.3 Mass Transport.......................................................................................................22

2.4.4 Water Balance ........................................................................................................25

2.5 Chapter Summary ..............................................................................................................26

CHAPTER 3 – Designing Gas Diffusion Layers with Pore Size Gradients via Electrospinning

for Polymer Electrolyte Membrane Fuel Cells .........................................................................28

3.1 Introduction ........................................................................................................................29

3.2 Methodology ......................................................................................................................33

3.2.1 eGDL Fabrication ..................................................................................................34

3.2.1.1 Electrospinning ........................................................................................34

3.2.1.2 Heat Treatment ........................................................................................37

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3.2.1.3 Hydrophobic Treatment ..................................................................... 38

3.2.2 eGDL Characterization ..........................................................................................38

3.2.2.1 eGDL Fiber Diameters and Pore Size Distribution .................................38

3.2.2.2 Graphitization of the Carbon Fibers ........................................................40

3.2.2.3 Electrical Conductivity ............................................................................40

3.2.2.4 eGDL Thickness ......................................................................................41

3.2.3 Fuel Cell Assembly and Testing ............................................................................41

3.2.3.1 Fuel Cell Hardware and Control ..............................................................41

3.2.3.2 𝒊 − 𝑽 curves .............................................................................................42

3.2.3.3 Electrochemical Impedance Spectroscopy ..............................................43

3.2.3.4 Synchrotron X-ray Radiography .............................................................46

3.3 Results ................................................................................................................................50

3.3.1 Structure and Material Properties of the eGDLs....................................................50

3.3.1.1 Fiber Diameter and Pore Size Distribution .............................................50

3.3.1.2 Effect of Fiber Diameter on Graphitization .............................................53

3.3.1.3 Effect of Fiber Diameter and Fiber Connectivity on Bulk In-Plane

Electrical Conductivity ............................................................................55

3.3.2 High Current Density Fuel Cell Performance ........................................................56

3.3.2.1 Improved Ohmic Performance with Smaller Pore Sizes and Fiber

Diameters .................................................................................................56

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3.3.2.2 Improved Mass Transport Performance with Pore Size Gradient ..... 60

3.3.2.3 Comparison to Commercial GDLs ..........................................................63

3.4 Chapter Conclusions ..........................................................................................................64

CHAPTER 4 – Degradation Characteristics of Electrospun Gas Diffusion Layers for Polymer

Electrolyte Membrane Fuel Cells .............................................................................................66

4.1 Introduction ........................................................................................................................67

4.2 Methodology ......................................................................................................................71

4.2.1 Fabrication of Graded Hydrophobic eGDLs ..........................................................72

4.2.1.1 Electrospinning and Heat treatment ........................................................72

4.2.1.2 Direct Fluorination Procedure .................................................................73

4.2.2 Accelerated Degradation Procedure ......................................................................76

4.2.3 Characterization of Degraded eGDLs ....................................................................77

4.2.3.1 Surface Contact Angle .............................................................................77

4.2.3.2 Electrical Conductivity ............................................................................77

4.2.3.3 Fuel Cell Hardware and Control ..............................................................78

4.2.3.4 Fuel Cell Performance Testing ................................................................79

4.2.3.5 Synchrotron X-ray Radiography .............................................................79

4.3 Results ................................................................................................................................83

4.3.1 Effect of Degradation Procedure on eGDL Surface Hydrophobicity ....................83

4.3.2 Increased Liquid Water Accumulation due to Loss of Surface Hydrophobicity ...86

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4.3.3 Increased Ohmic Losses due to Carbon Degradation ...................................... 88

4.4 Chapter Conclusions ..........................................................................................................90

CHAPTER 5 – Conclusions...........................................................................................................93

5.1 Summary of Findings .........................................................................................................93

5.2 Future Work .......................................................................................................................96

References ....................................................................................................................................100

Appendix A – Tafel Slope Measurement.....................................................................................108

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List of Tables

Table 1: Summary of the material properties of the tailored eGDLs. .......................................... 53

xi

List of Figures

Figure 1. Schematic of a polymer electrolyte membrane (PEM) fuel cell. .................................... 9

Figure 2. Scanning electron microscopy (SEM) images of a commercial Sigracet (SGL) Gas

Diffusion Layer (GDL). Surface SEM image of a) SGL GDL substrate and b) SGL microporous

layer (MPL). c) Cross-sectional SEM image of SGL 25BC GDL showing MPL and substrate

regions. .......................................................................................................................................... 12

Figure 3. PEM fuel cell polarization curve. .................................................................................. 17

Figure 4. Summary of eGDL manufacturing procedure. a) Schematic of the electrospinning

apparatus. b) Temperature profile used for the stabilization and carbonization of the electrospun

polymer fibers. c) Schematic of the direct fluorination treatment used to render the carbonized

substrates hydrophobic.................................................................................................................. 36

Figure 5. Modified Randle’s equivalent circuit used to quantify the mass transport resistance of

the fuel cell from the impedance spectra. ..................................................................................... 44

Figure 6. Sample images obtained via synchrotron X-ray radiography. a) Sample of raw X-ray

radiograph obtained in greyscale. b) Sample processed image of the cathode GDL and CCM

region. ........................................................................................................................................... 49

Figure 7. Fiber diameter and pore size distribution of the tailored eGDLs. a) Surface SEM image

of the 12wt.% eGDL. b) Surface SEM image of the 8wt.% eGDL. c) SEM cross-section image of

the Bi-Layer eGDL. d) Pore size distribution from SEM cross-section images. .......................... 52

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Figure 8. Raman spectra and electrical conductivity of the tailored eGDLs. a) Representative

Raman spectra obtained for 8wt.% and 12 wt.% electrospun carbon fibers. b) Bulk in-plane

electrical conductivity of the eGDLs. ........................................................................................... 54

Figure 9. PEM fuel cell performance with the tailored eGDLs. a) 𝑖 − 𝑉 curves obtained at 50%

RH. b) 𝑖 − 𝑉 curves obtained at 100% RH. .................................................................................. 57

Figure 10. High frequency resistance (HFR) and water content within the region of interest, 𝑉𝑊,𝑅𝑂𝐼,

at 50% RH. a) Average HFR at 50% RH. b) Total liquid water content, 𝑉𝑊,𝑅𝑂𝐼, in the region of

interest at 1.0 A/cm2 and 50% RH. ............................................................................................... 58

Figure 11. Mass transport resistance, RMT, at 100% RH. a) Representative Nyquist spectra obtained

at 1.5A/cm2 and 100% RH. b) Average RMT values obtained from the measured EIS spectra via

equivalent circuit modelling. ........................................................................................................ 61

Figure 12. Summary of experimental procedures. a) Direct fluorination treatment used to

functionalize the eGDLs. b) Apparatus used for accelerated degradation. ................................... 75

Figure 13. Sample images from synchrotron X-ray radiography. a) Sample radiograph obtained

from X-ray radiography. b) Processes image showing water thickness, 𝑡𝑊, of each pixel. ......... 82

Figure 14. Surface contact angle measurements of the eGDLs at various stages. a) Average contact

angles of pristine, post-fuel cell tested, and degraded eGDLs. Sample droplet image with b) the

pristine eGDL, c) the post fuel cell tested eGDL, and d) the degraded eGDL. ............................ 84

Figure 15. Fuel cell performance and water profiles at 100% RH. a) 𝑖 − 𝑉 curves obtained at 100%

RH. b) Through-plane liquid water profile at 100% RH at 0.5 A/cm2. ........................................ 87

xiii

Figure 16. Fuel cell performance at 50% RH and electrical conductivity. a) 𝑖 − 𝑉 curves obtained

at 50% RH. b) High frequency resistance (HFR) at 50% RH. c) Bulk in-plane electrical

conductivity of eGDLs. ................................................................................................................. 89

Figure 1A. Plot of 𝐸𝑖𝑅 − 𝑓𝑟𝑒𝑒 vs. 𝑖 at 100% inlet RH used to calculate the Tafel slope, 𝑏. ..... 109

xiv

List of Appendices

Appendix A – Tafel Slope Measurement.....................................................................................108

xv

Abbreviations and Nomenclature

Technical Abbreviations

PEM Polymer electrolyte membrane

IC Internal combustion

GHG Greenhouse gas

GDL Gas diffusion layer

CCM Catalyst coated membrane

CL Catalyst layer

HOR Hydrogen oxidation reaction

ORR Oxygen reduction reaction

MPL Microporous layer

SGL Sigracet® carbon group

RH Relative humidity

Chemical Abbreviations

𝐻2 Hydrogen

𝑂2 Oxygen

𝐻2𝑂 Water

𝐻+ Hydrogen ion

𝑒− Electron

PTFE Polytetrafluoroethylene

PAN Polyacrylonitrile

PEN Polyethylene naphthalate

xvi

𝐶 Carbon

𝐹 Fluorine

𝐻2𝑂2 Hydrogen peroxide

Chapter 2 Variables and Abbreviations

𝐸𝑐𝑒𝑙𝑙 Operating cell voltage (V)

𝑖 Current density (A/cm2)

𝐸𝑡ℎ Maximum thermodynamic cell potential (V)

OCV Open circuit voltage

∆𝐻 Enthalpy change of a reaction (J/mol∙K)

𝑇 Temperature (K)

𝑛 Number of electrons generated per mole of fuel (mol)

𝐹 Faraday’s constant (96485 C/mol)

𝐸𝑟𝑒𝑣 Maximum reversible cell potential (V)

∆𝐺𝑜 Gibbs free energy change of a reaction at standard temperature and pressure

𝑅 Universal gas constant (8.314 J/mol∙K)

𝑃𝑖 Partial pressure of species 𝑖

𝐸𝑂𝐶𝑉 Cell potential at open circuit (i.e. near 0.0 A/cm2)

𝑞 Heat generated within fuel cell (W/cm2)

𝑛𝑗 Flux of species 𝑗 (mol/s)

𝐷𝑗 Diffusion coefficient of species 𝑗 (m2/s)

𝐴 Cross sectional area (m2)

𝐶𝑗 Concentration of species 𝑗 (mol/m3)

xvii

𝐾𝑛 Knudsen number

𝑙 Mean free path length of a diffusing gas molecule (m)

𝑑 Pore diameter (m)

𝜙 Porosity

𝜏 Tortuosity

𝐷𝑒𝑓𝑓 Effective diffusion coefficient (m2/s)

𝐷𝑏𝑢𝑙𝑘 Bulk diffusion coefficient (m2/s)

𝑠 Saturation

𝐶𝑎 Capillary number

𝑀 Viscosity ratio

𝑢 Velocity of non-wetting fluid (m/s)

𝜇𝑛𝑤 Viscosity of non-wetting fluid (Ns/m2)

𝜎 Interfacial tension (N/m)

𝜇𝑤 Viscosity of wetting fluid (Ns/m2)

Chapter 3 and 4 Variables and Abbreviations

SEM Scanning electron microscopy

EIS Electrochemical impedance spectroscopy

eGDL Electrospun gas diffusion layer

𝐼𝐷 Peak D-band intensity of Raman spectra

𝐼𝐺 Peak G-band intensity of Raman spectra

𝑖 Current density (A/cm2)

HFR High frequency resistance (Ω∙cm2)

xviii

𝑅𝑀𝑇 Mass transport resistance (Ω∙cm2)

AC Alternating current

CPE Constant phase element

𝑍𝑇𝑂𝑇𝐴𝐿 Total impedance of equivalent circuit (Ω∙cm2)

𝑅𝛺 Ohmic resistance (Ω∙cm2)

𝑍𝐶𝑃𝐸 Impedance of electric double-layer at reaction interface (Ω∙cm2)

𝑅𝐶𝑇 Charge transfer resistance (Ω∙cm2)

𝑍𝑊 Warburg impedance (Ω∙cm2)

𝑗 Imaginary number (√−1)

𝜔 Frequency of alternating current input signal (rad/s)

𝜏 Time constant for diffusion process (s)

𝐶𝑑𝑙 Cathode double-layer capacitance (F/cm2)

𝛼 Phase angle of constant phase element (rad)

𝜂𝑎𝑐𝑡 Activation overpotential (V)

𝑏 Tafel slope (V/decade)

𝐸𝑖𝑅−𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 Internal resistance corrected cell voltage (V)

𝑡𝑊 Water content (cumulative thickness) (cm)

𝜇𝑊 Attenuation coefficient of liquid water (cm-1)

𝐼𝑂𝐶𝑉 Pixel intensity of reference radiograph

𝐼𝑤𝑒𝑡 Pixel intensity of radiograph during cell operation

𝑉𝑊,𝑅𝑂𝐼 Total water content in region of interest (cm3)

𝐴𝑅𝑂𝐼 Area of region of interest (cm2)

𝑁𝑡 Number of radiographs averaged over time

xix

𝑁𝑥 Number of pixels averaged in 𝑥-direction

𝑁𝑦 Number of pixels averaged in 𝑦-direction

𝜎 Bulk in-plane electrical conductivity (S/cm)

𝑡𝑊̅̅̅̅ Normalized average cumulative water thickness (cm/cm)

𝐿𝑧 Thickness of active area parallel to path of X-ray beam (cm)

SAM Self-assembled monolayer

XPS X-ray photoelectron spectroscopy

Other Nomenclature

DOE The United States Department of Energy

1

CHAPTER 1 – Introduction

1.1 Preamble

Low carbon energy solutions are required to mitigate the effects of anthropogenic climate

change [1]. The use of hydrogen and polymer electrolyte membrane (PEM) fuel cells can

significantly contribute to the decarbonization of various energy sectors including transportation,

heating, and power generation [1, 2]. A PEM fuel cell is an electrochemical device that converts

hydrogen and oxygen to produce electricity. The reaction emits zero local carbon emissions and

the only products are water and heat. Furthermore, PEM fuel cell systems are capable of cold start-

ups, and exhibit high energy efficiencies making them a particularly attractive alternative to fossil

fuel-based internal combustion (IC) engines in the transportation sector [2, 3]. State of the art PEM

fuel cells are up to 60% efficient in converting the stored energy in hydrogen to usable electricity,

whereas IC combustion engines are only 20 – 30 % efficient in converting gasoline to usable

power [4, 5]. When the higher efficiency of PEM fuel cells is coupled with the use of hydrogen

produced via renewable sources such as wind, an 85% reduction in greenhouse gas (GHG)

emissions is possible compared to IC engines and fossil fuels (the calculation for reduction in GHG

emissions considered life cycle of fuels from production to utility and indirect fossil fuel usage via

raw material consumption) [4]. Although PEM fuel cells have considerable environmental

advantages over conventional IC engines, the wide-spread commercialization of PEM fuel cell

systems are hindered by their high costs (stems from the cost of raw materials such as the platinum

catalysts) and comparatively lower durability [3].

2

1.2 Motivation and Objectives

The US Department of Energy (DOE) has set several technical targets to facilitate the

commercialization of PEM fuel cell systems [3]. The current technical targets for 2025 include a

fuel cell system cost target of $35/kW (based on 500,000 production units) and a powertrain

durability target of 8000 hours of operation (equivalent to 150,000 miles) with <10% loss in fuel

cell performance [3]. The 2017 status of the cost and durability of PEM fuel cell systems were

reported to be $45/kW and 4130 hours of operation with 10% performance loss, respectively [3].

Therefore, significant research and development is required to reduce the cost and improve the

durability of PEM fuel cell systems.

The first study presented in this thesis was motivated by the need to reduce fuel cell system costs.

An effective strategy to minimize system cost is to improve the power density of PEM fuel cells [6].

Specifically, operating a fuel cell stack at higher current densities can increase the power output

and thereby allow a reduction in the number of cells and materials required within a stack. Analysis

done by James et al. [7], in conjunction with the US DOE, indicated that a 50% improvement in

power density from 749 mW/cm2 can lead to approximately an $8/kW reduction in system cost.

However, when a fuel cell is operated at high currents, the product water from the electrochemical

reactions tends to accumulate within the cell and block pathways for reactant supply. The blockage

of reactants leads to performance losses and reduced power outputs. Therefore, to improve the

high current density performance, effective management of the product water is necessary [8]. The

gas diffusion layer (GDL) is a porous layer within the fuel cell that facilitates the transport of

product water. Specifically, the pore structure and material properties of the GDL have a

3

significant influence on water accumulation within the cell [9]. The objective of the first study was

to fabricate novel GDL materials with tailored microstructures to enhance the water management

and high current density performance of PEM fuel cells. Electrospinning was chosen as the

fabrication method for producing the tailored GDLs. The structure and properties of the novel

electrospun GDLs were thoroughly characterized and the effect of the electrospun GDLs on fuel

cell performance were systematically evaluated via in situ performance testing and in-operando

synchrotron X-ray radiography. The results of the first study provided valuable insight into the

effect of GDL microstructures on high current density PEM fuel cell performance. Furthermore,

the methods presented offer a platform for manufacturing tailored GDLs for next generation PEM

fuel cells.

When developing novel materials to reduce the cost of PEM fuel cell systems, the durability of the

novel materials must be simultaneously scrutinized to ensure they can meet the drivetime

requirements for practical applications [6]. Therefore, the second study presented in this thesis

investigated the degradation characteristics of the novel electrospun GDLs under long term fuel

cell operation. An ex situ accelerated degradation protocol was employed to test the electrospun

materials. The material properties and fuel cell performance of the degraded GDLs were compared

to pristine GDLs to elucidate the degradation characteristics of the electrospun materials. The

results from the second study provided insight into the degradation mechanisms of the novel GDLs

and highlighted areas for development for the successful implementation of robust tailored GDLs

for next generation PEM fuel cells.

4

1.3 Contributions

This thesis contains two studies which were prepared as first-authored journal manuscripts. The

two studies, listed below, encompass Chapters 3 and 4:

1. M. Balakrishnan, P. Shrestha, CH. Lee, Ge. N., K.F. Fahy, R. Zeis, V. Schulz,

B.D. Hatton, A. Bazylak, “Designing Gas Diffusion Layers with Pore Size Gradients via

Electrospinning for Polymer Electrolyte Membrane Fuel Cells” Small, (Submitted).

2. M. Balakrishnan, P. Shrestha, CH. Lee, Ge. N., K.F. Fahy, M. Messerschmidt, J. Scholta,

L. Eifert, R. Zeis, B.D. Hatton, A. Bazylak, “Degradation Characteristics of Electrospun

Gas Diffusion Layers for Polymer Electrolyte Membrane Fuel Cells” (In Preparation).

The studies above were completed with the support of the following personnel. Pranay Shrestha,

Chung Lee, Nan Ge, and Dr. Kieran Fahy of the Thermofluids for energy and advanced materials

(TEAM) lab provided support during the experiments conducted at the Canadian Light Source,

Canada, as well as valuable feedback and critiques throughout the study design and data analysis

process. Dr. Roswitha Zeis (Helmholtz Institute Ulm, Germany), Dr. Volker Schulz (Baden-

Württemberg Cooperative State University, Germany), and Dr. Benjamin Hatton (University of

Toronto, Canada) provided mentorship and guidance during the fabrication process and data

analysis for study 1. Similarly, Dr. Matthias Messerschmidt (Zentrum für Sonnenenergie- und

Wasserstoff-Forschung Baden-Württemberg (ZSW), Germany), Dr. Joakim Scholta (ZSW,

Germany), Dr. Roswitha Zeis, and Dr. Benjamin Hatton provided mentorship and guidance for the

experimental work conducted in study 2. László Eifert provided support with the experimental

5

setup and data acquisition for study 2. Furthermore, Dr. Adam Webb, Dr. Ning Zhu, Dr. Sergey

Gasilov, and Denise Miller were all staff at the Canadian Light Source who provided

administrative and operational support during the beamline experiments.

1.4 Co-authorship

In addition to the two journal manuscripts above, I also contributed as a co-author to the following

work over the duration of my thesis. As co-author, I provided experimental support during the in-

operando visualization experiments conducted at the Canadian Light Source, as well as the fuel

cell experiments conducted at the University of Toronto. Additionally, I contributed to the

discussion of the results and review of the final journal manuscript.

1. N. Ge, P. Shrestha, M. Balakrishnan, D. Ouellette, A.K.C. Wong, H. Liu, CH. Lee, J.K.

Lee, A. Bazylak, “Resolving the Gas Diffusion Layer Substrate Land and Channel Region

Contributions to the Oxygen Transport Resistance of a Partially-saturated Substrate”

Electrochimica Acta, (Submitted).

1.5 Thesis Organization

This thesis is organized into 5 chapters. Chapter 1 provides a high-level introduction to PEM fuel

cells and their role in a low carbon economy. The current limitations and technical targets for the

commercialization of PEM fuel cells are highlighted. Based on the technical targets, the

motivations and objectives for the thesis are provided.

6

The purpose of Chapter 2 is to a provide a thorough understanding of PEM fuel cell working

principles, the function and structure of key components within the fuel cell, and the characteristics

of PEM fuel cell performance. The transport mechanisms of the reactants and products within the

fuel cell are also described. The information presented in Chapter 2 serves as the technical

foundation for the two studies presented in this thesis.

The two principle studies conducted in this thesis are presented in Chapters 3 and 4, respectively.

Specifically, the study in Chapter 3 utilized electrospinning to produce novel GDL materials to

enhance the high current density performance of PEM fuel cells. The structural and material

properties of the electrospun GDLs were obtained via a suite of characterization techniques, and

the effects of the novel GDL on fuel cell performance were investigated via in situ fuel cell

performance testing and synchrotron X-ray radiography. The study in Chapter 4 investigated the

durability of the novel electrospun GDLs for robust PEM fuel cell application. The GDLs were

degraded via an ex situ degradation protocol, and the degradation characteristics of the electrospun

GDLs were elucidated via ex situ material characterization and in situ fuel cell performance testing

and visualization. Finally, Chapter 5 provides a summary of contributions from the two studies

presented in this thesis and proposes several promising future studies to further facilitate the

development of tailored GDLs for the improved performance and durability of next generation

PEM fuel cells.

7

CHAPTER 2 – Background

In this chapter, an overview of the working principles, architecture, and primary components of a

PEM fuel cell are described to serve as background information for the studies presented in

Chapters 3, and 4. Furthermore, the characteristics of PEM fuel cell performance, the transport

mechanisms encountered within the fuel cell, and the concept of water balance are described to

provide a foundation for the analysis techniques used in the remainder of the thesis.

2.1 The PEM Fuel Cell – An Introduction

The PEM fuel cell is a multi-layered structure with the following basic components: the catalyst

coated membrane (CCM), the gas diffusion layers (GDLs), and the bi-polar plates with integrated

flow fields. These layers are mechanically compressed together and facilitate the electrochemical

conversion of hydrogen and oxygen into electricity. A schematic of a single fuel cell is presented

in Figure 1. Hydrogen is supplied via the anode flow field, and oxygen (typically as air) is supplied

via the cathode flow field. The reactants diffuse from the flow field to the catalyst layer (CL) via

the GDL. The CL is the site of the electrochemical reactions. The polymer membrane is an

electrolytic barrier between the anode and cathode and maintains the electrochemical potential

difference between the two electrodes. The PEM fuel cell generates electricity via the following

half-reactions:

Half reaction at anode: 𝐻2 → 2𝐻+ + 2𝑒− (1)

Half reaction at cathode: 1

2𝑂2 + 2𝐻+ + 2𝑒− → 𝐻2𝑂 (2)

8

At the anode, hydrogen is oxidized to form two hydrogen ions and two electrons. This reaction is

referred to as the hydrogen oxidation reaction (HOR). The hydrogen ions are conducted through

the polymer electrolyte membrane towards the cathode due to the electrochemical gradient. The

electrons from the HOR are conducted via the GDLs, bi-polar plates, and then through an external

circuit where useful work can be extracted. At the cathode, the electrons and ions from the anode

reaction are recombined and react with the supplied oxygen to produce water. The cathode reaction

is referred to as the oxygen reduction reaction (ORR). The complete PEM fuel cell reaction with

hydrogen as the fuel can be described as:

𝐻2 +1

2𝑂2 → 𝐻2𝑂 (3)

9

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

10

The maximum reversible cell potential attainable with a single PEM fuel cell at 25 ○C and 1 atm

is 1.23V (based on Nernst equation described in Section 2.3.1). However, the operating potential

is often below 1.0 V per cell due to various performance losses (discussed in Section 2.3).

Therefore, to achieve the required power outputs for practical applications, PEM fuel cells are

constructed in a stack with each stack consisting of multiple cells in series [10]. The power output

of a fuel cell stack is a function of the number of cells present in a stack, the active area of each of

the cells (i.e. reaction area), and the operating current [10]. Cost reductions are possible with

improved high current density performance (i.e. improved cell voltages at high currents) as the

number of cells required within a stack can be reduced thereby reducing material cost [6].

2.2 PEM Fuel Components

In this section, the function, structure, and morphology of key PEM fuel cell components are

described.

2.2.1 Gas Diffusion Layer

The gas diffusion layer (GDL) within the PEM fuel cell is a porous structure that facilitates:

1) reactant transport from the flow field to the CL, 2) product water removal from the CL to the

flow field, 3) electron transport to and from the CL and bi-polar plates, and 4) heat dissipation

from the electrochemical reactions at the CL. In addition to functioning as a critical transport layer

for the reactants and products within the fuel cell, the GDL also provides mechanical support to

the polymer electrolyte membrane. Given the range of functions of the GDL, the design and

structure of the GDL has a large influence on PEM fuel cell performance [11].

11

2.2.1.1 Conventional Gas Diffusion Layers

To meet all of the necessary functions required of the GDL, commercial GDL substrates are

usually carbon-based porous materials composed of two unique layers: the macro-porous substrate

(adjacent to the flow field), and the microporous layer (adjacent to the CL) [12] (Figure 2). The

macro-porous substrate is made of graphitic carbon fibers bound within a carbon-based resin [8].

The carbon fibers are conventionally manufactured via melt-spinning or wet-spinning processes

and are graphitized to ensure effective electronic and thermal conductivity [13, 14]. The average fiber

diameter within the macro-porous substrate ranges between 7 – 10 μm [15], and average pore sizes

range between 10 – 30 μm [16]. The porosity of the macro-porous substrate ranges from 0.6 – 0.9

depending on the compression of the GDL within the fuel cell assembly and the GDL

manufacturing process [10, 17]. GDL compression is often defined as compression rate (%) [18, 19]:

𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 =𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 − 𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑒𝑑 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠

𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠∙ 100 (4)

where nominal thickness is the initial uncompressed thickness of the GDL.

12

Figure 2. Scanning electron microscopy (SEM) images of a commercial Sigracet (SGL) Gas

Diffusion Layer (GDL). Surface SEM image of a) SGL GDL substrate and b) SGL microporous

layer (MPL). c) Cross-sectional SEM image of SGL 25BC GDL showing MPL and substrate

regions.

13

The high porosity of the macro-porous substrate allows for the uniform distribution of reactants to

the CL and effective diffusion of gases from the flow field. The thickness of commercial macro-

porous substrates typically ranges from approximately 100 – 300 μm [10, 13, 20]. The thicker the

substrate, the more mechanical stability it can impart to the fuel cell; however, increased thickness

leads to higher electronic resistance and larger fuel cell stacks. Finally, macro-porous substrates

are often treated with polytetrafluorethylene (PTFE) coatings (ranging between 5 – 30 wt.%) [8, 10,

21]. The PTFE coating imparts a hydrophobic surface onto the carbon fibers, which helps to

effectively expel accumulated liquid water within the GDL.

The microporous layer (MPL) is typically coated onto the macro-porous substrates as a thin layer

ranging between 5 – 50 μm [8, 13, 22]. The MPL is comprised of a mix of carbon black particles

bound within a polymeric matrix (typically PTFE) [10, 22]. The PTFE content within the MPL ranges

between 20 – 30wt.% [23, 24]. The average size of carbon black particles in the MPL is

approximately 50 nm [23]; consequently, the mean pore size of the MPL is significantly smaller

than that of the macro-porous substrate and ranges between 0.1 – 0.5 μm [25]. The porosity of the

MPL is also lower than that of the substrate at approximately 0.5 – 0.6 [25, 26]. The fine porous

structure of the MPL enhances the electrical contact at the CL, promotes the hydration of the

polymer membrane by presenting a high capillary pressure barrier (due to submicron pore sizes),

and facilitates effective water removal from the CL interface upon water invasion due to the

presence of the PTFE binder [8, 27].

14

2.2.1.2 Electrospun Gas Diffusion Layers

Recently, electrospinning has been utilized to fabricate carbon fiber substrates for use as GDL

materials for PEM fuel cells [20, 28]. The electrospinning process involves slowly pumping a viscous

polymer solution via a syringe pump through a steel needle. A high voltage is then applied to the

needle tip which exerts an electrostatic force onto the polymer solution droplet. When the

electrostatic forces overcome the surface tension of the droplet, a jet of polymer is ejected from

the needle tip resulting in the formation of ultrafine polymer fibers. The charged fibers collect onto

a grounded surface to be extracted and converted to carbon fiber substrates via carbonization

processes [29].

A major advantage of electrospinning over conventional carbon fiber manufacturing processes

(e.g. melt spinning or wet spinning) is the ability to produce a wide range of carbon fiber diameters

and microstructures. Electrospun carbon fibers can exhibit fiber diameters as low as 10 nm to as

large as 10 μm [14, 30]. Whereas conventional spinning processes can only produce fibers greater

than ~5 μm as they rely on the mechanical extrusion of the polymer precursor [14]. Furthermore, as

electrospinning is a deposition-based fabrication procedure, the structure and properties of the

electrospun substrates can be varied throughout the thickness of the substrate. As the structure and

design of the GDLs have a large influence on fuel cell performance, electrospinning is particularly

attractive for the fabrication of tailored GDLs.

15

2.2.2 Catalyst Coated Membrane

When the catalyst is coated onto the polymer electrolyte membrane (PEM), the entire assembly is

referred to as the catalyst coated membrane (CCM). The CCM is the central layer of the fuel cell

(Figure 1) and is critical to the function of the cell. The catalyst layers (CLs), on either side of the

PEM, are the sites of the electrochemical reactions. The PEM acts as an electrolytic barrier

between the anode and cathode CLs and is responsible for the transport of hydrogen ions from the

anode to the cathode.

2.2.2.1 Polymer Electrolyte Membrane

The polymer electrolyte membrane (PEM), also referred to as the proton exchange membrane, is

made from an ionically conductive polymer, the most common of which is Nafion (DuPont).

Nafion is composed of a PTFE backbone with sulfonic acid side chains [31]. The hydrophobic and

inert PTFE backbone provides chemical stability, while the hydrophilic sulfonic acid groups

provide a pathway for ion transport across the membrane [10, 31]. The conductivity of the PEM is

dependent upon its hydration state and increases with water content. Furthermore, a minimum

level of hydration is required for the sulfonic acid groups to form a connected network across the

membrane for effective ionic transport [32]. Consequently, PEM fuel cells are often operated with

humidified reactants to maintain sufficient hydration of the PEM. The thickness of Nafion

membranes range from 20 – 250 μm, with thinner membranes exhibiting higher ionic conductivity

but lower durability [10].

16

2.2.2.2 Catalyst Layer

The PEM fuel cell catalyst layer (CL) is a complex porous matrix consisting of carbon supported

catalyst particles and Nafion. Platinum is the most widely used catalyst in PEM fuel cells. The

platinum catalyst particles range in size from 2-10 nm, and the carbon supports onto which the

catalyst is deposited usually have diameters between 40-90 nm [10, 33]. The complex porous

structure of the CL facilitates the electrochemical reactions of the cell by providing pathways for

the electrons (carbon supports), ions (Nafion matrix), and reactants (pore space). The thickness

and porosity of the CL ranges from 5 – 30 μm, and 0.4 – 0.6, respectively, depending on the

manufacturing technique [10, 34]. Due to the high cost and limited supply of platinum, the

development of non-precious metal catalysts is currently an area of active research [6, 34]. PTFE is

often mixed within the CL to impart hydrophobicity in order to minimize water accumulation at

the reaction sites (i.e. platinum particles).

2.3 Fuel Cell Performance

In this section, the performance characteristics of a PEM fuel cell are described. A polarization

curve represents the voltage versus current density (𝐸𝑐𝑒𝑙𝑙 (V) versus 𝑖 (A/cm2)) relationship for a

fuel cell and is a standard means to evaluate fuel cell performance (and other electrochemical

energy conversion devices in general). A generic polarization curve for a PEM fuel cell is

presented in Figure 3. The operating voltage, 𝐸𝑐𝑒𝑙𝑙, of a PEM fuel cell decreases with increasing

current density (as seen in Figure 3) due to various operational losses encountered within the cell.

The losses can be classified into four broad areas: 1) Thermodynamic and OCV losses, 2)

activation losses, 3) ohmic losses, and 4) mass transport losses.

17

Figure 3. PEM fuel cell polarization curve. Thermodynamic potentials calculated for 25 ○C and 1

atm.

18

2.3.1 Region I - Thermodynamic and OCV losses

When a fuel cell is operated near 0.0 A/cm2 (referred to as open circuit), the measured voltage is

always below that of the maximum thermodynamically achievable voltage (based on the enthalpy

change of the reaction), 𝐸𝑡ℎ (V). The drop in cell voltage from 𝐸𝑡ℎ (Region I) occurs due to

thermodynamic and open circuit voltage (OCV) losses, both of which are described below.

The difference between 𝐸𝑡ℎ and 𝐸𝑟𝑒𝑣 (V) (defined as the maximum reversible potential of the

electrochemical reaction based on the Gibbs free energy change) in Figure 3 is described as the

thermodynamic losses. 𝐸𝑡ℎ (V) is defined as:

𝐸𝑡ℎ = −∆𝐻(𝑇)

𝑛𝐹 (5)

where ∆𝐻 (J/mol∙K) is the enthalpy change of the reaction, 𝑇 (K), is the operating temperature,

the constant 𝑛 (mol/mol) represents the number of electrons generated per mole of fuel, and 𝐹 is

Faraday’s constant (96485 C/mol). In a PEM fuel cell, 𝑛 is equal to 2 as two moles of electrons

are generated per mole of hydrogen. 𝐸𝑟𝑒𝑣 at a given temperature and pressure is described by the

Nernst equation as:

𝐸𝑟𝑒𝑣(𝑇, 𝑃) = −∆𝐺𝑜(𝑇)

𝑛𝐹+

𝑅𝑇

𝑛𝐹ln [

𝑃𝐻2∙ (𝑃𝑂2

)12

𝑃𝐻2𝑂] (6)

where 𝐺𝑜(J/mol) represents the Gibbs free energy of the reaction at standard temperature and

pressure, 𝑅 (J/(mol∙K)) is the universal gas constant (8.314 J/mol∙K), and 𝑃𝐻2, 𝑃𝑂2

, and 𝑃𝐻20 (atm)

represent the partial pressures of each of the species at the electrode interface (i.e. the CL).

19

Thermodynamic losses arise due to irreversible heat loss that occurs due to the change in entropy

of the reaction. The difference between 𝐸𝑡ℎ and 𝐸𝑟𝑒𝑣 is constant through all current densities for a

given temperature and pressure.

OCV losses are defined as the difference in cell potential between 𝐸𝑟𝑒𝑣 and 𝐸𝑂𝐶𝑉 (i.e. cell potential

measured near 0.0 A/cm2). OCV losses occur due to a variety of reasons including the unwanted

crossover of reactant gasses through the PEM due to defects or degradation, or via the presence of

impurities within the reactant supply [35].

2.3.2 Region II – Activation Losses

Activation losses occur due to the kinetics of the electrochemical reactions at the anode and

cathode and correspond to the activation energy required for the electrochemical reactions to

proceed [10]. Activation losses are characterized by the steep, exponential drop in cell voltage

observed at low current densities (i.e. Region II). Higher operating temperatures facilitate faster

reaction kinetics and lower activation losses. Activation losses tend to dominate cell performance

at low current densities (i.e. Region II), however, as current density is increased, the activation

overpotential reaches a plateau and plays a lesser role in cell performance compared to the other

loss mechanisms [10].

2.3.3 Region III – Ohmic Losses

Ohmic losses occur within the cell due to the resistance to electron and ion transport within the

PEM fuel cell layers (transport discussed in Section 2.4.1). Ohmic losses are characterized by the

20

near linear decrease in cell voltage in Region III. Although a linear drop in cell voltage is only

observed in Region III, ohmic losses are prevalent at all current densities, as the conductivity of

electrons and ions are a material property of the individual components within the PEM fuel cell.

2.3.4 Region IV – Mass Transport Losses

Mass transport losses occur when the reactant supply to the CL (i.e. reaction sites) is hindered.

Reactant supply can be hindered via a variety of mechanisms including liquid water accumulation

within the GDL leading to reduced gas diffusion rates (discussed in detail in Section 2.3.4) or

liquid water flooding in the CL leading to blockage of active sites [27, 36]. Mass transport losses are

characterized by the steep drop in cell voltage observed at high current densities (i.e. Region IV).

To achieve high power outputs, mass transport losses associated with liquid water accumulation

at high current densities need to be minimized. GDL designs that facilitate effective water removal

from the CL and GDL can lead to improved high current density performance.

2.4 Transport Mechanisms within the PEM Fuel Cell

The following sections describe the various transport mechanisms encountered within the PEM

fuel cell and how these mechanisms affect the overall fuel cell performance. The section is divided

into three broad areas: 1) electronic and ionic transport, 2) heat transport, and 3) mass transport.

Finally, a critical concept known as water balance, which links all three areas and fuel cell

performance, is described.

21

2.4.1 Electronic and Ionic Transport

Electrons in the PEM fuel cell are conducted via the CL, GDL, and bi-polar plates. Therefore, the

total electrical conductivity of the fuel cell is dependent on the conductivity of each individual

layer, as well as the contact resistance between each layer. To minimize contact resistance and

ohmic losses, the components in a fuel cell are often compressed together. However, excessive

compression is undesirable as the pore space required for gas and water transport within the CL

and GDL can collapse leading to increased mass transport losses [18].

Ionic transport within the fuel cell occurs within the PEM and the CL. The medium for ionic

transport in both layers is the polymer electrolyte, which in a PEM fuel cell is Nafion. Therefore,

the ionic conductivity of the PEM fuel cell is dependent on the hydration state of Nafion [32].

Ionic conductivity of Nafion ranges from approximately 0.01 – 0.1 S/cm depending on its

hydration state [32], whereas the electronic conductivity of commercial GDLs range from

approximately 2-200 S/cm [13, 20]. Due to the large difference in conductivity between the two fuel

cell layers, the ohmic losses within a PEM fuel cell are typically dominated by the ionic

conductivity of the PEM and the CL. Therefore, there is a performance benefit to employing thin

membranes (reduced ionic resistance) and providing adequate hydration via external gas

humidification (improved ionic conductivity).

22

2.4.2 Heat Transport

The overall PEM fuel cell reaction described in Equation 3 is exothermic. Furthermore, heat is a

byproduct of various operational losses (e.g. activation losses and ohmic losses). Consequently,

during fuel cell operation, a considerable amount of heat is generated within the fuel cell. The heat

generated within the fuel cell during operation, 𝑞 (W/cm2), is described by following equation:

𝑞 = 𝑖(𝐸𝑡ℎ − 𝐸𝑐𝑒𝑙𝑙). (7)

As seen in Equation 7, heat generation within the cell increases with increasing current density

and operational losses (i.e. lower 𝐸𝑐𝑒𝑙𝑙). Excessive heat generation is detrimental to fuel cell

operation as it can lead to membrane dehydration and lower ionic conductivity, as well as the

degradation of the membrane [37]. The operating temperatures of PEM fuel cells are typically

maintained at 60 – 90 ○C. The overall maximum operating temperature of Nafion is 120 ○C [10].

To minimize the effects of membrane dehydration and degradation, heat generated at the CLs

needs to be effectively dissipated from the fuel cell. Analogous to electron transport, the overall

thermal conductivity of the fuel cell is a function of the thermal conductivity of each layer (CL,

GDL, and bi-polar plates). Typical thermal conductivity values for commercial GDLs range from

approximately 0.2 – 2.0 W/(m∙K) [10, 38].

2.4.3 Mass Transport

Mass transport refers to the transport of the reactant gasses and product water within the fuel cell.

The transport of reactant gasses through the GDL is driven by the concentration gradient between

23

the flow field (gas supply) and the CL (layer at which reactants are consumed). Diffusion of gas

within the GDL can be described by Fick’s law, which when written in 1-D is:

𝑛𝑗 = −𝐷𝑗𝐴𝜕𝐶𝑗

𝜕𝑥 (8)

where 𝑛𝑗(mol/s) is the flux of species 𝑗 (either oxygen or hydrogen in the fuel cell), 𝐷𝑗 (m2/s) is

the diffusion coefficient of species 𝑗 in a given medium, 𝐴 (m2) is the cross-sectional area through

which diffusion occurs, and 𝐶𝑗 (mol/m3) is the molar concentration of species 𝑗 at a given

location 𝑥.

The diffusion coefficient, 𝐷𝑗 , in Equation 8 for porous media, such as GDLs, is dependent on the

diffusion mechanism [39]. The two dominant diffusion mechanisms encountered within the GDL

are bulk diffusion and Knudsen diffusion [10, 40]. Bulk diffusion is governed by molecule – molecule

interactions, whereas Knudsen diffusion is governed by molecule – wall (e.g. GDL structure)

interactions [39, 41]. The relative importance of each type of diffusion is determined via the Knudsen

number, 𝐾𝑛, which compares the mean free path length of a diffusing gas molecule, 𝑙 (m), and the

pore diameter, 𝑑 (m), and is defined as:

𝐾𝑛 =𝑙

𝑑 . (9)

When 𝐾𝑛 is < 0.01 (pore spaces are much larger than the mean free path), bulk diffusion is the

primary mechanism of gas transport. When 𝐾𝑛 > 10 (pore space is much smaller than the free

path), Knudsen diffusion tends to dominate. If 𝐾𝑛 is between 0.01 – 10, then both diffusion

mechanisms can play a role. In the macro-porous substrate of the GDL, the pore diameter typically

ranges from 10 – 30 μm [16]. At this range, 𝐾𝑛 varies from to ~ 0.005 – 0.001 [10] and bulk diffusion

tends to dominate. However, within the MPL where the pore size ranges from 0.1 – 0.5 μm [16],

24

𝐾𝑛 varies from ~ 0.1 – 0.5, therefore both Knudsen diffusion and bulk diffusion can play a role

[10, 41].

Within the GDL substrates where bulk diffusion dominates, the bulk diffusion coefficient, 𝐷𝑏𝑢𝑙𝑘

(m2/s), is affected by the porosity, 𝜙, and tortuosity, 𝜏, of the GDL structure [42]. Additionally, the

bulk diffusion coefficient is further influenced by the presence of liquid water [42, 43]. To illustrate

the relationship between the three parameters (porosity, tortuosity, and saturation), a common

expression (empirically derived) used to calculate the effective diffusion coefficient, 𝐷𝑒𝑓𝑓 (m2/s),

of the GDL is shown below [44] (other expressions used in the literature also follow a similar form

[42, 43, 45]):

𝐷𝑒𝑓𝑓 = 𝐷𝑏𝑢𝑙𝑘

𝜙

𝜏2(1 − 𝑠)3 (10)

where 𝑠 is liquid water saturation which is defined as:

𝑠 =𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑤𝑎𝑡𝑒𝑟

𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑟𝑒 𝑣𝑜𝑙𝑢𝑚𝑒. (11)

A similar power-law relationship between water saturation, 𝑠, and effective diffusivity has also

been shown for the MPL were both Knudsen and bulk diffusion mechanisms play a role [41]. Since

effective diffusivity of gasses reduces with increasing water saturation within the GDL, the GDL

pore structure must be designed to minimize liquid water accumulation in order to reduce mass

transport losses.

Water transport in the GDL, due to the micron-scale porous structure, is governed by capillary

forces and viscous forces [9]. To minimize water retention within GDLs, hydrophobicity treatments

25

are often employed as described in Section 2.2.1.1. Therefore, water transport within the GDL can

be described as a drainage process whereby the product water is the non-wetting, invading fluid,

and air is the defending, wetting fluid. The dimensional numbers often used to characterize two-

phase water transport within the GDL (and other porous media) are the capillary number, 𝐶𝑎, and

viscosity ratio, 𝑀, which are defined as follows:

𝐶𝑎 =𝑢𝜇𝑛𝑤

𝜎 (12)

𝑀 =𝜇𝑛𝑤

𝜇𝑤 (13)

where 𝑢 (m/s) is the velocity of non-wetting fluid (water), 𝜇𝑛𝑤 (Ns/m2) is the viscosity of the non-

wetting fluid, 𝜎 (N/m) is the interfacial tension, and 𝜇𝑤 (Ns/m2) is the viscosity of the wetting fluid

(air). As the fluid velocities within the micron-scale GDL are expected to be very slow, the

capillary number has been reported to be on the order of 10-8 [9]. Furthermore, the viscosity ratio

between air and water is approximately 23.0 (calculated at 60 ○C). Based on the 𝐶𝑎 and

𝑀 encountered within the GDL and the descriptions of two-phase flow by Lenormand et al., water

transport within the GDL of the fuel cell is typically within the capillary fingering regime [9, 46].

The transport of water via capillary fingering is strongly dependent on the pore structure of the

GDL. Therefore, understanding the water transport regime within the PEM fuel cell enables the

effective design of GDL microstructures to facilitate improved water removal at high current

densities for enhanced fuel cell performance.

2.4.4 Water Balance

In a PEM fuel cell, water plays multiple crucial roles. On one hand, water is required for membrane

hydration and ionic conductivity. However, excessive water accumulation within the GDL leads

26

to lower oxygen diffusion and results in mass transport losses. The operating temperature and

relative humidity (RH) of the reactant gasses also affect the water balance within the fuel cell.

Higher operating temperatures can lead to lower water accumulation due to vapor phase diffusion

of the product water, but as mentioned in Section 2.4.2, higher temperatures lead to the dehydration

of the membrane and lower ionic conductivity. A similar trade-off exists with RH. For instance,

when the inlet gasses are supplied at a lower RH, the hydration of the membrane is reduced leading

to lower ionic conductivity; however, when gasses are supplied at high RH, water is more prone

to condense and accumulate within the GDL leading to mass transport losses. Therefore, an

optimal water balance is required within the fuel cell to facilitate membrane hydration while

minimizing mass transport losses.

2.5 Chapter Summary

This chapter provided an overview of the working principles, key components, and the overall

performance characteristics of a PEM fuel cell. Furthermore, the various transport mechanisms

encountered within the PEM fuel cell and the concept of PEM fuel cell water balance were

described. The information provided in this chapter serves as a foundation for the studies presented

in Chapters 3 and 4.

The introduction sections in Chapter 3 and 4 provide additional information related to the specific

studies presented in each chapter. Specifically, the introduction in Chapter 3 provides a literature

review for the development of novel GDL materials for the improved high current density

performance of PEM fuel cells. The introduction in Chapter 4 focuses on the literature associated

27

with the degradation characteristics of the GDLs and methods employed to study GDL

degradation.

28

CHAPTER 3 – Designing Gas Diffusion Layers with Pore Size Gradients via

Electrospinning for Polymer Electrolyte Membrane Fuel Cells

Abstract

This study presents a novel nano-fibrous electrospun gas diffusion layer (GDL) designed with a

pore size gradient (increasing pore sizes from the catalyst layer interface to the flow field interface)

to enhance the high current density performance of a polymer electrolyte membrane (PEM) fuel

cell. The graded GDL was developed to be highly robust over a wide range of inlet gas relative

humidities (RH). The performance of the graded electrospun GDL was compared to uniform

electrospun GDLs to elucidate the effect of the specific GDL microstructures on the underlying

transport mechanisms encountered during fuel cell operation. At low inlet RH (50% RH), the fuel

cell with the graded GDL exhibited lower ohmic resistance compared to the uniform GDL with

larger pores and fiber diameters. The graded GDL was prone to liquid water retention at the

catalyst layer (CL) interface due to the high capillary pressure inherent in the microstructure

design. The graded GDL also facilitated improved heat dissipation from the CL interface due to

enhanced graphitization and fiber connectivity compared to the GDL with larger fiber diameters.

Both effects resulted in improved membrane hydration and cell performance at high current

densities and 50% RH, where ohmic losses typically dominate. At high inlet RH (100% RH), the

fuel cell with the graded GDL exhibited lower mass transport resistance compared to the GDL

with a uniform distribution of small pores. The pore size gradient promoted effective and directed

removal of liquid water away from the fuel cell, which led to performance improvements at high

current densities and fully humid conditions where mass transport losses typically dominate.

29

3.1 Introduction

The design of the gas diffusion layer (GDL) has a significant influence on polymer electrolyte

membrane (PEM) fuel cell performance. While excessive water accumulation within the GDL is

undesirable, some water is required to hydrate the polymer electrolyte membrane of the cell for

sufficient ionic conductivity. Performance losses associated with liquid water accumulation are

dominant at high relative humidity (RH) operating conditions, whereas losses associated with

membrane dehydration and low ionic conductivity are dominant at low RH [24]. Fuel cell systems

for the transportation sector (e.g. passenger vehicles) need to operate effectively over a wide range

of RH due to varying load cycles and ambient conditions [6]. Consequently, GDLs need to be

designed to maintain an optimal water balance in the fuel cell across a range of humidity conditions

to achieve effective high current density PEM fuel cell performance.

Commercial GDL materials are highly porous and have a dual-component design composed of a

hydrophobic carbon fiber substrate and a thin microporous layer (positioned next to the catalyst

layer) made of dispersed carbon particles within a polymeric binding agent [8, 47]. The microporous

layer (MPL) provides a high capillary pressure barrier to facilitate membrane hydration, while the

hydrophobic carbon fiber substrate provides pathways for excess water egress from the cell. In

efforts to manage the transport of water within the PEM fuel cell, both the design of the fibrous

substrate and the MPL have received significant attention in recent years [27, 48-50]. Pore structure

and wettability of the MPL have both been tailored to improve water management at various

operating conditions [27, 51-53]. For instance, Shrestha et al. [52] applied a custom hydrophilic MPL

coating onto a commercial hydrophobic GDL for fuel cell operation without anode humidification.

The hydrophilic MPL coating was found to be effective at retaining liquid water at the catalyst

30

layer (CL)/MPL interface and led to improved membrane hydration and higher cell voltages. Chun

et al. [53] developed MPL materials with increasing porosity from the CL to the substrate using

thermally expandable graphite to improve water management at fully humid conditions. The use

of an MPL with a gradient in porosity promoted liquid water removal and led to improved cell

performance compared to uniform materials. While there is a significant body of work focused on

MPL design, the addition of new layers and additives increases the complexity of GDL production.

Novel designs that encompass a single component with tunable properties have the potential to

reduce manufacturing complexity while still addressing water management challenges.

The design of the GDL substrate also has a significant impact on water management in the

cell [48, 54-58]. To address the changing water balance needs at various RH, recent experimental

works have suggested designing GDL substrates with a pore size gradient (increasing pore size)

from the CL interface to the flow field interface [57, 58]. Oh et al. [57] manufactured a custom, layered

GDL (single MPL, and two substrates) with a pore size gradient from the MPL to the flow field

(pore diameter ranged from 30 – 90 μm). They reported improved cell performance with the use

of the graded GDL substrate compared to a uniform GDL substrate at both 50% and 100% RH. At

100% RH, they attributed the performance improvement to enhanced water removal enabled by

the increasing pore sizes of the GDL substrates. At 50% RH, they suggested that the relatively

smaller pores of the substrate near the MPL encouraged water retention, which led to enhanced

hydration of the membrane and improved cell performance. Ko et al. [58] also developed custom

GDLs with pore size gradients from the MPL to the flow field. Using in-operando synchrotron X-

ray radiography, they demonstrated that a GDL with a large pore size gradient (average porosity

gradient of -0.656 mm-1) can minimize liquid water accumulation and improve cell performance

31

at 100% RH, while the same GDL gradient can lead to excessive water removal, membrane

dehydration, and reduced cell performance at 25% RH [58]. Furthermore, they demonstrated that a

GDL with an intermediate pore size gradient (average porosity gradient of -0.217 mm-1) could be

used to improve the performance of the cell over a wide range of RH conditions. They attributed

the differences in cell performance to the changing water distribution observed within the cell with

varying GDL pore size gradients.

While it has been established that GDLs with a pore size gradient from the CL interface to the flow

field interface can improve high current density performance at a variety of RH, designing fine-

tuned gradients to maintain an optimal water balance requires a thorough understanding of the

graded material properties (such as pore size variations, and thermal/electrical conductivity

variations) and their effects on cell performance. Although there have been some insightful

numerical work on the topic of GDL optimization [56, 59], there is a scarcity of experimental work

in the literature due to the complexity of manufacturing GDLs with controlled pore size gradients

[48, 57, 59]. Therefore, a simple manufacturing technique for creating GDLs with tailored pore size

gradients is also required.

This study presents electrospinning as a controllable, versatile manufacturing technique to

fabricate GDLs with tunable pore size gradients for PEM fuel cells. Electrospinning generates

controlled fibrous layers and has been previously demonstrated to be a viable technique for

fabricating GDL substrates [20, 28, 60]. The process involves applying a high voltage to a droplet of

viscous polymer solution at the tip of a needle. Electrostatic forces cause the polymer droplet to

extrude and collect onto a grounded surface as an ultra-fine fibrous polymer substrate (fiber

32

diameters can range from tens of nanometers to a few micrometers). The polymer substrate can be

subsequently converted to a carbon fiber substrate via a carbonization process [29]. Many of the

properties of the electrospun substrates, such as fiber diameters, pore sizes, fiber alignment, and

surface properties, can be controlled by altering the process parameters [14, 30, 61, 62].

Considering the range of properties that can be controlled, electrospinning provides a powerful

fabrication platform to systematically alter and tune the GDL microstructure and study the effects

on PEM fuel cell performance. Furthermore, given the ability to produce nano-fibrous structures

with electrospinning, this technique can eliminate the need for a typical MPL coating, thereby

reducing manufacturing complexity. For example, Kaur et al., [63] manufactured electrospun

substrates for filtration applications with mean fiber diameters ranging from 0.067 ± 0.027 μm to

0.573 ± 0.225 μm, resulting in mean pore diameters of 0.54 μm to 7.75 um, respectively. For

comparison, commercial GDL materials have pore sizes that range from 0.1 – 0.5 μm in the MPL

and 10 – 30 μm in the substrate [16]. Finally, as electrospinning involves the deposition of fibrous

layers, the technique can be used to alter the porous structure from the first deposited layer to the

last by controlling the process parameters throughout the spinning process. This makes

electrospinning ideal for fabricating controlled pore size gradients; however, this has not been

previously attempted.

In this study, a novel electrospun GDL was developed with a pore size gradient from the CL

interface to the flow field interface. The electrospun GDL was rendered hydrophobic via a direct

fluorination treatment. The structure of the graded GDL was characterized using scanning electron

microscopy (SEM). The degree of graphitization and electrical conductivity of the eGDL was

33

quantified via Raman spectroscopy and 4 point-probe measurements, respectively. The effects of

the graded GDL on PEM fuel cell performance were systematically evaluated over a wide range

of RH conditions (50% and 100% RH) and compared to uniform GDLs. Finally, the effects of the

graded microstructure on water management was examined via in situ synchrotron X-ray

radiography, and electrochemical impedance spectroscopy (EIS). This study provides new insight

into the design of graded GDLs for effective high current density PEM fuel cell performance over

a range of operating RH. Furthermore, the methods employed present a versatile platform for

manufacturing GDLs with optimized pore size gradients.

3.2 Methodology

In this study, an electrospun GDL (herein referred to as eGDL) that exhibited a pore size gradient

was developed, characterized and tested over a range of inlet relative humidities (RH) to examine

the impact of the graded eGDL microstructures on high current density PEM fuel cell performance.

First, fibrous polymer substrates with uniform and graded pore sizes (increasing from catalyst layer

to flow field) were fabricated via electrospinning (Section 3.2.1.1). The electrospun polymer

substrates were then carbonized via a heat treatment process (Section 3.2.1.2) and rendered

hydrophobic via direct fluorination (Section 3.2.1.3).

Next, the material properties of the eGDLs were characterized (Section 3.2.2). Specifically, the

fiber diameters and pore size distribution of the uniform and graded eGDLs were characterized via

scanning electron microscopy (Section 3.2.2.1). The carbon structure of the eGDL was

characterized via Raman spectroscopy (Section 3.2.2.2), the electrical conductivity of the eGDL

34

was quantified via 4-point probe measurements (Section 3.2.2.3), and the thickness of the eGDL

was measured using a micrometer (Section 3.2.2.4).

Lastly, fuel cell experiments were performed to evaluate the effects of uniform and graded eGDLs

on high current density PEM fuel cell performance and water management (Section 3.2.3). A

custom fuel cell was utilized (Section 3.2.3.1) to perform constant current 𝑖 − 𝑉 curve

measurements (Section 3.2.3.2) and EIS (Section 3.2.3.3) with varying inlet RH. Concurrent

synchrotron X-ray radiography was performed to quantify the spatial distribution of liquid water

within the uniform and graded eGDLs in-operando (Section 3.2.3.4).

3.2.1 eGDL Fabrication

This section presents a detailed outline of the electrospinning, heat treatment, and direct

fluorination procedure used to fabricate tailored uniform and graded eGDLs.

3.2.1.1 Electrospinning

Porous, fibrous polymer substrates were first electrospun from a precursor solution of

polyacrylonitrile (PAN) (Sigma Aldrich, molecular weight: 150,000 g/mol) dissolved in N, N-

dimethylformamide (ACP chemicals) using an in-house electrospinning apparatus (schematic

shown in Figure 4a). The PAN concentration in the solution was switched between 8wt.% and

12wt.% during the electrospinning process. The use of the more viscous 12wt.% PAN solution

was expected to result in the formation of larger fiber diameters, and consequently larger pore sizes

compared to the 8wt.% PAN solution. The prepared solutions were fed through a stainless-steel

35

needle (21 gauge and 16 gauge for 8wt.% and 12wt.% PAN solutions, respectively) at a flow rate

of 1 mL/hour via a syringe pump (AL1000, New Era Pump Systems Aladdin). A high voltage

(25kV and 20kV for the 8wt.% and 12wt.% PAN solutions, respectively) was applied at the needle

tip using a power supply (SL30P10, Spellman), and the resulting polymer fibers were collected

onto a grounded drum wrapped in aluminum foil. The drum was rotated at 3000 RPM in order to

produce aligned fibers.

36

Figure 4. Summary of eGDL manufacturing procedure. a) Schematic of the electrospinning

apparatus. b) Temperature profile used for the stabilization and carbonization of the electrospun

polymer fibers. c) Schematic of the direct fluorination treatment used to render the carbonized

substrates hydrophobic. The carbonized substrates were first air-plasma treated and then exposed

to a fluorinated trichlorosilane (R represents 𝐶 − 𝐹 functional group) under vacuum.

37

In this study, three unique eGDLs were fabricated via electrospinning (summarized in Table 1).

First, to evaluate the influence of varying uniform fiber diameters and pore sizes on PEM fuel cell

performance, two uniform substrates were electrospun from 8wt.% and 12wt.% PAN solutions.

The uniform substrates were referred to as 8wt.% eGDL and 12wt.% eGDL, respectively. To

examine the effect of a pore size gradient, a layered third substrate was electrospun using 8wt.%

PAN solution followed by 12wt.% PAN solution. The resulting graded substrate was referred to

as the Bi-Layer eGDL. The thickness of each polymer substrate was controlled by the

electrospinning time. After electrospinning, the aluminum foil with the deposited fibers was

dipped in deionized water and the substrates were peeled from the foil and laid to dry on polished

stainless-steel plates prior to the heat treatment process. The thickness of each of the dried

substrates was approximately 350 μm (measurement procedure described in Section 3.2.2.4).

3.2.1.2 Heat Treatment

The electrospun polymer substrates were transformed to carbon substrates via a 2-step heat

treatment process: (1) stabilization and (2) carbonization (Figure 4b). The stabilization procedure

was performed at 240○C for 2 hours (with ramp rate of 1○C/min) in air using a box furnace (Blue

M, Lindberg). During the stabilization step, the polymer substrates were compressed between

stainless steel plates with 260 μm shims to maintain a flat surface and consistent thickness. The

final carbonization step was performed at 1400○C for 1 hour (with ramp rate of 5○C/min) under a

95% nitrogen, and 5% hydrogen environment using a high temperature tube furnace (Carbolite).

The substrates were not compressed during the carbonization process.

38

3.2.1.3 Hydrophobic Treatment

The carbonized substrates were rendered hydrophobic without altering the morphology of tailored

structures via a chemical vapor deposition based treatment referred to as direct fluorination (Figure

4c) [20]. First, the carbonized substrates were exposed to air plasma (PDC-001 Plasma Cleaner,

Harrick Plasma) for 2 minutes per side. The plasma was assumed to penetrate the porous substrate

and produce hydroxyl and carboxyl groups on the carbon fiber surfaces. The substrates were then

immediately placed in a vacuum chamber (Labconco) for 8 hours with a vial containing 40 μL of

trichloro(1H,1H,2H,2H-perfluorooctyl)silane (Sigma Aldrich). The fluorinated trichlorosilane

vapor reacts with the OH groups to form hydrophobic carbon fiber surfaces [20, 64].

3.2.2 eGDL Characterization

This section presents the comprehensive suite of characterization techniques used in this study to

quantify the structural and material properties of the eGDLs.

3.2.2.1 eGDL Fiber Diameters and Pore Size Distribution

The average fiber diameter of the eGDLs was quantified via scanning electron microscopy (SEM).

Specifically, a minimum of 20 fibers per eGDL were measured and averaged from surface SEM

(Supra 55VP, Carl Zeiss SMT Ltd.) images using Fiji®.

The pore size distribution of the eGDLs were quantified via cross-sectional SEM image analysis.

The eGDLs were first embedded in resin (EpoThin 2, Buehler) and cut with a diamond saw

39

(perpendicular to the fiber orientation) to expose the eGDL cross-section. The cut surface was

polished and cleaned to optimize image quality, and images were obtained via an SEM (SU5000,

Hitachi). A minimum of 4 cross sectional images were obtained per eGDL to calculate a mean

pore size value.

The cross-sectional images were post-processed and converted to segmented, binarized images of

void space and fibers using MATLAB (MathWorks®). The first step in post-processing consisted

of applying a median filter [65] to reduce “salt and pepper” noise in the SEM images. The

neighborhood size for the median filter was varied from 3x3 to 15x15 pixels depending on the

resolution of the image (82 pixel/μm for the 12wt.% eGDL to 366 pixel/μm for the 8wt.% eGDL).

A Gaussian filter [66] was then applied with a sigma, σ, between 3 and 10 to reduce the variance in

pixel intensity within the fibers and void space to improve the accuracy of the segmentation

process. Both the neighborhood size and σ value for the median and Gaussian filters were selected

by qualitatively assessing the accuracy of the resulting segmented images. The post-processed

images were then segmented into binary images of void space and fibers via an adaptive

thresholding algorithm with a sensitivity of 0.5 [67]. After segmentation, an area opening [68] and

image filling [69] algorithm was applied to the images to remove artefacts that resulted from

physical defects on the cross-section surface (residual polishing powder and imperfect surface

finishes). Finally, the pore size distribution was extracted from the segmented cross-sectional

images using the Sub-Network of an Over-segmented Watershed (SNOW) algorithm in PoreSpy

(open source software package for Python) [70, 71].

40

3.2.2.2 Graphitization of the Carbon Fibers

The graphitic structure of the eGDL carbon fibers were quantified by analyzing the Raman spectra

(inVia confocal Raman microscope, Renishaw) of the 8wt.% and 12wt.% eGDL surfaces. The

spectra were obtained via a diode laser (532 nm, 500 mV) from 800 cm-1 to 1900 cm-1 to capture

the characteristic carbon spectral peaks [72]. Particularly, the peaks of interest were the G-band

(~1580 cm-1), which corresponds to an ideal graphitic carbon structure, and the D-band

(~1340 cm-1), which corresponds to a disordered carbon structure [73, 74]. In this work, the ratio

between the D-band intensity and the G-band intensity was defined as 𝐼𝐷/𝐼𝐺, where 𝐼𝐷 and 𝐼𝐺

represent the peak values of the D-band and G-band intensities, respectively. A lower 𝐼𝐷/𝐼𝐺

indicates a more graphitic carbon structure [75]. Carbon fibers with lower 𝐼𝐷/𝐼𝐺 have been shown

to have improved electrical and thermal conductivity [76, 77], both of which can be beneficial for

fuel cell performance. A minimum of six spot measurements per sample were taken across the

eGDL surfaces to determine the mean 𝐼𝐷/𝐼𝐺 for the 8wt.% and 12wt.% eGDL carbon fibers.

3.2.2.3 Electrical Conductivity

The bulk in-plane electrical conductivity of the eGDLs was measured using a 4-point probe (Model

101C, Four Dimensions). A minimum of five samples per type of eGDL were measured and a

minimum of 4 measurements per sample were obtained. The measurements were conducted with

varying probe orientations with respect to the fiber alignment in order to calculate a bulk

conductivity value.

41

3.2.2.4 eGDL Thickness

The thickness of each eGDL was quantified by placing the samples between glass slides and

measuring the thickness of the assembly using a micrometer (Mitutoyo). The glass slides were

used to evenly distribute the pressure applied via the micrometer to minimize the effects of uneven

compression during measurements. A minimum of eight samples were measured per eGDL to

obtain a mean thickness value.

3.2.3 Fuel Cell Assembly and Testing

This section describes the fuel cell assembly used and in situ experiments performed to examine

the effects of tailored eGDLs on high current density PEM fuel cell performance and water

management.

3.2.3.1 Fuel Cell Hardware and Control

A custom fuel cell with an active area of 0.68 cm2 was employed for the in situ experiments and

synchrotron X-ray imaging [78, 79]. The fuel cell was composed of matching eGDLs and bi-polar

plates at both the anode and cathode. The flow fields were integrated within the bi-polar plates and

consisted of eight parallel channels (0.5 mm wide × 0.5 mm deep). The eGDLs were all

compressed to a thickness of 113 μm using polyethylene naphthalate (PEN) gaskets. Custom

catalyst coated membranes (CCMs) with a platinum loading of 0.3 mg/cm2 at the anode and

cathode (Nafion HP, Ion Power Inc.) were used.

42

An 850e Fuel Cell Test System (Scribner Associates Inc.) was used to operate and control the fuel

cell. Hydrogen and air were supplied at 1 L/min to the anode and cathode, respectively. The cell

back pressure at both outlets were maintained at 200 kPa (absolute), and the cell temperature was

maintained at 60 ○C via a circulating water bath (Isotemp™ 4100R20, Fisher Scientific Co.). The

anode and cathode gas inlet RH were controlled via gas humidifiers within the test station.

3.2.3.2 𝒊 − 𝑽 curves

Constant current density – voltage curves, i.e., 𝑖 − 𝑉 curves, were obtained to examine the effects

of the tailored eGDLs on high current density PEM fuel cell performance. Each eGDL was tested

at two inlet RH set points, 50% and 100%. The inlet RH at both anode and cathode were set to the

same value. At each RH, the fuel cell was tested from 0.0 A/cm2 to limiting current in increments

of 0.5 A/cm2 with a 15-minute hold at each current density. The constant current density hold was

performed to obtain a stable voltage response and water distribution [10]. In addition to the voltage

measurement, the high frequency resistance, HFR (Ω∙cm2), of the fuel cell was measured at 1 kHz

at each current density to quantify the ohmic resistance of the cell. The voltage and HFR

measurements were averaged over the last 60 seconds of each current density step to obtain a

single value during stable operation.

To ensure the repeatability of the results, each eGDL material (8wt.%, 12wt.%, and Bi-Layer

eGDL) was tested three times. Fuel cell builds for the first two tests used eGDLs from the first

fabrication batch. The cell builds for the third test used eGDLs from a second fabrication batch.

Each cell build was composed of a pristine CCM. Each measured value (voltage, HFR) and

calculated value (𝑅𝑀𝑇, described in Section 3.2.3.3) is presented as an average of the three tests

43

conducted (Note: the average cell voltage at 1.5 A/cm2 and 100% RH for the 8wt.% eGDL and the

HFR and 𝑅𝑀𝑇 values for the 8wt.% eGDL were calculated from two cell builds). The error bars of

the measured and calculated values represent ± 1 standard deviation.

3.2.3.3 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) measurements were performed immediately

following each current density step to calculate the mass transport resistance of the fuel cell. The

mass transport resistance was calculated to correlate the water management characteristics of the

tailored eGDLs to fuel cell performance. The alternating current (AC) frequency for the EIS

measurements ranged from 10 kHz – 0.1 Hz, and 10 data points were obtained per decade. The

amplitude of the AC frequency was set to be 10% of the direct current input.

A Randle-based equivalent circuit model (Figure 5) containing a Warburg element and a constant

phase element (CPE) was fitted to the obtained EIS spectra to calculate the mass transport

resistance.

44

Figure 5. Modified Randle’s equivalent circuit used to quantify the mass transport resistance of

the fuel cell from the impedance spectra.

45

The total impedance of the equivalent circuit, 𝑍𝑇𝑂𝑇𝐴𝐿 (Ω∙cm2), was expressed as

𝑍𝑇𝑂𝑇𝐴𝐿 = 𝑅𝛺 + (1

𝑍𝐶𝑃𝐸+

1

𝑅𝐶𝑇 + 𝑍𝑊)

−1

(14)

where 𝑅𝛺 (Ω∙cm2) represents the ohmic resistance of the cell, 𝑍𝐶𝑃𝐸 (Ω∙cm2) represents the

impedance of the electric double-layer at the porous cathode reaction interface, 𝑅𝐶𝑇 (Ω∙cm2)

represents the charge transfer resistance associated with the activation overpotential, and

𝑍𝑊 (Ω∙cm2) represents the Warburg impedance, which encompasses the mass transport resistance,

𝑅𝑀𝑇 (Ω∙cm2), due to the diffusion of oxygen through the cathode components.

To obtain 𝑅𝑀𝑇, the Warburg impedance, 𝑍𝑊, was fitted to

𝑍𝑊 = 𝑅𝑀𝑇

tanh(√𝑗𝜔𝜏)

√𝑗𝜔𝜏 (15)

where 𝑗 denotes the imaginary number √−1, 𝜔 (rad/s) denotes the frequency of the AC input

signal, and 𝜏 (s) denotes the time constant for the diffusion process [80].

To obtain the impedance of the CPE, 𝑍𝐶𝑃𝐸 was fitted to

𝑍𝐶𝑃𝐸 =1

𝐶𝑑𝑙(𝑗𝜔)𝛼 (16)

where 𝐶𝑑𝑙 (F/cm2) denotes the cathode double-layer capacitance, and 𝛼 (rad) denotes the phase

angle of the CPE which accounts for the heterogeneous porous electrode/electrolyte interface [80].

Lastly, in this study, 𝑅𝛺 and 𝑅𝐶𝑇 were experimentally measured and input as fixed values to reduce

the uncertainty in the fitted parameters that could arise during the fitting process [81, 82]. 𝑅𝛺 was

set to be the real component of the total impedance (𝑍𝑇𝑂𝑇𝐴𝐿) measured at 5 kHz, and 𝑅𝐶𝑇 was

46

calculated as a function of current density based on Tafel slope measurements [10, 81]. The

expression for 𝑅𝐶𝑇 was defined as

𝑅𝐶𝑇 =𝜕𝜂𝑎𝑐𝑡

𝜕𝑖=

𝑏

2.303𝑖 (17)

where 𝜂𝑎𝑐𝑡 (V) denotes the activation overpotential, 𝑖 (A/cm2) denotes the operating current

density, and 𝑏 (V/decade) denotes the Tafel slope. The Tafel slope was obtained as described in [10].

The calculation procedure along with the experimental data for the Tafel slope, 𝑏, is presented in

Appendix A – Tafel Slope Measurement.

The equivalent circuit model defined above was fitted to the obtained EIS spectra using the ZView

3.5e software (Scribner Associates Inc.). The linear response of the fuel cell system for each

current density presented in this study was verified by performing the Kramers–Kronig transforms

on the measured spectra at each respective current density [83, 84].

3.2.3.4 Synchrotron X-ray Radiography

In-operando synchrotron X-ray radiography was performed at the Biomedical Imaging Therapy

Bending Magnet (BMIT-BM) beamline at the Canadian Light Source (Saskatoon, Canada) [85] to

gain insight into the effect of uniform and graded eGDLs on liquid water distribution within the

fuel cell. The incident X-ray beam energy was maintained at 24 keV and the attenuated beam was

absorbed by an AA40 scintillator (Hamamatsu Photonics KK) coupled to a C11440-22CU CMOS

detector (Hamamatsu Photonics KK) to capture the raw radiographs. The pixel resolution of the

imaging set up was 6.5 μm/pixel, and the frame rate was 0.33 frames/second. The obtained

radiographs were post-processed using an in-house MATLAB algorithm to correct for background

47

noise and the intensity decay of the incident X-ray beam as described in [86]. Finally, the water

content (cumulative thickness), 𝑡𝑊 (cm), within each pixel of the radiographs was quantified as a

function of the x-and y-directions and time based on the Beer-Lambert law [87]:

𝑡𝑊(𝑥, 𝑦, 𝑡) =1

𝜇𝑊ln (

𝐼𝑂𝐶𝑉(𝑥, 𝑦)

𝐼𝑤𝑒𝑡(𝑥, 𝑦, 𝑡)) (18)

where 𝜇𝑊 (cm-1) represents the attenuation coefficient of liquid water measured via a calibration

procedure [88], 𝐼𝑂𝐶𝑉 represents the pixel intensity of the reference radiograph obtained at open

circuit voltage (OCV) when there is an absence of liquid water in the cell, and 𝐼𝑤𝑒𝑡 represents the

pixel intensity of the radiographs obtained during fuel cell operation (𝑖 > 0 A/cm2).

The water distribution within the cathode GDL (highlighted in Figure 6) was of particular interest

in this study. Specifically, the water content adjacent to the CCM has been demonstrated to have

a significant influence on the hydration of the membrane and consequently fuel cell

performance [52]. In this work, the 13 μm thick region adjacent to the CL interface, referred to as

the region of interest (ROI), was analyzed to elucidate the effect of the tailored eGDLs on the

hydration state of the membrane. Similar ROIs have been defined in the literature with synchrotron

X-ray radiography [22, 52]. The total water content in the ROI, 𝑉𝑊,𝑅𝑂𝐼 (cm3), was calculated as

𝑉𝑊,𝑅𝑂𝐼 =𝐴𝑅𝑂𝐼

𝑁𝑡 𝑁𝑥 𝑁𝑦∑ ∑ ∑ 𝑡𝑊(𝑥𝑖 , 𝑦𝑗 , 𝑡𝑘),

𝑁𝑦

𝑗=1

𝑁𝑥

𝑖=1

𝑁𝑡

𝑘=1

(19)

where 𝐴𝑅𝑂𝐼 (cm2) represents the area of the ROI in the x-y plane. Specifically, the width of 𝐴𝑅𝑂𝐼

consists of 8 channels and 7 lands, and the height is 13 μm. Thus, 𝐴𝑅𝑂𝐼 is equal to 0.75 cm × 13

μm (9.75 × 10-4 cm2) in Equation 19. 𝑁𝑡 is the number of radiographs averaged over time, and 𝑁𝑥

and 𝑁𝑦 represent the total number of pixels averaged in the x- and y-directions, respectively. The

48

last 20 radiographs (collected over 60 seconds with a frame rate of 3 seconds per frame) acquired

at the end of each constant current step were averaged; therefore, 𝑁𝑡 = 20. The error of 𝑉𝑊,𝑅𝑂𝐼

was calculated based on the instrumentation error and the spatial variation in the water thickness,

𝑡𝑊, as described in [89].

49

Figure 6. Sample images obtained via synchrotron X-ray radiography. a) Sample of raw X-ray

radiograph obtained in greyscale. b) Sample processed image of the cathode GDL and CCM

region. Color bar represents the water content, 𝑡𝑤 (in cm), of each pixel in the processed image.

50

3.3 Results

The development and implementation of the tailored eGDLs are presented in two parts. First, the

structure and material properties of the eGDLs are presented to obtain a thorough understanding

of the eGDL properties from the CL to the flow field.

Next, the impact of the uniform and graded eGDLs on high current density PEM fuel cell

performance and water management are discussed. Specifically, the 𝑖 − 𝑉 performance curves of

the fuel cell with the eGDLs are compared at 50% and 100% RH conditions. The EIS

measurements and liquid water distribution within cathode GDL are also presented to explain the

underlying physical mechanisms that caused the observed changes in fuel cell performance.

Finally, the peak power density attained with the Bi-Layer eGDL and commercial GDLs are

compared in order to benchmark the performance of the graded eGDL.

3.3.1 Structure and Material Properties of the eGDLs

3.3.1.1 Fiber Diameter and Pore Size Distribution

Higher PAN concentrations led to larger eGDL carbon fiber diameters and pore sizes (Figure 7

and Table 1). Specifically, the 8wt.% eGDL (from 8wt.% PAN solution) had a mean fiber diameter

and mean pore size of 174 ± 39 nm and 200 ± 30 nm, respectively. Whereas, the 12wt.% eGDL

(from 12wt.% PAN solution) had a mean fiber diameter and mean pore size of 687 ± 47 nm and

690 ± 40 nm, respectively. Finally, as prescribed, the Bi-Layer eGDL exhibited a graded structure

with increasing fiber diameters and pore sizes from the 8wt.% layer to the 12wt.% layer (each

layer was approximately 75 μm in thickness). Specifically, the fiber diameters increased from

51

168 ± 57 nm to 359 ± 68 nm, and the pore diameter increased from 160 ± 10 nm to 330 ± 10 nm.

In the fuel cell with the Bi-Layer eGDL, the 8wt.% fiber layer was adjacent to the CL, and the

12wt.% fiber layer was adjacent to the flow field, thereby forming a pore size gradient from the

CL interface to the flow field interface (Figure 7c). The controlled, distinct, and layered structure

of the Bi-Layer eGDL showcases the effectiveness of using electrospinning as a platform to

manufacture tailored GDLs.

52

Figure 7. Fiber diameter and pore size distribution of the tailored eGDLs. a) Surface SEM image

of the 12wt.% eGDL. b) Surface SEM image of the 8wt.% eGDL. c) SEM cross-section image of

the Bi-Layer eGDL. The bottom layer of the Bi-Layer eGDL was electrospun using 8wt.% PAN

solution. The top layer of the Bi-Layer eGDL was electrospun using 12wt.% PAN solution. d)

Pore size distribution from SEM cross-section images. Higher PAN solution concentrations led to

the formation of larger fiber diameters and pore sizes.

53

Table 1: Summary of the material properties of the tailored eGDLs.

8wt.% eGDL 12wt.% eGDL Bi-Layer eGDL

Fiber diameter near catalyst layer (nm) 174 ± 39 687 ± 47 168 ± 57

Fiber diameter near flow fields (nm) 174 ± 39 687 ± 47 359 ± 68

Pore diameter near catalyst layer (nm) 190 ± 30 690 ± 40 160 ± 10

Pore diameter near flow field (nm) 190 ± 30 690 ± 40 330 ± 10

𝐼𝐷/𝐼𝐺 (8wt.% and 12wt.% fibers) 0.71 ± 0.25 1.11 ± 0.02 -

In-plane electrical conductivity (S/cm) 23.3 ± 2.7 19.4 ± 3.8 21.5 ± 6.9

GDL uncompressed thickness (μm) 162 ± 21 130 ± 14 153 ± 17

GDL compressed thickness (μm) 113 113 113

Note: All error values represent ± 1 standard deviation.

3.3.1.2 Effect of Fiber Diameter on Graphitization

The Raman spectra of the 8wt.% fibers exhibited a significantly higher peak intensity at the G-band

(~1580 cm-1) compared to the 12wt.% fibers indicating a higher degree of graphitization (Figure

8a). Specifically, the mean 𝐼𝐷/𝐼𝐺 values were 0.71 ± 0.25 for the 8wt.% fibers and 1.11 ± 0.02 for

the 12wt.% fibers. The higher graphitization of the 8wt.% fibers was attributed to the smaller fiber

diameters. During heat treatment, the graphitization of carbon fiber occurs initially on the surface

and proceeds inward towards the bulk of the material [77, 90]. Therefore, since the smaller 8wt.%

fibers had a higher surface area to volume ratio compared to the 12wt.% fibers, a higher degree of

graphitization was expected in the bulk of the 8wt.% fibers under the same heat treatment

conditions. A similar trend has been observed in the literature with hollow carbon fibers [77].

54

Figure 8. Raman spectra and electrical conductivity of the tailored eGDLs. a) Representative

Raman spectra obtained for 8wt.% and 12 wt.% electrospun carbon fibers. The 8wt.% carbon

fibers exhibited a higher degree of graphitization compared to the 12wt.% carbon fibers. b) Bulk

in-plane electrical conductivity of the eGDLs. On average, the 8wt.% eGDL had a higher electrical

conductivity than the 12wt.% eGDL (p-value = 0.10 from T-test comparison). Error bars represent

± 1 standard deviation.

55

3.3.1.3 Effect of Fiber Diameter and Fiber Connectivity on Bulk In-Plane Electrical

Conductivity

The 8wt.% eGDL exhibited a higher average electrical conductivity compared to the

12wt.% eGDL (23.3 ± 2.7 S/cm compared to 19.4 ± 3.8 S/cm for the 8wt.% and 12wt.% eGDLs,

respectively) (Figure 8b). The p-value, calculated to be 0.10 between the 8wt.% and 12wt.% eGDL

conductivities, provides some statistical justification for this conclusion (i.e. ≤ 10% chance that

the calculated means between 8wt.% and 12wt.% eGDLs are the same). Furthermore, measured

attributes such as the higher degree of graphitization of the 8wt.% fibers (Section 3.3.1.2) and

higher fiber connectivity of the 8wt.% eGDL (observed via surface SEM images, Figure 7) provide

physical justifications for the difference in electrical conductivity. Carbon fibers with higher

graphitization have been extensively reported to have improved electrical conductivity [77, 91];

therefore, the more graphitic 8wt.% eGDL was expected to have a higher electrical conductivity

compared to the 12wt.% eGDL. Furthermore, the smaller fiber diameters and pore size distribution

of the 8wt.% eGDL resulted in reduced inter fiber distances compared to the 12wt.% eGDL. The

smaller inter fiber distances led to a higher number of contact points and connectivity which can

lead to a higher electrical conductivity due to more available pathways for electron transport. A

similar trend between fiber diameter, fiber connectivity and electrical conductivity with

electrospun GDLs was previously observed by Chevalier et al. [20].

The Bi-Layer eGDL exhibited a similar electrical conductivity to the uniform materials

(21.5 ± 6.9 S/cm). The similarity in conductivity was expected since the in-plane conductivity

values of the 8wt.% and 12wt.% layers within the Bi-Layer eGDL were expected to be comparable

to those of the uniform 8wt.% and 12wt.% eGDLs, respectively. The larger variance in the

56

measured data with the Bi-Layer eGDL compared to the uniform materials was attributed to the

inconsistent penetration depth of the 4-point probe tips into the Bi-Layer eGDL.

3.3.2 High Current Density Fuel Cell Performance

This section discusses the effects of the uniform and graded eGDLs on high current density fuel

cell performance and water management. The 𝑖 − 𝑉 curve measurements were conducted at 50%

and 100% inlet RH. The effects of the eGDLs on fuel cell performance and water management

were examined via synchrotron X-ray radiography and EIS.

3.3.2.1 Improved Ohmic Performance with Smaller Pore Sizes and Fiber Diameters

At 50% inlet RH, both the Bi-Layer and 8wt.% eGDL led to a 21% higher average cell voltage at

1.5 A/cm2 (high current density) compared to the 12wt.% eGDL (0.47 V with the Bi-layer and

8wt.% eGDL compared to 0.39 V with the 12wt.% eGDL; p-value of 0.11 between Bi-Layer and

12wt.% eGDL and p-value of 0.09 between 8wt.% and 12wt.% eGDL) (Figure 9a). As ohmic

losses associated with membrane dehydration were expected to dominate at 50% RH [24, 57], the

HFR (measure of ohmic resistance) of the fuel cell between the three eGDLs were compared

(Figure 10a). The Bi-layer and 8wt.% eGDLs led to more than a 33% reduction in HFR relative to

the 12wt.% eGDL at 1.5 A/cm2 (105.4, 95.6, and 140.0 mΩ∙cm2 with the Bi-Layer, 8wt.%, and

12wt.% eGDLs, respectively). The lower HFR with the Bi-Layer and 8wt.% eGDLs indicated

improved membrane hydration and ionic conductivity.

57

Figure 9. PEM fuel cell performance with the tailored eGDLs. a) 𝑖 − 𝑉 curves obtained at 50%

RH. The use of the Bi-Layer eGDL led to higher cell voltages at high current density (1.5 A/cm2)

compared to the 12wt.% eGDL (p-value = 0.11 from T-test comparison). b) 𝑖 − 𝑉 curves obtained

at 100% RH. The use of the Bi-Layer eGDL led to higher cell voltages at high current density

(2.5 A/cm2) compared to the 8wt.% eGDL (p-value = 0.09 from T-test comparison). Error bars

represent ± 1 standard deviation.

58

Figure 10. High frequency resistance (HFR) and water content within the region of interest, 𝑉𝑊,𝑅𝑂𝐼,

at 50% RH. a) Average HFR at 50% RH. Fuel cells with Bi-Layer and 8wt.% eGDLs had lower

HFR compared to fuel cells with the 12wt.% eGDL. Error bars represent ± 1 standard deviation.

b) Total liquid water content, 𝑉𝑊,𝑅𝑂𝐼, in the region of interest at 1.0 A/cm2 and 50% RH. Fuel cells

with Bi-Layer and 8wt.% eGDLs retained more liquid water adjacent to the CL interface compared

to fuel cells with the 12wt.% eGDL.

59

The lower HFR (Figure 10a) with the Bi-Layer and 8wt.% eGDLs (Figure 9a) was attributed to

the 8wt.% carbon fibers present in the region adjacent to the CL in both materials. As the eGDLs

were hydrophobic, the smaller pores of the 8wt.% layers (Figure 7d) presented a higher capillary

pressure barrier for the produced water compared to the 12wt.% eGDL. Specifically, the mean

pore diameter of the 8wt.% layers was approximately 4-times smaller than that of the 12wt.%

eGDL. The effect of the smaller pores and proportionally higher capillary pressure was directly

observed via in-operando synchrotron X-ray radiography (Figure 10b). The Bi-Layer and

8wt.% eGDL both led to higher water retention at the CL interface compared to the 12wt.% eGDL

at 1.0 A/cm2 (> 2 times the water content). The higher water retention at the CL interface was

postulated to increase the diffusion rate of product water from the cathode towards the anode and

thereby improve membrane hydration and ionic conductivity at 50% RH.

Furthermore, as discussed in Section 3.3.1.2, the smaller 8wt.% fibers exhibited a higher degree

of graphitization relative to the 12wt.% fibers (Figure 8a). Higher degrees of graphitization

correspond to higher thermal conductivity [76]. Additionally, increased fiber connectivity of the

8wt.% layers (evidenced by higher average electrical conductivity, Figure 8b) was also assumed

to improve the thermal conductivity by providing more pathways for heat transfer (analogous to

electrical conductivity). Therefore, in addition to the higher water retention observed in Figure

10b, the higher graphitization and higher connectivity of 8wt.% fiber layers was expected to lead

to enhanced heat dissipation from the CL interface compared to the 12wt.% eGDL. Enhanced heat

dissipation resulted in minimal dehydration of the membrane, which led to improved ionic

conductivity (and consequently lower HFR) and high current density cell performance at 50% RH.

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3.3.2.2 Improved Mass Transport Performance with Pore Size Gradient

The Bi-Layer eGDL led to a 120% higher average cell voltage at 2.5 A/cm2 and 100% inlet RH

compared to the 8wt.% eGDL (0.29 V with the Bi-Layer eGDL compared to 0.13 V with the

8wt.% eGDL; p-value of 0.09) (Figure 9b). As mass transport losses associated with excess water

accumulation in the GDL were expected to dominate at 100% RH [24, 57], the mass transport

resistance, RMT, of the fuel cell with the three eGDLs were compared (Figure 11). It was observed

that the Bi-Layer eGDL led to lower average RMT values compared to the 8wt.% eGDL at high

current densities (≥ 1.5 A/cm2) indicating effective water removal and oxygen diffusion.

61

Figure 11. Mass transport resistance, RMT, at 100% RH. a) Representative Nyquist spectra obtained

at 1.5A/cm2 and 100% RH. b) Average RMT values obtained from the measured EIS spectra via

equivalent circuit modelling. Fuel cells with the graded Bi-Layer eGDL were measured to have

lower mass transport resistance at 100% RH and high current densities compared to the fuel cells

with 8wt.% eGDL. Error bars represent ± 1 standard deviation.

62

The lower RMT and corresponding improvement in high current density cell performance with the

Bi-Layer eGDL compared to the 8wt.% eGDL at 100% RH was attributed to the graded pore

structure of the Bi-Layer eGDL. In the GDL of a PEM fuel cell, water transport is dominated by

capillary forces and is expected to follow a capillary fingering regime [9, 92]. In the capillary

fingering regime, water preferentially invades larger pores from smaller pores due to the lower

threshold capillary pressures of the larger pores [46]. Therefore, in the Bi-Layer eGDL, when the

produced water from the CL percolates to the 8wt.%-12wt.% layer interface, the water was

expected to preferentially invade the 12wt.% layer due to the larger pores. Upon breakthrough into

the 12wt.% layer, further water invasion in the 8wt.% layer would have been suppressed as water

continued to grow in the 12wt.% layer [93]. This mechanism is proposed to lead to directed water

transport toward the flow field, and consequently, more effective water removal. However, in the

8wt.% eGDL, due to its relatively uniform structure, liquid water was not directed toward the flow

field. Capillary fingering led to water growth in all directions, including towards the CL, which

led to higher levels of water accumulation before breakthrough. Consequently, higher water

accumulation in the 8wt.% eGDL led to higher RMT values and lower average cell voltage at 100%

RH compared to the Bi-Layer eGDL.

It should be noted that the performance between the 12wt.% and 8wt.% eGDLs at high current

densities and 100% RH was similar (<5% difference in cell voltage at 1.5 A/cm2, Figure 9b), and

this similarity was attributed to the trade-off between mass transport and ohmic properties of the

two eGDLs. Although larger pore sizes have been demonstrated to improve the effective

diffusivity of the GDL compared to smaller pores [94, 95], any potential improvements in mass

transport resistance at 100% RH with the 12wt.% eGDL compared to the 8wt.% eGDL were likely

63

offset by the larger ohmic losses. Specifically, even at the fully humidified inlet conditions, the

HFR of the fuel cell with the 12wt.% eGDL was 51% higher at 2.0 A/cm2 compared to the 8wt.%

eGDL (89.2 and 59.0 mΩ∙cm2 for the 12wt.% and 8wt.% eGDL, respectively).

In summary, the observed differences in average cell voltage at high current densities

(≥ 1.5 A/cm2) between the three eGDL structures (Figure 9) were accompanied by differences in

measured parameters such as HFR and water content at 50% RH (Figure 10) and mass transport

resistance, RMT, at 100% RH (Figure 11). These measured parameters provide physical evidence

of the impact the eGDL structure has on PEM fuel cell performance.

3.3.2.3 Comparison to Commercial GDLs

The peak power density obtained with the graded Bi-Layer eGDL was compared to a commercially

available Sigracet (SGL) 25BC GDL (SGL 25BC included an MPL) and a Toray® TGP-H-060

GDL (Toray GDL did not include an MPL). The commercial materials were tested using the same

custom fuel cell and CCM described in Section 3.2.3.1. The Bi-Layer eGDL led to a similar peak

power output compared to the SGL 25BC GDL (0.86 ± 0.12 W/cm2 with Bi-Layer eGDL

compared to 0.80 W/cm2 with SGL 25BC) and a 131% higher peak power output compared to the

Toray GDL (0.37 W/cm2 with Toray TGP-H-060 GDL). It should be noted that the eGDL

materials were applied and tested in the absence of an additional MPL coating; therefore, a direct

comparison with the Toray TGP-H-060 provides a valuable illustration of the significant

improvement in performance made possible by the Bi-Layer eGDL.

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3.4 Chapter Conclusions

In this study, a novel, tailored GDL with a pore size gradient (increasing from the CL interface to

the flow field interface) was fabricated via electrospinning to enhance the high current density

performance of PEM fuel cells. The performance of the graded eGDL (Bi-Layer eGDL) was

compared to two uniform eGDLs (8wt.% eGDL and 12wt.% eGDL) to elucidate the effects of the

eGDL microstructures on PEM fuel cell transport mechanisms and water management.

At 50% RH, where ohmic losses were expected to dominate cell performance, the Bi-Layer eGDL

led to lower ohmic resistance relative to the 12wt.% eGDL. The lower ohmic resistance was

attributed to the smaller pore sizes and fiber diameters of the 8wt.% PAN carbon fibers adjacent

to the CL interface. The smaller pores presented a higher capillary pressure barrier and led to

higher water retention (observed via synchrotron X-ray radiography). Furthermore, the smaller

fiber diameters was hypothesized to lead to improved heat dissipation due to enhanced

graphitization and fiber connectivity (measured via Raman spectroscopy and 4-point probe

measurements). Both effects led to improved membrane hydration and cell performance at 50%

RH.

At the 100% RH condition, where mass transport losses were expected to dominate cell

performance, the Bi-Layer eGDL led to improved high current density performance compared to

the 8wt.% eGDL. The improvement in performance was attributed to the graded pore structure of

the Bi-Layer eGDL which led to directed and effective water removal from the CL interface to the

flow field leading to lower mass transport resistance.

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This study demonstrates that a graded electrospun GDL can be used to improve high current

density fuel cell performance over a range of RH conditions compared to uniform materials.

Furthermore, by combining thorough material characterization with powerful in situ experimental

techniques such as EIS and in situ synchrotron X-ray radiography, valuable insight was gained

into the underlying transport mechanisms that led to the improved cell performance. The eGDLs

fabricated in this study also demonstrate the versatility of using electrospinning as a single

manufacturing platform to fabricate tailored GDL structures for PEM fuel applications. The

fabrication procedure outlined in this study and the insights gained from the material

characterization and in situ experiments can be used to develop the next generation of GDLs with

optimized pore size gradients from the CL interface to the flow field interface for the improved

high current density performance of PEM fuel cells.

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CHAPTER 4 – Degradation Characteristics of Electrospun Gas Diffusion

Layers for Polymer Electrolyte Membrane Fuel Cells

Abstract

This study reports the degradation characteristics of hydrophobic, electrospun gas diffusion layers

(eGDLs) with a pore size gradient for applications in polymer electrolyte membrane fuel cells. The

eGDLs were subject to ex situ accelerated degradation via submersion in a hydrogen peroxide

solution to mimic long-term fuel cell operation. After the degradation procedure, the surface

contact angle of the degraded eGDL (44○) was observed to be drastically lower than the pristine

eGDL (137○). The loss of hydrophobicity was attributed to the presence of defects within the

hydrophobic monolayer of the eGDLs. These defects allowed for the hydrolysis of the monolayer

from the carbon surface during long term exposure to the H2O2 solution. Fuel cell tests and

concurrent synchrotron X-ray radiography at 100% inlet relative humidity (RH) revealed that the

degraded eGDL was prone to higher liquid water accumulation and mass transport losses

compared to the pristine eGDL. The higher water accumulation was attributed to the loss of surface

hydrophobicity and subsequent transition from water drainage to imbibition within the eGDL. At

50% inlet RH, the degraded eGDL led to higher ohmic losses compared to the pristine eGDL. The

higher ohmic losses were attributed to the degradation of the carbon fibers as evidenced by the

lower electrical conductivity of the degraded eGDLs.

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4.1 Introduction

Electrospinning was utilized in Chapter 3 to develop a tailored gas diffusion layer (GDL) with

increasing pore sizes from the catalyst layer (CL) to the flow field to enhance the high current

density performance of polymer electrolyte membrane (PEM) fuel cells compared to uniform

materials. The tailored eGDL, referred to as the Bi-Layer eGDL, had unique properties compared

to typical commercial materials. First, the eGDL was a completely fibrous structure with an

average fiber diameter of ~ 250 nm. In contrast, commercial GDLs consist of two drastically

differing layers known as the macro-porous substrate and the microporous layer (MPL). The

macro-porous substrate (adjacent to the flow field) is made of carbon fiber (fiber diameters range

between 7-10 μm [15]), whereas the MPL (adjacent to the CL interface) is comprised of carbon

black particles (particle diameter is ~50 nm) bound together in a polytetrafluoroethylene (PTFE)

matrix [8]. Secondly, the Bi-Layer eGDL was rendered hydrophobic for effective water removal

via a chemical vapor deposition technique referred to as direct fluorination. The fluorination

procedure imparted a uniform hydrophobic monolayer onto the electrospun carbon fiber surfaces

without altering the tailored pore structure [20]. The fluorination procedure was starkly different

from typical PTFE coating procedures used to render commercial GDL materials hydrophobic.

Coating procedures lead to an uncontrolled and non-homogenous dispersion of PTFE through the

GDL thereby altering the pore structure of the carbon substrate [20, 26, 96, 97]. The uncontrolled

alteration of the pore structure is undesirable for eGDLs as the tailored structures could be

unintentionally modified to hinder water and oxygen transport. Consequently, hydrophobicity

treatments that induce minimal morphological alterations, such as the direct fluorination treatment,

are necessary and vital for the development of tailored GDLs.

68

Although the feasibility of using electrospinning and the direct fluorination treatment to develop

tailored eGDLs has been demonstrated (Chapter 3), the durability of new eGDL materials with

long-term fuel cell operation has not yet been examined. Understanding the degradation

characteristics of eGDLs and the effects of the degraded eGDL on fuel cell performance is a critical

step in the successful implementation of robust tailored GDLs with optimized pore structures for

next generation PEM fuel cells.

The degradation characteristics of the PEM fuel cell and its components have been extensively

studied in the literature, and fuel cell degradation continues to be a topic of ongoing research [37,

98]. There are a variety of degradation mechanisms encountered within the PEM fuel cell such as

the chemical degradation of the polymer membrane [37, 99], platinum dissolution within the CL [100,

101], corrosion of the carbon particles in the cathode CL and MPL due to the high, transient

potentials encountered during startup and shutdown [37, 101-103], and the oxidation of GDLs due to

the presence of accumulated liquid water in the fuel cell [104-106]. In the context of eGDL

development, the degradation of the GDL via oxidation and corrosion are of particular interest, as

these mechanisms affect the transport of liquid water, oxygen, and electrons through the GDL [37,

98].

To study the degradation of GDLs over long-term fuel cell operation, researchers have employed

drive-cycle based degradation tests (i.e. long-term testing protocols that mimic automotive

application) [37, 98, 104]. For instance, Hiramitsu et al. conducted a long-term fuel cell test

(6000 hours of fuel cell operation) to study the impact of the fuel cell operating environment on

the degradation of a typical GDL with a PTFE coating [104]. Throughout the 6000 hours of

69

operation, they observed increases in mass transport losses and attributed the losses to the

oxidation of the GDL fibers (observed post-mortem) associated with liquid water accumulation.

The oxidation of the GDL fibers led to a loss of hydrophobicity, ineffective liquid water removal,

and consequently reduced fuel cell performance [104]. Although in situ drive-cycle tests are

extremely insightful, the tests take a considerable amount of time making them impractical for the

rapid development of new materials such as the eGDLs [98, 105].

To minimize the testing times required for degradation studies, researchers have employed in situ

accelerated degradation testing protocols (i.e. test conditions that enhance/accelerate the

degradation of fuel cell components) [37, 98, 101, 107]. For instance, Fairweather et al. [101] examined

the effect of carbon corrosion on liquid water transport within the PEM fuel cell by imposing a

1.3V hold at the cathode (vs. H2 anode) for a total of 320 minutes to induce accelerated corrosion

of the cathode CL and GDL (H2 supplied at anode, N2 supplied at cathode). They demonstrated

that GDLs with an MPL led to increased mass transport losses after being subject to the

degradation protocol compared to a GDL without an MPL. They postulated that the corrosion of

the MPL along with the cathode CL led to the collapse of pore spaces within the CL and MPL,

which led to increased mass transport losses over time. However, with the use of the in situ

accelerated degradation protocols, it was difficult to deconvolute and isolate the contributions of

the MPL and CL degradation on fuel cell performance [101].

To overcome the challenges of in situ testing, ex situ accelerated degradation protocols have been

developed for the GDL [98, 105, 106, 108-110]. It has been demonstrated that the oxidation of the GDL

during long-term fuel cell operation occurs due to the accumulation of liquid water [104, 106].

70

Furthermore, it has been suggested that the oxidation of the GDL is enhanced by the presence of

hydrogen peroxide (H2O2) within the PEM fuel cell [99, 105, 111]. H2O2 formation within the fuel cell

occurs when the oxygen reduction reaction follows an alternative pathway as described by Liu et

al. [99]. Based on these oxidation mechanisms, ex situ accelerated degradation protocols for the

GDL often involve submerging GDL samples in a solution of H2O2 at elevated temperatures for

extended periods of time to mimic the oxidation/corrosion of the GDL during long-term fuel cell

operation [105, 106, 108, 109, 111].

For instance, Frisk et al. [108] degraded GDLs by immersing GDL samples within a solution of

15wt.% hydrogen peroxide (H2O2) at 180 ○F for extended periods of time (≥ 72 hours). The fuel

cell performance of the H2O2 degraded GDLs was then shown to be similar to the performance of

fuel cells that had undergone long term operation [108]. Chlistunoff et al. [109] also studied the

degradation of GDLs via immersion in a solution of H2O2 (30wt.% solution at 90oC for up to

15 hours). They demonstrated that the oxidation of the GDL via the ex situ degradation procedure

resulted in similar levels of acidic groups on the GDL fiber surfaces compared to GDL materials

that had undergone long term fuel cell operation (~1000 hours). The similarity in the surface

composition of the ex situ degraded GDLs and the in situ degraded GDLs (via long-term fuel cell

operation) implied the suitability of using H2O2 solutions to accelerate the degradation of GDL

components. Accelerated degradation via H2O2 is particularly advantageous for novel materials,

such as the eGDL, as their degradation characteristics can be evaluated in an isolated and timely

manner which allows for rapid material development while still simulating the effects of long-term

fuel cell operation.

71

In this study, the degradation characteristics of electrospun GDLs was examined for the first time

via an ex situ accelerated degradation protocol using H2O2 to mimic the effects of long-term fuel

cell operation. Following the degradation procedure, surface contact angle measurements, fuel cell

performance tests, in situ synchrotron X-ray radiography, and electrical conductivity

measurements were utilized to examine the hydrophobicity, water transport characteristics, and

ohmic properties of the degraded eGDLs. Based on the insights gained from the study, the

development of defect-free fluorination treatments was identified as a necessary step for the

successful implementation of robust GDLs with optimized, tailored pore structures for PEM fuel

cell applications.

4.2 Methodology

In this study, the degradation characteristics of hydrophobic eGDLs exhibiting a pore size gradient

(increasing pore size) from the CL interface to the flow field interface was examined. The eGDLs

were fabricated using an in-house electrospinning apparatus (Section 4.2.1.1) and the substrates

were rendered hydrophobic via a direct fluorination treatment (Section 4.2.1.2). The eGDLs were

then subjected to an accelerated degradation procedure via immersion in a solution of hydrogen

peroxide (H2O2) (Section 4.2.2).

The effects of the degradation procedure on the functionalization treatment and carbon structure

were evaluated through surface contact angle measurements (Section 4.2.3.1), electrical

conductivity measurements (Section 4.2.3.2), in situ fuel cell performance testing (Section

4.2.3.4), and synchrotron X-ray radiography (Section 4.2.3.5). The material properties and fuel

72

cell performance of the degraded eGDLs were compared to pristine materials to elucidate the

degradation characteristics of electrospun GDLs.

4.2.1 Fabrication of Graded Hydrophobic eGDLs

The procedure employed to fabricate hydrophobic eGDLs with a pore size gradient is summarized

in this section. The reader is directed to the previous study (Chapter 3) for the detailed fabrication

procedure.

4.2.1.1 Electrospinning and Heat treatment

The eGDLs were electrospun from a precursor solution of polyacrylonitrile (PAN) and N,N-

dimethylformamide. The precursor solution was fed through a stainless-steel needle via a syringe

pump at a flow rate of 1 mL/hr. A high voltage (20 – 25 kV) was applied at the needle tip via a

power source (SL30P10, Spellman). The electrostatic forces imposed by the high voltage caused

the polymer droplet to extrude and collect as ultra-fine polymer fibers onto an electrically grounded

collector drum. The drum was rotated at 3000 RPM to create aligned fibers.

A pore size gradient was achieved by controlling the PAN solution concentration during the

electrospinning process. Higher PAN concentrations were expected to lead to the formation of

larger fiber diameters and pore sizes [29, 63]. Thus, the graded eGDL, referred to as the

Bi-Layer eGDL, consisted of two electrospun layers (of approximately equal thickness) where the

first layer was electrospun from 8wt.% PAN solution, and the second layer was electrospun from

12wt.% PAN solution. The fibrous polymer substrates were converted to graphitic carbon fiber

73

substrates via a two-step heat treatment procedure. First, the polymer substrates were thermoset at

240 ○C in air for 2 hours (at a ramp rate of 1 ○C/min). The substrates were then carbonized at 1400

○C for 1 hour (at a ramp rate of 5 ○C/min) under a 95% nitrogen and 5% hydrogen environment.

Post carbonization, the Bi-Layer eGDL exhibited increasing fibers diameters and pore sizes from

the 8wt.% fiber layer to the 12wt.% fiber layer. Specifically, the mean fiber diameter increased

from 168 ± 57 nm to 359 ± 68 nm, and the mean pore diameter increased from 160 ± 10 nm to

330 ± 10 nm.

4.2.1.2 Direct Fluorination Procedure

The tailored eGDLs were rendered hydrophobic via a direct fluorination procedure as outlined in

Figure 12a. The fluorination treatment induced minimal morphological alterations; therefore, the

treatment was particularly advantageous for eGDLs as the tailored structure of the electrospun

substrate was preserved. The eGDLs were first exposed to air plasma (PDC-001 Plasma Cleaner,

Harrick Plasma) for 2.0 minutes per side to produce hydroxyl and carboxyl groups on the surfaces

of the carbon fibers. During the plasma treatment, it was assumed that the air plasma thoroughly

penetrated the porous substrate to uniformly oxidize the carbon surfaces. The plasma treated

eGDLs were then immediately placed in a vacuum chamber (Labconco) containing 40 μL of

trichloro(1H,1H,2H,2H-perfluorooctyl)silane (Sigma Aldrich) for 8 hours. Under vacuum, the

fluorinated trichlorosilane vapor was expected to covalently bond to the OH groups on the surfaces

of the carbon fibers to form a self-assembled hydrophobic monolayer as depicted in Figure

12a [64, 112]. A vapor phase deposition technique was chosen to form a hydrophobic layer with the

eGDLs due to the ability of vapor to evenly penetrate the small porous structures (~100 – 600 nm

74

pores) of the eGDL material and form a uniform hydrophobic surface. Liquid phase deposition

techniques are not as ideal for the eGDL as the solutions may not uniformly penetrate the material

due to presence of microscopic air bubbles [113]. The deposition of the trichlorosilane vapors on the

surface of the eGDLs was verified via surface contact angle measurements (described in

Section 4.2.3.1). In the future, the uniformity of the treatment procedure through the thickness of

the eGDL substrate could be verified via cross-sectional X-ray photoelectron spectroscopy (XPS)

analysis (not completed in this study). Specifically, the elemental composition of fluorine (present

in the functional group of trichlorosilane used) could be quantified across the thickness of the

eGDL cross-section to confirm the uniformity of the fluorination procedure.

75

Figure 12. Summary of experimental procedures. a) Direct fluorination treatment used to

functionalize the eGDLs. The substrate was first exposed to oxygen plasma for 2.0 minutes per

side to create surface hydroxyl and carboxyl groups. After the plasma treatment, the substrates

were exposed to fluorinated trichlorosilane vapors in vacuum for 8 hours (R denotes functional

group). b) Apparatus used for accelerated degradation. The eGDL samples were immersed in the

H2O2 solution which was maintained at a temperature of 90 ○C. The condenser was used to

maintain a constant solution concentration throughout the procedure.

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4.2.2 Accelerated Degradation Procedure

This section presents the ex situ procedure used to degrade the eGDLs. A schematic of the

degradation apparatus is shown in Figure 12b. The eGDL samples were immersed in a 1 L solution

of 35wt.% hydrogen peroxide (H2O2) (Sigma Aldrich) at a temperature of 90 ○C for 6 hours. The

relatively high temperature and concentration of the H2O2 solution was intentionally used to induce

the accelerated degradation of the eGDL samples. The effect of the H2O2 solution on the eGDL

was assumed to mimic the long-term degradation of a GDL in the aqueous and corrosive

environments encountered during PEM fuel cell operation [105, 108].

The H2O2 solution (with the samples) was contained in a glass reaction vessel. The vessel lid was

sealed using a steel clamp with silicon gaskets. The temperature of the solution was maintained

via a heating plate (Hei-Standard, Heidolph Instruments) coupled to a feedback controller and

thermocouple. The thermocouple was placed in a protective glass tube and immersed in the

solution to ensure the temperature reading was representative of the entire solution. The exhaust

from the reaction vessel was directed to a reflux condenser (Lauda, Germany) with circulating

water flowing at a temperature of 5 ○C. The condenser cooled and condensed majority of the water

vapor and H2O2 in the exhaust and returned the condensed fluid to the reaction vessel, thereby

maintaining a constant H2O2 concentration throughout the experiment. A compact refrigerated

circulator (DC10-K20, Thermo Haake®) was used to maintain the circulating water temperature.

After the degradation procedure, the samples were rinsed and soaked in deionized water for

24 hours to remove any residual H2O2 and then dried in an oven (Heratherm General Protocol

Oven, Thermo Scientific) at 70 ○C for 24 hours.

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4.2.3 Characterization of Degraded eGDLs

This section describes the ex situ and in situ tests conducted to evaluate the effect of the

degradation procedure on the hydrophobic eGDLs.

4.2.3.1 Surface Contact Angle

The effect of the degradation procedure on the direct fluorination treatment was determined by

comparing the surface contact angle of the pristine and degraded eGDL samples. The surface

contact angles of post-fuel cell tested samples (fuel cell testing procedure presented in

Section 4.2.3.4) were also measured and compared to rule out the degradative impact of initial fuel

cell testing on the fluorination treatment. The surface contact angles were obtained via sessile drop

measurements conducted using a custom-built goniometer system. The water droplets (volume of

15 μL and 2 s wetting time) were imaged using a digital microscope (Dino-lite Pro, Dino-lite). The

measurements were conducted on both sides of the samples to ensure that the effects of both the

initial fluorination treatment and the accelerated degradation treatment were uniform. Finally, the

contact angles of the imaged droplets were calculated via the contact angle plugin [114] available

with Fiji® image processing software (Fiji®). A minimum of six sessile drop measurements per

sample were obtained to calculate an average value.

4.2.3.2 Electrical Conductivity

The effect of the degradation procedure on the eGDL carbon microstructure was elucidated via

bulk in-plane electrical conductivity measurements of the eGDL. The in-plane conductivity was

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measured using a 4-point probe (Model 101C, Four Dimensions). A minimum of 4 samples per

eGDL were measured (pristine and degraded). A bulk conductivity value was calculated for each

sample by obtaining a minimum of 4 measurements per sample with varying probe orientations

with respect to fiber alignment.

4.2.3.3 Fuel Cell Hardware and Control

The fuel cell experiments with the degraded and pristine eGDLs were conducted using a custom

fuel cell with an active area of 0.68 cm2. The cell build consisted of catalyst coated membranes

(CCMs) with a platinum loading of 0.3 mg/cm2 at both the anode and cathode (Nafion HP,

IonPower Inc.). Fresh CCMs were used for each separate cell build. The CCMs were placed in-

between matching eGDLs and bi-polar plates with the 8wt.% PAN fiber layer of the Bi-Layer

eGDL (pristine and degraded) facing the CCM at both the anode and cathode. The pristine and

degraded eGDLs were measured to have similar thickness values: 153 ± 17 μm and 128 ± 6 μm,

respectively, therefore, the eGDLs in each cell build was compressed to 113 μm using polyethylene

naphthalate (PEN) gaskets. The flow fields for the inlet gasses were machined into the bi-polar

plates and consisted of eight parallel channels (0.5 mm deep × 0.5 mm wide) separated by 0.5 mm

wide lands.

The fuel cell was operated via a Fuel Cell Test System (850e, Scribner Associates Inc.). Hydrogen

and oxygen were fed at 1 L/min to the anode and cathode of the cell, respectively. The inlet relative

humidity (RH) of the gasses were controlled via gas humidifiers within the test station. The outlet

pressure at the anode and cathode were maintained at 200 kPa (absolute), and the cell temperature

was maintained at 60 ○C via a circulating water bath (Isotemp™ 4100R20, Fisher Scientific Co.).

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4.2.3.4 Fuel Cell Performance Testing

The effect of the degraded eGDLs on fuel cell performance was examined by comparing the

constant current – voltage curves (𝑖 − 𝑉 curves) between the pristine and degraded eGDLs. The

𝑖 − 𝑉 curves were obtained at two inlet RH conditions: 50% and 100% RH. Both the anode and

cathode inlet RH were set to the same value during the tests. The 100% inlet RH condition was

selected to examine the water transport characteristics of the degraded eGDL as humid conditions

were expected to induce water accumulation. The 50% inlet RH condition was selected to examine

the effect of the degradation procedure on the eGDL carbon structure and electrical conductivity

as ohmic losses typically dominate at low humidity conditions. To obtain an 𝑖 − 𝑉 curve, the input

current was raised from 0.0 A/cm2 to limiting current at 0.5 A/cm2 steps via the fuel cell test

station. A 15-minute current hold was performed at each current density step to obtain a stable cell

response and water distribution [78]. The voltage response (measured every second) and high

frequency resistance (HFR, measured every 2 seconds) of the cell were averaged over the last

60 seconds of each current step to obtain a single value during stable operation. The HFR was

measured at 1 kHz, and the amplitude of the swept frequency was set to be 10% of the direct

current input.

4.2.3.5 Synchrotron X-ray Radiography

The water transport characteristics of the degraded eGDL was investigated by obtaining the

through-plane liquid water distribution within the cathode GDL during fuel cell operation via in-

operando synchrotron X-ray radiography. The synchrotron experiments were conducted at the

Biomedical Imaging Therapy Bending Magnet (BMIT-BM) beamline at the Canadian Light

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Source in Saskatoon, Canada [85]. The incident X-ray beam energy for the beamline was set to 24

keV. The attenuated beam was absorbed by an AA40 scintillator (Hamamatsu Photonics KK),

which converted the X-ray spectrum into the visible light spectrum. The converted signal was

captured as radiographs by a C11440-22CU CMOS camera (Hamamatsu Photonics KK). The

radiographs were captured at a frame rate of 0.33 frames/second, and the pixel resolution of the

experimental setup was 6.5 μm/pixel.

The radiographs (shown in Figure 13) were corrected for background noise, camera noise, and the

intensity decay of the incident beam as described by Hinebaugh et al. using an in-house MATLAB

algorithm [86]. The liquid water thickness, 𝑡𝑊 (cm), of each pixel of the corrected radiographs was

determined as a function of the x-and y-directions and time based on the Beer-Lambert law as [87]:

𝑡𝑊(𝑥, 𝑦, 𝑡) =1

𝜇𝑊ln (

𝐼𝑂𝐶𝑉(𝑥, 𝑦)

𝐼𝑤𝑒𝑡(𝑥, 𝑦, 𝑡)) (20)

where 𝜇𝑊 (cm-1) denotes the attenuation coefficient of liquid water, which was measured as

described in [88]. The pixel intensity of the reference radiograph, 𝐼𝑂𝐶𝑉, was obtained at 0.0 A/cm2,

at which point there was an absence of liquid water within the fuel cell, and the parameter

𝐼𝑤𝑒𝑡 denotes the pixel intensity of the radiographs acquired at current densities greater than

0.0 A/cm2 (i.e., fuel cell operation).

The average, normalized (to the thickness of the active area along the beam path) through-plane

(y-direction) liquid water distribution, 𝑡𝑤̅̅ ̅ (cm/cm), within the fuel cell at each current density was

calculated by averaging the liquid water thickness, 𝑡𝑊, over the width of the image (x-direction)

at each through plane position as:

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𝑡𝑤̅̅ ̅(𝑦) =1

𝐿𝑧

1

𝑁𝑡

1

𝑁𝑥 ∑ ∑ 𝑡𝑤(𝑥, 𝑦, 𝑡)

𝑁𝑥

𝑖=1

𝑁𝑡

𝑘=1

(21)

where 𝐿𝑧 (cm) denotes the thickness of the active area parallel to the path of the X-ray beam

(0.8 cm), 𝑁𝑡 denotes the number of radiographs averaged over time, and 𝑁𝑥 denotes the number

of pixels across the width of the image (x-direction). The last 20 radiographs obtained at the end

of each current density step was averaged, therefore 𝑁𝑡 = 20 (equivalent to 60 seconds of

operation with a frame rate of 0.33 frames/second). The error associated with the through-plane

water distribution based on the instrumentation and spatial variation in liquid water was calculated

as described in [89].

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Figure 13. Sample images from synchrotron X-ray radiography. a) Sample radiograph obtained

from X-ray radiography. b) Processes image showing water thickness, 𝑡𝑊, of each pixel. The color

represents the liquid water thickness in cm.

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4.3 Results

The degradation characteristics of the hydrophobic eGDLs are presented in three parts. First, the

surface contact angle measurements are presented to analyze the effect of the degradation

procedure on the surface fluorination treatment and eGDL hydrophobicity. Next, the fuel cell

performance and through plane liquid water distribution of the pristine and degraded eGDLs are

compared at 100% RH to examine the effect of the degradation procedure on water transport

within the cell (humid conditions used to induce water accumulation). Finally, the electrical

conductivity and fuel cell performance between the pristine and degraded eGDLs are compared at

50% RH to investigate the effect of the degradation procedure on the electrospun carbon fibers.

4.3.1 Effect of Degradation Procedure on eGDL Surface Hydrophobicity

The measured surface contact angles of the Bi-Layer eGDL at the pristine, post-fuel cell tested,

and degraded states are presented in Figure 14. The pristine eGDL and post-fuel cell tested eGDL

exhibited similar contact angles of 137○ ± 6○ and 136 ± 8○, respectively. The >90○ contact angle of

the pristine eGDL confirmed that the fluorination treatment led to the formation a hydrophobic

monolayer on the carbon fiber surfaces. Furthermore, the minimal change in contact angle between

the pristine and post-fuel cell tested samples indicated that the monolayers (formed via the

fluorination treatment) exhibited short-term stability (>10 hours of fuel cell testing). The stability

of the fluorination treatment with initial fuel cell operation suggested that the treatment is a viable

option to functionalize tailored eGDLs while still maintaining the desired pore structure (i.e. a pore

size gradient).

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Figure 14. Surface contact angle measurements of the eGDLs at various stages. a) Average contact

angles of pristine, post-fuel cell tested, and degraded eGDLs. Error bars represent ± 1 standard

deviation. Measurements were conducted on both sides of the eGDLs, and minimal differences

were observed between the sides. Sample droplet image with b) the pristine eGDL, c) the post fuel

cell tested eGDL, and d) the degraded eGDL.

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After the accelerated degradation procedure, the surface contact angle of the eGDL reduced

significantly to 44○ ± 12○ indicating a severe loss of surface hydrophobicity. The loss of

hydrophobicity observed from the accelerated degradation procedure was attributed to the

degradation of the hydrophobic monolayer via hydrolysis. Ideally, chemical vapor deposition

methods, such as the direct fluorination treatment procedure used in this study, result in the

formation of ordered self-assembled monolayers (SAMs) onto the substrate being treated as shown

in Figure 12a [64]. However, defects (i.e. breaks in the Si-O-Si bonds between adjacent silane

molecules in Figure 12a) in the monolayer formed via vapor deposition methods can result due to:

1) the scarcity of hydroxyl groups on the substrate surface following plasma treatment, or 2) the

disordered covalent bonding of the silane molecules to the oxidized surface during vacuum

treatment (i.e. vertical polymerization) [112, 115]. When such monolayers are exposed to aqueous

environments for an extended period of time (e.g. long term fuel cell operation, mimicked by our

degradation process), the defects in the monolayer allow for the adsorption of water onto the

substrate surface and the subsequent hydrolysis of the Si-O-Substrate bonds leading to a loss of

hydrophobicity [64]. The loss of hydrophobicity due to hydrolysis of SAMs has been previously

observed with silicon and metal oxide substrates [64, 112, 116]. Furthermore, the increased hydrolytic

instability of SAMs has been reported for lower pH environments [112]. Therefore, the accelerated

degradation of the eGDL substrates in the dilute H2O2 solution (estimated pH of ~3) was postulated

to lead to the hydrolysis of the hydrophobic layer on the eGDL carbon fibers due to the presence

of surface defects. These findings necessitate the development of optimized fluorination treatment

procedures to minimize the presence of surface defects. Robust fluorination treatments that can

withstand long-term exposure to the aqueous and corrosive environments encountered during fuel

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cell operation will facilitate the successful implementation of tailored GDLs for next generation

PEM fuel cells.

The prevalence of defects in the hydrophobic monolayer may be verified for the eGDL surfaces

via XPS (not completed in this study). One proposed approach would be to compare the XPS

spectra of hydrophobic monolayers formed on the eGDL surfaces to monolayers formed on smooth

glassy carbon surfaces via the same direct fluorination procedure. Monolayer formation is

expected to be more uniform and ordered on smooth, flat surfaces (such as glassy carbon)

compared to porous surfaces such as the eGDL. Therefore comparison of the XPS spectra (e.g.

compare ratio of Si-O-Si and Si-O-C bonds between the smooth surface and porous eGDL, where

higher relative intensity of Si-O-Si bonds indicates the presence of defects due to vertical

polymerization) between the two materials could provide insight into how prevalent defects

(within the monolayer) are on a porous surface such as the eGDL compared to smooth surfaces.

4.3.2 Increased Liquid Water Accumulation due to Loss of Surface Hydrophobicity

At 100% RH, the degraded eGDL led to a severe drop in cell voltage after 0.5 A/cm2 (0.5 V lower

at 1.0 A/cm2) compared to the pristine eGDL (Figure 15a). The drop in cell voltage was attributed

to the higher levels of water accumulation observed within the degraded eGDL via synchrotron X-

ray radiography (Figure 15b). Specifically, on average, the normalized water thickness, 𝑡𝑤̅̅ ̅, within

the 8wt.% carbon fiber layer of the degraded eGDL was 16-times higher compared to the pristine

eGDL at 0.5A/cm2. Excessive water accumulation led to blocked pathways for oxygen transport

within the eGDL and increased mass transport losses, which were characterized by the severe drop

in cell voltage observed in Figure 15a.

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Figure 15. Fuel cell performance and water profiles at 100% RH. a) 𝑖 − 𝑉 curves obtained at 100%

RH. The error bars represent ± 1 standard deviation. Note: The dashed line for the degraded eGDL

represents the real-time voltage response with current (1 data point per second) and is shown to

highlight the sharp drop in voltage following the 0.5 A/cm2 current step. The severe drop in voltage

with the degraded eGDL at 0.5 A/cm2 indicates large mass transport losses at 100% RH. b)

Through-plane liquid water profile at 100% RH at 0.5 A/cm2. The degraded eGDL led to more

significant water accumulation compared to the pristine eGDL due to the loss of hydrophobicity.

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The higher level of liquid water accumulation within the degraded eGDL was attributed to the

reduced hydrophobicity of the degraded eGDL. The use of a pristine Bi-Layer eGDL was

previously demonstrated to lead to the effective removal of liquid water from the CL interface at

100% inlet RH (discussed in Chapter 3). Furthermore, the effective water removal mechanism was

proposed to be facilitated by the hydrophobic pore size gradient (increasing pore size from the CL

interface to the flow field). The degradation of the SAMs and the corresponding reduction in

hydrophobicity of the eGDL (Section 4.3.1) negated the water removal capabilities of the graded

Bi-Layer eGDL. Specifically, the transition from a hydrophobic to hydrophilic eGDL is

hypothesized to lead to the weak imbibition of product water into the eGDL pores in contrast to

strong drainage that would have otherwise been expected with pristine hydrophobic eGDLs. The

weak imbibition of water results in a more compact displacement pattern due to mechanisms such

as cooperative pore-filling [117] compared to strong drainage, which results in fractal-like

displacement patterns [118, 119]. Consequently, the compact pore-filling mechanisms induced by the

reduction in hydrophobicity led to higher water content within the eGDL as illustrated in Figure

15b.

4.3.3 Increased Ohmic Losses due to Carbon Degradation

The degraded eGDL led to consistently lower cell voltages compared to the pristine eGDL for

current densities above 0.5 A/cm2 and 50% inlet RH (Figure 16a). Higher HFR (ohmic losses)

values were also observed at 50% RH with the degraded eGDL as seen in Figure 16b. Specifically,

the HFR of the degraded eGDL was on average 138 mΩ·cm2 higher than the pristine eGDL at all

current densities.

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Figure 16. Fuel cell performance at 50% RH and electrical conductivity. a) 𝑖 − 𝑉 curves obtained

at 50% RH. The error bars represent ± 1 standard deviation. b) High frequency resistance (HFR)

at 50% RH. The error bars represent ± 1 standard deviation. c) Bulk in-plane electrical conductivity

of eGDLs. The error bars represent ± 1 standard deviation.

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To elucidate the effect of the degradation procedure on eGDL carbon fibers and the HFR of the

fuel cell, the in-plane electrical conductivity, 𝜎 (S/cm), of the pristine and degraded eGDLs were

compared (Figure 16c). The degraded eGDL exhibited a 53% lower bulk in-plane electrical

conductivity compared to the pristine eGDL. The reduction in electrical conductivity was

attributed to the oxidation of carbon fibers by the H2O2 solution during the accelerated degradation

procedure. Oxidation of eGDL carbon fibers can lead to the formation of defects within the

ordered graphitic layers at the surface of the fibers [120]. The presence of defects in the graphitic

layers were postulated to lead to reduced electrical conductivity of the carbon fibers [91] as

evidenced by the lower bulk conductivity values observed in Figure 16c with the degraded eGDL.

Consequently, the lower electrical conductivity of the degraded eGDL (proposed to be due to the

oxidation of carbon fibers) led to higher HFR and lower cell voltages at 50% RH. In the future,

the presence of defects within the graphitic layers of the eGDL carbon fibers after the degradation

procedure could be verified via Raman spectroscopy (e.g. comparison of spectral intensity between

disordered graphite and ordered graphite between the pristine and degraded eGDLs); however,

only the surface composition of the eGDLs could be verified via this technique.

4.4 Chapter Conclusions

In this study, the degradation characteristics of hydrophobic electrospun gas diffusion layers

(eGDLs) for PEM fuel cells were examined. The hydrophobic eGDLs underwent an accelerated

degradation procedure via immersion in a solution of H2O2 at elevated temperatures. The

degradation procedure was designed to simulate the long-term exposure of the eGDL to the

aqueous and corrosive environments encountered during fuel cell operation.

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Surface contact angle comparisons of the pristine and degraded eGDLs indicated a drastic loss of

surface hydrophobicity following the degradation procedure. The average contact angle of the

pristine sample was 137○, whereas the contact angle of the degraded eGDL was 44○. The lower

contact angle was attributed to the hydrolysis of the hydrophobic monolayers of the eGDL fibers

upon long-term exposure to the H2O2 solution. The hydrolysis of the monolayers was hypothesized

to be enabled by the presence of surface defects within the monolayers and further enhanced under

the acidic conditions of the degradation treatment. The effect of reduced surface hydrophobicity

on cell performance was elucidated via fuel cell performance tests and in situ synchrotron X-ray

radiography at 100% RH. At 100% RH, higher water accumulation and mass transport losses was

observed with the degraded eGDL compared to the pristine eGDL. The effective water removal

capabilities of the initially hydrophobic graded pore structure of the Bi-Layer eGDL were negated

by the loss of surface hydrophobicity. Higher water accumulation within the degraded eGDL was

attributed to the weak imbibition of product water that accompanied the loss of hydrophobicity.

At lower inlet RH conditions (50% RH), the degraded eGDL led to higher ohmic losses (HFR)

compared to the pristine eGDL at all tested current densities. The higher HFR was attributed to

the lower electrical conductivity of the degraded eGDL compared to the pristine eGDL due to

oxidation of the carbon fibers upon exposure to the H2O2 solution.

This study examined the degradation characteristics of eGDL materials. The fluorination treatment

employed was stable after initial fuel cell operation suggesting that the treatment is a viable method

to render GDLs hydrophobic while maintaining the desired pore structure. However, the loss of

hydrophobicity upon accelerated degradation and consequently ineffective water removal during

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fuel cell operation necessitates the development of robust treatment procedures with minimal

surface defects for the successful implementation of hydrophobic GDLs with tailored, optimized

pore structures.

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CHAPTER 5 – Conclusions

5.1 Summary of Findings

This thesis presented the development, characterization, performance, and degradation

characteristics of tailored electrospun GDL materials for PEM fuel cells. Specifically, Chapter 3

presented a novel graded electrospun GDL (eGDL) designed to enhance the high current density

performance of PEM fuel cells. Chapter 4 investigated the degradation characteristics of the novel

graded eGDL via an accelerated degradation procedure.

In Chapter 3, a novel eGDL was developed with a pore size gradient from the CL interface to the

flow field interface to enhance the high current density performance (≥1.5 A/cm2) of PEM fuel

cells compared to uniform materials. The structure and material properties of the graded eGDL

were thoroughly characterized via SEM imaging, Raman spectroscopy, and 4-point probe

measurements. Towards developing the eGDL for robust applicability, the effect of the graded

eGDL on high current density PEM fuel cell performance was examined over a wide range of RH

conditions via 𝑖 − 𝑉 curve measurements, in-operando synchrotron X-ray radiography, and EIS.

The findings of this study were as follows:

1. Post carbonization, the electrospun carbon fibers with smaller fiber diameters exhibited a

higher degree of graphitization and higher electrical conductivity compared to the carbon

fibers with larger fiber diameters.

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2. The fuel cell with the graded eGDL exhibited lower ohmic resistance and improved high

current density performance at 50% RH compared to the uniform GDL with relatively

larger pores and fiber diameters. Specifically, the graded eGDL led to higher liquid water

retention at the CL interface (increasing ionic conductivity) and enhanced heat dissipation

from the CL interface (reducing membrane dehydration).

3. The fuel cell with the graded eGDL exhibited lower mass transport resistance at 100% RH

compared to GDLs with a uniform distribution of small pores. A pore size gradient

promoted the directed removal of liquid water from the CL interface towards the flow field

resulting in improved high current density performance at 100% RH.

The findings and methods presented in Chapter 3 can be used by researchers and manufacturers to

fabricate tailored electrospun porous layers to enhance the high current density performance of

PEM fuel cell systems and thereby reduce system cost.

In chapter 4, the degradation characteristics of the graded eGDL were investigated. An ex situ

accelerated degradation protocol was utilized to mimic the effects of long-term fuel cell operation.

The effects of the degradation procedure on the direct fluorination treatment were analyzed via

surface contact angle measurements. The water transport characteristics of the degraded eGDL

was examined via in situ synchrotron X-ray radiography at 100% RH. Finally, the effect of the

degradation procedure on the eGDL carbon structure was elucidated via fuel cell performance tests

at 50% RH and electrical conductivity measurements. The results of this study were as follows:

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1. Ex situ accelerated degradation of eGDLs via hydrogen peroxide led to the loss of surface

functional groups over time as evidenced by lower surface contact angles. The degradation

of surface functional groups was proposed to occur due to the presence of defects within

the hydrophobic layer.

2. The degraded graded eGDL was prone to liquid water accumulation and led to mass

transport losses at 100% RH. Transition from a hydrophobic to hydrophilic substrate

negated the effective water removal mechanisms enabled by the pore size gradients.

Reduced hydrophobicity was hypothesized to promote water imbibition and a more

compact water displacement pattern as evidenced by higher water accumulation.

3. Degraded eGDLs exhibited a lower electrical conductivity and led to higher ohmic losses

at 50% inlet RH compared to pristine materials. The oxidation of carbon fibers via the

degradation procedure was hypothesized to introduce defects within the graphitic layers

leading to reduced electrical conductivity.

The findings of Chapter 3 informed researchers and manufacturers of the degradation mechanisms

of eGDL materials. Particularly, the development of robust hydrophobicity treatments that

preserve the tailored structure of the eGDLs was identified to be a critical step for the commercial

implementation of tailored GDLs for PEM fuel cell applications.

In conclusion, the contributions of this thesis advanced the development of tailored GDL materials

for PEM fuel cells. Application of robust tailored materials will enhance PEM fuel cell

performance, help to reduce cost, and facilitate the adoption of PEM fuel cells in the global market.

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5.2 Future Work

Based on the findings of this thesis, the following projects are proposed as promising future works

for further development of tailored GDL materials for PEM fuel cell applications.

1. Numerical optimization of eGDL structures

Numerical simulations could be used to predict optimal eGDL microstructures for improved PEM

fuel cell performance and water management. For instance, the fiber diameter, pore size

distributions, surface contact angles, and electrical conductivity measurements in this thesis could

be used to generate a numerical model of the Bi-Layer eGDL via stochastic modelling techniques,

such as those reported in [15, 121]. Techniques such as pore network modelling [122] could then be

used to simulate water, heat, and oxygen transport through the generated eGDL structures. Using

stochastic and pore network modelling, the thickness of each layer (i.e. the thickness of the 8wt.%

layer and 12wt.% layer) of the Bi-layer eGDL could be numerically optimized for water transport

and inform future manufacturing directions.

2. Gradual GDL pore size gradients via electrospinning to enhance the performance of PEM fuel

cells with low platinum loading electrodes

Reducing the platinum loading within the CL is an attractive option to reduce the cost of PEM fuel

cells. However, with low platinum loading, water flooding near the CL can lead to high mass

transport resistance and reduced cell performance due to the limited number of available reaction

sites [36, 123]. A gradual pore size gradient (increasing pore size from the CL interface to the flow

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field interface) could be beneficial for low platinum loading electrodes by minimizing water

accumulation near the CL by continuously directing water transport (via capillary fingering)

towards the flow field. Electrospinning is a powerful tool that could be used to fabricate such

gradual pore size gradients. For instance, the electrospinning apparatus described in this thesis

could be modified to gradually eject polymer solution with increasing PAN concentration (PAN

wt.%). The gradual increase in PAN concentration would result in a gradual increase in fiber

diameter and pore size (as opposed to the layered pore structure developed in this thesis). The

resulting GDL could then be used with low platinum loading electrodes to enhance fuel cell

performance and reduce cost.

3. Optimization of the direct fluorination treatment to enhance the durability of hydrophobic

monolayers

The findings of Chapter 4 necessitate the development of robust hydrophobicity treatments that

can endure long term fuel cell operation while still maintaining the tailored structure of the eGDL

materials. The durability of the fluorination treatment described in this thesis could potentially be

improved with the use of alternative surface functional groups and post processing techniques. For

instance, an annealing treatment was used by Gnanappa et al. [64] to improve the durability of self-

assembled monolayers (SAMs) on silicon substrates. The annealing treatment was reported to

minimize the effect of defects present in the monolayer and improve the hydrolytic stability of the

SAMs. Additionally, functional groups other than trichlorosilane molecules could also be

investigated to examine their hydrolytic stability with carbon surfaces under fuel cell operating

conditions. For example, Marcinko et al. [112] reported that phosphonic acid groups exhibited

enhanced hydrolytic stability compared to silane groups at low pH conditions on metal oxide

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surfaces. Improving the durability of the direct fluorination treatment will facilitate the

commercialization of tailored GDL materials for PEM fuel cell applications.

4. Alternative hydrophobic electrospun carbon fibers

The development of electrospun carbon fibers that exhibit intrinsic hydrophobic properties could

eliminate the need for a surface functionalization treatment and improve the durability of eGDLs.

For instance, Zhu et al. [124] used electrospinning to develop carbon nanofibers consisting of Fe3O4

particles (incorporated as FeAc2 in the polymer precursor solution). The fibers had rough surfaces

due to the embedded Fe3O4 particles (particles were 30 – 40 nm), and the resulting fibrous substrate

exhibited hydrophobic properties (surface contact angle of 156.6○). Since the electrospun fibers

were carbonized, they also exhibited a reasonable electrical conductivity for PEM fuel cell

applications (reported to be 3.4 S/cm measured via 4-point probe) [124]. Such electrospun carbon

fiber substrates are attractive for PEM fuel cells as they are both conductive and hydrophobic. The

diameters and pore sizes of the resulting substrates could still be controlled by tuning the viscosity

of the precursor solution. However, for effective performance, the electronic conductivity of such

fibers should be enhanced via higher graphitization temperatures or the use of additional additives

such as carbon nanotubes.

5. Use of passivating layer to enhance oxidation resistance of eGDL carbon fibers

A nanolayer of silica could be deposited onto the eGDL carbon fibers as described by

Hatton et al., [125] to minimize carbon oxidation during long term fuel cell operation. Specifically,

the silica would act as a passivating layer and protect the graphitic structure of the eGDL fibers

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from oxidation due to liquid water accumulation. Improving the oxidation resistance of carbon

fibers can be beneficial for long term fuel cell performance as ohmic losses associated with carbon

oxidation (as observed in Chapter 4) would be minimized. Furthermore, improving the oxidation

resistance via a passivating layer can also minimize instances of “pitting” on the carbon surface

(due to surface oxidation) which is expected to lower the electrical contact area between eGDL

fibers and thereby lower bulk electrical conductivity. The passive nanolayer of silica still allows

for the eGDLs to be rendered hydrophobic via direct fluorination, however, instead of the

trichlorosilane molecules bonding to the plasma treated carbon surfaces, the molecules would bond

to the silica (SiO2) nanolayer.

The future works presented above are promising research endeavors that will ultimately help

implement PEM fuel cells as an economical and low carbon alternative to internal combustion

engines in the transportation sector.

100

References

[1] E. S. Hanley, J. P. Deane, B. Ó Gallachóir, Renewable and Sustainable Energy Reviews 2018,

82, 3027.

[2] I. Staffell, D. Scamman, A. V. Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward,

Energy Environ. Sci. 2019, 12(2), 463.

[3] U.S. Department of Energy, Energy Efficiency & Renewable Energy, Fuel Cell Technologies

Office 2017.

[4] M. Granovskii, I. Dincer, M. A. Rosen, Int. J. Hydrogen Energy 2006, 31(3), 337.

[5] U.S. Department of Energy, Energy Efficiency & Renewable Energy, Fuel Cell Technologies

Office 2015.

[6] United States Driving Research and Innovation for Vehicle efficiency and Energy

sustainability Partnership, Fuel Cell Technical Team Roadmap 2017, 1.

[7] B. D. James, J. M. Huya-Kouadio, C. Houchins, D. A. DeSantis 2017.

[8] S. Park, J. Lee, B. N. Popov, Int J Hydrogen Energy 2012, 37(7), 5850.

[9] P. K. Sinha, P. P. Mukherjee, C. Wang, J. Mater. Chem. 2007, 17(30), 3089.

[10] M. M. Mench, Fuel cell engines, John Wiley & Sons2008.

[11] M. Andersson, S. B. Beale, M. Espinoza, Z. Wu, W. Lehnert, Appl. Energy 2016, 180, 757.

[12] L. Cindrella, A. Kannan, J. Lin, K. Saminathan, Y. Ho, C. Lin, J. Wertz, J. Power Sources

2009, 194(1), 146.

[13] R. Schweiss, C. Meiser, T. Damjanovic, I. Galbiati, N. Haak, SIGRACET® Gas Diffusion

Layers for PEM Fuel Cells, Electrolyzers and Batteries 2016, 1.

[14] B. Zhang, F. Kang, J. Tarascon, J. Kim, Progress in Materials Science 2016, 76, 319.

101

[15] J. Hinebaugh, A. Bazylak, Int J Hydrogen Energy 2017.

[16] H. Li, Y. Tang, Z. Wang, Z. Shi, S. Wu, D. Song, J. Zhang, K. Fatih, J. Zhang, H. Wang, Z.

Liu, R. Abouatallah, A. Mazza, J. Power Sources 2008, 178(1), 103.

[17] Z. Fishman, J. Hinebaugh, A. Bazylak, J. Electrochem. Soc. 2010, 157(11), B1643.

[18] M. B. Sassin, Y. Garsany, B. D. Gould, K. Swider-Lyons, J. Electrochem. Soc. 2016, 163(8),

F808.

[19] U. U. Ince, H. Markötter, M. G. George, H. Liu, N. Ge, J. Lee, S. S. Alrwashdeh, R. Zeis, M.

Messerschmidt, J. Scholta, A. Bazylak, I. Manke, Int J Hydrogen Energy 2018, 43(1), 391.

[20] S. Chevalier, N. Lavielle, B. Hatton, A. Bazylak, J. Power Sources 2017, 352, 272.

[21] Z. Fishman, A. Bazylak, Journal of The Electrochemical Society 2011, 158, 841.

[22] J. Lee, R. Yip, P. Antonacci, N. Ge, T. Kotaka, Y. Tabuchi, A. Bazylak, J. Electrochem. Soc.

2015, 162(7), F669.

[23] M. El Hannach, R. Singh, N. Djilali, E. Kjeang, J. Power Sources 2015, 282, 58.

[24] T. Kitahara, H. Nakajima, M. Inamoto, K. Shinto, J. Power Sources 2014, 248, 1256.

[25] A. Nanjundappa, A. Alavijeh, M. Hannach, E. Kjeang, Electrochim. Acta 2013, 110, 349.

[26] M. G. George, J. Wang, R. Banerjee, A. Bazylak, J. Power Sources 2016, 309, 254.

[27] D. Spernjak, R. Mukundan, R. L. Borup, L. G. Connolly, B. I. Zackin, V. De Andrade, M.

Wojcik, D. Y. Parkinson, D. L. Jacobson, D. S. Hussey, K. L. More, T. Chan, A. Z. Weber, I. V.

Zenyuk, ACS Appl. Energy Mater. 2018, 1(11), 6006.

[28] D. Todd, W. Mérida, Journal of Power Sources 2016, 311, 182.

[29] M. Inagaki, Y. Yang, K. Feiyu, Adv. Mater 2012, 24, 2547.

[30] S. Cavaliere, S. Subianto, I. Savych, D. J. Jones, J. Rozière, Energy Environ. Sci. 2011, 4(12),

4761.

102

[31] F. N. Büchi, G. G. Scherer, J. Electrochem. Soc. 2001, 148(3), A183.

[32] A. Kusoglu, A. Z. Weber, Chem. Rev. 2017, 117(3), 987.

[33] P. K. Mohanta, F. Regnet, L. Jorissen, Materials (Basel) 2018, 11(6), 10.3390/ma11060907.

[34] A. Kongkanand, M. F. Mathias, J. Phys. Chem. Lett. 2016, 7, 1127.

[35] B. Wu, M. Zhao, W. Shi, W. Liu, J. Liu, D. Xing, Y. Yao, Z. Hou, P. Ming, J. Gu, Z. Zou,

Int J Hydrogen Energy 2014, 39(26), 14381.

[36] A. Kongkanand, N. P. Subramanian, Y. Yu, Z. Liu, H. Igarashi, D. A. Muller, ACS Catal.

2016, 6(3), 1578.

[37] R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson,

F. Garzon, D. Wood, Chem. Rev. 2007, 107(10), 3904.

[38] G. Karimi, X. Li, P. Teertstra, Electrochim. Acta 2010, 55(5), 1619.

[39] J. M. Zalc, S. C. Reyes, E. Iglesia, Chemical Engineering Science 2004, 59(14), 2947.

[40] N. B. Carrigy, L. M. Pant, S. Mitra, M. Secanell, J. Electrochem. Soc. 2013, 160(2), F81.

[41] X. Zhang, Y. Gao, H. Ostadi, K. Jiang, R. Chen, Int J Hydrogen Energy 2014, 39(30), 17222.

[42] P. A. García-Salaberri, G. Hwang, M. Vera, A. Z. Weber, J. T. Gostick, Int. J. Heat Mass

Transfer 2015, 86(0), 319.

[43] T. Rosén, J. Eller, J. Kang, N. I. Prasianakis, J. Mantzaras, F. N. Büchi, J. Electrochem. Soc.

2012, 159(9), F536.

[44] R. Vetter, J. Schumacher, Comput. Phys. Commun. 2019, 234, 223.

[45] G. Hwang, A. Weber, J. Electrochem. Soc. 2012, 159(11), F683.

[46] R. Lenormand, E. Touboul, C. Zarcone, J. Fluid Mech. 1988, 189, 165.

[47] L. Cindrella, A. M. Kannan, J. F. Lin, K. Saminathan, Y. Ho, C. W. Lin, J. Wertz, J. Power

Sources 2009, 194(1), 146.

103

[48] J. Park, H. Oh, Y. I. Lee, K. Min, E. Lee, J. Jyoung, Appl. Energy 2016, 171, 200.

[49] T. Tanuma, M. Kawamoto, S. Kinoshita, J. Electrochem. Soc. 2017, 164(6), F499.

[50] C. Tseng, S. Lo, Energy Conversion and Management 2010, 51(4), 677.

[51] T. Kitahara, H. Nakajima, K. Okamura, ECS Trans. 2015, 69(17), 1313.

[52] P. Shrestha, R. Banerjee, J. Lee, N. Ge, D. Muirhead, H. Liu, A. K. C. Wong, D. Ouellette,

B. Zhao, A. Bazylak, J. Power Sources 2018, 402, 468.

[53] J. Chun, D. Jo, S. Kim, S. Park, C. Lee, E. Lee, J. Jyoung, S. Kim, Renewable Energy 2013,

58, 28.

[54] J. Hinebaugh, Z. Fishman, A. Bazylak, J. Electrochem. Soc. 2010, 157(11), B1651.

[55] J. Cho, H. Oh, J. Park, K. Min, E. Lee, J. Jyoung, Int J Hydrogen Energy 2014, 39(1), 495.

[56] Y. Huang, C. Cheng, X. Wang, J. Jang, Energy 2010, 35, 4786.

[57] H. Oh, J. Park, K. Min, E. Lee, J. Jyoung, Appl. Energy 2015, 149(0), 186.

[58] D. Ko, S. Doh, H. S. Park, M. H. Kim, Int J Hydrogen Energy 2018, 43(4), 2369.

[59] I. M. Kong, A. Jung, Y. S. Kim, M. S. Kim, Energy 2017, 120, 478.

[60] S. Chevalier, N. Ge, J. Lee, M. G. George, H. Liu, P. Shrestha, D. Muirhead, N. Lavielle, B.

Hatton, A. Bazylak, J. Power Sources 2017, 352, 281.

[61] S. Chan, J. Jankovic, D. Susac, M. S. Saha, M. Tam, H. Yang, F. Ko, J. Power Sources 2018,

392, 239.

[62] M. D. R. Kok, R. Jervis, D. Brett, P. R. Shearing, J. T. Gostick, Small 2018, 14(9), 1703616.

[63] S. Kaur, S. Sundarrajan, D. Rana, T. Matsuura, S. Ramakrishna, Journal of Membrane

Science 2012, 392-393, 101.

[64] A. K. Gnanappa, C. O'Murchu, O. Slattery, F. Peters, T. O'Hara, B. Aszalós-Kiss, S. A. M.

Tofail, Appl. Surf. Sci. 2011, 257(9), 4331.

104

[65] J. S. Lim, , Prentice Hall, Englewood Cliffs, NJ 1990.

[66] MathWorks, imgaussfilt, https://www.mathworks.com/help/images/ref/imgaussfilt.html,

accessed: September 2019.

[67] D. Bradley, G. Roth, Journal of Graphics Tools 2007, 12(2), 13.

[68] MathWorks, bwareaopen, https://www.mathworks.com/help/images/ref/bwareaopen.html,

accessed: September 2019.

[69] P. Soille, Morphological Image Analysis: Principles and Applications, 2nd edn., Springer,

Berlin 2004.

[70] J. T. Gostick, Z. A. Khan, T. G. Tranter, M. D. R. Kok, M. Agnaou, M. Sadeghi, R. Jervis,

The Journal of Open Source Software 2019, 4, 1296.

[71] J. T. Gostick, Phys Rev E. 2017, 96(2), 023307.

[72] Z. Zhou, C. Lai, L. Zhang, Y. Qian, H. Hou, D. H. Reneker, H. Fong, Polymer 2009, 50(13),

2999.

[73] T. Jawhari, A. Roid, J. Casado, Carbon 1995, 33(11), 1561.

[74] A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner, U. Pöschl, Carbon 2005, 43(8), 1731.

[75] D. Papkov, A. M. Beese, A. Goponenko, Y. Zou, M. Naraghi, H. D. Espinosa, B. Saha, G. C.

Schatz, A. Moravsky, R. Loutfy, S. T. Nguyen, Y. Dzenis, ACS Nano 2013, 7(1), 126.

[76] L. Qiu, X. H. Zheng, J. Zhu, G. P. Su, D. W. Tang, Carbon 2013, 51, 265.

[77] Shilpa, S. K. Das, M. A. F. Afzal, S. Srivastava, S. Patil, A. Sharma, Carbon 2016, 108, 135.

[78] R. Banerjee, N. Ge, J. Lee, M. G. George, S. Chevalier, H. Liu, P. Shrestha, D. Muirhead, A.

Bazylak, J. Electrochem. Soc. 2017, 164(2), F154.

[79] D. Muirhead, R. Banerjee, J. Lee, M. G. George, N. Ge, H. Liu, S. Chevalier, J. Hinebaugh,

K. Han, A. Bazylak, Int J Hydrogen Energy 2017, 42(49), 29472.

105

[80] S. M. Rezaei Niya, M. Hoorfar, J. Power Sources 2013, 240, 281.

[81] D. Malevich, E. Halliop, B. A. Peppley, J. G. Pharoah, K. Karan, J. Electrochem. Soc. 2009,

156(2), B216.

[82] S. Cruz-Manzo, R. Chen, J. Electroanal. Chem. 2013, 694, 45.

[83] M. Urquidi-Macdonald, S. Real, D. D. Macdonald, Electrochim. Acta 1990, 35(10), 1559.

[84] S. Ramanathan KKT – Validation Tool, http://www.che.iitm.ac.in/~srinivar/, accessed:

September 2019.

[85] T. W. Wysokinski, D. Chapman, G. Adams, M. Renier, P. Suortti, W. Thomlinson, Nucl.

Instrum. Methods Phys. Res. , Sect. A 2007, 582(1), 73.

[86] J. Hinebaugh, P. R. Challa, A. Bazylak, J. Synchrotron Radiat. 2012, 19(6), 994.

[87] J. Hubbell, Phys. Med. Biol. 1999, 44(1), R1.

[88] N. Ge, S. Chevalier, J. Hinebaugh, R. Yip, J. Lee, P. Antonacci, T. Kotaka, Y. Tabuchi, A.

Bazylak, J. Synchrotron Radiat. 2016, 23(2).

[89] S. Chevalier, N. Ge, M. G. George, J. Lee, R. Banerjee, H. Liu, P. Shrestha, D. Muirhead, J.

Hinebaugh, Y. Tabuchi, T. Kotaka, A. Bazylak, J. Electrochem. Soc. 2017, 164(2), F107.

[90] M. Kruk, K. M. Kohlhaas, B. Dufour, E. B. Celer, M. Jaroniec, K. Matyjaszewski, R. S.

Ruoff, T. Kowalewski, Microporous and Mesoporous Materials 2007, 102(1), 178.

[91] T. Maitra, S. Sharma, A. Srivastava, Y. Cho, M. Madou, A. Sharma, Carbon 2012, 50(5),

1753.

[92] P. P. Mukherjee, C. Y. Wang, Q. J. Kang, Electrochim Acta 2009, 54(27), 6861.

[93] N. Zhan, W. Wu, S. Wang, Electrochim. Acta 2019, 306, 264.

[94] S. Wang, Y. Wang, Int. J. Heat Mass Transfer 2016, 98, 541.

[95] S. Didari, A. Asadi, Y. Wang, T. A. L. Harris, Int J Hydrogen Energy 2014, 39(17), 9375.

106

[96] Z. Fishman, A. Bazylak, J. Electrochem. Soc. 2011, 158(8), B841.

[97] H. Sadeghifar, N. Djilali, M. Bahrami, J. Power Sources 2014, 248, 632.

[98] J. Park, H. Oh, T. Ha, Y. I. Lee, K. Min, Appl. Energy 2015, 155, 866.

[99] W. Liu, D. Zuckerbrod, J. Electrochem. Soc. 2005, 152(6), A1165.

[100] T. Aoki, A. Matsunaga, Y. Ogami, A. Maekawa, S. Mitsushima, K. Ota, H. Nishikawa, J.

Power Sources 2010, 195(8), 2182.

[101] J. D. Fairweather, D. Spernjak, A. Z. Weber, D. Harvey, S. Wessel, D. S. Hussey, D. L.

Jacobson, K. Artyushkova, R. Mukundan, R. L. Borup, J. Electrochem. Soc. 2013, 160(9), F980.

[102] N. Macauley, D. D. Papadias, J. Fairweather, D. Spernjak, D. A. Langlois, R. Ahluwalia, K.

L. More, R. Mukundan, R. L. Borup, J. Electrochem. Soc. 2018, 165(6).

[103] E. Brightman, G. Hinds, J. Power Sources 2014, 267, 160.

[104] Y. Hiramitsu, H. Sato, K. Kobayashi, M. Hori, Journal of Power Sources 2011, 196(13),

5453.

[105] H. Liu, M. G. George, M. Messerschmidt, R. Zeis, D. Kramer, J. Scholta, A. Bazylak, J.

Electrochem. Soc. 2017, 164(7), F695.

[106] P. K. Das, A. Grippin, A. Kwong, A. Z. Weber, J. Electrochem. Soc. 2012, 159(5), B489.

[107] T. Ha, J. Cho, J. Park, K. Min, H. Kim, E. Lee, J. Jyoung, Int J Hydrogen Energy 2011,

36(19), 12436.

[108] J. W. Frisk, M. T. Hicks, R. T. Atanasoski, W. M. Boand, A. K. Schmoeckel, M. J.

Kurkowski, "MEA Component Durability", presented at 2004 Fuel Cell Seminar, 2004.

[109] J. Chlistunoff, J. R. Davey, K. C. Rau, R. Mukundan, R. L. Borup, "PEMFC Gas Diffusion

Media Degradation Determined by Acid-Base Titrations", presented in ECS Transactions, 2012.

[110] G. Chen, H. Zhang, H. Ma, H. Zhong, Int J Hydrogen Energy 2009, 34(19), 8185.

107

[111] T. Arlt, M. Klages, M. Messerschmidt, J. Scholta, I. Manke, Energy 2017, 118, 502.

[112] S. Marcinko, A. Y. Fadeev, Langmuir 2004, 20(6), 2270.

[113] M. Adamkiewicz, T. O'Hara, D. O'Hagan, G. Hähner, Thin Solid Films 2012, 520(22), 6719.

[114] M. Brugnara, 2019.

[115] A. Y. Fadeev, T. J. McCarthy, Langmuir 2000, 16(18), 7268.

[116] N. S. Bhairamadgi, S. P. Pujari, F. G. Trovela, A. Debrassi, A. A. Khamis, J. M. Alonso, A.

A. Al Zahrani, T. Wennekes, H. Al-Turaif, C. van Rijn, Y. A. Alhamed, H. Zuilhof, Langmuir

2014, 30(20), 5829.

[117] B. Zhao, C. W. MacMinn, R. Juanes, Proc. Natl. Acad. Sci. U. S. A. 2016, 113(37), 10251.

[118] R. Lenormand, C. Zarcone, Transp. Porous Media 1989, 4(6), 599.

[119] K. J. Måløy, L. Furuberg, J. Feder, T. Jøssang, Phys. Rev. Lett. 1992, 68(14), 2161.

[120] Z. Wu, C. U. Pittman, S. D. Gardner, Carbon 1995, 33(5), 597.

[121] V. P. Schulz, J. Becker, A. Wiegmann, P. P. Mukherjee, C. Wang, J. Electrochem. Soc.

2007, 154(4), B419.

[122] J. Gostick, M. Aghighi, J. Hinebaugh, T. Tranter, M. A. Hoeh, H. Day, B. Spellacy, M. H.

Sharqawy, A. Bazylak, A. Burns, W. Lehnert, A. Putz, Comput. Sci. Eng. 2016, 18(4), 60.

[123] A. Z. Weber, A. Kusoglu, J.Mater.Chem.A 2014, 2(41), 17207.

[124] Y. Zhu, J. C. Zhang, J. Zhai, Y. M. Zheng, L. Feng, L. Jiang, Chem. Eur. J. of Chem. Phys.

2006, 7(2), 336.

[125] B. Hatton, V. Kitaev, D. Perovic, G. Ozin, J. Aizenberg, J. Mater. Chem. 2010, 20(29), 6009.

108

Appendix A – Tafel Slope Measurement

This appendix presents the procedure used to calculate the Tafel slope in Chapter 3 (Study 1). The

Tafel slope was measured for two separate cell builds to ensure repeatability. The first cell build

consisted of the 12wt.% eGDL and the second build consisted of the Bi-Layer eGDL. The GDLs

at both the anode and cathode were the same for each respective build. Each cell build also

employed a pristine catalyst coated membrane (CCM) (Nafion HP membrane, active area: 0.68

cm2, 0.3 mg/cm2 platinum loading at both the anode and cathode, IonPower Inc.).

A Scribner 850e fuel cell test station (Scribner Associates Inc.) was used to control the fuel cell.

The cell operating conditions (i.e. temperature, relative humidity, backpressure, and fuel flowrates)

were identical to the 𝑖 − 𝑉 curve measurements described in Section 3.2.3.1. For the Tafel slope

measurements, the cell input current, 𝑖 (A/cm2), was raised from 0.00 A/cm2 to 0.50 A/cm2 in the

following current step increments: 0.00, 0.03, 0.05, 0.075, 0.10, 0.20, 0.30, 0.40, and 0.50 A/cm2.

Each current step was held for 5 minutes to obtain a stable voltage response. The cell voltage, 𝐸𝑐𝑒𝑙𝑙

(V), and the internal resistance-corrected cell voltage (cell voltage corrected for ohmic losses),

𝐸𝑖𝑅−𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 (V), of the fuel cell was recorded every second using the fuel cell test station. The

last 60 seconds of each current hold was averaged to obtain a single value during stable operation.

To calculate the Tafel slope, 𝐸𝑖𝑅−𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 vs. 𝑖 was plotted as shown in Figure 1A. The Tafel

slope (i.e. slope of 𝐸𝑖𝑅−𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 vs. 𝑖 ) was obtained by fitting a line via the least squares method

to the semi-log linear region of Figure 1A (current steps of 0.03 to 0.10 mA/cm2). Current steps

above 0.10 A/cm2 were not used in the fitting process since the values deviated from the linear

trend (i.e. Tafel relationship) indicating the presence of mass transport losses.

109

Figure 1A. Plot of 𝐸𝑖𝑅−𝑓𝑟𝑒𝑒 vs. 𝑖 at 100% inlet RH used to calculate the Tafel slope, 𝑏. Tafel slope

was fitted to data points from 0.03 – 0.10 A/cm2.

The average Tafel slope, 𝑏, of the two cell builds was calculated to be 0.077 V/decade and was

used in Equation 17 in Section 3.2.3.3 to calculate the charge transfer resistance, 𝑅𝐶𝑇.