X-ray Investigations of PEMFC Gas Diffusion Layers (GDLs)...porous polymer electrolyte membrane fuel...

X-ray Investigations of PEMFC Gas Diffusion Layers (GDLs)

by

Pradyumna R. Challa

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Mechanical & Industrial Engineering University of Toronto

© Copyright by Pradyumna R. Challa, 2012

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X-ray Investigations of PEMFC Gas Diffusion Layers (GDLs)

Pradyumna R. Challa

Master of Applied Science

Mechanical & Industrial Engineering

University of Toronto

2012

Abstract

In this thesis, synchrotron radiography was utilized to image liquid water distributions in the

porous polymer electrolyte membrane fuel cell (PEMFC) gas diffusion layers (GDLs). GDLs

were compressed in an ex situ flow field apparatus with 1mm x 1mm channels, and injected with

liquid water to study the effect of current density and microstructure on through-plane GDL

liquid water distributions. The effect of the size of the water inlet on GDL liquid water

distribution was also investigated. Micro-computed tomography was employed to characterize

the effect of flow field compression on commercial and non-commercial GDLs. Porosity

distributions of compressed GDLs were compared with those of uncompressed GDLs, and the

effect of microstructure on the porosity was discussed. The experimental techniques documented

in this thesis will inform future research, while the results will help modellers generate realistic

GDL pore structures for multiphase flow simulations and validate their models.

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Acknowledgments

I am very grateful to my research advisor, Dr. Aimy Bazylak, for her never ending support and

encouragement throughout these past two years. I am also very grateful for the support of all my

good friends and colleagues at the microscale energy systems transport phenomena (MESTP)

laboratory – thank you for all the wonderful discussions and good memories. I would like to

thank James Hinebaugh, Jongmin Lee, and Ronnie Yip, in particular, for their help with

conducting experiments at the Canadian Light Source Inc., Saskatoon, SK. Additionally, I would

like to thank the research scientists, Dr. George Belev and Dr. Adam Webb of the Canadian

Light Source Inc., for their help with my imaging experiments. I would also like to thank Mr.

Shiang Law, a research associate at the Forestry department at the University of Toronto, for

accommodating my schedule on the CT scanner. Finally, I would never have been able to

complete my MASc without the vision and the emotional support of my parents, Subbarao BVR

Challa and Rukmini Challa, and my brother, Anirudh Challa. Thank you all.

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This thesis is dedicated to my parents, Dr. Subbarao BVR Challa and Rukmini Challa, for

inspiring me to pursue engineering.

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Table of Contents

Abstract ........................................................................................................................................... ii

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

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

List of Tables ............................................................................................................................... viii

List of Figures ................................................................................................................................ ix

1. Introduction ................................................................................................................................ 1

1.1 Motivation and Objectives ................................................................................................... 1

1.2 Organization of the Thesis ................................................................................................... 2

2. Background and Literature Review............................................................................................ 4

2.1 Pore Structure of a GDL ...................................................................................................... 4

2.2 Quantification of GDL Porosity ........................................................................................... 4

2.3 Effect of Compression on the Porosity of a GDL ................................................................ 6

2.4 Effect of Water Transport on the Porosity of a GDL ........................................................... 7

2.4.1 Review of Visualization Techniques for Water Transport ........................................ 7

2.4.2 Water Transport Investigations using X-ray Imaging ............................................... 8

2.5 Conclusion.......................................................................................................................... 10

3. Ex situ Synchrotron Investigations of Water Distribution in PEMFC GDLs – Flooded Inlet . 12

3.1 Introduction ........................................................................................................................ 12

3.2 Methods .............................................................................................................................. 13

3.3 Image Analysis ................................................................................................................... 14

3.3.1 Normalization .......................................................................................................... 14

3.3.2 Error Analysis .......................................................................................................... 15

3.4 Results and Discussion ....................................................................................................... 16

3.4.1 GDL Microstructure ................................................................................................. 16

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3.4.2 Surface Treatment of Felt GDLs .............................................................................. 17

3.4.3 Water Injection Rate ................................................................................................ 18

3.5 Conclusion.......................................................................................................................... 18

3.6 Figures and Tables ............................................................................................................. 20

4. Ex situ Synchrotron Investigations of Water Distribution in PEMFC GDLs – Single Point

Injection ........................................................................................................................................ 24

4.1 Introduction ........................................................................................................................ 24

4.2 Methods .............................................................................................................................. 24

4.2.1 Compression Calibration ......................................................................................... 25

4.2.2 GDL Edge Isolation ................................................................................................. 25


4.3.1 Repeatability ............................................................................................................ 26

4.3.2 Water Injection Rate ................................................................................................ 27

4.3.3 GDL Microstructure ................................................................................................. 29

4.4 Conclusion.......................................................................................................................... 29


5. Quantifying the Effect of Compression on the Through-plane Porosity Distributions of

PEMFC GDLs with Micro-computed Tomography (μCT) .......................................................... 39

5.1 Introduction ........................................................................................................................ 39

5.2 Methods .............................................................................................................................. 39

5.2.1 Compression Calibration ......................................................................................... 40

5.2.2 GDL Surface Isolation ............................................................................................. 40

5.2.3 Image Binarization ................................................................................................... 41

5.2.4 Image Processing ..................................................................................................... 41


5.3.1 Uncompressed Porosity Distributions ...................................................................... 43

5.3.2 Effect of Compression ............................................................................................. 43

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5.4 Conclusion.......................................................................................................................... 44


6.0 Conclusions & Future Work ................................................................................................... 55

6.1 Water distribution visualization ......................................................................................... 55

6.2 Microstructural Investigations ........................................................................................... 56

References ..................................................................................................................................... 59

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List of Tables

Table 3.1: Flooded Inlet Investigations: GDL material properties along with their breakthrough

pressures and average water contents.

Table 4.1: Single-point Injection Investigations: GDL material properties, experimental

conditions and the breakthrough pressures.

Table 5.1: Effect of rib/channel compression on GDL porosity.

Table 5.2: Effect of rib/channel compression on GDL thickness.

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List of Figures

Figure 1.1: A Schematic of a PEMFC. The membrane electrode assembly (MEA) consists of a

polymer electrolyte membrane (PEM) coated with catalyst layers (shown in black) and porous

cathodic and the anodic GDLs (black circles represent solid carbon fibers). Water (shown in

blue), which is generated at the cathodic GDL, hinders oxygen diffusion (shown in red).

Figure 1.2: SEM Images of various GDL microstructures: a) paper (Toray TGP-H-090), b) felt

(Freudenberg H2315), and c) cloth (AvCarb 1071).

Figure 3.1: Experimental setup: (a) A schematic of the apparatus with the following

components: compression plate (1), plastic gasket (2), GDL (3), rubber gasket (4), and base

plate (5). (b) This assembly of (1), (2), (3), (4), and (5) is bolted together with four bolts (20in-

lb/bolt). (c) An image of the apparatus and the experimental setup in the hutch at the Canadian

Light Source Inc., Saskatoon, Sk. The orientation of the beam is parallel to the channels.

Figure 3.2: Synchrotron radiographs: (a) An example radiograph obtained from synchrotron X-

ray radiography. This particular radiograph is of Freudenberg H2315 I6 when injected with

liquid water at 8μL/min. The region of interest is highlighted with a dashed line. Rib (R) and

channel (c) placement is also shown. (b) A normalized radiograph of 3.2(a) with the region of

interest highlighted (inverted for clarity).

Figure 3.3: 1D Water thickness distributions at breakthrough of (a) Toray TGP-H-090 (10 wt. %

PTFE), (b) Freudenberg H2315, (c) Freudenberg H2315 I6 (10 wt. % PTFE), (d) Freudenberg

H2315 I3 C1(10 wt. % PTFE with MPL), and (e) AvCarb 1071 (10 wt. % PTFE) at liquid water

injection rates of 1μL/min and 8μL/min.

Figure 4.1: Experimental setup: (a) A schematic of the apparatus with a close-up view of the

GDL set-up with the following components: (1) top plate, (2) GDL, (3) compression plate, and

(4) base plate. (b) This assembly of (1), (2), (3), and (4) is bolted together with four bolts. The

torque on the bolts can be adjusted to compress the GDL to a required pressure. The synchrotron

beam is incident into the plane of the page. (c) A schematic of the experimental setup.

Figure 4.2: Pressure-torque calibration curve for GDL A.

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Figure 4.3: GDL Edge Identification Process: (a) A typical radiograph obtained from imaging

the setup with synchrotron X-rays. This particular radiograph is of DURA-GDL ST400TC. (b)

The highlighted region of 4.3(a) used to determine the edges of the GDL. (c) Image after

applying the Sobel operator. (d) The edge profile of 4.3(c) with edges of the GDL identified as

points (denoted by asterisk) of maximum intensity.

Figure 4.4: Image normalization and analysis: (a) A typical radiograph obtained from imaging

the setup with synchrotron X-rays. This particular radiograph is of DURA-GDL ST400TC when

injected with liquid water at 2μL/min. (b) A normalized radiograph of 4.4(a) with the region of

interest highlighted. The GDL is highlighted along with the regions under the ribs. (Refer Image

Analysis Section for the normalization process). (c) 1D water thickness profile with error bars

obtained from the 2D water thickness map in 4.4(b).

Figure 4.5: Water thickness contour maps for GDL A (paper) obtained at the moment of

breakthrough: (a) water injection rate of 1μL/min, (b) water injection rate of 2μL/min.

Figure 4.6: Water thickness contour maps for Freudenberg H2315 I3 C1 (felt) obtained at the

moment of breakthrough: (a) water injection rate of 1μL/min, (b) water injection rate of

2μL/min.

Figure 4.7: Water thickness contour maps for DURA-GDL ST400TC (cloth) obtained at the

moment of breakthrough: (a) water injection rate of 1μL/min, and (b) water injection rate of

2μL/min.

Figure 5.1: A schematic of the (a) uncompressed and (b) compressed GDL sample holders for

μCT.

Figure 5.2: Compression calibration results: (a) pressure distribution on a Prescale pressure film

after applying a torque of 10 in-oz in the compression device. (b) average pressure calibration

curves as a function of applied torque for GDL B. The calibration was repeated twice to ensure

repeatability.

Figure 5.3: Performing edge detection on μCT images: (a) A portion of a typical through-plane

μCT slice obtained after reconstruction of GDL B compressed between the flow field and the

compression plate of the compression device. (b) The output of 4(a) after applying Canny’s edge

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detection technique. (c) Surface isolation based on the rate of change in the surface profile. The

black compounded line is the surface profile of the GDL, while the red line denotes the rate of

change in the surface profile. Refer 5.2.2 for a discussion on GDL surface isolation.

Figure 5.4: Through-plane uncompressed porosity profiles of investigated GDLs.

Figure 5.5: Through-plane porosity distributions of AvCarb GDS3250.

Figure 5.6: Effect of incremental compression: (a) Through-plane porosity distributions of GDL

B. (b) Figure that illustrates the effect of incremental compression on the thickness of the GDL,

and the average porosity under land and channel.

Figure 5.7: Through-plane porosity distributions of Dura-GDL ST400TC.

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1. Introduction

1.1 Motivation and Objectives

In the age of global warming, clean alternative energy solutions are critically needed for

achieving a sustainable energy future. Polymer electrolyte membrane fuel cells (PEMFCs), with

major applications in automotive propulsion and back-up power systems, provide a zero-local-

emission alternative to internal combustion engines and diesel generators [1-5]. PEMFCs are

electrochemical devices that can convert the stored chemical energy in hydrogen into electrical

energy. The reactions that liberate the chemical energy stored in fuels occur at the catalyst layer

in the presence of fuel and electrons. At the anode, hydrogen (H2) splits into protons and

electrons (Eq.1.1). The electrons travel to the cathode, where they combine with oxygen and

protons to form water (Eq.1.2).

(1.1)

(1.2)

(1.3)

The by-products of the chemical reactions are water and heat (Eq.1.3). A certain amount of water

generated in a PEMFC should be removed in order to facilitate the diffusion of oxygen through

the GDL (Figure 1.1), the prevention of which can lead to performance degradation. However,

excessive water removal can lead to membrane dehydration and increased protonic resistance

[6].

The porous gas diffusion layer (GDL) plays an important role in providing pathways for

reactants to reach the catalyst layer and product water to reach the exhaust channels. The GDL is

required to exhibit high electronic conductivity and provide structural stability for the membrane

electrode assembly (MEA) [7]. Although the GDL is typically treated to be hydrophobic to

enhance liquid water wicking, excess liquid water tends to accumulate and prevents reactants

from reaching the catalyst layer, thereby degrading the cell performance [7]. Specifically, these

mass-transport related voltage losses dominate other losses such as activation and ohmic losses

when the fuel cell is operated at higher current densities i.e. for applications with higher power

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requirements [7-10]. Through an improved understanding of GDL material properties, such as

porosity and hydrophobicity, new materials can be designed for enhanced water management

strategies. The motivation for this research is to understand the effect of compression and water

generation on GDL porosity. An important first step in understanding the effect of water

generation on the porosity of a GDL is visualizing the water distribution in a GDL, which is one

of the main objectives of this work. The second objective of this work is to characterize the

effect of GDL compression on the porosity of a GDL.

The direct visualization of liquid water in the GDL is needed to understand the influence of

liquid water flow rate and GDL microstructure on the overall liquid water content. Through ex

situ visualizations, water transport within the GDL can be studied independently of membrane

swelling which occurs in operating PEMFCs. Additionally, the outcomes of ex situ experiments

with adjustable inlet conditions can be compared with operating PEMFCs in order to extract

information about the microscale transport behaviour within the porous materials of the fuel cell

that are challenging to observe in situ. It is also important to note that though the conditions in a

fuel cell are transient, a study of flow through the porous electrode at steady state conditions is a

necessary first step to develop a technique to visualize water in the GDL.

1.2 Organization of the Thesis

This thesis is divided into six chapters. Chapter 1 provides a general introduction and motivation

for the research, in addition to listing the objectives of this research. Chapter 2 contains a

literature review of the water distribution visualizations in GDL, in addition to a literature review

on calculating the porosity and the effect of compression on the porosity of a GDL. The third and

fourth chapters provide a discussion of the ex situ experiments that were conducted to visualize

the water distributions in GDLs with varying microstructures using synchrotron X-ray

radiography. In Chapter 3, the liquid water invasion of a GDL from a flooded inlet condition is

reported, and in Chapter 4, the liquid water invasion of a GDL from a single-point injection is

presented. In Chapter 5, the effects of compression on GDL porosity are quantified using

computed micro-tomography. In addition to summarizing the contributions of this work,

suggestions to extend this work are presented in Chapter 6.

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Figure 1.1: A Schematic of a PEMFC. The membrane electrode assembly (MEA) consists of a

polymer electrolyte membrane (PEM) coated with catalyst layers (shown in black) and porous

cathodic and the anodic GDLs (black circles represent solid carbon fibers). Water (shown in

blue), which is generated at the cathodic GDL, hinders oxygen diffusion (shown in red).

V

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2. Background and Literature Review

2.1 Pore Structure of a GDL

The pore structure of a GDL is important as it provides pathways for reactants to reach the

catalyst layer and product water to reach the exhaust channels. Both compression and water

generation result in a decrease in the available GDL pore space for oxygen transport [7].

GDLs are typically made of polyacrylonitrile (PAN)-based carbon fibers. Commercially

available GDLs can be classified into three categories based on their microstructure: paper, felt

and cloth (Figure 2.1). In paper, individual carbon fibers are typically cut to lengths of 3–12mm,

bound using a polyvinyl alcohol (PVA) binder, impregnated with resin, and heat-treated at over

2000°C [7]. Felt GDLs are manufactured by replacing the above-mentioned binding step with a

hydro-entanglement step, resulting in fibers oriented in both the in-plane and the through-plane

directions. Cloth GDLs consist of interwoven strands of carbon fibres. The woven GDL structure

results in both macro-pores and micro-pores, whereas the random orientation of fibers in paper

and felt GDLs typically results in strictly micro-sized pores [7]. Pore sizes of both felt and paper

morphologies are typically between 10μm and 30μm [7], and the average pore diameter of the

cloth GDL is approximately 80μm [11]. The microstructure has been shown to influence the

transport properties of the GDL [12]; therefore, it is important to study the effect of compression

and water generation on these GDL microstructures.

2.2 Quantification of GDL Porosity

The porosity of a porous material can be measured using a variety of techniques, some of which

are destructive in nature. Destructive techniques including Capillary Flow Porometry (CFP) and

Mercury Intrusion Porosimetry (MIP) render samples unusable due to contamination.

Porosimetry refers to the technique of invading a porous material by a non-wetting fluid at high

pressures, whereas porometry refers to the technique of preferentially invading the pores with a

wetting liquid and subsequently forcing the liquid out of the sample with a high pressure gas [7].

Pore size distributions can be obtained from the tracked pressure and volume of the invading

fluid as pressure is increased as a function of time. MIP and CFP have been used to obtain the

characteristics of a GDL pore structure, such as pore size distribution and pore volume [7,13-18].

Although the information obtained from MIP and CFP can be used to estimate the porosity of a

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GDL, this information must be treated with caution, due to the underlying assumption in the

methodology that the porous material can be represented as a bundle of capillary tubes.

A non-destructive technique that can be used to measure the porosity of a material is gas

pyncometry [19,20]. In gas pycnometry, a sample of known solid volume ( = + ; where

is the volume of the void space of the sample, and is the volume of the material) is enclosed

in an evacuated container of known volume ( ). The evacuated container (along with the

sample) is connected to another container that is filled with a gas of known volume ( ) and

pressure ( ). When a valve connecting the two containers is opened, gas from the second

container expands into the evacuated container occupying the entire empty volume of the

container until a uniform pressure distribution ( ) is attained. Using ideal gas law, the volume

of the pores can be calculated as

(

) (2.1)

In prior investigations of the GDL, it was observed that the GDL porosity may be non-uniform

[15,21,22]. Though gas pyncometry allows us to calculate the porosity of a GDL without

destroying the sample, it does not enable the quantification of any spatial variation in the GDL

porosity.

Another non-destructive technique that has recently been successfully utilized to measure

porosity of a GDL is computed micro-tomography [23-32,32-34]. In this technique, a sample of a

GDL is imaged to obtain a three-dimensional (3D) reconstruction of its random microstructure,

which can subsequently be analyzed using image processing algorithms to quantify its porosity.

A significant advantage of employing this technique is that the information obtained from the

reconstructions can be used to study the heterogeneous pore structure. The information can also

be utilized to inform numerical models.

Ostadi et al. [32-34] utilized computed nano-tomography (nCT) to obtain a GDL reconstruction

at a spatial resolution of 680 nm and provided a detailed overview of the tomography analysis

process. They also calculated the bulk porosity of a GDL from the obtained reconstructions, in

addition to calculating the permeability and diffusivity in the in-plane and through-plane

directions. Büchi and coworkers [23,35,36] were the first group to report the use of computed

http://en.wikipedia.org/wiki/Ideal_gas

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micro-tomography (µCT) to measure through-plane porosity distributions of an uncompressed

paper GDL (Toray TGP-H-060 with 20. wt% PTFE). Through-plane porosity distribution refers

to the porosity distribution of the GDL along its thickness. They correlated the through-plane

porosity distribution with water saturation distribution in a GDL and observed that the water is

retained in the denser regions near the surfaces of the GDL.

Fishman et al. [28] employed µCT to compare the through-plane porosity distributions of paper,

felt, and cloth GDLs. They noticed that felt and paper GDLs possessed surface and core regions

in their through-plane porosity distributions, while the surface and core regions could not be

identified in a cloth GDL. When compared to a paper GDL, the felt GDL was observed to exhibit

a more uniform core region. Fishman and Bazylak [26,27,37], in their subsequent papers, also

investigated the effect of PTFE treatment and MPL application on the through-plane porosity

distributions. The PTFE treatment was observed to be non-uniform, and it predominantly

resulted in a decrease in the porosity at the surfaces of a GDL. The decrease in near-surface

porosity due to the MPL was also observed from their µCT investigations.

These investigations provided valuable insights into the heterogeneous porosity distributions of

uncompressed paper, felt, and cloth GDL materials. Although these heterogeneous porosity

distributions of uncompressed GDLs were subsequently utilized to model GDLs in numerical

models that predicted liquid water distributions in a GDL [38] or thermal conductivity of a PEM

GDL [39], the effect of compression on the through-plane porosity distributions is not yet

experimentally understood.

2.3 Effect of Compression on the Porosity of a GDL

While over-compression can damage the GDL [40,41], it is known that an optimal amount of

compression is needed to maximize the fuel cell performance [42-47]. Ge et al. [43] observed

that there is an optimal GDL compression ratio for maximizing the fuel cell performance in both

paper and cloth GDLs. They also noted that compression affected the performance of paper GDL

to a greater extent than that of the cloth GDL. Roshendal et al. [48] found that a decrease in the

average porosity led to a decreased oxygen consumption thereby decreasing the fuel cell

performance. Additionally, they noted that the change in porosity had a greater effect on the

performance of a fuel cell operating at a higher current density, compared to that at a lower

current density.

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Becker et al. [25] employed µCT imaging to investigate the effect of varying compression on the

GDL transport properties such as porosity, diffusivity, and permeability. They observed that the

porosity values decreased with compressed GDL thicknesses. However, the effect of

compression on spatially varying material properties has yet to be determined. Wang and Chen

[49] modelled the liquid water distribution in GDLs using spatially varying GDL properties and

uniform land compression ratio. However, direct experimental investigations to understand the

effect of flow field compression on the through-plane porosity are yet to be conducted.

2.4 Effect of Water Transport on the Porosity of a GDL

Water generation at the catalyst layer also has an impact on the effective porosity of a GDL. To

characterize the impact on the effective porosity of a GDL, it is important to visualize the liquid

water distribution in a GDL.

2.4.1 Review of Visualization Techniques for Water Transport

Various visualization techniques [50] including soft X-ray radiography [51,52], micro-computed

tomography [23,31,35,36,53-55] , neutron radiography [56-73], and synchrotron radiography

[54,56,74-83] have been utilized to understand water transport in GDLs. The refined focus of this

review is to discuss the use of X-ray imaging including synchrotron radiography, soft X-ray

radiography, conventional X-ray micro-tomography, and synchrotron X-ray micro-tomography.

Synchrotron radiography refers to the process of imaging using high intensity synchrotron X-

rays. These X-rays are produced from a cyclic electron accelerator (synchrotron), in which the

magnetic field and the electric field are synchronized with a travelling electron beam. Of the

methods mentioned above, synchrotron radiography has the highest intensity [56] of 1011

- 1015

photons/s/cm2 with monochromatic beams ideal for imaging fuel cells. Synchrotron radiation is

also capable of providing images with a high spatial resolution of up to 0.7μm [56]. The

temporal resolution of the images obtained using synchrotron radiography can be as low as 1s

[56], enabling researchers to gain a detailed insight of the dynamic liquid water transport

phenomena inside a gas diffusion layer (GDL).

Conventional X-ray radiography refers to the process of imaging using X-rays ranging in

energies from 100 – 225keV [51,52]. Conventional X-rays are released when an electron beam

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bombards a target (typically tungsten). Conventional X-ray radiography is an economical

imaging technique, which can be produced using a desktop machine. However their biggest

disadvantage when compared to synchrotron radiography is the conical nature of the produced

beam. In synchrotron radiography, a near-parallel beam is used for imaging. A conical beam

causes non-uniform magnification effects, which would be detrimental for investigating liquid

water in the GDL. The maximum spatial and temporal resolutions possible with conventional X-

rays are 1μm and 10s, respectively [56].

Soft X-ray radiography involves X-rays with energies less than 40keV [56]. Soft X-rays or low

energy X-rays are produced when an electron beam is bombarded onto a vanadium target. A

spatial resolution of 0.5μm and a temporal resolution of 1s have been reported with soft X-ray

radiography [51,52].

2.4.2 Water Transport Investigations using X-ray Imaging

Though various numerical models have been developed to predict the distribution of liquid water

within the PEMFC GDL [84-86], there is little availability of experimentally determined liquid

water distributions with microscale resolution due to the imaging challenges associated with the

opacity of fuel cell materials [50].

Using synchrotron X-ray micro-tomography, Flückiger and coworkers [35] visualized ex situ

water injection into a Toray TGP-H-060 GDL (20 wt. % PTFE) and obtained water saturation

distributions at a range of water invasion pressures. They observed that the water accumulated in

the denser regions of the GDLs, and a significant amount of water was retained within the GDL

even after purging. This retention of water was attributed to the non-homogenous porosity

distribution inside of a GDL, which led to the entrapment of water within the highly porous inner

layer [35]. Sinha et al. [55] also conducted an ex-situ experiment to quantify the amount of

residual water in a GDL using conventional X-ray micro-tomography. They noted that after

purging a saturated GDL with nitrogen, a drying-time of 25min was required to remove water

from the GDL. The rate of water removal was also observed to slow down at after 6min of

purging. This slowdown in the rate of water removal was attributed to the lack of evaporation

taking place from the surface of the GDL due to the lack of a temperature gradient between the

GDL and the surrounding environment.

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Manke and coworkers [78] visualized in-plane water evolution in the GDL of an operating

PEMFC by bombarding X-rays in the through-plane direction of the fuel cell. The authors

observed an eruptive water ejection mechanism, where water was ejected onto the surface of the

cathodic GDL in a cyclic manner from distinct pores. This localization of the eruptive water

ejection mechanism was attributed to the existence of a two-phase equilibrium between liquid

water and water vapour in those pores. Sasabe et al. [52] also visualized in-plane water evolution

using soft X-ray radiography. Though Sasabe et al. [52] did not report the occurrence of eruptive

water ejections; they observed that the spatial distribution of ejected water was uneven [52].

Hartnig et al. [74] observed two major liquid water agglomerations along the cross-section of the

GDL. The first accumulation was located next to the MPL, while the second agglomeration was

observed under the landing of the flow field. Water agglomeration near the MPL on the cathodic

side was attributed to the condensation of water vapour caused by the temperature gradient

across the MPL layer. Water agglomeration under the landing at the cathode was attributed to the

mass limitation of evaporation caused by excessive water accumulation. The occurrence of two

liquid agglomerations on both anodic and cathodic GDLs at higher operating current densities

has also been observed by Sasabe et al. [52].

Sasabe et al. [51,52] also suggested that the water generated at the catalyst layer was drained

from the surface cracks on the surface of the GDL. They also observed that the channel wall

wettability influenced the saturation profile of the GDL. According to the Sasabe and coworkers,

the copper flow field with hydrophilic walls resulted in increased capillary-dominated water

removal and reduced GDL water retention, thereby influencing the saturation profile of the GDL.

Lee et al. [77] investigated the dynamic distribution of water in the GDL using in-plane, in-situ

synchrotron X-ray imaging. They noted that the instant water generation at both the anodic and

the cathodic GDLs was cyclic, with approximately the same time period. Damping in the

frequency of cyclic instant water generation was observed at the cathodic GDL towards the end

of the time period for which the fuel cell was monitored. This damping phenomenon was

attributed to a decrease in the electrochemical activity at the cathode due to flooding.

Despite the temporal and spatial resolution advantages of synchrotron radiography, Schneider et

al. [81] observed that the performance of a fuel cell decreased after minutes of exposure to

synchrotron radiation. Although this effect was only observed when the entire active area of a

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fuel cell was exposed to synchrotron radiation, the authors noted that the localized effect of

synchrotron radiation on the GDL, the catalyst, and the membrane should be further investigated.

2.5 Conclusion

In this chapter, a critical literature review of microstructural investigations and water distribution

visualizations using X-ray radiography has been presented. Though significant insights into the

process of water evolution were obtained for some GDLs, a scarcity of experimental works exists

addressing the influence of GDL microstructure on porosity and liquid water transport behaviour.

Hence, the effects of compression, multiphase flows, and thermal gradients need to be

characterized using controlled experiments to understand the complex water transport

phenomena inside a fuel cell.

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Figure 2.1: SEM Images of various GDL microstructures: a) paper (Toray TGP-H-090), b) felt

(Freudenberg H2315), and c) cloth (AvCarb 1071).

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3. Ex situ Synchrotron Investigations of Water Distribution in PEMFC GDLs – Flooded Inlet

3.1 Introduction

Although a coating of polytetrafluoroethylene (PTFE) is generally applied to the GDL to

minimize water retention, liquid water accumulation is observed in operating PEMFCs

[23,35,87]. This accumulation of liquid water prevents reactants from effectively reaching the

catalyst layer and leads to performance degradation, which in turn affects the reliability of

PEMFCs. Though different microscopic modeling techniques such as pore network modeling

[38] and lattice Boltzmann modeling [88] have been developed to predict water saturation in

GDLs, experimental visualization of water distributions in GDLs is necessary for validating the

modeling results. Given that several assumptions regarding the size of the inlet are made while

modeling water transport through a GDL [89], it is also important to experimentally study the

effect of the size of the inlet on the GDL water distribution.

In this chapter, the investigations that were conducted to visualize water injected into a GDL

with a flooded inlet condition are presented. Synchrotron radiography was employed to visualize

liquid water injection through PEMFC GDLs in an ex situ flow field apparatus, to study the

dependence of liquid water content on GDL microstructure, surface treatment, and simulated

current density (water flow rate). In the microstructure study, liquid water was quantified after

injection via an ex situ apparatus for the following GDLs: paper (Toray TGP-H-090 with 10 wt.

% PTFE), treated felt (Freudenberg H2315 I6 with 10 wt. % PTFE), and cloth (AvCarb 1071

with 10 wt. % PTFE). In the surface treatment study, liquid water distributions of untreated felt

(Freudenberg H2315), treated felt (Freudenberg H2315 I6 with 10 wt. % PTFE), and treated felt

with an MPL (Freudenberg H2315 I3 C1 with 10 wt. % PTFE), were compared to determine the

effect of surface treatment on liquid water content in GDLs. In the flow rate study, the GDLs

were invaded at flow rates representative of current densities, 0.2A/cm2 and 1.6A/cm

2, to

determine the effect of water production rate on the liquid water accumulation in a GDL.

13

All visualizations were performed at the Biomedical Imaging and Therapy (05B1-1) beamline at

the Canadian Light Source Inc. (Saskatoon, Canada). Labview (National Instruments, Texas,

USA) was employed, along with a syringe pump (Harvard 11Plus: Harvard Instruments, MA,

USA) and a pressure transducer (PX309-005G5V Omega Engineering, Connecticut, USA), to

remotely control the liquid water injection into the GDL while measuring the liquid water

pressure during invasion. The liquid water pressure is measured to monitor the water invasion

process in the GDL. The X-ray beam was directed in parallel to the channels of the compression

plate, which facilitated the capture of through-plane liquid water distributions (beam orientation

into the page of Fig. 3.1(a)) with a pixel resolution of 4.4μm/pixel and a temporal resolution of

1s/frame. It has to be noted that pixel resolution is not equivalent to spatial resolution. Pixel

resolution corresponds to the width or height of the imaged region divided by the number of

pixels that span that orientation in the digital image; whereas, spatial resolution corresponds to

the minimum discernible length in the obtained raw radiographs. The obtained spatial resolution

was approximately 20μm.

3.2 Methods

The GDLs were placed in an ex situ polycarbonate sample holder, simulating compression from

a flow field plate and facilitating liquid water injection from the base of the GDL (Fig. 3.1(a)).

The compression plate consisted of parallel, 1mm square channels, spanning the width of the

GDL sample. As shown in Fig. 3.1(a), a compressible silicon gasket (375μm in thickness) and a

non-compressible clear polyethylene gasket (200μm in thickness) were included with the

material assembly in order to prevent leakage. The gasket-GDL assembly was compressed using

four bolts (diameter of 5.7mm, each torqued to 20in-lb). Liquid water was injected through the

base of the apparatus at two flow rates: 1μL/min and 8μL/min. The area of the GDL exposed to

the flooded base plate was 1cm2. These flow rates represented current densities of 0.2A/cm

2 and

1.6A/cm2, respectively, in an operating PEMFC.

Injected liquid water volume data, liquid water pressure measurements, and radiographic

visualizations were simultaneously acquired during invasion, from the initial injection of liquid

water into the apparatus until past breakthrough. A breakthrough event corresponds to the first

formation of a sample-spanning cluster of liquid-water invaded pores [18]. In total, five

commercially available GDLs (Toray TGP-H-090 with 10 wt. % PTFE, Freudenberg H2315,

14

Freudenberg H2315 I6, Freudenberg H2315 I3 C1, and AvCarb 1071 with 10 wt. % PTFE) were

injected with liquid water at two distinct flow rates of 1μL/min and 8μL/min. The results shown

below are obtained from a single parametric investigation dataset per GDL. Between each

invasion experiment, the apparatus was disassembled and dried using compressed air for

subsequent use.

3.3 Image Analysis

3.3.1 Normalization

Water content in the GDL can be quantified from the raw radiographs using Beer-Lambert’s law.

According to the Beer-Lambart equation, the intensity, I, of the beam after traveling a distance,

X, through a material is given by [77]:

(3. 1)

where I0 is the incident intensity of the beam, and μ is the attenuation coefficient of the material.

According to equation (3.1), the intensity of the beam, upon passing through the dry GDL, is

given by:

(3. 2)

where µnon-water is the attenuation coefficient of all materials combined without water present, and

Xnon-water is thickness of those materials.

Similarly, the captured intensity of the beam that passed through the wet GDL at a given instant,

j, after the initialization of invasion is given by:

(3. 3)

where µwater is the attenuation coefficient of water, and Xwater is the thickness of the water.

Combining equations (3.2) and (3.3), with the assumption that the incident intensity I0 is

constant, yields the following expression for water thickness (Xwater) within the GDL for each

pixel of our image:

15

[ ] (3. 4)

where μwater for monochromatic beams with energy of 23 keV is 0.644/cm [90].

Figure 3.2(a) is a raw radiograph of Freudenberg H2315 I3 C1 obtained during water injection at

a flow rate of 8μL/min. Figure 3.2(b) depicts the output of the normalization process from which

a 2D water thickness (Xwater) distribution in the GDL was obtained. To facilitate comparison

among the GDLs, these 2D water thickness distributions were converted to 1D water thickness

distributions which show the variation of average Xwater along the thickness of a GDL. In the

water thickness distributions, the GDL through-plane position of 0μm represents the inlet face of

the GDL, with the other end representing the flow field face of the GDL.

3.3.2 Error Analysis

Beam fluctuations and noise from the thermal fluctuations of the CCD camera can result in false

positive or negative 2D water thicknesses, which when converted into 1D water thickness

distributions, could potentially result in non-physical negative water thickness values.

Uncertainties in the reported water thickness distributions can be accounted for by quantifying

the effect of synchrotron beam fluctuations, methods of which were reported in detail by

Hinebaugh et al. [75]. The procedure outlined in that paper was employed for all images obtained

in this work; the technique is briefly summarized below.

An algorithm that matched the beam position of the raw wet-state radiograph to that of a raw

dry-state radiograph was developed and utilized. A raw dry-state radiograph refers to the

radiographs that were collected before the liquid-water/air interface reached the GDL surface,

and raw wet-state radiograph refers to the radiographs that were obtained after the water touched

the inlet face of the GDL surface. For every raw wet-state radiograph, five raw dry-state

radiographs were selected from a stack of raw dry-state radiographs with matching beam

positions, and an average of these five raw dry-state radiographs was utilized to normalize the

raw wet-state radiograph, as described in the Image Analysis section to obtain the 2D water

thickness map.

The confidence interval associated with the 1D water thickness distributions can be determined

by analyzing the dry-state radiographs separately. Through a similar process, water thickness

16

distributions can be obtained for every dry-state radiograph by matching its beam position to five

other radiographs in the dry-state stack. In the absence of noise, liquid water should not be

detected through this dry-state radiograph analysis. However, due to the noise and errors from

the beam position matching procedure employed [75], a finite water thickness was calculated and

is shown as the confidence intervals presented in the subsequent results.

3.4 Results and Discussion

The uncompressed thickness and the liquid water pressure at breakthrough for the GDLs

employed, and the average through-plane water content values of the GDLs are listed in Table

3.1. Average through-plane water content values were obtained by calculating the average 1D

water thickness values over the entire thickness of the GDL.

3.4.1 GDL Microstructure

Figures 3.3(a), 3.3(c), and 3.3(e) illustrate the water thickness distributions for Toray TGP-H-090

(10 wt. % PTFE), Freudenberg H2315 I6, and AvCarb 1071 (10 wt. % PTFE) at breakthrough

for water injection at a flow rate of 1μL/min. For this flow rate, the breakthrough pressures of

Toray TGP-H-090 (10 wt. % PTFE), Freudenberg H2315 I6, and AvCarb 1071 (10 wt. % PTFE)

were 0.98psi, 0.92psi, and 0.18psi, respectively (Table 3.1). Although Toray TGP-H-090 (10 wt.

% PTFE) and Freudenberg H2315 I6 exhibited comparable breakthrough pressures, the resulting

water thickness distributions varied significantly. At the inlet face of the GDL, Freudenberg

H2315 I6 (water thickness = 2.1mm) exhibited nearly double the water content of Toray TGP-H-

090 (10 wt. % PTFE) (0.9mm). On the other hand, the water content at the inlet face of AvCarb

1071 (10 wt. % PTFE) was approximately 30% of Toray TGP-H-090 (10 wt. % PTFE) and 15%

of Freudenberg H2315 I6.

Throughout the thickness of the GDL, Freudenberg H2315 I6 also exhibited a greater through-

plane average water content compared to both Toray TGP-H-090 (10 wt. % PTFE) and AvCarb

1071 (10 wt. % PTFE), as noted in Table 3.2. Similarly, the average through-plane water content

of Toray TGP-H-090 (10 wt. % PTFE) was 167% higher compared to AvCarb 1071 (10 wt. %

PTFE) at 1μL/min.

17

The average water content was highest in Freudenberg H2315 I6 (10 wt. % PTFE), followed by

Toray TGP-H-090 (10 wt. % PTFE) and AvCarb 1071 (10 wt. % PTFE). This could imply that

there was substantially more interconnected pores in Freudenberg H2315 I6 compared to Toray

TGP-H-090 (10 wt. % PTFE), and AvCarb 1071 (10 wt. % PTFE). It is possible that the binder

(polyvinyl alcohol [1]) used in the paper-making step of Toray TGP-H-090 (10 wt. %

PTFE) may have blocked pore connections. The presence of this binder may lead to fewer

interconnections in Toray TGP-H-090 (10 wt. % PTFE) as compared to those formed

in Freudenberg H2315 I6.

3.4.2 Surface Treatment of Felt GDLs

Figures 3.3(b)-3.3(d) illustrate the water thickness distributions for Freudenberg H2315,

Freudenberg H2315 I6 (10 wt. % PTFE), and Freudenberg H2315 I3 C1 (10 wt. % PTFE and

MPL) at breakthrough. As shown in Table 3.2, Freudenberg H2315 had the lowest average

through-plane water content of the three GDLs at a water injection rate of 1μL/min. Although the

breakthrough pressure at 1μL/min of Freudenberg H2315 (0.41psi) was approximately 25% of

that of the Freudenberg H2315 I3 C1 (1.66psi), the average through-plane water content was

similar: 0.41mm and 0.42mm, respectively.

When the 1D water thickness value at the inlet face of Freudenberg H2315 I3 C1 (10 wt. %

PTFE and MPL) was compared to that of Toray TGP-H-090 (10 wt. % PTFE) and AvCarb 1071

(10 wt. % PTFE) (0.26 mm), it was noted that the 1D water thickness value of non-MPL coated

GDLs, Toray TGP-H-090 (10 wt. % PTFE) and AvCarb 1071 (10 wt. % PTFE), were

respectively 21% and 79% of the MPL coated Freudenberg H2315 I3 C1. The counter-intuitively

high water contents observed at the inlet face of Freudenberg H2315 I3 C1 may be explained by

a delamination of the GDL from the rubber gasket, which allowed water to spread along the

surface of the rubber gasket. Since the MPL had a lower liquid water permeability, the injected

water spread laterally until there was enough pressure to penetrate the GDL, resulting in the

formation of a secondary reservoir, and therefore, a higher water content at the inlet face of

Freudenberg H2315 I3 C1. The unusually high 1D water thickness values at the inlet faces of the

other felt GDLs (Table 3.2) could have also resulted from the delamination of the GDL from the

apparatus. In this case, the cause of the delamination is attributed to the insufficient compression

18

of the untreated Freudenberg H2315 and Freudenberg H2315 I6 (10 wt. % PTFE), which

resulted in the formation of a secondary reservoir.

Among the felt GDLs, Freudenberg H2315 had the lowest average through-plane water content

(Table 3.1). More water accumulated at the inlet face of Freudenberg H2315 I6 (1.89mm)

compared to Freudenberg H2315 (1.00mm) and Freudenberg H2315 I3 C1 (1.22mm). Again, it

was counter-intuitive that the GDL with no treatment (most hydrophilic) had the lowest average

through-plane water content.

3.4.3 Water Injection Rate

The impact of increasing the liquid water injection rate from 1μL/min to 8μL/min at

breakthrough on water thickness profiles was also seen in Figs. 3.3(a)-3.3(e). Upon increasing

the rate of liquid water injection, Freudenberg H2315 showed the maximum decrease in the

average through-plane liquid water content at 51%, followed by AvCarb 1071 (10 wt. % PTFE)

(41%), Toray TGP-H-090 (10 wt. % PTFE) (24%), Freudenberg H2315 I6 (10%), and

Freudenberg H2315 I3 C1 (8%).

The water content at the inlet face (for the exception of Freudenberg H2315 I6 and AvCarb 1071

(10 wt. % PTFE)) was observed to decrease upon increasing the water injection rate from

1µL/min to 8µL/min (Figs. 3.3(a)-3.3(e)). This may be related to the variation in breakthrough

pressures at 1µL/min and 8µL/min. As can be seen from Table 3.1, the breakthrough pressures of

all the GDLs are higher at 8µL/min compared to 1µL/min. The increased pressure in the injected

water may lead to a decreased probability of liquid water spreading laterally within the GDL.

Recall that only a single GDL sample was injected at each flow rate and hence further

investigations should be conducted to conclusively identify the effect of flow rate on the GDL

saturation.

3.5 Conclusion

In this chapter, a systematic technique was outlined to obtain the 1D water thickness distributions

as a function of through-plane positions for various GDLs. Synchrotron radiography was

employed to measure the water content at breakthrough after invading GDL samples with liquid

water to simulate liquid water production in the PEMFC. The liquid water content at

19

breakthrough for five commercially available GDLs was presented, including: Freudenberg

H2315, Freudenberg H2315 I6 (10 wt. % PTFE), Freudenberg H2315 I3 C1 (10 wt. % PTFE

with MPL), Toray TGP-H-090 (10 wt. % PTFE), and AvCarb 1071 (10 wt. % PTFE). This is the

first comparison of liquid water distributions in commercially available GDLs.

Freudenberg GDLs exhibited greater average through-plane water content than Toray TGP-H-

090 (10 wt. % PTFE) and AvCarb 1071 (10 wt. % PTFE) at breakthrough. At a liquid water

injection rate of 1µL/min, the difference in the average through-plane water content between

AvCarb 1071(10 wt. % PTFE) and a) Toray TGP-H-090 (10 wt. % PTFE) was 167%, b)

Freudenberg H2315 was 340%, c) Freudenberg H2315 I6 was 545%, and d) Freudenberg H2315

I3 C1 was 352%. The average through-plane water content at 1µL/min was higher than that at

8µL/min. Upon injecting the GDLs with a higher water injection rate, Freudenberg H2315 (51%)

showed the maximum decrease in the average through-plane liquid water content, followed by

AvCarb 1071 (10 wt. % PTFE) (41%), Toray TGP-H-090 (10 wt. % PTFE) (24%), Freudenberg

H2315 I6 (10%), and Freudenberg H2315 I3 C1 (8%).

Among the felt GDLs, Freudenberg H2315 (no treatment or coatings) had the lowest average

through-plane water content. The difference in the average through-plane water content between

Freudenberg H2315 and Freudenberg H2315 I6 (10 wt. % PTFE treatment) was 46%, whereas

the difference in the average through-plane water content between Freudenberg H2315 and

Freudenberg H2315 I3 C1 (10 wt. % PTFE and MPL) was 2.7%. A larger amount of water also

accumulated at the inlet face of Freudenberg H2315 I6 (1.89mm) when compared to

Freudenberg H2315 (1.00mm) and Freudenberg H2315 I3 C1 (1.22mm).

20

3.6 Figures and Tables

Table 3.1: Flooded Inlet Investigations: GDL material properties along with their breakthrough

pressures and average water contents.

GDL Breakthrough Pressure

(psi)

Average Water Content

(mm)

Microstructure Substrate Treatment

GDL

Thickness

(µm) [19]

At

1µL/min

At

8µL/min At 1µL/min

At

8µL/min

Felt

Freudenberg

H2315

No PTFE,

No MPL 287 0.41 0.54 0.41 0.27

Freudenberg

H2315 I6

10 wt. %

PTFE 287 0.92 0.94 0.60 0.57

Freudenberg

H2315 I3 C1

10 wt. %

PTFE and

MPL

320 1.66 1.94 0.42 0.37

Paper Toray TGP-

H-090

10 wt. %

PTFE 300 0.98 1.06 0.25 0.17

Cloth AvCarb 1071 10 wt. %

PTFE 400 0.18 0.20 0.09 0.06

21

Figure 3.1: Experimental setup: (a) A schematic of the apparatus with the following

components: compression plate (1), plastic gasket (2), GDL (3), rubber gasket (4), and base

plate (5). (b) This assembly of (1), (2), (3), (4), and (5) is bolted together with four bolts (20in-

lb/bolt). (c) An image of the apparatus and the experimental setup in the hutch at the Canadian

Light Source Inc., Saskatoon, Sk. The orientation of the beam is parallel to the channels.

(a)

(b)

(c)

22

Figure 3.2: Synchrotron radiographs: (a) An example radiograph obtained from synchrotron X-

ray radiography. This particular radiograph is of Freudenberg H2315 I6 when injected with

liquid water at 8μL/min. The region of interest is highlighted with a dashed line. Rib (R) and

channel (c) placement is also shown. (b) A normalized radiograph of 3.2(a) with the region of

interest highlighted (inverted for clarity).

1mm

1mm

(a)

(b)

23

Figure 3.3: 1D Water thickness distributions at breakthrough of (a) Toray TGP-H-090 (10 wt. %

PTFE), (b) Freudenberg H2315, (c) Freudenberg H2315 I6 (10 wt. % PTFE), (d) Freudenberg

H2315 I3 C1(10 wt. % PTFE with MPL), and (e) AvCarb 1071 (10 wt. % PTFE) at liquid water

injection rates of 1μL/min and 8μL/min.

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400

Wate

r th

ickn

ess (

mm

)

GDL Through Plane-Position (μm)

(a)

1 ml/min

8 ml/min

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400

Wate

r th

ickn

ess (

mm

)


(b)

1 ml/min

8 ml/min

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400

Wate

r th

ickn

ess (

mm

)


(c)

1 ml/min

8 ml/min

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400

Wate

r th

ickn

ess (

mm

)


(d)

1 ml/min

8 ml/min

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400

Wate

r th

ickn

ess (

mm

)


(e)

1 ml/min

8 ml/min

1μL/min

8μL/min

1μL/min

8μL/min

1μL/min

8μL/min

1μL/min

8μL/min

1μL/min

8μL/min

24

4. Ex situ Synchrotron Investigations of Water Distribution in PEMFC GDLs – Single Point Injection

4.1 Introduction

Presented in this chapter are synchrotron radiographic visualization employed to visualize single

point liquid water injection through PEMFC GDLs in an ex situ flow field apparatus, for

studying the dependence of liquid water content on GDL microstructure and simulated current

density (water flow rate). In the microstructure study, liquid water was quantified after injection

via an ex situ apparatus for the following three MPL-coated 10 wt. % PTFE-treated GDLs: paper

(GDL A), felt (Freudenberg H2315 I3 C1), and cloth (DURA-GDL ST400TC). In the flow rate

study, the GDLs were invaded at flow rates of 1μL/min and 2μL/min, representative of current

densities of 0.8A/cm2 and 1.6A/cm

2, respectively. This work is motivated by recent numerical

modelling work performed in our group, where it was shown that the size of the inlet has a

significant impact on the liquid water distribution in a GDL [89].

4.2 Methods

Figure 4.1(a) shows an exploded view of the ex situ apparatus used to inject water into a GDL

from a single point, and Fig. 4.1(b) shows a collapsed view with the field of view highlighted.

The apparatus consisted of a: top plate, flow field compression plate, and base plate. A 5mm x

5mm GDL sample was placed in a 1.5mm-deep inset in the center of the top plate. The

compression plate features a configuration of alternating 1mm-wide ribs and 1mm-wide

channels, with an injection hole (0.8mm in diameter) oriented above the center landing. The top

plate and the compression plate were bolted together, and torque was applied to the bolts until a

compression of 250psi (~1.8MPa) was achieved. The bolted top/compression plates were slot

fitted into the base plate, which was secured to the stage inside the experimental hutch. Figure

4.1(c) illustrates the experimental setup. The thicknesses of the top plate (15mm) and

compression plate (15mm) were selected to minimize bowing during compression. The top plate

was machined out of optically clear polycarbonate, and the flow field compression plate and the

base plate were produced with 3D polyjet printing (Nova Product, Toronto, Canada).

All visualizations were performed at the Biomedical Imaging and Therapy (05B1-1) beamline at

the Canadian Light Source Inc. (Saskatoon, Canada). The absorption imaging technique was

25

used to measure the liquid water content in a GDL, using monochromatic X-rays at 18keV.

Labview (National Instruments, Texas, USA) was employed, along with a syringe pump

(Harvard 11Plus: Harvard Instruments, MA, USA) and a pressure transducer (PX309-005G5V

Omega Engineering, Connecticut, USA), to remotely control the liquid water injection into the

GDL while measuring the liquid water pressure during invasion. The liquid water pressure is

measured to monitor the water invasion process in the GDL, and the X-ray beam was directed

parallel to the channels of the compression plate, which facilitated the capture of through-plane

liquid water distributions with a pixel resolution of 4.4μm/pixel and a temporal resolution of

1s/frame. Radiographs were acquired from the initial injection of liquid water until the

breakthrough event had occurred.

4.2.1 Compression Calibration

To relate the applied torque to the average pressure under the ribs of a GDL, a pressure sensitive

film (Fujifilm) was inserted in between the inset surface of the top plate and the GDL. The

colour of the film changed with the applied torque, and it was analyzed using a software package

(Pressure Mapping System, FPD-8010E) developed by Fujifilm. Figure 4.2 shows the obtained

correlation between the applied torque and the average pressure under the ribs for all the GDLs

utilized.

4.2.2 GDL Edge Isolation

Edge identification in a 2D image is the first step in isolating the GDL edge from that of the

apparatus. Matlab has a number of in-built and well documented techniques such as Sobel [91],

Prewitt [92], Canny [93] etc., that can be employed to identify the solid-air interfaces in 2D

images. These techniques can be used to convert a gray scale image into a black and white

image, where white represents a solid-air interface and black represents void space. These edge

detection techniques find the edges by identifying gradients in pixel intensity values. A number

of factors, such as the surface features on the apparatus, X-ray attenuation coefficient of the

GDL-MPL, and the apparatus material, influence the selection of the edge identification

technique. Sobel and Prewitt techniques are commonly used to identify strong edges while

Canny’s technique has been proven useful when the edges are less prominent or weak [93].

26

As the edges are prominent in the raw radiograph (Fig. 4.3(a)), the Sobel operator was used to

extract the GDL edges from a raw radiograph [91]. Figure 4.3(b) shows a highlighted region of

Fig. 4.3(a) that was used to identify the edges of the GDL. Figure 4.3(c) is the resulting image

after applying the Sobel operator to identify the edges. After running the in-built edge

identification technique on the radiographs in Matlab, an edge intensity profile (Fig. 4.3(d)) i.e.,

the variation of the average pixel value along the thickness of the GDL was determined for the

edge identified image (Fig. 4.3(c)). Subsequently, the locations of local-maxima (denoted by

asterisk in Fig. 4.3(d)) in the edge intensity profile were identified as the edges of the GDL.


The uncompressed and compressed GDL thicknesses, along with liquid water breakthrough

pressures are listed in Table 4.1. This section is organized into three sub-sections: In the first

sub-section, the effect of invasion experiments at a single flow rate on distinct 5mm x 5mm

samples of a GDL is investigated. In the 2nd

sub-section, the effect of flow rate on liquid water

distribution is investigated, and in the 3rd

sub-section, the effect of GDL microstructure on the

liquid water distributions is discussed.

Using Equation 3.4 and the analysis described in section 3.3 of Chapter 3, 2D water thickness

maps were obtained for each region (Fig. 4.4(b)), which were subsequently converted into 1D

water thickness distributions (Fig. 4.4(c)). The 0μm-position in the 1D water thickness

distributions represents the inlet face of the GDL. As discussed in Chapter 3, it is important to

note that the negative water thickness values result from thermal noise in the detector, beam

fluctuations, and phase contrast effects. The error bars on the 1D water thickness distributions

(e.g. Fig. 4.4(c)) only account for the thermal noise and the beam fluctuations, and the error due

to phase contrast effects has not been quantified. The phase contrast effects are dominant at the

inlet and the outlet faces of the GDL due to the difference between the X-ray absorption

properties of the GDL and the polycarbonate top plate (Fig. 4.4(b)).

4.3.1 Repeatability

The average water content distribution for each GDL regions is shown in Fig. 4.5. For the GDLs

that have been invaded at the same water injection rate, reasonable repeatability was obtained

from the analysis of multiple samples; however, there were some exceptions, such as the

27

experiment Trial 2 at 2μL/min involving a cloth GDL (Table 4.1). Exceptions such as these can

be attributed to the non-homogenous MPL coating, which may have had a particular influence

since the GDL samples were cut from various locations of a larger supply sheet. Samples

showing high water content may have exhibited more instances of cracks in the surface.

4.3.2 Water Injection Rate

The effects of water injection rate and inlet size on liquid water distributions were investigated;

however, a statistically relevant distinction between injection rates was not observed in the

measured average water content, as shown in Table 4.1. Although limited beam time poses a

challenge, further trials would be needed to fully determine the impact of these particular flow

rates on water content. However, the widely ranging results presented in Table 4.1 suggest that a

closer examination of the characteristic flow regime expected in the GDL should be performed at

these flow rates, in the context of our empirical observations. This examination is presented

below in the form of a discussion of the non-dimensional numbers to identify the influential

forces acting in the system. The capillary number, defined as the ratio of the invading viscous

forces to the surface tension forces acting across the invading-defending fluid interface, is given

by:

(4.1)

where and V are the dynamic coefficient of viscosity and the mean velocity of the invading

fluid, respectively, and is the surface tension of the invading-defending fluid interface. The

Weber number is the product of the capillary number and the Reynolds number, which can be

written as:

(4.2)

where is the density of the invading fluid, and L is the characteristic length of the porous

media. The mean velocity, V, must be defined carefully, and since V is not known a priori, upper

and lower estimations were performed based on our experimental observations. The upper limit

of the mean velocity was determined from the thickness of a GDL (approximately t ~ 200μm

(Table 1)), its tortuosity (τ ~ 1.5 [37]) and the invasion time. The invasion time (T ~ 2s) was

defined by the shortest time over which liquid water penetrated the GDL to reach breakthrough.

28

The upper limit of the mean velocity can be obtained from:

(4.3)

The lower limit of the mean velocity was calculated from the injection flow rate (Q = 1μL/min),

and the cross-sectional area of the GDL (A = 25mm2). The lower limit of the mean velocity can

be obtained from:

(4.4)

From Equations 4.3 and 4.4, the upper and the lower limits of the mean velocity were between

the orders of 10-4

m/s and 10-6

m/s, respectively, resulting in Ca = 10-8

-10-6

and We = 10-9

-10-7

.

These values suggest that the inertial forces were negligible compared to the viscous and surface

tension forces. The viscosity ratio at room temperature in the experiments was M = 54 (log M =

1.72).

To qualitatively understand the underlying drainage mechanism occurring in these GDLs, the

capillary numbers and the viscosity ratio (viscosity ratio at room temperature in the experiments

was M = 54 or log M = 1.72) were superimposed onto the phase diagram for flow through porous

media, developed by Lenormand et al. [94]. Three zones with distinct drainage mechanisms,

namely capillary fingering regime, stable displacement regime, and viscous fingering regime,

were identified by Lenormand et al. [94]. In the capillary fingering regime, the capillary forces at

the pores dominated over the viscous forces in the invading fluid, while in the stable

displacement and the viscous fingering regime, the viscous forces dominated over the capillary

forces. With the experimental conditions employed in this thesis, the flows are positioned within

the capillary fingering regime, where the saturation is expected to be independent of flow rate. It

has to be noted that even though in an operating fuel cell (Δop ~ 90°C [6]) the viscosity ratio (M

= 17) is lower than the viscosity ratio (M = 54) at room temperature (Δexp ~ 20°C), the expected

regime according to the phase diagram developed by Lenormand et al. [94] does not change.

While in the literature, it is generally expected that liquid water transport in the GDL exists

within the capillary fingering regime of a porous media phase diagram [84], in the experiments

performed here, a closer inspection is required. Rebai and Prat [95] presented a phase diagram to

illustrate the limit of the invasion percolation regime for drainage in the GDL. If we consider a

29

network thickness of N=10 (a 200m-thick GDL with an average pore size of 20m) combined

with the estimated Ca values (10-8

-10-6

), according to the phase diagram presented by Rebai and

Prat [95], the drainage processes in this paper coincide with the limit of the invasion percolation

regime, where viscous effects may be non-negligible.

Lenormand et al. [94] noted in their work that the phase diagram is material specific, and that the

limits that determine the flow regime change from material to material. Also, the phase diagrams

developed by Lenormand et al. [94] and Rebai and Prat [95] were developed for 2D structured

pore networks, and hence may not precisely represent the flow regime in GDLs that have random

3D pore structures. Hence, further investigations are required to determine if there is an effect of

flow rate on average GDL saturation.

4.3.3 GDL Microstructure

Figures 4.5-4.7 illustrate the water thickness contour maps from a single invasion investigation

of GDL A (paper), Freudenberg H2315 I3 C1 (felt), and DURA-GDL® ST400TC (cloth) at

breakthrough for water injection rates of 1μL/min and 2μL/min, respectively. At a flow rate of

2μL/min, the breakthrough pressures of GDL A, Freudenberg H2315 I3 C1, and DURA-GDL

ST400TC were 3.34psi, 2.14psi, and 1.10psi, respectively (Table 4.1). The cloth GDL (DURA-

GDL ST400TC) had the lowest breakthrough pressure, which was expected due to its large pore

sizes. The lateral spread of water can be defined as the spread of water in the in-plane direction,

which is a good indicator of the interconnectivity in the GDL pore network. Compared

to DURA-GDL ST400TC (Fig. 4.7), the lateral spread was greater in both the Freudenberg

H2315 I3 C1 (Fig. 4.6) and the GDL A (Fig. 4.5). Also, from Figs. 4.5(a) and 4.6(a), it can be

seen that in both felt and paper GDLs, lateral spread was greater at the low flow rate when

compared to the higher flow rate.

4.4 Conclusion

In this chapter, the influences of current density and gas diffusion layer (GDL) microstructure on

liquid water distribution in a polymer electrolyte membrane fuel cell (PEMFC) GDL were

investigated using an ex situ study, where liquid water was introduced into a GDL via a single-

point injection and simultaneously imaged using synchrotron X-ray radiography. A water

injection device was developed to successfully inject water into a GDL from a single point, along

30

with a technique to apply the desired compression on the GDL. The liquid water contents at

breakthrough for three structurally distinct 10 wt. % PTFE-treated, MPL-coated GDLs were

presented, including: GDL A (paper), Freudenberg H2315 I3 C1 (felt), and DURA-GDL

ST400TC (cloth).

Although the average through-plane water content at a given flow rate was generally constant for

all GDLs investigated here, the lateral spread in the in-plane direction was the highest in the felt

and paper GDLs implying greater pore connectivity in their pore network, compared to the cloth.

The average water content in a GDL also remained generally constant when various samples

obtained from the same GDL manufacturing lot were injected with water. The variations

observed in liquid water distributions are expected, given the random nature of the GDL as well

as the heterogeneous MPL-coating.

Current densities of 0.8A/cm2 and 1.6A/cm

2 were simulated using flow rates of 1μL/min and

2μL/min, respectively. A definitive trend could not be identified between the average water

content in a GDL at an injection rate of 1µL/min and the average water content in the GDL

invaded at a flow rate of 2µL/min. Finally, it was also observed that the water content in the

GDL, when injected from a single point, was lower when compared to the observed average

water content, when injected from a flooded inlet condition, as reported in chapter 3. This is in

agreement with what has been seen in the modelling experiments [89].

31


Table 4.1: Single-point injection investigations: GDL material properties, experimental

conditions, and the breakthrough pressures.

GDL Experiment

#

Flow

Rate

(µL/min)

Uncompressed

thickness

(µm)

Compressed

thickness

(µm)

Breakthrough

Pressure

(psi)

Average

Water

Content

(mm)

GDL A

Trial 1 1 300 203 2.21 0.008

Trail 2 1 300 203 2.96 0.004

Trial 3 1 300 203 1.79 0.010

Trial 1 2 300 186 1.73 0.009

Trail 2 2 300 146 3.34 0.117

Trial 3 2 300 129 1.29 0.050

Freudenberg

H2315 I3 C1

Trial 1 1 280 195 1.61 -0.008

Trail 2 1 280 212 4.62 -0.009

Trial 3 1 280 186 2.91 0.061

Trial 1 2 280 186 2.14 0.018

Trail 2 2 280 199 2.13 0.059

Trial 3 2 280 168 3.12 0.080

DURA-GDL

ST400TC

Trial 1 1 400 202 1.30 0.117

Trail 2 1 - - - -

Trial 3 1 - - - -

Trial 1 2 400 185 1.10 0.041

Trail 2 2 400 176 0.74 0.138

Trial 3 2 400 194 1.16 0.043

32

Figure 4.1: Experimental setup: (a) A schematic of the apparatus with a close-up view of the

GDL set-up with the following components: (1) top plate, (2) GDL, (3) compression plate, and

(4) base plate. (b) This assembly of (1), (2), (3), and (4) is bolted together with four bolts. The

torque on the bolts can be adjusted to compress the GDL to a required pressure. The synchrotron

beam is incident into the plane of the page. (c) A schematic of the experimental setup.

(a) (b)

(c)

1

2

3

4

33

Figure 4.2: Pressure-torque calibration curve for GDL A.

y = 44.594x R² = 0.9358

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7

Ave

rag

e P

ressure

(p

si)

Applied torque (in-oz)

34

Figure 4.3: GDL Edge Identification Process: (a) A typical radiograph obtained from imaging

the setup with synchrotron X-rays. This particular radiograph is of DURA-GDL ST400TC. (b)

The highlighted region of 4.3(a) used to determine the edges of the GDL. (c) Image after

applying the Sobel operator. (d) The edge profile of 4.3(c) with edges of the GDL identified as

points (denoted by asterisk) of maximum intensity.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 50 100 150 200 250

Avera

ge P

ixel In

tensity

Through-plane Position, X (pixels)

GDL

Thro

ugh-p

lane P

ositio

n,

X

(a)

(b) (c)

(d)

35

Figure 4.4: Image normalization and analysis: (a) A typical radiograph obtained from imaging

the setup with synchrotron X-rays. This particular radiograph is of DURA-GDL ST400TC when

injected with liquid water at 2μL/min. (b) A normalized radiograph of 4.4(a) with the region of

interest highlighted. The GDL is highlighted along with the regions under the ribs. (Refer Image

Analysis Section for the normalization process). (c) 1D water thickness profile with error bars

obtained from the 2D water thickness map in 4.4(b).

-0.05

0.00

0.05

0.10

0.15

0 50 100 150 200

1D

Wate

r T

hic

kness (

mm

)

GDL Through-plane Position (m)

(a)

(b)

(c)

36

Figure 4.5: Water thickness contour maps for GDL A (paper) obtained at the moment of

breakthrough: (a) water injection rate of 1μL/min, (b) water injection rate of 2μL/min.

(a)

(b)

Water Thickness (mm)


GDL In-plane Position (μm)


GD

L T

hro

ugh

-pla

ne P

ositio

n (

μm

) G

DL T

hro

ugh

-pla

ne P

ositio

n (

μm

)

37

Figure 4.6: Water thickness contour maps for Freudenberg H2315 I3 C1 (felt) obtained at the

moment of breakthrough: (a) water injection rate of 1μL/min, (b) water injection rate of

2μL/min.

(a)

(b)



GD

L T

hro

ugh

-pla

ne

Positio

n (

μm

) G

DL T

hro

ugh

-pla

ne P

ositio

n (

μm

)



38

Figure 4.7: Water thickness contour maps for DURA-GDL ST400TC (cloth) obtained at the

instance of breakthrough: (a) water injection rate of 1μL/min, and (b) water injection rate of

2μL/min.

(a)

(b)





GD

L T

hro

ugh

-pla

ne P

ositio

n (

μm

) G

DL T

hro

ugh

-pla

ne P

ositio

n (

μm

)

39

5. Quantifying the Effect of Compression on the Through-plane Porosity Distributions of PEMFC GDLs with Micro-computed Tomography (μCT)

5.1 Introduction

The porosity of a GDL is an important design parameter for GDL manufacturers, as it has a

direct impact on the material permeability and effective diffusivity of reactants to the catalyst

layer [7]. In this chapter, the effect of compression on the pore structures of three MPL-coated

GDLs, paper (AvCarb GDS3250), felt (GDL B), and cloth (Dura-GDL STC 400ST) GDLs, is

investigated using microscale computed tomography (μCT). Through-plane porosity

distributions from this study may be used to inform future GDL models

5.2 Methods

GDL samples of size 5mm x 4mm were first scanned using μCT in the uncompressed state using

a sample holder, and subsequently scanned in a compressed state utilizing a compression device

that consisted of a base plate and two compression plates. The base plate had two channels that

were 4mm long, 1mm wide, and 0.5mm deep, which represented the gas channel in an operating

fuel cell. Figures 5.1(a) and 5.1(b) respectively show the uncompressed GDL apparatus and the

compression setup that consisted of a compression device, gaskets and GDLs. Silicon gaskets

(250μm in thickness) were used in the compression device to ensure an even distribution of

pressure throughout the apparatus. Due to high user fees, the devices were designed to maximize

the number of material samples imaged during each scan. The uncompressed GDL device was

rapid prototyped (Nova Product Development Services Inc., Toronto, Canada), while the

compression device was fabricated with polycarbonate using conventional machining. The

assembled devices were subsequently scanned in a μCT machine (Skyscan 1172, Belgium) to

acquire the raw radiographs necessary for 3D GDL reconstructions.

Fishman et al. [82] observed that the porosity of a GDL is batch dependent. Given the batch

dependent nature of the porosity, care was taken to obtain samples from the same sheet received

from the supplier. Even while sampling the GDL from a single sheet, care was taken to not

sample the GDL from the same region. This was done to obtain a more representative porosity

profile of a GDL.

40

5.2.1 Compression Calibration

Similar to the procedures described in Chapter 4, pressure sensitive films were placed between

the compression plate and the gaskets to obtain pressure distributions, which were analyzed

using the Fuji Digital Analysis System for Prescale (Tekscan Inc., Boston, MA) to obtain the

average compression on the GDL (Fig. 5.2(a)). Fig. 5.2(b) shows the relationship between

applied pressure and applied bolt torque. From Fig. 5.2(b), it can be seen that both the GDL

samples that were compressed together produced similar pressure distributions and that the

obtained results were repeatable.

5.2.2 GDL Surface Isolation

GDL surface isolation from the compressed device was a crucial step in quantifying the effect of

compression on GDL porosity. A number of factors such as the surface features on the apparatus,

and the X-ray attenuation coefficients of the MPL and the apparatus material, influenced the

selection of the surface identification technique. Ultem, Teflon, and polycarbonate were intially

considered as candidate materials, and ultimately, polycarbonate was chosen for its relatively

highest absorption contrast with the MPL.

To detect GDL surfaces, an algorithm has been developed in Matlab utilizing the built-in

Canny’s edge identification technique. Figure 5.3(a) is a portion of a single through-plane slice,

and Fig. 5.3(b) is the resulting image after the solid-air interfaces have been identified using

Canny’s edge identification technique. After identifying the edges in a through-plane slice, the

edge intensity profile was obtained for that slice by averaging the intensity along the through-

plane direction of the GDL. Fig 5.3(c) shows the edge intensity profiles of all the through-plane

slices in the digital GDL reconstruction (~750 through-plane slices). Subsequently, a surface

intensity profile (denoted by a black compounded line in Fig. 5.3(c)) is generated by averaging

all the individual edge intensity profiles shown in Fig. 5.3(c). The surface information profile

was further converted into a rate of change of intensity profile that denoted the instantaneous

change in the surface along the through-plane direction. This rate of change of intensity profile

(denoted by a red line in Fig. 5.3(c)) was utilized to identify the surfaces of the GDL that are in

contact with the polycarbonate device. The first maximum instantaneous change in the surface

(circled in Fig. 5.3(c)), which denoted a shift from a darker region into a brighter region (Fig.

5.3(b)), was identified as the MPL/compression plate interface. Similarly, the last minima

41

(circled in Fig. 5.3(c)) that denoted a shift from a brighter region into a darker region (Fig.

5.3(b)), was identified as the GDL/landing or GDL/channel interface.

5.2.3 Image Binarization

Various thresholding techniques have been proposed in the past to binarize a GDL: Fishman et

al. [96] used Otsu’s method, whereas Jhong et al. [97] used a visually determined threshold and

AMIRA’s filament tracing method to binarize a GDL image. In this work, Otsu’s method was

used to binarize the GDL. Otsu's method involves the use of an iterative algorithm to select a

threshold that minimizes the statistical intra-class variance of the histogram. A threshold divides

the histogram into foreground and background segments, while a statistical intra-class variance

of the histogram is defined as a weighted average of the statistical variances (square of the

standard deviation) of the foreground and background segments [98].

Three-dimensional GDL reconstruction can be binarized in two distinct ways: a) obtaining an

individual threshold for every voxel-thick in-plane slice using the Otsu’s method, or b) obtaining

a single threshold for the entire 3D GDL reconstruction using the Otsu’s method. Since the

greyscale histograms of the GDL substrate and the MPL substrate are not similar, in this work it

was necessary to binarize the in-plane slices individually to obtain a realistic porosity value of

both the MPL and the GDL.

5.2.4 Image Processing

A typical through-plane cross-section of a reconstructed GDL is shown in Fig. 5.3(a). Fishman et

al. [28] observed that the minimum representative cross-sectional area for determining the GDL

through-plane porosity distributions was 1mm2. This minimum representative area ensured that

porosity distributions of GDLs could be determined within a repeatability of 4%. Therefore, the

porosity profiles obtained from compressed GDLs with a cross-sectional area of 2mm2 were

considered reasonable for the calculations presented here.

While the uncompressed GDL reconstructions were analyzed using the technique described by

Fishman et al. [28] to obtain through-plane porosity distributions, the compressed GDL

reconstructions were first analyzed to identify the interfacial locations of the MPL/compression

plate and the GDL/land. An interface identification algorithm was developed using Matlab to

42

identify the GDL surfaces that were in contact with the polycarbonate compression device.

Subsequently, the isolated GDL was binarized based on a threshold greyscale value obtained

from Otsu's technique. Subsequently, the through-plane porosity distribution was obtained by

calculating the porosity values of voxel-thick in-plane slices as a function of their through-plane

position. The process of obtaining the through-plane porosity distributions from compressed

GDLs is summarized below:

1) Step 1: Reconstruction

a) The raw projections are reconstructed using software developed by Skyscan.

2) Step 2: GDL Isolation

a) Edge Identification: The edge identification algorithm was employed on all

through-plane slices to identify the solid/air interfaces.

b) Surface Profile: The edge profiles of all the through-plane slices were averaged to

obtain a surface profile.

c) The rate of change of the intensity of the surface profile was obtained and the

surfaces of the GDL were identified based on the first local maxima and the local

minima.

3) Step 3: Image Binarization

a) The in-plane slices of the isolated GDL were binarized individually using a

threshold obtained from Otsu’s algorithm

4) Step 4: Porosity Calculation

a) The porosities of in-plane slices were calculated based binarized images.

43


5.3.1 Uncompressed Porosity Distributions

Figure 5.4 contains the through-plane porosity distributions of uncompressed paper, felt, and

cloth GDLs, which shows that the through-plane porosity of paper (AvCarb GDS3250) was

consistently higher than felt (Freudenberg H2315 I3 C1) and cloth (Dura-GDL ST400TC). In

these results, the 0μm-position corresponds to the surface of the GDL that would be in contact

with the catalyst layer in an operating PEFMC. Although all the GDLs were coated with MPLs, a

distinct MPL region could only be identified for the felt GDL. For paper and cloth GDLs,

however, the MPL appeared impregnated into the GDL itself. Consequently, the MPL region

shown in Fig. 5.4 was defined only for the felt GDL. Average porosities of all GDLs, which can

be obtained by averaging the porosity along the thickness of a GDL, were summarized in Table

5.1. Paper had the highest average porosity (0.81), followed by cloth (0.79), and felt (0.71).

5.3.2 Effect of Compression

Figures 5.5-5.7 illustrate the effect of compression on the regions under the landings and the

channels of compressed paper, felt, and cloth, respectively. The reduction in the thickness of the

GDL (Table 5.2) at 250psi is the highest for paper GDL (50%), followed by cloth GDL (49%)

and felt GDL (17%). It is interesting to note that although paper GDL has the maximum

thickness change due to compression at 250 psi, the porosity of the compressed GDL in the bulk

regions remain fairly unchanged (Fig. 5.5). This is also the case for the felt GDL (Fig. 5.6(a)),

indicating that the surface regions of the GDL were more susceptible to compression than the

bulk region of the GDL, i.e. compression has a non-uniform effect on the porosity distribution of

paper and felt GDLs. For a cloth GDL, however, a uniform reduction in its through-plane

porosity distribution was observed at 250 psi (Fig. 5.7). Since cloth GDLs were comprised of

strands of woven carbon fibers, the compression was uniformly translated throughout the

thickness of the material, as opposed to the paper and felt GDLs that were comprised of

randomly oriented carbon fibers. Table 5.1 shows that the change in average porosity under the

landing at a compression of 250psi was maximum in cloth (37%), followed by felt (14%) and

paper (9%). Similarly, the maximum change in average porosity under the channel at a

compression of 250 psi was observed in cloth (36%), followed by felt (11%), and paper (1%).

44

The effect of compression (250psi) on the average porosity of paper, felt, and cloth GDLs was

also summarized in Table 5.1. In general, the average porosity was higher under the channel

compared to that under the landing. Moreover, in felt and cloth GDLs, the compressed porosity

distributions under the landing and channel (Figs. 5.6 and 5.7), track each other, i.e. the average

porosity under the land and the channel is within 3% of each other. However, for the paper GDL,

the average porosity under the channel is 8% higher than the average porosity under the landing.

This difference, which can also be seen in the through-plane porosity distributions (Fig. 5.5), can

be attributed to the delamination of a portion of the GDL surface under the channel due to the

applied landing-channel compression scheme. The hydro-entanglement step in felt GDL

manufacturing process and the weaving step in the cloth GDL manufacturing process make the

fibers in felt and cloth GDLs less susceptible for breakage.

Felt was also compressed to 175psi, 250psi, and 325psi to determine the effect of incremental

compression on the porosity. From Fig. 5.6(a) it can be seen that incremental compression did

not significantly affect the bulk or surface regions of the felt. As the compression increased from

175psi to 250psi, the GDL thickness decreased by 5%; thickness further decreased by 3% when

the same GDL was compressed from 250psi to 325psi (Table 5.2). Nevertheless, the average

porosities under the landing and channel (at 250psi and 325 psi) remained within 1% of the

average porosity at 175psi (Fig. 5.6(b)). This suggests that felt may be compressed for the benefit

of enhanced electrical conductivity without compromising the oxygen diffusivity, which can

potentially lead to increased performance of the PEMFC.

5.4 Conclusion

In this work, a systematic technique using micro-computed tomography was developed to

characterize the effect of compression on paper, felt, and cloth GDLs under the influence of a

channel and rib configuration that would be present in an operating fuel cell. Uncompressed

porosity profiles were obtained prior to carrying out the compression investigation with the aid

of a polycarbonate apparatus. A linear torque-compression correlation was employed to

determine the appropriate compression for each GDL. Polycarbonate was identified as a viable

material in order to maximize the absorption contrast between the MPL and the compression

device for ex-situ visualizations. A GDL surface isolating algorithm was developed in Matlab to

separate the GDL from the polymer surface of the device.

45

From the uncompressed scans, it was observed that the porosity of paper GDL was consistently

higher throughout its thickness when compared to the porosities of the felt and cloth GDLs. The

average porosities of uncompressed paper, felt, and cloth GDLs was 0.81, 0.79, and 0.71,

respectively. The uncompressed scans also showed that the effects of compression in a GDL are

not uniform throughout the thickness of the felt and paper GDLs. In paper and felt, the effect of

compression was greater on the surface regions than on the bulk regions of these GDLs. For the

cloth GDL, however, the effects of compression were uniform throughout its thickness. The

reduction in the thickness of the GDL at 250psi was highest for paper (50%), followed by cloth

(49%) and felt (17%). The maximum change in average porosity under the landing at a

compression of 250psi was observed in cloth (37%), followed by felt (14%) and paper (9%).

Similarly, the maximum change in average porosity under the channel at a compression of 250

psi was also observed in cloth (36%), followed by felt (11%), and paper (1%). In addition, in felt

and cloth GDLs, the compressed porosity distributions under the landing and channel tracked

each other, i.e. the average porosity under the landing is within 3% of the average porosity under

the channel. However, for the paper GDL, the average porosity under the channel is 8% higher

than the average porosity under the landing.

In the felt GDL, upon applying a compression of 175psi, there was a 13% decrease in the

thickness. Subsequent increases in applied compression yielded further decreases in GDL

thickness of 3% for 250psi, and 5% for 325psi. However, the average porosity under land and

channel remained within 1% of the average porosity at 175 psi. This suggests that felt may be

compressed with the benefit of enhanced electrical conductivity without compromising on the

oxygen diffusivity, which can potentially lead to increased performance of the PEMFC.

46


Table 5.1: Effect of rib/channel compression on GDL porosity.

GDL

Microstructure

GDL

Name

Average

Uncompressed

Porosity

(U)

Average Compressed

Porosity (250psi) % Change

Land

(L)

Channel

(C)

(C-

L)/L (L-U)/U (C-U)/U

Paper AvCarb

GDS 3250 0.81 0.74 0.80 8 -9 -1

Felt GDL B 0.71 0.62 0.63 3 -14 -11

Cloth Dura-GDL

ST400TC 0.79 0.50 0.51 3 -37 -36

47

Table 5.2: Effect of rib/channel compression on GDL thickness.

GDL

Microstructure

GDL

Name

Uncompressed

Thickness

(μm)

(UT)

Compressed Thickness

(μm) (CT)

% Change at

250psi

175psi 250psi 325psi (UT-CT)/UT

Paper AvCarb

GDS 3250 305 N/A 153 N/A 50

Felt GDL B 276 239 228 222 17

Cloth Dura-GDL

ST400TC 415 N/A 210 N/A 49

48

Figure 5.1: A schematic of the (a) uncompressed and (b) compressed GDL sample holders for

μCT.

Uncompressed Sample Holder

GDL

GDL

Base Plate

Gasket

Compression Plate

b) a)

49

Figure 5.2: Compression calibration results: (a) pressure distribution on a Prescale pressure film

after applying a torque of 10 in-oz in the compression device. (b) average pressure calibration

curves as a function of applied torque for GDL B. The calibration was repeated twice to ensure

repeatability.

y = 34.988x R² = 0.8509

y = 32.296x R² = 0.8285

0

50

100

150

200

250

300

350

400

0 5 10 15

Avera

ge P

ressure

(psi)

Torque Applied (in-oz)

Round 1Round 2Linear (Round 1)Linear (Round 2)

b) a)

Avera

ge

Pre

ssure

(psi)

50

Figure 5.3: Performing edge detection on μCT images: (a) A portion of a typical through-plane

μCT slice obtained after reconstruction of GDL B compressed between the flow field and the

compression plate of the compression device. (b) The output of 4(a) after applying Canny’s edge

detection technique. (c) Surface isolation based on the rate of change in the surface profile. The

black compounded line is the surface profile of the GDL, while the red line denotes the rate of

change in the surface profile. Refer 5.2.2 for a discussion on GDL surface isolation.

-0.02

-0.02

-0.01

-0.01

0.00

0.01

0.01

0.02

0.02

0.00

0.05

0.10

0.15

0.20

0.25

0 50 100 150 200 250 300 350 400 450 500

Rate

of

ch

an

ge in

th

e S

urf

acfe

Pro

file

Ed

ge I

nte

nsit

y P

rofi

le

Through-plane Position, X (m)

a) b)

c)

Thro

ugh-p

lane P

ositio

n,

X

GDL Surface

GDL Surface

Compression Plate

Base Plate

GDL

51

Figure 5.4: Through-plane uncompressed porosity profiles of investigated GDLs.

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 50 100 150 200 250 300 350 400

Po

rosit

y


AvCarb GDS3250 (Paper)

GDL B (Felt)

Dura-GDL ST400TC (Cloth)FELTMPL

52

Figure 5.5: Through-plane porosity distributions of AvCarb GDS3250.

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 50 100 150 200 250 300 350 400

Poro

sity


Average Uncompressed

Average under Land

Average under Channel

Uncompressed Compressed under Landing Compressed under Channel

53

Figure 5.6: Effect of incremental compression: (a) Through-plane porosity distributions of GDL

B. (b) Figure that illustrates the effect of incremental compression on the thickness of the GDL,

and the average porosity under land and channel.

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 50 100 150 200 250 300 350 400

Poro

sity

GDL Through-plane Position (mm)

Average UncompressedAverage under Land (175psi)Average under Channel (175psi)Average under Land (250psi)Average under Channel (250psi)Average under Land (325psi)Average under Channel (325psi)

0

60

120

180

240

300

0.50

0.60

0.70

0.80

0.90

1.00

175 250 325

Co

mp

ressed

GD

L T

hic

kn

ess (m

m)

Avera

ge P

oro

sity

Appiled Compression (psi)

Land Porosity

Channel Porosity

Thickness

b)

a)

Compressed Porosity under a Landing

Compressed Porosity under a Channel

Compressed Thickness

Uncompressed Compressed under Landing (175psi) Compressed under Channel (175psi) Compressed under Landing (250psi) Compressed under Channel (250psi) Compressed under Landing (325psi) Compressed under Channel (325psi)

54

Figure 5.7: Through-plane porosity distributions of Dura-GDL ST400TC.

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 50 100 150 200 250 300 350 400

Poro

sity


Average Uncompressed

Average under Land (250psi)

Average under Channel (250psi)

Uncompressed Compressed under Landing Compressed under Channel

55

6.0 Conclusions & Future Work

6.1 Water distribution visualization

A critical literature review of water distribution visualizations of GDLs using X-ray radiography

revealed a gap in the experimental work addressing the influence of a GDL microstructure and

current density on liquid water transport in the GDL. Additionally, the assumptions regarding the

size of the inlet while modeling water transport through a GDL needed to be verified as well.

Accordingly in this thesis, a systematic technique was outlined to obtain 2D water thickness

distributions and 1D through-plane water thickness distributions for commercial and non-

commercial GDLs with varying microstructures using synchrotron radiography.

In Chapter 3 the design of an ex situ water injection setup to simulate the compression and water

generation occurring in an operating PEMFC was reported. Water was injected into GDLs after

flooding the inlet face of the sample holder to investigate the effect of GDL microstructure and

current density on commercially available GDLs. The liquid water content at breakthrough for

five commercially available GDLs (Freudenberg H2315, Freudenberg H2315 I6, Freudenberg

H2315 I3 C1, Toray TGP-H-090, and AvCarb 1071), with varying microstructures and surface

treatments, was presented. Freudenberg GDLs exhibited greater average through-plane water

content than Toray TGP-H-090 and AvCarb 1071 GDLs at breakthrough. For all the GDLs, the

average through-plane water content at 1µL/min was higher than the average through-plane

water content at 8µL/min. Among the Fruedenberg GDLs, Freudenberg H2315 had the lowest

average through-plane water content. A larger amount of water also accumulated at the inlet face

of Freudenberg H2315 I6 (1.89mm) when compared to Freudenberg H2315 (1.00mm) and

Freudenberg H2315 I3 C1 (1.22mm). This was the first ever experimental comparison of liquid

water distributions in commercially available GDLs.

Subsequently, in Chapter 4, the development of a new water injection device to successfully

inject water into a GDL from a single point was reported. The objective again was to investigate

the influences of current density and GDL microstructure on liquid water distribution in the GDL

using synchrotron radiography. A technique was also developed to measure the applied

compression on the GDL. The liquid water contents at breakthrough for three structurally distinct

PTFE-treated, MPL-coated GDLs were presented, including: GDL A (paper), Freudenberg

56

H2315 I3 C1 (felt), and DURA-GDL ST400TC (cloth). Although the average through-plane

water content at a given flow rate was generally constant for all GDLs investigated here, the

lateral spread in the in-plane direction was the highest in the felt and paper GDLs implying

greater pore connectivity in their pore networks, compared to the cloth. For a given flow rate, the

variations observed in average water contents are expected, given the random nature of the GDL

as well as the heterogeneous MPL-coating. Current densities of 0.8A/cm2 and 1.6A/cm

2 were

simulated using flow rates of 1μL/min and 2μL/min, respectively. A definitive trend could not be

identified between the average water content in a GDL at an injection rate of 1µL/min and the

average water content in the GDL invaded at a flow rate of 2µL/min. The overall average water

content reported in Chapter 4 was significantly lower than the overall average water content

reported in Chapter 3. This variation in the water content was due to the change in the size of the

inlet, which is in agreement with modelling work by Hinebaugh et al. [89].

Although the experiments reported in this thesis were conducted at room temperature, they

provide valuable data that can inform the choice of a GDL microstructure. However, the addition

of thermal gradients across the GDL will help simulate realistic fuel cell operation. The results

presented in Chapters 3 and 4 were obtained after imaging GDLs using the absorption contrast

imaging technique. In the future, phase contrast imaging can be investigated for its applicability

to image water distributions in GDLs. Also in the future, water should be injected from multiple

injection points to better represent the conditions in a fuel cell. Finally, the phase diagrams found

in the literature used to characterize the flow through the GDLs were based on 2D structured

pore network models. Phase diagrams based on GDL materials should be used to determine

operating regimes.

6.2 Microstructural Investigations

A critical literature review of microstructural investigations on GDLs revealed a scarcity of

experimental investigations addressing the effect of compression on the through-plane porosity

distributions, which are necessary for accurately modelling multiphase flow through a PEMFC

GDL. In Chapter 5, the development of a non-destructive technique to characterize the effect of

compression on paper, felt, and cloth GDLs under the influence of a channel and rib

configuration was reported. Uncompressed porosity profiles were initially obtained by imaging

an uncompressed GDL. The same GDL sample was then compressed and imaged. Polycarbonate

57

was identified as viable material when looking to maximize the absorption contrast between

itself and MPL for ex situ visualizations. A GDL surface isolating algorithm was developed in

Matlab to distinguish the GDL from the polymer.

From the uncompressed GDLs, it was observed that the porosity of the paper GDL was

consistently higher throughout its thickness when compared to the felt and the cloth GDLs. The

average porosity of uncompressed paper, felt, and cloth GDLs was 0.81, 0.79, and 0.71,

respectively. From the compressed GDLs, it was observed that the effects of compression were

not uniform throughout the thickness of the felt and paper GDLs. In paper and felt, it was also

observed that the effect of compression was greater on the surface regions than on the bulk

regions of these GDLs. For the cloth GDL, however, the effects of compression were uniform

throughout the thickness of the GDL. Also, in felt and cloth GDLs, the compressed porosity

distributions under the landing and channel followed the same trend i.e., the average porosity

under the landing is within 3% of the average porosity under the channel. However, for the paper

GDL, the average porosity under the channel is 8% higher than the average porosity under the

landing.

Upon applying a compression of 175psi on the felt a 13% decrease in the thickness was

observed, followed by a 5% decrease for an additional 75psi, and a further 3% for subsequent

compression by another 75psi. The average porosity under land and channel, however, was

within 1% of the average porosity at 175 psi. This suggests that felt GDL may be compressed

with the added benefit of enhanced electrical conductivity without compromising the oxygen

diffusivity, which can potentially lead to increased performance of the PEMFC. The results

presented in Chapter 5 will provide the required insights for modelling the effect of compression

on spatially varying porosity of a GDL, and can be used for stochastic generation of GDLs to

model liquid water distributions in PEMFC GDLs.

It was also found that the porosity determinations in Chapter 5 were sensitive to thresholding.

Though Otsu’s technique has been utilized in the literature for binarizing GDL tomograms, the

method should be validated. To validate this thresholding method, the calculated porosity should

be compared to the porosity of the same GDL sample obtained from another non-destructive

technique such as gas pyncometry. Additionally, the intrusion of the GDL into the channel was

58

not fully studied in Chapter 5. The accurate description of GDL channel intrusion may be

informative to modellers studying the effect of GDL compression on PEMFC performance.

59

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