development of conductive polymer membranes for energy ......development of conductive polymer...

Development of Conductive Polymer Membranes for Energy Applications

by

Jingwen Wang

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

Department of Mechanical and Industrial Engineering University of Toronto

© Copyright by Jingwen Wang 2012

ii

Development of Conductive Polymer Membranes for Energy

Applications

Jingwen Wang

Master of Applied Science

Department of Mechanical and Industrial Engineering University of Toronto

2012

Abstract

In this thesis, three types of conductive membranes were fabricated and characterized for

potential energy applications such as fuel cells and solar photovoltaics. First, a single layer

conductive polypyrrole (PPy) membrane was synthesized and activated. Through image

analysis, surface pore geometry changes were analyzed. The single layer PPy membrane was

proposed as a possible additional layer or coating in polymer electrolyte membrane fuel cells.

Next, a novel adaptive trilayer PPy membrane was fabricated. The membranes were activated,

and characterized through changes in surface wrinkle, roughness and contact angle. A dynamic

range of surface properties were observed. Lastly, conductive fibrous membranes were

fabricated with electrospinning. Two methods were utilized to spin conductive fibers including

the incorporation of multi-walled carbon nanotubes (MWCNT) in polystyrene (PS) and the

utilization of vapor phase polymerization (VPP) to chemically synthesize PPy on electrospun

FeCl3/PS oxidant fibers. Properties including fiber morphology, thermal stability and

conductivity were characterized.

iii

Acknowledgments

I would like to show my sincere gratitude to my supervisors, Dr. Aimy Bazylak and Dr. Hani

Naguib, who have provided me with tremendous guidance and support through my studies. I

would like to thank Aaron Price, Linus Leung and Reza Rizvi; I would not have been able to

complete this work without you. I would also like to thank James Hinebaugh for all the help and

great ideas. I would like to acknowledge my undergraduate research student Steven Botelho for

his assistance on electrospinning. Everyone at nano-imaging and chem store at Chemistry

Department, University of Toronto, your help with chemistry knowledge and your inspirations

are greatly appreciated.

I would like to thank all MESTP and SAPL members for not only being good colleagues but

great friends. You made the two years pass by really fast. Without any one of you, it would not

have been the same! Many thanks to all my friends, your company, support and encouragement

meant a lot to me.

The Natural Sciences and Engineering Research Council of Canada (NSERC), Bullitt

Foundation, the Canada Foundation for Innovation (CFI), NSERC Canada Research Chair

(CRC), and University of Toronto are gratefully acknowledged for their financial support.

iv

Dedication

I would like to dedicate this thesis to my parents. Your love and support kept me going. Thanks

for everything that you have provided for me, I would not have gotten this far without you.

v

Table of Contents Abstract .......................................................................................................................................... ii

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

Dedication ...................................................................................................................................... iv

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

List of Tables ................................................................................................................................ vii

List of Figures ............................................................................................................................ viii

Abbreviations & Nomenclature ................................................................................................. xii

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

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

1.2 Motivation and Objective ......................................................................................................... 1

1.3 Organization of Thesis ............................................................................................................. 2

Chapter 2 Background and Literature Review ....................................................................... 3

2.1 Introduction .............................................................................................................................. 3

2.2 Polymer Electrolyte Membrane Fuel Cell ................................................................................ 3

2.2.1 Gas Diffusion Layer ................................................................................................ 4

2.2.2 Water Management .................................................................................................. 4

2.2.3 Water Management Solutions ................................................................................. 4

2.3 Conductive Polymers ............................................................................................................... 5

2.3.1 Polypyrrole .............................................................................................................. 6

2.4 Electrospinning ........................................................................................................................ 6

2.4.1 Electrospun Conductive Polymers ........................................................................... 7

2.5 Conductive Polymers in Energy Applications ......................................................................... 9

2.6 Figures .................................................................................................................................. 10

Chapter 3 Investigation of Electroactive Polymers for the PEMFC GDL .......................... 14

3.1 Introduction ............................................................................................................................ 14

3.2 Motivation and Objective ....................................................................................................... 14

3.3 Experimental .......................................................................................................................... 15

3.3.1 Experimental Materials .......................................................................................... 15

3.3.2 Membrane Fabrication ........................................................................................... 15

vi

3.3.3 Characterization Methodology .............................................................................. 15

3.4 Results & Discussion ............................................................................................................. 16

3.5 Conclusion .............................................................................................................................. 17

3.6 Figures .................................................................................................................................. 18

Chapter 4 Development of a Novel Electro-active Polypyrrole Trilayer Membrane ......... 24

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


4.3 Experimental .......................................................................................................................... 24


4.3.2 Membranes Fabrication ......................................................................................... 25

4.3.3 Characterization Methodology .............................................................................. 25

4.4 Results and Discussion ........................................................................................................... 28

4.5 Conclusion .............................................................................................................................. 30

4.6 Tables .................................................................................................................................. 32

4.7 Figures .................................................................................................................................. 33

Chapter 5 Development of Conductive Fibrous Polymer Film ............................................ 44

5.1 Introduction ............................................................................................................................ 44


5.3 Experimental .......................................................................................................................... 44


5.3.2 Sample Fabrication ................................................................................................ 45

5.3.3 Characterization ..................................................................................................... 46

5.4 Result and Discussion ............................................................................................................ 47

5.5 Conclusion .............................................................................................................................. 50

5.6 Tables .................................................................................................................................. 52

5.7 Figures .................................................................................................................................. 54

Chapter 6 Conclusions .............................................................................................................. 67

Chapter 7 Future Works .......................................................................................................... 69

References ..................................................................................................................................... 71

vii

List of Tables

Table 4.1 Summary of membrane fabrication conditions. All membranes

were polymerized for 6 hours.

Pg. 32

Table 5.1 Fabrication parameters employed in electrospinning two polymer

blends.

Pg. 51

Table 5.2 Thermal degradation temperature for MWCNT/PS fibers. Pg. 52

viii

List of Figures

Figure 2.1 Schematic of polymer electrolyte membrane fuel cell. Pg. 10

Figure 2.2 SEM images of carbon fiber based GDL materials: (a) Toray TGP-

H-120 paper, (b) SGL Sigracet 10AA felt, (c) AvCarb 1071HCB

cloth.

Pg. 11

Figure 2.3 PPy electrochemical redox reactions (a) Anionic system with

mobile anion. Anions are removed when reduced and incorporated

when oxidized (b) Cationic system with mobile cations. Cations

are removed when oxidized and incorporated when reduced.

Pg. 12

Figure 2.4 Electrospinning schematic that shows the three key components:

high voltage supply, syringe and grounded metallic collector.

Pg. 13

Figure 3.1 Proposed flow channel design incorporating electroactive polymer

thin film along the surface of the GDL in a PEMFC.

Pg. 18

Figure 3.2 Schematic of the electrochemical cell apparatus for the

electrochemical polymerization of PPy.

Pg. 19

Figure 3.3 Schematic showing the microscopy-based apparatus employed for

surface pore measurements. Raw images were collected by the

CCD camera and uploaded to the computer.

Pg. 20

Figure 3.4 Microscopic images of porous PPy thin film under 2 and 5 V

activation.

Pg. 21

Figure 3.5 Percentage of total surface pore area over a period of 30 min for 2

and 5 V electrical activation potentials.

Pg. 22

Figure 3.6 Histogram of pore size distribution before and after a 5 V

activation.

Pg. 23

Figure 4.1 Schematic of the electrochemical cell apparatus employed for the

electro-polymerization of PPy.

Pg. 33

ix

Figure 4.2 (a) Schematic showing the microscopy-based apparatus employed

for measuring surface wrinkles during material activation. Raw

images were collected by the CCD camera and uploaded to the

computer. (b) Image showing the placement of the electrodes on

PPy membrane that is fixed onto a glass slide.

Pg. 34

Figure 4.3 Schematic showing the apparatus employed for surface roughness

measurements.

Pg. 35

Figure 4.4 Schematic showing the microscopy-based apparatus employed for

contact angle measurements. Raw images were collected by the

CCD camera and uploaded to the computer.

Pg. 36

Figure 4.5 SEM images of PPy trilayer membranes fabricated with ip =

0.1mA/cm2 (Membrane B) and ip = 1.0 mA/cm2 (Membrane E).

(a) Membrane B at 200x and 500x magnification, (b) Membrane E

at 200x and 500x magnification. Membrane B is visibly smoother

with smaller nodules, compared to Membrane E, which has larger

nodules distributed heterogeneously.

Pg. 37

Figure 4.6 Raw optical images of a tri-layer PPy membrane fabricated with

an ip of 0.1 mA/cm2, showing the changes in surface morphology

associated with Va = (a) 0 V, (b) 0.5 V, (c) 1.0 V, (d) 1.5 V and (e)

2.0 V. Each activation took place for 1min before photo was taken.

The length bar represents 150 µm.

Pg. 38

Figure 4.7 Percentage of wrinkled area increase as a function of Va, where the

most significant increase in wrinkled area was observed for

Membrane B.

Pg. 39

Figure 4.8 Impact of activation potentials on surface roughness. The lightly

shaded region indicates that membranes A, D, and E provide an

overall increase (~70%) in surface roughness, Ra, with increasing

activation potential, Va. In contrast, the darker region indicates that

membranes B and C provide significant increases (~200%) in Ra

with increasing Va.

Pg. 40

x

Figure 4.9 Side view images obtained with fluorescence microscopy to

determine the impact of activation potential on wettability. Liquid

droplets (3 µL) were dyed with a dilute solution of fluorescence to

elucidate the liquid/gas interface. Contact angle at Va = 0 and 2 V

are shown for membranes fabricated with: ip = (a) 0.05mA/cm2,

(b) 0.1 mA/cm2, (a) 0.3 mA/cm2, (a) 0.5 mA/cm2 and (e)

1.0mA/cm2. The length bar represents 500 µm.

Pg. 41

Figure 4.10 Relationship between the contact angle, θ, and Va. An increase in θ

was observed for all 5 membranes, where membrane B had the

highest overall increase in θ measurements. The maximum

standard deviation for each θ is 3°.

Pg. 42

Figure 5.1 Schematic of electrospinning setup consisted of (A) Syringe

mounted on a syringe pump, (B) High voltage source, (C) Needle,

(D) Grounded current collector, and (E) Glove box for humidity

control.

Pg. 53

Figure 5.2 Schematic of vapor phase polymerization setup employed for

chemical polymerization of Py on FeCl3/PS fibers. An aspirator

was used to create vacuum. Samples were placed directly onto the

stainless steel wire mesh sample holder for polymerization.

Pg. 54

Figure 5.3 A screen shot of Universal Analysis software with percentage

weight and derivative of weight change curves with respect of

temperature, as well as the calculated Tonset.

Pg. 55

Figure 5.4 Schematic of a four point probe tip with four collinearly and

evenly spaced needles at 1 mm apart.

Pg. 56

Figure 5.5 SEM images and fiber diameter distribution histogram of

electrospun MWCNT/PS fibers with various MWCNT

concentrations: (a) pure PS fibers (b) 1% MWCNT/PS (c) 3%

MWCNT/PS, (d) 5% MWCNT/PS, and (e) 10% MWCNT/PS.

Pg. 57

Figure 5.6 TEM image of 10% MWCNT/PS fiber (a) along the fiber (b) cross

section view of the fibers.

Pg. 58

xi

Figure 5.7 TGA thermograms of MWCNT/PS fibers. All samples were

heated from 0 to 500 °C at a rate of 20 °C/min.

Pg. 59

Figure 5.8 Two point electrical conductivity measurements of MWCNT/PS

electrospun fibrous films as a function of MWCNT concentration.

Pg. 60

Figure 5.9 SEM images of (a) pure 40% FeCl3/PS oxidant fibers, (b) fiber

diameter distribution histogram of 40% FeCl3/PS fibers. High

resolution SEM images of PPy coated oxidant fibers at 20k ×

magnification fort different polymerization time (c) 60 min, (d)

100 min, and (e) 140 min.

Pg. 61

Figure 5.10 PPy SEM images of (a) pure 60% FeCl3/PS oxidant fibers, (b)

fiber diameter distribution histogram of 60% FeCl3/PS fibers.

High resolution SEM images of PPy coated oxidant fibers at 20k ×

magnification fort different polymerization time (c) 60 min, (d)

100 min, and (e) 140 min.

Pg. 62

Figure 5.11 TEM images of 60% FeCl3/PS fiber polymerized for 140 min. (a)

Cross sectional view of PPy/PS fiber. (b) Cross sectional view of

fibers joint together with the growth of PPy coating.

Pg. 63

Figure 5.12 Four point probe conductivity measurements with respect to

polymerization time for PPy coated 40% and 60% FeCl3/PS fibers.

Pg. 64

xii

Abbreviations & Nomenclature Chemical formulas (PPy3)+(DEHS)- Polypyrrole di(2-ethylhexyl) sulfosuccinate C Carbon DMF Dimethylformamide EDMA Ethylene glycol dimethacrylate FeCl3 Ferric chloride FeTS Ferric p-toluenesulfonate HEMA Hydroxyethyl methacrylate Li Lithium LiTFSI Bis(trifluoromethane)-sulfonimide lithium salt PANi Polyaniline PC Propylene carbonate PEO Polyethylene oxide PMMA Poly(methyl methacrylate) PPy Polypyrrole PPy(ClO4) Polypyrrole perchlorate PPy(DBS) Polypyrrole dodecylbenzenesulfonate PPy(SO3H)-DEHS) Polypyrrole sulfonic acid-bearing di(2-ethylhexyl) sulfosuccinate PS Polystyrene Pt Platinum PTFE Polytetrafluoroethylene PTh Polythiophene PVA Polyvinyl acetate Acronyms CNT Carbon nanotube DWCNTs Double-walled carbon nanotubes EAP Electroactive polymer GDL Gas diffusion layer MEA Membrane electrode assembly MWCNTs Multi-walled carbon nanotubes PEMFC Polymer electrolyte membrane fuel cell PVDF Polyvinylidene fluoride

xiii

SEM scanning electron microscopy SWCNTs Single-walled carbon nanotubes TEM Transmission electron microscopy TGA Thermal gravimetric analyzer VPP Vapor-phase polymerization Variables

ip Polymerization current density A Area P Perimeter ∆ε Change in % wrinkled area Va Activation potential εVa Percent wrinkled area at activation potential εo Percent wrinkled area at 0 V θ Contact angle Tonset Onset temperature

1

Chapter 1 Introduction

1.1 Preamble

The increasing global energy demand has driven the development of new energy technologies

over the past two decades. Technologies such as fuel cells, solar photovoltaics, supercapacitors,

and batteries are under active research for decreased production cost, higher efficiency and

improved performance. New materials are being investigated to address these barriers.

Conductive polymers are a class of materials with attractive and tunable properties. Properties

such as electrical conductivity, thermal conductivity and surface morphology can be tuned

through synthesis conditions, as well as with external stimuli. In recent years, an increasing

number of research projects have been performed on the application of conductive polymers for

various energy applications such as polymer electrolyte membrane fuel cells (PEMFC). PEMFC

is an energy conversion device that converts chemical energy from a fuel to electrical energy

through chemical reactions. The PEMFC is especially attractive due to its zero local green house

gas emissions, with only water and heat as by products, its high power density, and portability.

Recent research and development for PEMFCs has mainly focused on improving catalyst

utilization, mitigating water management issues and improving proton conductivity. PEMFC is

one of the main research areas where conductive polymers are playing an important role. For

example, conductive polymers have been studied as corrosion resistive coatings on metallic

bipolar plates [1].

1.2 Motivation and Objective

The main objective of this thesis is to develop micro-structured conductive, porous polymer

membranes for energy applications. Initially, a single layer electroactive membrane is

investigated. This work focuses on the application of an active coating on the gas diffusion layer

(GDL) for improved water management in PEMFCs. Next, an active trilayer conductive

polymer membrane is designed. The purpose of this work is to further investigate tunable

surface properties upon activation. Finally, electrospinning technique is used to fabricate high

porosity conductive polymer membranes with tunable properties such as fiber morphology,

thermal stability, and electrical conductivity.

2

1.3 Organization of Thesis

This thesis is organized into seven chapters. Chapter 1 includes the general introduction,

background and motivation. In-depth background, literature survey, fabrication methods, and

current energy applications of conductive polymers, as well as a brief background on PEMFC,

water management problem and current solutions are presented in Chapter 2. In Chapter 3, the

fabrication of single layer active polypyrrole (PPy) membrane is presented, and pore size change

upon activation of the membrane is investigated. The application of active polymer membrane

in the PEMFC is proposed. In Chapter 4, surface morphological changes of electroactive trilayer

PPy membrane are investigated through surface roughness, wrinkle formation, and contact angle

analysis. Electrospun conductive fibrous membranes are characterized in Chapter 5. Physical,

thermal, and electrical properties of the fibrous membranes are analyzed and compared. Finally,

conclusions and future work are presented in Chapter 6 and 7.

3

Chapter 2 Background and Literature Review

2.1 Introduction

As mentioned in the previous chapter, conductive polymers have been studied in various energy

applications such as PEMFC. In this chapter, PEMFC is first introduced as a potential energy

application for conductive polymers. Water management issues in the PEMFC and current

solutions with the incorporation of novel material are discussed. Background information on

conductive polymers, specifically PPy is then presented. Previous studies on the effects of

activation on PPy membrane morphology changes are examined. Next, electrospinning is

introduced as the methodology of fabricating conductive polymer fibrous membrane. Lastly,

current applications of conductive polymers in energy technologies are reviewed.

2.2 Polymer Electrolyte Membrane Fuel Cell

PEMFC is a promising clean energy technology that has attracted much attention in recent

years. It generates electrical energy through the electrochemical reaction of hydrogen and

oxygen. Therefore, only water and heat are generated as by products. PEMFC also has the

highest power density among other fuel cell technologies that makes it more attractive for a

variety of applications in portable power and automotives [2]. The PEMFC consists of: bipolar

plates, membrane electrode assembly (MEA) and gas diffusion layers (GDLs) [2] as shown in

Figure 2.1. Bipolar plates are typically made with graphite or metal blocks; they are engraved

with flow channels for hydrogen and oxygen distribution. The MEA is a sandwiched structure

with Nafion proton exchange membrane and platinum catalyst coated on both sides. The MEA

needs to be hydrated to ensure sufficient proton transport. The GDL is a carbon fiber based

product that exhibits a set of properties which will be discussed in the following subsection.

One of the major obstacles that the PEMFC faces is water flooding due to the accumulation of

product water in the GDL and flow channels. In the next three subsections, GDL functionalities

and its effect on water management, as well as current water management solutions are

discussed.

4

2.2.1 Gas Diffusion Layer

When PEMFC is in operation, water is generated at the reaction site, transported through the

GDL and into the flow channels predominantly via diffusion and capillary forces [3]. And it

finally gets purged out of the fuel cell. The GDL is a key component in the PEMFC and must

exhibit several properties to ensure fuel cell functionality. The GDL must be gas permeable to

allow reactant gas transport to reach the reaction site. It must be water permeable to allow

product water to transport away from the reaction site. It also needs to be electronically

conductive, thermally conductive and have structural integrity [4]. The most widely used GDL

materials are carbon fibre based products such as carbon paper, felt and cloth due to their high

porosity and high electrical conductivity as shown in Figure 2.2. GDLs are usually treated with

polytetrafluoroethylene (PTFE) to increase hydrophobicity. The understanding of GDL

properties and functionalities is very important for better understanding its effect on water

management in the PEMFC.

2.2.2 Water Management

In addition to the product water, water is supplied to the fuel cell with the humidified reactant

gas since MEA hydration is critical for proton exchange [3]. Water accumulation may lead to

flooding in the cathode flow channels and block the pores in the GDL. Cathode flooding hinders

oxygen transport through the GDL to the reaction sites, which leads to performance degradation.

Especially at high current density, water production rates are higher than their removal rate, and

water accumulation becomes more severe [3]. Therefore, liquid water management within and

on the surface of the GDL will benefit PEMFC operation.

2.2.3 Water Management Solutions

The incorporation of novel materials in the fuel cell to improve water management has only

been explored in a limited number of studies [5, 6]. Strickland et al. [5] polymerized ethylene

glycol dimethacrylate (EDMA) and hydroxyethyl methacrylate (HEMA) in the flow channels

creating thin porous hydrophilic wicks. They reported that a 62% increase in peak power was

achieved with polymer wicks in comparison to a cell without wicks [5]. Ge et al. [6] integrate

two strips of absorbent polyvinyl acetate (PVA) sponge into a flow field to redistribute water

throughout the cell. Since the PVA strips are placed at the gas inlets, the strips were mainly used

5

to eliminate pre-humidification of the reactant gases. These solutions were investigated in the

anticipation of guiding or absorbing water away from the flow channels without any alterations

to the existing fuel cell components. Chen et al. [7] reported that rough surfaces with anisotropic

grooves could increase surface hydrophobicity. Larger pores resulted in lower capillary

pressures, as reported in the model presented by Ghazanfari et al. [8]. The change in surface

roughness and pore geometry of the GDL can be critical in water management.

2.3 Conductive Polymers

Conductive polymers are a favorable class of materials mainly due to their ease of synthesis,

environmental stability and their intrinsic conductive properties [9]. Conductive polymers have

backbones with highly mobile delocalized electrons when “doped”. The doping agent, also

known as the dopant, is an impurity introduced to the polymer molecular structure during

synthesis that allows electrons to hop from one polymer chain to another. Conductive polymers

such as PPy, polyanline (PANi), and polythiophene (PTh) have molecular blocks that are able to

recognize environmental stimuli and respond to these stimuli in a repeatable manner [9]. This

subclass of conductive polymers is also referred to as electroactive polymers (EAPs).

A diverse range of synthesis and processing methods can be used in EAP fabrication which can

result in unique material properties and physical appearance [9]. Desired chemical, electrical

and mechanical properties are achieved by molecular organization, and can be further

manipulated and controlled by applied potential. Electrical stimulation causes the polymer to

change its oxidation states, which is induced by the injection or removal of electrons. In

response to the transfer of electrons to and from the polymer, dopant ions are inserted or

withdrawn from the polymer chain from an electrolytic source [10]. The transfer of dopant ions

results in the expansion or contraction of the polymer chain [11]. The ability to undergo redox

reactions repeatedly is a unique and attractive characteristic of EAPs. Since EAPs’ physical,

chemical, and electrical properties are interrelated [9], controlled redox reaction can potentially

allow the tuning for desired membrane surface properties such as roughness.

6

2.3.1 Polypyrrole

PPy is an ionic EAP of particular interest because it can be synthesized with a wide range of

methodologies, it exhibits high conductivity, environmental stability [9] and it is biocompatible

[12]. When PPy undergoes a redox reaction, both anions and cations can be mobile. When the

mobile ions are cationic, the PPy layer will expand during reduction and contract during

oxidation. The opposite effect occurs for a system with mobile anions. Figure 2.3 illustrates the

redox process of PPy, where PPy is synthesized with polymerized with n amount of connected

five-membered Py rings and doped with A-, m indicates the length of PPy chain. Figure 2.3(a)

describes the PPy electrochemical redox process in an anionic system. Where dopant anion A- is

incorporated during oxidation and removed from the polymer chain when reduced. When the

anions are too large to move or the movement is slow, cations X+ are incorporated during

reduction and removed upon oxidation as shown in Figure 2.3(b).

Active PPy membranes have been studied primarily for their actuation behaviour, their high

strain capabilities and fast redox responses [13-16]. Research has also focused on volume

changes of single layer PPy films [17, 18] and surface property changes of such single layer

films during activation [10, 19, 20]. Chainet et al. measured the surface roughness of lithium

perchlorate doped polypyrrole (PPy(ClO4)) film on a platinum electrode using scanning

tunnelling microscopy [20]. Upon application of activation potentials between -0.5 and 0.1 V,

the mean surface roughness decreased from 219 to 75 nm. The authors attributed this decrease

in surface roughness to uneven membrane swelling. Teh et al. measured the change in contact

angle of sodium dodecylbenzenesulfonate doped polypyrrole (PPy(DBS)) films due to applied

activation potentials ranging from -0.9 to 0.9 V, and they found that the contact angle increased

by 60° [10]. These works had illustrated the range of surface property changes that were

possible with PPy films; however, in these studies the immersion in an aqueous electrolyte for

activation was required. Also the effects of fabrication conditions on activation behaviours have

not been investigated.

2.4 Electrospinning

Electrospinning is a simple, cost effective and well established technique in fabricating highly

porous fibrous membranes. These fibrous membranes can exhibit similar appearances as carbon

7

paper GDL [21]. A schematic is shown in Figure 2.4 to demonstrate the basic working principle

of electrospinning. There are three key components: a high voltage source (in kV range), a

syringe filled with a polymer solution, and a grounded metallic collector. During electrospinning

a high voltage (5 - 30 kV) is applied between the needle tip and the collector. At a critical

voltage, a conical fluid structure known as Taylor cone is formed at the tip of the needle. The

repulsive force of the charged polymer overcomes the surface tension of the solution; as a result

a charged jet erupts from the tip [21]. The charged jet accelerates towards the region with lower

potential, the solvent evaporates, and dried polymer fibers are deposited onto the grounded

collector. Parameters including solution properties (polymer molecular weight, viscosity, and

solution conductivity), environmental properties (temperature and humidity), and operation

parameters (solution feed rate, voltage, distance between needle and collector) [21] need to be

carefully considered to produce fibers with desired properties.

2.4.1 Electrospun Conductive Polymers

Electrospun conductive polymers are promising material choices for a broad range of

applications, such as electronics, sensors, actuators [22], biomedical applications [23], as well as

the emerging energy applications including batteries, supercapacitors, fuel cells, and solar cells

[24, 25]. Conductive polymers cannot be dissolved in a solvent or melted by heat. Their poor

processibility results in difficulties in electrospinning conductive polymers on their own.

Therefore, a few techniques are commonly used to overcome this issue. These techniques

include the addition of a spinnable polymer [26, 27], the addition of conductive particles such as

carbon nanotubes in a polymer matrix [28-34], using polymer precursors and converting into

conductive polymer in the second step [35-37], and using a core-shell coaxial electrospinning

strategy and removing the non-conductive core or shell [38-41].

2.4.1.1 Electrospun Carbon Nanotubes in a Polymer Matrix

Carbon nanotubes (CNTs) are the most studied conductive filler in electrospun fibers for

improved electrical and mechanical properties [28-34]. Sung et al. [30] compared the electrical

conductivity between solvent casted and electrospun poly(methyl methacrylate) (PMMA) films

containing 1 to 5 wt% of MWCNTs. Well aligned MWCNTs were observed through scanning

electron microscopy (SEM) and transmission electron microscopy (TEM) imaging. However,

8

regardless of the MWCNT concentration, the conductivities of electrospun fibrous films were in

the order of 10E-10 S/cm, eight orders of magnitude lower than that of the chemically casted

films. The authors attributed the low conductivity measurements from electrospun films to their

high porosity and the fact that MWCNTs were fully embedded in the insulating PMMA matrix.

Mazinani et al. used styrene-butadiene-styrene as a co-polymer to improve CNT dispersion in

polystyrene (PS). Various types of CNTs were studied including, single-walled carbon

nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs) and MWCNTs [31].

Electrical and mechanical properties with the addition of co-polymer were enhanced under the

percolation threshold at 4%. The highest conductivity (3.7E-4 S/cm) was measured with 5%

SWCNTs without the addition of copolymers; however, large amount of beads were present.

2.4.1.2 Electrospun Fibers with PPy

Since PPy cannot be dissolved in a solvent or melted by heat, it cannot be electrospun on its

own. Vapor phase polymerization (VPP) is often used to produce conductive PPy fibers. Nair et

al. [38] studied electrospun PS fibers with two types of oxidant, ferric chloride (FeCl3) and

ferric p-toluenesulfonate (FeTS). A comparison of PPy growth and sheet conductivity were

presented. Highest conductivity achieved was 5E-3 S/cm with PS-TS-PPy fibers. Although PS-

TS-PPy has a much higher degree of cystallinity compare to PS-Cl-PPy, conductivities of the

two fibrous mats were similar. The author concluded that the overall connectivity of the fibers

within the mat was the limiting factor of sheet conductivity. Chronakis et al. [26] electrospun

polyethylene oxide (PEO) with water soluble di(2-ethylhexyl) sulfosuccinate doped PPy

([(PPy3)+(DEHS)-]) and sulfonic acid-bearing di(2-ethylhexyl) sulfosuccinate doped PPy

(PPy(SO3H)-DEHS)) blends. Similar conductivities were measured for PEO/[(PPy3)+(DEHS)-]

and PEO/ PPy(SO3H)-DEHS), both on the order of 10E-4 S/cm. Upon the removal of insulating

PEO, the non-conducting barriers in between PPy chain were eliminated. Thus, conductivity of

pure [(PPy3)+(DEHS)-] fibers were 2 orders of magnitude higher at 2.7E-02 S/cm. Ketpang et

al. [36] polymerized PPy on MWCNT reinforced polyvinylidene fluoride (PVDF) fibers. The

conductivities of PVDF/MWCNT, PPy/PVDF, PPy/PVDF/MWCNT fibers were measured to be

2.88E-08, 4.02E-02, 3.88E-01 S/cm respectively. The addition of MWCNT did not only

improve mechanical properties of the fibers, it also improved the conductivity of the fibrous

film by forming a conductive network in the composites.

9

2.5 Conductive Polymers in Energy Applications

Conductive polymers have been studied in various energy applications including: dye-sensitized

solar cell counter electrodes [42-46], and novel electrode materials for supercapacitors, and

batteries [47-51]. A number of studies have also incorporated conductive polymers such as PPy

and PANi in fuel cell applications including: modified Nafion membranes [52-55], platinum

catalyst supports [56], corrosion-resistive coatings for fuel cell bipolar plates [1, 57].

The ability to produce fibrous membranes with electrospinning further broadened conductive

polymers applicability in energy applications. These applications include fuel cell catalyst [58-

61] and catalyst supports [62-65], Li-ion battery electrodes [66-68], supercapacitors electrodes

[69-71], and hydrogen storage [72]. The utilization of electrospinning in these applications are

mainly due to the desirable properties of electrospun fibrous films such as, large specific area

which provides large reaction site, good durability, and compatibility with a wide range of

materials. In fuel cells, catalyst facilitates hydrogen and oxygen redox reactions. Traditional

catalyst is made of carbon black supported Pt particles (Pt/C). Pt/C has poor durability and can

be poisoned by carbon monoxide easily. Electrospun platinum/ruthenium, platinum/rhodium

and Pt nanowires [58] have been studied and compared with Pt/C and showed improved

durability and performance in a PEMFC. Combination of conventional Pt/C on electrospun Pt

nanowires was also studied showing improved oxygen reduction reaction activity [60]. In

addition, the traditional catalyst support, carbon black, suffers degradation especially under

harsh operation conditions. Electrospun PANi and carbon nano-fibers had been explored as

novel catalyst support [62]. The electrospun catalyst support provides large reaction site, high

electro-catalytic activity, and durability. Current research in energy technologies utilizes

conductive polymers’ electro-activity and environmental stability.

10

2.6 Figures

Figure 2.1: Schematic of a polymer electrolyte membrane fuel cell.

GDL MEA GDL

Electrical current e-

Excess fuel Water and heat

H+

H+

H+ H2

O2

H2O

e- e-

Fuel in Air in

Bipolar plate

11

Figure 2.2: SEM images of carbon fiber based GDL materials: (a) Toray TGP-H-120 paper, (b)

SGL Sigracet 10AA felt, (c) AvCarb 1071HCB cloth.

(a)

(b)

(c)

12

Figure 2.3: PPy electrochemical redox reactions (a) Anionic system with mobile anions. Anions

are removed when reduced and incorporated when oxidized (b) Cationic system with mobile

cations. Cations are removed when oxidized and incorporated when reduced.

(a)

(b)

13

Figure 2.4: Electrospinning schematic that shows the three key components: high voltage

supply, syringe and grounded metallic collector.

High voltage supply

Syringe

Grounded collector

14

Chapter 3 Investigation of Electroactive Polymers for the PEMFC

GDL

3.1 Introduction

In this chapter, PPy is presented as a novel porous material for the PEMFC, which can be

utilized as an active GDL coating. The fabrication procedures including the polymer solution

preparation and the electro-chemical deposition process for producing a thin and porous PPy

film are presented. The activation behaviour of PPy thin film is observed. Pore size change and

total pore area along the membrane surface are analyzed.


Water balance in a PEMFC is critical in ensuring good fuel cell performance. Current water

management strategies without any external components are passive approaches to redirect

excess water. If water movement at the cathode can be actively controlled, water accumulation

would be avoided. Conductive EAPs have dynamic properties that can be fabricated to suit

PEMFC environment. Either as a GDL, channel coating or as an additional layer, the activated

EAP layer has great potential to benefit PEMFC performance with its dynamically controlled

properties for water mitigation.

Recent studies that associated smart polymers with PEMFCs are focused on increasing catalyst

utilization [73] and lowering operation humidity levels [52, 55]. These applications exploit the

conductive property of smart polymers, but they do not actually take advantage of their inherent

redox switching properties. Electro-active PPy film is investigated in this chapter with the

motivation that it may be activated with the potential of the fuel cell, providing activation during

start-up and shutdown to facilitate liquid water purging. A proposed design is shown in Figure

3.1, where a thin PPy film is incorporated into a fuel cell flow channel on top of the GDL, so

that the GDL can be activated. Pore size increase can be beneficial during fuel cell operation,

since large pores have small capillary pressure and are less likely to trap water [8]. At shutdown,

the PPy layer will relax to its original state. Pore shrinkage may help flush out remaining water

from the GDL eliminating the accumulation of water, as a result, improving PEMFC

performance.

15

3.3 Experimental

3.3.1 Experimental Materials

Pyrrole (Py) monomer (MW: 67.09), bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI)

(MW: 287.09), propylene carbonate (PC) (99.7%) were purchased from Sigma Aldrich. The

Pyrrole monomer was purified and stored at 4°C in darkness prior to use.

3.3.2 Membrane Fabrication

Conductive PPy film is produced via an electrochemical polymerization process from an

electrolyte solution containing Py monomers. The process begins with an electrolytic solution

consisting of 0.2 M of Py monomer solution, and 0.2 M of LiTFSI in PC. The LiTFSI provides

TFSI- ions, which serve as the dopant ions for the PPy polymeric chains. Finally, 1% w/w

deionised water is added to the PC solution. The mixture is continuously stirred for two hours to

ensure solution homogeneity.

The Py monomer in the electrolyte solution changes from a liquid phase to a solid phase through

electrochemical polymerization. The electrochemical polymerization apparatus is the custom-

designed electrochemical cell chamber [11] shown in Figure 3.2. The anode collector substrate

employed is prepared by sputter-coating an acetate film with Pt on both sides to create a

conductive surface for polymer deposition. Acetate film is used as the substrate in this

experiment exclusively for the facile separation of the PPy film from the substrate. Two

stainless steel plates placed on either side of the chamber serve as the counter electrodes. The

electrolytic solution is then placed in the electrochemical cell chamber for the polymerization

step. A current density of 0.1 mA/cm2 is applied with a Keithley 2400 Sourcemeter (Keithley

Instrument Inc., Cleveland US). The potential across the electrolyte solution induces the

deposition of Py with the incorporation of TFSI- ions onto the anode collector in the form of

solid polymer chains. The electrochemical polymerization process is conducted at -30 ºC for 14

hours to form the polymer thin film.

3.3.3 Characterization Methodology

Conducting PPy thin film (20 mm × 20 mm × 107.6 µm) was activated by applying an electric

potential across the sample in the presence of a dopant reservoir, which was an insulating porous

16

polyvinylidine difluoride (PVDF) core. The PPy film was manually placed on top of the PVDF

core and stored in a new electrolyte solution containing 0.2 M of LiTFSI in PC prior to

activation. For the activation process, PPy and PVDF films are placed on a glass slide, and two

electrodes were placed on both sides of the PVDF core. Two electrical potentials (2 and 5 V)

were applied, during which the PPy film was imaged from above with a high resolution CCD

camera (PCO Pixelfly) combined with a stereoscope (Leica Z16 APO) as shown in Figure 3.3.

The microscopic images were taken at 10 minute intervals. One set of images were taken for

each activation voltage. The images were then processed using MATLAB. The microscopic

images were first converted to binary images with a threshold value of 0.35 (chosen to

maximize surface pore detection) where every pixel with brightness less than 0.35 were set to 0

(black).

3.4 Results & Discussion

Fig. 3.4 shows a set of microscopic images taken when the PPy film is under 2 and 5 V of

electrical activation after 0, 10 and 30 minutes. The darker regions in the images represent the

surface pores. For the 2 V activation, no significant changes are shown from image (a) to (c).

When the same sample is employed for the 5 V activation, from image (e) to (f), the

enlargements of pores are observed. Surface wrinkling of the thin film is also observed during

the 5V activation process even in the absence of microscopic magnification. The arrows in the

images point to the same pore on the film surface for both 2 and 5 V activations. This wrinkling

effect results in the shifting of pore locations on the material surface when 5 V is applied.

Figure 3.5 illustrates the percentage of surface pore area as a function of time when 2 and 5 V

are applied to the PPy film. The pore diameters are then obtained by calculating the equivalent

diameter using MATLAB. A stimulus of 2 V does not result in any significant changes to the

surface pore area for this sample over a period of 30 min. As seen in Figure 3.5, significant

increases to the surface pore area are observed for the 5 V activation. After a period of 30 min,

the total surface pore area increases by 45%. The increase in surface pore area combined with

the observed wrinkling effect indicates that the surface roughness of this material has increased.

17

Fig. 3.6 is a histogram of the pore size distribution before and after 30 min of 5 V activation. A

significant shift in pore size distribution is observed after this activation with the average pore

diameter changing from 61 to 67 µm. The number of smaller pores increasing may be a result of

surface wrinkling which generates higher pore density on the material surface.

3.5 Conclusion

In this work, conductive PPy is electrochemically polymerized as a standalone porous, smart

conductive polymer layer that has the potential to be employed as a modifying layer for the

PEMFC GDL. The thin film is then activated with an applied potential 2 and 5V. Surface pore

size change is investigated. Although a 2 V stimulus results in negligible changes to the surface

pore area after 30 min, a 5 V stimulus results in a 45% increase in surface pore area after 30 min

and an increase in average pore size from 61 to 67 µm. The surface roughness is also increased

by material wrinkling, although future work is required to measure the changes in surface

roughness, which can have desirable effects for liquid water management on the surface of the

GDL. The above changes in the material surface properties can be favourable by reducing the

tendency of water droplets to pin on the material surface, which can help reduce flooding in the

PEMFC. A proof-of-concept for the employment of smart polymers as porous activated

materials has been presented. It is important to note that due to the electrical potential required

to activate PPy/TFSI, the particular thin film used in this study may not be the most practical for

use in a working PEMFC. This work provides insight into how conductive smart polymers can

be employed to alter material surface properties for improved water management.

18

3.6 Figures

Figure 3.1: Proposed flow channel design incorporating electroactive polymer thin film along

the surface of the GDL in a PMEFC.

Electroactive polymer

membrane

Flow channel

GDL

19

Figure 3.2: Schematic of the electrochemical cell apparatus for the electrochemical

polymerization of PPy.

20

Figure 3.3: Schematic showing the microscopy-based apparatus employed for surface pore

measurements. Raw images were collected by the CCD camera and uploaded to the computer.

21

(a) 2V at 0 min (b) 2V at 10 min (c) 2V at 30 min

(d) 5V at 0 min (e) 5V at 10 min (f) 5V at 30 min

Figure 3.4: Microscopic images of porous PPy thin film under 2V and 5V activation. The length

bar represents 200µm.

200µ

m

22

Figure 3.5: Percentage of total surface pore area over a period of 30 min for 2 and 5 V electrical

activation potentials.

23

Figure 3.6: Histogram of pore size distribution before and after a 5 V activation.

24

Chapter 4 Development of a Novel Electro-active Polypyrrole

Trilayer Membrane

4.1 Introduction

In this work, a new adaptive PPy trilayer membrane is developed. The surface properties of

membranes fabricated at five polymerization current densities, ip, are compared. The membranes

are activated at potentials ranging between 0 and 2V, and the effects of ip and activation

potential on the following surface properties: roughness, wrinkling, and wettability of the PPy

trilayer membrane are determined.


Previous work (discussed in Section 2.3.1) has illustrated the range of surface property changes

that are possible with PPy films; however, in the studies discussed previously, PPy films

required immersion in an aqueous electrolyte for activation. To provide functionality in dry

environments, we have developed a novel active PPy trilayer membrane that can function

without electrolyte immersion. For the PPy trilayer membrane discussed in this paper, the

electrolyte is contained within the porous core layer, isolated between PPy layers from the

ambient environment. The main object of this chapter is to investigate the range of tunable

surface properties, such as surface roughness, wrinkling, and wettability (contact angle), which

the trilayer membranes are able to exhibit as a function of activation potential and

polymerization current density.

4.3 Experimental


Pyrrole (Py) monomer (MW: 67.09), bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI)

(MW: 287.09), propylene carbonate (PC) (99.7%), and polyvinylidine difluoride (PVDF)

membranes were purchased from Sigma Aldrich. The Pyrrole monomer was purified and stored

at 4 °C in darkness prior to use. It is important to note that the PVDF membrane serves as the

core of the PPy trilayer membrane.

25

4.3.2 Membranes Fabrication

To fabricate the trilayer membrane, a custom-built electrochemical cell shown in Figure 4.1 was

employed [11]. PVDF membranes were sputter-coated with gold (99.99% purity, ~21 nm thick)

on both sides using a sputter coater (EMS 7620, Electron Microscopy Sciences US) to create

conductive surfaces that served as the working electrodes for polymer deposition. Two stainless

steel plates placed on either side of the chamber were used as counter electrodes. An electrolytic

solution was prepared by combining 0.3 M Py monomer, 0.2 M of LiTFSI, and 1% w/w

deionised water in PC. This mixture was stirred for 15min until homogeneity was achieved and

then poured into the electrochemical cell chamber for polymerization.

Electrical current was applied to the working electrode with a Keithley 2400 Sourcemeter

(Keithley Instrument Inc., Cleveland US), where five current densities between 0.05 and 1.00

mA/cm2 were applied (Table 4.1) to fabricate membranes with distinct surface morphologies.

The electrochemical polymerization process was conducted at -30 ºC for six hours to form the

PPy(TFSI) trilayer membranes with dimensions of 50 mm x 50 mm. The resulted PPy(TFSI)

trilayer membrane consisted of a PVDF core layer saturated with electrolyte (containing TFSI-

mobile ions). The trilayer membranes were cut into sample sizes of 5 mm x 25 mm for

characterization purposes and stored in an electrolyte solution containing 0.2M LiTFSI in PC,

prior to use.

4.3.3 Characterization Methodology

To obtain high spatial resolution images of the membranes prior to activation, the fabricated

PPy(TFSI) samples were dried under vacuum (-30 inHg) at room temperature for ~12 hours

before being imaged with a scanning electron microscope (SEM) (JEOL-JSM-6060 Akishima

Tokyo, Japan). Nodule formations were analyzed with ImageJ software. Once samples were

imaged using the SEM, they were no longer employed for further activation-based

investigations. Fresh samples were employed for the activation-based investigations described

below.

Before each activation experiment, the PPy(TFSI) trilayer membrane was removed from the

electrolyte, and the surface was blotted dry prior to activation to remove excess electrolyte on

26

the surface. The bottom surface of the membrane was then adhered to a glass slide with double-

sided tape (3M Canada). The tape was employed to restrict the bending motion of the membrane

in order to isolate the changes in surface properties. Electrodes were placed on both sides of the

trilayer membrane as shown in Figure 4.2(b), and four activation potentials (Va = 0.5, 1.0, 1.5

and 2.0 V) were applied to the membrane in succession for approximately 1 min at each

potential. Positive potentials were applied to induce membrane expansion during oxidation

(mobile anions were TFSI- ions). These activation potentials were also chosen to be in the range

that is typically employed for ionic EAPs [74]. After each sample was activated for one minute,

it was again blotted dry due to the emergence of electrolyte at the surface.

The electrodes were removed from the membrane before the visualization was performed for

each of the studies described in detail below. It is important to note that all visualizations were

captured within a 10 minute time frame after ceasing activation, during which the materials

maintained their activated surface properties. After the 10 minute time frame, the materials

slowly relaxed to their unactivated states. This was verified through an optical image-based

sensitivity study where the measured surface properties were compared as a function of time.

Raw images captured during activation and 10 minutes after activation was ceased only varied

by 0.5% in a pixel-by-pixel based intensity comparison.

Surface wrinkle measurements

The surface wrinkle analysis demonstrates the in-plane activation effects on surface

morphology. The PPy membrane was imaged from above with a high-resolution CCD camera

(PCO Pixelfly, Kelheim Germany) combined with a compound microscope (Leica DM2500 M,

Wetzler Germany) as shown in Figure 4.2(a). Visualizations with a 20x objective were

performed prior to and after each activation. The surface wrinkle formation was analyzed using

a custom image processing procedure in Matlab. To analyze this wrinkling effect, the raw

images were first converted to binary images with a threshold value of 0.31 (chosen to

maximize the detection of wrinkles). Every pixel with brightness below the threshold was set to

0 (black). Since PPy membranes have nodular surface features, objects with circularity less than

0.2 were considered as wrinkles. Circularity is defined as: Circularity = 4π(A/P2). Where A and

P represent area and perimeter of the object of interest respectively, therefore, the circularity for

27

a circle is 1. The percentage increase of wrinkled area (∆ε) on each image was calculated with

∆ε = εVa – εo . Where εVa and εo represent wrinkled area at each activation potential and wrinkled

area at Va = 0 V respectively.

Surface roughness measurements

Surface roughness results from the through-plane activation effects on surface morphology.

Surface roughness values (Ra) were measured with a surface roughness tester (Times Inc.

TR200) as shown in Figure 4.3. The measurements were in compliance with ISO standards.

From a sensitivity analysis, a measurement length of 50 mm was chosen to best represent the

roughness of the membrane surface. Following the activation at each potential, the membrane

was placed under the surface roughness tester. A new sample was employed for each

measurement. The averaged Ra values from three sets of measurements were collected at each

Va and graphed.

Surface wettability measurements

To examine surface wettability changes during activation due to the in-plane and through-plane

effects, contact angles (θ) for liquid water droplets on the membrane surfaces were also

measured at each Va. The side views of the droplets were imaged with a macroscope (Leica Z16

APO, Wetzler Germany) as shown in Figure 4.4. The macroscope was employed for its

capability for imaging at long depth of fields. Liquid water was dyed with a dilute solution of

fluorescein (~1 mM) to enhance the detection of the liquid/gas and liquid/solid interfaces.

Fluorescein had been used in various studies as a tracer for water observation. It was found that

the addition of fluorescein causes negligible effect on water density, surface tension and

viscosity [79]. Water droplets (~3 µL) were dispensed with a micropipette (Finnpipette, Thermo

Fisher Scientific Inc., Waltham US) onto the membrane surface. At each activation potential, an

average contact angle was determined from measurements of 10 separate droplets within a

region of 5mm x 5 mm. The images were captured with a high resolution CCD camera (PCO

1600, Kelheim Germany) and processed with software developed in-house using MATLAB.

28

4.4 Results and Discussion

We investigated the surface property tunability of PPy trilayer membranes fabricated at various

current densities. These surface properties included: surface wrinkling, surface roughness, and

wettability over a range of applied potentials. Even in the absence of electrical activation, we

found that the current density applied during membrane fabrication made a significant impact on

the surface morphology of the material. In particular, membranes B and E, fabricated with

ip = 0.1 mA/cm2 and ip = 1.0 mA/cm2, respectively exhibited the most drastic differences in

nodule formation and distribution. SEM images of membranes B and E are shown in Figure 4.5.

The surface of membrane B shown in Figure 4.5(a) (ip = 0.1mA/cm2) was generally uniform,

exhibiting small and uniformly spaced nodules with diameters ranging from 1 to 3 µm. In

contrast, membrane E shown in Figure 4.5(b) (ip = 1.0mA/cm2) exhibited larger, cauliflower-

shaped nodules with diameters ranging from 2 to 8 µm. Non-homogeneously distributed larger

nodules with diameters from 10 to 20 µm were also observed on membrane E.

Effect of activation potential on planar wrinkling

Figure 4.6 shows optical images taken when membrane B was activated from 0 to 2 V, in

increments of 0.5 V. As seen in this figure, the degree of wrinkling increased with increasing

activation potential. Similar behaviour was observed for all other membranes. When a positive

potential was applied to the trilayer membrane, the PPy layer in contact with the positive

electrode became more positively charged. In order to maintain charge neutrality, mobile TFSI-

ions from the electrolyte were incorporated into the polymer chain, resulting in polymer chain

expansion. If the membrane was free to move, the swelling of the membrane would result in

trilayer membrane bending. Because the bottom surface of the membrane was fixed, we were

able to isolate activation effects as morphological changes, where the expansion of polymer

chains was translated into wrinkle formation.

Figure 4.7 illustrates the percentage increase in wrinkled area as a function of Va for all five

membranes. Six trials were analyzed for each membrane. Similar trends were observed in all six

trials, and a representative trial was presented in Figure 4.7. For all membranes, as Va increased,

the percentage increase of wrinkled area, ∆ε, increased. From Va = 0 to 2 V, membrane B

exhibited the largest increase in wrinkled area (37% increase), and membrane C had the next

29

largest increase in wrinkled area (25% increase). Membrane A, D, and E exhibited significantly

smaller changes (less than 10%).

From Figure 4.7, we can see that increasing the applied current density during membrane

fabrication, ip, beyond 0.3 mA/cm2 does not result in increased wrinkle tunability (membranes D

and E). This is attributed to the increase in membrane thickness that is associated with

increasing ip. When the membrane thickness increases, the membrane becomes stiffer and more

potential mobile ions become immobilized. This results in creating fewer available chains for

TFSI- ion attachment, thereby reducing the effect of activation on surface wrinkling. This

phenomenon is in agreement with the work of Dziewonski et al.[75], where they explained that

the film thickness is proportional to fabrication current density, and as the film becomes thicker,

more anions are immobilized. A small degree of wrinkling was also observed for ip values less

than 0.1 mA/cm2. This is attributed to the small amount of available polymer chains for ionic

diffusion during activation for thin membranes.

Effect of activation potential on surface roughness

Figure 4.8 illustrates the surface roughness values, RaThe averaged Ra values from three sets of

measurements at each activation potential are shown, where the error bars indicate the standard

deviation for each data point. Two regions of interest are identified. The dark shaded region

indicates significant increases in Ra values during activation. For membrane B and C, the

overall increase in Ra from 0 to 2V was approximately 200%. For the lightly shaded region

comprising membranes A, D and E, the increases in Ra values were approximately 60 – 90%.

As shown in Figure 4.8, larger standard deviations were associated with high Va, especially for

membranes fabricated with higher ip. This may be attributed to the non-uniform nodule

formation associated with higher ip. Prior to activation, the surface roughness increased with

increasing current density, which is consistent with the nodule sizes visualized in the SEM

images (Figure 4.5). The amount of change in Ra values also corresponded to planar wrinkle

formations for the membranes, where membrane B (ip = 0.1 mA/cm2) showed the highest degree

of surface morphology changes in roughness values and had the most wrinkle formation. It is

noteworthy that these changes were much greater in the through-plane direction (roughness)

compared to the in-plane direction (wrinkles).

30

Effects of activation potential on contact angle

Side views of liquid water droplets (~3 µL) at 0 and 2 V are shown in Figure 4.9 to illustrate the

increase in contact angle that is observed due to electrical activation. Figure 4.10 shows the

contact angle as a function of increasing activation potential, where the maximum standard

deviation of the 10 measurements for each data point was 3°. At 0 V, θ is increasing with ip. An

approximately linearly increasing trend is observed for the membrane contact angle during

activation. Membrane B has the largest increase in contact angle during activation, from 55° to

96°. The wetting properties of membrane B and E changed from hydrophilic to hydrophobic

after a 2 V activation. This increase in hydrophobicity is reasonable due to the observed increase

in surface roughness [76]. The increase in contact angle measurements also indicates a

decreasing surface energy of the polymer membranes with activation [80]. It is interesting to

note that the effects of activation at the molecular level extend to macro-scale effects on the

material that we have observed through changes in wrinkle formation, surface roughness, and

contact angle measurements.

4.5 Conclusion

In this chapter, a novel design of an electro-active membrane using PPy trilayer membrane was

developed and the tunability of surface property changes were analyzed through wrinkle

formation, surface roughness, and contact angle measurements. Five conductive PPy trilayer

membranes were electropolymerized from an electrolyte solution containing Py monomers onto

a porous PVDF core at current densities ranging from 0.05 to 1.00 mA/cm2. The trilayer

membranes were activated at applied activation potentials ranging from 0 to 2V. It was shown

that the applied current density during fabrication affected the structural formation of the PPy

membranes, which in turn influenced the surface property tunability of the membranes. It was

shown that the wrinkled area, surface roughness, and contact angles increased as the activation

potential increased for all membranes. In particular, membrane B (ip = 0.1 mA/cm2) exhibited

the greatest change in all three parameters with maximum increase of 37% in wrinkled area,

200% in surface roughness, and 41° in contact angle. Thicker membranes were fabricated at

current densities above 0.3 mA/cm2, resulting in membranes with higher stiffness, increased

numbers of immobilized ions, and an overall reduction in available reaction sites. Thinner

31

membranes fabricated at current densities below 0.1 mA/cm2 were also unfavourable for surface

property tunability due to their insufficient amount of reaction sites. For the range of fabrication

current densities investigated in this study, membrane B was the most desirable for exhibiting

highly tunable surface properties through electrical activation.

In this chapter, a strong relationship between polymerization current density and activation

potential on membrane tunability of surface wrinkling, roughness, and wettability were

identified. The result of which is expected to have strong impacts on the development of new

clean energy technologies where surface-based reactions influence device performance and

durability.

32

4.6 Tables

Table 4.1: Summary of membrane fabrication conditions. All membranes were polymerized for

6 hours.

Membrane ip (mA/cm2)

A 0.05 B 0.10 C 0.30 D 0.50 E 1.00

33

4.7 Figures

Figure 4.1: Schematic of the electrochemical cell apparatus employed for the electro-

polymerization of PPy.

34

Figure 4.2: (a) Schematic showing the microscopy-based apparatus employed for measuring

surface wrinkles during material activation. Raw images were collected by the CCD camera and

uploaded to the computer. (b) Image showing the placement of the electrodes on PPy membrane

that is fixed onto a glass slide.

(a)

(b)

35

Figure 4.3: Schematic showing the apparatus employed for surface roughness measurements.

(a)

Surface roughness tester

PPy(TFSI) trilayer

membrane on glass

slide

36

Figure 4.4: Schematic showing the microscopy-based apparatus employed for contact angle

measurements. Raw images were collected by the CCD camera and uploaded to the computer.

37

(a)

(b)

Figure 4.5: SEM images of PPy trilayer membranes fabricated with ip = 0.1mA/cm2 (Membrane

B) and ip = 1.0mA/cm2 (Membrane E). (a) Membrane B at 200x and 500x magnification, (b)

Membrane E at 200x and 500x magnification. Membrane B is visibly smoother with smaller

nodules, compared to Membrane E, which has larger nodules distributed heterogeneously.

38

(a)

(b)

(c)

(d)

(e)

Figure 4.6: Raw optical images of a tri-layer PPy membrane fabricated with an ip of 0.1mA/cm2,

showing the changes in surface morphology associated with Va = (a) 0V, (b) 0.5V, (c) 1.0V, (d)

1.5V and (e) 2.0V. Each activation took place for 1min before photo was taken. The length bar

represents 150 µm.

40

Figure 4.7: Percentage of wrinkled area increase as a function of Va, where the most significant

increase in wrinkled area was observed for Membrane B.

41

Figure 4.8: Impact of activation potentials on surface roughness. The lightly shaded region

indicates that membranes A, D, and E provide an overall increase (~70%) in surface roughness,

Ra, with increasing activation potential, Va. In contrast, the darker region indicates that

membranes B and C provide significant increases (~200%) in Ra with increasing Va.

42

0V

2V

(a)

(b)

(c)

(d)

(e)

Figure 4.9: Side view images obtained with fluorescence microscopy to determine the impact of

activation potential on wettability. Liquid droplets (3µL) were dyed with a dilute solution of

fluorescence to elucidate the liquid/gas interface. Contact angle at Va = 0 and 2V are shown for

membranes fabricated with: ip = (a) 0.05mA/cm2, (b) 0.1mA/cm2, (a) 0.3mA/cm2, (a) 0.

5mA/cm2 and (e) 1.0 mA/cm2. The length bar represents 500µm.

43

Figure 4.10: Relationship between the contact angle, θ, and Va. An increase in θ was observed

for all 5 membranes, where membrane B had the highest overall increase in θ measurements.

The maximum standard deviation for each θ is 3°.

44

Chapter 5 Development of Conductive Fibrous Polymer Film

5.1 Introduction

In this chapter, conductive fibrous films with MWCNTs and PPy are fabricated with

electrospinning. Electrospinning conditions are tailored according to MWCNT concentrations to

produce fibers with minimal or no beads. The effects of coating electrospun oxidant fibers with

PPy by the means of vapor phase polymerization (VPP) were then investigated. In both methods

the conductive fibers are characterized in terms of their morphologies, thermal stability, and

electrical conductivity.


Electrospun fibers have very similar macro structures as PEMFC GDLs. The advantage of

electrospun fibers is that fiber orientation, fiber diameter, and film porosity can be controlled.

Electrospun fibers can also be treated with other materials to modify fiber properties. One of the

important properties for GDLs is electrical conductivity. Despite the advantages of

electrospinning, to electrospin conductive polymer fibers is challenging since almost all

conductive polymers cannot be processed on their own. Previous studies (summarized in

Section 2.4.1) have demonstrated various methods in producing electrospun conductive fibers

using MWCNT and PPy. Electrospun fibers containing high concentration (more than 1 wt%)

MWCNT usually experience bead formation [31]. Even though beads can create large contact

areas which may potentially be beneficial to conductivity measurements, beads may not be a

desirable physical property in electrospun fibers. Various oxidants had been used for PPy coated

electrospun fibers. However, the effects of oxidant concentration and polymerization time on

PPy fiber conductivity and fiber surface morphology have not been studied. The objective of

this chapter is to fabricate MWCNT fibers with minimal beads or no beads. In addition, to study

the effects of PPy polymerization stage on PPy nano-patterns and fiber properties.

5.3 Experimental


Polystyrene (PS) pellets (Mw: 280,000), anhydrous grade dimethylformamide (DMF), iron (III)

chloride (FeCl3) and, pyrrole (Py) monomer (MW: 67.09) were purchased from Sigma Aldrich.

MWCNTs (20 – 40 nm in diameter, 1 – 10 µm in length, 97% purity) were purchased from

45

Cheap Tubes. Pyrrole monomer was purified and stored at 4°C in darkness prior to use. PELCO

Eponate 12 Kit was purchased from Ted Pella Inc. for TEM sample preparation.

5.3.2 Sample Fabrication

MWCNT/PS solutions blends were prepared by first adding 1, 3, 5 or 10% (wt% to PS) of

MWCNT to DMF. The MWCNT/DMF mixture was mechanically stirred for half an hour. 25%

(wt% to solution) PS was then added to this mixture. To prepare for FeCl3/PS solutions, 40 or

60% (wt% to PS) of FeCl3 were first added to DMF and mechanically stirred until FeCl3 was

fully dissolved (~10 min). 15% (wt% to solution) PS was then added to the mixture. All

polymer solutions were stirred at 50 °C overnight in a closed glass vial. The solutions were

cooled to room temperature before electrospinning.

5.3.2.1 Electrospinning of Polymer MWCNT Composites

Electrospinning setup employed in this work is shown in Figure 5.1. A glove box was used for

controlled humidity during electrospinning. MWCNT/PS fibers were spun at room temperature

(23 ± 2 °C) and humidity was kept at 19 ± 1 %RH. The aluminum collector was 10 cm away

from the tip of the needle. Since solution viscosity and conductivity varies with MWCNT

concentration. Voltage and feed rate were adjusted to eliminate beading for each solution

composition. Three samples were prepared for every MWCNT concentration.

5.3.2.2 Sample Fabrication of Conductive PPy Electrospun Fibers

To produce PPy fibers, two steps were involved. First, FeCl3/PS oxidant fibers were

electrospun. Humidity in the glove box was controlled with desiccant and maintained at 9 ± 2

%RH. Since high voltage and humidity speed up PS solidification process, when

electrospinning FeCl3/PS at higher humidity environment, the solution begins solidifying before

erupting from the needle tip. The semi solidified solution accumulates at the needle tip and

eventually detaches as large droplets. Therefore, humidity was kept low when spinning

FeCl3/PS fibers. Electrospinning was carried out at room temperature (23 ± 2 °C) with an

aluminum collector 15 cm away from the needle tip. Voltage and solution feed rate were kept

the same for both 40 and 60% fibers at 23kV and 1.0 ml/h.

46

Py was then chemically oxidized through VPP method onto the electrsospun FeCl3/PS fibers.

The schematic of customized VPP apparatus utilized in this work is shown Figure 5.2. Py

monomer solution (~3 ml) was placed at the bottom of the vial. The electrospun oxidant fibrous

film was placed onto the wired stainless steel sample holder. Once the oxidant fibers were

exposed to Py monomer vapor, Py was oxidized by FeCl3. As a result, a layer of PPy coating

was generated on top of the FeCl3/PS fibers. All samples were under active vacuum for 20

seconds, and then held under passive vacuum for 60, 100 and 140 minutes. Two samples were

prepared for each set of parameters.

Detailed fabrication parameters for each of the fibrous film produced in this work are

summarized in Table 5.1. All samples were dried in ambient condition overnight and removed

from the aluminum collector prior to characterization.

5.3.3 Characterization

Morphological characterization

Small samples were obtained from the electrospun fibrous film and imaged with a scanning

electron microscope (SEM) (JEOL-JSM-6060, Akishima Tokyo, Japan). Fiber diameters were

measured with ImageJ software. The measurements of 100 fibers from various locations on the

samples were collected. Fiber diameter distribution histograms were plotted for each polymer

compositions. PPy coated FeCl3/PS fibers were also imaged with an ultrahigh resolution SEM

(Hitachi S-5200) for fiber surface structure observation. Bright field transmission electron

microscopy (TEM) (Hitachi HD-2000, Tokyo Japan) was used to image MWCNT inside PS

fibers and the coating of PPy on FeCl3/PS fibers. To prepare TEM samples, the fibers were

embedded in Eponate resin. Once cured, the samples were cut with a microtome (Leica Ultracut

UCT, Wetzler Germany) into 200 nm thick slices for imaging.

Electrical characterization

Conductivities of MWCNT/PS fibers were measured with a two point measurement method

using two stainless steel electrodes. MWCNT/PS fibrous films were placed in between the two

electrodes (~1 mm x 1 mm). Pressure was applied to the two electrodes to ensure good contact

between the electrodes and the sample. The thickness of the compressed samples was measured

47

with a digital caliper (MasterCraft, US). One set of resistivity measurement was collected from

each of the three samples for each material composition. Conductivities of the PPy/PS were

measured with a four-point probe made in house. A schematic of the four-point probe is shown

in Figure 5.3 with four needles evenly spaced at 1 mm part. The probe was gently placed on the

film and pressed down to ensure good contact with all four needles. Three measurements at

different locations were taken from each sample. All measurements were collected with a

Keithley 2400 Sourcemeter (Keithley Instrument Inc., Cleveland, OH US) in LabView. Since

the reading fluctuates slightly, for each sample measured data was collected for 20 seconds, the

values were then averaged.

Thermal characterization

A thermalgravimetric analyzer (TGA) (Q50, TA Instruments, New Catsle, DE US) was used to

examine the thermal stability of the fibers. The weight of the samples used in TGA analyses

were 2 – 5 mg for MWCNT fibers and 10 – 15 mg for PPy fibers. All TGA analyses were

performed in a nitrogen environment at temperatures ranging from 0°C to 800°C with a heating

rate of 20 °C/min. Degradation temperature (Tonset), which is the temperature at where the slope

change of a TGA thermogram occurs, can be obtained with Universal Analysis software (TA

Instruments). An example of Tonset measurement is shown in Figure 5.4. The derivative of

weight change curve was first plotted. Two onset points were selected to define the start and

stop limits of the analysis. The start onset point was selected to be the start of the analysis when

weight percent was at 100%, the end onset point was selected to be at where derivative of

weight change curve peaks. Universal Analysis software then calculates Tonset automatically. For

PPy fibers with multiple peaks on derivative of weight change curve, the first peak was used.

5.4 Result and Discussion

Electrospinning of MWCNT/PS nanofibers

SEM images of electrospun MWCNT/PS fibers are shown in Figure 5.5. The average fiber

diameter for MWCNT concentrations of 0, 1, 3, 5 and 10% are 2.32, 2.51, 2.59, 2.41, and 3.46

µm respectively. Pure PS fibers have a smaller range of fiber diameters, the fibers geometry are

visually more uniform compared to other fibers. Fibers with 1, 3 and 5% MWCNT are very

similar in diameters, where 1 and 3% fibers are bead free. 5% fibers have very minimal amount

48

of small bead formation. 10% MWCNT fibers have a wide range of fiber diameters, and the

average diameter of the fibers is approximately 40% higher than fibers with lower MWCNT

concentrations. The fibers are visually non-uniform and less smooth, small beads are also

observed. Fiber morphology and diameters are directly affected by MWCNT concentration,

voltage, and solution feed rate. Since voltage and feed rate were adjusted in the fabrication

process to reduce bead formation, fiber diameters obtained here are not only dependent of

MWCNT concentration as reported in other literatures [31]. The increase in fiber diameters and

the non uniformity of fiber shape for 10% MWCNT/PS could be the result of poor alignment of

MWCNT inside the fibers and the formation of MWCNT agglomerates. TEM images of 10%

MWCNT/PS fibers are shown in Figure 5.6. Figure 5.6(a) is imaged along the fibers. The bright

spots on the image are Ni particles which were used in MWCNT synthesis. MWCNT can be

seen along the fibers. Figure 5.6(b) shows the cross section view of a fiber. Long strands of

MWCNT can be clearly observed. The TEM images indicate that the MWCNTs are dispersed;

however, poorly aligned with the formation of agglomerates.

TGA thermograms of electrospun MWCNT/PS fibers are shown in Figure 5.7. Carbon

nanotubes do not decompose at 500 °C. Therefore, the residual at 500 °C is the additional

MWCNT in PS fibers. The percentage residual matches well with the percentage MWCNT

added to PS solution, which further confirms the MWCNT dispersion in electrospun fibers.

Tonset are tabulated in Table 2, a slight increase in Tonset is shown from pure PS fibers up to 5%

MWCNT concentration, which indicates that the addition of MWCNT is delaying the thermal

degradation of PS fibers. However at 10% MWCNT concentration, the fibers degrade sooner

compared to pure PS fibers. This can be attributed to the high amount of impurities in the PS

fibers.

Due to the high contact resistance between fibers, 4 point probe conductivity measurements

were not successful. Therefore, a 2 point measurement method was employed. Figure 5.8 shows

the conductivity of MWCNT/PS films as a function of MWCNT concentration. MWCNT

loading lower than 3% had no significant effects on the conductivity of the fibrous films, an

increase was observed with higher MWCNT concentrations at 5% and 10%. This shows the

percolation threshold is at around 5% MWCNT. The highest conductivity value obtained was

49

1.86E-8 S/cm with 10% MWCNT/PS film. These low conductivity values were mainly due to

the contact resistance between fibers and the lack of MWCNT alignment. The large fiber

diameters may also contribute to the low conductivity values by wrapping larger amount of

insulating PS material around MWCNT which hinders the formation of a continuous conductive

network.

PPy coated electrospun FeCl3/PS fibers

Two oxidant concentrations (40 and 60%) and three polymerization time (60, 100, and 140 min)

were investigated for PPy coated FeCl3/PS fibers. SEM images and fiber diameter histogram of

pure 40 and 60% oxidant fibers are shown in Figure 5.9(a) and (b) and Figure 5.10(a) and (b)

respectively. 40% fibers are thinner with an average diameter of 1 µm. The fibers are smoother

and more uniform compare to 60% fibers which has an average fiber diameter of 1.81 µm. The

formations of spherical structures are observed for both fibers. As shown in Figure 5.10(a), the

formation of these spheres differs from beads. In these spheres, intertwined individual fibers can

be clearly differentiated. The formation of the spheres may be due to the high solution

conductivity and the high voltage applied to the solution during electrospinning.

When the electrospun oxidant fibers are exposed to Py monomer vapors, redox reaction occurs

where Py is oxidized while FeCl3 is reduced. While the oxidant fibers are in the VPP vessel, Py

vapor adds moisture to the fibers, therefore, the longer the polymerization time, the more

compact the fibers become. In addition, as PPy is polymerized on top the oxidant fibers,

neighboring fibers are likely to fuse together by the growth of PPy in between fibers as shown in

Figure 5.9(c) and Figure 5.10(c). As shown in Figure 5.9(e) and Figure 5.10(e), at 140 min,

almost all fibers are fused together forming a fibrous web. In the high magnification (20k ×)

SEM images, the progression of PPy growth can be observed. PPy nanostructures vary at

different polymerization stage. As shown in Figure 5.9(c), (d) and (e) (coated 40% oxidant

fibers), nodular structures were not formed until 140 min into polymerization. Whereas for the

coated 60% oxidant fibers, early stage nodule growth can be seen at 60 min (Figure 5.10(c)); at

100 min, more nodular structures are formed, and finally at 140 min, cauliflower shaped surface

nanostructures cover the entire fiber surface as shown in Figure 5.10(e). TEM images of the

cross section of 60% FeCl3/PS fiber polymerized for 140 min are shown in Figure 5.11. The

50

bright outer ring showing in Figure 5.11(a) indicates the highly structured PPy coating. Figure

5.11(b) shows the PPy grown in between two adjacent fibers.

Tonset of pure oxidant fibers and PPy coated fibers are obtained from TGA analysis and tabulated

in Table 5.2. Pure oxidant fibers with 40% FeCl3 are more thermally stable compare to 60%

FeCl3/PS fibers. However, after polymerization 60% FeCl3/PS fibers are coated with more PPy

and the larger amount of coating have improved the fibers’ thermal stability in comparison to

PPy coated 40% fibers. On the other hand, the longer the polymerization, the lower Tonset

becomes for both oxidant concentration.

Figure 5.12 shows the conductivity measurements with respect to polymerization time for 40%

and 60% FeCl3/PS fibers. Conductivity at 60 min is low with an average value of 8.42E-9 S/cm

for the 40% fibers and 7.61E-9 S/cm for the 60% fibers. The low conductivity further indicates

the low degree of polymerization at 60min, which does not provide a sufficient amount electron

hopping sites. The high contact resistance between fibers also contributes to low conductivity.

At 100 min, conductivity of 60% fibers increased 3 orders of magnitude to an average of 8.84E-

6 S/cm. As for the 40% fibers, it increased slightly to an average of 8.43E-8 S/cm. At 100 min,

nodules start forming on the 60% fiber surface, creating more conductive pathways compare to

the 40% fibers. Conductivities at 140 min are measured to be 9.50E-4 S/cm for 40% fibers and

6.9E-4 S/cm for 60% fibers. At 140 min, the fibers are compacted which forms a continuous

conductive path and significantly reduces the contact resistance between fibers. The

conductivity for both 40 and 60% fibers at 140 minutes are similar, which indicates that contact

resistance is the dominating factor in determining fibrous film conductivity. Once a conductive

pathway is formed, regardless the amount of PPy coating on the fibers, the electrical

conductivity can be significantly improved.

5.5 Conclusion

In this chapter, two methodologies for fabricating conductive fibrous film were investigated. PS

fibers with 1, 3, 5 and 10% MWCNT conductive fillers were electrospun. SEM and TEM were

used to image the fiber morphology and MWCNT dispersion inside the fibers. Electrospun

fibers with 5% or less MWCNT loadings were smooth and had improved thermal stability. The

51

10% MWCNT/PS had the highest electrical conductivity of 1.86E-8 S/cm. However, the

electrospun fibers were geometrically inconsistent and had the lowest degradation temperature.

MWCNT were clearly observed in 10% MWCNT/PS fibers with TEM showing good

dispersion. The unaligned MWCNT resulted in large fiber diameters which attributed to low

conductivity. Nano structured PPy were successfully polymerized on the surface of electrospun

FeCl3/PS fibers. 40% and 60% FeCl3/PS fibers with 60, 100, and 140 min Py polymerization

time were compared. The progression of PPy formation on the fiber surface was observed with

high magnification SEM images. At 140 min polymerization time, fibers were fused together

forming a continuous web where the highest conductivity 9.5E-4 S/cm and 6.9E-4 S/cm were

achieved for 40% and 60% oxidant fibers respectively.

To attain high conductivity in fibrous films, two important factors need to be considered:

providing conductive pathway for electron transfer and reducing contact resistance between

fibers. As shown with MWCNT/PS and PPy coated FeCl3/PS fibers, high conductivities are

often associated with low thermal stability and less favorable fiber morphology. A fine balance

in material composition and fabrication parameters need to be met in order to achieve desired

morphological, physical and electrical properties. Tradeoffs need to be made to obtain optimal

set of properties for a specific application.

The conductivity obtained from PPy coated fibers are about four orders of magnitude lower

compare to PEMFC GDLs. However, studies had been conducted on the effect of GDL

electrical resistance to PEMFC performance [77] through 3D modeling. The conclusion drawn

from the study showed that GDL conductivity was not the most critical factor that affects

PEMFC performance. If other properties such as thermal conductivity and structural integrity of

the electrospun films are compatible, the low conductivity may not be a limiting factor for the

applications in PEMFC.

52

5.6 Tables

Table 5.1. Fabrication parameters employed in electrospinning two polymer blends

Polymer Matrix

Additive Particles

Needle Size

Feed Rate Voltage

Distance from

collector Humidity Polymerization

Time

Gauge ml/h kV cm %RH min

25% PS

N/A

22

2.0 10

10 19±1 - 1% MWCNT 3.0 12.5 3% MWCNT 3.0 16 5% MWCNT 2.0 11

10% MWCNT 1.8 8

15% PS 40% FeCl3 20 1.0 23 15 9±2 60, 100, 140 60% FeCl3

53

Table 5.2: Thermal degradation temperature for MWCNT/PS fibers

Composition Polymerization Time

Tonset

(min) (°C) 25% PS -- 408.37

1% MWCNT + 25% PS -- 409.73 3% MWCNT + 25% PS -- 412.98 5% MWCNT + 25% PS -- 413.07 10% MWCNT + 25% PS -- 405.18

40% FeCl3 + 15% PS 0 266.94 60 175.75

100 162.85 140 150.38

60% FeCl3 + 15% PS 0 243.71 60 197.88

100 196.92 140 179.16

54

5.7 Figures

Figure 5.1: Schematic of electrospinning setup consisted of (A) Syringe mounted on a syringe

pump, (B) High voltage source, (C) Needle, (D) Grounded current collector, and (E) Glove box

for humidity control

55

Figure 5.2: Schematic of vapor phase polymerization setup employed for chemical

polymerization of Py on FeCl3/PS fibers. An aspirator was used to create vacuum. Samples were

placed directly onto the stainless steel wire mesh sample holder for polymerization.

56

Figure 5.3: Schematic of a four point probe tip with four collinearly and evenly spaced needles

at 1 mm apart.

57

Figure 5.4: A screen shot of Universal Analysis software with percentage weight and derivative

of weight change curves with respect of temperature, as well as the calculated Tonset

58

Figure 5.5: SEM images and fiber diameter distribution histogram of electrospun MWCNT/PS

fibers with various MWCNT concentrations: (a) pure PS fibers (b) 1% MWCNT/PS (c) 3%

MWCNT/PS, (d) 5% MWCNT/PS, and (e) 10% MWCNT/PS

Bead

Bead

(a)

(b)

(d)

(c)

(e)

59

Figure 5.6: TEM image of 10% MWCNT/PS fiber (a) along the fiber (b) cross section view of

the fibers.

(a)

(b)

MWCNTs

MWCNTs

MWCNTs

60

Figure 5.7: TGA thermograms of MWCNT/PS fibers. All samples were heated from 0 to 500°C

at a rate of 20 °C/min;

61

Figure 5.8: Two point electrical conductivity measurements of MWCNT/PS electrospun fibrous

films as a function of MWCNT concentration.

62

Figure 5.9: SEM images of (a) pure 40% FeCl3/PS oxidant fibers, (b) fiber diameter distribution

histogram of 40% FeCl3/PS fibers. High resolution SEM images of PPy coated oxidant fibers at

20k × magnification for three polymerization times (c) 60 min, (d) 100 min, and (e) 140 min.

(b)

(a)

(c)

(d)

(e)

63

Figure 5.10: SEM images of (a) pure 60% FeCl3/PS oxidant fibers, (b) fiber diameter

distribution histogram of 60% FeCl3/PS fibers. High resolution SEM images of PPy coated

(b)

(a)

(c)

(d)

(e)

64

oxidant fibers at 20k × magnification for three polymerization time (c) 60 min, (d) 100 min, and

(e) 140 min.

65

Figure 5.11: TEM images of 60% FeCl3/PS fiber polymerized for 140 min. (a) Cross sectional

view of PPy/PS fiber. (b) Cross sectional view of fibers joint together with the growth of PPy

coating.

(a)

(b)

PPy coating

1µm

66

Figure 5.12: Four point probe conductivity measurements with respect to polymerization time

for PPy coated 40% and 60% FeCl3/PS fibers.

67

Chapter 6 Conclusions

In this thesis, polymer membranes were investigated where the main objective was to develop

micro-structured conductive porous polymer membranes with tunable surface properties for

potential applications such as the PEMFC. Introduction on the current energy technologies such

as fuel cells, supercapacitors, batteries and solar cells were presented. Conductive polymer was

first introduced, specifically polypyrrole. Electrospinning and the utilization of electrospun

conductive polymers in energy applications were then discussed. As a potential energy

application, PEMFC was introduced including the functionality of GDL, water management

issues and water management solutions with the investigation of novel materials. The main

conclusions of this study are:

• Conductive polymer membranes were fabricated using electrochemical deposition

method from a monomer containing electrolyte.

• A single layer PPy membrane was first fabricated and investigated. Surface pores were

imaged during activation with an external potential.

• The average pore size increased from 61 to 67 µm, an overall 45% increase in total pore

area were also observed with a 5 V applied potential. The increase in pore size is

beneficial in reducing capillary pressure within the pores, which reduces the probability

of water trapping in the pores.

• An adaptive PPy trilayer membrane was fabricated using electrochemical deposition

method from a monomer containing electrolyte.

• PPy was directly deposited onto PVDF porous electrolyte containing a core layer, which

allowed it to perform in dry environment.

• Polymerization current density was found to have a strong influence on the activation

behaviors of the PPy membranes.

• The surface properties of these trilayer membranes changes in relation to activation

potential were investigated.

• The membrane fabricated at 0.1 mA/cm2 polymerization current density exhibited the

most increase for all three properties studied: 37% in wrinkled area, 200% in surface

roughness, and 41° in contact angle. The tunable surface properties of such system can

be very promising in the development of surface-based energy technologies.

68

• Electrospinning was used to fabricate a highly porous fibrous conductive MWCNT/PS

fibers and PPy coated FeCl3/PS fibrous membranes with similar macro-structures as

PEMFC GDL.

• MWCNT/PS fibers were electrospun with minimal beads and good thermal stability.

However, the conductivity measurement was low. Highest conductivity achieved with

10% MWCNT fibers was 1.86E-8 S/cm.

• A parametric study was conducted for PPy coated FeCl3/PS. It was found that PPy

nanostructure formation and conductivity were strongly correlated with oxidant

concentration and polymerization time.

• Highest conductivity measured was 9.5E-4 S/cm with 40% FeCl3/PS fibers polymerized

for 140 min.

Conductive polymers can be fabricated with a range of morphological, physical, thermal, and

electrical properties. These set of properties are often interrelated where tradeoffs need to be

made to compensate for a specific application. Conductive polymers have attractive properties

such as high specific area, high conductivity, and good environmental stability, and most

importantly the ability to be manipulated with external stimuli.

69

Chapter 7 Future Works

As a result of the work conducted in this thesis, there still exist many future directions for

research of conductive polymers in energy applications.

PPy was the conductive polymer membrane studied in this work. For single layer PPy

membrane, surface pore changes were observed upon activation with applied electrical

potential. To apply PPy layer as an active coating to PEMFC, a much lower activation potential

(~ 1 V) is required. Future work can include a parametric study on the effect of membrane

thickness, dopant, and other EAPs on the required activation potential.

The trilayer PPy membrane studied in this work demonstrated desirable surface property

changes when activated. The use of a liquid electrolyte and electrical activation are not

compatible with the operation of conventional PEMFCs. Future work may include the

development of an active membrane that can be activated with other sources of stimuli, such as

humidity.

Conductive polymer fibrous films have been fabricated in this work using PPy and MWCNT.

MWCNT/PS fibers demonstrated desirable morphology (no beads), thermal stability, but

electrical conductivity was low. In order to increase conductivity, the contact resistance between

the fibers need to be reduced. Further investigation on reducing fiber size while limiting the

amount of bead formation can be carried out to study the influence on conductivity. Other

methods such as introducing a conductive polymer binder which may enhance the conductivity

of the fibers can also be investigated in the future.

PPy coated FeCl3/PS fibers studied in this work demonstrated good thermal stability, and much

higher electrical conductivity compared to MWCNT/PS fibers. Electrospun FeCl3/PS fibers

exhibited fiber-intertwined spherical structures. Future work can include the investigation of

how these spherical structures affect material properties, such as thermal and electrical

conductivities.

70

As mentioned in the conclusion of Chapter 5, the low conductivity of electrospun PPy film may

not be an issue for potential applications in PEMFC. Future work on the characterization of

porosity, thermal conductivity, and structural integrity of PPy coated fibers can be investigated

for improved compatibility with PEMFCs.

To increase catalyst utilization in PEMFCs is another area under active research as discussed in

section 2.3. Conductive polymers exhibit high electro-catalytic activity [78], and the nano

structured PPy provides large reaction sites, which can potentially be beneficial as a catalyst

support. Future work on the investigation of electrochemical properties of electrospun PPy

fibers can also be explored for catalyst support applications.

71

References

[1] R. A. Antunes, M. C. L. Oliveira, G. Ett, V. Ett, Corrosion of metal bipolar plates for PEM fuel cells: A review, Int J Hydrogen Energy. 35 (2010) 3632-3647.

[2] R. O’Hayre, S.W. Cha, W. Colella, F. B. Prinz, Fuel cell fundamentals, New York, John Wiley & Sons, 2005.

[3] H. Li, Y. Tang, Z. Wang, Z. Shi, S. Wu, D. Song, J. Zhang, K. Fatih, J. Zhang, H. Wang, Z. Liu, R. Abouatallah and A. Mazza. A review of water flooding issues in the proton exchange membrane fuel cell, Journal of Power Sources. 178 (2008) 103-117.

[4] M. Mathias, Chapter 46: Diffusion media materials and characterisation. In Handbook of Fuel Cells - Fundamentals, Technology and Applications, by Mark Mathias. New York: John Wiley & Sons, 2003.

[5] D. G. Strickland and J. G. Santiago. In situ-polymerized wicks for passive water management in proton exchange membrane fuel cells, Journal of Power Sources. 195 (2010) 1667-1675.

[6] S. Ge, X. Li and I. Hsing. Water management in PEMFCs using absorbent wicks. Journal of The Electrochemical Society. 151 (2004) B523-B528.

[7] Y. Chen, B. He, J. Lee, N. A. Patankar. Anisotropy in the wetting of rough surfaces. Journal of Colloid and Interface Science. 281 (2005) 458–464.

[8] M. H. Ghazanfari, D. Rashtchian, R. Kharrat, S. Vossoughi. Capillary pressure estimation using statistical pore size functions. Chem. Eng. Technol. 30 (2007) 862–869.

[9] G. G. Wallace. “Condcutive electroactive polymers: intelligent materials system.” Boca Raton, Florida: CRC Press LLC, 2003.

[10] K. S. Teh, Y. Takahashi, Z. Yao and Y. Lu. Influence of redox-induced restructuring of polypyrrole on its surface morphology and wettability. Sensors and Actuators A. 155 (2009) 113-119.

[11] A. D. Price, V. C. Kao, J. X. Zhang and H. E. Naguib. Mechanical characterization of porous membrane core morphologies for conductive polymer trilayer actuators, Cansmart Smart Materials and Structures. Montreal: Cansmart, 2009. 247 - 256.

[12] C. Mao, A. Zhu, Q. Wu, X. Chen, J. Kim, J. Shen, New biocompatible polypyrrole-based films with good blood compatibility and high electrical conductivity, Colloids and Surfaces B: Biointerfaces. 67 (2008) 41-45.

[13] Y. Fang, X. Tan, Y. Shen, N. Xi, G. Alici, A scalable model for trilayer conjugated polymer actuators and its experimental validation, Materials Science and Engineering: C. 28 (2008) 421-428.

[14] A. D. Price, H. E. Naguib, Characterization of conductive polymer trilayer actuators for biomimetic robotics, Cansmart Smart Materials and Structures. Montreal: Cansmart, 2008.

72

[15] Y. Wu, G. Alici, G. M. Spinks, G. G. Wallace, Fast trilayer polypyrrole bending actuators for high speed applications, Synth Met. 156 (2006) 1017-1022.

[16] C. Chan, S. Chang, H. E. Naguib, Development and characterization of polypyrrole bi-layer and tri-layer thin porous films, Smart Mater Struct. 18 (2009) 104022.

[17] M. F. Suárez, R. G. Compton, In situ atomic force microscopy study of polypyrrole synthesis and the volume changes induced by oxidation and reduction of the polymer, J Electroanal Chem. 462 (1999) 211-221.

[18] E. Smela, N. Gadegaard, Volume change in polypyrrole studied by atomic force microscopy, The Journal of Physical Chemistry B. 105 (2001) 9395-9405.

[19] J. Li, E. Wang, M. Green, P. E. West, In situ AFM study of the surface morphology of polypyrrole film, Synth Met. 74 (1995) 127-131.

[20] E. Chainet, M. Billon, In situ study of polypyrrole morphology by STM: effect of the doping state, J. Electroanal. Chem. 451 (1998) 273-277.

[21] Z. Dong, S. J. Kennedy, Y. Wu, Electrospinning materials for energy-related applications and devices, Journal of Power Sources. 196 (2011) 4886–4904.

[22] Z. M. Huang, Y. Z. Zhang, M. Kotaki, S. Ramakrishan, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Composites Science and Technology. 63 (2003) 2223–2253.

[23] S. Agarwal, J. H. Wendorff, A. Greiner, Use of electrospinning technique for biomedical applications, Polymer. 49 (2008) 5603–5621.

[24] V. Thavasi, G. Singh, S. Ramakrishna, Electrospun nanofibers in energy and environmental applications, Energy & Environmental Science. 1 (2008) 205–221.

[25] S. Huang, P. Ganesan, B. N. Popov, Development of conducting polypyrrole as corrosion-resistant catalyst support for polymer electrolyte membrane fuel cell (PEMFC) application, Appl. Catal. B-Environ. 93 (2009) 75-81.

[26] I. S. Chronakis, S. Grapenson, A. Jakob, Conductive polypyrrole nanofibers via electrospinning: electrical and morphological properties, Polymer. 47 (2006) 1597–1603.

[27] T. S. Kang, S. W. Lee, J. Joo, J. Y. Lee, Electrically conducting polypyrrole fibers spun by electrospinning, Synthetic Metals. 153 (2005) 61-64.

[28] L. Y. Yeo, J. R. Friend, Electrospinning carbon nanotube polymer composite nanofibers, Journal of Experimental Nanoscience. 1 (2006) 177-209.

[29] Q. Zhang, Z. Chang, M, Zhu, X. Mo, D, Chen, Electrospun carbon nanotube composite fibers with uniaxially aligned arrays, Nanotechnology. 18 (2007) 115611.

[30] J. H. Sung, H. S. Kim, H. J. Jin, H. J. Choi, I. J. Chin, Nanofibrous membranes prepared by multiwalled carbon nanotube/poly(methyl methacrylate) composites, Macromolecules. 37 (2004) 9899-9002.

73

[31] S. Mazinani, A. Ajji, C. Dubois, Morphology, structure, and properties of conductive PS/CNT nanocomposite electrospun mat, Polymer. 50 (2009) 3329-3342.

[32] Y. Ji, C. Li, G. Wang, J. Koo, S. Ge, B. Li, J. Jiang, B. Herzberg, T. Klein, S. Chen, J. C. Sokolov, M. H. Rafailovich, Confinement-induced super strong PS/MWCNT composite nanofibers, A Letters Journal Exploring the Frongiers of Physics, 84 (2008) 56002.

[33] B. Sundaray, V. Subramanian, T. S. Natarajan, Preparation and characterization of polystyrene-multiwalled carbon nanotube composite fibers by electrospinning, Journal of Nanoscience and Nanotechnology, 7 (2007) 1793-1795.

[34] H. Hou, J. J. Ge, J. Zeng, Q. Li, D. H. Reneker, A. Greiner, S. Z. D. Cheng, Electrospun polyacrylonitrile nanofibers containing a high concentration of well-aligned multiwall carbon nanotubes, Chem. Mater. 17 (2005) 967-973.

[35] G. Han, G. Shi, Novel route to pure and composite fibers of polypyrrole, Journal of Applied Polymer Science. 103 (2007) 1490-1494.

[36] K. Keptang, J. S. Park, Electrospinning PVDF/PPy/MWCNTs conducting composites, Synthetic Metals. 160 (2010) 1603-1608.

[37] A. Laforgue, L. Robitaille, Production of conductive PEDOT nanofibers by the combination of electrospinning and vapor-phase polymerization, Macromolecules. 43 (2010) 4194-4200.

[38] S. Nair, E. Hsiao, S. H. Kim, Fabrication of electrically-conducting nonwoven porous mats of polystyrene–polypyrrole core–shell nanofibers via electrospinning and vapor phase polymerization, Journal of Materials Chemistry. 18 (2008) 5155-5161.

[39] Z. Sun, E. Zussman, A. Yarin, J. H. Wendorff, A. Greiner, Compound core-shell polymer nanofibers by co-electrospinning, Advanced Materials. 15 (2003) 1929-1932.

[40] C. Wang, K. W. Yan, Y. D. Lin, P. C. H. Hsieh, Biodegradable core/shell fibers by coaxial electrospinning: processing, fiber characterization, and its application in sustained drug release, Macromolecules. 43 (2010) 6389-6397.

[41] J. F. Zhang, D. Z. Yang, F. Xu, Z. P. Zhang, R. X. Yin, J. Nie, Electrospun core−shell structure nanofibers from homogeneous solution of poly(ethylene oxide)/chitosan, Macromolecules. 42 (2009) 5278-5284.

[42] J. Zhang, T. Hreid, X. Li, W. Guo, L. Wang, X. Shi, H. Su, Z. Yuan, Nanostructured polyaniline counter electrode for dye-sensitised solar cells: Fabrication and investigation of its electrochemical formation mechanism, Electrochim. Acta. 55 (2010) 3664–3668.

[43] G. M. Kumar, V. Raman, J. Kawakita, P. Ilanchezhiyan, R. Jayavel, Fabrication of polypyrrole/ZnCoO nanohybrid systems for solar cell applications, Dalton Trans. 39 (2010) 8325-8330.

74

[44] T. Makris, V. Dracopoulos, T. Stergiopoulos, P. Lianos, A quasi solid-state dye-sensitized solar cell made of polypyrrole counter electrodes, Electrochim. Acta. 56 (2011) 2004-2008.

[45] X. Feng, X. Huang, Z. Tan, B. Zhao, S. Tan, Application research of polypyrrole/graphite composite counter electrode for dye-sensitized solar cells, Acta Chim. Sin. 69 (2011) 653-658.

[46] J. Xia, L. Chen, S. Yanagida, Application of polypyrrole as a counter electrode for a dye-sensitized solar cell, J. Mater. Chem. 21 (2011) 4644-4649.

[47] L. Cui, J. Shen, F. Cheng, Z. Tao, J. Chen, SnO2 nanoparticles-polypyrrole nanowires composite as anode materials for rechargeable lithium-ion batteries, Journal of Power Sources. 196 (2011) 2195-2201.

[48] S. Paul, Y. Lee, J. Choi, Y.C. Kang, D. Kim, Synthesis and electrochemical characterization of polypyrrole/multi-walled carbon nanotube composite electrodes for supercapacitor applications, Bull. Korean Chem. Soc. 31 (2010) 1228-1232.

[49] G. A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, Journal of Power Sources. 196 (2011) 1-12.

[50] W. Sun, R. Zheng, X. Chen, Symmetric redox supercapacitor based on micro-fabrication with three-dimensional polypyrrole electrodes, Journal of Power Sources. 195 (2010) 7120-7125.

[51] Z. Guo, J. Wang, H. Liu, S. Dou, Study of silicon/polypyrrole composite as anode materials for Li-ion batteries. Journal of Power Sources. 146 (2005) 448–451.

[52] L. Cindrella, A.M. Kannan, Membrane electrode assembly with doped polyaniline interlayer for proton exchange membrane fuel cells under low relative humidity conditions, Journal of Power Sources. 193 (2009) 447-453.

[53] Q. M. Huang, Q. L. Zhang, H. L. Huang, W. S. Li, Y. J. Huang, J. L. Luo, Methanol permeability and proton conductivity of Nafion membranes modified electrochemically with polyaniline, Journal of Power Sources. 184 (2008) 338-343.

[54] M. A. Smit, A.L. Ocampo, M.A. Espinosa-Medina, P.J. Sebastian, A modified Nafion membrane with in situ polymerized polypyrrole for the direct methanol fuel cell, Journal of Power Sources. 124 (2003) 59-64.

[55] J. Yang, P.K. Shen, J. Varcoe, Z. Wei, Nafion/polyaniline composite membranes specifically designed to allow proton exchange membrane fuel cells operation at low humidity, Journal of Power Sources. 189 (2009) 1016-1019.

[56] S. M. Unni, V. M. Dhavale, V. K. Pillai, S. Kurungot, High Pt utilization electrodes for polymer electrolyte membrane fuel cells by dispersing Pt particles formed by a preprecipitation method on carbon “polished” with polypyrrole, The Journal of Physical Chemistry C. 114 (2010) 14654-14661.

75

[57] S. Joseph, J. C. McClure, P. J. Sebastian, J. Moreira, E. Valenzuela, Polyaniline and polypyrrole coatings on aluminum for PEM fuel cell bipolar plates, Journal of Power Sources. 177 (2008) 161-166.

[58] Y. S. Kim, S. H. Nam, H. S. Shim, H. J. Ahn, M. Anand, W. B. Kim, Electrospun bimetallic nanowires of PtRh and PtRu with compositional variation for methanol electrooxidation, Electrochemistry Communications. 10 (2008) 1016-1019.

[59] H. J. Kim, Y. S. Kim, M. H. Seo, S. M. Choi, W. B. Kim, Pt and PtRh nanowire electrocatalysts for cyclohexane-fueled polymer electrolyte membrane fuel cell, Electrochemistry Communications. 11 (2009) 446–449.

[60] H. J. Kim, Y. S. Kim, M. H. Seo, S. M. Choi, J. Cho, G W. Huber, W. B. Kim, Highly improved oxygen reduction performance over Pt/C-dispersed nanowire network catalysts, Electrochemistry Communications. 12 (2010) 32–35.

[61] J. Shui, J. C. M. Li, Platinum nanowires produced by electrospinning, Nano Letters. 9 (2009) 1307-1314.

[62] Z. Wu, L. Xu, W. Li, M. Waje, Y. Yan, Polyaniline nanofibre supported platinum nanoelectrocatalysts for direct methanol fuel cells, Nanotechnology. 17 (2006) 5254–5259.

[63] J. Huang, H. Hou, T. You, Highly efficient electrocatalytic oxidation of formic acid by electrospun carbon nanofiber-supported PtxAu100-x bimetallic electrocatalyst, Electrochemistry Communications. 11 (2009) 1281–1284.

[64] X. Liu, M. Li, G. Han, J. Dong, The catalysts supported on metallized electrospun polyacrylonitrile fibrous mats for methanol oxidation, Electrochimica Acta. 55 (2010) 2983–2990.

[65] M. Li, S. Zhao, G. Han, B. Yang, Electrospinning-derived carbon fibrous mats improving the performance of commercial Pt/C for methanol oxidation, Journal of Power Sources. 191 (2009) 351–356.

[66] Y. Ding, P. Zhang, Y. Jiang, D. Gao, Effect of rare earth elements doping on structure and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 for lithium-ion battery, Solid State Ionics. 178 (2007) 967 – 971.

[67] L. Wang, Y. Yu, P. C. Chen, D. W. Zhang, C. H. Chen, Electrospinning synthesis of C/Fe3O4 composite nanofibers and their application for high performance lithium-ion batteries, Journal of Power Sources. 183 (2008) 717–723.

[68] L. Ji, Z. Lin, R. Zhou, Q. Shi, O. Toprakci, A. Medford, C. R. Millns, X. Zhang, Formation and electrochemical performance of copper/carbon composite nanofiber, Electrochimica Acta. 55 (2010) 1605–1611.

[69] C. Kim, Electrochemical characterization of electrospun activated carbon nanofibres as an electrode in supercapacitor, Journal of Power Sources. 142 (2005) 382–388.

76

[70] C. Kim, Y. O. Choi, W. J. Lee, K. S. Yang, Supercapacitor performances of activated carbon fiber webs prepared by electrospinning of PMDA-ODA poly(amic acid) solutions, Electrochimica Acta. 50 (2004) 883–887.

[71] J. Li, E. Liu, W. Li, X. Y. Meng, S. T. Tan, Nickel/carbon nanofibers composite electrodes as supercapacitors prepared by electrospinning, Journal of Alloys and Compounds. 478 (2009) 371–374.

[72] A. D. Lueking, L. Pan, D. L. Narayannan, C. E. B. Clifford, Effect of Expanded Graphite Lattice in Exfoliated Graphite Nanofibers on Hydrogen Storage, J. Phys. Chem. B. 109 (2005) 12710-12717.

[73] M. Michel, F. Ettingshausen, F. Scheiba, A. Wolz and C. Roth. Using layer-by-layer assembly of polyaniline fibers in the fast preparation of high performance fuel cell nanostructured membrane electrodes. Physical Chemistry Chemical Physics. 2008.

[74] Y. Bar-Cohen, X.Q. Bao, S. Sherrit, S.S. Lih, Characterization of the electromechanical properties of ionomeric polymer-metal composite (IPMC), Smart Structures and Materials 2002: Electroactive Polymer Actuators and Devices (Eapad). 4695 (2002) 286-293.

[75] P. M. Dziewonski, M. Grzeszczuk, Impactof the Electrochemical Porosity and Chemical Composition on the Lithium Ion Exchange Behavior of Polypyrroles (ClO4

-, TOS-, TFSI-) Prepared Electrochemically in Propylene Carbonate. Comparative EQCM, EIS and CV Studies, The Journal of Physical Chemistry B. 114 (2010) 7158-7171.

[76] Z. Yoshimitsu, A. Nakajima, T. Watanabe, K. Hashimoto, Effects of Surface Structure on the Hydrophobicity and Sliding Behavior of Water Droplets, Langmuir. 18 (2002) 5818-5822.

[77] T. Zhou, H. Liu, Effects of the electrical resistances of the GDL in a PEM fuel cell, Journal of Power Sources. 161 (2006) 444–453.

[78] W. Martinez M. T. T. Thompson, M. A. Smit, Characterization and electrocatalytic activity of carbon-supported polypyrrole-cobalt-platinum compound, Int. J. Electrochem. Sci. 5 (2010) 931 – 943.

[79] D. R. Timmons, C. K. Mutchler, E. M. Sherstad, Use of fluorescein to measure the composition of waterdrop splash, Water Resources Research, 7 (1971) 1020-1023.

[80] P. G. de Gennes, Wetting: statics and dynamics, Rev. Mod. Phys. 57 (1985) 827-862.

development of conductive polymer membranes for energy ......development of conductive polymer...

Documents