Proceedings of the ASME 2010 Eighth International Fuel Cell Science, Engineering and Technology Conference FuelCell2010

June 14-16, 2010, Brooklyn, New York, USA

FuelCell2010-33168

Investigation of Electroactive Polymers for the PEMFC GDL

Jingwen Wang1, 2, Hani E. Naguib

2, Aimy Bazylak

1*

Microscale Energy Systems Transport Phenomena Laboratory1

Smart and Adaptive Polymer Laboratory2

Department of Mechanical & Industrial Engineering, University of Toronto 5 Kings College Road, Toronto, Ontario, Canada, M5S 3G8

ABSTRACT In this work, electroactive polymers (EAPs) are introduced

as novel materials for the polymer electrolyte membrane fuel

cell (PEMFC). Polypyrrole (PPy) is selected as a promising

EAP for the PEMFC. 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 behavior of PPy thin film is observed,

and the surface properties are analyzed.

INTRODUCTION Water Management

In the hydrogen-oxygen polymer electrolyte membrane

fuel cell (PEMFC), water is transported through the gas

diffusion layer (GDL) and into the flow channels

predominantly via diffusion and capillary forces [1]. Water

accumulation may lead to water flooding in the cathode flow

channels and block the pores of the GDL. Cathode flooding

hinders oxygen transport through the GDL to the reaction sites,

which leads to performance degradation.

The GDL must exhibit several properties that ensure a

functional fuel cell: gas permeability, water permeability,

electronic conductivity, thermal conductivity and structural

integrity [2]. The most widely used GDL materials are carbon

fibre based products such as carbon paper and carbon cloth due

to their high porosity and high electrical conductivity. The

GDL material is often treated with polytetrafluoroethylene

(PTFE) to increase hydrophobicity for enhanced water removal

rates [1].

The incorporation of novel materials in the fuel cell to

improve water management has only been explored in a limited

number of studies [3, 4]. Strickland et al. [3] polymerize

ethylene glycol dimethacrylate (EDMA) and hydroxyethyl

methacrylate (HEMA) in the flow channels creating thin porous

hydrophilic wicks. They report that a 62% increase in peak

power is achieved with polymer wicks in comparison to a cell

without wicks [3]. Ge et al. [4] 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 are mainly used to eliminate pre-

humidification of the reactant gases. These solutions are

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. [5] report that rough

surfaces with anisotropic grooves increase surface

hydrophobicity. Larger pores result in lower capillary pressures,

as reported in the model presented by Hossein et al. [6]. The

change in surface roughness and pore geometry of the GDL can

be critical in water management.

Conducting Electroactive Polymers Liquid water management within and on the surface of the

GDL stands to benefit from the ability to actively alter the GDL

properties within an operating PEMFC. The class of materials

that are sensitive to environmental stimuli is referred to as

“smart” or “intelligent" materials. Electroactive polymers

(EAPs) are a subcategory of smart materials that can recognize

environmental stimuli and respond to these stimuli in a

repeatable manner [7]. EAPs such as polypyrrole (PPy),

polyanline (PANI), and polythiophene are examples of

conductive smart materials. They can be produced with a

diverse range of properties and further manipulated by

electrical stimuli. EAPs have attracted much attention in the

past 15 years due to their environmental stability and redox

switching property [8]. The redox switching property is widely

studied for sensors [9], actuators [10], artificial muscles [11],

and biomimetic robotics [12]. PPy among all EAPs is one of

the most studied smart polymers because of its high

conductivity, ease of preparation, environmental stability and

favourable mechanical properties [13].

The conductive properties of EAPs are introduced by the

incorporation of dopant ions. Dopant, also known as the doping

agent, is an impurity introduced to the polymer molecular

structure during polymerization that allows electrons to hop

*Corresponding Author: A. Bazylak (abazylak@mie.utoronto.ca)

from one polymer chain to another. The dopant ion is critical in

this process to balance the charge on the polymer backbone and

it also affects the conductivity and polymerization process of

the conducting polymer.

The electrical stimulation of EAPs 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 [8] from an electrolytic source. The

transfer of dopant ions results in the expansion or contraction of

the polymer chain [14]. Generally, dopant ions influence the

mechanical, electrical and electro-activation properties of the

polymer, therefore, they need to be carefully considered for

each application [15, 16].

A number of studies have incorporated EAPs such as PPy

and PANI in fuel cell applications [17-24]. Mondal et al. [17]

use PANI coated Ni as anode catalyst layer to replace the use of

platinum (Pt). This application is limited to direct methanol fuel

cells (DMFC) using ascorbic acid as fuel. Huang et al. [18]

electrochemically modify a Nafion 117 membrane with PANI

to reduce methanol crossover in a DMFC. Joseph et al. [19]

utilize the conductivity of both PANI and PPy to coat

aluminum bipolar plates to address corrosion issues.

Recent studies that link smart polymers with PEMFCs are

focused on increasing catalyst utilization [20] and lowering

operation humidity level [21, 22]. These applications make use

of the conductivity but not the “smart” properties of these

EAPs. In other applications, the research of EAPs has been

mainly focused on actuators and artificial muscles [7, 10, 11,

12]. Furthermore, both the activation of porous electroactive

polymers and the exploitation of surface property changes

during activation have not been studied previously.

Figure 1: Proposed flow channel design incorporating electroactive polymer thin film along the surface of the

GDL.

For the first time, smart polymers are presented as novel

porous materials for the PEMFC, which can be utilized as an

activated GDL coating. In this work, the electro-chemical

deposition process for producing porous PPy film is presented,

along with details of the activation process. The motivation for

this work is the proposed design shown in Fig. 1, where a thin

film of PPy may be incorporated into a fuel cell flow channel

on top of the GDL, so that it may be activated for improved

water management. In this work, as a proof-of-concept, a thin

and porous layer of PPy is fabricated and activated.

EXPERIMENTAL Fabrication

In this work, conductive PPy film is produced via an

electropolymerization process from an electrolyte solution

containing pyrrole (Py) monomers. The process begins with an

electrolytic solution consisting of a 99% propylene carbonate

(PC) solution (solvent), 0.2M of Py monomer solution, and

0.2M of bis(trifluoromethane) sulfonimide lithium salt

(LiTFSI). 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 2 hours to ensure solution homogeneity.

The Py monomer in the electrolyte solution changes from

a liquid phase to a solid phase through electropolymerization.

The electropolymerization apparatus is the custom-designed

electrochemical cell chamber shown in Fig. 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 (Fig. 2) for the polymerization step. A current density

of 0.1mA/cm2 is applied with a Keithley 2400 Sourcemeter.

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

electropolymerization process is conducted at -30ºC for 14

hours to form the polymer thin film.

Figure 2: Schematic of the electrochemical cell apparatus for the electropolymerization of PPy.

Testing Conducting PPy thin film is activated by applying an

electric potential across the sample in the presence of a dopant

reservoir, which is an insulating porous polyvinylidene fluoride

(PVDF) core. The PPy film is manually placed on top of the

PVDF core and stored in a new electrolyte solution containing

0.2M of LiTFSI in PC prior to activation. For the activation

process, PPy and PVDF films are placed on a glass slide, and

two electrodes are placed on both sides of the PVDF core. A

potential ranging from 2V to 5V is then applied, during which

the PPy film is imaged from above with a high resolution CCD

camera (PCO Pixelfly) combined with a stereoscope (Leica

Z16 APO) as shown in Fig. 3. Two experimental trials are

carried out with voltage settings of 2V and 5V, and microscopic

images are taken at 10 minute intervals. Images are processed

using software developed in-house (MATLAB).

Figure 3: Schematic of the testing apparatus.

RESULTS & DISCUSSION The sample prepared has dimensions of 20mm × 20mm ×

107.6µm. Fig. 4 shows a set of microscopic images taken when

the PPy film is under 2V and 5V of electrical activation after 0,

10 and 30 minutes. The darker regions in the images represent

the surface pores. For the 2V activation, no significant changes

are shown from image (a) to (c). When the same sample is

employed for the 5V 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 2V

and 5V activations. This wrinkling effect results in the shifting

of pore locations on the material surface when 5V is applied.

Fig. 5 illustrates the surface pore area as a function of time

when 2V and 5V are applied to the PPy film. The microscopic

images are first converted to binary images with a threshold

value of 0.35 where every pixel with brightness less than 0.35

is set to 0 (black). The pore diameters are then obtained by

calculating the equivalent diameter using MATLAB. A stimulus

of 2V does not result in any significant changes to the surface

pore area for this sample over a period of 30min. As seen in

Fig. 5, significant increases to the surface pore area are

observed for the 5V activation. After a period of 30min, 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.

Fig. 6 is a histogram of the pore size distribution before

and after the 30min 5V activation. A significant shift in pore

size distribution is observed after this activation with the

average pore diameter changing from 61µm to 67µm. The

number of smaller pores increasing may be a result of surface

wrinkling which generates higher pore density on the material

surface.

(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 4: Microscopic images of porous PPy thin film under 2V and 5V activation.

200µm

Figure 5: Total surface pore area over a period of 30 min for sequential 2V and 5V electrical activation potentials.

Figure 6: Histogram of pore size distribution before and

after a 5V activation.

CONCLUSION In this study, PPy is fabricated and tested as a porous, smart

conductive polymer layer that has the potential to be employed

as a modifying layer for the PEMFC GDL. A conductive porous

PPy thin film is electropolymerized from an electrolyte solution

containing Py monomers. The thin film is then activated with

an applied potential ranging from 2V to 5V, and the mechanical

response is investigated. Although a 2V stimulus results in

negligible changes to the surface pore area after 30min, a 5V

stimulus results in a 45% increase in surface pore area after

30min and an increase in average pore size from 61µm to

67µm. The surface roughness is also increased by material

wrinkling, although future work is required to measure the

changes in surface roughness, which may also 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

: Total surface pore area over a period of 30 min for

sequential 2V and 5V electrical activation potentials.

: Histogram of pore size distribution before and

In this study, PPy is fabricated and tested as a porous, smart

conductive polymer layer that has the potential to be employed

as a modifying layer for the PEMFC GDL. A conductive porous

PPy thin film is electropolymerized from an electrolyte solution

ining Py monomers. The thin film is then activated with

an applied potential ranging from 2V to 5V, and the mechanical

response is investigated. Although a 2V stimulus results in

negligible changes to the surface pore area after 30min, a 5V

% increase in surface pore area after

age pore size from 61µm to

µm. The surface roughness is also increased by material

wrinkling, although future work is required to measure the

may also have desirable

effects for liquid water management on the surface of the GDL.

material surface properties can be

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 du

required to activate PPy/TFSI, the

this study may not be the most practical for use in a working

PEMFC. However, this work provides insight into how

conductive smart polymers can be employed to alt

surface properties for improved water management. The

activation potential is directly related to the EAP film thickness,

therefore thinner PPy/TFSI films will be investigated in the

future for activation potential reduction. Alici and Gunderso

[25] report a PPy/TFSI actuator with activation voltage as low

as 1V. The low voltage is achieved by a PPy/TFSI film that is

approximately 30µm in thickness. Hara et al. [26] also conduct

experiments with a thin PPy/TFSI actuator that is 35µm thick,

requiring an activation potential of 0.7V. In this work, the EAP

film is investigated 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.

Future work will include the investigation of

conductivity, permeability, and mechanical strength

film are affected by the fabrication and activation process

situ testing will also be performed to validate the compatibility

of EAP film in a fuel cell and its benefit on water management.

ACKNOWLEDGMENTS

The Natural Sciences and Engineering Research Council of

Canada (NSERC), Bullitt Foundation and University of Toronto

are gratefully acknowledged for their financial support.

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