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Effect of surface wettability of polymer composite bipolarplates on polymer electrolyte membrane fuel cellperformances

Fatma Gul Boyaci San*, Isil Isik-Gulsac

Energy Institute, TUB_ITAK Marmara Research Center, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey

a r t i c l e i n f o

Article history:

Received 22 November 2012

Received in revised form

16 January 2013

Accepted 19 January 2013

Available online 19 February 2013

Keywords:

Bipolar plate

Surface wettability

Fuel cell

Polymer composite

* Corresponding author. Tel.: þ90 262 677 27E-mail addresses: fatmagul.boyaci@tubita

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.01.1

a b s t r a c t

The effects of fluoropolymer based additive at different additive/binder and additive/filler

ratios on surface wettability, conductivity andmechanical properties of polymer composite

bipolar plates are investigated in this study. Fuel cell performance tests are performed at

different feed flow rates by using composite bipolar plates containing organic based hy-

drophobic and inorganic based hydrophilic additives to investigate the effect of surface

wettability properties on polymer electrolyte membrane fuel cell (PEMFC) performance.

The conductivity of the composite materials decreases with the increase in additive/filler

ratios, due to a decrease in the amount of conductive filler in the composite structure,

whereas conductivity increases with the increase in additive/binder ratios due to

a decrease in the amount of nonconductive binder. The surface hydrophobicity gets

stronger with increasing fluoropolymer/filler and fluoropolymer/binder ratio amounts,

related to the hydrophobic properties of both filler and fluoropolymer. In all feed flow rates,

at low current densities, the single cells exhibit almost the same performance. At inter-

mediate and high current densities, polymer composite without any additives shows

higher performance than the bipolar plates containing organic or inorganic based addi-

tives. Current and power densities show maxima at the bipolar plate contact angle of 80�.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction The fuel cell stack is mainly composed of bipolar plates,

Polymer electrolyte membrane fuel cells (PEMFCs) represent

one of the most promising power production technologies in

the near future; because of their zero emissions, low noise and

operating temperatures, relatively quick startup, rapid

response to varying loads, high efficiencies and power den-

sities. They find application in many areas, including power

supplies in cellular telephones, laptop computers and port-

able entertainment equipments, automobiles, residential

power, and military communication installations [1e5].

03; fax: þ90 262 642 35 54k.gov.tr, fgbs37@gmail.co2013, Hydrogen Energy P35

end plates and membrane electrode assembly (MEA). MEAs

have fuel electrode (anode) and oxidant electrode (cathode)

separated by an ion conducting electrolyte/membrane.

Hydrogen passes over one electrode and oxygen over the

other, generating electricity, water and heat. The bipolar plate,

which is one of the key components of PEMFC, contacts with

the surface of the cathode and the anode of the next cell [2e4].

A simplified structure of a PEMFC stack is given in Fig. 1.

The bipolar plate has five main functions: distribution of

gases homogeneously over anode and cathode, separation of

.m (F.G. Boyaci San).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Simplified structure of a PEMFC stack.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 84090

the fuel and oxidant gases, prevention of gas leakage, pro-

viding of electrical contact between the anodes and cathodes

and removing of heat and water.

Requirements applying to bipolar plates in PEMFC are (a)

high in plane electrical conductivity (>100 S cm�1); (b) chem-

ical stability in the presence of hydrogen fuel, oxygen, and

slightly acidic water (pH < 4), corrosion resistance < 16

mAcm�2; (c) chemical compatibility, such as no byproducts

affecting the MEA performance and no plate surface degra-

dations; (d) high thermal conductivity >10 W (mK)�1; (e) low

permeability to hydrogen and oxygen with hydrogen perme-

ability<2� 10�6 cm3 cm�2 s�1; (f) goodmechanical properties,

such as, tensile strength >41 MPa, and flexural strength

>59 MPa (g) thermal stability at fuel cell operating tempera-

tures (�40)e120 �C [2]. In addition, they must have suitable

surface properties to manage the water in the stack.

During the operation of PEMFCs, water is involved in the

cathode side via three processes, transport by the humidified

reactants, water generation due to the electrochemical re-

actions, and transport via the electro osmotic drag associated

with proton transport across thepolymer electrolytemembrane

[6]. In order to prevent back diffusion of water from the cathode

side to anode side, which generated at the cathode electrolyte

interface due to the electrochemical reaction, water must be

removed from the cathode electrode. Thus, water management

is one of the major issues in PEMFC technology. The ionic con-

ductivity will decrease if the membrane electrolyte is too dry,

whereas, when the cell is too wet; the flooding of the porous

agglomerate and gas diffusion layer (GDL) opposing affects the

performance of PEMFC, leading to concentration overpotential

[7]. Flooding and dehydration has been identified as the main

current limiting processes, thus, improving liquid water trans-

port throughout the cell is very important in increasing PEMFC

performance. Additionally, better water management can

reduce or eliminate the need for humidification [8]. It is very

important to control the gradientand the locationof thewater in

the fuel cell, without negatively affecting the conductivity of

membrane and porosity of the electrode or GDL [7e10].

There are several approaches specific to solve problems in

water management. Water management can be controlled by

changing the operation conditions of PEMFC such as humidity

of the feed streams, fuel cell temperature, back pressure and

air/fuel stoichiometric flow rates [7,11,12]. Another strategy is

to design a gas humidification system with high humidifica-

tion performance and low energy consumption [13].

Optimization and innovative design of the MEA material

properties and structures are also investigated in the liter-

ature. There are extensive studies on the development of

membranematerial [14,15], GDLmaterial and structure design

[8,16e22]. GDL materials are commonly treated with a hydro-

phobic agent such as polytetrafluorethylene (PTFE) to change

its wetting characteristics, thus, especially the product water

at the cathode can be efficiently removed fromGDL to the flow

field channels [23e28]. Treatment with such agents leads to

a mixture of hydrophilic and hydrophobic pores in GDL, the

hydrophobic pores allow a pathway for gas transport whereas

the hydrophilic pores facilitate liquid water transport [26].

Another approach is to design an appropriate flow channel

design, which plays a major role in the water management

problems, as the water transported from GDL must be

removed out of the cell system via flow channels [21,29,30].

Very few studies focused to change thematerial wettability

of polymer composite bipolar plates on the way to improve

water management in PEMFCs [31e41].

Ge at al. [31] incorporated hydrophilic polyvinyl alcohol

(PVA) sponge wicking material into graphite cathode flow

channels. They claimed that mounting the sponge wicks was

advantageous for the humidification of dry inlet air and for the

removal of liquid water in the cell. An accelerated water

removal was achieved at a current density of 1.2 A cm�2.

Owejan et al. [32] investigated effects of liquid distribution

in gold coated aluminum flow channels with and without

PTFE. It was observed that channel geometry and surface

property both have appreciable effects on the volume of

accumulated water and on the morphology of water droplets

retained in the flow field channels.

Yang et al. [33] suggested that a hydrophilic bipolar plate

surface might help to remove water from GDL. As the droplet

grew large enough to touch the more hydrophilic channel

walls, a liquid film was formed on the wall as a result of the

lower surface contact angle. Finally, the water gradually

migrated along the channel walls toward the exit.

Taniguchi et al. [6] deposited a hydrophobic thin filmon the

surface of the metallic gas flow channels by plasma poly-

merization of hexafluoropropylene in order to make the

channels hydrophobic. Sand blast pretreatment was applied

prior to plasma polymerization in order to increase water

contact angle of the coated surface. The peak power in the

PEMFC was improved with gas flow channels, which were

surface treated by sand blast pretreatment and the following

plasma polymerization. The waterproofing was effective in

the condition of lower oxidant flow rate.

Tang et al. [34] investigated the benefit of the super hy-

drophobic and super hydrophilic surface modification on the

aluminum flow channels to remove water by purging with

nitrogen gas. They found that while the super hydrophobic

coating helped to keep the water content low in the channels

and improved the purging of the water out of the channels,

the super hydrophilic coating helped to pull water out of the

landing area.

Turhan et al. [35] examined the through plane liquid stor-

age, transport and flooding mechanism inside a PEMFC as

a function of channel wall hydrophobicity with the use of high

resolution neutron imaging. The neutron images of hydro-

philic anode/PTFE coated cathode and PTFE coated anode/

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 8 4091

hydrophilic cathode, gold coated aluminum channel walls

illustrated thewater build upmechanism in the flow channels

based on surface treatment. Their results showed that un-

coated channels had less water on average, and liquid formed

a film layer around the walls which promoted steadier oper-

ation, but was more difficult to purge. Hydrophilic channel

walls enhanced the liquid suction from under the land loca-

tions whereas PTFE coated land channel interface suppressed

the liquid inside diffusionmedia resultingmore water storage

in diffusion media.

Bazylak et al. [36] employed experiments to study the

droplet stability of the droplet as a function of the bipolar plate

wetting properties and the potential for liquid entrapment in

GDL/land contact area. In order to vary the wettability of the

simulated PEMFC land area, three different solid plates were

employed in the study: untreated microscope slides, micro-

scope slides with a hydrophobic treatment, and a graphite

plate. Their results indicated that hydrophilic land areas may

be beneficial to increase the likelihood of GDL/land area

entrapment, whereas if water removal through gas flow was

preferred, then hydrophobic gas channels might enhance

droplet formation and detachment.

Jung et al. [37] manufactured ladder structure flow fields

with hydrophobic PTFE materials coated on different ladders

to investigate the effect of oxidant flow rate and oxidant hu-

midity on PEMFC performance. They found that the greatest

improvement in performance and durability was obtained

with hydrophobic PTFE coated on the second ladder.

Nowak et al. [38] developed electrically conductive and

hydrophilic coatings for PEMFC stainless steel bipolar plates

for enhanced fuel cell performance and water management.

They tested the coatings according to hydrophilicity retention

under wet and dry fuel cell conditions and determined that

the 1,2-bis(triethoxysilyl)ethane eCOOH coating remained

hydrophilic on stamped stainless steel bipolar plate pro-

totypes after greater than 1200 h.

Lu et al. [39] investigated the effects of channel surface

wettability, cross sectional geometry and orientation on the

two phase flow in parallel gas channels of PEMFCs. They found

that the hydrophilically coated gas channels are advanta-

geous over uncoated or slightly hydrophobic channels due to

uniform water and gas flow distribution and favoring film

flow. Also the higher performances were obtained by sinu-

soidal channel geometry and vertical channel orientation.

Zhang et al. [40] investigated the liquidwater transport and

removal from GDL and gas channel of a PEMFC. Two modes

were identified to remove of liquid water from GDL surface.

The first mode is droplet detachment by the shear force of the

gas flow followed by a mist flow in the gas channel. The

droplet detachment diameter is correlatedwell with themean

gas velocity in the channel. The second mode is capillary

wicking onto the more hydrophilic channel walls followed by

the annular film flow and/or liquid slug flow in the channel.

The film instability and channel cloggingwas observed in case

of insufficient corner flow to remove liquid water from the gas

channel.

Zhu et al. [41] used micro CT X-ray imagining to look

through the graphite flow channels. They determined that the

drops of the water in both hydrophobic and hydrophilic flow

channels distorted from the expected spherical shape before

they started to move. Drops in hydrophobic channels did not

remove from GDL before moving, whereas water in the hy-

drophilic channel formed a thin water layer at the bottom of

the flow channel.

To the best of our knowledge, there are no studies inves-

tigating the effectiveness and the amount of organic surface

modifier agent on surface wettability, electrical conductivity,

mechanical and electrochemical properties of thermoset

polymer/graphite based composite bipolar plates in PEMFC.

Graphite filled polymer composite bipolar plates offer

a combination of low cost material and economic processing,

moreover, exhibit performance comparable to metal plates.

Polymer matrix used not only plays a binder role in graphite

filled polymer composite bipolar plate production process, but

is also one of the main factors affecting performances of bi-

polar plate. Most of the properties, such as electrical conduc-

tivity, mechanical strength, permeability, porosity, etc. are

affected by the properties of the polymer itself. Polymer ma-

trix tolerates the incorporation of large amounts of additive

with no deterioration of its mechanical properties, thus fa-

voring the dimensional stability of the resulting composite

[42]. In this study, the effects of fluoropolymer based additive

at different additive/binder and additive/filler ratios on sur-

face wettability, conductivity and mechanical properties of

polymer composite bipolar plates are investigated. Addition-

ally, in order to examine the effect of surface wettability

properties on PEMFC performance, fuel cell tests are per-

formed at different feed flow rates by using the bipolar plates

containing inorganic based hydrophilic and organic based

hydrophobic additives.

2. Experimental

2.1. Composite bipolar plate preparation

Graphite based material as conductive filler (70-80 wt. %),

thermoset based polymeric material as a binder (10e20 wt. %),

mold release agent (1 wt. %) and additive (fluoropolymer)

(0e10 wt. %) are mechanically mixed with different additive/

filler (0e0.14) and additive/binder (0e1.11) ratios to prepare

the composites. The mixtures are poured into a mold having

20 cm� 20 cm � 5mm dimensions. Then themixture is cured

in a compression molding machine at 180 �C, 50 bar for 5 min.

In order to investigate the effect of surface wettability prop-

erties on PEMFC performance, polymer composites are pre-

pared with both organic and inorganic additives at 1.11

additive/binder and 0.14 additive/filler ratios, since the

stronger effects can be observed at high additive concentra-

tions. Fluoropolymer type additive is used as organic based

(OBA) and silicate typemineral is used as inorganic based (IBA)

additives, which are selected due to their hydrophobic and

hydrophilic characteristics. Polymer composites having 0.14

organic additive/filler, 1.11 organic additive/binder, 0.14 inor-

ganic additive/filler, 1.11 inorganic additive/binder ratios are

coded as, OBA1, OBA2, IBA1 and IBA2, respectively. The pol-

ymer composite without any additives is abbreviated as A0.

Anode and cathode flow fields with rectangular channels of

1 mm width and 1 mm depth are machined by computer

numerical control (CNC) machine. Isel FlatCom CNC machine

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 84092

is used at feed rate of 20 mm min�1 and spindle speed of

25 000 rpm with 1 mm carbon steel.

2.2. Characterization of bipolar plates

Surface wettability properties of the samples are examined by

measuring the contact angle of water drop with a contact

angle meter (CAM 200, Contact angle and surface tension

meter).

The through plane electrical conductivities of compression

molded bipolar plate are measured according to four point

probe (JANDEL RM3), using a current source meter. All mea-

surements are performed at room temperature and the aver-

age electrical resistance of each sample is obtained from five

repeated measurements at different locations on the sample

according to ASTM C611-98.

Tensile, flexural and compressive strengths of the samples

aremeasured according to ASTMD 638-03, ASTMD790-07 and

ASTM D695-02a, respectively. All mechanical tests are per-

formed using a computer controlled testing machine (Shi-

madzu AG-1 10 kN), at 23 �C and are reported to represent the

average of the results on at least five samples. The morphol-

ogy of the prepared polymer composite plates is examined by

Scanning Electron Microscope (SEM) JEOL 6510 LV.

2.3. Fuel cell performance tests

The electrocatalyst ink is prepared by mixing 2-propanol with

5 wt. % of Nafion solution and carbon supported 20 wt. % Pt

catalysts. The ink is coated on carbon paper (Avcarb GDS2120)

and actual Pt loading mass on anode and cathode is used as

1 mg cm�2. The Nafion 212 membrane is cleaned by boiling in

3 wt. % H2O2 and 3 wt. % H2SO4 for 1 h, followed by boiling in

ultrapure water for 2 h with the water being changed every

30 min. Anode/membrane/cathode unit is compressed be-

tween two polymer composite plates with a single serpentine

flow field with rectangular channels of 1 mmwidth and 1 mm

depth. Silicon gaskets are assembled in between the electrode

and the polymer composite plate. The PEMFC performance

tests are performed in a 50 cm2 active area. The hydrogen and

oxygen are humidified by passing through bubblers at 65 �C.Line temperature is kept at 70 �C. Single cell performance is

investigated by steady state measurements at the cell tem-

perature of 60 �C and atmospheric pressure. The cell is con-

nected to an Electrochem 400W fuel cell test station including

an ECL 150 electronic load, HAS humidifier and MTS 150 gas

control unit. The voltage versus current density data are col-

lected for each bipolar plate.

Fig. 2 e Effect of organic basedadditive/filler ratio onsurface

wettability and conductivity of polymer composite bipolar

plates.

3. Results and discussion

3.1. Effect of organic based additive/filler ratio onpolymer composite bipolar plate properties

Fig. 2 shows the surface wettability properties of the com-

posite plates with different additive/filler contents. The sur-

face hydrophobicity gets stronger with increasing additive/

filler amount, due to the strong hydrophobic properties of

fluoropolymer based additive as observed in Fig. 2. Surface

energy and contact resistance of bipolar plates are important

factors affecting cell performance particularly at high current

densities, since water produced by the cathode reaction

should be immediately removed to avoid flooding and power

degradation due to catalyst submergence [43]. Graphite is also

hydrophobic and has an ordered domain structure [44,45].

This characteristic is important for the improvement of fuel

cell system efficiency, because high flow rates result in low

oxidant utilization and large power consumption for driving

an air compressor or blower to supply air to the fuel cell in the

case of using air as an oxidant [46].

The variation of through plane conductivity with respect to

organic based additive/filler ratios is shown in Fig. 2. The

amount of the thermoset resin is fixed in all the composites

for comparison. As it can be seen in the Fig. 2, the conductivity

of the samples decrease with the increase in additive/filler

ratios, due to a decrease in the amount of conductive filler in

the composite structure.

The tensile, flexural and compressive strength of com-

posite plates with different amounts of additive/filler ratios

can be seen in Fig. 3. Experimental results indicate that the

change in tensile, and flexural strength with respect to addi-

tive/filler content is not significant since the same amounts of

binder is used.

3.2. Effect of additive/binder ratio on composite bipolarplate

Fig. 4 shows the surface wettability properties of the com-

posites with different additive/binder amounts. As it can be

seen in the Fig. 4, surface hydrophobicity increases as the

hydrophobic fluoropolymer amount increases in the com-

posite materials.

The variation of conductivity with different additive/

binder ratios is shown in Fig. 4. Graphite amount is fixed in the

composites for comparison. The conductivity of the samples

increases with the increase in additive/binder contents. This

is an interesting result as the graphite amount is kept con-

stant in all the composites. When electrically conductive filler

is incorporated into a polymer matrix, electrical and me-

chanical properties of the pure material may change

depending on the properties of individual components, the

shape, size, and amount of the filler, the morphology of the

system, and the interface between the components. When

Fig. 3 e Effect of organic based additive/filler ratio on

mechanical properties of polymer composite bipolar plates.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 8 4093

binary blends are considered, the conductivity depends

strongly on the morphology, and the distribution of the con-

ductive filler in the polymer blends. The presence of fluo-

ropolymer in the composite might affect the distribution of

graphite in a polymer matrix and makes conducting current

carrier flow easily as the conductivity of graphite depends on

density of conducting current carrier (electron and cavity) in

the crystal lattice [47e50].

The tensile, flexural and compressive strength of com-

posites with different amounts of additive/binder ratios can

be seen in Fig. 5. According to Fig. 5, strength values decrease

with increasing amounts of fluoropolymer and decreasing

amount of thermoset binder.

Graphite is a special material, consists of extended planes

of carbon atoms in which each carbon forms strong covalent

bonds to three other carbon atoms. These planes of atoms are

held together by relatively weak van der Waals forces. Ther-

moset resin can be able to coat the surface of graphite parti-

cles better as its content increases, and then bond the

interface by contacting with organic matter bonding by co-

valent bonds. Moreover, thermoset resin tends to form a three

dimensional network structure as its content increases, which

can raise the bonding force between graphite particles [51]. In

the present study, the fluoropolymer based additive may

disrupt the formation of three dimensional network structure.

Therefore, strength values of the composite materials would

Fig. 4 e Effect of organic based additive/binder ratio on

conductivity and surface wettability of thermoset/graphite

composite bipolar plates.

decrease with the increase in fluoropolymer content and with

the decrease in binder content.

3.3. Morphology of polymer composite bipolar platematerials

In order to examine the phase structure of the cured resin,

fracture surfaces of polymer/graphite composites with

organic and inorganic additives are analyzed by using SEM at

magnitudes of �500, �1000 and �2500, respectively. In Fig. 6,

A0 represents the fracture surfaces of polymer composite

without additive. The surface is featureless and layered

graphite can be seen in the micrographs. In the SEM micro-

graphs, OBA1 shows the micrographs of a polymer composite

having ratio of 1.11 fluoropolymer/graphite. It is seen that

fluoropolymer forms an immiscible phase in the thermoset

graphite matrix. IBA1 shows the micrographs of a polymer

composite having ratio of 1.11 silica/graphite. The images

clearly show that inorganic particles are uniformly distributed

in the composite, and many inorganic particles are present

between graphite layers.

3.4. Effect of surface wettability on fuel cell performance

The hydrophobic or hydrophilic properties of catalyst layers

and bipolar plates have a large impact on the water transport

and PEMFC performance since water removing ability de-

pends on the wettabiliy properties of these components. Up to

now, the effect of the contact angle of the catalyst layer on the

fuel cell performance is mainly investigated in the literature

[21,52e55]. Nevertheless, the contact angle of the bipolar plate

is important for the fuel cell behavior especially at high cur-

rent densities. The electrochemical oxidation and reduction

reactions take place at the three phase boundary interface,

which consists of reactant gases (oxygen and hydrogen), an

electron conductive catalyst and/or a catalyst support, and an

ion conducting phase. At low current densities, the electro-

chemical reaction is carried out uniformly, since reactants are

accessible in all surfaces. With an increasing current density,

the excessive amount of water is generated, condenses, and

accumulates (called as flooding) in the catalyst layer. The

Fig. 5 e Effect of organic based additive binder ratio on

mechanical properties of thermoset/graphite composite

bipolar plates.

Fig. 6 e SEM micrographs of polymer composites bipolar plates.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 84094

reaction rate is limited by the transport rate of oxygen to the

reaction sites. The oxygen transport properties of the catalyst

layer are affected by the accumulation of liquid water within

the cathode and mainly governed by the permeability,

porosity, the capillary pressure saturation relation and the

contact angle of surface of catalyst layer and bipolar plate. The

capillary forces are strongly related to the contact angle.

The perfect wettability is obtained with 0� contact angle;

that is the water suddenly spreads over a solid surface due to

high surface energy. Hydrophilic channel improve capillary

driven water flow but it can increase water retention in

channels. When the channel contact angle is greater than 90�,the water tends to bead up on the surface in the form of dis-

crete droplets. The partially wetting is obtained between 45

and 90� contact angles.Fig. 7 shows the effect of surface wettability on fuel cell

performance of polymer composite bipolar plates in partially

wetting region at different feed flow rates (a)

H2 ¼ 1 dm3 min�1, O2 ¼ 1 dm3 min�1 (b) H2 ¼ 1.7 dm3 min�1,

O2 ¼ 1 dm3 min�1 (c) H2 ¼ 2 dm3 min�1 O2 ¼ 1 dm3 min�1 for

the samples having 1.1 additive/binder ratios. The excess ox-

ygen without limitation was used. The performances data

belong to commercial pure graphite bipolar plate (CPG) are

also shown in Fig. 7 for comparison. Table 1 shows the contact

angle of polymer composite materials. In all feed flow rates, at

low current densities, the single cells exhibited almost the

same performance. At intermediate and high current den-

sities, composite without additives and commercial graphite

bipolar plate showed higher performance than the bipolar

plates containing organic or inorganic based additives. At high

current rates, bipolar plate with organic additive (i.e. with

higher contact angle) has higher performance than the bipolar

plate having inorganic additive (i.e. with lower contact angle).

The bipolar plate with inorganic additive and lower contact

angle could retain more water produced by the cathode re-

action and supplied through a humidifier, resulting in the

mass transfer polarization [56].

Fig. 8 shows the effect of surface properties on fuel cell

performance of polymer composite bipolar plates at different

feed flow rates (a) H2 ¼ 1 dm3 min�1, O2 ¼ 1 dm3 min�1 (b)

H2 ¼ 1.7 dm3 min�1, O2 ¼ 1 dm3 min�1 (c) H2 ¼ 2 dm3 min�1

O2 ¼ 1 dm3 min�1 for the samples having 0.14 additive/filler

ratios. The data are obtained also in partially wetting region.

The same trend in Fig. 7 is also observed in all feed flow rates.

Due to the easy removal of water within the flow channels of

the bipolar plate with increasing hydrophobicity, hydrophobic

bipolar plates show higher current and power densities. In

a fuel cell stack, gas flow channels blocked by condensed

liquid water and results in a serious degradation as electrode

area, reactant starvation and humidifying temperature in-

creases. Therefore, the better fuel cell performance is

obtained by partially hydrophilic composites than hydrophilic

ones. The results are consistent with the literature [57].

Fig. 9 and Fig. 10 show the effect of the bipolar plate contact

angles on current density and power density, respectively at

different feed flow rates (a) H2 ¼ 1 dm3 min�1,

O2 ¼ 1 dm3 min�1 (b) H2 ¼ 1.7 dm3 min�1, O2 ¼ 1 dm3 min�1 (c)

H2 ¼ 2 dm3 min�1 O2 ¼ 1 dm3 min�1 for the composite bipolar

0100200300400500600

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100

200

300

400

500

600

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100

200

300

400

500

600

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Fig. 7 e Effect of surface wettability on fuel cell

performance of polymer composite bipolar plates at

different feed flow rates (a) H2 [ 1 dm3 minL1, O2 [ 1 dm3

minL1 (b)H2[1.7dm3minL1,O2[1dm3minL1 (c)H2[2dm3

minL1, O2 [ 1 dm3 minL1 for the samples having 1.1

additive/binder ratios.

0100200300400500600

0.00.20.40.60.81.01.2

0 200 400 600 800 1000 1200 1400

Po

we

r d

en

sity

, m

Wc

m-2

Vo

lta

ge

,V

Current density, mAcm-2

(a)

CPG,Voltage A0,Voltage OBA2,Voltage IBA2,VoltageCPG,Power A0,Power OBA2,Power IBA2,Power

0100200300400500600

0.00.20.40.60.81.01.2

0 200 400 600 800 1000 1200 1400

Po

we

r d

en

sity

, m

Wc

m-2

Vo

lta

ge

, V

Current density, mAcm-2

(b)

CPG,Voltage A0,Voltage OBA2,Voltage IBA2,VoltageCPG,Power A0,Power OBA2,Power IBA2,Power

0

200

400

600

800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 200 400 600 800 1000 1200 1400

Po

wer d

en

sity, m

W cm

-2

Vo

ltag

e, V

Current density, mA cm-2

(c)

CPG,Voltage A0,Voltage OBA2,Voltage IBA2,VoltageCPG,Power A0,Power OBA2,Power IBA2,Power

Fig. 8 e Effect of surface wettability on fuel cell performance

of polymer composite bipolar plates at different feed

flow rates (a) H2 [ 1 dm3 minL1, O2 [ 1 dm3 minL1

(b) H2 [ 1.7 dm3 minL1, O2 [ 1 dm3 minL1 (c) H2 [ 2 dm3

minL1, O2 [ 1 dm3 minL1 for the samples having 0.14

additive/filler ratios.

400

600

800

1000

1200

rre

nt d

en

sit

y,m

Ac

m-2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 8 4095

plates having 0.14 additive/filler and 1.11 additive/binder ra-

tios. The all data belong to additive/filler and additive/binder

ratios show a common trend. The different behavior of OBA2

can depend on the other chemical and physical surface

properties of the bipolar plates such as roughness. The deep

investigation will be done in another study to explain this

behavior. Generally, current and power density versus contact

angle figures at 0.5 V showsmaxima. It is seen from the figures

Table 1 e Contact angle of polymer composite bipolarplates.

Polymer composite Contact angle (�)

CPG 87 [58]

A0 80

OBA1 90

IBA1 77

OBA2 87

IBA2 72

0

200

60 70 80 90 100

Cu

Contact angle,o

(a) (b) (c)

Fig. 9 e Effect of bipolar plate contact angle on current

density at different feed flow rates (a) H2 [ 1 dm3 minL1,

O2[ 1 dm3minL1 (b) H2[ 1.7 dm3minL1, O2[ 1 dm3minL1

(c) H2 [ 2 dm3 minL1, O2 [ 1 dm3 minL1 for the composite

bipolar plates having 0.14 additive/filler and 1.11 additive/

binder ratios.

0

100

200

300

400

500

600

700

60 65 70 75 80 85 90 95

Po

we

r d

en

sit

y, m

Wcm

-2

Contact angle,o

(a) (b) (c)

Fig. 10 e Effect of bipolar plate contact angle on power

density at different feed flow rates (a) H2 [ 1 dm3 minL1,

O2[ 1dm3minL1 (b) H2[ 1.7 dm3minL1, O2[1dm3minL1

(c) H2 [ 2 dm3 minL1, O2 [ 1 dm3 minL1 for the composite

bipolar plates having 0.14 additive/filler and 1.11 additive/

binder ratios.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 4 0 8 9e4 0 9 84096

that, with increasing hydrophobicity current and power den-

sities increase, since the high hydrophobic characters of bi-

polar plate are beneficial to a high current density operation

due to the easy removal of water within the flow channels of

bipolar plate. On the other hand, water is oriented to the

catalyst layer simultaneously. Moreover, water accumulates

as droplets that continue to expand because of the high hy-

drophobic property of the plate. As the size of the droplets

increases, the flow channel of plate is closed and the reactant

gas cannot force the water of the channel. Therefore,

a decrease is observed in the fuel cell performance at the

higher contact angle of bipolar plate. Increasing of the wett-

ability of the bipolar plate (hydrophilic character), increases

the water removing from catalyst layer due to spreading of

water on the landings of bipolar plate. If water is removed

easily in the flow field, the hydrophilic surface allows the fuel

cell to operate without flooding. Otherwise flooding is

observed at low flow rates. According to our results, the

highest performance data are obtained with the polymer

composite bipolar plates having a 80� contact angle.

4. Conclusions

The conductivity of the composite materials decreases with

the increase in organic based additive/filler ratios, due to

a decrease in the amount of conductive filler in the composite

structure, whereas conductivity increaseswith the increase in

organic based additive/binder ratios due to a decrease in the

amount of nonconductive binder. The presence of fluoropol-

ymer in the composite might affect the distribution of

graphite in a polymer matrix and makes conducting current

carrier flow easily as the conductivity of graphite depends on

density of conducting current carrier (electron and cavity) in

the crystal lattice. The surface hydrophobicity gets stronger

with increasing organic additive/filler and organic additive/

binder ratios, related to the strong hydrophobic properties of

both graphite and fluoropolymer. Experimental results indi-

cate that the change in tensile and flexural strength with

respect to organic additive/filler content is not significant

since the same amount of binder is used. The strength values

decrease with increasing amounts of organic based additive/

binder ratios. The fluoropolymer based additive may disrupt

the formation of three dimensional network structures. In all

feed flow rates, at low current densities, the single cells

exhibit almost the same performance. At intermediate and

high current densities, composite bipolar plate without addi-

tive and commercial graphite bipolar plate show higher per-

formance than the bipolar plates containing organic or

inorganic based additives. Current and power density versus

contact angle figures show maxima at 80� contact angle. The

hydrophobic character of a bipolar plate in partially wetted

region is beneficial to a high current density operation due to

the easy removal of water within the flow channels of bipolar

plate. The water droplet in the gas flow channels of more

hydrophobic plate can be swept away more easily by the gas

stream under relatively low flow rate conditions. Hydrophobic

bipolar plate channels require less time to remove water,

since the droplets are very unstable.

On the other hand, the fuel cell performance decreases

over the critical value of contact angle due to difficulty of

water removing.

Acknowledgments

The authors are highly grateful toMr. Selahattin Uysal andMr.

Aydin Canbasa for their technical assistance throughout this

study.

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