Kyushu University Global COE Program
Journal of Novel Carbon Resource Sciences, Vol. 3, pp. 21-26, Feb. 2011

Investigation of Liquid Water Behavior in PEFC Porous Media
Yunpeng Fan*1, Gen Inoue*2, Yosuke Matsukuma*2, Masaki Minemoto*2*1

Graduate School of Engineering, Kyushu University*2
Faculty of Engineering, Kyushu University(Received November 12, 2010; accepted January 21, 2011)
In order to improve the output performance of PEFC, the liquid water has to be removed smoothly from GDL
because it prevents gas diffusion. In this study, the relationship between the liquid water distribution and operating
condition of fuel cell was examined by experiments with visualized cell and numerical analysis. As a result, it was
found that the position and the amount of the liquid water in porous media made a great impact on the output performance. In the case of high temperature but low humidity operating condition, liquid water accumulated near Rib side
and caused higher performance.

1. Introduction

Polymer electrolyte fuel cell (PEFC) is a kind ofclean energy system that combines hydrogen and oxygen to produce electricity. It is expected as one of mostimportant energy in the next generation due to its highpower density, low operating temperature, quick startup, fast dynamic response and zero emission. Recently,
PEFC is widely used in vehicles and stationary cogeneration fields1) to solve the energy crisis and environmental
problems. As a result, the researches of PEFC are popular
now, and various studies focus on the fuel cell design,
new applications, alternative catalyst and liquid water
management to develop the capability and practical useof PEFC. However, PEFC system is very expensive yet,and the durability is not high enough. One of the approaches to reduce the cost is to decrease the amount of
Pt catalyst or to replace the catalyst with other cheapermaterials. And another approach is to increase the current density and the output density of single cell. If theoutput performance of each cell is increased, the numberof cells in stack system can be reduced, so the stack cost
will drop.
In order to improve the performance of single cell, it
is important to speed up the oxygen transfer to catalyst
surface in cathode catalyst layer (CL) in the case of highcurrent density condition, because the oxygen transfer
rate is dominant2-3). However, the oxygen transfer phenomena in gas diffusion layer (GDL) and micro porous
layer (MPL) have not be known and optimized enough.
As the principle of PEFC, the fuel cell has to be kept humid to make the proton conductivity of electrolyte membrane high. As a result, in an actual PEFC, liquid water
accumulated in GDL during electricity is generated, andthe accumulated water prevents the gas diffusion. Thisis the major reason of the degradation of performance.
It is obvious that an efficient way to improve the output
performance is to increase the gas diffusion rate of GDL.
On the other hand, it is also well recognized that the water condition in porous media makes large influence on
the gas diffusion rate. Therefore, in order to improve theoutput performance, the comprehension of liquid water
behavior is extremely necessary.
In this research, the liquid water behavior of GDL surface was investigated with several PEFC visualized cells

under different cell temperature and relative humidity
conditions. With the experimental data, the relationshipbetween the internal liquid water distribution andthe operating condition was considered. In addition,
temperature and vapor concentration distribution
was calculated by numerical analysis. By comparison
between experiment and calculation, the effect ofoperating condition on vapor concentration distribution
was examined.

2. Experiment

2.1 Experimental procedure

Instead of the actual fuel cell, the PEFC visualized
single cell (Fig. 1), which was based standard PEFC cell,
was used in these experiments. Through the acrylic plateof cell, the behavior of water droplets flowing out from
cathode GDL surface in gas channel was observed bydigital microscope. This behavior was captured to movie
for 30 minutes. At the same time the fluctuation of cellvoltage was also measured. From the movies, the flowing
points of liquid water were obtained by image processing method which will be explained in next section. The
size of cell components is shown in Table 1. The cathodeGDL was coated with micro porous layer (MPL) at thecatalyst layer side. MPL consists of very small carbon
particles with about 1 μm secondary diameter. MPL
can reduce contact resistance between MEA and GDL.
Fig. 2 shows the visualization experiment apparatus inthis study. Hydrogen and oxygen (air) were supplied
to visualized cell via humidifier. In PEFC, electrolyte
membrane has to be kept humid condition to keep thehigh proton conductivity. So humidification of supply
gas is necessary. And the operating relative humidity canbe controlled by the humidifier and thermostat. The cell
was set horizontally, and cathode side was set at upperside to observe by microscope. At the back side of anode,
rubber heater was set to keep the cell temperature. Thistemperature was controlled by external thermostat. As
the parameter, operating temperature and humidifier
temperature were changed. Experimental conditions areshown in Table 2. As a result, the effect of temperatureand relative humidity (RH) on liquid water condition was
examined.

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

Table 2

Experimental conditions

FactorCell temperature
Gas relative humidity
Cathode gas component
Current density
H2 and O2 utilization ratio

Condition
40 ºC, 50 ºC, 60 ºC
70%, 80%, 90%
N2:79% O2:21%
1.0 A/cm2
5.4% and 6.4%

2.2 Image processing and data processing
Fig. 1 Structure and components of visualized cell.
(a) Separator plated with gold, (b) Membraneelectrode assembly (MEA), (c) Gas channel(Graphite), (d) Acrylic plate, (e) Gas diffusion
layer (GDL), (e) Back plate

Table 1 Size of cell compone
Part nameSize and amount
Membranesize:
100 mm×100 mm
thickness:
30 μm
Electrodesize:
10 mm×10 mm
Pt content: 0.3 mg/cm2
Cathode GDL width:
2 mm, 3 mm, 4 mm
length:30 mm
thickness
190 μmAnode GDL
size:
10 mm×10 mmthickness:
190 μmGas channel
width:
1 mm, 2 mm, 3 mm
depth:
1 mm

Fig. 2 Visualization experiment apparatus.
1: H2, 2: Air, 3: Flow control, 4: Humidifier,
5: PC, 6: Cell, 7: Microscope, 8: Thermostat,
9: Air outlet, 10: H2 outlet

To know the distribution of flowing points of waterfrom GDL, all images of captured movie were changed
to binary images by image processing. Next, the droplet parts of each image were extracted by obtaining thedifference between initial image and local time image.
After this, all images were multiplied, and several clear
images of water droplets in gas channel were obtained.
Fig. 3 shows the binary image of observation result on
GDL surface, the gray part of the photo shows the GDL
surface in gas channel, and the bright points on it are
the generated water droplets. The other parts of image inblack are the ribs of gas channel. After image processing,
the clear image of droplet points is obtained and showed
in Fig. 4. In these experiments, the section of gas channel
was divided to 5 parts from channel center to Rib side.
This schematic image of partition of channel is shownin Fig. 5. rib is the solid wall of separator. By counting
droplet in each section, the droplets distribution was obtained.

Fig. 3Observation result of droplets on GDL surface.

Fig. 4

Points of droplet obtained by image processing.

Investigation of Liquid Water Behavior in PEFC Porous Media

l2

l1

0

ChannelL

Sectionl4

l3

Section

Rib

5
4
3
2
1
1
2
3
4
5

Fig. 8 shows the droplets distribution with different
channel width. Cell temperature is 40 ºC, relative humidity is 70%. The droplets distribution is more symmetricin narrow gas channel. There was an uneven distributionof generated droplets in wide gas channel.
15

Number of droplets

Rib

L

Fig. 5 Schematic image of partition of channel.

1mm
2mm

10

5

0
1

2.3 Experimental results

Fig. 6 shows the droplets distribution under different
RH conditions. Cell temperature is 50 oC, channel width
is 1mm. At rib side (section 5), much droplets flowed out
from GDL in the cases of all conditions. Moreover, theamounts of droplets not only at rib side but also at centerof channel increased by increasing RH.

Number of droplets

15
RH70%
RH80%
RH90%

10

5

0
1

2

3
Section

4

5

Fig. 6 Droplets distribution at different RH conditions.

2

3

4

5

SectionFig. 8Droplets distribution with each channel width.

From all of experimental results, generated droplets
were less in the center of the channel and increased in thearea close to the edge of the rib side. In the case of hightemperature and low humidity, much more droplets stayed
under rib side. Conversely, in the case of low temperatureand high humidity condition, more droplets flowed out
GDL surface widely. There was no excessive distribution. With this information, the liquid water condition
inside GDL was considered. Fig. 9 shows the inferential
image of the liquid water conditions inside and outside
GDL by using experimental results. It was guessed that
inside liquid water distribution in the direction of inplane was the same as the droplet distribution observedby these experiments. Next, in order to check the validityof this conjecture, temperature and vapor distribution
inside GDL were calculated.

ChannelFig. 7 shows the droplets distribution at different
temperature condition. Relative humidity is 80%, channel width is 1mm. From this graph, more droplets generated in the case of lower cell temperature. On theother hand, it showed an obvious uneven distribution ofgenerated droplets under high temperature condition.

Rib
GDL

GDL
surface

Number of droplets

15

50ºC
40ºC

10

Fig. 9Liquid water conditions inside and outside GDL.

3. Numerical analysis

3.1 Equations and conditions

5

0
1

2

3
Section

4

5

Fig. 7 Droplets distribution at each temperature condition.

In order to explain the experimental results and checkthe validity of suppositions, the liquid water condition
in GDL was examined by numerical analysis. In thiscalculation, the following assumptions were used. The
generated water at catalyst layer is vapor phase only. The
temperature and the concentrations at upper surface ofJournal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

GDL are constant. The gas flow and convection in GDL
can be ignored. The effect of liquid water on gas diffusion
performance can be ignored. The cell temperature andvoltage are kept stable in this system. The diffusionsof oxygen and vapor in GDL are assumed as molecular
diffusion, and there is only Knudsen diffusion exists in
MPL. The schematic image of calculation system and assumptions are showed in Fig. 10. Heat and water are generated in catalyst layer (CL) by cathode oxygen reduction
reaction, and then transfer from bottom side to upper sidethrough the GDL and MPL. On the other hand, oxygen
diffuses from gas channel to catalyst layer.

Tgas
Cgas :const Channel

Rib Tcell: const
GDLzHeat & Water transfer

MPL
x

CL

Fig. 10

Heat & Water generation

Schematic image of calculation system andassumptions.

In this calculation, it was assumed that the local
temperature and concentration did not affect each other,and these distributions were solved separately by thefollowing equation of energy and mass balance equations.
kx

∂ 2T
∂ 2T
=0
+
k
z
∂z 2
∂x 2

DOeff2 , x∂ 2 CO2∂x 2+ DOeff2 , z∂ 2 C H 2O

(1)
∂ 2 CO2
∂z 2

=0

(2)

∂ 2 C H 2O

(3)
=0
∂z 2
∂x 2
In these equations, T is temperature, k means
the thermal conductivity of GDL and MPL, C isconcentration, and Deff means effective diffusion
coefficient. Other parameters and their values as DO2and DH2O are showed in Table 3. Subscript O2 and H2Omean oxygen and water vapor respectively. As GDL isfibrous porous media, thermal conductivity and effective
diffusion coefficient are anisotropic properties. As aresult, all the calculations are taken along two directions
as show in Fig. 10, and signified in equations as subscript
x and z respectively. The effective diffusion coefficientsof GDL were obtained by following equations.
DHeff2O , x+ DHeff2O , z

DOeff2 ,d,GDL = (ε GDL / τ GDL,d )DO2

(4)

DHeff2 O,GDL, d = (ε GDL / τ GDL , d )DH 2 O(5)ε means the porosity of porous media, and τ is tortuosity.
Subscript d means the direction x or z. The diffusions in
MPL were assumed as Knudsen diffusion only, so the
diffusion coefficients were calculated as follows.

k xMPL= k zMPL= k c (1 − ε MPL )

(6)

1

(7)

τ MPL = ε MPL − 2
DOeff2 , MPL =DHeff2O, MPL =ε MPL Kn28 RT
D , DOKn = r
3 π MO
τ MPL
O2

(8)

2

2

ε MPL Kn28 RT
D , DHKnO = r
3 πMH O
τ MPL
H 2O

(9)

2

2

Table 3 Calculation conditions
Parameter
Value
Operating conditions
Cell temperature(Tcell)
40, 50 ℃
Relative humidity(RH)70, 90 %Average current density(i)1.0 A/cm 2

Components
GDL
porosity(ε GDL)
0.78
thickness(-)
170 μmtortuosity
X direction(τGDL, x) 1.205
Z direction (τGDL, z) 1.343
thermal conductivity
X direction(k x)
21.0 W/(m ・ K)
Z direction (k z)
1.7 W/(m ・ K)
MPLporosity(εMPL)
0.2
thickness(-)
30 μm
pores’ average radius(r) 1.0×10 -7 m
MEA
thickness(tm)
30 μm
Channel width
0.5 mmRibwidth
0.5 mmCoefficientsO2 diffusion coefficient(DO2)2.350×10 -5 m2/s
H2O diffusion coefficient(DH2O)
2.505×10 -5 m2/s
Voltage (overall enthalpy) (Emax)
1.481 VElectromotive Force (E0)1.23 VGas constant(R)8.314 J/(mol ・ K)
Faraday constant(F)96485.309 C/mol
Transfer coefficient(αt)
0.3
Thermal conductivity of carbon black (kc)
98.0 W/(m ・ K)
MPL is made by micro carbon particles, so the thermal
conductivities and effective diffusion coefficient of all
the directions are the same. kc is the thermal conductivity of carbon black. Bruggeman equation (7) shows
the relationship between the tortuosity and porosity of
MPL4). DKn means the Knudsen diffusion coefficient,r is the average radius of pores in MPL, R means gas
constant, and M is the molecular weight.
At the upper side of GDL, the boundary condition at
gas channel part and rib part are not same. Each conditionis showed as follows.
(Channel) T=Tgas, CO = COgas , C H O = C HgasO

(10)

∂CO2

(11)

2

(Rib)

T=Tcell,

∂z

2

= 0,

2

∂C H 2 O∂z

2=0Investigation of Liquid Water Behavior in PEFC Porous MediaAt right and left side in Fig. 10, wall condition wasused as the boundary condition in order to calculate
temperature and concentration distributions. As a result,the boundary conditions of CL-MPL surface were set
with the following equations5).
MPL ∂Tk=( EMax − Vcell )i
z
∂z

(12)

DOeff2 ,,zMPL

∂CO2
i
= −
∂z
4F

(13)

DHeff2O, MPL,z∂CH 2Oi= (1 + 2α )
2F
∂z

(14)

EMax means the voltage value estimated by enthalpy
change of overall reaction, Vcell means cell voltage, i is the
local current density, F is the Faraday’s constant, α is net
water transfer coefficient in membrane, and superscript
MPL means the value of MPL near CL. Cell voltage wascalculated by the following Butler-Volmer equation.
V=
E−
cell

ref
RTcell  iCO2 
ln ex e − Rohmi
α t 2 F  iO2 CO2 (15)

In this equation, E means effective motive force ofoverall reaction, αt is transfer coefficient of cathodeelectrode reaction, Cref is reference concentration, iex is
exchange current density, Ce is local concentration of
catalyst layer surface, and Rohm means membrane ohmic
resistance. The values of these coefficients were calculated by the method from other paper6). Net water transfer
coefficient was calculated by electro-osmotic effect andback diffusion as follows6).

α= nd −

c(m)

a(m)

F m C H 2O − C H 2O
DHOO
i
tm

(16)

Fig. 11

Vapor concentration distribution in GDL.

Fig. 12 shows the temperature distribution at the same
condition as Fig. 11. The temperature of catalyst layer
increased by generated heat of overvoltage. In particular,
current density under gas channel was higher than that ofother parts because of the high oxygen diffusion rate. So
the temperature at center of gas channel on the interfaceof catalyst layer and MPL was the highest. On the other
hand, the temperature under rib on the GDL surface was
the lowest because it attached cold solid wall. Moreover,though GDL has only 200 μm thickness, there wereabout 1.5 oC differences inside GDL.

n means the electro-osmotic coefficient, Cc(m) andmean the concentration of membrane surface inCcathode side and anode side respectively, and Dm is thewater effective diffusion coefficient inside membrane.
The values of nd and Dm are also obtained by otherpapers6,7). The calculation conditions and parameters areshown in Table 3. Furthermore, the vapor condensation
part was examined with local temperature and vapor
concentration distribution. In this study, the difference
between the local vapor partial pressure and the saturated
vapor pressure was calculated as Δpvapor. Saturated vapor
pressure was calculated with local temperature. Vapor iseasy to condense at the position which the Δpvapor value
is high. In this study, the effect of liquid water on gas
diffusion and heat transfer was ignored.
da(m)3.2 Calculation results

Fig. 11 shows the vapor concentration distribution when
the RH is 90%, cell temperature is 50 oC and cell voltage
is 0.5 V. Upper graph is two-dimensional distribution,and lower graph is vapor concentration distribution in
the direction of in-plane at each slice. In these figures, it
was found that vapor concentration was higher at under
rib area because the diffusion length under Rib is longer
than that under channel. So diffusion rate at this partdecreased and vapor accumulated here more easily.

Fig. 12 Temperature distribution in GDL.

Fig. 13 shows the vapor condensation distribution with
Δpvapor under each operating condition. In these figures,

Journal of Novel Carbon Resource Sciences, Vol.3, Feb. 2011

white parts mean the value of Δpvapor is negative and thevapor doesn’t condense there. From these graphs, in all
the cases, vapor is easy to condense under rib.
Comparing with (a) and (c) of Fig. 13, it can be
confirmed that the condensation part under channel
increased by increasing RH. Similarly comparing with(b) and (c) it can be confirmed that the condensation part
under channel increased by increasing cell temperature.
Therefore, these distributions of temperature and vapor
concentration strongly affect liquid water behavior in
GDL. In addition, the experimental results of droplets
distribution on GDL surface and the calculation resultsof vapor condensation part inside GDL were almost thesame qualitatively.

(a) Cell temperature 50 ºC, RH 90%

4. Conclusions

In this study, the generated droplets distributions on
GDL surface were observed with visualized polymer
electrolyte fuel cell during electricity generated under
different conditions. In addition, the vapor condensation
distribution was examined by heat and mass transfer
calculation in GDL. Results showed the cell temperature,
relative humidity and the width of gas channel strongly
affected the liquid water distribution in GDL. Especially,in the case of high temperature and low humidity, much
more droplets stayed under rib side. The experimental
results of droplets distribution on GDL surface and thecalculation results of vapor condensation part were almost
same qualitatively. This distribution can be controlled byoperating condition in order to improve gas diffusion
rate in two-phase porous media. And this knowledge canbe applied to improve PEFC output performance and to
reduce the stack cost.
In our future study, to know the effect of other factor
on gas diffusion performance in two-phase condition,
experimental method and calculation model have to be
improved with other devise and with other equations.
Acknowledgement: This research was financially supported by PEFC program of new energy and industrial
technology development organization (NEDO), Japan.
References

1)2)(b) Cell temperature 40 ºC, RH 70%

3)
4)
5)
6)
7)

(c) Cell temperature 50 ºC, RH 70%
Fig. 13 Δpwater distributions in GDL under different
temperature and humidity conditions.

C. Siegel, Energy, 33, 1331 (2008).
G. Inoue, Y. Matsukuma, M. Minemoto, ECS Trans.,
16(2), 769 (2008).
L. Zhang, Y. Liu, H. Song, S. Wang, Y. Zhou, S. J. Hu, J.
Power. Soc., 162(2), 1165 (2006).
A. Z. Weber, J. Newman, J.Electrochem.Soc., 152(4),A677 (2005).
J. S. Yi, Trung V. Nguyen, J. Electrochem. Soc., 146(1),38 (1999).
T. V. Nguyen, R. E. White, J. Electrochem. Soc., 140(8),2178 (1993).
A. Parthasarathy, S. Srinivasan, A. J. Appleby, C. R.
Martin, J. Electrochem. Soc., 139(9), 2530 (1992).