3. Methanol Crossover Reduction by Vapor Fed Direct Methanol Fuel Cell

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April 2011, Volume 2, No.2

International Journal of Chemical and Environmental Engineering

Methanol Crossover Reduction by

Vapor Fed Direct Methanol Fuel CellNur Hafizah Shaharom*, Katayama Noboru, Kogoshi Sumio

Department of Electrical Engineering, Tokyo University of Science, Chiba Ken – 278 8510 Japan*Corresponding author: [email protected]

AbstractThis paper deals with methanol crossover as one of the main problem in utilizing a direct methanol fuel cell (DMFC). The keyobjective of this work is to investigate the reduction of methanol crossover by a vapor fed DMFC compared to a liquid fed DMFC.

The reduction is shown by decreasing the output power in both vapor and liquid fed.

Keywords: Vapor Fed DMFC, Methanol Crossover Reduction

1. Introduction1.1 Research Background

A direct methanol fuel cell (DMFC) is one of the most

promising fuel cell which can be apply to portable devices

such as a mobile phone, a camera and a laptop due to

easy recharging via a fuel cartridge, high energy density

and small volume. In particular, a DMFC is more efficient

and is expected to be the new power source for portable

devices. However, the commercialization of a DMFC

faces a few problems such as methanol crossover, low

catalyst activity, slow anode reaction rate and so on.

Some of the previous studies have made improvement

regarding these problems; such as changing the catalyst

and so on. In this study, we focused on methanol

crossover reduction by changing the fuel fed method,

from liquid to gas. Methanol crossover is a phenomenon,

where methanol from a fuel electrode (anode) passes

through a polymer electrolyte membrane (PEM).

Methanol should be fully reacted at the anode, but since

methanol has almost the same characteristic as water,

some of it permeates through a PEM. This ends up with

loss of methanol which is the fuel for a DMFC. Besides,

oxidation reaction occurs for methanol which arrives at an

air electrode (cathode). Since oxidation reaction occurs

both at anode and cathode at the same time, this causes

the loss of voltage in the fuel cell itself. Fuqiang et al. (1)

stated that the open-circuit voltage (OCV) loss due to

methanol crossover is about 100 mV when supplying airat the cathode.

Previous studies in reducing methanol crossover have

been focused on optimization of fuel cell operating

condition (2) and designing a new type of a PEM (3). Yet,

the best way to reduce methanol crossover is still being

examined until now.

J. Kallo et al. (4) reported that the methanol crossover

decreases due to the decreased amount of water in the

membrane and due to the smaller methanol uptake

from a gas phase than from a liquid one. Taking this as

our main focus in the study, we examine the difference of

methanol crossover in vapor fed and liquid fed DMFC.

Besides, very little experimental work has been done in

comparing the exact value of methanol crossover in vapor

fed and liquid fed.

As stated before, the major objective of this study is to

measure the amount of methanol crossover of both liquid

and vapor fed DMFC, and the measurement is being held

at the same output power. Furthermore, the influence of

methanol vapor concentration on the efficiency of a

DMFC has been examined. Compared to a liquid fed

DMFC, the output power of a vapor fed DMFC is quitelow. However, a lower methanol crossover rate is shown

in a vapor fed DMFC.

1.2 Methanol Crossover Principles

As being stated before it is possible to make a smaller

and lighter DMFC, but practically, there are several

problems that still remain. One of the problems is the

methanol crossover. The first problem of this

phenomenon is the methanol loss which passes through

PEM. Thus, the loss of methanol also means the fuel is

lost.

As for the second problem, the methanol which

reaches at the air electrode oxidises and causes theelectric potential loss. Reduction should be the only

reaction that occurs at air electrode, but as methanol

oxidation happens at the same time, electric potential

drops to minus direction.

Chemical reactions which occur at anode, cathode

and methanol crossover equation are as follow.



98

Fuel electrode (anode): e H COO H OH CH 66223

Air electrode (cathode): O H e H O 22 366

2

3

Methanol crossover : 2223 2

2

1CO H OOH CH

2. Material and Method2.1 Experimental setup

The single fuel cell tested in this study has an active

area of 25 (cm2). The electrolyte membrane assembly

(MEA) consists of Nafion 117 loading Pt of 1 (mg/cm2)

and Ru of 0.5 (mg/cm2) at the anode, and Pt of 1

(mg/cm2) at the cathode. The gas diffusion layer for

anode is a carbon cloth (Teflon, EC-CC1-060T) and a

carbon paper (Teflon, EC-TP1-060T) for cathode. Themeasurement of voltage and current is controlled by an

electronic load device (KIKUSUI SPEC-40026S). The

anode mass flow rate is controlled by a fuel mass flow

meter (OMEGA FD-07), while the cathode mass flow rate

is controlled by an air mass flow meter (OMEGA FD-07).

During the experiment, the DMFC is put into the constant

operating chamber (Yamato DKN302), to ensure the

operating temperature remains constant. The amount of

CO2 concentration at the cathode side, which represents

the amount of methanol crossover, is measured using a

CO2 concentration meter (VAISALA GMP221-

D2710029). The experimental setup used in this study is

shown in Fig. 1.

2.2 Bubbling concept and the made of bubbler

Bubbling concept is a way to generate a carrier gas

and chemical vapor mixture to supply the device with

reactive chemical needed. Carrier gas (generally nitrogen,

argon, helium, hydrogen) bubbled through liquid source

chemical, and then being humidified together with the

chemical vapor. In other word, bubbler is a device where

carrier gas bubbles through the liquid chemical source

which is being stored inside it. A bubbler consists of a

container with constant temperature. The container should

be wrapped with heater, since it is important for the

temperature to keep constant.

A device which is called bubbler is made to producemethanol vapor. Since it is important to keep the

temperature constant, the container; a perfluoroalkoxy

(PFA) based transfer jar (Ono Science Co., Ltd) is winded

with a ribbon heater. During the experiment, the

temperature is set to 60 (oC ) by a temperature sensor

(FINE FHP-201).

2.2.1 Calculation for Methanol Vapor Flow

Methanol vapor flow which comes out from the

bubbler can be calculated using the following equation.

V HS

v

N vPP

PQQ 2

Eq. (1)

: Vapor pressure of liquid source chemical (Pa)

: Bubbler head-space pressure (Pa)

: Flow of output vapor (slpm)

: Flow of carrier gas (slpm)

2.2.1.1 Pv Vapor Pressure of Liquid Source Chemical

In this research, methanol solution is used as the

liquid source chemical. Therefore, Pv in Eq. (1) has to

consider both methanol vapor pressure and water vapor

pressure. Equations for methanol vapor pressure and

water vapor pressure are shown as follows.

Methanol vapor pressure

T P

230

113.147387863.7

10 Eq. (2)

P : Methanol vapor pressure (kPa)

T : Temperature (℃)

Water vapor pressure

T P

426.233

63.173007131.8

10Eq. (3)

P : Water vapor pressure (kPa)

T : Temperature (℃)

Pv as shown below is based on Raoult’s Law and Dalton’sLaw.

21 PPPv

2

'

21

'

1 xP xP Eq. (4)

)1( 1'

21'

1 xP xP

'

1P

,'

2P

: Vapor pressure of pure liquid (kPa)

1 x

, 2 x

2 : Mole fraction

2.2.1.2 P HS , Head Space Pressure



99

From Boyle-Charle’s Law, PHS can be expressed as below

(PHS is referred as P2).

2

22

1

11

T

V P

T

V P

Eq. (5)

By rearranging equation (5), the following equation maybe obtained.

21

211

2V T

T V PP

Eq. (6)

2.3 Experimental method

2.3.1 I-V, I-W Characteristic for Vapor Fed

I-V, I-W characteristic for vapor fed is measured

according to the conditions below.

Table 1 Measurement condition

Table .2 Measurement condition

2.3.2 Methanol Crossover Measurement

In order to compare the methanol crossover between

liquid phase and vapor phase, the measurement condition

is shown as follow.i. Anode flow rate : 0.3(l/min)

ii. Cathode flow rate : 0.6(l/min)

iii. Cell operating temperature : 60 (℃)

iv. Measuring time : 15 (min)

All the measuring devices are measured and controlled by

using LABView.

3. Results and discussions 3.1 I-V, I-W Characteristic for Different Anode and

Cathode Flow Rate

Results for Measurement Condition①are shown in Fig. 2

and 3. In Fig. 2, the graphs going down to the right is the

I-V characteristic, and the graphs going up to the rightis the I-W characteristic.

Fig.1 Experimental setup

Fig.2 I-V, IW characteristic for vapor fed

Methanol concentration (%) - 25,

Anode flow rate (slpm) - ◆,■:0.03; ▲, :0.04; ✴,●:0.05


Methanol concentration – 50 (%),N2 flow rate (slpm) - ◆,■:0.03; ▲, :0.04; ✴,●:0.05

It is known that power densities for methanol

concentration of 25 [wt%] is higher than 50 [wt%].

Besides, the power densities getting higher with the

increase of anode flow rate, for both condition. This

shows that anode catalyst reaction is restricted for denser

methanol. Therefore, it can be thought that the efficiency

is decreased by methanol crossover phenomenon, where

Methanol concentration (wt%)25、50

Anode flow rate (slpm) 0.03、0.04、0.05

Cathode flow rate (Air) (slpm) 0.5

Cell operating temperature (℃) 60

Methanol concentration (wt%) 25

Anode flow rate(slpm) 0.3、0.4、0.5、0.6、0.7、0.8

Cathode flow rate (Air) (slpm) 0.5、0.6

Cell operating temperature (℃) 60

Temperature

sensor

Flow controllerAi

r

N2

Methanolaqueous

solution

Flow controller

Electronic load

device

Constanttemperature

chamber

Anode

Cathode

CO2concentra

tion meter

Wate

rSilicael

Wat

er

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0

電流密度 [mA/cm2]

[

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

[

/ c

2

V o l t a e V

Current densit

P o w e r d e n s i t y

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5


[

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

[

/ c

2


Current density [mA/cm2]

V o l t a e [ V ]



100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12 14 16 18


電圧 [ V ]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

電力密度 [ m W / c m

2 ]

空気 0.5 空気 0.6 空気 0.5 空気 0.6

Current density

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12 14 16


電圧 [ V ]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

電力密度 [ m W / c m 2 ]

空気 0.5 空気 0.6 空気 0.5 空気 0.6

Current density

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12 14 16


電圧 [

V ]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

電力密度

[ m W / c m

2 ]

空気 0.5 空気 0.6 空気 0.5 空気 0.6

0.0

0.1

0.20.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12 14


[ ]

0.00.2

0.40.6

0.81.01.2

1.41.6

1.82.0

[ m

/ c m 2 ]

空気 0.5 空気 0.6 空気 0.5 空気 0.


P o w e r d e

n s i t y [ m W / c m

2 ]

V

o l t a g e [ V ]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12


[ V

0.0

0.20.4

0.6

0.81.0

1.2

1.4

1.6

1.8

2.0

[ m

/ c m

2

空気 0.5 空気 0.6 空気 0.5 空気 0.6



V o l t a g e [ V ]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10 12


[ V ]

0.0

0.2

0.40.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

[

2

空気 0.5 空気 0.6 空気 0.5 空気 0.


P o w e r d e n s i t

m W / c m

2

V o l t a e V

methanol which do not react, permeate from anode to

cathode, while the concentration increases at the same

time. However, in Fig. 3, it is understood that when N2

flow rate is 0.04 [slpm] and 0.05 [slpm], power densities

are almost the same. This can be assumed that during that

time, catalyst reaction has reached its limit.

Results for Measurement Condition ②are shown from

Fig. 4 until Fig. 9.


Anode flow rate (slpm) - 0.3,N2 flow rate (slpm) - ◆,■:0.5; ▲, :0.6

Fig.5 I-V, IW characteristic for vapor fedAnode flow rate (slpm) – 0.4,

N2 flow rate (slpm)- ◆,■:0.5; ▲, :0.6


Anode flow rate (slpm) – 0.5,

N2 flow rate (slpm)- ◆,■:0.5; ▲, :0.6


N2 flow rate (slpm)- ◆,■:0.5; ▲, :0.6


N2 flow rate (slpm)- ◆,■:0.5; ▲, :0.6


Anode flow rate (slpm) – 0.8,

N2 flow rate (slpm)- ◆,■:0.5; ▲, :0.6



101

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14

Current desity[mA/cm2]

e l l

o l t a g e [

0.0

0.5

1.0

1.5

2.0

2.5

o

e r d e n s i t y [

/ c

2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 200 400 600

Time [s]

C O 2 c o n c e n t r a t i o n [ % ]

y = 5.1939x + 0.2051

y = 0.1119x + 0.5731

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.00 0.05 0.10 0.15 0.20 0.25


O u t p u t p o w e r [ m W / c m 2 ]

0.48

0.50

0.52

0.54

0.56

0.58

0.60

0.62

M e t h a n o l c r o s s o v e r [ % ]

Looking into Fig.4 untuil Fig.9, when anode flow

rate 0.3~0.5 [slpm], power density for anode flow rate 0.6

[slpm] is higher than 0.5 [slpm], but it decreases with the

increase of anode flow rate. In addition, although power

density decreases continuously until anode flow rate is 0.8

[slpm], but power density is higher during lower air flow

rate when anode flow rate are 0.7 [slpm] and 0.8 [slpm].

It is understood that the optimum output power density

obtained when air flow rate 0.5 [slpm] is 1.4 [mW/cm2

].Furthermore, as being stated before, based on result from

anode flow rate 0.6 [slpm], output power density is lower

with the increase of air flow rate. This may be caused by

the block of pore electrode and gas diffusion layer with

water, which is the reaction product that prevents the flow

of fuel and air. Thus, optimum cathode flow rate exists,

and it can be considered that during that time voltage loss

at cathode becomes to the least value.

3.2 I-V, I-W characteristic for different methanol

concentration

I-V and I-W characteristics for different methanolconcentration of methanol vapor, which were 10(wt%)

and 25(wt%) were measured respectively. The result is

shown in Fig 10. It can be seen that with lower percentage

of methanol concentration, the output power shows even

better. This may be because denser methanol results in an

increase of methanol crossover. In this experiment, when

25(wt%) of methanol is being supplied, it can be thought

that the concentration supplied is higher than the optimum

concentration. Therefore, even though only 10(wt%) of

methanol is being supplied, the output power shows even

higher. By using low concentration, the amount of

methanol used can be reduced, where at the same time

higher output power can be gained.

Fig.10 I-V,I-W characteristic ( ▲, 10(wt%) , ▲, 25(wt%) )

3.3 Methanol crossover measurement and comparison

between vapor fed and liquid fed

In order to measure the percentage of methanol

crossover, the amount of CO2 which is produced at the

cathode side is measured. The existence of CO2 at the

cathode side proves that methanol crossover occurred.

This is because, according to chemical reaction only

water will be produced on the cathode side. Since

methanol which permeated through PEM is oxidized with

air which is being supplied at the cathode, this becomes

the reason why CO2 exist. The amount of CO2 is

measured by using a CO2 concentration meter for about

15 minutes and the data is collected through LABView

program.

Fig. 11 to Fig. 13 show the amount of CO2 producedat the cathode when the output power is 0.16, 0.23 and

0.37 (mW/cm2) respectively. Also shown in the graph is

the average amount of CO2 produced in each case.

Regarding these figures, it can be seen that the amount of

CO2 produced at the cathode side increased gradually

with the increase of output power. It seems reasonable to

say that as output power increases, the amount of

methanol that reaches at the anode also increases. As the

result, the increased amount of methanol at the cathode

has increased the quantity of CO2 produced. Hence, the

amount of methanol that reacts at the anode decreases

from time to time, and reduces the performance of a

DMFC.

Fig.11 Measurement of CO2 concentration for output power

: 0.16 [mW/cm2] (■: measured value, ■: average value)





102

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 200 400 600

Time [s]

C O 2

c o n c e n t r a t i o n [ % ]

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 200 400 600

Time [s]

C O 2 c o n c e n t r a t i o n [ % ]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.16 0.23 0.37

Output power [mW/cm2]

C O 2 c o

n c e n t r a t i o n [ % ]



The amount of methanol crossover CO2 produced bu

liquid fed and the output power are shown in Fig. 14.

Using both of the equations shown in Fig. 14, the amount

of CO2 produced by liquid fed at specified output power

can be easily calculated.The comparison of methanol crossover amount for

liquid fed and vapor fed is shown in Fig. 15. According to

the graph, in all three cases, it can be seen that the vapor

fed DMFC produced lower amount of CO2 concentration,

compared to the liquid fed. Quantitatively, the amount of

CO2 produced by vapor fed is reduced for about half than

the liquid fed. Since methanol is supplied in vapor phase,

and it is stated that the absorption rate of vapor on PEM is

slower than liquid, therefore it can be concluded that by

supplying methanol vapor in DMFC, the amount of

methanol that permeates from anode to cathode can be

decreased. Consequently, only a small amount of

methanol that does not react at the anode. Thus, theamount of CO2 produced is lower, as shown in Fig. 15.

Fig.14 Output power and methanol crossover amount for liquid fed

(■: methanol crossover, ■: output power)

Fig.15 Amount of methanol crossover for liquid fed and vapor fed (■: vapor fed, ■: liquid fed)

4. ConclusionFrom the above experiments and results lead to the

conclusion that for a vapor fed DMFC, lower methanolconcentration (10 (wt %)) shows even better performance

than a little bit higher methanol concentration (25 (wt

%)). Besides, when comparing the amount of methanol

crossover for both liquid fed and vapor fed DMFC, vapor

fed indicates lower amount of CO2 concentration for

almost half than liquid fed DMFC, at the same output

power. Thus, this support the statement [4] that the

methanol crossover decreases due to the decreased

amount of water in the membrane and due to the smaller

methanol uptake from a gas phase than from a liquid one.

ACKNOWLEDGMENT Especially deserving of acknowledgment are the people I

have learned from and worked with at the Kogoshi

Laboratory in Tokyo University of Science, Japan.

REFERENCES

[1] Fuqiang Liu and Chao-Yang Wang, “Mixed potential in a direct

methanol fuel cell – Modeling and experiments,” Journal of The

Electrochemical Society, 154 (6) B514-B522, 2007.

[2] Takeo Yamaguchi, Masaya Ibe, Balagopal N. Nair and Shin-ichi Nakao, “A pore-filling electrolyte membrane-electrode integrated

system for a direct methanol fuel cell application,” Journal of The

Electrochrmical Society, 149 (11) A1448-A1453, 2002

[3] S. Basri, S.K. Kamarudin and W.R.W. Daud, “Design and

optimization of direct methanol fuel cell,” Institute of Fuel Cell,

Universiti Kebangsaan Malaysia

[4] Josef Kallo, Wernez Lehnert and Rittmar von Helmolt,

“Conductance and methanol crossover investigation of nafionmembranes in a vapor-fed DMFC,” Journal of The

Electrochemical Society, 150 (6) A765-A769, 2003.

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3. Methanol Crossover Reduction by Vapor Fed Direct Methanol Fuel Cell