Development and characteristics of a 400 W-class direct methanol fuel cell stack

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 e n e r g y 3 3 ( 2 0 0 8 ) 7 1 5 3 – 7 1 6 2

Avai lab le a t www.sc iencedi rec t .com

j ourna l homepage : www.e lsev ier . com/ loca te /he

Development and characteristics of a 400 W-class directmethanol fuel cell stack

Han-Ik Joha,b, Sang Youp Hwanga, Jae Hyung Choa, Tae Jung Haa, Soo-Kil Kima,Sang Heup Moonb,**, Heung Yong Haa,*aCenter for Fuel Cell Research, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-dong, Sungbuk-gu,

Seoul 136-791, Republic of KoreabSchool of Chemical & Biological Engineering and Institute of Chemical Processes, Seoul National University, San 56-1,

Shillim-dong, Kwanak-ku, Seoul 151-744, Republic of Korea

a r t i c l e i n f o

Article history:

Received 19 May 2008

Received in revised form

28 July 2008

Accepted 2 August 2008

Available online 18 October 2008

Keywords:

Direct methanol fuel cell

Stack

CO2

pH

Distributions of voltage and

temperature

* Corresponding author. Tel.: þ82 2 958 5275** Corresponding author. Tel.: þ82 2 880 7409

E-mail addresses: [email protected]/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.08.016

a b s t r a c t

In this study, a 400 W-class direct methanol fuel cell (DMFC) stack is developed for large

size portable applications and its operating behaviors under the various conditions are

monitored. The DMFC stack comprising of 42-cells is assembled with graphite bipolar

plates and membrane–electrode assemblies (MEAs) having an active area of 138 cm2 per

each. The stack is operated by varying the concentrations of methanol, stoichiometry (l),

and the electric load. In addition, other associated factors, such as voltage and temperature

distributions along the individual unit cells, pressure drops inside the stack, voltage

behaviors in response to the dynamic change of the electric load and the pHs of the effluent

solutions from the outlets of both electrodes, are also studied in a detailed manner. The

stack produces a power of 400 W under an operating condition of feeding 0.8 M methanol

and 34 l/min air at 1 atm, and uniform distributions of temperature and voltage prevail in

all the 42 unit cells. A long-term operation coupled with performance restoration processes

shows that a typical single cell used in this stack is able to run with a good stability for

more than 500 h without any substantial degradation in the performance.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction hundred watts, which can find its application in small size

Direct methanol fuel cells (DMFCs) have been developed

mainly as power sources for small scale (less than 50 W)

applications because they have some drawbacks in gener-

ating high powers, due to their inherent sluggish electrode

kinetics and methanol cross-over from anode to cathode.

However, the continuous research efforts devoted to improve

the catalysts, membranes and stacking techniques have

resorted to develop the DMFC system with a power of several

; fax: þ82 2 958 5199.; fax:þ 82 2 875 6697.r (S.H. Moon), [email protected] Association for H

vehicles, emergency auxiliary powers, and military portable

powers [1].

Generally, three types of stack configurations are used with

DMFC; namely, mono-polar strip stack, bi-cell stack, and

bipolar stack [2]. The mono-polar strip stack can be assembled

by placing the electrodes with the same polarity on the same

plane of a membrane and subsequently, connecting the

electrodes in series with the electric wire [3–5]. In this

configuration, the reactants are supplied by means of

re.kr, [email protected] (H.Y. Ha).ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]



http://www.elsevier.com/locate/he

Fig. 1 – Photograph of a 400 W-class DMFC stack.


diffusion without using any pump or blower to simplify the

system. However, in this system, the current flow is possible

only through the strip of electrodes, which can cause a huge

internal resistance [6,7] and likely to limit its power to appli-

cation of 10 W-class. The bi-cell stack is an array of bi-cell

units that is composed of one common anode and two sepa-

rated cathodes. In this system, the fuel is supplied by means of

a pump through a main fuel supply line and the oxidant is

supplied by means of natural convection along the space that

exists between bi-cell units. This system has advantages, like

an easy replacement of failed bi-cell units without disjointing

the whole stack and easier removal of heat and products [8].

On the other hand, the bipolar stack, which is the most widely

used one, is composed of unit cells that has an alternate

polarity and faces each other through a bipolar plate. In this

system, the fuel and oxidant are supplied by a pump and

a blower, respectively. This type of stack has a small internal

resistance due to the serial connection between unit cells

through bipolar plates, and thus it is suitable for high power

applications over 100 W-class [9,10].

It is interesting to mention some of the recent reports that

deal with the operating behaviors of various sizes of DMFC

stacks. Chen et al. [11] investigated the effects of various

operating parameters on the DMFC stack performance. These

parameters include the concentration of methanol, rate of

the air flow, thickness of gasket, and depth of the channel. The

stack generated a power of 33 W by feeding 3% (0.621 M)

methanol at 70 �C. Kim et al. [10] developed a 50 W-class DMFC

stack, consisting of 6 cells with an active area of 106 cm2. The

stack was able to generate a maximum power of 54 W under

the condition of feeding 2 M methanol and dry air at the flow

rates of 25 ml/min and 12 l/min, respectively, at room

temperature and atmospheric pressure. In another approach,

Dohle et al. [12] developed a 500 W-class DMFC stack having

72 unit cells. They investigated the effects of PTFE amount in

the GDLs, current collector material, air and methanol flow

rates and concentration of methanol on the performance of

the stack.

Besides, researchers also investigated the effects of the

nature of material of bipolar plates and the structure of

manifold on the performance of stacks. For example, Jiang

et al. [9] employed Ni incorporated and Au coated stainless

steels as anode and cathode bipolar plates. The stack with

these plates could generate a power of about 30 W with

a maximum power density of 26 mW/cm2 by supplying 1 M

methanol and 3 psi air. Shudo and Suzuki [13] used a highly

porous and corrosion-resisting stainless steel flow filed and

observed a 25% improvement in DMFC performance over

a conventional grooved flow field. Lee et al. [14] developed

a 40 W-class DMFC stack composed of 6 unit cells with an

external manifold, which exhibited a maximum performance

of about 41 W on 2 M methanol and air at 65 �C. The stack was

hybridized with three types of batteries and then the load

sharing characteristics of both the stack and the batteries

were evaluated.

As mentioned above, almost all of the investigations have

focused mainly on the DMFC stack with a capacity below

50 W, where small-size MEAs of area around 50 cm2 were

installed. In fact, the studies related to the development of

large-size stacks with capacity more than 100 W are found to

be scarce in the literature. Therefore, an understanding on the

performance of the larger size stacks is highly required.

In this study, we have developed a 400 W-class DMFC stack

consisting of 42 unit cells with internal manifolds and

graphite bipolar plates. Before testing the stack performance,

single cells and a 5-cells short-stack have been tested to verify

the suitability of all components used in our study and to

determine a range of operating conditions to test the 42-cells

stack. Furthermore, the stack characteristics including pres-

sure drops, distributions of unit cell voltage and temperature

and pHs of the exhausting liquids out of the stack have been

monitored and analyzed to completely understand the

operating behaviors of the large size DMFC stacks.

2. Experimental

The DMFC stack built in this study comprised of 42 cells with

an electrode area of 138 cm2 per cell and the pitch of the cell

was only 2.4 mm. The membrane–electrode assemblies

(MEAs) used in the stack consisted of Nafion 115, anodic and

cathodic catalyst layers and gas diffusion layers. The catalyst

loadings both in the anode and the cathode were 6.0 mg/cm2

PtRu black and 4.0 mg/cm2 Pt black (Johnson–Matthey),

respectively. The flow field of bipolar plates has a serpentine

structure. Fig. 1 displays a real image of the stack and its

specifications are summarized in Table 1.

Before assembling the stack, some experiments have been

conducted to characterize single cells and a 5-cells short stack

by means of changing the operation variables, such as

Table 1 – Specification of 400 W DMFC stack fabricatedand investigated in this study

Number of cells 42

Electrode area 138 cm2

Stack dimensions 196 mm� 126 mm� 140 mm

Design values

Current (current density) 20 A (145 mA/cm2)

Voltage

(unit cell voltage)

20 V (0.476 V/cell)

Power (power density) 400 W (69 mW/cm2)

MEA

Membrane Nafion-115

Anode PtRu black, 6.0 mg-PtRu/cm2

Cathode Pt black, 4.0 mg-Pt/cm2

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 e n e r g y 3 3 ( 2 0 0 8 ) 7 1 5 3 – 7 1 6 2 7155

methanol concentration, air flow rate under non-back pres-

sure, cell temperature, etc. The single cell and stack perfor-

mances were measured with an automated test station

(Smart-II DM, Wonatech Co.). The effectiveness of the flow

field and manifold design was also tested by measuring the

pressure drops at both sides of the electrodes in the cell

compartment. The distributions of voltage and temperature

across the entire stack were measured at different operating

conditions, such as at an open circuit (OC) state, during the

measurement of I–V characteristics, and on applying a load

with constant current of 20 A.

The behavior of CO2 that was produced in the stack was

indirectly monitored by measuring the pHs of the outflowing

methanol solution from the anode compartment and the

condensed water from the cathode compartment. The long-

term operation and durability tests of the MEAs were carried

out with single cells under the various air supply modes.

3. Results and discussion

In order to set the optimal operating conditions of the stack,

a set of experiments was conducted to see the effects of

operating temperature, methanol concentration and air flow

rate on the performance of a single cell. Fig. 2 exhibits the

performance of a single cell by varying the temperatures of

Fig. 2 – The effects of air flow rate and cell temperature on

the performance of a single cell (l [ stoichiometry of

reactants at 150 mA/cm2, anode/cathode).

the cell and cathode air flow rates (stoichiometry, lair), while

maintaining the supply of 1 M methanol at a stoichiometry

(lMeOH) of 3. The air flow rates were not varied as with

increasing the current, but were fixed at the values calculated

based on the stoichiometry at a load of 150 mA/cm2. When the

lair was maintained at 3, the performance was gradually

increased with increasing temperature of the cell irrespective

of the current ranges. On the other hand, when the lair was

kept at 2, the cell exhibited the lowest performance, particu-

larly at higher current range, that is, over 170 mA/cm2 at the

operating temperature of 80 �C. Generally, an increase in

the cell temperature has both positive and negative effects on

the performance of a cell. The increase in temperature of the

cell can enhance the kinetics of the electrochemical reaction,

which in turn contributes to the improvement in the perfor-

mance of a cell. On the other hand, an increase of water vapor

pressure resulting from higher cell temperature can decrease

the oxygen partial pressure within the cell, and thus hold

responsible for the deterioration in the performance of a cell

[15]. Besides, the higher cell temperature could also induce the

higher rate of methanol crossover, which has a negative

impact on the performance of a cell. Therefore, in the case of

lair¼ 3, the supply of oxygen to the cathode is sufficiently

available enough to enhance the overall performance of a cell,

whereas in the case of lair¼ 2 the performance of the cell

declines with increasing the temperature particularly at

currents over 150 mA/cm2, due to increased methanol cross-

over and lowered oxygen partial pressure at elevated

temperature of a cell. However, in all the cases, the difference

in the performance of a cell is found to be around 10% at all the

operating temperatures.

The effect of methanol concentration on the performance

of a single cell was investigated and the results are shown in

Fig. 3. As shown in the figure, the performances of the cell

didn’t show any significant difference in a lower current

density region, that is, up to 130 mA/cm2 with change of

concentration of methanol. In this lower current density

range, the 0.5 M methanol exhibited relatively a better

performance with that of the 1 M methanol and this can be

due to a lower methanol crossover in this lower concentration

Fig. 3 – The effect of concentration of methanol on the

performance of a single cell. Stoichiometries were fixed at

3/3 for methanol and air.


of methanol. At higher current density region, an abrupt drop

in the voltage of the cell has been noted, and this might be due

to lower rate of methanol diffusion when feeding with a 0.5 M

methanol solution. According to the Wilke–Chang equation

[16], the diffusivity of some diluted liquid solution can

increase with solution concentration;

DV ¼ 7:4� 10�8ðjBMBÞ1=2T

mV0:6A

(1)

D1 M

D0:5 M¼ 1:52

T1 Mm0:5 M

T0:5 Mm1 M

(2)

where DV is the diffusivity of a solution, T is the absolute

temperature, m is the viscosity of a solution, VA is the molar

volume of the solute as liquid at its normal boiling point, jB is

the association parameter for a solution; the recommended

value of jB is 1.9 for methanol, and MB is the molecular weight

of a solvent. Assuming that the temperature and the viscosity

of methanol solutions are the same regardless of the

concentration, changing the concentration of methanol from

0.5 to 1.0 M could result in an increase in the diffusivity by 1.5

times according to the equations. The faster diffusion rate of

methanol has a positive impact on the performance of a cell

by means of bringing down the overpotential of the anode

owing to increased mass transport of the reactant at high

current density regions.

Here, another set of experiments has been carried out to

study the operational characteristics of a short stack

composed of 5-cells before dealing with the stack composed of

42-cells. Fig. 4 shows the short-stack performance as a func-

tion of temperature and methanol concentration. The stoi-

chiometries of the reactants, such as methanol and air, were

fixed at 3.6/3.6 (anode/cathode) for a convenience of con-

ducting the experiments, which are slightly different from

that of a single cell test (3.0/3.0). As shown in the figure, the

1.0 M methanol exhibited a better performance than the 0.5 M

at the two different operating temperatures. At 25�C,

a remarkable difference in the performance could be observed

depending on the concentration of methanol whereas at 70�C

the performance had only a slighter dependence on the

Fig. 4 – The effects of concentration of methanol and cell

temperature on the performance of 5-cells short-stack.

Stoichiometries were fixed at 3.6/3.6 for methanol and air.

concentration of methanol. The appreciable impact of the

methanol concentration at 25�C is probably due to the heat

released by oxidation of crossed methanol at the cathode that

increases with increasing the methanol concentration. In the

case of feeding of 1.0 M methanol, the inside temperature of

the short-stack rose up to 40�C (at a load of 20 A) and thus

improved the performance of the cell. On the other hand, in

the case of feeding 0.5 M methanol, the feed flow rate was

doubled to make the stoichiometry of the methanol in the

anode the same as the case of feeding 1.0 M methanol, and

this higher flow rate is thought to cool down the cell to lower

the performance when it was being operated at room

temperature.

By keeping the experimental results of the single cell as

well as the short stack with 5-cells (Figs. 2–4) as the base, we

have operated the stack with 42-cells and compared its

performance with those of the single cell and the short stack

(see Fig. 5). The single cell and the short-stack were tested by

feeding 1.0 M methanol at 70�C by means of externally heating

the cells, whereas the full-stack with 42-cells was operated by

feeding 0.8 M methanol at room temperature without

providing any external heating. The full-stack was operated at

room temperature and was heated-up autonomously by the

exothermic reactions at the cathode after the incubation

period. The stack could yield 97% performance of a single cell

at 150 mA/cm2, implying the fact that the stack system,

including bipolar plates, gaskets, manifolds and fastening

bolts, was well designed and assembled.

The temperature and OCV distribution across the entire

42-cells stack were monitored and the results are shown in

Fig. 6(a) and (b). In these experiments, the supply of methanol

and air was maintained at room temperature of 25�C. The

temperature was measured by inserting a thermocouple for

every 3 unit cells, and the OCV measurement was done by

placing the metal pins on every single cell. The stoichiometry

was fixed at 3.6/3.6 based on an electric load of 150 mA/cm2. In

the case of feeding 0.5 and 0.8 M methanol, the cell tempera-

tures and OCVs were found to be around 25 �C and 0.78–0.81 V,

and 29 �C and 0.76–0.79 V, respectively, and these profiles

(temperature and OCV) appeared to be very uniform. On the

Fig. 5 – Comparison of the performances of a single cell, 5-

cells short-stack, and 42-cells stack. The single cell and

short-stack were operated at 70 8C with external heating,

whereas 42-cells stack was operated at room temperature.

Fig. 6 – Distributions of (a) temperature in the unit cell and

(b) OCV throughout the stack at an OC condition.

Stoichiometries were fixed at 3.6/3.6 for methanol and air.Fig. 7 – (a) The pressure drop and (b) pH variation

depending on methanol stoichiometry at an OC condition.

The air stoichiometry was fixed at 3.6.


other hand, when feeding 1.0 M methanol, there was a signif-

icant increase in the temperature of unit cells, varying from

60 �C (at the methanol inlet) to 83 �C (at the middle of the

stack). The OCV values also varied from 0.74 to 0.79 V, which

are lower than those of the 0.5 and 0.8 M methanol feeds.

The prevailed higher temperatures with 1.0 M methanol can

be attributed to the heat generated by crossed methanol in the

cathode side. In the case of feeding 1.0 M methanol, the heat

generated by crossed methanol is relatively higher than the

heat removed from the stack, which can lead to accumulation

of heat and result in a rise of the temperature of the stack

when compared with those of feeding 0.5 and 0.8 M methanol

feeds. The increase in temperature of the cells, in turn, can

induce higher rate of methanol crossover to the cathode side.

From these results, it is understood that the concentration of

methanol is a critical factor that can determine the perfor-

mance as well as temperature of a stack.

Fig. 7(a) exhibits the pressure drop both at the anode and

cathode compartments of the stack as a function of methanol

stoichiometry under an open circuit (OC) condition and a fixed

air stoichiometry (lair¼ 3.6). The pressure drop at the anode

gradually increased with the increase of lMeOH. In these set of

experiments, the air flow rate was not varied, and therefore

the cathode pressure drop remained the same (around

20 kPa). Since the pressure drop was measured under an OC

condition, naturally, there was no contribution from the

products. Hence, the anode pressure drop at lower concen-

tration of methanol is higher than at higher concentration due

to an increased flow rate of the feed to make the methanol

stoichiometry the same.

Fig. 7(b) shows the pHs of the outflowing methanol from

the anode compartment and the water produced from the

cathode under the OC condition. As shown in the figure, the

pHs of water produced at the cathode are about 3.75–3.80,

which is much lower than that of the methanol solution at the

anode, and is independent of lMeOH. The reason for the lower

pH of the water out of the cathode is that the CO2 produced by

the oxidation of crossed methanol subsequently dissolves in

water and thereby converts into carbonic acid. Under the OC

condition, the amount of CO2 generated might be relatively

larger than its solubility in the produced water and, eventu-

ally, the water can be fully saturated with CO2. This is likely to

be the reason for maintaining a constant pH of the cathode

water regardless of concentration of methanol and stoichi-

ometry. In contrast, at the anode, usually no reaction takes


place under the OC condition and therefore lowering down of

pH from 5.30 to 5.37 (pH of the 0.5–1.0 M aqueous methanol

solution) to below 5 is solely owing to the back diffusion of CO2

from cathode to anode. The higher concentration of methanol

can induce the higher rate of methanol crossover and thus the

more CO2 generation in the cathode, which accelerates the

back-diffusion of CO2 to the anode and consequently results in

the lower pH of the anode solution. It is interesting to mention

that Ye et al. [17] also confirmed the occurrence of back-

diffusion of CO2 in the anode effluent gas under the OC

condition, of which 40% of the gas was proved to be CO2.

The effluent solutions with low pHs are likely to corrode

the metal bus plates that were used as current collectors. In

our stack, the gold-coated brass plates have been employed as

the bus plates at both ends of the stack. After running the

stack for 17 days, a severe corrosion had occurred, particularly

at the areas where the plates were in direct contact with the

methanol solution, as shown in Fig. 8. Therefore, we have

employed other types of end plates for the stack. These were

designed in such a way that the direct contact of the metal end

plates, including the bus plates, could be completely avoided

from the methanol solution and the water condensate.

A similar characterization for the stack under loaded

conditions was done, and the results are presented in Figs.

9–11. Fig. 9(a) illustrates the performance of the stack by

feeding methanol under different operating conditions: 0.5 M

at 25�C, 0.8 M at 25�C, and 0.8 M at 48 �C. As we have discussed

in Fig. 6(a), when 1.0 M methanol was fed to the stack, the

temperature of the stack rose up to 80 �C, even though

the methanol feed temperature was maintained at 25 �C, and

the stack was placed under an OC condition without providing

any external heating. Therefore, there might be a possibility

for the occurrence of failure (a temperature overshoot) to the

stack due to thermal shock when a 1.0 M methanol was being

used under a current loaded condition. Because of this reason,

Fig. 8 – Photographs of the corroded bus plates that were

affected by effluent solutions of the stack; (a) anode bus

plate and (b) cathode bus plate.

we have deliberately avoided using 1.0 M methanol, and

instead fed 0.8 M methanol to test the performance of the

stack. In addition, to produce an electric power in a real DMFC

system, the heated methanol solution coming out of a stack

has to be recirculated into the stack after adjusting the

concentration of methanol and temperature. In this case the

temperature of the recirculating methanol solution may be

controlled at the range of 40–60 �C in order to avoid over-

shooting of temperature in the stack. That is why we have

tested the cell with a methanol solution warmed at 48 �C. The

stoichiometries were kept at 3.6/3.6, both for the anode and

the cathode on the basis of the current density of 150 mA/cm2.

As shown in Fig. 9(a), when the methanol solution was fed at

25�C, the 0.8 M methanol exhibited a better performance than

that of the 0.5 M methanol. When we fed 0.8 M methanol at

different temperatures, we did not observe any significant

difference in the performance except at the lower current

ranges (<12 A), where the self-heating of the stack was

insufficient.

Fig. 9(b) and (c) illustrate distributions of temperature and

voltage across the stack when applying a load of 20 A. As

shown in the figures, when the methanol was fed at 48�C,

more uniform distribution of temperature as well as voltage of

the cells has occurred than with feeding at 25�C. Therefore,

one can see that in real DMFC systems with a methanol

recirculating loop, the temperature and voltage distribution in

the stack will be more uniform compared with the DMFC

system measured in a test station that does not have a meth-

anol recirculation facility.

The operational characteristics of the stack, such as pres-

sure drop, temperature, pH of the effluents at both the elec-

trodes and the amount of water produced at the cathode, were

studied by means of gradually increasing the electric load; the

results are shown in Fig. 10(a)–(c). As shown in Fig. 10(a), the

pressure drops at both electrodes exhibited different behav-

iors. For instance, the cathode pressure drop maintained

a constant value around 20 kPa, irrespective of using different

concentrations of methanol at the anode or under applying

varying electric loads. The existence of a slight difference in

the pressure drop values between 0.5 and 0.8 M methanol is

probably due to relatively higher amount of methanol cross-

over induced by the latter, which results in higher rate of CO2

generation at the cathode. On the other hand, the anode

pressure drop tended to gradually increase with increasing

the current, especially at a lower current region (up to 12 A). At

higher currents, the anode pressure drop for 0.5 M methanol

continued to increase, whereas for 0.8 M methanol the pres-

sure drop started to decrease at a load of around 12 A. The

primary reason for an increase in the pressure drop in both

cases can be attributed to the generation of CO2 bubbles,

which would build up in the anode flow field. This view is also

supported by the study of Wong et al. [18] who recently

reported that transient capillary blocking by the CO2 gener-

ated at the anode could increase the anode channel pressure

drop. As the current load increases, the amount of CO2

generation also increases, which can consequently block the

channels; this can explain the linear increase in the anode

pressure drop in the case of using 0.5 M methanol.

On the other hand, a sudden decrease in the anode pres-

sure drop for the 0.8 M methanol at a high current region that

Fig. 9 – Characteristics of the 42-cells stack; (a) performance of the stack, (b) temperature distribution across the unit cells at

20 A, and (c) voltage distribution across the unit cells at 20 A.

Fig. 10 – Characteristics of the 42-cells stack; (a) pressure drops, (b) temperature, and (c) pHs of the effluent solutions

depending on the electric loads.


Fig. 11 – Characteristics of the 42-cells stack; (a) the

performance and (b) pressure drop behaviors of the stack

with response to dynamic change of the stack current. A

20 A load was instantly applied and maintained.

Fig. 12 – The effect of air supply mode on the long-tem

operation; (a) a 20 A load, continuous air flow, 10 h/day

operation and (b) a 20 A load, ON/OFF air flow (3 min ON

followed by 2 s OFF), 500 h continuous operation.


has been confirmed by our repeated experiments may be

explained like this: There are two probable reasons that can be

associated with this phenomenon [17,19,20]. Firstly, the

average viscosity and density of the liquid phase decrease

with increase of temperature, which might decrease the fric-

tional and gravitational pressure drop. Secondly, the rise in

temperature of the stack can slow down the solubility of CO2

in methanol solution and this can further promote the chance

for the evaporation of methanol solution. Eventually, there is

a possibility for the formation of more gas phase in the

channel, which can lead to decrease in the gravitational

pressure drop. In Fig. 10(b) we can see that the stack temper-

ature dependency on the current varies depending on the

methanol feed concentrations. There is no change in

temperature for 0.5 M feed when comparing with a propor-

tional increase in temperature to current for 0.8 M feed. It is

therefore speculated that the high temperature at the currents

above 12 A is the main reason for lowering the pressure drop

for 0.8 M feed.

The pHs of the effluent solutions at both electrodes of the

stack under an OC condition (Fig. 7) and electrically loaded

condition (Fig. 10(c)) differed to a large extent. It is obvious

from the figure that the pH of the anode effluent solution

gradually decreases, and attains a saturation at pH 3.75 and

3.65 for 0.5 and 0.8 M methanol, respectively, with the

formation of CO2 resulting from the increase of current.

Contrarily, in the cathode, pH of the effluent solution

increases only slightly with the increase of current because

generation of CO2 at the cathode decreases with the increase

of current owing to the lowered rate of methanol crossover,

due to consumption of methanol at the anode.

The performance and pressure drop behaviors of the

42-cells stack in response to the dynamic change of the elec-

tric load (current steps with a rise/decline time¼ 1 s) were

monitored, and are presented in Fig. 11. For the dynamic

change of the stack current, a 20 A load was applied instantly

to the stack under an OC condition and the same load has

been maintained for 30 min. The stack voltage has dropped

sharply from an OCV and attained a voltage of 18.8 V imme-

diately after applying the load; then the stack exhibited

a capacitive increment and reaching a steady voltage of 19.3 V

(see Fig. 11(a)). The change of transient capacitive voltage

during the period of first 5 min can be attributed to the

increased overpotential owing to an instant depletion of

reactants in both the electrodes [21]. In addition, the oxidative

removal of intermediate poison species, such as, CH2OH and

CO from the surface of the catalyst might result in subsequent

recovery of performance of the stack [22].

The pressure drops at both electrodes after applying a load

of 20 A were continuously monitored and are presented in

Fig. 11(b). The cathode and anode pressure drop values were


found to be around 22 and 13 kPa, respectively, which are

similar to the values that were obtained at 20 A load applied to

the stack (see Fig. 10(a)). As discussed in the preceding section,

the anode internal pressure is mainly affected by CO2

behavior; therefore, the anode pressure tends to fluctuate,

whereas the cathode pressure remains to be very stable.

Durability of the MEAs that were used in the 42-cells stack

was evaluated by carrying out a long-term operation on

a single cell. Although several factors can influence the

durability of MEAs [23–30], here we have investigated the

effect of air supply mode on the long-tem operational char-

acteristics and the results are presented in Fig. 12. We have

adopted two different operational modes for the long-term

testing; (a) 20 A loads, continuous air flow, 10 h/day operation

and (b) 20 A loads, ON/OFF air flow (3 min ON followed by 2 s

OFF), continuous operation for 500 h. The continuous opera-

tion for 10 h without any interruption in the supply of air

exhibited a slight degradation in the cell voltage as shown in

Fig. 12(a). After shutting down the cell for 14 h everyday (the

shutdown time is not shown in the figure), the performance

could be restored to its original state. On the other hand, the

repeated air interruption (3 min ON and 2 s OFF) as shown in

Fig. 12(b) seems to be arresting any voltage degradation during

the continuous operation and only a slight degradation in the

voltage of the cell has been observed over the continuous

operation for 500 h. The degradation rate in cell voltage has

been calculated from Fig. 12(b) and is found to be about 1.9 mV/

h. Interestingly, the degradation rate of our cell appears to be

much lower than the reported values in the literature (10–

25 mV/h) [23].

4. Conclusions

In this study, we developed a 400 W-class DMFC stack

comprising of 42 unit cells with an active area of 138 cm2 per

cell, and investigated the operational characteristics in

a detailed manner. The stack was able to generate a power of

400 W (69 mW/cm2 at 0.46 V/unit cell) at 20 A, and it attained

a temperature of 78 �C. A single cell made of a similar MEA

employed in the stack has provided a maximum power

density of 100 mW/cm2 at 0.4 V. In this investigation, it has

been found that the stack temperature was primarily influ-

enced by the concentration of methanol and, in fact, 0.8 M

methanol was an optimal concentration on the basis of

considering the performance and thermal stability of the

stack. The pressure drops of the stack were mainly influenced

by the CO2 generated in the electrodes during the courses of

operation. The pressure drops both at the anode and the

cathode were found to be 12 and 20 kPa, respectively, under

conditions of applying a load of 20 A and the reactant stoi-

chiometry at 3.6. The CO2 generation also lowered the pHs of

the effluent solutions both in the anode and in the cathode by

the way of forming carbonic acid, which accelerated the

corrosion of the metal bus plates used in the stack. A long-

term operation of a single cell with air interruption control

(3 min air-ON followed by 2 s air-OFF) exhibited a stable

performance for more than 500 h with a voltage decay rate of

1.9 mV/h.

Acknowledgement

The authors acknowledge the financial support for this work

from Korea Institute of Science and Technology, under the

contract number of KIST-2E20184.

AppendixSupplementary material

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.ijhydene.2008.08.016

r e f e r e n c e s

[1] Agnolucci P. Prospects of fuel cell auxiliary power units inthe civil markets. Int J Hydrogen Energy 2007;32:4306.

[2] Qian W, Wilkinson DP, Shen J, Wang H, Zhang J. Architecturefor portable direct liquid fuel cells. J Power Sources 2006;154:202–13.

[3] Kim YJ, Bae BC, Scibioh MA, Cho EA, Ha HY. Behavioralpattern of a monopolar passive direct methanol fuel cellstack. J Power Sources 2006;157:253–9.

[4] Guo Z, Faghri A. Development of planar air breathing directmethanol fuel cell stacks. J Power Sources 2006;160:1183–94.

[5] Chen CY, Tsao CS. Characterization of electrode structuresand the related performance of direct methanol fuel cells. IntJ Hydrogen Energy 2006;31:391–8.

[6] Ghosh PC, Wuster T, Dohle H, Kimiaie N, Mergel J, Stolten D.In situ approach for current distribution measurement infuel cells. J Power Sources 2006;154:184–91.

[7] Jiang R, Chu D. Stack design and performance of polymerelectrolyte membrane fuel cells. J Power Sources 2001;93:25–31.

[8] Fletcher NJ, Lamont GJ, Basura V, Voss HH, Wilkinson DP. USpatent, No 5470671.

[9] Jiang R, Rong R, Chu D. Determination of energy efficiency fora direct methanol fuel cell stack by a fuel circulation method.J Power Sources 2004;126:119–24.

[10] Kim D, Lee J, Lim TH, Oh IH, Ha HY. Operationalcharacteristics of a 50 W DMFC stack. J Power Sources 2006;155:203–12.

[11] Chen CY, Shiu JY, Lee YS. Development of a small DMFCbipolar plate stack for portable applications. J Power Sources2006;159:1042–7.

[12] Dohle H, Schmitz H, Bewer T, Mergel J, Stolten D.Development of a compact 500 W class direct methanol fuelcell stack. J Power Sources 2002;106:313–22.

[13] Shudo T, Suzuki K. Performance improvement in directmethanol fuel cells using a highly porous corrosion-resistingstainless steel flow field. Int J Hydrogen Energy 2008;33:2850–6.

[14] Lee BD, Jung DH, Ko YH. Analysis of DMFC/battery hybridpower system for portable applications. J Power Sources2004;131:207–12.

[15] Ge J, Liu H. Experimental studies of a direct methanol fuelcell. J Power Sources 2005;142:56–69.

[16] McCabe WL, Smith JC, Harriott P. Unit operation of chemicalengineering. 6th ed. New York: McGraw-Hill; 2001 [chapter 17].

[17] Ye Q, Zhao TS, Yang H, Prabhuram J. Electrochemicalreactions in a DMFC under open-circuit conditions.Electrochem Solid State Lett 2005;8:A52–4.

[18] Wong CW, Zhao TS, Ye Q, Liu JG. Transient capillary blockingin the flow field of a micro-DMFC and its effect on cellperformance. J Electrochem Soc 2005;152:A1600–5.

http://dx.doi.org/doi:10.1016/j.ijhydene.2008.08.016


[19] Yang H, Zhao TS, Ye Q. Pressure drop behavior in the anodeflow field of liquid feed direct methanol fuel cells. J PowerSources 2005;142:117–24.

[20] Argyropoulos P, Scott K, Taama WM. Pressure dropmodeling for liquid feed direct methanol fuel cells Part II.Model based parametric analysis. Chem Eng J 1999;73:229–45.

[21] Yoo JH, Choi HG, Nam JD, Lee YK, Chung CH, Lee ES, et al.Dynamic behavior of 5-W direct methanol fuel cell stack. JPower Sources 2006;158:13–7.

[22] Kallo J, Kamara J, Lehnert W, Helmolt R. Cell voltagetransients of a gas-fed direct methanol fuel cell. J PowerSources 2004;127:181.

[23] Knights SD, Colbow KM, St-Pierre J, Wilkinson DP. Agingmechanisms and lifetime of PEFC and DMFC. J Power Sources2004;127:127.

[24] Eickes C, Piela P, Davey J, Zelenay P. Recoverable cathodeperformance loss in direct methanol fuel cells. J ElectrochemSoc 2006;153:A171–8.

[25] Uribe FA, Zawodzinski TA. A study of polymer electrolytefuel cell performance at high voltages. Dependence oncathode catalyst layer composition and on voltageconditioning. Electrochim Acta 2002;47:3799–806.

[26] Neergat M, Seiler T, Savinova ER, Stimming U. Improvementof the performance of a direct methanol fuel cell usinga pulse technique. J Electrochem Soc 2006;153:A997–1003.

[27] Taniguchi A, Akita T, Yasuda K, Miyazaki Y. Analysis ofdegradation in PEMFC caused by cell reversal during airstarvation. Int J Hydrogen Energy 2008;33:2323–9.

[28] Jianguo Liu ZZ, Zhao X, Xin Q, Sun GQ, Yi B. Studies onperformance degradation of a direct methanol fuel cell(DMFC) in life test. Phys Chem Chem Phys 2004;6:134–7.

[29] Colliera A, Wang H, Yuan XZ, Zhang J, Wilkinson DP.Degradation of polymer electrolyte membranes. Int JHydrogen Energy 2006;31:1838–54.

[30] Wang F, Tang T, Pan M, Li D. Ex situ investigation of theproton exchange membrane chemical decomposition. Int JHydrogen Energy 2008;33:2283–8.

Development and characteristics of a 400 W-class direct methanol fuel cell stack

Documents

Transcript of Development and characteristics of a 400 W-class direct methanol fuel cell stack