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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 9

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Operational characteristics of a passive air-breathing directmethanol fuel cell under various structural conditions

Wei Yuan a,b,*, Yong Tang a, Zhenping Wan a, Minqiang Pan a

aKey Laboratory of Surface Functional Structure Manufacturing of Guangdong Higher Education Institutes,

South China University of Technology, Guangzhou 510640, People’s Republic of Chinab School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia

a r t i c l e i n f o

Article history:

Received 10 September 2010

Received in revised form

29 October 2010

Accepted 16 November 2010

Available online 15 December 2010

Keywords:

Passive

Air-breathing

Direct methanol fuel cell

Operational characteristic

Structural adaptation

Overshoot/undershoot

* Corresponding author. Key Laboratory of SSouth China University of Technology, Guan

E-mail address: yuanweijob@yahoo.com.0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.11.067

a b s t r a c t

The operational characteristics of a small-scale passive air-breathing direct methanol fuel

cell (PAB-DMFC) are comprehensively investigated under both steady-state and dynamic

conditions. As the most important operating parameter, methanol concentration has

significant effects on the cell performance. For different methanol concentrations (e.g., 0.5

and 8 M), the structural adaptations are particularly discussed. The results show that the

structural factors are closely related to the influence degree of methanol concentration. In

this study, the characteristics of the open circuit voltage (OCV) under various structural

and methanol-concentration conditions are presented. Besides, the effects of other oper-

ating conditions such as running time, forced air convection and refueling action on the

cell performance are also evaluated. In addition, a series of dynamic operations of the PAB-

DMFC are conducted under different load cycles. Accordingly, the transient phenomena

such as voltage undershoot and overshoot are explored. A fundamental principle for

evaluating the operational characteristics of a PAB-DMFC is to simultaneously take into

account the mass transfer requirements such as reactant delivery, product removal,

methanol/water crossover control and so on.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction with an anode absorbing methanol from the built-in reser-

Direct methanol fuel cells (DMFCs) have attracted extensive

attentions and efforts of many academic institutions and

commercial enterprises all over the world. A DMFC is able to

directly convert the electrochemical energy into electricity by

using aqueous or vaporized methanol as the fuel at the

anode and air or pure oxygen as the oxidant at the cathode.

According to the reactant delivery modes, the DMFC can be

categorized into two types including the active and passive

ones. Typically, the latter has somewhat open characteristics

urface Functional Structugzhou 510640, People’s Rcn (W. Yuan).ssor T. Nejat Veziroglu. P

voir and a cathode breathing from the ambient atmosphere.

This special type is termed as a passive air-breathing direct

methanol fuel cell (PAB-DMFC). Such a fully passive config-

uration significantly facilitates system simplification and

operation flexibility since it no more depends on the auxiliary

devices such as heat exchangers, humidifiers, fuel pumps,

gas blowers/compressors, etc, thereby eliminating the para-

sitic energy losses and enhancing overall system efficiency.

As a result, the construction cost of the fuel cell system can

be also reduced to a great extent. Thus, it is regarded as one

re Manufacturing of Guangdong Higher Education Institutes,epublic of China. Tel./fax: þ86 2087114634.

ublished by Elsevier Ltd. All rights reserved.

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 92238

of the most promising candidates to provide sustainable

power output for portable applications due to its high energy

density, compact architecture, easy refueling and high cost

efficiency [1e7]. Unfortunately, the PAB-DMFC has to suffer

a relatively lower performance than the active DMFC because

it mainly operates under spontaneous conditions that may

substantially inhibit the cell performance. Thus, it seems

more competent wherever the power density is not consid-

ered the top criteria, e.g., the low-power consumer elec-

tronics market [3e7]. However, from the open literature

resources in the past two decades, it can be seen that most of

the R&D activities have been dedicated to active DMFCs,

while the published information concerning the PAB-DMFC

still appears insufficient. Although some groups have put

their PAB-DMFC prototypes into laboratory-scale or even

business demonstrations [8e11], there is still plenty of scope

to comprehensively explore the intractable challenges before

realizing full-scale commercialization of the PAB-DMFC.

In recent years, the mass transfer processes such as

reactant delivery and permeation, and product removal

under the influence of passive forces, have been increasingly

reported. These issues are all closely related to methanol,

water and gas (e.g., CO2 bubbles and air) management

[2,12e22]. As is well known, methanol crossover inevitably

happens and depresses the cell performance by forming a so-

called mixed potential due to oxidization of the permeated

methanol at the cathode [23]. In order to diminish this effect,

various vapor-feed techniques are developed for the PAB-

DMFC because they are more able to suppress methanol

crossover than the liquid-feed systems [8,16,24e27]. Some

other groups also tried to reduce methanol crossover by

using highly-resistant but permeable materials as methanol

barriers [14,28e31] or mixing viscid additives [32,33] into the

methanol reservoir, thereby making the methanol delivery

more sluggish before it permeates through the membrane.

Water crossover is also proved to be a critical issue exerting

great influence on methanol permeation and hydration

condition in the membrane electrode assembly (MEA). The

degree of water crossover flux can be characterized by water

transport coefficient (WTC). In general, a lower WTC is

favorable because the water back diffusion can significantly

mitigate methanol waste and cathode flooding

[13,18,20,34,35]. This explains why a thinner membrane (e.g.,

Nafion 112 and 1135) is also able to yield an acceptable cell

performance [20,34,35], although it tends to promote meth-

anol crossover. Product removal is another concerned issue

that deserves a serious consideration, especially for the PAB-

DMFC in which the residual water at the cathode and the CO2

bubbles at the anode is expelled simply by passive forces.

Since the PAB-DMFC mainly depends on passive mass

transport through diffusion and natural convection driven by

a concentration gradient or pressure difference, it is essential

to establish effective mechanisms controlling the fuel

feeding and product removal in an inactive mode. To this

end, a few groups developed their PAB-DMFCs based on self-

regulated techniques involving the micro valve [36], capillary

pump [37], surface tension [38] and bubble-induced motion

[39], etc. In addition to the above methods, effective

management of the mass transfer process can be also ach-

ieved by optimizing the structures and materials of the fuel

cell components, especially the backing layer (BL) and micro

porous layer (MPL) in the MEA. For instance, a popular way to

enhance the water back diffusion from the cathode to the

anode is to utilize a hydrophobic porous layer as a hydraulic

pressure regulator inside the cathode so as to passively

promote hydration compensation from the cathode to the

anode [2,4,12,18,20,24e35].

Methanol concentration is one of the most focused

operating parameters influencing the cell performance of

PAB-DMFCs [1,2,13e18,22,26,28,39e43]. Generally, a higher

methanol concentration tends to result in increased meth-

anol crossover, thereby reducing the cell performance. Thus,

most of the published work just targets low-concentration

(�4 M) operation in order to prevent excessive methanol

crossover. However, it is also worth noting that in a PAB-

DMFC, the methanol is only delivered to the anode catalyst

layer in a passive diffusion mode. Under this condition, if the

methanol is not supplied adequately and timely, severe

polarization of the cell voltage may occur due to methanol

starvation. This means that the slow methanol transport is

also a restricting factor related to the cell performance [2,22].

Therefore, there should be an optimum level of the meth-

anol concentration which not only helps feed enough reac-

tant but also keeps the methanol crossover to a reasonable

degree [13,41]. Nonetheless, the output of the PAB-DMFC at

lower methanol concentration is sometimes not quite

acceptable for the engineering practice. Besides, the oper-

ating time of the fuel cell is greatly reduced due to contin-

uous consumption of the limited methanol solution at

a lower concentration. In order to solve these problems, a lot

of contributions aimed at operating the PAB-DMFC with

highly-concentrated methanol solution or even neat meth-

anol [24e33,42e44]. As aforementioned, the researchers

attempted to validate the feasibilities of setting up methanol

transport barriers located at the feeding line or transforming

the liquid methanol into vapor phase to realize high-

concentration operation along with higher fuel efficiency

[10,11,14,15,24e33,42e44]. Some researchers attributed the

advantage of high-concentration operation to the increased

internal temperature that helps improve the cell perfor-

mance [13,39,40]. However, it should be also noticed that the

enhanced methanol crossover due to a higher methanol

concentration and an elevated temperature may possibly

overwhelm or offset this effect [41].

The above introduction briefly reviews the development

and challenges of the PAB-DMFCs. It can be seen that the

reactant and product management is still the most important

issue that ought to bewell addressed. However, it is also found

that the operational characteristics of the PAB-DMFC are

scarcely reported, probably because of its limited and rigorous

operating conditions. In view of this situation, it is essential to

gain a deeper understanding of the operational characteristics

of a PAB-DMFC, especially when the structural effects are

taken into account [41]. Therefore, this study throws light

upon the operating details of a small-scale PAB-DMFC with

different structural combinations. Both steady-state and

dynamic operations are included during the experiment.

Particularly, the cell performances under various operating

conditions, e.g., open circuit, forced air convection, fuel

refilling and dynamic load, are elaborated.

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2. Experimental

2.1. Preparation of the membrane assembly electrode(MEA)

Fig. 1 shows the photo of the in-situ tested PAB-DMFC. The

MEAs, with an active area of 3 cm � 3 cm, were fabricated by

means of catalyst coated membrane (CCM) method. In this

study, three types of Nafion� membranes (DuPont, Inc.) with

different thicknesses (i.e., Nafion 212, 115, and 117) were

tested. Before each membrane was ready for electrode

assembly, it was pretreated following the processes below: At

the beginning, the membrane was boiled in 3 wt.% H2O2

aqueous solution for 1 h and then in de-ionized (DI) water for

1 h. Then, it was boiled in 0.5MH2SO4 aqueous solution for 1 h.

Finally, it was purified again in boiling DI water to remove the

residual sulfuric acid. The catalyst slurry for the electrodes

was prepared by mixing the catalyst powders, Nafion� ion-

omer solution, and isopropyl alcohol together, and uniformly

coated on both sides of the membrane by using a spraying

machine. The catalyst (Johnson Matthey, Inc.) loadings were

prescribed with 4 mg cm�2 Pt-Ru (1:1, a/o) at the anode and

2 mg cm�2 Pt at the cathode, respectively. Fig. 2 shows the

morphologic images of the catalyst layers characterized by the

scanning electron microscopy (SEM) method. After attaching

the CLs, the CCMs were dried in a vacuum oven at around

80 �C for 2 h. The backing layer (BL) was made of the

commercial carbon paper TGP-H-060 (Toray, Inc.), while the

micro porous layer (MPL) was made of a mixture consisting of

Vulcan XC72 carbon blacks (E-TEK, Inc.) and 20 wt.% poly-

tetrafluorethylene (PTFE). Subsequently, the MPL was bonded

to the surface of the wet-proofed BL to compose a complete

diffusion layer (DL). In the present work, the DLs were

assembled in two different patterns: the non-boned DL (NBDL)

and hot-pressed DL (HPDL). The former is simply sandwiched

between the current collector (CC) and the CCM under the

assembly force. The latter is hot-pressed together with the

CCM under a pressure of 50 kg cm�2 at 140 �C for 3 min.

Fig. 1 e A photo of the tested passive air-breathing DMFC

with a visualized fixture.

Fig. 2 e SEM images of the catalyst layers: (a) the anode

side; (b) the cathode side; (c) an amplified view of the anode

side.

2.2. Single cell fixture and testing setup

For the convenience of visualization, the transparent material

named polymethyl methacrylate (PMMA) was used to fabri-

cate the anode compartment and cathode window frame by

using milling and polishing techniques. A built-in reservoir

with a volume of 10.8mLwasmachined to store themethanol

solution. Two through holes were drilled at the top of the

anode compartment for fuel injection and gas exhaust. Both

the anode and cathode CCs were made of 1 mm 316L stainless

steel plate with perforated hole-arrays distributed within the

active area. In this study, two types of CC geometries were

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designed: the first type has 49 (7 � 7) holes (d ¼ 3 mm) with

a higher open ratio of 38.5% (HCC); and the second type has

144 (12 � 12) holes (d ¼ 1.5 mm) with a lower open ratio of

28.3% (LCC). A silicon-rubber strip was used to seal the gaps

between CC and the end plate, while an in-house PTFE film

served as the gasket between CC and the edged membrane of

the MEA. A commercial electronic load was used to test the

fuel cell discharge performance, and a LabVIEW-based data

acquisition (DAQ) programwas used to record the current and

voltage outputs. Both the galvanodynamic and galvanostatic

methods were employed to characterize the cell performance.

Before each test, the MEA was activated under a constant load

for 12 h. In order to verify the effect of forced air supply, an

adjustable fan with a nominal voltage of 12 V was fixed

neighboring the cathode window frame. Over the experiment,

the fuel cells were tested at an ambient pressure and a room

temperature of approximately 28 �C. The relative humidity

was conditioned at around 85%.

3. Results and discussion

The operational characteristics of a PAB-DMFC are greatly

determined by its passive operating mode. Under this condi-

tion, most of the operating parameters cannot be controlled

Fig. 3 e Structural adaptations to 0.5 M methanol concentration

150 mA for 5 min, and Nafion 117 (c) IP and IV curves; (d) const

manually. Thus, the value of methanol concentration is

thought to be the most important operating parameter that

can be prescribed in advance. On the other hand, the cell

structures can be also controllably designed. Therefore, the

operational details related to these two factors are discussed

in the first two parts of this section. In the following subsec-

tions, other operating conditions such as the running time, air

blowing (i.e., semi-passive) and intermittent refueling are

included. Moreover, the effects of structures and methanol

concentrations on the values of open circuit voltages (OCVs)

are also explored. The last part describes the dynamic

behaviors of the PAB-DMFC under different operating cycles.

3.1. Structural adaptations to a lower/higher methanolconcentration

The structural adaptations can be more obviously distin-

guished at either a lower (0.5 M) or a higher (8 M) methanol

concentration. Fig. 3 compares the polarization curves of the

PAB-DMFC with different structural combinations at a meth-

anol concentration of 0.5 M. Meanwhile, the constant-load

performances at 150 mA were also presented to inspect the

effect of structures on the steady-state behaviors. When such

a lower methanol concentration is used, the methanol supply

is very limited especially when it is passively delivered. In this

when Nafion 212 (a) IP and IV curves; (b) constant load at

ant load at 150 mA for 5 min, are respectively used.

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 9 2241

case, the structural combinations should bemore profitable to

transport sufficient methanol to the reaction areas. Simulta-

neously, the issues related to methanol crossover should be

also considered. Specifically, when Nafion 212 is used (see

Fig. 3(a) and (b)), methanol crossover tends to excessively

happen due to its lower thickness, as is well known

[5,12,20,45]. It is also worth noting that a thinner membrane

benefits enhancement of water back diffusion that may

conversely depress the methanol crossover [12,20,34,35].

Under this condition, the HCC combined with HPDL yields the

highest performance. This is because a higher open ratio of

the current collector provides more effective paths for meth-

anol feeding, while the hot-pressed DL serves as a barrier to

resist methanol permeation due to its tight and compact

structure [41,46,47]. Therefore, the combination of HCC and

HPDL cooperates to supply enough reactant and concurrently

keeps the methanol crossover in a reasonable degree.

Consequently, they adapt the best to a lower methanol

concentration. This can be further confirmed by the results

shown in Fig. 3(b). It is evident that the combination of HCC

and HPDL yields the highest voltage output under a constant-

current condition. In addition, it is noticed that the perfor-

mance of HCC and NBDL is comparable with the other two

combinations, which implies again that the methanol cross-

over is actually not quite serious.

Fig. 4 e Structural adaptations to 8 M methanol concentration w

150 mA for 5 min, and Nafion 117 (c) IP and IV curves; (d) const

On the contrary (see Fig. 3(c)), Nafion 117 has a higherWTC

so that methanol crossover is still a key issue that should be

well addressed, although it owns a higher thickness [20,34,35].

Under this condition, considering the inevitable methanol

crossover and lower methanol concentration (0.5 M), the LCC

and NBDL together produce a higher cell performance than

others. This can be explained by the following reasons. On the

one hand, a lower open ratio of the current collector helps

limit the methanol transport to a certain extent. On the other

hand, a non-bonded DL helps keep a reasonable methanol

permeation rate due to its looser structure but longer feeding

paths [47]. As a result, a balance between methanol delivery

and methanol crossover control can be successfully achieved

by using this structural combination. Fig. 3(d) also proves that

the LCC and NBDL perform the best at a constant current. It

can be seen that the performance of HCC and NBDL gradually

decreases as the time goes on, while others are able to retain

a relatively steady-state. This phenomenon shows that the

methanol crossover may excessively happen if no effective

measures were taken to hold back the methanol permeation.

This is quite different from Fig. 3(a) and (b) due to the change

of the internal mass transfer environment. Therefore, the

structural adaptation accordingly varies in a renewed way.

Fig. 4 shows the performances of the PAB-DMFC with the

8 M methanol solution supplied. Apparently, such a higher

hen Nafion 212 (a) IP and IV curves; (b) constant load at

ant load at 150 mA for 5 min, are respectively used.

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 92242

methanol concentration promotes lower performances due to

enhanced methanol crossover that significantly drags down

the cell voltage. Meanwhile, it increases the electrochemical

reaction rate, thereby generating more gas bubbles at the

anode and produced water at the cathode within a short time.

If these products cannot be expelled in time, they may

increasingly accumulate so that the reactant feeding paths are

blocked. Thus, the issues related to the product removal also

have to be taken into account. For example,whenNafion 212 is

employed (see Fig. 4(a) and (b)), the combination of HCC and

HPDL produces the top performance. This result is similar to

that in Fig. 3, but the contributingmechanism differs from the

previous case. Here, the critical issues are methanol crossover

control and product removal, rather than reactant delivery.

Thus, at the anode, the HCCmostly contributes to gas removal

due to its smaller rib area, as shown in Fig. 5. It can be seen that

the CO2 bubbles progressively escape from the anode DL and

gather along theorifice edges.With the reactiongoingon,more

and more bubbles are produced. Then they gradually grow up

in position, and are finallymoved by buoyancy to the chamber

ceiling to form a large bubble cluster. Meanwhile, the HCC also

tends to accelerate methanol permeation, so the HPDLmainly

takespart inprovidingmethanol resistance to supplement this

effect. On the other hand, at the cathode, the HCCmanages to

provide more air diffusion passages to satisfy the oxygen

requirement especially when a large amount of methanol

arrives at the cathode. The HPDL helps to enhance the water

back diffusion to further alleviate the methanol crossover,

because its compact structure acts as a water barrier layer to

build up a hydraulic liquid pressure in the cathode [2]. There-

fore, these two structures combine to maximize the cell

performance when themethanol concentration is excessively

high. Likewise, they yield the highest power output when

Nafion 117 is used, as depicted in Fig. 4(c) and (d).

In summary, either a lower or a higher methanol concen-

tration brings forth rigorousmass transfer demands to the fuel

cell structures. The key issues such as reactant delivery,

product removal, andmethanol/water crossovermanagement

should be coordinately considered. The rules restricting the

mass balance can be quite different under different circum-

stances, thereby leading to different structural adaptations.

Generally, a better performance results from a compromise

Fig. 5 e Comparison between the anode bubble behaviors

among various mass transfer requirements. Therefore, an

overall evaluation of the structural adaptations is quite

necessary for optimization of the cell performance, although

the involvedmass transfer mechanisms are very complex and

sometimes even incompatible with each other.

3.2. Effects of the methanol concentrations undera constant load

Fig. 6 shows the constant-load performances of the PAB-DMFC

at different methanol concentrations. It can be seen that the

2 Mmethanol solution produces the highest power output, no

matter which structural combination is used. This suggests

that the general effect of the methanol concentration is

independent of the structural factors. But it does not mean

this effect has no interrelation with the structures. As is dis-

cussed in Section 3.1, the structural combinations influence

how much the methanol concentration affects the cell

performance. This explains why we focus on the structural

adaptations to the methanol concentration levels, typically at

0.5 and 8 M. In this study, 2 M is proved to be the optimal

methanol concentration that results in the highest perfor-

mance. When the methanol concentration is below this

optimal value, e.g., 0.5 M, the cell performance is mostly

limited for lack of methanol especially when the fuel cell

works in a passivemode.When themethanol concentration is

above 2 M, e.g., 4 M, the cell performance is mostly depressed

owing to the exaggerated methanol crossover. This explains

why the cell performances at 0.5 and 4 M methanol concen-

trations are both inferior to that at 2 M. Although the 0.5 and

4 M methanol solution seemingly produce comparable

performances, the dominant influence mechanisms are

totally different. Therefore, it can be concluded that the 2 M

methanol solution is favorable to not only ensure sufficient

methanol supply but also maintain the methanol crossover at

a reasonable degree.

3.3. Effects of the running time

Since themethanol solution in a PAB-DMFC is not fed through

an active fluid carrying line, it can be inferred that the amount

of the methanol may be increasingly reduced so that the

in different stages: (a) an earlier time; (b) a later time.

Fig. 6 e Effects of the methanol concentrations under

a constant load at 150 mA for 5 min: (a) HCC, NBDL and

Nafion 117; (b) LCC, HPDL and Nafion 117.

Fig. 7 e Effects of the running time when LCC, HPDL and

Nafion 115 are combined: (a) IV and IP curves; (b) under

a constant load at 300 mA for 30 min.

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methanol concentration may accordingly drops due to

continuous consumption at the reaction sites [48,49]. Thus, in

this study, we examined how the running time affects the cell

performance of the PAB-DMFC at 2 M methanol concentra-

tion, as shown in Fig. 7. For this test, Nafion 115 membrane is

typically used and assembled together with LCC and HPDL.

The performance curves and constant-load curves are recor-

ded with an interval of 30 min. It can be seen that, in the low-

current region (�15 mA cm�2), the cell performance after

30 min is higher than the others, while in the high-current

region (>15 mA cm�2), the cell performance tested after

60 min becomes the highest. Apparently, at a lower current

density, except for the consumed methanol at the anode, the

rest may permeate through the membrane and be oxidized at

the cathode. Thus, with the time going on, the cell voltage is

inevitably reduced due to increased methanol crossover. On

the other hand, at a higher current density, most of the

methanol participates in the anode reaction so that higher

methanol utilization efficiency can be obtained. Under this

condition, the methanol concentration is significantly

reduced, thereby decreasing the cell performance after

a relatively long time. In addition, the enhanced cell perfor-

mance may also benefit from the elevated cell temperature

caused by continuous exothermic reaction, which may offset

the effect of reduced methanol concentration to some extent

[39,40,49]. But an enhanced temperature tends to promote

more severe methanol crossover, which should be also

considered. In general, in the earlier stage, it takes more time

for the methanol to reach the anode CL. During this stage, the

key issue lies in a fact that more and more feeding paths are

built so as to more effectively transport the methanol to the

reaction area. Thus, the amount of the permeatedmethanol is

still not too large [49], so that the cell performance is under-

going a climbing period and ultimately reaches the peak value

in the middle stage, although the methanol crossover still

keeps increasing. This can be regarded as a pre-activation

stage. At a later time, methanol starvation on the anode side

may gradually turns into the main issue. Moreover, the

methanol increasingly accumulates at the cathode, which is

also a contributing factor depressing the cell performance. As

a result, the cell performance gradually falls due to both the

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 92244

consumption and waste of methanol. This can be termed as

a decay stage after the top performance is achieved. Here, it

should be mentioned that this time-dependent phenomenon

brings forth a question about the time point at which we can

treat the measured curve as a more accurate performance

curve. In our experiment, it is found that the cell performance

at about 60 min reaches the maximum. As shown in Fig. 7(b),

when the current is kept at 300 mA, the cell voltage after

60 min is about 10 and 20 mV higher than the results obtained

after 30 and 90 min, respectively. Although this difference is

quite slight or even negligible, the cell performance change

with the development of the running time can be clearly

identified.

3.4. Characteristics of the open circuit voltage

The open circuit voltage (OCV) of a PAB-DMFC is usually

a concerned technical target because its value more or less

reflects the methanol crossover degree. Fig. 8 illustrates the

OCV characteristics of the single cell with various structural

combinations when different methanol solutions are

supplied. It can be seen that the OCV of each combination

enters into a stable period after about 100 s. The combination

of LCC and NBDL yields the highest OCV at approximately

Fig. 8 e Effects of the structural parameters and methanol

concentrations on the values of open circuit voltages: (a)

during 2 min continuous testing; (b) at 0.5, 2, 4 and 8 M.

690 mV when Nafion 117 is used at both 0.5 and 2 M.

Correspondingly, the combinations of HCC and HPDL, and

LCC and NBDL yield the lowest OCVs at about 530 mV when

Nafion 212 is used at 2 M. Obviously, the structural factors

affect the OCV values in different ways because of their

different effect on the methanol permeation rate. In a general

sense, the LCC and HPDL are propitious towards enhance-

ment of the methanol transport resistance. Thus, these two

structures are able to act as methanol barriers, thereby

controlling the methanol crossover at a lower level. On the

contrary, the HCC and NBDL possibly aggravate the methanol

crossover because they both provide a higher permeability.

As another important structural parameter, the membrane

thickness also has a great effect on the methanol crossover,

as previously discussed. For example, Nafion 117 is able to

limit the methanol crossover due to its higher thickness,

while Nafion 212 also has its ability in strengthening water

back diffusion so as to indirectly reduce the methanol

crossover to some extent. But this does not mean the

combination of all the highly-resistant components is rec-

ommended because the demand of methanol delivery should

be also satisfied. Thus, the methanol transfer resistance

needs to be controlled within a reasonable range by adjusting

the structural arrangement. Additionally, the methanol

concentration also has a prominent effect on the values of

OCVs. As shown in Fig. 8(b), the OCV decreases with the

increase in methanol concentration, in spite of the structural

combinations. Especially, when the methanol concentration

is at 8 M, the OCV is reduced to 300 mV or even lower. This

demonstrates that the methanol concentration has a more

fatal effect on the value of OCV than the structural factors. In

other words, how a certain methanol concentration affects

the value of OCV is independent of the structural factors, but

its influence degree can be different when different struc-

tural combinations are applied. This conclusion is similar to

that made in Section 3.2.

3.5. Effects of the forced air-blowing (semi-passive)

In a PAB-DMFC, the cathode is directly exposed to the ambient

atmosphere. In view of the possibility that the fuel cell may be

placed and operated in an environment with strong air

convection, the effects of the forced air-feeding were investi-

gated in this work. Fig. 9 describes how an air-blowing fan

affects the cell performancewhen it is discharged at a constant

current of 150 mA. As can be seen, the best performance is

obtained when a 50% duty cycle is applied to the fan. Inter-

estingly, the lowest performance emergeswhen the fan is fully

powered (i.e., 100% duty cycle). This suggests that the cell

performance is very sensitive to the intensity of the forced air

supply. Theoretically speaking, the forced air-feeding mode

produces a faster air flow rate, thereby enhancing the oxygen

permeation and finally the cell performance, as imagined

previously. However, the result in this study shows that the

forced air supply does not definitely lead to a higher perfor-

mance of the PAB-DMFC. This is partially consistent with the

findings presented by Bae et al. [50] who found that the forced

air blowing could reduce the cell temperature so as to result in

a lower performance. In this study, it is shown that the active

air supply could improve the cell performance by enhancing

Fig. 10 e Effects of the forced air blowing with a 50% duty

cycle on the values of open circuit voltages when Nafion

115 is used: (a) LCC and HPDL at 0.5 M; (b) LCC and NBDL at

2 M; (c) HCC and NBDL at 8 M (Note: Iecontinuous blowing;

IIetransient blowing).

Fig. 9 e Effects of the forced air blowing under a constant

load at 150 mA for 5 min, when LCC, HPDL and Nafion 115

are together used at 2 M.

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 9 2245

the oxygen reduction reaction (ORR) with an appropriate air

stoichiometry. Meanwhile, since the PAB-DMFC operates

under a passive self-heating mode, more heat can be soon

dissipated if the air flow rate is beyond a certain value. So, it

can be inferred that the full-speed air supply may excessively

cool down the fuel cell, whichmay negatively degrade the cell

performance due to a reduced cell temperature. Therefore,

a conclusion can be drawn that, in a PAB-DMFC, a balance

between the effects of increased air flow rate and the reduced

cell temperature is necessarily required under the active air

supply mode (i.e., semi-passive mode).

In order to verify how the OCV is affected by the forced air

supply, we carried out two different air-blowing actions in

this study: one is continuous, denoted by “I”; the other is

transient, denoted by “II”. Fig. 10 illustrates the OCV varia-

tions under the influence of the 50% forced air supply when

different structural combinations are used at different

methanol concentration levels. Here, it is worth mentioning

that all these data was measured during the downward

period in which the OCV suffers from the increasing meth-

anol crossover once the fuel is filled into the reservoir. When

the fan is suddenly loaded on the fuel cell, the OCV experi-

ences a transient jump. After this, if the fan continues

blowing, the OCV increases very slowly with a slight magni-

tude of several millivolts. When the fan is unloaded, the OCV

suddenly drops to a lower level. It is evident that the forced

air supply mode helps stop the OCV from decreasing with

a limited recovery of the cell voltage. This is because the

enhanced air flow increased the local oxygen concentration

at the cathode reaction sites, indirectly alleviating the effect

of mixed potentials caused by the permeated methanol [49].

Moreover, the forced air is able to effectively carry away the

produced heat caused by the methanol oxidation reaction

(MOR) at the cathode, leading to a lower cell temperature that

may inversely inhibit the methanol crossover to some extent.

As a sequence, the above two effects combine to generate

a higher value of OCV.

3.6. Effects of the refueling action

Different from the active DMFC with a motional feeding line,

the PAB-DMFC needs to be refueled if the methanol is

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 92246

exhausted. Such a static operating mode opens up a question

that whether the cell performance is able to inherit the same

performance as before after the refueling action. In order to

make clear this problem, this study compared the perfor-

mance curves before and after the refueling action. The

refueling action happened at approximately 2 h after the fuel

cell went into operation. Furthermore, an additional polari-

zation test was also performed 10 min later after refueling to

inspect the subsequent development trend of the cell

performance. The LCC and HPDL were typically selected to

provide a higher methanol transfer resistance before it rea-

ches the anode CL. Fig. 11 shows the effects of the refueling

action on the performance of the PAB-DMFC. It can be seen

that the cell performance suffers severe degradation after the

2 M fresh solution is filled. For Nafion 115, the maximum

power density (MPD) decreases from 2.968 to 2.492 mW cm�2.

For Nafion 117, the MPD decreases from 5.236 to

4.048 mW cm�2. However, after a short time, the fuel cells

with Nafion 115 and 117 regain higher MPDs of 3.216 and

5.97 mW cm�2, respectively. This result reveals that the

performance decline is just a temporary phenomenon. The

reason may be ascribed to a fact that a temporary mass

Fig. 11 e Effects of the refueling action at 120 min on the

cell performances when LCC and HPDL are used at 2 M: (a)

Nafion 115; (b) Nafion 117.

transfer break-off occurs after the refueling action because

the methanol is simply transported to the anode CL in

a passive way. Thus, shortly after the refueling, the cell

performance goes down due to the polarization caused by

methanol insufficiency. But this effect may disappear soon

after the methanol feeding paths are fully established. This

explains why the performance recovery subsequently

happens. Even then, a new issue comes out that the meth-

anol concentration of the fresh solution is relatively higher

than that before refueling. Under this condition, more

methanols permeate through the membrane from the anode

to the cathode in a short time. During this period, more

oxygen directly reacts with the permeated methanol so as to

partially offset the positive effect of the sufficient methanol

supply [49]. For all this, the overall cell performance still gets

improved as a result of a compromise between the above two

mechanisms.

3.7. Dynamic behaviors and transient responses

Fig. 12 shows the dynamic characteristics of the target PAB-

DMFC when different structural combinations are used at

a methanol concentration of 2 M. It can be seen in Fig. 12(a)

that the cell voltage responds rapidly and instantly to the

current step-up change from 150 to 750 mA with an incre-

ment of 150 mA. For this case, the structural combination of

LCC, NBDL and Nafion 117 was tested. As is observed, the

voltage undershoot happens when the current changes from

a lower level to a next-step higher level in the low and

medium-current regions. This is because an abrupt current

jump leads to a temporary incident of reactant starvation at

the catalytic reaction sites. During this transience, the

methanol at the anode and the oxygen at the cathode are

both consumed sufficiently fast. Under this condition, the

reactant supply in a passive mass transfer mode cannot

immediately satisfy the reaction requirement so as to trigger

server voltage polarization. As a result, the transient load

change undershoots the cell voltage. Afterward, the voltage

gradually gets rejuvenated because of the subsequent

enhancement of the reactant delivery driven by a higher

concentration gradient, and finally reaches a stable value.

However, the undershoot behavior is not clearly captured

when the current increases in the high-current regions. This

is because the effect of the charge double-layer (CDL)

capacitance gets enhanced particularly when the fuel cell

operates at a higher current level. When the load current

changes, the charges in this conceptual capacitor accumulate

or dissipate in a short time and finally smoothen the

magnitude of the voltage change [51,52]. On the other hand,

a sudden drop of the current tends to result in voltage

overshoot, as can be seen in Fig. 12(a). The reason for this

behavior can be attributed to a fact that, when the current

changes from high to low, the methanol crossover increases

with a decrease in anodic methanol consumption, so that

the voltage is slowly dragged down by progressively in-

creased mixed potentials after the transient overshoot.

Fig. 12(b) and (c) also affirmed the transient phenomena of

the voltage undershoot and overshoot, when other different

structural types are tested under step-up and repeated load

changes, respectively. Besides, by vertical comparison, it can

Fig. 12 e Dynamic performances of the PAB-DMFC at 2 M

methanol concentration: (a) LCC, NBDL and Nafion 117; (b)

HCC, HPDL and Nafion 117; (c) LCC, HPDL and Nafion 212.

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 9 2247

be further noted that the voltage overshoot in Fig. 12(a) is

steeper than in Fig. 12(b) and (c) when the current decreases

from different initial value to 150 mA. This result proves that

the magnitude of the voltage overshoot depends on the

change magnitude and initial level of the current, which is

consistent with that reported by Yang and Zhao [51].

4. Conclusions

In the present work, the operational characteristics of

a typical passive air-breathing DMFC are explored in details by

carrying out a series of steady-state and dynamic experi-

ments. Based on the results and discussion, the conclusions

can be made point by point as follows:

i) The structural adaptations to different methanol

concentrations should be seriously considered, espe-

cially when a lower or a higher methanol concentration

is prescribed. The methanol concentration significantly

affects the cell performance, regardless of the structural

factors, but its influence degree can be quite different

when different structural combinations are used.

Particularly, the OCV decreases with the increase in

methanol concentration. However, this effect is also

closely related to structures and functions of the key

components.

ii) The cell performance also varies with the change of

running time. It is usually restricted by the sluggish

methanol delivery during the earlier stage, while by the

decreased methanol concentration during the later

stage. Thus, an optimum output is obtained during the

middle stage. The time point at which the top perfor-

mance is achieved needs to be captured when evalu-

ating the development of the fuel cell performance.

iii) The effect of forced air supply mainly lies in how to

maintain a balance between mass transfer enhance-

ment and cell temperature dissipation. Thus, a medium

air flow rate performs the best. Moreover, the air-

blowingmode enhances the voltage when the fuel cell is

in an open-circuit state. In addition, the refueling action

tends to temporarily drag down the cell performance.

But this is proved to be a transitory phenomenon

because a recovery of the cell performance occurs

shortly after refilling the methanol.

iv) The dynamic results show that the voltage undershoot

appears simply in the low/medium-current regions

when the load increases, because the CDL effect gets

enhanced in the high-current regions so as to smoothen

the voltage drop. On the other hand, the voltage over-

shoot also takes place when the current goes down. The

overshoot degree mainly depends on the change

magnitude and initial value of the current.

Acknowledgements

This research is supported by the key program of NSFC-

Guangdong Joint Funds of China (No. U0834002) and the key

program of Natural Science Foundation of Guangdong Prov-

ince (No. 07118064). The programs (No. 50930005 and

51075155) supported by the National Nature Science Founda-

tion of China are also acknowledged. The authors also would

like to express their gratitude to the Joint-training Program

(No. 2009615064) sponsored by China Scholarship Council and

the Doctorate Dissertation Innovation Funds (No. 201000008)

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 6 ( 2 0 1 1 ) 2 2 3 7e2 2 4 92248

supported by South China University of Technology. The

constructive suggestions by professor Jun Wang in The

University of New SouthWales are also gratefully appreciated.

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