Operation of a direct methanol fuel cell stack by self-heating at low temperatures

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 6 ( 2 0 1 1 ) 5 6 5 5 e5 6 6 5

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Operation of a direct methanol fuel cell stack by self-heating at low temperatures Young-Chul Park a,b, Dong-Hyun Peck a, Sang-Kyung Kim a, Seongyop Lim a,c,*, Doo-Hwan Jung a,c, Dok-Yol Lee b a

Korea Institute of Energy Research (KIER), 71-2 Jangdong, Yuseong, 305-343 Daejeon, Republic of Korea Department of Materials and Science Engineering, Korea University, 136-705 Seoul, Republic of Korea c Advanced Energy Technology, University of Science and Technology (UST), Daejeon, Republic of Korea b

article info

abstract

Article history:

The operation characteristics of a direct methanol fuel cell (DMFC) are investigated at low

Received 6 December 2010

temperatures of
5  C and
10  C by using a laboratory-made 10-cell stack. The stack is

Received in revised form

operated only by heat generation of internal exothermic reactions without any heating device

31 January 2011

and additional insulation means, to examine behaviors of the stack performance at low

Accepted 3 February 2011

temperatures. The self-heating stack is successfully operated in a stable manner at
10  C by

Available online 5 March 2011

control of the operation modes. An appropriate operation strategy using the fuel switching as well as selection of the operation modes is proposed, and possibility and limitation for

Keywords:

operation of DMFC stacks by self-heating under cold conditions are discussed, based on the

Direct methanol fuel cell (DMFC)

results.

Stack

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Low temperature

reserved.

Operation mode Self-heating

1.

Introduction

The direct methanol fuel cell (DMFC) has attracted a considerable amount of attention as a promising power source for portable devices because it does not require any fuel processing equipment [1,2]. In recent years, many companies and research groups have developed key components of the DMFC. Smart Fuel Cells in Germany has obtained quite successful accomplishment in commercialization of DMFC systems [3,4], and many global electronics companies such as Samsung, LG, Hitachi, Sony, MTI, etc. have announced prototypes and trial products of DMFC systems for devices such as laptop computers and cellular phones [5e10]. In Oct. 2009, Toshiba eventually started to distribute the portable

DMFC charger, Dynario in the market, and commercialization of DMFC is expected to be accelerated from now on [11]. DMFC systems in practical applications will be exposed to a wide range of temperatures. DMFCs must be able to operate under conditions that are found in low temperatures [12], such as freezing of water, slow startup due to low performance and heat loss etc. There are, however, some critical problems in operation of the system at low temperatures, basically caused by water existing in the system components such as membrane, catalyst layers, gas diffusion layers, channels of bipolar plates. Many studies have been done for operation of hydrogen fueled PEMFCs at low temperatures [13e21], whereas there are few reports on behaviors of a DMFC stack under such circumstances. A DMFC stack appear to have

* Corresponding author. Korea Institute of Energy Research (KIER), Fuel Cell Research Center, 71-2 Jangdong, Yuseong, 305-343 Daejeon, Republic of Korea. Tel.: þ82 42 860 3073; fax: þ82 42 860 3739. E-mail address: [email protected] (S. Lim). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.020

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more complicated phenomena during low-temperature operation compared to hydrogen fueled ones, since the fuel is liquid methanol. For operation of DMFC under low-temperature conditions, one method is external heating using electrical heating devices (by batteries) or chemical heating by catalytic combustion of methanol as separately equipped heating solution. These might be the easiest and most stable method, but need additional energy. Another one is anti-freeze mode, which prevents the fuel cell from freezing in advance by its self operation under prescribed critical temperature; this method might be most practical. The other method is internal heating by exothermic reactions in the fuel cell. The internal heat is as much as potential loss and crossover current in DMFC. This may not provide complete solution to heat up the stack sufficiently from very low temperatures, but it is necessary to examine a feasibility of internal heating. This study focuses on how to effectively utilize this internally generated heat for operations of DMFC at low temperatures. According to our previous study [22], the catalysts were more significantly affected by heat generated from the exothermic reaction in feeding of higher concentration methanol (5 M or 8 M). A performance loss of a DMFC stack after a series of the low temperature tests was mainly ascribed to electrochemical surface area (ECSA) loss of the catalysts in both electrode sides that was exposed to high concentration methanol. Therefore, in order to attain successful operation of DMFC under low-temperature conditions together with minimizing loss of the performance, an elaborate strategy should be established, even in the fuel feeding, for example, a fuel switching method with monitoring the stack temperature to lower use of high concentration methanol. In this paper, DMFC operation at low temperatures was investigated by a strategy for operation of DMFC controlling the concentration of feeding methanol as well as operation modes. Any heating devices or heat-insulation means were not equipped for the stack, to investigate performance of the stack genuinely exposed under low temperatures. The key idea for operation of DMFC was heat generation by methanol oxidation, which has been intuitively considered as a possible method, but has not been significantly studied so far. Although the self-heating strategy by methanol oxidation actually has limitation to full coverage for operation of DMFC at low temperatures below
20  C or lower, this study is believed to give information on the behaviors of DMFC at low temperatures.

2.

Experimental

2.1.

Preparation of the DMFC stack

For fabrication of MEAs, the procedure, which had been optimized in this laboratory to provide high performance of MEA, was applied. A MEA was fabricated using Nafion 115 as a membrane, PtRu/C (HISPEC 12100, Johnson Matthey, UK) as an anode catalyst, and Pt/C (Johnson Matthey, HISPEC 13100, Johnson Matthey, UK) as a cathode catalyst. The gas diffusion layers for the anode were Toray carbon paper were treated with 5 wt% PTFE (TGP-H-060, Toray Carbon, Japan), and the

gas diffusion layers for the cathode were SGL carbon paper with microporous layers (SIGRACET GDL 25BC, SGL Carbon, Germany), respectively. The electrode was formed via a barcoating procedure on the gas diffusion layer. The Pt loading was 1.8 mg cm
2 for the anode and 1.6 mg cm
2 for the cathode. The electrode and the Nafion membrane were hotpressed using a Carver Laboratory Press (Carver, Model M, USA). The hot-press process was carried out at less than 50 kg cm
2 for 1 min at 150  C. In order to examine the self-heating of DMFC under cold surroundings, a DMFC stack of over 30 W power density under normal conditions, which can be used for portable applications such as note PC power pack and portable power devices, was fabricated, as shown in Fig. 1. The DMFC stack used in this study consists of 10 cells and each cell has an electrode area of 30 cm2. The DMFC stack has internal manifolds for the supply of air and fuel. The flow field channel of the anode and cathode was designed as a serpentine type with two flow paths. Detailed specification of the stack is presented in Table 1.

2.2.

Set-up of the cold test equipment

The cold test equipment consisted of an environmental chamber (JEIO-TECH, TH-G 180, Korea) to control the temperature and humidity, an electrochemical test system with a DC electronic load (Smart II, WonA-Tech, Korea) to record the voltage, current and temperature of the stack, and a mass flow controller to provide a constant supply of methanol solution. The chamber was used to maintain low temperatures such as
5  C or
10  C for surroundings of the DMFC stack. Aside from temperature indicator equipped with the environmental chamber, additional thermocouple was closely attached to the inside of the stack end-plate to measure the inner temperature of the stack.

2.3.

Cold test

After the stack was cooled to a prescribed temperature (
5 or
10  C) in the chamber, the stack was then kept for 3hr at
5  C or
10  C to observe whether or not the methanol solution

Fig. 1 e Photograph of the DMFC stack used in this study (detailed specification is given in Table 1).

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Nafion 115 PtRu/C, 1.8 Pt mg cm
2 Pt/C, 1.6 Pt mg cm
2 30 cm2

Stack Number of cells Cell pitch Size Thickness of gasket Current collector

10 cell 2.0 mm 32.0 mm  80.0 mm  69.0 mm 200 mm Gold-coated copper

present in the stack and lines were frozen. Cold startup of the DMFC then commenced at a constant voltage or constant current mode with higher concentration of methanol (3e5 M) than that (w1 M) for normal operation. It should be noted that the words, ‘normal conditions or operation’, which are often used in this manuscript, indicate conditions of room temperature and feeding of a 1 M aqueous methanol solution and air. The fuel together with the DMFC stack was placed in the chamber. Therefore, the fuel temperature was equal to the temperature of the environmental chamber. As the stack temperature increased with the operating time, the methanol concentration was switched to 1 M at proper time. The stoichiometry ratio of methanol and air (no humidification) was 3.0 l and 3.5 l, respectively, and both of the compartments of the DMFC stack were purged with nitrogen gas after all cold operation tests. Evaluation of the startup and operation of the DMFC at low temperatures and an assessment of its performance were done using an electrochemical test system to record the voltage, current and temperature of the stack.

2.4.

MEA analysis

Any apparent change or damage in the MEA before and after the cold operation were observed using scanning electron microscopy (SEM, HITACHI, S-4700S, Japan), and change of the catalysts was examined using transmission electron microscope (TEM, Tecnai F20, FEI, Japan). The particle size of the electrocatalysts was calculated by the Scherrer equation using (220) plane of X-ray diffractometry (XRD, DMAX-2500, Rigaku Co.) patterns. t¼

0:9lka1 BcosqB

(1)

where t is the particle size of very small crystals, lka1 is the ˚ ), B is the width (radian) of the wavelength of X-ray (1.542 A peak at half-height, and qB is the angle at the Pt (220) peak maximum.

3.

Results and discussion

3.1. Performance dependency on the concentration of methanol solution DMFC is usually operated with a low concentration of methanol of 0.75e1.5 M to reduce irreversible performance loss due

-2

MEA Membrane Catalyst of anode Catalyst of cathode Active area of electrode

to methanol crossover through the perfluorosulfonic acid polymer membrane (e.g., a Nafion membrane, DuPont). A high concentration methanol solution, however, would be required for operation of DMFC at low temperatures, considering the freezing point of the solution and intensive self-heating of the cell. The freezing point of aqueous solution with a high methanol concentration is low enough to sustain the liquid state. The freezing points of aqueous methanol solutions are about
2,
7 and
12  C for 1, 3 and 5 M solutions, respectively, according to the literature [23]. Also, heat generation by the exothermic methanol oxidation can be accelerated with high methanol concentration. The crossover of methanol through membrane to the cathode cannot be avoided in the state-of-art technology, and appropriate methanol crossover would be helpful in terms of warming-up of a DMFC cell just at the initial operation stage under low-temperature conditions. First, the performance of a single cell was examined with various methanol concentrations of 1e10 M and air flow rates of 350 cc min
1 at several temperatures of 30, 40, 50, and 60  C (temperature-controlled by an exterior heating device), as shown in Fig. 2. At a temperature such as 30  C, the maximum power density was not significantly affected by the feeding methanol concentration, whereas the stack performance at temperatures such as 50 and 60  C drastically decreased according to increase of the methanol concentration. Using 5 or 10 M methanol solution, the performances at temperatures of 30 and 40  C were rather higher than those of temperatures of 50 and 60  C. Such a performance decrease would be mainly ascribed to a mixed potential and catalyst poisoning at the cathode by the methanol crossover [24], which can be more promoted by higher methanol concentration as well as higher temperatures. Highly concentrated methanol above 3 M, therefore, could not be used as a feeding solution for normal operation of DMFC. Fig. 3(a) shows the performance and the polarization curves of the DMFC stack manufactured for this study. It was tested with methanol solutions of 1, 2, and 3 M at room temperature (around 20  C). It should be noted that the best stoichiometry ratio of fuels (methanol/air stoich. ratio ¼ 3/3.5) was predetermined by a series of stack tests, and that the same mass flux of methanol was fed to the stack with different concentration of methanol solutions by adjusting the flow rate. When a 1 M methanol solution was supplied as the fuel, the DMFC stack had the maximum output power of

Power Density (mW cm )

Table 1 e Detailed specification of the DMFC stack.

160 o

30 C o 40 C o 50 C o 60 C

140 120 100 80 60 40 20 0

1

2 3 5 10 MeOH Concentration (mol)

Fig. 2 e Maximum power density of the single cell fed with different methanol concentrations from 1 M to 10 M at several temperatures of 30, 40, 50, and 60  C.

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1M methanol solution 2M methanol solution 3M methanol solution

2

4 6 8 10 12 Stack Current (A)

14

b

45 40 35 30 25 20 15 10 5 0

160

0.8

140

Voltage (V)

120 0.6

100 80

0.4 0.2 0.0 0

60 1M methanol solution 2M methanol solution 3M methanol solution

100 200 300 400 500 -2 Current Density (mA cm )

40 20 0 600

-2

9 8 7 6 5 4 3 2 1 0 0

Power Density (mW cm )

Stack Voltage (V)

a

Stack Power (W)

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Fig. 3 e Polarization curves of a DMFC stack with different methanol concentrations at room temperature (a) and polarization curves of a single cell measured at the stack operation conditions (b). 42.3 W (140 mW cm
2) and the nominal output power of 32.7 W (8.2A@4V, 109 mW cm
2). However, as 2 M and 3 M methanol solutions were fed into the stack, the maximum power of the stack was decreased to 27.7 W (92 mW cm
2) and 21.4 W (71 mW cm
2), and the nominal power of the stack became 25.8 W (6.4A@4V, 86 mW cm
2) and 20 W (5.0A@4V, 67 mW cm
2), respectively. During the polarization tests, the DMFC stack temperature was monitored. When 2 and 3 M methanol solutions were supplied to the DMFC stack, the stack temperature rapidly increased from ambient temperature to 67  C and 81  C at the end of the operation, respectively, whereas feeding of 1 M methanol solution increased the stack temperature to around 59  C. The temperature increase of the stack depended on concentrations of the feeding methanol solution as expected, although the exact temperature values can be different according to the surroundings. These experiments showed the possibility of tuning of the methanol concentration for operation of DMFC at low temperatures. In Fig. 3(b), the polarization curves of the single cell with methanol solutions of 1, 2, and 3 M are presented, when each temperature is adjusted to a similar value observed at the stack operation (Fig. 3(a)): 60  C for 1 M, 70  C for 2 M, and 80  C for 3 M. With the higher concentration of methanol solution, the polarization curve started at the lower cell voltage from the region of a low current density, and showed totally the lower performance. In the operation with 1, 2 and 3 M methanol solutions, the maximum power density of single cell was

152, 118 and 91 mW cm
2, respectively. The performance decrease with high concentration indicates that the methanol crossover governs the cell performance. Also, enhanced methanol crossover due to the operation with high concentration leads to fast increase of the stack temperature, at which the crossover can be promoted again. It should be noted that the polarization curves of the stack (Fig. 3(a)) and the single cell (Fig. 3(b)) appear to be different especially at a low current density (below 100 mA cm
2). It is because the stack temperature increased according to the operation, starting from ambient temperature, whereas the temperature conditions for the single cell tests were artificially controlled. In other aspect, increase of the stack temperature can be accelerated under cold conditions, when a high concentration methanol is properly utilized for heat generation.

3.2. Operation of the stack at low temperatures of
5  C and
10  C Fig. 4 shows the polarization curves and temperature profile of the DMFC stack starting at room temperature and
5  C with 3 M methanol solution and air. Under the surrounding of
5  C, a significant voltage drop was observed at the initial stage, but as the stack temperature increased rapidly, the overall power curve was similar with that at room temperature. The first voltage drop in the operation at
5  C could be simply expected due to a high overpotential of the electrocatalysts at a low temperature, and also mass transport problems in the MEA at a low temperature was probably involved in the activation loss [19,25,26]. Once the stack was heated by the exothermic reactions and the resistive heating due to internal ohmic losses [12], the stack returned almost to a normal operation. The stack temperature increased to 81  C under room temperature conditions. In the operation at
5  C, the stack temperature remained at 59  C due to heat loss in low-temperature circumstances, which however appeared to be high enough for normal operation of the stack. Although the slope of temperature profiles under
5  C conditions was slightly small at the initial stage, the overall temperature increase rates of the stack under room temperature and
5  C conditions were nearly identical as 0.158  C s
1 and 0.156  C s
1, respectively. The heat capacity of the stack is an intrinsic property, and the stack temperature is determined by the rate of heat accumulation in the stack, which is balanced by the exothermic reactions and the heat discharge from the stack. The temperature differences between the stack and surroundings were similar before and after the polarization tests. From the results, it seems to be possible to operate a DMFC stack normally, even under low temperature circumstances, if the stack temperature is quickly increased up to the normal operating range. The operation mode was examined for operation of the stack at
5  C. According to our previous study [22], the constant current mode was found to be not effective to start the stack up under low temperatures, irrespective of the concentration of methanol solutions. The constant current mode underwent a severe voltage drop at the beginning of DMFC’s cold operation and it can result in shutdown of the fuel cell. In contrast, the constant voltage mode was able to initiate the stack with increase of the temperature.

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10 5

at room temperature o at -5 C

1

2 3 4 5 6 Stack Current (A)

0 8

7

80

o

Stack Temperature ( C)

b 60 40 20 at room temperature o at -5 C

0 0

100

200 300 Time (sec)

400

Fig. 4 e Polarization curves (a) and temperature profiles (b) of a DMFC stack with 3 M methanol solution at room temperature and at L5  C.

60

3M MeOH 1M MeOH

35

o

Temperature( C)

30

40

25

Power(W)

30

20

20

15 10

10

Current(A)

0

5 0 0

50

O

40

Voltage(V)

5

10 15 Time (min.)

20

Stack Temperature( C)

Output (current, voltage, power)

Fig. 5 shows operation of the DMFC stack at
5  C under a constant voltage condition of 4 V. Initially, a 3 M methanol solution was used to operate the DMFC stack, since the freezing point of the solution is lower than
5  C. As shown in Fig. 5, the DMFC stack appeared to start without any difficulty at
5  C using the 3 M methanol solution. The DMFC stack reached to a normal temperature of 25  C after 190 s from the starting only by self-heating. The current and the power performance of stack were recovered with increase of the temperature, reaching to an output power of 20 W (67 mW cm
2) within 255 s, which is comparable to the performance of the stack at room

-10 25

Fig. 5 e Startup and operation behaviors of a DMFC stack under a constant voltage of 4 V at -5  C as 3 M and 1 M methanol solutions were supplied in turn.

40

3M MeOH

35 30 25 20 15 10 5 0 0

5

80 70 o Temperature( C) 60 50 Power(W) 40 30 20 Current(A) 10 0 Voltage(V) -10 10 15 20 25 30 Time (min.)

2M MeOH

1M MeOH

O

15

Stack Temperature( C)

20

temperature. However, while the stack temperature continued to increase, the DMFC stack experienced an abrupt performance drop. These phenomena were ascribed to the fact that enhanced methanol crossover accelerated increase of the stack temperature, while a significant mixed potential due to the methanol crossover at the cathode decreased the stack performance. In addition, the abrupt voltage drop after the fuel and operation mode switching would be caused by instantaneous oxygen depletion at the cathode (see Section 3.3). When the stack temperature reached around 40  C (after 5 min from the starting), the fuel was switched to 1 M solution still under the constant voltage of 4 V to prevent an irreversible loss of the performance due to severe methanol crossover. Just after the fuel switching, the current and power were radically increased to 34 W (113 mW cm
2) with some instant fluctuation and continuous temperature increase and then immediately the current, power and temperature followed a course of gradual decrease. A stepwise fuel switching (3 Me2 Me1 M) under a constant voltage of 4 V was applied to the stack operation at
5  C, as shown in Fig. 6. This was done to prevent the temperature drop of the DMFC stack due to an instant change of the methanol concentration. When the stack started with a 3 M solution, the performance and temperature profiles followed similar patterns with the results in Fig. 6. The 3 M feeding was continued until the stack temperature increased to around 60  C. The output power was around 20 W with a slightly decreasing profile, although the stack temperature increased continuously. As soon as the feeding solution changed from 3 M to 2 M around 450 s, the current initially fluctuated, and then rapidly increased to 27 W. The temperature and power profiles were quite stable, exhibiting similar patterns at this range. When switching the solution from 2 M to 1 M, the temperature sustained around 120 s, and then decreased from 60  C to 35  C for 15 min. On the other hand, the stack power rapidly increased to 35 W with an initial fluctuation similar as shown in every switching step of solutions, and then immediately decreased to 15 W for 15 min. From the results, a high performance of the stack, which can be comparable to that under normal conditions, was found to be difficult to obtain with 1 M methanol solution at
5  C when using a constant voltage mode, irrespective of the fuel switching methods. Another combination of the operation modes was attempted to achieve high and stable performance of the stack at

Output (current, voltage, power)

25

9 8 7 6 5 4 3 2 1 0 0

Stack Power (W)

Stack Voltage (V)

a

Fig. 6 e Startup and operation behaviors of a DMFC stack under a constant voltage of 4 V at -5  C as 3 M, 2 M and 1 M methanol solutions were supplied in turn.

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50

25

40

20

o

Temperature( C)

15

20

10

Current(A)

5

Voltage(V)

0 0

30

5

10 15 Time (min.)

20

10 0 -10 25

Fig. 7 e Startup and operation behaviors of a DMFC stack under a constant voltage of 4 V and a constant current of 7.5 A at L5  C as 3 M and 1 M methanol solutions were supplied in turn.

Analytical approach for heat generation in the stack

The heat generation in the stack was checked out by a simple calculation, based on the experimental data. First, the heat balance of DMFC can be expressed as follows [27]: Qrxn ¼ Qacc;stack þ Qcond þ Qconv

(2)

where Qrxn is the heat generated by the reactions, Qacc;stack is the heat accumulated in the stack, Qcond is the heat loss by conduction, Qconv is the heat change by mass flow, and all units for heat are watt (W ¼ J/s). The reaction heat ðQrxn Þ is related to the current and the departure of the cell voltage from the thermodynamic voltage,iðEth
Ecell Þ, which is caused by various overpotentials such as activation, ohmic and concentration, and also a significant amount of heat is generated from oxidation of the crossover methanol at the cathode,ixover Eth . Qrxn ¼ iðEth
Ecell Þ þ ixover Eth ¼   1 ¼
hV iEth hM

  i þ ixover Ecell iEth
i Eth (3)

where hM is the methanol efficiency, hV the voltage efficiency, i the load current, ixover the methanol crossover current, Eth the theoretical voltage (w1.21 V) and Ecell the cell voltage. The equation is rearranged with the efficiency expression, and the heat generation is thus confirmed to be governed by the current density, the methanol and voltage efficiency. Temperature and concentration dependency of the methanol crossover for the stack used in this study was examined through direct measurement of collected output mass and concentration at both electrodes and then calculation by the mass balance. The empirical equation to calculate ixover (A cm
2)

20

1M MeOH

5M MeOH o

Temperature( C)

15 Power(W)

10 Voltage(V)

5 0 0

Current(A)

5

10 15 20 Time (min.)

70 60 50 40 30 20 10 0 -10

O

60

Power(W)

3.3.

Stack Temperature( C)

1M MeOH

O

30

3M MeOH

significantly affect the performance for a time, because the methanol crossover is severe in feeding of a 5 M. The output power was around 14 W, which is 46% of the performance under normal conditions. Although the performance was significantly reduced, the stack was found to be able to operate at
10  C only by control of the operation method without any means for heat insulation.

Output (current, voltage, power)

70

35

Stack Temperature( C)

Output (current, voltage, power)

the low temperature. Fig. 7 shows operation of the DMFC stack at
5  C under a constant voltage condition of 4 V and then a constant current of 7.5 A. The changing point of the operation modes (including the fuel switching), at which the increasing rate of the power clearly reduced (around 6e7 min), was determined by monitoring the profiles of the current and the power. The current, 7.5 A at the constant current mode was chosen by the previous polarization tests (Fig. 3) to maintain the voltage level around 4 V for stability of the stack. For the startup, a 3 M methanol solution was fed to the stack under a constant voltage of 4 V, since a constant voltage mode was found to be more effective in terms of the warm-up of the stack than a constant current mode. The operation mode was then changed simultaneously with switching the solution from 3 M to 1 M, when the stack temperature reached over 60  C. As soon as the operation mode was changed, the stack temperature started to decrease gradually, and then appeared to sustain around 42  C. The power was instantaneously increased just as the current rose up, and then decreased gradually like the corresponding temperature profile, finally to sustain around 26.2 W (the stack voltage is also stable around 3.5 V), which is approximately 66% of 39.5 W under normal conditions (see Fig. 3). Hence, the stack was successfully operated at
5  C by means of the combination of operation modes as well as methanol solution switching. Operation of the stack at
10  C was investigated, using the combinational operation method, which was successful in the operation at
5  C (Fig. 7). Fig. 8 showed the results of the stack test through startup with a 5 M solution at a constant voltage mode of 4 V and then operation with an 1 M solution at a constant current mode of 3.5 A. The mode changing point and the current of the constant current mode were determined in the same fashion as the previous test (Fig. 7). Increase of the stack temperature (about 8 mine40  C) was slower than that in the startup at
5  C (about 5 mine40  C). The overall profiles of the temperature, current, voltage and power, however, were similar with those at the operation at
5  C. However, when the operation mode changed, the voltage and power drop temporarily to some extent, and then increased to be recovered around 4 V and 14 W, respectively. This behavior is not clearly understood for the present, but it appeared that the remaining methanol in the cathode might

25

Fig. 8 e Startup and operation behaviors of a DMFC stack under a constant voltage of 4 V and a constant current of 3.5A at L10  C as 5 M and 1 M methanol solutions were supplied in turn.

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depending on the stack temperature and methanol concentration is obtained as follows: (4)

where T is the stack temperature (K), and Cm the methanol concentration (M). The equation is applicable for the ranges of the temperature, 0e80  C and the methanol concentration, 1e10 M, and the calculated ixover would be close to the maximum value of the methanol crossover under corresponding conditions. Although the stoichiometric and electroosmotic drag effects according to the current density are not considered in the Equation (3), a valuable insight on the heat generation of the stack can be obtained. The temperature and corresponding performance of the stack can be predicted by the Equations (2)e(4) for prescribed current density, methanol concentration, and the surrounding temperature, if the parameters relating to the methanol crossover, the thermal conductivity and specific heat capacity of the stack (apparent values, at least) are given. The cell voltage is a function of temperature and methanol crossover (ixover ), and the methanol crossover is also a function of temperature, current density, and stoichiometric ratio (feeding rate). It is not actually easy to describe the temperature and performance profiles of the stack fully, but the stable operation point can be approximated for a given surrounding temperature at the steady state ðQacc;stack ¼ 0Þ Under several assumptions, the stack temperature at the steady state can be predicted as follows:

(6)

(5)

a o

where Kapp;stack is the apparent thermal conductivity of the stack (unit: W K
1), Tstack the stack temperature, and Tsurr the surrounding temperature. The assumption is: (i) methanol permeated to the cathode is completely consumed, and (ii) the conductive heat loss is dominant to the convective one [28]. The heat generation, Qrxn is calculated from the Equation (3), in which the cell voltage and crossover current are temperaturedependent functions. In case of the operation with combination of fuel and operation mode switching at
5  C, the profiles of performance (7.5 A at 4 V) and temperature (42  C) were confirmed clearly stable, as shown in Fig. 7. Using this result, Kapp;stack (1.73 W K
1) was obtained from the Equations (4) and (5). By applying Kapp;stack to the Equations (4) and (5), the temperature and performance of the stack at the steady state were estimated at sub-zero temperatures, when the operation was switched to the constant current mode with 1 M methanol solution as shown in Figs. 7 and 8. Fig. 9 shows the calculation results of the stack temperature (Fig. 9(a)) and cell voltage (Fig. 9(b)) at the steady state for the load current 1e8 A and the surrounding temperatures
5 to
35  C. According to the results, the constant current operation of 3.5 A with 1 M solution at
10  C as seen in Fig. 8 is expected to be stabilized at the stack temperature 9.4  C and the voltage 3.8 V (the power 13.4 W) after all. The stack temperature appears to increase almost linearly with the load current as shown in Fig. 9(a). As the surrounding temperature decreases, the current for the operation at the stack temperature above zero should become high as can be expected, for example, the load current at
35  C should be higher than 5 A

Stack temperature ( C)

Qrxn ðTstack Þ ¼ Kapp;stack ðTstack
Tsurr Þ

 iN n O2 ;feed ¼ l 4F

-2

Current density (A cm ) 0.05 0.10 0.15 0.20 0.25 50 40 30 20 10 0 -10 -20 -30

Tsurrounding o -5 C o -15 C o -25 C o -35 C

1

b Stack voltage (V)

ixover ¼ 1:032105 e
4560=T þ 0:0322ðCm
1Þ

for the stack temperature above zero. The stack voltage (Fig. 9 (b)) also decreased according to decrease of the surrounding temperatures, and severe voltage drop appears below
20  C. At a high load current, the voltage appears to increase, as the stack temperature becomes high, but in actual cases, it would be difficult to operate the stack in a stable manner below
20  C. To keep up the stack voltage around 4 V (0.4 V for a unit cell) and stable operation, the temperature around
15  C is supposed to be the limitation for the stack operation. It should be noted that in this calculation, a fixed value for the methanol crossover at the same temperature was applied irrespective of the current and that the convective heat loss by mass flow was ignored. Hence, the values from calculation is not exact, and also have no meanings for other types of DMFC stack, but the DMFC operational behaviors and trend depending on the surrounding temperature below zero would be a good reference. One more thing to take into account is the issue of oxygen depletion at the cathode, when high concentration methanol solutions are fed for startup of the stack at low temperatures. In this study, the stoichiometric ratio, l of methanol and air (no humidification) was adjusted to 3.0 and 3.5, respectively, with respect to the load current for the operations in Figs. 5e8.  The oxygen feeding rate ðn O2 ;feed Þ and oxygen consumption  rate ðn O2 ;cons Þ are as follows:

6

2

o

-10 C o -20 C o -30 C

3 4 5 6 Load current (A)

Tsurrounding o -5 C o -25 C

5

o

-10 C o -30 C

7

o

8

o

-15 C o -35 C

-20 C

4 3 2 1 0

1

2

3 4 5 6 Load current (A)

7

8

Fig. 9 e Calculation results of the stack temperature (a) and cell voltage (b) at the steady state for the load current 1e8 A and the surrounding temperatures L5 to L35  C.

5662

ði þ ixover ÞN 4F 

gfeed
cons ¼

n O2 ;feed 

n O2 ;cons

¼

(7)

li ¼ lhM  1 ði þ ixover Þ

(8)

where N is the cell number of the stack (30 cells), and F the Faraday constant (96,485 C mol
1). The ratio ðgfeed
cons Þ of feeding rate per consumption rate is related to the methanol efficiency under assumption of complete methanol consumption at the cathode, and it should be more than 1 to obtain the target current. For given i and l, the ratio is a function of Ixover , in other words, dependent on the stack temperature and methanol concentration. For the experiment conditions in this study (Figs. 7 and 8), the ratio, gfeed
cons was examined as shown in Fig. 10. In case of the conditions for Fig. 7, the ratio sustains over 2 at most of the temperature range and it indicates that the feeding of oxygen would be high enough for the load. On the other hand, it is smaller than 1.6 at the same range in case of the Fig. 8 experiment, and over 30  C, it decrease significantly, reaching to 1 around 60  C. It should be noted that the stack temperature in Fig. 8 reached to 64  C just before changing the methanol solution and the operation mode. The abrupt voltage drop after the fuel and operation mode switching must have been caused by instantaneous oxygen depletion at the cathode, since the current was quickly increased at the same point, and also the operation with 1 M recovered the stack performance after a few minutes. The same phenomena in Figs. 5 and 6 can be explained by the oxygen depletion due to the cathode methanol combustion at a high temperature.

3.4.

MEA analysis after the cold test

Fig. 11 exhibits comparison of the stack performances under normal conditions before and after operation of the DMFC stack at
5 and
10  C. After the tests, the maximum output power of the DMFC stack decreased from its initial performance of 42 W (140 mW cm
2) to 37 W (125 mW cm
2) and the nominal output power of the stack decreased from 33 W (109 mW cm
2 @ 4 V) to 30 W (99 mW cm
2 @4 V). The DMFC stack after a series of the operations showed the performance

3.5 -2

3.0

operation with 3M at 7.5 A (0.250 A cm ) -2 operation with 5M at 3.5 A (0.117 A cm )

rfeed-cons

2.5 2.0 1.5 1.0 0.5 260 270 280 290 300 310 320 330 340 Stack temperature (K) Fig. 10 e Ratio ðgfeedLcons Þ of feeding rate per consumption rate depending on the stack temperature at the applied load currents (3.5 A and 7.5 A) and methanol concentration (3 M and 5 M).

Stack Voltage (V)



n O2 ;cons ¼

9 8 7 6 5 4 3 2 1 0 0

Before cold operation After a series of the cold operation

2

4 6 8 10 12 Stack Current (A)

14

45 40 35 30 25 20 15 10 5 0

Stack Power (W)

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 6 ( 2 0 1 1 ) 5 6 5 5 e5 6 6 5

Fig. 11 e Performance change of a DMFC stack before and after cold operation at L5  C. loss of about 11.4%. Degradation of the stack can be caused by feeding of high concentration methanol (high crossover flux) as well as freeze-thaw cycles. The catalysts in both the anode and cathode can be damaged by local heat evolution of oxidation of concentrated methanol, and some structural and physical damages in the backing and catalyst layers as well as membranes also can be supposed to occur in the fuel cell during freeze-thaw processes [12,14]. After a series of the operation tests, several analyses such as electron microscopy and XRD have been performed to find any changes in MEA concerning the performance loss. First, any noticeable change or damage in the membrane and GDL, however, was not observed by SEM analyses before and after the cold test at
5  C and
10  C. The resistance of the DMFC stack was also almost identical before and after the cold test (the resistance of the stack was 98 mU). As shown in Fig. 12, SEM images of MEA after a series of the tests were similar with the results of Wilson et al. [13] that freezing at
10  C appears not to be so detrimental to the integrity of a membrane-electrode assembly (MEA) in the PEMFC, even with a high level of water content in the membrane. Effects of long-term operation or repetitive freeze-thaw cycles under prescribed conditions on the MEA structure however have yet to be investigated in further studies. The status of electrode catalysts before and after operation of DMFC at low temperatures was examined by TEM (Fig. 13) and XRD (Fig. 14). From TEM images (Fig. 13), any significant changes in both anode and cathode catalysts were not actually observed, whereas the particle size calculated from XRD data (Fig. 14) showed a little growth of the particle size in both anode and cathode catalysts. In the 2q range of 20 e80 , three peaks of the Pt face-centered cubic lattice are confirmed in all samples, as shown in Fig. 14. From the Pt (220) peak, the mean particle size was calculated using Scherrer’s equation (shape constant K ¼ 0.9). The mean particle size for the anode and cathode catalysts was found to increase from 2.4 nm to 2.6 nm and from 4.0 nm to 4.9 nm (Table 2), respectively, after a series of cold stack tests. Such growth of the particles resulted in a drop in the number of active sites and hence a decreased ECSA (electrochemical surface area), and this in turn led to performance deterioration in DMFCs. According to Wang’s review paper [29], the growth of catalyst particles in the fuel cells is accelerated by 1) undesirable temperature, 2) high potential control (0.9e1.2 V), 3) load cycling and 4) contamination. However, in this case, the potential was low below 0.9 V, and severe load

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 6 ( 2 0 1 1 ) 5 6 5 5 e5 6 6 5

5663

Fig. 12 e SEM images of MEA after a series of cold tests: (a) cross-sections of MEA after the cold tests, (b) interfaces of MEA after the cold tests.

cycling was not applied. Thus, increase of the particle size would be caused by use of concentrated methanol and local intensive heat generation (undesirable temperature). Considering convective heat loss associated with constant flow rate ðQconv Þ and conductive heat loss involved with constant temperature difference at the boundary ðQcond Þ (see Equation (2)),

the heat generation by methanol oxidation in the catalyst layer would be locally much more intensive than that measured by the exterior temperature (59e67  C). The intensive heat generation by high concentration methanol could bring out the particle growth by thermal degradation of the catalysts. These suggestions are supported by the previous studies [30e32].

Fig. 13 e TEM images of the catalysts before and after the cold tests: (a) anode catalysts before the cold tests, (b) anode catalysts after the cold tests, (c) cathode catalysts before the cold tests, (d) cathode catalysts after the cold tests.

5664

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 6 ( 2 0 1 1 ) 5 6 5 5 e5 6 6 5

Intensity

degradation of the cell (stack) performance, although it is difficult to arithmetically derive the portion of the ECSA loss in the total performance loss of 11.4% (Fig. 11).

(111)

8000

6000

(200)

After the cold operation

(220)

4000

4. Before the cold operation

2000

0

20

30

40

50

2

12000

60

70

80

degree

(111)

10000

Intensity

(200)

After the cold operation

(220)

8000 6000 4000

Before the cold operation

2000 0

20

30

40

2

50

60

70

80

degree

Fig. 14 e XRD patterns of the electrocatalysts before and after the cold operation of the DMFC stack: (a) anode and (b) cathode.

Borup et al. [30] concluded that the rate of Pt particle growth increased with increasing temperature. Dam et al. [31] reported that when temperature was increased from 60 to 80  C, the Pt dissolution rate increased from 0.87 ng/h cm2 to 1.58 mg/h cm2. Cai et al. [32] showed that the Pt surface area loss during 10 h of thermal degradation was comparable to the loss from electrochemical degradation after 500 cycles between 0 and 1.2 V. One thing should be noted that the degradation of the stack performance in this study definitely was not severer, compared to the previous study [22], in which high concentration methanol without the fuel switching was applied and the performance loss was about 45%. This also supports the fact that use of high concentration methanol at high temperatures would be dominant in the performance degradation. If the surface area of catalyst particles is geometrically calculated under assumption of a simple ball particle, the calculated losses of ECSA are 7.7 and 18.3% for the anode and cathode catalysts, respectively. The ECSA loss of catalysts due to growth of the particle size must be a major factor for

Conclusions

In this study, the operation behaviors of a DMFC stack were investigated at low temperatures. The stack was successfully operated in a stable manner at
5  C and
10  C only by selfheating of the stack (heat generation by the electrochemical reaction and methanol oxidation at the cathode), although there was no heating device nor means for heat insulation of the stack through the experiments. The stack performance, however, showed low values of 66% and 54% at
5 and
10  C, respectively, compared to that under normal conditions (for example, around 20  C with a 1 M methanol solution), since the stack temperature was not sufficiently increased high (over 45  C) in the operation under the low-temperature conditions. For operation of DMFC, a constant voltage mode with a high concentration methanol solution such as 3 M and 5 M was found to be more effective than a constant current mode. On the other hand, a constant current mode with a low concentration methanol solution such as 1 M should be applied to keep the performance and temperature of the stack at a stable state. Hence, careful control of the operation modes associated with the fuel switching is required for successful operation of the stack under low-temperature conditions. Based on the results of this study, self-heating operation of DMFC stacks appears to be limited around
20  C, and additional heating or heat-insulation devices may be adopted for operation at lower than
20  C. The performance can be improved more than these results, if the stack is adequately insulated, at least. Considering the degradation of stack as well as successful operation of DMFC under low-temperature conditions, use of highly concentrated methanol should be carefully optimized. Best performance of DMFC at the low temperature is believed to be obtained by a well-designed protocol including the fuel switching and operation modes.

Acknowledgments This work was supported by the Next Generation Military Battery Research Center program of Defense Acquisition Program Administration and Agency for Defense Development, Korea.

references Table 2 e Particle size of the catalysts before and after the series of cold tests.

Before the cold test After the cold test

Anode catalysts (nm)

Cathode catalysts (nm)

2.4 2.6

4.0 4.9

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