Energy Fuels 2010, 24, 6449–6455 Published on Web 11/09/2010
: DOI:10.1021/ef101101q
Experimental Investigation of the Dynamic Performance of a Microdirect Methanol Fuel Cell Stack for Practical Applications Yufeng Zhang,*,†,‡ Zhenyu Yuan,† Shibo Wang,† Hong He,† Youran Zhao,† and Xiaowei Liu†,‡ †
MEMS Center, Harbin Institute of Technology, Harbin 150001, China, and ‡Key Laboratory of Micro-Systems and Micro-structures Manufacturing, Ministry of Education, Harbin 150001, China Received August 17, 2010. Revised Manuscript Received October 16, 2010
In this paper, a self-breathing 10-cell microdirect methanol fuel cell ( μ-DMFC) stack with double-planar structure is designed, fabricated, and tested. With the help of microstamping technology, the current collectors are microfabricated on the 300 μm-thick stainless steel plate. First, the steady-state performance of the stack is tested. Fed with 2 mL min-1 of 1 M methanol solution, the maximum power output of the stack can reach 141.0 mW at room temperature with a density of 22.03 mW cm-2. The dynamic performance of this stack is also investigated under several loading modes in consideration of practical conditions. The stack performance represents good responding and reproducible ability in most cases, but deteriorates to varying degrees with the increment of loading cycles. Finally, the μ-DMFC stack proves its good performance in powering two electronic devices five times in 120 days successfully while performance deterioration behavior is also detected.
Currently, great progress has been achieved in improving the single μ-DMFC performance. However, in view of the fact that the output voltage of a single cell is limited, investigations relating to the μ-DMFC stack technology are quite essential to fulfill the requirements in practical applications. Cao et al.17 developed two air-breathing μ-DMFC stacks with different anode flow fields. Using the microelectro-mechanical system (MEMS) technology, the current collectors were microfabricated on the silicon substrate and deposited with a Ti/Pt/Au composite metal layer as the electrical conductor. Experimental results showed that the stack with the double serpentinetype flow field could generate a higher output power of 151 mW. Chan et al.18 reported a 6-cell monopolar DMFC stack with performance research under different methanol concentrations. Furthermore, this stack which could provide 350 mW at 1.8 V was successfully applied to power a seagull display kit. Hashim et al.19 presented a low-cost μ-DMFC stack with the active area of 1.0 cm2 and novel cathode structure, and the highest output power was 12.05 mW at 1.08 V. Besides, the dynamic characteristic is also an important index to evaluate the performance of the μ-DMFC stack. In most cases for practical applications, varying the load to satisfy the power supply for electronic products requires the μ-DMFC stack with high performance with respect to dynamic response and stability. However, most research concerning the dynamic performance was concentrated on single cells.20-23
1. Introduction The research on reliable, efficient, environmentally protective and economical power sources has been one of the key energy topics in recent years. Fuel cells, transforming the chemical energy into electrical energy with cheap fuels and green emission, have been realized as the most competitive candidate to substitute the conventional power sources. Several reasons indicate that a microdirect methanol fuel cell ( μ-DMFC) is the most desirable choice for microscale devices and systems.1-8 For one thing, compared with some polluting batteries, the reactants (CO2 gas and water) of the μ-DMFC have little impact on the environment. For another, as the consumed fuel, the methanol can be stored easily and safely. Moreover, high energy density, simple controllability and operation, etc. are also attractive. Anyway, considering the potential of μ-DMFC to overturn conventional power sources, many researchers have paid increasing attention to various aspects concerning μ-DMFC technology.9-16 *Corresponding author. Mailing address: Box 328, Harbin Institute of Technology, Harbin 150001, China. Tel.: þ86 451 86413451. Fax: þ86 451 86413441. E-mail address:
[email protected]. (1) Dyer, C. K. J. Power Sources 2002, 106, 31–34. (2) Kamarudin, S. K.; Achmad, F.; Daud, W. R. W. Int. J. Hydrogen Energy 2009, 34, 6902–6916. (3) Pichonat, T.; Gauthier-Manuel, B. Microsyst. Technol. 2007, 13, 1671–1678. (4) Wee, J. H. J. Power Sources 2006, 161, 1–10. (5) Morse, J. D. Int. J. Energy Res. 2007, 31, 576–602. (6) Kundu, A. J. Power Sources 2007, 170, 67–78. (7) Kamarudin, S. K.; Daud, W. R. W.; Ho, S. L.; et al. J. Power Sources 2007, 163, 743–754. (8) Nguyen, N. T.; Chan, S. H. J. Micromech. Microeng. 2006, 16, R1–R12. (9) Lu, G. Q.; Wang, C. Y. J. Power Sources 2005, 144, 141–145. (10) Wong, C. W.; Zhao, T. S.; Ye, Q.; et al. J Power Sources 2006, 155, 291–296. (11) Yang, H.; Zhao, T. S. Electrochim. Acta 2005, 50, 3243–3252. (12) Lu, G. Q.; Wang, C. Y.; Yen, T. J.; et al. Electrochim. Acta 2004, 49, 821–828. (13) Cha, H. Y.; Choi, H. G.; Nam, J. D.; et al. Electrochim. Acta 2004, 50, 795–799. (14) Ito, T.; Kunimatsu, M. Electrochem. Commun. 2006, 8, 91–94. r 2010 American Chemical Society
(15) Zhang, Y.; Lu, J.; Shimano, S.; et al. Electrochem. Commun. 2007, 9, 1365–1368. (16) Dai, H.; Zhang, H. M.; Luo, Q. T.; et al. J. Power Sources 2008, 185, 19–25. (17) Cao, J. Y.; Zou, Z. Q.; Huang, Q. H.; et al. J. Power Sources 2008, 185, 433–438. (18) Chan, Y. H.; Zhao, T. S.; Chen, R.; et al. J. Power Sources 2008, 178, 118–124. (19) Hashim, N.; Kamarudin, S. K.; Daud, W. R. W. Int. J. Hydrogen Energy 2009, 34, 8263–8269. (20) Argyropoulos, P.; Scott, K.; Taama, W. M. J. Power Sources 2000, 87, 153–161. (21) Argyropoulos, P.; Scott, K.; Taama, W. M. Electrochim. Acta 2000, 45, 1983–1998.
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: DOI:10.1021/ef101101q
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Figure 1. Configuration of the self-breathing μ-DMFC stack.
Figure 2. Photograph of the fabricated cathode current collector with the TiN layer.
The dynamic behavior investigation of the μ-DMFC stack seems obviously significant. This paper presented a self-breathing metal-based 10-cell μ-DMFC stack with double-planar structure. The steady-state performance of the stack was first investigated. Furthermore, to evaluate the practicability for prospective applications, the dynamic performance of the stack was also studied under various operating modes. In the end, the μ-DMFC stack was successfully applied to power two electronic devices five times in four months, and the deterioration performance was examined.
In the single cell design, both the anode and cathode current collectors share the same configuration with two portions, the flow field area and the interconnection area. The flow field area with an active area of 0.64 cm2 plays the role of transporting reactants to the membrane electrode assembly (MEA), while the interconnection area acts as the electrical connector for the adjacent two cells. By means of the microstamping technology, current collectors were fabricated on the 300 μm-thick stainless steel plate. To enhance the depth of the stamped channels, the fabrication process was conducted under a thermal hydromechanical conditions at 90 °C with the methylsilicone oil serving as the hydro-medium.26 In addition, some circular holes with the diameter of 600 μm were perforated on the channels by laser drilling to form the cathode air-breathing ventilators. Finally, with purpose of covering the stamped cracks and enhancing the corrosive resistance, a 500 nm-thick titanium nitride (TiN) layer was sputtered onto the surface of each current collector. Figure 2 displays the real features of the fabricated cathode current collector. In this work, a piece of 0.64 cm2 5-layered MEA fabricated by the catalyst coated membrane (CCM) approach was employed, with the catalyst loadings of 4.0 mg cm-2 (PtRu, anode) and 2.0 mg cm-2 (Pt, cathode). First, the hydrophilic catalyst layers were prepared using the decal transfer method to form the CCM. Second, carbon paper was prepared with the hydrophobic (10 wt % PTFE for anode and 30 wt % PTFE for cathode) and pore-formed (NH4HCO3) pretreatment to form the diffusion layers. Finally, the 5-layered MEA was achieved with the diffusion layers hot pressed on both sides of the CCM at 130 °C and 4 MPa for 120 s. 2.2. Fabrication of the Self-Breathing μ-DMFC Stack. To ensure the μ-DMFC stack to achieve the optimal performance and long-term durability, the stack feeding pattern is required to
2. Experimental Methods The basic structure of the metal-based self-breathing μ-DMFC stack with double planar arrays is shown in Figure 1. It consists of ten single cells, a distribution plate, two anode fixer plates, two cathode plates, and gaskets. 2.1. Preparation of the Single μ-DMFC. To minimize the performance differences among single cells from one another in the stack, it is required that each single cell obtains the reactants with uniform flow velocity and concentration. Previous study24 has proven that a parallel flow field has an advantage over the serpentine flow field for providing the stack with better stability and reliability. Therefore, a parallel flow field is adopted in the anode current collector. For the cathode current collector, a selfbreathing structure with a circular opening geometry25 is superimposed on the anode structure. (22) Sundmacher, K.; Schultz, T.; Zhou, S.; et al. Chem. Eng. Sci. 2001, 56, 333–341. (23) Kardash, D.; Korzeniewski, C.; Markovic, N. M. J. Electroanal. Chem. 2001, 500, 518–523. (24) Li, X. G.; Sabir, I. Int. J. Hydrogen Energy 2005, 30, 359–371. (25) Kim, S. H.; Cha, H. Y.; Miesse, C. M.; et al. Int. J. Hydrogen Energy 2009, 34, 459–466.
(26) Xu, Y. C.; Kang, D. C. J. Harbin Inst. Technol. 2003, 35, 1165–1167.
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Figure 3. Schematics of the self-breathing μ-DMFC stack anode feeding pattern.
Figure 5. Testing results (open circuit voltage and maximum power density) of the single cell performance with seven different anode flow rates.
Figure 4. Photograph of the assembled μ-DMFC stack.
distribute the fuels and remove the products uniformly and steadily. After a series of simulation and experiments, an optimal feeding pattern was designed and employed in another paper27 as shown in Figure 3. In this arrangement of the feeding pattern, every five single cells are placed side by side to make up of a planar array and the two arrays are combined together to form the stack. In the operating process, there is a flow of methanol solution into the distribution plate. This flow is first distributed by anodes of five paralleled single cells, namely 1 to 5. Each of these five single cells is in series with another cell, 6 to 10, and these five extra cells are connected in parallel. The methanol solution flowing out the parallel structure of cell 6 to 10 constitutes the output. Meanwhile, the ambient air accesses each cathode difussion layer via open windows of the cathode fixer plates. The feeding pattern can ensure each single cell sufficient methanol solution with uniform flow rate and concentration, and thereby, the behavior that the stack performance is encumbered with some abnormal single cells which will not be eliminated. Furthermore, the pattern can guarantee each cell to obtain a higher liquid flow rate at the same fuel consumption, which is meaningful for the improvement of the whole performance, CO2 gas removal, and fuel utilization. The distribution plate and fixer plates were micromachined on the polymethyl methacrylate (PMMA) plates with the heatresistant treatment. Separatory channels with optimized dimensions were fabricated in the distribution plate to achieve the anode feeding pattern of the stack (see Figure 1). Five chambers were machined on each anode fixer plate for the sake of mounting the single cells, while five square openings were formed by laser cutting on each cathode fixer plate to make each cathode open to the ambient air. The brief explanation of the assembling procedure is provided as follows. First, with the modified acrylate resin adhesive added around the edges as a sealant, two current collectors (anode and
Figure 6. Power density curves of the μ-DMFC stack at different anode flow rates.
cathode) with the MEA in between were pressed for more than 24 h to complete the single cell package. Second, every five cells were mounted into chambers of each anode fixer plate to form the planar array, with the adhesive filled in the void space to fasten the cells and prevent the liquid leakage, and the solidification time was about 12 h. Third, bolts and nuts were employed to electrically connect the single cells one by one, as sketched in Figure 1. In the end, two anode fixer plates with ten single cells, two cathode fixer plates, the distribution plate, and gasket layers were screwed together to complete the stack assembly. Figure 4 shows the photograph of the assembled stack, with a total size of about 95 mm 15 mm 23 mm.
3. Results and Discussion 3.1. Steady-State Performance of the Self-breathing μ-DMFC Stack. In this work, all the experiments were carried out at room temperature (20 ( 2 °C) and ambient pressure. Figure 5 indicates the influence of the anode flow rate on the single μ-DMFC performance by feeding a 1.0 M methanol solution at seven different flow rates (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 2.0 mL min-1). It can be found that the open circuit voltage (OCV) continuously decreased with the increment of the flow rates. This phenomenon justifies the idea that the increment of the anode flow rate can accelerate the methanol crossover rate.
(27) Liu, X. W.; Zhang, B.; Zhang, Y. F. J. Micromech. Microeng. 2010, 20, 104008.
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Table 1. Parameter Comparisons between the Stack and Its Corresponding Single Cell parameter -1
anode flow rate (mL min ) maximum power density (mW cm-2)
value stack single cella stack single cellb
maximum performance ratio (%)c
1.0 0.2 17.06 19.80 86.16
2.0 0.4 22.03 24.20 91.03
3.0 0.6 22.44 24.58 91.29
4.0 0.8 23.51 25.66 91.62
5.0 1.0 24.75 27.11 91.29
a Obtained from ref 27z. It is an approximation. b Obtained from Figure 5. It is a theoretical maximum power density of each single cell after receiving the methanol solution at a distributed flow rate. c It is defined as the ratio of the maximum power density of the stack to the theoretical maximum power density of the corresponding single cell.
Figure 7. Dynamic responses of the μ-DMFC stack voltage under the instantaneous load-on/instantaneous load-off mode.
Figure 8. Dynamic responses of the μ-DMFC stack voltage under the instantaneous load-up/instantaneous load-down mode.
The testing results come to an agreement that the optimal anode flow rate of the single cell is 1.0 mL min-1. In addition, from 0.4 to 1.2 mL min-1, the variation of the cell maximum power density is not significant from 24.0 to 28.0 mW cm-2. As illustrated in Figure 6, the performance of the μ-DMFC stack was evaluated under five different anode flow rates (1.0, 2.0, 3.0, 4.0, and 5.0 mL min-1) with a methanol concentration of 1.0 M. The stack obtained the highest maximum power density of 24.75 mW cm-2 (158.42 mW) at 5.0 mL min-1, while the lowest one of 17.06 mW cm-2 (109.20 mW) was found for a flow rate of 1.0 mL min-1. Since the performance of each single cell directly had great effects on the stack, the highest stack performance at different flow rates were compared with the maximum power densities which should be obtained by each corresponding single cell, as shown in Table 1. As seen from this table, the stack maximum performance could reach approximately 91% of the theoretically maximum performance obtained by the corresponding single cell, proving the superiority of the feeding pattern. It is worth pointing out that the stack performance was close to 2-5 mL min-1. To enhance the fuel utilization, it is recommended that the relatively lower flow rate (e.g., 2.0 mL min-1, the stack achieved its maximum power output of 141.0 mW at a voltage of 1.88 V) was fed to the stack. 3.2. Dynamic Performance Analysis of the Self-breathing μ-DMFC Stack. In practical applications, batteries are required to power the electronic products under different working conditions. By changing the external loads, we arranged the operating currents of the μ-DMFC stack to vary according to four types of waveforms, including the rectangular, triangular, and trapezoidal and step waves to simulate various conditions possibly occurring in the μ-DMFC applications.
The start-stop is the most common operating mode for batteries, and this characteristic of the μ-DMFC stack was tested under the “instantaneous load-on/instantaneous loadoff” mode. Figure 7 reveals the stack voltage response during the circular process with currents of 30 and 60 mA, and the loading and unloading time were 120 and 180 s, respectively. As seen from this figure, during different loading periods, the stack voltage showed with regularity that it decreased slightly and then increased gradually to a stable value, and the relaxation time increased with the increment of current. During the unloading periods, the voltage first increased to a maximum value which was much higher than the open circuit voltage and then decreased to some extent, and a higher operating current prior to unloading brought a higher maximum value. This is mainly because the electrochemical reaction during the load-on process resulted in a certain consumption of the methanol, which increased with the increment of the operating current, and hence, the methanol crossover effect was instantly weakened when the unloading process began. Besides, the distinction of the voltage fluctuation and relaxation time were also found in different unloading periods, which was probably caused by the nonuniform draining process of produced CO2 bubbles. Another rectangular wave loading mode was realized by the “instantaneous load-up/instantaneous load-down” method, in which the reference current was added by a periodical current with certain amplitude. This mainly simulates the working condition of the electronic device cycling between high and low loads (e.g., the conversion of the wireless communication sensor between the “sleep” mode and “active” mode). Figure 8 indicates the voltage response when the μ-DMFC stack operated between two loads of 10 and 40 mA. The two loading time were both 60 s for the first three cycles, 120 and 60 s, respectively, for the second 6452
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: DOI:10.1021/ef101101q
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Figure 11. Dynamic responses of the μ-DMFC stack voltage under the sustainable load-off/constant loading/sustainable load-off mode.
Figure 9. Dynamic responses of the μ-DMFC stack voltage under the sustainable load-on/instantaneous load-off mode.
The current wave is an isosceles triangle when the loading rate equals the unloading rate under the sustainable load-on/ sustainable load-off mode, as shown in Figure 10. A cycle is explained as enhancing the current linearly from zero to 60 mA and then unloading under the same but reverse pattern, in which the loading rate was 1, 2, and 4 mA s-1, respectively. The results show that the μ-DMFC stack represented superior dynamic characteristics. First, the voltage responded rapidly and smoothly without any apparent fluctuations in all cycles. Second, the response rates were basically consistent with the same loading rate, which represented good reproducibility. Third, the maximum voltage outputs of the stack were all in the field from 2.2 to 2.3 V and not affected by the loading (unloading) rate. However, the transient OCVs were relatively low, even lower than the former stable value, which was different from the regularity shown in Figure 9. The trapezoidal wave loading mode is realized by the “sustainable load-on/constant loading/sustainable loadoff” process with equivalent loading and unloading rates. This mode is mainly suitable for micromovable devices (e.g., microrobots). Figure 11 reveals the dynamic response of the μ-DMFC stack when the loading current varied according to the isosceles trapezoid wave. In the former four cycles, the current gradually increased from zero to 60 mA at 1 mA s-1, followed by a constant loading period of 60 s, and decreased to zero with the same but reverse pattern. Differently, the constant loading time increased to 120 s in the latter four cycles. As seen from Figure 11, the stack responded rapidly with good reproducibility. Comparing with the former and latter cycles, it is clearly observed that the output voltage kept almost the same when constantly discharging and the relaxation times were relatively short. In virtue of the sustained growth of the current, it was not found that the stack voltage decreased sharply due to the methanol starvation in the constant discharging area. The step wave loading mode is achieved with the operating current increasing to a maximum value by several steps, which is different from the triangular wave loading mode by reason of the longer discharging time and lower frequency. This mode mainly simulates the conversion of different operations of the electric device (e.g., the cell phone). Figure 12 shows testing results of the dynamic response when the μ-DMFC stack operated at different loads in succession with the same discharging time of 120 s. It can be seen that
Figure 10. Dynamic responses of the μ-DMFC stack voltage under the sustainable load-on/sustainable load-off mode.
three cycles, and both 120 s for the third three cycles. In the first three cycles, although the duration time was short, the stack voltage could reach the steady state rapidly during the load-up time. And, in the third three cycles, the voltage output started to reach a stable value at the end of the loadup time by reason that the duration time was enhanced and higher than that of the prior cycles. The triangular wave loading mode includes two types, which are the “sustainable load-on/instantaneous load-off” mode (right triangular wave) and “sustainable load-on/ sustainable load-off” mode (isosceles triangle wave). The former represents the process that the current increases to a maximum value with a certain acceleration and then decreases to zero instantaneously, taking the operating condition of a micro electric drill as an example; the variation of the latter is that the load-down process is sustainable, instancing the up-and-down flight of a microaerial vehicle (MAV). The dynamic response of the μ-DMFC stack under the right triangular wave mode is revealed in Figure 9, in which the peak currents were 30, 50, and 70 mA, respectively, and the increasing rate was 1 mA s-1. It can be observed that when the current increased at a uniform rate, the voltage response performed the same in all cycles that first decreasing and then a rapid increment. Furthermore, during different loading periods, the stack responded rapidly and showed good reproducibility with the same load. 6453
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Figure 12. Dynamic responses of the μ-DMFC stack voltage under the constant load/instantaneous load-up/constant load mode.
Figure 14. Durability testing results of the μ-DMFC stack when powering the electronic devices: (a) wireless communication module and (b) micromotor.
good response ability and reproducibility under the prior loading modes, the performance durability could not be ensured if the stack operated under the step mode frequently, which needs to be analyzed and solved in our future work. 3.3. Performance Deterioration Analysis of the Self-breathing μ-DMFC Stack. In addition, as revealed in Figure 13, the metal-based μ-DMFC stack was successfully applied to drive the wireless communication module (rated power 104 mW) and the micromotor (rated power 105 mW) five times within 120 days, each of which lasted for 8 h. Figure 14 provides the transient voltage curves of the stack when powering these two electronic devices. It can be seen from this figure that the stack performance deteriorated to various degrees in the two cases. The attenuation ratios were obtained by calculating the average power output during various operation periods, as shown in Table 2. It reveals that the stack performance deterioration was relatively apparent and aggravated with the increasing days, which can be proved by the fact that the stack performance in the 120th day was only 75.78% and 82.85% of that in the first day. We conclude that the performance deterioration of the stack was mainly attributed to the attenuation of some key components and the harm of the cathode water flooding. As the performance deterioration is an essential issue in the commercialization of the μ-DMFC
Figure 13. Photographs of the application of the μ-DMFC stack to the electronic equipments: (a) wireless communication module and (b) micromotor.
with the increment of cycle times, the stack performance decreased to various degrees. By calculating the average output voltage which was obtained at the current of 10, 30, 50, and 70 mA in every cycle (except for 10 mA in cycle-1), the greatest attenuation ratios were 2.24%, 9.08%, 24.96%, and 16.03%, respectively. Hence, though the stack presented 6454
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Table 2. Performance Attenuation Ratios of the μ-DMFC Stack at Different Days
wireless communication module micromotor
1st day
10th day
30th day
60th day
120th day
0% 0%
4.01% 5.43%
9.94% 7.62%
16.24% 13.76%
24.22% 17.15%
with repeated cycles. Moreover, the μ-DMFC stack exhibits long-term durability in powering two electronic devices of different rated power during four months, but with attenuation ratios of 24.22% and 17.15% on the 120th day, respectively.
system, the structure and materials of the stack should be further improved to prolong the lifetime. 4. Conclusions A self-breathing μ-DMFC stack consisting of ten single cells has been presented in this work. The microstamping technology was successfully applied to manufacture the current collectors on the 300 μm-thick stainless steel plate, and both the stainless steel mesh and the TiN layer were employed in the single cell configuration to improve the performance and stability. According to the testing results, the μ-DMFC stack shows good responding and reproducible characteristics under several operating modes designed for simulating practical applications, but performance degradation is also found
Acknowledgment. We thank the National Natural Science Funds of China (No. 60806037 and No. 61076105), the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT. NSRIF. 2009008) and Key Laboratory Opening Funding of Key Laboratory of Micro-Systems and MicroStructures Manufacturing, Ministry of Education (HIT. KLOF. 2009003) and National Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology of Chongqing University (2009MS03) for financially supporting this work.
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