A micro-direct methanol fuel cell stack with optimized design and microfabrication

Available online at www.sciencedirect.com

Sensors and Actuators A 143 (2008) 70–76

A micro-direct methanol fuel cell stack with optimized design and microfabrication Lingyan Zhong a , Xiaohong Wang a,∗ , Yingqi Jiang a , Qian Zhang a , Xinping Qiu b , Yan’an Zhou a , Litian Liu a a

Institute of Microelectronics, Tsinghua University, Beijing 100084, PR China b Department of Chemistry, Tsinghua University, Beijing 100084, PR China

Received 31 March 2007; received in revised form 29 June 2007; accepted 30 June 2007 Available online 17 July 2007

Abstract This paper presents a micro direct methanol fuel cell (␮DMFC) stack with the advantages of compact structure and simple fuel delivery. The ␮DMFC stack consists of two ␮DMFC cells stacked one by one with the anode plate shared. The dimensions of the flow channels are optimized by modeling and simulation. Microfabrication and PDMS assembly are used to implement the ␮DMFC stack. Experiment results show that the ␮DMFC stack generates a power density of 12.71 mW/cm2 , 39.3% higher than that of a ␮DMFC cell, at the cost of only 23% increase in volume. © 2007 Elsevier B.V. All rights reserved. Keywords: DMFC stack; DMFC modeling; Flow structure design; PDMS packaging; Micro fuel cell

1. Introduction The rapid development of portable electronic products requires micro power sources (MPS) with high performance. Among the candidates of the future MPS, DMFC is competitive for the advantages of potential high energy density, low pollution, quick recharging and room temperature operation, etc. [1–4]. Great progress has been made in the membrane electrode assembly (MEA), water and thermal management, reactant/product delivery, and fuel processing, etc. [5–12]. Unfortunately, the output power density of a ␮DMFC cell is still not high enough to meet the requirements of practical applications. Therefore, a ␮DMFC stack is a necessary and facile way to enhance the output performance to satisfy the requirements of most portable applications. Micro fuel cell arrays with the planar structure have been reported with the advantages of a shared proton exchange membrane (PEM) and the conventional assembly like a single fuel cell. In a fuel cell array, the single cells are placed side by side so that the cathode of one cell straps across the membrane to the anode of the next adjacent cell [13]. However, the planar struc-

∗

Corresponding author. E-mail address: [email protected] (X. Wang).

0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.06.045

ture also has the disadvantages of large area, difficult electrical interconnection, and lateral ionic conduction in the continuous PEM. Lee et al. reported an improved prototype of the planar structure called the “flip–flop” configuration. Reactant chambers alternate between fuel and oxidant in the “flip–flop” configuration so that the adjacent fuel cells can be laterally interconnected by thin-film metal layers on the plates and no interconnecting bridges are needed across or around the PEM [14]. The peak power density of a four-cell prototype has exceeded 40 mW/cm2 using hydrogen and oxygen as the reactants. Seo and Cho [15] fabricated a silicon-based portable DMFC and its stacks featured by microcolumn electrodes and a built-in fuel chamber. Platinum is sputtered directly on the microcolumn electrodes as the catalyst. A DMFC cell with the microcolumn electrodes shows 3.2 times larger power density than that without the microcolumn electrodes. A four-cell stack with the “flip–flop” structure generates the maximum power density of 329.8 ± 0.47 ␮W/cm2 . This paper presents the design and fabrication of a ␮DMFC stack which consists of two ␮DMFC cells piled up together [16]. Compared with the planar array, the stacked structure needs much smaller area which efficiently reduces the entire volume because the packaging is generally much thicker than the miniature fuel cell itself. In other words, as the packaging takes up much larger room than the MEA and plates, a ␮DMFC stack, which has a packaging almost as large as that of a single

L. Zhong et al. / Sensors and Actuators A 143 (2008) 70–76

71

cell, significantly enhances the volume utility ratio. Besides, the parasitic lateral ionic conduction is avoided because the MEAs are totally separated. The flow structure of the ␮DMFC is optimized by the DMFC modeling and simulation and implemented by microfabrication process. The assembly technology using PDMS and aluminum holders is employed to complete the ␮DMFC fabrication. 2. Design and optimization Fig. 1 illustrates the configuration of the ␮DMFC stack which consists of two ␮DMFC cells in electrically parallel connection. Aqueous CH3 OH solution and oxygen gas are supplied respectively by the peristaltic pumps to the catalyst surface of the MEAs where electrochemical reactions happen. CO2 , H2 O, protons, and electrons, etc. are produced by the electrochemical reactions. The electrons flow to the external circuit to form the electrical current as a result. To make the structure compact, the adjacent anode plates of the two ␮DMFC cells are combined into one shared plate with flow patterns on both sides. The two-sided flow patterns can be designed into various shapes. For the purpose of simplifying the supply equipments and achieving efficient product removal, series mass transport is adopted. For instance, serpentine flow channels are arranged in parallel on the two sides of the middle plate, and connected via a thru-etched hole as shown in Fig. 2. The upper and lower plates act as the cathodes, which have the same serpentine patterns as the anode has. The flow patterns of the upper and lower plates are connected by an external feeding tube so that oxygen gas can flow through the cathodes successively. Because the flow channels of the anode are totally covered by the cathode plates, the leading channels are designed to allow the aqueous methanol to flow into the effective channel area. The leading channels actually act as the extension of the feeding tubes. Experiments and modeling using mathematic or computational fluid dynamic methods have been investigated to optimize the fuel cell performances in recent years [17–20]. In this paper, a 3D mass transport and electrochemical model is set up to optimize the DMFC performance. The cathode over-potential is assumed to be constant, which is reasonable on the condition that sufficient pure oxygen gas is supplied, to simplify the computational complexity. The structure schematic of the 3D DMFC

Fig. 1. Schematic of the ␮DMFC stack.

Fig. 2. The flow patterns of the shared anode plate.

Fig. 3. The schematic of the 3D ␮DMFC model.

model is shown in Fig. 3. The parameters are defined and listed in Table 1. Assuming steady-state, isothermal operation, laminar flow in channels, isotropic porosity in MEA, and single-phase flow when the production CO2 is neglected, the mass transport in the flow channels can be expressed as follows according to Table 1 Nomenclature α γ ε ρ η cm c0 D Eth F I i0 Mm N R S r T u  V Yi Yj

Change transfer coefficient (0.5) Order of reaction (0.2) Porosity Density (kg m−3 ) Over-potential (V) Local methanol concentration (mol l−1 ) Reference methanol concentration (2 mol l−1 ) Diffusion coefficient (m2 s−1 ) Thermodynamically predicted voltage (1.2 V) Faraday constant (96 487 C mol−1 ) Current density (A m−2 ) Exchange current density (A m−3 ) Molar weight of methanol (3.2 × 104 kg mol−1 ) Molar ratio of reacting electrons to methanol (6) Universal gas constant (8.314 J mol−1 K−1 ) The methanol consuming rate (kg m−3 s−1 ) Resistance () Operating temperature (300 K) Velocity vector (m s−1 ) DMFC output voltage Methanol mass fraction in the methanol solution Water mass fraction in the methanol solution

72

L. Zhong et al. / Sensors and Actuators A 143 (2008) 70–76

the mass conversation law [21]: ∇(ρ uYi ) = ∇(ρDii ∇Yi ) + ∇(ρDij ∇Yj ) + S

(1)

The catalyst layers composed of catalyst particulates and the diffusion layers composed of porous materials are modeled as the porous media, where the mass transport equation is modified by Darcy’s law and becomes: ∇(ρε uYi ) = ∇(ρεDii ∇Yi ) + ∇(ρεDij ∇Yj ) + S

(2)

As the electrochemical reactions happen only on the catalyst surface, the current density can be derived by integrating the bulk current density in the catalyst layer in the form of the Butler–Volmer equation:  γ     cm αNF exp i0 η i= c0 RT catalyst layer   αNF η dz (3) − exp − RT The source term in the mass transport equation in the catalyst layer represents the consuming rate of the CH3 OH mass and can be mathematically described as a function of the bulk current density: Mm di S=− NF dz

(4)

In all the other places in the DMFC model except the catalyst layer, the source term is 0 because no CH3 OH is generated or consumed. The DMFC output voltage is often described by subtracting anode, cathode and ohmic over-potentials from the thermodynamically predicted voltage: V = Eth − ηanode − ηcathode − I × r

(5)

The operating voltage and the current density are calculated by simulating the aforementioned model under given anode and cathode over-potential values. Dirichlet and Neumann boundary conditions are used to solve the hydrodynamics transport equations. The stoichiometric proportion and the flow velocity of the reactants at the inlet are given previously as well. The DMFC performance is affected by the reactant delivery in the flow structure. Thus the simulation is focused on optimizing the flow structure dimensions in order to achieve uniform fuel distribution and fluent mass transport. The ␮DMFCs with different channel width have been numerically analyzed, all of which have serpentine flow patterns and the same effective area of 6.9 mm × 6.9 mm. The simulated performances are plotted in Fig. 4. It is indicated that 0.4 mm channel width has an excellent balance between fuel delivery and current collection and achieves the maximum peak power density within the channel width range of 0.2–0.5 mm. For the ␮DMFC stack that has the fuel delivered successively in series, the methanol concentration of the second ␮DMFC cell (labeled cell 2) is always lower than that of the first one (labeled cell 1). However, taking the inlet for example, the methanol concentration difference between the two ␮DMFC cells is less

Fig. 4. Plot of the simulated ␮DMFC power density. The numbers in the legend represent the channel width.

than 3%, and the maximum output power density of the ␮DMFC stack is 99.8% higher than that of cell 1, as Fig. 5 shows. 3. Fabrication The microfabrication process of the anode and cathode silicon plates is shown in Fig. 6. Flow channels and feeding holes were formed on both sides of a 400 ␮m thick 1 0 0 silicon wafer. The detailed steps are as followed: (a) thermal oxide and LPCVD Si3 N4 were deposited on both sides of the silicon wafer as the mask layers; (b) double-side lithography was used to transfer the flow patterns on both side of the silicon wafer. Reaction ion etch (RIE) and buffered HF solution were used to remove the Si3 N4 and SiO2 mask layers under the developed photoresist; (c) KOH wet etching was employed to anisotropically etch the substrate at the rate of 1 ␮/min until the thru-etched holes and the feeding ˚ Ti/Pt was sputtered on holes were thru-etched; (d) finally 2000 A both sides of the silicon wafer to improve the current collecting capability. The double-side lithography technology allows us to implement the complicated flow patterns of the anode and

Fig. 5. Plot of the simulated power density of the ␮DMFC cells and stack and the ratio of the input methanol concentrations of two ␮DMFC cells.

L. Zhong et al. / Sensors and Actuators A 143 (2008) 70–76

73

Fig. 6. MEMS fabrication process of the anode and cathode plates of the ␮DMFC stack. Fig. 8. PDMS assembly process.

cathode plates on the same wafer using exactly the same process, and greatly decreases the KOH etching time compared with our previous method [22]. A ␮DMFC cell was fabricated by the same process as the stack cathode plate. The microscopy and the SEM images of the fabricated plates are shown in Fig. 7. The MEA was fabricated by hot pressing two catalyst-coated (anode: 4.0 mg/cm2 Pt–Ru, cathode: 1.5 mg/cm2 Pt) wet-proof carbon papers together with the PEM, Nafion® 117 in between. 4. Assembly Polydimethylsiloxane (PDMS) is used as the packaging material for its chemical inertness (especially to CH3 OH) and excellent flexibility to keep the packaging hermetic which satisfies the critical requirements of the ␮DMFC assembly [23]. Before the assembly process, PDMS was molded into membranes at the thickness of about 200–300 ␮m and cut into small

pieces of proper shapes. The prefabricated PDMS pieces were then used in the packaging for three purposes. First the PDMS sealing layers were used to keep the MEAs, leading channels and feeding holes from leakage. Secondly the PDMS buffering layers were used to protect the fragile silicon plates against mechanical destruction. Finally the PDMS blocks were used to fix the feeding tubes. The assembly process of each parts of the ␮DMFC can be found in Fig. 8. PDMS is an excellent material for sealing and buffering. However, it is too flexible to press the components tightly together. Therefore, the aluminum holders were employed to provide a uniform pressure over the whole plate area. This solution not only makes the packaging more compact but also enhances the ␮DMFC performance because the contact between the MEA and the silicon plates is improved and the current collecting capability is raised. No leakage was found during the

Fig. 7. The anode plate of the ␮DMFC stack (a); the cathode plate of the ␮DMFC stack (b); SEM cross-section view of the stack anode plate (c). Table 2 Dimension comparison of the ␮DMFC cell and stack (unit: mm)

␮DMFC stack ␮DMFC cell a b

Total sizea

Plate area

Thicknessb

Channel area

Channel width

Channel depth

Rib width

22.8 × 25.0 × 6.4 22.8 × 25.0 × 5.2

22.8 × 15.9 20.8 × 12.8

2.8 1.6

6.8 × 8.0 6.8 × 6.8

0.4 0.4

0.2 0.2

0.4/1.2 0.4

Including holders and exposed lead pads. Plates and MEAs only.

74

L. Zhong et al. / Sensors and Actuators A 143 (2008) 70–76

Fig. 9. Components of the ␮DMFC stack (a); assembled ␮DMFC stack (b); assembled ␮DMFC cell (c).

daylong testing, indicating that this assembly method is effective and reliable. In addition, this assembly technology has the advantage of convenient reassembly. The ␮DMFC cell and stack made by the same fabrication and assembly process are shown in Fig. 9, and the detailed dimensions are listed in Table 2. 5. Results and discussion The assembled ␮DMFC stack was tested on an electrochemical interface, Solartron SI1287 under room temperature. One molar of aqueous methanol was supplied by a peristaltic pump, while sufficient pure oxygen gas was provided at the flow rate of 20 ml/min. The experimental data are obtained by the constant current method, which means that the ␮DMFC is forced to work under a constant current for a period until the output voltage is stable and recorded at the steady-state value. The output voltage and power density versus the current density of the ␮DMFC stack at aqueous methanol flow velocities of 0.1, 0.2, and 0.4 ml/min are plotted in Fig. 10. The experiment results indicate that the performance of the ␮DMFC stack

Fig. 10. Performance of the ␮DMFC stack at different CH3 OH flow velocities and pure O2 at 20 ml/min at 25 ◦ C.

improves at high methanol flow velocity. However, more power is consumed by the peristaltic pump to provide higher flow velocity, which will impair the advantage of the high flow velocity. According to I/V–I/P curves, the prototype has an open circuit voltage of 0.47 V and the maximum power density of 12.71 mW/cm2 at the flow rate of 0.40 ml/min. A ␮DMFC cell, which used the cathode plates of the ␮DMFC stack as the anode and cathode plates, was tested under the same operating conditions. Fig. 11 illustrates the comparison of the performances of the ␮DMFC cell and stack. It is indicated that the performance of the ␮DMFC stack is 39.3% higher than that of the ␮DMFC cell, which has the maximum power density of 9.12 mW/cm2 . A possible reason is that the dilution of CH3 OH solution is more severe than the simulated assumption. Fuel is not only consumed by the electrochemical reactions in ␮DMFC, but also depleted by concentration diffusion across the PEM, electro-osmosis, and side reactions, etc. Experimentally, the electrochemical reactions only consume 7.5% of the total amount of the consumed CH3 OH [23]. As a result, the performance of a real ␮DMFC stack would be lower than that

Fig. 11. Performance comparison of the ␮DMFC cell and stack with 1 M CH3 OH solution at 0.4 ml/min and pure O2 at 20 ml/min at 25 ◦ C.

L. Zhong et al. / Sensors and Actuators A 143 (2008) 70–76

of the simulation results, especially under the situation of high current density. The current leakage between the two ␮DMFC cells impairs the ␮DMFC stack performance too. The prototype has a better utilization factor of fuel compared with the ␮DMFC cell because of the double reaction area. According to Table 2, the ␮DMFC stack is only 23% larger than the ␮DMFC cell, because the compact structure and assembly introduces very little increase in the cell area and packaging volume. 6. Conclusions A silicon-based ␮DMFC stack with compact structure has been designed, fabricated, and characterized. The simulated results show that the flow channel width has an optimized value about 0.4 mm. Microfabrication technology was used to implement both the anode and cathode silicon plates on the same wafer. The assembly technology using PDMS and the aluminum holders was employed. Experiment results show that the ␮DMFC stack generated a power density of 12.71 mW/cm2 , 39.3% higher than the power density generated by a ␮DMFC cell at room temperature, while the volume of the ␮DMFC stack was only 23% larger than that of the single cell. The stack structure and PDMS assembly technology are expected applicable for stacking more cells to meet the demands of high-power electronic devices. Acknowledgement This project (No. 90607014) is supported by the National Natural Science Foundation of China.

75

[11] J.D. Holladay, E.O. Jones, M. Phelps, J. Hu, Microfuel processor for use in a miniature power supply, J. Power Sources 108 (2002) 21–27. [12] S. Tanaka, K.-S. Chang, K.-B. Min, D. Satoh, K. Yoshida, M. Esashi, MEMS-based components of a miniature fuel cell/fuel reformer system, Chem. Eng. J. 101 (2004) 143–149. [13] A. Heinzel, R. Nolte, K. Ledjeff-Hey, M. Zedda, Membrane fuel cells—concepts and system design, Electrochim. Acta 43 (1998) 3817–3820. [14] S.J. Lee, A. Chang-Chien, S.W. Cha, R. O’Hayre, Y.I. Park, Y. Saito, F.B. Prinz, Design and fabrication of a micro fuel cell array with “flip–flop” interconnection, J. Power Sources 112 (2002) 410–418. [15] Y.H. Seo, Y.-H. Cho, MEMS-based direct methanol fuel cells and their stacks using a common electrolyte sandwiched by reinforced microcolumn electrodes, in: Tech. Digest MEMS Conf., Maastricht, Netherlands, 2004, pp. 65–68. [16] L. Zhong, X. Wang, Y. Jiang, X. Qiu, Y. Zhou, L. Liu, A silicon-based micro direct methanol fuel cell stack with compact structure and PDMS packaging, in: Tech. Digest MEMS Conf., Kobe, Japan, January 21–25, 2007, pp. 891–894. [17] M. Kunimatsu, T. Shudo, Y. Nakajima, Study of performance improvement in a direct methanol fuel cell, JSAE Rev. 23 (2002) 21–26. [18] R. Sousa Jr., E.R. Gonzalez, Mathematical modeling of polymer electrolyte fuel cells, J. Power Sources 147 (2005) 32–45. [19] S.W. Cha, R. O’Hayre, Y. Saito, F.B. Prinz, The scaling behavior of flow patterns: a model investigation, J. Power Sources 13 (2004) 57–71. [20] K. Scott, P. Argyropoulos, K. Sundmacher, A model for the liquid feed direct methanol fuel cell, J. Electroanal. Chem. 477 (1999) 97–110. [21] T. Berning, D.M. Lu, N. Djilali, Three-dimensional computational analysis of transport phenomena in a PEM fuel cell, J. Power Sources 106 (2002) 284–294. [22] Y. Jiang, X. Wang, K. Xie, X. Qiu, L. Zhong, L. Liu, A micro direct methanol fuel cell using PDMS assembly technology, in: Tech. Digest Transducers ’05, vol. 1, Seoul, Korea, June 5–9, 2005, pp. 303–306. [23] Y. Jiang, X. Wang, L. Zhong, L. Liu, Design, fabrication and testing of a silicon-based air-breathing micro direct methanol fuel cell, J. Micromech. Microeng. 16 (2006) 233–239.

Biographies References [1] E. Sakaue, Micromachining/nanotechnology in direct methanol fuel cell, in: Tech. Digest MEMS Conf., Miami, USA, January 30–February 3, 2005, pp. 600–605. [2] R. O’Hayre, S.-W. Cha, W. Colella, F.B. Prinz, Fuel Cell Fundamentals, 1st ed., John Wiley & Sons, New York, 2006, pp. 3–6. [3] S.F.J. Flipsen, Power sources compared: the ultimate truth? J. Power Sources 162 (2006) 927–934. [4] G.Q. Lu, C.Y. Wang, T.J. Yen, X. Zhang, Development and characterization of a silicon-based micro direct methanol fuel cell, Electrochim. Acta 49 (2004) 821–828. [5] M. Umeda, H. Ojima, M. Mohamedi, I. Uchida, Methanol electrooxidation at Pt–Ru–W sputter deposited on Au substrate, J. Power Sources 136 (2004) 10–15. [6] S.J. Lee, S. Mukerjee, E.A. Ticianelli, J. McBreen, Electrocatalysis of CO tolerance in hydrogen oxidation reaction in PEM fuel cells, Electrochim. Acta 44 (1999) 3283–3293. [7] Z.Q. Ma, P. Cheng, T.S. Zhao, A palladium-alloy deposited Nafion membrane for direct methanol fuel cells, J. Membr. Sci. 215 (2003) 327–336. [8] R. Eckl, W. Zehtner, C. Leu, U. Wagner, Experimental analysis of water management in a self-humidifying polymer electrolyte fuel cell stack, J. Power Sources 138 (2004) 137–144. [9] X. Yang, Z. Zhou, H. Cho, X. Luo, Study on a PZT-actuated diaphragm pump for air supply for micro fuel cells, Sens. Actuator A 130–131 (2006) 531–536. [10] D.D. Meng, J. Kim, C.-J. Kim, A degassing plate with hydrophobic bubble capture and distributed venting for micro fluidic devices, J. Micromech. Microeng. 16 (2006) 419–424.

Lingyan Zhong is pursuing a MS degree at the Institute of Microelectronics, Tsinghua University. She received her BS degree from the Department of Electronic Engineering, Tsinghua University in 2004. Her main research interests include the fuel cell stack, the DMFC modeling and flow structure optimization. Xiaohong Wang received the PhD degree from Department of Precision Instruments and Mechanology, Tsinghua University, in 1998. She is an associate professor at the Institute of Microelectronics, Tsinghua University. She is focusing on the field of MEMS devices, micro-fuel cells and the new method of integrating MEMS. Yingqi Jiang received his BS and MS degrees in electrical engineering in Tsinghua University in 2003 and 2006, respectively. He is currently pursuing a PhD degree in MEMS/nano at University of California at Berkeley. His main research interests include the design and fabrication of MEMS devices/systems and nanotechnology. Qian Zhang is pursuing a MS degree at the Institute of Microelectronics, Tsinghua University, Beijing, China. He received his BS degree from the Department of Electronic Engineering, Tsinghua University in 2006. His main research interests include fuel cell thermal management, DMFC modeling and structure optimization. Xinping Qiu is an associate professor in the Department of Chemistry, Tsinghua University. His research is focused on the chemical sensors and the electrochemical power sources, including lithium ion battery and fuel cell. Yan’an Zhou is a PhD candidate at the Institute of Microelectronics, Tsinghua University, Beijing. He received his BS degree from the Department of Electronic Engineering, Tsinghua University in 2004. His main research includes the design

76

L. Zhong et al. / Sensors and Actuators A 143 (2008) 70–76

and fabrication of micro-DMFC system and the thermal and water management of micro-DMFC system. Litian Liu received the BS degree from the Department of Electronics Engineering, Tsinghua University in 1970. Since then, he has been working on the research and development of semiconductor devices and ICs. He is

currently a professor at the Institute of Microelectronics, Tsinghua University. His research fields include micro-electro mechanical systems (MEMS), micro-sensors, bio-chips and novel semiconductor devices. Prof. Liu has authored or co-authored more than 150 technical papers on peer-reviewed journals.