Fabrication of a miniature twin-fuel-cell on silicon wafer

Transcription

1 Electrochimica Acta 48 (2003) 1537/ Fabrication of a miniature twin-fuel-cell on silicon wafer Jingrong Yu a,b, Ping Cheng a, *, Zhiqi Ma a, Baolian Yi b a Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b Fuel Cell R&D Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , People s Republic of China Received 7 October 2002; received in revised form 10 January 2003 Abstract The fabrication and performance evaluation of a miniature twin-fuel-cell on silicon wafers are presented in this paper. The miniature twin-fuel-cell was fabricated in series using two membrane-electrode-assemblies sandwiched between two silicon substrates in which electric current, reactant, and product flow. The novel structure of the miniature twin-fuel-cell is that the electricity interconnect from the cathode of one cell to the anode of another cell is made on the same plane. The interconnect was fabricated by sputtering a layer of copper over a layer of gold on the top of the silicon wafer. Silicon dioxide was deposited on the silicon wafer adjacent to the copper layer to prevent short-circuiting between the twin cells. The feed holes and channels in the silicon wafers were prepared by anisotropic silicon etching from the back and front of the wafer with silicon dioxide acting as intrinsic etchstop layer. Operating on dry H 2 /O 2 at 25 8C and atmospheric pressure, the measured peak power density was mw/cm 2 at 270 ma/cm 2 for the miniature twin-fuel-cell using a Nafion 112 membrane. Based on the polarization curves of the twin-fuel-cell and the two single cells, the interconnect resistance between the twin cells was calculated to be in the range from V (at 10 ma/cm 2 )to V (at 300 ma/cm 2 ), which is relatively low. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Miniature fuel cell; Twin-fuel-cell; Microfabrication; Silicon wafer; Interconnect resistance 1. Introduction * Corresponding author. Tel.: / ; fax: / address: mepcheng@ust.hk (P. Cheng). The current collector as well as the fuel distributor of a fuel cell is usually made of graphite, carbon-polymer composites, or metals that is rather bulky in size. In the past several years, much attention has been given to the use of microelectronic fabrication technology for the fabrication of miniature fuel cells using silicon wafers as the current collector and flow distributor [1 /6]. The advantages of microelectronic fabrication technology include fine feature resolution, high repeatability, batch operation, integrated process sequences, and a variety of material transfer options [6,7]. However, all of the above-mentioned research work focused on using such technology for the fabrication of a single fuel cell. It has been shown that microelectronic fabrication techniques are promising for fuel cell miniaturization with practically no compromise in cell performance. In practice, a single fuel system can usually produce a low voltage of around 1.0 V. The conventional filter-press configuration of the cell stack overcomes the low-voltage problem by stacking up a number of cells one after another with bipolar plates. However, the large number of cells and components, as well as complicated gas distribution and stack geometry lead to low energy per unit mass or volume of the conventional system. In order to design a compact fuel-cell system, alternative approaches have been suggested [8 /12]. Heinzel et al. [8,9] proposed a fuel cell stack of banded membrane configuration (see Fig. 1a), where an interconnect was employed to conduct electricity from the cathode of one cell to the anode of the adjacent cell. Subsequently, Narayanan et al. [10] reported a flat-pack design while Hockaday et al. [11,12] developed a non-bipolar fuel cell stack configuration, both of which have a structure similar to those shown in Fig. 1a. However, Narayanan et al. [10] show that the flat-pack design has a higher electrical resistance compared to that of a bipolar plate stack with the same active electrode area and same number of cells /03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)

2 1538 J. Yu et al. / Electrochimica Acta 48 (2003) 1537/1541 Fig. 1. Fuel cell stack of banded and flip-flop configuration: (a) banded structure stack; (b) flip-flop structure stack. From manufacturing point of view, it is not so easy to fabricate the interconnect of the flat-pack design because it is a three-dimensional structure which flops from upside of one cell to downside of the adjacent cell (see Fig. 1a). Recently, Lee et al. [6] proposed a novel flip-flop interconnect configuration (see Fig. 1b) for multiple cells design, where the interconnect from the cathode of one cell to the anode of the next cell was located on the same plane. As far as the authors are aware, no data have been published in the literature to support the practicality of such a design. In this paper, we have successfully fabricated a twinfuel-cell with silicon wafers as the current collector and flow distributor, using the microelectronic fabrication technology. A flip-flop interconnect configuration was used to connect the cathode of one cell to the anode of the adjacent cell on the same plane. The performance of the twin-fuel-cell was measured and the resistance of such an interconnect configuration was calculated based on the polarization curves of the twin-fuel-cell and the two single cells. 2. Experimental The present design consists of fabricating a miniature fuel cell stack on silicon wafers and the use of sputtering metal conductor acting as the electricity interconnect between the two adjacent cells. Fig. 2 is a schematic diagram of a miniature twin-fuel-cell in series used in this experiment. Fig. 2. The structure of miniature twin-fuel-cell on silicon wafer Fabrication of twin-fuel-cell on silicon wafer In this work, the polar plate of the miniature twinfuel-cell made from a silicon wafer was prepared following a series of microelectronic fabrication steps similar to those discussed by Kelley et al. [3,4], Sim et al. [5], and Madou [7]. The original material was a 5259/25 mm thick p-type Ž1 00 silicon wafer with resistivity ranging from 15 to 25 V cm. The first step was to deposit silicon dioxide layer on the wafer for Si etch mask. To make feed holes, the silicon dioxide on the backside of the wafer was patterned by photolithography. Dry etching was first applied on silicon dioxide of the backside wafer and then the exposed surface of silicon was etched with tetramethylammonium hydroxide (TMAH) solution at about 90 8C. To make microchannels in alignment with feed holes, the photolithography machine with infrared ray was employed. The silicon dioxide on the frontside of the wafer was patterned, and the frontside wafer was etched using the same processes mentioned above. Subsequently, 1.0- mm layer of silicon dioxide was deposited on the entire surface of the silicon wafer again. There are two reasons for depositing silicon oxide layer for the second time: (1) to insulate the twin cells electrically because silicon

3 J. Yu et al. / Electrochimica Acta 48 (2003) 1537/ wafer is a semiconductor material (upside silicon wafer in Fig. 2), and (2) to ensure that there was no fresh silicon surface exposed in the channels and holes so that the metal layer would bond well in contact with the silicon wafer. Finally, three sputtering steps were sequentially applied to the frontside of the silicon wafer: (1) 50 nm of titanium/tungsten layer as adhesive, (2) 1.0 mm of a copper layer, and (3) 0.5 mm of a gold layer. Both the sputtered copper and gold layers were used to act as current collector and interconnect between the twin cells simultaneously. Note that the empty space of the metal composite layer on upside silicon wafer in Fig. 2 was formed by a lift-off step. In this twin-fuel-cell design, single-path channels were patterned to act as flow structure. The photograph of the feed holes on the backside of the silicon wafer is shown in Fig. 3a, and the channels and ribs on the frontside of the silicon wafer is shown in Fig. 3b. The geometry of the trapezoidal cross section (see the enlarged figure in Fig. 2) of the channels on silicon wafer was the result of anisotropic wet etching in TMAH solution. Using the surface profiler, geometric parameters were measured as follows: a/409.2 mm, b/ mm, c/207.7 mm, and d /591.8 mm, where a represents top width, b bottom width, c height of the trapezoidal, and d is rib width. The effective area of the single cell was 3 cm 2 (10 mm/30 mm). The electricity interconnect between the twin cells is on the same plane, which was fabricated by sputtering a layer of copper and a layer of gold on the top of the silicon wafer (downside silicon wafer in Fig. 2). Silicon dioxide was deposited on the silicon wafer adjacent to the composite metal layer to prevent short-circuiting between the two cells (upside silicon wafer in Fig. 2). The geometry of the interconnect between the twin cells was 10 mm in length and 30 mm in width. The membrane-electrode-assembly (MEA) components used in this system are similar to those typically used in proton exchange membrane fuel cell. The pretreatment of Nafion 112 membranes was performed by the following four sequential boiling steps [13]: in 3% H 2 O 2 solution, in deionized water, in 0.5 M H 2 SO 4 solution, and in deionized water, each of the above steps lasted for 1 h. The same anode and cathode electrodes consisting of catalysts loaded onto a supported carbon paper, with Pt loading of 1 mg/cm 2, were purchased from Electrochem, Inc. Prior to the hot-press procedure, both the anode and the cathode were sprayed with the Nafion solution with dry Nafion loading of 1 mg/cm 2 (geometric surfaces area). Two electrodes with effective area of 3 cm 2 (10 mm/3 mm) were hot-pressed to one piece of Nafion 112 membrane at the conditions of 135 8C and 30 MPa to form MEA. In the present design, two separated MEAs with the same structure and fabrication conditions were positioned between the two pieces of silicon wafer (see Fig. 2). All the void spaces around the two MEAs were filled by silicone rubber gaskets in order to seal the gas leakage. The entire assembly was then clamped mechanically for performance evaluation. The miniature twin-fuel-cell was installed in an Arbin fuel cell test station Cell performance evaluation Fig. 3. Photograph of the feed holes, channels, and ribs on silicon wafer: (a) feed holes on the backside; (b) channels and ribs on the frontside. To evaluate cell performance, pure hydrogen and oxygen without any humidification were used as fuel and oxidant under atmospheric pressure. The gas flow configuration was in series. That is, H 2 or O 2 was fed to cell 1, and the exhaust from cell 1 was fed to cell 2. The fuel cell was operated at the room temperature (25 8C). The flow rates of H 2 and O 2 were controlled at 40.0 and 60.0 ml/min, respectively. Before collecting data, the twin-fuel-cell was allowed to equilibrate for 48 h at a current density of 100 ma/cm 2. The discharge of the fuel cell was controlled with an electric load system (BT2000, Arbin Instrument, Inc.). This Arbin test system had three current ranges (10 ma, 1 A, and 10 A) and a sensitivity of 0.1 mv and 0.1 ma. The voltages of the twin-fuel-cell and the two single cells at various currents or current densities could be simultaneously collected from main and auxiliary channels, respectively.

4 1540 J. Yu et al. / Electrochimica Acta 48 (2003) 1537/1541 Fig. 4. Performance curves of the twin-fuel-cell and the two single cells: cell temperature at 25 8C, dry H 2 /O 2 gas pressure at 0.10/0.10 MPa, and gas flow rates of H 2 /O 2 at 40/60 ml/min. 3. Results and discussion Fig. 4 shows the performance curves of the twin-fuelcell and two single cells operating at 25 8C and 0.10/0.10 MPa of dry H 2 /O 2. The shape of potential versus current density curve is typical for a PEMFC. The initial drop of the polarization curve at very low current density was due to an electrochemical activating process, which was caused by the sluggish kinetics of oxygen reduction at the cathode surface. The subsequent linear decrease of the polarization curve was due to ohmic over-potential, which was attributed to the ion flow through the electrolyte membrane, the electron flow through the electrode materials, and current collector. A comparison of the performance of the two single cells is also presented in Fig. 4. It is shown that cell 1 had a slight better performance than that of cell 2 at low current density. This is because cell 1 was located near the inlet of reactant gases and had a higher gas pressure. With the increase of current density, however, cell 2 had a better performance than that of cell 1. This is because the water emitted from the cell 1 had a humidifying effect on cell 2, thus improving its performance. The measured peak power densities for cells 1 and 2 were 92.3 and mw/cm 2, respectively. Due to the interconnect resistance, the measured peak power density of mw/cm 2 for the twin-fuel-cell was slightly lower than the sum of those of cells 1 and 2. Fig. 5 shows the equivalent circuit of the twin-fuel-cell for the purpose of evaluating the resistance of the interconnect between the twin cells. In this figure, R in represents the interconnect resistance between the twin cells, R the resistance added from electronic loader and I the cell current, V 1 and V 2 denote the cell voltages of cells 1 and 2, respectively, and V represents the total Fig. 5. Equivalent circuit of the twin-fuel-cell design. voltage of the twin-fuel-cell unit. Due to the interconnect resistance, the value of V equals to the sum of V 1 and V 2 minus R in I. Therefore, the interconnect resistance (R in ) can then be calculated from the following equation: R in V 1 V 2 V : (1) I The values of R in at various current densities were calculated according to Eq. (1), and the results are presented in Fig. 6. It is shown that the value of R in ranges from V (at 10 ma/cm 2 ) to V (at 300 ma/cm 2 ). The interconnect resistance R in appears to have an upward trend as the current density is increased. It is likely that the contact resistance between the carbon paper electrode and the composite metal conductor increases as the current density is increased. At current densities less than 10 ma/cm 2, the calculated R in value fluctuated greatly because any inaccuracy in small current measurements in the denominator of Eq. (1) would result in a large error. Therefore, these values are not presented in Fig. 6.

5 J. Yu et al. / Electrochimica Acta 48 (2003) 1537/ Fig. 6. The value of R in at various current densities calculated according to Eq. (1). 4. Concluding remarks In this paper, a miniature twin-fuel-cell in series on silicon wafer has been successfully fabricated by microelectronic fabrication techniques including spin coating, photolithography, dry and wet etching, chemical and physical vapor deposition, etc. Operating on dry H 2 /O 2 at 25 8C and atmospheric pressure, the measured peak power density was mw/cm 2 for the miniature twin-fuel-cell using Nafion 112 membrane. Based on the cell polarization curves of the twin-fuel-cell and the two single cells, the interconnect resistance between the twin cells can be calculated to be in the range from to V, which is relatively low. Therefore, the flip-flop interconnect is promising for a practical multiple cell system in the low power range. Acknowledgements The authors acknowledge that this work was supported by the grants HKUST6014/02E as well as HIA 98/99.EG04. References [1] L. Mex, N. Ponath, J. Muller, Fuel Cells Bull. 39 (2002) 9. [2] J.P. Meyers, H.L. Maynard, J. Power Sources 109 (2002) 76. [3] S.C. Kelley, G.A. Deluga, W.H. Smyrl, AIChE J. 48 (2002) [4] S.C. Kelley, G.A. Deluga, W.H. Smyrl, Electrochem. Solid State Lett. 3 (2000) 407. [5] W.Y. Sim, G.Y. Kim, S.S. Yang, IEEE Int. Conf. Microelectron. Mech. Syst. 14 (2001) 341. [6] S.J. Lee, S.W. Cha, Y. Liu, R. O Hayer, F.B. Prinz, in: Proceedings of the 198th Electrochemical Society Meeting, Abstract 241, Phoenix, 22/27 October [7] M. Madou, Fundamentals of Microfabrication, CRC Press, Boca Raton, FL, [8] A. Heinzel, R. Nolte, K. Ledjeff-Hey, M. Zedda, Electrochim. Acta 43 (1998) [9] A. Heinzel, C. Hebling, M. Muller, M. Zedda, C. Muller, J. Power Sources 105 (2002) 250. [10] S.R. Narayanan, T.I. Valdez, F. Clara, Fuel Cell Seminar Abstracts, Vol. 795, Portland, OR, 30 October/2 November [11] R.G. Hockaday, M. Dejohn, C. Navas, P.S. Turner, H.L. Vaz, L.L. Vazul, Fuel Cell Seminar Abstracts, Vol. 791, Portland, OR, 30 October/2 November [12] R.G. Hockaday, US Patent No (2001). [13] X. Du, J. Yu, B. Yi, M. Han, K. Bi, Phys. Chem. Chem. Phys. 3 (2001) 3175.