Mems-based Design And Fabrication Of A New Concept Micro

Electrochemistry Communications 6 (2004) 562–565 www.elsevier.com/locate/elecom

MEMS-based design and fabrication of a new concept micro direct methanol fuel cell (l-DMFC) Shinji Motokawa a, Mohamed Mohamedi a,*, Toshiyuki Momma Shuichi Shoji c, Tetsuya Osaka a

a,b

,

a

c

Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan b CREST, Japan Science and Technology Agency, Japan Department of Electronics, Information and Communication Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-Ku, Tokyo 169-8555, Japan Received 25 March 2004; received in revised form 13 April 2004; accepted 13 April 2004 Available online 30 April 2004

Abstract A design for a novel micro direct methanol fuel cell (l-DMFC) of 0.018 cm2 active area is described. The l-DMFC was prepared using a series of fabrication steps from micro-machined silicon wafer including photolithography, deep reactive ion etching, and electron beam deposition. The novelty of this structure is that we have fabricated the anodic and cathodic micro-channels arranged in plane, dissimilar to the conventional bipolar structure. The first objective of the experimental trials was to verify the feasibility of this novel structure on basis of MEMS technology. Preliminary testing results show that this new concept l-DMFC generates electricity.
2004 Elsevier B.V. All rights reserved. Keywords: Micro-fuel cell; Micro-power; Methanol; Micro-DMFC; Electroplating

1. Introduction Several groups are actively engaged in the development of low power DMFCs for cellular phone, laptop computer, portable camera, or powering various microsystems. Small fuel cells with various degrees of microfabrication have been reported in the literature [1–8]. Medicine is also a demanding field for this kind of miniature fuel cells as an implantable micro-power source for medical devices, such as cerebrospinal fluid shunt pump and a micro-insulin pump [9]. The research cited shows that the adaptation of micro-electronic techniques to micro-electrochemical systems such as DMFC can be successful. We have to emphasize that while related work exists, the design and testing of a micro-fabricated silicon-base miniature methanol/oxygen polymer electrolyte fuel cell remains a novel concept. Since always, different applications imply different

system design characteristics, operation parameters, as well as materials employed in the device. Towards that aim, we embarked in an effort to develop a l-DMFC with good compatibility with waferlevel MEMS process for 1–100 mW class application, such as distributed micro-sensors and wireless MEMS. This communication will focus on the design, fabrication, and preliminary performance evaluation of a novel concept of l-DMFC. The novelty of the structure proposed in this work is that we have fabricated and arranged the anodic and cathodic micro-channels in plane fabricated onto a single silicon substrate. This concept is different from the conventional bipolar structure where the anode and cathode channels are made on two separate substrates. 2. Experimental 2.1. l-DMFC electrode fabrication

*

Corresponding author. Tel./fax: +81-3-5286-2745. E-mail address: [email protected] (M. Mohamedi).

1388-2481/$ - see front matter
2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2004.04.007

The l-DMFC was prepared using a series of fabrication steps tailored from MEMS techniques. This

S. Motokawa et al. / Electrochemistry Communications 6 (2004) 562–565

procedure is shown in schematic form in Fig. 1. Beginning with 20 mm
25 mm h1 0 0i oriented silicon (ptype, 1–10 X cm, 200  20 lm thick) polished on both front and backsides, a 500 nm layer of silicon dioxide was grown thermally (wet oxidation at 1100 C for 1 h and 30 min). To make feedholes and channels, the silicon dioxide on the front and backside of the wafer were patterned by photolithography. Windows were then opened using buffered hydrogen fluorides etch. To complete the fabrication process, the channels and feedholes were then etched using Deep Reactive Ion Etching (D-RIE) process. D-RIE was performed with a STS Multiplex ICP Deep Reactive Ion Etcher. Etch rate was typically 2.5–3.0 lm/min. The feedholes are circulars, 1 mm diameter, and with a hole spacing of 8 and 10 mm horizontally and vertically, respectively. The width and the depth of the micro-channel are both equal of 100 lm, while the clearance between channels is 100 lm. In this single cell design and fabrication, only two microchannels were fabricated, one for the anode and one for the cathode. Subsequently, a 50 nm layer of silicon dioxide was grown thermally (dry oxidation at 1100 C for 12 min) on the entire surface of the silicon wafer again. Finally, a photosensitive dry film for printed wiring boards was applied to prepare for selective deposition of metal film. Ti/Au for current collectors were formed by electronbeam deposition and lift-off method. A 100 nm gold layer was deposited in an ULVAC CRTM-6000 electron beam evaporation chamber, preceded by 20 nm titanium layer just beneath the gold to promote adhesion.

563

The actual mass loading of Pt was 2.4 mg/cm2 . The Pt– Ru for methanol oxidation was obtained from a solution containing 20 mM H2 PtCl6  xH2 O + 20 mM RuCl3  xH2 O. The deposition was performed at )0.15 V vs. Ag/AgCl for 5 min. The actual mass loading of Pt– Ru was 2.85 mg/cm2 . The electroplating process was carried out at 25 C for both electrodes. Energy dispersive X-ray (EDX) analysis showed a platinum/ruthenium of 90/10 atomic ratio. 2.3. l-DMFC assembly DuPontTM Nafion 112 (thickness: 50 lm, equivalent weight 1100 g/ml, ionic conductivity 0.083 X1 cm1 ) has been used as the proton exchange membrane of the l-DMFC. The membrane is pretreated as follows. It was boiled in 1 M H2 O2 solution for 1 h and then rinsed in boiling in ultra pure water for 1 h to remove any organic compounds, and finally treated with boiling 1 M H2 SO4 for 1 h. Finally, the l-DMFC was assembled by placing the pretreated Nafion 112 membrane between the patterned silicon and a glass substrate and the whole was clamped mechanically for testing. Performance testing was conducted with an electrochemical workstation HZ-3000 HAG-1512l (Hokuto Denko, Japan).

3. Results and discussion The design of the l-DMFC and its working principle are depicted in Fig. 2. The fuel solution (CH3 OH/

2.2. Cathode and anode catalyst deposition The methanol anode and oxidant cathode were prepared by electroplating either Pt–Ru or Pt and Pt, respectively, onto the Ti/Au electrodes. The electroplating solution for Pt was 20 mM H2 PtCl6  6H2 O and 0.5 mM (CH3 COO)Pb  3H2 O. The deposition was carried by applying a current density of 30 mA/cm2 during 10 min.

Fig. 1. Schematic of the l-DMFC chip fabrication process.

Fig. 2. Schematic diagrams of our proposed silicon-based methanol/ oxygen micro-fuel cell: (a) top-view of design; (b) cross-sectional schematic with working principle. Drawings not to scale. CC: current collector.

564

S. Motokawa et al. / Electrochemistry Communications 6 (2004) 562–565

H2 SO4 /H2 O) is fed to the anode side of the unit cell where the catalyst promotes the CH3 OH to release electrons, carbon dioxide, and protons. The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where the oxidant solution (O2 -sat./H2 SO4 /H2 O) has been fed. On the other hand, molecular oxygen reacts at the cathode with proton being transported through the membrane from the anode and produces water, thereby completing the oxidation–reduction process. The anode to cathode contact is the thickness of the Nafion (50 lm) as illustrated in Fig. 2. Here, the path length from anode to cathode is the clearance between the two micro-channels, which is 100 lm. Connecting both the anode and the cathode to an external load makes it possible to produce electric power. The approach shown in Fig. 2 has several advantages and differs in many ways from previous designs: • It is of planar structure and essentially an unfolded fuel cell as shown in Fig. 2, which integrates the anode and cathode onto a single Si surface. Whereas,

the bilayer design uses separate Si wafers with channels for the anode and cathode. • The fuel and oxidant are supplied to the cell in isolated, separate micro-channels. Both the fuel and oxidant are distributed in micro-channels throughout the wafer, and they both possess an exhaust. The isolation of fuel and oxidant precludes one from crossing the fuel and oxidants streams. • The characteristic length of the system that is the distance that the protons must travel from anode to cathode is very short. This makes the system less sensitive to ohmic impedance effect. • The efficiency of the current collectors is high, because the catalyst layers are supported on the metal directly. In addition, the current collectors are directly deposited in the micro-channels. The current does not need to be pulled out by relatively large metal lines. • Catalyst electrodes are directly fabricated in the bottom and sidewalls of the micro-channels. Therefore, this design offers simplicity in the stepwise integration of substrate, electrodes, and membranes.

Fig. 3. Picture of the fabricated l-DMFC prototype. One-cent coin is shown for scaling reference.

Fig. 4. l-DMFC testing using 2 M methanol solution with methanol flow and oxygen flow both equal to 10 lL/min: (a) Voltage–current density profiles; (b) power density plots. Measurements were made at ambient temperature and under atmospheric pressure.

S. Motokawa et al. / Electrochemistry Communications 6 (2004) 562–565

The plane view structure of prototype l-DMFC fabricated using MEMS technology is shown in Fig. 3. The performance of the l-DMFC was assessed at ambient temperature using 2 M CH3 OH/0.5 M H2 SO4 / H2 O as the fuel and O2 -sat./0.5 M H2 SO4 /H2 O as the oxidant. The O2 saturated solution was prepared by using oxygen bubbling into 0.5 M H2 SO4 /H2 O solution. The supply test of fuel was made by means of a microsyringe pump connected to the fabricated l-DMFC unit. Fig. 4(a) shows cell polarization curves operated at Pt and Pt–Ru anode electrodes using 2 M methanol solution at ambient temperature and atmospheric pressure. When a 2 M methanol solution was supplied, the voltage of the unit cell increased to OCV in 2 s and maintained until the fuel was exhausted. The OCV for Pt cell was 300 mV while for Pt–Ru cell of 400 mV. The maximum power density is 0.44 mW/cm2 at 3 mA/cm2 at Pt electrode. While, the maximum power density reached 0.78 mW/cm2 at 3.6 mA/cm2 for cell with Pt–Ru anode. When it is compared with other macro DMFC unit cell, the output voltage (400 mV) of the fabricated lDMFC unit cell is 1/1.25. The reasons for the low output voltage may be described as follows: (1) The Nafion membrane was just placed between the patterned silicon and the glass substrate, i.e., we do not use any high pressure bonding. This brings a decrease in reaction area relatively. (2) The composition of the Pt–Ru anode catalyst used in our cell is not optimal. The composition used in most of the technical works is Pt–Ru: 1:1 atomic ratio. It is still necessary to optimize the electroplating conditions to obtain the desired composition of the Pt– Ru catalyst so as to increase the l-DMFC performance.

4. Conclusions Aspects of the design, materials and fabrication of a micro-fabricated methanol fuel cell have been presented. Our concept of a novel structure lies in that the anodic and cathodic micro-channels arranged in a plane, unlike

565

the conventional bipolar design. The first objective of the experimental trials was simply to verify the feasibility of this novel structure on basis of MEMS technology. Thus a l-DMFC on a silicon wafer has been successfully fabricated using photolithography, deep reactive ion etching, and electron beam deposition. Test results were able to confirm that this new concept of l-DMFC generates electricity. The performance of the cell was measured at ambient temperature was of mW/ cm2 class. The laboratory aims further to improve the performance of this l-DMFC and establish the assembling technique for practical applications. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on the priority area of DMFC (Grant no. 13134204), COE Research ‘‘Molecular Nano-Engineering’’, and 21C-COE Programme ‘‘Practical NanoChemistry’’ from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We would like to thank Dr. Mizuno of Nanotechnology Laboratory for suggestions on fluid injection system.

References [1] C.S. Kelly, G.A. Deluga, W.H. Smyrl, Electrochem. Solid-State Lett. 3 (2000) 407. [2] C.S. Kelly, G.A. Deluga, W.H. Smyrl, AIChE J. 48 (2002) 1071. [3] S.J. Lee, A. Chang-Chien, S.W. Cha, R. O’Hayre, Y.I. Park, Y. Saito, F.B. Prinz, J. Power Sources 112 (2002) 410. [4] J.P. Meyers, H.L. Maynard, J. Power Sources 109 (2002) 76. [5] J. Yu, P. Cheng, Z. Ma, B. Yi, J. Power Sources 124 (2003) 40. [6] J. Yu, P. Cheng, Z. Ma, B. Yi, Electrochim. Acta 48 (2003) 1537. [7] K-B. Min, S. Tanaka, M. Esashi, in: Proceedings of the IEEE Sixteenth Annual International Conference on MEMS, 2003, p. 379. [8] J.S. Wainright, R.F. Savinell, C.C. Liu, M. Litt, Electrochim. Acta 48 (2003) 2869. [9] W.Y. Sim, G.Y. Kim, S.S. Yang, in: Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems, 2001, p. 341.