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Design and fabrication of MEMS-based monolithic fuel cells
Available online at www.sciencedirect.com
Sensors and Actuators A 145–146 (2008) 354–362
Design and fabrication of MEMS-based monolithic fuel cells Nariaki Kuriyama a,∗ , Tadahiro Kubota a , Daisuke Okamura a , Toshifumi Suzuki b , Jun Sasahara a a
Fundamental Technology Research Center, Honda R&D Co., Ltd., Wako-shi, Saitama, Japan b Automobile R&D Center, Honda R&D Co., Ltd., Haga-machi, Tochigi, Japan Received 3 July 2007; received in revised form 10 October 2007; accepted 21 October 2007 Available online 26 December 2007
Abstract This paper describes the design, fabrication and performance of small fuel cells using micro electromechanical systems (MEMS) technology. In this study, one of our biggest challenges was to integrate the carbon material widely used (as gas diffusion and catalyst layers) in fuel cells with all other components in the MEMS process. First, as a unique feature of our prototype, multi-wall carbon nanotube (MWCNT) was employed as a promising material suitable for both the gas diffusion and catalyst support layers. Second, our MEMS-based fuel cell prototype consisted of only three component layers – electromechanically integrated anode and cathode, and proton exchange membrane (PEM). The prototype was successfully demonstrated with maximum power output of 75 mW/cm2 at current density of approximately 0.2 A/cm2 without any pressurizing mechanism. The demonstration of the prototype indicated that MEMS-based components were sufficient for fuel cell applications, and that MEMS technology could help in realizing mass-produced small fuel cells with uniform specifications, such as integrated circuits. © 2007 Elsevier B.V. All rights reserved. Keywords: Fuel cell; MEMS; Carbon nanotube; Platinum catalyst; Gas diffusion; Hydrogen
1. Introduction Due to no emissions, high efficiency and high energy density, the fuel cell has been examined for mobile applications such as automobiles and personal electronics devices. In the future, the demand for portable energy sources is expected to increase dramatically and the Li-based battery is one of the strong candidates for this demand. However, in practical use a fuel cell is superior to a battery from the viewpoint of theoretical energy density. Fig. 1 shows a schematic of a proton exchange membrane (PEM) fuel cell. Fuel cells for mobile applications require compactness and low manufacturing cost with minimum number of components [1,2]. The fuel cell is an electrochemical device that converts chemical energy into electrical energy (Fig. 1). The keys to achieving high-performance fuel cells are: catalytic properties; electrical conductivity of the material of the electrodes, various interconnections and electrolyte; and the fuel transport [3,4]. Conventional fuel cells consist of many components, includ-
∗
Corresponding author. Tel.: +81 48 461 2511; fax: +81 48 462 5330. E-mail address: nariaki [email protected] (N. Kuriyama).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.078
ing structural components such as gaskets, fasteners and end plates, and their assembly is complicated (Fig. 1). These structural components hinder reducing the size of the fuel cells. Therefore, novel design and manufacturing approach have been expected to realize the significant reduction of components and simplification of assembly of the fuel cells [2]. Recent studies [5–14] have suggested that, in order to satisfy electromechanical integration of the fuel cell structure with high precision, repeatability and productivity required, micro electromechanical systems (MEMS) technology is the most attractive fabrication process for small fuel cells for mobile applications, compared with other conventional techniques like machining, molding and fastening, and is expected to realize simple and mass-producible fuel cells with uniform specifications, like integrated circuits [5]. MEMS technology is able to provide the following improvements in the fuel cell: significant reduction of the amount of catalyst and higher power output due to the controlled microstructure of threephase boundary; lower contact resistance at the layer interface and controlled gas permeable structure due to electromechanically integrated fabrication; and flexible connection design of multiple in-plane cells. In addition, by using bonding technologies including anodic and eutectic bonding, which are
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Fig. 1. Schematic of conventional PEM fuel cell.
used in MEMS devices such as pressure sensors [15] and yaw rate sensors [16], all components of fuel cells are monolithically integrated. Localized bonding [17,18] which can be conducted at a low temperature and a solid state electrolyte [19,20] could minimize bonding area and maximize the active area available in the fuel cell. As a result, components such as gaskets, fasteners and end plates can be eliminated. Furthermore, by integrating the auxiliary devices such as micro-pumps, valves, connectors and controllers on chips, small fuel cell systems with high power density could be realized. There have been a number of reports [5–14] on fuel cells fabricated by MEMS-based technology. Their research presented various designs and fabrication methods to miniaturize and simplify the fuel cell. However, it has not been possible to meet the needs of high power output, because the integration of carbon material, which is widely used for fuel cells as the gas diffusion and catalyst support layers, in the MEMS process, is difficult. Some papers [21,22] have already discussed platinum catalyst supported on CNTs and its activity. In those studies, the CNTs have been used simply as catalyst supporting material. In this paper, we selected MWCNT as a promising carbon material which has electrical conductivity, gas permeability and catalyst support properties, and is suitable for MEMS process integration. The MWCNTs were employed for both the gas diffusion and catalyst support layers integrated as a single layer, in which they are in electromechanical contact with other components. Recently, several prototypes of miniature fuel cells have been reported [2,23]. Some of these use methanol as fuel instead of hydrogen. Liquid methanol can have energy density higher than hydrogen (H2 ) gas in a defined volume and be instantly refueled. In general, it is well known that the current direct methanol fuel cells (DMFC) still present issues, such as methanol crossover through the PEM and catalyst lifetime. In this study, hydrogen gas was used as fuel in order to avoid such methanol-related issues and focus primarily on the study of fuel cell structures.
Fig. 2. Schematic design of MEMS-based three-component fuel cell. PEM is sandwiched by the integrated anode and cathode. The three components are assembled using a bonding technique without any fasteners.
collection electrode, gas diffusion layer, catalyst electrode and so on – have been electromechanically integrated. They are deposited and grown on the silicon wafer substrate that serves as the flow channel structure. In a conventional fuel cell, electromechanical contact between all components is controlled by applying external mechanical pressure. However, uniform pressure control in small fuel cells requires more complicated assembly and results in higher cost. In contrast, as all electrode components in this study are deposited and grown layer-by-layer with good electromechanical contact, it is expected to eliminate the pressurizing components such as gaskets, fasteners and end plates, and decrease cell resistance. 2.2. Three-component fuel cell Our fuel cell consists of three components – PEM sandwiched by the integrated anode and cathode (Fig. 2). The three components are assembled using a bonding technique without any fasteners. 2.3. Honeycomb-shaped support structure As the integrated anode/cathode consists only of fragile perforated metal and MWCNT films, a support structure is required. Therefore, a honeycomb-shaped structure was designed and incorporated on each channel to support the bridge structure of thin films as shown in Figs. 2 and 3.
2. Design features 2.1. Integrated anode/cathode The key concept of this study is the integrated anode/cathode, all components of which – the flow channel structure, current
Fig. 3. Cross-sectional diagram of integrated anode/cathode. All components of the integrated anode/cathode – the flow channel structure, current collection electrode, gas diffusion layer, catalyst electrode and so on – have been electromechanically integrated.
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2.4. Current collection electrode While multiple functions such as high conductivity, gas permeability and support of the MWCNT layer are required for the current collection electrode, there are some constraints on the fabrication process. Therefore, we designed the current collection electrode with two layers: one for distant-area and the other for local-area current collection (Figs. 2 and 3). 2.5. CNT gas diffusion and catalyst electrode layers In conventional fuel cells, the catalyst electrode is made of carbon black, which has an extremely high surface area-tovolume ratio, and a gas diffusion layer of carbon paper or carbon cloth. However, an external mechanical pressure using pressurizing components such as gaskets, fasteners and end plates is necessary to establish the electromechanical contact between the carbon material and other components. Using MWCNTs for gas diffusion layer and catalyst support material allows the fuel cell structure to be electromechanically integrated from catalyst layer through flow channel structure. It is important to grow MWCNTs in ohmic contact with molybdenum silicide (MoSi), which serves as a foundation of MWCNTs (Figs. 2 and 3).
Fig. 5. Integrated anode/cathode. The two black sites on the reaction side were active areas, and with a total active area of 1 cm2 are the MWCNT gas diffusion and catalyst layer. On the gas flow side, there were 40 channels and five via-hole electrodes.
3. Prototype fabrication 3.1. Fuel cell prototype In this section, we explain the specifications of our fuel cell prototype (Fig. 4), based on the design description in the previous section. The fuel cell prototype was a 2 cm × 2 cm square, 0.2 cm thick, 2.2 g in weight, and consisted of the integrated anode, cathode and PEM. Fig. 5 shows the integrated anode/cathode. The two black sites on the reaction side were active areas and, with a total active area of 1 cm2 , are the MWCNT gas diffusion and catalyst layers. On the gas flow side, there were 40 channels and five via-hole electrodes. Figs. 6–9 show further details, in particular, on the design features of the integrated anode/cathode. Fig. 6 shows a SEM image of the cross-sectional view of flow channels with honeycombshaped support structures of approximately 30 m height. Each flow channel was 200 m wide and 200 m deep including the honeycomb-shaped support structure. Fig. 7 shows a SEM image
Fig. 4. Outer view of completed fuel cell prototype. The integrated anode/cathode is a 2 cm × 2 cm square, 0.2 cm thick, 2.2 g in weight, and consisted of the integrated anode, cathode and PEM.
Fig. 6. SEM image of cross-sectioned flow channels with support structure. Each flow channel is 200 m wide and 200 m deep, including the 30 m-high honeycomb-shaped support structure.
Fig. 7. SEM image of current collection electrode, on reaction side, serving as a foundation of MWCNT growth (before MWCNT growth). The electrode consists of two layers: gold distant-area current conduction electrode and molybdenum silicide local-area current collection electrode. Gold surface is covered with molybdenum silicide for the following carbon nanotube growth.
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Fig. 8. SEM image of MWCNT gas diffusion and catalyst support layer. The MWCNTs grew on the perforated electrode and SiO2 film, and the surface profile of MWCNTs replicates the substrate pattern shown in Fig. 7.
of the current collection and conduction electrode. The electrode consists of local area current collection electrode made of MoSi with circular holes and gold honeycomb-shaped distant-area current conduction electrode. The two-layer electrode was lying on the silicon dioxide (SiO2 ) film that served as a substrate for MWCNT growth. Fig. 8 shows a SEM image of MWCNT gas diffusion and catalyst support layer. The MWCNTs grew on the perforated electrode and SiO2 film, shown in Fig. 7, at uniform height. Fig. 9 shows a TEM image of platinum catalyst supported on MWCNTs. The platinum catalyst was approximately 5 nm in diameter. 3.2. Fabrication process One of the most unique aspects of our fabrication processes is that the silicon dioxide layer, which is serving as the foundation of MWCNTs growth, is employed as a sacrificial layer to make the MWCNTs suspend by themselves over the circular holes of
Fig. 9. TEM image of sputtered platinum catalyst supported on MWCNT. Approximately 5 nm in diameter platinum particles distribute on the MWCNT.
MoSi current collection electrode. Fig. 10 shows the step-bystep fabrication process of the integrated anode/cathode. 3.2.1. Integrated anode/cathode The distant-area current conduction electrode with 30 mholes was patterned by a lift-off process of 0.07 m-thick titanium and 3 m-thick gold film deposited by evaporation. The local area current collection electrode with 5 m-holes was patterned by plasma etch sputtered 0.1 m-thick MoSi. The substrate was a thermally oxidized silicon wafer of 100 mm diameter and 200 m thickness. All gold film in the active area was covered with MoSi for the MWCNT growth process, as the MWCNTs could not grow directly on the gold film (Fig. 10(a), (b)). A 2.5 nm-thick iron film, serving as MWCNT growth catalyst, was deposited by electron beam evaporation and patterned
Fig. 10. Fabrication process of integrated anode/cathode.
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by a lift-off process on the perforated metal and SiO2 film (Fig. 10(c)). The flow channels were fabricated in two steps using deep reactive ion etching (DRIE). In the first step, the honeycomb shape was etched 30 m deep using a resist mask (Fig. 10(d)). In the following step, the flow channels were then formed 200 m deep using SiO2 mask (Fig. 10(e)). MWCNTs were synthesized by the thermal chemical vapor deposition on the perforated metal and SiO2 sacrificial layer, in a gas mixture of acetylene (C2 H2 ), hydrogen (H2 ) and helium (He) (Fig. 10(f)). The growth temperature and pressure were 700 ◦ C and atmospheric pressure, respectively [24]. The growth time was 10 min. Platinum catalyst supported by MWCNTs was formed using either sputtering or the alcohol reduction method [25]. In the latter case, a di-nitro di-amine platinum nitric acid (cis-[Pt (NH3 )2 (NO2 )2 ]/HNO3 ) solution, containing 6.5 g/l of platinum, was heat-treated at 50 ◦ C for 2 h. SiO2 film that served as the growth substrate for MWCNTs at the MoSi etch matrix was removed using reactive ion etching (Fig. 10(g)), and glass substrate with windows for the via-electrode was bonded by anodic bonding on the flow channel side of the integrated anode/cathode (Fig. 10(h)). Finally, flexible printed circuit (FPC) board was bonded to the glass, and the integrated anode/cathode assembly was completed. 3.2.2. Cell assembly A commercially available membrane (GORE-SELECT® , 30 m thick) was used for an electrolyte. This electrolyte was sandwiched between the integrated anode and cathode and the three layers were bonded together using silicon adhesive. Fig. 11
Table 1 MWCNT layer characteristics Measured characteristic
Values CNT
Length (m) Diameter (nm) Area density (/cm2 ) Contact resistivity with MoSi (m cm2 ) H2 permeability (cm2 /(Pa s)) Platinum surface area (m2 /g)
40 20 1011 2.4 4.5 × 10−4 17.2
Commercial sample CP + Pt/C
Pt/C (TEC10V40E)
– – – –
– – – –
11.0 × 10−4
–
–
40.0
shows a SEM image of the cross-sectional cell assembly. This confirmed that all components of the cell were successfully connected. 4. Characterization of MWCNT layer for fuel cell As previously described, we selected MWCNTs as a promising carbon material for gas diffusion and catalyst support layers. In order to verify the concept, the characteristics of MWCNTs for fuel cells such as electron conductivity, contact resistivity, gas permeability and catalytic activity were examined. High electrical conductivity of the material of the electrodes, various interconnections and electrolytes decrease ohmic losses, which degrade the electrical performance of the fuel cell [3,4]. Better gas permeability is required to maintain mass transport of fuel and oxidants and prevent concentration loss. Platinum catalyst is an essential component of fuel cells, the amount, size and distribution of which determine overall catalytic activity. Minimum use of the catalyst is expected to reduce the cost of the fuel cell. The results of characterization are summarized in Table 1. 4.1. MWCNT configuration The volume fraction and thickness of carbon paper, which is used as gas diffusion layer in a conventional fuel cell, are approximately 30–50% and 100–300 m, respectively. This is because the gas diffusion layer requires sufficient thickness to transport gases such as fuel and oxidants, from flow channels evenly throughout the entire reaction sites. The 50 m-wide flow channel spacing of the prototype was several times narrower than that of conventional fuel cells. Therefore, we estimated 40 mthick MWCNTs to be sufficient to allow effective gas diffusion. The length, diameter and area density of grown MWCNTs were approximately 40 m, 20 nm and 1011 /cm2 , respectively.
Fig. 11. SEM image of sectioned prototype. PEM is sandwiched by the integrated anode and cathode, which consists of the flow channel structure, current collection electrode, gas diffusion layer, catalyst electrode and so on. All components of the cell were successfully connected as designed.
4.2. Contact resistivity with molybdenum silicide The cross-bridge Kelvin resistor (CBKR) was used to measure the contact resistivity between MWCNTs and MoSi. Fig. 12
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4.3. Hydrogen permeability The differential pressure method was used to measure H2 permeability. The relationship between mass flow (Q) and pressure difference (P) was defined by Eq. (1). By measuring the mass flow of H2 (Q) and P, the permeability (A) was calculated, where S, L and α were the cross-sectional area, length of the flow channel and the correction coefficient, respectively. H2 permeability from the MWCNTs and a commercially available carbon paper with carbon-supported platinum as the reference material are severally measured in the same experimental setup. The size of the square cross-sectional area was 0.01 mm2 and the thickness of MWCNTs was 20.3 m. The measurement time was 3 s. Fig. 14 shows a SEM image of the cross-sectional view of a sample that has the same shape and structure as that of the measured sample but with different MWCNT thickness and the experimental setup of the differential pressure method. The results show that H2 permeability from the MWCNTs and a commercially available carbon paper with carbon-supported platinum are 4.5 × 10−4 cm2 /(Pa s) and 11 × 10−4 cm2 /(Pa s), respectively. Q = α × A × P ×
S L
(1)
4.4. Platinum surface area Fig. 12. Measurement of contact resistivity between MWCNT and MoSi thin film. (a) Schematic drawing of CBKR setup. (b) SEM image of CBKR setup (top view, dark area: MWCNT). The size of the square overlapping area was 1600 m2 and the thickness of MWCNTs was 4.5 m.
Cyclic voltammetry was used to measure the platinum catalyst activity on the MWCNTs. The electrolyte solution was
shows a schematic and SEM image of the CBKR structure. The two right angle patterns made of MoSi and MWCNTs were respectively overlapped at the corner. The size of the square overlapping area was 1600 m2 and the thickness of MWCNTs was 4.5 m. While electric current was applied between electrode pads A and C, the voltage between electrode pads B and D was measured. CBKR data were plotted in Fig. 13. As a result, the MWCNTs’ contact resistivity with MoSi was 2.4 m cm2 .
Fig. 13. The result of MWCNT-MoSi contact resistivity (square: current; circle: resistivity).
Fig. 14. Test setup for MWCNT gas permeability measurement. (a) SEM image of cross-sectioned MWCNTs sample that has the same shape and structure as that of the measured sample but with different MWCNT thickness. The size of the square cross-sectional area was 0.01 mm2 and the MWCNT thickness of the measured sample was 20.3 m. (b) Schematic of experimental setup of the differential pressure measurement.
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Fig. 17. Effect of external mechanical pressure on cell potential with different carbon gas diffusion layers. Fig. 15. Example of measured cyclic voltammogram used to calculate the platinum catalyst surface area (conversion coefficient: 210 C/cm2 ; platinum weight: 0.1 g; voltage sweep rate: 50 mV/s).
0.1 M sulfuric acid (H2 SO4 ) at room temperature. The reference electrode was the saturated calomel electrode. The electrode window of the MWCNTs is 5 mm in diameter, masked with silicon rubber. The measurement was done after N2 bubbling for 1 h, in order to remove residual oxygen. The platinum surface area was calculated from the amount of integral charge transferred by the adsorbed proton. The platinum surface area from the platinum catalyst supported on MWCNTs and commercially available carbon-supported platinum (Tanaka Precious Metals, TEC10V40E) as the reference material are severally measured in the same experimental setup. The cyclic voltammetry data from the platinum catalyst supported on the MWCNTs are plotted in Fig. 15. The results show that the platinum surface area from the platinum catalyst supported on the MWCNTs and commercially available carbon-supported platinum are 17.2 m2 /g and 40.0 m2 /g, respectively. 5. Fuel cell characteristics The prototype performance was characterized by I–V measurements with H2 (99.99999%) as fuel and air (nitrogen [N2 ] 80%, oxygen [O2 ] 20%, 99.99%) as oxidant at room temperature without humidification and back-pressure. The performance is
Fig. 16. I–V and power density characteristic curves of the prototype (active cell area: 1 cm2 ; electrolyte: GORE-SELECT® , 30 m thick; H2 : 8.0 sccm; air: 8.0 sccm; no humidification and back-pressure).
shown in Fig. 16. The maximum power output of 75 mW/cm2 at a current density of approximately 0.2 A/cm2 was obtained. The effect of the integrated anode/cathode on contact resistance in the prototype was examined by measuring the relationship between external mechanical pressure and cell potential at current density of 0.1 A/cm2 as shown in Fig. 17. The integrated anode/cathode clearly succeeded in operating the prototype at a pressure 90% lower than that of a conventional fuel cell using carbon paper. 6. Discussion The purpose of this study was to demonstrate a novel design and fabrication approach of small fuel cells for mobile applications. Our results confirmed that MEMS technology would be an attractive fabrication process, as all components of the assembled fuel cell prototype were successfully connected electromechanically (Fig. 11). Though the I–V performance of the prototype has not reached that of conventional fuel cells, it is noted that ohmic contact with the substrate in MEMS-based components such as integrated MWCNTs might be of a comparable range with that of conventional components. Although carbon paper and carbon black are widely used for gas diffusion layer, current collection electrode and catalyst support, a pressurizing mechanism is required due to stiffness of the carbon, so that carbon material and other components are electromechanically in good contact. Our experiments showed that an equivalent cell performance was achieved with the external pressure 90% lower than that of conventional fuel cells using carbon paper, and suggested that all the components in the fuel cell prototype were electromechanically integrated and the pressurizing components could be eliminated. These results are very encouraging for designing a small fuel cell that has critical space and volume constraints. The MWCNT properties characterized in this study were sufficient for use in fuel cell applications and we confirmed its function in reality in the fuel cell prototype. Although the MWCNTs’ gas permeability was approximately half that of the commercially available carbon paper with carbonsupported platinum, it was considered acceptable for use in fuel cell applications because sufficient gas transport could be
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achieved due to the MWCNT gas diffusion layer being several times thinner than the carbon paper in conventional fuel cells. In order to achieve the ideal MEMS-based fuel cell, further study will be required in the following areas: advanced bonding technique for electrolyte membrane and electrode layers without damaging electrolyte property to realize a fully monolithic fuel cell, a controlled microstructure of the three-phase boundary including appropriate catalyst distribution to reduce the amount of catalyst and increase power output, and the integration of auxiliary devices such as micro-pumps and controller to realize a mass-produced small fuel cell system with high power density. 7. Conclusions As the demand for portable energy sources is expected to increase dramatically in the future, a small fuel cell is a preferable candidate over the Li-based battery, due to its higher energy density for mobile applications. In this study, the design, fabrication and performance of small fuel cells using micro electromechanical systems technology were reported and summarized as follows: 1. Multi-wall carbon nanotube (MWCNT) was employed as a promising material suitable for both the gas diffusion and catalyst support layers. MWCNT layer was characterized in gas permeability, contact resistivity with substrate and catalyst support. The measured cell performance suggested that MWCNT was successfully integrated electromechanically and performed similarly to conventional carbon paper and carbon black. 2. MEMS-based fuel cell prototype consisted of only three component layers: electromechanically integrated anode and cathode, and proton exchange membrane (PEM). Without any pressurizing mechanism, the prototype generated maximum power output of 75 mW/cm2 at current density of approximately 0.2 A/cm2 . 3. The demonstration of the prototype indicated that MEMSbased components were sufficient for fuel cell applications, and that MEMS technology could help in realizing massproduced small fuel cells with uniform specifications, like integrated circuits. Acknowledgements The authors would like to thank Prof. Friedrich B. Prinz and the members of the Rapid Prototyping Laboratory at Stanford University for their technical support and helpful suggestions on the prototype design and fabrication, and Prof. Yoshikazu Nakayama of Osaka University for his helpful suggestions on carbon nanotube growth. References [1] C.K. Dyer, Fuel cells for portable applications, J. Power Sources 106 (1–2) (2002) 31–34.
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Biographies
in the research of materials science for electronics and micro electromechanical systems (MEMS) devices.
Nariaki Kuriyama received his BE degree in electrical and electronics engineering from Kagoshima University in 1990. He joined Honda R&D Co., Ltd. in 1990 and has been engaged in the research of electronics and micro electromechanical systems (MEMS) devices.
Toshifumi Suzuki received his BS and MS degrees in electronic engineering from Tohoku University in 1986 and 1988, respectively. He joined Honda R&D Co., Ltd. in 1993 and has been engaged in the research of electronics and micro electromechanical systems (MEMS) devices.
Tadahiro Kubota received his BS degree in materials science and engineering from Tohoku University in 1985. He joined Honda R&D Co., Ltd. in 1989 and has been engaged in the research of materials science for sensor devices.
Jun Sasahara received his BE and ME degrees in electronics engineering from Hokkaido University in 1984 and 1986, respectively. He joined Honda R&D Co., Ltd. in 1986 and has been engaged in the research of materials science for sensor devices.
Daisuke Okamura received his MS degree in materials science form Kyoto University in 2003. He joined Honda R&D Co., Ltd. in 2003 and has been engaged
Sensors and Actuators A 145–146 (2008) 354–362
Design and fabrication of MEMS-based monolithic fuel cells Nariaki Kuriyama a,∗ , Tadahiro Kubota a , Daisuke Okamura a , Toshifumi Suzuki b , Jun Sasahara a a
Fundamental Technology Research Center, Honda R&D Co., Ltd., Wako-shi, Saitama, Japan b Automobile R&D Center, Honda R&D Co., Ltd., Haga-machi, Tochigi, Japan Received 3 July 2007; received in revised form 10 October 2007; accepted 21 October 2007 Available online 26 December 2007
Abstract This paper describes the design, fabrication and performance of small fuel cells using micro electromechanical systems (MEMS) technology. In this study, one of our biggest challenges was to integrate the carbon material widely used (as gas diffusion and catalyst layers) in fuel cells with all other components in the MEMS process. First, as a unique feature of our prototype, multi-wall carbon nanotube (MWCNT) was employed as a promising material suitable for both the gas diffusion and catalyst support layers. Second, our MEMS-based fuel cell prototype consisted of only three component layers – electromechanically integrated anode and cathode, and proton exchange membrane (PEM). The prototype was successfully demonstrated with maximum power output of 75 mW/cm2 at current density of approximately 0.2 A/cm2 without any pressurizing mechanism. The demonstration of the prototype indicated that MEMS-based components were sufficient for fuel cell applications, and that MEMS technology could help in realizing mass-produced small fuel cells with uniform specifications, such as integrated circuits. © 2007 Elsevier B.V. All rights reserved. Keywords: Fuel cell; MEMS; Carbon nanotube; Platinum catalyst; Gas diffusion; Hydrogen
1. Introduction Due to no emissions, high efficiency and high energy density, the fuel cell has been examined for mobile applications such as automobiles and personal electronics devices. In the future, the demand for portable energy sources is expected to increase dramatically and the Li-based battery is one of the strong candidates for this demand. However, in practical use a fuel cell is superior to a battery from the viewpoint of theoretical energy density. Fig. 1 shows a schematic of a proton exchange membrane (PEM) fuel cell. Fuel cells for mobile applications require compactness and low manufacturing cost with minimum number of components [1,2]. The fuel cell is an electrochemical device that converts chemical energy into electrical energy (Fig. 1). The keys to achieving high-performance fuel cells are: catalytic properties; electrical conductivity of the material of the electrodes, various interconnections and electrolyte; and the fuel transport [3,4]. Conventional fuel cells consist of many components, includ-
∗
Corresponding author. Tel.: +81 48 461 2511; fax: +81 48 462 5330. E-mail address: nariaki [email protected] (N. Kuriyama).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.078
ing structural components such as gaskets, fasteners and end plates, and their assembly is complicated (Fig. 1). These structural components hinder reducing the size of the fuel cells. Therefore, novel design and manufacturing approach have been expected to realize the significant reduction of components and simplification of assembly of the fuel cells [2]. Recent studies [5–14] have suggested that, in order to satisfy electromechanical integration of the fuel cell structure with high precision, repeatability and productivity required, micro electromechanical systems (MEMS) technology is the most attractive fabrication process for small fuel cells for mobile applications, compared with other conventional techniques like machining, molding and fastening, and is expected to realize simple and mass-producible fuel cells with uniform specifications, like integrated circuits [5]. MEMS technology is able to provide the following improvements in the fuel cell: significant reduction of the amount of catalyst and higher power output due to the controlled microstructure of threephase boundary; lower contact resistance at the layer interface and controlled gas permeable structure due to electromechanically integrated fabrication; and flexible connection design of multiple in-plane cells. In addition, by using bonding technologies including anodic and eutectic bonding, which are
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Fig. 1. Schematic of conventional PEM fuel cell.
used in MEMS devices such as pressure sensors [15] and yaw rate sensors [16], all components of fuel cells are monolithically integrated. Localized bonding [17,18] which can be conducted at a low temperature and a solid state electrolyte [19,20] could minimize bonding area and maximize the active area available in the fuel cell. As a result, components such as gaskets, fasteners and end plates can be eliminated. Furthermore, by integrating the auxiliary devices such as micro-pumps, valves, connectors and controllers on chips, small fuel cell systems with high power density could be realized. There have been a number of reports [5–14] on fuel cells fabricated by MEMS-based technology. Their research presented various designs and fabrication methods to miniaturize and simplify the fuel cell. However, it has not been possible to meet the needs of high power output, because the integration of carbon material, which is widely used for fuel cells as the gas diffusion and catalyst support layers, in the MEMS process, is difficult. Some papers [21,22] have already discussed platinum catalyst supported on CNTs and its activity. In those studies, the CNTs have been used simply as catalyst supporting material. In this paper, we selected MWCNT as a promising carbon material which has electrical conductivity, gas permeability and catalyst support properties, and is suitable for MEMS process integration. The MWCNTs were employed for both the gas diffusion and catalyst support layers integrated as a single layer, in which they are in electromechanical contact with other components. Recently, several prototypes of miniature fuel cells have been reported [2,23]. Some of these use methanol as fuel instead of hydrogen. Liquid methanol can have energy density higher than hydrogen (H2 ) gas in a defined volume and be instantly refueled. In general, it is well known that the current direct methanol fuel cells (DMFC) still present issues, such as methanol crossover through the PEM and catalyst lifetime. In this study, hydrogen gas was used as fuel in order to avoid such methanol-related issues and focus primarily on the study of fuel cell structures.
Fig. 2. Schematic design of MEMS-based three-component fuel cell. PEM is sandwiched by the integrated anode and cathode. The three components are assembled using a bonding technique without any fasteners.
collection electrode, gas diffusion layer, catalyst electrode and so on – have been electromechanically integrated. They are deposited and grown on the silicon wafer substrate that serves as the flow channel structure. In a conventional fuel cell, electromechanical contact between all components is controlled by applying external mechanical pressure. However, uniform pressure control in small fuel cells requires more complicated assembly and results in higher cost. In contrast, as all electrode components in this study are deposited and grown layer-by-layer with good electromechanical contact, it is expected to eliminate the pressurizing components such as gaskets, fasteners and end plates, and decrease cell resistance. 2.2. Three-component fuel cell Our fuel cell consists of three components – PEM sandwiched by the integrated anode and cathode (Fig. 2). The three components are assembled using a bonding technique without any fasteners. 2.3. Honeycomb-shaped support structure As the integrated anode/cathode consists only of fragile perforated metal and MWCNT films, a support structure is required. Therefore, a honeycomb-shaped structure was designed and incorporated on each channel to support the bridge structure of thin films as shown in Figs. 2 and 3.
2. Design features 2.1. Integrated anode/cathode The key concept of this study is the integrated anode/cathode, all components of which – the flow channel structure, current
Fig. 3. Cross-sectional diagram of integrated anode/cathode. All components of the integrated anode/cathode – the flow channel structure, current collection electrode, gas diffusion layer, catalyst electrode and so on – have been electromechanically integrated.
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2.4. Current collection electrode While multiple functions such as high conductivity, gas permeability and support of the MWCNT layer are required for the current collection electrode, there are some constraints on the fabrication process. Therefore, we designed the current collection electrode with two layers: one for distant-area and the other for local-area current collection (Figs. 2 and 3). 2.5. CNT gas diffusion and catalyst electrode layers In conventional fuel cells, the catalyst electrode is made of carbon black, which has an extremely high surface area-tovolume ratio, and a gas diffusion layer of carbon paper or carbon cloth. However, an external mechanical pressure using pressurizing components such as gaskets, fasteners and end plates is necessary to establish the electromechanical contact between the carbon material and other components. Using MWCNTs for gas diffusion layer and catalyst support material allows the fuel cell structure to be electromechanically integrated from catalyst layer through flow channel structure. It is important to grow MWCNTs in ohmic contact with molybdenum silicide (MoSi), which serves as a foundation of MWCNTs (Figs. 2 and 3).
Fig. 5. Integrated anode/cathode. The two black sites on the reaction side were active areas, and with a total active area of 1 cm2 are the MWCNT gas diffusion and catalyst layer. On the gas flow side, there were 40 channels and five via-hole electrodes.
3. Prototype fabrication 3.1. Fuel cell prototype In this section, we explain the specifications of our fuel cell prototype (Fig. 4), based on the design description in the previous section. The fuel cell prototype was a 2 cm × 2 cm square, 0.2 cm thick, 2.2 g in weight, and consisted of the integrated anode, cathode and PEM. Fig. 5 shows the integrated anode/cathode. The two black sites on the reaction side were active areas and, with a total active area of 1 cm2 , are the MWCNT gas diffusion and catalyst layers. On the gas flow side, there were 40 channels and five via-hole electrodes. Figs. 6–9 show further details, in particular, on the design features of the integrated anode/cathode. Fig. 6 shows a SEM image of the cross-sectional view of flow channels with honeycombshaped support structures of approximately 30 m height. Each flow channel was 200 m wide and 200 m deep including the honeycomb-shaped support structure. Fig. 7 shows a SEM image
Fig. 4. Outer view of completed fuel cell prototype. The integrated anode/cathode is a 2 cm × 2 cm square, 0.2 cm thick, 2.2 g in weight, and consisted of the integrated anode, cathode and PEM.
Fig. 6. SEM image of cross-sectioned flow channels with support structure. Each flow channel is 200 m wide and 200 m deep, including the 30 m-high honeycomb-shaped support structure.
Fig. 7. SEM image of current collection electrode, on reaction side, serving as a foundation of MWCNT growth (before MWCNT growth). The electrode consists of two layers: gold distant-area current conduction electrode and molybdenum silicide local-area current collection electrode. Gold surface is covered with molybdenum silicide for the following carbon nanotube growth.
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Fig. 8. SEM image of MWCNT gas diffusion and catalyst support layer. The MWCNTs grew on the perforated electrode and SiO2 film, and the surface profile of MWCNTs replicates the substrate pattern shown in Fig. 7.
of the current collection and conduction electrode. The electrode consists of local area current collection electrode made of MoSi with circular holes and gold honeycomb-shaped distant-area current conduction electrode. The two-layer electrode was lying on the silicon dioxide (SiO2 ) film that served as a substrate for MWCNT growth. Fig. 8 shows a SEM image of MWCNT gas diffusion and catalyst support layer. The MWCNTs grew on the perforated electrode and SiO2 film, shown in Fig. 7, at uniform height. Fig. 9 shows a TEM image of platinum catalyst supported on MWCNTs. The platinum catalyst was approximately 5 nm in diameter. 3.2. Fabrication process One of the most unique aspects of our fabrication processes is that the silicon dioxide layer, which is serving as the foundation of MWCNTs growth, is employed as a sacrificial layer to make the MWCNTs suspend by themselves over the circular holes of
Fig. 9. TEM image of sputtered platinum catalyst supported on MWCNT. Approximately 5 nm in diameter platinum particles distribute on the MWCNT.
MoSi current collection electrode. Fig. 10 shows the step-bystep fabrication process of the integrated anode/cathode. 3.2.1. Integrated anode/cathode The distant-area current conduction electrode with 30 mholes was patterned by a lift-off process of 0.07 m-thick titanium and 3 m-thick gold film deposited by evaporation. The local area current collection electrode with 5 m-holes was patterned by plasma etch sputtered 0.1 m-thick MoSi. The substrate was a thermally oxidized silicon wafer of 100 mm diameter and 200 m thickness. All gold film in the active area was covered with MoSi for the MWCNT growth process, as the MWCNTs could not grow directly on the gold film (Fig. 10(a), (b)). A 2.5 nm-thick iron film, serving as MWCNT growth catalyst, was deposited by electron beam evaporation and patterned
Fig. 10. Fabrication process of integrated anode/cathode.
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by a lift-off process on the perforated metal and SiO2 film (Fig. 10(c)). The flow channels were fabricated in two steps using deep reactive ion etching (DRIE). In the first step, the honeycomb shape was etched 30 m deep using a resist mask (Fig. 10(d)). In the following step, the flow channels were then formed 200 m deep using SiO2 mask (Fig. 10(e)). MWCNTs were synthesized by the thermal chemical vapor deposition on the perforated metal and SiO2 sacrificial layer, in a gas mixture of acetylene (C2 H2 ), hydrogen (H2 ) and helium (He) (Fig. 10(f)). The growth temperature and pressure were 700 ◦ C and atmospheric pressure, respectively [24]. The growth time was 10 min. Platinum catalyst supported by MWCNTs was formed using either sputtering or the alcohol reduction method [25]. In the latter case, a di-nitro di-amine platinum nitric acid (cis-[Pt (NH3 )2 (NO2 )2 ]/HNO3 ) solution, containing 6.5 g/l of platinum, was heat-treated at 50 ◦ C for 2 h. SiO2 film that served as the growth substrate for MWCNTs at the MoSi etch matrix was removed using reactive ion etching (Fig. 10(g)), and glass substrate with windows for the via-electrode was bonded by anodic bonding on the flow channel side of the integrated anode/cathode (Fig. 10(h)). Finally, flexible printed circuit (FPC) board was bonded to the glass, and the integrated anode/cathode assembly was completed. 3.2.2. Cell assembly A commercially available membrane (GORE-SELECT® , 30 m thick) was used for an electrolyte. This electrolyte was sandwiched between the integrated anode and cathode and the three layers were bonded together using silicon adhesive. Fig. 11
Table 1 MWCNT layer characteristics Measured characteristic
Values CNT
Length (m) Diameter (nm) Area density (/cm2 ) Contact resistivity with MoSi (m cm2 ) H2 permeability (cm2 /(Pa s)) Platinum surface area (m2 /g)
40 20 1011 2.4 4.5 × 10−4 17.2
Commercial sample CP + Pt/C
Pt/C (TEC10V40E)
– – – –
– – – –
11.0 × 10−4
–
–
40.0
shows a SEM image of the cross-sectional cell assembly. This confirmed that all components of the cell were successfully connected. 4. Characterization of MWCNT layer for fuel cell As previously described, we selected MWCNTs as a promising carbon material for gas diffusion and catalyst support layers. In order to verify the concept, the characteristics of MWCNTs for fuel cells such as electron conductivity, contact resistivity, gas permeability and catalytic activity were examined. High electrical conductivity of the material of the electrodes, various interconnections and electrolytes decrease ohmic losses, which degrade the electrical performance of the fuel cell [3,4]. Better gas permeability is required to maintain mass transport of fuel and oxidants and prevent concentration loss. Platinum catalyst is an essential component of fuel cells, the amount, size and distribution of which determine overall catalytic activity. Minimum use of the catalyst is expected to reduce the cost of the fuel cell. The results of characterization are summarized in Table 1. 4.1. MWCNT configuration The volume fraction and thickness of carbon paper, which is used as gas diffusion layer in a conventional fuel cell, are approximately 30–50% and 100–300 m, respectively. This is because the gas diffusion layer requires sufficient thickness to transport gases such as fuel and oxidants, from flow channels evenly throughout the entire reaction sites. The 50 m-wide flow channel spacing of the prototype was several times narrower than that of conventional fuel cells. Therefore, we estimated 40 mthick MWCNTs to be sufficient to allow effective gas diffusion. The length, diameter and area density of grown MWCNTs were approximately 40 m, 20 nm and 1011 /cm2 , respectively.
Fig. 11. SEM image of sectioned prototype. PEM is sandwiched by the integrated anode and cathode, which consists of the flow channel structure, current collection electrode, gas diffusion layer, catalyst electrode and so on. All components of the cell were successfully connected as designed.
4.2. Contact resistivity with molybdenum silicide The cross-bridge Kelvin resistor (CBKR) was used to measure the contact resistivity between MWCNTs and MoSi. Fig. 12
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4.3. Hydrogen permeability The differential pressure method was used to measure H2 permeability. The relationship between mass flow (Q) and pressure difference (P) was defined by Eq. (1). By measuring the mass flow of H2 (Q) and P, the permeability (A) was calculated, where S, L and α were the cross-sectional area, length of the flow channel and the correction coefficient, respectively. H2 permeability from the MWCNTs and a commercially available carbon paper with carbon-supported platinum as the reference material are severally measured in the same experimental setup. The size of the square cross-sectional area was 0.01 mm2 and the thickness of MWCNTs was 20.3 m. The measurement time was 3 s. Fig. 14 shows a SEM image of the cross-sectional view of a sample that has the same shape and structure as that of the measured sample but with different MWCNT thickness and the experimental setup of the differential pressure method. The results show that H2 permeability from the MWCNTs and a commercially available carbon paper with carbon-supported platinum are 4.5 × 10−4 cm2 /(Pa s) and 11 × 10−4 cm2 /(Pa s), respectively. Q = α × A × P ×
S L
(1)
4.4. Platinum surface area Fig. 12. Measurement of contact resistivity between MWCNT and MoSi thin film. (a) Schematic drawing of CBKR setup. (b) SEM image of CBKR setup (top view, dark area: MWCNT). The size of the square overlapping area was 1600 m2 and the thickness of MWCNTs was 4.5 m.
Cyclic voltammetry was used to measure the platinum catalyst activity on the MWCNTs. The electrolyte solution was
shows a schematic and SEM image of the CBKR structure. The two right angle patterns made of MoSi and MWCNTs were respectively overlapped at the corner. The size of the square overlapping area was 1600 m2 and the thickness of MWCNTs was 4.5 m. While electric current was applied between electrode pads A and C, the voltage between electrode pads B and D was measured. CBKR data were plotted in Fig. 13. As a result, the MWCNTs’ contact resistivity with MoSi was 2.4 m cm2 .
Fig. 13. The result of MWCNT-MoSi contact resistivity (square: current; circle: resistivity).
Fig. 14. Test setup for MWCNT gas permeability measurement. (a) SEM image of cross-sectioned MWCNTs sample that has the same shape and structure as that of the measured sample but with different MWCNT thickness. The size of the square cross-sectional area was 0.01 mm2 and the MWCNT thickness of the measured sample was 20.3 m. (b) Schematic of experimental setup of the differential pressure measurement.
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Fig. 17. Effect of external mechanical pressure on cell potential with different carbon gas diffusion layers. Fig. 15. Example of measured cyclic voltammogram used to calculate the platinum catalyst surface area (conversion coefficient: 210 C/cm2 ; platinum weight: 0.1 g; voltage sweep rate: 50 mV/s).
0.1 M sulfuric acid (H2 SO4 ) at room temperature. The reference electrode was the saturated calomel electrode. The electrode window of the MWCNTs is 5 mm in diameter, masked with silicon rubber. The measurement was done after N2 bubbling for 1 h, in order to remove residual oxygen. The platinum surface area was calculated from the amount of integral charge transferred by the adsorbed proton. The platinum surface area from the platinum catalyst supported on MWCNTs and commercially available carbon-supported platinum (Tanaka Precious Metals, TEC10V40E) as the reference material are severally measured in the same experimental setup. The cyclic voltammetry data from the platinum catalyst supported on the MWCNTs are plotted in Fig. 15. The results show that the platinum surface area from the platinum catalyst supported on the MWCNTs and commercially available carbon-supported platinum are 17.2 m2 /g and 40.0 m2 /g, respectively. 5. Fuel cell characteristics The prototype performance was characterized by I–V measurements with H2 (99.99999%) as fuel and air (nitrogen [N2 ] 80%, oxygen [O2 ] 20%, 99.99%) as oxidant at room temperature without humidification and back-pressure. The performance is
Fig. 16. I–V and power density characteristic curves of the prototype (active cell area: 1 cm2 ; electrolyte: GORE-SELECT® , 30 m thick; H2 : 8.0 sccm; air: 8.0 sccm; no humidification and back-pressure).
shown in Fig. 16. The maximum power output of 75 mW/cm2 at a current density of approximately 0.2 A/cm2 was obtained. The effect of the integrated anode/cathode on contact resistance in the prototype was examined by measuring the relationship between external mechanical pressure and cell potential at current density of 0.1 A/cm2 as shown in Fig. 17. The integrated anode/cathode clearly succeeded in operating the prototype at a pressure 90% lower than that of a conventional fuel cell using carbon paper. 6. Discussion The purpose of this study was to demonstrate a novel design and fabrication approach of small fuel cells for mobile applications. Our results confirmed that MEMS technology would be an attractive fabrication process, as all components of the assembled fuel cell prototype were successfully connected electromechanically (Fig. 11). Though the I–V performance of the prototype has not reached that of conventional fuel cells, it is noted that ohmic contact with the substrate in MEMS-based components such as integrated MWCNTs might be of a comparable range with that of conventional components. Although carbon paper and carbon black are widely used for gas diffusion layer, current collection electrode and catalyst support, a pressurizing mechanism is required due to stiffness of the carbon, so that carbon material and other components are electromechanically in good contact. Our experiments showed that an equivalent cell performance was achieved with the external pressure 90% lower than that of conventional fuel cells using carbon paper, and suggested that all the components in the fuel cell prototype were electromechanically integrated and the pressurizing components could be eliminated. These results are very encouraging for designing a small fuel cell that has critical space and volume constraints. The MWCNT properties characterized in this study were sufficient for use in fuel cell applications and we confirmed its function in reality in the fuel cell prototype. Although the MWCNTs’ gas permeability was approximately half that of the commercially available carbon paper with carbonsupported platinum, it was considered acceptable for use in fuel cell applications because sufficient gas transport could be
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achieved due to the MWCNT gas diffusion layer being several times thinner than the carbon paper in conventional fuel cells. In order to achieve the ideal MEMS-based fuel cell, further study will be required in the following areas: advanced bonding technique for electrolyte membrane and electrode layers without damaging electrolyte property to realize a fully monolithic fuel cell, a controlled microstructure of the three-phase boundary including appropriate catalyst distribution to reduce the amount of catalyst and increase power output, and the integration of auxiliary devices such as micro-pumps and controller to realize a mass-produced small fuel cell system with high power density. 7. Conclusions As the demand for portable energy sources is expected to increase dramatically in the future, a small fuel cell is a preferable candidate over the Li-based battery, due to its higher energy density for mobile applications. In this study, the design, fabrication and performance of small fuel cells using micro electromechanical systems technology were reported and summarized as follows: 1. Multi-wall carbon nanotube (MWCNT) was employed as a promising material suitable for both the gas diffusion and catalyst support layers. MWCNT layer was characterized in gas permeability, contact resistivity with substrate and catalyst support. The measured cell performance suggested that MWCNT was successfully integrated electromechanically and performed similarly to conventional carbon paper and carbon black. 2. MEMS-based fuel cell prototype consisted of only three component layers: electromechanically integrated anode and cathode, and proton exchange membrane (PEM). Without any pressurizing mechanism, the prototype generated maximum power output of 75 mW/cm2 at current density of approximately 0.2 A/cm2 . 3. The demonstration of the prototype indicated that MEMSbased components were sufficient for fuel cell applications, and that MEMS technology could help in realizing massproduced small fuel cells with uniform specifications, like integrated circuits. Acknowledgements The authors would like to thank Prof. Friedrich B. Prinz and the members of the Rapid Prototyping Laboratory at Stanford University for their technical support and helpful suggestions on the prototype design and fabrication, and Prof. Yoshikazu Nakayama of Osaka University for his helpful suggestions on carbon nanotube growth. References [1] C.K. Dyer, Fuel cells for portable applications, J. Power Sources 106 (1–2) (2002) 31–34.
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Biographies
in the research of materials science for electronics and micro electromechanical systems (MEMS) devices.
Nariaki Kuriyama received his BE degree in electrical and electronics engineering from Kagoshima University in 1990. He joined Honda R&D Co., Ltd. in 1990 and has been engaged in the research of electronics and micro electromechanical systems (MEMS) devices.
Toshifumi Suzuki received his BS and MS degrees in electronic engineering from Tohoku University in 1986 and 1988, respectively. He joined Honda R&D Co., Ltd. in 1993 and has been engaged in the research of electronics and micro electromechanical systems (MEMS) devices.
Tadahiro Kubota received his BS degree in materials science and engineering from Tohoku University in 1985. He joined Honda R&D Co., Ltd. in 1989 and has been engaged in the research of materials science for sensor devices.
Jun Sasahara received his BE and ME degrees in electronics engineering from Hokkaido University in 1984 and 1986, respectively. He joined Honda R&D Co., Ltd. in 1986 and has been engaged in the research of materials science for sensor devices.
Daisuke Okamura received his MS degree in materials science form Kyoto University in 2003. He joined Honda R&D Co., Ltd. in 2003 and has been engaged