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Solid oxide fuel cell membranes supported by nickel grid anode
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Solid State Ionics 179 (2008) 1497 – 1500 www.elsevier.com/locate/ssi
Solid oxide fuel cell membranes supported by nickel grid anode Samuel Rey-Mermet ⁎, Paul Muralt Ecole Polytechnique Fédérale de Lausanne EPFL, STI-IMX Laboratoire de Céramique, Station 12, 1015 Lausanne, Switzerland Received 13 July 2007; received in revised form 7 January 2008; accepted 14 January 2008
Abstract Microfabricated solid oxide fuel cells (SOFC) have been integrated onto silicon substrates. The SOFC membrane contained a sputter deposited 8YSZ electrolyte layer with a thickness of 750 nm. It is supported on the anode side by a nickel grid grown by electroplating. The grid has either a hexagonal or a spiderweb pattern with maximal free standing length of 80 μm and a height comprised between 4 and 6 μm. The SOFC membranes have diameter up to 5 mm and are mechanically stable up to 550 °C. Completed with porous platinum anode and cathode, they show a maximal open circuit voltage (OCV) of 280 mV at 450 °C, under a mixture of H2 and Ar at the anode. © 2008 Elsevier B.V. All rights reserved. Keywords: Microfabrication; Thin film; Nickel grid; Silicon substrate; YSZ
1. Introduction Solid oxide fuel cells scaled down to the micrometer range and based on microfabrication processes have found an increasing interest in the recent years [1,2]. The combination of thin film electrolytes (about 1 μm thick) and new materials permits to reduce the operation temperature to 400 °C when operated with hydrogen [3]. This new technology potentially enables small portable power sources with superior autonomy as compared to lithium ion batteries. Most of the previous works in the field present very small membranes in the range of hundreds of micrometers or less with a thickness of approximately 1 μm [4–6]. These cells can reach power densities up to 400 mW/cm2 [3], but their smallness limits their total output power. In the future, the output power will be increased either by multiplying the number of these cells on the wafer or by increasing the size of the cell itself. The disadvantage of the first solution is a bad exploitation of available surface. Nevertheless arrays of small cells are mechanically stronger than large free standing membranes [5,7]. Large membranes can be supported by stiffeners [7] or by porous substrates [8,9] to reinforce their structure. But porous substrates and porous layers such as films ⁎ Corresponding author. Tel.: +41 21 693 29 88; fax: +41 21 693 58 10. E-mail address: [email protected] (S. Rey-Mermet). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.007
deposited by spray pyrolysis [10] are not compatible with clean room equipment and do not allow to obtain pore free electrolyte layer of micrometer thickness. In this paper we propose a solution combining stiffeners and large membranes in order to enhance the output power [11]. We use a nickel grid grown by electroplating that assures the mechanical and thermal stability of the Positive electrode–Electrolyte–Negative electrode (PEN) membrane up to 550 °C at least. Nickel is widely used as anode for SOFC [12,13] thus this grid is an active part of the device as it serves as anode as well as current collector. The grid structure allows for a larger active surface of the cell area than with the use of silicon nitride or other non conductive materials [5,7]. We fabricated micro-SOFCs (μSOFC) having free standing membranes with diameters of up to 5 mm and a thickness of less than 1 μm for the cathode–electrolyte–anode layers stack The device is supported by a nickel grid with a maximal aperture of 80 μm. These cells resist thermal cycling up to 550 °C and have an OCV of 280 mV at 450 °C in H2/Ar gas mixture. 2. Experimental results and discussion 2.1. Processing and microfabrication Our μSOFC fabrication is based on double side polished (100) silicon wafers. In a first step, the wafers are wet oxidized
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to obtain a 1.5 μm thick layer of silicon dioxide. It is patterned by dry etching on the back side of the wafer to serve later as a mask for the liberation of the membranes by silicon deep etching. On the front side, the oxide layer is thinned down to 200 nm in a buffered HF bath. This layer serves as electrical insulation from the Si wafer. A platinum layer of 50 nm is deposited by sputtering on the front side serving as current collector for the cathode. This layer is dry etched through a photoresist mask in a chlorine atmosphere. Then the YSZ electrolyte is deposited through a metallic hard mask to leave the contact to the cathode open. A platinum layer of 100 nm is deposited by sputtering on the YSZ. This layer is patterned through a photoresist mask and will serve as seed layer for the nickel electroplating. Finally the silicon wafer is dry etched through the backside oxide mask. The insulating oxide layer remaining at the bottom of the holes is dry etched afterwards. Finally porous Pt electrodes are deposited on both sides to serve as anode and cathode. 2.2. μSOFC design The μSOFC are 5 mm wide circular membranes. The thickness of the membrane, comprising the electrolyte, the patterned current collectors on anode and cathode sides and porous anode and cathode is comprised between 700 and 900 nm (Fig. 1). The supporting grid has an hexagonal or spider web pattern with maximal free standing dimension of 80 μm. According to the calculations of Tang [14], the maximal length Lmax of the free standing membrane is limited by the thermal stress induced by the thermal expansion coefficient mismatch Δα between the supporting wafer (Si) and the electrolyte (YSZ). h Lmax ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DT ð1þmÞDa 1:22
For a heating ramp ΔT of 500 °C and for a thickness h of the YSZ electrolyte of 750 nm, the maximal length before buckling or cracking is 20 μm. The fabricated cells resist thermal cycling up to 550 °C. But the corrugated shape of the grid helps to increase the thermal resistance of the membrane [14]. Inside a
Fig. 1.
Table 1 List of the symbols Symbols Description and units Lmax ΔT h Δα Ν C C' Tanδ Rl Rp F ω
Maximal free standing length of a membrane before cracking or buckling [μm]. Difference between room temperature and the temperature of operation of the μSOFC [K]. Thickness of the SOFC membrane [nm]. Thermal expansion coefficient mismatch between the μSOFC and the silicon substrate [ppm/K]. Poisson's coefficient of the free membrane. Complex capacity of the thin film [F]. Real part of C [F]. Dielectric losses [Ω]. Leakage resistance [Ω]. Total parallel resistance of the film capacity [Ω]. Frequency [Hz]. Pulsation [Hz].
Ni grid cell, stress is small as Ni and YSZ have similar thermal expansion coefficients. Therefore, grid cells with a maximal free standing dimension of 80 μm support thermal cycling up to 500 °C (Table 1). 2.3. Supporting grid The nickel grid is deposited in a photoresist mould from a commercial nickel speed bath with the following composition: nickel sulfamate 600 g/l, nickel chloride 10 g/l, boric acid 40 g/l and additives. The grid height is comprised between 4 and 6 μm with a linewidth of 10 μm. 2.4. Electrodes The anode and the cathode are made of porous platinum films deposited by magnetron sputtering at 100 mT under pure argon flow at room temperature and with a DC power density of 1.3 W/cm2. The rate of deposition amounts to 2.5 nm/min and the thickness is 25 nm. The film has columnar grains spaced by 10–20 nm. SEM observation shows that the film is percolated and that the mean grain size is approximately 50 nm (Fig. 2).
Fig. 2.
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2.6. SOFC characterization The maximal obtained value of open charge voltage is 200 mV with the anode exposed to an reducing atmosphere (ratio Ar/H2 equal to 4:1) and the cathode exposed to pure O2 at 400 °C. Two types of defects can lower the OVC:
Fig. 3.
1) Pinholes leading to metallic shorts: They show large leakage currents even at room temperature. Such pinholes are much rarer on smaller devices. Bad devices with pinholes can be removed from further process before Ni grid plating and membrane liberation. 2) Thermally activated grain boundary diffusion of electrons: This effect can only be observed at high temperature and thus only detected at the end of the fabrication. Measurements of the capacity and losses (tanδ) of the YSZ films at room temperature have been carried out. The capacity C can be written in complex form as:
2.5. Electrolyte The 8YSZ electrolyte is sputter deposited from a metallic target (16% mol. yttrium) at 5 mT under a gas ratio O2/Ar of 1:2 at 500 °C with an RF power density of 2.6 W/cm2. After deposition the film is cooled down for 2 h under a pure O2 flow. The film is 750 nm thick and the X-ray diffraction pattern shows a preferential (201) orientation (Fig. 3). After deposition, the electrolyte is annealed at 500 °C for 1 h to reduce stresses in the film. The conductivity of the YSZ film has been measured at different frequencies (100 Hz to 1 MHz) as a function of temperature (RT to 500 °C) (Fig. 4). These measurements have been done on a cylindrical capacitor Pt/YSZ(700 nm thick)/Au. The top electrode has a diameter of 5 mm (i.e. the same as the largest fabricated μSOFC). At 100 Hz the activation energy amounts to 0.92 eV. Measurements as a function of frequency help to distinguish between the dielectric losses (increasing with the frequency) and the electronic leakage (constant with the frequency) in the film. At high frequencies, the dielectric losses are predominant making the conductivity artificially high even at low temperatures, as seen in Fig. 4.
Fig. 4.
C ¼ C Vð1 itandÞ The total parallel resistance Rp of the film capacity C' can be expressed by the dielectric losses 1/ωC'tanδ (linearly decreasing with frequency) and the leakage Rl (constant with frequency). Rp ¼
Rl 1 þ Rl xC Vtand
If Rp is plotted against the frequency f (2πω) for films deposited on platinum, Rp linearly decreases with the frequency showing only dielectric losses (Fig. 5). For bad devices, Rp is constant with f indicating an electronic leakage through the film and thus the cells present a very low OCV. For better devices, Rp is constant at low frequencies and then decreases linearly with f. In this case, the cell OCV reaches 200 to 280 mV. As we use the same deposition parameter for YSZ on Pt test sample and on devices, we suspect that leakage in the films is due microcracks induced by the patterned cathode mesh on which YSZ is grown.
Fig. 5.
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3. Conclusion The principle of fabrication of μSOFC has been demonstrated. Membranes with a diameter of 5 mm, a thickness of less than 1 μm that are supported by a nickel grid resist thermal cycles from room temperature to 550 °C. The functionality of the fuel cell has been demonstrated by the measurement of an OCV of 280 mV with the anode exposed to a reducing atmosphere (ratio of H2/N2 of 1:4) and the cathode exposed to pure oxygen gas at 400 °C. The reason for the low OCV still needs further investigation. These defects can be detected at room temperature by dielectric measurements. Platinum porous electrodes with a fine microstructure have been prepared and deposited on the both electrodes. Acknowledgments The authors want to acknowledge the ONEBAT project for the funding.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14]
J.D. Morse, International Journal of Energy Research 31 (6–7) (2007) 576. MEMSentry, 16, 2007, p. 1. H. Huang, et al., Journal of the Electrochemical Society 154 (1) (2007) B20. C.D. Baertsch, et al., Journal of Materials Research 19 (9) (2004) 2604. A.F. Jankowski, et al., Proceedings of the Materials Research Symposium, 2002. X. Chen, et al., Applied Physics Letters 84 (14) (2004) 2700. V.T. Srikar, et al., Journal of Power Sources 125 (2004) 62. S. Kang, et al., Journal of the Electrochemical Society 153 (3) (2006) A554. Y.-I. Park, et al., Journal of The Electrochemical Society 153 (2) (2006) A431. D. Beckel, et al., Journal of Electroceramics 16 (2006) 221. S. Rey-Mermet, P. Muralt, J. Baborowski, Metallic Supporting Grid or Thin Electrolyte Membrane in Solid Oxide Fuel Cells, 2006 PCT/EP2006/ 069688. B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. S.P. Jiang, S.H. Chan, Journal of Materials Science 39 (2004) 4405. Y. Tang, et al., Journal of Micromechanics and Microengineering 15 (9) (2005) S185.
Solid State Ionics 179 (2008) 1497 – 1500 www.elsevier.com/locate/ssi
Solid oxide fuel cell membranes supported by nickel grid anode Samuel Rey-Mermet ⁎, Paul Muralt Ecole Polytechnique Fédérale de Lausanne EPFL, STI-IMX Laboratoire de Céramique, Station 12, 1015 Lausanne, Switzerland Received 13 July 2007; received in revised form 7 January 2008; accepted 14 January 2008
Abstract Microfabricated solid oxide fuel cells (SOFC) have been integrated onto silicon substrates. The SOFC membrane contained a sputter deposited 8YSZ electrolyte layer with a thickness of 750 nm. It is supported on the anode side by a nickel grid grown by electroplating. The grid has either a hexagonal or a spiderweb pattern with maximal free standing length of 80 μm and a height comprised between 4 and 6 μm. The SOFC membranes have diameter up to 5 mm and are mechanically stable up to 550 °C. Completed with porous platinum anode and cathode, they show a maximal open circuit voltage (OCV) of 280 mV at 450 °C, under a mixture of H2 and Ar at the anode. © 2008 Elsevier B.V. All rights reserved. Keywords: Microfabrication; Thin film; Nickel grid; Silicon substrate; YSZ
1. Introduction Solid oxide fuel cells scaled down to the micrometer range and based on microfabrication processes have found an increasing interest in the recent years [1,2]. The combination of thin film electrolytes (about 1 μm thick) and new materials permits to reduce the operation temperature to 400 °C when operated with hydrogen [3]. This new technology potentially enables small portable power sources with superior autonomy as compared to lithium ion batteries. Most of the previous works in the field present very small membranes in the range of hundreds of micrometers or less with a thickness of approximately 1 μm [4–6]. These cells can reach power densities up to 400 mW/cm2 [3], but their smallness limits their total output power. In the future, the output power will be increased either by multiplying the number of these cells on the wafer or by increasing the size of the cell itself. The disadvantage of the first solution is a bad exploitation of available surface. Nevertheless arrays of small cells are mechanically stronger than large free standing membranes [5,7]. Large membranes can be supported by stiffeners [7] or by porous substrates [8,9] to reinforce their structure. But porous substrates and porous layers such as films ⁎ Corresponding author. Tel.: +41 21 693 29 88; fax: +41 21 693 58 10. E-mail address: [email protected] (S. Rey-Mermet). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.007
deposited by spray pyrolysis [10] are not compatible with clean room equipment and do not allow to obtain pore free electrolyte layer of micrometer thickness. In this paper we propose a solution combining stiffeners and large membranes in order to enhance the output power [11]. We use a nickel grid grown by electroplating that assures the mechanical and thermal stability of the Positive electrode–Electrolyte–Negative electrode (PEN) membrane up to 550 °C at least. Nickel is widely used as anode for SOFC [12,13] thus this grid is an active part of the device as it serves as anode as well as current collector. The grid structure allows for a larger active surface of the cell area than with the use of silicon nitride or other non conductive materials [5,7]. We fabricated micro-SOFCs (μSOFC) having free standing membranes with diameters of up to 5 mm and a thickness of less than 1 μm for the cathode–electrolyte–anode layers stack The device is supported by a nickel grid with a maximal aperture of 80 μm. These cells resist thermal cycling up to 550 °C and have an OCV of 280 mV at 450 °C in H2/Ar gas mixture. 2. Experimental results and discussion 2.1. Processing and microfabrication Our μSOFC fabrication is based on double side polished (100) silicon wafers. In a first step, the wafers are wet oxidized
1498
S. Rey-Mermet, P. Muralt / Solid State Ionics 179 (2008) 1497–1500
to obtain a 1.5 μm thick layer of silicon dioxide. It is patterned by dry etching on the back side of the wafer to serve later as a mask for the liberation of the membranes by silicon deep etching. On the front side, the oxide layer is thinned down to 200 nm in a buffered HF bath. This layer serves as electrical insulation from the Si wafer. A platinum layer of 50 nm is deposited by sputtering on the front side serving as current collector for the cathode. This layer is dry etched through a photoresist mask in a chlorine atmosphere. Then the YSZ electrolyte is deposited through a metallic hard mask to leave the contact to the cathode open. A platinum layer of 100 nm is deposited by sputtering on the YSZ. This layer is patterned through a photoresist mask and will serve as seed layer for the nickel electroplating. Finally the silicon wafer is dry etched through the backside oxide mask. The insulating oxide layer remaining at the bottom of the holes is dry etched afterwards. Finally porous Pt electrodes are deposited on both sides to serve as anode and cathode. 2.2. μSOFC design The μSOFC are 5 mm wide circular membranes. The thickness of the membrane, comprising the electrolyte, the patterned current collectors on anode and cathode sides and porous anode and cathode is comprised between 700 and 900 nm (Fig. 1). The supporting grid has an hexagonal or spider web pattern with maximal free standing dimension of 80 μm. According to the calculations of Tang [14], the maximal length Lmax of the free standing membrane is limited by the thermal stress induced by the thermal expansion coefficient mismatch Δα between the supporting wafer (Si) and the electrolyte (YSZ). h Lmax ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DT ð1þmÞDa 1:22
For a heating ramp ΔT of 500 °C and for a thickness h of the YSZ electrolyte of 750 nm, the maximal length before buckling or cracking is 20 μm. The fabricated cells resist thermal cycling up to 550 °C. But the corrugated shape of the grid helps to increase the thermal resistance of the membrane [14]. Inside a
Fig. 1.
Table 1 List of the symbols Symbols Description and units Lmax ΔT h Δα Ν C C' Tanδ Rl Rp F ω
Maximal free standing length of a membrane before cracking or buckling [μm]. Difference between room temperature and the temperature of operation of the μSOFC [K]. Thickness of the SOFC membrane [nm]. Thermal expansion coefficient mismatch between the μSOFC and the silicon substrate [ppm/K]. Poisson's coefficient of the free membrane. Complex capacity of the thin film [F]. Real part of C [F]. Dielectric losses [Ω]. Leakage resistance [Ω]. Total parallel resistance of the film capacity [Ω]. Frequency [Hz]. Pulsation [Hz].
Ni grid cell, stress is small as Ni and YSZ have similar thermal expansion coefficients. Therefore, grid cells with a maximal free standing dimension of 80 μm support thermal cycling up to 500 °C (Table 1). 2.3. Supporting grid The nickel grid is deposited in a photoresist mould from a commercial nickel speed bath with the following composition: nickel sulfamate 600 g/l, nickel chloride 10 g/l, boric acid 40 g/l and additives. The grid height is comprised between 4 and 6 μm with a linewidth of 10 μm. 2.4. Electrodes The anode and the cathode are made of porous platinum films deposited by magnetron sputtering at 100 mT under pure argon flow at room temperature and with a DC power density of 1.3 W/cm2. The rate of deposition amounts to 2.5 nm/min and the thickness is 25 nm. The film has columnar grains spaced by 10–20 nm. SEM observation shows that the film is percolated and that the mean grain size is approximately 50 nm (Fig. 2).
Fig. 2.
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2.6. SOFC characterization The maximal obtained value of open charge voltage is 200 mV with the anode exposed to an reducing atmosphere (ratio Ar/H2 equal to 4:1) and the cathode exposed to pure O2 at 400 °C. Two types of defects can lower the OVC:
Fig. 3.
1) Pinholes leading to metallic shorts: They show large leakage currents even at room temperature. Such pinholes are much rarer on smaller devices. Bad devices with pinholes can be removed from further process before Ni grid plating and membrane liberation. 2) Thermally activated grain boundary diffusion of electrons: This effect can only be observed at high temperature and thus only detected at the end of the fabrication. Measurements of the capacity and losses (tanδ) of the YSZ films at room temperature have been carried out. The capacity C can be written in complex form as:
2.5. Electrolyte The 8YSZ electrolyte is sputter deposited from a metallic target (16% mol. yttrium) at 5 mT under a gas ratio O2/Ar of 1:2 at 500 °C with an RF power density of 2.6 W/cm2. After deposition the film is cooled down for 2 h under a pure O2 flow. The film is 750 nm thick and the X-ray diffraction pattern shows a preferential (201) orientation (Fig. 3). After deposition, the electrolyte is annealed at 500 °C for 1 h to reduce stresses in the film. The conductivity of the YSZ film has been measured at different frequencies (100 Hz to 1 MHz) as a function of temperature (RT to 500 °C) (Fig. 4). These measurements have been done on a cylindrical capacitor Pt/YSZ(700 nm thick)/Au. The top electrode has a diameter of 5 mm (i.e. the same as the largest fabricated μSOFC). At 100 Hz the activation energy amounts to 0.92 eV. Measurements as a function of frequency help to distinguish between the dielectric losses (increasing with the frequency) and the electronic leakage (constant with the frequency) in the film. At high frequencies, the dielectric losses are predominant making the conductivity artificially high even at low temperatures, as seen in Fig. 4.
Fig. 4.
C ¼ C Vð1 itandÞ The total parallel resistance Rp of the film capacity C' can be expressed by the dielectric losses 1/ωC'tanδ (linearly decreasing with frequency) and the leakage Rl (constant with frequency). Rp ¼
Rl 1 þ Rl xC Vtand
If Rp is plotted against the frequency f (2πω) for films deposited on platinum, Rp linearly decreases with the frequency showing only dielectric losses (Fig. 5). For bad devices, Rp is constant with f indicating an electronic leakage through the film and thus the cells present a very low OCV. For better devices, Rp is constant at low frequencies and then decreases linearly with f. In this case, the cell OCV reaches 200 to 280 mV. As we use the same deposition parameter for YSZ on Pt test sample and on devices, we suspect that leakage in the films is due microcracks induced by the patterned cathode mesh on which YSZ is grown.
Fig. 5.
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3. Conclusion The principle of fabrication of μSOFC has been demonstrated. Membranes with a diameter of 5 mm, a thickness of less than 1 μm that are supported by a nickel grid resist thermal cycles from room temperature to 550 °C. The functionality of the fuel cell has been demonstrated by the measurement of an OCV of 280 mV with the anode exposed to a reducing atmosphere (ratio of H2/N2 of 1:4) and the cathode exposed to pure oxygen gas at 400 °C. The reason for the low OCV still needs further investigation. These defects can be detected at room temperature by dielectric measurements. Platinum porous electrodes with a fine microstructure have been prepared and deposited on the both electrodes. Acknowledgments The authors want to acknowledge the ONEBAT project for the funding.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14]
J.D. Morse, International Journal of Energy Research 31 (6–7) (2007) 576. MEMSentry, 16, 2007, p. 1. H. Huang, et al., Journal of the Electrochemical Society 154 (1) (2007) B20. C.D. Baertsch, et al., Journal of Materials Research 19 (9) (2004) 2604. A.F. Jankowski, et al., Proceedings of the Materials Research Symposium, 2002. X. Chen, et al., Applied Physics Letters 84 (14) (2004) 2700. V.T. Srikar, et al., Journal of Power Sources 125 (2004) 62. S. Kang, et al., Journal of the Electrochemical Society 153 (3) (2006) A554. Y.-I. Park, et al., Journal of The Electrochemical Society 153 (2) (2006) A431. D. Beckel, et al., Journal of Electroceramics 16 (2006) 221. S. Rey-Mermet, P. Muralt, J. Baborowski, Metallic Supporting Grid or Thin Electrolyte Membrane in Solid Oxide Fuel Cells, 2006 PCT/EP2006/ 069688. B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. S.P. Jiang, S.H. Chan, Journal of Materials Science 39 (2004) 4405. Y. Tang, et al., Journal of Micromechanics and Microengineering 15 (9) (2005) S185.