3. RESULTS AND DISCUSSIONS
3.3 Current-voltage (I-V) characteristics and power density
Formic acid is used as the fuel to obtain current-voltage (I-V) characteristics and power density curve. The current-voltage (I-V) characteristics of the proposed membrane electrode assembly can provide valuable information about the mechanism as described in Section 1.4.2.
The current and voltage are measured by an external resistor and an ammeter in series and a voltmeter in parallel, respectively. Measurements are carried out at room temperature. The current-voltage (I-V) characteristics of the single membrane electrode assembly are operated in 5M formic acid/air breathing with and without 0.5M sulfuric acid as shown in Figure 3-6 for various value of resistance.
Original After
(a) (b)
Si Si
Original After
(a) (b)
Si
Si SiSi
Fig.3-5. The optical microscope imaging of (a) the original membrane (b) after immersing into DI water for three days. The schematic cross-section diagram as below.
Fig.3-6. 5M formic acid/air and 5M formic acid with sulfuric acid/air (a) cell potential and (b) power density curves.
(a)
(b) 0
0.1 0.2 0.3 0.4 0.5 0.6
0 20 40 60 80 100 120 140 160
Cell potential (V)
Current density (mA/cm2)
5M FA (10ml)
5M FA (10ml) + 0.5M H2SO4 (2.5ml)
0 5 10 15 20 25
0 20 40 60 80 100 120 140 160
Power density (mW/cm2)
Current density (mA/cm2)
5M FA (10ml)
5M FA (10ml) + 0.5M H2SO4 (2.5ml)
In Figure 3-6, the open circuit potential and current density are increased by adding few amount of sulfuric acid into formic acid. The probable reason is that the V-shaped channels of silicon membrane may not be filled with Nafion® entirely, so the proton conductivity is improved a lot after adding few amount of sulfuric acid. With the condition for adding sulfuric acid, we obtain a current density of about 145 mAcm−2 in minimal charge and 600 mV for open circuit potential, and the maximal power density can reach 23 mWcm-2.
The current-voltage (I-V) characteristics of the single membrane electrode assembly are operated in 5M, 8M, and 12M formic acid with 0.5M sulfuric acid/air breathing as shown in Figure 3-7.
Fig.3-7. 5M, 8M, and12M formic acid with sulfuric acid/air current density vs.
feed concentration: (a) cell potential and (b) power density curves.
0 12M FA (10ml) + 0.5M H2SO4 (2.5ml)
(b) 12M FA (10ml) + 0.5M H2SO4 (2.5ml)
(a)
Figure 3-7 shows the comparison of current-voltage curves and power density curves in different concentrations of formic acid. When the concentration of formic acid is increased, the open circuit potential is decreased. A possible explanation is that the dehydration of Nafion® membrane results in the increase of resistance in fuel cell, due to the fact that the higher feed concentrations are almost devoid of water.
Chapter 4
Conclusions and Future works
4.1 Conclusions
In this study, we successfully fabricate a silicon based porous membrane with the V-shaped nano-size channels for portable fuel cell applications. One of its advantages is the total compatibility with silicon microfabrication process; which will allow us to reduce the cell size furthermore and mass produce it at a low cost in the future. The inverse pyramid shaped channels in silicon membrane is advantageous to avoid surface tension and swelling problem. The micro fuel cell had been characterized by feeding formic acid with sulfuric acid and the max power density is around 23 mWcm-2.
4.2 Future works
We will functionalize the surface of each pore with acidic functional groups using various surface chemistry. By controlling the aspect ratio of our V-shaped channel and reducing its tip diameter down to the order of 10 nm, we hope to manage the fuel crossover problem for future micro fuel cells.
References
[1] G. Hoogers, ed., “Fuel Cell Technology Handbook” CRC Press, 2003.
[2] J. Bockris, S. Srinivasan, “Fuel Cells, Their Electrochemistry, 1st Ed.”, McGraw-Hill, 659 pages, New York, USA, 1969.
[3] S. Møller-Holst, “Solid Polymer Fuel Cells: Electrode and Membrane Performance Studies”, Department of Physical Chemistry, Norwegian University of Science and Technology, Trondheim, Norway, doctoral thesis, 1996.
[4] S. Srinivasan, E. Ticianelli, C. Derouin, A. Redondo, “Advanced in solid polymer electrolyte fuel cell technology with low platinum loading electrodes”, J. Power Sources 22 , pp. 359-375, 1988.
[5] E. Ticianelli, C. Derouin, A. Redondo, S. Srinivasan, “Methods to Advance Technology of Proton Exchange Membrane Fuel Cells”, J. Electrochem. Soc. 135, pp. 2209-2214, 1988.
[6] S. Srinivasan, D. Manko, H. Koch, M. Enayetullah, A. Appleby, “Recent Advances in Solid Polymer Electrolyte Fuel Cell Technology with Low Pt Loading Electrodes”, J.
Power Sources 29, pp. 367-387, 1990.
[7] M. Wilson, S. Gottesfeld, “Thin-film catalyst layers for polymer electrolyte fuel cell electrodes”, J. Appl. Electrochem. 22, pp. 1-7, 1992.
[8] X. Ren, T.E. Springer, S. Gottesfeld, “Water and methanol uptakes in Nafion membranes and membrane effects on direct methanol cell performance”, J. Electrochem. Soc. 147 (1), pp. 92–98, 2000.
[9] P. Dimitrova, K.A. Friedrich, U. Stimming, B. Vogt, “Modified Nafion® based membranes for use in direct methanol fuel cells”, Solid State Ionics 150, pp. 115–122, 2002.
[10] N. Caretta, V. Tricoli, F. Picchioni, “Ionomeric membranes based on partially sulfonated poly(styrene): synthesis, proton conduction and methanol permeation”, J. Membr. Sci.
166, pp. 189–197, 2000.
[11] D.H. Jung, S.Y. Cho, D.H. Peck, D.R. Skin, J.S. Kim, “Performance evaluation of a Nafion/silicon oxide hybrid membrane for direct methanol fuel cell”, J. Power Sources 106, pp. 173–177, 2002.
[12] T. Yamaguchi, M. Ibe, B.N. Nair, S. Nakao, “A pore-filling electrolyte membrane-electrode integrated system for a direct methanol fuel cell application”, J.
Electrochem. Soc. 149 (11), pp. 1448-1453, 2002.
[13] S. C. Kelley, G. A. Deluga, and W. H. Smyrl, “Miniature Fuel Cells Fabricated on Silicon Substrates”, AIChE J. 48, pp. 1071-1082, 2002.
[14] J. Yu, P. Chenga, Z. Ma, B. Yi, “Fabrication of miniature silicon wafer fuel cells with improved performance”, J. Power Sources 124, pp. 40-46, 2003.
[15] J. P. Meyers, H. L. Maynard, “Design considerations for miniaturized PEM fuel cells”, J.
Power Sources 109, pp. 76–88, 2002.
[16] S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, “MEMS-based design and fabrication of a new concept micro direct methanol fuel cell (µ-DMFC)”, Electrochem.
Commun. 6, pp. 562-565, 2004.
[17] T. Pichonat, B. Gauthier-Manuel, “Realization of porous silicon based miniature fuel cells”, J. Power Sources 154, pp. 198-201, 2006.
[18] G.D’Arrigo, C. Spinella, G. Arena, S. Lorenti, “Fabrication of miniaturised Si-based electrocatalytic membranes”, Materials Sci. and Eng. C23, pp. 13–18, 2003.
[19] J. Yeom, G.Z. Mozsgai, B.R. Flachsbart, E.R. Choban, A. Asthana, M.A. Shannon, P.J.A.
Kenis, “Microfabrication and characterization of a silicon-based millimeter scale, PEM fuel cell operating with hydrogen, methanol, or formic acid”, Sensors and Actuators B107, pp. 882–891, 2005.
[20] T. Pichonat, B. Gauthier-Manuel, D. Hauden, “A new proton-conducting porous silicon membrane for small fuel cells”, Chem. Eng. J. 101, pp. 107-111, 2004.
[21] G. Alberti, M. Casciola, “Solid state protonic conductors, present main applications and future prospects”, Solid State Ionics 145, pp. 3-16, 2001.
[22] A. Appleby, F. Foulkes, “Fuel Cell Handbook”, Van Nostrand Reinhold, 762 pages, New York, USA, 1989.
[23] M. Wilson, S. Gottesfeld, “Thin-film catalyst layers for polymer electrolyte fuel cell electrodes”, J. Appl. Electrochem. 22, pp. 1-7, 1992.
[24] T. Mennola, “Design and Experimental Characterization of Polymer Electrolyte Membrane Fuel Cells”, Helsinki University of Technology, Department of Engineering Physics and Mathematics, Licentiate’s thesis, 2000.
[25] D. Davies, P. Adcock, M. Turpin, S. Rowen, “Stainless steel as a bipolar plate material for solid polymer fuel cells”, J. Power Sources 86, pp. 237-242, 2000.
[26] D. Davies, P. Adcock, M. Turpin, S. Rowen, “Bipolar plate materials for solid polymer fuel cells”, J. Appl. Electrochem. 30, pp. 101-105, 2000.
[27] T. Besmann, J. Klett, J. Henry Jr., E. Lara-Curzlo, “Carbon/carbon Composite Bipolar Plate for Proton Exchange Membrane Fuel Cells”, J. Electrochem. Soc. 147, pp.
4083-4086, 2000.
[28] D. Busick, M. Wilson, “Low-Cost Composite Bipolar Plates for PEFC Stacks”, Proc. - Electrochem. Soc. Vol. 98-27, (Proton Conducting Membrane Fuel Cells II), pp.
435-445.
[29] I. Levine, “Physical Chemistry”, McGraw-Hill, Inc., 847 pages, 1979.
[30] J. Koryta, J. Dvořák, L. Kavan, “Principles of Electrochemistry”, 2nd ed., John Wiley &
Sons, Ltd., 486 pages, Chichester, UK, 1993.
[31] E. Wakefield, “History of the electric Automobile, Battery-Only Powered Cars”, 1st ed., Society of Automobile Engineers, Inc., Warrendale, PA, USA, 1994.
[32] T. Springer, M. Wilson, S. Gottesfeld, “Modeling and Experimental Diagnostics in Polymer Electrolyte Fuel Cells”, J. Electrochem. Soc. 140, pp. 3513-3526, 1993.
[33] W. Wruck, R. Machado, T. Chapman, “Current Interruption-Instrumentation and Applications”, J. Electrochem. Soc. 134, pp. 539-546, 1987.
[34] F. Büchi, A. Marek, G. Scherer, “In Situ Membrane Resistance Measurements in Polymer Electrolyte Fuel Cells by Fast Auxiliary Current Pulses”, J. Electrochem. Soc.
142, pp. 1895-1901.
[35] T. Springer, T. Zawodzinski, M. Wilson, S. Gottesfeld, “Characterization of Polymer Electrolyte Fuel Cells Using AC Impedance Spectroscopy”, J. Electrochem. Soc. 143(2), pp. 587-599, 1996.
[36] T. Zawodzinski, T. Springer, J. Davey, R. Jestel, C. Lopez, J. Valerio, S. Gottesfeld, “A comparative study of water uptake by and transport through ionomeric fuel cell membranes”, J. Electrochem. Soc. 140, pp. 1981-1985, 1993.
[37] G. Janssen, M. Overvelde, “Water transport in the proton-exchangemembrane fuel cell:
measurements of the effective drag coefficient”, J. Power Sources 101, pp. 117-125, 2001.
[38] J. Kolde, B. Bahar, M. Wilson, T. Zawodzinski, S. Gottesfeld, “Advanced composite polymer electrolyte fuel cell membranes”, Proc. - Electrochem. Soc. Vol. 95-23, (Proton Conducting Membrane Fuel Cells I), pp. 193-201.
[39] M. Watanabe, H. Uchida, Y. Seki, M. Emori, & P. Stoneheart, “Self- Humidifying Polymer Electrolyte Membranes for Fuel Cells”, J. Electrochem. Soc. 143(12), pp.
3847-3852, 1996; M. Watanabe, “Solid polymer electrolyte fuel cell”, European Patent Application EP 589 535 A1, Bulletin 94/13, 1994.
[40] A. West, T. Fuller, “Influence of rib spacing in proton-exchange membrane electrode assemblies”, J. Appl. Electrochem. 26, pp. 557-565, 1996.
[41] N. Djilali, D. Lu., “A multi-component, multi-dimensional model for heat and mass transport in proton-exchange membrane fuel cells”, In: Proceedings of the 1998 Fuel Cell Seminar 16.-19.11, Palm Springs, California, USA, 1998.
[42] T. Zawodzinski, S. Gottesfeld, S. Stoichet, T. McCarthy, “The contact angle between water and the surface of perfluorosulfonic acid membranes”, J. Appl. Electrochem. 23(1), pp. 86-88, 1993.
[43] Donald A. Neamen, “Semiconductor Physics and Devices”, McGraw-Hill Higher Education, 2003.
[44] L. Walter, “Silicon microstructuring technology”, Materials science and engineering, R17, pp. 1-55, 1996.
[45] D. B. Lee, “Anisotropic etching of silicon “, Journal of Applied physics, Vol. 40, No. 11, pp. 4569-4574, 1969.
[46] P. J. Hesketh, C. Ju, and S. Gowda, “Surface free energy model of silicon anisotropic etching”, J. Electrochem. Soc., Vol.140, No.4, pp. 1080-1084, 1993.
[47] H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, “Anisotropic etching of crystalline silicon in alkaline solution-Part I. Orientation dependence and behavior of passivation layer”, J. Electrochem. Soc., Vol. 137, No. 11, pp. 3612-3626, 1990.
[48] H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, “Anisotropic etching of crystalline silicon in alkaline solution-Part II. Influence of dopants”, J. Electrochem. Soc., Vol. 137, No. 11, pp. 3626-3632, 1990.
[49] D. R. Ciarlo, “Corner compensation structures for (110) oriented silicon”, IEEE Micro Robots and Teleoperators Workshop, pp. 6/1-4, 1987.
[50] Seidel H., “The Mechanism of Anisotropic , Electrocheical Silicon Etching in Alkaline Solution” IEEE Solid –State Sensor and Actuator Workshop, Hilton Head Island, pp.
86-91, 1990.
[51] E. D. Palik, O. J. Glembocki, I. Heard, P. S. Burno, L. Tenerz, “Etching roughness for (100) silicon surfaces in aqueous KOH”, J. Appl. Phys. 70 (6), pp. 3291-3300, 1991.
[52] T. Baum, D. J. Schiffrin, “AFM study of surface finish improvement by ultrasound in the anisotropic etching of Si<100> in KOH for micromachining applications”, J. Micromech.
Microeng. Number 4, pp. 338-342, 1997.
[53] S. S. Tan, M. L. Reed, H. Han, R. Boudreau, “Mechanisms of etch hillock formation”, J. Micro Electro Mechanical Systems, Vol. 5, No. 1, pp. 66-72, 1996.
[54] Y. K. Bhatnagar and A. Nathan, “On pyramidal protrusions in anisotropic etching of
<100> silicon”, Sensors and Actuators A, Vol. A36, pp. 233-240, 1993.
[55] W. K. Choi, J. T. L. Luo, P. Tan, C. M. Chua, T. H. and Y. Bai, “Characterisation of pyramid formation arising from the TMAH etching of silicon”, Sensors and Actuators A, Vol. A71, pp. 238-243, 1998.
[56] L. M. Landsberger, S. Naseh, M. Kahrizi and M. Paranjape, “On hillocks generated during anisotropic etching of Si in TMAH”, Journal of Microelectromechanical System, Vol. 5, No. 2, pp. 106-116, 1996.