INVESTIGATION OF ELECTROCHEMICAL STABILITY OF THE CATALYSTS AND CATALYST SUPPORTS FOR OXYGEN REDUCTION IN PEMFC.

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1 INVESTIGATION OF ELECTROCHEMICAL STABILITY OF THE CATALYSTS AND CATALYST SUPPORTS FOR OXYGEN REDUCTION IN PEMFC. PROPERTIES OF MESOPOROUS TITANIA BASED PLATINUM CATALYST FOR OXYGEN REDUCTION. Victor Multanen Ariel University Center of Samaria Department of Biological Chemistry Laboratory of fuel cells and electrochemistry Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are emerging as promising candidates in the portable electronics and automobile industries 1. In contrast to the early development of PEMFCs, current research focuses extensively on the improvement of fuel cell reliability and durability. It has been shown that several factors can reduce the lifetime of PEMFCs, including: platinum-particle cathode catalysts dissolution and sintering 2, carbon support corrosion, and membrane thinning. 3 Carbon is the typical support material in fuel cell due to its large surface area, high electrical conductivity, and well-developed pore structures. However, carbon support in the oxygen electrode is subjected to severe corrosion in the present of water at potentials above 0.9V via the following reaction 4 : C+2H 2 O CO 2 + 4H + + 4e - (0.207 vs. NHE at 25 0 C) [1] Agglomeration of the Pt catalyst on the carbon surface increases as carbon corrosion becomes more severe. This effect causes the performance of the catalysts to degrade quickly, resulting in shortening of the fuel cell's lifetime beyond the requirement of most applications. The main characteristics of an ideal support for PEMFC electrocatalysts are: high electrical conductivity; high surface area; cooperative metal-support interaction. Thus maximize the triple-phase interface improve good water management capability on the mesoporous support structure to avoid electrode flooding and improve corrosion resistance, especially at the cathode side. 2-72

2 Various alternative support materials have been synthesized and investigated. 5-8 all with specific advantages and limitations Research Objective Investigate corrosion stable supports based on metal oxides (e.g. TiO2, SnO 2 ) and Pt based catalysts for oxygen reduction in acidic polymer electrolyte membrane fuel cells (PEMFC). Experimental Durability is the ability of a PEMFC or stack to resist permanent change in performance over time 9. The requirements of US Department of Energy for lifetime performance of fuel cell are very clear: 5000 h for cars to 20,000 h for buses, and 40,000 h of continuous operation for stationary applications. For example, normal degradation targets require less than 10% loss in the efficiency of the FC system by the end of its life, and a degradation rate of 2 10 μv h 1 is commonly accepted for most applications 10 We have used a catalyst stability protocol of the US department of energy (DOE) protocol for Cathode durability tests. The protocol uses two techniques to determine losses in catalytic activity using electrochemical surface area and polarization curve measurements. The electrochemical surface area (ECSA) of the electrode is estimated based on the relationship between the surface area and the H desorption charge from the electrode, as determined from the CV (cycling voltammetry) measurement. The hydrogen atom desorption charge on a smooth Pt electrode has been measured to be 210 C/cm 2 of Pt loading in the CL. The ECSA of electrodes is then calculated using the following equation 11 ECA(cm 2 Pt/gPt) =[ charge( C/cm 2 )]/[ 210( C/cm 2 Pt) *catalyst loading(gpt/cm 2 )] [2] A plot of cell potential against current density under a set of constant operating conditions (pressure, H 2 /O 2 flow rate, humidity and temperature), known as a polarization curve, is the standard electrochemical technique for characterizing the performance of fuel cells. It yields information on the catalytic activity losses in the cell or stack. The expedite stability tests proposed by the DOE includes performing set of potential steps applied to the tested electrode, followed by measurements of ECA and polarization curve in the fuel cells. 2-73

3 Results and discussions Experimental conditions The cathode durability on home-made gas humidifier fuel cell testing station and commercial 5cm 2 "Electrochem" single cell stack were obtained. The commercial unsupported Pt (Johnson Matthey) catalyst with ratio 1:1 to carbon Vulcan (specific area is 250m 2 /g) was prepared with Pt loading 2 mg/cm 2, Temp of cell is 80 0 C, relative humidity 100% and the H 2 flow rate through anode is 45 cc/min and of N 2 through the cathode is 95 cc/min. ECSA on a cell before and after applying potential working cycles was obtained with potential steps volt vs. NHE during 30 sec (full step is 1min).The results were measured and presented in table 1 and figure 1. Table 1. Properties of the cathode in the durability test for Pt 50%wt. on C fuel cell Number of cycles Electrochemical surface area m 2 /g Pt Double layer capacitance m 2 /g Pt

4 Figure 1. Polarization curve of :a)new fuel cell, after b) 1000 cycles c) 2000cycles d)3000cycles e) 3500 cycles f) 4000 cycles, surface area of the electrode 5cm 2. Synthesis and characterization of Pt /TiO 2 catalyst The mesoporous titania can be prepared by using the sono-chemistry method with surfactant, that allows you to control the size of the created pores (5-50nm diameter) 12,13 In prior studies, has been developed the techniques for this type of Titania particles, as well as to create a catalyst but rather directly to titania. In this study, was investigated the catalitic activity of the titania based platinum catalyst in comparison with different loading of platinum on titania. Surface of catalyst was measured by using of BET method. To comprise surface of catalyst was obtained an electrochemical cyclic voltammetry (CV) experiment with 3-electrode electrochemical cell. Working electrode is Glassy-carbon electrode, counter electrode is 2-75

5 platinum wire, reference electrode is normal hydrogen electrode.h 2 SO M is electrolyte. Electrochemical window was 0-1Volt. From CV measurements with scan rate 20, 50, 100,200 mv/sec was build a graph of Δi (anodic current cathodic current) at 0.5 V vs. scan rate to obtane a calculation of Double Layer Capacitance (DLC). 14 The electrochemical surface area (ECSA) of the electrodes is then calculated using the equation [2]. Table 2. Physical and electrochemical properties of Pt on TiO2 catalyst in comparison with commercial Pt on C catalyst material Surface Area BET method DLL cm 2 /g ECSA cm 2 /g Pt cm 2 /g 10:90 Pt:TiO :50 Pt:TiO :30 Pt:TiO Commercial Pt 50% wt. on Carbon Vulcan % TiO % Carbon Figure 2. Micrographs of the a) mesoporous titania and b)pt 10% wt. on mesoporous titania a b 2-76

6 Future experiments The physical and electrochemical characterization of the new materials will be obtained by the techniques: XPS, XRD, FTIR, BET, CV, ORR, FC durability tests and polarization curves. References (1) Knights, S. D.; Colbow, K. M.; St-Pierre, J.; Wilkinson, D. P. J. Power Sources 2004, 127, 127. (2) P. J. Ferreira, G. J. la O, Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha and H. A. Gasteiger, J. Electrochem. Soc., 2005, 152, (11), A2256 (3) (a) Chalk, S. G.; Miller, J. F. J. Power Sources 2006, 159, 73. (b) Wilson, M. S.; Garzon, F. H.; Sickafus, K. E.; Gottesfeld, S. J. Electrochem. Soc.1993, 140, (4). K. Kinoshita, Carbon: Electrochemical and Physicochemical Properties, p. 319, Wiley, New York, (1988). (5) Chhina, H.; Campbell, S.; Kesler, O. J. Power Sources 2007, 164,431. (6) Ganesan, R.; Ham, D. J.; Lee, J. S. Electrochem. Commun. 2007,9, (7) Travitsky, N.; Ripenbein, T.; Golodnitsky, D.; Rosenberg, Y.;Burshtein, L.; Peled, E. J. Power Sources 2006, 161, 782. (8) S. Shanmugam, A. Gedanken J. Phys. Chem. C 2009, 113, (9) Haijiang Wang, Xiao-Zi YUan, Hui Li PEM Fuel Cell Diagnostic Tools, p. 9, CRC Press, Boca Raton, (2012) (10) Wu, J., Yuan, X. Z., Wang, H., Blanco, M., Martin, J. J., and Zhang, J. 2008b., Part I, Electrochemical techniques. Int. J. Hydrogen Energy 33: (11) T. R. Ralph, G. A. Hards, and J. E. Keating J. Electrochem. Soc., Volume 144, Issue 11, pp (1997). (12) Yang-Qin Wang, Si-Guang Chen, Xiang-Hai Tang, Oleg Palchik, Arie Zaban,* Yuri Koltypin, and Aharon Gedanken Mesoporous titanium dioxide: sonochemical synthesis and application in dye-sensitized solar cells, J. Mater. Chem., 2001, 11,

7 (13) Sangaraju Shanmugam and Aharon Gedanken, Synthesis and Electrochemical Oxygen Reduction of Platinum Nanoparticles Supported on Mesoporous TiO2, J. Phys. Chem. C 2009, 113, (14) Eliezer Gileadi, Electrode kinetics for chemists, chemical engineers and material scientists, p