Philosophiae Doctor - PhD / Direct methanol fuel cells (DMFCs) are attractive power sources as they offer high conversion efficiencies with low or no pollution. One of the major advantages DMFCs has over PEMFCs is that methanol is a liquid and can be easily stored where in the case for PEMFCs storing hydrogen requires high pressures and low temperatures. However, several challenging factors especially the sluggish oxygen reduction reaction (ORR) and the high cost of Pt catalysts, prolong their commercialization. With the aim to search for more active, less expensive more active ORR catalysts and methanol tolerant catalysts than pure Pt, this dissertation focuses on the development of low loading Pt electrocatalyst and the understanding of their physical and electrochemical properties. Pt-Sn/C electrocatalsyts have been synthesized by a modified polyol reduction method. The effect of temperature, pH, water, sonication and addition of carbon form were studied before a standard polyol method was established. From XRD patterns, the Pt-Sn/C peaks shifted slightly to lower 2Ө angles when compared with commercial Pt/C catalyst, suggesting that Sn is alloying with Pt. Based on HRTEM data, the Pt-Sn/C nanoparticles showed small particle sizes well-dispersed onto the carbon support with a narrow particle distribution. The particle sizes of the different as-prepared catalysts were found to be between 2-5 nm. The Pt-Sn/C HA Slurry pH3 catalysts was found to be the best asprepared catalyst and was subjected to heat-treatment in a reducing atmosphere at 250-600 °C which led to agglomeration yielding nanoparticles of between 5-10 nm. The Methanol Oxidation Reaction (MOR) on the as-prepared Pt-Sn/C HA Slurry pH3 catalyst appeared at lower currents (+7.11 mA at 860 mV vs. NHE) compared to the commercial Pt/C (+8.25 mA at +860 mV vs. NHE) suggesting that the Pt-Sn/C catalyst has „methanol tolerance capabilities‟. Pt-Sn/C HA Slurry pH3 and Pt-Sn/C 250 °C catalysts showed better activity towards the ORR than commercial Pt/C with specific and mass activities higher than Pt/C at +0.85 V vs NHE. The Tafel slopes of Pt-Sn/C HA Slurry pH3 catalyst was -62 and -122 mV dec-1 for the low and high current regions respectively and suggests that the ORR mechanism is similar to that of commercial Pt/C indicating that the ORR kinetics was not negatively influenced by the addition of tin. It was found that the electrochemical oxidation reduction reaction follows first order kinetics of a multi-electronic (n=4ē) charge transfer process producing water. All the Pt-Sn/C catalysts showed resistance towards MOR and it was found for the heat-treated catalysts that an increase in temperature resulted in an increase in methanol tolerance. The synthesized Pt-Sn/C HA Slurry pH3 catalysts were also tested in a fuel cell environment. Electrodes were prepared by either spraying on Toray carbon paper with the Asymtek machine or by VI spraying directly on the membrane with a hand spray gun the catalysts coated membrane (CCM) technique. Polarization curves obtained in DMFC with CCM showed superior performance than electrode prepared by spraying on the carbon paper with the machine. In our study, the Pt-Sn/C catalyst appears to be a promising methanol tolerant catalyst with activity towards the ORR in the DMFC.

Importance of clean, sustainable and renewable energy sources are increasing gradually because of either being environmental friendly or being alternative for fossil fuels. Hydrogen energy system will let the utilization of alternative energy sources. Fuel cells are the most suitable energy conversion devices while passing through the hydrogen economy. The cost of the fuel cell systems need to be reduced in order to achieve commercialization of these systems. One of the most important cost items is platinum which is used as catalyst both in anode and cathode sides of the proton exchange membrane (PEM) fuel cells. Not only is the cost of the platinum, but also the limited reservoir of the platinum is a handicap. Therefore, the utilization of the cheap replacements of platinum catalysts will accelerate the process of commercialization. Because of their highly conjugated structure and high chemical stability metalo phthalocyanines have been encouraging electrocatalytic activity for oxygen reduction. Therefore, electrocatalytic activity for oxygen reduction in fuel cells was studied with some metalo phthalocyanines and some positive effects have been observed. In this study, phthalocyanines of cobalt, iron and nickel were synthesized via phthalic anhydride-urea method and characterized by IR Spectrophotometry, X-Ray Diffractometry and Thermal Gravimetry (TGA). Catalyst materials were prepared by impregnation method such that they contain either 4% cobalt, 4% or 10% iron or 4% nickel phthalocyanines on carbon black (Vulcan XC72) structure. Impregnated catalysts were pyrolyzed at 600oC or 1000oC and cathode electrodes were prepared by these catalysts as well as unpyrolyzed ones by spraying technique. The impregnated catalysts were characterized by scanning electron microscopy (SEM) and pore structures were analyzed by surface area analyzer (by BET and BJH techniques). All of the anode electrodes were prepared by using 20% Pt containing commercial catalyst by the same technique applied for cathode electrodes. A membrane electrode assembly was also prepared by 20% Pt containing commercial catalyst on the cathode electrode. Performance characteristics of the manufactured membrane electrode assemblies were determined by means of a test station, built in Middle East Technical University Chemical Engineering Department, having a 5 cm2 test cell. The highest performance observed with the commercial membrane electrode assembly was 0.40W/cm2 at 0.5 V. Whereas, the power density obtained from the MEA manufactured at the laboratory having 0.4 mg Pt/cm2 loading both on the anode and cathode was 0.18 W/cm2 at 0.5 V. For the phthalocyanine cathodic MEAs, the highest power reached was 0.04W/cm2 which was obtained from the MEA having a loading of 0.28mg Co/cm2 prepared by using the CoPc/C catalyst pyrolyzed at 1000 oC.

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