Synthesis and characterization of pt-sn/c cathode catalysts via polyol reduction method for use in direct methanol fuel cell
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.