Catalyst Layers for Liquid Fuel Cells

PEMFCs can be design to be fed with liquid fuels this method could lead to a very simple system with a fuel that has a relatively high energy density and is a liquid under ambient conditions. 

The meehanism of methanol oxidation on the Pt surface has been investigated extensively for decades. According to a well-described mechanism [80], methanol oxidation is a dehydronation process involving the formation of the CO-like intermediates. These intermediates adsorb on the surface of Pt to reduce the catalytic activity. The Pt-Ru alloy has been found to be the most active antipoisoning anode catalyst for DAFCs. The enhanced activity of the Pt-Ru catalyst for methanol oxidation has been attributed to a bi-functional mechanism [81]. This bi-functional mechanism involves the adsorption of oxygen-containing species on Ru atoms at lower potentials, thereby promoting the oxidation of CO to CO2, which can be summarized as follows [82]  

The catalytic activity of die Pt-Ru catalyst is strongly dependent on the composition, structure, morphology, particle size and alloying degree. 

The contact between the metal nanoparticles and Nafion micelles in the CL of a DMFC is affected by carbon-support-particle pore size and distribution [79]. Rao et al. investigated the effect of carbon porosity on the specific activity of the Pt-Ru/C catalyst for methanol oxidation [83]. They found that a higher content of small pores ( 20 nm) containing metal particles, where die Nafion ionomer could not easily enter, resulted in poor contact between die metal nanoparticles and 

Seott et al. pointed out that earbon dioxide gas will be generated in the anode strueture at a position dependent upon loeal eonditions of pressure, temperature, earbon dioxide solubility and bubble nueleation [85]. Due to the moderate solubility of earbon dioxide in the aqueous methanol solution, bubble generation will oeeur away from the thin anode eatalyst surface. In practice, the methanol solution is recycled to the cell from a reservoir and thus will contain a significant amount of dissolved carbon dioxide. Thus, it is likely, and is assumed here, that gas bubbles will be formed in the diffusion layer and flow counter-current to the mass transport of liquid. 

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