Multiphase Anode Catalysts for Ethanol Oxidation in Direct Ethanol PEM Fuel Cells (DEFCs)

Fuel cells convert chemical energy stored in fuels directly into electricity. The hydrogen-oxygen proton exchange membrane (PEM) fuel cell is limited by the need to store a gaseous fuel; the development of a liquid-fed fuel cell like the direct ethanol fuel cell (DEFC) is therefore pursued. A catalyst that oxidizes ethanol completely to CO2 - releasing 12 electrons - has not been developed. Once an effective catalyst is identified, all higher order alcohols will become potential fuels, dramatically increasing the available volumetric energy density for power generation. To this end, Pt-based catalysts that were prepared using microwave-assisted polyol synthesis, a method that enables rapid synthesis of multicomponent catalysts, were characterized using electron microscopy and x-ray diffraction techniques. Ethanol oxidation activity was investigated using both electroanalytical techniques and fuel cell tests. Results show that PtRhSnO2 and PtSnO2TiO2 have higher activities than PtSnO2, the best bimetallic system. The current-voltage response was found to have a strong dependence on ethanol concentration. Analysis of reaction products of ethanol oxidation at 130°C showed that the CO2 yield decreases significantly with increasing concentration for all catalysts, suggesting that ethanol oxidation is self-inhibiting. On pure Pt, acetaldehyde is the dominant product, while acetic acid is favored for metal oxide-containing catalysts, suggesting that the metal oxide facilitates the oxidation of acetaldehyde to acetic acid. The oxidation of acetaldehyde and acetic acid was also studied, and it was found that acetaldehyde is easily oxidized to CO2 and acetic acid, with CO2 being the preferred product on Pt. It was also found that acetic acid can be oxidized to CO2 on all catalysts, and that it is not a dead end product in ethanol oxidation. A detailed mechanism for ethanol oxidation was then proposed.