Kinetics and Stoichiometry of Methanol and Ethanol Oxidation in Multi-Anode Proton Exchange Membrane Cells

Abstract

The anode catalyst of a direct alcohol fuel cell (DAFC) influences its energy efficiency through both its effect on the cell potential and the reaction stoichiometry (average number of electrons released per fuel molecule; nav). A method for determining these parameters simultaneously from a polarization curve (current vs. cell potential) is reported and various catalysts have been evaluated in a multi-anode cell. The cell was operated in crossover mode, in which fuel flows though the cathode chamber and diffuses though the membrane to be oxidized at the anode, to provide controlled mass transport conditions. Tafel analysis at low potentials provides kinetic information, while currents at high potentials provide nav values and their potential dependence. The method allows a number of catalysts to be compared under the same conditions, and provides characteristic parameters that could be compared across research groups. It is shown that while PtRu alloy catalysts provide faster kinetics than Pt for both methanol of ethanol oxidation, Pt can provide much higher stoichiometries for ethanol oxidation. The value of this methodology for catalyst screening is demonstrated with mixed Pt/C + PtRu/C anodes which show a pronounced synergistic effect relative to the individual Pt/C and PtRu/C catalysts.