Evaluation of RuCORE@PtSHELL Structure for the Oxygen Reduction Reaction (ORR) with Methanol-Crossover during DMFC Operation

Abstract

Direct Methanol Fuel Cells (DMFCs) is a promising energy conversion device operating in the low temperature operating range, and is especially appropriate for portable applications. Nevertheless, there are still many problems that need to be solved before any market penetration can be expected. Electrodes with high loadings of expensive platinum-based electrocatalysts, such as PtRu/C and Pt/C, are found as the best catalysts for methanol electrooxidation and oxygen reduction, respectively. However, while the anodic electrocatalyst suffer a serious deactivation due to the strong adsorption of methanolic residues, mainly CO, the ORR on platinum is limited by its slow kinetics. In addition, non-reacted methanol can diffuse through the solid (polymer) electrolyte (typically, a Nafion® membrane) towards the cathode due to the relative high concentration of methanol at the anode (the so-called methanol-crossover). Thus, methanol competes with oxygen for the platinum adsorption sites on the cathode and causes mixed potentials which reduce the open-circuit potential and, as a result, a serious decrease in the performance of the DMFC-device.To solve the above setbacks, one approach is to develop membranes with lower permeation for alcohols, as well as the design of cathodic electrocatalysts which operates successfully with the presence of methanol.RucorePtshell/C has shown a superior CO-tolerance than single platinum, which is assigned to an electronic effect towards the Pt shell from the covered Ru cores[1]. Additionally, it also exhibit higher stability on the anodic performance for the methanol oxidation reaction (MOR) at elevated temperatures in a single fuel cell, compared with PtRu/C[2]. Previous experiments suggest changes in the reaction mechanism for both the MOR and the ORR for the core-shell structure, related to changes induced in the energy of the d-band of the Pt shell. Results so far indicate that the Ru@Pt core-shell structures would be useful in anode fed with carbon monoxide and hydrogen (reformate), DMFC (methanol-fed) anodes, and DMFC cathodes (exposed to methanol-crossover), whereas in hydrogen-oxygen fuel cells little is gained by introducing Ru@Pt.This contribution simulates the cathodic behavior of the RucorePtshell/C catalyst, by conventional electrochemical methods and, tested in a single cell directly fed with methanol. We demonstrate results, showing a much lower increase in the overpotential for the core-shell structure over a Pt/C catalyst with similar total catalyst loading when methanol is present compared to no methanol present.The presence of ruthenium in the structure thus improves the tolerance towards methanol-crossover along the ORR, even when this ruthenium is not present in the catalyst surface, which indicates the purely electronic nature of the effect of core ruthenium on shell platinum.