COMBINATORIAL SCREENING OF ANODE AND CATHODE ELECTROCATALYSTS FOR DIRECT METHANOL FUEL CELLS

摘要

Progress in several important electrochemicaltechnologies, including batteries, fuel cells, sensors, andelectrosynthesis, is currently materials-limited. A commonfeature of all electrode reactions is tic imbalance (i.e., loss orgeneration) of ions at the electrode surface. We describe in thispaper a method by which excess ions in the electrode diffusionlayer can be imaged, and used to identify the best electrodematerials from a combinatorial array of compositions.Although in principle this method can be applied to manyelectrochemical problems, we have focused on finding betterelectrocatalysts for direct methanol fuel cells (DMFCs). TheDMFC performs two half-cell reactions: oxidation of methanol,and reduction of oxygen. Two of the most important problems inDMFCs are the poor performance of the electrocatalysts, and thecrossover of methanol from the anode to the cathode side of thecell. An ideal situation would be the simultaneous developmentof two new catalysts: an anode that oxidizes methanol at lowoverpotential, and a "methanol-tolerant" cathode that reducesoxygen without oxidizing methanol.Based on previously developed rules for predicting the activityof ternary alloy catalysts (Ley, et al., J Elecfrochem. Soc. 1997,144, 1543), we began searching quaternary combinations ofnoble metals for the anode, and ruthenium selenide-typematerials for the cathode reaction. The anode and cathodereactions generate and consume protons, respectively, creating asubstantial pH gradient at the electrode surface. Changes in localpH are imaged by means of an appropriate fluorescent indicator:Ni-PTP for the anode and Eosin Y for the cathode. DMFCtesting confirms the utility of the screening method, in that aPt/Ru/Os/Ir quaternary catalyst was substantially superior to thebest binary and ternary catalysts prepared under similar conditions.