Reactivity of Bimetallic Nanoclusters Toward the Oxygen Reduction in Acid Medium

Abstract

The oxygen reduction reaction (ORR) on Pt and Pt-alloys, the slowest of the two electrode reactions of low temperature fuel cells, has been studied for a long time in an effort to fully understand its mechanism and therefore be able to develop improved catalyst materials which may significantly contribute to enhance the overall fuel cell efficiency [1, 2]. The standard potential for the four-electron reduction of oxygen in acid medium is 1.23V with respect to the standard hydrogen electrode. However, a negative overpotential – for the ORR – of about 0.3–0.5V is needed to start the reaction on a Pt electrode, Pt being the best catalyst known so far for this reaction [3]. This overpotential is usually attributed both to kinetic and mass transport limitations at the cathode electrode. Nanoscale proton-exchange membrane (PEM) electrocatalysts have been used since the 1960s [1]; however they are in most cases the result of lucky trial and error experimentation, and there is no assurance that they correspond to the optimal materials [2]. There are advantages and challenges due to the nature of the nanometer-scale regime for the electrocatalytic system; nanocatalysts not only provide enhanced reaction rates with respect to those obtained from catalysis on extended surfaces [4, 5, 6, 7, 8, 9], but most importantly, they may be suitable for alternative reaction paths that are available only because of the electronic characteristics at nanodimensions. Besides, the feature size of nanoscale systems allows a theory-guided and a controlled atomic manipulation that should enable the fabrication of nanosystems with very precise characteristics. Currently used electrode-catalysts (anode and cathode) consist of an assembly of metallic nanoparticles usually deposited on an electronic conducting substrate and embedded in a hydrated membrane [10, 11], which is the polymer electrolyte proton-conductive material (Figure 17.1). What differs between cathode and anode is the catalyst material, and also the significantly slow kinetics of the cathode oxygen reduction reaction compared to that of the anode hydrogen oxidation reaction. For this reason, several