THE MORPHOLOGICAL AND SURFACE-PROPERTIES AND THEIR RELATIONSHIP WITH OXYGEN REDUCTION ACTIVITY FOR PLATINUM-IRON ELECTROCATALYSTS

Abstract

A series of iron-platinum bimetallic and pure platinum catalysts supported on carbon were prepared and characterized by means of H-2-O-2 titration, X-ray diffraction, TEM, XPS and cyclic voltammetry. Their catalytic activities for the electrochemical reduction of oxygen in 100% phosphoric acid were related to their physical properties. When the bimetallic catalysts were heated at 750 degrees C, a disordered alloy phase was formed. An almost completely ordered superlattice structure, Pt,Fe, was achieved when heated at 900 degrees C. The mass activity showed a maximum value as a function of Pt particle size for both pure platinum and platinum-iron catalysts, and no noticeable enhancement in the activity was found by alloying platinum with iron. The specific activity increased as the particle size was increased for pure platinum catalysts, whereas it was nearly independent of the particle size for Pt-Fe alloy catalysts. The specific activities of Pt alloys were much higher than those of pure platinum over the tested range of particle size. Therefore, it is expected that a Pt alloy catalyst having a superior mass activity can be made only if metal particles are kept extremely small; ie appreciately smaller than 3 nm. The increase in the specific activity is caused by increased chemisorption strength of hydrogen and simultaneous decrease in the oxygen chemisorption strength in the cathode.