SURFACE AND CATALYTIC PROPERTIES OF IRON-PLATINUM CARBON ELECTROCATALYSTS FOR CATHODIC OXYGEN REDUCTION IN PAFC

Abstract

A Fe-Pt bimetallic catalyst was prepared by impregnating iron nitrate solution into a carbon-supported platinum catalyst. The catalyst was heat treated in flowing Ar (nontreated) at 450-degrees-C (denoted as PF450), 750-degrees-C (PF750), and 900-degrees-C (PF900), or this process was followed by an acid-treatment in 1M H2SO4 solution to leach out surface-enriched iron (acid-treated). The surface and catalytic properties of these catalysts were studied using H2-O2 titration, XRD and TEM measurements, and oxygen reduction tests in PAFC. With increased heat-treatment temperature, the Pt surface area measured by hydrogen chemisorption (S(CHEM)) decreases rapidly owing to particle sintering, alloying effect, and enrichment of iron on the surface. The mass activity (mA/g Pt) of the alloyed catalyst is about the same as that of pure Pt catalyst due to the particle sintering in the alloyed catalyst. However, the specific activity (mA/m2 Pt, based on S(CHEM)) of the alloyed catalyst is estimated at twice that of pure Pt catalyst. The acid-treatment leads to dissolution of surface-enriched iron and results in an increase of the Pt surface area due to surface roughening. Except for PF450, the specific activities of both nontreated and acid-treated catalysts are the same, indicating that S(CHEM) is a good measure of active site for the reaction. After acid treatment of the partially alloyed PF750, a two-fold increase in the Pt surface area (therefore, twice the increase in the MA) was observed when compared with the nontreated Pt-Fe alloy catalyst.