EFFECT OF SURFACE ROUGHENING ON THE CATALYTIC ACTIVITY OF PT-CR ELECTROCATALYSTS FOR THE OXYGEN REDUCTION IN PHOSPHORIC-ACID FUEL-CELL

A Pt-Cr bimetallic catalyst with an atomic ratio of 3 to 1 was prepared by the impregnation method using a Pt/C catalyst (Pt loading: 10 weight percent). The catalyst was subjected to heat-treatment from 400 to 1200 degrees C. Its physical properties were characterized by H-2-O-2 chemisorption, x-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy. Changes in the catalytic activity for oxygen reduction in PAFC were also examined. In particular, surface roughening caused by acid pretreatment and/or potential excursion was investigated to see its effect on the activity. Acid treatment (in 1M H2SO4 for 1 day) and/or potential excursion at a mild condition (with a limited upper potential of 0.9 V vs. RHE) effectively created surface roughening without showing particle growth via sintering. The surface roughening increased the Pt surface area and consequently mass activity (catalytic activity based on mass of Pt) of the catalysts due to the selective leaching of surface-enriched chromium species. Concerning the specific activity (catalytic activity based on the Pt surface area), Pt-Cr having a smaller lattice parameter than Pt-Fe or Pt showed better performance, and this rule could be extended for other alloy systems. The mass activity, which is more important for practical applications, could be enhanced substantially when the surface roughening was brought on the catalyst surface while keeping the metal particle size small. The largest enhancement in the mass activity was observed when a partially alloyed or disordered alloy of Pt-Cr catalyst was subjected to the acid treatment or to the potential excursion.