HIGH-PERFORMANCE, LOW PT CONTENT CATALYSTS FOR THE ELECTROREDUCTION OF OXYGEN IN POLYMER-ELECTROLYTE FUEL-CELLS

Pt-included and Pt-supported catalysts have been synthesized using gra phite and carbon black supports of various specific areas. The graphic s are KS6 (20 m(2)/g), HS100 (110 m(2)/g), and HS300 (305 m(2)/g) from Lonza, and the carbon blacks are Vulcan (254 m(2)/g) and Black Pearls (1475 m(2)/g) from Cabot. The Pt-included and Pt-supported catalysts were used at the cathode of a H-2/O-2 fuel cell, and their polarizatio n curves were compared to each other and to those of various Pt-suppor ted catalysts from E-TEK. In the high current region of interest to fu el cell developers, it is shown that Pt-supported catalysts perform be tter than Pt-included ones when the specific area of the support is sm all. The contrary is true when the specific area of the support is lar ge. The best catalysts are HS300-Pti [8.3 weight percent (w/o) Pt incl uded in HS300 graphite] and Vu-Pti (6.1 w/o Pt included in Vulcan XC-7 2R). These catalysts display very high mass and specific activities fo r O-2 reduction. Furthermore, the iR-corrected polarization curves of both HS300-Pti (with a Pt loading of 0.110 mg/cm(2)) and Vu-Pti (with a Pt loading of 0.070 mg/cm(2)) cross at high current the polarization curve of the cm electrode prepared with E-TEK20 (20 w/o of supported Pt, with a Pt loading of 0.287 mg/cm(2)). Pt inclusion in graphite or carbon black is therefore an interesting way of reducing the Pt loadin g of fuel cell cathodes without lowering electrochemical performance. HS300-Pti and Vu-Pti have been characterized by x-ray diffraction, tra nsmission electron microscopy, and x-ray photoelectron spectroscopy. T hese analyses indicate that they both contain metallic Pt and Pt(II an d IV) oxides and/or hydroxides.