Influence of bismuth on the structure and activity of Pt and Pd nanocatalysts for the direct electrooxidation of NaBH4

In the past few years, borohydrides have gathered a lot of attention as an energy carrier for fuel cell application. Numerous investigations on both hydrogen generation and direct oxidation of NaBH4 have been published. Nonetheless, in our knowledge, only a few catalysts are capable to completely perform the direct oxidation of NaBH4 at low potentials without hydrogen evolution. In this work, carbon supported Pd1-xBix/C and Pt1-xBix/C nanocatalysts were synthesized by a water in oil microemulsion method. The influence of surface modifications of Pt and Pd by Bi on the electrooxidation of sodium borohydride in alkaline media was evaluated. Physical and electrochemical methods were applied to characterize the structure and surface of the synthesized catalysts. It was verified that bismuth is present at the surface of the bimetallic catalysts and that hydrogen adsorption/desorption reactions are strongly limited on Pt and Pd surfaces with high bismuth coverage. Although the onset potential for NaBH4 oxidation on PdxBi1-x/C catalysts is ca. 0.2 V higher than that for Pd/C, the presence of bismuth on palladium surface influences the reaction mechanism, limiting hydrogen evolution and oxidation in the case of Pd0.8Bi0.2 catalyst. On Pt0.9Bi0.1 catalyst the onset potential remains unchanged comparing to Pt/C and negligible hydrogen evolution was observed in the whole potential range where the catalyst is active. The number of exchanged electrons was calculated using the Koutecky-Levich equation and it was found that for Pt0.9Bi0.1 catalyst, ca. 8 electrons are exchanged per BH4- ion at low potentials. The presented results are remarkable evidencing that NaBH4 can be directly oxidized at low potentials with high energy efficiency. (C) 2010 Elsevier Ltd. All rights reserved.