Determination of boron in fuel cell catalysts using prompt gamma activation analysis

The Palladium--Cobalt alloy system was studied for potential Proton Exchange Membrane Fuel Cell catalyst applications. The focus of this study was the determination of the effects of boron content on the structure and electrochemical behavior of Pd--Co alloys. Boron is taken up by the alloy system during the borohydride reduction of the catalyst to nanoparticle size. The boron contents of the catalysts were determined using Prompt Gamma Activation Analysis, while alloy structure and electrochemical behavior were determined using X--Ray Diffraction (XRD) and Cyclic Voltammetry (CV), respectively. Alloys were tested at three heat treatment levels: as--prepared, 500 °C and 900 °C. The as--prepared samples retained the greatest amount of boron, but the samples heat treated at 500 °C revealed minimal losses in the boron content. A significant change in the boron content was only observed for samples heat treated at 900 °C. A comparison of the prepared alloys showed that the boron content increased with increasing cobalt content of the alloy for the alloys and heat treatments tested. Several other preparation methods were also tested for effects on catalyst properties. The XRD measurements showed substantial alloy formation and particle growth with increasing heat treatment temperature. CV measurements revealed the best performance for the Oxygen Reduction Reaction (ORR) in the as--prepared samples. The catalyst prepared with the reverse microemulsion method also showed increased reaction kinetics over catalysts prepared by the conventional borohydride reduction. These facts led to the conclusion that particle size has a dominant effect over boron content for ORR catalyst activity.