Preparation of Pt-based Bimetallic Catalysts for the Oxygen Reduction Reaction in PEM Fuel Cells

Abstract

This work includes the preparation and characterization of carbon supported Pt-based bimetallic electrocatalysts and their subsequent electrochemical evaluation for the oxygen-reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. Monometallic Pt/C was modified using a surface organometallic chemistry (SOMC) route to prepare a series of PtFe/C, PtCr/C, PtNi/C and PtCo/C bimetallic electrocatalysts. The catalysts were studied as prepared, and after a range of different heat treatments. Physical characterization of unmodified Pt/C and the PtM/C bimetallic electrocatalysts was performed using a range of techniques. UV-visible spectroscopy, ICP, TGA and DSC were used to analyse the composition of the electrocatalysts and to postulate a reaction mechanism for the SOMC reaction during the electrocatalyst preparation. TEM/BDX, BET, chemisorption and X-ray diffraction examined particle size, showing that electrocatalysts prepared using the SOMC method sinter less readily than those prepared by conventional methods, and also provided evidence that a selective reaction had occurred between the reduced surface Pt of the Pt/C catalyst and the organometallic precursor of the second metal. XRD and EXAFS indicated that an alloy was formed between the surface Pt and the second metal in many of the heat-treated samples. EXAFS, TPR and XPS were used to examine the metal oxidation states in the PtM/C bimetallic electrocatalysts. The PtM/C electrocatalysts were evaluated for the oxygen-reduction reaction (ORR) in PEM fuel cells. The optimum electrocatalyst, was found to be 0.67 PtCr/C after heat treatment at 750 °C; this had high catalytic activity compared with the commercial electrocatalyst, PtCr/C(JM), provided by Johnson Matthey Technology Centre.