An investigation into stability of palladium-based catalysts for oxygen reduction

Abstract

Although platinum-based catalysts are regarded as the most active and stable catalysts for oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cell (PEMFC), platinum’s prohibitive cost and scarcity limits its widespread use for commercial applications. Therefore, the cheaper and more available palladium has been extensively researched as an alternative to platinum. Although palladium has also been reported to have a lower stability than platinum under acidic fuel cell conditions, most of the research on palladium-based catalysts have focussed on improving its ORR activity rather than its stability. In this research, the ORR activity and stability of palladium-based catalysts were studied with a view of enhancing both. Palladium-based catalysts were synthesised by modifying palladium with gold and iridium well as metal oxides such as TiO2 and WO3 known to be stable in acid. The catalysts investigated were PdAu/C, Pd3Au/C, PdIr/C, PdIrAu/C, PdIr/TiO2-C and PdIr/WO3-C. The catalysts were evaluated for ORR by voltammetry in a three-electrode cell and were characterised with techniques such as XRD, TEM and XPS to relate their physical properties to their electrochemical behaviour; before and after durability studies. The catalysts were synthesised using a polyol method with ethylene glycol as a mild reducing agent that yielded small nanoparticles around 5 nm. The electrochemical surface areas obtained were 15.5, 6.2 and 2.5 m2g-1 for the Pd/C, Pd3Au/C and PdAu/C catalysts respectively. The stability tests indicated that gold-modified PdAu/C and Pd3Au/C had higher stability than unmodified Pd/C catalyst as they retained 38 %, 31% and 11 % of their initial chronoamperometric currents respectively over a two-hour period. In comparison to Pd/C, PdIr/C exhibited at least a two-fold increase in stability; measured by the percentage of initial electrochemical surface area (ECSA) retained by the catalysts after potential cycling. Ex-situ XPS and TEM analyses after the potential cycling revealed that the alloying interaction between palladium and iridium reduced palladium’s surface oxidation and particle agglomeration and hence improved PdIr/C’s stability. The use of TiO2-modified support on palladium-iridium nanoparticles resulted in an enhancement of activity and stability, with PdIr/TiO2-C having a specific activity of 0.52 mAcm-2 and a 61 % ECSA retention; a twoand a three -fold increase compared to PdIr/C’s 0.23 mAcm-2 and 20 %. In summary, this study found the modification of palladium nanoparticles with gold and iridium could enhance its ORR stability in acid environment and that a further stability enhancement could be obtained by modifying the carbon support with acid-stable oxides such as TiO2 and WO3.