Oxygen reduction reaction on palladium-cobalt alloy catalysts for polymer electrolyte fuel cells

The Oxygen Reduction Reaction (ORR) activity in acid medium on Pd-Co was studied in this work. The catalysts were synthesized by two techniques; physical vapor deposition technique and ultrasonic spray reaction technique. The last technique was developed for the first time in our laboratory for the supported electro catalyst preparation and direct deposition onto the carbon paper or gas diffusion electrode the for PEMFC applications. The electrochemical properties such as the amount of hydrogen adsorption/desorption, the oxide formation/reduction of Pd-Co alloy catalyst have not been sufficiently studied before. Therefore these electrochemical properties were investigated by using the Pd-Co thin films prepared by sputtering method. A thin film catalyst cannot be directly used as an electrode of working PEMFCs, however the sputtering method is very useful since the chemical composition of alloy and surface area of the electrode can be controlled easily. Thus the fundamental electrochemical properties such as the amount of hydrogen adsorption/desorption, oxide formation/reduction and oxide reduction peak position on thin films of Pd-Co alloy, Pd and Pt catalysts were determined and their correlations to ORR catalytic activities in acid medium were studied. Enhancements of the catalytic activities for ORR by Pd-Co binary alloys were found to be in agreement with results obtained in previous studies.;Ultrasonic spray reaction method was developed for the first time in our laboratory for carbon supported nano-scale catalyst for PEMFC application. Fine catalyst particles supported on high surface area carbon powder are required to apply the catalyst as the PEMFC cathode materials for the commercialization, but none of the studies done before were able to successfully obtain the Pd-Co fine particles which are comparable with the existing carbon supported platinum catalyst (ϕ2-4nm). Therefore the establishment of the catalyst synthesis method for Pd-Co fine particles are required to use the catalyst for PEMFCs. By using this method established in this study, carbon supported Pt, Pd and Pd-Co catalysts were synthesized and characterised for ORR activity. TEM images indicate that this technique is very useful for preparing carbon supported nano-scale catalysts having the dominant particle size of 2.5-4.5 nm. XRD showed diffraction peaks consistent with face-centered cubic (fcc) structure for Pt. XRD of the synthesized Pd and Pd-Co catalysts by ultrasonic spray reaction also indicated fcc crystal system. All diffractograms of the samples are similar to the structure of fcc Pd, but the Co-origin peaks cannot be found. The Pd-Co system is substitutional solid solutions where some Pd atoms are replaced by Co atoms. The solid solution system is considered to be stable in acid media.;We developed this new technique of the ultrasonic spray reaction method not only for nano-scale carbon supported catalyst synthesis but also fabricate directly catalyzed GDE. By using carbon paper as a filter of ultrasonic spray reaction method as described above, carbon supported catalyst are directly deposited onto the carbon paper. It directly forms catalyzed GDE. This technique saves some step of the GDE fabrication and will leads lower cost of PEMFC.;Catalytic activity enhancements of ORR for the synthesized Pd-Co catalysts by ultrasonic spray reaction method are confirmed. This result has good agreement with the ORR activity enhancement of the thin film synthesized by PVD. For a given potential, the ORR current of Pd3Co1/C and Pd 2o1/C were almost the same and show the highest values among the values of all the other alloys. After, the value of the current decreases from Pd5o1/C to Pd1o1/C and Pd/C. All Pd based catalysts have around 45mV/dec of tafel slope and Pd mass corrected exchange current around 10-11 mA mg-1. It is assumed that the ORR kinetics in this potential region are the same among the Pd and Pd based catalysts and the addition of Co into Pd have small effect on the ORR kinetics. We conclude that ultrasonic spray reaction method with piezoelectric transducer is applicable for Pd-Co binary alloy catalyst synthesis and the activity enhancement effect caused by alloying with Co was confirmed on the synthesized catalyst by this technique. (Abstract shortened by UMI.).