ABSTRACTBORON DOPED ULTRANANOCRYSTALLINE DIAMOND POWDER:ALTERNATIVE SUPPORT MATERIAL FOR PEMFCSByAyten AyRecent studies have showed that the corrosion of carbon support in fuel cells can greatly affect the cell performance and durability. On this basis, novel corrosion resistant materials have attracted much interest as electrocatalyst support. The challenges are to develop supports with high surface area (100m2/g), good electrical conductivity (~1S/cm), chemical and thermal stability up to 200°C, morphological and microstructural stability at high potentials (e.g., ~1 V vs. the reversible hydrogen electrode (RHE)) and not too expensive. Ultrananocrystalline diamond (UNCD) powder is an advanced carbon material that has high surface area (200-600 m2/g), superb mechanical strength, chemical and thermal stability, and corrosion resistance. However, electrical conductivity is a technical challenge that must be overcome before electrically conducting diamond can become a viable electrocatalyst support material. In this dissertation, we suggested the way to overcome conductivity problem for UNCD powders. The preparation of higher specific surface area (50 and 170 m2/g) and electrically conducting (1 S/cm) diamond powders prepared by coating 500 nm, 100 nm and 3-6 nm diam. particles with a layer of boron-doped nanocrystalline diamond is reported. The physical, chemical, and electrochemical properties of boron doped UNCD (B-UNCD) powder with respect to sp2-bonded carbons were studied using electrochemical methods, SEM, HRTEM, Raman spectroscopy, and XRD. Electrical conductivity measurements and Raman spectroscopy confirmed the successful deposition of a boron-doped UNCD overlayer. All powders exhibited electrochemical responses for that were comparable to typical responses seen for high-quality, boron-doped nanocrystalline diamond thin-film electrodes. The deposition of Pt electrocatalyst particles on the conductive diamond powder surfaces were second part of the work in this dissertation. The particle size, dispersion and electrochemical activity of Pt/B-UNCD-diamond powders were characterized. The catalytic activity of conductive diamond powder supported Pt particles towards oxygen reduction reaction (ORR) was investigated. To calculate the kinetic parameters (Tafel slope, exchange current density, mass activity and specific activity) RDE was prepared and cyclic voltammetry and linear sweep voltammetry were performed. In the last part of the dissertation, the performance of membrane electrode assemblies (MEA) prepared by using the metal decorated B-UNCD powders were discussed. A 200 h life test in a single cell test assembly was carried out and open circuit potential and catalyst active area were monitored as a function of time. SEM images were obtained to characterize the MEAs before and after accelerated degradation test and results compared with Vulcan XC-72 powder supported MEAs. Excellent stability of the B-UNCD powder supported catalysts over the Vulcan XC-72 commercial support was observed. These results prove the potential of B-UNCD powder as a support to replace carbon black.
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