Degradation mechanisms of carbon-based electrocatalyst support materials and development of an advanced support based on electrically conducting diamond

In this dissertation, the degradation mechanisms of sp 2-bonded carbon electrocatalyst supports were studied under potential and temperature conditions relevant to the polymer electrolyte membrane fuel cell (PEMFC). In addition, an alternative support was fabricated in two forms: electrically conducting diamond powder and paper to overcome current material stability issues in the PEMFC.; Two structurally well-characterized sp2-bonded carbon powders, graphite (structurally well-ordered) and glassy carbon (GC, structurally disordered) were studied under potentiostatic polarization from 1.0 to 1.6 V vs. Ag/AgCl at 25, 50, and 80°C. Characterization of the surface oxidation and microstructural changes (i.e., increase in the exposed edge plane density) provided evidence for the so-called order/disorder mechanism where structurally disordered carbons corrode more severely because of oxidation and gasification of the exposed edge plane. Microstructural changes for graphite were heterogeneously distributed across the electrode surface. This is indicative of a nucleation and growth process, where disordered regions and defects serve as active sites for electrochemical corrosion, while other, more structurally ordered regions do not corrode. Preliminary results for a high-surface-area carbon black, Vulcan XC-72, are presented that show changes in the surface oxide content and also discuss the effect of polarization potential on Pt activity.; The physical and electrochemical properties of two commercial boron-doped diamond thin-film electrodes were compared with microcrystalline and nanocrystalline boron-doped diamond thin film deposited in our laboratory. The electrochemical response for Fe(CN)63-/4-, Ru(NH3)6 3+/2+, IrCl62-/3-, 4-methylcatechol, and Fe3+/2+ was quite reproducible from electrode type-to-type and from film-to-film for a given type. DeltaEp, ipox, and ip red values for Fe(CN)63-/4-, Ru(NH 3)63+/2+ on all electrodes were relatively unaffected by pH.; Electrically conducting diamond powder was prepared by coating insulating diamond powder (8-12 mum diam) with a thin boron-doped diamond layer using microwave-assisted chemical vapor deposition (CVD). Increases in the electrical conductivity after growth confirmed that a conductive diamond overlayer formed. The charge passed during anodic polarization at 1.6 V vs. Ag/AgCl and 25°C for 1 h was largest for GC powder (0.88 C/cm2) and smallest for conductive diamond powder (0.18 C/cm2), illustrating the dimensional stability of diamond powder compared to sp2-bonded carbon powder.; Boron-doped nanocrystalline diamond (BND) was coated on Toray RTM carbon paper (TCP) via microwave-assisted CVD. Pt nanoparticles were deposited on TCP and BND using a pulsed galvanostatic method. The stability of the bare TCP and BND substrates and the composite Pt/TCP and Pt/BND electrodes were studied using potentiostatic polarization in 0.1 M HClO4. The BND electrode exhibited superior morphological and microstructural stability over TCP at 1.6 V vs. Ag/AgCl. Evidence was found for dissolution and redeposition of Pt on composite electrodes, particularly for Pt/TCP.