An ex-situ and in-situ evaluation of carbides as potential electrocatalysts.

摘要

One of the most prominent challenges facing the commercialization of the direct methanol fuel cell (DMFC) is the high cost of its electrocatalyst components, particularly the anode. The anode typically requires a high loading of precious metal electrocatalyst (Pt-Ru) to obtain a useful amount of electrical energy from the electrooxidation of methanol (CH3OH). The complete electrooxidation of methanol on these catalysts produces strongly adsorbed CO on the surface, which reduces the activity of Pt. The presence of Ru in these electrocatalysts assists with the decomposition of H2O to more efficiently remove the poisoning CO species as CO2(g). The primary disadvantage of these electrocatalyst components is the scarcity and consequently high price of both Pt and Ru.; A series of surface science studies ultrahigh vacuum (UHV) have identified molybdenum and tungsten carbide materials as potential alternative DMFC anode electrocatalysts. Both of these materials demonstrated activity towards the decomposition of methanol and water molecules. The purpose of this research was to extend these investigations by the synthesis and characterization of more realistic carbide materials. This was accomplished by a combination of surface science and electrochemical experiments. The electrochemical studies were performed both in-situ and ex-situ in order to better address the "materials gap" and "pressure gap" that often separate findings in UHV studies from results in more realistic environments.; Thin film surfaces of molybdenum carbide could be produced on various carbon substrates in a vacuum system by physical vapor deposition (PVD). When modified with low coverages of Pt, MoC phase molybdenum carbides were found to be more active towards the electrooxidation of hydrogen in an acidic electrolyte than Ptmodified carbon substrates in cyclic voltammetry (CV) studies. These surfaces demonstrated a limited range of electrochemical stability in this acid solution. Mo2C surfaces have previously shown hydrogen electrooxidation activity, but demonstrated a nearly identical stability range to MoC in an identical electrolyte. Within these stable ranges of operation, neither surface demonstrated activity towards methanol electrooxidation. These surfaces are also found to undergo rapid decomposition at higher operating potentials, which could be disadvantageous for use in DMFC's.; Despite these findings for molybdenum carbides, in-situ CV studies reveal that tungsten monocarbides (WC) show significant activity towards methanol oxidation in acidic solution and a larger range of stability. Steady-state Chronoamperometry (CA) measurements show an enhanced performance for methanol electrooxidation on WC and sub-monolayer Pt-modified WC surfaces by comparison with Pt surfaces. Surface science studies demonstrate that the WC and Pt-modified WC surfaces remained stable during the CA measurements.; To further bridge the materials and pressure gaps mentioned earlier, polycrystalline thin films of WC were synthesized on various carbon substrates commonly used in fuel cell applications. The activity of WC and Pt-modified WC PVD films surfaces towards methanol and adsorbed CO species in ex-situ CV experiments enabled a discussion of the advantages and limitations of the WC electrocatalyst when produced using larger scale synthesis methods. To further aid this investigation, WC nanomaterials with and without Pt-modification were integrated as the anode electrocatalyst in DMFC devices. These fuel cells were used in a preliminary study to identify the most basic performance characteristics of the anode. Additionally, these findings motivate a discussion of the relative ease with which WC-based electrocatalysts may be integrated into fuel cells using proven fabrication techniques.