Bogdan Gurau

Deuterium isotope analysis of methanol oxidation on mixed metal anode catalysts

The widely accepted mechanism for methanol oxidation on Pt based catalyst surfaces has held that the rate determining step is activation of water, and/or oxidation of surface-bound CO to CO2. In fact on pure Pt, water activation is always rate limiting at potentials negative of 0.6 V. Anode potentials greater than 0.4 V are outside the useful potential window of direct methanol fuel cells when using Nafion 117 at 60 °C. Enhancement of the water activation kinetics on Pt has been effected by the use of oxophilic transition metal promoters including Ru, W and Sn. For decades the search for improved methanol oxidation electrocatalysts has focused on water activation. A systematic deuterium isotope study on Pt black and two active mixed metal catalysts (PtRu and PtRuOsIr) shows that for each catalyst there is a characteristic transition potential above which the primary reaction in the rate-determining step changes from water activation to CH bond activation. On the mixed metal catalysts, this crossover potential is ca. 0.35 V, which is within the direct methanol fuel cell potential window (0–0.400 V). This study confirms that on these active catalysts there is a potential above which further improvements in water activation must be concomitant with acceleration of CH bond activation. Thus the catalyst search strategy involving Pt promoter metals must also consider the kinetic importance of CH bond activation.

In Situ 50°C FTIR Spectroscopy of Pt and PtRu Direct Methanol Fuel Cell Membrane Electrode Assembly Anodes

A tandem surface-transmission Fourier transform infrared (FTIR) method permits the simultaneous investigation of adsorbed and desorbed species formed at the electrode surface of direct methanol fuel cell (DMFC) membrane electrode assemblies (MEAs). Potential dependent, in situ specular reflectance spectra and on-line transmission FTIR spectroscopy of adsorbed and desorbed species on working fuel cell electrode surfaces, confirm that linear hound CO, the primary intermediate on unsupported Pt and PtRu surfaces of fuel cell MEAs, have lower Stark tuning rates than the corresponding on arc-melted alloys. Two peaks corresponding to CO stretching modes were observed on a PtRu fuel cell anode surface prepared with Johnson Matthey catalysts. The higher frequency peak is ascribed to a Pt-rich alloy and the low frequency peak is ascribed to either pure Ru or an Ru-rich phase. The tandem technique confirms that a peak easily misassigned as a linear bound CO stretching mode (2083 cm -1 ) is is a methanol overtone peak. In addition to CO 2 , methylformate is a major product of a gas fed DMFC which is detected at the anode exhaust by on-line FTIR.

Spectroscopic study of NEMCA promoted alkene isomerizations at PEM fuel cell Pd-Nafion cathodes

Abstract We recently reported the electrochemical promotion of the heterogeneously catalyzed isomerization of 1-butene to cis - and trans -butene with ρ values of 38 and 46, respectively, at the remarkably low temperature of 70°C using Nafion as the electrolyte in a fuel cell configuration. This was the first reported case of a NEMCA promoted unimolecular and non-redox reaction concomitant with the reduction of butene to butane. We now present isotopic mass spectral and FTIR data confirming the mechanism involves abstraction of a proton from the catalytic surface for Markovnikov addition to the C-1 carbon concomitant with removal of a proton from C-3 to yield the isomer. This striking example of an acid catalyzed reaction at a metal surface is facilitated by the use of the super acidic Nafion electrolyte.

Combinatorial Screening of Anode and Cathode Electrocatalysts for Direct Methanol Fuel Cells

Progress in several important electrochemical technologies, including batteries, fuel cells, sensors, and electrosynthesis, is currently materials-limited. A common feature of all electrode reactions is the imbalance (i.e., loss or generation) of ions at the electrode surface. We describe in this paper a method by which excess ions in the electrode diffusion layer can be imaged, and used to identify the best electrode materials from a combinatorial array of compositions. Although in principle this method can be applied to many electrochemical problems, we have focused on finding better electrocatalysts for direct methanol fuel cells (DMFCs). The DMFC performs two half-cell reactions: oxidation of methanol, and reduction of oxygen. Two of the most important problems in DMFCs are the poor performance of the electrocatalysts, and the crossover of methanol from the anode to the cathode side of the cell. An ideal situation would be the simultaneous development of two new catalysts: an anode that oxidizes methanol at low overpotential, and a “methanol-tolerant” cathode that reduces oxygen without oxidizing methanol. Based on previously developed rules for predicting the activity of ternary alloy catalysts (Ley, et al., J. Electrochem. Soc. 1997, 144, 1543), we began searching quaternary combinations of noble metals for the anode, and ruthenium selenide-type materials for the cathode reaction. The anode and cathode reactions generate and consume protons, respectively, creating a substantial pH gradient at the electrode surface. Changes in local pH are imaged by means of an appropriate fluorescent indicator: Ni-PTP for the anode and Eosin Y for the cathode. DMFC testing confirms the utility of the screening method, in that a Pt/Ru/Os/Ir quaternary catalyst was substantially superior to the best binary and ternary catalysts prepared under similar conditions.

Electrochemical Energy Storage: Current and Emerging Technologies

This chapter includes theory based and practical discussions of electrochemical energy storage systems including batteries (primary, secondary and flow) and supercapacitors. Primary batteries are exemplified by zinc-air, lithium-air and lithium thionyl chloride batteries. Secondary batteries are exemplified by recombination, lithium ion and high temperature batteries. The state-of-the-art in supercapacitors and pseudo capacitors are discussed. The chapter concludes with battery and capacitor emerging technologies. This chapter is suitable as a reference for professionals and for classroom education.