Peter Urban

Impedance studies on direct methanol fuel cell anodes

Abstract The processes taking place in direct methanol fuel cell anodes are characterized by ac impedance spectroscopy. Under conditions of practical interest, i.e., low methanol stoichiometry factors, the kinetic and the mass-transport resistance give rise to two well-resolved semicircles in the Nyquist plot. When mass-transport limitations are excluded, inductive loops occur in the complex plane which are interpreted in terms of the most widely accepted reaction mechanism for methanol electrooxidation. A simple equivalent circuit is used to model this impedance behaviour.

Characterization of direct methanol fuel cells by ac impedance spectroscopy

The processes taking place in direct methanol fuel cells (DMFC) are characterized by ac impedance spectroscopy under realistic operating conditions. This method allows the separate examination of anode kinetics, anode mass transport, cathode kinetics, cathode mass transport, and membrane conductivity, making it a valuable diagnostic tool for DMFC development.

Electro‐oxidation of Dimethyl Ether in a Polymer‐Electrolyte‐Membrane Fuel Cell

The possibility of using dimethyl ether (DME) as a fuel in direct oxidation polymer‐electrolyte‐membrane (PEM) fuel cells is investigated. A mechanism for DME electro‐oxidation is proposed based on the results of half‐cell experiments using cyclic voltammetry combined with gas‐chromatographic (GC) analyses of a single direct DME fuel cell. It is shown that, as a consequence of this mechanism, there is an additional overpotential at the anode of a direct DME fuel cell which is related to the initial adsorption step on the catalyst surface. DME is typically not oxidized at the cathode of a PEM fuel cell. This minimizes unwanted effects of fuel crossover, leading to improved fuel‐cell efficiencies compared to direct methanol fuel cells, especially at low‐to‐medium current densities.

Catalytic processes in solid polymer electrolyte fuel cell systems

Abstract Industrial application of fuel cell technology requires suitable electrocatalysts. This is true for all different types of fuel cells. These catalysts are responsible for the oxidation of the fuel (i.e. hydrogen, hydrogen rich gases or methanol) as well as for the oxygen reduction. This paper focuses on solid polymer electrolyte fuel cell systems for mobile applications. Here the demands on the catalyst are most severe due to the low-temperature operating regime. Two system configurations are possible: either the carbon containing fuel is processed by a fuel converter to a hydrogen rich gas mixture and this in turn is fed into the fuel cell. Alternatively fuels such as methanol can be supplied directly into the fuel cell. In the first case, contaminations of CO in the feed gas have to be taken into account. These strongly absorbs on the surface of the catalysts (carbon-supported Pt or Pt-alloys) thereby inhibiting the hydrogen oxidation reaction. Electro-oxidation mechanisms of adsorbed CO as well as methanol—as an example for a direct fuel cell system—is discussed on Pt-Ru and other catalysts. Finally catalysts for methanol and hydrocarbon reforming reactions as well as for the shift reaction are reviewed.