A. J. McEvoy

Analysis of performance losses in polymer electrolyte fuel cells at high current densities by impedance spectroscopy

The physico-chemical origins of the performance loss in polymer electrolyte fuel cells operated at high current densities are investigated. This effect could be attributed to a shortage of water in the proton conducting membrane leading to an increase of the membrane proton-conduction resistance as well as to a increase of the activation overpotential for the hydrogen oxidation reaction due to a decrease in the number of active sites on the anode side. To fortify this thesis, we present measurements of the fuel cell impedance, varying the membrane thickness, the ionic density, and the humidification conditions of the reactants.

A study on the La1-xSrxMnO3 oxygen cathode

The impedance responses and current-overpotential characteristics were studied for oxygen reducing cathodes, stoichiometric La1 − xSrxMnO3 (x = 0.16 − 0.20) in solid oxide fuel cells, under varying conditions of temperature (700–900 °C) and oxygen partial pressure (1 − 10−4atm) for two distinctly different electrode morphologies (porous and dense structure). An electrode densified at the interface with the yttria-stabilized zirconia solid electrolyte was more efficient in this study. The reaction at the finely porous LSM proceeds via the triple phase boundary and ineffectively uses the mixed conductivity property of the material. Reaction mechanisms for both cases (porous and dense structure) are discussed and compared with the literature. A new, two-layered cathode structure is proposed.

Lanthanide co-doping of solid electrolytes: AC conductivity behaviour

Abstract Solid electrolytes of cubic structure employed in Solid Oxide Fuel Cells (SOFC) like ceria (CeO 2 ) and zirconia (ZrO 2 ) are typically doped with a single selected element from the (Y, lanthanide)-series. Co-doping of ceria with several elements is investigated here in terms of its influence on ionic conductivity. It is found that, for a same total dopant concentration, mixtures of dopants give a higher total ionic conductivity (by 10–30%) than the best singly doped material.

Thin SOFC electrolytes and their interfaces–: A near-term research strategy

In the apparent impasse concerning the identification of more promising new materials for intermediate temperature solid oxide fuel cells, and the imperative for the credibility of the technology that applications be proven in the short term, all relevant information from earlier work should be exploited to secure the stable and efficient operation of SOFC systems with the conventional established materials, stabilised zirconia, perovskite cathodes and cermet anodes. In a retrospective, seminal work of the past is revisited and guidelines for ongoing work established on that basis.

Oxygen diffusion through silver cathodes for solid oxide fuel cells

Abstract The oxygen reduction mechanism at the interface O2,Ag/YSZ (yttria-stabilized zirconia) was investigated by impedance spectroscopy and current-overpotential recording. Oxygen partial pressure was varied between 10−4 and 1 atm, and temperature between 600 and 850°C. Both dense and porous silver electrodes were studied. Bulk diffusion of oxygen atoms through the solid silver was identified as the rate-determining mechanism. A second, smaller contribution is ascribed to dissociative adsorption of oxygen molecules on the silver electrode. Good agreement with known data for the diffusion and solubility of oxygen in silver is obtained.

Conductivity measurements of various yttria-stabilized zirconia samples

Samples of yttria-stabilized zirconia manufactured by the following fabrication procedures, were obtained from commercial sources: (i) hot isostatic pressing; (ii) tape casting; (iii) vacuum plasma spraying, and (iv) calendering. The ionic conductivities of these samples were measured by (a) impedance spectroscopy; (b) the four-point probe method; (c) the current-interruption technique, and (d) the van der Pauw technique. The tape-cast and hot pressed samples showed good and very reproducible conductivity values. The vacuum plasma sprayed samples showed an anisotropy in their conductivity, with the cross-plane value being several times lower than the in-plane value. A simple model based on the porous microstructure of these samples can explain this observation. Sintering of the plasma sprayed samples minimized the anisotropy and significantly improved their conductivity values. The calendered samples also showed a similar anisotropy in their conductivity data when they were inadequately sintered.

Materials for high-temperature oxygen reduction in solid oxide fuel cells

Solid state ionic devices such as fuel cells and oxygen separation membranes require the adsorption of oxygen molecules, their dissociation into oxygen atoms, oxidation by charge exchange and entry of the resultant ion into the solid phase. The cathodes capable of sustaining these processes must themselves be stable in the high temperature environment of air with a significant water vapour content, and compatible chemically and mechanically with the contacting solid phase, normally an electrolyte. As charge transfer materials obviously a high electronic conductivity is imperative, and some degree of ionic conductivity can serve to delocalise the oxidation process, thus reducing polarisation. In the present review the evolution of these cathode materials and their present status will be presented.

Morphology and growth rate of polyaniline films modified by surfactants and polyelectrolytes

Abstract The rate of electropolymerization of aniline can be significantly accelerated by the addition of small amounts of bifunctional species, polyelectrolytes or surfactants to the monomer solution. In the first case, using para-phenylene diamine (PPDA) for example, the morphology of the polyaniline (PANI) film is densely branched and fibrous. In the latter cases, the mechanism of growth enhancement appears to be quite different, controlled by physical rather than chemical processes. Scanning electron micrographs reveal that the typical fibrous PANI morphology is suppressed and that the granular structure of the initial nucleation is maintained. Cyclic voltammetry confirms that these modified aniline polymers still demonstrate the electrochemical behavior characteristic of pure PANI. Elemental analysis by X-ray photoelectron spectroscopy (XPS) shows that, based on sulfur-to-nitrogen (S/N) ratios, some of the polyelectrolyte or surfactant additive is incorporated into the polymer surface depending on the initial relative concentrations. A model of the effect of polyelectrolyte or surfactant during the electropolymerization of aniline is presented.

Electrocatalysis in solid oxide fuel cell electrode domains

Reference LPI-ARTICLE-1995-024View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Activation processes, electrocatalysis and operating protocols enhance SOFC performance

As the current density on a solid oxide fuel cell element increases, the voltage is reduced from that, close to the Nernst potential, obtained under open-circuit conditions due to loss mechanisms, the ohmic resistance of the materials and polarisation effects at the interfaces, as well as any effect of fuel depletion. The ohmic losses are determined by the materials selected and the dimensions of the component, particularly the electrolyte thickness. Minimisation of polarisation effects requires detailed interface structural control, and advances in this aspect of SOFC engineering are reviewed. Additionally note is taken of variation in performance of SOFC devices consequent on their operational history.