Nikolaos Bonanos

Geometric Requirements of Solid Electrolyte Cells with a Reference Electrode

A range of solid electrolyte cell geometries has been evaluated in terms of their suitability for measurements of the working electrode polarization resistance in a three-electrode configuration. The potential and current density distributions in the cells are calculated numerically. If the current density at the working electrode is not uniform, the standard procedure of evaluating the electrode polarization resistance from the experimental data is inadequate and may lead to erroneous results. Simulations are presented for cells with thin electrolytes and for pellet-based geometries with large electrolyte thickness.

A Method to Separate Process Contributions in Impedance Spectra by Variation of Test Conditions

Many processes contribute to the overall impedance of an electrochemical cell, and these may be difficult to separate in the impedance spectrum. Here, we present an investigation of a solid oxide fuel cell based on differences in impedance spectra due to a change of operating parameters and present the result as the derivative of the impedance with respect to ln(f). The method is used to separate the anode and cathode contributions and to identify various types of processes.

Proton and oxide ion conductivity of doped LaScO3

Abstract The conductivity of La 0.9 Sr 0.1 Sc 0.9 Mg 0.1 O 3 has been studied by impedance spectroscopy in controlled atmospheres. The material was found to be a mixed conductor with p-type conduction at high oxygen partial pressures and a combined proton and oxide ion conductor at low oxygen partial pressures. At temperatures below 800°C and low partial pressure of oxygen, proton conduction was dominant. Above this temperature, the ionic conductivity is dominated by oxide ion transport. The protonic transport number was estimated from the conductivities measured in dry and in water-moisturised gas. An isotope effect was observed using D 2 /D 2 O in place of H 2 /H 2 O. The oxygen transport number was estimated from this effect and from measurements in a two-chamber electrochemical cell.

High Performance Cathodes for Solid Oxide Fuel Cells Prepared by Infiltration of La0.6Sr0.4CoO3−δ into Gd-Doped Ceria

Cathodes prepared by infiltration of La0.6Sr0.4CoO3-delta (LSC40) into a porous Ce0.9Gd0.1O1.95 (CGO10) backbone have been developed for low temperature solid oxide fuel cells. The CGO10 backbone has been prepared by screen printing a CGO10 ink on both sides of a 180 mu m dense CGO10 electrolyte-tape followed by firing. LSC40 was introduced into the CGO10 porous backbone by multiple infiltrations of aqueous nitrate solutions followed by firing at 350 degrees C. A systematic study of the performance of the cathodes was performed by varying the CGO10 backbone firing temperature, the LSC40 firing temperature and the number of infiltrations. The cathode polarization resistance was measured using electrochemical impedance spectroscopy on symmetrical cells in ambient air, while the resulting structures were characterized by scanning electron microscopy (SEM) and high temperature X-ray diffraction (HT-XRD). The firing temperature of 600 degrees C for the LSC40 infiltrate was found to provide a balance between LSC40 material formation and high surface area micro/nanostructure. The lowest polarization resistances measured at 600 and 400 degrees C were 0.044 and 2.3 Omega cm(2) in air, respectively. During degradation tests at 600 degrees C, the cathode polarization resistance levels out after about 450 h of testing, giving a final polarization resistance of 0.07 Omega cm(2)

Effect of electrode material on the oxidation of H2 at the metal-Sr0.995Ce0.95Y0.05O2.970 interface

Abstract The effect of electrode material on the electrode kinetics of a metal–proton conductor interface has been investigated in a quasi point-contact configuration for four metals: Ni, Ag, Au and Pt. The current–voltage behaviour depends on the nature of the electrode, the hydrogen partial pressure and on temperature. For all systems studied, the anodic part of the polarisation curves displayed limiting current behaviour. For Pt at low anodic polarisation, an additional process determines the overall reaction rate. The apparent reaction order ( q app ) is found to be strongly dependent on electrode type, suggesting that the reaction mechanism is decisively determined by the electrode, rather than by electrolyte surface. The electrocatalytic activity of the metal tested has been classified according to the limiting current density in a humidified atmosphere of N 2 –1% H 2 . The highest i lim has been detected for Ni, followed by Pt, Au and Ag.

Considerations of defect equilibria in high temperature proton-conducting cerates

Acceptor doped perovskites such as strontium cerate form a variety of point defects through reaction with surrounding gases at high temperature, namely protons by dissolution of water vapour, electron holes by dissolution of oxygen and electrons by loss of oxygen. The defect equilibria can be described by three equilibrium constants coupled with electroneutrality and site conservation constraints. This work describes a numerical solution of these equations for arbitrary oxygen and water vapour partial pressures, without the need to neglect minority defects. It further examines the charge compensation mechanisms that dominate under the different regimes and their implications for transport properties.

Ionic conductivity and thermal stability of magnetron-sputtered nanocrystalline yttria-stabilized zirconia

Thermally stable, stoichiometric, cubic yttria-stabilized zirconia (YSZ) thin-film electrolytes have been synthesized by reactive pulsed dc magnetron sputtering from a Zr-Y (80/20 at. %) alloy targ ...

Structural and electrical characterisation of SrCe1−xYxOξ

Abstract The acceptor-doped perovskite proton conductor SrCe 1− x Y x O ξ ( x =0.025 to 0.20, ξ =3− x /2) has been prepared and characterised using X-ray diffraction and AC impedance spectroscopy, and the effect of the yttrium dopant concentration on structure and electrical properties has been investigated. X-ray diffraction studies show a decrease in lattice volume with increasing yttrium content. Electrical conductivity studies have been made as a function of oxygen partial pressure, and at a partial pressure of water vapour of 0.007 atm. The total conductivity has been separated into different components by fitting procedures and regions of ionic and p-type conduction have been identified. At 800°C and at the water vapour partial pressure of 0.007 atm, the ionic conductivity showed a maximum at a doping level of x =0.10, reaching a value of 5 mS/cm. The conductivity component appearing at low oxygen partial pressure, which according to recent studies may be regarded as protonic rather than n-type, decreased with doping, while the p-type component at high oxygen partial pressure increased. The relationship between the effect of doping on the conductivity and unit cell volume is discussed.

Highly oriented δ-Bi2O3 thin films stable at room temperature synthesized by reactive magnetron sputtering

We report the synthesis by reactive magnetron sputtering and structural characterization of highly (111)-oriented thin films of δ–Bi2O3. This phase is obtained at a substrate temperature of 150–200 °C in a narrow window of O2/Ar ratio in the sputtering gas (18%–20%). Transmission electron microscopy and x-ray diffraction reveal a polycrystalline columnar structure with (111) texture. The films are stable from room temperature up to 250 °C in vacuum and 350 °C in ambient air.

Effects of the fabrication process on the grain-boundary resistance in BaZr0.9Y0.1O3−δ

This paper reports on the effect of the fabrication process on the conductivity of BZY10 (BaZr0.9Y0.1O3−δ). The dense specimens were prepared by four methods: (1) solid-state reactive sintering (SSRS), (2) conventional sintering using powder prepared by solid-state reaction and NiO as sintering aid, (3) conventional sintering using powder prepared by solid-state reaction followed by high-temperature annealing (HT), and (4) spark plasma sintering (SPS). The four specimens crystallize in a cubic structure, without any observable secondary phases. The AC conductivities of these four specimens were measured by impedance spectroscopy in moist reducing atmosphere from 600 to 200 °C; the grain boundary and bulk contributions were distinguished by the analysis of the low-temperature spectra. The grain-boundaries of the sample prepared by solid-state reactive sintering exhibited a resistance typical of the bulk material, while the three other specimens had more resistive grain boundaries. Similar activation energies for proton transport were obtained for the bulk resistance of the four specimens (0.39–0.42 eV). The activation energy for the grain boundaries increased from 0.45 eV for the solid-state reactive sintered BZY10 to 0.84 eV for the conventional solid-state reaction using NiO as sintering aid. This study highlights the potential of the solid-state reactive sintering process as a time and cost-effective method for producing dense ceramic with lower resistance BZY10 grain boundaries.