Armelle Ringuedé

Oxygen reaction on strontium-doped lanthanum cobaltite dense electrodes at intermediate temperatures

Abstract La 0.7 Sr 0.3 CoO 3− δ (LSC) powder was characterized by thermogravimetry. Thin, dense layers of LSC were deposited by RF magnetron sputtering on YSZ pellets. The electrochemical behavior was studied as functions of temperature (300–530°C) and oxygen partial pressure (0.21–2×10 −5 bar) by impedance spectroscopy. Impedance diagrams were decomposed into two elements, characteristic of the electrode polarization. The low frequency contribution can be described either as a limiting Warburg diffusion loop or as a semi-circle. These results can be interpreted in terms of a progressive evolution of the limiting process as functions of oxygen pressure and temperature, i.e. a finite diffusion process in series with dissociative sorption process. Under low oxygen pressure, at temperatures higher than 475°C, gas-phase diffusion polarization becomes significant.

Characterisation of thin films of ceria-based electrolytes for IntermediateTemperature — Solid oxide fuel cells (IT-SOFC)

At the present, a major technological challenge for the development of solid oxide fuel cells (SOFC) is the reduction of their operation temperature in order to reduce the costs and increase the fuel cell lifetime. Nevertheless, decrease in the operating temperature leads to losses in cell performance mainly due to the ohmic drop through the electrolyte. Therefore, several approaches are currently under investigation to overcome the electrolyte problem and the use of oxygen ion conductor thin films seems to be the most promising solution. In this respect, the well-known electrolyte CeO2-Gd2O3 (CGO) was investigated. Thin layers of less than 5 µm of CGO were deposited using two different techniques: RF magnetron sputtering and Atomic Layer Deposition (ALD). Physicochemical properties of the thin films obtained were characterised by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Furthermore, impedance measurements were carried out in order to determine the electrical properties of the CGO films, in particular their ionic conductivity.

Input of atomic layer deposition for solid oxide fuel cell applications

The development of a new generation of solid oxide fuel cells (SOFCs) operating at lower temperatures with competitive performances requires the use of high-quality thin layers, either as electrolytes, electrodes or interlayers, such as catalysts, diffusion barriers, bond or protective layers. Atomic layer deposition (ALD) is a sequential chemical vapour deposition (CVD) technique allowing processing of one mono-atomic layer after another, conformal, adherent and homogeneous nano-scaled films which are often crystalline as-deposited without the need of high-temperature annealing treatments. Moreover, the scalability of ALD offers an important prospect for industrial applications. In this work, the literature dealing with ALD applied to SOFCs is thoroughly analysed, showing the present achievements as well as the numerous advantages of this technique. New developments for the future are currently in progress extending the potential use of ALD to other high-temperature devices such as proton electrolyte fuel cells, high-temperature water electrolysis, HTWE (with reversed SOFC-type systems) and molten carbonate fuel cells (MCFCs).

Use of Ni electrodes chronoamperometry for improved diagnostics of diabetes and cardiac diseases

Low DC active current through nickel electrodes applied on skin in places like hands, forehead and feet provides a novel non invasive diagnostic tool using reverse iontophoresis. This work describes the electrochemistry reactions involved and some medical applications, such as diagnostics of neuropathy, diabetes and cardiac diseases. Finally we provide evidence for the utility of this method through large clinical studies.

A comprehensive DFT investigation of bulk and low-index surfaces of ZrO2 polymorphs

The bulk structure, the relative stability, and the electronic properties of monoclinic, tetragonal, and cubic ZrO2 have been studied from a theoretical point of view, through periodic ab initio calculations using different Gaussian basis sets together with Hartree–Fock (HF), pure Density Functional Theory (DFT), and mixed HF/DFT schemes as found in hybrid functionals. The role of a posteriori empirical correction for dispersion, according to the Grimme D2 scheme, has also been investigated. The obtained results show that, among the tested functionals, PBE0 not only provides the best structural description of the three polymorphs, but it also represents the best compromise to accurately describe both the geometric and electronic features of the oxide. The relative stability of the three phases can also be qualitatively reproduced, as long as thermal contributions to the energy are taken into account. Four low‐index ZrO2 surfaces [monoclinic (−111), tetragonal (101 and 111), and cubic (111)] have then been studied at this latter level of theory. Surface energies, atomic relaxations, and electronic properties of these surfaces have been computed. The most stable surface is the cubic one, which is associated to small relaxations confined to the outermost layers. It is followed by the monoclinic (−111) and the tetragonal (101), which have very similar surface energies and atomic displacements. The tetragonal (111) was instead found to be, by far, the less stable with large displacements not only for the outermost but also for deeper layers. Through the comparison of different methods and basis sets, this study allowed us to find a reliable and accurate computational protocol for the investigation of zirconia, both in its bulk and surfaces forms, in view of more complex technological applications, such as ZrO2 doped with aliovalent oxides as found in solid oxide fuel cells.

ZrO2–In2O3 thin layers with gradual ionic to electronic composition synthesized by atomic layer deposition for SOFC applications

To achieve the processing of high performance solid oxide fuel cells (SOFCs) operating in the intermediate-temperature range (600–750 °C), either a thin layer electrolyte configuration or development of a new electrolyte material with high ionic conductivity is needed. In this work, atomic layer deposition, ALD, was used to process at 300 °C ZrO2–In2O3 thin layers which can be mixed ionic and/or electronic conductors depending on the amount of indium oxide. Single thin layers with different compositions and a thin multilayer with a composition gradient were deposited. The structural and morphological properties were analyzed by SEM/EDX and XRD. The deposits were well-crystallized without post-deposition annealing. Impedance spectroscopy measurements showed that the ZrO2–In2O3 gradient of composition seems to improve the interface properties.

Understanding crystallization processes of NiO/Ce0.9Gd0.1O2−δ sol–gel processed thin films for the design of efficient electrodes: an in situ thermal ellipsometry analysis

We describe a simple, non-destructive method, in situ thermal ellipsometry analysis (TEA), for understanding the different processes (decomposition of organics, crystallization, and sintering) occurring upon heating hybrid organic–inorganic films. According to these studies, a thermal treatment was tailored in order to obtain robust, nanocrystalline inorganic mesoporous 100–150 nm thick films with efficiently connected porosity surrounded by a crystalline inorganic network. Polymodal porous, nanocrystalline NiO/Gd-doped Ceria composites or Ni/Gd-doped Ceria films, interconnected network of open pores ranging from macro- to micro-pores, have been synthesized. The inorganic network is built from connected crystalline nanoparticles with mean diameters of 12 ± 3 nm, whose small size is still preserved even at 800 °C. We also show that the thermal ellipsometry analysis is readily extendable to MO/Gd-doped Ceria with M = Cu, Ni, Co, etc., therefore demonstrating the interest of this technique in understanding thermal phenomena in complex ceramic and composite systems. This is trivial for designing electrodes with efficient microstructure.

Structural and Electrical Properties of Gadolinia-doped Ceria Mixed with Alkali Earth Carbonates for SOFC Applications

The properties of composite materials based on mixtures of gadolinium-doped ceria (GDC) and Li(2)CO(3)-K(2)CO(3) are analyzed as potential SOFC electrolytes. In a temperature range higher than 500 degrees C, their ionic conductivity is significantly higher than for single GDC. Discontinuities were found in the conductivity Arrhenius diagram (sigma vs. 1/T) around the melting point of the carbonate mixture (490 degrees C), showing, at least partially, the contribution of molten carbonates. At this stage, precise mechanisms are still under analysis.

Revealing the properties of the cubic ZrO2 (111) surface by periodic DFT calculations: reducibility and stabilization through doping with aliovalent Y2O3

A detailed theoretical study concerning the formation of oxygen vacancies on the clean (111) surface of cubic ZrO2 and the structural and electronic properties of the (111) surface of yttria-stabilized zirconia (YSZ, 8 mol% Y2O3) was carried out using DFT methods in a periodic approach. For the formation of oxygen defects on the clean (111) surface, two different oxygen vacancy positions and two possible spin states for each position were investigated. Large vacancy formation energy, small relaxation and the presence of a highly localized state in the gap characterize the formation of oxygen defects on this surface. Regarding the yttria-stabilized surface, a systematic study of the stability, geometry and electronic structure of seven different configurations for Y atoms and oxygen vacancies on the surface was performed. The doping with Y2O3 stabilizes the cubic (111) ZrO2 surface and is accompanied by large relaxations of the O atoms NN to the vacancies. In addition, Y atoms preferentially occupy positions NNN to the defect. Despite the presence of vacancies in YSZ, no mid-gap states have been observed in any of the studied arrangements. This study allowed identifying an accurate computational protocol and a suitable model of the (111) surface of YSZ, through the characterization of its structural and electronic properties. Both could be used to further elucidate the role of YSZ as electrolyte in SOFC applications, with a view to better clarifying the basic operating principles of low temperature solid oxide fuel cells (LT-SOFCs).

Microstructure — Electrical properties relationship of YSZ thin films reactively sputter-deposited at different pressures

In order to decrease the operating temperature of Solid Oxide Fuel Cells (SOFC) from about 1000 °C to around 700 °C, the thickness of commonly used electrolytes such as Yttria Stabilised Zirconia (YSZ) must be decreased for about one order of magnitude in the range 1–10 µm. In this paper, we investigate the suitability of reactive magnetron sputtering for deposition of about 1 µm-thick YSZ films dedicated to SOFC.The coatings are synthesised by sputtering a metallic Zr-16 at.%Y target in the presence of a reactive argon-oxygen discharge. The deposition stage is controlled by Optical Transmission Interferometry (OTI) in order to guarantee the film transparency and its thickness. The influence of the deposition pressure on the chemical, structural and morphological properties of the coatings is studied in order to establish relationships with their ionic conductivity, determined by impedance spectroscopy.