J. Fouletier

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.

Alternative anode material for gradual methane reforming in solid oxide fuel cells

Abstract This study reports the catalytic and electrochemical characteristics of La 0.8 Sr 0.2 Cr 0.97 V 0.03 O 3 (LSCV). Catalytic properties of LSCV for the methane steam reforming have been carried out at 800°C. This oxide exhibits a low activity. The most important fact is that no carbon deposit was detected. We have investigated electrochemical behaviour of a LSCV–YSZ composite (YSZ: yttria stabilised zirconia) in H 2 –H 2 O and in CH 4 –H 2 O atmosphere by impedance spectroscopy. Impedance diagrams are composed of two contributions at high-frequency (HF, typically 100 Hz) and at medium-frequency (MF, typically 1 Hz). At 800°C, the normalised value of the MF resistance is only three-times higher than that obtained with an optimised Ni–YSZ cermet. In fact, the large difference comes from the HF resistance which is lower for the Ni–YSZ cermet by a factor of 20. This could be explained by the microstructure of the interface of the LSCV–YSZ composite layer and the electrolyte pellet which has to be improved. In CH 4 –H 2 O, an additional semicircle at low-frequency (noted as LF, typically 0.1 Hz) is observed. This LF contribution might be associated with the catalytic rate of the steam reforming of methane reaction. Ruthenium, as a steam reforming catalyst, was added to the LSCV–YSZ electrode. At 800°C, in pure methane mixed with 5% of steam, this anode material shows similar results to those observed in H 2 –H 2 O without ruthenium. Moreover, electrochemical behaviour was stable with time and no carbon deposit was detected after 100 h of operation. These results demonstrated that the gradual methane reforming can be implemented.

Structure and conductivity of Cu and Ni-substituted Bi4V2O11 compounds

The partially Cu- or Ni-substituted compounds (Bi4V2(1−x)M2xO11−3x;M=Cu, Ni) are highly oxygen-conducting. Three phases (α, β, γ) are observed in the unsubstituted compound; α is the low-conducting room temperature phase and γ the high-conducting phase at high temperature. Structure and conductivity are studied as a function of the substitution on the vanadium sites. Between 0 and 6% at room temperature, the Cu compound remains in the orthorhombic α phase and its ionic conductivity increases. A strong anisotropic conductivity is observed. For 0.07≤x≤0.12, the average structure is tetragonal (γ-type) at room temperature. The conductivity is very high and does not vary very much over this substitution range. Impedance spectroscopy measurements have also been carried out on the x=0.07 Ni-substituted compound. Commensurate or incommensurate superstructures are observed for all of these compounds.

Electrochemical properties of Ni–YSZ cermet in solid oxide fuel cells: Effect of current collecting

The effects of various current collectors on the electrochemical property measurements of a Ni-YSZ cermet have been investigated. Electrochemical characteristics were determined by impedance spectroscopy (IS) at open-circuit voltage in H -H O atmosphere at 8008C using a symmetrical cell. We selected various current collectors made of gold, platinum and 22 nickel and with two different structures, paste and mesh. When using paste, the electrochemical response, recorded with the same Ni-YSZ cermet sample, depends greatly on the nature of the current collector. Polarization resistance values follow the order: Ni , Pt , Au. This is the same order as reported in the literature for the hydrogen electrochemical oxidation activities of these metals. This demonstrates that paste current collectors interfere with the electrochemical properties of the anode material. Therefore, in order to compare the electrochemical activities of various anode materials, the same current collector has to be used. We have also observed that the polarization resistance increases with time, due to metal particle diffusion through the Ni-YSZ cermet. When using a nickel mesh, the electrochemical properties do not vary with time. Moreover, the structure of this current collector is most similar to the interconnect materials, which are the current collectors in planar SOFCs. It is, therefore, the best current collector for the study of the electrochemical properties of Ni-YSZ cermet.

Catalytic properties of new anode materials for solid oxide fuel cells operated under methane at intermediary temperature

Abstract The recent trend in solid oxide fuel cell concerns the use of natural gas as fuel. Steam reforming of methane is a well-established process for producing hydrogen directly at the anode side. In order to develop new anode materials, the catalytic activities of several oxides for the steam reforming of methane were characterized by gas chromatography. We studied the catalytic activity as a function of steam/carbon ratios r . The methane and the steam content were varied between 5 and 30% and between 1.5 and 3.5%, respectively, corresponding to r -values between 0.07 and 0.7. Catalyst (ruthenium and vanadium)-doped lanthanum chromites substituted with strontium, gadolinium-doped ceria (Ce 0.9 Gd 0.1 O 2 ) referred as to CeGdO 2 , praseodymium oxide, molybdenum oxide and copper oxide were tested. The working temperature was fixed at 850°C, except for 5% ruthenium-doped La 1− x Sr x CrO 3 where the temperature was varied between 700 and 850°C. Two types of behavior were observed as a function of the activity of the catalyst. The higher steam reforming efficiency was observed with 5% of ruthenium above 750°C.

Thermal behaviour of Bi4V2O11 : X-ray diffraction and impedance spectroscopy studies

Abstract Bi 4 V 2 O 11 powdered samples and single crystals were studied by high temperature X-ray diffraction and impedance spectroscopy to characterize the phase transitions. From high temperature X-ray diffraction on powders and single crystals, the α ⇆ β and β ⇆ γ reversible phase transitions were observed. The β ⇆ γ one is ferroelastic ⇆ paraelastic but surprisingly the α ⇆ β transition also exhibits a ferroelastic character, with a 90 ° switching of the a and b axis on cooling and/or, more scarcely, on heating. Impedance spectroscopy measurements were carried out using platelet shaped single crystals with well developed (001) faces. The corresponding σ ∥ (001 plane) and σ ⊥ ( c direction) bulk conductivities were obtained and compared with values from ceramic pellets, σ ∥ values are close to those characterizing the pellets, and the anisotropy of the conductivity is evidenced by σ ∥ values about 2 orders of magnitude larger than σ ⊥ ones. Slope changes observed in Arrhenius plots are in agreement with the phase transitions.

Electrode materials for zirconia sensors working at temperatures lower than 500 K

Abstract With Bi2Ru2O7 and Bi3Ru3O11 as electrode materials, the zirconia sensor can be used at a temperature as low as 450 K. Chemical gaseous pre-treatments, efficient over several weeks, significantly shorten the response times down to a few seconds.

Low-temperature oxygen electrode reaction on bismuth ruthenium oxides/stabilized zirconia

Abstract Bi2Ru2O7 and Bi3Ru3O11, have been tested as electrodes on stabilized zirconia at temperatures as low as 500 K. The oxides have been characterized by thermogravimetry under controlled oxygen partial pressure at high temperature and by potentiometric measurements after controlled cathodic polarization. The oxygen electrode reaction as functions of oxygen pressure and temperature has been studied by impedance spectroscopy. At low temperature, the dependence of the polarization resistance on the oxygen pressure was analyzed in terms of charge transfer as a rate controlling step.

La0.8Sr0.2Cr0.95Ru0.05 (LSCRu) as a new anode material for SOFC using natural gas

The strontium and ruthenium doped lanthanum chromite was synthesized by the Pyrosol technique. This method has allowed to insert ruthenium in the perovskite powder. The catalytic behavior for the methane reforming was determined using a catalytic bench. In the presence of a few percent of water vapor, this material transforms the methane into hydrogen without generating carbon deposition. The electrochemical behavior of this material as anode has been studied by impedance spectroscopy under Ar/H2O, H2/H2O and CH4/H2O mixtures. The overpotential resistance values are of the same order of magnitude under hydrogen and under methane. An additional contribution at low frequencies is observed under argon, resulting probably of the electrode polarization.

Physico-chemical characterization of highly pure nanocrystalline doped TZP

Abstract Yttria and scandia doped nanocrystalline tetragonal zirconia ceramics have been prepared from sintering of spray-pyrolysis powders. The average primary crystallite size after sintering has been found equal to 70 nm from X-ray diffraction line broadening while aggregated particle size of 300 nm has been characterized by scanning electron microscopy (SEM). No trace of segregated Si has been detected in the grain boundaries using wavelength dispersion scanning (WDS). Impedance diagrams show two well-resolved components in the 300–700°C range. Small size of crystallites and existence of nanoporosity give rise to a large grain boundary contribution compared to microcrystallized zirconia.