U. Guth

Oxide ion conducting solid electrolytes based on Bi2O3

The high oxide ion conductivity of solid solutions of bismuth oxide was initially discovered by Takahashi and coworkers. The bismuth oxide based compounds are much better solid electrolytes than the well-known stabilized zirconia. The only difficulty which has prevented their use in high temperature fuel cells and gas sensors up to now is their instability against reduction at low oxygen partial pressures. In this article, we review the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions of bismuth oxide with face centred cubic, rhombohedral, tetragonal or layer structures.

Gas analysis with arrays of solid state electrochemical sensors: implications to monitor HCs and NOx in exhausts

Abstract Solid-state electrochemical sensors based on stabilized zirconia may be used for monitoring gas mixtures of several electro-active components. Concepts of multi-component analysis with potentiometric and amperometric sensors are discussed with respect to the simultaneous detection of O2 and HCs or O2 and NOx in exhausts. Recent results demonstrate the possibilities of continuously detecting oxygen, HCs and NOx with arrays of potentiometric or arrays of amperometric electrodes. Amperometric arrays are preferable because of the possibilities connected with fixing oxygen activities and reaction conditions by the applied potentials. The results also demonstrate that system solutions have to be found for each application.

Au–oxide composites as HC-sensitive electrode material for mixed potential gas sensors

In potentiometric zirconia-based gas sensors, gold electrodes show a high sensitivity for hydrocarbons (HC) when the measurements take place in nonequilibrated oxygen containing gas mixtures at temperatures <700 °C. That behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To clarify the correlation between the potential behaviour of electrodes and their catalytic activity for the hydrocarbon combustion, we investigated composite electrodes Au–oxide/YSZ with Ga2O3, Y2O3, Al2O3, ZrO2, In2O3 and Nb2O5 as oxides and 70 mass percent gold in C3H6–O2 gas mixtures using a platinum air reference electrode. n nGold composite electrodes using Nb2O5, Ga2O3, In2O3 as oxides show, as compared with pure gold, an enhanced propylene sensitivity and a time-independent characteristic at high concentrations of C3H6. Gold composite electrodes with Y2O3, Al2O3, ZrO2 tend to the equilibrium behaviour. The catalytic activity of the Au-composites for the HC combustion correlates very well with the cell voltage response.

A novel thick film sensor for simultaneous O2 and NO monitoring in exhaust gases

Abstract A miniaturized two-electrode sensor for the simultaneous detection of oxygen and nitrogen monoxide in oxygen, nitrogen, NO gas mixtures has been developed by using thick film technology. The solid state electrochemical sensor, based on yttria-stabilized zirconia, operates in the amperometric mode. Oxygen concentrations ranging up to 5xa0vol.% and NO concentrations ranging up to 2500xa0ppm can be detected separately and simultaneously. The diffusion properties of the diffusion barrier of the sensor have been determined by the current dependence on the working temperature.

A parallel analysis of oxygen and combustibles in solid electrolyte amperometric cells

Abstract A parallel analysis of oxygen and combustible components is a common problem for control of exhausts of automobiles, boilers, power plants, and so on. Multi-electrode solid electrolyte amperometric sensors are considered as an effective instrument for such measurements in hazardous working conditions. This paper presents basic principles and experimental results on the analysis of combustibles in the presence of oxygen by means of zirconia electrolyte amperometric cells. Experimental studies are concerned with electrochemical oxidation of carbon monoxide, methane and propene in the presence of oxygen in different working conditions at different types of electrodes. Results show diverse opportunities for selective performance of different electroanalytical reactions at certain electrodes.

Modified Au/YSZ electrodes — preparation, characterization and electrode behaviour at higher temperatures

Abstract The influence of the oxidic boundary layer on the electrode potential behaviour of gold electrodes with the solid electrolyte Zr 0.86 Y 0.14 O 1.93 (YSZ) at temperatures ≤ 700 °C in gases containing oxygen and combustible components was investigated. The interface between gold and the solid electrolyte was chemically modified with TiO 2 which was produced in two states (as a surface coating and dissolved in the YSZ boundary layers) after annealing. This was proved by X-ray diffraction with various incidence angles. In the oxygen excess region, the sensitivity to oxidizable gases increases by TiO 2 -containing interlayers compared to that of the pure Au/YSZ electrode.

Materials for electrodes based on rare earth manganites

Abstract For application in high temperature fuel cells and oxygen sensors, alkaline earth doped lanthanoide manganites have been studied systematically with regard to preparation conditions, structures, electrical conductivities and thermal expansion coefficients, respectively. La 1- x Sr(Ca) x MnO 3 , ( x =0–0.5) show a cubic perovskite structure for Ca-containing at x =0.2−0.5 and for Sr-containing at x =0.5. For Gd(Nd) 1- x Ca x MnO 3 ( x =0–0.5) an orthorhombic distorted perovskite structure was found. Yb(Y) 1- x Ca x MnO 3 shows in the range of x =0.3–0.5 an orthorhombic structure, whereas for the pure Yb(Y)MnO 3 the hexagonal structure was determined. A correlation between the conductivity of doped lanthanum manganites and the concentration of Mn 4+ could be found. The suitability of these materials as an electrode for zirconia cells has also been studied in detail. At temperatures under 1073 K the polarization resistivity of the best electrodes based upon La 0.7 Ca 0.3 MnO 3 is much smaller than such made of active platinum. Electrodes were tested in solid electrolyte fuel cells.

Tubular amperometric high-temperature sensors: simultaneous determination of oxygen, nitrogen oxides and combustible components

Abstract Tubular amperometric sensors based on stabilized zirconia solid electrolyte with two working electrodes have been studied experimentally with respect to the simultaneous measurements of oxygen and nitrogen oxides, and of oxygen and combustible gas mixtures. Sensors with different types of working electrodes have been optimized for specific gas analytical problems. An almost complete separation of the sensor signals to defined gas components has been achieved by optimization of the working parameters of the electrodes. Cross-sensitivities to oxygen-containing and to combustible gas components have been measured. Cross-interference effects from CO2 and H2O at O2/NOx sensors were minimized by properly adjusting the electrode potentials. Cross-effects on O2/HCx-sensors have been found to be even lower. The results demonstrate a new level of performance for new applications of multi-electrode solid electrolyte amperometric sensors.

Multi-electrode zirconia electrolyte amperometric sensors

Abstract The concept of multi-component gas sensors based on stabilized zirconia amperometric cells has been proposed and tested using cells with two working electrodes. Variations of the cell structure, geometric parameters, materials of working electrodes and working regimes are carried out for optimal adaptation to the particular gas analytic application. Experiments have been conducted in gas mixtures containing nitrogen, oxygen, nitrogen oxide and some combustible gases. Optimization of cell design and working parameters has had a result in a good separation of signals from the measured gas components at the particular electrode. Cross-effects from different electrochemically active gas components have been studied as well. These effects are negligible in measurement of oxygen and combustible components. Cross-effects from H 2 O and CO 2 have been found at large potential difference applied to the working electrodes. These effects are eliminated by optimal selection of electrode potentials and temperatures.

Chemical modifications of au-electrodes on YSZ and their influence on the non-Nernstian behaviour

The interface between gold electrodes and the solid electrolyte Zr0.86Y0.14O1.93 (YSZ) was chemically modified in several ways. Nb2O5 was obtained in two states: as an X-ray amorphous surface layer after vacuum evaporation and dissolved in YSZ after annealing at 1150 °C. This was confirmed by X-ray diffraction with grazing incidence and Bragg-Brentano technique. On the metal side the modification was made by potentiostatic deposition of the electronically conducting perovskite La1-xSrxCrO3 and subsequent annealing at 1000 °C.The influence of these modifications on the electrode potential in gases containing oxygen and propylene at temperatures ≤700 °C was investigated. The known sensitivity of the Au electrode to propylene is nearly maintained after both Nb2O5 modifications of the solid electrolyte. After modification with the chromite layer, the sensitivity to propylene disappeared completely in gas mixtures with excess oxygen.Strong differences in the sensitivity caused by chemical modifications can possibly be utilized in hydrocarbon sensors.