Götz Reinhardt

Trends in the development of solid state amperometric and potentiometric high temperature sensors

Abstract High temperature sensors based on solid electrolytes are well established in many applications. In this context, stabilized zirconia as an oxygen ion conductor plays the key role to monitor oxygen. Recent trends in the use of stabilized zirconia aim at the detection of other exhaust or environmental gases like NO x , CO or hydrocarbons. This is possible by, e.g. monitoring currents of specific electrode reactions at a given voltage (‘amperometric devices’) or monitoring voltages between different electrodes which result from specific electrode reactions including the kinetically determined formation of surface oxygen during different catalytic reactions [‘mixed-potential (non-Nernstian) devices’]. In all applications, the catalytic and electrochemical properties of the electrode materials play a key role for achieving reproducible sensor signals. This requires controlled microstructures down to the atomic scale. Therefore, extensive work now focuses on the structural as well as functional optimization of known, and on the development of new electrode materials. In this promising trend particular emphasis is put on the in-situ spectroscopic and microscopic characterization of the various interfaces under sensor operation conditions.

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.

Sensing small molecules with amperometric sensors

Abstract Amperometric sensors based on stabilized zirconia are well established with respect to the detection of oxygen. Recent trends aim at the detection of oxygen containing gases like NO x or combustible gases such as CO, H 2 and hydrocarbons. Concepts with multi-electrode amperometric sensors are able to detect O 2 and NO or O 2 and combustibles at the same time. However, the clear separation of different electrode reactions is often a problem. Highly selective electrodes for specific reactions are desired to improve sensor performance. In this context, the understanding of the mechanisms of electrode reactions is important for the optimization of sensor properties. Electrochemical as well as catalytic reactions determine the electrode properties. NO-reduction at La 1− x Sr x MnO 3 -electrodes proceeds via electrochemically generated oxygen vacancies. Silver under the same conditions has no activity for NO-reduction. In situ surface spectroscopic investigations combined with electrochemical methods under applied potentials provide deeper insight in the processes at the electrode surfaces. Work function changes detected with ultraviolet photoelectron spectroscopy (UPS) on evaporated Ag electrodes could be attributed to the potential dependent formation of subsurface oxygen. The latter was identified by X-ray photoelectron spectrosopy (XPS). Electrochemical investigations reveal an influence of oxygen desorption kinetics from the surface. Frequency dependent measurements further indicate that the desorption kinetics is further modified by a slow process.

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.

Thick film device for the detection of NO and oxygen in exhaust gases

Diffusion limited amperometric yttria stabilised zirconia sensors with two consecutive electrodes can be used as a combined broad band λ/NO sensor for the analysis of automotive exhausts. At an appropriately low pumping voltage, the first electrode removes all oxygen from the gas, but not the NO, which can only be pumped using higher cathodic potentials. The NO is then reduced at the second electrode. The pumping currents at the two electrodes are connected to the amounts of oxygen and NO. This paper reports on the fabrication and performance of a screen printed version of such a sensor. The sensors are self-heated with dimensions ready for use in real exhausts.

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.

Electrode reactions of La0.8Sr0.2MnO3±δ-electrodes on stabilized zirconia with oxygen and the nitrogen oxides NO and NO2

The kinetics for the electrode reactions with oxygen and with NO and NO2 in the presence of oxygen has been studied for La0.8Sr0.2MnO3±δ-electrodes on stabilized zirconia (8 mol% Y2O3=YSZ) in the temperature range between 500°C and 900°C for oxygen partial pressures between 1 kPa and 20 kPa by means of electrochemical methods (impedance, I-U characteristics) and temperature programmed desorption (TPD). For oxygen reduction below 900°C a mechanism is proposed which describes the formation of peroxidic ions at the electrode surface and a subsequent rate-determining electron transfer at the three-phase-boundary. At temperatures below 650°C the electrode reaction between NO and NO2 is much faster than the oxygen reduction. The results for the NO2-reduction to NO can be explained by a two-step mechanism consisting of a fast one-electron transfer to adsorbed NO2 at the electrode surface and a subsequent rate-determining transfer of the second electron to NO2 at the three-phase-boundary.

Solid Electrolytes For Gas Sensing At High Temperatures. Multi Electrode Setup To Analyze Gas Mixtures

Diffusion limited amperometric cells based on stabilized zirconia with a sequential arrangement of several electrodes have been developed for the analysis of exhaust gas mixtures. Such a multi electrode setup allows a spatial separation of different electrode reactions which may be used to monitor the concentrations of several gases at the same time. First results obtained with a cell with two electrodes operating in NO/O/sub 2/ gas mixtures demonstrate the feasibility of this approach by a simultaneous detection Of O/sub 2/ and NO.

Selectivity-optimization of planar amperometric multi-electrode sensors: identification of O2, NOx and combustible gases in exhausts at high temperatures

Abstract Experimental studies and numerical calculations have been performed to describe and optimize the response of planar amperometric multi-electrode sensors. These sensors may be used to detect simultaneously oxygen and NO or oxygen and combustible components. The experimentally determined kinetics of the electrode reaction and the gas phase diffusion have been included in the numerical calculation to optimize the geometry and the operation parameters. An excellent agreement was obtained between modeling and experimental results. The sensor response signal depends strongly on the kinetics of the electrode reaction given by the materials, temperatures, and voltages as well as on the geometry of the cell.

Separation of electrode reactions in multi electrode amperometric sensors

Multi electrode amperometric sensors based on stabilized zirconia have been studied with respect to the simultaneous detection of oxygen and NO. Both gases are of particular interest in oxygen rich exhaust gases. With a setup consisting of two subsequent electrodes it was possible to separate the reduction of oxygen and the reduction of NO spatially. Hence the currents of the two electrodes are directly correlated with the oxygen and the NO concentrations. Combustible gases like CO are oxidized at the oxygen electrode, thus lowering the effective oxygen concentration in the cell. H2O and CO2 are partly reduced at the NO electrode. This effect strongly depends on the applied potential and may be eliminated by an appropriate selection of the working parameters. The results demonstrate the potential of such multi electrode cells to the simultaneous detection of several gas components.