Daniela Schönauer-Kamin

Half-Cell Potential Analysis of an Ammonia Sensor with the Electrochemical Cell Au | YSZ | Au, V2O5-WO3-TiO2

Half-cell potentials of the electrochemical cell Au, VWT | YSZ | Au are analyzed in dependence on oxygen and ammonia concentration at 550 °C. One of the gold electrodes is covered with a porous SCR catalyst, vanadia-tungstenia-titania (VWT). The cell is utilized as a potentiometric ammonia gas sensor and provides a semi-logarithmic characteristic curve with a high NH3 sensitivity and selectivity. The analyses of the Au | YSZ and Au, VWT | YSZ half-cells are conducted to describe the non-equilibrium behavior of the sensor device in light of mixed potential theory. Both electrode potentials provide a dependency on the NH3 concentration, whereby VWT, Au | YSZ shows a stronger effect which increases with increasing VWT coverage. The potential shifts in the anodic direction confirm the formation of mixed potentials at both electrodes resulting from electrochemical reactions of O2 and NH3 at the three-phase boundary. Polarization curves indicate Butler-Volmer-type kinetics. Modified polarization curves of the VWT covered electrode show an enhanced anodic reaction and an almost unaltered cathodic reaction. The NH3 dependency is dominated by the VWT coverage and it turns out that the catalytic properties of the VWT thick film are responsible for the electrode potential shift.

NO Detection by Pulsed Polarization of Lambda Probes-Influence of the Reference Atmosphere

The pulsed polarization measurement technique using conventional thimble type lambda probes is suitable for low ppm NOx detection in exhaust gas applications. To evaluate the underlying sensor mechanism, the unknown influence of the reference atmosphere on the NO sensing behavior is investigated in this study. Besides answering questions with respect to the underlying principle, this investigation can resolve the main question of whether a simplified sensor element without reference may be also suitable for NO sensing using the pulsed polarization measurement technique. With an adequate sensor setup, the reference atmosphere of the thimble type lambda probe is changed completely after a certain diffusion time. Thus, the sensor response regarding NO is compared with and without different gas atmospheres on both electrodes. It is shown that there is still a very good NO sensitivity even without reference air, although the NO response is reduced due to non-existing overlying mixed potential type voltage, which is otherwise caused by different atmospheres on both electrodes. Considering these results, we see an opportunity to simplify the standard NOx sensor design by omitting the reference electrode.

Pulsed Polarization-Based NOₓ Sensors of YSZ Films Produced by the Aerosol Deposition Method and by Screen-Printing

The pulsed polarization technique on solid electrolytes is based on alternating potential pulses interrupted by self-discharge pauses. Since even small concentrations of nitrogen oxides (NOx) in the ppm range significantly change the polarization and discharge behavior, pulsed polarization sensors are well suited to measure low amounts of NOx. In contrast to all previous investigations, planar pulsed polarization sensors were built using an electrolyte thick film and platinum interdigital electrodes on alumina substrates. Two different sensor layouts were investigated, the first with buried Pt electrodes under the electrolyte and the second one with conventional overlying Pt electrodes. Electrolyte thick films were either formed by aerosol deposition or by screen-printing, therefore exhibiting a dense or porous microstructure, respectively. For screen-printed electrolytes, the influence of the electrolyte resistance on the NOx sensing ability was investigated as well. Sensors with buried electrodes showed little to no response even at higher NOx concentrations, in good agreement with the intended sensor mechanism. Electrolyte films with overlying electrodes, however, allowed the quantitative detection of NOx. In particular, aerosol deposited electrolytes exhibited high sensitivities with a sensor output signal ΔU of 50 mV and 75 mV for 3 ppm of NO and NO2, respectively. For screen-printed electrolytes, a clear trend indicated a decrease in sensitivity with increased electrolyte resistance.

NOx-detection by pulsed polarization of lambda probes

Conventional thimble type lambda sensors combined with a pulse discharge technique were used for NOx detection in a simulated combustion exhaust gas with varying oxygen and moisture levels. Open circuit discharge characteristics after defined polarization pulses with alternating polarity show strong dependencies on NOx (112 mV / decade) in the lowest ppm range (0.5 – 50 ppm). Increase in NOx concentration accelerates sensor discharge, whereby the discharge curves following negative pulses are more affected by the NOx content compared to positive pulses. The sensitivities to NO and NO2 are equal and measurement of total NOx is possible. The discharge curves are affected by the oxygen and the water content. An increase in O2 concentration results in a shift of the discharge curves to negative voltages. Additionally a decreased sensor response to NOx is obtained for both polarization polarities. The water concentration has the opposite effect. All discharge curves are shifted to positive voltages with increasing H2O content. As a result, the sensor response increases strongly. The voltage shifts due to O2 and H2O are expected from Nernstian behavior of a lambda probe, whereby moisture influence is stronger compared to O2. The feasibility of the method as an exhaust gas total NOx measurement system is confirmed.

P1GS.1 - Room Temperature UV-Enhanced NO2-Gas Sensing of Doped and Undoped Sol-Gel-Synthesized ZnO

ZnO is of interest for many applications, e.g. also for gas sensing. Here, sol-gel synthesized, nanosized ZnO is investigated with regard to its room temperature NO2 sensing behavior. It is shown that both doping and UV exposure shorten the response and the recovery times of the sensors. NO2 measurements were carried out with undoped, Sn-doped, and Al-doped ZnO sensors, respectively. The Al-doped samples provide the highest NO2-response. Furthermore, the effect of dry or humid atmosphere on the sensor response was investigated. The strong humidity influence on the sensor signal almost disappears with UV exposure. Hence, UV-light, nano-sized structure, and proper doping may be the key for room temperature NO2 sensing.

P1GS.21 - Dosimeter for Low-Level NOx Detection – Influence of the Deposition Method of the NOx Storage Film

The detection of low-level NOx concentrations and the dose of NOx for air-quality monitoring (AQM) is still a huge task and a widely discussed topic. Dosimeter-type NOx sensors detect directly the NOx dose and are advantageous considering mean value detection, linearity of sensor responses, and drift phenomena. The electrical properties of a NOx storage film, here potassium permanganate impregnated on alumina powder, depend linearly on the amount of sorbed NOx. The electrical resistance correlates very well with the amount of formed nitrate and nitrite species. The influence of the deposition method of the sensitive NOx storage material on the characteristic behavior of the dosimeter-type sensors is investigated. Aerosol-deposited dense films (ADM films) behave like porous thick-films, but typical sensor characteristics like detection limits and relative resistance changes seem to be different.

2D SnS2—A Material for Impedance-Based Low Temperature NOx Sensing?

The sensor signal of tin disulfide (SnS2), a two-dimensional (2D) group-IV dichalcogenide, deposited as a film on a conductometric transducer is investigated at 130 °C. The focus is on the detection of the total NOx concentration. Therefore, the sensor response to NO and NO2 at ppm- and sub-ppm level at low operating temperature is determined. The results show that the sensing device provides a high sensor signal to NO and NO2 even at concentrations of only 390 ppb NOx. Both nitrous components, NO and NO2, yield the same signal, which offers the opportunity to sense the total concentration of NOx.

Half-cell characterization of a novel NH3 gas sensor

The function of a novel electrochemical NH3 gas sensor for application in SCR-systems is investigated. It provides a semi-logarithmic characteristic curve with a high NH3-sensitivity and marginal NOx cross interference at 550 °C. The electrochemical cell of the sensor device can be defined as Au | YSZ | Au, VWT. It is assumed that the sensing mechanism is based on nonequilibrium conditions (mixed potential theory) including electrochemical kinetics. This paper describes the investigation of electrode potentials and polarization curves of the half-cells Au | YSZ and Au, VWT | YSZ in dependence of NH3, which provides essential information about electrochemical reactions at the three-phase boundary. All electrode potentials depend on reactive gas concentration, whereby the electrode potential of the VWT-covered Au-electrode shows a stronger dependency on the NH3 concentration. The formation of mixed potentials at both electrodes is confirmed. Additionally, the influence of the VWT-catalyst coating on the sensing mechanism and the sensitivity is demonstrated. The sensitivity increases with increasing coverage of the Au-electrode with VWT catalyst. Voltage-current curves help to analyze the kinetics of electrochemical reactions at the TPB. A clear shift in cathodic direction (to more negative potentials) can be observed with increasing NH3 concentration and the current increases at a fixed potential due to an enhanced electrochemical NH3 oxidation.

SCR-Catalyst Materials for Exhaust Gas Detection

The application of SCR-catalyst materials like vanadia-doped tungsten-titania (VWT) and ironexchanged zeolites (Fe-ZSM-5) for analyte detection in exhaust gases is investigated. Three detection modes are covered by this overview: the detection of certain gas concentrations with sensing devices comprising SCR-materials, the measurement of the electrical properties of the catalyst materials itself, and the direct determination of the catalyst status during operation by microwave method (an inoperando method). In this work, an overview on the suitability of SCR materials for gas detection at high temperatures (500 – 600 °C) is given. Various sensing principles, like mixed-potential, impedimetric, or resistive sensing, are discussed with respect to NH3 and SO2 response. A promising NH3 sensor behavior was found in case of a mixed-potential sensor with a VWT catalyst layer. Additionally, an impedimetric sensor with a VWT functional layer provides a selective NH3 response with marginal NO cross interference. Initial results of an impedimetric Fe-ZSM5 sensor indicate a strong NH3 dependency but a high NOx interfering effect is observed. For SO2 detection in coal combustion processes, two promising sensor setups utilizing VWT are investigated. Furthermore, the applicability of contactless radio-frequency method for direct determination of the amount of stored ammonia in a zeolite-based catalyst is shown.

P2.8.5 Sensing of NO, NO2, and NH3 with Zeolite-Based Impedimetric Gas Sensors

In this study, the suitability of zeolites for two different sensor principles is investigated. Zeolites can change their electrical properties with the sorption of gas molecules in their microporous network structure. Therefore, they are potential materials for impedimetric gas sensors. The screened zeolites H-ZSM-5, Fe-ZSM-5, Pt-loaded H-ZSM-5, and Pt-loaded Fe-ZSM-5 are tested for their sensitivity towards NO, NO2, and NH3 at varying temperatures. The two different sensor principles are considered: the integratingor accumulating-type sensing behavior at low temperatures and a conventional gas sensor behavior at high temperatures. At high temperatures, only the expected response towards NH3 occurs, whereas at low temperatures a cross effect towards NOx could be observed for H-ZSM-5 and Fe-ZSM-5. The sensor behavior seems to be an integrating one at 200 °C for NO and NO2. By inserting elementary platinum the sensitivity towards NO and NO2 was eliminated due to a higher catalytic activity of the Pt-loaded zeolite. At temperatures of 400 °C and 500 °C, the Ptloaded zeolites show no response even not towards NH3.