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Two-step electrochemical reduction of CO2 is investigated as an alternative to increase selectivity towards C2 and C3 products. In this type of proposed cascade electrocatalytic operation, CO is produced in a first step and subsequently reduced to multi-carbon products in a second step with significantly higher Faradaic efficiencies compared to a one-step process. Research efforts have been focused on the feasibility of the isolated second step with pure CO as reactant, however the interdependencies of both steps need to be considered. Accordingly, two-step electrochemical reduction of CO2 is studied in this work as an integrated system. Taking into account that the study of this technology at high current densities is crucial for industrial applicability, gas diffusion electrodes and flow-cells were used for operation at current densities above −200 mA cm−2 . Firstly, each step was characterized separately, the first using a silver gas diffusion electrode to generate a mixture of humidified CO, H2, and unreacted CO2; the second step using copper nanoparticles on a carbon-based gas diffusion structure to obtain C2 and C3 products. This step was studied using synthetic mixtures of CO2 and CO with different ratios. Furthermore, experiments with isotope labeled 13CO2 and 13CO were performed in order to obtain some insights on the (electrochemical) reaction path of gas mixtures containing CO2 and CO. Subsequently, the two units were integrated into a system, where the full gas output of the first unit was directly fed to the second unit. The total Faradaic efficiency towards multi-carbon products of this initial system was limited to 20% at total current density of −470 mA cm−2. These initial results together with the isotopic labeling studies indicate that the presence of significant amounts of unreacted CO2 from the first step is detrimental for the second step. A significant improvement was achieved by introducing a CO2 absorption column between the two units and after splitting the overall charge flow applied in each cell in accordance with the main reaction at each step. With this set-up a total Faradaic efficiency towards C2 and C3 products of 62% at a total current density of −300 mA cm−2 was achieved. The results confirm the need for a gas separation technique between the two steps for a feasible two-step electrochemical reduction of CO2.

The electrochemical conversion of CO2 to value-added chemicals is a technology gaining broader interest as society moves towards a carbon-neutral circular economy. Nonetheless, there are still several challenges to overcome before this technology can be applied as an industrial process. In the reaction path of the electrochemical reduction of CO2 with Cu as an electrocatalyst, it is known that carbon monoxide is the key intermediate to chemicals such as ethylene, ethanol, and n-propanol. However, a better understanding of the electrochemical reduction of CO is still necessary to improve selectivity and efficiency at high current densities. In this work, the electrochemical reduction of CO2 and CO towards C2 and C3 products is investigated using gas diffusion electrodes in a flow cell. Thereby the electrochemical reaction is not limited by the solubility of the feed gas in the electrolyte, and current densities of industrial relevance can be achieved. The electrodes are prepared using commercial Cu-powders consisting either of nano- or microparticles that are deposited on gas diffusion layers. Potentiostatic experiments show that with CO as the reactant, higher current densities for C2 and C3 products can be achieved at lower working electrode potentials compared to CO2 as the reactant. Galvanostatic CO electrochemical reduction at −300 mA cm−2 with Cu-nanoparticles (40–60 nm) results in a cumulative Faradaic efficiency of 89% for C2 and C3 products. This represents a two-fold increase in selectivity to ethylene and a three-fold increase towards ethanol and n-propanol compared to the selectivity obtained with CO2 as the reactant. This enhancement of selectivity for C2 and C3 products at current densities of industrial relevance with CO as reactant provides a new perspective regarding a two-step electrochemical reduction of CO2.

In n–i–p-type conventional perovskite solar cells (PSCs) using a doped 2,2′,7,7′-tetrakis (N,N′-di-p-methoxyphenylamine)-9,9′-spirofluorene (spiro-OMeTAD) hole transport layer (HTL), the issues of ...

With the transfer of the electrochemical CO

For the upscaling of the electrochemical conversion of CO2 to CO using Ag catalyst in water, gas diffusion electrodes (GDE) are needed for a sufficient supply of CO2 to obtain currents in the 100 mA cm−2 range. The effects of an upscale of one order of magnitude starting with 10 cm2 GDE size are presented. The penetration of electrolyte through the GDE needs to be regulated to balance the positive effects (avoid salt deposition) and the detrimental impacts (access blocking of CO2 to the GDE). Using a control of the partial pressure at the GDE to monitor the electrolyte penetration and enhancing CO2 feed by recirculation and turbulence promoters at the larger GDE. The first step of scale-up could be achieved without loss in performance. With three-compartment cells and 0.4 M K2SO4 electrolytes, the process is run at a current density of 150 mA cm−2 over a couple of hundred hours with a Faradaic efficiency for CO (FECO) of approximately 60% on 100 cm2 electrode area. It is discussed how to improve the performance by a management of the perspiration rate through the GDE to lay the scientific foundations for an industrial use of this technology.

It is still a challenge to find catalysts for the selective electrochemical reduction of CO 2 to value added products. We present a new method for the in-situ modification of a copper catalyst by adding oxygen to the reactant gas. With an appropriate process procedure consisting of a sequence of electrolysis and purge steps, a Cu 2 O layer was formed during the purge steps. When it was reduced again during the next electrolysis step, Cu + species remained stable on the surface and let to a higher faradaic efficiency for C 2 H 4 formation and even a lower onset potential for CO 2 reduction in general. This effect was observed for a wide range of O 2 content from 10 % to 60 % in the reactant gas. In comparison to electrolysis with pure CO 2 , formation of methane was largely suppressed.

Stabilized power output at maximum power point (mpp) has been considered as one of the most reliable parameters as it provides a key performance indicator for perovskite solar cells (PSCs) revealing the operational stability of the photovoltaic device. Here, we show the effect of selective contact on the power output change under mpp tracking, which closely correlates with the charge recombination dynamics with a time scale of minutes. The normal n-i-p cell architecture comprising cp-TiO2/mp-TiO2/perovskite/spiro-MeOTAD (doped by either Li-TFSI or Zn-TFSI2) and the inverted p-i-n structure, NiOx/perovskite/PCBM, are examined to investigate the specific effect of the nature of the interface on operational stability. The normal structure with Li-TFSI shows a gradual performance decrease at mpp owing to the enhanced recombination at the interface between the perovskite and the spiro-MeOTAD, becoming the dominant recombination process, although the bulk-related recombination is suppressed. On the other hand, the inverted structure demonstrates an improved photocurrent at mpp due to the effectively suppressed recombination both in bulk and at the interface. Remarkably, the deteriorating performance of the normal structure with Li-TFSI at mpp is successfully avoided by replacing Li-TFSI with Zn-TFSI2, leading even to an increased power output with stable performance at mpp.

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With the unprecedented rise of renewable energies, we will experience a profound change of our energy system, moving away from an unsustainable unidirectional energy system to a stable cyclic energy system. Carbon dioxide (CO2) is a product unavoidably coupled to the energy production for electricity generation or transport based on fossil fuels and the emission need to be reduced. Economic solutions for storage or conversion possibilities of large quantities of energy are essential in the future due the volatility of renewable electricity. This article will look into the industrial aspects of a new technology that converts collected CO2 into fuel precursors using renewable energy, thus opening a path for CO2 neutral transportation keeping combustion engines or hybrid concepts.

By applying the pulsed polarization technique, a thimble-type lambda probe can be used as a NOx sensor in the low ppm range. Due to the robustness of the sensor in harsh exhaust gas environments, this approach has many opportunities for application. The temperature operating range for best NO sensing properties is a crucial parameter. The sensor temperature changes with the ambient gas temperature, but can be stabilized actively by internal heating in a certain temperature range. This study evaluates in detail the temperature influence on NO sensitivity, so that an optimum operating point can be derived from these results using a dynamic measurement technique. Stepwise NO concentration changes between 0 and 12.5 ppm in synthetic exhausts demonstrate the potential of the concept.

WOS: 000448339100022 Li-TFSI is the most common p-dopant for the hole conductor spiro-MeOTAD in the normal structure (n-i-p) of perovskite solar cells (PSCs), which consistently yield the highest power conversion efficiency (PCE) albeit at the risk of lower long-term operational stability. Here we successfully replace conventional Li-TFSI with Zn-TFSI2, which not only acts as a highly effective p-dopant but also enhances considerably both the photovoltaic performance and long-term stability. The incorporation of Zn-TFSI2 as a dopant for spiro-MeOTAD leads to an increase by one order in the hole mobility compared to Li-TFSI from 3.78 x 10(-3) cm(2) V-1 s(-1) to 3.83 x 10(-2) cm(2) V-1 s(-1). Furthermore, the device with Zn-TFSI2 showed an 80 mV higher built-in voltage and a bigger recombination resistance than the one with Li-TFSI, which were responsible for the striking increase in both the open-circuit voltage and fill factor, leading to a stabilized PCE of 22.0% for the best cells. Remarkably, the device employing Zn-TFSI2 demonstrated superb photo-stability, showing even a 2% increase in the PCE after 600 h light soaking at the maximum power point (mpp) under full sun, while the PCE of the device with Li-TFSI decreased by 20% under the same conditions. Similarly, the device with Zn-TFSI2 showed better operational stability at 50 degrees C resulting in a 21% decrease in the PCE after 100 h aging at the mpp under full sun while the Li-TFSI based one showed a 55% decrease. Moreover, the Zn-TFSI2 based device was capable of effectively resisting humidity compared to the one based on Li-TFSI from shelf stability monitoring (R.H. 40%) in the dark. Coporate Research and Technology Siemens AG, Germany; GRAPHENE Flagship Core 2 project - European Commission H2020 Programme [785219]; TUBITAK - 2214-A International Doctoral Research Fellowship Programme; NRP 70 "Energy Turnaround" This work was supported by a grant from Coporate Research and Technology Siemens AG, Germany. H.-S. K thanks the financial support from the GRAPHENE Flagship Core 2 project supported by the European Commission H2020 Programme under contract 785219 is gratefully acknowledged. J.-Y. S thanks the financial support from the NRP 70 "Energy Turnaround". S. A. would like to thank TUBITAK - 2214-A International Doctoral Research Fellowship Programme, for supporting his research at EPFL.

The direct electro-reduction of CO2 to functional molecules like ethene is a highly desirable variant of CO2 utilization. The formation of, for example, ethene from CO2 is a multistep electrochemical process going through various intermediates. As these intermediates are organic species, the CO2 reducing electro-catalyst has to be competent for a variety of organic functional group transformations to yield the final product. In this work, the activity of an in situ-grown nano-structured copper catalyst towards a variety of organic functional group conversions was studied. The model reagents were selected from the product spectrum of actual CO2 reduction reaction (CO2RR) experiments and from proposals in the literature. The CO2 bulk electrolysis benchmark was conducted at 170 mAcm−2 current density with up to 43% Faradaic Efficiency (FE) for ethene and 23% FE for ethanol simultaneously. To assure relevance for application-oriented conditions, the reactivity screening was conducted at elevated current densities and, thus, overpotentials. The found reactivity pattern was then also transferred to the CO reduction reaction (CORR) under benchmark conditions yielding additional insights. The results suggest that at high current density/high overpotential conditions, also other ethene formation pathways apart from acetaldehyde reduction such as CH2 dimerization are present. A new suggestion for a high current density mechanism will be presented, which is in agreement with the experimental observations and the found activity pattern of copper cathodes toward organic functional group conversion.

The sensing characteristic of a non-Nernstian ammonia sensor, which can be described as an electrochemical cell Au | YSZ | Au, V 2 O 5 –WO 3 –TiO 2 , is analyzed in dependence on the V 2 O 5 -content of the porous SCR-catalyst layer on top of one gold electrode. The ammonia sensitivities are determined at 550 °C for sensors with different amounts of V 2 O 5 (0–3 wt.%). The slope of the semi-logarithmic characteristic curve increases with the vanadia content. The analysis of the half-cell electrode potentials of the half cells YSZ | Au, VWT confirms this behavior. All electrode potentials increase with increasing NH 3 concentration and with the vanadia content of the catalyst layer. The V 2 O 5 content of the catalyst layer affects the electrochemical processes at the three-phase boundary Au | YSZ | gas and the resulting voltage of the mixed potential. The gas composition at the three phase boundary can be modified by the catalyst layer. Both the NH 3 sensitivity and the electrode potential shifts increase with the ammonia oxidation efficiency. The formation of an activated ammonia species on the catalyst layer, which is electrochemically oxidized, seems to be responsible for the electrode potential shift. According to this suggestion, more activated species are generated with increasing vanadia content. It is then subsequently electrochemically converted with oxygen ions from the YSZ to N 2 and H 2 O.

Pulsed polarization is a measurement technique based on alternating voltage pulses with intermediate pauses for self discharging. The possibility of NO detection by this dynamic measurement technique using conventional lambda sensors is shown in prior investigations. The discharging after polarization depends strongly on NO content in the low ppm range. Due to faster discharging at NO containing atmosphere compared to base gas, the voltage difference between the discharge curves can act as a characteristic value; this voltage change shows a logarithm dependency on NO concentration. The feasibility of NO sensing by use of a planar, more cost-effective sensor (Pt electrodes on yttria stabilized zirconia) is evaluated in detail. The optimum range for NO detection is found at 400 °C by comparing the NO sensitivities at temperatures from 300 °C up to 550 °C.

To detect NO in the low ppm range in exhaust gases is of high interest due to the ever tighter emission and immission regulations. Applying the pulsed polarization technique, Pt|YSZ sensors like the classical thimble type lambda probes for automotive applications or planar sensor elements (Pt electrodes on both sides of YSZ) show very good sensitivities toward NO. To further improve the sensor with respect to simplicity and robustness for exhaust gas applications, a one-sided design is preferred, especially to implement a heater on the reverse side of the sensor element. For that purpose, a design that bases on platinum interdigital electrodes is investigated. Again, by using this polarization technique very strong signals towards NO in the lowest ppm range have been achieved.

The toxicity of chromium ions was investigated using mammalian cell cultures on impedance sensors as well as physiological in vitro sensor systems. The performance of commercially available systems like the 2500 Analyzing System (Bionas), xCELLigence (Roche) and Cytosensor Microphysiometer (Molecular Devices) was compared with a novel CMOS-based impedance-to-frequency converter device. Cell-based sensor systems are shown to be powerful tools to detect Cr(VI) pollutions within several hours in the range of multinational drinking water regulations. The ability to distinguish between toxic Cr(VI) and non-toxic Cr(III) species is one advantage of these integral sensor systems. Impedance only devices are not sufficient for the fast detection of toxic chromium species as rapid cellular changes occur only in the respiration system and the cell physiology. © 2013 Elsevier B.V. ispartof: Sensors and Actuators B, Chemical vol:182 pages:58-65 status: published

Pulsed polarization is employed for NO x detection using thimble-type standard lambda probes. The model system Pt l YSZ is investigated to understand the reactions occurring at electrodes during alternating pulsed voltages and self discharge. For that purpose, the atmosphere is varied and different pulse parameters like pulse duration and pause between the pulses with opposite sign are evaluated. During polarization in oxygen rich atmosphere, it appears that platinum oxide is formed at one electrode and reduced due by the following pulse with opposite sign at this electrode. At the other electrode, this reaction also takes place, but one pulse delayed. With increasing pulse duration, more platinum is oxidized. According to this, the polarization current of the subsequent pulse grows clearly due to the reduction of the formed oxide and therefore more charge is transferred in contrast to a short polarization pulse duration. Higher oxygen content affects the polarization current in the same way. The theory of platinum oxidation and reduction during alternating polarization is in good agreement to cyclic voltammetry investigations.

Pollution of drinking water sources represents a continuously emerging problem in global environmental protection. Novel techniques for real-time monitoring of water quality, capable of the detection of unanticipated toxic and bioactive substances, are urgently needed. In this study, the applicability of a cell-based sensor system using selected eukaryotic cell lines for the detection of aquatic pollutants is shown. Readout parameters of the cells were the acidification (metabolism), oxygen consumption (respiration) and impedance (morphology) of the cells. A variety of potential cytotoxic classes of substances (heavy metals, pharmaceuticals, neurotoxins, waste water) was tested with monolayers of L6 cells (rat myoblasts). The cytotoxicity or cellular effects induced by inorganic ions (Ni(2+) and Cu(2+)) can be detected with the metabolic parameters acidification and respiration down to 0.5 mg/L, whereas the detection limit for other substances like nicotine and acetaminophen are rather high, in the range of 0.1 mg/L and 100 mg/L. In a close to application model a real waste water sample shows detectable signals, indicating the existence of cytotoxic substances. The results support the paradigm change from single substance detection to the monitoring of overall toxicity.

In the past years, whole cell gas toxicity assays have been developed in order to study the harmful effects of gaseous chemicals. Cells are incubated with nutrition medium supplemented with the gaseous substances to be tested. These methods are not feasible for the examination of toxic gases, which are insoluble in water. Carbon monoxide (CO), a poorly soluble and toxic gas (lethal dose 1 vol% for some minutes) has been chosen as a model gas. Besides its binding to hemoglobin, it also leads to an inhibition of cytochrome c oxidase in the mitochondrial respiratory chain of living cells. Thus, a decrease in the respiratory behavior of eukaryotic cells indicates the presence of CO in a gaseous phase. Based on the Bionas® 2500 analyzing system, a device to measure the metabolic and morphological effects, caused by direct gas exposure on eukaryotic cells was established. The gaseous substances directly contact the eukaryotic cell line V79 (Chinese hamster lung fibroblast) without interfering liquid phase in between. Beside the measurement of oxygen consumption also acidification and impedance changes of the cell culture are detected by a metabolic sensor chip. In order to increase the stability different chip surface coatings like collagen A and poly-L-lysin (PLL) were used. Adhesion coating leads to a tighter junction of the cells to the sensor chip surface and thus enables higher stability of the confluent cell monolayer towards the gaseous flow during the gas exposure. The method reported here has the potential to become a valuable means for rapid monitoring of toxic compounds in gaseous phases.

Presented is a measurement approach to differentiate reliably exhaust gas components using standard zirconia-based thimble-type potentiometric lambda sensors. A self-discharge characteristic after applying different voltage pulses between the sensor electrodes is used as a measurement parameter which depends on the gas component and its concentration in the exhaust. The detection of NO in the lower ppm range is demonstrated. In addition, this approach allows significant improvements in the response and selectivity to NO compared to other investigated gases such as H2, C2H5OH, and a mixture of hydrocarbons. Therefore, this technique seems to be capable of enabling NO sensing in combustion exhaust with improved selectivity using standard lambda sensors.

Typically, carbon nanotube field effect transistors (CNFETs) based sensors exhibit substantial gate hysteresis and therefore, prior to measurements, require electrical initialization to erase electrical history of their threshold voltage (Vth(t)). Particularly, for CNFET gas sensors, which respond with small Vth shifts to analyte concentration changes, electrical initialization is necessary for reliable sensor operation. Here, we propose and demonstrate electrical initialization of CNFETs for sensing applications by applying triangular ambipolar pulse (TAP) sequences to the gate electrode., Procedia Engineering, 5, Eurosensor XXIV Conference

A laser spectroscopic oxygen sensor aiming at combustion optimization is developed. The sensor allows for self-monitored operation and has a response time of 300 ms. The accuracy for the oxygen concentration is better than ±0.2 vol%. The laser optical sensor is inherently calibration-free and does not age compared to the conventional ZrO2 probes. The spectroscopic oxygen sensor is based on a diffuse reflective geometry, which minimizes possible optical interference and also alignment problems caused by the thermal cycles of the furnace and an efficient evaluation algorithm that allows for long-term stable operation.

A miniaturized optical carbon-monoxide sensor based on tunable diode laser spectroscopy is presented and tested for various applications. It utilizes recently developed Vertical-Cavity Surface-Emitting Lasers (VCSELs) emitting at 2.3 μm as light source and the photodetector housing as integrated gas reference cell. Together with a wide/narrow spectral scan algorithm for wavelength stabilization, a compact, self-monitored and microcontroller-based sensor is realized. In contrast to conventional line-locking, the full wavelength scale of the spectral scans is inherently calibrated. The achieved sensing resolution (1 sigma) is 3 ppm for 1 second averaging.

We investigate the possibility of applying rectangular voltage pulses to the gate electrode of carbon nanotube field effect transistors (CNFETs) for NO2 detection. CNFET gate characteristics measured using pulsed gate voltages show a reduction of hysteresis by about 20% compared to measurements with linearly swept gate voltages. Moreover, an enhancement of the measurement range is observed in the pulsed regime. The characterized CNFET devices are based on individual single-walled carbon nanotubes (SWNTs), which act as the sensing elements.

Novel selective ammonia sensors with high potential for long-term stability in harsh exhaust environments are introduced. The sensor bases on the mixed potential effect. In contrast to common sensors, the electrode functionalities electrical conductivity, selective catalytic activity, and electrochemical activity combined with long-term stability are separated. For that reason, one of the two electrodes is covered by a well-known porous vanadia–tungstenia–titania-based SCR catalyst material, which has been developed for exhaust gas SCR applications. The resulting sensor signal depends semi-logarithmically on ammonia. The NOx cross-sensitivity is marginal. If enough oxygen is in the exhaust, the sensor signal is independent of the oxygen concentration. Tests downstream of an SCR catalyst show that very small ammonia slips can be determined.

A qualitative comparison of the NO 2 sensitivity of carbon nanotube field effect transistors (CNFETs) has been performed on CNFETs with 1) suspended channels, and 2) channels that are in contact with the SiO 2 substrate. No significant response difference to N0 2 is apparent in the two CNFET configurations. Therefore, the Si0 2 substrate acting as a dominant indirect sensing mechanism can be ruled out. Additionally, two measurement modes in CNFETs are presented and discussed. The measurement modes are 1) chenfiresistor response, and 2) the shift of the CN-FET gate characteristic toward positive gate voltages. A method is proposed to acquire the shift in the gate thresh-old for future CNFET sensor applications, and examples of such transduction curves of two sensors are presented.

In this contribution, a multilayer resistive sensor based on catalytically activated and non-activated SrTiO 3 is presented as a device for lean hydrocarbon sensing. This set-up should enable a compensation of oxygen cross interference. Whereas the catalytically activated part merely detects a variation in the oxygen equilibrium concentration in presence of hydrocarbons, the non-activated part strongly interacts with the reducing gases. Promising results with respect to stable sensor signals covering a wide hydrocarbon concentration range have been obtained using ethane and propane as hydrocarbon species. The sensor response is found to depend on the chain length of the used hydrocarbons. Using both sensor signals, an evaluation method for oxygen compensation has been studied. In addition, the cross interference of other common exhaust components has been investigated.

Simultaneous Hall and conductivity measurements were performed in situ between 650° and 1050°C on n-type semiconducting BaSnO3ceramics. The variation of the Hall mobility and the Hall carrier density as a function of oxygen partial pressure between 102 and 105 Pa and of temperature was investigated. At temperatures below 900°C the conductivity exhibits a dependence on temperature and oxygen partial pressure which is mainly determined by variations of the Hall mobility. Above 900°C most of the significant dependence is due to a variation in carrier density. Furthermore, a simple defect model assuming doubly ionized oxygen vacancies and acceptor impurities is discussed for BaSnO3.

The gas sensitivity of Ga2O3 thin-film n-type conductors was investigated at temperatures of 500–1000°C. Palladium dispersions whose particle sizes are dependent on the preceding annealing processes were deposited by a wet-chemical technique onto Ga2O3 thin-film ceramics. The palladium clusters and their temperature-dependent growth were detected using scanning electron microscopy micrographs and X-ray photoemission spectroscopy measurements. The effect of the palladium dispersions on the gas-sensitive behavior of the Ga2O3 ceramics was investigated in various O2/H2 mixtures in the N2 carrier gas at 700°C. The conductivity of the ceramics treated in this way was dependent on the O2 partial pressure, as well as on the H2 partial pressure of the surrounding gas atmosphere. The ceramic conductivity can be described as a function of the O2:H2 ratio, in accordance with the relation σ(pO2/pH2/)−1/3.

The characteristic NIR-absorption lines of methane and ethane are being acquired simultaneously and their concentrations are determined using a tunable DFB laser diode. This paper reports on the development of a hand-held and low-power sensor, based on direct absorption spectroscopy for leak detection (methane), natural gas identification and lower-explosion-limit (LEL) measurements (methane and ethane). The algorithm corrects for ambient temperature and pressure. At normal conditions and constant operation a battery-lifetime of 8+ h is achieved.

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.

It is known that barium carbonate can act as CO2 sensitive material in work function type gas sensors in ambient condition. The reaction is due to the formation of bicarbonate species at the surface. The cross-sensitivity to other acidic gases as NO2 was up to now regarded and described in terms of similar reactions on the carbonate surface. In this work it was found that the NO2 sensitivity is due to reactions on the back metalisation of the carbonate film. Using different layer systems different sensitivities can therefore be switched on and off according to the needs in special applications. The design of specialised sensor systems is possible on the base of the results found in this work.

The CO sensitivity of Pt(NH3)2(NO2)2 and Pd(NH3)2(NO2)2 impregnated SnO2 thick layers was measured by following the work function change, at 90 °C. The complexes were decomposed by heat treatments in air, in the temperature range of 150–350 °C. The composition of the surface layer was studied by XPS. The maximum CO sensitivity, the optimal response and recovery times-measured by Kelvin probe—were found, if PtO or PdO were present on the surface.

High temperature Ga 2 O 3 -gas sensors show some differences to other sensors based on other metal oxides like SnO 2 . Among the advantages of Ga 2 O 3 -based sensors good long-term stability, fast response and recovery times, good reproducibility, low cross sensitivity to humidity and short pre-ageing times have to be mentioned. The good stability in sulphur-containing atmospheres makes them suitable for use in domestic burner controls. On the other hand, is the high operation temperature of about 600–800°C which means that power consumption is comparably high ( 2 O 3 -sensors to CO and NO 2 is lower compared to other metal oxide-based sensors. In this article measurements are presented in order to compare the performances of high temperature Ga 2 O 3 -gas sensors and SnO 2 -sensors. Electronic conductivity models are proposed explaining the above mentioned differences and recommendations for the different application areas are given.

Metal oxide gas sensors in general, show high sensitivity but poor selectivity if pure sensor materials are used. The performance in terms of reproducibility may be enhanced by using very stable materials, which may be operated at quite high temperature but this does not help in the problem of selectivity. This paper discusses the use of gas pretreatment using filters operated at elevated temperatures to overcome this problem for special applications. Three different types of gas filters are discussed: a physical filter which is permeated by hydrogen only, a catalyst filter which removes disturbing solvent vapours by oxidation, and a gas conversion filter which ensures a defined NO/NO2 equilibrium. These filters relate to the applications of selective indoor CH4, H2, and automotive exhaust gas nitrogen oxide detection.

In this work, it was found that exposure of TiN to ammonia (400 ppb–100 ppm) in wet synthetic air at room temperature leads to a reversible decrease of Δ Φ . There is a change of the work function of −30 meV/decade of increasing ammonia concentration at room temperature. The response time ( t 90 ) to 1 ppm is ≤1 min. The sensitivity is a typical room temperature effect as it decreases at higher temperatures up to 100°C. The reaction has a low cross sensitivity to humidity, as — first — the sensitivity to ammonia does not depend on the relative humidity and — second — the work function change due to humidity changes is as low as −2 meV for an increase of 10% in relative humidity. The Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFT-spectra) indicate that the adsorption of ammonia is correlated with a decrease of OH species on the surface. All data are compared to the gas reactions of TiO 2 .