Zirconia Oxygen Sensor Research Articles

Evaluation of Hydrogen Ions Flowing through Protonic Ceramic Fuel Cell Using Ytterbium-Doped Barium Zirconate as Electrolyte

Protonic ceramic fuel cells (PCFCs) are clean power generation devices that do not emit carbon dioxide, and generate electricity when protons (hydrogen ions) pass from the fuel electrode through the electrolyte to form water vapor at the air electrode. As a fuel cell, PCFC is expected to be a highly efficient fuel cell because it can operate at medium to low temperatures of around 600°C and no fuel dilution occurs. However, proton-conducting ceramics can conduct not only protons but also a small amount of holes, which can cause electron leakage and lower the power generation efficiency. Therefore, it is very important to understand the ionic and hole conductivity in a fuel cell.To evaluate the electronic leakage of PCFC, the amount of proton current flowed in PCFC can be measured by an apparatus developed based on the electromotive force of an electrochemical cell using 10 mol% In-doped CaZrO3 (hydrogen sensor) and Y stabilized zirconia (oxygen sensor). Water vapor pressure on cathode was change when hydrogen ion passes thorough PCFC. The current of hydrogen ion can be determined by monitoring water vapor pressure. We measured the current of hydrogen flowing through protonic ceramic fuel cell using ytterbium-doped barium zirconate as electrolyte. Hydrogen ion and hole were found to leak at OCV at 700°C. Then electronic leakage was estimated form the current of hydrogen ion and external current. The electronic leakage was found to be suppressed with increasing electrolyte film thickness. On the other hand, no leakage of hydrogen and holes was observed at 500°C.

Evaluation of Hydrogen Ions Flowing through Protonic Ceramic Fuel Cell Using Ytterbium-Doped Barium Zirconate as Electrolyte

To measure the proton current in a PCFC (protonic ceramic fuel cell), the proton current detector was developed using a proton conducting oxide. The amount of proton current that flowed in the PCFC can be measured by an apparatus developed based on the electromotive force of an electrochemical cell using 10 mol% In-doped CaZrO3 (hydrogen sensor) and Y-stabilized zirconia (oxygen sensor). The water vapor pressure on the cathode changed when hydrogen ions pass through the PCFC. The hydrogen ion current can be determined by monitoring the water vapor pressure. The electronic leakage was then estimated from the hydrogen ion current and external current. We measured the current of hydrogen flowing through protonic ceramic fuel cell using ytterbium-doped barium zirconate as the electrolyte. Hydrogen ions and holes were found to leak under the OCV at 700 °C. The electronic leakage was found to be suppressed with the increasing electrolyte film thickness. On the other hand, no leakage of hydrogen and holes was observed at 500 °C.

Evaluation of Hydrogen Ions Flowing through Protonic Ceramic Fuel Cell Using Ytterbium-Doped Barium Zirconate as Electrolyte

To measure the proton current in a PCFC (protonic ceramic fuel cell), the proton current detector was developed using a proton conducting oxide. The amount of proton current that flowed in the PCFC can be measured by an apparatus developed based on the electromotive force of an electrochemical cell using 10 mol% In-doped CaZrO3 (hydrogen sensor) and Y-stabilized zirconia (oxygen sensor). The water vapor pressure on the cathode changed when hydrogen ions pass through the PCFC. The hydrogen ion current can be determined by monitoring the water vapor pressure. The electronic leakage was then estimated from the hydrogen ion current and external current. We measured the current of hydrogen flowing through protonic ceramic fuel cell using ytterbium-doped barium zirconate as the electrolyte. Hydrogen ions and holes were found to leak under the OCV at 700°C. The electronic leakage was found to be suppressed with the increasing electrolyte film thickness. On the other hand, no leakage of hydrogen and holes was observed at 500°C.

Effect of Partial Equilibrium and Redox Reactions in a Zirconia Solid Electrolyte on the Electromotive Force in a Zirconia Oxygen Sensor

A zirconia oxygen sensor is an electrochemical device for rapidly measuring oxygen concentrations in molten metals at high temperatures. It is desirable to make the sensors capable of continuous, long-time measurements. Here, the effect on the electromotive force of a partial equilibrium state on the surface of a zirconia solid electrolyte in a zirconia oxygen sensor was investigated to reveal the factors preventing long-time measurements. The reference electrode was a mixture of W and WO2. Continuous measurements were performed in molten iron containing carbon for deoxidization. The electromotive force between the positive reference electrode and the negative sample electrode of the sensor decreased from a positive value to zero over time. This was because a metallic tungsten layer formed on the surface of the zirconia electrolyte by the reduction of the tungsten oxide via oxygen diffusion through the zirconia tube. In addition, the equilibrium in the reference electrode was disturbed during the measurements. A direct electrical current was applied to the sensor to examine the relationship between the surface and measured values. The electromotive force during the current was maintained at a higher value than that without the current because of induced re-oxidation at the zirconia surface. However, a long-time application could cause over-oxidation and dissolution of the reference electrode surface.

A review of zirconia oxygen, NOx, and mixed potential gas sensors – History and current trends

Zirconia-based electrochemical sensors have revolutionized oxygen monitoring and combustion control. These robust, solid-state devices are found in virtually every internal combustion engine today providing feedback information for tight air to fuel ratio control thus minimizing unwanted emissions and improving engine efficiency. Variations on the sensor design and operational mode enable the measurement of nitrogen oxides to part per million levels, providing control information for exhaust after-treatment systems. Emerging designs include the development of "gas analyzers on a chip" capable of monitoring oxygen, carbon monoxide, nitrogen oxides, ammonia and hydrocarbons. • Zirconia based electrochemical gas sensors can detect wide variety of pollutants. • Potentiometric, amperometric, impedance-based, and pulsed polarization methods are reviewed. • Modeling of sensor behavior can predict sensor response. • Machine learning techniques can be applied for sensor signal decoding.

Non-Faradaic Electrochemical Modification of Catalytic Activity: LSCF As Electrocatalyst for Nitrous Oxide Electrochemical Reduction

Nitrous oxide (N2O) mitigation is of high importance as this gas is responsible for several environmental problems, such as photochemical smog, acid rain, ozone layer depletion and global warming. While N2O is not the principal contributor to global warming (~ 6 %), it has 310 and 21 times the Global Warming Potential (GWP) of the most common anthropogenic greenhouse gases, CO2 and CH4, respectively [1]. European Union directives currently aim at a reduction in the emissions of these gases by 42 % from their 2013 levels by 2030 [2].The current work aims to bridge solid-state chemistry and heterogenous catalysis, by studying the electrochemical promotion of catalysis (EPOC), also known as non-Faradaic electrochemical modification of catalytic activity (NEMCA), of the N2O reduction reaction. Here, the catalytic activity of an electrode surface is radically changed upon polarization, typically applying large electrical potentials (> 2 V) that lead to considerable changes in the conversion and the selectivity of the products [3].For this work, the well-known mixed electronic and ionic conductor La0.6Sr0.4Co0.2Fe0.8O3-δ LSCF) was utilised as the electrocatalyst, due to its high availability and excellent performance as a cathode in solid oxide fuel cells (SOFCs). This composition exhibits a very high oxygen surface exchange coefficient (K* = 1.1 x 10-3cm s-1 at 800 °C) as well as high electronic (230 S cm-1 at 900 °C) and ionic conductivities (0.2 S cm-1 at 900 °C), essential requisites for this application [4].A 3-probe cell-based reactor was developed, composed by a solid oxide-ion-conducting electrolyte substrate made of 8 % mol Y2O3 – stabilized ZrO2, a working electrode made of LSCF and counter and reference electrodes made of inert gold (Au), to avoid any influence of the latter parts in the electrocatalytic activity of the main LSCF electrocatalyst. A buffer layer of Ce0.9Gd0.1O2-δ (CGO) was additionally used between the working electrode and the substrate, to prevent the inter-reaction between the YSZ and LSCF.The key aspect of this work was the use of an integrated zirconia oxygen sensor in the studies, to determine variations in the values of oxygen partial pressure (pO2 ) upon polarisation, which were subsequently correlated with the extent of N2O conversion. This methodology proved to be highly effective and avoids the need for additional techniques, such as gas chromatography.From the experiments, it was found that by applying cathodic polarisation (i.e -0.05 and -0.25 A), an increase of the catalytic rate (r O2 mol s-1) was observed, resulting in a significant increase of N2O conversion. An electrophobic behaviour of the electrocatalyst, i.e., a reaction rate that increases upon cathodic polarization, was observed for all temperatures with a maximum rate enhancement ratio (ρ=r/r0) up to 3.47 was achieved at -0.05 A at 600 ºC. The Faradaic efficiency (Λ) was also calculated for the tested experimental conditions, and showed values that exceed unity, evidencing non-Faradaic electrochemical behaviour. The highest Λ value of 8.8 was obtained at the lower temperature and currents tested at 600 °C. The nature of the mechanism was also assessed, showing a completely reversible effect, where the electrochemically promoted reaction rate returns to its open circuit value upon removal of polarisation. This work, therefore, highlights that non-Faradaic activation of heterogenous catalysis can be an effective tool to promote the reductive treatment of this greenhouse gas, with great environmental importance.

Electro-thermo-mechanical modelling of automotive exhaust sensors

The development of automotive exhaust sensors is generally based on a time and resources consuming empirical approach. Building numerical models to study the behavior of the sensors is therefore an important key to reduce the development time and improve sensor quality. Zirconia sensor accuracy and response are dependent from temperature, so temperature is generally controlled in the desired range through a heating element. In the present study an electro-thermo-mechanical model of a heated zirconia oxygen sensor with planar structure has been provided. The numerical model has been used as a tool to study and compare different geometries of heaters. Several heater configurations have been modelled and compared in terms of: temperature rise in different points on the surface of the sensor both on the electrode side and heater side, average and maximum temperature, thermal stress and time for the sensor to be considered functional. The improved heater provided a lower peak temperature, but higher average temperature, more uniform temperature distribution, lower thermal stress and lower time than the base heater to reach the prescribed operational conditions.

Effect of Molybdenum Layer Formed at the Inner Surface in Mo/MoO<sub>2</sub>–type Zirconia Oxygen Sensor on Continuous Measurement of Oxygen Content in Molten Steel

Effect of Molybdenum Layer Formed at the Inner Surface in Mo/MoO<sub>2</sub>–type Zirconia Oxygen Sensor on Continuous Measurement of Oxygen Content in Molten Steel

Zirconia-Based Electrochemical Oxygen Sensor for Accurately Determining Water Vapor Concentration

By varying the control voltages of commercially available zirconia-based automotive exhaust gas oxygen sensors, the concentration of other oxygen-containing molecules such as H2O and CO2 can be measured. Typical water concentration sensors that measure relative humidity via swelling of certain polymers have a narrow temperature range of operation (typically < 120°C) and limited accuracy of about ± 0.4% absolute. Initial measurements with modified zirconia oxygen sensors, both in the laboratory and on engine dynamometers, have shown that water vapor concentration in a gas stream can be measured accurately to ± 0.1% over the range from 0-20 vol% H2O at temperatures up to 800°C.

Gas Composition Analysis Using Yttria-stabilized Zirconia Oxygen Sensor during Dry Reforming and Partial Oxidation of Methane

Gas Composition Analysis Using Yttria-stabilized Zirconia Oxygen Sensor during Dry Reforming and Partial Oxidation of Methane

Preparation and Resistivity of Zirconia Oxygen Sensor Probe

A ZrO2/Al2O3 (molar ratio of 1: 1, ZA) composite ceramic was formed by slip casting as solid electrolyte in a zirconia oxygen sensor. The bonding of the ZA ceramic green body to 95% Al2O3 ceramic green body support tube was carried out at 900 oC for 60 min by the gradient joining technique using pure Al2O3 slurry as the interlayer. Subsequently, the Pt slurry was coated on the surface of the pre-sintered composite ceramic, and then co-sintered at 1550 oC for 60 min to fabricate a Pt/ceramic composite probe. The interface microstructure and bonding mechanism were briefly investigated, and the electrical conductivity of the probe was tested. The experimental results show that the two high-quality ZA/Al2O3 and ZA/Pt interfaces were obtained. In particular, the relationship between the logarithm of conductivity (lnρ) and the reciprocal of temperature was well in accord with the Arrhenius equation. The Pt/ceramic composite probe presented the typical characteristic of high-temperature ionic conduction.

Natural circulation experiments in a non-uniform diameter lead bismuth loop and validation of LeBENC code

Experimental studies are carried out on natural circulation in a Lead Bismuth Eutectic (LBE) loop. The loop mainly consists of a heated section, air heat exchanger, valves, various tanks and argon gas control system. All the components and piping are made of SS316L. The dissolved oxygen in the LBE is monitored online by an Yttria Stabilised Zirconia (YSZ) oxygen sensor and controlled during the operation of the loop. In this paper the details of the loop and experimental studies carried out with heater power levels varying from 900 W to 5000 W are described. The temperature range of LBE during the experiments was 200 °C–500 °C. The maximum heat loss in the piping is kept less than 20% of the main heater power. Steady state experimental studies are carried out at different power levels and the LBE flow rate was found to be varying from 0.095 kg/s to 0.135 kg/s. The analysis and results of the performance of the heat exchanger with air and water as the secondary coolants are also discussed in the paper. Transient studies were carried out to simulate various events like heat sink loss, step power change and secondary side coolant flow rate change and reported in the paper. In the start up experiments, where the flow is started from stagnant condition of LBE, the time required for starting of natural circulation is found to be 600 s, 400 s and 240 s with power level of 1200 W, 2400 W and 3000 W respectively. The results are compared with available correlation and prediction of computer code LeBENC.

Simple method for determining metal power oxidation kinetics with a zirconia sensor

A basic zirconia oxygen sensor was utilized to monitor the gas–solid oxidation reaction characteristics of metal/metal-oxide powder beds. The method described allowed for simple determination of chemical reaction rate constants for the oxidation reactions. The signal output of the sensor was analyzed for oxidation of the powder bed and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed, and a discussion of relevant thermodynamic and kinetic theory was provided. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor. Combining the experimental data with relevant kinetic theory, chemical reaction rate constants were calculated from plots of sensor ocv versus time for each reaction step during the metal oxidation sequences.

Surface tension of binary Al–Si liquid alloys

In the present study, the surface tensions of pure liquid Al, Si, and liquid Al–Si alloys were systematically investigated by means of the oscillating drop method in combination with electromagnetic levitation. Prior to the measurement, the ambient oxygen partial pressure in the chamber was measured using an yttrium-stabilized zirconia oxygen sensor to estimate the surface oxygen partial pressure based on the thermodynamic assesssment. Under the experimental Ar gas with oxygen partial pressure of 10–1 Pa, the oxygen-reduced surfaces of pure liquid Al and Si can be obtained. However, the surface tensions of alloys in Al-rich composition are strongly affected by the oxygen adsorption under the same oxygen partial pressure. With the increasing Si concentration, the surface tension of the alloy approaches to the values of the oxygen-reduced surface.

Temperature-programmed Desorption of Oxygen from La–Sr–Co–Fe Perovskite in Atmospheres with Varying Oxygen Partial Pressure

Abstract A temperature-programmed desorption technique under atmospheres with variable partial pressure of oxygen has been developed using a homemade fixed-bed flow reactor equipped with a yttria-stabilized zirconia (YSZ) oxygen sensor as a detector of oxygen desorbed from a sample. Its significance has been verified for the particular catalytic material of La–Sr–Co–Fe–O perovskite-type oxide.

Thick-film amperometric zirconia oxygen sensors: influence of cobalt oxide as a sintering aid*

Amperometric thick-film zirconia oxygen sensors are electrochemical devices in which the zirconia thick-film acts as both electrolyte and diffusion barrier. Their preparation involves temperatures of 1300–1400 °C to sinter the thick-film and reduce the through-porosity to a sufficiently low value to restrict oxygen diffusion rates and hence sensor limiting currents. The resulting sensors normally require an operating temperature of 800 °C to enable operation in the percentage oxygen concentration range. In this work sensors were prepared with and without doping with cobalt oxide. Thick-films were characterized using scanning electron microscopy to view a fracture edge and by plotting current–voltage curves of the prepared sensors operated at 600–800 °C in oxygen concentrations up to 21% at atmospheric pressure. It was found that doping with cobalt oxide markedly increased the sintering rate and enabled a reduction of 100–110 °C in sintering temperature for a given final through-porosity. As a result it was possible to operate doped sensors at a temperature around 200 °C lower than otherwise identically-prepared undoped sensors. This is expected to have a beneficial effect on sensor life and reduce operating power.

Zirconia Sensor Device for In Situ Monitoring of Metalpowder Oxidation for Energy Storage Applications

A zirconia oxygen sensor was designed to monitor the gas-solid oxidation reaction characteristics of metal/metal-oxide powder beds for energy storage applications. Energy is stored in the metallic state and released during oxidation. This in-situ monitoring device allows for determination of the oxidation state of the metal oxides during reactions in real time, as well as calculation of the chemical reaction rate constants. The signal output of the sensor was analyzed for oxidation of and diffusion into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the sensor. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor for actual energy storage applications.

Zirconia Sensor Device for In Situ Monitoring of Metalpowder Oxidation for Energy Storage Applications

A zirconia oxygen sensor was designed to monitor the gas-solid oxidation reaction characteristics of metal/metal-oxide powder beds. This in-situ monitoring device allows for determination of the oxidation state of the metal oxides during reactions in real time, as well as calculation of the chemical reaction rate constants. The signal output of the sensor was analyzed for oxidation of and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed. A model metal/metal-oxide system is examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility.

A stoichiometric titration method for measuring galvanic hydrogen flux in ceramic hydrogen separation membranes

A method is described for quantitative measurement of the hydrogen flux through protonic ceramic hydrogen separation membranes under practical operating conditions. The method uses a simple stoichiometric titration technique to determine the quantity of oxygen required in the membrane permeate stream to reach the lambda transition point, as determined by a down-stream zirconia oxygen sensor. As such, the method does not require the use of gas chromatographs or mass spectrometers, which are difficult to calibrate for hydrogen under high pH2O conditions. The theory of operation and application to yttrium-doped barium cerate/zirconate, BCZY, membranes is laid out in detail in consideration of the unique transport properties of these perovskite proton conductors under galvanic operation in asymmetric atmosphere conditions. The Faradaic efficiency toward hydrogen in these membranes was found to be 95% at 700°C, and the H2 flux was 1.4sccm/cm2 (0.95μmol/scm2) when pumped galvanically at 8V.

Oxygen Measurement System using Optical Communication Devices

Concentration measurement of oxygen gas is required for better monitoring and control of various types of combustion systems. As compared with conventional zirconia oxygen sensors, tunable diode laser absorption spectroscopy can provide faster response, with smaller interference of inflammable gases. In this paper we report the concentration measurement of oxygen molecules using the absorption band (A-band) of 760 nm wavelength. A periodically poled LiNbO3 crystal (PPLN) is employed for the frequency doubling of 1520 nm light from a distributed feedback laser, resulting in the 760 nm output power of 1.8 mW. The wavelength modulation spectroscopy with dual beam configuration enables the cancellation of the common mode noise from diode laser as well as excess etalon noise that arises from optical paths including the PPLN module. With the cell length of 200 mm, the absorption measurement using the R11Q12 line at 760.445 nm has led to a minimum detectable absorbance of 3.7×10-4 m-1. A dynamic response faster than 1 s was seen when the oxygen concentration was changed between 0% and 24% while the laser wavelength was fixed at the absorption peak.