YSZ-based Sensor Research Articles

YSZ-based NO2 Mixed Potentiometric Sensor with Three-Dimensional Ordered Porous Structure

This study presents the preparation of a nitrogen dioxide (NO2) gas sensor based on yttrium-stabilized zirconia (YSZ) using a hard-template method and evaporation self-assembly technology. The sensing electrode employed is nickel oxide, and the sensor exhibits an efficient three-phase boundary. The study systematically investigates the effects of changing the three-phase reaction boundary on the gas-sensing performance of the sensor. A three-dimensional (3D) ordered macroporous (3DOM) structure was deposited on a YSZ substrate using the hard template method to obtain an efficient three-phase boundary. At 450 °C and 100 ppm NO2, the response value of the sensor with a 3D-ordered porous structure was 38.71 mV, which is 1.91 times the response value observed for the untreated YSZ-based sensor (20.3 mV), and the sensitivity significantly improved. In addition, a sensor with a 3D-ordered structure exhibits good 2 selectivity, and long-term stability. The improvement in the sensitivity of the sensor can be attributed to an increase in the area of the three-phase reaction boundary and an increase in the number of active sites in the electrochemical reaction.

Dual chamber YSZ-based sensor for simultaneous measurement of methane and water vapor concentrations in CH4 + H2O + N2 gas mixtures

Dual chamber YSZ-based sensor for simultaneous measurement of methane and water vapor concentrations in CH4 + H2O + N2 gas mixtures

Development of a selective ammonia YSZ-based sensor and modeling of its response

In the present study, Au-V2O5 sensing material was tested as a sensing electrode for developing mixed-potential ammonia gas sensors. The results of gas sensing measurements indicated selective responses to NH3 while this selectivity was highly dependent on the temperature. Different V2O5 contents were tested in the sensing electrode. The results showed that by increasing V2O5 content from 15 wt.% to 50 wt.%, selective ammonia sensors could be achieved at 550 °C. The selectivity of Au-50 %V2O5 sensor was also confirmed in gas mixtures of CO, NH3, NO, and NO2 gases. Modeling of the sensor responses in the ammonia concentration range 2–40 ppm at four oxygen concentrations was performed based on mixed-potential theory. Nernst and Butler-Volmer equations with an electron transfer assumption were used for data modeling.

Gas phase reaction combined light-regulated electrochemical sensing technique for improved response selectivity and sensitivity to hydrocarbons

Yttria-stabilized zirconia (YSZ)-based potentiometric gas sensor has been widely utilized for detecting non-methane hydrocarbons (NMHCs) that derived from exhaust gases. Nevertheless, poor selectivity and sensitivity still remained a challenging issue. Herein, we reported an efficient strategy to sense NMHCs (e.g., C3H6) at high temperature through the gas phase reaction combined light-regulated sensing technique. When the YSZ-based sensor that was attached with ZnO sensing electrode and Mn-based reference electrode was operated without illumination, significant mutual interference that derived from CO was witnessed for sensing C3H6. On the contrary, enhanced sensitivity and selectivity are observed by simply illuminating the sensor. The low detection limit of the sensor to C3H6 extends to 0.768 ppm with the response/recovery rate of 27 s/30 s. These pilot results clearly indicate the validity of employing gas phase reaction combined light-regulated sensing technique in tailoring the response selectivity and sensitivity for future exhaust gas sensing.

Compact Yttria-Stabilized Zirconia Based Total NOx Sensor with a Dual Functional Co3O4/NiO Sensing Electrode.

Yttria-stabilized zirconia (YSZ) based potentiometric gas sensors have been widely utilized for detecting NOx (NO and NO2). Nevertheless, it is still remains challenging issue for YSZ-based sensors to sense total NOx due to the opposite response signals to NO and NO2. Herein, we report an efficient strategy to sense total NOx at high temperature (above 300 °C) by designing a dual functional sensing electrode (SE); namely, the SE will simultaneously convert NO (in NOx mixture) to NO2 and electrocatalyze all of the obtained NO2 to generate the response signal of total NOx. In comparison with those previously reported total NOx sensors, the proposed total NOx sensor will be featured with a simplified sensor configuration and desirable long-term stability. To confirm the practicability of the proposed strategy, the NO conversion rate of several metal oxides and their composites have been measured and it turns out that the Co3O4/NiO shows relatively high NO conversion rate. Further study indicates a YSZ-based sensor consisting of (Co3O4 + 20 wt % NiO)-SE and Mn-based RE demonstrates satisfactory performance in detecting total NOx. For instance, analogous response magnitude to NO and NO2 as well as the mixture of NO/NO2 (within 35 ppm) is witnessed for the sensor; particularly, the sensor gives acceptable stability and response/recovery rate at the operating temperature of 500 °C within the examined period. In summary, the use of dual functional SE (e.g., Co3O4/NiO composite SE) indeed addressed those issues of concern in monitoring the level of total NOx and has provided a promising alternative way for designing future high-performance total NOx sensor.

CO sensing performances of YSZ-based sensor attached with sol-gel derived ZnO nanospheres

Zinc oxide nanospheres (ZnO) were synthesized by modified sol-gel route using zinc nitrate as precursor and citric acid as gelling agent. Yttria-stabilized zirconia (YSZ)-based sensor was fabricated using the sol-gel derived ZnO nanospheres as sensing electrode and Pt as reference electrode. The sensing characteristics were examined in the temperature range of 600–800 °C under various O2 concentrations (5, 10, 15 and 21 vol.%). It turned out that the sensor attached with the sol-gel derived ZnO-SE exhibited selective and sensitive response to carbon monoxide (CO) at 700 °C in all measured O2 concentrations. The sensor using sol-gel derived ZnO-SE showed high response and selectivity to CO at 700 °C in 5 vol.% O2. The response of the sensor varied linearly with CO concentrations on the logarithmic scale. The sensing mechanism was confirmed to be mixed-potential type. The sensor retained good stability even for about 40 days. The nano features associated with the sol-gel derived ZnO led to high response and selectivity to CO.

YSZ-based sensor using Cr-Fe-based spinel-oxide electrodes for selective detection of CO

A selective carbon monoxide (CO) sensor was developed by the use of both of CuCrFeO4 and CoCrFeO4 as the sensing electrode (SE) for yttria-stabilized zirconia (YSZ)-based potentiometric sensor. The sensing-characteristic examinations of the YSZ-based sensors using each of spinel oxides as the single-SE sensor showed that CuCrFeO4-SE had the ability to detect CO, hydrocarbons and NOx gases, while CoCrFeO4-SE was sensitive to hydrocarbons and NOx gases. Thus, when both SEs were paired as a combined-SEs sensor, the resulting sensor could generate a selective response to CO at 450 °C under humid conditions. The sensor was also capable of detecting CO in the concentration range of 20–700 ppm. Its sensing mechanism that was examined via polarization-curve measurements was confirmed to be based on mixed-potential model. The CO response generated by the combined-SEs sensor was unaffected by the change of water vapor concentration in the range of 1.3–11.5 vol% H2O. Additionally, the sensing performance was stable during 13 days tested.

Mixed-potential NH3 sensor based on Ce0.8Gd0.2O1.9 solid electrolyte

Gadolinia-doped ceria (GDC) prepared by the oxalate coprecipitation method was used as an oxygen-ion conducting solid electrolyte for a mixed-potential ammonia gas sensor with a WO3-sensitive electrode. The sensing performance of this sensor was compared with a sensor based on yttria-stabilized zirconia (YSZ) electrolyte. The influence of different oxygen-ion electrolytes on the performance of the gas sensor was evaluated. The test results show that the sensor device is much more sensitive (140.44mV/decade) than the YSZ-based sensor device. Furthermore, the also shows good repeatability and stability. The electrochemical impedance measurements indicate that the GDC-based sensor’s high performance may benefit from the high conductivity of GDC.

Impedancemetric YSZ-based oxygen sensor using BaFeO3 sensing-electrode

This paper deals with an impedancemetric sensor based on an yttria-stabilized zirconia (YSZ) using an oxide sensing-electrode (SE) for measuring O2 concentration. Fourteen oxides were examined as the SE materials for selective detection of O2. Among the sensors examined, the YSZ-based sensor using BaFeO3-SE was found to be insensitive to interfering gases such as NH3, H2, C3H6, C3H8, CH4, NO2, NO or CO, while sensitive to only O2 at 600°C. The impedance value |Z| at 10Hz for the sensor using BaFeO3-SE decreased linearly with increasing O2 concentration on a logarithmic scale in the range of 0.05–21 vol.%. The present sensor exhibited quick and repeatable responses to the change in O2 concentration.

Improvement in Response/Recovery Characteristics of Mixed-Potential-Type Zirconia-Based CO Sensor Using ZnCr2O4 Added with Au Particles-Sensing Electrode

In order to fulfill strict environmental regulations regarding air pollutions such as carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2), high performance gas sensors have been widely investigated for the last decade. Among air pollutions, CO is well known as colorless and toxic pollutant. Currently, the sensitive and selective detection of CO at high operating temperatures remains a challenging task. Recently, we have reported that the CO response of a mixed-potential-type yttria-stabilized zirconia (YSZ)-based sensor using a ZnCr2O4-sensing electrode (SE) was augmented by the dispersion of nano-sized Au (nano-Au) particles into the SE layer. However, the 90% response and recovery times against 800 ppm CO at 550°C for the sensor using ZnCr2O4 (+nano-Au)-SE were more than 40 and 160 s, respectively. From a practical point of view, it is very important to improve such poor response and recovery characteristics to CO and to achieve fast response and recovery behavior. In this study, therefore, to improve the response and recovery characteristics against CO for the sensor using ZnCr2O4 (+ nano-Au)-SE, Au particles with three different sizes (nano-Au, subμ-Au, and μ-Au) were separately added to ZnCr2O4-SE, and then the CO sensing performances of the resulting sensors were examined. The SE materials were prepared by mixing commercial ZnCr2O4 powder (Kojundo Chemical Lab. Co., Ltd., Japan) and one of the following Au particles (nano-Au: 1-4 nm; subμ-Au: 0.3-0.6 μm; μ-Au: 2-5 μm) in an agate mortar with ethanol, which was subsequently dried to evaporate the dispersant solution. The sensors were assembled in a tubular configuration using a commercial hemispherically-terminated YSZ tube (8 mol.% Y2O3-doped ZrO2, Nikkato Corp., Japan). A YSZ layer was firstly formed on the outer surface of the YSZ tube using a YSZ paste, which was made by mixing a commercial YSZ powder (Tosoh Corp., Japan) and α-terpineol, to improve the mechanical and electrochemical stability of the interface between the SE layer and the YSZ tube. After drying the YSZ layer, the SE material pastes, which were also made by mixing with α-terpineol, were applied on the YSZ layer to form SE. A commercial Pt paste was applied on the inner surface of the closed-end YSZ tube to form a Pt/air-reference electrode (RE). Finally, the painted and assembled YSZ tube was sintered at 1100°C for 2 h in air to fabricate the final sensing device. Gas sensing measurements were carried out in a conventional gas-flow apparatus equipped with a furnace operating at 550°C. The SE was exposed to the humidified base gas (synthetic air + 5 vol.% H2O) or sample gases with a flow rate of 100 cm3/min. The Pt/air-RE was always exposed to an atmospheric air. The difference in electromotive force (emf) between SE and RE was measured as the sensing signal by means of a digital electrometer. The effect of the addition of Au particles with different sizes into the ZnCr2O4−SE on the CO sensing characteristics of the mixed-potential-type YSZ-based sensors was investigated by measuring the emf response against the inner Pt/air−RE after exposure to 20 - 800 ppm CO. The response transients indicate that the sensitivity toward CO decreases with increasing Au particle size. Among the three SEs tested, the ZnCr2O4(+subμ-Au)-SE gave relatively high sensitivity and good response/recovery against CO. Although CO sensitivity of the ZnCr2O4(+subμ-Au)-SE became lower than that of the ZnCr2O4(+nano-Au)-SE, the 90% recovery time to 800 ppm CO was improved and it was approximately 80 s. Additionally, to examine the influence of Au additive on the sensitivity and response/recovery behavior to CO, the content of subμ-Au additive to ZnCr2O4 was varied from 1 to 10 wt.%. As a result, the CO sensitivity increased with increasing content of subμ-Au, as expected. Examinations of the detailed CO sensing characteristics for the present sensor are now in progress.

Stabilized zirconia-based selective NO2 sensor using sol-gel derived Nb2O5 sensing-electrode

Niobium pentoxide (Nb2O5) powder was synthesized by citrate-based sol-gel method using niobium pentachloride as precursor and citric acid as gelling agent. The synthesized powder was characterized by X-ray diffraction (XRD), Fourier transformed infra-red spectroscopy (FT-IR) and scanning electron microscope (SEM). Yttria-stabilized zirconia (YSZ)-based electrochemical sensor was fabricated using the Nb2O5 powder as sensing electrode (SE) and Pt as reference electrode (RE), and its sensing characteristics were examined in the temperature range of 750–850°C under each of various O2 concentrations (5, 10, 15 and 21vol%). It was observed that the YSZ-based sensor attached with Nb2O5-SE exhibited selective and sensitive response to NO2 at 800°C in all the measured O2 concentrations. The sensor showed the highest sensitivity and selectivity to NO2 in 5vol% O2. The NO2 sensing mechanism of the present sensor was confirmed to be mixed potential-type. The sensor exhibited stable response to NO2 for the examined period of more than a month.

Selective CO Detection Using YSZ-based Sensor with a Combination of CuCrFeO4 and CoCrFeO4 Electrodes

A solid-state potentiometric sensor using yttria-stabilized zirconia (YSZ) as a solid electrolyte and oxide electrodes has been developed for selective CO detection. At 450oC, the YSZ-based sensor using CuCrFeO4 sensing-electrode (SE) was found to have similar gas sensing characteristic with the sensor using CoCrFeO4-SE, except toward CO. When both SEs was paired on a YSZ tube, the responses to various gases except for CO could be cancelled out for the resulting combined-SEs sensor. Thus, this sensor could generate a sensitive and selective response to CO at 450oC even under humid conditions.

YSZ-based NO2 sensor utilizing hierarchical In2O3 electrode

Mixed-potential type YSZ-based sensor utilizing In2O3 as sensing electrode (SE) was fabricated and examined for detection of NO2 at 700°C. The hierarchical In2O3 oxide material was synthesized via a facile hydrothermal process, and In2O3 sintered at different temperatures were characterized using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The present study mainly focused on the effect of sintering temperature of SE materials (800°C, 1000°C, 1200°C) on NO2 sensing characteristics. As a result, the sensor attached with In2O3-SE calcined at 1000°C exhibited the largest sensitivity to NO2, which the response to 100ppm NO2 was as large as126mV and excellent long-term stability for 30 days tested at 700°C. It is noteworthy that the NO2 sensitivities for present device showed a small influence by the change of relative humidity at 700°C. Additionally, the present devices also displayed good repeatability, the excellent sensitivity and relatively good selectivity in conjunction with various interfering gases before and after 30 days high-temperature-aging of 700°C. It was speculated that the largest sensing performance for the sensor attached with In2O3-SE sintered at 1000°C was contributed to the special micro-structure of the SE and the highest electrochemical catalytic activity for cathodic reaction of NO2 at the triple phase boundary (TPB).

Mixed-potential-type YSZ-based sensor with nano-structured NiO and porous TPB processed with pore-formers using coating technique

In order to fabricate a high performance triple-phase-boundary (TPB) for a mixed potential type NOx sensor based on stabilized zirconia and oxide electrode, a porous surface of the yttria-stabilized-zirconia substrate (YSZ) was formed with pore-formers (PMMA, graphite and starch) using coating technique. SEM observation result indicated that the porous morphology in the final ceramic was related to the category and amount of the pore formers, so that the pore size and microstructure of YSZ layer could be controlled. The NO2 gas sensors using above-mentioned modified YSZ substrate with porous surface and nano-structured NiO electrode were fabricated and evaluated. The electric potential difference (ΔV) of the sensors varied almost linearly with the logarithm of NO2 concentrations in the examined range of 10–500ppm at 800°C. The sensor based on 10wt% starch-added YSZ matrix showed the largest response with the ΔV of 114mV to 100ppm NO2. It was also seen that the sensors showed good response-recovery transient, selectivity and repeatability to NO2.

Mixed potential type acetone sensor using stabilized zirconia and M3V2O8 (M: Zn, Co and Ni) sensing electrode

A series of mixed potential type gas sensors using stabilized zirconia and M3V2O8 (M: Zn, Co and Ni) sensing electrode were fabricated and applied for detecting acetone at 600°C. Among the sensors utilizing these composite oxides (Zn3V2O8, Co3V2O8 and Ni3V2O8)–SEs prepared via a facile sol–gel method, the sensor attached with Zn3V2O8–SE exhibited the highest response to acetone at 600°C. Therefore, the present study mainly focused on acetone sensing performance for YSZ-based sensor using Zn3V2O8–SE at 600°C. Results showed that the sensor attached with Zn3V2O8–SE could detect even 1ppm acetone with an acceptable response. Moreover, ΔV of the sensor attached with Zn3V2O8–SE exhibited segmentally linear relationship to the logarithm of acetone concentration in the ranges of 1–10ppm and 10–400ppm, which the sensitivities were −16 and −56mV/decade, respectively. The present device also displayed good repeatability, small drifts in 30 days measured period, and the excellent sensitivity and relatively good selectivity in the presence of various interfering gases before and after 30 days high-temperature-aging of 600°C. Additionally, polarization curve was measured to further demonstrate the mixed potential mechanism.

Sensing characteristics of YSZ-based oxygen sensors attached with BaxSr1 - xFeO3 sensing-electrode

Perovskite-type oxides of BaxSr1-xFeO3 (x=0, 0.1, 0.5, 0.9 and 1.0) were examined as a sensing electrode (SE) material of potentiometric yttria-stabilized zirconia (YSZ)-based oxygen sensors. The sensors using each of BaxSr1-xFeO3-SEs were found to be insensitive to the various examined gases (100ppm each) such as NH3, H2, C3H6, C3H8, CH4, NO2, NO and CO, while they were sensitive to oxygen in the concentration range of 0.05-21vol.% at 600°C. The number of electron (n) in the electrochemical reaction of oxygen for the present sensors was estimated to be about 4.2 based on Nernst equation in the examined concentration range. The estimated n values for the present sensors are consistent with that (n=4.2) for the conventional YSZ-based oxygen sensor using Pt-SE, being almost same as the theoretical value (n=4). The YSZ-based sensor attached with BaFeO3-SE further showed excellent responses at temperatures of 450–600°C. The electric potential difference (ΔV) value of the present sensor was rather stable under the operating conditions (relative humidity: 20–100%, 400ppm CO2, 600°C) for the examined period of 14days. In addition, it was found that the sensor using BaFeO3-SE could work well as a lambda (λ) sensor at 600°C in the combustion gas of O2/C3H8, as is the case with the sensor using Pt-SE.

Highly selective zirconia-based propene sensor attached with sol–gel derived NiO nanospheres

An YSZ-based electrochemical sensor was fabricated using sol–gel derived NiO nanospheres. The sensor exhibited high sensitivity and selectivity to C3H6 (propene) with excellent long-term stability.

A review of mixed-potential type zirconia-based gas sensors

A robust and reliable gas sensing device is considered as a convenient and practical solution for gas concentration monitoring that has become a mandatory requirement in different field of applications. For in situ hazardous gases detection, a mixed-potential type gas sensor has been regarded as a promising solid-state gas sensor. For the past three decades, there has been a significant progress in achieving high performance in mixed-potential type sensors. Therefore, this review is focused on reporting the development of mixed-potential type gas sensors with combined yttria-stabilized zirconia (YSZ) as the base solid electrolyte material and various classes of electrode materials for their potential utilization as a high-performance sensing electrode. The underlying sensing mechanism of a mixed-potential type YSZ-based sensor is elaborated here in detail. Transformation in design and configuration of this type of sensor is also covered in this report. In addition, recent progresses on mixed-potential type gas sensors development for detection of several target gases, such as carbon monoxide, hydrocarbons, nitrogen oxides, hydrogen, and ammonia, are reviewed. Strategies to improve the sensing characteristic, particularly gas sensitivity and selectivity, are also reported. Based on the understanding of the fundamental sensing mechanism and the requirements for high-performance gas sensors, challenges and future trends for this type of gas sensor development are discussed.

Sensing Properties of YSZ-Based NO Sensors with Double-Perovskite (La<sub>0.8</sub>Sr<sub>0.2</sub>)<sub>2</sub>FeNiO<sub>6-δ</sub> Sensing Electrodes

The powder of (La0.8Sr0.2)2FeNiO6-δ (LSFN) oxide with double-perovskite structure was synthesized by polymeric precursor method. Then the YSZ-based NO sensors with LSFN sintered at different temperatures (1000, 1100, 1200 and 1300 °C) as sensitive electrode (SE) were fabricated. All samples were characterized by XRD. The morphologies of the LSFN-SEs were observed with ESEM. The NO sensing properties of the sensors were investigated in the operating temperature range of 350-650 °C in 10 vol. % O2. Results demonstrated that the sensor with LSFN-SE sintered at 1300 °C exhibited highest response to 500 ppm NO at 400 °C, which was about 85 mV. A linear relationship was obtained between the emf and the logarithm of NO concentration from 500 to 800 ppm at 400 and 500 °C. Moreover, both magnitude and slope to NO response decreased as operating temperature increased. And both the response time and recovery time shortened as temperature increased. But the recovery rate was slower than the response rate, especially at and below 450 °C. The optimal sensor response was obtained at 500-550 °C.

The effects of sintering temperature of MnCr2O4 nanocomposite on the NO2 sensing property for YSZ-based potentiometric sensor

In this paper, MnCr2O4 with spinel structure has been prepared by using polymeric precursor method. The thermal stability, grain size and surface morphology of MnCr2O4 calcined at different temperatures were studied using thermogravimetric analysis and differential scanning calorimeter analysis (TG-DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM). Then the YSZ-based sensors using MnCr2O4 sintered at different temperatures (800, 900, 1000, 1100 and 1250 °C) as electrode were fabricated and their NO2 sensing properties were investigated. Results demonstrate that MnCr2O4 sample calcined at 1000 °C exhibits higher response to 100 ppm NO2, which is about 73 mV. It is believed that the calcined temperature affect the material's sensing properties. In addition, the sensing characteristics such as selectivity, repeatability were investigated.