Oxide Sensing-electrode Research Articles

Performance Tailoring of Mixed-Potential Hydrogen Sensor with Perovskite Oxide Sensing Electrode

Introduction Mixed-potential gas sensors based on solid electrolyte have attracted much attention due to their simple structure, robust stability, and high sensitivity [1]. As the sensor performance is mainly dominated by the (electro-)catalytic activity of the sensing materials [2], materials screening has been adopted as the most important approach to developing high performance sensors. Unfortunately, the criteria for the materials design remain still unclear. One major reason for this is that for usual materials the compositions cannot be finely varied, which prevents a clear understanding how the composition governs the sensing property as well as the (electro)catalytic activities.Perovskite oxides with a general formula ABO3 can accommodate large composition variations, which renders them abundant electrocatalytic properties and thus possibility to tackle the above problem [3]. In this work, La0.8Sr0.2CrxB1-xO3-δ (x=0, 0.5, 1; B = Fe, Mn) perovskite series was selected as a model system to investigate the composition - activity - performance relationship of the sensing electrode. The perovskite powders were synthesized via a sol-gel method. YSZ was used as the solid electrolyte and Pt as the reference electrode. The H2 response as a function of composition will be discussed in combination with results of polarization curves, impedance spectroscopy, and catalytic conversion. Results and Conclusions Fig. 1a shows the dependence of response values on H2 concentrations at 450 ᵒC, indicating an order of LSFe > LSCr > LSCrFe > LSCrMn > LSMn. The sensor response increased linearly with logarithmic H2 concentration in the range of 100-1000 ppm. The sensitivities, i.e. the slopes of the linear fittings, are -115, -77, -90, -68, and -49 mV/decade, respectively. These results showed clearly that variation of the B-site composition in the perovskite indeed has a great impact on the sensing properties. Fig. 1b compares the selectivity of the devices. All the sensors showed the largest response to H2. The corresponding selectivity, defined as the response ratio for H2 over the second most responsive gas, presented an order of LSCrMn > LSCrFe ~LSFe > LSMn > LSCr.Fig. 2a displays the tafel curves of the sensors tested in air at 450 ᵒC. The exchange current density, which is indicative of the electrocatalytic activity to the O2 reduction reaction (ORR), was found to be LSFe > LSMn >> LSCr~ LSCrFe >LSCrMn. Impedance spectroscopy also presented a similar order of the polarization resistance. Fig. 2b compares the heterogeneous catalytic conversion rate (normalized to the BET surface area) of H2 for the materials, revealing LSFe and LSMn as the most active ones while LSCr and LSCrFe as the least active ones. Apparently, the most responsive composition, LSFe, exhibited the highest ORR and catalytic activities, whilst the second most responsive composition, LSCr, presented low ORR and catalytic activities.Our results showed that LSFe exhibited the best overall performance in the series, including high response, large sensitivity, fast speed, and good selectivity, which is also among the best of state-of-the-art mixed potential H2 sensors. The sensing performance as a function of the perovskite composition and the governing factors will be discussed in terms of the mixed-potential theory.

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.

Sensing characteristics of aged zirconia-based hydrogen sensor utilizing Zn–Ta-based oxide sensing-electrode

We report here the enhanced sensing characteristics to H2 for a potentiometric sensor using an yttria-stabilized zirconia (YSZ) solid electrolyte and a ZnO(+84wt.% Ta2O5) sensing electrode (SE) after aging at 500°C. The emf response toward 400ppm H2 was found to gradually increase up to −800mV after 40days operation (aging) and was stabilized at this value until the 90th day. The aged and stabilized sensor exhibited highly sensitive response to H2, with minor responses toward other examined gases such as NOx and HCs. The 90% response time toward 100ppm H2 was approximately 70s. The H2 sensitivity of the stabilized sensor was hardly affected by changes in water vapor as well as O2 concentration, with repeatable and reproducible responses to H2.

Effects of Sr Addition to La-Based Perovskite Sensing-Electrode on YSZ-Based Amperometric-Type NOx Sensor

Sensing characteristics of amperometric NOx sensor using yttria-stabilized zirconia (YSZ) and La-based perovskite-type oxide sensing-electrode (SE) was examined. La0.8Sr0.2MO3(M = Co, Mn, Fe) and LaMnO3 was synthesized by means of spray pyrolysis method. The sensor attached with La0.8Sr0.2MnO3 as a SE showed the highest occupancy rate of NO2 response in total current as well as low current response to CO, C3H6, NH3 at 600°C in O2 (21%). The response was increased with increasing NO2 concentration in the examined range between 50 to 800 ppm. The comparison of sensing property between LaMnO3-SE and La0.8Sr0.2MnO3-SE showed the substitution Sr for La improved the NO2 response for the sensors.

Potentiometric YSZ-based sensor using NiO sensing electrode aiming at detection of volatile organic compounds (VOCs) in air environment

The potentiometric yttria-stabilized zirconia (YSZ)-based sensors using each of various oxide sensing-electrodes (SEs) were fabricated and examined for detection of toluene (C 7H 8) in several tens ppb level. As a result, the sensor using NiO-SE was found to exhibit relatively high sensitivity and selectivity to toluene at 450 °C under the wet condition (1.35 vol.% H 2O). The present sensor could respond well to toluene in the concentration range of 10–150 ppb. The response transients to 50 ppb toluene were stable and repeatable, accompanying with the response/recovery time acceptable for an actual environmental monitoring. In addition, the toluene sensitivity was hardly affected by the interference of the other co-existing gases examined.

Impedancemetric zirconia-based sensor attached with laminated-oxide sensing-electrode aiming at highly sensitive and selective detection of propene in atmospheric air

Aiming at a sensitive and selective detection of very low concentrations of propene (C 3H 6) in an atmospheric air, the impedancemetric yttria-stabilized zirconia (YSZ)-based sensor was fabricated and examined. Among the different oxide sensing-electrodes (SEs) studied, SnO 2-SE was found to give the highest sensitivity to C 3H 6 at 600 °C even in the presence of 1.35 vol.% H 2O and 400 ppm CO 2. Then, the selective response to C 3H 6 was achieved by coating of the additional oxide layer (ZnO) onto SnO 2-SE, giving the laminated-type ZnO/SnO 2-SE. Moreover, to improve further the sensing performances of the present sensor, ZnO powder (0–30 wt.%) was subsequently added to the SnO 2 layer of the laminated ZnO/SnO 2-SE. As a result, the sensor using ZnO/SnO 2 (+ 20 wt.% ZnO)-SE exhibited highly selective and sensitive response to C 3H 6 at 500 °C with minor cross sensitivities to other co-existing gases, such as NO x , H 2, CO and CH 4. The sensitivity of the present sensor varied linearly with the logarithm of C 3H 6 concentration in the range of 20–100 ppb and was found to be hardly affected by the change in water vapor concentration.

Sensing characteristics of mixed-potential-type zirconia-based sensor attached with NiO-based sensing electrode prepared by ball-milling

A stabilized zirconia-based sensor attached with an oxide sensing-electrode (SE) was developed for the selective detection of propylene (C3H6), representative hydrocarbon (HC), at high temperature. Among six kinds of commercial oxides examined, NiO-SE gave the highest sensitivity to C3H6 at 600°C under the wet condition. However, other sensing characteristics, such as the C3H6 selectivity, the response and recovery times, of this sensor were not satisfactory. Thus, to improve the C3H6 sensing characteristics, NiO-SE was successively mixed with different oxides. For better reproducibility of response, all the single- or mixed-oxide SEs were prepared by ball-milling. As a result, the sensor attached with NiTiO3-SE (the initial mixture of 50 mol% NiO and 50 mol% TiO2) exhibited highly selective and sensitive response to C3H6 at 650°C, accompanying with relatively quick response and recovery. In addition, by the comparison of sensing performances of NiTiO3-SEs fabricated by the different methods, the ball-milling method was found to be the most appropriate from the viewpoint of obtaining high selectivity and sensitivity to C3H6.

Measurement of Total Concentration of Hydrocarbon by Using Zirconia-based Sensor Attached with ZnCr2O4 Sensing-Electrode

A mixed-potential-type sensor was fabricated by using yttria-stabilized zirconia (YSZ) and various zinc-based oxide sensing-electrodes (SEs) aiming at detection of hydrocarbons (HCs) in automobile exhausts. Among the various zinc-based oxide-SEs examined, ZnCr2O4 was found to give highly selective and sensitive response to C3H6 (propene) at 550oC even in the presence of 5 vol.% H2O. The Δemf (electromotive force) of the sensor showed almost linear dependence with the logarithm of C3H6 concentration. The sensitivity to 100 ppm C3H6 was found to hardly affect by the change in concentration of water vapor or O2. In addition, it was found that the sensitivity of the present sensor varied linearly with the carbon number of various HCs examined.

Mixed-Potential-Type YSZ-Based Sensor Capable of Detecting Propene at Several Tens ppb Level

The mixed-potential-type yttria-stabilized zirconia (YSZ)-based sensors using each of several oxide sensing-electrodes (SEs) were fabricated and examined for detection of propene (C 3 H 6 ) at its very low concentration level. Among five oxide-SEs tested, the In 2 O 3 -SE was found to give the highest sensitivity to C 3 H 6 with an acceptable response rate even at 450°C. The A electromotive force value of the sensor varied logarithmically with the C 3 H 5 concentration in the examined range of 10-200 ppb. In addition, the sensor exhibited a relatively good selectivity to C 3 H 6 over the other gases, such as NO 2 , CO, H 2 , and CH 4 , in the respective environmentally existing level.

Zirconia-Based Sensor Using ZnCr[sub 2]O[sub 4] Sensing Electrode for Measurement of Total Concentration of Various Hydrocarbons

A mixed-potential-type sensor was fabricated using yttria-stabilized zirconia and various zinc-based oxide sensing electrodes (SEs) with the goal of detecting hydrocarbons (HCs) in automobile exhausts. Among the various zinc-based oxide SEs examined, ZnCr 2 O 4 was found to give highly selective and sensitive response to C 3 H 6 at 550°C even in the presence of 5 vol % H 2 O. The Aemf (electromotive force) of the sensor showed almost linear dependence with the logarithm of C 3 H 6 concentration. In addition, it was found that the sensitivity of the present sensor varied linearly with the carbon number of various HCs examined.

Highly sensitive impedance-based propene sensor using stabilized zirconia and zinc oxide sensing-electrode

Aimed at an environmental monitoring of hydrocarbons (HCs), a new-type impedancemetric sensor was fabricated by using an yttria-stabilized zirconia (YSZ) tube and ZnO sensing-electrode (SE). The fabricated tubular sensor was examined for detection of low concentrations of propene (C 3H 6) in the presence of 1.35 vol.% H 2O and 400 ppm CO 2 at 600 °C. As a result, it was found that the present sensor could detect the low concentrations of C 3H 6 in the range of 0.05–0.8 ppm and the sensitivity varied linearly with C 3H 6 concentration. In addition, the C 3H 6 sensitivity was almost invariant with the changes in the concentration of water vapor and the interferences to other gases, such as NO 2, NO, H 2, CO and CH 4, were insignificant.

Dependence of NO2 sensitivity on thickness of oxide-sensing electrodes for mixed-potential-type sensor using stabilized zirconia

Dependence of NO2 sensitivity on thickness of oxide-sensing electrodes for mixed-potential-type sensor using stabilized zirconia

Detection of propene by using new-type impedancemetric zirconia-based sensor attached with oxide sensing-electrode

We report here that an impedancemetric sensor based on yttria-stabilized zirconia (YSZ) can detect propene (C 3H 6) sensitively and selectively under wet condition, if the sensing-electrode (SE) material and the operating condition are selected properly. Among the various single-oxide SEs examined, ZnO was found to give rather high sensitivity to C 3H 6 as well as negligible sensitivity to CH 4 at 600 °C in the presence of 1 vol.% H 2O. In addition, when 1.5 wt.% Pt was loaded on ZnO-SE and the SE was polarized at +50 mV versus Pt counter-electrode (CE), the present sensor was found to detect C 3H 6 rather sensitively with little interference of other gases, such as CO, NO, NO 2, H 2, CH 4, CO 2, H 2O and O 2, at 600 °C.

NO[sub x] Sensing Characteristics of Mixed-Potential-Type Zirconia Sensor Using NiO Sensing Electrode at High Temperatures

A stabilized-zirconia-based NO x sensor attached with oxide sensing-electrode (SE) was tested for the detection of NO x at temperatures higher than 800°C for car-emission control applications. Among various oxide SEs tested, NiO was found to give the highest sensitivity to NO 2 even at high temperatures. It was found that the sensor using NiO-SE showed quicker recovery in the presence of water vapor, compared with that in the dry sample gas. Electromotive force values of 57 and 75 mV were observed, respectively, under dry and wet (5 vol % water vapor) conditions with 400 ppm NO 2 at 850°C.

Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NO x at high temperature

A solid-state electrochemical NO x sensor was fabricated by using a closed-one-end yttria-stabilized zirconia (YSZ) tube, an oxide sensing electrode (SE) and a Pt counter electrode (CE). The complex impedance of the device using each of several oxide SEs was measured in the frequency and the temperature ranges of 0.1 Hz to 100 kHz and 600–700 °C, respectively. In most cases, a large semicircular arc was observed in complex impedance spectra (Nyquist plots) in the lower frequency range examined in dry-air flow. Only in the case of the device using ZnCr 2O 3 SE, the semi-arc shrank to some extent upon exposure to NO x gas. The resistance value ( Z′) at the intersection of the semi-arc with the real axis at lower frequencies (around 0.1 Hz) varied with concentration of both NO and NO 2 in the sample gas. The impedance value at 1 Hz of the present device was found to vary almost linearly with the concentration of NO (or NO 2) from 50 to 400 ppm. The 90% response and 90% recovery times were as short as less than few seconds at 700 °C. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO 2. This indicates that the present device can detect the total NO x at higher temperatures.

Mixed potential type sensor using stabilized zirconia and ZnFe2O4 sensing electrode for NOx detection at high temperature

Electrochemical sensors using tubular yttria-stabilized zirconia (YSZ) and the spinel-type oxide sensing-electrode (SE) were fabricated and examined for NOx detection at high temperatures. For a mixed-potential type NOx sensor, ZnFe2O4 has been found to give the highest NOx sensitivity among the other spinel-type oxides tested and reported to date. The ZnFe2O4-attached device gave a linear correlation between emf and the logarithm of NO2 (NO) concentration from 50 to 436ppm in the high temperature range 600–700°C. The sensing mechanism of the sensor has been discussed on the basis of the gas adsorption–desorption behavior and the catalytic activity for the oxides examined.

Stabilized Zirconia-Based NO[sub x] Sensor Using ZnFe[sub 2]O[sub 4] Sensing Electrode

A tubular sensor using yttria-stabilized zirconia (YSZ) and a spinel-type oxide sensing-electrode (SE) was fabricated and examined for detection at high temperatures. Among spinel-type oxides tested and reported in the literature to date, has the highest sensitivity to in air at temperatures of 550-700°C. In this temperature range, the electromotive force (emf) response of the sensor was almost linear to the logarithm of NO or concentration in the range from 25 to 436 ppm. Scanning electron microscopy and X-ray diffraction analysis were used to examine the morphology and crystalline structure of the film. Investigation of the sensor stability showed that the output emf to 100 ppm have been gradually increased within ∼20% range after ∼20 day operation up to 200 days examined at 700°C in dry synthetic air. The air base has been stable during the whole test period. © 2001 The Electrochemical Society. All rights reserved.

High-temperature sensors for NO and NO2 based on stabilized zirconiaand spinel-type oxide electrodes

G. Lu, N. Miura and N. Yamazoe, J. Mater. Chem., 1997, 7, 1445 DOI: 10.1039/A701708A