Yuki Fujio

Stabilization of sensing performance for mixed-potential-type zirconia-based hydrocarbon sensor

The recently reported sensing characteristics of the mixed-potential-type yttria-stabilized zirconia (YSZ)-based hydrocarbon (HC) sensor attached with ZnCr(2)O(4)-sensing electrode (SE) were found to be changed after the 10-day operation at 550°C under the wet condition (5 vol.% water vapor). To improve the stability of the present sensor, the several modifications of the SE material by adding YSZ powder were examined. As a result, the sensor using the laminated (ZnCr(2)O(4)/YSZ)-SE gave the stable electromotive force (emf) response against 100 ppm C(3)H(6) at 550°C for about one month examined. Based on the scanning electron microscopy (SEM) observation and the AC complex-impedance measurements, it was concluded that the stable behavior of the sensor using the laminated (ZnCr(2)O(4)/YSZ)-SE was provided by the stabilization of the interface between ZnCr(2)O(4) grains and YSZ particles. The fabricated sensor exhibited the linear dependence of sensitivity on the logarithm of either C(3)H(6) concentration (in the range of 20-800 ppm) or mixtures of various hydrocarbons (HCs) (in the range of 90-2600 ppmC). In addition, the emf response was not altered by the change of O(2) (2-20 vol.%), H(2)O (0-10.8 vol.%) and CO(2) (0-20 vol.%) concentrations, and no interference of other gases (CO, NO, NO(2), H(2), and CH(4)) was observed.

Construction of sensitive and selective zirconia-based CO sensors using ZnCr2O(4)-based sensing electrodes.

The carbon monoxide (CO) sensitivity of a mixed-potential-type yttria-stabilized zirconia (YSZ)-based tubular-type sensor utilizing a ZnCr(2)O(4) sensing electrode (SE) was tuned by the addition of different precious metal nanoparticles (Ag, Au, Ir, Pd, Pt, Ru and Rh; 1 wt % each) into the sensing layer. After measuring the electromotive force (emf) response of the fabricated SEs to 100 ppm of CO against a Pt/air-reference electrode (RE), the ZnCr(2)O(4)-Au nanoparticle composite electrode (ZnCr(2)O(4)(+Au)-SE) was found to give the highest response to CO. A linear dependence on the logarithm of CO concentration in the range of 20-800 ppm at an operational temperature of 550 °C under humid conditions (5 vol % water vapor) was observed. From the characterization of the ZnCr(2)O(4)(+Au)-SE, we can conclude that the engineered high response toward CO originated from the specific properties of submicrometer sized Au particles, formed via the coalescence of nanosized Au particles located on ZnCr(2)O(4) grains, during the calcining process at 1100 °C for 2 h. These particles augmented the catalytic activities of the gas-phase CO oxidation reaction in the SE layer, as well as to the anodic reaction of CO at the interface; while suppressing the cathodic reaction of O(2) at the interface. In addition, the response of the ZnCr(2)O(4)(+Au)-SE sensor toward 100 ppm of CO gradually increased throughout the 10 days of operation, and plateaued for the remainder of the month that the sensor was examined. Correlations between SEM observations and the CO sensing characteristics of the present sensor were suggestive that the sensitivity was mostly affected by the morphology of the Au particles and their catalytic activities, which were in close proximity to the ZnCr(2)O(4) grains. Furthermore, by measuring the potential difference (emf) between the ZnCr(2)O(4)(+Au) and a ZnCr(2)O(4) electrode, sensitivities to typical exhaust component gases other than CO were found to be negligible at 550 °C.

Improvement in Propene Sensing Characteristics by the Use of Additives to In2O3 Sensing Electrode of Mixed-Potential-Type Zirconia Sensor

In an effort to improve propene (C 3 H 6 ) selectivity and long-term stability of the mixed-potential-type yttria-stabilized zirconia (YSZ)-based tubular sensor attached with In 2 O 3 sensing electrode (SE), each of the nanosized noble metals (Pt, Ru, Pd, Rh, or Ir) and YSZ powder was added to In 2 O 3 SE. It was found that the addition of nanosized Pt (0.3 wt %) gave considerable improvement in C 3 H 6 selectivity, and the addition of YSZ powder (10 wt %) exhibited great improvement in long-term stability of the sensor. Thus, the sensor attached with composite SE consisting of In 2 O 3 , 0.3 wt % Pt, and 10 wt % YSZ was fabricated and its sensing characteristics to C 3 H 6 were examined. As a result, the present sensor exhibited high selectivity and excellent long-term stability to C 3 H 6 at 450°C under the wet condition for the examined period of about 1 month. Based on the results obtained from the measurement of catalytic activity for CO oxidation to CO 2 , the observation of morphology, and the analysis of composition of SEs, it was speculated that the added Pt nanoparticles could promote the oxidation of CO and H 2 and then improve the C 3 H 6 selectivity, while the added YSZ powder could stabilize the morphology of the SE/YSZ interface and give better long-term stability.

Tunable NO2-Sensing Characteristics of YSZ-Based Mixed-Potential-Type Sensor Using Ni1 − x Co x O -Sensing Electrode

Tunable NO 2 -sensing characteristics using Ni 1-x Co x O-sensing electrodes (SEs) at high temperature are reported. Various compositions of cobalt (Co)-doped NiO solid solutions (Ni 1-x Co x O, 0 ≤ x ≤ 0.3) were synthesized by the citrate-gel method, and each of the Ni 1-x C Ox O samples obtained was utilized as SE for yttria-stabilized zirconia (YSZ)-based planar sensor for detection of NO 2 at 800°C under wet condition. As a result, the NO 2 sensitivity increased with increasing cobalt content (x) in Ni 1-x Co x O, and the maximum sensitivity was observed for Ni 0.9 Co 0.1 O. The sensitivity of the sensors was almost invariant to the change in water vapor concentration in the sample gas. Moreover, the sensor exhibited excellent long-term stability for about 6 months. Interestingly, the sensor attached with Ni 0.8 Co 0.2 O SE showed excellent selectivity to NO 2 . The effect of Co substitution in NiO on the rates of electrochemical reactions at the Ni 1-x Co x O/YSZ interface and the catalytic activity for the gas-phase conversion of NO 2 to NO was evaluated, and the sensing mechanism of the present sensor was discussed.

Zirconia-Based Sensor Using ZnCr2O4 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.

Gas sensing characteristics of Au sensing electrode fabricated on YSZ single-crystals

The nano-structured gold sensing electrodes (SEs) were fabricated on (100) and (111) surfaces of single-crystal (sc) yttria-stabilized zirconia (YSZ) plates utilizing colloidal gold solution of 5 nm. After annealing at 1000°C, Au was found to form the large square-like domains (ca. 5×5 µm) on (100) surface of YSZ, whereas the triangular-like domains with the size of 2–3 µm were observed on YSZ(111). Such a discrepancy in morphology of nano-Au SEs resulted in the different sensing properties of the fabricated sensors as well as their short-term stability. While the sensor based on YSZ(100) treated with Au showed high emf responses to the non-methane hydrocarbons (HCs) at 700°C under the wet condition, the sensor based on YSZ(111) gave highly selective and sensitive responses to NH3.

Mixed-potential-type zirconia-based sensor using Ni-Ti-O sensing electrode for detection of propylene

The mixed-potential-type sensors using yttriastabilized zirconia (YSZ) and various oxide-based sensing electrodes (SEs) were examined for detection of propylene (C<inf>3</inf>H<inf>6</inf>). As a result, the sensor attached with NiO-SE was found to exhibit highly sensitive but not well selective response to C<inf>3</inf>H<inf>6</inf> at 600°C under the wet condition (5 vol.% H<inf>2</inf>O). To improve the C<inf>3</inf>H<inf>6</inf> selectivity, NiO-SE was further mixed with each of several oxides (50 mol.%) which can form mechanical mixtures or mixed oxides. It was found that the sensor attached with NiTiO<inf>3</inf>-SE (initial mixture: 50 mol.% NiO + 50 mol% TiO<inf>2</inf>) gave highly selective and sensitive response to C<inf>3</inf>H<inf>6</inf> at 650°C. The electromotive force (emf) response of the present sensor varied linearly with the logarithm of C3H6 concentration in the range of 10 – 800 ppm.