Mixed-potential NH3 Sensor Research Articles

Mixed potential NH3 sensor based on Ag-Doped La2NiO4+δ sensing electrode

Perovskite-type oxides have attracted extensive focus as sensing electrodes (SE) for NH3 sensors due to their excellent gas-sensing and tunability. The mixed-potential sensor was developed using yttrium stabilized zirconia (YSZ) as the solid electrolyte and non-stoichiometric perovskite oxides La2AgxNiO4+δ (x = 0, 0.025, 0.05, 0.1) as the SE. Due to the Ag doping in the SE, the electrochemical activity was enhanced, and the adsorption capacity of NH3 was further improved. Compared with the La2NiO4+δ-SE-based sensor, the Ag doping in SE significantly improved the response value and sensitivity of the sensor, and the sensor based on La2Ag0.05NiO4+δ-SE had the best capabilities. The response to 200 ppm NH3 was 61.2 mV, and the sensitivity was 8.12 mV/decade (5–25 ppm) and 73.77 mV/decade (50–200 ppm) at 500 ℃. The sensor had a strong anti-interference ability to H2, NO2, NO, CO2, CH4, and the O2 concentrations had little effect on the sensing performance.

The mixed-potential NH3 sensor using BiVO4/Bi4V2O11 heterojunction sensing electrode prepared by self-phase separation

BiVO4/Bi4V2O11 heterojunction sensing materials for the mixed-potential NH3 sensor were prepared by the hydrothermal method and sequent heat treatment, accompanying self-phase separation. The influence of the Bi3+/V5+ mole ratios on phase composition, microstructure of sensing materials and the performances of the sensor was investigated. The samples with different Bi3+/V5+(1:1–2:1) calcined at 500 °C form non-stoichiometric Bi4V2O11 and Bi2VO5.5 and further calcination at 750 °C, all samples change into BiVO4/Bi4V2O11 heterojunction materials. The mixed potential sensors based on above-mentioned heterojunction materials and Ce0.8Gd0.2O1.9 electrolyte show excellent performance. The sensor based on sensing electrode with Bi3+/V5+=1.75:1 can stably work at 450–600 °C due to good porous structure of sensing electrode, which is conducive to gas diffusion and adsorption. At 500 °C, the sensor has the highest sensitivity and the lowest detection limit. The relationship between ΔV and logarithm of NH3 concentrations is piecewise linear in the range of 1–25 ppm and 25–300 ppm, and the sensitivity is −6.9 and −38.2 mV/decade, respectively. The cross-sensitivity test of the sensor shows that co-existent gas CO2, CH4, H2 or H2S makes the response value change −3.7%, −3.0%, 0.9% and −6.8%, showing good anti-interference performance to these gases. While NO or NO2 causes serious interference, making the response value reduce by −9% and −27%. CuO instead of Pt as the reference electrode nearly eliminates the interference of NO and NO2. In addition, sensor following the mixed potential mechanism was confirmed by experiments.

Mixed-potential ammonia sensor using Ag decorated FeVO4 sensing electrode for automobile in-situ exhaust environment monitoring

High-performance YSZ-based mixed potential NH3 sensor has been developed and utilized with the prospect of excellent applications in in-vehicle SCR (Selective Catalytic Reduction) systems. In order to further lower the detection limit and improve the sensitivity to NH3, we prepared Ag decorated FeVO4 as sensing electrode material through the NaBH4 reduction method in this work. Electrochemical methods such as cyclic voltammetry (CV), linear sweep voltammetry (LSV) and I-t curves are used to explore and compare the electrochemical catalytic activity of electrode materials. 3 mol% Ag decorated FeVO4 was considered as the best sensing electrode material because of its highest electrochemical catalytic activity and reliable stability. Also, the reaction product under the sensor’s working condition was confirmed by the online mass spectrometer. The sensor attached with 3 mol% Ag decorated FeVO4-SE could give a response of − 57 mV to 50 ppm NH3 in 3s and recover to base line in 16s, exhibiting its rapid detection ability at high temperature of 550 °C. Moreover, it displayed good selectivity and a low detection limit of 200 ppb. Additionally, the stability of the sensor under different oxygen concentration and humidity was studied in detail.

Mesoporous tungsten trioxide for highly sensitive and selective detection of ammonia

The sensing performance of a mixed-potential NH3 sensor is dependent on the microstructure of its electrode, which can be optimized to enhance the adsorption and diffusion of target gases. Mesoporous tungsten trioxide was prepared using an effective hard template method and applied as a sensitive electrode in a yttria-stabilized zirconia electrolyte-based mixed-potential NH3 gas sensor. The NH3 detection of the sensor was investigated in a temperature range of 250–650 °C. The sensor exhibited a much better NH3 sensing performance than another sensor based on commercial bulk WO3 under the same conditions. Furthermore, the NH3 sensor based on mesoporous WO3 exhibited a logarithmic response to the NH3 concentration (30–250 ppm). To investigate this response, the specialized microstructure of mesoporous WO3 was observed by transmission electron microscopy. Specifically, the high surface area and well-defined mesopores of the material significantly contributed to the excellent sensing performance. This report provides the first demonstration of the excellent response and sensitivity of an NH3 sensor based on mesoporous WO3 at low temperatures (250–400 °C).

Mixed potential NH3 sensor based on La9.95K0.05Si5Al1O26.45 electrolyte and Ag doped BiVO4 sensing electrode

Mixed potential NH3 sensors with La9.95K0.05Si5AlO26.45 (LKSAO) as the solid electrolyte and Bi1-xAgxVO4-δ as the sensing electrode (SE) were developed. Bi1-xAgxVO4-δ was produced by solid state reaction and then characterized by XRD, XPS, SEM and EDS. As revealed from the results, The BiVO4 samples exhibit an unchanged crystalline structure after Ag doping molar amount x≤0.2, accompanied by the growth of the particle. The sensor based on Bi1-xAgxVO4-δ exhibit the noticeably enhanced sensing performance to NH3 in contrast to the sensor using BiVO4, including its larger response potential (ΔV) and higher sensitivity. For instance, the sensors based on BiVO4 and Bi0.8Ag0.2VO4-δ at 400 °C exhibit the sensitivities of -28.1 and -77.3 mV/decade, respectively. Nevertheless, by Ag doping, there is no effective improvement of the anti-interference ability of the sensor against NO and NO2. To address the mentioned problem, CuO, NiO and La0.8Sr0.2MnO3 (LSM) were used as reference electrodes (RE). As suggested from the results, using CuO and NiO as the reference electrodes can obviously reduce the cross-sensitivity of the sensor to NO and NO2, whereas the sensitivity slightly decrease.

Synergistic enhancement in the sensing performance of a mixed-potential NH3 sensor using SnO2@CuFe2O4 sensing electrode

A mixed-potential type NH3 sensor equipped with CuFe2O4 and SnO2@CuFe2O4 sensing electrode is presented. The CuFe2O4 spinel-oxide and SnO2@CuFe2O4 composites were synthesized by a modified-Pechini route. The electrode materials were characterized for the physical properties by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS) analysis. It was found that the sensing characteristics were critically dependent on the extent of Triple-phase boundary (TPB) lengths and operating conditions of the sensor. Furthermore, the sensing performance of CuFe2O4 spinel-oxide was enhanced by compositing with SnO2 nanocrystals resulting in a synergistically enhanced response (ΔV) of −40 mV towards 80 ppm NH3, almost double and quadruple of the response of bare CuFe2O4 and SnO2 electrodes at 650 ℃, respectively. The sensor also displayed excellent stability towards oxygen and humidity variations, along with low cross-sensitivities towards interfering gases; e.g. NO, CO, CH4, and NO2. The complex impedance spectra (EIS) and dc polarization (I–V) measurements were performed for an insightful analysis of the sensing mechanism conforming to the mixed-potential model.

Improvement of Al3+ ion conductivity by F doping of (Al0.2Zr0.8)4/3.8NbP3O12 solid electrolyte for mixed potential NH3 sensors

New Al3+ ion conducting solid electrolytes (Al0.2Zr0.8)4/3.8NbP3O12-xF2x(0 ≤x ≤ 0.4) with Nasicon-structure are successfully prepared by solid state reaction method. The influences of the doped F- content on the properties of the (Al0.2Zr0.8)4/3.8NbP3O12-xF2x samples are investigated using X-ray powder diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The results show that F- doping can effectively improve the sinterability and the total conductivity of the (Al0.2Zr0.8)4/3.8NbP3O12-xF2x samples. Among the solids series, (Al0.2Zr0.8)4/3.8NbP3O11.7F0.6 shows the highest conductivity of 1.53 × 10−3 S cm−1at 500 °C, which is approximately 7.9 times higher than that of the undoped (Al0.2Zr0.8)4/3.8NbP3O12. The ion transference number of the samples is higher than 0.99 at 300–700 °C. On the basis of the promising properties, a mixed-potential type NH3 sensor based on (Al0.2Zr0.8)4/3.8NbP3O11.7F0.6 electrolyte and In2O3 sensing electrode has been developed. The sensing performance of the sensor is evaluated. The mixed-potential type sensor can work at relatively low temperatures of 200–350 °C and an excellent sensitivity of 99.71 mV/decade at 250 °C is obtained. The sensor also displays excellent stability and reproducibility, accompanied by low cross-sensitivities to CO2, CH4 and H2.

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.

Response Characteristics of Stable Mixed-Potential NH3 Sensors in Diesel Engine Exhaust

A mixed-potential, electrochemical sensor platform is extended to NH3 sensing by the introduction of a new gold alloy working electrode. A planar, pre-commercial NH3 sensor utilized LANL’s controlled interface approach, and a Pd-Au alloy working electrode was tested in exhaust of a GM 1.9 L diesel engine downstream of a diesel oxidation catalyst through a slipstream arrangement. A fraction of the exhaust was pulled across the sensor with a pump at 20 L/min. In order to simulate NH3 slip inside of a full SCR emissions control system, NH3 was injected immediately upstream of the sensor using a calibrated mass flow controller. The sensor response quantitatively tracked the NH3 as measured via Fourier transform infrared (FTIR) analyzer. A calibration curve was obtained in the exhaust from an ammonia staircase response with the engine running at steady-state engine conditions resulting in low background concentrations of NOx and HC (<20 ppm) during calibration. Exhaust gas recirculation (EGR) switching and sweeps were used to evaluate the NH3 sensor response under different amounts of total background NOx. The calibration curve was used to directly compare the [NH3] calculated from sensor response to the gas phase composition measured via FTIR. In general, there was excellent quantitative agreement between the sensor response and the actual NH3 in the exhaust gas, and fast response time such that transients (<5 ppm) could be easily discerned from baseline. A LANL pre-commercial NOx sensor was tested simultaneously with the NH3 sensor and the extent of cross-sensitivity between the two sensors will be discussed.

Effects of CoFe2O4 electrode microstructure on the sensing properties for mixed potential NH3 sensor

Mixed potential ammonia sensors based on 5YSZ (ZrO2 doped 5mol% Y2O3) were prepared, with ferrite CoFe2O4 as sensing electrode material. Co-sintering of electrode and electrolyte has been realized. Sensors with different electrode microstructure characterized by Environmental Scanning Electron Microscope (ESEM) were made by different sintering process, and its influences on the sensors’ performance were investigated. The co-sintered sensor, with a porous three dimensional network structure electrode and great adhesion with electrolyte, exhibits the largest response (about 81mV for 320ppm NH3) and the highest NH3 sensitivity (55mV/decade) with the response time (8–10s) and recovery time (10–12s) at 450°C, it also shows satisfying NH3 selectivity compared with NOx, and O2 has few influence on the sensor responses. The enhancement of the NH3 sensitivity may owe to the porous three dimensional network structures, which is beneficial to adsorption and diffusion of the target gas, and better bonded interface between the electrode and electrolyte also provides more quantity of triple-phase boundary (TPB).

A novel mixed potential NH3 sensor based on TiO2@WO3 core–shell composite sensing electrode

The TiO2@WO3 core-shell composite with mass ratio of core and shell4:1 was prepared by a hydrothermal synthesis method using sodium tungstate dehydrate, nitric acid and commercial TiO2 powder as raw materials. A novel mixed potential NH3 sensor was fabricated by using above-mentioned TiO2@WO3 as sensing electrode and La10Si5.5Al0.5O27 as solid electrolyte. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and structure of the samples. The sensor response to NH3 was examined at 400∼550°C. The experimental results indicated that the sensor based on TiO2@WO3 sensing electrode possessed greatly enhanced NH3 sensing properties including higher and more stable response value and faster response rate compared to the sensor using TiO2, WO3 or TiO2-WO3 mixture sensing electrode under the same conditions. The responding potential values of the sensor with TiO2@WO3 sensing electrode exhibited a linear dependence on the logarithm of the NH3 concentrations. The highest NH3 sensitivity of 74.8mV/decade was achieved at 450°C. In the meantime, the sensors also showed well anti-interference capability to CH4, CO2 and H2, but noticeable cross sensitivity toward NO2 was observed. O2 effect on responding signal could be calibrated by predetermining O2 content.

Mixed potential NH3 sensor based on Mg-doped lanthanum silicate oxyapatite

A mixed-potential type NH3 sensor was fabricated using La10Si5MgO26 (LSMO) as electrolyte with both a dense layer and a porous layer and nano-structured CoWO4 as the sensing electrode. The dense LSMO electrolyte was prepared by solid state reaction method at 1500°C via introducing Y2O3 as the sintering aid. The nano-structured CoWO4 powders were synthesized by hydrothermal method and then screen printed on the porous LSMO layer as the sensing electrode. The sensor exhibited well response-recovery characteristics to NH3 at 400–700°C. The response and recovery time were 11 and 13s when the NH3 concentration was changed between 200 and 300ppm, respectively. Good linear correlations between the ΔV values of the sensor and the logarithm of the NH3 concentrations for 30–300ppm were obtained. Compared with the sensor without LSMO porous layer, the sensor with LSMO porous layer exhibited larger ΔV values and higher sensitivity due to the enhanced TPB length. The mixed-potential-model of the sensor was identified by the polarization curves in different atmospheres. The sensitivity of the sensor decreased with increasing the sintering temperature of the CoWO4 electrode, due to the growth of the CoWO4 particles. Furthermore, the present sensor also displayed small cross-sensitivities to H2, CH4 and CO2 present in the gas mixture.

Mixed potential NH3 sensor with LaCoO3 reference electrode

Abstract Mixed potential NH 3 sensors using an yttria stabilized zirconia (YSZ, 8 mol% Y 2 O 3 ) electrolyte were prepared and evaluated at 700 °C. Various oxide materials were tested as a sensing electrode, among which In 2 O 3 sensing electrode showed the highest sensitivity to NH 3 with good stability. However, the EMF of the sensor decreased to 50% of its original value when 150 ppm NO 2 was added to gas containing NH 3 . In an effort to eliminate interference from NO 2 , a new concept of a mixed potential sensor using an oxide reference electrode of LaCoO 3 instead of a noble metal is introduced. It was found that the interfering effect of the new sensor to 150 ppm NO 2 was reduced from 50% to 10%. The EMF showed little change upon exposure to 1000 ppm CO and 150 ppm NO with a logarithmic dependence of the EMF on the NH 3 concentration.