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Abstract: In this article, the independent testing and validation of a packaged, electrochemical prototype hydrogen sensor at the National Renewable Energy Laboratory (NREL) is reported. Custom electronics were developed to be compatible with the data acquisition system at NREL. The specialized hydrogen sensor-testing laboratory at NREL used a variety of standardized test protocols to assess sensor performance. The system controlled and monitored humidity, pressure, and hydrogen gas concentration and introduced interference gases such as methane, carbon dioxide, carbon monoxide and ammonia. The hydrogen sensor was made of Indium Tin Oxide and Platinum electrode and Yttria-Stabilized Zirconia (YSZ) electrolyte. NREL testing showed a fast response to H2 with exceptional low-level sensitivity (0.005–2%) and high signal-to-noise ratio. A minimal deviation in sensor response to changes in ambient conditions such as humidity was observed along with minimal response to interference gases. The hydrogen prototype sensor was operational and insensitive to variations in oxygen partial pressure down to approximately 0.2% oxygen, while anaerobic conditions caused the sensor to become inoperable. The NREL tests indicate that the sensor prototype will effectively function and trigger event alarms and continue to function unless anaerobic conditions are established. It is envisioned in future to develop a different working electrode based on a more robust material such as La1−x Sr x CrO3 for applications where substantially harsher environments (absence of oxygen supply, higher altitudes, and higher atmospheric pressure) may be experienced. [Copyright &y& Elsevier]

Abstract: In this article, the influence of perovskite electrode composition on an electrochemical based mixed potential sensor response is reported. Specifically, various stoichiometry of Strontium doped Lanthanum Chromite materials were used as the sensing electrode. Yttria-Stabilized Zirconia was used as the solid electrolyte and Pt as the counter electrode. The effect of varying doping levels of Strontium on the sensor response was studied. NO, NO2, C3H6, and C4H10 were used as test gases. The sensor response was studied under open-circuit and current biased conditions. Based on sensitivity and selectivity evaluations, the device configuration ‘La0.9Sr0.1CrO3/YSZ/Pt’ was found to behave as a selective total hydrocarbon sensor at 500°C when operated under open-circuit conditions. On the other hand, ‘La0.7Sr0.3CrO3/YSZ/Pt’ exhibited minimal cross-sensitivity to hydrocarbons facilitating its use as a selective NO x sensor when operated under a current bias. The variation in the sensor response and selectivity for different electrode materials indicate that it might be possible to develop multi-electrode sensor platform for identifying a suite of analytes with improved selectivity and sensitivity. [Copyright &y& Elsevier]

Abstract: In this investigation, an electrochemical mixed potential type gas sensor was characterized using impedance spectroscopy. Specifically, the effect of operating temperature (435–610°C) on sensor response and response time was studied. Propylene was used as the analyte to test the ‘La0.8Sr0.2CrO3/YSZ/Pt’ sensor configuration. Two-electrode AC impedance measurement was performed with a frequency sweep from 13MHz down to 10mHz and excitation voltage of 10mV. For a fixed propylene concentration, the bulk and interfacial resistances was seen to decrease with increase in the sensor operating temperature. An Arrhenius behavior of the bulk and interfacial resistance was observed. The activation energy for O2 ion conduction and charge transfer was found to be 0.94eV and 1.54eV respectively. For a 150°C rise in operating temperature from 485 to 585°C, a 26-fold improvement in response rise time was observed while an 82% reduction in sensor response was recorded. It is postulated that the increase in operating temperature results in faster reaction kinetics, faster oxygen reduction and greater heterogeneous catalysis. [Copyright &y& Elsevier]

Abstract: In this article, testing of an electrochemical, potentiometric hydrogen safety sensor is reported within a proposed packaging scheme. Device packaging under static and flow testing conditions are presented. During the static volume sensor testing, the sensors response is compared against a calibrated Gas Chromatography (GC) measurement. Also, a commercial H2 sensor is incorporated into the test chamber to act as a benchmark for the sensor prototype. In the testing phase, H2 selectivity is demonstrated using pulsed discharge technique. The packaging schemes for static and flow condition testing permit easy handling and exchange of the laboratory prototype sensors with other agencies for independent testing/performance validation. Upon testing, for a fixed inlet concentration of 2% (20,000 ppm) H2 and within the duration of the typical experiment conducted within a non-sealed test chamber, the sensor sees only around 6200 ppm as confirmed by the GC measurements. The sensitivity is around 105 mV for 6200 ppm of H2. While a logarithmic correlation of the sensor output with H2 concentration is observed in laboratory based H2 sensor, the commercial device suffered from an elevated baseline (due to reduced barometric pressure conditions of 2195 m elevation), humidity and uncorrelated sensor output for different H2 concentrations. The pulsed discharge scheme for achieving hydrogen selectivity holds a great potential for tackling cross-sensitivity problems that affect many H2 sensor technologies. [Copyright &y& Elsevier]

Abstract: In this article, selective and sensitive detection of trace amounts of pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX) is demonstrated. The screening system is based on a sampling/concentrator front end and electrochemical potentiometric gas sensors as the detector. Preferential hydrocarbon and nitrogen oxide(s) mixed potential sensors based on lanthanum strontium chromite and Pt electrodes with yttria stabilized zirconia (YSZ) solid electrolyte were used to capture the signature of the explosives. Quantitative measurements based on hydrocarbon and nitrogen oxide sensor responses indicated that the detector sensitivity scaled proportionally with the mass of the explosives (1–3μg). Moreover, the results showed that PETN, TNT, and RDX samples could be discriminated from each other by calculating the ratio of nitrogen oxides to hydrocarbon integrated area under the peak. Further, the use of front-end technology to collect and concentrate the high explosive (HE) vapors make intrinsically low vapor pressure of the HE less of an obstacle for detection while ensuring higher sensitivity levels. In addition, the ability to use multiple sensors each tuned to basic chemical structures (e.g., nitro, amino, peroxide, and hydrocarbon groups) in HE materials will permit the construction of low-cost detector systems for screening a wide spectrum of explosives with lower false positives than present-day technologies. [Copyright &y& Elsevier]

Abstract: In this article, the development and testing of an electrochemical hydrogen (H2) sensor prototype based on ‘indium tin oxide (ITO)/yttria-stabilized zirconia (YSZ)/platinum (Pt)’ configuration is detailed. The device fabricated on an alumina substrate integrates a resistive Pt heater to achieve precise control of operating temperature while minimizing heterogeneous catalysis. Targeting fuel cell powered automotive applications, the safety sensor was subjected to interference studies, temperature cycling, operating temperature variations, and long-term testing over 2000h. The sensor responded in real-time to varying concentrations of H2 (1000–20,000ppm). Among the interference gases tested such as nitric oxide (NO), nitrogen dioxide (NO2), ammonia (NH3), carbon monoxide (CO), and propylene (C3H6), the sensor showed cross-sensitivity to C3H6. Analyzing the overall device performance over 2000h of testing for 5000ppm of H2, (a) the sensitivity varied between 0.135 and 0.167V, (b) the baseline signal ranged from 0 to 0.04V, and (c) the response rise time fluctuated between 3 and 7s. The salient features of the H2 sensor prototype developed by Los Alamos National Laboratory (LANL) are (a) the low power consumption, (b) compactness to fit into critical areas of application, (c) simple operation, (d) fast response, (e) a direct voltage read-out circumventing the need for any additional conditioning circuitry and (f) conducive to commercialization. [Copyright &y& Elsevier]

Abstract: In this article, the influence of yttria-stabilized zirconia (YSZ) sintering temperature on a Pt/YSZ/La0.8Sr0.2CrO3 mixed potential sensor performance is reported. The sintering temperature of YSZ was varied from 1000 to 1200°C. Mercury porosity measurements were performed to estimate the porosity and tortuosity of the YSZ sample as a function of sintering temperature. Further, the surface area of YSZ was computed by the BET method. After YSZ characterization, the unbiased and biased sensor response was recorded. The 1000°C sintered YSZ sample was taken as the reference for comparison purposes. Experimental results indicated a 30% reduction in porosity for the 1200°C sintered YSZ sample, resulting in a 14-fold increase in the sensor response rise time. In addition, for the same sample, a 13-fold increase in sensitivity was observed upon exposure to propylene (100ppm), associated with a 76% reduction in surface area. The slow response time of the sensor with YSZ sintered at higher temperatures has been attributed to higher tortuosity (delay in gas permeation to the three-phase interface). Whereas, reduced heterogeneous catalysis induced by lower surface area accounts for the rise in sensitivity levels. The optimum YSZ sintering temperature was found to lie between 1100 and 1150°C. [Copyright &y& Elsevier]

Abstract: The article details the application of commercial manufacturing methods towards the development of NO x /NH3 sensors for vehicle on-board emissions control. These sensors possess a unique mixed potential sensor design. This unique LANL (Los Alamos National Laboratory) design results in improved sensitivity, selectivity and response time over conventional mixed potential sensors incorporating a stable three-phase interface using a porous electrolyte coated over a dense electrode. A prototype gas sensing platform conducive to large-scale manufacturing and commercialization is also been presented. The observed results indicate the possibility of evolution towards a combined NO x /NH3 sensor on the same platform. Finally, critical challenges towards developing field deployable mixed potential NO x /NH3 sensors are identified and discussed. [Copyright &y& Elsevier]

Abstract: Mixed potential sensors using dense, thin film metal oxide working electrodes, Pt counter electrodes, and thin film YSZ electrolytes on Al2O3 polycrystalline and sapphire substrates were prepared and studied between 450 and 650°C. Their response to NO, NO2, CO, and C1 and C3 hydrocarbons in 10.4% O2/N2 balance and in air atmospheres was characterized. The lanthanum chromite-based sensors showed preferential sensitivity to NO2 with cross sensitivity to CO and non-methane hydrocarbons such as C3H6 and C3H8 with the highest NO x sensitivity and minimal CO/HC cross sensitivity exhibited by a sensor prepared with a La0.8Sr0.2CrO3 working electrode. Studies of the Mg-doped LaCrO3 devices conducted for up to 800h at 600°C showed minimal aging in these devices. In contrast, sensors fabricated with spinel NiCr2O4 or ZnFe2O4 working electrodes showed preferential sensitivity to HCs and minimal sensitivity to NO2. Sensors fabricated with Y0.16Tb0.3Zr0.54O2−∂ working electrodes required operating temperatures in excess of 600°C and, like the spinels, showed minimal NO x sensitivity. A prototype three-electrode mixed potential device for the detection of multiple gases was also demonstrated. The use of a heated Pt black pre-catalyst upstream to the sensor was demonstrated to effectively remove the CO and hydrocarbon response. [Copyright &y& Elsevier]

Thin films of LaCrO3 and La0.8Sr0.2CrO3 were prepared using RF magnetron sputtering, electron beam evaporation and pulsed laser deposition (PLD). The films were prepared from both oxide targets and from targets consisting of Cr metal and lanthanum and strontium fluorides. Subsequent anneal of the fluoride/metal composite films in humidified Ar produced an oxide film. Direct growth of the perovskite from oxide targets via sputtering was usually accompanied by second phases with Cr in valance states higher than 3+. Single-phase La–Sr–Cr–O films could not be prepared from a single oxide target electron beam evaporation. However, thick, stoichiometric films were prepared at 400 °C using an electron beam/thermal evaporation dual source method but inconsistently. Single-phase lanthanum chromate films, based on the limited results obtained in this work, could not be prepared using PLD from either oxide or fluoride precursors under conditions similar to those used in the other techniques. However, films prepared by the intermediate fluoride process via off-axis RF magnetron sputtering process produced high quality thin films with density, microstructure and phase purity very similar to bulk La0.8Sr0.2CrO3 using a film growth temperature as low as ambient (Tsub<100 °C). The deposition rate was typically low at approximately 1500 A˚/h. SEM analysis showed that the morphology of films produced via fluoride sputtering was nearly identical to bulk La0.8Sr0.2CrO3 prepared by sintering ceramic powders at 1650 °C in air and, based on previous work, should be suitable for mixed potential sensor applications. [Copyright &y& Elsevier]

We have demonstrated a mixed potential, zirconia electrolyte-based sensor that utilizes dense, thin films of LaMnO3 and Y0.16Tb0.30Zr0.54O2⋅− as electrodes to generate an EMF that is proportional to the oxidizable gas species (CO, C3H6, and C3H8) concentration in a gas stream containing oxygen. The devices reported in this work were tested at 600 and 700 °C and in gas mixtures with O2 concentrations between 0.86 and 1.75%. An Au/LaMnO3/YSZ/Y0.16Tb0.30Zr0.54O2⋅−/Au device was tested for a total of 3000 h at 600 and 700 °C and was subject to periodic excursions to 10−10 atm PO2 to simulate reducing conditions. SEM analysis indicated that the aging effects measured in the device response over the course of this experiment may be linked to the sintering of the Au pad used for current collection on the metal oxide electrode. Three-terminal devices were fabricated so that the magnitude of the mixed potential generated from CO and C3H6 exposure on the Au/Y0.16Tb0.30Zr0.54O2⋅−, Pt/Y0.16Tb0.30Zr0.54O2⋅−, Au/LaMnO3, and Pt/LaMnO3 metal/metal oxide electrode combinations could be studied at 600 °C and in 1% O2. It was found that the catalytic properties of the Au or Pt used for current collection played a significant role in the generation of the mixed potential for both metal oxides tested. The combination of Pt on LaMnO3 gave an equilibrium response whereas a significant mixed potential was generated on both Au/Y0.16Tb0.30Zr0.54O2⋅− and Au/LaMnO3 electrodes at 600 °C. [Copyright &y& Elsevier]