Rangachary Mukundan

SOLID-STATE MIXED POTENTIAL GAS SENSORS: THEORY, EXPERIMENTS AND CHALLENGES

Abstract Solid-state mixed potential electrochemical sensors sense gases using differential electrocatalysis on dissimilar electrode materials. The response theory is typically expressed in terms of models invoking Butler–Volmer kinetics at high overpotentials (Tafel behavior). This model is not adequate for describing all types of mixed potential sensor responses. For low concentrations of analyte gas, mass transport limitations must also be considered. Experiments with sensors with air reference electrodes also demonstrate the importance of low overpotential oxygen reduction kinetics in establishing the device response. A sensor response model that predicts a linear relationship between response voltage and analyte gas concentration is derived. The development of oxide electrode based devices offers improved long-term response stability over metal electrode based devices.

Sulfur Tolerant Anodes for SOFCs

Solid oxide fuel cells (SOFCs) using yttria-stabilized zirconia (YSZ) electrolytes, lanthanum strontium manganate cathodes, and La 1 - x Sr x BO 3 /YSZ anodes (where B = Mn, Cr, and Ti) were fabricated. The sulfur tolerance of the various perovskite-based anodes was examined at 1273 K in a H 2 /H 2 O fuel. The Sr 0 . 6 La 0 . 4 TiO 3 /YSZ (50/50 wt %) anode showed no degradation in the presence of up to 5000 ppm of H 2 S in a hydrogen fuel. This anode was also able to operate for 8 h with 1% H 2 S as a fuel and showed no degradation when the fuel was switched back to hydrogen.

CO/HC sensors based on thin films of LaCoO3 and La0.8Sr0.2CoO3−δ metal oxides

Abstract We have demonstrated a new type of mixed potential, zirconia-based sensor that utilizes dense, thin films of either La–Sr–Co–O or La–Co–O perovskite transition metal oxide vs. a Au counter electrode to generate an EMF that is proportional to the oxidizable gas species (carbon monoxide (CO), C3H6, and C3H8) concentration in a gas stream containing oxygen. The devices reported in this work were tested at 600°C and 700°C and in gas mixtures containing 0.1% to 20% O2 concentrations. The metal-oxide-based sensors exhibited an improvement in operating temperature and level stability at elevated temperatures compared to Au–zirconia–Pt mixed potential devices already reported in the literature. However, as with Au–zirconia–Pt devices previously reported, the response behavior reproducibility from device to device was dependent on the Au morphology, which could vary significantly between samples under identical thermal histories. The changing Au morphology on both the Au counter electrode and the Au current collector on the metal oxide electrode were responsible for sensor aging and changes in device response over time. No change in the crystal structure of the perovskite thin film could be seen from XRD. A significant hysteresis in sensor response was found as the background oxygen concentration was cycled through stoichiometry, and this may be attributed to a change in the oxidation state of the cobaltate-based metal oxide electrode. In an effort to mitigate device aging, we replaced the Au counter electrode with a second metal oxide thin film, doped LaMnO3, and demonstrated the operation of a mixed potential sensor based on dual metal oxide electrodes.

Development of ceramic mixed potential sensors for automotive applications

Mixed potential sensors that utilize Gd{sub 0.2}Ce{sub 0.8}O{sub 2} electrolytes and patterned dense 1 {micro}m-thick LaMnO{sub 3} thin films were studied at 600 C and 1%O{sub 2}. The response to C{sub 3}H{sub 6} and CO of two different sensor configurations were studied continuously for 1000 hrs versus an air reference. Although two different current collection schemes and two different metal oxide electrode geometries were employed, the magnitude of the mixed potential generated by both sensors was remarkably similar. From previous work with Au-ceria-Pt mixed potential sensors, this behavior is attributed to precisely controlling the metal oxide electrode/solid electrolyte interface unlike the random interface produced when Au electrodes are used. Although doped ceria is not a suitable electrolyte for automotive exhaust gas applications, this work serves to illustrate design goals for zirconia-based sensors.

Mixed Potential Hydrocarbon Sensors based on a YSZ Electrolyte and Oxide Electrodes

The mixed potential response of perovskite-type oxide electrodes on a yttria stabilized zirconia (YSZ) electrolyte have been examined. The effects of varying the electrode and electrolyte composition and morphology on the sensor response have been studied in detail. La 0.8 Sr 0.2 CrO 3 was found to be an excellent electrode material for a highly selective nonmethane hydrocarbon sensor. Moreover, the sensor response was greatly improved by decreasing the heterogeneous catalysis on the YSZ electrolyte. Optimizing the electrode and electrolytes resulted in a sensor response of up to 100 mV for 500 ppm of propylene in a 1% O 2 stream at 550°C. The response times of these sensors were found to be strongly dependent on the porosity in the YSZ electrolyte and could be optimized to yield a sensor response time as low as one second.

A mixed-potential sensor based on a Ce{sub 0.8}Gd{sub 0.2}O{sub 1.9} electrolyte and platinum and gold electrodes

A Pt/Ce{sub 0.8}Gd{sub 0.2}O{sub 1.9}/Au mixed-potential sensor was constructed and its response to reducing gases in an oxygen containing stream was measured at T = 550 and 600 C. The sensor response was maximum for H{sub 2} and negligible for methane; other gases followed the trend methane < propane < CO, propylene < hydrogen. The mixed potentials developed at the Au and Pt electrodes were also independently monitored with respect to Pt air-reference electrodes. The mixed potential at the Au electrode was always higher (more negative) than that at the Pt electrode irrespective of the type of reducing gas used. The polarization curves of the two electrodes during oxygen reduction were also measured. These measurements revealed that the mixed potential at the electrodes was dependent on both the amount of electrochemical oxidation of reducing gas and the overpotential for oxygen reduction. While the response of the Pt electrode was found to be stable at both 600 and 550 C, the Au electrode had stability problems at 600 C due to the changing morphology of the gold film.

Accurate measurement of the through-plane water content of proton-exchange membranes using neutron radiography

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeders paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. ...

Trace detection and discrimination of explosives using electrochemical potentiometric gas sensors

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 (1-3 μg). 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.

Nitrogen Oxide Sensors Based on Yttria-Stabilized Zirconia Electrolyte and Oxide Electrodes

Mixed potential sensors for the detection of hydrocarbons and carbon-monoxide have been previously studied at Los Alamos National Laboratory (LANL). The LANL sensors have a unique design incorporating dense ceramic-pellet/metal-wire electrodes and porous electrolytes. When these sensors are exposed to various gases, in addition to their mixed-potential response, their resistance changes. This change in resistance is probably associated with the oxygen reduction reaction at the electrode/electrolyte interface and can be used to yield a total NO, response. The NO x sensors are operated in a current bias mode where the voltage response is related to the total NO, concentration.

Parametric Study of the Morphological Proprieties of HT-PEMFC Components for Effective Membrane Hydration

A 1D non-isothermal model has been developed to study the optimum morphological properties of HT-PEMFC components that will help the catalyst layer retain water vapor generated by the electrochemical reaction and that delivered by the feed gases. The use of a microporous layer (MPL) helps retain water generated in the catalyst layer (CL), and the effectiveness of the MPL in retaining water vapor in the CL increases the MPL pore size and porosity are reduced. Reducing the GDL porosity is found to help retain more water in the CL but the pore sizes and porosity of the PEMFC components should not be too small, so as to avoid increased O 2 concentration overpotentials. The optimum values of MPL and GDL porosity depend on the operating conditions such as the cell voltage, operating pressure, and inlet relative humidity.