Active Reaction Sites for Oxygen Reduction in La0.9Sr0.1,MnO3/YSZ Electrodes

Active reaction sites for 02 reduction in La0.~Sr01MnO3 electrode have been characterized by addressing the origin of the cathodic polarization effects on this electrode material. Cathodic polarization (up to - 1.2 V vs. Pt reference electrode} had several effects on O2 reduction kinetics. First, the O2 reduction rate was favorably increased when the perovskite electrode was cathodically polarized. Second, in situ x-ray photoelectron spectroscopy results indicated that the Mn ions are electrochemically reduced and concomitantly the oxygen stoichiometry decreases. Reduction of Mn ions was further demonstrated in the cyclic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was observed in the cyclic voltammograms of the perovskite/YSZ/Pt system, and the hysteresis phenomena were more prominent at higher O~ pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies participate in the O2 reduction reaction. However, it has been explained from the Po2-dependent hysteresis phenomena that, even though those surface sites are active in the O2 reduction~ their activity is less than that of the three-phase boundary sites since additional diffusional processes are required for the former sites. Consequently, the three-phase boundary sites are the major reaction sites at lower O2 pressure, which leads to a small hysteresis. However, at higher 02 pressure, the surface sites also participate in the reaction, resulting in a larger hysteresis.Funding for this work was provided by the R&D Management Center for Energy and Resources (Korea). S. M. Oh gratefully acknowledges the financial support from the Alexander yon Humboldt Foundation

Effective Use of Molecular Recognition in Gas Sensing: Results from Acoustic Wave and In-Situ FTIR Measurements

To probe directly the analyte/film interactions that characterize molecular recognition in gas sensors, we recorded changes to the in-situ surface vibrational spectra of specifically fictionalized surface acoustic wave (SAW) devices concurrently with analyte exposure and SAW measurement of the extent of sorption. Fourier-lmnsform infrared external- reflectance spectra (FTIR-ERS) were collected from operating 97-MH2 SAW delay lines during exposure to a range of analytes as they interacted with thin-film coatings previously shown to be selective: cyclodextrins for chiral recognition, Ni-camphorates for Lewis bases such as pyridine and organophosphonates, and phthalocyanines for aromatic compounds. In most cases where specific chemical interactions-metal coordination, "cage" compound inclusion, or z stacking-were expected, analyte dosing caused distinctive changes in the IR spectr~ together with anomalously large SAW sensor responses. In contrast, control experiments involving the physisorption of the same analytes by conventional organic polymers did not cause similar changes in the IR spectra, and the SAW responses were smaller. For a given conventional polymer, the partition coefficients (or SAW sensor signals) roughly followed the analyte fraction of saturation vapor pressure. These SAW/FTIR results support earlier conclusions derived from thickness-shear mode resonator data

Using bias superposition to test a thick film conductance sensor.

A novel on-line monitoring technique for a range of MEMS and integrated sensor systems is presented based on the injection of a test stimuli into the bias structure of transducer functions. The technique `Bias Superposition' utilises both signal injection and signal extraction techniques to achieve an indication of structural integrity of the transducer and interface. The technique has been successfully applied to a thick film conductance sensor

Gas identification by modulating temperatures of SnO2-based thick film sensors

A new method is presented to identify the presence of two gases in the ambient atmosphere. The method employs only one SnO2-based gas sensor in a sinusoidal temperature mode to perform the quantitative analysis of a binary gas mixture (CO/NO2) in air. (C) 1997 Elsevier Science S.A