To improve the stability of a NASICON ͑Na 3 Zr 2 Si 2 PO 12 ; Na + conductor͒-based potentiometric CO 2 sensor under humid conditions, a composite of BiCuVO x ͑Bi 2 Cu 0.1 V 0.9 O 5.35 ͒ and perovskite-type oxide ͑La 0.6 Sr 0.4 Co 0.78 Ni 0.02 Fe 0.2 O 3 ͒ was used as a solidreference electrode. The CO 2 sensing properties and stability of a NASICON-based planar device fitted with Li 2 CO 3 -BaCO 3 ͑auxiliary phase͒ and the composite reference electrode were examined under humid conditions. The planar device mounted on an alumina substrate with a Pt heater showed stable electromotive force ͑emf͒ responses to changes in the CO 2 concentration ͑100-400 ppm͒ at 400 and 450°C in humid air without degradation. The sensor device also exhibited a good warming-up characteristic, i.e., the emf of the device quickly reached a steady and constant value when the sensor operation was restarted even after the sensor was exposed to humid air ͑86% relative humidity at 25°C͒ at room temperature for a long time.

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Mixed-potential-type gas sensors are promising solid-state devices for the in-situ detection of ppm concentrations of air pollutants in the atmosphere. In this study, we designed electrode structures of mixed-potential-type sensors using a BiCuVOx (Bi 2 V 0.9 Cu 0.1 O 5.35 ) solid electrolyte to control the responses of electrodes to organic gases. Devices fitted with sensing and counter electrodes made of a composite of BiCuVOx and a perovskitetype oxide (La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 ) were fabricated and tested for their ethanol-sensing properties. Sensor devices with a three-electrode configuration, in which a reference electrode was exposed to a standard air mixture, were fabricated to separately measure the potentials of the sensing and counter electrodes relative to the reference electrode. When a thin (30 µm) sensing electrode and a thick (68 µm) counter electrode were attached to a planar device, the sensing electrode exhibited high responses to ethanol at 350 and 400 °C. However, the overall signal was still small because the counter electrode also responded to ethanol. In another planar device in which the counter electrode was coated with a Pt/Al 2 O 3 catalyst, the counter electrode exhibited almost no response to ethanol resulting in the generation of large sensor signals. This is because the catalyst layer efficiently decomposed ethanol and impeded its diffusion to the reaction interface between the counter electrode and the solid electrolyte. The device coated with the combustion catalyst also showed good response and recovery speeds and high stability under humid conditions indicating its feasibility as a practical, compact ethanol sensor.