Chemical sensors protonic conductor

Dr. Hui has worked on various projects, including chemical sensors, solid oxide fuel cells, magnetic materials, gas separation membranes, nanostruc-tured materials, thin film fabrication, and protective coatings for metals. He has more than 80 research publications, one worldwide patent, and one U.S. patent (pending). He is currently leading and involved in several projects for the development of metal-supported solid oxide fuel cells (SOFCs), ceramic nanomaterials as catalyst supports for high-temperature PEM fuel cells, protective ceramic coatings on metallic substrates, ceramic electrode materials for batteries, and ceramic proton conductors. Dr. Hui is also an active member of the Electrochemical Society and the American Ceramic Society. 

Sensing mechanism of the modified sensor. The sensing mechanism in this modified sensor should be essentially the same as that of the unmodified one. It is noteworthy that a stationary short circuit current was obtained in spite of such sensor construction that the counter electrode was covered with Epoxy resin. Since the sensing electrode is placed in the same situation as the unmodified sensor, this fact indicates that the cathodic reaction is allowed to take place stationarily at the counter electrode. The proton conductor membrane is as thin as 0.2 mm, so that the reactant 02 and the produced H20 will permeate the membrane as shown in Figure 11. A part of H2 will naturally also permeate through the membrane, but the transfered H2 will be consumed by the reaction with 02 electro-chemically or catalytically at the counter electrode. 

Some proton conductors have relatively high conductivities at room temperature. Introduction of these materials into electrochemical cells brings about attractive chemical sensors workable at room temperature. Potentiometric or amperometric detection of chemical components at room temperature would create new fields of application for sensors especially in bioprocess control and medical diagnosis. With an all-solid-state structure, the sensors would be compatible with micro-fabrication and mass production, and small power consumption associated with their ambient-temperature operation would be intrinsically suited for cordless or portable sensors. 

As listed in Table 36.1, a good deal of research has been carried out so far on proton conductor-based gas sensors workable at ambient temperature. Various inorganic and organic ion exchangers, such as hydrogen uranyl phosphate (HUP), zirconium phosphate (ZrP), antimonic acid (AA), and NAFION membrane, have been utilized in the form of a disc, thick- or thin-film. The ionic conductivities of these proton conductors, in the range lO " 10 S cm are modest but seem to be still sufficient for chemical sensing devices. 

In this section, development of proton conductor-based chemical sensors is briefly described by focusing on room temperature operation type ffj sensors. 

Most proton conductor sensors have aimed at sensing of Hj in inert gases or air, as mentioned above. However, other chemical components are also sensitive whenever they produce or consume protons through electrode reactions. Typical of such examples are and... 

Miura N, Yamazoe N (1992a) Development of new chemical sensors based on low-temperature proton conductors. SoUd State Ionics 53-56 975-982... 

Miura N, Yamazoe N (1992b) Solid-state gas sensors operating at room temperature. In Colomban P (ed) Chemistry of soUd state materials 2 proton conductors. Cambridge University Press, Cambridge, p 527 Miura N, Kato H, Yamazoe N, Seiyama T (1983) Mixed potential type NO sensor based on stabihzed zirconia. In Proceedings of international meeting of chemical sensors, Fukuoka, Japan. Kodansha/Elsevier, Tokyo/Amsterdam, p233... 

A protonic conductor such as CaZro 9lno.i Os- , can be applied to sense hydrocarbon gases. In this case, two different electrodes were used. One is silver metal which is a material inert to hydrocarbons and the other is a perovskite-type oxide (Lao.6Bao.4Co03) which contains rare earth and accelerates the combustion of hydrocarbons. The chemical reaction occurring in the sensor is shown in fig. 69 (Hibino and Iwahara 1994). Hydrocarbons in the ambient atmosphere do not react on the Ag electrode. On the contrary, hydrocarbons react with oxygen and form water vapor and carbon dioxide on the oxide electrode surface. 

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