Effects of Pt loading onto SnO2 electrodes on CO-sensing properties and mechanism of potentiometric gas sensors utilizing an anion-conducting polymer electrolyte

Gas-sensing properties of electrochemical CO sensors utilizing N-loaded SnO2 electrodes and an anion-conducting polymer electrolyte have been investigated mainly at 30 degrees C in wet synthetic air (57%RH), and the effects of the N loading onto SnO2 on the CO-sensing properties and their CO-sensing mechanism have been discussed in this paper. The amount of N loaded onto SnO2 (0.5-5.0 wt%) and the subsequent heat-treatment at 500 degrees C in air were effective in enhancing the CO responses and the CO selectivity against H-2. The sensing-electrode potential was governed by mixed potential resulting from electrochemical CO (or H-2) oxidation and O-2 reduction, and all the results obtained indicated that the oxidation rate of CO molecules was electrochemically quite faster than that of H-2 molecules on the mono-dispersive and oxidized N species as an active site, which were doped at the surface SnO2 lattice. On the other hand, the heat treatment at 250 degrees C in H-2 after the N loading reduced the surface of N-loaded SnO2 and drastically enhanced both CO and H-2 responses and thus decreased the CO selectivity against H-2. This effect arose probably from the reduced N species with metallic surface, which were quite active against both CO and H-2 anodic reactions.