Ambient temperature oxygen sensors based on fluoride solid electrolyte: The roles of the constituents in the sensing electrode mixtures containing phthalocyanines

The short-circuit-current response properties of ambient temperature oxygen sensors of the type, Ag\Ag6I4WO4\PbSnF4(PSF)\sensing electrode (SE), O-2; SE: mixtures of Pt-black/Pc's/PSF, carbon/Pc's/PSF, Pc's/PSF, Pt-black/PSF, carbon/PSF (phthalocyanines, Pc's: FePc, CoPc, CuPc, H2Pc) have been examined to elucidate the roles of the incorporated Pc's, Pt-black and carbon. FePc and CoPc act as the catalyst for the SE-reaction involving the two-electron reduction of oxygen, whereas CuPc and H2Pc have not such a catalytic action. The difference is related to the types of the first oxidation of Pc's, i.e. the central metal oxidation and the ligand oxidation. Ln addition, the sensitivity (S) and 90%, response time (t(90)) depend on the oxidation potentials. FePc is the best of the Pc's used here in terms of t(90) (i.e. 40 s for the SE-mixtures of Pt-black/FePc/PSF and carbon/FePc/PSF). Although Pt-black acts as the catalyst, it tends to give rise to hysteresis, drifts and slow response. The undesirable effects are lessened by incorporating Pc's. For the Pt-black/FePc/PSF sensor, the response properties are almost dominated by the incorporated FePc and hence are comparable to those of the C/FePc/PSF sensors. This indicates that Pt-black can be substituted by carbon materials. The incorporated carbon has no such catalytic action. The role is to make the SE-mixtures more electronically conductive, resulting in the improvement of S and t(90). (C) 1997 Published by Elsevier Science B.V.