Tomoyasu Eguchi

Optimum design for the sensing electrode mixtures of PbSnF4-based oxygen sensors for fast response at ambient temperature

Abstract The amperometric oxygen sensors of the type, Ag|Ag 6 I 4 WO 4 |PbSnF 4 |sensing electrode (a mixture of iron(II) phthalocyanine (FePc), PbSnF 4 and a whisker material) have been fabricated and effects of the incorporated whiskers on response properties have been examined to design the optimum sensing electrode mixture. The selected whiskers were several 9Al 2 O 3 ·2B 2 O 3 and TiO 2 whiskers different in length and conductivity (with or without conductive SnO 2 coating film). Fast response comparable to that of galvanic-cell-type sensors was realized with the sensing electrode mixture designed on the basis of the following two points: (1) the sensing electrode should be a ternary mixture of whiskers, FePc and PbSnF 4 with a volume ratio of 1:7:2; (2) the whiskers should have a length around 20 μm, an aspect ratio of about 20 and a good electronic conductivity. The mean response time, 25 s, was to date the shortest of those of known solid-state electrochemical sensors at room temperature. The incorporation also greatly improved other response properties such as transient behavior, drifts and hysteresis. The roles of the incorporated conductive whiskers are to form the conductive whisker networks in the sensing electrode mixture and to make the reduction sites of oxygen molecules electrochemically uniform by localizing the sites along the networks.

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|Ag(6)I(4)WO(4)|PbSnF(4)(PSF)| sensing electrode (SE), O(2); SE: mixtures of Pt-black/Pcs/PSF, carbon/Pcs/PSF, Pcs/PSF, Pt-black/PSF, carbon/PSF (phthalocyanines, Pcs: FePc, CoPc, CuPc, H(2)Pc) have been examined to elucidate the roles of the incorporated Pcs, 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 H(2)Pc have not such a catalytic action. The difference is related to the types of the first oxidation of Pcs, i.e. the central metal oxidation and the ligand oxidation. In addition, the sensitivity (S) and 90% response time (t(90)) depend on the oxidation potentials. FePc is the best of the Pcs 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 Pcs. 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).

Response mechanism of amperometric oxygen sensors with PbSnF4 electrolyte and whisker-containing sensing electrode

Abstract The sensor, Ag|Ag 6 I 4 WO 4 |PbSnF 4 |sensing electrode (SE: a mixture of conductive whiskers, iron phthalocyanine and PbSnF 4 ), shows a linear I sc (short-circuit current)–{ P O 2 (O 2 partial pressure)} 1/2 relation and a Nernst-type emf– P O 2 relation (the number of the associated electrons≈2). The SE-reaction thus occurs as follows: O 2 →2O, O+e − →O − . Under short-circuit (sc) condition, the rate-determining step is the charge-transfer step of the reduction, because the I – E characteristics of the SE exhibit a Tafel relation in −400 to +100 mV vs. Ag|Ag 6 I 4 WO 4 . The SE-potential is always fixed at that of the Ag electrode under SC condition. In addition to this, the very low level of I sc permits the reproducible response with a long-term stability.