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A series of pyrochlore-type lead-ruthenium oxide (Pb 2 Ru 2 O 7-δ ) containing various amounts of oxygen vacancies was prepared by varying the heat-treatment temperature, and the oxygen reduction reaction (ORR) activity as well as the electrochemical redox property in acid and alkaline solution was evaluated using rotating disk electrode. The specific ORR activity was increased with increasing the amount of oxygen vacancies, and linearly correlated to the charge amount of electrochemical redox of Pb 2 Ru 2 O 7-δ both in acid and alkaline solution. We concluded that the surface oxygen vacancy participates in the electrochemical redox of Pb 2 Ru 2 O 7-δ , and at the same time, behaves as the active site for ORR. The ORR activity in alkaline solution was much higher than that in acid solution, possibly because of the difference of the facility of electrochemical desorption of adsorbed oxygen species in the ORR process. Additionally, the effect of humidity on the ORR activity was investigated by single cell test. Proton exchange membrane fuel cells (PEMFCs) are promising alternative power sources for home co-generation system and transportation applications due to advantages such as low emission and high efficiency. There are, however, still many problems remaining for widespread commercialization of PEMFCs. In the current PEMFCs, platinum based catalyst is used as the cathode electrocatalyst for oxygen reduction reaction (ORR). From the view of cost and availability, alternative electrocatalyst to platinum is strongly required. While many types of non-platinum electrocatalyst have been reported, such as metal-oxides, metal macrocyclic complex, metal chalcogenides, and nitrogen-containing carbon, none of them has been applied to practice due to poor performance or durability. 1-12We have been focusing on lead-ruthenium oxide (Pb 2 Ru 2 O 7-δ ) as a candidate of non-platinum electrocatalyst for ORR. [13][14][15][16] Pb 2 Ru 2 O 7-δ has a pyrochlore-type structure with some oxygen vacancies, and shows high electronic conductivity. [17][18][19][20][21][22][23][24][25][26] Horowitz et al. 20 have firstly reported that Pb 2 Ru 2 O 7-δ which is prepared by co-precipitation method in aqueous solution and post heat-treatment at 300-750• C shows high ORR activity in alkaline solution. Afterwards, several researches aimed at application of Pb 2 Ru 2 O 7-δ to electrochemical devices such as fuel cell, metal-air battery and oxygen sensor have been conducted. 20,21,[27][28][29][30][31][32][33][34] For example, Goodenough et al. 31,35 have reported that Pb 2 Ru 2 O 7-δ shows ORR activity and high stability not only in alkaline solution but also in acid solution and proton exchange membrane such as Nafion. Also, we have found that partial substitution of other cations such as Bi, In, or Sb with Pb site in Pb 2 Ru 2 O 7-δ have a positive effect on the ORR activity.13 Nevertheless, the ORR activity of Pb 2 Ru 2 O 7-δ is needed to be drastically enhanced for practical use as the cathode electrocatalyst of PEMFC. For this purpose, it ...

A cobalt-iron alloy thin-film electrode-based electrochemical hydrogen-phosphate-ion sensor was prepared by electrodepositing on an Au-coated Al₂O₃ substrate from an aqueous solution of metal-salts. The use of a cobalt-iron alloy electrode greatly improved the hydrogen-ion sensor response performance, <i>i.e.</i>, the sensor worked stably for more than 7 weeks and showed a quick response time of several seconds. Among the cobalt and iron alloy systems tested, the electrodeposited Co₅₈Fe₄₂ thin-film electrode showed the best EMF response characteristics, <i>i.e.</i>, the sensor exhibited a linear potentiometric response to hydrogen-phosphate ion at the concentration range between 1.0 × 10^–5 and 1.0 × 10^–2 M with the slope of –43 mV/decade at pH 5.0 and at 30℃. A sensing mechanism of the Co-based potentiometric hydrogen-phosphate ion sensor was proposed on the basis of results of instrumental analysis

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Detecting exact amounts of hydrogen-phosphate ion, one of the main causes of eutrophication of closed natural system, has become important for the protection of the global environment. The importance of a hydrogen-phosphate ion sensor is also increasing in the fields of electroplating of metals, dyeing processes, food additives, and bio-related processes. For the practical monitoring of hydrogenphosphate ion concentration, chemical analysis based on analytical instruments such as UV absorption or chemiluminescence is commonly used. The method has good accuracy; however, the system is still complicated and not suited for an application in feedback control. So far, many kinds of compact hydrogen-phosphate ion-sensors, such as ion-selective electrodes, 1-8 bio-related systems, 9 hydroxyapatite-based electrodes, 10 and electrochromic devices 11-13 have been investigated. However, they have some problems with respect to selectivity, stability, and sensing conditions for practical applications.Some perovskite-type oxides exhibit remarkable activities as the electrocatalyst for oxygen electrodes 14-16 or H 2 O 2 sensor. 17 In this study, carbon electrodes loaded with lanthanum cobaltite showed high catalytic properties of anodic reaction including those involving HPO 4 2Ϫ ion and are applicable as amperometric HPO 4 2Ϫ sensors. A high performance, thin film-type sensor element using a perovskitetype oxide prepared by a sol-gel method was also developed.Experimental Preparation of perovskite-type oxide powder.-Perovskite-type oxide powders (La 1Ϫx AЈ x BO 3 : AЈ ϭ Ca, Sr, Ba, Ce, Ag; B ϭ Cr, Mn, Fe, Co, Ni, Co 0.8 Fe 0.2 ; 0 Յ x Յ 0.4) with high specific surface area were prepared by an amorphous malate precursor method. 18 The precursors prepared from malic acid and the nitrates of constituent metal were heated at 650ЊC for 2 h in an ambient atmosphere. X-ray diffraction (XRD) analysis revealed that the oxide powders thus prepared showed well-crystallized and almost single-phase perovskitetype oxides, when the contents of the AЈ site cations of the perovskite-type oxides of La 1Ϫx AЈ x BO 3 systems were at x Յ 0.4 for (AЈ ϭ) Ca, x Յ 0.2 for Sr and Ba, x Յ 0.1 for Ce, or x Յ 0.05 for Ag.Preparation of ion-sensor elements.-Hydrogen-phosphate ion sensor elements were based on poly(tetrafluoroethylene) (PTFE)-bonded carbon-based electrodes (types A, B) or a thin film-based electrode (type C) as shown in Fig. 1. PTFE-bonded electrodes were prepared by a sintered process (type A) or a hot-press process (type B). A sintered process (type A): carbon black (50 wt %, AKZO Ltd., Ketjen black 600DJ), perovskite-type oxide (30 wt %), and PTFE dispersion (PTFE content 20 wt %, Daikin Kogyo Co., Ltd., Polyflon D-1) were kneaded into a paste with water. The paste was painted on a Ni mesh (100 mesh, a current collector) to form an electrode, 1 cm 2 in area and ca. 0.5 mm thick. The electrode thus obtained was dried at ca. 100ЊC for 6 h and finally sintered at 300ЊC for 60 min in air. A hot-pressed process (type B): a mixed powder of per...