Standard Gibbs energy of formation of Pb 2 Ru 2 O 6.5
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Y. Waseda Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan (Received 29 May 2006; accepted 24 August 2006)
Lead ruthenate is used as a bifunctional electrocatalyst for both oxygen evolution and reduction and as a conducting component in thick-film resistors. It also has potential applications in supercapacitors and solid oxide fuel cells. However, thermodynamic properties of the compound have not been reported in the literature. The standard Gibbs energy of formation has now been determined in the temperature range from 873 to 1123 K using a solid-state cell incorporating yttria-stabilized zirconia (YSZ) as the electrolyte, a mixture of PbO + Pb2Ru2O6.5 + Ru as the measuring electrode, and Ru + RuO2 as the reference. The design of the measuring electrode is based on a study of phase relations in the ternary system Pb–Ru–O at 1123 K. For the reaction, 2PbO 共ortho兲 + 2RuO2 共rut兲 Ⳮ 1⁄4O2共g兲 = Pb2Ru2O6.5共pyro兲 ⌬Gro Ⲑ J mol−1 共Ⳳ524兲 = −80224 + 75.763共T Ⲑ K兲 − 5.873共T Ⲑ K兲ln共T Ⲑ K兲 , the standard enthalpy of formation and standard entropy at 298.15 K are estimated from the high-temperature measurements. An oxygen potential diagram for the system Pb–Ru–O is composed based on data obtained in this study and auxiliary information from the literature.
I. INTRODUCTION
Lead ruthenate, Pb 2 Ru 2 O 6.5 , with cubic defectpyrochlore structure is used as a conducting component in thick film resistors because of its high electronic conductivity.1,2 The solid solution Pb2(Ru2–xPbx)O6.5 (0 艋 x 艋 1) is an excellent bifunctional electrocatalyst for both reduction and evolution of oxygen gas.3 Electrochemical capacitors fabricated with Pb2Ru2O6.5 electrodes exhibit high pseudo-capacitance and improved specific energy, and are cost effective compared to pure precious metal oxide electrodes such as RuO2.4 Lead ruthenate has been investigated as a cathode material in solid oxide fuel cells.5,6 The endurance and pulse switching behavior of lead zirconate titanate (PZT) films in ferroelectric capacitors can be improved with the use of conductive oxide electrodes like RuO2 instead of Pt.7 However, the high leakage current observed in some cases with RuO2 electrodes was attributed to the formation of conducting pyrochlores.8 RuO2 reacts with high-Zr PZT films to form Pb2Ru2O6.5.9 A temperature–composition (T-X) phase diagram for the system PbO–RuO2 in air was composed by Shaplygin a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0171 1174 J. Mater. Res., Vol. 22, No. 5, May 2007 http://journals.cambridge.org Downloaded: 14 Mar 2015
et al.10 based on x-ray diffraction (XRD) analysis of quenched samples. They identified one intermediate compound at equimolar composition with lattice parameters a ⳱ 1.0257 (±0.0005) nm. The composition of this was designated Pb2Ru2O6 based on the earlier report of Longo et al.,11 who synthesized an oxygen-deficient cubic pyrochlore Pb2Ru2O7-x with x ≈ 1. On the basis of differential thermal analysis (DT
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