Electrochemical properties of doped ceria electrolyte under reducing atmosphere: Bulk and grain boundary
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Electrochemical properties of doped ceria electrolyte under reducing atmosphere: Bulk and grain boundary Eduardo C. C. Souza
Received: 23 January 2013 / Accepted: 17 April 2013 # Springer Science+Business Media New York 2013
Abstract The electrochemical behavior of a symmetrical cell, Pt/Ce0.8Sm0.2O1.9−δ/Pt, under reducing conditions and wide temperature range (250 – 600 °C) is detailed. In terms of the charge carriers transport through the electrolyte microstructure, AC impedance spectroscopy has been applied to address useful concerns about the transport properties over electrolytic and mixed conduction regimes. The impedance spectra at lower temperature and oxygen partial pressure show the electrochemical response of separated bulk and grain boundary contributions. The increase in the electronic conductivity from 250 to 400 °C shows that the electrochemical reduction Ce4+/Ce3+ is as kinetic as thermodynamically favorable in the experimental conditions. In a typical Nyquist plot of an impedance diagram, until temperatures as low as 400 °C, the high and low frequency arcs can be accessed and the influence of reducing atmosphere over both the components is presented. The apparent activation energy for the electronic process (ΔE) extracted from the total conductivity is 2.54 eV. Distinguished bulk (2.34 eV) and grain boundary (2.63 eV) activation energies point the latter as an energetic barrier in the redox reaction. The oxygen partial pressure dependence of individual capacitances suggests storage of electrical charge along grain boundaries which can potentially behave as a chemical capacitor. Keywords Doped ceria electrolyte . Mixed conduction . Defect chemistry . Impedance spectroscopy
1 Introduction Including cerium oxide related compounds in a short list of the most extensively studied functional ionic materials to a
E. C. C. Souza (*) Energy and Nuclear Research Institute – CCTM, Cidade Universitaria, Av. Prof. Lineu Prestes, 2242, São Paulo, 05508-000, SP, Brazil e-mail: [email protected]
wide range of technological applications, it may lead one to imagine that these conductors may be limited in shedding new lights on the chemical and physical properties. However, especially in the case of components for SOFC, oxygen sensors and catalytic convertors, over the last few years there have been observations of unusual conductivity behavior, enhanced photochemical activity for renewable fuel production and an improvement of the electrochemical oxidation has been also highlighted [1–3]. Although the dynamic properties of cerium based compounds in a fairly high temperature range seem to be of major applicable interest, new findings in advanced electrochemical performance associated to the non trivial size effects may lead those materials to be highly desirable for operation conditions with minor energy required. There is an extensive literature on the transport properties of doped ceria as a function of temperature and oxygen partial pressure [4–9]. Particularly for investigation of the electrical properties in r
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