Electrochemical properties of doped ceria electrolyte under reducing atmosphere: Bulk and grain boundary

Abstract

The electrochemical behavior of a symmetrical cell, Pt/Ce_0.8Sm_0.2O_1.9−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.