Publication date: 10th April 2024
In Solid Oxide Fuel Cells, oxygen electrode polarization related to electrochemical reactions at the gas/solid interface is often the dominant flux limiting mechanism. The most common materials used as oxygen electrodes are metal oxides with mixed ionic and electronic conductivity. Their performance as an oxygen electrode is a function of the kinetics of exchange between lattice oxygen defects and molecular oxygen in the gas phase. As such, there has been tremendous efforts to characterize the rate of this oxygen exchange on mixed conducting oxides, with a wide variety of techniques.
One technique uses electrical conductivity as tool to track the stoichiometry changes within the oxide. With stepwise changes of oxygen pressure in the atmosphere, the stoichiometry goes through a transient state and stabilize to a value that equilibrate the oxygen chemical potential with that of the gas phase. This technique, called electrical conductivity relaxation, is usually performed on highly dense ceramics, and carries information of oxygen surface exchange kinetics as well as bulk ionic diffusion of oxygen defects. The latter can often be predominant in the total conductivity transient, which make the determination of surface exchange kinetics rates less accurate.
In a recent work1, we have introduced a variation of the electrical conductivity relaxation technique, which uses porous ceramics to improve the sensitivity to surface exchange processes. In this presentation, we will present this technique in-depth, and discuss the relevant parameters to consider for accurate measurements of surface exchange coefficients. The interpretation of relaxation profiles measured on porous ceramics requires an accurate description of the microstructure. The procedure proposed in this work features a simple determination of microstructural parameters from 2D images recorded by scanning electron microscopy. Then, those parameters are directly accounted for in the fitting procedure of the relaxation profiles with an adapted analytical model. The accuracy of the procedure is demonstrated on a wide range of mixed conducting oxides and microstructures.
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