Publication date: 10th April 2024
Oxygen exchange kinetics is a critical parameter for the improvement and understanding of fuel cells and solid oxide electrolysers. To enhance those kinetics, electrode materials require good ionic and electronic transport properties. However, other parameters can also influence these kinetics, such as the redox properties of the transition metal that make up the electrode material. Indeed, transition metals are directly involved in the charge transfer reaction that turns gaseous O2 into lattice O2-. However, studying of redox properties is not straightforward, because attempts at modifying the redox properties, for example by doping, will lead to a change in the transport properties. Therefore, it is difficult to separate the influence of redox and transport properties on oxygen reaction kinetics.
This work proposes a new approach to study redox effects during oxygen reaction with an electrode without changing the transport properties of the electrode material. First, a mixed conducting oxide is selected to provide charge transport (here Pr0.1Ce0.9O2-δ)[1], while a transition metal oxide with a spinel type structure is infiltrated at its surface to control the redox properties of the system. Then, the oxygen exchange kinetics of the composite is measured by electrical conductivity relaxation. By varying the spinel composition, it is possible to modify the redox properties without changing the transport properties of the system. The method is applied to the MgAl2-xFexO4 spinel system, in which only Fe is redox active, while Mg and Al act as indirect force to modify the redox properties of the Fe centres while maintaining the spinel structure. Variations of Oxygen exchange kinetics are discussed as a function of composition, site distribution, and oxidation states of the iron in the spinel infiltrate[2] [3]. The analysis of site distribution and oxidation states is performed via Electron Energy Loss Spectroscopy (EELS) and X-ray diffraction.
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