Study of the oxygen reduction reaction : application of impedance spectroscopy to an innovative system derived from Ca3Co4O9+δ. ; Étude de la réaction de réduction de l'oxygène : application de la spectroscopie d'impédance à un système innovant dérivé de Ca3Co4O9+δ.

In the context of energy transition towards carbon neutrality by 2050, Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs) offer real potential for use via hydrogen as an energy carrier. The aim of this thesis is to understand the electrochemical processes in these systems, with a view to improving their performance and durability. The technique chosen is impedance spectroscopy to study the oxygen reduction reaction. This is a complex reaction involving several stages: diffusion of molecular oxygen, dissociation of molecular oxygen at the electrode surface, diffusion of oxygen or partially ionized atoms at the solid surface or their incorporation into the solid, charge transfer, diffusion of ions into the solid, etc. Whereas gaseous diffusion is a slow process, ionic diffusion in solids is rapid. The detailed study of impedance spectra measured on symmetrical cells enables us to define the steps that limit the reaction and identify the directions to take to optimize the systems. This requires the measurement of reliable data. The Kramers-Krönig test is used to check the quality of the data. From these data, it is possible to calculate the distribution function of the relaxation times characteristic of the phenomena involved within the cell, but as the number of data is finite, solving the equation associated with this function is not straightforward. The aim of this thesis was first to define a methodology for the rigorous processing of impedance spectra measured on symmetrical cells consisting of a gadolinium-doped ceria electrolyte on which a model electrode based on Ca3Co4O9+δ, an innovative electrode material studied for several years at UCCS, has been deposited. Unlike state-of-the-art materials, calcium cobaltites have the advantage of not containing rare earths and, above all, of presenting an expansion coefficient of the same order of magnitude as that of the electrolytes used for these applications, giving rise to the hope of increased durability. Initially used as a model ...