Limiting the scale and effects of global warming requires our societies to drastically reduce our energy consumption and electrify our production in order to decarbonise it. However, the development of solar and wind energy is hindered by their main drawback: their intermittency. Seasonal energy storage in the form of hydrogen is a promising solution to overcome this limitation. EPISTORE is a European research project aimed at developing a reversible, all thin-film, relatively low-temperature (< 500°C) solid oxide cell (SOC). These devices are particularly relevant for power-to-gas and gas-to-power conversion.

This thesis, within the framework of EPISTORE, funded by the European Union’s Horizon 2020 research and innovation program, which aims to develop thin layers of yttria-stabilised zirconia (YSZ) to serve as electrolytes for these SOCs. YSZ containing 8 mol% yttria (8YSZ) is a material already widely used as an electrolyte in ceramic electrochemical devices. Its properties enable it to efficiently transport oxygen ions while forming an electronic barrier. The study of the thin YSZ layers produced during this thesis should make it possible to assess the relevance of reducing the amount of yttria in the material. Indeed, published work on the subject suggests that a concentration of 3 mol% or 4 mol% yttria could improve the mechanical properties, which are vital for thin-film devices.

This work details the optimisation of the synthesis of these films by metal organic chemical vapour deposition (MOCVD) and then the advanced characterisation of these samples, not only structurally and morphologically (XRD, TEM, SEM, Raman, etc.) but also in terms of their electrical properties via electrochemical impedance spectroscopy (EIS) and mechanical properties via nanoindentation and bulge tests. A new technique for direct observation of ion diffusion involving isotopic tracers and Raman spectroscopy has also been adapted to this material for the first time.

The results obtained suggest that the use of 3YSZ and 4YSZ at low temperatures (< 500 °C) allows electrochemical performances equivalent or superior to 8YSZ to be obtained. However, a positive effect of this change in concentration on the mechanical properties of the films has not been demonstrated
 

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