As part of the France 2030 Plan, hydrogen technologies and fuel cells are currently enjoying a major boom in both industry and research. Among the electrochemical systems being considered, ceramic technologies are particularly promising. Whether Solid Oxide Fuel Cells (SOFC) or Solid Oxide Electrolysis Cells (SOEC), also known as High Temperature Steam Electrolysers (HTSE), their high operating temperatures enable them to achieve high conversion efficiencies (Gas to Power and Power to Gas). What's more, these devices do not use precious metal catalysts such as platinum (Pt) or iridium oxide (IrO2). Although highly efficient in the short term, current cells are not sufficiently durable. In particular, a degradation rate of the order of 0.1%/k hour is targeted in the near future (which can be estimated at an operating life of the order of 10 years).
Although charge transfer and ion transfer properties at the interfaces are very important to ensure good electrochemical cell behavior, material stability is also crucial. At present, the main reasons for premature cell ageing are related to parasitic reactions between the constituent materials and a certain chemical instability of the latter with respect to the gases used. In the case of SOFCs and SOECs based on an O2- conductive electrolyte made of Yttria Stabilized Zirconia (YSZ), a so-called 'barrier' layer is usually interposed between the electrolyte and the oxygen electrode to ensure proper transfer of O2- ions through the cell, but also to prevent diffusion of cations from the electrode and/or the interconnector metal material. In particular, this means avoiding reaction with ions such as La3+, Sr2+, Fe3+, Co3+ (in the case of La1-xSrxFe1-yCoyO3-d type electrodes) or others, or Cr3+, Ni2+ cations in the case of the interconnector metal.
In this context, gadolinium ceria barrier layers - Cerium Gadolinium Oxide (CGO) - are frequently used. This oxide crystallizes in a fluorine structure such as YSZ, which accommodates CGO/YSZ interfaces, and has good oxygen ionic conductivity thanks to the presence of vacancies. What's more, this material slows down the diffusion of cations into the electrolyte. However, the ionic conductivity of Zr1-x-y'-y 'YxM'yM''y 'O2-d mixed phases (where M 'and M'' are the metal cations) is poorly understood. In addition, the structural and microstructural parameters of this interfacial layer remain to be defined in order to optimize this interface and increase cell lifetime: grain size, thickness, porosity, etc.
The aim of this thesis will be to study and develop new barrier layers in order to improve their performance (stability, ionic resistance) and reduce the quantity of critical elements such as Gd. Magnetron sputtering, which enables the production of dense layers significantly thinner than those traditionally obtained by tape casting, will be chosen here as the synthesis process. This study will comprise 4 main components: (i) the synthesis of films by magnetron sputtering, (ii) their in-depth physico-chemical and structural characterization, (iii) the production of interfaces and architectural electrodes and (iv) the study of the influence of the coating on the electrochemical behavior of the oxygen electrode and the evolution of the interfaces over time. This will require the use of various characterization techniques, including SEM/EDS, SEM/FIB, X-ray diffraction, electrochemical impedance spectroscopy (EIS), confocal optical microscopy, ToF-SIMS, Auger nanoprobe.
This work will be carried out as part of the European SustainCell Project, which brings together 10 partners and aims to support European industry in developing the next generation of electrolyzers and fuel cell technologies (low and high temperature) by developing a sustainable European supply chain of materials, components and cells, with significantly lower dependence on critical raw materials (CRMs), a smaller environmental footprint and lower costs, and superior performance and durability to existing technologies. They will be carried out jointly at two laboratories in the Nouvelle Aquitaine region, in Pessac (CEA Tech's Plateforme Batterie and Bordeaux's Institut de Chimie de la Matière Condensée (ICMCB)).