Optimization of Interfaces in High-Temperature Fuel Cells (SOFC) and Electrolyzers (SOEC) by Magnetron Sputtering Deposition

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)).

High-throughput experimentation applied to battery materials

High throughput screening, which has been used for many years in the pharmaceutical field, is emerging as an effective method for accelerating materials discovery and as a new tool for elucidating composition-structure-functional property relationships. It is based on the rapid combinatorial synthesis of a large number of samples of different compositions, combined with rapid and automated physico-chemical characterisation using a variety of techniques. It is usefully complemented by appropriate data processing.
Such a methodology, adapted to lithium battery materials, has recently been developed at CEA Tech. It is based, on the one hand, on the combinatorial synthesis of materials synthesised in the form of thin films by magnetron cathode co-sputtering and, on the other hand, on the mapping of the thickness (profilometry), elemental composition (EDS, LIBS), structure (µ-DRX, Raman) and electr(ochim)ical properties of libraries of materials (~100) deposited on a wafer. In the first phase, the main tools were established through the study of Li(Si,P)ON amorphous solid electrolytes for solid state batteries.
The aim of this thesis is to further develop the method so as to enable the study of new classes of battery materials: crystalline electrolytes or glass-ceramics for Li or Na, oxide, sulphides or metal alloys electrode materials. In particular, this will involve taking advantage of our new equipment for mapping physical-chemical properties (X-ray µ-diffraction, Laser-Induced Breakdown Spectroscopy) and establishing a methodology for manufacturing and characterising libraries of thin-film all-solid-state batteries. This tool will be used to establish correlations between process parameters, composition, structure, and electrochemical properties of systems of interest. Part of this work may also involve data processing and programming the characterisation tools.
This work will be carried out in collaboration with researchers from the ICMCB and the CENBG

Combination study of high throughput screening techniques and artificial intelligence (AI) to identify innovative materials for next generation of battery

In recent years, the CEA has set up an experimental high-throughput screening (HTS) activity for lithium battery materials, based on combinatorial synthesis by sputtering and various high-throughput characterisation techniques on large substrates (typically 4 inches). Optimisation of material compositions is traditionally carried out by analysing experimental designs. In the framework of this thesis, we propose to compare the results of this conventional method with the Artificial Intelligence tools developed at CEA-LIST (symbolic AI) and CEA-CTREG (connectionist AI). The objetive is to demonstrate that AI can advantageously replace standard experimental design in order to offer an innovative, high-performance high-throughput screening tool.

Study of co-integrated TeraHertz source arrays in Silicon and III-V photonics technology

TeraHertz (THz) radiation is of growing interest for imaging and spectroscopy in various application fields such as safety, health, environment and industrial control, since in this frequency range many dielectric materials are transparent and many molecules present unique spectral signatures for their identification. However, the limitations of the current sources, required for this active Imaging, hinder its use over long distances or through thick materials.

This thesis proposes to develop a widely tunable THz power source in the form of an array of photoconductor sources excited by photomixing two infrared lasers. The aim is to integrate several dozen or even hundreds of sources on a single component, by co-integrating components made of III-V materials on a silicon photonic substrate, in order to offer an innovative solution to power and tunability problems.

This thesis work, shared between the Bordeaux and Grenoble sites, is positioned in fields with strong industrial potential: integrated photonics and silicon integration technologies. Several items will be addressed , including the study of the architecture of the complete photonic system using simulation tools, the choice of structures and materials, technological development on CEA LETI platforms, and performance characterization. A proof-of-concept with a small number of sources is planned, followed by the design of a large-scale matrix system.

The project represents a major technological challenge, but its success would pave the way for a significant improvement in the penetration capacity of THz radiation, and would also contribute to the broadening of THz application fields.

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