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)).

Experimental study and thermo-hydraulical modelling of a heat and cold storage prototype coupling thermocline and latent heat technologies

Heating and cooling in residential buildings hold a 28% share in the total energy consumption of Europe, out of which 75% of the energy is still generated from fossil fuels, while only 19% comes from renewable energy sources. To increase the share of renewable energy in the near future, the French energy commission has identified 4th generation district heating networks as a plausible option. Key hardware components for next generation smart urban heating networks are heat and cold storages, which allows a shift between production and consumption as necessary.
The prototype that will be studied in this thesis couples in a same component heat and cold storage, in order to obtain significant gains in terms of compactness and cost. Cold storage is based on ice-water phase change transition around finned tubes in charging mode (-6°C), and on direct contact between water and ice in discharging mode (direct contact = water flow through the ice and directly exchange heat without any wall between water and ice). Heat storage is based on thermocline technology with water (60-70°C) as coolant.
The prototype is currently under manufacturing in the framework of a European Project and will be operational at the beginning of the thesis. The objective of the thesis is on the one hand to experimentally characterize the performances of the storage, on the other hand to work on a numerical modelling of the storage. The thermos-hydraulical modeling of the discharge in cold mode, with direct contact between ice and water, is particularly challenging. The study of the addition of Phase Change Material capsules for heat storage (50-60°C), in order to boost energy and power, will also be studied with potential implementation in prototype during the thesis.

Development of a multi-criteria comparison tool for electrochemical stationary storage systems

Use of stationary storage systems is now essential to keep pace with changes in the electricity grid and the growing integration of intermittent renewable energies such as solar and wind power. The choice of a storage solution is based on a number of criteria, including performance, lifetime, environmental impact, safety, regulatory constraints and, of course, economics.
The laboratory possesses comparative data on these different criteria, via experimental studies and feedback on existing systems. In addition, an initial software tool has been developed to assess environmental impact using LCA (Life Cycle Assessment). The aim of this thesis work is to integrate these different components into a broader comparison tool with a multi-criteria approach, targeting specific case studies and a limited number of storage technologies that have reached sufficient maturity for the available data to be reliable.

Hierarchically conducting polymer coated on 3D ALD/Silicon nanostructures for integrated solid and flexible micro-supercapacitors

The objective of this thesis is to develop and high performance and durable all-solid state flexible micro-supercapacitors (micro-SC). These new solid-state micro-generators will operate over a wide temperature range (-50°C to +120°C) and exhibit exceptional lifetime and performance. The ?-SCs proposed in this thesis project are based on i) the elaboration by CVD growth of electrodes composed of silicon nanowires and nanotrees followed by a nanometric deposition of a dielectric and new electronically conductive polymer, ii) the elaboration and characterization of new copolymers based on n-type EDOTs derivativesiii) the synthesis of polymeric solid electrolytes (ESPs) based on poly(siloxane)s and ionogels iv) performance tests of different electrodes and electrolytes in three-electrode system configuration, v) elaboration of nanocomposites based on EDOT-based electronically conductive polymers and silicon nanowires covered with nanometric layers of alumina and HfO2, and v) assembly and tests of devices in rigid and flexible sandwich type configuration

Cooling and quench in HTS winding packs for tokamaks

The magnets used in current tokamak-type fusion machines are made of superconducting cables, and they will necessarily also be superconducting in future tokamaks aiming at generating electricity, copper coils being no longer useable due to the huge level of required electrical power. The use of high temperature superconductor (HTS) materials is envisaged instead of present low temperature superconductor (LTS) materials, because they give access to higher magnetic fields and open the way to more compact and potentially more performing machines. The aim of the thesis will be to propose designs for HTS windings, taking into account their specific features, notably the large operation domain in temperature and the phenomenology of incidental transition to the resistive state, known as quench, very different from LTS superconductors. Addressing electrical, thermal and thermohydraulic issues, the doctoral project will aim to study different HTS designs (stacked or twisted tapes), develop the associated simulation tools, design and carry out experiments to characterize the thermophysical properties on a suitable HTS sample, and use the obtained results to propose one or more HTS winding designs.

Understanding flamability of Li-ion thermal runaway vent gases

The objective of the thesis is to characterize the ignition of vent gases resulting from the thermal runaway of lithium-ion cells. During the venting phase, the cell emits electrolyte vapours that mix with the air, and then during thermal runaway, a hot jet of gases and particles is formed that can enrich the mixture and ignite it. The underlying scientific questions concern the evolution of the fundamental characteristics of combustion (flame velocity and auto-ignition delay) with temperature, pressure and composition, ignition mechanisms and the impact of the environment on the preponderance of one of the mechanisms.
To answer this question, we will first use an approach based on the identification of a reaction kinetic model using existing results in the literature and complementary tests to be performed in the thesis at the cell level. Then, we will experimentally characterize the conditions of ignition of mixtures by a hot jet of gas and particles in a shock tube coupled to a combustion chamber. Finally, the mapping obtained on the basis of the knowledge acquired on the mechanisms and conditions of inflammation will be reinterpreted.

Numerical and experimental studies of an ejector designed for a cold or heat production cycle

The ejector has been the research subject in the literature as the main component of refrigeration cycles using “thermal compression” thanks to its simplicity without moving parts. It uses a high-pressure fluid called “primary fluid” to drive and compress a low-pressure fluid, which is called “secondary fluid”. The performance of the ejector is defined by the entrainment ratio, which is the mass-flow ratio between the secondary and primary flows; as well as the critical pressure, which limits the operating range of the ejector. Most of the numerical and experimental studies have been conducted on water vapor ejectors. The studies showed that the geometry optimization is crucial in order improve the ejector performance. Moreover, experiments showed that the flow inside an ejector is often supersonic and highly compressible therefore inducing strong pressure variation. This can induce strong temperature variations and the apparition of liquid water and ice in ejectors have already been witnessed.

Numerical studies carried out previously have shown the importance of accurately modeling the liquid-vapor phase changes in order to establish consistent and accurate numerical models for flows hydrodynamics within the ejector. However, these studies give little or no consideration to the temperature field distribution within the ejector. The main difficulty here are the huge pressure variations that happen inside the ejector which lead to liquid vapor phase changes in a highly compressible flow. In this PhD project, we aim to investigate innovative solutions with ejector integrated into thermodynamic cycles working with natural fluids (ammonia, water, CO2 …) in order to improve the global performances. For this, it is important to understand the local physical phenomena of the flows inside an ejector, especially the impact of liquid-vapor phase change as well as the impact of the operating conditions.

Based on the strong research background of both CEA and INSA Lyon, we will conduct numerical and experimental works about the ejector and the thermodynamic cycles with the following research plan:
* Numerical work:
_ Development of a 1D model and perform the CFD simulations for comparison
_ Modelling and simulations of the identified thermodynamic cycles integrated the appropriate ejector
_ Design of ejector for tests
*Experimental work : fabrication of test ejector and perform measurements for model validation and analysis

For more than 15 years, CEA has conducted extensive research on thermodynamic cycles in order to develop innovative solutions for production of heat, cold and electricity. Recently, we have developed a new model of ejector for integration into a thermodynamic cycle . To bring new insight about the local phenomena of the flows inside an ejector considering the liquid-vapor phase, we have investigated and performed CFD simulations. INSA Lyon has strong research background on the topics related to CO2 such as heat pump cycles, heat exchangers as well as ejector. The test bench of ejector at INSA Lyon together with the INES platform at CEA will be served for the experimental work of this project.

Hydrogen explosions within geometrically-tailored porous media : fluid-solid coupling and safety challenges


Hydrogen is a key asset for the energy transition, but it still poses major scientific and safety challenges. Colorless and odorless, hydrogen leaks easily, ignites at low concentrations and temperatures, and can lead to the propagation of rapid deflagrations as well as detonations, a dangerous type of supersonic combustion. Understanding the mechanisms involved in the transition from deflagration (slow flame) to detonation (supersonic flame accompanied by a shock wave) is therefore vital to the safety of hydrogen production facilities (electrolyzers) and the nuclear industry. In the accidental scenario of loss of cooling and core meltdown, oxidation of uranium rod cladding can lead to the release of hydrogen. It was the subsequent explosion that led to the loss of containment and release of radioactive material at Fukushima and Three Mile Island. Hydrogen risk management is therefore one of the major challenges for nuclear safety.

The main mechanism behind the deflagration -> detonation transition is the presence of obstacles along the flame path. These generate vorticity, which increases the surface area of the flame and accelerates the reactive wave. When obstacles are in sufficient number and proportion, a runaway effect and wave reflections can lead to a shock-chemical reaction coupling: detonation is born, propagating at several kilometers per second. Unfortunately, it's impossible to avoid the fact that industrial plants are cluttered with obstacles: pipes, buildings, machines, walkways, structures... and present this type of scenario.

Conversely, a very densely-packed, porous environment can, on the contrary, smother a too-rapid flame and allow the reverse transition from detonation to deflagration, which is less dangerous in nature. For example, detonation can be attenuated by passage through a porous matrix, or when a porous medium is placed along the walls during propagation in a tube. A crucial safety question then arises: under what circumstances does an obstacle accelerate or slow down a flame? Can porous media be designed to stop dangerous flames?


The aim of this thesis is to approach this question from three angles:

1/ on the one hand, via the preparation and execution of experimental tests on the CEA Saclay hydrogen explosion test bench (SSEXHY). These include:
- exploring different geometries for porous media, based on parameterizable topologies. These porous matrices will then be 3D printed via metal additive manufacturing;
- prepare instrumentation for the SSEXHY explosion test bench, featuring a visualization section using a Schlieren technique coupled with an ultra-fast camera capable of several million images per second;
- post-process the results of the shock and pressure sensors and the OH*-filtered photomultipliers.

2/ secondly, via numerical simulations of the DNS or LES type on research calculation codes. For example, we might be interested in :
- the influence of porous obstacle geometry (shape, porosity, hydraulic diameter, etc.) on flame propagation speed and deflagrationdetonation transitions;
- the influence of the 2/3D character of porous materials;
- the proposal of new criteria for selecting mesh refinement levels to capture the phenomena of interest.

3/ Finally, theoretical modelling of the problem from the point of view of volume-averaged equations will be carried out, with the aim of developing simplified, predictive models of the behavior of porous flame arresters.

Influence of the chemical composition of lamellar materials on the safety of Li-Ion batteries

The thesis focuses on the development of secure and high-performance lithium-ion batteries. The first part of the research will concentrate on understanding the phase change mechanisms of the NMC cathode, examining parameters such as lithiation rate, temperature, and crystal structure.

The thesis will stand out by evaluating various strategies, including synthesis, crystal morphology, coating, and doping, to identify the most promising approach. The advantages of this approach lie in access to sophisticated characterization tools such as the ESRF, enabling a detailed analysis, and the ability to test new materials in real conditions through the creation of high-capacity cells. By leveraging these strengths, the thesis aims to significantly contribute to the development of lithium-ion batteries that combine high performance with increased safety.

Multi-block and non-conformal domain decomposition, applied to the 'exact' boundary coupling of the SIMMER-V thermohydraulics code

This thesis is part of the research required for the sustainable use of nuclear energy in a decarbonized, climate-friendly energy mix. Sodium-cooled 4th generation reactors are therefore candidates of great interest for saving uranium resources and minimizing the volume of final waste.

In the context of the safety of such reactors, it is important to be able to precisely describe the consequences of possible core degradation. A collaboration with its Japanese counterpart JAEA allows the CEA to develop the SIMMER-V code dedicated to simulating core degradation. The code calculates sodium thermohydraulics, structural degradation and core neutronics during the accident phase. The objective is to be able to represent not only the core but also its direct environment (primary circuit) with precision. Taking this topology into account requires partitioning the domain and using a boundary coupling method. The limitation of this approach generally lies in the quality and robustness of the coupling method, particularly during fast transients during which pressure and density waves cross boundaries.

A coupling method was initiated (Annals of Nuclear Energy 2022, Implementation of multi-domains in SIMMER-V thermohydraulic code https://doi.org/10.1016/j.anucene.2022.109338) at LMAG, which consists of merging the different decompositions of each of the domains, with the aim of constituting a unique decomposition of the overall calculation. This method was developed in a simplified framework where the (Cartesian) meshes connect in a conformal manner at the boundary level. The opportunity that opens up is to extend this method to non-conform meshes by using the MEDCoupling library. This first step, the feasibility of which has been established, will make it possible to assemble components to constitute a 'loop' type system. The second step will consist of extending the method so that one computational domain can be completely nested within another. This nesting will then make it possible to constitute a domain by juxtaposition or by nesting with non-conforming domain meshes and decompositions. After verifying the numerical qualities of the method, the last application step will consist of building a simulation of the degradation of a core immersed in its primary tank ('pool' configuration) allowing the method followed to be validated.

This job will enable the student to develop knowledge in numerical techniques and modeling for complex physical systems with flows. He or she will apply techniques ranging from method design to validation, as part of a dynamic, multidisciplinary team at CEA Cadarache.