Medium temperature PEMFC: impact of the drying processes of catalyst layers on their microstructure and performance

- The Proton Exchange Membrane Fuel Cell (PEMFC, using H2 and air as fuels) is a relevant solution for the production of low-carbon electrical energy. However, it is necessary to further improve its performance and durability, and reduce its cost.
- In this spirit, the national French project PEMFC95 aims at developing and characterizing PEMFC materials able to operate sustainably at 95°C (standard is 80°C) and thus more suitable and attractive for Heavy-Duty application (buses, trucks, trains…). It is supported by the French ‘Programme et Equipements Prioritaires de Recherche sur l’hydrogène décarboné’ (PEPR-H2).
- The component considered in this thesis is the catalyst layer (CL) which is a mixture of Pt/C (platinum onto carbon particles), H+ conductive ionomer, and solvents. The optimization of the CL in terms of spatial distribution of Pt/C, ionomer and pores is crucial for improving performance and durability. This is directly linked to the ink formulation and to the manufacturing process used to produce the CL. Nevertheless, the relation between the CL manufacturing process and parameters, its structure and components’ distribution, and the performance and durability of the PEMFC, is still an open question. The aim of your Ph.D. thesis is to progress on this, focusing on the drying step of the bar coater manufacturing process.
- You will contribute to the PEMFC95 project thanks to your scientific/technological developments to understand the impact of the drying process and parameters on the microstructure of CL and make the link with the performance and durability of PEMFC.
- You will have interactions and meetings with the partners of the project and with CNRS/IMFT (Toulouse), specialist of transport phenomenon in porous media.
You will be hired by CEA-Grenoble and work with permanent and non-permanent staff in the laboratory, (male and female) engineers and technicians, to discuss your ideas, perform your experiments and analyze the data. You will be managed by Joël Pauchet as your thesis director, specialist of porous media and their modeling for PEMFC, and Christine Nayoze-Coynel for her knowledge on the CL and MEA manufacturing.

More information are accessible in the attached file and/or Under request.

Development of thin film negative electrodes for Li-free all-solid-state batteries

The aim of this work is to develop 'Li-free' negative electrodes for new generations of high energy density all-solid-state lithium batteries. The function of this type of electrode is to provide a significant gain in energy density in the battery, to facilitate its manufacture by eliminating the need to handle lithium metal and, most importantly, to enable the formation of a homogeneous, dendrite-free lithium film when the battery is charged.
These electrodes will be based on the functionalisation of a metal collector with thin-film materials comprising at least one lithiophilic material (typically a compound that can be alloyed with lithium) and an inorganic ionic conductor. These electrodes are prepared by physical vacuum deposition processes such as sputtering or thermal evaporation. It will therefore be necessary to study the influence of the composition and structure of the lithiophilic layer on the nucleation and growth mechanism of the lithium film and on the evolution of the electrode during charge/discharge cycles. The role of chemical/mechanical interactions with the ionic conducting layer will also be investigated.
This work, which is part of a national CEA/CNRS joint project, will be carried out at the CEA Tech site in Pessac, which has a full range of vacuum deposition and thin film characterisation equipment, in close collaboration with ICMCB CNRS in Bordeaux. It will benefit from the many characterisation resources (confocal optical microscopy, SEM/cryo FIB, ToF-SIMS, SS-NMR, µ-XRD, AFM,...) available in the various partner laboratories involved in the project.

Hybrid Generic EMC Filter

In the field of embedded applications, power converter specifications are crucial. They must not only be efficient and compact, but also meet strict electromagnetic compatibility (EMC) standards. Understand that these converters may be susceptible to their own interference (autoimmunity), cause or experience disturbances in their environment, primarily from common mode currents.
Even low-power power converters can generate high-frequency electromagnetic emissions, which can interfere with other nearby equipment or even disrupt radio signals.
Traditionally, to meet EMC requirements, we rely on shielding and passive filtering techniques, which add significant weight, volume and cost to the system. Around 20% of these costs and constraints are attributed to passive EMC filters.
We note the arrival of new converters (based on large gap SiC/GaN components) whose switching frequencies approach, or even encroach on, the frequency ranges of EMC standards. In order to counter this problem, a new alternative is emerging: active EMC filters. The latter offer at least similar performance while considerably reducing bulk and weight.
As part of this thesis, we will explore these active CEM filters through different stages. We will start with a state of the art, followed by the estimation of common mode and differential mode noise of switching components. Then, we will simulate and compare the most relevant solutions, whether active or passive. We will also get hands-on by performing electromagnetic compatibility tests on common filters and converters.
Finally, we will design and test a prototype active filter for a specific converter. To successfully complete this thesis, it is necessary to master both analog and digital electronics, as well as electronic simulation software (LTspice, Pspice or PSIM) and printed circuit design tools (Altium). Additionally, knowledge of embedded programming would be a valuable asset.

Enhancing micro-mobility battery pack lifetime by using proper and cost effective thermal management

Urban mobility options are diversifying, notably with the deployment of micro-mobility vehicles (electric bikes, cargo bikes, scooters, electric carts). Their usage poses increasing challenges for powertrains, especially for batteries. Li-Ion batteries are temperature-sensitive and require effective thermal regulation to maximize their lifespan. This constraint is well-considered in the automotive sector, which often employs onboard cooling systems to cool or warm batteries.

Micro-mobility vehicles often incorporate battery packs with a simple and cost-effective design, featuring minimal components and lacking a thermal management system. If these batteries are used in severe weather conditions or under high electrical loads, their lifespan can be significantly affected, directly impacting their overall environmental footprint. The development of new thermal management solutions tailored to usage patterns and industrial viability will be the central theme of this thesis.

We aim to achieve the following objectives:

- Investigation of typical usage profiles for these mobility devices (current profiles, external thermal constraints, charging and discharging conditions, integration constraints within the vehicle).
- Design and development of innovative thermal management systems.
- Prototyping and characterization of one or more solutions.
- Integration of thermal results into an aging model to analyze the effects on lifespan.

This thesis proposes an approach combining design, simulation/modeling, and prototyping/characterization. The laboratory is equipped with a dedicated platform for battery pack assembly, featuring rapid prototyping tools (3D printing, laser cutting and welding, mechanical workshop), as well as a testing platform enabling advanced characterizations of battery systems.

Development of high-halogen argyrodites for all-solid all-sulfide systems

All-solid-state batteries have been enjoying renewed interest in recent years, as this technology offers the prospect of higher energy densities due to the use of lithium as a negative electrode, as well as increased battery safety compared with Li-ion technology. The use of sulfides as positive electrode materials coupled with argyrodite as solid electrolyte are interesting systems to develop. The argyrodites achieve ionic conductivities close to those of liquid electrolytes. Moreover, the electrochemical stability window of sulfides is close to that of argyrodite, making all-sulfide technology a promising one for the development of all-solid batteries.
In order to improve the conduction properties of argyrodites, recent studies have shown that ionic conductivity is highly dependent on their local structure. Solid-state NMR thus appears to be a promising technique for probing the local environments of the nuclei mentioned, and in particular for quantifying the variety of different local environments favoring an increase in ionic conductivity. Some compositions enriched in halides appear to promote ionic conduction, and the synthesis of corresponding materials and their structure will be studied.
The thesis will focus on two main areas: the study of all-sulfide batteries and the fine characterization of argyrodite with controlled local structures. Halogen-rich argyrodites will be developed and studied to determine the influence of different local environments on conduction properties.

Modeling catalyst layer degradation in fuel cells

The lifetime of fuel cells is one of the limiting factors in their large-scale deployment. A good understanding of the mechanisms involved in material degradation is a prerequisite for the development of these solutions, particularly catalyst degradation.
We propose to develop a comprehensive model coupling all the phenomena required to simulate catalyst degradation during cycling at potentials representative of cell use. Studies on the effect of cycling frequency and amplitude will enable us to define a validation experiment.
A first existing degradation model will be validated, then coupled to an oxidation model. We will also study the relevance/necessity of taking other reaction paths into account. The complete model will be implemented in a 2D model capable of simulating cells representative of an operational fuel cell.
The subject is therefore mainly numerical simulation with an experimental component. The thesis will be supervised and accompanied by three experts respectively in electrochemistry, numerical simulation and experiment on fuel cells.

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.

Relationships between surface reactivity, composition and deformation of Silicon-based negative electrodes for sulfide-type solid electrolyte batteries

All-solid-state batteries using sulfide-based electrolytes are among the most extensively studied, in order to improve energy density, safety and fast charging. Although lithium metal was initially the preferred choice for the anode, the difficulties encountered in its implementation and the performance achieved suggest that alternatives should be proposed. Silicon offers an interesting compromise in terms of energy density and lifetime. However, further improvements are still needed. An initial thesis on the subject highlighted the benefits of using silicon nanomaterials in combination with argyrodite L6PS5Cl. This work also enabled us to switch from 0.8 mAh cells made from compacted powders to 16 mAh cells made from coated electrodes, while significantly reducing cycling pressure from over 125 MPa to 1 MPa and improving lifetime (90% capacity retention after 160 cycles). However, several questions remain unanswered. The reactivity between argyrodite and silicon, which depends on the surface chemistry, and the mechanisms that enable coated electrodes to cycle at pressures as low as 1 MPa need to be elucidated
To answer these questions, we propose to use XPS to characterize the interfaces between the electrolyte and various silicon materials during the life of the battery. Secondly, to measure cell deformation during cycling. These characterizations, coupled with standard physico-chemical and electrochemical characterizations, will help to improve cell performance. These improvements will be based on the use of high-performance silicon nanowire/graphite composites synthesized at IRIG for the anode, NMC with a coating for the cathode, and electrode formulation development work. Initial tests with silicon/graphite composites have been conclusive, but the impact of these materials' characteristics on performance remains to be assessed, in particular wire diameter, silicon content, surface chemistry and choice of graphite. The production of coated electrodes, initiated in the thesis of M. Grandjean in collaboration, remains to be developed. In particular, there is a need to increase surface capacity and power performance, and to do this we need to increase the proportion of active material and evaluate different types of carbon for the electrical conductive network.
This work will help maintain CEA's momentum on the subject and propose a solution for generation 4a batteries, which could succeed current batteries thanks to a better understanding of operating and degradation mechanisms.

Development of catalysts based on sustainable and non-critical materials for AEMWE

Hydrogen production by electrolysis is the only process that enables hydrogen to be produced without carbon by-products. Electrolysis using anionic membranes is attracting increasing attention, as this technology makes it possible to consider electrodes without noble metals and non-fluorinated membranes.
At the anode, the kinetics of the OER reaction are the most limiting. It occurs under highly basic, high-potential conditions. Carbon is therefore not recommended as a support, as it is susceptible to oxidative degradation at the high potentials applied to the anode, or to nucleophilic OH ions in alkaline media.
The synthesis of non-noble catalysts on conductive supports such as fibers or foams would increase the electrical conductivity of the catalyst as well as the anchoring of the active site in order to increase the electronic active site/support interaction and the durability of the electrode.
On the cathode side, although HER catalysis is faster than that of OER, it remains a major obstacle to electrolysis reactions in alkaline media. Indeed, the overpotential of non-noble materials is on average 100 mV higher than that of platinum. However, our experience suggests that molybdenum-based catalysts hold great promise for the development of PGM-free catalysts. In order to optimize these catalysts, we plan to improve electrical conductivity by using carbonaceous supports and to work on the shape structure of these catalysts to improve HER kinetics.
The aim of this project is to provide the scientific community with new knowledge on materials that could be as efficient as the noble catalysts usually used in anionic electrolysis. The use of manufacturing and shaping processes proven in the field of PEM fuel cells offers a good chance of success. Another major contribution to AEM electrolysis will be the exploration of electrode material degradation mechanisms, about which very little is known at present.

Synthesis and electrochemical characterization of p-type organic electrode materials for anion-ion battery

Nowadays Li-ion batteries use mainly inorganic compounds as electrode materials especially transition metal based ones. Although their performances are satisfying, they present several important drawbacks. Indeed these compounds are expensive and leads to large environmental footprint because they are prepared due to energy-consuming techniques from rare mineral precursors. Moreover, this technology is based on the use of lithium leading to geostrategic issues.

Some organic redox compounds such as viologen based derivatives can reversibly react with anions. Consequently they appear as an interesting alternative to conventional active materials especially for negative electrode for battery in anion-ion configuration which not use metallic counter ions. Interestingly these organic molecules can be easily prepared using simple organic chemistry techniques from low cost precursors. However their redox potential is too high (~2-2.5V vs Li+/Li) for the development of high energy density batteries.

The work of this thesis will firstly focus on the synthesis of new insoluble structure based on viologen derivatives presenting a redox potential below 2V vs Li+/Li. Some fine characterizations in particular Electron Paramagnetic Resonance (EPR) will be applied in order to better understand their electrochemical mechanisms.

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