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.

Develoment of lithium mediated ammonia electrolyzer

Recent developments in electrochemical ammonia (NH3) synthesis using lithium (Li) metal deposition in THF-based electrolytes in the presence of protic species, reinvigorated the research interest in direct NH3 electrolyzes technology thanks to its surprisingly high performance in terms of synthesis rate and faradaic efficiency. However, the main drawback is poor energy efficiency due to minimum voltage requirements associated to Li metal deposition and H2 oxidation reactions on the opposite electrodes. In this project, we propose to study the nitridation reaction of Li-alloy forming metals that can enable the decrease in electolyzer voltage. This study will be performed using a 3-electrode electrochemical pressure cell and differential scanning calorimetry – thermogravimetric analysis under N2, H2 pressures. The goal here is to couple existing knowledge in chemical looping synthesis of ammonia with electrochemical synthesis. Porous (carbon or steel tissue) electrodes will be developed with nanoparticles of Li-alloy forming metals and their performance will be studied in an electrolyzer. The assumed 3-step reaction mechanism to form NH3 is as follows: Li deposition > nitridation > protonation. This mechanism is already a subject of discussion for pure Li metal which will be further complicated with the use of alloy forming metals. Therefore, we propose an in-depth study using x-ray photoemission spectroscopy. The ultimate objective of the project is to accelerate the direct NH3 electrolysis technology and address the Power-to-X needs of renewable electricity sources.

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.

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

CONTEXT

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?

OBJECTIVES

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.

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.

Elucidation of the Correlation between the Electrochemical Activity of Oxygen Reduction and the Molecular Structure of the Platinum/Ionomer Interface in Proton Exchange Membrane Fuel Cells

This thesis focuses on the Proton Exchange Membrane Fuel Cell (PEMFC), used in the transportation sector to generate electricity and heat from hydrogen and oxygen. Although promising for reducing CO2 emissions through the use of green hydrogen, the PEMFC needs to enhance its performance and durability to compete with combustion engines and batteries. The electrode plays a crucial role, but the molecular complexity of the electrochemical interface between the platinum-based catalyst and the ionomer makes characterization challenging. Currently, the qualitative understanding of this interface is limited, impeding progress and model predictability. The thesis aims to establish a correlation between the molecular structure of the electrochemical interface and the electrochemical kinetics, focusing on platinum oxidation and ionomer adsorption. A unique device developed at CEA allows simultaneous electrochemical and spectroscopic characterizations. The novelty lies in using Atomic Force Microscopy (AFM) coupled with Raman spectroscopy and synchrotron-based micro-infrared spectroscopy as original techniques to obtain crucial information for PEMFC applications.

Glass gaskets sealing characterization and modeling for High-temperature steam electrolysis technologies

Carbon free hydrogen production is a key challenge for the energy mix of the future. One of the technologies identified is based on high-temperature steam electrolysis (HTE). The operating conditions of this process require the development of specific glass gaskets to seal the electrolysis cells. The technical issues with these gaskets are directly related to the loss of seal occurring because of interface adhesion problems or material cracking during HTE thermal cycling.

The objective of this PhD work is to study the sealing performance of the glass gasket. Firstly, leakage tests will be carry out to discriminate the origin of seal loss according to the selected glasses. Then, mechanical characterization of the glass at high temperature will be performed in order to build the constitutive equation of the material. The overall PhD work will establish a link between the physico-chemical properties of glass and its mechanical and sealing properties. The results of the experimental tests and modeling will issue recommendations on the glass gasket to ensure the proper electrolyzer operation at industrial scale.

The thesis is part of the development of HTE technologies in sight of an industrial-scale production. The project is based on a close collaboration between GENVIA (CIFRE thesis)and CEA.

Applicant must hold a master’s degree or an engineering degree in material sciences. The student will have to acquire extensive knowledge in mechanic, a first experiment in this field will be highly appreciated. Applicant is expected to show good synthesis and communication skills in order to collaborate with the various teams involved in the project.

The expertise developed in glass mechanics and the experience acquired in the HTE field will be an asset for the future PhD. It is a great opportunity for the student to take advantage of his scientific knowledge to support the energy transition.

New sustainable electrode materials for High Temperature Electrolysis

High temperature electrolysis is considered as the high efficiency technology for hydrogen production with low carbon emissions. The electrolysis reaction occurs in a solid oxide cell (SOC) composed of a dense electrolyte of yttria stabilized zirconia (YSZ), sandwiched between two porous electrodes. The most common hydrogen electrode material is a cermet of Ni and YSZ, and the oxygen electrode is a perovskite La0.6Sr0.4Co0.2Fe0.8O3 (LSCF).
To make the high temperature electrolysis more sustainable to better support the European eco-system towards the achievement of the Sustainable Development Goals and the objectives of the Paris Agreement, there is a critical need to reduce reliance on critical raw materials (CRM).
The objective of the thesis is therefore to limit the use of CRM in the oxygen electrode material. Critical elements such as cobalt will be substituted by new cations on the A and/or B site of the crystal lattice, while maintaining equivalent performance and long-term stability. At the same time, in order to limit losses during synthesis, a part of the work will be carried out on the synthesis process efficiency and on the increase in capacity of the synthesis method.
After a bibliographic study on oxygen electrode materials for high temperature electrolysers, the proposed work will initially be focused on the synthesis by chemical routes as well as on fine characterization of the perovskites. The thermal and chemical compatibility with the other materials constituting the cell will be studied, then this work will lead to the shaping of the materials with the most interesting properties in order to test them electrically and electrochemically. The electrochemical behaviour of the electrodes will be analysed in order to understand the influence of substitutions and to determine the electrochemical performance of the different compositions studied.

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