Issue 56 of Science@ifpen - PhD projects and Yves Chauvin Prize

The gradual loss of autonomy of lithium-ion batteries, used in our cell phones and electric vehicles, is notably linked to the formation of a layer known as a Solid Electrolyte Interphase (SEI), which builds up between one of the electrodes and the battery electrolyte. The formation of this SEI layer (Figure 1) was studied theoretically in this thesis using molecular modeling to improve the understanding of its thermodynamic and kinetic aspects. The PhD research initially focused on compiling a database of the principal degradation reactions involved by calculating the reaction and activation energies using DFT. The results used in KMC simulations then revealed that salts such as Li2CO3 and Li2O, derived from electrolyte degradation, play a crucial role. Simulations are also used to predict the loss of battery capacity as a function of the initial composition of the SEI layer (Figure 2) and hint the importance of considering other salts such as LiF resulting from degradation...

To maximize the contribution of hybrid electric vehicles (HEV) to the decarbonization of the transport sector, their energy performance is a key factor that needs to be considered from the design phase. For an HEV, optimizing the design in order to reduce  consumption involves not only optimizing a sizing  aspect but also the EMS. This co-optimization of sizing and energy management is generally tackled either by nesting  optimal control algorithms within an optimization algorithm dedicated to sizing, or by using convex optimization to simultaneously optimize the design levels. However, the former approach is known to induce computational constraints (for example relating to calculation time), while the latter can affect modeling fidelity due to the constraints inherent in convex optimization...

For the last ten years or so, geologists have been fascinated by the natural hydrogen (H2) emitted within “cratons”, in the heart of continents. While its presence in subsurface has been identified in many places around the globe, it remains to be ascertained whether the use of this decarbonized energy source is likely to be economically viable. To answer this question and then identify the drilling sites with the greatest potential, large-scale quantification of this resource requires the use of modelling. (...) This PhD research set about examining the issue, taking as its subject the North American Midcontinent Rift System (MRS) composed of ancient rocks (over 1 billion years old) and extending over thousands of kilometers.

Biomass conversion into chemical products and intermediates is increasingly being adopted to reduce the carbon footprint of the industry concerned. Among biomass-based resources, sugars are extremely attractive since they contain a lot of functional groups enabling their conversion to products of interest (alcohols, acids, etc.). For example, through fructose dehydration, it is possible to obtain 5-hydroxymethylfurfural (5-HMF), a molecule that can be used to produce polymers. Since the conversion of fructose to 5-HMF is catalyzed by Brønsted acids1, liquid acid catalysts in solution are currently used, but without the possibility of recycling this catalyst. The development of a solid acid catalyst is thus crucially important...

In electric vehicle powertrains, it is necessary to convert the DC voltage supplied by the battery into three-phase AC voltages to power the vehicle's electric motor. This conversion is carried out by the vehicle's on-board power electronics, known as an inverter. This inverter is also responsible for controlling the electric motor, varying its speed or torque (...) These voltage drops, be they intrinsic or of external origin, will reduce the performance of the machine in question. Initially, this PhD research made it possible to study and model in detail the physical phenomena behind intrinsic voltage drops in converters....

In the current drive to find ways to produce inexpensive, green hydrogen, the natural hydrogen found underground is attracting growing interest as a component of a new energy mix. In Mali, the Bourakebougou field is the emblematic benchmark for natural underground hydrogen accumulations of this type today: 25 exploratory boreholes have demonstrated its presence in high concentrations (98 mol%), both in a geological reservoir located around one hundred meters below the surface and in other reservoirs at greater depths.  In order to better characterize these reservoirs and the hydrogen retention and preservation processes at play, PhD research was conducted at IFPEN, in partnership with the field operator Hydroma...

Understanding the chemical properties of the supports (alumina-gamma in particular) and active phases of heterogeneous catalysts is a challenge that requires a detailed atomic scale description of systems and the quantification of events that are rare on this scale: chemical reactions. Quantum simulation appears to be a suitable tool for trying to overcome this challenge. However, continuous improvement of numerical methodologies and atomistic models is required to determine the complex structure of active sites, on the one hand, and their reactivity (rate constant), on the other. By covering all these aspects, this PhD research has provided answers to this dual challenge, while exploring the contributions of machine learning (ML) to the understanding of the active sites...

Coastal systems such as deltas are vulnerable to sea-level rise and erosion. The stability of deltas depends on the sedimentary input produced in the watershed, which is strongly impacted by climatic and anthropogenic factors, whose variations and increasing pressure lead to problems of resource management (aquifers) and land-use planning (bank instability, flood management, etc.). In order to predict the impact of different environmental change scenarios and thus enable the implementation of appropriate local policies, it is essential to have modeling tools capable of integrating the various hydroclimatic and anthropogenic parameters and their temporal evolution...

In urban zones, road traffic is responsible for a high proportion of pollutant emissions, with a significant impact on air quality, which represents a major public health issue. Atmospheric dispersion can be comprehensively taken into account using CFD. However, random uncertainties, of both natural and anthropogenic origin, can affect its predictive capacity. Concerning this prediction, the PhD research was aimed at quantitatively evaluating the impact of uncertain parameters: firstly, meteorological and, secondly, those related to road traffic. To do this, “high-fidelity” simulations of pollution dispersion on a micro-urban scale (neighborhood) were conducted...

The photocatalytic reduction of CO2 to “C1/C2 solar fuels” is an attractive conversion reaction since it has the advantage of eliminating this greenhouse gas while providing a usable product. However, despite extensive research efforts, the level of CO2 conversion remains very low, even for the most active catalysts on the market, and the reaction mechanisms involved is still poorly understood. In order to remedy this, this PhD research focused on the study of a model Pt/TiO2-type photocatalytic system. For the purpose of the study, an operando methodology was deployed combining several techniques...

Injectivity in geological formations is a well documented factor in fields related to new energies and the climate (geothermal energy, underground CO2 storage, etc.). The clogging of geological formations, a recurrent problem causing injectivity losses, is associated with the fact that reinjected water frequently carries a high concentration of suspended organic and mineral elements, in the form of colloidal particles. To attempt to overcome this clogging, or at the very least minimize it, it is important to have a clear understanding of the mechanisms at work. Such was the aim of this PhD research conducted using a microfluidics approach (Figure 1) combining two visualization techniques: optical imaging and laser-induced fluorescence.