Name of Project Leader: Professor SGHAIER Nour

Names of the involved researchers:        

• BEN NASRALLAH ABDI Samia
• KHEMILI FAYCEL
• BEN AMARA Mohamed Amine
• EL OUKABI Houda

Names of PhD students:

• BAHRINI Imen
• FEKIH Oumeima
• LASSOUED Norhene

 1. Summary

The manufacturing process of active and diffusion layers in hydrogen fuel cells (PEMFC) is based on drying and dispersion techniques (referred to as inks). The project is structured around two themes: the first aims to study the functionalization of fibrous substrates for the diffusion layer. Its goal is to identify the drying conditions that allow for the deposition of PTFE and controlled hydrophobicity. The second theme will focus on the drying of inks for catalytic layers and microporous layers. The objective of this part is the fundamental study of the relationships between the properties of the nanoporous layers of PEMFC, the catalytic layer, and the microporous layer, as well as the formulation of inks and drying conditions.

We will develop numerical simulations of the deposition of PTFE particles on the fibres of the GDL substrate during drying. This entire work will constitute a first step toward the numerical generation of a PEMFC single cell and the prediction of its performance based on the composition of the inks.

2. Methodology:

• Investigating PTFE particle deposition on a carbon support, monitoring drying kinetics through weighing, and characterizing final deposition using SEM and AFM.
• Verifying that acquired knowledge leads to effective control over GDL functionalization, including observations of water distribution using X-ray microtomography and capillary pressure measurements.
•  Exploring the relationship between nanoporous layer properties in PEMFCs, catalytic layers, microporous layers, and ink formulation and drying conditions.
• Analyzing solvent evaporation kinetics and dispersions of increasing complexity, with varied parameters such as initial dispersion volume, temperature, relative humidity, and substrate wettability.
• Using various techniques (SEM, X-ray microtomography, optical profilometry, AFM, pycnometry, mercury porosimetry) to analyze the nanoporous layers. The most promising layers will be imaged using FIB-SEM for 3D microstructure analysis.
• Ultimately, the results will connect drying conditions and ink composition with the structural properties of the resulting nanoporous layers.

3. Cooperation

Collaboration with the Institute of Fluid Mechanics in Toulouse, particularly with the GEMP group. 

4. Research results

This study investigates the deposition of PTFE particles by drying a solution within a porous medium, in relation to the hydrophobization of gas diffusion layers to limit flooding caused by water production in the fuel cell. Drop drying experiments on different substrates show that deposition conditions significantly influence the film morphology. With a 60% PTFE mother solution, the nature and roughness of the substrate (glass vs. carbon) affect the shape and frequency of cracks, which are more numerous and non-radial on rough carbon (Fig. IV.1.1). On smooth carbon (Fig. IV.1.2), cracks appear only during the slow drying phase, concentrated in the first 10 minutes, i.e., 1/6 of the total drying time.

Glass                                                      Smooth carbon                            Rough carbon

Figure IV-1-1. Initial drops and final deposits on different substrates using a 60% PTFE mother solution.

Successive immersion/drying cycles of smooth carbon plates in PTFE solutions were carried out. Drying kinetics were monitored at each cycle (Fig. IV-1-2), and SEM observations were performed after each treatment (Fig. IV-1-3). Deposit quantification is ongoing.

In the modeling component, a pore-scale drying code was developed on structured networks. Influencing effects such as capillarity, viscosity, wettability, gravity, porosity, and external conditions are accounted for and controlled.

Figure IV-1-2. Drying kinetics of four treatments, with pure water in blue as reference.

Figure IV-1-3. Evolution of deposited PTFE after successive treatments: (a) First treatment (b) Second treatment (c) Third treatment (d) Fourth treatment

Figure IV-1-4. Invasion patterns in pore networks of size 36/41. Liquid phase in blue, gas phase in white: (a) Hydrophilic medium with porosity ε = 0.8899  (b) Hydrophobic medium with ε = 0.8899 (c) Hydrophilic medium with ε = 0.1367

Name of Project Leader: Professor MHIMID Abdallah

Names of the involved researchers:        

• TOUATI Hazem
• BEN CHIEKH Maher
• MERGHENI Mohamed Ali
• CHEKIB Ghabi
• BOUTOUB Ahmed

Names of PhD students:

• BDIOUI Hadhemi

1. Summary

Injecting green hydrogen into the existing natural gas network presents an opportunity for reducing pollutant emissions since the combustion of hydrogen produces no carbon. However, there are several challenges to overcome before implementing this solution:

• Assess the interchangeability between this hybrid fuel and natural gas in existing installations.
• Determine the thermodynamic and chemical properties of this fuel.
• Prepare a numerical model for simulating combustion chambers using a hydrogen-natural gas mixture. This model must be capable of predicting:
• The impact of adding hydrogen on the flow dynamics within the chamber.
• The efficiency of combustion.
• The combustion products and pollutant emissions.
• An experimental study remains feasible to ensure the reliability of the model.

2. Methodology

This research theme comprises two parts: an experimental section and a numerical section.

Experimental Section: Using the burner designed in the laboratory during previous work dedicated to the study of non-premixed jet flames to analyse the impact of hydrogen injection in a combustion chamber under inert and reactive conditions. The fuel supply circuits will be modified for the use of a hybrid fuel (methane-hydrogen). The oxidizer can be air or oxygen.

The experiment will study the influence of:

• The composition of the mixture
• The geometry of the injector, which has been modified
• The flow rates of the fuel and the oxidizer
• On combustion and flow within the burner.

Numerical Study: The numerical section for the simulation is the parametric study of combustion (hydrogen-methane), which requires:

• The development of a numerical model addressing the chemical kinetics of the hybrid fuel
• The integration of this numerical model into the CFD codes already developed within the laboratory
• Taking into account the influence of the presence of hydrogen on turbulence and thermal transfers in the model.

3. Research results

The addition of hydrogen to natural gas was studied for two types of gas used in Tunisia: Algerian natural gas (ANG) and Miskar natural gas (MNG). This injection results in a decrease in the higher heating value (HHV) and the Wobbe Index (WI), limiting the acceptable proportion of hydrogen without compromising compatibility with existing combustion equipment: up to 26% by volume for ANG and 18% for MNG (Fig. IV-2-1.a-b).

Figure IV-2.1. Wobbe Index (solid line) and Higher Heating Value – HHV (dashed line) as a function of hydrogen content in natural gas blends: (a) ANG, (b) MNG.

An experimental component was dedicated to the rehabilitation and reactivation of the existing combustion facility in the laboratory (Fig. IV-2-2). Several types of burners were tested to ensure safety and verify the feasibility of the planned experimental investigations. In parallel, a numerical study was carried out. A first series of simulations, incorporating detailed chemical mechanisms, made it possible to analyze the effect of hydrogen addition on the fundamental characteristics of natural gas flames. The results show a significant increase in flame speed (Fig. IV-2-3), a broadening of flammability limits, and a modification of emission profiles.

Figure IV-2.2. Methane flame stabilized by oxygen enrichment in a coaxial burner

Figure IV-2-3. Effect of hydrogen addition on flame speed

Name of Project Leader: Professor ASKRI Faouzi

Names of the involved researchers:        

• JEMNI Abdelmajid
• DHAOU Mohamed Houcine
• MELLOULI Sofien
• BOUZGAROU Fatma
• BEN SALAH Yasser
• MAATALLAH Taher
• SAKLY Ahlem

Names of PhD students:

• SAIDI Sirine
• MAYOU Ishak  

​​​​1.Objectives

The objective of this project is to study numerically and experimentally the dynamic behaviour of metal-hydrogen reactors and to optimize their operation for application in energy systems. We also propose to study multiple energy production systems (polygeneration) based on renewable energies and green hydrogen. An environmental and economic impact study of green hydrogen will be conducted. To achieve this, we propose to:

• Conduct numerical and experimental studies on Metal-Hydrogen Reactors (RMHs) to enhance hydrogen storage performance.
• Characterize new materials for hydrogen absorption/desorption.
• Model and optimize multiple electric power generation systems based on renewable energies and green hydrogen.
• Assess the environmental and economic impacts of green hydrogen.

2. Methodology:

The methodology for the implementation of the research program is as follows:

For the theoretical part:

• Numerical simulation of the operation of metal-hydrogen reactors, integrating heat pipes and/or the IEM.
• Numerical modeling of a multiple energy production system based on renewable energies and green hydrogen.

For the experimental part:

• Design and construction of a metal-hydrogen reactor to conduct tests on new materials.
• Use of heat pipes for the heating and/or cooling of metal-hydrogen reactors.

3. Research results:

In applications where the hot water demand peaks during the day, such as residential heating, the collector using a phase change material (PCM) is more suitable. Conversely, the metal hydride (MH)-based collector reaches higher temperatures at night, indicating better thermal storage efficiency. MH systems are therefore particularly recommended for nighttime heating applications (see Fig. IV-3-1).

Figure IV-3-1. Comparison between the two solar collectors as a function of water flow rate.

The electrochemical and thermodynamic modeling of a PEM electrolyzer's performance shows that diffusion overpotential increases with current density (see Fig. IV-3-2), leading to a rise in cell voltage and a higher electricity demand. This increase in overpotentials lowers the electrolyzer's efficiency by generating irreversible heat losses.

Figure IV-3-2. Comparison of thermal energy demand and heat production due to overpotential losses.

The experimental study of hydrogen adsorption by the compound Ni₀.₆Mg₀.₄Fe₂O₄ began with the synthesis of the material, followed by the characterization of its morphological and structural properties using X-ray diffraction and SEM, as well as the determination of adsorption kinetics and isotherms. The numerical study enabled the identification of stereographic parameters (number and density of adsorption sites, energy parameters) and adsorption energies. The results reveal that hydrogen adsorbs onto two types of interstitial sites via an exothermic process (see Fig. IV-3-3).

Figure IV-3-3. Hydrogen adsorption isotherms fitted using a dual-energy monolayer model.