Le dihydrogène se présente comme un vecteur énergétique d'avenir pour la diversification des sources de production d'énergie. L'électrolyse de l'eau dans le système PEMWE (Proton Exchange Membrane Water Electrolyzer) permet l'obtention de dihydrogène de grande pureté. Les atouts de cette technologie induite par l'utilisation d'assemblage membrane électrode (AME) permettent son couplage aux énergies renouvelables. Toutefois, l'amélioration de l'activité catalytique des matériaux anodiques et leur stabilité pour baisser la tension de cellule et la diminution de la teneur en métaux nobles dans la composition des matériaux sont nécessaires.Lors de ces travaux de thèse, une voie de synthèse a été élaborée pour préparer des nanomatériaux à base de ruthénium. L'ajout d'iridium a permis dans un premier temps de prévenir l'oxyde de ruthénium de la dissolution tout en maintenant l'activité du catalyseur initial. Les meilleures performances catalytiques des AMEs en termes de densité de courant, de tension de cellule et de durabilité ont été délivrées avec les matériaux anodiques dont la composition molaire en Ru est supérieure à 70 %. La substitution partielle des métaux précieux (Ru et Ir) par du cérium et du niobium dans le but de proposer des catalyseurs à moindre coût a été aussi réalisée. Contrairement au niobium qui apporte une phase amorphe dans la structure du matériau, le cérium jusqu'à une teneur de 10 % permet de conserver les performances de l'anode telles que obtenues dans le matériau bimétallique. Le cérium se présente donc comme un métal prometteur à intégrer de manière appropriée dans la composition des matériaux anodiques. / Hydrogen seems to be the most promising energetic vector in order to diversify the sources of energy production. Water splitting in a Proton Exchange Membrane Water Electrolyzer (PEMWE) provides a sustainable way of producing clean hydrogen. One of the main advantages of this technology based on the utilization of Membrane Electrode Assembling (MEA) is its potential coupling with renewable energy sources. However, the improvement of the catalytic activity of anode materials, their stability and the reduction of the noble metal content in their composition are required.During this thesis work, a new synthesis approach that consists in hydrolysis of metallic precursors in ethanol medium has been undertaken to prepare non-supported ruthenium-based nanomaterials. The addition of iridium to the nanocatalysts composition prevents the ruthenium oxide from dissolution without decreasing the initial activity of the anode catalyst. The best catalytic performance of MEAs in terms of current density, cell voltage and durability were observed with anode materials whose ruthenium molar composition is higher than 70 %. The partial substitution of precious metals (Ru and Ir) either by cerium or niobium with the purpose of decreasing the catalysts cost was also attempted. While the substitution with niobium introduces an amorphous phase in the material structure, the trimetallic materials containing cerium were shown to be crystalline. Furthermore, cerium contents up to 10 % allows maintaining the catalytic activity of the trimetallic anode close to that obtained with the bimetallic oxide material. Thus cerium appears as a promising metal to include in a suitable way on the composition of anode materials.

The electrocatalytic water decomposition has been investigated in this thesis by means of its two half standard reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These reactions occur in different locations in a typical electrochemical cell: the anode and the cathode, respectively. Motivated by the lack of understanding about the reaction mechanisms occurring at the anodes and cathodes, we have proposed first: novel representations of typical TiO2 surfaces, based on small cluster systems, which can be used for a quick and more detailed assessment of the OER activities at modified TiO2 surfaces, and secondly we investigated the HER in two sets of model surfaces which represent recently synthesized materials, based on Mo2C and graphene with promising activities toward the HER. We have employed Density Functional Theory (DFT) based methods within both localized and extended basis sets, as implemented in GAMESS and VASP packages, respectively, to examine the structural, electronic and vibrational properties of the proposed models. We propose new reaction mechanisms for the OER on a number of molecular representations of TiO2 electrodes. For each reaction pathway, the free energy profile is computed, at different biases, from the DFT energies, the entropic and the zero-point energy contributions. The mechanisms explored in this thesis are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on molecular representations of the TiO2 surfaces. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. We subsequently investigate the influence of doping TiO2 surfaces with transition metals (TMs) on the performance of TiO2 -based electrodes for the water splitting electrochemical reaction. Two cluster models of the TM-doped active sites which resemble both the TiO2 anatase (001) and rutile (110) surfaces, respectively, are considered for the evaluation of the water decomposition reaction when a Ti is replaced by a TM atom. A set of TMs spanning from Vanadium to Nickel is considered. The late TMs explored here: Fe, Co and Ni are found to reproduce the observed experimental trends for the overpotentials in TiO2-doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. The vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO2-based cluster models. The last part of this work is devoted to the theoretical study of the HER on recently found materials based on the synergistic combination of molybdenum carbide and graphene layers. We propose two major structural models to describe the HER mechanism within the framework of DFT: Mo2C-based clusters adsorbed on carbon nanosheets and the Mo2C (001) surface covered by pure and nitrogen-doped graphene layers. The former system evaluates the influence of Mo2C nanoparticles adsorbed on carbon nanosheets towards the HER. The second one is employed to gain insight about the high HER activity observed in molybdenum carbide anchored on nitrogen-doped porous carbon nanosheets (Mo2C@2D-NPC), recently synthesized. The H-adsorption free energy has been used as a principal descriptor to asses the HER activity at the proposed model active sites. It resembles the value for the best state of the art catalyst for the HER (i.e., platinum at carbon substrate Pt@C) in some of the proposed structural models. Furthermore, a pH-correction is added within a simplified model, to the H-adsorption free energy barrier in every proposed structure. The pH dependence of the H-adsorption free energy barriers allows the assessment of the HER at acidic and alkaline conditions simultaneously. An overall agreement with experimental results is found and further predictions, promoting the development of better HER catalysts, have been done.

The electrocatalytic water decomposition has been investigated in this thesis by means of its two half standard reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These reactions occur in different locations in a typical electrochemical cell: the anode and the cathode, respectively. Motivated by the lack of understanding about the reaction mechanisms occurring at the anodes and cathodes, we have proposed first: novel representations of typical TiO2 surfaces, based on small cluster systems, which can be used for a quick and more detailed assessment of the OER activities at modified TiO2 surfaces, and secondly we investigated the HER in two sets of model surfaces which represent recently synthesized materials, based on Mo2C and graphene with promising activities toward the HER. We have employed Density Functional Theory (DFT) based methods within both localized and extended basis sets, as implemented in GAMESS and VASP packages, respectively, to examine the structural, electronic and vibrational properties of the proposed models. We propose new reaction mechanisms for the OER on a number of molecular representations of TiO2 electrodes. For each reaction pathway, the free energy profile is computed, at different biases, from the DFT energies, the entropic and the zero-point energy contributions. The mechanisms explored in this thesis are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on molecular representations of the TiO2 surfaces. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. We subsequently investigate the influence of doping TiO2 surfaces with transition metals (TMs) on the performance of TiO2 -based electrodes for the water splitting electrochemical reaction. Two cluster models of the TM-doped active sites which resemble both the TiO2 anatase (001) and rutile (110) surfaces, respectively, are considered for the evaluation of the water decomposition reaction when a Ti is replaced by a TM atom. A set of TMs spanning from Vanadium to Nickel is considered. The late TMs explored here: Fe, Co and Ni are found to reproduce the observed experimental trends for the overpotentials in TiO2-doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. The vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO2-based cluster models. The last part of this work is devoted to the theoretical study of the HER on recently found materials based on the synergistic combination of molybdenum carbide and graphene layers. We propose two major structural models to describe the HER mechanism within the framework of DFT: Mo2C-based clusters adsorbed on carbon nanosheets and the Mo2C (001) surface covered by pure and nitrogen-doped graphene layers. The former system evaluates the influence of Mo2C nanoparticles adsorbed on carbon nanosheets towards the HER. The second one is employed to gain insight about the high HER activity observed in molybdenum carbide anchored on nitrogen-doped porous carbon nanosheets (Mo2C@2D-NPC), recently synthesized. The H-adsorption free energy has been used as a principal descriptor to asses the HER activity at the proposed model active sites. It resembles the value for the best state of the art catalyst for the HER (i.e., platinum at carbon substrate Pt@C) in some of the proposed structural models. Furthermore, a pH-correction is added within a simplified model, to the H-adsorption free energy barrier in every proposed structure. The pH dependence of the H-adsorption free energy barriers allows the assessment of the HER at acidic and alkaline conditions simultaneously. An overall agreement with experimental results is found and further predictions, promoting the development of better HER catalysts, have been done.

The main focus of this master thesis is the investigation of the anode catalysts for the Proton Exchange Membrane Water Electrolyzers (PEM-WEs). PEM-WEs play a pivotal role in the hydrogen economy concept as they allow water decomposition into oxygen and hydrogen. However, their operation requires expensive noble metal catalysts, i.e., iridium or platinum. This issue has yet to be solved to mass-produce PEM-WEs. Consequently, our main objective is to reduce the amount of iridium on the anode of PEM-WEs. We addressed this objective by two distinct approaches: morphological and chemical. With the morphological approach, plasma etching of the membrane and the magnetron sput- tering of CeO2 served to increase the membrane's active surface. Hence we improved the catalysts utilization. With the chemical approach, we focused on the catalyst itself. Thus, we replaced the pure iridium catalyst with a bimetallic iridium-ruthenium one. Therefore, the activity of the catalyst was enhanced while its price got reduced. To ex- plain and describe the catalyst's behavior, we used various electrochemical methods and surface analysis techniques. Finally, we combined both approaches to obtain one active, stable, and low-iridium-loading anode catalyst for PEM-WE.

No description available.

No description available.

Le dihydrogène (H2) se présente comme le futur vecteur énergétique pour une économie basée sur des ressources propres et respectueuses de l'environnement. Il est le combustible idéal de la pile à combustible régénérative constituée de deux entités : un électrolyseur pour sa production, et une pile à combustible pour sa conversion directe en énergie électrique. Ce système présente l'avantage d'être compact et autonome. Cependant, l'amélioration de l'activité catalytique des matériaux, leur stabilité et l'élimination de métaux nobles dans leur composition sont nécessaires. Des catalyseurs bifonctionnels à base de métaux de transition associés au graphène ont alors été synthétisés. L'interaction oxyde-graphène a été étudiée sur un catalyseur Co3O4/NRGO. À faible teneur en cobalt, l'interaction entre les atomes de cobalt de l'oxyde et les atomes d'azote greffés sur les plans de graphène a été observée par voltammétrie cyclique. Cette interaction est responsable d'une diminution de la taille des nanoparticules de cobaltite et de l'activité de celles-ci vis-à-vis de la réaction de réduction du dioxygène (RRO). La substitution du cobalt par le nickel dans des structures de type spinelle (NiCo2O4/RGO) obtenu par voie solvothermale, a permis d'améliorer les performances électrocatalytiques vis-à-vis de la RRO et de la RDO. Ce matériau et un autre de type Fe-N-C préparé en collaboration avec un laboratoire de l'Université Technique de Berlin ont servi de cathode dans des études préliminaires réalisées en configuration pile à combustible alcaline à membrane échangeuse d'anion (SAFC). / Hydrogen, as an environmentally friendly future energy vector, is a non-toxic and convenient molecule for regenerative fuel cell, which connects two different technologies: an electrolyzer for H2 production, and a fuel cell for its direct conversion to electric energy. This kind of system possesses many advantages, such as lightness, compactness and more autonomy. However, improvement of activity and durability of electrode materials free from noble metals in their composition is needed. Thereby, bifunctional catalysts composed of transition metals deposited onto graphene-based materials were synthesized. The interaction between the metal atom of the oxide and the graphene doped heteroatom in the Co3O4/NRGO catalyst was investigated physicochemically. With a low cobalt loading, the interaction between cobalt and nitrogen was characterized by cyclic voltammetry, which revealed that it was responsible for decreasing the oxide nanoparticle size, as well as increasing the material activity towards the oxygen reduction reaction (ORR). The substitution of Co by Ni in the spinel structure (NiCo2O4/RGO) obtained by solvothermal synthesis, allowed the enhancement of the electrocatalytic performances towards the ORR and OER. Moreover, this catalyst as well as another material prepared in collaborative program with a lab from Technical University of Berlin were used as cathode in preliminary studies undertaken on solid alkaline fuel cell (SAFC).

Face à la nécessité d’une réduction drastique des émissions de gaz à effet de serre, le déploiement des piles à combustibles est présenté comme une solution d’avenir. La production d’hydrogène décarbonée est un des enjeux futurs pour permettre une transition énergétique efficace. Dans cette optique, l’électrolyseur à membrane échangeuse de proton (PEMWE), combiné aux sources énergétiques renouvelables, est une technologie intéressante. De nombreux défis sont encore à relever pour permettre une commercialisation de cette technologie, en particulier côté anodique. L’oxyde d’iridium, matériau coûteux et très rare, est utilisé à l’anode pour sa capacité à catalyser le dégagement d’oxygène tout en résistant aux conditions acide et oxydante. Il subit néanmoins des dégradations au cours de son utilisation.Dans ce travail, différentes surfaces modèles d’iridium pour le dégagement d’oxygène ont été étudiées pour comprendre les mécanismes mis en jeu lors des premières étapes d’oxydation de la surface et du dégagement d’oxygène. Après caractérisations par spectroscopie d’impédance électrochimique dynamique (DEIS), technique innovante permettant d’analyser les systèmes dynamiques, les relations structure-activité-stabilité lors du dégagement d’oxygène ont été étudiées en comparant des surfaces modèles d’iridium ((111), (210) et (210) nanostructurée). Les résultats obtenus mettent en évidence qu’après quelques heures à haut potentiel (> 1,6 V vs. Electrode Réversible à Hydrogène), ces surfaces, de structures et compositions chimiques initiales différentes, tendent vers le même état. Enfin, l’étude de films minces d’iridium et de nickel@iridium, modélisant des particules cœur@coquille, a montré qu’après dissolution du nickel initialement présent, une couche poreuse active pour le dégagement d’oxygène est formée. Ces résultats sont prometteurs pour la synthèse de catalyseurs à base d’iridium pour le dégagement de dioxygène. / With the need for a drastic reduction of greenhouse gases, the deployment of fuel cells is one of the considered solutions. Decarbonated hydrogen production is subsequently a major challenge to enable an efficient energetic transition. From this perspective, Proton Exchange Membrane Water Electrolyser (PEMWE) is a technology of interest, especially if coupled with renewable energy sources. Key challenges are still to be addressed before commercializing this technology, in particular at the anode. Iridium oxide, a costly and rare material, is implemented in anodic catalytic layers to catalyse the Oxygen Evolution Reaction (OER) while being resistant to harsh acidic and oxidative conditions. It nonetheless undergoes some degradations.In this work, different iridium model surfaces for the OER where studied to understand mechanisms involved during the first oxidations step and oxygen evolution. After characterisations by Dynamic Electrochemical Impedance Spectroscopy (DEIS), an innovative technique used to study dynamic systems, structure-activity-stability relationships towards the OER were studied by comparing iridium model surfaces ((111), (210) and nanostructured (210)). Results showed that after few hours at high potential (> 1.6 V vs. Reversible Hydrogen Electrode)), these surfaces, with different initial chemical compositions and structures, tend to the same state. Finally, iridium and nickel@iridium thin films were studied, to model core@shell particles. Results indicate that the nickel dissolution lead to the formation of a porous layer more active towards the OER. These findings could help to design active iridium catalysts for the OER.

Monolayer two-dimensional (2D) materials have been regarded as a hot topic in the fields of condensed matter physics, materials science, and chemistry due to their unique physical, chemical, and electronic properties. However, the research on the preparation method and properties understanding of the 2D monolayer are inadequate. In this dissertation, taking 2D nickel-iron layered double hydroxides (NiFe LDHs) and molybdenum disulfide (MoS2) as examples, the practicability of the direct synthesis of NiFe LDHs monolayer and the thermal enhancement catalytic performance of 2D MoS2 monolayer (MoS2 ML) are discussed. First, a one-pot synthetic strategy (bottom-up method) is presented to synthesize 2D NiFe-based LDHs monolayers, including NiFe, Co-, Ru-, doped, and Au-modified NiFe LDHs. The prerequisite and universality of this strategy are investigated and confirmed. The features of LDHs are characterized by advanced technologies. The obtained LDH bulks own a large interlayer spacing up to 8.2 Å, which can be facilely exfoliated into monolayers in water by hand-shaking within 10 s. As a result, the as-prepared NiFe-based LDH monolayers display a good electrocatalytic oxygen evolution reaction (OER) performance. This facile strategy paves the way for designing easily exfoliated LDHs for highly active catalysts and energy conversion devices based on other monolayer LDHs. Second, with gold-modified tape, 2D MoS2 ML is exfoliated from the bulk crystal through a micromechanical exfoliation method (top-down strategy). The thermal effects of MoS2 ML are confirmed by Raman and photoluminescence (PL) spectra. Moreover, an on-chip MoS2 ML hydrogen evolution reaction (HER) reactor is designed and fabricated. The thermal effects generate efficient electron transfer in the MoS2 ML and at the electrolyte-catalyst (MoS2 ML) interface, leading to an enhanced HER performance. Compared to the results obtained at room temperature, the MoS2 ML shows a direct thermal enhanced HER performance at higher temperatures. In summary, the findings and understandings, the direct synthesis and direct thermal enhancement catalytic performance, of 2D monolayers offer a guideline for synthesizing and catalyst application of other 2D monolayers.

<p dir="ltr">Insights from research into the natural photosynthetic processes are applied to inform the rational design of inorganic catalysts. The study of these synthetic systems – artificial photosynthesis – will lead towards the development of a device able to absorb light, convert and store the energy in the form of chemical bonds. The water-splitting reaction, a bottleneck of the photosynthetic process, is a key barrier to overcome in this endeavor. Thus, the focused study of water-oxidation catalysts able to facilitate this difficult reaction is performed, in order to develop a green-energy solution in the form of an artificial photosynthesis system.</p>