Keywords: CO2 photoelectrochemical reduction, molecular catalyst, CIGS, cobalt quaterpyridine, cobalt phthalocyanine

CO2 reduction is an environmentally friendly and economically meaningful strategy with significant benefits, but it is still a challenge. The use of abundant solar energy to convert CO2 into value-added chemicals through photo-electrocatalysis technology has attracted widespread attention from researchers in chemistry, materials, and environmental science. Photoelectrochemical (PEC) reduction of CO2 can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In this doctoral work, I have: (i) combined a ptype Cu(In,Ga)Se2(CIGS) semiconductor material with cobalt quaterpyridine molecular catalyst. Pure carbon dioxide dissolved in aqueous solution (pH 6.8) is converted to carbon monoxide under visible light illumination with partial current density above 3 mA cm -2 and 97% selectivity, showing good stability over time. (ii) developed a new system, consisting in a CIGS unit linked to a carbon electrode loaded with cobalt phthalocyanine as molecular catalyst, able to achieve the CO2 reduction to CO and then to methanol in aqueous media. Under ideal conditions, the CO2-to-CH3OH reaction shows a global Faradaic efficiency of 28 % and chemical selectivity of 11.6 %.

Résumé en français

Le climat de la Terre a changé au cours de l'histoire. Les preuves scientifiques du réchauffement du système climatique sont sans équivoque. La tendance actuelle au réchauffement de la planète est le résultat de l'activité humaine depuis le milieu du 20e siècle et se poursuit à un rythme sans précédent. Une étude récente souligne que l'augmentation des niveaux de CO2 entraînera une hausse de la température globale et aura un impact sévère sur la survie de tous les êtres vivants. Par conséquent, afin d'éviter les pires effets du changement climatique, le monde doit de toute urgence réduire les émissions de CO2. Cependant, la manière dont cette responsabilité est partagée entre les régions, les pays et les individus a toujours été un point de discorde sans fin dans les discussions internationales. La dépendance excessive à l'égard des combustibles fossiles n'a pas seulement accéléré la surexploitation des ressources naturelles, elle a également interrompu le cycle du carbone. C'est avec reconnaissance que la plupart des pays se sont tournés vers des sources d'énergie à faible teneur en carbone -technologies nucléaires et renouvelables. Les énergies renouvelables et durables joueront un rôle clé dans la décarbonisation de nos systèmes énergétiques au cours des prochaines décennies. Le captage et l'utilisation du CO2 (CCU) ont suscité beaucoup d'intérêt.

La réduction du CO2 présente des avantages considérables, mais reste un défi difficile à relever. Des progrès ont été réalisés ces dernières années. Parmi eux, l'utilisation de l'énergie solaire en abondance pour convertir le CO2 en produits chimiques à plus haute valeur ajoutée grâce à la technologie de la photoélectrocatalyse a attiré l'attention de nombreux chercheurs en chimie, en matériaux et en sciences environnementales.

L'énergie solaire est la source d'énergie renouvelable et propre la plus abondante.

Elle peut être utilisée à grande échelle en convertissant efficacement l'énergie solaire en énergie électrique par la production de paires électron-trou séparées. Cependant, le stockage de l'électricité à grande échelle reste un défi, tandis que son transport est associé à d'importantes pertes. Par le biais de la photosynthèse naturelle, une autre stratégie a vu le jour : la conversion de l'énergie solaire en combustibles chimiques (stockage de l'énergie dans des liaisons chimiques). Cela peut se faire en utilisant l'électricité produite à partir de l'énergie solaire pour mettre en oeuvre des réactions électrocatalytiques de manière indirecte ou en générant des produits chimiques par des réactions photoélectrocatalytiques de manière directe.

La réduction photoélectrochimique (PEC) du CO2 est alimentée par l'électricité et l'irradiation lumineuse. La PEC présente plusieurs avantages, comme la sélectivité contrôlable des produits et l'utilisation écologique de l'énergie solaire renouvelable. Elle peut atteindre une efficacité et une sélectivité élevées grâce à l'intégration et à l'optimisation des avantages de l'électrocatalyse et de la photocatalyse.

L'élément clé du processus de réduction du CO2 par la CEP est la photoélectrode.

Une photoélectrode se compose d'un photoabsorbeur et d'un catalyseur pour réduire le CO2. Des canaux efficaces de migration des électrons entre le photoabsorbeur et le catalyseur peuvent améliorer considérablement les activités de réduction du CO2. La réduction du CO2 par la CEP en est encore à ses débuts. Les catalyseurs à base de métaux nobles, tels que les nanoparticules ou les alliages à base d'Au, restent les cocatalyseurs les plus étudiés en raison de leur activité et de leur sélectivité élevée pour le CO. En outre, les catalyseurs moléculaires à base de Ru et de Re ont également été largement étudiés. Outre les métaux nobles, peu de travaux ont été consacrés aux catalyseurs non nobles. Les matériaux à base de cuivre sont les catalyseurs non nobles les plus développés en raison de leur grande abondance et de leurs caractéristiques uniques pour la production d'hydrocarbures. L'utilisation de matériaux semiconducteurs comme photoélectrodes combinés à un catalyseur moléculaire à base de métaux non nobles pour obtenir une réduction hautement efficace et sélective du CO2 dans l'eau reste un défi de taille. Dans mon travail, j'ai voulu contribuer à la conception de systèmes PEC plus efficaces pour la réduction du CO2 en solution aqueuse avec des photoélectrodes stables et l'utilisation de complexes de métaux non nobles. Les matériaux photovoltaïques (PV) convertissent l'énergie lumineuse en électricité en utilisant des matériaux semi-conducteurs qui présentent un effet photovoltaïque.

Les dispositifs CIGS (cellules polycristallines de chalcopyrite Cu(In,Ga)Se2) sont considérés comme faisant partie des dispositifs à couches minces les plus prometteurs. Tout d'abord, les dispositifs à couches minces CIGS sont assez efficaces. En outre, le coût des modules CIGS a encore diminué grâce à la réduction des coûts de fabrication et à l'augmentation de la production. Le CIGS est un semi-conducteur multi-éléments qui peut être déposé sur des substrats flexibles en métal ou en polyimide. Il est largement utilisé dans les cellules solaires en raison de sa grande résistance aux rayonnements. La cellule CIGS est généralement constituée d'une couche de CIGS de type p qui joue le rôle de principal absorbeur de lumière, en contact avec une couche de CdS de type n pour former une jonction p-n. Sur la face avant, une électrode transparente généralement basée sur une bicouche ZnO/ZnO:Al collecte les électrons.

Une électrode en molybdène est fabriquée sur la face arrière du CIGS pour collecter les trous. Cette structure n'est pas fixe et pourrait être ajustée pour s'adapter à des problèmes spécifiques ou pour atteindre des rendements élevés.

Dans la première partie de mon travail, nous avons démontré avec succès l'application de la photocathode CIGS pour la réduction catalytique du CO2 par voie solaire. Pour le couplage avec le CIGS, un catalyseur moléculaire de quaterpyridine de cobalt a été fonctionnalisé avec deux groupes d'acide phosphonique (Co-qPyH). Cela permet l'immobilisation sur les surfaces d'oxyde métallique. Il a été démontré que l'acide phosphonique est l'un des groupes de liaison les plus stables à la surface d'un oxyde métallique. Des couches de TiO2 mésoporeux ont été déposées sur des substrats en verre conducteur FTO par une méthode de sérigraphie, ce qui est désigné comme m-TiO2|FTO. Ensuite, le catalyseur Co-qPyH a été immobilisé sur la surface du TiO2. Des mesures détaillées ont été effectuées pour étudier l'immobilisation du catalyseur. Des spectres d'absorption UV-visible ont été réalisés sur la surface du m-TiO2|FTO avant et après l'immobilisation du catalyseur sur les couches de TiO2, ce qui a confirmé que la surface semi-conductrice avait été modifiée. En outre, seule une petite quantité d'absorption a été observée dans la région de la lumière visible, rendant la surface presque transparente à la majorité de la lumière entrante. D'autres analyses par spectroscopie infrarouge à réflectance totale atténuée (ATR-IR) sur la poudre de Co-qPyH et l'électrode m-TiO2 chargée de Co-qPyH (CoqPyH|m-TiO2) indiquent une immobilisation réussie du catalyseur sur la surface du TiO2. Le résultat de la cartographie EDX révèle une distribution homogène du catalyseur sur la surface de l'électrode. En outre, la cartographie de l'élément Co a indiqué qu'il n'y avait pas d'agrégation du cobalt. Le rapport atomique constant du phosphate et du cobalt est de 2:1, confirmant à la fois la présence du catalyseur et son intégrité après immobilisation.

Une électrolyse à long terme a été réalisée à -0,51 V vs. RHE pendant 2 h, ce qui a montré un courant catalytique moyen de 1,2 mA.cm -2 . Les produits ont été étudiés par chromatographie en phase gazeuse (GC), indiquant la production de CO avec une sélectivité de 80% et une efficacité faradique de 63%. Un seul sous-produit, le H2, a également été détecté. L'intégrité du catalyseur avant et après l'électrolyse (1 h) dans du KHCO3 0.1 M saturé de CO2 (pH 6.8) à -0.56 V vs. RHE a été étudiée par l'analyse ATR-IR de l'électrode, ne montrant presque aucun changement des signaux caractéristiques. Ceci indique une bonne stabilité du catalyseur et une immobilisation efficace. Compte tenu des bonnes performances de l'électrode Co-qPyH|m-TiO2 dans les conditions de la CE, nous avons alors pensé à combiner ce matériau comme couche supérieure avec le CIGS comme photocathode pour réduire le CO2 sous irradiation lumineuse. Nous avons proposé une stratégie étape par étape de la configuration PV-EC à la configuration PEC. Nous avons d'abord conçu une stratégie qui consiste à coupler une cellule PV CIGS externe (matériau ZnO:Al|ZnO|CdS| CIGS|Mo) avec l'électrode Co-qPyH|m-TiO2 ci-dessus. Le couplage d'une cellule PV à un électrolyseur (PV-EC) est une méthode pratique pour utiliser les charges photogénérées dans les dispositifs de la cellule et activer un substrat en solution. Dans cette configuration, le matériau photoactif n'est pas directement en contact avec la solution d'électrolyte, ce qui résout en partie le problème de la stabilité. Des expériences d'électrolyse ont été réalisées dans du KHCO3 0,1 M saturé de CO2 (à pH 6,8) à 0,14 V vs RHE pendant 2 h avec une densité de courant moyenne d'environ 1 mA.cm -2 (par surface géométrique de TiO2). Une autre électrolyse a été réalisée à -0.06 V vs. RHE dans les mêmes conditions d'électrolyte. Une densité de courant moyenne de 3,25 mA.cm -2 (par surface géométrique de TiO2) a été obtenue. L'analyse par GC indique que le produit principal est le CO avec une sélectivité de 87 % à un potentiel de polarisation de -0,06 vs RHE, correspondant à un potentiel plus négatif que le potentiel thermodynamique (surpotentiel formellement négatif l η = -50 mV). Une électrolyse de plus longue durée, 7 h, a ensuite été réalisée dans un électrolyte de KHCO3 0,5 M avec un flux continu de CO2 (pH 7,2) à un potentiel de -0,03 V vs RHE correspondant à un surpotentiel formellement négatif (η = -80 mV) pour la réaction CO2 vers CO. Une densité de courant moyenne de 1,78 mA.cm -2 a été observée et la sélectivité pour la production de CO a été maintenue entre 82 et 85% au cours de l'expérience. Dans la deuxième approche, nous avons greffé le catalyseur directement sur la photocathode à base de CIGS afin de réduire le CO2 dans un processus PEC entièrement intégré. Plus précisément, une couche de TiO2 a été déposée directement sur le dispositif CIGS complet (ZnO:Al/ZnO/CdS/CIGS/Mo), puis greffée avec le catalyseur Co-qPyH. La stabilité de la photoélectrode a été évaluée par une expérience de photoélectrocatalyse à long terme réalisée dans un électrolyte de KHCO3 0,1 M saturé en CO2 (pH 6,8) à un potentiel de polarisation de -0,06 V par rapport au RHE. La densité de courant moyenne était de 0,8 mA.cm -2 avec seulement une légère décroissance au cours de 2 h. Les produits ont été analysés par GC, qui montre une sélectivité de 97% pour le CO et 3% pour le seul sous-produit H2. Avec cette configuration, il est possible d'obtenir une sélectivité élevée pour la génération de CO pendant plusieurs heures. Le TON pour le CO supérieur à 8000 a été obtenu, correspondant à un TOF de

Résumé

La réduction du CO2 est une stratégie respectueuse de l'environnement et économiquement pertinente, qui présente des avantages considérables, mais elle reste un défi. L'utilisation de l'abondante énergie solaire pour convertir le CO2 en produits chimiques à valeur ajoutée grâce à la photoélectrocatalyse. a attiré l'attention des chercheurs en chimie, en matériaux et en sciences environnementales, et de l'environnement. La réduction photoélectrochimique (PEC) du CO2 peut intégrer et optimiser les avantages de la photocatalyse et de l'électrolyse pour améliorer l'efficacité et la sélectivité de la conversion. Dans ce travail de doctorat, j'ai : (i) combiné une structure de type p-Cu(In,Ga)Se2(CIGS) de type p avec un catalyseur moléculaire de quaterpyridine de cobalt. Le dioxyde de carbone pur dissous dans une solution aqueuse (pH 6.8) est converti en monoxyde de carbone sous l'éclairage de la lumière visible avec une densité de courant partielle supérieure à 3 mA cm -2 et une sélectivité de 97%, montrant une bonne stabilité dans le temps. (ii) développé un nouveau système, consistant en une unité CIGS liée à une électrode de carbone chargée de phtalocyanine de cobalt comme catalyseur moléculaire, capable de réaliser la réduction du CO2 en CO et ensuite en méthanol en milieu aqueux. Dans des conditions idéales, la réaction du CO2 au CH3OH présente une efficacité faradique globale de 28 % et une sélectivité chimique de 11,6 %. unprecedented [START_REF] Santerm | Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes[END_REF][START_REF] Westerhold | An astronomically dated record of Earth's climate and its predictability over the last 66 million years[END_REF]. Recent studies from NASA have shown that most of the warming occurred in the past 40 years, and the years 2016 and 2020 have been the warmest years [3]. The average surface temperature of our planet has risen about 2.12 degrees Fahrenheit since the late 19th century, which is mainly driven by increased carbon dioxide emissions into the atmosphere [4]. Recent study points out that anthropogenic CO2 levels could reach ∼590 ppm by the year 2100, which will lead to an increase in the global temperature of 1.9°C and would severely impact the survival of all living beings [5].

Mots clefs :

Therefore, in order to avoid the worst effects of climate change, the world needs to urgently reduce CO2 emissions. However, the way this responsibility is shared among regions, countries and individuals has always been an endless point of contention in international discussions. Over-reliance on fossil fuels has not only accelerated the over-exploitation of natural resources, it has also interrupted the carbon cycle [6].

Nature converts carbon dioxide into carbohydrates through photosynthesis to maintain the carbon cycle, but this is greatly affected by deforestation, industrialization and urbanization. It is grateful that most countries have been shifting towards low-carbon sources of energy -nuclear and renewable technologies [6]. Renewable and sustainable energy will play a key role in the decarbonization of our energy systems in the coming decades. CO2 capture and utilization (CCU) has attracted plenty of attention. Various research efforts have been made in the past decades. Consequently, the recycling or conversion of CO2 into fuels and other valuable products is an attractive strategy to help mitigating the global warming and energy crisis.

Principles and advantages of Photoelectrochemical reduction of CO2

Solar energy is the most abundant renewable and clean energy source. It can be used in a large scale by efficiently converting solar energy into "electronic" energy through producing separated electron-hole pairs. One direct method to achieve this conversion is collecting the photogenerated electrons and holes to produce electricity with photovoltaic cells. However, it is still a challenge to store electricity on a massive scale, while its transport is associated to important losses. Thus, electrical energy is not ideal because of the problems of large-scale use and long-distance transportation.

Enlightened by natural photosynthesis, another strategy has emerged: converting solar energy into chemical fuels (storing energy in chemical bonds). This can be achieved by utilizing the electricity produced from solar energy to drive electrocatalytic reactions in an indirect way or generating chemicals through photoelectrocatalytic reactions in a direct way (Figure 1.1). A key element for PEC CO2 reduction process is the photoelectrode. A photoelectrode consists in a photo absorber and a catalyst for reducing CO2. Efficient electron migration channels between photo absorber and catalyst could greatly enhance the activities of CO2 reduction.

The PEC reduction of CO2 at a photoelectrode with a co-catalyst (for example a metal complex) needs multiple electron transfers and various products could be generated according to the transfer of electrons and protons and specific reaction route.

Carbon monoxide, formic acid, formaldehyde, methanol, and methane have all been detected in some systems even at ambient temperature and pressure [START_REF] Kumaravel | Photoelectrochemical Conversion of Carbon Dioxide (CO2) into Fuels and Value-Added Products[END_REF][START_REF] Eggins | Factors affecting the photoelectrochemical fixation of carbon dioxide with semiconductor colloids[END_REF]. The product yield of PEC reduction of CO2 is mainly governed by the choice of photoelectrode materials, catalyst and electrolytes. Figure 1.2 illustrates the principle scheme of PEC CO2 reduction using solar light as the main energy source. Therefore, semiconductors are considered as attractive materials for lightabsorbing applications. When a photon has an energy higher or equal to the band gap, it could be absorbed. Once absorbed, an electron would be excited from the VB to the CB, thereby leaving behind an empty state that is called a hole. This means that, in a semiconductor, the number of both electrons and holes under light illumination are larger than in the equilibrium state. After the separation, the electrons and holes can recombine or they may transfer to the surface of the semiconductors and eventually migrate to adsorbed molecules, and then initiate the reduction or oxidation reactions (Figure 1.4).

Recombination of Charge Carriers

After light excitation, the formation of charge carriers, electron and hole pairs, is usually followed by some deactivation routes (Figure 1.5). The effective donation of the electrons or holes to the acceptor molecules at the surface will lead to the desired redox reactions (routes 1 and 2), while these processes compete with several recombination pathways. The carriers can be trapped and consumed by their counterparts with opposite charge on the surface (routes 3), or recombine with other carriers inside the bulk of the material (routes 4) [START_REF] Habisreutinger | Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors[END_REF]. These recombination reactions reduce the efficiency of the catalytic reaction. Several factors have been showed to affect the recombination rates, e.g. transfer and trapping of charges, defect (doping) density of the crystal [START_REF] Fujishima | TiO2 photocatalysis and related surface phenomena[END_REF][START_REF] Shen | Identification of the Active Species in Photochemical Hole Scavenging Reactions of Methanol on TiO2[END_REF] as well as possible combination with another material which may play the role of an electron or a hole sink [START_REF] Kamat | Manipulation of Charge Transfer Across Semiconductor Interface. A Criterion That Cannot Be Ignored in Photocatalyst Design[END_REF]. Both recombination processes have negative effects on the efficiency of the photocatalytic reaction. An example of metal catalyst supported on the semiconductor is described here (as shown in Figure 1.4). In a material, the chemical potential is defined by Fermi level (EF), which is a thermodynamic quantity and can be used for analysis of electron transfer at material interfaces. In the case in Figure 1.4, the photoexcited electrons are transferred towards the metal because the EF energy lies below the CB states. This process finally changes the EF of the composite. Therefore, the electrons can be transported towards the adsorbate more easily compared with semiconductor itself. In another words, the composite is more reductive. This strategy could promote the separation of electron-hole pairs by promoting migration of electrons towards the metal, while the holes are still in the semiconductor. Finally, improved separation of charge carriers would lead to less recombination and an increased efficiency of the reactions. [START_REF] Habisreutinger | Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors[END_REF]

p-n Junction in semiconductor materials

Introducing p-n junction is an efficient way to improve the separation of charge carriers. n-type semiconductors are semiconductors doped with an electron donor element during preparation. The term n-type refers to the negative charge of the electron.

In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers (Figure 1.6 a). On the contrary, p-type semiconductors are doped with an electron acceptor element and the term p-type comes from the positive charge of a hole.

In p-type semiconductors, holes are the majority carriers with a larger hole concentration than electron concentration (Figure 1.6 b) [START_REF] Sah | Carrier Generation and Recombination in P-N Junctions and P-N Junction Characteristics[END_REF]. A p-n junction is the interface or the boundary between a p-type and a n-type semiconductor. As described above, the p-type side contains an excess of holes, while the n-type side contains an excess of electrons as the main carriers. When illuminated with light, the p-n junction can drive a current which could pass through the junction in one direction only [START_REF] Chen | Preparation of a Novel TiO2-Based p-n Junction Nanotube Photocatalyst[END_REF]. Specifically, there are two different types of current generated at the p-n junction: the drift current and the diffusion current, which flow in opposite directions. Under "dark" conditions (e.g., without illumination), these currents balance each other and no net current flows. At the microscopic level, the drift current originates from the internal electric field created at the interface of the p-type semiconductor and the n-type semiconductor. The diffusion current originates from the disparate carrier concentrations in the n-type and p-type regions, with electrons tending to diffuse from the higher electron concentration region (n-type semiconductor) towards the lower electron concentration region (p-type semiconductor). Similar action, though in the opposite directions, occurs for the holes. Under illumination, the drift current becomes greater than the diffusion current, thus facilitating the separation of electron-hole pairs.

Improved separation contributes to slower recombination rates and an increased efficiency of the drift current. When a bias potential is applied to the semiconductor with a p-n junction, such as in solar cell, in a band theory model, more carriers would get into the CB and then enter the region where the electric field is located [START_REF] Sah | Carrier Generation and Recombination in P-N Junctions and P-N Junction Characteristics[END_REF][START_REF] Smith | Introduction to semiconductor processing: Fabrication and characterization of p-n junction silicon solar cells[END_REF]. On one hand, considering a high efficiency, it is beneficial that the semiconductor material absorbs light in the visible range, so a small band gap is necessary for this condition. On the other hand, the band gap must span the range of the oxidation or reduction potentials corresponding to the catalyst and catalyzed reaction. These requirements regarding the width of the band gap actually limit the application of semiconductor material, while optimization approaches are explored through sensitization or band-gap engineering.

Regarding specifically PEC reduction of CO2, semiconductors primarily act as the photo absorber and are functionalized with cocatalyst, such as metal nanoparticles or molecular catalyst. An ideal semiconductor photocathode will efficiently harvest solar light and transfer charge carriers to the catalytic sites for efficient reduction reactions.

Hence, a semiconductor with relatively small band gap and highly efficient electronmigration channel between photo absorber and catalysts will be beneficial for CO2 reduction.

Thermodynamics and chemical pathways of PEC CO2 reduction

The electrochemical standard reduction potentials for C1 products and ethanol are summarized below in Table 1, and are in the same order of magnitude than the proton reduction reaction. Moreover, the potentials are less negative than the CB of various semiconductor materials [START_REF] Tran | Recent advances in hybrid photocatalysts for solar fuel production[END_REF] (as illustrated in Figure 1.9), so these reactions are theoretically feasible. In PEC systems, the energy of photoexcited electrons is normally determined by the conduction band edge (CBE) of the semiconductor photoelectrode.

Thermodynamically, CO2 photoreduction is favorable if the energy of the CBE is more negative than that of the reduction potential. The single-electron process from CO2 to its anion radical CO2 • ‾ has a quite negative reduction potential of -1.9 V vs. standard hydrogen electrode (SHE) at pH 7 (Figure 1.8). This step is highly unfavored, because nearly no semiconductor material could provide sufficient potential to transfer a photogenerated electron to a CO2 molecule [START_REF] Tran | Recent advances in hybrid photocatalysts for solar fuel production[END_REF]. Utilization of a multiple protoncoupled electron transfer process significantly lowers the potential required for CO2 reduction, and gives various products that depend on the number of electrons involved. The multistep reduction into C1 products involves up to eight electrons and protons (for methane), with the breaking of C-O bonds and the formation of C-H bonds, and may lead to various products depending on the specific pathway taken. Besides, considering the reduction of CO2 is generally performed in the presence of water, the competing generation of molecular hydrogen would compete with carbon species for the photoexcited electrons, so it is quite important to suppress this concurrent pathway.

Thus, an in-depth understanding of the mechanisms is mandatory to minimize the relevant thermodynamic and kinetic barriers of the intermediate reactions, thereby increasing the overall rate of CO2 reduction.

CO 2 + 2H + + 2e -→ HCOOH -0.61 V (1)
CO 2 + 2H + + 2e -→ CO + H 2 O -0.53 V (2)
CO 2 + 4H + + 4e -→ HCHO + H 2 O -0.48 V (3)
CO 2 + 6H + + 6e -→ CH 3 OH + H 2 O -0.38 V (4)
CO 2 + 8H + + 8e -→ CH 4 + 2H 2 O -0.24 V (5)
2CO 2 + 12H + + 12e -→ CH 3 CH 2 OH + 3H 2 O -0.08 V (6)
2H + + 2e -→ H 2 -0.41 V (7)
The variation of the band structure at the semiconductor-electrolyte interface is also an important aspect. Specifically, at a semiconductor-electrolyte solution interface, if the Fermi level (EF) of the semiconductor is not equal to those of the electrolyte, then electrons will be transferred from high EF to low EF. As an example, we may take a ptype semiconductor, with EF lower than that of the electrolyte solution. When the photocathode is immersed in the electrolyte, electrons will transfer from the electrolyte to the semiconductor. Therefore, an electric field in the semiconductor near the interface is built up, which will cause a band bending of the semiconductor's energy bands structure. As a result, the semiconductor's CB and VB will bend downward, (Figure 1.10). This band bending promote electrons transport to the photocathode-electrolyte interface. Under light irradiation, some electrons in the VB will be excited to the CB, leaving holes in the VB. The semiconductor's CB/VB bending near the interface supplies a force on electron-hole pairs in opposite directions, driving electrons toward the interface and holes away from that. In others words, this bending facilitates electron-hole pairs separation, which is a critical parameter for photoconversion efficiency that hinges on irreversible separation of charge carriers. This p-type photoelectrode will promote reduction reactions. An n-type semiconductor will act as a photoanode, leading a bending in an opposite direction to the energy bands. This band bending promotes hole transport to the photoanode-electrolyte interface, achieving oxidation of electrolyte molecules (e.g. H2O). As described above, a PEC system could contain both a p-type photocathode and a n-type photoanode as photoelectrodes simultaneously in a tandem way, or one photoelectrode only with a dark counter electrode. To achieve an efficient photon-induced reduction reaction on a semiconductor photocathode, it is essential to focus on the following aspects:

1) Ensure sufficient lifetime of the photoexcited electrons;

2) Ensure fast transport of photoexcited electrons and holes in opposite directions, which is essential to photoconversion efficiency;

3) Optimize band gap energy: for the photocathode used in the PEC CO2 reduction, we need to best utilize the solar spectrum and have a sufficiently high CB energy to drive the desired reduction reaction.

As discussed earlier (Figure 1.3), metal NPs could reduce charge recombination and enhance the separation at the interface of metal-semiconductor. They may also play an important role in catalyzing the subsequent steps and determining the selectivity of the products. [START_REF] Peterson | Activity descriptors for CO2 electroreduction to methane on transition-metal catalysts[END_REF] Coupling semiconductor with a molecular complex has also been shown to be beneficial for the performance of PEC reduction of CO2.

[19]

The nature of the CO2 reduction product and selectivity are determined by the chemical composition of the electrocatalyst. The two-electron-reduction product CO is a very important raw material in the chemical industry, especially for the famous Fischer-Tropsch process (synthesis of hydrocarbon fuels). Besides, formic acid (HCOOH) is also a useful compound since it is a hydrogen carrier which got much attention recently. It could be easily decomposed to H2 and CO2 in the presence of a suitable catalyst, even under mild conditions [START_REF] Casado-Rivera | Electrocatalytic activity of ordered intermetallic phases for fuel cell applications[END_REF][START_REF] Hull | Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures[END_REF].

Multi-electron CO2 reduction is also promising. With a few rare exceptions, molecular catalysts are not yet capable to make such highly reduced products (methanol, CH4, ethanol, …). A suitable photoelectrocatalytic CO2 reduction system should operate in response to visible light and near-infrared light, because these domains account for the majority of the solar spectrum. As in electrocatalytic systems, side reactions should also be suppressed, such as the reduction of protons to H2 (see Equation 14).

Ideally, water is going to be the electron donor. Since the apparent standard potential associated with the oxidation of water (see Equation 8) is +0.82 V vs. SHE at pH 7, the minimum photon energy required to cause the photocatalytic reduction of CO2 on a thermodynamic basis (for the generation of HCOOH, for instance) and to oxidize water is 1.43 eV, which corresponds to a wavelength of approximately 870 nm.

Thus, the total range of visible light (400 < λ < 800 nm) can theoretically be used. [START_REF] Maeda | Metal-Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light[END_REF] O

2 + 4H + + 4e -→ 2H 2 O E o′ = +0.82 V vs. SHE (pH 7) (8)
Metal complexes associated to semiconductors are promising photocatalysts toward CO2 reduction to formate, CO, and maybe to various hydrocarbons. In the following sections, some representative examples of photocatalytic CO2 reduction using metal complexes and semiconductors are introduced.

Photoelectrochemical CO2-reduction at semiconductor/ metal-complex systems

PEC CO2 reduction using a p-type semiconductor cathode in the presence of a metal-complex catalyst has been studied since the 1980s, either with a freely diffusing catalyst or upon anchoring the complex to the photoelectrode surface [START_REF] Bradley | p-Type silicon based photoelectrochemical cells for optical energy conversion: electrochemistry of tetra-azomacrocyclic metal complexes at illuminated[END_REF][START_REF] Parkinson | Photoelectrochemical pumping of enzymatic CO2 reduction[END_REF][START_REF] Petit | Molecular catalysts in photoelectrochemical cells: study of an efficient system for the selective photoelectroreduction of CO2: p-GaP or p-GaAs/Ni (cyclam) 2+ , aqueous medium[END_REF][START_REF] Jo'm | On the photoelectrocatalytic reduction of carbon dioxide[END_REF]. The performance of such semiconductor/metal-complex hybrid systems in aqueous condition could potentially be improved by anchoring water-resistant metal-complex catalysts on the surface of photoelectrode to prevent undesirable aggregation.

Obviously, appropriate choice of a semiconductor material is also of great importance.

Group IV Materials

Silicon (Si) has a narrow bandgap of 1.1 eV, allowing the absorption of light to span from the UV region to the near-IR region. Many early studies of PEC reduction of CO2 have been performed on Si photoelectrodes also because of its abundance.

Kubiak et al. reported the selective reduction of CO2 to CO using hydrogenterminated p-type Si as the photocathode in the presence and homogeneous catalyst Re(bipy-Bu t )(CO)3Cl (bipy-Bu t =4,4′-di-tert-butyl-2,2′-bipyridine) in 0.1 M tetrabutyl ammonium hexafluorophosphate (TBAH) acetonitrile solution (Figure 1.11). [START_REF] Kumar | Photoreduction of CO2 on p-type Silicon Using Re (bipy-Bu t )(CO)3Cl: Photovoltages Exceeding 600 mV for the Selective Reduction of CO2 to CO[END_REF] The reduction potential required to drive CO2 to CO conversion on p-Si was found to be 600 mV lower than that for the same reaction on a Pt electrode, with 97 ± 3% Faradaic efficiency at -1.4 V vs. Ag/AgCl. A current density of 31 mA cm -2 for CO2 reduction under a monochromatic illumination of 95 mW cm -2 is noteworthy. Both the H-Si and the rhenium catalyst showed excellent stability, lasting for hours without any sign of degradation. In a later study, Torralba-Peñalver et al. reported three Mn-based complexes as homogeneous catalysts for the selective conversion of CO2 to CO, in combination with a p-type hydrogen-terminated silicon nanowires (SiNWs-H) photocathodes under illumination without losing the intrinsic electrocatalytic activity of the complex. [START_REF] Torralba-Peñalver | Selective Catalytic Electroreduction of CO2 at Silicon Nanowires (SiNWs) Photocathodes Using Non-Noble Metal-Based Manganese Carbonyl Bipyridyl Molecular Catalysts in Solution and Grafted onto SiNWs[END_REF] The photocurrent densities measured at -1.0 V vs SCE for [Mn(bpy) (CO)3(CH3CN)](PF6) at SiNWs-H exceeded 1 mA cm -2 in CO2-saturated CH3CN + 5% v/v H2O for 3h with a TOF number of 8.

Group III-V Materials

Much efforts have been made in the research of III-V semiconductors used for CO2 reduction. Among all of the III-V family members, InP, GaP, and GaAs have been the most commonly studied.

Experimental observation of pyridine (Py)-catalyzed CO2 PEC reduction at p-GaP electrodes was originally reported by Bocarsly and coworkers. [START_REF] Barton | Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell[END_REF] They measured almost 96% Faradaic efficiency of CO2 reduction to methanol at a potential of -0.2 V vs. SCE. It should be noted that it has never been demonstrated that the methanol is issued from CO2. Moreover, the absence of follow-up studies causes some doubt on the reproducibility of the results.

p-InP is another example of a III-V semiconductor that possesses a bandgap matching the solar spectrum and has thus been studied extensively. A Zn-doped InP (InP-Zn) single crystal modified with a polymerized ruthenium-complex (Figure 1.12)

has been reported to serve as a photocathode for the PEC reduction of CO2, generating formate in aqueous solution under visible-light irradiation. [START_REF] Arai | Photoelectrochemical reduction of CO2 in water under visible-light irradiation by a p-type InP photocathode modified with an electropolymerized ruthenium complex[END_REF] Through the photoelectropolymerization of the mononuclear Ru complex, the Ru complex was immobilized onto the InP-Zn surface. Under visible light irradiation, this Rupolymer/InP-Zn photocathode could operate in water at a lower potential than that is required in conventional electrocatalysis, with a selectivity for formate of 62.3% at a bias potential of -0.6 V vs. Ag/AgCl (~0.01 vs. RHE) in water. and Cu2ZnSnS4, can act as competent photocathode for generating formate when paired with a polymerized Ru complex synthesized by chemical polymerization using pyrrole units and FeCl3 as initiators. [START_REF] Sato | Selective CO2 conversion to formate conjugated with H2O oxidation utilizing semiconductor/complex hybrid photocatalysts[END_REF][START_REF] Arai | Selective CO2 conversion to formate in water using a CZTS photocathode modified with a ruthenium complex polymer[END_REF] Upon polymerization, polypyrrole chains were formed that were stable in electrochemical conditions. [START_REF] Fan | High-performance polypyrrole electrode materials for redox supercapacitors[END_REF] Cyclic voltammetry measurements revealed that the Ered values of these Ru polymers were almost identical to those of the monomeric analogs. [START_REF] Sekizawa | Solar-driven photocatalytic CO2 reduction in water utilizing a ruthenium complex catalyst on p-type Fe2O3 with a multiheterojunction[END_REF] Because of the presence of propyl chains in the Ru polymers, it is believed that this result is caused by negligible electronic interactions between the bpy ligands and the polypyrrole. Therefore, selecting the optimal Ru catalyst is key to maximizing catalytic performance.

Oxide Semiconductors

In general, titanium dioxide (TiO2) is the most extensively used oxide semiconductors, due to its high photocatalytic stability conditions, low cost, and use in a variety of photocatalytic processes, including water splitting, CO2 reduction and degradation of various organic species. Consequently, this material has been widely studied over the past few decades. [START_REF] Tran | Recent advances in hybrid photocatalysts for solar fuel production[END_REF] In PEC CO2 reduction system, it is commonly combined with other semiconductor materials.

For instance, a Zn and N (Fe2O3:Zn,N) co-doped Fe2O3 has been reported to be an efficient photocathode device for CO2 reduction with a p-type Cr2O3 underlayer and an n-type TiO2 used as protecting layer. [START_REF] Sekizawa | Solar-driven photocatalytic CO2 reduction in water utilizing a ruthenium complex catalyst on p-type Fe2O3 with a multiheterojunction[END_REF] The combination of Cr2O3 and TiO2 layers dramatically enhanced the cathodic photocurrent in the Fe2O3:Zn,N and preventing self-reductive deactivation of the material. By coupling the optimized hybrid photocathode with a SrTiO3 photoanode, both CO2 reduction and water oxidation could be achieved without an external electrical bias under light driven (AM1.5G). In this system, for both cathode and anode reactions, nearly 100% overall Faradaic efficiencies could be achieved with formate, CO and H2 as reduction products and O2 as the oxidation product. The use of the Ru complex with an electronic network provided by polypyrrole improved the performance and resulted in a stable photocurrent of 150 μAcm -2 for 1 h. HCOOH, CO and a small amount of H2 were generated under 1 sun irradiation with application potential of 0.1 V vs RHE. The selectivity for formate could reach 79% and the solar to chemical energy conversion efficiency was 0.15%, which is close to that obtained with a Ru catalyst/single-crystalline InP:Zn photocathode.

CO2 Reduction on Multi-semiconductor Systems

In the long run, earth-abundant elements-based p-type semiconductors will be preferable to materials made up of noble metals or high-quality Si. Accordingly, it appears that p-type Cu2O is an attractive semiconductor with a bandgap of 2 eV and a negative CB edge. However, its inherent instability upon reduction conditions limits the use of Cu2O for PEC applications. By atomic-layer-deposition (ALD) technique, a heterojunction device made of TiO2/ZnO:Al/Cu2O layers was obtained and showed a good stability in a water-splitting PEC cell. [START_REF] Paracchino | Highly active oxide photocathode for photoelectrochemical water reduction[END_REF] In this photocathode, the p-type Cu2O layer acts as a light absorber and forms a p-n junction with the n-type ZnO:Al layer, while the top TiO2 layer serves as a protective layer to keep the Cu2O from photocorrosion.

This hetero-junctioned TiO2/ZnO:Al/Cu2O device was also reported in reduction of CO2 to CO under light illumination. In 2015, Grätzel and colleagues applied the Re( t Bu-bpy)(CO)3Cl catalyst, introduced by Kubiak et al., [START_REF] Smieja | Re(bipy-t Bu)(CO)3Cl-improved catalytic activity for reduction of carbon dioxide: IR-spectroelectrochemical and mechanistic studies[END_REF] and a TiO2-protected Cu2O photocathode in PEC reduction of CO2 (Figure 1.15). [START_REF] Schreier | Efficient and selective carbon dioxide reduction on low cost protected Cu2O photocathodes using a molecular catalyst[END_REF] According to the nature of the p-n junction devices, [START_REF] Walter | Solar water splitting cells[END_REF] the electron transfer from photocathode to the acceptor (e.g., a molecular catalyst) is dominated by the energy difference between the CB of TiO2 and the reduction potential of the acceptor. Through the covalent bond linkages on the surface, the nanostructured photocathode allowed more immobilization of the Re(I) catalyst, leading to a higher photocurrent and a larger TON value. [START_REF] Maeda | Visible-light CO2 reduction over a ruthenium (ii)-complex/C3N4 hybrid photocatalyst: the promotional effect of silver species[END_REF] Eventually, a high photovoltage of 560 mV was achieved, and a photocurrent density of 2.1 mA cm -2 for CO evolution was obtained with a FE of nearly 100%. A further observation was made that the transfer of electrons, from the surface of the photocathode to the Rebase molecular catalyst, was greatly limited, so protic electrolyte additives, such as methanol and n-propanol, are required. Research about photovoltaics has been dominated by crystalline silicon materials for a long time. The current needs for PV is directed towards highly efficient devices that produce solar electricity at a low price. CIGS devices are considered to be among the most promising thin-film devices. Firstly, CIGS thin-film devices are quite efficient. [START_REF] Palm | White paper for CIGS thin film solar cell technology[END_REF] The conversion efficiencies could reach up 22.6% for 0.5 cm 2 cells. [START_REF] Jackson | Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6 %[END_REF] Besides, the cost of CIGS modules, calculated according to the levelized price of CIGS to collect the holes. Notably, this structure is not fixed and could be adjusted to adapt specific issues or to reach high efficiencies. This configuration is named substrate configuration: under light irradiation, the light goes inside the cell from the top layer (ZnO:Al). Each layer is described below, in terms of its role and details:

Motivation and outline of this thesis

a. Substrate

The substrate is made of a soda-lime glass, which is used for the deposition of other layers. This substrate is especially well suited because of the following factors:

• its low cost; [START_REF] Horowitz | Cost and potential of monolithic CIGS photovoltaic modules[END_REF] • good stability under high temperature (glass transition temperature: 550-600°C). The fabrication temperature of the CIGS could be higher than 350°C. [START_REF] Rudmann | Sodium incorporation strategies for CIGS growth at different temperatures[END_REF]. Generally, a high deposition temperature will result in a higher conversion efficiency. [START_REF] Rudmann | Sodium incorporation strategies for CIGS growth at different temperatures[END_REF] • a thermal expansion of 9-9.5 ppm/K which matches with those of Mo and CIGS layers;

• Na impurities. Hedström and coworkers have proved that Na can diffuse from the glass to the CIGS layer under a high temperature, which is of benefit to the CIGS [START_REF] Hedstrom | ZnO/CdS/Cu(In,Ga)Se2 thin film solar cells with improved performance[END_REF].

Additionally, one may note that recent research also found other good candidates, such as polyimide substrate, [START_REF] Kessler | Technological aspects of flexible CIGS solar cells and modules[END_REF][START_REF] Chirilă | Potassiuminduced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells[END_REF] metallic substrate [START_REF] Miasolé | MiaSolé -Home[END_REF] and ultra-thin glass. [START_REF] Gerthoffer | CIGS solar cells on flexible ultra-thin glass substrates: Characterization and bending test[END_REF]

b. Mo back-contact

The back-contact is made from a molybdenum layer because of the following factors:

• its high electron affinity (~4.5 eV). An interfacial thin layer of MoSe2 will be formed under high temperature [START_REF] Kohara | Electrical properties of the Cu(In,Ga)Se2/ MoSe2/Mo structure[END_REF][START_REF] Abou-Ras | Formation and characterisation of MoSe2 for Cu(In,Ga)Se2 based solar cells[END_REF] providing an ohmic contact with the CIGS;

• permeability and diffusion of Na element from the glass substrate to CIGS though channels among the Mo grains [START_REF] Tomassini | Synthèse de couches minces de molybdène et application au sein des cellules solaires à base de Cu(In,Ga)Se2 co-évaporé[END_REF]. Na is also good for the deposition of MoSe2 layer. respectively [START_REF] Klinkert | Differential in-depth characterization of co-evaporated Cu(In,Ga)Se2 thin films for solar cell applications[END_REF]. The band-gap of CIGS varies with this structure variation, especially with the Ga content. From CuInSe2 to CuGaSe2, the band-gap ranges from about 1.01 eV to 1.63 eV [START_REF] Paulson | Optical characterization of CuIn1-xGaxSe2 alloy thin films by spectroscopic ellipsometry[END_REF][START_REF] Alonso | Optical functions of chalcopyrite CuGaxIn1-xSe2 alloys[END_REF]. The energy value of band-gap is given by the following equation:

E g (GGI) = 1.01 + 0.626 • GGI -0.167 • GGI • (1 -GGI) GGI = [Ga]/([Ga] + [In])
The faction of Cu in CIGS is usually small ([Cu]/([Ga]+[In]<1) [START_REF] Siebentritt | The electronic structure of chalcopyrites-bands, point defects and grain boundaries[END_REF][START_REF] Scheer | Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices[END_REF]. It is reported that the doping level is 5•10 15 -5•10 16 cm -3 . In Cu-poor structures, there are plenty of defects in CIGS, which will enhance the recombination of charge carrier. So that the growth of Cu percentage would decrease the recombination [START_REF] Siebentritt | Why do we make Cu(In,Ga)Se2 solar cells non-stoichiometric?[END_REF].

Moderate Na concentration in the CIGS layer has proved to improve the performance of the cells [START_REF] Hedstrom | ZnO/CdS/Cu(In,Ga)Se2 thin film solar cells with improved performance[END_REF][START_REF] Granata | Quantitative incorporation of sodium in CuInSe2 and Cu(In,Ga)Se2 photovoltaic devices[END_REF][START_REF] Rockett | Na incorporation and diffusion in CuIn1-xGaxSe2[END_REF]. In small concentrations, Cu and In sites could be occupied by Na [START_REF] Wei | Effects of Na on the electrical and structural properties of CuInSe2[END_REF]. The effective hole density would be promoted, and the recombination would be decreased. CIGS thin films usually have an average thickness of few micrometers with large grains. As shown in Figure 2.5, CIGS layers typically have many grain boundaries across their thickness, which appear to have a mild effect on the performance of the cells. [START_REF] Rau | Grain boundaries in Cu(In, Ga)(Se, S)2 thin-film solar cells[END_REF] It was observed that the surface recombination velocity was low at the grain boundaries, as determined by electron-beam induced current (EBIC) and various other methods. [START_REF] Abou-Ras | Grain boundary types in chalcopyrite-type thin films and their correlations with film texture and electrical properties[END_REF] It has also been demonstrated that some composition variations such as a decreased Se vacancy, the presence of Na or Cu enrichment or oxygen enrichment will lead to a limitation of the recombination at the grain boundaries. [START_REF] Baier | Properties of electronic potential barriers at grain boundaries in Cu(In,Ga)Se2 thin films[END_REF] There are several deposition techniques for the materials. The high temperature physical vapor deposition (PVD) of the metallic element is the most important and main deposition technique. [START_REF] Jackson | Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%[END_REF] Other alternative methods include the sputtering of metallic precursors combined with annealing under the atmosphere of Se or S and high temperature; [START_REF] Kamada | New world record Cu(In,Ga)(Se,S)2[END_REF] alternatively, non-vacuum method including electro-deposition of metallic or oxide precursors [START_REF] Broussillou | IEEE 42nd Photovoltaic Specialist Conference (PVSC)[END_REF] has been developed.

d. CdS buffer layer

Chemical bath deposition is usually well adapted for the deposition of a CdS layer on the surface of the CIGS with a controlled composition. This CdS layer is an n-type semiconductor forming an p-n junction with the p-type CIGS layer. Upon the intermixing at the interface, the CdS layer helps passivating the CIGS surface, and protects the absorbing surface during sputtering. [START_REF] Scheer | Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices[END_REF] For Cu-poor CIGS devices, a thin layer with Cu deficiency is formed at the CIGS/CdS interface, named ordered vacancy compound (OVC). [START_REF] Schmid | Chalcopyrite/defect chalcopyrite heterojunctions on the basis of CuInSe2[END_REF] This thin layer could widen the band-gap near the interface. As a result of the valence energy band offset, holes are repelled and recombination at the CIGS/CdS interface is reduced. [START_REF] Scheer | Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices[END_REF][START_REF] Siebentritt | Why do we make Cu(In,Ga)Se2 solar cells non-stoichiometric?[END_REF] Likewise, the band-gap (2.4 eV) of the CdS is too low for use at the front-window, which result in optical losses. It should also be noted that CdS is a toxic compound, which may limit further use in devices. Other buffer layers including Zn(O,S) have been also investigated. [START_REF] Siebentritt | The electronic structure of chalcopyrites-bands, point defects and grain boundaries[END_REF] e. ZnO/ZnO:Al front layer Above the CdS layer, a thin layer of ZnO is deposited on the surface. Due to its intrinsic doping, this ZnO layer promotes electron conduction through the CB. [START_REF] Contreras | Wide bandgap Cu(In,Ga)Se2 solar cells with improved energy conversion efficiency[END_REF] Aluminum doped zinc oxide (ZnO:Al) is generally deposited to form the front-contact layer by sputtering. Optimization of the ZnO:Al layer is important to balance its transparency and its conductivityin order to avoid resistive loss. In addition, materials including MgF2 could be deposited on the top of the ZnO:Al to reduce the light reflection.

Overall, a high cell efficiency may be obtained by optimizing the substrate composition, CIGS deposition techniques and buffer/front-contact materials.

The Mo/CIGS/CdS/ZnO heterojunction

The structure of Mo/CIGS/CdS/ZnO/ZnO:Al hererojunction is a junction combined semiconductors with different band-gaps. The CIGS layers is poorly p-doped (4 10 15 cm -3 ), and the CdS layer is highly n-doped (10 18 cm -3 ). Charge diffusion at the CIGS/CdS interface results in positive charges in the n-part and negative charges in the p-part. According to the Poisson equation, an electric field is induced at the CIGS/CdS interface across a distance of ca. 300 nm due to the neutrality condition. This region lacks in free carriers, which is referred to as Space Charge Region (SCR). Free carrier density is close to the net doping density in the region extending from the SCR to the back contact layer, resulting in a neutral condition. This region is called the Quasi-Neutral Region (QNR).

Quaterpyridine complexes with CO2 reduction

In the 1930s, Morgan and Burstall firstly isolated the tetradentate 2,2':6',2'':6'',2'''-quaterpyridine (qpy). [START_REF] Gorczyński | Quaterpyridines as Scaffolds for Functional Metallosupramolecular Materials[END_REF] but the tedious synthetic process and the poor yield have greatly limited its development. proved to be one of the most effective molecular catalysts. [START_REF] Cometto | Highly Selective Molecular Catalysts for the CO2-to-CO Electrochemical Conversion at Very Low Overpotential. Contrasting Fe vs Co Quaterpyridine Complexes upon Mechanistic Studies[END_REF] In aqueous solutions, qpy complex could also work efficiently. We appended Co(qpy)Cl2 complexes on the surface of multi-walled carbon nanotubes (MWCNT) to form a heterogeneous catalyst (Co(qpy)Cl2@MWCNT) for electrochemical reduction of CO2 with high selectivity, rate and low overpotential. [START_REF] Wang | A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water[END_REF] Specifically, a selectivity of 100% for CO and a FE of 100% were obtained at very low catalyst loading (ca. 8.5 nmol cm -2 ) in water (pH = 7.3). The current density reached 0.94 mA cm -2 with a low overpotential of 240 mV (at -0.35 V vs. RHE). At the overpotential of 340 mV, the current density was up to 9.3 mA cm -2 with a good catalyst stability of 4.5 h and a TON of 89095. A current density for CO of 19.9 mA cm -2 was obtained at 425 mV overpotential.

Quaterpyridine complexes for photocatalytic reduction of CO2

In addition to their performance in electrochemical CO2 reduction, quaterpyridine complexes have also shown impressive performance for photocatalytic reduction of CO2. In 2016, our group reported that [M(qpy)(H2O)2] 2+ (M = Co, Fe) complexes are highly active catalysts for visible light driven photocatalytic reduction of CO2 to CO.

With Ru(bpy)3 2+ (bpy = 2,2'-bipyridine) worked as photosensitizer, 1,3-dimethyl 2phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) functioned as sacrificial reductant, and [Co(qpy)(H2O)2] 2+ as catalyst, a TONCO of 2660 as well as 98% selectivity for CO were obtained. Additionally, with [Fe(qpy)(H2O)2] 2+ as catalyst, a TONCO of 4300 and a selectivity of 95% were obtained. [START_REF] Guo | Highly Efficient and Selective Photocatalytic CO2 Reduction by Iron and Cobalt Quaterpyridine Complexes[END_REF] Excellent results were also obtained with an organic dye as sensitizer.

In a following work, it was shown that [Fe(qpy)(H2O)2] 2+ also showed good performance in photocatalytic reduction of CO2 to CO combined with mesoporous graphitic carbon nitride (mpg-C3N4) as the photosensitizer with a selectivity of 97% for CO and a TON of 155 under visible light irradiation for 17 hours. [START_REF] Cometto | A Carbon Nitride/Fe Quaterpyridine Catalytic System for Photostimulated CO2-to-CO Conversion with Visible Light[END_REF] Furthermore, coupling an excellent molecular catalyst with a solid sensitizer may yield a highly selective and very stable photocatalytic CO2 reduction system. Recently, our group reported a novel molecular-hybrid material, Coqpy@mpg-C3N4, which is formed by attaching Coqpy-Ph-COOH to mpg-C3N4 through an amide linkage. [START_REF] Ma | Efficient Visible-Light-Driven CO2 Reduction by a Cobalt Molecular Catalyst Covalently Linked to Mesoporous Carbon Nitride[END_REF] Under visible light irradiation, CO was generated with a selectivity of 98% and a TON of 128 in a acetonitrile solution, using BIH as sacrificial reductant and phenol as proton donor. This Coqpy@mpg-C3N4 material showed impressive stability during 4 days with no degradation.

Coupling quaterpyridine complex with PV materials could be a promising way to reduce CO2 under solar light, especially with a p-n junction material allowing for working near or under thermodynamic potential for CO2-to-CO conversion, thanks to the photopotential generated by the semiconductor material.

Functionalization of cobalt quaterpyridine 2.3.1 Cobalt quaterpyridine synthesis and functionalization

Cobalt quaterpyridine catalyst was modified with two phosphonic acid groups

Immobilization of modified cobalt quaterpyridine on TiO2

Among all the semiconductors applied in photocatalysis, TiO2 is the most common one which is widely used for dye bleaching, pollutant degradation and water The EDX-mapping experiment (Figure 2.12) reveals homogeneous distribution of the catalyst across the surface of the electrode. In addition, mapping of Co element indicated that there was no aggregation of cobalt. The consistent atomic ratio of phosphate and cobalt is 2:1, confirming both the presence of the catalyst as well as its intactness after immobilization. Compared to previously reported data (85 nmol cm -2 for a Re complex on mesoporous TiO2 [START_REF] Shen | High-Yield and Selective Photoelectrocatalytic Reduction of CO2 to Formate by Metallic Copper Decorated Co3O4 Nanotube Arrays[END_REF]), this value is approximately one-third lower, suggesting that there is still potential to improve.

Electrochemical performance for CO2 reduction

In order to test the catalytic performance of Co-qPyH|TiO2 electrode, CV was first recorded in 0.1 M KHCO3 water condition saturated with CO2 (pH = 6.8). Catalytic current was observed as shown in Figure 2.13. A long-term electrolysis was performed at -0.51 V vs. RHE for 2 h, which showed an average catalytic current of 1.2 mA cm -2 (Figure 2.14). The products were investigated through gas chromatography (GC) analysis indicating the production of CO with a selectivity of 80% and FE of 63%. The only by-product H2 was also detected.

Within the first 15 min, the current density had reached a steady-state. Over the next two hours, it remained stable with no significant changes. For comparison, a blank experiment is also performed in water with the same pH condition (0.1 M phosphate buffer) under argon at the same potential and did not yield any CO. XPS measurements of the electrodes were performed before and after a 2 h electrolysis in 0.1 M KHCO3 saturated with CO2 (pH 6.8) at -0.51 V vs. RHE (Figure

2.11).

There was a slight decrease in signal intensity, which could be attributed to a slight desorption of the catalyst rather than decomposition of the catalyst. This is probably due to the pH value being very close to the upper limit of the stability of the phosphonate group [START_REF] Jang | Unbiased Sunlight-Driven Artificial Photosynthesis of Carbon Monoxide from CO2 Using a ZnTe-Based Photocathode and a Perovskite Solar Cell in Tandem[END_REF].

According to the above results, the cobalt quaterpyridine complex kept its catalytical activity upon immobilization on a quasi-transparent m-TiO2 electrode after functionalization with phosphonic acid. In our previous work (as introduced in Chapter 2.2.1), the Co(qpy)Cl2 catalyst was immobilized on carbon nanotubes in water (pH = 7.3) showing a larger current density up to 19.9 mA cm -2 [START_REF] Wang | A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water[END_REF]. Although the current density is lower here, but the TiO2 inorganic layer offers the potential to be used as a support layer on photoelectrode surfaces for solar-driven reduction of CO2.

Photovoltaic-electrochemical performance of CIGS and functionalized TiO2

CIGS for hydrogen evolution has been reported [START_REF] Palm | White paper for CIGS thin film solar cell technology[END_REF][START_REF] Jackson | Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6 %[END_REF], but they have not yet been used for CO2 reduction, despite their attractive properties. Considering the good performances of Co-qPyH|m-TiO2 electrode in EC conditions as described in Chapter 2.3, we then thought about combining this material as a top layer with CIGS as the photocathode to reduce CO2 under light irradiation.

Photocathode assembly and characterizations

We prepared a CIGS component with a p-n junction made up of a p-type CIGS To combine the Co-qPyH catalyst and CIGS devices, we firstly devised a strategy (as illustrated in Figure 2), which coupling an external CIGS PV cell (ZnO:Al|ZnO|CdS| CIGS|Mo material) with the above Co-qPyH|m-TiO2 electrode (Figure 2.17). The coupling of a PV cell to an electrolyzer (PV-EC) is a practical method of utilizing the photogenerated charges in the cell devices and activating a substrate in solution. The photoactive material in this configuration is not in directly in contact with the electrolyte solution, which partially addresses the stability issue. An average current density of 1.78 mA cm -2 was observed and the selectivity for CO production was maintained between 82 and 85% over the course of the experiment. A labeled experiment under 13 CO2 atmosphere is compulsory to conclude on the origin of the product of CO. Electrolysis experiment was repeated with 13 CO2. The products were analyzed via GC/MS. In the meantime, the result was compared with that under 12 CO2 forcefully confirming that CO originated from CO2 (Figure 2.26).

Conclusions and perspectives

In this chapter, we successfully demonstrated the application of CIGS photocathode for solar-driven CO2 catalytic reduction. For coupling with CIGS, cobalt quaterpyridine molecular catalyst was functionalized and homogeneously immobilized on a TiO2 upper layer without structural damage and loss of activity confirmed by a series of complementary measurements. Our step-by-step strategy from PV-EC configuration eventually to PEC configuration was proposed to combine CIGS with Co-qPyH, and good activity, selectivity and negative overpotential were obtained under visible-light irradiation in neutral water condition. It is worth mentioning that this study led to the development of the first PEC device with high photoelectrocatalytic activity in aqueous solution and a 97% of selectivity for CO. In contrast to other previously reported PEC CO2 reduction systems, including those with noble metals (see entries 1, 5, and 6 in Table 2), our work shows pioneering performance. For instance, with a Ru catalyst grafted onto a hematite photocathode [START_REF] Sekizawa | Solar-driven photocatalytic CO2 reduction in water utilizing a ruthenium complex catalyst on p-type Fe2O3 with a multiheterojunction[END_REF], a maximum photocurrent density for CO of 45 μA cm -2 was obtained at 110 mV overpotential, which is approximately one order of magnitude less than that of the Co-qPyH|f-TiO2|CIGS photocathode.

In this study, we present the first report of CIGS semiconductor as a component of photocathode with molecular catalyst for CO2 reduction in aqueous medium. It will open up the possibility of better-performing photocathode for PEC reduction of CO2. A higher catalyst loading would further contribute to the enhancement of this system by using a strategy such as nano-structuration of the upper layer. A fully unassisted solardriven fuel production system could be achieved when coupled with an appropriate photoanodic reaction.

CO2 reduction to methanol

Electrochemical and photochemical conversion of CO2 into liquid fuels is one of the most difficult and important chemical challenge. Although CO and formate could be interesting molecules, further reduced products, such as methanol, are particularly appealing. Methanol is an excellent energy carrier because of its high volume-specific energy density and it isan intermediate for many bulk chemicals. From CO2 to CH3OH, it is a six-electron six-proton catalytic reduction process. Efforts have been devoted to the investigation of various catalysts such as metal oxides, metal alloys, or chalcogenide-based materials. [START_REF] Sun | Molybdenum-Bismuth Bimetallic Chalcogenide Nanosheets for Highly Efficient Electrocatalytic Reduction of Carbon Dioxide to Methanol[END_REF][START_REF] Albo | Production of methanol from CO2 electroreduction at Cu2O and Cu2O/ZnO-based electrodes in aqueous solution[END_REF][START_REF] Navarro-Jaén | Highlights and challenges in the selective reduction of carbon dioxide to methanol[END_REF] Most of these work either focus on rare metals, or report low selectivity for methanol with the formation of cogenerated products including methane or C2 molecules. [START_REF] Hatsukade | Carbon Dioxide Electroreduction using a Silver-Zinc Alloy[END_REF][START_REF] Low | Enhanced Electroreduction of Carbon Dioxide to Methanol Using Zinc Dendrites Pulse-Deposited on Silver Foam[END_REF] In addition, Mul et. coworkers reported that the thermodynamic stability window of metal oxides is limited [START_REF] Kas | Electrochemical CO2 reduction on Cu2O-derived copper nanoparticles: controlling the catalytic selectivity of hydrocarbons[END_REF] while reduced metal electrodes lead to a lower activity for CH3OH generation. Consequently, molecular catalysts could be an interesting alternative to achieve high selectivitythanks to the control of the active site structure, as previously done in our group for the CO2to-CO reduction process. [START_REF] Azcarate | Through-space charge interaction substituent effects in molecular catalysis leading to the design of the most efficient catalyst of CO2to-CO electrochemical conversion[END_REF][START_REF] Costentin | A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst[END_REF] There are still few examples of molecular catalyst driving the CO2 reduction up CH3OH. Some transition-metal complexes with the help of heterogeneous co-catalyst showed some methanol evolution. [START_REF] Ogura | Conversion of carbon monoxide into methanol at room temperature and atmospheric pressure[END_REF][START_REF] Ogura | Electrocatalytic reduction of carbon dioxide to methanol. Part 2. Effects of metal complex and primary alcohol[END_REF][START_REF] Ogura | Electrocatalytic reduction of CO2 to methanol: Part 9: Mediation with metal porphyrins[END_REF] Besides, a few papers reported methanol as CO2 reduction product using molecular catalysts, but the exact source of the alcohol was not identified. [START_REF] Abdinejad | Enhanced electrochemical reduction of CO2 catalyzed by cobalt and iron amino porphyrin complexes[END_REF] As described in Chapter 1.2.2 (Figure 1.11) [START_REF] Arai | Photoelectrochemical reduction of CO2 in water under visible-light irradiation by a p-type InP photocathode modified with an electropolymerized ruthenium complex[END_REF], pyridine complex was used in a PEC CO2 reduction process and showed trace of methanol. Nevertheless, no evidence proved that the generated methanol was issued from CO2. In contrast, our previous work reported demonstrated for the first time that cobalt phthalocyanine (CoPc) is able to electroreduce CO2 into CH3OH in aqueous media through a CO intermediate. [START_REF] Boutin | Aqueous Electrochemical Reduction of Carbon Dioxide and Carbon Monoxide into Methanol with Cobalt Phthalocyanine[END_REF] Since the CO2-to-CO step is very efficient at a neutral pH, a good strategy would be to combine an optimized CO2-to-CO step in a first electrolyzer with the following step of CO reduction to methanol in a second electrolyze. In basic media (pH 12-13), pure CO was used as a substrate and the Faradaic efficiency (FE) towards methanol generation up to 14.3% was detected. an overall FE from CO2 to CH3OH of 19.5% may be achieved.

Cobalt phthalocyanine complexes

Phthalocyanine (Pc) are a well-known family of catalysts which is industrially used as dyes for painting [START_REF] Lomax | Phthalocyanine and quinacridone pigments: their history, properties and use[END_REF] and are thus particularly appealing as easily accessible compounds. Among them, CoPc has been extensively studied in electrochemistry, [START_REF] White | Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes[END_REF][START_REF] Tran | Recent advances in hybrid photocatalysts for solar fuel production[END_REF] and used as a dye at a large scale in the painting industry due to its low cost. An early study of Lieber et al. demonstrated the reduction of generation of CO2 to CO on a carbon electrode modified by absorbed CoPc at an applied potential of -1.15 V vs.

SSCE (silver-silver chloride electrode) in an aqueous solution (pH = 5), with a turnover number (TON) of 3.710 5 . [START_REF] Lieber | Catalytic reduction of carbon dioxide at carbon electrodes modified with cobalt phthalocyanine[END_REF] The faradaic efficiency (FE) of CO was 55 ~ 60% with H2, oxalate and formate as by-products. It is notable that this study showed a good stability attributed to immobilized CoPc on the electrode compared with the homogeneous case. Choi et al. [START_REF] Choi | Steric Modification of a Cobalt Phthalocyanine/Graphene Catalyst to Give Enhanced and Stable Electrochemical CO2 Reduction to CO[END_REF] developed a sterically hindered CoPc that led to electrochemical CO2-to-CO competion with up to 85% FE in aqueous solution (pH 6.8), at a potential of -0.79 V vs. RHE. In this study, CoPc π-stacked onto chemically converted graphene which was deposited on a carbon paper electrode. In a recent study, [START_REF] Roy | Electrocatalytic and Solar-Driven Reduction of Aqueous CO2 with Molecular Cobalt Phthalocyanine-Metal Oxide Hybrid Materials[END_REF] Reisner et al. reported the CO2-to-CO conversion on a hybrid electrode composed of a CoPc complex bearing four phosphonic acid anchoring groups immobilized on mesoporous TiO2. Our previous work reported that cobalt phthalocyanine was used as a catalyst for the electrochemical conversion of CO2 to methanol in aqueous electrolytes at ambient conditions of temperature and pressure. [START_REF] Boutin | Aqueous Electrochemical Reduction of Carbon Dioxide and Carbon Monoxide into Methanol with Cobalt Phthalocyanine[END_REF] We set a simple procedure for preparing the catalytic electrode, and the low loading amounts of the catalyst (a few nmol cm -2 ) make the process versatile and easy to implement. It allowed to identify formaldehyde as a key intermediate and pH effect on selectivity. This can help for establishing asequential process where CO2 is first converted to CO, which would be further used as areactant to produce methanol. 14.3% methanol (FE) was generated at -0.64 V vs. RHE in aqueous solusion (pH = 13) with pure CO as substrate. This study opens up possibilities for developing earth-abundant metal-based molecular complexes as electrocatalysts for CO2 reduction to liquid fuel in water.

Photo-assisted reduction of CO2 to methanol

Considering the excellent performance of the CIGS-based semiconductors in PEC reduction of CO2-to-CO (as described in Chapter 2) and the good capacity of CoPc for CO2 reduction to methanol (as introduced in Chapter 3.1), it would be an interesting strategy to drive CO2 reduction to methanol under solar light through combining a CIGS-based PV material with CoPc complex as catalyst.

Thus, we decided to combine photovoltaics and electrochemistry in PV-EC configuration. Specifically, we coupled the light conversion and the chemical process by using a CIGS-based PV device generating electrons and an EC cell employing these electrons to carry on the electrochemical reactions. This approach has two advantages.

Firstly, both PV and EC parts could benefit from optimization independently concerning their separated requirements. Secondly, the normally encountered photodegradation of PV devices in contact with an electrolyte is bypassed. Therefore, a PV-EC coupling cell (Figure 3.2) is also an interesting approach to utilize solar generated electrons to sustainably activate and convert CO2 in aqueous solutions. [START_REF] Cheng | CO2 Reduction to CO with 19% Efficiency in a Solar-Driven Gas Diffusion Electrode Flow Cell under Outdoor Solar Illumination[END_REF][START_REF] Creissen | Solar-Driven Electrochemical CO2 Reduction with Heterogeneous Catalysts[END_REF] Figure 3.2 Scheme of the PV-EC system combining a CIGS PV cell linked to a CoPc loaded carbon paper used as the working electrode for CO2 reduction in the EC unit.

CIGS cell characterizations

The structure of the CIGS PV cell used in this work is shown in Figure 3.2. As introduced in Chapter 2, the p-tpye CIGS thin films were synthesized following a so called CuPRO-process (described in Chapter 4). The ratio of gallium to indium within the films was large enough to yield an absorber band gap of 1.6 eV, measured from absorption threshold of the external quantum efficiency (Figure 3.3).

Preparation of porous carbon-CoPc electrode

To make the catalyst film, a catalytic ink was prepared by mixing CoPc with multiwalled carbon nanotubes (MWCNT) and Nafion resin in a mixture of ethanol and ethylene glycol, to obtain a stable colloidal solution (see Chapter 6 Experimental Section, for details). Ink deposition onto porous carbon paper was made by drop casting, and the operational catalytic film was obtained after drying at 100°C. This preparation is similar to the method used previously in the laboratory.

Photo-assisted electrolysis cell assembly

On the external CIGS PV cell, a front (-) and a back (+) lateral contacts were taken on AZO and Mo layers with copper wire, respectively. The front side (AZO layer) was connected to a carbon paper electrode with copper wire, and the back side was connected to an Autolab potentiostat. Regarding the EC part, a standard three-electrode configuration was employed with a Pt mesh as the counter electrode, which was separated from the working compartment by a glass frit bridge filled with the same electrolyte, and a saturated calomel electrode (SCE) as reference electrode (Ref). basic conditions, as described in our initial electrochemical study. [START_REF] Boutin | Aqueous Electrochemical Reduction of Carbon Dioxide and Carbon Monoxide into Methanol with Cobalt Phthalocyanine[END_REF] Electrolysis experiments were performed in 0.1 M KOH (pH = 13) and at different applied bias voltage (Figure 3.7). Notably, during the first 200 s, a chopped-light experiment was conducted to check for the photo-response of the system, which explains the shape of the current signal (Figure 3.8). Very interestingly, methanol was produced in significant amount, further illustrating that CO is an intermediary step to further reduction. During 2 h electrolysis, a quite stable current of 1.18, 1.32 and 2.10

Photovoltaic-Electrochemical performance for CO2 reduction

mA was observed at bias voltage of -0.09, -0.14 and -0.19 V vs. RHE respectively (Table 3.1, entries 3-5). The more negative the bias voltage the higher the total current, with H2 as main product and in increasing proportion. A maximum 19% FE for methanol was obtained at -0.14 V vs. RHE (Table 3.1, entry 4), while at a positive bias voltage of +0.01 V vs. RHE, no current was observed (Table 3.1, entry 6), with no CO conversion occurring.

Labeled experiments

After electrolysis under CO atmosphere at -0.14 V vs. RHE, a 1 H NMR spectrum of the electrolyte (see Figure 3.9 Grey) shows a series of peaks at both side of the broad -OH peak (4.79 ppm). Among them, two could be attributed to CO reduction products:

formate (singlet at 8.44 ppm) a commonly reported product, and methanol (singlet at 3.34 ppm). Then isotope labelled experiments under 13 CO atmosphere were conducted to confirm that CH3OH originated from CO.

A labeled experiment under 13 CO atmosphere is indeed compulsory to conclude on the nature and the origin of this peaks as shown in Figure 3.9 magenta (see details in Chapter 4). In such conditions, both the 3.34 and 8.44 ppm peaks are splitted with a respective JCH strength characteristic of methanol and formate, definitively proving the

Effects of area ratio of Scarbon electrode and SCIGS

To optimize the EC part, we investigated the influence of the surface area of the carbon paper electrode. At a fixed CIGS surface area of 0.9 cm², three carbon paper electrode areas, 0.5, 1 and 1.5 cm² were tested (Figure 3.10). While the current was multiplied by a factor of. 2 when going from 0.5 (Table 3.1, entry 4) to 1 cm² (entry 7), almost no current increase was noticed with a further increase to 1.5 cm² (entry 8). This seems to indicate that an insufficient amount of electrons was provided by the CIGS PV cell to fully feed an electrode of such area. It is worth noting that, when the surface area of carbon paper electrode reached 1 cm 2 , the FE for methanol got to a maximum value of 21 %. Blank experiment was conducted under argon atmosphere (Table 3.1, entry 9) with 1 cm 2 Scarbon paper, and we observed no CO or methanol production, only H2 was detected in a similar amount than under CO atmosphere, suggesting an independent reaction pathway to hydrogen.

Long-term stability investigation

To evaluate the stability of the PV-EC assembly and the evolution of the products generated with time, we conducted an extended electrolysis under CO atmosphere for 7 h at -0.14 V vs. RHE in 0.1 M KOH (pH = 13). Both liquid and gaseous reduction products were analyzed periodically (Table 3.2). As can be seen in Figure 3.11, even if a little bit fluctuating, the current remained quite constant over the electrolysis course, at a average value of -1.75 mA, together with a linear evolution of the charge passed. Gas chromatography and 1 H NMR analysis of the products revealed that, first, FE for methanol was progressively lowered from 16% after the first electrolysis hour to 10% after 7 h (Table 3.2). Hydrogen FE was concomitantly increased from 65 to 70% FE. However, cumulated molar concentration of methanol in the reaction mixture evolved linearly over the whole process up to a total amount of 2.4 mM. Additionally, methanol production occurs at a very low overpotential. The reasons for progressive decrease of methanol production (catalyst leaching from the electrode, catalyst degradation in the film) remain unclear and are currently investigated in our laboratory.

Overall efficiency approach

According to our previous work, in terms of application, the choice of going sequentially, CO2 reduction to CO in a first electrolyzer followed by CO reduction into methanol in a second one, rather than using a single reactor will be mainly cost-driven.

Concerning these two electrolyzers in series, operating the theoretical cathodic reaction described in equation 3. with a Faradaic efficiency of 21%.

Conclusions and perspectives

In this chapter, we have showed promising results with the combination of a CIGS PV cell with a molecular catalyst deposited on a carbon electrode to achieve the solar assisted reduction CO2 to CO and then to methanol reduction in aqueous media. The origin of carbon products was asserted by isotope labelled experiments. In this PV-EC system, the electrochemical part is only based on abundant elements (carbon, cobalt complex) and is operational at ambient pressure and temperature, the employed Step 1

Step 2 molecular catalyst belonging to an already industrially used compounds (cobalt phthalocyanine).

CIGS unit showed high efficiency and supplies a large photovoltage to feed the EC unit. The formation of mM amounts of methanol was reached after few hours of electrolysis at very moderate applied bias.

First stability tests are highly encouraging, with no great loss of activity during several hours. Our results indicate that the performance of the system depend on the surface area matching between the PV cell and the electrode. More work is under way to gain both in understanding and in productivity. photocorrosion. [START_REF] Zeng | CO2 Reduction to Methanol on TiO2-Passivated GaP Photocatalysts[END_REF] In these studies, no experimental evidence showed that methanol is issued from CO2 reduction. Despite these interesting early results, high efficient and long-term stable system towards CO2 reduction has not yet been designed.

Hydrogen evolution reaction at bare GaP photoelectrode

The wide bang gap of GaP semiconductor theoretically provides enough energy to directly split water in a PEC cell. The Cronin group reported photocatalytic water splitting on TiO2 passivated GaP. The TiO2 passivation layer prevents corrosion of the GaP surface, making it stable in pH = 0 solution.

In order to check the preparation of the photoelectrode and the PEC properties of this obtained GaP material, a simple HER reaction was explored in 0.5 M H2SO4.

Irradiation were performed with the light of a Xe lamp (150 W) combined with a Schott GG435 long pass filter and 2-cm-long glass (OS) cell filled with deionized water to cut off infrared and low ultraviolet incident photons. PEC experiments were performed in a three-electrode system using a Pt counter electrode and a SCE reference electrode separated from the working electrode compartment by bridges. For the TiO2 passivated GaP, we get a smaller current density of 6 µA cm -2 . The products were investigated through gas chromatography analysis. In both conditions, no products were detected. As already pointed out [START_REF] Qiu | Plasmon-enhanced water splitting on TiO2passivated GaP photocatalysts[END_REF], p-type GaP acts as a photocathode due to the direction of band bending at its interface with the solvent, promoting photoelectron flow to the water. For n-type semiconductors, the bending is usually upward, resulting in photohole flow to the water, as illustrated in Figure 4.7 (a). When the TiO2 thickness is further increased, there is enough donor impurities to bend the band upward, hindering the electron flow to the water, as shown in Figure 4.7 (b). It is, therefore, somewhat surprising that when n-type TiO2 with 5 nm thick is coated on top of p-GaP, the current density decreased. Further investigation will be done in our group to pursue these first unsuccessful tests.

Preliminary experiments in acetonitrile

Having in mind the good catalytic performance of CoPc molecular catalyst, we also preliminary explored a strategy using GaP as photoelectrode and CoPc as catalyst to drive PEC reduction of CO2 in a homogeneous system. We adapted a new method to prepare to photoelectrode, on account of the corrosion of tape in organic solvent. Firstly, an ohmic contact was made to the back of the crystal using an indium-gallium solder and connected to an external lead with copper wire. Then we use an epoxy resin to expose only the front face of the semiconductor to solution.

According to the work of Sauvage et al., a small photocurrent could probably be due to a passive film formation on the top of GaP crystal. [START_REF] Sauvage | Photoassisted electro-reduction of CO2 on p-GaAs in the presence of Ni cyclam 2+[END_REF] A pretreatment was thus conducted before being used as photocathodes. The GaP crystals were etched in HF + HNO3 + H2O (1: 3 : 4), and an ohmic contact (Au + Zn) was made on their back side by vapor deposition under vacuum, followed by 20 min heating at 400°C under an N2 atmosphere. Before each experiment, the semiconducting electrode was successively enched in 1 M KOH and HF + HNO3 + H2O (1: 3 : 4). LSV was performed in acetonitrile in the presence of phenol (PhOH, 1 M) on a GaP photoelectrode under chopped light illumination (Figure 4.9), which showed a photo-response with an onset potential of -0.6 V vs. SCE and small photocurrent as well. A blank experiment was also conducted under Ar exhibiting negligible photocurrent in the same conditions (Figure 4.9). At that stage, and due to the small current density, electrolysis and product identification have not been performed.

-

Perspectives

This work was just a first step. Beside the pretreatment process of the GaP crystals to increase the PEC performance, the thickness of TiO2 protective layer needs to be optimized since it has a significant effect on the performance (e.g. resistances).

Considering our previous work (described in Chapters 2 and 3), it should be interesting also to graft molecular molecular catalyst on the surface of GaP based photoelectrode.

for photoelectrochemical reduction of CO2. Cobalt quaterpyridine complexes were functionalized and homogeneously immobilized on a TiO2 upper layer on CIGS.

Various spectroscopic characterization techniques showed no structural damage and loss of activity of the catalyst. A PV-EC configuration was designed with a carbon paper electrode to load the catalyst connected to a CIGS device. Average current density of 3.25 mA cm -2 and a selectivity of 87% for CO were obtained with a bias potential of -0.06 V vs. RHE in water, corresponding to a formally negative overpotential relatively to the thermodynamic potential (η = -50 mV). A long-term electrolysis for 7 h showed a good stability and the selectivity for CO was maintained between 82% and 85% over the whole experiment course. Then a PEC configuration was developed to combine CIGS photocathode with functionalized catalyst, leading to a selectivity of 97% for CO2 (and 3 % for H2 as the only by-product) at -0.06 V vs. RHE, corresponding gain to a formal overpotential of -50 mV in pure aqueous media. The photocathodes were stable for a couple of hours. At low overpotential (less than 100 mV), current density up to 3-4 mA cm -2 could be obtained in chopped-light, short time experiment. When compared to previously reported PEC CO2 reduction systems, even those employing rare metal complex as catalyst, this work shows excellent performance.

2. Encouraged by these results, we further investigated decoupling the process of CO2 reduction to methanol in order to maximize efficiency, upon first realizing CO2to-CO reduction in optimized conditions in an electrochemical cell and then using CO as a reactant in a second photoelectrochemical cell, in aqueous solution (basic conditions). In a PV-EC system, we successfully combined a carbon paper electrode risk of decomplexation, so the following procedure was instead followed. Co-qPyE (50 mg, 0.06 mmol) was taken in an oven-dried three-neck round-bottom flask kept under Ar atmosphere. About 6 mL of extra dry DMF was added followed by 0.25 mL of distilled triethylamine added to the mixture. Then 0.15 mL of SiMe3Br was added to the reaction mixture dropwise, and the reaction mixture was stirred for 48 h at 45°C.

The reaction mixture was cooled, and solvent was removed through reduced pressure.

The residue was diluted with very dilute aqueous solution of HCl (pH = 6.8) to remove quaternary ammonium salt formed during reaction and protonation of phosphonic acid.

Immobilization of Co-qPyH on TiO2

A 0.2 mM solution of Co-qPyH was prepared in methanol (10 mL). Two drops of ultra-pure water were added, and the mixture was sonicated for 10 min with continuous degassing by Ar. Then electrodes were dipped in the solution, and was heated at 45°C in the oven for 48 h. The electrodes were completely rinsed with methanol and dried under nitrogen atmosphere.

CIGS electrode preparation

Mo back contact layer was deposited by DC sputtering, with total thickness of 500 nm. CuIn0.1Ga0.9Se2 (CIGS) layers were grown onto Mo coated soda-lime glass substrates by co-evaporation from elemental sources. The soda-lime glass consists of 1-mm-thick microscope slides. The process used for CIGS deposition is known as CuPRO process. [START_REF] Kessler | Cu(In,Ga)Se2 thin films grown with a Cu-poor/rich/poor sequence: growth model and structural considerations[END_REF] Specifically, keep In and Ga fluxes constant and change Cu flux so that nominal composition of the growing film undergoes Cu-poor/Cu-rich transitions needed for high-performance devices. [START_REF] Barreau | Recrystallization of CIGSe layers grown by three-step processes: A model based on grain boundary migration[END_REF] Substrate temperature was kept at 580°C

during the whole deposition.

The n-type CdS layer was grown by chemical bath deposition. The p-CIGS/n-CdS heterojunction was finally covered with Radio Frequency (RF) sputtered ZnO/ZnO:Al bilayer. Here the applied ZnO:Al-layer thickness was ca. 300 nm (Rsheet = 20 Ω/sq).

Characterization Methods

Solar simulator

Irradiation was simulated with a 150 W Xenon lamp from Oriel Instrument with light source positioned at 20 cm from the PV cell for PV-EC configuration and 25 cm from the photoelectrode in PEC configuration. UV light was filtered at wavelength below λ = 435 nm with a low-pass filter (Schott GG435). Part of the light above 1000 nm was also filtered via a water filter to avoid electrolyte warmup. In PEC configuration of Chapter 2, the cell was equipped with a quartz window to avoid parasite light absorption.

UV-visible spectroscopy

UV-visible absorption measurement was conducted on a Agilent Cary 60 spectrometer.

Aliquots of the solution were poured into a quartz cell (1 cm length). Typical spectral region ranged from 200-800 nm and it was swept at a rate of 720 nm s -1 .

and AVANCE 400 BRUKER at room temperature. Chemical shifts for 1 H and 13 C NMR spectra are referenced relative to residual protium and 13 C in the deuterated solvent (CDCl3 δ= 7.26 ppm for 1 H and δ= 77.16 ppm for 13 C). Chemical shifts are given in ppm and coupling constants in Hz.

Gas chromatography (GC)

GC analyses of gas during the electrolysis (H2, O2, N2, CO, and CO2) were conducted with an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector and a capillary column (CARBOPLOT P7), with Argon as carrier gas.

Calibration curves for H2 and CO were determined separately by injecting known volumes of pure gas. These apparent TON value can be regarded as a lower estimation of the real number of catalytic cycles because the electrolysis is not performed until the total deactivation of the catalyst/photoelectrode. Moreover, in most cases, the amount of active molecular catalyst is significantly smaller (ca. up to an order of magnitude) than For PV-EC system in Chapter 2, the external CIGS PV cell is connected in series with a Co-qPyH|m-TiO2 electrode. Front (-) and back (+) lateral contacts of the cell were taken on ZnO:Al and Mo layers, respectively. When the geometric surface of TiO2 is different from the surface of irradiated CIGS, current density is reported per surface of electrode and per surface of PV. When only one single value is reported for current density, it means that both CIGS and TiO2 surfaces have been intentionally set to the same value. In PEC configuration, working electrode was covered with an insulating tape covering the edges to avoid contact between underlayers and electrolyte. A window was previously cut in this tape to define the working area. Working surface was between 0.15 and 0.25 cm 2 .

6.2.10 J(V) curve measurement.

J(V) measurements were performed in conditions as close as possible to those defined by standard testing conditions, namely at 25°C, under AM1.5 (using filtered light source) illumination normalized at 1000 Wm -2 . The size of the investigated solar cells was about 1 × 0.5 cm 2 .The actual value of Jsc was deduced from the integration of EQE measured from a laboratory-built setup. The surface shaded by the grids, accounted in the Jsc calculation, is about 3%.

  photoélectrochimique, en solution aqueuse (conditions basiques). Dans un système PV-

  Réduction photoélectrochimique du CO2, catalyseur moléculaire, CIGS, cobalt quaterpyridine, phtalocyanine de cobalt Contents Ré sumé én français ...................................................................................................... Abstract .......................................................................................................................... Résumé ......................................................................................................................... Chapter 1 -Introduction ............................................................................................... 1.1 Photoelectrochemical reduction of CO2: towards a sustainable future ........... 1.1.

Figure 1 . 1

 11 Figure 1.1 Two pathways for the conversion of solar energy-to-chemical fuels.

Figure 1 . 2 1 . 1 . 2

 12112 Figure 1.2 Scheme of the PEC CO2 reduction into chemicals fuels under solar

Figure 1 . 3

 13 Figure 1.3 Energy bands structure of a semiconductor material.

Figure 1 . 4

 14 Figure 1.4 Migration of a photoexcited electron from a semiconductor to a catalyst.

Figure 1 . 5

 15 Figure 1.5 Recombination of photoexcited electron-hole pair in a semiconductor

Figure 1 . 6

 16 Figure 1.6 Band structure of an (a) n-type semiconductor and an (b) p-type

Figure 1 . 7

 17 Figure 1.7 Principle of the p-n junction in semiconductors. Dark dots are

Figure 1 . 8

 18 Figure 1.8 Thermodynamic constraints on the migration of electron and hole to

Figure 1 . 9

 19 Figure 1.9 CB (white squares) and VB (gray squares) potentials of some

Figure 1 . 10

 110 Figure 1.10 Scheme of band bending at the p-type semiconductor/electrolyte

Figure 1 . 11

 111 Figure 1.11 Scheme of p-type Si photocathode with Re(bipy-Bu t )(CO)3Cl.[START_REF] Kumar | Photoreduction of CO2 on p-type Silicon Using Re (bipy-Bu t )(CO)3Cl: Photovoltages Exceeding 600 mV for the Selective Reduction of CO2 to CO[END_REF]

Figure 1 . 12

 112 Figure 1.12 The structure of [Ru II (L-L)(CO)2Cl2] complex, deposited at a Zn-

Figure 1 . 13 Figure 1 . 14

 113114 Figure 1.13 Scheme of CO2 reduction over a Ru Complex/TiO2/N,Zn-Fe2O3/Cr2O3

Figure 1 . 15 Figure 1 . 16

 115116 Figure 1.15 Scheme of the PEC CO2 reduction involving TiO2-protected Cu2O

1

 1 Structure and properties of Cu(In,Ga)Se2 materials Nowadays, semiconductor industry steps into the solar Photovoltaics (PV) market. PV materials convert light energy into electricity using semiconductor materials which exhibit photovoltaic effect. Since the 1950s, the development of crystalline silicon with p-n junction made much progress in the field of PV, and devices are no more restricted to silicon materials anymore. Various PV materials have come into the market, including organic semiconductors and each of them have advantages and drawbacks. Solar PV devices are usually named according to the primary light-absorbing material. As shown in figure 2.1, PV devices are typically classified into two categories[55]: wafer-based and thin-film cells. Wafer-based cells are made up of semiconductor wafers and normally covered with glass for protection and mechanical stability, which could be handled with no additional substrate. While thin-film cells are fabricated with semiconductor films deposited onto a substrate such as glass, plastic, or metal. What's more, thin-films devices can further be sub-classified into commercial and emerging thin-film cells. The commercial availability of PV devices has also made progress. Several of them get much attention: the crystalline or multi-crystalline silicon devices (c-Si) and the thin-film based devices including the cadmium-telluride (CdTe) cells, amorphous silicon cells (a-Si) and polycrystalline chalcopyrite Cu(In,Ga)Se2 cells (CIGS).

Figure 2 . 1

 21 Figure 2.1 Typical solar PV device structures. Primary absorber layers are labeled in

  electricity, has further decreased due to the reduction of fabrication cost and up-scaling of the production, making them more competitive. According to a survey from 2016, Solopower, Avancis, Hanergy, Solar Frontier, Stion, Silva Power, and Manz are among the major CIGS manufacturers.[START_REF]Photovoltaics Report[END_REF] In Figure2.2, the market share of CIGS devices in the PV market is illustrated. In 2015, the thin-film modules represented ca. 7% on the whole production of PV devices and the CIGS production was accounting for nearly half of this percentage. There has been a rapid increase since 2010.

Figure 2 . 2 2 . 2 . 1 .Figure 2 . 3

 2222123 Figure 2.2 Annual Global PV Module Production[START_REF]Photovoltaics Report[END_REF]

[70, 71 ]Figure 2 . 4

 7124 Figure 2.4 Unit cell of chalcopyrite for stoichiometric Cu(In,Ga)Se2. a and c are the

Figure 2 . 5

 25 Figure 2.5 Cross sectional SEM image of a complete CIGS device.[83]

Figure 2 .

 2 6 shows the simulated band-diagram of a CIGS cell on SCAPS (a software invented by Prof. Burgelman et al. version 3.2.01[START_REF] Burgelman | Modelling polycrystalline semiconductor solar cells[END_REF][START_REF] Burgelman | Modeling thin-film PV devices[END_REF]) according to the materials parameters utilized by Chirilă et al.[START_REF] Chirilă | Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films[END_REF]. As can be seen from his diagram, the VB and CB exhibit discontinuities as a result of variations in band-gap, electron affinities and doping concentration for each layer.At the CIGS/CdS interface, a negative VB offset and a positive CB offset are obtained.Recombination at the interface is thus reduced.

Figure 2 . 6

 26 Figure 2.6 Band-diagram for the Mo/Cu(In0.7Ga0.3)Se2/CdS/ZnO/ZnO:Al junction

( 6 (Figure 2 . 7 a.

 627 Figure 2.7 a. Synthetic route for Co-qPyH, b. CV in 0.1 M KHCO3, saturated with

Figure 2 . 9 Figure 2 .

 292 photochromic and electrochromic devices. Several factors explain the popularity of

Figure 2 .

 2 Figure 2.11 XPS analysis of Co-qPyHm-TiO2. Before (blue trace) and after (orange trace) 2 h electrolysis at -0.51 V vs. RHE in 0.1 M KHCO3 electrolyte saturated with CO2 (pH = 6.8). Data show binding energies for Co2p (a), N1s (b) and P2p (c).

Figure 2 .

 2 Figure 2.12 EDX-mapping images of a Co-qPyHm-TiO2 electrode.

Figure 2 .

 2 Figure 2.13 CV in EC conditions at a Co-qPyHTiO2 (red) electrode and a bare TiO2

Figure 2 .

 2 Figure 2.14 Long-term electrolysis in EC conditions at Co-qPyHm-TiO2 electrode

  layer and a n-type CdS layer, and further layers of ZnO and ZnO:Al (AZO) were used to protect the material ( see details in Chapter 6). The structure of the CIGS used in this chapter is shown in Figure 2.15 (composition is Cu In0.1Ga0.9Se2).

Figure 2 . 15

 215 Figure 2.15 Schematic view of the CIGS based layered material used in

Figure 2 . 16

 216 Figure 2.16 Scanning electron microscope (SEM) cross section image of the CIGS layer.

Figure 2 . 17

 217 Figure 2.17 Illustrative scheme of the PV-EC systems investigated in this study.

Figure 2 .

 2 Figure 2.18 J-V curve of one of the CIGS solar cell without TiO2 layer (front contact

Figure 2 . 20 1 M

 2201 Figure 2.20 Electrolysis in PV-EC conditions on Co-qPyHm-TiO2 electrode in a 0.1

Figure 2 . 21 Figure 2 . 22

 221222 Figure 2.21 Long term electrolysis in PV-EC conditions at a Co-qPyHm-TiO2

Figure 2 . 23

 223 Figure 2.23 Illustrative scheme of the PEC systems investigated in this study.

Figure 2 .

 2 Figure 2.24 LSV in PEC conditions at a Co-qPyH|f-TiO2|CIGS electrode in 0.1 M

Figure 2 . 25

 225 Figure 2.25 Electrolysis in PEC conditions at Co-qPyHf-TiO2CIGS electrode in a

Figure 2 .

 2 Figure 2.26 GC/MS chromatograms at the retention time of carbon monoxide. Specific traces of m/z = 28 value (blue) and m/z = 29 value (orange) after 2h electrolysis at illuminated Co-qPyHf-TiO2CIGS (Ebias = -0.06 V vs. RHE), in 0.1 M KHCO3 electrolyte saturated with 12 CO2 (a) and 13 CO2 (b).

a

  Under continuous gas flow. b No catalyst. c Lowest estimation (see Methods). NA: Not applicable. NR: Not reported (experiments under gas flow).

Chapter 3 -

 3 Photo-assisted CO2 reduction to methanol on a porous carbon-cobalt molecular catalyst electrode with the assistance of CIGS photovoltaic material

Figure 3 . 1

 31 Figure 3.1 Structure of cobalt phthalocyanine (CoPc), a molecular electrocatalyst for

Figure 3 . 3

 33 Figure 3.3 External quantum efficiency (EQE) of the CIGS solar cell. The blue arrow

Figure 3 . 4

 34 Figure 3.4 Current-voltage (J-V) response of the CIGS solar cell, with corresponding

Figure 3 . 6 2 3. 4 . 2

 36242 Figure 3.6 Controlled potential photo-electrolysis at -0.36 (orange) and -0.46 (green)

Figure 3 . 7

 37 Figure 3.7 Controlled potential photo-electrolysis at -0.09 (red), -0.14 (blue) and -0.19

Figure 3 . 8 Figure 3 . 7 .

 3837 Figure 3.8 Chopped light experiment of the first 200 s of the electrolysis shown in

Figure 3 . 10

 310 Figure 3.10 Controlled potential photo-electrolysis at Eapplied = -0.14 V vs. RHE of the

Figure 3 . 11

 311 Figure 3.11 Extended controlled potential photo-electrolysis at -0.14 V vs. RHE

Figure 3 . 12

 312 Figure 3.12 Evolution of the Faradaic efficiency in H2 (green) and methanol (blue) and

Figure 4 . 1

 41 Figure 4.1 Energy band alignment of GaP and TiO2.[START_REF] Zeng | CO2 Reduction to Methanol on TiO2-Passivated GaP Photocatalysts[END_REF]

Figure 4 . 2

 42 Figure 4.2 Bare p-GaP semiconductors

Figure 4 .

 4 Figure 4.3 p-GaP photoelectrode as prepared in this work.

Figure 4 .Figure 4 . 4 4 . 3 Homogeneous PEC reduction of CO2 on GaP based photoelectrode with molecular catalyst 4 . 3 . 1 Figure 4 . 5 )

 4444343145 Figure 4.4 CV in PEC conditions at a GaP electrode in 0.5 M H2SO4 under air

Figure 4 . 5

 45 Figure 4.5 Structure of iron tetra-phenyl porphyrin (Fe-o-TMA) catalyst.

Figure 4 . 6

 46 Figure 4.6 Photo-electrolysis at -0.35 V vs. RHE in 0.1M KHCO3 saturated with

Figure 4 . 7

 47 Figure 4.7 Energy band diagrams for (e) thin and (f) thick TiO2 layers.[137]

Figure 4 . 9

 49 Figure 4.9 LSV in PEC conditions at a GaP electrode in acetonitrile under chopped

  Hz, 4H), and 7.38 (dt, J= 4.8, 1.2 Hz, 2H); HRMS m/z calcd. for C32H21Br2N4 [M+ H] + 619.0133, found 619.0137.

Figure 6 .1 1 H NMR spectrum of compound 2 . 3 :

 6123 Figure 6.1 1 H NMR spectrum of compound 2. Compound 3: tetraethyl ([2,2′:6′,2″:6″,2″′-quaterpyridine]-4′,4″-diylbis(4,1-

Figure 6 .

 6 Figure 6.2 1 H NMR spectrum of compound 3. Cobalt[tetraethyl([2,2′:6′,2″:6″,2″′-quaterpyridine]-4′,4″-diylbis(4,1phenylene))

Figure 6 . 3

 63 Figure 6.3 13 C NMR spectrum of compound 3. Cobalt[2,2 ′ :6′,2″:6″,2″′-quaterpyridine]-4′,4″-diylbis(4,1-phenylene)bis

Figure 6 .

 6 4 and 6.5 show the calibration curves of H2 and CO, respectively. A typical gas chromatogram in Figure 6.6 as an example indicates the peaks of H2 and CO.

Figure 6 . 4 Figure 6 . 5

 6465 Figure 6.4 Calibration curve of H2.

Figure 6 . 6

 66 Figure 6.6 Gas chromatogram

  catalyst present at the electrode.[START_REF] Wang | A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water[END_REF] 143] Finally, the amount of electrochemically active catalyst in the film is decaying over time, so the TOF is changing with time t. The TOF calculated here is a mean value over the course of the electrolysis.6.2.9 EC and PEC measurements EC and PEC experiments were conducted on a CHI 760 or a PGSTAT 302N Autolab potentiostat-galvanostat, and every experiment was conducted at least three times to make sure the reproducibility of the results. Electrolytes were saturated with the gas of interest by bubbling more than 20 min prior to measurement. In a standard three-electrode EC setup, counter electrode (CE) was a platinum grid, separated from the working compartment by a glass frit bridge filled with the same electrolyte. Potentials were measured relative to a saturated calomel electrode (SCE) as reference electrode (Ref) and converted into relative hydrogen electrode (RHE) using Eq. 6-3: E (V vs. RHE) = E (V vs. SCE) + 0.242 + 0.059 ⨯ pH

-Introduction 1.1 Photoelectrochemical reduction of CO2: towards a sustainable future

  1 Principles and advantages of Photoelectrochemical reduction of CO2 . 1.1.2 Semiconductors as photoelectrodes ...................................................... 1.1.3 Thermodynamics and chemical pathways of PEC CO2 reduction ........ 1.2 Photoelectrochemical CO2-reduction at semiconductor/ metal-complex systems .................................................................................................................. 1.2.1 Group IV Materials ............................................................................... 1.2.2 Group III-V Materials ........................................................................... Motivation and outline of this thesis ............................................................... Chapter 2 -Photocathode functionalized with a cobalt-based molecular catalyst for 2electron CO2 reduction in water ................................................................................... Photovoltaic-electrochemical performance of CIGS and functionalized TiO2 ............................................................................................................................... Photo-electrochemical performance of CIGS with Cobalt Quaterpyridine .... 2.5.1 Photocathode assembly and characterizations ...................................... 2.5.2 Photo-assisted CO2 reduction performance .......................................... 2.6 Conclusions and perspectives ......................................................................... Chapter 3 -Photo-assisted CO2 reduction to methanol on a porous carbon-cobalt molecular catalyst electrode with the assistance of CIGS photovoltaic material ........ Earth's climate has changed throughout history. Scientific evidence for warming of the climate system is unequivocal. This current global warming trend is the result of human activity since the mid-20th century and is proceeding at a rate that is

1.2.3 Oxide Semiconductors .......................................................................... 1.2.4 Chalcogenide Semiconductors .............................................................. 1.2.5 CO2 Reduction on Multi-semiconductor Systems ................................ 1.4 2.1 Structure and properties of Cu(In,Ga)Se2 materials ....................................... 2.2.1. Structure of a standard CIGS solar cell ................................................ 2.1.2 The Mo/CIGS/CdS/ZnO heterojunction ............................................... 2.2 Quaterpyridine complexes with CO2 reduction .............................................. 2.2.1 Quaterpyridine complexes for electrocatalytic reduction of CO2 ......... 2.2.2 Quaterpyridine complexes for photocatalytic reduction of CO2........... 2.3 Functionalization of cobalt quaterpyridine ..................................................... 2.3.1 Cobalt quaterpyridine synthesis and functionalization ......................... 2.3.2 Immobilization of modified cobalt quaterpyridine on TiO2 ................. 2.3.3 Electrochemical performance for CO2 reduction .................................. 2.

4 2.4.1 Photocathode assembly and characterizations ...................................... 2.4.2 PV-EC system for CO2 reduction .......................................................... 2.5 3.1 CO2 reduction to methanol.............................................................................. 3.2 Cobalt phthalocyanine complexes .................................................................. 3.3 Photo-assisted reduction of CO2 to methanol ................................................. 3.3.1 CIGS cell characterizations................................................................... Chapter 1

Table 1 .

 1

1 Apparent standard redox potentials (E o ') relative to the Standard Hydrogen Electrode (SHE) at pH 7 Reaction E o ' vs SHE (V)

Table 1 . 2

 12 reduction in water remains a high challenge. At part of my work, I intend to contribute to the design of more effective PEC systems for CO2 reduction in aqueous solution with stable photoelectrodes and the use of non-noble metal complexes. Recent advances in semiconductor materials for PEC CO2 reduction.

	Entry	Photocathode	Catalyst	Solvent	Illumination	Irrad.	Bias	Products	Performance	Ref.
				conditions		Time	potential
						(h)	(V vs. RHE)
	1	p-Si	Nonporous Au film	0.2 M KHCO3	1 sun	1	-0.03	CO	FE of 91% for CO	[44]
	2	p-Si photocathode	Particulate Ag film	0.5 M KHCO3	0.5 sun	1	-0.5	CO	FE > 90% for CO	[45]
	3	p-Si nanowire arrays	Sn NPs	0.1 M KHCO3	AM 1.5G	3	-0.85	HCOOH	FE of 40% for HCOOH	[46]
	4	B-doped p-Si	Fe porphyrin	MeCN/5% DMF	90 mW cm -2 , 650 nm	6	-1.11 V vs.	CO	FE of over 80% for CO	[47]
			complex	(v/v)	illumination		SCE
	5	p-GaN	Au NPs	0.05 m K2CO3	> 495 nm illumination	50	-1.8	CO	Selectivity of 80% for CO	[48]
	6	Zn-doped InP	Ru complex	0.1 M NaHCO3	SrTiO3 photoanode in AM			HCOOH	Solar-to-chemical efficiency of	[49]
			polymer	in water	1.5G, > 400 nm illumination				0.08% for HCOOH
	7	p-GaP	Pyridine	0.1 M acetate buffer	6.5 mW cm -2 , 365 nm	6-30	0.35	CH3OH	FE of 96% for CH3OH	[30]
				(pH 5.2)	illumination
	8	TiO2-protected n + p-Si	Au3Cu NPs	0.1 M KHCO3
		nanowire

Despite the fact that PEC CO2 reduction is at an early stage, it has the potential to utilize solar energy and convert CO2 into chemical fuels. As shown in Table

1

.2, noble metals catalysts, such as Au-based nanoparticles or alloys, remain the most extensively studied cocatalysts owing to their high activity and selectivity for CO. Besides, Ru and Re based molecular catalysts have also been widely studied. Apart from noble metals, few work has been devoted to non-noble catalyst. Cu-based materials are the most developed non-noble catalysts due to their high abundance and unique characteristics for hydrocarbon production. Using semiconductor materials as photoelectrodes combined with non-noble-metal-based molecular catalyst to achieve highly efficient and selective CO2

Table 2 .

 2 1 A summarizing table of all the electrolysis in this chapter:

	stem Sy	#	Electrolyte	h)	( t	vs. RHE) (V ed Eappli	jtotal,Ti

Table 2 . 2

 22 Summary of the most efficient molecular photocathodes for CO2 reduction reported in the literature.

	Entry	Light absorbing electrode	Catalyst	Eapplied	jtotal (mA cm -2 ) @ Eapplied	jCO (mA cm -2 ) @ Eapplied	FE total (%) @Eapplied	FECO (%)@ Eapplied	Selectivity Eapplied for CO@	Electrolyte	Illumination	Ref.
	1	Fe2O3 co-doped with N and Zn| TiO2	Ru(MeCN)CO2C 3Py-p	0.1 V vs. RHE	0.15	0.045	100 13h	30 13h	30 13h	0.1 M KHCO3(aq.)	100 mW cm -2 , AM1.5G	[35]
	2	Cu2O | TiO2	ReCl(bpy)(CO)3	-2.05 V vs. Fc + /Fc	2.5 * @-2.0 V vs.Fc + /Fc	2~2.4 @-2.0 V vs.Fc + /Fc	NR	NR	80~95 1.5h	0.1 M TBAPF6, MeCN	100 mW cm -2 , AM1.5G	[43]

  1, overall cathodic Faradaic efficiency expression is shown in equation 3.2. Application of this overall Faradaic efficiency calculation to the reactions involved in this manuscript is pictured in Figure3.13 It shows that sequential reduction of CO2 to methanol allows to increase the overall Faradaic efficiency up to 28.4%.

	CO 2 α1=2 → CO Figure 3.13 Overall Faradaic efficiency (FE) and chemical selectivity for sequential α2=4 → CH 3 OH (3.1)
	FE sequential = reduction of CO2 to CH3OH, taking into account a two steps procedure with first CO2-α1 + α2 α1 FE1 (3.2) α2 + FE2 to-CO conversion with 95% Faradaic efficiency and then reduction of CO to CH3OH

Table 3 .

 3 1 PV-EC CO2 and CO electrochemical reduction performance with CoPc decorated carbon paper electrode powered by a CIGS PV cell.

	ntry	E	Subs trate	te	Electroly	ime (h)	T	E applied ( V vs. RHE)	arbon paper m 2 )	Sc (c	IGS m 2 )	SC (c	mc m MCNTs at.:	a mV)	η (	total mA)	i (	E %)	CH3OH F ( i ( mA)	E %)	F (	CO mA)	i (	FE %)	(	H2 mA)	i (
		1 2	CO2	0.1 KHCO3 pH 6.8	M		1 2	0.36 0.46	--	5 5	0. 0.	9 9	0. 0.	15 15	1: 1:	90 90	3 4	.35 .83	2 2	.2 .3	1 0	.03 .01	0 0	05 08	1 1	.35 .83	2 2	7 8	.16 .23	0 0
		3						2	0.19	-	5	0.	9	0.	15	1:	90	2	.10	2	2	1	.25	0		-		-	7	8	.83	1
		4						2	0.14	-	5	0.	9	0.	15	1:	40	2	.32	1	9	1	.25	0		-		-	5	6	.86	0
		5 6	CO	0.1 KOH pH 13	M		2 2	0.09 0.01	-+	5 5	0. 0.	9 9	0. 0.	15 15	1: 1:	90 0	1 9	.18	1 -	.5	8 0	.10	0 -		--		--	0	6 -	.71	0 -
		7						1	0.14	-		1	9	0.	15	1:	40	2	.75	2	1	2	.58	0		-		-	3	7	.01	2
		8						1	0.14	-	5	1.	9	0.	15	1:	40	2	.85	2	5	1	.43	0		-		-	8	7	.22	2
		9	Ar					1	0.14	-		1	9	0.	15	1:	40	2	.39	3		-		-		-		-	3	7	.47	2

Table 3 . 2

 32 Current (i) and faradaic efficiency (FE) during controlled potential photo-electrolysis at Eapplied = -0.14 V vs. RHE, with CoPc:MWCNT mass ratio of 1:15, in 0.1 M KOH (pH 13) electrolyte, under CO atmosphere, with SCIGS = 0.3 cm 2 and SCarbon paper = 1 cm 2 .

			CH3OH		H2
	Time (h)	itotal (mA)	FE (%)	i (mA)	FE (%)	i (mA)
	1	1.74	16.2	0.28	65.9	1.15
	3	1.75	15.6	0.27	63.3	1.11
	4	1.74	13.6	0.24	66.2	1.15
	5	1.73	12.1	0.21	69.2	1.2
	6	1.73	10.7	0.19	69.3	1.20
	7	1.73	10.5	0.18	70.2	1.21

  ).1 H NMR characterization is shown in Figure6.2 (ppm, 400 MHz, 298 K, CDCl3) δ 8.95 (d, J= 1.6 Hz, 2H), 8.80 (J= 1.6 Hz, 2H), 8.75 (m, 2H), 8.70 (dt, J= 8.0, 0.8 Hz, 2H), 8.01-8.03 (m, 8H), 7.93 (td, J= 7.6, 1.6 Hz, 2H), 7.38 (dt, J= 6.0, 1.2 Hz, 2H), 4.13-4.26 (m, 8H), and 1.38 (t, J= 6.8 Hz, 12H);

13

C NMR characterization is shown in Figure

6

.3 (ppm, 100 MHz, 298 K, CDCl3) δ 156.5, 156.3, 156.1, 149.5, 149.4, 143.0, 137.2, 132.8, 132.7, 127.8, 127.6, 124.3, 121.6, 119.61, 119.56, 62.5, 62.4, 16.6, 16.5; HRMS m/z calcd. for C40H41N4O6P2 [M+ H] + 735.3501, found 735.3497.

  The brown precipitate was filtered and washed with water and dried under vacuum to produce 36 mg (yield = 82%) of Co-qPyH. Anal. calcd. for C32H24Cl2CoN4O6P2•3.1H2O: C, 47.56; H, 3.77; N, 6.93. Found: C, 48.07; H, 4.23; N, 6.93. HRMS m/z calcd. for C32H24CoN4O6P2 [M-2Cl -] 2+ 681.0503, found 681.0494.

over a wide pH range. [START_REF] Hernández-Alonso | Development of alternative photocatalysts to TiO2: Challenges and opportunities[END_REF] A majority of TiO2 exists as four polymorphs (crystallographic phases): rutile (tetragonal), anatase (tetragonal), brookite (orthorhombic), and TiO2 (B) (monoclinic). [START_REF] Chen | Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications[END_REF] The results of thermodynamic calculations showed that rutile is the most stable polymorph at all temperatures and pressures up to 6 kbar. In this chapter, mesoporous TiO2 layers were deposited on FTO-conductive glass substrates by a screen-printing method (described in Chapter 6), which is labeled as m-TiO2|FTO. Then Co-qPyH catalyst was immobilized on the surface of TiO2 as shown in Figure 2.8. Careful detailed measurement was performed to investigate the immobilization of the catalyst. UV-vis absorption spectra were conducted on m-TiO2|FTO surface before and after immobilization of catalyst on TiO2 layers (Figure 2.9), which confirmed that the semiconducting surface had been modified. Besides, only a small amount of absorbance was observed in the visible light region, making the it almost transparent to the majority of incoming light.

PV-EC system for CO2 reduction

In order to measure the photocurrent, a linear scan voltammetry (LSV) was recorded in 0.1 M KHCO3 electrolyte saturated with CO2 (pH 6.8) with the Co-qPyH|m-TiO2 electrode connected to a CIGS solar cell (ZnO:Al/ZnO/CdS/CIGS/Mo).

Photocurrent was observed, up to approximately 2 mAcm -2 . Electrolysis experiments were performed in 0.1 M KHCO3 saturated with CO2 (at pH 6.8) at 0.14 V vs. RHE for 2 h with an average current density of ca. 1 mA cm -2 (per geometric surface of TiO2). Another electrolysis was conducted at -0.06 V vs. RHE in the same electrolyte conditions. An average current density of 3.25 mA cm -2 (per geometric surface of TiO2) was obtained (Figure 2.20). Analysis by GC indicates the main product is CO with a selectivity of 87% at a bias potential of -0.06 vs. RHE, corresponding to a potential more negative than the thermodynamic potential (overpotential formally negative l η = -50 mV). details in Chapter 6).

CO2 as reactant

We first explored the ability of the PV-EC assembly to drive the reduction of CO2.

In Figure 3.5, linear sweep voltammetry (LSV) measurements show that no catalytic wave was observed in the absence of illumination, illustrating the need for photons and photovoltage from the CIGS cell to activate the catalyst and to reach the required potential to reduce CO2. Moreover, Chopped-light experiments clearly showed that a significant 2-3 mA photocurrent (corresponding to 4-6 mA cm -2 relatively to the carbon electrode surface) can be obtained upon illumination, associated with a typical photovoltage from the CIGS cell of ca. 660 mV. presence and the origin of these products. [START_REF] Chatterjee | Manifesto for the routine use of NMR for the liquid product analysis of aqueous CO2 reduction: from comprehensive chemical shift data to formaldehyde quantification in water[END_REF] Note that the small amount of formate may arise from reaction of CO with OH -and from the disproportionation of small amount of formaldehyde form during electrolysis. [START_REF] Boutin | Aqueous Electrochemical Reduction of Carbon Dioxide and Carbon Monoxide into Methanol with Cobalt Phthalocyanine[END_REF][START_REF] Boutin | On the Existence and Role of Formaldehyde During Aqueous Electrochemical Reduction of Carbon Monoxide to Methanol by Cobalt Phthalocyanine[END_REF]

Chapter 4 -Preliminary investigation of PEC CO2

reduction on p-GaP semiconductors associated with molecular catalysts

PEC reduction of CO2 on GaP semiconductors

GaP is a semiconductor with a band gap of about 2.3 eV. [START_REF] Levinshtein | Handbook Series on Semiconductor Parameters[END_REF] The wide bang gap theoretically provides enough energy to split water directly in a PEC cell. Additionally, it is a promising electrode because of its stability in electrolytes under cathodic conditions. This is caused by the slow kinetics of the cathodic dissociation reaction of GaP and by the rapid formation of a thin oxide layer. [START_REF] Ginley | Interfacial Chemistry at p-GaP Photoelectrodes[END_REF] Therefore, GaP has been quite intensively studied for PEC-applications. In 1978, Halmann's group first reported CO2 reduction on p-GaP under 365 nm illumination with an applied potential of -1.4 V (vs. SCE). [START_REF] Halmann | Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells[END_REF] In 1979, Fujishima and Honda demonstrated photoelectrocatalytic reduction of CO2 to formaldehyde and methanol by irradiating TiO2 and GaP with the UV light at a potential of -1.5 V (vs SCE). [START_REF] Inoue | Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders[END_REF] Petit et al. later reported PEC reduction of CO2 to CO on p-GaP semiconductor associated with molecular Ni(cyclam) 2+ in aqueous medium. The Faradaic efficiency for CO was 83% at a potential as early as 0.46 V vs. RHE with H2 as the only byproducts. [START_REF] Petit | Molecular catalysts in photoelectrochemical cells, study of an efficient system for the selective photoelectroreduction of CO2: p-GaI or p-GaAs/Ni(cyclam) 2+ , aqueous medium[END_REF] Bocarsly's group reported pyridinium-catalyzed CO2 reduction on GaP photocathodes with applied potentials between -0.7 V and -0.2 V (vs SCE) under UV light. [START_REF] Barton | Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell[END_REF] More recently, Cronin group reported PEC reduction of CO2 to methanol using TiO2

Chapter 5 -General conclusions and perspectives

General conclusions

The main objective of this thesis was to explore the use of semiconductor materials associated to molecular catalysts for the visible-light driven photoelectrochemical reduction of CO2 in aqueous solutions. CIGS semiconductor was introduced because of its high efficiency and low cost in producing solar electricity. This thin-film device has been well studied and characterized for several PV applications. In this work, efficient CIGS solar devices were fabricated and characterized, and the CIGS electrodes have been successfully used for CO2 catalytic reduction in different configurations with the assistance of molecular catalysts. Cobalt-based molecular catalysts, such as the quaterpyridine complexes used in this work, are capable of driving CO2 reductions in heterogeneous water conditions with several supporting materials. The results in this thesis have illustrated the combination of CIGS devices and functionalized cobalt quaterpyridine molecular catalyst. Most importantly, the photoelectrochemical reduction of CO2-to-CO was achieved at the photocathodes, under visible-light in water.

In a complementary approach, thin films made of Co phthalocyanine and multi-walled carbon nanotubes were used to drive reduction of CO2-to-methanol with a photovoltaic CIGS device in a PV-EC system, again in water. Both high catalytic efficiency, selectivity as well as stability could be obtained. More precisely, the main results of this work can be summarized as follows:

1. We successfully demonstrated the application of CIGS photovoltaic materials loaded with cobalt phthalocyanine catalyst and a CIGS PV cell to drive CO reduction to methanol. CIGS PV cell supplies a large photovoltage to the EC unit. A Faradaic efficiency of 21% for methanol was obtained with a bias potential of -0.06 V vs. RHE, corresponding to an overpotential of 240 mV. The performances of the system depend on the surface area matching between CIGS and the carbon paper electrode. A longterm electrolysis for 7 h showed progressive albeit small loss of activity during the time, and the origin of methanol product was asserted by isotope labelled experiments.

Future perspectives

This work paves the way to the development of more efficient systems for PEC reduction of CO2. Because of the complexity of solar-driven reduction system, the performance is controlled by several factors, including light absorption, charge transfer, catalytic activity, and engineering techniques as well. The ability to design an efficient and stable photoelectrode integrating molecular catalysts, allowing to properly synchronize in time and space all steps of the process remains a challenge.

A highly active catalyst is essential to get high efficiency. Considering the advantages of molecular catalyst, developing earth-abundant metal based efficient molecular catalyst is still very promising. A higher catalyst loading would also significantly improve the performance. Improve the specific surface area eg. upon nano-structuration of the surface is certainly an approach to be followed. Besides, it is still a challenge to design molecular catalyst to drive CO2 reduction beyond twoelectron process, so as to produce liquid fuels (e.g. methanol).

Different semiconductor materials may be explored for PEC reduction of CO2 to expand light absorbers, produce large current densities and photovoltage. As described in Chapter 4, we did a start to investigate GaP semiconductor as photoelectrode.

A thorough understanding of the catalytic pathways and the deactivation process (such as the reasons for catalyst leaching and/or degradation, (un)stability of protective oxide layers) is essential to improve the catalytic activity and long-term stability. Finally, designing a fully unassisted solar-driven fuel production system may also be investigated when coupled with an appropriate photoanodic reaction.

Currently, this work has led to two publications: All chemicals were used as received without any further purification.

Liquids and solvents

Acetonitrile (99.8%), ethylene glycol (EG, reagent plus > 99%) and Nafion© (5 wt% solution in low aliphatic alcohol/H2O) were purchased from Sigma-Aldrich; ethanol (EtOH, analysis grade) was purchased from Merck.

Electrolyte solutions were prepared with ultra-pure water (TKA MicroPure, 0.055 µS cm -1 ) or electrochemical grade acetonitrile. Gases 12 CO2 (> 99.7%), 12 CO (> 999.7) and Argon (> 99.998) gases were purchased from Air Liquide. 13 CO2 (99% content in atom 13 C) was purchased from Sigma-Aldrich.

Attenuated Total Reflection (ATR-IR) spectroscopy

AT R-IR spectra were recorded on BRUKER Tensor 27.

Inductively coupled plasma -Optical Emission Spectroscopy (ICP -

OES)

The loading of the catalyst was measured by ICP-OES. Cobalt catalyst was desorbed from the electrode in a 69% HNO3 solution overnight. This solution was then diluted to a 1.38% HNO3 solution for analysis. ICAP 6300 (Thermo Sci.) ICP -OES CID spectrometer was used with detection of cobalt in different samples (238.8 nm band that is the most intense).

X-ray photoelectron spectrometer (XPS)

The composition (content in carbon, nitrogen, oxygen, phosphorus and cobalt elements) od samples were determined by XPS spectrometer analyses, which were performed with a THERMO-VG ESCALAB 250 spectrometer accompanied by a microfocalized X-ray source (Kα AI 1486.6 eV).

pH measurements

pH value of the electrolyte was tested on a Hanna pH210 and HI221 instruments and 6 mm microelectrodes (Fisher Scientific).

Nuclear magnetic resonance (NMR) spectroscopy

1 H and 13 C NMR spectra were recorded on an AVANCE 300 Ultra Shield BRUKER