Dissertationen zum Thema „Photo-electrochemical cells“

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 130-142).
Fossil fuels depletion and climate change are driving the need for sustainable development and renewable energy sources globally [1]. Solar being the most abundant and widespread source of renewable energy is resulting in a rapidly growing, with a growth rate more than 35% annually for the past 10 years [4]. Hydrogen is an ideal energy carrier for next generation given its high efficiency, environmental friendliness, wide application as well as several attractive methods for storage and distribution [17]. The hydrogen economy, a proposed system of producing, delivering and employing energy by using hydrogen, is under intensive research and development, and is projected to be realized at the end of this century as one of the leading suppliers [60]. Photo-electrochemical (PEC) cells connect the solar energy and hydrogen economy together by directly converting solar energy into chemical energy in the form of hydrogen gas. The metal oxide based PEC cell has advantages of low cost, high stability and durability and environmental friendliness [14], a good option for commercialization. With the rapid development of nanotechnology in recent years, novel nano-structured metal oxide PEC cells can have higher efficiency and better performance due to the effects of quantization, large surface areas, improved charge transport, etc. In this thesis, the current status and future development of the hydrogen economy in terms of identifying the markets, opportunities and risks of solar-hydrogen has been reviewed and accessed. The technology review of PEC cells in terms of the working mechanism and efficiency determining factors has been studied. The current research efforts on metal oxide based PEC cells for optimizing the performances and processing methods have also been studied. A case study and cost modeling in the context of scenario has been conducted; the analysis showed the cost of PEC cells was still very high mainly due to the high materials and processing costs. Thus, future research development should focus on the technological approaches with low materials and processing costs and high energy conversion efficiency for earlier commercialization of PEC cells. Besides, hydrogen storage, distribution, safety codes and standards, education and training as well as fuel cell technology must also require intensive research and development to insure the realization of solar-hydrogen economy.
by Jianfeng Zhu.
M.Eng.

Mon projet de thèse porte sur l'étude comparative de deux concepts de générateurs de carburant solaire : une cellule PEC basée sur une photoanode de vanadate de bismuth (BiVO4), et une cellule intégrée (IPEC) combinant une cellule solaire à un cellule d’électrolyse. Les deux dispositifs réalisent le processus de photosynthèse artificielle consistant à capturer l'énergie solaire et à la stocker dans les liaisons chimiques. L’objectif de mes travaux est d'identifier les verrous au déploiement à grande échelle de ces dispositifs photo-électrochimiques et de proposer des solutions pour les lever. Deux questions scientifiques sont en particulier au cœur de mes travaux de recherche: 1. Quel est l’impact des hétérogénéités, inhérentes aux procédés d’élaboration à grande échelle des matériaux photo-actifs, sur leurs performances ? 2. Quel rôle peut jouer l’intégration dans la transposition, à l’échelle pilote, des performances obtenues en laboratoire ? Pour répondre à la 1ère question j’ai mis en place une étude paramétrique expérimentale sur des photo-anodes à base de BiVO4, un matériau modèle pour la photosynthèse artificielle. En parallèle, afin de répondre à la 2ème question, j’ai réalisé et testé une cellule IPEC : la combinaison d'une cellule solaire tandem PK/Si et d'un électrolyseur à membrane échangeuse de protons. Dans une deuxième étape, cette démarche a été poursuivie par la conception d’un module monolithique original, intégrant 9 cellules IPEC. La réalisation de 5 de ces modules a permis l’assemblage du démonstrateur EASI Fuel (European Autonomous Solar Integrated fuel station) pour la conversion continue d’hydrogène (produit sous ensoleillement par les IPEC) et du CO2, en CH4 au sein d’un bioréacteur de méthanation. Grace à ce couplage innovant, le prototype EASI Fuel a été sélectionné et testé avec succès pendant 72h en continu et en totale autonomie, lors de la finale du concours Horizon Prize - Fuel from the Sun: Artificial Photosynthesis, qui s'est déroulée à la fin du 2ème année de thèse
This PhD project focuses on the comparative study of two types of solar fuel generators: a PEC cell based on a bismuth vanadate (BiVO4) photoanode, and an integrated photoelectrochemical (IPEC) cell combining a solar cell with an electrolyser. Both devices achieve the artificial photosynthesis process of capturing solar energy and storing it in chemical bonds. The goal of the work is to identify the limiting parameters that hinder the large-scale deployment of these photoelectrochemical devices and to propose solutions in order to facilitate research in this field. Two specific scientific questions are at the heart of this research: 1. What is the impact of heterogeneities, inherent to large-scale preparation processes, on the performance of photoactive materials? 2. What role can integration play in transferring performance from the laboratory to the pilot scale? To answer the first question, an experimental parametric study was undertaken on BiVO4 photoanodes, considered as photoactive semiconductors representative for artificial photosynthesis. In parallel, in order to answer to the second question, an IPEC cell, the combination of a PK/Si tandem solar cell with a proton exchange membrane electrolyser, was realized, tested and optimized. In a second phase, this approach was followed by the design of a monolithic module, integrating 9 IPEC cells. The realization of 5 of these modules enabled the assembly of the EASI Fuel device (European Autonomous Solar Integrated fuel station) for the continuous conversion of H2 (produced under sunlight by the IPEC cells) and CO2 into CH4 within a methanogenesis Archaea-based bioreactor. Thanks to this innovative coupling, the EASI Fuel device was selected and successfully tested for 72 hours of continuous operation in total autonomy, during the final of the Horizon Prize - Fuel from the Sun: Artificial Photosynthesis competition, which took place at the end of the 2nd year of the thesis

En raison des problèmes d'instabilité à moyen termes des cellules solaires à colorant (DSSC), l'électrolyte liquide à base d'iodure a été remplacé par plusieurs types de matériaux solides transport de trous (HTM) pour obtenir des DSSCs à l'état solide (s-DSSCs). Parmi ces matériaux, l’utilisation des polymères conducteurs(PC) a attiré une attention considérable en raison de leur bonne stabilité, de leur haute conductivité et de la facilité de leur dépôt sur le semi-conducteur mésoporeux TiO2. Dans ce travail de thèse, plusieurs s-DSSCs basées sur des PC utilisés comme HTM ont été développés dans le but d'améliorer leurs performances photovoltaïques en tenant compte des deux objectifs suivants: (i) l'optimisation des processus de transfert inter facial de charge dans la cellule solaire, et (ii) l'optimisation du transport de charge dans le semi-conducteur d'oxyde de type n. Pour atteindre ces objectifs, chaque composant de la s-DSSC a été modifié afin d'étudier son effet sur les performances du dispositif final. En première tentative, une étude analytique est réalisée en faisant varier le sensibilisateur afin de déterminer les fragments de la structure du colorant, qui ont un effet important sur le processus de photopolymérization électrochimique in-situ (PEP) à la fois en milieu organique et en milieu aqueux mais aussi sur les performances des s-DSSCs. Sur la base de ces résultats, un nouveau concept a été développé et consiste en la suppression totale de l'interface entre le colorant et le HTM. Ceci est obtenu par la synthèse de nouveaux colorants liés de façon covalente à un monomère électroactif qui est co-polymérisé par la PEP in-situ. Le copolymère résultant, utilisé comme HTM, est lié de manière covalente au colorant. En outre, la nature de la liaison chimique, reliant le résidu triphénylamine TPA au monomère, est également étudiée comme un facteur clé dans les performances de s-DSSC. En outre, et pour optimiser les processus de transport de charges dans ce type de s-DSSC, de nouvelles s-DSSC basées sur ZnO ont été réalisées et étudiées
Due to instability problems of dye sensitized solar cells (DSSCs) in longtime uses, the iodine based liquidelectrolyte has been replaced by several types of solid hole transporting materials (HTM) to perform solidstate DSSCs (s-DSSCs). Among them, the substitution by conducting polymers (CP) has attractedconsiderable attention because of their good stability, high hole-conductivity and simple deposition withinthe mesoporous TiO2 semiconductor. In this thesis work, several s-DSSCs based on CPs used as HTM havebeen developed in order to improve their photovoltaic performances taking into account the following twoobjectives: (i) the optimization of the interfacial charge transfer processes within the solar cell, and (ii) theoptimization of the charge transport within the n-type oxide semiconductor. To reach these goals, eachcomponent that constitutes the device was varied in order to investigate its effect on the device’sperformances. As first attempt, an analytical study is carried out by varying the sensitizer in order todetermine the fragments of the dyes structures, that have an important effect on the in-situ photoelectrochemical polymerization process (PEP) both in organic and in aqueous media and hence on theperformances of the s-DSSCs. Based on these results, a new concept of removing completely the interfacebetween the dye and the HTM is developed. This is achieved by the synthesis of new dyes covalently linkedto an electroactive monomer which is co-polymerized by in-situ PEP. The resulting co-polymer, used asHTM, is covalently linked to the dye. In addition, the nature of the chemical bond linking the triphenylamineresidue TPA to the monomer is also investigated as a key factor in the s-DSSCs performances. Besides, andto optimize the charge transport processes within this type of s-DSSC, the elaboration of novel ZnO baseds-DSSCs has been achieved and investigated

Tesis para optar al grado de Doctor en Ciencias de la Ingeniería, mención Ingeniería Química y Biotecnología
Las fuentes de energía limpias y sostenibles deben ser consideradas una base importante para el futuro crecimiento y desarrollo económico de cualquier país. Actualmente, el suministro mundial de energía depende en gran medida de los combustibles fósiles. Esto conlleva a que tecnologías tales como las celdas foto-electroquímicas se vuelvan especialmente atractivas, ya que permiten usar energía solar para producir hidrógeno. El funcionamiento de las celdas foto-electroquímicas se basa en el uso de semiconductores como electrodos, que al ser irradiados generan pares hueco-electrón, los cuales pueden migrar en la superficie del semiconductor y reaccionar con las especies adsorbidas o recombinarse entre sí. El hueco electrónico generado por la migración de un electrón puede oxidar una molécula de agua para producir oxígeno en el ánodo, mientras que los electrones generados viajan hacia el cátodo para reducir los protones presentes en el agua formando hidrógeno El presente trabajo de tesis tuvo como objetivo general la sintesis (a través de los métodos de electrodeposición y de spin-coating) y caracterización del desempeño de foto-ánodos basados en óxidos de molibdeno para la producción de hidrógeno en una celda foto-electroquímica a partir de electrólisis de agua. Películas de óxido de molibdeno dopadas con niquel y sin dopar se electrodepositaron aplicando un potencial de -1,377 V vs Ag / AgCl (KCl 3 M) durante 3 horas en un vidrio de cuarzo cubierto con dióxido de estaño dopado con flúor - FTO - sumergido en soluciones acuosas de molibdato-citrato a pH 9. Por otra parte, se depositaron peliculas de MoOx, WO3 y MoOx dopado con W sobre vidrio de aluminoborosilicato recubierto con óxido de estaño dopado con flúor. Este proceso se realizó mediante spin-coating a 4000 rpm durante 40 segundos. La caracterización de los foto-ánodos fabricados a través de electrodeposición y spin-coating sugiere que presentan propiedades semiconductoras y catalíticas que los hacen atractivos para su uso en una celda foto-electroquímica para la hidrólisis del agua. Sin embargo, aunque los foto-electrodos sintetizados a través de las técnicas mencionadas tienen un intervalo de banda prohibida óptimo para aprovechar eficientemente la luz solar, la caracterización foto-electroquímica mostró que estos electrodos no exhiben una estabilidad en solución acuosa y que son susceptibles a la foto-corrosion, que son factores limitantes para el uso de semiconductores convencionales como foto-electrodos.

En raison des problèmes d'instabilité à moyen termes des cellules solaires à colorant (DSSC), l'électrolyte liquide à base d'iodure a été remplacé par plusieurs types de matériaux solides transport de trous (HTM) pour obtenir des DSSCs à l'état solide (s-DSSCs). Parmi ces matériaux, l’utilisation des polymères conducteurs(PC) a attiré une attention considérable en raison de leur bonne stabilité, de leur haute conductivité et de la facilité de leur dépôt sur le semi-conducteur mésoporeux TiO2. Dans ce travail de thèse, plusieurs s-DSSCs basées sur des PC utilisés comme HTM ont été développés dans le but d'améliorer leurs performances photovoltaïques en tenant compte des deux objectifs suivants: (i) l'optimisation des processus de transfert inter facial de charge dans la cellule solaire, et (ii) l'optimisation du transport de charge dans le semi-conducteur d'oxyde de type n. Pour atteindre ces objectifs, chaque composant de la s-DSSC a été modifié afin d'étudier son effet sur les performances du dispositif final. En première tentative, une étude analytique est réalisée en faisant varier le sensibilisateur afin de déterminer les fragments de la structure du colorant, qui ont un effet important sur le processus de photopolymérization électrochimique in-situ (PEP) à la fois en milieu organique et en milieu aqueux mais aussi sur les performances des s-DSSCs. Sur la base de ces résultats, un nouveau concept a été développé et consiste en la suppression totale de l'interface entre le colorant et le HTM. Ceci est obtenu par la synthèse de nouveaux colorants liés de façon covalente à un monomère électroactif qui est co-polymérisé par la PEP in-situ. Le copolymère résultant, utilisé comme HTM, est lié de manière covalente au colorant. En outre, la nature de la liaison chimique, reliant le résidu triphénylamine TPA au monomère, est également étudiée comme un facteur clé dans les performances de s-DSSC. En outre, et pour optimiser les processus de transport de charges dans ce type de s-DSSC, de nouvelles s-DSSC basées sur ZnO ont été réalisées et étudiées
Due to instability problems of dye sensitized solar cells (DSSCs) in longtime uses, the iodine based liquidelectrolyte has been replaced by several types of solid hole transporting materials (HTM) to perform solidstate DSSCs (s-DSSCs). Among them, the substitution by conducting polymers (CP) has attractedconsiderable attention because of their good stability, high hole-conductivity and simple deposition withinthe mesoporous TiO2 semiconductor. In this thesis work, several s-DSSCs based on CPs used as HTM havebeen developed in order to improve their photovoltaic performances taking into account the following twoobjectives: (i) the optimization of the interfacial charge transfer processes within the solar cell, and (ii) theoptimization of the charge transport within the n-type oxide semiconductor. To reach these goals, eachcomponent that constitutes the device was varied in order to investigate its effect on the device’sperformances. As first attempt, an analytical study is carried out by varying the sensitizer in order todetermine the fragments of the dyes structures, that have an important effect on the in-situ photoelectrochemical polymerization process (PEP) both in organic and in aqueous media and hence on theperformances of the s-DSSCs. Based on these results, a new concept of removing completely the interfacebetween the dye and the HTM is developed. This is achieved by the synthesis of new dyes covalently linkedto an electroactive monomer which is co-polymerized by in-situ PEP. The resulting co-polymer, used asHTM, is covalently linked to the dye. In addition, the nature of the chemical bond linking the triphenylamineresidue TPA to the monomer is also investigated as a key factor in the s-DSSCs performances. Besides, andto optimize the charge transport processes within this type of s-DSSC, the elaboration of novel ZnO baseds-DSSCs has been achieved and investigated

The aim of this thesis is to enhance the extracellular electron transfer rates (exoelectrogenesis) in cyanobacteria, to be utilised for photo-bioelectricity generation in biophotovoltaics (electrochemical cell). An initial cross comparison of the cyanobacterium Synechococcus elongatus PCC7942 against other exoelectrogenic cultures showed a hindered exoelectrogenic capacity. Nonetheless, in mediatorless biophotovoltaics, it outperformed the microalgae Chlorella vulgaris. Furthermore, the performance of S. elongatus PCC7942 was improved by constructing a more efficient design (lower internal resistance), which was fabricated with carbon fibres and nitrocellulose membrane, both inexpensive materials. To strategically obtain higher exoelectrogenic rates, S. elongatus PCC7942 was conditioned by iron limitation and CO2 enrichment. Both strategies are novel in improving cyanobacteria exoelectrogenesis. Iron limitation induced unprecedented rates of extracellular ferricyanide reduction (24-fold), with the reaction occurring favourably around neutral pH, different to the cultural alkaline pH. Iron limited cultures grown in 5% and 20% CO2 showed increased exoelectrogenic rates in an earlier stage of growth in comparison to air grown cultures. Conveniently, the cultural pH under enriched CO2 was around neutral pH. Enhanced photo-bioelectricity generation in ferricyanide mediated biophotovoltaics was demonstrated. Power generation was six times higher with iron limited cultures at neutral pH than with iron sufficient cultures at alkaline pH. The enhanced performance was also observed in mediatorless biophotovoltaics, especially in the dark phase. Exoelectrogenesis was mainly driven by photosynthetic activity. However, rates in the dark were also improved and in the long term it appeared that the exoelectrogenic activity under illumination tended to that seen in the dark. Proteins participating in iron uptake by an alleged reductive mechanism were overexpressed (2-fold). However, oxidoreductases in the outer membrane remain to be identified. Furthermore, electroactive regions in biofilms of S. elongatus PCC7942 were established using cyclic voltammetry. Double step potential chronoamperometry was also successfully tested in the biofilms. Thus, the electrochemical characterisation of S. elongatus PCC7942 was demonstrated, implying that the strategies presented in this thesis could be used to screen for cyanobacteria and/or electrode materials to further develop systems for photo-bioelectricity generation.

Afin de relever le défi énergétique et climatique du 21ième siècle qui s’annonce, une solution consiste, pour valoriser la ressource solaire, à mettre au point des procédés de production de vecteurs énergétiques stockables par photosynthèse artificielle permettant la synthèse de carburants solaires, en particulier l’hydrogène. La compréhension de ses procédés et l’obtention de performances cinétiques et énergétiques élevées nécessitent le développement de modèles de connaissance génériques, robustes et prédictifs considérant le transfert de rayonnement comme processus physique contrôlant le procédé à plusieurs échelles mais aussi les différents autres phénomènes intervenant dans la structure ou la réification du modèle.Dans le cadre de ce travail de doctorat, le procédé photo-réactif au cœur de l’étude était la cellule photo-électrochimique. D’un fonctionnement plus complexe que le simple photoréacteur, avec une photo-anode et une (photo)cathode, la cellule photo-électrochimique dissocie spatialement les étapes d’oxydation et de réduction. En se basant à la fois sur la littérature existante (essentiellement dans le domaine de l’électrochimie) et en déployant les outils développés par l’équipe de recherche sur le transfert de rayonnement et la formulation du couplage thermocinétique, il a été possible d’établir des indicateurs de performance des cellules photo-électrochimiques.En parallèle de l’établissement de ce modèle, une démarche expérimentale a été entreprise en se basant tout d’abord sur une cellule commerciale de type Grätzel (DS-PEC) indiquant les tendances générales de tels convertisseurs de l’énergie des photons avec en particulier une chute de l’efficacité énergétique en fonction de la densité incidente de flux de photons. Un dispositif expérimental modulable (Minucell) a aussi été développé et validé afin de caractériser des photo-anodes de différentes compositions comme des électrodes de TiO2 imprégnées de chromophore pour un fonctionnement en cellule de Grätzel ou bien des électrodes d’hématite Fe2O3 (SC-PEC) où le semiconducteur joue à la fois les fonctions d’absorption des photons et de conduction des porteurs de charges. Surtout, le dispositif Minucell a permis de tester, caractériser et modéliser le comportement d’une cellule photo-électrochimique de type bio-inspiré pour la production d’H2 utilisant à la photo-anode un catalyseur moléculaire Ru-RuCat (développé par ICMMO Orsay/CEA Saclay) et à la cathode un catalyseur CoTAA (développé par LCEMCA Brest). Minucell a été utilisé pour caractériser chaque élément constitutif d’une cellule photo-électrochimique puis la cellule dans son ensemble, confirmant les tendances et observations obtenues sur les efficacités énergétiques.Ce travail préliminaire ouvre de très nombreuses perspectives de recherche, il pose des bases communes entre électrochimie et génie des systèmes photo-réactifs et donne des pistes quant à la conception et l’optimisation cinétique et énergétique des cellules photo-électrochimiques pour la production d’hydrogène et de carburants solaires
In order to meet the energy and climate challenge of the coming 21st century, one solution consists of developing processes for producing storable energy carriers by artificial photosynthesis to synthesize solar fuels, in particular hydrogen, in order to valorize the solar resource. The understanding of these processes and the achievement of high kinetic and energetic performances require the development of generic, robust and predictive knowledge models considering radiative transfer as a physical process controlling the process at several scales but also including the various other phenomena involved in the structure or reification of the model.In this PhD work, the photo-reactive process at the heart of the study was the photo-electrochemical cell. More complex than the simple photoreactor, with a photo-anode and a (photo)cathode, the photo-electrochemical cell spatially dissociates the oxidation and reduction steps. Based both on the existing literature (mainly in the field of electrochemistry) and by deploying the tools developed by the research team on radiative transfer and thermokinetic coupling formulation, it was possible to establish performance indicators of photo-electrochemical cells.In parallel to the establishment of this model, an experimental approach was undertaken based first on a commercial Grätzel-type cell (DS-PEC) indicating the general trends of such photon energy converters with in particular a drop in energy efficiency as a function of the incident photon flux density. A modular experimental device (Minucell) has also been developed and validated in order to characterize photo-anodes of different compositions such as chromophore impregnated TiO2 electrodes for operation in Grätzel cells or Fe2O3 hematite electrodes (SC-PEC) where the semiconductor plays both the functions of photon absorption and charge carrier conduction. Above all, the Minucell device allowed to test, characterize and model the behavior of a bio-inspired photo-electrochemical cell for H2 production using at the photo-anode a Ru-RuCat molecular catalyst (developed by ICMMO Orsay/CEA Saclay) and at the cathode a CoTAA catalyst (developed by LCEMCA Brest). Minucell was used to characterize each constituent element of a photo-electrochemical cell and then the cell as a whole confirming the trends and observations obtained on energy efficiencies.This preliminary work opens up a wide range of research prospects, lays common ground between electrochemistry and photo-reactive systems engineering, and provides insights into the design and kinetic and energy optimization of photo-electrochemical cells for the production of hydrogen and solar fuels

Les réserves de combustibles fossiles diminuent et leur utilisation illimitée depuis la révolution industrielle a généré de profonds changements du climat, notamment des cycles de la température atmosphérique. Stocker l'énergie solaire sous forme d'hydrogène produit par dissociation de l'eau est un moyen idéal pour combattre le réchauffement climatique. Les matériaux de la famille des «metal organic framework» (MOF) commencent à être utilisés comme photo-électrocatalyseurs, notamment pour la photo-dissociation de l'eau. Leur porosité extrêmement élevée et leur grande polyvalence, tant chimique que structurelle, les désignent comme des candidats potentiels pour faciliter l'absorption du rayonnement solaire et catalyser la dissociation de l'eau dans les cellules photoélectrochimiques. En contrôlant la composition chimique et le dopage du linker utilisé dans le MOF, il est possible d'ajuster l'énergie de la bande interdite, de favoriser la fonctionnalisation sur des substrats très variés ou encore d'ajuster leur résistance à la corrosion dans divers environnements chimiques. Ce sont donc des matériaux d'un grand intérêt pour la catalyse, l'électrocatalyse ou la photo-électro-catalyse. D'autre part, le TiO₂ nano-structuré, par exemple sous forme de tapis d’épaisseur micrométrique de nanotubes ou de nanofils, parfois appelé TNA, est un matériau bien adapté à la construction de photoanodes pour le dégagement d'oxygène en milieu aqueux. Il a déjà été largement étudié et décrit dans la littérature. Au cours de notre thèse, nous avons fabriqué des matériaux composites constitués de MOF de métaux de transition (Ni, Co, Fe) déposés sur TNA (TDNR et TNTA). Pour cela, nous avons utilisé une méthode électrochimique d'électrodéposition. Cela nous a permis de déposer des nanoparticules métalliques sur du TNA à potentiel fixe - 1,0 V puis de les transformer par réaction chimique avec des ligands organiques (BTC, BDC, et 2MZ) par voie thermo-thermique. Les matériaux obtenus présentent une activité électrocatalytique significative et une excellente durabilité photoélectrochimique. Ces matériaux composites ont été utilisés avec succès comme phase active dans des photo-électrodes pour la réaction de dégagement d'oxygène moléculaire (OER)
The fossil fuel reserves are dwindling and their unrestricted use has generated profound changes in Earth's surface temperature and climate. Storing solar energy in the form of hydrogen produced by dissociation of water is an ideal way to mitigate global warming. Materials from the “metal organic framework” (MOF) family are starting to be used as photo-electrocatalysts, especially for photo-dissociation of water. Their extremely high porosity and their great versatility, both chemical and structural, designate them as potential candidates to facilitate the absorption of solar radiation and catalyze the dissociation of water in photoelectrochemical cells. By controlling the chemical composition and doping of the linker used in the MOF, it is possible to adjust the band gap energy, to favor the functionalization on very varied substrates or even to adjust their resistance to corrosion in various chemical environments. They are therefore materials of great interest for catalysis, electrocatalysis or photo-electro-catalysis. On the other hand, nano-structured TiO₂, for example in the form of nanotube or nanowire mats, sometimes called TiO₂ nanoarray (TNA), is a material very suitable for the construction of photoanodes for the evolution of oxygen in aqueous medium. It has already been extensively studied and described in the literature. During our thesis, we manufactured composite materials made up of MOFs of transition metals (Ni, Co, Fe) deposited on TNA (network of nanotubes or nanowires). For this we used an electrochemical method of electrodeposition (cyclic voltammetry). This allowed us to deposit metallic nanoparticles on TNA with fixed potential - 1.0 V and then transform them by chemical reaction with organic ligands (1,3,5-benzenetricarboxylic acid, BTC, 1,4-benzenedicarboxylic acid, BDC and imidazole, 2MZ) by thermal-thermal route. The materials obtained exhibit significant electrocatalytic activity and excellent photoelectrochemical durability. These composite materials have been successfully used as an active phase in photo-electrodes for the oxygen release reaction (OER)

The optimisation of existing chemistries by the introduction of environmentally friendly materials and the simplification of the device production process are intriguing challenges to promote the future widespread diffusion of LIBs. Moreover, the recent development of the next-generation electronic devices promoted a new research field for the modification of the current systems into light, flexible and/or micro-sized device. The enhancement of mechanical properties through the introduction of flexible electrodes will enable LIBs to be embedded into various functional systems in a wide range of innovative products such as smart cards, displays and implantable medical devices. Moreover, the optimisation of the electrolyte by moving towards an all-solid-state configuration will offer adaptability to various designs and stressful mechanical handling, as well as enhance cell safety and reliability. During the three years of the Ph.D. course, the attention was focused on the optimisation of innovative materials for Li-ion batteries as well as the development of easily up-scalable procedures for the production of electrodes and polymer electrolytes. The basic idea was to start from eco-friendly materials to develop simple, low-cost and easily adaptable processes in order to propose innovative solutions for LIBs with a wide range of possible applications. Moreover, during my experimental activities, I considered the performances and the cycling stability of Li-ion batteries, by studying the mechanisms related to the capacity fade of lab-scale batteries and also by analysing commercial Li-ion batteries for automotive application. The results of the research work are presented in this thesis (Chapters 4-7) following an introductory section that provides the general information needed to follow the discussions (Chapters 1-3). The experimental research work presented in Chapter IV was carried out in collaboration with the Laboratory of Pulp and Paper Science and Graphic Arts (LGP2) in Grenoble (France). A well-known natural material such as cellulose was exploited for the production of innovative low-cost and easily recyclable electrodes for Li-ion batteries. A simple aqueous filtration process, based on a well-known industrialised paper-making technology, was developed and the electrodes (graphite-based anodes and LiFePO4-based cathodes) produced and partly characterized in Grenoble by Dr. Lara Jabbour were electrochemically studied in our Labs in Politecnico di Torino. In particular, cellulose fibres (FBs) were used as natural binder for the production of paper-like electrodes obtained without addition of any synthetic binder and/or solvent and showing electrochemical performance comparable to those produced with the same active materials by a standard process. In Chapter V, results are reported regarding a newly developed procedure where a methacrylic-based polymer electrolyte is directly formed in situ at the interface with the electrodes. Exploiting the versatile nature of UV-induced free-radical photo-polymerisation, novel ready-to-use multiphase electrode/electrolyte composites (MEEC) were developed in which the electrode is conformally coated by the polymer electrolyte. This “one-shot” process was successfully applied to enhance the cycling performances of two nanostructured materials conceived for microbattery application, such as Cu2O (in collaboration with CSHR@Polito IIT research institute in Torino) and V2O5 (in collaboration with Prof. Mustarelli’s group in University of Pavia), prepared in the form of thin films and proposed respectively as anode and cathode. The proposed one-shot process, thanks to the intimate interfacial contact between electrodes surface and electrolyte obtained by in situ process, induced a huge effect of stabilization thus improving the cycling stability of both the nanostructures. All along Chapter VI, the problems related to the assembling of complete Li-ion cells, starting from two well performing electrodes, are progressively discussed and valuable solutions are proposed. A strong capacity fade was initially found, thus the possible causes were studied also considering the failure mechanisms proposed in the literature. Several measures were adopted to improve the cycling stability, considering the effect of all the different cell components as well as the effects of both charging protocol and cell apparatus. Moreover, due the knowhow progressively achieved on the intimate characteristics of complete Li-ion cells and their assembly, even thanks to a three months stage at ENEA Casaccia Research Centre of Rome, the installation of a 10 m2 dry room was personally followed at our Electrochemistry Research Group Labs in Politecnico di Torino and the results obtained are presented in the same Chapter VI. These results include the realisation of an all-paper Li-ion battery with the cellulose-based electrodes and paper hand-sheets as separator. Finally, the cycling stability and the failure prediction issue was studied for a 53 Ah commercial battery. The results obtained, by means of different standard reference tests, are reported in Chapter VII. The commercial battery was also disassembled in the controlled atmosphere of an Ar-filled dry box in order to study the system structure and characterise the various components. A testing protocol was personally developed and the results obtained allowed to evaluate the commercial battery based on the performances requested for HEV and EV application. In particular, an easy measure of the internal resistance was developed, by opportunely modulating the measured parameters, and the obtained results were found to be very useful in directly predicting the cell failure which is fundamental in practical application.

博士
國立中央大學
機械工程研究所
96
The technique of electrochemical impedance spectroscopy (EIS) diagnosis has been used to investigate the electrochemical kinetics in the systems of (1) direct methanol fuel cell (DMFC) and (2) photo-electrochemical etching on silicon. The results and contributions of this work were summarized as follows. 1. EIS was carried out to monitor the performance of DMFC under a variety of current densities. Based on analysis of the EIS data that depend upon the performing conditions, an innovative model including the qualitative sketch and its quantitative description relying on postulated equivalent circuit (EQC) was established to delineate the reaction mechanism of DMFC on the membrane electrode assembly (MEA). This model provides a satisfactory diagnosis in the performance of DMFC in terms of the EQC sets. One EQC sets comprises elements such as the internal resistance (Rs) at the highest frequency, the high-frequency impedance (Rif /Cif) that is a parallel combination of the interfacial resistance (Rif) and interfacial capacitance (Cif) resultant from the interfaces in the cell, the medium-frequency impedance (Rrxn /Crxn) that is a parallel combination of the resistance (Rrxn) and capacitance (Crxn) resultant from electrochemical reactions, and the low-frequency impedance (LCO /RCO) that is a parallel combination of the resistance (RCO) and inductance (LCO) resultant from the adsorption and relaxation of CO. This postulated model provides a useful tool to diagnose the degradation mechanism for a cell subject to a test of accelerating degradation. Through the diagnosing and the evidences supported by the examinations through instruments such as the electron probe microanalyzer (EPMA), transmission electron microscope (TEM) and X-ray photoelectron spectroscope (XPS), the degradation is major attributed to (a) the increase of Rif and Rrxn resultant from catalytic degradation that may arise from a series of processes including the dissolution of Ru from the anodic catalyst Pt-Ru, the migration of Ru ions to be reduced on the membrane nearby the cathode. The Ru-dissolution leads to a decrease of catalytic activity on the anode that could be confirmed by the technique of CO stripping in company with the observation through EPMA and XPS. The particles reduced on the membrane nearby the cathode were verified by the examination through TEM and EPMA. (b) The increase of internal resistance (Rs) is ascribed to the loss of sulfonic-acid group from the graded membrane near the anode. Membrane degradation possibly arisen from the heat accumulation in a severely acidic environment near the anode derived from cell reactions. The loss of sulfonic acid group was verified by EPMA and XPS analyses. 2. The photo-electrochemical etching on Si (100) surface reveals different SEM morphologies depending on whether or not the HF solution contains ethanol. Finer smooth pores (around 4 μm in diameter) were formed in the presence of ethanol but larger rough pores (around 8 μm in diameter) formed in 2 M HF solution alone during silicon etched at 0.250 V (vs. SCE) under 50W-illumination for 3 h. The characteristic potentials and current such as transition potential (Etrans), half-wave potential (Ep/2), and limiting current density (jlimit), resulted from dc anodic polarization, were the major parameters used in EIS to diagnose the etching system. There appears an extra low-frequency inductive loop in the Nyquist plot for the etching system in the presence of ethanol. This loop is attributed to relaxation of the adsorption of ethanol in the pores. The contact angle between the etching solution and the silicon decreases with increasing the ethanol concentration. Accordingly, ethanol plays a wetting role in the etching process thus forming fine smooth pores.

碩士
國立高雄應用科技大學
化學工程與材料工程系
99
Various morphologies polyaniline (PANI) deposited on the interface betweenTiO2 and Dye (TiO2-Dye/PANI) and ITO counter electrode (PANI-ITO C.E.) were prepared by photo-chemical and electrochemical polymerization method, respectively, where deposited PANI used as the electro hole transport layer material (HTM) in the part of solid state dye-sensitized solar cell (DSSC). The solid state TiO2-Dye/PANI/ITO and TiO2/Dye/PANI-ITO solar cells were further fabricated by using sandwich assembling technique. As a result, in spite of the photo-chemical polymerization method can effectively improve the electric contact between TiO2 and PANI, leading to increase in current density, but the PANI chains may penetrate into the TiO2-Dye layer and growth on the surface of ITO, results in the total photoelectric conversion efficiency () is apparently depressed. As the total charge is 0.3 coulomb, the deposited PANI exhibits the best surface roughness, conjugated degree and electrochemical activity than those of applied coulombs. When 4-tert-Butylpyridine (TBP) added into HTM, the electron-hole recombination between TiO2 and HTM is significantly reduced; the Voc hence increases to 0.52 V. As TBP and LiI added into HTM, LiI acts as the dopant of PANI and the redox couple in the solar cell, leading to the improvement of photovoltaic characteristics. At 0.7M TBP and 0.1 M LiI, the η of TiO2/Dye/PANI-ITO solar cells is 0.643 %.

The possibility of exploiting solar energy for the direct production of fuels and chemicals (e.g. hydrogen, hydrocarbons, alcohols) represents a future challenge to move towards a new green economy, recently defined as “solar-driven chemistry”. In this view, this PhD thesis focuses on the development of a new approach to convert solar energy, through the synthesis of innovative photoactive materials/electrodes for the production of solar fuels. By assembling these photo-electrodes in a photo-electrochemical (PEC) cell, designed on purpose to mimic what nature does with the leaves, solar energy can be captured and used to produce hydrogen from water (by water photo-electrolysis) or to generate value-added carbon products (by reducing atmospheric CO2) in a one-step process, like an “artificial leaf”. Thus, the main objective of the present PhD work was to develop photocatalytic thin films able to work as photoanodes in efficient PEC devices, especially for the production of hydrogen. The PhD activities were carried out at the laboratory CASPE/INSTM (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. During the three years of activity, all the aspects concerning the performances of the photocatalysts and the related PEC electrodes and cell, have been carefully evaluated. Initially, the research activity focused on the preparation of titania (TiO2) nanotubes synthesized by controlled anodic oxidation technique. The peculiarity of this method is the possibility to “tailor” the morphology and the nanostructure of the catalyst by modulating some parameters during the synthesis (such as the electrolyte composition, the pH, the applied voltage, the anodization time). In general, the use of titanium dioxide as a photocatalyst, despite many advantages (low cost, non-toxicity, resistance to photocorrosion, high quantum yield), has two main drawbacks: i) the low absorption of light in the visible region, due to the high band gap (in the range of 3.0-3.2 eV) and ii) the fast charge recombination, which usually occurs at the grain boundaries of the particles. The latter can be mitigated by the realization of nanostructures such as nanotubes or nanorods, which may improve the vectorial transport of electrons to the collector layer. VI Different characterization techniques (SEM-EDX, TEM, XRD, UV-vis Diffuse Reflectance Spectroscopy) were used to investigate the properties of the as-prepared TiO2 nanotube arrays, as well as to evaluate their electrochemical behaviour (Cyclic voltammetry, Chronoamperometry). Part of the characterization by electron microscopy was carried out in collaboration with the Department of Chemical Sciences of the University of Padua. The main aim was to obtain a correlation between synthesis parameters, nanostructure properties and photo-catalytic performances. Moreover, particular attention was given to the evaluation of the efficiency of the PEC cell. To pursue this aim, titania nanotubes of different lengths (from 0.5 to 6 m) were synthesized by varying the anodization time from 30 min to 5 h. A monochromator and a spectroradiometer were used to evaluate the light irradiance at different wavelengths directly inside the PEC device. These measurements allowed for the calculation of different kinds of efficiencies: i) the photoconversion efficiency, also called solar-to-hydrogen efficiency (STH), which takes into account the amount of energy supplied in terms of light and the products obtained (i.e. hydrogen); ii) the Faradaic efficiency (η), which relates the photo-generated current to the produced hydrogen; iii) the quantum efficiency, expressed as IPCE (incident photon to current efficiency) and APCE (absorbed photon to current efficiency). The most important results (reported in detail in Chapter 3) showed that, for use in a PEC cell, the 45- min-anodized nanotube arrays (tube length of about 1 μm) provided the best performance, with a H2 production of 22.4 mol h-1 cm-2 and a STH efficiency as high as 2.5%. These values are among the best ever reported insofar as undoped TiO2 photoanodes are used and in absence of external bias or sacrificial agents. The final part of Chapter 3 was dedicated to the preparation of 3D-type meso/macro porous structured photoanodes based on Ti mesh, working as a hierarchical structure (consisting of Ti mesh macropores and TiO2 nanotube mesopores) to improve the mass and charge transport within the PEC cell. In order to improve the light absorption in the visible region, it is necessary to dope the nanostructured TiO2 materials with heteroatoms or decorate their surface with metal nanoparticles. In this direction, nanoparticles of gold (Au) were deposited on the surface of TiO2 nanotubes by optimizing three different techniques (wet impregnation, photo-reduction and electrodeposition) and their performances were studied by using a gas photo-reactor (GP) VII and a photo-electrochemical (PEC) cell, both homemade. Furthermore, with the aim of exploiting earth-abundant and low-cost materials, photocatalysts based on Cu-doped TiO2 nanotubes were also synthesized and successfully tested in the PEC cell for H2 production in water-photo-electrolysis and ethanol photo-reforming. This part of the work was carried out in collaboration with the Institute of Chemistry in Araraquara (Brazil). The CuO nanoparticles were deposited by adopting two different techniques, dip-coating and electrodeposition. The results (reported in detail in Chapter 4) showed that the presence of small metal (Au and Cu) nanoparticles strongly increased H2 production rate in a gas photo-reactor, with a maximum of about 190 mol in 5 h of light irradiation obtained for Au-doped TiO2 nanotubes prepared by electrodeposition. However, in the PEC cell (with oxidation/reduction half reactions separated in two different chambers of the cell) it was observed that the presence of metal nanoparticles on TiO2 surface at the photo-anode can create a counter-circuited current, diminishing the H2 production at the cathode side. However, this phenomenon was successfully minimized by preparing very small CuO nanoparticles (lower than 2 nm) decorating the internal walls of the TiO2 nanotubes by controlled dip-coating technique. Finally, nanostructured tantalum oxynitride (Ta-oxy-N) electrodes were synthesized through controlled anodic oxidation technique, by adapting the synthesis conditions previously optimized for TiO2. The advantages of these tantalum-based materials refer to their lower band-gap (1.9-2.5 eV) with respect to titania (3.0-3.2 eV), thus improving light absorption in the visible region. After the anodization, a high temperature nitridation process (600-900 °C) was needed to replace partially oxygen with nitrogen in the Ta2O5 lattice. The results (reported in detail in Chapter 5) allowed to obtain a clear correlation between the parameters using during the synthesis (i.e. applied voltage, anodization time) and the Ta-oxy- N nanostructures (nanotube diameter and length, wall thickness and grade of voids). The best photocurrent response was obtained for the Ta-oxy-N sample anodized at 40 V for 1 min and then thermally treated with ammonia at 800°C. However, further investigation is needed to improve the mechanical resistance of these photo-catalysts.