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A simple and useful experimental alternative to field-effect transistors for measuring electrical properties free electron concentration nd, electrical mobility , and conductivity in individual nanowires has been developed. A combined model involving thermionic emission and tunneling through interface states is proposed to describe the electrical conduction through the platinum-nanowire contacts, fabricated by focused ion beam techniques. Current-voltage I-V plots of single nanowires measured in both two- and four-probe configurations revealed high contact resistances and rectifying characteristics. The observed electrical behavior was modeled using an equivalent circuit constituted by a resistance placed between two back-to-back Schottky barriers, arising from the metal-semiconductor-metal M-S-M junctions. Temperature-dependent I-V measurements revealed effective Schottky barrier heights up to BE= 0.4 eV.

This paper deals with the analysis of the vibrational and crystallographic properties of CuInC2 sC=S,Sed chalcogenides. Experimentally, evidence on the coexistence in epitaxial layers of domains with different crystalline order—corresponding to the equilibrium chalcopyrite sCHd and to CuAu sCAd—has been obtained by cross section transmission electron microsopy sTEMd and high resolution TEM sHREMd. Electron diffraction and HREM images give the crystalline relationship f110gCHif100gCA and s112dCHis011dCA, observing the existence of a s112dCHis001dCA interphase between different ordered domains. The vibrational properties of these polytypes have been investigated by Raman scattering. Raman scattering, in conjunction with XRD, has allowed identifying the presence of additional bands in the Raman spectra with vibrational modes of the CA ordered phase. In order to interpret these spectra, a valence field force model has been developed to calculate the zone-center vibrational modes of the CA structure for both CuInS2 and CuInSe2 compounds. The results of this calculation have led to the identification, in both cases, of the main additional band in the spectra with the total symmetric mode from the CuAu lattice. This identification is also supported by first-principles frozenphonon calculations. Finally, the defect structure at the interphase boundaries between different polymorphic domains has also been investigated.

A novel NO2 sensor based on (CdO)x(ZnO)1-x mixed-oxide thin films deposited by the spray pyrolysis technique is developed. The sensor response to 3-ppm NO2 is studied in the range 50°C-350°C for three different film compositions. The device is also tested for other harmful gases, such as CO (300 ppm) and CH4 (3000 ppm). The sensor response to these reducing gases is different at different temperatures varying from the response typical for the p-type semiconductor to that typical for the n-type semiconductor. Satisfactory response to NO2 and dynamic behavior at 230°C, as well as low resistivity, are observed for the mixed-oxide film with 30% Cd. The response to interfering gas is poor at working temperature (230°C). On the basis of this study, a possible sensing mechanism is proposed.

The details of the sulphurisation reaction of Cu/In metallic precursors layers for the preparation of CuInS2 solar cells have been investigated. For this purpose, we have designed an experiment using a combined sequence of sulphurisation and selenization steps. Analysing the resultant layers by Raman Spectroscopy and Auger Electron Spectroscopy (AES) we could study the details of the chemical reaction. By this method we have been able to identify where the chemical reaction between the metallic species and the reactive atmosphere takes place. Besides, we have developed an experimental system to analyse the sulphurisation process in-situ by Raman spectroscopy. With this system we studied the evolution of the material structure during the sulphurisation process. The experimental results prove that the reaction takes place in the surface, forming in the firsts steps CuInS2 in two different structures (chalcopyrite and CuAu ordering). Afterwards, the out-diffusion of the metallic atoms towards the surface allows the reaction between them and the sulphur vapour. Together with the sulphurisation process the improvement of the crystalline quality of the CuInS2 forming a chalcopyrite CuInS2 layer has been observed.En este trabajo se presenta un estudio detallado de los procesos implicados en la sulfurización de capas metálicas de Cu-In para la fabricación de células solares de CuInS2. Con este objeto, se ha desarrollado un experimento de sulfurización parcial de las capas, que han sido sometidas posteriormente a un tratamiento de selenización. El estudio de estas estructuras mediante Espectroscopía Raman y Espectroscopía de Electrones Auger (AES) ha permitido conocer algunos de los detalles de la reacción química, en concreto la identificación del frente de crecimiento de la reacción de sulfurización. Paralelamente, se ha desarrollado un sistema experimental que ha hecho posible investigar in-situ la reacción de sulfurización por Espectroscopía Raman, lo cual ha permitido un seguimiento preciso de la evolución estructural del material durante el proceso. Los resultados experimentales demuestran que la reacción de sulfurización se inicia en la superficie de la capa, dando lugar a la formación de CuInS2, coexistiendo dos estructuras cristalinas polimórficas (calcopirita y orden catiónico CuAu). Posteriormente la reacción química continúa asistida por la difusión de los metales hacia la superficie, que reaccionan con la atmósfera de azufre, de forma simultánea se produce una transformación de la fase CuAu del compuesto en la estructura calcopirita.

En este trabajo se presenta un estudio químico y estructural de las capas metálicas de Pt y TaSix utilizadas como puerta catalítica en sensores de gas de alta temperatura basados en dispositivos MOS de SiC. Para ello se han depositado capas de diferentes espesores sobre substratos de Si. Los resultados muestran que con la reducción del espesor de Pt y con un recocido se consigue aumentar la rugosidad de las capas de puerta, lo que debería aumentar la sensibilidad y la velocidad de respuesta de los dispositivos que las incorporasen. Otro efecto del recocido es la transformación química del material de la puerta que, para capas delgadas de Pt con TaSix, produce la transformación total Pt en Pt2Ta, lo que podría afectar a las características catalíticas de la puerta. Los primeros resultados eléctricos indican que, a pesar de que las capas de Pt empleadas son gruesas y compactas, los diodos MOS túnel de SiC son sensibles a los gases CO y NO2, aunque presentan una velocidad de respuesta bastante lenta.

Liquid pyrolysis is presented as a new production method of SnO2 nanocrystalline powders suitable for gas sensor devices. The method is based on a pyrolytic reaction of high tensioned stressed drops of an organic solution of SnCl4·5(H2O). The main advantages of the method are its capability to produce SnO2 nanopowders with high stability, its accurate control over the grain size and other structural characteristics, its high level of repeatability and its low industrialization implementation cost. The characterization of samples of SnO2 nanoparticles obtained by liquid pyrolysis in the range between 200ºC and 900ºC processing temperature is carried out by X-ray diffraction, transmission electron microscopy, Raman and X-ray photoelectron spectroscopy. Results are analyzed and discussed so as to validate the advantages of the liquid pyrolysis method.

InAlAs/InGaAs/InP based high electron mobility transistor devices have been structurally and electrically characterized, using transmission electron microscopy and Raman spectroscopy and measuring Hall mobilities. The InGaAs lattice matched channels, with an In molar fraction of 53%, grown at temperatures lower than 530¿°C exhibit alloy decomposition driving an anisotropic InGaAs surface roughness oriented along [1math0]. Conversely, lattice mismatched channels with an In molar fraction of 75% do not present this lateral decomposition but a strain induced roughness, with higher strength as the channel growth temperature increases beyond 490¿°C. In both cases the presence of the roughness implies low and anisotropic Hall mobilities of the two dimensional electron gas.

El crecimiento autoorganizado de puntos cuánticos de InSb sobre substratos de InP mediante epitaxia de haces moleculares ha sido caracterizado mediante microscopía electrónica de transmisión y microscopía de fuerzas atómicas a fin de estudiar la morfología y establecer el mecanismo de relajación de las estructuras. Medidas de difracción de electrones de alta energía durante el crecimiento de las muestras indican una transición en el modo de crecimiento de bidimensional a tridimensional a partir de un espesor total equivalente depositado de 1.1 monocapas atómicas de InSb. Los puntos cuánticos tienen en general, una buena calidad cristalina así como una distribución superficial homogénea, pero una alta anisotropía en las dimensiones entre las direcciones [110] y [110]. Así mismo, se encuentra una rugosidad superficial, también afectada de un alto grado de anisotropía, siendo las crestas y los valles paralelos a la dirección de elongación de las islas. Las imágenes de microscopía electrónica revelan un facetado de las estructuras según planos (001) que las limitan por la parte superior y planos {111}, {113} y {114}, que las limitan lateralmente. Las tensiones son eficazmente relajadas por la presencia de una red de dislocaciones situada en la interficie InSb/InP.

Se ha n realizado implantaciones de silicio y de carbono + silicio en matrices aislantes de SÍO2 térmico, las cuales, después de un recocido a alta temperatura precipitan en forma de nanocristales de tamaños comprendidos entre 30 y 60 Á. Estas estructuras presentan una intensa fotoluminiscencia en el rojo profundo (1.4-1.6 eV) y el verde (2.0-2.2 eV). La energía e intensidad de las bandas depende fuertemente de la temperatura y duración del recocido. Diferentes comportamientos se han encontrado para las bandas roja y verde, incluyendo la cinética de desexcitación y el origen estructural. Los experimentos de absorción infrarroja, Raman y microscopía electrónica demuestran que los nanocristales son los responsables de la banda roja mientras que agregados amorfos de carbono son los responsables de la verde.

Se ha analizado por microscopía electrónica en transmisión (TEM) la estructura de transistores HEMT basados en un pozo cuántico tensionado de InGaAs/InAlAs crecido sobre un sustrato {111} de InP. Se han observado dislocaciones filiformes y defectos planares que cruzan la capa superior hacia la superficie, así como maclas paralelas a la interficie y grandes complejos defectivos en forma de V que se nuclean unos pocos nanometres por encima de la interficie entre el pozo cuántico y la capa superior que lo confina. La estructura de los defectos es muy diferente de la observada en heteroestructuras similares crecidas sobre sustratos {100}, hecho que sugiere que hay que tener en cuenta consideraciones sobre el proceso mismo de nucleación de los defectos junto con las convencionales relacionadas con el desajuste de redes.

Optical-absorption measurements have been carried out on tensile and compressive In x Ga 1 − x As/InP strained layers. It is shown that the energetic dispersion of the heavy-hole relative to the light-hole subband σ HH / σ LH is the key to knowing the origin of the microscopic inhomogeneities. So, σ HH / σ LH

Un dels principals objectius de la recerca en electrònica és millorar les prestacions de velocitat dels equips. Als circuits basats en dispositius fabricats amb silici, l'augment de la velocitat de funcionament s'ha aconseguit mitjançant la reducció de les dimensions dels dispositius, és a dir, la disminució del camí que han de recórrer els portadors. Aquesta reducció ha estat possible gràcies als avenços en les tecnologies de creixement i processament dels materials, però encara que el ritme de progrés en aquestes àrees ha estat constant en els darrers anys en algunes àrees està proper a la saturació imposada per les pròpies limitacions intrínseques del material. En conseqüència, per tal de fer un salt qualitatiu en l'augment de velocitat, s'ha enfocat la recerca darrerament a aconseguir materials amb majors velocitats inherents de portadors.

Transmission electron microscopy studies have been performed to characterize InxAl1−xAs layers grown by molecular beam epitaxy on (100) InP substrates. The first observations of compositional nonuniformities in strained InAlAs layers are reported. The coarse quasiperiodic structure present in each sample has been found to be dependent upon the growth parameters and the sample characteristics.

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron¿hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron¿hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron¿hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

En este trabajo se investiga la síntesis de estructuras SiC/Si mediante implantación iónica de carbono en Si. Las implantaciones se han realizado a energías entre 25 y 300 keV y las dosis en el rango lO^^ylO^^ cm , manteniendo el substrato a temperatura ambiente o 500°C. Algunas estructuras han sido recocidas a 1150°C. Los resultados indican que implantando a temperatura ambiente se forma una capa de SiC amorfa y de composición gradual, que recristaliza formando precipitados de ß-SiC con orientaciones aleatorias después del recocido. Además se forma un capa superficial rica en carbono, debida a la difusión del carbono hacia la superficie durante la implantación, y que desaparece con el recocido. Implantando a 500°C se forma directamente una capa con una muy alta densidad de precipitados de ß-SiC orientados preferencialmente con la matriz de silicio. Dada la estabilidad térmica y química de dicha capa se han realizado membranas de SiC mediante técnicas fotolitográficas y ataque químico selectivo, cuya rugosidad superficial es inferior a 6 nm. Estas membranas muestran unos gradientes de tensiones residuales, que prácticamente desaparecen después del recocido. Los resultados confirman la potencialidad de la implantación iónica para la formación de estructuras microme-cánicas de SiC sobre Si.

[eng] Lithium-sulfur batteries (LSBs) are regarded as the most promising candidate to replace Lithium-ion batteries (LIBs) in next-generation energy storage systems. Compared with LIBs, LSBs are characterized by a sixfold higher theoretical energy density, and a potentially lower cost and environmental impact for commercialization. Despite these attractive advantages, the electrically insulating character of sulfur/Li2S and the shuttle effect of lithium polysulfides (LiPS) greatly limit the practical application of LSBs. Additionally, the serious volume changes (∼80%) and slow redox kinetics during the charging/discharging process also reduce the cycling life and power density. The rational design and engineering of the cathode host can effectively overcome the above challenges. In Chapter 1, I summarize the state of the art on advanced hosts for LSBs and detail the targeted requirements from three points of view: material, architecture, and heterogeneous interface. In Chapter 2, I detail my work on the design and engineering of u-NCSe nanostructures as efficient sulfur hosts to overcome the limitations of LSBs. u-NCSe provide a beneficial hollow structure to relieve volumetric expansion, a superior electrical conductivity to improve electron transfer, a high polarity to promote adsorption of LiPS, and outstanding electrocatalytic activity to accelerate LiPS conversion kinetics, which were confirmed by experiments and theoretical calculation. Owing to these excellent qualities as LSB cathode, S@u-NCSe electrodes delivered outstanding rate performance of 626 mAh g−1 at 5 C, and a very low capacity decay rate of only 0.016% per cycle during cycling. This work probed that transition metal selenides can be promising candidates as sulfur host, and was published in Advanced Functional Materials in 2019. In Chapter 3, I explain my work on the design and production of multifunctional Ag/VN@Co/NCNTs nanocomposite with multiple adsorption and catalytic sites within hierarchical nanoreactors as a robust sulfur host for LSB cathodes. In this hierarchical nanoreactor, heterostructured Ag/VN nanorods serve as a highly conductive backbone structure and provide internal adsorption and catalytic sites for LiPS conversion. Interconnected NCNTs in situ grown from the Ag/VN surface, greatly improve the overall specific surface area for sulfur dispersion and accommodate volume change in the reaction process. Owing to their high LiPS adsorption ability, outer Co nanoparticles at the top of the NCNTs catch escaped LiPS, thus effectively suppressing the shuttle effect and enhancing kinetics. Benefiting from the multiple adsorption and catalytic sites of the developed hierarchical nanoreactors, Ag/VN@Co/NCNTs@S cathodes display outstanding electrochemical performances, including a superior rate performance of 609.7 mAh g−1 at 4 C and good stability with a capacity decay of 0.018% per cycle after 2000 cycles at 2 C. This work demonstrated the great advantages of designing the host architecture, and it was published in ACS Nano in 2021. In Chapter 4, in view of the complexity and difficulty in the synthesis of superlattice materials, I detail a simple solution-based method to efficiently produce organic-inorganic PVP-WSe2 superlattices and demonstrate that the pyrolysis of the PVP compound enables to continuously adjust their interlayer space in the range from 10.4 Å to 21 Å, resulting in NG/WSe2 superlattices with superior electrical conductivities. Both experimental results and theoretical calculations further demonstrate that NG/WSe2 superlattices are excellent sulfur hosts for LSB, being able to effectively reduce the LiPS shuttle effect by dual-adsorption sites and accelerating the sluggish Li-S reaction kinetics. Consequently, S@NG/WSe2 electrodes delivered high sulfur usages, superior rate performance, and outstanding cycling stability. Overall, this work not only establishes a cost-effective strategy to produce artificial superlattices but also pioneers their application in the field of LSBs. This work has been published in Advanced Functional Materials in 2022., [spa] Lithium-sulfur batteries (LSBs) are regarded as the most promising candidate to replace Lithium-ion batteries (LIBs) in next-generation energy storage systems. Compared with LIBs, LSBs are characterized by a sixfold higher theoretical energy density, and a potentially lower cost and environmental impact for commercialization. Despite these attractive advantages, the electrically insulating character of sulfur/Li2S and the shuttle effect of lithium polysulfides (LiPS) greatly limit the practical application of LSBs. Additionally, the serious volume changes (∼80%) and slow redox kinetics during the charging/discharging process also reduce the cycling life and power density. The rational design and engineering of the cathode host can effectively overcome the above challenges. In Chapter 1, I summarize the state of the art on advanced hosts for LSBs and detail the targeted requirements from three points of view: material, architecture, and heterogeneous interface. In Chapter 2, I detail my work on the design and engineering of u-NCSe nanostructures as efficient sulfur hosts to overcome the limitations of LSBs. u-NCSe provide a beneficial hollow structure to relieve volumetric expansion, a superior electrical conductivity to improve electron transfer, a high polarity to promote adsorption of LiPS, and outstanding electrocatalytic activity to accelerate LiPS conversion kinetics, which were confirmed by experiments and theoretical calculation. Owing to these excellent qualities as LSB cathode, S@u-NCSe electrodes delivered outstanding rate performance of 626 mAh g−1 at 5 C, and a very low capacity decay rate of only 0.016% per cycle during cycling. This work probed that transition metal selenides can be promising candidates as sulfur host, and was published in Advanced Functional Materials in 2019. In Chapter 3, I explain my work on the design and production of multifunctional Ag/VN@Co/NCNTs nanocomposite with multiple adsorption and catalytic sites within hierarchical nanoreactors as a robust sulfur host for LSB cathodes. In this hierarchical nanoreactor, heterostructured Ag/VN nanorods serve as a highly conductive backbone structure and provide internal adsorption and catalytic sites for LiPS conversion. Interconnected NCNTs in situ grown from the Ag/VN surface, greatly improve the overall specific surface area for sulfur dispersion and accommodate volume change in the reaction process. Owing to their high LiPS adsorption ability, outer Co nanoparticles at the top of the NCNTs catch escaped LiPS, thus effectively suppressing the shuttle effect and enhancing kinetics. Benefiting from the multiple adsorption and catalytic sites of the developed hierarchical nanoreactors, Ag/VN@Co/NCNTs@S cathodes display outstanding electrochemical performances, including a superior rate performance of 609.7 mAh g−1 at 4 C and good stability with a capacity decay of 0.018% per cycle after 2000 cycles at 2 C. This work demonstrated the great advantages of designing the host architecture, and it was published in ACS Nano in 2021. In Chapter 4, in view of the complexity and difficulty in the synthesis of superlattice materials, I detail a simple solution-based method to efficiently produce organic-inorganic PVP-WSe2 superlattices and demonstrate that the pyrolysis of the PVP compound enables to continuously adjust their interlayer space in the range from 10.4 Å to 21 Å, resulting in NG/WSe2 superlattices with superior electrical conductivities. Both experimental results and theoretical calculations further demonstrate that NG/WSe2 superlattices are excellent sulfur hosts for LSB, being able to effectively reduce the LiPS shuttle effect by dual-adsorption sites and accelerating the sluggish Li-S reaction kinetics. Consequently, S@NG/WSe2 electrodes delivered high sulfur usages, superior rate performance, and outstanding cycling stability. Overall, this work not only establishes a cost-effective strategy to produce artificial superlattices but also pioneers their application in the field of LSBs. This work has been published in Advanced Functional Materials in 2022.

[eng] One of the most serious challenges facing society is the need to reduce the emissions of greenhouse gases such as carbon dioxide (CO2), mainly caused by the widespread use of fossil fuels. The energy transition to renewable energies is currently in its initial stages of its implementation. Due to the need to increase renewable energy sources, it is necessary to develop new technologies that can adapt the fluctuating generation of renewables with the energy consumption. The conversion of CO2 into synthetic fuels using electrical energy, technologies known as Power to Gas, presents a solution to the energy storage. In the specific case of the methanation reaction, CO2 is converted to methane (CH4, synthetic natural gas), which can be stored and distributed in large quantities. In this way, synthetic natural gas is used as an energy vector, providing a solution as energy storage and as a method of CO2 valorization. In the last years, Power to Gas technologies are being implemented in several pilot plants with conventional thermochemical technologies for the methanation reaction. In this regard, alternative methods to thermal catalysis are being studied, which could potentially have advantages in terms of conversion or reduction of energy costs, among others. The use of plasmas in the conversion of CO2 is particularly promising due to the ability of plasma to activate stable molecules such as CO2 or N2. In this context, the aim of this thesis is the development of a dielectric barrier discharge plasma reactor for the conversion of CO2 to CH4, the development of catalysts for this reaction and the optimization of both the catalysts and the plasma reactor. Following these objectives, the thesis is structured in 6 main chapters. The first chapter presents the general context, the motivation of the research regarding CO2 conversion technologies and a basic introduction of plasma technologies. In chapter 2, all experimental setup and material synthesis are explained. The following chapters are focused on the main results obtained during the development of the thesis, which can be divided into two parts. The first part is focused on the catalyst evaluation for plasma-CO2 methanation (chapter 3 and chapter 4). The second part is focused on the optimization of the plasma reactor (chapter 5 and chapter 6). In the catalyst evaluation for plasma-CO2 methanation, different types of catalyst were synthesized, physicochemical characterized and evaluated in thermal and plasma methanation. Operation temperature was reduced to 100-200 ºC in the case of plasma methanation. The effect of different catalysts composition and structure was analyzed. The incorporation of cerium in nickel based catalyst boosted the conversion and efficiencies. The different role of plasma activation and catalyst activity were evaluated. In the second part, DBD reactor optimization, different reactor configurations were tested. The role of temperature was analyzed in pseudo-adiabatic and adiabatic DBD reactor. The use of adiabatic reactors allows to increase the energy efficiency. Finally, a new approach was evaluated based on using DBD-plasma as reaction ignitor, rather than the classical approach of continuous operation. After plasma ignition, the operation in autothermal conditions without any external energy input was evaluated. The new methodology allowed to faster start the reaction, due to the plasma activation at a lower temperature, minimizing the start-up time and the energy cost., [cat] Un dels reptes més importants al qual s’enfronta la societat és la necessitat de reduir les emissions de gasos d’efecte hivernacle, principalment el diòxid de carboni (CO2), causades per l’ús generalitzat dels combustibles fòssils. Atesa la necessitat d’augmentar les energies renovables, és necessari desenvolupar noves tecnologies que puguin adaptar la generació fluctuant de les renovables amb el consum energètic. La conversió de CO2 a combustibles sintètics emprant energia elèctrica, tecnologia que es coneix com a Power to Gas, presenta una solució a l’emmagatzematge d’energia. En el cas concret de la reacció de metanització, el CO2 és convertit a metà (CH4, gas natural sintètic), el qual es pot emmagatzemar i distribuir en grans quantitats. En aquest context es presenta la tesi amb l’objectiu de desenvolupar un reactor de plasma de descàrrega de barrera dielèctrica per a la conversió de CO2 a CH4, el desenvolupament de catalitzadors per a aquest reacció i la optimització tant dels catalitzadors com del reactor de plasma. Seguint aquests objectius, la tesi s’estructura en 6 capítols. El primer capítol presenta el context general, i una introducció bàsica de les tecnologies de plasma. En el segon capítol s’explica tota la configuració experimental del reactor de plasma i la síntesi de catalitzadors. En els següents capítols es mostren els principals resultats obtinguts durant el desenvolupament de la tesi, els quals es divideixen en dues parts. La primera part comprèn els capítols 3 i 4 i se centra en l’avaluació de catalitzadors per a la reacció de metanització del CO2. Mitjançant l’activació per plasma, la temperatura de treball dels catalitzadors es va poder reduir fins als 100-200 ºC. Tot seguit es va estudiar l'efecte de la composició i l'estructura de diferents catalitzadors. La incorporació de ceri en els catalitzadors de níquel augmentà considerablement els nivells de conversió i eficiència energètica. La segona part se centra en l’optimització del reactor de plasma (capítols 5 i 6). En aquest apartat es va analitzar el rol de la temperatura en configuracions pseudo-adiabàtica i adiabàtica, demostrant que l’ús de reactors adiabàtics ha permès augmentar l’eficiència energètica. Finalment, es va avaluar un nou enfocament basat en l’ús del plasma DBD com a iniciador de la reacció, en lloc de l’enfocament clàssic d’operació contínua. Després de l’encesa del plasma, es va avaluar el funcionament en condicions autotèrmiques, sense cap aportació externa d’energia. La nova metodologia permet iniciar més ràpidament la reacció, gracies a l’activació del plasma a baixes temperatures, minimitzant el temps i el cost energètic.

La combustió excessiva de combustibles fòssils té com a resultat l’emissió de diòxid de carboni (CO2), que està desencadenant problemes ambientals creixents, com ara l’escalfament global, l’augment del nivell del mar, el clima extrem i l’extinció d’espècies. Per tant, les tecnologies per a la conversió de CO2 en altres productes de valor estan jugant un paper vital per eliminar la concentració de CO2 a l’atmosfera. En aquest sentit, la conversió electroquímica de CO2, alimentat per energia renovable, en productes químics útils es considera una solució elegant per aconseguir el cicle del carboni. Tanmateix, a causa de la interioritat de les molècules de CO2 i de la reacció competitiva d’evolució d’hidrogen (HER), els principals reptes de CO2 RR són l’elevat requeriment de sobrepotencial associat a una termodinàmica desfavorable i una baixa eficiència faradaica (FE) per a un producte concret. Per tant, buscar un electrocatalitzador d’alta eficiència i econòmic és raonable i necessari per a aplicacions pràctiques. En les darreres dècades, els marcs metal·lorgànics (MOF) van absorbir les enormes consideracions en el camp de l’electrocatàlisi a causa de la seva gran superfície específica, una rica estructura de porus i llocs actius uniformement dispersos. Tot i que tenen un gran potencial en electrocàlisi, la majoria dels materials MOF encara pateixen una activitat insuficient, baixa conductivitat i poca estabilitat, cosa que dificultaria les seves aplicacions pràctiques. Especialment, en el camp del CO2 RR, s’han de tenir en compte molts paràmetres importants, inclosa una alta eficiència faradaica (FE), l’excessiu baix sobrepotencial, una gran densitat de corrent i una estabilitat robusta, entre d’altres. Per tant, el disseny racional dels MOF per complir els requisits anteriors tant com sigui possible és crucial per explotar el seu futur en aplicacions de CO2 RR. Per tant, en aquesta dissertació, vam fer molts esforços per desenvolupar catalitzadors basats en MOFs/derivats amb una eficiència, activitat i estabilitat superiors per augmentar el rendiment del CO2 RR. Aquesta dissertació es divideix en 5 capítols: El capítol 1 presenta les idees sobre els conceptes fonamentals sobre CO2 RR electroquímic, que inclou la cèl·lula fonamental de CO2 RR electroquímica, que revisa els productes de reducció comuns i les seves vies senzilles. En aquest capítol també es presenta la visió general de paràmetres importants que afecten el CO2 RR, inclosos diferents catalitzadors dels darrers anys i electròlits, i les mètriques rellevants que avaluen els electrocatalitzadors, així com les limitacions de la reducció electroquímica de CO2. El capítol 2 tracta de la fabricació de ZIF-8 modificat a la superfície com a elèctrode basat en MOFs per a un CO2 RR electroquímic per generar CO. En aquest treball, hem modificat la superfície del MOF ZIF-8 a partir d’introduir un petita proporció d’àcid 2,5-dihidroxyterephthalic (DOBDC), aconseguint una densitat de corrent de CO 2,5 vegades superior i una eficiència faradaica augmentada. Al capítol 3, s’utilitza una ruta fàcil per introduir grups que contenen O enllaçats axialment en un catalitzador Fe-N-C mitjançant piròlisi de marcs orgànics metàl·lics basats en Zn dopats amb Fe (IRMOF-3), formant àtoms individuals de Fe molt dispersos amb llocs actius de HO-FeN4. A causa de la modulació de l’entorn local induïda per aquests grups -OH, el catalitzador D-Fe-N-C presenta una activitat CO2 RR millorada, que inclou una alta selectivitat amb una eficiència faradaica de CO, i una estabilitat robusta , que és superior a la dels llocs FeN4 normals reportats sense grups -OH. Al capítol 4, vam proposar la introducció d’àtoms de Fe en catalitzadors de Ni-N-C per produir catalitzadors amb àtoms individualitzats (Ni/Fe-N-C) de doble metall (bimetàl·lics) de cara al CO2 RR per aconseguir una alta selectivitat i activitat simultàniament. Finally, Chapter 5 summarizes the general conclusions. La combustión excesiva de combustibles fósiles da como resultado la emisión de dióxido de carbono (CO2), que desencadenó crecientes problemas ambientales, como el calentamiento global, el aumento del nivel del mar, el clima extremo y la extinción de especies. Por lo tanto, las tecnologías para la conversión de CO2 en otros productos de valor jugaron un papel vital para eliminar la concentración de CO2 en la atmósfera. En ese sentido, la conversión electroquímica de CO2 alimentado por energía renovable en productos químicos útiles se considera una solución elegante para lograr el ciclo del carbono. Sin embargo, debido a la interioridad de las moléculas de CO2 y la reacción competitiva de evolución de hidrógeno (HER), los principales desafíos en el campo CO2 RR son el alto requerimiento de sobrepotencial que representa la termodinámica desfavorable y la baja eficiencia faradaica (FE) para los productos objetivo. Por lo tanto, la búsqueda de un electrocatalizador económico y de alta eficiencia es sensato y necesario para aplicaciones prácticas. En las últimas décadas, las estructuras organometálicas (MOF) absorbieron las enormes consideraciones en el campo de la electrocatálisis debido a su gran área de superficie específica, rica estructura de poros y sitios activos uniformemente dispersos. Aunque con grandes potenciales en electrocatálisis, la mayoría de los materiales MOF todavía sufren de actividad insuficiente, baja conductividad y poca estabilidad, lo que dificultaría sus aplicaciones prácticas. Especialmente, en el campo de CO2 RR, se deben considerar muchos parámetros importantes, incluida la alta eficiencia faradaica (FE), bajo sobrepotencial, gran densidad de corriente y estabilidad robusta, etc. Por lo tanto, el diseño racional de MOF para cumplir con los requisitos anteriores tanto como sea posible es crucial para explotar sus futuras aplicaciones de CO2 RR. Por lo tanto, en esta disertación, hicimos muchos esfuerzos para desarrollar catalizadores basados en MOFs / derivados de MOF con eficiencia, actividad y estabilidad superiores para aumentar el rendimiento de CO2 RR. Esta disertación se divide en 5 capítulos: El capítulo 1 es la información sobre los conceptos fundamentales sobre la CO2 RR electroquímico, que incluye la celda fundamental de la CO2 RR electroquímico, revisa los productos de reducción comunes y sus vías simples. Mientras tanto, la descripción general de los parámetros importantes que afectan la CO2 RR, incluidos los diferentes catalizadores en los últimos años y el electrolito, y las métricas relevantes que evalúan los electrocatalizadores. El Capítulo 2 trata de la fabricación de ZIF-8 modificado en superficie como electrodo basado en MOF para CO2 RR electroquímico para generar CO. En este trabajo, se preparó un ZIF-8 modificado en superficie mediante la introducción de una proporción muy pequeña de ácido 2,5-dihidroxitereftálico (DOBDC) en ZIF-8, logrando una densidad de corriente de CO mayor. En el Capítulo 3, se utiliza una ruta fácil para introducir grupos que contienen O con enlaces axiales en un catalizador de Fe-N-C a través de la pirólisis de estructuras orgánicas metálicas a base de Zn dopado con Fe (IRMOF-3), formando átomos únicos de Fe altamente dispersos con sitios activos HO-FeN4. Debido a la modulación del ambiente local inducida por tales grupos -OH, el catalizador D-Fe-N-C exhibe una actividad CO2 RR mejorada, incluida una alta selectividad con alta eficiencia Faradaica de CO y una estabilidad sólida. En el capítulo 4, proponemos que la introducción de átomos de Fe en catalizadores de Ni-N-C fabrica catalizadores de un solo átomo de metal doble (Ni/Fe-N-C) hacia CO2 RR para lograr una alta selectividad y actividad simultáneamente. El catalizador de doble metal optimizado mostró excelentes rendimientos, obteniendo una alta selectividad con eficiencia faradaica CO a un bajo sobrepotencial, superior a las contrapartes de un solo metal. Finalmente, el Capítulo 5 resume las conclusiones generales. The excessive combustion of fossil fuels results in the emission of carbon dioxide (CO2), which triggers increasing environmental problems, such as, global warming, rising sea levels, extreme weather, and species extinction. Therefore, the technologies for conversion of CO2 into other value products plays a vital role in order to eliminate the CO2 concentration in atmosphere. Thereinto, electrochemical conversion of CO2 powered by renewable energy to useful chemicals is considered as an elegant solution to achieve the carbon cycle. However, due to the innerness of CO2 molecules and competitive hydrogen evolution reaction (HER), the main challenges in the field CO2 RR are the high overpotential requirement that represents the unfavourable thermodynamics and low Faradaic efficiency (FE) for the target products. Therefore, searching for a high-efficient and cost-friendly electrocatalyst is sensible and necessary for practical applications. In the past decades, metal-organic frameworks (MOFs) engrossed the enormous considerations in the field of electrocatalysis because of their large specific surface area, rich pore structure, and uniformly dispersed active sites. Although they have a great potential in electrocatalysis, most MOFs materials still suffer from insufficient activity, low conductivity, and poor stability, which would hinder their practical applications. Especially, in the field of CO2 RR, many important parameters, including high FE, low overpotential, large current density and robust stability among others, should be considered. Thus, the rational design of MOFs to fulfil the above requirements as much as possible is crucial for exploiting their future in CO2 RR applications. Therefore, in this dissertation, we made many efforts to develop MOFs-based/derived catalysts with superior efficiency, activity, and stability for boosting the CO2 RR performance. This dissertation is divided into 5 chapters: Chapter 1 is the insights on the fundamental concepts about electrochemical CO2 RR, which includes the fundamental cell of electrochemical CO2 RR, reviews the common reduction products and their simple pathways. Meanwhile, the overview of important parameters affecting CO2 RR, including different catalysts over the past years, electrolyte, and the relevant metrics evaluating the electrocatalysts as well as limitations of electrochemical CO2 reduction are also presented in this chapter. In addition, this chapter summarizes the fundamental concepts about MOFs materials and their high-temperature pyrolysis derived materials as the electrocatalysts. Chapter 2 deals with the fabrication of surface modified ZIF-8 as MOFs-based electrode for electrochemical CO2 RR to generate CO. In this work, a surface modified ZIF-8 has been prepared through introducing a very small proportion 2,5-dihidroxyterephthalic acid (DOBDC) into ZIF-8, achieving a higher current density of CO and a boosted Faradaic efficiency. In Chapter 3, a facile route is used to introduce axial bonded O-containing groups into a Fe-N-C catalyst through pyrolysis of Fe-doped Zn-based metal organic frameworks (IRMOF-3), forming highly dispersed Fe single atoms with HO-FeN4 active sites. Due to the local environment modulation induced by such -OH groups, the D-Fe-N-C catalyst exhibits an enhanced CO2 RR activity, including a high selectivity with CO Faradaic efficiency, and a robust stability, which is higher than that of the reported normal FeN4 sites without -OH groups. In Chapter 4, we proposed that introducing Fe atoms into Ni-N-C catalysts fabricates double metal (bimetallic) single-atom catalysts (Ni/Fe-N-C) towards CO2 RR to achieve a high selectivity and activity simultaneously. The optimized double-metal Ni/Fe-N-C catalyst showed an excellent performance, obtaining a high selectivity with a high CO Faradaic efficiency at a low overpotential. The performance obtained is superior to both single metal counterparts and other state-of-the-art M-N-C catalysts, proving that regulating single active sites with a second metal site potentially breaks the single metal-based activity benchmark to obtain the high selectivity and activity in CO2 RR, simultaneously. Finally, Chapter 5 summarizes the general conclusions. Universitat Autònoma de Barcelona. Programa de Doctorat en Ciència de Materials

La combustió excessiva de combustibles fòssils té com a resultat l'emissió de diòxid de carboni (CO2), que està desencadenant problemes ambientals creixents, com ara l'escalfament global, l'augment del nivell del mar, el clima extrem i l'extinció d'espècies. Per tant, les tecnologies per a la conversió de CO2 en altres productes de valor estan jugant un paper vital per eliminar la concentració de CO2 a l'atmosfera. En aquest sentit, la conversió electroquímica de CO2, alimentat per energia renovable, en productes químics útils es considera una solució elegant per aconseguir el cicle del carboni. Tanmateix, a causa de la interioritat de les molècules de CO2 i de la reacció competitiva d'evolució d'hidrogen (HER), els principals reptes de CO2 RR són l'elevat requeriment de sobrepotencial associat a una termodinàmica desfavorable i una baixa eficiència faradaica (FE) per a un producte concret. Per tant, buscar un electrocatalitzador d'alta eficiència i econòmic és raonable i necessari per a aplicacions pràctiques. En les darreres dècades, els marcs metal·lorgànics (MOF) van absorbir les enormes consideracions en el camp de l'electrocatàlisi a causa de la seva gran superfície específica, una rica estructura de porus i llocs actius uniformement dispersos. Tot i que tenen un gran potencial en electrocàlisi, la majoria dels materials MOF encara pateixen una activitat insuficient, baixa conductivitat i poca estabilitat, cosa que dificultaria les seves aplicacions pràctiques. Especialment, en el camp del CO2 RR, s'han de tenir en compte molts paràmetres importants, inclosa una alta eficiència faradaica (FE), l'excessiu baix sobrepotencial, una gran densitat de corrent i una estabilitat robusta, entre d'altres. Per tant, el disseny racional dels MOF per complir els requisits anteriors tant com sigui possible és crucial per explotar el seu futur en aplicacions de CO2 RR. Per tant, en aquesta dissertació, vam fer molts esforços per desenvolupar catalitzadors basats en MOFs/derivats amb una eficiència, activitat i estabilitat superiors per augmentar el rendiment del CO2 RR. Aquesta dissertació es divideix en 5 capítols: El capítol 1 presenta les idees sobre els conceptes fonamentals sobre CO2 RR electroquímic, que inclou la cèl·lula fonamental de CO2 RR electroquímica, que revisa els productes de reducció comuns i les seves vies senzilles. En aquest capítol també es presenta la visió general de paràmetres importants que afecten el CO2 RR, inclosos diferents catalitzadors dels darrers anys i electròlits, i les mètriques rellevants que avaluen els electrocatalitzadors, així com les limitacions de la reducció electroquímica de CO2. El capítol 2 tracta de la fabricació de ZIF-8 modificat a la superfície com a elèctrode basat en MOFs per a un CO2 RR electroquímic per generar CO. En aquest treball, hem modificat la superfície del MOF ZIF-8 a partir d'introduir un petita proporció d'àcid 2,5-dihidroxyterephthalic (DOBDC), aconseguint una densitat de corrent de CO 2,5 vegades superior i una eficiència faradaica augmentada. Al capítol 3, s'utilitza una ruta fàcil per introduir grups que contenen O enllaçats axialment en un catalitzador Fe-N-C mitjançant piròlisi de marcs orgànics metàl·lics basats en Zn dopats amb Fe (IRMOF-3), formant àtoms individuals de Fe molt dispersos amb llocs actius de HO-FeN4. A causa de la modulació de l'entorn local induïda per aquests grups -OH, el catalitzador D-Fe-N-C presenta una activitat CO2 RR millorada, que inclou una alta selectivitat amb una eficiència faradaica de CO, i una estabilitat robusta , que és superior a la dels llocs FeN4 normals reportats sense grups -OH. Al capítol 4, vam proposar la introducció d'àtoms de Fe en catalitzadors de Ni-N-C per produir catalitzadors amb àtoms individualitzats (Ni/Fe-N-C) de doble metall (bimetàl·lics) de cara al CO2 RR per aconseguir una alta selectivitat i activitat simultàniament. Finally, Chapter 5 summarizes the general conclusions. La combustión excesiva de combustibles fósiles da como resultado la emisión de dióxido de carbono (CO2), que desencadenó crecientes problemas ambientales, como el calentamiento global, el aumento del nivel del mar, el clima extremo y la extinción de especies. Por lo tanto, las tecnologías para la conversión de CO2 en otros productos de valor jugaron un papel vital para eliminar la concentración de CO2 en la atmósfera. En ese sentido, la conversión electroquímica de CO2 alimentado por energía renovable en productos químicos útiles se considera una solución elegante para lograr el ciclo del carbono. Sin embargo, debido a la interioridad de las moléculas de CO2 y la reacción competitiva de evolución de hidrógeno (HER), los principales desafíos en el campo CO2 RR son el alto requerimiento de sobrepotencial que representa la termodinámica desfavorable y la baja eficiencia faradaica (FE) para los productos objetivo. Por lo tanto, la búsqueda de un electrocatalizador económico y de alta eficiencia es sensato y necesario para aplicaciones prácticas. En las últimas décadas, las estructuras organometálicas (MOF) absorbieron las enormes consideraciones en el campo de la electrocatálisis debido a su gran área de superficie específica, rica estructura de poros y sitios activos uniformemente dispersos. Aunque con grandes potenciales en electrocatálisis, la mayoría de los materiales MOF todavía sufren de actividad insuficiente, baja conductividad y poca estabilidad, lo que dificultaría sus aplicaciones prácticas. Especialmente, en el campo de CO2 RR, se deben considerar muchos parámetros importantes, incluida la alta eficiencia faradaica (FE), bajo sobrepotencial, gran densidad de corriente y estabilidad robusta, etc. Por lo tanto, el diseño racional de MOF para cumplir con los requisitos anteriores tanto como sea posible es crucial para explotar sus futuras aplicaciones de CO2 RR. Por lo tanto, en esta disertación, hicimos muchos esfuerzos para desarrollar catalizadores basados en MOFs / derivados de MOF con eficiencia, actividad y estabilidad superiores para aumentar el rendimiento de CO2 RR. Esta disertación se divide en 5 capítulos: El capítulo 1 es la información sobre los conceptos fundamentales sobre la CO2 RR electroquímico, que incluye la celda fundamental de la CO2 RR electroquímico, revisa los productos de reducción comunes y sus vías simples. Mientras tanto, la descripción general de los parámetros importantes que afectan la CO2 RR, incluidos los diferentes catalizadores en los últimos años y el electrolito, y las métricas relevantes que evalúan los electrocatalizadores. El Capítulo 2 trata de la fabricación de ZIF-8 modificado en superficie como electrodo basado en MOF para CO2 RR electroquímico para generar CO. En este trabajo, se preparó un ZIF-8 modificado en superficie mediante la introducción de una proporción muy pequeña de ácido 2,5-dihidroxitereftálico (DOBDC) en ZIF-8, logrando una densidad de corriente de CO mayor. En el Capítulo 3, se utiliza una ruta fácil para introducir grupos que contienen O con enlaces axiales en un catalizador de Fe-N-C a través de la pirólisis de estructuras orgánicas metálicas a base de Zn dopado con Fe (IRMOF-3), formando átomos únicos de Fe altamente dispersos con sitios activos HO-FeN4. Debido a la modulación del ambiente local inducida por tales grupos -OH, el catalizador D-Fe-N-C exhibe una actividad CO2 RR mejorada, incluida una alta selectividad con alta eficiencia Faradaica de CO y una estabilidad sólida. En el capítulo 4, proponemos que la introducción de átomos de Fe en catalizadores de Ni-N-C fabrica catalizadores de un solo átomo de metal doble (Ni/Fe-N-C) hacia CO2 RR para lograr una alta selectividad y actividad simultáneamente. El catalizador de doble metal optimizado mostró excelentes rendimientos, obteniendo una alta selectividad con eficiencia faradaica CO a un bajo sobrepotencial, superior a las contrapartes de un solo metal. Finalmente, el Capítulo 5 resume las conclusiones generales. The excessive combustion of fossil fuels results in the emission of carbon dioxide (CO2), which triggers increasing environmental problems, such as, global warming, rising sea levels, extreme weather, and species extinction. Therefore, the technologies for conversion of CO2 into other value products plays a vital role in order to eliminate the CO2 concentration in atmosphere. Thereinto, electrochemical conversion of CO2 powered by renewable energy to useful chemicals is considered as an elegant solution to achieve the carbon cycle. However, due to the innerness of CO2 molecules and competitive hydrogen evolution reaction (HER), the main challenges in the field CO2 RR are the high overpotential requirement that represents the unfavourable thermodynamics and low Faradaic efficiency (FE) for the target products. Therefore, searching for a high-efficient and cost-friendly electrocatalyst is sensible and necessary for practical applications. In the past decades, metal-organic frameworks (MOFs) engrossed the enormous considerations in the field of electrocatalysis because of their large specific surface area, rich pore structure, and uniformly dispersed active sites. Although they have a great potential in electrocatalysis, most MOFs materials still suffer from insufficient activity, low conductivity, and poor stability, which would hinder their practical applications. Especially, in the field of CO2 RR, many important parameters, including high FE, low overpotential, large current density and robust stability among others, should be considered. Thus, the rational design of MOFs to fulfil the above requirements as much as possible is crucial for exploiting their future in CO2 RR applications. Therefore, in this dissertation, we made many efforts to develop MOFs-based/derived catalysts with superior efficiency, activity, and stability for boosting the CO2 RR performance. This dissertation is divided into 5 chapters: Chapter 1 is the insights on the fundamental concepts about electrochemical CO2 RR, which includes the fundamental cell of electrochemical CO2 RR, reviews the common reduction products and their simple pathways. Meanwhile, the overview of important parameters affecting CO2 RR, including different catalysts over the past years, electrolyte, and the relevant metrics evaluating the electrocatalysts as well as limitations of electrochemical CO2 reduction are also presented in this chapter. In addition, this chapter summarizes the fundamental concepts about MOFs materials and their high-temperature pyrolysis derived materials as the electrocatalysts. Chapter 2 deals with the fabrication of surface modified ZIF-8 as MOFs-based electrode for electrochemical CO2 RR to generate CO. In this work, a surface modified ZIF-8 has been prepared through introducing a very small proportion 2,5-dihidroxyterephthalic acid (DOBDC) into ZIF-8, achieving a higher current density of CO and a boosted Faradaic efficiency. In Chapter 3, a facile route is used to introduce axial bonded O-containing groups into a Fe-N-C catalyst through pyrolysis of Fe-doped Zn-based metal organic frameworks (IRMOF-3), forming highly dispersed Fe single atoms with HO-FeN4 active sites. Due to the local environment modulation induced by such -OH groups, the D-Fe-N-C catalyst exhibits an enhanced CO2 RR activity, including a high selectivity with CO Faradaic efficiency, and a robust stability, which is higher than that of the reported normal FeN4 sites without -OH groups. In Chapter 4, we proposed that introducing Fe atoms into Ni-N-C catalysts fabricates double metal (bimetallic) single-atom catalysts (Ni/Fe-N-C) towards CO2 RR to achieve a high selectivity and activity simultaneously. The optimized double-metal Ni/Fe-N-C catalyst showed an excellent performance, obtaining a high selectivity with a high CO Faradaic efficiency at a low overpotential. The performance obtained is superior to both single metal counterparts and other state-of-the-art M-N-C catalysts, proving that regulating single active sites with a second metal site potentially breaks the single metal-based activity benchmark to obtain the high selectivity and activity in CO2 RR, simultaneously. Finally, Chapter 5 summarizes the general conclusions.

[eng] The transformation of the current energy model towards a more sustainable mix, independent of fossil fuels, requires the exploration of new technologies that are capable of taking advantage of excess electricity derived from renewable energy sources and to use new alternative sources of carbon for the generation of clean fuels. An alternative that combines both is the Power-to-Gas (P2G) technology, whose concept is based on a two-stage process. In the first stage, excess electricity from renewable energies is converted to hydrogen by electrolysis. Then, in a second stage, the H2 produced is transformed to CH4 through methanation with CO2. The CH4 produced is referred to as synthetic natural gas (SNG) and allows large amounts of renewable energy to be distributed from the energy sector to the end-use sectors. The thermo-chemical CO2 methanation process is considered the most efficient route for large-scale SNG production. However, developing a cost-effective CO2 methanation technology is one of the biggest challenges facing the P2G concept. In this context, this thesis focused on the catalyst and reactor design for CO2 methanation. The thesis objectives were addressed in three main aspects, which are: i) design a high-performance catalyst based on metal oxide-promoted Ni/γ-Al2O3 and determine its reaction mechanism; ii) evaluate the stability of the catalyst and the tolerance to sulfur for its implementation in a relevant industrial environment (CoSin project); and finally, iii) develop a CFD model based on experimental kinetic data to understand the role of operating conditions and propose a new reactor configuration. In the first Chapter of this thesis it is presented a general introduction of the SNG production through CO2 methanation process. In the second Chapter, the addition of a promoter (X) on a system composed by Ni and γ-Al2O3 microspheres was studied as the design strategy to develop a micro-sized Ni-X/γ-Al2O3 catalyst. The catalysts based on Ni-CeO2/γ-Al2O3 was proposed as the most feasible due to its high catalytic performance in relation to its economic competitiveness. The optimal composition of each component of the Ni-CeO2/γ-Al2O3 was found through a systematic experimental design. The catalyst composed by 25wt.%Ni, 20wt.%CeO2 and 55wt.%γ-Al2O3 proved to be the most active and stable thanks to its enhanced Ni dispersion and reduction, its high metallic area, and the formation of moderate base sites. In Chapter three, the thermal stability and tolerance to sulfur impurities on the Ni-CeO2/γ-Al2O3 catalyst was further studied using high temperatures and the presence of H2S on the reactants. The strong metal-promoter interaction and the favourable formation of Ce2O2S were revealed as the main causes of its high stability and tolerance to H2S, respectively. Additionally, the implementation of Ni-CeO2/γ-Al2O3 in a two-stage industrial methanation process was performed to evaluate its technical feasibility. The desired gas composition (≥92.5%CH4) was successful obtained using a decreasing temperature profile (T=450-275°C) and P=5bar·g. The high stability recorded during the 2000h of experimentation demonstrated that Ni-CeO2/γ-Al2O3 can be a competitive catalyst for CO2 methanation. Regarding to reactor design, in Chapter four, the design of a fixed-bed multitubular reactor on a Ni-CeO2-Al2O3 catalyst was evaluated for mid-scale SNG production. A CFD mathematical model based on experimental kinetic data was developed. A reactor tube with a diameter of 9.25mm and a length of 250mm was proposed, which should be operated at Tinlet=473K, Twall=373K, GHSV=14,400h-1 and P=5atm to achieve XCO2=99% with Tmax of 673K. On the other hand, a reactor tube (di=4.6mm and L=250mm) with a heat management approach based on free convection was proposed for small-scale SNG production. The optimal conditions were found at GHSV=11,520h-1, Tinlet=503K, P=5atm, and Tair=298K. The feasibility of the simulated reactor proposal was experimentally validated over the micro-sized Ni-CeO2/γ-Al2O3 (XCO2=93% and T=830-495K)., [spa] Power-to-Gas (P2G) es una tecnología prometedora para el almacenamiento de combustibles bajos en carbono. El concepto P2G implica la conversión de energía renovable en hidrógeno mediante electrólisis con la posibilidad de combinarlo con CO2 para producir metano (gas natural sintético, SNG). La producción de SNG mediante el proceso termoquímico de metanación de CO2 es particularmente interesante porque ofrece un combustible fácilmente transportable con un amplio mercado probado para aplicaciones de uso final de energía, calor y movilidad. Sin embargo, el desarrollo de una tecnología de metanación de CO2 rentable es uno de los mayores desafíos que enfrenta el concepto P2G. En este contexto, esta tesis se centró en el desarrollo de un catalizador y un reactor para la metanación de CO2. Los objetivos de la tesis se abordaron en tres aspectos principales, que son: i) diseñar un catalizador de alto rendimiento basado en Ni/The transformation of the current energy model towards a more sustainable mix, independent of fossil fuels, requires the exploration of new technologies that are capable of taking advantage of excess electricity derived from renewable energy sources and to use new alternative sources of carbon for the generation of clean fuels. An alternative that combines both is the Power-to-Gas (P2G) technology, whose concept is based on a two-stage process. In the first stage, excess electricity from renewable energies is converted to hydrogen by electrolysis. Then, in a second stage, the H2 produced is transformed to CH4 through methanation with CO2. The CH4 produced is referred to as synthetic natural gas (SNG) and allows large amounts of renewable energy to be distributed from the energy sector to the end-use sectors. The thermo-chemical CO2 methanation process is considered the most efficient route for large-scale SNG production. However, developing a cost-effective CO2 methanation technology is one of the biggest challenges facing the P2G concept. In this context, this thesis focused on the catalyst and reactor design for CO2 methanation. The thesis objectives were addressed in three main aspects, which are: i) design a high-performance catalyst based on metal oxide-promoted Ni/γ-Al2O3 and determine its reaction mechanism; ii) evaluate the stability of the catalyst and the tolerance to sulfur for its implementation in a relevant industrial environment (CoSin project); and finally, iii) develop a CFD model based on experimental kinetic data to understand the role of operating conditions and propose a new reactor configuration. In the first Chapter of this thesis it is presented a general introduction of the SNG production through CO2 methanation process. In the second Chapter, the addition of a promoter (X) on a system composed by Ni and γ-Al2O3 microspheres was studied as the design strategy to develop a micro-sized Ni-X/γ-Al2O3 catalyst. The catalysts based on Ni-CeO2/γ-Al2O3 was proposed as the most feasible due to its high catalytic performance in relation to its economic competitiveness. The optimal composition of each component of the Ni-CeO2/γ-Al2O3 was found through a systematic experimental design. The catalyst composed by 25wt.%Ni, 20wt.%CeO2 and 55wt.%γ-Al2O3 proved to be the most active and stable thanks to its enhanced Ni dispersion and reduction, its high metallic area, and the formation of moderate base sites. In Chapter three, the thermal stability and tolerance to sulfur impurities on the Ni-CeO2/γ-Al2O3 catalyst was further studied using high temperatures and the presence of H2S on the reactants. The strong metal-promoter interaction and the favourable formation of Ce2O2S were revealed as the main causes of its high stability and tolerance to H2S, respectively. Additionally, the implementation of Ni-CeO2/γ-Al2O3 in a two-stage industrial methanation process was performed to evaluate its technical feasibility. The desired gas composition (≥92.5%CH4) was successful obtained using a decreasing temperature profile (T=450-275°C) and P=5bar·g. The high stability recorded during the 2000h of experimentation demonstrated that Ni-CeO2/γ-Al2O3 can be a competitive catalyst for CO2 methanation. Regarding to reactor design, in Chapter four, the design of a fixed-bed multitubular reactor on a Ni-CeO2-Al2O3 catalyst was evaluated for mid-scale SNG production. A CFD mathematical model based on experimental kinetic data was developed. A reactor tube with a diameter of 9.25mm and a length of 250mm was proposed, which should be operated at Tinlet=473K, Twall=373K, GHSV=14,400h-1 and P=5atm to achieve XCO2=99% with Tmax of 673K. On the other hand, a reactor tube (di=4.6mm and L=250mm) with a heat management approach based on free convection was proposed for small-scale SNG production. The optimal conditions were found at GHSV=11,520h-1, Tinlet=503K, P=5atm, and Tair=298K. The feasibility of the simulated reactor proposal was experimentally validated over the micro-sized Ni-CeO2/γ-Al2O3 (XCO2=93% and T=830-495K).-Al2O3 promovido por óxido metálico y determinar su mecanismo, ii) evaluar la estabilidad del catalizador y la tolerancia al azufre para su implementación en un entorno industrial relevante (proyecto CoSin), and iii) desarrollar un modelo CFD basado en datos cinéticos experimentales para comprender el papel de las condiciones de operación y proponer una nueva configuración de reactor. En línea con estos objetivos, un catalizador ternario basado en 25wt.%Ni-20wt.%CeO2-55wt.%The transformation of the current energy model towards a more sustainable mix, independent of fossil fuels, requires the exploration of new technologies that are capable of taking advantage of excess electricity derived from renewable energy sources and to use new alternative sources of carbon for the generation of clean fuels. An alternative that combines both is the Power-to-Gas (P2G) technology, whose concept is based on a two-stage process. In the first stage, excess electricity from renewable energies is converted to hydrogen by electrolysis. Then, in a second stage, the H2 produced is transformed to CH4 through methanation with CO2. The CH4 produced is referred to as synthetic natural gas (SNG) and allows large amounts of renewable energy to be distributed from the energy sector to the end-use sectors. The thermo-chemical CO2 methanation process is considered the most efficient route for large-scale SNG production. However, developing a cost-effective CO2 methanation technology is one of the biggest challenges facing the P2G concept. In this context, this thesis focused on the catalyst and reactor design for CO2 methanation. The thesis objectives were addressed in three main aspects, which are: i) design a high-performance catalyst based on metal oxide-promoted Ni/γ-Al2O3 and determine its reaction mechanism; ii) evaluate the stability of the catalyst and the tolerance to sulfur for its implementation in a relevant industrial environment (CoSin project); and finally, iii) develop a CFD model based on experimental kinetic data to understand the role of operating conditions and propose a new reactor configuration. In the first Chapter of this thesis it is presented a general introduction of the SNG production through CO2 methanation process. In the second Chapter, the addition of a promoter (X) on a system composed by Ni and γ-Al2O3 microspheres was studied as the design strategy to develop a micro-sized Ni-X/γ-Al2O3 catalyst. The catalysts based on Ni-CeO2/γ-Al2O3 was proposed as the most feasible due to its high catalytic performance in relation to its economic competitiveness. The optimal composition of each component of the Ni-CeO2/γ-Al2O3 was found through a systematic experimental design. The catalyst composed by 25wt.%Ni, 20wt.%CeO2 and 55wt.%γ-Al2O3 proved to be the most active and stable thanks to its enhanced Ni dispersion and reduction, its high metallic area, and the formation of moderate base sites. In Chapter three, the thermal stability and tolerance to sulfur impurities on the Ni-CeO2/γ-Al2O3 catalyst was further studied using high temperatures and the presence of H2S on the reactants. The strong metal-promoter interaction and the favourable formation of Ce2O2S were revealed as the main causes of its high stability and tolerance to H2S, respectively. Additionally, the implementation of Ni-CeO2/γ-Al2O3 in a two-stage industrial methanation process was performed to evaluate its technical feasibility. The desired gas composition (≥92.5%CH4) was successful obtained using a decreasing temperature profile (T=450-275°C) and P=5bar·g. The high stability recorded during the 2000h of experimentation demonstrated that Ni-CeO2/γ-Al2O3 can be a competitive catalyst for CO2 methanation. Regarding to reactor design, in Chapter four, the design of a fixed-bed multitubular reactor on a Ni-CeO2-Al2O3 catalyst was evaluated for mid-scale SNG production. A CFD mathematical model based on experimental kinetic data was developed. A reactor tube with a diameter of 9.25mm and a length of 250mm was proposed, which should be operated at Tinlet=473K, Twall=373K, GHSV=14,400h-1 and P=5atm to achieve XCO2=99% with Tmax of 673K. On the other hand, a reactor tube (di=4.6mm and L=250mm) with a heat management approach based on free convection was proposed for small-scale SNG production. The optimal conditions were found at GHSV=11,520h-1, Tinlet=503K, P=5atm, and Tair=298K. The feasibility of the simulated reactor proposal was experimentally validated over the micro-sized Ni-CeO2/γ-Al2O3 (XCO2=93% and T=830-495K).-Al2O3 se propone como el más factible debido a su alto rendimiento catalítico en relación a su competitividad económica. La fuerte interacción metal-promotor y la formación favorable de Ce2O2S se revelaron como las principales causas de su alta estabilidad y tolerancia al H2S, respectivamente. Adicionalmente, su exitosa implementación en un proceso de metanación industrial de dos etapas demostró su viabilidad técnica. Finalmente, se propone un reactor multitubular para la producción de SNG a mediana escala. Por otro lado, para la producción de SNG a pequeña escala, se propone un nuevo diseño de reactor con un enfoque de gestión del calor basado en la libre convención.

Treballs Finals del Màster d’Energies Renovables i Sostenibilitat Energètica, Facultat de Física, Universitat de Barcelona, Curs: 2020-2021, Tutors: María Isabel Díez García, Juan Ramón Morante Lleonart, Climate change and environmental degradation have forced humanity to intensify efforts to enhance the production of clean energy without emits greenhouse gas emissions, and one of the fundamental pillars for achieving this objective is the generation of green hydrogen by water electrolysis. Water electrolysis requires catalysts to efficiently separate of its hydrogen and oxygen components in gas phase. Currently, the best catalysts for the hydrogen evolution reaction are the noble metals of the platinum group, scarce and high‐cost elements, which complicates the development of this technology and raises its costs. As a result, the interest in the research of electrodes and catalysts for the hydrogen and oxygen evolution reaction has increased significantly, intending to improve their performance and are composed of materials that are abundant in the earth's crust, at a reasonable cost. The research aims to conduct a study of iron phosphide (FeP) electrodes for the hydrogen evolution reaction in a carbon felt substrate, through the formation of FeOOH by electrodeposition (FeOOH / CF) and subsequent gas phase phosphidation for the final obtaining of the FeP electrode (FeP / CF). These elements have been chosen for their abundance in the earth's crust, affordable costs, high current densities at low overpotentials, faster kinetics, high stability under strong acid conditions, and because there are simple processes for its synthesis.

[eng] Storing the fluctuating renewable energy into synthetic fuels or in batteries is meaningful due to the emerging energy crisis. In this thesis, four nanostructured catalysts based on two kinds of metal sulfides, namely Cu2S and SnS2, were produced and optimized to improve their performance towards three key electrochemical energy conversion processes, namely electrochemical oxygen evolution, photoelectrochemical water splitting and lithium-ion batteries. Chapter 1 presented a general introduction to explain the motivation of the thesis topic. In chapter 2, a metallic copper substrate was used as current collector and chemical template to produce Cu2S nanorod arrays for electrochemical oxygen evolution reaction (OER). Suitable characterization tools were applied to investigate the chemical, structural and morphological transformation in OER operation, during which the initial Cu2S nanorod arrays would perform as a “pre-catalyst” that in-situ changed to CuO nanowires. Notably, the Cu2S-derived CuO showed significant improved OER performance compared with that of CuO prepared by directly annealing a Cu(OH)2 precursor, in terms of both activity and stability. Thus obtained electrocatalyst can be ranked among the best Cu-based OER catalysts reported so far. To take advantage of the unlimited solar energy, an ultrathin SnS2 NPL with a suitable band gap around 2.2 eV was produced via a hot-injection solution-based process in chapter 3. The unsatisfied photoelectrochemical (PEC) performance of bare SnS2 motivated me to deposit Pt NPs on its surface as cocatalyst via in-situ reduction of a Pt salt. The resulting SnS2-Pt heterostructures with optimal Pt amount showed significant improvement (six fold) towards PEC water oxidation. Mott-Schottky analysis and PEC impedance spectroscopy (PEIS) were used to analyze in more detail the effect of Pt on the PEC performance. The optimal SnS2-Pt heterostructure presented acceptable performance towards PEC water splitting. However, it still suffered from a moderate stability due to the peel-off of the catalyst layer from the FTO surface. To solve this problem, in chapter 4 we detailed a simple, versatile and scalable amine/thiol- based molecular ink to grow nanostructured SnS2 layers directly on conductive substrates such as FTO, stainless steel and carbon cloth. Such layers on FTO were characterized by excellent photocurrent densities. The same strategy was used to produce SnS2-graphene composites, SnS2-xSex ternary coatings and even phase pure SnSe2 layers. Finally, the potential of this precursor ink to produce gram scale amounts of unsupported SnS2 was also investigated. Apart from the application as a photocatalyst, SnS2 can also be a promising anode material for Li-ion batteries (LIB). In chapter 5, nanostructured SnS2 with different morphologies produced in chapter 3 were tested as LIB anodes firstly to find that thin SnS2 NPLs provided the highest performance. Thereafter, a colloidal synthesis strategy to grow the same SnS2 NPLs within a matrix of porous g-C3N4 (CN) and graphite plates (GPs) was developed and the obtained materials were tested for LIB application. Such hierarchical SnS2/CN/GP composites using SnS2-NPL as active materials, porous CN to provide avenues for electrolyte diffusion and ease the volumetric expansion of SnS2, and GP as “highways” for charge transport displayed excellent rate capabilities (536.5 mAh g-1 at 2.0 A g-1) and an outstanding stability (~99.7 % retention after 400 cycles), which were partially associated with a high pseudocapacitance contribution (88.8 % at 1.0 mV s-1). The excellent electrochemical properties of these nanocomposites were ascribed to the synergy created between the three components. Overall, four nanostructured catalysts based on Cu2S and SnS2 were prepared, and proper optimizations/treatments were defined to improve their catalytic performance. The results shown in this thesis demonstrate the promising application of non-toxic, low cost metal sulfides in electrochemical energy conversion technologies., [spa] En esta tesis, se produjeron y optimizaron cuatro catalizadores nanoestructurados basados en Cu2S y SnS2 para mejorar su rendimiento hacia la conversión de energía electroquímica. El Capítulo 1 presentó una introducción general para explicar la motivación del tema de tesis. En el capítulo 2, las matrices de las nanovarillas de Cu2S se sintetizaron in situ sobre un sustrato de cobre metálico para la reacción electroquímica de evolución de oxígeno (OER). Se aplicaron herramientas de caracterización adecuadas para investigar la transformación en la operación OER, durante la cual las matrices iniciales de las nanovarillas Cu2S in situ cambió a nanohilos de CuO. En particular, el CuO derivado de Cu2S mostró un rendimiento de OER significativamente mejor cuando comparado al de CuO preparado mediante el recocido. En el capítulo 3, se detalló un proceso basado en una solución de inyección en caliente para producir nanoplacas ultrafinas SnS2 (NPL). Posteriormente, se cultivóPt en su superficie mediante la reducción in situ de una sal de Pt. Posteriormente se probó el rendimiento fotoelectroquímico (PEC) de los fotoanodes hacia la oxidación del agua. Los fotoanodes de SnS2-Pt optimizados proporcionaron densidades de fotocorriente significativamente más altas que el SnS2 desnudo (seis veces). Se analizó el efecto de Pt. En el capítulo 4, se informó una tinta molecular simple para cultivar capas de SnS2 nanoestructuradas directamente sobre sustratos conductores. Tales capas nanoestructuradas en FTO se caracterizaron por excelentes densidades de fotocorriente. Se utilize la misma estrategia para producir compuestos de grafeno-SnS2, recubrimientos ternarios SnS2-xSex, capas de SnSe2 de fase pura e incluso polvo de SnS2 a gran escala. En el capítulo 5, el SnS2 nanoestructurado con diferentes morfologías se probaron como ánodos LIB en primer lugar para encontrar que los NPL de SnS2 delgados proporcionaban el mayor rendimiento. Posteriormente, se desarrolló una estrategia de síntesis coloidal para cultivar los mismos NPL de SnS2 dentro de una matriz de g-C3N4 (CN) poroso y placas de grafito (GP) y se probaron para la aplicación LIB. Tales compuestos jerárquicos SnS2/CN/GP mostraron excelentes propiedades electroquímicas, lo que se atribuye a la sinergia creada entre los tres componentes como se investigó.

[eng] Along this Thesis dissertation book, which is focused on the topic of Redox Flow Batteries, many efforts have been done in order to improve different aspects of the all-Vanadium Redox Flow batteries (VRFBs) technology, as monitoring each battery compartment, increasing operational temperature range, enhancing negative electrode to reduce side reactions and charge transfer towards V3+/V2+ redox reaction and also modifying positive electrode to obtain a faster VO2 /VO redox reaction. Vanadium technology was chosen over all redox flow technologies due to its mature development reaching the barrier to commercial breakthrough. The main targeted aspects about VRFBs have been: • Reference electrode implementation into a single cell device (battery) to study separately anolyte and catholyte in real working conditions. This set- up allows following the contribution of each one of the electrodes separately, and consequently knowing the limiting factor in the battery, in order to improve them. • Electrolyte modifications with catalytic quantities of an additive allowing a larger vanadium ion concentration being able to be solved into the electrolyte, as well as increase the operational temperature window. These improvements are done in order to increase the energy density of the system, and also allow the battery to work in a wider temperature range to adapt this system to broader climate areas without temperature control. • Electrodes enhancement: Initially, we will focus our attention into electrode modifications to enhance their electrochemical properties. Firstly, increasing functional groups on the electrode’s surface, which make them more electroactive towards vanadium redox reactions. Secondly, different catalysts are deposited to obtain faster vanadium redox reactions on a carbon-based structure (as graphite felt or carbon felt). o Anode, main lacks are large ohmic overpotential due to competing side reaction. - It has been done an exhaust study of different structures of the same material, as it is nanoparticles (NPs), single- nanorods (SNRs) and multi-nanorods assembly (MNRs). - Graphite and/or carbon felt enhancement using the synthesis of different catalyst (TiO2, TiO2:H, TiO2:iN, O and N groups) which help not only to make the negative redox reaction (V3+/V2+) faster and reduce the voltage drop, but also avoid side reactions (gas evolution) as can be hydrogen evolution reaction (HER). All of these help to elevate the reachable energy and power densities of the battery. o Cathode, which lacks of a fast kinetics. - Deposition of a catalyst over graphite felt, as it is ceria (CeO2), to aid the positive redox reaction (VO2 /VO ) making it faster, as well as improve the efficiencies and accessible capacity of the battery. Despite the fact that the systems described previously were already proposed several decades ago, they are still the subject of current research. These systems show several inconvenient related to the vanadium abundance, the cost of it, as well as the geopolitical impact caused by its mining. As a consequence, the implementation of organic redox active species is a first step in order to avoid these disadvantages. Organic molecules are abundant, tunable by synthetic pathways and an improved kinetic with the possibility of having two-electron transfer process (as happens to quinones)1,2. Such a battery has the potential to meet the demanding cost, durability, eco-friendliness, and sustainability requirements for grid-scale electrical energy storage. Furthermore, this system has been studied targeting on a Quinone-based redox flow battery. After the selection of the catholyte (benzoquinone-based molecules as p-benzoquinone, o-benzoquinone and disodium 4,5-dihydroxy-1,3- benzenedisulfonate) and anolyte (Anthraquinone-2,7-disulfonic acid disodium salt) in a methanosulfonic acid solvent, the next step has been improving the electrode technology on both single cell compartments to promote positive and negative redox reactions. In order to do that surface modification has been done, by means of nitrogen and oxygen functionalization using different methods. Finally, it has been commented the present of all-Vanadium and Aqueous Organic Redox Flow Batteries, as well as some future perspective of both technologies., [spa] A lo largo de esta tesis doctoral, centrada en el tema de baterías de flujo, se ha puesto mucho empeño en la mejora y desarrollo de diferentes aspectos de la tecnología referente a baterías de flujo de vanadio (VRFBs). En este aspecto, se ha realizado el seguimiento de cada compartimento de la batería por separado durante el ciclado de la batería, el aumento del rango de temperatura de trabajo de la batería, la mejora del electrodo negativo mediante la inhibición de reacciones parasitas y reducción de los valores de transferencia de carga para la reacción redox relativa a V3+/V2+, al igual que la modificación del electrodo positivo para obtener un aumento de la velocidad de reacción del par redox VO2+/VO2+. Se eligió la tecnología referente a vanadio respecto a otras tecnologías de baterías de flujo como consecuencia de su madurez en cuanto a desarrollo, rozando la barrera de la expansión comercial a gran escala. Los principales aspectos tratados sobre las baterías de flujo de vanadio han sido: • Implementación de un electrodo de referencia no-comercial dentro de una batería con el objetivo de estudiar por separado el anolito y el catolito a lo largo del ciclado de la batería. Esta configuración permite seguir la contribución de cada uno de los electrodos a la reacción por separado, y en consecuencia saber cuál es el factor limitante dentro de la batería con el objetivo de mejorarlo. • Mejoras del electrolito mediante aditivos en cantidades catalíticas con el objetivo de permitir una mayor cantidad de iones vanadio capaz de ser disueltos y también aumentar el rango de temperatura donde la batería es capaz de trabajar. Estas mejoras se han realizado para aumentar la densidad de energía del sistema, y también ampliar la temperatura operacional de la batería con el fin de poder trabajar en un mayor número de zonas climáticas. • Mejoras de electrodo: Inicialmente la atención se centra en la modificación de electrodos para mejorar sus propiedades electroquímicas. Primero aumentando el número de grupos funcionales en la superficie del electrodo, lo que los hace estos electrodos más electro activos hacia las reacciones redox de vanadio. Seguidamente también se realiza el depósito de diferentes catalizadores para obtener reacciones redox de vanadio más rápidas sobre un substrato carbonoso, como pueden ser los fieltros de carbono o grafito. o Ánodo, cuya principal deficiencia es el elevado sobrepotencial óhmico como consecuencia de las reacciones parásitas. - Se ha realizado el estudio de diferentes estructuras del mismo material, como es el óxido de titanio en su fase rutilo, como nanopartículas (NPs), nanorods simples (SNRs) y ensamblaje de nanorods multiples (MNRs). - Mejora de sustratos de fieltro de grafito y de carbono mediante diferentes catalizadores (TiO2, TiO2:H, TiO2:N), además de mediante grupos funcionales (grupos O y N), lo cuales no solo facilitan la mejora de la reacción redox V3+/V2+ aumentando la velocidad de dicha reacción y reduce la caída de voltaje de la batería, sino que es capaz de inhibir las reacciones parásitas de evolución de gases, como puede ser la reacción de evolución de hidrógeno (HER). Todo esto ayuda a aumentar la densidad de energía y la densidad de potencia de la batería. o Cátodo, cuya principal deficiencia es la baja velocidad de reacción comparativamente con la reacción negativa. - Depósito de un catalizador, como es la Ceria (CeO2), sobre un fieltro de grafito, con el objetivo de mejorar la reacción redox positiva VO2+/VO2+ haciéndola más rápida, además de mejorar las eficiencias de la batería y la capacidad que esta puede alcanzar. A pesar que los sistemas de baterías de flujo de vanadio descritos han sido estudiados desde hace varias décadas, son aún objeto de estudio actual. Sin embargo, estos sistemas muestras diversos inconvenientes relacionados con la abundancia de vanadio, su elevado coste, además del impacto geopolítico causado por su extracción. Como consecuencia de estos hechos, la implementación de baterías de flujo con especies orgánicas como pares redox activos es un primer paso con el objetivo de evitar las desventajas de las baterías de flujo previamente descritas. Al contrario que el vanadio, las moléculas orgánicas son abundantes, pueden ser modificadas mediante diferentes caminos de síntesis y poseen una cinética mejorada que además incluye una transferencia de dos electrones en el proceso, como se produce en las quinonas, mientras que en las baterías de flujo de vanadio solo se da la transferencia de un único electrón. Dichas baterías orgánicas tienen el potencial de satisfacer las demandas de coste, durabilidad, medioambientalmente respetuoso, y requerimientos de sostenibilidad para el almacenamiento energético dentro de la red eléctrica. Más allá, este sistema se ha estudiado centrándose en moléculas como base quinona en las baterías de flujo, seleccionando como catolito moléculas con base benzoquinona (p-benzoquinone, o-benzoquinone y 4,5-dihidroxi-1,3- benzenedisulfonato) y la sal disódica del ácido antraquinone-2,7-disulfónico como anolito. Ambos materiales activos se han disuelto en una solución con el ácido metanosulfónico como disolvente que ayuda a mejorar las eficiencias y capacidad alcanzable por la batería, además de su estabilidad a lo largo de los ciclos. Paralelamente, con el objetivo de mejorar las reacciones de las moléculas orgánicas previamente mencionadas, diferentes modificaciones en la superficie de electrodos de fieltro de carbono se han realizado, como puede ser la funcionalización mediante grupos nitrógeno y/o oxígeno mediante varios métodos. Finalmente, una breve perspectiva de futuro de las tecnologías estudiadas, como son las baterías de flujo de vanadio y las nuevas baterías de flujo con compuestos orgánicos activos, para ello se ha tenido en cuenta el citado trabajo de tesis como además los datos presentes en la literatura al respecto de estos temas.

[eng] The excessive consumption of non-renewable energy sources such as fossil fuels has lead the world to a global climate change, urging for new energy consumption habits together with developing cost- effective alternative renewable technologies. Photoelectrochemical (PEC) water splitting allows for direct conversion of solar light and water into hydrogen and oxygen, storing energy into chemical bonds, solving the storage problem of photovoltaic technology. It has demonstrated to produce pure hydrogen and oxygen in significant efficiencies, although this technology is not ready for market implementation due to lack of efficient, stable and scalable photoelectrodes. In this work, we undertake a journey from improving the efficiency of stable metal-oxide-based photoanodes to stabilizing efficient photovoltaic materials by the introduction of protective, transparent, conductive and catalytic layers. Efforts have focused on using cost-effective and scalable materials and techniques. Metal oxide candidate TiO2 is reported stable in alkaline electrolytes and at anodic potentials, but they present low photon to current conversion efficiencies. This is due to excessively large band gap, absorbing small part of the visible spectra, and small electron and hole mobility. Its efficiency is increased both by microstructuring the substrate and nanostructuring the thin film into nanorods, and by modifying the electronic structure with a reductive H2 treatment, enhancing potential drop inside the nanorods. The strategy is shifted into stabilizing highly efficient short band gap semiconductor materials used by the photovoltaic industry. Silicon based photocathodes are protected from acidic electrolyte corrosion by TiO2 overlayers grown by atomic layer deposition (ALD). Temperature is found to play a key role for both efficient film conductivity and stability, being this caused by polycrystalline films formation. ALD enabled high thickness control and pinhole-free layers, together with lower crystallization temperatures than other techniques. Copper-indium-gallium-selenide (CIGS) solar cells fabricated on flexible stainless steel substrates are also protected from corrosion by TiO2 ALD protective layers. The transparent conductive oxide (TCO) already used in solar cells is found necessary for efficient p-n junction formation and charge transport to the hydrogen evolution reaction. Copper-zinc-tin- sulfide/selenide (CZTS/Se) solar cells, where scarce indium and gallium are substituted by tin and zinc, are implemented for PEC devices with TiO2 overlayers too. By modifying the S/Se ratio, band gap can be tuned, an especially interesting characteristic to design tandem PEC devices. ALD deposited protective layers are also studied in anodic polarizations and alkaline electrolytes. By varying the deposition temperature of TiO2, completely amorphous, mixed amorphous and crystalline and fully crystalline films are deposited, and a clear conductivity increase is observed correlated to crystallization. Preferential conductivity paths are observed inside crystalline grains, proposed to be related to crystalline defects and grain boundaries. Few hundred hours stability tests reveals significant photocurrent decrease, with no observed dissolution of the Si photoabsorber. This is attributed to oxidative potentials and electrolyte hydroxides diminishing the n-type semiconductor behavior of TiO2 and forming a barrier to charge injection into the oxygen evolution reaction. UV superimposed illumination partially recovered conductivity. NiO films are ALD-deposited on Si photoanodes and conductivity is found to decrease when temperature is increased from 100 to 300 ºC, simultaneous to a change in preferential crystal growth direction. Higher stoichiometric film, being formed when increasing temperature, decreases Ni2+ vacancies, responsible of the p-type semiconductor behavior. Impressive 1000 hours stability measurements are obtained. Although, this is only attained under periodic cyclic voltammetries, avoiding partial deactivation of the photoanodes. This is attributed to chemical modifications at the surface in such highly oxidative conditions.

[eng] The thesis is focused on the development and enhancement of the electrochemical properties of the carbon based supercapacitors and pseudocapacitors. To overcome the capacitance loss at the condition of fast charging in the carbon-based supercapacitors, a metal-oxide embedded porous carbon nanofiber with a 3-D electrode architecture is designed. This electrode reduces the electrode resistance and at the same time increases the associated values of capacitance at high rates. The investigation also indicates an essential role in the concentration of the metal oxide precursor towards the electrochemical behavior of the electrodes. This correlation could be useful to design better electrodes for supercapacitor, functioning with better energy and power density capabilities. Whereas, in the case of the water-based pseudocapacitors, it is shown that they suffer from low voltages. Two strategies were used to overcome this issue. (i) Exploring and improving the electrode material based non-carbon materials. In this regard, new materials from the family of MXenes are introduced, to achieve higher cell voltages. Under this frame, a new 2-D MXene based on Molybdenum Vanadium Carbide is proposed and its electrochemical characteristics were investigated. According to its characteristics, its coupling with 2-D Titanium Carbide MXene exhibits a higher cell voltage. The investigation reveals that the charge storage in 2-D molybdenum vanadium carbide MXene has the dependence on the type of electrolyte cations. For the case in point, small size monovalent cations, such as lithium and sodium ions, demonstrate lower hindrance to the charge storage, while large size monovalent potassium ions and bivalent magnesium ions suffer from hindrance effect, causing them to have lower charge storage than lithium and sodium ions. Therefore, the selection of appropriate electrolyte ions especially in the case of MXene based materials appears to be important, which is here found to be with the protonic and sodium ion based electrolytes. (ii) the proposed approach is based on the use of water-based super-concentrated salt solutions which are promising electrolytes to contribute to widening the cell voltage of aqueous pseudocapacitors. Likewise, besides this, it is also proposed that the coupling of 2-D Titanium Carbide MXene with the tunnel structures of Manganese Oxide using this super-concentrated electrolyte water in salt can enable a high voltage aqueous pseudocapacitive energy storage device. The investigation using this approach reveals that the concentration of the salt electrolyte plays a significant role in the values of charge storage in 2-D titanium carbides. Although an extremely high concentration of salt electrolytes widens the potential window, the electrolyte ions in such high concentration face difficulty to insert within the 2-D layers of titanium carbide MXene. On the contrary, the use of low concentrated salt solutions is not recommended, as they provide narrow potential windows. Consequently, during the cell assembling using super-concentrated electrolytes, a moderate concentration of salt electrolyte needs to be taken into attention. On this way, both wider potential window and high charge storage, can be achieved with pseudocapacitive materials like 2-D titanium carbides MXenes. The crystallographic tunnel size of manganese oxide plays a vital role in the charge storage. For instance, tunnel structures, both smaller and larger than the size of the electrolyte ions store fewer charges. As both of these tunnel phases of manganese oxide face difficulty for the insertion of the electrolyte ions. Therefore, manganese oxide with adequate tunnel size needs to be taken into account. Besides this, it is also essential to consider the electronic conductivity of the manganese oxide phase, as high electronic conductivity allows it to store more charges during the condition of fast charging. In regards of the cell assembly, after considering the above-mentioned understanding the practice of applying the voltage-hold test to determine the realistic cell voltage is helpful, as the cell assembled with such realistic voltages permits the cell to have long cycle life. Besides these understanding, remarkable performances were witnesses with the technologies developed in this thesis. For example: (i) the carbon-based electric double layer supercapacitor shows faster responses than the existing carbon-based supercapacitors, (ii) the pseudocapacitors shows high volumetric capacitances (> 35 F cm-3) than carbon-based supercapacitors. Besides this, pseudocapacitors also exhibit higher cells voltages than the existing pseudocapacitors. The pseudocapacitor cells developed in this exhibits high electrochemical stability (> 95 %) over thousands of cycles. Furthermore, the pseudocapacitor is more favorable than EDLCs in applications as they provide slower self-discharges than EDLCs. The above understanding, such as the selection of the electrode, electrode processing and the cell assembly is a tool for designing better supercapacitors., [spa] La tesis se centra en el desarrollo del conocimiento orientado y conducido a la mejora de las propiedades electroquímicas de los supercapacitores, ya que sufren bajos valores de densidad de energía. Este inconveniente limita a los supercapacitores en las aplicaciones donde son necesarios tanto alta potencia como densidad de energía. Entonces, en este escenario, se identificaron dos problemas principales importantes: (a) las limitaciones de rendimiento del supercapacitor debido a la condición de carga rápida, y (b) el bajo voltaje de celda de los pseudocapacitores en electrolitos acuosos en comparación con los electrolitos orgánicos. Para superar la limitación de rendimiento en el primer problema, se muestra una alternativa original a través del electrospinning para diseñar nanofibras de carbono porosas con incrustaciones de óxido metálico con una arquitectura de electrodo 3D que contribuyen a reducir la resistencia del electrodo y al mismo tiempo aumentan los valores asociados de capacidad. La investigación indica un papel esencial en la concentración del precursor de óxido metálico hacia el comportamiento electroquímico de los electrodos. Esta correlación podría ser útil para diseñar mejores electrodos para supercapacitadores, funcionando con mejores capacidades de densidad de energía y potencia. En lo que respecta al problema relacionado con los bajos voltajes celulares en el pseudocapacitor acuoso, en lugar de utilizar materiales basados en carbono más estándar, se toma una metodología en términos de exploración y mejora basada en las propiedades del material del electrodo. Así, se introducen nuevos materiales de la familia de MXenes, para lograr voltajes de celda más altos. Bajo este marco, se propone un nuevo MXene 2-D basado en carburo de vanadio y molibdeno y se han investigado sus características electroquímicas. De acuerdo con sus características, su acoplamiento con carburo de titanio 2-D MXene exhibe un voltaje más alto en una celda pseudocapacitiva todo en MXene. Además de esto, el problema del bajo voltaje de la celda también se resuelve aplicando otro enfoque basado en la modificación del electrólito. El enfoque propuesto se basa en el uso de soluciones salinas superconcentradas a base de agua que son electrolitos prometedores en la ampliación del voltaje celular de los pseudocapacitores acuosos. Del mismo modo, también se propone que el acoplamiento del carburo de titanio 2-D MXene con las estructuras del túnel de óxido de manganeso utilizando este electrolito súper concentrado o agua en sal permite lograr una celda de pseudocapacitador acuoso de alto voltaje. En conjunto, la estrategia presentada a través de esta tesis en términos de preparación de electrodos, selección de materiales, ensamblaje celular y su evaluación de las propiedades electroquímicas es una herramienta para diseñar supercapacitores con mejores capacidades de energía y potencia.

[eng] The need of substituting the current energetic model by a system based on clean Renewable Energy Sources (RES) have gained more importance in the last decades due to the environmental issues related to the use of fossil fuels. These energy sources are site-specific and intermittent, what makes essential the development of Energy Storage Systems (ESS) that allows the storage of the electricity generated by renewable energies. Among the technologies under development for the storage of electrical energy, Solid Oxide Electrolysis Cells (SOECs) have been proposed in the last decades as a promising technology. Achieving efficiencies higher than 85%, SOEC technology is able to convert electrical energy into chemical energy through the reduction of H2O, CO2 or the combination of both; generating H2, CO or syngas (H2 +CO). The implementation of this technology based on renewable electrical energy, combined with fuel cells would allow closing the carbon cycle. The work presented in this thesis has been devoted to enhance the performance of SOEC. The approach that is presented for that propose is based on the implementation of high surface area and thermally stable mesoporous metal oxide materials on the fabrication of SOEC electrodes. High performance and stability of the electrodes was expected during its characterization. Structural and electrochemical characterization techniques have been applied during the development of this thesis for this purpose. The thesis is organized in eight chapters briefly described in the following: Chapter 1 briefly analyses the current energy scenario presenting electrolysers as a promising technology for the storage of electrical energy. Besides, basic principles of SOECs operation and the state-of-the-art materials of SOECs are reviewed. Chapter 2 describes all the experimental methods and techniques employed in this thesis for the synthesis and characterization of synthesised materials and fabricated cells. Chapter 3 presents the results obtained from the structural characterization of the mesoporous materials and fabricated electrodes, revealing the successful implantation of the hard-template method for obtaining Sm0.2Ce0.8O1.9 (SDC), Ce0.8Gd0.2O1.9 (CGO) and NiO mesoporous powders, and the fabrication of SDC-SSC (Sm0.5Sr0.5CoO3-δ), CGO- LSCF (La0.6Sr0.4Co0.2Fe0.8O3) and NiO-SDC electrodes based on mesoporous materials. The attachment of the mesoporous scaffold for the fabrication of oxygen electrodes has been optimized at 900 °C. Chapter 4 compares electrolyte- and fuel electrode-supported cell configurations based on the same oxygen electrode. The electrochemical performance and the microstructural characterization of these cells are considered for that purpose. Showing a maximum current density of -0.83 and -0.81 A/cm2 on electrolysis and co- electrolysis modes respectively, fuel electrode-supported cells are considered more suitable for SOEC fabrication. Chapter 5 presents a study focused on analysing the influence of the oxygen electrode interface on the SOEC performance. The electrochemical and microstructural characterization of barrier layers and oxygen electrodes fabricated applying different methods are discussed in this chapter. The combination of a barrier layer fabricated by Pulsed Laser Deposition (PLD) with an oxygen electrode based on mesoporous materials resulted on the injection of up to -1 A/cm2, what allows concluding that this interface microstructure is directed related with the best performing SOECs in this thesis. Chapter 6 shows the performance of SOEC cells on co-electrolysis mode containing the optimized oxygen electrode, fabricated by infiltration of mesoporous scaffolds. The long-term stability of infiltrated mesoporous composites have been demonstrated during 1400 h, registering degradation rates of 2%/kh and, [spa] Una de las principales desventajas de las fuentes de energías renovables es que producen energía eléctrica de forma discontinua. Los electrolizadores de alta temperatura basados en óxidos sólidos (SOEC) se presentan como una tecnología prometedora para el almacenamiento de energía eléctrica. Alcanzando eficiencias mayores de un 85%, los electrolizadores SOEC permite convertir energía eléctrica en energía química mediante la reducción de las moléculas de agua (H2O), dióxido de carbono (CO2), o la combinación de ambas; generándose hidrógeno (H2), monóxido de carbono (CO) o gas de síntesis (H2 +CO) como producto. El trabajo que se presenta en esta tesis tiene como objetico mejorar el rendimiento de los electrolizadores SOEC mediante la utilización de óxidos metálicos mesoporosos, caracterizados por poseer alta área superficial y ser estables a altas temperaturas. Esta tesis está organizada en ocho capítulos. Los capítulos 3, 4, 5, 6 y 7 presentan los resultados alcanzados: El capítulo 3 presenta la caracterización estructural de los materiales mesoporosos y de los electrodos fabricados. Además, la temperatura de adhesión del material mesoporoso ha sido optimizada y se ha fijado a 900 °C. El capítulo 4 compara electrolizadores fabricados soportados por el electrodo de combustible y por el electrolito. Los resultados muestran que las densidades de corriente más altas fueron inyectadas en los electrolizadores soportados por el electrodo de combustible, considerándose esta configuración la más apropiada. El capítulo 5 presenta la influencia de la microstructura de la intercara del electrodo de oxígeno en el rendimiento de los electrolizadores SOEC. La caracterización electroquímica, apoyada por la caracterización microestructural, ha demostrado que la máxima densidad de corriente ha sido inyectada por el electrolizador cuya barrera de difusión ha sido depositado por láser pulsado (PLD) y la capa funcional del electrodo de oxígeno mediante infiltración de materiales mesoporosos. El capítulo 6 estudia el electrodo de oxígeno optimizado. Durante 1400 h de operación continua y caracterización microstructural, se ha demostrado la estabilidad de este electrodo. Por último, el capítulo 7 muestra los resultados obtenidos del escalado de los electrodos mesoporosos en celdas de mayor área (25 cm2). La caracterización electroquímica muestra alta flexibilidad ante las composiciones de gases utilizadas, y estabilidad de los electrodos mesoporosos propuestos.

L’energia que s’origina de combustibles fòssils ha permés avenços molt remarcables a la nostra civilització durant el segle passat. No obstant, els combustibles fòssils no son il·limitats i suposen una font d’increment del diòxid de carboni a l’atmòsfera, amb els seus conseqüents efectes ambientals nocius. Millorar la eficiencia dels dispositius d’enmagatzematge d’energia i la conversió d’energia solar a hidrògen mitjançant la dissociació de l’aigua són tecnologies clau per encarar problemes energètics i ambientals. Els semiconductors que es presenten en abundància i són beneficiosos pel medi ambient han estat en el punt de mira durant els últims anys donades les seves característiques especifiques com a supercapacitors i dispositius per la dissociació de l’aigua. És conegut que les propietats capacitives dels semiconductors están molt afectades per la seva estructura a la nanoescala i la seva baixa conductivitat, limitant les densitats d’energia i potencia. Així doncs, entendre i manipular l’estructura jeràrquica a la nanoescala és essencial per dissenyar materials nanocompostos per l’emmagatzematge d’energia amb millores en la transferència de càrrega i habilitat de transportar ions electrolítics. Per la dissociació d’aigua fotoelectroquímica (PEC), la recombinació electró-forat al “bulk” i les interfícies juguen un paper molt determinant en l’actuació catalítica. La investigació sobre la modulació de la dinámica de transferencia de càrrega així com el nivell d’energia i la densitat d’estats de superfície sobre la modificació d’un segon semiconductor o catalitzadors per dissociació de l’oxígen (OEV) podrien ser de gran interés. Per altra banda, pels catalitzadors de evolució d’hidrògen (HEC), com la identificació de defectes estructurals, transmisió de fase i les vacants presents en materials 2D juguen un paper de vital interès per optimitzar els catalitzadors per la reacció d’evolució de l’hidrògen (HER) en la dissociació de l’aigua. Aquest treball està dividit en 7 capítols: El Capítol 1 és la part introductòria, que inclou els principis bàsics dels supercapacitors i la dissociació de l’aigua i comenta els factors limitants de les propietats electroquímiques dels semiconductors per aplicacions en supercapacitors i dissociació de l’aigua. El Capítol 2 resumeix les metodologies emprades en aquest treball. Aquest capítol inclou els detalls sobre les configuracions experimentals del TEM, STEM i EELS, processament de dades, simulacions i una introducció a les tècniques electroquímiques com la voltimetria cíclica, espectre d’impedància electroquímica i els models de circuits electrònics per il·lustrar els estats de superficie. La síntesi i els resultats experimentals es presenten en els Capítols 3-6. El Capítol 3 tracta sobre la fabricació de nuclis embrancats de nanocompostos de Fe2O3/PPy com a electrodes negatius per aplicacions en supercapacitors, així com la investigació dels mecanismes de creixement de nanofulles de PPy sobre flocs d’hematita. Al Capítol 4, hem optimitzat les condicions de síntesi, incloent el gruix de ITO, gruix de TiO2, càrrega de dipòsit de FeNiOOH i la temperatura de post-sinterització dels nanofils de ITO/Fe2O3/Fe2TiO5/FeNiOOH com a fotoànodes per la dissociació de l’aigua en electrolits alcalins. Els detalls de l’estructura s’han investigat principalment mitjançant TEM i STEM-EELS, mentre que la transferència de càrrega i els mecanismes de la dinàmica de reacció han estat investigats sistemàticament per PEIS. Al Capítol 5 hem optimitzat les condicions del bany químic per sintetitzar CoFe PBA suportat sobre fotoànodes basats en nanofils de Fe2O3/Fe2TiO5 per la dissociació de l’aigua en electròlits àcids. Els detalls de l’estructura han estat investigats principalment per TEM i STEM-EELS mentre que la transferència de càrrega i els mecanismes de la dinàmica de reacció han sigut investigats sistemàticament per PEIS. Al Capítol 6, ens hem centrat en la caracterització de defectes estructurals, transmissió de fase, vacants en materials 2D per HER per la dissociació de l’aigua amb un STEM dedicat amb correcció d’aberracions, incloent HAADF, ABF, EELS-STEM, GPA i simulacions d’HAADF. Finalment, al Capítol 7 es resumeixen les conclusions generals d’aquest treball, juntament amb les projeccions futures d’aquests. The energy originated from fossil fuels has enabled the remarkable advancement of civilization over the past century. However, fossil fuels are not infinite in supply and they are a source of increasing atmospheric carbon dioxide and the associated abominable environmental effects. Improving the efficiency of the energy storage devices and conversion of solar energy into hydrogen energy via water splitting are key technologies to tackle the serious energy and environmental problems. Earth-abundant, environmental-friendly semiconductors for supercapacitor and water splitting applications have received great attention due to their specific characteristics. It is well established that the capacitive properties of semiconductors are greatly affected by their nanostructure and poor conductivity, leading to a limited energy and power densities. Thus, understanding and manipulating the hierarchical structure at the nanoscale is essential to design composite materials for energy storage with enhanced charge transfer and electrolyte ions transportation abilities. On one hand, in photoelectrochemical water splitting (PEC), the electron-hole recombination in the bulk interfaces plays a determinative role in the catalytic performance. The investigation about modulation of the charge transfer kinetics as well as the energy level and density of surface state upon the modification of a second semiconductor or oxygen evolution catalysts (OEC) could be of great interest. On the other hand, for hydrogen evolution catalysts (HEC), as the identification of structural defects, phase transmission and vacancies presented in the 2D materials play a vital role in optimizing the catalyst for hydrogen evolution reaction (HER) in water splitting. This dissertation is divided into 7 chapters: Chapter 1 is the introduction part, which includes the background of supercapacitors and water splitting and reviews the limited factors affecting the electrochemical properties of semiconductors for supercapacitor and water splitting applications. In Chapter 2 summarizes the applied methodologies in this dissertation. This chapter includes the details about the TEM, STEM, EELS experimental setups, data processing, simulations and general introductions to the electrochemical techniques, such as cyclic voltammetry, electrochemical impedance spectrum as well the electrical circuit model for illustrating the surface states. Specific synthesis procedures and experimental results for every one of the studied nanosystems are presented in Chapters 3-6. Chapter 3 deals with the fabrication of core-branch Fe2O3/PPy nanocomposites as negative electrode for supercapacitor applications as well as the investigation of PPy nanoleaves growth mechanism onto the hematite nanoflakes. In Chapter 4, we have optimized the synthesis conditions, including the ITO thickness, TiO2 thickness, FeNiOOH deposition charge and the post-sintering temperature of ITO/Fe2O3/Fe2TiO5/FeNiOOH nanowire-based photoanodes for water splitting in alkaline electrolyte. The detailed structure has been mainly investigated by TEM and STEM-EELS, while, the charge transfer and reaction kinetic mechanisms were systematically investigated by PEIS. In Chapter 5, we have optimized the chemical bath conditions for synthesising CoFe PBA supported onto Fe2O3/Fe2TiO5 nanowire-based photoanodes for water splitting in acidic electrolyte. The detailed structure has been mainly investigated by TEM and STEM-EELS, while, the charge transfer and reaction kinetic mechanisms were investigated by PEIS. In Chapter 6, we moved the characterization of structural defects, phase transmission, vacancies in 2D materials for HER in water splitting with advanced aberration-corrected dedicated STEM, including HAADF, ABF, EELS-STEM, GPA and HAADF simulation. Finally, Chapter 7 summarizes the general conclusions of this dissertation, along with a brief outlook.

[eng] Growing global emission of carbon dioxide gas (CO2) reflects the world’s energy dependence on fossil fuels. The conversion of CO2 emission into value-added products, like fuels completes a circular CO2 economy which requires a renewable energy conversion and storage system. Amongst a few, photo/electrochemistry has been particularly appealing thanks to its energy efficiency and enormous potential for industrial applications. Formic acid (HCOOH) production from CO2 reduction appears as an alternative energy storage option based on the commercialization of this process. Herein, stable and selective catalysts working at low overpotential are needed to reduce CO2. Likewise, cell design is critical to have improved CO2 mass transport for obtaining high conversion efficiencies and to achieve feasible production yields. The initial work was conducted on the design and understanding of operational parameters of an electrochemical flow cell (ECf-cell) such as flow rates and electrode potentials. For CO2 reduction at the cathode site, two different gas diffusion electrodes were produced by electrodeposition: Sn-GDE and Cu-GDE. An optimum potential range was established to control HCOOH selectivity. The complementing reaction at the anode site, oxygen evolution reaction (OER), was studied using Mn-Co oxide nanoparticles to replace expensive DSA: Ir-Ta oxide catalyst. Subsequent efforts were devoted on the assembly of a photoelectrochemical flow cell (PECf-cell) which enabled coupling of Sn-GDE as cathode vs. TiO2 nanorods as photoanode. This led to nearly 1/3 reduction in overall cell voltage reaching an energy efficiency up to 70 %. The solar-to-fuel (STF) conversion efficiency was 0.25% which was one of the highest efficiencies reported amongst the data obtained from a cell in device level. The results proved that optimized system efficiency could be achieved with a large bandgap photoanode having superior stability and a GDE cathode with improved CO2 mass transfer. The deployment of renewable energy sources will require new technologies to emerge. The photoelectrochemical flow cell developed in this work can store energy from intermittent electricity sources (i.e. wind and solar) in a sustainable manner. This may pave the way for commercialization of this process and moving towards a circular CO2 economy., [spa] La conversión de CO2 en productos de valor añadido con energías renovables resulta interesante para mitigar las emisiones de este. La conversión foto/electroquímica es atractiva por su eficiencia energética y su enorme potencial para aplicaciones industriales. La producción de ácido fórmico (HCOOH) a partir de la reducción de CO2 aparece como una vía alternativa para su comercialización. Sin embargo, se requieren catalizadores estables y selectivos que trabajen a bajo sobre potencial. Además, el diseño de la celda es crítico para mejorar el transporte de masa de CO2 y obtener elevadas eficiencias de conversión. En este trabajo se estudió en un primer lugar el diseño y la comprensión de los parámetros operativos de una celda de flujo electroquímica: caudales y potenciales de electrodo. Para la reducción de CO2 sobre el cátodo, se emplearon dos electrodos diferentes de difusión de gas preparados por electrodeposición: Sn-GDE y Cu-GDE. Se estableció un valor de operación óptimo para controlar la selectividad a HCOOH. Se estudió también la reacción complementaria en el ánodo (evolución de O2), empleando nanopartículas de óxido de Mn-Co para reemplazar el elevado coste del catalizador de óxido de Ir-Ta. Finalmente, se montó una celda fotoelectroquímica de flujo que permitió la inclusión de TiO2 nanorods como fotoánodo. El voltaje total de la celda se redujo alrededor 1/3 alcanzando una eficiencia energética del 70 %. El rendimiento de conversión de energía solar a combustible (STF) fue de 0,25%. Los resultados demuestran que se puede lograr una eficiencia optimizada del sistema con un fotoánodo que tiene una buena estabilidad y un cátodo que favorece la transferencia de masa de CO2. La celda de flujo fotoelectroquímica desarrollada en este trabajo permite almacenar energía de fuentes de electricidad intermitentes (eólica y/o solar) de una manera sostenible, con el consiguiente avance en una economía circular de CO2.

La interacció de la llum amb els òxids metàl.lics (MOX's) semiconductors, sempre i quan l'energia dels fotons sigui superior a l'amplada de la banda prohibida, produeix un increment significatiu de la densitat de portadors de càrrega en el semiconductor, ja que cada fotó incident genera, de forma molt eficient, un parell electró—forat (e- — h+). Aquests portadors de càrrega fotogenerats poden migrar a la superfície del semiconductor i interactuar amb espècies donadores o acceptadores d'electrons, propiciant les reaccions de tipus redox. Aquest fenomen rau en la base de la fotocatàlisi heterogènia i fa dels òxids metàllics semiconductor uns materials molt atractius com a catalitzadors. L'objectiu d'aquesta tesi és l'estudi de dues aplicacions directes de la interacció entre gasos i la superfície dels òxids metàl.lics semiconductors activats mitjançant radiació lumínica: els sensors de gasos i la fotoreducció de CO2. • Sensors de gasos Un dels principals inconvenients dels sensors de gasos és que, per tal de propiciar i accelerar les reaccions d'adsorció i desorció de les molècules de gas sobre la superfície del semiconductor, han d'operar a alta temperatura. La necessitat d'un sistema calefactor en el dispositiu limita les seves aplicacions, per exemple, en medis amb gasos explosius. Substituir l'activació tèrmica del sistema per una activació lumínica permetria augmentar-ne el ventall de possibles aplicacions. Tradicionalment, el Sn02, el WO3 i el In203, entre d'altres, s'han emprat en els sensors químics resistius. En el present treball s'estudia el comportament d'aquests òxids metàl.lics sota il•luminació UVA. Per tal d'augmentar-ne l'estabilitat i la resposta, s'utilitzen estructures nanomètriques (Nanofils de Sn02) i mesoporoses (W03 i In203amb estructures KIT-6). s’estableix una relació entre el flux de fotons que incideixen sobre la superfície dels òxids metàl·lics i el comportament dels sensors (resposta, temps de transició, estabilitat, etc.). Els mecanismes de detecció s’associen a la competència entre el NO2 i el O2 per adsorbir-se sobre els mateixos llocs actius, i com la llum influeix en aquesta competència. De l’altra banda, s’estudia el fenomen de la fotoconductivitat i de la seva persistència (PPC per Persistent Photoconductivity) en diferents condicions de treball i se suggereix que el seu origen està relacionat no només amb l’augment de portadors de càrrega induït per la radiació, sinó també amb la desorció directa dels O2 adsorbits en superfície i la fotoreducció de l’òxid metàl·lic. • Fotoreducció de CO2 La fotoreducció de CO2 assistida per H2O és una alternativa al emmagatzemament geològic força prometedora. Aprofitant la interacció de la radiació UV i solar, es pot aconseguir reduir el CO2 per convertir-lo en hidrocarburs que puguin ser reutilitzats en el cicle energètic (hidrogen, metà, età, metanol, etanol...). La selectivitat i eficiència del procés depèn en bona part del material fotocatalític emprat. En aquest treball, amb l’objectiu de millorar aquests materials, els esforços s’han centrat en semiconductors fotosensibles nanoestructurats (nanopartícules de TiO2) modificats amb additius altament dispersats (Mg, Ca, In i Pt). L’activitat fotocatalítica dels material s’ha testat en un reactor bifàsic (sòlid–gas) emprant un simulador solar com a font de llum i amb una atmosfera humida. Les mostres de Pt-TiO2 exhibeixen una alta selectivitat cap a la producció de H2. En canvi, les mostres de Mg-TiO2, Ca-TiO2 i In-TiO2 amb baixes concentracions d’additiu (0.2 i 0.5 wt%) mostren una millora en la productivitat d’hidrocarburs (especialment, CH4), respecte de les mostres de TiO2 pur. La caracterització òptica i estructural dels catalitzadors relaciona la 3+ productivitat amb els estats superficials, en concret amb l’abundància de Ti i grups hidroxils presents a la superfície. Els resultats també suggereixen que la selectivitat cap a la producció de un o altre hidrocarbur pot estar relacionada amb la basicitat del catalitzador i els additius emprats., The interaction of light with metal oxides (MOX's) semiconductors, as long as photons have enough energy, produces an increase in the density of charge carriers in the semiconductor. The photogenerated charge carriers can migrate to the surface of the semiconductor and interact with electron acceptors or donors species, leading to redox reactions. This phenomenon lies at the basis of the heterogeneous photocatalysis. The aim of this thesis is to study two direct applications of the interaction between gases and the surfaces of metal oxide activated by light radiation: gases sensors and CO2 photoreduction. * Gas sensors One of the main drawbacks of gas sensors is that they must operate at high temperatures, which limits the range of possible applications. Light activation could help to overcome this limitation. In the present work, Sn02, WO3 and In203 were studied as chemiresistor gas sensors under UV illumination. To improve their stability and response, nanometric (Sn02) and mesoporous (W03 and In203) structured materials were used. At room temperature, photoactivated sensors show a reversible response to oxidizing gases (NO2) not seen in dark conditions. A relation between the photon flux and the sensor behaviour is established. Detection mechanims are determined by the competition between NO2 and 02 to adsorb on the same active sites, and light modulate the adsorption rates. The photoconductivity and its persistence were also studied. Results suggest that these phenomena are related to charge carrier generation, to the direct desorption of surface 02 species and to the photoreduction of the MOX. * CO2 photoreduction Artificial photosynthesis is a promising alternative to the geological sequestration. Using the interaction between solar radiation and MOX’s, CO2 could be reduced to obtain reusable hydrocarbons. The selectivity and the efficiency of the process depend on the photocatalyst. In the present work, to improve the photocatalysts, efforts were focused on photosensitive semiconductors nanostructures (TiO2) modified by highly dispersed additives (Mg, Ca, In and Pt). The photocatalytic activity of these materials was tested in a biphasic reactor using a solar simulator as light source. Pt‐TiO2 samples exhibit a high selectivity towards the production of H2. Mg‐TiO2, Ca‐TiO2 and In‐TiO2 samples with low amount of additive show an improvement in the hydrocarbons productivity compared to pure TiO2. According to the optical and the structural characterization, the productivity is related to the surface states of the catalyst, particularly 3+ with Ti and hydroxyl groups. Results also suggest that the selectivity of the by‐products could be related to the basicity of the catalysts.

[cat] La interacció de la llum amb els òxids metàl.lics (MOX's) semiconductors, sempre i quan l'energia dels fotons sigui superior a l'amplada de la banda prohibida, produeix un increment significatiu de la densitat de portadors de càrrega en el semiconductor, ja que cada fotó incident genera, de forma molt eficient, un parell electró—forat (e- — h+). Aquests portadors de càrrega fotogenerats poden migrar a la superfície del semiconductor i interactuar amb espècies donadores o acceptadores d'electrons, propiciant les reaccions de tipus redox. Aquest fenomen rau en la base de la fotocatàlisi heterogènia i fa dels òxids metàllics semiconductor uns materials molt atractius com a catalitzadors. L'objectiu d'aquesta tesi és l'estudi de dues aplicacions directes de la interacció entre gasos i la superfície dels òxids metàl.lics semiconductors activats mitjançant radiació lumínica: els sensors de gasos i la fotoreducció de CO2. • Sensors de gasos Un dels principals inconvenients dels sensors de gasos és que, per tal de propiciar i accelerar les reaccions d'adsorció i desorció de les molècules de gas sobre la superfície del semiconductor, han d'operar a alta temperatura. La necessitat d'un sistema calefactor en el dispositiu limita les seves aplicacions, per exemple, en medis amb gasos explosius. Substituir l'activació tèrmica del sistema per una activació lumínica permetria augmentar-ne el ventall de possibles aplicacions. Tradicionalment, el Sn02, el WO3 i el In203, entre d'altres, s'han emprat en els sensors químics resistius. En el present treball s'estudia el comportament d'aquests òxids metàl.lics sota il•luminació UVA. Per tal d'augmentar-ne l'estabilitat i la resposta, s'utilitzen estructures nanomètriques (Nanofils de Sn02) i mesoporoses (W03 i In203amb estructures KIT-6). s’estableix una relació entre el flux de fotons que incideixen sobre la superfície dels òxids metàl·lics i el comportament dels sensors (resposta, temps de transició, estabilitat, etc.). Els mecanismes de detecció s’associen a la competència entre el NO2 i el O2 per adsorbir-se sobre els mateixos llocs actius, i com la llum influeix en aquesta competència. De l’altra banda, s’estudia el fenomen de la fotoconductivitat i de la seva persistència (PPC per Persistent Photoconductivity) en diferents condicions de treball i se suggereix que el seu origen està relacionat no només amb l’augment de portadors de càrrega induït per la radiació, sinó també amb la desorció directa dels O2 adsorbits en superfície i la fotoreducció de l’òxid metàl·lic. • Fotoreducció de CO2 La fotoreducció de CO2 assistida per H2O és una alternativa al emmagatzemament geològic força prometedora. Aprofitant la interacció de la radiació UV i solar, es pot aconseguir reduir el CO2 per convertir-lo en hidrocarburs que puguin ser reutilitzats en el cicle energètic (hidrogen, metà, età, metanol, etanol...). La selectivitat i eficiència del procés depèn en bona part del material fotocatalític emprat. En aquest treball, amb l’objectiu de millorar aquests materials, els esforços s’han centrat en semiconductors fotosensibles nanoestructurats (nanopartícules de TiO2) modificats amb additius altament dispersats (Mg, Ca, In i Pt). L’activitat fotocatalítica dels material s’ha testat en un reactor bifàsic (sòlid–gas) emprant un simulador solar com a font de llum i amb una atmosfera humida. Les mostres de Pt-TiO2 exhibeixen una alta selectivitat cap a la producció de H2. En canvi, les mostres de Mg-TiO2, Ca-TiO2 i In-TiO2 amb baixes concentracions d’additiu (0.2 i 0.5 wt%) mostren una millora en la productivitat d’hidrocarburs (especialment, CH4), respecte de les mostres de TiO2 pur. La caracterització òptica i estructural dels catalitzadors relaciona la 3+ productivitat amb els estats superficials, en concret amb l’abundància de Ti i grups hidroxils presents a la superfície. Els resultats també suggereixen que la selectivitat cap a la producció de un o altre hidrocarbur pot estar relacionada amb la basicitat del catalitzador i els additius emprats., [eng] The interaction of light with metal oxides (MOX's) semiconductors, as long as photons have enough energy, produces an increase in the density of charge carriers in the semiconductor. The photogenerated charge carriers can migrate to the surface of the semiconductor and interact with electron acceptors or donors species, leading to redox reactions. This phenomenon lies at the basis of the heterogeneous photocatalysis. The aim of this thesis is to study two direct applications of the interaction between gases and the surfaces of metal oxide activated by light radiation: gases sensors and CO2 photoreduction. * Gas sensors One of the main drawbacks of gas sensors is that they must operate at high temperatures, which limits the range of possible applications. Light activation could help to overcome this limitation. In the present work, Sn02, WO3 and In203 were studied as chemiresistor gas sensors under UV illumination. To improve their stability and response, nanometric (Sn02) and mesoporous (W03 and In203) structured materials were used. At room temperature, photoactivated sensors show a reversible response to oxidizing gases (NO2) not seen in dark conditions. A relation between the photon flux and the sensor behaviour is established. Detection mechanims are determined by the competition between NO2 and 02 to adsorb on the same active sites, and light modulate the adsorption rates. The photoconductivity and its persistence were also studied. Results suggest that these phenomena are related to charge carrier generation, to the direct desorption of surface 02 species and to the photoreduction of the MOX. * CO2 photoreduction Artificial photosynthesis is a promising alternative to the geological sequestration. Using the interaction between solar radiation and MOX’s, CO2 could be reduced to obtain reusable hydrocarbons. The selectivity and the efficiency of the process depend on the photocatalyst. In the present work, to improve the photocatalysts, efforts were focused on photosensitive semiconductors nanostructures (TiO2) modified by highly dispersed additives (Mg, Ca, In and Pt). The photocatalytic activity of these materials was tested in a biphasic reactor using a solar simulator as light source. Pt‐TiO2 samples exhibit a high selectivity towards the production of H2. Mg‐TiO2, Ca‐TiO2 and In‐TiO2 samples with low amount of additive show an improvement in the hydrocarbons productivity compared to pure TiO2. According to the optical and the structural characterization, the productivity is related to the surface states of the catalyst, particularly 3+ with Ti and hydroxyl groups. Results also suggest that the selectivity of the by‐products could be related to the basicity of the catalysts.

[cat] La demanda d'energia dels dispositius electrònics portàtils augmenta exponencialment any rere any, però la tecnologia actual amb les bateries d'ió liti no progressa suficientment. Aquesta divergència energètica és una oportunitat per poder desenvolupar noves tecnologies. En aquest sentit, micro dispositius que operin de forma contínua mitjançant l'ús d'un combustible i que proporcionin una potència compresa entre 1 i 20 W reben una especial atenció per part de la comunitat científica. Probablement, l'alternativa més prometedora són les micro piles de combustible, ja que tenen una llarga vida útil, una alta densitat de potència i són fàcils d'integrar. Dels diferents tipus, les piles de combustible d'òxid sòlid (mi-SOFCs) són les que tenen una major densitat específica d'energia (per unitat de massa i de volum). A més a més, treballen a una temperatura més alta i aquesta característica permet utilitzar hidrocarburs com a combustible. Una de les configuracions més esteses de les mi-SOFCs és la que es basa en membranes electrolítiques en suspensió, integrades amb la tecnologia de silici. Aquesta configuració permet una producció elevada, barata i fiable. La present tesi té com a objectiu dissenyar, fabricar i validar experimentalment els principals components d'un nou dispositiu generador. Els resultats obtinguts són un primer pas per desenvolupar en un futur proper un dispositiu complet basat en l'ús de MEMS (sistemes micro electromecànics) i hidrocarburs d' alta densitat d'energia. El dispositiu mi-SOFC ha estat ideat per subministrar 1W de potència elèctrica i ocupar un volum entre 10 i 20 cm3. La tesi consta de sis capítols. En el primer es descriuen els conceptes generals relacionats amb un dispositiu mi-SOFC i els principals reptes associats que presenta el seu desenvolupament. El segon es centra en els procediments experimentals i les tècniques de caracterització utilitzades al llarg de la tesi. El tercer capítol presenta l'anàlisi tèrmica del sistema proposat mitjançant simulacions termofluídiques. El quart mostra la fabricació i caracterització d'una unitat de processament de combustible capaç de produir hidrogen a partir del reformat d'etanol i metà. El cinquè descriu una unitat catalítica de combustió. I l'últim capítol, es centra en les capes funcionals d'una pila de combustible mi-SOFC completament ceràmica., [eng] In the last decades, energy requirements of portable devices are exponentially increasing while the capacity of the current battery technology is not progressing accordingly. This energy gap claims for the development of new technologies beyond Li-ion. Novel miniaturized devices able to efficiently operate on the low power regime (1 — 20 W) in continuous mode by using a fuel are receiving increased attention. Due to their long lifetime, high power density and integrability, probably the most promising alternative is the development of micro fuel cells. Amongst them, micro Solid Oxide Fuel Cells mi-SOFCs) present the highest values of specific energy densities (by unit mass and volume), mainly due to their higher operating temperature and their capability of operating directly on hydrocarbon fuels. One of the most promising approaches for the mi-SOFCs is based on the monolithic integration of functional free-standing electrolyte membranes in silicon technology. This approach ensures high reproducibility and reliability, cheap mass production and easy integration to mainstream technology. This thesis encompasses the design, fabrication and test of the main components of a novel mi-SOFC power generator as a first step to develop a complete device in the near future. The adopted approach is based on the use of MEMS fabrication methods to miniaturize mi-SOFCs in silicon technology and high energy density hydrocarbons as fuels. The mi-SOFC power generator is designed to supply 1W of electrical energy in a small volume (10-20 cm3). The work developed is divided into six chapters. The first chapter introduces the basics and challenges of a mi-SOFC power generator. The second chapter focuses on the experimental procedures and characterization techniques used. In the third chapter, the thermal analysis of a new mi-SOFC power generator with finite-volume simulations is presented. The fourth chapter shows the fabrication and characterization of a fuel processing unit capable to produce hydrogen from ethanol steam reforming and methane dry reforming. The next chapter is related to a catalytic micro-machined combustor. Finally, chapter six presents the development of a full ceramic mi-SOFC.

This thesis is devoted to prove the concept of the CO(2) reduction to CH(4) with a decreasing in the voltage requirements using a photocatalytic mechanism. Subsequently, part of the solar energy is transferred to the reaction, obtaining an improvement in the total energy balance. The work developed intends first, to take advantage of the know features of the photoactive nanostructured materials obtained by anodization and hydrothermal synthesis (allowing to obtain better surface areas and improving the photon collection, light photosynthetic reactions). Second investigate the copper and copper oxide cathodes for the CO(2) electroreduction activity to CH(4) (dark photosynthetic reactions) using a complete cell to understand the parameters involved in the process and the products selectivity for each cathodes. And third the implementation of the photoanode and cathode in a photoelectrochemical complete cell. Respect the photoactive materials we are going to talk about TiO(2) based nanostructured materials for water splitting. The first TiO(2) nanostructuration under study are nanotubes obtained by anodization of a Titanium foil using organic electrolytes. The TiO(2) crystal phase obtained by this technique was anatase. The next step in this material was the surface modification to improve the efficiency. To obtain this improvement the anodization process was done using two electrolytes in different steps. As sequence a porous surface with an increment in the surface area was obtained. After, the photoelectrochemical measurements were done in 1 M of sodium hydroxide (NaOH) under AM 1.5G illumination source to observe the photoactivity of these samples. The second nanostructured materials under study were TiO(2) nanorods obtained by hydrothermal synthesis over a conductive glass substrate, Fluorine Tin Oxide (FTO). The nanorods using this technique have rutile structure. An optimization of two parameters involved in the hydrothermal synthesis was studied: (1) initial titanium precursor concentration and (2) increasing the chlorine concentration to obtain larger and thinner rods. To enhance the photoactivity of TiO(2) we try to incorporate other materials inside the structure. The materials selected were: tin which improves the charge carriers, vanadium which allows the absorption in the visible range and nitrogen doping to enhance the efficiency in the photoactivity of the material. Concerning to methane production study, a discussion about the electrochemical CO(2) reduction activity over a copper based electrode using a hydrogen carbonate as supporting electrolyte was done, where the positive ions used are sodium and potassium. The first electrode selected is a pristine copper due to the interest of the methane production. The samples were characterized by scanning electronic microscopy (SEM) and X-Ray diffraction (XRD) to visualize the surface morphology and the crystal structure of the electrode. Afterwards, the electrochemical process is studied to understand the activity of these electrodes. Chronopotentiostatic (CP) experiments were done at different current densities to observe the activity as a function of the reached potential. The second electrode under study was a copper oxide cathode. In the electrochemical experiments an effect was studied related to the electrochemical reduction of the different copper oxide layers generated during the thermal synthesis, leading to a catalytically active copper that enables carbon dioxide reduction. With this type of electrodes a time- dependence test was done to carefully study these crystallographic changes. Finally, another important variable for CO(2) conversion was studied, the humidification of the CO(2) gas stream before the introduction in the electrochemical cell with the impact on the faradaic efficiency of the process. In the implementation in the PEC cell an evaluation of the photoanode and cathode was done. In this evaluation, the external potential requirements were studied concerning about the energy consumption and the benefit from the photoactivated process., Esta tesis se ha desarrollado con el objetivo de probar el concepto de la reducción del dióxido de carbono a metano, mediante una reducción de los potenciales necesarios usando un mecanismo fotocatalítico. Parte de la energía solar es transferida a la reacción obteniendo una mejora en el balance energético total. El trabajo desarrollado se focaliza primero en el estudio de materiales nanoestructurados fotoactivos basados en dióxido de titanio obtenidos por anodización, generando nanotubos, y por síntesis hidrotermal obteniendo nanohilos sobre un sustrato conductor transparente, los cuales permiten obtener mayores superficies activas mejorando la colección de fotones, similar a las reacciones luminosas en la fotosíntesis. En segundo lugar, se ha estudiado la electroreducción del dióxido de carbono a metano usando cátodos de cobre y oxido de cobre (similar a las reacciones oscuras de la fotosíntesis). Usando el cobre como cátodo, se ha observado la obtención de metano a diferentes densidades de corriente aplicadas para poder observar la productividad respecto al potencial medido. Para el caso de los cátodos de óxido de cobre, no se ha encontrado producción de metano pero si de etileno. En estos cátodos se ha observado un efecto proveniente de la reducción de las capas de los diferentes óxidos de cobre, generados en la síntesis térmica, hacia un cobre catalíticamente activo para la reacción de reducción del dióxido de carbono. Este efecto se ha estudiado profundamente mediante un estudio de los cambios cristalográficos y superficiales a determinados tiempos. Finalmente, se ha estudiado el efecto de la humidificación del dióxido de carbono (gas) previa a la entrada a la celda electroquímica. Como parte final se ha realizado una evaluación energética de los fotoánodos generados por síntesis hidrotermal y de los cátodos basados en cobre estudiados, para poder implementar ambos en una celda fotoelectroquímica completa. En esta parte se ha estudiado los valores de los potenciales externos necesarios para que se pueda dar la reacción, asumiendo un 100% de eficiencia hacia la producción de metano para los cátodos de cobre y de etileno para los de óxido de cobre.