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[EN] The development of energy storage devices with higher energy efficiency is necessary to achieve a sustainable energy system based in renewable energy sources. Vanadium redox flow batteries (VRFB) are standing out among the variety of energy solutions for stationary storage, nevertheless, one of the main device performance limiting factors are the electrodes, which need to be optimized. The main goal of this thesis is the modification of carbon fibre felts, typically used as electrodes in VRFB, which in turn improve their electrochemical performance towards the vanadium redox reactions during the battery charge and discharge. Two different pathways to improve the carbon fibre felt have been proposed in order to obtain novel, efficient and easily scale-up materials. The first method consisted of the incorporation of carbon nanomaterials (graphene materials or multiwalled carbon nanotubes) on the fibre surface by electrophoretic deposition (EPD). The comparison of the electrochemical performance between the modified electrodes and an electrode modified under the same experimental conditions but without nanomaterial deposition, revealed that the key factor for the improvement of the felt electrocatalytic activity is the underlying oxidation suffered by fibres during the EPD treatment., These results promoted the investigation on optimised electrochemical modification of carbon felt electrodes. In a second study, carbon felts were modified by electro-oxidation and electro-reduction treatments in different acidic media, seeking to study the influence of the developed oxygen containing functional groups on the electrochemical performance of the electrodes. The results concluded that the depth and the oxidation degree of the fibres are key parameters that affect the catalysis of vanadium reactions. Another challenge faced by the scientific community working in this field is the need to define a standardized method for the electrochemical characterization of electrode materials. The limited information regarding the used experimental conditions together with the lack of similitudes on cell geometries and electrode configuration impede the comparison between different results published. In this thesis a reproducible and reliable protocol for the electrochemical characterization of carbon-felt materials has been proposed. The compression degree of the felt and the electrode configuration setup have been studied, evaluating the influence of both parameters on the resistance and the electrochemical performance of graphitized carbon felts towards vanadium reactions redox, finally demonstrating a clear impact on both parameters., [ES] El cambio hacia un modelo energético más sostenible, basado en el uso de fuentes de energía renovable, hace necesario el desarrollo de dispositivos de almacenamiento con mayores eficiencias energéticas. Las baterías de flujo redox de vanadio (BFRV) se abren camino como parte de la solución energética para el almacenamiento estacionario, sin embargo, uno de los principales factores que limitan su rendimiento son los electrodos que integran estos dispositivos, siendo necesaria su mejora. Esta Tesis Doctoral tiene como objetivo principal la modificación de fieltros de carbono, empleados como electrodos en BFRV, buscando mejorar su actividad electroquímica frente a las reacciones involucradas en la carga y descarga de la batería. Se han propuesto dos vías de mejora de los electrodos para la obtención de nuevos materiales fácilmente escalables y eficientes. Una primera vía consiste en la incorporación de nanomateriales de carbono (materiales grafénicos o nanotubos de pared múltiple) sobre fieltros de carbono comerciales mediante depósito electroforético (EPD). La comparación del comportamiento electroquímico de estos materiales con un electrodo sometido a las mismas condiciones experimentales, pero sin depósito de nanomaterial, reveló que el factor determinante en la mejora de la actividad electrocatalítica de los fieltros es la oxidación subyacente que sufren las fibras durante el tratamiento de EPD., Los resultados obtenidos en el estudio anterior motivaron la investigación sobre modificación electroquímica de fieltros de carbono. En un segundo trabajo los electrodos fueron sometidos a tratamientos de electro-oxidación y electro-reducción en diferentes medios ácidos, evaluando la repercusión de los grupos funcionales oxigenados desarrollados en el rendimiento electroquímico de los electrodos. Los resultados obtenidos permitieron concluir que la profundidad y el grado de oxidación de las fibras son parámetros clave que afectan a la catálisis de las reacciones de vanadio. Otro de los desafíos al que se enfrenta la comunidad científica es la necesidad de definir un método estandarizado para la caracterización electroquímica de materiales de electrodo. La falta de información acerca de las condiciones experimentales empleadas, junto con la falta de uniformidad en la geometría/configuración de celda ensayada limita la comparación cuantitativa entre resultados publicados. En la presente Tesis se propone un protocolo reproducible y fiable para la caracterización electroquímica inicial de materiales de electrodo. Se ha estudiado tanto el efecto del grado de compresión del fieltro como la configuración espacial de los electrodos en la celda de medida, y se ha analizado su influencia en la resistencia del sistema y en la respuesta electroquímica de fieltros de carbono frente a las reacciones redox de vanadio, demostrándose una clara influencia de ambos parámetros

Los supercondensadores (SC) son dispositivos importantes para el almacenamiento de energía debido a las ventajas que presentan sobre las baterías, como su mayor densidad de potencia y su excelente ciclabilidad. El principal reto de la investigación aplicada a estos SC consiste en aumentar su densidad de energía. Los líquidos iónicos (LIs) tienen un gran potencial como electrolitos en dispositivos electroquímicos por presentar una amplia ventana de estabilidad electroquímica, lo que permitiría obtener SC de alta densidad de energía. No obstante, hoy en día, aún no hay información suficiente acerca de los diferentes mecanismos de almacenamiento de energía implicados en la formación de la doble capa en estos supercondensadores de carbono con líquidos iónicos como electrolito. Por ello, en la presente memoria se estudian las interacciones de los iones de dos líquidos iónicos ampliamente utilizados con diferentes materiales de carbono de alta superficie.

We have designed a lab made extruder for direct ink writing (DIW) to be incorporated into a conventional fused deposition modelling (FDM) 3D printer. The STP and STL files required for printing the different parts of such extruder can be downloaded here. Instructions for their assembly and functioning are detailed in the paper.

The energy storage mechanism operating in carbon-based supercapacitors using ionic liquids as electrolytes is not yet fully understood. In this paper, the interactions of ions of two widely used ionic liquids, i.e. EMImTFSI and EMImBF4, with a high specific surface area microporous carbon are investigated. Galvanostatic cycling experiments performed on each electrode and in the full cell demonstrate that a specific energy close to 46 Wh kg-1 can be achieved with this carbon. More interestingly, impedance spectroscopy studies reveal the presence of some unusual behavior, such as the presence of inductive elements in some of the electrodes, either in the positive or in the negative electrode. These inductive elements are identified, for the first time, as another type of possible experimental evidences of some phenomena previously proposed, such as ion-swapping or co-ions desorption, both phenomena leading to the formation of the superionic state.

Hydrogen as an energy carrier plays a significant role in tackling energy transition challenges. Its production via water electrolysis can be powered by renewable sources of energy and it has been identified as the key to a secure and sustainable energy system. Therefore, the development of efficient and abundant electrocatalysts is very important to realize the required large-scale production. In this context, transition metals have been postulated as promising alternatives to noble metal oxides for water splitting in alkaline media. The efficient incorporation of these metals into supports can improve their distribution and particle size control, making carbon supports an ideal choice due to their high conductivity and electrochemical stability. In this work, reduced graphene oxide 3D aerogels doped with very low concentrations of nickel were prepared by freeze-casting. A fully water-based approach that enables an outstanding integration of nickel hydroxide precursor in a colloidal graphene oxide (GO) aqueous suspension was developed. The as-prepared Ni/graphene 3D networks were extensively characterized showing a low loading of nickel (<10 wt%), and ultradispersed, and nanosized Ni-based particles (15-40 nm). Electrochemical experiments show that Ni/graphene 3D networks exhibit very good catalytic properties towards the oxygen evolution reaction (OER) and outstanding Fe-ion activation from the impurities present in the alkaline (KOH 1 M) electrolyte media. Previous studies have shown that Fe incorporation can increase two-fold the activity of Ni-based electrocatalysts. In contrast, the ultradispersed Ni/graphene hybrid prepared here exhibits an impressive over ten-fold performance increase, highlighting the remarkable influence of Fe in these materials. The high-dispersion and surface availability of Ni species promotes the in situ formation of highly active Ni-Fe oxyhydroxide on the surface of the catalysts.

The energy efficiency of vanadium redox flow batteries (VRFB) greatly depends on the electrochemical performance of the electrodes that constitute these devices, so it is essential to develop more efficient electrode materials with optimized structure and physicochemical properties. Carbon-based felts are commonly used as electrodes in such devices due to their high chemical stability in acid media, high electrical conductivity and their three-dimensional structure. However, they show low reversibility and electrocatalytic activity, especially for the VO2+ / VO2+ redox couple. In the quest for improving the electrochemical behaviour of carbon-based felts, it has been proposed to modify them with functionalized carbon nanotubes or graphene oxide by electrophoretic deposition. To elucidate if the improvements achieved on these electrodes can be really ascribed to the presence of the carbon nanomaterials or to the oxidation undergone by the fibers during this treatment, the electro-oxidation of a pristine rayon-based graphitized carbon felt was performed in parallel. The electro-oxidized felt exhibits the best behaviour, suggesting that the improved performance obtained for the electrodes modified with nanocarbons are mainly influenced by the oxidation of the fibers.

A hybrid SC prepared with mesoporous carbon as the negative electrode, LiFePO4 as the positive electrode, and a LiTFSI/imidazolium ionic liquid solution as electrolyte is presented. The cell was conceived on the basis that it offers all of the safety features of ionic liquids (IL) and LiFePO4, in addition to the advantages of a high energy density device. Most of the high performance hybrids so far reported in the literature employ aqueous or organic electrolytes, whereas studies of hybrid cells based on IL are still rare. Here, a fundamental study was conducted to understand how the different interfaces and mechanisms operate in a hybrid system based on IL electrolyte and how this affects cell performance. This device was mainly characterized using cyclic chronopotentiometry that allows cell voltage and electrode potentials to be simultaneously recorded. By means of this technique, it was possible to evaluate the overall behavior of the hybrid cell and the faradaic and capacitive electrodes simultaneously and to compare it with the performance of selected standard cells. The results show that the cell is able to attain an energy density of 43.3 W h kg–1 at 0.010 A g–1 (C/5 in relation to LiFePO4), while maintaining a good cycling performance.

Activated carbons (AC) with excellent textural properties have been obtained for the first time from waste polystyrene foam (PF), without any previous treatment, following a simple and conventional two-step procedure (formation of char followed by chemical activation). Even considering that the PF is not a graphitizable material, the best AC produced from this precursor has a very high BET surface area larger than 2700 m2 g−1 and a pore volume of 1.2 cm2 g−1, with a significant contribution of small mesopores. As a consequence, this AC reveals a surprising capacity to adsorption of relatively large molecules and a high specific capacitance when applied as a supercapacitor electrode. The maximum amount of adsorbed methylene blue obtained by batch equilibrium experiments are greater than 1 g g−1. In the context of the technical difficulties and low economic return of the reuse and recycling of waste PF, this work offers a strategic destination for this environmentally unfriendly residue.

The mechanism of operation of a supercapacitor based on methylene blue (MB) as redox shuttle in cells containing activated carbon as electrodes and aqueous electrolyte has been studied. Attention is focused on the instability of these molecules which can undergo secondary reactions that impact negatively on the cell, generating fluctuations in its performance and reducing its cycling life. It is demonstrated that MB interacts strongly with activated carbons and modifies their structure, decreasing the resistance of the electrodes and increasing the energy density of the cell by more than 40%. It is also shown that MB is not stable in acidic electrolyte at the high potentials attained by the electrodes of the supercapacitor. Demethylation byproducts and oxidized derivates of the MB were detected after cycling by means of electrospray ionization mass spectrometry. These products were observed to cause a constant change in the equilibrium potential of the cell, substantially modifying the storage mechanism and the ranges of operation potential of the electrodes.

This study aims to increase the knowledge on the interactions that occur at the electrode/electrolyte interface in carbon-based electric double-layer capacitors (EDLCs) when solvent-free ionic liquids are used as electrolytes. Many previous studies found in the literature are conducted using theoretical approaches, and they are unable to model all the variables and the complexity of an actual device with a complex carbon surface and an ionic liquid (IL). Here, the compatibility between imidazolium ionic liquids and different carbon materials-an activated carbon (AC), a mesoporous carbon (MES), multiwalled carbon nanotubes (MWCNTs), and reduced graphene oxide (RGO)-is empirically investigated applying synchronous chronopotentiometric tests to various symmetrical EDLCs. The study of the simultaneous evolution of the cell and electrode potentials of the various carbon/ILs cells, monitoring the evolution of specific capacitances and electrical resistances for each independent electrode, allows inferring about the ion-electrode compatibility, the limiting factors for charge accumulation, and its impacts on the performance of the global cell. The results indicate that the sp2 structures of MWCNTs and RGO favor interactions with the EMI+ cation on the negative electrode. In the positive electrodes, MES and AC favor interactions with the BF4- and TFSI- anions, respectively, yielding a higher specific capacitance and lower resistance.

Biological membranes are currently used in Ophthalmology in order to treat different ocular disorders. These membranes have different properties such as cellular biocompatibility and promoting wound healing. Moreover, intrinsic antimicrobial properties could also be desirable because it would allow their use reducing the risk of infections. Graphene and its derivatives are promising biomaterials that already proved their bactericidal effect. However, their clinical use is limited due to the controversial results regarding their toxicity. In this work, we have developed and characterized a reduced graphene oxide membrane (rGOM) for its use in ocular Regenerative Medicine, and studied its in vitro and in vivo biocompatibility and genotoxicity with different types of human ocular cells. We proved that rGOM allowed the growth of different ocular cells without inducing in vitro or in vivo cytotoxicity or genotoxicity in the short-term. These results indicate that rGOM may be a promising candidate in Regenerative Medicine for the treatment of different ocular pathologies.

A novel strategy for the development of high energy density supercapacitors (SC) is explored. The new devices contain two electrolytes that are different in nature, an aqueous electrolyte in one half-cell and an ionic liquid in the other. These systems, referred to as biliquid SCs, make it possible to combine the best electrode/electrolyte in terms of individual electrode capacitance and cathodic/anodic stability limits. Furthermore, other asymmetries that combine different electrodes, electrolytes and storage mechanisms can be incorporated. Synchronous cyclic chronopotenciometry was used to monitor the performance of these novel complex systems. It is expected that this approach will help scientists working in this field to develop new devices that overcome the energy density limitations of current SCs, thereby extending the scope of their application.

The presence of nanomaterials in our everyday life is ever increasing, and so too are concerns about the possible health consequences of exposure to them. While evidence of their biological activity is growing, there is still scant knowledge of the epigenetic mechanisms that could be at play in these processes. Moreover, the great variability in the chemical and physical structures of these compounds handicaps the study of their possible health risks. Here we have synthesized reduced graphene oxide (rGO) through the thermal exfoliation/reduction of graphite oxide, and characterized the resulting material. We have then made use of Illumina's MethylationEPIC arrays and bisulphite pyrosequencing to analyse the genome-wide and global DNA methylation dynamics associated with the medium-term exposure of human lung epithelial cells to rGO at concentrations of 1 and 10 µg/mL. The results show no genome-wide or global DNA methylation changes associated with either condition. Our observations thus suggest that medium-term rGO exposure does not have significant effects on the DNA methylation patterns of human lung epithelial cells.

[EN] The aim of this doctoral thesis is to increase the energy stored in carbon based supercapacitors (CBSCs). For this end, asymmetric cell configurations were developed and combined with the use of inorganic electroactive species either dissolved in aqueous electrolytes (redox electrolytes) or deposited on the active electrode materials have been used. This made it possible to actuate simultaneously on the capacitance and operating voltage of the systems, the two parameters that most influence the amount of energy stored. While the energy storage mechanism in SCs is purely capacitive, in these systems there is also a faradaic contribution due to the development of the redox reactions of the electroactive species. Consequently, the systems studied present a hybrid energy storage mechanism: one of the electrodes shows a typically capacitive behavior (capacitor type), while on the other one the redox reactions occur (battery type). First, the use of Ce2(SO4)3 as inorganic redox electrolyte in asymmetric CBSCs was studied. The development of the redox reactions related to Ce+3/Ce+4, together with the optimization of the configuration of the device (active materials, mass ratio between electrodes, use of exchange-ion membranes) allowed the operating cell voltage to be increased up to 1.9 V, resulting in a significant increase in the amount of energy stored, comparable to values obtained in asymmetric SCs with organic electrolytes (20 W h kg- 1). In addition, these systems showed excellent cyclability, more than 20,000 charge/discharge cycles being necessary to observe significant changes in their electrochemical behavior. Moreover, the behavior of carbon materials modified with Prussian Blue as positive electrode in asymmetric CBSCs using a neutral electrolyte (KCl) was evaluated. In this case, the development of the redox reactions of the Prussian Blue deposit led to operating voltages of up to 1.4 V. As in the previous case, this voltage enhancement, together with th, [ES] Esta tesis doctoral aborda la mejora de la energía almacenada en supercondensadores basados en materiales de carbono (CBSCs). Para ello se desarrollaron configuraciones de celda asimétricas y se usaron especies electroactivas inorgánicas disueltas en electrolitos acuosos (electrolitos redox) o en forma de depósito sólido sobre los materiales de electrodo. Esto permitió actuar de forma simultánea sobre la capacidad de almacenamiento y el voltaje de operación del sistema, los dos parámetros que más influencia tienen sobre la energía almacenada por el mismo. Si bien el mecanismo de almacenamiento de energía de los SCs es puramente capacitivo, en estos sistemas existe también una contribución faradaica debida al desarrollo de las reacciones redox de las especies electroactivas. En consecuencia los dispositivos investigados presentan un mecanismo de almacenamiento híbrido: uno de los electrodos presenta un comportamiento típicamente capacitivo (tipo condensador), mientras que en el otro tienen lugar las reacciones de oxidación/reducción (tipo batería)., En primer lugar, se estudió el uso de Ce2(SO4)3 como electrolito redox inorgánico en CBSCs asimétricos. El desarrollo de las reacciones redox asociadas al Ce+3/Ce+4, junto con la optimización de la configuración de los dispositivos (materiales activos, relación de masas de los electrodos, empleo de membranas de intercambio iónico) permitió incrementar el voltaje de operación hasta 1,9 V, lo que se tradujo en un notable aumento de la energía, alcanzándose valores comparables a los obtenidos en SCs asimétricos con electrolitos orgánicos (20 W h kg-1). Además, estos sistemas mostraron una excelente ciclabilidad, siendo necesarios más de 20.000 ciclos de carga/descarga para observar cambios significativos en su comportamiento electroquímico. Además, se evaluó el comportamiento de materiales de carbono modificados con Azul de Prusia como electrodos positivos en CBSCs asimétricos empleando un electrolito neutro (KCl). En este caso, el desarrollo de las reacciones redox del depósito de Azul de Prusia permitió alcanzar voltajes de operación de hasta 1,4 V. Como en el caso anterior, este aumento de voltaje, junto con la mejora en los valores de capacidad, supuso un incremento en los valores de energía del sistema de 1,5 a 8,0 W h kg-1.

N-enriched activated carbon fibers were successfully prepared from a coal derived pitch blended with urea-formaldehyde resin. The mixture was melt-spun, stabilized, carbonized and activated with KOH, giving rise to microporous carbon fibers with high nitrogen content (up to 2.2%). Nitrogen functional groups were mostly identified as quaternary N, and the content in this nitrogen group decreased with the degree of activation. N-enriched fibers activated under mild conditions (KOH/fiber = 5/1) showed the highest CO2 uptake (3.4 mmol CO2 g-1 at 25 °C and 1 atm). Due to their high alkalinity, the N-enriched fibers showed high CO2 adsorption rates and good selectivities for CO2/N2 and CO2/CH4 separation. After saturation with CO2, the fibers could be easily regenerated and maintained their performance over several adsorption-desorption cycles.

The direct chemical activation of pitch-based stabilized carbon fibers (uncarbonized) with a conventional activating agent (KOH) has been studied for the first time. Unlike the traditional activation method, this shorter route merges the carbonization and activation steps into one simple thermal treatment. It also requires up to a 70% less activating agent for a given porous structure, which can reach values of SBET>1700m2g-1. In addition, the overall yield of the activation process is increased. The advantages obtained open up the way to a novel environmentally friendly industrial method of preparing activated carbon fibers.

Structural flexibility in a petroleum pitch-derived carbon material has been indirectly evaluated using X-ray diffraction (XRD), immersion calorimetry and inelastic neutron scattering (INS) measurements. Exposure of the carbon material to an organic solvent (e.g., n-nonane) gives rise to a large internal rearrangement, associated with a drastic re-ordering of the graphitic microdomains. These structural changes are also associated with a high flexibility of the internal porous network, as observed by inelastic neutron scattering measurements. The internal rearrangement and the structural flexibility could be responsible for the excellent performance of this kind of activated carbons in a wide variety of adsorption processes. Last but not least, the structural characteristics of these carbon materials composed of graphitic microdomains has been used to synthesize graphene “egg-like” flakes following a simple procedure based on exfoliation with organic solvents.

This work describes the preparation of isotropic carbon fibres from three anthracene oil-derived pitches of different softening points. The total absence of solid particles (e.g., primary quinoline insoluble (QI) particles) and the high aromatic composition turn these precursors into a promising raw material for the preparation carbon fibres, as they can be used without any pre-treatment. The carbon fibres were prepared by a melting process that involved the spinning of the pitches and subsequent oxidative stabilization with air and carbonisation. Scanning electron microscopy was used to monitor the surface appearance and diameter of the fibres. The results showed that softening point by itself cannot explain the spinning behaviour of the pitches. Therefore, detailed characterisation of the samples by means of size exclusion chromatography, UV-fluorescence spectroscopy, and laser desorption time-of-flight mass spectrometry was used to elucidate the composition of the pitches, especially their molecular weight distributions. The combination of the characterisation of the precursors with the characterisation of the fibres provides an overall vision of the carbon fibre preparation from the pitches and will certainly permit a pre-selection of the most appropriate precursor for this application. © 2010 Elsevier Ltd. All rights reserved.

The suitability of new environmentally-friendly coal-based products for the preparation of cokes with a highly oriented microstructure is investigated. These products (AO-A and AO-B) are obtained from two different stages of the processing of an industrial anthracene oil. The composition and degree of polymerization determine their ability to produce cokes with highly oriented textures. The transformation of AO-A into a highly oriented coke requires pyrolysis temperatures of 450-460°C, residence times of 8-20 h and a pressure of 5 bar, whereas AO-B, which is more polymerized, requires lower temperatures (420-440°C) and shorter residence times (8 h) without the application of any external pressure. The larger extent of highly oriented microstructures present in cokes from AO-A is explained from the greater hydrogen transfer capacity during the coking process. © 2012 Elsevier Ltd. All rights reserved.

A coal-tar-derived mesophase was chemically activated to produce a high surface area (∼3200 m2/g) carbon with a porosity made up of both micropores and mesopores. Its adsorption capacities were found to be among the highest ever reported in literature, reaching values of 860 mg/g and 1200 mg/g for the adsorption of benzene and toluene, respectively, and 1200 mg/g for the combined adsorption of benzene and toluene from an industrial wastewater. Such high values imply that the entire pore system, including the mesopore fraction, is involved in the adsorption process. The almost complete pore filling is thought to be due to the high relative concentrations of the tested solutions, resulting from the low saturation concentration values for benzene and toluene, which were obtained by fitting the adsorption data to the BET equation in liquid phase. The kinetics of adsorption in the batch experiments which were conducted in a syringe-like adsorption chamber was observed to proceed in accordance with the pseudo-second order kinetic model. The combined presence of micropores and mesopores in the material is thought to be the key to the high kinetic performance, which was outstanding in a comparison with other porous materials reported in the literature.

The wetting behaviour of a petroleum pitch (modified with a surfactant) and a binder coal-tar pitch was studied using different substrates. The results show that both the pitch and the substrate have a significant influence on wetting behaviour, and consequently, on their mutual interactions during the mixing stage. Low values of surface tension and viscosity in the pitches lead to lower wetting temperatures. It was found that with petroleum coke and magnesia as substrates, the wetting occurs at lower temperatures than with graphite and carbon black for all pitches. Moreover, experiments carried out with amorphous and crystalline alumina revealed that the crystalline order affects the wetting behaviour considerably. Thus, whereas the pitch wets the corundum (crystallized alumina), it does not wet the amorphous alumina (basic, acid and neutral alumina). © 2010 Elsevier B.V.

Los compuestos de carbenos N-heterocíclicos (NHC) de iridio son capaces de reducir enlaces C=O y C=N para generar alcoholes y aminas en condiciones suaves siguiendo los mecanismos de la transferencia de hidrógeno. Las excelentes propiedades de los nanotubos de carbono (CNTs) los vuelven adecuados candidatos para el soporte de dichos compuestos organometálicos. Sin embargo, los diferentes métodos de producción de los CNTs generan impurezas metálicas y carbonosas que deben ser eliminadas. Generalmente se emplean tratamientos ácidos oxidantes, los cuales modifican los CNTs desarrollando una química superficial oxigenada. Estos grupos funcionales se pueden emplear en la unión covalente de los compuestos NHC a los materiales. En este trabajo se purificaron CNTs aplicando cuatro tratamientos de severidad oxidativa creciente, como son ácido clorhídrico, una mezcla de hidróxido amónico y peróxido de hidrógeno, ácido nítrico diluido y peróxido de hidrógeno, y una mezcla de ácidos nítrico y sulfúrico concentrados en caliente. La oxidación gradual generó una progresiva funcionalización oxigenada, junto con la introducción de defectos, a lo largo de las paredes de los CNTs. En los grupos carboxilo se anclaron compuestos NHC de iridio mediante uniones acetilo a las unidades N-metilimidazolio, con posterior fijado del metal. Se obtuvo un grado de funcionalización proporcional al grado de oxidación de los nanotubos, tanto en el contenido de nitrógeno (análisis elemental) como en el de iridio (ICP-MS). Los análisis microscópicos y de EXAFS descartaron la presencia de nanopartículas, revelando además la existencia de un enlace directo entre la química oxigenada del nanotubo y el metal. Los materiales híbridos desarrollados se ensayaron como catalizadores en la reducción de ciclohexanona a ciclohexanol siguiendo los mecanismos de la transferencia de hidrógeno, siendo la fuente de hidrógeno un medio de 2-propanol/KOH. Todas las muestras fueron activas, pero la actividad catalítica se

CVD-grown multiwalled carbon nanotubes were purified by applying four different treatments with increasing oxidation severity. The growing severity of the treatment results in progressive oxygen functionalization of the surface along with introduction of an increasing quantity of defects on the carbon nanotube walls. Iridium-N-heterocyclic carbene complexes were covalently anchored to those oxidized surfaces through their surface carboxylic acids via acetyl linkers. The carbon nanotube-based iridium-NHC hybrid materials developed are active in the hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source but with rather different activity. The catalytic activity of the hybrid catalysts is strongly influenced by the type and amount of oxygenated functionalization resulting from the treatment applied to the support, being the most active and the most oxidized material.

The development of more efficient electrode materials is essential to obtain vanadium redox flow batteries (VRFBs) with enhanced energy densities and to make these electrochemical energy storage devices more competitive. A graphene-modified graphite felt synthesized from a raw graphite felt and a graphene oxide water suspension by means of electrophoretic deposition (EPD) is investigated as a suitable electrode material in the positive side of a VRFB cell by means of cyclic voltammetry, impedance spectroscopy and charge/discharge experiments. The remarkably enhanced performance of the resultant hybrid material, in terms of electrochemical activity and kinetic reversibility towards the VO2+/VO2+, and mainly the markedly high energy efficiency of the VRFB cell (c.a. 95.8% at 25 mA cm−2) can be ascribed to the exceptional morphological and chemical characteristics of this tailored material. The 3D-architecture consisting of fibers interconnected by graphene-like sheets positively contributes to the proper development of the vanadium redox reactions and so represents a significant advance in the design of effective electrode materials.

Hybrid catalysts based on Iridium N-heterocyclic carbenes anchored to carbon nanotubes (CNT) have been studied by XAFS spectroscopy. Oxidation of CNT yields a large amount of functional groups, mainly hydroxyl groups at the walls and carboxylic groups at the tips, defects and edges. Different kinds of esterification reactions were performed to functionalize oxidized CNT with imidazolium salts. Then, the resulting products were reacted with an Ir organometallic compound to form hybrid catalysts efficient in hydrogen transfer processes. XANES spectroscopy agree with the presence of Ir(I) in these catalysts and the EXAFS spectra detected differences in the local structure of Ir atoms between the initial Ir organometallic compound and the Ir complexes anchored to the CNT. Our results confirm that the halide atom, present in the Ir precursor, was replaced by oxygen from -OH groups at the CNT wall in the first coordination shell of Ir. The lability of this group accounts for the good recyclability and the good efficiency shown by these hybrid catalysts.

A proper design of heterogeneous molecular catalysts supported on carbon materials requires a systematic study of >metal-carbon support interactions> and their influence on catalytic activity. In this study, hybrid materials containing covalently anchored iridium N-heterocyclic carbene (NHC) organometallic complexes have been successfully prepared from oxidized and partially reduced carbon nanotubes (CNTs). The preparation method for these supported materials relies on the selective functionalization of the superficial hydroxylic groups using the imidazolium salt, 1-(3-hydroxypropyl)-3-methyl-1H-imidazol-3-ium chloride. The hydrogen transfer catalysis activity of these nanotube-based hybrid catalysts was tested by the reduction of cyclohexanone to cyclohexanol with 2-propanol, and the results of the tests were compared with those obtained using similar hybrid graphene-based catalysts. While EXAFS analysis revealed a common first coordination shell of the iridium atom for all the hybrid materials examined, independently of whether they were either supported on carbon nanotubes or graphene materials, catalytic activity in all the reduced materials was significantly superior. Moreover, catalytic systems based on reduced CNTs exhibited a better performance than those based on reduced graphene materials. Both these facts suggest there is a positive correlation between hydrogen transfer catalytic activity, reconstruction of the aromatic carbon structure and the smaller amount of oxygen functional groups.

The mechanisms of pitch/coke interactions at the mixing stage were studied by a spreading drop test, using a bed of calcined petroleum coke as substrate. For this purpose, three petroleum pitches and a binder coal-tar pitch, which was selected as a reference, were used. The results show that pitch wetting behaviour is related to both pitch surface tension and pitch viscosity. Low values of surface tension and viscosity are required for the pitch to spread and penetrate into the coke bed at temperatures below 160 °C. The non-wetting behaviour observed in some petroleum pitches was ascribed to oxidation processes, as demonstrated by the fact that the non-wetting pitch behaviour can be improved by using an inert atmosphere and/or increasing the heating rate during the spreading drop test. Additionally, the use of blends of wetting and non-wetting pitches and the addition of active surface agents contributes significantly to modifying the wetting behaviour of non-wetting pitches. From the results obtained, a relationship between surface tension-viscosity and pitch wetting behaviour was established. © 2005 Elsevier Ltd. All rights reserved.

Carbon nanotubes and graphene have been extensively applied as proactive supports to generate heterogeneous catalysts. Both exhibit a carbon structure composed by carbon atoms with sp2 hybridization and both are used in similar applications, as in catalysis. However, the role of each particular substrate, determined by its structural peculiarities, can be differentvshould be studied for each particular catalytic reaction and in their structure.The aim of this work is to study the differences between oxidized carbon nanotubes (CNT) and graphene oxides (GO) as proactive supports of iridium Nheterocyclic carbene (NHC) catalyst. The effect of their inherent structure in the catalytic activity in hydrogen transfer reactions was studied in detail.

The effect of different N-doping treatments on the CO2 adsorption capacity and kinetics of activated carbons prepared from the same carbonaceous precursor was studied. Non-doped and N-doped activated carbon fibers with similar shape and diameter were considered in this study in order to discard the influence of particle size and morphology. The N-enriched fibers were obtained following different techniques, which led to activated fibers with different content, type and distribution of nitrogen. The porous texture was the main factor determining the CO2 adsorption capacity of the fibers, and treatment with ammonia at high temperature did not exert any positive influence. In contrast, fibers prepared from a pitch/nitrogenized resin blend showed a slightly enhanced CO2 adsorption capacity, which was ascribed to their homogenous distribution of nitrogen in the bulk material occupying internal positions of the carbon matrix. The kinetics of adsorption remarkably increased with the pH of the activated fibers, and the amount of CO2 adsorbed at short time (100s) by the N-enriched fibers was, regardless the methodology used for their N-enrichment, twice higher than that observed for the N-free fibers. These findings prove the suitability of N-enriched activated carbons for use in swing adsorption systems operating at short time cycles.

A multi-step thermal reduction process to produce graphene from graphite oxide was developed to optimize the degree of reduction, its BET surface area and its suitability as electrode in electrochemical energy storage devices. The procedure combines an initial flash pyrolysis to a temperature below 460ºC and a subsequent ramp-heating treatment up to the selected final temperature. With this methodology, the morphology of the graphene materials obtained, which have a great influence in its properties as electrodes, is controlled.

The energy density of carbon based supercapacitors (CBSCs) was significantly increased by the addition of an inorganic redox species [Ce2(SO4)3] to an aqueous electrolyte (H2SO4). The development of the faradaic processes on the positive electrode not only significantly increased the capacitance but also the operational cell voltage of these devices (up to 1.5 V) due to the high redox potentials at which the Ce3+/Ce4+ reactions occur. Therefore, in asymmetric CBSCs assembled using an activated carbon as negative electrode and MWCNTs as the positive one, the addition of Ce2(SO4)3 moderately increases the energy density of the device (from 1.24 W h kg−1 to 5.08 W h kg−1). When a modified graphite felt is used as positive electrode the energy density of the cell reaches values as high as 13.84 W h kg−1. The resultant systems become asymmetric hybrid devices where energy is stored due to the electrical double layer formation in the negative electrode and the development of the faradaic process in the positive electrode, which acts as a battery-type electrode.