Diseño de condensadores asimétricos híbridos en electrolito acuso mediante ZTC y carbones activados utraporosos

The use of two different materials as electrodes allows the construction of asymmetric and hybrid capacitors cells with enhanced energy and power density. This approach is especially well-suited for overcoming the limitations of pseudocapacitive materials that provide a huge capacitance boost, but in a limited potential window. In this work, we introduce the concepts and protocols that are required for a successful design of such systems, which is illustrated by the construction of an asymmetric hybrid cell where a zeolite-templated carbon and an ultraporous activated carbon have been combined.La combinación de dos electrodos de distintos materiales permite la construcción de celdas de superconsandores híbridas y asimétricas, obteniéndose notables mejoras en cuanto a su energía y potencias específicas. Esta estrategia de montaje es especialmente útil en el caso de usar un material pseudocapacitivo, cuya capacidad es notoria, pero limitada a un rango de potencial reducido. En este trabajo, introducimos los conceptos y protocolos necesarios para un correcto diseño de estos sistemas, lo que ilustramos con un ejemplo de la construcción de una celda híbrida asimétrica que combina un electrodo de carbón nanomoldeado con zeolitas y un carbón activado ultraporoso.The authors want to acknowledge the financial support of Ministerio de Economía y Competitividad (MINECO, Spain) through the research projects CTQ2012-31762, MAT2013-42007 and PRIPIBJP-2011-0766

Preparación de nanofibras de carbono y cerámica para aplicaciones en Ingeniería Química

Esta tesis presenta el uso de las técnica de electrospinning para preparar fibras y tubos de carbono y/o cerámica con diámetros submicrométricos, pudiéndose soportar nanopartículas de metales en sus superficies en el mismo paso de preparación. Se obtuvieron mallas de submicrofibras de óxidos de zirconio, silicio, de carbono y composites carbono-cerámico, estudiándose su aplicación como catalizadores y adsorbentes en reacciones de interés en la biorrefinería y en la eliminación de contaminantes

High temperature treatments of porous activated carbon

The use of biomass waste for the preparation of activated carbon is of great industrial interest for reducing costs and increasing the sustainability, especially in the field of energy storage. A high temperature treatment is required to obtain a more ordered carbon material, thus increasing its conductivity. However, this high temperature treatment entails as a disadvantage a significant reduction in porosity. Therefore, a method to prepare activated carbons with a high porosity development as well as high conductivity could be of great interest for many applications. The aim of this work is to analyze the possible influence of phosphorus compounds on the physical-chemical properties of different carbon materials thermally treated at relatively high temperatures (1600 ºC). With this goal, it has been prepared activated carbons from different precursors (olive stone, lignin and hemp) and different conformations (powder, fibers and monoliths) by physical and chemical activation, with CO2 and H3PO4, respectively. Once the different activated carbon materials were prepared, they were thermally treated at 1600 ºC under inert atmosphere. The different samples were characterized by N2 and CO2 adsorption at 77 and 273 K, respectively, XPS, XRD and Raman techniques. The oxidation resistance was also evaluated in a thermogravimetric balance. High temperature treatments of activated carbon without the presence of P surface groups produced an important contraction of the porosity (from 900 to 150 m2 g-1). However, temperature treatments of phosphorus-activated carbon allowed for preparing carbon materials with a relatively high structural order and a well-developed porosity (c.a. 1100 m2 g-1), with a significant contribution of mesoporosity. These results suggest that these P-surface groups are responsible for the low contraction observed for the porous structure, avoiding, in a large extent, its collapse.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO (CTQ2015-68654-R). MINECO (PTA2015-11464-I)

Modification of the morphology, porosity and surface chemistry of lignin-based electrospun carbon materials

Lignin is a biopolymer that can be found as the main component of plants. It is obtained as a coproduct in the papermaking and biofuel industries. Owing to its high carbon and aromatic content, high availability and reduced cost, it is an excellent precursor for the preparation of highly valued carbon materials. Electrospinning is a suitable top-down technique for the preparation of polymeric fibers using high voltage electrical fields and polymer solutions of proper viscosity and conductivity. Organosolv lignins, which are extracted from lignocellulosic biomass using organic solvents, are soluble in ethanol, obtaining a solution that matches the requirement of the electrospinning process. In this way, it is possible to produce lignin-based porous carbon fibers using a coaxial electrospinning device [1]. This contribution summarizes our findings about the preparation of carbon materials with different morphologies and composition by processing lignin using electrohydrodynamic forces. Lignin spheres, beaded fibers, straight fibers, beaded tubes and straight tubes are obtained by using coaxial and triaxial spinnerets that allows the electrospinning of two or three different solutions at once [1], Fig. 1. Thermal stabilization in air is needed in order to avoid melting of lignin fibers during carbonization. Stabilization times of 48-96 hours are usually required in this step, decreasing the sustainability of the production process. Phosphoric acid can be added in small amounts in the lignin solution, shortening the time for achieve a successful thermostabilization of the fiber [2]. The carbonized materials show narrow microporosity and large surface area values (SBET from 600 to 1000 m2g-1) and additional pore size and volume can be developed by controlled gasification.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. This work was supported by the Spanish Ministry of Economy and Competitiveness and FEDER (CTQ-2015-68654-R)

A perspective on the preparation of value-added carbon materials from lignin

The thermochemical conversion of lignin into different added-value carbon materials constitutes an alternative approach for valorization of this co-product that can be integrated in pulping and biorefinery processes. Such approach is based on the relatively high carbon content and the abundance of aromatic rings in the structure of raw and technical lignins. In this way, our research group have been preparing carbon molecular sieves, activated carbons, electrospun nanofibers, nanostructured and highly ordered carbons from different types of lignins during the last three decades.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Methanol dehydration on carbon-based acid catalysts

Methanol dehydration to produce dimethyl ether (DME) is an interesting process for the chemical industry since DME is an important intermediate and a promising clean alternative fuel for diesel engines. Pure or modified γ-aluminas (γ-Al2O3) and zeolites are often used as catalysts for this reaction. However, these materials usually yield non desirable hydrocarbons and undergo fast deactivation. In this work, we study the catalytic conversion of methanol over an acid carbon catalyst obtained by chemical activation of olive stone with H3PO4. A significant amount of phosphorus remains over the catalyst surface after the activation process, mostly in form of C-O-PO3 and C-PO3 groups, which provide the carbon a relatively high surface acidity and oxidation resistance. Methanol decomposition on this catalyst yields selectivities to DME higher than 82% at 623K and methanol conversion of 34%, under the operating conditions studied. The activated carbon catalytic activity and stability, under inert and oxidant atmospheres, as well as different regeneration procedures, were studied. In the absence of oxygen, the catalyst suffers a progressive deactivation by coke deposition on the active acid sites (Fig. 1). The presence of oxygen modifies the carbon surface chemistry, probably through oxygen spillover on the catalyst surface, where the availability of labile oxygen avoids catalyst deactivation. A reaction mechanism has been proposed where methanol dehydration seems to proceed through an Eley-Rideal mechanism, which assumes the adsorption of water and oxygen spillover on the acid active sites, avoiding coke deposition

Highly conductive microporous carbon fibers by electrospinning of lignin/phosphoric acid/ethanol solutions

This contribution reports the preparation of electrospun lignin-based carbon fibers at different carbonization temperatures and the influence of heat treatments at temperatures ranging from 900 to 1600 ºC. The influence of the addition of phosphoric acid in the initial electrospinning solution on the structural ordering, electrical conductivity and porosity development of the final carbon fibers is studied in detail. Alcell lignin fibers were electrospun using a coaxial electrospinning device following the procedure previously reported by our research group. Electrospun H3PO4-lignin fibers were prepared in the same device by addition of phosphoric acid to the lignin solution using mass ratios of 0.1 and 0.3. The electrospun fibers were stabilized in air at 200 ºC, using a slow heating rate and carbonized under inert atmosphere at temperatures between 500 and 900 ºC. In addition, the fibers carbonized at 900 ºC were heat treated at temperatures between 1200 and 1600 ºC. The high temperature heat treatment removes most of the heteroatoms (O, P) for both carbon fibers. However, the surface area of the phosphorous containing carbon fibers is mostly preserved after the heat treatment, while a large porosity shrinkage is observed for the pure lignin-derived fibers. Thus, microporous carbon fibers with large electrical conductivity values have been obtained by heat treatment at 1600 ºC of P-containing electrospun carbon fibers.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Highly porous and conductive functional carbon fibers from electrospun phosphorus-containing lignin fibers

Functional carbon fibers were prepared by carbonization of thermostabilized electrospun lignin-fibers at 500–900 °C followed by a high temperature thermal treatment at 1200–1600 °C. The effect of the preparation temperature on their surface chemistry, structural order, textural properties and electrochemical behavior has been stablished. Maximum porosity development is obtained at 900 °C. The addition of phosphoric acid in the electrospinning lignin solution shortens the stabilization time of the fibers, increases the carbonization yield, generates phosphorus functional groups (P content as high as 3% wt.) and increases the BET surface area of carbon fibers from 840 to 1143 m2 g−1. Interestingly, when phosphorus-containing carbon fibers are treated even at very high temperature, 1600 °C, most of the porosity is preserved (ABET = 822 m2 g−1). XPS depth profile reveals the presence of reduced phosphorus in the core of carbonized fibers. XRD and TEM analysis make evident that the presence of phosphorus induces curvature of the graphitic layers, which seems to hinder the stacking of the graphene layers, explaining the preservation of microporosity after the thermal treatment at high temperature. However, Raman and XRD analyses point out that the presence of phosphorus does not affect the lateral growing of the crystallites. Thus, phosphorus preserves the porosity and allows the development of the electrical conductivity after the thermal treatments. Gravimetric capacitances of 79 F g−1 and capacitance retention of 63% at 68 A g−1 have been determined for phosphorus-containing carbon fibers prepared at 1200 °CWe gratefully thank MICINN (RTI2018-097555-B-100) and Junta de Andalucía (UMA18-FEDERJA-110 and P18-RT-4592) for financial support. Funding for open access charge: Universidad de M ́alaga / CBUA

Electroadsorption of arsenic from natural water in granular activated carbon

The adsorption and electroadsorption of arsenic from a natural water has been studied in a filter-press electrochemical cell using a commercial granular activated carbon as adsorbent and Pt/Ti and graphite as electrodes. A significant reduction of the arsenic concentration is achieved when current is imposed between the electrodes, especially when the activated carbon was located in the vicinity of the anode. This enhancement can be explained in terms of the presence of electrostatic interactions between the polarized carbon surface and the arsenic ions, and changes in the distribution of most stable species of arsenic in solution due to As(III) to As(V) oxidation. In summary, electrochemical adsorption on a filter-press cell can be used for enhancement the arsenic remediation with activated carbon in the treatment of a real groundwater.The authors gratefully thank the economic support from the Ministerio de Economía y Competitividad and FEDER (Projects MAT2013-42007-P and CTQ2012/31762) and Generalitat Valenciana (PROMETEO/2013/038 and PROMETEOII/2014/010). Ramiro Ruiz-Rosas also thanks MINECO for support through a “Juan de la Cierva” contract (JCI-2012-12664)

Bentonite Modified Carbon Paste Electrode as a Selective Electrochemical Sensor for the Detection of Cadmium and Lead in Aqueous Solution

This work describes the use of a carbon paste electrode (CPE) modified with a Bentonite clay (Maghnite) as an electrochemical non-pollutant, selective and low cost sensor for Cadmium and Lead detection in aqueous solution. The physico-chemical properties of the clay mineral were analysed by X-Ray Fluorescence analysis (XRF), Powder X-ray diffraction (XRD), Thermogravimetric analysis (TG) and N2 adsorption isotherm. The square wave anodic stripping voltammetry (SWASV) has been used in which the different electrochemical parameters have been studied. The optimal preconcentration pH and Maghnite-CPE content were found to be 3.4 and 14% w/w respectively. Under these optimized conditions and at a preconcentration time of 5 min, the response of the electrode was linear with analytes concentration in the ranges from 1 to 30 μmol/L for Cadmium and 0.1 to 30 μmol/L for Lead with limit of detection (LOD) values of 0.16 μmol/L and 0.30 μmol/L for Cadmium and Lead respectively.We acknowledge the MESRS and DG–RSDT (Ministère de l’Enseignement Supérieur et de la Recherche Scientifique et la Direction Générale de la Recherche - Algérie) for financial support. Financial support from the Spanish Ministerio de Economía y Competitividad and FEDER funds (MAT2016-76595-R) is also gratefully acknowledged