Ricardo Santamaría

Towards a Further Generation of High‐Energy Carbon‐Based Capacitors by Using Redox‐Active Electrolytes

This work was supported by MICINN (Project MAT2007-61467). S.R. thanks MICINN for an FPI doctoral grant.

Surface area measurement of graphene oxide in aqueous solutions.

Graphene oxide (GO) forms persistent dispersions in aqueous solutions up to concentrations of 0.2 mg mL(-1). Addition of methylene blue (MB) to these aqueous dispersion of GO gives rise to the observation in optical spectroscopy of new absorption bands that are indicative of the formation of MB/GO conjugates. Four new absorption maxima have been characterized, and their intensity varies depending on the relative concentration of MB with respect to GO. Two of these bands appearing at 677 and 757 nm correspond to individual MB molecules adsorbed on neutral or acid sites of GO, respectively. Two other bands at 615 and 580 nm are attributable to adsorbed MB molecules showing interaction with other neighbor dye molecules at incomplete (615 nm) or complete (580 nm) surface coverage. Complete coverage of GO surface by MB causes the formation of a precipitate and the separation of the MB/GO conjugate. EDS mapping of carbon and sulfur atoms of MB/GO conjugate indicates the homogeneous distribution of MB molecules coating GO sheets. A simple and reliable protocol for surface area measurement and determination of the level of aggregation for GO dispersions in water has been proposed by determining the amount of MB that leads to the maximum intensity of the 580 nm band and precipitation of the MB/GO conjugate. Specific surface area as high as 736.6 m(2) g(-1) in the range of the theoretical value for GO has been experimentally measured for diluted GO solutions, but aggregation levels of 15% were estimated for GO concentration of 50 μg mL(-1).

A comparative study of air-blown and thermally treated coal-tar pitches

Abstract A commercial impregnating coal-tar pitch was thermally treated at 430°C for times varying between 2 and 6 h and the same pitch was air-blown at 275°C for between 10 and 30 h. Both series of pitches were characterized and their properties compared. The pitches from the thermal treatments contained mesophase, the amount increasing from 10 vol.% to 65 vol.% with increasing time of treatment, whereas those from the air-blowing were completely isotropic. The study of the pitches revealed that the thermally treated pitches still contained a considerable amount of light compounds together with the large planar molecules generated. On the contrary, light components polymerized to a considerable extent during air-blowing to form cross-linked molecules. Evidence of cross-linked structures is provided by a decrease in the iodine uptake and also by the X-ray diffraction results. Furthermore, extensive removal of aliphatic hydrogen took place during pitch air-blowing, as confirmed by the increase in the aromaticity indices of the pitches.

Electrochemical, textural and microstructural effects of mechanical grinding on graphitized petroleum coke for lithium and sodium batteries

Abstract A graphitized coke material obtained from petroleum residua was mechanically ground at different milling times between 0 and 100 h. Electrochemical reactions with both lithium and sodium are significantly altered as a function of grinding time. Short-time ball milling of graphite (1 and 5 h) induces a limited decrease in particle size and an increase in microstrain content. Simultaneously, alkali metal intercalation and electrolyte decomposition are hindered, and thus the irreversible and reversible capacities decrease. For longer milling time (up to 100 h), average crystallite size decreases and particles adopt a lamellar shape. Simultaneously, the irreversible capacity increases and correlates with an increase of the resistance, as obtained by impedance spectroscopy. Ex-situ XRD shows that extensively ground graphite samples need a higher discharge specific capacity to reach the formation of n-stages as compared to non-ground graphite, this being indicative of lithium incorporation in energetically different sites to the interlayer space. Sodium storage capacity increases with prolonged grinding time. This effect is shown here for the first time for graphitized cokes.

A study of pitch-based precursors for general purpose carbon fibres

Abstract The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres. Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation temperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softening temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone–60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature.

Graphite Felt Modified with Bismuth Nanoparticles as Negative Electrode in a Vanadium Redox Flow Battery

A graphite felt decorated with bismuth nanoparticles was studied as negative electrode in a vanadium redox flow battery (VRFB). The results confirm the excellent electrochemical performance of the bismuth modified electrode in terms of the reversibility of the V(3+) /V(2+) redox reactions and its long-term cycling performance. Moreover a mechanism that explains the role that Bi nanoparticles play in the redox reactions in this negative half-cell is proposed. Bi nanoparticles favor the formation of BiHx , an intermediate that reduces V(3+) to V(2+) and, therefore, inhibits the competitive irreversible reaction of hydrogen formation (responsible for the commonly observed loss of Coulombic efficiency of VRFBs). Thus, the total charge consumed during the cathodic sweep in this electrode is used to reduce V(3+) to V(2+) , resulting in a highly reversible and efficient process.

High performance activated carbon for benzene/toluene adsorption from industrial wastewater

A coal-tar-derived mesophase was chemically activated to produce a high surface area (~3200 m(2)/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.

Pitch-based carbon composites with granular reinforcements for frictional applications

Abstract Four pitches (a commercial impregnating coal-tar pitch, an air-blown pitch and two thermally treated coal-tar pitches) and four granular carbons (graphite, anthracite, petroleum coke and foundry coke) were used to prepare carbon composites. A preliminary study of the main parameters involved in the preparation process of the carbon composites was carried out for the selection of the most suitable conditions. Mixtures with 30 wt.% of pitch and a moulding pressure of 80 MPa were selected and used for the preparation of all carbon composites. The behaviour of the resultant materials during carbonisation influenced their final properties. Composites obtained from the commercial pitch and air-blown pitch combined with anthracite and foundry coke deformed on carbonisation, presenting a high porosity and low mechanical properties. Materials obtained from the thermally treated pitches, foundry coke and anthracite were the ones with the highest compressive strength. The frictional behaviour of the materials was mainly governed by the type of granular carbon used.

Preparation of low toxicity pitches by thermal oxidative condensation of anthracene oil.

This article describes a novel industrial procedure for producing new pitches of low toxicity from anthracene oil, a byproduct of coal tar distillation. The procedure involves oxidative treatment in order to polymerize and condense the anthracene oil components followed by thermal treatment and distillation in order to obtain a pitch with the desired parameters. This sequence (oxidative treatment/thermal treatment/distillation) was repeated four times under reaction conditions of increasing severity in four cycles of anthracene oil processing to obtain the four pitches. The pitches had similar characteristics to those of standard binder coal tar pitches (e.g., softening point and wetting capacity). Because of the inherent composition of the parent anthracene oil, the pitches were found to be totally free of solid particles, i.e., primary quinoline insolubles and metals. The gas chromatography and gas chromatography/mass spectroscopy results revealed a consecutive decrease in toxicity with successive cycles of anthracene oil processing. Thus, the benzo[a]pyrene content decreased from 11.2 mg/g for the pitch in cycle one to 1.5 mg/g for the pitch with four processing cycles. The carcinogenicity of the pitches, evaluated on the basis of benzo[a]pyrene toxic equivalency factors, also followed the same tendency. The final carcinogenity values are nearly all lower than those of standard binder coal tar pitches.

Chemical and rheological characterization of air-blown coal-tar pitches

A commercial impregnating coal-tar pitch was air-blown at 275 °C for 10, 18, 25 and 30 hours. Isotropie pitches were produced with softening points ranging from 140 to 210 °C. The parent and treated pitches were characterized by elemental analysis, Fourier transform infrared, solubility in toluene and N-methylpyrrolidone, iodine adsorption and reflected light optical microscopy. The viscoelasticity of the pitches also was investigated using transient shear and controlled strain oscillatory rheometry. The chemical characterization of the pitches suggested that, as air-blowing proceeded, larger molecules formed through dehydrogenative polymerization (aromatization/condensation) and cross-linking of the pitch molecules. This is consistent with the rheological characterization of the pitches. The results obtained from transient shear and oscillatory rheometry showed that the parent impregnating pitch exhibited a viscous behavior (typical of fluids composed of small molecules). In contrast, the air-blown pitches, which contained large aromatic molecules, showed viscoelastic properties.