An approach to classification and capacitance expressions in electrochemical capacitors technology.

The proliferation of novel types and designs of electrochemical capacitors makes it necessary to obtain a better understanding of the behavior of these systems together with a more systematic classification of them. In this study a rational classification of supercapacitors based on the charge storage mechanism and the active material of each electrode is proposed. The internationally accepted terminology - the terms symmetric, asymmetric and hybrid - is also clarified in an attempt to standardize the current definitions and facilitate the systematic classification of each device. Additionally, the selection of suitable mathematical expressions to calculate the capacitance of each kind of system is rationalized throughout the discussion taking into account the behavioral characteristics of each electrode. An examination of the potential evolution profile of each electrode during the galvanostatic cycling of the supercapacitor is presented as a key tool for understanding the fundamental behavior of these devices.

Proactive Effect of Algae-Based Graphene Support on the Oxygen Evolution Reaction Electrocatalytic Activity of NiFe.

The preparation of graphene materials from biomass resources is still a challenge, even more so if they are going to be employed as supports for electrocatalysts for water splitting. Herein, we describe the preparation and characterization of graphene oxides (GOs) from solid macroalgae waste obtained after processing an agar-agar residue. The structural and morphological characterization of the obtained GO confirm the presence of a lamellar material that is composed of few layers with an increased number of heteroatoms (including nitrogen) if compared with those observed in a GO obtained from graphite (reference). Three-dimensional electrodes were prepared from these GOs by depositing them onto a fibrous carbon paper, followed by electrodeposition of the catalyst, NiFe. The electrocatalytic performance of these hybrid systems for the oxygen evolution reaction (OER) showed a proactive effect of both graphene materials toward catalysis. Moreover, the electrode prepared from the algae-based graphene showed the highest electrocatalytic activity. This fact could be explained by the different structure of the algae-based graphene which, due to differences in the nucleation growth patterns and electroactive sites developed during the electrodeposition process, produced more reactive NiFe species (higher oxidation state).

Matrix-iron interactions in carbon-embedded iron oxide nanoparticles.

This study deals with the interaction between iron oxide nanoparticles and a carbon-rich source (pitch) during the carbonization process, resulting in a carbon material of high added value. The iron oxide nanoparticles were prepared by different procedures in order to obtain Fe2O3 and Fe3O4 with different particle sizes. The results show that the presence of iron oxide modifies the thermal behaviour of the pitch, especially in the temperature range of 400-500 degrees C when the pre-graphitic order in the carbon matrix is formed. The crystalline microstructure of the materials changes from domains to mosaics when iron oxide is added. The oxidation state of the iron is also modified during carbonization due to the presence of the carbon.

The adsorption of chromium (VI) from industrial wastewater by acid and base-activated lignocellulosic residues.

This study deals with the adsorption of Cr(VI) from synthetic and industrial wastewater, produced by a sewage plant. The activated carbons were prepared from a lignocellulosic raw material by thermal treatment at 450 and 650 degrees C in the presence of acid (AlCl(3), HCl, H(3)PO(4) and H(2)SO(4)) and base (NaOH) agents. To optimize the adsorption of Cr(VI), the chemical modifications caused by each activating agent (related to the capability of Cr(VI) removal), and the optimal experimental conditions of the pH, Cr(VI) concentration, adsorbent dose and residence time, were studied. Thus, treatment with H(3)PO(4) gives rise to carbons with a high surface area and high efficiency for Cr(VI) removal at short equilibrium times. In contrast, the generation of active surface sites by means of NaOH requires longer equilibrium times, the adsorption being less effective than in the former case. The adsorption isotherms obey the Langmuir equation only in the first stages of the reaction but fit the Freundlich equations over the whole range studied, so the heat of adsorption can be easily calculated. The results also show that the activated carbons obtained can be recovered by filtration with an efficiency of 30% in the third cycle.

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.

Tailoring micro-mesoporosity in activated carbon fibers to enhance SO2 catalytic oxidation.

Enhanced SO2 adsorption of activated carbon fibers is obtained by tailoring a specific micro-mesoporous structure in the fibers. This architecture is obtained via metal catalytic activation of the fibers with a novel precursor, cobalt naphthenate, which contrary to other precursors, also enhances spinnability and carbon fiber yield. In the SO2 oxidation, it is demonstrated that the combination of micropores and large mesopores is the main factor for an enhanced catalytic activity which is superior to that observed in other similar microporous activated carbon fibers. This provides an alternative way for the development of a new generation of catalytic material.

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.

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.