Large-scale conversion of helical-ribbon carbon nanofibers to a variety of graphene-related materials

Helical-ribbon carbon nanofibers produced on an industrial scale were successfully converted in highly functionalized graphene nanoplatelets by using a slight modification of the Hummers oxidation method. The duration of the oxidative process severely affected the interlayer spacing in the resulting nanoplatelets and, consequently, they showed very different exfoliation behavior. Therefore, it was possible to obtain a variety of graphene-related products through their ultrasonication or thermal treatments such as exfoliation-reduction by flash-pyrolysis in air at temperatures between 400 and 1000 °C or standard activation with CO2 at 800 °C. Detailed comparison of the functionalized carbon nanoplatelets, graphene oxides, reduced graphene oxides and activated carbon nanoplatelets reveals the wide spectrum of their properties with specific surface areas in the range of 4–500 m2 g−1, oxygen content from 38 to 5 wt% and different structural ordering. This study also underlines the impact of the structural, textural and chemical changes experienced by the carbon nanofibers along the various processes on the performance as supercapacitor electrodes. This preliminary study, based on cyclic voltammetry in 2 M H2SO4 aqueous electrolyte, is a summary of the strengths and weaknesses of the different graphene-related materials for this application. The helical-ribbon carbon nanofibers displayed only 10 F g−1, the capacitance of the functionalized graphene nanoplatelets greatly rose to 104 F g−1 with clear contributions from pseudocapacitance. Values around 100–120 F g−1 were found for the graphene oxides and activated graphene nanoplatelets although a marked resistive character is detected. Flash-pyrolysis at 1000 °C leads to lower capacitance (79 F g−1) but much quicker charge propagation. Among all these materials, the lower-cost functionalized graphene nanoplatelets displayed the better behavior for aqueous supercapacitors.Financial support from EU 7FP (Project Electrograph- 266391) and EU FP7- ICT-2013-FET-FGRAPHENE Flagship Project (Nr. 604391) is gratefully acknowledged.Peer reviewe

A Vettonian rubbish pit at the Cerro de la Mesa (Alcolea de Tajo, Toledo)

La existencia de fosas rellenas con tierra, cenizas, fragmentos cerámicos y fauna denominadas genéricamente como cenizales, basureros o vertederos, es un hecho bien conocido en la Meseta Norte durante la Segunda Edad del Hierro, vinculándose con los grupos Vacceos y Vettones. Sin embargo, su distribución debe ampliarse al occidente de la Meseta Sur y oriente extremeño, como se evidencia tanto en el Cerro de la Mesa como a través de indicios documentados en otros yacimientos más occidentales. Se presenta aquí un detallado estudio de un sector de la fosa que actuó como vertedero para la retirada de ciertos restos domésticos con ocasión de la reurbanización en profundidad del poblado vettón que tuvo lugar en la transición del siglo III al II a.C.The existence of pits filled with earth, ashes, pottery fragments and fauna, usually known as ash pits, rubbish pits or dumps, has been recognized on many Second Iron Age settlements of the Spanish Northern Plateau, generally linked with Vaccean and Vettonian groups. However, its distribution should be extended to the west of the Southern Plateau and eastern Extremadura, as evidenced by the example of the Cerro de la Mesa village and indications coming from other western sites. We offer here a detailed study of part of a rubbish pit where certain domestic remains were withdrawn in connection with a new project of urbanisation of the Vettonian village held on the transition from the 3rd to the 2nd centuries BC.Este trabajo se enmarca en el proyecto HAR2011/25191: “Identidad y territorio en el valle medio del Tajo durante el Bronce Final y la Edad del Hierro” del Ministerio de Economía y Competitivida

Assessment of graphene-related materials in supercapacitors

Tesis con mención internacional. Tesis doctoral por el sistema de compendio de publicacionesLos supercondensadores (SC) están experimentando una creciente demanda como dispositivos avanzados para el almacenamiento de energía eléctrica. Entre la gran variedad de materiales de carbono existentes, el grafeno ha surgido en los últimos años como el mejor candidato como material activo para electrodos en SC. Su elevada superficie teórica y su excelente conductividad eléctrica son características clave para esta aplicación. A pesar del potencial del grafeno, su implementación en SC comerciales presenta actualmente muchas incertidumbres. Esto es debido a que el mercado de los supercondensadores es extremadamente sensible al precio y la producción de este material a escala industrial y con un bajo coste todavía se enfrenta a numerosos desafíos. En la actualidad, la ruta más prometedora para obtener grafeno para esta aplicación implica una severa oxidación química de grafito para, posteriormente, ser sometido a procesos de exfoliación y reducción. En este contexto, el objetivo principal de esta investigación es evaluar el potencial de los productos intermedios de bajo coste, generados en las primeras etapas de dicho proceso, como electrodos para estos dispositivos. Esta tesis se centra en el efecto que tienen los precursores y procedimientos de síntesis en los materiales tipo grafeno resultantes, así como en el impacto de sus características estructurales, texturales y químicas en el funcionamiento del supercondensador. Adicionalmente, se ha llevado a cabo una comparación sistemática con los perfiles generales encontrados para los supercondensadores basados en materiales de carbono. Los materiales de carbono intermedios generados durante la síntesis de grafeno mediante oxidación de precursores grafíticos pueden ser una alternativa competitiva como materiales activos en electrodos de supercondesadores. La capacidad gravimétrica de la mayoría de ellos en electrolitos acuosos y orgánicos es comparable con la de carbones activados y otros materiales porosos de carbono con una superficie específica alrededor de 1000 m2 g-1. Adicionalmente, los óxidos de grafito pueden alcanzar una capacidad volumétrica mucho mayor lo que resulta un factor clave para la miniaturización de los dispositivos. La estructura expandida derivada de la intercalación de grupos funcionales en la estructura grafítica da lugar a un aumento en la superficie disponible al electrolito acuoso y facilita la interacción con los mismos. Sin embargo, la movilidad iónica se ve obstaculizada por lentas reacciones redox que limitan la respuesta del SC. Los procesos de oxidación tienen que ser mejorados para lograr que el óxido de grafito sea adecuado para supercondensadores orgánicos. Para ello se requiere un espaciado mayor entre capas que facilite así el movimiento de los iones más voluminosos. El enfoque estándar basado en la determinación de la superficie específica para evaluar el potencial de los materiales de carbono en supercondensadores resulta excesivamente simple. La adsorción de gases en materiales tipo grafeno no siempre proporciona el área involucrada en el almacenamiento de energía. Adicionalmente, este tipo de materiales de carbono pueden experimentar una caída en su porosidad durante su procesamiento en electrodos con un espesor y una masa similar a los de los dispositivos comerciales. SUMMARY (in English) Supercapacitors (SCs) are under an increasing demand as advanced devices for electrical energy storage. Among the wide variety of carbon materials, graphene has recently emerged as the best candidate for SC electrodes. Its high theoretical surface area and outstanding electrical conductivity result key features for this application. In spite of the potential of graphene, its implementation in commercial SCs currently presents many uncertainties. Particularly, the supercapacitor market is extremely sensitive to price and the low cost production of graphene at industrial scale still faces numerous challenges. Up-to-now the most promising route to obtain graphene for SC involves the harsh chemical oxidation of graphite and subsequent exfoliation and reduction. In this context, the main goal of this research is the assessment of the potential of low-cost intermediates generated in the early stages of this process as supercapacitor electrodes. The study focuses on the effect of precursors and synthesis procedures on the resulting graphene-related materials and the impact of their structural, textural and chemical features on the supercapacitor performance. A systematic comparison with the general patterns found for carbon based-SC is also carried out. It is found that intermediate carbons generated during graphene synthesis by wet oxidation of graphitic precursors may be competitive alternatives for supercapacitor electrodes. The gravimetric capacitance of most of them in aqueous and organic electrolytes compares with those of activated carbons and other porous carbons with specific surface area around 1000 m2 g-1. Besides, graphite oxides may achieve much higher volumetric capacitance, which is a key factor for the development of small devices. An expanded network resulting from the intercalation of oxygen functionalities into graphitic structure increases the available surface for the aqueous electrolyte and facilitates the interaction with the oxygen-surface groups. By contrast, ionic mobility is hindered by slow redox processes which limit the SC response rate. Oxidation processes have to be improved to achieve suitable graphite oxides for organic supercapacitors. A nanoengineered interlayer spacing is required to facilitate the movement of large ions. The popular approach based on the specific surface area to assess the potential of carbons in SC cannot be used in a straightforward manner. Standard gas adsorption on graphene-related materials may not always provide the area involved in the energy storage. Additionally, this type of carbons may experience a drop in their porosity during the processing of electrodes matching the thickness and the carbon loading of commercial devices

Graphite Oxide: An Interesting Candidate for Aqueous Supercapacitors

A graphite oxide, obtained on a large scale at low cost as an intermediate in the graphene production, achieves specific capacitances (159 Fg−1 in H2SO4 and 82 Fg−1 in (C2H5)4NBF4 in acetonitrile) that compete with those of activated carbons and largely surpass the values obtained with graphene nanoplatelets. More promising, the high electrode density leads to volumetric capacitances of 177 and 59 F cm−3 in the aqueous and the organic electrolytes, respectively, which are above most data reported for carbons. In the aqueous electrolyte, the graphite oxide stands out on energy density when compared to graphene nanoplatelets and on power capability if compared to an activated carbon commercialized for supercapacitors, whereas in the organic electrolyte, the limited interlayer spacing restricts the mobility of the larger ions into the expanded graphitic structure. This study also illustrates that the specific surface of carbons measured by standard gas adsorption may not be a relevant parameter as it does not always match the electrochemically active area involved in the energy storage.Financial support to T.A.C. from EU 7FP (Project Electrograph- 266391) and MICINN (project MAT 2011-25198) is gratefully acknowledged. Financial support for graphene oxide development was obtained from the Research Council of Norway, through grant No 73146. The authors thank the Instituto Universitario de Investigación en Nanociencia de Aragón-Universidad de Zaragoza for the HRTEM images.Peer reviewe

The influence of pore size and surface area of activated carbons on the performance of ionic liquid based supercapacitors

[EN] This study analyses and compares the behaviour of 5 commercial porous carbons in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and its mixture with propylene carbonate (PC) as electrolytes. The results of this investigation show that the existence of a distribution of pore sizes and/or constrictions at the entrance of the pores leads to significant changes in the specific capacitance of the investigated materials. The use of PYR14TFSI as an electrolyte has a positive effect on the EDLC energy storage, but its high viscosity limits the power density. The mixture 50 : 50 wt% propylene carbonate–PYR14TFSI provides high operative voltage as well as low viscosity and thus notably enhances EDLC operation.A. Balducci wishes to thank the University of Münster, the Ministry of Innovation, Science and Research of North Rhine-Westphalia (MIWF) for the financial support. Funds to T.A.Centeno from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n°266391-Electrograph and MICINN (project MAT 2011-25198) are gratefully acknowledged. The authors also thank the Unit of Information Resources for Research (URICI-CSIC) for the co-financing of this publication in Open Access.Peer reviewe

Constant capacitance in nanopores of carbon monoliths

The results obtained for binder-free electrodes made of carbon monoliths with narrow micropore size distributions confirm that the specific capacitance in the electrolyte (C2H5)4NBF4/acetonitrile does not depend significantly on the micropore size and support the foregoing constant result of 0.094 ± 0.011 F m−2.We thank Dr Jose M. Rojo (ICMM-CSIC) for his useful assistance. Financial support from Spanish MICINN (project MAT 2011-25198) is gratefully acknowledged. G. Moreno-Fernandez thanks Spanish MINECO for a FPI fellowship. We also acknowledge institutional support from the Unit of Information Resources for Research at the “Consejo Superior de Investigaciones Científicas” (CSIC) for the article-processing charges contribution.Peer reviewe

Reduced graphite oxide in supercapacitor electrodes

The current energy needs have put the focus on highly efficient energy storage systems such as supercapacitors. At present, much attention focuses on graphene-like materials as promising supercapacitor electrodes. Here we show that reduced graphite oxide offers a very interesting potential. Materials obtained by oxidation of natural graphite and subsequent sonication and reduction by hydrazine achieve specific capacitances as high as 170 F/g in H2SO4 and 84 F/g in (C2H5)4NBF4/acetonitrile. Although the particle size of the raw graphite has no significant effect on the physico-chemical characteristics of the reduced materials, that exfoliated from smaller particles (<75 μm) result more advantageous for the release of the stored electrical energy. This effect is particularly evident in the aqueous electrolyte. Graphene-like materials may suffer from a drop in their specific surface area upon fabrication of electrodes with features of the existing commercial devices. This should be taken into account for a reliable interpretation of their performance in supercapacitors.Financial support from EU 7FP [Project Electrograph-266391], the project VEGA 1254/12 (M.H.) and MICINN [MAT 2011-25198] (T.A.C.) is gratefully acknowledged. The authors are grateful to Prof. Viera Skákalová for the useful discussions and critical remarks.Peer reviewe

Distillation of granulated scrap tires in a pilot plant

[EN] This paper reports the pyrolytic treatment of granulated scrap tires (GST) in a pilot distillation unit at moderate temperature (550 °C) and atmospheric pressure, to produce oil, char and gas products. Tire-derived oil is a complex mixture of organic C5–C24 compounds, including a very large proportion of aromatic compounds. This oil has a high gross calorific value (∼43 MJ kg−1) and N and S contents of 0.4% and 0.6%, respectively, falling within the specifications of certain heating fuels. The distillation gas is composed of hydrocarbons; methane and n-butane are the most abundant, investing the distillation gas with a very high gross calorific value (∼68 MJ N m−3). This gas is transformed into electric power by a co-generation turbine. The distillation char is mostly made of carbon but with significant inorganic impurities (∼12 wt%). The quality of the solid residue of the process is comparable to that of some commercial chars. The quantity of residual solids, and the qualities of the gas, liquid and solid fractions, are similar to those obtained by conventional pyrolytic treatments of waste tires. However, the simplicity of the proposed technology and its low investment costs make it a very attractive alternative.This work was carried out with the financial support of ENRECO 2000 Ltd.Peer reviewe

The GRAUTHERMIC-Tyres process for the recycling of granulated scrap tyres

[EN] The influence of thermolysis temperature (range 500–700 °C) on the products produced from granulated scrap tyres (GST) (2–6 mm Ø) in a semi-batch pilot plant that uses several fixed bed reactors in parallel (GRAUTHERMIC-Tyres® process) was examined. This pilot unit consisted of six vertical fixed bed reactors in parallel, each with a capacity of 12 kg GST. Thermolysis proceeded in a normal air atmosphere. The solid (char), liquid (tyre-derived oil [TDO]) and gaseous products generated at each temperature were characterised. The incondensable gas fraction was used to produce electricity via a gas turbine. The yields of the gas, TDO and char fractions were 15–22 wt.%, 34–46 wt.% and 39–44 wt.% respectively, values that represent major recoveries from the GST feedstock. The TDOs were mainly a mixture of aliphatic and aromatic hydrocarbons and had very high GCVs (41–43 MJ/kg), higher than those of conventional liquid fuels. A large increase was seen in the polycyclic aromatic hydrocarbon content above 550 °C. The non-condensable gases obtained had a high GCV value (64–83 MJ/Nm3), and produced 4.1–6.5 kWh electricity/g of GST in a gas turbine. The chars had a GCV of 27–28 MJ/kg, high ash (17–21 wt.%) and zinc contents, and an essentially macroporous texture with a low BET surface area (46–78 m2/g). The thermolysis of GSTs with co-generation of gases by the GRAUTHERMIC-Tyres® method would appear to be a viable means of valorising GST.This work was performed with the financial support of ENRECO 2000 Ltd.Peer reviewe