Biomass-derived carbon materials for energy storage applications

Energy storage systems are an essential link in the implementation of renewable energies and in the development of electric vehicles, which are needed to reduce our dependence on fossil fuels and the emission of greenhouse gases. Various technologies have been proposed for energy storage based on different working principles, including lithium-ion batteries, emerging sodium-ion batteries and electric-double layer capacitors. Besides the quest for improving key aspects such as energy and power densities, current research efforts are devoted to foster the manufacturing of more environmentally friendly devices using sustainable materials. Carbon-based electrodes hold considerable promise in such terms due to their low cost, tailorable morphology and microstructure, and the possibility of processing them by direct carbonization of eco-friendly and naturally-available biomass resources. The main goal of this thesis is to develop carbon materials from biomass resources and study their applications as electrode for lithium-ion batteries, sodium-ion batteries and electric-double layer capacitors. En route towards that goal, it also aims at expanding our understanding of the microstructural changes of biomass-derived carbons with varying processing conditions and their effect on the electrochemical performance for each of these technologies. The first part of this work reports on the synthesis of graphitized carbon materials from biomass resources by means of an Fe catalyst, and the study of their electrochemical performance as anode materials for lithium-ion batteries (LIBs). Peak carbonization temperatures between 850 °C and 2000 ºC were covered to study the effect of crystallinity, surface and microstructural parameters on the anodic behavior, focusing on the first-cycle Coulombic efficiency, reversible specific capacity and rate performance. Reversible capacities of Fe-catalyzed biomass-derived carbons were compared to non-catalyzed hard carbon and soft carbons materials heated up to 2800 ºC. Moreover, in-situ characterization experiments were carried out to advance our understanding of the mechanisms responsible for catalytic graphitization. The second part of this work reports a comprehensive study on the structural evolution of hard carbons from biomass resources as a function of carbonization temperature (800 - 2000 ºC), and its correlation with electrochemical properties as anode materials for sodium-ion batteries (SIBs). Synchrotron X-ray total scattering experiments were performed and the associated atomic pair distribution function (PDF) extracted from the data to access quantitative information on local atomic arrangement in these amorphous materials at the nanoscale, as well as its evolution with increasing processing temperature. Then, electrochemical properties and the storage mechanisms involved on Na ions insertion into hard carbon structures at each characteristic potential regions were elucidated and correlated with microstructural properties. Finally, the third part of this work reports on the synthesis of nanostructured porous graphene-like materials from biomass resources using an explosion-assisted activation strategy by nitrate compounds and Ni as a graphitization catalyst. The thermal behavior during carbonization as well as the resulting microstructural and surface properties were evaluated at two different processing temperatures, 300 and 1000 ºC. Finally, their application as electrode materials for electric-double layer capacitors (EDLCs) and LIBs is investigated, with a view to their performance under high charge/discharge specific current densities experiments.Premio Extraordinario de Doctorado U

Dislocation density and graphitization of diamond crystals

Two sets of diamond specimens compressed at 2 GPa at temperatures varying between 1060 K and 1760 K were prepared; one in which graphitization was promoted by the presence of water and another in which graphitization of diamond was practically absent. X-ray diffraction peak profiles of both sets were analyzed for the microstructure by using the modified Williamson-Hall method and by fitting the Fourier coefficients of the measured profiles by theoretical functions for crystallite size and lattice strain. The procedures determined mean size and size distribution of crystallites as well as the density and the character of the dislocations. The same experimental conditions resulted in different microstructures for the two sets of samples. They were explained in terms of hydrostatic conditions present in the graphitized samples

Spontaneous graphitization of ultrathin cubic structures: A computational study

Results based on {\em ab initio} density functional calculations indicate a general graphitization tendency in ultrathin slabs of cubic diamond, boron nitride, and many other cubic structures including rocksalt. Whereas such compounds often show an energy preference for cubic rather than layered atomic arrangements in the bulk, the surface energy of layered systems is commonly lower than that of their cubic counterparts. We determine the critical slab thickness for a range of systems, below which a spontaneous conversion from a cubic to a layered graphitic structure occurs, driven by surface energy reduction in surface-dominated structures.Comment: 5 pages, 3 figure

Porous Graphene-like Carbon from Fast Catalytic Decomposition of Biomass for Energy Storage Applications

A novel carbon material made of porous graphene-like nanosheets was synthesized from biomass resources by a simple catalytic graphitization process using nickel as a catalyst for applications in electrodes for energy storage devices. A recycled fiberboard precursor was impregnated with saturated nickel nitrate followed by high-temperature pyrolysis. The highly exothermic combustion of in situ formed nitrocellulose produces the expansion of the cellulose fibers and the reorganization of the carbon structure into a three-dimensional (3D) porous assembly of thin carbon nanosheets. After acid washing, nickel particles are fully removed, leaving nanosized holes in the wrinkled graphene-like sheets. These nanoholes confer the resulting carbon material with ≈75% capacitance retention, when applied as a supercapacitor electrode in aqueous media at a specific current of 100 A·g–1 compared to the capacitance reached at 20 mA·g–1, and ≈35% capacity retention, when applied as a negative electrode for lithium-ion battery cells at a specific current of 3720 mA·g–1 compared to the specific capacity at 37.2 mA·g–1. These findings suggest a novel way for synthesizing 3D nanocarbon networks from a cellulosic precursor requiring low temperatures and being amenable to large-scale production while using a sustainable starting precursor such as recycled fiberwood.Spanish Government Agency Ministerio de Economí a y Competitividad (MINECO) (grant number MAT2016-76526-R)

Micro-beam and pulsed laser beam techniques for the micro-fabrication of diamond surface and bulk structures

Micro-fabrication in diamond is involved in a wide set of emerging technologies, exploiting the exceptional characteristics of diamond for application in bio-physics, photonics, radiation detection. Micro ion-beam irradiation and pulsed laser irradiation are complementary techniques, which permit the implementation of complex geometries, by modification and functionalization of surface and/or bulk material, modifying the optical, electrical and mechanical characteristics of the material. In this article we summarize the work done in Florence (Italy) concerning ion beam and pulsed laser beam micro-fabrication in diamond.Comment: 14 pages, 5 figure

A new database program installed at the SUERC radiocarbon laboratory

The SUERC Radiocarbon Dating Laboratory has recently replaced its spreadsheet-based record keeping with a new database program, custom designed to help laboratory staff manage the high throughput of nearly 5000 cathodes in the past year. The system can accept data from a variety of sources in addition to manual entry; experimental results can be uploaded from spreadsheets, while integration with graphitisation lines means that graphite yields are automatically recorded. The system is able to pass radiocarbon results directly to OxCal 4 for calibration, with the resulting plots incorporated into the dating certificates issued to submitters. There are also benefits to submitters, with electronic sample submission both eliminating transcription errors and speeding up the logging-in process which keeps turnaround times down. For bone samples, data on collagen yields are now stored electronically and are more readily obtainable from the laboratory. The new SUERC Radiocarbon Dating Laboratory database will make a significant contribution to maintaining the high quality of results produced by the laboratory, aiding staff in tracking sample progress and monitoring quality assurance (QA) samples going through the laboratory, eliminating transcription errors and making communication easier between laboratory staff and sample submitters