Sintesi e caratterizzazione di nanostrutture di carbonio e loro applicazione nel campo delle celle a combustibile

Nella tesi di dottorato viene descritta la sintesi di nano strutture di carbonio, in particolare “MWNT: Multi-wall carbon Nanotubes”, nanotubi a parete multipla e “SWNH: Single-Wall carbon Nanohorns”, strutture a parete singola, mediante la tecnica di scarica ad arco in ambiente liquido e gassoso. E’ stato messo a punto un sistema per la produzione sia in corrente continua (DC), convenzionale, che corrente alternata (AC), di nuova concezione, per la sintesi di queste nano strutture di carbonio. I materiali prodotti sono stati caratterizzati con diffrattometro ai raggi X e analisi termo gravimetrica. In particolare è stato messa a punto una metodologia per la misura quantitativa del contenuto di nanotubi di carbonio in una polvere di materiale appena prodotto che utilizza entrambe le tecniche. Le nano strutture di carbonio sono state utilizzate come supporto per il platino per poter realizzare elettrodi per celle a combustibile a elettrolita polimerico (PEM) o a metanolo diretto (DMFC). Il platino è stato depositato mediante un processo chimico di impregnazione a partire da un precursore del platino, acido esacloroplatinico, e successiva riduzione. La resa di deposizione del platino sulle due nano strutture di carbonio prodotte e sul Vulcan XC 72 commerciale è stata valutata. Inoltre un’analisi approfondita con il SEM, il TEM e il diffrattometro per polveri ha permesso di conoscere le caratteristiche delle particelle di catalizzatore depositate. Per concludere il lavoro della tesi di dottorato il composito Platino-Carbonio sintetizzato è stato utilizzato per produrre elettrodi mediante un processo a inchiostro. Misure elettrochimiche, voltammetria ciclica, ha permesso di valutare l’area specifica superficiale del catalizzatore depositato e di valutare l’attività catalitica di questo materiale verso il processo di ossidazione del metanolo.Carbon has been under deep investigation since half part of the 20th century. In particular the branches of micro and nanotechnology received a great attention due to the discovery of new allotropes form of carbon: fullerenes and carbon nanotubes. Carbon is used under different forms which have peculiar properties: graphite is a lubrificant and electrical conductor, diamond is the hardest material known, carbon black is used as filler in rubber, in plastic composites and as support material in the electrodes for fuel cells, carbon fibres exhibit very high tensile and compressive strength and they are widely used in automotive and aerospace industries, fullerenes are thought to be useful as vehicle for pharms, carbon nanotubes have extraordinary electrical, mechanical and thermal properties and some laboratory applications have just been realized, single-walled nanohorns are known to have very high reactive surface. These different forms of carbon reported introduce us to the world of nanometric objects. Only to have an idea, the diameter of a fullerene is of 0.71 nm. Nanomaterials show unexpected features and properties. These aspects represent one of the most important subject of investigation for the future. Materials with typical dimensions of tens of nanometers have just been synthesised and studied during the 19th century, i.e. colloids, but new methods of characterization and a systematic approach gave a great development to the knowledge of nanometric materials. The synthesis and characterization of these new forms of carbon have been under deep study since 1985 (discovery date of fullerene). Some theses are reported in literature about the mechanism of growth of carbon nanostructures, but this is far from being completely understood. Many techniques are used to characterize these carbon nanostructures: electron microscopy, AFM, STM, X-ray diffraction, Raman, NMR. Moreover in the last few years the possibility of manipulating nanostructures, in particular carbon nanostructures, has been exploited. Contemporary to the study of the properties of these nanostructures, their possible applications have been investigated by many authors. The topics of this PhD thesis concern the synthesis of carbon nanostructures by arc discharge, with the purpose of exploring new approaches respect to those reported in literature. Multi wall carbon nanotubes and single wall carbon nanohorns have been synthesised by an arc discharge powered respectively by direct current (DC)alternating current (AC). The quantitative evaluation of multi wall carbon nanotubes in the raw material produced by arc discharge has been carried out by X-ray diffraction and thermo-gravimetric analysis supported by transmission electron microscopy. A suitable application of these carbon nanostructures has been exploited. In particular the feasible use of these carbon nanostructures as supports for catalysts for fuel cells application has been carried out by the mean of cyclic voltammetry. Pt nano-particles have been successfully deposited on three different carbon supports, multi wall carbon nanotubes, single wall nanohorn and Vulcan XC-72, by the mean of an impregnation process, in order to compare their performances as anodes in PEFC (polymer electrolyte fuel cell) and DMFC (direct methanol fuel cell). The relationship between the structure of the different carbon supports and their electrochemical behaviour is discussed