Since the beginning of the 19th century, scientists have recognized that a world dominated by fossil fuels is no longer a sustainable condition for humankind to thrive; the huge damages that this kind of energy source is dealing both to our health and to the environment can no longer be ignored and must be addressed. The urge to go to a desirable and drastic reduction of both world-wide greenhouse gas emission and non-renewable resource exploitation is moving the scientific community toward the study of more environmental-friendly and renewable energy sources. The idea of using hydrogen as an energy carrier was noticeably strengthened after the global crisis of 1974. The outstanding properties of hydrogen as a fuel (high energy density, low flashpoint, huge flammable range and octane number), bolstered by the fact that its only combustion product is water (thus preventing further environmental pollution), opened the doors to a possible development of a global “hydrogen economy” based on the hypothesis that hydrogen could play a basilar role as a future energy carrier, chiefly as innovative fuel in automotive field, in order to flank or (hopefully) replace the traditional fossil fuels. Obviously it couldn’t be so easy. There are two major drawbacks that are currently hindering the application of hydrogen as a fuel: first of all its production, and second its storage. As a secondary energy source, hydrogen must be obtained using a primary energy source; unfortunately, fossil based fuels are still the main resource used for industrial mass scale hydrogen production, and this fact is absolutely and totally in contradiction with the very idea of a green and sustainable energy cycle. The electrochemical splitting of water does not have a large share in global hydrogen production due to low conversion efficiency and high electrical power expenses. Then, once hydrogen is produced, the second main limitation comes in, i.e.: the choice of the hydrogen storage method. In current industrial applications hydrogen is stored either as a compressed gas or as a cryogenic liquid; both techniques have their flaws: storing hydrogen as a gas, due to the high pressures required, poses a serious threat to personal safety (the most striking accident occurred to the German airship Hindenburg in 1937, when 36 people lost their lives); while storing hydrogen as a cryogenic liquid isn’t dangerous, it is very expensive and not cost-effective on a large scale distribution due to the extremely low temperatures needed. The third storage option involves solid-state compounds (such as metal hydrides) that are capable of reversibly absorbing large amounts of hydrogen. The goal of the thesis project is to find an efficient way to achieve Hydrogen storage in a solid state compound, in order to overcome the major drawbacks that are preventing the use of Hydrogen as a fuel. We used metal borohydrides [Mg(BH4)2] and Reactive Hydrides Composites (RHC, LiH+MgB2), systems that are able to exchange hydrogen in a reversible way in suitable temperature and pressure conditions. These materials suffer from three major limitations: the temperatures of both the hydrogenation and dehydrogenation are still high (from 270°C to 390°C) for practical applications, the kinetics are slow (6h for the desorption, and from 12h up to 24h for the absorption), and after some cycle the materials stops being completely reversible. We are trying to overcome these drawbacks by adding substances acting like catalysts and destabilizing agents (nanometric TiB2 and ZrB2) and by confinement in porous matrices such as SiO2 aerogel, in order to improve the reaction’s kinetics and the thermodynamic properties.
Attualmente il settore energetico è in un periodo di transizione: le riserve di combustibili fossili stanno gradualmente diminuendo e il loro impiego è reso sempre più difficile da questioni ambientali ed economiche. A fronte di indubbi vantaggi pratici ed economici, l’utilizzo massiccio delle fonti fossili comporta una serie di problematiche gravi: dal punto di vista socio-politico il problema principale è costituito dal fatto che le fonti fossili non sono equamente distribuite sulla superficie terrestre, e pertanto i Paesi che non possiedono giacimenti sono fortemente vincolati a quelli che invece ne hanno grande disponibilità; dal punto di vista ambientale il problema è dato dal rilascio in atmosfera di sostanze inquinanti a seguito della combustione, tra le quali si annoverano il monossido di carbonio, ossidi di azoto e zolfo, composti organici volatili, polveri sottili e particolato, e anidride carbonica. Appare dunque evidente che il futuro dell’industria energetica dipende dallo sviluppo di nuovi metodi di produzione dell’energia, economici e non inquinanti, come ad esempio l’utilizzo e la commercializzazione dell’idrogeno. L’idea di utilizzare l’idrogeno come combustibile ha fatto presa dopo la crisi energetica globale del 1973; il drastico aumento del prezzo del carburante, unito alla presa di coscienza del grave impatto ambientale che deriva dal crescente utilizzo dei combustibili fossili, è servito come catalizzatore ed ha aperto la strada alla ricerca di fonti energetiche alternative e “green”. I due principali svantaggi che ne limitano la diffusione come combustibile sono i processi di produzione e stoccaggio. Essendo un vettore energetico, l’idrogeno deve essere prodotto utilizzando fonti energetiche primarie; ad oggi, il metodo più conveniente e comunemente usato per produrre idrogeno è tramite il processo dello steam reforming partendo da idrocarburi, e questo fatto è ovviamente incompatibile con la ricerca di un combustibile “green”. Un’ulteriore limitazione risulta essere la scelta della fase in cui utilizzare l’idrogeno come combustibile e il conseguente problema dello stoccaggio della fase scelta. Nelle moderne applicazioni industriali l’idrogeno viene immagazzinato o come gas compresso o come liquido criogenico; nel primo caso le elevate pressioni richieste pongono seri rischi nell’utilizzo on board, mentre nel secondo caso la bassa temperatura di stoccaggio richiede contenitori con tecniche di isolamento sofisticate, troppo costosi per una distribuzione su larga scala. Lo scopo del lavoro di tesi è pertanto quello di trovare un metodo efficiente che permetta lo stoccaggio dell'idrogeno in composti a stato solido, nella speranza che possa diventare un carburante alternativo nell'industria automobilistica. In questo progetto di tesi sono stati studiati due diversi metodi di stoccaggio in fase solida: il confinamento in una matrice di SiO2 del Mg(BH4)2 e l’utilizzo di Reactive Hydrides Composites (RHC), ovvero una miscela di LiH e MgB2; entrambi questi sistemi sono in grado di scambiare reversibilmente idrogeno in opportune condizioni di temperatura e pressione. I problemi riscontrati con questi materiali sono principalmente tre: le temperature di idrogenazione e deidrogenazione sono troppo alte (tra 270°C e i 390°C), le cinetiche sono lente (6 ore per il desorbimento e da 12 a 24 ore per l’assorbimento), e dopo qualche ciclo il materiale non risulta più in grado di scambiare idrogeno in maniera completamente reversibile. Abbiamo cercato di limitare questi problemi e di migliorare le proprietà cinetiche e termodinamiche del processo aggiungendo catalizzatori nanometrici (quali TiB2 and ZrB2) alla miscela RHC e sperimentando un diverso approccio per quanto riguarda l’infiltrazione del boroidruro in matrice di SiO2.
Preparazione e caratterizzazione chimico-fisica di materiali innovativi a base boroidruri per lo stoccaggio di idrogeno allo stato solido
GIOVENTU', MARTA
2016/2017
Abstract
Since the beginning of the 19th century, scientists have recognized that a world dominated by fossil fuels is no longer a sustainable condition for humankind to thrive; the huge damages that this kind of energy source is dealing both to our health and to the environment can no longer be ignored and must be addressed. The urge to go to a desirable and drastic reduction of both world-wide greenhouse gas emission and non-renewable resource exploitation is moving the scientific community toward the study of more environmental-friendly and renewable energy sources. The idea of using hydrogen as an energy carrier was noticeably strengthened after the global crisis of 1974. The outstanding properties of hydrogen as a fuel (high energy density, low flashpoint, huge flammable range and octane number), bolstered by the fact that its only combustion product is water (thus preventing further environmental pollution), opened the doors to a possible development of a global “hydrogen economy” based on the hypothesis that hydrogen could play a basilar role as a future energy carrier, chiefly as innovative fuel in automotive field, in order to flank or (hopefully) replace the traditional fossil fuels. Obviously it couldn’t be so easy. There are two major drawbacks that are currently hindering the application of hydrogen as a fuel: first of all its production, and second its storage. As a secondary energy source, hydrogen must be obtained using a primary energy source; unfortunately, fossil based fuels are still the main resource used for industrial mass scale hydrogen production, and this fact is absolutely and totally in contradiction with the very idea of a green and sustainable energy cycle. The electrochemical splitting of water does not have a large share in global hydrogen production due to low conversion efficiency and high electrical power expenses. Then, once hydrogen is produced, the second main limitation comes in, i.e.: the choice of the hydrogen storage method. In current industrial applications hydrogen is stored either as a compressed gas or as a cryogenic liquid; both techniques have their flaws: storing hydrogen as a gas, due to the high pressures required, poses a serious threat to personal safety (the most striking accident occurred to the German airship Hindenburg in 1937, when 36 people lost their lives); while storing hydrogen as a cryogenic liquid isn’t dangerous, it is very expensive and not cost-effective on a large scale distribution due to the extremely low temperatures needed. The third storage option involves solid-state compounds (such as metal hydrides) that are capable of reversibly absorbing large amounts of hydrogen. The goal of the thesis project is to find an efficient way to achieve Hydrogen storage in a solid state compound, in order to overcome the major drawbacks that are preventing the use of Hydrogen as a fuel. We used metal borohydrides [Mg(BH4)2] and Reactive Hydrides Composites (RHC, LiH+MgB2), systems that are able to exchange hydrogen in a reversible way in suitable temperature and pressure conditions. These materials suffer from three major limitations: the temperatures of both the hydrogenation and dehydrogenation are still high (from 270°C to 390°C) for practical applications, the kinetics are slow (6h for the desorption, and from 12h up to 24h for the absorption), and after some cycle the materials stops being completely reversible. We are trying to overcome these drawbacks by adding substances acting like catalysts and destabilizing agents (nanometric TiB2 and ZrB2) and by confinement in porous matrices such as SiO2 aerogel, in order to improve the reaction’s kinetics and the thermodynamic properties.È consentito all'utente scaricare e condividere i documenti disponibili a testo pieno in UNITESI UNIPV nel rispetto della licenza Creative Commons del tipo CC BY NC ND.
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https://hdl.handle.net/20.500.14239/20787