Hydrolysis of metals is a promising technique to generate pure hydrogen without the emission of CO2 and other polluting agents. It is also a safe and efficient way of H2 storage. Currently, the most commonly used metal is micrometric aluminum. Such material presents stable properties with time, low cost, relatively low toxicity and high availability. However, contact between water and aluminum is prevented by its protective oxide layer on the surface, reducing dramatically the reactivity of the powder and then the hydrogen generation capability. The augment of reactivity for aluminum powders plays a key role. Mechanical activation via ball milling is a possible way to increase metal powder reactivity. In the literature, interesting results were achieved reducing aluminum size, down to the nanometric scale. Unfortunately, long activation times are often required. To favor the reaction between aluminum and water, several strategies were employed in the past. The use of strong acid or alkaline aqueous solutions instead of pure water was one of the most used. However, such method increases dramatically health and safety hazards. Another promising way is the mechanical activation with aluminum additivated to salts or other metals. Good results were obtained, but they are still insufficient for industrial applications on large scale. This work explores some strategies to favor the aluminum corrosion in pure water to find the best way for an efficient hydrogen generation. Tap water, pure water and water-NaCl solution were evaluated as possible environment for the reaction. NaCl, Bi, Sn and Zn were employed as additives in the milling process. The powders were obtained through mechanical activation with a planetary mill and an innovative milling technique (NHEMA). An optimal combination of milling parameters was found. The Al/Bi/Zn powders featured interesting behavior in tap water with a significant high hydrogen generation and high yields (> 90%). Short activation times were required for the production of these materials (20 min). A deep characterization of a selection of such powders to relate the behaviour in water and their microstructure and composition was done. A possible mechanism of corrosion was proposed and discussed.
L’idrolisi dei metalli è un metodo promettente per la generazione di idrogeno puro senza l’emissione di CO2 e altri agenti inquinanti. È anche un metodo sicuro ed efficiente di stoccaggio. Attualmente il metallo più comunemente usato è l’alluminio, in forma di polvere sferica micrometrica. Questo materiale presenta proprietà stabili nel tempo, bassi costi, tossicità relativamente bassa ed è facilmente reperibile. Tuttavia, il contatto tra l’acqua e l’alluminio è bloccato dal suo strato di ossido protettivo presente sulla superficie, riducendo drasticamente la reattività della polvere. L’aumento della reattività delle polveri di alluminio è quindi un aspetto chiave al fine di garantire un'efficiente produzione di idrogeno. L’attivazione meccanica per mezzo di macinazione con sfere è uno dei possibili modi per incrementare la reattività della polvere. Nella letteratura, risultati considerevoli sono stati raggiunti riducendo la dimensione della polvere, fino anche alla scala nanometrica. Sfortunatamente, di solito sono necessari tempi di attivazione molto lunghi. Per incentivare la reazione tra alluminio e acqua, differenti strategie sono state impiegate in passato. Un sistema è stato l’uso di soluzioni acquose fortemente acide o alcaline. Tuttavia in questo modo i rischi per la salute e la sicurezza aumentano enormemente. Vie promettenti sono l’attivazione meccanica di alluminio additivato con sali o altri metalli. I risultati ottenuti sono stati buoni, ma ancora insufficienti per applicazioni industriali su larga scala. Questo lavoro esplora alcune strategie innovative per la produzione di polveri in grado di generare idrogeno. Come ambiente di reazione sono state valutati acqua potabile, acqua pura e una soluzione di acqua e NaCl. NaCl e i metalli Bi, Zn e Sn sono stati usati come additivi per l’alluminio nel processo di macinazione. Le polveri sono state ottenute attraverso attivazione meccanica usando un mulino planetario e una tecnica innovativa di macinazione (NHEMA). Una combinazione ottimale dei parametri di macinazione è stata trovata. Le polveri di Al/Bi/Zn hanno raggiunto ottimi risultati reagendo in acqua potabile con evoluzioni di idrogeno e rendimenti superiori al 90%. I tempi di attivazione richiesti per la produzione di queste polveri sono inferiori ai 20 min. Un’approfondita caratterizzazione di una selezione di tali polveri è stata eseguita per relazionare il comportamento in acqua e descrivere la loro microstruttura e composizione. Un possibile meccanismo di corrosione è stato proposto.
Mechanically activated Al-based powders for efficient hydrogen generation from water
SGARZI, RAFFAELE
2018/2019
Abstract
Hydrolysis of metals is a promising technique to generate pure hydrogen without the emission of CO2 and other polluting agents. It is also a safe and efficient way of H2 storage. Currently, the most commonly used metal is micrometric aluminum. Such material presents stable properties with time, low cost, relatively low toxicity and high availability. However, contact between water and aluminum is prevented by its protective oxide layer on the surface, reducing dramatically the reactivity of the powder and then the hydrogen generation capability. The augment of reactivity for aluminum powders plays a key role. Mechanical activation via ball milling is a possible way to increase metal powder reactivity. In the literature, interesting results were achieved reducing aluminum size, down to the nanometric scale. Unfortunately, long activation times are often required. To favor the reaction between aluminum and water, several strategies were employed in the past. The use of strong acid or alkaline aqueous solutions instead of pure water was one of the most used. However, such method increases dramatically health and safety hazards. Another promising way is the mechanical activation with aluminum additivated to salts or other metals. Good results were obtained, but they are still insufficient for industrial applications on large scale. This work explores some strategies to favor the aluminum corrosion in pure water to find the best way for an efficient hydrogen generation. Tap water, pure water and water-NaCl solution were evaluated as possible environment for the reaction. NaCl, Bi, Sn and Zn were employed as additives in the milling process. The powders were obtained through mechanical activation with a planetary mill and an innovative milling technique (NHEMA). An optimal combination of milling parameters was found. The Al/Bi/Zn powders featured interesting behavior in tap water with a significant high hydrogen generation and high yields (> 90%). Short activation times were required for the production of these materials (20 min). A deep characterization of a selection of such powders to relate the behaviour in water and their microstructure and composition was done. A possible mechanism of corrosion was proposed and discussed.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/150006