Activation of aluminum particles to react with water for the purpose of hydrogen generation

Abstract

Aluminum can react with water and produce hydrogen. Researchers have developed different methods to promote the reaction of aluminum with water for hydrogen generation. Most of these methods considered ball milling of aluminum necessary prior to the reaction. In spite of numerous works on activation of aluminum powder to react with water, the activation process of aluminum powders is not optimized, and there is not enough knowledge on the kinetics and mechanism of the reaction. This research is to fill this gap. Considering the energy consumption in ball milling, firstly, we optimized the milling time based on the highest rate of hydrogen generation. Then, contributions of milling process to activation of the aluminum powder were evaluated. We found that microstructural refinement has a significant contribution in promoting the reaction. Therefore, we studied the mechanism of grain refinement of aluminum particles during ball milling. We used electrochemical tests to better understand the reaction of aluminum with hot water and effect of addition of water-soluble salts was also studied. The shrinking core model was modified to predict the kinetics of the reaction. It was found that ball milling promotes the reaction in two ways: a) increasing the instability of the microstructure (by refining the microstructure) and, b) decreasing the particle size of the powders. A considerable increase in amount of the grain boundaries was found as the reason for instability of the microstructure. Deformation banding and subgrain rotation were identified as the mechanisms responsible for introducing new boundaries during milling. For the pure aluminum, the small size and the laminated structure of particles at the medium stage of milling increased the rate of the reaction, and further milling destroyed the laminated structure and consequently decreased the reaction rate. For the aluminum-salt mixtures, there is no optimum milling time as it was observed for the pure aluminum powder. However, more milling after a certain time does not have any significant influence on the reaction rate of aluminum-salt mixture. The addition of water-soluble salts (potash or salt) considerably increased the hydrogen generation rate. Comparison of different distributions of the salt in the aluminum particles revealed that chemical aspect of the presence of salt is negligible compared to the structural modifications. Finally, considering the changes in thickness and porosity of the hydroxide layer formed on the aluminum particles, the traditional shrinking core model was modified for the reaction of aluminum particles with hot water.