On the Ignition of Mechanically Activated Al Powders for Hydrogen Production

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This study investigates the influence of material aging on the hydrogen generation and ignition behavior of aluminum-based powders designed for water-reactive applications. Two batches of Al/Bi/NaCl (85/5/10 wt%) powders in fresh and aged conditions were studied in terms of hydrogen release and ignition tests under both dry and wet conditions. The results show that aging reduces the active aluminum content from 80.5 to 73.7% due to surface oxidation and hydroxide formation during prolonged exposure to moisture. Consequently, aged powders exhibit lower hydrogen yields (88%) compared to fresh samples (96%) and slower reaction kinetics, with the fresh powder producing 40% more hydrogen within the first 30 s of reaction. Ignition tests revealed that aging and water exposure have opposing effects. Under dry conditions, aged powders ignited more easily (≈532°C), while under wet conditions, ignition was delayed (≈745°C) due to the formation of a passivation hydroxide layer acting as a thermal barrier. Fresh powders showed more uniform, predictable performance, whereas aged materials exhibited greater variability and reduced efficiency. These findings highlight the strong interdependence among powder aging, environmental conditions, and ignition safety, emphasizing the need for strict storage and handling protocols to minimize risks associated with hydrogen generation and reactive powder ignition.