Effects of Mg and Si Doping on Hydrogen Generation via Reduction of Aluminum Alloys in Water

Harnessing the oxidation-reduction reaction between aluminum and water is an attractive option for hydrogen generation and storage. Using liquid metals as a method of circumventing aluminum's native oxide layer has been gaining popularity, yet the underlying reaction mechanism and various effects on hydrogen generation remain elusive. This article reports the investigation of liquid-metal-activated aluminum that has been doped with commonly employed alloying elements. We observe that the eutectic Ga-In penetrates through subgrain boundaries in aluminum, providing experimental evidence that the eutectic permeates as a line dislocation front. We also demonstrate the opposing effects that magnesium and silicon alloying elements have on the hydrogen-generating properties of liquid-metal-activated commercial aluminum alloys. Compared to pure aluminum, the addition of 0.6 wt % silicon to aluminum gives rise to significantly higher reaction rates and hydrogen yields (+20%). The addition of 1 wt % magnesium to aluminum significantly lowers the rate and yield, especially following 72 h of eutectic Ga-In permeation (-30%). When both magnesium and silicon are present in aluminum such that intermetallic particles are formed like in the commercial alloy AA6061, the rate and extent of the aluminum-water reaction are highly dependent on permeation time. After 96 h of permeation, Al + Mg, Si produces the same amount of hydrogen as pure aluminum produces following only 48 h of permeation. These experimental observations do not agree well with historical studies based on pure corrosion of aluminum, indicating that the reaction mechanism involving eutectic Ga-In is more complex and hence requires additional investigations to understand. Using these discoveries, we can now better predict the reaction rate of the aluminum with water when considering Mg-doped and/or Si-doped scrap aluminum as a fuel source, giving rise to more applications in which aluminum can be harnessed as a hydrogen production and storage technology.