Reactivity of commercial aluminum alloys in high-temperature water for hydrogen and heat generation

Aluminum is a promising energy carrier due to its ability to store renewable energy in a compact and safe manner. It has been shown to react completely with supercritical water, producing heat, hydrogen, and aluminum (hydr)oxide. In addition to aluminum, other metals, such as zinc and magnesium, have also shown good reactivity under these conditions. However, the effect of alloying aluminum with other elements on its reactivity in supercritical water remains largely unexplored. This work investigates the reactivity of six commercial aluminum alloys, comparing each to pure aluminum at three different operating temperatures: below (350 ∘ C ), around (380 ∘ C ) and above (410 ∘ C ) the critical point of water. Results show that pure aluminum exhibits the highest reactivity at lower temperatures, for which some alloys showed almost no sign of oxidation. At 410 ∘ C , pure aluminum oxidation rate decreased, resulting in a lower reactivity compared to the commercially pure alloy 1100 and alloy 3003. All alloys exhibited significant reactivity at the highest temperature, suggesting that scrap could be a viable feedstock under these conditions. However, it appears that reactivity can be sensitive to small variations in some elements, such as silicon, as two samples of the same alloy 1100 displayed oxidation rates differing by an order of magnitude. Finally, PXRD characterizations of the oxides produced at 410 ∘ C did not show the presence of α − Al 2 O 3 , as expected from prior results. Instead, a crystalline structure similar to that of smelter grade alumina was obtained, suggesting that the experimental conditions—including reactor design—may enable the synthesis of a more easily recyclable product. • Aluminum’s reaction with supercritical water is sensitive to alloying elements. • Reactivity does not increase with aluminum purity in all cases. • Low-Si-content alloys all generated H 2 within 30 min above 400 ∘ C . • In this setup, oxides formed at 410 ∘ C display a crystal structure similar to SGA.