# Producing Hydrogen from aluminum, really ?

A friend recently asked me if it makes sense to use aluminum scrap to generate green hydrogen via the following reaction:

$1Al(s) + 3H_2O(l) \rightarrow 1Al(OH)_3(s) + 1.5H_2(g)$

with

$\Delta H (300K) = -418.8 kJ$

The combustion of this produced hydrogen:

$1.5H_2(g) + 0.75O_2(g) \rightarrow 1.5H_2O(l)$

releasing:

$\Delta H (300K) = -428.7 kJ$

Apart from the challenging aspect of being able to react aluminum with water (because of the presence of a protective alumina coating at the surface of the metal), it looks like the potential internal energy available in metallic aluminum is not fully harvested with this strategy… So, is there any plan to valorize this energy release from this oxidation reaction ?

As a comparison basis, let’s evaluate the energy release from the direct oxidation of aluminum:

$1Al(s) + \frac{3}{4} O_2(g) \rightarrow \frac{1}{2} Al_2O_3(s)$

with

$\Delta H (300K) = -837.8 kJ$

Therefore, we are loosing about 50% of the potential energy stored in metallic aluminum when using the reaction process involving water without valorizing the heat generated from this reaction… When comparing to modern water electrolysis processes (energetic efficiency of 70-80%) using green electricity, it does not seem like a promising option… And I am not talking about the use of the Ga-In eutectic mixture to clean the surface from its protective oxide…

Direct oxidation of aluminum scrap is perhaps an option to consider if one wants to harvest the energy (via power cycles) available in this metal (assuming the scrap is not easy to recycle because of the presence of contaminants such as copper, iron, etc.)… This would produce alumina which could then be recycled (hopefully) in the conventional (or inert-anode) Hall-Heroult process…

Of course, direct oxidation of aluminum is not an easy task (maybe in the liquid state ?)… Who knows…