Hydrogen from Aluminum, coffee and seawater
In this little write-up, I am questioning the largely misleading press coverage of a cell.com article by some MIT authors quoted below, which reports the production of hydrogen from seawater using elemental aluminium activated with some rare metals, which we are going to ignore (The rare metals, I mean). In all fairness, the title of the paper itself talks about the enhanced recovery of exactly those rare activation metals. But the press department of MIT already converted that into a revolutionary method to produce hydrogen from seawater.
The cell.com article is here:
https://www.cell.com/cell-reports-physical-science/fulltext/S2666-3864(24)00399-0
And this is some example of a typical news coverage. There are hundreds such news items.
Part of problem why popular press might misunderstand things is of course that they never read the primary article but just cite that press department statement. But in this case, the way the original authors talk about energy efficiency in their paper is also a problem. They write: “Estimations indicate that the energy inputs required for various subprocesses involved in treating aluminum for fuel production constitute approximately 2% of the total energy output from the reaction.” This is of course only true if you think that aluminium lies around on the street in elemental form, which indeed it does in the form of thrown away soda cans, which, as we all know, grow on soda can trees.
But here are some basics to reconsider what has been reported. Those “basics” are a result of some afternoon of googling and back-of-the-envelope calculations. There might be a major flaw and I am happy to stand corrected.
- Aluminum is produced from bauxite using the Bayer Process. It is one of the most energy-intensive industrial processes on the planet.
- 55% of all aluminium is produced with coal as an energy source, 10% from gas, the rest is hydropower and other renewables..
- The production of 1 Kg of Aluminum from bauxite needs at least 13 kWh electrical energy.
- The energy density of 1 Kg of H2 is 33 kWh.
- The atomic weight of Al is 13, the molecular weight of H2 is 2.
Here is the chemical equation of the reaction of elemental aluminium with water to produce hydrogen:
2 Al + 3 H2O -> Al2O3 + 3 H2
This means that 26 kg of aluminium produces 6 kg of hydrogen. In energetic terms, 338 kWh is needed to produce roughly 200 kWh worth of energy in the hydrogen synthesised, an efficiency of approximately 60%!
Conventional electrolysis systems that produce hydrogen from water have an efficiency of 60% – 85%. But then of course you need to compress, transport, etc, the H2.
If you recycle the aluminium, only 5% of the energy from the Bauxite process is needed – a far better energy efficiency than when burning (oxidising it to Al2O3 again) it.
This might all be fine. The aluminium could be produced using completely from renewable energy. Then the authors’ argument that “aluminum holds twice the energy density per unit volume of diesel and approximately 40 times that of lithium-ion batteries.” might be relevant.
Currently, a large amount of fossil energy would be invested to produce hydrogen with a moderate efficiency of 60%, which is then used to propel ships, presumably with a fuel cell with an efficiency of again 60%. This is clearly not what the authors of the many press items about this paper have understood and conveyed in their articles.
Categorised as: Open Science
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