Hallo everybody,
Let me comment on several aspects which I see crucial, and apologize the length of this contribution which I have to split into several parts. This is the first part.

1) Aluminum as a high energy density storage medium

The authors of the disputed article have the same point as many contributors herein – that a cheap, economically feasible energy storage may be the crucial „missing link“ between electricity from renewable energy sources and a more sustainable „carbon neutral“ economy.
Indeed, with respect to energy density – which may be roughly taken as an estimate for the costs of energy storage in the chosen medium – aluminum looks pretty favourable.
Please note that the volumetric energy density value given in the disputed article for hydrogen pertains to liquid hydrogen which is the most dense practically applicable form of elemental hydrogen at normal pressure. Obviously the vessels for reliable storage of cryogenic liquid hydrogen are more expensive than (pretty expensive) pressure vessels chosen as a more feasible technical solution by automobile manufacturers developing hydrogen cars. Therein, volumetric density slightly above 1 kWh/L is a current standard with hydrogen pressurized to 70 MPa (700 bars).

2) Energy losses from irreversibilities in energy storage and/or recovery

High level of reversibility in the respective energy storage and recovery is the main advantage of batteries. Unfortunately, there is at least one fundamental reason for which critical disadvantage of batteries - high price for a unit capacity – is basically incurrable even with cheapest materials.

This fundamental reason is the ratio between volume and mass of the electrochemically active materials which can be put into a reasonably operable battery on one hand and the overall volume and mass of all other parts necessary for this reasonable function.
This unfavourable ratio can significantly, in several orders of magnitude, improve in „flow“ batteries, and even more if we split the function of the flow battery into a separate electrolyzer designed solely for „charging“ our storage system and a separate fuel cells designed solely for „discharging“ (electricity recovery from the storage system).

Unfortunately, the only electrochemically active medium that is more-less applicable in such „ideal“ electricity storage systems (that enable both high capacity and high storage efficiency by combining the high efficiency of direct electrochemical energy conversion with high energy density of a neat „fuel“ used as the energy storage medium), is still hydrogen – which, however, suffers from the costly storage due to low volumetric energy density in comparison with conventional fuels.
As soon as we try to improve hydrogen energy density by converting it chemically into a form which is more favourable form the viewpoint of the cheap storage and transport (e.g. into liquid ammonia having energy density above 4 kWh/L, or, by carbon dioxide reduction, into carbon-based synthetic fuels), we will necesarily lose a significant part of the energy originally conserved in hydrogen, as a Gibbs energy of the respective reaction dissipated on the expense of a thermodynamically „spontaneous“ chemical conversion.

For example, by simple hydrogen conversion into ammonia by its reaction with atmospheric nitrogen that is achievable on an industrial scale by well-known Haber-Bosch process, you unavoidably lose about 30 % of the renewable electricity which you originally conserved in the „green“ hydrogen. Very similar numbers apply for carbon dioxide conversion into synthetic methane by well-known Sabbatier process, and you can take as a rule of thumb that the more complicated conversion, the more of the originally conserved energy you have to sacrifice.

Even worse, none of the synthetic fuels currently considered for large scale energy storage can even approach hydrogen in technical maturity of its direct electrochemical re-conversion into electricity. For example, known ammonia fuel cells are still at least order of magitude worse in their performance than hydrogen fuel cells. And, to my best knowledge, reliable and highly efficient hydrogen fuel cells are - after decades of development - still impossible without use of precious and therefore pretty expensive platinum metals.

Finally, as soon as you resort to simply burning your synthetic fuel in an internal combustion engine or any similar equipment, the efficiency of electricity recovery from the respective fuel drops to some 40 %.

The second part will follow.