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I am searching for a composition which could host the most hydrogen atoms in a specific volume. Note that this density is sought for justifiable conditions in terms of pressure, temperature, ... and should be realizable in a reasonably equipped lab in quantities of a few kilograms. As for hydrogen itself, besides its very low melting point (which is a problem in certain respects), the density itself is 0.086 g.cm-3 which is lower that water (nearly 0.110 g.cm-3), so that it is evident that a composition should be sought after. Any help would be appreciated.

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    $\begingroup$ I think you need to be more specific about what you want. Are you looking for compounds that can release hydrogen easily? Like, for example, palladium. And BTW you have understated the density of water bay a factor of 10: it is close to 1g/mL not 0.1g/mL. $\endgroup$ – matt_black Jun 19 at 12:45
  • $\begingroup$ What is the context? Are you trying to generate hydrogen gas? For what purpose? $\endgroup$ – Michael Lautman Jun 19 at 14:03
  • $\begingroup$ @matt_black Note that the effective density of hydrogen mass is of importance in my problem which for water is about 2/18 of the original density. $\endgroup$ – Seyed Mohsen Ayyoubzadeh Jun 19 at 14:11
  • $\begingroup$ @SeyedMohsenAyyoubzadeh, then you should have written: "which is lower than the density of the hydrogen in water (nearly 0.110 g.cm-3)." Have you consulted en.wikipedia.org/wiki/Hydrogen_storage? $\endgroup$ – DrMoishe Pippik Jun 19 at 18:14
  • $\begingroup$ @DrMoishePippik Yes, you are correct but that could be clearly inferred from the question itself. $\endgroup$ – Seyed Mohsen Ayyoubzadeh Jun 20 at 7:49
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A few binary metal hydride candidates:

Magnesium hydride gives about 0.11 grams of hydrogen per cubic centimeter, and unlike water it can release its hydrogen leaving the element behibd on simple heating. It does, however, require 287°C to decompose, limiting reversibility.

Lithium hydride offers nearly the same hydrogen density as the magnesium compound but decomposes at a much higher temperature, limiting its utility.

Beryllium hydride approaches approaching 0.12 grams hydrogen per cubic centimeter. But it has several drawbacks; beryllium is expensive, it's a serious health hazard and the element does not react directly with hydrogen, making an element/hydride cycle impossible.

Aluminum hydride, with a favorable stoichiometry, comes inat nearly 0.15 grams hydrogen per cubic centimeter and also releases its hydrogen at 150°C. Like magnesium hydride it has been considered for hydrogen storage. However, reversible cycling between the element and the hydride is again inhibited because aluminum and hydrogen do not react directly except at very high pressure (10GPa for aluminum hydride versus 20 MPa for a direct synthesis of magnesium hydride). As with beryllium hydride, getting the aluminum back to hydride form requires indirect methods with other hydride sources.

A ternary compound with 0.15 grams of hydrogen per cubic centimeter is $\ce{Mg2FeH6}$, which can be made from the elements at 2-12 MPa pressure after compressing the metals into pellets. This compound can also release its hydrogen thermally and is one of many ternary hydride candidates for hydrigen storage.

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  • $\begingroup$ Thanks, is there a theoretical reason why the hydrogen density is in such a narrow range for these different kind of materials, e.g. water and Magnesium hydride? In other words, is 120 mgH/cm3 some kind of upper bound related to STP conditions? $\endgroup$ – Seyed Mohsen Ayyoubzadeh Jun 20 at 7:47
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    $\begingroup$ Found one with nearly 150, see edit. But yes, there is a limit to how much any atom can be squeezed by chemical bonding unless assisted by high pressure. $\endgroup$ – Oscar Lanzi Jun 20 at 9:36
  • $\begingroup$ Please add Mg2[FeH6] pubs.acs.org/doi/abs/10.1021/ic00181a032 $\endgroup$ – permeakra Jun 20 at 10:03
  • $\begingroup$ Done (with some delay b/c my first edit somehow got lost). $\endgroup$ – Oscar Lanzi Jun 20 at 18:26

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