> per volume ammonia holds more hydrogen than does liquid hydrogen

How is this possible? Suspect the answer would be way over my head!

Intramolecular bonds are quite a bit smaller than intermolecular distances, so if you pack atoms into longer molecules you get a denser result.

I'm guessing very long molecules become problematic (because they don't pack well), but ammonia is a small molecule so way below that.

Liquid ammonia has a density of ~690 kg/m3 and a molar mass of 17 g/mol so ~40k mol / m3, as NH3 that's ~120k atoms of hydrogen per cubic meter. Meanwhile liquid hydrogen has a density of 71g/L and a molar mass of 2.02 g/mol so a very similar ~35k mol/m3 but as H2 that's only ~70k atoms of hydrogen per cubic meter.

And liquid hydrogen aside from being extremely flammable, can't exist above 30K and degrades storage material (https://en.wikipedia.org/wiki/Hydrogen_embrittlement), ammonia is much more forgiving and liquid at ambient temperature above 1MPa (10 times atmospheric pressure), not innocuous by any means but way easier to transport and store (storage requirements are similar to propane).

Probably also because ammonia is polar, three positive protons and an electron pair in a tetrahedral arrangement. Whereas H2 is two protons hiding an electron pair in a linear arrangement.

spot on

~120k mol/m^3 and ~70k mol/m^3

Armchair chemistry ahead:

Molecular hydrogen is unpolarized while ammonia molecules can establish hydrogen bonds, thus packing them more densely under the same conditions.

For really dense hydrogen we would need its metallic form but that requires pressures you might find in the core of jupiter. The electromagnetic force is more convenient than gravity.

I would expect this is due to the fact that each ammonia molecule contains three hydrogen atoms, whereas a hydrogen molecule contains only two. So liquid ammonia would be expected to contain more hydrogen per volume than liquid hydrogen.

That would be true of their gases.

Wouldn't it be true of both liquid and gas?

Ammonia is 10x more dense than hydrogen gas at the same temperature/pressure, so you can pack more of it into the same volume.

"liquid hydrogen"

It applies to liquids as well.

What is the typical energy density of the ammonia using this process?

Also what does the chemical reaction look like when harvesting hydrogen from ammonia?

"The raw energy density of liquid ammonia is 11.5 MJ/L,[64] which is about a third that of diesel."

https://en.wikipedia.org/wiki/Ammonia#As_a_fuel

(Note this is energy density by volume, which is the metric most care about. Energy density by weight of H2 gas is great, but the volume is enormous in comparison.)

https://en.wikipedia.org/wiki/Hydrogen_storage

Ammonia has been long recognized as a great medium for energy storage and you can generate at the site of electric generation. But the challenge has been extracting the hydrogen from the ammonia. My understanding is that hydrogen crackers exist, but have only been successful commercially at large scale. A portable cracker that you can put on a car that extracts hydrogen on demand from an ammonia storage tank is the innovation we need to see. Apparently there is work in Denmark that looks promising... (And now there is this new membrane technology from Australia.)

https://www.mvsengg.com/products/hydrogen/ammonia-cracker/

http://www.ammoniaenergy.org/ammonia-cracking-to-high-purity...

Personally, I am cheering for ammonia as a storage means. I am not a fan of batteries found in today's electric vehicles because there are too many conflict minerals in them. Maybe Tesla will succeed mining colbat in Colbat Ontario Canada... But until then, it is probably coming from the Congo or Bolivia.

https://www.washingtonpost.com/graphics/business/batteries/c...

https://www.bloomberg.com/news/features/2017-10-31/the-canad...

> "The raw energy density of liquid ammonia is 11.5 MJ/L, which is about a third that of diesel."

Yes, but that's assuming you'd burn it, I suppose (not a chemist). In this case, you're extracting H atoms using a different process, and I imagine that this wouldn't utilize the full raw energy density of ammonia.

Looking at the process as a whole, it is burning - or at least oxidation. Starts with NH3 and ends up with H2O and N2 (preferably not NO2). The energy liberated from the bonds is the same. The amount of energy available at the wheels depends on a lot of fine details of how it's used.

"what does the chemical reaction look like when harvesting hydrogen from ammonia?"

Reverse Haber process maybe?

2NH3 -> N2 + 3H2

I wonder, does converting ammonia to hydrogen absorb energy or release it? (Ie is it endothermic or exothermic?)

It's highly endothermic. Replacing the hydrogen with oxygen is exothermic.

So one can create some ammonia-air fuel cell. But reducing it first to push the hydrogen into a fuel cell will almost surely net a negative amount of energy because of the losses.

Exothermic as the reverse is endothermic.

How would this work in an ammonia-powered battery?

It doesn't. The membrane "cracks" the ammonia into N2 and H2. The N2 is released to the atmosphere. The H2 is used in a hydrogen fuel cell to power an electrical drivetrain or in an internal combustion engine with a mechanical drivetrain.

how do you regulate the membrane? does it degrade?