Interestingly, the little question of Hydrogen storage for fuel consumption purposes is actually revision for me right now! Believe it or not… So the US government has deemed that 62kg m-3 is the threshold storage density for hydrogen to be viable as a fuel. Hydrogen is quite a logical fuel to use really, seeing as it has a higher energy density than pretty much any other fuel, and it’s easy to come by. After all, over 90% of the baryonic matter in the universe is Hydrogen. On Earth, it’s child’s play to produce, as all that needs to be done is the electrolysis of water, giving hydrogen and oxygen. Storing hydrogen as plain old hydrogen, however, isn’t the safest of things to do. It needs to be pressurised an cooled well below freezing point to turn it into liquid. Even though it’s what’s commonly referred to as rocket fuel, I personally wouldn’t like to put that much liquid hydrogen anywhere near a combustion engine! On the plus side, however, liquid hydrogen is extremely light, at around 70kg m-3 (compare that with water, which by definition weighs 1000kg m-3).
How is this revision? Because a logical solution would be to store it through adsorption within a mesoporous material. Mesoporous materials contain pores ranging between 5 and 20 nanometres in diameter, making them effectively a molecular sponge. Personally, I’d suggest a pillared Layered Double Hydroxide material. LDHs (as they’re colloquially known) are type II layered materials, consisting of positively charged layers of metal ions, terminated by hydroxy groups. Intercalated between these layers (in a stage 1 order) are anions which balance the charge. Pillaring these LDHs with large anions (such as Ta6O18OH7- ) is a process to hold the layers apart (with tiny molecular ‘Pillars’), increasing the pore sizes, thus increasing the potential for hydrogen adsorption within the material.
These pillared LDHs could provide viable storage for molecular hydrogen for a number of reasons — each hydroxy groupis capable of capturing two hydrogen molecules using Van der Waals interactions. The large negative anions would also help attract and bind hydrogen, while being large enough to (hopefully) prevent the formation of actual chemical bonds. Then it would be a matter of testing to plot the BET isotherm for adsorption of molecular hydrogen onto the material. Slightly backwards, admittedly, as BET isotherms are usually used to determine the surface area of a material… I don’t know if a sufficient volume could be adsorbed, but theoretically, it’s possible. However, it’s also likely that the storage medium would weight significantly more than the hydrogen stored therein. Maybe an aerogel of some kind might be better…
I guess all this solid state chemistry revision has paid off!