As you may have read in my previous article on energy for the future (), hydrogen is considered a top candidate to replace oil in the near future. It's high energy capacity (120 MJ/kg compared to 44 MJ/kg for gasoline), makes it a viable replacement. But there is more. It produces a clean exhaust product (water vapor without CO2 or NOx), and can be derived from a variety of primary energy sources. All in favor of hydrogen. But there is one big issues. Hydrogen is a volatile gas. It is difficult to store and can escape out of most common containers. That makes it difficult to handle and transport. But if this is the only issue keeping us from using hydrogen, maybe some research to better storage means can be done?
Reading this article means that they've done that already. With some success, but not quite perfect. I'll try to give an overview. Let's examine the criteria for our ideal hydrogen storage:
1) operate on room temperature
2) contain a lot of gas
3) atmospheric pressure
For oil, this is simply named a barrel. For a gasous substance, that's more difficult, and that's why we can allow some deviations from the ideal container. We could use some cooling if needed, and we could also put some pressure on it, but that makes it more dangerous to operate. And we don't want danger. At least, I don't.
Pressurized gas
We know this guy already. A canister of gas under pressure. It's not ideal because it's under pressure (800bar!). You need a safety valve that brings the pressure down and you're good to go. If we take this simple method our reference, we can measure up the others.
Liquid hydrogen in cryo tanks
Instead of raising the pressure, we can also drop the temperature. It contains twice as much hydrogen as the pressurized gas can, our reference, and it does not need extra pressure. What it needs is a cooling mechanism that makes the container cumbersome and the very low temperature (-252°C) makes it a hazard.
Complex compounds
Another idea is to produce the hydrogen in another molecule. For example within metal complexes. It's produced at high temperatures, because the absorption is high, but released at low temperatures, when the desorbtion is high. Playing with the temperatures would create a system of absorbtion and desorption. The container would not be that heavy, it could operate at atmospheric pressure, hold 4 times as much gas (and thus energy) as a presurized cylinder, but playing with the temperature makes it as impractical as all the others.
Absorbing Hydrogen
This is where it gets interesting. Gas can adhere to a surface, so given sufficient surface, you can make it into a storage medium. Sort of. For the same amount of volume you can only store half of what our reference would do and you still need both the low temperature (-80°C) and high pressure (100 bar). Not a good candidate at all, but at least it's a good idea.
Absorbing Hydrogen to porous metals
Absording hydrogen seems to work better if we engineer the adhering material better. We can construct a metal surface, covered in little caveats. The hydrogen adheres better, and it has a larger surface to adhere to. We are getting better results here. This technique offers 4 times as much hydrogen storage for the same volume, operates around room temperature and at atmospheric pressure. Seems like this thing has it all? Not quite, it's not robust enough. And it's heavy. Very heavy. It's volume is large, but the container is just stone heavy.
Metal-organic frameworks
So could we create a big surface area for the hydrogen to adhere to, that is lightweight itself? Well, a promising technology is the metal-organic frameworks (MOF). The idea is that you create a 3 dimensional structure of metals (to adhere the hydrogen) and small organic molecules (to bond the metals). The choice of metals and organic molecules is almost endless, so research is being done to optimize this technology. But the latest develloped MOFs are capable of storing more than 4 times the volume of hydrogen without comparable container weight while operating at room temperature and atmospheric pressure.
Conclusion
What is holding us back to fully deploy the latest technology? Well, it is not as cost effective as the current common containers. And as long as there is no dire need, investment in more expensive technology is not economical driven. It's good to know that researchers continue to refine this technology so that it will be top notch when it becomes a necessity.
