Feb 15 2022

LOHCs for Hydrogen Storage

The “coming hydrogen economy” never came, and may never come, because of the critical problem of hydrogen storage. Hydrogen can be a great fuel because it is light and burns clean with oxygen, creating only water as a byproduct. But these features of hydrogen, while appealing, are not enough. The angle that popular reporting on such scientific advances rarely take is that, in order to be useful, hydrogen fuel (or any similar technology) must work as an entire system. We therefore have to imagine the entire industrial cycle of a hydrogen economy – where will the hydrogen come from, how will it be stored, how will it be distributed, how will it be burned, what are all the byproducts of this entire system, and what other materials are required to make it work? Every piece of the system must be scalable, safe, cost competitive, efficient, and convenient. Further, and significant new infrastructure requirements will present a hurdle to adoption.  The potential number of deal-killer “gotchas” are enormous.

What often happens, however, is that articles report on a scientific advance in one aspect of the entire cycle and couples that with the implication that now the hydrogen economy is really almost here, we just need to work out a couple of details. They rarely mention how the whole system will work, or the fact that those remaining “details” are, in face, deal-killers. They also rarely put it into context of other competing technologies. In the case of hydrogen fuel for cars that competition is battery electric vehicles, which are clearly winning the race to replace internal combustion engines.

With all that in mind, I read the following article: “Department of Energy’s “Fairly Simple” Breakthrough Makes Accessing Stored Hydrogen More Efficient.” The entire article was framed as a breakthrough leading us down the road to hydrogen fuel cars. There are many quotes like:

“This research will positively impact the target of reducing carbon dioxide emission,” Huang said, “and we will need to develop more efficient catalytic systems.”

So now let’s put this into context. The discovery is definitely a good one that may have industrial applications, I just don’t think it will impact the auto industry. The researchers developed a catalyst (a compound that makes a chemical reaction go faster) that can liberate hydrogen from a liquid organic hydrogen compound (LOHC) where it is being stored. LOHCs are carbon-based compounds that have lots of receptor sites for hydrogen. So they have a dehydrogenated state (with lots of double bonds between the carbon atoms), and a hydrogenated state, where hydrogen atoms bind to all those carbon atoms. Toluene is one such compound that is often used as an LOHC.

How would an LOHC be used as part of a hydrogen economy? The article never makes it explicit, but in my further searching it’s clear that the primary use would be for the transportation and storage of hydrogen, but not for use as a fuel in a car. The idea is that you have a factory that makes hydrogen, say by splitting water using electricity produced by wind farms or nuclear power or through direct electrolysis using solar energy. That hydrogen can then be bound to an LOHC for long term storage at normal temperature and pressure. Because it’s a liquid it can also be easily transported by truck or even piped. A the other end the hydrogen is then released from the LOHC for use in a hydrogen fuel cell.

That’s a lot of steps, and each step adds inefficiency (energy is used) and potential expense. In order for the whole system to work, each step, therefore, needs to have all the nice features I listed above. This new catalyst addresses one step – the release of hydrogen from the LOHC. Existing catalysts require metals, usually platinum group metals that are rare and expensive (no bueno). This new catalyst does not include any metals, and is also cheap and can be mass produced. Further still it can operate at normal temperature and pressure. That’s very good, because if any step in the process requires either high temperature or high pressure, that costs a lot of energy. So this is a nice advance, make no mistake, it’s just not a gamechanger for a hydrogen economy focused on transportation. We still have the problem of how we are going to store the hydrogen in the vehicle.

Could an LOHC be used directly as a fuel? There are proposed systems for this (meaning specific LOHC molecules and their hydrogen storage cycles). One uses acetone/isopropanol (the first chemical is the dehydrogenated molecule and the second is the hydrogenated form). However this system requires high pressure (300 kPa) and high temperature (85-100 degrees C). High temperature like that is not a big deal as car engines will tend to produce waste heat, so once the engine is running it can be self-sustaining.

There is also the matter of energy density. Ideally LOHCs used as fuel in cars will be more than 6.5% hydrogen by weight when hydrogenated. Right now the new catalyst has not been tested on any such LOHC, so again, we don’t have a full proposed system for using the LOHC. The reporting just glosses over that point –

“Qi and Huang explained that based on DOE goals for vehicle technologies, the hydrogen storage capacity needs to be close to 6.5% by weight. They are optimistic about the future of their research to meet the goal with molecules that have a larger capacity.”

That is always how such drawbacks are framed – the researchers are confident that in the future their system will solve the remaining issues. But they rarely make explicit that such problems may not be solved, and until they are the technology is dead in the water.

Ultimately I think that LOHCs may serve some niche but important role in industry where storing hydrogen is needed or can be useful. I just don’t like the chances of this kind of technology as a fuel system for vehicles. There are too many moving parts, too many opportunities for “gotchas”, and too much of an infrastructure hurdle. Researchers are also still working out the details, which optimistically will likely take at least 10-20 years to sort out of everything goes well. In that time battery EVs will continue to advance their lead and build infrastructure (charging stations).

No responses yet