A novel system can potentially eliminate the need to store hydrogen in vehicles under high pressure in massive tanks. Instead, it can be chemically stored using ionic liquids (salts) and a catalyst that can convert CO2 and hydrogen into a liquid, and the hydrogen can be released as it is needed.
Hydrogen offers great potential within the green transition but is difficult to work with because of its special characteristics.
Hydrogen-powered vehicles must store compressed hydrogen at high pressure in tanks that are heavy and difficult to fill. Since hydrogen is also highly flammable, special safety considerations are required.
However, hydrogen and CO2 can be combined to create formic acid, which can be stored very easily under negligible vapour pressure.
New research shows how hydrogen can be bound to (hydrogenation) or separated from CO2 (dehydrogenation) easily and elegantly using a catalyst in a patented system, thereby enabling hydrogen to be used more economically and practically in the transport sector.
“Our patent is the result of unique interdisciplinary research collaboration. We combined our knowledge of ionic liquids and catalysts for CO2 activation to solve a problem that many have tried to solve before us. In addition, this system can be easily used in vehicles or elsewhere in the energy sector,” explains a researcher behind the project, Associate Professor Martin Nielsen, , Department of Chemistry, Technical University of Denmark, Kongens Lyngby.
The research has been published in the Journal of the American Chemical Society.
Using formic acid in the transport sector
The method combines hydrogen with CO2 to create formic acid, which enables hydrogen to be stored under conditions that make sense in the transport sector: easily hydrogenating CO2 into formic acid and then dehydrogenating the formic acid when the hydrogen is needed.
Research in this field has been intense for many years, and solutions exist for hydrogenating CO2 and dehydrogenating the formic acid. For example, formic acid can be dehydrogenated by heating it to 300°C with a catalyst, but this is unsuitable for vehicles.
Another possibility is to employ an additive that promotes the reaction at lower temperatures. However, using an additive to dehydrogenate formic acid renders the previous hydrogenation more difficult, which makes the transition to a more sustainable transport sector more complicated and expensive.
Using ionic liquids (salts) that melt at room temperature
Martin Nielsen and colleagues developed a novel system to hydrogenate CO2
and to dehydrogenate the resulting formic acid without using high temperatures or additives. Instead, the researchers used a small amount of a ruthenium catalyst (Ru-PNP) and ionic liquids.
Ionic liquids are a family of compounds composed entirely of ions that melt at low temperature, similar to how table salt melts at 800°C.
Combining the ionic liquids and the catalyst is ideal for moving electrons around in CO2, hydrogen, and formic acid to make the components join or separate under exceedingly mild conditions.
Another researcher behind the study, Professor Anders Riisager, says that the Department of Chemistry at the Technical University of Denmark has investigated ionic liquids for more than 20 years and that various industrial applications of these liquids with their special properties have begun to emerge, also for large-scale applications.
“The Ru-PNP/ionic liquid system keeps the chemical reaction time relatively short and enables repeat reactions. The system enables both hydrogenation of CO2 into formic acid and dehydrogenation of the formic acid without expending much energy or consuming the catalyst. Our tests have shown that the system can be used at least 18 million times, an extremely high number, and that it is very stable, which is relevant for real-world applications such as in vehicles.”
New energy infrastructure not required
Martin Nielsen and Anders Riisager envision a future in which hydrogen-powered vehicles fill up with hydrogen at filling stations.
The system in the vehicle then converts the hydrogen into formic acid and stores it in a tank under normal pressure and temperature.
As you press the accelerator, the system splits the formic acid into CO2 and hydrogen, so that the hydrogen can then propel the vehicle using a fuel cell.
“The good thing about the technology we have developed is that we do not need to invent a completely new energy infrastructure. Hydrogen filling stations are being built in more and more places globally, so we are taking advantage of infrastructure that already exists and is constantly being expanded,” explains Martin Nielsen.
Patent rights sold
Martin Nielsen and Anders Riisager also think that their invention can have other commercial applications, such as making hydrogen batteries, which can help solve some energy challenges in parts of the world with poor energy infrastructure.
The good thing is that the system does not need hydrogen and can just as efficiently use formic acid, which is much easier and safer to transport.
“An advantage of our system is that we have shown that we can convert formic acid into hydrogen as it is needed. Other systems convert all the formic acid at once, whereas our system only converts the hydrogen required to power a vehicle’s engine,” says Anders Riisager.
The researchers say that a patent has been filed for the invention, and the rights have already been bought by a major actor in the chemical industry that has interests in the automotive industry.