In the future, a combination of two simple chemicals could help to reversibly store hydrogen and absorb carbon dioxide at the same time. Such a chemical hydrogen storage facility enables the safe transport and storage of the energy gas – a crucial prerequisite for the energy transition. The new hydrogen “battery” is made possible by a chemical cycle in which a manganese catalyst, formic acid and the amino acid lysine play a key role.
Hydrogen is considered an important energy carrier of the future. It can be obtained by electrolysis of water using electricity from wind and sun and does not emit any CO2 when burned. In fuel cells, hydrogen can also serve as an electrochemical drive. The catch, however, is that hydrogen is not easy to transport in gaseous form because of its low density and the risk of explosion. For mobile applications, it must therefore be liquefied or chemically bound.
A reversible chemical hydrogen storage
This is exactly where the new hydrogen storage device by Duo Wei from the Leibniz Institute for Catalysis in Rostock and his colleagues comes in. They have developed a catalytic system that chemically stores hydrogen and can release it again in a highly pure form and with a high yield. The system thus follows the principle of an electric battery, except that the hydrogen battery is charged and discharged with hydrogen instead of electricity.
There are already some concepts for such chemical hydrogen storage. “However, most of them require expensive catalysts based on noble metals such as ruthenium, rubidium or iridium,” explains the research team. In addition, the reactions for storing and releasing the hydrogen each require different conditions, which prevents an effective “accumulator” function.
Combination of formic acid and amino acid as core components
In contrast, Wei and his colleagues have now developed a hydrogen battery that works with a comparatively cheap manganese complex as a catalyst and can absorb and release hydrogen under uniform conditions – in a real cycle. In addition to the manganese catalyst, the central element of this cycle is the simple organic molecule formic acid (HCOOH), which serves as a storage medium for the hydrogen.
Formic acid is formed when the second core component, the amino acid lysine, reacts under the influence of the manganese catalyst with carbon dioxide from the air and the supplied hydrogen to form formic acid. Alternatively, the reaction to form a formate, the salt of formic acid, is also possible. The hydrogen is now chemically bound. For re-release, the formate is dehydrogenated again in the presence of the two reaction aids lysine and manganese complex, resulting in CO2 and hydrogen.
Lysinate binds CO2 and closes the cycle
In the first tests of this cycle, the researchers have already achieved a yield of more than 80 percent, and after ten cycles in a row it was still a good 72 percent. The problem, however: “The goal of a practically usable, rechargeable hydrogen battery is not achieved,” write Wei and his colleagues. Because the CO2 is released with every dehydrogenation, it has to be refilled and a closed circuit, in which only hydrogen goes in and out, is not possible.
The chemists therefore optimized their system by using the potassium salt of this amino acid instead of lysine. Tests showed that the potassium lysinate can absorb 99.9 percent of the CO2 released during the reactions, thus closing the CO2 cycle. “We keep the CO2 permanently in our reaction system,” explains Wei’s colleague Matthias Beller. The hydrogen battery only has to be filled with air once at the beginning, the rest then runs in the cycle.
This optimization also increased the yield of recoverable hydrogen: The system can bind hydrogen as formate with 93 percent efficiency and releases it again at 99 percent. Calculated over ten cycles, the overall efficiency for hydrogen storage and release is more than 80 percent, the team reports. The hydrogen gas recovered when discharging this hydrogen battery is also of high purity.
“This method thus represents the most productive combination of CO2 binding and formate dehydrogenation based on a non-noble metal catalyst,” write Wei and his colleagues. The system paves the way for the development of CO2-neutral chemical hydrogen storage based on non-toxic components. The team has already applied for a patent for their hydrogen battery. (Nature Energy, 2022; doi: 10.1038/s41560-022-01019-4)
Source: Leibniz Institute for Catalysis