In the future, a combination of two simple chemicals could help reversibly store hydrogen and absorb carbon dioxide at the same time. Such a chemical hydrogen storage facility enables the safe transport and storage of 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 amino acid lysine play a fundamental role.
Hydrogen is considered an important energy carrier of the future. It can be obtained by electrolysis of water using electricity from the wind and the sun and does not emit CO2 when burned. In fuel cells, hydrogen can also act as an electrochemical engine. The problem, however, is that hydrogen is not easy to transport in gaseous form due to its low density and risk of explosion. For mobile applications, it must therefore be liquefied or chemically bonded.
A reversible chemical hydrogen deposit
This is where the new hydrogen storage device by Duo Wei of the Leibniz Institute for Catalysis in Rostock and his colleagues comes into play. They have developed a catalytic system that chemically stores hydrogen and can release it again in a highly pure and highly efficient form. The system therefore follows the principle of an electric battery, except that the hydrogen battery is charged and discharged with hydrogen rather than 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. Furthermore, the hydrogen storage and release reactions each require different conditions, which prevent an effective “accumulator” function.
Combination of formic acid and amino acid as main components
In contrast, Wei and his colleagues have now developed a hydrogen battery that works with a relatively inexpensive 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 of formic acid (HCOOH), which acts as a storage medium for hydrogen.
Formic acid is formed when the second component of the nucleus, the amino acid lysine, reacts under the influence of the manganese catalyst with the carbon dioxide of 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. Hydrogen is now chemically bonded. For the new release, the formate is again dehydrogenated in the presence of the two reaction aids of the lysine and manganese complex, producing CO2 and hydrogen.
The lysinate binds the CO2 and closes the cycle
In the first tests of this cycle, the researchers have already achieved a yield of over 80 percent, and after ten consecutive cycles it was still a whopping 72 percent. The problem, however: “The goal of a practically usable rechargeable hydrogen battery is not achieved,” write Wei and his colleagues. Since CO2 is released with each dehydrogenation, it must be recharged and a closed circuit is not possible, in which only hydrogen enters and exits.
The chemists then optimized their system by using the potassium salt of this amino acid instead of lysine. Tests have shown that potassium lysinate can absorb 99.9 percent of the CO2 released during reactions, thus closing the CO2 cycle. “We keep CO2 permanently in our reaction system,” explains Wei’s colleague Matthias Beller. The hydrogen battery only needs to be filled with air once in the beginning, the rest is then done in the cycle.
This optimization has also increased the yield of recoverable hydrogen: the system can bind hydrogen as formate with an efficiency of 93% and release it again at 99%. Calculated over ten cycles, the overall efficiency for hydrogen storage and release is more than 80 percent, the team reports. The hydrogen gas recovered during the discharge of this hydrogen battery is also of high purity.
“This method therefore represents the most productive combination of CO2 bonding and formate dehydrogenation based on a non-noble metal catalyst,” Wei and his colleagues write. 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. (Natural energy, 2022; doi: 10.1038 / s41560-022-01019-4)
Source: Leibniz Institute for Catalysis