The research team jointly developed a homogeneous catalyst system with which they can bind hydrogen (H2) to potassium bicarbonate and thus store it chemically in a safe and stable manner. Bicarbonate is a salt of carbonic acid, commonly known as baking powder or baking soda.
The hydrogen in the system described reacts with bicarbonate in the presence of a ruthenium catalyst to formate, another harmless salt, namely that of formic acid. “We can release the hydrogen stored in the formate at any time - with the same catalyst, in the same system,” explain Dr. Rui Sang and doctoral student Carolin Stein, both first authors of the scientific publication.
According to research group leader Dr. Henrik Junge, the system works stably at temperatures of around 60 degrees Celsius. The reaction takes place in a solution containing all the chemical substances involved: Hydrogen and bicarbonate as well as the catalyst, which makes the reaction possible in the first place and is not consumed in the process itself. In the case of the latest publication, it is based on ruthenium and is commercially available. In the end, this solution also contains the newly formed formate - the actual hydrogen storage medium.
Easy to store and transport
Hydrogen plays a key role in alternative energy supply scenarios. Methanol, ammonia and methane are being discussed as storage media for a future hydrogen economy. Formic acid salts have an advantage over these storage media in terms of toxicity and energy consumption. Formate could be easily stored in plastic containers and transported in tankers. Henrik Junge says: “Basically like milk, beer or diesel.”
Together with the bicarbonate, the formate forms an energy system that is charged or discharged via hydrogen like a battery. Such a system is actually suitable for use in local, rural areas in particular. There, wind power or solar energy can produce green hydrogen via electrolysis in phases where more electricity is provided than is consumed, which is then stored as formate.
40 cycles for a climate-neutral process
The authors emphasize that the process is carbon dioxide neutral. Normally, when hydrogen is recovered, some of the bicarbonate is decomposed into carbon dioxide and released, explains Carolin Stein. “Our system, on the other hand, permanently retains the carbon dioxide.” This means that pure hydrogen can be obtained from this storage system, which can be used directly in a fuel cell without further purification.
In their scientific article, the authors report 40 consecutive cycles of hydrogen storage and release over a period of six months. Using minimal amounts of the ruthenium catalyst in the ppm range, the chemists produced 50 liters of hydrogen with an average purity of 99.5 percent with their laboratory system.
Future demonstrator at the technical center
The research results will serve the participating company as a basis for the construction of a larger demonstrator. The company will use the institute's technical center for this purpose. The system is scheduled for completion by the end of 2025.
