The search for carbon-neutral energy sources is becoming more vital as the struggle against climate change intensifies. Green hydrogen is a promising fuel in a carbon-neutral economy, and initiatives to manufacture it are now underway. One of the difficulties is safely storing and transporting the potentially explosive gas.
Now, a group of microbiologists from Goethe University Frankfurt has succeeded in utilizing bacteria to store and release hydrogen in a regulated manner — a significant step forward in the search for carbon-neutral energy sources in the cause of climate change mitigation. Researchers discovered an enzyme in bacteria that lives in the absence of oxygen and produces formic acid by binding hydrogen straight to CO2. The procedure is fully reversible, which is a prerequisite for hydrogen storage.
These acetogenic bacteria eat carbon dioxide and convert it to formic acid with the help of hydrogen. Normally, formic acid is merely an intermediate result of their metabolism, which is digested further to produce acetic acid and ethanol. However, the team has altered the bacteria in such a way that it is now able to not only stop but also reverse this process at the formic acid stage. Since 2013, the core principle has been patented.
“The measured rates of CO2 reduction to formic acid and back are the highest ever measured and many times greater than with other biological or chemical catalysts; in addition, and unlike chemical catalysts, the bacteria do not require rare metals or extreme conditions for the reaction, such as high temperatures and high pressures, but instead do the job at 30 °C and normal pressure.”
Professor Volker Müller
The team has now achieved a further milestone: the construction of a biobattery for hydrogen storage using the same bacteria.
During the day, a photovoltaic unit generates electricity, which is subsequently used to fuel water hydrolysis. The hydrogen generated by the bacteria binds to CO2, resulting in the synthesis of formic acid. This reaction is completely reversible, and the concentration of the starting ingredients and end products determines the reaction’s direction. The hydrogen concentration in the bioreactor drops over the night, and the bacteria begin to release hydrogen from the formic acid once more. The hydrogen can then be used as a source of energy.
To test their bacterial hydrogen storage mechanism, the researchers supplied hydrogen to the bacteria for eight hours before putting them on a hydrogen diet for 16 hours. The bacteria subsequently released all of the hydrogen back into the atmosphere. With the use of genetic engineering procedures, it was feasible to abolish the unwanted generation of acetic acid.
The system ran extremely stably for at least two weeks,” explains Fabian Schwarz, who is pleased that this work has been accepted for publication in “Joule,” a prestigious journal for chemical and physical process engineering. “That biologists publish in this important journal is somewhat unusual,” says Schwarz.
The proven process architecture, according to the researchers, might be used as a future “bio-battery” for the reversible storage of electrons in the form of hydrogen in formic acid.