At the heart of the energy storage device is a powerful and also sustainable graphene hybrid material that has comparable performance data to currently utilized batteries.
Usually, energy storage is associated with batteries and accumulators that provide energy for electronic devices but in laptops, cameras, cellphones or vehicles, so-called supercapacitors are now increasingly being installed.
While unlike batteries they can quickly store large amounts of energy and put it out just as fast one problem has been their lack of energy density. While lithium accumulators reach an energy density of up to 265 Kilowatt hours (KW/h), supercapacitors have only been delivering a tenth of that.
The team at TUM has now developed a powerful as well as sustainable graphene hybrid material for supercapacitors. It serves as the positive electrode in the energy storage device. The researchers are combining it with a proven negative electrode based on titan and carbon.
The new energy storage device does not only attain an energy density of up to 73 Wh/kg, which is roughly equivalent to the energy density of an nickel metal hydride battery, but also performs much better than most other supercapacitors at a power density of 16 kW/kg. This supercapacitor comprises of a combination of different materials – hence, chemists refer to the supercapacitor as "asymmetrical."
The idea of combining basic materials has resulted in the combination of a novel positive electrode of the storage unit with chemically modified graphene to create a nano-structured metal organic framework, a so-called MOF.
Decisive for the performance of graphene hybrids are on the one hand a large specific surface and controllable pore sizes and on the other hand a high electrical conductivity.
"The high performance capabilities of the material is based on the combination of the microporous MOFs with the conductive graphene acid," explained first author Jayaramulu Kolleboyina.
A large surface is important for good supercapacitors. It allows for the collection of a respectively large number of charge carriers within the material which is the basic principle for the storage of electrical energy.
Through skillful material design, the researchers achieved the feat of linking the graphene acid with the MOFs. The resulting hybrid MOFs have a very large inner surface of up to 900 square meters per gram and are highly performant as positive electrodes in a supercapacitor.
However, in order to achieve a chemically stable hybrid, strong chemical bonds between the components are required. The bonds are apparently the same as those between amino acids in proteins, according to Fischer: "In fact, we have connected the graphene acid with a MOF-amino acid, which creates a type of peptide bond."
The stable connection between the nano-structured components has huge advantages in terms of long term stability: The more stable the bonds, the more charging and discharging cycles are possible without significant performance impairment.
For comparison: A classic lithium accumulator has a useful life of around 5,000 cycles. The new cell developed by the TUM researchers retains close to 90 percent capacity even after 10,000 cycles.
