A team at the Technical University Munich has developed a highly efficient graphene- based supercapacitor. The basis of the energy storage device is a novel, powerful and sustainable graphene hybrid material that has comparable performance to current battery technology. 

Usually, energy storage is associated with batteries and accumulators that provide energy for electronic devices. However, in laptops, cameras, cellphones or vehicles, so-called supercapacitors are increasingly competitive.

Unlike batteries, they can store large amounts of energy very quickly and offload it just as fast. If, for instance, a train brakes when entering the station, supercapacitors can store the energy and provide it again when the train needs a lot of energy very quickly while starting up.

However, one problem with supercapacitors to date is their lack of energy density. 

The team at the Technical University Munich has now developed a 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 titanium and carbon.

The new energy storage device not only attains an energy density of up to 73 kWh/kg, which is roughly equivalent to the energy density of a nickel-metal hydride battery but also performs much better than most other supercapacitors at a power density of 16 kWh/kg.  The secret of the new supercapacitor is the combination of different materials - hence, chemists refer to the supercapacitor as “asymmetrical.”

The abstract idea of combining basic materials was transferred to supercapacitors by the research team. They used the novel positive electrode of the storage unit with chemically modified graphene and combined it with a nano- structured metal-organic framework, a so-called MOF.

The performance of graphene hybrids is based on their large specific surface and controllable pore sizes as well as their high electrical conductivity. A large surface is important for good supercapacitors.  It allows for the collection of a 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. This provides a very large inner surface of up to 900 square meters per gram.

However, that is not the only advantage of the new material. To achieve a chemically stable hybrid, one needs strong chemical bonds between the components. 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.