High-performance graphene hybrid material for highly efficient energy storage systems: Supercapacitors challenge batteries

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A highly efficient supercapacitor has been developed by a team led by Roland Fischer, Professor of Inorganic and Organometallic Chemistry at the Technical University of Munich (TUM).

A novel, high-performance and at the same time sustainable graphene hybrid material that has comparable performance data to the batteries currently used is the basis of the energy storage unit.

Energy storage is usually associated with batteries and accumulators that provide electronic devices with electricity. Nowadays, however, in computers, cameras, mobile phones and cars, so-called supercapacitors are increasingly being mounted.

They can store vast quantities of energy easily, unlike batteries, and release it just as quickly.

For instance, when a train brakes when it enters a station, supercapacitors store the energy and release it again when it starts up very quickly when the train needs a lot of energy.

However, their lack of energy density has been one problem with supercapacitors.

Although the energy density of lithium accumulators is up to 265 kilowatt hours (KW/h), supercapacitors have only delivered a tenth of that so far.

Sustainable content guarantees elevated efficiency
For supercapacitors, the team led by TUM chemist Roland Fischer has now created a new graphene hybrid material that is as efficient as it is sustainable.

It acts as a positive electrode in the system for energy storage.

The researchers combined it with a tested titanium and carbon-based negative electrode.

In addition to achieving an energy density of up to 73 Wh/kg, which is approximately equal to the energy density of a nickel-metal hydride battery, the new energy storage system also performs significantly better than most other 16 kW/kg power density supercapacitors.

The new supercapacitor’s secret lies in the mixture of various materials, so chemists refer to the supercapacitor as “asymmetric.”

Hybrid materials: The model is Nature
The researchers rely on a new method to address the performance constraints of traditional materials – they use hybrid materials. Nature is full of hybrid materials that are highly complex, evolutionarily optimized – bones and teeth are examples.

By combining various materials, their mechanical properties, such as hardness and elasticity, have been optimized by nature,” says Roland Fischer.

The abstract concept of mixing basic materials was transferred to supercapacitors by the research team.

They used the novel positive electrode of the memory with chemically altered graphene as a basis and combined it with a so-called MOF, a nanostructured metal-organic system.

Mighty and stable
A large specific surface area and controllable pore sizes on the one side and high electrical conductivity on the other are critical to the efficiency of the graphene hybrids. Jayaramulu Kolleboyina, a former visiting scientist at Roland Fischer, explains: “The high performance of the material is based on the combination of the microporous MOFs with the conductive graphene acid,”

For good supercapacitors, a wide surface area is important.

It allows the aggregation inside the material of a correspondingly large number of charge carriers – this is the basic concept for electrical energy storage.

The researchers managed the feat of connecting the graphene acid with the MOFs by clever material design.

The resulting hybrid MOFs have up to 900 square meters per gram of very wide inner surface area and are very efficient as positive electrodes in a supercapacitor.

Stability for the long term
That is not the only benefit of the new content, however.

Strong chemical bonds are needed between the components to achieve a chemically stable hybrid.

“We linked the graphene acid to a MOF amino acid, creating a kind of peptide bond.”We bound graphene acid to a MOF amino acid, generating a kind of peptide bond.

In terms of long-term stability, the stable relation between the nanostructured components has tremendous advantages: the more stable the bonds are, the more cycles of charge and discharge are possible without significant performance loss.

By contrast, there is a service life of around 5,000 cycles for a classic lithium battery.

Even after 10,000 cycles, the new cell formed by the TUM researchers retains almost 90 percent of its energy.

The Foreign Expert Network
During the production of the new supercapacitor, Fischer stresses the importance of the complete international cooperation that the researchers themselves have managed.

For instance, Jayaramulu Kolleboyina developed the team. Invited by the Alexander von Humboldt Foundation, he was a visiting scientist from India, who

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