Chemist Lars Henning Heß building electrochemical cells in a glovebox filled with argon.

High power from waste products

Chemists develop efficient and sustainable supercapacitors from biowaste
Chemist Lars Henning Heß building electrochemical cells in a glovebox filled with argon.
Image: Anne Günther (University of Jena)
Prof. Dr Andrea Balducci and his team are looking for ways to make sustainable raw materials usable for energy storage. Prof. Dr Andrea Balducci and his team are looking for ways to make sustainable raw materials usable for energy storage. Image: Anne Günther (University of Jena)

Supercapacitors are energy storage devices that – unlike rechargeable batteries – can be charged within seconds and have an almost unlimited lifespan. They are already being used today wherever high power is needed within a short period of time: for example in defibrillators, when switching the automatic start-stop system, and in vehicle service brakes. To make supercapacitors also available for other applications, teams from the Friedrich Schiller University Jena and the Ruhr University of Bochum are researching new materials for these energy storage devices. In a study, they are presenting a concept with which the manufacturing process for supercapacitors can be made much more sustainable than before.

To this end, the research teams from Jena and Bochum are not only using renewable raw materials as starting material for the electrodes of the supercapacitors. »We have also further developed the manufacturing process in such a way that it can operate with practically no waste because any by-products that arise are not discarded but are used in the subsequent production process«, explains Prof. Dr Andrea Balducci. Upcycling by-products increases the productivity of the process – experts speak of atomic efficiency – by a factor of 15, according to the Professor of Applied Electrochemistry at the Friedrich Schiller University Jena.

Carbon in biowaste is a high-grade material

Supercapacitors, also known as electrical double layer capacitors, consist of two activated carbon electrodes. Between the two carbon layers there is a liquid electrolyte with mobile charge carriers. »The special thing about activated carbon is its enormous inner surface area«, explains Prof. Balducci. One gram of the porous material has an area of up to 2,500 square metres – this corresponds to a size of ten tennis courts. This large electrode surface is decisive for the capacitance of supercapacitors, and it determine their energy and power density.

The chemists use carbon from organic waste such as expired dairy products or from paper production waste as the starting material for the activated carbon electrodes, but coconut shells or plant fibres are also suitable. »These wastes are otherwise simply composted or incinerated«, says Lars Henning Heß from Balducci's team. »But these carbons are high-grade material that can be used more sensibly«, the doctoral candidate, who is the main author of the study together with the young chemist Desiree Leistenschneider, makes clear.

Sample holder with inserted carbon electrode in front of the electron microscope. The screen in the background shows the measurement of carbon. Sample holder with inserted carbon electrode in front of the electron microscope. The screen in the background shows the measurement of carbon. Image: Anne Günther (University of Jena)

Waste products become starting materials

For the realization of sustainable supercapacitors, the chemists use chemical and thermal processes to extract carbonaceous material from the biowaste, which is then activated in a chemical reaction. The chemicals used in this process accumulate in the numerous pores of the activated carbon. »Previously, these substances in excess and the side reaction products were washed out of the activated carbon before it was used as electrode material«, says Andrea Balducci. However, this process generates enormous quantities of liquid waste that must be disposed of at great expense.

The team has therefore further developed the activation process in such a way that the resulting substances, which are created as »waste« in the pores of the activated carbon, are not lost but are directly reused as the starting point for the electrolyte solution. In the paper now presented, the researchers present the details of their »in-situ electrolyte process«: From the water-soluble by-products in the pores of the activated carbon, organic salts, the basic building blocks of the electrolyte solution, are formed through various reaction steps. »The activated carbon electrodes therefore bring their electrolyte solution with them«, says Andrea Balducci.


By Ute Schönfelder

Lars Henning Heß during the fine adjustment of a so-called squeegee. The tool is used for the exact preparation of the electrode films. Lars Henning Heß during the fine adjustment of a so-called squeegee. The tool is used for the exact preparation of the electrode films. Image: Anne Günther (University of Jena)
Notice

Original publication:

Leistenschneider D. et al. Solid-state transformation of aqueous to organic electrolyte -Enhancing the operating voltage window of ‘in situ electrolyte’ supercapacitors, Journal of the Royal Society of Chemistry (2020); DOI: https://doi.org/10.1039/D0SE00180E.

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