Solar charged Redox Flow Batteries

Redox flow batteries:

An emerging technology

Investigation of redox couples for the batteries are being carried out in the labs of the Chemical Engineering Group in Aarhus, whereas photoanodes and cathodes will be investigated in Porto. The photo shows PhD Fellow Kristina Wedege in the laboratory (photo Lars Kruse, AU)

As the need for large-scale energy storage increases with the development of renewable energy sources such as wind power or solid state solar cells, research into redox flow batteries has increased dramatically.

In these batteries, redox couples in two separate chambers are charged electrochemically by changing their oxidation states. During discharge, the reverse redox reaction takes place and charge balance is kept by ion-exchange through a selective membrane between the chambers. Lately the development of commercial redox flow batteries have been successful and pilot plants are being build. State-of-the-art redox flow batteries are based on aqueous solutions of various Vanadium-species and uses carbon-based anodes and cathodes.

Direct solar charging of redox couple
In one hour the radiation from the sun supplies about the total annual energy consumption on Earth, which naturally causes enormous interest in finding ways to harvest this energy. The possibility of direct charging of redox couples using solar energy has been explored earlier, mostly with a well-known redox couple: Water. Termed photo-electrochemical water splitting, this idea is similar to the objective of the current project, but as water develops hydrogen and oxygen gas upon electrochemical reaction, other redox couples have to be found and investigated, if such a capture of the energy of the sun are to be used in a battery.

Photoanodes and cathodes: Harvesting solar radiation
Solar charging of chemical species requires either a photoanode or a photocathode, which are semiconductors that facilitate electron transport to and from the solution of redox couples upon solar radiation. If the right combination of energy levels in these semiconductors and the energy levels of the redox couples are made, so-called photocurrents can be observed, meaning that the solar energy has been captured and stored.
At the Engineering Faculty of University of Porto another research group is working on developing stable, cheap and effective photoanodes and cathodes to be used both for water splitting, and with redox couples that are being investigated in the Chemical Engineering Group in Aarhus. Thus, a close collaboration is needed between the two groups to develop and optimize the necessary parts.    

Responsible energy storage
In traditional battery technology, toxicity has not always been considered, and batteries generally have to be collected and destroyed responsibly after their lifetime. By careful choice of redox couples that can e.g. be derived from rhubarb, it may be possible to create a safer technology. In addition to this, the materials must be relatively cheap and all process parameters optimized in order to answer the research question: Is it possible to experimentally show high efficiencies (hc > 10 %) of solar charging of redox couples?