Redox flow batteries store energy in flowing media, allowing energy and power to be scaled almost freely. This makes it possible to create potentially inexpensive batteries with inexpensive energy storage materials, if the storage times amount to several hours. This makes this type of battery a candidate for medium time storage and for applications where energy has to be shifted from day to night (peak-shift). To this end, research activities and commercialization efforts have increased significantly in recent years. To date, more than 60 different types of redox flow batteries have been investigated, with vanadium redox flow batteries and zinc/bromine redox flow batteries being the most widely commercialized. The vanadium redox flow battery (VRFB) was developed in the 1980s at the UNSW and is today the most studied type of redox flow battery.
The physical and chemical properties of the electrolyte have a significant influence on the characteristics of the battery. Comprehensive chemical, physical and electrochemical analysis methods are available to optimize electrolyte properties or to develop new electrolytes. Using half-cell measurements, the electrochemical properties of electrolytes, active materials and the electrode can be investigated and optimized with regard to reaction rates, side reactions and aging effects. The half-cell measurements can also be coupled with spectroscopic methods such as UV/VIS, IR or RAMAN (spectroelectrochemistry) to obtain additional information on changes in the chemical composition. However, existing synthesis possibilities also allow the development and optimization of manufacturing processes or regeneration processes for electrolytes. Furthermore, stability tests according to standardized procedures offer the possibility of testing the stability of materials in contact with the medium.
Most redox flow batteries use carbon-based electrodes. The electrodes must allow high reaction speeds, high conductivity and good mechanical properties. For this purpose, commercial electrode materials can be investigated in different test cells or in half cells, or novel materials can be developed. For example, compounds can be produced and then processed into suitable electrode materials using different processes. Furthermore, there are processes that allow thermoplastic materials to be joined to form liquid-tight cells and stacks.
The membrane used is often a high cost and efficiency factor. Commercial membranes can be characterized and compared using test cells and other methods. There are competences for ion exchange membranes, microporous separators, ion-con-ducting glasses and MEAs (membrane electrode assemblies). Particularly for MEAs for hydrogen- and oxygen-based cells and recombination units there are possibilities for the development and investigation of alternative catalysts and composite units.
Using coupled simulations, optimized cell geometries can be investigated and developed. All the necessary procedures are available to develop and test efficient single cells and stackable cells for industrial scale. The cells can be tested for their electrical properties using multi-channel battery testers in climatic chambers at different temperatures. In addition, with the aid of reference electrodes and modern potentiostats, half-cell measurements and redox potential measurements can be carried out at the same time in order to identify and solve problems. Impedance analyzers can also be used to determine the resistances of cells, anodes and cathodes, which allows statements to be made about the loss of materials such as membranes, electrodes and electrolytes as well as active materials.
Different technical facilities with different manufacturing processes allow the construction and investigation of prototype stacks. Battery test systems with several hundred amperes of current are available for electrical tests, as well as the possibility to carry out electrochemical impedance spectroscopy up to 100 V. For vanadium redox flow batteries, process engineering test rigs up to a power of approx. 10 kW are available, which allow isothermal measurements to be carried out and thus, for example, heat and pressure losses to be determined at different flow velocities. Computer tomography (CT) allows stacks to be mapped three-dimensionally to identify design problems. Time-resolved measurements can also be performed to visualize the flow rate.
Depending on customer requirements, complete battery systems can be built as prototypes or tested according to different standards such as IEC 61427-2. For the construction of prototypes, location-optimized simulations for the design of heat management systems can be carried out in order to achieve high efficiency. The prototypes can be built up flexibly as DC or AC systems in any size by means of memory-programmed controls and self-developed battery management.
Due to our experience in building Europe‘s largest vanadium redox flow battery with 2 MW / 20 MWh we can also support larger battery projects. The testing of battery systems can be carried out on AC or DC test station with several hundred kilowatts of power up to the container scale and, if required, on a small grid in the MW range with wind turbine, photovoltaic and vanadium redox flow batteries.
Our experience covers a wide range of different chemistries. Beside V/V, V/Br, V/O2, Fe/Fe, H/Br, Zn/Br, Fe/Cr, organic RFBs and others have also been examined.