Jens Noack1 2

1, Fraunhofer ICT, Prinztal, , Germany
2, University of New South Wales (UNSW), Sydney, New South Wales, Australia

Redox flow batteries (RFBs) have become indispensable in the debate on the storage of renewable energies. To date, over 60 different combinations of active materials have been released, but only a small number are in commercialization. The reasons for this are complex but superficially determined by the specific chemical and physical properties of the active materials, as well as the production technologies used. The supposedly simple electrochemical reactions ultimately result in a complex system for reducing undesirable effects. Currently the most important commercial systems are mainly V/V-, Zn/Br-, Zn/Fe- and H/Br-RFBs, as well as the significantly increased number of water-soluble organic RFBs. VRFBs have today reached a stage of development where costs can be reduced, notably through alternative materials such as microporous separators and low-cost electrode materials, as well as optimized production while increasing life time. However, it should also be noted that side reactions such as hydrogen evolution and oxidation by atmospheric oxygen possibly require more complex balancing measures through the use of recombination or regeneration cells and thus makes the system more expensive, especially if service lives of 20 years are to be achieved. In addition, the question of the electrode material must be clarified, i.e. how an inexpensive electrode material must be designed to achieve a favorable power density/cost ratio and, in this context, the influence of the structure and composition of the electrode surface. Thermal stability of the electrolyte solution plays more than higher energy density an important role to reduce costs. With the use of additives or alternative solvents, the interplay of physical and electrochemical properties and side reactions is a challenge that is still in need of improvement. Organic compounds as active material represent an interesting alternative to potentially more expensive inorganic redox pairs. However, some fundamental questions have to be clarified. Pure organic electrolytes have low conductivities, low solubilities and thus low power densities which leads to high storage costs. Water-soluble organic compounds circumvent this problem, but it is necessary to add conductive salts that alter the properties of the electrolytes. However, one of the biggest challenges is the possibility of side reactions in the potential range of the other active material and generally the stability of the radicals over a longer period of time at higher states of charge and in particular at higher concentrations of active material. Electro-migration effects, the impact of real concentrations and real storage times must not be disregarded when evaluating a new electrolyte in order to be able to assess the application potential. In this talk, we give an overview of today's general challenges to RFBs, as well as in detail to the main commercial representatives VRFB and Zn / Br-RFB, as well as organic based RFBs.