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Storage & smart power side reactions, in particular hydrogen development at platinum electrodes, were reported, he ruled out suitability. Pieper later used carbon electrodes, however, that standard material, which is also used today for vanadium redox flow batteries, in his experiments with other active materials. A vanadium redox flow battery would have been possible with his experiments as early as the 1950s. However, Pieper favoured a titanium/iron redox flow battery, which was later further developed by NASA [11]. The Ti/Fe-RFB uses Fe2+/Fe3+ as negative and Ti3+/TiO2+ as positive redox pair. The reactions of Fe2+/Fe3+ are very fast, but the reactions of Ti3+/TiO2+ are much slower, which makes the energy efficiency and power density relatively low. The Ti/ Fe-RFB has the disadvantage of a low cell voltage of about 0.8V and a low concentration of active materials. The maximum achievable energy density is thus approximately 10Wh/L and practically much lower. For these reasons the Ti/Fe-RFB has never been successful. With the beginning of the first oil price crisis in 1973, a rethinking of the energy supply began. Investments in regenerative energy sources and the necessary research and development of storage systems for fluctuating energy producers led to the development of the iron/chromium redox flow battery at NASA by Thaller [12]. Thaller was also the first to use the term “Redox Flow Cell”. In the Fe/Cr-RFB, as in the Ti/Fe-RFB, the redox pair Fe2+/Fe3+ is used, but on the positive electrode. As already mentioned above, the reaction rate of the redox pair is high and thus are the achievable power density and energy efficiency. Iron is also an extremely inexpensive material for energy storage. By far the greatest challenges occur with the reactions of chromium ions at the negative electrode. The redox reactions of Cr2+/Cr3+ are very slow and are close to hydrogen generation, so the efficiency of the reactions is very low. NASA’s work was therefore primarily concerned with these reactions and their acceleration by catalysts and the suppression of hydrogen formation by inhibitors. Prior to this, however, Iron/iron redox flow battery there was again a screening of possible candidates as active materials for redox flow batteries. Again, vanadium was considered on a theoretical basis, but ultimately due to the cost was not further studied. Iron and chromium were selected because of potentially low costs [14]. NASA’s work led to a demonstration system with an output of 1kW/13kWh as a domestic storage system coupled with a PV system [15] and lasted until around the mid-1980s. With the lowering of crude oil prices, the general interest in renewable energies and storage facilities decreased so that no commercialisation took place. It was not until the mid-2000s that various companies attempted to commercialise Fe/Cr-RFBs again, but these were discontinued. In 1981, Hruska and Savinell published an article about a hybrid redox flow battery that only uses iron as an active material [16]. The motivation was the use of an energy storage material that was as inexpensive as possible, which is almost unsurpassable with iron. One kilogram of iron corresponds to approximately 500Wh, or 1kWh would cause approximately US$5 in active material costs. The Fe/Fe-RFB uses the soluble redox pair Fe2+/Fe3+ at the positive electrode but the redox pair Fe/Fe2+ at the negative electrode just like the two iron-based RFBs discussed above. The initial solution is a relatively simple and widely available Fe(II) salt solution, similar to that used on a large scale in wastewater treatment. Solid Technical Briefing 108 | November 2019 | www.pv-tech.org Colours of different oxidation states of vanadium from left to right: VO2+, VO2+, V3+, V2+ Credit: University of New South Wales Credit: Fraunhofer ICTPDF Image | Redox flow batteries for renewable energy storage
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