text: The zinc–bromine flow battery (Zn-Br2) is the oldest flow battery chemistry, with John Doyle's patent US224404 filed on September 29, 1879. Zn-Br2 batteries have relatively high specific energy, and they were demonstrated as power sources for electric cars in the 1970s.
  Walther Kangro, an Estonian chemist working in Germany, was the first to demonstrate in the 1950s flow batteries based fully on dissolved transition metal ions: Ti–Fe and Cr–Fe. After some initial experimentations with Ti–Fe RFB  NASA and some other groups in Japan and elsewhere selected Cr–Fe chemistry for further development. In order to reduce the effect of time-varying concentration during RFB cycling, mixed solutions (i.e. comprising both chromium and iron species in the negolyte and in the posolyte) were used. Among disadvantages of the Cr–Fe chemistry are: hydrate isomerism (i.e. the equilibrium between electrochemically active Cr3+ chloro-complexes and inactive hexa-aqua complex, which can be alleviated by adding chelating amino-ligands) and hydrogen evolution on the negode (which is mitigated by adding Pb salts for increasing the H2 overvoltage and Au salts for catalyzing the chromium electrode reaction).
  In the late 1980s, Sum, Rychcik and Skyllas-Kazacos at the University of New South Wales (UNSW) in Australia demonstrated the advantages of all-vanadium RFB chemistry, such as the existence of four oxidation states within the electrochemical voltage window of the graphite-aqueous acid interface, and thus the elimination of the mixing dilution, detrimental in Cr–Fe RFBs. UNSW filed several patents related to VRFBs, that were later licensed to Japanese, Thai and Canadian corporations, which tried to commercialize this technology with variable success.
  In 2022, Dalian, China began operating a 400 MWh, 100 MW vanadium flow battery, then the largest of its type.