TECHNOLOGY FOCUS - ELECTRONICS - Preferred Components: The redox flow battery further comprises a catholyte solution tank and an anolyte solution tank, where each of tanks is fluid communicated to the cathode cell and the anode cell.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The metal of the metal-ligand coordination compound is nickel, cobalt, iron, ruthenium, zinc, manganese, yttrium, zirconium, titanium, chromium, magnesium, cerium, copper, lead or vanadium.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The supporting electrolyte is: at least one salt comprising spiro-(1,1')-bipyrrolidinium (SBP), spiro-(1,1')-bipiperidinium (SBPI), piperidine-1-spiro-1-pyrrolidinium (PISP), 1-ethyl-1-methylpyrrolidinium (EMP), dimethyl pyrrolidinium (DMP) and diethylpyrrolidinium (DEP) or their counter anions; and at least one compound comprising spiro-(1,1')-bipyrrolidiniumtetrafluoroborate (SBP-BF4), spiro-(1,1')-bipiperidiniumtetrafluoroborate (SBPI-BF4), piperidine-1-spiro-1-pyrrolidiniumtetrafluoroborate (PISP-BF4), 1-ethyl-1-methylpyrrolidiniumtetrafluoroborate (EMPBF4), dimethylpyrrolidiniumtetrafluoroborate (DMP-BF4), or diethylpyrrolidiniumtetrafluoroborate (DEP-BF4). The supporting electrolyte is present at a concentration of 0.1-2 M in the electrolyte solution. The ligand of the metal-ligand coordination compound is dipyridyl, tert-pyridyl, ethylenediamine, propylenediamine, phenanthroline or N-heterocyclic carbine (NHC). The NHC is 2,6-bis(methylimidazole-2-ylidene)pyridine, 1,3-dimesitylimidazole, 1,3-bis(2,5-diisopropylphenyl)imidazole, 1,3-dimethylimidazole, 1,3-di-tert-butylimidazole, 1,3-dicyclohexylimidazole or 1 -ethyl-3-methylimidazole. The metal-ligand coordination compound is 39 compounds e.g. metal complexes of formulae (a)-(j). The redox flow battery further comprises boron tetrafluoride (BF4-), hexafluorophosphate (PF6-), trifluoromethanesulfonic acid (CF3SO3-) or bis(trifluoromethane)sulfonimide ((CF3SO2)2N-) as a counter anion of the metal-ligand coordination compound. The non-aqueous solvent is dimethyl acetamide, diethyl carbonate, dimethyl carbonate, acetonitrile, gamma -butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), fluoroethylene carbonate or N,N-dimethylacetamide. The metal-ligand coordination compound is subject to reversible redox reactions.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The ion exchange membrane is an anion exchange membrane. The anion exchange membrane is an anion exchange membrane aminated by addition of a chloromethyl group to a styrene-divinylbenzene copolymer base, an anion exchange membrane obtained by addition of a quaternary pyridinium to a vinylpyridine-divinylbenzene copolymer, or an anion exchange membrane aminated by addition of a chloromethyl group to an aromatic polysulfone copolymer base. Preferred Definitions:    Y1- = a counter anion comprising BF4-, Cl-, Br-, I-, SO42-, PF6-, ClO4- and CF3SO3-.