Water-soluble pH-switchable cobalt complexes for aqueous symmetric redox flow batteries

A water soluble octahedral Co(II) complex, BCPIP-Co(II), with 4 appended carboxylic groups on the ligand periphery is utilized as both posolyte and negolyte in an aqueous, symmetric redox flow battery (RFB). The full RFB demonstrates Coulombic efficiencies >99% for up to 100 cycles.