Cobalt and Vanadium Trimetaphosphate Polyanions: Synthesis, Characterization, and Electrochemical Evaluation for Non-aqueous Redox-Flow Battery Applications.

An electrochemical cell consisting of cobalt ([CoII/III(P3O9)2]4-/3-) and vanadium ([VIII/II(P3O9)2]3-/4-) bistrimetaphosphate complexes as catholyte and anolyte species, respectively, was constructed with a cell voltage of 2.4 V and Coulombic efficiencies >90% for up to 100 total cycles. The [Co(P3O9)2]4- (1) and [V(P3O9)2]3- (2) complexes have favorable properties for flow-battery applications, including reversible redox chemistry, high stability toward electrochemical cycling, and high solubility in MeCN (1.09 ± 0.02 M, [PPN]4[1]·2MeCN; 0.77 ± 0.06 M, [PPN]3[2]·DME). The [PPN]4[1]·2MeCN and [PPN]3[2]·DME salts were isolated as crystalline solids in 82 and 68% yields, respectively, and characterized by 31P NMR, UV/vis, ESI-MS(-), and IR spectroscopy. The [PPN]4[1]·2MeCN salt was also structurally characterized, crystallizing in the monoclinic P21/c space group. Treatment of 1 with [(p-BrC6H4)3N]+ allowed for isolation of the one-electron-oxidized spin-crossover (SCO) complex, [Co(P3O9)2]3- (3), which is the active catholyte species generated during cell charging. The success of the 1-2 cell provides a promising entry point to a potential future class of transition-metal metaphosphate-based all-inorganic non-aqueous redox-flow battery electrolytes.