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

An electrochemical cell consisting of cobalt ([Co II/III (P 3 O 9 ) 2 ] 4–/3– ) and vanadium ([V III/II (P 3 O 9 ) 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(P 3 O 9 ) 2 ] 4– ( 1 ) and [V(P 3 O 9 ) 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 31 P NMR, UV/vis, ESI-MS(−), and IR spectroscopy. The [PPN] 4 [ 1 ]·2MeCN salt was also structurally characterized, crystallizing in the monoclinic P 2 1 / c space group. Treatment of 1 with [( p -BrC 6 H 4 ) 3 N] + allowed for isolation of the one-electron-oxidized spin-crossover (SCO) complex, [Co(P 3 O 9 ) 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.