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

摘要

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.