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

Abstract

An electrochemical cell consisting of cobalt ([Co(PO)]) and vanadium ([V(PO)]) 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(PO)] (1) and [V(PO)] (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][1]·2MeCN; 0.77 ± 0.06 M, [PPN][2]·DME). The [PPN][1]·2MeCN and [PPN][2]·DME salts were isolated as crystalline solids in 82 and 68% yields, respectively, and characterized by P NMR, UV/vis, ESI-MS(-), and IR spectroscopy. The [PPN][1]·2MeCN salt was also structurally characterized, crystallizing in the monoclinic P2/c space group. Treatment of 1 with [(p-BrCH)N] allowed for isolation of the one-electron-oxidized spin-crossover (SCO) complex, [Co(PO)] (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.

Sustainable Phosphorus Chemistry: A Silylphosphide Synthon for the Generation of Value‐Added Phosphorus Chemicals

“…[4] Their discovery is based on the use of commercially available sodium trimetaphosphate (Na 3 [P 3 O 9 ]), which can be obtained in ao ne-pot procedure by the dehydration of phosphoric acid using sodium chloride (600 8 8C, 2h), with hydrochloric acid (HCl) as the only byproduct. [9] Cummins and Geeson now developed the direct conversion of trimetaphosphate,[ P 3 O 9 ] 3À ,i nto valuable phosphorus compounds.N amely,[ TBA] 3 [P 3 O 9 ]·2 H 2 Oc an be reduced in as teel pressure reactor with trichlorosilane as the solvent (HSiCl 3 ;3 3:1S i/P molar ratio) at 110 8 8Cf or 72 hours to afford the novel bis(trichlorosilyl)phosphide anion in the form of its tetrabutylammonium salt [TBA][P-(SiCl 3 ) 2 ]( 65 %; d( 31 P) = À171.7 ppm, 1 J P, Si = 156 Hz; Scheme 1). [4, 8b] Tr imetaphosphate is an interesting anionic k 3 -O ligand for organometallic chemistry, [8b] and was recently applied, also by Cummins and co-workers,f or the synthesis of cobalt and vanadium trimetaphosphate polyanions,w hich have favorable properties for non-aqueous redox-flow battery applications.…”

“…[4, 8b] Tr imetaphosphate is an interesting anionic k 3 -O ligand for organometallic chemistry, [8b] and was recently applied, also by Cummins and co-workers,f or the synthesis of cobalt and vanadium trimetaphosphate polyanions,w hich have favorable properties for non-aqueous redox-flow battery applications. [9] Cummins and Geeson now developed the direct conversion of trimetaphosphate,[ P 3 O 9 ] 3À ,i nto valuable phosphorus compounds.N amely,[ TBA] 3 [P 3 O 9 ]·2 H 2 Oc an be reduced in as teel pressure reactor with trichlorosilane as the solvent (HSiCl 3 ;3 3:1S i/P molar ratio) at 110 8 8Cf or 72 hours to afford the novel bis(trichlorosilyl)phosphide anion in the form of its tetrabutylammonium salt [TBA][P-(SiCl 3 ) 2 ]( 65 %; d( 31 P) = À171.7 ppm, 1 J P, Si = 156 Hz; Scheme 1). [4] Note that the industrial-scale production of HSiCl 3 requires elemental silicon, which is,j ust like P 4 , prepared in ah igh-energy process.…”

Zukunftsfähige Phosphorchemie: ein Silylphosphid‐Synthesebaustein für die Entwicklung hochwertiger Phosphorverbindungen

“…Trimetaphosphat ist ein interessanter anionischer κ 3 ‐O‐Ligand für die Organometallchemie und wurde so kürzlich von Cummins und Mitarbeitern für die Synthese von Cobalt‐ und Vanadiumtrimetaphosphat‐Polyanionen eingesetzt, die günstige Eigenschaften für die Anwendung in nichtwässrigen Redox‐Flüssigbatterien haben . Cummins und Geeson haben nun eine direkte Umsetzung von Trimetaphosphat, [P 3 O 9 ] 3− , in wertvolle Phosphorverbindungen entwickelt.…”

Dithiolene Complexes of First‐Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries

“…[3][4][5] As energy is stored externally to the electrochemical reactor,t he capacity can be increased independently of the battery power.A tp resent,c ommercial RFBs utilize aqueous electrolyte solutions of inorganic metal salts, however, despite continualp rogress in powero utputs and efficiencies being made, the cell potentiali si nherently limitedb y the narrow (1.23 V) electrochemical window of water.I nstead, the developmento fn onaqueous RFBs, which use organic solvents with wide electrochemical windows,i sa nticipated to improve the voltage outputs. [12][13][14][15] Indeed, several metal coordination complexes have been tested as electrolytes for nonaqueous RFBs with cell potentials in excess of 1.23 V; these contain acetylacetonate, [12,[16][17][18][19][20][21] bipyridine, [13,15,[22][23][24][25] phenanthroline, [26,27] terpyridine-like, [14,15] trimetaphosphate, [28] and macrocyclic [29,30] ligands. [11] Metal-ligandc oordination complexes are good candidates for nonaqueous RFBelectrolytes as they can be stable in multiple oxidation states and have high solubility in organic solvents.F urthermore, careful choice of metal ion as well as modification of the ligand scaffold (e.g.,s olubilizing groups, denticity,d onorg roups) can allow for fine tuning of the desired properties for RFB applications.…”

“…[11] Metal-ligandc oordination complexes are good candidates for nonaqueous RFBelectrolytes as they can be stable in multiple oxidation states and have high solubility in organic solvents.F urthermore, careful choice of metal ion as well as modification of the ligand scaffold (e.g.,s olubilizing groups, denticity,d onorg roups) can allow for fine tuning of the desired properties for RFB applications. [12][13][14][15] Indeed, several metal coordination complexes have been tested as electrolytes for nonaqueous RFBs with cell potentials in excess of 1.23 V; these contain acetylacetonate, [12,[16][17][18][19][20][21] bipyridine, [13,15,[22][23][24][25] phenanthroline, [26,27] terpyridine-like, [14,15] trimetaphosphate, [28] and macrocyclic [29,30] ligands.…”