Complexes Containing Redox Noninnocent Ligands for Symmetric, Multielectron Transfer Nonaqueous Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) hold promise for use in large-scale energy storage applications, but new electrolyte chemistries are needed to significantly enhance their energy densities and lower their cost. The energy density is governed by the cell voltage, active species concentration and number of electrons transferred at each electrode. Non-aqueous solvents offer wider voltage windows than water; however, most if not all of the previously reported active species have low solubilities and/or are limited to single electron transfer at each electrode. This paper describes the design, synthesis, and characterization of metal coordination complexes containing non-innocent ligands that demonstrate enhanced solubilities at different oxidation states along with multiple electron transfers. In particular, a series of ester functionalized chromium bipyridine complexes are demonstrated that afford six reversible redox couples over ~2 V and solubilities approaching 1 M. These characteristics allow the same complex to be used at the negative and positive electrodes. Using an electrolyte consisting of the tris(4,4'(bis(2-(2-methoxyethoxy)ethyl)ester-2,2'-bipyridine)chromium complex ([Cr(L3) 3 ]) in acetonitrile, we demonstrate two reversible electron transfers at each electrode in an unoptimized, symmetric Hcell with efficiencies of ~70%. With further enhancements in the electrolyte chemistry and cell design, this approach could lead to the demonstration of highly energy dense RFB chemistries for grid-scale storage applications. INTRODUCTION With increasing efforts to incorporate renewable energy sources such as

Linked Picolinamide Nickel Complexes as Redox Carriers for Nonaqueous Flow Batteries

“…Metal coordination complexes, in particular, have been a focal point for redox carrier development with those using the tris acetylacetonate ligand set thoroughly studied for V, Cr, and Mn variations, along with extensive ligand modification . Non‐innocent ligands have been prominently targeted of late, with examinations of bipyridine complexes, modified versions, and three‐coordinate hybrids . Recently, polyoxometalate complexes have been revised for nonaqueous systems, exhibiting excellent reversibility albeit with a relatively small potential …”

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.…”

Characterization of Structural and Electronic Transitions During Reduction and Oxidation of Ru(acac)3 Flow Battery Electrolytes by using X‐ray Absorption Spectroscopy

“…Cycling solutions of metal acetylacetonates in acetonitrile reveals significant capacity fade that is often attributed to irreversible ligand shedding and/or irreversible oxidation reactions . Chromium(III) bipyridine complexes have six redox couples over a 2 V window and solubilities approaching 1 m . The multielectron transfer capabilities have been linked to non‐innocence (redox activity) of the bipyridine ligands.…”

“…The multielectron transfer capabilities have been linked to non‐innocence (redox activity) of the bipyridine ligands. When cycled in an H‐cell, however, solutions of Cr(bpy) 3 in acetonitrile reach less than 30 % of their theoretical capacities, and significant capacity fade occurs . Wei et al.…”

“…[26] Cycling solutions of metal acetylacetonates in acetonitrile reveals significant capacity fade that is often attributed to irreversible ligand sheddinga nd/or irreversible oxidation reactions. [17-19, 21, 26] Chromium(III) bipyridine complexes have six redox couples over a2Vw indow and solubilitiesa pproaching 1 m. [10] The multielectron transfer capabilitiesh ave been linked to non-innocence (redox activity) of the bipyridine ligands. When cycled in an H-cell,h owever, solutionso f Cr(bpy) 3 in acetonitriler each less than 30 %o ft heir theoretical capacities, and significant capacity fade occurs.…”