Maximizing Vanadium Deployment in Redox Flow Batteries Through Chelation.

By tailoring the coordination sphere of vanadium to accommodate a 7-coordinate geometry, a highly soluble (>1.3 M) and reducing (-1.2 V vs Ag/AgCl) flow battery electrolyte is generated from [V(DTPA)]2-/3- (DTPA = diethylenetriaminepentaacetate). Bulk spectroelectrochemistry is performed in situ to assess material properties in both oxidized and reduced states. Flow batteries are assembled in near neutral pH conditions and operated with discharge energy densities of 12.5 Wh L-1 and high efficiency. Further, the first chelated flow battery using the same aminopolycarboxylate ligand for both electrolytes is generated. The presented batteries demonstrate comparable performance to the iron-vanadium and all-vanadium flow batteries while doubling the effective discharge energy of vanadium (Wh per mol V) and minimizing safety and operating risks, offering grid-scale energy storage alternatives.

High-Energy-Density Chelated Chromium Flow Battery Electrolyte at Neutral pH.

High-concentration operation of redox flow batteries (RFBs) is essential for increasing their energy-storage capacity, but non-acidic electrolytes struggle to achieve the high concentrations of metal ions dissolved in acid, limiting the development of energy-dense neutral pH electrolytes. We report neutral pH RFB operation of chromium 1,3-propylenediaminetetraacetate (CrPDTA) at concentrations of 1.2 M at room temperature and 1.6 M at 40 °C, demonstrating 60% higher negolyte capacity, up to 42.9 Ah L-1 , than previously reported for non-additive-utilizing solutions of this promising material. With extended full cell cycling, we demonstrate the importance of buffer selection and pH when using the Fumasep E-620(K) membrane. Finally, we expand the pH operation range of CrPDTA to pH 7, which when cycled at 100 mA cm-2 against a ferrocyanide posolyte demonstrated excellent coulombic efficiencies >99.7% and energy efficiencies >87%, while operating at almost 700 mV more negative than the thermodynamic hydrogen evolution window.

Isolation and Characterization of a Highly Reducing Aqueous Chromium(II) Complex.

The highly reducing CrII-(1,3-propylenediaminetetraacetate) (CrPDTA) complex (-1.1 V vs SHE) has been isolated from aqueous solution and the solid-state structure is described. The reduced CrIIPDTA complex is characterized by single-crystal X-ray diffraction, elemental analysis, infrared spectroscopy, UV-vis spectroscopy, magnetic moment, and density functional theory calculations. The concentration profile, state of charge, and pH of CrPDTA electrolyte were monitored in a flow battery system in situ by absorption spectroscopy and a pH probe. The stability of CrIIPDTA in aqueous environments is demonstrated by the ability to isolate CaCrPDTA, despite the common misconception that water spontaneously evolves hydrogen at such potentials. The reduced CrIIPDTA prevents water from coordinating to the metal center by maintaining the same coordinatively saturated pseudo-octahedral structure as the oxidized CrIIIPDTA, despite experiencing an increased geometric strain from a Jahn-Teller distortion of the high-spin CrII ion. The important difference between solvent reactivity and solvent thermodynamic window is examined by comparing the electrochemical behavior of the reduced species of CrPDTA in various organic solvents to its behavior in aqueous solution. When examined in tetrahydrofuran (THF), the reduction potential of CrPDTA is observed to be -1.19 V vs cobaltocene (-2.52 V vs ferrocene). Reduced CrPDTA in aqueous solution is also exposed to atmospheric O2 without exhibiting any decomposition of the Cr(III) or Cr(II) species. The techniques detailed provide a higher standard method of characterization for flow battery electrolyte species.

Open for Bismuth: Main Group Metal-to-Ligand Charge Transfer.

The synthesis, characterization, and photophysical properties of 4- and 6-coordinate Bi3+ coordination complexes are reported. Bi(bzq)3 (1) and [Bi(bzq)2]Br (2) (bzq = benzo[h]quinoline) are synthesized by reaction of 9-Li-bzq with BiCl3 and BiBr3, respectively. Absorption spectroscopy, electrochemistry, and DFT studies suggest that 1 has 42% Bi 6s character in its highest-occupied molecular orbital (HOMO) as a result of six σ* interactions with the bzq ligands. Excitation of 1 at 450 nm results in a broad emission feature at 520 nm, which is rationalized as a metal-to-ligand charge transfer (MLCT) and phosphorescent emission resulting from bismuth-mediated intersystem crossing (ISC) to a triplet excited state. This excited state revealed a 35 µs lifetime and was quenched in the presence of oxygen. These results demonstrate that useful optoelectronic properties of Bi3+ can be accessed through hypercoordination with covalent organobismuth interactions that mimic the electronic structure of lead perovskites.

Evaluating aqueous flow battery electrolytes: a coordinated approach.

Here, we outline some basic pitfalls in the electrochemical investigation of aqueous metal complexes and advocate for the use of bulk electrolysis in redox flow cells for electrolyte analysis. We demonstrate the methods of operation and performance of a lab scale redox flow battery (RFB), which is assembled from unmodified, commercially available material and cycled with a vanadium electrolyte in order to provide a comparative baseline of expected performance. Common misconceptions about the thermodynamic window for water splitting are addressed and further express the need to develop next-generation aqueous redox flow battery electrolytes. Although non-aqueous electrolytes are a popular approach, they suffer from distinct challenges that limit energy and power density in comparison with aqueous electrolytes. Expanding the scope of aqueous electrolytes to include metal-chelate complexes allows electrolytes to be as tailorable as organic species, while maintaining robust metal-based redox processes. A flow battery assembly and operation guide is provided to help facilitate the use of flow battery testing in the evaluation of next-generation electrolytes.

Copper(II) as a Platform for Probing the Steric Demand of Bulky ß-Diketonates.

The synthesis and characterization of a series of copper bis(ß-diketonate) complexes, functionalized with sterically hindered o-biphenyl and m-terphenyl functional groups, are reported. X-ray structural analysis reveals that the ligands exhibit several modes of flexibility in order to accommodate the steric demand. Increased steric bulk of the ligands influences the CuII/I electrochemical reduction, which is likely due to inhibited ligand rotation. Chemical reduction of CuII forms CuI, which disproportionates to Cu0 and CuII. The CuI species could be quantitatively trapped using triphenylphosphine to form Cu(ß-diketonate)(PPh3)2 (7), which is also characterized. The catalytic ability of these complexes, along with several common precatalysts, was determined for the reaction of bromobenzene and 2-naphthol, an Ullmann-type C-O bond coupling reaction. Control experiments in toluene show no catalytic ability in the absence of ß-diketonates, suggesting involvement of the ligand in catalytic turnover.

Sterically encumbered ß-diketonates and base metal catalysis.

Metal coordination complexes of the sterically hindered ß-diketonate, 2,6-dimesitylbenzoyl pinacolone (esac), are reported for Co, Ni, Cu, and Zn. All four form ML2-type complexes with typical coordination behavior for late-metal ß-diketonates, however the effects on established electrochemistry and reactivity vary somewhat per metal. For example, the striking chemical and electrochemical inertness of CoII(esac)2 to oxidation and disproportionation is atypical. Conversely, the behavior of CuII(esac)2 is rather typical relative to other CuII(ß-diketonate)2 complexes. These data suggest a relative disfavoring of certain reaction pathways, and represent an important step in modulating the catalysis of the base metals via sterically hindered ß-diketonates.

Synthesis of Sterically Hindered ß-Diketones via Condensation of Acid Chlorides with Enolates.

Bulky ß-diketones have rarely exceeded dipivaloylmethane (DPM) in steric demand, largely due to synthetic limitations of the Claisen condensation. This work demonstrates hindered acid chlorides to be selective electrophiles in noncoordinating solvents for condensations with enolates. An improved synthesis of DPM is described (90% yield), and crowded ß-diketones featuring bulky o-biphenyl or m-terphenyl fragments were prepared in good to excellent yields. These compounds are anticipated to have a steric profile far greater than that of DPM. General reaction conditions and mechanistic considerations are included.

Bulky ß-Diketones Enabling New Lewis Acidic Ligand Platforms.

The synthesis of a sterically encumbered ß-diketone ligand (Aracac) substituted with 2,6-(2,4,6-Me3C6H2)2C6H3 is described. Coordination complexes of the type M(Aracac)2Cl(solv) (M = Ti, V, Cr; solv = THF, CH3CN) were prepared by the reaction of Aracac with MCl3 (M = V, Cr) or with TiCl4 to generate Ti(Aracac)2Cl2, followed by reduction. These complexes show a trend of alternating the cis/trans geometric preference with increasing dn electron count (n = 0, 1, 2, 3), which is rationalized in part by the unusual ability of ß-diketonates to behave as either a weak π donor or a π acceptor in the cis and trans geometries, respectively. In this way, the bis-ß-diketonate platform can accommodate the varying electronic demands of the coordinated metal ion. These results demonstrate the ability to limit the coordination of ß-diketonates on metal complexes for the first time, providing a chemically robust and coordinatively versatile platform for mechanistic investigations, metal functionalization, and improved catalyst design.

Alkaline quinone flow battery.

Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liquid electrolytes stored outside of the battery. We report an alkaline flow battery based on redox-active organic molecules that are composed entirely of Earth-abundant elements and are nontoxic, nonflammable, and safe for use in residential and commercial environments. The battery operates efficiently with high power density near room temperature. These results demonstrate the stability and performance of redox-active organic molecules in alkaline flow batteries, potentially enabling cost-effective stationary storage of renewable energy.

Computational design of molecules for an all-quinone redox flow battery.

Inspired by the electron transfer properties of quinones in biological systems, we recently showed that quinones are also very promising electroactive materials for stationary energy storage applications. Due to the practically infinite chemical space of organic molecules, the discovery of additional quinones or other redox-active organic molecules for energy storage applications is an open field of inquiry. Here, we introduce a high-throughput computational screening approach that we applied to an accelerated study of a total of 1710 quinone (Q) and hydroquinone (QH2) (i.e., two-electron two-proton) redox couples. We identified the promising candidates for both the negative and positive sides of organic-based aqueous flow batteries, thus enabling an all-quinone battery. To further aid the development of additional interesting electroactive small molecules we also provide emerging quantitative structure-property relationships.

A metal-free organic-inorganic aqueous flow battery.

As the fraction of electricity generation from intermittent renewable sources--such as solar or wind--grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output. In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form. Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious-metal electrocatalysts. Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br2/Br(-) redox couple, yields a peak galvanic power density exceeding 0.6 W cm(-2) at 1.3 A cm(-2). Cycling of this quinone-bromide flow battery showed >99 per cent storage capacity retention per cycle. The organic anthraquinone species can be synthesized from inexpensive commodity chemicals. This organic approach permits tuning of important properties such as the reduction potential and solubility by adding functional groups: for example, we demonstrate that the addition of two hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and we describe a pathway for further increases in cell voltage. The use of π-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realizing massive electrical energy storage at greatly reduced cost.

Chromium(IV) siloxide.

The reaction of Na(OSi(t)Bu(2)Me) with CrCl(3) yields solid [Cr(OSi(t)Bu(2)Me)(3)](n) (1), which can be crystallized in the presence of excess Na(OSi(t)Bu(2)Me) to yield [Na(THF)][Cr(OSi(t)Bu(2)Me)(4)] (2). This complex is oxidized to yield Cr(OSi(t)Bu(2)Me)(4) (3), a crystalline chromium(IV) siloxide complex that is air- and moisture-stable. Electronic spectroscopic analysis of the absorption spectrum of 3 indicates a particularly weak ligand field (Δ(T) = 7940 cm(-1)) and covalent Cr-O bonding. 3 provides the first structural and spectroscopic characterization of a homoleptic chromium(IV) siloxide complex and provides a benchmark for tetrahedral chromium(IV) ions residing in solid oxide lattices.

Cobalt in a bis-ß-diketiminate environment.

The reaction of Co(2)(mesityl)(4) with acetonitrile leads to the formation of a planar, low spin, bis-ß-diketiminate cobalt(II) complex, (1-mesitylbutane-1,3-diimine)(2)Co (1). EPR spectroscopy, magnetic studies, and DFT calculations reveal the Co(II) ion to reside in a tetragonal ligand field with a (2)B(2)(d(yz))(1) ground state electronic configuration. Oxidation of 1 with ferrocenium hexafluorophosphate furnishes (1-mesitylbutane-1,3-diimine)(2)Co(THF)(2)PF(6) (2). The absence of significant changes in the metal-ligand bond metrics of the X-ray crystal structures of 1 and 2 supports ligand participation in the oxidation event. Moreover, no significant changes in C-C or C-N bond lengths are observed by X-ray crystallography upon oxidation of a ß-diketiminate ligand, in contrast to typical redox noninnocent ligand platforms.

Thermodynamics, kinetics, and mechanism of (silox)3M(olefin) to (silox)3M(alkylidene) rearrangements (silox = tBu3SiO; M = Nb, Ta).

Olefin complexes (silox)(3)M(ole) (silox = (t)Bu(3)SiO; M = Nb (1-ole), Ta (2-ole); ole = C(2)H(4), C(2)H(3)Me, C(2)H(3)Et, C(2)H(3)C(6)H(4)-p-X (X = OMe, H, CF(3)), C(2)H(3)(t)Bu, (c)C(5)H(8), (c)C(6)H(10), (c)C(7)H(10) (norbornene)) rearrange to alkylidene isomers (silox)(3)M(alk) (M = Nb (1=alk), Ta (2=alk); alk = CHMe, CHEt, CH(n)Pr, CHCH(2)C(6)H(4)-p-X (X = OMe, H, CF(3) (Ta only)), CHCH(2)(t)Bu, (c)C(5)H(8), (c)C(6)H(10), (c)C(7)H(10) (norbornylidene)). Kinetics and labeling experiments suggest that the rearrangement proceeds via a delta-abstraction on a silox CH bond by the beta-olefin carbon to give (silox)(2)RM(kappa(2)-O,C-OSi(t)Bu(2)CMe(2)CH(2)) (M = Nb (4-R), Ta (6-R); R = Me, Et, (n)Pr, (n)Bu, CH(2)CH(2)C(6)H(4)-p-X (X = OMe, H, CF(3) (Ta only)), CH(2)CH(2)(t)Bu, (c)C(5)H(9), (c)C(6)H(11), (c)C(7)H(11) (norbornyl)). A subsequent alpha-abstraction by the cylometalated "arm" of the intermediate on an alpha-CH bond of R generates the alkylidene 1=alk or 2=alk. Equilibrations of 1-ole with ole' to give 1-ole' and ole, and relevant calculations on 1-ole and 2-ole, permit interpretation of all relative ground and transition state energies for the complexes of either metal.