Organic–Inorganic Hybrid Polyoxotungstates As Configurable Charge Carriers for High Energy Redox Flow Batteries

We describe the synthesis and electrochemical analysis of the phenyl siloxane-hybridized phosphotungstate Keggin-type polyoxometalate (POM) TBA3[PW11O39(SiC6H5)2O] (TBA3W11SiPh) and its performance as the charge carrier in nonaqueous redox flow batteries (RFBs). The hybridized POM is synthesized by modification of the parent POM [PW12O40]3–, increasing its saturation concentration in acetonitrile by 2 orders of magnitude over that of the parent compound (600 mmol dm–3 for TBA3W11SiPh vs –3 for TBA3[PW12O40]). Electrochemical analysis of TBA3W11SiPh reveals four one-electron, quasi-reversible, redox couples between −0.60 and −2.50 V vs Ag+|Ag, prompting us to explore its application as a dual-function charge carrier in symmetric RFBs. The stability of TBA3W11SiPh is investigated in several symmetric RFBs, and the system demonstrates high coulombic efficiency (98%), voltage efficiency (89%), and energy efficiency (87%) during redox cycling. We show that capacity fade due to oxidation-state imbalance can be counteracted by rereduction of electrolytes. These results demonstrate that organic–inorganic hybridization of POMs offer opportunities for the development of highly soluble multielectron redox electrolytes that operate across a wide range of potentials, expanding the available range of charge carriers for high-energy RFBs

Four Redox Isomers of a [3 × 3] Copper–Iron Heterometal Grid

A mixed-valence heterometallic nonanuclear [3 × 3] grid complex, [CuI2CuII6FeIII(L)6]­(BF4)5·MeOH·9H2O (1; MeOH = methanol), was synthesized by a one-pot reaction of copper and iron ions with multidentate ligand 2,6-bis­[5-(2-pyridinyl)-1H-pyrazol-3-yl]­pyridine (H2L). 1 showed five quasi-reversible one-electron redox processes centered at +0.74, +0.60, +0.39, +0.27, and −0.13 V versus SCE, assignable to four CuI/CuII processes and one FeII/FeIII couple, respectively. The two-electron-oxidized species [CuII8FeIII(L)6]­(PF6)7·4MeOH·7H2O (12eOx), the two-electron-reduced species [CuI4CuII4FeIII(L)6]­(PF6)3·2H2O (12eRed), and the three-electron-reduced species [CuI4CuII4FeII(L)6]­(PF6)2·5MeOH·H2O (13eRed) were isolated electrochemically. The four redox isomers were characterized by single-crystal X-ray analysis, SQUID magnetometry, and Mössbauer spectroscopy

Trading Templates: Supramolecular Transformations between {Co^II₁₃} and {Co^II₁₂} Nanoclusters

“Nuclearity switching” from a {Co13} supercluster to a {Co12} species via the addition of CO32- anions is reported and can be traced in solution using electrospray MS techniques. In addition, cryospray MS can be used to identify the entire cluster in solution despite the relative lability of its constituents

Trading Templates: Supramolecular Transformations between {Co^II₁₃} and {Co^II₁₂} Nanoclusters

“Nuclearity switching” from a {Co13} supercluster to a {Co12} species via the addition of CO32- anions is reported and can be traced in solution using electrospray MS techniques. In addition, cryospray MS can be used to identify the entire cluster in solution despite the relative lability of its constituents

Enflurane Additive for Sodium Negative Electrodes

Development of sodium anodes, both hard carbon (HC) and metallic, is dependent on the discovery of electrolyte formations and additives able to stabilize the interphase and support Na+ transport. Halogen salt additives are known to lower the energy barrier for the Na-ion charge transfer at the interface and facilitate stable Na plating/stripping in a symmetric cell configuration. Here, a halogen-rich additive for the sodium-ion battery electrolyte, 2-chloro-1,1,2-trifluoroethyl difluoromethyl ether (enflurane), is reported. Enflurane offers a simple molecular alternative to salt-based additives. The additive is also shown to improve the cycling performance of sodium metal electrodes. Our analysis demonstrates that enflurane is preferentially reduced at the HC electrode over propylene carbonate and is incorporated into the solid electrolyte interphase (SEI). The result is a thin, halogen-rich SEI that offers better charge transport properties and stability during cycling compared to that formed in the additive-free electrolyte. Additionally, enflurane inhibits polarization of metallic sodium electrodes, and when included in HC half-cells at 10 v/v %, it improves the reversible specific capacity and stability

Multiredox Active [3 × 3] Copper Grids

A nonanuclear copper grid complex, [CuII9(L)6]­(BF4)6·1-PrOH·5H2O (1·1-PrOH·5H2O; L = 2,6-bis­[5-(2-pyridinyl)-1H-pyrazol-3-yl]­pyridine), was synthesized with a [3 × 3] grid structure consisting of nine CuII ions and six deprotonated ligands and displayed four-step quasi-reversible redox behavior from [CuII9] to [CuI4CuII5]. The corresponding heterovalent complex [CuI2CuII7(L)6]­(PF6)4·3H2O (2·3H2O) was successfully isolated and had a distorted core structure that radically changed the intramolecular magnetic coupling pathways

Chiral Single-Chain Magnet: Helically Stacked [Mn^III₂Cu^II] Triangles

The one-dimensional complex [Mn^III₂Cu^II(μ₃-O)­(Cl-sao)₃(EtOH)₂]·EtOH (Mn₂Cu) was obtained by the metal replacement reaction of the trinuclear manganese complex (Et₃NH)­[Mn^III₃(μ₃-O)­Cl₂(Cl-sao)₃(MeOH)₂(H₂O)₂] with [Cu­(acac)₂]. The Mn₂Cu chain exhibits single-chain-magnet behavior with finite-size effects due to its large magnetic anisotropy

Voltammetric Evidence of Proton Transport through the Sidewalls of Single-Walled Carbon Nanotubes

Understanding ion transport in solid materials is crucial in the design of electrochemical devices. Of particular interest in recent years is the study of ion transport across 2-dimensional, atomically thin crystals. In this contribution, we describe the use of a host–guest hybrid redox material based on polyoxometalates (POMs) encapsulated within the internal cavities of single-walled carbon nanotubes (SWNTs) as a model system for exploring ion transport across atomically thin structures. The nanotube sidewall creates a barrier between the redox-active molecules and bulk electrolytes, which can be probed by addressing the redox states of the POMs electrochemically. The electrochemical properties of the {POM}@SWNT system are strongly linked to the nature of the cation in the supporting electrolyte. While acidic electrolytes facilitate rapid, exhaustive, reversible electron transfer and stability during redox cycling, alkaline-salt electrolytes significantly limit redox switching of the encapsulated species. By “plugging” the {POM}@SWNT material with C60-fullerenes, we demonstrate that the primary mode of charge balancing is proton transport through the graphenic lattice of the SWNT sidewalls. Kinetic analysis reveals little kinetic isotope effect on the standard heterogeneous electron transfer rate constant, suggesting that ion transport through the sidewalls is not rate-limiting in our system. The unique capacity of protons and deuterons to travel through graphenic layers unlocks the redox chemistry of nanoconfined redox materials, with significant implications for the use of carbon-coated materials in applications ranging from electrocatalysis to energy storage and beyond

Contrasting Magnetism of [Mn^III₄] and [Mn^II₂Mn^III₂] Squares

Two tetranuclear manganese distorted square-shaped clusters, [MnIII4(L1)4(μ2-OMe)4]·2.5H2O (1) and [MnII2MnIII2(L2)4(H2O)2](PF6)2·CHCl3·CH3OH·1.5H2O (2) (H2L1 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid methyl ester; H2L2 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid ethyl ester), exhibit antiferromagnetic and ferromagnetic interactions between neighboring manganese ions, respectively

Contrasting Magnetism of [Mn^III₄] and [Mn^II₂Mn^III₂] Squares

Two tetranuclear manganese distorted square-shaped clusters, [MnIII4(L1)4(μ2-OMe)4]·2.5H2O (1) and [MnII2MnIII2(L2)4(H2O)2](PF6)2·CHCl3·CH3OH·1.5H2O (2) (H2L1 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid methyl ester; H2L2 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid ethyl ester), exhibit antiferromagnetic and ferromagnetic interactions between neighboring manganese ions, respectively