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1. Development of high-voltage and high-energy membrane-free nonaqueous lithium-based organic redox flow batteries

   https://doi.org/10.1038/s41467-023-40374-y  

   Gautam, Rajeev K. ; Wang, Xiao ; Lashgari, Amir ; Sinha, Soumalya ; McGrath, Jack ; Siwakoti, Rabin ; Jiang, Jianbing “Jimmy” ( August 2023 , Nature Communications)

   Lithium-based nonaqueous redox flow batteries (LRFBs) are alternative systems to conventional aqueous redox flow batteries because of their higher operating voltage and theoretical energy density. However, the use of ion-selective membranes limits the large-scale applicability of LRFBs. Here, we report high-voltage membrane-free LRFBs based on an all-organic biphasic system that uses Li metal anode and 2,4,6-tri-(1-cyclohexyloxy-4-imino-2,2,6,6-tetramethylpiperidine)-1,3,5-triazine (Tri-TEMPO), N-propyl phenothiazine (C3-PTZ), and tris(dialkylamino)cyclopropenium (CP) cathodes. Under static conditions, the Li||Tri-TEMPO, Li||C3-PTZ, and Li||CP batteries with 0.5 M redox-active material deliver capacity retentions of 98%, 98%, and 92%, respectively, for 100 cycles over ~55 days at the current density of 1 mA/cm²and a temperature of 27 °C. Moreover, the Li||Tri-TEMPO (0.5 M) flow battery delivers an initial average cell discharge voltage of 3.45 V and an energy density of ~33 Wh/L. This flow battery also demonstrates 81% of capacity for 100 cycles over ~45 days with average Coulombic efficiency of 96% and energy efficiency of 82% at the current density of 1.5 mA/cm²and at a temperature of 27 °C.

    

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2. A Stable, Low Permeable TEMPO Catholyte for Aqueous Total Organic Redox Flow Batteries

   https://doi.org/10.1002/aenm.202102577  

   Hu, Bo ; Hu, Maowei ; Luo, Jian ; Liu, T. Leo ( December 2021 , Advanced Energy Materials)

   Aqueous organic redox flow batteries (AORFBs) are highly attractive for large‐scale energy storage because of their nonflammability, low cost, and sustainability. (2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl (TEMPO) derivatives, a class of redox active molecules bearing air‐stable free nitroxyl radicals and high redox potential (>0.8 V vs NHE), has been identified as promising catholytes for AORFBs. However, reported TEMPO based molecules are either permeable through ion exchange membranes or not chemically stable enough for long‐term energy storage. Herein, a new TEMPO derivative functionalized with a dual‐ammonium dicationic group,N¹, N¹, N¹, N³, N³, 2, 2, 6, 6‐nonamethyl‐N³‐(piperidinyloxy)propane‐1,3‐bis(ammonium) dichloride (N₂‐TEMPO) as a stable, low permeable catholyte for AORFBs is reported. Ultraviolet–visible (UV–vis) and proton nuclear magnetic resonance (¹H‐NMR) spectroscopic studies reveal its exceptional stability and ultra‐low permeability (1.49 × 10⁻¹² cm² s⁻¹). Coupled with 1,1′‐bis[3‐(trimethylammonio)propyl]‐4,4′‐bipyridinium tetrachloride ((NPr)₂V) as an anolyte, a 1.35 VN₂‐TEMPO/(NPr)₂V AORFB with 0.5 melectrolytes (9.05 Wh L⁻¹) delivers a high power density of 114 mW cm⁻²and 100% capacity retention for 400 cycles at 60 mA cm⁻². At 1.0 melectrolyte concentrations, theN₂‐TEMPO/(NPr)₂V AORFB achieves an energy density of 18.1 Wh L⁻¹and capacity retention of 90% for 400 cycles at 60 mA cm⁻².

    

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3. Development of High Energy Density Diaminocyclopropenium‐Phenothiazine Hybrid Catholytes for Non‐Aqueous Redox Flow Batteries

   https://doi.org/10.1002/ange.202111939  

   Yan, Yichao ; Vogt, David B. ; Vaid, Thomas P. ; Sigman, Matthew S. ; Sanford, Melanie S. ( November 2021 , Angewandte Chemie)

   This report describes the design of diaminocyclopropenium‐phenothiazine hybrid catholytes for non‐aqueous redox flow batteries. The molecules are synthesized in a rapid and modular fashion by appending a diaminocyclopropenium (DAC) substituent to the nitrogen of the phenothiazine. Combining a versatile C‐N coupling protocol (which provides access to diverse derivatives) with computation and structure‐property analysis enabled the identification of a catholyte that displays stable two‐electron cycling at potentials of 0.64 and 1.00 V vs. Fc/Fc⁺as well as high solubility in all oxidation states (≥0.45 M in TBAPF₆/MeCN). This catholyte was deployed in a high energy density two‐electron RFB, exhibiting >90 % capacity retention over 266 hours of flow cell cycling at >0.5 M electron concentration.

    

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4. Development of High Energy Density Diaminocyclopropenium‐Phenothiazine Hybrid Catholytes for Non‐Aqueous Redox Flow Batteries

   https://doi.org/10.1002/anie.202111939  

   Yan, Yichao ; Vogt, David B. ; Vaid, Thomas P. ; Sigman, Matthew S. ; Sanford, Melanie S. ( November 2021 , Angewandte Chemie International Edition)

   This report describes the design of diaminocyclopropenium‐phenothiazine hybrid catholytes for non‐aqueous redox flow batteries. The molecules are synthesized in a rapid and modular fashion by appending a diaminocyclopropenium (DAC) substituent to the nitrogen of the phenothiazine. Combining a versatile C‐N coupling protocol (which provides access to diverse derivatives) with computation and structure‐property analysis enabled the identification of a catholyte that displays stable two‐electron cycling at potentials of 0.64 and 1.00 V vs. Fc/Fc⁺as well as high solubility in all oxidation states (≥0.45 M in TBAPF₆/MeCN). This catholyte was deployed in a high energy density two‐electron RFB, exhibiting >90 % capacity retention over 266 hours of flow cell cycling at >0.5 M electron concentration.

    

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5. Single-Electron Redox Chemistry on the [Cp*Rh] Platform Enabled by a Nitrated Bipyridyl Ligand

   https://doi.org/10.3390/molecules23112857  

   Moore, William ; Henke, Wade ; Lionetti, Davide ; Day, Victor ; Blakemore, James ( November 2018 , Molecules)

   [Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) are attracting renewed interest in coordination chemistry and catalysis, but these useful compounds often undergo net two-electron redox cycling that precludes observation of individual one-electron reduction events. Here, we show that a [Cp*Rh] complex bearing the 4,4′-dinitro-2,2′-bipyridyl ligand (dnbpy) (3) can access a distinctive manifold of five oxidation states in organic electrolytes, contrasting with prior work that found no accessible reductions in aqueous electrolyte. These states are readily generated from a newly isolated and fully characterized rhodium(III) precursor complex 3, formulated as [Cp*Rh(dnbpy)Cl]PF6. Single-crystal X-ray diffraction (XRD) data, previously unavailable for the dnbpy ligand bound to the [Cp*Rh] platform, confirm the presence of both [η5-Cp*] and [κ2-dnbpy]. Four individual one-electron reductions of 3 are observed, contrasting sharply with the single two-electron reductions of other [Cp*Rh] complexes. Chemical preparation and the study of the singly reduced species with electronic absorption and electron paramagnetic resonance spectroscopies indicate that the first reduction is predominantly centered on the dnbpy ligand. Comparative cyclic voltammetry studies with [NBu4][PF6] and [NBu4][Cl] as supporting electrolytes indicate that the chloride ligand can be lost from 3 by ligand exchange upon reduction. Spectroelectrochemical studies with ultraviolet (UV)-visible detection reveal isosbestic behavior, confirming the clean interconversion of the reduced forms of 3 inferred from the voltammetry with [NBu4][PF6] as supporting electrolyte. Electrochemical reduction in the presence of triethylammonium results in an irreversible response, but does not give rise to catalytic H2 evolution, contrasting with the reactivity patterns observed in [Cp*Rh] complexes bearing bipyridyl ligands with less electron-withdrawing substituents. 

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