High-Voltage Symmetric Nonaqueous Redox Flow Battery Based on Modularly Tunable [Ru₂M(μ₃-O)(CH₃CO₂)₆(py)₃] (M = Ru, Mn, Co, Ni, Zn) Cluster Compounds with Multielectron Storage Capability

Abstract

Redox flow batteries (RFBs) provide an attractive solution for large-scale energy buffering and storage. This report describes the development of nonaqueous RFBs based on trimetallic coordination cluster compounds: [Ru2M­(μ3-O)­(CH3CO2)6(py)3] (M = Ru, Mn, Co, Ni, Zn). The all-ruthenium complex exhibited stable battery cycles in anolyte–catholyte symmetric operation, with rarely observed multielectron storage in a single molecule. Moreover, the complex holds modularly tunable synthetic handles for systematic improvements in solubility and redox potentials. An optimized battery stack containing [Ru3(μ3-O)­(CH3CO2)6(py)3]+ anolyte and [Ru2Co­(μ3-O)­(CH3CO2)6(py)3] catholyte yielded stable cycles with a discharge voltage of 2.4 V, comparable to the state-of-the-art nonaqueous RFBs. Explanation for the exceptional stability of the charged states and prediction of systematic tunability of the redox potentials of the cluster compounds were assisted by DFT calculations.

Controlling Charge Percolation in Solutions of Metal Redox Active Polymers: Implications of Microscopic Polyelectrolyte Dynamics on Macroscopic Energy Storage

“…Increasing the energy density of NaRFBs can be accomplished by extending the redox potentials of the anolyte and catholyte, , and increasing their solubility. Furthermore, it is highly desirable that these species are chemically stable in the charged state. , To date, a selection of nonaqueous redox active materials have been studied, ranging from coordination complexes, ionic liquids, small organic molecules, and redox active polymers (RAPs). − ,,− The use of polyelectrolytes is presented as an important and novel tool for RFB design because they enable the construction of size-exclusion NaRFBs by pairing them with inexpensive and chemically stable microporous separators. − Redox active polymers using a polyvinyl benzylic backbone chain with viologen derivatives as redox pendants have been used in such strategies. ,− , The bulky nature of RAPs compared to other small molecules makes it difficult for them to cross over to the reservoir of other redox active species. − Also, the large size of polymers facilitates the creation of networks for electron transfer (ET) as is the case for redox-active colloids. , …”

Recent Developments in Materials and Chemistries for Redox Flow Batteries

“…11 Another attractive approach is to tailor the coordination ligands or functional groups in metalcentered organic compounds, which can enable the anolytecatholyte symmetric reaction in the same structure. 13,14 Furthermore, computational methods (e.g., machine learning, density functional theory) have been explored to accelerate the discovery of potential redox-active materials with desired properties. 15,16 Another direction for improving the energy density is the development of semisolid flow batteries, including suspending solid active materials and conductive carbon into liquid electrolytes 17 or utilizing soluble redox mediators to achieve redox-targeting reactions of solid active materials.…”

“…Similar methods can also be adopted to design high-potential catholytes with tuning other important properties such as solubility and electrochemical stability. , For instance, as the oxygen analogue of phenothiazine, phenoxazine can show the higher potential in the N-diaminocyclopropenium-substituted form (1.35 V vs SHE) than N -alkyl ones (∼0.9 V vs SHE) . Another attractive approach is to tailor the coordination ligands or functional groups in metal-centered organic compounds, which can enable the anolyte-catholyte symmetric reaction in the same structure. , Furthermore, computational methods (e.g., machine learning, density functional theory) have been explored to accelerate the discovery of potential redox-active materials with desired properties. , Another direction for improving the energy density is the development of semisolid flow batteries, including suspending solid active materials and conductive carbon into liquid electrolytes or utilizing soluble redox mediators to achieve redox- targeting reactions of solid active materials …”