Jeffrey A. Kowalski

A subtractive approach to molecular engineering of dimethoxybenzene-based redox materials for non-aqueous flow batteries

The development of new high capacity redox active materials is key to realizing the potential of non-aqueous redox flow batteries (RFBs). In this paper, a series of substituted 1,4-dimethoxybenzene based redox active molecules have been developed via a subtractive design approach. Five molecules have been proposed and developed by removing or reducing the bulky substituent groups of DBBB (2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene), a successful overcharge protection material for lithium-ion batteries. Of these derivatives, 2,3-dimethyl-1,4-dimethoxybenzene (23DDB) and 2,5-dimethyl-1,4-dimethoxybenzene (25DDB) are particularly promising as they demonstrate favorable electrochemical characteristics at gravimetric capacities (161 mA h g−1) that approach the stability limit of chemically reversible dimethoxybenzene based structures. Diffusivity, solubility, and galvanostatic cycling results indicate that both 23DDB and 25DDB molecules have promise for non-aqueous RFBs.

High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cells low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

Recent Developments and Trends in Redox Flow Batteries

Stationary energy storage systems are needed to facilitate the widespread integration of intermittent renewable electricity generators, such as solar photovoltaic and wind turbines, and to improve the energy efficiency of the electrical grid. While no single technology can meet all needs, redox flow batteries (RFBs) have shown a favorable balance of cost, safety, and performance for many high-value applications. RFBs are rechargeable electrochemical devices that utilize the reversible redox reactions of two soluble electroactive species for energy storage. A compelling feature of the RFB configuration is the independent scaling of power and energy which enables cost-effective implementation of electrochemical couples with low energy density. Aqueous RFBs have been the subject of the vast majority of research efforts to date, which have yielded industry-level demonstrations. By comparison, non-aqueous RFBs are still in their infancy but have the potential for high energy density due to the extended stability window of non-aqueous electrolytes and the enriched selection of redox materials due to the broad variety of organic solvents. This chapter aims to introduce emerging, potentially transformative, strategies for enhancing RFB technologies through molecular design, electrolyte development, and cell-level engineering. Detailed discussions focus on recent developments in redox active materials (inorganic – aqueous, organic – aqueous, inorganic – non-aqueous, and organic – non-aqueous) and in system design (interdigitated flow fields, semi-solid flow cells, and hybrid flow cells). Future research directions and key challenges for RFB technologies are also highlighted.

A stable two-electron-donating phenothiazine for application in nonaqueous redox flow batteries

Stable electron-donating organic compounds are of interest for numerous applications that require reversible electron-transfer reactions. Although many organic compounds are stable one-electron donors, removing a second electron from a small molecule to form its dication usually leads to rapid decomposition. For cost-effective electrochemical energy storage utilizing organic charge-storage species, the creation of high-capacity materials requires stabilizing more charge whilst keeping molecular weights low. Here we report the simple modification of N-ethylphenothiazine, which is only stable as a radical cation (not as a dication), and demonstrate that introducing electron-donating methoxy groups para to nitrogen leads to dramatically improved stability of the doubly oxidized (dication) state. Our results reveal that this derivative is more stable than an analogous compound with substituents that do not allow for further charge delocalization, rendering it a promising scaffold for developing atom-efficient, two-electron donors.