Collaborative Research: Establishing Design Principles for Molecular Engineering of High Concentration Redox Electrolytes

Overview: Electrochemical energy storage will play a pivotal role in decarbonization of the global power system by enabling the wide-scale penetration of intermittent non-dispatchable renewable resources as well as by extending the lifetime and improving the energy efficiency of the current electricity grid. The proposed collaborative project focuses on enabling high capacity nonaqueous redox electrolytes that enable energydense cost-effective flow batteries through the systematic development of concentrated redox electrolytes. Realization of high concentration redox electrolytes is key to the economic viability of nonaqueous flow batteries and an enabling component for a number of other relevant electrochemical technologies. However, fundamental knowledge gaps exist both in (1) the molecular engineering of stable concentrated redox active solutions and (2) the electrochemical characterization of these concentrated electrolytes as, at present, most investigations focus on molecular discovery and electrochemical characterization under dilute conditions followed by direct integration into unoptimized proof-of-concept flow cells for preliminary cycling analysis. Thus, this collaborative research proposal focuses on the establishment of molecular design principles that enable the development of high performance concentrated nonaqueous redox electrolytes for use in electrochemical energy storage. We hypothesize that phenothiazines, metallocenes, and nitroxides can be employed as stable molecular scaffolds with well-defined physical and electrochemical properties that, through systematic derivatization based on simple syntheses, can provide diverse training sets to inform generalized structure-activity-mechanism models, can serve as redox probes to characterize complex electrochemical environments, and can guide the synthesis of novel multifunctional materials that enable the development of undiluted rechargeable electrochemical fluids. Intellectual Merit: Design principles based on molecular composition and electrolyte formulations have yet to be established for concentrated redox electrolytes. Current approaches rely on low-concentration studies (millimolar) to predict, largely unsuccessfully, high concentration (molar) behavior. Furthermore, there is dearth of electrochemical techniques for evaluating non-aqueous solutions with high concentrations of redox active materials and low ionic conductivities. Our key innovations are (i) the utilization of a redox-active materials with well-defined characteristics as platforms for multi-property optimization (solubility, stability, reversibility) and (ii) the advancement of versatile electroanalytical methods for evaluating concentrated redox electrolytes. Fundamental knowledge of kinetic, ohmic, and transport properties in concentrated redox electrolytes will enable design and operation of advanced nonaqueous flow batteries. Moreover, quantification of concentration-dependent undesirable reactions will advance scientific understanding of coupled molecule and electrolyte design, which is relevant to other electrochemical technologies including redox mediators / catalysts in dye-sensitized solar cells, advanced lithium-oxygen batteries, and indirect electrochemical processing of alternative feedstocks Broader Impacts: Knowledge gained from this project will enable a new generation of non-aqueous chemistries for redox flow batteries, creating new pathways to low cost grid storage and expanding the flow battery application space through the increased energy density and smaller system footprint. The development of such grid storage solution is potentially transformative with far-reaching economic and societal benefits. The proposed research and educational activities conceive an integrated strategy to train students, including women and underrepresented minorities, in subjects related to sustainable energy, synthetic organic chemistry, and electrochemical engineering, while also disseminating the results to the broader community through outreach programs and curriculum development.