CAREER: Investigation of Non-Aqueous Single-Metal Redox Flow Batteries through Experiment and Modeling

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Intellectual Merit: This proposal focuses on experimental and theoretical studies of a new class of non-aqueous, single-metal redox flow battery (RFB). These RFBs rely on disproportionation of metal coordination complexes into oxidized and reduced products. Use of non-aqueous liquids to support redox reactions makes it possible to raise cell voltages well above the range where water decomposes. Our preliminary experiments using first-row transition-metal complexes demonstrate some equilibrium voltages more than twice as large as aqueous systems permit, suggesting that development of high-conductivity solvent/support systems and highly soluble active-complex structures that undergo multiple electron transfer could improve energy density by 200%-300%. Non-aqueous, single-metal, disproportionation-based RFBs promise significantly improved performance over aqueous chemistries, and our preliminary cycling results suggest competitive coulombic efficiencies. We see comparatively low energy and power efficiencies in non-aqueous RFB systems during cycling, issues this research will address. The proposed research will perform and rationalize experimental observations of non-aqueous RFBs. Our goal is to guide reactor design and materials selection for next-generation nonaqueous RFBs on the basis of sound fundamental principles. Three complementary projects will be executed in parallel during the five-year research term. (1) Equilibrium electrochemical properties of various non-aqueous RFB systems will be investigated by potentiometric and spectroscopic methods. Several trivalent β-diketonate metal active complexes will be studied in various support and solvent systems. In addition to elucidating how activity and potential depend on an RFB system?s state of charge, this work will inform future system design by showing how solute/solute and solute/solvent interactions impact efficiency of RFBs in general.(2) Transport phenomena in the liquid and separator phases will be characterized with methods including DC conductimetry, AC impedance spectroscopy, and UV-vis spectroscopy. The results will be matched to advanced theoretical multicomponent transport models, and used to underpin a property database for computer simulations of RFB operation. RFB cells will be fabricated to corroborate numerical results with experimental charge/discharge data. The model will also be used to predict how control schemes and cell design can be varied to optimize RFB performance. (3) Reaction rates and mechanisms in single-metal RFBs will be probed by electrochemical experiments. Theoretical methods will be created to establish mechanisms for several active-complex redox reactions. Kinetic models will be included in RFB cell simulations. Ultimately, this research program will deliver a continuum-scale model that rationalizes and predicts the transient current/voltage response of RFB cells during practical charging and discharging between various states of charge, and at various charge and discharge rates. Simulations will permit studies of various electrode configurations, cell designs, and electrolyte flow schemes. We will take a bottom-up approach, using studies of bulk, interfacial, and phase exchange processes to support a holistic, microscopically informed model of the entire RFB.Broader Impact: Outreach activities will foster the natural connection that I believe to exist between research progress and pedagogical activity. My central outreach objective is to advocate multidisciplinary, collaborative electrochemical engineering education in a diverse research environment. This objective will be addressed by: (1) continuing development of graduate-level electrochemical engineering courses that incorporate RFB research problems; (2) encouraging undergraduate interest in energy-system design by participating in service activities related to electrochemical engineering, offering undergraduate research opportunities, and developing forums outside the classroom for the exchange of ideas; and (3) taking part in the MI-LSAMP outreach program, which aims to increase participation by under-represented minorities and women in cutting-edge science, technology, engineering, and mathematics. I hope to move electrochemistry away from the periphery of chemical engineering research.Increasing the exposure of pre-college, undergraduate, and graduate students from all walks of life to electrochemical engineering will facilitate the innovation necessary to support a national shift to more diverse, sustainable energy production.