Novel Electrolyte Energy Storage Systems

We seek an approach to enable widespread deployment of grid-based storage by drastically lowering the cost of such a system. We are doing so by reexamining the fundamentals of flow battery technology and engaging in an effort in which the active redox couples, the materials that separate the couples, and the flow characteristics that dictate the rate of delivery are optimized, thereby allowing system-level solutions with high efficiency and with capital costs that are much lower than if each aspect of the system were optimized with the other aspects left unchanged. Informed by a deep understanding of fuel cell design, we are developing materials in concert with an understanding of the ultimate device behavior, and engineering electrodes that ensure high power per unit area, drastically lowering the cost per unit of power delivered. Tin(IV) bromide is a promising candidate for flow batteries because both redox couples, Sn(IV)/Sn(II) and Br2/Br, are dissolved in same solution, which eliminates concerns of cross contamination. With a view toward discovering ways to improve poor electrochemical reversibility of Sn(IV)/Sn(II), progress is being made toward a complete understanding of the mechanism of the Sn(IV)/Sn(II) redox reaction. We have detected the Sn(III) intermediate through electrochemical analysis methods, and estimated a rate constant for disproportionation of Sn(III). In related work, the first studies are underway of the electrochemistry of Sn(IV)Cl4 liquid, both undiluted and in solution with another solvent, toward development of a metal halide liquid redox flow battery. Additional redox couples are also under investigation. Introduction Efficient, cost-effective energy storage is vital to the effort to fully integrate renewable power sources into the electric utility grid. While compressed-air and pumped-hydro storage plants hold the promise of large-scale economical storage, they both require special sites. To date, redox flow batteries (RFB) have shown promise, but are presently far too expensive to be effectively deployed. This research offers an integrated approach to identify new electrolyte systems and cell designs that allow drastic cost reductions (removing this key barrier) while maintaining the high efficiency and ease of operation that are the hallmarks of RFB systems. Our approach brings together expert researchers with skills in chemistry, material science and characterization, electrochemical engineering, and mathematical modeling. We will advance new materials and electrochemistry for RFB systems, optimize cell designs, and develop guidelines for scaling up to utility-scale configurations. The requirements for large-scale electrical energy storage systems are quite different from existing battery systems. While batteries for portable and transportation applications place a premium on weight and volume, stationary energy storage systems have considerably less stringent requirements. Backup power systems support telecommunications and data centers, but are generally not expected to survive large numbers of charge/discharge cycles. Time-of-day pricing on the grid, mandates for renewable power sources, and the accompanying intermittency of those renewable sources are creating demand for electrical energy storage. Energy needs to be stored efficiently, and to accommodate several hours of continuous energy accumulation and release to the grid. For flow battery systems, one can specify independently the size of the electrochemical reactor (power capacity) and the size of the storage tanks of the freeflowing electrolyte streams (energy capacity). The ability to deliver the active material to the electrode surface by convection ensures that one can bypass mass-transport limitations that curtail the energy density of conventional batteries with solid-phase active materials. We seek to identify new electrolyte systems and cell designs that we expect will allow drastic cost reductions while maintaining the high efficiency and ease of operation that are the hallmarks of RFB systems. Background Flow batteries have been cited as a potential for grid-based energy storage, optimized over a very different set of metrics from conventional portable or transportation batteries. Several companies are developing flow battery technology to improve both the efficiency and stability of the smart grid. In recent months, in addition to flow schemes in which the active material is comprised of dissolved species in solution, researchers have also proposed a slurry based configuration, and a liquid metal halide electrolyte, either of which could drastically increase energy content per unit volume.[1] We seek to understand the limitations of such systems, and use that to benchmark the required performance of electrolyte systems. Results Semi-solid flow cell The semi-solid flow cell (SSFC) [1] represents the first reported application of lithium chemistries to the flow battery concept. It uses low volume fractions of active materials (20-25%) and carbon (0.5-2%) with no polymeric binder, creating slurries that can be flowed through the electrode chambers as in a typical flow battery. The SSFC provides the ease of manufacturing and the scalability of flow batteries along with energy densities approaching that of solid-state lithium-ion batteries. This project is intended to verify the SSFC concept and its electrochemical performance and to provide some insight into possible engineering concerns, especially the design of the electrodes and the rate capability of the battery. To explore these engineering problems, we created a discharge model based on porous electrode theory.[2] The porous electrode model is modified [3] by separating the continuous solid phase into two phases, one consisting of active material particles and the other consisting of carbon, such that the volume fractions of these two solid phases can be altered independently of each other. The coupled set of governing equations and boundary conditions are solved using a modified Newton-Raphson method in a MATLAB simulation environment. As of this time, the model is under development. To provide parameters for the model and to verify the electrochemical performance of the SSFC, we have begun to create a variety of semi-solid suspensions and insert them into coin cells, instead of creating a flow cell as was done in [1]. Cyclic voltammetry can be done to validate the model and to verify the performance of the SSFCs reported in [1]. Electrochemical impedance spectroscopy can be done to provide important conductivity values, which can be used in the model and can be compared to lithium-ion batteries with higher amounts of solid materials. Currently, we have successfully created half-cells by using Viton rubber O-rings to contain the semi-solid suspensions. Although the cells exhibit the correct open-circuit potentials, proving that there are no shorts or leaking of the slurries, we have been unable to obtain current-voltage data that agrees with standard data for the relevant active materials or with the results reported in [1]. Mechanistic studies of the tin-bromine system for redox flow batteries The Bard group is focusing first on tin-based systems for redox flow batteries. Tin(IV) bromide is a promising candidate because both redox couples, Sn(IV)/Sn(II) and Br2/Br are dissolved in same solution, eliminating concerns of cross contamination. The system has been limited by poor electrochemical reversibility of Sn(IV)/Sn(II). With a view toward discovering ways to improve that, progress is being made toward a complete understanding of the mechanism of the Sn(IV)/Sn(II) redox reaction. A key question has been whether (1) two electrons are simultaneously transferred through tunneling interface or (2) each stepwise one-electron transfer via a Sn(III) intermediate is preferred reaction pathway. Jinho Chang has now detected the Sn(III) intermediate through two electrochemical analysis methods, fast scan cyclic voltammetry and scanning electrochemical microscopy (SECM). Furthermore, a rate constant for the disproportionation of Sn(III) was estimated from a collection efficiency measured in SECM.[4] Pure liquid reactants for redox flow batteries Redox flow batteries tend to have relatively low specific energy density. For example, the energy density of a vanadium redox flow battery is 25-35 Wh/kg. Energy density is related to the concentration of redox species in solution. In the case of an allvanadium redox flow cell, the maximum concentration of the vanadium ion is ~ 2 M in sulfuric acid solution. Sn(IV)/Sn(II) is a promising redox couple for flow batteries in part because it transfers two electrons. Sn(IV)Cl4 is a special case among tin halides because it is liquid at room temperature. Potentially, it could be used as a redox electrolyte without dilution (Sn concentration is ~ 8.5 M). Pure liquid Sn(IV)Cl4 has low conductivity (dielectric constant, εr = 3.014), so its electrochemistry had not been reported. The Bard group is now working on the first studies of the electrochemistry of Sn(IV)Cl4 liquid, both undiluted and in solution with dichloromethane, as the first step in studying the metal halide liquid redox flow battery.[5] Challenges include passivation due to bulk precipitation of Sn(II)Cl2, and iR drop due to low conductivity. Progress We expect that the fundamental leaps in chemistry, materials, and engineering knowledge provided by this work will unlock the ability to build and operate commercial grid-scale storage for intermittent renewable energy sources. This type of large-scale storage is a natural complement to intermittent renewable generation methods, and to the degree we can enable those methods, we can directly reduce greenhouse gas emissions. Our initial modeling effort, which is nearly complete, will provide a basis for understanding whether a slurry concept can deliver the required cost per kWh for grid storage applications. We should be able to provide key material figures of merit for considering a slurry system versus a true electrolyte-based flow battery concept. This modeling effort should be complete by late May. Future Plans We seek to accomplish the following: (1) synthesize new ligands and complexes as electrolytes with tailored thermodynamic potentials, (2) screen these candidate materials under a variety of conditions on carbon electrodes, (3) characterize the most promising materials in larger electrochemical cells, (4) quantify the transport properties of ionomeric membranes in the presence of redox couples and supporting electrolyte, (5) test and simulate candidate systems as a means of down-selecting lab-scale tests to understand performance issues, (5) develop transient models based on conservation principles, incorporating the fundamental modes of transport for the charged species and solvent, including transport and reaction kinetics to simulate the performance of various RFB systems, and (7) facilitate the design of large-scale (>10 MW, and on the order of 1 GWh) RFB systems. References 1. M. Duduta, B. Ho, V. C. Wood, P. Limthongkul, V. E. Brunini, W. C. Carter, and Y.-M. Chiang, “Semi-Solid Lithium Rechargeable Flow Battery,” Adv. Energy Mater., vol. 1, no. 4, pp. 511–516, 2011. 2. J. Newman and K. Thomas-Alyea, Electrochemical Systems, 3rd ed. New York: Wiley and Sons, 2004. 3. D. E. Stephenson, E. M. Hartman, J. N. Harb, and D. R. Wheeler, “Modeling of Particle-Particle Interactions in Porous Cathodes for Lithium-Ion Batteries,” J. Electrochem. Soc., vol. 154, no. 12, p. A1146–A1155, Dec. 2007. 4. J. Chang and A. J. Bard, " Detection of Intermediate Sn(III) on Gold electrode through Fast Scan Cyclic Voltammetry and Scanning Electrochemical Microscopy," manuscript in preparation, 2012. 5. J. Chang, S. K. Cho, and A. J. Bard, " Electrochemical studies of Sn(IV)Cl4 Liquid," manuscript in preparation, 2012. Contacts Jeremy P. Meyers: [email protected] Allen J. Bard: [email protected] Brent Bennett: [email protected] Jinho Chang: [email protected] Annual Report for University of Tennessee GCEP Program: