Novel Electrolyte Energy Storage Systems

To enhance reliability of the electric grid while simultaneously incorporating renewable power sources into it, there is a pressing need for electrical energy storage, to increase the capability for dispatch and to accommodate the variable nature of those resources. There is, at present, very little energy storage on the grid, due in part to the high capital costs associated with electrochemical energy storage and the lack of flexibility in siting for other technologies. In this project, we approach enabling widespread deployment of grid-based storage by lowering the cost of such a system. We are reexamining the fundamentals of redox flow battery (RFB) technology and engaging in an effort in which new active redox couples are discovered and optimized, in pursuit of high efficiency and lower capital costs. We seek transformative changes in the construction and composition of the RFB. Major research accomplishments during the past year include: a) a description of the total reaction pathway for the two-electron Sn(IV)/(II) redox reactions in bromide solutions, including identification of the main species and Sn(III) intermediates; and b) the first evidence of reaction intermediates in the bromide/bromine reaction. Both of these accomplishments advance the fundamental science necessary for pushing tinbromine and other bromine-based RFB systems toward commercialization. c) The application of very high energy-density RFB systems, based on systems with very high concentrations of suitable redox couples, including a solvent-bromine system and the first investigations of the electrochemistry of Sn(IV)Cl4 liquid, both undiluted and in solution with another solvent and d) fundamental studies of how potentials of redox couples (e.g. of Fe, Co, and Mn) can be shifted to increase voltage output with various novel ligands, and the bench-scale demonstration of a novel alkaline Fe/Co redox flow battery. 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 at present far too expensive to be effectively deployed. The objective of this research it 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 and develop guidelines for scaling up to utility-scale configurations. 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. 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. 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 free-flowing 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. Moreover, since the charge and discharge cycles of most RFBs do not involve phase changes, the cycle life and stability is much higher than conventional batteries. Redox couples for flow-battery applications, represented by reaction in Equation 1,