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and a significant Improvement i n electrochemical energy efficiency, with no significant increase i n projected system cost. I n the remainder of this report, the development of the iron-chromium Redox storage system technology i s traced t o i t s present state. The basic sys- tem concept i s discussed i n some d e t a i l . The development of the enabling tech- nologies for the basic concept (e.g., membranes and electr~des)i s reviewed, with considerable reference to existing reports. The discussion of the effort during the final several years ~f the project Is somwhat expanded, to place proper emphasis on the significant advancements I n Redux system performance during those years. THE REDOX SYSTEM I n i t s original form, the iron-chromlum Redox system represents the clas- sic flow battery. The most significant characteristic of such a system i s the complete solubility of reactants and products at a l l states of charge. This characteristic allows the storage of the reactants as acidified aqueous solu- tions i n tankage that I s external t o the power-proLucing portion of the sys. ,n. In the *two tanku version of a flow battery shown schematically in figurt 1, each reactant I s continuously circulated from i t s storage tank, through the groupings of single cells that make up the power-producing subsystem, and back to i t s storage tank. The flow rate of each reactant i s always higher than the stoichiometric flow requirement, which would result i n total reactant utlllza- tion in one pass through the battery cells. Typically, the cells would be zrranged I n series-parallel configurations of multicell stacks. I n each stack the cells would be series connected in the bipolar mode, and the reactants would flow I n parallel through the individual cells. As with acy battery, each stack has terminals that allow for dlscharge into a load or for the acceptance of charge from an external power supply. In the iron-chromium Redox system the positive reactant i s an aqueous solutlon of the.ferric-ferrous redox couple. acidified with hydrochloric acid. The negative reactant i s slmilar solution of the chromus-chromic couple. For both reactants the charge and discharge reactions involve simple one-electron transfers: Pssitlve electrode Negative electrode +3 Fe + e 4 charge dlscharge +Fe crt2- @ crt3 t e dlscharge t2 charge I n figure 1, the power-producing portion of the system i s depicted as a single cell. In each such cell an anion exchange membrane separates the two flowing reactant solutions. The membrdne ideally prevents cross diffusion of the iron and chromium ions, which would result in a loss of capacity, yet per- mits free passage of chloride and hydrogen ions for completion of the electri- cal circuit through the cell. These two desired characteristics of an ideal membrane - good selectivity and low resistivity - represent a stringent set of conditlons. In the Redox cell, each electrode i s a porous carbon felt or graphite felt pad that i s compressed between the membrane and the respective terminal plate.PDF Image | NASA Redox Storage System Development Project
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