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and system design. Otherwise the Inefficiencles tha+ are also ir,herent in such systems can fully negate the advantages. In a later section a 1-kW Redox flow battery system designed, assembled, and tested a t Lewis I s discussed. The final design of this system gave attention to all of the characteristics of flow batteries that have been discussed here. DEVELOPMENT OF AMBIENT-TEMPERATURE REDOX STORAGE SYSTCM The basic Redox system was conceived at Lewis i n 1973 (ref 9). A t that time it was specifled that system operation would be at or near ambient tem- perature. During the next 7 years the concept was brought from a very small proof-of-concept demonstration cell to a fully functicnal 1-kW, 13-kwh system that operated in conjunction with a photovoltaic array. The development proc- ess involved both In-house and contract f f o r t s i n the areas of electrodes, membranes, system componenxs, c e l l and stack scaleup, system analyses and cost studies. This development process i s traced herein. Electrodes Several factors had to be considered w;th regard to flow-cell electrode development. The f i r s t obviously was the choice of the redox couples t o be used. In parallel with this determination though was the selection of the electrode substrate material, 1:; physical form, and the associated c e l l struc- ture and reactant flow regime. Finally it was necessary to ascertain whether catalysts would be required for any of the redox reactions and, if so, to b;;ng about+the1r development. Redox cou les. - The earliest cell testing at Lewis was based on the ~e+3/~ecouple as the posit:ve reactant and the T I ~ ~ / T ~ oco+u~ple as the neg- ative (ref. 10). This pair soon was snown to be less than ideal, primarily because of low cell voltage, ooor oegative electrode kinetics, and TiO? solu- bility ity problems. Nonetheless, it did serve as a stat.ting point for cell development olt~dfor establishing testing procedures. In the meantime, a con- tract effort at Giner, Inl:., undertock the systematic screening of redox couples by the rotating-disk procedure (ref. 11). using gold and vitrfgus t2 carbon as th-e working electrodes. The results of this work+qhowpg Fe /Fe and Br . ,r3 to be attractive as positive reactants, and Cr /Cr and Cu(NH, ) "/CU(NH ) " to be attractive as negative reactants, when the meas- ured dxthange cugrants were used as criteria. The iron couple was selected by Lewis over the bromide couple for reasons of safety, reactant vapor pressure, and materials compatibility. This choice 3 e n precluded the copper couple as the negative electrode because of i t s alkallnlty. The copper couple also suf- fered from poor solubility. Therefore it was decided to undertake the develop- ment of an Iron-chromium Redox flow battery. Substrates. - Another aspect of the initial Giner contract was tho eval- uation of several different electrode structures and cell configurations. The electrode structures included metal screens and sheets, carbon and graphlte blocks and cloths, and screens a?d carbon papers activated with Teflon cata- l y s t mixtures. The c e l l configurations Included electrodes adjacent t o the membrane with flow behind; electrodes adjacent to the terminal blocks with flow in front; and cavity-filling electrodes through which the reactants flowed. Redox couple screening at Giner, Inc., indicated that each couple had a favored 10PDF Image | NASA Redox Storage System Development Project
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