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compares the constant--voltagecharge acceptance rates of a cell havlng room temperature-equlllbrated chromium re2ccant and a cell havlng chromlum reactant equilibrated at elevated temperature. The dramatlc ,uperlorlty of the latter case Is revealed i n the abillty to charge at high rates to nearly 100 percent state of charge. Subsequent In-cell tests (ref. 35) verlfled an additlonal advantage ~f elevated-temperature operation, a reduction I n membrane and elec trolyte reslstlvity. Also, from a systems standpoint, i t 1s obvlous that hlgher temperatures would greatly sjmpllfy the rejc?ctlon of waste heat. A l l of this experlmental work overwhelmingly lndlcated that it was lmpera- t l v e t o operate the iron-chromlum Redox system a t elevated temperature. The benefits were too great to do otherwise. However, there was a slgnlflcant detrlment - the selectlvlty of the anlon exchange membranes belng used or eval- uated at that tlm became worse by factors of 4 to more than 10 (table IV) when the temperature was ralsed to 65" C. As stzted earlier, the best anlon exchange membrane avallable for the arnblent-temperature Redox system, the COIL, wdi miy~iar~inaatc;c~eptab;~'In 'Its aS'Il'Ity t~ keep the reactant cat:on; separate at 25" C. Wlth thls same membrane belng used i n cells at 65" C, com- plete cross-mlxlng of the reactaets occurred rapidly. The llkellhood of developing a new membrane wlth low reslstlvlty and good selectlvlty for use at 65" C was much less even than that for 25" C operatlon. For these reasons the posslblllty of operatlng wlth mlxed reactants at elevated temperature became the next f x a l point of development for the Iron-chromlum Redox storage system. REDOX STORAGE SYSTEM AT ELEVATED TEHPERATUSE As was explained in the prevlous section, the equlllbrtum between actlve and inactlve chromic ion species dictated that the iron-chromium flow battery be oper8tcd at about 65" C. Although thls Increase from the prevlous 25" C operatlng point solved the then-exlsting operatlonal dlfflcultles, lt also Introduced the posslblllty of protlems wlth materials compatlblllty and ccr- roslon. It was therefore necessary to reevaluate at 65" C the cells and cell components that had been developed for use at25"C. It soon became obvlous that no state-of -the-ar - alon exchange membrane could provlde adequate selectlvlty at the hlgher te~nperatures. Thus the near- perfect separation of the reactant metal catlons, as called for by the classlc flow battery concept, ceased t o be a viable optlon. This fact lead t o the realization that, for the Iron-chrcmium battery to functlon acceptably at 65" C, It would have to be able to operate with reactants that had undergone complete cross-mlxing. Mixed-Reactant Wode of Operation I n t h i s operatlng mode, both reactant solutions, when f u l l y discharged, are ldentlcal. Typically they would be 1 H FeCl2, 1 H CrC13, and 2 to 3 N HC1. The f u l l y charged positive reactant stream would be 1 N FeC13, 1 H CrC13, and 2 to 3 N HCl. Slmllarly, the fully charged negatlve reactant stream would be 1MFeC12, 1 HCrC12, and 2 to 3 N HC1. Once t h i s concept was considered, severa: p o t e n t l a l advantages became apparent. For example, since selectlvlty would no longer be a slgnlflcant crlterlon for membrane selection, It would be posslble to stress low reslstivltyPDF Image | NASA Redox Storage System Development Project
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