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NASA REOOX STORAGE SYSTEM DEVELOPHEN1 PROJECT - FINAL REPORT Norman H. Hagedorn Natlonal Aeronautics and Space Admlnistratlon Lewls Research Center Cleveland, Ohio 44135 ' The NASA Redox Storage System Technology Project recelved NASA and U.S. Department of Energy support, the l a t t e r belng under Interagency Agreement DE-A104-80AL12726 (prev;oosly EC-77-A-31-1002). S t a r t l n g i n 1979, p r o j e c t management was by the Sandia Natlonal Laboratories. The overal: objectives of the project were to develop the Redox flow battery technology and to verlfy Its sultzblllty for utlllty and stand-alone appllcatlons. rrom 1973 to 1982 the Redox system concept was based on 25" C operatlon and the use of anlon exchange membranes as separators to prevent ffilxlng of the a c l d l f l e d aqueous reactant so1u:lons. New classes of such membranes were developed that, although not meeting the r e s l s i l v l t y and selectivity targets established for economlc viability. were adequate to show that the basic system concepts were valld. By 1978 the Iron and chromlum redox couples had been selected as the posltlve and negatlve reactants. respectively. A catalyst system, based on a gold-lead comblnatlon, was developed for the chromlurn elec trode. Thls catalyst, i n addltlon t o belng active for the chromlum redox re- action. also inhibited hydrogen evolutlon. Analytlcal capahllltles were developed t 6 suppart the deslgn o f complete systems. These were used t o study the various tradeoffs between shunt currents, reactant flow maldlstrlbutlon, pumping pouer, reactant flow rates, and c e l l performance. System-level func- tlons such as rebalanclng, state-of-charge monitoring, and the use of trlm cells were reduced to practice. This work culminated In the deslgn, fabrlca- tlon, and testlng of a 1-kW, 13-CWh system. Thls system successfully valldated the many desirable characterlstlcs clalmed for flow batteries, of which the iron-chrmlum system I s a c1ass:c example. Between 1980 and 1982 it was determlned that certaln cell performance problems were related to the existence of a chromlc ton species that was not electrochoinlcally active. I t was further shown that lncreaslng the temperature to 65' C would cause an equlllbrlum shift of thls species to an acttve form. The remainder of the project effort was devoted to the exploltatfon of these new lnslghts. The higher temperatures requlred for these reasons reduced mem- brane se1ect:vlty to the polnt that the use of separate reactants could no longer be considered. Thls, then, led t o the mlxed-reactant concept of opera- tlon, whlch in turn permltted the use of membranes wlth very low reslstlvities (-0.5 ohm-cm2!. It then became possible to routinely opera:. slng?ecells at current densities as hlgh as 80 m~/cm* wlth energy efflclenctes greater than 80 percent. Early system analyses lndlcated that thls level of performance, ccvpled wlth the mass production of system components, would result in an eco nomlcally vlable Redox system. The actual achlevement of these hlgh levels of cell performance has greatly increase0 confldence that the Redox system, at a cost of about $7S/kWh, could be a valld contender for storage appllcatlons. A new catalyst system uslng bismuth aisc was developed for the chromium electrode at elevated temperature. Thls materlal has several advantages over the gold- lead comblnatlon and has increased the operatlonal flexibility of the system.PDF Image | NASA Redox Storage System Development Project
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