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Therefore, early I n the program Ionlcs. Inc., was contracted to develop m r a n e s speclflcally for the Redox system. It was assumed from the begln- nlng that such nedranes would requlre an anion exchange character In order to lnhlb't cross dlffuslon of the reactant catlons. Glvrng the ReRbranes a high ion-exchange capaclty (IEC) would also enhance d r a n e conductlvlty. There- fore Ionlcs, Inc., f i r s t investigated and evaluated 15 membrane systems, each coraprislng a different cornblnatlon of resln and ton exchange group (ref. 22). These c&lnatlons possessed a wlde range of both strong-base and weak-base ion exchange groups, the latter lnclud'ing examples of prlroary, secondary, and ter- tlary amlnes. Membrane constituents Included 2-vlnyl pyrldlne, ethylene glycol dlraethacrylate, vinylbenzyl chlorlde (VBC) post-amlnated wlth dlethylenetrla- nine and dlmethylamloethyl methacrylate (DHAEHA). Methods of membrane fabrl- catlon Included bulk polymerlzatlon followed by actlvatlon, I f needed; fllrn castlng; and dlrect actlvatlon of preformed films. These i n l t l a l evaluations (discussed l n detall I n ref. 22) resulted I n conslderable lnq~rovecrentsi n a l l of the crltlcal membrane characterlstlcs. #onet;leless conslderable lmprove- ments were s t i l l required. The next level of develo-nr was therefore to select the most promising candidate membrane systems and endeavo:. t o further i ~ p r o v ethem (ref. 23). These systems were prepared by bulk ~olyraerlzationof the llquld monomers on synthetic fabrqc backing. Variat'ons l n the c r i t i c a l synthesis parameters of cross-llnk density, RonoRler ratlo, and solvent composition were exanlned. Thls advanced screening effort trtentlfled the two most effectlve candidate Alerabrane types, deslgnated .CF41.' anu aCDIL.m As these membranes were further refined and ch~1racteriz~4r.he contractor was more and more forced to come to grips wlth the basic technical problem: improvements I n selectivlty generally are accompanied by increases I n reslstlvlty (ref. 24). For the CP4L type of mem- brane, not only was the reslstlvlty relatively hlgh upon initial exposure to acldlfled ferrlc chlorlde solutlons, but It proceeded to Increase wlth extended exposure. Prirnarlly on the basis of thls characteristic of the CP4L, the CDlL was selected as the focus for further developmental work. This membrane Is a copolymer of VBC as the backbone and DRAEHA as the cross-llnker, on a woven mdacrt: c substrate. The VBC/OCIAEMA ratio was 1/2 and the nonpolymerizable (NP) ,~lvent content was about 25 percent. Subsequent efforts by Ionlcs, Inc., consisted largely of varlatlonal studles and fine tuning of the CDlL composltlon and fabrlcatlon techrilque to optlmlze the nembrane characterlstlcs (refs. 25 and 26). The resulting mem- brane, whlch became the standard f o r use a t Lewls, was deslgnated CD1L-AAS-LC (27.5 NP). !ts pertinent performance characterlstlcs (ref. 2) were resistlvl- tles of 4.2 chm-cd tn 0.1 N HC1 and and 2.6 ohm-cm2 In a worklng Iron-chromlum Qedox ce, and a selectlvlty of about 20 pg fet3/h cm2 4. Thls membrane type tlad e?.allent stablllty In the amblent-temperature Redox cell environment, as evA ?wed by analyses performed on a 320-cm2 membrane after 18 months of ln- cell cycling at Lewls (refs. 2 and 26). There was virtually no change !n water content, IEC, or reslstlvlty. In parallel wlth its membrane development work, Ionlcs, Inc., evolved fabrlcatlon techniques that led to yields of 95 percent for COlL membranes as large as '07 by 51 cm (42 by 20 in.) and for batches as large as the 200 46 by !b cm (18 by 18 In.) membranes prepared for the 1-kW system a t Lewls. The selectivlty and resistivity given here for the optimum CDlL membrane represent the best posslble tradeoff between these two important characterts- tlcs and made the CDlL superior to all other types evaluated durlng most of the ISPDF Image | NASA Redox Storage System Development Project
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