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natlve electrocatalysts and full-cell testing. Electrode performance changes as a result of the crossmlxing of reactants through the membrane and the impuritles found ln technical-grade chromlum chlorlde were also studied. Work on the current contract (DEN3-198) began ln mld-1980 wlth the prlmary objectives of characterlzation of the carbon/graphite felt substrate materlal; optlmlzatlon of the gold-lead catalysis procedure; preparation of full-slze, assembly-ready electrodes; and the study of alternative catalysts. Approach. - Cyclic voltammetry was used to qualitatlvely and quantita- tively evaluate varlOUS carbon and graphite felt substrate materlals for electrochemical actlvlty, wlth and without catalyst additlons. Chromium oxidation and reductlon, hydrogen lon reduction, and lead oxidatlon and re- ductlon reactions were measured to characterlze the electrodes. Surface actlvity of the felt materials was modlfled through treatment in acid or base Solutlons ln an attempt to "normalize" all of the felt lots prior to catalyst appllcations. Water absorption and retention propertles of each felt were determined so that gold solutions could be added quantita- tively. Several methods for gold application were evaluated for uniformity of gold dlstributlon on the felts. Electrochemlcal actlvity as a functlon of gold loading was also investigated. Studies were made of blsmuth and bismuth-gold catalyzed felts as alternative catalyst systems. Status. - The research performed at Giner, Inc., disclosed slgnlflcant varlablllty among the lots of carbon felt recelved from the vendor. Some lots of felt had strong chemlcal reduction properties that affected the gold deposition onto the felt matrlX. Treatlng thlS type of carbon felt wlth nltrlc acid changed the surface activity, giving better gold distrlbution and performance. Poor gold dlstrlbution generally results in a hlgh hydrogen evolution rate and in a sudden change in discharge performance as the lead deplates and the electrode reverts to a less-actlve carbon surface. A catalyst applicatlon method using an alcohol-gold solution was suggested. Alcohol greatly improved fiber wettabllity and resulted in a more homogeneous gold dlstributlon. A Joint patent dlsclosure was filed wlth NASA for this catalysls procedure. ThlS procedure was used successfully to prepare the 156 chromium electrodes used in the NASA 1-kW Redox storage system. Several other gold catalyst applicatlon methods were studied but clid not appear to provide unlform gold dlstributlon. A vacuum method was devel- oped that used reduced pressure to remove air from the felt and give better wettablllty for the cleaning treatment and for addlng the gold solution without the use of alcohol. Nonuniform gold dlstrlbutlon was eVldent wlth thlS procedure because gold mlgrated to the outer surface of the felt plece during the long drying period. Four different lots of carbon and graphlte felt~ each catalyzed at four different gold loadlngs (3, 6, 12.5, and 25 ~g Au/cm ) were evaluated by cyclic voltammetry. The felts were treated in hot 1.0 N KOH and then actl- vated by addlng a methanol-gold solution, followed by thermal reduction of 0 the gold salt to gold at 260 C. Cyclic voltammetry testing gave no strong, consistent correlation with gold loadlng. Graphite felt performance was generally lower than that of carbon felt. 20PDF Image | NASA Redox Storage System Development Project 1980
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