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The gold-lead catalyst can be formed In several ways, Including sequen- t i a l electrodeposltlon of the constltuents. sllaultaneous el.ectrodeposltlon, and thermal decoclposltlon of a gold salt followed by electrodeposltlon of lead. Nork by Lewis and by Glner tended to show that the last technique produced electrodes of acceptable performance most conslstently. Subsequent work was dlrected toward reflnlng the gold application technique and evaluatlng varla- tlons In the constituent loadings. The net result of these efforts was a stan- dardlzed process for the preparatlon of gold-lead-catalyzed felts for use as chromlum electrodes (refs. 13, and 15 to 17). Uork by Yeager and colleagues at Case Western Reserve Unlverslty has shown the probable structure of the gold- lead cornblnatlons to be the result of underpotentlal deposltlon of the lead on the gold. One major crlterlon by uhlch catalyzed chrmlum electrodes were evaluated was the rate of hydrogen evolutlon durlng the charqlng (cathodlc) reaction. Flgure 8 glves an lndlcatlon of the progress that was made durlng these studies I n reducing hydrogen evolutlon. The varlous curves show the effects of the f e l t substrate, the cleanlng process, and the gold appllcatlon technlque on hydrogen evolutlon. In a wlde range of tests the gold-lead catalyst proved to be stable and durable. One 320-c& c e l l uas cycled f o r 18 months, undergolng 20 000 acceler- ated cycles and 3000 standard cycles wlth no apparent loss of electrode actlv- lty (flg. 9). Durlng thln testing perlod the cell was several tlms drlven lnto deep reversal wlthout damage (ref. 17). Other tests. though, showed that the ablllty of the catalyst to lnhlblt hydrogen evolu:!on could be lost lf the chrmlum electrode were exposed to a chemtcally oxidizing envlromnt after some perlod of normal usage. Such oxldatlon could result from air (oxygen) or ferric Ion Intrusion lnto the chroinlurn system (ref. 19). Nelther of these occurrences would be expected I n normal operation. The gold-lead corablnatlon thus became the standard chromlum electrode catalyst for the amblent-temperature Redox system. Electrodes catalyzed as prevlously dencrlbed were used successfully In the 1-kW Redox system. Crlterla also were developed by whlch cycllc voltammetry could be used to evaluate these electrodes before thelr lnstallatlon I n the cell stacks (ref. 16). Membranes The membrane t o be used I n the amblent-temperature Redox c e l l had t o per- form two crltlcal functions: prevent, almost totally, the cross mlxlng oi the two reactant species and at the same tlme allow free passage of other tons such as protons and chlorlde Ions to complete the electrlc clrcult through the cell. These requlred propertles - good selectivity and low reslstlvlty - unfortunately tend to be mutually excluslve: steps taken to improve one prop- erty lnevltably cause the other to deterlorate. Other requirements were that the membrane be physlcally strong, chernlcally inert i n the cell envlronment, and lnexpenslve. At the very beglnnlng of the Redox project, several co~nercla1ly avallable membranes were evaluated. Many of them were lncorrtgatlble wlth the Redox cell envlronment. Those that were physlcally and chemjcally stable failed to pro- vide acceptable reslstlvlty and selectlvlty (refs. 20 and 21).PDF Image | NASA Redox Storage System Development Project
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