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Evaluation of bismuth and gold-blsmuth as possible catalysts for the chromium electrode showed that electroplated bismuth on carbon felt has a high hydrogen overvoltage and deflnlte actlvlty for chromous oxidatlon and chromic reduction. This material will be further examined during 1981. Summary. - The major issue in the chromium electrode development work is to conslstently prepare electrodes with good electrochemical performance. Varlations in carbon felt propertles (lot to lot and withln lot) appear to account for most of the varlations in performance. Speciflc cleaning treat- ments and catalyzing procedures can generally be identifled for each lot of carbon felt. However, a "normalization" step for all lots of felt has not been developed. A jOlnt effort wlth a carbon felt manufacturer will be started to systematlcally identify a carbon felt fabrication procedure to manufacture a "Redox-grade" carbon felt. Lewis Research Center - Electrode Preparation and Screening Background. - Redox flow cell performance is largely determined by the electrochemlcal characterlstlcs of the inert electrodes. The requlrements for a reversible lron electrode have been met satisfactorlly by uSlng an uncatalyzed carbon felt. However, studles have shown that a lead-gold catalyst is needed for the chromium electrode to increase the rates of chromlum reduction and oxidatlon and to provide a hydrogen evolutlon over- potentlal. (Thermodynamlcally hydrogen should be evolved before chromium is reduced.) Appreciable coevolution of hydrogen not only reduces the current efficiency of the system but, over a perlod of many cycles, also allows the system to become chemlcally out of balance and thus to lose effective capaclty. The performance of these carbon felt electrodes depends on the production history of the felt, the cleanlng procedure, any surface activa- tlon treatment, and the methods of depositlon of the gold and lead. Other materlals that have been tested as chromium electrodes are carbon and purified-graphite plates. ' Approach. - A certain lack of consistency in the physical character- lStlCS of the several batches of carbon felt purchased in 1980 for electrode materlal has required a unlque pretreatment and catalysis procedure for each batch. Much of the in-house effort thlS year has therefore involved the screenlng of methods for electrode pretreatment and catalysis in terms of subsequent electrode performance. Pretreatments have lncluded acid and caustic washes, heating in an 0 oxidjzing atmosphere to 800 F and heatlng ln a reducing atmosphere to 1600 F. Dlstrlbutlon of gold catalyst on the chromlum electrodes has been accomplished by thermal decomposltlon following soaklng in aqueous and/or organlc solvent solutions of gold salts. Another approach has been to electroplate the gold from a gold salt solution. The lead catalyst has been either electroplated after the gold deposition or coplated wlth the gold. Evaluatlon of these electrodes has taken place in complete Redox cells of 14.5- and 320-cmf Slzes. Generally the cells are repetltively cycled between depth-of-dlscharge llmlts of 10 and 90 percent at some nomlnal cur- rent density. These cycles can be normal or accelerated, the latter result- ing from turnlng off the reactant pumps and limltlng the capacity to the 21PDF Image | NASA Redox Storage System Development Project 1980
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