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c e l l configuration. A t Lewis, several electrode structures and c e l l configura tions were also being evaluated. These included a catalyst that was solvent bonded to the membrane, activated planar surfaces (ref. 12), and cavity-filling carbon/graphite cloths and felts. General1j cell structures i n which the re- actant streams flowed entirely within the electrodes gave superior performance because of the lnducea turbulence and enhanced mass transport arising from such a flow environment. At Lewis, multiple layers of carbon or graphite cloth were i n i t i a l l y used i n laboratory cells, but these cloths were soon replaced wlth carbon or graphite felts. With regard to the iron redox couple, there were never any dlfflculties with the use of carbon or graphite felts as electrodes. These materials, after moderate cleaning i n acid or caustic, provided active surfaces for the highly reversfble iron redox reactions. Such wis not the case for the chromium redox reactions. As catalyst systems were being developed and evaluated for the chromlum reactant (discussed later). i t became apparent that the successful catalysis of an electrode was highly dependent on the character- Istics of the felt substrate. The major difflculty arose from variations between felt lots that ostensibly were the same, and even from variationswith- in a given lot. Cdnsiderable effort was expended during the subsequent years by Lewis and Giner, Inc., to develop speclflcations for carLon and graphite felts that would form acceptable chromium electrodes when catalyzed. This endeavor was made easier by the cooperation of the suppl'er of the felts, Fiber Materials, Inc. (FMI). The major result of this effort was to show that the optimum felt resulted when the rayon precursor was pyrolyzed at about 1650" C. This determination was based on the measured rate of hydrogen evolu- tion from the chromlum electrode of a working cell during a standardized cycle (fig. 7). This work I s discussed i n detail i n references 2, 13, and ;4. Another aspect of the endeavor to assure reproducible production of well- bepaved chromlum electrodes as the development of pretreatments to further normalize the felt substrates before catalysis. Such pretreatment was deemed necessary to counter felt hydrophobicity and to deal with possible active sur- face radlcals that could affect the subsequent catalysis processes. Nitrlc acid and potassturn hydroxide were evaluated as cleaning agents, and heat and vacuum lrpregnation wer? tested as aids to the pretreatment process. This work, discussed i n detail i n references 13 t n 17, resulted l n standardized pro- cedures that grea,ly enhanced the ability to fabricate catalyzed chromium elec- trodes of reproducible performance. Catalysts. - As mentioned earlier, carbon and graphite felts were shown to provide excellent activity for the iron redox reactions. The chromlum reac- tions on carbon and graphite, hodever, required hlgh polarizations i n order to proceed i n working cells. Therefore Giner, Inc., was contracted to screen possible catalyst materials for the chromium reactions. These materials, 26 i n a l l (ref. 18), included metals and metarlolds, alloys, plated surfaces, and Teflon-bonded structures. Also f i v e organic materials were evaluated as pos- sible hydrogen evolution inhlbitors, but these proved to be ineffectual. Eval- uations were b l l based on linear sweep voltamtry In stirred solutions. No slngle material tested gave both acceptable anodic and acceptable cathodlc performance f o r the chromium redox reactions. However, gold served well as an anodic electrocatalyst and lead was a good catalyst for the cathodic reaction, having i n addition a hlgh hydrogen overpotential. Gold- lead comblnatlons were therefore evaluated, and they exhibited the good char- acteristics of the two constituents without the d4sadvantages shown by them individually ( r e f . 18).PDF Image | NASA Redox Storage System Development Project
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