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quantity of reactant trapped with1n the cell. Per10dically the cell 1S charged or discharged to 50 percent depth of discharge, and polar1zat10n data (voltage vs. current density) are obtained dur1ng both charging and d1scharging. During normal cycles the variat10n of open-circuit voltage as a function of state of charge is monitored. The degree of hysteres1s between the charge and d1scharge portions of these data is a measure of the abil1ty of the chrom1um electr~de to catalyze the reaction of, or the equ1l1br1um between, the varlOUS Cr+ complexes present. (This is discussed in a following sect10n on fundamental studies.) The rate of hydrogen evolution from the chromium electrode is monitored by oX1d1zing the hydrogen electrochemically aga1nst FeC13 in a rebalance cell and measur1ng the ampere-hours of charge transferred. Cycl1C voltam- metry has been used as a tool for predicting the catalyt1c and gassing character1stics of electrodes. Electrode stability is generally defined in terms of the effect of cycling on performance. An additional test, quite severe, also has been occas1onally used. It involves driving the Redox cell negative as much as 1 V and hold1ng it there for an extended per10d. Subsequent polar1zation, cycl1ng, and gas evolution data def1ne the electrode stability. Another aspect of electrode stab1lity manifests itself as a dip in the voltage- versus-time curve during a discharge at constant current (fig. 20). This dip 1S generally assumed to result from an initially incomplete deposition of gold on the carbon (or graphite) felt substrate, followed by lead depo- sit10n on both the gold and the bare felt. Dur1ng d1scharge the lead deplates from the felt, and subsequent d1scharge performance reflects the poor catalyt1c act1vity of the felt. Status. - Table 15 presents the pretreatment and catalysis techn1ques appl1ed to the various carbon and graph1te chromium electrode substrates. The KOH-cleaned FMI carbon felt lots 8/79 and 1/80 are compared in figure 21. The var1ab1l1ty 1n the propert1es of d1fferent felt lots 1S shown by the fact that, although treated 1dentically, the sample from the 1/80 lot did not beg1n to gas untll a h1gher state of charge and then at a lower rate than the 8/79 sample. Acell containing this better electrode could be charged at 1.3 V, w1th a hydrogen evolut1on rate equi- valent to only 0.3 mA/cm2 • Performance data for a 320-cm2 cell conta1n1ng a chromium electrode taken from the 1/80 lot and optlmally prepared are presented ln figure 22. Init1ally, when at 50 percent s~ate of charge, this cell could be charged and discharged out to 100 mA/cmf w1th no polarization except that attri- butable to IR. Only after 16 000 accelerated cycles and several reversals to -1.1 Vdid the cell develop additional polarizations dur1ng charge. An ent1rely different type of chromium electrode 1S represented by the UTC carbon plate 1n table 15. Carbon plates obtained from United Tech- nologies Corp. were treated to roughen the surface and then prepared as indlcated. In cells containing such electrodes the flow of electreolyte is 22PDF Image | NASA Redox Storage System Development Project 1980
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