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percent as the hexahydrate. Interconvers10n between chromium (II) species takes place rapidly, but interconversion between chromium (Ill) species takes place extremely slowly, requiring several months to reach equilibrium. Provided that the behavior of the gold-lead catalyst is sim1lar to that of mercury electrodes, the sequence of events tak1ng place in charging and d1scharging Redox cells would be as follows: (1) The in1tial blue-green solution consists of roughly equal amounts of the hexahydrate and pentahydrate. When charged, the pentahydrate is reduced more readily. Because of the slow rate of interconversion, it is greatly depleted at 50 to 60 percent state of charge. The rema1ning blue Solut1on, mostly hexahydrate, requires a higher potent1al for continued reduction, and this also increases the dr1ving force for hydrogen evolution. (2) In the charged state the predominant 10n 1S the blue chromium (II) pentahydrate. Since the chrom1um (Ill) pentahydrate is produced on dis- charge, the Solut10n quickly turns back to a blue-green color. CYC11C voltammetry and spectrophotometry were used to confirm this hypothes1s about the behavior of the solutions at gold-lead electrodes and to develop methods for examining the effects of d1fferent electrodes on the Solut10n equ1libria. Status of CYC11C voltammetry studies of electrodes. - The cyclic voltammetry apparatus 1S shown schematically in f1gure 28. Voltage was var1ed from 0 to -1 V versus an Ag/AgCl reference electrode. Chromium (III) should start being reduced at just past -0.5 V. Numerous electrodes were examined under var10US cond1tions. A sweep rate of 10 mV/s, a chromium concentration of 0.05 M, and an HCl concentration of 2 Mwere found to be the best conditions for differentlat1ng between chromium activity and hydrogen evolut1on activity. Figure 29(a), for uncatalyzed felt, shows low activ1ty for both chrom1um and hydrogen re- duct10n. Figure 29(b) shows a "good" electrode with sat1sfactory chromium act1vity (peak current, 45 mAl and suff1c1ently low hydrogen activity. (Hydrogen current at -1 V 1S less than the chrom1um peak current.) A cyclic voltammogram of a poor electrode is also shown (fig. 29(c)). This electrode has suff1cient activ1ty for chrom1um but exceSS1ve hydrogen activity, as eV1denced by a m1n1mum greater than 15 rnA and a hydrogen peak larger than the chrom1um peak. These electrode acceptance cr1teria were selected by comparing CYC11C voltammetr~ scans with measured performance 1n laboratory-scale cells. Over 200 1/3-ft electrodes were 1ndlvidually screened for the l-kW preproto- type system. These criter1a have proved quite satisfactory for th1S appll- cation. Status of cyclic voltammetry and spectrophotometric studles of chromlum complexes. - CyCl1C voltammetry swe~ps have been carried out on solutions conta1nlng predominantly Cr(H20)6 + ions prepared from Cr(C104)3. These data show lesser chrom1um act1vity and greater hydrogen evolut10n with this species, as compared w1th [Cr(H20)5Cll+2t which 1S cons1stent with the electrochemical behavior on mercury ana wlth the proposed equllibrium model. 24PDF Image | NASA Redox Storage System Development Project 1980
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