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over the surface of the electrode instead of through the structure as 1S the case with felt electrodes. In sp1te of a flow reg1me that should be much more susceptible to mass transport losses, it can be seen in figure 23 that such polarizat10ns do not develop unt~l a dlscharge rate of 70 mA/cm2• Charging was possible out to 90 rnA/cm with no losses other than those attrlbutable to IR polar1zation. These results offer the possibility of completely new cell des1gn concepts, which could also have a favorable effect on system-level considerations such as pump power and shunt current losses. Lewis Research Center - Fundamental Studies Background. - Two operational difficulties with chromium Redox solutions are the hysteres1s in the open-circuit voltage (fig. 24) and the difficulty of charging a cell 1n the latter stages of charge. A sizable drop in the charging rate occurs at about 50 to 60 percent state of charge when the cell is belng charged at a constant voltage. If the cell 1S be1ng charged at a constant current density, a substantlal increase 1n voltage lS needed at that point to maintain the same current (fig. 25). In addltion to reducing the voltage effic1ency, thlS also 1ncreases the amount of hydrogen evolutlon. Changes ln color in the chromium Solut10n from blue-green to blue are observed at thlS pOlnt. However, durlng discharge, the color change from blue back to the orig1nal blue-green does not occur at 50 to 60 percent state of charge but at about 85 to 90 percent state of charge. The hyster- eses ln both the color change and open-clrcuit voltage and the assoc1ated charg1ng difficulties were believed to be due to the two complexed chromlc lons predom1nant in the Redox solutions (fig. 26) and to the slow rates of lnterconverSlon between these two ion speCles. Approach. - Electrochemical data from the literature are summarized in figure 27. It 1S ObV10US that from a thermodynamic standpolnt hydrogen will be evolved before chrom1um is reduced. To have a m1nimum of hydrogen evolved ln the Redox cell, the electrode therefore must have low actlvity for hydro- gen reduction (hlgh hydrogen overvoltage) as well as high activlty for chromium reductlon. Cyclic voltammetry was used to study electrodes from different felt lots, and prepared ln various ways, in terms of chromium activity and hydrogen act1vlty. Acceptance criterla for lndividual elec- trodes were also developed. Slnce the charglng characteristics and the open-circuit voltage hyster- esis are assoc1ated with the behavior of the different chromium (III) ions, lt is necessary to examine what is known about these species. The equili- brium potentials 1n figure 27(a) show that the hexahydrate wlll not be reduced unt1l a more negative potent1al 1S reached than for the pentahydrate. F1gure 27(b) shows that the rate of reductlon on mercury electrodes of the pentahydrate species is much more rapid than that of the hexahydrate. Thus the reduction of the pentahydrate is favored not only thermodynamically, but also kinetlcally. It can also be calculated from literature data that in Redox solutions at equll1br1um at 25° C the relative amounts of the chromium (III) hexa- hydrate and pentahydrate are about equal but that in chromium (II) solutions about 85 percent of the chromium is present as the pentahydrate and 15 23PDF Image | NASA Redox Storage System Development Project 1980
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