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How these results relate to ,ctual working cell performance i s unclear, especially in light of the fact that the concentrattsn of the spec!es being reduced and oxidlred I n each case was only 50 d,whlle that of tne 'cross mixed' species was varied to 500 RH. Also these tests were carrted out at 25" C. In any event, the observed effects are probably just a very ;mall part of the overall thermdynarrlc effect of cross-mlxing discussea earlier. A t Lewis the possible effect of temperature on electrode perforwnce was investigated by using a standard laboratory c e l l containing an RAI anion exchange membrane and the gold-lead catalyst (ref. 19) with onmlxed react an^^ After 104 electrical cycles, which were accompanied by thermal cycllng to 65' C, polarlzatl~r!tests indlcated no loss of re ersibllity. Burino +he testqng of this cell there also was no increase in the rate of hydrogzn evulu- tlan. Hasaver, the evolutlc? r ? ' e was about 6 percent of the charge capacity, rh+chwas three or four tfmes greater than normal for room-terrrper?+!~reopera- ttot~. In general, the testlng of seveidl cells or'lth z9e geld-lead catalyst revealed no loss of reverslbillty nor increase !A trr!ro~enevolution rate during continued operation at elevated temperature. However, the hydrogen rate dld vary considerably from cell to cell. ranging from a low of about 1 percent to the earlier mentioned 6 percent of charge capac ty. This lr~corisls- tency was considered to be slqnlficant and led to the evaluatio~cf other can- didate catalysts for the chromium electrode. The f i r s t question addressed was whether, a t elevated temperature, any catalyst was indeed requtred. It was hoped that at 65" C the carbon felt elec- trode substrate would have sufficient intrinsic electrochemical actlvlty for the chromlum redox reactjons. Therefore a cell was assembled with a bare car- bon f e l i chromium electrode and cycled a t 65' C. Unmixed reactants were used, and the standard quantity of saturated Pbd12 solution was addea to the chromium solution so that during chargtrig a thin lead deposit would lessen the tendency toward hydrogen evolution. The cell did charge with very low hydrogen genera-. tion. However, polarization curves at 50 percent state of charge (fig. 18) revealed irreverstble behavior for both charge and discharge. Next a planar carbon surface was ion etched to increase I t s surface area. Tested at room temperature, the ion-etched plate unfortunately showed far more activity for hydrogen evolution than for the chromium redox reactions. Heated to 55" C, the plate performed even more poorly. Adding bismuth and lead salts to the chro- mlum reactant solution in order to suppress hydrogen evolution was beneficial, but the net Improvement left much to be desired. Probably the attempt to de- posit materials on this surface resulted In an uneven distribution, with high spots being plated while recessed regions were not. Also, the Ufuzzytl charac-- ter of the ion-etched surface undoubtedly led to the growth of an appreciable mass-transport boundary layer that inhibited performance. An analog of the gold-lead catalyst, using silver instead of the gold, was next evaluated. The rationale for thls selection was that silver, having a lower reduction potential than gold would be more easIly stripped and redepos- ited, making possible a restoration of performance aft?r any deterioration evi- denced by increasing hydrogen evolution rates. At 65' C with mixed reactants, the initial hydrogen rate from a cell using thls chromium electrode catalyst was low. However, the electrode was irreversible for the chromium redox reac- tions and i t s current-voltage curves showed several steps during constant-load discharges.PDF Image | NASA Redox Storage System Development Project
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