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The evaluation of scaled-up single cells, multicell stacks, and complete systems revealed no inherent difficulties with the basic Pedox flow battery concept. Rather, the testing served t o verify that the system was uncompll- cated, simple to operate, and extremely flexlble In terms of the available design options and the applications that night be accommodated. Rebalance Cells The development of rebalance cells was not a major part of the Redox project. As discussed earlier, the purpose of these cells is to restore the two reactant systems to the same state of charge after imbalance has occurred. This rebalancing was easily accomplished by using two very well-behaved electrodes - an iron cathode identical to that In the Redox working cell, and a hydrogen anode typical of hydrogen-oxygen fuel cell technology. However. the hydrogen electrode was always subject t o a slow loss of performance dae t o flooding, because the membrane allowed osmosis of solvent from the adjacent flowing aqueous Iron reactant stream. Generally, this performance loss was reversible through the occasional use of an acid flush of the hydrogen elec- trode cavity followed by drying with a nitrogen purge. A more serious difficulty arose with regard to the stability of the plat- inum catalyst of the hydrosen cell. Small rebalance cells were used routinely in the testing of Redox single cells in the laboratory to monitor the hydrogen evolution from the chromium electrodes. It was noted that often, after about 4 months of testing, thts hydrogen evolution rate would start to increase sig-- nlficantly. This was always attrlbuted to a deterioration of the Redox cell's chromium electrode. However, when testing at 65" C began (discussed i n later sections), i t was noted that the hydrogen evolution began t o increase alrnost imnediately. An x-ray analysis of the component parts of the working cells revealed the presence of platinum. Thus it was learned that it was pcssible for the hydrogen electrode catalyst, platinum, to dissolve anodically, migrate into the flowing iron stream, and then work its way through the working cell membrane t o the chromium half-cell, where i t was reduced. The platinum metal could then continuously catalyze the reduction of water by chromous ions, evolving hydrogen. The anodic dissolutlon of the platinum has several causes. One i s exces- sive polarlzaticn of the hydrogen half-cell during discharge of the rebalance cell. Another is the diffusion of ferric ions through the membrane when the cell i s I n a standby mode. I n principle, these conditions are easily avoided. The use of a reference electrode would make i t possible t o monitor the perform- ance of the hydrogen electrode and avoid excessive polarization. When i n standby, a continuous t r i c k l e charge of the rebalance c e l l would cathodically prctect the hydrcgen electrode catalyst. The effect of these steps on system cost or efficiency would be negligible, but they would represent an additional operational requlrement. approaches were evaluated t o t o t a l l y remove the platlnum from the f l r y t was to replace the platinum electrode catalyst wlth tungsten Ihls material dld show some inltial electrochemlcal actlvity a rebalance cell but soon passivated, probably because of elec- tr;;~ flooding. It i s believed that a small development effort would result I n a WC-catalyzed hydrogen electrode of sufficient activity for rebalancirg purposes. Several system. The carbide (WC) when used jnPDF Image | NASA Redox Storage System Development Project
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