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since the primary purpose of the membrane then would be merely to prevent con- vective (bulk) cross-mixing of the reactant streams. The new, low-resistance d r a n e s would result in operatlon at hlgh voltage efficiencies, but with the penalty of lower coulomblc efficiency, because of increased diffusional self- dischargeacrossthemembrane. However.thelossofcapacityduetothis cross-diffusion, plus any osmotlc solvent transfer, could be countered easily by occas7onally discharging the system fuily, reblending the twu reactants. and then repartitloninq them to their respective tankage systems. Thls. plus proper rebalanclny to compensate for hydrogen evolution (or air intrusion), would make the system capacity virtually Invariant. As S~IOW In following sections, the actdallty of these advantages and others for ralxed-reactant oper- ation has beeq verified. In addltion to the operational advantages perceived for the mlxed- reactant, elevated-temperature mode of operation, several economic benefits were aibu a~tieipateri: i o n membrane resistance would allow opert n at hlgh current densities. thus greatly reduclng cell and stack stze and therefore cost. Also, lou-resistivity membranes uith poorer selectivity would te expec- ted to be much less expensive than their highly selective counter~arts. On the other hand, th2 cost of reactants would be doubled for the mlxed-reactant mode. Another negative aspect, mntloned earlier, is t9e reduction in coulom- bic efficiency. Open-circuit voltages would also be reduced at zll states of char~eby the thermodynamic effects of elevated temperature and reactant activ- ity changes. Thest! effects (fig. 15, ref. 36) resulted in a penalty of about 50mV. Subsequentevaluationoftheelevated-temperature,mlxed-reactantoper- ating mode has show the advantages to considerably outwelgh the disadvantages. Electrodes Because of the importance to system operation of proper chromium electrode performance, it was of iranedlate concern that the elevated temperature or the mixing of reactants might prove deieterlous to the performance of the chromlum electrodecatalyst. Severalevaluationswerecarri2doutin-houseandzt Glner, Inc., to examine these possibilities. With regard to the mixing of reactants, per se, several tests were carried out at Glner, Inc. (ref. 14), uslng cyclic voltamnetry. The effect of ferrous chloride (FeC12) on the electrochemical performance of a gold-catalyzed elec- trodeInachromicionsolutionwasfirstdetermined. TheFeC12concentration of the chromium solution was Incrementally increased from 0 to 0.5 molar. The results (fig. 16) show that the chromic ion reductios reaction was shifted to slightly more negative potentials and that the total quantity of chromium reactedincreasedastheFeC12concentrationIncreased. Inaddition,hydrogen evolutiondecreasedwithincreasingFeC12concentration. ThusItseemsthat the presence of FeC12 in the chrmium reactant solution has a beneficial ef- fect,perhapsbecauseofashlftlngofthechrom.: ionequilibriumtofavorthe electrochemically active monochloropentaaquo species. The effect of chromic chloride (CrC13) on the iron redox reactions was observedinasimilarway(fig.17). TheincrementaladditionsnfCrC13 increasingly depressed the level of the ircn redox reactions. From these unexpected results It is apparent that the presence of CrC13 interferes with the iron redox reactions, perhaps by increasing the chloride complexes of the ferric Ion.PDF Image | NASA Redox Storage System Development Project
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