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the titanium was anodized before coating with the graphite. Repeated anodizing was found to be both inconvenient and of limited value. Also, the thin graphite films sometimes contained pinholes or were slowly permeated by the electrolyte which caused the film to lose adherence. No problems have been encountered with the thicker graphite coatings and, as the last three control tests show, the average control efficiency is higher. In addition, the standard deviation is lower for the last three tests. These results indicate less substrate interference and better test reliability. The procedural change does not affect the interpretation of impurity test results prior to the change. The results for the impurity testing are given in alphabetical order in Table 37-6. Again, the electrolyte composition for all the tests was 0.5-molar zinc chloride and potassium chloride. A round trip coulombic efficiency of less than 94% is considered a positive test for hydrogen promotion on zinc due to the presence of an impurity. Test results show antimony to be the most detrimental impurity tested. At the 1 mg/f level, Co, Ge, Ni, and Sn also cause considerable hydrogen evolution. Both iron and copper are detrimental at somewhat higher levels. Somewhat surprisingly, arsenic is not observed to be a catalyst for hydrogen evolution on zinc as indi cated by some literature references. Although elements such as chromium, manganese, and vanadium do not affect the zinc coulombic efficiency by hydrogen evolution, their presence in a battery is un desirable. They would cause coulombic inefficiencies because they undergo redox reactions with the dissolved chlorine and the zinc electrode. Although most, if not all, of these impurity effects may be referenced in the literature, their evaluation under the conditions in which a zinc-chlorine battery operates is necessary due to the high acidity and chloride ion strength. Hydrogen Evolution Due to Graphite Degradation Products In the previous section, a detailed account was presented about metallic impurity effects on hydrogen evolution at the zinc electrode. Because the design of the technique permits conditions to be rigidly controlled and experiments may be repeated to insure reliability, unequivocal results may be obtained. In this section, it is demonstrated again that reproducible and reliable results are 37-11PDF Image | Development of the Zinc-Chlorine Battery for Utility
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