Nickel strongly acidic electrolytes

The electrolytic separation of copper and nickel from cupronickel alloy can be achieved by controlling the pH and electrolytic conditions. Copper is determined in strongly acidic solution at a potential not exceeding 4V (generally 2-3 V) as above this potential nickel may also plate out. The solution should contain a mixture of sulfuric and nitric acids for complete deposition. However, a high concentration of acid is not recommended as it may lead to incomplete deposition of copper or the deposit may not adhere satisfactorily to the cathode. 

Anions of common strong acids, such as C104, S04, CF, NOa , etc. exhibit as a rule only weak complexing interactions, if any. Nevertheless, even weak complexation may be of importance in electrode kinetics if the complex ion undergoes electrode reaction more easily than the free metal ion, as is often the case, especially with chlorides. In such cases, the complex takes the role of an electroactive species, as already discussed for the hydroxo complexes. Thus, e.g., nickel can hardly be anodically dissolved at all if chloride ions are not present in the solution. In sulfate electrolytes, the oxidation product (some oxygen-containing species) forms a passive film and further dissolution is blocked soon after an anodic overpotential is imposed upon the electrode. The phenomenon of passivity is discussed elsewhere (cf. Volume 4). At this point, one should note that passivity is absent in the presence of chlorides. 

It is seen that at the higher concentrations the equivalent conductance is very low, which is the characteristic of a weak electrolyte, but in the more dilute solutions the values rise with great rapidity the limiting equivalent conductance of acetic acid is known from other sources to be 390.7 ohms cm. at 25 , and so there must be an increase from 131.6 to this value as the solution is made more dilute than 10 equiv. per liter. The plot of the results for acetic acid, shown in Fig. 20, may bo regarded as characteristic of a weak electrolyte. As mentioned in Chap. I, it is not possible to make a sharp distinction between electrolytes of different classes, and the variation of the equivalent conductance of an intermediate electrolyte, siu h as trichloroacetic, cyanoacetic and mandelic acids, lies between that for a weak electrolyte, e.g., acetic acid, and a moderately strong electrolyte, e.g., nickel sulfate (cf. Fig. 20). 

The conventional cathode material is lithiated nickel oxide that is formed by in situ oxidation during cell conditioning. The porosity and thickness are also around 50 % and 1 mm, respectively. The cathode polarization and dissolution/ deposition, which are very important factors of the performance and lifetime, are strongly influenced by the cathode pore structure, electrolyte composition, and operating conditions. The nickel oxide slowly dissolves into the electrolyte as Ni " with an acidic dissolution mechanism as follows ... 

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