Activated cathode coatings

The coatings used for diaphragm and membrane electrolyzers differ because of the different substrates (carbon steel and nickel, respectively) and the different operating conditions. The weak 11 % caustic in diaphragm cell liquor is less corrosive than the strong 33% caustic of a membrane electrolyzer. The less expensive and more fragile 

The advantages and disadvantages of the three chlor-alkali processes are summarized in Table 18. The three chlor-alkali processes can be compared in respect to the quality of the chlorine and caustic produced, and the equipment and operating costs. 

Today the membrane process is the state of the art for producing chlorine and sodium hydroxide or potassium hydroxide. All new plants are using this technology. The production capacity of chlor-alkali plants using the membrane process reached about 21% of total world production capacity in 1995 and is predicted to increase to about 28% by 2001 (Table 19) [133]. The diaphragm cell capacity remains constant and there is a decline in mercury cell capacity. 

The conditions for a conversion from the mercury and the diaphragm process to the membrane process are discussed below. 

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The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Ni can be taken as the reference material against which all other materials should be evaluated. On the average, the operating overpotential of untreated Ni electrodes is about 0.4 V at 0.2 A cm-2 [5], Beyond Ni, we deal with activated cathodes , which in fact derive from the idea of activated anodes such as the DSA . By activated electrodes we mean that the surface has been subjected to some treatments aimed at increasing its catalytic activity. This can be a treatment which modifies the surface structure and the morphology of the base metal, but more often the treatment is aimed at coating the base metal with a more active material [31]. 

Depth progression depends on the size and activity of the cathode surface. Pitting in cases of adsorption of inhibitors and in the salt layers formed under woiking conditions in corrosive media is due to an incomplete protective layer on the material surface. The intensity of corrosion depends on the area ratio between anode, uncoated surface, and cathode, coated surface. 

In Situ Activation. The various cathode coatings examined for application in industrial chlor-alkali cells, and discussed above, have to be applied on to a nickel or steel cathode substrate by electroplating or thermal procedures. This step involves significant cost, depending on the technique employed for laying the active coating. 

Studies have shown that Cr, Ni, Hg, Fe, and organic compounds containing N or S adversely affect the cathode potential [186,190] of steel. Iron was foimd to be particularly harmful to the performance of activated cathodes (Fig. 4.6.17). Pt-Ru coated cathodes [191] are affected at Fe levels as low as 3 ppm Fe in the solutioru However, with smooth Ni or Ti cathodes, Fe was fotuid to lower the cathode potential [181,182], as the deposits formed on the cathode had a large surface area. 

The cathode coating and substrate should also be resistant to hypochlorite and the reverse currents encountered during shutdowns. Whenever the current is interrupted, a reverse current flows through the cell because of the presence of the active chlorine species in the anolyte. As a result, the cathode becomes anodic for a brief period until all the active chlorine species are reduced to chloride. Also, following a shutdown, the hypochlorite in the anolyte is transported into the catholyte, raising its hypochlorite level to 5-30 ppm in membrane cells, and 300-5,000 ppm in diaphragm cells. Reduction of... 

The cathode material is carbon steel in diaphragm cells, and nickel, often with a catalytic coating in membrane cells. As discussed in Section 4.6.6, exposure to anolyte containing active chlorine (CI2. HOCl, and OCl ) without cathodic protection is the primary reason for the corrosion of these components, unless the cathode coating is pore-free and noble metal based. Another species contributing to the corrosion of iron and nickel is the hydroxyl ion in the catholyte. 

Hypochlorite-resistant cathodes. Commercialization of activated cathodes for diaphragm cells has been hampered because hypochlorite corrodes steel under open-circuit conditions (Section 4.6). In the process, the coating spalls off or loses its activity. Development of resistant coatings for steel or other substrates could render the cathode immune during shutdowns. 

Certain proprietary zinc and aluminum filled polymers and some ion vapor deposited aluminum coatings, applied to steel fasteners may actually produce more damage than the untreated steel fastener itself. This was attributed to two possible causes either the high surface area involved with each of the particulate coatings or the contamination of the particulate surface with active cathodic contaminants such as iron, nickel, or graphite ... 

Multiple cathode materials are currently utilized in commercial lithium-ion cells. Table 1.2 lists the most common active cathode materials along with their most important cycling performance characteristics. Derivatives of these cathodes also exist including materials that have been doped with additional ions, surface coated, or otherwise modified. The examples of such derivatives are elaborated in the following sections, along with the rationale for a given modification. 

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