LEAD DIOXIDE ANODE
The lead dioxide anode is an electrode of massive lead dioxide in an extremely hard and dense form. It is produced by electrodeposition on a suitable base material from an aqueous lead salt bath. This form of lead dioxide is of increasing interest as an inert anode in electrochemical processes. Its many favorable properties make it a promising substitute for the platinum anode.
Other forms of lead dioxide, such as thin flash coatings which may be fanned on lead anodes in sodium hydroxide solution, or the pasted lead dioxide anodes of the lead storage battery, are not included in this discussion.
Background
Platinum has long been the favored anode material for electrochemical processes because of its high oxygen overvoltage and low erosion and dissolution rates. However, platinum requires a large capital investment, and its anodic losses, while small in volume, may be significant in dollar value. Also, with rising requirements for platinum in the United States in the period after World War II, it was felt that the supply might not be adequate in times of national emergency. Therefore, interest in other inert and high current efficiency anode materials was aroused.
The production of sodium perchlorate by the electrolysis of a concentrated sodium chlorate solution is of particular interest and importance in relation to this need for inert anode material. In the period of 1951-1954, Pennsalt Chemicals Corporation under a research contract from the Office of Naval Research explored alternate anodes to replace platinum in the electrolytic oxidation of sodium chlorate to perchlorate. A lead dioxide anode made by electrodeposition of lead dioxide from a lead nitrate bath on tantalum and nickel metal cores was developed. Based on this research effort, American Potash and Chemical Corporation (Western Electrochemical Company) carried out a development study under a Navy Bureau of Ordnance contract during 1954-1956. A pilot plant cell was designed, constructed, and successfully operated for the conversion of sodium chlorate to perchlorate in which lead dioxide anodes were used. In 1962, Pacific Engineering and Production Company of Nevada was reported to be producing lead dioxide anodes commercially and using them in electrolytic cells for the production of sodium perchlorate.
The technical literature shows a strong, continuing interest, beginning about 1934, in lead dioxide anodes and their use in electrochemical oxidations. Recent work has been concentrated on improvements in the process for electro-depositing lead dioxide and on means for increasing the anode current efficiency in process use.
Forming Lead Dioxide Anodes
Several types of baths from which lead dioxide may be plated have been proposed, icluding those based on alkaline lead tartrate, neutral lead perchlorate, and acid lead nitrate. The lead nitrate bath is preferred, since it may readily, be controlled over long plating periods and gives a high-quality deposit over a relatively wide range of operating conditions. A typical bath composition is:
| Lead nitrate, Pb(NO3)2 | 250-350 gpl
|
| Copper nitrate, Cu(-NO3)2'H,O | 1.5-4.0 gpl
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| *Surface-active agent | 0.5-2.0 gpl
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* Preferably a nonionic of the class of alky1 phenoxy polyoxyethylene ethanol, as for example"Igepal CO-880" (Trademark General Dye Corporation).
The addition of copper nitrate serves to suppress lead deposition on the cathode, which may be carbon, graphite, or copper. A suitable surface-active agent ensures deposition of lead dioxide of high strength, density, and surface smoothness. Other additives, such as nickel nitrate and sodium fluoride, also have been proposed. Periodic additions of 35 per cent hydrogen peroxide are claimed to aid in maintaining anode plating efficiency above 85 percent. Regeneration of the plating bath to maintain production of high-quality lead dioxide deposits has also been effected by additions of n-amyl alcohol to used cell effluent. The n-amyl alcohol removes residual surface-active agents, and its altered products in a separated liquid layer which is decanted. A fresh amount of surface-active agent is then added to the regenerated plating solution.
Initial bath pH is about 3.5 and during anodic lead dioxide plating the pH is usually controlled in the range of 2 to 4 by the frequent addition of lead oxide. Strong, dense deposits of lead dioxide in thicknesses of 2.5 cm or more have been obtained by operating the bath at an anode currrent density of 0.016 to 0.032 amp/sq cm (15 to 30 amp/sq ft) and a temperature of 70'C. Impurities in the bath, such as iron or cobalt compounds, can drastically reduce the strength, density, and surface smoothness of lead dioxide deposits.
Lead dioxide deposits on iron or steel base surfaces are difficult to form in the acid nitrate bath because of dissolution of the base. This dissolution effect may be minimized by maintaining a bath pH of 4 or above. With careful control of the bath at 2 to 4 pH, nickel is a satisfactorv anode base. Anode bases which are essentially independent of bath acidity are tantalum, platinum-clad tantalum, carbon, and graphite. Tantalum metal as a plating base has the further advantage that in subsequent electrolytic use of the lead dioxide anode the tantalum rapidly polarizes and then acts as an inert, noneroding filler. Other metal bases, such as nickel and steel, are gradually dissolved during electrolytic use of the lead dioxide anode.
Nodular-free deposits are most easily formed on wire or rod-form cores or bases. Where flat, rectangular anodes are desired, deposits may be formed on both sides of screen or expanded metal mesh. In this case, inert, nonconducting baffles closely spaced about the anode edges permit formation of nodular-free deposits within close dimensional tolerances.
Properties
Two modifications of lead dioxide have been reported, the orthorhombic alpha-PbO, and the tetragonal beta-PbO,. The massive form electrodeposited from the lead nitrate bath is beta-PbO, which has a higher oxygen overvoltage than the alpa-PbO. This oxygen overvoltage is comparable to that of platinum. When dried at 80 to 90'C the water content is about 0.2 per cent. With a resistivity as low as 40 X 10- ohm-cm, the dense lead dioxide is a better electrical conductor than many metals and a much better conductor than carbon or graphite. Chemically it is inert to most oxidizing agents and resistant to many strong acids. While massive, electrodeposited lead dioxide is rather brittle, it has adequate strength for ordinary handling when plated in rod or plate form to thicknesses of 0.5 cm or greater. It is extremely hard, dense, and metallic in appearance with a gray to black coloration.
Current Contacts
For successful use of the lead dioxide anode, a durable, low-resistance current contact must be made. When a conventional clamp-type current contact of copper or other usual conductor metal is made directly to the lead dioxide, a high contact resistance and local heating results. This high contact resistance is also observed when current contact is made to the core when this is a metal, such as nickel, iron, or steel. However, when a thin spray coating, 0.002 cm or less in thickness, of a noble metal such as silver is first applied to the top of the lead dioxide anode, an extremely low-resistance current contact may be made by any of the usual means. Preferably, the thin silver coating is protected by a further spraying with a heavy layer of copper to form a jacket about the top end of the anode. Where the lead dioxide is plated on a carbon or graphite core, the same unplated top portion of the core may be successfully utilized as current contact in both the electrodeposition of the lead dioxide and in the subsequent eleettochemical use of the lead dioxide anode.
Applications
Lead dioxide has been proposed as an insoluble anode for the electrolysis of aqueous solutions containing such anions as Cl-, Br-, l-, F-, CIO-, CIO4-, CIO3-, SO4-, NO3-, CO3-, and C2H3O2-. Lead dioxide has been used in sodium chloride electrolysis for chlorine production, where it is reported to have a low chlorine overvoltage. Also, it is claimed to be efficient in very dilute hvdrochloric acid, allowing chlorine formation at almost reversible potential. Lead dioxide, however, is attacked by concentrated hydrochloric acid. In the oxidation of iodic acid, HIO3 to periodic acid, HIO4, with an anode of lead dioxide, the current efficiency is 100 percent as compared to onlv 1 percent at smooth platinum. This is an example of the catalytic activity of the anode. The electrolytic oxidation of starch has also been carried out at a lead dioxide anode. A perchloric acid power cell from which high currents may be drawn at temperatures as low as -20'C has positive plates of- lead dioxide plated on an inert conducting grid, such as palladium. The negative plates of this cell are lead metal, and the electrolyte is aqueous perchloric acid.
Greatest interest for lead dioxide anode use has been in cells for electrochemical oxidation of sodium chlorate to perchlorate. Perchlorate cells using lead dioxide anodes are very similar in structural details and operating conditions to those using platinum anodes. One striking difference is that whereas in the platinum-anode cell, 0.5 to 5 gpl of sodium dichromate is added to improve current efficiency, in the lead dioxide-anode cell presence of chromate reduces current efficiency. As a result, the mild steel cathodes in the platinum-anode cell which are protected by the chromate ion are replaced bystainless steel or nickel in the chromate-free lead dioxide cell. The most effective additives for improving current efficiency in the lead dioxide- anode cell are sodium fluoride and potassium persulfate used at concentrations of 0.5 to 2.5 gpl. Other typical cell operating conditions are:
| Sodium chlorate | 500-600 gpl
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| Anode current density | 0.16-0.32 amp/sq cm (150-300 amp/sq ft)
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| Voltage | 5-6.5
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| Temperature | 25-40C
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| pH (adjusted with HCI) | 6.0-6.8
|
Cumulative current efficiency with suitable additive approaches that obtained with the platinum anode, being 80 percent or better down to a chlorate concentration of 100 gpl. Below this concentration current efficiency drops rapidly with both platinum and lead dioxide anodes.
Although judgment must be made on a rather limited experience to date beyond the laboratorn stage, electrodeposited lead dioxide shows promise of development to a high efficiency as an inert and insoluble anode of long, indefinite life in electrochemical processes.
References
1. GRIGGER, J. C., MILLER, H. C., AND Loobus, F. D., J. Electrochem. Soc., 105, 100 (1958).
2. NARASIMHAM, K. C., SUNDARARAJAN, S., AND UDUPA, H. V. K., J. Electrochem. Soc., 108, 798 (1961).
3. SCHUMACHER, J. C., STERN, D. R., AND GRAHAM, P, R., J. Electrochem. Soc., 105, 151(1958).
4. SHIBASAKI, Y., J. ElectTOchem. Soc., 105, 624 (1958).
5. SUGINO, K., Bull. Chem. Soc. Japan, 23, 115 (1950).
J. C. GRIGGER
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