The chemical and electrical properties of PbO2 suggest that it should be an ideal material for anodes in electrolytic processes. With a resistivity as low as 40 to 50 x 10-10 ohm-cm, it is a better electrical conductor than many metals, and a much better conductor than carbon or graphite. Chemically, PbO2 is inert to most oxidizing agents and strong acids. Although it has been suggested as an anode material for several electrolytic processes (1-7), up to the present time no commercially practical anode has been advanced. Electrodes reported to date have been weak, they have been formed in odd shapes difficult to adapt to commercial cells, and methods of making the electrical contact have not been satisfactory.
The purpose of this investigation was to develop a practical PbO2 anode that could be used in industrial electrolytic processes. It was hoped that a suitable electrode would be developed that would replace Pt in the perchlorate cell.
Several baths (3, 8, 9) are known for the electro deposition of PbO2 on common metals. The composition of three bath types modified to give improved PbO 2 deposits are shown in Table 1.
| Alkaline lead tartrate. |
|---|
| 100g potassium sodium Tartarate, KNaC4H4
O6.4H2O 50g sodium hydroxide, NaOH 96g lead oxide, PbO Dissolve in the order listed in distilled water to make 2 liters of solution. Heat to 60C to complete solution of lead oxide. Cool and filter through sintered glass. Bath pH is about 13. |
| Lead perchlorate. |
|
108ml of 60% perchloric acid (100g HCl04) 167ml distilled water 111.0g lead oxide, PbO Dissolve the lead oxide in the diluted percholric acid. Make up to 2 liters with distilled water. Heat to boiling for 2-3 minutes to dissolve any white precipitate. Cool and use. Bath pH is about 5. |
| Lead nitrate. |
|
269ml of 69.9% nitric acid (266.5g HNO3) 1000ml distilled water 472g lead oxide, PbO Add the lead oxide slowly to the diluted nitric acid with stirring. Dilute to 2 liters and heat to 75C with stirring. Cool and filter through sintered glass. To this bath add: 0.75g per liter Copper nitrate, Cu(NO3)2 .3H2O 0.75g per liter Igepal CO-880 (surface active agent) The bath pH is about 3.5 |
In this work, the lead nitrate bath was
preferred because it gives the highest quality of deposit. The addition of copper nitrate to
this bath serves to suppress lead deposition on the cathode, which is preferably carbon or
graphite. In order to deposit lead dioxide of high strength, density, and surface smoothness
, an addition agent is necessary such as Igepal CO-880 (Trade mark of Antara Chemical Division
of General Dyestuff Corp.) which is a non-ionic surface-active agent of the class "alkyl
phenoxy polyoxyethylene ethanol." Addition to the bath of a natural hydrophilic colloid such as
gelatin resulted in the formation of a lead dioxide deposit with a high surface smoothness , but
which was very weak and was laced throughout its cross section with many fine fissures.
Using the acid baths mentioned above, it is difficult to form good deposits on thin attackable
base sheets because of the serious anodic dissolution of the metal base. This problem was
overcome by using Tantalum as the base metal. Sound, adherent deposits of lead dioxide 2cm or
more in thickness could be formed without any signs of erosion of the base material. This
plating on Ta was unexpected, since Ta polarizes in most electrolytes when operated as the
anode.
Electro deposits of lead dioxide were made readily on Ta wire, rod, and sheet without any nodular
growth, using the lead nitrate bath at an anode current density of 0.016-0.032 amp/cm squared
(15-30 amp/foot squared) at a bath temperature of 70C. A rod of massive lead
dioxide 8 inches long by 0.5 inch diameter was formed on a single wire of #20 B&S. The wire core
was withdrawn by a sharp pull with pliers.
In plating flat base-free deposits by blanking off one side of the starting sheet and stripping
away this base after a thick deposit had formed, it was difficult to secure unbroken specimens.
Therefor, this approach was discontinued in favor of plating on permanent base sheets.
Flat, massive lead dioxide deposits of surprising strength were made by plating on both sides
of rectangular sections of Ta screen in the mesh range of 10-50.
The use of baffles around the edges of flat, rectangular anodes permitted the formation of
nodular
free deposits to within rather close tolerances. Using a 14 mesh (0.064 cm
wire) Ta screen, a lead dioxide electrode measuring 36.8 by 8.9 by 1.6 cm and weighing 4500g
was plated in 142.5 hours from the lead nitrate bath. Current was maintained at 0.016 amp/cm
squared on the anode and the bath temperature at 70C throughout the electrolysis.
If the pH of the nitrate plating bath is not carefully controlled, the bath pH drifts strongly
acid during electrolysis and is very corrosive to all of the common metals. However, by
careful maintenance of the pH in the range of about 2-4 during electrolysis by the frequent
addition of lead oxide, and by protecting the base metal at the surface of the electrolyte, it is
possible to plate lead dioxide on such metals as Nickle and Iron. Even with these precautions,
the base is slowly eroded away and by the time a thick plate was formed most of the base metal
(in contrast to Ta) will have eroded away, leaving voids (which are not always objectionable) in
the center of the lead dioxide deposit.
Whenever lead dioxide with a conventional Cu current contact is used as anode in electrolytic cells, severe heating is observed in the contact area. If Silver current contacts are used, no heating occurs. The contact resistance between a number of the common metals and lead dioxide was measured by spraying 2.5cm of each end of electro deposited PbO2 rods about 1cm in diameter and 10cm long with the given metal. The rods were clamped at the metal coated ends and 1 amp of current was passed from a DC source. The potentials across the metal-lead dioxide contact were measured on a potentiometer using manual pressure test probes. It was found that all metals tested with the exception of silver showed a high contact resistance to the lead dioxide as shown in table 2.
Table 2. Contact resistance of electro deposited PbO2 to metals sprayed thereon.
| Metal | Contact potential at 1 amp. Volts |
|---|---|
| Tin | 0.65 |
| Lead | 0.52 |
| Copper | 0.04 |
| 18-8 Stainless steel | 0.69 |
| Zinc | 0.5 |
| Aluminum | 0.19 |
| Silver | 0.0002 |
| Copper over silver | 0.0002 |
| Tin over silver | 0.0002 |
| Aluminum over silver | 0.0002 |
It is suggested that the resistance is caused by an oxide layer forming between the contact metal
and the PbO2. Most metal oxides being poor conductors show high
resistance. Silver, on the other hand, forms a conducting oxide and therefore has a low contact
resistance.
A coating of Ag only 0.0002cm or less in thickness applied by a metal spray technique was
sufficient to produce low resistance and to overcome completely the heating previously
observed in these electrode connections. In order to protect the Ag and to provide a rugged
electrical contact to the PbO2, the Ag-coated area was sprayed with
a heavy coat of Cu, 0.16cm or more in thickness. Preferably, the Ag and the Cu are sprayed to
form a jacket over the top end of the PbO2 electrode. The
combination is sufficiently adherent to the base oxide so that it can be machined to fit in
a mechanical current contact or it can be soldered directly to the power bus without injury to
the PbO2.
Table 3. Current efficiencies in electrolysis of NaClO3 with PbO2 and Pt anodes (no additives)
| NaClO3 conc. range over
which efficiency is calculated: |
||||
|---|---|---|---|---|
| Anode | Test No. | Initial g/l | Final g/l | Current
efficency,% |
| Pt | 1 | 602 | 100 | 85.2 |
| 293 | 39.8 | 82.4 | ||
| 2 | 602 | 100 | 87.4 | |
| 197.6 | 3.9 | 65.4 | ||
| PbO2 | 1 | 606 | 100 | 75.0 |
| 198 | 1.8 | 27.1 | ||
| 2 | 612 | 100 | 61.2 | |
| 186 | 49.1 | 33.9 | ||
Table 4. Effect of K2S2O8 addition on current efficiency in electrolysis of NaClO3 with PbO2 anode.
| NaClO3 conc. range over
which efficiency is calculated: |
||||
|---|---|---|---|---|
| g K2S2O8 per liter of electrolyte |
Test No. | Initial g/l | Final g/l | Current efficiency,% |
| 2.08 | 1 | 606 | 7.1 | 73.3 |
| 204 | 7.1 | 52.0 | ||
| 2 | 606 | 30.3 | 68.2 | |
| 200 | 44.9 | 49.2 | ||
| None | 1 | 606 | 28.9 | 46.5 |
| 200 | 28.9 | 27.1 | ||
| 126 | 28.9 | 20.3 | ||
| 2 | 606 | 31.0 | 43.4 | 200 | 31.0 | 30.5 | 128.4 | 31.0 | 22.9 |
The large lead dioxide electrode formed on the Ta screen, and described above, was used with a sprayed Cu over Ag contact in a 100 amp perchlorate cell at a current density of 0.28amp/cm squared and a temperature of 30-50C. The cathodes were type 430 stainless steel and the electrolyte was 5 liters of NaClO3 solution having an initial concentration of 600g/l. This cell was operated for 24 batches for a total running time of 860 hr without noticeable erosion of the anode, and with less than 0.25 ppm of Pb in the recovered NaClO4.
The current efficiency of PbO2 anodes in the conversion of
chlorate to perchlorate, although less than that of Pt, is reasonably high when the
concentration of NaClO3 in the electrolyte is above 100g/l.
Below this concentration of chlorate, the current efficiency drops sharply. In table 3 the
current efficiencies of PbO2 and Pt anodes are compared for
various chlorate concentration ranges when operated in 10-amp cells.
In order to obtain higher current efficiencies with the PbO2 anode,
especially in the lower chlorate concentration range, the use of additives becomes necessary.
Sugino (10) has reported using NaF additive at a concentration of 2g/l. In the present work,
K2S2O8 was found (11) to be even better , and the increase
in current efficiency due to this additive is shown in table 4.
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