Porous Substrate Anodes

The ceramic substrate anode has its problems. It will need a good coating of Lead Dioxide in order to carry the heavy currents need in the (Per)Chlorate cell. It also needs a good flawless coating of Lead Dioxide at the top end so that a good electrical connection can be made to the anode. The porous substrate conducts electrolyte (wicks) up itself and will corrode the connection at the top if the Lead Dioxide coating at the top is not of a high quality. You may need to seal the substrate close to the top. All of the possible advantages of the ceramic substrate (only a poor quality coating of Lead Dioxide would suffice) anode are not realizable in reality. (IMHO)

The inspiration for the anode described here was originally taken from U.S. patent No. 4,008,144 (See elsewhere on this page) which describes an anode having a ceramic substrate. Since ceramic is not conductive you cannot electrodeposit lead dioxide onto it directly, the ceramic must first be made conductive. This was achived in the patent by depositing lead dioxide onto/into the ceramic by a chemical technique described in the patent which involved soaking the ceramic in an oxidizable lead (II) salt (eg.lead nitrate, lead acetate) and then oxidizing the lead nitrate/acetate crystals in the ceramic to lead dioxide using an oxidizer (persulphate or hypochlorite at a pH of ~12).
This method was succussful but was laborious as the cycle of soaking and oxidizing had to be done a number of times. The cycling process can be eliminated altogether by simply applying a paste of lead dioxide + water to the ceramic and then electrodepositing on a coating of lead dioxide.

One of the biggest problems with lead dioxide anodes is getting a long term, reliable, heavy duty current connection to the anode when it goes into service as an anode in a chlorate or perchlorate cell.
Graphite is a material that is compatible with lead dioxide but it is difficult to obtain a good, low resistance contact between the two materials. The only way to make a reliable contact between graphite and the lead dioxide of an anode proper is to electrodeposit the lead dioxide onto the graphite connection and the ceramic substrate at the same time. This is why I would refere to this type of connection as a grafted graphite connection (as opposed to simply clamping graphite to lead dioxide). It can be problamatic to get Lead Dioxide to adhere to graphite unless you have a well controlled plating bath together with surfactant in the bath or spinning.
If using a metal connection to the finished anode, there is the problem of oxide buildup between the lead dioxide and the metal which you choose to make the connection with. The metal oxide always forms because of electronic interactions between the metal and the lead dioxide. Silver (Ag) metal is the only easily available metal that has been shown to be succusseful as a connection because silver oxide is conductive and will continue to form a low resistance contact between the silver metal and lead dixoide. ( JES Vol. 105, No.2 page 100 (1958) ) Copper can then be used on top of the silver. All metal connections will corrode if exposed to salts. The substrate is inclined to conduct (if porous,ie. ceramic) the salts from the Chlorate/Perchlorate cell by capillary action up to the metal connection. The best way to combat the problem is to use the (ceramic substrate) anode 'upsidedown' (relative to the way it was plated) in the Chlorate/Perchlorate cell. There will be continous coating of lead dioxide on the end of the anode that has the metal connection which will act as a barrier to the salts in the substrate.
If the substrate you choose is non porous such as plastic, there will not be such a tendency for the substrate to conduct the salts from the Chlorate/Perchorate cell up to the connection. There may still be some capillary action between the electrodeposited lead dioxide and the substrate itself but I have seen little.

The ceramic substrate Anode has been use for 5 month continuous operation and was still ok. The electrical connections had to be renewed about 3 times. This simply entailed unwrapping off the coper braid and resilvering the top of the anode and putting on new copper braid.

A problem with this type of Anode it the fact that the porous ceramic substrate will carry up electrolyte to the connection outside the Chlorate cell and corrode the connection. Therefor you need a perfect coating of Lead Dioxide at one end (the connection end) of the anode, this is hard to achive. Therefor his type of anode in not recommended.

Choosing a substrate

A question that arises at this stage is: What size of an anode do I want?
Most lead dioxide anodes are used at a current density of about 100 to 200mA/cm^2. Therefor if you want to make an anode to suit a supply that can produce 20 amps, you will need an anode that has a usable surface area of between, 20,000/(100 or 200) = 200 or 100 cm^2. Remember that you must have some 'extra anode' for placement of the current connections to it outside the cell.
The first task is to obtain a suitable substrate. Porous ceramic materials are a suitable candidate for the task. Plastics (non porous) substrates are discussed elsewhere.
[DIAGRAM OF TWO DRESSING/SHARPENING STONES BEING JOINED] Silicon carbide or Aluminium oxide can be obtained at the local machine tool store or welding supply shop in the form of dressing or sharpening stones. Getting the shape that is desirable for an anode can be difficult but it should be available by order. The most suitable shape is a cylinder shape (broom handle shape) that is about 1/2 to 1 inch (1.3 to 2.5 CM) in diameter and about 5 to 10 inches (13 to 26 CM) long. A square section stone is also suitable but the corners must be rounded. The grit size should be from about 30 to 100 and it is desirable to have a rough surface on the stone. If the stone has a smooth surface, sandblasting would be desirable.

If you can't obtain a stone that is long enough they can be joined by drilling a hole in the end of each stone to be joined using a tungsten rod (available from welding store, and used in TIG welders) that has had a chisel shape ground on to one end. You will need to renew the chisel shaped head a number of times before the hole is complete. When both holes are make the rods can be joined using polyester resin and some fiber glass. The fiberglass and resin can be purchased at the car accessory store.

Another type of ceramic substrate that is being suitable with this type of anode is fired potters clay. You should make up your own mix from ingredients purchased from the ceramics store. The advantage with this substrate is that you can make it whatever shape you want but you have the extra trouble of making and fireing the substrate. See Casting your own substrate. for further information regarding making your own substrate. All sharp corners on the substrate should be rounded off before making the substrate conductive and plating.
There are a range of other inert materials that could be used for the substrate. Quarry tiles, flagstones, tiles, asbestos cement, and other acid resistant cements. Plastic can be make conductive by coating with lead dioxide powder, (see elsewhere on this page).

Making the porous substrate conductive

The porous substrate must now be made conductive so that the lead dioxide can be electrodeposited onto the substrate. This is easily accomplished by making a paste of lead dioxide and water. The paste is applied using a small piece of cloth and is rubbed into the substrate with gusto. You may need to dampen the lead dioxide paste if it gets too dry on the anode. Lead dioxide that is pasted onto the substrate in this step will convert to hard plate-type lead dioxide when the plating step begins. A good pair of rubber gloves should be worn as lead dioxide is toxic. See Obtaining lead dioxide. for to make powdered lead dioxide.
See also here for some observations regarding 'rubbed on' lead dioxide.
The porous substrate can also be made conductive by the method described in the patent (No. 4,008,144) or variations thereof. One metod is to gas the substrate that the has been soaked in the oxidizable lead compound using ClO2 gas (chlorine dioxide) which is generated using an acid and a chlorate. ClO2 is explosive in > 10% concentrations and is toxic.