THIS INVENTION relates to an improved electrode construc¬ tion suitable for use in operations involving electroly¬

^" \ sis.

Electrodes for use in electrolysis operations at present are not satisfactory because they are relatively expensive (involving the use of precious metals such as platinum, gold and silver and titanium rhodium, ruthenium and palla¬ dium) and have a relatively short life when due to expense factor the electrodes are kept small and suffer high current density.

The object of the present invention is to provide an electrode construction which substantially overcomes these problems.

It has been found that a satisfactory electrode may be constructed by bonding an inert electrically conductive material with a high ^' surface area ^" to an electrically conductive sheet.

In one form the invention resides in a composite electrode comprising a substrate of electrically conductive material one face of which has large surface area and the other face of which is formed of or is covered with a highly conductive material.

The substrate is formed by loading a suitable synthetic resin with a highly conductive material such as carbon black, carbon fibres or finely divided metal particles. The carbon black and/or carbon fibres are to be preferred because of their inertness. Suitable resins include polypropylene and high density polyethylene. The sub¬ strate may be formed by moulding or extrusion techniques as required.

The exposed surface of the substrate having a high surface area may be formed by applying a coating or layer of carbon fibre or carbon black to the surface, or it may be accomplished by etching or otherwise treating the surface to expose the particle of carbon black or the carbon fibres. The carbon black or carbon fibres may be applied to the surface during the moulding or extrusion operation whilst the resin is in a suitable condition to effect satisfactory bonding. Alternatively it may be applied with an electrically conductive adhesive such as a solu¬ tion of the conductive resin in a volatile solvent. Suitable carbon blacks or carbon fibres have a surface area within the range of 20 m ² /g to 1600 m ² /g and a micro- pore structure with a diameter of the order of 5-100 Angstrom units. The carbon fibre may be in the form of a paper or felt or as a knitted cloth.

The other surface of the substrate is coated with a layer of a suitable metal such as nickel which may be applied using an electroless technique such as that described by Feldstein (R.L.A. Review 31-317 [1970]).

In some cases it may be desirable to use a metal sheet or the like as the substrate in which case a layer of carbon black and/or carbon fibres is applied to one surface of the metal substrate.

A suitable terminal is mounted on the metal surface to enable the electrode to be connected to an electrolysis circuit.

The electrodes of the present invention are particularly suitable for use in a chlorine generating unit of the type used in salt water swimming pools. One such embodiment is shown in the accompanying diagrammatic drawings wherein:-

Fig. 1 is a sectional elevation; and

Fig. 2 is an enlarged fragmentary section of the portion circles in Fig. 1.

As shown in the drawing the electrode 11 is in the form of a cylinder which is coupled to a T-piece 12 incorporated in the return flow line leading from the swimming pool filter to the pool. The lower part 13 of the unit pro¬ jects into the flow line to form a restriction which acts as a venture so that water is caused to flow through opening 14 into the cylinder and to the top thereof and into the return pipe 15 and then back to the flow line through opening 16 into the area of reduced pressure created in the flow line by the venture effect of the restriction. The wall of the cylinder is constructed in accordance with the invention and as will be seen from Fig. 2 comprises a layer 17 of carbon fibres and/or carbon particles bonded to a substrate 18 formed of a plastic loaded with carbon black and/or carbon fibre.

A terminal for connecting the electrode in the circuit comprises a terminal 20 formed of copper or brass or other suitable metal bonded to two plates 21 and 22 of copper separated by a layer 23 of fibre glass such as that used in the construction of printed circuit boards. The assem¬ bly is covered by the electrically conductive plastic 24 during the formation of the electrode. An outer layer 19 of nickel or other suitable metal (not shown) is then applied to the outer surface in accordance with the inven¬ tion. An anode 25 of stainless steel titanium or other suitable material projects into the cylinder as does a gas detecting probe 26.

Whilst the invention has been described with particular reference to a chlorine generating unit it will be appre¬ ciated that the electrode of the present invention may be

used in any electrolyte system where it is desirable that the current density on the electrode be relatively small.