Metal Electrode With an Active Cover Layer for Electrochemical Purposes

The present invention relates to a metal electrode with an active cover layer comprising metal platinates, metal palladates, or the like.

It is known in the art to utilize metal electrodes with an active cover layer on the basis of Me(I) approx. 0.5Pt.sub.3 O.sub.4, with Me(I) representing Li and Na for the electrolyses of NaCl, KCl, chlorate and HCl, as shown by the German published application No. 1,813,944. As shown by U.S. application Ser. No. 450,834, the active cover may also contain substances of the type Me.sub.A (I) approx. 0,4Me.sub.B (I) approx. 0,1Pt.sub.3 O.sub.4, with Me.sub.A representing Li, Na, K and Me.sub.B representing Tl and Ag. These electrodes stand out for a high reliability in operation, long life and high constancy of the cell voltage.

It is, therefore, an object to be solved by the invention to provide an embedding material for these metal electrodes positioned between the alloyed-on-surface of the carrier member of the electrode, consisting i.e. of titanium, and the above mentioned electrochemically active substances.

It is a further object of the invention to provide an embedding material between the alloyed-on-surface of the carrier member and the electrochemically active substances, which embedding material does not impair the effectiveness of the active materials.

According to a further object of the invention the embedding material between the alloyed-on surface of the carrier member of the electrode and the electrochemically active substances should, in areas where it is exposed, protect the electrode in a chemically resistant manner without showing complete inactivity, in order to increase the current yield of the anode.

According to the invention, this problem is solved by the use of compounds of the type Ba (M.sub.x Ti.sub.1.sub.-x)O.sub.3 alone or in combination with other additives, as embedding substances to constitute the cover layer. These compounds have been prepared in pure forms and examined for the first time by J. G. Dickson, L. Katz and R. Ward (J. Am. Chem. Soc. 83, page 3026, 1961).

It turned out that these compounds quite well embed the active compounds Me(I) approx. 0.5Pt.sub.3 O.sub.4 and Me.sub.A (I) approx. 0.4Me.sub.B (I) approx. 0,1Pt.sub.3 O.sub.4 and also adhere very well to the alloyed-on titanium surface of the electrode carrier member. In contrast to many other co-adhesives, these substances also show a specific electrical conductivity and, though small, an electrochemical activity.

1 Mol BaCO.sub.3, 0.1 Mol PtO.sub.2, 0.9 Mol TiO.sub.2 each are finely ground and are heated for about 1 hour in air in a silica crucible, the sinter material is again finely ground and in the course of 24 hours is under vacuum brought to 1300.degree. C. A homogeneous, micro-crystalline, amber compound is obtained. This compound (70%) is sintered together with Li.sub.0,5 Pt.sub.3 O.sub.4 (30%) to platinized-on titanium at approx. 600.degree. C. In extreme cases, the content of Li.sub.0,5 Pt.sub.3 O.sub.4 can be lowered for the electrochemical use down to 10%, but normally a proportionate amount of about 40% is preferred. The thus obtained electrode showed current yields of 97-98 %.

The reaction is conducted according to the following equation:

it is not expedient to use values below 0.1 Mol PtO.sub.2, because otherwise the pure barium titanate is already present, which shows much too low a chemical activity and too poor electrical conductivity. At values of 0.1-0.5 PtO.sub.2 in the compound a complete reaction is no longer taking place and, simultaneously with the compound formation, decomposition of the PtO.sub.2 takes place, wherein metallic platinum is set free which makes the compound impure. This is of advantage for the burning-in process of the titanium surface, so that it is preferred to operate within this range. In the compound group Ba (M.sub.x Ti.sub.1.sub.-x)O.sub.3, Ru, Rh, Pd, Os, Ir may be used as metals.

It can be considered an example for the preparation:

0.3 mol Ru, 1 Mol BaCO.sub.3 and 0.7 Mol TiO.sub.2 are slowly heated in finest powdered form to 1050.degree.-1100.degree. C in air in a silica crucible. Small, almost colourless crystals result. They are applied together with a mixture of 30 % Li.sub.0.5 Pt.sub.3 O.sub.4, 5 % Na.sub.0.4 Tl.sub.0.1 Pt.sub.3 O.sub.4 and 5 % Li.sub.0.5 Ag.sub.0.1 Pt.sub.3 O.sub.4 onto a titanium surface alloyed with thallium. The cell voltage at 10.000 A/m.sup.2 amounts to 4.3 V, the electrodes so far have operated in the NaCl-cell for 15 months without difficulties.

A particular variation of the combination of active substance of the cover layer and embedding material of the invention consists in selecting in the embedding material the same precious metal component as in the active substance. If, for example, Li.sub.0.5 Pt.sub.3 O.sub.4 is used as active substance, Ba(Pt.sub.0.2 Ti.sub.0.8)O.sub.3 may be used as embedding material. If the active component consists of TlPd.sub.3 O.sub.4, then Ba(Pd.sub.0.25 Ti.sub.0.75)O.sub.3 may be used as embedding material.

Also the embedding material may be selected such that the doping component of the titanate consists of the metal that is also alloyed into the surface of the electrode carrier member consisting of titanium. For example Pt, Rh, Pd and Ir come into consideration therefor.

It has also turned out expedient to use the same precious metal component for the embedding substance, the active substance and the alloy of the surface of the electrode carrier member consisting of titanium.

It is, of course, to be understood that the present invention is, by no means, limited to the particular examples, but also comprises any modifications within the scope of the appended claims.