The metal-doped tin oxide precursor solid obtained by the wet chemical synthesis is subjected to a thermal treatment. The thermal treatment can be carried out at a relatively low temperature just for removing residual solvent from the wet chemical synthesis. However, in a preferred embodiment, the thermal treatment includes heating to a temperature within the range of from 400 to 800° C., more preferably 500 to 700° C. If a solid additive such as carbon has been added to the reaction mixture, said solid additive can be burnt off or decomposed to gaseous decomposition products by a thermal treatment at relatively high temperature.

However, the metal-doped tin oxide to be coated with a noble metal oxide layer can be obtained by other preparation methods as well.

As indicated above, a noble-metal-containing precursor compound is dissolved in the aqueous medium so as to come into contact with the tin oxide particles.

Appropriate noble-metal-containing precursor compounds are e.g. a noble metal salt and a noble-metal-containing acid. Typically, the oxidation state of iridium and/or ruthenium in the noble-metal-containing precursor compound is +III or +IV. The salt of iridium or ruthenium is e.g. a halide salt, a chloro complex, a nitrate salt, or an acetate salt. A noble-metal-containing acid is e.g. H₂IrCl₆.

The concentration of the tin oxide particles in the aqueous medium can be varied over a broad range. Typically, the tin oxide particles are present in the aqueous medium at a concentration of from 0.05 to 50 wt %, more preferably from 0.1 to 20 wt %. The noble-metal-containing precursor compound is added in an amount which is sufficient for obtaining the desired iridium and ruthenium content in the final composition.