The electron transfer kinetics in Fe(CN)_6~(3-/4-) redox system at thin-film electrodes of amorphous diamond-like carbon with admixtures of platinum (up to 15 at. %) is studied. The wide-gap diamond-like carbon is not in itself electrochemically active; however, it acquires the electrochemical activity upon introducing platinum into the DLC bulk during the film deposition. The unusual feature of the effect of platinum is its threshold nature: the electrochemical current appears at ca. 3 at. % and kind of saturates at ca. 10 at. % of Pt. By contrast, the differential capacitance increases continuously with increasing Pt content; and the film bulk resistivity practically does not change. The observed effects are explained in terms of a model assuming nonuniform character of both electric conductance in the DLC bulk and the facilitation of electron transfer by Pt clusters on the electrode/solution interface.
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