Boron doped diamond/titanium composite electrodes for electrochemical gas generation from aqueous electrolytes

Titanium substrates were coated by 2-5mu m thick boron doped diamond layers through the HFCVD-process from an activated H-2/CH4 gas phase. The doping level was 50-230 atom ppm B. The surface was saturated with drop CH2 groups. A TIC interphase was formed in situ. Stationary current-voltage curves up to (+/-) = 0.1 cm (-2) were measured in three aqueous electrolytes, 1 M H2SO4, 1M NaOH and 3 M NaCl. Very high overvoltages of eta=1-2V at 0.1 A cm (-2) were found for the relevant gas evolution reactions. An appreciable, potential dependent portion of the Galvani voltage seems to be due to a voltage drop in a space charge layer in the semiconductor. Tafel plots yield no straight lines therefore. An additional reason is the inertness of the electrode surface due to the drop CH2 groups in the negative and drop C=O groups in the positive region. Electrochemical transformations between these surface structures seem to be possible.