Electrooxidation of methanol and ethylene glycol on gold and on gold modified with an electrodeposited polyNiTSPc film

The electrooxidation of methanol and ethylene glycol on Au and on polynickeltetrasulphophthalocyanine-modified gold (polyNiTSPc/Au/Q) electrodes in a pH 11 carbonate/hydrogen carbonate buffer electrolyte was studied by cyclic voltammetry (CV) and with an electrochemical quartz crystal microbalance (EQCM). Au shows negligible activity for methanol oxidation, in agreement with the fact that methanol does not adsorb on Au, since it does not affect the potential at which the mass increase due to Au oxidation starts. On the contrary, ethylene glycol (EG) is electrooxidized on Au at a significant rate, probably because it adsorbs rather strongly on Au, as evidenced by the positive shift by 0.35 V of the mass increase attending Au monolayer oxidation. The polyNiTSPc-modified Au electrodes are hydrophobic, as inferred from the disappearance of the large mass decrease in the double-layer region typical of the bare Au electrode, a decrease which is due to the desorption of the water molecules adsorbed at the negative potential limit. On the polyNiTSPc/Au/Q electrodes the current at the positive potential limit (at which only Ni(III) exists) of methanol oxidation, and the peak current of EG oxidation (at a potential at which only Ni(II) exists), show a Langmuir dependence on the concentration, which indicates that in both cases the reactive species are adsorbed, and that their oxidation rate is much lower than the adsorption rate. As is well known, the oxidation of Ni(II) to Ni(III) in the polyNiTSPc film is accompanied by a large mass decrease, due mostly to the expulsion of water from the film by the hydrophobic Ni(III). This mass decrease is independent of the scan rate, but in the presence of methanol or ethylene glycol it increases with increasing scan rate, which indicates that the oxidation of the alcohols involves a chemical reaction of the alcohols with Ni(III) ions, the extent of which decreases with increasing rate. Consequently, the amount of the hydrophobic Ni(III) will increase, and so will the mass loss. The mass decrease of Ni(II) oxidation decreases with increasing concentration of methanol or ethylene glycol, which again shows that there is a chemical reaction between the alcohols and Ni(III) ions, since the concentration of the latter would decrease with increasing alcohol concentration.