Electrochemical behavior of acidic acetonitrile solutions at platinum and gold electrodes

Hydrogen evolution reactions from acidic MeCN solutions were studied at Pt and Au electrodes. The acids studied were HCl and HPic in a wide range of concentrations. LiClO₄ and Et₄NClO₄ (0.4 M) were used as supporting electrolytes. Quasi steady state polarization curves show that the reaction is irreversible with a Tafel slope of 2 RT/F after diffusion polarization and pseudo-ohmic drop have been corrected. Non-stationary techniques reveal that the electrochemical reaction is preceded by a chemical reaction attributed to slow dissociation of ACN·2 HCl and ACN·3 HPic species. Equilibria and chemical rate constants for both species were evaluated by chronopotentiometry. The values obtained explain satisfactorily the results accomplished under potentiodynamic conditions. Data at the RDE confirm the existence of a previous chemical reaction. MeCN electroadsorption in the double layer potential region at Pt accounts for the solvent levelling effect towards H adsorption between Pt and Au. Furthermore, an inhibitory effect over hydrogen evolution and hydrogen oxidation is reported at more negative potentials at both Pt and Au electrodes.