Electrochemical quartz crystal impedance study on the electrodeposition of LiOH onto a gold electrode in acetonitrile containing LiClO4 · 3H2O and its application in preparing a Pt-plated porous polypyrrole thin film for the catalytic electrooxi

The electrochemical quartz crystal impedance (EQCI) analysis method was used for the first time to quantitatively examine the precipitation of LiOH onto a gold electrode at potentials negative to ca. −0.7 V vs. SCE during the cathodic sweep reduction of dissolved oxygen and coexisting water in acetonitrile (ACN) containing LiClO4 · 3H2O, as a result of the poor solubility of electrogenerated LiOH in the ACN medium. The suggested LiOH-precipitation mechanism was supported by comparative experiments conducted in ACN containing NaClO4 · 2H2O (or tetrabutyl ammonium chloride/bromide), since large EQCI responses implying a similar precipitation of electrogenerated NaOH were also obtained in the NaClO4 · 2H2O system, but the quartz crystal impedance responses were negligibly small in the two systems of quarternary ammonium salts. The effects of concentrations of LiClO4 · 3H2O and the foreign water added in ACN on the EQCI responses were individually examined, and the maximum frequency shift induced by the LiOH precipitation was found to be as large as about −5 kHz. The cyclic voltammetric growth of polypyrrole (PPY) films at several Au electrodes in fresh ACN solutions of 1 mol L−1 pyrrole + 0.1 mol L−1 LiClO4 · 3H2O were comparatively conducted over three potential-sweep ranges, 0 to 0.85 (A, PPYA), −1.6 to 0.85 (B, PPYB) and −2.0 to 0.85 V vs. SCE, respectively, giving that the accompanying precipitation of LiOH notably influenced the polymer growth and porosity. Compared with the normal PPY film (PPYA), the PPYB after removing the LiOH precipitate formed during the cyclic voltammetric growth of the polymer was more porous, as examined by EQCI and SEM techniques, which resulted in a larger Pt dispersion when Pt particles were electrodeposited on the PPYB/Au electrode in acidic chloroplatinic solution and a higher electrocatalytic activity toward methanol oxidation in aqueous H2SO4. The proposed protocol of increasing the PPY porosity by introducing removable electrodeposits during PPY’s growth may be of some general interests for other polymers using other removable precipitates.