Electropreparation and characterization of polyNiTSPc films. An EQCM study

Thin films of polyNi(II)tetrasulfophthalocyanine (polyNiTSPc) electrodeposited on a gold electrode by potential cycling in 0.1 M NaOH solutions containing 1 mM Ni(II) tetrasulfophthalocyanine were studied by cyclic voltammetry, electrochemical quartz crystal microbalance and impedance techniques. Both the mass increase due to the electrodeposited film, and the current of the Ni(II)/Ni(III) process in the film, increase monotonically with the number of electrodeposition cycles, a linear correlation existing between these two parameters. AFM micrographs show that the film is rugose and porous, this porosity being confirmed by the fact that the voltammetric features of gold oxide formation and reduction are unaffected by the film. The cyclic voltammogram of the film in a pH 11 carbonate-hydrogencarbonate buffer shows the characteristic anodic and cathodic peaks of the Ni(II)⇔Ni(III) process, with a large mass decrease upon Ni(II) electrooxidation and an equally large mass increase upon Ni(III) electroreduction. The sign of the mass change is compatible with a β-Ni(OH)2-character of the film. The large slopes of the plots of mass change vs. charge in the Ni(II)/Ni(III) region clearly show that large fluxes of water and/or ions accompany the electron transfer. The fairly low activation energy of Ni(II) electrooxidation, 38 kJ mol−1, is in agreement with the moderately non-Nernstian behaviour of the Ni(II)/Ni(III) process. The super-Nernstian dependence on pH of the peak potentials of the Ni(II)/Ni(III) process indicates that the species involved are non-stoichiometric, similarly to the Ni1 − xHx(OH)2 species in nickel hydroxide.