In situ observation of electrochemical formation and degradation processes of polyaniline by Fourier-transform infrared spectroscopy

The electrochemical polymerization of aniline on a platinum electrode in aqueous HClO4 and H2SO4 solutions has been studied by means of in situ Fourier-transform infrared (FTIR) reflection spectroscopy. The spectra, taken at early stage of the polymerization in both solutions, exhibited the absorption characteristic of polyaniline (PAn) in an emeraldine state, even at a highly positive potential (greater than or equal to+0.7 V vs. Ag/AgCl (saturated KCl)). Polymer growth in H2SO4 was accompanied by an increase in the molar ratio of [HSO4-]/[SO42-], indicating that the protons generated during the oxidation of aniline react with SO42- to afford HSO4-, and that polymer growth is facilitated. In contrast, such proton-scavenging behavior was not observed in a HClO4 solution, and the growth of PAn was suppressed owing to the accumulation of protons. Under the condition that aniline was extremely consumed, the produced PAn was subjected to oxidative degradation, which was competitive with the growth process of PAn. The major degradation species was identified to be p-benzoquinone.