Sulfonated polyaniline (SPAN) was synthesized by sulfonation of polyaniline (PANI) base with fuming sulfuric acid. Thin films were cast from polymer solutions in basic media. The polymer films were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible-near-infrared spectroscopy, scanning electron microscopy (SEM) and cyclic voltammetry. XPS in combination with FTIR showed that the preparation procedure led to ca. 47% sulfonation of an otherwise unchanged polyaniline backbone. The NIR spectra of SPAN films showed a polaron band at higher energies than with polyaniline. This is in agreement with the lower conductivity of SPAN as compared with polyaniline. SEM micrographs of the SPAN films showed a compact globular morphology. Electrodes modified with thin SPAN films exhibited two redox steps, both in aqueous and in non-aqueous electrolytes. The specific charge stored in SPAN films was found to be ca. 37 A h kg-' in aqueous solution (only the first redox step) and ca. 68 A h kg-' in non-aqueous media (both redox steps). A practical SPAN-Li battery could have 50% more specific energy than a PANI-Li battery. The optical spectra of SPAN films exhibited bands at 310, 450 and 750 nm, the intensities of which changed during the redox process. The absorption coefficients of SPAN (emeraldine base state) solutions had values of a = 41 0 at 31 3 nm and a = 239 at 563 nm. The suitability of SPAN for use as a cation-insertion material for battery and electrochromic applications is discussed.
Probe beam deflection (PBD) was used to monitor the ion exchange between polymer film and bulk electrolyte during the redox reaction in polyaniline in aqueous electrolyte (HC1, HC104, H2SO4). The PBD results clearly show that protons as well as anions are exchanged during the first oxidation process. Protons are exchanged at potentials below 0.55 Vrh~. The relative contribution of the proton to the overall process depends not only on the pH of the electrolyte but also on the nature of the specific anion present. While the participation ofH § could be observed in HC1 for a concentration of0.5M a concentration of 1M was needed in H2SO4. A simple model based on the protonation equilibria of leucomeraldine species is used to analyze the results quantitatively. The results are discussed in terms of previously proposed mechanisms of redox processes in polyaniline.
Spectroscopic methods are used to investigate the formation of low molecular mass intermediates during aniline (ANI) oxidation and polyaniline (PANI) degradation. Studying ANI anodic oxidation by in situ Fourier transform infrared spectroscopy (FTIRS) it is possible to obtain, for the first time, spectroscopic evidence for ANI dimers produced by head-to-tail (4-aminodiphenylamine, 4ADA) and tail-to-tail (benzidine, BZ) coupling of ANI cation radicals. The 4ADA dimer is adsorbed on the electrode surface during polymerization, as proved by cyclic voltammetry of thin PANI films and its infrared spectrum. This method also allows, with the help of computational simulations, to assign characteristic vibration frequencies for the different oxidation states of PANI. The presence of 4ADA retained inside thin polymer layers is established too. On the other hand, FTIRS demonstrates that the electrochemically promoted degradation of PANI renders p-benzoquinone as its main product. This compound, retained inside the film, is apparent in the cyclic voltammogram in the same potential region previously observed for 4ADA dimer. Therefore, applying in situ FTIRS is possible to distinguish between different chemical species (4ADA or p-benzoquinone) which give rise to voltammetric peaks in the same potential region. Indophenol and CO(2) are also detected by FTIRS during ANI oxidation and polymer degradation. The formation of CO(2) during degradation is confirmed by differential electrochemical mass spectroscopy. To the best of our knowledge, this is the first evidence of the oxidation of a conducting polymer to CO(2) by electrochemical means. The relevance of the production of different intermediate species towards PANI fabrication and applications is discussed.
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