Characterization of copper manganite oxide-polypyrrole composite electrodes cathodically polarized in acidic medium

We have studied the electrochemical behaviour induced by polarization in sandwich-type composite electrodes with the structure GC/PPy/PPy(Ox)/PPy where GC stands for glassy carbon, PPy for polypyrrole and Ox for Cu1.4Mn1.6O4 nanoparticles. The electrodes were polarized at -0.45 V/SCE in 0.15 M KCl aqueous solution at pH 2.2 either saturated in Ar or O-2 at 25 degrees C. The changes occurring on these electrodes were studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (EXAFS and XANES) techniques. In previous work we have shown that when the oxide particles are incorporated into the PPy matrix the Cu+ present in the initial oxide suffers dismutation to give Cu2+ and metallic Cu. In this work we show that the polarized electrodes also reveal the presence of metallic Cu and Cu2+. The data also show that the oxide particles embedded in the polarized electrodes contain Mn3+ and Mn4+, although the Mn3+/Mn4+ ratio is different from that found in the fresh electrodes. The Cl 2p XPS data show that in the electrode polarized in O-2 there is an enhancement of the Cl covalent contribution that appears at 200.8 eV (which is already present in the fresh electrode although with a very small intensity). This result suggests that the oxygen reduction reaction leads to an increase of the OH- concentration inside the composite electrode that explains the charge transport in PPy at negative potentials. (C) 2007 Elsevier Ltd. All rights reserved.