Abstract
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 Cu 1.4Mn 1.6O 4 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 °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 Cu 2+ and metallic Cu. In this work we show that the polarized electrodes also reveal the presence of metallic Cu and Cu 2+. The data also show that the oxide particles embedded in the polarized electrodes contain Mn 3+ and Mn 4+, although the Mn 3+/Mn 4+ 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.
Highlights
Nanoparticle transition metal oxide electrodes are important materials due to their low cost, high-surface area, potential electrocatalytic activity for the oxygen reduction reaction and for being suitable materials to be used in fuel cell vehicles and light batteries
The voltammograms recorded on composite electrodes previously polarized at À0.45 V/SCE prior to the cyclic voltammetry experiment (Fig. 1, curve 2), in the same conditions of the fresh one, showed that after a prolonged polarization at negative potentials almost all ClÀ anions are expulsed from the PPy matrix leading to an important loss of conductivity
The composite electrode is stable in acidic solution because the H+ ions are consumed by the orr favouring the increase of local pH and stabilizing the oxide
Summary
Nanoparticle transition metal oxide electrodes are important materials due to their low cost, high-surface area, potential electrocatalytic activity for the oxygen reduction reaction (orr) and for being suitable materials to be used in fuel cell vehicles and light batteries. During the orr on the Cu–Mn oxide covered by a PPy film taking place at negative potentials, two important features were observed: (i) the conservation of the electrical conductivity of the composite film in conditions at which generally the polymer PPy is insulator and (ii) the occurrence of a remarkable electrochemical stability of the electrocatalyst at acidic pHs, a situation where the oxide is normally destroyed by electroreduction. This experimental behaviour can be explained by the local formation, by the orr, of OHÀ ions in the PPy(Cu1.4Mn1.6O4) inner layer, which arises from the consumption of the H+ ions. We report here on the use of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (EXAFS and XANES) to determine the influence of the polarization conditions in oxygenated and non-oxygenated KCl aqueous solutions (pH 2.2) on the bulk and surface structure of GC/PPy/PPy(Cu1.4Mn1.6O4)PPy composite electrodes
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