Developing a reversible oxygen electrode is a major technical challenge in electrochemistry. In fuel cells, the oxygen electrode reduces oxygen to water, the oxygen reduction reaction (ORR). In electrolysis cells, the oxygen electrode oxidizes water to oxygen, the oxygen evolution reaction (OER). Currently, the best known catalysts for the ORR and the OER are platinum and ruthenium oxide, respectively. Neither material is a particularly active catalyst for the other reaction; thus a reversible oxygen electrode is far from reality. In this paper, we will discuss our approach in developing such an electrode, drawing inspiration from a biological OER catalyst that is highly active. In nature, an oxygen electrode reaction is catalyzed by oxygen evolving center (OEC) in Photosystem II. OEC, which is a cubane-like CaMn4O4, contains four manganese atoms and has been predicted to be very close to an ideal oxygen electrode . Manganese oxide surfaces have also been shown to be good surface catalysts for oxygen reduction and evolution reactions [2-6]. Drawing inspiration from nature and building upon previous work on manganese oxide surfaces, manganese oxide films were synthesized and studied as oxygen electrode catalysts. Special attention was paid to the nanostructuring of the surface which modifies the energetics for binding surface intermediates involved the ORR and the OER. In this study, manganese oxide thin films were electrodeposited onto glassy carbon electrodes (0.196 cm2, SigradurG HTW Hochtemperatur-Werkstoffe GmbH). Synthesis was performed using Bio-Logic potentiostat (VMP3) in a 3-electrode electrochemical cell in a rotating disk electrode (RDE, Pine Instruments) configuration, using a modification of a procedure developed by Pang et al8. Thin films were systematically studied in an effort to understand relationships between structure, composition and function. A variety of methods were employed: catalyst morphology was studied using scanning electron microscopy (SEM, FEI XL30 Sirion), the oxidation state was studied using x-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe), and electrochemical activity was studied using cyclic voltammetry (CV) in a 3-electrode electrochemical cell in a RDE configuration. Oxygen electrode activities were measured in oxygen saturated 0.1M KOH electrolyte, at 23°C and 1600rpm, with a sweep rate of 20mV/s, using a platinum wire counter electrode and Hg/HgO reference electrode. The potential scale was calibrated to reversible hydrogen electrode (RHE), using a hydrogen reduction/evolution experiment with a platinum electrode. Manganese oxide thin films were found to be active for both the ORR and the OER. For the ORR, the manganese oxide thin film is nearly as active as platinum; for the OER, its activity is close to that of ruthenium oxide. SEM and TEM images showed a highly nanostructured surface. Spectroscopic investigations employing x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) revealed a mixture of manganese (IV) oxide and manganese (III) oxide. The surface stoichiometry was found to change substantially during operating conditions the ORR or the OER. The observed changes in surface structure and stoichiometry indicate ways in which improved catalysts can be developed – ones that operate more closely to the equilibrium potential.
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