Catalytic Activities of Perovskite Oxide Electrodes for Oxygen Reduction in Alakline Solutions

Introduction

There has been increasing interest in fuel cells using alkaline electrolytes. In the research of alkaline fuel cells, much attention has been paid to oxide electrocatalysts since their costs are much lower than those of noble metal catalysts. As the results of many researchers, it is found that perovskite oxide is one of the promising candidates for electrocatalysts for oxygen reduction in alkaline solutions. Since most perovskite oxides have inadequate electron conductivities, it is necessary to use additives such as carbon black for providing electron paths for oxides in the electrodes. While the using of additives offers better catalytic performance of oxides, the additives make it difficult to observe the catalytic activities of oxides. Therefore, we have investigated the catalytic activities of perovskite oxides using thin-film electrodes prepared. Through electrochemical measurements, we found that carbon black had catalytic activities for the initial 2e oxygen reduction, and perovskite oxides were active for the following decomposition reaction of peroxide. In this study, we report the catalytic behaviors of thin-film electrodes of perovskite oxides.

Experimental

Thin-film electrodes of perovskite oxides were prepared by the pulsed laser deposition (PLD) method. Pellets prepared with oxide powders of La_0.8Sr_0.2MnO₃, La_0.8Sr_0.2CoO₃, La_0.5Ca_0.5MnO₃, and La_0.5Ca_0.5CoO₃ were used as PLD targets. Resultant perovskite thin-films were characterized by X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). A three-electrode cell was used in electrochemical measurements, which consisted of perovskite thin-film, Ag/AgCl, and Pt wire as working, reference, and counter electrodes.

Results and discussion

XRD patterns of resultant perovskite thin films were in good agreement with previous data of powder diffraction. Therefore, we successfully prepared thin films of polycrystal perovskites. The results of ICP-AES showed PLD method copied the target composition of oxides. Linear sweep voltammograms showed small current densities for oxygen reduction on perovskite thin-film electrodes. Electronic conductivities of perovskite thin films were evaluated by cyclic voltammetry of [Fe(CN)₆]^3-/[Fe(CN)₆]^4-, and we confirmed the prepared electrodes had adequate conductivities for electrochemical measurements. As a result, we concluded that perovskite oxides needed an assisting catalyst such as carbonaceous materials to exhibit the catalytic activities of oxygen reduction.