Oxygen reduction activities of praseodymium manganites in alkaline solution

Abstract

A series of praseodymium manganites, Pr1-xAxMnO3 (A=Ca, Sr, Ba, 0≤x≤1.0), were examined for their electrode-catalytic activities as incorporated into a gas diffusion-type oxygen reduction electrode working in an alkaline solution. Among the oxides tested, Pr0.6Ca0.4MnO3 gave the highest electrode performance (320 mA·cm-2 at -150 mV vs. Hg/HgO) in 8 mol·dm-3 KOH aqueous solution at 60°C under air flow. The electrode performances were found to be well correlated with the activities of the oxides for the 4-electron reduction of oxygen (O2+2H2O+4e-→40H-) evaluated from a rotating ring-disk electrode analysis. The oxidation state of Mn ions of the oxides was investigated by means of iodometry, X-ray photoelectron spectroscopy, temperature-programmed oxygen desorption and electron spin resonance. The results obtained suggest that the active sites for the 4-electron reduction of oxygen involve Mn3+ ions and that their activities increase profoundly as the Mn3+ ions neighbor Mn4+ to form Mn3+-Mn4+ pairs. The electrode loaded with Pr0.6Ca0.4MnO3 was confirmed to continue to work rather stably for a test period of 200h under the galvanostatic condition of 300 mA·cm-2, the oxide being kept intact as revealed by X-ray diffraction analysis after the test. The same electrode was shown to exhibit a promising performance when incorporated into a zinc-air cell.