OXYGEN REDUCTION ACTIVITIES OF PRASEODYMI UM MANGANITES IN ALKALINE-SOLUTION

A series of praseodymium manganites, Pr(1-x)A(x)MnO(3) (A = Ca, Sr, Ba , 0 less than or equal to x less than or equal to 1.0), were examined for their electrode-catalytic activities as incorporated into a gas di ffusion-type oxygen reduction electrode working in an alkaline solutio n. Among the oxides tested, Pro(0.6)Ca(0.4)MnO(3) gave the highest ele ctrode performance (320 mA . cm(-2) at -150 mV vs. Hg/HgO) in 8 mol . dm(-3) KOH aqueous solution at 60 degrees C under air flow. The electr ode performances were found to be well correlated with the activities of the oxides far the 4-electron reduction of oxygen (O-2 + 2H(2)O + 4 e(-) --> 4OH(-)) evaluated from a rotating ring disk electrode analysi s. The oxidation state of Mn ions of the oxides was investigated by me ans of iodometry, X-ray photoelectron spectroscopy, temperature-progra mmed oxygen desorption and electron spin resonance. The results obtain ed suggest that the active sites for the 4-electron reduction of oxyge n involve Mn3+ ions and that their activities increase profoundly as t he Mn3+ ions neighbor Mn4+ to form Mn3+-Mn4+ pairs. The electrode load ed 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 bq X-ray diffractio n analysis after the test. The same electrode was shown to exhibit a p romising performance when incorporated into a zinc-air cell.