The Reduction of Oxygen on Teflon‐Bonded Perovskite Oxide Electrodes

Abstract

The electrochemical reduction of oxygen on Teflon-bonded Nd0.sSr0.sCoO3 electrodes in 45% KOH was studied as a function of temperature and oxygen partial pressure. The activation energy was 10.75 kcal/mole and the relationship between i and Po2 obeys the relationship d In i/d In Poe = ~, suggesting that the rate of oxygen chemisorption is the rate-controlling step, Galvanostatic oxygen stripping showed that the surface coverage of oxygen was only 1% for both Sr-doped LaCoO3 and NdCoOs, and that the coverage was independent of temperature in the range 25~176 Earlier work by Tseung and Bevan (1) shows that Sr-doped LaCoO3 exhibits reversible behavior toward oxygen reduction in 45% KOH solution at room temperature. The performance at room temperature is low ~2 mA/cm 2, 500 mV vs. DHE (Dynamic Hydrogen Electrode) but it increases linearly with temperature and surpasses that of platinum black at 170~ (250 mA/cm 2 as compared to 180 mA/cm 2 for Pt black, 75% KOH, 900 mV vs. DHE). It is suggested that the performance is controlled by the number of Co3+/Co 4+ couples free to interact with the oxygen molecule and that the numbers of such couples are low at 25~ but increase in number as the temperature rises. Studies by Kudo et al. (2, 3) on Lal-xSrxCoOa and Ndl-xSrzCoO3 (x = 0.5) suggest that the electrochemical reduction of 02 on these oxides proceed via two parallel routes I/z 02 + H20 -4-2e = 2 OH-They also suggest that the relatively low performance at room temperature is due to the slow rate of oxygen ion diffusion in the lattice. They reach this conclusion mainly from cyclic voltammetric studies on Teflonbonded La0.sSr0.sCoOa electrodes. When the potential is swept from an anodic potential more positive than the reversible oxygen electrode potential to the cathodic region, the cathodic current increases almost linearly with decreasing potential until an overvoltage of 400 mV is reached. Thereafter, the cathodic current decreases and in the reverse sweep the cathodic current is markedly lower. They claim that this shows that oxide is being reduced and that the limiting step is the rate of oxygen ion diffusion in the oxide lattice. This paper presents further cyclic voltammetric and pulse measuremnts on the reduction of oxygen on perovskite oxides. ExperimentalLa0.sSr0.sCoO3 and Nd0.sSr0.sCoO8 were prepared by freeze drying of the mixed nitrate solutions (1), followed by vacuum decomposition at 250~ for 5 hr. The powders were then heated in air at 600~ for 5 hr. The formation of the perovskite phase was confirmed by x-ray powder diffraction, though it is not certain whether all the material had fully reacted since the lower limit for x-ray detection is only about 5%. The BET surface area for Lao.sSr0.~CoO3 was 17 M2/g and Nd0.sSr0.sCoOa was 20 M2/g. The powders were fabricated into Teflon-bonded electrodes on 100 mesh Pt screens (5). The catalyst: Teflon ratio was fixed at 10:3. The electrodes were tested in floating half-cells (6) using a dynamic hydrogen reference electrode (DHE). A piece of gold foil was used as th...

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