Overpotential-Induced Introduction of Oxygen Vacancy in La_0.67Sr_0.33MnO₃ Surface and Its Impact on Oxygen Reduction Reaction Catalytic Activity in Alkaline Solution

Abstract

Oxygen reduction reaction (ORR) catalytic activity of La0.67Sr0.33MnO3 epitaxial thin films was investigated in a KOH solution by using a rotating-disk electrode. We found that while the films exhibit ORR current, the current is not limited by oxygen transport resulting from the film electrode rotation and shows the large hysteresis against the potential sweep direction. This behavior is in stark contrast to the oxygen reduction reaction activity of an electrode ink made from LSMO bulk powder, whose ORR current is oxygen-transport limited. In situ synchrotron X-ray absorption spectroscopy also reveals that the valence state of Mn in the LSMO film surface is lowered under the reducing atmosphere caused by the overpotential. This indicates the overpotential-induced introduction of oxygen vacancies in the film surface. We also show that the ORR current of the LSMO films exposed to the reducing atmosphere is lowered than that of the original surface. These results indicate that the ORR catalytic activity of LSMO surfaces is strongly influenced by oxygen vacancies.

Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

“…All Mn-containing perovskites showed catalytic activity but the weak dependence on rotational speed in RDE measurements [127,128] suggests that oxygen is not directly reduced to water on these surfaces, which is in agreement with microkinetic analysis of composite electrodes [97]. LaCoO 3 [60,129] [96,131] showed no activity but could be activated by addition of carbon [96].…”

“…Direct evidence of the active valence by X-ray based methods is rare for the ORR in alkaline media. A notable exception is the recent XAS study at the Mn-K edge of Kan et al [127] on (001)-oriented La 0.67 Sr 0.33 MnO 3 in 0.1 M KOH, where Mn reduction of the thin film surface was found at 0.7 and 0.5 V vs. RHE relative to the film at open-circuit. While the spectra clearly support reduction, unfortunately, the analysis was not performed quantitatively.…”

“…While the total ORR current of the simulation is sigmoidal as frequently observed in experiments, the individual contributions have local minima between 0.5 and 0.6 V vs. RHE and diverge at lower voltages (higher overpotential). The ORR has also been studied on rotated carbon-free crystalline films of La 1−x Sr x MnO 3 [127,128] [96,128]. Cyclic voltammetry (CV) of these films does not show the expected sigmoidal shape and saturation currents in contrast to comparable measurements of composite electrodes containing these perovskites mixed with carbon [96].…”

Peroxide Yield of the (001) La0.6Sr0.4MnO3 Surface as a Bifunctional Electrocatalyst for the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Alkaline Media

“…The combination of these properties makes the La 1–x Sr x MnO 3±δ perovskite system an excellent candidate material for the oxygen electrode of a metal‐air or metal‐O 2 cell. Single crystalline and polycrystalline La 1–x Sr x MnO 3 films with x between 0.0 and 0.7 have been previously studied for the oxygen reduction, and oxygen evolution,, reactions or both . However, the reproducibility of the CV traces and the stability of the material during the harsh conditions of both reactions must also be considered to realize practical devices.…”

“…In contrast to powders, the limiting current was not reached, and the rotation rate had little influence on the measured currents. Kan et al . designed a holder to rotate epitaxial (001)‐oriented La 0.67 Sr 0.33 MnO 3 thin films on conventional square substrates, yet their film behaved similar to that of Miyahara et al .…”