(Invited) Oxygen Evolution and Reduction on La0.5Sr0.5Co0.8Fe0.2O3- Δ Perovskites with Tunable Structural Features

Abstract

Oxygen evolution (OER) and oxygen reduction (ORR) reactions are the fundamental processes occurring at the air electrode of metal-air batteries and fuel cells. Presently, large overpotentials are required for these reactions to occur at an appreciable rate. Therefore, development of catalysts capable of reducing the activation losses at the oxygen electrode is highly desirable. Perovskite oxides are one of the most interesting low-cost electrocatalysts for air electrodes, because they can adopt a wide variety of chemical compositions and crystal structures, thereby enabling correlative studies between the electrocatalytic activity and their electronic structure (1, 2). Recently, we reported a possible relation between the cell volume of La0.6Sr0.4Fe0.6Mn0.4O3-δ perovskites and their ORR activity and selectivity (2 e- vs ‘’2+2’’ e- pathway), with lower cell volumes corresponding to higher number of exchanged electrons (3). It was also reported by others that some structural features in the perovskite influence the activity of these mixed oxides for both ORR and OER (4-6). In this presentation, we will discuss our recent studies on the ORR and OER activity of a series of La0.5Sr0.5Co0.8Fe0.2O3- δ electrocatalysts prepared by solution combustion synthesis. The choice of diverse experimental conditions allowed us to tune oxygen deficiency, microstrain and other structural properties. As exemplified in Figure 1, for one of the investigated electrocatalysts, thermal annealing in argon reduces the bond angle between the oxygen atom and the two B-site metal cations, resulting in a higher electrocatalytic activity for both OER and ORR. In addition to structure–electrocatalytic activity correlations, we will complement our comparative studies with analytical scanning electron microscopy and spatially resolved elemental mapping of selected electrode surfaces subjected to accelerated stability tests. Figure 1. Crystal structures of La0.5Sr0.5Co0.8Fe0.2O3- δ prepared from solution combustion using citrate as fuel and thermal annealed in Ar (top right) and in Air (bottom left). Differences in the Co-O-Co bond angle are highlighted. Oxygen evolution (OER, top left) and oxygen reduction (ORR, bottom right) polarization curves recorded in 0.1 M KOH at 5 mVs-1 and 1600 rpm.