Alkaline Stability of LaBO3 (B = Co, Ni, Mn, and Fe) Perovskites and Their Application as Bifunctional Oxygen Electrocatalysts for Electrochemical Devices.

Transition metal perovskites and derived compositions have long been suggested as electrocatalysts for the oxygen electrochemical reactions in alkaline media. This paper presents a study of the alkaline stability of LaBO3 perovskites (B = Co, Ni, Mn, and Fe) by exposing powdered samples to a 2M NaOH solution with pH > 14 for variable periods of time. The elemental analysis of the supernatant test solution reveals the presence of Fe only after 48 h reaction time, whereas Co is apparent within the first 12 h. Ni and Mn are detected after 24 h in quantities like those obtained for Co after 12 h. These data suggest that the initial dissolution is primarily determined by the nature of the transition metal, in excellent agreement with calculated Pourbaix equilibrium diagrams. The analysis of the powders by electron microscopy combined with energy-dispersive spectroscopy and X-ray photoelectron spectroscopy confirms the underlying compositional changes, which are particularly important on the surface of the particles, where the transition metal cations tend to be depleted, thereby forming lanthanum-enriched regions. It is concluded from these tests that the chemical stability increases in the series Co < Ni < Mn < Fe. A degradation of kinetic parameters for oxygen reduction and evolution reactions (ORR and OER), assessed by linear scanning voltammetry and time-dependent galvanostatic measurements, is observed with variable magnitude depending on the transition metal, following the same compositional trend of the chemical stability series. The effect of the transition metal on ORR and OER performance is well explained by the eg orbital occupation model. The dissolution of these types of materials in strong alkaline media, potentially aggravated in compositions where lanthanum is partly substituted by alkaline earths, underlies complex compositional changes that may determine the performance and stability of the incorporating device, e.g., fuel cells, metal-air batteries, electrolyzers, or supercapacitors.