Evaluating the electronic structure and stability of epitaxially grown Sr-doped LaFeO3 perovskite alkaline O2 evolution model electrocatalysts

Abstract

In this work, we have investigated the relationships between surface stability, electronic structure and O₂ evolution reaction (OER) activity for epitaxial thin film La_1−xSr_xFeO₃ (x = 0, 0.33, 0.8) model electrocatalysts before and after different electrochemical treatments. Cyclic voltammetry (CV) between +1.22 V and +1.92 V vs. RHE results in the continuous enhancement of OER performance of LaFeO₃, while for La_0.67Sr_0.33FeO₃ and La_0.2Sr_0.8FeO₃ a gradual decrease of OER performance with increasing number of CV cycles was observed. A combination of atomic force microscopy, X-ray diffraction and X-ray reflectivity reveals that the surfaces of La_1−xSr_xFeO₃ (x = 0, 0.33, 0.8) undergo surface morphology changes during OER treatment. Synchrotron ex situ X-ray photoemission spectroscopy data show a gradual down-shift of the Fermi level (E_F) of LaFeO₃ with increasing number of CV cycles, while near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the Fe L-edge and O K-edge shows the presence of surface Fe⁴⁺ species as well as new hole states near the conduction band minimum upon electrochemical treatment, leading to a further enhancement of the electrochemical activity of LaFeO₃. The newly formed hole state in LaFeO₃ that appeared after 3 CV cycles remained constant upon progressing OER treatment. On the contrary, the decrease of OER performance of La_0.67Sr_0.33FeO₃ and La_0.2Sr_0.8FeO₃ with increasing CV cycles is attributed to an up-shift of E_F along with a decrease of Fe⁴⁺ and hole state content after OER treatment. Furthermore, we found that the stability of the OER performance of La_1−xSr_xFeO₃ is closely related to the leaching of Sr during OER, and the stability deteriorates with increasing Sr doping concentration in the pristine samples.