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.