K2NiF4 type oxides, Ln2-xSrxNiO4+δ (Ln = La and Pr; x = 0–1.4) as an oxygen electrocatalyst for aqueous lithium–oxygen rechargeable batteries

Abstract

The structural and electrochemical characteristics of the Ln2-xSrxNiO4+δ (Ln = La and Pr; x = 0–1.4) system were investigated to clarify the relationship between the electrocatalytic activity and various characteristics such as the structure, composition, electrical conductivity, and oxygen content. Rietveld analysis of the powder X-ray diffraction (XRD) data showed that the tetragonal distortion of the NiO6 octahedra decreased monotonically with an increase of x, while the Ln (Sr)–O (//c-axis) in the rock-salt block was expanded with an increase in x, although the tetragonality, c/a showed a maximum at x = 0.6. Cyclic voltammetry (CV) measurements showed that the activity for the oxygen evolution reaction (OER) increased with x, while that of the oxygen reduction reaction (ORR) was independent of x. The characteristic peak due to oxygen insertion into the lattice was observed at ca. 1.45 V vs. RHE, the intensity of which increased with the Sr content, which reflects the increase in the internal chemical diffusion of oxygen species. X-ray photoelectron spectroscopy (XPS) measurements for the constituent elements confirmed the presence of OH− groups inside the catalyst particles, especially in the Sr-rich region. It is suggested that the reversible intercalation ability of the OH− group into the rock-salt layer in the Ruddlesden-Popper (RP) phase (An+1BnO3n+1) contributes significantly to the improvement of the OER activity.