Deciphering the role of Fe impurities in the electrolyte boosting the OER activity of LaNiO3

Abstract

Perovskites have emerged as potential catalysts for the alkaline oxygen evolution reaction. Iron impurities in the electrolyte play an important role in enhancing the catalytic activity of Ni centres, but the nature of active sites is elusive. In this article, we report a detailed study of iron incorporation dynamics and provide direct spectroscopic evidence for surface re-construction and dynamic active site evolution in LaNiO₃ perovskite using identical location scanning transmission electron microscopy, electron energy loss spectroscopy and in situ electrochemical Raman spectroscopy. We demonstrate that the electrocatalytic activity is enhanced up to an amount of 7.5 ppm Fe traces in the electrolyte by lowering the Tafel slope from 115 to 49 mV dec⁻¹. The iron impurities in the electrolyte enter the perovskite structure, leading to the dissolution of the A-site, vacancy formation, and amorphization of surface layers. The origin of activity arises from these amorphous layers which are ∼5 nm thick and rich in Ni oxyhydroxides, Fe oxyhydroxides, and a Ni–O–Fe coordinated environment. Together with pH-dependent studies, we confirm the lattice oxygen mechanism in the presence of Fe impurities. Our work provides new insights into the design and a deeper understanding of Ni–Fe synergetics in perovskite-based catalysts for alkaline OER.