Self-Terminated Surface Reconstruction of Lanthanum Nickelates Promotes Alkaline Oxygen Evolution

Abstract

Oxygen evolution reaction (OER) itself is expected to have a strong impact on the state of the surface of the electrocatalyst, yet the actual structure of the surface under operating conditions is rarely considered while formulating structure-activity correlations in electrocatalysis. In this study, changes in the surface structure of Ruddlesden-Popper-type lanthanum nickelate, La2NiO4+δ, and the perovskite LaNiO3 are probed and contrasted using a series of complementary surface sensitive techniques, including XPS, HRTEM and O K-edge XAS. We found noticeable surface changes to occur on La2NiO4+δ after exposure its exposure to the alkaline electrolyte that is accompanied by the emergence of a strong Nin+/n+1 redox feature and the coincides with a 45-fold enhancement of the OER activity (at η = 370 mV) in 0.1 M KOH, as compared to the pristine La2NiO4+δ phase. We observe that the in situ formation of an amorphous nickel (oxy)hydroxide layer on the surface increases the electrochemically-active surface area of La2NiO4+δ and imparts a high stability of the catalyst under OER conditions (the formation of the amorphous layer self-terminates at ca. 3 nm). In addition, the transition from octahedral Ni sites (O h ) to square-planar Ni sites (D 4h ) in the reconstructed surface was observed using O K-edge X-ray absorption spectroscopy. Further, we found no evidence for any surface transformations in the perovskite-based catalyst LaNiO3 that also did not demonstrate any improvement in its activity over cycling. This work highlights the importance of a detailed surface analysis of the catalysts under operation to build robust structure-activity correlations for the metal oxides under electrochemical reaction conditions. Figure 1