Tuning synergy between nickel and iron in Ruddlesden–Popper perovskites through controllable crystal dimensionalities towards enhanced oxygen-evolving activity and stability

Abstract

Ni–Fe-based oxides are among the most promising catalysts developed to date for the bottleneck oxygen evolution reaction (OER) in water electrolysis. However, understanding and mastering the synergy of Ni and Fe remain challenging. Herein, we report that the synergy between Ni and Fe can be tailored by crystal dimensionality of Ni, Fe-contained Ruddlesden–Popper (RP)-type perovskites (La0.125Sr0.875)n+1(Ni0.25Fe0.75)nO3n+1 (n = 1, 2, 3), where the material with n = 3 shows the best OER performance in alkaline media. Soft X-ray absorption spectroscopy spectra before and after OER reveal that the material with n = 3 shows enhanced Ni/Fe–O covalency to boost the electron transfer as compared to those with n = 1 and n = 2. Further experimental investigations demonstrate that the Fe ion is the active site and the Ni ion is the stable site in this system, where such unique synergy reaches the optimum at n = 3. Besides, as n increases, the proportion of unstable rock-salt layers accordingly decreases and the leaching of ions (especially Sr2+) into the electrolyte is suppressed, which induces a decrease in the leaching of active Fe ions, ultimately leading to enhanced stability. This work provides a new avenue for rational catalyst design through the dimensional strategy.