Mixed-Cation Perovskite La0.6Ca0.4Fe0.7Ni0.3O2.9 as a Stable and Efficient Catalyst for the Oxygen Evolution Reaction

The La0.6Ca0.4Fe0.7Ni0.3O2.9 perovskite was synthesized using a modified ultrasonic spray pyrolysis technique with sorbitol as the fuel and ozone as the oxidizer, resulting in chemically homogeneous hollow spheres with a specific surface area as high as ∼15 m2 g–1. The crystal structure and the chemical composition were determined with powder X-ray diffraction, electron diffraction, aberration-corrected scanning transmission electron microscopy, energy-dispersive X-ray mapping, 57Fe Mössbauer spectroscopy, iodometric titration, and X-ray photoelectron spectroscopy. Being employed as a catalyst for the oxygen evolution reaction (OER) in 1 M NaOH, La0.6Ca0.4Fe0.7Ni0.3O2.9 demonstrates a mass activity of ∼400 A g–1oxide at 1.61 V vs RHE and a low 52 ± 2.6 mv dec–1 Tafel slope without noticeable degradation. The superior activity of La0.6Ca0.4Fe0.7Ni0.3O2.9 compared to that of undoped LaFe0.7Ni0.3O3 was rationalized by the comparison of DFT-calculated electronic structures. The Ca doping increases Ni and Fe oxidation states, enhances covalency of the Ni/Fe-O bonds, shifts the center of the O 2p band closer to the Fermi level thus decreasing formation energy of the oxygen vacancies, and activates the lattice oxygen mechanism of the OER, which enhances the catalytic activity. Yet, an optimal balance between stability and activity ensures that the thin and stable active layer of Ni-Fe (oxy)hydroxide is supported by the preserved perovskite structure.