Intermediate Sr₂Co_1.5Fe_0.5O_6-δ Tetragonal Structure between Perovskite and Brownmillerite as a Model Catalyst with...

The generation of hydrogen in an environmentally benign way is highly essential to meet future energy demands. However, in the process of splitting water electrochemically, sluggish kinetics of the oxygen evolution reaction (OER) curtails its applicability, as it drags energy input. Herein, we synthesized Sr-Co-Fe-O oxides to optimize their OER activity by varying the Co/Fe ratio. Among them, Sr₂Co_1.5Fe_0.5O_6-δ exhibited the best OER catalytic activity in the series, with an overpotential of 318 mV at 10 mA cm^-2 and Tafel slope of 44.8 mV dec^-1. High-resolution neutron powder diffraction analysis identified an intermediate structure between the perovskite and brownmillerite, with alternating layers of disorderly orientated oxygen-deficient tetrahedra and fully stoichiometric octahedra. The unique stacking of tetrahedral and octahedral units facilitates desired interactions between the electrode surface and electrolyte. Theoretical calculations revealed that increased covalency of Co 3d and O 2p in Sr₂Co_1.5Fe_0.5O_6-δ oxide is another primary contributor to its augmented water oxidation ability. As a model for developing catalysts with such an intermediate structure, the synergetic effect of oxygen vacancy and hybridization between Co 3d and O 2p assured the Sr₂Co_1.5Fe_0.5O_6-δ oxide as a better catalyst for its enhanced OER activity.