Boosting Oxygen Evolution Reaction by Creating Both Metal Ion and Lattice-Oxygen Active Sites in a Complex Oxide

Developing efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance to many chemical and energy transformation technologies. The diversity and flexibility of metal oxides offer numerous degrees of freedom for enhancing catalytic activity by tailoring their physicochemical properties, but the active site of current metal oxides for OER is still limited to either metal ions or lattice oxygen. Here, a new complex oxide with unique hexagonal structure consisting of one honeycomb-like network, Ba₄ Sr₄ (Co_0.8 Fe_0.2 )₄ O₁₅ (hex-BSCF), is reported, demonstrating ultrahigh OER activity because both the tetrahedral Co ions and the octahedral oxygen ions on the surface are active, as confirmed by combined X-ray absorption spectroscopy analysis and theoretical calculations. The bulk hex-BSCF material synthesized by the facile and scalable sol-gel method achieves 10 mA cm^-2 at a low overpotential of only 340 mV (and small Tafel slope of 47 mV dec^-1 ) in 0.1 m KOH, surpassing most metal oxides ever reported for OER, while maintaining excellent durability. This study opens up a new avenue to dramatically enhancing catalytic activity of metal oxides for other applications through rational design of structures with multiple active sites.