Tailored brownmillerite oxide catalyst with multiple electronic functionalities enables ultrafast water oxidation

A sluggish oxygen evolution reaction (OER) is a central issue for many chemical and energy transformation technologies, necessitating the development of cost-effective electrocatalysts to accelerate the reaction rate. A recent study has demonstrated that the intrinsic activity of catalysts is strongly associated with diverse electronic structure parameters; therefore, designing an effective strategy to optimize electronic structures in multiple aspects is highly attractive for realizing high-performance OER catalysts. Here, we report a facile A/B-site cosubstitution strategy to regulate the electronic structures of oxygen-deficient brownmillerite oxides for optimizing the OER performance. Cosubstitution of strontium and cobalt into Ca2Fe2O5 parent oxide is found to be very effective in enhancing the OER catalytic performance. Combined soft X-ray absorption spectroscopy analysis and density functional theory calculations show that optimized CaSrCoFeO6-? (CSCF) is endowed with multiple advantageous electronic structure features for OER catalysis including favorable valence, orbital, and spin states of metal ions and an upshifted O 2p-band center as well as high metal-oxygen covalency. Benefiting from these ideal electronic structure parameters, CSCF exhibits ultrahigh OER activity with a low overpotential of only 330 mV at 10 mA cm-2 in 0.1 M KOH, outperforming noble metal RuO2 and various state-of-the-art metal oxides ever reported while maintaining robust stability as evidenced by operando spectroscopy.