A′-B Intersite Cooperation-Enhanced Water Splitting in Quadruple Perovskite Oxide CaCu3Ir4O12

Developing highly efficient electrochemical catalysts and exploring the basic mechanisms for the oxygen evolution reaction (OER) are key issues for the large-scale commercialization of environmentally friendly electrolytic hydrogen energy. Compared with a simple ABO(3) perovskite, the A-site-ordered quadruple structure AA'3B4O12 shows enhanced OER activity, but the underlying mechanisms remain unknown. Herein, we find that the quadruple perovskite oxide CaCu3Ir4O12 has stable and superior electrochemical activity with a very low overpotential of 252 mV to achieve the current density of 10 mA.cm(-2) in alkaline solution. Operando X-ray absorption spectroscopy reveals that the B-site Ir is an OER active site with a variable valence state from the initial Ir4+ approach to Ir5+, while the A'-site Cu is inactive with a constant valence state during the OER process. Density functional theory calculations demonstrate that the A'-B intersite cooperation synergistically enhances OER activity via the corner-sharing Cu-O-Ir framework owing to the strong 3d-2p-5d orbital hybridizations, regardless of the inactive Cu site. In the structural constitution of CaCu3Ir4O12, a small Cu-O-Ir bond angle (110.7 degrees) forms. The special orbital symmetry as well as the delicate 3d-5d levels enhance the orbital overlap and therefore promote the charge transfer, favoring the superior OER activity of CaCu3Ir4O12.