Importing Antibonding-Orbital Occupancy through Pd-O-Gd Bridge Promotes Electrocatalytic Oxygen Reduction

The active-site density, intrinsic activity, and durability of Pd-based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd₂ O₃ , Sm₂ O₃ , Eu₂ O₃ , and CeO₂ ) by using RE metal-organic frameworks to tune the ORR performance of the Pd sites through the Pd-RE_x O_y interface interaction. Taking Pd-Gd₂ O₃ /C as a representative, it is identified that the strong coupling between Pd and Gd₂ O₃ induces the formation of the Pd-O-Gd bridge, which triggers charge redistribution of Pd and Gd₂ O₃ . The screened Pd-Gd₂ O₃ /C exhibits impressive ORR performance with high onset potential (0.986 V_RHE ), half-wave potential (0.877 V_RHE ), and excellent stability. Similar ORR results are also found for Pd-Sm₂ O₃ /C, Pd-Eu₂ O₃ /C, and Pd-CeO₂ /C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd₂ O₃ promotes electron transfer through the Pd-O-Gd bridge, which induces the antibonding-orbital occupancy of Pd-*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd-Gd₂ O₃ is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd-Gd₂ O₃ /C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.