Pd@SmMn₂O₅ as an Efficient Oxygen Reduction Reaction Catalyst via Triggering the Synergistic Effect between Dual Crystal Fields in Mullite

The cooperation among different surface coordination environments is beneficial to reach a moderate interaction with the oxygen intermediates and therefore achieve an optimal electrochemical oxygen reduction reaction (ORR) activity. A facilely effective strategy is essential to regulate the electronic structure and then the ratio of Mn3+ to Mn4+ in the intrinsically strong Mn–O bonding SmMn2O5. In this work, a two-step photochemical reduction method was adopted to load the Pd nanoparticles on the SmMn2O5 nanorods to form an atomic interface contact. The optimized catalyst 7.5 wt % Pd@SmMn2O5 shows an excellent activity with the half-wave potential 0.83 V (vs RHE) and the charge-transfer resistance ∼38 Ω smaller than that of commercial platinum. The electrons transferring from Pd to p-type SmMn2O5 at the interface contribute to the moderate d-band center and Mn valence and then the neither too strong nor too weak interaction with oxygen intermediates. The accumulated electrons on the conduction band of mullite occupy the anti-bonding states of oxygen in mullite and then activate more oxygen, which favors the labile oxygen participant adsorbate evolving mechanism (LAM) for the ORR process. The assembled Zn–air battery exhibits a high peak power density of ∼236 mW/cm2 and a large open-circuit potential of ∼1.43 V. This work provides insights into the activity optimization of the intrinsically stable catalysts from the aspect of the metal–semiconductor interface.