Activating the PdN₄ single-atom sites for 4 electron oxygen reduction reaction via axial oxygen ligand modification

The absence of Pd ensemble sites and weak affinity for O-intermediates in tetracoordinate planar PdN₄ sites inhibit O-O bond dissociation and restrict 4e^- oxygen reduction reaction (ORR) pathway. Optimizing the adsorption energy between O-intermediates and PdN₄ sites is a rational approach for selectively promoting 4e^- ORR electrocatalysis. Herein, we propose a molten-salt strategy for introducing axial oxygen ligand into the PdN₄ sites to manipulate the electronic structure and create catalytic O-PdN₄ sites (one subsurface axial Pd-O configuration on PdN₄ plane) on N-doped graphene nanosheets (O-PdN₄-NGS). Density functional theory calculations revealed that the axial Pd-O coordination can induce charge redistribution, downshift the d-band center of Pd and enhance the adsorption affinity of PdN₄ site for O-intermediates, thereby leading to superior electrocatalysis activity and selectivity for 4e^- ORR process compared to conventional PdN₄ sites in 0.1 M KOH (with a positive half-wave potential E_1/2 of 0.90 V vs RHE). Moreover, zinc-air battery featuring with O-PdN₄ sites delivers a peak power density of 178 mW cm⁻² at 227.5 mA cm⁻² and stable long-term cycling performance, effectively showcasing the advantages of the axial O ligand modification in practical application. This work provides a constructive strategy to activate the reaction activity and specificity of PdN₄ single-atom sites by precisely-tuning the local coordination environment of Pd.