Axial Ligand-Modified PdN4 as Efficient Electrocatalysts for the Two-Electron Oxygen Reduction Reaction: Insights from DFT

Unlocking efficient and nontoxic electrocatalysts for the two-electron oxygen reduction reaction (2e– ORR) is essential for advancing clean energy technologies. The 2e– ORR is crucial for applications such as fuel cells and metal-air batteries, highlighting the need for effective catalysts. Palladium-based single-atom catalysts are widely used in various reactions due to their high activity, selectivity, cost-effectiveness, and stability. However, the tetracoordinate planar PdN4 complex struggles with low O2 affinity, limiting O–O bond cleavage and 2e– ORR performance. Herein, density functional theory was employed to explore the activity of PdN4 modified with various axial ligands. Our findings reveal that adding the axial ligand OH significantly redistributes charge in the catalyst, leading to a downward shift in the d-band center of PdN4–OH. This modification facilitates the desorption of intermediate *OOH from the surface of PdN4–OH, thereby optimizing the 2e– ORR selectivity. Projected density of states was calculated to determine the impact of coordination between the axial ligand and the metal on the 4d orbital of Pd and 2p orbital of O2 hybridization following O2 adsorption on the catalyst surface. This modification enhances cleavage of the O–O bond during the initial stage of the 2e– ORR, endowing it with remarkably high catalytic activity demonstrated by PdN4–OH with an overpotential as low as 0.02 V. This work offers insights into controlling the local environment around Pd atom through axial ligands, improving catalytic performance, and reducing deactivation. The findings have important implications for the design of advanced catalysts for clean energy applications and for understanding catalytic processes at the atomic level.