Mechanistic insight into the synergetic catalytic effect of Pd and MnO₂ for high-performance Li–O₂ cells

Although noble metals, metal oxides and their combinations are the most studied catalysts for Li−O₂ cells, it is unclear yet about their intrinsically synergetic catalytic mechanism. Here, a hybrid nanostructure composed of two-dimensional (2D) δ-MnO₂ nanosheets and Pd nanocrystals (NCs) was prepared and investigated as a promising catalyst for Li−O₂ cells. Li−O₂ cells with the Pd/δ-MnO₂ hybrid catalyst exhibit low polarization (terminal charge/discharge voltages 4.2 V/2.58 V at 1600 mA g⁻¹), good rate capability (2400 mAh g⁻¹ at a high current density of 1600 mA g⁻¹), and long cycle life (133 cycles/247 cycles with limited capacities of 1000 mAh g⁻¹/500 mAh g⁻¹) due to the synergetic catalytic effect between δ-MnO₂ and Pd. Density functional theory (DFT) calculations clarify that the charge transfer between δ-MnO₂ and Pd underlies their synergetic catalytic mechanism, where the presence of Pd on the one side of δ-MnO₂ sheets facilitates the formation of Li₂O₂ on the opposite side of the δ-MnO₂ sheets. The DFT calculations also indicate that it is energetically possible for the formation of stable, electronically conductive LiO₂ on the δ-MnO₂ sheets with Pd on the opposite side. This work gives a mechanistic insight into the synergetic catalytic effect between noble metal and metal oxide, and provides an effective strategy for designing high-performance catalysts especially with a 2D configuration.