α-MnO2 nanoneedle-based hollow microspheres coated with Pd nanoparticles as a novel catalyst for rechargeable lithium-air batteries

The hollow alpha-MnO2 nanoneedle-based microspheres coated with Pd nanoparticles were reported as a novel catalyst for rechargeable lithium-air batteries. The hollow microspheres are composed of alpha-MnO2 nanoneedles. Pd nanoparticles are deposited on the hollow microspheres through an aqueous-solution reduction of PdCl2 with NaBH4 at room temperature. The results of TEM, XRD, and EDS show that the Pd nanoparticles are coated on the surface of alpha-MnO2 nanoneedles uniformly and the mass fraction of Pd in the Pd-coated alpha-MnO2 catalyst is about 8.88%. Compared with the counterpart of the hollow alpha-MnO2 catalyst, the hollow Pd-coated alpha-MnO2 catalyst improves the energy conversion efficiency and the charge-discharge cycling performance of the air electrode. The initial specific discharge capacity of an air electrode composed of Super P carbon and the as-prepared Pd-coated alpha-MnO2 catalyst is 1220 mA.h/g (based on the total electrode mass) at a current density of 0.1 mA/cm(2), and the capacity retention rate is about 47.3% after 13 charge-discharge cycles. The results of charge-discharge cycling tests demonstrate that this novel Pd-coated alpha-MnO2 catalyst with a hierarchical core-shell structure is a promising catalyst for the lithium-air battery.