RuO₂ nanoparticles supported on MnO₂ nanorods as high efficient bifunctional electrocatalyst of lithium-oxygen battery

Highlights: • The np-RuO₂/nr-MnO₂ were firstly synthesized via a two-step hydrothermal reaction. • The np-RuO₂/nr-MnO₂ as cathode of Li–O₂ battery exhibits high bifunctional electrocatalytic activity. • In-situ synchrotron HEXRD illustrated the formation process of Li₂O₂. RuO₂ nanoparticles supported on MnO₂ nanorods (denoted as np-RuO₂/nr-MnO₂) were synthesized via a two-step hydrothermal reaction. SEM and TEM images both illustrated that RuO₂ nanoparticles are well dispersed on the surface of MnO₂ nanorods in the as-prepared np-RuO₂/nr-MnO₂ material. Electrochemical results demonstrated that the np-RuO₂/nr-MnO₂ as oxygen cathode of Li–O₂ batteries could maintain a reversible capacity of 500 mA h g⁻¹ within 75 cycles at a rate of 50 mA g⁻¹, and a higher capacity of 4000 mA h g⁻¹ within 20 cycles at a rate as high as 200 mA g⁻¹. Moreover, the cell with the np-RuO₂/nr-MnO₂ catalyst presented much lower voltage polarization (about 0.58 V at a rate of 50 mA g⁻¹) than that measured with only MnO₂ nanorods during charge/discharge processes. The catalytic property of the np-RuO₂/nr-MnO₂ and MnO₂ nanorods were further compared by conducting studies of using rotating disk electrode (RDE), chronoamperommetry and linear sweep voltammetry. The results illustrated that the np-RuO₂/nr-MnO₂ exhibited excellent bifunctional electrocatalytic activities towards both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Furthermore, in-situ high-energy X-ray diffraction was employed to trace evolution of species on the np-RuO₂/nr-MnO₂ cathode during the discharge processes. In-situ XRD patterns demonstrated the formation process of the discharge products that consisted of mainly Li₂O₂. Ex-situ SEM images were recorded to investigate the morphology and decomposition of the sphere-like Li₂O₂, which could be observed clearly after discharge process, while are decomposed almost after charge process. The excellent electrochemical performances of the np-RuO₂/nr-MnO₂ as cathode of Li–O₂ battery could be contributed to the excellent bifunctional electrocatalytic activities for both the ORR and OER, and to the one-dimensional structure which would benefit the diffusion of oxygen and the storage of Li₂O₂ in the discharge process of Li–O₂ battery.