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

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-O-2 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-O-2 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-O-2 battery. (C) 2016 Elsevier Ltd. All rights reserved.