Hierarchical Ru nanospheres as highly effective cathode catalysts for Li-O2 batteries
Carbon-free, hierarchical Ru nanospheres are reported as an effective cathode catalyst for Li-O2 batteries. The hierarchical Ru nanospheres were synthesized by a simple one-step hydrothermal method. As evaluated by electrochemical measurements, it was found that the carbon-free hierarchical Ru nanosphere cathode can significantly reduce discharge and charge overpotential (approx. 0.3 V with the capacity limited to 1000 mA h g-1) with a high discharge capacity of 3445 mA h g-1 at a current density of 200 mA g-1 (ORR). This superior oxygen evolution reaction (OER) and oxygen reduction reaction catalytic performance can be maintained over long time cycling at different current densities. Ex situ scanning electron microscopy, X-ray diffraction, and Raman measurements reveal that leaf-shaped Li2O2 is the only reaction product during the discharge process, and the hierarchical Ru nanospheres can effectively decompose the Li2O2 discharge product, facilitate the OER, and promote a high round-trip efficiency. Therefore, the carbon-free hierarchical Ru nanospheres are a promising cathode catalyst for rechargeable Li-O2 batteries with low charge overpotential, long cycle life, and high specific capacity.