Tailored MnO2 nanoparticles as cathode electrocatalysts for rechargeable Li-air batteries

Because of their high theoretical specific energy, metal-air batteries have attracted much attention as next generation high capacity batteries. In order to achieve a compact cell and a high energy density, the air electrode used is desired to be bifunctional: the same electrode should be used for both the Oxygen Reduction Reaction (ORR) and the Oxygen Evolution Reaction (OER). Gas Diffusion Electrodes (GDEs), made of a two-layer or a three-layer structure, are used as cathode electrodes in Li-air batteries. However, one of the main problems for the cathodic reaction (ORR) is the overpotential loss (about 0.3-0.4 V) under operation condition. Lots of efforts were undertaken to inhibit the voltage loss requiring an effective ORR catalyst. The most promising material, in terms of performances and costs, seems to be manganese dioxide, MnO2. According to the literature, MnO2 would ensure capacities comparable to those of platinum, letting higher capacity retention to be reached even in the presence of non-aqueous electrolytes, widely used to prevent Li decomposition. In the present work, the electrochemical performances of different GDEs for oxygen reduction are evaluated by using Linear Sweep Voltammetries (LSVs). Different kinds of bare and doped MnO2 electrocatalytic nanopowders are synthesized through hydrothermal methods. The crystal structure and the surface properties of the present materials are examined by means of XRPD, BET, SEM/EDX and XPS analyses. Correlations between the physico-chemical characteristics of MnO2 employed and the final electrical GDE performances are drawn. Experimental results reveal that the air-cathode electrodes have excellent electrochemical properties both in aqueous and organic electrolytes. Thus these GDEs may represent a highly potential candidate for Li-air batteries.