Ad hoc tailored electrocatalytic MnO2 nanorods for the oxygen reduction in aqueous and organic media

Abstract Metal-air batteries are one of the most promising electrochemical systems for energy storage and conversion. Herein we report promising results by exploiting manganese dioxide nanoparticles as ORR electrocatalysts. MnO 2 nanorods were prepared through a hydrothermal synthesis, i.e. by varying both the salt precursors ( i.e. manganese sulphate or chloride) and the oxidizing agents ( i.e. ammonium persulfate or potassium permanganate). All the nanopowders were finely characterized on structural, morphological and surface points of view. Then, their electrocatalytic power was tested either in aqueous 0.1 M potassium hydroxide or in Tetra Ethylene Glycol Dimethyl Ether (TEGDME)/LiNO 3 0.5 M electrolytes, by using Gas Diffusion Electrodes (GDEs) and Glassy Carbon (GC) as cathodes, respectively. All the nanoparticles promoted the ORR by causing a shift of the onset potential up to 100 mV in both solvents. Nevertheless, this shift was different according to the solvent/electrolyte used: in the case of the ether-based solvent, different values are obtained by adopting the synthesized MnO 2 powders. Thus, we hypothesized that the structural/surface properties of MnO 2 samples are leveled in the aqueous medium ( i.e. in a OH rich solvent, the hydroxyls can interact with the homologs on the MnO 2 surface), contrary to what occurs in the organic solvent. Furthermore, a different behavior was observed also on the kinetic point of view thus leading to diverse interpretations of the oxygen reduction mechanism, especially in TEGDME.