Bare and titanium-doped manganese dioxide nanoparticles : their pivotal role in energetic and sensoristic applications

The most prominent feature of metal-air batteries is the combination of a metal anode and an air electrode with open structure to draw cathode active materials from air. Oxygen reduction is the main cathodic process, but unfortunately it is affected by overpotential loss under operation conditions. Thus, an effective catalyst is required. Owing to their structural flexibility and versatility, manganese oxides have been widely applied in several research fields. Since MnO2 possesses quite a lot polymorphic phases, its controlled synthesis is pivotal. Hence, this Thesis work was focused on i) the hydrothermal synthesis of bare and novel Ti-doped MnO2 nanopowders, ii) their deep physico-chemical characterization and iii) their application as electrocatalysts for the Oxygen Reduction Reaction (ORR). In the synthetic route, the oxidizing agent was varied (i.e. (NH4)2S2O8 for samples labelled as MH_N and KMnO4 for MH_K ones) to study the correlation between the physico-chemical properties and the electrochemical performances of the nanopowders. Indeed, it has been possible to observe that both the oxidants cations and Ti-dopant ions play a pivotal role in modifying the nanopowders structural, morphological and surface properties. For the electrochemical tests, Linear Sweep Voltammetries (LSVs) have been carried out in KOH (-1.0 – 0.0 V vs SCE). Gas Diffusion Electrodes (GDEs), prepared by adding the synthesized uncalcined MnO2 to the air-cathode slurry, have performed explicitly better than GDEs made of only carbonaceous matrixes, proving synthesized MnO2 to be good electrocatalysts for the ORR (4). Furthermore, GDE(MH_K) seems to have less diffusive limitations probably due to the much higher O2 permeability, which is connected to the greater MH_K pore volume (confirmed by BET analyses). On the kinetic point of view (Tafel elaborations), high values of exchange current densities have been determined for GDEs with Ti-doped nanopowders. In particular, the presence of 5% Ti-doped MH_N has led to an increase of almost six orders of magnitude. Parallel to the above investigation, a preliminary forefront study about the use of MH_K sample (chosen for its highest BET surface area) for CHCl3 detection was developed. Thus, Glassy Carbon Electrodes (GCEs) were modified by drop casting of a suspension of the adopted MnO2 in DMF (20 μL, 0.5 g cm-3). Contrary to the response obtained with only GCE (no peaks evidence), the presence of MnO2 (at neutral pH) has caused the appearance of two peaks in the Cyclic Voltammetry (CV) anodic scan (ascribable to two characteristic reactions of MnO2) and a further broad peak in the cathodic scan (at -0.5 V vs SCE) that can be due to the reduction of O2. The presence of chloroform has led to a linear decreasing of the specific CV peaks currents, because of the adsorption of the pollutant molecules onto the electrode surface (indirect detection method). In conclusion, the present Thesis work focused on the deep investigation of bare and Ti-doped MnO2 electrocatalytic nanoparticles showing novel and promising results.