Ad hoc MnO2-based cathode electrocatalysts for rechargeable metal-air batteries

Due to the remarkably high theoretical energy and light weight, metal–air batteries represent one class of promising power sources for applications in next-generation electronics, electrified transportation and energy storage of smart grids. The most prominent feature of a metal–air battery is the combination of a metal anode with high energy density and an air electrode with open structure to draw cathode active materials (i.e. oxygen) from air [1]. In particular, the air electrode is desired to be bifunctional, i.e. it should be used for both the Oxygen Reduction Reaction (ORR) and the Oxygen Evolution Reaction (OER) [2]. Gas Diffusion Electrodes (GDEs), made of a two-layer or a three-layer structure, are widely used as cathode in metal-air devices [3]. However, one of the main drawbacks related to the cathodic reaction (ORR) is the overpotential loss (about 0.3-0.4 V) under operation condition. Thus, lots of efforts were undertaken to inhibit the voltage loss requiring an effective ORR catalyst [1,4]. One of the most promising materials, in terms of both 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 [5,6]. In the present work, the electrochemical performances of different GDEs are evaluated by using several electrochemical analyses, such as 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, TEM, 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 (i.e. TEGDME). Hence, they may represent a promising candidate for metal-air batteries.