Oxygen Reduction Properties of Bifunctional α-Manganese Oxide Electrocatalysts in Aqueous and Organic Electrolytes

Several different preparation and morphologies of α-manganese oxide catalysts were synthesized, and their physical properties were characterized. These catalysts were also characterized by electrochemical means (RDV and CV) for oxygen reduction in both aqueous and organic electrolytes. The solvent-free preparation yielded catalysts with ideal physical properties: low average oxidation state (3.73) of manganese, small crystallite size (8.1 nm), small particle size ( d = 10 nm, l = 30–100 nm), high surface area (144 m 2 g –1 ), and pore volume (0.470 cm 3 /g). This catalyst displayed the highest activity in both aqueous ( i L = 3.66 ± 0.12 mA cm –2, i 0 = 1.0 × 10 –5 mA cm –2 ) and organic electrolytes ( i L = 2.25 ± 0.15 mA cm –2, α n = 0.51). Doping the solvent-free preparation with Ni 2+ slightly improved its oxygen reduction capabilities in aqueous ( i L = 3.78 ± 0.15 mA cm –2, i 0 = 2.3 × 10 –5 mA cm –2 ) and organic electrolytes ( i L = 2.43 ± 0.17 mA cm –2, α n = 0.47). A large increase in the ability of the catalysts to decompose hydrogen peroxide was observed (2.7 ± 0.26 vs 0.53 ± 0.03 s –1 g –1 ), which is a good indicator of enhanced bifunctionality for application in secondary lithium air batteries. Stability tests in the presence of Li 2 O 2 at elevated temperatures (100 °C) indicate a decrease in catalyst crystallinity as exposure time is increased.