Abstract
Manganese oxides (MnO<inf>x</inf>) in α-, β-, γ-, δ-MnO<inf>2</inf> phases, Mn<inf>3</inf>O<inf>4</inf>, Mn<inf>2</inf>O<inf>3</inf>, and MnOOH are synthesized for systematically comparing their electrocatalytic activity of the oxygen reduction reaction (ORR) in the Zn-air battery application. The optimal MnO<inf>x</inf>/XC-72 mass ratio for the ORR is equal to 1 and the oxide crystalline structure effect on the ORR is compared. The order of composites with respect to decreasing the ORR activity is: α-MnO<inf>2</inf>/XC-72 > γ-MnO<inf>2</inf>/XC-72 > β-MnO<inf>2</inf>/XC-72 > δ-MnO<inf>2</inf>/XC-72 > Mn<inf>2</inf>O<inf>3</inf>/XC-72 > Mn<inf>3</inf>O<inf>4</inf>/XC-72 > MnOOH/XC-72. The textural properties of MnO<inf>x</inf> are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N<inf>2</inf> adsorption/desorption isotherms with Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Electrochemical studies include linear sweep voltammetry (LSV), rotating ring-disk electrode (RRDE) voltammetry, and the full-cell discharge test. The discharge peak power density of Zn-air batteries varies from 61.5 mW cm<sup>-2</sup> (α-MnO<inf>2</inf>/XC-72) to 47.1 mW cm<sup>-2</sup> (Mn<inf>3</inf>O<inf>4</inf>/XC-72). The maximum peak power density is 102 mW cm<sup>-2</sup> for the Zn-air battery with an air cathode containing α-MnO<inf>2</inf>/XC-72 under an oxygen atmosphere when the carbon paper is 10AA. The specific capacity of all full-cell tests is higher than 750 mAh g<sup>-1</sup> at all discharge current densities.