Synthesis and characterization of carbon black/manganese oxide air cathodes for zinc–air batteries: Effects of the crystalline structure of manganese oxides

• α-MnO2/XC-72 shows the highest ORR activity among 7 types of MnOx/XC-72.
• The mean electron transfer number of the ORR on MnO2 crystallites is above 3.8.
• The ORR activity of MnO2-based composites is higher than other MnOx-based ones.
• The maximum peak power density of Zn–air battery is 102 mW cm−2.
• The full-cell discharge capacity is 798 ± 20 mAh g−1 by using MnO2/XC-72 cathodes.

Manganese oxides (MnOx) in α-, β-, γ-, δ-MnO2 phases, Mn3O4, Mn2O3, and MnOOH are synthesized for systematically comparing their electrocatalytic activity of the oxygen reduction reaction (ORR) in the Zn–air battery application. The optimal MnOx/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-MnO2/XC-72 > γ-MnO2/XC-72 > β-MnO2/XC-72 > δ-MnO2/XC-72 > Mn2O3/XC-72 > Mn3O4/XC-72 > MnOOH/XC-72. The textural properties of MnOx are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 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−2 (α-MnO2/XC-72) to 47.1 mW cm−2 (Mn3O4/XC-72). The maximum peak power density is 102 mW cm−2 for the Zn–air battery with an air cathode containing α-MnO2/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−1 at all discharge current densities.