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Synthesis and Characterization of ORR Electrocatalysts for Rechargeable Zn-Air Batteries
[2017/04/13]
Synthesis and Characterization of ORR Electrocatalysts for Rechargeable Zn-Air Batteries

Due to the poor electric conductivity but the excellent catalytic ability for the oxygen reduction reaction (ORR), manganese oxides (MnOx) in various phases anchored onto carbon black powders (XC72) has been synthesized by wet chemistry methods. Manganese oxides in a-, b-, g-, d-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. The order of composites with respect to decreasing the ORR activity is: a-MnO2/XC-72>g-MnO2/XC-72> b-MnO2/XC-72> d-MnO2/XC-72> Mn2O3/XC-72> Mn3O4/XC-72> MnOOH/XC-72. The specific surface area and electric conductivity of the composites are generally enhanced by increasing the XC72 content while the high XC72 content will induce the formation of MnOOH which shows a worse ORR catalytic ability than a-MnO2. By using this optimized cathode under the air atmosphere, the quasi-steady-state full-cell discharge voltages are equal to 1.353 and 1.178 V at 2 and 20 mA cm-2, respectively. Due to the usage of ambient air rather than pure oxygen, this Zn-air battery shows a modestly high discharge peak power density (67.51 mW cm-2) meanwhile the power density is equal to 47.22 mW cm-2 and the specific capacity is more than 750 mAh g-1 when this cell is operated at 1 V.

A novel configuration by pressing two electrodes containing electrocatalysts for the oxygen reduction and evolution reactions (ORR and OER) into a bi-functional air electrode is designed for rechargeable Zn-air batteries. MOC/25BC carbon paper (MOC consisting of a-MnO2 and XC-72 carbon black) and Fe0.1Ni0.9Co2O4/Ti mesh on this air electrode mainly serve as the cathode of the ORR and the anode of the OER, respectively. Electrochemical studies include linear sweep voltammetry (LSV), rotating ring-disk electrode (RRDE) voltammetry, and the full-cell charge-discharge- cycling test. The discharge peak power density of a Zn-air battery with this unique air electrode reaches 88.8 mW cm-2 at 133.6 mA cm-2 and 0.66 V in the alkaline electrolyte under the ambient condition. After 100 discharge-charge cycles at 10 mA cm-2, an increase in 0.3 V between charge and discharge cell voltages is found. The long-time discharge-charge-cycling curve (10 h in each step) shows that the cell voltages of discharge (1.3 V) and charge (1.97 V) keep constant during the entire process. The performances of this rechargeable Zn-air battery are superior to most reports in recent literature.

Time: 3:10 PM, 14th Apr 2017

93406, 4F, Dept. Chem. Eng.


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