Development and characterization of bi-functional air electrodes for rechargeable zinc-air batteries: Effects of carbons

In spite of high mean transfer number and catalytic ability of the oxygen reduction reaction (ORR), alpha-MnO2 is lack of electric conductivity and specific surface area to fully exert the performance of rechargeable Zn-air battery. Here, carbons in various forms are chosen as substrates for uniform dispersion of alpha-MnO2 to form air electrode catalysts to evaluate the influences of carbon types on the catalytic activities of the ORR and OER (oxygen evolution reaction). The morphology and physicochemical properties of various alpha-MnO2/carbon composites are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Electrochemical studies include rotating ring-disk electrode (RRDE) voltammetry of catalysts, linear sweep voltammetry (LSV) of air electrodes, and the charge-discharge-cycling test of full cells. The discharge peak power density of Zn-air batteries varies from 66.3 (alpha-MnO2/carbon nanotubes with diameter approximate to 10 nm, denoted as alpha-MnO2/CNT10) to 40.5 mW cm(-2) (alpha-MnO2/super fine mesophase graphite powder) in 6 M KOH under ambient condition. The rechargeable Zn-air battery with the air electrode containing alpha-MnO2/CNT10 is stably operated for 100 cycles at 10 mA cm(-2), which shows that an increase in 0.09 V between charge (decayed ca. 0.05 V) and discharge (decayed ca. 0.04 V) cell voltages. (C) 2016 Elsevier Ltd. All rights reserved.