Controllable syntheses of composite catalysts Co3O4/(δ-, β-) MnO2 and α-MnO₂ as oxygen reduction catalyst for zinc-air battery

Manganese dioxide is considered to be the most promising oxygen reduction catalyst. In order to improve the conductivity of MnO₂, the composite catalyst MnO₂/Co3O₄ was prepared by hydrothermal method, and the composite of Co₃O₄ improved the conductivity of MnO₂. The complex electrocatalysts Co₃O₄/δ-MnO₂ and Co₃O₄/β-MnO₂ have been successfully synthesized by a simple one-step hydrothermal way involving KMnO₄, Co(NO₃)₂∙6H₂O, MnSO₄∙H₂O, and HNO₃. The products were characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy), EDS (energy-dispersive spectrometer), FTIR (Fourier transform infrared spectrum), and BET (Brunauer-Emmett-Teller). The results showed that Co²⁺ could not be used as a template for the formation of the [2×2] tunnel structure of α-MnO₂, while α- and δ-MnO₂ were obtained by changing the concentration of Mn²⁺. The cyclic voltammograms (CV) results were in accordance with the order of specific surface area of the prepared catalysts. The results of the AC impedance showed that the composite of Co₃O₄ significantly reduced the charge transfer resistance of the catalysts δ-MnO₂ and β-MnO₂ compared to single pattern of α-MnO₂. In the prepared composite electrocatalyst Co₃O₄/δ-MnO₂, compared with the prepared α-MnO₂ samples, the current of oxygen reduction of the prepared composite catalyst of Co₃O₄/δ-MnO₂ was increased by 25% at −0.35V. The varies scan rate of CV analysis confirmed that the composite of Co₃O₄ proved the synergistic coupling effect with MnO₂, and from the constant potential test, the current retention rate kept stable after 2 h and was still as high as 87.9% after 15 h, proving good stability. Therefore, Co₃O₄/δ-MnO₂ was proved to be a promising catalyst towards oxygen reduction in zinc-air battery.