Fundamental And Applied Research On The Problems Of Zinc-air Battery Systems

Due to its high energy density, superior discharge performances, low cost and ecological benefits, Zinc-air battery has attracted considerable research attention during past decades. However, the battery systems have to operate in open air and therefore suffer severely from the exchange with environmental gases, which usually results in serious degradation in the power capability and service life. This also brings about a number of difficulties in the design and construction of practical batteries and imposes a great hindrance for commercial application. This Ph. D work is focused on some important fundamental and technological problems concerning the development of a practical zinc/air battery and intended to establish new preparation methods for making the cathodic catalyst, high efficiency zinc anode and the manufacturing techniques for cylindrical types of zinc/air battery. In addition, the effects of environmental conditions on the performance of the zinc/air battery are examined and the resolutions to suppress the environmental influences are proposed. The main experimental results and conclusions are summarized as follows: 1. The mechanism of oxygen reduction at MnO₂-catalyzed air cathode was investigated by steady-state polarization curves, linear voltammetry and intermediate characterization. It is found that oxygen reduction at MnO₂ catalyst is accompanied with the reduction of MnO₂ and the rate of oxygen reduction is proportional to the generating rate of Mn³⁺ ions on the surface of MnO₂ catalyst. Based on the experimental results, we propose a chemical oxidation mechanism for the catalytic oxygen reduction, in which oxygen reduction proceeds through chemical oxidation of the discharge product of MnO₂. The polarization expression derived from this mechanism can very well explain the observed experimental facts and the electrocatalytic model given in this work provides a new means for the prediction and evaluation of the cathodic catalysts. 2. The surface passivation of zinc anode in dilute alkaline solution is studied by passivation curves, impedance analysis and scanning microscopy. It is found that with addition of 2% sodium dodecyl benzene sulfonate (SDBS) in 20% KOH solution, the discharge capacity of zinc anode increases 35% than that in blank electrolyte. Based on the electrochemical and morphological observation of the anodic passivation behaviors of zinc electrode, this effect is revealed that due to the SDBS adsorption, the passive layer formed on the zinc surface has a loose and porous structure. This type of surface layer facilitates the diffusive exchange of the solution reactant and discharged product through the surface deposit layer and therefore effectively enhances the utilization of zinc anode. 3. The manufacturing technique for industrial scale production of cylindrical zinc/air battery is explored. The gas diffusion layer of the cylindrical air cathode is fabricated by mould pressing method and the catalyst layer in the inner wall of the air cathode is coated by hot spraying method. The performance test of a large number of cell samples shows that the processing technique can not only produce the batteries of good electrochemical performances, but also have advantages of processing convenience. Besides, the manufacturing technique has wider adaptability easily applicable for other types of air cathodes. 4. The effects of environmental conditions on the discharge and storage performance of zinc/air batteries were quantitatively analyzed. The methods to enhance the environmental tolerance of the batteries were studied. The experimental results show that the water vapor transfer is the dominant factor for the performance degradation of zinc/air batteries and the entrance of CO2 and O2 in the cell can greatly accelerate the performance degradation. For most applications,the diffusion-controlled design is the most effective way to reduce the influences of water exchange on the cell performances. In comparison, the use of permeable membrane could provide a selected diffusion for oxygen, but it also reduces the diffusion flux of oxygen seriously. In our experiments, we found that the use of polymerized gel electrolyte or less-concentrated alkaline solution seems to be a better way not only to improve the environmental tolerances, but also to prevent the leakage of the battery.