Abstract
The purpose of this study is to investigate the effects of electrode chemistry, electrolyte concentration, and manufacturing process on the performance of the primary zinc-air cell. The investigation is via the utilization of familiar electrochemical methods including the constant-current discharging measurement, the cyclic voltammetry, the half-cell potential measurement, and conductivity measurement. Systematic experiments have been made by varying all important factors in manufacturing the primary zinc-air cells in this study.The porous electrodes are made of slurry with various kinds of compositions. With a high specific surface area of the reactive materials in the electrodes, the polarization of porous electrodes is low and the mass transportation in the cells is high. Manufacturing processes and optimal conditions for both the zinc anode and the air cathode are described as follows. Anodic slurry composed of 94 wt % Zn, 2 wt % Bi2O3, and 4 wt % PTFE powders is coated on a stainless wire net which acts as the anode current collector. Air cathode is a dual diffusion-catalytic layer structure. The diffusion layer, a water-proof and air diffusion layer, is comprised of 60 wt % carbon black and 40 wt % PTFE powders. The catalytic layer, an electrochemical reaction supply layer, is made of 30 wt % carbon black, 30 wt % graphite, 30 wt % MnO2, and 10 wt % PTFE powders. The diffusion layer and the catalytic layer are pressed together with a stainless wire net to form the air cathode. Both the zinc anode and the air cathode are then hot-pressed at 350oC for 10 min. Thus the PTFE powders in both electrodes bind the reactive materials together to form stable electrodes as they are submerged in KOH electrolyte. The maximal conductivity is 55 S．m-1 at 8 M KOH.The half-cell potential measurements show fast and low reaction rates for the zinc anode and the air cathode, respectively. Experiments in this study manifest that the MnO2 air cathode has a potential difference of 0.3 V for the measured and the thermodynamic values, while the zinc anode has no difference in this aspect. The cyclic voltammetry indicates that as the electrolyte concentration increases from 6 to 8 M, the peak potential has an increase of 0.07 V. The cyclic voltammetry also indicates that for an effective self-discharge inhibitor such as Bi2O3 there is a Bi reduction reaction on the zinc anode. However, there is none for ineffective additives. The reduction effect of the inhibitor increases the overvoltage of zinc oxidation and inhibits the self-discharge of the cell. The constant-current discharging measurement of a typical cell assembly in this study shows that the depth of discharging of a cell increases from 34% to 59% as the discharged current decreases from 0.2 to 0.05 A.The efficiency of zinc reduction study is performed with various kinds of electrode metals and different amounts of charged current density. The obtained results indicate that for all cases the reduction structure of zinc is a loose dendritic one in shape. This explains both the drastic shape change of zinc anode after each discharge-charge cycle of a cell and the short cycle life of zinc-air battery. When Ag sheets are used as electrodes, the typical zinc reduction efficiency increases from 14 to 87% as the charged current density increases from 0.025 to 0.1 A/cm2. At 0.05 A/cm2, the reduction efficiency in this experiment is 89, 35, and 22% for Pt, Ag, and Ni electrodes, respectively.