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bar with one type of electrode. It should be noted that in this concept both faces of each zinc electrode are plated, while only the two outside faces of each chlorine-electrode pair are active. Each unit cell is comprised of multiple single cells, i.e. one side of a zinc- electrode substrate facing a chlorine-electrode substrate. A schematic of a sin gle cell is shown in Figure 31-2.Immediately after initiation of charge, the current density is approximately uniform because of the similarity of the sheet resistance of both graphite substrates. However, in the early stages of charge, the current- density distribution becomes more asymmetrical because of the electrodeposition of conductive zinc. At the end ofcharge, the zinc .film is consequently thickest near the chlorine-electrode bus-bar. This tends to increase the probability of edge- dendrite formation. Zinc-dendrite formation is favored at this location because of the higher local current density. The distribution of current density across the zinc and chlorine electrodes is a function of the chlorine-electrode substrate width, thickness, conductivity, the electrolyte resistivity, the inter-electrode gap, the apparent current density, and the non-ohmic ("IR-free") cell polarization; i.e. the total cell polarization less the ohmic polarization components. Research and development on cell tech nology have resulted in: (a) lowering of chlorine-electrode polarization by improved activation techniques, (b) reduction of the electrolyte resistivity by use of supporting electrolytes and an increased temperature of cell operation, (c) qualification of a more conductive porous graphite, and (d) increased charge and discharge apparent current densities by use of zinc-plate leveling agents. Developments (a), (b), and (c) have led to improved voltaic efficiencies. Devel opment (d) not only improved coulombic efficiency but also raised the stack energy density, thereby reducing the specific cost ($/kWh) of the stack. However, such changes also affect the distribution of current density, as described in this paper. Initially, a circuit model is developed for the single cell. A differential seg ment of this circuit is then integrated over the width of the cell, and an expres sion for the current-density distribution is derived. The current-density distri bution is calculated for a "standard case", considered to represent present tech nology. Current-density distributions are presented also for changes in cell width, electrode polarizations, chlorine-electrode resistivity and thickness,inter electrode gap, and electrolyte resistivity. Next, the calculated zinc-deposit 31-2PDF Image | Development of the Zinc-Chlorine Battery for Utility
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