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Static Water Feed Process The static water feed process employed in the SFWEM uses water both in liquid and in vapor form. Liquid static water feed is used from an external reservoir into the individual water cavities of the module's cells. Static water vapor feed occurs from these individual water feed cavities, across the hydrogen (H2) cavity, to the electrolysis site. Figure 1 is a functional schematic of a cell designed for the SFWEM. The overall static water feed concept operates as follows. Initially, the water feed cavity, the water feed matrix and the cell matrixelectrodes contain an aqueous solution of KOH electrolyte at equal concentrations. Both the H and 0 cavities are void of liquid. An equilibrium condition exists prior io star? of electrolysis. When power is applied to the electrodes, water from the cell electrolyte is decomposed. As a result, the concentration of the cell electrolyte increases and, therefore, its water vapor pressure decreases to a level below that of the feed compartment electrolyte. This water vapor pressure differential is a driving force causing water vapor to diffuse from the liquid gas interface within the water feed matrix, through the H2 cavity and cathode electrode into the cell electrolyte. This process establishes a new equilibrium condition based on the water requirements for electrolysis and humidification of the product gases and continues as long as electrical power is applied to the cell electrodes. As water diffuses from the feed matrix and is removed from the water feed compart- ment, it is statically replenished from an external source to maintain a constant pressure, volume and electrolyte concentration within the feed compartment. Upon interruption of electrical power, water vapor will continue to diffuse across the H2 compartment until the electrolyte concentration in the cell matrix is equal to that of the water feed matrix and compartment. At this point, the original equilibrium condition is regained with the electrolyte retained in the cell matrix and electrodes equal to the initial charge volume and concentration. The increase in the concentration of the electrolyte in the cell matrix during electrolysis is, of course, accompanied by an electrolyte volume decrease, since only a fixed amount of salt is present from the initial charge. This volumetric decrease is a direct function of current density and resistance to water transport from the water feed cavity to the cell electrolyte. The cell must therefore be designed to prevent electrolyte volume decrease to the point where gas crossover or precipitation occurs. Some of the major consider- ations to lessen the effect of electrolyte volume decrease are: 1. Optimum initial electrolyte concentration. 2. High electrode-to-matrix thickness ratio. 3. Low resistance to water vapor diffusion across the H2 compartment. 4. Low resistance to electrolyte and water diffusion within the water feed compartment and water feed matrix. 9 Zice Sscems, Jc.PDF Image | WATER ELECTROLYSIS MODULE
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