Accumulation and continuous removal of impurities in fuel cells: I. One‐dimensional model

Abstract

A mode of operation and o design technique hove been developed which permit the attainment of continuous purging of impurities directly from the gas compartments of a fuel cell-either anode, cathode, or both-with the realization of minimum reactont loss, most stable voltage and current output, and operating conditions with respect to reactant gas flow and electrolyte inventory. We have found o way to eliminate the complex periodic purge valves and attendant electronics by o fuel cell system which i s both simple in structure and operation and which has a high degree of reliability.The technique was suggested by the observation that in dead-ended fuel cell gas compartments, the inert impurities present i n reactant gases tend to accumulate at the dead-ended portion of the cell. Hence a small amount of bleed should be placed there. This observation has been made onolyticolly by solving numerically a system of two partial differential equations simultaneously.When hydrogen and oxygen or similarly pure reactants are used as fuel and oxidant and where the product of reaction i s a condensable gas such as water vapor, it i s common practice to make either the anode gas chamber or cathode gas chamber dead-ended. Such designs have been used recently, for example, on the oxygen or cathode chamber of the Apollo (1) fuel cell and for both the oxygen and hydrogen chamber of the matrix-type ( 2 ) fuel cell which may be used in the U S . Air Force's manned orbiting laboratory (MOL) fuel cell.Even though the hydrogen and oxygen used in these fuel cells are of high purity, there are, nevertheless, some impurities present in these reactants. These impurities are in the form of noncondensable gases, such as nitrogen, helium, or argon or similar such gases. During the course of operation of these dead-ended cells, the fuel and oxidant are continuously consumed and converted into water, which i s then removed from the cell in some fashion. In the case of the above-mentioned Apollo fuel cell, the product water i s removed by recirculation of the hydrogen gas through a water condensor. In the case of the MOL fuel cells, the water is removed through a water transport membrane located within the anode chamber of the fuel cell, When the cell i s dead-ended, the noncondensable impurity accumulates, that is, its concentration becomes higher in the gas compartments of the cell. The accumulation of these impurities causes a drop in operating voltage of the cell and a nonuniform current distribution within the cell. 80th features are most undesirable. A s a result, it i s common practice to purge the electrode chambers periodically to allow the impurity to escape from the cell. This periodic purging causes undesirable voltage transients, possible temporary interruption of power, and permanent loss of electrolyte from the cell, as well as other damaging transients, which, it i s believed, ultimately lead to total cell failure. Moreover, to accomplish the periodic purging, a complicated set of purge valves and accompanying electronics equipment...

Two‐dimensional inert accumulation in space hydrogen‐oxygen fuel cells

“…W e finite difference Equation (21) using a twodimensional analog of the computational scheme given in Part 1 (1 ) . We again satisfy the sufficient stability conditions of Keller (9).…”

“…But even ultrapure fuels and oxidants have some impurities which accumulate and cause a drop in voltage due to dilution of the reactants. In Part I of this study (1) we obtained concentration profiles as a function of time for the case when the flow in the fuel cell can be considered one-dimensional. We also suggested a method of improved operation of a fuel cell involving bleeding of inerts.…”