Carbon dioxide anode contamination

Fuel cells must carry the costs of conditioning the two reactant gases as well as their own capital charges. Hydrogen requires transport to the anode side of the fuel cells. This is usually by rotary blower, but it also should be possible to operate membrane cells at some positive pressure and then to deliver the hydrogen without mechanical aid. The temperature and water content of the hydrogen must be considered in the overall heat and mass balance. Air and oxygen are candidates for use at the cathodes. The classical balance between cost and efficiency determines the choice. Wth alkaline fuel cells, the carbon dioxide in the air is of concern. It can consume the hydroxide value and contaminate the end product. It is possible to scrub the air to remove the CO2 before... 

The metal is produced on a massive scale by the Hall-Heroult method in which aluminum oxide, a nonelectrolyte, is dissolved in molten cryolite and electrolyzed in a large cell. The bauxite contains iron oxide and other impurities, which would contaminate the product, so the bauxite is dissolved in hot alkali, the impurities are removed by filtration, and the pure aluminum oxide then precipitated by acidification. In the cell, molten aluminum is tapped off from the base and carbon dioxide evolved at the graphite anodes, which are consumed in the process. The aluminum atom is much bigger than boron (the first member of group 13) and its ionization potential is not particularly high. Consequently aluminum forms positive AT ions. However, aluminum also has nonmetallic chemical properties. Thus, it is amphoteric and also forms a number of covalently bonded compounds. 

A variation on the concept has been introduced by Dionex as the EG40 module [6]. In this case, KOH contained in a reservoir (labeled in Figure 2.2 as K Electrolyte Reservoir) is used rather than an NaOH feed solution. The process is the same however, is generated from the anode, because its counterion OH is consumed in the production of H. migrates across the cation-exchange membrane to combine with OH formed at the cathode. Carbon dioxide is removed from the eluent stream en route to the EG40, to prevent contamination by carbonate. The electrolyte reservoir must be changed when the K Electrolyte is depleted. 

Numerical models of the contamination of the anode by CO2 and H2S are not as abundant as their counterparts for CO poisoning. The numerical models found in literature, which take into account the presence of carbon dioxide, are focused on understanding the reverse water-gas shift reaction (WGSR) and its effects on the performance of the cell. The effect of hydrogen sulfide presence on the polarization curve and liquid water distribution in the cell is also studied numerically. It is found that with trace amounts of H2S, the cell performance can be dramatically decreased. The effects of H2S poisoning can be sometimes very severe and cannot be reversed, unlike their counterparts of CO poisoning. 

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