Cathode contamination oxygen concentration

Corrosion resistance of metallic coatings is dependent on the composition and nature of the electrolyte, oxygen concentration, polarization characteristics, ratio of cathodic to anodic area and the surface contaminants. If the corrosion potentials of two metals such as iron and aluminum are close to each other in a particular environment there may be reversal of the galvanic couple. 

Continuous and semicontinuous electrochemical reactors are normally employed for effluent metal ion remediation, where the anode reaction is usually oxygen evolution from water [compare with Equation (26.4)]. After the metal contaminant is captured on the cathode, the cathode can be discarded, the collected metal can be resold, or the deposited metal can be chemically or elecfro-chemically etched into a small volume of a suitable leaching liquor (e.g., water) so as to increase its concentration substantially. 

While cathode reactions tend to be quite efficient, low concentrations of mercury and oxygen may be objectionable. Sections 9.2.5.1 and 9.2.5.2 deal with their removal. Volatile impurities in the catholyte may also contaminate the hydrogen. This is most likely in diaphragm cells, and Section 7.5.8.5 gave an example in which the removal of ammonia from brine reduced the concentrations of chloramines and other nitrogen compounds in the hydrogen. 

Narusawa et al. assessed the allowable concentrations of air contaminants on platinum cathodes [96]. While CO did not lead to any measurable poisoning at the cathode, presumably because the oxygen present oxidizes CO at a high rate, NO2 and SO2 do lead to a loss in performance, albeit reversible. The allowable concentration of the air contaminants, defined as the concentration of a contaminant leading to a performance loss equal to 2 ppm of CO in the anode feed when using a Pt-Ru anode, is 257 ppm for CO, 2.6 ppm for NO2, and 1.8 ppm for SO2. 

Figure 23.14. Impact of ruthenium on oxygen reduction performance (a) CO stripping scans for the cathode and anode, (b) steady-state anode polarization plots before and alter contamination of the eathode, (c) H2-air steady-state polarization curves, and (d) DMFC steady-state polarization curves. Methanol concentration 0.3 M, anode potential during contamination 1.3 V vs. hydrogen counter/quasi-reference electrode, cell temperature 75 °C [65]. (Reprinted by permission of ECS— The Electrochemical Society, from Piela P, Eickes C, Brosha E, Garzon F, Zelenaya P. Ruthenium crossover in direct methanol fuel cell with Pt-Ru black anode.)...

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