Cathode contamination discussion

The cathode contaminants sources and general mechanisms are described in chapter 2, and a more detailed discussion on mechanisms, experimental results, and mitigation is provided in chapter 3. 

In practical applications, a PEM fuel cell cathode will be exposed to air, and all of the impurities in that air may enter the fuel cell system if it has no effective filtration. The major air contaminants are CO (CO2, CO), SO (SO2, SO3), NO (NO2, NO), and NH3. In this section, we will discuss each of these cathodic contaminants in detail. 

Besides the kinetic models we discussed above, the other cathode contamination models available in the literature are the empirical model and the competitive adsorption model [4,57]. Empirical models have been successfully used to describe FEM fuel cell performance at different temperatures and pressures [55,56]. Equation (6.73) was proposed by Kim et al. [56] ... 

Agemian and Bedak [42] have described a semi-automated method for the determination of total arsenic in soils. Chappell et al. [43] have described an inexpensive but effective method for the quantitative determination of arsenic species in contaminated soils. Chappell found that the extraction efficiency varied with the ratio of soil to acid and with the concentration of the acid. Rurikova and Beno [346] accomplished speciation of arsenic(III) and arsenic(V) in soils by cathodic stripping voltammetry. Wenclawiak and Krah [347] used reactive supercritical fluid extraction in speciation studies of inorganic and organic arsenic in soils. In this method, derivatisation with thioglycollic acid methyl ester was performed in supercritical carbon dioxide. Various other workers have discussed the determination of arsenic in soils [44-46]. 

It is well known that ACN reacts with active metals (Li, Ca) to form polymers [48], These polymers are products of condensation reactions in which ACIST radical anions are formed by the electron transfer from the active metal and attack, nucleophilically, more solvent molecules. Species such as CH3C=N(CH3)C=N are probably intermediates in this polymerization. ACN does not react on noble metal electrodes in the same way as with active metals. For instance, well-re-solved Li UPD peaks characterize the voltammograms of noble metal electrodes in ACN/Li salt solutions. This reflects a stability of the Li ad-layers that are formed at potentials above Li deposition potentials. Hence, the cathodic limit of noble metal electrodes in ACN solutions is the cation reduction process (either TAA or active metal cations). As discussed in the previous sections, with TAA-based solutions it is possible that the electrode surfaces remain bare. When the cations are metallic (e.g., Li+), it is expected that the electrode surfaces become covered with surface films originating from atmospheric contaminants reduction if the electrodes are polarized below 1.5 V (Li/Li+). As Mann found [13], in the presence of Na salts the polarization of metal electrodes in ACN solutions to sodium deposition potentials leads to solvent decomposition, with evolution of H2, CH4 and sodium cyanide (due to reaction with metallic sodium). 

With TAA salts of small alkyl groups (e.g., ethyl, methyl), cation reduction is usually the limiting cathodic reaction. The anodic limiting reaction for ammonium ions is their oxidation to nitrogen and protons. It should be emphasized that atmospheric contaminants are supposed to influence the above cathodic and anodic limits of liquid ammonia, as they do for the other nonaqueous systems discussed in the previous sections. 

Surfactants influence electrokinetic remediation in various ways. They adsorb to soils and alter their surface properties, and as a result, EOF and sorption of hydro-phobic organics to the soils are affected. Surfactants also increase the aqueous phase concentration of organics via micellar solubilization. Depending on the type of surfactants, micelles, and therefore organic contaminants within the micelles, may be transported toward the anode or cathode. A simphfied conceptual model of a surfactant-enhanced electrokinetic process is presented in Figure 11.1. The effects of surfactants on these processes are discussed below in more detail. 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

As discussed above, currently the modeling work on PEMFC contamination is mainly focused on the anode side. The only work on cathode catalyst modeling was published by Knight et al. [21], who presented an equation to fit the fuel eell performance caused by NO2 and SO2 contamination at the eathode, as shown in Equation 6.26 [21] ... 

Degradation of the cathode catalyst is the main contributor to performance degradation in the fuel cell in the absence of contamination effects. This will be discussed in terms of degradation of the catalyst metal itself as well the corrosion of the carbon catalyst support. 

The general behavior of the membrane with cation contamination, with and without the water effects, is presented and discussed based on predictions from the model. The model is extended to include the potential from the metal on each side into and through the ionomer in the electrodes. Because the work is focused on the membrane, this aspect is restricted to only include a hydrogen pump cell with hydrogen electrodes on both the anode and cathode sides. 

Thus, NO2 would compete with O2 for electrons on the cathode and the NHJ thus formed would further contaminate the fuel cell, causing performance degradation by poisoning the Nation membrane and ionomer in the catalyst layer, as discussed in section 2.2.3. 

When contaminant presents in the fuel cell, its concentration at the catalyst layer varies with both the inlet contaminant concentration and the current density, as discussed in Shi et al. [18]. Furthermore, the contaminant adsorption (desorption) rate constant is also related to the electrode potential. This variation of the contaminant concentration can be obtained by introducing the CGDL and cathode flow field into the model, which definitely increases its complexity. For simplicity here, we considered the product of the contaminant adsorption (desorption) rate constant and the contaminant concentration at the CCL, as a fimction of current density and contaminant inlet concentration (kCp-- f Cpr J))/ where Cp is the contaminant inlet concentration in the cathode charmel. Based on the experimental data at current densities of 0.2, 0.5, 0.75, and 1 A/cnP, and contaminant inlet concentrations of... 

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