Anode contamination carbon formation

Solid Particulates These contaminants can originate from a variety of sources, and their presence is a major concern because they can block gas passages and/or the anode surface. Carbon deposition and conditions that can be used to control its formation have been discussed earlier in this section. Solid particles such as ZnO, which is used for sulfur removal, can be entrained in the fuel gas leaving the desulfurizer. The results by Pigeaud (72) indicate that the tolerance limit of MCFCs to particulates larger than 3 pm diameter is <0.1 g/1. 

Similar to the behavior of nonactive metal electrodes described above, when carbon electrodes are polarized to low potentials in nonaqueous systems, all solution components may be reduced (including solvent, cation, anion, and atmospheric contaminants). When the cations are tetraalkyl ammonium ions, these reduction processes may form products of considerable stability that dissolve in the solution. In the case of alkali cations, solution reduction processes may produce insoluble salts that precipitate on the carbon and form surface films. Surface film formation on both carbons and nonactive metal electrodes in nonaqueous solutions containing metal salts other than lithium has not been investigated yet. However, for the case of lithium salts in nonaqueous solvents, the surface chemistry developed on carbonaceous electrodes was rigorously investigated because of the implications for their use as anodes in lithium ion batteries. We speculate that similar surface chemistry may be developed on carbons (as well as on nonactive metals) in nonaqueous systems at low potentials in the presence of Na+, K+, or Mg2+, as in the case of Li salt solutions. The surface chemistry developed on graphite electrodes was extensively studied in the following systems ... 

Nonaqueous electrolyte solutions can be reduced at negative electrodes, because of an extremely low electrode potential of lithium intercalated carbon material. The reduction products have been identified with various kinds of analytical methods. Table 3 shows several products that detected by in situ or ex situ spectroscopic analyses [16-29]. Most of products are organic compounds derived from solvents used for nonaqueous electrolytes. In some cases, LiF is observed as a reduction product. It is produced from a direct reduction of anions or chemical reactions of HF on anode materials. Here, HF is sometimes present as a contaminant in nonaqueous solutions containing nonmetal fluorides. Such HF would be produced due to instability of anions. A direct reduction of anions with anode materials is a possible scheme for formation of LiF, but anode materials are usually covered with a surface film that prevents a direct contact of anode materials with nonaqueous electrolytes. Therefore, LiF formation is due to chemical reactions with HF [19]. Where does HF come from Originally, there is no HF in nonaqueous electrolyte solutions. HF can be produced by decomposition of fluorides. For example, HF can be formed in nonaqueous electrolyte solutions by decomposition of PF6 ions through the reactions with H20 [19,30]. 

We have demonstrated that LSCF exhibited high activity for the direct CH4 fuel cells for more than 72 h [59]. Transient response studies revealed that the electrochemical oxidation of CH4 on the anode produced electricity, CO2, and CO through a parallel pathway C + 20 C02 C + 0 C0, where the intrinsic rate constant for the formation of CO2 is greater than that of CO. This study and many subsequent studies with various perovskites revealed that perov-skite materials are promising for the direct CH4 SOFC [60-66]. Contaminants such as H2S have been foimd to decrease oxidation activity of perovskites and lead to the increase in carbon deposition [67]. One approach to enhance oxidation activity of perovskites is through the addition of metals such as Ag, Au, and Cu that exhibit good oxidation activities. 

The complexity of SEI formation is topped off with reactions of the electrolyte with contaminants and additives. Because of different reaction rates of all reactive components with lithium, which yield surface films of different quality, additives can be used to modify the surface films to highly conductive lithium films, preventing the components of the electrolyte from further decomposition. There are many successful examples of this approach in the open and patent literature, both for lithium anodes and also for lithiated carbon electrodes. 

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