Anode contamination reduction

The slow kinetics of the cathode oxygen reduction reaction (ORR) plays the key role in limiting PEMFC performance when pristine hydrogen is used as the fuel. Therefore, improving the catalytic activity for the ORR has drawn most of the research attention in catalysis studies. Cathode contamination has attracted less attention compared with anode contamination, and only a limited number of papers have been published. Pollutants in air include NOx (NO2 and NO), SOx (SO2 and... 

Cathodic protection with impressed current, aluminum or magnesium anodes does not lead to any promotion of germs in the water. There is also no multiplication of bacteria and fungi in the anode slime [32,33]. Unhygienic contamination of the water only arises if anaerobic conditions develop in the slurry deposits, giving rise to bacterial reduction of sulfate. If this is the case, HjS can be detected by smell in amounts which cannot be detected analytically or by taste. Remedial measures are dealt with in Section 20.4.2. 

It must be noted that impurities in the ionic liquids can have a profound impact on the potential limits and the corresponding electrochemical window. During the synthesis of many of the non-haloaluminate ionic liquids, residual halide and water may remain in the final product [13]. Halide ions (Cl , Br , I ) are more easily oxidized than the fluorine-containing anions used in most non-haloaluminate ionic liquids. Consequently, the observed anodic potential limit can be appreciably reduced if significant concentrations of halide ions are present. Contamination of an ionic liquid with significant amounts of water can affect both the anodic and the cathodic potential limits, as water can be both reduced and oxidized in the potential limits of many ionic liquids. Recent work by Schroder et al. demonstrated considerable reduction in both the anodic and cathodic limits of several ionic liquids upon the addition of 3 % water (by weight) [14]. For example, the electrochemical window of dry [BMIM][BF4] was found to be 4.10 V, while that for the ionic liquid with 3 % water by weight was reduced to 1.95 V. In addition to its electrochemistry, water can react with the ionic liquid components (especially anions) to produce products... 

The fourth factor is the current density. At an inert anode and for 100% Faradaic efficiency for water oxidation, the density of the current controls the flux of H+ ions. The cathodic current density and the species available in its vicinity establish the efficiency of the reduction processes (Pb2+ —> Pb). These vary to a greater extent than the anode process, because the pH and the species reaching the cathode vary with processing time. Thus, control of the current density is critical to ensure optimal EO efficiency and contaminant removal. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

Robustness requires a low sensitivity to what might cause permanent damage or degradation of the SOFC system and, hence, the cell - for excursions outside the normal operating window, contaminants in the fuel and the air, thermal and reduction-oxidation (redox) cycling of the anode. 

C—l lie impure silver must be oxidized so it will go into solution. Oxidation occurs at the anode. Reduction is required to convert the silver ions to pure silver. Reduction occurs at the cathode. The cathode must be pure silver, otherwise it could be contaminated with the cathode material. 

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. 

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 ... 

Ammonia, produced due to the coexistence of H2 and N2 at high temperatures in the presence of catalyst, was estimated to be in the concentration range of 30 to 90 ppm [37, 38], Uribe et al. [39] examined the effects of ammonia trace on PEM fuel cell anode performance and reported that a trace in the order of tens of parts per million could lead to considerable performance loss. They also used EIS in their work. By measuring the high-frequency resistance (HFR, mainly contributed by membrane resistance) with an operation mode of H2 + NH3/air (feeding the anode with hydrogen and ammonia), they obtained some information related to membrane conductivity, and found that conductivity reduction due to ammonia contamination is the major cause of fuel cell degradation. 

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]. 

The effluent salt is the radioactively contaminated calcium chloride saturated with calcium oxide which can not be discarded as-is. Thus it is desirable to cathodically reduce calcium oxide to calcium which can then be recycled back to the direct oxide reduction process. The primary difficulty in obtaining a cathodic calcium deposit is due to the evolution of carbonaceous anodic gases which encourage various reverse reactions in the cell leading to loss of deposited calcium. 

The advantages of the electrochemical pathway are that contamination with byproducts resulting from chemical reduction agents are avoided and that the products are easily isolated from the precipitate. The electrochemical preparation also provides a size-selective particle formation. Reetz et al. have conducted several experiments using a commercially available Pd sheet as the sacrificial anode and the surfactant as the electrolyte and stabilizer. Analysis of the (C8Hi7)4N Br-stabilized Pd ° particles produced have indicated that the particle size depends on such... 

Many researchers have attempted to determine mercury levels in the blood, urine, tissues, and hair of humans and animals. Most methods have used atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), or neutron activation analysis (NAA). In addition, methods based on mass spectrometry (MS), spectrophotometry, and anodic stripping voltametry (ASV) have also been tested. Of the available methods, cold vapor (CV) AAS is the most widely used. In most methods, mercury in the sample is reduced to the elemental state. Some methods require predigestion of the sample prior to reduction. At all phases of sample preparation and analysis, the possibility of contamination from mercury found naturally in the environment must be considered. Rigorous standards to prevent mercury contamination must be followed. Table 6-1 presents details of selected methods used to determine mercury in biological samples. Methods have been developed for the analysis of mercury in breath samples. These are based on AAS with either flameless (NIOSH 1994) or cold vapor release of the sample to the detection chamber (Rathje et al. 1974). Flameless AAS is the NIOSH-recommended method of determining levels of mercury in expired air (NIOSH 1994). No other current methods for analyzing breath were located. 

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