Cathode contamination electrochemical reaction

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

High electrochemical activity and contamination tolerance. An eleetroeatalyst should be of high intrinsic activity, to diminish electrochemical reaction polarization and enhance energy conversion effieiency. More specifieally, the anode catalyst should have the ability to resist detrimental by-products such as CO and other intermediates that poison the active reaction surface the cathode catalyst should have the ability to restrain crossover methanol oxidation when liquid methanol is seleeted as the fuel. 

Ft (platinum) catalysts supported on a conductive matrix, such as carbon, to provide electron conduction and (3) a hydrophilic agent, such as polytet-rafluoroethylene (PTFE) to provide sufficient porosity and adjust the hydro-phobicity/hydrophilicity of the CL for gaseous reactants to be transferred to active sites [2,3]. With each of those elements optimized to provide the best overall performance, the CL functions as a place for electrochemical reactions. The processes occurring in a CL include mass transport of the gaseous reactants, interfacial reactions of the reactants (e.g., H2 at anode and O2 at cathode) at the electrochemically active sites, proton transport in the electrolyte phase, and electron conduction in the electronic phase. When contaminants are present in the reactant streams, one or more of the above processes can be adversely affected, causing degradation in fuel cell performance or even fuel cell failure. 

Currently, several air-side contamination models have been published in the literature, ranging from simple empirical and adsorption models to general kinetic models. These models have been applied to simulate and predict SO2, NO2, NH3, and toluene contamination. The kinetic model is a very general one based on the associative oxygen reduction mechanism. It takes into account contaminant reactions, such as surface adsorption, competitive adsorption, and electrochemical oxidation, and has the capability of simulating and predicting both transient and steady state cell performance. The model can be applied to other cathode contaminants, e.g., SO2 and NO2. 

The electrochemical treatment of contaminated groundwater technology uses direct electrical current applied between two immersed electrodes to produce oxidation-reduction reactions in aqueous solutions. Positively charged metal ions are attracted to the negatively charged electrode (the cathode), where they are reduced. 

A coastal beach in California is polluted with heavy metals. Since it is a protected wildlife habitat, a minimally intrusive electrochemical method is selected for cleanup. Assume that a constant current density of 125 pA cm-2 in a 40 x 6-foot cross section is used in the contaminant pit, which is 40 x 20 x 6 feet deep, (a) What is the total current and voltage required if the pore fluid conductivity is 21.9 mS cm-1 (approx, equivalent to 0.2 M KC1) (b) If the soil is saturated and approx. 50% pore fluid and 50% solids by volume, how long would it take to pass a charge equivalent to the ionic content of the pore fluid (c) How much acid should be added to depolarize the cathode in this time in order to ensure reaction (A) below, instead of water electrolysis, reaction (B) ... 

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. 

Although there is little evidence for auto-catalysis in dechlorination by Fe , it is still possible that localized corrosion contributes to the remediation of contaminants in environmental applications. Various investigators have postulated that localized corrosion contributes through increased surface area (44) and creation of corrosion cell domains (49-51). The corrosion cell model works on the same principle as the electrochemical model described above (Figure 3), but invokes additional effects such as the reduction of protons as the major cathodic reaction, and the creation of an electrical double layer between the anode and cathode that permits transport due to electrical migration as well as diffusion. Although many aspects of these models are plausible, there are not yet any data that specifically support them, and a study that systematically addresses the role of localized corrosion in remediation applications of Fe remains to be done. 

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