Cathode contamination contaminant surface

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

Ensure a clean metal surface free from cathodic contaminants. 

Ohmic losses can result from a variety of causes resistance to ion flow in the electrolyte, resistance in the bus bars, and resistance in membranes used to separate anode and cathode electrolytes. The magnitude of the resistances may change with time as films build up on electrode surfaces or as membranes become contaminated. Surface overpotentials can be estimated from rate expressions such as the Tafel equation, or they can be evaluated from em-... 

These can cause short circuits or entrapment of the electrolyte. Entrapment of electrolyte in the deposit, with or without enrichment of additives, will lower the cathode quality. Codeposition of electrolyte impurities can lead to cathode contamination. A nodule forms when a conductive or semiconductive particle remains on the cathode surface. The deposit will then grow around the particle encasing it. The nodular growth can be initiated by several factors. The nodule becomes larger with the continuation of electrolysis and more contaminated with slime particles. Nodules can also lead to short circuits. The local current efficiency can vary depending on the inhibitors and local current density. This can also result in rough deposits, if the current efficiency increases with current density. 

Eventually, as the anodic process continues, a hard, dense, protective layer of Pb02 is formed on the anode surface. Once this protective film has been formed, cathode contamination decreases and the amount of sludge generated by the anode decreases as well. This process (called conditioning) may take 30-60 days or more depending on the anode composition and current density (1). Because of the difficulty in conditioning anodes, operators of zinc cellhouses are very reluctant to replace an entire cell of used, conditioned anodes with new, unconditioned anodes. Operators will normally replace only one or two anodes per cell or try to condition the anodes prior to use in the cells. 

Certain performance losses of fuel cells during steady-state operation can be fully or partially recovered by stopping and then restarting the life test. These recoverable losses are associated to reversible phenomena, such as cathode catalyst surface oxidation, cell dehydration or incomplete water removal from the catalyst or diffusion layers [85]. Other changes are irreversible and lead to unrecoverable performance losses, such as the decrease in the ECSA of catalysts, cathode contamination with ruthenium, membrane degradation, and delamination of the catalyst layers. 

Certain proprietary zinc and aluminum filled polymers and some ion vapor deposited aluminum coatings, applied to steel fasteners may actually produce more damage than the untreated steel fastener itself. This was attributed to two possible causes either the high surface area involved with each of the particulate coatings or the contamination of the particulate surface with active cathodic contaminants such as iron, nickel, or graphite ... 

Contamination in a PEM fuel cell directly affects the kinetics, conductivity, and mass transport properties of the cell. In particular, the blocking of electrocatalysts by impurity adsorption can drastically reduce the effective surface area of the catalysts and, thus, slow down the kinetics and hinder cell performance. This chapter is devoted to cathode contamination modeling. 

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. 

Calcium metal was produced in 1855 by electrolysis of a mixture of calcium, strontium, and ammonium chlorides, but the product was highly contaminated with chlorides (1). By 1904 fairly large quantities of calcium were obtained by the electrolysis of molten calcium chloride held at a temperature above the melting point of the salt but below the melting point of calcium metal. An iron cathode just touched the surface of the bath and was raised slowly as the relatively chloride-free calcium solidified on the end. This process became the basis for commercial production of calcium metal until World War II. 

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