Cathode contamination

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. 

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. 

The resultant cathodes must undergo a further refining step as small levels of tin and antimony can often be deposited along with the lead. These elements are removed by oxygen softening and/or caustic dressing from the remelted cathodes. To limit the level of cathode contamination, some lead refineries apply partial softening to the lead bullion before the anodes are cast. 

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 ... 

At the cathode Contaminants with a strong interaction with the cathode catalyst might accumulate over time and lead to performance loss after reaching a certain threshold. 

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... 

Fuel cell cathode contamination caused by chemical warfare gases is disastrous. With 1780 ppm HCN in air, the power output of a PEMFC was only 13% of the original value, and the degraded performanee was only partly recoverable. After 30 minutes of purging with neat air, the output reeovered to 45% of the original value [36]. 

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. 

Ammonia in the hydrogen fuel originates from the hydrogen production process. The impact on fuel cell performance is as described for the cathode contamination, and is similar whether it is introduced in the cathode or anode. 

Contamination modeling is an important aspect of fuel cell development. It is required to interpolate and extrapolate experimental results to expected conditions in real-world operation, as it is impractical to test all combinations of reactant concentrations and fuel cell operating conditions. Modeling also assists in the development and validation of hypothesized contamination mechanisms. Model development for the anode is more extensive than that for the cathode contamination. The majority of the modeling deals with the kinetic effects associated with adsorption of contaminant species on the cathode and anode catalysts. 

Cathodic Contaminants/Impurities and Their Basic Chemistry.66... 

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. 

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