Deposits cell operations

Catalytic cathodes in membrane cell operations exhibit a voltage savings of 100—200 mV and a life of about 2 + yr using ultrapure brine. However, trace impurities such as iron from the caustic recirculation loop can deposit on the cathode and poison the coating, thereby reducing its economic life. 

Two sections of steel condenser tubing experienced considerable metal loss from internal surfaces. An old section contained a perforation the newer section had not failed. A stratified oxide and deposit layer overlaid all internal surfaces (Fig. 5.14). Corrosion was severe along a longitudinal weld seam in the older section (Fig. 5.15). Differential oxygen concentration cells operated beneath the heavy accumulation of corrosion products and deposits. The older tube perforated along a weld seam. 

As stated, one of the fundamental problems encountered in the direct oxidation of hydrocarbon fuels in SOFCs is carbon deposition on the anode, which quickly deactivates the anode and degrades cell performance. The possible buildup of carbon can lead to failure of the fuel-cell operation. Applying excess steam or oxidant reagents to regenerate anode materials would incur significant cost to SOFC operation. The development of carbon tolerant anode materials was summarized very well in several previous reviews and are not repeated here [7-9], In this section, the focus will be on theoretical studies directed toward understanding the carbon deposition processes in the gas-surface interfacial reactions, which is critical to the... 

In the EPD process, a DC electric field is used to deposit charged particles from a colloidal suspension onto an oppositely charged substrate, as illustrated in Figure 6.8. The graphite rod used for the deposition substrate is later burned out prior to cell operation, leaving a hollow tube. The other fuel cell layers can be deposited by a similar process onto the anode support tube. 

To determine how many moles of metal have been deposited, you need to determine how many electrons have flowed through the circuit to fuel this reaction. With the following relationship, you can determine the amount of charge passing through the circuit (measured in coulombs, C, the standard unit of charge) by using the current provided by the power source and the amount of time the cell operates ... 

FIGURE 4.6.1 A Daniell cell. The rectangular box labeled P represents a potentiometer. Anode and cathode are shown for spontaneous operation of the cell, wherein Zn enters the solution and Cu is deposited on the right-hand electrode. The cell operates as shown when the potentiometer emf is slightly below the emf of the cell. If the opposing potentiometer emf exceeds that of the Daniell cell, the cell operation is reversed. 

It was concluded that most of the ionic liquid from the external surface of the membrane disappeared during cell operation. However, comparison of the SEM-EDX spectra taken from membranes before (Fig. 11.3) and after (Fig. 11.5) immersion in the n-hexane/n-hexane solution showed similarity. The EDX spectra taken from a sample of np to a few micrometres thick/depth demonstrate the contribution of ionic liquid within the membrane pores which is more important than the accumulated liquid found on the surface. Consequently, from the SEM study, it was deduced that only the ionic liquid deposited on the external membrane surface has been stripped off during operation. The amount of ionic liquid retained in the membrane pores, however, was apparently kept constant, and consequently, the membrane was stable against the possible solvent action of n-hexane. 

Microfabrication processes have been used successfully to form micro-fuel cells on silicon wafers. Aspects of the design, materials, and forming of a micro-fabricated methanol fuel cell have been presented. The processes yielded reproducible, controlled structures that performed well for liquid feed, direct methanol/Oj saturated solution (1.4 mW cm ) and direct methanol/H O systems (8 mA cm" ). In addition to optimizing micro-fuel cell operating performance, there are many system-level issues to be considered when developing a complete micro power system. These issues include electro-deposition procedure, catalyst loading, channel depth, oxidants supply, and system integration. The micro-fabrication processes that have... 

To avoid getting into diffusion-limited regimes arising from the solubility limitations, the amalgam should be stirred sufficiently to reduce the thickness of the diffusion layer. However, impurities such as Mg2+ in the brine can deposit on the amalgam cathode as Mg(OH)2 retarding the convection of the amalgam. Hence, brine purity becomes an important issue in mercury cell operations. 

Bipolar electrolysis systems are characterized by the type of electrolyte. The proton exchange membrane (PEM) system, developed by the General Electric Compare (GE), uses as the electrolyte a thin membrane of sulfonated fiuorocaibon (Nation ) that conducts electricity when saturated with water. Electrodes are formed by depositing a thin platinum film on opposite sides of the merrtbrane to form a bipolar cell. An electrolyzer is made by stacking 50-200 cells in series, with srritably formed separators to direct the exhaust gases into charmels at the sides. Since the membrane is the electrolyte, only pine water needs to be supphed to the cell. When the cell oper-... 

The next question is which of the above-discussed scenarios is most Kkely to be found at the catalyst/hydrated membrane interface. It is generally accepted that the proton conductivity of the membrane depends on the characteristics of ionic clusters formed surrounding the polymer hydrophilic sites, both within the bulk polymeric structure and at the interface with the catalyst [79]. The ionic clusters located at the membrane/catalyst interface are the ones that close the circuit of this electrochemical system. That is, these ionic clusters act as bridges through which protons and other hydrophilic reactants and products may pass from the membrane to the catalyst surface and vice versa during fuel cell operation. To get some insights into the possible formation of ionic clusters, we have analyzed the conformation of a hydrated model nation membrane over Pt nanoparticles deposited on a carbon substrate via classical MD simulations [80] at various degrees of hydration. 

The hydrogen-PEM cell operates at around 80 °C. At this temperature the electrochemical reactions would normally occur very slowly, and so small islands of platinum are deposited on each electrode to catalyze the reactions. The high cost and relative scarcity of platinum is one factor that limits wider use of hydrogen-PEM fuel cells. 

Another promising way to improve the activity and durability of Pd-based nanocatalysts is to deposit a Pt layer on them. Recently, Pd/C and PdM/C catalysts modified by a Pt monolayer were found to possess higher activity than that of Pt/C due to the strain and electronic effects from the Pd-based cores, and the durability of the catalysts is improved significantly and comparable to Pt/C [70, 93-95]. The Pd-based core materials are expected to be partially dissolved xmder the fuel cell operation conditions due to some defects in the Pt monolayer. In the meantime, the diffusion of Pt atoms on the surface results in a more compact shell. Thus, further dissolution of Pd-based core is greatly reduced. 

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