Membrane cells construction materials

Both cells either undivided or as membrane-separated cell. Construction material is selected for corrosion stability. 

Sensitivity to contaminants, both in fuel and oxidant and in fuel cell construction materials, surprisingly has not been studied enough. There is a very corrosive environment inside a fuel cell (hot, humid, and presence of sulfuric acid) which limits the choice of materials. Both catalyst and polymer membrane may be extremely sensitive to contaminants, particularly metal ions. More research in this area is required. 

Ionics, Inc., electrodialysis cell, two-three compartment cells, approx. 2 sq ft cathode area. 2 Nickel coated steel cathodes — one common center TiSOx coated A1 alloy anode. Membranes 2 — Ionics CR-6170 (cathode side) 2 — Ionics CR-61-CZL-183 (anode side) Cell construction material CPVC. 

Many chlorine producers are having to tolerate impurities originating from standard plastic materials used in membrane technology. Flot chlorine and hot anolyte emerging from the anodic compartment of the cell place tremendous stresses on these plastic construction materials. 

Another potential material for cell construction purposes is nickel. For instance, Eka Chemicals has employed nickel cartridges with standard GORE-TEX membranes to remove graphite particles directly from a caustic decomposer. The result was promising. Not only was the graphite removed but the quantities of mercury were reduced to levels of <10 pg. Aside from the cleaner caustic, the volume of sludge was drastically reduced since no pre-coat was required in this procedure. 

Chrysotile is a noncombustible fibrous solid that has been widely used as a fireproof thermal insulator, for brake linings, in construction materials, and for filters under the name of asbestos. It decomposes with loss of water at 600-800 °C, eventually forming forsterite and silica at 810-820 °C. Because it is more resistant to attack by alkalis than are the amphibole asbestoses, chrysotile has been used in chloralkali cell membranes and in admixture with Portland cement for making sewer pipes (Chapter 11). 

Cell construction is mainly confined to two types, using either pocket plate electrodes (vented cells) or sintered , bonded or fibre plate electrodes (vented and sealed cells). In the former, the active materials are retained within pockets of finely perforated nickel-plated sheet steel which are interlocked to form a plate. Positive and negative plates are then interleaved with insulating spacers placed between them. In sintered plate electrodes, a porous sintered nickel mass is formed and the active materials are distributed within the pores. In sintered plate vented cells, cellulose or other membrane materials are used in combination with a woven nylon separator. In sealed or recombining cells, special nylon separators are used which permit rapid oxygen diffusion through the electrolyte layer. 

Planar SOFCs are composed of flat, ultra-thin ceramic plates, which allow them to operate at 800°C or even less, and enable less exotic construction materials. P-SOFCs can be either electrode- or electrolyte- supported. Electrolyte-supported cells use YSZ membranes of about 100 pm thickness, the ohmic contribution of which is still high for operation below 900°C. In electrode-supported cells, the supporting component can either be the anode or the cathode. In these designs, the electrolyte is typically between 5-30 pm, while the electrode thickness can be between 250 pm - 2 mm. In the cathode-supported design, the YSZ electrolyte and the LSM coefficients of thermal expansion are well matched, placing no restrictions on electrolyte thickness. In anode-supported cells, the thermal expansion coefficient of Ni-YSZ cermets is greater than that of the YSZ... 

To drive the volatile products directly from the electrode surface via a suitable inlet system into the vacuum chamber of the mass spectrometer, a plain gas-permeable membrane (e.g. PTFE or polyethylene) on which the electrode material is deposited by sputtering or vacuum evaporation is usually used. Alternatively, the material can be deposited onto a metal gauze that is fixed close to the gas-permeable membrane. DBMS cell constructions such as that shown in Big. 12.39 have been described that allow the use of massive electrodes, which are more easily available. 

In membrane-cell plants fed with well brine, evaporation of depleted brine can be used to remove the excess water and maintain a water balance. This approach does not require a crystallizing evaporator. The solids-recovery section of a salt evaporator also is not required, and evaporator design is simpler. However, the problem of sulfates in the brine remains, and the materials of construction mentioned above may not be suitable. Free chlorine should be scrupulously removed from the depleted brine before it enters an evaporator, and even the chlorate that forms in the cells can be a problem. The working materials may have to be upgraded to types with higher nickel contents. Thus, the evaporator bodies and liquor-handling components may be upgraded from Monel to one of the Incoloys, which contain about 20% chromium and up to 45% nickel. 

With brine at or near its full process temperature, the operating pressure is reduced to about a third or a half of an atmosphere. Nearly all the chlorine in the depleted brine is recovered and can be returned to the process. The resulting brine is not suitable for return to a membrane-cell process. There is need for further dechlorination, which is the topic of Section 7.S.9.3. In a mercury-cell process, on the other hand, there actually are advantages to incomplete dechlorination. The presence of free chlorine in the brine returned to the salt dissolver, given suitable materials of construction, helps to keep mercury in solution and prevents its deposition on the brine sludge that will be removed from the process. Typical concentrations are 10-50 ppm CI2. Especially given the inherently lower solubility of chlorine, conditions used to dechlorinate mercury-cell brines therefore can and should be less rigorous. 

Both feed brine and depleted brine are subject to contamination by compounding agents and processing aids contained in plastic piping. In membrane-cell plant design, materials must be chosen with this in mind. Section 7.5.5.3 has already covered this subject in the comments on materials of construction and the possibility of recontamination. 

The standard materials of construction of membrane cells are titanium on the anode side and nickel on the cathode side. Painting in a manner similar to that employed for diaphragm cells protects the external surfaces of these materials. 

The use of nonaqueous electrolytes in more recent work has considerably extended the range of electrode potentials available for oxidation and reduction of a variety of organic molecules. Also, the advent of superior materials for cell construction, electrodes, separators, and ion exchange membranes, e.g., Nafion , and the application of computer technology to control of electrochemical systems and process engineering, are further factors that have contributed to the renewed interest in organic electrode processes in the last two decades, t See p. 667. 

The above requirements lead to the need for relatively simple geometries usually based on tank cells where the anodes are surrounded by porous bags, baskets or membranes to contain anode debris and slime. There are two traditional cell designs which, with modern improvements in engineering design and constructional materials, persist today. The Moebius cell uses a vertical electrode arrangement, while the Balbach cell (later improved by Thum) uses horizontal electrodes. 

In contrast to all other techniques, which use an ion-selective electrode and a reference electrode, for this technique a concentration cell must be built using two identical ion-selective electrodes or one ion-selective membrane and two identical reference electrodes. Fig. 45 shows a few possible concentration cell constructions. The principle of this method rests on the fact that the EMF of a concentration cell is zero if the measured ion activity is the same in both electrode compartments (Null point poten-tiometry). One can start with a measured ion activity of zero in the reference compartment (or more precisely, of an activity corresponding to the solubility of the active phase in this solution), and gradually increase the concentration of indicated ion through the precisely measured addition of a solution of the indicated ion of known concentration. This process is continued until a cell EMF of zero is obtained. At this point the known concentration of the reference compartment is identical to that in the adjacent sample solution. With this procedure no calibration of the electrodes is necessary. The slope of the Nernst response is also unimportant it only affects the observed sensitivity of the EMF change per indicated ion addition. This technique is recommended if only a small amount of sample material is available, or with samples which cannot be contaminated. As Fig. 45B shows, even microliter samples can be measured accurately with this technique. 

The unit of construction of all living organisms on Earth is the cell. Some organisms consist of a single cell others contain many cells. Cells range in size from less than 1 pm (10 m) to more than 500 pm in diameter. All cells have the same basic structures a bounding cell membrane, a nucleus or nuclear material, and cytoplasm in which most biochemical reactions take place. 

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