Cells design

A wide range of cells can be used for corrosion experiments. As mentioned, the selection of the cell is connected to the form of the sample and the masking scheme. It is often possible to simply immerse a sample into a beaker. Fig. 1. The water line can be managed with a flag electrode, or by masking the sample to expose only a submerged area. 

A round-bottomed multinecked flask is a common approach when a sealed cell is required. Fig. 2(a). The multiple ports allow for insertion of tubes for deaeration, thermometers, and the various electrodes. As mentioned above, cells that press the sample against a window are called flat cells. Flat cells are easily sealed and convenient to use. Fig. 2(b). A clamp-on cell is a type of flat cell commonly used for coated samples. Fig. 2(c). Large areas are typically needed for clamp-on cells, and crevice corrosion is not a problem. The clamp-on cell uses a common glassware 

There are several issues related to the reference and counterelectrodes. Reliable reference electrodes are commercially available. The saturated calomel electrode (SCE) is extremely robust and is commonly used for studies in chloride solutions. For studies in which chloride is to be avoided, the mercurous sulfate electrode (MSB) is suitable. The location of the reference electrode is critical in cells in which large ohmic potential drops exist. In these cases, a Luggin capillary should be used to bring the sensing location of the reference electrode close to the working electrode 

Counterelectrodes should be made from an inert material such as Pt or graphite, and should have a large enough area to 

The droplet cell. Fig. 2(d), has uniform current distribution and shrunken dimensions that allow resistive electrolytes to be used [5]. This approach was developed for the use of pure water as an electrolyte as a means to mimic atmospheric corrosion, but it can be used with any electrolyte. An area of a flat sample is exposed through a hole in a piece of protective tape. Electroplater s tape is a very resistant tape with good adhesion that is useful for this and other masking applications in corrosion. If the hole in the tape is made with a round punch, the same punch can be used to make circular dots from pieces of filter paper. One such dot is placed securely into the exposed hole. A small (typically 10-20 gl) droplet of soluhon is placed on the filter paper using a calibrated pipette. This wet filter paper acts as the electrolyte. A piece of woven Pt mesh is placed on top of the wet filter paper, and a reference electrode is held against the back of the Pt counterelectrode. As mentioned, the small dimensions allow the use of even very pure water. This simulates atmospheric corrosion, in which a thin water layer forms on the surface. As in atmospheric corrosion, soluble species on the sample surface and pollutant gases in the air are dissolved into the water droplet, which provides some conductivity. This technique has been used 

FIGURE 3-22 Coimnon detector configurations (a) thin-layer (channel) and (b) wall-jet flow cells. WE = working electrode. 

FIGURE 3-23 Schematic of a carbon-fiber amperometric detector for capillary electrophoresis A, fused silica capillary B, eluent drop C, stainless steel plate RE, reference electrode WE, working electrode, AE, auxiliary electrode. (Reproduced with permission from reference 58.) 

Well-defined hydrodynamic conditions, with high rate of mass transport, are essential for successful use of electrochemical detectors. Based on the Nemst approximate approach, the thickness of the diffusion layer ( 5) is empirically related to the solution flow rate (U) via 

FIGURE 3-24 Electrophoretic separation of catechols with end-column detection. Detection potential, +0.8 V separation capillary, 20 kV The peaks correspond to 4.6 fmol dopamine (1), 4.1 fmol isoproterenol (2), and 2.7 fmol catechol (3). (Reproduced with permission from reference 60.) 

A more rigorous treatment takes into account the hydrodynamic characteristics of the flowing solution. Expressions for the limiting currents (under steady-state conditions) have been derived for various electrodes geometries by solving the three-dimensional convective diffusion equation  

Overwhelming majority of publications [6-9] concerning ECL reveals that it has been widely used as a detector in FIA, LC, CE, and microchip CE. For ECL equipped with FIA, various types of thin-layer flow-through ECL cells have been constmcted some of them are exemplifled in Fig. 3.1 [10]. 

Development of miniaturized analytical systems, the so-called lab on a chip, typically consist of a multi-layer glass, silicon, or polymer cell with small channels for the passage of reagents moved by conventional FI methods off-chip , or by electro-osmosis on-chip [13]. Combined with such systems, ECL has great prospective as a detection technique. The electrodes are substituted as miniaturized 

Printed circuit board (PCB) technology was employed for the fabrication of electrodes to be employed for luminol-H202 solutions. These electrodes were used with two microfluidic ECL cells that combine transparent polydimethylsiloxane (PDMS) microchannels [16]. The differences between the two cells were the working electrode size (10 mm Au vs 0.09 mm Au) and the ECL detection volume (4 pL vs 4 nL). No counter electrode was used in this system while Ag/AgCl reference electrode was obtained by electrodeposition of Ag on Au, 

An ECL detection cell used for CE separation was described in a recent publication (Fig. 3.4), in which the joint was fabricated by etching the capillary wall with hydrofluoric acid after removal of half of the circumference of the polyimide coating in a 2-3-cm section [18]. The present ECL cell offers some advantages over previously reported ECL cells, such as the joint is quite strong and whole system is fabricated with no need to fix the joint on a plate. Band broadening effect is decreased by using a very short detection capillary of 7 mm, hence increasing the CE efficiency. Moreover, the sample loss is small, and the part replacement is relatively easy [23]. 

Flow through electrochemical detectors based on a cylindrical geometry, as opposed to a planar geometry, have also been developed Three cell designs using cy- 

Both cell designs permit positioning of the second electrode downstream of the first working electrode (Fig. 11), which is known as the series configuration. This electrochemical transducer is used in the same manner as the classic ring-disk electrode. Products generated at the upstream electrode are detected (or collected) at the downstream electrode Selectivity is enhanced when the products of the upstream 

The problem of gas bubbles is to be added to the resistive effect of mechanical separators [12-14]. H2 and O2 are formed at the surface of the electrodes facing the separator. Hence the solution between electrode and diaphragm becomes saturated with gas bubbles that reduce the volume occupied by the electrolyte, thus incrementing the electrical resistance of the solution. In the conventional cell configuration, IR can be minimized, once the electrolyte and the separator are fixed, only by minimizing the distance between the anode and cathode. However, a certain distance between the electrode and separator must be necessarily maintained. 

Electrode materials in principle should not bear on ohmic drop problems. In practice, they can do, if the conductivity is poor and the thickness of the active film is sizable. Thus, although in principle IR should not depend on electrode materials but only on cell design, in practice catalytically active materials with poor electrical performance cannot be used industrially since they would unacceptably increase the energy consumption for the product unit. 

The problem of ohmic drops has recently been tackled by modifying the cell design. This is a matter of engineering, not of chemistry. The conventional design of divided cell has been replaced with a so-called zero-gap configuration [15], where the 

The new configuration calls for a modification also of the design of electrodes. These cannot be solid plates otherwise there would be no surface for electrical contact between anodic and cathodic electrolytes. Electrodes consist of perforated or stretched plates or meshes leaving free areas of separator available for electrical contacts. 

With the SPE configuration, electrodes are pressed against the membrane so that the thickness of the latter fixes the interelectrode gap. A membrane can be thiimer than a physical separator, thus reducing such a kind of contribution to IR. The ensemble of electrode and membrane is usually referred to as a membrane-electrode assembly (MEA). This term is used irrespective of the operation of the cell, that is, for both electrolyzers and fuel cells. 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

Ultramicroelectrodes can also greatly benefit modem microseparation techniques such as open-tube liquid chromatography or capillary-zone electrophoresis (CZE) (73). For example, cylinder-shaped carbon or copper fibers can be inserted into the end of the CE separation capillary (e.g., see Fig. 3.26). Such alignment of the working electrode with the end of the capillary represents a challenge in combining electrochemistry with CZE. 

Electrochemical detection offers also great promise for CZE microchips, and for other chip-based analytical microsystems (e.g., Lab-on-a-Chip) discussed in Section 6.3 (77-83). Particularly attractive for such microfluidic devices are the high sensitivity of electrochemical detection, its inherent miniaturization of both the detector and control instrumentation, low cost, low power demands, and compatibility with micromachining technologies. Various detector configurations, based on different capillary/working-electrode 

Experimental studies of the membrane transport by confocal Raman microspectrometry can be carried out only in some specific cases. Amongst various 

---

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

This reaction has a positive free energy of 422.2 kj (100.9 kcal) at 25°C and hence energy has to be suppHed in the form of d-c electricity to drive the reaction in a net forward direction. The amount of electrical energy required for the reaction depends on electrolytic cell parameters such as current density, voltage, anode and cathode material, and the cell design. 

Other Cell Designs. Although not used in the United States, another important cell is based on designs developed by ICl (90). Cells of this type are used by British Nuclear Fuels pic and differ from the cells shown in Figures 2 and 3 in two ways (/) the anodes used are made of the same hard, nongraphitized carbon, but are more porous and 2) the cathodes are formed from coiled tubes and provide additional cooling (91). 

Ethylene glycol can be produced by an electrohydrodimerization of formaldehyde (16). The process has a number of variables necessary for optimum current efficiency including pH, electrolyte, temperature, methanol concentration, electrode materials, and cell design. Other methods include production of valuable oxidized materials at the electrochemical cell s anode simultaneous with formation of glycol at the cathode (17). The compound formed at the anode maybe used for commercial value direcdy, or coupled as an oxidant in a separate process. 

Fig. 11. Solid polymer electrolyte (SPE) fuel cell (a) cell design and (b) power curve at 25°C.

Because of limited commercial experience with anode coatings in membrane cells, commercial lifetimes have yet to be defined. Expected lifetime is 5—8 years. In some cases as of this writing (ca 1995), 10-years performance has already been achieved. Actual lifetime is dictated by the membrane replacement schedule, cell design, the level of oxygen in the chlorine gas, and by the current density at which the anode is operated. 

The latitude that titanium affords the cell designer has made a wide variety of monopolar and bipolar membrane cell designs possible. 

Producers have developed specific cell configurations to optimise electricity consumption, cell capital, and operating costs. Pacific Engineering Corp., Kerr-McGee Chemical Corp., Chedde Pechiney, Cardox Corp., Electrochemie Turgi, American Potash and Chemical, and I. G. Earbenindustrie each has a unique cell design. 

Flotation. Flotation (qv) is used alone or in combination with washing and cleaning to deink office paper and mixtures of old newsprint and old magazines (26). An effective flotation process must fulfill four functions. (/) The process must efficiently entrain air. Air bubble diameter is about 1000 p.m. Typically air bubbles occupy 25—60% of the flotation cell volume. Increa sing the airRquid ratio in the flotation cell is said to improve ink removal efficiency (27). (2) Ink must attach to air bubbles. This is primarily a function of surfactant chemistry. Air bubbles must have sufficient residence time in the cell for ink attachment to occur. (3) There must be minimal trapping of cellulose fibers in the froth layer. This depends on both cell design and surfactant chemistry. (4) The froth layer must be separated from the pulp slurry before too many air bubbles coUapse and return ink particles to the pulp slurry. 

Other commercial cells designed for the electrolysis of fused sodium chloride iaclude the Danneel-Lon2a cell and the Seward cell, both used before World War I. The former had no diaphragm and the sodium was confined to the cathode 2one by salt curtains (ceramic walls) the latter utili2ed the contact-electrode principle, where the cathode was immersed only a few millimeters ia the electrolyte. The Ciba cell was used over a longer period of time. 

Separator s a physical barrier between the positive and negative electrodes incorporated into most cell designs to prevent electrical shorting. The separator can be a gelled electrolyte or a microporous plastic film or other porous inert material filled with electrolyte. Separators must be permeable to ions and inert in the battery environment. 

Discharging to this lower cell voltage usually results ia shorter cycle life. Enough excess iron should be provided ia the cell design to avoid this problem. Active iron ia the metallic state is slowly attacked by the alkaline electrolyte according to... 

However, the generation and migration of water in the half-ceU reactions must be considered in the cell design. At the nickel electrode ... 

Li—Al/FeS cells have demonstrated good performance under EV driving profiles and have deUvered a specific energy of 115 Wh/kg for advanced cell designs. Cycle life expectancy for these cells is projected to be about 400 deep discharge cycles (63). This system shows considerable promise for use as a practical EV battery. 

The Na—S system is expected to provide significant iacreases ia energy density for sateUite battery systems (69). In-house testing of Na—S cells designed to simulate midaltitude (MAO) and geosynchronous orbits (GEO) demonstrated over 6450 and over 1400 cycles, respectively. 

A battery system closely related to Na—S is the Na—metal chloride cell (70). The cell design is similar to Na—S however, ia additioa to the P-alumiaa electrolyte, the cell also employs a sodium chloroalumiaate [7784-16-9J, NaAlCl, molten salt electrolyte. The positive electrode active material coasists of a transitioa metal chloride such as iroa(Il) chloride [7758-94-3] EeQ.25 or nickel chloride [7791-20-0J, NiQ.25 (71,72) in Heu of molten sulfur. This technology is in a younger state of development than the Na—S. 

Current efficiency depends on operating characteristics, eg, pH, temperature, and cell design, and is generally in the 90—98% range. The cell voltage is a function of electrode characteristics and electrolyte conductivity and can be expressed as... 

Fig. 4. Sodium chlorate cell designs (a) the horizontal bipolar cells used by Huron having narrow gap vertical plates or horizontal mesh where (-)...

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Electrode materials and shapes may have a profound effect on cell designs. Anode materials encountered ia electrochemical processes are... 

Membrane cells are the state of the art chlor-alkah technology as of this writing. There are about 14 different membrane cell designs in use worldwide (34). The operating characteristics of some membrane cells are given in Table 3. The membranes are perfluorosulfonate polymers, perfluorocarboxylate polymers, and combinations of these polymers. Membranes are usually reinforced with a Teflon fabric. Many improvements have been made in membrane cell designs to accommodate membranes in recent years (35,36). 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

---