Porous separator

Separator backweb refers to the porous separating membrane. It is of uniform thickness and has a macroscopically uni-... 

C19-0135. Consider an electrochemical cell consisting of two vessels connected by a porous separator. One vessel contains 0.500 M HCl solution and an Ag wire electrode coated with AgCl solid. The other vessel contains 1.00 M MgCl2 solution and an Mg wire electrode, (a) Determine the net reaction, (b) Calculate E for the cell (see Appendix F). (c) Draw a molecular picture showing the reactions at each electrode. 

In porous separators the pore radii are large compared to the size of molecules. Hence, interaction between the electrolyte and the pore walls has practically no qualitative effects on the ionic current through the separator the transport numbers of the individuaf ions have the same vafues in the pores as in the bulk electrolyte, hi swollen membranes the specific interaction between individuaf ions and macromofecufes is very pronounced. Hence, these membranes often exhibit sefectivity in the sense that different ions are affected differentfy in their migration. As a resuft, the transport numbers of the ions in the membrane differ from those in the efectrofyte outside the membrane. In the limiting case, certain types of ion are arrested completely, and the membrane is called permselective (see Chapter 5). 

Influence on Electrolyte Conductivity In porous separators the ionic current passes through the liquid electrolyte present in the separator pores. Therefore, the electrolyte s resistance in the pores has to be calculated for known values of porosity of the separator and of conductivity, o, of the free liquid electrolyte. Such a calculation is highly complex in the general case. Consider the very simple model where a separator of thickness d has cylindrical pores of radius r which are parallel and completely electrolyte-filled (Fig. 18.2). Let / be the pore length and N the number of pores (all calculations refer to the unit surface area of the separator). The ratio p = Ud (where P = cos a > 1) characterizes the tilt of the pores and is called the tortuosity factor of the pores. The total pore volume is given by NnrH, the porosity by... 

Real porous separators differ from the model above. It was found experimentally that in real porons systems the relation... 

Two basically different types of cell separators are available porous separators... 

Decisive characteristics of porous separators ( diaphragms ) are porosity, pore diameter, and thickness. For practical use, other aspects such as mechanical strength (brittle or flexible), constant dimensions (swelling in the solvent), and chemical stability are important. 

The mostly applied porous separator in laboratory cells is sintered glass ( fritted glass ). It can be simply and leakproof mounted by the glassblower into the walls of glass cells and then it is very easy to handle. It is available - like similar ceramic materials, for example, sintered aluminum oxide or unglazed porcelain... 

Mkaline Fuel Cell The electrolyte for NASA s space shnttle orbiter fuel cell is 35 percent potassinm hydroxide. The cell operates between 353 and 363 K (176 and I94°F) at 0.4 MPa (59 psia) on hydrogen and oxygen. The electrodes contain platinnm-palladinm and platinum-gold alloy powder catalysts bonded with polytetraflnoro-ethylene (PTFE) latex and snpported on gold-plated nickel screens for cnrrent collection and gas distribution. A variety of materials, inclnding asbestos and potassinm titanate, are used to form a micro-porous separator that retains the electrolyte between the electrodes. The cell structural materials, bipolar plates, and external housing are nsnally nickel-plated to resist corrosion. The complete orbiter fuel cell power plant is shown in Fig. 24-48. 

For many processes, how ever, it is necessary to employ a divided cell in which the anode and cathode compartments are separated by a barrier, allowing the diffusion of ions but hindering transfer of reactants and products between compartments. This prevents undesirable side reactions. Good examples of the need for a divided cell are seen in the reduction of nitjobenzenes to phenylhydroxylamines (p. 379) or to anilines (p. 376). In these ca.scs the reduction products are susceptible to oxidation and must be prevented from approaching the anode. The cell compartments can be divided with a porous separator constructed from sintered glass, porous porcelain or a sintered inert polymer such as polypropene or polytetra-... 

The silver oxide and mercuric oxide button cells used in cameras and other devices requiring a miniature source of EMF consist of a zinc disk, which serves as the anode, and, on the other side of a porous separator, a paste of Ag20 or HgO. The reaction products are zinc hydroxide and metallic silver or mercury. Inert metal caps serve as the current collectors. 

Fig. 5.6 (a) Tubular plates for lead-acid cells, (b) Cross-section showing central lead current collector, active material and porous separators... 

Figure 2.4 SEM micrograph of a cross-section of a hollow-fiber dialysis membrane (Polyflux, Cambro) with an anisotropic structure and macrovoids in the support layer (left), and details of the inner porous separation layer in two different magnifications (right reprinted from [12], with permission from Wiley-VCH, 2003).

FIGURE 1.26 Three-electrode laboratory cell. (A) Porous carbon electrode film (carbon + binder) (B) treated A1 current collectors (C) porous separator (D) Teflon plates (for stack pressure) and (E) stainless steel clamps (not shown, see arrows). (From Gamby, J., et al., J. Power Sources, 101,109, 2001.)... 

Liquid junction potentials are the result of different cation and anion mobilities under the influence of an electric field. The potential manifests itself in the interface between two different solutions separated by a porous separator or by a membrane. These junctions can be classified into three distinct types ... 

When it comes to practical application, the actual conductance (the inverse of resistance R) instead of specific conductivity is the important variable. This is the reason why polymer electrolytes have drawn so much attention as a potential component of allmli-metal batteries aithough their specific conductivities are usually low ( 10 S cm ) compared with those at nonaqueous eiectrolytes (-10 S cm ). Calculate the conductance of 1.0 A/ LiSOjCFj in poiy(ethyiene oxide) and propylene carbonate, respectively. The former is fabricated into a fiim of 10 Tm thickness and the latter is soaked with porous separator of 1 mm thickness. (Xu)... 

Membranes can be either dense or porous. Separation by dense membranes relies on physicochemical interaction between the permeating components and the membrane material. Porous membranes, on the other hand, achieve separation by size exclusion, where the rejected material may be either dissolved or suspended depending on its size relative to that of the pore (3). 

Both the anode and the cathode are composed of a coating of the electrochemically active material onto a current collector (copper or aluminum). Another key component of the battery is the separator that physically separates the two electrodes and prevents contact between them. In the case of a liquid technology battery, a polyolefin separator is typically used and a liquid electrolyte is used to transport the Li ions from one side of the porous separator to the other. In the case of a polymer Li ion battery, a polymer, such as PVDF, is used to form a porous structure, which is then swollen with a Li" " conducting liquid electro-lyte. " This results in a gel-type electrolyte, which plays the dual role of electrolyte and separator, with no free liquid present. 

---