Electrochemical cells, membrane separator

Figure 2.115 shows a schematic representation of the DEMS apparatus. In essence, the electrochemical cell is separated from a mass spectrometer by a porous, non-wetting PTFE membrane of very small pore size. The working electrode is then deposited as a porous metal layer on the thin... 

The type of apparatus described here has been used by many workers since the early 1900s, but was not used much until the work of Devanathan and Stachurski emphasized its potential in the early 1960s. The principle is quite simple and is illustrated in Fig. 25. Two electrochemical cells are separated by a metallic membrane, which acts as the working electrode in each of the cells. 

In the second method to produce ADN, known as electrohydrodimerization, two moles of acrylonitrile [107-13-1] are combined and hydrogenated in an electrochemical cell where the two half-cells are separated by a membrane. 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

Although ED is more complex than other membrane separation processes, the characteristic performance of a cell is, in principle, possible to calculate from a knowledge of ED cell geometry and the electrochemical properties of the membranes and the electrolyte solution. 

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

Electrochemical cells are of two types power cells and sensors. In an ideal power cell, the ionic current through the electrolyte inside the cell matches an electronic current through an external load. The solid electrolyte is in the form of a membrane of thickness L and area A that separates electronically the two electrodes of the cell. Any internal electronic current across the electrolyte reduces the power output. The internal resistance to the ionic current is... 

Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new separators. Similarly, not much attention has been given to separators in publications reviewing batteries.A number of reviews on the on cell fabrication, their performance, and application in real life have appeared in recent years, but none have discussed separators in detail. Recently a few reviews have been published in both English and Japanese which discuss different types of separators for various batteries. A detailed review of lead-acid and lithium-ion (li-ion) battery separators was published by Boehnstedt and Spot-nitz, respectively, in the Handbook of Battery Materials. Earlier Kinoshita et al. had done a survey of different types of membranes/separators used in different electrochemical systems, including batteries."... 

The earliest concerted effort in the research and development of Nafion perfluorosulfonate ionomers was directed toward their use as a permselective membrane separator in electrochemical cells used in the large scale industrial production of NaOH, KOH, and CI2. In short, the membrane in this application, in addition to keeping CI2 and H2 gases separated, prevents the unfavorable back migration of hydrated OH ions from the catholyte (concentrated aqueous NaOH or KOH) chamber, while allowing for the transport of hydrated Na+ ions from the anolyte chamber in which is aqueous NaCl. 

The principles of the fuel cell are illustrated in Figure 1.1. The electrochemical cell consists of two electrodes, an anode and a cathode, which are electron conductors, separated by an electrolyte [e.g. a proton exchange membrane (PEM) in a PEMFC or in a DAFC], which is an ion conductor (as the result of proton migration and diffusion inside the PEM). An elementary electrochemical cell converts directly the chemical... 

Styrene and Vinyl Monomer, Polymer, and Copolymer Sulfonates. The incorporation of sulfonates into polymeric material can occur either after polymerization or at the monomer stage. The sulfonic acid group is strongly acidic and can therefore be used to functionalize the polymer backbone to the desired degree. The ability of sulfonic acids to exchange counterions has made these polymers prominent in industrial water treatment applications, separators in electrochemical cells, and selective membranes of many types. 

Electrochemical cells may consist of two electrodes of the same type, but with different concentrations of the electroactive species in the electrolyte. Such cells are known as concentration cells. For example, two platinum electrodes operate in two H+/H2 solutions of different activity, separated by a membrane. The equilibrium cell voltage is defined by Equation (21a). As the standard potential is the same for both electrode reactions, the measurable cell voltage will depend only on the activity ratios, Equation (21b). If in this system both electrolytes were in equilibrium with the same EE pressure, the measured E would respond linearly to the pH difference between the two electrolytes, Equation (21c) (i.e. a pH electrode). 

The electrochemical 2-chlorophenol and 2,6-dichlorophenol removal from aqueous solutions using porous carbon felt (Polcaro and Palmas 1997) or a fixed bed of carbon pellets (Polcaro et al. 2000) as three-dimensional electrodes was investigated by Polcaro s group. The group s experimental setup consisted of a two-compartment electrochemical cell separated by an anionic membrane where the carbon felt or pellets could be lodged and the solution was recirculated by peristaltic pumps. Both carbon-based anodes effectively removed the chlorophenols as well as their reaction... 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Thiele, W. and Foerster, H.-J. (2006) Progress in electrochemical ozone generation and disinfection of ultra-pure water using new electrochemical cell with polymer membrane separators (in German). Proceedings of the Annual GDCh Meeting, Bayreuth 2006. 

The quantity of ambipolar conductivity is widely used for the analysis of -> electrolytic permeability of -> solid electrolytes, caused by the presence of electronic conductivity. Other important cases include transient behavior of electrochemical cells and ion-conducting solids, dense ceramic membranes for gas separation, reduction/ oxidation of metals, and kinetic demixing phenomena [iv]. In most practical cases, however, the ambipo-... 

Divided cells — Electrochemical cells divided by sintered glass, ceramics, or ion-exchange membrane (e.g., - Nafion) into two or three compartments. The semipermeable separators should avoid mixing of anolyte and - catholyte and/or to isolate the reference electrode from the studied solution, but simultaneously maintain the cell resistance as low as possible. The two- or three-compartment cells are typically used a) for preparative electrolytic experiments to prevent mixing of products and intermediates of anodic and cathodic reactions, respectively b) for experiments where different composition of the solution should be used for anodic and cathodic compartment c) when a component of the reference electrode (e.g., water, halide ions etc.) may interfere with the studied compounds or with the electrode. For very sensitive systems additional bridge compartments can be added. 

A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electricity. The cell consists of three main parts the fuel compartment, the oxidant compartment, and an electrolyte membrane separating the fuel and oxidant. At the fuel side, the fuel is oxidized and electrons are released. At the oxidant side, the oxidant is reduced by accepting the electrons released from the fuel side. The electrons flowing through the fuel side to the oxidizer side can be harnessed, producing electric power. For an H2/air fuel cell, the reactions are ... 

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