Membranes and Electrochemical Cells

This is an elementary example of how the expression for entropy production can be used to obtain linear relations between thermodynamic forces and flows, which often turn out to be empirically discovered laws such as Ohm s law. In section 10.3 we shall see that similar consideration of entropy production due to diffusion leads to another empirically discovered law called the Pick s law of diffusion. Modern thermodynamics enables us incorporate many such phenomenological laws into one unified formalism. 

Just as equilibrium with a semipermeable membrane produced a difference in pressure (the osmotic pressure) between the two sides of the membrane, equilibrium of ions across a membrane that is permeable to one ion but not another results in an electric potential difference. As an example, consider a membrane separating two solutions of KCl of unequal concentrations (Fig. 10.4). We assume that the membrane is permeable to K ions but is impermeable to the larger Cl ions. Since the concentrations of the K ions on the two sides of the membrane are unequal, K+ ions will begin to flow to the 

From this equation it follows that the potential difference, the membrane potential = ((j) — j ) across the membrane, can now be written as 

In electrochemistry the concentrations are generally measured using the molality scale, as discussed in Chapter 8. In the simplest approximation, the activities may be replaced by molalities mK+, i.e. the activity coefficients are assumed to be unity. Hence one may estimate the membrane potential using 

In an electrochemical cell the reactions at the electrodes that transfer electrons can generate an electromotive force (EMF). An electrochemical cell generally has different phases that separate the two electrodes (Fig. 10.5). By considering entropy production due to the overall reaction and the electric current flowing through the system, we can derive a relationship between the electrochemical activity and the EMF. In an electrochemical cell the reactions at the two electrodes can generally be written as 

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A porous anode and cathode are attached to each surface of the membrane, forming a membrane-electrode assembly, similar to that employed in SPE fuel cells. Electrochemical reactions (electron transfer-l-hydrogenation) occur at the interfaces between the ion exchange membrane and electrochemically active layers of electrodes. Electrochemical reductive HDH occurred at the interfaces between the ion exchange membrane and the cathode catalyst layer when an electrical current is applied between the electrodes ... 

Membrel cell — (membrane electrolysis) Electrochemical cell developed by BBC Brown Boveri Ltd, now joined with ASEA AB, to ABB Asea Brown Boveri Ltd) for water electrolysis. A polymeric cation exchange membrane acting as -> solid electrolyte is placed between a catalyst-coated porous graphite plate acting as cathode and a catalyst-coated porous titanium plate acting as anode. 

Most of the current applications of perfluorosulfonate membranes involve electrochemical cells in which concentrated electrolyte solutions are employed, often at elevated temperatures. Relatively little diffusion data are available under these conditions, although a larger amount of membrane resistance and other operating data have been published. Sodium ion self-diffusion coefficients have been measured in various Nafion membranes in concentrated NaOH solutions at elevated temperatures (23). This... 

W. G. Grot, G. Rajendran (E. 1. du Pont de Nemours and Company) Membranes containing inorganic fillers and membrane and electrode assemblies and electrochemical cells employing same. US Patent 5919583, July 1999. 

Al-air fuel cells, Zn-Mn02 and Al-Mn02 cells, were assembled with anodes, cathodes and alkaline solid polymer electrolyte membranes. The electrochemical cells showed excellent cell power density and high electrode utilization. Therefore, these PVA-based solid polymer electrolyte membranes have great advantages in the applications for all-solid-state alkaline fuel cells. Some other potential applications include small electrochemical devices, such as supercapacitors and 3C electronic products. 

Many solid electrolytes are known today and it can be expected that their importance will further increase, especially for electrochemical devices. For example, beta-alumina solid electrolyte (BASE) is a fast ion conductor, which is used as a membrane in electrochemical cells. It can contain small ions like sodium, which show a high mobility. More classical examples are electrolytes based on lithium or silver iodide where the small cations are very mobile [13]. Note that solid polymer electrolytes are also a rapidly growing field [14]. 

The concept of the reversed fuel cell, as shown schematically, consists of two parts. One is the already discussed direct oxidation fuel cell. The other consists of an electrochemical cell consisting of a membrane electrode assembly where the anode comprises Pt/C (or related) catalysts and the cathode, various metal catalysts on carbon. The membrane used is the new proton-conducting PEM-type membrane we developed, which minimizes crossover. 

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. 

Electrochemical processes require feedstock preparation for the electrolytic cells. Additionally, the electrolysis product usually requires further processing. This often involves additional equipment, as is demonstrated by the flow diagram shown in Figure 1 for a membrane chlor-alkali cell process (see Alkali AND chlorine products). Only the electrolytic cells and components ate discussed herein. 

For a profitable electrochemical process some general factors for success might be Hsted as high product yield and selectivity current efficiency >50%, electrolysis energy <8 kWh/kg product electrode, and membrane ia divided cells, lifetime >1000 hours simple recycle of electrolyte having >10% concentration of product simple isolation of end product and the product should be a key material and/or the company should be comfortable with the electroorganic method. 

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). 

With eveiy change in ion concentration, there is an electrical effect generated by an electrochemical cell. The anion membrane shown in the middle has three cells associated with it, two caused by the concentration differences in the boundaiy layers, and one resulting from the concentration difference across the membrane. In addition, there are ohmic resistances for each step, resulting from the E/I resistance through the solution, boundary layers, and the membrane. In solution, current is carried by ions, and their movement produces a fric tion effect manifested as a resistance. In practical applications, I R losses are more important than the power required to move ions to a compartment wim a higher concentration. 

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. 

Ren, X. Springer, T. E. and Gottesfeld, S. (1998). Direct Methanol Fuel Cell Transport Properties of the Polymer Electrolyte Membrane and Cell Performance. Vol. 98-27. Proc. 2nd International Symposium on Proton Conducting Membrane Euel Cells. Pennington, NJ Electrochemical Society. 

Voltage-gated Ca2+ channels are Ca2+-selective pores in the plasma membrane of electrically excitable cells, such as neurons, muscle cells, (neuro) endocrine cells, and sensory cells. They open in response to membrane depolarization (e.g., an action potential) and permit the influx of Ca2+ along its electrochemical gradient into the cytoplasm. 

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. 

Potassium channels are a diverse and ubiquitous family of membrane proteins present in both excitable and nonexcitable cells that selectively conduct K+ ions across the cell membrane along its electrochemical gradient at a rate of 106-108 ions/s. 

As described above, some solutes such as gases can enter the cell by diffusing down an electrochemical gradient across the membrane and do not require metabolic energy. The simple passive diffusion of a solute across the membrane is limited by the thermal agitation of that specific molecule, by the concentration gradient across the membrane, and by the solubility of that solute (the permeability coefficient. Figure 41—6) in the hydrophobic core of the membrane bilayer. Solubility is... 

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