Cell voltage and its components

Cell Voltage and Its Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, E°. E° is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

We have developed two-dimensional CE systems for the characterization of proteins and biogenic amines. The use of this technology for chemical cytometry is similar to the use of onedimensional electrophoresis a cell is aspirated into the column, lysed, and its components labeled with FQ. For two-dimensional electrophoresis, components are separated based on CSE in the first-dimension capillary. Fractions are then transferred across an interface to a second capillary, where they undergo additional separation based on micellar electrokinetic chromatography (MECC) before detection by fluorescence. The voltage drop across the first capillary is set to zero during the second dimension separation, holding components stationary. In a typical experiment, 300 fractions are transferred between capillaries under computer control. 

In the discussion of the Daniell cell, we indicated that this cell produces a voltage of 1.10 V. This voltage is really the difference in potential between the two half-cells. The cell potential (really the half-cell potentials) is dependent upon concentration and temperature, but initially we ll simply look at the half-cell potentials at the standard state of 298 K (25°C) and all components in their standard states (1M concentration of all solutions, 1 atm pressure for any gases and pure solid electrodes). Half-cell potentials appear in tables as the reduction potentials, that is, the potentials associated with the reduction reaction. We define the hydrogen half-reaction (2H+(aq) + 2e - H2(g)) as the standard and has been given a value of exactly 0.00 V. We measure all the other half-reactions relative to it some are positive and some are negative. Find the table of standard reduction potentials in your textbook. 

Based on the present understanding of the effect of sulfur on MCFCs, and with the available cell components, it is projected that long-term operation (40,000 hr) of MCFCs may require fuel gases with sulfur " levels of the order 0.01 ppm or less, unless the system is purged of sulfur at periodic intervals or sulfur is scrubbed from the cell burner loop (76). Sulfur tolerance would be approximately 0.5 ppm (see Table 6-3) in the latter case. Considerable effort has been devoted to develop low-cost techniques for sulfur removal, and research and development are continuing (80,81). The effects of H2S on cell voltage are reversible if H2S concentrations are present at levels below which nickel sulfide forms. 

As with the other cell types, it is necessary to stack SOFCs to increase the voltage and power being produced. Because there are no liquid components, the SOFC can be cast into flexible shapes (Figure 8-1). 

One crifical paramefer fhaf affecfs fhe fhickness of fhe diffusion layer is fhe compression force used in fhe fuel cell in order fo avoid any gas leaks and to assure good contact between all the components. However, this compressive force can deform the diffusion layer and hence affect the performance of the cell. More information regarding how the compression forces affect the diffusion layer is discussed in Section 4.4.5. Ideally, the material used as the DL should be able to resist this compression force or pressure without affecting most of its parameters. Figure 4.21 shows a schematic of the cell voltage (performance) at a given current density, resistance, and DL porosity as a function of the cell s compression. 

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