Cell voltage components

Cell voltage components under realistic conditions. 

Table 8. Components of Chlor—Alkali Cell Voltages...

The components of the diaphragm, membrane, and mercury cell voltages presented ia Table 8 show that, although the major component of the cell voltage is the term, ohmic drops also contribute to the irreversible energy losses duting the operation of the cells. 

The individual components of smelting-cell voltage and energy are shown in Figure 5. The electrical energy required to decompose aluinina Fj = 2.233 V in a cryoHte bath 65% saturated with alumina is given by... 

To date, as indicated in Fig. 17.3, the effort to reduce cell voltage has been focused on the membrane and the electrodes. In current commercial operations at 4 kA m-2 with a 2 mm electrode gap, the voltage loss of the membrane, anode and cathode has been reduced to approximately 350 mV, 50 mV and 100 mV, respectively, and thus a total of approximately 500 mV for these cell components, or less than one-third the voltage loss of these components in the early years of the commercial membrane process. 

Typical PAFCs will generally operate in the range of 100 to 400 mA/cm at 600 to 800 mV/cell. Voltage and power constraints arise from increased corrosion of platinum and carbon components at cell potentials above approximately 800 mV. 

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. 

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. 

Another kinetic aspect is observed if a component other than the electroactive species is predominantly mobile. The electroactive species are in this case made available to the electrolyte by the motion of the other components in the opposite direction. In a binary compound this does not make a difference to the electrode performance. But in the case of a compound with more than two components the composition is changed to values which are not expected from a thermodynamic point of view for the variation of the concentration of the electroactive species. Other phases are formed which may provide a lower or higher activity of the electroactive species than that expected thermodynamically. This has an influence both on the current and the cell voltage. Upon discharging and charging a galvanic cell, the composition of the electrode at the interface with the electrolyte may follow very different compositional pathways (Weppner, 1985). 

The cell voltage depends on the variation of the Gibbs energy of formation of the electrode compound with the stoichiometry of the electroactive species. Energy is accordingly only stored if chemical work is necessary to add or take away the electroactive component. 

The phase diagram of an electrode material may be determined from the slope of the coulometric titration curve. An electrode of N components shows activities which are independent of the composition as long as the maximum number of N phases are in equilibrium with each other. Relative changes in the amounts of the different phases do not change the activities of the components and therefore keep the cell voltage constant. This causes voltage plateaux for any region of the equilibrium of the maximum number of phases. 

Also, the phases formed in the course of discharge of an electrode with three or more components may be readily detected by reading the equilibrium cell voltage. As an example, the determination of the quite complex ternary phase diagram of the system Li-In-Sb is shown in Fig. 8.9. In this case, plateaux are observed in the presence of three-phase equilibria. In order to obtain the complete phase diagram it is necessary... 

As discussed in Section 8.2 the relation between the chemical diffusion coefficient and diffusivity (sometimes also called the component diffusion coefficient) is given by the Wagner factor (which is also known in metallurgy in the special case of predominant electronic conductivity as the thermodynamic factor) W = d n ajd In where A represents the electroactive component. W may be readily derived from the slope of the coulometric titration curve since the activity of A is related to the cell voltage E (Nernst s law) and the concentration is proportional to the stoichiometry of the electrode material ... 

We have seen in Section 5.2 that one can determine the relative electrode potential by measuring cell voltage. To form a series of relative electrode potentials, one has to select a reference electrode and standard conditions of components of an electrode/ electrolyte interphase. 

In electrical communication, changes in membrane potential are used to conduct a stimulus within a nerve cell. Changes in membrane potential can also be used for intercellular commimication. In this case, communication between the cells takes place via electrical synapses at which the potential change can be directly passed on to neighboring cell. Central components of electrical communication are voltage-dependent ion channels with open states regulated by changes in the membrane potential. 

The double-cell voltage-clamp setup (as used in the author s laboratory) consists of the following components ... 

Calculate the cell voltage for each of the cells constructed in Problem 1, assuming (i) all components at unit activity, or (u)the negative half-cell ionic concentrations to be i0-4molar and the positive half-cell ionic concentrations... 

Irreversible losses result in the difference of the efficiency between the reversible and the real processes. These losses can be described by the irreversible entropy production within the components however the system structure itself might be reversible. The consideration of the ohmic losses shows that the irreversible entropy production at a high temperature is smaller than at a low temperature. The effects of the irreversible mixing of reactants and products lead to an irreversible entropy production as well that reduce the cell voltage. 

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