Cell voltage, overall

In the sodium—sulfur storage battery above 300°C, the overall chemical reaction occurs between molten sodium metal and sulfur to form sodium polysulfide. The cell voltage is related to the activity of the sodium ( Aia) sulfide relative to its activity in the metal. 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

In practice, the decomposition potential for this overall reaction is found to be about 1.5 V this somewhat high value probably results from polarization and contact resistances. It could be seen that the electrochemical decomposition of alumina to deposit aluminum, using an inert anode, would require a theoretical cell voltage of 2.21 V as opposed to that of 1.18 V when carbon is used as the anode. Thus the participation of carbon in the cell reaction brings down the theoretically required cell voltage by almost 50%. 

A cell in this type of battery has electrodes made of lead and spongy lead impregnated with Pb02. Sulfuric acid is the electrolyte, and the cell voltage is approximately 2.0 V. Depending on the number of cells linked in series, the overall voltage can be 6 or 12 (the more common type of battery). 

In chlorate production the EMOS system has also been used to determine the formation of deposits on the electrodes, either the anode or cathode and combined with the information on process and electrolyte composition the system determines the need for cell cleaning or acid rinsing. The close monitoring of individual cell voltages has allowed plant engineers to establish the most appropriate current density for production lines dependent upon the state of the anode coatings. This allows for the same overall production capacity while permitting the operation of two different cell lines in the cell room at different current densities based upon the state of the anodes and cathodes in the cell. 

A clear advantage of alkaline electrolysers is the use of nickel-based electrodes, thus avoiding the use of precious metals. Catalytic research is aimed at the development of more active anodes and cathodes, primarily the development of high surface area, stable structures. Nickel-cobalt spinel electrodes for oxygen evolution and high surface area nickel and nickel cobalt electrodes for hydrogen evolution have been shown at the laboratory scale to lead to a decrease in electrolyzer cell voltage [47]. More active electrodes can lead to more compact electrolysers with lower overall systems cost. 

The ideal performance of a fuel cell is defined by its Nemst potential represented as cell voltage. The overall cell reactions corresponding to the individual electrode reactions listed in Table 2-1 are given in Table 2-2, along with the corresponding form of the Nemst equation. The Nemst... 

Cell Voltage - For an identical stack the overall cell voltage will be lower as temperature decreases due to the decreased kinetics, diffusion, and ionic conductivity versus the improved electrical conductivity which typically does not dominate the cell polarizations. This is partially but not fully offset by the increased theoretical open circuit voltage of the electrochemical reaction at the lower temperature. 

We also found that in the presence of ultrasound that not only was there a drop in overall cell voltage from 8.3 V to 7.3 V but the reaction approached completion in a shorter timescale despite the apparent switch to the two-electron process. Overall, ultrasound appeared to favour the two-electron mechanism, and the greatest effect of sonication upon product distribution was the substantial enhancement of alkene formation. 

Bromide ions serve as a catalyst, although in practice there is some loss due to the formation of bromate in side reactions. The process has also been run with a feed of oxygen through a porous cathode, which eliminates the liberation of hydrogen and decreases the overall cell voltage. 

Notice that when the silver reaction is multiplied by 2 its E° value stays the same. The potential of a half-cell does not depend on the coefficients of the equation because the potential of the cell is independent of the quantities of reactants. Adding the two half reactions gives the overall reaction and the cell voltage ... 

The question now arises as to what factors are responsible for determining the rates at which the various cell processes occur. Thermodynamic arguments permit the feasibility of overall cell reactions to be predicted, but give no information on rates. To understand the latter it is necessary to consider the effects on various parts of the cell of forcing the cell voltage to assume a value different from that of the equilibrium emf. It has been shown above that in the Daniell cell at equilibrium, charge transfer across the zinc/solution interface can be described in terms of processes... 

For spontaneous discharge, the overall cell voltage must be reduced from its equilibrium value, as would happen if a load resistor were connected to the terminals. If a potential difference greater than the emf were applied (i.e. one making the cathode more positive and the anode more negative), the net result would be a current flow in the reverse direction, causing a net charging of the cell. 

This equation has a standard electrode potential of -0.447 V. Thus, the solution containing mercuric and chloride ions in contact with iron forms a battery. The reduction of the complex ions to metallic mercury is the cathodic reaction. The dissolution of iron is the anodic reaction. The overall reaction in the battery is given by the addition of Equation (13.42) and Equation (13.43). Due to the high value of its reversible cell voltage under standard conditions (0.85 V), it is expected that a very low equilibrium concentration of the complex ion can be achieved. 

The equivalent circuit corresponding to this interface is shown in Fig. 6.1b. The charge-transfer resistances for the exchange of sodium and chloride ions are very low, but the charge-transfer resistance for the polyanion is infinitely high. There is no direct sensing application for this type of interface. However, it is relevant for the entire electrochemical cell and to many practical potentiometric measurements. Thus if we want to measure the activity of an ion with the ion-selective electrode it must be placed in the same compartment as the reference electrode. Otherwise, the Donnan potential across the membrane will appear in the cell voltage and will distort the overall result. 

The deviation of the flux from a purely radial configuration can lead to several consequences on the operational conditions of the fuel cell, thus affecting the resultant performance. First of all, if the gas is not well distributed within the cell surface, the reaction rate varies from area to area. This implies the existence of preferred zones for the electrochemical reaction, and, consequently, of local high current density, thus reducing the overall cell voltage. Secondly, different reaction rates throughout the cell causes temperature gradients and, consequently, thermal stresses, which can cause mechanical failure of the cell [2-4], Finally, the existence of (some) preferred zones for the electrochemical reaction implies that part of... 

Each tube is a sequence of cells electrically connected in series and this justifies the hypothesis that the current is the same for all the cells. Since the fuel flows in series through the various cells of the tube, fuel compositions vary from one cell to the next. As a consequence, cell voltage also varies from cell to cell, and this can be evaluated through a detailed evaluation of the local electrochemical kinetics of the reactor, as described in Section 6.2 of this work. When the model of the electrochemical kinetics is included into the overall model of the tube, an additional type of resistance is taken into consideration, i.e. the contact resistance (an approach... 

In closing, we see that certain simplified models are beneficial, especially when they permit analytical solutions to be derived showing the influence of all key governing parameters. As seen in the above analysis, however, certain information is unavailable such as information on the transient cell voltage. In addition, effort is needed to independently assess the overall cell resistance, R(I). which, in general, is also a function of other parameters (reactant concentration, temperature, and pressure). This can be remedied by adding additional equations as the following section shows. 

The maximum cell voltage, which varies in the range of 0.8-1.5 V/cell under the current density recommended by the manufacturers, tends to increase with time as the charged groups in the electromembranes vanish with use as a result of their chemicophysical reactions with the feed contaminants. Beyond such potential difference limits, it is generally advisable to replace the membranes to limit the overall electric power consumption. 

This deviation from the thermodynamic reversible behavior leads to a decrease of the cell voltage by ca. 0.4-0.6 V, which reduces the energy efficiency of the fuel cell by a factor eE = E(j)/ Eeq, called voltage efficiency (eE = 0.80/1.23 = 0.65 for a cell voltage of 0.80 V), so that the overall energy efficiency at 25°C for the H2/02 fuel cell becomes 0.83 x 0.65 = 0.54. 

Various authors consider appropriate flow sheets for fuel cell systems. Fellows [5] proposes to employ a cascade of fuel cells, operating at different cell voltages, to increase the overall electric power output. According to this concept the exhaust gas... 

If you look at the effect this has on the overall equation, when the value of log Q is negative, the value of °° 92 log Q will be added to the cell voltage. In other words, it increases over standard conditions. When the value of log Q is positive, the value of °° 92 log Q will be subtracted from the cell voltage. In other words, it decreases over standard conditions. 

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