Calculating Cell Voltage

Fig. 7.21 Influence of edge rounding radius on the secondary current density distribution along the surface of plate electrode (parameter x) between location a and b (kinetics obtained from regional tap water study, cti = 47 ppm, average current load —50 A m 2 calculated cell voltage for all radius nearly constant value of 6.04 V, electrolyte conductivity 667 p,S cm-1).

Identify conditions that lead to corrosion and ways to prevent it. Calculate cell voltage from a table of standard electrode potentials. 

From a purely electrokinetic point of view (i.e. for single-phase flow), the calculated cell voltage equal to 0.732 V is mainly attributed to S02 oxidation kinetics which is responsible for the anodic overvoltage (fig. 3). 

Fig. 33 Calculated cell voltage as function of current density based on the model described by Eq. (24-27). Dashed curves correspond to coverage-independent energy of adsorption of CO on Pt, whereas solid curves, which better fit experimental data, correspond to adsorption energy falling with coverage according a Temkin-type isotherm [42].

Therefore, the calculated cell voltage for a system that could produce hydrogen at the cathode is... 

The second type of internal resistance is electrical resistance—r2- The influences of temperature and electrolyte thickness on electronic internal resistance of the electrolytes are not well known. The electronic conductivity values of solid oxide electrolytes are spread across a very wide range. They do not have a major impact on calculated cell voltage for high fuel utilization factors. It is hard to measure the electronic resistance of solid oxide electrolytes since they have both conductivities (ionic and electronic) simultaneously, which gives total electrical resistance. It should be noted that decreasing electrolyte thickness reduces ionic resistance (positive effect), but also probably reduces electronic resistance (negative effect). 

An electrodialysis cell has the following dimensions (110 cm X 60 cm x 0.04 cm (thickness), and is used to treat water with a throughput velocity of 10 cm/sec. The product concentration is 0.0092 eq/Liter. The cell current efficiency is 0.892. Resistance across the cell is 0.205 ohm. The influent concentration is 125 mg/Liter of NaCl. Calculate the following (a) cell current, (b) cell power output, (c) the cell voltage, and (d) the energy consumption per equivalent of product transferred. 

One of the most important characteristics of a cell is its voltage, which is a measure of reaction spontaneity. Cell voltages depend on the nature of the half-reactions occurring at the electrodes (Section 18.2) and on the concentrations of species involved (Section 18.4). From the voltage measured at standard concentrations, it is possible to calculate the standard free energy change and the equilibrium constant (Section 18.3) of the reaction involved. 

In chemistry, the most important use of the Nemst equation lies in the experimental determination of the concentration of ions in solution. Suppose you measure the cell voltage and know the concentration of all but one species in the two half-cells. It should then be possible to calculate the concentration of that species by using the Nemst equation (Example 18.7). 

Using the reaction free energy AG, the cell voltage Aelectrons exchanged during an electrode reaction must be determined from the cell reaction. For the Daniell element (see example), two moles of electrons are released or received, respectively ... 

The temperature dependence of the equilibrium cell voltage forms the basis for determining the thermodynamic variables AG, A//, and AS. The values of the equilibrium cell voltage A%, and the temperature coefficient dA< 00/d7 which are necessary for the calculation, can be measured exactly in experiments. 

These relationships can be used to obtain thermodynamic data otherwise difficult to get. Vice versa they can be used to calculate the temperature coefficient of a cell voltage respectively an electrode potential based on known thermodynamic data. 

Table 1. Electrochemical characteristics of some asymmetric supercapacitors. U is the maximum available cell voltage. Cs is the specific capacitance of a pellet electrode calculated from Cs = Ct 4/M, where Q is the capacitance of the asymmetric supercapacitor, M is the total mass of both electrodes.

When E°cs is directly calculated from AG°, the cell voltage is inversely proportional to the number of electrons passing through the cell. 

Since the final cell voltage is negative, the disproportionation reaction will not occur spontaneously under standard conditions. Alternatively, we can calculate Keq by using ln/Cq = -AG°/RT and AG° = -nFE°. This method gives a Keq = 8.6 x 10 17. Clearly, the reaction will not go to completion. 

FIGURE 3.20 Cell voltage drop calculated from 1-V curves at 0.3 Acm 2 before and after tested for 1000 h. Operating temperature temperature during the long-term test characterization temperature temperature at which the I-Vcurves before and after the long-term tests were measured. (From Mai, A. et al., Solid State Ionics, 177 1965-1968, 2006. With permission.)... 

C) The question concerns the effect of changing standard conditions of a cell to nonstandard conditions. To calculate the voltage of a cell under nonstandard conditions, use the Nernst equation... 

Generally, a continuous recording of electrically available data - for example, current, cell voltage, electrode potentials, temperatures - is beneficial to supervise the proper procedure of each experiment. Especially in case of a failure this will be a valuable help to find the reason. Today, the best way is to use a data acquisition system in a computer that offers the results directly for further calculations, for example, integration of the consumed current (converted charge). For continuously operated experiments the addition of scales, which acquire the weight of input and output reservoirs, will be advantageous in order to supervise the mass balances continuously. 

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