Open-cell voltage

Figure 4. Variations of the a.c. impedance with cycling measured with C/Li(NiojCoo,2Mn0f02 cells at the open cell voltage of 3.7V...

La photovoltaic cells, the same redox reaction, OX + e = KED, may be used for both the anode and the cathode. Figure 10-33 shows an eneigy diagram of an operating photovoltaic cell this cell consists of a metallic cathode and a photoexcited n-type semiconductor anode. The electromotive force (the open cell voltage), ph > approximately equals the difference between the flat band potential of... 

The equilibrium situation in an electrochemical cell is obtained, if the electrical current is interrupted, if all local actions (e.g. transport in the electrode) have come to an end and no internal short circuits occur. Then, as mentioned (Figure 3.5.10), the cell voltage is determined by the difference in the lithium potential (chemical potential of lithium) between the left-hand side (Ihs) and right-hand side (rhs) of the electrochemical cell (E - open cell voltage, F - Faraday constant) ... 

An often very helpful strategy therefore is to modify existing materials. Compared to new compounds, such attempts would primarily not address thermodynamic quantities, such as open cell voltage. Instead, kinetic parameters can be optimized, and quantities such as practical cell voltage, electrode capacity, and power density can be drastically improved. 

It can be shown that (see App. 7D) when a solid is placed between two electrodes with chemical potentials and /zn, as shown schematically in Fig. 7.19 , the open-cell voltage V of such a cell is given by... 

There is evidence for a more complex discharge mechanism via lithium intercalation into (CF,) , yielding ternary compounds, (CLiyFJ 0,34.36 -pjjg interplanar distance in (CFj) increases during reduction and may reach 935 pm . Moreover, the open-cell voltage of the redox couple (CFj) /Li is only 3 V, whereas 4.57 V is calculated from thermodynamic data via Eq. (b) . ... 

Put the power "on" for 0.5 s in the copper wire electrodes and measure the decay in the open cell voltage after the power(applied potential) is "off" using high impedance millivoltmeter. 

Another effect due to the presence of water vapor in the fuel gas is observed in the open cell voltage. The observed differences of the open cell voltage, depending on the water vapor content in hydrogen, can be explained by the Nemst eqnation. The gas diffnsion overvoltage (%) is given by the Nemst-equation ... 

This diffnsion overvoltage leads to the Nemst impedance. Operating the fuel cell with oxygen (po, = 1), no diffusion overvoltage for X = 0.5 occnrs. Therefore, the open cell voltage, given by Eq. (86), is very sensitive to the water vapor content in the fuel gas and can be nsed to determine the water vapor in the fnel gas quantitatively. 

Temperature Dependency of EIS Measured in SOFC at Open Cell Voltage. In order to calculate the activation energy of the oxygen reduction reaction at the cathode, hydrogen oxidation reaction at the anode and ionic conduction... 

Figure 6.2 Typical polarization curves that can be recorded from a BES. OCV, open cell voltage OCV, open cell voltage used to determine current-dependent internal resistance. Sections A, activation overpotentials are the dominant voltage loss in this region B, current-dependent voltage loss...

The most interesting parameter here is the exchange current density jo, which describes a hypothetical limit of how much current could be produced by the catalyst in the absence of aU mass transport effects (i.e., very close to the open cell voltage). The value ofjo depends on the oxygen concentration at the catalyst surface and the temperature. The concentration dependence can be expressed as follows [62-64] ... 

We shall see in Chapter 6 that energy consumption during industrial synthesis increases in importance with the decrease of the unit price of a particular product. In this section we shall address the fundamental concepts of thermodynamics to the calculation of minimum energy requirements and open-cell voltages. We shall consider energy balances and heat transfer requirements, since both can be an essential part of practical reactor design. 

In engineering calculations one is rarely concerned with systems in their standard states and in consequence one has to use further thermodynamic calculations (see below) to determine the effect of say temperature on the open-cell voltage. The question is whether the required data are available in the literature. 

Returning to Eq. 2.41, many interrelated parameters govern the required values of the open-cell voltage and ohmic resistances. We will mention the third component of the total cell voltage the overpotential or overvoltage—but polarization and electrode kinetics are the subject of Chapter 3. 

Fig. IV.21 also contains the efficiency for storage in a solar redox battery with an open cell voltage of 1 eV. The latter efficiency can be obtained by replacing in equ. (1) the factor (E -C)...

Polarization by hydrogen is minimized by the PbOa electrode, which is also a depolarizer. The discharge of the battery consumes acid and forms insoluble lead sulfate and water that is, the density of the solution decreases from about 1.28 g/cm in the fully charged condition to about 1.1 g/cm in the discharged state. The overall open cell voltage (when no current is being drawn) depends on the... 

The open-cell voltage (OCV) in hydrogen-oxygen SOFCs at a temperature of 900°C has a value of about 0.9 V, the exact value depending on reactant composition. The relation between current density and voltage of an operating cell, Ui versus i, is practically linear that is, the cell s apparent internal resistance / app has a constant value that does not depend on the current density ... 

In the case of reversible systems the cell voltage may be derived from the thermodynamic data of the cell reactions according to Eq. (5). But often this equilibrium voltage cannot exactly be measured (even not at an open circuit), since the electrode process is not quite reversible, as in the case of the nickel electrode or since secondary reactions cause a slight deviation (cf. Fig. 1.18). Then the open cell voltage (OCV) actually is measured. 

Equation 5.12 indicates that sulfuric acid has a strong effect on the cell voltage of the lead-acid battery. Table 5.10 lists the calculated and measured open cell voltages (or electromotive force) as a function of sulfuric acid concentration [11]. It can be seen that the cell voltage increases with an increasing sulfuric acid concentration, which is exactly the tendency predicted by Equation 5.12. 

F. 4.2 Open cell voltage versus the O/C ratio for SOFC at 1000°C... 

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