Cells terminal voltage

When the cell terminal voltage is low (lower than a critical value called critical voltage, typically between 20 and 30 V), traditional electrolysis occurs... 

FIGURE 10.24 Changes in cell terminal voltage vs. time. 

For the Daniell element in Fig. 3, a potential difference A is obtained by calculation from the values in Fig. 5 according to Eq. (11) under equilibrum conditions the potential difference corresponds to the terminal voltage of the cell. 

During charging and discharging of the cell, the terminal voltage U is measured between the poles. It should also be possible to calculate directly the thermodynamic terminal voltage from the thermodynamic data of the cell reaction. This value often differs slightly from the terminal voltage measured between the poles of the cell because of an inhibited equilibrium state or side reactions. 

An important experimentally available feature is the current-voltage characteristic, from which the terminal voltage ([/v ) supplied by the electrochemical cell at the corresponding discharge current may be determined. The product of current / and the accompanying terminal voltage is the electric power P delivered by the battery system at a given time. 

An obvious extension of the bipolar design idea presented in the previous section is the induction of NEMCA using multi-stripe or multi-dot Pt catalysts placed between two terminal Au electrodes, as shown in Figs. 12.8 and 12.9. Both designs have been successfully tested as shown in these figures.10 Larger terminal voltages are applied here between the two Au electrodes, so that the potential difference in each individual cell formed between the Pt stripes or dots is of the order of IV.10... 

The cell tests were carried out by measuring the terminal voltage between the two electrodes during cell discharge by a Solartron 1287 electrochemical interface. 

EP is the terminal voltage measured in a cell where a current, I, is flowing... 

It can be seen that the electrolytic cell must accept 11 679 cal. from the surroundings per each mole of decomposed water to keep a constant temperature (when working adiabatically the electrolytic cell would cool down). This heat is also covered by electric energy. Therefore, to achieve an isothermal decomposition of a mole of water a total amount of not only 56 693 cal. but 68 372 cal. in the form of electrical energy is necessary which corresponds to the minimum terminal voltage across the electrolytic cell ... 

If an electrolytic cell operates with current 1 at terminal voltage E it accepts a total amount of 0.86 El keal. per hour. A part of this energy equalling 0.86 Eg)t I = 0.86 1.48 I is consumed in the isothermal decomposition of water and the remnant Qu is the heat liberated within the cell in the form of Joule s heat ... 

Relations between current density and terminal voltage (I-V curves) of the proton-conducting ceramic of Sr( c were determined for application to a fuel cell working at 600 - 800°C. In... 

Here, V and Eq are cell potentials or terminal voltage at arbitral current density I and 1 = 0. Usually, E(-) is called a tliermodynamic electromotive force, EMF. The notations of A and 5 are a surface area and thickness of the ceramic electrolyte, and s is an electric conductivity. Since measurement was carried out by a direct current method, the s value corresponded to an overall one including the contributions of anode and cathode polarities as well as the protonic conductivity of ceramic electrolyte. The dependence of Eq on temperature was very small and almost independent of it. On the other hand, the Eq values slightly depended on the input CH4/H2O ratio. 

The potential difference between the two electrodes is called the terminal voltage or the cell voltage U. By convention it is often measured as ... 

When no current is flowing in the cell (at thermodynamic equilibrium) the terminal voltage can be computed using the Nernst equations for the two electrodes ... 

If the cell is connected to an external power source, electrolysis takes place. The system, called an electrochemical cell, is now driven away from the thermodynamical equilibrium by an imposed flux, the electrical current. Internal entropy is produced. With increasing current, the terminal voltage will increase (Fig. 3.2(b)). 

The over potential plays a central role in electrochemistry as it controls the electrochemical reactions. By convention it is generally measured as a positive value for reactions where electrons are transferred to the electrode. The associated current is also counted positively. In this case the electrode is called an anode. If electrons are transferred from, the electrode to the ions of the electrolyte, the over potential and the associated current are measured as negative values. The electrode is termed cathode. Using the definition of the over potential, the terminal voltage for an electrochemical cell is given by (see Fig. 3.2(b)) ... 

In order to be able to apply this model, the nucleation probability p has to be linked to the electrochemical, electrode, and cell properties such as the current density and the terminal voltage. Due to the lack of knowledge in this field, in particular the knowledge about the activation of nucleation sites, this is not a straightforward task. In the next section, a simplified approach that is able to explain the important effects is presented. 

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