Theoretical cell voltage

Theoretical cell voltage Fraction of current required for desired reaction Actual cell voltage Actual current... 

Theoretical cell voltage Actual cell voltage... 

In electrorefining and in electroplating, where the same metal is used as a soluble anode, the theoretical cell voltage is zero or almost zero therefore, the energy efficiency, as per this definition, is expected to be zero or close to zero, whatever may be the current efficiency. The actual cell voltage depends on the current density, the temperature, the electrolyte... 

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%. 

For the hydrogen-oxygen couple, the corresponding theoretical cell voltage at 25°C is 1.23 volt if liquid coaler is formed or 1.18 volt if the product water is vaporized. 

The theoretical cell voltage of a DMFC at standard conditions is 1.20 V. The materials used in DMFCs are similar to those in PEMFCs. Pt, PtRu, and Nafion membrane are used as cathode catalyst, anode catalyst, and proton transfer membranes, respectively. However, the catalyst loading in a DMFC is much higher than the loading used in H2/air fuel cells, because both side reactions are slow (Pt loadings 4 mg/cm2 for a DMFC, 0.8 mg/cm2 for a H2/air fuel cell). 

In H2/02 fuel cells, an increase in temperature leads to a decrease in theoretical cell voltage. 

In a fuel cell, the H2 and 02 pressures are not limited to 1 atm. Change in these pressures leads to deviation from the theoretical cell voltage. The relationships between the gas pressures, the anode and cathode potentials, and the cell voltage... 

It can be seen that increasing either oxygen or hydrogen pressure leads to an increase in theoretical cell voltage. For example, in the case of H2/air fuel cells, at room temperature and if the pressure is 1 atm on both sides, the theoretical cell voltage is 1.22 V. If the pressure for both sides is 3 atm, the theoretical cell voltage becomes 1.24 V, 20 mV higher. 

Under theoretical cell voltage conditions, for both half-cell reactions (HOR and ORR) there is no net reaction. In other words, both half-electrochemical reactions are in equilibrium, and no net current passes through the external circuit. The cell voltage can be considered the OCV. At 25 °C, if the pressures of both H2 and 02 are 1 atm, the OCV should be 1.23 V. However, in reality the OCV is normally lower and an OCV of 1.23 V is never observed. This is due to the mixed potential at the cathode side, and hydrogen crossover from the anode side to the cathode side [22, 23], At 1.23 V, Pt is not stable so oxidation of Pt occurs ... 

The voltage efficiency i)v = C/cell/ theoretic reflects voltage losses due to polarization phenomena at the electrodes and ohmic potential drops at the interfaces in the electrolyte, and in the interconnectors (the theoretic cell voltage for water production is 1.23 V, voltages between 0.6 and 0.9 V are usually observed in real systems depending on current densities). 

In recent years, biofuel cells have gained tremendous attention. The use of methanol instead of dihydrogen as the oxidizable substance offers special advantages as it is readily available and easy to store and handle. At the same time, the theoretical cell voltage of an MeOH/02 cell (1.19 V) is near that of H2/O2 (1.23 V). 

If we take the Ha oxidation as an example, the theoretical cell voltage is ... 

Metal Electrochemical equivalent (Ah/g) Theoretical cell voltage (V) Theoretical specific energy (Wh/g) Operating voltage (V)... 

The theoretical cell voltage of the reaction is 1.214 V according to the free Gibbs energy of the reaction of ArG = —702.5 kJ/mol. In reality various effects lead to a reduction of the achievable open circuit voltage. The formation of CO-species as intermediate at the anode [12, 13] and the methanol permeation through the membrane are well known causes for reduced power density and the formation of mixed potentials. Special anode catalysts for lower sensitivity towards... 

The overall electrochemical reaction, with its theoretical cell voltage at 298.15 K, is ... 

In a zinc/chlorine RFB, the theoretical cell voltage of 2.12 V is defined according to the following reactions ... 

Obviously, E is the eell voltage, Eo is the theoretical cell voltage, I is the current density, bi is the Tafel slope, C is the contaminant concentration, and R is the membrane resistanee. However, the physical meaning of the other parameters was not given. Most possibly, these are kinetie parameters related to NO2 and SO2 adsorption on the Pt surfaee. 

Figure 21.2 shows a typical polarization curve (or current-voltage curve) of PEMFCs. This curve results from both the anodic HOR and the cathodic ORR reactions. The actual celt voltage is much lower than the ideal celt voltage and the theoretical cell voltage. When the current is drawn from a fuel cell, the actual cell voltage will drop from its ideal due to several types of irreversible losses, as shown in Figure 21.2. The drop is mainly caused by mixed potential and fuel crossover, activation overpotential, ohmic overpotential, as well as mass transfer (concentration) overpotential.

The efficiency of a fuel cell is primarily determined by the thermodynamics of electrochemical hydrogen combustion. Only Gibbs free enthalpy can be converted into electricity. As shown in Fig. 4.26, additional losses in the fuel cell stack are encountered maiifly by limitations in the electrochemical kinetics of oxygen reduction, ohmic resistances in the electrolyte membrane and the current collectors as well as mass transport losses in the electrodes. All these losses cause the operating voltage of the fuel cell to deviate from the theoretical cell voltage. In the... 

Therefore, the equilibrium potential of the cathode will then be 1.23 V more positive than formerly, and the cell s thermodynamic EMF will be 0.94 V instead of 2.17 V. Under the assumption that other losses (ohmic losses and polarization) will not be much higher than now, then theoretically, cell voltages would be expected to decrease by almost 1 V. 

The overall discharge reaction mechanism for various lithium primary batteries is shown in Table 14.4, which also lists the theoretical cell voltage. The mechanism for the discharge of the lithium anode is the oxidation of lithium to form lithium ions (Li ) with the release of... 

In fact, the theoretical cell voltage, is the fuel cell open circuit... 

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