Thermodynamic electrolysis voltage

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

In Eq. 2, F is the Faraday constant (96485 C mof ) and the negative sign denotes the thermodynamically non-spontaneous nature of the water splitting process. The actual voltage required for electrolysis will depend on the fugacities of the gaseous products in Reaction 1 as well as on the electrode reaction kinetics (overpotentials)... 

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)). 

Thermodynamic and over potential region For a terminal voltage smaller than the water decomposition potential Ud (Ud 2 V), no significant electrolysis happens and no current flows between the electrodes. 

Electrochemical methods can be divided into two classes those involving no net current flow ( potentiometric ), and all others. In potentiometry, one measures the equilibrium thermodynamic potential of a system essentially without causing electrolysis or current drain on the system—because this would affect the existing equilibrium. In all other methods, a voltage or current is applied to an electrode... 

Up to now, reversible thermodynamics has been assumed. The electrical energy demand for water electrolysis under real conditions is significantly higher than the theoretical minimum energy derived above. The total voltage of an electrolysis cell imder operation depends on the current in the cell, the voltage drop caused by Ohmic resistance and the anodic and cathodic overvoltages (see Eq. (5.16)). 

The Gibbs-Helmholtz equation dG = dH — T dS yields the thermodynamics of chemical reactions. In the case of a negative free enthalpy dG, a spontaneous reaction occurs. Water splitting means a positive dH and dS, respectively. So it is only for very large T that spontaneous water decomposition occurs. As mentioned above, this means temperatures of about 2000 °C. For electrolysis, an electric potential in line with the free enthalpy dG is applied so the reaction can take place. The equation demonstrates that the required voltage sinks with higher temperatures. 

On the other hand, when the cell voltage V exceeds the thermodynamic voltage Vq, the direction of the cell current (i.e., that of electrons and ions) and that of the electrodes and the OR (Eqn (15.1)) is reversed. In other words, for the case of Figure 15.4, electrolysis of water (Choi, Bessarabov, Datta, 2004) occurs to produce the H2 and O2. Then the power density, P = V i < 0, is negative and must be supplied externally from a power source, as per the first law of thermodynamics. 

Secondly, reference was made to the effects of increasing the temperature of the electrolysis of water. Whilst the thermodynamic gains might be modest, there were significant improvements in the kinetic performance. Finally Professor Nurnberg referred to the current-voltage curves cited in the paper. He stressed that the performance of the ambient temperature alkaline cells developed at Julich was already considerably imposed. 

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