Gibbs water electrolysis

One of the most extensively examined gas evolution reactions, next only to the H2 evolution reaction, is the O2 evolution reaction (OER) (209) as it is one of the main electrochemical reactions in water electrolysis, metal electrowinning, and recharging of metal-air cells. The standard electrode potential for the oxygen evolution reaction at 25°C calculated from the standard Gibbs energy of formation of H2O and OH ions (/) is 1.299 V [versus normal H2 electrode (NHE)] and 0.401 V (versus NHE) in alkaline media. The oxygen evolution reactions are... 

Two conclusions arise from Eq. (10). First, it is seen that efficiency can be maximized if electrochemical losses can be made small as compared with the Gibbs free energy change for water electrolysis, AGhjO- Second, it is seen that AT/hjO. the enthalpy of water electrolysis (with little temperature dependence), is involved in determining the overall efficiency as well as A//ph, the enthalpy of photogenerated charge carriers. If these two can be properly matched, a maximum overall efficiency may be accomplished. 

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. 

The change in Gibbs energy associated with the electrolysis of one molecule of water may be written... 

The standard Gibbs energy and enthalpy of water splitting reaction (A G°, A Fl°) are shown in Fig. 2 as a function of temperature. A G° decreases with temperature. According to (Eq. 4), the theoretical steam electrolysis voltage can be expressed as Afi°l2F when all the gas species taking part in the water splitting reaction... 

FIGURE 3.21 A calculated Gibbs energy profile for the ORR shown at different values of the electrode potential relative to the SHE. Close to equilibrium (dashes), the profile contains two uphill sequences, corresponding to step 1 and steps 3 and 4. Below an electrode potential of 0.78 V all steps in the sequence are downhill, as shown by the profile in the middle (short dashes). (Adapted from Chem. Phys., 319(1), Rossmeisl, J., Logadottir, A., and Nprskov, J. K. Electrolysis of water on (oxidized) metal surfaces, 178-184, Figure 2, Copyright (2005) Elsevier. With permission.)... 

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