Chlorine overpotential

The electrochemical characterisation studies, discussed in the previous section, showed that a 40 at.% Ru electrode, when subjected to extended electrolysis or potential or current cycling in NaCl solutions and when the chlorine overpotential reaches 300-400mV, behaves like a fresh, low at.% Ru (about 5 at.%) electrode. This strongly suggests that Ru losses from the Ru/Ti oxide coating occur during electrolysis. To determine whether or not the Ru losses in failed anodes take place by uniform dissolution across the entire coating or whether only localised surface... 

In the literature we often come across the statement that the chlorine overpotential is independent of pH (see, for example, [348,363]). All these results, however, refer to electrodes with quite thick layers of active mass for which the dependence on pH can be masked owing to two effects. Firstly, even a weak evolution of oxygen at elevated pH values results in a noticeable accumulation of... 

PbOj has a low overpotential for the liberation of oxygen from and KOH solutions, and for chlorine . 

The relative proportions of oxygen and chlorine evolved will be dependent upon the chloride concentration, solution pH, overpotential, degree of agitation and nature of the electrode surface, with only a fraction of the current being used to maintain the passive platinum oxide film. This will result in a very low platinum consumption rate. 

Current-voltage profiles for chlorine evolution, obtained at various times between periods of square-wave potential cycling (1.35 to — 0.32 V versus SCE, 60 s cycle-1), all at the same 40 at. % Ru electrode, are shown in Fig. 5.4. It can be seen that the activity of these electrodes increases at the beginning of the deactivation period, as revealed by the decrease in the overpotential at approximately 5 h. This may arise, at least in part, from... 

To further understand and characterise the oxide deactivation process, a.c. impedance studies were carried out, primarily with a 30 at.% Ru/Ti electrode, at various stages during deactivation. These data were compared to those obtained for freshly formed Ru/Ti oxide films, ranging in Ru content from 5 to 40 at.%. Impedance data were collected at the oxide OCP (approximately 0.9 V versus SCE) in fresh NaCI solutions. Under these conditions, no chlorine reactions can occur and the OCP is defined by the equilibria of the redox states on the Ru oxide surface. Deactivation was generally accomplished by square-wave potential cycling, using overpotentials versus the chlorine/chloride potential of 1.59 to — 0.08 V (60 s cycle-1) in 5 M NaCI + 0.1 M HC1 solutions at room temperature. 

The photoelectrochemical production of chlorine at nanocrystalline titanium dioxide thin film electrodes exposed to U V light has been reported [96]. In this process, the energy from photons substantially reduces the overpotential required for the chlorine evolution process and therefore less harsh conditions are required. Metal doping of the Ti02 photoelectrocatalyst was explored but found to be not beneficial for this process. In future, this kind of process could be of practical value, in particular, for water treatment and disinfection applications requiring low levels of chlorine. 

Passage of 1.0 mol of electrons (one faraday, 96,485 A s) will produce 1.0 mol of oxidation or reduction—in this case, 1.0 mol of Cl- converted to 0.5 mol of Cl2, and 1.0 mol of water reduced to 1.0 mol of OH- plus 0.5 mol of H2. Thermodynamically, the electrical potential required to do this is given by the difference in standard electrode potentials (Chapter 15 and Appendix D) for the anode and cathode processes, but there is also an additional voltage or overpotential that originates in kinetic barriers within these multistep gas-evolving electrode processes. The overpotential can be minimized by catalyzing the electrode reactions in the case of chlorine evolution, this can be done by coating the anode with ruthenium dioxide. 

To reverse this reaction and oxidize chloride ions, we have to supply at least 1.36 V. Because only 0.82 V is needed to force the oxidation of water, but 1.36 V is needed to force the oxidation of CL, it appears that oxygen should be the product at the cathode. However the overpotential for oxygen production can be very high, and in practice chlorine might also be produced. 

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