Anode surface depletion

Thus, the polarisation data, cyclic voltammetric results and the a.c. impedance measurements all suggest that, when an Ru02/TiC>2 anode exhibits a high overpotential, this is a direct consequence of the surface depletion of Ru. This is also consistent with the estimated Re values of approximately 20 Q for the failed electrodes, in contrast to the known, much higher specific resistivity of Ti02 of... 

Another option to lower the Ru losses is to dope the anode coatings with IrO2, which has been shown [53, 54] to markedly decrease the Ru corrosion rate during electrolysis in NaCl solutions. This would minimise the surface depletion of Ru, as shown by the SIMS analysis of the TiC>2 + RuC>2 + IrCU coatings (Fig. 5.19), and thus extend the operating life of the anodes. 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

Assume that a crevice is exposed to a differential in acidity between the bottom of the crevice and its outer surface in the absence of chlorides or other dissolved oxidizers. The crevice bottom acts as an anode, consuming hydrogen ions through active metal corrosion. The anodic reaction depletes acid concentration in the crevice. The outer crevice suffice is passivated and acts as a cathode. On the cathode, hydrogen evolution occurs by hydrogen reduction in solution. Assume the cathode surface area is 10 times larger than the anode suffice area. 

The rate of discharge of OH at the anode surface may be greater than the rate of migration of OH ions from the bulk of the electrolyte, resulting in a depleted concentration at the interface, which gives rise to concentration polarization. 

Limiting currents are usually associated with cathodic reactions (e.g., in metal deposition), although anodic reactions are by no means excluded. Whenever the supply of a dissolved species from the solution to the electrode surface becomes the rate-limiting factor, limiting-current phenomena may be observed. Anodic limiting currents can be obtained, for example, in the oxidation of ferrous to ferric ion, or ferro- to ferricyanide ion (El). Diffusion of H20 limits 02 evolution in fused NaOH (A2). In these examples the limiting current is caused by depletion of the reactant species at the anode. 

For forced-convection studies, the cathodic reaction of copper deposition has been largely supplanted by the cathodic reduction of ferricyanide at a nickel or platinum surface. An alkaline-supported equimolar mixture of ferri- and ferrocyanide is normally used. If the anolyte and the catholyte in the electrochemical cell are not separated by a diaphragm, oxidation of ferrocyanide at the anode compensates for cathodic depletion of ferricyanide.3... 

Heraeus markets a multicathode metal depletion system. In the system, one or two anodes face a large number of permeable cathodes, e.g. in form of copper strips similar to expanded metal. [90,234a], Plattner and Comninellis have developed large scale reactors total plate electrode surface ca. 20 m2, cf. ref. [140]. 

Recent studies performed with deactivated anodes show [55] that electroless or electrolytic platinum deposition on failed anodes, not only lowered the polarisation behaviour of these anodes (see Fig. 5.20), but also demonstrated an equivalent lifetime as that of a new anode in accelerated life tests in the sulphuric acid solution (see Fig. 5.21). These results unequivocally demonstrate that the deactivation of anodes, for which the Ru loading is still high, is a direct consequence of the depletion of Ru from the outer region of the anode coating. Note that this process of surface enrichment by conducting electroactive species will not lead to reactivating a failed anode, if there is a TiC>2 build-up at the Ti substrate/coating interface. 

We consider, now, an electron-depleted space charge layer that is gradually polarized in the anodic direction. As long as the Fermi level is located away from the surface state, the interfacial capacity is determined by the capacity of the depletion layer that obeys a Mott-Schottlsy relation as shown in Fig. 5-61. 

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