Coated anodes lifetime

Alternative anodes may be used, particularly the various dimensionally stable anodes (DSA) coatings based on precious metals and their oxides. The correct choice of such materials may allow a longer anode lifetime and low oxygen overpotetUials. 

Cathodic protection to the steel substrate (the zinc acts as a sacrificial anode). This takes place at the beginning of the coating s lifetime and naturally disappears with time [86]. 

The initial attemps to replace the graphite anodes with activated titanium anodes began as early as 1957 with platinized titanium and Pt/lr-coated anodes. However because of the short lifetimes of the anodes, they were not economic. The use of mixed metal oxides was first patented by Beer in 1965 and 1967 [150]. The initial patent described a coated metal electrode in which the active material was a mbced metal oxide coating containing one or more of the platinum metal group oxides. The second patent described coatings in which mfred metal oxide crystals contained a non-platinum metal oxide in addition to the platinum metal oxide (including Ti, Ta, and Zr oxides). 

Adequate anode lifetime is obviously also an important factor related to the magnitude and imiformity of current flow. A variety of anode systems have evolved for cathodic protection of reinforcing steel, each with certain advantages and limitations. Continuous surface anodes have been based on conductive bituminous overlays and conductive surface coatings. The former are suited only to horizontal surfaces. In general, good current distribution is achievable with such systems. Discrete anodes have been used without overlays and with cementitious overlays. For horizontal surfaces, anodes without overlays can be recessed in the concrete surface. Nommiform current distribution is a... 

Because of limited commercial experience with anode coatings in membrane cells, commercial lifetimes have yet to be defined. Expected lifetime is 5—8 years. In some cases as of this writing (ca 1995), 10-years performance has already been achieved. Actual lifetime is dictated by the membrane replacement schedule, cell design, the level of oxygen in the chlorine gas, and by the current density at which the anode is operated. 

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. 

Despite quite some progress reported in improving the performance and lifetime of anode materials, a great deal of research needs to be dedicated to the improvement of the cathode in Li-ion batteries. This task was addressed by hydrothermal carbon coating techniques. Thus, Olivine LiMP04 (Me = Mn, Fe, and Co) cathodes with a thin carbon coating have been prepared by a rapid, one-pot, microwave-assisted hy-... 

In many cases the surface of the anode is oxidized to lead oxide, which thus is the real anode material. Titanium anodes coated with lead oxide have been found to have a long lifetime [154,155]. In some cases the lead dioxide, a strong oxidant in acid solution, reacts chemically with the substrate [156] and is continuously regenerated whereas it acts as an inert electrode in other oxidations. A review of the basic electrochemistry of Pb02, mainly in relation to the lead battery, has been published [157]. 

The anode and cathode should be stable in the electrolysis medium, allow the desired oxida-tion/reduction reactions at the highest possible rates with miiumal by-product formation, and be of reasonable cost. In actuality, the electrodes may corrode or undergo physical wear during reactor operation, which may limit their lifetime. Often, if an expensive electrode material is needed for a given reaction, it can be plated or physically coated on a less costly, inert, and electronically conducting substrate. Common anode and cathode materials are listed in Table 26.8. 

Corrosion control technology is a mainstay of automobile coatings as well as household appliances for example, the lifetime of water heaters is extended and often governed by the presence of a magnesium sacrificial anode that represents a small fraction of the appliance price. 

An example of the protective value of polypyrrole is to be found in the work of Skotheim et al (43) who stabilized n-Si with thin films of this material. These conductive films were photo-electrochemically generated and they extended the operational lifetime of illuminated n-Sl from about four hours to 6 days in the presence of l2 /I couple in aqueous solution. The cell showed no decay after running continuously at anodic current densities of 9 ma/cm2 for this long period. Noufl et al (42) found similar enhanced stability of n-Si coated with polypyrrole in the presence of the Fe3+/2+ couple, and for n-GaAs in CH3CN solutions (45). 

Wang, X., Rundle, R, Bale, M., and Mosley, A. 2003. Improved operating lifetime of phosphorescent OLED by novel anode coating. Synth. Met. 137 1051. 

The second layer is anodic compared both to the underlying layer and to the layer above the lifetime of the coating depended on its thickness its chemical composition was not important its compacmess and ductility mattered little or not at all. 

Another advantage with the new ISO standard (ISO 15589-1/2 2004) is that it does not specify any maximum distance between the anodes. Instead it requires calculation of maximum potential midway between two anodes based on the current density used and the actual coating breakdown factor in the end of the lifetime. Figure 19.16 shows a schematic presentation of the situation and the equivalent current flow loop with all actual resistors, where =anode potential (V versus Ag/AgCl) =potential on the pipe surface (cathode) (V versus Ag/AgCl) = anode resistance (Q) Rs= resistance for current flow in seawater outside the pipe (i2) Rc= resistance for current entering the pipe surface (Q) R f= resistance for current flowing in the pipe metal (Q) and 4= total protection current in the loop (A). 

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