Dimensional Stable Anode

K. J. O Leary and T. J. Navin, "Morphology of Dimensionally Stable Anodes," paper presented at the Chlorine bicentennial Symposium San Erancisco, Calif., May 1974. 

The most common oxidation states and the corresponding electronic configuration of mthenium are +2 and +3 (t5 ). Compounds are usually octahedral. Compounds in oxidations states from —2 and 0 (t5 ) to +8 have various coordination geometries. Important appHcations of mthenium compounds include oxidation of organic compounds and use in dimensionally stable anodes (DSA). 

Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) ofIr02 ate stable electrochromic materials (191). 

A dimensionally stable anode consisting of an electrically conducting ceramic substrate coated with a noble metal oxide has been developed (55). Iridium oxide, for example, resists anode wear experienced ia the Downs and similar electrolytic cells (see Metal anodes). 

Anode Applications. Graphite has been used as the primary material for electrolysis of brine (aqueous) and fused-salt electrolytes, both as anode and cathode. Technological advances, however, have resulted in a dimensionally stable anode (DSA) consisting of precious metal oxides deposited on a titanium substrate that has replaced graphite as the primary anode (38—41) (see Alkali and chlorine products). 

High yields of NaOCl are obtained electrolyticaHy by oxidation of CT at dimensionally stable anodes (219). Sodium hypochlorite is prepared using small diaphragmless or membrane cells, with a capacity of 1—150 kg/d of equivalent CI2, which produce a dilute hypochlorite solution of 1—3 and 5—6 g/L from seawater and brine, respectively (see Chemicals from brine). They are employed in sewage and wastewater treatment and in commercial laundries, large swimming pools, and aboard ships. 

It has now gained acceptance as an impressed current anode for cathodic protection and has been in use for this purpose since 1971. The anode consists of a thin film of valve and precious metal oxides baked onto a titanium substrate and when first developed was given the proprietary name dimensionally stable anode , sometimes shortened to DSA. Developments on the composition of the oxide film have taken place since Beer s patent, and this type of anode is now marketed under a number of different trade names. 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

Dimethyl sulphoxide has also been oxidized electrochemically, using either a platinum anode or a dimensionally stable anode containing iridium and selenium in 1 M sulphuric acid solution158. The former electrode requires a potential close to that required for oxygen evolution whilst the latter needed a potential 0.5 volts lower. Thus the dimension-... 

Appreciable interest was stirred by the sucessful use of nonmetallic catalysts such as oxides and organic metal complexes in electrochemical reactions. From 1968 on, work on the development of electrocatalysts on the basis of the mixed oxides of titanium and ruthenium led to the fabrication of active, low-wear electrodes for anodic chlorine evolution which under the designation dimensionally stable anodes (DSA) became a workhorse of the chlorine industry. 

Ru02 is an important electrode material for industrial anodic processes. Special attention is deserved by the so-called dimensionally stable anodes (DSA) invented by H. B. Beer in 1968. These are formed by a layer of a microcrystalline mixture of Ti02 and Ru02 (crystallite size less thn 0.1 jum) on a titanium support (Fig. 5.26). This material is suitable as anode for chlorine and oxygen evolution at high current densities. For industrial chlorine production, it replaced the previously used graphite anodes. These... 

Technical electrodes usually consist of a mixture of Ru02 and TiC>2 plus a few additives. They are called dimensionally stable anodes because they do not corrode during the process, which was a problem with older materials. These two substances have the same rutile structure with similar lattice constants, but RuC>2 shows metallic conductivity, while pure TiCU is an insulator. The reaction mechanism on these electrodes has not yet been established the experimental results are not compatible with either of the two mechanisms discussed above [4]. 

Ru02-based coatings on Ti show excellent properties as dimensionally stable anodes (DSA). 

DSA-02 oxide-coated Ti anode (DSA = Dimensionally Stable Anode) is placed, thus creating a large cathode volume. The effluent solution flows perpendicularly through the electrodes with a typical flow rate of 0.5 dm3 s-1. The flowthrough metal electrodes have an active area approximately 15 times their geometric area. The cell allows air sparging to increase the mass-transfer. The current efficiency is about 40% when the inlet concentration of the metal ions is 150 to 1500 ppm and the concentration at the out-let is about 50 ppm. The cell is currently used for the treatment of recirculated wash-waters from acid copper, copper cyanide, zinc cyanide, zinc chloride, cadmium sulphate, cadmium cyanide and precious metal plating and washwaters from electroless copper deposition. Since the foam metal electrodes are relatively expensive the electrodes... 

The laboratory unit was based on a ElectroCell MP cell with an elec-trode/membrane area of 0.01 m2. A DSA (Dimensionally Stable Anode) anode served as oxygen electrode, Ni as cathode. Anion exchange membranes = Neosepta ACM (Tokuyama Soda) AMH cation ex-change membranes = Nafion 324 Nation 902. 

Chlorate, production, dimensionally stable anodes, 40 102-103 Chlorides catalysts, 40 50... 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

In 1958, Henry Beer invented the dimensionally stable anode (19. 20), which consisted of a titanium anode covered with a catalytic layer composed mainly of a mixture of Ti02 and Ru02. Due to the joint efforts of Beer and of the De Nora Company, the dimensionally stable chlorine anode became accepted everywhere and its introduction revolutionized the technology (4) of chloralkali electrolysis. 

Beck and Schulz have been working since 1984 on titanium anodes on whose surface chromate is anchored by the usual baking process, which produces dimensionally stable anodes tor chloralkali elektrolysis (e.g., (185, 196, 197)]. 

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