Ceramic anodes

Sintered and sprayed ceramic anodes have been developed for cathodic protection applications. The ceramic anodes are composed of a group of materials classified as ferrites with iron oxide as the principal component. The electrochemical properties of divalent metal oxide ferrites in the composition range 0- lA/O-0-9Fe2O3 where M represents a divalent metal, e.g. Mg, Zn, Mn, Co or Ni, have been examined by Wakabayashi and Akoi" . They found that nickel ferrite exhibited the lowest consumption rate in 3% NaCl (of 1 56 g A y at 500 Am and that an increase in the NiO content to 40mol 7o, i.e. O NiO-O-bFejO, reduced the dissolution rate to 0-4gA y at the expense of an increase in the material resistivity from 0-02 to 0-3 ohm cm. 

Ceramic anodes may be cast or sintered around a central steel core which acts as the electrical conductor. However, anodes produced in this form are brittle and susceptible to mechanical shock. 

Gross MD, Vohs JM, and Gorte RJ. A strategy for achieving high performance with SOFC ceramic anodes. Electrochem. Solid State Lett. 2007 10 B65-B69. 

Chen, G., Betterton, E.A. and Arnold, R.G. (1999) Electrolytic oxidation of trichloroethylene using a ceramic anode. J. Appl. Electrochem. 29, 961-970. 

A. Kumar, J.B. Bushman, J.H. Fitzgerald, A.E. Brown and T.M. Kelly, Impressed Current Cathodic Protection Systems Utilizing Ceramic Anodes, US Army Res. Labs, Champaign, Inc., 1990. 

The Ebonex ceramic anode has been used to study the electrolytic oxidation of trichloroethylene to CO2, CO, Cl , and ClOa [19]. Ebonex and Pb02-coated Ebonex electrodes were used for the electrochemical treatment of phenolic pollutants in water [20]. 

H. Kawamura, et al., Electrical Conductive Ceramic Anodes in Sulfur-based Hybrid Cycle for Hydrogen Production , Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2005. 

Recently, sensational papers about direct oxidation of methane and hydrocarbon in solid oxide fuel eells (SOFC) at relative low temperatures about 700°C were published. Even though the conversion of almost dry CH4 on ceramic anodes were demonstrated more than 10 years ago " at 1000°C, the reports about high current densities for methane oxidation at such low temperatures are indeed surprising. 

Metal-thennal reduction of tantalum from Ta205 during electrolysis of CaClj-CaO melt with a ceramic anode of TiOj... 

CaClj was chosen as a melt for the metal-thermal reduction process, as much CaO can be dissolved in it, which is required for an anode process with oxygen evolution. In addition electrolysis of CaCl2 can be conducted at high temperatures without danger of considerable evaporation and high temperatures are necessary for operation of a TiOj anode. The possibility of electrolysis of CaCl2-CaO melt with a ceramic anode was shown earlier [3]. 

In several runs, e.g. run 17, Table 4, stable operation of the anode was obtained at a rather low voltage. This was due to the effect of the anode construction (the inner current lead had been pressed into a hole in the anode body) and also that the temperature in this run was high, which resulted first in a relatively low electrical resistance of the ceramic anode and secondly in a more complete and faster metal-thermal reduction reaction. As a result tantalum powder was not spread over the container volume, but was concentrated on the cathode rod as a compact conglomerate which could be removed from the melt together with the cathode. In addition there was practically no Ta205 powder in the container and after the experiment it was not difficult to separate the tantalum powder from that of TajOj. The low current efficiency in this run is due to the non-optimized operating conditions of electrolysis in particular that an excess amount of electricity had been passed. 

The high Ni impurity content is significant and results from the dissolution of the inner current lead made from Ni which diffused through pores of the ceramic anode body. The increased content of Ca in tantalum produced in electrolysis of CaCl2 CaO melt is connected with the entrainment of complex oxides containing CaO. A great advantage of the electrolysis process with a ceramic anode over that with a carbon anode is the production of tantalum with a relatively low carbon content. 

Fonseca FC, Muccillo ENS, Mnccillo R, de Floiio DZ (2008) Synthesis and electrical characterization of the ceramic anode Lai xSrxMno.5Cro 503. J Electrochem Soc 155 B483-B487... 

In addition, direct oxidation has also been investigated on novel ceramic anodes for SOFCs made of mixed ionic- and electronic-conducting materials. A good example is the work reported by Perry Mnrray et al. (1999) on ceria-doped zirconia anodes in low-temperature SOFCs. Many other materials are also being investigated and a recent review has been published by Irvine and Sauvet (2001). 

Although the bulk electrolyte temperature might be only 50 °C, the local temperature in the plasma zone would probably be in excess of 1000 °C, which can lead to formation of glassy or ceramic anodic coatings. The local high temperature can speed up reactions, such as deposition of the coating, gas evolution and oxidation of Mg alloy. It can also cause high-temperature decomposition of the components in the bath solution. 

One important conclusion from the work on infiltrated electronically conducting ceramic anodes is the great similarities in the reported data. The data in each case seem to suggest that the ceramic component simply provides electronic conductivity and that the (electro) catalytic function is supplied by the materials added separately via infiltration. However, this also means that there is certainly room for performance improvement by (i) further optimization of the amount and type of infiltrated material, (ii) further optimization of the microstructure of the porous backbone, and (iii) further improvement of the current collection (electronic conductivity) of the porous backbone and support structure. 

Surfaces roughened by very fine shot blasting or by application of porous coatings (electrode-posited porous metals, ceramics, anodizing, or phosphatizing) can better retain lubricants and thus help to prevent corrosion. 

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