Electrodes for Industrial Applications HELMUT VOGT, Beuth University of Applied Technology, Berlin, Germany SUBRAMANYAN VASUDEVAN, CSIR-Central Electrochemical Research Institute, Karaikudi, India 1. 2. 3. Electric Circuiting. . . . . . . . . . . . . . . . Electrode Qualities . . . . . . . . . . . . . . . Electrode Materials. . . . . . . . . . . . . . . 1 1 2 4. Electrode Wear. . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . 4 4 1. Electric Circuiting 2. Electrode Qualities Electrodes are electrically connected unipolar (monopolar) or bipolar. Such circuiting is generally independent of the particular electrode shape and the application of a diaphragm or a membrane separating the anodic compartment from the cathodic one. In unipolar circuits of electrodes, the total electrode is either anode or cathode (Fig. 1A). The potential of the electrochemical reactors is that of one cell. The current in the connecting bus bars to all reactors within the plant are the sum of the current of each cell and requires large cross-sectional area of the bars. In bipolar circuits, the reactor contains several cells electrically interconnected in series (Fig. 1B). Each electrode (except for the outer ones) operates on one side as anode, on the other side as cathode. The current of the total reactor is the current of each cell. The total potential of each reactor is the sum of the cell potentials (with no regard to possible deviations in ohmic drop). The current in the bus bars is that of each cell. The potential at the rectifier attains large values and is limited by its reverse voltage. A substantial shortcoming of cells with bipolar electrodes is the vagrant current in connecting pipes between the cells of different potential. This current circumvents cells and does not contribute to an electrochemical reaction. Circuiting several reactors in one plant permits interconnecting a group of reactors in series, and another group in parallel to manage the rectifier blocking voltage. Certain requisites are applicable to each electrode material as specified below. Various characteristics must be balanced with respect to smooth process operation, moderate maintenance expenditure and general economic requirements. Selecting a material meeting all desired characteristics is often problematic. In industrial field, the paramount parameter to appraise electrode performance is the overall cost per unit product (see Section 3). The development of electrocatalysis has helped to define the main requisites of electrodes materials for industrial applications. # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007 High electrical conductivity. The electrode material should have a high electric conductivity to keep the ohmic potential drop within the electrodes at tolerable values. This requisite is extraordinarily important for reactors with unipolar electrodes where the current paths through the electrode are often long, and current density is high. Efficient electrocatalytic activity of the electrode material for any particular reaction generally indicated by a low overpotential plays an important role. The overpotential depends on the current density and on the electrode material and often necessitates a compromise with capital costs. Any potential drift with time towards larger values should be prevented. Sufficient selectivity. Electrodes must be selected to enable the desired reaction in 2 Electrodes for Industrial Applications Figure 1. Electric circuiting of electrodes. A) Unipolar electrodes; B) Bipolar electrodes such a way that any competing side reactions are most widely suppressed. Long-term mechanical stability is complied with by most electrode materials, above all by metals. However, some materials such as graphite tend to form a brittle structure during operation possibly leading to ruptures, under unfavorable conditions to short-circuits. Corrosion resistance. Competing reactions may transform the electrode material into substances solved in the electrolyte liquid or transferred into the gaseous phase. The wear of material may result in a loss of the original electrode shape. A characteristic example is the oxidation of graphite anodes during operation where apart from the wanted product traces of oxygen are generated reacting with the electrode material to form CO2. Dimensional stability. Inevitable wear of electrode material may be small enough to keep the original shape. The so-called dimensionally stable electrodes compose of electroactive coating material on an inert substrate. An adjustment of electrodes becomes dispensable. Long-term chemical stability. An essential requisite is low sensitivity to poisoning by impurities contained in the electrolyte liquid or resulting during operation. They may form deposits on the electrode surface hampering the desired reaction, i.e. leading to increased overpotential. The same effect may result during current shut down where unwanted reactions may proceed. In such cases, a small protective current must be applied to the reactor at any production interruption. Availability and low investment cost. Capital expenditure for the initial or repeated investment of electrodes must meet economic demands. Electrode materials must be available to a sufficient extent and available at economically reasonable price. Health safety. During storage and operation, the electrode must not release substances harmful for health. A striking example is mercury, a cathode material previously used in chlor-akali electrolysis for long time until health implications entered the focus of interest. Unproblematic flow of electrolyte liquid. To counteract the depletion of electrolyte in course of operation and to keep the concentration of electrolyte at a sufficiently high level, a sufficient flow rate of liquid must be ensured. The shape of the electrodes contributes essentially to the flow conditions. Operation and shape of electrodes evolving gases require particular attention. 3. Electrode Materials A great variety of electrode materials is employed in industry. A survey of the most important materials and applications is given here. A valuable overview on various electrode materials and applications including extraordinary ones can be found in . Carbon and graphite have been the main anodic material in industrial electrochemical processes for a long time. Graphite is more widely used as anode material in electrolysis than carbon, owing to its fair chemical resistance, good conductivity, considerable mechanical strength, satisfactory surface-to-volume Electrodes for Industrial Applications ratio, and is rather cheap. But it also undergoes chemical attack by active oxygen and highly oxidizing products. Due to the disintegration, the interelectrode gap continues to increase with time and this causes a penalty in the total energy consumption and/or requires discontinuous adjustment of the interelectrode distance. Moreover, there is an accumulation of sludge in the cell and disposal becomes a problem. Sometimes, the disintegrated graphite sludge causes discoloration of the product or may lead to short-circuiting. Regarding the electrocatalytic activity of graphite, it exhibits higher chlorine overpotential even to platinum at high current densities. Graphite also exhibits low oxygen overpotential so that simultaneous oxygen evolution can occur [2–4]. Platinum is an excellent anode material due to its extraordinarily small overpotentials. Platinum is stable for most of the reactions. In compact form it is preferably applied in laboratory experiments. Industrial usage is limited due to its large capital costs. However, platinum is a valuable coating material on an inert support material. In the production of perchlorate, the loss is 5–7 mg/kg of the product. Platinum coated on titanium used in Cl2 production lowers wear to 0.5g mg/kg . Magnetite was introduced into electrochemical processes some 90 years ago and has been used as anode material in aqueous solutions but exhibits poor acid resistance. Industrial application has suffered from poor electric conductivity (20 times poorer than graphite) and inherent brittleness. The industrial preparation results in forms that are poorly compatible with engineering requisites of modern electrochemical processes [6, 7]. Coatings. Anodes made of thermally prepared oxides of noble metals, so-called as dimensionally stable anodes (DSA), had better activities than those of the parent metals, and triggered a technological revolution in the history of electrode materials. Thus RuO2 with TiO2 and/or IrO2 became major constituents of many of the catalytic compositions. Even though the individual oxides possess extraordinary catalytic activity towards O2 and Cl2 evolution reactions, the loss of activity within time and corrosion induced the applications of their mixed oxides with valve metal and nonnoble metal oxides. Generally, these mixed oxide 3 coating compositions are (i) a stabilizer, selected from the valve metal oxides, (ii) a catalyst for the desired reaction, selected from the noble metal group, (iii) a catalyst diluent, i.e. dioxides of group 4 and 7 metals, and (iv) a doping agent to enhance the selectivity and activity of the main catalyst and at the same time retarding the undesirable reactions. As examples, coating containing RuO2, TiO2 with IrO2 or SnO2 or PdO2 has been found to yield longer service life in the production of chlor-alkali, hypochlorite, chlorate, bromate, iodate, chlorine substitute organic compounds, eosin and in cathodic protection with impressed current. The preparation of DSA is carried out by taking chlorides of Ru, Ti, Ir, or Sn in an organic vehicle (like amyl alcohol or isopropyl alcohol) applying over the pretreated titanium substrate and then thermally decomposing to form the oxide. The advantages of these electrodes are: good chemical and mechanical stability, catalytically highly active, lower Cl2 overpotential and consequently saving in energy, employment of higher current density, maintenance of same interelectrode gap, purer Cl2, relative resistance to mercury and increase of diaphragm life and advent of membrane cell [8, 9]. Lead Dioxide (PbO2). Lead is an easily available and cheap electrode material. Substitutes for the noble metal anodes for industrial applications have led to the development of lead dioxide anodes, especially in the form of graphite or titanium substrate lead dioxide. They are relatively stable from mechanical, chemical, and electrochemical point of view. The interesting aspect of a lead dioxide anode is that the anodic oxygen evolution takes place at more positive potential. Although lead dioxide could be deposited anodically from different baths, lead nitrate bath containing copper nitrate (to prevent the deposition of lead at the cathode) is preferred for getting an adherent and smooth deposit. In order to get pore-free deposit, different techniques (i) rotation, (ii) fluidizing an inert material with flow of electrolyte, (iii) ultrasonic, and (iv) addition of surfactants, both ionic and nonionic have been employed. These anodes allow the preparation of light-weight electrodes of desired dimensions to handle large currents on a commercial scale. However, lead dioxide is a poor catalyst both for O2 and Cl2 evolution . 4 Electrodes for Industrial Applications Low-carbon steel electrodes exhibit low capital costs and low hydrogen overpotentials in hydrogen formation. They are widely used as cathodes in industrial reactors for production of chlorine in membrane or diaphragm cells and in production of chlorate. At any reactor shutdown, the electrodes must be kept at a sufficient catholic potential to prevent corrosion. Mercury is an extraordinary liquid electrode material at all industrial operation temperatures. Its high overpotential in hydrogen evolution allows decomposition of sodium in chlor-alkali electrolysis with subsequent formation of an amalgam to be easily released from the reactor. However, the large quantities of mercury required for each reactor and the enormous capital costs together with environmental and health problems have favored the competing diaphragm and membrane process rid of mercury. 4. Electrode Wear All electrode materials exhibit more or less undesired wear. In some cases, wear in course of industrial application is small enough to consider the electrodes stable or insoluble, in industrial use particularly referring to suitable coatings. In other applications, wear is tolerable or may be intended. The dissolution of metal anodes is widely employed in electroplating and inherent in electroforming and electrorefining [11, 12] to balance anode and cathode current efficiencies. Cadmium is employed as soluble anode in protective electroplating from either cyanide bath or fluoroborate bath although in extraction and recovery of the metal, sulfate and chloride baths are employed. Copper anodes are used in electroplating of copper from either acid sulfate bath or alkaline cyanide bath. In cathodic protection, the sacrificial anodes, which are selected to have a minimal tendency to passivity and mainly to protect the steel in various environments, are limited to zinc, aluminum, and magnesium . Aluminum, magnesium, or iron are preferred as soluble anodes in reactions where electroflotation principle is adopted. Thus, in the removal of inorganic contaminants from water aluminum, magnesium, or its alloys as anode are employed dissolving to form aluminum/magnesium hydroxide. In synthesis of very pure chemicals, application of soluble anodes is attractive. Thus, high purity potassium stannate, silver nitrate, antimony trioxide, lead and tin fluoroborates, aluminum hydroxychloride are manufactured in a membrane cell by the anodic dissolution of tin, copper, antimony, lead tin, silver, and aluminum, respectively. The inevitable wear as stated above must be distinguished from the intended wear as with carbon in the Hall–Heroult electrolysis for production of aluminum. In this case, the anodic generation of CO and CO2 provides continuous consumption of the anodes and necessitates their frequent replacement but provides a decrease in cell potential. Anode consumption is substantial and requires frequent replacement of prebaked carbon anodes or self-baking S€oderberg anodes but contributes to lower the cell potential. References 1 F. Cardarelli: “A Concise Desktop Reference”, in Materials Handbook, Springer Verlag, Berlin 2008. 2 M.M. Jaksic, J. Appl. Electrochem. 3 (1973) 219–225. 3 V.I. Eberil, L.M. Elina, Sov. Electrochem. 6 (1970) 758–761. 4 G. Wranglen, B. Sj€odin, B. Wallen, Electrochim. Acta 7 (1962) 577–587. 5 K.C. Narasimham, H.V.K. Udupa, Electrochim. Acta 15 (1970) 1615–1622. 6 B.V. Tilak, J. Electrochem. Soc. 126 (1979) 1343–1348. 7 M. Hayes, A.T. Kuhn, J. Appl. Electrochem. 8 (1978) 327–332. 8 X. Li, D. Pletcher, F.C. Walsh, Chem. Soc. Rev. 40 (2011) 3879–3894. 9 S. Pushpavanam, K.C. Narasimham, J. Mater. Sci. 29 (1994) 939–942. 10 K.C. Narasimham, H.V.K. Udupa, J. Electrochem. Soc. 123 (1976) 1294–1298. 11 K.C. Narasimham, S. Silaimani, K.I. Vasu, Bull. Electrochem. 5 (1989) 109–112. 12 K.C. Narasimham, S. Silaimani, Bull. Electrochem. 8 (1992).
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