Electrochemical Engineering Richard C. Alkire Department of Chemical Engineering, University of Illinois, Urbana, IL 61801 The availability of new materials for electrodes, membranes, and other cell components has exerted an enormous influence on industrial electrolysis during the recent past. Additional influences of feedstock and energy availability, and of wastewater treatment orocedures. have also led to chanees in " industrial practice. Many processes have thus been reoptimized to new constraints, while other entirelv new technoloeies have emerged to fill needs. Activities such as these req;ire effective engineering methodologies for accurate design, . .. scaleup and ;ptimiz&n. Electrochemical processes are complex because they involve many different phenomena simultaneously. Included among these, for example, would be ohmic resistance effects, mass transport limitations on reactants and products, and charge transfer rate processes. The relative importance of such processes depends upon geometry and current density. Because the reaction rate along a surface is generally not uniform, the relative imwortance of such orocesses can therefore varv strongly with position inside a cell. As a consequence, it is usually difficult to predict behavior of electrolysis cells by intuition alone. Effective engineering methodologies are needed for transformine understandina of fundamental principles into properly designed hardware. Electrochemical engineering consists of applying scientific principles to understand and improve technological devices. Fundamental principles upon which the field draws heavily include Thermodynamics, which describes the equilibrium state of the
electrodeleiectrolyte interface; Kinetics, whieh relates the rate of passage of current through t h e interface to the driving forces across the interface; Transport Phenomena, which determines the rate at whieh species and energy can become available to the interface region; Current and Potential Field Distribution, which determine the flow of current between electrodes,and the variation of potential
along the electrode surfaces. While these areas of studv are included in the earlv training of chemists and chemical engineers, their application to electrochemical examples is often not emphasized. When teaching electrochemical concepts, i t is relatively easy to describe fundamentals in language which is familiar to students with one important except~on:the role of the electric potential. The potential enters into every one of the four areas listed above. The extension of chemical understanding to electrochemical understanding requires mainly an appreciation of the role of the potential. 'The voltage of any electrochemical cell consists of several components. At equilibrium the cell voltage would exhibit a thermodynamic potential. In order to pass current, i t is necessary that driving forces in excess of the thermodvnamic potential be supplied. The overpotential caused by irreversible processes arises from several regions in the cell. Ohmic resistance, for example, arises throughout the entire region, both in the electrolyte between electrodes, and in the electrodes and connecting wires. In the diffusion layers near electrodes, concentration differences arise which beget concentration overpotentials. Within a few angstroms of the electrodeelectrolyte interface, additional overpotentials arise in order to drive charge-transfer reactions at the surface. These various overpotentials depend upon the level of current which passes
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Journal of Chemical Education
through the cell following, as is nature's way, the path of least resistance. The s i m ~ l act e of seoaratinr the overall cell voltaee into several components (the;modyn&ic potential and I'rreversible overpotentials) provides an i m ~ o r t a nconce~tual t basis for the s&dy of complex electrochekical systems. A number of other papers in this symposium have emphasized a number of these component areas. Therefore, in the paragraphs below, emphasis will be placed on engineering ramifications and examples. Key among these are the distribution of current and potential within a cell, the evaluation of trade-offs between the influences of different phenomena, the use of dimensionless numbers to assist in scale-over to new operating conditions, and economics. Current and Potential Distribution The central point to be recognized is that the rate of an electrochemical reaction varies from point to point along an electrode surface. In hardly any case is the reaction rate uniform, usually because the shape of the electrode doesnot lend itself to thatresult, or else the process economics do not permit it. For example, a platinum flag electrode attached to a wire and immersed in a beaker of solution and facing another similar flag electrode may he considered. In such a cell, electrochemical reactions will occur at a high rate on the surfaces which face each other in comparison with the surface on the "hack side" of the electrodes. For the same reason, the reaction rate will be higher near the edge of the flag than in the central region of the surface because, from potential field considerations, the current finds an easier path to the edges than to the central region. The verv corners of the flae have the highest reaction rates since they are the most exposed to the volume of electrolvte throueh which current flows. Thus. simple platinum fl& electrodks are not very simple at all and are rarely used in modern laboratories. Engineering Tradeolfs In addition to the effect of cell geometry, the current and potential distribution is influenced by mass transfer and by charge transfer. Engineering tradeoffs are therefore required in order to balance the need for cell operation against economic considerations. For example, a uniform distribution of current can be achieved over a surface, but only by operating a t very small currents. Also, mass transfer complications can be avoided by stirring the solution vigorously. While these wrocedures are easv to accomwlish in a beaker, they mav not be profitable in an-industriaiprocess. Operating at lowcurrents means that the manufacturing rate is low: stirrinr on a large scale requires pumps and special cells designed toguide the fluid over the electrode surface effectively. Electrochemical engineering consists in large part of evaluating different possible courses of action on the basis of their effect on cell operation and upon process economics. A key step in this process, at least when the design of the cell is involved, is evaluation of the potential distribution to determine what factors influence it to the greatest extent. Economics I t is common, in undergraduate science courses, to sneak of "energy balances" or "mass balances." By application of these balances, one can evaluate what goes into a system, and
in what form it comes out. In the same spirit, one can talk of a "buck balance" where dollars are used to buy and operate equipment, from which product, unwanted waste, and old equipment emerge. Economic halances are used to identify costlv elements in an overall urocess. so that imnrovements can be made which will have an economic impact. Economic balance eauations can also be made dimensionless in order to evaluate how profit, return on investment, or other economic indicators scale with changes in process operation. Applied research and development activities are always carried out in the presence of economic awareness. A Survey of Applications
The following examples of electrochemical engineering activity were selected to illustrate the foregoing general comments and to nrovide soecific examnles for classroom "
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"
details may he obtained, as well as a guide to the literature in each examole area. A second volume in this series of tutorial lectures isblanned for publication in late 1983. Batteries
There are an enormous variety of batteries for different applications, many of which can be found at any corner store or gas station. Of these, the lead acid storage battery is well known. The electrode reactions which occur are Negative Pole: Positive Pole:
Pb
+ HzSOa = PbSOa
+ 2Ht + 2ePbOz + 2Ht + HzSOa = PbS04 + 2H20
The overall reaction is thus P b + P b 0 2 + ZHzSOd = 2PbS06 2Hz0
+
Many tradeoffs have been made in arriving at present battery designs. The negative material, for example, consists of highly porous lead in order to promote high reaction rates from small volumes; additives are used to maintain the porous structure during cycling so that it stays in place. The positive active material is Pb02, an n-type semiconductor. Metal support grids are used on both electrodes to keep active material in place and to conduct current to reactive sites. An excellent discussion of details is available." The sulfuric acid in the battery is consumed during discharge. Diffusion of Hi304 within the porous electrodes plays an important role on discharge rate. In cold winter days, for examole, a few hard cranks from a weak batterv and it seems to die; after ten minutes, however, i t seems to recover and be able to crank the motor again. Why? The sulfuric acid was the situation does not occur in summertime Aluminum Manufacture
The mechanism of electrolysis in aluminum production is still imperfectly understood after nearly a century of commercial practice. The prohahle reactions 6AbOFec2 + 36F- + 3C = 12 A~FC-; 3C02 12e12~= - 4AI 24FCathode: 4AIFs-" 2A120a+ 3C = 4A1+ 3C02 Overall:
Anode:
+
+
+
'
"Tutorial Lectures in Electrochemical Engineering and Technology" Alkire, R., and Beck, T., (Editors),A.i.Ch.E Symp. Series, 204, 77 (1981).
Bade, H., "Lead Acid Batteries." John Wiley and Sons, New York, 1977.
The process operates in a molten electrolyte at high temperature in which the aluminum oxide is soluble. Carbon anodes are consumed in the process and give copious amounts of gaseous products. Modern cells are rated at around 250,000 A. Manv dozens of cells are coupled to rectifiers in a mannfacturing site. Because the prodo&m of aluminum consumes nearly 4.5% of the total electric power generated in the US., it is obvious that cells must be optimized with careful attention to the price of electricity. Electroplating
Electronlatine is an old field which has been nracticed as long as there has been a source of electricity. Nearly everyone carries on their oersons items which were electroolated: a belt buckle, purse snaps and jewelry, watch cases, etc. Modern communications and computer technoloeies depend w o n authmobile industry. A modern electroplating facility is carefully engineered to provide proper surface preparation, high speed metal deposition with careful process control, thorough rinsing and water re-use, fume exhaust, and waste treatment. For economic reasons, it is usually advantageous to plate at hieh vol" rates and therebv to a achieve laree - oroduction . umes. However, high rates (large current densities) can lead to depletion of metal ions near the plating surface, a condition which often gives deposits of poor quality. Therefore, agitation of the solution is an important area of concern. In addition, large current densities can lead to nonuniform current distribution. Thus, attempts to increase the current density can lead to highly nonuniform deposition along the surface, and of course, to large variations in plated thickness as a result. As can be seen, the application of fundamental principles must be made in view of economic constraints in order to achieve the best production arrangement. Corrosion
The cost of materials lost by corrosion is truly enormous. A recent study by the National Bureau of Standards indicated that, in 1975, the cost of corrosion to the U.S. economy was $70,000,000,000or 4'Ia% of the GNP. Corrosion proceeds in a wide variety of routes, of which the following electrochemical reactions are typical
+
2Fe = 2Fe+% 4eAnode. Cathode: .02 2Hz0 + 4 e = 40HOverall: 2Fe 2Hz0 Oz = 2Fe(OH)x
+
+
+
That is, iron rusts in moist air. T o protect against corrosion, special alloys are often used such as stainless steel. There are many types of stainless steels, but these generally contain Ni and Cr along with Fe because these elements promote formation of a protective surface film which slows down the rate of anodic dissolution by an enormous amount. The US., however, produces neither Ni nor Cr. In fact, Cr is strategic material of which a large fraction is produced hy South Africa and the Soviet Union. Thus, careful engineering of corrosive systems is critically important. Crevice corrosion occurs under gaskets, under a leaf in arain gutter, or in recesses in automotive trim. Transport by diffusion of reactants and products within the crevice region is slow. Thus, the composition inside the crevice can become significantly different from that outside the crevice region and, under certain conditions, can lead to extremely high corrosion rates. The initiation of crevice corrosion is thus attributed to transport processes in the crevice along with metallurgical features of the metal. Understanding" corrosion svstems requires a multidisciplinary background. Summary
Compelling trends toward the future indicate that it will become important to give increased emphasis on electroVolume 60
Number 4
April 1983
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chemical education. Chief among these is the shift in the primary form of energy to electricity, away from chemical fossil fuel. Chemists and chemical engineers must learn to operate chemical processes without a fossil fuel based econ-
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omy. Electricity will be inexpensive with respect to chemical agents for oxidation'and reduction. New routes will be required for synthesis of chemical materials and for beneficiation of increasingly lean natural resources. The education of students should include electrochemical fundamentals.
