OVERVOLTAGE AS A FUNCTION O F CURRENT DENSITY AND T H E EFFECTS OF TIME, TEMPERATURE, STIRRING, PRESSURE, NATURE OF SL'RFACE, AND O F A SCPERIhiIPOSED ALTERNATING CURRENT* BY WILLIAM D. HARKINS AND H.
s. A D A M B ~
1. Introduction In 1905 Tafe12 showed that overvoltage is in some cases nearly a linear function of the logarithm of the current density, which indicates a probable relation to a concentration effect. One year later Lewis and Jacksons proved that when the cathodic material is mercury, the overvoltage is represented as a straight line over a limited range, when it is plotted against the logarithm
FIG.I Effect of Current Density on Orervoltage (Current in Billionths of an Ampere for an Area of 84 sq. mm.).
of the current density. The present paper shows that at 2 0 O C . this is true over a very much more extensive range, between 3 billionths and a thousandth of an ampere per square centimeter, when the cathode consists of carefully prepolarized m e r ~ u r y . ~ Figure I presents these relations in a graphic form. In this particular instance the current and not the current density, is *Contribution from the Kent Chemical Laboratory of the University of Chicago. Presented in the year 1914 to the University of Chicago by H. S. Adams, in partial fulfillment of the requirements of the Ph.D. degree. Practically the only changes made in the thesis as presented are due to the inclusion of references to work done since it was written and to a considerable abbreviation. Tafel: Z. physik, Chem. 50, 641 (1905). Lewis and Jackson: Ibid., 56, 193 (1906). Between current densities of 0.08 and 1 2 milliamperes per sq. cm. the overvoltage of tantalum is represented by a remarkably straight line, when plotted against the logarithm of the current density.
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WILLIAM D. HXRKlKS AND H. S. ADAMS
given, for a cathode which consisted of a mercury meniscus in a circular tube of 0.526 cm. internal diameter. 2. Apparatus for determining Overvoltage under Pressure
The apparatus used for the determination of the effect of pressure upon overvoltage was designed for work below I atmosphere and up t o 3 atmospheres. It consisted of a 700 cc. bottle of heavy glass,-fitted with a rubber stopper supported by a metal case which surrounded the bottle,-half filled by electrolyte, into which the two electrodes dipped. The anode was a sheet of fine platinum gauze 10x50 cm., made in the form of a spiral, over which hydrogen passed continuously, and which also served as the hydrogen elctrode. The hydrogen was made from zinc amalgam according to the method of Cooke and Richards. The flow of hydrogen was regulated to any desired speed by putting the requisite resistance or potential into the circuit of the hydrogen generator. The acid, 2 0 7 0 hydrochloric, was boiled and cooled in an atmosphere of hydrogen before it was used. KO rubber connections were used in any of the work described in this paper. The electrolyte used in the overvoltage cell and in all of the later work was K / I Osulfuric acid which had been redistilled in a concentrated form and was later diluted by conductivity water. The mercury was purified by the method used a t the Bureau of Standards. The potential measurements were made by a sensitive potentiometer, and the apparatus was in all cases protected by an equipotential shield, such as that described by White. 3.
The Effect of Pressure upon the Overvoltage of a Mercury Cathode
When this work was begun, the intention was to determine the effect of pressure upon the overvoltage of cathodes with both high and low voltages, but the work was interrupted after the measurements on a single metal, mercury, had been made. Through the apparatus set up as described above, and filled with pure hydrogen, a current of low current density was passed for four days. The current was then regulated to the desired current density and kept at this density until a steady overvoltage was obtained. Then the pressure was raised in steps to 3 atmospheres, lowered in steps to 1/36 atmosphere, and finally raised again to one atmosphere. Each run of this nature was completed in a period of about 14 hours, and although the e.fect w a s investigated at widely varying current densities, no appreciable variation due to pressure w a s found1. It must not be lost sight of that what is being measured here is not the change of potential on the cathode with the change of pressure, but the Since the completion of this work, some related results have been published by Newbery: J. Chem. SOC.,105,2419 (1914),who found no appreciable effect to be produced upon the overvoltage of oxygen upon platinum by pressures as high as IOO atmospheres. Recent work by Bircher and Harkins: J. Am. Chem. SOC.,45, 2890-98 (1923); is practically in agreement with that of the present paper, but shows that there is an extremely small rise of overvoltage a t low pressures when mercury, lead, or nickel is used. I t gives definite evidence that the extremely rapid rise with lowering pressure sometimes found is fictitious.
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difference between this change and that produced on the hydrogen electrode, where the reaction is assumed to be reversible. Foi this reason, the experiments of Tammann and Nernst’ on the liberation of hydrogen by metals under pressure, cited by Lewis and Jackson in this connection, seem not to be strictly pertinent.
TEMPERATURE FIG.2 Effect of Temperature on the Overvoltage of a Mercury Cathode. Temperature changed (A) rapidly and continuously, (B) sloivlp and continuously (C) ten degrees in % hour and then kept constant ?.i hour. Current (A) 6 x 10-3 milliamperes. (B and C) 3. j x IC-3 milliamperes per sq. em. 1
Z. physik. Chem., 56, 193 (1906).
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WILLIAM D. HARKIXS AND H. S. ADAMS
4. The Effect of Temperature upon Overvoltage At the time this work was begun, there was a general impression that overvoltage decreases with rise of temperature, though some investigators expressed the opinion that temperature has little effect. Thus this was the conclusion of Lewis and Jackson1 who found that at the higher temperatures there is even a tendency for the current to decrease, which implies an increase in overvoltage. After the completion of the present work, Ridea12found that the overvoltage of a number of metals remains constant as the temperature is changeda. The apparatus used by us in investigating the temperature effect was the same as that used in the work on the effect of pressure. It was kept in a thermostat whose temperature could be varied rapidly, and then kept constant to 0.01~. The temperature was determined by the use of a thermometer kept in the overvoltage cell. The overvoltage of the mercury cathode was lowered about 2 millivolts per degree of increase of temperature between o and 80 degrees (Fig. 2 ) whether the temperature was varied rapidly and continuously (A) with a current of 6 X IO-^ milamperes; slowly and continuously (B) with a current of 3.5 X IO-$ milamperes; or changed I O degrees in 1 / 2 hour and then kept constant 1/2 hour (C) with the same current. All of the curves exhibit considerable hysterisis. In Fig. 2 C, the values obtained during the half hour at which the temperature was kept constant are represented by crosses, while the final values a t the end of each period are shown by circles. It may be noted that during each constant temperature period the overvoltage increases on the descending branch, and decreases on the ascending branch of the curve. This tendency of the overvoltage to “recover” after it has been disturbed is a somewhat general phenomenon.
5 . Apparatus for the Determination of Overvoltage In order to extend the work to an investigation of overvoltage phenomena on various metals a second apparatus was constructed, the planning of which was directed by the following considerations. The anode and cathode were to be separated in such a way that the possibility of contaminating the cathode with the anode material or with electrolyte from the anode compartment should be reduced to a minimum. The hydrogen electrode against which the cathode potential was to be measured was to be so constructed as to make it possible to change the cathode without disturbing the hydrogen electrode or admitting air to it. A device for rotating the cathode was to be provided, to which different cathodes might be fitted interchangeably. The manner in which these details were worked out is shown in the accompanying diagram, Fig. 3. Rideal: J. Am. Chem. Soc., 42, 94 (1920). Z. physik. Chem., 9, I (1892). 3 Recent work by Bircher and Harkins indicates that there are two types of overvoltage: ( I ) The ordinary type such as is investigated in the present paper, which is lowered by a rise of temperature, and (2) overvoltage which is determined largely by the single potential of the metal. Overvoltage of this type increases as the temperature rises.
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The anode compartment is represented by (A). The anode (a) was a hydrogen electrode of platinized platinum, made up in the form of a spiral and mounted in a glass tube which passed out through the rubber stopper of the cell. Through a second glass tube (b) hydrogen passed into the cell, and a third tube (c) connected with a small wash bottle provided for its escape. A tube (d) sealed into the side of the cell made possible the withdrawal of electrolyte. A large glass tube (e) bent in the manner indicated and provided with a stopcock, connected the anode compartment with the cathods compartment (C). The cathode compartment was fitted with a rubber stopper in the center of which was a piece of glass tubing (f) of 1 2 mm. internal diameter. This opening provided for the introduction and removal of the various cathodes
FIG. 3 Apparatus for the Determination of Overvoltage.
under observation without disturbing any part of the apparatus. The rubber stopper carried also a glass tube (g) through which hydrogen passed from the hydrogen electrode compartment (H) into the cathode compartment, and a second tube (h) provided for the escape of the gas through a. wash bottle. The third tube (i) drawn out into a Luggin capillary formed one arm of the bridge which made electrical connection with the hydrogen electrode. This bridge was fitted with two stopcocks, and between them was sealed in a tube (k) through which fresh electrolyte might be syphoned from a reservoir. The hydrogen electrode compartment (H) was fitted also with a rubber stopper, carrying in the center a glass tube in which was mounted the hydrogen electrode ( I ) , a rectangular piece of platinized platinum gauze. Through the stopper passed also one arm of the bridge (i), a tube through which hydrogen passed into the cell (m), and another (g) through which the gas passed out and over to the cathode cell. Still another tube (n) made it possible to withdraw electrolyte from this compartment. The apparatus was immersed in a thermostat kept a t 25.0"C. Hydrogen for the hydrogen electrode and the cathode, was furnished by the electrolysis of 10% sodium hydroxide. The electrodes were of monel
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WILLIAM D. HARKINS AR'D H. S . ADAMS
metal, and the cathode enclosed in a porous cup. The gas was passed over palladium asbestos heated to 175'C. in an electric tube furnace, and washed subsequently through N / I O sulfuric acid. No rubber connections were used, the entire train being of glass. A small electrolytic generator, of the U-tube type and electrodes of pure, previously unused, scrap platinum, furnished hydrogen for the anode. The electrolyte was N / I O sulfuric acid, made as described previously. It was stored in a five-liter bottle under hydrogen, and a syphon of glass tubing provided for its withdrawal through the bridge either into the hydrogen electrode or cathode compartments. After the experiments on a metal had been completed and the cathode removed, the electrolyte from the cathode compartment was drawn into the anode compartment and thence out of the apparatus. The two compartments were then washed out by running in fresh electrolyte through the bridge and withdrawing it as before, and finally the apparatus was filled with fresh electrolyte. A cathode of another metal could now be introduced and measurements commenced. The source of current consisted of a battery of five storage cells and twenty dry cells, with a device for adding or cutting out the cells one or more a t a time, and a decade box in series with the circuit for more closely regulating tjhe current. In series also could be placed resistances of IOOOOO, IOOOO, or 1000ohms, from the drop of potential across which the current was calculated. The fall of potential between the hydrogen electrode and the cathode represented the overvoltage. Measurements were made, as described above, with a Leeds and Northrup Type K Potentiometer. Fig. 4 gives the details of the rotating device. The bearings (b,b) were made by drilling a brass rod, brazing it to the frame (f), and subsequently cutting the opening to receive the pulley (p). In this way perfect alignment was secured. The steel rod by which the bearings were drilled was used as the shaft (s) of the stirrer. All vibration was thus reduced to a minimum, and an air-tight bearing secured. The lower end of the shaft was threaded to receive the various cathodes. The upper end was drilled to form a cup in which mercury was placed and electrical connection thus secured. The pulley was of hard rubber and was slotted to allow the passage downward of oil. The lower bearing carried a rubber stopper (x) which made an air-tight joint with the glass tube in the stopper of the cathode cell. The frame was bolted to the edge of the thermostat, from which it was insulated by a plate and bushings of hard rubber, the details of which are not shown in the diagram. The stirrer was driven by an electric motor, and a speed of 5,600 revolutions per minute was obtained with complete smoothness of operation and freedom from vibration. Fig. 4, (B) represents one of the interchangeable cathodes. It consisted of a brass cylinder (c) tapped a t one end to receive the shaft of the rotating device, and a t the other to receive the cathode. Fitted tightly around it was a glass tube (t, t) which served to protect it from the splashing of the electrolyte and formed a cup to catch any oil which might work down from the bear-
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ings. The electrode (e) was threaded tn fit the brass holder, except in the case of some of the softer metals, where connection was made by soldering. A glass tube (t', t') surrounded the electrode for part of its length. Joints between this tube and the electrode, as well as the holder, were made by the use of de Khotinsky cement. The electrolyte in the cathode cell was adjusted to such a height that it covered the exposed part of the electrode and part of the glass tube surrounding it. The area of the electrode accordingly underwent no change during rotation. In general the cathodes were approximately 20 mm. in diameter and 300 mm. in length, and were made by turning or by drawing the metal. The final polishing was made with rouge paper and oil, after which the cathode was washed with benzene, ether, alcohol and distilled water. 6 . Effect of Rotating the Cathode
I n so far as overvoltage is due to a concentration effect which may be influenced by a diffusion of the hydrogen into the solution itmay beexpected to fall when the cathode is rotated. Experiments with some nine different metals, under supposedly identical conditions, show that in general this is what occurs. Figures 5 to 8 show the effect upon the overvoltage of stirring the electrolyte by means of a stirrer (Fig. 5), of rotating the cathode a t a speed of 5600 revolutions per minute, or of FIG.4 bubbling hydrogen gas over the Rotating Cathode. cathode. I n general any of these procedures gave a considerable lowering of the overvoltage. In the case of a mercury or a tin cathode, the lowering was much greater at the lower current densities then at the higher. With a cathode of polished tin at a current density of 50 x IO-^ milamperes, stirring by means of hydrogen gas lowered the overvoltage from 660 to 140 inillivolts and additional rotation of the cathode at high speed did not affect the minimum value (Fig. 7 ) . Rotation of the cathode lowered the overvoltage, but much less, when cathodes of polished gold, or polished or rough platinum were used (Fig. 6). Obviously stirring the electrolyte has a marked effect in lowering the overvoltage. Thus in the case of tin cited above, at the lowest current densities as much as 807~~ but a t the highest, only I ~7~ of the overvoltage disappears when the cathode is rotated. Rotation of the cathode removes many of the gas bubbles
1"
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WILLIAM D. HARKINS AND H. S. BDBhlS
and it also brings near to the surface of the electrode a layer of liquid which is not supersaturated with hydrogen. 7. The Time Effect
Data already presented in the paper illustrate the fact that the overvoltage on a cathode increases in general with the time when the current density is kept constant. Figure g shows that the overvoltage of a monel metal cathode was increased 2 0 millivolts at a current density of 13X IO-^ amperes, and 40 millivolts a t I 1.8roX IO-^ amperes, in a period of 2 4 hours.
FIG.5 Effect of stirring the Electrolyte on the Overvoltage of a Mercury Cathode a t Higher and Lower Current Density.
8. The Effect of a Superimposed Alternating Current Experiments on tin, cadmium, and platinum (Figs. 8, IO, 11), and on tungsten, and molybdenum, show that the general effect of superimposing an alternating current upon an electrode which is a cathode for a direct current is to lower the overvoltage. It was found that with platinum, tungsten, and molybdenum, metals of low overvoltage, the alternating current changes the overvoltage to an undervoltage except a t the higher current densities for the direct current. While this effect may be due in part in some cases to an asymmetry of the alternating current, this is not the underlying cause of the effect, since it persists when the alternating current is reversed. The effectiveness of the alternating current in lowering the overvoltage seems to decrease as its frequency increases.
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IO
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i
70
TIMF:
IN
MINUTES
FIG.6 Effect of Rotating Cathodes of Gold and Platinum on Overvoltage. (Current in Milliamperes).
I
I
FIG.7 Effect of Rotating Cathode on Overvoltage. (Current in Milliamperes per 84 sq. mm.).
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WILLIAM D. HARKINS AND H . S. BDAMS
FIG.8 Effect of a Rotating Cathode and of a Superimposed Alternating Current on Overvoltage (Current in IO* Amperes).
It may be assumed that the negative phase of the alternating current raises, and the positive phase lowers the overvoltage, since it has a strong depolarizing action and probably oxidizes the hydrogen atom before they forms molecular hydiogen. It is of interest to note that the lowering produced is the greatest on platinum, a metal of extremely high oxygen overvoltage.
FIG.9 Effect of Time on Overvoltage. (Current in
IC-@
Amperes).
OVERVOLTAGE
FIG.IO Effect of a Superimposed Alternating Current on Overvoltage. (Logarithm of Current in Terms of Billionths of an Ampere for an Electrode of 84 sq. mm. Area. C =Current in Milliamperes),
That there are exceptions to the above rule is shown by the fact that the overvoltage of rough copper (Fig. IZ), of smooth copper, and of copper deposited from acid and from alkaline solutions, was raised by a superimposed alternating current except a t the lowest current densities used. This seemE to indicate that the positive phase of the current is not particularly effective as a depolarizing agent on copper, and that the negative phase acts to increase the effectiveness of the direct current in increasing the overvoltage.
LOGRR\THM OT CURRENT FIG. I1
Effect of a Superimposed Alternating Current on Overvoltage (Log of (Liirrent Amperes. C =Current in 10-3 Amperes).
111 10-9
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W1LLI.IM D. HARKIXS AND H. S . ADAMS
9.
Overvoltage and Current Density
The relation between the overvoltage and the current density was determined for 1 2 metals a t z5"C. I n general order of increasing overvoltage these were platinum, tungsten, molybdenum, monel metal, nickel, gold, silver, copper, tantalum, mercury, cadmium, and tin. Fig. 13 gives the overvoltage as plotted against the logarithm of the current density, as obtained by in-
Effect of a Rotatlnp Cathode and of a Superimposed -4lternating Current on Overvoltage (Current in 10 Amperes).
creasing the current. The curves obtained when the current was again decreased lie slightly above these in every case, but are omitted to avoid confusion in the diagram. Thus there is a hysteresis comparable t o that obtained by raising and lowering the temperature. Similar data were obtained from rough cathodes for all of the metals except mercury, tungsten, tantalum, molybdenum, and monel metal. I n every case the rough cathode gives a lower overvoltage than the smooth a t the same apparent current density. Roughening the surface has two effects: it increases the actual area for a definite apparent area, and it supplies points from which hydrogen is more easily liberated. Fig. 14 illustrates this effect for I polished copper, I1 copper
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FIG.13 A Effect of Current Density on Overvoltage (Current in Billionths of an Ampere on nn Electrode of 84 sq. mm. Area).
FIG.13 B Effect of Low Current Densities on the Overvoltage of Gold. (Current in
10-9
Amperes).
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WILLIAM D. HARKINS AND H. S. AD.4MS
from ammoniacal solution, and I11 copper from acid solution. The overvoltages given by I11 are from IOO to zoo millivolts lower than those given by I. The curves of Fig. 13 indicate that a t zjo and with the 1 2 metals represented, the overvoltage is in general not far from a linear function of the logarithm of the current density, though there are in some cases considerable variations from linearity which indicate that there are conditions under which the relation does not hold. It is important in this connection that similar experiments should be carried out at different temperatures.' It is apparent that in general the overvoltage increases as the cohesion in the metal decreases.
Fig. 14 Effect of Nature of Surface on Overvoltage. (Current in Billionths of an Ampere).
10.
Relation to the Periodic System
An earlier paper by H a r k i d indicates that overvoltage is related to the periodic system of the elements in the sense that members of the same family of elements have nearly the same overvoltage. Thus it was found that arsenic, antimony, and bismuth, have overvoltages which differ little, and the same was found true of copper, silver, and gold, of zinc, cadjmum and mercury, and of tin and lead. The present paper shows that this is true for copper silver, and gold, and for cadmium and mercury, over the whole range of current density. In addition it is found that the same relation holds for molybdenum and tungsten, metals for which the overvoltage increases very slowly as the current density rises, so slowly that a t high current densities they have a lower overvoltage than platinum. The overvoltage curve for monel metal is almost the same as that €or molybdenum and tungsten, and it should be noted that its slope is much less than that of the curves for its components, copper and nickel, and in general lies far below both of them. This has been done by Bircher and Harkins. J. Am. Chem. SOC., 32, 518-30 (1910).
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Summary I. The overvoltage on a metal cathode is found for inactive metals to be on the whole a linear function of the logarithm of the current density, this line being specially straight in the cases of mercury and tantalum. ,4t low current densities the overvoltage falls off much more rapidly, than corresponds to this relation. The overvoltage of molybdenum and tungsten increases very slowly, while that of platinum, nickel, and mercury, increases rapidly with the current density. Curves for tin, cadmium, copper, silver, gold, and monel metal, in addition to those cited above, are given. 2. The overvoltage of mercury, a metal of high overvoltage, is practically constant between 1/36 and 3 atmospheres pressure. 3 . The overvoltage of a metal with a high overvoltage (mercury) is found, contrary to the deduction made by Rideal on the basis of a desorption theory, to decrease rapidly by 2 millivolts for each degree increase of temperature, which indicates that the effect is related to the changes of surface energy. 4. Rotating the cathode at high speed, 2 0 0 0 to 6000 revolutions per minute, or bubbling hydrogen nitrogen, carbon dioxide, air 01 oxygen, over the electrode, reduces the overvoltage greatly at low, and to some extent a t high, current densities. The extent of the lowering depends also on the surface, being, for example, much larger for smooth tin than for rough copper. The overvoltage of rough nickel was greatly raised by rotation. 5 . Alternating currents with frequencies of from 1 5 to 1800and current densities of from I to 2 0 0 , were found to give rise to undervoltages on platinum, tungsten, and molybdenum at low current densities of the direct current, and to lower the overvoltage in all cases except on a smooth or rough copper cathode on which the overvoltage was raised. The general effect is what would be expected if the hydrogen were oxidized, presumably while still in the atomic state. 6. The overvoltage increases with the time during which the metal is used continuously as a cathode. 7. Roughening the surface is found to decrease the overvoltage in all cases. The effects cited in this and the preceding paragraph were already known when this work was begun, but the present paper shows the extent of the variations which occur. 8. The paper illustrates the relation between overvoltage and the ordinary periodic system. The object of the paper has been to present facts concerning the phenomena of overvoltage. Many theoretical papers on this subject have been written. Some idea of the various theories may be obtained by consulting the papers of Bennett', Bancroft2, MacInnes, hdler, and Contieria, and Newbery4. l
Bennett: J. Phys. Chem., 20,296-322 (1916). Bancroft: J. Phys. Chem. 20, 396-401 (1916). MacInnes, Adler and Contieri: J. Am. Chem. Soc., 41. 194, 2013 (1919). Newbery: J. Chem. Soc., 105, 2428 (1914).
