TAIC CELLS BY ALLEN D.
GARRISON^
Introduction The terms “Becquerel effect” and “photo-voltaic effect” have been used to distinguish between the light-sensitive systems of the electrode-electrolyte type and the well known “Hallwachs effect’’ or “photo-electric effect.” Cells having one or more light-sensitive electrodes of the former type are able to convert radiant energy into electrical energy and have been called “photo-voltaic cells.” Photo-voltaic cells may be divided for convenience into two general classes: (I) those having. the light-sensitive material in the electrodes, in which the electrodes enter into the chemical reaction of the cell and (11) those having the lightsensitive material in the electrolyte, in which the electrodes may be inert. Since the experiments of Becquerel a large number of photo-voltaic cells have been described. Hans Kochan2 has given a summary of the literature on the subject up to the year 1905. The most complete experimental data on the first class of cells have been recently obtained by Goldmann and B r o d ~ k y ,and ~ Samsonow4 using cells consisting of oxidized copper electrodes in various electrolytes. A theoretical explanation of their results was offered by Goldmann5 based upon the theories of the photo-electric effect. Baur,6 Titlestad,‘ and Samsonows have studied the cell consisting of inert electrodes of platinum and an electrolyte conl
National Research Fellow, in Chemistry. Kochan: Jahrb. der Rad. und Elect., 2, 186 (1905). Goldmann and Brodsky: Ann. Phys., [4] 44, 849 (1914). Samsonow: Zeit. wiss. Phot., 18, 141 (1918). Goldmann: ‘Ann. Phys., [4] 44, 901 (1914). Baur: Zeit. phys. Chem., 66, 683 (1908); 72, 323 (1910). Titlestad: Ibid., 72, 257 (1910). Samsonow: Zeit. wiss. Phot., 9, 12 (1910); 11, 35 (1912).
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taining uranyl and uranous salts, which is the best known example of the second class of cells. It has been found that the-behavior of the cells of the first class is more complicated than it appeared to be from the results previously obtained. The theories advanced have also been found inadequate. It is the purpose of this report first, to present the experimental results which have been obtained using cells of the first class having oxidized copper electrodes and second, to offer an explanation of their behavior.
Experimental Results
A copper electrode which has been coated with a film of cuprous oxide and placed in a neutral solution of some salt such as an alkali chloride, sulphate or nitrate, has the property of altering its potential relative to the electrolyte on illumination. A cell consisting of two such electrodes in an electrolyte will set up an e.m.f. and produce a current if one electrode is illuminated while the other is kept in the dark. This is a photo-voltaic cell of the first class since the electrodes contain the light-sensitive material and enter into the chemical action of the cell. Goldmann and Brodsky (loc. cit.) found that the behavior of the cell was independent of the nature of the electrolyte with the exception of certain active ions. It was previously thought that the cuprous oxide electrode always becomes positive when illumined but it has been found that the direction of the current is dependent on the manner in which the oxide coating is prepared and may be changed by certain changes in the electrolyte and by a variation in the intensity or color of the light. The cuprous oxide electrodes used by Goldmann and others were prepared by the oxidation of copper in a flame. The following method has been devised which produces a more uniform and dense coating of oxide having different properties. The polished copper electrodes were placed in a solution of cupric chloride in which a cuprous chloride coating was
Cir prous Oxide Photo- Voltaic Cells
603
quickly formed on the copper. This was hydrolyzed in water to the cuprous oxide. By repeating this process several times a very dense and uniform coating was produced. With such dense cuprous oxide films in a neutral electrolyte the effect of illumination was found to be in the opposite direction, that is the electrode becomes negative to the solution. By varying the treatment electrodes of both kinds have been made in this way, their properties depending on the condition of the oxide coating. Electrodes which on illumination become more positive with respect to the electrolyte will be spoken of as having a “positive light effect” and those which become more negative as having a “negative light effect.” The term “photo-potential” will be used to express the change in potential of the light-sensitive electrode or the change in voltage of the photovoltaic cell on illumination when no current flows. Where photo-potentials are compared, the same intensity of white light was used for illumination unless otherwise stated. Many cuprous oxide electrodes have been found which ultimately have a positive light effect under prolonged illumination although at first they have a large negative light effect. The maximum of the negative effect is attained more rapidly than that of the positive effect so that electrodes which have intermediate properties have first a negative light effect followed by a recoil in the positive direction which may or may not cross the value of the original potential, that is the initial light effect is negative; the final effect may be either negative or positive. The tendency t o a negative effect appears to increase as the density of the oxide film over the copper is increased, at the same time the extent of the positive recoil is diminished. The following experimental facts have been established in addition to those given by the authors cited above: 1. A cuprous oxide electrode which when illuminated becomes positive with respect to a neutral solution may be made to become negative by increasing the density of the oxide coating.
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Goldmann and Brodsky (loc. cit.) concluded from their experiments that the positive light effect was not influenced by varying the thickness of the oxide coating. They, however, produced all their coatings by oxidation in the flame and it has been found that this method while producing varying thicknesses of cuprous oxide does not give a dense and uniform layer. If the copper is first coated with cuprous chloride by dipping in cupric chloride solution, then hydrolyzed to the oxide in water a very uniform oxide coating is produced. After a single treatment the electrode ordinarily behaved as though it had been oxidized in a flame, but after several treatments, by applying thin layers of chloride and by slow hydrolysis of each coating in cold water a dense, red, coherent deposit was produced which had a large negative light effect. Electrodes have been made in this way which had a negative photo-potential of 0.30 volt in the light of a 500 candle power lamp a t a distance of 25 cm. These are the largest light effects which have ever been obtained. The positive photo-potentials are small compared to these values rarely exceeding 0.05 volt. 2. A cuprous oxide coating is slowly formed on copper when it is left standing in a neutral solution for a long time in contact with air or oxygen and as a result electrodes which have only a positive effect a t first slowly develop the negative effect on contact with oxygen. A polished copper electrode was placed in a neutral K 8 0 4 solution and left standing in contact with it for a long time. Its potential with respect to a normal calomel electrode was recorded at intervals and also the effect of light on the potential. The electrode was approximately -0.093 volt with respect to the calomel electrode a t the start and illumination of the electrode produced no marked change in this value. After standing for several days the copper was found to have become coated with a thin, irregular, cuprous oxide layer and its potential slightly increased. This electrode had a slight positive light effect. After standing for a week or so the potential had increased slightly and the effect of illum-
Cuproz6s Oxide Photo-Voltaic Cells
605
ination was found to be first a small negative effect followed by a larger positive effect. These changes were found to be brought about in a shorter time by causing a discharge of oxygen a t the copper electrode by electrolysis. The final state of the electrode is one in which the copper is entirely covered by a dense red crust of oxide, its potential relative to the calomel electrode having become approximately -0.027 volt and its light effect strongly negative. Without electrolysis it requires several weeks to reach this condition. Cuprous oxide electrodes which were prepared in other ways and had positive light effects were found to develop ,also a negative effect on standing where oxygen was available. 3. When the hydrogen ion concentration is not too high in the electrolyte the illumination of a cuprous oxide electrode causes just enough chemical action to alter the potential of. the electrode and no further action takes place until a current flows through the cell. Chemical changes which go on when current passes, produce a current in the opposite direction when the light is removed. Two cuprous oxide electrodes were prepared of approximately 10 cm'.2 area and placed in a normal KC1 solution. Electrodes which were to be exposed to light in the photovoltaic cells were coated on the back with paraffin or some other inert insulator so that only the portion of the electrode which was uniformly illuminated was in contact with the electrolyte. This was necessary in order to reduce local currents in the electrode to a minimum. This photo-voltaic cell having identical electrodes produced no e.m.f. in the dark. It was arranged so that one electrode could be illuminated from a 500 candle power tungsten lamp while the other electrode was kept in the dark. The electrodes were first connected through a low resistance galvanometer. When light was allowed to fall on one electrode a current passed in a direction which indicated that the exposed electrode had become positive with respect to the electrolyte. If this current was permitted to pass for several minutes and the light then turned off, the current dropped to zero and reversed its direction
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indicating that the exposed electrode had now become negative to the one which had not been exposed. The extent of this reversal depended on the duration of the illumination. However, when a potentiometer was substituted for the galvanometer and the e.m.f. of the cell in light balanced so that no current passed, the tendency to reverse after illumination was entirely destroyed. No perceptible action took place on illumination other than the change in potential of the electrode. When the light was turned on or off the electrode changed its potential at a moderate rate and was found to come into equilibrium in 15 to 30 seconds. 4. A high concentration of hydrogen ions in solution causes a destruction of cuprous oxide with the formation of copper and cupric ions and a higher concentration is required in the dark than is required in light. It is a well known fact that the addition of a dilute acid . to cuprous oxide decomposes it with the formation of copper and cupric ions in equivalent amounts as follows: (21120 2Hf+ CU CU++ H2O Two electrodes were coated with CuzO and placed in distilled water. One was exposed to intense white light and the other shaded. The liquid was kept well stirred while very dilute sulphuric acid was added slowly a drop at a time. When the concentration of the hydrogen ions had reached a certain value the coating on the electrode which was illuminated was decomposed, while the other remained unattacked. It required the addition of a few more drops of acid to destroy the coating on the electrode in the dark. If the electrodes were already in a solution which contained cupric ions a higher concentration of hydrogen ions was required to cause the decomposition. This fact may be deduced from the above reaction by the application of the mass law but its real significance may be seen when the reaction is considered from the electrochemical standpoint. 5 . The current produced by the light flows in such a direction that the cuprous oxide coating at the exposed electrode is destroyed.
+
+
+
Cup~ousOxide Photo-Voltaic Cells
607
A cuprous oxide electrode was placed in a neutral KzS04 solution where the oxide was stable. The electrode was illuminated strongly on one side while the other side was left shaded. This was a photo-voltaic cell producing a current through the electrode itself. The current was found to destroy gradually the cuprous oxide on the illuminated side. 6. A cuprous oxide electrode which has a negative light effect in a neutral electrolyte such as a solution of K2SO4 or KC1, may be made t o have a positive effect by increasing
1
TABLE I
Current
+3.0
+4.0 -0.5 -1.0 -3.0 -4.0 -4.0 -4.0 -2.5 -0.5
-0.5 +1.0
+I
+
3 . 0 (neutral) + 3 . 5 (acid) Coating destroyed
I
KOH
0.o'cc. 0.1 0.2 0.3 1.o 2.0 3.0 4.0 7.5 10.0 HCl 2.0 5.0
5.25 5.5 5.6
TABLEII
Current
-10.0 - 5.0 - 3.0 - 2.5 - 2.0 - 1.0 - 0.5 4.0 4.5 5 . 0 (neutral) 5 . 0 (acid) Coating destroyed
+ + + +
0.0 cc. 0.5 0.75 1.0 1.5 2.0 2.5 2.6 2.7 2.8 2.9 3.0
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contains the data obtained with this arrangement. I n the first column will be found the deflections of the galvanometer when one electrode was illuminated the sign indicating the direction of the change in potential of that electrode. I n the second column will be found first the relative amount of KOH solution added and second the .amount of HC1 added. I n Table I1 are the results of adding H2S04to a cell whose electrolyte a t the start consisted of K2S04 containing 1.6 grams of KOH per 100 cc. These results are similar to those obtained with other electrodes. The law holds only for electrodes having a fairly dense cuprous oxide deposit. Electrodes having a very thin deposit do not always develop a negative light effect on the addition of alkali. Also, electrodes having dense and uniform coatings as has been stated before already have a large negative light effect in neutral solutions. I n this case the effect is not increased by the addition of alkali. Thus an electrode which had a maximum negative photo-potential of 0.16 volt in neutral normal KzS04 solution retained this photo-potential even after the solution had been made strongly alkaline with KOH The same peculiar behavior was found on varying the concentration of the cupric ions in the solution. 7 . I n the case of electrodes having very dense CuzO coatings, those having large negative light effects, an increase in cupric ion concentration in solution reduces the negative effect but does not produce a positive effect. I n the case of electrodes having positive effects the addition of cupric ions to the solution causes the appearance of a negative effect. A photo-voltaic cell was prepared having two electrodes of dense cuprous oxide in normal K2S04solution. The photopotential of this cell was -0.20 volt. Increasing amounts of solid CuS04 were added to the cell and the initial negative photo-potential was found to diminish to values which appeared from these preliminary experiments to be proportional to the logarithm of the cupric ion concentration, that is the potential dropped less rapidly than the concentration rose.
.
Cuprous Oxide Photo- Voltaic Cells
609
Table I11 contains the results of experiments on a photovoltaic cell having electrodes with less dense cuprous oxide coatings. The electrodes were connected through a galvanometer and in column one will be found the initial current produced on illumination after concentrated CuS04 solution had been added as recorded in the second column.
TABLE 111 The Influence of Cut+Ion Concentration Current Scale divisions
+6.0
+3.5
+l.S +0.5 1st. - 1 . 0 then 0 1st. - 2 . 0 then 0 -3.0 -4.0 -4.5
+ . 2 slowly + .1 slowly
I
CuSOd added cc
0.0 0.5 1.0 1.5 2.2 3.0 4.0 5.0 6.0
The behavior of this electrode is typical of all electrodes when their condition is such that they are near the change from a positive to a negative effect. The initial effect of light is a quick change in the negative direction. If the illumination be kept constant, however, the negative potential is not maintained but approaches again the original dark value sometimes crossing this value and developing a steady positive photo-potential. This phenomenon must not be confused with the reversal of the e.m.f. after illumination which was shown to be the result of chemical action of t h e current during illumination. 8. The initial maximum negative photo-potential is approximately proportional to the intensity of the light for low intensities but beyond a certain intensity is constant and no longer increases with the intensity. The initial negative photo-potential attained on constant illumination is not maintained but after the maximum is reached the photo-potential diminishes to an extent which depends on the intensity.
a
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An electrode having a dense cuprous oxide coating was placed in a 0.1 formal K2S04 solution and its potential was compared with that of a 0.1 formal calomel electrode by the use of a potentiometer. Its maximum changes in potential relative to the calomel electrode and also the potential after some fixed time had elapsed were observed while the electrode was being illumined with a known intensity of white light. The intensity of the light was measured with a thermopile and galvanometer, the thermopile being placed as close as possible to the light-sensitive electrode of the photo-voltaic cell. One way of varying the intensity without varying the distribution of energy in the spectrum of the light was to place the light a t different distances from the cell and thermopile. A more convenient method was to place a large lens in the path of the light at a distance from the source less than the focal length of the lens. This produced a diverging cone of light and the cell and thermopile were placed in the center of this cone. By a relatively small variation in the distance from the lens to the light source a large variation in the angle of the cone was obtained and a resultant large change in the intensity of the light a t the point desired, without any appreciable change in the distribution of the energy. The intensities of the light expressed in the deflections of the galvanometer and the corresponding changes in the e.m.f. of the photo-voltaic cell were recorded a t the same time. Table IV contains the experimental results which were obtained with an electrode having a large negative light effect. The results are typical of such electrodes the nature of the effect being always the same although the values of the photo-potentials vary from one electrode to another. The light intensities are given in the first column. I n the second column are the maximum values of the photo-potentials which were reached in a few seconds after the light was turned on. Five minutes after the maximum was reached the potential relative to that a t the start was recorded in column four. The positive recoil thus obtained increased indefinitely with the intensity. Up to the value of the intensity marked 100.0 in column
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611
one it may be seen from column three that the maximum photo-potential was almost proportional to the intensity but beyond this point it became independent of the intensity. Electrodes having less dense oxide coatings were found to have relatively less negative effects and larger positive recoils. In those electrodes which had only one thin coating of oxide the positive effect was the only one noticed.
TABLE IV The Influence of Intensity on the Negative Effect Intensity
(L)
6.2 7.0 8.0 10.0 16.5 40.0 52.0 71.0 77.0 93.0 100.0 183.0 260.0 339.0 490.0
--I-
I
Maximum Photo-potential (dv)
-0.0131 -0.0140 -0.0146 -0.0188 -0.0367 -0.082 -0.105 -0.119 -0.121 - 0.152 -0.157 -0.206 -0.200 -0.202 -0.205
(do)
After 5 min.
0.0021 0.0020 0.0017 0.0019 0.0022 0.0020 0.0020 0.0017 0.0017 0.OOlG 0.0016 0.0011 0.0007 0.0005 0.0004
,
-0.012 -0.013 - 0.0135 -0.016 -0.032 -0.032 -0.035 '-0.029 -0.019 -0.012 -0.007 -0.005 +o . O O l +o .005 +o .010
9. With increasing intensity of illumination the positive effect increases approximately as the logarithm of the intensity. 'The maximum positive photo-potential remains constant with constant illumination. In Table V are the results of experiments in which varying intensities of light were used with an electrode having a single coating of cuprous oxide and consequently having only the positive effect. I n the first column are the light intensities and in the second column the corresponding values of the positive photo-potentials. Unlike the negative light effects the positive photo-potentials increase more slowly when the
,
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Allen D . Garrison
light is turned on to a constant value which is maintained as long as the intensity remains constant and no current flows. I n the third and fourth columns are the values of the photo-potentials divided by the corresponding intensities and logarithms of the intensities, respectively.
TABLE V The Influence of Intensity on the Positive Effect Intensity
Photo-potential
(L)
(do)
52.0 56.5 75.0 80.0 157.0 395.0 477.0
'
+O ,024 volts +O. 025 +O. 029 +O .030 3-0.052 +0.056
$0.057
(d4 /L
4.7 X 4.5 4.27 3.7 3.3 1.3 1.2
(dv)llog L
0.0140 0.0146 0.0175 0.0158 0.0231 0.0212 0.0213
From this it may be seen that the relation is nearly the logarithmic one. Although he did not arrive a t this relation Goldmann (loc. cit.) found that direct proportionality did not hold for electrodes oxidized in the flame and that the photo-potential did not increase as rapidly as the intensity. 10. I n the case of electrodes having both a positive and a negative effect, it has been found by using a monochromatic illuminator that light of long wave-length produced the negative effect, that light of short wave-length produced the positive effect and that the light of intermediate wavelengths produced both effects, the negative effect appearing first as has been described. It was also found that an increase' in the concentration of the hydrogen ions in the solution shifted the intermediate position toward the longer wave-lengths, and that a decrease shifted the position toward the shorter wave-lengths. However, the data are at present incomplete on this phase of the problem chiefly due to the fact that the intensity of the radiation throughout the spectrum of the illuminator was not uniform, the maximum intensity being in the infra red and the shorter waves having relatively small
Cuprous Oxide Photo- Voltaic Cells
613
energy. I n view of the complicated changes introduced by variations in intensity the effects thus far obtained cannot be attributed entirely to variations in wave-length. For this reason the experiments on this point will be reported when they are in a more satisfactory condition.
Theoretical Part It is difficult to explain the complicated behavior of the cuprous oxide cells by application of the theory of electron emission as has been attempted by Goldmann. It cannot be clearly seen from this theory how the light may cause either a positive or a negative effect or both determined by the condition of the oxide or the concentration of the hydrogen ions or the cupric ions in solution. There is also additional experimental evidence that the values of the absolute electrode potentials used by Goldmann were not the most probable ones. Consequently the problem will be attacked in this report from the photo-chemical rather than the photo-electric standpoint. By' the application of the established laws of electrochemical equilibrium to the cuprous oxide electrode it is possible to isolate a single light-sensitive reaction and to explain all the known characteristics of the photo-voltaic cells by attributing certain properties to this single photo-chemical equilibrium. If this theory proves itself to be the most probable explanation of the photo-voltaic cells it becomes of interest to those who are studying thermal and photo-chemical reaction velocities or photo-phenomena in general since it offers a new and direct method of measuring the effects of light. For simplifying the explanation of electrode behavior it has been found convenient to classify equilibria in a more precise manner than is usually done in electro-chemical textbooks. Reactions are described as reversible, irreversible, polarizable or nonpolarizable. In a number of instances, Garrison: Jour. Am. Chem. SOC.,45, 37 (1923).
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Allen D. Garrison
however, the idea of physical reversibility has taken the place of that of thermodynamic reversibility and polarization has not been properly connected with irreversibility. As in any thermodynamically irreversible process the energy relations of an irreversible electrochemical reaction are indefinite in that the available work term may have any value between its maximum value and zero. The requirements of reversibility in an electrochemical reaction are, first, the potential of each electrode will not vary from its normal equilibrium value by more than an infinitely small amount, and second, diffusion of any substance in the system will take place only between points a t an infinitely small difference in pressure. It appears advisable to use these terms only in this strict sense and to introduce other terms which express the physical conditions of reversible equilibria. The term polarization has sometimes been used to describe this physical characteristic but in its more rigorous definition, polarization is always irreversible. A “primary” reversible electrochemical reaction will be defined as a reaction which may proceed reversibly to an appreciable extent at an electrode a t constant electrode potential. A “secondary” reversible electrochemical reaction will be defined as one which may proceed reversibly but only as the electrode potential continually increases or decreases. This distinction may be made clearer by an example. Pure mercury in contact with an electrolyte containing a mercury salt in solution and also some of the solid salt has frequently been called a “nonpolarizable” electrode, and when the solid salt is absent a “polarizable” electrode. These terms are misleading in that they convey the idea that the passage of current in the first case is reversible and in the second case irreversible because of the physical nature of the electrode. For, quoting a recent and accurate text book of physical chemistry : “The name ‘polarization’ [is] used in general to denote ’ the production by the passage of the current of any change in the solution adjoining the electrode or in the surface of the
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electrode which makes its potential deviate from its norvnal value.” I n polarization therefore the concentration of some substance is locally changed in the neighborhood of the electrode and after a time the concentrations become equalized by the irreversible process of diffusion and the electrode potential returns to its normal value. The mercury electrode in either of the above cases may be polarized by high current densities or may behave reversibly at low current densities. The mercury ion equilibrium in the first case is primary, since during reaction the mercury ion concentration is kept constant as long as there is any solid salt left. It is secondary in the second case for as reaction proceeds reversibly the mercury ions throughout the solution change their concentration. This change is not due to polarization since the potential of the electrode does not vary from its normal value at any time. Changes in electrode potential in a secondary reaction are analogous to changes in pressure in the reversible isothermal expansion of a gas. Assuming that an electrode has reached equilibrium and behaves reversibly some conclusions may now be drawn from the established laws of electrochemistry. 1. The concentrations of the reactants of a primary reaction determine the potential of the electrode by the well known logarithmic relation, and 2. The potential of the electrode as thus determined fixes the concentrations of the reactqnts of all the secondary equilibria. A certain change in the solubility of the mercury salt in the above example alters the potential of the electrode accordingly, since by definition the primary reaction may be displaced without appreciable change in concentration thus proceeding until all the secondary reactions have come into equilibrium. On the other hand a change in the concentration of one of the reactants of a secondary equilibrium does not alter the potential of the electrode in the presence of a primary reaction, unless it indirectly shifts the primary equilibrium. If in the above example the solid salt were mercurous chlo-
Allen D. Garrison
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ride then an increase of hydrogen ions in solution by the addition of sulphuric acid would not alter the electrode potential, the mercury equilibrium would be shifted slightly to bring the hydrogen and oxygen pressures into equilibrium with the electrode potential. 3. When the potential of the electrode is changed from the outside by an infinitely small value (dv)the primary equilibrium is displaced quantitatively in accordance with Faraday’s laws and 4. The secondary reactions because of their rapid changes in pressure require only a negligible fraction of the current to bring them again into complete equilibrium with the new electrode potential. 5 . If no primary reactions are possible then on altering the normal electrode potential from the outside by a small amount (dv) the reaction which proceeds with the smallest changes in pressure, that is the reaction which resembles a primary one most closely, is displaced quantitatively in accordance with Faraday’s laws with the exception of the small amount of current necessary to keep the other secondary reactions in equilibrium. Unlike the electrode having a primary reaction the potential must be repeatedly changed from the outside in order that the reaction may continue. To apply this classification to the reactions of the cuprous oxide electrode it is necessary to know at least in a relative way the changes in the concentrations of each substance in equilibrium with the electrode. Let us consider a copper electrode coated with cuprous oxide and placed in a normal aqueous KzS04 solution. All the ions in the solution must establish an equilibrium a t the surface of the electrode, thus (1)
(2) (3)
2H+-Hz 2O-c--+02 K+++K
(gas) (gas)
-t 2 (+>
+ 4 (-) + (+)
(metal)
For the sake of simplicity it is assumed that the sulphate ion is not capable of an equilibrium. Under these conditions (3) as an ideal secondary reaction for the concentration of
Cuprous Oxide Photo-Voltaic Cells
617
potassium metal is infinitely small compared to the massive metal at any electrode potentials we shall consider. The gas equilibria (1) and (2) are also secondary since the electrode is not supposed to be in equilibrium with a gas phase a t constant pressure. Cuprous oxide has a finite,solubility and as a result the following equilibria are established, (4)
CuzO(so1id)++ CuzO(disso1ved) +-+
(5)
cu+++cu Cu-Cu+++
(6)
+ (+)
2Cu++O=
(=)
It may readily be seen that reaction (5) is a primary reaction as long as solid cuprous oxide and massive copper are present. Reaction (6) although dependent on (5) is a secondary reaction since the cupric ion concentration may vary in the solution. However, as long as massive copper is present it has only one variable reactant and therefore resembles a primary reaction more closely than the gas equilibria. If the normal electrode potential were displaced from the outside, reactions (5) and (4) would proceed according to Faraday’s laws. If this potential were higher than the equilibrium potential (4) and (5) would proceed to the left and Cu20 would be formed at the expense of copper. This would continue as a primary reaction at the same electrode potential until the copper were exhausted or until it became coated with such dense cuprous oxide that it no longer behaved as massive copper toward the solution. Some data on the variation of solution pressure with amount of metal have been obtained by Oberbeckl and Konigsberger and in which they show that the solution pressure of a metal is independent of the amount of metal present until the amount becomes very small and then its apparent concentration varies rapidly as the amount decreases. Therefore a t the point where copper begins to be exhausted Wied. Ann., 31, 336 (1887). 849 (1905).
* Phys. Zeit., 6, 847,
618
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Allen D. Garrison
and the massive metal is cut off from contact with the electrolyte by the cuprous oxide coating, the concentration of the copper becomes variable and reaction (5) is no longer primary. This explains the experimental fact that the equilibrium potential is higher for very dense cuprous oxide coatings than for others. It was also found experimentally that this condition was brought about by passing a current in such a direction that the cuprous oxide coating was formed at the expense of copper. A simple calculation is all that is necessary to explain the formation of cuprous oxide a t the expense of copper when in contact with atmospheric oxygen. At the electrode potential of -0.09 volt with respect to the normal calomel element and in a solution where OH- ions approximate one mole per liter, the pressure of oxygen gas in equilibrium must be approximately 1/2D00 of an atmosphere. When brought in contact with oxygen a t 1/5 atmosphere this reaction is polarized and as oxygen diffuses through the solution to the electrode and ionizes, the electrode becomes more positive. This is equivalent to a slight current forming cuprous oxide. It was found experimentally that a pure copper electrode in K2S04solution is not sensitive to visible radiation. This electrode has necessarily set up all the equilibria (l), (2), (3), (5), and (6). The light-sensitive reaction must therefore be introduced by the presence of solid cuprous oxide. From experiments Nos. 4 and 5 we may conclude that reaction (4) is shifted to the right by the radiation. If we start with this assumption all the characteristics of the influence of light which have been found for these cells may be explained by the established laws of electrochemistry. I n the case of an electrode containing massive copper, equilibrium (5) was found to be primary, and reaction (2) secondary. If the reaction (4) were displaced to the right' by the radiation the concentrations of both the cuprous ions and the oxygen ions increase simultaneously. The electrode potential will be determined by the cuprous ion concentration since it is primary. This accounts for the positive effect.
C u p ~ o u sOxide Photo-Voltaic Cells
e19
If the velocity of formation of ions is proportional to the intensity of the light then the concentration of cuprous ions at equilibrium is proportional to the intensity and the photopotential should increase with the logarithm of the intensity. This maximum photo-potential would also remain constant with constant illumination provided no current was permitted to flow. Therefore the properties which we must attribute t o the photo-chemical reaction are in agreement with the Einstein photo-chemical law in that the number of quanta available for shifting the reaction constant is proportional t o the light intensity. It was proven that reaction (5) is secondary when the cuprous oxide becomes of sufficient density to limit the amount of copper available for reaction. At this point all the reactions of the electrode are secondary. To trace the effect of an increased ionization of cuprous oxide it is necessary to know how the reactions come again into equilibrium and which reaction resembles a primary one most closely. I n this case it is possible that the oxygen equilibrium behaves more like a primary one than does the copper equilibrium, making the light effect negative. There are two ways in which the cupric ions may adjust themselves t o the new concentration of cuprous ions, as follows : (7) 2 c u + + 2 c u 2(+)+Cu Cu++ (8)
CU++CU
+ + + (+)+CU++ + (-)
As the cuprous ions increase they deposit on the electrode as copper but this greatly increases the concentration of the copper. This necessitates the solution of cupric ions as in (7) or (8). Suppose that the first small amount of cuprous ions liberated reacts as in (7). Although equivalent quantities of copper and cupric ions are formed, under the conditions specified the copper concentration may increase faster than the cupric ion concentration for an addition of the same amount of material. As a result the products of reaction (7) accelerate reaction (8), which results in a more negative electrode potential.
.
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Allen
D.Garrison
The increased oxygen ion concentration also tends toward a negative photo-potential the reaction being now relatively (+). less secondary than the reaction Cu++Cu Therefore when the concentration of copper is very small a t the start and the light increases the concentrations of the cuprous and oxygen ions simultaneously, the electrode must become negative in order to establish the other secondary equilibria. If the concentration of copper were infinitely small it can easily be seen how the negative photo-potential may become large. With increasing intensity of the light more cuprous oxide ionizes and enough copper forms through reaction (7) to behave as massive copper. After this point reaction ( 5 ) is again primary, the products of reaction (7) no longer accelerate reaction (8) and a further increase in intensity produces the positive recoil. This explains those electrodes which have a temporary negative light effect followed by a permanent positive effect. By reaction (4) the cuprous ion concentration is dependent on the oxygen ion concentration and by reaction with water the oxygen and hydrogen ions are interdependent. Thus by adding hydrogen ions cuprous ions are liberated which react according to (7) and (8) and by the reverse cuprous ions and copper are removed. This explains the decomposition of cuprous oxide in acid solution and the effects on the positive and negative light effects. By applying the mass law to reaction (8) it is evident that in the case of large negative light effects cupric ions reduce the maximum negative photo-potential. I n other cases the additions of cupric ions may reduce the amount of copper and develop the negative effect by reversing reaction (7). It appears from experiment 10 above that an increase in the frequency of the light influences either the rate or the amount of oxide ionized, This point requires more experimental data. I n connection with this theory of cuprous oxide cells it is interesting to note that cuprous chloride electrodes as well
+
Cuprous Oxide Photo-Voliaic Cells
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as silver halide electrodes behave in some ways like the cuprous oxide electrodes. It has been known for a long time that moist cuprous chloride decomposes in light as follows : 2CUCl+CU
+ CUClZ
but the mechanism of this reaction in which an equivalent of copper and cupric chloride are formed has not been satisfactorily explained. An explanation is evident, however, by applying this theory and substituting chlorine for oxygen. Dry cuprous chloride and silver halides are stable in light, and according to this theory the “catalytic” action of moisture is the action of a solvent which must moisten the solid to permit electrochemical equilibrium. From this we may conclude that the influence of the radiation in such cases is primarily a separation of the charged elements as ions followed by the establishment of an electrochemical equilibrium. I n some instances the limit of solubility from an electrochemical standpoint is reached and a spontaneous discharge of ions takes place with the liberation of the e1ements.l
Summary The formation of cuprous oxide electrodes and the conditions under which they have both a positive and a negative light effect have been described and the general characteristics of each effect discussed. The positive photo-potential was found approximately proportional to the log of the light intensity and increased with increasing hydrogen ion concentration. The maximum negative photo-potential is proportional to the light intensity for low intensities and constant a t high intensities, while the positive recoil following the negative maximum, increased with the intensity. The maximum negative photo-potential decreased with increasing hydrogen ion concentration. For large effects the negative photoNernst: Ber. deutsch. chem. Ges., 30, 1547 (1897); Bodlander: Zeit. phys. Chem., 27, 55 (1898).
Allen D. Garrison
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potential decreased and for small effects increased with increasing cupric ion concentration. The characteristics of the cuprous oxide electrodes which are now known were all explained from the proven laws of electro-chemistry by the assumption that the light shifts the equilibrium constant of the reaction CUZO(solid) ++CUZO(dissolved) ++2Cu+
+ O3
causing the cuprous oxide to behave as though its solubility was increased in the light. The results are what would be expected if the Einstein photo-chemical law applied to this reaction in a qualitative manner. The active frequencies are as yet undetermined. The theory is also applicable to the silver halide electrodes as well as the copper electrodes since they behave in the same way and is opposed to the theory that the primary effect of radiation is either photo-electric in its nature or causes a direct separation of the uncharged elements. The theory that the effect of the light is a separation of the charged elements as ions or thereby an increase in the solubility of the salt and that the decomposition is made complete only by electrochemical equilibrium is supported by the experimental fact that the perfectly dry salts are stable in light. The author wishes to thank Prof. H. A. Wilson, F.R.S., for his suggestions and interest in this work. The Rice Institute Houston, Texas
