- Email: [email protected]
Incorporation of alkali metals into solid cathodes
Electrochimica
Acta.
1968. Vol.
13. pp.
19 to 25.
INCORPORATION
Institute of Electrochemistry,
Pergatnon
Press.
Printed
in Northern
Ireland
OF ALKALI METALS INTO SOLID CATHODES* B. N. KABANOV Academy of Sciences of the U.S.S.R.,
Moscow, U.S.S.R.
Abstract-In explaining the peculiarities of the hydrogen-evolution process and of some other cathodic processes occurring in the solutions containing alkali cations, it is necessary to take into consideration the incorporation of alkali metals into the electrode metal. The cathodic incorporation of alkali metals into lead, tin, zinc, cadmium, silver ahuninium, bismuth and other metals has been investigated. An explanation is suggested for the slowness of initial incorporation and the increase in the rate of incorporation when the electrode “develops”. The incorporation rate is determined by the number of vacancies in the crystal lattice of the electrode metal near its surface. The process of anodic removal of alkali metal from the electrode is discussed. R&sum&11 convient de tenir compte de l’incorporation de m&al alcalin au m&al de l’&ctrode pour expliquer les particularit& de processus d%volution de l’hydrogkne ainsi que de certains autres processus se manifestant dans les solutions contenant des cations alcalins. Recherches sur cette incorporation cathodique de m&aux alcalins aux plomb, &in, zinc, cadmium, argent, aluminium, bismuth et autres m&aux. Une explication est sugg&& pour la lenteur de l’incorporation i&ale et l’accroissement de sa vitesse au fur et & mesure que 1’6lcctrode se “d&eloppe”. La vitesse d’incorporation est dttermink par le nombre de vacances dans le rbseau cristallin de 1’6lectrode metallique au voisinage de sa surface. Discussion de processus d’elimination de m&al alcalin de Nlectrode. Zusammenfassung-Bei der ErklLung der Bcsonderheiten des Wasserstoff-Abscheidungsvorganges und einiger anderer kathodischer Vorgsnge, welche sich in Alkali-kationen enthaltenden LGsungen abspielen, ist es notwendig, such die Einlagerung von Alkalimetallen in das Elektrodenmetall in Betracht zu ziehen. Man untersuchte die kathodische Einlagerung von Alkalimetallen in Blei, Zinn, Zink, Kadmium, Silber, Aluminium, Wismuth und andere Metalle. Ftir die Langsamkeit des anfsbglichen Einbaus und die Geschwindigkeitszunahme bei sich “formierender” Elektrode wird eine Erkllrung vorgeschlagen. Die Gcschwindigkeit des Einbaus ist durch die Zahl der Leerstellen im Kristallgitter des Elektrodenmetalls nahe der OberfXche bcstimmt. Man diskutiert den Vorgang der anodischen Auflijsung des Alkali-metalls aus der Elektrode. to explain the cathodic dispersion of metals in alkalis, Haber,l Bredig2 and others,8 and more recently Angerstein, suggested a hypothesis that in aqueous solutions alkali metals form intermetallic compounds with solid cathode metals: but the majority of electrochemists have considered this hypothesis to be unlikely.5 There are now, however, many new facts that cannot be accounted for without recourse to the above hypothesis. Thus, the author and co-worker9 observed cathodic superactivation of aluminium and magnesium in alkalis and assumed it to be due to the cathodic formation of a surface compound of aluminium and magnesium with the alkali metal. The assumption was based on the dependence of superactivation of aluminium upon the cation nature (in the series from lithium to tetramethylammonium). Investigating the electroreduction of acetone in alkaline solutions, Antropov IN ORDER
* Presented at the 17th meeting of CITCE, Tokyo and Kyoto, received 24 March 1967. 19
September
1966; manuscript
B. N.
20
I(ABANOV
and others’ observed anomalies in the current efficiency in the case of lead and two other solid cathodes, the anomalies being of the same nature as those encountered with amalgam cathodes. These authors also explained the results obtained by the formation of compounds between the alkali metals and the metal of the cathode. Changes in the equilibrium potential of silver electrodes are observed in alkalichloride melts, and are attributed by some authors to the formation of half-valent silver and by others to the formation of intermetallic compounds with alkali metals. A spontaneous formation of a silver alloy with an alkali metal is also known to take place when silver is immersed in a melt of alkali-metal salts.8 Unaccountable phenomena have been observed time and again in hydrogenovervoltage measurements on various metals in alkalis, such as a slow change in the overvoltage with time, hysteresis, increased Tafel slopes, and poor reproducibility. Attempts have been made to explain the change in the overvoltage with time by the formation of surface hydrides,9 even in the case of metals which are known not to form stable hydrides. Zinc, cadmium and lead give no hydrides and adsorb hydrogen very poorly: in such cases there can scarcely be any question of a cathodic formation of hydrides in the quantities of the order of a monolayer. We have consequently studied hydrogen overvoltage on zinc, silver and other solid metals in alkaline solutions.lO The measurements were carried out in hydrogen atmosphere. The results of measurements on a silver electrode are shown in Fig. 1. In slow measurements, on reaching a certain value, the potential begins to rise with time (Fig. 1, a). In some tens of minutes the potential shifts in the negative direction by
-1.0
1
I -3
log
i.
-2
A
/cm2
FIO. 1. Polarization curves of hydrogen evolution on silver in 1 N NaOH.
0~2-0~3V. With a sufficiently reduced polarization of the electrode, the potential shift with time in the positive direction at constant current (b). The change in the hydrogen overvoltage with time was assumed to be due to the formation as the result of incorporation, a, or to the decomposition, b, of compounds of the cathode metal with the alkali metal atoms. As a rule, the hydrogen overvoltage on these compounds is by 0.243 V higher than on the metal without the alkali metal, which seems to be due to the lesser adsorption energy of hydrogen. If the hydrogen overvoltage in alkalis is determined by the presence of intermetallic compounds, it must be also dependent on the nature of the alkali metal. Indeed, the overvoltage proved to increase with the radius of the unhydrated alkali cation: for instance, when we pass from lithium to cesium, the overvoltage on a silver electrode increases by O-25 V. On the other hand, the formation of an intermetallic compound occurs the faster, the smaller is the crystallographic radius of the alkali ion, being the
Incorporation
of alkali metals into solid cathodes
21
fastest in the case of lithium. This conclusion is drawn on the basis of measurements of the time necessary for a constant value of hydrogen overvoltage to be established. The explanation of all these phenomena by the incorporation of alkali metals in the cathode metal however, could not be considered as conclusive without direct evidence in favour of the formation of such compounds and without the elucidation of the mechanism of their formation. It was natural to expect that incorporation should occur into such metals which form intermetallic compounds by alloying with alkali metals, eg Zn, Cd, Ag, Al, Pb, Sb, etc. It is not impossible, however, that some compounds which cannot exist at high temperature should prove stable at room temperature and could be obtained by incorporation. We shall not consider here the co-deposition of some metals with alkali metals, which may proceed faster than incorporation. For instance, no incorporation appears to be proved to take place in the case of iron electrode, for which no intermetallic compounds can be obtained by melting, whereas co-deposition, eg of iron with an alkali metal, has been observed to occur.ll To prove that the alkali metals are incorporated in the electrodes, Astakhov’O carried out some experiments in which the alkali metal was caused to pass through a membrane of zinc foil. On one side of the membrane was an alkaline solution and cathodic polarization was applied, on the other was a dilute potassium sulphate solution. The incorporation and passage of potassium through the membrane was established from the change in pH of the second solution. Cesium and tetramethylammonium ions practically do not diffuse through the membrane at all. These results are in agreement with the expected dependence of the incorporation on ionic radii. The dependence of the rate of incorporation of potassium into zinc upon the electrode potential is shown in Fig. 2. The polarization potential of the membrane is shown on the abscissa and the logarithm of the amount of potassium that has diffused through the membrane in 22 h is shown on the ordinate. The incorporation rate increases exponentially with electrode potential. The rate of bulk self-diffusion of zinc at room temperature is known to be about
d o 0
-6.0
-6.5
9-e
V
FIG. 2. Dependence of the logarithm of the amount of potassium that has diffused in 22 h through a 20 ,u zinc membrane upon the polarization potential a, of the membrane in 1 N KOH.
B. N.
22
KABANOV
a million times less than that at the intercrystalline boundaries. Probably, the rates of heterodiflusion in the bulk and at the boundary differ as much as those of self-diffusion. Thus, at room temperature the transfer of the alkali metal incorporated into the electrode metal must occur mainly along the boundaries of the crystal grains. To investigate the incorporation of alkali metal, potential/time curves were measured.13*14 The measurements were carried out in an inert gas or hydrogen atmosphere. The arrests on the cathodic curves correspond to the formation of intermetallic compounds (Fig. 3, b) and those on the anodic curves to their decomposition (Fig. 3, a). From the position of the arrests, the equilibrium potentials of the intermetallic -1.60
FIG. 3. (a) Anodic and (b) cathodic potential/time curves measured on cadmium in 1 N LiOH after polarization at Q = -1.65 V.
compounds were determined. The equilibrium potential of the compound obtained by alloying lead with sodium proved equal to that determined from the charging curve of lead in sodium hydroxide solution. I5 Sometimes the anodic curves show two or three arrests. This means that several intermetallic compounds with different compositions can be formed upon cathodic polarization of a given metal. Sometimes the potential arrests are not well defined and only in time the solid solution of alkali metals in the electrode metal passes into such a state where the arrests become more distinct. Previously for this and similar intermetallic compounds the equilibrium potential measurements were carried out in non-aqueous electrolyte solutions.ls All the attempts to explain the behaviour of the electrodes in alkalis by incorporation could not meet with success, unless it were possible to explain the main inconsistency-the high value of the overvoltage observed in the incorporation as opposed to a low one observed in the deposition of an alkali metal on mercury cathodes in aqueous alkaline solutions and melts. The slowness of the process of incorporation is evidenced by the extremely slow change in the hydrogen overvoltage (on pure metal in alkaline solutions) as well as by the fact that the anodic arrest corresponding to a monolayer of the alkali metal can be detected only after a prolonged cathodic polarization, whereas in the lead-sodium alloy obtained by melting, the same amount of an alkali metal is incorporated over lo6 times as fast. Figure 4 shows the polarization curves for the lead-sodium alloy. Lead previously subjected to a prolonged cathodic polarization behaves in a similar way. The rate of incorporation into a fresh lead electrode is lower by many orders of magnitude (approximately 7) than that for a previously cathodically polarized one (or for a lead-sodium alloy).15
Incorporation
of alkali metals into solid cathodes
23
In solid metals-lead, silver, zinc, e&-the atoms are closely packed, All intermetallic compounds with alkali metals are formed by substitution. Therefore, incorporation can occur only into vacancies. If the energy of interaction of the ions in the metal with its environment is assumed to be proportional to the ionic co-ordination number, it is evident that in point of energy the transfer of the discharging alkali ion to a vacancy on the electrode surface is at most advantage, since its co-ordination
-2
-3 log FIG. 4.
i,
A/cm2
Polarization curves of (a) anodic decomposition and (b) cathodic formation of an intermediate compound of lead with sodium obtained by alloying. Solution, 1 N NaOH.
number will be 9. The co-ordination number of the ion transferred to the edge of a screw dislocation12 is only 5 or 6. If the intermetallic compound formed under cathodic polarization was not confined to the surface monolayer, but had penetrated beyond it into the neighbouring metal layers, and was later partially decomposed on the surface by the passage anodic current, we found the electrode to have changed. We call such an electrode a “developed” electrode. Due to a large number of vacancies on it, new formation of the intermetallic compound occurs faster and the diffusion rate increases. The electrode can remain in such a developed state only for a limited period, until the excessive vacancies have diffused to sinks.* The rate of many electrochemical processes is known to be strongly dependent on adsorption on the electrode surface. The factors affecting only the electrode surface state, are of some importance in our case as well. But their effect is negligible compared with that due to the structural changes in the bulk of the electrode. It is evident from Fig. 5 that the presence of arsenic and mercury compounds in the solution affects the incorporation rate but little.15
-6
-6
-7 log C,
FIG. 5. Dependence * Eg, to grain-boundaries,
-5
M
of the length of the anodic arrest Q upon the concentration 1, AsBOs; 2, HgO in 1 N NaOH. voids or to a re-arranged
surface--Ed.
c of
24
B. N.
KABANOV
The ratio of the reaction rate of incorporation to that of the transfer of the incorporated atoms into the bulk, as well as the ratio of the rates of the back processes, determines the concentration of these atoms in the surface layer of the cathode metal. Kiseleva14 has studied the dependence of the length of the arrest on the anodic potential/time curves upon the cathodic polarization time for electrodes of lead, silver, gold, cadmium and other metals in 1 N LiOH and NaOH solutions. The length of the arrest corresponds to the amount of akali metal removed from the electrode under anodic polarization. * As is clear from Fig. 6, at first this amount increases with the time of the preliminary cathodic polarization, but later, rather unexpectedly, it drops nearly to the initial value.14
25
‘K I
FIO. 6. Dependence
75
50 min
of the length of the anodic arrest Q in 1 N NaOH upon the time of preliminary cathodic polarization. 1, Au; 2, Cd; 3, Pb; 4; In.
Evidently, the presence of the maximum shows that the shape of the curve is determined by different processes, with one of them prevailing first and the second later. The rise of the curve should be due to the accumulation of the alkali metal on the cathode surface in the process of incorporation: actually it is accompanied by an increase in the hydrogen overvoltage. Judging by the fact that the hydrogen overvoltage no longer changes, further polarization does not affect the concentration of the intermetallic compound on the electrode surface. The drop of the curve, corresponding to the decrease in the amount of the alkali metal removed, can be assumed to result from the decrease in the rate of diffusion from the cathode metal to the surface. Ditfusion in a solid body is a structure-sensitive process and may change upon recrystallization of the solid solution, or upon separation of intermetallic compounds from it. Therefore, it was necessary to study the dependence of the amount of the alkali metal removed anodically upon the “rest” time, ie upon the time the incorporated alkali-metal atoms reside in the bulk of the cathode after switching off or decrease of the cathodic current. In fact, the resulting curves proved to be similar to those obtained with different cathodic polarization times (Fig. 7). The shape of the curves was found to be dependent on microstructure, in the case of l The measurements were carried out both on the same electrode after “development’* and on fresh electrodes made of identically treated metal samples.
Incorporation of alkali metals into solid cathodes
I,
25
min
FIG. 7. Dependence of the length of anodic arrest Q on a gold electrode in 1 N LiOH upon the time of “rest” 1, rest at q = -0.9 V; 2, rest at ‘p = -0.2 V.
cadmium. In the case of the fine-gained structure, the change occurs faster and sharp maxima appear on the curves. If the size of the grains is ten times as large, the maxima are diffuse. This is in agreement with the concept that the grain boundaries of the electrode metal play a large part in the incorporation and diffusion of the alkali metal. REFERENCES 1. G. BREDIGand F. HABER,Ber. Dts. Chem. Ges. 31, 2741 (1898). 2. G. BREJXG,2. uqew. Chem. 16,951 (1898). 3. F. HABERand M. SACK,Z. Elektrockem. 8,245 (1902); M. SACK, Z. anorg. Chem. 34,286 (1903). 4. H. ANGERSTEIN,Bull. Acad. Pal. Sci. 3,443 (1957). 5. H. SALZBERG, J. electrochem. Sot. 100,146 (1953); G. DUBPJXNELLand H. SALZBJZRG, J. electrothem. Sot. 100, 588 (1953); L. W. GASTWIRTand H. SALZBJZRG, J. electrochem. Sot. 104, 710 (1957). 6. B. KMANOV and A. ZACK, Dokl. Akad. Nauk SSSR 72,531
(1950); E. BARELKOand B. KAJP Doki. Akad. Nauk SSSR 90,1039 (1953). 7. M. SMIRNOVA,V. SMIRNOVand L. ANTRO~V, Trudy novocherk. politekn. Inst. 79, 43 (1959). 8. T. -DY, J. electroanal. C/tern. 11, 77 (1966). 9. V. PASTand 2. JOFA,Zh.fiz. Khim. 23,913, 1230 (1959); Dokl. Akad. Nauk SSSR 106, 1050 ANOV,
(1956). 10. B. KABANOV, D. LEIKIS,I. KISELEVA,I. ASTAKHOV and D. ALEXANDROVA, Dokl. Akud. Nuuk SSSR 144, 1085 (1962); D. ALEXANDROVA, I. KISELEVA and B. KABANOV,Dokl. Akad. Nauk SSSR 38,1493 (1964); A. ZACKand B. KABANOV, Elektrokhimiya 1,68 (1965).
11. K.
GORBUNOVA and
L. LJ~A.
Electrochim. Acta 11,457 (1966). Moscow (1965); I. KISJZLEVA. N. TOMASHOVA and B. KABANOV. ’ Zh. ’fiz Khim. 38.1188 (1964). . , I. KISELEVA; Elektrokhimiya 3,275 (1967). B. KABANOV, I. KISELEVA, I. ASTAKHOVand N. TOMASHOVA, Elektrokhimiyu 1, 1023 (1965). R. KRJWANN et al., Z. Metullk. 12, 185,257, 273,414, 444 (1920); 13,19, 66 (1921).
12. I. &TAKHOV, private communickion,
13. 14. 15. 16.
Acta.
1968. Vol.
13. pp.
19 to 25.
INCORPORATION
Institute of Electrochemistry,
Pergatnon
Press.
Printed
in Northern
Ireland
OF ALKALI METALS INTO SOLID CATHODES* B. N. KABANOV Academy of Sciences of the U.S.S.R.,
Moscow, U.S.S.R.
Abstract-In explaining the peculiarities of the hydrogen-evolution process and of some other cathodic processes occurring in the solutions containing alkali cations, it is necessary to take into consideration the incorporation of alkali metals into the electrode metal. The cathodic incorporation of alkali metals into lead, tin, zinc, cadmium, silver ahuninium, bismuth and other metals has been investigated. An explanation is suggested for the slowness of initial incorporation and the increase in the rate of incorporation when the electrode “develops”. The incorporation rate is determined by the number of vacancies in the crystal lattice of the electrode metal near its surface. The process of anodic removal of alkali metal from the electrode is discussed. R&sum&11 convient de tenir compte de l’incorporation de m&al alcalin au m&al de l’&ctrode pour expliquer les particularit& de processus d%volution de l’hydrogkne ainsi que de certains autres processus se manifestant dans les solutions contenant des cations alcalins. Recherches sur cette incorporation cathodique de m&aux alcalins aux plomb, &in, zinc, cadmium, argent, aluminium, bismuth et autres m&aux. Une explication est sugg&& pour la lenteur de l’incorporation i&ale et l’accroissement de sa vitesse au fur et & mesure que 1’6lcctrode se “d&eloppe”. La vitesse d’incorporation est dttermink par le nombre de vacances dans le rbseau cristallin de 1’6lectrode metallique au voisinage de sa surface. Discussion de processus d’elimination de m&al alcalin de Nlectrode. Zusammenfassung-Bei der ErklLung der Bcsonderheiten des Wasserstoff-Abscheidungsvorganges und einiger anderer kathodischer Vorgsnge, welche sich in Alkali-kationen enthaltenden LGsungen abspielen, ist es notwendig, such die Einlagerung von Alkalimetallen in das Elektrodenmetall in Betracht zu ziehen. Man untersuchte die kathodische Einlagerung von Alkalimetallen in Blei, Zinn, Zink, Kadmium, Silber, Aluminium, Wismuth und andere Metalle. Ftir die Langsamkeit des anfsbglichen Einbaus und die Geschwindigkeitszunahme bei sich “formierender” Elektrode wird eine Erkllrung vorgeschlagen. Die Gcschwindigkeit des Einbaus ist durch die Zahl der Leerstellen im Kristallgitter des Elektrodenmetalls nahe der OberfXche bcstimmt. Man diskutiert den Vorgang der anodischen Auflijsung des Alkali-metalls aus der Elektrode. to explain the cathodic dispersion of metals in alkalis, Haber,l Bredig2 and others,8 and more recently Angerstein, suggested a hypothesis that in aqueous solutions alkali metals form intermetallic compounds with solid cathode metals: but the majority of electrochemists have considered this hypothesis to be unlikely.5 There are now, however, many new facts that cannot be accounted for without recourse to the above hypothesis. Thus, the author and co-worker9 observed cathodic superactivation of aluminium and magnesium in alkalis and assumed it to be due to the cathodic formation of a surface compound of aluminium and magnesium with the alkali metal. The assumption was based on the dependence of superactivation of aluminium upon the cation nature (in the series from lithium to tetramethylammonium). Investigating the electroreduction of acetone in alkaline solutions, Antropov IN ORDER
* Presented at the 17th meeting of CITCE, Tokyo and Kyoto, received 24 March 1967. 19
September
1966; manuscript
B. N.
20
I(ABANOV
and others’ observed anomalies in the current efficiency in the case of lead and two other solid cathodes, the anomalies being of the same nature as those encountered with amalgam cathodes. These authors also explained the results obtained by the formation of compounds between the alkali metals and the metal of the cathode. Changes in the equilibrium potential of silver electrodes are observed in alkalichloride melts, and are attributed by some authors to the formation of half-valent silver and by others to the formation of intermetallic compounds with alkali metals. A spontaneous formation of a silver alloy with an alkali metal is also known to take place when silver is immersed in a melt of alkali-metal salts.8 Unaccountable phenomena have been observed time and again in hydrogenovervoltage measurements on various metals in alkalis, such as a slow change in the overvoltage with time, hysteresis, increased Tafel slopes, and poor reproducibility. Attempts have been made to explain the change in the overvoltage with time by the formation of surface hydrides,9 even in the case of metals which are known not to form stable hydrides. Zinc, cadmium and lead give no hydrides and adsorb hydrogen very poorly: in such cases there can scarcely be any question of a cathodic formation of hydrides in the quantities of the order of a monolayer. We have consequently studied hydrogen overvoltage on zinc, silver and other solid metals in alkaline solutions.lO The measurements were carried out in hydrogen atmosphere. The results of measurements on a silver electrode are shown in Fig. 1. In slow measurements, on reaching a certain value, the potential begins to rise with time (Fig. 1, a). In some tens of minutes the potential shifts in the negative direction by
-1.0
1
I -3
log
i.
-2
A
/cm2
FIO. 1. Polarization curves of hydrogen evolution on silver in 1 N NaOH.
0~2-0~3V. With a sufficiently reduced polarization of the electrode, the potential shift with time in the positive direction at constant current (b). The change in the hydrogen overvoltage with time was assumed to be due to the formation as the result of incorporation, a, or to the decomposition, b, of compounds of the cathode metal with the alkali metal atoms. As a rule, the hydrogen overvoltage on these compounds is by 0.243 V higher than on the metal without the alkali metal, which seems to be due to the lesser adsorption energy of hydrogen. If the hydrogen overvoltage in alkalis is determined by the presence of intermetallic compounds, it must be also dependent on the nature of the alkali metal. Indeed, the overvoltage proved to increase with the radius of the unhydrated alkali cation: for instance, when we pass from lithium to cesium, the overvoltage on a silver electrode increases by O-25 V. On the other hand, the formation of an intermetallic compound occurs the faster, the smaller is the crystallographic radius of the alkali ion, being the
Incorporation
of alkali metals into solid cathodes
21
fastest in the case of lithium. This conclusion is drawn on the basis of measurements of the time necessary for a constant value of hydrogen overvoltage to be established. The explanation of all these phenomena by the incorporation of alkali metals in the cathode metal however, could not be considered as conclusive without direct evidence in favour of the formation of such compounds and without the elucidation of the mechanism of their formation. It was natural to expect that incorporation should occur into such metals which form intermetallic compounds by alloying with alkali metals, eg Zn, Cd, Ag, Al, Pb, Sb, etc. It is not impossible, however, that some compounds which cannot exist at high temperature should prove stable at room temperature and could be obtained by incorporation. We shall not consider here the co-deposition of some metals with alkali metals, which may proceed faster than incorporation. For instance, no incorporation appears to be proved to take place in the case of iron electrode, for which no intermetallic compounds can be obtained by melting, whereas co-deposition, eg of iron with an alkali metal, has been observed to occur.ll To prove that the alkali metals are incorporated in the electrodes, Astakhov’O carried out some experiments in which the alkali metal was caused to pass through a membrane of zinc foil. On one side of the membrane was an alkaline solution and cathodic polarization was applied, on the other was a dilute potassium sulphate solution. The incorporation and passage of potassium through the membrane was established from the change in pH of the second solution. Cesium and tetramethylammonium ions practically do not diffuse through the membrane at all. These results are in agreement with the expected dependence of the incorporation on ionic radii. The dependence of the rate of incorporation of potassium into zinc upon the electrode potential is shown in Fig. 2. The polarization potential of the membrane is shown on the abscissa and the logarithm of the amount of potassium that has diffused through the membrane in 22 h is shown on the ordinate. The incorporation rate increases exponentially with electrode potential. The rate of bulk self-diffusion of zinc at room temperature is known to be about
d o 0
-6.0
-6.5
9-e
V
FIG. 2. Dependence of the logarithm of the amount of potassium that has diffused in 22 h through a 20 ,u zinc membrane upon the polarization potential a, of the membrane in 1 N KOH.
B. N.
22
KABANOV
a million times less than that at the intercrystalline boundaries. Probably, the rates of heterodiflusion in the bulk and at the boundary differ as much as those of self-diffusion. Thus, at room temperature the transfer of the alkali metal incorporated into the electrode metal must occur mainly along the boundaries of the crystal grains. To investigate the incorporation of alkali metal, potential/time curves were measured.13*14 The measurements were carried out in an inert gas or hydrogen atmosphere. The arrests on the cathodic curves correspond to the formation of intermetallic compounds (Fig. 3, b) and those on the anodic curves to their decomposition (Fig. 3, a). From the position of the arrests, the equilibrium potentials of the intermetallic -1.60
FIG. 3. (a) Anodic and (b) cathodic potential/time curves measured on cadmium in 1 N LiOH after polarization at Q = -1.65 V.
compounds were determined. The equilibrium potential of the compound obtained by alloying lead with sodium proved equal to that determined from the charging curve of lead in sodium hydroxide solution. I5 Sometimes the anodic curves show two or three arrests. This means that several intermetallic compounds with different compositions can be formed upon cathodic polarization of a given metal. Sometimes the potential arrests are not well defined and only in time the solid solution of alkali metals in the electrode metal passes into such a state where the arrests become more distinct. Previously for this and similar intermetallic compounds the equilibrium potential measurements were carried out in non-aqueous electrolyte solutions.ls All the attempts to explain the behaviour of the electrodes in alkalis by incorporation could not meet with success, unless it were possible to explain the main inconsistency-the high value of the overvoltage observed in the incorporation as opposed to a low one observed in the deposition of an alkali metal on mercury cathodes in aqueous alkaline solutions and melts. The slowness of the process of incorporation is evidenced by the extremely slow change in the hydrogen overvoltage (on pure metal in alkaline solutions) as well as by the fact that the anodic arrest corresponding to a monolayer of the alkali metal can be detected only after a prolonged cathodic polarization, whereas in the lead-sodium alloy obtained by melting, the same amount of an alkali metal is incorporated over lo6 times as fast. Figure 4 shows the polarization curves for the lead-sodium alloy. Lead previously subjected to a prolonged cathodic polarization behaves in a similar way. The rate of incorporation into a fresh lead electrode is lower by many orders of magnitude (approximately 7) than that for a previously cathodically polarized one (or for a lead-sodium alloy).15
Incorporation
of alkali metals into solid cathodes
23
In solid metals-lead, silver, zinc, e&-the atoms are closely packed, All intermetallic compounds with alkali metals are formed by substitution. Therefore, incorporation can occur only into vacancies. If the energy of interaction of the ions in the metal with its environment is assumed to be proportional to the ionic co-ordination number, it is evident that in point of energy the transfer of the discharging alkali ion to a vacancy on the electrode surface is at most advantage, since its co-ordination
-2
-3 log FIG. 4.
i,
A/cm2
Polarization curves of (a) anodic decomposition and (b) cathodic formation of an intermediate compound of lead with sodium obtained by alloying. Solution, 1 N NaOH.
number will be 9. The co-ordination number of the ion transferred to the edge of a screw dislocation12 is only 5 or 6. If the intermetallic compound formed under cathodic polarization was not confined to the surface monolayer, but had penetrated beyond it into the neighbouring metal layers, and was later partially decomposed on the surface by the passage anodic current, we found the electrode to have changed. We call such an electrode a “developed” electrode. Due to a large number of vacancies on it, new formation of the intermetallic compound occurs faster and the diffusion rate increases. The electrode can remain in such a developed state only for a limited period, until the excessive vacancies have diffused to sinks.* The rate of many electrochemical processes is known to be strongly dependent on adsorption on the electrode surface. The factors affecting only the electrode surface state, are of some importance in our case as well. But their effect is negligible compared with that due to the structural changes in the bulk of the electrode. It is evident from Fig. 5 that the presence of arsenic and mercury compounds in the solution affects the incorporation rate but little.15
-6
-6
-7 log C,
FIG. 5. Dependence * Eg, to grain-boundaries,
-5
M
of the length of the anodic arrest Q upon the concentration 1, AsBOs; 2, HgO in 1 N NaOH. voids or to a re-arranged
surface--Ed.
c of
24
B. N.
KABANOV
The ratio of the reaction rate of incorporation to that of the transfer of the incorporated atoms into the bulk, as well as the ratio of the rates of the back processes, determines the concentration of these atoms in the surface layer of the cathode metal. Kiseleva14 has studied the dependence of the length of the arrest on the anodic potential/time curves upon the cathodic polarization time for electrodes of lead, silver, gold, cadmium and other metals in 1 N LiOH and NaOH solutions. The length of the arrest corresponds to the amount of akali metal removed from the electrode under anodic polarization. * As is clear from Fig. 6, at first this amount increases with the time of the preliminary cathodic polarization, but later, rather unexpectedly, it drops nearly to the initial value.14
25
‘K I
FIO. 6. Dependence
75
50 min
of the length of the anodic arrest Q in 1 N NaOH upon the time of preliminary cathodic polarization. 1, Au; 2, Cd; 3, Pb; 4; In.
Evidently, the presence of the maximum shows that the shape of the curve is determined by different processes, with one of them prevailing first and the second later. The rise of the curve should be due to the accumulation of the alkali metal on the cathode surface in the process of incorporation: actually it is accompanied by an increase in the hydrogen overvoltage. Judging by the fact that the hydrogen overvoltage no longer changes, further polarization does not affect the concentration of the intermetallic compound on the electrode surface. The drop of the curve, corresponding to the decrease in the amount of the alkali metal removed, can be assumed to result from the decrease in the rate of diffusion from the cathode metal to the surface. Ditfusion in a solid body is a structure-sensitive process and may change upon recrystallization of the solid solution, or upon separation of intermetallic compounds from it. Therefore, it was necessary to study the dependence of the amount of the alkali metal removed anodically upon the “rest” time, ie upon the time the incorporated alkali-metal atoms reside in the bulk of the cathode after switching off or decrease of the cathodic current. In fact, the resulting curves proved to be similar to those obtained with different cathodic polarization times (Fig. 7). The shape of the curves was found to be dependent on microstructure, in the case of l The measurements were carried out both on the same electrode after “development’* and on fresh electrodes made of identically treated metal samples.
Incorporation of alkali metals into solid cathodes
I,
25
min
FIG. 7. Dependence of the length of anodic arrest Q on a gold electrode in 1 N LiOH upon the time of “rest” 1, rest at q = -0.9 V; 2, rest at ‘p = -0.2 V.
cadmium. In the case of the fine-gained structure, the change occurs faster and sharp maxima appear on the curves. If the size of the grains is ten times as large, the maxima are diffuse. This is in agreement with the concept that the grain boundaries of the electrode metal play a large part in the incorporation and diffusion of the alkali metal. REFERENCES 1. G. BREDIGand F. HABER,Ber. Dts. Chem. Ges. 31, 2741 (1898). 2. G. BREJXG,2. uqew. Chem. 16,951 (1898). 3. F. HABERand M. SACK,Z. Elektrockem. 8,245 (1902); M. SACK, Z. anorg. Chem. 34,286 (1903). 4. H. ANGERSTEIN,Bull. Acad. Pal. Sci. 3,443 (1957). 5. H. SALZBERG, J. electrochem. Sot. 100,146 (1953); G. DUBPJXNELLand H. SALZBJZRG, J. electrothem. Sot. 100, 588 (1953); L. W. GASTWIRTand H. SALZBJZRG, J. electrochem. Sot. 104, 710 (1957). 6. B. KMANOV and A. ZACK, Dokl. Akad. Nauk SSSR 72,531
(1950); E. BARELKOand B. KAJP Doki. Akad. Nauk SSSR 90,1039 (1953). 7. M. SMIRNOVA,V. SMIRNOVand L. ANTRO~V, Trudy novocherk. politekn. Inst. 79, 43 (1959). 8. T. -DY, J. electroanal. C/tern. 11, 77 (1966). 9. V. PASTand 2. JOFA,Zh.fiz. Khim. 23,913, 1230 (1959); Dokl. Akad. Nauk SSSR 106, 1050 ANOV,
(1956). 10. B. KABANOV, D. LEIKIS,I. KISELEVA,I. ASTAKHOV and D. ALEXANDROVA, Dokl. Akud. Nuuk SSSR 144, 1085 (1962); D. ALEXANDROVA, I. KISELEVA and B. KABANOV,Dokl. Akad. Nauk SSSR 38,1493 (1964); A. ZACKand B. KABANOV, Elektrokhimiya 1,68 (1965).
11. K.
GORBUNOVA and
L. LJ~A.
Electrochim. Acta 11,457 (1966). Moscow (1965); I. KISJZLEVA. N. TOMASHOVA and B. KABANOV. ’ Zh. ’fiz Khim. 38.1188 (1964). . , I. KISELEVA; Elektrokhimiya 3,275 (1967). B. KABANOV, I. KISELEVA, I. ASTAKHOVand N. TOMASHOVA, Elektrokhimiyu 1, 1023 (1965). R. KRJWANN et al., Z. Metullk. 12, 185,257, 273,414, 444 (1920); 13,19, 66 (1921).
12. I. &TAKHOV, private communickion,
13. 14. 15. 16.