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Formation of crystalline intermetallic compounds and solid solutions in electrochemical incorporation of metals into cathodes
Ekrmckimico &I Per#amon
Acre. Vol. 24, pp, 167-171 Press Ltd. 1979. Primd in Grest
au9-4686/79@!0l~167
$oz.mlo
Britain.
FORMATION OF CRYSTALLINE INTERMETALLIC COMPOUNDS AND SOLID SOLUTIONS IN ELECTROCHEMICAL INCORPORATION OF METALS INTO CATHODES* B.N.
KABANOV,~.
Institute of Electrochemistry
1. ASTAKHOV
and I. G. KEELEVA
of the Academy of Sciences of the U.S.S.R.,
Moscow, U.S.S.R.
(Received 24 April 1978) Abstract -The elementaryact of cathodic incorporation of metal into metal is an electrochemical transition of a metal ion from solution(or melt) into the cathode with formation of a solid chemical compound between the Iwo metals. Unlike chemisorption the elementary act of incorporation occurs not only at interface but also inside the solid metal phase and depends strongly on its crystalline structure. Electron transfer in incorporation is always complete (not partial). It follows from nonequilibrium thermodynamics that when the incorporation rate is limited by crystallizationof an intermetalliclayer,it is linearly related to potential within the homogeneity region of intermetallic compounds. If the rate is limited by incorporated metal diffusion in the intermetallic compound, the square of the rate is linearly related to potential. Incorporated metal can react chemically with a solution component, reducing it. If the rate of this reaction is less than that of incorporation and not limited by oxidant diffusion in solution, in the casesinvestigated (solid intermetallic compounds of alkali metals) the chemical reaction rate is linearly related to potential. This follows from the activated complex theory. The equilibrium potentials of intermetallic compounds formed in incorporation are more positive than those of single metals being incorporated (sometimes by 1.5 V). The electrochemical incorporation of metals has numerous applications.
phase. Therefore incorporation depends strongly on the structure of the electrode metal. The subsequent stages of the overall incorporation process also occur inside the electrode, in particular, crystallization of the intermetallic compound, Finally, incorporation differs from cathodic formation of liquid amalgams and other liquid alloys in that the elementary act terminates in formation of a solid intermetallic system. Of course, under specific conditions intermediate cases are possible between any types of processes. Thus in the classification of electrochemical processes, the incorporation of metals into solid metals fills a special place among the electrocrystallization processes (including codeposition of metals), the chemisorption processes and the processes of cathodic deposition of liquid metals. The possibility of cathodic formation of surface alloys on solid electrodes was first pointed out by Gaber at the turn of the century when he attempted to explain the cathodic spraying of tin and lead in aqueous solutions of alkalis[3]. He did not study experimentally the phenomenon of incorporation of one metal into another. No suggestions were made at that time regarding the mechanism of cathodic formation of an alloy and, for a long time, the fact of the participation of alkali cations in the spraying reaction was considered questionable in the literature. In electrochemistry of melts the cathodic alloy formation at the “negative” overpotential was described, eg in[4]. however the regularities and mechanism of this phenomenon were not studied. This process called by us “incorporation” and the reverse one called “anodic extraction” have been studied at our laboratory both experimentally and
INTRODUCTION
a new line of research has made its appearance in electrodeposition of metals. The authors called the process involved in this research “cathodic incorporation of metals”[l]. An elementary act of the process of incorporation of a metal into metal can be defined basically as an electrochemical transition of a metal ion from solution (or melt) into a cathode with simultaneous formation of a solid chemical compound of the metal being incorporated with the cathode metal. An example of the incorporation reaction isE_Z]:
In recent
years
Na+
+ 3Pb + e- ---*NaPb,
Incorporation is distinguished from such electrochemical processes as electrocrystallization of a metal or its electrodeposition on a liquid cathode from the same metal in that in this case the metal being incorporated combines chemically with the cathode metal. At the expense of the energy of this reaction, the equilibrium potential of the intermetallic compound being formed becomes much more positive relative to the potential of the equilibrium deposition of the pure phase of the metal. Eg the equilibrium potential of the system Na*/NaPb, is 1400mV more positive than that of the system Na+/Na (met). Incorporation differs essentially from chemisorption of metals on a foreign electrode in that the elementary act occurs not only at the interface, but also inside the solid metal ’ Presented at the 2Bth Meeting of the International Society of Electrochemistry, Bulgaria, Varna. September 20 (1977). 167
B. N. KABANOV,I. I.
168
A.WAKHOV AND I. ct. KISELJWA
theoretically and more than 40 papers have been published since 1961 on the subject. This paper reports only some brief conclusions based on these studies. The incorporation process has proved to be universal, It has been experimentally found that alkali metals may be incorporated into electrodes of more than 30 different metals the incorporation rate being frequently very large. Thus even at the potentials, eg by 1.5 V more positive than the equilibrium potential of an alkali metal at room temperature, tens and hundreds of monolayers (on conversion to a monomonoatomic intermetallic compound) become incorporated into the electrode in several minutes. Briefly, the qualitative characteristics of the incorporation process are as follows. At more positive potentials than the equilibrium potential of the intermetaltic compound, incorporation may involve formation of a solid solution of metals*. The rate of incorporation into a pure foreign electrode depends on the structural state of the electrode, oiz on the surface concentration of vacancies and other defects. Thus incorporation of alkali metals into a flowed tin electrode does not start until a long period of time has passed[S]. This fact is taken into consideration in measuring the hydrogen overvoltage on tin. The formation of an intermetallic compound in the case of a pure foreign electrode will start only if the overvoltage is high enough, similar, as believed by Melendres[6] to that at which electrocrystallization of pure metal starts. In the case of anodic galvanostatic extraction of an electropositive (eg alkali) metal from an alloy, the electrode potential remains nearly constant for a long time if there is an intermediate compound present on the surface, and shifts continuously in the direction of positive values if there is a solid solution of metals on it. If an anodic extraction process from solid solution is limited by diffusion at constant current as much as 50”/, of incorporated metal can be extracted[7]. In a quantitative description of the kinetics of electrochemical incorporation we proceed From the principles of linear thermodynamics .of irreversible processes and assume the flux to be linearly related to the motive force value. In (I) J is the flux of atoms undergoing incorporation, X is themotive force and L is a coefEcient[8]. J=LX
(1)
Depending on which stage limits the overall process rate, (I) ieads to different relations, which were verified experimentally. When the process is limited by an electrochemical act, the incorporation rate depends exponentially on potential. This is exemplified by the incorporation of sodium into an electrode from a sodium-lead alloy, which has been studied on great detail[9]. When the incorporation is limited by the formation of a crystalline intermetallic compound or the solid solution of both metals on the surface the activity of the metal being incorporated is determined by the equilibrium potential of the electrode and when the diffusion process is fast enough, the chemical afinity is
0
03
w
43
i, mA/cm
Fig. 1. Incorporation under chemical control. i-rate of lithium incorporation at 34°C: O-into sotid gallium, a-into liquid gallium. q-potential of gallium vs Li+/Li electrode. 0.6 N solution LiCIO, in propylene carbonate. the motive force of the flux. In the case in question the rate should be linearly related to the electrode potential[lO]. In practice this holds for instance when lithium incorporated into gallium from a lithium perchlorate solution in propylene carbonate[lO]. Figure 1 illustrates this case. When the process rate is limited by the diffusion of the incorporated eIement in the growing layer of the intermetallic compound and there exists an equilibrium at the layer boundaries, the gradient of the chemical potential is the motive force of the diffusion flux : J=L,f.!!
dx This form of the dependence of the flux on the motive force has an advantage over Fick’s law in that it does not contain in an explicit form the diffusion coefficient, which in the case of compounds characterized by strong interaction between components is not a constant even if the change in the concentration of a component is small. At constant electrode potential the current undergoes decay with time, characteristic of nonsteady-state diffusion i = Kt- l/Z (2) Here K is the rate constant depending on potential. It is shown in[&, 1l] that the incorporation rate should be related to the electrode potential by a square-law dependence. In accordance with (2) this can be written as follows : KZ - cp
(3)
However, in the general case the factor of proportionality L in the diffusion equation depends on the diffusing substance concentration, and in the particular case of diffusion in an intermetallic compound, this dependence is the stronger, the greater is the relative change of the concentration. The linearity of the diffusion (1) and the accuracy of (2) and (3) hold the better the narrower the homogeneity region of the intermetallic compound. For thecase when an intermetallic compound with a narrow homogeneity region is formed the validity of (2) and (3) was confirmed by experiments on the incorporation of lead into platinum from a melt to form the compound PtPb[l 11, Figure 2 presents the dependence of the rate constant squared on potential. * Of course, at a more negative potential than the equilibThe quadratic equation (3) holds also for inrium potential for the solid solution of given concentration.
Formation
K2-104 Afs.cm
30
-2
1
20 -
10 -
Fig. 2. Incorporation under diffusion control. K-rate constant of lead incorporation into platinum at 500°C from PbCl,-KCI-NaCI melt electrolite[ll]. q-potential of platinum vs Pb2+/Pb electrode.
termetallic compounds with a relatively wide homogeneity region, as in the case of the incorporation of aluminium into nickel from melts at 600°C to form the compounds Ni,Al, NiAl and Ni,AI, with the homogeneity region from 3 to 5 atom %. In Fig. 3 the electrode potential of relatively pure aluminium is plotted along the abscissa, on which the potentials of formation of some Ni-Al compounds are indicated, and the square of the incorporation rate constant is
plotted along the ordinate. In this case as well, the value of KZ in the region of formation of any one of the compounds depends linearly on potential. Recently studies were made also on the incorporation of alkali metals into tin, lead and other metals from nonaqueous solutions at room temperature. In these cases equations (2) and (3) also proved to he valid. The solid intermetallic compound formed by incorporation on the electrode surface can .interact chemically with the solution components, eg with water[lZ]. The rate of this interaction was determined from the decrease in the amount of the intermetallic compound on the electrode surface in a definite period of time in the absence of current with allowance for the loss through diffusion into the bulk of the metal[ 133. A study was made of the interaction of lithium and its intermetallic compounds lithiumnickel and lithium-lead with water and ethyl alcohol in propylene carbonate (Fig. 4). The kinetic parameters of these reactions (rate constant and reaction order) are listed in the table. They were determined from the dependence of the chemical interaction rate on the concentration of the solution components undergoing reduction. As can be seen from the Table, the equilibrium potentials of the system Li/Li+ and Li,Ni/Li * differ by a whole volt, whereas the interaction rates of Li and Li,Ni with alcohol differ only by several times and are approximately linearly related to the potential. It follows from the activated complex theory that the rate of a heterogeneous reaction on a filled surface can be proportional not to the activity but to the con-
-2
Fig. 3. Incorporation under diffusion control. K-rate constante of aluminium incorporation into nickel at 550°C from KCI-AICI, melt electrolite. rp-potential ofnickel vs AI”+/AI electrode.
-1
Electrode potential,V, referred to
Reaction + Ni
Li + &H,OH Li,Pb + H,O
= C,H,OLi
+ 1/2H,
= C,H,OLi + 1/2H, = LiOH + 1/2Hz + Pb
0
egc,
M/ii
Fig. 4. Chemical interaction of electrodeposited lithium (1) and lithium-nickel alloy (2) with ethyl alcohol and of lithium-lead alloy with water (3) in propylene carbonate-based electrolyte. i--chemical interaction rate in electrical units, CPreacting substance concentration in solution.
Table 1. Kinetic parameters ofthe chemical reactions occurring during incorporation LiCIO, in propylene carbonate
Li,Ni + C,H,OH
169
of crystalline intermetalliccompounds
Reaction order
of Li into Pb from 2 M
Rate constant
(W&St)
tl
mA .cm mol-’
-2.0
1.0 1.2 1.1
1 30 80
-3.0 - 2.4
B. N. KABANOV, I. 1. ASTAKHOV AND I. G. KISELEW
170
centration IQ) of the reacting component. In this case the activity changes by many orders of magnitude, while the concentration 8 changes only a few times. This shows that the alcohol reduction process follows a chemical not electrochemical mechanism. The well-known phenomenon of cathodic spraying of metals, which has long been studies by a number of of alkali authors, is based on the chemical interaction and alkali-earth metals as the components of intermetallic compounds with water. The cathodic dissolution of aluminium in alkalis, proportional to the cathodic current density was explained by the chemical interaction with water of the intermetallic compound formed by aluminium and alkali metal [14]. The cathodic hydrogen evolution from alkali solutions through the chemical interaction with water of the alkali metal being electrodeposited is called the This secondary mechanism of hydrogen evolution. problem has been already discussed by LeBlanc. For the case of an alkali metal amalgam A. N. Frutnkin, V. N. Korshunov et al[15] proved this mechanism to be present on Hg at high current densities. According to the data of Matsuda, the cathodic hydrogen evolution in alkaline solutions is the result of the interaction of water with adsorbed sodium not only in the case of mercury cathode but also on some solid metals such as platinum or nickel[16]. There is another effect of incorporation which is of importance for electrochemists. This is a change in the properties of the electrode surface. For instance, the electrochemical hydrogen overvoltage increases strongly when a layer of an intermetallic compound is formed on the electrode surface[I7]. Thus, the electrochemical hydrogen overvoltage at the intermetallic K, Al compound is by 0.6V higher than at pure aluminium[18]. The potential of zero charge of a lead - 1% sodium alloy has a 0.2 V more negative value than-that of lead (Fig. 5)[19]. There are wide nossibilities for the practical use of the electrochemical incorporation of elements. One of them is to utilize the chemical interaction with the solution components of the intermetallic compounds formed during incorporation. Incorporation can find application in organic electrosynthesis[20] and in the preparation of catalysts. Thus Yu. D. Kudryavtsev
developed a method of preparation of a highly active platinum catalyst by spraying platinum by cathodic polarization in solutions containing alkali or alkaliearth metals[21]. Another practical application of the incorporation is based on the fact that intermetallic compounds possess very valuable properties which are absent in their components. By forming intermetallic compounds it is possible to increase the hardness, strength, heat-and corrosion resistance of a material, to endow its surface with magnetic, catalytic, emission, semiconducting and superconducting properties. It is often required that only a thin surface layer of the metal shoutd have some of these properties. With the use of incorporation it is possible to obtain layers of intermetallic compounds on the electrode surface while keeping its bulk properties unchanged. A method of increasing the heat- and corrosion resistance of a vanadium surface by cathodic incorporation into it of aluminium from a melt at 1000°C in England in 1970[22]. A similar was patented intermetallic compound was obtained on vanadium surface at room temperature by incorporating into it aluminium from a nonaqueous salution[23]. By incorporating manganese into bismuth from an aqueous solution, an intermetallic compound MnBi was obtained on the bismuth electrode surface which showed pronounced magnetic properties[24]. Some other cases are reported of preparing by incorporation materials of practical value. Thus the General Electric Company uses a method called “Metalliding”[25], which is based on the application of cathodic incorporation. Electrochemical incorporation can be used to prepare compounds difficult to obtain by other methods. For example, LiGa can be obtained by incorporation[lO]. By alloying the components it can be obtained only at 7OO”C[26]. By incorporation it is possible to effect the interaction of elements which otherwise do not interact, eg, the interaction of alkali- and transition metals[27]. Finally, as was recently shown[6], cathodic incorporation plays an important role in the application of a promising electrode from the intermetallic compound LiAl in battery cells with molten electrolyte.
REFERENCES
D. Leikis, I. Kiseleva, I. Astakhov and D. Alexandrova, DAN SSSR t44, 1085 (1962). 2. 3. Kabanov, I. Kiseleva and I. Astakhov, Elekrrokhimiya 8.955 (1972). B. Kabanov, I. Kiseleva, I. Asrakhov and N. 1. B.
L
-q5
-qa
Tomashova, Sashchita merallou 11, 131 (1975). 3. 0. Bredig and F. Haber, Ber. Deutsch. Chem. Ges. 31, 2741(1898). F. Haberand M. Sack, 2. ebktrochem. 8,245 (1902). 4. Yu. Delimarskiy and I. Panchenko. DAN SSSR 91, 115 (t953) ; Yu. Delimarskiy and V. Kuz’movich, J. Neorgnnicheskoy Chem. 4, 2732 (1959). 5. M. Haga. Yu. Kukk and V. Past, Double Layer and Adsorbtion on Solid Electrodes. Tartu Gas. University,
t&v
Fig. 5. Dependence of the differential capacity on potential (us nhe) in O.OlN Na2SG4: a-Pb; Y -Pb + 1%
(atom) Na.
Kabaaov,
Tartu (l%S]. 6. C. Melendres, J. electrochem. Sot. 124, 650 (1977). 7. 1. Astakhov. Yu. Filinovsky and G. Teplitskaya. Etekrrokhimiyo 13, 566 (1977). 8. I. Astakhov, Elekrrolrhimiyo 9, 521 (1973).
Formation
af crystalline intermetallic
9. B. Kabanov. I. Kiseleva, I. Astakhov and N. Tomaachova, Elekrrokhimiyo 1. 1023 (1%5). 10. 3. Kabanov, I. Kiseleva. I. Astakhov, N. Tomashova and 10, 765 (1974). P. Petukhova, Elektrokhimiyn Il. N. Chovnyk and A. Fomichev, Elektrokhimiya 7, 130 (1971); 10, 284 (1974). 12. F-4.Tomaschova, I. Kiseleva and B. Kabanov, Elektrokhimiya 7, 438 (1971); 8, 112 (1972). 13. 1. Avrutskaya, 1. Kiseleva, A. Chekavtsev, V. Shinkarev, B. Kabanov and M. Fioshin, Elektrokhimiya 13, 1516 (1977). 14. B. Kabanov and A. Zak, DAN SSSR 72, 531 (1950). 15. V. Korshunov, A. Frumkin and T. Ivanova, Elektrokhimiya 4, 1120 (1968). 16. A. Matsuda, R. Notoya, T. Ohmori,K. Kunimatsuand T. Kushimoto, Efektrokhimiya 13,726 (1977); A. Matsuda, R. Notoya, T. Ohmori, K. Kunimatsu and T. Kushimoto, J. Res. Catalysis 24, 187 (1976). 17. D. Alexandrova, I. Kiseleva and B. Kabanov, J. Phis.
compounds
171
Chem. (Russian) 38, 1493 (1964). 18. A. Zak and B. Kabanov, Elektrekhimiya 1,68 (1965). 19. I. Kiseleva, D. Leikis and B. Kabanov, Elektrokhimiya 8, 250 (1972). 20. I. Kiseleva, I. Avrutskaya, N. Tomashova, B. Kabanov and M. Fioshin. Double Layer and Adsorption on Solid Electrodes. Tartus Gos. University Tartu (1975). 21. Yu. Kudr’avtsev, D. Semchenko, K. Bin, Trudy now ocherk. politekh. Inst. 141 (sektsi’a tekhnologii elektrokhim. proizvodstv i analit. himii), p. 49 (1964). 22. British Patent N119335, kl C 7 B (1970). 23. A. Chekavtsev, N. Matveuva, 1. Kiseleva and B. Kabanov. Elektrokhimiva 13. 1153 (1977). 24. V. Gaiushko, E. Vd&iko.and V.‘Tro&enko, Ukr. khim. Zh. 33, (6) 574 11967). 25. J. Smiih: Scient.’Am. ‘9, 105 (1967). 26. E. Zint) and G. Brauer, Z. phis. Chemie BZO, 245 (1933). 27. B. Kabanov, I. Kiseleva, N. Tomashova and I. Schvartsman, Efektrokhimiya 9, 381 (1973).
Acre. Vol. 24, pp, 167-171 Press Ltd. 1979. Primd in Grest
au9-4686/79@!0l~167
$oz.mlo
Britain.
FORMATION OF CRYSTALLINE INTERMETALLIC COMPOUNDS AND SOLID SOLUTIONS IN ELECTROCHEMICAL INCORPORATION OF METALS INTO CATHODES* B.N.
KABANOV,~.
Institute of Electrochemistry
1. ASTAKHOV
and I. G. KEELEVA
of the Academy of Sciences of the U.S.S.R.,
Moscow, U.S.S.R.
(Received 24 April 1978) Abstract -The elementaryact of cathodic incorporation of metal into metal is an electrochemical transition of a metal ion from solution(or melt) into the cathode with formation of a solid chemical compound between the Iwo metals. Unlike chemisorption the elementary act of incorporation occurs not only at interface but also inside the solid metal phase and depends strongly on its crystalline structure. Electron transfer in incorporation is always complete (not partial). It follows from nonequilibrium thermodynamics that when the incorporation rate is limited by crystallizationof an intermetalliclayer,it is linearly related to potential within the homogeneity region of intermetallic compounds. If the rate is limited by incorporated metal diffusion in the intermetallic compound, the square of the rate is linearly related to potential. Incorporated metal can react chemically with a solution component, reducing it. If the rate of this reaction is less than that of incorporation and not limited by oxidant diffusion in solution, in the casesinvestigated (solid intermetallic compounds of alkali metals) the chemical reaction rate is linearly related to potential. This follows from the activated complex theory. The equilibrium potentials of intermetallic compounds formed in incorporation are more positive than those of single metals being incorporated (sometimes by 1.5 V). The electrochemical incorporation of metals has numerous applications.
phase. Therefore incorporation depends strongly on the structure of the electrode metal. The subsequent stages of the overall incorporation process also occur inside the electrode, in particular, crystallization of the intermetallic compound, Finally, incorporation differs from cathodic formation of liquid amalgams and other liquid alloys in that the elementary act terminates in formation of a solid intermetallic system. Of course, under specific conditions intermediate cases are possible between any types of processes. Thus in the classification of electrochemical processes, the incorporation of metals into solid metals fills a special place among the electrocrystallization processes (including codeposition of metals), the chemisorption processes and the processes of cathodic deposition of liquid metals. The possibility of cathodic formation of surface alloys on solid electrodes was first pointed out by Gaber at the turn of the century when he attempted to explain the cathodic spraying of tin and lead in aqueous solutions of alkalis[3]. He did not study experimentally the phenomenon of incorporation of one metal into another. No suggestions were made at that time regarding the mechanism of cathodic formation of an alloy and, for a long time, the fact of the participation of alkali cations in the spraying reaction was considered questionable in the literature. In electrochemistry of melts the cathodic alloy formation at the “negative” overpotential was described, eg in[4]. however the regularities and mechanism of this phenomenon were not studied. This process called by us “incorporation” and the reverse one called “anodic extraction” have been studied at our laboratory both experimentally and
INTRODUCTION
a new line of research has made its appearance in electrodeposition of metals. The authors called the process involved in this research “cathodic incorporation of metals”[l]. An elementary act of the process of incorporation of a metal into metal can be defined basically as an electrochemical transition of a metal ion from solution (or melt) into a cathode with simultaneous formation of a solid chemical compound of the metal being incorporated with the cathode metal. An example of the incorporation reaction isE_Z]:
In recent
years
Na+
+ 3Pb + e- ---*NaPb,
Incorporation is distinguished from such electrochemical processes as electrocrystallization of a metal or its electrodeposition on a liquid cathode from the same metal in that in this case the metal being incorporated combines chemically with the cathode metal. At the expense of the energy of this reaction, the equilibrium potential of the intermetallic compound being formed becomes much more positive relative to the potential of the equilibrium deposition of the pure phase of the metal. Eg the equilibrium potential of the system Na*/NaPb, is 1400mV more positive than that of the system Na+/Na (met). Incorporation differs essentially from chemisorption of metals on a foreign electrode in that the elementary act occurs not only at the interface, but also inside the solid metal ’ Presented at the 2Bth Meeting of the International Society of Electrochemistry, Bulgaria, Varna. September 20 (1977). 167
B. N. KABANOV,I. I.
168
A.WAKHOV AND I. ct. KISELJWA
theoretically and more than 40 papers have been published since 1961 on the subject. This paper reports only some brief conclusions based on these studies. The incorporation process has proved to be universal, It has been experimentally found that alkali metals may be incorporated into electrodes of more than 30 different metals the incorporation rate being frequently very large. Thus even at the potentials, eg by 1.5 V more positive than the equilibrium potential of an alkali metal at room temperature, tens and hundreds of monolayers (on conversion to a monomonoatomic intermetallic compound) become incorporated into the electrode in several minutes. Briefly, the qualitative characteristics of the incorporation process are as follows. At more positive potentials than the equilibrium potential of the intermetaltic compound, incorporation may involve formation of a solid solution of metals*. The rate of incorporation into a pure foreign electrode depends on the structural state of the electrode, oiz on the surface concentration of vacancies and other defects. Thus incorporation of alkali metals into a flowed tin electrode does not start until a long period of time has passed[S]. This fact is taken into consideration in measuring the hydrogen overvoltage on tin. The formation of an intermetallic compound in the case of a pure foreign electrode will start only if the overvoltage is high enough, similar, as believed by Melendres[6] to that at which electrocrystallization of pure metal starts. In the case of anodic galvanostatic extraction of an electropositive (eg alkali) metal from an alloy, the electrode potential remains nearly constant for a long time if there is an intermediate compound present on the surface, and shifts continuously in the direction of positive values if there is a solid solution of metals on it. If an anodic extraction process from solid solution is limited by diffusion at constant current as much as 50”/, of incorporated metal can be extracted[7]. In a quantitative description of the kinetics of electrochemical incorporation we proceed From the principles of linear thermodynamics .of irreversible processes and assume the flux to be linearly related to the motive force value. In (I) J is the flux of atoms undergoing incorporation, X is themotive force and L is a coefEcient[8]. J=LX
(1)
Depending on which stage limits the overall process rate, (I) ieads to different relations, which were verified experimentally. When the process is limited by an electrochemical act, the incorporation rate depends exponentially on potential. This is exemplified by the incorporation of sodium into an electrode from a sodium-lead alloy, which has been studied on great detail[9]. When the incorporation is limited by the formation of a crystalline intermetallic compound or the solid solution of both metals on the surface the activity of the metal being incorporated is determined by the equilibrium potential of the electrode and when the diffusion process is fast enough, the chemical afinity is
0
03
w
43
i, mA/cm
Fig. 1. Incorporation under chemical control. i-rate of lithium incorporation at 34°C: O-into sotid gallium, a-into liquid gallium. q-potential of gallium vs Li+/Li electrode. 0.6 N solution LiCIO, in propylene carbonate. the motive force of the flux. In the case in question the rate should be linearly related to the electrode potential[lO]. In practice this holds for instance when lithium incorporated into gallium from a lithium perchlorate solution in propylene carbonate[lO]. Figure 1 illustrates this case. When the process rate is limited by the diffusion of the incorporated eIement in the growing layer of the intermetallic compound and there exists an equilibrium at the layer boundaries, the gradient of the chemical potential is the motive force of the diffusion flux : J=L,f.!!
dx This form of the dependence of the flux on the motive force has an advantage over Fick’s law in that it does not contain in an explicit form the diffusion coefficient, which in the case of compounds characterized by strong interaction between components is not a constant even if the change in the concentration of a component is small. At constant electrode potential the current undergoes decay with time, characteristic of nonsteady-state diffusion i = Kt- l/Z (2) Here K is the rate constant depending on potential. It is shown in[&, 1l] that the incorporation rate should be related to the electrode potential by a square-law dependence. In accordance with (2) this can be written as follows : KZ - cp
(3)
However, in the general case the factor of proportionality L in the diffusion equation depends on the diffusing substance concentration, and in the particular case of diffusion in an intermetallic compound, this dependence is the stronger, the greater is the relative change of the concentration. The linearity of the diffusion (1) and the accuracy of (2) and (3) hold the better the narrower the homogeneity region of the intermetallic compound. For thecase when an intermetallic compound with a narrow homogeneity region is formed the validity of (2) and (3) was confirmed by experiments on the incorporation of lead into platinum from a melt to form the compound PtPb[l 11, Figure 2 presents the dependence of the rate constant squared on potential. * Of course, at a more negative potential than the equilibThe quadratic equation (3) holds also for inrium potential for the solid solution of given concentration.
Formation
K2-104 Afs.cm
30
-2
1
20 -
10 -
Fig. 2. Incorporation under diffusion control. K-rate constant of lead incorporation into platinum at 500°C from PbCl,-KCI-NaCI melt electrolite[ll]. q-potential of platinum vs Pb2+/Pb electrode.
termetallic compounds with a relatively wide homogeneity region, as in the case of the incorporation of aluminium into nickel from melts at 600°C to form the compounds Ni,Al, NiAl and Ni,AI, with the homogeneity region from 3 to 5 atom %. In Fig. 3 the electrode potential of relatively pure aluminium is plotted along the abscissa, on which the potentials of formation of some Ni-Al compounds are indicated, and the square of the incorporation rate constant is
plotted along the ordinate. In this case as well, the value of KZ in the region of formation of any one of the compounds depends linearly on potential. Recently studies were made also on the incorporation of alkali metals into tin, lead and other metals from nonaqueous solutions at room temperature. In these cases equations (2) and (3) also proved to he valid. The solid intermetallic compound formed by incorporation on the electrode surface can .interact chemically with the solution components, eg with water[lZ]. The rate of this interaction was determined from the decrease in the amount of the intermetallic compound on the electrode surface in a definite period of time in the absence of current with allowance for the loss through diffusion into the bulk of the metal[ 133. A study was made of the interaction of lithium and its intermetallic compounds lithiumnickel and lithium-lead with water and ethyl alcohol in propylene carbonate (Fig. 4). The kinetic parameters of these reactions (rate constant and reaction order) are listed in the table. They were determined from the dependence of the chemical interaction rate on the concentration of the solution components undergoing reduction. As can be seen from the Table, the equilibrium potentials of the system Li/Li+ and Li,Ni/Li * differ by a whole volt, whereas the interaction rates of Li and Li,Ni with alcohol differ only by several times and are approximately linearly related to the potential. It follows from the activated complex theory that the rate of a heterogeneous reaction on a filled surface can be proportional not to the activity but to the con-
-2
Fig. 3. Incorporation under diffusion control. K-rate constante of aluminium incorporation into nickel at 550°C from KCI-AICI, melt electrolite. rp-potential ofnickel vs AI”+/AI electrode.
-1
Electrode potential,V, referred to
Reaction + Ni
Li + &H,OH Li,Pb + H,O
= C,H,OLi
+ 1/2H,
= C,H,OLi + 1/2H, = LiOH + 1/2Hz + Pb
0
egc,
M/ii
Fig. 4. Chemical interaction of electrodeposited lithium (1) and lithium-nickel alloy (2) with ethyl alcohol and of lithium-lead alloy with water (3) in propylene carbonate-based electrolyte. i--chemical interaction rate in electrical units, CPreacting substance concentration in solution.
Table 1. Kinetic parameters ofthe chemical reactions occurring during incorporation LiCIO, in propylene carbonate
Li,Ni + C,H,OH
169
of crystalline intermetalliccompounds
Reaction order
of Li into Pb from 2 M
Rate constant
(W&St)
tl
mA .cm mol-’
-2.0
1.0 1.2 1.1
1 30 80
-3.0 - 2.4
B. N. KABANOV, I. 1. ASTAKHOV AND I. G. KISELEW
170
centration IQ) of the reacting component. In this case the activity changes by many orders of magnitude, while the concentration 8 changes only a few times. This shows that the alcohol reduction process follows a chemical not electrochemical mechanism. The well-known phenomenon of cathodic spraying of metals, which has long been studies by a number of of alkali authors, is based on the chemical interaction and alkali-earth metals as the components of intermetallic compounds with water. The cathodic dissolution of aluminium in alkalis, proportional to the cathodic current density was explained by the chemical interaction with water of the intermetallic compound formed by aluminium and alkali metal [14]. The cathodic hydrogen evolution from alkali solutions through the chemical interaction with water of the alkali metal being electrodeposited is called the This secondary mechanism of hydrogen evolution. problem has been already discussed by LeBlanc. For the case of an alkali metal amalgam A. N. Frutnkin, V. N. Korshunov et al[15] proved this mechanism to be present on Hg at high current densities. According to the data of Matsuda, the cathodic hydrogen evolution in alkaline solutions is the result of the interaction of water with adsorbed sodium not only in the case of mercury cathode but also on some solid metals such as platinum or nickel[16]. There is another effect of incorporation which is of importance for electrochemists. This is a change in the properties of the electrode surface. For instance, the electrochemical hydrogen overvoltage increases strongly when a layer of an intermetallic compound is formed on the electrode surface[I7]. Thus, the electrochemical hydrogen overvoltage at the intermetallic K, Al compound is by 0.6V higher than at pure aluminium[18]. The potential of zero charge of a lead - 1% sodium alloy has a 0.2 V more negative value than-that of lead (Fig. 5)[19]. There are wide nossibilities for the practical use of the electrochemical incorporation of elements. One of them is to utilize the chemical interaction with the solution components of the intermetallic compounds formed during incorporation. Incorporation can find application in organic electrosynthesis[20] and in the preparation of catalysts. Thus Yu. D. Kudryavtsev
developed a method of preparation of a highly active platinum catalyst by spraying platinum by cathodic polarization in solutions containing alkali or alkaliearth metals[21]. Another practical application of the incorporation is based on the fact that intermetallic compounds possess very valuable properties which are absent in their components. By forming intermetallic compounds it is possible to increase the hardness, strength, heat-and corrosion resistance of a material, to endow its surface with magnetic, catalytic, emission, semiconducting and superconducting properties. It is often required that only a thin surface layer of the metal shoutd have some of these properties. With the use of incorporation it is possible to obtain layers of intermetallic compounds on the electrode surface while keeping its bulk properties unchanged. A method of increasing the heat- and corrosion resistance of a vanadium surface by cathodic incorporation into it of aluminium from a melt at 1000°C in England in 1970[22]. A similar was patented intermetallic compound was obtained on vanadium surface at room temperature by incorporating into it aluminium from a nonaqueous salution[23]. By incorporating manganese into bismuth from an aqueous solution, an intermetallic compound MnBi was obtained on the bismuth electrode surface which showed pronounced magnetic properties[24]. Some other cases are reported of preparing by incorporation materials of practical value. Thus the General Electric Company uses a method called “Metalliding”[25], which is based on the application of cathodic incorporation. Electrochemical incorporation can be used to prepare compounds difficult to obtain by other methods. For example, LiGa can be obtained by incorporation[lO]. By alloying the components it can be obtained only at 7OO”C[26]. By incorporation it is possible to effect the interaction of elements which otherwise do not interact, eg, the interaction of alkali- and transition metals[27]. Finally, as was recently shown[6], cathodic incorporation plays an important role in the application of a promising electrode from the intermetallic compound LiAl in battery cells with molten electrolyte.
REFERENCES
D. Leikis, I. Kiseleva, I. Astakhov and D. Alexandrova, DAN SSSR t44, 1085 (1962). 2. 3. Kabanov, I. Kiseleva and I. Astakhov, Elekrrokhimiya 8.955 (1972). B. Kabanov, I. Kiseleva, I. Asrakhov and N. 1. B.
L
-q5
-qa
Tomashova, Sashchita merallou 11, 131 (1975). 3. 0. Bredig and F. Haber, Ber. Deutsch. Chem. Ges. 31, 2741(1898). F. Haberand M. Sack, 2. ebktrochem. 8,245 (1902). 4. Yu. Delimarskiy and I. Panchenko. DAN SSSR 91, 115 (t953) ; Yu. Delimarskiy and V. Kuz’movich, J. Neorgnnicheskoy Chem. 4, 2732 (1959). 5. M. Haga. Yu. Kukk and V. Past, Double Layer and Adsorbtion on Solid Electrodes. Tartu Gas. University,
t&v
Fig. 5. Dependence of the differential capacity on potential (us nhe) in O.OlN Na2SG4: a-Pb; Y -Pb + 1%
(atom) Na.
Kabaaov,
Tartu (l%S]. 6. C. Melendres, J. electrochem. Sot. 124, 650 (1977). 7. 1. Astakhov. Yu. Filinovsky and G. Teplitskaya. Etekrrokhimiyo 13, 566 (1977). 8. I. Astakhov, Elekrrolrhimiyo 9, 521 (1973).
Formation
af crystalline intermetallic
9. B. Kabanov. I. Kiseleva, I. Astakhov and N. Tomaachova, Elekrrokhimiyo 1. 1023 (1%5). 10. 3. Kabanov, I. Kiseleva. I. Astakhov, N. Tomashova and 10, 765 (1974). P. Petukhova, Elektrokhimiyn Il. N. Chovnyk and A. Fomichev, Elektrokhimiya 7, 130 (1971); 10, 284 (1974). 12. F-4.Tomaschova, I. Kiseleva and B. Kabanov, Elektrokhimiya 7, 438 (1971); 8, 112 (1972). 13. 1. Avrutskaya, 1. Kiseleva, A. Chekavtsev, V. Shinkarev, B. Kabanov and M. Fioshin, Elektrokhimiya 13, 1516 (1977). 14. B. Kabanov and A. Zak, DAN SSSR 72, 531 (1950). 15. V. Korshunov, A. Frumkin and T. Ivanova, Elektrokhimiya 4, 1120 (1968). 16. A. Matsuda, R. Notoya, T. Ohmori,K. Kunimatsuand T. Kushimoto, Efektrokhimiya 13,726 (1977); A. Matsuda, R. Notoya, T. Ohmori, K. Kunimatsu and T. Kushimoto, J. Res. Catalysis 24, 187 (1976). 17. D. Alexandrova, I. Kiseleva and B. Kabanov, J. Phis.
compounds
171
Chem. (Russian) 38, 1493 (1964). 18. A. Zak and B. Kabanov, Elektrekhimiya 1,68 (1965). 19. I. Kiseleva, D. Leikis and B. Kabanov, Elektrokhimiya 8, 250 (1972). 20. I. Kiseleva, I. Avrutskaya, N. Tomashova, B. Kabanov and M. Fioshin. Double Layer and Adsorption on Solid Electrodes. Tartus Gos. University Tartu (1975). 21. Yu. Kudr’avtsev, D. Semchenko, K. Bin, Trudy now ocherk. politekh. Inst. 141 (sektsi’a tekhnologii elektrokhim. proizvodstv i analit. himii), p. 49 (1964). 22. British Patent N119335, kl C 7 B (1970). 23. A. Chekavtsev, N. Matveuva, 1. Kiseleva and B. Kabanov. Elektrokhimiva 13. 1153 (1977). 24. V. Gaiushko, E. Vd&iko.and V.‘Tro&enko, Ukr. khim. Zh. 33, (6) 574 11967). 25. J. Smiih: Scient.’Am. ‘9, 105 (1967). 26. E. Zint) and G. Brauer, Z. phis. Chemie BZO, 245 (1933). 27. B. Kabanov, I. Kiseleva, N. Tomashova and I. Schvartsman, Efektrokhimiya 9, 381 (1973).