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Electrical double layer on renewed iron electrodes in solutions based on a number of organic solvents
Acta, Vol. 42, No. 4, pp. 67S687, 1997 Copyright 0 1996 Elsevier Science Ltd. Printed in Great Britain. All rights reserved
Elecfrochimica
Pergamon PII: soo13-46sq96)00228-9
0013-4686/97$17.00+ 0.00
Electrical double layer on renewed iron electrodes in solutions based on a number of organic solvents* V. A. Safonov,? L. Yu. Komissarov and 0. A. Petrii Department
of Electrochemistry,
(Received 17 January
Moscow State University,
Moscow, 119899 Russia
1996; in revised form 17 April 1996)
Ah&act-Dependences were obtained of the current and impedance component transients on the potential for Fe electrodes renewed by cutting in the solutions based on certain organic solvents: tetra-methylurea, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, hexamethylphosphortriamide, dimethylsulfoxide (DMSO), and acetonitrile (AN). It was found that the renewed iron electrode in LiC104 or KBK solutions in a certain potential region behaves in the first approximation as an ideally polarizable electrode (with the exception of DMSO and AN solutions). The values of potentials of zero charge (pzc) of Fe were determined from the capacitance minimum in dilute electrolyte solutions. The relationship between the double-layer capacitances corresponding to the positive and negative charges strongly depends on the solvent nature. A similarity was found in the behaviour of Fe and Pt electrodes, which is associated with the chemisorptive interaction between these metals and organic solvent molecules. Literature data on the Fe pzc in aqueous solutions are analyzed in comparison with the data for organic solvents and from the standpoint of the correlation with the work functions W of metals. It is concluded that the values of Fe pzc obtained earlier by Leikis, Rybalka, and Zelinskii for renewed Fe electrodes in aqueous electrolyte solutions correspond most closely to the active iron surface free from any oxides. Copyright 0 1996 Elsevier Science Ltd Key words: Renewed iron electrode, double-layer
structure, organic solvents, chemisorption.
INTRODUCTION In view of the considerable progress made in studies of the electrical double layer on mercury-like metals (Hg, Bi, Ga, Cd, Sn, Sb, Zn, Tl-Ga, etc. [l-3], Ag, Au [I, 4]), and platinum group metals in aqueous [ 1,5] and nonaqueous [6,7] media, the achievements in this direction for practically important corrosionactive light metals (Li, Mg, Al, erc.) and the iron-group metals look quite modest. The reason for this is that it is difficult to produce well-characterized and reproducible clean surfaces of these metals in electrolyte solutions. As a result, the chosen surface pretreatment conditions and the method of studying strongly affect the measured double layer parameters. One glowing illustration of this is the data on the potentials of the zero free charge (pzc) of iron summarized recently in [8]. *Dedicated to Professor A. N. Frumkin Centenary. These results were presented at the Poster Session of the 6th International Frumkin Symposium (Moscow, August 20-24, 1995). tCorresponding Author
From our viewpoint, the most promising results can be obtained by using the method of cutting the surface of mentioned metals immediately in the studied solution and comparing the data obtained by this method in different solvents. In particular, this exact method was used for obtaining reproducible and reliable results on the double layer capacitance for Fe in aqueous solutions [9, lo], and also for Al in nonaqueous media [1 1, 121. Although the method of the surface renewal was first applied to an iron electrode about 20 years ago [9, lo], no systematic measurements have yet been carried out. EXPERIMENTAL In this work, we used the method of electrochemical measurements on electrodes, the surfaces of which were periodically renewed immediately in the studied solution by cutting off a thin layer of the metal (about 10 pm) with a sapphire tool. The set-up for these studies was described in detail in [ 131. The end of a wire of OS-O.8 mm diameter made of iron of high purity (99.995%) served as the working electrode [14]. 615
616
V. A. Safonov et al.
The results obtained on these electrodes in aqueous solutions by the method of surface renewal confirmed the data obtained earlier by Leikis and coworkers [9, lo]. The minimal time required for the surface to be cut and the tool to be removed from the electrode (to put an end to the screening effect) was - 0.5-l s. Iron, even in its annealed state, is a substantially hard metal. Therefore, when we cut its surface with a sapphire tool, the cutting edge of the latter is soon damaged. As a result, the electrode surface changes uncontrollably from one cutting to another, and so the measured impedance values change correspondingly. All this makes detailed experimental study of the interfaces of a renewed Fe electrode with a large number of solvents rather difficult. Therefore, in a search for new systems, the latter were not analyzed in as much detail as the systems based on dimethylsulfoxide (DMSO) and tetramethylurea (TMU), and certain conclusions were drawn on the basis of model calculations. When using the method of the surface renewal by cutting, a question arises about the equilibrium degree of the obtained surface state. To answer it unambiguously, additional studies specific for a given metal and a given cutting method are necessary. For example, in [15], by using the quick potentiostatic method, the authors showed that the state of the platinum surface immediately after cutting was practically identical to the state achieved by the anodic-cathodic cycling. This means that the relaxation processes in the surface layer proceed sufficiently quickly. On the other hand, it can be assumed that the properties of surfaces obtained in our experiments at relatively slow renewal rates differ from the properties of surfaces obtained by the quick permanent scraping [ 161. We obtained the major part of electrochemical results using the potentiostatic method and automatic impedance measurements. For these purposes, we used a P-5848 potentiostat and an automatic IKS-5 device to measure complex resistances. The time dependences of current and electrode impedance measured from the moment of the surface renewal at a fixed E, were used as initial data for plotting the I us E curves (I is the current, E is the potential), and also X us E and R us E curves (X = l/Cc+ where C and o are differential capacitance and the angular frequency o = Zrcf, respectively, and R is the resistance; frequency f is changed in the range from 20 to 1000 Hz). This method of simultaneously recording the Z us E, X us E, and R us E dependences corresponding to different times after the surface renewal allows one to select quite easily the metal-solvent systems promising for more accurate studies of the double layer structure (selection of the potential region characterized by small currents of faradaic processes and a low frequency dispersion of the electrode impedance). Dimethylsulfoxide, acetonitrile (AN), and also a number of amides: tetramethylurea, dimethylfor-
mamide (DMFA), dimethylacetamide (DMAA), n-methylpirrolidone (N-MP), and hexamethylphosphortriamide (HMPTA) were used as the solvents. As electrolytes, LiC104 and KBF4 were selected. The initial solvents of the reagent grade purity were additionally purified using the known procedures [17]. The salts were twice crystallized and dried. The solutions were prepared out of contact with the atmosphere. For this purpose, the dried salt portion was placed into an ampoule, which was connected with a set-up for the vacuum rectification of the respective solvent. A necessary amount of the solvent was rectified immediately into the ampoule. Then the ampoule with the obtained solution was separated from the vacuum set-up, and transferred without its exposure to the atmosphere to the cell preliminary dried and filled with argon. An aqueous saturated calomel electrode served as the reference electrode. The junction between aqueous and nonaqueous solutions was created in accordance with [ 181.The potential drop between the aqueous solution and the studied solvent was not taken into acount. The current densities and capacitance values (unless specially mentioned) were referred to the geometrical surface area of electrodes. The temperature was 20 + 2°C.
RESULTS
AND DISCUSSION
1. The interface of a renewed iron electrode with DMSO Figure 1 shows the i us E dependences for a Fe electrode in a DMSO solution of 0.1 M LiC104 which correspond to different moments from the iron surface renewal. Beginning with small deviations from the steady-state potential, these curves are characterized by a relatively weak current us potential dependence. With increasing time, the currents of anodic and cathodic processes decrease, the former, apparently, to a greater extent, which leads to an increase in the steady-state potential (Fig. la). The open-circuit potential shifts immediately after cutting in the cathodic direction to the values of about -0.8 V and then quickly (in ca. 10 s) relaxes to the steady-state value of about -0.35 V. A detailed study of the nature and the mechanism of anodic and cathodic processes was beyong the scope of our work. However, it is probable that a substantial role is played by the adsorption combined with the destructive phenomena chemisorption of the solvent. This is confirmed by the high current values observed practically throughout the potential region on a renewed Fe electrode in DMSO solutions (Fig. l), as well as by the independence of the i us E curves from the solution stirring by the inert gas, and the virtual absence of the electrolyte concentration effect (from 0.003 to 0.1 M LiClO4) on the steady-state potential and its shift after the surface renewal.
Electrical double layer on renewed iron electrodes in solutions of organic solvents Figure 2 shows the dependences of the differential capacitance C on the potential at different frequencies. The extrapolation of C to the infinite frequency in the C us l/f> coordinates gives the capacitance values C, which are practically potential-independent. It was assumed in these calculations that the simplest equivalent circuit-the parallel-connected double-layer capacitance and the resistance corresponding to a faradaic process-an be used. The estimated values of C, proved to be low (_ ll12 pF/cm’), especially if one takes into account that the electrode roughness factor can substantially exceed 1. The presented results can be explained in the terms of the destructive irreversible adsorption of DMSO molecules on the iron surface [19]. This correlates with a number of studies. For instance, in [20] and several other works, evidence is given of the fact that metallic iron, especially in the presence of its bivalent ions catalyzes the rapid breakdown of DMSO molecules. It is probable that a newly formed surface is an even more active catalyst than the surface introduced into the solution after being in contact with air, and the adsorption on such a surface can be destructive. The adsorption layer formed under these conditions apparently undoes the potential effect on the double layer structure. Recently, Kruger et al. [21] reconfirmed that the supposition about the stability of nonaqueous solvents in contact with the iron cannot be considered as reliable. The estimates of the charge passed between the cutting moment and the beginning of measurements at different potentials show that at the potential of -0.5 + 0.1 V the charge necessary for the formation
I
log i (i /
617
of a monolayer coverage on iron (calculated for a one-electron process) manages to pass in ca. O.l0.2 s, and at the potential - 1.0 + 0.2 V this takes 0.024.05 s. Hence, immediately after the cutting, during the time that, because of the procedure specifics, cannot be measured, the chemisorption layers are formed on the electrode surface, which are practically independent of the potential changes. We also carried out similar studies in the system Fe/acetonitrile solutions of LiClOd. The results obtained point to the fact that the Fe/AN interface is characterized by the same behaviour as that mentioned above for the FeDMSO interface. 2. The interface of the renewed iron electrode/TMU Tetramethylurea pertains to stable solvents and exhibits very interesting properties [22]. In LiC104 solutions in TMU, the steady-state potential is established on a Fe electrode in the vicinity of -0.35 V. After the metal surface was renewed, the open-circuit potential (Fig. 3a) shifts sharply to - 0.8 V and then slowly, in approximately 15 min, relaxes to the steady-state value. Polarization curves plotted on the basis of current transients and corresponding to a fixed t are shown in Fig. 3. The effect of the electrolyte concentration on the time dependences of the open-circuit potential is small, so, with an increase in the LiC104 concentration by an order, the currents at fixed potentials increase by only S-10% depending on E and are indifferent to the solution stirring with an inert gas. A comparison with similar curves measured in DMSO solutions (Fig. 1) shows that the rates of electrochemical processes in
p.kcm4) .EIV
a
I
-1.6
I -1.2
I -0.8
I -0.4
I 0
I 0.4
I 0.8
EIV I 1.2
Fig. 1. Polarization curves of Fe electrode in a solution of 0.1 M LiClOo in DMSO at different moments after surface renewal: I-0.3, 2-3, and 3-100 s. inset (a) shows the time dependence of open-circuit potential of renewed Fe electrode.
V. A. Safonov et al.
678
60
50
40
30
20
10
EIV -i
I
I
I
I
I
I
I
0.4
0
-0.4
-0.8
-1.2
-1.6
Fig. 2. Potential dependence of differential capacitance of renewed Fe electrode (12 s after cutting) in solution of 0.03 M LiClO4 in DMSO at different frequencies: l-70, 2-140, 3-210, and 4-S10H.z. Curve 5 was obtained as a result of extrapolation of measured capacitance values in C us l/f* coordinates for f- co.
TMU
solutions
magnitude
are at least 1.5 - 2 orders lower than in DMSO solutions.
of
The sufficiently low current densities of electrochemical processes observed on a renewed iron surface in Lie104 solutions in TMJJ result in the fact that the electrode impedance components weakly depend on 1. In 1 min after cutting, the values R and f/Co change by no more than 5-?%, and the frequency dispersion of the impedance components in the ac frequency region of 70-1000 Hz does not exceed 7-l 0%. Figure 4 shows (taking into account the roughness factor determined as is shown below) the experimentally measured dependences of C values on the Fe electrode potential in solutions of different concentration of LiC104 recorded in 10-15 s after cutting. In these curves, with the solution dilution, a characteristic minimum appears, the potential of which does not depend on the electrolyte concentration. The
dependences of 1JC on potential
l/cd
of the minimum
(Fig. 4a) are linear at the (C is the experimental
capacitance and Cd is the diffuse layer capacitance calculated according to the Gouy-Chapman theory), which indicates that the Parsons-Zobel criterion [23] is fulfilled and the Grahame model can be used for describing the double layer on Fe in this solvent. When calculating Cd, we used the buIk value of dielectric constant of TMU E = 23.06. The roughness factor of the Fe electrode determined from the slope of the l/C us l/C’, Iine [I, 241 was about 5.0, which is greater than similar values observed on Pt and Pd electrodes in solutions based on AN and DMSO [6,7J and obtained by the same method of the metal surface renewal, and also the values for Al in aprotic solvents [ll, 121. Figure 4 also demonstrates the C ZJ,S6 curve calculated according to the Grahame method for 3 x 10e3 M solution in TMU, when the roughness
679
Electrical double layer on renewed iron electrodes in solutions of organic solvents factor of Fe was taken into account. The values measured in a 0.1 M LiClO4 solution were taken as the dense layer capacitance. It can be seen that there is a good agreement between theoretical and experimental C us E dependences. Thus, on the basis of the analysis performed we can conclude that the double layer structure on a renewed Fe electrode in TMU solutions of LiClO4 obeys the Gouy-Chapman-Grahame theory in the potential region of 0 to - 1.4 V, and the potential at which the capacitance minimum in diluted solutions is observed corresponds to the zero charge potential of iron. In this case Li+ and ClO; do not adsorb specifically on iron from TMU solutions. The C us E shown curves correspond to sufficiently short contacts of a renewed Fe surface with electrolyte (10-l 5 s). However, the capacitance values at fixed potentials gradually decrease and in l&l5 min attain their steady-state values, which no longer appear to be strongly dependent on the potential. In C us E curves corresponding to such t, no characteristic minima are observed with the solution dilution. We believe that the time changes of the capacitance values are first of all caused by the faradaic processes which slowly proceed on the electrode in this medium and are accompanied by the formation of chemisorption products, and also by the adsorption of uncontrollable impurities from the solution bulk. A substantial evidence in support of the abovementioned assumption is the comparison of the results for TMU with the data on the behaviour of a renewed Fe electrode in DMSO solutions. Since the rates of electrochemical processes on a renewed Fe
I
surface in solutions based on TMU are much lower, the effects similar to those observed in DMSO are, naturally, observed in this system at longer contacts. It was of interest to compare the properties of the interfaces of Fe and Hg electrodes with TMU. Figure 5 shows a C us E curve measured on a dropping Hg electrode in 0.1 M solution of LiClO4 in TMU. The zero charge potential was determined by the method of jet electrode [25] and was equal to - 0.29 V. The curve of the double layer capacitance of a Hg electrode in 3 x 10e3 M LiClO4 in TMU obtained by calculating according to the Grahame method is shown by the dashed line in Fig. 5. As can be seen from Figs 4 and 5, in TMU solutions of LiClO4 the capacitance values (with regard to the roughness factor) are much lower on Fe electrode than on Hg electrode, and, as a consequence, Fe electrode is characterized by a much weaker dependence of the capacitance on the potential as compared with Hg electrode. Figure 6 shows the dependence of the surface charge c on the potential calculated by integrating C us E curves for Hg and Fe electrodes in 0.1 M LiClO4 solutions in TMU. The shape of the 0 us E curve for Hg is qualitatively the same as in the cases of the interfaces of this metal with water [26], AN [27], and DMSO [28]. If we suppose that the solvent does not interact with the metal at high negative potentials of the mercury surface, then we can estimate from these data the adsorption potential drop of TMU molecules [29]. This value is -0.17 V, which is close to the corresponding values for the interfaces of a Hg electrode with aqueous and acetonitrile solutions [26, 271. This makes it possible to conclude that the
log i (i / l&cm?
3 t
-1.8
,EIV
-1.2
a
-0.8
-0.4
0
0.4
0.8
Fig. 3. Polarization curves of Fe electrode in solution of 0.1 M LiClO4 in TMU at different moments after surface cutting: I- 0.3, 2-3,
and 3-100 s. Insertion (a) shows the time dependence of open-circuit potential of renewed Fe electrode.
V. A. Safonov et al. C / p.FTm-* 2
a
0.1
0.0
-6
\6
I
I
0.4
0.0
I -0.4
I -0.8
I -1.2
EIV -1.8
Fig. 4. Potential dependence of differential capacitance (referred to the real surface area) of renewed Fe electrode (I 2 s after cutting) in solutions of different LiClO4 concentrations in TMU: 14.1, 24.05, 34.03, u.01, S4.005, and GO.003 M; f= 210 Hz. Dashed line is the C c’s E curve calculated for 0.003 M solution. Inset (a) is the Parsons-Zobel plot at the potential of capacitance minimum.
uncharged Hg surface interacts rather weakly with TMU molecules. At the same time, for Fe electrodes, Q depends on E more weakly than in the case of Hg electrodes, and the slope of a u us E dependence is roughly constant throughout the potential region. If, at high negative surface charges, an Fe electrode did not interact with TMU molecules, then the cr us E curve would be shifted with respect to the curve of a Hg electrode by difference between the electron work functions. This theoretical dependence is shown in Fig. 6 by the dashed line. The value of the electron work function We for Fe was taken in accordance with the recommendations of [30] and is equal to 4.31 eV. The adsorption potential drop of TMU molecules on Fe
calculated using this value is equal to: AEi;iTMu = EF” TMUa = 0 - EF,iTMu r-0 - we/e0 + pg/eo + AEaHd811TMU = -0.8 + 0.29 - 4.31 + 4.5 - 0.17 = -0.49 V. The negative value of AEi$TMU points to the orientation of adsorbed solvent molecules by the negative end of dipole to the metal surface. It should be noted that there is a wide scatter in the values of the electron work function (by more than 0.5 eV) for Fe in literature [30]. Earlier, basing on the analysis of a large number of data, Trasatti [31] used the value of 4.65 V for WF’. In this case AE$iTMu would be substantially more negative (-0.83 V) and close to the values obtained for the interfaces of Pt with organic solvents ]6,71.
Electrical
double
layer on renewed
iron electrodes
681
in solutions of organic solvents
As an example of lithium halide salts and alkali metal perchlorates, the effect of the electrolyte composition on the double layer structure on Fe in TMU solutions was also studied. If the Cl- ion concentration does not exceed 10e2 M, then, as is evident from Fig. 7, at not too negative surface charges, the capacitances in LiCl solutions are higher than in LiC104 solutions, which is apparently due to the specific adsorption of Cl- anions on Fe. Starting from certain positive charges, the capacitances in chloride solutions are, on the contrary, lower than in perchlorate solutions. Such effects of halide ions were
A substantial difference between the G us E curves on Hg and Fe electrodes in solutions based on TMU correlates with those observed when comparing the G us E curves on mercury-like metals (Hg, Bi, etc.) and d-metals such as Pt and Pd in AN and DMSO solutions [6, 71. To explain these differences, the concept of the chemisorptive interaction of the molecules of these solvents with Pt and Pd was used. Apparently, this conclusion on the chemisorptive interaction with solvents (in our case-with TMU) can be also extended to Fe electrode.
4c
3c
2c
1c
0.6
1
I
I
I
0.4
0
-0.4
-0.6
I -1.2
EIV I -1.6
Fig. 5. C OSE dependence for Hg electrode in solution of 0.1 M LiC104 in TMU. Dashed line is the C us E curve calculated for 0.003 M solution.
682
V. A. Safonov et al. al p3cm”
I’
-10
/’
1
,’
s’
,’ d’
l’ 4’ ,’
a’
0
:/.
10
I
I
I
1
0
-0.5
-1.0
-1.5
E/V
Fig. 6. Charge (referred to the real surface area) us potential dependences for (1) Hg and (2) Fe electrodes in the solution of 0.1 M LiClO4 in TMU. Dashed line is the hypothetical curve for Fe.
observed earlier for the interfaces of certain metals with aqueous solutions (the data for Ni [32], Cu [33], and Pt [34]).
0.8
I
I
0.4
0
I
-0.4
At the further increase in the Cl- ion concentration in solution (> 10m2M), even at quite negative potentials of a Fe electrode, the steady-state anodic currents probably caused by the metal dissolution increase dramatically, which leads to the increase in the measured C values. Br- and I- anions strongly accelerate the anodic process even at minor concentrations. Due to the increase in the contribution of faradaic processes (Fig. 8), the potentials of the sharp capacitance rise shift in the cathodic direction in the sequence Cl- < Br- < I-. Such an effect of halide anions allows one to make an assumption that their interaction with Fe increases in the same sequence. These results can bear witness to the mechanism of the direct participation of anions in the metal dissolution [35]. Changes in the nature of cations (Li+, K+, and Cs+]) in perchlorate solutions do not affect the curves of the differential capacitance us potential, which points to the absence of any noticable specific adsorption of these ions on Fe from TMU solutions. It is important, especially in view of the experimental procedure, to consider the problem of the influence of the water content in the initial solvent on the structure of the Fe/TMU solutions interface. It was found that in dilute LiC104 solutions the minimum that corresponds to the maximum
I
-0.8
I
-1.2
I
-1.6
EIV Fig. 7. C us E dependences for Fe electrode (I2 s after cutting) in 0.003 M solutions of (I) LiClO4 and (2) LiCl in TMU. f= 210 Hz.
Electrical double layer on renewed iron electrodes in solutions of organic solvents
683
26
16
4
6
I
I
I
I
I
0.4
0
-6.4
-6.6
-1.2
I
-1.6 EIV
Fig. 8. C USE dependences for a Fe electrode (12 s after cutting) in 0.1 M solutions of (1) LiC104, (2) LiCI, (3) LiBr, and (4) LiI in TMU. f= 210 Hz.
diffuseness of the double layer also keeps at the introduction of up to 1 ~01% of water to the solvent. In this case, only a small shift of the minimum in the anodic direction is observed (by no more than 50 mV), as compared with water-free solutions; the currents of the polarization curves increase in this case by lO-15%. The addition of more than 5 vol % water leads to a sharp acceleration of faradaic to a dramatic processes and, correspondingly, deformation of the capacitance curves as a result of the pseudo-capacitance contribution.
3. Interface of a renewed Fe electrode with solutions based on dimethylacetamide (DMAA), N-methylpyrrolidone (N-MP), dlmethylformamide (DMFA), and hexamethylphosphortrlamide (HMPTA) It was found that, for organic aprotic solvents of the amide nature, such as DMFA, DMAA, N-MP, and HMPTA, the potential regions can be determined where the electrochemical processes on the interface with a renewed Fe electrode proceed very slowly and which can lx considered, in the first
V. A. Safonov et al.
684
7 / pF*cm”
a 30
b
b
1
2
d
w 7
2
,EIV
I
I
-0.8
0
-1.6
5
I 0
EIV
I -0.8
-1.6
Fig. 9. C OSE dependences for a Fe electrode (12 s after cutting) in (1)0.1and (2) 0.003 M solutions of LiClO4 in (a) DMAA, (b) N-MP, (c) DMFA, and (d) HMPTA. C values refer to an apparent electrode surface.
approximation as the regions of the ideal polarizability. Figure 9 shows C us E curves on a renewed Fe electrode measured in solutions of 0.1 and 0.003 M LiC104 in the mentioned solvents. In the curves, the capacitance minimums are observed, which, as in TMU solutions, correspond to the zero charge potentials of a Fe electrode in solutions of surface inactive electrolytes in these solvents. The table presents the potential regions corresponding to the ideal polarizability of these systems. It also compares the values of Em=0 for Fe and Hg electrodes. the latter were measured by the method of the jet electrode.
Table 1. Regions of ideal polarizability of a renewed Fe electrode in a number of aprotic solvents and zero charge potentials of Fe and Hg
Solvent TMU DMFA DMAA N-MP HMPTA
Region of ideal polarizability, v (aqueous SCE) 0.0 -0.1 -0.1 0.0 -0.4
to to to to to
-1.4 -1.3 -1.2 -1.3 -1.4
&=o, u (aqueous SCE) Fe
Hg
-0.80 -0.65 -0.55 -0.85 w -0.80
- 0.29 -0.26; -0.18 -0.32 -0.34
*From the electrocapillary curve maxima [36].
A number of characteristic qualitative specific features of the curves should be noted. Usually, in the potential region of C us E curves where negative surface charges change to positive, a rise of the capacitance is observed, and, by the large, the capacitance values at positive charges are greater than at negative charges. As can be seen from Figs 4 and 9, such a shape of dependences is observed for TMU and N-MP solutions, while for DMFA solutions the values of C at positive and negative charges are approximately the same, and in the solutions based on DMAA the capacitance values are higher at negative charges than at positive. Apparently, the observed specific features are caused by the differences in the ion solvation in these solvents. The other cause is likely to be a difference in the structure of chemisorption layer molecules of solvents at positive and negative surface charges. To give an ultimate answer to these questions, it is appropriate to carry out further studies in the systems based on the mentioned solvents on Hg and mercury-like metals. The capacitance curves shown in Fig. 9 are referred to the geometrical surface area of the electrode. The data obtained are insufficient for plotting the Parsons-Zobel dependences and calculating the real surface area of the electrode. However, the estimate of real surface area can be carried out on the basis of the Gouy-Ghapman-Grahame model [1] from the data on the bending points near the minima in the
Electrical double layer on renewed iron electrodes in solutions of organic solvents C us E curves in dilute solutions. For simplicity, we assume that the capacitance of the double layer dense part CI is independent of the electrode potential. [In the general case, the Cl us Q curve can be approximated, eg by some polynomial, and following equations (lt(3) can be used in numerical calculations. At the same time, condition C, = const in the first approximation is valid for systems studied in the potential range close to E, _,I (at more cathodic or more anodic potentials than E,,o depending on the nature of the solvent).] Then the overall capacitance C and the potential cp (with respect to the potential of zero charge) can be represented as c =
C,F&A%
+ a2/(2RT) (1)
C, + FJm/(2RT)’ 2RT cp = 7 arcsinh
(2)
where A = ,,/e, E is the dielectric constant of the solvent; EO= 8.854 x lo-l2 F/m; c is the electrolyte concentration; 0 is the electrode charge, which acts as a parameter in equations (1) and (2); and the other values have their usual meaning. The condition of the bending on C, E-curve corresponds to d2C _...-
dq2 -
cp’c”- cp“C’ = o
(cp’)’
.
If we introduce the expression z = Jw, then, after transformations, equation (3) takes the form:
-5!G&o.
(4)
Therefore, by solving equation (4) with respect to z, we can calculate 0 values that correspond to the bending points in the C us E dependences. The values (pknd themselves are calculated from equation (2). Figure 10 shows the results of numerical calculations of (Pknd in C us E curves according to equations (4) and (2) as a function of C, at different electrolyte concentrations and E values. The values E = 23.06 and E = 78.3 model the solutions based on TMU and water, respectively. As follows from the figures, a decrease in E, all other conditions being equal, leads to a decrease in the difference lEbend- E,=o~. A comparison of the presented dependences with experimental data of Fig. 4 for TMU solutions (the position of the bending point with respect to E, =0) showed that such a method can be used for making approximate estimates of the electrode real surface area. The corresponding calculations for the other systems made it possible to conclude that the roughness factor of the renewed Fe surface ranges from 3.5 to 5.
685
In Fig. 11, by the analogy with the data for TMU solutions, the dependences of the surface charge of Fe, Hg,and Bi electrodes on the potential in DMFA solutions calculated from C us E curves are shown. The data for Hg and Bi are taken from [36] and [37], respectively. The dashed line in this figure represents a hypothetic 0 us E dependence for a Fe electrode calculated under the assumption of a weak interaction of this metal with DMFA molecules. The behaviour of this system is for the major part the same as the above-discussed behaviour of Pt and Pd electrodes in DMSO and AN solutions, and also that of Fe electrode in solutions based on TMU. A comparison of the behaviour of a Fe electrode in contact with studied solutions allows one to make a supposition, in our view quite reasonable, on the nature of the chemisorptive interaction between this metal and organic solvents. The case in point is that the obtained data can be qualitatively interpreted as a result of a strong donor-acceptor interaction [38] between the renewed metal surface and solvent molecules in the very beginning of their contact. However, after longer contact, the observed effects point to the destruction of the chemisorbed solvent molecules. This can explain the observed time dependences of the measured capacitance and the absence of any characteristic double-layer effects after a long contact of as Fe electrode with solutions based on organic solvents. Under these conditions, the behaviour of iron resembles that of a Pt electrode in aqueous solutions with organic substance additives. For solvents such as DMSO and AN, the destructive adsorption proceeds at considerable rates practically in the first seconds after the contact of the renewed Fe electrode surface with the solvent. For solvents of the amide nature, which are probably more resistive to the contact with a renewed Fe electrode surface, the effects of destructive adsorption manifest themselves at much longer times of contact. 4. Comparison of the experimental results in aqueous and nonaqueous solutions and the pzc values of Fe in water.
The values of E,=o for Fe in aqueous solutions presented in literature range from +0.03 V (in early works) to -0.7 to -0.9 V [8]. From our viewpoint, the reliability of the latter values is to a certain extent confirmed by their comparison with the results of the above-mentioned studies in solutions based on organic solvents. First of all, the values of E, =o obtained above for Fe electrodes in all the solvents lie at potentials more negative than those for a Hg electrode. This is in agreement with the lower [30] or close 1311electron work function for iron, as compared with mercury. Secondly, the difference between the zero charge potentials of iron and mercury in the studied solvents is about 0.5 V. For aqueous solutions, if we use the
V. A. Safonov et al.
686
b
I
0
I
I
I
I
5
10
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20
C, / l.tF.cm-*
%Ud/V
0.3
a
0.2
0.1
I
I
I
I
I
I
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15
20
25
C, / pF*cm-* Fig. 10. Dependences of (phd on CI in the 1, I-valent electrolyte solutions of different concentrations. l- 0.01, 2- 0.003, and 3-0.001 M; (a) E= 78.3 and (b) E = 23.06.
data of [9, 10, 391, this difference is somewhat lower (-0.3-0.4 V), and this effect should be associated with the organic molecule chemisorption on iron which proceeds with the negative end oriented to the metal. As follows from [8], the authors suppose that the values of E, _ o for iron in aqueous solutions are more positive than those for mercury: from - 0.1 to -0.15 V at pH l-8 in 0.01 M NarS04 (at higher pH values the authors observed a substantial shift of E ,,=o in the negative direction). Under these conditions, bearing in mind the difference between the electron work functions of these metals, it follows that the water dipoles should adsorb on iron with the positive end to the metal. Apparently, such an orientation of water molecules at the zero charge
potential on iron that has a strong affinity to oxygen is highly improbable. The main explanation for the positive shift of the values of E., = 0 for Fe observed in [8], can lie in the electrode pretreatment used by the authors. It follows from a number of studies (for example, see [40,41]) devoted to investigating the effect of different factors on the electron work function (heating at high temperatures, adsorption of oxygen, hydrogen, water vapours, etc.) that, depending on the kind of pretreatment, the value of W can substantially change. The other explanation can nevertheless consist in the presence of adsorbed oxygen on the surface, despite all the precautions taken [8] to prevent the oxidation of Fe in the course of experiments.
Electrical
double layer on renewed
iron electrodes
in solutions
of organic
solvents
687
9. L. E. Rybalka and D. I. Leikis, Elektrokhimiya 11, 1619 (1975).
10. L. E. Rybalka, D. I. Leikis and A. G. Zelinskii, Elektrokhimiya 12, 1340 (1976). 11. V. A. Safonov, S. A. Sokolov and V. M. Gerovich, Dokl. Akad. Nauk SSSR 299, 1438 (1988). 12. V. A. Safonov and S. A. Sokolov, Elektrokhimiya 21, 1317 (1991). 13. A. G. Zelinskii and R. Yu. Bek, Elektrokhimiya 21, 66 (1985). 14. D. S. Kamenetskaya, I. B. Piletskaya and V. I. Shiryaev, Zhelezo vysokoy stepeni chistoty (Ultra-high Purity Iron), Metallurgiya, Moscow (1978).
l.I/:.
Fig. 11. Charge (referred to potential dependences for (1) electrodes in solutions of 0.1 M line is the hypothetic curve for
the real surface area) us Hg, (2) Bi, and (3) Fe LiC104 in DMFA. Dashed Fe.
CONCLUSION The obtained data taken together show that the electrical double layer formed on a Fe electrode in
solutions based on TMU and a number of other solvents of a similar nature is characterized by certain specific features. The strong chemisorption interaction of solvent molecules with the surface can be explained in terms of these features. A comparison of the pzc values of Fe in nonaqueous solutions with those of Fe in aqueous solutions makes it possible to conclude that the pzc of a clean oxide-free Fe surface should lie in the potential interval of -0.7 to -0.9 V (SHE).
15. 0. A. Petrii, L. Yu. Luk’yanycheva and A. G. Zelinskii, Elektrokhimiya 24, 103 (1988).
16. T. N. Andersen, J. L. Anderson and H. Eyring, J. Phys. Chem 73, 3562 (1969). 17. Recommended Methods for Purification of Solvents and Test for Impurities (Edited by J. F. Coetzee), IUPAC Editions, Pergamon Press (1982). 18. Techniques in Electrochemistry (Edited by E. Yeager and A. J. Solkind), Wiley-Interscience, New York, Vol. 1 (1972). 19. V. A. Safonov, L. Yu. Komissarov and 0. A. Petrii, Zashchita metalloo 22, 212 (1986). 20. T. R. Agladze, Itogi nauki i tekhniki. Korroziya i zashchita ot korrozii (Achievements in Science and Technology. Corrosion and Corrosion Protection),
VINITI, Moscow, 9, 3 (1982). 21. J. Kruger, D. A. Shifler, J. F. Scanlon and P. J. Moran, Elektrokhimiya 31, 1087 (1995). 22. J. A. Riddic and W. B. Bunger, Organic Solvents, Wiley-Interscience, New York (1972). 23. R. Parsons and F. G. K. Zobel, J. Electroanal. Chem. 9, 333 (1965).
24. D. I. Leikis, K. V. Rybalka and E. S. Sevast’yanov, Adrorbtsiya i dvoinoi elektricheskii sloy u elektrokhimii (Adsorption and Double Electrical Layer in Electrochemistry), Nauka, Mowcow, p. 5 (1972). 25. D. C. Grahame and E. M. Coffin, J. Amer. Chem. Sot.
74, 1207 (1952). 26. D. C. Grahame, J. Amer. Chem. Sot. 74,4819 (1954). 27. I. A. Bagotskaya and A. M. Kalyuzhnaya, Elektrokhimiya 12, 1043 (1976).
28. L. M. Doubova and I. A. Bagotskaya, Elektrokhimiya 13, 64 (1976).
REFERENCES I. A. N. Frumkin, Potentsialy nulevogo zaryada (Potentials of Zero Charge). Nauka, Moscow (1979). 2. S. Trasatti, in Modern Aspects of Electrochemistry
(Edited by B. E. Conway and J. GM. Bockris), Plenum Press, N.Y. 13, 81 (1979). 3. I. A. Bagotskaya, Itogi nauki i tekhniki. Eiektrokhimiya (Achievements in Science and Technology. Electrochemistry), VINITI, Moscow, 23, 60 (1986). 4. A. Hamehn, in Modern Aspects of Electrochemistry
(Edited by B. E. Conway, R. E. White and J. GM. Bockris), Plenum Press, N.Y., 16, Ch.1 (1985). 5. 0. A. Petrii. Itoai nauki i tekhniki. Elektrokhimiva (Achievements in Science and Technology. Electrochemistry), VINITI, Moscow, 12, 56 (1977). 6. 0. A. Petrii and I. G. Khomchenko, J. Electroanal. Chem. 106, 277 (1980).
7. E. Yu. Alekseeva, V. A. Safonov and 0. A. Petrii, Elektrokhimiya 20, 945 (1984).
8. M. Turowska and J. Sokolowski, Elektrokhimiya 30, 821 (1994).
29. S. Trasatti, J. Electroanal. Chem. 33, 351 (1971). 30. V. S. Fomenko, Emissionnye svoistva materialov (Emission Properties of Materials), Naukova Dumka, Kiev (1981). 31. S. Trasatti, Chim. Ind. Milan 53, 559 (1971). 32. E. I. Mikhailova and Z. A. Iofa, Elektrokhimiya 6, 231 (1970).
33. V. V. Batrakov,
J. Dittrich
and
A. N. Popov,
Elektrokhimiya 8, 640 (1972).
34. V. A. Safonov, K. Jackowska and 0. .4. Petrii, Elektrokhimiya 11, 1628 (1975). 35. Ya. M. Kolotyrkin, Zashchita Metallov 3, 131 (1967). 36. V. D. Bezuglyi and L. A. Korshikov, Elektrokhimiya 1, 1422 (1965). 37. E. K. Petyarv and U. V. Palm, Elektrokhimiya 11, 313 (1975). 38. B. B. Damaskin and V. A. Safonov, Electrochim. Acta, (in press). 39. V. V. Batrakov and N. I. Naumova, Elektrokhimiya 15, 551 (1979). 40. G. F. Voronina, L. A. Larin and T. V. Kalish, Elektrokhimiya 16, 172 (1980).
41. M. E. Belyaeva, Elektrokhimiya 30, 611 (1994).
Elecfrochimica
Pergamon PII: soo13-46sq96)00228-9
0013-4686/97$17.00+ 0.00
Electrical double layer on renewed iron electrodes in solutions based on a number of organic solvents* V. A. Safonov,? L. Yu. Komissarov and 0. A. Petrii Department
of Electrochemistry,
(Received 17 January
Moscow State University,
Moscow, 119899 Russia
1996; in revised form 17 April 1996)
Ah&act-Dependences were obtained of the current and impedance component transients on the potential for Fe electrodes renewed by cutting in the solutions based on certain organic solvents: tetra-methylurea, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, hexamethylphosphortriamide, dimethylsulfoxide (DMSO), and acetonitrile (AN). It was found that the renewed iron electrode in LiC104 or KBK solutions in a certain potential region behaves in the first approximation as an ideally polarizable electrode (with the exception of DMSO and AN solutions). The values of potentials of zero charge (pzc) of Fe were determined from the capacitance minimum in dilute electrolyte solutions. The relationship between the double-layer capacitances corresponding to the positive and negative charges strongly depends on the solvent nature. A similarity was found in the behaviour of Fe and Pt electrodes, which is associated with the chemisorptive interaction between these metals and organic solvent molecules. Literature data on the Fe pzc in aqueous solutions are analyzed in comparison with the data for organic solvents and from the standpoint of the correlation with the work functions W of metals. It is concluded that the values of Fe pzc obtained earlier by Leikis, Rybalka, and Zelinskii for renewed Fe electrodes in aqueous electrolyte solutions correspond most closely to the active iron surface free from any oxides. Copyright 0 1996 Elsevier Science Ltd Key words: Renewed iron electrode, double-layer
structure, organic solvents, chemisorption.
INTRODUCTION In view of the considerable progress made in studies of the electrical double layer on mercury-like metals (Hg, Bi, Ga, Cd, Sn, Sb, Zn, Tl-Ga, etc. [l-3], Ag, Au [I, 4]), and platinum group metals in aqueous [ 1,5] and nonaqueous [6,7] media, the achievements in this direction for practically important corrosionactive light metals (Li, Mg, Al, erc.) and the iron-group metals look quite modest. The reason for this is that it is difficult to produce well-characterized and reproducible clean surfaces of these metals in electrolyte solutions. As a result, the chosen surface pretreatment conditions and the method of studying strongly affect the measured double layer parameters. One glowing illustration of this is the data on the potentials of the zero free charge (pzc) of iron summarized recently in [8]. *Dedicated to Professor A. N. Frumkin Centenary. These results were presented at the Poster Session of the 6th International Frumkin Symposium (Moscow, August 20-24, 1995). tCorresponding Author
From our viewpoint, the most promising results can be obtained by using the method of cutting the surface of mentioned metals immediately in the studied solution and comparing the data obtained by this method in different solvents. In particular, this exact method was used for obtaining reproducible and reliable results on the double layer capacitance for Fe in aqueous solutions [9, lo], and also for Al in nonaqueous media [1 1, 121. Although the method of the surface renewal was first applied to an iron electrode about 20 years ago [9, lo], no systematic measurements have yet been carried out. EXPERIMENTAL In this work, we used the method of electrochemical measurements on electrodes, the surfaces of which were periodically renewed immediately in the studied solution by cutting off a thin layer of the metal (about 10 pm) with a sapphire tool. The set-up for these studies was described in detail in [ 131. The end of a wire of OS-O.8 mm diameter made of iron of high purity (99.995%) served as the working electrode [14]. 615
616
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The results obtained on these electrodes in aqueous solutions by the method of surface renewal confirmed the data obtained earlier by Leikis and coworkers [9, lo]. The minimal time required for the surface to be cut and the tool to be removed from the electrode (to put an end to the screening effect) was - 0.5-l s. Iron, even in its annealed state, is a substantially hard metal. Therefore, when we cut its surface with a sapphire tool, the cutting edge of the latter is soon damaged. As a result, the electrode surface changes uncontrollably from one cutting to another, and so the measured impedance values change correspondingly. All this makes detailed experimental study of the interfaces of a renewed Fe electrode with a large number of solvents rather difficult. Therefore, in a search for new systems, the latter were not analyzed in as much detail as the systems based on dimethylsulfoxide (DMSO) and tetramethylurea (TMU), and certain conclusions were drawn on the basis of model calculations. When using the method of the surface renewal by cutting, a question arises about the equilibrium degree of the obtained surface state. To answer it unambiguously, additional studies specific for a given metal and a given cutting method are necessary. For example, in [15], by using the quick potentiostatic method, the authors showed that the state of the platinum surface immediately after cutting was practically identical to the state achieved by the anodic-cathodic cycling. This means that the relaxation processes in the surface layer proceed sufficiently quickly. On the other hand, it can be assumed that the properties of surfaces obtained in our experiments at relatively slow renewal rates differ from the properties of surfaces obtained by the quick permanent scraping [ 161. We obtained the major part of electrochemical results using the potentiostatic method and automatic impedance measurements. For these purposes, we used a P-5848 potentiostat and an automatic IKS-5 device to measure complex resistances. The time dependences of current and electrode impedance measured from the moment of the surface renewal at a fixed E, were used as initial data for plotting the I us E curves (I is the current, E is the potential), and also X us E and R us E curves (X = l/Cc+ where C and o are differential capacitance and the angular frequency o = Zrcf, respectively, and R is the resistance; frequency f is changed in the range from 20 to 1000 Hz). This method of simultaneously recording the Z us E, X us E, and R us E dependences corresponding to different times after the surface renewal allows one to select quite easily the metal-solvent systems promising for more accurate studies of the double layer structure (selection of the potential region characterized by small currents of faradaic processes and a low frequency dispersion of the electrode impedance). Dimethylsulfoxide, acetonitrile (AN), and also a number of amides: tetramethylurea, dimethylfor-
mamide (DMFA), dimethylacetamide (DMAA), n-methylpirrolidone (N-MP), and hexamethylphosphortriamide (HMPTA) were used as the solvents. As electrolytes, LiC104 and KBF4 were selected. The initial solvents of the reagent grade purity were additionally purified using the known procedures [17]. The salts were twice crystallized and dried. The solutions were prepared out of contact with the atmosphere. For this purpose, the dried salt portion was placed into an ampoule, which was connected with a set-up for the vacuum rectification of the respective solvent. A necessary amount of the solvent was rectified immediately into the ampoule. Then the ampoule with the obtained solution was separated from the vacuum set-up, and transferred without its exposure to the atmosphere to the cell preliminary dried and filled with argon. An aqueous saturated calomel electrode served as the reference electrode. The junction between aqueous and nonaqueous solutions was created in accordance with [ 181.The potential drop between the aqueous solution and the studied solvent was not taken into acount. The current densities and capacitance values (unless specially mentioned) were referred to the geometrical surface area of electrodes. The temperature was 20 + 2°C.
RESULTS
AND DISCUSSION
1. The interface of a renewed iron electrode with DMSO Figure 1 shows the i us E dependences for a Fe electrode in a DMSO solution of 0.1 M LiC104 which correspond to different moments from the iron surface renewal. Beginning with small deviations from the steady-state potential, these curves are characterized by a relatively weak current us potential dependence. With increasing time, the currents of anodic and cathodic processes decrease, the former, apparently, to a greater extent, which leads to an increase in the steady-state potential (Fig. la). The open-circuit potential shifts immediately after cutting in the cathodic direction to the values of about -0.8 V and then quickly (in ca. 10 s) relaxes to the steady-state value of about -0.35 V. A detailed study of the nature and the mechanism of anodic and cathodic processes was beyong the scope of our work. However, it is probable that a substantial role is played by the adsorption combined with the destructive phenomena chemisorption of the solvent. This is confirmed by the high current values observed practically throughout the potential region on a renewed Fe electrode in DMSO solutions (Fig. l), as well as by the independence of the i us E curves from the solution stirring by the inert gas, and the virtual absence of the electrolyte concentration effect (from 0.003 to 0.1 M LiClO4) on the steady-state potential and its shift after the surface renewal.
Electrical double layer on renewed iron electrodes in solutions of organic solvents Figure 2 shows the dependences of the differential capacitance C on the potential at different frequencies. The extrapolation of C to the infinite frequency in the C us l/f> coordinates gives the capacitance values C, which are practically potential-independent. It was assumed in these calculations that the simplest equivalent circuit-the parallel-connected double-layer capacitance and the resistance corresponding to a faradaic process-an be used. The estimated values of C, proved to be low (_ ll12 pF/cm’), especially if one takes into account that the electrode roughness factor can substantially exceed 1. The presented results can be explained in the terms of the destructive irreversible adsorption of DMSO molecules on the iron surface [19]. This correlates with a number of studies. For instance, in [20] and several other works, evidence is given of the fact that metallic iron, especially in the presence of its bivalent ions catalyzes the rapid breakdown of DMSO molecules. It is probable that a newly formed surface is an even more active catalyst than the surface introduced into the solution after being in contact with air, and the adsorption on such a surface can be destructive. The adsorption layer formed under these conditions apparently undoes the potential effect on the double layer structure. Recently, Kruger et al. [21] reconfirmed that the supposition about the stability of nonaqueous solvents in contact with the iron cannot be considered as reliable. The estimates of the charge passed between the cutting moment and the beginning of measurements at different potentials show that at the potential of -0.5 + 0.1 V the charge necessary for the formation
I
log i (i /
617
of a monolayer coverage on iron (calculated for a one-electron process) manages to pass in ca. O.l0.2 s, and at the potential - 1.0 + 0.2 V this takes 0.024.05 s. Hence, immediately after the cutting, during the time that, because of the procedure specifics, cannot be measured, the chemisorption layers are formed on the electrode surface, which are practically independent of the potential changes. We also carried out similar studies in the system Fe/acetonitrile solutions of LiClOd. The results obtained point to the fact that the Fe/AN interface is characterized by the same behaviour as that mentioned above for the FeDMSO interface. 2. The interface of the renewed iron electrode/TMU Tetramethylurea pertains to stable solvents and exhibits very interesting properties [22]. In LiC104 solutions in TMU, the steady-state potential is established on a Fe electrode in the vicinity of -0.35 V. After the metal surface was renewed, the open-circuit potential (Fig. 3a) shifts sharply to - 0.8 V and then slowly, in approximately 15 min, relaxes to the steady-state value. Polarization curves plotted on the basis of current transients and corresponding to a fixed t are shown in Fig. 3. The effect of the electrolyte concentration on the time dependences of the open-circuit potential is small, so, with an increase in the LiC104 concentration by an order, the currents at fixed potentials increase by only S-10% depending on E and are indifferent to the solution stirring with an inert gas. A comparison with similar curves measured in DMSO solutions (Fig. 1) shows that the rates of electrochemical processes in
p.kcm4) .EIV
a
I
-1.6
I -1.2
I -0.8
I -0.4
I 0
I 0.4
I 0.8
EIV I 1.2
Fig. 1. Polarization curves of Fe electrode in a solution of 0.1 M LiClOo in DMSO at different moments after surface renewal: I-0.3, 2-3, and 3-100 s. inset (a) shows the time dependence of open-circuit potential of renewed Fe electrode.
V. A. Safonov et al.
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50
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EIV -i
I
I
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Fig. 2. Potential dependence of differential capacitance of renewed Fe electrode (12 s after cutting) in solution of 0.03 M LiClO4 in DMSO at different frequencies: l-70, 2-140, 3-210, and 4-S10H.z. Curve 5 was obtained as a result of extrapolation of measured capacitance values in C us l/f* coordinates for f- co.
TMU
solutions
magnitude
are at least 1.5 - 2 orders lower than in DMSO solutions.
of
The sufficiently low current densities of electrochemical processes observed on a renewed iron surface in Lie104 solutions in TMJJ result in the fact that the electrode impedance components weakly depend on 1. In 1 min after cutting, the values R and f/Co change by no more than 5-?%, and the frequency dispersion of the impedance components in the ac frequency region of 70-1000 Hz does not exceed 7-l 0%. Figure 4 shows (taking into account the roughness factor determined as is shown below) the experimentally measured dependences of C values on the Fe electrode potential in solutions of different concentration of LiC104 recorded in 10-15 s after cutting. In these curves, with the solution dilution, a characteristic minimum appears, the potential of which does not depend on the electrolyte concentration. The
dependences of 1JC on potential
l/cd
of the minimum
(Fig. 4a) are linear at the (C is the experimental
capacitance and Cd is the diffuse layer capacitance calculated according to the Gouy-Chapman theory), which indicates that the Parsons-Zobel criterion [23] is fulfilled and the Grahame model can be used for describing the double layer on Fe in this solvent. When calculating Cd, we used the buIk value of dielectric constant of TMU E = 23.06. The roughness factor of the Fe electrode determined from the slope of the l/C us l/C’, Iine [I, 241 was about 5.0, which is greater than similar values observed on Pt and Pd electrodes in solutions based on AN and DMSO [6,7J and obtained by the same method of the metal surface renewal, and also the values for Al in aprotic solvents [ll, 121. Figure 4 also demonstrates the C ZJ,S6 curve calculated according to the Grahame method for 3 x 10e3 M solution in TMU, when the roughness
679
Electrical double layer on renewed iron electrodes in solutions of organic solvents factor of Fe was taken into account. The values measured in a 0.1 M LiClO4 solution were taken as the dense layer capacitance. It can be seen that there is a good agreement between theoretical and experimental C us E dependences. Thus, on the basis of the analysis performed we can conclude that the double layer structure on a renewed Fe electrode in TMU solutions of LiClO4 obeys the Gouy-Chapman-Grahame theory in the potential region of 0 to - 1.4 V, and the potential at which the capacitance minimum in diluted solutions is observed corresponds to the zero charge potential of iron. In this case Li+ and ClO; do not adsorb specifically on iron from TMU solutions. The C us E shown curves correspond to sufficiently short contacts of a renewed Fe surface with electrolyte (10-l 5 s). However, the capacitance values at fixed potentials gradually decrease and in l&l5 min attain their steady-state values, which no longer appear to be strongly dependent on the potential. In C us E curves corresponding to such t, no characteristic minima are observed with the solution dilution. We believe that the time changes of the capacitance values are first of all caused by the faradaic processes which slowly proceed on the electrode in this medium and are accompanied by the formation of chemisorption products, and also by the adsorption of uncontrollable impurities from the solution bulk. A substantial evidence in support of the abovementioned assumption is the comparison of the results for TMU with the data on the behaviour of a renewed Fe electrode in DMSO solutions. Since the rates of electrochemical processes on a renewed Fe
I
surface in solutions based on TMU are much lower, the effects similar to those observed in DMSO are, naturally, observed in this system at longer contacts. It was of interest to compare the properties of the interfaces of Fe and Hg electrodes with TMU. Figure 5 shows a C us E curve measured on a dropping Hg electrode in 0.1 M solution of LiClO4 in TMU. The zero charge potential was determined by the method of jet electrode [25] and was equal to - 0.29 V. The curve of the double layer capacitance of a Hg electrode in 3 x 10e3 M LiClO4 in TMU obtained by calculating according to the Grahame method is shown by the dashed line in Fig. 5. As can be seen from Figs 4 and 5, in TMU solutions of LiClO4 the capacitance values (with regard to the roughness factor) are much lower on Fe electrode than on Hg electrode, and, as a consequence, Fe electrode is characterized by a much weaker dependence of the capacitance on the potential as compared with Hg electrode. Figure 6 shows the dependence of the surface charge c on the potential calculated by integrating C us E curves for Hg and Fe electrodes in 0.1 M LiClO4 solutions in TMU. The shape of the 0 us E curve for Hg is qualitatively the same as in the cases of the interfaces of this metal with water [26], AN [27], and DMSO [28]. If we suppose that the solvent does not interact with the metal at high negative potentials of the mercury surface, then we can estimate from these data the adsorption potential drop of TMU molecules [29]. This value is -0.17 V, which is close to the corresponding values for the interfaces of a Hg electrode with aqueous and acetonitrile solutions [26, 271. This makes it possible to conclude that the
log i (i / l&cm?
3 t
-1.8
,EIV
-1.2
a
-0.8
-0.4
0
0.4
0.8
Fig. 3. Polarization curves of Fe electrode in solution of 0.1 M LiClO4 in TMU at different moments after surface cutting: I- 0.3, 2-3,
and 3-100 s. Insertion (a) shows the time dependence of open-circuit potential of renewed Fe electrode.
V. A. Safonov et al. C / p.FTm-* 2
a
0.1
0.0
-6
\6
I
I
0.4
0.0
I -0.4
I -0.8
I -1.2
EIV -1.8
Fig. 4. Potential dependence of differential capacitance (referred to the real surface area) of renewed Fe electrode (I 2 s after cutting) in solutions of different LiClO4 concentrations in TMU: 14.1, 24.05, 34.03, u.01, S4.005, and GO.003 M; f= 210 Hz. Dashed line is the C c’s E curve calculated for 0.003 M solution. Inset (a) is the Parsons-Zobel plot at the potential of capacitance minimum.
uncharged Hg surface interacts rather weakly with TMU molecules. At the same time, for Fe electrodes, Q depends on E more weakly than in the case of Hg electrodes, and the slope of a u us E dependence is roughly constant throughout the potential region. If, at high negative surface charges, an Fe electrode did not interact with TMU molecules, then the cr us E curve would be shifted with respect to the curve of a Hg electrode by difference between the electron work functions. This theoretical dependence is shown in Fig. 6 by the dashed line. The value of the electron work function We for Fe was taken in accordance with the recommendations of [30] and is equal to 4.31 eV. The adsorption potential drop of TMU molecules on Fe
calculated using this value is equal to: AEi;iTMu = EF” TMUa = 0 - EF,iTMu r-0 - we/e0 + pg/eo + AEaHd811TMU = -0.8 + 0.29 - 4.31 + 4.5 - 0.17 = -0.49 V. The negative value of AEi$TMU points to the orientation of adsorbed solvent molecules by the negative end of dipole to the metal surface. It should be noted that there is a wide scatter in the values of the electron work function (by more than 0.5 eV) for Fe in literature [30]. Earlier, basing on the analysis of a large number of data, Trasatti [31] used the value of 4.65 V for WF’. In this case AE$iTMu would be substantially more negative (-0.83 V) and close to the values obtained for the interfaces of Pt with organic solvents ]6,71.
Electrical
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layer on renewed
iron electrodes
681
in solutions of organic solvents
As an example of lithium halide salts and alkali metal perchlorates, the effect of the electrolyte composition on the double layer structure on Fe in TMU solutions was also studied. If the Cl- ion concentration does not exceed 10e2 M, then, as is evident from Fig. 7, at not too negative surface charges, the capacitances in LiCl solutions are higher than in LiC104 solutions, which is apparently due to the specific adsorption of Cl- anions on Fe. Starting from certain positive charges, the capacitances in chloride solutions are, on the contrary, lower than in perchlorate solutions. Such effects of halide ions were
A substantial difference between the G us E curves on Hg and Fe electrodes in solutions based on TMU correlates with those observed when comparing the G us E curves on mercury-like metals (Hg, Bi, etc.) and d-metals such as Pt and Pd in AN and DMSO solutions [6, 71. To explain these differences, the concept of the chemisorptive interaction of the molecules of these solvents with Pt and Pd was used. Apparently, this conclusion on the chemisorptive interaction with solvents (in our case-with TMU) can be also extended to Fe electrode.
4c
3c
2c
1c
0.6
1
I
I
I
0.4
0
-0.4
-0.6
I -1.2
EIV I -1.6
Fig. 5. C OSE dependence for Hg electrode in solution of 0.1 M LiC104 in TMU. Dashed line is the C us E curve calculated for 0.003 M solution.
682
V. A. Safonov et al. al p3cm”
I’
-10
/’
1
,’
s’
,’ d’
l’ 4’ ,’
a’
0
:/.
10
I
I
I
1
0
-0.5
-1.0
-1.5
E/V
Fig. 6. Charge (referred to the real surface area) us potential dependences for (1) Hg and (2) Fe electrodes in the solution of 0.1 M LiClO4 in TMU. Dashed line is the hypothetical curve for Fe.
observed earlier for the interfaces of certain metals with aqueous solutions (the data for Ni [32], Cu [33], and Pt [34]).
0.8
I
I
0.4
0
I
-0.4
At the further increase in the Cl- ion concentration in solution (> 10m2M), even at quite negative potentials of a Fe electrode, the steady-state anodic currents probably caused by the metal dissolution increase dramatically, which leads to the increase in the measured C values. Br- and I- anions strongly accelerate the anodic process even at minor concentrations. Due to the increase in the contribution of faradaic processes (Fig. 8), the potentials of the sharp capacitance rise shift in the cathodic direction in the sequence Cl- < Br- < I-. Such an effect of halide anions allows one to make an assumption that their interaction with Fe increases in the same sequence. These results can bear witness to the mechanism of the direct participation of anions in the metal dissolution [35]. Changes in the nature of cations (Li+, K+, and Cs+]) in perchlorate solutions do not affect the curves of the differential capacitance us potential, which points to the absence of any noticable specific adsorption of these ions on Fe from TMU solutions. It is important, especially in view of the experimental procedure, to consider the problem of the influence of the water content in the initial solvent on the structure of the Fe/TMU solutions interface. It was found that in dilute LiC104 solutions the minimum that corresponds to the maximum
I
-0.8
I
-1.2
I
-1.6
EIV Fig. 7. C us E dependences for Fe electrode (I2 s after cutting) in 0.003 M solutions of (I) LiClO4 and (2) LiCl in TMU. f= 210 Hz.
Electrical double layer on renewed iron electrodes in solutions of organic solvents
683
26
16
4
6
I
I
I
I
I
0.4
0
-6.4
-6.6
-1.2
I
-1.6 EIV
Fig. 8. C USE dependences for a Fe electrode (12 s after cutting) in 0.1 M solutions of (1) LiC104, (2) LiCI, (3) LiBr, and (4) LiI in TMU. f= 210 Hz.
diffuseness of the double layer also keeps at the introduction of up to 1 ~01% of water to the solvent. In this case, only a small shift of the minimum in the anodic direction is observed (by no more than 50 mV), as compared with water-free solutions; the currents of the polarization curves increase in this case by lO-15%. The addition of more than 5 vol % water leads to a sharp acceleration of faradaic to a dramatic processes and, correspondingly, deformation of the capacitance curves as a result of the pseudo-capacitance contribution.
3. Interface of a renewed Fe electrode with solutions based on dimethylacetamide (DMAA), N-methylpyrrolidone (N-MP), dlmethylformamide (DMFA), and hexamethylphosphortrlamide (HMPTA) It was found that, for organic aprotic solvents of the amide nature, such as DMFA, DMAA, N-MP, and HMPTA, the potential regions can be determined where the electrochemical processes on the interface with a renewed Fe electrode proceed very slowly and which can lx considered, in the first
V. A. Safonov et al.
684
7 / pF*cm”
a 30
b
b
1
2
d
w 7
2
,EIV
I
I
-0.8
0
-1.6
5
I 0
EIV
I -0.8
-1.6
Fig. 9. C OSE dependences for a Fe electrode (12 s after cutting) in (1)0.1and (2) 0.003 M solutions of LiClO4 in (a) DMAA, (b) N-MP, (c) DMFA, and (d) HMPTA. C values refer to an apparent electrode surface.
approximation as the regions of the ideal polarizability. Figure 9 shows C us E curves on a renewed Fe electrode measured in solutions of 0.1 and 0.003 M LiC104 in the mentioned solvents. In the curves, the capacitance minimums are observed, which, as in TMU solutions, correspond to the zero charge potentials of a Fe electrode in solutions of surface inactive electrolytes in these solvents. The table presents the potential regions corresponding to the ideal polarizability of these systems. It also compares the values of Em=0 for Fe and Hg electrodes. the latter were measured by the method of the jet electrode.
Table 1. Regions of ideal polarizability of a renewed Fe electrode in a number of aprotic solvents and zero charge potentials of Fe and Hg
Solvent TMU DMFA DMAA N-MP HMPTA
Region of ideal polarizability, v (aqueous SCE) 0.0 -0.1 -0.1 0.0 -0.4
to to to to to
-1.4 -1.3 -1.2 -1.3 -1.4
&=o, u (aqueous SCE) Fe
Hg
-0.80 -0.65 -0.55 -0.85 w -0.80
- 0.29 -0.26; -0.18 -0.32 -0.34
*From the electrocapillary curve maxima [36].
A number of characteristic qualitative specific features of the curves should be noted. Usually, in the potential region of C us E curves where negative surface charges change to positive, a rise of the capacitance is observed, and, by the large, the capacitance values at positive charges are greater than at negative charges. As can be seen from Figs 4 and 9, such a shape of dependences is observed for TMU and N-MP solutions, while for DMFA solutions the values of C at positive and negative charges are approximately the same, and in the solutions based on DMAA the capacitance values are higher at negative charges than at positive. Apparently, the observed specific features are caused by the differences in the ion solvation in these solvents. The other cause is likely to be a difference in the structure of chemisorption layer molecules of solvents at positive and negative surface charges. To give an ultimate answer to these questions, it is appropriate to carry out further studies in the systems based on the mentioned solvents on Hg and mercury-like metals. The capacitance curves shown in Fig. 9 are referred to the geometrical surface area of the electrode. The data obtained are insufficient for plotting the Parsons-Zobel dependences and calculating the real surface area of the electrode. However, the estimate of real surface area can be carried out on the basis of the Gouy-Ghapman-Grahame model [1] from the data on the bending points near the minima in the
Electrical double layer on renewed iron electrodes in solutions of organic solvents C us E curves in dilute solutions. For simplicity, we assume that the capacitance of the double layer dense part CI is independent of the electrode potential. [In the general case, the Cl us Q curve can be approximated, eg by some polynomial, and following equations (lt(3) can be used in numerical calculations. At the same time, condition C, = const in the first approximation is valid for systems studied in the potential range close to E, _,I (at more cathodic or more anodic potentials than E,,o depending on the nature of the solvent).] Then the overall capacitance C and the potential cp (with respect to the potential of zero charge) can be represented as c =
C,F&A%
+ a2/(2RT) (1)
C, + FJm/(2RT)’ 2RT cp = 7 arcsinh
(2)
where A = ,,/e, E is the dielectric constant of the solvent; EO= 8.854 x lo-l2 F/m; c is the electrolyte concentration; 0 is the electrode charge, which acts as a parameter in equations (1) and (2); and the other values have their usual meaning. The condition of the bending on C, E-curve corresponds to d2C _...-
dq2 -
cp’c”- cp“C’ = o
(cp’)’
.
If we introduce the expression z = Jw, then, after transformations, equation (3) takes the form:
-5!G&o.
(4)
Therefore, by solving equation (4) with respect to z, we can calculate 0 values that correspond to the bending points in the C us E dependences. The values (pknd themselves are calculated from equation (2). Figure 10 shows the results of numerical calculations of (Pknd in C us E curves according to equations (4) and (2) as a function of C, at different electrolyte concentrations and E values. The values E = 23.06 and E = 78.3 model the solutions based on TMU and water, respectively. As follows from the figures, a decrease in E, all other conditions being equal, leads to a decrease in the difference lEbend- E,=o~. A comparison of the presented dependences with experimental data of Fig. 4 for TMU solutions (the position of the bending point with respect to E, =0) showed that such a method can be used for making approximate estimates of the electrode real surface area. The corresponding calculations for the other systems made it possible to conclude that the roughness factor of the renewed Fe surface ranges from 3.5 to 5.
685
In Fig. 11, by the analogy with the data for TMU solutions, the dependences of the surface charge of Fe, Hg,and Bi electrodes on the potential in DMFA solutions calculated from C us E curves are shown. The data for Hg and Bi are taken from [36] and [37], respectively. The dashed line in this figure represents a hypothetic 0 us E dependence for a Fe electrode calculated under the assumption of a weak interaction of this metal with DMFA molecules. The behaviour of this system is for the major part the same as the above-discussed behaviour of Pt and Pd electrodes in DMSO and AN solutions, and also that of Fe electrode in solutions based on TMU. A comparison of the behaviour of a Fe electrode in contact with studied solutions allows one to make a supposition, in our view quite reasonable, on the nature of the chemisorptive interaction between this metal and organic solvents. The case in point is that the obtained data can be qualitatively interpreted as a result of a strong donor-acceptor interaction [38] between the renewed metal surface and solvent molecules in the very beginning of their contact. However, after longer contact, the observed effects point to the destruction of the chemisorbed solvent molecules. This can explain the observed time dependences of the measured capacitance and the absence of any characteristic double-layer effects after a long contact of as Fe electrode with solutions based on organic solvents. Under these conditions, the behaviour of iron resembles that of a Pt electrode in aqueous solutions with organic substance additives. For solvents such as DMSO and AN, the destructive adsorption proceeds at considerable rates practically in the first seconds after the contact of the renewed Fe electrode surface with the solvent. For solvents of the amide nature, which are probably more resistive to the contact with a renewed Fe electrode surface, the effects of destructive adsorption manifest themselves at much longer times of contact. 4. Comparison of the experimental results in aqueous and nonaqueous solutions and the pzc values of Fe in water.
The values of E,=o for Fe in aqueous solutions presented in literature range from +0.03 V (in early works) to -0.7 to -0.9 V [8]. From our viewpoint, the reliability of the latter values is to a certain extent confirmed by their comparison with the results of the above-mentioned studies in solutions based on organic solvents. First of all, the values of E, =o obtained above for Fe electrodes in all the solvents lie at potentials more negative than those for a Hg electrode. This is in agreement with the lower [30] or close 1311electron work function for iron, as compared with mercury. Secondly, the difference between the zero charge potentials of iron and mercury in the studied solvents is about 0.5 V. For aqueous solutions, if we use the
V. A. Safonov et al.
686
b
I
0
I
I
I
I
5
10
15
20
C, / l.tF.cm-*
%Ud/V
0.3
a
0.2
0.1
I
I
I
I
I
I
5
10
15
20
25
C, / pF*cm-* Fig. 10. Dependences of (phd on CI in the 1, I-valent electrolyte solutions of different concentrations. l- 0.01, 2- 0.003, and 3-0.001 M; (a) E= 78.3 and (b) E = 23.06.
data of [9, 10, 391, this difference is somewhat lower (-0.3-0.4 V), and this effect should be associated with the organic molecule chemisorption on iron which proceeds with the negative end oriented to the metal. As follows from [8], the authors suppose that the values of E, _ o for iron in aqueous solutions are more positive than those for mercury: from - 0.1 to -0.15 V at pH l-8 in 0.01 M NarS04 (at higher pH values the authors observed a substantial shift of E ,,=o in the negative direction). Under these conditions, bearing in mind the difference between the electron work functions of these metals, it follows that the water dipoles should adsorb on iron with the positive end to the metal. Apparently, such an orientation of water molecules at the zero charge
potential on iron that has a strong affinity to oxygen is highly improbable. The main explanation for the positive shift of the values of E., = 0 for Fe observed in [8], can lie in the electrode pretreatment used by the authors. It follows from a number of studies (for example, see [40,41]) devoted to investigating the effect of different factors on the electron work function (heating at high temperatures, adsorption of oxygen, hydrogen, water vapours, etc.) that, depending on the kind of pretreatment, the value of W can substantially change. The other explanation can nevertheless consist in the presence of adsorbed oxygen on the surface, despite all the precautions taken [8] to prevent the oxidation of Fe in the course of experiments.
Electrical
double layer on renewed
iron electrodes
in solutions
of organic
solvents
687
9. L. E. Rybalka and D. I. Leikis, Elektrokhimiya 11, 1619 (1975).
10. L. E. Rybalka, D. I. Leikis and A. G. Zelinskii, Elektrokhimiya 12, 1340 (1976). 11. V. A. Safonov, S. A. Sokolov and V. M. Gerovich, Dokl. Akad. Nauk SSSR 299, 1438 (1988). 12. V. A. Safonov and S. A. Sokolov, Elektrokhimiya 21, 1317 (1991). 13. A. G. Zelinskii and R. Yu. Bek, Elektrokhimiya 21, 66 (1985). 14. D. S. Kamenetskaya, I. B. Piletskaya and V. I. Shiryaev, Zhelezo vysokoy stepeni chistoty (Ultra-high Purity Iron), Metallurgiya, Moscow (1978).
l.I/:.
Fig. 11. Charge (referred to potential dependences for (1) electrodes in solutions of 0.1 M line is the hypothetic curve for
the real surface area) us Hg, (2) Bi, and (3) Fe LiC104 in DMFA. Dashed Fe.
CONCLUSION The obtained data taken together show that the electrical double layer formed on a Fe electrode in
solutions based on TMU and a number of other solvents of a similar nature is characterized by certain specific features. The strong chemisorption interaction of solvent molecules with the surface can be explained in terms of these features. A comparison of the pzc values of Fe in nonaqueous solutions with those of Fe in aqueous solutions makes it possible to conclude that the pzc of a clean oxide-free Fe surface should lie in the potential interval of -0.7 to -0.9 V (SHE).
15. 0. A. Petrii, L. Yu. Luk’yanycheva and A. G. Zelinskii, Elektrokhimiya 24, 103 (1988).
16. T. N. Andersen, J. L. Anderson and H. Eyring, J. Phys. Chem 73, 3562 (1969). 17. Recommended Methods for Purification of Solvents and Test for Impurities (Edited by J. F. Coetzee), IUPAC Editions, Pergamon Press (1982). 18. Techniques in Electrochemistry (Edited by E. Yeager and A. J. Solkind), Wiley-Interscience, New York, Vol. 1 (1972). 19. V. A. Safonov, L. Yu. Komissarov and 0. A. Petrii, Zashchita metalloo 22, 212 (1986). 20. T. R. Agladze, Itogi nauki i tekhniki. Korroziya i zashchita ot korrozii (Achievements in Science and Technology. Corrosion and Corrosion Protection),
VINITI, Moscow, 9, 3 (1982). 21. J. Kruger, D. A. Shifler, J. F. Scanlon and P. J. Moran, Elektrokhimiya 31, 1087 (1995). 22. J. A. Riddic and W. B. Bunger, Organic Solvents, Wiley-Interscience, New York (1972). 23. R. Parsons and F. G. K. Zobel, J. Electroanal. Chem. 9, 333 (1965).
24. D. I. Leikis, K. V. Rybalka and E. S. Sevast’yanov, Adrorbtsiya i dvoinoi elektricheskii sloy u elektrokhimii (Adsorption and Double Electrical Layer in Electrochemistry), Nauka, Mowcow, p. 5 (1972). 25. D. C. Grahame and E. M. Coffin, J. Amer. Chem. Sot.
74, 1207 (1952). 26. D. C. Grahame, J. Amer. Chem. Sot. 74,4819 (1954). 27. I. A. Bagotskaya and A. M. Kalyuzhnaya, Elektrokhimiya 12, 1043 (1976).
28. L. M. Doubova and I. A. Bagotskaya, Elektrokhimiya 13, 64 (1976).
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(Edited by B. E. Conway, R. E. White and J. GM. Bockris), Plenum Press, N.Y., 16, Ch.1 (1985). 5. 0. A. Petrii. Itoai nauki i tekhniki. Elektrokhimiva (Achievements in Science and Technology. Electrochemistry), VINITI, Moscow, 12, 56 (1977). 6. 0. A. Petrii and I. G. Khomchenko, J. Electroanal. Chem. 106, 277 (1980).
7. E. Yu. Alekseeva, V. A. Safonov and 0. A. Petrii, Elektrokhimiya 20, 945 (1984).
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29. S. Trasatti, J. Electroanal. Chem. 33, 351 (1971). 30. V. S. Fomenko, Emissionnye svoistva materialov (Emission Properties of Materials), Naukova Dumka, Kiev (1981). 31. S. Trasatti, Chim. Ind. Milan 53, 559 (1971). 32. E. I. Mikhailova and Z. A. Iofa, Elektrokhimiya 6, 231 (1970).
33. V. V. Batrakov,
J. Dittrich
and
A. N. Popov,
Elektrokhimiya 8, 640 (1972).
34. V. A. Safonov, K. Jackowska and 0. .4. Petrii, Elektrokhimiya 11, 1628 (1975). 35. Ya. M. Kolotyrkin, Zashchita Metallov 3, 131 (1967). 36. V. D. Bezuglyi and L. A. Korshikov, Elektrokhimiya 1, 1422 (1965). 37. E. K. Petyarv and U. V. Palm, Elektrokhimiya 11, 313 (1975). 38. B. B. Damaskin and V. A. Safonov, Electrochim. Acta, (in press). 39. V. V. Batrakov and N. I. Naumova, Elektrokhimiya 15, 551 (1979). 40. G. F. Voronina, L. A. Larin and T. V. Kalish, Elektrokhimiya 16, 172 (1980).
41. M. E. Belyaeva, Elektrokhimiya 30, 611 (1994).