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Nickel adsorption on a platinized platinum electrode
Surface
530
NICKEL ADSORPTION Krzysztof
FRANASZCZUK
Chemist<~~ Depurtmenf, Received
ON A PLATINIZED
PLATINUM
ELECTRODE
and Jerzy SOBKOWSKI
Wursaw Unioersit.v, Zwirki
6 May 1988; accepted
Science 204 (1988) 530-536 North-Holland. Amsterdam
for publication
I Wiguty 101. 02489
Warsaw. Poland
2 June 19X8
The electrodeposition of nickel on a platinum electrode from a divalent nickel ion solution under UPD conditions has been studied by radiometric and voltammetric methods. The dependencies of surface concentration of adsorbed nickel on the electrode potential, nickel ion concentrations in acid and neutral solutions have been determined. From the radiometric and voltammetric data the electrosorption valency, y. has been calculated. The change of y with 8 has been discussed.
1. Introduction The formation of thin metal films on metallic or semiconducting electrodes is a convenient way for modifying electrode surfaces with respect to their electrocatalytic activity. In general, there are two models of the formation of a monolayer in underpotential deposition conditions. The first one is the adsorption model which takes into account the particle-substrate (ion-metal surface) interactions [l-6]. The second one is the model of nucleation and growth [7-111 based upon the possibility of two-dimensional nucleation leading to the phase formation energetically feasible at potentials more positive than the reversible Nernst potential for the bulk phase deposition [12]. It is recognized that both models are correct for an individual couple of metals but the phenomena controlling the rate of the process have to be identified for each studied system. The thermodynamic and structural properties of the underpotentially deposited metal monolayers have been studied for many systems based on the concept of a “submonolayer equilibrium potential” [13.14]. The Nernst-like expression describes the relation between the electrode potential and surface coverage:
where E( 0) is the submonolayer potential, cM-. is the concentration of ions in the solution, fM-+ and fM,: lta are the activity coefficients of ions in the 0039-6028/X8/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
K. Franasrczuk, J. Sobkowski / Nickel adsorphon on a plutmum electrode
531
solution and of atoms or ions deposited on the surface, respectively, E, is the standard potential when the formation of the monolayer is completed (B = l), i.e. the potential for the bulk deposition, and y is the electrosorption valency. It has been demonstrated [15] that at constant potential the electrosorption valency can describe the charge flow during the UPD process:
where q and r are the charge and surface concentration, respectively. The values of y vary from zero to the value of the ionic charge z. When y is very close to z the adsorbed species are almost completely discharged and covalently bounded to the substrate, whereas values of y lower than z indicate adsorption with a partial charge transfer [16]. Since the electrosorption valency determination requires experimental methods capable of separating the UPD mass flux from the flowing charge, an independent method of surface concentration determination of electrodeposited metal is needed. This requirement can be easily fulfilled by a radiometric method. The mechanism of electrochemical deposition of nickel on platinum, gold and glassy carbon was studied by Bozhkov and Rashkov [17]. It was found that (at pH = 4) nickel was deposited on platinum within the hydrogen adsorption potentials range, starting at a potential 0.22 V more positive than the Nernst potential of Ni. The degree of surface coverage was equal to 0.5. The electrodeposition of nickel is often accompanied by hydrogen evolution, formation of nickel hydroxides, etc. The problem is even more complicated in the case of electrodeposition on rough electrodes because of the change of the pH value in the vicinity of the electrode during its polarisation [18]. The aim of this study was to give some more information on the mechanism of nickel electrodeposition by the use of electrochemical as well as radiochemical methods.
2. Experimental The measurements have been carried out in OSM H,SO, and 0.5M Na,SO, solutions prepared from fourfold distilled water, with the third distillation carried out from permanganate solution. The nickel ion concentration was varied from 10e5M to 10p2M. The surface concentration of nickel, r, was determined by a radiometric technique using p--emitting “Ni. The equipment, procedure and calculation of y were similar to the experiments with tritium [19] since 63Ni emits ,L- radiation of low energy (Epm_11’=67 keV). The electrochemical activity of the platinized electrode depends on the conditions of platinum deposition [20]. Therefore, the electrode was platinized at a
532
K. Franaszczuk,
J. Sobkowski
/ Nickel adsorption
on u platinum
electrode
controlled potential of 0.25 V. The roughness factor of the electrode was equal to - 110. At the beginning of each experiment, the electrode was cycled repeatedly to obtain reproducible voltammograms. The electrodeposition of nickel on platinum was achieved by holding an electrode for a given time at a desired potential. All the r values are related to the real area of the electrode as determined from the charge required for hydrogen adsorption [21]. The values of the surface concentration of nickel have been calculated with an accuracy of +15%. All electrode potentials are referred to the hydrogen electrode in OSM H,SO, solution. The measurements have been carried out at ambient temperature (20 + 2” C).
3. Results and discussion The voltammograms of a platinum electrode in solutions of two different concentrations of nickel ions are given in fig. 1. For comparison, the voltammogram of the same electrode recorded for a OSM H,SO, solution free of any nickel species is also given. Only a slight shift of hydrogen adsorption and desorption peaks can be observed when nickel is present in the solution but the charges involved in both processes are the same as in the absence of nickel. No further changes, after holding the electrode at various potentials and for various times, in the shape of voltammograms have been registered. The radiometric data indicating the presence of labeled nickel on the Pt surface are given in fig. 2. Since no charge transfer accompanies the adsorption process it is clear that only divalent nickel ions are adsorbed. It is seen that the onset of nickel ion adsorption occurs at 0.4 V and increases rapidly as the electrode potential becomes more cathodic. Nickel ion adsorption is reversible with
) and Fig. 1. Cyclic voltammograms of a platinum electrode in a solution of OSM H,SO, (___ in the presence of 10e2M NiSO, (. .. .) and 10m3M NiSO, (- ~ -). Sweep rate: 50 mV s ‘.
K. Franaszcruk, J. Sobkowski / Nickel adsorption on a platinum electrode
533
Fig. 2. The dependence of the surface concentration of adsorbed 63Ni species in acid solution (0.5M H2S04) on adsorption time and electrode potential. Step by step procedure. 63NiS04 bulk concentration was equal to lo-*M.
respect to the electrode potential as well as to their concentration in the solution. The last conclusion follows from the data in fig. 3, where the decrease is shown of the surface concentration of adsorbed 63Ni ions after addition of an excess of non-radioactive nickel ions to the solution. This isotope dilution is possible only when the labeled 63Ni ions are replaced by the non-radioactive ions coming from the bulk of the solution and is an evidence of the reversibility of the adsorption process. To study the nickel reactions on the Pt electrode in more cathodic potentials the measurements were carried out in a 0.5M Na,SO, solution. The voltammetric curve of the Pt electrode in 0.5M Na,SO, differs markedly from the curve registered in 0.5M H,SO, (fig. 4). There are two peaks of oxygen (or Pt oxide) reduction. This is not the case when a smooth Pt electrode is used. The phenomenon is likely connected with the local change of pH in the vicinity of the rough electrode which can be treated as a porous one [lg]. r/cpm
t /min Fig. 3. The plot of the surface concentration of adsorbed 63Ni species versus time. The electrode potential was 50 mV. 63NiS04 bulk concentration was 10 -‘M. Arrow indicates the addition of non-labeled NiSO,.
534
K. Franasrcruk,
J. Sohkowski
/ Nickel adsorption on a plattnum
electrode
Fig. 4. Cyclic voltammograms of platinum electrode in 0.5M Na2S0, solution (-) and after addition of lO_‘M NiSO, (- - -). Basic curve in 0.5M H,SO, is marked with dotted line. Sweep rate for all curves was 50 mV s _ ’
In the presence of nickel the current during an anodic scan is markedly enhanced (see fig. 4). It is supposed that the extra charge observed is due to the oxidation process of the chemisorbed nickel species present on the Pt surface, although it cannot be excluded that this charge is connected with the oxidation of additionally adsorbed hydrogen on previously deposited nickel on the Pt surface. However, from the radiometric data shown in fig. 5 it follows that nickel is removed from the surface at these potentials where the net charge appears on the voltammetric curve. It is also seen that the adsorption
Fig. 5. The dependence solution on adsorption
of the surface concentration of adsorbed “Ni species in 0.5M Na,SO, time and electrode potential. Step by step procedure. 63NiS0, bulk concentration was equal to IO-‘MM.
K. Franaszczuk, J. Sobkowskr / Nickel adsorption on a platrnum electrode
535
of nickel species on the Pt surface is a reversible process with respect to the electrode potential. To estimate the electrosorption valency a set of experiments were carried out. At a given potential and after a definite time of nickel adsorption the solution had been replaced several times by the supporting electrolyte (during this washing procedure the electrode potential was kept constant). After this, voltammetric curves were recorded. The charge of oxidation of nickel species adsorbed on the electrode surface were calculated for various adsorption times, i.e. for different surface coverages. Simultaneously, the surface concentrations were determined from radiometric measurements. The surface coverage of the electrode, 8, was calculated assuming that the maximum surface concentration is equal to the number of adsorption sites on the surface of polycrystalline platinum electrode i.e. 1.3 X lOI Pt atoms/cm* [21]. The y values change with 8 and applied potential as shown in fig. 6. At positive potential (e.g. 0.1 V) the value of y is constant and independent of the electrode surface coverage 0. The value of y = 1 differs from the nickel ion valency and suggests that nickel is deposited in the form of hydroxyor sulphate complexes (NiOH+ or/and NiHSO:). For negative potentials closer to the Nernst potential of the Ni2+/Nio couple, the plots of y versus 8 are more complex. E.g. for E = - 0.3 V (see fig. 6) at first the y value decreases with 8, probably due to the increasing electrostatic repulsion between adions. This would imply that at higher coverages nickel adions are located somewhat further away from the platinum surface being weaker bound to it and stronger interacting with the solvent dipoles. Similar arguments were given by Schultze to explain the adsorption of copper on platinum [22]. The further increase in 0 causes an increase of y. This effect can be explained if a multilayer adsorption is assumed. However, further experimental data are necessary to elucidate this question.
1.3 1.2 1.1 20 a9 OB
0.7
eo.3
”
116 a5 1
0.5 e
Fig. 6. The plots of the electrosorption valency against coverage under different potentials. (OSM Na,SO, solution). 0 = ~/rcalculared, rCalculated = 1.3 X 10’s molecules
electrode cmd2.
536
K. Franaszczuk, J. Sohkowki
/ Nickel adsorptwn on a platinum electrode
Acknowledgement This work has been supported by scientific program CPBP 01.15. We are grateful for the gift of glass scintillators from the Nuclear Enterprises Ltd. (Edinburgh, Scotland).
References [l] A.K. Vijh. Surface Sci. 47 (1975) 709. 121 W.J. Lorenz, H.D. Herman, N. Wtitherich and F. Hilbert, J. Electrochem. Sot. 121 (1974) 1167. [3] K. Jiitner and W.J. Lorenz, Z. Phys. Chem. 122 (1980) 163. [4] K. Jtitner, G. Staikov, W.J. Lorenz and E. Schmidt, J. Electroanal. Chem. 80 (1977) 67. [5] K. Engelsmann, W.J. Lorenz and E Schmidt, J. Electroanal. Chem. 114 (1980) 11. [6] W.J. Lorenz, E. Schmidt, G. Staikov and H. Bort, Faraday Symp. Chem. Sot. 12 (1977) 14. [7] A. Bewick and B. Thomas, J. Electroanal. Chem. 65 (1975) 911. (81 A. Bewick and B. Thomas, J. Electroanal. Chem. 70 (1976) 239. [9] A. Bewick and B. Thomas, J. Electroanal. Chem. 84 (1977) 127. [lo] A. Bewick and B. Thomas, J. Electroanal. Chem. 85 (1977) 329. [II] A. Bewick, J. JoviCeviC and B. Thomas, Faraday Symp. Chem. Sot. 12 (1977) 24. [12] D.M. Kolb, in: Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Eds. H. Gerischer and C.W. Tobias (Wiley, New York, 1978). [13] S. Fletcher, J. Electroanal. Chem. 118 (1981) 419. [14] J.A. Harrison and H.R. Thirsk, Electroanal. Chem. 5 (1971) 67. [15] J.W. Schultze and K.J. Vetter, J. Electroanal. Chem. 44 (1973) 63. [16] G. Kokkinidis, J. Electroanal. Chem. 201 (1986) 217. (171 C. Bozhkov and S. Rashkov, Extended Abstract of the 37th ISE Meeting, Vol. 2, Vilnius, 1986, p. 49. [18] Y.C. Chang and G. Prentice, Electrochim. Acta 31 (1986) 579. [19] A. Wieckowski, J. Electrochem. Sot. 122 (1975) 252. [20] B.I. Podlovchenko and R.P. Petukhova, Elektrokhimija 6 (1970) 198. [21] R. Woods, in: Electroanalytical Chemistry. Vol. 9, Ed. A.J. Bard (Dekker. New York. 1976). [22] J.W. Schultze, Ber. Bunsenges. Phys. Chem. 74 (1970) 705.
530
NICKEL ADSORPTION Krzysztof
FRANASZCZUK
Chemist<~~ Depurtmenf, Received
ON A PLATINIZED
PLATINUM
ELECTRODE
and Jerzy SOBKOWSKI
Wursaw Unioersit.v, Zwirki
6 May 1988; accepted
Science 204 (1988) 530-536 North-Holland. Amsterdam
for publication
I Wiguty 101. 02489
Warsaw. Poland
2 June 19X8
The electrodeposition of nickel on a platinum electrode from a divalent nickel ion solution under UPD conditions has been studied by radiometric and voltammetric methods. The dependencies of surface concentration of adsorbed nickel on the electrode potential, nickel ion concentrations in acid and neutral solutions have been determined. From the radiometric and voltammetric data the electrosorption valency, y. has been calculated. The change of y with 8 has been discussed.
1. Introduction The formation of thin metal films on metallic or semiconducting electrodes is a convenient way for modifying electrode surfaces with respect to their electrocatalytic activity. In general, there are two models of the formation of a monolayer in underpotential deposition conditions. The first one is the adsorption model which takes into account the particle-substrate (ion-metal surface) interactions [l-6]. The second one is the model of nucleation and growth [7-111 based upon the possibility of two-dimensional nucleation leading to the phase formation energetically feasible at potentials more positive than the reversible Nernst potential for the bulk phase deposition [12]. It is recognized that both models are correct for an individual couple of metals but the phenomena controlling the rate of the process have to be identified for each studied system. The thermodynamic and structural properties of the underpotentially deposited metal monolayers have been studied for many systems based on the concept of a “submonolayer equilibrium potential” [13.14]. The Nernst-like expression describes the relation between the electrode potential and surface coverage:
where E( 0) is the submonolayer potential, cM-. is the concentration of ions in the solution, fM-+ and fM,: lta are the activity coefficients of ions in the 0039-6028/X8/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
K. Franasrczuk, J. Sobkowski / Nickel adsorphon on a plutmum electrode
531
solution and of atoms or ions deposited on the surface, respectively, E, is the standard potential when the formation of the monolayer is completed (B = l), i.e. the potential for the bulk deposition, and y is the electrosorption valency. It has been demonstrated [15] that at constant potential the electrosorption valency can describe the charge flow during the UPD process:
where q and r are the charge and surface concentration, respectively. The values of y vary from zero to the value of the ionic charge z. When y is very close to z the adsorbed species are almost completely discharged and covalently bounded to the substrate, whereas values of y lower than z indicate adsorption with a partial charge transfer [16]. Since the electrosorption valency determination requires experimental methods capable of separating the UPD mass flux from the flowing charge, an independent method of surface concentration determination of electrodeposited metal is needed. This requirement can be easily fulfilled by a radiometric method. The mechanism of electrochemical deposition of nickel on platinum, gold and glassy carbon was studied by Bozhkov and Rashkov [17]. It was found that (at pH = 4) nickel was deposited on platinum within the hydrogen adsorption potentials range, starting at a potential 0.22 V more positive than the Nernst potential of Ni. The degree of surface coverage was equal to 0.5. The electrodeposition of nickel is often accompanied by hydrogen evolution, formation of nickel hydroxides, etc. The problem is even more complicated in the case of electrodeposition on rough electrodes because of the change of the pH value in the vicinity of the electrode during its polarisation [18]. The aim of this study was to give some more information on the mechanism of nickel electrodeposition by the use of electrochemical as well as radiochemical methods.
2. Experimental The measurements have been carried out in OSM H,SO, and 0.5M Na,SO, solutions prepared from fourfold distilled water, with the third distillation carried out from permanganate solution. The nickel ion concentration was varied from 10e5M to 10p2M. The surface concentration of nickel, r, was determined by a radiometric technique using p--emitting “Ni. The equipment, procedure and calculation of y were similar to the experiments with tritium [19] since 63Ni emits ,L- radiation of low energy (Epm_11’=67 keV). The electrochemical activity of the platinized electrode depends on the conditions of platinum deposition [20]. Therefore, the electrode was platinized at a
532
K. Franaszczuk,
J. Sobkowski
/ Nickel adsorption
on u platinum
electrode
controlled potential of 0.25 V. The roughness factor of the electrode was equal to - 110. At the beginning of each experiment, the electrode was cycled repeatedly to obtain reproducible voltammograms. The electrodeposition of nickel on platinum was achieved by holding an electrode for a given time at a desired potential. All the r values are related to the real area of the electrode as determined from the charge required for hydrogen adsorption [21]. The values of the surface concentration of nickel have been calculated with an accuracy of +15%. All electrode potentials are referred to the hydrogen electrode in OSM H,SO, solution. The measurements have been carried out at ambient temperature (20 + 2” C).
3. Results and discussion The voltammograms of a platinum electrode in solutions of two different concentrations of nickel ions are given in fig. 1. For comparison, the voltammogram of the same electrode recorded for a OSM H,SO, solution free of any nickel species is also given. Only a slight shift of hydrogen adsorption and desorption peaks can be observed when nickel is present in the solution but the charges involved in both processes are the same as in the absence of nickel. No further changes, after holding the electrode at various potentials and for various times, in the shape of voltammograms have been registered. The radiometric data indicating the presence of labeled nickel on the Pt surface are given in fig. 2. Since no charge transfer accompanies the adsorption process it is clear that only divalent nickel ions are adsorbed. It is seen that the onset of nickel ion adsorption occurs at 0.4 V and increases rapidly as the electrode potential becomes more cathodic. Nickel ion adsorption is reversible with
) and Fig. 1. Cyclic voltammograms of a platinum electrode in a solution of OSM H,SO, (___ in the presence of 10e2M NiSO, (. .. .) and 10m3M NiSO, (- ~ -). Sweep rate: 50 mV s ‘.
K. Franaszcruk, J. Sobkowski / Nickel adsorption on a platinum electrode
533
Fig. 2. The dependence of the surface concentration of adsorbed 63Ni species in acid solution (0.5M H2S04) on adsorption time and electrode potential. Step by step procedure. 63NiS04 bulk concentration was equal to lo-*M.
respect to the electrode potential as well as to their concentration in the solution. The last conclusion follows from the data in fig. 3, where the decrease is shown of the surface concentration of adsorbed 63Ni ions after addition of an excess of non-radioactive nickel ions to the solution. This isotope dilution is possible only when the labeled 63Ni ions are replaced by the non-radioactive ions coming from the bulk of the solution and is an evidence of the reversibility of the adsorption process. To study the nickel reactions on the Pt electrode in more cathodic potentials the measurements were carried out in a 0.5M Na,SO, solution. The voltammetric curve of the Pt electrode in 0.5M Na,SO, differs markedly from the curve registered in 0.5M H,SO, (fig. 4). There are two peaks of oxygen (or Pt oxide) reduction. This is not the case when a smooth Pt electrode is used. The phenomenon is likely connected with the local change of pH in the vicinity of the rough electrode which can be treated as a porous one [lg]. r/cpm
t /min Fig. 3. The plot of the surface concentration of adsorbed 63Ni species versus time. The electrode potential was 50 mV. 63NiS04 bulk concentration was 10 -‘M. Arrow indicates the addition of non-labeled NiSO,.
534
K. Franasrcruk,
J. Sohkowski
/ Nickel adsorption on a plattnum
electrode
Fig. 4. Cyclic voltammograms of platinum electrode in 0.5M Na2S0, solution (-) and after addition of lO_‘M NiSO, (- - -). Basic curve in 0.5M H,SO, is marked with dotted line. Sweep rate for all curves was 50 mV s _ ’
In the presence of nickel the current during an anodic scan is markedly enhanced (see fig. 4). It is supposed that the extra charge observed is due to the oxidation process of the chemisorbed nickel species present on the Pt surface, although it cannot be excluded that this charge is connected with the oxidation of additionally adsorbed hydrogen on previously deposited nickel on the Pt surface. However, from the radiometric data shown in fig. 5 it follows that nickel is removed from the surface at these potentials where the net charge appears on the voltammetric curve. It is also seen that the adsorption
Fig. 5. The dependence solution on adsorption
of the surface concentration of adsorbed “Ni species in 0.5M Na,SO, time and electrode potential. Step by step procedure. 63NiS0, bulk concentration was equal to IO-‘MM.
K. Franaszczuk, J. Sobkowskr / Nickel adsorption on a platrnum electrode
535
of nickel species on the Pt surface is a reversible process with respect to the electrode potential. To estimate the electrosorption valency a set of experiments were carried out. At a given potential and after a definite time of nickel adsorption the solution had been replaced several times by the supporting electrolyte (during this washing procedure the electrode potential was kept constant). After this, voltammetric curves were recorded. The charge of oxidation of nickel species adsorbed on the electrode surface were calculated for various adsorption times, i.e. for different surface coverages. Simultaneously, the surface concentrations were determined from radiometric measurements. The surface coverage of the electrode, 8, was calculated assuming that the maximum surface concentration is equal to the number of adsorption sites on the surface of polycrystalline platinum electrode i.e. 1.3 X lOI Pt atoms/cm* [21]. The y values change with 8 and applied potential as shown in fig. 6. At positive potential (e.g. 0.1 V) the value of y is constant and independent of the electrode surface coverage 0. The value of y = 1 differs from the nickel ion valency and suggests that nickel is deposited in the form of hydroxyor sulphate complexes (NiOH+ or/and NiHSO:). For negative potentials closer to the Nernst potential of the Ni2+/Nio couple, the plots of y versus 8 are more complex. E.g. for E = - 0.3 V (see fig. 6) at first the y value decreases with 8, probably due to the increasing electrostatic repulsion between adions. This would imply that at higher coverages nickel adions are located somewhat further away from the platinum surface being weaker bound to it and stronger interacting with the solvent dipoles. Similar arguments were given by Schultze to explain the adsorption of copper on platinum [22]. The further increase in 0 causes an increase of y. This effect can be explained if a multilayer adsorption is assumed. However, further experimental data are necessary to elucidate this question.
1.3 1.2 1.1 20 a9 OB
0.7
eo.3
”
116 a5 1
0.5 e
Fig. 6. The plots of the electrosorption valency against coverage under different potentials. (OSM Na,SO, solution). 0 = ~/rcalculared, rCalculated = 1.3 X 10’s molecules
electrode cmd2.
536
K. Franaszczuk, J. Sohkowki
/ Nickel adsorptwn on a platinum electrode
Acknowledgement This work has been supported by scientific program CPBP 01.15. We are grateful for the gift of glass scintillators from the Nuclear Enterprises Ltd. (Edinburgh, Scotland).
References [l] A.K. Vijh. Surface Sci. 47 (1975) 709. 121 W.J. Lorenz, H.D. Herman, N. Wtitherich and F. Hilbert, J. Electrochem. Sot. 121 (1974) 1167. [3] K. Jiitner and W.J. Lorenz, Z. Phys. Chem. 122 (1980) 163. [4] K. Jtitner, G. Staikov, W.J. Lorenz and E. Schmidt, J. Electroanal. Chem. 80 (1977) 67. [5] K. Engelsmann, W.J. Lorenz and E Schmidt, J. Electroanal. Chem. 114 (1980) 11. [6] W.J. Lorenz, E. Schmidt, G. Staikov and H. Bort, Faraday Symp. Chem. Sot. 12 (1977) 14. [7] A. Bewick and B. Thomas, J. Electroanal. Chem. 65 (1975) 911. (81 A. Bewick and B. Thomas, J. Electroanal. Chem. 70 (1976) 239. [9] A. Bewick and B. Thomas, J. Electroanal. Chem. 84 (1977) 127. [lo] A. Bewick and B. Thomas, J. Electroanal. Chem. 85 (1977) 329. [II] A. Bewick, J. JoviCeviC and B. Thomas, Faraday Symp. Chem. Sot. 12 (1977) 24. [12] D.M. Kolb, in: Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Eds. H. Gerischer and C.W. Tobias (Wiley, New York, 1978). [13] S. Fletcher, J. Electroanal. Chem. 118 (1981) 419. [14] J.A. Harrison and H.R. Thirsk, Electroanal. Chem. 5 (1971) 67. [15] J.W. Schultze and K.J. Vetter, J. Electroanal. Chem. 44 (1973) 63. [16] G. Kokkinidis, J. Electroanal. Chem. 201 (1986) 217. (171 C. Bozhkov and S. Rashkov, Extended Abstract of the 37th ISE Meeting, Vol. 2, Vilnius, 1986, p. 49. [18] Y.C. Chang and G. Prentice, Electrochim. Acta 31 (1986) 579. [19] A. Wieckowski, J. Electrochem. Sot. 122 (1975) 252. [20] B.I. Podlovchenko and R.P. Petukhova, Elektrokhimija 6 (1970) 198. [21] R. Woods, in: Electroanalytical Chemistry. Vol. 9, Ed. A.J. Bard (Dekker. New York. 1976). [22] J.W. Schultze, Ber. Bunsenges. Phys. Chem. 74 (1970) 705.