- Email: [email protected]
Rotating ring disk electrode in molten chloride
ROTATING
RING
HIDEKI Department
of Nuclear
YABE,
Engineering,
(Received
DISK ELECTRODE CHLORIDE
7 December
EMA and YASUHIKO
KEIKO
Faculty
IN MOLTEN
of Engineering, Japan
Kyoto
ITO
University,
1988, in reuised form 23 January
Sakyo-ku,
Kyoto
606,
1989)
Abstract-A rotating ring disk electrode (rrde) assembly for use in molten chloride at 723 K has been constructed. The electrode is made of nickel and high purity alumina. The rotating electrode sealing is made of glass cloth. By the use of this rotating ring disk electrode, cathodic reduction of CdCI, in molten LiCI-KC1 eutectic melt at 723 K has been investigated in detail. The rotating ring disk system voltammograms have been obtained at rotation velocities of SO&3000 ‘pm and composition ranging from 0.002 to 0.1 M. As a result of n-de experiments, the existence of monovalent cadmium ion has been confirmed in LiCI-KCl. The standard potential of Cd(II)/Cd(O) is 1.98 50.07 V, and that of Cd(II)/Cd(I) is 2.3 +O. 1 V us Li/Li + , respectively. From the dependence of the limiting current on the rotation velocity and concentration a diffusion coefficient of Cd(H) , ion has been estimated to be D,,,,,, = 2.1 (k 0.3) x lo- 5 cm% i (2.1( + 0.3) x 10-9 m*s-‘).
INTRODUCTION rotating ring disk electrode (rrde) technique is popular in electrochemical studies[l-31. It is one of the few convective systems for which the equations of fluid mechanics have been solved rigorously for steady state conditions. However, few rrde works have been devoted to the study of molten salt systems. In the present work, an n-de assembly for use in molten halide has been constructed and by the use of this assembly, cathodic reduction of CdCl, in molten LiCl-KC1 eutectic melt has been studied in detail. The
F
(
EXPERIMENTAL Experimental apparatus is shown in Fig.1. The vertical and horizontal reduction rate is identical. Scheme of cell assembly is shown in Fig. 2. The vessel is made of nickel, and kept gas-tight. As an electrolyte, LiCl-KC1 eutectic melt was used. All chemicals were reagent grade (Wako Chemical Co., Ltd). They were contained in a high purity alumina crucible (99.5% Also,, Nippon Kagaku Togyo Co., Ltd SSA-S), and were dried under vacuum at 573 K for a few days. After that, the eutectic mixture was melted under dry argon atmosphere and kept at the experimental temperature. By this procedure, initial oxide ion concentration can be reduced to about l-4 x 10m4 mole fraction. This value was determined by a rest potential measurement of the zirconia-air electrode at the initial condition. In this case, the cathodic residual current density by potential sweep method was less than 5 mA cm - * at scan rate of 500 mV s I. This result was almost the same as that obtained by conventional method[4]. Thus, this purification procedure was satisfactory enough in this experiment. As a quasi-reference electrode, zirconia-air electrode with large zirconia constant K’ (> 10 000)[5] was used. When the constant K’ is large, this electrode does not respond to oxide ion concentration, and it showed almost con-
t
’
’
I
30cm
Fig. 1. Experimental apparatus: (A) rrde; (B) holder; (C) electrodes; (D) jack; (E) motor; (F) cooling fan; (G) electric furnance; (H) cooling water
stant potential during the measurements[S]. Whereas, when the constant IS is small, it responds to the oxide ion concentration obeying modified Nernst equation[S]. For potential calibration of the quasi-reference electrode, a lithium electrode Li/Li+ in the same electrolyte was used, for which a little amount of lithium (about 1 C) was electrodeposited on the nickel wire before addition of CdCl,. This procedure did not give any noticeable impurity effect on an electrolyte condition. The lithium electrode thus prepared showed good reproducibility, which potential is 2.55 V us Ag/Ag+ (1 mol % AgCl). As an anode, glassy carbon was used. A zirconia-air electrode with small zirconia constant K’ (= MOO) was used as an oxide ion indicator. Figure 3 shows schematic section of ring disk electrode and sealing. The vertical and horizontal
1479
1480
H. YABE et al.
cloth (98.92% SiOr, Nihon Muki Co., Ltd Siliglass cloth BCS-665), stainless steel cylinder, glass wool (98.92% SiO,, Nihon Muki Co., Ltd Siliglass 9~ wool), ceramic paper (50% alumina-50% silica, Taiyo Chemical Co., Ltd., NSP-025) and Teflon cap. After melting the salt, cadmium chloride was added at the concentrations ranging from 0.002 to 0.10 M (:mol 1- ‘) (0.0067-0.33 mol%). For another experiment, cadmium chloride and cadmium metal were added together in the melt. The rotating ring disk system voltammograms have been obtained at rotation velocities ranging from 500 to 3000rpm (52.4-314.1 rad s-l). Sweep rates were usually 10 mV s-i. By this sweep rate, polarization curve was almost the same as that obtained by constant potential electrolysis method. Furthermore, the other workers[3,6,7] also used this scan rate. Thus, this sweep rate is satisfactory enough to obtain steady state polarization curves. All experiments were conducted at 723 K.
Fig. 2. Scheme of cell assembly: (A)rotating ring disk electrode; (B) counter electrode;(C) reference electrode; (D)nickel wire; (E) zirconia electrode; (F) thermocouple, (G) gas inlet, (H) gas outlet
RESULTS
AND
DISCUSSION
Figure 4 shows the disk current-disk potential curve and the corresponding ring current-disk potential curve obtained with the electrode rotation velocity of 3000 rpm (314.1 rads-i) at 0.002 M CdCl,. In Fig. 4, ohmic drop and residual current are corrected. The disk electrode was polarized from 2.39 to 1.29 V us Li/Li+ with a potential scan rate of 10 mVs_’ while the potential of the ring electrode was kept at 2.455 V to oxidize the cadmium. The disk current-disk potential curve in Fig.4 clearly shows two plateaux, and the ring current-disk potential curve also shows two plateaux. Here, we call the plateau between 2.0 and 2.4 V as first plateau and the plateau between 1.7 and 2.0 V as second plateau. Equation (1) is a theoretical relation among diffusion limiting current density i,, number of electrons
Fig. 3. Schematic section of ring disk electrode and sealing:
(A) disk electrode; (B) ring electrode; (C) alumina insulator;
(D) nickel holder; (E) glass cloth, (F)stainless steel cylinder; (G) glass wool; (H) ceramic paper; (I) teflon cap
reduction rate is identical. The electrodes are made of nickel ring, nickel disk, and high purity alumina insulator (99.5% Al,O,, Nippon Kagaku Togyo Co., Ltd SSA-S). The inner diameter of nickel ring is 1.10 cm and outer diameter is 1.50 cm. The diameter of nickel disk is 0.70 cm. The electrode was mechanically polished with silicon carbide paper and a-alumina particle (Baikowski International Corp.) which mean size was 1.0 pm. The rrde and rotating stem are made of nickel and high purity alumina insulator (99.5% Al,O,, Nippon Kagaku Togyo Co., Ltd SSA-S). By a careful observation after the experiment, it was confirmed that there was no problem of salt creeping up the rotating stem of the electrode and solidifying. The rotating electrode sealings were completed using glass
kb ., P-q 1.0
Disk
2.0
1.5
Potential
ED /
2.5
V vs. Li/Li+
Fig. 4. The disk current-disk potential (ID-ED) curve and the corresponding ring current+lisk potential (Is-E,) curve. (3000 rpm (314.1 rad s-l); 0.002M CdCI,; 723 K; ring potentia112.455 V vs Li/Li+).
Rotating ring disk electrode in molten chloride
1481
H, Faraday constant F, diffusion coefficient D, kinematic viscosity v, angular velocity of the disk w and bulk concentration C[S]: i, = 0.62nFD213v
-1’6w’/2C.
(11
From this equation, diffusion limiting current density is proportional to the square root of rotation velocity and bulk concentration. Figure 5 shows the relation between square root of rotation velocity &*(rad”* s-1/z) and the limiting current density i, (mA cm-*) at first plateau obtained at several definite concentration C(M)” of CdCI,. The limiting current density is proportional to the square root of rotation velocity. Figure 6 shows the relation between square root of rotation velocity &z and the limiting current density i, at second plateau obtained at several definite concentrations C of CdCl,. The limiting current density here is also proportional to the square root of rotation velocity. These results show that this rotating ring disk electrode assembly is available for use in molten chloride. Figure 7 shows concentration dependence of the limiting current density at first plateau obtained at several definite rotation velocity, and Fig. 8 shows
0
: 0.015
.
: 0.010
.f
-0
0.02
0.01
Concentration
of
CdCl2
C
/ M
Fig. 7 Limiting current density vs concentration of CdCI, for different rotation velocity at first plateau.
M CdCIZ
A : 0.005 0
: 0.002
Concentration
.3
10
5
0
(Rototi on Velocity
IA)“’
15 / rod”’
20
0
: 0.015 : 0.010
M CdCl2
A : 0.005 0
CdCl2
C
/ M
Fig. 8. Limiting current density us concentration of CdCl, for different rotation velocity at second plateau.
s-‘/z
Fig. 5. Limiting current density us square root of rotation velocity for different concentration of CdCI, at first plateau.
.
of
: 0.002
concentration dependence of the limiting current density at second plateau. These two limiting current densities are both proportional to the concentration of Cd(I1). From Fig. 4, the value of the collection efficiency of first plateau estimated from Equation (3) is 0.38kO.02, which is in good agreement with the calculated values of 0.386 obtained from the given geometry[9]. However, the value of the collection efficiency of second plateau is 0.244.26. The collection efficiency of metal particle and gas is not good. Therefore, second plateau is attributed to the reaction: Cd(H) + 2e- = Cd(O).
(2)
The standard potential of Cd(II)/Cd(O) estimated from half-wave potential E,,, is 1.98kO.07 V us Li/Li+. The value for Cd(II)/Cd(O) is in good agreement with the value 2.004 V reported by Laitinen et aZ.[lO]. The first plateau can be explained by the reaction: (Rotation
Velocity
w)“’
/ rod”’
~~‘1’
Fig. 6 Limiting current density us square root of rota&n velocity for different concentration of CdCI, at second plateau.
Cd(I1) + e- = Cd(I).
(3)
The occurrence of reaction (3) in LiCI-KC1 was first proposed by Delimarskii et aZ.[ll]. The existence of Cd(I) in chloride melts were also reported in AlCl,-NaCl and AlCl,-NaCl-KC1 by Potts ef aI.[12]
1482
H. YABE
and Hames et aZ.[13], respectively. Our experimental results support their conclusions. Those papers suggest that the probable type of Cd(I) in chloride melts is Cdz2+ Therefore, the type of Cd(I) in our case is also probably Cd:+, though further studies are necessary to confirm it. The standard potential of Cd(II)/Cd(I) is thus estimated to be 2.3 *O.l V vs Li/Li+. Figure 9 shows the disk currentdisk potential curve and the corresponding ring current-disk potential curve obtained with the electrode rotation velocity of 1000 rpm (104.7 rads-‘) at 0.1 MCdCl,, when 0.05 M (calculated) Cd was added in the melt together. The figure shows the result obtained 16 days after addition of CdCl, and Cd. The disk electrode was polarized from 2.35 to 1.24 V with a potential scan rate of 10 mVs- ’ while the potential of the ring electrode was kept at 2.468 V to oxidize the cadmium. The disk current-disk potential curve clearly shows three plateaux, however the ring currentdisk potential curve does not show clear three plateaux. Furthermore, collection efficiencies of second and third plateau are not good. Thus, the third plateau might be attributed to the reaction: Cd(I) + e- = Cd(O). Cd(I) is considered reaction.
to be produced
(4) by the following
Cd(I1) + Cd(O) = 2Cd(I)
(5)
Figure 10 shows the relation between square root of rotation velocity 0112 and the normalized limiting current density i,C- ’ obtained at first and second plateau, respectively. The diffusion coefficient of Cd(I1) ion calculated from first plateau (Cd(I1) + e- = Cd(I), n= 1) by Equation (1) (v=O.O148 cm2 s- ‘, LiClKC1,723 K[14]) is 2.1(&0.3)x 10-5cm2s-‘, and that calculated from second plateau (Cd(I1) + 2e = Cd(O), n = 2) is 2.2( k 0.3) x lo-’ cm’ s- I, respectively. The results are in fairly good agreement with 1.68-2.08 x lo-’ cm2 s-l at 723 K reported by Laitinen et aI.[15, 163. 01
I
I
et al.
(Rotation
Velocity
w) l/z
/ &l/z
s-‘/2
Fig. 10. Normalized limiting current density us square of rotation velocity for first and second plateau.
root
CONCLUSIONS The main results obtained from the above ments are summarized as follows.
experi-
(1) A rotating ring disk electrode assembly for use in molten chloride at 723 K has been constructed in success. (2) The voltammogram showed two or three plateaux. These can be explained by the existence of Cd(I). Thus, the existence of monovalent cadmium has been confirmed in LiCl-KCl.
(3) The standard potential of Cd(II)/Cd(I) is 2.3fO.l VvsLi/Li+ (-0.25+0.1 Vvs Ag/Ag+), and that of Cd(II)/Cd(O) is 1.98 kO.07 V us Li/Li + (-0.57 f 0.07 V z)s Ag/Ag+), respectively. (4) From the dependence of the limiting current on the rotation velocity and concentration a diffusion coefficient of Cd(I1) ion has been estimated to be 2.1(&0.3)x 10ms cm2s-‘(2.1( +0.3)x 10m9 m’s_‘). Acknowledgement-This work was carried out under the support of a Grant-in-Aid from Japanese Ministry of Education, Science and Culture.
REFERENCES
-100 I
-E
Disk
Potential
ED /
I
V vs. Li/Li+
Fig. 9. The disk current-disk potential (I,&,) curve and the corresponding ring current-disk potential (I,PE,) curve. (1000 rpm (104.7rad s-l); 0.1 M CdCI,; 723 K; ring potential = 2.468 V us Li/Li * ; 16 days after addition of 0.05 M Cd)
1. V. Jovancicevic and J. O’M. Bockris, J. electrochem. Sot. 133. 1797 (1986). 2. J. R. White, J. appl. Elecrrochem. 17, 977 (1987). 3. M. Shirkhanzadeh and G. E. Thompson, Electrochim. Acra 33, 939 (1988). 4. H. A. Laitinen, W. S. Ferguson and R. A. Osteryoung, J. electrochem. Sot. 104, 516 (1957). 5. Y. Ito, H. Yabe, T. Nakai, K. Ema and J. Oishi, Electrochim. Acta 31, 1579 (1986). 6. R. M. Machado and T. W. Chapman, J. electrochem. Sot. 134, 385 (1987). 7. R. Kniidler, J. electrochem. Sot. 134, 1419 (1987). 8. B. Levich, Acta Physicochim. U.R.S.S. 17, 257 (1942). 9. W. J. Albery and S. Bruckenstein, Trans. Faraday Sot. 62,
1920 (1966). 10. H. A. Laitinen (1958).
and C. H. Liu, J. Am. Chem. Sot. 80, 1015
Rotating ring disk electrode in molten chloride II. Yu. K. Delimarskii, N. Kh. Tumanova and M. U. Prikhodko, Electrokhimiya 6, 556 (1970). 12. R. A. Potts, R.D. Barnes and J. D. Corbett, Inorg. Chem. 7, 2558 (1968). 13. I$ A. Hames and J. A. Plambeck, Can. J. Chem. 46,1727 (1.986). 14. G. J. Jam, R. P. T. Tomkins, C. B. Allen, J. R. Downey,
1483
Jr., G. L. Gardner, U. Krebs and S. K. Singer, J. phys. Chem. Rex Data 4, 871 (1975). 15. H. A. Laitinen and W. V. Ferguson, Anaf. Chem. 29, 4 (1957). 16. H. A. Laitinen and H. C. Gaur, Anal. Chem. Acta 18, l(1958).
RING
HIDEKI Department
of Nuclear
YABE,
Engineering,
(Received
DISK ELECTRODE CHLORIDE
7 December
EMA and YASUHIKO
KEIKO
Faculty
IN MOLTEN
of Engineering, Japan
Kyoto
ITO
University,
1988, in reuised form 23 January
Sakyo-ku,
Kyoto
606,
1989)
Abstract-A rotating ring disk electrode (rrde) assembly for use in molten chloride at 723 K has been constructed. The electrode is made of nickel and high purity alumina. The rotating electrode sealing is made of glass cloth. By the use of this rotating ring disk electrode, cathodic reduction of CdCI, in molten LiCI-KC1 eutectic melt at 723 K has been investigated in detail. The rotating ring disk system voltammograms have been obtained at rotation velocities of SO&3000 ‘pm and composition ranging from 0.002 to 0.1 M. As a result of n-de experiments, the existence of monovalent cadmium ion has been confirmed in LiCI-KCl. The standard potential of Cd(II)/Cd(O) is 1.98 50.07 V, and that of Cd(II)/Cd(I) is 2.3 +O. 1 V us Li/Li + , respectively. From the dependence of the limiting current on the rotation velocity and concentration a diffusion coefficient of Cd(H) , ion has been estimated to be D,,,,,, = 2.1 (k 0.3) x lo- 5 cm% i (2.1( + 0.3) x 10-9 m*s-‘).
INTRODUCTION rotating ring disk electrode (rrde) technique is popular in electrochemical studies[l-31. It is one of the few convective systems for which the equations of fluid mechanics have been solved rigorously for steady state conditions. However, few rrde works have been devoted to the study of molten salt systems. In the present work, an n-de assembly for use in molten halide has been constructed and by the use of this assembly, cathodic reduction of CdCl, in molten LiCl-KC1 eutectic melt has been studied in detail. The
F
(
EXPERIMENTAL Experimental apparatus is shown in Fig.1. The vertical and horizontal reduction rate is identical. Scheme of cell assembly is shown in Fig. 2. The vessel is made of nickel, and kept gas-tight. As an electrolyte, LiCl-KC1 eutectic melt was used. All chemicals were reagent grade (Wako Chemical Co., Ltd). They were contained in a high purity alumina crucible (99.5% Also,, Nippon Kagaku Togyo Co., Ltd SSA-S), and were dried under vacuum at 573 K for a few days. After that, the eutectic mixture was melted under dry argon atmosphere and kept at the experimental temperature. By this procedure, initial oxide ion concentration can be reduced to about l-4 x 10m4 mole fraction. This value was determined by a rest potential measurement of the zirconia-air electrode at the initial condition. In this case, the cathodic residual current density by potential sweep method was less than 5 mA cm - * at scan rate of 500 mV s I. This result was almost the same as that obtained by conventional method[4]. Thus, this purification procedure was satisfactory enough in this experiment. As a quasi-reference electrode, zirconia-air electrode with large zirconia constant K’ (> 10 000)[5] was used. When the constant K’ is large, this electrode does not respond to oxide ion concentration, and it showed almost con-
t
’
’
I
30cm
Fig. 1. Experimental apparatus: (A) rrde; (B) holder; (C) electrodes; (D) jack; (E) motor; (F) cooling fan; (G) electric furnance; (H) cooling water
stant potential during the measurements[S]. Whereas, when the constant IS is small, it responds to the oxide ion concentration obeying modified Nernst equation[S]. For potential calibration of the quasi-reference electrode, a lithium electrode Li/Li+ in the same electrolyte was used, for which a little amount of lithium (about 1 C) was electrodeposited on the nickel wire before addition of CdCl,. This procedure did not give any noticeable impurity effect on an electrolyte condition. The lithium electrode thus prepared showed good reproducibility, which potential is 2.55 V us Ag/Ag+ (1 mol % AgCl). As an anode, glassy carbon was used. A zirconia-air electrode with small zirconia constant K’ (= MOO) was used as an oxide ion indicator. Figure 3 shows schematic section of ring disk electrode and sealing. The vertical and horizontal
1479
1480
H. YABE et al.
cloth (98.92% SiOr, Nihon Muki Co., Ltd Siliglass cloth BCS-665), stainless steel cylinder, glass wool (98.92% SiO,, Nihon Muki Co., Ltd Siliglass 9~ wool), ceramic paper (50% alumina-50% silica, Taiyo Chemical Co., Ltd., NSP-025) and Teflon cap. After melting the salt, cadmium chloride was added at the concentrations ranging from 0.002 to 0.10 M (:mol 1- ‘) (0.0067-0.33 mol%). For another experiment, cadmium chloride and cadmium metal were added together in the melt. The rotating ring disk system voltammograms have been obtained at rotation velocities ranging from 500 to 3000rpm (52.4-314.1 rad s-l). Sweep rates were usually 10 mV s-i. By this sweep rate, polarization curve was almost the same as that obtained by constant potential electrolysis method. Furthermore, the other workers[3,6,7] also used this scan rate. Thus, this sweep rate is satisfactory enough to obtain steady state polarization curves. All experiments were conducted at 723 K.
Fig. 2. Scheme of cell assembly: (A)rotating ring disk electrode; (B) counter electrode;(C) reference electrode; (D)nickel wire; (E) zirconia electrode; (F) thermocouple, (G) gas inlet, (H) gas outlet
RESULTS
AND
DISCUSSION
Figure 4 shows the disk current-disk potential curve and the corresponding ring current-disk potential curve obtained with the electrode rotation velocity of 3000 rpm (314.1 rads-i) at 0.002 M CdCl,. In Fig. 4, ohmic drop and residual current are corrected. The disk electrode was polarized from 2.39 to 1.29 V us Li/Li+ with a potential scan rate of 10 mVs_’ while the potential of the ring electrode was kept at 2.455 V to oxidize the cadmium. The disk current-disk potential curve in Fig.4 clearly shows two plateaux, and the ring current-disk potential curve also shows two plateaux. Here, we call the plateau between 2.0 and 2.4 V as first plateau and the plateau between 1.7 and 2.0 V as second plateau. Equation (1) is a theoretical relation among diffusion limiting current density i,, number of electrons
Fig. 3. Schematic section of ring disk electrode and sealing:
(A) disk electrode; (B) ring electrode; (C) alumina insulator;
(D) nickel holder; (E) glass cloth, (F)stainless steel cylinder; (G) glass wool; (H) ceramic paper; (I) teflon cap
reduction rate is identical. The electrodes are made of nickel ring, nickel disk, and high purity alumina insulator (99.5% Al,O,, Nippon Kagaku Togyo Co., Ltd SSA-S). The inner diameter of nickel ring is 1.10 cm and outer diameter is 1.50 cm. The diameter of nickel disk is 0.70 cm. The electrode was mechanically polished with silicon carbide paper and a-alumina particle (Baikowski International Corp.) which mean size was 1.0 pm. The rrde and rotating stem are made of nickel and high purity alumina insulator (99.5% Al,O,, Nippon Kagaku Togyo Co., Ltd SSA-S). By a careful observation after the experiment, it was confirmed that there was no problem of salt creeping up the rotating stem of the electrode and solidifying. The rotating electrode sealings were completed using glass
kb ., P-q 1.0
Disk
2.0
1.5
Potential
ED /
2.5
V vs. Li/Li+
Fig. 4. The disk current-disk potential (ID-ED) curve and the corresponding ring current+lisk potential (Is-E,) curve. (3000 rpm (314.1 rad s-l); 0.002M CdCI,; 723 K; ring potentia112.455 V vs Li/Li+).
Rotating ring disk electrode in molten chloride
1481
H, Faraday constant F, diffusion coefficient D, kinematic viscosity v, angular velocity of the disk w and bulk concentration C[S]: i, = 0.62nFD213v
-1’6w’/2C.
(11
From this equation, diffusion limiting current density is proportional to the square root of rotation velocity and bulk concentration. Figure 5 shows the relation between square root of rotation velocity &*(rad”* s-1/z) and the limiting current density i, (mA cm-*) at first plateau obtained at several definite concentration C(M)” of CdCI,. The limiting current density is proportional to the square root of rotation velocity. Figure 6 shows the relation between square root of rotation velocity &z and the limiting current density i, at second plateau obtained at several definite concentrations C of CdCl,. The limiting current density here is also proportional to the square root of rotation velocity. These results show that this rotating ring disk electrode assembly is available for use in molten chloride. Figure 7 shows concentration dependence of the limiting current density at first plateau obtained at several definite rotation velocity, and Fig. 8 shows
0
: 0.015
.
: 0.010
.f
-0
0.02
0.01
Concentration
of
CdCl2
C
/ M
Fig. 7 Limiting current density vs concentration of CdCI, for different rotation velocity at first plateau.
M CdCIZ
A : 0.005 0
: 0.002
Concentration
.3
10
5
0
(Rototi on Velocity
IA)“’
15 / rod”’
20
0
: 0.015 : 0.010
M CdCl2
A : 0.005 0
CdCl2
C
/ M
Fig. 8. Limiting current density us concentration of CdCl, for different rotation velocity at second plateau.
s-‘/z
Fig. 5. Limiting current density us square root of rotation velocity for different concentration of CdCI, at first plateau.
.
of
: 0.002
concentration dependence of the limiting current density at second plateau. These two limiting current densities are both proportional to the concentration of Cd(I1). From Fig. 4, the value of the collection efficiency of first plateau estimated from Equation (3) is 0.38kO.02, which is in good agreement with the calculated values of 0.386 obtained from the given geometry[9]. However, the value of the collection efficiency of second plateau is 0.244.26. The collection efficiency of metal particle and gas is not good. Therefore, second plateau is attributed to the reaction: Cd(H) + 2e- = Cd(O).
(2)
The standard potential of Cd(II)/Cd(O) estimated from half-wave potential E,,, is 1.98kO.07 V us Li/Li+. The value for Cd(II)/Cd(O) is in good agreement with the value 2.004 V reported by Laitinen et aZ.[lO]. The first plateau can be explained by the reaction: (Rotation
Velocity
w)“’
/ rod”’
~~‘1’
Fig. 6 Limiting current density us square root of rota&n velocity for different concentration of CdCI, at second plateau.
Cd(I1) + e- = Cd(I).
(3)
The occurrence of reaction (3) in LiCI-KC1 was first proposed by Delimarskii et aZ.[ll]. The existence of Cd(I) in chloride melts were also reported in AlCl,-NaCl and AlCl,-NaCl-KC1 by Potts ef aI.[12]
1482
H. YABE
and Hames et aZ.[13], respectively. Our experimental results support their conclusions. Those papers suggest that the probable type of Cd(I) in chloride melts is Cdz2+ Therefore, the type of Cd(I) in our case is also probably Cd:+, though further studies are necessary to confirm it. The standard potential of Cd(II)/Cd(I) is thus estimated to be 2.3 *O.l V vs Li/Li+. Figure 9 shows the disk currentdisk potential curve and the corresponding ring current-disk potential curve obtained with the electrode rotation velocity of 1000 rpm (104.7 rads-‘) at 0.1 MCdCl,, when 0.05 M (calculated) Cd was added in the melt together. The figure shows the result obtained 16 days after addition of CdCl, and Cd. The disk electrode was polarized from 2.35 to 1.24 V with a potential scan rate of 10 mVs- ’ while the potential of the ring electrode was kept at 2.468 V to oxidize the cadmium. The disk current-disk potential curve clearly shows three plateaux, however the ring currentdisk potential curve does not show clear three plateaux. Furthermore, collection efficiencies of second and third plateau are not good. Thus, the third plateau might be attributed to the reaction: Cd(I) + e- = Cd(O). Cd(I) is considered reaction.
to be produced
(4) by the following
Cd(I1) + Cd(O) = 2Cd(I)
(5)
Figure 10 shows the relation between square root of rotation velocity 0112 and the normalized limiting current density i,C- ’ obtained at first and second plateau, respectively. The diffusion coefficient of Cd(I1) ion calculated from first plateau (Cd(I1) + e- = Cd(I), n= 1) by Equation (1) (v=O.O148 cm2 s- ‘, LiClKC1,723 K[14]) is 2.1(&0.3)x 10-5cm2s-‘, and that calculated from second plateau (Cd(I1) + 2e = Cd(O), n = 2) is 2.2( k 0.3) x lo-’ cm’ s- I, respectively. The results are in fairly good agreement with 1.68-2.08 x lo-’ cm2 s-l at 723 K reported by Laitinen et aI.[15, 163. 01
I
I
et al.
(Rotation
Velocity
w) l/z
/ &l/z
s-‘/2
Fig. 10. Normalized limiting current density us square of rotation velocity for first and second plateau.
root
CONCLUSIONS The main results obtained from the above ments are summarized as follows.
experi-
(1) A rotating ring disk electrode assembly for use in molten chloride at 723 K has been constructed in success. (2) The voltammogram showed two or three plateaux. These can be explained by the existence of Cd(I). Thus, the existence of monovalent cadmium has been confirmed in LiCl-KCl.
(3) The standard potential of Cd(II)/Cd(I) is 2.3fO.l VvsLi/Li+ (-0.25+0.1 Vvs Ag/Ag+), and that of Cd(II)/Cd(O) is 1.98 kO.07 V us Li/Li + (-0.57 f 0.07 V z)s Ag/Ag+), respectively. (4) From the dependence of the limiting current on the rotation velocity and concentration a diffusion coefficient of Cd(I1) ion has been estimated to be 2.1(&0.3)x 10ms cm2s-‘(2.1( +0.3)x 10m9 m’s_‘). Acknowledgement-This work was carried out under the support of a Grant-in-Aid from Japanese Ministry of Education, Science and Culture.
REFERENCES
-100 I
-E
Disk
Potential
ED /
I
V vs. Li/Li+
Fig. 9. The disk current-disk potential (I,&,) curve and the corresponding ring current-disk potential (I,PE,) curve. (1000 rpm (104.7rad s-l); 0.1 M CdCI,; 723 K; ring potential = 2.468 V us Li/Li * ; 16 days after addition of 0.05 M Cd)
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