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Electrode processes of lead halides
J. Electroanal. Chem., 89 (1978) 87--95 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
87
ELECTRODE PROCESSES OF LEAD HALIDES PART I. PbCl2 and PbBr2
R.W. BONNE *, L. BOON and J. SCHOONMAN
Solid State Department, Physics Laboratory, State University o f Utrecht, Sorbonnelaan 4, Utrecht -- De Uithof (The Netherlands) (Received 9th May 1977; in revised form 15th July 1977)
ABSTRACT With cyclic voltammetry, scanning electron and optical microscopy, electrode reactions have been studied in symmetric and asymmetric cells of the type Pb IPbX 2 IPb or C [PbC121C, and Pb IPbX21M (X = Cl, Br and M = C, Pt), respectively, in a nitrogen, air and oxygen atmosphere. In nitrogen the electrode reaction at negative potentials is given by 2Vxj+ pbpXb+ 2e'-> Pb(s) This electrode reaction occurs irrespective of the electrode material employed, which makes blocking electrodes rapidly reversible. If oxygen is present the reaction Pb(s) + 102(g ) ~ PbO(s) interferes. The electrode reaction at positive potentials above the deposition voltage can be represented by 2XXx ~ X2(g ) + 2V x + 2e r for an inert electrode. If lead is present the electrode reaction Pb(s) + 2XXx ~ (PbX2)surface + 2V k + 2e' results at any voltage.
INTRODUCTION
The electrical conductivity of nominally pure and doped anion conducting PbC12 and PbBr2 has been studied extensively in the past [1--5]. Usually ionically blocking electrodes have been employed in these studies. To overcome interracial polarization effects at such electrodes, conductivities have been studied at audiofrequencies. These studies have provided detailed information about the temperature dependence of the mobilities and the intrinsic concentrations of the point defects in these halides. The thermal generation of the point defects can be described by means of the Schottky mechanism. In order to obtain information about electrode reactions, several techniques have been developed during the past few years [6--8]. Both a.c. bridge measurements and transient d.c. methods have been employed frequently in solid-state * Present address: Philips Research Laboratories, Eindhoven, The Netherlands.
88 electrodics. With these techniques the bulk and interfacial phenomena can be studied separately. In this work we have studied electrode reactions by means of cyclic voltammetry in symmetric and asymmetric cells composed of PbX2 (X = C1, Br) crystals with Pb, C, and Pt as electrodes. With the use of a lead electrode, or a pseudoreference electrode, anodic and cathodic reactions were studied separately. From the cyclic voltammograms of these cells we deduce in conjunction with scanning electron and optical micrographs possible electrode reactions. (2) EXPERIMENTAL
(2.1) Materials preparation The preparation, purification through zone-refining, and the crystal growth of PbC12 and PbBr2 have been reported in the literature [9,10]. The cleaved surfaces of these halide crystals were covered with either graphite (Aquadag) or platinum paint (Leitplatin 308), which are inert ionically blocking, electronic conducting electrodes, or they were covered with evaporated lead as a reversible electrode. In order to facilitate the microscopic study platinum meshelectrodes were used on several crystals. Measurements were performed on symmetric and asymmetric cells of the t y p e PbIPbX2IPb (X = C1, Br), CJPbC12[C, and MIPbX21Pb (M = C, Pt), respectively. A thin platinum wire was partly embedded in several crystals for use as a pseudo-reference electrode. The platinum wire (0 0.6 mm) was coated with graphite paint and then inserted into a drilled hole (0 0.7 mm). Crystals up to about 0.4 cm thick with surface areas of a b o u t 0.5 cm 2 were used. The cell systems were springloaded between platinum discs in evacuable conductivity equipment. Nitrogen, air and oxygen were employed as ambients.
(2.2) Electrical equipment In two-electrode cell systems a stabilized voltage source provided a potential ramp of known sweep rate across the cell, the sweep rate being adjustable by means of a motor-driven potentiometer. The current through the cell was measured with an automatic ranging digital pico-ammeter (Keithley 445). Sweep rates of 2 mV s-1 to 0.2 V s-1 were used. The voltage across the cell was measured with a digital electrometer with input impedance greater than 1014 ~2 (Keithley 616). An XY recorder (Bryans 29000} was used to record the voltammograms. In three-electrode systems a potentiostat (PAR 373) in conjunction with a function generator (Wavetek Model 144) was applied to control the electrode potential. The pseudo-reference electrode was connected with the potentiostat via an electrometer-probe. The current was measured b y the voltage drop over a known resistor. Admittance data referred to here will be published in detail elsewhere. The measuring technique has been published before [11]. The crystal surface studies were made with a Stereoscan $4 scanning electron microscope. An Olympus stereozoom SZ 111, and an Olympus metallurgical microscope MF/BIN were used to record the optical micrographs.
89 (3) RESULTS AND DISCUSSION
(3.1) Symmetric cells o f the type Pb lPbX2[Pb (X = Cl, Br) In the temperature range 300--500 K, and in nitrogen, air and oxygen atmospheres, this cell arrangement reveals an ohmic behaviour between the applied voltage and the measured current. At low temperatures and the lowest sweep rates small hysteresis effects were sometimes observed in the voltammograms. The slopes of the curves equalled at all temperatures the frequency-independent a.c. conductances. The admittances of these cells are independent of frequency for frequencies higher than 1 Hz. In general the lead electrodes are reversible at positive and negative potentials, thermodynamically as well as kinetically. The electrode reactions may, therefore, be represented by
Pb(s) anodic 2Vk
+
pbpXb+ 2e'
(1)
cathodic
since mass transport in these lead halides occurs via anion vacancies. These reactions account for a virtual lead transport. The cathodic reaction indicates a decreasing number of lattice sites and a deposit of lead. Scanning electron micrographs demonstrate a shrinkage of the crystal, especially at macroscopic scratches present on the crystal surface. With an optical microscope the growth of lead dendrites into the bulk was observed from the cathode surface, and especially from the macroscopically damaged sites. Figure 1 shows a scanning electron micrograph of lead anode. The formation of new material o n both PbC12 and PbBr~ is observed. The anodic reaction Pb(s) + 2X~ -~ (PbX2)su~face + 2 V k + 2e'
(2)
is therefore preferred. The small interfacial polarization that is sometimes observed may be caused by the growth of new material or possibly b y contact deterioration. As will be outlined in the next section, the reaction of cathodically formed lead with oxygen cannot be ruled out.
(3.2) Asymmetric cells o f the type PbiPbXzlM (X = Cl, Br and M = C, Pt) In Fig. 2 voltammograms, recorded in nitrogen and at different temperatures, are presented for the cell PbIPbBr2tC. Similar voltammograms were observed for cells with either PbCI2 or PbBr 2 and with painted Pt, and C electrodes in the temperature range 300--600 K. The influence of oxygen on the voltammograms is shown in Fig. 3. In the range from zero to positive potentials versus the lead electrode, the observed current is strongly time-dependent up to the deposition voltage. The details of polarization experiments in this regime will be reported elsewhere [121. The slope of the current-potential curves for the lead electrode operating at positive potentials equalled the bulk conductance. It is evident from Fig. 2 that a negatively biased blocking electrode is rapidly converted to a non-blocking electrode. This is accounted for by a deposit of lead
90
Fig. 1. Scanning electron micrograph of an evaporated lead electrode on PbBr2 operated at positive potentials. T = 400 K, Y a p p l i e d = 0.5 V.
on the blocking electrode. Sometimes dendritic growth of lead from the blocking electrode is observed. In the subsequent sweeps the non-blocking electrode is reconverted at positive potentials to a blocking electrode through reaction (2). The height of the wave depends on the electrolysis duration. In Fig. 4 the influence of the sweep rate on a voltammogram is shown for PbC12. From the duration of the electrolysis, the a m o u n t of lead deposit on the blocking electrode can be calculated. In our experiments the a m o u n t of transported charge Q, during electrolysis, can be calculated with the simplified relation VRblt 2, where v is the sweep rate (V s-l), RD the bulk resistance, and t half of the electrolysis time. When the potential is reversed the graphite or platinum paint electrode converts to a blocking electrode; this requires an a m o u n t of transported charge Q of 0.5 vR~lt 2, t now being the conversion time. With these relations the fraction x of the virtually transported a m o u n t of lead that is converted to lead halide to yield a blocking electrode can be calculated. In Fig. 5 the fraction x is shown as a function of the total a m o u n t of lead deposit and the
91 i/l~A I
a
/
150l
,
L1
2 5
.first scan
~ _
-10C i/IIA Fig. 2. Voltammogram of the asymmetric ceil Pb IPbBr2 IC. Sweep rate 83 m V s- 1 . (a) T = 385 K, (b) T --- 478 K. The sweep direction has been marked w i t h arrows.
JiA ~ ~5
r,~
75
,"~ ,,,y/'
5.0
a
5C
7
z,'/'
25
~ ~ -2. '
,,
..f'~
~
2--~
E/V
25~
50~
- 5.o:
Fig. 3. The influence of the ambient on the voltammogram of the cell Pb [PbCl2 IC. T -- 411 K, sweep rate 83 mV s-1. Fig. 4. Voitammogram of the cell PbIPbCl21C for different sweep rates. T = 411 K, ambient N2. (a) 167 mV s-1, (b) 83 mV s-1, (c) 33 mV s-1, (d) 17 mV s-1, (e) 8.3 mV s-1, (f) 3.3 mV s~ I
92 X
0.75
050
0.25
~
N2
~ 25
air °2
50
7.5
- - ~ . . , . 10 4Q/C
Fig. 5. The fraction x of the virtually transported a m o u n t of lead as a f u n c t i o n of the total a m o u n t of charge for different ambients, Cell Pb IPbC12 [C, T = 404 K.
ambient. This fraction is strongly dependent on the ambient used. Within the experimental error, the same results were obtained for platinum paint electrodes. For PbBr2 a similar observation was made. The virtual transport of a fraction x of the total amount of lead deposit will take a time (2xQRb)l/2v -1/2. The peak current ip can then be expressed by ip(A)
= ( 2 v x Q ) 112 R b -112
(3)
In Fig. 6 the peak current is shown as a function of the sweep rate. The differences between observed and calculated ip values originate from the oversimplified model and can be noticed immediately in the voltammogram {Fig. 4). The squareroot dependence as predicted b y relation (3) is however observed. From Fig. 5 it follows that the atmosphere has a great influence. A possible explanation is the reaction of the lead deposit with oxygen, i.e. Pb(s) + 1 0 2 ( g ) --> PbO(s)
(4)
This reaction accounts for the difference between the inferred a m o u n t of total
Iog0~/A)
Fq ~
r -5~
0c
-25
-20
i
-I 5 -10 --------~m~ log(v/Vs ~)
Fig. 6.,The observed peak current ip as a f u n c t i o n o f the sweep rate in nitrogen for the cell PblPbC12]C. T = 411 K. The slope has the value ! 2"
93 lead deposit and the a m o u n t of lead converted to lead halide. The fact that below some threshold lead deposit cannot be converted at all indicates that initially formed lead reacts immediately. It was also noticed that at positive potentials lead dendrites cannot be converted to lead halide. This observation contributes to the aforementioned difference. The passage of about 1.5 X 10 -4 C is required to deposit one monolayer of lead on the blocking electrode of the crystals used here. From measurements of the transported charge we conclude that several monolayers of lead would have been deposited. However, as has been mentioned before, the deposit of lead is not distributed uniformly over the surface area involved. In spite of this the e.m.f, of the cell Pb IPbX21M after electrolysis with the blocking electrode operating at negative potentials vanishes. Moreover, this electrode becomes blocking, even though the dendrites are still present. If lead oxide is present the anode reaction can be expressed by PbO(s) + 2X x -* PbX2(s) + ~O-(g) + 2Vk + 2e'
(5)
The total cell reaction then requires 0.92 V at 411 K. The voltammograms in oxygen indicate a decomposition of lead oxide (see Fig. 3). In/3-PbF2 this process could be studied in more detail [13]. From measurements on this cell arrangement, the deposition voltage can be inferred. In Fig. 7 the inferred voltages are plotted versus temperature for graphite and platinum paint electrodes on both PbC12 and PbBr2 in each ambient. The observed temperature dependence indicates a single process, depending on the nature of the electrode material. We are inclined to believe that the adsorption of halogen on the electrode material governs this behaviour. The scanning electron micrographs clearly revealed a difference between the structure of painted platinum and graphite electrodes, the latter being more dense.
o
Edepl'V
~
Edep/V
~o
22
2,0!
~ Pt
1,8
"~..C,.. ~ d',~ ~o 1.6- "---~. ~..~ 1.4
PbCi 2
1.8
~ Pt
1.6
~ ',C
1.4 - "k-.
"%'"
1
.
o',it
1.o
, io , 300 4oo 500 o o o - - - T / k
0,8
1.21-
I
1.0,
PbBr 2
9 \
2
~
3()0 4()0 5(X) 600---'-T/K
observed deposition voltage for the cells Pb IPbX2 [M (X = Cl, Br and M = C, Pt). ~ig. 7." -)TheTheoretical decomposition voltage [14,15 ]. ( . . . . . . . ) Pb IPbX2 IC. (---- --) Pb IPbX2 IPt.
94
40
~
20-
r
,2
Fig. 8. Voltammogram for PbC12 provided with two graphite electrodes and a pseudo-reference electrode, T = 424 K. Sweep rate 17 mV S- 1 (a) Total cell response. (b) Response of o n e electrode versus the probe.
(3.3) Symmetrical cells of the type MIPbXeIM (M = C, Pt) Some PbC12 crystals in this cell arrangement have been provided with a probeelectrode. Its potential could be considered as being practically constant. Figure 8 presents the cyclic voltammogram measured in nitrogen over the total cell (Fig. 8a) and over one of the electrodes versus the probe (Fig. 8b). The slope in the voltammogram represents again the bulk conductance. The behaviour of one electrode versus the probe is similar to that of the asymmetric cell PblPbX21M, indicating that the probe-electrode in fact behaves as a reversible electrode. Moreover, deposition voltages as inferred from the cell probe ]PbX21M fit the experimental relations depicted in Fig. 7. From Fig. 8 it is noticed that up to the deposition voltage all the applied voltage appears across the anode interface. Several crystals were provided with platinum mesh-electrodes, Pt *, in order to facilitate microscopical investigations. For voltages higher than the deposition voltage dendritic lead forms at the cathode surface, while at the anode as well as at the cathode a shrinkage of the crystal occurs. The electrode reaction at the anode can be expressed by 2X~ -~ 2Vk + X2(g) + 2e'
(6)
95 Micrographs revealed furthermore that the Pt*-electrode behaves identically at positive and negative potentials in the cell arrangements PblPbX21Pt* and Pt* IPbX2 IPt*. Positive and negative behaviour of the Pb-electrode in PbBPbX2 IPt* and PblPbX21Pb is also identical. ACKNOWLEDGEMENTS
The authors are grateful to Prof. Dr. G. Blasse for discussing the text in detail and for offering valuable criticisms in the course of the preparation of the manuscript. We are indebted to Mr. J. Pieters of the Molecular Cellbiology Department of this University, who recorded the scanning electron micrographs, and to Drs. H.A. Harwig, of the Inorganic Chemistry Department of this University, who kindly made available the potentiostat, and optical microscope. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
K.J. de Vries and J.H.van Santen, Physica, 30 (1964) 2051. H. Hoshino, M. Yamazaki, Y. Nakamttra and M. Shimoji, J. Phys. Soc. Jap., 26 (1969) 1422. W.E. van den Brom, J. S c h o o n m a n and J.H.W. de Wit, J. SoUd State Chem., 4 (1972) 475. J. Schoonman, J. Solid State Chem,, 4 (1972) 466. H. Hoshino, S. Yokose and M. Shimoji, J. Solid State Chem., 7 (1973) 1. J. Bauerle, J. Phys. Chem. Solids,30 (1969) 2657. J. Bert, J.L. Picot and J. Dupuy, Phys. Status Solidi A, 19 (1973) 119. A.D. Franklin, J. Amer. Ceram. Soc., 58 (1975) 465. B. Willemsen, J. Solid State Chem., 3 (1971) 567. J.F. Verwey and J. Schoonman, Physica, 35 (1967) 386. R.W. Bonne and J. Schoonman, Solid State C o m m u n . , 18 (1976) 1005. R.W. Bonne and J. Schoonman, to be published. R.W. Bonne and J. Schoonman, Pa~t If, to be Published. W.J. Hamer, M.S. Malmberg and B. Rubin, J. Electrochem. Soc., 103 (1956) 8. W.J. Hamer, M.S. Malmberg and B. Rubin, J. Electrochem. Soc., 112 (1965) 750.
87
ELECTRODE PROCESSES OF LEAD HALIDES PART I. PbCl2 and PbBr2
R.W. BONNE *, L. BOON and J. SCHOONMAN
Solid State Department, Physics Laboratory, State University o f Utrecht, Sorbonnelaan 4, Utrecht -- De Uithof (The Netherlands) (Received 9th May 1977; in revised form 15th July 1977)
ABSTRACT With cyclic voltammetry, scanning electron and optical microscopy, electrode reactions have been studied in symmetric and asymmetric cells of the type Pb IPbX 2 IPb or C [PbC121C, and Pb IPbX21M (X = Cl, Br and M = C, Pt), respectively, in a nitrogen, air and oxygen atmosphere. In nitrogen the electrode reaction at negative potentials is given by 2Vxj+ pbpXb+ 2e'-> Pb(s) This electrode reaction occurs irrespective of the electrode material employed, which makes blocking electrodes rapidly reversible. If oxygen is present the reaction Pb(s) + 102(g ) ~ PbO(s) interferes. The electrode reaction at positive potentials above the deposition voltage can be represented by 2XXx ~ X2(g ) + 2V x + 2e r for an inert electrode. If lead is present the electrode reaction Pb(s) + 2XXx ~ (PbX2)surface + 2V k + 2e' results at any voltage.
INTRODUCTION
The electrical conductivity of nominally pure and doped anion conducting PbC12 and PbBr2 has been studied extensively in the past [1--5]. Usually ionically blocking electrodes have been employed in these studies. To overcome interracial polarization effects at such electrodes, conductivities have been studied at audiofrequencies. These studies have provided detailed information about the temperature dependence of the mobilities and the intrinsic concentrations of the point defects in these halides. The thermal generation of the point defects can be described by means of the Schottky mechanism. In order to obtain information about electrode reactions, several techniques have been developed during the past few years [6--8]. Both a.c. bridge measurements and transient d.c. methods have been employed frequently in solid-state * Present address: Philips Research Laboratories, Eindhoven, The Netherlands.
88 electrodics. With these techniques the bulk and interfacial phenomena can be studied separately. In this work we have studied electrode reactions by means of cyclic voltammetry in symmetric and asymmetric cells composed of PbX2 (X = C1, Br) crystals with Pb, C, and Pt as electrodes. With the use of a lead electrode, or a pseudoreference electrode, anodic and cathodic reactions were studied separately. From the cyclic voltammograms of these cells we deduce in conjunction with scanning electron and optical micrographs possible electrode reactions. (2) EXPERIMENTAL
(2.1) Materials preparation The preparation, purification through zone-refining, and the crystal growth of PbC12 and PbBr2 have been reported in the literature [9,10]. The cleaved surfaces of these halide crystals were covered with either graphite (Aquadag) or platinum paint (Leitplatin 308), which are inert ionically blocking, electronic conducting electrodes, or they were covered with evaporated lead as a reversible electrode. In order to facilitate the microscopic study platinum meshelectrodes were used on several crystals. Measurements were performed on symmetric and asymmetric cells of the t y p e PbIPbX2IPb (X = C1, Br), CJPbC12[C, and MIPbX21Pb (M = C, Pt), respectively. A thin platinum wire was partly embedded in several crystals for use as a pseudo-reference electrode. The platinum wire (0 0.6 mm) was coated with graphite paint and then inserted into a drilled hole (0 0.7 mm). Crystals up to about 0.4 cm thick with surface areas of a b o u t 0.5 cm 2 were used. The cell systems were springloaded between platinum discs in evacuable conductivity equipment. Nitrogen, air and oxygen were employed as ambients.
(2.2) Electrical equipment In two-electrode cell systems a stabilized voltage source provided a potential ramp of known sweep rate across the cell, the sweep rate being adjustable by means of a motor-driven potentiometer. The current through the cell was measured with an automatic ranging digital pico-ammeter (Keithley 445). Sweep rates of 2 mV s-1 to 0.2 V s-1 were used. The voltage across the cell was measured with a digital electrometer with input impedance greater than 1014 ~2 (Keithley 616). An XY recorder (Bryans 29000} was used to record the voltammograms. In three-electrode systems a potentiostat (PAR 373) in conjunction with a function generator (Wavetek Model 144) was applied to control the electrode potential. The pseudo-reference electrode was connected with the potentiostat via an electrometer-probe. The current was measured b y the voltage drop over a known resistor. Admittance data referred to here will be published in detail elsewhere. The measuring technique has been published before [11]. The crystal surface studies were made with a Stereoscan $4 scanning electron microscope. An Olympus stereozoom SZ 111, and an Olympus metallurgical microscope MF/BIN were used to record the optical micrographs.
89 (3) RESULTS AND DISCUSSION
(3.1) Symmetric cells o f the type Pb lPbX2[Pb (X = Cl, Br) In the temperature range 300--500 K, and in nitrogen, air and oxygen atmospheres, this cell arrangement reveals an ohmic behaviour between the applied voltage and the measured current. At low temperatures and the lowest sweep rates small hysteresis effects were sometimes observed in the voltammograms. The slopes of the curves equalled at all temperatures the frequency-independent a.c. conductances. The admittances of these cells are independent of frequency for frequencies higher than 1 Hz. In general the lead electrodes are reversible at positive and negative potentials, thermodynamically as well as kinetically. The electrode reactions may, therefore, be represented by
Pb(s) anodic 2Vk
+
pbpXb+ 2e'
(1)
cathodic
since mass transport in these lead halides occurs via anion vacancies. These reactions account for a virtual lead transport. The cathodic reaction indicates a decreasing number of lattice sites and a deposit of lead. Scanning electron micrographs demonstrate a shrinkage of the crystal, especially at macroscopic scratches present on the crystal surface. With an optical microscope the growth of lead dendrites into the bulk was observed from the cathode surface, and especially from the macroscopically damaged sites. Figure 1 shows a scanning electron micrograph of lead anode. The formation of new material o n both PbC12 and PbBr~ is observed. The anodic reaction Pb(s) + 2X~ -~ (PbX2)su~face + 2 V k + 2e'
(2)
is therefore preferred. The small interfacial polarization that is sometimes observed may be caused by the growth of new material or possibly b y contact deterioration. As will be outlined in the next section, the reaction of cathodically formed lead with oxygen cannot be ruled out.
(3.2) Asymmetric cells o f the type PbiPbXzlM (X = Cl, Br and M = C, Pt) In Fig. 2 voltammograms, recorded in nitrogen and at different temperatures, are presented for the cell PbIPbBr2tC. Similar voltammograms were observed for cells with either PbCI2 or PbBr 2 and with painted Pt, and C electrodes in the temperature range 300--600 K. The influence of oxygen on the voltammograms is shown in Fig. 3. In the range from zero to positive potentials versus the lead electrode, the observed current is strongly time-dependent up to the deposition voltage. The details of polarization experiments in this regime will be reported elsewhere [121. The slope of the current-potential curves for the lead electrode operating at positive potentials equalled the bulk conductance. It is evident from Fig. 2 that a negatively biased blocking electrode is rapidly converted to a non-blocking electrode. This is accounted for by a deposit of lead
90
Fig. 1. Scanning electron micrograph of an evaporated lead electrode on PbBr2 operated at positive potentials. T = 400 K, Y a p p l i e d = 0.5 V.
on the blocking electrode. Sometimes dendritic growth of lead from the blocking electrode is observed. In the subsequent sweeps the non-blocking electrode is reconverted at positive potentials to a blocking electrode through reaction (2). The height of the wave depends on the electrolysis duration. In Fig. 4 the influence of the sweep rate on a voltammogram is shown for PbC12. From the duration of the electrolysis, the a m o u n t of lead deposit on the blocking electrode can be calculated. In our experiments the a m o u n t of transported charge Q, during electrolysis, can be calculated with the simplified relation VRblt 2, where v is the sweep rate (V s-l), RD the bulk resistance, and t half of the electrolysis time. When the potential is reversed the graphite or platinum paint electrode converts to a blocking electrode; this requires an a m o u n t of transported charge Q of 0.5 vR~lt 2, t now being the conversion time. With these relations the fraction x of the virtually transported a m o u n t of lead that is converted to lead halide to yield a blocking electrode can be calculated. In Fig. 5 the fraction x is shown as a function of the total a m o u n t of lead deposit and the
91 i/l~A I
a
/
150l
,
L1
2 5
.first scan
~ _
-10C i/IIA Fig. 2. Voltammogram of the asymmetric ceil Pb IPbBr2 IC. Sweep rate 83 m V s- 1 . (a) T = 385 K, (b) T --- 478 K. The sweep direction has been marked w i t h arrows.
JiA ~ ~5
r,~
75
,"~ ,,,y/'
5.0
a
5C
7
z,'/'
25
~ ~ -2. '
,,
..f'~
~
2--~
E/V
25~
50~
- 5.o:
Fig. 3. The influence of the ambient on the voltammogram of the cell Pb [PbCl2 IC. T -- 411 K, sweep rate 83 mV s-1. Fig. 4. Voitammogram of the cell PbIPbCl21C for different sweep rates. T = 411 K, ambient N2. (a) 167 mV s-1, (b) 83 mV s-1, (c) 33 mV s-1, (d) 17 mV s-1, (e) 8.3 mV s-1, (f) 3.3 mV s~ I
92 X
0.75
050
0.25
~
N2
~ 25
air °2
50
7.5
- - ~ . . , . 10 4Q/C
Fig. 5. The fraction x of the virtually transported a m o u n t of lead as a f u n c t i o n of the total a m o u n t of charge for different ambients, Cell Pb IPbC12 [C, T = 404 K.
ambient. This fraction is strongly dependent on the ambient used. Within the experimental error, the same results were obtained for platinum paint electrodes. For PbBr2 a similar observation was made. The virtual transport of a fraction x of the total amount of lead deposit will take a time (2xQRb)l/2v -1/2. The peak current ip can then be expressed by ip(A)
= ( 2 v x Q ) 112 R b -112
(3)
In Fig. 6 the peak current is shown as a function of the sweep rate. The differences between observed and calculated ip values originate from the oversimplified model and can be noticed immediately in the voltammogram {Fig. 4). The squareroot dependence as predicted b y relation (3) is however observed. From Fig. 5 it follows that the atmosphere has a great influence. A possible explanation is the reaction of the lead deposit with oxygen, i.e. Pb(s) + 1 0 2 ( g ) --> PbO(s)
(4)
This reaction accounts for the difference between the inferred a m o u n t of total
Iog0~/A)
Fq ~
r -5~
0c
-25
-20
i
-I 5 -10 --------~m~ log(v/Vs ~)
Fig. 6.,The observed peak current ip as a f u n c t i o n o f the sweep rate in nitrogen for the cell PblPbC12]C. T = 411 K. The slope has the value ! 2"
93 lead deposit and the a m o u n t of lead converted to lead halide. The fact that below some threshold lead deposit cannot be converted at all indicates that initially formed lead reacts immediately. It was also noticed that at positive potentials lead dendrites cannot be converted to lead halide. This observation contributes to the aforementioned difference. The passage of about 1.5 X 10 -4 C is required to deposit one monolayer of lead on the blocking electrode of the crystals used here. From measurements of the transported charge we conclude that several monolayers of lead would have been deposited. However, as has been mentioned before, the deposit of lead is not distributed uniformly over the surface area involved. In spite of this the e.m.f, of the cell Pb IPbX21M after electrolysis with the blocking electrode operating at negative potentials vanishes. Moreover, this electrode becomes blocking, even though the dendrites are still present. If lead oxide is present the anode reaction can be expressed by PbO(s) + 2X x -* PbX2(s) + ~O-(g) + 2Vk + 2e'
(5)
The total cell reaction then requires 0.92 V at 411 K. The voltammograms in oxygen indicate a decomposition of lead oxide (see Fig. 3). In/3-PbF2 this process could be studied in more detail [13]. From measurements on this cell arrangement, the deposition voltage can be inferred. In Fig. 7 the inferred voltages are plotted versus temperature for graphite and platinum paint electrodes on both PbC12 and PbBr2 in each ambient. The observed temperature dependence indicates a single process, depending on the nature of the electrode material. We are inclined to believe that the adsorption of halogen on the electrode material governs this behaviour. The scanning electron micrographs clearly revealed a difference between the structure of painted platinum and graphite electrodes, the latter being more dense.
o
Edepl'V
~
Edep/V
~o
22
2,0!
~ Pt
1,8
"~..C,.. ~ d',~ ~o 1.6- "---~. ~..~ 1.4
PbCi 2
1.8
~ Pt
1.6
~ ',C
1.4 - "k-.
"%'"
1
.
o',it
1.o
, io , 300 4oo 500 o o o - - - T / k
0,8
1.21-
I
1.0,
PbBr 2
9 \
2
~
3()0 4()0 5(X) 600---'-T/K
observed deposition voltage for the cells Pb IPbX2 [M (X = Cl, Br and M = C, Pt). ~ig. 7." -)TheTheoretical decomposition voltage [14,15 ]. ( . . . . . . . ) Pb IPbX2 IC. (---- --) Pb IPbX2 IPt.
94
40
~
20-
r
,2
Fig. 8. Voltammogram for PbC12 provided with two graphite electrodes and a pseudo-reference electrode, T = 424 K. Sweep rate 17 mV S- 1 (a) Total cell response. (b) Response of o n e electrode versus the probe.
(3.3) Symmetrical cells of the type MIPbXeIM (M = C, Pt) Some PbC12 crystals in this cell arrangement have been provided with a probeelectrode. Its potential could be considered as being practically constant. Figure 8 presents the cyclic voltammogram measured in nitrogen over the total cell (Fig. 8a) and over one of the electrodes versus the probe (Fig. 8b). The slope in the voltammogram represents again the bulk conductance. The behaviour of one electrode versus the probe is similar to that of the asymmetric cell PblPbX21M, indicating that the probe-electrode in fact behaves as a reversible electrode. Moreover, deposition voltages as inferred from the cell probe ]PbX21M fit the experimental relations depicted in Fig. 7. From Fig. 8 it is noticed that up to the deposition voltage all the applied voltage appears across the anode interface. Several crystals were provided with platinum mesh-electrodes, Pt *, in order to facilitate microscopical investigations. For voltages higher than the deposition voltage dendritic lead forms at the cathode surface, while at the anode as well as at the cathode a shrinkage of the crystal occurs. The electrode reaction at the anode can be expressed by 2X~ -~ 2Vk + X2(g) + 2e'
(6)
95 Micrographs revealed furthermore that the Pt*-electrode behaves identically at positive and negative potentials in the cell arrangements PblPbX21Pt* and Pt* IPbX2 IPt*. Positive and negative behaviour of the Pb-electrode in PbBPbX2 IPt* and PblPbX21Pb is also identical. ACKNOWLEDGEMENTS
The authors are grateful to Prof. Dr. G. Blasse for discussing the text in detail and for offering valuable criticisms in the course of the preparation of the manuscript. We are indebted to Mr. J. Pieters of the Molecular Cellbiology Department of this University, who recorded the scanning electron micrographs, and to Drs. H.A. Harwig, of the Inorganic Chemistry Department of this University, who kindly made available the potentiostat, and optical microscope. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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