J. Electrochem. Soc., Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc.
Pb(IV) detected is at a m i n i m u m for concentrations of
Pb(II) concentration >ca. 0.1M and HC104 < ca. 1M, which
correlates with conditions for a m i n i m u m value of to and
the m a x i m u m rate of the deposition process (i.e., large
idlSS)It has been demonstated (1) that evolution of 02 is required to enable the anodic deposition of PbOz(s). It was
demonstrated also that the induction period was short for
surfaces at which PbO2(s) had previously been deposited
and then stripped. An ad-layer of Pb(II) was concluded (1)
to remain at the Au electrode surface following dissolution
of PbO2(s) at 0.3V which serves as an array of nucleation
sites following oxidation to an ad-layer of Pb(IV) (probably
PbO~) with the step of potential from 0.3V to the deposition value. Data obtained with the R R D E demonstrated a
significantly lower quantity of soluble Pb(IV) being produced at the start of the induction period w h e n the adlayer was left intact following cathodic dissolution of bulk
deposits of PbO2(s).
It also is concluded that very small quantities of soluble
Pb(III) and Pb(IV) are produced with the onset of cathodic
dissolution of PbO2(s) for a negative potential scan. Conditions which favor the rapid deposition of PbO2(s) favor the
production of soluble Pb(III) over Pb(IV) during the dissolution process.
Acknowledgment
X P S data were obtained by J a m e s W. Anderegg.
Manuscript submitted March 21, 1988; revised manuscript received May 12, 1988.
Iowa State University assisted in meeting the publication costs of this article.
27
REFERENCES
1. H. Chang and D. C. Johnson, This Journal, 136, 17
(1989).
2. E. Hameenoja, T. Laitinen, and G. Sundholm, Electrochim. Acta, 32, 187 (1987).
3. 1VLSkyllas-Kazacos, J. Power Sources, 13, 55 (1984).
4. M. Skyllas-Kazacos, This Journal, 128, 817 (1981).
5. R. F. Amile and T. A. Berger, J. Electroanal. Chem., 36,
427 (1972).
6. H. Bode and E. Voss, Electrochim. Acta, 6, 1 (1962).
7. H. A. Laitinen and N. H. Watkins, This Journal, 123,
804 (1976).
8. J. P. Pohl and W. Scholz, J. Power Sources, 16, 293
(1985).
9. V. A. Volgina, E. A. Nechaev, and N. G. Bakhchisaraits'yan, Sov. Electrochem., 9, 984 (1973).
10. E. E. Littauer and L. L. Schreir, Electrochim. Acta., 12,
465 (1967).
11. J. P. Pohl and H. Rickert, Power Sources, 6, 59 (1976).
12. P. Reutschi, J. Power Sources, 2, 3 (1977/1978).
13. Z. Takehara and K. Kanamura, Electrochim. Acta, 29,
16~3 (1984).
14. S. R. Ellis, N. A. Hampson, F. Wilkinson, and M. C.
Ball, This Journal, 134, 2388 (1987).
15. W. J. Albery and S. Bruckenstein, Trans. Faraday
Soc., 62, 1920 (1966).
16. S. H. Cadle and S. Bruckenstein, Anal. Chem., 46, 16
(1974).
17. W. J. Albery and M. L. Hitchman, "Ring-disk Electrodes," Clarendon Press, Oxford (1971).
18. A. Kozawa, in "Batteries," Vol. 1, K. V. Kordesch, Editor, p. 387, Marcel Dekker Inc., New York (1974).
19. I. H. Yeo, Ph.D. Dissertation, Iowa State University,
Ames, IA (1987).
Mechanism of Action of Sn on the Passivation Phenomena in the
Lead-Acid Battery Positive Plate (Sn-Free Effect)
D. Pavlov* and B. Monakhov
Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria
M. Maja* and N. Penazzi
Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, 10129 Torino, Italy
ABSTRACT
When the lead-acid battery positive plate is made of pure Pb or of low-antimony non-Sn lead alloys, during low current polarization or stay at an open circuit the positive plates are passivated. This passivation is due to the formation of an
uninterrupted corrosion layer of tet-PbO with high resistance. It was established that Sn keeps down the passivation phen o m e n a in the plate. In the present paper, through SLV and photoelectrochemical investigations, it was found that Sn facilitates the process of oxidation of PbO to PbOn (1 < n < 2) and lowers the potential at which this reaction starts. This
p h e n o m e n o n is explained using a model of the oxidation of PbO to PbOn based on the semiconductive properties of PbO.
Sn creates hole conductivity in the PbO layer allowing the oxidation reaction to proceed at the PbO/solution interface. The
oxygen which has diffused into the oxide oxidizes PbO to PbOn. Above a certain n value the oxide conductivity becomes
equal to that of PbO2. The corrosion layer resistance decreases dramatically thus suppressing the passivation p h en o me na
on the positive plate grid.
When the positive plate grids are made of pure lead or
low-antimony non-Sn alloys, under certain conditions the
lead-acid battery exhibits some unpleasant behavior. First,
during charging at constant voltage, the battery shows
very low charge acceptance. Second, w.hen the positive
plates after formation are dried at temperatures higher
than 80~ or the batteries are stored for a longer period of
time, their voltage during discharge is very low (thermopassivation and storage passivation). Third, when keeping these batteries in floating condition, if the positive
plate potential is in the range between 1.1 and 1.3V (vs. Hg/
Hg2SO4), at discharge these batteries have a strongly reduced power. These phenomena are due to the formation
* Electrochemical Society Active Member.
on the grids of a thin tet-PbO layer whose electrical properties condition the behavior of the plates (1).
It was established that an addition of tin (in quantities
greater than 0.2%) to the lead alloys with non- or lowantimony content improves significantly the charge acceptance (2). Thermopassivation and storage passivation
are strongly slowed down by tin-containing alloys (3). Sn
prevents the formation of a tet-PbO layer (Sn-free effect).
The mechanism of this effect of Sn remains still unexplained. This will be the purpose of the present paper.
Experimental
The anodic oxidation of the positive plate lead grid proceeds in three successive stages which may be expressed
in general as follows (4.5)
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28
P b --> tet-PbO
[1]
tet-PbO -~ t e t - P b O n w h e r e 1 < n < 1.5
[2]
t e t - P b O n --+ ~-PbO2
[3]
The three reactions p r o c e e d at different rates. D e p e n d ing on the ratio b e t w e e n these rates, corrosion sublayers
w i t h different stoichiometric coefficients can be formed. If
the rate of reaction [1] is greater t h a n that o f reaction [2], a
tet-PbO layer will be f o r m e d on the grid surface. The electric c o n d u c t i v i t y of the lead o x i d e layers d e p e n d s strongly
on the s t o i c h i o m e t r i c coefficient (6). T h e specific electric
c o n d u c t i v i t y of tet-PbO is 10 -l~ 12 9 cm. W h e n t h e stoichiom e t r i c coefficient of t h e o x i d e grows f r o m 1.2 to 1.5, the
c o n d u c t i v i t y increases f r o m 10 -l~ to 102 l~ 9 cm. It is e v i d e n t
that t h e f o r m a t i o n of tet-PbO is critical for the battery.
The p u r p o s e of the p r e s e n t paper is to elucidate the
m e c h a n i s m of action of S n on the f o r m a t i o n of a tet-PbO
layer.
The i n v e s t i g a t i o n was carried out w i t h a P b / t e t - P b O
layer/PbSO4 membrane/H2SO4 electrode s y s t e m w h o s e
properties are well k n o w n (5, 7). The effect of Sn on the oxidation rate of P b O to P b O n was investigated. This elect r o d e is t h e r m o d y n a m i c a l l y stable in the potential range
f r o m - 0 . 4 to +0.95V. (All electrode potentials h a v e b e e n
m e a s u r e d w i t h r e s p e c t to a Hg/Hg2SO4 r e f e r e n c e electrode.) The Pb/PbO/PbSO4 electrode was o b t a i n e d by potentiostatic o x i d a t i o n at +0.6 or +0.4V for 16h in 1N H2SO4
solution. T h e electrodes, cylindrical s p e c i m e n s of metals,
w e r e inserted in Teflon holders so that only the c y l i n d e r
b a s e w i t h an area of 1.0 c m 2 was s u b j e c t e d to oxidation.
T h e o x i d a t i o n of P b O to PbO2 m a y be carried out in two
ways: (i) in the dark, by increasing P b / P b O / P b S O 4 elect r o d e potential d e e p into the PbO~ potential r e g i o n
(E > 950 mV) (5), and (ii) by illuminating the P b / P b O /
PbSO4 e l e c t r o d e at potentials m o r e positive t h a n 0 m V
(8, 9). T h e irradiation of the s y s t e m w i t h w h i t e light causes
a d e c r e a s e by 1V in the potential of P b O oxidation.
Experimental Results
Formation of Pb/PbO/PbS04 electrode system.--Using an
x-ray t e c h n i q u e , the effect of S n on the ratio b e t w e e n the
phases in the anodic layer was d e t e r m i n e d . The electrode
s y s t e m was o b t a i n e d by polarization of t h e electrode at
0.60V in the dark for 15h. A n x-ray diffractogram of t h e
anodic layer was made.
F i g u r e 1 p r e s e n t s the relative intensities of the phases
d e t e c t e d in the anodic layer f o r m e d on P b and Pb-0.5% Sn
electrodes.
T h e t h i c k n e s s of t h e anodic layer can be e s t i m a t e d from
t h e x-ray diffraction line intensity of Pb, d = 2.86A. F i g u r e
1 s h o w s that S n has not essentially c h a n g e d the t h i c k n e s s
of the anodic layer. The 3.00/k line is characteristic for
PbSO4. S n does not affect the PbSO4 layer formation, either. B e t w e e n the P b surface and the PbSO4 layer a tetP b O layer is formed. The latter is oriented along the 110
face (d = 2.80A) d u r i n g the e l e c t r o c h e m i c a l o x i d a t i o n of
the P b electrode (7). F i g u r e 1 shows that the intensity of
the 2.80A line for Pb-0.5% Sn alloy is l o w e r t h a n that for P b
electrode. T h e reverse is o b s e r v e d w i t h the 3.12A line.
T h e r e f o r e S n w e a k e n s the orientation of the tet-PbO layer
along t h e 110 face leaving the t h i c k n e s s of the oxide layer
unaffected. This p h e n o m e n o n m a y be d u e to the incorporation of S n ions into the o x i d e crystal lattice.
Oxidation of PblPbOIPbS04 electrode to PblPbOnlPbS04
one in the dark.--The o x i d a t i o n was carried out using linear potential s w e e p s to an u p p e r potential v a l u e d e e p into
t h e PbO2 potential range (E > 950 mV). This potential
s h o u l d not be too h i g h neither should the electrode stay in
the PbO2 potential range for too l o n g in order to avoid the
PbSO4 o x i d a t i o n to P b Q . After the u p p e r potential limit
was reached, a cathodic potential s w e e p followed to indicate t h e f o r m a t i o n of P b O n and PbO2 phases. The elect r o d e stayed for 10 m i n at 0.60V before the n e x t s w e e p w a s
started.
S o m e results of the e l e c t r o c h e m i c a l P b - S n alloy oxidation w e r e already p r e s e n t e d in an earlier p a p e r of ours (10).
F i g u r e 2 r e p r e s e n t s a series o f potential s w e e p s in t h e dark
of P b and Pb-0.5% Sn/PbO/PbSO4 electrodes f o r m e d
u n d e r the conditions d e s c r i b e d in the section on F o r m a tion of Pb/PbO/PbSO4 electrode system. S w e e p rate was
10 mV/s. T h e serial n u m b e r of the s w e e p is encircled. The
arrows m a r k the start of the P b O o x i d a t i o n reaction.
A c o m p a r i s o n of the potentials at w h i c h the process of
P b O o x i d a t i o n starts for p u r e P b and Pb-0.5% Sn electrodes s h o w s that Sn causes the o x i d a t i o n reaction to start
at a l o w e r potential.
F i g u r e 3 presents the d e p e n d e n c e of the potential at
w h i c h t h e o x i d e stoichiometric coefficient starts to grow
on t h e n u m b e r of sweeps.
The a d d i t i o n of tin to the lead alloy decreases the
starting potential of the P b O to P b O n o x i d a t i o n reaction in
the dark by a p p r o x i m a t e l y 0.2V. It m a y be e x p e c t e d that
this potential d e c r e a s e is due to t h e action of tin ions incorp o r a t e d in the o x i d e layer.
The a b o v e s w e e p s w e r e carried out w i t h electrodes previously o x i d i z e d at +0.6V for 15h. It w o u l d be interesting
to establish w h e t h e r the Sn influence is felt f r o m the v e r y
b e g i n n i n g of t h e f o r m a t i o n of a P b O layer at these h i g h
positive potentials. We s u b j e c t e d Pb and Pb-l.0% Sn elect r o d e s to o x i d a t i o n at c o n s t a n t potentials w i t h i n the
0.7-1.0V range (with 100 m V increments) for lh. A f t e r ' t h a t
50
__-I~ -~
@
4O
50
[11o]
t e t - Pb0
40
/
30
D -Pb
/
.->---"
2O
L~-Pb-0.5% Sn
10
0
PbS0~
30
30
~0
2O
Pb
2O
10
lt~
0
o
d=
2.80A
2B6A
3.00A
3.12
Fig. 1. Relative intensities of the phases in the anadic layer formed
on Pb and Pb-0.5% Sn electrodes oxidized at 0.60V for 15h. d is the interplanar space of the compound crystal lattice.
I
0.8
i
I
10
1.2
PotentiaL. V
Fig. 2. 1 st, 6th, and 14th voltammograms for Pb-0.5% Sn/PbO/PbS04
electrode at up to 1.2V, and Pb/PbO/PbS04 electrode at up to 1.3V.
The arrow marks the start of PbO oxidation. Sweep rate 10 mV/s.
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1.4
29
W h a t is t h i s v e r y w e l l - d i s t i n g u i s h e d e l e c t r o c a t a l y t i c effect o f S n d u e to? T h e a n s w e r to t h i s q u e s t i o n w a s l o o k e d
for i n s t u d y i n g t h e s e m i c o n d u c t i v e b e h a v i o r of t h e electrode by photoelectrochemical methods.
Photoelectrochemical oxidation of the PbO layer of Pb/
PbO/PbSO system to PbOn.--Photoactivation of Pb/PbO/
PbS04 electrode.--Photoelectrochemical o x i d a t i o n o f
1.2-
t-
~10
o
0.8
--0.
I
0
I
5
10
Number ol= sweeps
0
I
15
20
Fig. 3. Dependence of the potential at which PbO oxidation starts on
the number of sweeps for different Pb-Sn alloy electrodes.
t h e c i r c u i t was o p e n a n d t h e p o t e n t i a l c h a n g e s w i t h t i m e
were registered. Figure 4 represents the obtained trans i e n t s at o p e n circuit.
T h e d e c a y of c u r v e s p e r m i t s u s to d r a w t h e f o l l o w i n g
c o n c l u s i o n s : (i) W h e n p u r e P b e l e c t r o d e is o x i d i z e d at pot e n t i a l s m o r e n e g a t i v e t h a n +0.9V, a f t e r o p e n i n g o f t h e circuit, t h e p o t e n t i a l d r o p s t o t h e p l a t e a u o f t h e P b / P b O elect r o d e . T h a t m e a n s t h a t u p to +0.9V t h e P b O l a y e r d o e s n o t
u n d e r g o o x i d a t i o n . (it) W h e n P b - l . 0 % S n e l e c t r o d e s are oxi d i z e d at +0.8V after o p e n i n g of t h e circuit, a p o t e n t i a l arr e s t is o b s e r v e d in t h e t r a n s i e n t , w h i l e w h e n t h e o x i d a t i o n
is c a r r i e d o u t at +0.9V, after o p e n i n g t h e circuit, a p l a t e a u
a p p e a r s at +0.5 to +0.4V. T h e l a t t e r is m o r e clearly outl i n e d a f t e r o x i d a t i o n at +I.0V. T h i s p l a t e a u is d u e to t h e
f o r m a t i o n o f a P b O n layer. I t m u s t n o t b e r e l a t e d to t h e form a t i o n of PbO2 s i n c e t h e l a t t e r gives a p l a t e a u at +0.9V,
a n d it a p p e a r s a f t e r o x i d a t i o n at 1.0V.
T h e a b o v e e x p e r i m e n t p r o v e d t h a t f r o m t h e v e r y beg i n n i n g of o x i d e f o r m a t i o n S n c a u s e s c h a n g e s i n t h e r a t e s
of t h e o x i d a t i o n r e a c t i o n s i n s u c h a w a y t h a t a P b O n l a y e r
is f o r m e d a l r e a d y a t +0.8V.
1.0
0.6
0.4
02
After I h oxidation
in the dark
1.0
Determination of the bandgap of Sn-containing PbOn
layer.--The o x i d a t i o n of P b O to P b O n is a s s o c i a t e d w i t h
"PbO2/PbS04 (~)
After 1 h oxidation
in the dark
[Pb-l,0%Sn I
~,~
0,4
\
Pb-0.5% S n / P b O / P b S O 4 e l e c t r o d e f o r m e d at 0.6V for 15h
w a s c a r r i e d o u t t h r o u g h a series of l i g h t p u l s e s 'with 3 m i n
d u r a t i o n e a c h a n d 15 m i n d a r k p e r i o d s b e t w e e n t h e m . T h e
p h o t o c u r r e n t / t i m e c u r v e s at 1, 2, 3, a n d 6 l i g h t p u l s e s are
p r e s e n t e d in Fig. 5.
W h e n t h e l i g h t is s w i t c h e d on, a p h o t o c u r r e n t is~ flows.
During the illumination the photocurrent grows reaching
a m a x i m u m ism. I t w a s e s t a b l i s h e d t h a t t h i s p r o c e s s is d u e
to a n i n c r e a s e of t h e s t o i c h i o m e t r i c coefficient n of t h e
P b O n o x i d e ( w h e r e b y 1 < n < 1.5) (8). T h e d e c r e a s e of t h e
p h o t o c u r r e n t w a s r e l a t e d to t h e f o r m a t i o n of a n o x y g e n
l a y e r at t h e P b O n / s o l u t i o n i n t e r f a c e (8). W h e n t h e l i g h t is
s w i t c h e d off, t h i s o x y g e n as well as p a r t o f t h e P b O n is red u c e d , w h e r e b y a c a t h o d i c c u r r e n t id flows. A t t h e first m o m e n t t h i s c u r r e n t is id~ A f t e r s e v e r a l i l l u m i n a t i o n s of t h e
Pb-0.5% S n e l e c t r o d e w i t h i n c r e a s e of t h e n u m b e r of
p u l s e s t h e p h o t o c u r r e n t id~ d e c r e a s e s (Fig. 5 c o m p a r e s is~
at t h e 3rd a n d 6th pulse). F r o m Fig. 5 it is e v i d e n t t h a t : t h e
p h o t o c u r r e n t o f t h e S n c o n t a i n i n g e l e c t r o d e is h i g h e r duri n g t h e initial p u l s e s t h a n t h a t of p u r e lead electrode. Obviously, S n a c c e l e r a t e s t h e p r o c e s s o f P b O o x i d a t i o n a n d
makes the PbOn layer more electroconductive.
T h i s e l e c t r o c a t a I y t i c effect of S n w a s e x a m i n e d t h r o u g h
p h o t o a c t i v a t i o n of a series o f e l e c t r o d e s w i t h a d i f f e r e n t S n
c o n t e n t . F i g u r e 6 i l l u s t r a t e s t h e d e p e n d e n c i e s of i=~ a n d id~
o n t h e n u m b e r of l i g h t pulses. T h e s e v a l u e s are r e f e r r e d to
the maximum and minimum values of the respective
curves.
I n t h e c a s e of p u r e P b e l e c t r o d e s t h e m a x i m u m v a l u e of
i, ~ is r e a c h e d a f t e r 13 l i g h t pulses. With P b - S n alloys t h i s
v a l u e is r e a c h e d after less l i g h t pulses. T h e S n c o n t e n t in
t h e alloy affects s t r o n g l y p h o t o a c t i v a t i o n . A f t e r t h e m a x i m u m t h e c u r r e n t is~ d e c r e a s e s w i t h i n c r e a s i n g t h e a m o u n t
of l i g h t e n e r g y fallen o n t h e electrode. A t t h e Pb-0.5% S n
e l e c t r o d e t h e c u r v e r e a c h e s a s t a t i o n a r y v a l u e w h i c h is
a b o u t 30% of t h e m a x i m u m one. T h e b e h a v i o r of t h e id~
n u m b e r o f p u l s e s c u r v e s is s i m i l a r to t h a t o f t h e c u r v e s a t
illumination.
W h a t m i g h t b e t h e r e a s o n for p h o t o p a s s i v a t i o n of t h e
e l e c t r o d e ? We c a n s u p p o s e t h a t it is d u e to t h e f o r m a t i o n of
a photoinactive Pb-Sn oxide with the composition
Pb=SnO,~. F o r P b ~ S n O , , to b e f o r m e d a c e r t a i n a m o u n t o f
t i n h a s to e n t e r t h e oxide, a n d t h e s t o i c h i o m e t r i c coeffic i e n t of t h e o x i d e h a s to r e a c h a c e r t a i n v a l u e m. F i g u r e 6
s h o w s t h a t t h e h i g h e r t h e S n c o n t e n t in t h e oxide, t h e
f a s t e r t h i s critical a m o u n t of t i n is o b t a i n e d . T h e l i t e r a t u r e
r e f e r s to s o m e l e a d - t i n o x i d e s as lead o r t h o s t a n n a t e ,
Pb2SnO4, a n d lead m e t a s t a n n a t e , P b S n O 3 (11).
k~ ~...",--~
~
1.0 V
./_...__.._. PbO_____~n
~
"'"
PbO
,~,0.9 V
02
~ \\ lb
-02}
- o,
-0,61
\ 10v
v_&.._
t,min
\ o8v
Fig. 4. Potential decay of Pb and
Pb-1% Sn electrodes after lh oxidation at constant potentials in
the dark.
\t, min
"--
Pb
_0,6/o.7v
--
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J. Electrochem. Soc., VoI. 136, No. 1, January 1989 9 The Etectrochemicat Society, tnc.
30
Pb
Pb-Q5% Sn
(i,~).=
200
0
, , ,g'I
400
"0
Is
200 -I
0
-200"
i
I
i
I
=k
.,J
6oo
|
t-
f,=.
I=..
:3
i~(r)
400
0
Fig. 6. Dependence of the initial photocurrent (in relative units) and
the initial cathodic current in the dark (in relative units) on the number
of light pulses during photooctivation of Pb/PbO/PbS04 and Pb-Sn/PbO/
PbS04 electrodes.
20O
~ :i
12 l i g h t i r r a d i a t i o n s . F i g u r e 7 r e p r e s e n t s t h e s p e c t r a l
c u r v e s of P b a n d Pb-0.5% S n e l e c t r o d e s .
T h e b a n d g a p for Pb-0.5% S n e l e c t r o d e c o i n c i d e s w i t h
t h a t for p u r e P b , n a m e l y , 1.90 eV, w h i c h is v e r y close to t h e
v a l u e d e t e r m i n e d i n (12). It c a n b e c o n c l u d e d t h a t S n d o e s
n o t affect t h e b a n d g a p w i d t h .
A c c o r d i n g to s o m e r e s u l t s r e p o r t e d in l i t e r a t u r e a 0.5%
S n O c o n t e n t in t h e P b O S n O p o w d e r n a r r o w s t h e b a n d g a p
300
-2oob ':I
200
100
f
"
0,4
0
-100
0,3
-400
,
,
o123
,
t, min
,
-200
~ ,, ,
0123
t,rnin
Fig. 5. Photocurrent determined during the ist, 2nd, 3rd, and 6th
light pulses of the photoactivation of Pb and Pb-O.5% Sn electrodes.
Oxidation potential 0.6V. Intervals between two light pulses 15 min.
e l e c t r o n t r a n s f e r f r o m t h e P b ~+ ions of t h e P b O crystal lattice o v e r t h e b a n d g a p of t h e oxide. S n d e c r e a s e s t h e p o t e n tial at w h i c h P b O n f o r m a t i o n starts. Is t h i s n o t d u e to a
n a r r o w i n g of t h e b a n d g a p ? T h e optical b a n d g a p (Eg) w a s
d e t e r m i n e d f r o m t h e s p e c t r a l c u r v e m e a s u r e d a c c o r d i n g to
t h e c o n d i t i o n s d e s c r i b e d in (12).
A Pb-0.5% S n electrode, o x i d i z e d in t h e d a r k at +0.400V
for 2h, w a s p h o t o a c t i v a t e d t h r o u g h 3 l i g h t p u l s e s w i t h 3
m i n d u r a t i o n e a c h a n d 15 m i n d a r k p e r i o d s b e t w e e n t h e m .
After that the spectral curve was registered. The photoact i v a t i o n of t h e P b e l e c t r o d e w a s c a r r i e d o u t u n d e r t h e s a m e
c o n d i t i o n s b u t s u b j e c t i n g t h e P b / P b O / P b S O 4 e l e c t r o d e to
o,z
Pb-O,S%Sn / ]"
Pb
~
I
I
I
2.4
7A
2.2
2.3
E, eV
Fig. 7. Spectral curves of photoactivated Pb/PbOn/PbS04and
Pb-0.5% Sn/PbOn/PbS04electrodes formed at +400 mV.
1.7
I
1.8
1.9
I
2D
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J. Electrochem. Soc., Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc.
of the latter (13). In this case Pb(l_=)SnxO was formed by
mixing a solution of SnO in HC104 with Pb(CH3COO)2,
after which the obtained voluminous precipitate was
washed in 15N ammonia and then dried at ll0~ for 24h
(14).
It may be expected that the lead-tin oxide we obtained
-electrochemically will differ in composition from the
above-mentioned one, which may be the reason for the observed difference in the behavior of the bandgap.
Effect of Sn on the photocurrent/potential curve.--Electrodes, polarized at +0.400V in the dark for 15h, were
photoactivated using 3 min light pulses with 15 min dark
periods between them. Photoactivation was carried out
using 16 light pulses for Pb electrodes. 10 light pulses for
Pb-0.1% Sn ones, and 3 pulses for Pb-0.5% Sn electrodes.
After photoactivation the photocurrent/potential dependence was determined through the following procedures.
For each potential the electrode was polarized for 5 rain in
the dark, 3 min under illumination, and another 5 rain in
the dark. Thereafter the potential value was changed. Figure 8 presents the dependence of i~~ and id~ on the potential.
The following conclusions can be drawn from these results:
1. With increasing the potential, the photocurrent curve
of the pure Pb electrode passes through a m a x i m u m at
+0.95V and then drops due to the formation of PbO2. With
the Pb-0.1% Sn electrode the m a x i m u m is lower and appears at +0.85V, and with the Pb-O.5% Sn electrode it appears at about 0.55V after which a potential arrest follows
at about 0.85V. It can be concluded that Sn facilitates the
formation of a high-valency lead-tin-oxide at more negative potentials. The formation of such an oxide is evident
on Fig. 4, too, where the potential at open circuit draws a
plateau at about 0.5V.
2. When the light is switched off, the cathodic current ia~
at the Pb-Sn electrode is m u c h stronger than that at the
pure Pb one. This implies that Sn facilitates the formation
and arrest in the oxide layer and on its surface of considerably higher concentration of oxidized particles, O and O-,
which accept electrons causing a cathodic current flow.
7
9
/ "LPb-O'l%Sn
5-
//
,_
31
3. For Pb-Sn electrodes the potential at which i, ~ = O is
about 0.2V more negative than that for pure Pb electrodes.
Discussion of Results
Oxidation of PbO to PbOn.--On the basis of the photoelectrochemical behavior of Pb/PbO/PbSO4 electrode a
model of PbO oxidation has been developed (9). A scheme
presenting this model is depicted in Fig. 9.
At potentials higher than 0 V, light generates electronhole pairs
PbO + hv = PbOh + + e-
[4]
The electrons move towards the metal, while the hole towards the PbO/solution interface, where they react with
water forming OH radicals
PbOh + + H20--~ H + + OH + PbO
[5]
The OH radicals recombine between themselves or react
with OH ions to form O atoms or O radicals which enter
into the oxide crystal lattice forming PbOn
OH
4- OH
--~ H20
+ O
[6]
OH + OH- -~ H20 + O-
[7]
kPbO + m/2 0--~ kPbOn
[8]
P b O n contains Pb 3§ ions (S-centers) which readily exchange electrons thus creating a p-type conductivity in the
oxide. With increase of the amount of light energy fallen
on the electrode the amount of the oxygen formed and the
stoichiometric coefficient n of PbOn increase. As a result
of this process the oxide layer becomes more conductive.
This is in accordance with Lappe's experimental data (6).
The photocurrent is a function of the potential (Fig. 8).
This relation is represented by the expression
i - k(Eo~F - EO~/OH F)/e
[9]
where: K is a constant, e is the charge of a single electron,
EOH/OH-F is the Fermi level of the reaction OH-= OH + e-. It
is only the electrochemical stage of the reaction of PbO to
PbOn oxidation. When the OH/OH- redox electrode is in
equilibrium or near to that state, EOH/OHff is equal to the
equilibrium potential of the OH/OH- electrode. The value
of that potential is about -0.45V. EoxF is the Fermi level of
the oxide. The difference between the Fermi levels gives
the value of electrode polarization at the oxide/solution interface where the electrochemical reaction proceeds. The
position of the Fermi level in the oxide determines this polarization. At high concentrations of Pb 3+ ions the oxide
conductivity is also. high, the space charge and hence the
Pb
PbSO~
membr.a,.ne
PbOn
E,eV
~~
~
~=~
-I.0
0
e-
/
'
-05
Pb
E~-(OH/OH-)
0
h++OH;d
-IOC ~-
EF(M)
+10
-20(:
-- " ' ~
kPbO+mO:PbOn
1"
E F(PbOn 1. ~ - .
OHa4
20Had
0+
l
0
~
///
~,~
~./6
I
I
-o.,
I
;.2
I
o
I
G.2' d,~'
d.6'
I
& ' ,.o
Fig. 8. Dependence of/,~ and /d ~ on the potential for Pb, Pb-O.1%Sn,
Pb-0.5% Sn/lead oxide/lead sulfate electrodes.
PbO+hv=e'-~
Fig. 9. Model of theenergy band scheme of Pb/PbOn/PbS04electrode at polarization between 0 and 900 mV (6).
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32
J. E l e c t r o c h e m .
Soc.,.
Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc.
potential drop in t h e o x i d e layer are small. T h e potential
i m p o s e d on t h e e l e c t r o d e t h r o u g h t h e m e t a l of t h e elect r o d e system Pb/PbO/PbSO4 membrane/H2SO4 is transferred w i t h insignificant losses to the oxide/solution interface w h e r e the e l e c t r o c h e m i c a l reaction takes place. T h e
F e r m i level of the oxide d e p e n d s on t h e c o n c e n t r a t i o n of
P b 3+ ions in the o x i d e crystal lattice, a c c o r d i n g to t h e
equation
Eoxv = Ev - R T In
[Kh+/Nvb3+]
h+
- - P b - - 0 -- Pb-- 0 - - S r ~ O - - ' ~ P b - - O
--Pb--
h
I
- - P b - - 0 - - P b - - 0 - - S n - - O - - Pb-- 0 -- Pb--
- Sn--O--Pb--O--Pb--O--Sn--O-I
,
-- Pb-- 0 -- P b -
Pb-~h
0
+
"....
0 -- S n -
PbOn layer
9
Pb02 A.M.
H2SO~
solution
E eV
- 1.0 -
[10]
(where Ev is t h e v a l e n c y b a n d e d g e , K is a c o n s t a n t w h i c h
d e p e n d s on the effective mass of t h e holes). The concentration of P b 3+ (Npb3§ can g r o w up to a certain limit defined
by the condition: Eoxr = Ev, e.g., the o x i d e F e r m i level
reaches the v a l e n c y bandedge. When this c o n d i t i o n is fulfilled, a m a x i m u m is obtained. The s e m i c o n d u c t o r turns
into a d e g e n e r a t e d one and attains a c o n d u c t i v i t y e q u a l to
that of metals.
S u c h a high c o n c e n t r a t i o n of defects in the o x i d e howe v e r causes d i s p l a c e m e n t of the ions in the crystal lattice
t r a n s f o r m i n g it f r o m a tet-PbO into an ~-PbO2 one. As a result of t h e s e p h e n o m e n a , the p h o t o c u r r e n t decreases rapidly. This b e h a v i o r of the P b e l e c t r o d e is o b s e r v e d on Fig.
8, w h e n t h e potential e x c e e d s 0.9V.
L e t us n o w see in w h a t w a y will Sn affect this process?
W h e n a tin electrode is e l e c t r o c h e m i c a l l y o x i d i z e d in solutions w i t h ~0H from 2 to 12, Sn(OH)~ is formed. This comp o u n d is u n s t a b l e and is further d e h y d r a t e d (15), or oxidized to SnOa (15-17), w h e r e b y oxides with m i x e d v a l e n c y
are f o r m e d (16, 17). The s t a n d a r d e q u i l i b r i u m potentials of
S n O / S n and S n O d S n electrodes are v e r y near to one another, -0.784 and -0.786V, r e s p e c t i v e l y (15). On the basis
of this e l e c t r o c h e m i c a l b e h a v i o r t h e a s s u m p t i o n can be
m a d e that d u r i n g t h e o x i d a t i o n of P b - S n alloys Sn 2+, S n 3+,
and S n 4§ ions are f o r m e d and i n c o r p o r a t e d into the o x i d e
crystal lattice.
S i n c e tet-PbO and S n O are i s o m o r p h o u s , it m a y be exp e c t e d that the Sn ions are i n c o r p o r a t e d in the lead sublattice of t h e o x i d e and will create the s a m e t y p e of conductivity. Tin ions readily e x c h a n g e valencies b e t w e e n
t h e m s e l v e s and p r o b a b l y with P b 3+ ions. This can be illustrated by the s c h e m e on Fig. 10.
The potential curves on Fig. 4 s h o w that with the
Pb-0.5% Sn electrode a high-valency lead-tin-oxide is
f o r m e d in t h e dark at O.8V, w h i l e the p u r e P b e l e c t r o d e
P b O is not f o r m e d up to 0.95V. This potential difference
m a y be due to t h e following p h e n o m e n o n . The holes
created by the S n ions during o x i d a t i o n of t h e P b - S n grid
pass t h r o u g h the o x i d e corrosion layer and react with
w a t e r at the oxide/solution interface. As a result of this O
and O- radicals are formed. T h e y diffuse into t h e crystal
lattice of the o x i d e t u r n i n g it into a h i g h - v a l e n c y one,
w h i c h is illustrated by the potential arrest at o p e n circuit
0
Ev
0 -- Pb-- 0 -- Sn--
Fig. 10. Two-dimensional scheme of a PbO crystal lattice containing
Sn2§ and Sn3+ dopant ions.
O-
Eg
+I.0- M
EF
EF{PbO.)
=l(n) J
Charge
carriers
e-
h+
e-
(Ns+ Nsn~+)
Fig. 1 I. Model of the energy band scheme of Pb/PbO/PbO2/H2S04
electrode.
in the dark (Fig. 2, 4). In this case the c o n c e n t r a t i o n of
holes created by the tin ions m u s t also be i n c l u d e d in t h e
F e r m i level e q u a t i o n
EoxF = Ev + R T ln[Kh+/(Npb3+ + Ns,3+)]
[11]
Tin ions shift the F e r m i level towards the v a l e n c y band.
This will cause the reaction of P b O o x i d a t i o n to start at a
potential m o r e n e g a t i v e t h a n that for p u r e P b electrode,
w h i c h can be seen on Fig. 2, 3, and 5. Evidently, t h e concentration of holes in the o x i d e will d e p e n d on the concentration of Sn in the alloy. This a s s u m p t i o n is c o n f i r m e d by
Fig. 6.
Plate p a s s i v a t i o n . - - T h e positive plate consists of a porous active mass and a corrosion layer w i t h porous and
c o m p a c t sublayers. D u r i n g charge and discharge the corrosion layer e l e c t r o c o n d u c t i v i t y is realized by the following c h a r g e carriers (Fig. 11).
T h r o u g h the P b O n corrosion layer the current is carried
by holes, w h i l e in the P b Q active mass by electrons.
D u r i n g floating charge of t h e stationary battery the
charge current is m a i n t a i n e d at the lowest possible level.
A l t h o u g h at a low rate P b is o x i d i z e d to PbO. This process
d e c r e a s e s the stoichiometric coefficient of the o x i d e corrosion layer. T h u s its F e r m i level m o v e s a w a y from the val e n c y band. The c o n d u c t i v i t y of the corrosion layer decreases. On t h e othe r hand, at t h e interfaces P b Q active
mass/solution and corrosion layer/solution, o x y g e n is
f o r m e d w h i c h penetrates into the P b O n layer increasing
the v a l u e of n. The ratio b e t w e e n the rates of t h e s e two processes d e t e r m i n e s the v a l u e of n and the F e r m i level position.
As was m e n t i o n e d in the i n t r o d u c t i o n u n d e r certain conditions of battery p r o d u c t i o n and operation, tet-PbO is
f o r m e d on the grid surface.
W h e n t h e grid is m a d e of a tin-containing alloy, in the
process of its corrosion S n 3+ ions are i n c o r p o r a t e d in the
o x i d e crystal lattice. These ions bring a b o u t an increase in
h o l e c o n c e n t r a t i o n and, probably, in hole mobility, too.
This, on t h e one hand, causes the P b O to P b O n o x i d a t i o n
to start at m o r e n e g a t i v e potentials, and, on the other hand,
increases the o x i d a t i o n rate. In this w a y Sn changes the
ratio b e t w e e n t h e rates of o x i d a t i o n of ]Pb and P b O and of
P b O to P b O n in favor of the second reaction. Thus the form a t i o n of a tet-PbO layer is avoided. The passivation phen o m e n a c a u s e d by tet-PbO are eliminated.
M a n u s c r i p t r e c e i v e d Feb. 12, 1988.
Politecnico di Torino assisted in meeting the p u b l i c a t i o n
costs o f this article.
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J. Electrochem. Soc., Vol. 136, No. 1, J a n u a r y 1989 9 The Electrochemical Society, Inc.
REFERENCES
1. K. R. Bullock and M. A. Butler, This Journal, 133, 1085
(1986).
2. H. K. Giess, in "Advances in Lead-Acid Batteries,"
K.R. Bullock and D. Pavlov, Editors, p. 241, The
Electrochemical Society Softbound Proceedings
Series, PV 84-14, Pennington, NJ (1984).
3. K. Wiesener, J. Garche, and N. Anastasijevic, Power
Sources, 9, 17 (1983).
4. D. Pavlov and T. Rogachev, Electrochim. Acta, 23, 1237
(1978).
5. D. Pavlov and Z. Dinev, This Journal, 127, 855 (1980).
6. F. Lappe, J. Phys. Chem. Solids, 23, 1563 (1962).
7. D. Pavlov, C. N. Poulieff, E. Klaja, and N. Iordanov,
This Journal, 116, 316 (1969).
8. D. Pavlov, S. Zanova, and G. Papazov, ibid., 124, 1522
(1977).
33
9. D. Pavlov, J. Electroanal. Chem., 118, 167 (1981).
10. D. Pavlov, S. Zanova, B. Monakhov, M. Maja, and E.
Angelini, Ext. Abstr. 32 Meeting ISE, Dubrovnik,
Yugoslavia, Vol. I, p. 142 (1981).
11. D. Greniger, V. Kollonitsch, and Ch.H. Klein, "Lead
Chemicals," pp. l l l - l l 3 , ILZRO, New York (1975).
12. M. Dimitrov, K. Kochev, and D. Pavlov, J. Electroanal.
Chem., 183, 145 (1985).
13. J. van den Brock and A. Netten, PhiIips Res. Rep., 25,
145 (1970).
14. W. Kwestroo, J. H. van den Biggelaar, and C. Langereis, Mater. Res. Bull., 5, 307 (1970).
15. C. I. House and G. H. Kelsall, Electrochim. Acta, 29,
1459 (1984).
16. S.N. Shah and D. E. Davies, ibid., 8, 663 (1963).
17. R. O. Ansell, T. Dickinson, A. F. Povey, and P. M. A.
Sherwood, This Journal, 124, 1360 (1977).
Application of Anisotropic Graphite to Sealed Lead-Acid
Batteries
A. Tokunaga, M. Tsubota, and K. Yonezu
Japan Storage Battery Company, Limited, Nishinosho, Kisshoin, Minami-ku, Kyoto, Japan
ABSTRACT
Anisotropic graphite was used as an additive to the positive paste to improve the discharge performance of sealed
lead-acid batteries. The discharge capacity increased with the a m o u n t of graphite, particularly at high discharge rates and
at low temperatures. This is attributed to the increased amount of electrolyte retained in the pores of the swollen positive
plates that result from the addition of the graphite.
Anisotropic graphite has a u n i q u e property that it forms
intercalation compounds and expands during anodic oxidation in sulfuric acid. We have succeeded in improving
the discharge performance of pasted-type lead-acid batteries with flooded electrolyte by adding graphite as de-
I
I
I
I
scribed in a previous paper (1). This improvement was attributed to the increase in porosity of the positive active
mass owing to the expansion of the graphite.
On the other hand, retainer-type lead*acid batteries with
an oxygen recombination reaction have a limited amount
of electrolyte, which is immobilized in the plates and absorbent separator materials.
The diffusion of acid in these types of batteries is rather
restricted compared with conventional flooded ones (2, 3).
1.6
I
I
I
I
I
I
3.0 - ~
1.5
<5
L4
"~
0~
o_
I
$7
1.5
c.)
(D
~
o
._>
rw
2.0
-
@
1.2
re
#3
as
4
/.55
I.I
O
~
2
"
1
1.0
0
I
Q2
I
0.4
I
0.6
I
0.8
I.O
1.0
A m o u n t of graphite(W/o)
Fig. 1. Variation in relative capacity vs. amount of graphite. Discharge rate: curve 1, 0.05C; curve 2, 0.2C; curve 3, 1.0C; curve 4,
3 . 0 C ; curve 5, 5.0C. Temperature: 25~
I
-50
I
- 20
0
I
- I0
~
0
~
I
0
0
1
I
IO
I
20
I
50
T e m p e r a t u r e (~
Fig. 2. Variation in relative capacity at the 5C rate vs. discharge
temperature. Graphite: curve 1, 0.1 (w/o); curve 2, 0.3 (w/o); curve 3,
0.5 (w/o); curve 4, 1.0 (w/o).
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