Cyclic voltammetry on lead electrodes in sulphuric acid
solution
Citation for published version (APA):
Visscher, W. (1977). Cyclic voltammetry on lead electrodes in sulphuric acid solution. Journal of Power Sources, 1(3), 257-266. https://doi.org/10.1016/0378-7753(76)81003-8
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10.1016/0378-7753(76)81003-8 Document status and date: Published: 01/01/1977 Document Version:
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Journal of Power Sources, 1 (1976/77) 257 - 266 257 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
CYCLIC VOLTAMMETRY ON LEAD ELECTRODES IN SULPHURIC ACID SOLUTION
W. VISSCHER
Laboratory for Electrochemistry, Eindhoven University o f Technology, Postbox 513, Eindhoven (The Netherlands)
(Received June 26, 1976; in revised form September 19, 1976) S u m m a r y
The oxidation o f lead in 5 M H2SO4 was studied by cyclic v o l t a m m e t r y . When a potential scan is applied from --1.0 V to 2.6 V vs.
R.H.E. the P b S O J P b O 2 o x i d a t i o n peak can be observed in the anodic voltammogram provided the scan rate is 0.16 mV/s or lower. When t h e potential scan is restricted to the potential range +0.6 V to +2.6 V t h e anodic voltammogram shows two peaks which were assigned to t h e f o r m a t i o n o f ~-PbO2 and /3-PbO 2 respectively. This ~-PbO2 is formed u n d e r n e a t h t h e PbSO4 film. During the reverse sweep the main reduction peak at 1.65 V corresponds with the reduction o f ~-PbO2 to PbSO4. ~- PbO2 is n o t reduced at a definite potential b u t it is reduced to n-PbO. PbSO4 with n increasing from the o x i d e - e l e c t r o l y t e interface towards the interior of the electrode and with more cathodic potential. At potentials below 0 V, the basic lead sulphates are reduced.
The effect o f the addition o f small a m o u n t s o f H3PO 4 to t h e H~SO4 electrolyte during t h e potential scanning results in an increase o f the a- PbO2 peak and a disappearance o f the ~-PbO2 peak.
I n t r o d u c t i o n
The o x i d a t i o n of Pb in H2SO 4 to PbSO4 and PbO2 and t h e corresponding reduction reactions have been extensively studied b o t h at Pb metal electrodes and at b a t t e r y plates [1, 2]. Insight into the role of the PbSO4 film during t h e anodic oxidation process was given b y Ruetschi
[3, 4]. Using PbSO4 membranes Ruetschi [3] showed t h a t t h e PbSO4 film is impermeable for SO24 - and HSO4 ions b u t permeable for H ÷ ions. This accounts for t h e setting up o f a potential difference across the PbSO4 layer in such a way t h a t during anodic oxidation a region of high pH is created u n d e r n e a t h the PbSO4 film. In this region lead oxide and basic lead sulphates can be formed. This explains w h y several basic lead sulphates are observed in b a t t e r y plates.
Linear sweep v o l t a m m e t r y has been applied by Panesar [ 5 ] , Cart e t a l .
tion behaviour o f Pb, antimonial Pb and also of pure a-PbO2 and ~-PbO2 electrodes. The studies show t h a t only one peak (Pb/PbSO4) is observed when the potential is swept from --0.7 V to +2.4 V vs. R.H.E. at scan rates of 0.5 mV/s and 58 mV/s. Evidence o f the subsequent f o r m a t i o n o f PbO2 a n d / o r PbOte t could only indirectly be found from t h e cathodic
voltammogram. At low speed rates (10 mV/s) Cart e t al. [6] observed a current arrest corresponding to PbO2 formation. When the potential sweep is restricted to t h e potential region 1.5 to 2.4 V Panesar obtained t h e PbSO4/ PbO2 oxidation peak in the anodic voltammogram.
This paper reports results obtained b y linear sweep v o l t a m m e t r y on Pb electrodes in 5 M H2804 and the effect o f t h e addition o f small amounts of HsPO4 to this electrolyte.
Experimental
A pure lead rod (99.999%) was embedded in a Perspex holder such t h a t an area o f 0.64 cm 2 was exposed. The electrode was placed in the usual electrochemical cell with a Pt c o u n t e r electrode and a Pt hydrogen electrode as reference electrode. The electrolyte was 5 M H2804 (prepared from p.a. H2804) and flushed with N2. All experiments were carried o u t at r o o m tem- perature.
The potential scan could be programmed with a p o t e n t i o s t a t (Wenking) and voltage scan generator (Wenking SMP 69 or VSG 72). The p o t e n t i a l - current response was recorded on a XY recorder (Philips PM 8120). Before the experiments the electrode was polished and t h e n cathodically polarized in t h e same cell and electrolyte with 350 m A / c m 2
Results and Discussion
P o t e n t i a l range - - 1 . 0 V to +2.6 V
When a cyclic sweep is performed from --1.0 V to 2.6 V at sweep rates of 50 mV/s, t h e v o l t a m m o g r a m (Fig. 1) shows one p r o n o u n c e d anodic peak (A) at --0.28 V and three cathodic peaks: at +1.65 V (peak d), at --0.32 V (peak b), and at --0.44 V (peak a). The small anodic peak ~ is observed during the cathodic sweep at 1.6 V. In the high anodic potential region t h e current increases steeply owing to the 02 evolution reaction. During the reverse sweep this current is higher t h a n during t h e preceding anodic sweep, due to an apparently higher catalytically active surface. This change of the surface properties is also indicated by the occurrence o f peak d; peak d is due to the reduction o f PbO2 to PbSOa*, thus during the anodic sweep PbO2 f o r m a t i o n takes place simultaneously with 02 evolution. The
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I I ¸ l a b d i i i i i i I i i - 0 . 8 -Off 0 0.~ 0.8 1.2 1.6 2.0 2,£ " P O T E N T I A L / V .Fig. 1. V o l t a m m o g r a m o£ a Pb anode (0.69 em 2) in 5 M H2SO 4 in the p o t e n t i a l range --1.0 V t o +2.6 V. Sweep rate: 50 m V / s . - . . . , A n o d i e part o f the v o l t a m m o g r a m f o r a sweep rate o f 0.16 m V / s .
voltammogram is strongly d e p e n d e n t u p o n sweep rate and potential range (lower limit and upper limit) e.g. application o f a potential scan, at the same scan rate, from --1.0 V to +2.2 V does n o t show the reduction peak d; however, when t h e potential is held at 2.2 V for a few minutes, peak d is observed.
This shows t h e slow f o r m a t i o n o f PbO2. Evidence o f f o r m a t i o n of PbO 2 in the anodic voltammogram can only be observed at very slow scan rates; at 0.16 mV/s an anodic peak C is seen at 2.05 V (Fig. 1).
Peak A corresponds with the oxidation o f Pb to PbSO4 (reaction 1). The peak b in t h e reduction sweep is present only if the upper limit o f t h e potential has been more anodic t h a n 1.0 V. For potential sweeps to values between 0.1 and 1 V and at sweep rates from 0.5 to 50 mV/s only reduc- tion peak a is observed b u t n o t peak b (Fig. 2).
P o t e n t i a l range +0.6 to +2.6 V
When the cyclic sweep is performed f r o m 0.6 to 2.6 V
(after a preced-
ing cycle -- 1.0 to 2.6 V t o 0.6 V), the peak corresponding with PbSO4 oxida- tion is gradually built up with repeated cycling. The anodic diagram (Fig. 3) shows 2 anodic peaks B and C. Depending upon the sweep rate, peak B andSCALE(I} SCALE(2)
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Ill ~ T 2 -0.8 -0.6 -0." \,./~" I -0.2 7.5 5 2.S 2.5 5 I 0 - - ~ POTENTIAL / VFig. 2. V o l t a m m o g r a m o f a Pb e l e c t r o d e in 5 M H2SO 4. 1, P o t e n t i a l range --1.0 V to +1.0 V, s w e e p rate 0.5 m V / s ; 2, p o t e n t i a l range --1.0 V t o +1.77 V, sweep rate 25 m V ] s (only
c a t h o d i c part).
C are f o u n d at 1.67 and 1.94 V for v = 0.5 mV/s and at 1.96 and 2.20 V for v = 50 mV/s. Reduction peak d is much less d e p e n d e n t upon the sweep Velo- city: 1.68 V at v = 0.5 mV/s and 1.60 V at v = 50 mV/s.
During the reverse sweep a small anodic peak ~ occurs at 1.58 V during the reduction cycle. After a few cycles this peak decreases and finally disappears. With prolonged cycling peak B decreases while peak C increases.
If t h e lower limit o f the potential scan is set at 1.22 V instead o f 0.6 V the same results are obtained. In our opinion b o t h peaks B and C must corres- pond with PbO2 formation. This is concluded from the fact t h a t it has been well established [1, 2] t h a t f o r m a t i o n of a-PbO2 or ~-PbO2 depends on the pH o f the electrolyte. Even in strong acid solution a-PbO2 can be f o r m e d be- cause u n d e r n e a t h t h e PbSO4 film on Pb a film of different pH can exist [ 4 ] . This leads t o f o r m a t i o n o f lead oxide [9 - 11]. Therefore, the o x i d a t i o n o f lead oxide to a-PbO2 (under the PbSOa film) must be indicated by peak B. Peak B occurs o n l y if during the potential scan the electrode is n o t reduced to potentials at which also PbSO4 can be reduced, hence peak B is manifest o n l y in the scan range 0.6 to 2.6 V. At peak C PbSO4 is oxidized to/~-PbO2. If the o x i d a t i o n cycle is started at --1.0 V, peak C is the o n l y peak present as shown by the broken line of Fig. 1. So, it must be concluded t h a t under these circumstances alkaline conditions c a n n o t be built up sufficiently u n d e r the PbSO4 film, since this latter is t o o thin and no a-PbO2 can be f o r m e d underneath.
On pure ~-PbO2 electrodes Panesar [5] observed in the anodic sweep (scan range 1.50 to 2.40 V; 0.5 mV/s) one anodic peak at 2.1 V, corresponding
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_A~'~., I j r I i 0 3 6 0 ,'.0 ?0 20 -'0 60 1.2 1.6 2.0 2.~ - ~ p P O T E N T I A L / VFig. 3. V o l t a m m o g r a m o f a Pb electrode in 5 M H2SO4, potential range +0.6 V to 2.6 V. 1, First cycle recorded after applying the potential program - - 1 . 0 V to 2.6 V to 0.6 V; s w e e p rate 50 m V / s . With repeated cycling b e t w e e n 0.6 and 2.6 V the peaks B and C are built up; 2, recorded after 8 cycles; s w e e p rate 5 m V / s .
to peak C in this work. On pure ~-PbO2 electrodes Panesar obtained a wave {plateau) at 2.2 V, which shifted to less anodic potentials with repeated cycling.
Peak d corresponds t o the reduction o f PbO 2. The reversible potentials o f a - P b O 2 / P b S O 4 and ~ - P b O 2 / P b S O 4 in H 2 S O 4 differ only by 8 mV, (see eqns. 10 and 11), therefore distinct reduction peaks can only be expected if the rate processes o f the reduction differ considerably and/or if the local pH is different. With freshly prepared ~ - P b O 2 electrodes we observed a reduction peak at 1.7 V. Panesar [5] found a broad reduction peak with a preceding plateau. After a f e w cycles this plateau has developed into the main reduc- tion peak. Cart e t al. [6] observed for a-PbO2 a shoulder following the re- duction peak, using a higher scan rate (58.3 mV/s). With repeated cycling, the main peak decreased and the shoulder increased, which they attributed to a decrease in the a peak and an increase in ~-PbO2. For pure ~-PbO2 a re- duction peak at about the same potential was observed. The height o f peak d depends on the upper limit o f the anodic sweep. With increasing potential limit peak d increases.
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iO t5 2O s d I I I J I I I 1.0 1.2 I,~ 1.6 1.8 2.0 2.2 ~ P O T E N T I A L / VFig. 4. Voltammogram o f a Pb electrode in 5 M H2SO 4 for the potential range 0.85 to 2.12 V (curve 1) and 0.85 to 1.90 V (curve 2), sweep rate 25 mV/s.
If the anodic sweep is reversed at potential limits situated b e t w e e n peak B and C then peak d becomes very small (Fig. 4). This clearly indi-
cates that peak C and peak d correspond to the same reaction, i.e. the ~-
PbO2/PbSO4 reaction. The reduction process of ~-PbO2 begins at the ~- P b O 2 / H 2 8 0 4 electrolyte interface and gradually moves inwards towards the Pb electrode. Part o f the underlying a-PbO2 is reduced together with ~-PbO 2
to PbSO4. The PbSO4 layer impedes the diffusion o f H 2 S O 4 to the inner part
of the electrode, so, depending on the availability of H 2 8 0 4 and the pH, -PbO2 is reduced to n-PbO. PbSO4 with n increasing towards the electrode and with more cathodic potential. During potential scanning this conversion process t a k e s place gradually until finally the potential is reached at which basic lead sulphate can be reduced.
The potential at which ~-PbO2 is reduced to n-PbO-PbSO4 varies from 1.468 V for n = 1 to lower values (see eqns. 12 - 15). In the voltammogram no reduction peak is observed which could be attributed to ~-PbO2 r e d u c t i o n The reason for this must be that the a m o u n t o f the basic lead sulphates is small and strongly d e p e n d e n t u p o n pH.
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II/ / ! BI d I I i I J I 0 2 0.6 LO I./-, 1.8 2.2 2.6 ~ P O T E N T I A L / V /Fig. 5. Effect of H3PO 4 addition on the vol.tammogram of a Pb electrode in 5 M H2SO 4. Potential range 0.6 V to 2.6 V; sweep rate, 25 mV/s. 1, Recorded after 10 sweeps; 2, recorded after addition of 0.01 M H2PO 4.
D u r i n g t h e first c a t h o d i c s w e e p t h e r e d u c t i o n p e a k d is f o l l o w e d b y an a n o d i c p e a k ~ (Fig. 3). This p e a k s h o u l d b e assigned t o t h e o x i d a t i o n o f n e w l y e x p o s e d Pb t o P b O and basic s u l p h a t e s , as well as t o t h e o x i d a t i o n o f t h e s e l a t t e r p r o d u c t s t o a-PbO2. With r e p e a t e d c y c l i n g 5 disappears.
At p e a k b, P b O is r e d u c e d t o Pb, while at p e a k a PbSO4 is r e d u c e d t o Pb. Panesar argues t h a t a t b, a-PbO2 is r e d u c e d , b e c a u s e a close c o r r e s p o n - d e n c e was o b s e r v e d b e t w e e n t h e p e a k s d a n d b. H o w e v e r , his results s h o w t h a t p e a k d is also f o u n d w h e n t h e e l e c t r o d e is h e l d f o r 1 6 h at 4 5 0 m V . At this p o t e n t i a l a-PbO2 c a n n o t b e f o r m e d [cf. eqns. (8), (11) - ( 1 4 ) ] . Like- wise R u e t s c h i [3] d i s p u t e s Panesar's c o n c l u s i o n regarding p e a k b. On t h e basis o f his film t h e o r y , R u e t s c h i calculates t h a t at p H 9 . 3 4 P b O is r e d u c e d t o Pb at - - 0 . 1 5 V.
~o 4O < E \ ~_ 3 0 i1: 20 _¢2 o o v 10 z LO 2o I I I I I I I _ _ 0.6 1.0 I." 1.8 2.2 2.6 ---~" P O T E N T I A L / V Fig. 6. V o l t a m m o g r a m o f Pb in I M H3PO 4.
Effect o f addition o f H3PO 4
Because it is well k n o w n t h a t small a m o u n t s of phosphoric acid, improve the cycle life of b a t t e r y plates [2, 12, 1 3 ] , t h e effect of the addition o f H3PO 4 was studied.
Small quantities o f H3PO4 were added to the sulphuric acid electrolyte after the Pb electrode had been subjected to several sweeps in the potential region 0.6 to 2.6 V (v = 50 mV/s). Addition of 0.01 M H3POt to 100 ml of electrolyte at first slightly increases peak B (Fig. 5), while peak C disappears in the anodic sweep; in t h e cathodic sweep peak d is reduced. With repeated cycling peak B shifts in t h e anodic direction (after m a n y cycles peak B' = 1.95 V), and peak d is reduced further.
The effect depends u p o n the a m o u n t o f t h e H3PO4 addition. Under t h e experimental conditions used here, C had completely disappeared after t h e addition o f 0.007 M H3PO4, peak d is t h e n reduced to 50% o f t h e value be- fore t h e H3PO t addition. Prolonged cycling w i t h o u t H3PO 4 addition does n o t effect peak d while B decreases and C increases somewhat.
For comparison a v o l t a m m o g r a m on Pb was run in 1 M H3PO 4 in t h e potential range 0.6 to +2.6 V (v = 50 mV/s) and this shows (Fig. 6) a broad m a x i m u m at 1.8 V and two reduction m a x i m a at 1.5 and 0.85 V.
The results o f Fig. 5 suggest t h a t in the presence o f small a m o u n t s of H3PO 4 t h e f o r m a t i o n of/3-PbO2 on a Pb electrode is inhibited. This could
265
t h e n i m p l y increased a - P b O 2 f o r m a t i o n w h i c h is in a g r e e m e n t w i t h t h e o b s e r v e d f a c t [ 5 ] t h a t increase in a - P b O 2 f a v o u r s c y c l e life. S o m e w o r k e r s
[ 2 ] suggest t h a t in t h e p r e s e n c e o f H s P O 4 p l u m b o u s p h o s p h a t e is f o r m e d at t h e e l e c t r o d e w h i c h is t h e n o x i d i z e d t o p l u m b i c p h o s p h a t e . In excess s u l p h u r i c acid h y d r o l y s i s leads t o t h e s e p a r a t i o n o f t h e t w o lead d i o x i d e m o d i f i c a t i o n s .
R e f e r e n c e s
1 J. Burbank, A. C. Simon and E. Willihnganz, Adv. Electrochem. Electrochem. Eng., 8 (1971) 157.
2 J. P. Carr and N. A. Hampson, Chem. Rev., 72 (1972) 679. 3 P. Ruetschi, J. Electrochem. Soc., 120 (1973) 331.
4 P. Ruetschi and R. T. Angstadt, J. Electrochem. Soc., 111 (1964) 1323.
5 H. S. Panesar, in D. H. Collins (ed.), Power Sources, Vol. 3, Oriel Press, Newcastle upon Tyne, 1970, p. 79.
6 J. P. Carr, N. A. Hampson and R. Taylor, J. Electroanal. Chem., 33 (1971) 109. 7 M.P. Brennan, B. N. Stirrup and N. A. Hampson, J. Appl. ElectroChem., 4 (1974} 49. 8 T. F. Sharpe, J. Electrochem. Soc., 122 (1975) 845.
9 J. Lander, J. Electrochem. Soc., 98 (1951) 213; 103 (1956) 1. 10 D. Pavlov, Ber. Bunsenges. Phys. Chem., 71 (1967) 398. 11 D. Pavlov, Electrochim. Acta, 13 (1968) 2058.
12 S. Tudor, A. Weistuch and S. H. Davang, J. Electrochem. Technol., 3 (1965) 90; 4, (1966) 406.
13 A. H. Taylor, F. Goebel and J. Giner, in D. H. Collins (ed.), Power Sources, Vol. 4, Oriel Press, Newcastle upon Tyne, 1972, p. 561.
14 P. Ruetschi, J. Skarchuk and R. T. Angstadt, Electrochim. Acts, 8 (1963) 333.
A p p e n d i x (1) Pb + H S O l -* P b S O 4 + H ÷ + 2e e = 0 . 3 0 2 - - 0 . 0 2 9 5 p H - - 0 . 0 2 9 5 log aHSO; Ref.
[2]
(2) Pb + SO~-
-~ PbSO4 + 2e e = - 0 . 3 5 6 - - 0 . 0 2 9 5 log aso~- (3) 2 Pb + S O ~ - + H 2 0 -~ P b O ' P b S O 4 + 2 H ÷ + 4e e = - - 0 . 0 9 9 - - 0 . 0 2 9 5 p H - - 0 . 0 1 4 8 log asoI- 0 . 1 1 3 - - 0 . 0 2 9 5 p H - - 0 . 0 1 4 8 logaso~-
(4) 4 Pb + SO~- + 4 H 2 0 - ~ 3 P b O ' P b S O 4 . H 2 0 + 6 H ÷ + 8e e = + 0 . 0 3 7 - - 0 . 0 4 4 3 p H - - 0 . 0 0 7 4 log aso~- + 0 . 0 3 0 - - 0 . 0 4 4 p H - - 0 . 0 0 7 4 log a s o ~- [ 2 ] [ 4 ] [ 2 ] [41 [21 (5) 5 Pb + 7 H 2 0 - ~ 5 P b O - H 2 0 + 10 H ÷ + 1 0 e e = + 0 . 2 6 0 - - 0 . 0 5 9 1 p H [ 4 ] (6) Pb + H 2 0 - * P b O + 2 H + + 2e e = 0 . 2 4 8 - - 0 . 0 5 9 1 p H e = 0 . 2 4 2 - - 0 . 0 5 9 1 p H[2]
[lO]
( 7 ) 5 P b + S O 2 - + 4 H 2 0 -* 4 P b O ' P b S O 4 + 8 H ÷ + 1 0 e e = 0 . 1 1 5 - - 0 . 0 4 7 p H - - 0 . 0 0 6 l o g a s o ~-
