Determination of the phase composition of anodic lead(II) film formed in sulfuric acid solution

Journal of Electroanalytical Chemistry, 368 (1994) 43-46

43

Determination of the phase composition of anodic lead( II) film formed in sulfuric acid solution Jun Han, Cong Pu and Wei-Fang Zhou

l

Department of Chemistry, Fudan Vniversi@, Shanghai 200433 (China) (Received 6 May 1993; in revised form 2 June 1993)

Abstract The phase composition of the anodic film formed on a Pb electrode at 0.9 V (vs. Hg/Hg,SO,J in 4.5 M HrSO, (25°C) was analyzed by photocurrent spectroscopy, linear sweep voltammetry, and open-circuit decay curves. The experimental results show that the film is composed of o-PbO, t-PbO, PbO . PbSO, and PbSO,. The growth rates of the components of the film are discussed.

1. Introduction

X-ray diffraction (XRD) techniques have been used traditionally to identify the phase composition of the anodic lead(R) film formed in sulfuric acid solution. However, some XRD lines overlap with those of different lead compounds. For example, the XRD lines of 0.295 nm and 0.333 nm are common to o-PbO and PbO . PbSO,, and to PbSO, and PbO * PbSO, respectively. Moreover, XRD is not a good method for quantitative analysis of this phase compostion [l]. In the present work, we tried to use the photocurrent method in conjunction with linear sweep voltammetry (LSV) to analyze quantitatively the said composition. 2. Experimental The experimental set-up for measuring the photoelectrochemical currents consisted of the following units: a stabilized 250 W halogen-tungsten lamp, a Suzhou WDGOS-III monochromator, an EG& G PARC 197 chopper, an EG&G PARC 5209 lock-in analyzer, a Shanghai JH2C potentiostat, and a photoelectrochemical cell (PEC). The light beam was chopped at a frequency of 15 Hz. The intensity of the monochromatic light at the position of the quartz window of the PEC was measured by a Hamamatsu R955 photomulti-

l

To whom correspondence

0022-0728/94/$7.00 SSDI 0022-0728(93)03074-Y

should be addressed.

plier tube (spectral response range 160-900 nm), which was connected to a Hamamatsu DP type socket assembly C956-06 (output voltage range - 200 to - 1250 V), and the above-mentioned lock-in analyzer. Corrections [2] were made for reflection losses at both the window of the PEC and the working electrode, as well as for absorption losses in the electrolyte. The incident photon fluxes were controlled to be less than 4 x lOi cmp2 s-l in order to avoid changes in the growth rate of the given film. The photoelectrochemical current, measured by the lock-in analyzer, was converted into the quantum yield with respect to the incident radiation. Linear sweep voltammetry was carried out using an EG&G PARC 273 potentiostat-galvanostat and a Shanghai LZ3-204 recorder. The electrode under investigation was a section of lead rod (99.99% pure). This was sealed with epoxy resin at the lower part of an L-shaped glass tube, exposing a circular area of 0.283 cm2 to the electrolyte. A flat working-electrode surface was obtained by mechanical polishing with emery paper of successively decreasing grain size down to about 10 pm. Afterwards, the working electrode was rinsed with distilled water, and then placed in the PEC behind an optically flat quartz window. The electrode surface was vertical to that of the electrolyte. The counter-electrode was a platinum wire surrounding the working electrode concentrically for photocurrent spectroscopy, and was a platinum plate facing the working electrode for LSV. An Hg/Hg,S0,/4.5 M H,SO, electrode served as the 0 1994 - Elsevier Sequoia. All rights reserved

J. Han et al. / Pb(ll) film formed in H,SO,

44

solution

reference electrode. All potentials are reported with respect to this electrode. The electrolyte was a 4.5 M H,SO, solution prepared from AR H,SO, and distilled water. All the measurements were performed in a shielded room at a temperature of 25 f 2°C. 0.9 V was chosen as the anodizing condition for forming the film. The Pb(I1) species dissolving in the 4.5 M H,SO, from the anodic film were determined using a Shanghai 301 atomic absorption spectrophotometer.

Table 1. Thickness of t-PbO film d, and o-PbO film d, in the anodic PM10 film formed in 4.5 M H,SO, (25°C) on Pb at 0.9 V for different periods of time t

3. Results and discussion

sections A and B can be attributed to t-PbO film, and to both t-PbO film and o-PbO film respectively. The ratio of the amounts of t-PbO and o-PbO in the anodic film increases with increasing anodizing time (vide infra). Hence, the curve in Fig. 1 changes to a straight line if the anodizing time is long enough for t-PbO to become the main component of the anodic film (Fig. 1, curve 4). It can be concluded from Fig. 1 that eqn. (1) is valid for t-PbO between 2.3 and 2.9 eV. If the quantum yield measured, Y,, with photon energy corresponding to that of section B, is much smaller than unity it may be expressed by

3.1. Determination of t-PbO and o-PbO using photocurrent spectroscopy 3.1.1. Quantum yields of the t-PbO film o-PbO film Y,

Y,, and

If the minority carrier diffusion length is much less than l/a, the reciprocal of the optical absorption coefficient of the film for photons of energy hv, we have 131 [ -hv

ln(1 - Y)lm =B(hv

-Eg)

(1)

where B is a constant and E, is the band gap energy. The exponent m is l/2,2/3 or 2 when the transition is indirect, forbidden direct or allowed direct respectively. Figure 1 shows [ -hv ln(1 - Y)]‘.’ vs. hv plots for the anodic film formed on Pb at 0.9 V in 4.5 M H,SO, (25°C) for different periods of time. For shorter anodizing periods, the curves in Fig. 1 can be divided into two sections A and B. It has been suggested [4] that the

(2.30 eV)

(2.95 eV)

0.0030 0.0075 0.0127 0.0199

Y, =

31 76 128 201

0.0431 0.0654 0.0920 0.1033

72 109 154 172

r, + Y,

(2)

Here yt can be obtained by extrapolating section A, and Y, can be obtained using eqn. (2) accordingly (Table 1). Figure 2 shows the [ - hv ln(1 - Y,)]‘.’ vs. hv plots for anodic films formed on lead at 0.9 V in 4.5 M H,SO, (25°C) solution. The intercepts of the two curves in Fig. 2 are both at 2.59 eV, close to the E, value of o-PbO, 2.61 eV [5]. The intercepts of sections A of the

0.60

2.0 0

n

0

1.6 1

/

4

0.48 -

B.

N2

n/

2

3

0

i

0

_c;-

0.36-

n

A’

0

0

0 urn

q

mm m@oOO

2.08

I s c 0.24-

0°

n

n

2

2

n

0

0

f L

I

0

l l

0.12 -

.a’

2.36 2.64 hv/eV

2.92

3.20

Fig. 1. [ - hv In0 - Y )I’/’ us. hv plots for the anodic films formed on Pb at 0.9 V in 4.5 M H,SO, solution (25°C) for different periods of time: (1) 2 h, (2) 7 h, (3) 20 h, (4) 40 h.

0.001 2.40

I

I

2.52

IY I 2.64

I

I

2.76

I

I

2.88

I

3.00

hy/eV

Fig. 2. [ - hv In(l - Y,)11/2 vs. hv plots for the anodic films formed on Pb at 0.9 V in 4.5 M H,SO, solution (25°C) for different periods of time: (1) 2 h, (2) 7 h.

J. Han et al. / Pb(II) film formed in H,SO,

solution

45

curves in Fig. 1 range from 1.94 to 2.03 eV, close to the EB value of t-PbO, 1.94 eV [6]. Thus, it can be con-

cluded that the anodic films contain photoactive o-PbO and t-PbO. 3.1.2. Thickness of the t-PbO film d,, and o-PbO film d0

When the thickness of the anodic film d is much smaller than l/a, the Gartner equation can be simplified to 171 Y=2ad

(3)

Table 1 lists d, and d, obtained using eqn. (3) and values of a reported in the literature [8].

-3.00

5

-1.20

3.1.3. Quantity of electricity required to form t-PbO film Qt-mo and o-PbO film Qo_PbO As the film is dense [9], d may be given as d = QV,/AnF

-1.04

-0.88

-0.72

-0.56

-0.40

E/V(vs.Hg/Hg2S0,) Fig. 3. Voltammogram recorded at 1 mV s-t.

for Pb at 0.9 V in 4.5 M HaSO,

after

1 h,

(4)

where V, is the molar volume of PbO, A is the electrode area, F is the Faraday constant, n is the number of electrons required to form a PbO molecule from Pb and Q is the quantity of electricity required to form the PbO film. From eqns. (3) and (41, we have Q =AnFY/2aT/,

(5)

Values of Qt+,,o and Qo_rbo obtained from the above equation, are listed in Table 2 adopting the values of V, reported in the literature [lo]. 3.2. Determination of PbSO, using linear sweep voltammetry (LSV)

Figure 3 shows the voltammogram for Pb at 0.9 V in 4.5 M H,SO, (25°C) after 1 h. As the peak A in Fig. 3 corresponds to the reduction of PbSO, to Pb, it is simple to calculate the area of peak A to obtain the quantity of electricity required to form the PbSO, film from Pb Q,,,,,, see Table 2.

TABLE 2. Various times (25°C)

quantities

of electricity

for different

Qt-pbo /mC cm-*

Qo-pbo /mC cm-*

Qmso4 /mC cm-’

Qpbo.pbso4 QB

1 2 3 4

25 63 106 166

50 76 106 119

201 216 226 226

278 402 482 572

cm-2

3.3.1. Open-circuit decay curve for partially reduced anodic film

Figure 4 shows the open-circuit decay curve for Pb anodized at 0.9 V in 4.5 M H,SO, (25°C) for 1 h, followed by cathodic reduction using LSV to -0.85 V, the potential of peak B, then in open circuit. The steady potential shown in Fig. 4 is -0.443 V. This can be explained by the reduction of PbO * PbSO, to Pb for peak B as follows.

‘.OOr------

anodizing

t/h

/mC

3.3. Determination of PbO *PbSO,

/mC 357 485 621 728

cm-’

o.400.0

t /min

5.1 3

Fig. 4. Open-circuit decay curve for a Pb electrode anodized at 0.9 V for 1 h, followed by cathodic reduction using LSV (u = 1 mV s-r) to - 0.85 V, then in open circuit.

.I. Han et al. / Pb(II) film formed in H,SO,

46

The electrode given by [ll] E/V

= -0.713

potential of PbO * PbSO,/Pb

can be

- 0.0296 pH

- 0.0148 1ogaso42-

(25°C)

(6)

The PbO . PbSO, film is covered with a semi-permeable PbSO, film, thus its potential E, may be written as E,/V

= - 0.713 - 0.0296 pH, - 0.0148 loguso42-

+-%I E,/V

(25°C)

(7)

= O.O592(pH, - pH,)

(25°C)

(8)

where E, is the membrane potential film [12], pH, and pH, are the pH PbSO, I PbO - PbSO, interface and of respectively. If PbSO, and PbO * PbSO, coexist

of the PbSO, values of the 4.5 M H,SO, in equilibrium

ml, pH, = 8.4 + 0.5 loga,,;-

(25°C)

(9)

The pH of 4.5 M H,SO, is -0.52, then from eqns. (7)-(9) E, can be calculated to be -0.434 V, which is very close to the steady potential, -0.443 V, mentioned above (Fig. 4). 3.3.2. Quantity of electricity required to form PbO . PbW

from

Pb QPb0.PbS04

solution

mined by atomic absorption spectrophotometry is nearly constant at 35 mC cm-’ after anodizing Pb for 1 h. Table 3 lists values of qPbO.PbSO, calculated using eqns. (10) and (11). The value of Qpbo. rbSo4 obtained using eqn. (11) is more accurate than that obtained using eqn. (lo), since PbO * PbSO, and PbO may not both be reduced completely under peak B, especially when the film is thick. This is illustrated in Table 3. 3.4. Growth rates of the components of the film From Table 2, it can be seen that the growth rate of the PbSO, film is nearly zero after anodizing lead for about 1 h. This is due to the increase in pH of the solution at the PbSO, film I PbO * PbSO, film interface during anodization. At high pH values PbO - PbSO, rather than PbSO, is stable [ll], hence the PbO - PbSO, film can grow continuously instead of the PbSO, film, as shown in Table 2. The solution of higher pH may favor growth of the t-PbO film, but not of the o-PbO film [13]. Thus, as the formation of PbO . PbSO, from PbO and H,SO, increases the pH in the film, the growth rate of the t-PbO film increases with the increasing amount of PbO . PbSO, film, while that of the o-PbO film decreases, see Table 2. Acknowledgement

From the above findings, it can be concluded that peak B corresponds to the reduction of PbO * PbSO, to Pb, followed by the reduction of PbO (o-PbO and t-PbO) to Pb. Therefore, QPb0.Pbso4 may be expressed by

Financial support of this project by the National Natural Science Foundation of China is gratefully acknowledged.

QP~o+~,so~ = QB- Qo-PM, - Qmo

1 R.J. Hill, J. Power Sources, 11 (1984) 19. 2 T.X. Chen, in Z.W. Tian, SM. Zhou and Z.G. Lin (Eds.), Progress in Electrochemical Experimental Methods, Xiamen University Press, Xiamen, 1988, p. 151 (in Chinese). 3 Yu.V. Pleskov and Yu.Ga. Gurevich, Semiconductor Photoelectrochemistty, Consultants Bureau, New York, 1986. 4 Z.L. He, C. Pu, H.T. Liu and W.F. Zhou, Acta Chim. Sinica, 50 (1992) 118 (in Chinese, with English abstract). 5 G.H. Brilmyer, in K.R. Bullock and D. Pavlov (Eds.), Advances in Lead-Acid Batteries, Proc. Vol. 84-14, The Electrochemical Society, Pennington, NJ, 1984, p. 142. 6 J. van den Broek, Philips Res. Rep., 22 (1967) 36. 7 J.S. Buchanan and L.M. Peter, Electrochim. Acta, 33 (1988) 127. 8 J.S. Buchanan, N.P. Freestone and L.M. Peter, J. Electroanal. Chem., 182 (1985) 383. 9 Z.L. He, C. Pu and W.F. Zhou, J. Power Sources, 39 (1992) 225. 10 R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1988-1989, 69th edn. 11 S.C. Barnes and R.T. Mathieson, in D.H. Collins (Ed.), Batteries 2, Pergamon, Oxford, 1965, p. 41. 12 P. Ruetschi, J. Electrochem. Sot., 120 (1973) 331. 13 P. Veluchamy, M. Sharon and D. Kumar, J. Electroanal. Chem., 315 (1991) 293.

(10)

where Qn is the quantity of electricity corresponding to peak B. Alternatively,

QPI,O.P~SO,= Q - QPLSO~Qt-mo - Qom,- Qs (11) where Q is the total quantity of electricity passing the Pb electrode during anodization, Q, is the quantity of electricity corresponding to the Pb(I1) species dissolved in the solution from the film. The value of Q, deter-

TABLE 3. Calculated Qpbo.p,,sod using eqns. (10) and (11) t/h 1 2 3 4

&bo.Pbsos/mC

~-2

Eqn. (10)

Eqn. (11)

282 347 409 444

278 402 482 572

References