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Electrochimica Acta 54 (2009) 7382–7388

Contents lists available at ScienceDirect

Electrochimica Acta

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QCM study of electrodeposited PbO2: Investigation of the gel formation andischarge mechanisms

avid Pecha,b, Thierry Broussec, Daniel Bélangera, Daniel Guayb,∗

Département de Chimie, Université du Québec à Montréal, case postale 8888, succursale centre-ville, Montréal, Québec, H3C 3P8, CanadaINRS-Énergie, Matériaux et Télécommunications, 1650 Blvd Lionel Boulet, case postale 1020, Varennes, Québec, J3X 1S2, CanadaLaboratoire de Génie des Matériaux et Procédés Associés EA 2664, Polytech Nantes, Université de Nantes, BP50609, 44 306 Nantes Cedex 3, France

r t i c l e i n f o

rticle history:eceived 18 April 2009eceived in revised form 23 July 2009ccepted 26 July 2009

a b s t r a c t

Lead dioxide thin films were electrodeposited on gold substrates and characterized by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The mass change occurring upon immersion in a H2SO4

electrolyte and during electrochemical reduction was observed in situ by electrochemical quartz crystal

vailable online 3 August 2009

eywords:QCMead dioxide

microbalance (EQCM). A hydrated PbO2 gel-type layer is formed at the surface of electrodeposited PbO2.The concentration of the H2SO4 electrolyte does not affect the composition of the gel nor the amount oflead dioxide involved in the hydration process. It is established that 1.3 × 10−7 mol cm−2 of �-PbO2 arehydrated at the surface of an electrodeposited film and that the hydration reaction occurs according tothe following reaction: PbO2

(crystal) + xH2O ↔ (PbO(OH)2·(x − 1)H2O)(gel), where x = 8.1. The mass changeand sugel) ar

lectrodepositionydration

occurring during the firstand PbO(OH)2·(7.1)H2O)(

. Introduction

Lead acid batteries accumulate over 100 years of technolog-cal development and remain the most important rechargeablelectrochemical storage systems in many areas with the best com-romise between performance and cost [1,2]. The prediction ofhe lifetime of lead batteries requires a detailed interpretation ofhe discharge mechanisms occurring on the positive lead diox-de electrode. Ageing processes have been thoroughly investigated3,4], but most studies are mainly based on electrochemistry only,nd new approaches are needed to fully understand the reductionrocess. The electrochemical quartz crystal microbalance (EQCM)rovides a convenient in situ tool to observe the mass changes asso-iated with the discharge reaction. This technique has proved to beery efficient for the investigation of the electrochemical deposi-ion mechanism of lead dioxide thin film from solutions containingb2+ ions [5]. Although the EQCM could provide useful informationn the elementary processes occurring during an electrochemicaleaction, it has seldom been used for the study of lead dioxideeduction itself [6].

In this paper, we report the results obtained simultaneouslyrom EQCM and cyclic voltammetry discharge of lead dioxide thinlms. PbO2 was deposited onto gold-coated quartz crystal from.1 M lead nitrate in 1 M nitric acid and the resulting electrode

∗ Corresponding author.E-mail address: guay@emt.inrs.ca (D. Guay).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.07.070

bsequent discharge of PbO2 was recorded. It is shown that both PbO2(crystal)

e reduced to PbSO4 during the first discharge.© 2009 Elsevier Ltd. All rights reserved.

was subsequently characterized in aqueous sulfuric acid electrolyte[7]. A mass increase was observed that is attributed to the forma-tion of hydrated lead dioxide, according to the gel-crystal model.Its evolution for different masses of deposited films allowed usto establish the ratio between the hydrated gel and the crystalzones. Finally, the mechanisms occurring during electrochemicalreduction of lead dioxide was investigated. EQCM measurementssupported by scanning electron microscope (SEM) observationsenabled us to distinguish the different phenomena occurring duringthe first and the subsequent discharge of PbO2.

2. Experimental

Gold-coated quartz crystals (9 MHz, 0.2 cm2 surface area) wereused as working electrodes. They were first cycled in 1 M HNO3between −0.2 and 1.8 V vs. Ag/AgCl until a reproducible cyclicvoltammogram (CV) was obtained. Then, the desired mass of PbO2was electrodeposited by potentiostatic oxidation (E = 1.75 V vs.Ag/AgCl) of a deaerated 0.1 M Pb(NO3)2/1 M HNO3 aqueous solution[8].

The relationship between the shift of the resonant frequency(�f) and the mass change (�m) is given by the Sauerbrey equa-tion: �f = − K�m, where K is the experimental sensitivity factor.

The value of K (K = 883 Hz �g−1) was determined independentlyfrom the reduction of lead acetate in an acetic acid/sodium acetatebuffer solution [9]. The effect of the viscosity and density of theH2SO4 solution on the resonant frequency was taken into accountby relying on the equation derived by Kanazawa and Gordon [10].

ica Acta 54 (2009) 7382–7388 7383

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he frequency of the gold-coated quartz crystals was determinedsing a QCA 922 quartz crystal analyzer (Seiko EG&G).

The structure of the film was characterized by X-ray diffractionXRD) measurements using a Bruker AXS D8 Advance diffrac-ometer equipped with Cu K� radiation. All measurements wereerformed at a fixed grazing incidence angle (�inc = 2◦). The surfaceorphology of the electrodes was assessed through scanning elec-

ron microscopy (SEM) using a JEOL JSM-6300F scanning electronicroscope.The electrochemical characterization of the electrode was per-

ormed with a Solartron 1470 potentiostat using a three-electrodelectrochemical cell with a platinum gauze counter-electrode andsaturated K2SO4 mercury/mercurous sulfate reference-electrode

MSE) in deaerated 1.5 M H2SO4 electrolyte.

. Results and discussion

.1. Electrodeposition of PbO2

Fig. 1 displays the cyclic voltammogram and the simultane-us mass change occurring when a gold-coated quartz crystal ismmersed in 0.1 M Pb(NO3)2/1 M HNO3 solution. A large anodiceak which corresponds to the oxidation of PbII to PbIV is observedt ∼1.65 V vs. Ag/AgCl. This current peak is accompanied by a massncrease of the gold-coated quartz crystal indicating that depositionf lead dioxide is occurring at the surface of the substrate. On theeverse sweep, lead dioxide deposition continues until ∼1.48 V vs.g/AgCl, an electrode potential smaller than the one correspond-

ng to the initiation of lead dioxide deposition on freshly preparedold substrate. This profile is similar to the ones observed in liter-ture [5,11,12]. Also, on the positive going sweep, there is a smallxidation peak at ∼0.95 V vs. Ag/AgCl corresponding to the oxida-ion of gold. In that potential range, a monolayer thick film of goldxide/gold hydroxide is formed but does not result in a significantass change in comparison with the mass change involved when

ead dioxide is deposited.On the negative going sweep, reduction of PbO2 does not

ccur before the electrode potential reaches ∼1.31 V vs. Ag/AgCl.he reduction of PbO2 is accompanied by a simultaneous loss ofass, indicating that its reduction to PbII led to the dissolution of

he oxide. This dissolution is complete as the mass of the gold-

oated quartz crystal at the end of the first CV is identical tohat at the beginning. The total charge under the cathodic peak is72.5 mC cm−2 (the electrode surface is 0.2 cm2) and would corre-pond to a mass change of 338.0 �g cm−2, assuming the dissolution

ig. 1. Cyclic voltammogram and simultaneous mass measurements during thelectrodeposition of PbO2 on a gold-coated quartz crystal. The electrolyte was 0.1 Mb(NO3)2 in 1 M HNO3 solution and the sweep rate was 20 mV s−1.

Fig. 2. X-ray diffraction pattern of an electrodeposited PbO2 film from a 0.1 MPb(NO3)2 in 1 M HNO3 solution. The deposition charge was 161.5 mC cm−2.

of PbIV to PbII is the only reaction taking place. This value is closed tothe total mass loss (322.5 �g cm−2) recorded with the quartz crystalmicrobalance. This corresponds to a current efficiency of 95%. Thisvalue, lower than 100%, is due to the contribution of the chargeassociated to the reduction of gold oxides species at the electrodesurface.

In the following, potentiostatic polarization will be used todeposit PbO2 (1.75 V vs. Ag/AgCl) films by varying the deposi-tion charge and simultaneously recording the deposited massthrough the shift of the resonant frequency of the gold-coatedquartz crystal. The X-ray diffraction patterns of PbO2 (growth rate17.7 �g cm−2 s−1) prepared in these conditions is shown in Fig. 2.It displays the characteristic diffraction peaks of �-PbO2 with atetragonal structure. Electrochemical deposition of PbO2 in acidicmedium gives preferably the � phase, while � phase is favoured inalkaline medium. A mixture of � and � phase is sometimes foundat pH 1.0 (see for example ref [13], where deposition of PbO2 isachieved at 0.1 M HNO3), but deposition at lower pH has beenshown to consistently yield to the deposition of almost pure �-PbO2[14,15].

3.2. Mass change of the PbO2 electrode upon immersion inaqueous H2SO4 electrolyte

�-PbO2 thin films of various masses were immersed in H2SO4electrolyte of various concentrations ranging from 0 to 2.25 M.A reversible change of the resonant frequency is observed uponimmersion in the electrolyte. This change is rapid and completeafter ∼1 min of immersion. For the thickest films, the resonantfrequency then stays constant, while it increases steadily for thethinnest films (see below). The mass change was calculated fromthe resonant frequency using the equation derived by Kanazawaand Gordon to account for the change in the viscosity and densityof the electrolyte as the H2SO4 concentration is varied [10], andFig. 3 displays the percent variation of mass with the H2SO4 con-centration of the electrolyte. As seen in Fig. 3, the percent variationof mass is independent of the electrolyte concentration, indicating

that sulfuric acid is not involved in the modification of the film.

It has been suggested that lead dioxide immersed in a solu-tion reacts partially to form hydrated surface layers consisting ofhydrated polymers chains of lead dioxide. This reaction can be rep-

7384 D. Pech et al. / Electrochimica Acta 54 (2009) 7382–7388

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it is deposited on gold. This hypothesis is confirmed by theresults of Fig. 5, showing how the mass of PbO2 varies with timeafter immersion in 1.5 M H2SO4 electrolyte. As seen in that fig-ure, the rate of mass variation decreases from −3.4 × 10−1 to−3.9 × 10−3 �g cm−2 s−1 as mi increases from 5 to 20 �g cm−2.

ig. 3. Relative mass change (�m/m) of electrodeposited �-PbO2 thin films as aunction of the concentration of sulfuric acid.

esented as follows:

bO2(crystal) + xH2O ↔ (PbO(OH)2·(x − 1)H2O)(gel) (1)

herefore, lead dioxide particles would consist of crystalline,bO2

(crystal), and hydrated gel zones (PbO(OH)2·(x − 1)H2O)(gel). Theel-crystal model has become widely accepted to describe thetructure of PbO2 and its discharge mechanism [16,17]. Pavlov andalkanov have shown that the crystalline and amorphous gel zonesre in equilibrium [18]. The occurrence of these hydrated gel zonesas based on studies involving transmission electron microscopy,-ray diffraction and X-ray photoelectron spectroscopy (XPS) mea-urements [19,20].

The analysis of the quartz crystal microbalance data allowsetermining the number of water molecules and the number ofoles of PbO2 that are involved in the hydration process. For this,

he specific adsorption of HSO4− ions has been neglected and it was

ssumed that the hydrated gel has a uniform composition. So, toelp the following discussion, let us assume that electrodepositionnd hydration of PbO2 are occurring on two time scales. So, the totalumber of moles of PbO2, nt, that is corresponding to mi, the initialass of PbO2 deposited on the substrate, is given by the sum of ni

nd nr, with ni the number of moles of PbO2 that is not hydratednd nr the number of moles of PbO2 that is reacting according to theydration reaction described by Eq. (1). Then the various masses areiven by the following equations:

i = nt · MWPbO2= ni · MWPbO2

+ nr · MWPbO2(2)

nd

f = ni · MWPbO2+ nr · (MWPbO2

+ x · MWH2O) (3)

here mi is the initial mass of PbO2, mf is the final mass of PbO2,nd MWPbO2

and MWH2O are the molecular masses of PbO2 and2O, respectively.Hence

m = mf − mi = nr · x · MWH2O (4)

and

�m

m≡ nr · x · MWH2O

m≡ nr · x · MWH2O

n · MW(5)

i i t PbO2

o, the slope of the curve (�m/mi) vs. (1/mi) will depend on both nr

the number of moles of PbO2 that are reacting), and x (the numberf water molecules that are participating in the reaction). In thease where nr = nt, that is when the number of PbO2 moles that are

Fig. 4. Evolution of the relative mass change (�m/mi) as a function of the reciprocalof the deposited mass (1/mi) of �-PbO2.

reacting is equal to the number of PbO2 moles deposited on thesubstrate, Eq. (5) becomes:

�m

mi≡ x · MWH2O

MWPbO2

≡ cte (6)

and x, the number of water molecules that are participating in reac-tion (1) can be unequivocally determined.

Fig. 4 shows the evolution of the relative mass change (�m/mi)as a function of the reciprocal of the initially deposited mass (1/mi).For 1/mi < 0.036 �g−1 cm2, the relative mass change increases lin-early with 1/mi, while it decreases for 1/mi > 0.036 �g−1 cm2. Thelinear relationship observed for 1/mi < 0.036 �g−1 cm2 is consis-tent with the fact that only the surface of the film is modifiedby the electrolyte. The drastic change in the behaviour of �m/miat 1/mi = 0.036 �g−1 cm2 (corresponding to mi ∼ 25 �g cm−2) isattributed to the mechanical instability of hydrated PbO2 when

Fig. 5. Evolution with time of the mass change (�m) of electrodeposited �-PbO2

thin films following their immersion in deaerated 1.5 M H2SO4. Films of differentinitial masses are displayed and the slopes of the curves are indicated.

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or the thickest films studied here (200 �g cm−2), the slope ofhe curve is small (slope = +2.1 × 10−4 �g cm−2 s−1), indicating thathick films of PbO2 do not show the instability observed with thehinner films.

The intersection between the straight line and the dotted linehown in Fig. 4 occurs at 1/mi = 0.036 �g−1 cm2, which corre-ponds to mi = 27.8 �g cm−2, and would correspond to the massf a fully hydrated film. This value is consistent with the resultsf Fig. 5 showing that the rate of mass loss of PbO2 film withi > 20.0 �g cm−2 becomes negligible. On a gold substrate, theechanical instability of hydrated PbO2 offers a unique way of

ssessing nr. So, assuming that the mass equivalent to the num-er of moles of PbO2 that are reacting with water molecules is7.8 �g cm−2, it can be determined through the use of Eq. (6) that= 8.1 water molecules are involved in the gel formation. Moreover,nce x is known, nt can be computed from the slope of the linear sec-ion of the (�m/mi) vs. (1/mi) curve determined at low (1/mi). Fromig. 4 and x = 8.1, one can determine that nr = 1.3 × 10−7 mol cm−2

f PbO2. To the best of our knowledge, no quantitative data havever been published concerning the molar ratio of the crystal andel zone, as well as the degree of hydration of the gel zone.

.3. First PbO2 discharge

The mechanisms occurring during the discharge of PbO2 in2SO4 have been analyzed by simultaneous measurements of theurrent and the resonant frequency of the quartz crystal as the elec-rode potential is swept from 1.40 to 0.00 V vs. MSE. Fig. 6A showshe CV and the simultaneous mass change occurring during therst discharge of a 200 �g cm−2 electrodeposited PbO2 thin film at0 mV s−1. The CV is characterized by a sharp peak with a currenteduction peak occurring at 0.76 V vs. MSE. On the reverse sweep,

here is a small anodic peak occurring at 0.95 V vs. MSE associated tohe oxidation of the gold substrate. At higher potential, the currentncreases due to the oxidation of PbSO4 to PbO2. The charge asso-iated with the reduction peak is 67.5 mC cm−2 and corresponds todepth of discharge of 41.8%.

ig. 6. (A) Variation of mass (dashed line) and current (full line) during the discharge of0 mV s−1. (B) Variation of mass (�m) with the charge (�Q) calculated from the curves sh

ta 54 (2009) 7382–7388 7385

As seen in Fig. 6A, a negative mass variation (mass loss) occursat the very beginning of the reduction process. Then, the sign of themass variation is reversed (mass gain) and, upon completion of thereduction reaction, there is a net mass gain of ∼39.0 �g cm−2.

A detailed analysis of the various processes occurring during thereduction of PbO2 is best performed by looking at how the massvariation is related to the reduction charge [21,22]. This is doneby converting the data of Fig. 6A and plotting �m vs. the reductioncharge as shown in Fig. 6B. This curve displays two distinct regions:(i) in the first one, the mass decreases linearly with the charge (slopeof −1.70 �g mC−1, see the insert), (ii) in the second one, the massincreases and the slope of the curve varies continuously from 2.69to 0.21 �g mC−1.

According to the literature, the reduction of lead dioxide pro-ceeds according to the following reactions:

PbO(OH)2 + 2H+ + 2e− → Pb(OH)2 (7)

Pb(OH)2 → Pb2+ + 2OH− (8)

(Pb2+, 2OH−) + HSO4− + H+ → PbSO4 + 2H2O (9)

Reduction of PbIV would occur in active centers located in thehydrated gel zones. The gel zone acts as an ionic conductor allowingthe diffusion of H+ ions from the solution, while the electrons aretransported from the crystal zones towards the active centers alongthe hydrated polymer chains. Once formed, Pb(OH)2 is mainly dis-solved as Pb2+ ions in the electrolyte but may also be decomposed toorthorhombic PbO [23]. Once in solution, Pb2+ ions diffuse and pre-cipitate to PbSO4 in the presence of sulfuric acid electrolyte. Thereduction of PbIV to PbSO4 (or to PbO) is a two electron process.Therefore, the various slopes observed in Fig. 6B can be convertedfrom �g mC−1 to g mol−1 by using the Faraday law and assumingthat two electrons are involved in the reduction process.

As discussed before, hydration of PbO2 occurs at the elec-

trode/electrolyte interface. This results in the formation of ahydrated gel and, according to the results presented above, a200 �g cm−2 film of PbO2 is composed of 15.6% (PbO(OH)2, 7.1H2O)(gel) and 84.4% PbO2

(crystal). As seen in the insert to Fig. 6B,the slope of the first section of the mass change vs. charge curve

a 200 �g cm−2 electrodeposited �-PbO2 thin film in 1.5 M H2SO4 at a scan rate ofown in (A).

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386 D. Pech et al. / Electrochim

s −1.70 �g mC−1, which amounts to 328 g mol−1. The dissolutionf PbO2

(crystal) to Pb2+ is equivalent to a change of 239.2 g mol−1,hile that of (PbO(OH)2, 7.1 H2O)(gel) to Pb2+ would amount to

85.2 g mol−1. The slope value in the first section of the mass changes. charge curve is between these two values, which would indicatehat both PbO2

(crystal) and (PbO(OH)2, 7.1 H2O)(gel) are dissolved athe very beginning of the discharging process. Indeed, the slope inhe first part of the mass change vs. charge is almost the averagef those expected if either PbO2

(crystal) or (PbO(OH)2, 7.1 H2O)(gel)

ere reacting (the slopes would then be −1.24 and −2.00 �g mC−1,espectively), indicating that both species must have similar reac-ion rates.

The slope in the second section of Fig. 6B varies continuouslyrom 2.69 to 0.21 �g mC−1, which corresponds to molar massesarying from 519 to 41 g mol−1. However, unlike in the first sec-ion where the mass change vs. charge curve follows a well definedinear curve and a mechanism responsible for that change can benequivocally identified, it is not possible to resolve the mecha-isms occurring in this section. It is well known that the solubilityf Pb2+ in H2SO4 is limited and precipitation of PbSO4 is occurring.o, precipitation of Pb2+ dissolved in the first section of the curveust occur. Also as shown previously, hydration of PbO2 is fast and

t can be expected that the hydration of PbO2 will proceed furthernto the bulk of the film as initially formed (PbO(OH)2, 7.1 H2O)(gel)

s reduced. So, the fact that the slope in the second section of Fig. 6Baries continuously might result from a combination of several phe-omena including dissolution to Pb2+ ions, hydration of PbO2

(crystal)

nd precipitation of PbSO4. The fact that the kinetic of the reductionrocess might vary continuously with the applied potential is also aomplicating factor. In Fig. 6B, the slope at the end of the reductionrocess is 0.21 �g mC−1, corresponding to 41 g mol−1. This valueorresponds to the molar mass difference between PbO2

(gel+crystal)

nd PbSO4, and would indicate that the predominant reaction athe end of the reduction process is the direct transformation ofbO2

(gel+crystal) to PbSO4.From a macroscopic point of view, the reduction charge and the

ass variation occurring when a freshly prepared �-PbO2 layer∼200 �g cm−2) is cycled in 1.5 M H2SO4 depends on the sweep

ate. This is illustrated in Fig. 7A, where a linear relationship isbserved between the depth of discharge and the scan rate, with aecrease from 58.6 to 21.9% as the scan rate is increased from 5 to00 mV s−1. The mass change occurring concomitantly is plotted in

ig. 7. Evolution of (A) the depth of first discharge and (B) the gain of mass involvedn the discharge of an electrodeposited �-PbO2 thin film in 1.5 M H2SO4 as a functionf the scan rate. In each case, a freshly prepared film was used.

cta 54 (2009) 7382–7388

Fig. 7B. As expected, there is a net gain of mass following the firstdischarge. However, the gain of mass decreases as the sweep rateis increased from 5 to 100 mV s−1.

Part of the observed mass change is related to the fact that leaddioxide is reduced to PbSO4. So, considering the electrode is initiallycomposed of 15.6% (PbO(OH)2, 7.1 H2O)(gel) and 84.4% PbO2

(crystal),one can calculate the expected gain of mass based on the depth ofdischarge. As seen in Fig. 7B, the gain of mass expected on this basisaccounts for only half of the total mass change observed. However,it must be reminded that hydration of PbO2 occurs on a short timescale and that, since (PbO(OH)2, 7.1 H2O)(gel) is reduced (see before),rehydration of previously unreacted PbO2

(crystal) must occur by theend of the measurements. So, it is hypothesized that the differ-ence between the experimental and the theoretical gain of massis due to the rehydration of previously unreacted PbO2

(crystal). Thishypothesis is consistent with the fact that, in the limit of very slowscan rate, the experimentally measured gain of mass (+21.6% cor-responding to 41.0 �g cm−2) can be accounted for as the sum oftwo components, namely 22.0 �g cm−2 arising from the dischargeof the electrode (96.80 mC cm−2), and 19.0 �g cm−2 arising fromthe hydration of PbO2

(crystal) to 15.6% (PbO(OH)2, 7.1 H2O)(gel) and84.4% PbO2

(crystal) which, according to the previous results, shouldbe given by the product x · MWH2O · nt. As seen in Fig. 7B, the gain ofmass due to rehydration of PbO2

(crystal) decreases with the scan rate,indicating that rehydration of previously unreacted PbO2

(crystal)

would be less complete, and at 100 mV s−1, only 8.6% of PbO2 wouldbe rehydrated compared to 15.6% at low scan rate.

3.4. Following PbO2 discharges

Fig. 8 shows the first, second and third CV of an electrodeposited�-PbO2 thin film in 1.5 M H2SO4. In the first scan, the current peakpotential related to the reduction of lead dioxide to lead sulfate isobserved at 0.76 V vs. MSE (74.4 mC cm−2). On the anodic sweep,the oxidation charge is only 15.7 mC cm−2. This charge differencearises as a consequence of the fast sweep rate (50 mV s−1) and slowkinetics of the oxidation reaction. In the following scan, a newreduction peak appears at higher voltage (0.88 V vs. MSE). The total

−2

charge of the reduction peak is 37.1 mC cm that corresponds tohalf of that measured during the first discharge. The oxidation ofPbSO4 is known to yield to the formation of �-PbO2 [24], whichis the same crystallographic form as that formed initially duringthe electrodeposition process (see Fig. 2). So, the occurrence of a

Fig. 8. First, second and third voltammogram obtained with an electrodeposited�-PbO2 thin film on a gold substrate in 1.5 M H2SO4 at 50 mV s−1.

D. Pech et al. / Electrochimica Acta 54 (2009) 7382–7388 7387

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ig. 9. Scanning electron microscopy micrographs of an electrodeposited �-PbOeduction/oxidation cycle, and (D) after two complete reduction/oxidation cycles.

ew reduction peak in the second and third CV (total reductionharge of 26.4 mC cm−2) is not related to a shift of the current peakotential that would accompanied a transformation from � to �bO2. It can be hypothesized that a difference in local acid concen-ration might appear between the first and the successive cyclesue to a larger porosity of the film that would lead to a positivehift in the PbIV/PbII equilibrium potential. However, according toevilliers et al., the current peak observed at lower potential in therst CV is related to the reduction of bulky (as-deposited) PbO2,

hile the appearance of a new current peak at higher potentialuring cycling is related to the formation of smaller PbO2 crys-allites resulting from the re-oxidation of lead sulfate [13]. Thisnterpretation is consistent with the scanning electron microscopy

ig. 10. Relationship between the mass change (�m) and the quantity of charge�Q) obtained from in the second reduction cycle of an electrodeposited PbO2 in.5 M H2SO4 at 50 mV s−1.

film in H2SO4: (A) as-deposited, (B) after reduction, (C) after the first complete

micrographs of Fig. 9, showing how the morphology of an elec-trodeposited PbO2 film evolves upon cycling. The electrodeposited�-PbO2 has a typical rice-shape morphology (Fig. 9A), character-istic of lead dioxide deposited at high current density [25]. Thepartial reduction of these primary PbO2 crystallites results in theformation of large and compact PbSO4 crystallites (Fig. 9B). After acomplete reduction-oxidation cycle (Fig. 9C), the surface morphol-ogy is composed of the previous PbSO4 crystallites which have notbeen oxidised, and small PbO2 particles resulting from the oxida-

tion of PbSO4. Following Devilliers et al. [13], it is hypothesized thatthese small PbO2 crystallites are more accessible, which explainswhy reduction of PbO2 occurs at a higher potential (is more facile)on the second and successive scans. After two complete reduction-

Fig. 11. Experimental molar mass difference determined at 50 mV s−1 in 1.5 MH2SO4.

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xidation cycles (Fig. 9D), the surface is composed of small PbO2rystallites, and both large and small PbSO4 crystallites.

As shown in Fig. 10, a single linear relationship is observedetween the mass change, �m, and the quantity of charge, �Q,uring the second discharge of the electrode. This behaviour dif-ers markedly from that observed during the first discharge of thelectrode (see Fig. 6B). A similar behaviour is observed for all theubsequent cycles. As the electrochemical reduction of small leadioxide crystallites is much easier (it occurs at higher potential),he dissolution of PbO2 to Pb2+ ions is not observed anymore. The

olar mass �M estimated from the slope of the curve is close to0 g mol−1 and does not vary after the first discharge is completedFig. 11). This value is in excellent agreement with the molar massifference between PbSO4 and PbO2

(gel+crystal) (41 g/mol).

. Conclusions

�-PbO2 thin films were electrodeposited on gold-coated quartzrystals. Upon immersion in aqueous H2SO4, a hydrated gel isormed at the surface of the electrode. It has been established thathe composition and thickness of the amorphous layer formed athe surface of the PbO2 particles is not affected by the concentrationf the sulfuric acid in the electrolyte. The number of moles of PbO2hat is participating in the crystal-to-gel transformation was deter-

ined (nt = 1.3 × 10−7 mol cm−2), as well as the degree of hydrationf PbO2 in the gel zone (x = 8.1 water molecules). The mechanismsccurring during the successive cycles in sulfuric acid were stud-ed by combined cyclic voltammetry and in situ electrochemicaluartz microbalance measurements. It has been shown that, dur-

ng the first cycle, the hydrated gel zone that is consumed in the

ischarge process is regenerated from the remaining PbO2

(crystal)

n contact with the electrolyte. This rehydration is complete at lowcan rate, but occurs partially at higher scan rates. Finally, it haseen shown that PbO2 crystallites resulting from the oxidation ofbSO4 are much smaller, and give also smaller PbSO4 crystallites.

[

[[[[

cta 54 (2009) 7382–7388

Acknowledgments

This work was made with the financial support (strategic grant)of the National Science and Engineering Research Council (NSERC)of Canada, NanoQAM (http://www.nanoqam.uqam.ca) and thesupport of Axion Power International.

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