A Comparative Kinetic Study of Pb and Pb-Sb Electrodes in Sulfuric Acid Solutions

As respostas eletroquímicas de eletrodos de Pb e de Pbs-3.4%Sb em H2SO4 5 M foram examinadas combinando as técnicas de varredura e saltos de potencial. A presença do antimônio evita o bloqueio completo da superfície ativa do eletrodo por cristais de PbSO4 du rante a eletrooxidação, mudando a composição e a estrutura da camada passiva. A eletrooxidação do Sb tem sido interpretada em termos de um mecanismo de adsorção-dissolução onde um elétron é transferido na etapa determinante da velocidade. A cinética de crescimento da camada interna de PbO também tem sido estudada. No eletródo de Pb-Sb, a velocidade de crescimento do PbO diminui devido a uma mudança no mecanismo de transporte dos íons através da camada anódica.


In tro duc tion
In a lead-acid bat tery, the per for mance of pos i tive plates de pends on the grid cor ro sion pro cesses and the soften ing and shed ding of PbO 2 elec trode ac tive ma te rial 1 .Anti mony is widely used as a grid al loy ing el e ment for this kind of bat tery.How ever, the ex act in flu ence of Sb on the chem i cal na ture of the re ac tion prod ucts and on the morphol ogy of the pos i tive elec trode ac tive ma te rial is still open to dis cus sion.
There is gen eral agree ment on some stages in volved in the over all re ac tion of Pb and Pb-Sb elec trodes in sul fu ric acid elec tro lytes.Dur ing an odic po lar iza tion, the re ac tion be tween Pb 2+ and SO 4  2-ions oc curs, and a semi-permeable PbSO 4 crys tal layer is formed.In or der to main tain electroneutrality, the so lu tion in the pores is al ka lized as a re sult of wa ter dis so ci a tion and the mi gra tion of H + ions from the in ner layer to the bulk so lu tion, whereby a pH gradi ent ap pears.At an odic po ten tials higher than 0.23 V vs. NHE , the elec tro-oxidation of Sb and the for ma tion of tet-PbO be gin, and sev eral chem i cal re ac tions take place, re sult ing in the for ma tion of sub sti tuted lead an ti mony oxides and SbO + ions 2 .Thus, the an odic layer is com posed of PbSO 4 and an in ner film in volv ing ba sic Pb(II)-and Sb(III)-con tain ing com pounds (like ba sic sul fates, hy droxides and ox ides) which builds up pro gres sively be neath the ini tial PbSO 4 po rous layer struc ture 3 .
The pur pose of the pres ent re search is to study the elec tro chem i cal be hav ior of Pb and Pb-Sb elec trodes in 5 M H 2 SO 4 un der con trolled con di tions in or der to quan tita tively eval u ate the pres ent phases in the an odic layer.

Ex per i men tal
The ex per i men tal setup has been de scribed in pre vi ous pub li ca tions 4,5 ."Specpure" lead and lead-antimony (3.4 wt% Sb) al loy disks (John son Matthey Chem i cals, 0.30 cm 2 ap par ent area) em bed ded in PTFE hold ers were used as the work ing elec trodes in 5 M H 2 SO 4 at 25 °C, un der puri fied ni tro gen gas sat u ra tion.The elec trodes were me chani cally pol ished with 600 and 1200 grade em ery pa pers and thor oughly rinsed in four fold dis tilled wa ter.Po ten tials were mea sured and re ferred to in the text, against a Hg/Hg 2 SO 4 , K 2 SO 4 (sat.)ref er ence elec trode.
Prior to the elec tro chem i cal runs, work ing elec trodes were cathodized for 5 min at -1.30 V, i.e. in the hy dro gen evo lu tion re ac tion (HER) po ten tial range, to achieve a repro duc ible electroreduced elec trode sur face.Sub sequently, two types of ex per i ments were made: i) char ac ter is tic cur rent den sity/po ten tial (j/E) curves un der dif fer ent hy dro dy namic con di tions were re corded by apply ing ei ther sin gle (STPS) or re pet i tive (RTPS) tri an gu lar po ten tial sweeps be tween pre set cath odic (E s,c = -1.30V) and an odic (E s,a ) switch ing po ten tials at scan rate v, in cluding a po ten tial hold ing at E s,a for dif fer ent times τ; ii) combined lin ear po ten tial scans with po ten tial steps were used to study the elec tro-oxidation pro cess of an ti mony dur ing the for ward po ten tial sweep.

Re sults and Dis cus sion
The voltammogram of Pb in 5 M H 2SO 4 so lu tion at v = 0.04 V s -1 run be tween E s,c = -1.30V and E s,a = 1.40 V (Fig. 1) shows an an odic cur rent peak (A1) at ca. -0.96 V fol lowed by a wide pas sive re gion dur ing the for ward poten tial sweep 6 .Dur ing the re verse scan, two cath odic cur -rent peaks, C1 and C2, are ob served at -1.11 V and -0.95 V, re spec tively.Con ju gated cur rent peaks A1 and C1 cor respond to the elec tro-oxidation of Pb to PbSO 4 and to the electroreduction of PbSO 4 7,8 , whereas peak C2 may be asso ci ated with the electroreduction of PbO which builds up pro gres sively be neath the ini tial PbSO 4 po rous layer 9 .Elec tro chem i cal mea sure ments per formed us ing the Pb-3.4%Sbelec trode (Fig. 1) re veal ad di tional cur rent contri bu tions: two an odic cur rent peaks at -0.37 and -0.15 V (A' and A'') lo cated in the re gion of sta ble Pb(II) sur face com pounds, and two cath odic peaks (C' and C'') at -0.69 and -0.86 V pre ced ing the PbSO 4 electroreduction po tential range.Cur rent peaks A' and C' may be re lated to the electroformation and electroreduction of the Sb(III) species, whereas the broad peaks A'' and C'' seem to be as soci ated with the for ma tion of Sb(III) com plexes and the com plex electroreduction of ba sic Pb(II)-Sb(III) con taining sur face com pounds (C2-C''), re spec tively.The whole potentiodynamic re sponse is prac ti cally in de pend ent of the so lu tion stir ring con di tions.
The anal y sis of peak pa ram e ters dur ing con sec u tive potentiodynamic charge/dis charge cy cles be tween Fig ure 1. Com par a tive voltammograms of Pb (full curve) and Pb-Sb (dashed curve) elec trodes in 5 M H 2SO 4 so lu tion run at v = 0.040 V s -1 be tween E s,c = -1.30V and E s,a = 1.40 V.
Fig ure 2. The dependences of j p,A1 and j p,C1 on the num ber of cy cles for Pb and Pb-Sb elec trodes.Data ob tained from cy clic voltammograms run be tween E s,c = -1.30V and E s,a = 1.40 V at v = Es,c = -1.30V and E s,a = 1.40 V at v = 0.04 V s -1 on both Pb and Pb-Sb elec trodes, re veals grad ual struc tural changes of the pas si vat ing layer (Fig. 2).The height of con ju gated current peaks A1 and C1 de creases abruptly in the ini tial cycles, reaches a min i mum cur rent value, and then in creases slowly to at tain a sta bi lized re sponse af ter about twenty cycles.More over, the peak po ten tials E p,A1 and E p,C1 re main prac ti cally con stant dur ing cy cling.On the other hand, peak C2 grad u ally tends to wards more neg a tive po ten tials and its charge den sity (q C2 ) de creases to ap proach the station ary value af ter pro longed cy cling (Fig. 3).The voltammetric charge in volved in cur rent peaks A1, C1, and C2 for mea sure ments per formed us ing Pb elec trodes is smaller than that re corded em ploy ing Pb-Sb elec trodes (Figs. 2 and 3b).The dis so lu tion of the Sb(III) spe cies increases the per me abil ity of the pas sive layer.The Pb ions can then dif fuse more eas ily through the an odic film and the pH in side the pas sive layer de creases.This ex plains the fact that the charge and re duc tion overpotentials of ba sic Pb(II) com pounds de crease more quickly for al loy elec trodes during cy cling.
The height of cur rent peaks A' and C', which are obtained with Pb-Sb elec trodes and are re lated to the electroformation/electroreduction of Sb(III)-con tain ing sur face spe cies, grad u ally di min ishes dur ing cy cling to attain sta ble val ues at about twenty cy cles (Fig. 4).The charge den sity ra tio of peaks A' and C' is prac ti cally constant (q A' /q C' » 4) with both the num ber of cy cles and po - ten tial scan rate, at least up to the 50 th po ten tial cy cle and for v val ues within the 0.02 V s -1 ≤ v ≤ 0.20 V s -1 range, respec tively.The same elec tro chem i cal be hav ior for E s,a = 0.00 V was also ob served.This sug gests that the for ma tion and re duc tion pro cesses of Sb(III)-con tain ing spe cies take place at pref er en tial sur face sites lim ited to the ar eas unblocked by re sid ual lead sul fate, which seems to be highly re sis tant to pro longed electroreduction 10,11 .More over, these re sults im ply that only the ions near est to the electrode sur face can be electroreduced, i.e. those as so ci ated with the ad sorp tion mech a nism.There fore, for high cy cle num ber, the dis so lu tion pro cess ap pears to be in hib ited by chemisorption of [Sb(SO 4)2] -or [SbOSO 4] -an ions.On the other hand, the change of j p,A' and j p,C' dur ing ini tial po tential cy cles may be as so ci ated with the grad ual for ma tion of a more reg u lar struc ture of the sur face layer ( i.e. a more homo ge neous size of the PbSO 4 crys tals) and, ac cord ingly, the de crease of voltammetric charge den sity of peaks A', C' and C2 can be re lated to pro cesses which con trib ute to en hance sur face layer po ros ity.These con clu sions agree with scan ning elec tron mi cros copy (SEM) re sults ob tained on Pb and Pb-Sb elec trodes un der sim i lar ex per i men tal con di tions by Bialacki et al. 12 and We bster et al. 13 .Webster et al. con cluded from their ob ser va tions that an ti mony al loy elec trodes have sur face films of higher po ros ity and that the po ros ity in creases with cy cling.
The elec tro-oxidation of Sb to the Sb(III) spe cies at poten tials be tween -0.50 V and 0.00 V was stud ied by po tential sweep and po ten tial step com bin ing mea sure ments.Figs.5a and 5b pres ent the de pend ence of E p,A ' on log v a and j p,A' on v a 1/2 af ter τ = 1 min at E s,a = -0.50V.These linear re la tion ships are typ i cal of an ir re vers ible sys tem.This is con sis tent with the fact that j p,A' >> j p,C' for the whole scan rate range.Also, these re la tion ships may be as so ci ated with an EC re ac tion where the elec tron trans fer is re vers -Fig ure 3. The dependences of (a) E p,C2 and (b) q C2 on the number of cy cles in both elec trode sys tems, un der the same con di tions as in Fig 2 Fig ure 4. The dependences of j p,A' and j p,C' on the num ber of cy cles for Pb-Sb elec trodes, un der the same con di tions as in Fig 2 .ible and the chem i cal re ac tion is ir re vers ible 14 .Thus, E p,A' var ies with the sweep rate as shown by the the o ret i cal equation: where K is in de pend ent of v a.
The ex per i men tal slope value is ( ∂E p,A' /∂log v a ) = 0.0495.Thus, the cal cu lated value of αn α is 0.6, where n α , the num ber of elec trons in volved in the rate-determining step, can be as sumed to be 1.
Taking into ac count the above con sid er ations, the electro-oxidation of Sb can be in ter preted as two con sec u tive elec tronic trans fer steps and one chem i cal re ac tion, which cor re sponds to the fast dis so lu tion of an elec tro-oxidation prod uct.Ac cord ingly, the electroreduction charge dur ing the neg a tive po ten tial sweep is smaller than that de vel oped in the elec tro-oxidation pro cess.
The pH de crease re sult ing from steps ( 2) and (3) contrib utes to the dis so lu tion of the an odic layer ba sic compounds, ox ides and hy drox ides of Sb(III), as well as those of Pb(II).
To in ves ti gate the electroformation of PbSO 4 and PbO lay ers, the Pb and Pb-Sb elec trodes were an od ized at sev eral E s,a in the pas sive po ten tial range for dif fer ent polar iza tion times ( τ).From the anal y sis of electroreduction peak C1, it was found that the amount of PbSO 4 formed on Pb elec trodes is al most in de pend ent of po ten tial and increases slightly with τ (Fig. 6a,b), in agree ment with reported data 15 .On Pb-Sb elec trodes, on the other hand, the Fig ure 5.The dependences of (a) E p,A' and (b) j p,A' on v a. Data obtained from voltammograms re corded in the po ten tial re gion of peaks A' and A'', af ter τ = 1 min at E s,a = -0.50V.
Fig ure 6.The dependences of q C1 and q C2 on E s,a for dif fer ent an odic po lar iza tion times: peak C1 charge de pends on Es,a, and is higher than that obtained on Pb elec trodes.
Figs. 6c and 6d show that the PbO electroreduction charge in creases lin early with the po ten tial, but the rel a tive amounts for Pb and Pb-Sb elec trodes shown an in ver sion with τ.This can be ex plained by a dis so lu tion of the small crys tals of ba sic Sb(III) com pounds dur ing an od iza tion.
From the log qC2 vs. log τ plot (Fig. 7), lin ear re la tionships for each Es,a in the 0.0 to 0.6 V po ten tial range are obtained.Higher PbO growth rates on Pb than those on Pb-Sb are ob tained, and the ra tio be tween both PbO charge den sities, that ob tained on Pb and that on Pb-Sb, gives re sults pro por to τ 1/3 .As the charge den sity q C' is prac ti cally in de pend ent of Es,a and τ, these re sults im ply that the dis solu tion pro cess of Sb(III)-con tain ing spe cies within the pores mod i fied the trans port mech a nism of the ions (i.e.H + , OH -) through the an odic layer and the growth ki net ics of the PbO in ner film.

Con clu sions
Voltammetric re sults ob tained on Pb-Sb al loy electrodes show that the or der within the an odic layer struc ture in creases with potentiodynamic cy cling, and the electro-oxidation/electroreduction of Sb spe cies oc curs at prefer en tial sites on the sur face.The electrooxidation of Sb may be as so ci ated with an ad sorp tion-dissolution mech anism where one elec tron is trans ferred in the rate-determining step.The growth rate of the PbO in ner film on Pb-Sb elec trodes is smaller than that on Pb electrodes, due to a change in the trans port mech a nism of ions (OH -or H + ) through the an odic layer.

Ac knowl edg ments
This re search pro ject was fi nan cially sup ported by the Consejo Nacional de Investigaciones Científicas y Técnicas and the Comisión de Investigaciones Científicas de la Provincia de Bue nos Ai res.