Models of the Electroreduction of Passive Layers on Cobalt

Técnicas de transientes de corrente potenciostáticos foram empregadas no estudo de processos de eletroredução de camadas superficiais formadas no cobalto policristalino em soluções contendo íons carbonato-bicarbonato. As análises dinâmicas do sistema, realizadas através de procedimentos de investigação paramétrica e de rotinas de ajuste não-lineares, demonstraram que os da dos obtidos sob numa grande variedade de condições experimentais podem ser interpretados considerando a participação de várias espécies superficiais contendo Co(II)/Co(III). A influência do tempo de anodização no potencial de formação do óxido pode ser discutida na base dos modelos de eletroredução aqui propostos.

An odic ox ide films formed on co balt in al ka line so lutions have been in ves ti gated in the past in or der to de rive the most likely struc ture of the ox i da tion prod ucts, as well as their prop er ties.Elec tro chem i cal and ellipsometric studies were used to pro pose the over all mech a nisms for the elec tro-oxidation re ac tions [1][2] .The pas sive films that are formed in the po ten tial range of the sec ond ary pas siv ity pres ent a bilayer struc ture and ag ing pro cesses may be produced by proper po lar iza tion pro grams 3 .By con trast, the electroreduction of these sur face lay ers has re ceived compar a tively less at ten tion.More over, it should be ex pected that the cor re spond ing re ac tion mech a nisms are con sis tent with the pre vi ously ad vanced elec tro-oxidation path ways.The anal y sis of electroreduction us ing po ten tial-step techniques may yield sig nif i cant in for ma tion con cern ing not only the ki net ics of the re ac tion of this pro cess, but also the na ture of the pas sive film and the ki net ics of its growth and dis so lu tion 4,5 .
In this ar ti cle, a study of the electroreduction of co balt pas sive films formed through po ten tial con trolled an od ization in car bon ate-bicarbonate buff ers is pre sented.Fur thermore, the po ten ti al i ties of the electroreduction po ten tial-steps tech nique in the study of pas sive state proper ties will be dis closed.

Ex per i men tal
The ex per i men tal setup has been de scribed in pre vi ous pub li ca tions 4,6 ."Specpure" co balt in the form of both static and ro tat ing disks (John son Matthey Chem i cal, 0.30 cm 2 ap par ent area), em bed ded in PTFE hold ers was used for the work ing elec trodes.Po ten tials were mea sured against a sat u rated cal o mel elec trode (SCE) (0.242 V in the NHE scale), but in the text they are re ferred to the NHE.The elec tro lyte so lu tion con sisted of x M KHCO 3 + y M K 2 CO 3 Ar ti cle (0.0075 ≤ x ≤ 0.75 ; 0.0005 ≤ y ≤ 1.5) cov er ing the 8.4-11.6 pH range.The so lu tions were pre pared from an a lyt i cal grade (p.a.Merck) re agents and tri ply-distilled wa ter, previ ously boiled un der in ert at mo sphere to re move CO 2 .The work ing elec trode sur face was finely pol ished with γ-alumina (1.0 and 0.3 µm), rinsed in tri ply-distilled wa ter, and fi nally cath od i cally po lar ized for τ0 = 3 min at po ten tials suf fi ciently neg a tive to pro duce a net hy dro gen evo lu tion.
Ex per i ments were made un der pu ri fied N 2 gas sat u ration at 25 °C by em ploy ing the fol low ing per turb ing po tential pro grams: i) sin gle (STPS) tri an gu lar po ten tial scans be tween pre set cath odic (E s,c ) and an odic (E s,a ) switch ing po ten tials at a scan rate of v = 0.025 V s -1 ; ii) com bined poten tial scans and potentiostatic steps, usu ally two po ten tial steps, cov er ing dif fer ent po ten tial re gions.The first po tential step (E i) was ap plied for a cer tain time τ to mod ify the an od i cally formed layer in or der to change the to tal amount of an odic sur face prod ucts and to pro duce ag ing ef fects which may vary the com po si tion of the an odic layer constit u ents.Most of these cur rent tran sient ex per i ments were per formed by set ting E i = E s,a .The sec ond po ten tial step (E f) was fixed suf fi ciently neg a tive to electroreduce the anodic layer ei ther par tially or com pletely, and the cor respond ing cur rent tran sients were sys tem at i cally re corded.

Re sults and Dis cus sion
It has al ready been es tab lished that the re gion of primary pas siv ity of co balt elec trodes in al ka line me dia covers the 0.4-0.6V range for po ten tials re ferred to the hy dro gen elec trode in the same elec tro lyte (U Hss ) 7 .Co(II) ox ide or the re spec tive hy drox ide is the sta ble phase in this po ten tial range.Be yond that, i.e. in the re gion of sec ond ary pas siv ity, a spinel phase be comes sta ble and the film consists of an in ner layer of CoO and an outer layer of Co 3 O 4 8 .As such, set ting E s,a = 0.14 V ( vs. NHE) ren ders a film corre spond ing to the sec ond ary pas siv ity, i.e.E s,a ≥ 0.6 V ( vs. UHss ), in the pH range stud ied in this work.
The voltammogram of co balt in 0.75 M KHCO 3 + 0.05 M K 2 CO 3 so lu tion, at pH 8.9, and run at v = 0.025 V s -1 shows (Fig. 1) one main an odic peak at c.a. -0.17 V (peak II) and two sat el lite humps, one at c.a. -0.33 V (peak I) and the other at 0.07 V (peak II*), which pre cedes the on set of the sec ond ary pas sive cur rent re gion.The re verse scan pres ents a re ac ti va tion of the elec tro-oxidation pro cess followed by a lim it ing cur rent re gion prior to a cath odic peak at -0.63 V and a cath odic cur rent at po ten tials lower than -0.75 V, largely due to the hy dro gen evo lu tion re ac tion.The pre sen ta tion of Fig. 1 is sim ply in tended to show the po ten tial re gions where the main elec tro chem i cal re ac tions take place.A de tailed de scrip tion may be found in Ref. 2.
The electroreduction of co balt ox ide lay ers was in ves tigated through the potentiostatic cur rent tran sient method.In the case of nu cle ation and e growth-controlled pro cesses, experimental stud ies us ing this method are usu ally preferred, be cause then the elec tro chem i cal rate pa ram e ters are con stant for t > 0. In a pre vi ous pa per 6 , the sys tem atic anal y sis of po ten tial steps for the electroreduction re ac tion were de scribed for the same sys tems con sid ered in this work.It was con cluded that for a fixed value of τ = 30 s, the cur rent tran sients can be well de scribed ac cord ing to the fol low ing the o ret i cal model: where and k 1 (mol cm -2 s -1 ) and k 2 (mol cm -2 s -1 ) stand for the new phase growth rate con stants re spec tively per pen dic u lar and par al lel, to the elec trode sur face; M (g mol -1 ) and ρ (g cm -3 ) are the mo lec u lar weight and spe cific weight of the growth phase, re spec tively; A (nu clei cm -2 s -1 ) is the nu cle ation rate con stant; D (cm -2 s -1 ) and ∆c (mol cm -3 ) are re spectively, the dif fu sion co ef fi cient and the con cen tra tion gradi ent of the dif fus ing spe cies across the Nernst dif fu sion layer.This global model in volves the sum of two terms, a sim ple dif fu sion law and a re la tion ship ac cord ing to pro -Fig ure 1. Voltammograms run at v = 0.025 V s -1 be tween E s,c = -0.86V and E s,a = 0.22 V in 0.75 M KHCO 3 + 0.05 M K 2CO 3 so lu tion, pH gres sive nu cle ation and three-dimensional (3D) growth of nu clei un der charge trans fer con trol 9 .The model also includes the ap pear ance of an in duc tion time (P 4 ) for the forma tion of 3D nu clei.Fig ure 2 shows ex per i men tal cur rent tran sients ob tained in 0.075 M KHCO 3 + 0.005 M K 2 CO 3 so lu tion, at pH 8.9, with a po ten tial step be tween E i = 0.14 V and E f = -0.74V, and at times τ = 8 s and 30 s.A good agree ment can be ob served be tween the ex per i men tal values and the the o ret i cal curves cal cu lated with Eq. 1.However, when E f is set ≥ -0.57V, the ex per i men tal cur rent tran sients lack the max i mum typ i cal for a nu cle ation and growth con trolled pro cess.Fig ure 3 de picts an ex per i mental cur rent tran sient re corded at E f = -0.57V in 0.75 M KHCO 3 + 0.05 M K 2 CO 3 so lu tion, at pH 8.9, with τ = 3 s, to gether with the cor re spond ing the o ret i cal curve de rived through fit ting to the ini tial dif fu sion term in Eq. 1.This type of ki netic law at first sight sug gests that un der these work ing con di tions, the electroreduction re ac tion, cor responds to a pro cess con trolled solely by dif fu sion.The pro posed the o ret i cal model in Eq. 1 also fails to prop erly de scribe the ex per i men tal tran sients ob tained at lon ger times τ.Thus, Fig. 4 shows two cur rent tran sients re corded un der the ex per i men tal con di tions de scribed in Fig. 2, but with τ = 90 s (Fig. 4 a) and τ = 180 s (Fig. 4 b).The ex per i men tal re sults at a lon ger τ can be sat is fac to rily de scribed ac cord ing to the fol low ing re la tion ship: h is the height of the growth layer when N 0 nu clei are instan ta neously formed, and the rest of the sym bols have already been de fined.In this case, the ini tial dif fu sion term re mains un changed, while the sec ond term cor re sponds to an in stan ta neous nu cle ation and 2D growth pro cess un der charge trans fer con trol 10 .Fig ure 4 con tains best-fit curves ac cord ing to Eqs. 1 and 6.A com par i son of these re sults makes it clear that a tran si tion from a 3D to a 2D growth model can be mainly as so ci ated with lon ger ag ing times τ.
Ex per i men tal cur rent tran sients re corded at dif fer ent τs in a 0.75 M KHCO 3 + 1.5 M K 2CO 3 so lu tion, at pH 10.5, are de picted in Fig. 5, to gether with the cor re spond ing theo ret i cal curves de rived through fit ting to Eq. 6.Even for very short times τ the nu cle ation and growth model cor responds to a 2D pro cess.The val ues of the pa ram e ters, as derived from the fit ting pro ce dure, are as sem bled in Ta ble 1, which also in cludes the height of the growth phase h and the electroreduction charge den sity q com prised in the nu cleation and 2D growth pro cesses.These val ues were cal culated as fol lows: The val ues of h cal cu lated with Eq. 9 de crease with increas ing times τ, and are in good agree ment with the thickness of the pri mary pas sive layer of CoO elec tro formed in other elec tro lytes at com pa ra ble pH val ues 8 .
An en hanced chem i cal dis so lu tion of the in ner CoO layer is to be ex pected in so lu tions with in creased con centra tions of car bon ate and bi car bon ate ions, like the el ec tr olyte used in the ex per i ments in Fig. 5 2 .The ear lier ex haus tion of electroactive spe cies in the ox ide film in the per pen dic u lar di rec tion im poses a phys i cal limit for fur ther ver ti cal 3D growth and de ter mines the ob served 2D be havior.This is a con se quence of a thin ning of the film by its enhanced dis so lu tion af ter lon ger times in con tact with the elec tro lyte.
The re ac tions tak ing place dur ing the electroreduction of pas sive co balt elec trodes formed in the sec ond ary passiv ity re gion in car bon ate-bicarbonate so lu tions can be put as fol lows: Re ac tion 11 cor re sponds to an electroreductive dis so lution of the outer Co 3O4 layer with the for ma tion of sol u ble Co(II) spe cies, rather than an ox ide layer with lower valency.This re ac tion may ex plain the electroreduction charge in the tran sients as so ci ated with a dif fu sion controlled pro cess and the lim it ing cur rent de tected in the voltammogram of Fig. 1.More over, Re ac tion 11 has pre viously been pro posed to ex plain the electroreduction of cobalt ox ide lay ers formed in elec tro lytes con tain ing bo rate ions by means of mea sur ing tech niques dif fer ent from those de scribed here 1 .
Re ac tion 12 takes into ac count the for ma tion of a complex ion of Co(II) which has been shown to be the sta ble form in car bon ate-bicarbonate con tain ing so lu tions 12 .Although the dif fu sion cur rent in the tran sients was re lated, in a dif fer ent fash ion, to a re ac tion in volv ing the com plex ion Ta ble 1. Pa ram e ters used for fit ting the electroreduction cur rent de cay data in 0.75 M KHCO 3 + 1.5 M K 2CO 3, at pH 10.5, ac cord ing to Eq. 6 for P 4 ′ 0; the height of the growth phase h cal cu lated with Eq. 9 and the electroreduction charge den si ty q, cal cu lated with Eq. 10.
τ/s P3/mAcm -2 s 1/2 P5/mAcm -2 s -1 P6/s -2 q/mCcm -2 h/nm  The the o ret i cal curves were de rived from the fit ting of these data ac -Co(CO 3 ) 2 2-6 , no clear ex pla na tion was given for the Co(III) → Co(II) trans for ma tion, which should be de tected in the tran sients dur ing the electroreduction of Co 3 O 4 films.As the po ten tial ap plied to the elec trode en ters the poten tial win dow of the voltammetric cath odic peak, the reduc tion of the in ner CoO to yield an electroreduced co balt layer also takes place (Re ac tion 13).Re ac tions 11 and 13 prob a bly in volve mul ti ple steps.

Con clu sions
The pro posed electroreduction path way of co balt oxides formed in the po ten tial range of the sec ond ary pas sivity in car bon ate-bicarbonate buff ers is con sis tent with the pre vi ously pro posed struc ture of the pas sive films.As such, the mech a nisms pre sented de scribe the electroreduction kinet ics of both the outer Co 3 O 4 layer and the in ner CoO layer.The tran si tion of the growth mech a nism for the new phase from a 3D to a 2D pro cess may be re lated to the time of potentiostatic ag ing τ.The height of the growth phase h can be cor re lated to the thick ness of the an odic film, as previ ously mea sured us ing other tech niques.The electroreduction po ten tial step tech nique ap plied to an odic films has the abil ity not only to al low dis crim i na tion among dif fer ent ki netic mech a nisms, but also to pro vide in for mation about the na ture of these films.

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

Fig ure 2 .Fig ure 3 .
Fig ure 2. Ex per i men tal cur rent tran sients [•] re corded at E f = -0.74V in 0.075 M KHCO 3 + 0.005 M K 2CO 3 so lu tion, at pH 8.9, at dif fer ent τs.The the o ret i cal curves were de rived from the fit ting of these data ac cord ing to Eq. 1 [-].

Fig ure 5 .
Fig ure5.Ex per i men tal cur rent tran sients [•] re corded at E f = -0.79V in 0.75 M KHCO 3 + 1.5 M K 2CO 3 so lu tion,at pH 8.9, at dif fer ent τs.The the o ret i cal curves were de rived from the fit ting of these data ac -