Current oscillations in the reduction or oxidation of some anions involving convection mass transfer

ELSEVIER

Journal of Electmanalyticai Chemistry 430 (It~7) 195-201

Current oscillations in the reduction or oxidation of some anions involving convection mass transfer Zelin Li ,,.h., Jiale Cat

Shaomin Zhou h

* Del~lrtlll¢lll of Che.,nisl~:v, Itumm Normal lhlil'cr.~#|', Ctumg,'~'h. 4100 t2,, Chim~ I, o~l~il¢ Key I~thor~tlorv fbr Phv.~i¢~# (?h¢oli,~lry ~i Solid Sio:li~'e aim I)¢l~O'lmenl ~t' Cll~,mt~'oW, Xiamen Uni~,er,~itv, Xiame. 36 I I~IS, ('hm.

Re¢¢~ived I11 April It)~)?: r¢~'e~ived in r~'.vi,,cd [orm 24 Ju~t: 1~)~)'7

At~Ir~tct

We have discovered a new kind (||i~ cnrrcnl uscillalinn durin~ lii~ redu¢linn or nx~dt41it~n of ,~olne {iniOll,~, which al:)lx~ared over Illc

limiting |¢dticlion or oxidalion current el ° Ihe anions and was loaded on the currenl e': hydrogen or oxygen evolution. The overlap of a posilive i~¢dback (convection mass transfer of the anions induced by hydrogen or c~ygen evolulion, and Ihe increase el Ihe [r)tential difference in Ihc c¢11 when the eurr¢lll is slnalleO and a negative feedback (depiction of the surl~ce concentration of the anioB~ by reduction or oxidation, and the decrease of the potential difference in the cell when the current is larger) between the billable ~u~le~ accounts for Ih¢ oscillations, and a 'hidden' negative imt~d~mce in the second ascer~ding branch of the polarization curve was oh, erred lbr Ihe firsl lime. Our model has also shed some light on Ihe other systems reporte¢ in the lilerature, in which convection mas~ t,an~f¢l also plays an important role in Ihe cunent oscillations. In addition, the potential oscillations during Ihe oxidati,n of Fc~CN);~, are rel~)rtcd here f,r Ihe firsl tin|¢, a~ld are similar Io the potential oscillations during the reduction of Fe(CN),~ rcfa~ncd ill olir previou~

pawr [Z. Li. J. Cat. S. Zhou. J. Elects|anal. Chem.. in press]. ~:'~1997 Elsevier Science S.A. Kevwo~l,s: Nonlinear ph¢iloiii~iltliK (~'ur[¢llloscillations; Polenlial oscillations; Reduclion; OxidilllOii;Binl,d:Pilily;Negali~¢ impedai)~¢

1. Intrmluelion Current oscillations appearing dunng electrochemical reduction reactions on stationary or dropping mercury electrodes have been investigated intensively both expert° mentally and theoretically. Current oscillau,,ns in lh¢ reduction of anions like S ~ O ~ . Fe(CN)~ ~. Pt(CI4): and CrO,~ ~ were studied early by Gohkslein and Frumkin [I.2] and Frumkin et al. [3]. Wolf ~t al. [4] have recently developed a dynamic model for the reduction of S : O ~ . Jehring and Kuerschner [5] observed the current oscillations during the reduction of Cu 2~' and Bi 3. in the presence of trihenzylamine. A systematic study of current oscillations during the polarographic reduction of C u : ' . Cd "~' and TI + on dropping mercury electrode in the presence of various inhibitor was made by Dtirfler and MUller [6]. The current oscillations of In(Ill) reduction in

" Corresponding author. [email protected].

Fax:

+ 86-592-2188054:

e°mail:

0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights re,,erved. PI! S0022-0728(97 )00344-6

whe pre~enc¢ c,l' SCN on dropping mercury or hanging drop mercury electrode were fully studied hy !aku~ew~ki and Tumwska [7]. de Levie and Itusovsky [8]. de Levie and Pospisil [0], Pospi.ql and de L~.'vie [10]. de Levie t i l l Moreila and de Levie [I 2]. Different model.5 to explain the eurren~ oscillations have been proposed by de Levie [I I]. Keizer and Scherson [13], Koper ¢t al. [14.15]. Three conditions at which these oscillation~ gener~dly

occur are well established: (i) a region of negative differ° ¢ntiai resistance, i.e.. negative faradaic impedance, exi~l~ in the current-voltage curve, and the oscillation~ occur

precisely in that region: (it) a high external resistor, in series with the electrochemical cell. is required: (iii) Ihe occurrence of concentration polarization near the ¢l¢ctro(l~: surface due to the electr(~hemical reaction, In additiolt to the potential o~cillation,~ iz~ file electro chemical oxidation of Fe(CN)~ '~, which ~rc simil~ir lo the potential oscillations in the reduction of [e((:N)~, re.~, ported in our r)revious paper [16]. we repot| here a new kind of current oscillation in the electrochemical reduc|io~ or oxidation of some anions. It is interesting thai the

I~

Z Li et al. / Journal oj Eiectrmmalv#c,I Chemist~" 43¢5¢1997) 295-20i

curreut o~illations appeared above the limiting current pla~au for the reduction or oxidation of the anions and were "loack..~' on the current for hydrogen or oxygen evolution, but di~p~ared by agitation. In contrast with ~he,r current o~illmions cited above, the convection mass transfer, which is induced by hydrogen or oxygen evolution, plays an important role in the o,~illadons. Our study may ~ n a ~ w field for electrochemical o~illaiions ~ a u ~ t~re are certainly many systems that can undergo +imilar os¢ill:ations m ~hose in our report, and may shed ~)m¢ li:ghton sot~ ot~r systems reported in the literature.

VN

-2

-4

-6

-8

-10

a

3 I

An Hoty~ glas~ cell [16] was u.~ed, of which the two ¢~Ip~:lllen~ h~ve diffe~e~ ~i~e~: the ~maller one wi~h a 2,~ cm inner diameter for the counter electrode, a large ~trip of platinum: and the larger one with a 5 cm inner di~¢ter tbr the working electr~, a platinum disk, total diam¢lcr 4.~ ms, 3 mm diameter in platinum, insulmed with ~ ; x y reran. TI~ two electrifies were placed vertically faceoloot~ce at the two ends of the channel (I cm inner gameter and 5 cm long) connecting the two como partments, and the cell was connected in ~ries with a decade resistance bex adjustable from 0 m l0 k fl. So the vohage was impo~d ~)th on the cell and on the series resistance. An el~tr~xte of Hg(I)tHgO(s)l I sol d m ~ NaOH, w;th a Luggin capillary,served as the reference electro,de when the polarization curves, the iml~dance spectra and the !~tential o~illations were measured, T1~e working electro,~ was sm~x+d;ed with ah+mina ~+w~t~ th+wn to 0,3 #m and was c l e m ~ with ultrasonic waves. A solution ( 1 ~ cm ~) f~shly I~l~-~d ihHll doubly distill~I w~ter and ~ l y t i c a l g ~ chemicals was u ~ each t i ~ , S i ~ low dissolved oxygen has no ell~t ~ the o~illathm p h c ~ n o a , not..hingwas ~me to eliminate it fimn the ,,~d:u~iotl,Experiments we~ conducted at room tem~ratu~ ¢m0und 16~, Impedance s p ¢ . c t r ~ y was m e a s u ~ on ZAHNER Elek~k IM6 (Gem~ny), O~her electrochemical experi~n~s were c a ~ out with a CHI 660 elec~hemical stalioa (USA), whkh was interfaced with a computer for ~ u i d n g and a~tyzing data,

3, RcsMts and dlscasslo~

.~.LL C~ral.fea~es ~" ¢he c~rrent oscillatio~s F ~ I-3 ~ w the currenl oscillations for the t~luction otO.I sol ~m --~ IOf, I sol dm --~ S,O~- and 0.8 sol ~ - ~ Fe(CN)~- on "the platinum e l e c t , respectively. In Figs. la-.~, current-voltage curves, were obtained by

S~O ~ 0

~0

SO0

Fig, 1 Cun.¢n~ o~ill~|~ow, fi~' 0 1 ~ 1 dm ~ IOi w~luc|io~ o~ ~h|imm~ ia ~ I n~d dm + ~ NetOlol ~,dutiem. (a) cta~?nbovollagc cu~x,~s, Volla~Ic

voltage scmmmg al 10 mV s * without or with different ex|emal ~ries resislance (R~); Figs, !b-+3b. time evoluliras of U~ current, at difterent applied vo!t~ge while fixing the R,,; and Figs. ! c ~ , also tin~ evolutions of the eun~nt, but at differcn! R~ while fixing the applied voltage. When the current value is smaller, some inducing lin~ is required ~fore !he oscillations can ~ fully deveb o ~ . We show only ~he steady oscillations in Figs. I b,c to 3b,c. All o~ilhtions are pre.~nted in their absolule Ioca. lion (including the voltage, the cur~nt and the time). There are son~ common features in these systems. First, [he systems Io.~ suability during voltage scanning in Figs, la-3a when the current is beyond the limiting curo rent (labeled with dolled lines). At the minimum ~ak cun~nts of the oscillations, hydrogen evolution can be ob~rved; and at the maximum peak currents of the oscillalions, hydro[~n evolution is repres~d by the reduction reactions of the anions replenished through the convection induced by the hydrogen evolution, i.~., a bistability exists b~cau~ at a given applied voltage there are two different reaction processes (with or without hydrogen evolution) corresponding to two different currents. Experimentally, at a given applied voltage where the oscillations occur, if a constant agitation, with a strength equal or larger than that by the hydrogen evolution, is employed instead of the

197

Z Li et al./.h~ur~d ,)f Electr,~mal~1i,'a! Chemistry 436 ~i ~ T J 1 ~ - 2 0 1 VN -4

-2 .........

6 -

-6

!

-8

'1" e

-10

t

ai

higher the solution resis:ance (75 li, 90 gl) and 48 [! in Fig. l, Fig. 2 and Fig. 3 respectively), the larger the oscillation amplitude. Third, the exact location in which the current oscillations occur depends on the applied voltage (in Figs. la.b to 3a,b) and the exte~,,,I series resistance (in Figs. la,c to 3a,c).

4-

2 i . . . . . .

4 3

2[$0

300

3~11)

2tt0

300

3~0

i::ip, 2, Ciiffciil i!,~¢ilhiliOli~ for I ilitil dill ~ S~O[ redliclion lilt phil ° ilililil hi ii I niol lllll +~ NaOtl .~ohlliOli. (at clirrl~lil:volliige t:ilrves, Volllig¢ ~¢iiliilin I Ill 10 inV s-i with Ihe ¢Xlel'niil series r~sisliili¢¢ R,~ t i l t : (1: 2(XI: 400: 600: 1000: 14ll0., 18(XI: 2200 (from left io right!. The tcsi~iiill¢¢ of Ih¢ ~olulion I R,) is 90 11. Tile dolled line represcni.~ the Ihifiling curr~nl pliileliu 12.38 niA). Ih) lime scri¢.,i ill diffcrcnl volliigt~" IV): tl<5; 8; 7: tl: $.75 (friun top io boilonl) while fixing the R,. ill 1000 t), It)lilli0 ~ri~'~ ~li dilh~ri;nl R,, (11)'. 3~1): 400: ,'i00: (lO0:700 (l~roin top Io houonl) while tixing Ihe volliige ai 5 %/.

3. i.2. Theoo, mid mechanism of the current oscillations in addition to the bistability discussed a ~ v e , we surprisingly R~und that the negative impedance theory [18] can also be applicable to this kind of oscillation. The impedance spectra in Fig. 4 were taken at the second ascending branch of the inserted current-potential curve for Fe(CN)~ .... reduction. The 'hidden' negalive real impedances in a range of nonzero frequencies were oho served experimentally lt~r the first time in thai h~ation where current oscillations occur. II lla~ I~en i~poried lhat lli~ negative impedance exisls in Ihe descending I~ra.ch (with a negative ~lope) [11] or 'hide~' in the first a~cendi.g brauch (with a po.~ilive slope) [18.19] and in the limiting currenl plateau (with a zero slope) [16] of the currenipotential curve, a n d the oscilhitioris appear in those i~:o lions. The convection mass transfer of the anioll~ induced by the hydrogen evolution plays an important role in Ihe

VN

-2

4

........

convection effect induced by hydrogen evolution, no depletion of the anions at the electrode surface can occur, so the oscillations stop and the current stabilizes at the maximum current state where only the anions are reduced. This is a simple way similar to that in Ref. [17] to show the single stable state of the bistability in the oscillatory region, and it explains why the oscillations disappear by agitation as we have mentioned in Section I. Removing the agitation, the current drops to the minimum with the surface concentration depletion of the anions by reduction under th~ control of the limited diffusion mass transfer, and hydrogen evolution t~tkes place again, which is die other single stable state of the bistability, so the oscillao tions are restored between the bistable staecs. Below the limiting current, only the stable state for the reduction of anions occurs: while the current is high enough, the hydrogen evolution becomes the predominant reaction and cannot be repressed by the reduction of the anions, so another stable state appears. Second, the current oscillations in all cases only appear above the limiting current and are 'loaded' on the current of hydrogen evolution, The amplitude increases and then decreases when the current of hydrogen evolution incleases steadily in Figs, I-3, or when the external series resistance increases in Figs. l a,c3a,c. Small amplitude oscillations can be discerned even without external series resistance in Figs. l a-3a, and the

6-

•6

:8

.10

|

a

2 4,S

c

4.0 3.5 3.0 2.5

420 ,440 460 48O

420 440 460 480

f/s Fig, 3. Corre.I o~cilhltion~ t~)r 0,14 lllol dm ~ I:~(CN), ~,.... r(:(hlcii,til :.l philillUm ill ;i I inol dlil ~ N~iO|t ~lhlliOll, (~l) ctirf~lll:will~l~e ~:uivq~. (!1): 6: 2~i'. 400; (~00; tl(X): 10{X)'. 1400'0 I~(X)'0 2~00'0 ~f~(l~l Iffltlii I~ti lit rithi), Tile rt~i~iail~:¢ of II1¢ ~oluiioll (R,) i~ 4114 11, TII~ &iliad Ii~i~ rl~pr~.~t~lil,~ lhl; Iiiililillg curri~iil plaleau (2,d4 iliA), (hi lii~i~ ~ f i ~ ~it difl'ereni voltage (V): 8,~: 8: l: 6,2 (from top Io belie!!l) while I]~iii~ lhe R< al 14(X) [l, (¢) iinl(~ ~erie~ at (lif|~r¢lil R~ (1|): (~(XI: 7(~): I~(XI; (J~(t (tiom lop to boilOln) while fixing the vollale at $ V.

I~

Z. Li et aL / Jnurmd O/'b.'h,t'o,~m.lyth'.l Chemistry 43t~ ~1907~ 2t:5-201 i

i

ii

i

.t000

o ,o !

i

i

I"

iI

t¸000

~I000

0

I000

f~d~¢lh~!1 ifl ~ I tool dm ~ N~()H mohllJOll o11 ~i pl~mm,,m de¢Ir~J~, [~r~qll¢iI¢,~r~ille: I(,XII~AIJI,~I,I~, The i~1_~d ph. i~ lh~ l'~d~ri~,..,~Ion

~x~illations, We give a qualitative explanation here. When a constant voltage (V) is i m t ~ d in the circuit [20], it is divided into two parts: !( R~ + R,). in the external ~ries resi~ta~e (R~) and the solution resistance (R,): and E, in the cell. If the voltage is high enough, and so is the E at the beginning, a sudden imposition of a larger E will drive the c u ~ n t over tile limiting current of the anions in a statio|laoj state..~(, th~~ surface concentration of ti~ anions depletes to ~ero and hydrogen evolution takes place. Be~ cau~ the growth, de:achment, and movement of I~bbles pcoduce a G~ed conv~tion, which ~plenisbes the surt~ce ¢or~mratio~ of the anions a~d raise the limiting c u ~ n l pla~au over the s,tationacyone to the level of the pe~ks (m~ximum cutrenO like a ~4ating el~tr~e, the hydrogen evolution i~ thus re~.~sed, Tb~t may be call~ a positive t ~ d ~ k (higher ~ ~,nd convection mass transl~,r of the ankhs) be,eau~ it i~reases the current, There is a negative feedback ~ m p a n y i n g that, which a l ~ comes them two a ~ t s : ~ is tt~ ~p/etion of the surface concentration of the unions by ~ t i ~ : m ~ and the other is the decrca~ in E becau,~ of the increase of I and so the I( R~ + R,) under ot~stant voltage, and it ~ u c e s the curren! to the mini~ mum until hydrogen evolution takes ph~-e again, It is the overlap [21] of the t~sitive and negative f e c d ~ k s het~,~n th~ bistoble states th~t ~cotmls for the o~illalions, C ~ r l y , the ¢ff~t of the external series resistance is in i~en:sit~i~ the unstable nature of 11~ systems,

3,/,3, ~ i t ' m / ~ , ~ ~ IRe nuclei to orh~r ,~y,~tems |t :sh~Jld be pointed out that not all reactions involving hymn evolution can generate this kited of o~illation unk,~ the reduction potential of the species is more postfive ~ that for hydrogen evolution and the reaction is diffusi~-Iimiled, or simply, there should be a limiting

current plateau for the species. For example, we did not find o~illations (except small noise like that which appeared at the end part of the current-voltage curve in Figs. ia-3a) during the reduction of CrO~- in an alkaline solution on a solid electrode (platinum) because hydrogen evolution is always favol~ible and uo bistability exists, which is consistent with the literature [22,23]. Recently. the current o~illations during zinc deposition have been explained by the formation and reduction of the zinc hydroxi~ and have been u~d to explain the anomalous codept~sidon of Zn~Co alloy [24]. The explanation t'or the curl~enI oscillations~ we think, is basically inco~ct. Whereas our model is applicable to the phenomenon alo though the reactant is cations (Zn ~~ ions) there, noticing that the I~dari~.ation curve tbr ~in¢ deposition in Fig, I(d) ot° Rel: [24] is similar io thai G~r I.¢'{{N)~, r,~duclion inse~ed in Fig. 4 of Ibis report, i,e., a first ascending branch and then a limlling cu~lii plateau ibr zinc del~Si: lion. and a second a~:~lding hriinch where hydrogen evoo lution takes place. Fuahenno[~, the current oscillations for zinc deposition appear al'a)ve the limiting c u ~ n t plateau (Fig, ,~ in ReC [24|), which is one of the typical character~ istiC~ for tile osc~Hations we rer~n here. Our model, i.~. the overlap of tile two opposite I~¢dbacks between ~he bistable's states, may also be applicable to the currenl ~scillations on a n~rcury electrt~le, although hydrogen evolution is ah~nt there. Tile only difference, i.e., one factor of the p~silive feedback (convection mass tr~msfer) in the latter c a ~ is due to the movements of the mercury surface, which can be cau,~d by inhomogeneous ~darization and inhomogencous adsorption [25], as well as, we think, by Ihe variation of the potential (difference) under cons|am applied voltage.

We have discussed the potential oscillations and the ~.~r~n~ os,:i!lations during the reduction of FdCN)~ ~= ions in a previous p a ~ r [16] and in Section 3.1 of this pa~r. ~sp~tively. With the understanding of the mechanism, we have found a series of similar oscillators on the cathode [26]. The ~zme principle is applicable to the anodic prot e s t s in which oxygen evolution takes place instead. We repoa hear a typical nonlinear phenomenon, both current and potential oscillations, which appeared in the anodic oxidation of Fe(CN)~. Although its mechanism is similar to that for Fe(CN)~ ~ reduction, this example may be instructive for finding another series of oscillators on the am~le. 3.2./. Potemial oscillations in the oxidation of I"e(CN)~ ~ ions

Fig. 5 is a typical current-potential curve of Fe(CN) 4oxidation on a platinum electrode in a I mol dm-3 N a O H solution, which was obtained by current scanning. The curve shows the bistability and the oscillation region

Z Li el ~aL/lou.~al ~fl:7¢ctr~m,d~'th'al Chemislrv 4.¢6 ~!997) 195~201

4

2

0.2

0.4

0.6

0.8

1.0

£N

Fip, 5, A t~,plc~d ~0.l!~mullopr~ilU li)r Ihe oxidl|lion ~|' (I,{~ reel d m Fd('N)~ ill ~i I 11101 d n t i N~IOH ~ohltlOll till ii phltinull~ electri~le IIiIdl~l • i~'u[[~lll ~¢tlllliilt~ ((),i)()~ IlIA ~ I ),

clearly, When the applied cun'ent is less than the limiling c u ~ n t , the, system stabilizes at the lower potential side of the limiting current plateau. While at the limiting current° a bistability appears, i.e,, the Fe(CN)~ oxidation at the lower potential side of the plateau and the oxygen evolution at the higher potential side of the plateau. Then sustained oscillations follow in addition to the bistability with the continued increase of the applied current, which can span some range of currents because, at these currents, the convection induced by the oxygen evolution can raise the limiting curt'eat plateau over the stationary one to a series of higher levels. As a current larger than the limiting current (stationary) is imposed, the system is far from equilibrium, and shortly alter, the surfilce concentration of Fe(CN)~ ..... ions depletes to zero due to the limited supply rate by diffusion. We call the depletion a negative feed° back. Meanwhile, the potential moves to the higher side of the plateau with the decrease of the Fe(CN)~- surface concentration until oxygen evolution takes place to keep up the applied current. Because growth, detachment and movement of bubbles produce a forced convection [27] that replenishes the surface concentration of Fe(CN)~ .... ions, i.e., a positive feedback, oxygen evolution is thus repressed completely and the potential drops again. It is a typical period-one potential oscillation. In brief, it is the overlap [21] of the negative and positive feedbacks between the bistability that accounts t'or the potential oscillations. A stronger agitation can stop the potential oscillations as well, and, experimentally, the bistability under a given applied current in the oscillatory region c~n be shown in a similar way as we have discussed in Section 3. I. I, i.e., imposing an agitation, the potential stabilizes at the lower potential side of the plateau where only the Fe(CN)64- ions are oxidized because no surface concentration depletion of the Fe(CN)~- can occur; and removing the agitation, the potential shifts to the higher potential side of the plateau with the depletio,: of the Fe(CN)~- at

t~)

the electrode surface by oxidation controlled by diffusion until oxygen evolution takes place again, and the potential oscillations are restored between the bistable states. While the value of the applied current is over about twice that of the limiting current, oxygen evolution predominates and cannot be repressed by the replenishment of the Fe(CN) 4 ..... ions through the limited convection effect, so the system enters into the other stable state (with some noise). The depletion and replenishment can be seen also from the cyclic voltammograms of Fig. 6. An N-shaped curve appears by forward scan between 0 ~ 1. I V in (a), and the descending branch is due to the depletion of the Fe(CN) 4 ~ surface concentration hy oxidation with a limited supply rate by diffusion although the ~tential is keeping up a steady increase. While by backward scan ~1crossing cycle occurs in (a), whe~ the reverse scan current is larger Illan the lbrward scan current, and the occun'ence of this cross° ing cycle can be explained by the enhanced convection mass transfer of the Fe(CN)~~, ions induced by oxygen evolution I~cause o n l y a n o r d i n a r y cyclic voltammogram appears while there is no oxygen evolution in (b) (rever~e scan at 0.7 V), or no Fe(CN)~ ions in (c). A typical potential oscillation sequence is given in Fig. 7, which we got by increasing the applied current monotonously in the oscillatory region of Fig. 5. The amplitude increases and then decreases with the incrcase of the current from (a) to (g) of Fig. 7. The former is due to the increased polarization for oxygen evolution (Fig. 5). i.e., oxygen evolution moves to more positive I~tential at a larger applied current, and the latter is due to the blockage effect of the gas film [27], formed at still larger curt~nt, to the oxidation el' Fe(CN)4, . Aperiodic behavior (a. I'. g) appears near the lower and the upper current boundaries (Fig. 5) for the oscillations, whirl1 result~ lYom the nonuni° fonnity of the oxygen evolution in bubble sizes and in nucleation sites with a weaker agitation when the current is

10 a u m

0,~

0,4

0,6

0,8

t,0

Fig. 6. Cyclic voltammogt~lmr~. Higher limit in at I, I V excep! (h) w|li¢tl is at 0.7 V. l : d f N ) ~ ° Concenlr=ttion iv= =J I tool dm ~ NaOlt ~¢~luli=tI) ir~ 0.6 reel dm + + except (c) which if zero). Potential ~:~m ~lt I(K) mV r+ P

Z Li el aL/Journal ~g'Electr~malytical Cllemist~, 436 (1997) 295-201 t.0

,

'

0.6 0,1 3

0.8 | 0

O,O

SO

t~

1SO

100

F'iT, 0 Witvt'l~rlll~ tff eunt~nl o~illaiioli~ tilt li~i~ illol l l l i l t ['"('"i-t,.... l'~~t~'l. oxidalilm in a 1 nlol tJlli '~~ N~OH ~oliillon ill difl~p~lll ¢,ll~llllll ~cll~ rc~i~tilll¢C ( i l l : (Ill lillt', (hi I~iltll |el I l t i l : {dl I¢~11tl',(~) Iitltl) whil~ Iixin 7 Ih~ al~pli~d ~'~lli~g~ iti ~ V,

0,$ O,O 0,0 OAI

of the c u ~ n t region tbr the oscillalion ~cause of Ihe unil~orm oxygen evohi!ion which r e s l o ~ the F~CN)~* surfi~ce concentration completely.

0,8~ O,O

0,40

3,~,~., Clqrrem osHIhalions in lhe f~xidciliou ~f t:t~CN)~

SO

100

150

iO0

Fig, ?, Wlivehwm,~ of t~i-,l~mi~l o~ill~lii~a.~ fi~i O,i~ mid d m ~ FcICN)~ ~i,d~liol~ i~ a I 111ol dill i N~OIt ~ohiliotl ill ditl¢i~ol iqlti~d ~ilri\,~lll (illAl~' (ill ~,(L (hi ~,~ it) ~,$~ (d) .llji, It) 3,~ ~,(fi .1,8, I l l 4>It,

sillilller (a), or ih~nl formhlg tll~ ~ll~iog

Ih¢ !,in~llll~l~ ga~ f i l m i~xcssani!y w h e n Ih¢ curix~ni i~ I m ' l c r (f, l )

Cuffenl o~_'illalions in Ihe oxidation o f F e ( C N ) ~ - ¢an oh!llhli¢~l eilher by voltag~: ,scan (Fig. 8) or by conslanl

collage contrtd (Fig. 9) wilh different external series restslance. !I is inler~siing Io hole IhaI in bolh c a ~ Ihe current o~¢i!lalions als~ appear only a!~lve Ihe limiting current and a ~ "h~ided' on Ihe currenl of oxygen evolution, and lhe tipper permilled curreni for Ihe oscillalions i~ also aboul lwic~ o f Ihe limiting current Ihai is consisieni with the

I~lenlial (scllhlllOns (Fig, ~), Periodic OX)llen evolution was ob,~rved as well during l ~ o,~illaliorls, i,e,, oxygen evolution lakes place at the m i n i m a o f the current peaks lind is r~pr~ssed lit the m a y ima of lh¢ currenl I~aks, which means there is a bistability similar to the reduction of the l~vions discussed in Section 3,1, in which hydrogen evohidon lakes place instead. Because the :nechanism in Section 3. I is applicable to this current oscillation on the anode, only by replacing the reduction wilh oxidation, and the hydrogen evolution with the oxygen evolution, more discussion is unnecessary.

/

References .

l

4

.

.

.

.

.

|

6

$

10

~N

F~. ~ ~ ctil~ili-~x~ll~k~ cur~ in a soluihm of O,b rnol din- ~ I~CN)~ ~ p~us ! ~ I din- ~ N~I~I~Hby voltage man ai I0 rnV s- * with I ~ if) III~,#fill2400: (h) 3000. E i~ilanc~ of the .solutionit, is 55 i}. ~ ~ liite ~ f s , the Iinfiling currenl plateau {i,94 mA),

[I] A,Y, ~ h s l e i n , A.N, Frumkin, Dokl. Akad, Nauk SSSR 132 (1960) 388, [2] A,Y, Gokhstein, A.N. Frumkin, Dokl. Akado Nauk SSSR 144 (1962) 821, [3] A,N, Frumkin, O.A. Petrii, N.V. Nikolaeva-Fedorovich, Dokl. Akad. Nauk SSSR 136 (1961) 1158. [4] W, Wolf, J, Ye, M. Purgand, M. Eiswirth, K. Doblhofer, Bet. Bunmnges, Phys, Chem. 96 11992) 1797.

Z Li el aL/Journal ¢~fEleclr~an
H. Jt,hring, U. Kuerschner, J. Electro,anal. Chem. 75 (1977) 799. H.D. I:Nirfler,E, MUller, J. Eiectroanal. Chem. 135 (1982)37. B, Jakuszewski, M, Turowska, Rocz, Chem. 43 (1969) 2003. R. de Levie, A,A, Husovsky, J. Electroanal. Chem, 22 (1969) 20, R, de Levie, L, Pospisil, J. Etectroanal, Chem. 22 (1969) 277. L, Pospisil, R, de Levie, J, l~lectrt~mal.Chem. 25 (1970) 245. R, de Lcvi¢, J. Electroanal, (:hem. 25 (1970) 257. H. Moreira, R, de Levie, J. Eilectroanal. Chem. 29 (1971) 353. J, Keizer, D, Scherson, J. 'Phys. Chem, 84 (1980) 2025. M,T,M, K e r r , P, Gaspard, J,H. Sluyters, J, Phys. Chem. 96 (1992)

5675, [15] M,T,M. Kopcr, P, Gaspard, J,H, Sluyters,J. Chem. Phys. 97 (1992) 8250, [16] Z, Li, J. Cai, S. Zhou, J. Ele,:troanal. Chem. 432 (1997) 115. [17] M, Orb~in, I,R. Epstein, J. Am, Chem. St~. 103 (1981) 3723, [18] MTM, Koper, J, Elcctroanal, Chem, 409 (1~)06)175.

201

[19] M.TM. Koper, J.H. Sluyters, J. Electroanai. Chem. 371 (1994) 149. [20] M,T.M. Koper, J,H. Sluyters, J. Eiectroanal. Chem. ~ 3 (1991) 73. [21] W. Wolf', K. I~~'i~her, M. LUbke, M. Eiseirth. G. Ertl, J Electn~nai. Chem. 385 (1995) 85. [22] A. Franczak, J. Matysik. M. Korolczuk, J. Electroanal. Chem. IO7 (1980) 189. [23] L. Jiang, D. Pletcher, J. Appi. Electrochem. 13 (1983) 245. [24] H. Yah, J. Downes. P.,I. Boden. SJ. Harris, J. Electrochem. Soc. 143 (1996) 1577. [25] G. Ginzburg, J,Y. Becker. E. Lederman. Electrochim. Act~ 26 (1981) 851. [26] Z. Li, J. Cal. S. Zhou, submitted. [27] H. Vogt. in: E. Yeager, J. O'M Bockris. B,E. Conway. S. Saranga° pani (Eds.). Comprehensive Treatise of Eleetr~hcml.~try, Vol, 6, Chup. 7, Plenum, New York. 1983.