ELSEVIER Journal of Eleclroanalytical Chemistry 434 (It)~)7) 129-137

Voltammetric studies of electrochemical processes at the interface PtlYSZ between 300 and 600°C

M.W. Breiter *, K. Leeb, G. Fafilek In~litul j'tir Technisch¢ 1".T¢~tn~chemi¢, TU |Vh'n, ~,J Getreidemarkt, Aol060 Wiell, Auso'i.

Received 19 Septeml~r Iq~q6; revised 14 Fehnmry 1997

ASstrat, t

Elecln~chemic~d pn~:es~c~ occun'ing ,*kt fire inlerf~lce platinnmly|lria==~,~l~lbilized zirconia were investigaled by cyclic vol|aml)]¢lry and polentiostalic currenlo~poleatial curves, laken n|lder ~leatly~slate condilions, in different mixtures of oxygen and nitrogen at ~e|nperalures between 3tX) and fi00°C. Point or paste electrt~des were used~ The exchange ctnxenl of |be rate-determining slep of the oxygen rednclion I~comes very small below 500°C. Theretbre, the equilibriuin of the oxygen electrode is no longer established. A layer of chemisorbed oxygen aa~ms is lbrmed anodically on the contact surface of the point electrode, not by the dissociation of O, a~ the thrce.pha~e boundary. The shape of the cyclic voitannmograms is discussed. © 1997 Elsevier Science S,A.

gc~yw, rd.~: Inlerhice; ionic conductivity; S|ahilized zirconia; Cyclic vohammetty

1. Introduction

The properties of the interface metallstabilized zirconia or metal oxidelzirconia and the electrochemical processes at these interfaces determine the satisfactory operation of solid oxide fuel cells and oxygen sen~ors. Voltammetric techniques were applied successlully under stationary or cyclic conditions in previous papers [1-7] on this subject, While Refs. [I,4] dealt with investigations at relatively large temperatures (T ~ 700°C), the temperature range was extended down to about 300°C in Refs. [2,3,5-7]. This allowed investigation of the formation and reduction of the oxygen layer as well as the reduction and production of O 2 [6]. Here the word 'oxygen layer' designates chemisorbed oxygen or an oxide.

The present study of electrochemical processes at the interface platinumlyttria-stabilized zirconia (PtlYSZ) was not started to repeat some of the experiments described in Refs. [2,3,5-7]. The original purpose was different: the comparison of results, obtained under the same conditions for the interfaces PtlYSZ and PtlBICUVOX.10. Here BICUVOX.IO stands [8,9] for the oxygen ion conductor BizV0.oCu0 iO,i.35. However, it turned out that some of the important questions for such a comparison were not cove

" Corresponding author. E-mail: sekr158O~fbch.tuwien.ac.at.

0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0022-0728(97 )00 i 23-X

ered adequately in Refs. [2,3,5-7], making it difficult Io use the interface PtlYSZ as a model interface. In particular. the question of the establishment of the equilibrit~m of the reaction

O~+4e ~ 2 0 : - ( I )

at temperatures below 600°C was not considered in the previous work, although it was already concluded in Ref. [10] that the potentiometric response of certain oxygen sensors with the interthce PIIYSZ is non*Nernstian below 600°C. The latter conclusion was confirmed by our previo ous investigation [1 I] of the open-circuit behavior of the interface PtlYSZ at 400°C, using different Pt electrodes, Therefore, a detailed study of electrochemical precesse~ at th~ interface PIIYSZ in various gas atmospheres ((!0t) X )% N~ + X% O:)) was carried out in a temperature range of between 300 and 600°C in our laboratory. The conclu- sions in this paper are restricted to the ~,L~we temperature range. It cannot be said without additional studie~ whelher the same conclusions hold at higher temperatures (600 to 1200°C). A large number of investigations of electrochem- ical processes are devoted to this higher range of temi~ra= tures.

Following the suggestion in Ref, [ 12], Pt elect[~Jd¢~ with point contacts were mainly u~ed here to ovezcome difficuJo ties in the reproducibility of the electrode properlie~, Such diMcnlties were encountered in our early work when

I~,~ M, W, Bre#er et aL / Jour~ml of Electroanalytical Chemisto, 434 (1997) 129-137

producing the electrodes by sputtering or by paste. The point electrode has the additional advantage that the contri- imtion of t ~ ohmic potential drop IR¢I between the work- ing electrode (WE) and the reference electrode (RE) can be kept small in compari:~m to the contribution of the polarization resistance Rp. The situation is similar to that of studies [13] of e l e c ~ kinetics with the microelec- trode in unbuffe~d aqueous media where a large IR~ is ~ t . The i n f l u e ~ which the arrangement of the WE. RE and counter electrode (CE) exert on the shape of cyclic voltam~gramg because of a different current distribution was dimtly demonstrated in Ref, [14]. The results in Ref. [14] suggest that the distortion (zero line fi)r current is inclined vs. the ab~issa) of the cyclic voltammogoms in the re~l~ctivo figures of Ref, [3] is caused by the inhume= geneous c a ~ n t distribution.

However, the use of point electrt×les has a n a m e r of disadvantages, caused by the smallncs~ of the contact area~ It is hard to estimate the geometric area of the contact, The ~ i t ion ing of the point electrode at the same spot on the suO~e of the ~l id el~trolyte is difficult in our arrange- meat, ~casionally a small hole is *burned' through the ~l id electrolyte at the tip during mea~,;urements at higher ~mperatures and larger oxygen partial pressures, probably wl~n the tip teaches a defect of the polycrystalline oxygen to. conductor.

The following designations proved useful in the discu~ stun of porous electrodes consisting of small panicles of electrecatalyst: total area of active eiectrocatalyst, contact area between electr(~ta!yst and solid electrolyte and re~ gion of the three-phase boundary, A large number of ~ n is devoted to the question of where a given el~tro~ chemical reaction takes place and to what extent these t h ~ r~gions are involved, The total area can be deter° ndned by the BET method Or odt¢|' techniques, The ratio between ~ ~ and geometric area of the electrode r e , seals th~ roughness t~ctor, The c o , ~ t area and the ~gioa of th~ t h ~ p h ~ s e boandar~ are more difficult to c ~ e , The ~ n t ~ t ~ a ~u~l the thr~-pha~ Ixmnd- aey, trteasurett as I ~ sam of the circumferences of the contact areas of the electrocatalyst particles, were deter~ m i ~ by microscopy in Ref. [5] fivt the interfi~,ce YSZ sing~ crys|aJ~ p ~ e~trode.

The measuretr~nts were made in a fully automated ~ t ~ d e ~ i l ~ l ia ReE [ i i], This ~Vup allows one to take steatglyos~te o l t r c a t = ~ a t i a i curves in the potentioo ~t~ti¢ ~r g a l ~ t i c ~ and to carry out cyclic vol~m.caeW~,, coulometry at a constant potential or the ~..~.sm~raeat of ~ lx~ntial-time dependence at a cow stmtt c-arreat, The Solattroa 1286A electrochemical inter-

served as a programmable potentiostat controlled by a P ~ / ~ d b ~ compal~r which produces the desired

potential-time profiles and also establishes the tempera- ture by the use of two thermocouples and a special algo- rithm. The maximum time resolution is determined by the fastest data- acquisition rate of the Solartron 1286A (15 readings s-t) . The data are transferred in digital form to the computer, the memory of which was increased. McasuremenLs and data processing can be done simultane- ously by the ,same computer.

The ce~t is kept by a special ,sample holder (see fig. 2 of Ref. [l l]) in a cloud-end tube of quartz glass which is flushed by the given gas inside a tubular furnace heated by a direct current from a power supply, Three or four electrodes can be attached to tile cell.

Diffcreqt temperature ramps are t~asible for which the start, stop and the magnitude of the lempe|~ture step can be programmed. Steps of ~0°C we~ used in the present study. The c h e l a type of elect~chemical measm~ement is made at a constant ;emperature, Befi~re a measuremem is statled, the potential difference E,, between the RE aitd WE at open circuit is measured. A waiting period is applied until a stable value of this voltage is achieved. The level of suability (millivolts ~ r minute) can be set by the program,

Samples of YSZ were cul from bars (Friatec AG, 8%Y~O~) of rectangular cross-~etion with a thickness of 0. I to O. 12cm. TIv~ two large surf=lees were polished to I0 pm and cleaned in an ultrasonic hath, The ammgemem of the electives is shown schematically in Fig. I. Tbe CE, produced by unfluxed Pt paste, c o v e ~ the lower side of the YSZ disk and was eomact~d by a Pt foil. The RE consisted of a PI foil pres:ved by a spring outside the holder onto the solid elect~dyte. If t ~ WE had a point contact, it was cut from a Pl foil of O.i cm Ihickness. Aa electrode with roughly the sha~ of a flint-sided pyramtd was oh~ t ~ n ~ by grinding, polishing and cleaning in an ultrasonic kath, The contact area was estimated by ophcal rain croscopy and amounted to about 6 × 10°4era ~ if dle determination was carried out at the completion of the measurements. The point of the electrode is pressed by a spring against the solid electrolyte. For steady<ante mea- surements the currents of the point electrode became very small and widely scattered, 111erelbre a Pt electrode, pro° duced by paste in the shape of a small di~k, was used. It had a geometric surface of 0,05 cm ~.

Measurements wc~: usually, but not always, started after keeping the WE at 700°C for I h. It will be demon-

WE RE

~ ..... PI foil

~. ~r Pt paste

CE ~ R foil Fig. I. Schematic diagram of the arrangement of the electrodes.

?00 65O I '

-2.0

-2.5

" " "3.0 . . J

-35

~ .4,o

r'5~0

M. I1~ Bre~ter et at./Journal of Electroanalytic~d Chemistry 434 t 1997 ~ 129- t37

,,GO 10

T i ' C

600 550 5 ~ 450 400 350 300 250

-1.5

11 1 ~2 tr3 14 1,~ 10 1.7 ~1,8 19 2 0

10~r ''l / K ''~

Fig. 2. Sezrdlogarithmic plot of 7'/R~, vs. I()O0/T.

131

strated that such a pretreatment procedure is not only necessary for sputtered or paste electrodes, but also lbr the solid electrodes used in this investigation. As with the experiments in R~,fs. [5.6] the three electrodes were ex.~ posed to the same gas atmosphere.

The deteiTaination of the resistance Rea was carried out for some of the cells with point electrodes in a separate sebup by a four-probe a.c. technique in the way outlined in Ref. [11]. It was found that the cell had to be kept in the same holder for the R~ determination and the cyclic voltammetry. It could not be removed from the holder between these experiments without changing the value of R~. The results of a n|easurement of R¢l for increasing and decreasing temperatures are given in Fig. 2 in a semilogarithmic plot of T/Rel vs. IO00/T. A straight line (Arrhenius type) is obtained. The slope of this line leads to an activation energy of 1.03eV, agreeing well with the activation energy of conductivity determined indepeno dently for our solid electrolyte. This result confirms the correctness of the IR~ determination since R~ is inversely proportional to the conductivity of the solid electrolyte. The correction of some of the cyclic voltammograms for the IR¢~ drop was done numerically by computer to evalu- ate the influence of the IRe~ drop on the shape of the I -E curves, taken at 50 mV s -~ . It was found that corrections above 500°C affect the 1-E curves only in the region of

oxygen evolution (compare Fig. 5 for the general feature~ of the cyclic wfltammograms). Below 500°C the correco tions become larger with decreasing temperature, mainly having an influence in the vicinity of the cathodic peak and at potentials of oxygen evolution, At 300°C the cao thodic peak i~ shifted by about 0, I V in our cell because of the IR¢l drop.

3. Results

The open-circuit voltage E,, was practically the same at the start and at the end of the meas~rements of I-E curve~ by cyclic voltammetry. E,, is shower as a function of the temperature at different oxygen partial pressures in Fig. 3 for increasing and decreasing temperatures. In thi~ case a pretreatment at 700°C was n ~ done and the measurements were carried out from low to high temperatures and back. The E,, values for the second heating cycle agreed practb cally with those of the preceding cooling cycle and are not shown for this reason in Fig. 3.

The following experiments were made to obtain infbr~ mation about the oxygen layer after the pretreatmenl at 700°C. The temperature was allowed to cool to the d~ired value with the WE at open circuit, Then a cathodic ,~weep was started at 50mVs j from E o to -0 ,7V, followed at first by a positive one back to E,, and then by another

132 M. W. Breiter er .I. / Journal oJ Electroanalytical Chemistry 434 (1997) 129-137

>

.~=.-- = ~ o - - -

.... IV- 100%02

4).1 o - -'~--" 1%02 -- ~--- 0.101o02

.0.2

"03 !'

4),4

T/'C l:t$, 3, Ot~'el,t-cicui[ voltage E. at tiler ~Iart of tile I.-I;. CUIVC'* a! d i l i c r e m p~olial pr¢~surm, o f O~

cyclic sweep. The results are presented in Fig, 4 for two tam.futures.

If the cyclic vt~l~mmogram is taken at 44X)~C withoul a prcccdit~g p r¢lrc~tmeat at 7I~Y'C. the inner curve of Fi~, 5 is obtainS, In co|!trast, the ouler curve is meastm~d after th~ pt~trcatmerll,

Measurements in pure N~ arc ,~hown for diflereot 1¢m,~ ~ratu~s in Fig, 6, It should ~ Ivoimcd out d~at fl~e RE is t~t welt ~flncd for the:~ measu~ments.

The dependence of shape of the cyclic voltammog~ms upo~ oxygen I~,tlial ~ s s a ~ was ~latively small, in ~ g ~ c ~ n t with the r~sults in Rat\ [,S].. A similar t',ehaviot, llke that in Ref. [5], was fmtml with r e ~ t to the depeno ~ of the i~E curves upon sweep rates between l0 and 200mV s ~ ~, Theorem, the respective curves ate not shown i'~, Hog, ever, the ~nder~ . ,¢ of the ~ape of the curves ~ m the ar~xlic [',~eatial of ~ v e ~ ix illustrat~xt in Fig. ? f ~ O, I% O~ in the temperature range around 500~C,

S~dy-st~tc i=E curves were taken in the potentiostatic m~xie on a paste eto=~xie at first from g,, to 0, I V and ~ k ~ a given parliid pressure of O~, Suhmqucmly, the o~rew~m from ~,, to ~0.1 V and I~:k to g,, for M~ed Vhe lowest Iemper~ture t~r whieh this was |k2tksibl¢ wi~ ~'ept~b!¢ scattering ot" ~ currem was 400~C. He~e

steady-state was d e f i ~ ~ i t r ad ly as that poim at ~ k : h the cb,~m~ t)( the current with fin~ at a constant

was smaller than I%min-L The I -E curves in vk:inity of E,, are presented as an example for ! % O~

U diff<a~em temperatures in Fig. 8. The I -E curves at other pa~kfl pressures I~x~ similar end are not shown for this

t)0

-¢~0,0

~8 ,0~ -07 O0

E ~V

i j I i I I I -0.6 -0 4 4) 2 O~

E l Y

Fig, 4, Negadve sweep from E o to -0.7V at 50mrs- a followed by a positive one Io Eo (curve (a)) and another cyclic sweep between E o and -O.7V (curve (b)) for 350 and 450°C at 100% O z.

1.0xl0 ~

8.0x10 'a

5"0x10"B t

,~ o.o _-

ot 0 ~ 1 0 ~ ~ = ' ~ ' - ~ J . . . . . ~ . . . . " . . . . . . . ~0~0 ~()pa ~012 0 0 0r ~ ()'4

Fig~ 5, l=~t! ~'orvcs ollt~ii.ed at 4110"(? anti 511 i|lV ,', ill i % 0 2 withoul (iimeF c°t|r~.'e) aild wilh (ouler cur~e) i~Jctreatmen|,

reason. The s lope A E / A ! was de t e r . n ined f rom I - E

curves in the v ic in i ty o f E,,..'~ is

R~, = ( A E/~I)~,, (2)

The exchange cur i 'cnt may he comp t i t ed ;!ccoF(ling I(}

I , , = RT/ FR,, (3)

under the a s s u m p t i o n that the r a t e -de te rmin ing step of tile

react ion in Eq. (1) is a oqe-e lec t ron t ransfer reaction, Rp is

6.0xi0 a

40x10 s

< -.. 2.0x10 °

0,0

.2.0x10 ~

,4.0x10 'e

400"C 450"C 500'C

M, tl e, Breiter ¢,t ¢d. /Jormlal ~f Electr¢nmalytic.l Chemist!3' 434 t 1~TJ 12~- I37 i 33

o * I . . . . i , , * , J~_ p i ~ I I i . A ~ ? ~ i ~ ~ _ _ ~ 1 _ i ' ' : i ~ ' - O - ~ - L ~

-0.5 -0.4 -0,3 0 2 ~0 1 0,0 0,1 0 2 0,3

E I V

Fig. 6. I - E curves taken in pure N z at 50mY ,~ ' ;it difli:renl tempcr~,ture~.

134 M.W. Be~eiter et al / Journal of Electroanalytical Chemisto" 434 (1997) 129-137

O0

O0

[ I V

20xlO'* ~,,

O0 L

t ~ V

Fig, 7, I~E ,~ttf~s obtaltt~*d in 0,1% O~ at 5 0 m V s ~ ~ fiw dif le~nl am~li¢ i~lemhtl~ o| re~,e~d in the |emp~r~ltu~ r~ng¢ at't~and $00~'C,

Or~

W

1%0,

9 ;:;~ J 7 ̧

,/

400" C 450'C

/ ~ " C .... 550'C

/ o,~' '/

C

/

/ J

-~6 -0.4 -02. 0.0 0.2 0.4 0.6 U ~

F~. 8. S ~ . ~ l y - s ~ l - E curces in the vicinity or" Eo, me.asu~d pozentiostatically at different temperatures for I% 02 on the paste electrode.

M. W. Breiter et al. / Journal of Electr~nmalytic.I Chemistry 434 { ! 997') 129-o 137 135

lO'

10 7

lo '

a 20*/,o, o 1%o, r~ o,1~ o=

O

lx

1.1 1.2 1,3 1.4 1,5

10 ~ T ' / K '

Fig, 9, Slope of tile I~E curves ill th,~' viehlity of I~,, i~)r th|ve dil]er¢lll oxygen partml I)i'e,~sures ils ~l fllllCiiolJ of IO(IO/T m a ~t~mih~g[u'ithllli¢ ph}1: /,,,

plotted vs. IO00/T in a semilogari~hmic plot for different partial pressures of 02 in Fig. 9.

4. Discussion

4. !. Pretreaonent of point electrodes

The necessity of a high-temperature pretreatment for paste electrodes has already been pointed out [1-6]. Re- moval of organic residues and recrystallization of the Pt fihn are given as the reason. The results in Figs. 3 and 5 demonstrate that such a pretreatment is also required lbr solid electrodes. If the pretreatment is missing, the value of Eo is large at low temperatures (Fig. 3). The cyclic voltam- mogram at 400°C is barely recognizable.

It is suggested that the removal of impurities from the Pt surface and an increase of the contact area between the point electrode and the solid electrolyte during the pretreato ment are responsible for the observed effects. The cold~ worked point is pressed against a relatively hard substrate. At 700°C the stresses in the Pt electrode are annealed. This leads to enlargement of the contact area by about a factor of two, as found by comparison of the contact area before and after the experiment in the microscope. A sharp point cannot be maintained for this reason, as proved by a special experiment, it was shown in Ref. [5] that the area under the cathodic peak of tiae voltammogram taken at temperatures up to 400°C with an anodic reversal potential, before a strong contribution of the O~ evolution sets in, is proportional to the contact area. Although there is an uncertainty in this result because of the diffieulty in the determination of the peak area (see the subsequent discus- sion), the conclusion is considered valid. The geometry of

the point electrode favors the participation of the contact area over that of the region of the threeophase boundary. The voltammograms obtained on our point electrodes are very similar to those obtained on paste electrodes in Ref. [5] if the measurement is done under the same condition~.

4.2. Oxygen layer at E,, after pretreatment at 700~C

The curves in Fig. 4 were taken under conditions of the maximum influence of the oxygen partial pressure (100% O2). While the formation of an oxide is not expected to occur at an open circuit at 700°C according to the thermo~ dynamic estimate in Ref. [5], an oxide might be produced during the subsequent cooling to a lower temperature, taking about ! h at an open circuit in our set-up.

The first negative sweep at 350°C in Fig. 4 does not display a cathodic peak due to the reduction of an oxygen layer. In contrast, a small cathodic peak is recognizable at 450°C. This result is in agreement with similar measure- meats in air [3]. The cyclic voltammogram, taken subse- quently at 350°C, does not show either the formation of the oxygen layer or its reduction. However, small peaks are visible during the positive and negative sweeps at 450'~C..

The results in Fig. 4 demonstrate that the formation of the oxygen layer from 02 at an open circuit is very small at temperatures below about 560°C on ;:~ur point electrode.~. Other evidence for this statement is given by p comparison of the vo!tammograms with the sat.¢ ut)p~, poi~a6al oJ ~ reversal in Fig. 6 and Fig. 7. The shape of the re!famine- grams in the region of the anodi¢ formatio~J aJ~d ¢~lhodic reduction of the oxygen layer i,~ :~imi!ar, keeping in mind that the E-scale is not the same since the RE for the curve~ in Fig. 6 is not well defined. The above results are in disagreement with the conclusions about the formation of

1~6 M W. Breitcr el aL / Jnttnud ~] Eh,ctn~tmalytical ChemL~tO' 434 (1997) 129-137

the oxygen layer by the dissociation of 02 at the three- pha~ boundary in Refs. [2.6]. Similar statements about the formation of the oxygen layer have already been given in Retd. [.~.51,

4.3. F~mblishma~t of the equilibrium potential ~¢" the reac- tion in Eq. (I

The results in Fig. 9 confirm the conclusions in Refs. [10,1 I] that the equilibrium of the reaction in Fxl. ( I ) is no longer e.~tablished ~ low about 500°C~ It should ~ pointed o~ hc~'c ~h~t earlier work [15] on the oxygen surlace ¢~¢ha~ge and diffusions in fl~.~t ionic conductors denton° ~trated that the reaction in Eq~ (I) i~ dominated by the heterogeneou~ kinetic~ of oxygen exchange al file eleco tro!yl~ sari'ace. The rate constant i~ below 10 ~tcm~ tbr t~]YSZ at 7(~)"C. So it is to be expected that equilibo fium with oxygen will not be established I~low about 51~FC,

The I~)ints I~r the hi~her three temperatures lie on ~traighl lines with the same slope. The straight line is ~hifted to lower R~ values with an increa~ of the oxygen partial pressure. Such a temperature dependence of R is !o be expected if the ~ame ~tep of the reaction in Eq. ( I] is rate-determining and involves a dependence upon the oxyo gen partial pressure in some fi~shion. Systematic deviations occur at 450 and 40(}°C. The deviations will still be larger at 350 and 300°C. These deviations suggest the presence of additional reactions. The values of R~, ~come large a~ wm}m:r~ature~ below 50I?'C. Using Eq. (3). I, has values t~tween 6 x 10" and 6 × I0 ~A at 5(Xt~C. Such v.dues are no! sufficient tbr the eslahlistunent of the equilibrium of the reaction in F~|. ( I ) below 501)'~'.

11ae large values of Rp also explain why the reaction m Eq. (I) does no| e ~ b u t e to a noticeable exlent during the eatl~dic s~x~ep of cyclic vohammogt~nas at tempera° tu~s ~low 5t~Y'C in the range of the sweep rates em- l~oyed I ~ . The reduction of the oxygen layer is seen

above conclusion is not in disagrtwment with the resuhs of the ~ l i a g cycle in Fig. 3. Heating to 7(?a/°C prt~,eex the same surface st~e fiw the WE and RE. The I ~ f i a l difference ~tween the ele, ctrc~Je and solid dec- troly~ is practically the ~me for the WE and RE. even when it is delermia~.~l by a mixed eleemx.te process txqow 500~C.

P~vi~S ~terllli~ll~lons of ki~tic w~fameter,x, for in~ st~m'~ of II~ exchaa~ cu~n t density [6]. or ','alues of fl~e ~qu~|fb4~un_~ poleniia| of oxides. @termh~ed them themao~ dy~r~c 0a~a [5]. have to be considered with c~ution

Vae ~at:,e of cyclic voltammograms, taken with an u~rper potenlial of reversal of 0.3 V at temperatures up to

450°C. can be interpreted by the anodic formation and cathodic reduction of eithe," a thin layer of oxides [5] according to

Pt + +~O~'+ = PtO, + 2xe + (4)

or of a layer of chemisorbed oxygen [2.63] accoldiug to

Pt + 0 2- = P t - O + 2e ++ ( 5 )

A distinction ia favor of oxides was made in Ref. [5] on the basis of the charge/(geometric contact area) deter- mined from the area under the cathodic peak and subseo quently convened to charge/(ta~.ai contact area). The latter quantity was t~und to be sufficient to fern1 five to seven layers of PrO

It was not ~lal.ed [5] if the delemfiuation of charge/a~a was tan'ted out in the same way as h~ Refs. [2.6]. using the dashed line of the outer curve in Fig. 5 as ~ero llne. There is an uncelaainty in the procedure of Ref.s. [2.6] because of the residual cathodic ¢unvut during the uegalive sweep. This uncertainty was already recognized in Rei\ [3] and becomes larger with increasing {emperature al~we 450°C.

The area under the catht~lic peak was obtained for the point e!ectmde by integration with a zer~ line draw~ as in Fig. 5. The results, given in milliconlombs/{square cen~ timeter of ~eometfic area), were: O. 14 at 3110'~C. O. H5 at 400~C and O, 15 at 500°C, lrllese values c o , s p e n d [ 16[ m les~ than a monolayer of chemisorhed oxygen atoms (P1-O) on solid P| electrifies in aqueous systems. It is conceivable Ihat out' charge densities are somewhat t ~ small..just as 0~e values determined in other papers by the same prate+ dur¢. becau.~e tl~e residual current may also originate fi'om the reduction of the oxygen layer. However. they are considerably smaller tllan Ihe respective values in Ref. [5] fiw a paste electric|e. Our dala. which do not involve a detem~inaiion of the ~a! surfi~ce of the contact a~a, are in l~avor of the electn~chemical process according to Eq. {5).

It is useful for a detailed discussion of the shape of cyclic vollammograms to consider two temperaun~ ranges: ~ low and ab~we 450°C. The shape of cyclic voltammo- grams below 450°C has a cerlain similarity to those taken [16] at t~lycrystalline PI eleclrodes in 2.3M II~SO~ at temperatures belweeu - 7 and 70°C and in I M HCIO., at to, am temperature: there is a broad wave. due to the formation of the oxygen layer, before O~ evolution sets in daring the pt~sitive sweep. A relatively narrow reduction wave of the oxygen layer is seen during the negative sweep. A closer comparison of the cyclic voltammograms in the different systems reveals a large difference for the width of the anodic w~we. In the said acids !he anodic wave starts about 0.4 V below the potential of the oxygen electrode. The beginning of this wave occurs [16] even earlier in alkaline solutions, in contrast, the anodic wave starts at about the potential of the oxygen electrode at the interface PtlYSZ. Its width amounts to about 0.3 V to the beginning of O 2 evolution at temperatures below 450°C.

M. W. Breiler el al. / Jminial o.f Electro{malyt~cat Ckemistr 3, 434 ( f997~ 129-137 | 37

A possible mechanism for the formation of the oxygen layer at the interface PtlYSZ was given in Ref. [6]:

Pt + O 2- = PI -O- + e- (6)

P t - O - = P t - O + e ~ ( 7 )

It is suggested here that the intermediate Pt -O- is not seen in the cyclic voltammograms below 450°C. The inter- pretation for the different shapes of the anodic and ca- thodic waves of the cyclic voltammograms, advanced for the first time in Ref. [16] for aqueous systems, can also be applied to the above reaction mechanism involving a two- electron tnmsfer. Our conclusion is not in agreement with Pt-O ~ as the more strongly bonded intermediate [6] deter- mining the shape of the voltammograms with Olte cathodic peak, Spectroscopic evidence [171 is cited in Ref, [6] in support of Pt--O . However. this evidence is no| strong. A dhx'ct peak, corresponding Io P t ~ O , was nol observed, Instead |he presence of such a peak wi~s inferred on the basis of a dift~rcuce spectrnm, constructed f ionl IWO spec- Ira lot YSZ The~e Iwo speclra were taken Ihrotlgh the porous Pt eleclmde at open ch'cuit and at an anodic potential. The additional assumptions, made in the cone stmction of the difference spectrum, are not discussed in Ref. [I 71.

The shape of the cyclic voltammograms begins Io change with incre:tsi,~g temperature above 450°C (Fig. 7), The anodic currents become hirger, This is attributed to the contribution of the partial cun'ent of O~ evolution. The hindrance of this reaction decreases with temperature. Simultaneously the cathodic wave becomes broader. A comparison with the curves in a nitrogen atmosphere (Fig, 6) shows the same effect for the anodic wave. and to a le.~ser exlent for |he cathodic wave. 0 2 evolution occurs also during |l~c negative sweep at positive potentials. The cutTent, measured in this potential region, is ti~e difR'xence between the anodic current of O~ evolution and that of Pt-O reduction. The broadening of the cathodic wave may result from this effect. A conclusion on the existence [7] of two cathodic peaks in a broad wave can only be consid- ered certain if the voltammograms, centered for the partial curt'eat of O2 evolution, display two peaks.

Although the main part of this i,westigation deals with the temperature range between 300 and 600°C. a few cyclic voltammograms were run on paste electrodes at temperatures of 700 and 800°C. These curves had a shape like that of the respective curves in Ref. [3] and are not shown for this reason. The reader is referred to Ref. [3].

A c k n o w l e d g e m e n t s

Financial support by the Austrian Fends zur Fiirderung der wissenschaftlichen Forschung is acknowledged.

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