Indian Journal of ChemistryVoL 34A, January 1995, pp. 22-26

Influence of supporting electrolytes on photoelectrochemicalstability of lead oxide in KtFe(CN)6 solution t

G Srinivasulu Reddy! & Maheshwar Sharon-Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400 076, India

Received 27 January 1994; revised 22 June 1994; accepted 5 August 1994

Photoelectrochemical (PEC) stability oflead oxide (PbOn) in Fe(eN)64 solution with alkali halides at pH9.2 has been studied. Among the alkali halides studied, KI (0. 1M) enhances the stability of Pbf); by 80%. Thisstability enhances further if solution is made from a saturated solution of PbO. A model is developed toexplain the role of KI in suppressing the photocorrosion of PbOn. A special arrangement is designed forilluminating disc/ring electrode assembly of rotating rind disc electrode (RRDE) as well for electrode of aPEe cell to minimize the loss oflight intensity due to its absorption by coloured solution. Alkali halides MX(M = Li, Na, K and Cs; X = Cl, Dr, and I), have been used for this study.

Lead is one of the cheapest material, whose oxideshave been extensively studied for their electro-chemical and photoelectrochemical properties 1- 3.

Amongst the oxides, PbOn (a mixture of Pb02 andPbO)1 and Pb304 (ref.4) have been found to show aphotosemiconducting . behaviour. These non-stoichiometric oxides (PbOn & Pb304) are, however,prone to photo-oxidation resulting into adegenerated semiconductor, Pb02 (ref.5).

The present paper gives an account of studies madeto minimize the photocorrosion ofPbO thin films byintroducing suitable electrolytes into the reactionsystem. A rotating ring disc electrode (RRDE)technique has been preferred to study the influence ofsupporting electrolytes on the photocorrosion ofleadoxide electrode.

Materials and MethodsAn RRDE (Pine, AFRDE 4, USA) in conjunction

with a speed controller (Pine, MSRX) was used tomeasure separately the anodic photocurrentgenerated at the illuminated lead oxide discs and thecathodic current at the ring electrode. The lattercurrent is due to the reduction of redox electrolyteoxidized at the disc electrode. A rotation speed of1000 rpm was found to give a satisfactory result.

A thin film of lead oxide was prepared over lead

t This paper was read at the 2nd International Conference on NewTrends in Photoelectrochemistry, held at Tokyo University,22-24 March 1994.t Present address: Department of Applied Chemistry, Faculty ofEngineering, Osaka University, Osaka, Japan.

discs of RRDE (in situ) by the method describedelsewhere I. This condition gives a thin film of leadoxide containing a mixture of PbO and Pb02(referred as PlrO, henceforth). This oxide gives thehighest photocurrent so far achieved. The workingsurface oflead disc (prior to its oxidation to get a thinfilm ofPbOn), was polished with fine emery paper (250& 1000) and chemically polished in a solutioncontaining hydrogen peroxide, glacial acetic acid andmethanol (0.5:1:10). Pt ring was pretreated bypolishing with fine alumina powder, washed withdeionised water and electrochemically cleaned bycycling it under potentials for oxygen and hydrogenevolution in H2S04 solution.

A design of the RRDE assembly for illuminationpurpose

A special arrangement has been developed toilluminate only the discs and ring electrode and notthe entire solution (Fig. I). A solid quartz tube (l emdiam) was fused with araldite to the flask containingthe redox electrolyte. The position of quartz tube (x)was set at about 1 em below the disc electrode(RRDE). The illuination was done from other end ofthis tube (y) by focussing halogen quartz lamp. A lightchopper was placed in between the lamp and the tube(y). The intensity of light was measured near thequartz tube end (x). The solid quartz tube behavedlike an optical fiber and critically reflected light from"Y" to "X" with minimum loss of light intensity.PbO, disc and platinum ring electrodes of RRDEassembly were dipped in aqueous solution containinga redox electrolyte ~Fe(CN)6 (0.1 M, pH 9.2).

~DDY et 01.: PHOTOELECTROCHEMICAL STABILITY OF PbO IN K4Fe(CN)h SOLUTION 23

PRDE

8lightIIIciz:»Chopper

Fig. I-A schematic' sketch of the arrangement to illuminate thedisc electrode of RRDE via a quartz glass rod designed to transmitincidence light from "y" to "x" under almost critical angle of

reflectance condition

Fig. 2-Current vs potential curve of disc electrode (constitutinga cell: Pb/PbOn/Fe(CN)t - (0.1 M), pH 9.20jPt) and ring electrode(constituting a cell: Pt/Fe(CN)~- pH 9.2jPt). Disc electrode isilluminated under chopping condition and its effect is observed atboth, disc and ring electrodes. Dark current is constant upto 0.65V

vs SCE, beyond which corrosion starts (Venrr)

Photocurrent of PhO; disc electrode under choppedilluminated condition was measured by sweeping itspotential at the rate of 10 mVs-1 in the range of- 0.80V to +0.80V vs SCE. The potential of the ringelectrode was fixed at - 0.2V, because Fe(CN)6"3 canbe reduced at this potential. The magnitude ofphotocurrent and reduction current at the discelectrode and platinum ring electrode respectivelywere measured at the disc potential wherephotocurrent was maximum and its dark currentgave almost a constant value (Vear, Fig. 2). The ratioof ring current to disc current (designated at cxtY at thisselected disc potential was measured as a function oftime of illumination of the disc electrode and, wasplotted as a function of time of illumination (Fig.3a).The entire experiment was then repeated by adding asupporting electrolyte into the RRDE reactionassembly. Results are shown in Fig.3 for KCI, KBr

20 4 0 80illumination time in min

Fig. 3-Ratio of disc/ring photocurrent (i.e. cx,) vs lime ofillumination curves for Pt/PbOn in: (a) 0.1 M K4 Fe(CN)h.pH 9.2(i.e .. without any supporting electrolyte); (b) a + 0.1 M KCl: (c) a+ 0.1 M KBr and (d) a + 0.1 M KI. Light intensity &0 mW'cm2

Ring potential was fixed at - 0.2V vs SCE during biasing the diskpotential between - O.SV and + O.SV vs SCE. Sweep rate 10

mVs-'

20 40 60 80'IIImininatlon time In min

Fig. 4-Ratio of disc/ring current (a.) vs time of illuminatiorcurves for Pt/PbOn/electrolyte/Pt with (a) 0.1 M K4 Fe(CN)6 (PH9.2); (b) a + 0.1 M Lil.tc) a + 0.1 M Nal; (d) a + 0.1 M KI and (e) a+ O.IM Cal. Light intensity &0 mWcm-z. Ring potential wasfixed at - 0.2V vs SCE during biasing the disk potential between

-O.gVand + O.SV vs SCE. Sweep rate 10 mVs-l

and Kl (to observe the effect of halide ions) and inFigA for Lil, NaI, KI & CsI (to observe the effect ofcations). In both figures values of CXI with onlyFe(CN)r is shown for a comparison (a). In theseexperiments, concentration of supporting electro-lyte, K4Fe(CN)6 and pH were maintained at O.lM,0.1 M and 9.2 respectively. These concentrations werefound to give the best reproducible results.

24 INDIAN J CHEM. SEe. A, JANUARY 1995

o

...---+-- [ )1--4-- ......•E

Fig. 5-A schematic sketch of a PEC cell designed to focus lightonly on the working electrode via a quartz glass rod. This methodensures transmission oflight to the electrode under almost criticalangle of reflection and with minimum loss of intensity due to

presence of electrolyte in the solution

The Clt = 0 of PbO, with the redox electrolyte(Fe(CN)r), in presence as well as in absence of thesupporting alkali halides were calculatedimmediately after the illumination of disc electrodeand this value was taken as the collection efficiencybecause photocurrent with PbO, electrode does notchange during first 10minutes of illumination. A lightsource of 250 W tungsten-halogen lamp was used.

A design of photoelectrochemical cell for colouredsolution

Photocurrent for this work was measured with aspecial type of cell (Fig.5). This cell is found to showno effect of coloration of solution due to Fe(CNl: - /3-on the photocurrent of the electrode. The cell Econtains two electrodes (the lead oxide (C) and acarbon (D) electrode) and the redox electrolyte. Along solid quartz glass tube (A) (I em dia & 10em inlength) was used for illuminating the semiconductorelectrode.

Results and DiscussionPhotocurrent produced by the illumination of

PbOn (ref.6) disk electrode under chopped lightcondition (with 0.1 MFe(CN)r redox electrolyte atpH 9.2 and rpm 1000)shows a small dark current witha maximum photocurrent at a biased potential 0.65V(Vc or Fig.2). Moreover, the onset of the anodicphotocurrent (of the disc current) begins at - 0.60Vsuggesting the conduction band edge of PbO, to bethis value.

Dependence of the ratio of ring current to thecorresponding disc current (i.e. Clt) at potential

+ 0.65, on the time of illumination (Figs 3&4)suggests that the entire concentration ofphotogenerated holes at PbOn surface (reaction-I) isnot fully utilized for the oxidation of the redoxelectrolyte (reaction-2); otherwise, the ratio Clt

should have been independent of time of illumina-tion. Because the concentration of hole (if'Pbf); is notcorroding and if intensity of illumination is constant)would not change with time (equation-I) and hencethe concentration of oxidised form of redoxelectrolyte would also be constant (reaction-2),which would in turn make the ring current alsoconstant (reaction-3), making the Clt valueindependent of time of illumination.

n - PbOn + hv -+ h ~'urrace + e hulk · .. (I)

h+'urrace + Fe(CN)r -+ Fe(CN)r · .. (2)

(i.e, oxidation of redox electrolyte). This reaction isresponsible for the photocurrent at the disc electrode,photogenerated electron being consumed at the cou-nter electrode.

Fe(CN)r + e -+ Fe(CN)r · .. (3)

(responsible for ring current at - 0.2V).

But Clt is found to change with time of illumination(Figs' 3&4) suggesting that concentration ofFe(CN)r produced by reaction-2 depends uponconcentration ofh.~urfacewhich must be changing withtime due to reaction-4.

A mathematical model for photo-corrosion reactionThe magnitude of photo current being dependent

on the intensity oflight, and which was kept constantthroughout the experiment, suggests that utilizationof photogenerated holes (Eq.l) may be followingboth paths, shown by Equations 2&4.

PbOn + 20H - + 2h + surface-+ Pb02 + H20

· .. (4)If this be so, then the magnitude of oxidation currentdue to reaction-2 should be dependent on the rate ofreaction-4. In other words, if PbOn is undergoingphotocorrosion (i.e., self oxidation), then magnitude.of discs current due to reaction-2 will decrease withtime of illumination, which can be noticed at the ringelectrode (due to reaction-3).

Efforts, therefore, were made to calculate theportion of photo currents responsible for each of theabove two speculated reactions equations 2&4. Forthis purpose, the collection efficiency due to

REDDY et al: PHOTOELECTROCHEMICAL STABIUTY OF PbO IN K4Fe(CN)6 SOLUTION 25

20 40 60 80Illumination time in min

Fig. 6-A set of graphs generated from Figs 2&4,behaviour of % inhibition of corrosion of PbO" vs time ofillumination and effect of cations of the supporting electrolyte

thereon

Fe(CN)r at the Pbf); disc/Pt ring electrode wascalculated from the disc/ring current at almost zerotime of illumination (say, (Xt=o)' Let us assume that ata time "t" the magnitude of disc. current is B, and itscorresponding ring current as 'fl' The magnitude ofdisc current WI) which could have produced a ringcurrent (Y/). can be calculated from its collectionefficiency a, by equation 5:

'fl~t ... (5)

(Xt

If <Ptotalphoto and CPcorro'ion be the total photocurrent andphotocorrosion currents produced at the PbOn discselectrode at a time "t" respectively, then

tPcorrosion = tPlota/photo - PI ... (6)

Because ~t is that part of the current which isresponsible for reaction-2 and its magnitude dependsupon the time dependent 'ft (Eq.4).

With the help of Eq.6, we can thus calculate theeffect of added supporting electrolyte on themagnitude of the <Pcnrro.'ion' Therefore, if CPcorrosinn iscalculated for PEC cells of configurations:

(i) PbjPbOnjFe(CN)r, pH 9.20jPt and(ii) PblPbOn/Fe(CN)~-, MX (O.1M), pH 9.0IPt,

whereM= Li+Na ' K+, Q+andX =Cl-, Br , 1-; set ci<Pcorrosion (i.e. portion of photogenerated hole currentwhich was consumed for oxidation of PbOn) values

o1+------2~0------'40------~6~0----~80lIIuminqtion time in min

Fig, 7-A set of graphs generated from Figs 2&3. showing theeffect of anions of the supporting electrolyte on the 'X, inhibition of

corrosion vs time of illumination

for corresponding time of illumination, can betabulated for PEC cells containing (a) KI, KBr, KCl(to show the effect of halide ions) and (b) for PEC cellscontaining Lil, Nal, KI & CsI (to show the effect ofcations).

In order to get a quantitative estimation ofpercentage inhibition of the photocorrosion WI, a setofvalues were generated from this tabulated data (notshown here) by calculating the ratio of III (corrosioncurrent at time " [" in presence of added supportingelectrolytes) to the value of III (corrosion current attime "t" at disc electrode without any supportingelectrolyte). This ratio (WI) is plotted versus time ofillumination to get information about the percentageinhibition of photocorrosion due to the addition ofsupporting electrolytes: (i) LiI, Nal, KI, and Csi(Fig.6) and (ii) KCl, KBr & KI (Fig. 7). This exerciseclearly suggests that addition of KI suppresses thephotocorrosion to the extent of80% as compared toother systems. In the beginning there seems to be asmall deviation in the trends of graphs (Figs 6&7)perhaps because it takes a few minutes to stabilize thesystem while being illuminated.

These calculations reveal that these cations areable to suppress the photocorrosion and follow theorder KI > NaI > CsI > LiI (Fig.6) while halide ionsfollow the order of KI > KBr > KCl (Fig.7).However, between cations and anions, iodide,specially KI, seems to show maximum inhibition tocorrosion of PbO; (Eq.4).

Effect of dissolved PbO on the stability of Pbt);When photocurrent with a cell of configuration

PbjPbOn/Fe(CN)r (0.1 M), pH 9.20 (0.1 M) KI/C

26 INDIAN J CHEM. SEe. A,JANUARY 1995

20 40 80Illumination time in min.

Fig. 8 A set of graphs showing behaviour of % inhibition ofcorrosion vs time of illumination for system containing KIsolution (0.1 M prepared in distilled water) and with electrolytecontaining KI (0.1 M) but entire solutions prepared from a

saturated solution of PbOn

was measured using the cell as shown in Fig.S withsolar radiations for a prolonged illumination (for aperiod over a month and about 8 hr/day), even KI wasnot able to prevent the corrosion completely. Thephotocurrent fell down gradually to a very smallvalue.

Since PbO is slightly soluble in alkaline solution, itwas thought that perhaps PbOn dissolves in solutionover a longer period. Hence an experiment wasperformed with the cell configuration as stated abovebut using solutions made from a saturated solution oflead oxide (prepared by dissolving PbOn at SO'C andthen cooling to room temperature). The photo-current with this cell was found to remain constant fora period of about one month but thereafter it startedslowly decreasing.

Like before, ¢co/'I'(JS;OI/ was also calculated for thissystem (using RRDE) and the percentage photo-corrosion inhibition (0/ was plotted versus time ofillumination (Fig.S). These results substantiate theobservation that the stability improves with theapplication of saturated solution of PbOn. Thisbehaviour is supported by the fact that afterprolonged illumination, thesurface of Pbf), shows adeposition of some whitish substance (we have notbeen able to analyze this material due to its negligibleamount). When the electrode is washed and dried, itstarts showing its original photocurrent, suggestingoccurrence of some dissolution process at theelectrode. When the solution was made out ofsaturated solution of PbO, deposition of whitishlayer was found to be extremely small.

Role or supporting electrolytes in stabilising PbO"PbO has a structure comprising a layer of oxygen

and lead, followed by another layer of/ead separated

by 2.76 A, which is followed by oxygen layer2•3•5 -7. IfPhO is to be oxidized to PhO (Eq.4), oxygen ion has todiffuse through the gap between two lead layers.However, if the space created by the two layers ofleadis blocked to prevent oxygen diffusion into the lattice,photogenerated hole cannot be utilized for oxidationof Pb2 + to Pb4 + (i.e., reaction-4 is minimized). Ionic sizeof iodide ion (2.20A) being smaller than 2.76A, it cantake up the role of blocking this space, thus enhancingthe stability of PtrO, (and hence favouring thereaction-2) by disallowing the diffusion of 02 -. orOH - ions into Pbf), lattice (reaction-4).

This hypothesis is supported by the fact that othertypes of halide ions are not very effective in stabilizingPbOn because their ionic radii are much smaller, e.g.CI (1.S1 A), Br (1.96 A) compared to (2.20 A).

ConclusionThe present studies show that \ead oxide electrode

prepared by the method discussed elsewhere', can bestabilized in Fe(CN)r at pH 9.2 with 0.1 M KI(prepared in saturated solution of ex-PbO) for aperiod of about a month, Effect of cations and anionson the stability of the oxide electrode has been studiedand it is found that KI inhibits corrosion to about80%.

AcknowledgementThis work was supported by a grant-in-aid from

the CSIR, New Delhi. One of the authors (GSR) isthankful to CSIR for providing him the fellowshipand necessary grant to carry out this work.

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