Indian Journal of ChemistryVol. 33A, April 1994, pp. 291-296

Solvent and electrode kinetic effects on the anodic oxidation of Br - in waterand some dipolar aprotic solvents on bare and platinized Pt and C surfaces t

Jayati Datta, Subrata Sinha & Kiron K KuIi.du*

Physical Chemistry Laboratories,Jadavpur University, Calcutta 700 032, India

Received 19 May 1993; revised 23 August 1993; accepted 5 October 1993

The solvent effects on the exchange current densities (io) for anodic oxidation of Br " on platinum(Pt),-graphite (C) and their platinized surfaces, (Pt)Pt or (C)Pt at 15, 25'and 35°C have been determinedin water as well as in some pure non-aqueous solvents like acetonitrile (ACN), N,N-dimethylformamide(DMF) and propylene carbonate (PC) by potentiostatic polarization technique under near equilibriumcondition. For all the electrodes except C, the observed order of io values in different organic solventsfollowed the sequence: ACN> DMF> PC and that on C electrode: ACN > PC > DMF. On the otherhand, for any solvent except water, the observed io values on different electrodes are in the order:(C)Pt~(Pt)Pt>Pt»C and that in water: (Pt)Pt>Pt~(C)Pt>c. The kinetic solvent effect (KSE) dataat 25°C relative to that in water have been analyzed in the light of relative stabilization of the initialstate (Br "] and the transition state (TS), as well as the extent of free surface (1 - 0,), the coverage, 0"being guided by 'image interactions' inducted by dipolar and dispersion interactions of the cosolventswith the surfaces. The observed virtual heats of activation at the reversible potentia. (!1 HU •• ) which ref-1ect intrinsic electrocatalytic activities of the electrodes, being of the order C ~ Pt > (Pt )Pt > > (C)Pt, sug-gest that graphite (C) electrode is intrinsically a weaker electrocatalyst than both the (C) Pt and (Pt)ptor even bare Pt electrode and that the superiority of (C)Pt over (Pt)Pt is seemingly due to the porousnature of C substrate.

Nonaqueous photoelectrochemical (PEe) solarcells"? have immense potentiality in converting so-lar energy to electrical energy as most of the com-monly used photo anodes are likely to be free fromphotocorrosion which usually limits the stability ofPEC solar cells in aqueous media':", As a prereq-uisite to develop such cells, systematic studies onthermodynamic and kinetic as well as other rele-vant electrochemical properties of some potentialredox couples in some suitable electrochemicalsolvents are required. As a part of our systematicstudies on these aspects we have recently reportedthe thermodynamic=? and electrode kinetic'v" be-haviour of 12/1; and I; II -, Br./Br ; and Br ; IBr - redox couples in some potential electrochemi-cal solvents like DMF, ACN and PC using brightPt as indicating electrode. Also we have report-ed" the electrocatalytic activities of some cheaperelectrode substrates for 1/1 - redox couples insuitable non-aqueous solvents as well as their bi-nary mixtures by potentiostatic polarization stud-ies under near equilibrium condition.

In the present paper we are reporting the ex-

'Presented in the Proc. Fourth National Convention of Elec-trochenusts, Madras, July 5-6, 1993.

change current densities (io) of the galvanic reac-tion 2Br - ~ Br 2 + 2e(M) and hence the overallreaction 3Br- ~ Br ; + 2e(M) at 25°C on M = bareplatinum (Pt), graphite (C) and their platinized sur-faces, (Pt)Pt and (C)Pt in water as well as in ACN,DMF and PC by polarization studies at near equi-librium conditions. Attempts have also been madeto understand the kinetic solvent effect (KSE) ofthe reaction in terms of the previously reported?'transfer free energies (!1 G:~of the initial state(Br-) and the !1G:' values of the transition state(TS) derived herein.

Moreover, in view of the fact that the virtualheat of activation under reversible condition(!1HO"), obtainable from -(blogio/bT-I) (ref. 9)is free from the effect of roughness factor ~H.IIJ ofthe electrodes, the studies have also been ex-tended to 15° and 35°C with the hope that the re-sults will help to gain insight to the intrinsic elec-trocatalytic activities of the electrode surfaces inthe solvents concerned.

Materials and MethodsThe solvents ACN (AR, BDH) DMF (E

Merck) and PC (E. Merck) were purified by the

292 INDIAN J CHEM. SEe. A, APRIL 1994

methods described elsewhereI 1.12. KBr (GR, E.Merck) was dried by keeping it in a vacuum de-siccator for about a week. Br2 (GR, E. Merck) wasused as such.

Solutions of Br - and Br 3 of known strengthsin a given solvent were prepared by mixing the ap-propriate amounts of the standard stock solutionsof tetrabutylammonium bromide and Br2 in thatsolvent. The concentration of Br : was checked bypotentiometric titration with a standard aqueousAgNOJ solution and that of Br2 by iodornetricti-tration with standard aqueous thiosulphate solu-tion both in water. The concentrations of the solu-tions used in polarization studies wereCl:lr.- = CBr; = O.05Min all the solvents.

The cell, preparation of the electrodes, experi-mental set up and the procedures used for the po-larization studies under near equilibrium conditionwere exactly similar to that described in our previ-ous paper" with Izi I - system.

The primary data for Pt in PC and for (Pt)Pt inDMF could not be realized because of experimen-tal difficulties.

Results and DiscussionFigure 1 shows the linear portions of some ty-

pical ohmic current density (i) versus overpotential(l]) plots for the Br- IBr2 reaction. As in the case1112 system", portions of the plots for aU the elec-trodes in different solvents are similar in natureand indicate the extent of reversibility of the reac-tion under consideration.

The exchange current densities (io) based onapparent geometric area of 1 ern? of the electrodesin different solvents have been determined by useof the ohmicrelation':'

. RTv CI )10= oF Il]l]-O ... (1)

where i is the current density, l] the overpotential,and l] and v are the number of electrons involved

in the overall reaction and in the rate determiningstep (rds) respectively, which are 2 in each case(vide reaction Scheme 1).

Table 1 presents the observed io values of theelectrodes at different temperatures for the differ-ent solvents. These results show that io values inwater are much higher than those in the organicsolvents. For all the electrodes except C, the orderof io values in different organic solvents followsthe sequence: ACN > DMF> PC and that on C;

•••".~-e•

~ 505!•

-100

Fig. I-Plots of ix 102 (mA·cm-'2) versus ,,/(mV) for theanodic solution of 3Br- - Br 3 + 2e - on (Pt)C electrodes in

HP (0),ACN ("'), DMF (0) and PC (e) solvents at 25°C

Table I-Exchange current densities (io)1A cm - 2 at different temperatures and the virtual heat of activation (alfl", kJ mol") in dif-ferent solvents

T-1 Solvent H2O ACN PC DMFx 103 Electrodes

Pt (Pt)Pt (C)Pt C Pt (Pt)Pt (C)Pt C (Pt)Pt (C)Pt C Pt (C)Pt C3.470 iox 105 116 120 108 17.6 37.2 54.8 58.6 18 3.3 7.4 5.4 14.0 17.8 2.73.354 iox 105 148 146 134 30 49.4 61.6 64.6 36 5.3 10.3 8.6 19.1 19.8 5.93.254 ioX105 216 246 180 52 66.2 7515 76.2 52.6 7.4 13.7 10.6 27.6 21.2 13.8

aHo" 2.9 2.9 2.3 5.5 3.3 2.4 1.6 5.0 3.4 2.8 3.1 2.8 1.5 7.2(kJ mol-I)

DATTA et aL: SOLVENT & ELECTRODE KINETIC EFFECTS ON ANODIC OXIDATION OF Br- 293

ACN >PC >DMF. On the other hand, for any sol-vent except water, the observed io values on differ-ent electrodes are in the order: (C}Pt ~ (Pt )Pt >Pt > > C and that in water:(Pt )Pt > Pt > > (C}Pt > C. Evidently various factorsare responsible for the observed orders and hencethe understanding of their overall effects shouldbe an involved problem.

Since CVG's of Br " - Br 3"- Br~c system, likethose for I - ~ I 3"- 12 systems", on different noblemetal metallized electrodes over Pt and C sub-strates in all these solvents are found to be moreor less similar in nature with widely spaced peaks,they correspond to the overall reactions.

Thus the exchange current density io (based onthe apparent geometric area of 1 cm/) for theanodic oxidation reaction (2a) is given by

... (3)

where k, is the specific rate constant dependingupon the solvent and the nature of the electrodesurface, ~ the symmetric factor and other termsare having their usual significance.

In view of the fact that the equilibrium potentialof the redox couple, ~~e' is dependent upon thenature of the solvent the kinetic solvent effectfKSE) on io values can be written as

3Br- ~Br3" + 2e

and

. .. (2) RT In (sial )0) = RT In (.k/ wks)

+RT In (,CBrjwCBr-)

2Br3" ~3Br2 + 2e ... (2')

In the present near equilibrium polarization stud-ies with solutions where CBr- = CBr, =O.05M,oxidation of Br : IBr3" system given in Eq. (2) maybe written in the following sequences as suggestedby Giordano and others'v" and also supported byParsons and co-worker!"

(_ rds )2 Br +S cr (S)Br+e ... (2a)

(S)Br+(S)Br~Br2 + 2S ... (2b)

... (2c)Scheme 1

where the charge transfer (CT) rate determiningstep (rds) (2a) is followed by a fast dimerization(2b) and then a fast chemical step with the forma-tion of Br 3" and (S) stands for the surface of thesubstrates.

Since several species are involved in the abovemultistep reaction, their relative solvation wouldlikely to dictate the rates of the reaction to someextent. At the same time the extent of coverage ofthe electrode surface (e sl by the respective sol-vents is also an involved problem.

In order to understand the kinetic solvent effectof the anodic reaction (2) it should be useful toevaluate the rate constant (ks) values and the relat-ed free energies of activation ~ G'" of the reac-tion, which will be helpful in deducing the transferfree energies (~G?) of the transition state (TS), asthe corresponding data for the initial state (IS), i.e.,Br ", are known= for the solvents with respect tothe reference state, water.

... (4)

Also, expressing ~~e values of the redox couple interms of the Nernst equation, one has

(1 - ~)F [s~~e - w~~el= (1 - ~)F(s~~~ - w~~~)

+ (1- ~)RT In ,(CBr,!CB;-)I w( CBr~1 CB;-)

'" (5)

where subscripts sand w refer to solvent and thereference solvent water respectively. From our ex-perimental condition, since CBr; = CBr- = O.05Mfor all the solvents including water, the last termof Eq. (5) and also the second term of Eq. (4) be-come zero. As a result Eq. (4) becomes

RT In (sk/ wks = RT In (siol wio)

... (6)

Now, considering the solvent effect on the overallreaction (2) we have

2F(s~~~- w~~~)= - [s~Go - w~Gol

=3~G?(Br-)-~G?(Br3") ... (7)

Replacing the last term of Eq. (6) by Eq. (5) andtaking ~ = 112, Eq. (6) becomes

RT In (sk/ wks) = RT 1n (siol wio)- i~G?(Br-)

... (8)

As the transfer free energies of Br -, Br 3"from thereference solvent water to these solvents areknown=, the values of KSE on the reaction rateRT In (sk/ wks)' for the system were computed us-

294 INDIAN J CHEM. SEC. A, APRIL 1994

Table 2 Values of RT In (w~o), RT In (,k,), A G? (Br 3), AG?(Br-) and AG?(TS) for the anodic oxidation of Br : on pretreatedPt,.graphite (C),(Pt)Pt and (qPt in some dipolar aprotic solvent systems

Solvent RTIn(,io)/..,io) 6G?(BrJ} 6G?(B~-) -b(6G.") 6G~(TS)(kJ mol- I) (kJ mol- I) =RT In (,k,! wk,) (kJ mol- I)

Pt electrodes at 25°C

HPACNPC

DMF

o-2.72

o-4.7-17.0- 31.8-5.07

o31.8

29.730.1

0 0-27.8 59.6

-35.6 65.7

0 0-27.2 59.0-42.2 71.9

HPACNPC

DMF

o-2.14-8.21

(Pt)Pt electrodes at 25°Co 0

-4.7 31.8-17.0 29.7- 31.8 30.1

C electrodes at 25°Co 0

-4.7 31.8-17.0 29.7- 31.8 30.1

(C)Pt electrodes at 25°Co 0

-4.7 31.8-17.0 29.7- 31.8 30.1

o0.59

- 3.11-4.04

HPACNPC

DMF

o-3.79-6.36-4.74

0 0-24.4 56.2-29.6 59.3-34.6 64.7

0 0-28.8 60.6-32.9 62.6-35.3 65.4

ing Eq. (6) and are presented' in Table 2.

Moreover, in view of the fact that KSE on theelectrode reaction in different solvents are under-stood from the relative behaviour of IS and TS, at-tempts have also been made to compute ~ G?(TS)values for the solvents using the following expres-sion,

RT In (sk/wks)= -o(~G")= -[(s~G")]-(w~G")]

=~G~'(Br-)-~G~)(TS) ... (9)

In Table 2 are presented the values of transferfree energies of Br - and Br ; and those of- o(~ G") i.e. RT In (sk/ wks) as well as ~ G? (TS)as computed by the use of Eq. (9) for all the elec-trodes.

A perusal of the relatively large magnitudes of~ G~) of Br " (IS) and TS species in the solventssuggests that Br : gets largely desolvated in thesedipolar aprotic solvents as compared to water andthe TS species gets destabilized more than Br - inthe respective solvents. Thus, although increaseddesolvation of Br ' would have facilitated oxida-tion in the solvents as compared to water, thelarger destabilization of TS in the respective sol-vents compared to water should be at least partly

responsible for the observed decrement of the rateof anodic oxidation of Br ' as compared to that inwater.

Now, although Br- is stabilized more in waterthan in organic solvents, [~G? (Br "] values arepositive in all the solvents], Br : gets also specif-ically adsorbed more in aqueous media than inthese organic solvents. In fact, because of largerimage intersections'!' guided by dispersion anddipolar intersections of the organic solvents withthe electrode surface, the extent of surface cover-age by organic solvents, (8s) is larger than that (8w)

of water. This diminishes not only the specific ad-sorption of Br " but also the rate of charge trans-fer reaction through the smaller extent of free sur-face (1 - 8 sl in the organic solvents than (1 - 8w) inwater. This latter feature of the surface again par-tially dictates the relative stabilization of TS. So,the observed fastness of the reaction in watercould be attributed partly to the specific adsorp-tion of Br ' in water and partly to the larger stabil-izatiion of TS in water as compared to other sol-vents [~G? (TS) values are highly positive in allthese solvents].

From the data in Table 2, it can be seen thatthe positive magnitudes of ~ G~) (Br ") and hence

DATTA et 01.:SOLVENT & ELECfRODE KINETIC EFFECfS ON ANODIC OXIDATION OF Br- 295

the desolvation of the reactant Br - relative to thatin water, are nearly the same in ACN, DMFandPc. This might have caused more or less same en-hancement of the reaction as compared to that inwater. Evidently the larger positive magnitudes of~ G? (TS) and hence the larger destabilization ofTS should be the prime cause of the observed or-der of - o~G "'or RT In ski wks values in the sol-vents which is ACN> DMF> PC irrespective ofthe nature of the electrode surfaces. Increased des-tabilization of TS relative. to that in water on anyelectrode depends not only opon the relative sol-vation of TS but also through the extent of freesurfaces (1 - a.) of the electrode in the respectivesolvents.

If one assumes that relative solvation of TS aswell as as are guided by dipolar and dispersion in-teractions with the cosolvents the relative orderwill be PC> DMF> ACN the guiding factors ofthese interactions viz. the dipole moments" (~)and polarizability'? (a) of the solvent moleculesPC, DMF and ACN being 4.98, 3.96 .and 3.02Debye and 8.56, 7.9 and 4.41 (x 10-24 crrr') re-spectively. Thus, while the order of relative solva-tion of TS is PC> DMF> ACN, that of (1 - as) isPC < DMF < ACN. Since the relative stabilizationof TS is likely to be dictated by the combinedcontributions of these two effects, the observedorder will be an involved one and hence difficultto predict.

Now, in view of the fact that the 'virtual heat ofactivation at the reversible potential' ~ H"' (ref. 9)are free from the effect of roughness factor (Rf) ofthe different electrodes's!", as

~F'"- 2.3R = -{~Hog(sio)/OT-l}c=const

= [0 log(sis)tr. 1s:. -1 RfuT c=const

= [010g(~i~)trloT C<consr

... (10)

for the R, factor could be taken as independent ofT, we have computed the same.

Plots of log(sio) versus T- 1 for the reaction at aparticular concentration of the reacting species indifferent solvents are found to be linear. Theslopes of the lines yielded the ~ 1f1'" values. Thevalues so obtained are presented in Table1. Fromthe perusal of the values it may be noted thatthese values lie in the order: (C)Pt < {Pt)Pt ""Pt < Cin water, (C)Pt«Pt)Pt «Pt<C in ACN, (C)Pt <C «Pt) Pt in PC and (C)Pt<Pt <C in DMF.

Thus the fairly large values of ~ 1f1'" on thegraphite (C) electrode imply that it is the least effi-cient electrocatalyst for the reaction as comparedto other electrodes. Also, the catalytic effect of Ptsurface compared to C .surface may be attributedto the fact that sp' hybridized electron of graphiteatom interacts more strongly with the non-bondingand electron clouds of the donor - C == or)C= 0 groups of the solvent molecules than withthe 4s electron of Br atom involved in step 2a ofScheme 1. This again indicates that the relative va-lues of as play an important role in dictating therelative catalytic activity of the electrode surfaces.Notably however, ~Ho~ values being dependenton relative solvation of IS, TS and (1 - e,), solventeffect on ~Ho" valu'es is an involved problem.

Moreover, while the ~ 1f1" values for both thebright Pt and (Pt) Pt electrodes are nearly equal,the comparatively low values of ~ HO'" on {C)Ptelectrode are indicative of the best electrocatalyticefficiency of this electrode amongst the others forthis reaction. Seemingly, the larger electrocatalyticeffect of (C)Pt than that of (Pt)Pt may be attribut-ed to the porous nature of the substrate as com-pared to Pt substrate.

Thus, one may conclude that as in the case of1- - Ii - 12 system, Br " - Br; -Brz system fol-lows the same kinetic picture irrespective of metalsurfaces and solvents, which is however slightlydifferent from that 1- - I; - 12 system in respectof rds. But the most effective electrocatalyst forthe two systems is however the same cost effective(C)Pt electrode rather than costly bright Pt or(Pt)Pt electrodes.

AcknowledgementThanks are due to UGC, New Delhi.for a Re-

search Associateship Ito J.D. and to the CSIR,New Delhi for a Research Associateship to S. Sin-ha.

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