silver loaded iridium oxide electrodes in acid electrolytes

J E!ectroanal Chem. 185 (1985) 109-117 Elsevler Sequoia S A, Lausanne - Pnnted

109 III The Netherlands

THE ELECI’ROCPEE~CAL BEHAVIOUR OF IRIDIUM/SILVER IRIDIUM OXIDE ELECTRODES IN ACID ELECTROLYTES

RO

LEZNA.

N R DE TACCONI

LOADED

and A J ARVIA

Imrrruto de Investrgacrones Fsrcoquimrcas Ter;rrca.sy Aphcadas INIFTA, Card/a de Corrco 16. Sucursal 4. I900 La Plara (Argentma) (Recclved

19th March

1984,

m final form 17th October

1934)

ABSTRACT Changes m ekctrochenucal response brought about by the electrodeposItion of small amounts of sdver on elude covered m&urn electrodes are exammed m retatlon to the sermconductmg propertles of the oxide The charge mvolved m both the sdver and hydrogen adatom underpotenbai deposits (UPD) was found to remam pracucally constant and Independent of the oxide th&ness uhen the latter was gradually mcrcased by potenuodynanuc cychng up to Wferent anod~c swnchmg potentials Thk result suggests the charge transfer for both pr -es takes place ar the bare metal The anslderable ncrease m the film conducttvlty when the potential IS above ca 0 68 V IS shown by the fact that under certam controlled cond1tion.s the salver elatroreducuon/elcctrooudatlon reactions are shtftcd 10 a more posmve potential r-on where the omde becomes a conductor and paruapates m the charge transfer process Sdver loadmg mcreases the electrical conductivity of the oxide provldmg a large cross secuon for the electrooxldatlon of odium_ particularly m a regon where the oxide conductlmty IS poor (E c 0 68 V) The proposed mechamsm accounts for the enhanced growth rate of the m&urn oxide III the presence of the sdver 10ns m solution

INTRODUCTION

Many reported

electrochenucal studies on m&urn III the

hrerature

[l],

partxularly

both in actd and base electrolyte are concerning the behaviour of lzldlum

by relatively thick hydrous oxrde layers that can be grown through potentmdynamrc cychng 111the potentral range between the threshold potentrals corresponding to the hydrogen and oxygen evolutron reactions, respectrvely. These oxrde layers exh.tbrtprormsing electrocatalytic properties, interestmg electrochromic effects and electrical conductrvity characteristics which can be modifted by loadtng with an electrodeposited metal such as silver from an acrd electrolyte contammg a soluble stlver salt. The present paper refers to the potentiodynarmc response of rridium/srlver loaded hydrous irickm oxrde elecrrodes m acrd solution. Results are compared to those avarlable in the literature and interpreted in terms of conducttvity changes tn the oxide layer resulting from the deposrtion of small amounts of stlver, which can influence the charge transfer rate through the electrode/electrolyte boundary as occurs in the case of semiconducting oxide electrodes [2]. covered

OO22-0728/85/503

30

6 1985 Ekevler

Sequoia S.A

’

110 EXPERIMENTAL

Cychc

voltammetry

expenments

were conducted

in the conventronal

way. The

workmg electrode was made from ~ndnu-n wu-e (Spec pure, Johnson Matthey, 0.69 cm’ geometnc area) axtally embedded m a PTFE rod The counter electrode was a vitreous carbon rod. Potentials were measured against the mercurous sulphate electrode but m the text are referred to the reversible hydrogen electrode (RHE). Solutrons of 1 M H,SO, and 1 M H,SO, + xM Ag,SO, (10m5 < x < 10W3) were prepared from Merck AR grade sulphuric acid, Baker AR grade srlver sulphate and trtply drsttlled water. Iridtum oxide layers were grown by potenhal cychng at u = 0.1 V s-t between the swrtchmg potentials E, (0.0 V) and Es, (1.5 V). The average thtckness of the tndntm oxide layer was estrmcted from the number of potenttal cycles [3] Runs were performed at room temperature RESULTS

AND

DISCUSSION

(I) Sduer eiecrrodeposrttonon an mdrum elecrrode The voltammogram

of u-rdium rn 1 M H,SO,

at 0.1 V s-t (Frg. 1) between u-t the literature [l] On the other hand, the voltammogram tn 1 M H,SO, wtth the addition of 10m4 M Ag,SO, (broken line) using an uidtum electrode wluch has been subjected only to a single potential sweep between the same swrtchtng potenhals, so that the oxrde IS at the monolayer stage, shows coutnbutlons related to the srlver eizctrodeposrtton m the 0 76-O 31 V range, and to stlver stripptng tn the 0.51-1.01 V range. The elec trodeposrtron of stlver rnhrbrts both the anodic and cathodic H-adatom current contnbuttons on tndrum The voltammetnc charge of the same systems at 0.1 V s-’ after 500 cycles (Frg 2) shows that u-rdmm oxtde has grown to a tluckness of ca 70 nm In thrs case the current peaks correspondmg to the reductlon/oxrdatron of srlver Es, = 0.08 V and Es, = 1 15 V, 1s s~rmlarto those reported

0

02

04

06

08 Potenttal/

IO V

Fig 1 Voltammograms for mdlum run at 100 mV s-’ m, full he, 1 M H,SC;,-ad. broken lme, 1 M H,SO, +10-j M Ag,SO,, The reversible potentA of the Ag/Ag+ (10m4 M) redox couple IS mdvsatzd

111

are obscured by the large charge values related to the reversrb!a redox couples operating witinn the oxide layer [4]. As was previously reported [5], the hydrous iridium oxrde conductivrty tncrease exponentially vnth the potential applied to the electrode. Thus, for a layer 150 nm thick, the conducttvity changes by a factor of 10’ when the potenttal vanes from 0.65 V to 125 V. Hence, the voltammogram run m a silver sulphate-contammg electrolyte covenng the 0.0 V to 1.5 V range can be approximately dlvrded into three potenttal regtons, namely, one regron extendtng from 0.65 V downwards, where the oxtde layer IS bleached, another one from 1.25 V upwards which corresponds to a highly conductmg and coloured oxrde layer, and a transitton region in the intermediate potential range. The mfluence of the oxide thickness on the electrodepositron of silver on radium was exanuned systemattcally by changmg independently both E,, (E, = 0.0 V) and the number of cycles n. The average tluckness of the indmm oxide layer tncreases accordmg to both Es, and n. For a given E,, eg. 1.5 V, the oxide was made to grow to different tlucknesses by cycling in a cell contammg only 1 M H,SO,. The stnpping charge of hydrogen, en, was measrired from the final voltammograms by integratmg to the inflexron wluch marks the commencement of hydrogen evolutron and dtvrding by the fracttonal coverage at thrs potenttal, 0.65 [6]. No double layer corrections were made, Values are quoted per unrt of geometric area. The electrode was then transferred to a separated cell wrtb 1 M M2S0, + 1O-4 M Ag,SO, where the stnpping charge of stlver, QA, was measured and mtegrated between the potential hmrts at which the correspondmg voltamrnetnc peak edges merged with the blank run, (Frg 1). To check on stlver readsorptton dunng stnpping at 0.1 V s-r rn the potenttal range between E, = 0.0 V and Esa = 1.5 V the charge of the stlver monolayer was determmed m erther the same silver-contaming solutton or, after the electrode had been transferred to another cell, m a 1 M H,SO, stlver-free solution.

2

-1 -

-2

I 0

a2

1 Or.

Q6

08

1.0

1.2 IL Potential/V

Fig. 2 Vokammogram oblarned after the mdmm e!ectrode has been cycled 500 tunes at 100 mV s-’ M H,SO, +10-4 M Ag2S04.

m 1

2

1

0

3

4

In [number

5

6

of cycles)v=lV 2

Fig 3 Senuloganthrmc plots of charge vs number of repetmve Inangular potenual cycles ai 1 v S-’ boih 1 M H,SO, (PH. Qcu) and 1 M H,SO, + 1O-4 bf Ag,SO, (Q,,)

rn

As tl--e szme electrochemical response was found m both cases readsorptlon was proved neghlble under the gven condltlons. In these runs the UPD sliver anodlc strippmg charge, QAgr always appears greater than the hydrogen adatom anodx 200 &SOb 2 c’

0

p a

a

a

e

’

o

1208O-a

0

2

L

6

8

10

12

1L

, 16

18

20

t/1ci2s Fig 4 Dependence A&SO,

of the slnpprng charge of silver on adsorpuon

time al 0 65 V m 1 M HISO

+ IO-’

bf

113

charge, QH, and both charges remain practically constant and independent of the indlum oxide thickness (Fig. 3). These results suggest the charge transfer related to the UPD deposltlon of both Ag and H, takes place at the inner indlum surface whxh 1s reached by diffusion cf the correspondmg reactants through the irilum omde layer. The UPD of silver on -&urn, mth the oxide at the monolayer stage, was stuled as a function of both potenrlal and depositIon time. The UPD charge of sdver was evaluated from the stnppmg voltammograms and was shown to reach a lmuting value for a Bven constant potenhal (Fig. 4). This mdxates the adsorpuo_r?-desorptlon equllibnum and the completion of the monolayer are attained at z = 30 mm The sdver electrochenucal adsorption isotherm (Fl g. 5) appears as a continuous hne m the 0.65-1.0 V range m good agreement with the smgle strippmg peak voltammetncally observed. In the same potential range a Langmulr-type Isotherm IS apparently obeyed and the departure from the straght line observed at more negative potent&s can be attnbuted to the oxide adsorptlon isotherm, a fact that prevents the drawms of further rehable conclusions. (2) Sliver UPD on a thick mdrum oxrde covered rrrdum electrode An mterestmg change in the voltammogram run with a semlb!eached oxld, covered lndlum electrode in 1 M H2S0, + xM Ag2S0, solutions 1s notlced when a

1

I

05

06

08

07

09

Potential/V

Fig

5 JZkcIroch~rmcal

formatton Isotherm

charge,

curve

adsorption III

Isotherm

for

sliver electroadsorptton

sliver

Curve

charge

I

total

The stmght

anodlc

charge,

lrne corresponds

curve

II oxide to a J-angmux

114

4 E >

e L s ”

loo

075

050

025

C

-Cl25

-II50 1

02

04

I

0.6

08

1

1

10

12

IL.

Potent+

Fig 6 Voltammograms for uxlmm covered with a rhrck layer of uxhum oxide m 1 M H,SO, + 10m3 M Ag2S0, at 100 mV s-’ The full lme corresponds to a contmuous run between 005 V and 1 5 V The broken hne shows the anod~c profile after holdmg the potential for 40 s at be cathodx end. E, = 0 55 V

Fig 7 Dependence of the suxppmg H,SO, + 1O-3 M A:,SO,

charge

of

salver on

loadmg

tune 7 at E,,= 055

V III1 M

115

1

2

3 log

I v/mV C!:,

Fig 8 Dependence of the anod~c peak he&a on log u ca-respondmg to the stnppmg of salver loaded on lndmm oxide m 1 M H,SO, + 10m3 M Ag2S0,

negative potenttal going scan, at 0.1 V s-l, mcludes a potenttal holcimg at E, (Fig 6). The latter IS slightly more positive than Eig,Ag-, the potential of bulk silver electrodeposttron. The dashed curve in the ftgure corresponds to a sweep terminated at Ep, held at this potential, and then swept in the positive direction In thrs case the voltammogram shows a smgle arm&c peak at 1.3 V (Frg. 6, dash@ line). At low sweep rates no complementary cathodic peak can be recorded. This response can be accounted for by assummg the UPD stlver monolayer electrodepositron/stnppmg IS

v mV<’ / Rg 9 Dependence of the anod~c current peak on u correspondmg to the stappmg of sliver loaded on u-~&urnoade m 1 M H,SO, +10m3 M Ag,SO,

116

occurrmg at the hydrous indmm layer. The auodtc charge related to the peak at 1.3 V is asstgned to the electrochermcal stlver loadtng at Et, of the u-tdium oxide layer. For a gven E,, the stlver loading/stripping charge mcreases wtth T, the lapse of ttme the potenttal is held at E, (Ftg 7) and qualitatively, It changes inversely with the oxtde layer thickness On the basis of a semiconductor model [73, the urdmm-oxygen d-r* band IS gradually ftlled when the potenttal moves in the negative dtrection. The density of the most cathodic levels IS relattvely small with a low mob&ty of tamers Hence, m tlus case, low rates of both charge injection at the oxrde/electrolyte Interface and charge transport through the bulk of the oxrde are expected. On redox couple is not large enough the other hand, the oxldismg power of the Ag/Ag+ for hole mJectton to take place into the lower lymg, more extended, occupted electrontc levels m the oxtde. The stnppmg dependence of both potentral (Ftg. 8) and current herght (Fig 9) on sweep rate (u) are srrmlar to those expected for an trreversrble surface electrochemical reactton (3) influence of electrodeposlted sliver on the mdmm oxide growth rate Indmm electrodes ut aqueous electrolytes gradually develop thtck oxide layers when they are either cycled or pulsed between smtable potential lin-uts, 0.0 V to 1.5 V betng the opttmum values The mechamsm responsible for thus charge accumulatton has not yet been fully estabhshed The presence of silver ions m solunon produces a constderable enhancement of the growth rate of rndmm oxide by potenttal cyclmg. Starting wttb electrodes covered by about a monolayer of oxide, after 500 cycles (tluckness ca. 70 mn), the electrode unmersed in 1 M H,SO, + x M Ag,SO, produces about 4 tunes the charge of the oxrde grown in a separate cell contauung only 1 M H,SOa. The comparison was made by measuring the height of the auodic peak at 0.95 V. This increase m the oxide layer growth rate IS mdependent of the stlver ion concentration when the latter 1s vaned by a factor of seven (6.7 x lo-’ to 4.8 x 10m4). The enhancement can be linked to an tncrease in electrical conductrvtty in the two-dimensional plane over the potential range m which UPD silver IS laid down on the surface. In tlus potential range the oxide layer exlubits a low mtrinstc conductivity as against the high values reported at more positive potentials Thus, the stlver layer on the surface may offer a large cross sectton for indium oxidation and the UPD layer IS stnpped off in a potentral region correspondmg to the mam oxidation peak of the redox couples. On the other hand, no silver is on the surface dunng the cathodic sweep in the range corresponding to oxtde electroreductron, enabhng, therefore, the unreduced oxide to be mcorporated mto the redox equthbria. The mcrease in conductrvtty. however, rmght partially explam the overall behavrour of the stlver loaded indtum oxrde layer. The presence of electrodeposited stlver may create new states m the electronic structure of u-rdium oxide at energies corresponding to the oxide bleached state, aIIowmg fast carrier transport through the oxtde

117 CONCLUSIONS

Under controlled condltlons, silver can be electrodeposited on either the bare metal or on the extended oxide-electrolyte interface, the electrochenucal response being different in each case. The propertles of electrodeposiced sliver on u-idlum where a thick oxide layer has been electrochemisally grown depend on the mtnnslc conductivity of the oxide layer which changes accordmg to the potential apphed to the electrode A model based on the changes of the oxide propertles brought about by sdver loadmg qualitatively zcounts for the observed enhancement m the ~ndnml orlde growth rate. in the prc- .ence of silver the behavlour of the electrochermcal system can be ratlonahzed on the basis of the band structure and electromc conflguratlcns of uidrum oxide. ACKNOWLEDGEMENTS

INIFTA IS sponsored by the Consgo Naclonal Tkucas, the Umversldad Naclonal de La Plats clones Clentificas de la Provmcia de Buenos Aires

de Investigaclones and the CormsIon

REFERENCES 1 2 3 fl 5 6 7

J h’ozota and B E Ccnway, Ekctrochun Acta, 28 (1983) 1 T Kobayashl. H Yoneyama and H Tamura. J Ekctrochem Sot , 130 (1983) 1706 ; C;ottesTeld and S Snru~asan. J Electroanal Chem , 86 (1978) 89 D AJ Rand and R Woods. J Ekctroanal Chem , 55 (1974) 375 S H Glarum and J H Uarshall. J Electrochem Sot, 127 (1980) 1467 R Woods, J Ekctroanal Chem, 49 (1974) 217 S Gottesfeld. J Electrochem Sot , 127 (1980) 1922

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