1995 - los - the polymer electrolyte-electrode interface

8/12/2019 1995 - Los - The Polymer Electrolyte-electrode Interface

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PergamonElecrrochimica Am. Vol. 40. No. 13 14. pp. 2159 -2164. 1995

Copyrighl Q 1995 Elswicr Science Ltd.Printed in Great Britain. All rights reserved

0013~4686/95 9.50 + 0.000013~4686( )00157-3

THE POLYMER ELECTROLYTE ELECTRODE INTERFACEL. CHRISTIE,P. Los and P. G. BRUCE*

Centre for Electrochemical and Materials Sciences, School of Chemistry, University of St. Andrews, St.Andrews, Fife KY 16 9ST, ScotlandReceived 6 April 1995)

Abstract-The electrochemical reaction of the Fe+/Fe+ couple in high molecular weight, solid,poly(ethylene oxide) has been studied. The kinetic parameters (including the electrochemical rate constantfor charge transfer) of the system under investigation have been obtained by employing uc impedancemeasurements at ultramicroelectrodes (disk of 12.5pm radius). AC impedance at a microelectrode hasproved to be a powerful method with which to study redox reactions in solid polymer systems character-ised by low conductivity and diffusion coefficients.Key words: ultramicroelectrodes, ac impedance, poly(ethylene oxide), cyclic voltammetry, electrontransfer.

INTRODUCTIONThe importance of the interface between a solidpolymer electrolyte and an electrode cannot be over-stated. In the context of applications, while it isimportant to develop highly conducting polymerelectrolytes it is also essential to ensure facile kineticsat the electrode/electrolyte interface in any polymerelectrolyte based electrochemical device. Whileattention has been given to the interface betweenpolymer electrolytes and lithium metal electrodes[l,21 as well as the interface between polymer electro-lytes and intercalation electrodesC3, 43, much lessemphasis has been given to redox reactions in poly-mers at metal electrode interfaces, with some notableexceptions[& 63. This is regrettable in view of thefact that these reactions are amongst the simplestelectrode reactions, significantly less complex thenmetal-metal ion electrode reactions such as lithiumor intercalation electrodes, and as such form the beststarting point from which to develop a fundamentalunderstanding of electrode reactions at the polymer/electrolyte interface in general.Electrochemical redox reactions in solid polymerelectrolytes are important for two basic reasons.First they afford an opportunity to study redox reac-tions in a solid solvent. Second by comparing suchmeasurements with those in liquid solvents of asimilar chemical composition, the rates of redoxreactions in the same chemical environment, differ-ing only in the molecular weight of the solvent, maybe obtained. In this way the fundamental influence ofthe solvent dynamics, as we pass from liquid to asolid solvent, on the kinetics of electron transfer canbe probed[7]. Electron transfer processes lie at thecore of all electrochemical reactions as well as beingof great importance in biology, semiconductors and

* Author to whom correspondence should be addressed.

many other scientific fields. The crucial role that thesolvent plays in controlling electron transfer hasbeen described by the pioneering work of R. A.Marcus et aI.[8-123. However, our understanding ofelectron transfer and in particular the influence ofthe solvent dynamics remains incomplete. Moststudies have relied on using a variety of liquid sol-vents. The opportunity to compare liquid with solidsolvents offers a potentially much more powerfulhandle with which to probe the factors controllingelectron transfer.As part of a wider study of redox reactions andelectron transfer in solid and liquid solvents wereport here on the electrochemistry of the Fez+/+couple in long chain solid polyethers. The electro-chemical reactions are studied using ac impedancespectroscopy at ultramicroelectrodes. This involves

making ac measurements from 0.1 Hz to 100 kHz atup to lo9 R. As far as we are aware these are the firstvariable frequency nc measurements at an ultra-microelectrode in a solid polymer electrolyte.EXPERIMENTAL

The solid polymer electrolytes containing the elec-troactive species were prepared by the hot pressingtechnique[ 133. Lithium perchlorate (Aldrich,LiClO,, 99.5 ) was employed as the supportingelectrolyte; this was used without further purificationafter drying under dynamic vacuum at 130C for48 h. Poly(ethylene oxide) (Aldrich, m.wt. 5 million)was dried at 50C for 48 h also under dynamicvacuum. Fe(CF,SO&, and Fe(CF,SO,), were usedas the sources of Fe+ and Fe3+ ions. These saltswere prepared as described elsewhere[ 141. They weredried under vacuum at 60C for 48 h. A mixture ofthe dried salts were ground in a pestle and mortarand then transferred, along with the PEO, to a stain-less steel test tube containing ball bearings. The test

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2160 L. CHRISTIE t al

- 12.5 pm radius Pt microelectrodePt foil counter/reference electrode

0.7mm thick polymer film

Glass case enclosing microelectrodeFig. I. The two electrode cell employed for measurementson the solid polymer electrolyte.

tube was sealed, then removed from the dry-box andshaken in liquid nitrogen for 20min. At these tem-peratures, the polymer becomes brittle and fracturesunder the impact of the ball bearings. An intimatemixture of the polymer and salts was thus produced.After grinding, the tube was left for approximately2 h to return to room temperature, after which it wastransferred to the dry-box.

A small sample (x 70mg) of the cryogroundmixture was pressed (5 tons) for 30sec between twoTeflon disks in a 13 mm pellet die. This was heatedto 8OC, without applying pressure using a bandheater and the temperature maintained for at least5 h. Upon cooling to 35C, a pressure of 3 tons wasapplied. The sample was allowed to cool to roomtemperature over 24 h under the applied pressure.The films thus produced were typically between0.5mm and 1 mm. In all cases the electrolyte con-tained sufficient LiClO, to yield a 20: 1 ratio ofether oxygens to LiClO,. Equimolar amounts ofFe3+ and Fe+ were used throughout and four con-centrations were studied 7, 20, 50 and 2OOmM. Solu-bility of the iron salts in the polymer was confirmedby powder X-ray crystallography, no peaks fromcrystalline iron(H) triflate were observed up to a con-

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Fig. 2. The apparatus used for the ac impedance measure-ments at ultramicroelectrodes.

centration of 1.4 M and iron(II1) triflate was found tobe soluble up to a concentration of 2.7 M.

A simple two-electrode cell was employed for mea-surements on the solid polymer electrolyte (Fig. 1).The working electrode was a platinum disk of12.5 pm radius and the counter electrode a platinumdisk of area _ l.3cm2. The currents measured inthese ultramicroelectrodes studies were sufficientlysmall (typically less than 1 nA) such that the plati-num counter electrode also acts as a very satisfac-tory reference, ie there is no need for a thirdelectrode.

Ac measurements were carried out using theapparatus shown in Fig. 2. The cell was enclosed in aFaraday cage and the working electrode was con-nected to the input of a EG&G PAR preamplifier181 the output of which was connected directly tothe input of a Schlumberger-Solartron 1255 Fre-quency Response Analyser. The generator output ofthe 1255 was connected to both the voltage inputand the counter electrode. Coaxial cables are used toconnect the EG&G preamplifier to the 1255 and tothe cell, the sheaths of the coaxial cables were


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The polymer electrolyte/electrode interface 2161grounded. Measurements were under the control of a cated functions of a dimensionless parameter (~a~/Zenith microcomputer. An ac signal of between 5 D). They found that at high frequencies the diffusion-and 10 mV rms was employed in all measurements al impedance resembles a Warburg impedance andand we have demonstrated that impedances of up to at low frequencies a steady-state mass transport limitSOOMn may be reliably measured within the fre- is obtained due to the spherical diffusional fieldquency range 0.01 Hz to 250kHz. Data were around the disk. On the basis of this finding the dif-analysed using a modified version of the complex fusional impedance at a microdisk can be approx-non-linear least squares (CNLS) fitting program imated by a parallel combination of a Warburgwritten by MacDonald et aI.[15]. impedance, Z and non-linear resistance, R,,[ 18,191. Consequently it is possible to use for the micro-

RESULTS AND DISCUSSION disk an equivalent circuit identical to that for animpedance of the hemispherical electrode modifiedComparing typical liquid and solid solvents there only by a geometrical factor. Although this approachare two main differences which are important in the to the impedance of a microdisk is only an approx-context of measuring interfacial kinetics. The first imation, the hemispherical approximation enablesconcerns the large electrolyte resistance associated one to easily obtain all kinetic parameters (includingwith the solid solvent; generally this can be three the diffusion coefficient) from analysis of the UCorders of magnitude higher than in comparable impedance response of a microdisk. The completeliquids. One way to reduce this is to employ an equivalent circuit which approximates a microdisk atultramicroelectrode the radius of which should not which a simple one-step reaction is occurring isshown in Fig. 3.exceed a few micrometers. Current densities compa-rable to those used on electrodes of normal dimen- Complex impedance plots for a solid electrolyte,sions are associated with currents in the pA to nA PEO,, : LiClO,, containing iron(H) and (III) arerange with ultramicroelectrodes. As a consequence of shown in Figs 4 and 5. The ac impedance responsethis small current the iR, drop associated with the of the system is a function of the concentration of theresistance is much reduced. The second difficulty electroactive species.arises from the slower diffusion of electroactive At concentrations lower than 20mM a simple UCspecies to and from the electrode in a solid solvent impedance response is obtained (Fig. 4) and thecompared with a liquid. Typically diffusion coeffl- equivalent circuit for a one-step charge transfercients are at least three orders of magnitude lower in process at a microdisk electrode (Fig. 3) can be used

the solid state. In principle microdisk electrodes canto interpret the experimental results. The real and

also help here in that upon electrolysis the planar imaginary components 2 and Z obtained for allsemi-infinite diffusion obtained at short times (high frequencies were analysed using the CNLSfrequencies) gives way to near spherical diffusion and program[ 151 modified in order to extract the experi-as a consequence enhanced flux compared with an mental values of the following parameters: uncom-electrode of normal dimensions at which the linear pensated solution resistance R,, double-layerdiffusion continues to grow and the current decays capacitance C,, , apparent diffusion coefficient D andratecontinuously with a t - I2 (w) dependence. An esti- standard apparentRTIn2F2R,,G,,,,constant k,, k, =

mate of the frequency above which semi-infinite dif- A, where A is the surface area offusion to a microdisk dominates can be obtained the microdisk and all other symbols have their usualfrom the following equation w - 100D/a2[16-181, meaning[20]). The solid line in Fig. 4 represents thewhere D is the diffusion coefficient of the electro- best fit, which is very good. The following param-active species, a is the radius of the microdisk and w eters extracted from this procedure were obtained atthe angular frequency. Similarly, the frequency below 76C and for a 7 mM concentration of both iron(H)/which hemispherical diffusion dominates is given by (III) triflate: apparent diffusion coefficientw x 0.1D/a2[16-181. The intermediate region of fre-quencies is characterised by the mixed linear andhemispherical diffusion. For disks with a radius ofseveral microns in solid polymers the semi-infinite r-Illlinear region dominates down to frequencies around0.15 Hz in which case flux enhancement due to theuse of a microelectrode may not be obtained within -the frequency range of the interfacial processes. ,Nevertheless the combination of a microelectrodewith a small iR, drop (and low R, C,, time constant),where i is the current, R, the uncompensated resist-ance of the electrolyte and C,, the double layer L Ret -capacitance of the electrode/electrolyte interface, andac impedance spectroscopy offers the best tool withwhich to probe the nature of redox reactions inpolymer electrolytes at metal electrodes.Fleischmann and Pons[16] considered the diffu- Fig. 3. The equivalent circuit used in the present paper tosional impedance at a microdisk and proposed fit UC mpedance plots obtained at a microdisk, where R, isexpressions describing the real and imaginary part of the uncompensated resistance, R,, is the charge transferthe microdisk impedance using relatively compli- resistance, Z is the Warburg impedance and R is the..-non-hnear dlttusion reslstance.


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1I1 I I , 6Cl 30 10 60 80 100 1 20 1110

T/ MOFig. 4. C impedance, Nyquist, plot obtained at 76C for the solid electrolyte PEO,, : LiCIO, containing7 mM Fe(II)/Fe(III) triflate. The solid line corresponds to the best fit for the hemispherical approximation(Fig. 3) and open squares to the experimental results.

D = 1 x lo-scms-, apparent standard rate con-stant k 0 . 0 00 5 cm s- , uncompensated solutionresistance R, = 1.2 MQ and double layer capacitancearound .5pFcm-. From the value of the solutionresistance R, = l/4 ~a[211 a specific conductivity ofthe solid electrolyte K 1.7 x 10S4 Scm- can beobtained. This value is in good agreement with liter-ature data for the PEO,, : LiClO, system[22]. It isinteresting to note that since the mode of diffusion tothe microdisk is a function of the frequency, semi-infinite linear diffusion persists down to 0.1 Hz. Thisis a consequence of the relatively high value of theratio a2/D. This shows experimentally, that for theFe(II)/Fe(III) system in a polyether, the advantage ofusing microelectrodes in highly resistive media isrelated to the low values of the iR, drop. However,reduction of the size of the microdisk would shift the

limit of spherical diffusion to significantly higher fre-quencies. The UC impedance results are in goodagreement with those obtained by cyclic voltam-metry. It is shown in Fig. 6 that the typical steady-state response of a microdisk is obtained for sweeprates in the range of a few mV/s for solid PEO. Theapparent diffusion coefficient calculated from thevalue of the steady-state current ia, is3.5 x lo-cms_ at 94C for a concentration of20mM Fe(H) and Fe(II1) triflate. This is in goodagreement with the value obtained from the acimpedance measurements at the same concentrationand temperature (4.6 x lo- cm2 s - I).At high concentrations (> 50mM) of iron(II)/(III)triflate the experimental data do not agree with thesimple equivalent circuit of Fig. 3. In this case theequivalent circuit similar to that proposed by

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Fig. 5. C impedance, Nyquist, plot obtained at 75C for the solid electrolyte PEO,, : LiCIO, containing2OOmM Fe(I1) and Fe(W)) trilIate. Solid line correspond to the fit and symbols to the experimentalresults.


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The polymer electrolyte/electrode interface 2163

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Fig. 6. Cyclic voltammogram at a microdisk (12.5 pm radius) for 20mM Fe(H) and Fe(II1) triflate dis-solved in solid PEO,, : LiClO, ; sweep rate 1 mV s and temperature 94C.

Gerischer[23] and Armstrong et aI.[24], Fig. 7, wasfitted to the data. This equivalent circuit assumes thepresence of an adsorbed intermediate. Detailedstudies of the system with a high concentration ofthe electroactive species are under way in order tounderstand the true origin of this more complex elec-trochemical reaction.The results presented here must at this stage beregarded as preliminary. They demonstrate that LICimpedance spectroscopy can be carried out suc-cessfully, at microelectrodes in contact with solid

polymer electrolytes and that such measurementsrepresent a powerful technique for probing electro-chemistry in a solid solvent. However, as indicatedabove a number of important questions remainunanswered and work is continuing to better under-standing the behaviour of the Fe(II)/Fe(III) systemreported here.Acknowledgements-PGB is grateful to the Royal Societyfor the award of a Pickering Research Fellowship and tothe EPSRC for financial support.

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Fig. 7. The equivalent circuit used to fit ac impedance plots obtained in the solid electrolyte containingmore than 50mM iron(II)/(lII) triflate, where M is the microdisk diffusional impedance, C, is a parallelcapacitance and R, is a parallel resistance.


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REFEREN ESD. Fateaux, Sol id Stat e lonics, 17, 133 (1985).P. G. Bruce and F. Krok, Solid State lonics, 36, 171(1989).P. G. Bruce and F. Krok, Electrochimica Acta, 33, 1669(1988).P. G. Bruce, E. S. McGregor and C. A. Vincent, Pro-ceedi ngs of the 2nd I nc. Symposium on Pol ymer Elect ro-lytes, (Edited by B. Scrosati) p. 357 (1989).T. T. Wooster, M. L. Longmire, M. Watanabe and R.W. Murray, J. Phys. Chem. ,5315 (1991).D. Bard, Thesis, Institut Nationale Polytechnique deGrenoble, France, 1993.A. J. Bard, H. D. Abruiia, C. E. Chidsey, L. R. Faul-kner, S. W. Feldberg, K. Itaya, M. Majda, 0. Melroy,R. W. Murray, M. D. Porter, M. P. Soriaga and H. S.White, J. Phys. Chem. 97, 7147 (1993).R. A. Marcus, J. Chem. Phys. 43,679 (1965).R. A. Marcus and H. Sumi, J. Electr oana l . Chem. 204,59 (1986).P. G. Wolynes, J. Chem. Phys. 86, 5133 (1987).P. G. Dzhavakidze, A. A. Kornyshev and L. I. Krishta-lik, J. Electroanal. Chem. 228, 329 (1987).D. K. Phelps, A. A. Kornyshev and M. J. Weaver, J.Phys. Chem. 94, 1454 (1990).

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F. M. Gray, J. R. MacCallum and C. A. Vincent, SolidState fonics 18/19,282 (1986).J. R. MacCallum and C. A. Vincent, (Eds) PolymerElectrolyte Reviews, ~01s. 1 and 2. Elsevier, London(1989).J. R. MacDonald, J. Schoonman and A. P. Lehner, J.Hectroanal Chem. 131, 77 (1982).M. Fleiachmann and S. Pons, J. Electroanal. Chem.250,277 (1988).S. Sarangapani and R. de Levie, J. Electroanal. Chem.102, 165 (1979).A. S. Baranski, J. Elect rochem. Sot. 133,93 1986).P. G. Bruce, A. Lisowska-Oleksiak, P. Los and C. A.Vincent, J. Electroanal. Chem. 367, 279 (1994).M. Sluyters-Rehbach and J. H. Sluyters, Electroanalyti-cal Chemistry Vol. 4, Marcal Dekker, Inc., New York(1970).M. I. Montenegro, M. A. Queiros and J. L. Daschbach(Eds.), M icroelectr odes: Theory and Appl icat ion s,NATO AS1 Series, Series E: Applied Science, Vol. 197,Kluwer Academic Publishers, Dordrecht (1991).C. D. Robitaille and D. Fauteux, J. electr ochem. Sot .133, 315 1986).H. Gerischer, Z. Phys. Chem. 201, 55 (1952).R. D. Armstrong, R. E. Firman and H. R. Thirsk,Faraday Discuss. them. Sot. 56,244 (1973).

1995 - los - the polymer electrolyte-electrode interface

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