simple potentiostat

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Simple computercontrolled potentiostat for the characterization of electrochromic

films

L. Kirkup, J. M. Bell, D. C. Green, G. B. Smith, and K. A. MacDonald

Citation: Review of Scientific Instruments 63, 2328 (1992); doi: 10.1063/1.1143795

View online: http://dx.doi.org/10.1063/1.1143795

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Simple computer-controlled potentiostat for the characterizationof electrochromic filmsL. Kirkup, J. M. Bell, D. C. Green, G. B. Smith, and K. A. MacDonaldDepartment of Applied Physics, University of Technology, Sydney, P. 0. Box 123, Broadway,N. S. W. 2007, Australia(Received 9 October 1991;accepted or publication 9 December 1991)We describe a simple and inexpensivepotentiostat, incorporating current boosting andfiltering circuitry for use n the study of coloration and bleaching n electrochromic thin films.The system is sufficiently flexible to permit utilization in other electrochemicalapplications.

Three electrode potentiostats are widely used in elec-trochemical studies o establish he potentials at which spe-cific reactions occur. A potentiostat may be used o studythe coloration and bleaching hat occurs n an electrochro-mic film when charged speciesare injected into, or re-moved rom, a film. This application requires hat a linearramp voltage be applied to the potentiostat so that thepotential of the reference electrode with respect to theworking electrode varies within the range 3 to - 3 V witha period of the order of 100 s. Though it is possible toacquire expensiveproprietary potentiostats to accomplishthis task, the availability of low cost, high performance,integrated circuits and microcomputers permits the con-struction of a high quality, inexpensive electrochemicalanalysis system.Figure 1 shows he potentiostat circuit designed or theelectrical characterization of electrochromic films. When avoltage Vi is applied to the noninverting input of the fOPA2 111 operational amplifier (op-amp), labeled ICI A,the feedback oop around that op-amp (which contains theelectrochemicalcell) ensures hat the referenceelectrode sheld at the same potential. The other half of this dualop-amp ( IClB) acts as a current to voltage converter per-mitting the current through the electrochemical cell to beestablished. n this configuration the working electrode ofthe potentiostat is held at virtual ground. The currentthrough the cell is given by -V&Z/, where Rf is thefeedback esistor n the circuit containing IClB. V0may bemeasuredusing a chart recorder or input to an analog todigital converter. The very high input impedance of theOPA2111 ( > 10 a) ensures hat no current is drawn bythe referenceelectrodeand the offset voltage ( < 0.3 mV) issufficiently low that no offset adjustments are required.The amount of current that must flow betweencounterelectrodeand working electrode o force the referenceelec-trode to the samepotential as Vi, dependsupon the surfacearea of the film (which is attached to the working elec-trode). In some situations this current may exceed10 mAwhich is the limit of the current that a precision op-amp,such as the OPA 2111, can supply. To provide currents tothe electrochemical cell in excessof 10 mA, and in orderthat thesecurrents may be measured, simple push-pullarrangement of complementary bipolar transistors is usedat both outputs of the OPA2111. The current boosting

circuitry shown in Fig. 1 is capableof supplying up to 0.1A. Such a push-pull circuit is susceptible to cross overdistortion at high frequencies.3n the application discussedhere signal frequencies re of the order of 0.01 Hz and suchdistortion is negligible.As frequenciesof interest are close to dc, a simple firstorder low pass ilter is used o attenuate frequencies boveabout 20 Hz. At low levels of current through the cell, 50Hz interferencewas found to be a problem. This is associ-ated with the distributed nature of the feedback patharound the ICl A op-amp and the fact that no attempt hasbeenmade to shield any part of the circuit from the intru-sive effect of electrical mains interference. The effect ofsuch interference s reduced dramatically by the use of anactive notch filter which discriminates strongly againstsig-nals at or close o the notch frequency.4To adjust the filterfor optimum performance,a signal frequency of 50 Hz isapplied to Vi,. The 20 kfi potentiometer is adjusted untilthe 50 Hz component of the output is minimized. In thesituation where it is desired o monitor the referenceelec-trode directly with a conventional voltmeter or oscillo-scope, t is important to buffer the electrode with a highinput impedanceop-amp such as the 071 ( IC2) shown inFig. 1 otherwise the electrode will be loaded by the mea-suring device. The OP77 (IC3) performs a similar buffer-ing function to the output of the current to voltage con-verter.Though the circuit shown in Fig. 1 may be usedwith asignal generator o provide a linear ramp voltage Vi, and achart recorder used o plot the current t hrough the cell asa function of potential at the working electrode, greaterflexibility in terms of control, data storage and data ma-nipulation is offered by using a multipurpose interfacingcard in conjunction with a microcomputer. For example,the amount of charge injected into, or extracted from, afilm can be establishedby simply integrating with respectto time the current supplied to the film. A SunsetLabora-tory interfacing board5situated within an IBM compatiblePC is used o provide digital to analog functions so that aramp voltage may be generatedand analog to digital func-tions so that the current t hrough the cell may be measured.The hardware is controlled by software written inQuick-Basic.6The circuit was designed o operate rom anexternal bipolar power supply ( f 15 V at 0.5 A) or fromthe f 12 V supply of a microcomputer.

2328 Rev. Sci. Instrum. 83 (4), April 1992 9034-8748/92/042328-02 92.00 0 1992 American Institute of Physics 2328article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to

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Figure 2 shows a cyclic voltammogram obtained withthe system. An Ag/AgCl electrode was used as a referenceelectrode, a platinum rod as counter electrode and theworking electrode consisted of a tungsten oxide film depos-ited by the sol-gel technique2 on an indium-tin oxide sub-

FIG. 2. Cyclic voltammogram for coloring and bleaching of a WOs sol-gel film on indium-tin oxide. The inset of this figure shows the variationof the potential of the working electrode with respect to the referenceelectrode and the current density through the film with respect to time.

FIG. 1. Circuit diagram for poten-tiostat.

strate. The feedback resistor, RP shown in Fig. 1 was cho-sen to be 1 kfi. The electrolyte is LiClO, in propylenecarbonate. The curve represents a single cycle from - 1.5to 1.5 V and back. As can be seen, he drift of the systemis minimal and the noise intrusion negligible. The poten-tiostat described here is suitable for other electrochemicalapplications such as the study of corrosion in metals7 orthe investigation of reaction rates.The authors gratefully acknowledge Mr. Alan Coelhoand Mr. Robert Graves for their assistance n computingand electronic matters, respectively. D. D. MacDonald, Transient Techniques in Electrochemistry (Plenum,New York, 1977).5. M. Bell, D. C. Green, A. Patterson, G. B. Smith, K. A. MacDonald,K. Lee, L. Kirkup, J. D. Cullen, B. 0. West, L. Spiccia, M. J. Kenny,and L. S. Wielunski, Proc. SPIE Vol. 1536 (in press).3G. B. Clayton, Operational AmpIifirs, 2nd ed. (Butterworths, London,1979), Chap. 9.4P. Horowitz and W. Hill, The Art of Electronics, 2nd ed. (CambridgeUniversity, Cambridge, 1989).5Sunset Laboratory, Forest Grove, Oregon, 1989.Microsoft BASIC version 7.0 ( 1990).7M. G. Hocking and V. Vasantasree, Br. Corros. J. 10, 160 (1975).5. A. Fraunhofer and C. H. Banks, Potentiostat and its Applications(Butterworths, London, 1972), Chap. 9.

2329 Rev. Sci. lnstrum., Vol. 83, No. 4, April 1992 Notes 2329article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to163.10.198.122 On: Wed, 26 Feb 2014 12:16:13

simple potentiostat

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