Voltammetric Behavior of a Ferrocene Derivative A Comparison Using Surface-Confined and Diffusion-Controlled Species Marielle E. Gomez and Angel E. ~aifer' University of Miami, Coral Gables, FL33124
In this article we describe a new laboratory experiment that is suitable for instrumental analysis courses. The experiment examines the voltammetric behavior of a cationic ferrocene derivative in two very different situations: in aqueous solution where the voltammetric behavior is diffision-controlled, and inside an anionic polyelectrolyte film that has been cast on the electrode surface in which the voltammetric behavior approaches that expected from a surface-confinedelectroactive species.
This experiment allows students to grasp the nature of the differences in the elcctmchemical behavior of the two svstems (diffusional vs. surface-confined). I t also dLmonstrates the importance of electrostatic effectson the thermodynamics of a n electrochemical system.
Cyclic Voltammetry Cyclic voltammetry (CV) has become one of the most wide-spread and important electrochemical techniques. During the last ten years, several excellent laboratory experiments about CV have appeared in this Journal (1-9). The main focus of these experiments has been to familiarize the students with the basic features of the technique and to demonstrate its usefulness in probing the redox chemistry of freely diffusing solution species.
Nafion
Voltammetric Behavior using Modified Electrodes
To the best of our knowledre. no ~reviousundereraduate experiments have dealt witcthe kstinct voltamketric behavior ofspecies immobilized at the electrode surface. Thls is particularly surprising in view of the tremendous effort that recentlv has been devoted to the investieation of modified electrohes, that is, electrodes with derivkized surface structures that c o n h e molecules, ions, or polymer chains that are intended to alter the reactivity of the electrode in specific ways. Modified electrodes can hardly be considered as research curiosities anymore; some of these systems have already entered the realm of practical applications (10). The following laboratory experiment that explores the voltammetric behavior of a modified electrode is long overdue. Although many procedures for the modification of electrode surfaces are experimentally demanding, a few are simple and easily adapted for undergraduate instruction. In this context, electrodes modified with films of Nafion2-a perfluorinated anionic polyelectrolyte-can be readily prepared and have convenient properties, such as stability, roughness, and permselectivity towards different cations (11-14). Therefore, we describe here a simple experiment, which we originally developed for our instrumental analysis laboratory, that contrasts the electrochemical behavior of a water-soluble ferrocene derivative [11[PF81 (or [ZIBr) in two different situations: in aqueous solution in which the 111+/T1l2+ . . . . redox c o u ~ l eexhibits diffusion-controlled voltammetric behavior, and inside an anionic polyelectrolvte (Nation, film cast un the electrode surface in which h e [ll+/[l12+'coupleshows the voltammetric behavior expected from a surface-confined redox couple. 'Author to whom correspondence should be addressed. 2~afionis a trademark registered by E. I. Dupont de Nemours, inc.
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Theory
Cyclic voltammetry (1-4)is carried out by scanning the potential of the working electrode, measured vs. an appropriate reference electrode, while measuring the resultant current as a function of the applied potential. In a typical experiment the applied potential is varied linearly with time in the positive or negative direction, until a given potential value--the switching potential-is reached. Then the direction of potential scanning is reversed to return to the initial potential value. Although this cycle can be repeated many times, it is commonly carried out only once. The cyclic voltammogram is obtained by recording the current that flows through the working and counter electrodes during the potential cycle. The scan rate is defined as the rate of variation of the applied potential vs. time. Confined Species The voltammetric behavior expected from reversible redox cou~lesin solution has been described several times in this ~ d u r n a l( I d )and will not be repeated here. However, the voltammetric features of a redox couple in which both redox forms are confined to the electrode surface are quite different (15,161. For instance, the theoretical potential difference between the anodic and cathodic peaks is zero for a reversible redox couple. As with a fully reversible redox couple in solution, the i,li,, ratio should also be 1. Due to the confinement ofthe electroactive species at the electrode surface, diffusion ~ l a vno s role in current control. Thus, peak currents will be directly proportional to the scan rate as indicated by the following equation. i, = (9.37 x 1 0 5 ) n 2 u ~ ~
(at 298 K)
(1)
where i,, is the peak current (amperes); n is the number of electrons involved in the electrochemical reaction; v is the scan rate (Vls); A is the surface area of the electrode (cm2); and r is the surface coverage (mol/cm2). The Nature of the Electrochemical Response
The behavior of a reversible (fast) electrochemical system may be then identified as diffusion-controlledor surface-cokned, based on the appearance of the voltammograms. The diffusion-controlled system will show a separation of the oxidation and reduciion peaks and characteristic diffusional tails. The surface-controlled system will show little peak separation and no tailing of the baseline. Nevertheless, i t is far more accurate to assess the nature of the electrmhemical response by simply plotting the peak current vs. the scan rate or the square root ofthe scan rate, as the linearity of the plot identifies which behavior is predominant. Alternatively, a plot of log (peak current) vs. log (scan rate) will have a slope of 1.0 for a redox couple confined to the electrode surface or 0.50 for a diffisioncontrolled system. Experimental Caution: Benzene has been reported to be carcinogenic. This reaction must be camied out in a fume hood. Materials
The orange ferrocene derivative methylferrocenetrimethylammonium iodide [l]+I- can be obtained from Strem Chemicals (Newburyport, MA). Alternatively, it can be easily prepared according to published procedures (17). Anion exchange of iodide for hexafluorophosphate should be carried out to avoid the easily oxidized iodide ion. This exchange was performed by adding a filtered aqueous solution of excess ammonium hexafluorophosphate (Fluka) to an aqueous solution of [ll+I- to obtain a pale yellow-orange precipitate of [1ltPFsl-. Budget-minded instructors may wish to substitute the expensive ammonium hexafluorophosphate by either sodium tetrafluoroborate or sodium perchlorate to obtain the corresponding tetrafluoroborate or perchlorate salts of [l]+.However, we have not tried the experiment with these salts. An alternative electroactive material, which provides better data as a surface-confined electroactive species, is [2]+Br-.Although i t is not commercially available, i t can be made easily by refluxing equimolar amounts of (dimethylaminomethy1)ferrocene (Aldrich)and 1-bromoheptane (Aldrich) in benzene for 24 h. The resulting orange precipitate is collected by filtration, recrystallized from acetone-ther, and dried under vacuum. Asufficient amount of either ferrocene derivative ([I]' or [21+)can be prepared with little effort at the beginning of the semester because each student uses only a few milligrams. Alternatively, the students can be asked to prepare the electroactive material if the time available allows it. An interesting experiment for an integrated laboratoly course would be the preparation, structural characterization, and investigation of the voltammetric behavior of [lIi or [2It. The polyelectrolyte Nafion (equiv wt: 1,100) is available from Aldrich as a 5% solution in lower alcohols and water. The supporting electrolyte was freshly prepared using analytical-grade 0.10 M NaCl. All solutions were prepared using distilled water that had been further purified with a four-cartridge Barnstead system. However, high-quality distilled water is suitable for the experiment.
Equipment
The electrochemical cell was a simple 3-neck round-bottom flask, with openings for the reference electrode (saturated calomel electrode, SCE), a BAS (West Lafayette, IN) platinum-disk working electrode, and a large platinumflag counter electrode. (The area of the counter electrode must be greater than that of the working electrode.) The active electrode area was 0.0201 cm2.The area of the working electrode's tip, including active surface plus insulator, was 0.30 cm2. Although it is not electroactive in the potential range surveyed, oxygen should be purge from all solutions using a n N2 line attached to fine teflon tubing that may be inserted through any of the three ports, with an electrode. Any triangular potential function generator/potentiostat combination can be used to drive and control the potential of the working electrode. Currents can be simply recorded with any X-Y recorder. The data presented here were obtained with a Princeton A ~ ~ l i Research ed 1751173 combination. Our undereraduacstudents have obtained similar data sets using: BAS CV-1R ootentiostat. which incoroorates ootential scannine 0&nigrap6ic Series 100 X-q capabikies, and a recorder.
ousto on
Procedures
The working electrode was polished with 0.05-pm alumina and rinsed thoroughly with purified water. Students were encouraged to obtain flat background scans a t sevne solution eral scan rates in the Dure s u ~ ~ o r t ielectrolvte (0.10 M NaCl) to verify the &enceof elect;oactive imourities. The diffusion-controlledstudies were performed using a 1.0 mM solution of . I1.lW'F~l-in . ". 0.10 M NaCI. Althoueh the concentration need not be exactly the same, the precise concentration used is important in determining the diffision coefficient. Thus, students should take care to accurately record the exact gram amount used. ARer having verified a flat background in pure supporting electrolyte in the range 0.0 to 0.7 V vs. SCE, cyclic voltammograms of [I]' were obtained in the 25-250 mVls range in increments of 25 mVls. Anodic peak currents were measured from the extrapolated baseline (see Fig. 1). To confine the electroactive species a t the electrode surface, we prepared a solution containing the anionic p l y electrolvte and [I]+ (or [Zl*). Deposition on the electrode surfaced carefully measured voiumes of this solution, followed by solvent evaporation, yields cast films that contain the electroactive spkcies. his procedure allows for controlled loading of the Nafion fdms, so only a fraction of the sulfonic groups will be compensated by the cationic ferrocene derivative. From previous studies on the system, we selected a loading level of 20% (18). The following sample calculation illustrates the conversion factors that must be used to obtain the volume of Nafion solution, as received from Aldrich, needed to accomplish t h e 5 : l stoichiometry for sulfonic groups to electroactive cations for 1.0 mg of [ll+[PFsT. 1.0 x lo4 g [lI[PF61 1mol [ll[PFsl X 403 g 100 equiv Nafion 1,100 g Nafion 20 mol [lI[PF6] 1equiv Nafian
I
100 mL soh = 0.27 mL 5 g Nafion
Dissolving 1.0 mgof [lIIPFsl in 0.27 mL of the 5%Naiion solution yields a solution with 4.5 x 10" equiv of NafioniL and 9.0 x lo3 mol of [l]+/L. This solution is too concentrated to yield films of suitable thickness on the electrode surface. Therefore, i t was diluted 100-fold with absolute ethanol to yield a solution containing 4.5 x lo4 equiv of Volume 69 Number 6 June 1992
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Nafion and 9.0 x 10" mol of [lIt/L. This solution may be prepared in advance and yields good results for 2-3 weeks. Casting of the Nafion film on the electrode surface was accomplished by carefully depositing 3.25 KLof the dilute [ll+/Nafionsolution on the tip ofthe electrode. (A10-KLGC syringe is rewmmended for the deposition step.) The deposited volume was spread homogenously over the whole tip area on both the active Pt and the inactive Kel-F surface of the electrode tip with the syringe needle tip. The solvent was then allowed to evaporate in air to yield a cast Nafion fdm with 5.2 x 10"equiv of Nafion and 1.0 x lo4 mol of [ll+/cm2.The thickness of this film can be estimated as approximately 3 0 4 0 n m using the reported wet density of the Nai-form of 1100 EW Nafion (1.58 g/cm3)(19). Throughout the film-casting steps the electmde should be clamped in a vertical position. Once dry, the electrode can be immersed in the pure supporting electrolyte and allowed to equilibrate a t open circuit for no more than 3 min. After equilibration, the time used to record all the voltammograms should be minimized. Scan-rate studies should be carried out in the same range as above, beginning with faster rather than slower scans to minimize the effect of the electroactive species leaving the Nafion film for the solution. Although this is not a serious problem when the voltammograms are recorded according to the protocol given above, the use of the bulkier derivative Dl+ guarantees a slower migration of the electroactive species from the Nafion film and provides more time in which to carry out the experiment.
60 mV, as expected for a reversible electmn-transfer process, and Em = +0.375 V vs. SCE. The anodic peak current is directly proportional to the square mot of the scan rate as shown by the linear plot in Figure 3a. This demonstrates clearly that the anodic current is controlled by the diffusion of the electroactive species to the electrode surface. A diffusion coefficient of 6.8 x lo4 cm2/s was calculated using the RandlesSevcik equation (3) from the slope of the straight line. AE, was found to be independent of scan rate in the range surveyed (20250 mV/s). Voltammogram Using Nafion A series of cyclic voltammograms in 0.1 M NaCl for the same electrode modified with a cast Nafion film with 5.2 x lo4 equiv of Nafion and 1.0 x 10" mol of [ll+/cm2are shown in Figure 2. The anodic waves observed in this case correspond to the oxidation of the small amount of [lItretained in the Nafion film because there is no [I]+ in the solution. At 50 mVls the anodic and cathodic peak potentials are a t +0.304 and +0.326 V vs. SCE, respectively, yielding a small AE, value of 22 mV, and a E m value of +0.315 V vs. SCE. The plot of peak current vs. scan rate is
Results Typical Voltammogram A typical cyclic voltammogram that shows the reversible oxidation of 111' in 0.1 M NaCl solution is shown in Pieure 1. The anodic and cathodic peak potentials are i t +0.405 and +0.345 V vs. SCE, respectively, yielding AE, =
Sq. Fit. of Scan Rate.
',-
0.0
0.25
0.50
(V/s)1/2
0.70
POTENTIAL V nSSCE Figure 1 (left).CV response (50 mV,s)of a Pt electrode immersed n a 1.0 mM salmon of [l].PF; n 0.1 M NaCI. Figure 2 (right).CV response of a Pt electrode modified with a cast Nafion film with 5.2 x lo4 equiv of Nafion and 1.0 x l o 4 mol of [l]+/cm2 immersed in a 0.1 M NaCl solution. The scan rates are 50, 100, 150, 200, 250, and 300 mVIs. 504
Journal of Chemical Education
Scan Rate. V/s Figure 3. (a) Scan-rate dependence of the anodic peak current recorded with the electrode of Figure 1. (b) Scan rate dependence of the anodic peak current recorded with the electrode of Figure 2.
Scan Rate, Vls Rg~re4. Scan rate depenoence of tne peak-to-peakpotental differ2. ence recorded witn the electrode of F~g~re linear (see Fig. 3B) as expected for an electroactive species confined to the electrode surface. Interestingly, the formal potential for the oxidation of 1' in the Nafion film is about 60 mV lower than that found in 0.1 M NaCl solution. This shows that a formal potential reflects the relative stabilities of all the species involved in the electron-transfer process. Thus, in the Nafion fdm, the negatively charged sulfonic groups stabilize the oxidized form of the ferrocene derivative [112+more than the singly charged reduced form [I]'. This eases up the oxidation process and causes a shiR of the formal potential to lower values. Comparisons with Theoretical Values
The observed AE, values for the electrode modified with a fdm of [ l l + - ~ a d nare larger than the theoretical value of 0 mV. This is almost alwavs the case for the voltammetric response of surface-confihed electroactive species, due to either resistance in the electrode coatings or interactions among neighboring redox centers. A plot of mpvalues as a function of the scan rate reveals that the gap between the peaks widens as the scan rate increases (Fig. 4). This suggests that resistance within the Nafion film may be responsible for the observed effects. It is important to point out clearly to the students that the AE, dependence on scan rate is eaused by nonidealities in the system and does not wnform to theoretical predictions. The surface coverage of Ill+can be determined either using the scan-rate dependence of the anodic peak current
(via eq 1)or by the direct integration of the anodic wave. The f r s t procedure yields 5.7 x 10-"moVcm2, whereas the second procedure yields 8.8 x lo-" moVcm2,based on the active surface area of the electrode. Both values are lower than the nominal value of 1.0 x 104 moVcm2 calculated from the casting data, suggesting that some of the ferrocene subunits in the Nafion film do not undergo electron transfer. Since the value based on peak currents may be affected by broadening of the voltammetric peaks, it is less reliable than the value based on internation. In these calculutionfi we are assumlng that the e&e thickness of'the film is oxidized in the timescale of the experiment, which is reasonable given the thickness of the ~ a f i o layer. n Models for the propagation of charge across polymermodified electrod& (i6';take into acciunt two contributions: actual diffusion of the electroactive species through the layer and electron hopping between neighboring el&troactive sites. No assumptions concerning the predominance of either mechanism are needed for data treatment in this experiment. Previous results (18)suggest that actual diffusion is the predominant mechanism in this type of modified electrode. Conclusions
This laboratory experiment clearly exemplifies the types of voltammetric behavior to be expected from a reversible redox couple, both in solution and immobilized at the electrode surface. The contrast between the two situations should help the students understand the factors predominant in each type of current-potential response. The experiment can be completed in a four-hour laboratory session. In our instrumental analysis course, 30 junior or senior undergraduate students have already performed the experiment obtaining acceptable results. Literature Cited 1. Evans,D.H.;QComell,KM.;Peteraen,R.A.;Kelly,M.J.J. Chem,Edue 1885,60, 290.
2. MabbotZ G.A. J. Chem. E d v e 1885,60,697. 3. Kissinge~P. T ; Heineman, W R. J Chem. Edue. I983.60,702. 4. Van Benachoten. J. J.: Leais. J. Y;Heineman, W. R.; Raston. D.A: Kissinger,P. T. J. Cham. Ed=. 1963,60,712. 5. Haldw%n. .R.P:.Ravichadran..K.:.Johneon. R. K J. Chem. Educ. 1984.61.820. . . 6. Brillas, E.: Garrido, J. A ; Rodriwez, R M.; hmeneeh, J. J Chem. Educ 1987,M, 189 -~~
7. Plszczek, L.: Ignatowiez, A,:Kiebalsa, J.J Cham. Educ. IS@ 65, 171. 8. Camedo, G. A. J.Chem. Edue. 198865,1020. 9. Pamemy, R. S.; h n f o n , M. B.;Armabong, N. R. J Cham. Edue 1988.66.877. 10. FEW, J. E.: Ha,H . A . O A n a l Cham. 1987.59.933A. 11. But-, D.A; Anson,F C. J A m . Chem. Soc. lm, 104,4824. 12. White.H.S.:Iaddv.J.:Bard.A . . . J. J.Am. Chem. Sm.1982.104.4811. . . 13. Martin, C. R.; Rubinstein, I.: Bard,A J. J.Am. Chem. Soc 1883,IM, 4817. 14. Seentirmay, M. N.:Martin, C. R . A d Chem. 1984,56,1898. 15. Ba1d.A. J. J. Chrm. Edue. 1983.60,302. IS. M-Y. R.W.I ~ E I P ~ ~ ~chakrstv; Y . I ~ B~ ~ IF A. ~~ JI,., ~ d .M; B B B L D D NNW ~~~: York. 1983: Vol. 13.0.191. . 17. h b & , A; Bieber, T. I. J. C h . Educ. 1983,60,1080. 18. G s d a , 0.;Kaifer,A E. J. Elechoowl. Chsm. 1090,279,79. 19. Maudz, K A ; Hora, C. J.;Hopfmger, A. J. Polym. P r e p . (Am. Chem. Sm.Div. Pdym. Cham.) 1978,19,324.
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