Cyclic Voltammetry Dennis H. Evans.' Kathleen M. O'Connell.. Raloh . A. Petersen. and Michael J. Kelly University of Wisconsin-Madison, Madison, WI 53706
In cyclic voltammetry, the potential of a small, stationary working electrode is changed linearly with time starting from a potential where no electrode reaction occurs and moving to potentials where reduction or oxidation of a solute (the material being studied) occurs. After traversing the potential region in which one or more electrode reactions take place, the direction of the linear sweep is reversed and the electrode r~acriun-.I I I intt.rmtdiatr+ ; ~ n dl~r,rlucti,fmned during I he torward sc3n. oiten can l ~ eaetected. Tlre time scale o i he experiment, controlled hy the scan (or sweep) rate and the total potential traversed, can he varied over the range of 102-10-5 s though quantitative experiments are usually restricted to 10-10-3 s. A supportmi: electrolyte is present to repress migration of chargkd reactants and products. This simple experiment has become extremely popular in chemical research because it can nrovide useful information about redox reactions in a form which is easily obtained and interpreted. In this paper we will present the principles of the method and illustrate its use in the study of four electrode reactions. Basic Experiment Cyclic voltammetry is a simple and direct method for measuring- the .formal potential of a half reactlon when both oxidized and reducedforms are stable during the time required to obtain the uoltammogram (current-potential curve). Cyclic voltammograms (CV) for two such reactions are shown in Figure 1. Consider first the CV of 0% (Fig. 1A). The forward scan commences a t the nitla1 potential of -0.75 V and verylittle current is ohtained until about -1.15 V where oxygen begins to be reduced to the product, the oxygen radical anion (superoxide) (as in reaction 1). The current increases as the
notentials but rate of reduction increases at more neeative " eventually a maximum is reached (-1.25 V) and thereafter the current decreases steadilv. The cathodic ~ e a in k the CV results from the competition of two factors, the increase in the (net) rate of reduction as the potential is made more negative mV past the peak,2 the reactant concentration at the electrode surface is small compared to the concentration far from the electrode and the current is controlled hv the rate of diffusion of reactant through the depletion layer idiffusion-controlled current). The Scan direction is reversed a t -1.50 V (switching potential) and the diffusion-controlled reduction current continues until about -1.25 V where net oxidation of 0; back to Oz occurs. The layer from which 0 2 has been depleted is an accumulation layer for 0; and some of this 0; can diffuse hack
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Author to whom correspondence should be directed. This research was supported by the National Science Foundation (Grant number CHE81-11421).R. A. P. holdsan IncoGraduate Fellowship in Electrochemistry (1980-83). Potentials reported pertain to 298 K and must be scaled linearly for lower or higher temperatures. 290
Journal of Chemical Education
Figwe 1. C)cl~cvo tammograms at a mere-r) eiectrooe n acetan I, e Scan rate 100 4 s. Temperature 20 O°C Reference e ectroae: Ag 0 01 M AghO, 0 10 M ,C7h~l.hCIO~ :n acetonitr e Cmer exper.menta S~mooi; theoretical (digitalsimulation ( 11)).A) -1 mMOn; 0.10 M ( C ~ H ~ ) ~ NelecCIO~ trolyte: electrode area: 0.016 cm2:simulation parameters: E,,, = -1.221 V: kJ& 0.10 M(BC,H,J.NCIO. . - " = 354 s - ' ' ~ . 6) 0.60 mM 2-nitrooroome: . . . . ... ~. e ectrolfl~ eleclroa? area 0 025 cm2 s m. =!.on paramewrs E . ,= - 1 973 V k, \ Do = 0 192 5.' ' = 0 4 7 du aE = 0 23 V - ' 1121 ~
12
to the electrode and be oxidized. An anodic peak in the CV is ohtained for reasons analogous to those underlying the cathodic neak: the surface concentration of 0.;becomes small and the current is limited by the rate of re& of 0; to the electrode. The CV is characterized by several important parameters ( I ) : the cathodic (E,,) and anodic (E,,) peak potentials, the cathodic (i,,) and anodic (i,,) peak currents, the cathodic half-peak potential (Ep/z) and the half-wauepotential (EI,~). The definition of El12 has been borrowed from classical polarography (according to eqn. (2)). In eqn. (2) E" is the for-
ma1 potential pertaining to the ionic strength of the solution used, Do and DR are the diffusion coefficients of the oxidized and reduced forms and n is the number of electrons in the half reaction. Because Do - D H , El/* is usually within a few mV of EO' 12) ~ -- ,
The reduction of oxygen is an example of a reuersible reaction. In a reversihle process the ratio of surface concentrations of 0 and R (02 and 0;) as calculated from the Nernst equation for a given potential, differs insignificantly from the actual ratio. In other words, the electron transfer reaction at the electrode surface is so rapid that equilihrium conditions are maintained even with a substantial net current and a rapidly changing potential. The criteria of reversibility (over a given range of conditions) are AE, = ED,- E,, = 57ln mV (see (1) for dependence on switching potent~al)andEp/2 - Epc = 56.5ln mV, values which must he independent of scan rate and concentration. The Ellz is situated exactly (within 2ln mV) midway between E,, and Epc The reduction of oxygen is also diffusion-controlled (no other processes limit the current). The criterion for diffusion control is that i,Jul" must be constant (u is the sweep rate)." If the reaction is also reversible, the dimensionless group i,,lnFAC;(DonFulRT)'/2 (the current function) will equal 0.446. In this expression, A is the electrode area and Ci is the concentration of reactant. A quasi-reuersible or irreuersible reaction (see below) will give as much as a 50%smaller current function ( I ) . Although reversible behavior for oxygen reduction is observed at 100 Vls, the rate constants, kfand hb in eqn. (11, of the heteroeeneous electron transfer reaction are finite so a l i i ~ h t -t.,w r rate \YIII S:> kb for the cathodic peak and kb >> kr for the anodic peak). The nitropropane reaction is irreversible at 100 Vls. Irreversibility manifests itself through AE, > 57In mV, AE, increasing with increasing u and (EP12- EPc) > 56.51~1mV ( 3 , 4). The value of AE, obtained at any gwen u can he used to obtain k,, the standard heterogeneous electron transfer rate constant (i.e., the common value of kr and k b a t E = EO'). The oxygen reaction is inherently faster (k, > 1 cmls) than the 2-nitropropane reaction (k, = 1.0 X 1OWcmls). The value of n , as defined in the various CV criteria for reversibility, is usually 1, occasionally 2 (reduction of Cd(II)), and rarely 3 (e.g., reduction of Bi(II1)). When n = 2,3,4. . .for an overall reaction, the CV usually consists of overlappinr m e electron processes and the peakshapes are dictated by the differences among the En' values of the separate steps (5-
-1.0
POTENTIAL, VOLTS
kf
Figure 2. Cyclic voltammograms of 1.42 mM2.6di-tefi4utyi-4-ethyiphenolate, 1. at a platinum electrode (0.36 cmZ)in acetonitrile with 0.10 M(C2Hd,NCi0, electrolyte. Temperature: 18'C Reference electrode: same as Figure 1. Symbols: experimentai. Smooth curves: simuiation. Simulation parameters: K3 = 1.4 X lo5 M-'. ks/DI"2 a, ka (L/moi-s).respectiveiy: A) 3.9. 0.54. 1 . 8 X 1 0 6 ; B ) 4 . 5 . 0 . 5 1 , 2 . 6 X 1O6:C)5.8,O.5S, 1.6): lo6. (
s
C
'
'
~
~
metry is a powerful technique for detection and characterization of such coupled chemical reactions. 0; and the 2-nitro~rooaneanion radicals are relativelv is lost via chemical reactionire particularly easy to identify hy CV. The return peak will he reduced in magnitude if some chemical reaction occurs, and it will be completelyabsent if the reaction half-life is much less than the scan duration (1).
occur prior to electron transfer, following electron transfer or interposed between electron transfer steps. Cyclic voltam-
Such a reaction scheme is designated EC, where E signifies the electrochemical step, and C the following chemical reaction. The C step can he first- or second order, irreversible or reversible. More complex reaction schemes are denoted hy a string of letters In the order of the steps in the reaction scheme, e.g., CE, ECE, ECEC, EEC, etc. Subscripts are sometimes included to denote reversibility, reaction order or other special features.
"his is strictly true for shielded, planar electrodes. Spherical. cylindrical, or unshielded circular disk electrodes will show significant positive deviations (10%)when (DaRT/nFvr2)"22 0.1 where ris the electrode radius.
in the overall electrode reaction is the reversible oxidation df the phenolate ion, 1, to the phenoxyl radical, 2, which dimerizes forming 3 causing the return peak in the CV to be correspondingly small (Fig. 2C). By contrast, no dimerization
8).
Detection of Chemical Steps in an Overall Electrode Reaction
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Figure 3. Voltammograms ot 2.0 mMtrans-1.2-dibromo~y~I0he~ane, 4, at a frozen mercury drop electrode in butyronitrile with 0.10 M ( R C ~ H S ) ~ N Celectrolyte. IO~ Scan rate: 1.00 Vls. Symbols: experimental. Smooth cvwes: best fit simulations. Temperature, amount of solution resistance compensated, and simulation parameters K4, k4,u,,and a . : A)-60°C,3.2 ka, 0.60, 14.2s-'. 0.115, 0.175 8)-70% 4.4 kQ. 0.59. 5.0s-', 0.10, 0.16 C)-80% 6.1 k n , 0.57. 2.7s-', 0.10, 0.16.
occurs for larger para suhstituents such as i-CaH7 and t-C4H9 and the rednction peaks are always of normal size (9).
single rednction peak is observed as all molecules of 4 are reduced via 4aa.
0
+ 2 Br-
However, the reaction is reversible and the small hut significant rate of dissociation of 3 provides some phenoxyl radical, 2, for reduction to 1during the return sweep and this process becomes more and more important at lower scan rates (Figs. 2A and 2B) where large return peaks are seen. The data in Figure 2 (points) may he compared to calculated CV's (smooth curves) based on reaction (4) and values of the rate and equilibrium constants giving good agreement with experiment for a wide ranee of scan rates have been found. There are many instances where the CV also affords important qualitative information about the reaction pathway. Such information is usually obtained from a CV recorded over a n extended potential range which will reveal additional peaks for oxidation or reduction of the ultimate product of the electrode reaction or, in ideal cases, of an intermediate in the reaction. The reduction of the dimer 3 can he seen when the return scan is extended to -2.3 V (insets, Fig. 2). The cathodic peak a t -2.2 V is caused by the irreversihle reduction of 3 to 1. In this case the assignment of the new peak to 3c is based on comparison with the reduction potentials of structurally similar . . species and the fact that the peak behaves as it should, 1.e.., ~tis nresent at fast scan rates and ahsent at slow rates. In cases where the suspected product is an isolable compound (unlike 3), a direct comparison should be made between the peak potential of the authentic material and the peak potential of the process to which it is beine assiened. V characOur final example also illustrates t h e k e U ~ C to terize an electrode reaction with a coupled chemical reaction, hut it involves totally irreversihle E steps and somewhat unusual experimental conditions (low temperatures). In Figure 3 are shown results for the reduction of trans-1,2-dihromocyclohexane, 4, at three different temperatures (10).Solutions of 4 contain two rapidly interconverting conformations, one with equatorial bromine atoms, 4ee, and one with axial hromines. 4aa. The irreversihle rednction of 4aa occurs at less negative potentials than that of 4ee. At room temperature the interconversiou of the two chair forms is so rapid that only a
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292
Journal of Chemical Education
However, as the temperature is lowered the rate of the 4ee +4aa reaction decreases to a point where a new peak for reduction of 4ee begins to appear (-3.0 V, Fig. 3A). As the temperature is lowered still more (Fig. 3B and 3C) the new peak grows at the expense of the first peak because the 4ee 4aa reaction is imneded. At -80°C (Fie. 3C) the two neaks represent approximately the equilibrium concentrations of 4aa and 4ee.,resnectivelv. At constant temnerature. the second . peak is larger at the faster scan rates. Once again. - . the exnerimental data mav he comnared to theoretical calculations and values of equilibrium and rate constants can be inferred. Close agreement was found with equilibrium and rate results from other (nonelectrochemical) studies of the conformational interconversion. Note in Figure 3 that the reverse scan has been omitted because the electrode reactions are totally irreversible so no return peaks are ever
+
...--.
A short paper may he more noteworthy by its omissions than its content. We have discussed only four examples from the numerous cases of reaction schemes for which theoretical treatments are available and our choice of examples may be thought to imply (incorrectly) that inorganic compounds, biologically relevant species, or aqueous solutions cannot be studied. There are also important variations of the basic experiment (ac cyclic voltammetry, thin layer cells) or data treatment (convolutive or semi-integral techniques) which h a w not heen touched on. Finallv. " , there are numerous cases in which the reactant andlor product are confined on or near the electrode surface (adsorbed species, oxide layers, covalently attached species, redox polymer films) and these have not been covered. It is hoped, however, that the foregoing will provide an understanding of the principles of the technique and a glimpse of the range of useful applications. ~
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Literature Cited (1) Nichulson, R. S., and Shain, 1.. Anal. Chem., 38, 706 (19641. This paper summarizes earlier t h e o r e t d work aud presenlr theoretical renllt? for r numhei at important
' Chrm S o c f ' e r k m Trona.2.755
.
.
.
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(18%).
ciraanoi. Chrm., 108, 107
181 Lrlanaean. J. B.. Mareel. S..Bard. A. J.. and A w n , F.C., J. A m r r Chem Sac, 100,4248 (9) Richards, J.A.,and Evans, D. H., J. Electronno1 Chmt,81,171 (1977). (10) O'Connell. K. M..and Evans. D. H., J. Amar Chem Suc., in press. (11) Feldhne, S. W., in "Electroanalvtkal Chemistry: Bsrd, A. J., (Editor) Marcel Dekker, New York,Vol. 4 , 1969.00.199-296. (18) Corrigan, D.A.,and Evans, D. H., J. Elrclioonol. Chrm., 106,287 (lY80).
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