Anal. Chem. 1985, 57, 1117-1121
1117
Temperature Dependence of the Voltammetric Response of Thin Electroactive Polymer Films J a m e s Q.Chambers* a n d Gyorgy Inzelt'
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600
The temperature dependence of the electroactive polymer film voltammetric response Is considered under conditions of surface or thin-layer behavior. Existing theory Is used to show how Information regarding the variation of the electrode process with temperature and the transltion between thin-layer and diffuslon behavior can be obtained from experimental data. Poly(vinyiferrocene) (PVF) and tetracyanoquinodimethane (TCNQ) polymer modified electrodes have been studied under the following conditions: PVF/CH,CN/O. 1 M (C,H,),NCIO,, -30 to 50 O C ; and TCNQ/aqueous pH 7 bulfer, 2.5 to 55 OC. For the PVF electrodes the interaction parameter extracted from the wave shape of the surface voitammograms decreased markedly at the lower temperatures. For both systems it Is suggested that the temperature dependence of the voitammetrlc response Is strongly influenced by solvent swelling of the polymer film.
The modification of electrode surfaces with electroactive polymeric materials, first demonstrated by several groups (1-3), continues to be an active research area ( 4 , 5 ) . Much effort has been spent on the development and characterization of new polymers for the modification process, on the theory and study of electrocatalysis using these electrodes, and on understanding the charge transport process in the polymer films. The authoritative review by Murray ( 4 ) summarizes recent progress in these and other areas related to chemically modified electrodes. An often fruitful strategy in the study of any chemical system is the variation of temperature over a wide range in order to extract activation or thermodynamic parameters. In many cases it is possible to alter rates of specific steps in complex systems such that an experiment is brought into a time-temperature domain where simplifying behavior is observed. This is especially true in organic electrochemistry (64,but to date few examples have been reported where polymer film modified electrodes were studied over a wide temperature range. A notable exception is the study of Daum et al. (9) on the low-temperature voltammetry of poly(vinylferrocene) (PVF)f i s in contact with butyronitrile solvent. The temperature dependence of linear-sweep voltammetry of electroactive polymer films is considered below. Existing theory in the literature is employed in the analysis, which is applied to the well-studied PVF electrode and to tetracyanoquinodimethane (TCNQ) polyester films. Owing to the well-known equivalence of surface and thin-layer electrochemical processes (lo),the data analysis methods developed below are applicable to reversible thin-layer and surface electrode processes, as well as the polymer film experiment. EXPERIMENTAL SECTION Chemicals. The (TCNQ), polymer used in this study was synthesized by treatment of the 2,5-bis(2-hydroxyethoxy)-
Present address: Department of Physical Chemistry and Radiology, L. Eotvos University, Budapest, Hungary. 0003-2700/85/0357-1117$01.50/0
7,7,8,8-tetracyanoquinodimethane monomer (11)with stoichiometric amounts of adipoyl chloride in N,N-dimethylacetamide as described previously (12,13). The PVF polymer was prepared by the method of Smith et al. (14).Analytical grade recrystallized NaH2P04and NaOH (Reanal) were used for preparation of the pH 7 buffer solutions. Spectrograde acetonitrile (Aldrich) and recrystaUzed (C2H,)4NC104(0.1 M) were used for the PVF studies. Purified tetrahydrofuran (THF) and distilled CH2C12were used for casting films of (TCNQ), and PVF, respectively. All solutions were purged with oxygen-free nitrogen or argon before use, and an inert gas blanket was maintained throughout the cyclic voltammetric experiments. Film Preparation. The dip and evaporation coating procedures for polymer film preparation have been used as described previously (12). Films of different thicknesses (20-200 nm) were obtained by variation of the concentration and immersion time in (TCNQ),/THF or PVF/CH2C12solutions. For optical measurements on (TCNQ), films, the substrates were Pt or Au transparent f i h s on quartz disks. The (TCNQ), films were baked at 80 OC for 5 min before use. The PVF films were dip-coated on a Beckman Pt disk electrode (geometrical area, 0.21 cm2)and baked at approximately 120 OC for several minutes. Film thickness was measured on dry films before electrolysis using an Alpha step profiler and optically flat Pt or Au on quartz substrates as described previously (15). The resulting total charge (QT)vs. thickness plots, which were approximately linear in the range of 0 to 100 nm, were used to estimate apparent film thickness on the disk electrodes. Apparatus. The cells used have been described (12,16,17). The reference electrode for the (TCNQ), study was a saturated sodium calomel (SCE) or a 0.5 M Ag/Ag2S04electrode using a Luggin capillary. Two different reference electrodes were used to check whether or not the temperature dependence of the reference electrode potential affected the features of the cyclic voltammograms. For the same purpose, experiments were carried out with thermostated and nonthermostated (i.e., room temperature) reference electrodes. Neither the peak potentials nor the features of the cyclic voltammograms varied significantly using different reference electrodes after correcting for AEref= 410 mV and a small difference (10 mV maximum) between the thermostated and room-temperature reference electrodes. The reference electrode for the PVF study was a Ag/AgCl quasi-reference electrode immersed directly into the acetonitrile solution. Simultaneous spectrophotometry and cyclic voltammetry were carried out with a Teflon cell placed in the sample compartment of a Specord (Zeiss) recording spectrometer. The reference beam was attenuated by a window of the same size as that of the cell and a Pt or Au uncoated transparent electrode. The working electrode area in the spectroelectrochemical measurements was 0.785 cm2,while in the usual three-compartment cell, Pt electrodes of 1cm2or 2 cm2area were used. Platinum electrodes were used as counter electrodes. A Tacussel potentiostat and an Electroflex GMK (Szeged) potentiostat with ohmic drop compensation in combination with an Electroflex (Szeged) sweep generator and Hewlett-Packard 7046B X-Y recorder were used for the (TCNQ), studies. The ohmic drop compensation was accomplished using the interruption technique which has been found appropriate in cases where the resistance varies with time (18,19). Interestingly, it was found that the use of the interruption technique greatly improved the reproducibility of the peak currents on successive sweeps in multicycle voltammograms. A BAS 100 electroanalyzer was used for the PVF study. Temperature was measured with a thermocouple using a digital thermometer (Fluke Model No. 2100A). 63 1985 American Chemical Soclety
1118
ANALYTICAL CHEMISTRY, VOL. 57, NO. 0, MAY 1985
RESULTS AND DISCUSSION Temperature Dependence of Voltammetric Surface Waves. Before presenting experimental results for poly(vinylferrocene) and TCNQ polyester film electrodes, we briefly consider the effect of temperature on reversible surface voltammetric processes. We employ the theory for species confined to surfaces or thin layers of solution that has been developed by several authors and reviewed recently by Laviron (20). The treatment of Brown and Anson (21) has been followed in the data analyses presented below. These authors introduced an “interaction parameter”, r, which can be extracted from the shape of the voltammetric surface wave. In the treatment of Brown and Anson, waves that are too fat, i.e., AElI2 > 3.53RT/nF, arise from repulsive interactions between electroactive surface species that destabilize reactants and are characterized by negative values of r. (In general interaction parameters can be assigned to both Ox and R; in the equations below, an average value is employed.) The term AE1l2is the peak width a t i = iPl2for a Symmetrical “surface wave”. For the process [Ox + ne-
* Rlsurface
(1)
the current is given by eq 8 of ref 21
n2FArTUf(1 - f ) i = RT(1 - fI’T2r( 1 - f ) )
(2)
where A is the electrode area, r T is the total surface concentration of Ox and R, u is the sweep rate, f is the fraction electrolyzed, r is the interaction parameter, and the other terms have their usual significance. For a symmetrical Nernstian wave f = 0.5 a t the peak current, (i,) and eq 2 becomes
zp =
n2FArTU R T ( 4 - 2rTr)
(3)
This equation for the peak current is used below to interpret the temeprature dependence of the polymer film electrochemistry experiment where deviations from ideal behavior occur. For r = 0, eq 3 predicts that the peak current at constant sweep rate will decrease with increasing temperature for a given value of rT.This is opposite to the usual situation for diffusion controlled processes where the temperature dependence of the diffusion coefficient dominates the temperature dependence of the peak current. A negative or approximately zero temperature coefficient of the peak current accordingly is a sensitive indicator that the experiment is being performed in the thin-layer or surface time domain. The temperature dependence of the peak current predicted by eq 3 is complicated by the possible temperature dependence of the interaction parameter. However for a given surface wave at a given temperature, r can be obtained from the shape of the wave. In order to demonstrate the data analysis procedure, we cast eq 3 in a more convenient form by noting the equivalence of the surface voltammetry and uniform thin-layer electrochemistry experiment under conditions where concentration gradients are negligible. Here we have QT = n F A r T = nFACTd (4) where QT is the total charge consumed, CT is the thin-layer or thin-film concentration, and d is the film thickness. Substitution into eq 3 and rearrangement give
nF i, = -
QTU
RT ( 4 - 2 r T r )
(5)
Since the terms in eq 5 are readily derived from an experi-
Table I. Peak Width at i = i,/2 for a Surface Voltammetric Wave as a Function of rTr
rTr -2.0 -1.6 -1.4 -1.2 -0.8 -0.6 -0.4
A.EI12nFJRT 7.850 6.980 6.544 6.110 5.242 4.810 4.380
rTr -0.2
0.0 0.2 0.4 0.6 0.8 1.0
AEII,nFJRT 3.952 3.524 3.104 2.686 2.274 1.872 1.480
mental voltammogram without knowledge of the absolute amount of electroactive material on the electrode, the electrode area, or the film thickness, this form of the peak current equation has some diagnostic utility. Variation in the value of n over a temperature range or the influence of mass transfer effects would be reflected in a non-Nernstian temperature coefficient (i.e., # F I R ) and lack of agreement with eq 5. The temperature dependence of the interaction parameter can dominate the T1 term in eq 5, or in the equivalent eq 3, and govern whether i, will increase or decrease with temperature. This is the situation described below for the behavior of poly(vinylferrocene) f i electrodes in contact with acetonitrile solution. It is worth noting that the only term in the above peak current expression that will be affected by solvent swelling of the polymer film is the interaction parameter. When r = 0, this model predicts that the peak current will not be influenced by polymer film swelling. Furthermore, if repulsive interactions exist between electron transfer sites or species in a polymer film, one intuitively expects that solvent swelling would increase the distance between these centers and thereby increase the value of r (Le., make r less negative). Thus in this case, solvent swelling would result in increased peak currents and narrower peak widths. Determination of the Interaction Parameter. The values of r were determined from the peak width, AElI2, of the surface waves a t i = i,I2. This term is strongly dependent on the value of the interaction parameter; fat waves give negative and thin waves positive values for r. Others have fit the entire voltammetric curve to calculated voltammograms in order to obtain an estimate of the interaction parameter. The method adopted here, which uses the readily measured value of AEl12,has the advantage of convenience and would provide an initial estimate of r in more refined analyses. Substitution of i = iPl2at f = flI2 into eq 2 and combination with eq 3 lead by simple algebra to
fl12 =
f f $1
-
;)lI2
where B = 8 - 2rTr and fl12 is the fraction oxidized at i = iPl2. With the values of flI2 in hand, nFAEl12/RTcan be calculated for given values of r T r using the form of the Nernst equation due to Brown and Anson (21)
where E = EPl2a t i = ip12and E, is equated to Eo. The calculated values of nFAElI2/.RT are given in Table I. An equivalent analysis has been given previously by Smith et al. (22) who present the data of Table I (at 25 O C ) in graphical form. Experimental values of rTr can be obtained graphically from a working curve of the data in Table I and refined by iteration using eq 6 and 7 and the expression for B. Equations 6 and 7 relate to an oxidation process; extension to reduction is straightforward.
ANALYTICAL CHEMISC3Y, VOL. 57, NO. 6, MAY 1985
1119
Table 11. Low-Temperature Voltammetric Parameters of a Poly(viny1ferrocene) Film in Contact with CH&N, 0.1 M (C2H5)4NC104 Solution
(ilQu) X
(4 - 2rTr)/V
T/T
0
IC
21.0 -3.8 -19.3 -30.8 -17.6 -19.3 -12.3 -0.5 21.3 49.5
mV - ")I -40 -43.5 -50.2 -55 -51 -50.8 -48.8 -45.2 -38.8 -42
rTr
exptl
theory
0.174 -0.104 -0.500 -0.812 -0.517 -0.522 -0.382 -0.153 0.223 0.239
37.3 37.8 41.0 43.3 41.3 41.6 41.6 38.6 35.0 37.7
39.5 43.1 45.7 47.9 45.4 45.7 44.5 42.6 39.4 36.0
/
EIUOLTI
/
Flgwe I. Cyclic voltammograms of poly(v1nylfenocene)in contact with 0.1 M (CpH5),NCIO4,CH,CN: sweep rate, 0.02 VIS; (A) -0.5 "C,(B) -30.8 "C;E vs. AglAgCl quasi-reference electrode.
Low-Temperature Voltammetry of Poly(viny1ferrocene). The above analysis was developed in order to aid in the interpretation of the effect of temperature on the cyclic voltammetry of TCNQ polyester films (see below). However, in order to demonstrate the applicability of the theory to a simplier test case, voltammograms of the wellstudied material poly(viny1ferrocene) (PVF) were obtained over the temperature range of -30 to 50 "C. The PVF was studied as thin films cast on a platinum substrate in contact with 0.1 M (C2H5)4NC104/CH&Nsolution. Murray and co-workers have previously studied PVF electrochemistry a t low temperature in butyronitrile solvent (9) under conditions such that diffusion control was observed for the voltammetric waves. The peak currents reported were considerably attenuated at low temperature and these workers were able to extract activation energies for diffusion from their data. The results described below are entirely consistent with those of ref 9, but pertain to thinner PVF films. In order to test the above theory we chose conditions, i.e., very thin films (less than 10 nm) and slow sweep rates, such that the time frame of the experiment w a in the "thin-layer time domain". Figure 1shows representative voltammograms of a thin PVF film (rT = 1.3 X mol cm-2, where the geometric area of the platinum substrate was assumed) at -0.5 and -30.8 "C. Close to surface wave behavior is indicated by the symmetrical wave shapes and the lack of a diffusional tail even at -30 "C and a sweep rate of 50 mV/s. Some asymmetry is evident between the anodic and cathodic segments which could be due to slow charge transfer processes or, more likely, uncompensated iR drop in the electrochemical cell. For all voltammograms the first anodic segment had a larger peak current than the second, although the cathodic peak currents were approximately equal on the first and second cycles at a given temperature. The symmetrical wave shapes are an indication that the theory of Brown and Anson (21) can be employed using an average value of the interaction parameter. Data are presented in Table I1 for the temperature range of 21 to -30 O C . At least two voltammograms were obtained at each temperature; peak widths, Epiz- Ep,were reproducible to within fl mV. At higher temperatures (30-50 "C)irreversible changes were noted in the voltammetric response. The total charge under the waves decreased on successive cycles and diffusion-like wave shapes developed. Deactivation of the oxidized PVF film in contact with solvent/electrolyte
3.0
3.5
I
4 .O
T-' 8 IO' / IC'
Flgure 2. Plot of peak current function for a surface voltammetric wave vs. 1/T for PVF in contact with 0.1 M (C,H5),NCi04/CH3CN: points, experimental; solid line, theory according to eq 5.
is suspected at the higher temperatures, but this phenomenon was not studied further. For the lower temperature range, however, the peak current at constant sweep rate decreased slightly, and the peak width increased with decreasing temperature. The close agreement with eq 5, or the equivalent eq 3, shown in Figure 2 indicates that the experiment was indeed carried out in the thin-layer time domain and that the PVF electrode process did not vary over this temperature range. While incorporation of the experimentally determined r value into eq 5 gives excellent agreement with the slope in Figure 2, exact agreement with the peak current is not obtained. The experimental values are about 10% low in the temperature range, -30 to 21 "C. This discrepancy corresponds to an apparent n value in eq 5 of 0.91. The most likely explanation for this result is that the peak currents are depressed due to the uncompensated iR effects noted above, although it is also possible that the measured Q values were not properly corrected for background charging or faradaic processes due to impurities. It is more difficult to explain the significant temperature dependence of the interaction parameter. The decrease of r with temperature is believed to be real and not due to an experimental artifact, although an increase in the iR drop at low temperature would lead to negative r values. The latter is unlikely since the peak separation, Ep*-E
