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Electrodes modified with Nafion films incorporating amphiphilic ferrocene derivatives
79
J. Electroanal. Chem., 279 (1990) 79-94 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Electrodes modified with Nafion films incorporating amphiphilic ferrocene derivatives Orlando Garcia and Angel E. Kaifer * Department (Received
of Chemistry, 31 March
University of Miami, Coral Gables, FL 33124 (U.S.A.)
1989; in revised form 2 October
1989)
ABSTRACT
The electrochemical behavior of electrodes modified with Nafion@ films containing several ferrocene derivatives was investigated by cyclic voltammetry. The derivatives used were alkyldimethyl (methylferrocene) ammonium salts, where the alkyl group was methyl (I ), heptyl (2) or hexadecyl (3). The rate of charge propagation across the Nafion film was found to decrease as the length of the alkyl chain increased. This finding was confirmed by rotating disk electrode experiments in which the Nafion-bound ferrocene derivatives mediated the oxidation of ferrocyanide ions in solution. On the other hand, the longer chain derivatives 2 and 3 were retained inside Nafion films more strongly (or for longer times than the methyl derivative I. Therefore, 2 and 3 are suited for use as electrochemical probes to monitor microenvironmental changes in the polyelectrolyte matrix resulting from the incorporation of other species. This was demonstrated using 3 as the probe and tetrabutylammonium ions as microenvironment modifiers.
INTRODUCTION
Nafion ** is a perfluorinated anionic polyelectrolyte that provides an ionically conductive matrix into which many electroactive cations can be incorporated. Electrode surfaces can be readily modified with cast Nafion films that become electroactive upon exposure to solutions of electroactive cations. These electrodes have been investigated extensively [l-20] because the modification procedure is quite simple and provides substantial versatility regarding the types of redox species that can be concentrated in the vicinity of the electrode surface.
* To whom correspondence should be addressed. ** Nafion is a trademark registered by E.I. DuPont
0022-0728/90/$03.50
0 1990 Elsevier Sequoia
de Nemours,
S.A.
Inc.
80
Nafion films have remarkable selectivity properties for cations of rather hydrophobic nature. Martin and co-workers [15] have performed extensive measurements of selectivity coefficients and reported values as high as lo6 for hydrophobic cations (using Nat as the reference). It is generally accepted that a Nafion matrix consists of segregated phases with hydrophobic pockets, formed by the fluorocarbon backbone chains, and hydrophilic regions where the anionic sulfonic sites group together [21]. Common hydrophobic electroactive cations, such as M(bpy):+ (where M = Co, Fe, OS, or Ru and bpy = 2,2’-bipyridine) and methylviologen, are retained strongly by Nafion films but the rates of charge propagation across the film are typically sluggish. However, more hydrophilic cations, such as Ru(NH,)z+ , are more loosely bound to the Nafion matrix while exhibiting faster rates of charge propagation [18]. In this paper, we report results obtained with electrodes modified with Nafion films containing the amphiphilic redox-active materials 1, 2 and 3.
2, n = I,
x- = Br-
3, n = 16, X. = Br-
The results show that these electroactive materials can be incorporated inside Nafion films to which they confer lasting electroactive properties. As expected due to their substantial hydrophobic character, these redox substrates give rise to slow charge propagation rates through Nafion films. We demonstrate here that an increase in the hydrophobic character of the electroactive cation, as measured by the length of the covalently attached hydrocarbon chain, results in films having lower hydration levels and, hence, slower rates of charge propagation. However, the lasting stability of the incorporation into Nafion of the bulkier and more hydrophobic cations 2 and 3 enables the assessment of the effects that other species may exert on the environment sensed by these cations inside the polyelectrolyte matrix. EXPERIMENTAL
Materials Compounds 2 and 3 were synthesized according to a published procedure [22] by treatment of dimethylaminomethylferrocene (Aldrich) with bromoheptane and bromohexadecane, respectively. The crude products were recrystallized from acetone/ether. Their structure and purity were verified by ‘H-NMR spectroscopy and elemental analyses. The methyl analog 1 was prepared as the iodide salt by the same procedure utilizing methyliodide as the alkylating agent. Hydrogen ion-form Nafion (Unit molar mass (EW = 1100 g) was obtained from Aldrich as a 5 wt% solution in lower aliphatic alcohols and water. All other reagents were of the best commercial quality available.
81
All solutions were prepared with distilled water passed Barnstead Nanopure water purification system.
through
a four-cartridge,
Equipment The electrochemical instrumentation has been already described [23]. Rotating disk voltammetric experiments were performed with a Pine rotator assembly using gold disk electrodes also supplied by the Pine company. Procedures Modification of the electrode surface was performed by casting the Nafion film in the usual manner [8,9,11-141. Electroactive materials were incorporated in the Nafion solution before casting the film. Usually, l-2 mg of electroactive material was dissolved in a volume of 5 wt% Nafion solution calculated so that 20% of the available sulfonic sites were compensated by the cationic electroactive material. This loading level (the concentration of electroactive probe inside the cast films is estimated at 0.28 M based on the Nafion unit molar mass and its reported wet density [24]) was kept constant throughout our experiments. A O.lO-ml aliquot of this solution was then diluted with 1.90 ml of ethanol to yield the solutions that were actually used to cast the Nafion films unless specified otherwise. A small volume of the diluted solution (7.0 ~1) was deposited with a syringe on the tip of a Au electrode (0.196 cm2) while rotating it simultaneously. After evaporation of the solvent for at least 15 min under continuous spinning (500 rpm), the electrode was equilibrated in the supporting electrolyte solution (0.2 M Na *SO,) for 30 min prior to any electrochemical analysis. We have made no attempt to measure the thickness of the resulting Nafion films. Estimates based on the wet density of 1100 EW Na+-form Nafion (1.58 g/cm3 [24]) yield a thickness of about 500 nm; however, the actual thickness in solution is probably larger due to the presence of the rather bulky electroactive cations. Electrodes prepared following this general procedure will be referred to as Au/Nafion(x), where x stands for the particular electroactive material incorporated in the Nafion film. The surface coverage of these electrodes is nominally calculated as 8.1 x lo-’ molar units of Nafion per cm2. Since only 20% of the polyelectrolyte sulfonic sites were initially occupied by the electroactive ferrocene derivatives, their surface coverage is estimated to be 1.6 X 10K8 mol/cm2. All potentials were measured and reported using sodium chloride saturated calomel electrodes (SSCE) as the reference electrode. RESULTS
AND DISCUSSION
A typical cyclic voltammogram obtained with a Au/Nafion(l) electrode is given in Fig. 1A. It shows the expected reversible oxidation behavior characteristic of ferrocene derivatives. The observed peak potentials are 0.34 and 0.31 V; the small potential difference between both peaks is in excellent correspondence with the general shape of the voltammogram which suggests clearly that the electrode surface
82
,
da
0
POTENTIAL/
V(‘JS
SSCE
C
I33 I lTENTIAL/
V (vs
SSCE 1
*PA
I
La8
POTENTIAL
/
V (vs
SSCE 1
Fig. 1. Cyclic voltammograms in 0.2 M Na,SO, of Au electrodes modified with films of Nafion containing (A) 1, (B) 2, and (C) 3. All the Nafion films contained the same concentration of ferrocene derivative initially. The voltammograms were recorded at 10 mV/s after equilibration in the supporting electrolyte solution for 30 min.
addresses all the accessible redox sites contained in the Nafion film during the time scale of the experiment. This is as expected due to the rather low thickness (ca. 500 nm) estimated for the Nafion film. Thus, the observed voltammetric behavior is similar to that resulting from a ferrocene derivative confined at the electrode surface. A similar experiment performed with a Au/Nafion(Z) electrode yields the cyclic voltammogram of Fig. 1B. In this case, the peak potentials are clearly more separated (0.35 and 0.28 V) and the shape of the voltammogram suggests that the redox sites are not immediately addressable by the electrode surface as they were in the Au/Nafion(l) electrode. In other words, charge propagates more slowly across the Nafion film containing 2 than it does across the Nafion film containing an initially equivalent concentration of 1 and, thus, the voltammetric behavior of the electrode modified with the heptyl derivative is essentially diffusion-controlled. The cyclic voltammogram of Fig. lC, obtained with a Au/Nafion(3) working electrode, clearly suggests diffusion control of the electrochemical process in the film. In this case, the peak potentials (0.40 and 0.27 V) are separated by 130 mV
83
probably indicating that the polyelectrolyte film has a substantially larger ionic resistance than the Nafion(2) film. All three voltammograms in Fig. 1 were obtained using electrodes modified with identical surface coverages of Nafion and electroactive cation. Since the hydrophobicity of the ferrocene derivatives varies as a function of the length of the alkyl tail, different levels of hydration may be present in these films. In principle, the degree of film hydration is expected to decrease as the length of the alkyl chain increases. This factor, as well as the varying molecular volumes of the three ferrocene derivatives, may cause differences in the actual thickness of the films. However, these differences cannot be very large because only 20% of the exchanging capacity of the polyelectrolyte is actually used to bind the ferrocene derivatives; most of the sulfonic sites (80%) remain in the Na+ form. Therefore, the different electrochemical responses can hardly be ascribed to changes in the film thickness. More likely, the changes in electrochemical behavior recorded in Fig. 1, reflect different rates of charge propagation across the Nafion films. Experiments to assess the stability and durability of the electrochemical response of Au/Nafion(l-3) electrodes were also performed. In these experiments, an electrode was modified as described in the experimental section and immersed in a quiescent supporting electrolyte solution for 30 min. At this point a voltammogram was recorded using a scan rate slow enough to insure complete oxidation across the entire film thickness. Then, the electrode was rotated at 500 rpm for 10 min and another voltammogram was recorded at the same scan rate with the electrode and the solution completely stationary. These steps were repeated at least for 1 h. The rotation of the electrode enhances the rate of mass transfer from the solution to the electrode, thus increasing the leaving rate of any species that may not be strongly retained inside the film. Plots of the surface coverage determined from the charge consumed in the voltammetric oxidation of the ferrocene derivative as a function of time provide information on the stability and durability of the electrochemical response of the Nafion films incorporating derivatives 1, 2 and 3. The data are shown in Fig. 2 and demonstrate a very interesting trend. The electroactive surface coverage for Au/Nafion(l) electrodes decreases rather quickly as a function of time. After the initial 30 min of electrode equilibration in a quiescent solution, the electrode exhibits a coverage of 6.2 nmol/cm’ which corresponds to about 40% of the nominal ferrocene coverage as calculated from the casting data. Lower than nominal coverage values for electroactive cations in Nafion films have been found experimentally before [9]. They can be rationalized by postulating the existence of environments within the Nafion film in which the electroactive cations are rendered inert and prevented from undergoing oxidation/reduction. In this case, owing to the rapid decrease of coverages as a function of time which is observed during the first 30 min of electrode rotation (see Fig. 2) it seems reasonable to assume that a fraction of the ferrocene cations which are not detected by voltammetry may have been lost from the film during the equilibration period. That is, not all the ferrocene sites which are not detected electrochemically are assumed to occupy environments which render them electrochemically inactive. The data obtained with Au/Nafion(2) electrodes show a stable ferrocene surface
84
lo-
I
5-A
2-A UETHYL
n HEPTYL
l--
o-
: 0
20
40
80
IO
TIME
/
100
120
140
min
Fig. 2. Measured surface coverage for Nafion-bound ferrocene derivatives 1 and 2 as a function of time. Voltammograms for derivatives 1 and 2 were recorded at 10 and 2 mV/s, respectively, to insure complete oxidation of the film in each cycle. The electrode was continuously rotated at 500 rpm except when voltammograms were being recorded. All electrodes were initially equilibrated for 30 min in a quiescent 0.2 M Na,SO, solution before starting any electrochemical cycling.
coverage. Thus, the initial coverage of 8.9 nmol/cm2 remains essentially unaltered after more than 2 h of electrode rotation. This coverage value represents 55% of the nominal surface coverage as determined from the film casting data. This value is in good agreement with data reported by Martin et al on the fraction of incorporated Bu(bpy) : + sites that were detected electrochemically in 970 g EW Nafion films cast on glassy carbon electrodes [9]. It is also noteworthy that a very slow scan rate of 2 mV/ s was needed to insure the complete electrochemical oxidation of the Nafion(2) film as evidenced by the recording of surface-confined voltammetric responses. Films containing similar loading levels of the methyl derivative 1 shows surface-confined responses at much faster scan rates (2-200 mV/s). By contrast, clear surface-confined voltammetric behaviour is not observed with Au/Nafion(3) electrodes having similar film thickness, even at scan rates as low as 1 mV/s. Because of this we did not include surface coverage data for Nafion(3) films on Fig. 2. However, the hexadecyl derivative seems to be retained strongly inside the polyelectrolyte matrix because the observed voltammetric response remains essentially unaltered during 2 h or longer periods of time. Therefore, our data clearly suggest
85
that ferrocene derivatives 2 and 3 are retained strongly inside Nafion films cast on Au electrodes under the same experimental conditions in which the methyl derivative 1 leaves the film at a measurable rate. The scan rates necessary to insure the complete oxidation of Nafion films containing ferrocene derivatives 1, 2, and 3 become increasingly slower as the length of the derivative’s alkyl chains increases. This is probably due to a decreasing rate of charge propagation across the Nafion film resulting from the more lipophilic character of the electroactive cation. Increasing the length of the alkyl chain also results in better retention of the electroactive probe inside the Nafion layer. Charge propagation across Nafion films has been the subject of many studies [5,6,8,9,25,26]. Two mechanisms can be responsible for the propagation of an electrochemical conversion inside the polyelectrolyte matrix: (i) actual diffusion of the electroactive probes or (ii) electron hopping among neighboring probes. The level of hydration of the Nafion film may also seriously influence the rate of charge propagation since it affects the movement of ions inside the film that are related to both mechanisms. Many research efforts have addressed the relative importance of these two mechanisms in Nafion. Most results indicate that the overall diffusional behavior depends on the nature of the electroactive species involved. This is indeed expected in terms of the Dahms-Ruff model [27,28] in which the electron hopping rate depends on the magnitude of the electron self-exchange constant for the redox couple in question. Thus, generally speaking, the diffusional behavior of electroactive species inside polymer films is expected to be influenced by electron hopping if the redox couple has a large electron self-exchange rate. Conversely, the behavior of electroactive redox species having relatively slow electron-self exchange should be dominated by its actual diffusion (molecular motion) inside the polyelectrolyte matrix. Ferrocene-containing species are predicted to exhibit diffusional behavior because of the moderate rate for electron self-exchange (1.6 X lo6 M-i s-‘) shown by the ferrocene moiety [29]. Our data lends support to this view due to the effects observed upon lengthening the alkyl chain of the ferrocene derivative. In the modified electrodes described here the Nafion layer always contains the same initial concentration (loading level) of electroactive probe. Therefore, if the electron hopping mechanism were to predominate, one would expect to see approximately the same rate of charge propagation regardless of the length of the alkyl tail in the ferrocene derivative. Our results demonstrate a strong dependence on the length of the akyl chain suggesting that diffusion of the ferrocene derivatives, rather than electron hopping, is the predominant charge propagation mechanism in these Nafion films. However, since the lipophilicity of the ferrocene derivative increases with the length of the pending alkyl tail, the level of hydration of the Nafion films probably decreases throughout the series l-3. As it has been indicated before, a decreased level of hydration will also tend to decrease the rate of charge propagation across the film owing to the hindrance imposed on ionic movements within the polyelectrolyte matrix. The apparent diffusion coefficients of the ferrocene derivatives can be determined from plots of anodic peak current vs. the square root of the scan rate.
86
0
10
5 V
l/2 ,m"l/2
IS
5-1/2
Fig. 3. Anodic peak current vs. square root of scan rate plots for the methyl (I), heptyl(2) and hexadecyl (3) ferrocene derivatives in thick Nafion films on Au. The contacting solution was 0.2 M Na,SO,.
These data were obtained using Nafion films 20 times thicker than in all other experiments to insure that the diffusion layer was confined within the polyelectrolyte matrix. All derivatives give rise to linear plots for scan rates ranging from 20 to 200 mV/s (see Fig. 3). From the slope of these plots, Da values of 1.4 X lo-“, 5.9 x lo-l2 and 1.2 X lo-l2 cm2/s were determined for 1, 2 and 3, respectively. Thus D, values decrease as the length of the alkyl chain increases, reflecting the slower motion inside the Nafion layer of the bulkier cations. This parallels the trend observed with the diffusion coefficients of compounds l-3 in solution [30] *. The diffusion coefficient of 1 in 970 g EW Nafion was measured by White et al. in a variety of experimental conditions [B]. They report values in the range l-3 x lo-” cm2/s that are about an order of magnitude higher than our Do value for the same compound. However, this disagreement is not particularly surprising if one considers that these authors utilized Nafion of lower EW (970 g) which is expected to hydrate more extensively than the 1100 EW Nafion used in our experiments.
* Diffusion coefficients of 1 and 2 in aqueous solution have been accurately determined from Levich plots as 4.8X10W6 and 4.1 X 10e6 cm’/s, respectively. The diffusion coefficient of 3 could not be obtained because its electrochemical behavior is not diffusion-controlled due to micellization and adsorption effects.
From our diffusion coefficient data we conclude that the observed rates of charge propagation across Nafion films containing ferrocene derivatives 1 to 3 are essentially controlled by the actual diffusion of the electroactive cations within the polyelectrolyte. The apparent diffusion coefficient is indeed very sensitive to the length of the attached alkyl chain. As predicted on the basis of size considerations and the relative hydrophobic nature of 1-3, the observed D, values decrease as the length of the chain increases. Other factors may also be partially responsible for the observed trends. For instance, it is conceivable that an increase in the hydrophobic nature of the electroactive probe might decrease the overall content of water in the Nafion films. A lower degree of film hydration would hamper the movement of ions across the Nafion matrix resulting in slower charge propagation rates. The effects of film dehydration are specifically addressed later in this work. The oxidation of Fe(CN)zmediated by Nafion (2 or 3) films One of the main ideas behind the development of modified electrodes is to accomplish the derivatization of the electrode surface with redox active species that can mediate (or, even better, catalyze) the oxidation/reduction of solution species [31,32]. Therefore, we addressed the mediation properties of Nafion electrodes incorporating 2 and 3, the more hydrophobic derivatives in our series. We did not attempt mediation experiments with Nafion(1) films since the methyl derivative is not strongly retained by the polyelectrolyte matrix under the conditions of these experiments (see Fig. 2). Therefore, we decided to investigate the mediated oxidation of ferrocyanide by the Nafion-bound heptyl and hexadecyl ferrocene derivatives. The overall process can be simply represented by the following equation Cp,Fe+
+ Fe(CN)z-
+ Cp,Fe
+ Fe(CN$
(1)
where Cp,Fe represents any of the ferrocene derivatives used in this work. The reaction is thermodynamically downhill because the oxidation potential of ferrocyanide is lower than those of the ferrocene derivatives. The selection of this reaction also responds to the known fact that the negatively charged ferrocyanide ion cannot permeate the Nafion film due to electrostatic repulsive forces [12]. This prevents the direct electrochemical reaction of ferrocyanide at the electrode surface and insures that any ferrocyanide oxidation must be mediated by the Nafion-bound ferrocene derivative. The mediated oxidation reaction is quickly verified by the shape of the voltammograms in Fig. 4. The cyclic voltammogram of Fig. 4A corresponds to a Au/ Nafion(3) electrode immersed in 0.2 M Na,SO,. The only salient feature of the voltammogram is the reversible oxidation of 3 as discussed before. When 1.0 mM Fe(CN)iis present in the contacting solution the oxidation peak is clearly enhanced (see Fig. 4B) while the reduction peak is substantially decreased in comparison to the response obtained in the absence of Fe(CN)z-. These changes are exactly as predicted if the process given by eqn. (1) is taking place. When the same experiment is carried out with the electrode rotating at 500 rpm, the voltammogram of Fig. 4C is obtained. Again, the sigmoidal shape of this voltammogram
C
B
A
1
I
POTENTIAL/h’
0
I
,
I
0.6
vs.SSCE)
Fig. 4. Cyclic voltammograms at 10 mV/s recorded with a Au/Nafion (3) electrode in (A) 0.2 M Na,SO,, (B) 0.2 M Na2S0,+1.0 mM K,Fe(CN),, (C) 0.2 M Na,SO,+ 1.0 mM K,Fe(CN),. Voltammogram (C) was recorded at 500 rpm while (A) and (B) were obtained at 0 ‘pm.
reveals that more ferrocyanide is being oxidized by the ferricinium form of the Nafion-bound 3 because the rotation of the electrode increases the rate of mass transport of Fe(CN)zions from the bulk solution to the polyelectrolyte-solution interface. Similar cyclic voltammograms were obtained with Au/Nafion(2) electrodes. Since the rotation rate of the electrode can be varied, the rate of transport of Fe(CN)iions to the Nafion-solution interface can be controlled to demand variable electron transport rates from the polymer film. This affords another method to estimate charge propagation rates across the Nafion layers. If the rotation rate can be increased to a level at which the Nafion film can no longer support the oxidation of all the Fe(CN)zions reaching the interface, then one would expect to observe invariant currents for rotation rates equal or higher than this threshold value. This is exactly what we observed (see Fig. 5) in mediation experiments using Nafion films containing either 2 or 3. The plots have a slow rotation region in which the anodic current is limited by Fe(CN)zmass transfer from the solution to the interface. But the current levels off at higher rotation rates because the oxidation of ferrocyanide is now limited by charge transport across the Nafion layer. In good agreement with our previous results, the onset of the current plateau takes place at slower rotation rates with Au/Nafion(3) than with Au/Nafion (2) electrodes. This indicates that faster charge transport rates can be established across the latter. Furthermore, the plateau current for the Nafion layer containing the hexadecyl derivative is lower than that obtained for the Nafion layer containing the heptyl derivative (for the two Fe(CN)zconcentrations surveyed), again indicating faster charge propagation across the latter. The behaviour of polymer-coated rotating disk electrodes has been analyzed by Saveant and co-workers [33-351. In their model they propose several processes, other than mass transport in solution, that could become rate limiting in mediation experiments: (i) diffusion of the solution species in the film, (ii) charge propagation in the film via the mediator couple, and (iii) the redox reaction between the
89
50
0:
0
l0
10
40
i
a0
50
I ROTATION RATE )1” /
nk-“’ Fig. 5. Rotation rate dependence of the limiting anodic current (at 0.6 V vs SSCE) observed for the mediated oxidation of ferrocyanide on (A) Au/Nafion(Z) and (B) Au/Nafion(J) electrodes. (m) 1.0, (0) 0.3 mM K ,Fe(CN),.
of the substrate ion. To confirm this point further, we performed control experiments with Au/Nafion (without any ferrocene derivative inside the film) in ferrocyanide solutions and obtained the expected result, that is, the film prevents the oxidation of Fe(CN)%- in the potential window utilized in this work. The results in Fig. 5 show that, at high rotation rates, charge transport across the Nafion film becomes rate limiting. The reaction between the ferricinium form of the Nafion-bound mediator and Fe(CN)zseems to be fast and does not affect the overall rate in the range of rotation rates surveyed. For cases in which charge propagation through the film is the limiting process, Saveant and co-workers [34] have proposed that the characteristic current mediator
product
in the film is ruled
and the solution out owing
species. In our case, the diffusion
to the high negative
charge
of the ferrocyanide
90
i, would be given by i, = FAc;D,/D
(2)
where c,” is the mediator concentration in the film, D stands for the film thickness, is the effective diffusion coefficient for the diffusion-like propagation of electrons across the film, A is the electrode surface, and F is the Faraday constant. Using the average currents in the high rotation rate region, we obtained 9.2 x 10-l’ and 6.9 X lo-” cm2/s as D, values from eqn. (2) for the heptyl and the hexadecyl derivatives, respectively. These values are clearly larger than the diffusion coefficients determined from the voltammetric experiments (5.9 x lo-i2 and 1.2 x lo-i2 cm2/s, respectively). Perhaps an explanation for this difference can be found in the dissimilar film thicknesses employed in these two types of experiments. Thus, while the mediation experiments were carried out with thin Nafion films (D = 500 nm), the peak current vs. (scan rate) ‘/2 data points were obtained with films 20 times thicker. In very thin films the overall charge transport rate may be inordinately affected by interfacial phenomena (taking place either at the Au-Nafion or at the Nafion-solution interfaces). Alternatively, heterogeneous distribution of the ferrocene derivative in the Nafion film may also be responsible for these differences. Nonetheless, both sets of data indicate that charge transport across Nafion films containing the heptyl derivative is faster than with the hexadecyl derivative.
D,
Effects of the incorporation
of l-3 on the degree of hydration of Nafion films In order to assess the effects of the lipophilicity of the various ferrocene derivatives on the extent of hydration of the polyelectrolyte matrix we performed experiments in which a Au/Nafion(x) electrode was exposed to a solution containing 1.0 mM methylviologen (MV2’). This resulted in the exchanging of MV2+ into the film, a process which was allowed to proceed for 30 min. After the loading of methylviologen, the voltammetric behavior of the electrode was monitored in pure supporting electrolyte solution. During this last step, the amount of methylviologen detected electrochemically in the film decreased steadily as a function of time. The experiments with ferrocene derivative 1 were not very informative since both electroactive cations demonstrated a marked tendency to leave the Nafion film. However, experiments with Nafion(2) and Nafion(3) were more illustrative. Figure 6 shows a voltammogram showing the response of a Au/Nafion(3) electrode 5 min after transfer from the loading solution to a 0.2 M Na,SO,. At this point the observed coverage of MV 2+ is similar to that of 3 as measured from the integration of their respective waves. However, the peak potential separation of the ferrocene/ ferricinium couple is 96 mV while that of the MV2+/MV+ couple is only 20 mV. That is, both electroactive cations exhibit quite different electrochemical properties even though their apparent concentrations within the film are similar. Voltammograms recorded with the same electrode of Fig. 6 showed gradually decreasing waves for the MV2+/MV+ couple while those corresponding to the ferrocene derivative remained stable. The potential difference between the two peaks of the viologen couple was always about 20 mV whereas that of the ferrocene couple was always
91
1
-0.0
1
1
-0.6
-0.4
1
-0.2
POTENTIAL
/
0.0
I
0.2
0.4
0.6
V (vs. SSCE)
Fig. 6. Cyclic voltammogram (at 10 mV/s) recorded with a Au/Nafion(3) electrode in 0.2 M Na,SO,. The electrode had previously been immersed for 30 min in a solution containing 1.0 mM MV*+. The response shown was recorded 5 min after transferring the electrode from the MV*+ loading solution to the pure supporting electrolyte solution.
near 100 mV. Similar results were obtained with Nafion(2) electrodes. In this case, the observed peak potential differences were smaller (about 14 mV for the MV2+/ MV+ couple and 55 mV for the ferrocene couple). These data indicate clearly that the incorporation of hydrophobic ferrocene cations, such as 2 and 3, at loading levels equivalent to 20% of the stoichiometric exchanging capacity of the Nafion film does not cause an extensive dehydration of the film as evidenced by the sharp waves observed for methylviologen. This finding reinforces our contention that the charge propagation rates observed with the ferrocene derivatives are essentially controlled by their diffusional motion within the film. Nafion(3) films are seen to be slightly less hydrated than Nafion(2) films, as indicated by the larger MV’+/MV+ peak splitting observed in the former, but the difference in water content does not appear to be large enough to explain by itself the changes seen in the behavior of the ferrocene derivatives. Use of 2 and 3 as electrochemical probes to monitor microenvironmentaE changes in Nafon films This work demonstrates that amphiphilic ferrocene derivatives, such as 2 and 3, confer lasting electrochemical properties to Nafion films. Because the leaving rates of these electroactive probes are very slow, it is possible to use them as reporters of the microenvironmental changes that the incorporation of other cationic species may cause in Nafion. For instance, Martin and coworkers reported that tetrabutyl-
92
ammonium (TBA+) cations exchange readily into Nafion membranes eliciting a concomitant decrease in the water content inside the polyelectrolyte matrix [36]. Because of this report we have investigated the electrochemistry of several Nafionbound substrates in the presence of TBA+ ions in the contacting solution. Cyclic voltammetry with Nafion modified electrodes incorporating methylviologen or 1 shows that the presence of TBA+ ions (0.1-l mM) in the contacting solution disrupts notably the electrochemical behavior. Initially, a decrease in the peak currents and an increase in the potential difference between the reduction and oxidation peaks of the couple are observed. Then, the electroactivity essentially disappears ?nd only non-faradaic current responses can be recorded. These changes can certainly be interpreted as resulting from the partitioning and ion exchange of the TBA+ ion into the Nafion film. As indicated by Martin et al, the hydrophobic TBA+ ions decrease the degree of hydration of the Nafion film so that the electrochemistry in the resulting polyelectrolyte matrix becomes more difficult because the ion movements are hampered by the lack of water molecules. While this explains the observed changes in our experiments, it must be recognized that the TBA’ ions might be displacing the electroactive cations from the Nafion film and the observed changes may be also due to the loss of electroactive probes from the film. This problem can be addressed nicely using amphiphilic electroactive derivatives, such as 2 and 3, because these compounds are retained strongly inside Nafion membranes and can report on microenvironment changes while maintaining a stable concentration in the film. This idea is supported by the results shown in Fig. 7. The cyclic voltammogram in Fig. 7A corresponds again to a Au/Nafion(3) electrode in contact with a 0.2 M Na,SO, solution. Figure 7B shows the behavior of the same electrode 1 min after the addition of TBA+Br(0.5 mM) to the solution. The series of voltammograms in Figs. 7C to 7E demonstrate the quick changes in electrochemical behavior that the presence of TBA+ brings about. As previously stated, these changes reflect the decreasing water content of the Nafion film as the concentration of TBA+ increases. It is noteworthy that, after 15 min, essentially no faradaic response is detected with this electrode. In order to prove that this response is not due to the replacement of 3 by TBA+ ions, we transferred the electrode of Fig. 7E to pure 0.2 M Na,SO, solution and recorded voltammograms as a function of time. This experiment turned out to be quite long as expected from the high selectivity of Nafion for hydrophobic cations. However, after 18 h, we recorded the voltammogram of Fig. 7F, which exhibits again very clearly the oxidation and reduction peaks due to the ferrocene derivative. This proves that the voltammetric changes observed (Figs. 7B to 7E) in the presence of TBA+ ions take place while 3 remains inside the Nafion film. Therefore, the behavior recorded in Fig. 7E(for instance) does not result from depletion of 3 but from the film’s decreasing water content which, in turn, is the consequence of the exchange of TBA+ ions in the polyelectrolyte matrix. Hence, amphiphilic electroactive derivatives, such as 2 and 3, appear to be excellent electrochemical probes to monitor microenvironment changes inside Nafion films or similar systems.
93
I
0
1
I
I
0.6
I
0
I
0.6
Fig. 7. (A) Cyclic voltammogram (at 50 mV/s) recorded with a Au/Nafion(3) electrode in 0.2 M Na,SO,. (B) Same electrode 1 min after the addition of 0.5 mM TBA+ Br- to the contacting solution. (C) After 3 min. (D) After 5 min. (E) After 15 mm. (F) Same electrode after 18 h immersed in a TBA+-free, 0.2 M Na,SO, solution.
ACKNOWLEDGEMENTS
The authors are grateful to Jodi M. Schuette for performing some preliminary experiments. Financial support from the University of Miami Research Council is acknowledged. REFERENCES 1 2 3 4 5 6 7
D.A. Buttry and F.C. Anson, J. Electroanal. Chem., 130 (1981) 333. D.A. Buttry and F.C. Anson, J. Am. Chem. Sot., 104 (1982) 4824. H.Y. Liu and F.C. Anson, J. Electroanal. Chem., 158 (1983) 181. D.A. Buttry and F.C. Anson, J. Am. Chem. Sot., 106 (1984) 59. D.A. Buttry, J.-M. Saveant and F.C. Anson, J. Phys. Chem., 88 (1984) 3086. Y.-M. Tsou and F.C. Anson, J. Phys. Chem., 89 (1985) 3818. I. Rubinstein and A.J. Bard, J. Am. Chem. Sot., 102 (1980) 6641.
94 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
H.S. White, J. Leddy and A.J. Bard, J. Am. Chem. Sot., 104 (1982) 4811. C.R. Martin, I. Rubinstein and A.J. Bard, J. Am. Chem. Sot., 104 (1982) 4817. I. Rubinstein and A.J. Bard Anal. Chem., 53 (1981) 102. J.G. Gaudiello, P.K. Ghosh and A.J. Bard, J. Am. Chem. Sot., 107 (1985) 3027. M. Krishnan, X. Zhang and A.J. Bard, J. Am. Chem. Sot., 106 (1984) 7371. J. Leddy and A.J. Bard, J. Electroanal. Chem., 189 (1985) 203. A.E. Kaifer and A.J. Bard, J. Phys. Chem., 90 (1986) 868. M.N. Szentirmay and C.R. Martin, Anal. Chem., 56 (1984) 1898. CR. Martin, Anal. Chem., 54 (1982) 1639. R.B. Moore III and C.R. Martin, Anal. Chem., 58 (1986) 2569. C.R. Martin and K.A. Dollard, J. Electroanal. Chem., 159 (1983) 127. I. Rubinstejn, J. Electroanal. Chem., 188 (1985) 277. C.M. Lieber, M.H. Schmidt and N.S. Lewis, J. Phys. Chem., 90 (1986) 1002. A. Eisenberg and Y.L. Yeager (Eds.), Perfluoro Sulfonate Ionomer Membranes, ACS Symposium Series 180, American Chemical Society, Washington, DC, 1982. A. Lombard0 and T.I. Bieber, J. Chem. Ed., 60 (1983) 1080. P.A. Quintela and A.E. Kaifer, Langmuir, 3 (1987) 769. K.A. Mauri&, C.J. Hora and A.J. Hopfinger, Polym. Prepr., Am. Chem. Sot. Div. Polym. Chem., 19 (1978) 324. T.P. Henning, H.S. White and A.J. Bard J. Am. Chem. Sot., 103 (1981) 3937. A.J. Bard, T.P. Henning and H.S. White, J. Am. Chem. Sot., 104 (1982) 5362. H. Dahms, J. Phys. Chem., 72 (1968) 362. I. Ruff and V.J. Friedrich, J. Phys. Chem., 75 (1971) 3297. E.S. Yang, M.S. Chan and A.C. Wahl J. Phys. Chem., 84 (1980) 3094. R. Isnin and A.E. Kaifer, to be submitted. R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 191. L.R. Faulkner, Chem. Eng. News, 27 Feb. (1982) 28. C.P. Andrieux, J.M. Dumas-Bouchiat and J.M. Saveant, J. Electroanal. Chem., 131 (1982) 1. C.P. Andrieux and J.M. Saveant, J. Electroanal. Chem., 134 (1982) 163. C.P. Andrieux and J.M. Savtant, J. Electroanal. Chem., 142 (1982) 1. M.N. Szentirmay, N.E. Prieto and C.R. Martin, J. Phys. Chem., 89 (1985) 3017.
J. Electroanal. Chem., 279 (1990) 79-94 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Electrodes modified with Nafion films incorporating amphiphilic ferrocene derivatives Orlando Garcia and Angel E. Kaifer * Department (Received
of Chemistry, 31 March
University of Miami, Coral Gables, FL 33124 (U.S.A.)
1989; in revised form 2 October
1989)
ABSTRACT
The electrochemical behavior of electrodes modified with Nafion@ films containing several ferrocene derivatives was investigated by cyclic voltammetry. The derivatives used were alkyldimethyl (methylferrocene) ammonium salts, where the alkyl group was methyl (I ), heptyl (2) or hexadecyl (3). The rate of charge propagation across the Nafion film was found to decrease as the length of the alkyl chain increased. This finding was confirmed by rotating disk electrode experiments in which the Nafion-bound ferrocene derivatives mediated the oxidation of ferrocyanide ions in solution. On the other hand, the longer chain derivatives 2 and 3 were retained inside Nafion films more strongly (or for longer times than the methyl derivative I. Therefore, 2 and 3 are suited for use as electrochemical probes to monitor microenvironmental changes in the polyelectrolyte matrix resulting from the incorporation of other species. This was demonstrated using 3 as the probe and tetrabutylammonium ions as microenvironment modifiers.
INTRODUCTION
Nafion ** is a perfluorinated anionic polyelectrolyte that provides an ionically conductive matrix into which many electroactive cations can be incorporated. Electrode surfaces can be readily modified with cast Nafion films that become electroactive upon exposure to solutions of electroactive cations. These electrodes have been investigated extensively [l-20] because the modification procedure is quite simple and provides substantial versatility regarding the types of redox species that can be concentrated in the vicinity of the electrode surface.
* To whom correspondence should be addressed. ** Nafion is a trademark registered by E.I. DuPont
0022-0728/90/$03.50
0 1990 Elsevier Sequoia
de Nemours,
S.A.
Inc.
80
Nafion films have remarkable selectivity properties for cations of rather hydrophobic nature. Martin and co-workers [15] have performed extensive measurements of selectivity coefficients and reported values as high as lo6 for hydrophobic cations (using Nat as the reference). It is generally accepted that a Nafion matrix consists of segregated phases with hydrophobic pockets, formed by the fluorocarbon backbone chains, and hydrophilic regions where the anionic sulfonic sites group together [21]. Common hydrophobic electroactive cations, such as M(bpy):+ (where M = Co, Fe, OS, or Ru and bpy = 2,2’-bipyridine) and methylviologen, are retained strongly by Nafion films but the rates of charge propagation across the film are typically sluggish. However, more hydrophilic cations, such as Ru(NH,)z+ , are more loosely bound to the Nafion matrix while exhibiting faster rates of charge propagation [18]. In this paper, we report results obtained with electrodes modified with Nafion films containing the amphiphilic redox-active materials 1, 2 and 3.
2, n = I,
x- = Br-
3, n = 16, X. = Br-
The results show that these electroactive materials can be incorporated inside Nafion films to which they confer lasting electroactive properties. As expected due to their substantial hydrophobic character, these redox substrates give rise to slow charge propagation rates through Nafion films. We demonstrate here that an increase in the hydrophobic character of the electroactive cation, as measured by the length of the covalently attached hydrocarbon chain, results in films having lower hydration levels and, hence, slower rates of charge propagation. However, the lasting stability of the incorporation into Nafion of the bulkier and more hydrophobic cations 2 and 3 enables the assessment of the effects that other species may exert on the environment sensed by these cations inside the polyelectrolyte matrix. EXPERIMENTAL
Materials Compounds 2 and 3 were synthesized according to a published procedure [22] by treatment of dimethylaminomethylferrocene (Aldrich) with bromoheptane and bromohexadecane, respectively. The crude products were recrystallized from acetone/ether. Their structure and purity were verified by ‘H-NMR spectroscopy and elemental analyses. The methyl analog 1 was prepared as the iodide salt by the same procedure utilizing methyliodide as the alkylating agent. Hydrogen ion-form Nafion (Unit molar mass (EW = 1100 g) was obtained from Aldrich as a 5 wt% solution in lower aliphatic alcohols and water. All other reagents were of the best commercial quality available.
81
All solutions were prepared with distilled water passed Barnstead Nanopure water purification system.
through
a four-cartridge,
Equipment The electrochemical instrumentation has been already described [23]. Rotating disk voltammetric experiments were performed with a Pine rotator assembly using gold disk electrodes also supplied by the Pine company. Procedures Modification of the electrode surface was performed by casting the Nafion film in the usual manner [8,9,11-141. Electroactive materials were incorporated in the Nafion solution before casting the film. Usually, l-2 mg of electroactive material was dissolved in a volume of 5 wt% Nafion solution calculated so that 20% of the available sulfonic sites were compensated by the cationic electroactive material. This loading level (the concentration of electroactive probe inside the cast films is estimated at 0.28 M based on the Nafion unit molar mass and its reported wet density [24]) was kept constant throughout our experiments. A O.lO-ml aliquot of this solution was then diluted with 1.90 ml of ethanol to yield the solutions that were actually used to cast the Nafion films unless specified otherwise. A small volume of the diluted solution (7.0 ~1) was deposited with a syringe on the tip of a Au electrode (0.196 cm2) while rotating it simultaneously. After evaporation of the solvent for at least 15 min under continuous spinning (500 rpm), the electrode was equilibrated in the supporting electrolyte solution (0.2 M Na *SO,) for 30 min prior to any electrochemical analysis. We have made no attempt to measure the thickness of the resulting Nafion films. Estimates based on the wet density of 1100 EW Na+-form Nafion (1.58 g/cm3 [24]) yield a thickness of about 500 nm; however, the actual thickness in solution is probably larger due to the presence of the rather bulky electroactive cations. Electrodes prepared following this general procedure will be referred to as Au/Nafion(x), where x stands for the particular electroactive material incorporated in the Nafion film. The surface coverage of these electrodes is nominally calculated as 8.1 x lo-’ molar units of Nafion per cm2. Since only 20% of the polyelectrolyte sulfonic sites were initially occupied by the electroactive ferrocene derivatives, their surface coverage is estimated to be 1.6 X 10K8 mol/cm2. All potentials were measured and reported using sodium chloride saturated calomel electrodes (SSCE) as the reference electrode. RESULTS
AND DISCUSSION
A typical cyclic voltammogram obtained with a Au/Nafion(l) electrode is given in Fig. 1A. It shows the expected reversible oxidation behavior characteristic of ferrocene derivatives. The observed peak potentials are 0.34 and 0.31 V; the small potential difference between both peaks is in excellent correspondence with the general shape of the voltammogram which suggests clearly that the electrode surface
82
,
da
0
POTENTIAL/
V(‘JS
SSCE
C
I33 I lTENTIAL/
V (vs
SSCE 1
*PA
I
La8
POTENTIAL
/
V (vs
SSCE 1
Fig. 1. Cyclic voltammograms in 0.2 M Na,SO, of Au electrodes modified with films of Nafion containing (A) 1, (B) 2, and (C) 3. All the Nafion films contained the same concentration of ferrocene derivative initially. The voltammograms were recorded at 10 mV/s after equilibration in the supporting electrolyte solution for 30 min.
addresses all the accessible redox sites contained in the Nafion film during the time scale of the experiment. This is as expected due to the rather low thickness (ca. 500 nm) estimated for the Nafion film. Thus, the observed voltammetric behavior is similar to that resulting from a ferrocene derivative confined at the electrode surface. A similar experiment performed with a Au/Nafion(Z) electrode yields the cyclic voltammogram of Fig. 1B. In this case, the peak potentials are clearly more separated (0.35 and 0.28 V) and the shape of the voltammogram suggests that the redox sites are not immediately addressable by the electrode surface as they were in the Au/Nafion(l) electrode. In other words, charge propagates more slowly across the Nafion film containing 2 than it does across the Nafion film containing an initially equivalent concentration of 1 and, thus, the voltammetric behavior of the electrode modified with the heptyl derivative is essentially diffusion-controlled. The cyclic voltammogram of Fig. lC, obtained with a Au/Nafion(3) working electrode, clearly suggests diffusion control of the electrochemical process in the film. In this case, the peak potentials (0.40 and 0.27 V) are separated by 130 mV
83
probably indicating that the polyelectrolyte film has a substantially larger ionic resistance than the Nafion(2) film. All three voltammograms in Fig. 1 were obtained using electrodes modified with identical surface coverages of Nafion and electroactive cation. Since the hydrophobicity of the ferrocene derivatives varies as a function of the length of the alkyl tail, different levels of hydration may be present in these films. In principle, the degree of film hydration is expected to decrease as the length of the alkyl chain increases. This factor, as well as the varying molecular volumes of the three ferrocene derivatives, may cause differences in the actual thickness of the films. However, these differences cannot be very large because only 20% of the exchanging capacity of the polyelectrolyte is actually used to bind the ferrocene derivatives; most of the sulfonic sites (80%) remain in the Na+ form. Therefore, the different electrochemical responses can hardly be ascribed to changes in the film thickness. More likely, the changes in electrochemical behavior recorded in Fig. 1, reflect different rates of charge propagation across the Nafion films. Experiments to assess the stability and durability of the electrochemical response of Au/Nafion(l-3) electrodes were also performed. In these experiments, an electrode was modified as described in the experimental section and immersed in a quiescent supporting electrolyte solution for 30 min. At this point a voltammogram was recorded using a scan rate slow enough to insure complete oxidation across the entire film thickness. Then, the electrode was rotated at 500 rpm for 10 min and another voltammogram was recorded at the same scan rate with the electrode and the solution completely stationary. These steps were repeated at least for 1 h. The rotation of the electrode enhances the rate of mass transfer from the solution to the electrode, thus increasing the leaving rate of any species that may not be strongly retained inside the film. Plots of the surface coverage determined from the charge consumed in the voltammetric oxidation of the ferrocene derivative as a function of time provide information on the stability and durability of the electrochemical response of the Nafion films incorporating derivatives 1, 2 and 3. The data are shown in Fig. 2 and demonstrate a very interesting trend. The electroactive surface coverage for Au/Nafion(l) electrodes decreases rather quickly as a function of time. After the initial 30 min of electrode equilibration in a quiescent solution, the electrode exhibits a coverage of 6.2 nmol/cm’ which corresponds to about 40% of the nominal ferrocene coverage as calculated from the casting data. Lower than nominal coverage values for electroactive cations in Nafion films have been found experimentally before [9]. They can be rationalized by postulating the existence of environments within the Nafion film in which the electroactive cations are rendered inert and prevented from undergoing oxidation/reduction. In this case, owing to the rapid decrease of coverages as a function of time which is observed during the first 30 min of electrode rotation (see Fig. 2) it seems reasonable to assume that a fraction of the ferrocene cations which are not detected by voltammetry may have been lost from the film during the equilibration period. That is, not all the ferrocene sites which are not detected electrochemically are assumed to occupy environments which render them electrochemically inactive. The data obtained with Au/Nafion(2) electrodes show a stable ferrocene surface
84
lo-
I
5-A
2-A UETHYL
n HEPTYL
l--
o-
: 0
20
40
80
IO
TIME
/
100
120
140
min
Fig. 2. Measured surface coverage for Nafion-bound ferrocene derivatives 1 and 2 as a function of time. Voltammograms for derivatives 1 and 2 were recorded at 10 and 2 mV/s, respectively, to insure complete oxidation of the film in each cycle. The electrode was continuously rotated at 500 rpm except when voltammograms were being recorded. All electrodes were initially equilibrated for 30 min in a quiescent 0.2 M Na,SO, solution before starting any electrochemical cycling.
coverage. Thus, the initial coverage of 8.9 nmol/cm2 remains essentially unaltered after more than 2 h of electrode rotation. This coverage value represents 55% of the nominal surface coverage as determined from the film casting data. This value is in good agreement with data reported by Martin et al on the fraction of incorporated Bu(bpy) : + sites that were detected electrochemically in 970 g EW Nafion films cast on glassy carbon electrodes [9]. It is also noteworthy that a very slow scan rate of 2 mV/ s was needed to insure the complete electrochemical oxidation of the Nafion(2) film as evidenced by the recording of surface-confined voltammetric responses. Films containing similar loading levels of the methyl derivative 1 shows surface-confined responses at much faster scan rates (2-200 mV/s). By contrast, clear surface-confined voltammetric behaviour is not observed with Au/Nafion(3) electrodes having similar film thickness, even at scan rates as low as 1 mV/s. Because of this we did not include surface coverage data for Nafion(3) films on Fig. 2. However, the hexadecyl derivative seems to be retained strongly inside the polyelectrolyte matrix because the observed voltammetric response remains essentially unaltered during 2 h or longer periods of time. Therefore, our data clearly suggest
85
that ferrocene derivatives 2 and 3 are retained strongly inside Nafion films cast on Au electrodes under the same experimental conditions in which the methyl derivative 1 leaves the film at a measurable rate. The scan rates necessary to insure the complete oxidation of Nafion films containing ferrocene derivatives 1, 2, and 3 become increasingly slower as the length of the derivative’s alkyl chains increases. This is probably due to a decreasing rate of charge propagation across the Nafion film resulting from the more lipophilic character of the electroactive cation. Increasing the length of the alkyl chain also results in better retention of the electroactive probe inside the Nafion layer. Charge propagation across Nafion films has been the subject of many studies [5,6,8,9,25,26]. Two mechanisms can be responsible for the propagation of an electrochemical conversion inside the polyelectrolyte matrix: (i) actual diffusion of the electroactive probes or (ii) electron hopping among neighboring probes. The level of hydration of the Nafion film may also seriously influence the rate of charge propagation since it affects the movement of ions inside the film that are related to both mechanisms. Many research efforts have addressed the relative importance of these two mechanisms in Nafion. Most results indicate that the overall diffusional behavior depends on the nature of the electroactive species involved. This is indeed expected in terms of the Dahms-Ruff model [27,28] in which the electron hopping rate depends on the magnitude of the electron self-exchange constant for the redox couple in question. Thus, generally speaking, the diffusional behavior of electroactive species inside polymer films is expected to be influenced by electron hopping if the redox couple has a large electron self-exchange rate. Conversely, the behavior of electroactive redox species having relatively slow electron-self exchange should be dominated by its actual diffusion (molecular motion) inside the polyelectrolyte matrix. Ferrocene-containing species are predicted to exhibit diffusional behavior because of the moderate rate for electron self-exchange (1.6 X lo6 M-i s-‘) shown by the ferrocene moiety [29]. Our data lends support to this view due to the effects observed upon lengthening the alkyl chain of the ferrocene derivative. In the modified electrodes described here the Nafion layer always contains the same initial concentration (loading level) of electroactive probe. Therefore, if the electron hopping mechanism were to predominate, one would expect to see approximately the same rate of charge propagation regardless of the length of the alkyl tail in the ferrocene derivative. Our results demonstrate a strong dependence on the length of the akyl chain suggesting that diffusion of the ferrocene derivatives, rather than electron hopping, is the predominant charge propagation mechanism in these Nafion films. However, since the lipophilicity of the ferrocene derivative increases with the length of the pending alkyl tail, the level of hydration of the Nafion films probably decreases throughout the series l-3. As it has been indicated before, a decreased level of hydration will also tend to decrease the rate of charge propagation across the film owing to the hindrance imposed on ionic movements within the polyelectrolyte matrix. The apparent diffusion coefficients of the ferrocene derivatives can be determined from plots of anodic peak current vs. the square root of the scan rate.
86
0
10
5 V
l/2 ,m"l/2
IS
5-1/2
Fig. 3. Anodic peak current vs. square root of scan rate plots for the methyl (I), heptyl(2) and hexadecyl (3) ferrocene derivatives in thick Nafion films on Au. The contacting solution was 0.2 M Na,SO,.
These data were obtained using Nafion films 20 times thicker than in all other experiments to insure that the diffusion layer was confined within the polyelectrolyte matrix. All derivatives give rise to linear plots for scan rates ranging from 20 to 200 mV/s (see Fig. 3). From the slope of these plots, Da values of 1.4 X lo-“, 5.9 x lo-l2 and 1.2 X lo-l2 cm2/s were determined for 1, 2 and 3, respectively. Thus D, values decrease as the length of the alkyl chain increases, reflecting the slower motion inside the Nafion layer of the bulkier cations. This parallels the trend observed with the diffusion coefficients of compounds l-3 in solution [30] *. The diffusion coefficient of 1 in 970 g EW Nafion was measured by White et al. in a variety of experimental conditions [B]. They report values in the range l-3 x lo-” cm2/s that are about an order of magnitude higher than our Do value for the same compound. However, this disagreement is not particularly surprising if one considers that these authors utilized Nafion of lower EW (970 g) which is expected to hydrate more extensively than the 1100 EW Nafion used in our experiments.
* Diffusion coefficients of 1 and 2 in aqueous solution have been accurately determined from Levich plots as 4.8X10W6 and 4.1 X 10e6 cm’/s, respectively. The diffusion coefficient of 3 could not be obtained because its electrochemical behavior is not diffusion-controlled due to micellization and adsorption effects.
From our diffusion coefficient data we conclude that the observed rates of charge propagation across Nafion films containing ferrocene derivatives 1 to 3 are essentially controlled by the actual diffusion of the electroactive cations within the polyelectrolyte. The apparent diffusion coefficient is indeed very sensitive to the length of the attached alkyl chain. As predicted on the basis of size considerations and the relative hydrophobic nature of 1-3, the observed D, values decrease as the length of the chain increases. Other factors may also be partially responsible for the observed trends. For instance, it is conceivable that an increase in the hydrophobic nature of the electroactive probe might decrease the overall content of water in the Nafion films. A lower degree of film hydration would hamper the movement of ions across the Nafion matrix resulting in slower charge propagation rates. The effects of film dehydration are specifically addressed later in this work. The oxidation of Fe(CN)zmediated by Nafion (2 or 3) films One of the main ideas behind the development of modified electrodes is to accomplish the derivatization of the electrode surface with redox active species that can mediate (or, even better, catalyze) the oxidation/reduction of solution species [31,32]. Therefore, we addressed the mediation properties of Nafion electrodes incorporating 2 and 3, the more hydrophobic derivatives in our series. We did not attempt mediation experiments with Nafion(1) films since the methyl derivative is not strongly retained by the polyelectrolyte matrix under the conditions of these experiments (see Fig. 2). Therefore, we decided to investigate the mediated oxidation of ferrocyanide by the Nafion-bound heptyl and hexadecyl ferrocene derivatives. The overall process can be simply represented by the following equation Cp,Fe+
+ Fe(CN)z-
+ Cp,Fe
+ Fe(CN$
(1)
where Cp,Fe represents any of the ferrocene derivatives used in this work. The reaction is thermodynamically downhill because the oxidation potential of ferrocyanide is lower than those of the ferrocene derivatives. The selection of this reaction also responds to the known fact that the negatively charged ferrocyanide ion cannot permeate the Nafion film due to electrostatic repulsive forces [12]. This prevents the direct electrochemical reaction of ferrocyanide at the electrode surface and insures that any ferrocyanide oxidation must be mediated by the Nafion-bound ferrocene derivative. The mediated oxidation reaction is quickly verified by the shape of the voltammograms in Fig. 4. The cyclic voltammogram of Fig. 4A corresponds to a Au/ Nafion(3) electrode immersed in 0.2 M Na,SO,. The only salient feature of the voltammogram is the reversible oxidation of 3 as discussed before. When 1.0 mM Fe(CN)iis present in the contacting solution the oxidation peak is clearly enhanced (see Fig. 4B) while the reduction peak is substantially decreased in comparison to the response obtained in the absence of Fe(CN)z-. These changes are exactly as predicted if the process given by eqn. (1) is taking place. When the same experiment is carried out with the electrode rotating at 500 rpm, the voltammogram of Fig. 4C is obtained. Again, the sigmoidal shape of this voltammogram
C
B
A
1
I
POTENTIAL/h’
0
I
,
I
0.6
vs.SSCE)
Fig. 4. Cyclic voltammograms at 10 mV/s recorded with a Au/Nafion (3) electrode in (A) 0.2 M Na,SO,, (B) 0.2 M Na2S0,+1.0 mM K,Fe(CN),, (C) 0.2 M Na,SO,+ 1.0 mM K,Fe(CN),. Voltammogram (C) was recorded at 500 rpm while (A) and (B) were obtained at 0 ‘pm.
reveals that more ferrocyanide is being oxidized by the ferricinium form of the Nafion-bound 3 because the rotation of the electrode increases the rate of mass transport of Fe(CN)zions from the bulk solution to the polyelectrolyte-solution interface. Similar cyclic voltammograms were obtained with Au/Nafion(2) electrodes. Since the rotation rate of the electrode can be varied, the rate of transport of Fe(CN)iions to the Nafion-solution interface can be controlled to demand variable electron transport rates from the polymer film. This affords another method to estimate charge propagation rates across the Nafion layers. If the rotation rate can be increased to a level at which the Nafion film can no longer support the oxidation of all the Fe(CN)zions reaching the interface, then one would expect to observe invariant currents for rotation rates equal or higher than this threshold value. This is exactly what we observed (see Fig. 5) in mediation experiments using Nafion films containing either 2 or 3. The plots have a slow rotation region in which the anodic current is limited by Fe(CN)zmass transfer from the solution to the interface. But the current levels off at higher rotation rates because the oxidation of ferrocyanide is now limited by charge transport across the Nafion layer. In good agreement with our previous results, the onset of the current plateau takes place at slower rotation rates with Au/Nafion(3) than with Au/Nafion (2) electrodes. This indicates that faster charge transport rates can be established across the latter. Furthermore, the plateau current for the Nafion layer containing the hexadecyl derivative is lower than that obtained for the Nafion layer containing the heptyl derivative (for the two Fe(CN)zconcentrations surveyed), again indicating faster charge propagation across the latter. The behaviour of polymer-coated rotating disk electrodes has been analyzed by Saveant and co-workers [33-351. In their model they propose several processes, other than mass transport in solution, that could become rate limiting in mediation experiments: (i) diffusion of the solution species in the film, (ii) charge propagation in the film via the mediator couple, and (iii) the redox reaction between the
89
50
0:
0
l0
10
40
i
a0
50
I ROTATION RATE )1” /
nk-“’ Fig. 5. Rotation rate dependence of the limiting anodic current (at 0.6 V vs SSCE) observed for the mediated oxidation of ferrocyanide on (A) Au/Nafion(Z) and (B) Au/Nafion(J) electrodes. (m) 1.0, (0) 0.3 mM K ,Fe(CN),.
of the substrate ion. To confirm this point further, we performed control experiments with Au/Nafion (without any ferrocene derivative inside the film) in ferrocyanide solutions and obtained the expected result, that is, the film prevents the oxidation of Fe(CN)%- in the potential window utilized in this work. The results in Fig. 5 show that, at high rotation rates, charge transport across the Nafion film becomes rate limiting. The reaction between the ferricinium form of the Nafion-bound mediator and Fe(CN)zseems to be fast and does not affect the overall rate in the range of rotation rates surveyed. For cases in which charge propagation through the film is the limiting process, Saveant and co-workers [34] have proposed that the characteristic current mediator
product
in the film is ruled
and the solution out owing
species. In our case, the diffusion
to the high negative
charge
of the ferrocyanide
90
i, would be given by i, = FAc;D,/D
(2)
where c,” is the mediator concentration in the film, D stands for the film thickness, is the effective diffusion coefficient for the diffusion-like propagation of electrons across the film, A is the electrode surface, and F is the Faraday constant. Using the average currents in the high rotation rate region, we obtained 9.2 x 10-l’ and 6.9 X lo-” cm2/s as D, values from eqn. (2) for the heptyl and the hexadecyl derivatives, respectively. These values are clearly larger than the diffusion coefficients determined from the voltammetric experiments (5.9 x lo-i2 and 1.2 x lo-i2 cm2/s, respectively). Perhaps an explanation for this difference can be found in the dissimilar film thicknesses employed in these two types of experiments. Thus, while the mediation experiments were carried out with thin Nafion films (D = 500 nm), the peak current vs. (scan rate) ‘/2 data points were obtained with films 20 times thicker. In very thin films the overall charge transport rate may be inordinately affected by interfacial phenomena (taking place either at the Au-Nafion or at the Nafion-solution interfaces). Alternatively, heterogeneous distribution of the ferrocene derivative in the Nafion film may also be responsible for these differences. Nonetheless, both sets of data indicate that charge transport across Nafion films containing the heptyl derivative is faster than with the hexadecyl derivative.
D,
Effects of the incorporation
of l-3 on the degree of hydration of Nafion films In order to assess the effects of the lipophilicity of the various ferrocene derivatives on the extent of hydration of the polyelectrolyte matrix we performed experiments in which a Au/Nafion(x) electrode was exposed to a solution containing 1.0 mM methylviologen (MV2’). This resulted in the exchanging of MV2+ into the film, a process which was allowed to proceed for 30 min. After the loading of methylviologen, the voltammetric behavior of the electrode was monitored in pure supporting electrolyte solution. During this last step, the amount of methylviologen detected electrochemically in the film decreased steadily as a function of time. The experiments with ferrocene derivative 1 were not very informative since both electroactive cations demonstrated a marked tendency to leave the Nafion film. However, experiments with Nafion(2) and Nafion(3) were more illustrative. Figure 6 shows a voltammogram showing the response of a Au/Nafion(3) electrode 5 min after transfer from the loading solution to a 0.2 M Na,SO,. At this point the observed coverage of MV 2+ is similar to that of 3 as measured from the integration of their respective waves. However, the peak potential separation of the ferrocene/ ferricinium couple is 96 mV while that of the MV2+/MV+ couple is only 20 mV. That is, both electroactive cations exhibit quite different electrochemical properties even though their apparent concentrations within the film are similar. Voltammograms recorded with the same electrode of Fig. 6 showed gradually decreasing waves for the MV2+/MV+ couple while those corresponding to the ferrocene derivative remained stable. The potential difference between the two peaks of the viologen couple was always about 20 mV whereas that of the ferrocene couple was always
91
1
-0.0
1
1
-0.6
-0.4
1
-0.2
POTENTIAL
/
0.0
I
0.2
0.4
0.6
V (vs. SSCE)
Fig. 6. Cyclic voltammogram (at 10 mV/s) recorded with a Au/Nafion(3) electrode in 0.2 M Na,SO,. The electrode had previously been immersed for 30 min in a solution containing 1.0 mM MV*+. The response shown was recorded 5 min after transferring the electrode from the MV*+ loading solution to the pure supporting electrolyte solution.
near 100 mV. Similar results were obtained with Nafion(2) electrodes. In this case, the observed peak potential differences were smaller (about 14 mV for the MV2+/ MV+ couple and 55 mV for the ferrocene couple). These data indicate clearly that the incorporation of hydrophobic ferrocene cations, such as 2 and 3, at loading levels equivalent to 20% of the stoichiometric exchanging capacity of the Nafion film does not cause an extensive dehydration of the film as evidenced by the sharp waves observed for methylviologen. This finding reinforces our contention that the charge propagation rates observed with the ferrocene derivatives are essentially controlled by their diffusional motion within the film. Nafion(3) films are seen to be slightly less hydrated than Nafion(2) films, as indicated by the larger MV’+/MV+ peak splitting observed in the former, but the difference in water content does not appear to be large enough to explain by itself the changes seen in the behavior of the ferrocene derivatives. Use of 2 and 3 as electrochemical probes to monitor microenvironmentaE changes in Nafon films This work demonstrates that amphiphilic ferrocene derivatives, such as 2 and 3, confer lasting electrochemical properties to Nafion films. Because the leaving rates of these electroactive probes are very slow, it is possible to use them as reporters of the microenvironmental changes that the incorporation of other cationic species may cause in Nafion. For instance, Martin and coworkers reported that tetrabutyl-
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ammonium (TBA+) cations exchange readily into Nafion membranes eliciting a concomitant decrease in the water content inside the polyelectrolyte matrix [36]. Because of this report we have investigated the electrochemistry of several Nafionbound substrates in the presence of TBA+ ions in the contacting solution. Cyclic voltammetry with Nafion modified electrodes incorporating methylviologen or 1 shows that the presence of TBA+ ions (0.1-l mM) in the contacting solution disrupts notably the electrochemical behavior. Initially, a decrease in the peak currents and an increase in the potential difference between the reduction and oxidation peaks of the couple are observed. Then, the electroactivity essentially disappears ?nd only non-faradaic current responses can be recorded. These changes can certainly be interpreted as resulting from the partitioning and ion exchange of the TBA+ ion into the Nafion film. As indicated by Martin et al, the hydrophobic TBA+ ions decrease the degree of hydration of the Nafion film so that the electrochemistry in the resulting polyelectrolyte matrix becomes more difficult because the ion movements are hampered by the lack of water molecules. While this explains the observed changes in our experiments, it must be recognized that the TBA’ ions might be displacing the electroactive cations from the Nafion film and the observed changes may be also due to the loss of electroactive probes from the film. This problem can be addressed nicely using amphiphilic electroactive derivatives, such as 2 and 3, because these compounds are retained strongly inside Nafion membranes and can report on microenvironment changes while maintaining a stable concentration in the film. This idea is supported by the results shown in Fig. 7. The cyclic voltammogram in Fig. 7A corresponds again to a Au/Nafion(3) electrode in contact with a 0.2 M Na,SO, solution. Figure 7B shows the behavior of the same electrode 1 min after the addition of TBA+Br(0.5 mM) to the solution. The series of voltammograms in Figs. 7C to 7E demonstrate the quick changes in electrochemical behavior that the presence of TBA+ brings about. As previously stated, these changes reflect the decreasing water content of the Nafion film as the concentration of TBA+ increases. It is noteworthy that, after 15 min, essentially no faradaic response is detected with this electrode. In order to prove that this response is not due to the replacement of 3 by TBA+ ions, we transferred the electrode of Fig. 7E to pure 0.2 M Na,SO, solution and recorded voltammograms as a function of time. This experiment turned out to be quite long as expected from the high selectivity of Nafion for hydrophobic cations. However, after 18 h, we recorded the voltammogram of Fig. 7F, which exhibits again very clearly the oxidation and reduction peaks due to the ferrocene derivative. This proves that the voltammetric changes observed (Figs. 7B to 7E) in the presence of TBA+ ions take place while 3 remains inside the Nafion film. Therefore, the behavior recorded in Fig. 7E(for instance) does not result from depletion of 3 but from the film’s decreasing water content which, in turn, is the consequence of the exchange of TBA+ ions in the polyelectrolyte matrix. Hence, amphiphilic electroactive derivatives, such as 2 and 3, appear to be excellent electrochemical probes to monitor microenvironment changes inside Nafion films or similar systems.
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I
0
1
I
I
0.6
I
0
I
0.6
Fig. 7. (A) Cyclic voltammogram (at 50 mV/s) recorded with a Au/Nafion(3) electrode in 0.2 M Na,SO,. (B) Same electrode 1 min after the addition of 0.5 mM TBA+ Br- to the contacting solution. (C) After 3 min. (D) After 5 min. (E) After 15 mm. (F) Same electrode after 18 h immersed in a TBA+-free, 0.2 M Na,SO, solution.
ACKNOWLEDGEMENTS
The authors are grateful to Jodi M. Schuette for performing some preliminary experiments. Financial support from the University of Miami Research Council is acknowledged. REFERENCES 1 2 3 4 5 6 7
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