Synthesis and Redox Characterization of Phosphazene Terpolymers

Apr 23, 2012 - Department of Chemistry, Duke University, Durham, North Carolina ... with adsorption, depending on the degree of substitution with Fc g...
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Synthesis and Redox Characterization of Phosphazene Terpolymers with Pendant Ferrocene Groups Daniel C. Kraiter,† Patty Wisian-Neilson,*,‡ Cuiping Zhang,‡ and Alvin L. Crumbliss*,† †

Department of Chemistry, Duke University, Durham, North Carolina 27708, United States Department of Chemistry, Southern Methodist University, Dallas, Texas 75275, United States

‡

S Supporting Information *

ABSTRACT: The synthesis, characterization, and electrochemistry of a series of five ferrocene-containing phosphazene terpolymers derived from the poly(alkyl/arylphosphazene) random copolymer, [(Ph)(Me)PN]0.6[Me2PN]0.4, 5, are described. The new terpolymers, 6 {[(Ph)(Me)PN] 0.6 [Me 2 PN] 0.4 } x {[(Ph){FcCH(OH)CH 2 }PN] 0.6 [(Me){FcCH(OH)CH2}PN]0.4}y, where Fc = (η5-C5H5)Fe(η5-C5H4) (6a, x = 0.50, y = 0.50; 6b, x = 0.60, y = 0.40; 6c, x = 0.65, y = 0.35; 6d, x = 0.70, y = 0.30; and 6e, x = 0.85, y = 0.15), were prepared by deprotonation−substitution reactions and were characterized by NMR spectroscopy, elemental analyses, gel permeation chromatography, and thermal analysis. The redox properties are described by electrochemistry performed in dichloromethane solutions and evaporatively cast films. In solution, cyclic voltammograms of these polymers display a one-electron reversible (ΔEp ca. 60 mV) wave in the range of 433−492 mV (vs Ag/AgCl) combined with adsorption, depending on the degree of substitution with Fc groups. The adsorption of the oxidized polymer on a Pt electrode was established using double potential step chronocoulometry. A mechanism for electron diffusion is discussed in terms of physical diffusion and electron hopping. The apparent electron diffusion coefficient was calculated from a combination method of chronocoulometry and rotating disk voltammetry. The observed diffusion coefficients are larger than those of corresponding polymers that contain a larger number of phenyl side groups and are largely independent of the degree of Fc substitution. A second, less intense wave about 220−250 mV more positive than the main wave was also observed and attributed to the internal oxidation of a secondary alcohol to a ketone group. Reversible electrochemical behavior of surface bound species is also observed for the ferrocene/ferricenium couple in evaporatively cast films of the phosphazene polymers. Cyclic voltammetry and chronoamperometry were used to determine the electrochemical characteristics and charge transport quantities (Γ and CD01/2) of the terpolymer films. These quantities vary with the degree of substitution with Fc groups of the phosphazene terpolymers.

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INTRODUCTION Macromolecules containing metallocene units have been actively studied for multiple applications.1−3 Among these systems, the redox behavior of several inorganic phosphazene polymers and related cyclic small molecules have been examined.4−7 The ease of structure modification and resultant property variation of the polyphosphazenes provides unusual opportunity for structure−property studies including redox behavior. Polyphosphazenes are accessible by several methods that include ring-opening of [Cl2PN]n followed by nucleophilic substitution, ring-opening of partially substituted cyclic phosphazenes (e.g., (ferrocenyl)(Cl)5P3N3),8 condensation polymerization of N-silylphosphoranimines,8,9 Me3SiN PRR′(X) (e.g., where R = R′ = alkyl, aryl and X = OCH2CF3, OPh, or Cl), or modification of methyl groups in condensation polymers such as [(Ph)(Me)PN]n, 1.9 The ring-opening of partially substituted rings was used to obtain ferrocenesubstituted polymers 2 and 3,10,11 while modification of methyl groups on 1 was used to obtain a series of polymers, 4.12 Electrochemical analysis of these ferrocenyl-containing polyphosphazenes indicated that the mode of attachment of © 2012 American Chemical Society

the ferrocene group to the polyphosphazene backbone significantly influences the electrochemical behavior of the polymers. For example, both polymers 2 and 3 exhibit symmetrical cyclic voltammetry traces, indicating good redox stability and reversibility.4 However, the charge-transport rate for polymer 2, which contains transannular ferrocene groups, is greater than that of polymer 3 with pendant ferrocene units.10 This is significant since the pendant ferrocene polymer is expected to display greater mobility and, hence, a greater through-space electron self-exchange rate. Polymer 2 has a higher degree of electrochemical irreversibility than 3 and exhibits two voltammetric peaks, suggesting that the ferrocenyl groups experience two different compositional environments. Electrochemical studies of the series of polymers, 4, in solution and as-cast films, where a varying number of ferrocene units were attached by a more flexible two-carbon spacer unit,12 showed reversible electrochemistry and improved chargeReceived: February 20, 2012 Revised: April 7, 2012 Published: April 23, 2012 3658

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RESULTS AND DISCUSSION Terpolymer Synthesis and Characterization. The new ferrocene terpolymer derivatives 6a−e were prepared using procedures analogous to the deprotonation−substitution reactions of the homopolymer, [(Ph)(Me)PN] n 12,14 (Scheme 1). The 31P NMR spectra of the new terpolymers, 6, contained broad signals between δ 1 and δ 8, and the 1H NMR spectra contained the appropriate signals for PCH2, ferrocenyl, P−Ph, and P−Me groups. Because of the close proximity of the PMe and PCH2 NMR spectroscopic signals, it was not possible to determine whether substitution occurred preferentially at the PMe2 or P(Me)Ph methyl sites. However, relative integration of the phenyl, ferrocenyl, and aliphatic regions was useful in determining the approximate degree of substitution of the ferrocenyl moieties. Elemental analyses also showed the expected trends for incorporation of varying degrees of ferrocene groups. As observed with previous deprotonation− substitution reactions,14,15 the molecular weights of the new terpolymer derivatives 6 were generally no smaller than the parent polymers, thus indicating that no chain degradation occurred during the deprotonation−substitution process. As expected, the Tg values for the new ferrocene-containing terpolymers increase with the degree of substitution of ferrocene, suggesting a decrease in torsional mobility. Figure 1 shows a comparison between the Tg values of our ferrocene-

transport efficiency relative to 3 as well as a dependency on the concentration of ferrocene units. Moreover, the standard potentials for 4 were shifted only slightly positive of the redox potential of a similar small molecule, hydroxymethylferrocene,12 indicating that the electron-withdrawing influence of the P−N backbone is significantly diminished by the twocarbon spacer group in the polymers. Related studies of the electrochemistry of several types of ferrocene-containing cyclic phosphazenes have shown a marked positive shift in the oxidation potentials when ferrocene is directly attached to PN rings11 as compared to cyclic phosphazenes in which the ferrocene group is attached by various spacer groups.5,7 Since the flexibility of the spacer group between the ferrocene moieties and the polymer backbone affects the redox behavior of ferrocene-containing polymers, increased flexibility in the polymer backbone could also influence chargetransport efficiency. Here, we report the first deprotonation− substitution reactions on a poly(alky/arylphosphazene) random copolymer, [(Ph)(Me)PN]x[Me2PN]y, 5,13 for the preparation of a new series of ferrocene-containing polymers. Copolymer 5 has a more flexible backbone as indicated by a significantly lower glass transition temperature than the parent homopolymer, 1, which was used to prepare polymers 4. This is explained by the fact that almost half of the phenyl groups in 4 have been replaced with smaller methyl groups in 5. With the goal of understanding the mechanism of charge transport and the influence of polymer structure on charge transfer, electrochemical studies were conducted both in solution and with evaporatively cast films of five new derivatives of the copolymer 5.

Figure 1. Glass transition temperatures (a) parent copolymer, 5, and new terpolymer ferrocene derivatives, 6a−e; (b) parent homopolymer, 1, and its ferrocene derivatives, 4a−d.12

substituted phosphazene copolymer derivatives (6a−e) and those of the ferrocene-substituted phosphazene homopolymer derivatives (4a−d) previously reported.12 Although the

Scheme 1

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influences the redox potential of the ferrocene in a systematic manner, with electron-withdrawing groups shifting the potential toward more positive values. One can infer from this that the polyphosphazene backbone acts as an electronwithdrawing group on the ferrocene moiety. Similar results were obtained for homopolymers 4a−d.12 Other phosphazene polymer systems4,10 show a shift in redox potential to values 280−780 mV more positive than those for the hydroxymethyl ferrocene unit, (η5-C5H5)Fe(η5-C5H4CH2OH). In those cases, the ferrocene group was directly bound to the polymer backbone. Polymers 6a−e have a two-carbon spacer group between the backbone and the ferrocene unit, which diminishes the electron-withdrawing influence of the backbone on the ferrocene substituent. For all polymers, the peak current ratios, ip,a/ip,c are less than one, suggesting that the product of the electron transfer is weakly adsorbed on the electrode surface (Table 1).16 In each case, the plot of anodic peak current (first CV scan) vs square root of the scan rate is linear over a large range of scan rates (5−200 mV/s), indicating that the charge transfer may be described by the Randles−Sevcik equation.16 The peak current decreases as the degree of substitution with ferrocene groups drops from 50% to 15%, consistent with the decrease in the number of electroactive groups per polymer molecule. For all terpolymers, the peak currents increase with the number of scans (maximum values around the 10th scan), the peak current ratio increases to unity, and the ΔEp increases up to 200 mV as shown in Figure 2B. This is indicative of a film build up to a steady state, which may also contribute to an overall ohmic drop in the electrochemical cell. Diffusion coefficients for the polymer−background electrolyte systems were calculated using a combination of rotating disk voltammetry (RDV) and chronocoulometry (CC).17 Representative Levich plots (RDV) and Anson plots (CC) are shown in Figures 3 and 4, respectively. The apparent electron diffusion coefficients are 2−3 orders of magnitude smaller than that of a small molecule analogue, hydroxymethylferrocene, with no apparent trend with respect to the degree of Fc substitution (Table 1). The shapes of the cyclic voltammetry waves show evidence of the formation of a polymeric adsorbed layer on the surface of the electrode. The film formation is likely governed by the thermodynamics of electrode−polymer interaction during the potential sweep. Double potential step chronocoulometry (DPSCC) was used to calculate the degree of electrode coverage with active ferrocene groups for each terpolymer. The reported values (Γ in Table 1) were slightly higher for higher substituted polymers, suggesting that Fc groups play a minor role in the adsorption of the polymer molecules on the electrode surface. Representative charge vs time correlations for three of the terpolymers investigated are shown in Figure 5. For comparison purposes, the redox characteristics (E0′, ΔEp, ip,a/ip,c) and diffusion coefficient of hydroxymethylferrocene, the small molecule analogue of the pendant redox unit, are also included in Table 1. Our results are in good agreement with literature values.12,18 Hydroxymethylferrocene does not adsorb on the electrode surface, and therefore no degree of coverage was determined in this case. After about four current−voltage scans, a second electrochemical process appears at potentials 220−250 mV more positive than the primary ones. This current signal is much smaller than the primary current signal and has the characteristics of a diffusion-limited process. The position of the new

unsubstituted parent copolymer (5) has higher torsional mobility (Tg = 2 °C) than the unsubstituted homopolymer (1, Tg = 37 °C), this difference diminishes with ferrocene substitution. Thus, it appears that the effect of the smaller methyl substituents (relative to phenyl), which allow more torsional mobility, is eventually overcome by the steric bulk of the ferrocenyl moieties and OH group hydrogen bonding. It is noted that the ferrocene derivatives of the very flexible homopolymer, poly(dimethylphosphazene), [Me2PN]n, which has a significantly lower glass transition temperature of −40 °C (ca. 80 °C lower than homopolymer 1 and ca. 42 °C lower than parent copolymer 5),13 would facilitate the complete removal of phenyl groups, thus maximizing the torsional mobility of the polymer backbone. However, the semicrystallinity of [Me2PN]n and its lack of solubility in coordinating solvents such as THF prohibit the deprotonation reaction. The solubility problem was circumvented by using the random copolymer 5, which remains soluble in THF, but still has a significantly lower Tg (2 °C) than parent polymer 1 (37 °C).13 Electrochemical Studies. Electrochemistry in Solution. Cyclic voltammetric waves (CV) showing both oxidation and reduction peaks were obtained for all five polymers, 6a−e, in TBAP/CH2Cl2 solutions. Figure 2A shows the first current/

Figure 2. Cyclic voltammograms of phosphazene terpolymer 6b (41% Fc) in CH2Cl2 solution. Conditions: polymer concentration 1 mg/mL, 0.1 M TBAP, scan rate 10 mV/s, Pt electrode, voltage scan range 0.2− 0.85 V vs Ag/AgCl. Current scale is given in parentheses: (A) first scan (u = 1 μA); (B) tenth scan (u = 5 μA).

voltage scan for 6b at 10 mV/s, and Table 1 shows the electrochemical parameters for all five polymers. The ferrocene/ferricenium (Fc/Fc+) redox couple appears over the range of 464−492 mV (vs Ag/AgCl). All half-potentials, E10′, of the polymers 6a−e were higher than the corresponding value of the hydroxymethylferrocene redox unit, in the same background electrolyte−solvent system (Table 1). This is an indication that the substituent at the cyclopentadienyl ring 3660

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Table 1. Electrochemical Parameters of the Ferrocene-Substituted Phosphazene Polymers in Dichloromethane Solutiona polymer (% Fc)

Mw (g/mol)

E10′b (mV)

ΔEpb (mV)

ip an/ip cat

E20′b (mV)

107Dapp (cm2/s)c

109Γ (mol/cm2)d

6a (48) 6b (41) 6c (33) 6d (29) 6e (15) FcCH2OHe

111 000 208 000 101 000 154 000 103 000 216

464(4) 491(12) 479(8) 492(6) 484(12) 467(5)

44(3) 54(6) 41(2) 61(10) 35(9) 74(1)

0.279(1) 0.44(6) 0.42(6) 0.42(8) 0.75(19) 0.98

689(3) 728(1) 725(2) 713(1)

0.44 2.25 0.099 2.21 0.089 204

5.3 5.91 3.5 4.46 1.4

a Background electrolyte: 0.1 M TBAP; polymer concentration 1 mg/mL. bValues for the first scan, v = 20 mV/s, Pt disk electrode, ref Ag/AgCl; values are averages of at least three independent measurements with standard deviation in parentheses. cDiffusion coefficient of charge transport in solution as calculated by RDV and CC. dDegree of coverage with electroactive species as calculated by DPSCC. eHydroxymethylferrocene unit; concentration 0.04 mg/mL. Literature values: E0′ = 467 mV, ΔEp = 77 mV at 10 mV/s, D = 2.3 × 10−5 cm2/s; Pt disk electrode, ref Ag/AgCl.12,23

Figure 3. Rotating disk voltammetry. Plots of anodic limiting current as a function of electrode rotation speed. Conditions: E1 = 0.2 V, E2 = 0.65 V (vs Ag/AgCl), polymer concentration 1 mg/mL, 0.1 M TEAP/ CH2Cl2, Pt electrode. Although some curvature is observed, the data are analyzed according to a best fit linear regression of all data points.

Figure 5. Double potential step chronocoulometry. Plots of charge as a function of time as described in eq 2 (forward step) and charge as a function of the time function as described in eq 3 (backward step). Conditions: Ein= 0.2 V, Estep = 0.65 V, Efin = 0.2 V, Pt electrode, 0.1 M TEAP/CH2Cl2. The lines represent the linear regression of the experimental points. Although some curvature is observed, the data are analyzed according to a best fit linear regression of all data points.

peak may be the effect of an electron-withdrawing substituent that appears adjacent to the ferrocene side chain during the potential sweep (Table 1 and Figure 2B). Electrochemistry in Evaporatively Cast Films. The films were characterized by cyclic voltammetry in acetonitrile (AN)

containing 0.1 M TEAP or TBAP. No major differences in polymer electrochemistry were noticed using these two background electrolytes. Representative CV’s are shown in Figure 6. The redox potentials of the ferrocene moiety in evaporatively cast films (Table 2) were similar to those obtained in solution. The lack of any significant differences between E10′ of the polymer in solution and E0′ of the polymer in immobilized films demonstrates the absence of substantial differences in the electronic interactions of the electroactive species in the two environments. For each polymer, the first CV was very different from the subsequent CVs. After the “break-in” period of 4−5 scans the film reaches a steady state (constant peak currents), suggesting that specific film modifications took place during the first voltage−time sweeps. Background electrolyte migration/ diffusion accompanied by solvent penetration through the film may be responsible for these modifications. The electrochemical signal is consistent with the electrode surface bound redox behavior of the Fc groups (ΔEp < 40 mV, ΔEp,1/2 ca. 91 mV). The peak-to-peak separation is not exactly zero, probably due to a small ohmic drop in the film. The behavior of the evaporatively cast polymer films is also a function of the scan rate. Figure 7 shows the complex nature of the logarithmic dependence of the anodic peak current vs scan rate for films deposited from a 1 mg/mL polymer solution 0.1

Figure 4. Chronocoulometry. Plots of the charge as a function of the square root of time for the polymers in solution. Conditions: E1 = 0.2 V, E2 = 0.65 V, Pt electrode, 0.1 M TEAP/CH2Cl2. Although some curvature is observed, the data are analyzed according to a best fit linear regression of all data points. 3661

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Figure 7. Logarithmic plots of the anodic peak currents (ipa) vs scan rate (v) for ferrocene-substituted phosphazene terpolymers. Conditions: glassy carbon electrode (GC) in contact with 0.1 M TEAP/ CH3CN. Curve assignments are given in the legend.

Figure 6. Cyclic voltammograms of phosphazene terpolymer 6a (48% Fc) as an evaporatively deposited film. Conditions: polymer concentration of the deposition solution: 1 mg/mL, 0.1 M TBAP in CH3CN, scan rate 10 mV/s, glassy carbon electrode, voltage scan range: 0.0−1.0 V vs Ag/AgCl. Current scale is given in parentheses: (A) first scan (u = 20 μA for the positive direction and u = 5 μA for the negative direction, respectively); (B) cyclic voltammogram recorded after the break-in period of five scans (u = 2 μA).

Randles−Sevcik equation16 due to the particularities of the deposition procedure and the complex morphology of the film. However, at high scan rates, one can calculate the product CD01/2 from the slope of the correlation peak current vs square root of scan rate and assuming n = 1. This quantity is referred to as the charge transport rate and represents the flux of charge diffusion through the film (Table 2). At low scan rates (2−50 mV/s), polymers 6a and 6b show a clear tendency toward surface bound redox species.16 The polymer with the lowest degree of Fc substitution (6e) also exhibits a surface bound redox behavior, but for the entire range of scan rates. In these instances, the slope of log ipa vs log scan rate is ca. 1.0. This slope can be used to calculate the electrode surface coverage (Γ) with electroactive species if we assume n = 1 (Table 2). For polymers with intermediate degree of Fc substitution (6c and 6d) the slopes of the log ipa vs log scan rate (at high scan rates) are situated between 0.6 and 0.9, suggesting a mixed mechanism of charge transport (Table 2).

M TBAP/CH2Cl2 and scanned in a solution containing 0.1 M TEAP/AN. Polymers with higher degree of Fc substitution, 6a and 6b show a pronounced tendency toward diffusion limited behavior at high scan rates (60−250 mV/s). The slope of log ipa vs log scan rate is ca. 0.5. The bulky ferrocene side chains may make the film less compact, which allows the penetration of solvent and background electrolyte. The electrochemical response may be controlled by the diffusion of the background electrolyte counterion through the deposited film, which accompanies the redox at the ferrocene centers. Neither molar concentration, C, nor diffusion coefficient of the electroactive species, D0, can be directly estimated from the

Table 2. Influence of Degree of Substitution with Fc Groups on the Cyclic Voltammogram Characteristics of the Polymeric Filma polymer (% Fc)

E0′b (mV)

ΔEpb (mV)

ΔEp,1/2b,c (mV)

109Γ (mol/cm2)d

slope of the log correlatione

109CD01/2 (mol/(cm2 s1/2))f

slope of the log correlationg

6a (48%) 6b (41%) 6c (33%) 6d (29%) 6e (15%)

465(1) 476(1) 448(4) 455(6) 457(0)

36(5) 32(2) 28(2) 32(9) 26(0)

80(7) 90(6) 82(4) 91(7) 72(8)

1.59(19) 1.83(59) 0.0633(15) 0.0793(187) 0.00710(60)

0.918 0.948 0.973 0.953 1.023

0.90(29) 1.90(70) −h −h −h,i

0.497 0.533 0.633 0.731 1.023

a Evaporatively cast films scanned in 0.1 M TEAP on glassy carbon electrode; films were deposited from solutions containing 1 mg/mL polymers in 0.1 M TBAP in CH2Cl2; steady-state values. bData at v = 10 mV/s; the E1/2 values are relative to Ag/AgCl reference and are an average of at least three independent determinations with standard deviations in parentheses. cPeak width at half-height for the first anodic scan (at least three independent determinations with standard deviations in parentheses). dDegree of coverage with electroactive groups expressing surface concentration of redox sites calculated according to the equation of adsorbed species [ref 16], from data obtained over a scan rate range of 2−40 mV/s. eSlopes of the logarithmic correlation of anodic peak current vs scan rate at low scan rates for the degree of coverage with electroactive groups, Γ × 109 (mol/cm2). fCharge transport rates calculated according to the Randles−Sevcik equation,16 from data obtained over a scan rate range of 60− 250 mV/s. gSlopes of the logarithmic correlation of anodic peak current vs scan rate at high scan rates, for charge transport rates, 109CD01/2 (mol/ (cm2 s1/2)). hCharge transport rates, CD01/2, could not be calculated as the slope of the logarithmic correlation of anodic peak current vs scan rate was not 0.5. iThe degree of coverage with electroactive groups Γ (mol/cm2) was calculated from the logarithmic correlation of anodic peak current vs scan rate since the slope of this correlation was ∼1 for all scan rates.

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Film Morphology Investigation. The influence of film morphology on the electrochemical response of the evaporatively deposited polymers on the surface of the glassy carbon electrode indicates that these films have a different morphology than the adsorbed films observed in our solution studies. The adsorbed films formed during the cyclic voltammetric experiments in CH2Cl2 solution are saturated with background electrolyte and solvent while the evaporatively cast films do not contain the solvent, initially. These films were soaked for 15 min, allowing for some diffusion of solvent and background electrolyte into the film. During the voltage−time sweeps, there is a flux of electrolyte counterion in and out of the film to compensate for changes in charge on the pendant ferrocene groups (Fc/Fc+). From results obtained in solution, adsorbed film redox properties appear to be more sensitive to changes in electrolyte anion than cation (Supporting Information and Figure S-1). Extrapolating these results to the evaporatively cast films, we suggest that the limiting factor is the rate with which the polymer film changes its morphology as a result of the electrolyte anion flux that occurs with the voltage−time sweeps. The ease with which this anion flux occurs may depend on the degree of swelling of the film. This results in the “break-in” period. All of the above suggest that any voltage-induced morphological changes are correlated with the degree of polymer swelling and charge transport rates, where the electrode kinetics is limited by counterion diffusion. Scanning Electron Microscopy of Adsorbed Films from Solution and Evaporatively Cast Films. The shape of cyclic voltammograms of these polymers in solution suggests the formation of an adsorbed layer on the surface of the electrode during the voltage−time sweep.16 In order to detail this observation, the Pt electrode was carefully cleaned before a normal CV experiment. It was then placed in a dichloromethane solution containing the dissolved polymer 6b and background electrolyte (TBAP) and subjected to 10−12 potential sweeps (v = 10 mV/s) between 0.2 and 0.8 V. The electrode was then removed from solution, washed with a small amount of acetonitrile (a solvent for TBAP, but not for the polymer), and allowed to dry. The formation of the adsorbed layer on the electrode surface was probed by scanning electron microscopy (SEM). Figure S-2A,B (Supporting Information) presents two micrographic pictures of this adsorbed film. The front view (Figure S2-A) shows a smooth and uniform layer. From the side view (Figure S2B) the thickness of the film was determined to be 101−102 nm, which can be used to calculate the actual concentration of electroactive species. For an average value of 100 nm film thickness, and a degree of coverage Γ = 5 × 10−9 mol/cm2 (determined by DPSCC), a value of 0.5 M in ferrocene units was obtained, which is comparable with concentrations of electroactive sites in other polymer films.3d Another SEM experiment was performed to probe the surface of a film evaporatively cast on a glassy carbon electrode and to estimate the thickness of this film after the “break-in” period. Figure S-2C−E shows three micrographs (two front views and one side view) of polymer 6b after 5 scans in a solution containing 0.1 M TEAP/AN. The front view (Figure S-2C) reveals a rough film surface with ridges, valleys, and amorphous polymer aggregates. At a higher magnification, pinholes and regions where the film has been ruptured (Figure S-2D) are evident. The side view (Figure S-2E) shows large differences in film thickness (hundreds to thousands of nanometers). An average value of 1000 nm was used to

estimate the concentration of electroactive species. Based on this film thickness (l) and the electrochemically determined surface coverage, Γ = 1.83 × 10−9 mol/cm2 (Table 2), the concentration of ferrocene species (C = Γ/l) was estimated to be 3 mM. This value is small in comparison with other published values,3d,18 indicating that only a fraction of the ferrocene species are electroactive. Using the charge transport rates (CD1/2), the diffusion coefficient through film was calculated to be ca. 10−7 cm2/s. This relatively large value for an immobilized species is consistent with the charge transport being controlled by counterion diffusion in and out of the film. Mechanism of Charge Transport. The ability to vary the polymer lattice properties is important in understanding and exploiting the long-range charge transport in both solution and cast films. In addition, variation in the redox potential of ferrocene groups can shed light on the polymer environment in which these electroactive groups are embedded. In solution, it is generally accepted that electron diffusion represents a combination of physical diffusion and diffusion via electron hopping. However, the relative contribution of the two mechanisms is often difficult to determine.19−21 Physical diffusion of the bulky polymer chains may be decreased by the substitution with hydroxymethylferrocenyl groups, possibly by hydrogen bonding between hydroxyl groups. Additionally, the overall diffusion of the uncharged polymer (Fc) toward the electrode occurs in a boundary layer that also contains outwardly diffusing charged polymers (Fc+). If physical diffusion is slow enough in such mixed valence systems and the electron self-exchange is rapid (ca. 106−107 M−1 s−1 at 25 °C),22 then the charge transport may be enhanced by electron hopping from Fc to Fc+ groups. Dahms23 and Ruff24 proposed the following relationship for the mixed physical diffusion/ electron hopping situation, where Dapp is the observed diffusion coefficient, Dphys is the contribution of physical diffusion, and Deh is the contribution from electron hopping:

Dapp = Dphys + Deh The observed electron diffusion coefficients, Dapp, obtained for 6a−e in solution (Table 1) are unexpected considering previous results obtained for the phosphazene copolymers, 4.12 For copolymers, 4, a change from 44% to 6% in degree of substitution with hydroxymethylferrocenyl groups modified the diffusion coefficient by 5 orders of magnitude (10−8−10−13).12 The diffusion coefficients for 6a−e are generally higher than those of the copolymers 4 and relatively independent of the degree of ferrocene substitution as shown in Figure 8. The physical diffusion contribution to Dapp is probably constant as the copolymers and the terpolymers have similar molecular weights. The differentiating factor may be in the polymer composition and its effect on electron hopping through selfexchange. The bulky phenyl groups as phosphorus substituents in 4 (twice as many as in the terpolymers 6) provide an increased rigidity to the polymer backbone. This is reflected in the glass transition temperatures, which are 10−15 °C higher12 relative to those of the new polymers, 6 (Figure 1). This means that the torsional mobility of the terpolymers, 6, is larger, which will allow for an easier approach of ferrocene moieties. When one of these groups is in the oxidized state (Fc+) and the other is in its reduced state (Fc), the electron transfer may occur via through-space hopping between the redox centers. Thus, the electron hopping term, Deh, will be enhanced for the polymers 6 (fewer bulky phenyl groups), leading to an overall increase of the apparent diffusion coefficient, Dapp. The fact that the Dapp 3663

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due to a subsequent chemical reaction of the ferricenium ion produced in the first electrochemical step to generate another redox active species. This process is more evident in films adsorbed from solution than in evaporatively cast films. This is probably due to the increased mobility of the polymer in a film formed in situ, which is already saturated in background electrolyte and solvent as opposed to a film that is cast and dried on the electrode prior to the electrochemical experiment. An internal oxidation of the secondary hydroxyl group to a ketone may occur.18 An authentic sample of ferrocene carboxaldehyde in 0.1 M TBAP/dichloromethane has a redox potential measured by CV of 794 mV. This value is situated in the same potential region where the second electrochemical process appears in the CV for 6a−d (Table 1). Independent evidence for oxidation of the polymer side chain results from FTIR spectroscopy data. For this, 5 mL of 6b (1 mg/mL) in 0.1 M TBAP/CH2Cl2 was electrolyzed (potentiostatically) in a bulk cell at 0.85 V until the current dropped to near zero. At this point the electrolysis was stopped, the solvent was evaporated, and the polymer was thoroughly washed with acetonitrile, a solvent for TBAP but not for the polymer. Then the electrolyzed polymer was dried and subjected to an FTIR analysis in a KBr pellet. Another FTIR experiment was performed with the starting material (for reference), and the two spectra were compared (Figure 9). In

Figure 8. Semilogarithmic plot of diffusion coefficients as a function of % ferrocene substitution for (a) terpolymers, 6a−e; (b) copolymers, 4a−d. The diamond corresponds to the measured diffusion coefficient of monomeric hydroxymethylferrocene (HMF) (η5-C5H5)Fe(η5C5H4CH2OH).

for 6a−e does not appear to depend on the degree of substitution with ferrocene groups (unlike Dapp for 4a−d) suggests that in this case an upper limit for the diffusion coefficient is approached. In evaporatively cast films, the surface coverage, Γ, of 6a−e on the glassy carbon electrode spans from 10−9 to 10−12 mol/ cm2 (Table 2), which suggests that multiple layers are present. For the highest substituted polymer (6a, 48% Fc), the density of the ferrocene groups is high (almost every other unit substituted with a Fc pending group), the film is less compact (due to more bulky side chains), which suggests that the electron hopping mechanism has a significant contribution to charge transport. On the other hand, for the lowest substituted polymer (6e, 15% Fc), the film may be more compact and the distance between two neighboring ferrocene groups is large enough to considerably reduce the probability of through space electron transfer. According to Ruff,24a,b the diffusion coefficient of the electroactive species decreases with the second power of the distance between the electron transfer sites. In this case, not all of the Fc groups may be redox active, leading to a significant decrease in charge transport and degree of coverage with electroactive groups (Figure 7 and Table 2). In conclusion, mobility of the polymer in solution enables long-range transport of the charge by both physical diffusion and electron hopping mechanisms. The fact that diffusion coefficients are large and relatively constant indicates that the electron diffusion rates are at maximum by both transport mechanisms. In contrast, the charge transport in evaporatively cast films may be conceived as proceeding by a multistep mechanism.25 Since the physical diffusion is restricted by polymer immobilization in the film, the charge can be transmitted through the film by electron hopping between adjacent ferrocene groups. This mechanism is extensively influenced by polymer and film characteristics such as concentration of electroactive species, polymer torsional mobility (reflected in Tg), and film morphology. The change in the oxidation state of Fc groups requires diffusion of compensating charges through the film. This can occur mainly through physical diffusion of background electrolyte counterions, which must be associated with solvent flow into the polymer lattice. Mechanism of Redox Transformations. The appearance of the second current signal during the voltage−time scan may be

Figure 9. FTIR transmission spectra (KBr pellet) of the starting material and electrolyzed material from a solution containing 1 mg/ mL of polymer 6b (41% Fc) in 0.1 M TEAP/CH2Cl2 at the Pt electrode. The arrow indicates the appearance of an absorption band characteristic of a carbonyl group.

the electrolyzed material, there is a new band around 1650 cm−1 that is characteristic of a ⟩CO symmetrical stretch. These data indicate that during the electrochemical process some of the hydroxymethylferrocenyl groups are oxidized to carbonyl species, such as acyl ferrocenyl groups. We propose that an internal redox takes place between two oxidized species probably in the adsorbed layer on the electrode surface. The iron centers are reduced to iron(II) simultaneously with the oxidation of the ⟩CH−OH to a ⟩CO group. This process is more likely to occur in polymers with a high degree of ferrocene substitution, such as 6a and 6b, where the Fc groups are in close proximity. This is in contrast to polymers with low degree of ferrocene substitution, such as 6e, where presumably the Fc groups are too far away from each other for 3664

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Figure 10. Proposed reaction mechanism for the electrochemistry of phosphazene terpolymers. The main electrochemical process is represented by E0′1, while the second electrochemical process is represented by E0′2. A few side-chain units undergo an internal oxidation in which the ⟩CHOH group is converted to a ⟩CO group simultaneously with the reduction of two Fe(III) to Fe(II).

The redox behavior of the terpolymer films deposited from solution may result in the internal oxidation of a small fraction of the available hydroxyl groups to ketone groups in the hydroxymethylferrocenyl side chain of the polymer that yields a new electrochemically active species. In evaporatively cast films, the degree of coverage with electroactive groups decreases by over 2 orders of magnitude when the degree of substitution with Fc groups decreases from 50 to 15%. This suggests that a lower number of Fc groups are redox active due to the morphology of the cast film and increased separation distance between them, which lowers electron diffusion via through space electron hopping. Scanning electron microscopy revealed that the evaporatively cast films and the films deposited during redox cycling in solution have different morphologies.

this secondary process to occur. In agreement with these results, we propose a reaction scheme for the overall redox process that is illustrated in Figure 10. The main process is the oxidation−reduction of the ferrocene moiety. For the higher substituted polymers (6a, 6b) some of the Fc+ groups can undergo a reduction at the expense of an internal oxidation of an adjacent ⟩CH(OH) to a ⟩CO. This process will create another electroactive species with a redox potential shifted positive due to the electron-withdrawing nature of the carbonyl group.

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CONCLUSIONS A series of new ferrocene-containing polyphosphazene terpolymers 6a−e were prepared by deprotonation−substitution reactions on phosphazene copolymer, 5, which is comprised of nearly equal portions of randomly distributed [(Ph)(Me)PN] and [Me2PN] monomer units. The torsional mobility of these terpolymers, as reflected by lower Tg values, is somewhat greater than related ferrocenyl copolymers 4a−d in which all phosphorus atoms contain one bulky phenyl group rather than smaller Me groups. Reversible electrochemistry was observed for the ferrocene moiety in polymers 6a−e with the phosphazene backbone acting as an electron-withdrawing group. In solution, the diffusion coefficients of the terpolymers 6 appear to be largely independent of the degree of substitution with ferrocene groups and are larger than those of related copolymers 4a−d that contain more bulky phenyl groups. This may be due to the increased torsional mobility of the terpolymer backbone (lower Tg) in which smaller methyl side groups replace approximately half of the phenyl side groups.

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EXPERIMENTAL SECTION

Materials. Poly(methylphenylphosphazene)-co-poly(dimethylphosphazene), [(Me)(Ph)PN]0.6[Me2PN]0.4, 5, was prepared by a published procedure13 via the condensation polymerization of approximately equal portions of Me3SiNP(OPh)(Ph)(Me) and Me3SiNP(OPh)Me2. Ferrocene carboxaldehyde and n-BuLi (2.5 M in hexane) were purchased from Aldrich and used as received. THF was freshly distilled from Na/benzophenone and hexane was distilled from CaH2. The background electrolytes tetraethylammonium perchlorate, TEAP (S.A. Chem.), tetrabutylammonium perchlorate, TBAP (Aldrich), tetraethylammonium tetrafluoroborate, TEABF (Aldrich), and tetrabutylammonium tetrafluoroborate, TBABF (Aldrich), were free of any electrochemical impurities and were dried under vacuum at 25 °C and stored under a pressure of 1 mmHg. Hydroxymethylferro3665

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cene (97% purity, Aldrich), characterized by its melting point (80 °C, literature value 80−82 °C26), and ferrocene carboxaldehyde (98% purity, Aldrich) were free of any electrochemical impurities. Dichloromethane, CH2Cl2 (Mallinckrodt), HPLC grade, was used as received. Acetonitrile, AN (Mallinckrodt), was stored over 0.4 nm molecular sieves at least 24 h before use. Doubly distilled water was used when necessary. Polymer Characterization. The 1H, 13C, and 31P NMR spectra were recorded in CDCl3 on a Bruker WP 200 SY FT NMR spectrometer or on a SGI/Bruker DRX-400 sb spectrometer. Positive 1 H and 13C NMR shifts and 31P NMR shifts are downfield from the external references Me4Si and H3PO4, respectively. Elemental analyses were carried out using a Carbo Erba Strumentazione 1106 CHN elemental analyzer. Gel permeation chromatography measurements were performed on a Waters Associates GPC II instrument using 104, 105, and 106 μstyragel columns maintained at a temperature of 30 °C. The operating conditions consisted of a flow rate of 1.5 mL/min of unstabilized HPLC-grade THF containing 0.1% tetra-n-butylammonium bromide, [(n-Bu)4NBr]. The samples were dissolved in the mobile phase and filtered through 0.5 μm Teflon filters prior to injection. The injection volume was typically 0.05 mL of a 0.1% solution. The system was calibrated with a series of narrow molecular weight polystyrene standards in the molecular weight range of ca. 103− 106. The glass transition temperatures were determined using a Dupont DSC Model 910 instrument equipped with a TA Operating Software Module and Data Analysis system against an aluminum. Samples of 5−10 mg were crimped in a small aluminum boat and were heated from −80 to 120 °C at 10 °C/min. Glass transition temperatures listed in this study were the average of data obtained from second and third heating cycles. FTIR experiments were performed on a Bruker IFS88 continuous scanning spectrophotometer with a DTGS detector and a Globar source and spectral resolution set at 4.0 cm−1. The terpolymer samples were incorporated in KBr pellets. Polymer Synthesis. The parent copolymer 5 (3.0 g) was placed in a two-neck flask and dried under vacuum at 50 °C for at least 24 h immediately before use. The flask was filled with N2 and then equipped with a nitrogen inlet, a rubber septum, and a magnetic stir bar. THF (30 mL) was added to dissolve the polymer, and the solution was cooled to 0 °C. Then n-BuLi (different volumes, varying from 6.45 mL for polymer 6a to 2.26 mL for polymer 6e) was added via syringe, and the mixture was warmed to room temperature and stirred for 1 h. After recooling to 0 °C, ferrocene carboxaldehyde (3.5 g for polymer 6a to 1.2 g for polymer 6e, dissolved in ca. 20 mL of THF) was added. After stirring overnight at room temperature, 1 mL of saturated aqueous NH4Cl was added, and the mixture was stirred for ca. 1 h. Then more water was added and the volatile organic solvents were removed on a rotary evaporator to fully precipitate the polymer. The new terpolymers were further purified by dissolution in THF and precipitation into hexane, a process that was repeated three times, followed by drying under vacuum at 50 °C for ca. 24 h. Polymer, 5 (parent): Anal. Calcd C, 53.48; H, 6.46; N, 12.47. Found: C, 53.24; H, 6.35; N, 12.47. Tg = 2 °C. Mw =103 000, Mw/Mn = 2.0. Polymer 6a: Anal. Calcd for x = 0.5, y = 0.5; C, 57.50; H, 5.61; N, 6.39. Found: C, 56.70; H, 5.74; N, 6.34. Tg = 92 °C. Mw = 111 000, Mw/Mn = 1.8. Yield, 87%. Polymer 6b: Anal. Calcd for x = 0.6, y = 0.4; C, 57.05; H, 5.70; N, 7.08. Found: C, 56.76; H, 5.90; N, 6.19. Tg = 73 °C. Mw = 208 000, Mw/Mn = 3.56. Yield, 87%. Polymer 6c: Anal. Calcd for x = 0.65, y = 0.35; C, 56.78; H, 5.76; N, 7.48. Found: C, 54.92; H, 5.91; N, 7.42. Tg = 70 °C. Mw = 101 000, Mw/Mn = 1.8. Yield, 86%. Polymer 6d: Anal. Calcd for x = 0.7, y = 0.3; C, 56.48; H, 5.74; N, 7.94. Found: C, 54.93; H, 5.96; N, 7.18. Tg = 67 °C. Mw = 154 000, Mw/Mn = 1.8. Yield, 60%. Polymer 6e: Anal. Calcd for x = 0.85, y = 0.15; C, 55.31; H, 6.17; N, 9.70. Found: C, 53.18; H, 6.14; N, 9.50. Tg = 43 °C. Mw = 103 000, Mw/Mn = 1.8. Yield, 80%. Electrochemical and Physicochemical Measurements. Chronocoulometry (CC), cyclic voltammetry (CV), and double potential step chronocoulometry (DPSCC) were performed using a BAS-CV 27 voltammograph or a PAR Model 173 potentiostat coupled to a PAR 175 programmer, and the data were recorded on an Omnigraphic 100

Recorder (Houston Instrument). Rotating disk voltammetry (RDV) measurements were performed on a Pine AFM SRX modulated-speed rotator coupled to the BAS-CV 27 voltammograph. The electrochemical cell was a three-electrode cell with unseparated compartments. The auxiliary electrode was a Pt wire, and the reference electrode was a BAS Ag/AgCl electrode in 3 M NaCl. The working electrodes were Pt disks 2 mm in diameter for CV and DPSCC and 7 mm in diameter for RDV and CC. The Pt electrodes were prepared chemically and electrochemically before performing the measurements as follows. First, they were polished on a Buhler microcloth pad, then soaked in 0.1 M HNO3 for about 15 min, washed with deionized water, and dried. The electrochemical cleaning consisted of a potential step to −0.27 V (vs Ag/AgCl) with the electrode soaked in a 1 M solution H2SO4 (ca. 2 min) followed by slowly bringing the potential to +0.05 V, which was maintained until the current dropped to zero. Then the electrode was scanned in the same solution (v = 50 mV/s) until a reproducible cyclic voltammogram was obtained.27 For the film study, a glassy carbon electrode (BAS, 3 mm diameter) was employed as a working electrode. The electrode was chemically conditioned in the same manner as the platinum electrode. The electrochemical measurements were performed in CH2Cl2 solutions of polymer and background electrolyte and on evaporatively cast films in CH3CN and background electrolyte. For CV studies in solution, the electrode was kept in solution for ca. 10−20 scans in order to record and analyze the adsorption of the electroactive species. CC and DPSCC also show an adsorbed layer building up on the electrode surface. Redox potentials, E0′, were calculated from the anodic (Epa) and cathodic (Epc) peak potentials:

E 0′ =

Epa + Epc 2

(1)

All potentials are reported vs the Ag/AgCl (3 M NaCl) reference electrode. All experiments were performed at ambient room temperature (23 ± 2 °C). No resistance compensation was attempted due to the unknown nature of the polymer film contribution to the cell resistance. Solution densities (d) were obtained using the pycnometer method. Dynamic viscosity (η) measurements of polymer solutions were taken at 25 ± 2 °C using the thin capillary method.28 The density and dynamic viscosity were utilized to calculate the kinematic viscosity (υ) according to the equation υ = η/d. We found that υ is not significantly different from the kinematic viscosity of the solvent, and therefore for most of the polymer solutions we used the kinematic viscosity of the solvent (υ = 3.11 × 10−3 cm2/s).26 Scanning electron microscopy was performed on a Phillips 501 microscope (electron beam at 72 kV). The electrodes were subjected to a Au−Pd D/C plating procedure (in a Hummer V sputter coater) prior to the SEM experiment. For front and side views, the specimens were tilted at 55 and 0 °C, respectively. Polymer Film Preparation. The preparation of the polymer film coated electrodes was based on a previously published procedure12 where the deposition method was optimized with respect to the longevity of the film on the electrode surface. Dichloromethane solutions of each terpolymer in the presence of 0.1 M TBAP were employed as casting solutions. The working electrode was polished using a Buehler Microcloth pad on a spinning wheel for about 3 min followed by sonication (ca. 15 min) in absolute ethanol, washed thoroughly with CH2Cl2, and allowed to dry. Six drops (ca. 30−40 μL) of polymer solution (ca. 1 mg/mL) were placed on the clean electrode surface and allowed to evaporate (3−24 h). The electrode was then soaked for 15 min in a solution of 0.1 M background electrolyte/ solvent and subsequently scanned in a fresh solution of the same composition. This was done in order to avoid any dissolution of the polymer film in the scanning solution and to allow the polymer to swell with solvent and background electrolyte prior to any electrochemical experiment. The lifetime of the polymer film on the GC electrode was ∼2 h. The electrochemical behavior of the film was not influenced by the time of drying but was influenced by the time of soaking in the background/solvent solution. Calculations of the Diffusion Coefficients and Surface Coverage with Electroactive Groups. Diffusion coefficients of 3666

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the terpolymers in solution were calculated by a combination method of RDV and CC measurements as described previously.17 This was done in order to eliminate problems arising from uncertainties in the number of electrons transferred per polymer molecule, area of the electrode, and molar concentration of the polymer in the bulk solution (due to the wide range in molecular weights arising from the polymer polydispersity). For the determination of surface coverage in solution, a modification of Heineman’s DPSCC method was used.29 In our case for the forward (positive) step, the charge passed (Qfw) is given by

Q fw = 2π −1/2nFACD01/2t 1/2 + Q dl

ACKNOWLEDGMENTS A.L.C. thanks Duke University and P.W.-N. The Welch Foundation (N-1181) for generous financial support of this project at Southern Methodist University. We gratefully acknowledge Karl Koch for obtaining elemental analyses, Prof. Palmer, Duke University, for assistance with the FTIR experiments, and the Duke Biology Department for the SEM measurements.

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ABBREVIATIONS AN, acetonitrile; CC, chronocoulometry; CV, cyclic voltammetric waves; DPSCC, double potential step chronocoulometry; Fc, ferrocene group; Fc+, ferricenium ion; HMF, hydroxymethylferrocene; RDV, rotating disk voltammetry; TBAP, tetra-n-butylammonium perchlorate; TEAP, tetraethylammonium perchlorate; Tg, glass transition temperature; v, scan rate (cyclic voltammetry).

(2)

where Qdl is the double layer charge, n the number of electrons transferred, A the geometric surface area of the working electrode, F Faraday’s constant, D0 the diffusion coefficient of the electroactive species, C the molar concentration of polymer in solution, and t the time. The plot Qfw vs t1/2 is linear and has an intercept equal to Qdl. In the reverse step the charge passed during the potential step is

Q rev = 2π −1/2nFACD01/2f (t ) + Q ads + Q dl

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(3)

In agreement with the formula for adsorbed species

(4) 16

Q ads = nFA Γ

(5)

one can calculate the degree of electrode coverage with active (n = 1) ferrocenyl groups, Γ. In evaporatively cast films, cyclic voltammetry was used in order to calculate the degree of coverage of the electrode with electroactive groups (Γ) from the slope of ip vs v correlation at low scan rates (2−50 mV/s),12 assuming n = 1. Charge transport rates (DC01/2) were calculated from the slope of ip vs v1/2 correlation, using cyclic voltammetry at high scan rate (60−250 mV/s).12 Error assignments for the degree of coverage and charge transport rates are calculated based on uncertainties in the slope of the corresponding linear regression. Error assignments for the redox potential, peak-to-peak separation, and peak current ratios are the standard deviation of an average of at least three independent determinations unless otherwise mentioned.

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ASSOCIATED CONTENT

S Supporting Information *

Scanning electron micrographs and an additional figure. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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where Qrev is the charge for the reduction reaction, f(t) the time function, f(t) = t1/2 − τ1/2 + (t − τ)1/2, τ the time for the potential reverse step (potential switch), and Qads the charge for the adsorbed species. For the reverse step, there is a linear dependence of charge vs f(t). The intercept of this plot is Qads + Qdl. Thus, the difference between the absolute value of the intercept for the reverse step and for the forward step gives the charge consumed for the adsorption of the product on the electrode surface, provided the double layer charge is the same for both steps:

Q ads = |Q rev| − Q fw

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.L.C.), pwisian@smu. edu (P.W.-N.). Notes

The authors declare no competing financial interest. 3667

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