- Email: [email protected]
Structural effects on the electrochemistry of ferrocene in Nafion films on electrodes
J. Electroanal. Chem., 176 (1984) 359--362
359
Elsevier Sequoia S A., Lausanne -- Printed in The Netherlands Preliminary note STRUCTURAL EFFECTS ON THE ELECTROCHEMISTRY OF FERROCENE IN NAFION FILMS ON ELECTRODES
ISRAEL RUBINSTEIN Department of Plastics Research, The Weizmann Institute of Science, Rehovot 76100 (Israel)
(Received 18th June 1984;in revised form 23rd July 1984)
INTRODUCTION Nafion-coated electrodes*, originally introduced by Rubinstein and Bard [1, 2], were shown to immobilize electroactive cations strongly onto electrode surfaces by ion-exchange action (electrostatic binding) [3--9]. It is widely accepted that the structure of Nafion involves interconnected ionic clusters in a bulk of hydrophobic fluorocarbon phase. Yeager and Steck [ 10] proposed a threeregion structure, i.e. a hydrophobic phase, ionic clusters, and an interfacial region. Although the electrochemistry of incorporated species should be influenced by the chemical environment in the polymer, no unusual redox potentials were observed with Nafion-coated electrodes. These experiments, however, always involved positively-charged immobilized species. Since the determining factor is the relative stability of the oxidized and reduced forms in different regions with different electrostatic and hydrophobic interaction [3,8], one expects much greater structural effects with electrode couples comprising neutral and positively-charged forms. Such a case is the ferrocene--ferricinium couple (Fc/Fc÷), which was f o u n d to display a very unusual electrochemical behaviour in Nafion films on electrodes, as described below. EXPERIMENTAL The solution used was aqueous 0.1 M Na2SO4, saturated with Fc (Aldrich Chemical Co.; recrystallized from petroleum ether). A ferricinium-free solution was obtained by pre-electrolysing at -0.450 V during Fc dissolution. The saturation concentration of Fc in aqueous solution is ~1.7 X 10 -s M [11]. Trim e t h y l a m i n o m e t h y l ferrocene (Me3N÷MeFc) perchlorate was obtained by metathesis o f the corresponding iodide salt (Strem Chemicals). All solutions were prepared with triply
360 The film thickness was ~0.2 p m. A mercurous sulfate reference electrode (MSE; +0.405 V vs. saturated calomel electrode) was used. RESULTS AND DISCUSSION Figure 1 presents the electrochemical behaviour of the Fc/Fc ÷ system. When the Nation-coated electrode (Au/Naf) is cycled in the Fc solution (curve B), the Fc/Fc ÷ peaks which gradually increase are, at steady state, larger with respect to the bare electrode (curve A), which is c o m m o n for incorporated species; but the peaks are also shifted negatively by ~ 9 0 mV. If the Au/Naf electrode is held at a constant potential of +0.07 V for 3 min in a stirred solution and then cycled, curve C is obtained, with considerably larger initial oxidation--reduction peaks, separated by ~ 1 3 0 mV. The area under the initial peaks corresponds to ~6% loading. Upon repetitive cycling, the peaks decrease continuously, and it becomes evident that the oxidation involves two distinct, well-defined peaks (curve 4). The more positive peak eventually disappears, while the other, similar to curve 8, remains at steady state. Note that in background solution (no Fc) both oxidation peaks of Fig. 1C eventually disappear. Qualitatively similar results are also obtained with glassy carbon electrodes, indicating little or no effect of the electrode material on these results.
A
A
I4t'LA
C
I
I
i
I
i
I
-06 - 0 4 -02 E / V ( v s MSE)
a
I
O0
I
I
I
-04
M
I
i
I
i
I
-02 O0 0 2 E / V (vs MSE)
l
I
04
Fig. 1. Current--potential curves in aqueous 0.1 M Na2SO4 saturated with Fc (scan rate, 0.1
V/s). (A) Bare Au electrode. (B) Au/Naf, at steady state (after 5 min cycling). (C) Au/Naf, held 3 min at +0.07 V in a stirred solution; (curve 1) 1st cycle; (2--6) cycle Nos. 8, 16, 24, 32 and 68 (steady state), respectively. Fig. 2. Current--potential curves in aqueous 0.1 M Na2SO4 containing 1.7 × 10-S M Me3N+MeFc-C10~ (scan rate, 0.1 V/s). (A) Bare Au electrode. (B) Au/Naf; (curves 1--5) cycle Nos. 4, 48,104, 210 and 420 (steady state), respectively.
361 For comparison, Fig. 2 presents current--potential curves in a solution 1.7 X 10 -s M in Me3N÷MeFc perchlorate. Here, both the oxidized and reduced forms are cations, and indeed the usual behaviour reported previously for cationic couples in Nation is observed. We attribute the unusual behaviour of Fc in Nation-coated electrodes to the markedly different interactions of the neutral and ionic forms in different regions of the polymer. These probably include the interfacial region and the ionic clusters (denoted regions B and C in ref. 10), since it appears unlikely that the hydrophobic fluorocarbon phase is electrochemically active. The large initial oxidation/reduction peaks in Fig. 1C are nearly ideal thinlayer peaks, but separated b y ~ 1 3 0 mV. Supported by the later appearance of the second, "hidden" oxidation peak, this appears to be a clear case of a "square scheme" [6,13 ], with two different redox potentials corresponding to species residing in chemically different sites of the polymer. It can be reasonably assumed that the two types of sites are the interracial region and the ionic clusters. Fc ÷ (the initially incorporated species in Fig. 1C) is thus reduced in the aqueous region at a more negative potential, while the neutral and non-polar Fc is oxidized in the more hydrophobic interfacial region at a more positive potential; and a rapid mass transfer occurs between the regions u p o n oxidation or reduction. This assignment of redox potentials is consistent with the relative stabilities of the neutral and ionic forms in the two regions. Evidently this represents a non-steady-state situation, with gradual loss of neutral Fc from the polymer u p o n cycling. From Figs. 1B and 1C it is apparent that the small a m o u n t of Fc/Fc ÷ incorporated in the polymer at steady-state cycling remains in the aqueous region in both forms, with the Fc probably being stabilized by the small number of fluorocarbon chains found in the ionic cluster [ 1 0 ] . This results in the unusual observation of both oxidation peaks appearing in the same scan (Fig. 1C, curve 4). Furthermore, we have a unique case of a "square scheme" in which at least one complete couple can be observed independently. The model described above has other implications. For example, it requires continuity o f the two phases, i.e. in the interconnecting "channels" as well as in the clusters. Also, probing of the different regions with Fc/Fc ÷ m a y provide information on the partition of other species in the polymer. An interesting question is w h y the redox potential assigned to the ionic cluster (Fig. 1B) is shifted negative with respect to the bare electrode (Fig. 1A). Experimental indications relate this to strong association of Fc ÷ with --SO; groups, of which a remarkably high concentration is found in the clusters. Intentional reduction of the number of free sulfonate groups shifts the peaks in Fig. 1B positively, even b e y o n d those in Fig. 1A. This and other aspects of t h e system and the proposed model will be described in detail in forthcoming publications.
REFERENCES 1 I. Rubinstein and A.J. Bard, J. Am. Chem. Soc., 102 (1980) 6641. 2 I. Rubinstein and A.J. Bard, J. Am. Chem. Soc., 103 (1981) 5007. 3 C.R. Martin, I. Rubinstein and A.J. Bard, J. Am. Chem. Soc., 104 (1982) 4817.
362 4 5 6 7 8 9 10 11 12 13
H.S. White, J. Leddy and A.J. Bard, J. Am. Chem. Soc., 104 (1982) 4811. T.P. Henning, H.S. White and A.J. Bard, J. Am. Chem. Soc., 104 (1982) 5862. T.P. Henning and A.J. Bard, J. Electrochem. Soc., 130 (1983) 613. D.A. Buttry and F.C. Anson, J. Am. Chem. Soc., 104 (1982) 4824. D.A. Buttry and F.C. Anson, J. Am. Chem. Soc., 105 (1983) 685. N. Oyama, T. Ohsaka, K. Sato and H. Yamamoto, Anal. Chem., 55 (1983) 1429. H.L. Yeager and A. Steck, J. Electrochem. Soc., 128 (1981) 1880. I.M. Kolthoff and F.G. Thomas, J. Phys. Chem., 69 (1965) 3049. C.R. Martin, T.A. Rhoades and J.A. Ferguson, Anal. Chem., 54 (1982) 1639. P.J. Peerce and A.J. Bard, J. Electroanal. Chem., 114 (1980) 89.
359
Elsevier Sequoia S A., Lausanne -- Printed in The Netherlands Preliminary note STRUCTURAL EFFECTS ON THE ELECTROCHEMISTRY OF FERROCENE IN NAFION FILMS ON ELECTRODES
ISRAEL RUBINSTEIN Department of Plastics Research, The Weizmann Institute of Science, Rehovot 76100 (Israel)
(Received 18th June 1984;in revised form 23rd July 1984)
INTRODUCTION Nafion-coated electrodes*, originally introduced by Rubinstein and Bard [1, 2], were shown to immobilize electroactive cations strongly onto electrode surfaces by ion-exchange action (electrostatic binding) [3--9]. It is widely accepted that the structure of Nafion involves interconnected ionic clusters in a bulk of hydrophobic fluorocarbon phase. Yeager and Steck [ 10] proposed a threeregion structure, i.e. a hydrophobic phase, ionic clusters, and an interfacial region. Although the electrochemistry of incorporated species should be influenced by the chemical environment in the polymer, no unusual redox potentials were observed with Nafion-coated electrodes. These experiments, however, always involved positively-charged immobilized species. Since the determining factor is the relative stability of the oxidized and reduced forms in different regions with different electrostatic and hydrophobic interaction [3,8], one expects much greater structural effects with electrode couples comprising neutral and positively-charged forms. Such a case is the ferrocene--ferricinium couple (Fc/Fc÷), which was f o u n d to display a very unusual electrochemical behaviour in Nafion films on electrodes, as described below. EXPERIMENTAL The solution used was aqueous 0.1 M Na2SO4, saturated with Fc (Aldrich Chemical Co.; recrystallized from petroleum ether). A ferricinium-free solution was obtained by pre-electrolysing at -0.450 V during Fc dissolution. The saturation concentration of Fc in aqueous solution is ~1.7 X 10 -s M [11]. Trim e t h y l a m i n o m e t h y l ferrocene (Me3N÷MeFc) perchlorate was obtained by metathesis o f the corresponding iodide salt (Strem Chemicals). All solutions were prepared with triply
360 The film thickness was ~0.2 p m. A mercurous sulfate reference electrode (MSE; +0.405 V vs. saturated calomel electrode) was used. RESULTS AND DISCUSSION Figure 1 presents the electrochemical behaviour of the Fc/Fc ÷ system. When the Nation-coated electrode (Au/Naf) is cycled in the Fc solution (curve B), the Fc/Fc ÷ peaks which gradually increase are, at steady state, larger with respect to the bare electrode (curve A), which is c o m m o n for incorporated species; but the peaks are also shifted negatively by ~ 9 0 mV. If the Au/Naf electrode is held at a constant potential of +0.07 V for 3 min in a stirred solution and then cycled, curve C is obtained, with considerably larger initial oxidation--reduction peaks, separated by ~ 1 3 0 mV. The area under the initial peaks corresponds to ~6% loading. Upon repetitive cycling, the peaks decrease continuously, and it becomes evident that the oxidation involves two distinct, well-defined peaks (curve 4). The more positive peak eventually disappears, while the other, similar to curve 8, remains at steady state. Note that in background solution (no Fc) both oxidation peaks of Fig. 1C eventually disappear. Qualitatively similar results are also obtained with glassy carbon electrodes, indicating little or no effect of the electrode material on these results.
A
A
I4t'LA
C
I
I
i
I
i
I
-06 - 0 4 -02 E / V ( v s MSE)
a
I
O0
I
I
I
-04
M
I
i
I
i
I
-02 O0 0 2 E / V (vs MSE)
l
I
04
Fig. 1. Current--potential curves in aqueous 0.1 M Na2SO4 saturated with Fc (scan rate, 0.1
V/s). (A) Bare Au electrode. (B) Au/Naf, at steady state (after 5 min cycling). (C) Au/Naf, held 3 min at +0.07 V in a stirred solution; (curve 1) 1st cycle; (2--6) cycle Nos. 8, 16, 24, 32 and 68 (steady state), respectively. Fig. 2. Current--potential curves in aqueous 0.1 M Na2SO4 containing 1.7 × 10-S M Me3N+MeFc-C10~ (scan rate, 0.1 V/s). (A) Bare Au electrode. (B) Au/Naf; (curves 1--5) cycle Nos. 4, 48,104, 210 and 420 (steady state), respectively.
361 For comparison, Fig. 2 presents current--potential curves in a solution 1.7 X 10 -s M in Me3N÷MeFc perchlorate. Here, both the oxidized and reduced forms are cations, and indeed the usual behaviour reported previously for cationic couples in Nation is observed. We attribute the unusual behaviour of Fc in Nation-coated electrodes to the markedly different interactions of the neutral and ionic forms in different regions of the polymer. These probably include the interfacial region and the ionic clusters (denoted regions B and C in ref. 10), since it appears unlikely that the hydrophobic fluorocarbon phase is electrochemically active. The large initial oxidation/reduction peaks in Fig. 1C are nearly ideal thinlayer peaks, but separated b y ~ 1 3 0 mV. Supported by the later appearance of the second, "hidden" oxidation peak, this appears to be a clear case of a "square scheme" [6,13 ], with two different redox potentials corresponding to species residing in chemically different sites of the polymer. It can be reasonably assumed that the two types of sites are the interracial region and the ionic clusters. Fc ÷ (the initially incorporated species in Fig. 1C) is thus reduced in the aqueous region at a more negative potential, while the neutral and non-polar Fc is oxidized in the more hydrophobic interfacial region at a more positive potential; and a rapid mass transfer occurs between the regions u p o n oxidation or reduction. This assignment of redox potentials is consistent with the relative stabilities of the neutral and ionic forms in the two regions. Evidently this represents a non-steady-state situation, with gradual loss of neutral Fc from the polymer u p o n cycling. From Figs. 1B and 1C it is apparent that the small a m o u n t of Fc/Fc ÷ incorporated in the polymer at steady-state cycling remains in the aqueous region in both forms, with the Fc probably being stabilized by the small number of fluorocarbon chains found in the ionic cluster [ 1 0 ] . This results in the unusual observation of both oxidation peaks appearing in the same scan (Fig. 1C, curve 4). Furthermore, we have a unique case of a "square scheme" in which at least one complete couple can be observed independently. The model described above has other implications. For example, it requires continuity o f the two phases, i.e. in the interconnecting "channels" as well as in the clusters. Also, probing of the different regions with Fc/Fc ÷ m a y provide information on the partition of other species in the polymer. An interesting question is w h y the redox potential assigned to the ionic cluster (Fig. 1B) is shifted negative with respect to the bare electrode (Fig. 1A). Experimental indications relate this to strong association of Fc ÷ with --SO; groups, of which a remarkably high concentration is found in the clusters. Intentional reduction of the number of free sulfonate groups shifts the peaks in Fig. 1B positively, even b e y o n d those in Fig. 1A. This and other aspects of t h e system and the proposed model will be described in detail in forthcoming publications.
REFERENCES 1 I. Rubinstein and A.J. Bard, J. Am. Chem. Soc., 102 (1980) 6641. 2 I. Rubinstein and A.J. Bard, J. Am. Chem. Soc., 103 (1981) 5007. 3 C.R. Martin, I. Rubinstein and A.J. Bard, J. Am. Chem. Soc., 104 (1982) 4817.
362 4 5 6 7 8 9 10 11 12 13
H.S. White, J. Leddy and A.J. Bard, J. Am. Chem. Soc., 104 (1982) 4811. T.P. Henning, H.S. White and A.J. Bard, J. Am. Chem. Soc., 104 (1982) 5862. T.P. Henning and A.J. Bard, J. Electrochem. Soc., 130 (1983) 613. D.A. Buttry and F.C. Anson, J. Am. Chem. Soc., 104 (1982) 4824. D.A. Buttry and F.C. Anson, J. Am. Chem. Soc., 105 (1983) 685. N. Oyama, T. Ohsaka, K. Sato and H. Yamamoto, Anal. Chem., 55 (1983) 1429. H.L. Yeager and A. Steck, J. Electrochem. Soc., 128 (1981) 1880. I.M. Kolthoff and F.G. Thomas, J. Phys. Chem., 69 (1965) 3049. C.R. Martin, T.A. Rhoades and J.A. Ferguson, Anal. Chem., 54 (1982) 1639. P.J. Peerce and A.J. Bard, J. Electroanal. Chem., 114 (1980) 89.