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Electrochemistry in hydrophobic Nafion gels
129
J. Electroanai.
Chem., 2% (1990) 129-139
Elsevier Sequoia S.A., Lausanne
Electrochemistry
in hydrophobic Nafion gels
Part 2. Electrochemical behaviour and catalytic properties of electrodes modified by hydrophobic Nafion gels loaded with 9-phenylac~di~ium salts and ~thra~uinone C.P. Andrieux, P, Audebert and P. Hapiot Laboratoire d’Electrochimie 75005 Paris (France)
Mokdaire,
U.A. C.N. R.S. No. 438, Universitcl’ de Paris 7, 2 PI. Jiusieu,
B. Divisia-Blohom Eguipe ~El~tr~himie ~~l~~laire, ~RF/LC~-EM, 85X 38041 Grenoble Cedex (France)
C.E.N.G., Avenue de Martyrs,
P. Aldebert Groupe de Physi~o-chimie ~ol~cuiaire, 85X 38041 Grenoble Cea’ex (France)
~RF/~Ph-PCM,
C. E.N.G.. Avenue des Martyrs,
(Received 24 May 1990; in revised form 20 July 1990)
Abstract
The new and very promising kind of electrode modified by hydrophobic Nafion gels, the general type of which has been described in Part 1 of this series, has been loaded with an~raquinone and 9-phenyl acridinium salts and tested for activity towards the catalysed reduction of oxygen. The gels loaded with acridinium species were found to exhibit a fair catalytic activity, the rate of which was enhanced when compared with the solution values, due to the affinity of the ~rfluoronated chains for oxygen.
INTRODUCTION
In a previous communication [l] and Part 1 of this series [Z], we demonstrated that Nafion @ gel coatings are of special interest because they ally the chemical stability of this perfluorinated polymer to the unique property of behaving macroscopically like solids and microscopically like liquids, and thus allow us to prepare a wide range of modified electrodes by loading with various electroactive compounds such as ferrocenes. However, another interesting characteristic of Nafion is its affinity towards dioxygen solvation, which makes Nafion-coats electrodes suitable for 0, reduction catalysis. We wish to reported here further electrochemical results 0022-0728/90/$03.50
0 1990 - Etsevier Sequoia S.A.
130
& 0
000
lj
I-
Ii
Fig. 1. Formula of the 9-ph~yl-lo-~yl-ac~di~um
salts. R = CH, or C,H,.
on microelectrodes with gels loaded with anthraquinone or 9-phenyl-lo-alkyl acridinium iodides, both of which were chosen for their ability to catalyse under various conditions the reduction of oxygen. In addition, 9-phenyl-lo-alkyl acridinium cations possess the interesting peculiarity of providing upon oxidation some of the rare neutral radicals which are moderately stable in slightly to strongly acidic conditions 131, which make them good candidates for our study, if one considers the relatively strong ambient acidity of the H+ exchanged Nafion gel [4]. In this paper we show that the electrochemical study performed by cyclic voltammetry and chronoamperometry confirms the electroactivity of the loaded gels in these cases with very fast diffusion of the electr~he~cal catalysts; that is, electrons are transferred to dioxygen by an outer-sphere mechanism [5]. Preliminary attractive results had already been obtained [6] on the basis of similarities to the previously studied viologen [7]. Therefore, this property of the acridinium radical was investigated in the case of both classical TBP solutions and loaded gels (Fig. 1). While oxygen diffuses from the supernatant electrolyte into the gel where it can be reduced normally by an el~tr~he~cally irreversible process at potentials well below 1 V, its reduction wave disappears when the gel is loaded with 9-phenyl-lomethyl-acridinium. The similar dependence of the acridinium wave provides evidence of the occurrence of a catalytic process. The kinetics of the catalysis have been explored and will be presented here comparatively, both in pure homogeneous conditions (in the TBP + HClO, solution) and in heterogeneous conditions, with the help of a Nafion gel-coated electrode.
EXPERIMENTAL
Ant~aquinone was purchased (Fluka) and used without further p~fication. Propyl and methyl acridinium were prepared and tested. 9-Phenyl-lO-methylacridinium was prepared by heating 9-phenylacridine in an autoclave [8] at 110 o C with a large excess of methyl iodide (dark red needles filtered off the remaining methyl iodide, yield SOW). 9-Phenyl-lo-propyl-acridinium was prepared in low yields (dark red needles, 5-10%) by simply refluxing 9-phenylac~dine in excess propyl iodide for 12 h. As described in Part 1 of this series [2], a typical stiff gel was made by carefully mixing together lo-20 mg of cryocrushed Nafion powder (prepared according to
131
ref. 9), 3 to 5 times more tributylphosphate (TBP) *, and l-2 mg of the electroactive compound. Acetonitrile (about 0.5 ml) was added to homogenize and fluidize the mixture into a processable solution. A few microlitres of this solution were then micropipetted onto a 3 mm vitrous carbon microdisk. The acetonitrile was allowed to evaporate (20 min) to leave a stiff gel containing only the polymer, the heavy hydrophobic TBP and the electroactive compound. After the gel had dried, the modified electrode was simply transferred to a classical two-compartment cell filled with 0.3 M HClO, aqueous electrolyte, a sodium saturated calomel electrode and a platinum counter-electrode which was connected to an electrochemical device. For the experiments performed in TBP, an acetonitrile + HClO, bridge was used between the reference electrode and the solution. Except for the catalytic studies, all solutions were carefully degassed with nitrogen prior to the experiments. The potentiostat was a PAR 273 for large cell experiments and a home-built one for those with microelectrodes, fitted with a Sefram Plotter and a Nicolet digital oscilloscope for short time experiments. RESULTS
AND DISCUSSION
As shown previously, the main advantage of our system is that a hydrophobic solvent which is polar enough to prepare ionically conducting media is used to prepare the gel so that a layer of controlled thickness of gel can be used. The rest of the cell is filled with an aqueous electrolyte, which does not participate in the electrochemical reaction, which occurs solely inside the gel, and accordingly may contain a substrate which can pass in and out of the gel and undergo a catalytic reaction inside it. Electrochemical
behaviour of the loaded gels
Cyclic voltammetiy Figure 2 shows the voltammograms of 9-phenyl-lo-propyl-acridinium iodide in solution in DMF (a) and of 9-phenyl-lo-methyl-acridinium iodide in TBP (b) in the presence of perchloric acid. Both compounds exhibit typical reversible behaviour with a fast one-electron transfer at low and medium sweep rates. This result was previously reported in the case of 9-phenyl-lo-methyl-acridinium iodide in acetonitrile [3], and was explained by the formation and subsequent reoxidation of the corresponding neutral radical. Figure 3 shows the cyclic voltammograms obtained at different scan rates on a microelectrode of Nafion gels loaded with the same compounds. The same electrochemical behaviour is found both in solution and in the gels, showing no major perturbation induced by the Nafion itself. Since, in addition, except for the 50 mV difference between the redox potentials, there were no appreciable differences in the electrochemical reactivity between 9-phenyl-lo-
l
The preparation of the Nafion gels has been described in detail in Part 1 of this series [2].
132
Fig. 2. Cyclic voltammograms of (a) 9-phenyl-lo-propyl-acridinium iodide in dimethyl formamide with 0.1 M perchloric acid and (b) 9-phenyl-lO-methyl-actidinium iodide in TBP with the same electrolyte. The substrate concentration was 2 x 10m3 M and the scan rate 100 mV/s in both cases.
0
@
I
lOO/.tA
6-
~OSmA
E/V
E/V I
-0.5
I
0
I
-0.5
I
0
Fig. 3. Cyclic voltammograms of TBP+Nafion gels loaded with 0.5 M 9-phenyl-lo-propyl-acridinium iodide (a) and 0.5 M 9-phenyl-lo-methylacridinium iodide (b, c). The electrolyte above was 0.1 M aqueous perchloric acid. Scan rate for curves a and b: 5 V/s; curve c, from top to bottom: 100, 50, 20,lO and 5 V/s.
133 TABLE
1
Electrochemical data of the electroactive compounds studied Compound AcMe+ AcPr+ Anthraquinone Ferrocene
E" (TBP)/ V
E” (gel)/ V
lo6 D (gel)/ cm’ s-l
-0.55 - 0.50 0.02
-0.57 - 0.50 - 0.03
1 0.2 a
0.33
0.27
2
a The plots obtained did not show enough linearity.
propyl-acridinium iodide and 9-phenyl-lo-methyl-acridinium iodide in the gels, we decided to perform most of the work presented here, and especially the catalysis experiments, only with the second compound, which will simply be designated by “acridinium” in the following. It is clear that typical diffusion behaviour with rapid electron transfer is always obtained in the gels between 100 and 1 V/s with well-defined reversible peaks. However, this study was started with heavily loaded gels (above lo-’ M in substrate), and we found that the reoxidation peak diminishes and finally disappears when the scan rate falls to values below 1 V/s, showing that it is likely that the acridinium radical has unexpected reactivity in this case. Yet parallel investigations on the behaviour of the acridinium simply dissolved in TBP at classical concentrations of organic electrochemistry (2 X 10e3 M with perchloric acid as the electrolyte) showed ideal behaviour; therefore, its concentration in the gel was diminished to come closer to classical solution conditions. It could be noticed then (Fig. 7) that reversible behaviour of the acridinium occurs up to scan rates as slow as 0.1 V/s, and that at such concentrations similar electrochemical behaviour is observed both in the gel and in the TBP + HClO, medium. These results enable us to confirm that there is no perturbation due to the Nafion itself in the acridinium system and that the behaviour observed at high concentrations is due to the unexpected, but avoidable reactivity of the acridinium radical on itself or on the starting acridinium. Table 1 summarizes the electrochemical data on the electroactive compounds studied in addition to the results for ferrocene, which were obtained in the first part of the series. In the case of anthraquinone-loaded gels, the classical two-electron voltammogram can be observed, but in this case it is associated, as expected, with a much slower electron transfer. In Fig. 4 are represented the cyclic voltammograms at various scan rates obtained upon cycling an anthraquinone-loaded gel. It is apparent that the difference between the peak potentials AEr, equal to 150 mV at 50 mV/s, still increases as the scan rate increases further, featuring a typical very slow electrochemical couple. The apparent redox potential of the quinone system is -0.03 V, quite a positive value but explainable by the acidic surroundings of the Nafion gel. This value may be used to estimate the ambient pH of such a medium after a reasonable evaluation of the redox potential of anthraquinone in acidic TBP electrolytes. In order to evaluate this, a series of measurements of the anthraquinone
Fig. 4. Cyclic voltammogram of a Nafion gel loaded with anthraquinone at different scan rates; from top to bottom: 1, 0.5, 0.2,O.l and 0.05 V/s. Same electrolyte as for Figs. 1 and 2.
potential was performed in TBP in the presence of various concentrations of perchloric acid. It was found that the variation of the redox potential followed, as would be expected, the relationship E = E * - 0.06pH (taking pH =i -log(H+), and between 0.02 and 2 M HClO, to avoid the addition of another electrolyte salt), with E" = 0.03 V. Keeping the previously given average value of 0.05, V determined in the case of the ferroeenes for the difference between the redox potential of a given electroactive species in the gel compared with a TBP + HCIO, classical electrolyte, one finds E = - 0.02, - 0.06 pH for the apparent reduction potential of anthraquinone in a TBP -I- Nafion gel: from the value found for E in our gels (see Table 1) a pH of almost 0 is found which is in accordance with the very acidic character of an H+ exchanged gel f8].
Kinetic measurements Chronoamperometry was performed to extract the diffusion coefficients for the two acridiniums under study. Typical Cottrell plots are shown in Fig. 5, the linearity of which ~nfirms our earlier conclusion [1,2] about the stations by effusion just as in solution. The diffusion coefficients given in Table 1 are again in the range found in solution unlike those of classical Nafion-coated electrodes which are 103-lo4 lower, due to the strong electrostatic bonding between the electroactive
135
2
10
20
30
40
Fig. 5. Cottrell plots obtained from Nafion gels loaded with (a) 0.5 M 9-pbenyl-lo-methyl-acridinium iodide and (b) 0.3 M 9-phenyl-lo-propyl-acridinium iodide. Same conditions as before.
sites and the polymeric skeleton. This result is even more striking in this case, since our former studies treated Nafion gels loaded mostly with neutral molecules like ferroeene, while acridinium is a positively charged species which could be bound to the sulphonic group of the Nafion gel. As expected, there is a correlation between the steric hindrance of the electroactive molecules in the gel and their respective diffusion coefficients. Taking into account that this fast diffusion was also noticed with two positively charged ferrocenes, it can be concluded that in such gels even cationic species can move, if not completely freely, at least without strong interactions with the polymer network.
The stability of the gels was tested by repetitive cycling at 1 V/s in the case of acridinium and at 100 mV/s in the case of anthraquinone. In all cases, no more than a 10% decrease was observed in 3 h showing that very little desorption occurred, which is not unexpected with such hydrophobic compounds. In fact, we found that instead of releasing them, TBP + Nafion gel rather tends to concentrate organophilics [lo], so that the slight diminution of the signal observed in some cases should more properly be attributed to chemical evolution of the reactants rather than to their transfer into the aqueous solution. Catalysis
As stated in the Introduction, this study aimed mainly at the recognization and investigation of the catalytic properties of such coatings towards dioxygen reduc-
136
a
E/v b
E/V I
-1.5
1
-1
I
-a5
0
Fig. 6. Cyclic voltammograms registered on (a) a glassy carbon electrode in TBP+HClO, electrolyte under nitrogen and air saturated at 0.1 V/s and (b) a gel-coated electrode in 0.1 M aqueous perchloric acid under the same conditions.
tion. Figure 6 shows the comparative voltammograms, under nitrogen and airsaturated, obtained respectively on a naked glassy carbon electrode in TBP and on an electrode coated with an unloaded gel: that is, in the absence of any added electroactive substance. Just as in several previously described experiments, while no current was obtained under argon, a strongly irreversible wave was observed with air-saturated solutions corresponding to the two-electron reduction of the solvated oxygen, both in TBP and in the gel. The relative magnitude of the waves is almost the same, the preconcentration of the oxygen by the Nafion gel (compared with the solution) being co~terbalan~ probably by the slightly lower diffusion coefficient in this case. Turning to the behaviour of the loaded gels, we noticed that, unfortunately, anthraquinone-loaded gels appeared not to be affected by the presence of oxygen, the oxygen wave being too negative relative to the an~raquinone in this system at the ambient pH of the gel. We therefore turned our interest to the acridinium-loaded gels, and found that when the gel was cycled in the presence of oxygen (Fig. 7), the first cycle showed a clear improvement in the electrochemical signal, which in addition became totally irreversible behaviour, which is typical of a catalytic effect.
137
I
lop
@
A
L
I
I
0
I
40gA
1
to
20pA
I
Q-
EIV
El”
I
0
-0.5
Fig. 7. Catalytic properties of loaded Nafion gels. (a, b) Cyclic volt-ograms of two identical gels (prepared starting from the same solution) loaded with 3 x 1O-3 M 9-phenyl-lo-methyl-actidinium iodide at respectively 1 and 0.1 V/s under nitrogen, in a nitrogen-saturated electrolyte (as for Figs. l-4). (c, d) Same feature under air in an air-saturated electrolyte.
i
Q
I
-0.5
@
T
I
0
I
-0.5
I
0
Fig. 8. Homogeneous catalysis by 9-phenyl-lo-methyl-acridinium iodide in TBP. (a, b) Cyclic voltammograms obtained from 2 x 10K3 M 9-phenyl-lo-methyl-acridinium’iodide solution in TBP+ HClO, electrolyte saturated with nitrogen. (c, d) Same feature in the same electrolyte saturated with air.
138 TABLE 2 ip/ip” values at different scan rates
Scan rate/ VS-’
0.1 1 10 100
‘#p” Catalysis: homogen~us 1.94 1.32 B B
(TBP)
Catalysis: with the gel 5.26 3.85 2.76 1.18
a The peak shape was ill defined and prevented accurate measurement of its height.
Furthermore, no wave was observed at the potential of oxygen reduction, which is additional ~nfirmation of its previous reaction with the acridinium radical cation. The same phenomenon occurs similarly in the TBP solution on a naked glassy carbon electrode, as shown in Fig. 8. Therefore, two parallel studies of the catalysis in the gel (Fig. 6) and in a classical TBP + HClO, electrode (Fig. 8) were carried out. In the two series of experiments, the rate of the catalytic process could be estimated by analysis of the i,/i,” ratio at different scan speeds, i, and i; representing the current with and without the dioxygen substrate added, respectively [9]. It is clear from Table 2 that the catalysis, which is very weak at 100 V/s, becomes more and more important as the scan rate diminishes. It may also be noted that the catalytic effect is about two to three times greater when the catalyst is inside the gel rather than in solution in TBP. Comparison of the ip/i,” ratios when the catalysis is close to total, that is, at a scan rate of 100 mV/s, allows us to estimate by inference the partition of the oxygen in the gel, since nothing but the differences between the 0, concentrations can account for the discrepancies observed between the catalysis in the TBP solution and that in the gel. From the current values, it may thus be deduced that Oz is about 2.5 times more concentrated in a Nafion + TBP gel exposed to an air-saturated aqueous solution than in the air-saturated TBP + HClO, solution, therefore demonstrating once again the interest of the perfluoronated chains for oxygen solvation.
CONCLUSION
As demonstrated previously in the case of several substituted ferrocenes, we have shown again in this short study the closeIy ideal electrochemical behaviour of hydrophobic Nafion gels loaded with various electroactive compounds. In particular, it was observed that the fast kinetics typical of such systems are retained even when the electroactive species included a& cationic. The catalysis of the oxygen reduction by the acridinium species, demonstrated in the case of homogeneous conditions, was not only retained, but also enhanced in the gel system, most probably due to the known affinity of the perfluorated chains for oxygen salvation.
139
It may be believed that such systems could find applications in devices such as fuel cells, provided that they present the required stability required for such applications. REFERENCES 1 P. Audebert, P. Aldebert, B. Divisia-Blohom and J.M. Kern, J. Chem. Sot., Chem. Commun., (1989) 939. 2 C.P. Andrieux, P. Audebert, B. Divisia-Blohom, P. Aldebert and F. Michalak, J. Electroanal. Chem., 296 (1990) 117. 3 N.W. Koper, S.A. Jonker, J.W. Verhoeven and C. van Dick, Reck Trav. Chim. Pays-Bas, 104 (1985) 296. 4 P. Aldebert, personal communication, 1989. 5 J.-M. Savt+ant, Mechanisms and Reactivity in Organic Electrochemistry. Recent Advances. Advances in Electrochemistry, The Welsh Foundation, Houston, 1986, pp. 289-336. 6 P. Hapiot, unpublished results 7 C.P. Andrieux, P. Hapiot and J.-M. Savtant. J. Electroanal. Chem., 189 (1985) 121. 8 F.D. Popp, J. Org. Chem., 27 (1962) 2658. 9 C.P. Andrieux, C. Blocman, J.M. Dumas-Bouchiat, F. M’Halla and J.M. Sadant, J. Electroanal. Chem., 113 (1980) 19. 10 P. Audebert, B. Divisia-Blohom and F. Michalak, to be submitted.
J. Electroanai.
Chem., 2% (1990) 129-139
Elsevier Sequoia S.A., Lausanne
Electrochemistry
in hydrophobic Nafion gels
Part 2. Electrochemical behaviour and catalytic properties of electrodes modified by hydrophobic Nafion gels loaded with 9-phenylac~di~ium salts and ~thra~uinone C.P. Andrieux, P, Audebert and P. Hapiot Laboratoire d’Electrochimie 75005 Paris (France)
Mokdaire,
U.A. C.N. R.S. No. 438, Universitcl’ de Paris 7, 2 PI. Jiusieu,
B. Divisia-Blohom Eguipe ~El~tr~himie ~~l~~laire, ~RF/LC~-EM, 85X 38041 Grenoble Cedex (France)
C.E.N.G., Avenue de Martyrs,
P. Aldebert Groupe de Physi~o-chimie ~ol~cuiaire, 85X 38041 Grenoble Cea’ex (France)
~RF/~Ph-PCM,
C. E.N.G.. Avenue des Martyrs,
(Received 24 May 1990; in revised form 20 July 1990)
Abstract
The new and very promising kind of electrode modified by hydrophobic Nafion gels, the general type of which has been described in Part 1 of this series, has been loaded with an~raquinone and 9-phenyl acridinium salts and tested for activity towards the catalysed reduction of oxygen. The gels loaded with acridinium species were found to exhibit a fair catalytic activity, the rate of which was enhanced when compared with the solution values, due to the affinity of the ~rfluoronated chains for oxygen.
INTRODUCTION
In a previous communication [l] and Part 1 of this series [Z], we demonstrated that Nafion @ gel coatings are of special interest because they ally the chemical stability of this perfluorinated polymer to the unique property of behaving macroscopically like solids and microscopically like liquids, and thus allow us to prepare a wide range of modified electrodes by loading with various electroactive compounds such as ferrocenes. However, another interesting characteristic of Nafion is its affinity towards dioxygen solvation, which makes Nafion-coats electrodes suitable for 0, reduction catalysis. We wish to reported here further electrochemical results 0022-0728/90/$03.50
0 1990 - Etsevier Sequoia S.A.
130
& 0
000
lj
I-
Ii
Fig. 1. Formula of the 9-ph~yl-lo-~yl-ac~di~um
salts. R = CH, or C,H,.
on microelectrodes with gels loaded with anthraquinone or 9-phenyl-lo-alkyl acridinium iodides, both of which were chosen for their ability to catalyse under various conditions the reduction of oxygen. In addition, 9-phenyl-lo-alkyl acridinium cations possess the interesting peculiarity of providing upon oxidation some of the rare neutral radicals which are moderately stable in slightly to strongly acidic conditions 131, which make them good candidates for our study, if one considers the relatively strong ambient acidity of the H+ exchanged Nafion gel [4]. In this paper we show that the electrochemical study performed by cyclic voltammetry and chronoamperometry confirms the electroactivity of the loaded gels in these cases with very fast diffusion of the electr~he~cal catalysts; that is, electrons are transferred to dioxygen by an outer-sphere mechanism [5]. Preliminary attractive results had already been obtained [6] on the basis of similarities to the previously studied viologen [7]. Therefore, this property of the acridinium radical was investigated in the case of both classical TBP solutions and loaded gels (Fig. 1). While oxygen diffuses from the supernatant electrolyte into the gel where it can be reduced normally by an el~tr~he~cally irreversible process at potentials well below 1 V, its reduction wave disappears when the gel is loaded with 9-phenyl-lomethyl-acridinium. The similar dependence of the acridinium wave provides evidence of the occurrence of a catalytic process. The kinetics of the catalysis have been explored and will be presented here comparatively, both in pure homogeneous conditions (in the TBP + HClO, solution) and in heterogeneous conditions, with the help of a Nafion gel-coated electrode.
EXPERIMENTAL
Ant~aquinone was purchased (Fluka) and used without further p~fication. Propyl and methyl acridinium were prepared and tested. 9-Phenyl-lO-methylacridinium was prepared by heating 9-phenylacridine in an autoclave [8] at 110 o C with a large excess of methyl iodide (dark red needles filtered off the remaining methyl iodide, yield SOW). 9-Phenyl-lo-propyl-acridinium was prepared in low yields (dark red needles, 5-10%) by simply refluxing 9-phenylac~dine in excess propyl iodide for 12 h. As described in Part 1 of this series [2], a typical stiff gel was made by carefully mixing together lo-20 mg of cryocrushed Nafion powder (prepared according to
131
ref. 9), 3 to 5 times more tributylphosphate (TBP) *, and l-2 mg of the electroactive compound. Acetonitrile (about 0.5 ml) was added to homogenize and fluidize the mixture into a processable solution. A few microlitres of this solution were then micropipetted onto a 3 mm vitrous carbon microdisk. The acetonitrile was allowed to evaporate (20 min) to leave a stiff gel containing only the polymer, the heavy hydrophobic TBP and the electroactive compound. After the gel had dried, the modified electrode was simply transferred to a classical two-compartment cell filled with 0.3 M HClO, aqueous electrolyte, a sodium saturated calomel electrode and a platinum counter-electrode which was connected to an electrochemical device. For the experiments performed in TBP, an acetonitrile + HClO, bridge was used between the reference electrode and the solution. Except for the catalytic studies, all solutions were carefully degassed with nitrogen prior to the experiments. The potentiostat was a PAR 273 for large cell experiments and a home-built one for those with microelectrodes, fitted with a Sefram Plotter and a Nicolet digital oscilloscope for short time experiments. RESULTS
AND DISCUSSION
As shown previously, the main advantage of our system is that a hydrophobic solvent which is polar enough to prepare ionically conducting media is used to prepare the gel so that a layer of controlled thickness of gel can be used. The rest of the cell is filled with an aqueous electrolyte, which does not participate in the electrochemical reaction, which occurs solely inside the gel, and accordingly may contain a substrate which can pass in and out of the gel and undergo a catalytic reaction inside it. Electrochemical
behaviour of the loaded gels
Cyclic voltammetiy Figure 2 shows the voltammograms of 9-phenyl-lo-propyl-acridinium iodide in solution in DMF (a) and of 9-phenyl-lo-methyl-acridinium iodide in TBP (b) in the presence of perchloric acid. Both compounds exhibit typical reversible behaviour with a fast one-electron transfer at low and medium sweep rates. This result was previously reported in the case of 9-phenyl-lo-methyl-acridinium iodide in acetonitrile [3], and was explained by the formation and subsequent reoxidation of the corresponding neutral radical. Figure 3 shows the cyclic voltammograms obtained at different scan rates on a microelectrode of Nafion gels loaded with the same compounds. The same electrochemical behaviour is found both in solution and in the gels, showing no major perturbation induced by the Nafion itself. Since, in addition, except for the 50 mV difference between the redox potentials, there were no appreciable differences in the electrochemical reactivity between 9-phenyl-lo-
l
The preparation of the Nafion gels has been described in detail in Part 1 of this series [2].
132
Fig. 2. Cyclic voltammograms of (a) 9-phenyl-lo-propyl-acridinium iodide in dimethyl formamide with 0.1 M perchloric acid and (b) 9-phenyl-lO-methyl-actidinium iodide in TBP with the same electrolyte. The substrate concentration was 2 x 10m3 M and the scan rate 100 mV/s in both cases.
0
@
I
lOO/.tA
6-
~OSmA
E/V
E/V I
-0.5
I
0
I
-0.5
I
0
Fig. 3. Cyclic voltammograms of TBP+Nafion gels loaded with 0.5 M 9-phenyl-lo-propyl-acridinium iodide (a) and 0.5 M 9-phenyl-lo-methylacridinium iodide (b, c). The electrolyte above was 0.1 M aqueous perchloric acid. Scan rate for curves a and b: 5 V/s; curve c, from top to bottom: 100, 50, 20,lO and 5 V/s.
133 TABLE
1
Electrochemical data of the electroactive compounds studied Compound AcMe+ AcPr+ Anthraquinone Ferrocene
E" (TBP)/ V
E” (gel)/ V
lo6 D (gel)/ cm’ s-l
-0.55 - 0.50 0.02
-0.57 - 0.50 - 0.03
1 0.2 a
0.33
0.27
2
a The plots obtained did not show enough linearity.
propyl-acridinium iodide and 9-phenyl-lo-methyl-acridinium iodide in the gels, we decided to perform most of the work presented here, and especially the catalysis experiments, only with the second compound, which will simply be designated by “acridinium” in the following. It is clear that typical diffusion behaviour with rapid electron transfer is always obtained in the gels between 100 and 1 V/s with well-defined reversible peaks. However, this study was started with heavily loaded gels (above lo-’ M in substrate), and we found that the reoxidation peak diminishes and finally disappears when the scan rate falls to values below 1 V/s, showing that it is likely that the acridinium radical has unexpected reactivity in this case. Yet parallel investigations on the behaviour of the acridinium simply dissolved in TBP at classical concentrations of organic electrochemistry (2 X 10e3 M with perchloric acid as the electrolyte) showed ideal behaviour; therefore, its concentration in the gel was diminished to come closer to classical solution conditions. It could be noticed then (Fig. 7) that reversible behaviour of the acridinium occurs up to scan rates as slow as 0.1 V/s, and that at such concentrations similar electrochemical behaviour is observed both in the gel and in the TBP + HClO, medium. These results enable us to confirm that there is no perturbation due to the Nafion itself in the acridinium system and that the behaviour observed at high concentrations is due to the unexpected, but avoidable reactivity of the acridinium radical on itself or on the starting acridinium. Table 1 summarizes the electrochemical data on the electroactive compounds studied in addition to the results for ferrocene, which were obtained in the first part of the series. In the case of anthraquinone-loaded gels, the classical two-electron voltammogram can be observed, but in this case it is associated, as expected, with a much slower electron transfer. In Fig. 4 are represented the cyclic voltammograms at various scan rates obtained upon cycling an anthraquinone-loaded gel. It is apparent that the difference between the peak potentials AEr, equal to 150 mV at 50 mV/s, still increases as the scan rate increases further, featuring a typical very slow electrochemical couple. The apparent redox potential of the quinone system is -0.03 V, quite a positive value but explainable by the acidic surroundings of the Nafion gel. This value may be used to estimate the ambient pH of such a medium after a reasonable evaluation of the redox potential of anthraquinone in acidic TBP electrolytes. In order to evaluate this, a series of measurements of the anthraquinone
Fig. 4. Cyclic voltammogram of a Nafion gel loaded with anthraquinone at different scan rates; from top to bottom: 1, 0.5, 0.2,O.l and 0.05 V/s. Same electrolyte as for Figs. 1 and 2.
potential was performed in TBP in the presence of various concentrations of perchloric acid. It was found that the variation of the redox potential followed, as would be expected, the relationship E = E * - 0.06pH (taking pH =i -log(H+), and between 0.02 and 2 M HClO, to avoid the addition of another electrolyte salt), with E" = 0.03 V. Keeping the previously given average value of 0.05, V determined in the case of the ferroeenes for the difference between the redox potential of a given electroactive species in the gel compared with a TBP + HCIO, classical electrolyte, one finds E = - 0.02, - 0.06 pH for the apparent reduction potential of anthraquinone in a TBP -I- Nafion gel: from the value found for E in our gels (see Table 1) a pH of almost 0 is found which is in accordance with the very acidic character of an H+ exchanged gel f8].
Kinetic measurements Chronoamperometry was performed to extract the diffusion coefficients for the two acridiniums under study. Typical Cottrell plots are shown in Fig. 5, the linearity of which ~nfirms our earlier conclusion [1,2] about the stations by effusion just as in solution. The diffusion coefficients given in Table 1 are again in the range found in solution unlike those of classical Nafion-coated electrodes which are 103-lo4 lower, due to the strong electrostatic bonding between the electroactive
135
2
10
20
30
40
Fig. 5. Cottrell plots obtained from Nafion gels loaded with (a) 0.5 M 9-pbenyl-lo-methyl-acridinium iodide and (b) 0.3 M 9-phenyl-lo-propyl-acridinium iodide. Same conditions as before.
sites and the polymeric skeleton. This result is even more striking in this case, since our former studies treated Nafion gels loaded mostly with neutral molecules like ferroeene, while acridinium is a positively charged species which could be bound to the sulphonic group of the Nafion gel. As expected, there is a correlation between the steric hindrance of the electroactive molecules in the gel and their respective diffusion coefficients. Taking into account that this fast diffusion was also noticed with two positively charged ferrocenes, it can be concluded that in such gels even cationic species can move, if not completely freely, at least without strong interactions with the polymer network.
The stability of the gels was tested by repetitive cycling at 1 V/s in the case of acridinium and at 100 mV/s in the case of anthraquinone. In all cases, no more than a 10% decrease was observed in 3 h showing that very little desorption occurred, which is not unexpected with such hydrophobic compounds. In fact, we found that instead of releasing them, TBP + Nafion gel rather tends to concentrate organophilics [lo], so that the slight diminution of the signal observed in some cases should more properly be attributed to chemical evolution of the reactants rather than to their transfer into the aqueous solution. Catalysis
As stated in the Introduction, this study aimed mainly at the recognization and investigation of the catalytic properties of such coatings towards dioxygen reduc-
136
a
E/v b
E/V I
-1.5
1
-1
I
-a5
0
Fig. 6. Cyclic voltammograms registered on (a) a glassy carbon electrode in TBP+HClO, electrolyte under nitrogen and air saturated at 0.1 V/s and (b) a gel-coated electrode in 0.1 M aqueous perchloric acid under the same conditions.
tion. Figure 6 shows the comparative voltammograms, under nitrogen and airsaturated, obtained respectively on a naked glassy carbon electrode in TBP and on an electrode coated with an unloaded gel: that is, in the absence of any added electroactive substance. Just as in several previously described experiments, while no current was obtained under argon, a strongly irreversible wave was observed with air-saturated solutions corresponding to the two-electron reduction of the solvated oxygen, both in TBP and in the gel. The relative magnitude of the waves is almost the same, the preconcentration of the oxygen by the Nafion gel (compared with the solution) being co~terbalan~ probably by the slightly lower diffusion coefficient in this case. Turning to the behaviour of the loaded gels, we noticed that, unfortunately, anthraquinone-loaded gels appeared not to be affected by the presence of oxygen, the oxygen wave being too negative relative to the an~raquinone in this system at the ambient pH of the gel. We therefore turned our interest to the acridinium-loaded gels, and found that when the gel was cycled in the presence of oxygen (Fig. 7), the first cycle showed a clear improvement in the electrochemical signal, which in addition became totally irreversible behaviour, which is typical of a catalytic effect.
137
I
lop
@
A
L
I
I
0
I
40gA
1
to
20pA
I
Q-
EIV
El”
I
0
-0.5
Fig. 7. Catalytic properties of loaded Nafion gels. (a, b) Cyclic volt-ograms of two identical gels (prepared starting from the same solution) loaded with 3 x 1O-3 M 9-phenyl-lo-methyl-actidinium iodide at respectively 1 and 0.1 V/s under nitrogen, in a nitrogen-saturated electrolyte (as for Figs. l-4). (c, d) Same feature under air in an air-saturated electrolyte.
i
Q
I
-0.5
@
T
I
0
I
-0.5
I
0
Fig. 8. Homogeneous catalysis by 9-phenyl-lo-methyl-acridinium iodide in TBP. (a, b) Cyclic voltammograms obtained from 2 x 10K3 M 9-phenyl-lo-methyl-acridinium’iodide solution in TBP+ HClO, electrolyte saturated with nitrogen. (c, d) Same feature in the same electrolyte saturated with air.
138 TABLE 2 ip/ip” values at different scan rates
Scan rate/ VS-’
0.1 1 10 100
‘#p” Catalysis: homogen~us 1.94 1.32 B B
(TBP)
Catalysis: with the gel 5.26 3.85 2.76 1.18
a The peak shape was ill defined and prevented accurate measurement of its height.
Furthermore, no wave was observed at the potential of oxygen reduction, which is additional ~nfirmation of its previous reaction with the acridinium radical cation. The same phenomenon occurs similarly in the TBP solution on a naked glassy carbon electrode, as shown in Fig. 8. Therefore, two parallel studies of the catalysis in the gel (Fig. 6) and in a classical TBP + HClO, electrode (Fig. 8) were carried out. In the two series of experiments, the rate of the catalytic process could be estimated by analysis of the i,/i,” ratio at different scan speeds, i, and i; representing the current with and without the dioxygen substrate added, respectively [9]. It is clear from Table 2 that the catalysis, which is very weak at 100 V/s, becomes more and more important as the scan rate diminishes. It may also be noted that the catalytic effect is about two to three times greater when the catalyst is inside the gel rather than in solution in TBP. Comparison of the ip/i,” ratios when the catalysis is close to total, that is, at a scan rate of 100 mV/s, allows us to estimate by inference the partition of the oxygen in the gel, since nothing but the differences between the 0, concentrations can account for the discrepancies observed between the catalysis in the TBP solution and that in the gel. From the current values, it may thus be deduced that Oz is about 2.5 times more concentrated in a Nafion + TBP gel exposed to an air-saturated aqueous solution than in the air-saturated TBP + HClO, solution, therefore demonstrating once again the interest of the perfluoronated chains for oxygen solvation.
CONCLUSION
As demonstrated previously in the case of several substituted ferrocenes, we have shown again in this short study the closeIy ideal electrochemical behaviour of hydrophobic Nafion gels loaded with various electroactive compounds. In particular, it was observed that the fast kinetics typical of such systems are retained even when the electroactive species included a& cationic. The catalysis of the oxygen reduction by the acridinium species, demonstrated in the case of homogeneous conditions, was not only retained, but also enhanced in the gel system, most probably due to the known affinity of the perfluorated chains for oxygen salvation.
139
It may be believed that such systems could find applications in devices such as fuel cells, provided that they present the required stability required for such applications. REFERENCES 1 P. Audebert, P. Aldebert, B. Divisia-Blohom and J.M. Kern, J. Chem. Sot., Chem. Commun., (1989) 939. 2 C.P. Andrieux, P. Audebert, B. Divisia-Blohom, P. Aldebert and F. Michalak, J. Electroanal. Chem., 296 (1990) 117. 3 N.W. Koper, S.A. Jonker, J.W. Verhoeven and C. van Dick, Reck Trav. Chim. Pays-Bas, 104 (1985) 296. 4 P. Aldebert, personal communication, 1989. 5 J.-M. Savt+ant, Mechanisms and Reactivity in Organic Electrochemistry. Recent Advances. Advances in Electrochemistry, The Welsh Foundation, Houston, 1986, pp. 289-336. 6 P. Hapiot, unpublished results 7 C.P. Andrieux, P. Hapiot and J.-M. Savtant. J. Electroanal. Chem., 189 (1985) 121. 8 F.D. Popp, J. Org. Chem., 27 (1962) 2658. 9 C.P. Andrieux, C. Blocman, J.M. Dumas-Bouchiat, F. M’Halla and J.M. Sadant, J. Electroanal. Chem., 113 (1980) 19. 10 P. Audebert, B. Divisia-Blohom and F. Michalak, to be submitted.