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Boletín de la Sociedad Chilena de Química

Print version ISSN 0366-1644

Bol. Soc. Chil. Quím. vol.46 n.3 Concepción Sept. 2001

http://dx.doi.org/10.4067/S0366-16442001000300003 

"EFFECT OF THE CONDITIONS OF ELECTROPOLYMERIZATION
ON THE ELECTROCATALYTIC RESPONSE OF NON-METALLED-
POLY-TETRAAMINOPHTHALOCYANINE-MODIFIED ELECTRODES
TOWARD THE REDUCTION OF OXYGEN"

GALO RAMÍREZ1, EJNAR TROLLUND1, JUAN C. CANALES1, MARÍA J.
CANALES2, FRANCISCO ARMIJO1 AND MARÍA J. AGUIRRE1.

1Departamento de Química de los Materiales, Facultad de Química y Biología,
Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile.

2 Departamento de Ciencias Químicas, Facultad de Ingeniería, Ciencias y Administración,
Universidad de La Frontera, Casilla 54-D, Temuco, Chile.

(Received:March 8, 2001 - Accepted: May 4, 2001)

ABSTRACT

The macrocyclic ligand, tetraaminophthalocyanine, was electropolymerized on a glassy carbon electrode by continuously potentiodynamic scans, using two different solvents, dimethylsulfoxide and dimethylformamide. In the first case, an over-oxidation can takes place, which partially destroy the film and inhibits its electrocatalytic activity for the reduction of oxygen compared to the polymer grown in dimethylformamide. To avoid this over-oxidation, two possible ways can be used: to achieve the electropolymerization during a lower number of potentiodynamic scans or using a positive potential limit shifted to more negative potentials. If a narrow potential interval is used, the electrocatalytic behavior of the polymer is similar to that obtained in dimethylformamide. However, if a wide potential interval is used but only during few potentiodynamic scans, it is possible to obtain a polymer with a new redox signal that presents a very interesting electrocatalytic activity for the reduction of molecular oxygen.

KEY WORDS: Poly-tetraaminophthalocyanine, conducting polymers, electroreduction of molecular oxygen, dimethylsulfoxide, dimethylformamide.

RESUMEN

En este trabajo se realizó la electropolimerización por barrido continuo de potencial, de tetraaminoftalocianina sobre electrodos de carbón vítreo. Para esto, se utilizaron dos solventes, dimetilformamida y dimetilsulfóxido. En el caso del dimetilsulfóxido, se observa que a medida que transcurre la electropolimerización, empieza a ocurrir un proceso de sobre-oxidación que finalmente destruye al polímero, modificando e inhibiendo sus propiedades electrocatalíticas en la reducción de oxígeno, comparado con el polímero preparado en dimetilformamida. Para evitar esta sobre-oxidación, pueden utilizarse dos caminos: acortar el límite positivo de barrido de potencial, o bien, realizar la electropolimerización con un menor número de ciclos. En el primer caso, se logra evitar la destrucción parcial del polímero obteniéndose un electrodo de características electrocatalíticas similares al crecido en dimetilformamida. En el segundo caso, se obtiene un polímero que presenta una nueva señal redox y que es el de mayor electroactividad hacia la reducción de oxígeno.

PALABRAS CLAVES: Poli-tetraaminoftalocianina, polímeros conductores, electroreducción de oxígeno molecular, dimetilformamida, dimetilsulfóxido.

INTRODUCTION

In the last decade, a new kind of conducting polymer has been developed (1-16). It is made by transition complexes with macrocyclic ligands, that permit the electronic conductivity. The metal centers are used in electrocatalysis for many interesting reactions as the electroreduction of molecular oxygen (2, 5, 16). Generally, the complexes have a symmetry predominantly planar, in order to favor the interactions between the p cloud and the electrodic surface. These complexes need to have oxidizable substituent groups in the periphery of the ring to permit the electropolymerization (9). The way of polymerization growing is very different when the metal center is changed. For example, the polymerization of Co-tetraaminophthalocyanine is very easy and a large polymer is obtained with 100-potentiodynamic cycles. However, using the same conditions, the Fe-tetraaminophthalocyanine difficulty grows (4, 14-16). In some cases, the solubility of the complexes is different and solvents need to be changed in order to achieve the polymerization. The change in solvent involves changes in the other conditions of the polymerization. In this way, to interpret the differences in the behavior of the different polymerized complexes is difficult because the changes in the conditions of the polymerization can modify its structures. In this way, we have investigated the electropolymerization of the macrocyclic ligand of tetraaminophthalocyanine (H2PcTA) in dimethylsulfoxide (DMSO) and dimethylformamide (DMF) on glassy carbon electrodes using different conditions during the potentiodynamic scans. The polymer-modified electrodes have been used as electrocatalysts for the reduction of molecular oxygen. Although, its electrocatalytic activity is lower than the poly-iron-tetraaminophthalocyanine-modified electrodes, it is not negligible (16). We have found that the changes in solvent and in the conditions used during the electropolymerization modify the nature and the electrocatalytic properties of the polymers. In all the cases, the modified-electrodes obtained have a stable voltammetric response in aqueous pH 13 solution, at least during 100 cycles of repetitive scans.

EXPERIMENTAL

The ligand, (H2PcTA), was synthesized using a modified procedure reported by Achar et al. (17, 18). The glassy carbon-rotating electrode, from Radiometer (EDI 101), had a geometrical area of 0.049 cm2 and was polished with 0.25-mm alumina and washed in dimethylsulfoxide (DMSO) or dimethylformamide (DMF) in an ultrasonic bath during five minutes before each experiment. The electrosynthesis of the polymers (p-H2PcTA/DMSO and p-H2PcTA/DMF) on the glassy carbon surface was performed by continuously cycling the electrode potential between ­0.6 and +1.15 V or ­0.6 and +0.9V versus Ag/AgCl, at 200 mVs-1, for 100 or 20 potentiodynamic cycles. The electrolyte used in the electropolymerization consisted of a 0.1M-tetrabutylammonium perchlorate-DMSO or 0.1M-tetrabutylammonium perchlorate-DMF solution containing the free ligand (10-3 M). The electrolyte was purged with nitrogen and kept at room temperature. After the polymerization, the modified electrodes were rinsed with DMSO or DMF, ethanol and deionized twice-distilled water. Solvents and electrolytes were analytical grade. The electrochemical experiments were performed in a three-compartment glass cell, one for each of the electrodes: the working electrode connected to a speed control unit (Radiometer, CTV 101), the reference electrode, saturated Ag/AgCl, and the counter electrode, a Pt coil (0.14 cm2). The electrolyte used for the aqueous solutions was 0.1M NaOH (pH 13). For the oxygen reduction, O2 (ultra pure, AGA) was bubbled before and during the measurement. An AFCBP1 Pine bipotentiostat, along with PineChem 2.5 software was used for the electropolymerization and to obtain data for the electroreduction of molecular oxygen.

RESULTS AND DISCUSSION

Figure 1 shows the structure of a tetraaminophthalocyanine ligand, where the four amino groups could be located in different positions of the terminal benzenes of the ring. Probably, there is a random distribution of isomers with the 4,4',4'',4'''-tetraaminophthalocyanine being the main (17, 18). These amino groups are the responsible of the electropolymerization of the molecule, due to its irreversible anodic oxidation that takes place during the potentiodynamically scan of an electrode in a solution containing the molecule. Although this electropolymerization resembles that of poly-aniline, in our case probably the amino radical cation formed attacks an orto position (respect an amino group) of a neutral molecule, as informed by Li and Guarr (9). The tetraaminophthalocyanine belongs to D4h symmetry (19) only when a transition metal is located in the central cavity. In the case of a non-metalled molecule, there are two hydrogen atoms in this position that distorts the planarity. However, the molecule has an extensive p _system that could permit to obtain an electroconductive polymer even in absence of a central metal (16, 20).)


Fig.1. Structure of a tetraaminophthalocyanine ligand.

In previous work (14, 15, 16, 20, 21), we have investigated the electrocatalytic behaviour of poly-metal-etraaminophthalocyanine toward the electrooxidation of hydrazine (14, 20) and 2-mercaptoethaol (15, 21) and the reduction of oxygen (16, 22). In these cases, we also have found a little (14, 15, 20, 21) or interesting (21) activity of the polymerized ligand. However, the activity of the polymerized ligand strongly depends on the conditions of electropolymerization (16). For that reason, in this work we have prepared the free base, tetraaminophthalocyanine-modified electrodes with different solvents, DMSO and DMF and different potential limits in order to compare these modified electrodes as electrocatalysts for the reduction of oxygen. Figure 2 shows the voltammetric responses of the 100-potentiodynamic cycles of electropolymerized p-H2PcTA/DMSO and p-H2PcTA/DMF. The behavior resembles each other, but the potential of the cathodic peak is different in both solvents. However, the graph of current, I, versus time, t, corresponding to these voltammetric profiles shows a very different behavior (see Figure 3). In the case of DMSO, in the beginning of the polymerization, an increase in charge can be observed but the charge at the end of the polymerization, shows a very strong decrease. In the case of the DMF, the same increase in charge can be observed at the beginning of the polymerization, but instead of a decrease at the end, the charge stays almost constant after the first growing. The observed difference probably is due to the higher capacity of the DMSO to give electronic density to the macrocycle (22). Then, the more "negative" ligand is easier to be oxidized and then, after a few cycles of electrosynthesis, a loss of charge takes place due to an over-oxidation phenomenon. The second cycle of polymerization in both solvents (see Figure 4) depicts this behavior as shown in Figure 3. In this Figure it is clear that the redox oxidation processes in DMSO requires less potential than in DMF and the reduction waves appear at more negative potentials. Both polymers show a very different voltammetric response in aqueous solution. Figure 5 depicts these profiles. The p-H2PcTA/DMSO does not show redox peaks but only ill-defined shoulders. Its charge, compared with the glassy carbon electrode, is higher but in the same order of magnitude. On the other hand, the p-H2PcTA/DMF presents two clear anodic processes and a big cathodic peak. Its charge is two orders of magnitude higher than the bare glassy carbon. The couple in DMF is not very reversible probably because the oxidation takes place on a neutral polymer (without OH- groups) but after the loss of the electrons, the hydroxyl groups interact with the positive centers of the polymer. Then, the reduction process takes place on a different system. On the other hand, the two anodic peaks can be explained in terms of different spatial conformations of the redox sites that, after a hydroxylation, become similar. In order tohinder the over-oxidation that occurs when the polymerization is carried out in DMSO, a polymer was obtained with the same solvent and conditions, but using a lower positive potential limit. Figure 6 shows the voltammetric response of this polymer in aqueous solution at pH 13. When the electropolymerization takes place between ­0.6 and + 0.9V, the over-oxidation is avoided. The profile I versus t (not shown) does not show the loss in charge at the end of the 100-potentiodynamic cycles but resembles the behavior of the polymerization in DMF. Also, the profile in aqueous solution shows a better defined redox signals and lower resistivity. If the polymerization in DMSO is carried out only during the first 20 cycles of the potentiodynamic scans, in spite of using a potential limit of +1.15V as in the first case, a very defined voltammetric response in aqueous solution is obtained. Figure 7 shows this profile, with a strong, defined and reversible redox couple is obtained and a new little cathodic peak appears, close to the main. This new cathodic peak does not show a clear corresponding anodic signal and its difficult to assign it. However, it is very stable and does not disappear when repetitive scans are carried out. This profile, with the little cathodic peak is not obtained in DMF, neither when a lower number of potentiodynamic cycles are used nor when a lower potential limit is used during the electropolymerization. This little cathodic peak could be attributed to the response of a different structural isomer (obtained under these polymerization conditions), which depicts its anodic signal hidden by the main anodic signal. On the other hand, the response of the modified electrodes with different kinds of poly-tetraaminophthalocyanine for the electroreduction of oxygen is very different. Figure 8 depicts the voltammetric response of all the polymers to the reduction of molecular oxygen. The charge is higher in DMF but it is due to its own charge and the increasing in current is not as high as in the case of DMSO. The first reduction peak (at more positive potentials) appears in the same potential for the polymer grown in DMF and the polymers obtained in DMSO with short limits or 20 cycles (at ca. ­0.4V). This peak appears at more negative potential for the 100 cycles-polymer grown in DMSO, with large limits (at ca. ­0.55V). On the other hand, only one system shows a second process for the reduction of molecular oxygen in the potential limits studied. That is the case of the 20 cycles-polymer, grown in DMSO. This anomalous result could be interpreted in terms of two different redox sites in this polymer, due to different morphology obtained during the electropolymerization. The molecule, tetraaminophthalocyanine, when synthesized, is principally obtained as a 4,4',4'', 4'''-tetraaminocomplex, because it is the most stable of the isomers (18, 19). Then, we suggest two ways (with different lifetimes) of radical attack during the electropolimerization: an attack of the radical amino to the carbon two or to the carbon three in the benzene ring. If these two attacks occur, then two oligomers can be formed, with different morphology and properties. Probably, the attack to the carbon two is slower or hindered by steric factors. The solvent, DMSO, which is able to stabilize positive charge (22), could permit that it takes place. Then, a 20 cycles-DMSO polymer could be formed by two kinds of oligomers. The less stable oligomer should be dissolved when an over-oxidation takes place during the electropolymerization. Due to the current obtained, the best catalysts is the polymer grown in DMSO with 20-cycles of potentiodynamic scans. In all cases, the increasing in current (compared with the voltammetric profile obtained in the presence of nitrogen) is higher for the DMSO-polymers and the 20-cycles polymer shows two different active sites with electroactivity toward the reduction of molecular oxygen. The fairly modified electrode is that obtained in DMSO with a positive potential limit of +1.15V. It depicts only one cathodic signal at very negative potentials. It is not easy to find an explanation of these facts. However, some considerations can be done. The loss in charge obtained when the polymerization takes place in DMSO between ­0.6 and +1.15V probably is due to the higher sigma donor capacity of this solvent that interacts with the macrocycle giving electronic density to the p cloud. The higher negative charge in this case permits that the oxidation occurs at lower potentials than in the case of DMF. But, as occurs in polyaniline (23), when the first oligomers are formed, they need lower potentials to be oxidized compared to the monomers. Then, the polymer begins to be over-oxidized. This over-oxidation could be responsible of the formation of species positively charged that can diffuse to the solution. The polymer is partially destroyed and a loss in charge is obtained (see Figure 3, at the end of the polymerization, that shows a lower charge than in the beginning of the polymerization). This over-oxidation can be avoided if a lower positive potential limit or if a lower number of potentiodynamic scans are used during the electropolymerization. The appearance of the little cathodic peak in the case of the polymer obtained in DMSO with 20-cycles of polymerization is difficult to explain. But, it is possible that, in a similar way as occurs with the polymerization of polyaniline, when the adsorbed monomer is oxidized, it diffuses to the solution and reacts with a neutral molecule. When the oligomer is sufficiently massive, it falls on the electrodic surface. This process is favored in the case of DMSO than in the case of DMF, because DMSO has a higher capacity to stabilize positive charges. In this case, other interactions, different than the "orto" attack can be considered. These interactions, as the attack to a "meso" carbon or the formation of a di-azo bridge, are slower than the "orto" attack, but the stability conferred by the DMSO, can favored them. The little cathodic peak, then could corresponds to a structure not favored, formed by the attack of the radical cation to a neutral molecule in a site different than the "orto" position. It is very difficult to proof this assumption because the structural analysis of the adsorbed polymers is not easy to do. It is possible that the spectroscopy of electrochemical impedance can give some information about the number and nature of the redox sites. On the other hand, we do not have explanations for the absence of clear anodic signals corresponding to the cathodic peaks. The anodic charge is lower than the cathodic charge and the cathodic peaks are very noticeable. They do not correspond to the response of the polymer to traces of oxygen because they are very stable and reproducible in aqueous solution bubbled with pure nitrogen. Contrarily, in the presence of oxygen, the cathodic peaks decrease with the repetitive cycles.



Fig.2. Cyclic voltammograms corresponding to 100 cycles of electropolymerization from a purged solution containing 1mM of monomer and 0.1M TBAP. Scan rate: 200 mVs-1. Potential limits: -0.6 and +1.15V versus Ag/AgCl. A) DMSO, B) DMF. The arrows indicate the sense of the scan and the decreasing in the current of the peaks as the polymerization takes place.



Fig.3. Graphs I versus time corresponding to the polymerization shown in Figure 2, up to an extent of 60 % of the process. A) DMSO, B) DMF


Fig.4. Second voltammetric scan corresponding to the 100-cycles polymerization shown in Figure 2. Continuous line: DMSO and dashed line: DMF.



Fig.5. Voltammetric response in aqueous purged solution at pH 13 of the two polymers obtained in Figure 2. Scan rate: 100 mVs-1. Potential limits: -0.9 and +0.2V versus Ag/AgCl. A) p-H2PcTA/DMSO, B) p-H2PcTA/DMF. In each case GC represent the response of the bare glassy carbon.


Fig.6. Voltammetric response in aqueous purged solution at pH 13 of the p-H2PcTA/DMSO obtained by cycling the electrode between ­0.6 and +0.9V (continuous line) compared with that corresponding to the polymer grown between ­0.6 and +1.15V (open squares). Scan rate: 100 mVs-1. Potential limits: -0.9 and +0.2V versus Ag/AgCl.


Fig. 7. Voltammetric response in aqueous purged solution at pH 13 of the p-H2PcTA/DMSO obtained by cycling the electrode between ­0.6 and +1.15V during 20 potentiodynamic cycles (continuous line) compared with the 100 cycles-polymer (open squares). Scan rate: 100 mVs-1. Potential limits: -0.9 and +0.2V versus Ag/AgCl.

Fig. 8. Voltammetric response in aqueous oxygenated solution at pH 13 of
1: p-H2PcTA/DMF (100 cycles, large limits of potential),
2: p-H2PcTA/DMSO (100 cycles, large limits of potential):
3: p-H2PcTA/DMSO (20 cycles, large limits of potential) and
4: p-H2PcTA/DMSO (100 cycles, short limits of potential).
Scan rate: 100 mVs-1. Potential limits: -0.9 and +0.2V versus Ag/AgCl.

CONCLUSIONS

The conditions used during the electropolymerization, specially the solvents, are the key for obtaining different structures and hence, different behavior when the macrocycles are used as electrocatalysts. Appropriate solvents can stabilize the radical cation formation that takes place when the monomer is oxidized and it is possible to obtain structures less favored. These structures can give origin to new redox couples with interesting electrocatalytic properties.

ACKNOWLEDGEMENTS

This work has been supported by Fondecyt Project 1980837 and Dicyt-Usach. F.A. grateful to a Fondecyt doctoral scholarship.

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